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In order to investigate the structure-specific bioactivity of Cd(II) in a cellular environment and delve into the associated biomolecular interactions, binary systems of that metal ion in the presence of the physiological naturally-occurring α-hydroxycarboxylic acid quinic acid were examined synthetically in a pH-specific mode. Two discrete polymeric compounds [Cd(C7H11O6)2]n·n/2H2O (1) and [Cd(C7H11O6)2]n·nH2O (2) were isolated and characterized through elemental analysis, FT-IR, NMR, TGA, and X-ray crystallography. Despite their discrete solid state metal-binding ligand coordination modes and lattice architecture, ESI-MS studies on 1 and 2 showed that both give rise to the same species upon dissolution in water. In vitro biological investigation of the toxicity profile of 2 in two cell lines, 3 T3-L1 and Saos-2, point out the influence of the coordination environment of Cd(II) on cell survival, migration and cellular morphology. The in vitro cytotoxic work showed a concentration- and tissue-specific pattern for the Cd-quinate system. Finally, the cytoprotective role of the chelator agent EDTA was investigated in these cell lines at various concentrations of 2, thereby providing insight into Cd(II) interactions with cellular targets compromising cellular integrity. The collective results, signify the a) importance of the aqueous chemistry of Cd(II) with biologically significant substrates, leading to discrete and isolable architectures reflecting binary Cd(II)-quinate composition, b) formulation of the cytotoxicity profile, based on well-defined Cd(II) species, and c) selection and use of appropriately configured organic chelators into future research linked to the development of preventive and/or therapeutic approaches pertaining to Cd(II) (de)toxification processes.

Among transition and non-transition metals, thallium is a unique case of an element which, despite its known toxicity, provides interesting challenges through its biology and chemistry linked to diagnosis of human pathophysiologies. Poised to investigate in-depth the structural and electronic aspects of thallium involvement in physiological processes, the synthetic exploration of aqueous binary systems of Tl(I) with physiological binders from the family of hydroxycarboxylic acids (glycolic, lactic, mandelic and citric acid) was pursued in a pH-specific fashion. The isolated crystalline coordination polymers, emerging from that effort, were physicochemically characterized through elemental analysis, FT-IR, ESI-MS, 1H-/13C-NMR, and X-ray crystallography. The coordination environment of thallium in each molecular Tl(I) assembly, along with lattice dimensionality (2D 3D), reflects the contributions of the ligands, collectively exemplifying interactions probed into though BVS and Hirshfeld surface analysis. The results portray a well-defined solid-state and solution profile for all species investigated, thereby providing the basis for their subsequent selection into in vitro biological studies involving the (patho)physiological cell lines 3T3-L1, Saos-2, C2C12, and MCF-7. Biotoxicity profiles, encompassing cell viability, morphology, and cell growth support clearly a concentration-, time-, and cell tissue-specific behavior for the chosen Tl(I) compounds in a structure-specific fashion. Collectively, the chemical experimental data support the biological results in formulating a structure-specific behavior for Tl(I)-hydroxycarboxylato species with respect to biotoxicity mechanisms in a (patho)physiological environment. The accrued knowledge stands as the foreground for further investigation into the relevant biological chemistry of Tl(I) and molecular technologies targeting its sequestration and removal from cellular media.

Poised to investigate the association of vanadium with physiological substrates in materials capable of exerting anticancer biological activity, pH-specific synthetic chemical reactivity between vanadium and triethyl ammonium acetate/trigonelline in aqueous and mixed organic-aqueous media led to the isolation of three new binary composite materials, namely K2[(MeN(+)C5H4COOH)2][V10O28H2]·2H2O (1), [(H2N(+)Me2)4][V10O28H2][Me3N(+)CH2COO(–)]2 (2), and [(Me3N(+)CH2COOH)4][V10O28H2][Me3N(+)CH2COO(–)]2·2H2O (3). 1–3 were characterized by elemental analysis, FT-IR and UV/Vis spectroscopy, Cyclic voltammetry, TGA-DTG, and X-ray crystallography. In all three compounds, the [V10O28H2]4– core unit is a common cluster assembly, with the vanadium in the +5 oxidation state. The counteracting cationic assembly in each composite material originates in the betaine starting reagent or the solvent out of which the materials crystallized. Biological activity studies of 1 and 3 in vitro in MCF-7 breast epithelial and A549 lung adenocarcinoma cell cultures show that both materials inhibit the viability of both cell lines in a dose-dependent fashion, in juxtaposition to the behavior of the betaine components present in these materials. Collectively, the herein studies a) reveal the uniquely defined synthetic methodologies and physicochemical properties of the variably assembled [V10O28H2]4– core units crystallized into the composite binary polyoxovanadate-betaine lattice structures, b) unravel the distinct cytotoxicity profile of the composite materials toward MCF-7 and A549 cells, and c) attest to their future potential as metallodrugs of pharmacological significance in anticancer activity.

Poised to understand the influence of O- and S-heteroatoms on the chemical reactivity of dicarboxylic acids toward Pb(II), leading to crystalline metal–organic hybrid materials with distinct lattice architecture, dimensionality, and spectroscopic properties, the synthesis and physicochemical properties of binary/ternary Pb(II)–(O,S)-dicarboxylic acid–(phenanthroline) systems was investigated in aqueous media. pH-specific hydrothermal reactions of Pb(II) with O- and S-dicarboxylic acid ligands and phenanthroline (phen) afforded the variable dimensionality metal–organic Pb(II) polymers [Pb3(oda)3]n (1), [Pb(phen)(oda)]n (2), [Pb(tda)]n (3), and [Pb(phen)(tda)]n (4). The choice of O- vs S-ligands in the aqueous systems of Pb(II) and phenanthroline is linked to the emergence of distinct lattice composition–dimensionality (2D–3D) changes at the binary and ternary level, bestowing spectroscopic fingerprint identity to Pb(II) coordination and luminescence activity.

Chromium is widespread in the environment and is used in ample industrial applications in abiotic and biological systems. Its ability to influence biological processes through interactions with biomolecules underscores its positive role as well as its toxic manifestations in higher organisms. In an attempt to understand its (bio)chemistry, research efforts were undertaken to explore the aqueous chemistry of Cr(III) with the low molecular mass physiological ligand, quinic acid. pH-Specific synthesis in the binary Cr(III)–quinato system led to the isolation of Na[Cr3O(C7H11O6)6(H2O)3]·(NO3)1.5·(OH)0.5·6H2O (1). 1 crystallizes in the monoclinic space group C2, with a = 32.16(1) A, b = 14.076(7) A, c = 15.083(5) A, β = 95.53(1)°, V = 6796(5) A3, and Z = 4. The structure of 1 reveals a trinuclear assembly of Cr(III) with quinate. The assembly consists of a [Cr3O] core containing a triply bound oxido O2− ligand holding the three Cr(III) ions together and six coordinated quinato ligands linking contiguously the three Cr(III) centers in a peripheral cyclic fashion. Each ligand binds two abutting Cr(III) metal ions through the carboxylato oxygens. The alcoholic groups of the quinates are not deprotonated and do not coordinate to the Cr(III) ions. The collective binding of Cr(III) with quinates in a 1:2 ratio leads to a distorted octahedral geometry around each metal center, with the sixth coordination site occupied by a water ligand. 1 was characterized by elemental analysis, spectroscopic (UV–Vis, FT-IR, ESI-MS, EPR), structural, thermal, cyclic voltammetric, and magnetic susceptibility studies. The collective structural and spectroscopic techniques point out the distinct properties of 1 in the solid state and in solution, thus exemplifying the physicochemical profile of that compound in the overall structural speciation scheme of the binary Cr(III)–quinato system. Solid state-solution structural correlation suggests retention of the trinuclear core complex of 1 in solution, also supported by ESI-MS, thereby lending credence to the detailed aqueous speciation studies in the binary Cr(III)–quinato system investigated in the employed pH range. The unique behavior of 1 in comparison to other known trinuclear [Cr3O]-containing compounds (a) signifies the importance of binary interactions of Cr(III) with hydroxycarboxylic acids, and (b) necessitates further perusal of analogous binary systems of Cr(III), that will enable understanding of the factors influencing the role of the specific metal ion in the development of (bio)chemical reactivity in biological systems and abiotic materials applications.

