Search

Formation and accumulation of extracellular amyloid-beta ( $\text{A}\beta )$ peptide aggregates in the brain are associated with Alzheimer’s disease, a widespread age-related form of dementia, for which there is no effective treatment at present. It is essential to discover ways of visualizing and monitoring the progress of aggregation toward amyloidic plaques while at the same time therapeutic methods inhibiting the aggregation can be explored. Iron oxide nanoparticles Fe3O4 with size ~10 nm have been coated with silica SiO2 and Thioflavin-T (ThT), a non-toxic fluorescent dye binding to the amyloid- $\beta $ structures with a possible effect on fibril formation, and then added to $\text{A}\beta _{\mathrm {1-42}}$ phosphate-buffered saline solution, incubated for 30 min and extracted by magnetic separation. The magnetic ThT silica-coated nanoparticles with size ~20 nm have been characterized structurally and magnetically by powder X-ray powder diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and vibrating sample magnetometer. Their affinity to $\text{A}\beta _{\mathrm {1-42}}$ peptide, examined by fluorescence microscopy and protein concentration measurements using the BCA Protein Assay Kit has been verified. Successful binding of magnetic silica nanoparticles, doped with ThT, to $\text{A}\beta _{\mathrm {1-42}}$ peptide provides a nanocarrier potentially exemplifying multiple functions, such as magnetic imaging, magnetic handling, drug delivery, fluorescence imaging, $\text{A}\beta $ binding, and $\text{A}\beta $ aggregation inhibition.

The pursuit of metal–organic materials involving In(III) with specified architecture and properties stands as a challenge in contemporary chemistry of hybrid inorganic–organic materials. Efforts in that direction have focused on the pH-specific synthesis of new compounds bearing a distinct family of organic ligands, i.e. α-hydroxycarboxylic citric, malic and lactic acid. The emerging new binary materials [In7(C6H4O7)8(H2O)]·(CH6N3)11·5H2O (1), [In(C4H4O5)2]n·nNH4·2nH2O (2) and [In(C3H5O3)2(C3H5.5O3)2] (3) reveal unique structural characteristics influencing lattice and spectroscopic properties. The physicochemical investigation of 1–3 (elemental analysis, FT-IR, NMR, X-ray crystallography, TGA) unravels variable nuclearity species of discrete and polymeric nature, essentially reflecting on the form of In(III) inorganic synthon and organic acid chelator properties interacting into various modes under pH-specific conditions. Mononuclear as well as polymeric composition, variable coordination number and ligand deprotonation state in the In(III) coordination sphere earmark the impact of the (a) coordination attributes of organic acids chelating In(III), with malic acid ostensibly possessing minimal requirements, beyond which multinuclear assemblies arise, and (b) use of appropriate inorganic/organic base to provide stability in the arisen lattice in the solid state. The architectural arrangement of the molecular assemblies in 1–3 project the importance of hydrogen-bonding interactions, which provide access to 2D lattices on the assembled metal–organic In(III) units through intervening water or counter ions. Collectively, the synthetic endeavor at the structural speciation level in binary aqueous In(III)-hydroxycarboxylic acid systems reveals a wealth of information, supporting key correlations on physicochemical profiling to be used further in the design and synthesis of new binary-ternary In(III) metal–organic materials in advanced applications.

Efforts to comprehend the structural–spectrochemical correlations of crystalline metal–organic framework materials of Pb(II) with (a)cyclic and aromatic chelators linked to photoluminescent applications led to the hydrothermal pH-specific synthesis of crystalline materials [Pb{H2BTC}(phen)(H2O)]n·2nH2O(1), [Pb2{CBTC}]n(2), [Pb4(phen)8{CBTC}2(H2O)4]3·70.3H2O(3), and [Pb{HCTA}(H2O)2]n·nH2O(4). X-ray studies showed that 1–4 exhibit unique architectures linked to 2D–3D coordination polymers formulated by Z-type units composed of Pb2O2 cores, unusually high number of lattice–water molecules, and π–π and H-bond interactions. The contribution of the nature–structure properties of the aliphatic-(a)cyclic organic (poly)carboxylic/aromatic chelators-ligands to binary-ternary Pb(II) reactivity weaves into the assembly of supramolecular networks, thereby providing clear structural–spectroscopic inter-relationships exemplifying the observed photoluminescent activity in a distinct MOF-linked fashion.

