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Square planar complexes of Ni(II) and Pd(II) of a new redox-active pentadentate azo-appended 2-aminophenol ligand (H4L = N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino-ortho-azobenzene) in three accessible redox levels [amidophenolate(2−), semiquinonate(1−) π radical, and quinone(0)] were synthesized. The coordinated HL(3−) ligand provides four donor sites [two N(iminophenolates), an N′(azo), and an O(phenolate)], while the phenolic OH group remains free in the three complexes. Cyclic voltammetry on complex [Ni(L)] 1 and its corresponding Pd(II) analogue [Pd(L)] 2 in CH2Cl2 displayed three redox responses (two oxidative at E1/2 = 0.06 V and Epa (anodic peak potential) = 0.80 V and one reductive at −0.77 V for 1 and at E1/2 = 0.08 V and Epa = 0.85 V and at −0.74 V for 2vs. Fc+/Fc). The chemical oxidation of 1 with AgSbF6 afforded [Ni(L)]SbF6·2CH2Cl2 (3·2CH2Cl2). Complex [Pd(L*)] 4, which is coordinated by a phenoxazinyl derivative of L(4−), was obtained via intraligand cyclization in the parent complex 2 under basic oxidizing conditions. The molecular structures of 1, 2, 3·2CH2Cl2 and 4 were elucidated through X-ray crystallography at 100 K. Characterization using 1H NMR, X-band EPR, and UV–VIS–NIR spectroscopy established that the complexes have [NiII{(LISQ)˙2−}] 1, [PdII{(LISQ)˙2−}] 2, [NiII{(LIBQ)−}]SbF6/1+SbF6−(3), and [PdII{(L*AP)˙2−}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic St (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (St = 0). Density functional theory (DFT) electronic structural calculations at the B3LYP level rationalized the observed experimental results. Time-dependent (TD)-DFT calculations allowed us to identify the nature of the observed absorption spectra. [ABSTRACT FROM AUTHOR]

Abstract: Pyridine-2-carboxamide and pyridine-2,6-dicarboxamide-based chelating ligands form a variety of coordination complexes with a number of metal ions, providing varying coordination geometry and nuclearity. Recent years have seen considerable interest in the designing of this class of ligands and to study their structural properties to serve specific stereochemical requirement of a particular metal-binding site. Notably, this class of ligands has been extensively utilized by Mascharak and co-workers to provide low-molecular-weight representations of metallo-proteins/enzymes such as bleomycins, nitrile hydratase. Moreover, the transition metal complexes of this class of ligands are being used as various exogenous nitric oxide (NO) donors. Using over 60 this class of chelating carboxamide ligands, the stereochemical properties of over 150 discrete coordination complexes, studied by single-crystal X-ray crystallography have been analyzed. Various bonding modes for a given chelating ligand are involved, and are reviewed with reference to ligand structure and the resulting coordination complexes. It is shown that the complexes synthesized have served to address notable issues such as effect of ligand structure/donor atom on metal-centered redox potentials, change of spin-state of iron(III), ligand-radical coordinated metal-complexes, interesting chemical reactivity studies, and catalytic potential. The ligands are introduced systematically as a function of their denticity, making easy access to information on specific type of ligands and coordination complexes thereof. X-ray crystallographically determined bond lengths of various donor atoms/groups are collected in a table, thus providing an accessible source for reference purposes. Source material for the review amounts to about 90 references. [Copyright &y& Elsevier]

It is the intention of this Perspectiveto review the literature involving the synthesis of coordination polymers of varying dimensionality utilizing a class of chelating ligands comprising of (2-pyridyl)alkylamine-appended carboxylates performed in our group and by others. From the standpoint of functional aspects, we will focus on the magnetic properties of the chosen systems. In essence, we will provide a snapshot of this research field. [ABSTRACT FROM AUTHOR]

It is the intention of this Perspectiveto review the literature involving the synthesis of coordination polymers of varying dimensionality utilizing a class of chelating ligands comprising of (2-pyridyl)alkylamine-appended carboxylates performed in our group and by others. From the standpoint of functional aspects, we will focus on the magnetic properties of the chosen systems. In essence, we will provide a snapshot of this research field. [ABSTRACT FROM AUTHOR]