Hydrothermal pH-specific reactivity in the binary/ternary systems of Pb(II) with the carboxylic acids N-hydroxyethyl-iminodiacetic acid (Heida), 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid (Dpot), and 1,10-phenanthroline (Phen) afforded the new well-defined crystalline compounds [Pb(Heida)](n)·nH(2)O(1), [Pb(Phen)(Heida)]·4H(2)O(2), and [Pb(3)(NO(3))(Dpot)](n)(3). All compounds were characterized by elemental analysis, FT-IR, solution or/and solid-state NMR, and single-crystal X-ray diffraction. The structures in 1-2 reveal the presence of a Pb(II) center coordinated to one Heida ligand, with 1 exhibiting a two-dimensional (2D) lattice extending to a three-dimensional (3D) one through H-bonding interactions. The concurrent aqueous speciation study of the binary Pb(II)-Heida system projects species complementing the synthetic efforts, thereby lending credence to a global structural speciation strategy in investigating binary/ternary Pb(II)-Heida/Phen systems. The involvement of Phen in 2 projects the significance of nature and reactivity potential of N-aromatic chelators, disrupting the binary lattice in 1 and influencing the nature of the ultimately arising ternary 3D lattice. 3 is a ternary coordination polymer, where Pb(II)-Dpot coordination leads to a 2D metal-organic-framework material with unique architecture. The collective physicochemical properties of 1-3 formulate the salient features of variable dimensionality metal-organic-framework lattices in binary/ternary Pb(II)-(hydroxy-carboxylate) structures, based on which new Pb(II) materials with distinct architecture and spectroscopic signature can be rationally designed and pursued synthetically.

Cu(II) is a metal ion, the aqueous chemistry of which with carboxylic acids draws intense interest, targeting new materials and exemplifying diverse and unique structure–reactivity correlations. Driven by the need to explore the interplay of the chemical interactions of Cu(II) with polycarboxylic acid substrates and the association of such reactivity with lattice architecture and physicochemical properties, binary and ternary systems of Cu(II) with 1,2,3,4-cyclobutane-tetracarboxylic acid (H4CBTC) and bipy (2,2′-bipyridine) were investigated. To this end, aqueous synthetic reactions of Cu(II) with H4CBTC, under pH-specific conditions (pH 3), led to the isolation of the first species in the aforementioned binary system [Cu2(CBTC)(H2O)4)]n·2nH2O (1). Aqueous synthetic chemical reactivity in the ternary Cu(II)–H4CBTC–bipy system led to the isolation of the 1D polymer [Cu(NO3)2(bipy)]n (2). Complexes 1 and 2 were characterized by elemental analysis, spectroscopic techniques (EPR, FT-IR, UV–Vis and luminescence (2)), magnetic susceptibility, cyclic voltammetry (2) and thermogravimetric studies, and X-ray crystallography. The molecular lattice in 1 reveals the presence of Cu(II) units bound to (a) 1,2,3,4-cyclobutane-tetracarboxylate, and (b) water molecules, in a tetragonal pyramidal geometry, thereby projecting the unique chemical reactivity in the requisite system leading to a 3D lattice assembly. The presence of two types of channels in the solid state lattice of variable hydrophilicity/hydrophobicity signifies their unique nature in the coordination polymer and projects the importance of H2O and its H-bonding ability in the assembly of 1. The molecular lattice of 2 reveals the presence of Cu(II) ions bound to nitrate ions and 2,2′-bipy in an octahedral fashion, collectively leading to a 1D lattice assembly. The magnetic susceptibility and solid-state EPR data on 1 and 2 are consistent with the presence of Cu(II) in a tetragonal pyramidal and octahedral environment, respectively. Collectively, the physicochemical profiles of coordination polymers 1 and 2 earmark: (a) the influence of the polycarboxylic acid nature of the ligand on the chemical reactivity in binary and ternary systems, and (b) the critical nature of interactions in binary and ternary discrete Cu(II)–(O,N) species emerging in aqueous media and influencing the lattice assembly of Cu(II)–carboxylato materials of variable dimensionality (1D–3D), architecture and physicochemical properties.

Diverse vanadium biological activities entail complex interactions with physiological target ligands in aqueous media and constitute the crux of the undertaken investigation at the synthetic level. Facile aqueous redox reactions, as well as nonredox reactions, of V(III) and V(V) with physiological citric acid and hydrogen peroxide, under pH-specific conditions, led to the synthesis and isolation of a well-formed crystalline material upon the addition of ethanol as the precipitating solvent. Elemental analysis pointed to the molecular formulation (NH4)4[(VO2){VO(O2)}(C6H5O7)2]·1.5H2O (1). Complex 1 was further characterized by Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR), Raman spectroscopy, cyclic voltammetry, and X-ray crystallography. The crystallographic structure of 1 reveals the presence of the first dinuclear V(V)-citrate complex with non-peroxo- and peroxo-containing V(V) ions, concurrently present within the basic VV2O2 core. The nonperoxo unit VO2+ and the per...

Cobalt is an abundant metal ion present in the abiotic and biological world. The chemical reactivity of Co(II) is exemplified through complex interactions with variable molecular mass ligands, including amino acids, peptides, variable nature organic ligands, and/or phospho(nate)-derivatives thereof. Poised to gain insight into the chemical reactivity of Co(II) toward the family of mixed (carboxy)phosphonate-containing ligands, pH-specific aqueous reactions were carried out between Co(II) and N,N-bis(phosphonomethyl)-glycine (NTA2P), leading to a new pH-structural variant species (NH4)3[Co(C4H6O8NP2)(H2O)2]·4H2O (1) at pH 8. Compound 1 was characterized analytically, spectroscopically (FT-IR, UV–Vis, EPR), and magnetically. X-ray crystallography reveals a mononuclear complex of Co(II) in an NO5 octahedral environment. The solid state magnetic and EPR data on 1 suggest the presence of a high-spin Co(II) in a distorted octahedral geometry, with a ground state of an effective spin S = 1/2. The solution UV–Vis and EPR data suggest retention of the integrity of 1, consistent with the magnetization measurements. Detailed aqueous speciation studies on binary Co(II)–carboxylate (NTA) and all Co(II)–(carboxy)phosphonate (NTAxP; x = 1–3) systems reveal the aqueous distributions of all species involved in the respective systems and project a mononuclear species not unlike that of 1 in the Co(II)–NTA2P system. The structural and chemical attributes of the title complex reflect the (a) pH-dependent chemical reactivity in the binary Co(II)–NTA2P system, and (b) structure–activity correlations in the aqueous media linking both high and low pH-structural variants. To this end, fundamental structural properties influence the reactivity of Co(II) toward phosphonate and mixed carboxyphosphonate ligands and are ultimately exemplified as a function of phosphonate-containing moieties in NTA derivatives. The variably configured species in such binary Co(II)–ligand systems define the pH dependence and nature of interactions between the two reagents, and could serve further as precursors in the design and discovery of new Co(II)–organophosphonate materials of specific structural lattice, spectroscopic, and magnetic properties.

In an attempt to understand the aqueous interactions of Cr(III) with the low molecular mass physiological ligand citric acid, the pH-specific synthesis in the binary Cr(III)–citrate system was pursued, leading to the new complexes Na3[Cr(C6H5O7)2]·8.5H2O (1) and (Hdmphen)6[Cr(C6H5O7)2]·(NO3)3·14H2O (2). Complexes 1 and 2 were characterized by elemental analysis, spectroscopic, structural, thermal, and magnetic susceptibility studies. The structures of 1 and 2 reveal a mononuclear octahedral complex of Cr(III) with two citrate ligands bound to it. Albeit of the same deprotonation state, the disposition of the two citrate ligands with respect to Cr(III) differs between 1 and 2 in the solid state, thus reflecting the presence of pH-structural variants in the requisite binary system. This conformational difference is lifted in aqueous solution, thus providing (a) comparative information on the distribution and diversity of species in the binary Cr(III)–citrate system, and (b) insight into the nature of interactions developing in the binary Cr(III)–hydroxycarboxylate systems in abiotic and biological applications.

Nickel can play numerous roles in biological systems and in advanced abiotic materials. Supporting the diverse roles of Ni(II) in biological media are, among others, metal ion binding amino acids, their variably phosphorylated forms and/or exogenously administered organophosphonate drug substrates. In an effort to comprehend the aqueous chemistry of interactions between Ni(II) and organophosphonate substrates, research efforts were launched involving the ligands imino bis(methylenephosphonic acid) (H4IDA2P), H2O3P–CH2– NH 2 + –CH2–PO3H−, and N-(phosphonomethyl)glycine (glyphosate–H3IDAP), HOOC–CH2– NH 2 + –CH2–PO3H−. pH-Specific reactions of Ni(II) with H4IDA2P and H3IDAP led to the isolation of [Ni(C2H8O6NP2)2(H2O)2] (1) and [Ni(OOC–CH2–NH–CH2–PO3H)2]·[Ni(H2O)6]·3.3H2O (2), respectively. Compound 1 was characterized by analytical, spectroscopic techniques (UV–Vis, FT-IR), cyclic voltammetry, magnetic susceptibility measurements and X-ray crystallography. Compound 2 was characterized by elemental analysis, FT-IR spectroscopy, and X-ray crystallography. The structures of 1 and 2 reveal mononuclear octahedral Ni(II) assemblies bound by H3IDA2P− and water (1), and glyphosate (HIDAP2−) and water molecules (2), respectively. Magnetic susceptibility studies on 1 support the presence of high-spin octahedral Ni(II) in an oxygen environment, consistent with X-ray crystallography. The collective physicochemical features of the discrete Ni(II) assemblies in both species (a) shed light on aqueous binary Ni(II)–phosphoderivative interactions potentially influencing cellular physiology or toxicity, and (b) define the fundamental properties essential for the employment of such species in advanced materials synthesis and applications.