Efforts to probe and delineate intricate structure–property relationships key to the development of crystalline Pb(II)-containing metal–organic framework materials led to the design and pH-specific hydrothermal synthetic investigation of binary/ternary Pb(II)-(di)tricarboxylate ligand (succinic, glutaric, tricarballylic acids) systems in the presence of variable-nature aromatic N,N′-chelators bipyridine (bpy)/phenanthroline (phen). The arisen crystalline materials [Pb(phen)(suc)]n (1), [Pb3(phen)3(glu)3]n·7nH2O (2), [Pb3(tca)2]n (3), and [Pb2(phen)2(tcaH)2]n·nH2O (4) provide evidence for structural correlations linking the nature of ligands with Pb(II) chemistry and the emerging crystalline-polymeric assemblies. Detailed physicochemical characterization (Fourier transform infrared spectroscopy, 13C-,207Pb-cross polarization magic angle spinning NMR, thermogravimetric analysis, luminescence) reveals distinct architecture, lattice dimensionality (2D-3D), and luminescence property correlations and identifies...

The chemical reactivity of binary and ternary ZrIV/hydroxycarboxylic acid/(N,N-aromatic chelator) systems (hydroxycarboxylic acid = citric, malic acid; N,N-aromatic chelator = 1,10-phenanthroline) has been investigated in a pH-specific manner, giving rise to the new binary and ternary crystalline materials [Zr2F6(C6H4O7)2](CH6N3)6·H2O (1), [Zr2F6(C4H3O5)2](NH4)4·3H2O (2), [Zr2(C4H3O5)2(C4H4O5)2(C4H5O5)2](CH6N3)4·4.5H2O (3), and [Zr2(C4H3O5)2(C4H4O5)2(C4H5O5)2](C12H8N2)(C12H9N2)2(CH6N3)2·6H2O (4). Compounds 1–4 were characterized by elemental analysis, FTIR spectroscopy, TGA, 13C-CPMAS, 19F-DPMAS, and 1H CRAMPS NMR spectroscopy, and X-ray crystallography. The structures of 1–4 reveal the influence of fluoride in the starting ZrIV reagents on the identity of the Zr2O2-containing dinuclear assemblies in the arisen 3D architectures. The uniquely defined spectroscopic data, including the NMR data, corroborate the X-ray structures of 1–4 and reflect the contribution of the intra/intermolecular interactions to the structural speciation of the investigated systems. Collectively, the work 1) depicts the factors that determine the nature and properties of the emerging synthetic materials in the investigated systems, 2) formulates structural architecture–lattice dimensionality and spectroscopic fingerprint correlations, and 3) projects the framework through which new ZrIV materials with defined physicochemical properties can arise at the binary and ternary level.

To probe and understand the structural and coordinative flexibility of Group 13 ions with α-hydroxycarboxylic acids, leading to crystalline inorganic–organic hybrid materials with distinct lattice architecture, dimensionality, and spectroscopic properties, the systematic synthesis and physicochemical properties of binary and ternary B(III), Al(III), Ga(III), In(III), and Tl(I)-benzilic acid-(phenanthroline) systems were investigated in water–alcohol mixtures. Stoichiometric reactions of Group 13 ions with benzilic acid and phenanthroline (phen) afforded the new materials [B(C14H10O3)2](C3H5N2)·H2O (1), [Al(C14H11O3)3]·0.5C2H5OH·4.5H2O (2), [Ga(C14H11O3)3]·CH3OH·3H2O (3), [In(C14H11O3)4]·C3H5N2·C2H5OH·H2O (4), [Tl(C14H11O3)]n (5), [Tl2(C14H11O3)2(phen)2] (6), and [Tl(C14H11O3)(phen)(H2O)](C14H12O3)(phen) (7). All materials were characterized by elemental analysis, Fourier transform infrared spectroscopy, 13C, 11B, 27Al, 71Ga, and 205Tl cross-polarization/magic-angle spinning NMR, thermogravimetric analysis...

The structure of the title centrosymmetric compound, [Zn(C9H6NO)2(H2O)2], has already been solved three times [Merritt, Cady & Mundy (1954).Acta Cryst.7, 473–476; Palenik (1964).Acta Cryst.17, 696–700; Chen, Zhang, Shi, Huang, Liang & Zhou (2003).Acta Cryst.E59, m814–m815]. The authors of the two most recent papers state that they attained lowerR1 values than that obtained in the 1954 paper, but they do not mention that Merrittet al.had derived the structural model from a twinned crystal. Also, from a structural point of view, there are strong indications that the most recent report is in fact the isostructural CuIIcomplex already reported by Okabe & Saishu [Acta Cryst.(2001), E57, m251–m252] and not the ZnIIcomplex. The structure of the title compound is reported here based on data obtained from a twinned crystal.