Abstract: Structural analysis of a previously reported half-sandwich complex having three-legged “piano-stool” geometry [(η6-C6H6)RuII(L1)Cl][PF6] (1) (L1 = 2-(pyrazol-1-ylmethyl)pyridine) is described. Treatment of 1 with (i) Ag(CF3SO3) in CH3CN and (ii) NaN3 in CH3OH, and (iii) the reaction between [(η6-C6H6)Ru(L2)Cl]-[PF6] (2) (previously reported) and NaCN in C2H5OH led to the isolation of [(η6-C6H6)Ru(L1)(CH3CN)][PF6]2 (3), [(η6-C6H6)Ru(L1)(N3)][PF6] (4), and [(η6-C6H6)Ru(L2)(CN)][PF6] (5), respectively (L2 = 2-(3,5-dimethyl-pyrazol-1-ylmethyl)pyridine). The complex [(η6-C6H6)Ru(L4)Cl][PF6] (6) with a new ligand (L4 = 2-[3-(4-fluorophenyl)pyrazol-1-ylmethyl]pyridine) has also been synthesized. The structures of 3–6 have been elucidated (1H NMR spectra; CD3CN). The molecular structures of 1, 4, and 6·C6H5CH3 have been determined. Notably, the crystal-packing in these structures is governed by C–H⋯X (X = Cl, N) interactions, generating helical architectures. [Copyright &y& Elsevier]

The spin-transition (1A1↔5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2′-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom. [ABSTRACT FROM AUTHOR]

The title compound [FeII(L)2][ClO4]2·C7H8 (L = 2-[3-(2ʹ-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [FeII(L)2][ClO4]2, published earlier (Dalton Trans. 2003, 3392-3397). Magnetic susceptibility measurements, as well as Mössbauer and calorimetric investigations on polycrystalline samples of [Fe(L)2][ClO4]2·C7H8 revealed the occurrence of an abrupt HS (5T2) ↔ LS (¹A1) transition with steep and narrow (2 K) hysteresis at ∼232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale. [ABSTRACT FROM AUTHOR]

Reaction of N-(phenyl)-2-pyridinecarboxamide (HL¹) and N-(p-tolyl)-2-pyridinecarboxamide (HL²) ligands with MnCl2·4H2O affords complexes [(HL¹)2MnCl2] 1 and [(HL²)2MnCl2] 2. The structures of 1 and 2 were determined by three-dimensional X-ray crystallography revealing that the MnII ions assume distorted octahedral geometry with coordination by two HL¹/HL² ligands providing two pyridine N and two amide O and two chloride ions. Notably, secondary interactions [C-H…Cl (pyridine 3-H hydrogen) and N-H…Cl (amide NH hydrogen)] triggered by MnII-coordinated chloride ions acting as hydrogen bonding acceptors generate self-complementary dimeric tectons, which lead to 2D supramolecular architectures. [ABSTRACT FROM AUTHOR]

Abstract: Reaction of the ligand N-methyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylethyl)amine (mpppa) with equimolar amounts of [Cu(H2O)6][ClO4]2 or CuCl2 ·2H2O in MeCN afforded mononuclear copper(II) complexes [Cu(mpppa)][ClO4]2 (1) and [Cu(mpppa)Cl2] (2). Crystal structure analysis reveals CuN3O (two pyridyl, an aliphatic amine, and an amide oxygen) coordination in 1 and CuN3Cl2 (two pyridyl, an aliphatic amine, and two chlorides) coordination in 2. Crystal packing diagram of 1 reveals that one of the perchlorate counteranions provides weak coordination to copper(II) centers and in turn the copper(II) centers assume pseudo-six-coordination, generating 1D chain. Notably, one of the copper(II)-coordinated chloride ions in 2 participates in an intramolecular N–H⋯Cl interaction. Intermolecular C–H⋯Cl interactions in the solid state generate helical structure. Spectroscopic (IR, UV–Vis, and EPR) and redox properties of the two complexes have been investigated and compared. [Copyright &y& Elsevier]

Abstract: Synthesis and crystal structure of two coordination polymers of composition [MnII(H2bpbn)1.5][ClO4]2 ·2MeOH·2H2O (1) and [CoII(H2bpbn)(H2O)2]Cl2 ·H2O (2) [H2bpbn= N,N′-bis(2-pyridinecarboxamido)-1,4-butane], formed from the reaction between [Mn(H2O)6][ClO4]2/CoCl2 ·4H2O with H2bpbn in MeCN, are described. In 1 each MnII ion is surrounded by three pyridine amide units, providing three pyridine nitrogen and three amide oxygen donors. Each MnII center in 1 has distorted MnN3O3 coordination. In 2 each CoII ion is coordinated by two pyridine amide moieties in the equatorial plane and two water molecules provide coordination in the axial positions. Thus, the metal center in 2 has trans-octahedral geometry. In both 1 and 2, the existence of 1D zigzag network structure has been revealed. Owing to π–π stacking of pyridine rings from adjacent layers 1 forms 2D network; 2 forms 2D and 3D network assemblies via N–H⋯Cl and O–H⋯Cl secondary interactions. Both the metal centers are high-spin. [Copyright &y& Elsevier]