Iron is an essential metal ion in plant growth and development. Mobilization and further use of that metal by cellular structures in metabolic pathways entails the existence of soluble forms complexed with indigenous organic substrates, such as the low molecular mass d-(-)-quinic acid. In an effort to understand the relevant aqueous chemistry involving well-defined forms of iron, research efforts were carried out on the binary Fe(III)-quinic acid system. pH-specific reactions of FeCl(3).6H(2)O with quinic acid in a molar ratio 1:3 led to the isolation of the mononuclear Fe(III)-quinate complexes, K[Fe(C(7)H(11)O(6))(3)].(OH).3H(2)O (1), (NH(4))[Fe(C(7)H(11)O(6))(3)].(OH) (2), and Na[Fe(C(7)H(11)O(6))(3)].(OH).8H(2)O (3). Compounds 1-3 were characterized by analytical, spectroscopic techniques (UV/vis, FT-IR, Electron Paramagnetic Resonance (EPR), and Mossbauer spectroscopy), cyclic voltammetry, and magnetic susceptibility measurements. Compound 1 crystallizes in P2(1)3, with a = 15.1693(9) A, V = 3490.6(4) A(3), and Z = 4. Compound 2 crystallizes in P2(1)3, with a = 15.2831(9) A, V = 3569.7(4) A(3), and Z = 4. Compound 3 crystallizes in P2(1)3, with a = 15.6019(14) A, V = 3797.8(6) A(3), and Z = 4. The X-ray crystal structures of 1-3 reveal a mononuclear Fe(III) ion bound by three quinates in an octahedral fashion. Each quinate ligand binds Fe(III) through the alpha-hydroxycarboxylate group as a singly deprotonated moiety, retaining the alcoholic hydrogen. EPR measurements in solution suggest that 1 dissociates, releasing free quinate. Solution speciation studies of the binary system (a) unravel the aqueous species distribution as a function of pH and reagent molar ratio, and (b) corroborate the EPR results proposing the existence of a neutral Fe(III)-quinate complex form. The collective physicochemical properties of 1-3 formulate a well-defined profile for the Fe(III) assembly in aqueous media and project structural features consistent with solubilized Fe(III)-hydroxycarboxylate binary forms potentially mobilized into plant (bio)chemical processes.

Vanadium participation in cellular events entails in-depth comprehension of its soluble and bioavailable forms bearing physiological ligands in aqueous distributions of binary and ternary systems. Poised to understand the ternary V(V)-H(2)O(2)-amino acid interactions relevant to that metal ion's biological role, we have launched synthetic efforts involving the physiological ligands glycine and H(2)O(2). In a pH-specific fashion, V(2)O(5), glycine, and H(2)O(2) reacted and afforded the unusual complexes (H(3)O)(2)[V(2)(O)(2)(mu(2):eta(2):eta(1)-O(2))(2)(eta(2)-O(2))(2)(C(2)H(5)NO(2))] x 5/4 H(2)O (1) and K(2)[V(2)(O)(2)(mu(2):eta(2):eta(1)-O(2))(2)(eta(2)-O(2))(2)(C(2)H(5)NO(2))] x H(2)O (2). 1 crystallizes in the triclinic space group P1, with a = 7.805(4) A, b = 8.134(5) A, c = 12.010(7) A, alpha = 72.298(9) degrees, beta = 72.991(9) degrees, gamma = 64.111(9) degrees, V = 641.9(6) A(3), and Z = 2. 2 crystallizes in the triclinic space group P1, with a = 7.6766(9) A, b = 7.9534(9) A, c = 11.7494(13) A, alpha = 71.768(2) degrees, beta = 73.233(2) degrees, gamma = 65.660(2) degrees, V = 610.15(12) A(3), and Z = 2. Both complexes 1 and 2 were characterized by UV/visible, LC-MS, FT-IR, Raman, NMR spectroscopy, cyclic voltammetry, and X-ray crystallography. The structures of 1 and 2 reveal the presence of unusual ternary dinuclear vanadium-tetraperoxo-glycine complexes containing [(V(V)=O)(O(2))(2)](-) units interacting through long V-O bonds and an effective glycinate bridge. The latter ligand is present in the dianionic assembly as a bidentate moiety spanning both V(V) centers in a zwitterionic form. The collective physicochemical properties of the two ternary species 1 and 2 project the chemical role of the low molecular mass biosubstrate glycine in binding V(V)-diperoxo units, thereby stabilizing a dinuclear V(V)-tetraperoxo dianion. Structural comparisons of the anions in 1 and 2 with other known dinuclear V(V)-tetraperoxo binary anionic species provide insight into the chemical reactivity of V(V)-diperoxo species in key cellular events such as insulin mimesis and antitumorigenicity, potentially modulated by the presence of glycinate and hydrogen peroxide.

Efforts to delineate the interactions of Al(III), a known metallotoxin, with low molecular mass physiological substrates involved in cellular processes led to the investigation of the structural speciation of the binary Al(III)–quinic acid system. Reaction of Al(NO3)3 · 9H2O with d -(−)-quinic acid at a specific pH (4.0) afforded a colorless crystalline material K[Al(C7H11O6)3] · (OH) · 4H2O (1). Complex 1 was characterized by elemental analysis, FT-IR, DSC–TGA, 13C-MAS NMR, solution 1H and 13C NMR, and X-ray crystallography. The structure of 1 reveals a mononuclear octahedral complex of Al(III) with three singly ionized quinate ligands bound to it. The three ligand alcoholic side chains do not participate in metal binding and dangle away from the complex. The concurrent study of the aqueous speciation of the binary Al(III)–quinic acid system projects a number of species complementing the synthetic studies on the binary system Al(III)–quinic acid. The structural and spectroscopic data of 1 in the solid state and in solution emphasize its physicochemical properties emanating from the projections of the aqueous structural speciation scheme of the Al(III)–quinic acid system. The employed pH-specific synthetic work (a) exemplifies essential structural and chemical attributes of soluble aqueous species, arising from biologically relevant interactions of Al(III) with natural α-hydroxycarboxylate substrates, and (b) provides a potential linkage to the chemical reactivity of Al(III) toward O-containing molecular targets influencing physiological processes and/or toxicity events.

Outstanding among the plethora of roles of vanadium in biological systems is its ability to act as an insulin mimetic agent, counteracting hyperglycemia. Poised to comprehend the interactions of V(V) with physiological substrates of low molecular mass, research efforts were launched to investigate the aqueous synthetic chemistry of the ternary V(V)–citric acid–H 2 O 2 system. In a pH-specific fashion, reaction of VCl 3 with citric acid and H 2 O 2 in aqueous ammonia led to the isolation of a unique assembly of ternary dinuclear complexes in (NH 4 ) 6 [V 2 O 2 (O 2 ) 2 (C 6 H 5 O 7 ) 2 ] · [V 2 O 2 (O 2 ) 2 (C 6 H 6 O 7 ) 2 ] · 6H 2 O ( 1 ). This unique assembly of dinuclear species was characterized by elemental analysis, FT-IR spectroscopy and X-ray crystallography. The structure of 1 reveals two dinuclear V 2 O 2 core complexes, bearing peroxo moieties and bound citrates of variable deprotonation state and coordination mode. The physicochemical data: (a) present an unusual “instant picture” of the pH-specific aqueous speciation of the investigated system, attest to the presence of known species and clearly suggest the existence of a new discrete species in the aqueous structural speciation of the ternary system, (b) exemplify the usefulness of the synthetic strategy in the discovery of new discrete V(V)–citrate–peroxo species, and (c) offer insight into the ternary interactions of V(V) toward O-containing substrates relevant to insulin mimetic activity.

Cobalt involvement in chemical and metallobiological processes entails largely unknown reactivity pathways with a variety of ligands. Such ligands include phosphonate and carboxylate-containing metal ion binders. In an attempt to investigate the nature and properties of species arising from aqueous interactions between Co(II) and N , N -bis(phosphonomethyl)-glycine (H 5 NTA2P), reactions between the two led to an assembly of species in (NH 4 ) 4 [Co(H 2 O) 6 ][(H 2 O) 2 Co(HNTA2P)Co(NH 3 ) 2 (H 2 O) 3 ] 2 [Co(NTA2P)(H 2 O) 2 ] 2 · 10H 2 O · 1.36CH 3 CH 2 OH ( 1 ) at pH ∼ 5.5. The analytical, spectroscopic and X-ray data on 1 reveal mononuclear and dinuclear complexes of Co(II) surrounded by oxygens, belonging to terminal carboxylates, phosphonates and bound water molecules, and nitrogen atoms from coordinated ammonia and H x NTA2P q − ( x = 1, q = 4; x = 0, q = 5) ligands. Worth noting is the variable protonation state of the bound diphosphonate ligand and its ability to bridge two Co(II) centers with ostensibly differing coordination spheres. The assembly of three Co(II) species of variable nuclearity and composition attests to the importance of pH-specific conditions, under which “capturing” of more than one species can be achieved for a given Co(II):H 5 NTA2P stoichiometry in the presence of ammonia. Collectively, 1 provides a rare glimpse of a “slice” of the aqueous speciation of the binary Co(II)–H 5 NTA2P system, while its lattice composition projects key structural features in Co(II)-carboxyphosphonate materials.