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.

Co(II) and Zn(II) ions exhibit variable reactivity toward O-containing ligands in aqueous media, affording isolable materials with distinct solid-state lattice properties. d-(-)-quinic acid is a cellular α-hydroxycarboxylate metal ion binder, which reacts with Co(II) and Zn(II) under pH-specific hydrothermal conditions, leading to the isolation of two new species [Co(2)(C(7)H(11)O(6))(4)](n)·nH(2)O (1) and [Zn(3)(C(7)H(11)O(6))(6)](n)·nH(2)O (2). Compound 1 was characterized by elemental analysis, spectroscopic techniques (FT-IR, UV-visible, EPR), magnetic studies, and X-ray crystallography. Compound 2 was characterized by elemental analysis, spectroscopic techniques (FT-IR, ESI-MS), and X-ray crystallography. The 2D molecular lattices in 1 and 2 reveal the presence of octahedral M(II) units bound exclusively to quinate in a distinct fashion, thereby projecting a unique chemical reactivity in each investigated system. The magnetic susceptibility and solid-state/frozen solution 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 of S = 1/2. Concurrent aqueous speciation studies on the binary Zn(II)-quinate system unravel the nature and properties of species arising from Zn(II)-quinate interactions as a function of pH and molar ratio. The physicochemical profiles of 1 and 2, in the solid state and in solution, earmark the importance of (a) select synthetic hydrothermal reactivity conditions, affording new well-defined lattice dimensionality and nuclearity M(II)-quinate materials, (b) structural speciation approaches delineating solid state-aqueous solution correlations in the binary M(II)-quinate systems, and (c) pH-specific chemical reactivity in binary M(II)-quinate systems reflecting structurally unique associations of simple aqueous complexes into distinctly assembled 2D crystalline lattices.

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.

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.

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)

Among the biotargets interacting with vanadium is the calcium pump from the sarcoplasmic reticulum (SR). To this end, initial research efforts were launched with two vanadium(V)-citrate complexes, namely (NH(4))(6)[V(2)O(4)(C(6)H(4)O(7))(2)].6H(2)O and (NH(4))(6)[V(2)O(2)(O(2))(2)(C(6)H(4)O(7))(2)].4H(2)O, potentially capable of interacting with the SR calcium pump by combining kinetic studies with (51)V NMR spectroscopy. Upon dissolution in the reaction medium (concentration range: 4-0.5mM), both vanadium(V):citrate (VC) and peroxovanadium(V):citrate (PVC) complexes are partially converted into vanadate oligomers. A 1mM solution of the PVC complex, containing 184microM of the PVC complex, 94microM oxoperoxovanadium(V) (PV) species, 222microM monomeric (V1), 43microM dimeric (V2) and 53microM tetrameric (V4) species, inhibits Ca(2+) accumulation by 75 %, whereas a solution of the VC complex of the same vanadium concentration, containing 98microM of the VC complex, 263microM monomeric (V1), 64microM dimeric (V2) and 92microM tetrameric (V4) species inhibits the calcium pump activity by 33 %. In contrast, a 1 mM metavanadate solution, containing 460microM monomeric (V1), 90.2microM dimeric (V2) and 80microM tetrameric (V4) species, has no effect on Ca(2+) accumulation. The NMR signals from the VC complex (-548.0ppm), PVC complex (-551.5ppm) and PV (-611.1ppm) are broadened upon SR vesicle addition (2.5mg/ml total protein). The relative order for the half width line broadening of the NMR signals, which reflect the interaction with the protein, was found to be V4>PVC>VC>PV>V2=V1=1, with no effect observed for the V1 and V2 signals. Putting it all together the effects of two vanadium(V)-citrate complexes on the modulation of calcium accumulation and ATP hydrolysis by the SR calcium pump reflected the observed variable reactivity into the nature of key species forming upon dissolution of the title complexes in the reaction media.

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 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.