Abstract: A new distorted square planar (two CuN2 planes making an angle of ∼43°) copper(II) complex [Cu(L4)]·0.5EtOH·0.5MeOH (1) of a deprotonated tetradentate pyridine amide ligand [H2L4 = N,N′-bis(2-pyridinecarboxamide)-2,2′-biphenyl] has been synthesized and structurally characterized. Absorption and EPR spectroscopic properties have also been studied. The E 1/2 values (CuII/CuI redox process) of the title complex along with a selected group of structurally characterized CuN4 pyridine amide complexes with systematically varied structural, electronic/steric, and chelate-ring size effects, imposed by the coordinating ligands, have been determined and the observed trend has been rationalized. [Copyright &y& Elsevier]

Abstract: Four half-sandwich ruthenium(II) complexes [(η6-C6H6)Ru(L1-O)][PF6] (1), [(η6-C6H6)Ru(L2-O)][PF6] (2), [(η6-C6H6)Ru(L3-O)][PF6] (3), [(η6-C6H6)Ru(L4-O)][PF6] (4a), and [(η6-C6H6)Ru(L4-O)][BPh4] (4b) [L1-OH, 4-nitro-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L2-OH, 2,4-di-tert-butyl-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L3-OH, 2,4-di-tert-butyl-6-{[2′-((pyridin-2-yl)benzylamino)methyl}-phenol; L4-OH, 2,4-di-tert-butyl-6-{[(2′-imethylaminoethyl)methylamino]methyl}-phenol (L4-OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported. The molecular structures of 1–3, 4a, and 4b have been elucidated in solution using 1H NMR spectroscopy and of 1, 3, and 4b in the solid state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central amine nitrogen also becomes chiral. The 1H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely diastereoselective [1: S Ru,S N/R Ru,R N; 2: R Ru,R N/S Ru,S N; 3: S Ru,R N/R Ru,S N; 4b: S Ru,R N/R Ru,S N]. The crystal packing in 1 and 3 is stabilized by C–H…O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a one-electron oxidized phenoxyl radical coordinated species [2]2+ , [3]2+ , and [4a]2+ , respectively. The radical species are characterized by CV, UV–Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant (UV–Vis spectroscopy). [Copyright &y& Elsevier]

Abstract: We have prepared and structurally characterized six-coordinate Fe(II), Co(II), and Ni(II) complexes of types [MII(HL1)2(H2O)2][ClO4]2 (M=Fe, 1; Co, 3; and Ni, 5) and [MII(HL2)3][ClO4]2 ·MeCN (M=Fe, 2 and Co, 4) of bidentate pyridine amide ligands, N-(phenyl)-2-pyridinecarboxamide (HL1) and N-(4-methylphenyl)-2-pyridinecarboxamide (HL2). The metal centers in bis(ligand)-diaqua complexes 1, 3 and 5 are coordinated by two pyridyl N and two amide O atoms from two HL1 ligands and six-coordination is completed by coordination of two water molecules. The complexes are isomorphous and possess trans-octahedral geometry. The metal centers in isomorphous tris(ligand) complexes 2 and 4 are coordinated by three pyridyl N and three amide O atoms from three HL2 ligands. The relative dispositions of the pyridine N and amide O atoms reveal that the pseudo-octahedral geometry have the meridional stereochemistry. To the best of our knowledge, this work provides the first examples of structurally characterized six-coordinate iron(II) complexes in which the coordination is solely by neutral pyridine amide ligands providing pyridine N and amide O donor atoms, with or without water coordination. Careful analyses of structural parameters of 1–5 along with that reported in the literature [MII(HL1)2(H2O)2][ClO4]2 (M=Cu and Zn) and [CoIII(L2)3] have allowed us to arrive at a number of structural correlations/generalizations. The complexes are uniformly high-spin. Spectroscopic (IR and UV/Vis) and redox properties of the complexes have also been investigated. [Copyright &y& Elsevier]