In order to delineate the interactions of Cr(III) with low molecular mass ligands, often involved in chemistries of toxic and/or biologically significant processes, the aqueous structural speciation of the binary Cr(III)–heida (2-hydroxyethyliminodiacetic acid) system was investigated. The reaction of Cr(NO3)3 · 9H2O with heida at a specific pH (5.5) led to the isolation of a red crystalline (NH4)[Cr{HOCH2CH2N(CH2COO)2}2] · 2H2O (1), which was characterized by elemental analysis, spectroscopic, structural, thermal, and magnetic susceptibility studies. The structure of 1 reveals a mononuclear octahedral complex of Cr(III) with two doubly ionized heida ligands bound to it. The ligand alcoholic side chains do not participate in metal binding and dangle away from the complex. The structural and spectroscopic data emphasize the physicochemical properties of 1 in the solid state and in solution, thus reflecting the profile of 1 in the overall aqueous structural speciation scheme of the Cr(III)–heida system. The employed pH-specific synthetic endeavor exemplifies (a) the potential utility of the strategy in the exploration of key structural and chemical attributes of soluble aqueous species, arising from the biologically relevant interactions of Cr(III) with the O,N-containing ligands, and (b) the potential linkage of the chemical reactivity of Cr(III) toward the O,N-containing substrates of a variable structural composition with physiological processes or toxicity events.

The presence of cobalt in biological systems has been associated with a variety of regulatory roles as an inorganic cofactor. Its involvement in (bio)chemical processes, where it contributes to the integrity of the physiology of humans, underscores its ability to promote (bio)chemistry with molecular targets of variable mass and biological properties. Given the fact that organic phosphonates are widespread substrates/ligands in biological fluids, we sought to investigate the relevant chemistry with CoII. With the organophosphonate ligand N,N-bis(phosphonomethyl)glycine (H5NTA2P) as the main metal ion binder, aqueous reactions with CoII were examined, ultimately leading to the isolation of the complex [Co(C4H9O8NP2)(H2O)2]·2H2O (1) at pH 1.5. This complex was characterized analytically, spectroscopically (FT-IR, UV/Vis, EPR), magnetically, and by cyclic voltammetry. X-ray crystallography shows that 1 is a compound with a molecular type of lattice. The complex consists of a mononuclear, octahedral CoII center coordinated by oxygen atoms belonging to the terminal phosphonate and carboxylate groups of H3NTA2P2–, the ligand nitrogen, and two bound water molecules. It should be emphasized that similar structural features have been observed in other metal organophosphonate lattices of materials with potential catalytic and chemical reactivity. The magnetic and EPR data for 1 suggest the presence of a high-spin octahedral CoII, with a ground state of an effective spin S = 1/2. The solution UV/Vis and EPR spectra suggest retention of the integrity of 1, which is consistent with the magnetization measurements in the solid state. Collectively, the data suggest a soluble CoII–organodiphosphonate species, not unlike those expected in bio-fluids containing the specific ligand or ligands structurally akin to H3NTA2P2–.The biological relevance of 1 to species in bio-fluids and the structural association with CoII–organophosphonate materials is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Manganese is a metal with diverse (bio)chemical reactivity in aqueous media in the presence of biologically relevant ligands. Key to that reactivity is the existence of soluble and bioavailable forms of MnII across the physiological range of pH. To probe the interactions arising between MnII and the low-molecular-mass ligand D-(–)-quinic acid, research efforts were targeted at the synthetic aqueous chemistry in the requisite binary system. The pH-specific synthetic reaction of MnII with D-(–)-quinic acid led to the isolation of the new species [Mn2(C7H11O6)4]n·nH2O (1). Complex 1 was characterized by elemental analysis, spectroscopic techniques (EPR, FTIR), thermal and magnetic studies, and X-ray crystallography. The molecular lattice of 1 reveals the presence of dimeric units of MnII centers, with the quinate ligands coordinated to the metal ions through variable binding modes, indicating the diversity of chemical reactivity in the system. The octahedral MnII centers in the molecular assembly of 1 serve as chemical prototypes of the forms of interactions that most likely develop in biologically relevant fluids, similar to those encountered in plant exudates and plant cellular structures. Concurrently, the physicochemical features of 1 (from the 3D hydrogen-bonding network to the magnetic susceptibility properties and EPR spectral results) exemplify the key features that constitute the rudiments of MnII-organic hybrid lattices of specific functions and (bio)chemical reactivity patterns. The collective properties of 1 are discussed in the context of their occurrence in natural binary MnII-influenced systems and advanced metal-hybrid materials of potential applications. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Aqueous reactions of V2O5 or VCl3 in the presence of the physiological citric acid and hydrogen peroxide, in a pH specific fashion, afforded a new vanadium(V)-peroxo-citrate material isolated in a pure crystalline form. Elemental analysis pointed to the molecular formulation (NH4)6[V(V)2O2(O2)2(C6H4O7)2].4.5H2O (1). Complex 1 was further characterized by UV-vis, FT-IR, and X-ray crystallography. Compound 1 crystallizes in the monoclinic space group C2/c with a = 12.391(5) A, b = 15.737(7) A, c = 17.102(7) A, beta = 110.84(1) degrees, V = 3117(1) A3, and Z = 4. The structure of the anionic assembly consists of a planar V(V)2O2 core with two fully deprotonated citrates bound to it through the central carboxylate and alkoxide moieties as well as one of the terminal carboxylate groups. The presence of one peroxide group attached to each vanadium(V) renders the geometry around each metal center pentagonal bipyramidal. Key structural and spectroscopic features of 1 correlate with those seen in the peroxo congener and low-pH analogue (NH4)2[V(V)2O2(O2)2(C6H6O7)2].2H2O (3), in which all terminal carboxylate groups are protonated. In solution, simple pH-dependent transformation of 1 to 3 attests to their participation in the requisite speciation and potentiates the presence of other similar peroxo analogues not yet isolated and characterized. The reactivity of 1 through transformation reactions, yielding a plethora of well-characterized species, establishes a linkage among various species with the same or different vanadium oxidation states. Collectively, the data reflect soluble forms of vanadium with peroxide and citrate that contribute to the requisite pH-dependent distribution of that metal ion and likely influence biological processes.

The wide use of titanium in applied materials has prompted pertinent studies targeting the requisite chemistry of that metal's biological interactions. In order to understand such interactions as well as the requisite titanium aqueous speciation, we launched investigations on the synthesis and spectroscopic and structural characterization of Ti(IV) species with the physiological citric acid. Aqueous reactions of TiCl(4) with citric acid in the presence of H(2)O(2) and neutralizing ammonia afforded expediently the red crystalline material (NH(4))(4)[Ti(2)(O(2))(2)(C(6)H(4)O(7))(2)].2H(2)O (1). Complex 1 was further characterized by UV-vis, FT-IR, FT- and laser-Raman, NMR, and finally by X-ray crystallography. Compound 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.360(4) A, b = 10.226(4) A, c = 11.478(6) A, beta = 107.99(2) degrees, V = 1156.6(9) A(3), and Z = 2. The X-ray structure of 1 reveals a dinuclear anionic complex containing a Ti(IV)(2)O(2) core. In that central unit, two fully deprotonated citrate ligands are coordinated to the metal ions through their carboxylate moieties in a monodentate fashion. The central alkoxides serve as bridges to the two titanium ions. Also attached to the Ti(IV)(2)O(2) core are two peroxo ligands each bound in a side-on fashion to the respective metal ions. NH(4)(+) ions neutralize the 4- charge of the anion in 1, further contributing to the stability of the derived lattice through H-bond formation. The structural similarities and differences with congener vanadium(V)-peroxo-citrate complexes may point out potential implications in the chemistry of titanium with physiological ligands, when the former is present in a biologically relevant medium.

The interactions of Al(III) with the dipeptide AspAsp and the tripeptide AspAspAsp in aqueous solutions were studied by pH-potentiometry and multinuclear 1H- and 13C- nuclear magnetic resonance (NMR) spectroscopy. Their numerous negatively charged COO(-) functions allow these ligands to bind Al(III) even in weakly acidic solutions. Various mononuclear 1:1 complexes are formed in different protonation states. 13C-NMR spectroscopy unambiguously proved participation of the COO(-) functions in a monodentate or chelating mode in Al(III) binding, however, the terminal-NH(2) group seems to be excluded from the coordination. Depending on the metal ion to ligand ratio precipitation occurs at pH approximately 5 to 6. This indicates that the COO(-) groups at the low level of preorganization in such small peptides are not sufficient to keep the Al(III) ion in solution and to prevent the precipitation of Al(OH)(3) at physiological pH. To achieve this, a more specific arrangement of the side-chain donors seems necessary.