Iron is an essential metal ion with numerous roles in biological systems and advanced abiotic materials. D-(-)-quinic acid is a cellular metal ion chelator, capable of promoting reactions with metal M(II,III) ions under pH-specific conditions. In an effort to comprehend the chemical reactivity of well-defined forms of Fe(III)/Fe(II) toward α-hydroxycarboxylic acids, pH-specific reactions of: (a) [Fe3O(CH3COO)6(H2O)3]·(NO3)·4H2O with D-(-)-quinic acid in a molar ratio 1:3 at pH 2.5 and (b) Mohr's salt with D-(-)-quinic acid in a molar ratio 1:3 at pH 7.5, respectively, led to the isolation of the first two heptanuclear Fe(III)-quinato complexes, [Fe7O3(OH)3(C7H10O6)6]·20.5H2O (1) and (NH4)[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O (2). Compounds 1 and 2 were characterized by analytical, spectroscopic (UV-vis, FT-IR, EPR, and Mössbauer) techniques, CV, TGA-DTG, and magnetic susceptibility measurements. The X-ray structures of 1 and 2 reveal heptanuclear assemblies of six Fe(III) ions bound by six doubly deprotonated quinates and one Fe(III) ion bound by oxido- and hydroxido-bridges (1), and hydroxido-bridges (2), all in an octahedral fashion. Mössbauer spectroscopy on 1 and 2 suggests the presence of Fe(III) ions in an all-oxygen environment. EPR measurements indicate that 1 and 2 retain their structure in solution, while magnetic measurements reveal an overall antiferromagnetic behavior with a ground state S = 3/2. The collective physicochemical properties of 1 and 2 suggest that the (a) nature of the ligand, (b) precursor form of iron, (c) pH, and (d) molecular stoichiometry are key factors influencing the chemical reactivity of the binary Fe(II,III)-hydroxycarboxylato systems, their aqueous speciation, and ultimately through variably emerging hydrogen bonding interactions, the assembly of multinuclear Fe(III)-hydroxycarboxylato clusters with distinct lattice architectures of specific dimensionality (2D-3D) and magnetic signature.

Synthetic efforts linked to the design of defined lattice dimensionality and architecture materials in the binary/ternary systems of Cu(II) with butylene diamine tetra(methylene phosphonic acid) (H8BDTMP) and heterocyclic organic chelators (pyridine and 1,10-phenanthroline) led to the isolation of new copper organophosphonate compounds, namely, Na6[Cu2(BDTMP)(H2O)4]·[Cu2(BDTMP)(H2O)4]0.5·26H2O (1), [Cu2(H4BDTMP)(py)4]·2H2O (2), and [Cu2(H4BDTMP)(phen)2]n·6.6nH2O·1.5nMeOH (3). 1-3 are the first compounds isolated from the Cu(II)-BDTMP family of species. They were characterized by elemental analysis, spectroscopic techniques (FT-IR, UV-vis), magnetic susceptibility, TGA-DTG, cyclic voltammetry, and X-ray crystallography. The lattice in 1 reveals the presence of discrete dinuclear Cu(II) units bound to BDTMP(8-) and water molecules in a square pyramidal geometry. The molecular lattice of 2 reveals the presence of ternary dinuclear assemblies of Cu(II) ions bound to H4BDTMP(4-) and pyridine in a square pyramidal environment. The molecular lattice of 3 reveals the presence of dinuclear assemblies of Cu(II) ions bound to H4BDTMP(4-) and 1,10-phenanthroline in a square pyramidal environment, with the organophosphonate ligand serving as the connecting link to abutting dinuclear Cu(II) assemblies in a ternary polymeric system. The magnetic susceptibility data on 1, 2, and 3 suggest that compounds 1 and 3 exhibit a stronger antiferromagnetic behavior than 2, which is also confirmed from magnetization measurements. The physicochemical profiles of 1-3 (a) earmark the influence of the versatile H8BDTMP ligand as a metal ion binder on the chemical reactivity in binary and ternary systems of Cu(II) in aqueous and nonaqueous media and (b) denote the correlation of ligand hydrophilicity, aromaticity, denticity, charge, and H-bonding interactions with emerging defined Cu(II)-H8BDTMP structures of distinct lattice identity and spectroscopic-magnetic properties. Collectively, such structural and chemical factors formulate the interplay and contribution of binary and ternary interactions to lattice architecture and specified properties of new Cu(II)-organophosphonate materials with defined 2D-3D dimensionality.