Vanadium(V) involvement in interactions with physiological ligands in biological media prompted us to delve into the systematic pH-dependent synthesis, spectroscopic characterization, and perusal of chemical properties of arising aqueous vanadium(V)–citrate species in the requisite system. To this end, facile reactions led to dinuclear complexes (NH 4 ) 4 [V 2 O 4 (C 6 H 5 O 7 ) 2 ]·4H 2 O ( 1 ) and (NH 4 ) 6 [V 2 O 4 (C 6 H 4 O 7 ) 2 ]·6H 2 O ( 2 ). Complex 1 and 2 were characterized by elemental analysis, FT-IR and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group C 2/ c with a =16.998(5) A, b =16.768(5) A, c =9.546(3) A, β =105.22(1)°, V =2625(1) A 3 , and Z =4. Complex 2 crystallizes in the triclinic space group P 1 , with a =9.795(4) A, b =9.942(4) A, c =9.126(3) A, α =90.32(1)°, β =111.69(1)°, γ =108.67(1)°, V =774.5(5) A 3 , and Z =1. The structures of 1 and 2 were consistent with the presence of a V V 2 O 2 core, to which citrate ligands of differing protonation state were bound in a coordination mode consistent with past observations. Ultimately, the aqueous pH dependent transformations of a series of three dinuclear complexes, 1 , 2 and (NH 4 ) 2 [V 2 O 4 (C 6 H 6 O 7 ) 2 ]·2H 2 O ( 3 ), all isolated at pH values from 3 to 7.5, were explored and revealed an important interconnection among all species. Collectively, pH emerged as a determining factor of structural attributes in all three complexes, with the adjoining acid–base chemistry unfolding around the stable V V 2 O 2 core. The results point to the participation of all three species in aqueous vanadium(V)–citrate speciation, and may relate the site-specific protonations–deprotonations on the dinuclear complexes to potential biological processes involving vanadium(V) and physiological ligand targets.

The contact of Al(III) with biological components in human physiology has increased significantly over the years, due to a number of factors, prominent among which stands the rapid acidification of the environment and the concomitant introduction of that abundant metal ion in human biological fluids. As a result, pathophysiological aberrations in humans have arisen due to Al(III) (neuro)toxicity. Among the efforts targeting the elucidation of the factors responsible for Al(III) toxicity is the exploration of the requisite Al(III)-carboxylate chemistry in aqueous media, and its relevance to soluble, potentially bioavailable species capable of exerting toxic effects. A detailed synthetic, structural, and spectroscopic account of the Al(III)-carboxylate complexes, purported to exist as components in aqueous Al(III)–carboxylic acid speciation, is presented. The structures described are classified as mononuclear, dinuclear, trinuclear, tetranuclear, and polynuclear species, arising from various aqueous and non-aqueous Al(III)-carboxylate ligand reactions. Moreover, the solution chemistry and kinetic behavior of the so far reported complexes is given, with the specific aim of comparing their solid state and solution chemical and structural properties. In this sense, a comprehensive picture on the Al(III) speciation, in the presence of various physiological or biologically relevant carboxylate ligands, appears to emerge, which is expected to contribute to the understanding of Al(III) (neuro)toxicity and its consequence(s) in a multitude of human diseases. Carboxylate containing low and high molecular mass components stand prominent in their chemical preference to react with Al(III) in biological fluids. In this context, factors considered to influence the aqueous low molecular mass Al(III)-carboxylate chemistry, thus affecting the solubility and possibly the bioavailability of the resulting species, are discussed as potential research links to the ultimate manifestation of Al(III) toxicity at the cellular level.

The aqueous chemistry of vanadium with physiologically relevant ligands constitutes a subject of burgeoning research, extending from bacterial metalloenzymic functions to human-health physiology. Vanadium, in the form of VCl3 and V2O5, reacted expediently with citric acid, in a 1:2 molar ratio in water at pH∼4, and, in the presence of various cations, afforded crystalline materials bearing the general formula (Cat)2[V2O4(C6H6O7)2]·nH2O (A) (Cat+=Na+, NH4 +, n=2; Me4N+, K+, n=4). Exploration of the reactivity of A toward H2O2 yielded the peroxo-containing complexes (Cat)2[V2O2(O2)2(C6H6O7)2]·2H2O (B) (Cat+=K+, NH4 +). Both classes of compounds were characterized analytically and spectroscopically. The X-ray structures of complexes A and B emphasize the exceptional stability of the dimeric rhombic unit VV 2O2, which is retained upon H2O2 reaction, and the preserved mode of coordination of the citrate ligand as a doubly deprotonated moiety. In these complexes, typical six and eight coordination numbers were observed for the Na+ and K+ counter-ions, respectively. The variety of synthetic approaches leading to A, along with the stepwise and direct assembly and isolation of peroxo-compounds (B), denotes the significance of reaction pathways and intermediates in vanadium(III–V)–citrate synthetic chemistry. Hence, a systematic investigation of reactivity modes in aqueous vanadium–citrate systems emerges as a crucial tool for the establishment of chemical interconnectivity among low MW complex species, potentially participating in the intricate biodistribution of that metal ion in biological fluids.

The interaction of lead with biologically relevant ligands, such as citrate, constitutes a major research thrust relating to toxic effects exerted on humans. Reaction of Pb(NO3)2 with citric acid in water at pH 2 and various ratios (1:1–1:3) yielded the first Pb–citrate species, which was characterized analytically and spectroscopically (FT-IR). X-ray crystallographic studies were carried out and showed the novel compound to be [Pb(C6H6O7)]n·nH2O. The geometry around Pb2+ appears to be distorted trigonal bipyramidal with PbO distances ranging from 2.397(7) to 2.527(6) A and the lone pair on Pb2+ occupying one of the three basal plane positions. Citrate binds Pb2+ forming rhomboidal Pb2O2 units, while it subtends its terminal carboxylate onto a proximal Pb2+, thus linking adjacent Pb2O2 units into a chain along the a axis. The involvement of citrate in hydrogen bonds with water molecules and abutting Pb–citrate moieties leads to a stable structure. The stability of the species formed and eventually isolated from aqueous solutions as well as its reactivity toward bases present useful information on the chemistry in biological media and its relevance to the known biotoxicity of lead.

Vanadium involvement in cellular processes requires deep understanding of the nature and properties of its soluble and bioavailable forms arising in aqueous speciations of binary and ternary systems. In an effort to understand the ternary vanadium-H(2)O(2)-ligand interactions relevant to that metal ion's biological role, synthetic efforts were launched involving the physiological ligands betaine (Me(3)N(+)CH(2)CO(2)(-)) and H(2)O(2). In a pH-specific fashion, V(2)O(5), betaine, and H(2)O(2) reacted and afforded three new, unusual, and unique compounds, consistent with the molecular formulation K(2)[V(2)O(2)(O(2))(4){(CH(3))(3)NCH(2)CO(2))}]·H(2)O (1), (NH(4))(2)[V(2)O(2)(O(2))(4){(CH(3))(3)NCH(2)CO(2))}]·0.75H(2)O (2), and {Na(2)[V(2)O(2)(O(2))(4){(CH(3))(3)NCH(2)CO(2))}(2)]}(n)·4nH(2)O (3). All complexes 1-3 were characterized by elemental analysis; UV/visible, FT-IR, Raman, and NMR spectroscopy in solution and the solid state; cyclic voltammetry; TGA-DTG; and X-ray crystallography. The structures of 1 and 2 reveal the presence of unusual ternary dinuclear vanadium-tetraperoxido-betaine complexes containing [(V(V)═O)(O(2))(2)] units interacting through long V-O bonds. The two V(V) ions are bridged through the oxygen terminal of one of the peroxide groups bound to the vanadium centers. The betaine ligand binds only one of the two V(V) ions. In the case of the third complex 3, the two vanadium centers are not immediate neighbors, with Na(+) ions (a) acting as efficient oxygen anchors and through Na-O bonds holding the two vanadium ions in place and (b) providing for oxygen-containing ligand binding leading to a polymeric lattice. In 1 and 3, interesting 2D (honeycomb) and 1D (zigzag chains) topologies of potassium nine-coordinate polyhedra (1) and sodium octahedra (3), respectively, form. The collective physicochemical properties of the three ternary species 1-3 project the chemical role of the low molecular mass biosubstrate betaine in binding V(V)-diperoxido units, thereby stabilizing a dinuclear V(V)-tetraperoxido dianion. Structural comparisons of the anions in 1-3 with other known dinuclear V(V)-tetraperoxido binary anionic species provide insight into the chemical reactivity of V(V)-diperoxido systems and their potential link to cellular events such as insulin mimesis and anitumorigenicity modulated by the presence of betaine.