The interaction among the hydroxylase component of methane monooxygenase (MMO) from Methylococcus capsulatus (Bath), the coupling protein of the MMO enzyme system (component B), and substrate has been investigated by using Fe K-edge X-ray absorption spectroscopy (XAS). Fe K-edge extended X-ray absorption fine structure (EXAFS) studies of the semimet form of the hydroxylase in the presence of the coupling protein, 1-bromo-1-propene, and both the coupling protein and 1-bromo-1-propene revealed small differences in the appearance of the EXAFS above k = 8 {Angstrom}{sup {minus}1} as compared to the noncomplexed hydroxylase. No dramatic change in the Fe coordination was seen in fits to the data. The average first shell Fe-O/N distance for the complexed forms of the semimet hydroxylase ranged between 2.06 and 2.08 {Angstrom}, which is comparable to the distance found for the noncomplexed form, 2.06-2.09 {Angstrom}. Although the average first shell coordination was similar for all samples, a difference was seen in the distribution of long vs short distance contributions to the first shell coordination sphere for samples with component B present. This difference was accompanied by a small but consistent decrease in the Fe-Fe distance of the B-complexed hydroxylase samples, from 3.42 to 3.39 {angstrom}.

The interactions of bovine serum albumin (BSA) with ionic surfactants (sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB) and β-cyclodextrin (β-CD) have been investigated by electron paramagnetic resonance (EPR) and circular dichroism measurements. The spin probe selected to report on the interaction of albumin with surfactants and/or β-CD was 4-N,N-dimethyl hexadecyl ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT16), on account of (a) its balance between electrostatic and hydrophobic character and (b) the ability of BSA to form complexes with various organic molecules. The distribution of the spin probe among different environments in solutions containing only BSA was confirmed by the existence of two components in the EPR spectra: one revealing a restricted mobility of the spin probe, attributed to the protein-spin probe complex, and another one showing free movement, attributed to the spin probe in solution. The presence of surfactants and/or β-CD alters the distribution of CAT16 between various compartments in each system. Formation of protein aggregates as a result of thermal denaturation was evidenced by the appearance of an immobilized component in the EPR spectrum. This component is not present in the EPR spectra of CAT16 in protein/surfactant or protein/cyclodextrin solutions. Circular dichroism spectra of BSA provided information about changes in the secondary structure of the protein induced by the presence of surfactants and/or cyclodextrin in solution. The results demonstrate that β-CD hinders the interaction between the employed surfactants and the protein. The cationic surfactant (CTAB) induces changes in protein conformation at a lower concentration compared to the anionic surfactant (SDS).

The synthesis, characterization, and reactions of the new «cubane»-type clusters (ET 4 N) 3 [Fe 3 S 4 L 3 M(CO) 3 ], M=MO, L=EtS - , BzS - , p-OMePhS - , PhO - , Cl - ; M=W, L=p-OMePhS - , are described in detail. These clusters are obtained in a good yield as (ET 4 N) + salts, by the reaction of the [Fe 3 S 4 L 4 ] 3- linear trimer with M(CO) 3 (MeCn) 3 , M=Mo, W

In a pH-specific fashion, V(2)O(5) and citric acid in the absence and presence of H(2)O(2) reacted and afforded, in the presence of NaOH and (CH(6)N(3))(2)CO(3), two new dinuclear V(V) binary non-peroxo (CH(6)N(3))(6)[V(2)O(4)(C(6)H(4)O(7))(2)].2H(2)O (1) and ternary peroxo (CH(6)N(3))(4)[V(2)O(2)(Omicron(2))(2)(C(6)H(5)O(7))(2)].6Eta(2)Omicron (2) species, respectively. Complexes 1 and 2 were further characterized by elemental analysis, UV/Vis, FT-IR, NMR (solution and solid state Cross Polarization-Magic Angle Spinning (CP-MAS)) and Raman spectroscopies, cyclic voltammetry, and X-ray crystallography. Both 1 and 2 are members of the family of dinuclear V(V)-citrate species bearing citrate with a distinct coordination mode and degree of deprotonation, with 2 being the missing link in the family of pH-structural variants of the ternary V(V)-peroxo-citrate system. Given that 1 and 2 possess distinct structural features, relevant binary V(III), V(IV) and V(V), and ternary V(V) species bearing O- and N-containing ligands were tested in in vitro cell cultures to assess their cellular toxicity and insulin mimetic capacity. The results project a clear profile for all species tested, earmarking the importance of vanadium oxidation state and its ligand environment in influencing further binary and ternary interactions of vanadium arising with variable mass cellular targets, ultimately leading to a specific (non)toxic phenotype and glucose uptake ability.

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.

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.