Cobalt(II) is an essential metal ion, which can react with biologically relevant substrates in aqueous media, affording discrete soluble forms. D-(-)-quinic acid is a representative metal ion binder, capable of promoting reactions with Co(II) under pH-specific conditions, leading to the isolation of the new species K[Co(C(7)H(11)O(6))(3)] x 3 CH(3)CH(2)OH (1), Na[Co(C(7)H(11)O(6))(3)] x 3 CH(3)CH(2)OH x 2.25 H(2)O (2), and [Co(C(7)H(11)O(6))(2)(H(2)O)(2)] x 3 H(2)O (3). Compounds 1-3 were characterized by elemental analysis, spectroscopic techniques (Fourier-transform infrared, UV-visible, electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry), magnetic studies, and X-ray crystallography. Compound 1 crystallizes in the cubic space group P2(1)3, with a = 15.3148(19) A, V = 3592.0(8) A(3), and Z = 4. Compound 2 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 14.9414(8) A, b = 15.9918(9) A, c = 16.0381(9) A, V = 3832.1(4) A(3), and Z = 4. Compound 3 crystallizes in the monoclinic space group P2(1)/m, with a = 13.2198(10) A, b = 5.8004(6) A, c = 15.3470(12) A, beta = 108.430(7), V = 1116.45(17) A(3), and Z = 4. The lattices in 1-3 reveal the presence of mononuclear Co(II) units bound exclusively to quinate (1 and 2) or quinate and water ligands (3), thus projecting the unique chemical reactivity in each investigated system and suggesting that 3 is an intermediate in the synthetic pathway leading to 1 and 2. The octahedral sites of Co(II) are occupied by oxygens, thereby reflecting the nature of interactions between the divalent metal ion and quinic acid. The magnetic and EPR data on 1 and 3 support the presence of a high-spin octahedral Co(II) in an oxygen environment, having a ground state with an effective spin of S = 1/2. The significance of 3 is further reflected into the aqueous speciation of the binary Co(II)-quinic acid system, in which 3 appears as a competent participant linked to the solid state species 1. The physicochemical profiles of 1-3, in the solid state and in solution, earmark the importance of aqueous structural speciation, which projects chemical reactivity pathways in the binary Co(II)-quinate system, involving soluble Co(II) forms emerging through interactions with low molecular mass O-containing physiological substrates, such as quinic acid.

Efforts to delineate the interactions of neurotoxic Al(III) with low molecular mass substrates relevant to neurodegenerative processes, led to the investigation of the pH-specific synthetic chemistry of the binary Al(III)-[N-(phosphonomethyl) iminodiacetic acid] (Al-NTAP), Al(III)-[nitrilo-tris(methylene-phosphonic acid)] (Al-NTA3P), and Al(III)-[1-hydroxy ethylidene-1,1-diphosphonic acid] (Al-HEDP) systems, in correlation with solution speciation studies. Reaction of Al(NO 3 ) 3 ·9H 2 O with NTAP at pH 7.0 and 4.0 afforded the new species (CH 6 N 3 ) 4 [Al 2 (C 5 H 6 NPO 7 ) 2 (OH) 2 ]·8H 2 O ( 1 ) and (NH 4 ) 2 [Al 2 (C 5 H 6 NPO 7 ) 2 (H 2 O) 2 ]·4H 2 O ( 2 ), while reaction of Al(NO 3 ) 3 ·9H 2 O with NTA3P led to K 8 [Al 2 (C 3 H 6 NP 3 O 9 ) 2 (OH) 2 ]·20H 2 O ( 3 ). Complexes 1 – 3 were characterized by elemental analysis, FT-IR, 13 C, 31 P, 1 H NMR (for 1 – 2 solid state and solution NMR where feasible), and X-ray crystallography. The structures of 1 – 3 reveal the presence of uniquely defined dinuclear complexes of octahedral Al(III) bound to fully deprotonated phosphonate ligands, water and hydroxo moieties. The aqueous solution speciation studies on the aforementioned binary systems project a clear picture of the binary Al(III)–(carboxy)phosphonate interactions and species under variable pH-conditions and specific Al(III):ligand stoichiometry. The concurrent solid state and solution work (a) exemplifies essential structural and chemical attributes of soluble aqueous species, reflecting well-defined interactions of Al(III) with phosphosubstrates and (b) strengthens the potential linkage of neurotoxic Al(III) chemical reactivity toward O,N-containing (carboxy)phosphate-rich cellular targets.

Vanadium has been amply established as a competent inorganic cofactor in biological systems. Among vanadium's various functions its insulin mimetic capacity stands out and constitutes a challenge to bioinorganic chemists. Prompted by the need to comprehend the insulin mimetic action of vanadium and its interaction with biomolecular targets, extensive synthetic reactivity studies were carried out with V(IV,V) in the presence of low molecular mass ligands in aqueous solutions. Through meticulous synthetic pH-dependent efforts, new V(IV,V) soluble binary and ternary species arose that delineate the aqueous speciation and reflect the chemical reactivity of insulin mimetic vanadium involved in interactions with physiological substrates.

The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entails interactions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueous media, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid. Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition of ammonia at pH approximately 7 led to the new complexes [Cd3(C6H5O7)2(H2O)5] x H2O (1) and (NH4)[Cd(C6H5O7)(H2O)] x H2O (2), respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 18.035(6) A, b = 10.279(4) A, c = 12.565(4) A, beta = 109.02(1) degrees, V = 2202(2) A3, and Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1), with a = 9.686(4) A, b = 8.484(4) A, c = 7.035(3) A, beta = 110.28(1) degrees, V = 542.3(4) A3, and Z = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around one Cd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2 contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, the participating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueous structural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structural diversity validate past solution speciation studies, projecting the existence of mononuclear species such as the one in the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the information emerging from synthetic studies that otherwise would not have been revealed through conventional solution studies, while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity of Cd(II).

In an attempt to understand the aqueous interactions of Cr(III) with the low-molecular-mass physiological ligand citric acid, the pH-specific synthesis in the binary Cr(III)-citrate system was explored, leading to the complex (NH4)4[Cr(C6H4O7)(C6H5O7)].3H2O (1). 1 crystallizes in the monoclinic space group I2/a, with a = 19.260(10) A, b = 10.006(6) A, c = 23.400(10) A, beta = 100.73(2) degrees , V = 4431(4) A3, and Z = 8. 1 was characterized by elemental analysis and spectroscopic, structural, thermal, and magnetic susceptibility studies. Detailed aqueous speciation studies in the Cr(III)-citrate system suggest the presence of a number of species, among which is the mononuclear [Cr(C6H4O7)(C6H5O7)]4- complex, optimally present around pH approximately 5.5. The structure of 1 reveals a mononuclear octahedral complex of Cr(III) with two citrate ligands bound to it. The two citrate ligands have different deprotonation states, thus signifying the importance of the mixed deprotonation state in the coordination sphere of the Cr(III) species in aqueous speciation. The latter reveals the distribution of numerous species, including 1, for which the collective structural, spectroscopic, and magnetic data point out its physicochemical profile in the solid state and in solution. The importance of the synthetic efforts linked to 1 and the potential ramifications of Cr(III) reactivity toward both low- and high-molecular-mass biotargets are discussed in light of (a) the quest for well-characterized soluble Cr(III) species that could be detected and identified in biologically relevant fluids, (b) ongoing efforts to delineate the aqueous speciation of the Cr(III)-citrate system and its link to biotoxic Cr(III) manifestations, and (c) the synthetic utility of convenient Cr(III) precursors in the synthesis of advanced materials.

Titanium is a metal frequently employed in a plethora of materials supporting medical applications. In an effort to comprehend the involvement of titanium in requisite biological interactions with physiological ligands, synthetic efforts were launched targeting aqueous soluble species of Ti(IV). To this end, aqueous reactions of TiCl(4) with citric acid afforded expediently, under pH-specific conditions, the colorless crystalline materials Na(6)[Ti(C(6)H(4.5)O(7))(2)(C(6)H(5)O(7))].16H(2)O (1) and Na(3)(NH(4))(3)[Ti(C(6)H(4.5)O(7))(2)(C(6)H(5)O(7))].9H(2)O (2). Complexes 1 and 2 were characterized by elemental analysis, FT-IR, (13)C-MAS solid state and solution NMR, cyclic voltammetry, and X-ray crystallography. 1 crystallizes in the triclinic space group P, with a = 15.511(9) A, b = 15.58(1) A, c = 9.848(5) A, alpha = 85.35(2) degrees, beta = 76.53(2) degrees, gamma = 61.97(2) degrees, V = 2042(2) A(3), and Z = 2. 2 crystallizes in the triclinic space group P, with a = 12.437(5) A, b = 12.440(5) A, c = 12.041(5) A, alpha = 83.08(2) degrees, beta = 81.43(2) degrees, gamma = 67.45(2) degrees, V = 1697(2) A(3), and Z = 2. The X-ray structures of 1 and 2 reveal the presence of a mononuclear complex, with Ti(IV) coordinated to three citrate ligands in a distorted octahedral geometry around Ti(IV). The citrates employ their central alkoxide and carboxylate groups to bind Ti(V), while the terminal carboxylates stay away from the Ti(IV)O(6) core. Worth noting in 1 and 2 is the similar mode of coordination but variable degree of protonation of the bound citrates, with the locus of (de)protonation being the noncoordinating terminal carboxylates. As a result, this work suggests the presence of a number of different Ti(IV)-citrate species of the same nuclearity and coordination geometry as a function of pH. This is consistent with the so far existing pool of mononuclear Ti(IV)-citrate species and provides a logical account of the aqueous speciation in the requisite binary system. Such information is vital in trying to delineate the interactions of soluble and bioavailable Ti(IV) forms promoting biological interactions in humans. To this end, chemical properties, structural attributes, and speciation links to potential ensuing biological effects are dwelled on.

The chemistry of aluminum was explored in the presence of the physiological ligand citric acid and in low-pH aqueous media. As a result, the first dinuclear aluminum-citrate complex (NH4)4[Al2-(C6H4O7)(C6H5O7)2].4H2O was isolated at low pH (approximately 3.5), and was characterized by FT-IR spectroscopy and X-ray crystallography. The structural analysis reveals the presence of a dinuclear assembly of two aluminum ions octahedrally coordinated to three citrate ligands of differing protonation state. The NMR solution behavior of this complex emphasizes its time-dependent transformation into a number of variable nature species, ultimately leading to the thermodynamically stable trinuclear species. It also establishes the participation of the dinuclear complex as a viable component of the aqueous Al(III)-citrate speciation. The chemical and structural features of this novel low molecular mass species provide considerable insight into citrate's ability, as a natural ligand, to influence the chemistry of aluminum in a pH-dependent fashion, and potentially affect aluminum's (bio)distribution, absorption, accumulation, and biotoxicity at sensitive biological sites.

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)-citrate complex [Cd(C(6)H(6)O(7))(H(2)O)](n)() (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and (113)Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 6.166(2) A, b = 10.508(3) A, c = 13.599(5) A, V = 881.2(5) A(3), and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). (113)Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)-citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.

Synthetic efforts targeting soluble species of Co(II) with the low molecular mass physiological ligand citric acid led to the isolation of the first dinuclear complex [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-), at pH approximately 5, in the form of its K+ (1) and Na+ (2) salts. Both 1 and 2 were characterized analytically, spectroscopically (FT-IR, UV/visible, EPR), and magnetically. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.348(5) A, b = 11.578(6) A, c = 12.138(6) A, beta = 112.62(2) degrees, V = 1342(1) A(3), and Z = 2. Complex 2 crystallizes in the monoclinic space group P2(1)/c, with a = 9.234(4) A, b = 11.913(4) A, c = 11.728(6) A, beta = 99.93(2) degrees, V = 1271(1) A(3), and Z = 2. X-ray crystallography on 1 and 2 reveals the presence of two Co(II) ions, in a dinuclear assembly, octahedrally coordinated by two citrate ligands in a tridentate fashion. The octahedral environment around each Co(II) is complemented by another singly bonded citrate belonging to the adjacent Co(II) unit and two water molecules. Magnetic susceptibility and EPR studies on 1, in the solid state, corroborate the X-ray results, indicating a weak interaction between the two Co(II) ions. Moreover, EPR and UV/visible studies in solution suggest that 1 does not retain its dimeric structure, yielding a mononuclear octahedral Co(II)-citrate species. Detailed speciation studies suggest the presence of a number of species including the mononuclear complex [Co(C(6)H(5)O(7))](-), optimally present around pH approximately 5. In consonance with EPR and UV/visible spectroscopy, [Co(C(6)H(5)O(7))](-) is likely the scaffolding unit on the basis of which the dimer [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-) is isolated from aqueous solutions. Collectively, this comprehensive study offers significant structural insight into the Co(II)-citrate speciation and the elucidation of the role of Co(II) in biological fluids.

Vanadium interactions with low molecular mass binders in biological fluids entail the existence of vanadium species with variable chemical and biological properties. In the course of efforts to elucidate the chemistry related to such interactions, we have explored the oxidative chemistry of vanadium(III) with the physiologically relevant tricarboxylic citric acid. Aqueous reactions involving VCl(3) and anhydrous citric acid, at pH approximately 7, resulted in blue solutions. Investigation into the nature of the species arising in those solutions revealed, through UV/visible and EPR spectroscopies, oxidation of vanadium(III) to vanadium(IV). Further addition of H(2)O(2) resulted in the oxidation of vanadium(IV) to vanadium(V), and the isolation of a new vanadium(V)-citrate complex in the form of its potassium salt. Analogous reactions with K(4)[V(2)O(2)(C(6)H(4)O(7))(2)].6H(2)O and H(2)O(2) or V(2)O(5) and citrate at pH approximately 5.5 afforded the same material. Elemental analysis pointed to the molecular formulation K(4)[V(2)O(4)(C(6)H(5)O(7))(2)].5.6H(2)O (1). Complex 1 was further characterized by FT-IR and X-ray crystallography. 1 crystallizes in the triclinic space group P(-)1, with a = 11.093(4) A, b = 9.186(3) A, c = 15.503(5) A, alpha = 78.60(1) degrees, beta = 86.16(1) degrees, gamma = 69.87(1) degrees, V = 1454.0(8) A(3), and Z = 2. The X-ray structure of 1 reveals the presence of a dinuclear vanadium(V)-citrate complex containing a V(V)(2)O(2) core. The citrate ligands are triply deprotonated, and as such they bind to vanadium(V) ions, thus generating a distorted trigonal bipyramidal geometry. Binding occurs through the central alkoxide and carboxylate groups, with the remaining two terminal carboxylates being uncoordinated. One of those carboxylates is protonated and contributes to hydrogen bond formation with the deprotonated terminal carboxylate of an adjacent molecule. Therefore, an extended network of hydrogen-bonded V(V)(2)O(2)-core-containing dimers is created in the lattice of 1. pH-dependent transformations of 1 in aqueous media suggest its involvement in a web of vanadium(V)-citrate dinuclear species, consistent with past solution speciation studies investigating biologically relevant forms of vanadium.

Cobalt is an essential metal ion involved in key biomolecules, regulating processes in human physiology. As a metal ion, Co(II) assumes forms, which are dictated by the nature of organic binders in biological fluids, and the conditions under which metal ion ligand interactions arise. Among the various low molecular mass metal ion binders in biological fluids are amino acids, organic acids, as well as their variably phosphorylated forms. As a representative metal ion binder, the organophosphonate ligand H(2)O(3)P-CH(2)-NH(2)(+)-CH(2)-PO(3)H(-) was employed in aqueous reactions with Co(II), ultimately leading to the isolation of complex [Co(C(2)H(8)O(6)NP(2))(2)(H(2)O)(2)] (1) at pH 2. The complex was characterized analytically, spectroscopically (FTIR, UV-vis, EPR), and magnetically. Compound 1 crystallizes in the monoclinic space group P2(1)/n, with a = 7.361(3) A, b = 8.133(3) A, c = 14.078(5) A, beta = 104.40(1) degrees, V = 816.3(5) A(3), and Z = 2. X-ray crystallography reveals that 1 is a compound with a molecular type of lattice. In it, there exist mononuclear octahedral sites of Co(II) surrounded by oxygens, belonging to terminal phosphonates and bound water molecules. Both ends of the ligand zwitterionic form are involved in binding to adjacent Co(II) ions, thus creating tetranuclear 32-membered rings, with cavities extending in two dimensions and holes in the third dimension throughout the lattice of 1. Similar structural features were observed in other metal organophosphonate lattices of potential catalytic and chemical reactivity. The magnetic and EPR data on 1 support the presence of a high-spin octahedral Co(II) in an oxygen environment, having a ground state with an effective spin S = (1)/(2). The solution UV-vis and EPR data suggest retention of the high-spin octahedral Co(II) ion, consistent with the magnetization measurements on 1. Collectively, the data reflect the existence of a soluble Co(II)-iminodiphosphonate species not unlike those expected in biological fluids containing the specific ligand or ligands similar to that. Both biologically relevant perspectives and a synthetic outlook into Co(II)-organophosphonate materials are discussed.

Citrate is abundantly encountered in biological fluids as a natural metal ion chelator. Vanadium participates in biological processes as a catalyst in the active sites of metalloenzymes, as a metabolic regulator, as a mitogenic activator, and as an insulin-mimicking agent. Thus, vanadium chemistry with natural chelators, such as citrate, may have immediate implications on its role in a cellular milieu, and its action as a biological agent. In an effort to comprehend the aqueous chemistry of one of vanadium's oxidation states, namely, V(IV), implicated in its biological activity, reactions of VCl(3) and citric acid were pursued in water and led to V(IV)-citrate complexes, the nature and properties of which depend strongly on the solution pH. Analytical, FT-IR, UV/vis, EPR, and magnetic susceptibility data supported the formulation of X(4)[[VO(H(-1)Cit)](2)] x nH(2)O (H(-1)Cit = C(6)H(4)O(7)(4-); X = K(+), n = 6 (1); X = Na(+), n = 12 (2); X = NH(4)(+), n = 2 (3)) (pH approximately 8) and X(3)[[V(2)O(2)(H(-1)Cit)(Cit)]] x nH(2)O (X = K(+), n = 7 (4)) (pH approximately 5). Complex 2 crystallizes in space group P2(1)/c, a = 11.3335(9) A, b = 15.788(1) A, c = 8.6960(6) A, beta = 104.874(3) degrees, V = 1503.8(2), Z = 2. Complex 3 crystallizes in space group P one macro, a = 9.405(1) A, b = 10.007(1) A, c = 13.983(2) A, alpha = 76.358(4) degrees, beta = 84.056(4) degrees, gamma = 66.102(4) degrees, V = 1169.2(3), Z = 2. Complex 4 crystallizes in space group P2(1)nb, a = 9.679(4) A, b = 19.618(8) A, c = 28.30(1) A, V = 5374.0(4), Z = 8. The X-ray structures of 1-4 are V(2)O(2) dimers, with the citrate displaying varying coordination numbers and modes. 1 exhibits a small ferromagnetic interaction, whereas 4 exhibits an antiferromagnetic interaction between the V(IV) ions. 1 and 4 interconvert with pH, thus rendering the pH a determining factor promoting variable structural, electronic, and magnetic properties in V(IV)-citrate species. The observed aqueous behavior of 1-4 is consistent with past solution speciation studies, and contributes to the understanding of significant aspects of the biologically relevant vanadium(IV)-citrate chemistry.

The established biochemical potential of vanadium has spurred considerable research interest in our lab, with specific focus on pertinent synthetic studies of vanadium(III) with a biologically relevant, organic, dicarboxylic acid, malic acid, in aqueous solutions. Simple reactions between VCl3 and malic acid in water, at different pH values, in the presence of H2O2, led to the crystalline dimeric complexes (Cat)4[VO(O2)(C4H3O5)]2*nH2O (Cat = K+, n = 4, 1; Cat = NH4+, n = 3, 2) and K2[VO(O2)(C4H4O5)]2*2H2O (3). All three complexes were characterized by elemental analysis, FT-IR, and UV/visible spectroscopies. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.380(5) A, b = 9.252(5) A, c = 13.714(8) A, beta = 93.60(2) degrees, V = 1061(1) A3, and Z = 4. Compound 2 crystallizes in the triclinic space group P1, with a = 9.158(4) A, b = 9.669(4) A, c = 14.185(6) A, alpha = 104.81(1) degrees, beta = 90.31(1) degrees, gamma = 115.643(13) degrees, V = 1085.0(7) A(3), and Z = 2. Compound 3 crystallizes in the monoclinic space group P2(1)/c, with a = 9.123(8) A, b = 9.439(8) A, c = 10.640(9) A, beta = 104.58(3) degrees, V = 887(1) A3, and Z = 2. The X-ray structures showed that, in 1 and 2, the dimers consist of two (V(V)=O)2O2 rhombic units to which two malate ligands are attached. The ligands are triply deprotonated and, as such, they coordinate to vanadium(V), promoting a pentagonal bipyramidal geometry. In 3, the dimeric (V(V)=O)2O2 rhombic unit persists, with the two doubly deprotonated malate ligands coordinated to the vanadium(V) ions. UV/vis and EPR spectroscopic studies on the intermediate blue solutions of the synthesis reactions of 1-3 support the existence of vanadyl-containing dimeric species. These species further react with H2O2 to yield oxidation of V(IV)2O2 to V(V)2O2 and coordination of the peroxide to vanadium(V). From the collective data on 1-3, it appears that pH acts as a decisive factor in dictating the structural features of the isolated complexes. The details of the introduced structural differentiation in the reported complexes, and their potential relevance to vanadium(V) dicarboxylate systems in biological media are dwelled on.

Reactions of Al(III) and Ga(III) with citric acid in aqueous solutions, yielded the complexes (NH(4))(5)[M(C(6)H(4)O(7))(2)].2H(2)O (M(III) = Al (1), Ga (2)) at alkaline pH, and the complexes (Cat)(4)[M(C(6)H(5)O(7))(C(6)H(4)O(7))].nH(2)O (M(III) = Al (3), Ga (4), Cat. = NH(4)(+), n = 3; M(III) = Al (5), Ga (6), Cat. = K(+), n = 4) at acidic pH. All compounds were characterized by spectroscopic (FT-IR, (1)H, (13)C, and (27)Al NMR, (13)C-MAS NMR) and X-ray techniques. Complex 1 crystallizes in space group P1, with a = 9.638(5) A, b = 9.715(5) A, c = 7.237(4) A, alpha = 90.96(1) degrees, beta = 105.72(1) degrees, gamma = 119.74(1) degrees, V = 557.1(3) A(3), and Z = 1. Complex 2 crystallizes in space group P1, with a = 9.659(6) A, b = 9.762(7) A, c = 7.258(5) A, alpha = 90.95(2) degrees, beta = 105.86(2) degrees, gamma = 119.28(1) degrees, V = 564.9(7) A(3), and Z = 1. Complex 3 crystallizes in space group I2/a, with a = 19.347(3) A, b = 9.857(1) A, c = 23.412(4) A, beta = 100.549(5) degrees, V = 4389(1) A(3), and Z = 8. Complex 4 crystallizes in space group I2/a, with a = 19.275(1) A, b = 9.9697(6) A, c = 23.476(1) A, beta = 100.694(2) degrees, V = 4432.8(5) A(3), and Z = 8. Complex 5 crystallizes in space group P1, with a = 7.316(1) A, b = 9.454(2) A, c = 9.569(2) A, alpha = 64.218(4) degrees, beta = 69.872(3) degrees, gamma = 69.985(4) degrees, V = 544.9(2) A(3), and Z = 1. Complex 6 crystallizes in space group P1, with a = 7.3242(2) A, b = 9.4363(5) A, c = 9.6435(5) A, alpha = 63.751(2) degrees, beta = 70.091(2) degrees, gamma = 69.941(2) degrees, V = 547.22(4) A(3), and Z = 1. The crystal structures of 1-6 reveal mononuclear octahedral complexes of Al(III) (or Ga(III)) bound to two citrates. Solution NMR, on both 4- and 5- species, reveals rapid intramolecular exchange of the bound and unbound terminal carboxylates. Upon dissolution in water, the complexes, through a complicated reaction cascade, transform to oligonuclear 1:1 species that, in agreement with previous studies, represent the thermodynamically stable state in solution. The data provide, for the first time, structural details of low MW, mononuclear complexes of Al(III) (or Ga(III)) with citrate that are dictated, among other factors, by pH. The properties of 1-6 may provide clues relevant to their biological association with humans.

The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.

The significance of the interaction between aluminum and citrate in biological media is exemplified by the synthesis, isolation, structural characterization, and solution behavior of the first mononuclear aluminum citrate complex. The chemical and structural features of the low molecular weight aluminum citrate species characterized help gain insight into citrate's ability, as a natural ligand, to enhance aluminum's absorption and thus affect its accumulation and biotoxicity at biological sites.

Citric acid represents a class of carboxylic acids present in biological fluids and playing key roles in biochemical processes in bacteria and humans. Its ability to promote diverse coordination chemistries in aqueous media, in the presence of metal ions known to act as trace elements in human metabolism, earmarks its involvement in a number of physiological functions. Cobalt is known to be a central element of metabolically important biomolecules, such as B12, and therefore its biospeciation in biological fluids constitutes a theme worthy of chemical and biological perusal. In an effort to unravel the aqueous chemistry of cobalt in the presence of a physiologically relevant ligand, citrate, the first aqueous, soluble, mononuclear complex has been synthesized and isolated from reaction mixtures containing Co(II) and citrate in a 1:2 molar ratio at pH approximately 8. The crystalline compound (NH4)4[Co(C6H5O7)2] (1) has been characterized spectroscopically (UV/vis, EPR) and crystallographically. Its X-ray structure consists of a distorted octahedral anion with two citrate ligands fulfilling the coordination requirements of the Co(II) ion. The magnetic susceptibility measurements of 1 in the range from 6 to 295 K are consistent with a high-spin complex containing Co(II) with a ground state S=3/2. Corroborating this result is the EPR spectrum of 1, which shows a signal consistent with the presence of a Co(II) system. The spectroscopic and structural properties of the complex signify its potential biological relevance and participation in speciation patterns arising under conditions consistent with those employed for its synthesis and isolation.