Abstract: The ligand 1,3-bis[(2-dimethylaminoethyl)iminomethyl]benzene (baib) reacts with [Cu(MeCN)4][ClO4] to form a binuclear copper(I) complex . Crystal structure analysis reveals that the distorted tetrahedral coordination of each copper(I) center is satisfied by one bidentate arm of each ligand. The complex undergoes ready aromatic ring hydroxylation at position 2 of the phenyl ring when reacted with molecular oxygen in MeCN/MeOH/CH2Cl2, producing a four-coordinate μ-phenoxo- and μ-hydroxo-bridged dicopper(II) complex, [Cu2(baib-O)(OH)(OClO3)2]·1.5H2O (2) (baib-OH: 1,3-bis[(2-dimethylaminoethyl)iminomethyl]phenol). This reaction mimics the reactivity of the copper monooxygenase tyrosinase. A trend is observed for the extent of aromatic ring hydroxylation (25°C): MeCN>MeOH>CH2Cl2. Cyclic voltammetric experiment of 1 in MeCN reveals an appreciably high redox potential (anodic peak potential, E pa =0.69V versus SCE) for the redox process. Complex 2 has been characterized by single-crystal X-ray crystallography. Variable temperature (60–300K) magnetic susceptibility measurements on 2 establish that the copper(II) centers in 2 are antiferromagnetically coupled (2J =−280cm−1). [Copyright &y& Elsevier]

Abstract: New series of half-sandwich ruthenium(II) complexes supported by a group of bidentate pyridylpyrazole and pyridylimidazole ligands [(η6-C6H6)Ru(L2)Cl][PF6] (1), [(η6-C6H6)Ru(HL3)Cl][PF6] (2), [(η6-C6H6)Ru(L4)Cl][PF6] (3), and [(η6-C6H6)Ru(HL5)Cl][PF6] (4) [L2, 2-[3-(4-chlorophenyl)pyrazol-1-ylmethyl]pyridine; HL3, 3-(2-pyridyl)pyrazole; L4, 1-benzyl-[3-(2′-pyridyl)]pyrazole; HL5, 2-(1-imidazol-2-yl)pyridine] are reported. The molecular structures of 1–4 both in the solid state by X-ray crystallography and in solution using 1H NMR spectroscopy have been elucidated. Further, the crystal packing in the complexes is stabilized by C–H⋯X (X=Cl and π), N–H⋯Cl, and π–π interactions. [Copyright &y& Elsevier]

Abstract: Two new coordination polymers [(L6)CoCl2]∞ (1) and [(L6)CdCl2]∞ (2) (L6 =α,α′-bis(pyrazolyl)-m-xylene) have been synthesized and characterized by X-ray crystallography. Structural analysis of 1 (space group P21) revealed that each Co(II) center is coordinated by two chloride ions and two pyrazole nitrogens from two different L6 ligands, giving rise to pseudo-tetrahedral coordination environment around Co(II). For this structure 1D helical polymeric chain running along crystallographic 21 axis is observed. While for 2 (space group ), each Cd(II) center is coordinated by two chloride ions, two pyrazole nitrogens from two different L6 ligands, and a chloride ion from the adjacent layer. It provides distorted trigonal bipyramidal geometry around Cd(II), which leads to the formation of a 2D zigzag polymer. In continuation of our recent activity on inorganic crystal engineering, from the standpoint of metal–ligand coordination chemistry and C–H⋯Cl2MII hydrogen bonding, in this work we have investigated the potential of coordination tectons 1 and 2 in bringing about generality in helix/zigzag network formation, strengthened primarily by C–H⋯Cl hydrogen-bonding interactions. [Copyright &y& Elsevier]

Acyclic pyridine-2-carboxamide- and thioether-containing hexadentate ligand 1,4-bis[o-(pyridine-2-carboxamidophenyl)]1,4-dithiobutane (H2bpctb), in its deprotonated form, has afforded purple low-spin (S = 0) iron(II) complex [Fe(bpctb)] (1). A new ligand, the pyrazine derivative of H2bpctb, 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2bpzctb), has been synthesized which has furnished the isolation of purple iron(II) complex [Fe(bpzctb)]· CH2Cl2 (4) (S = 0). Chemical oxidation of 1 by [(η5-C5H5)2Fe][PF6] or [Ce(NO3)6][NH4]2 led to the isolation of low-spin (S = ½) green Fe(II) complexes [Fe(bpctb)][PF6] (2) or [Fe(bpctb)][NO3]·H2O (3), and oxidation of 4 by [Ce(NO3)6][NH4]2 afforded [Fe(bpzctb)][NO3]·H2O (5) (S = ½). X-ray crystal structures of 1 and 4 revealed that (i) in each case the ligand coordinates in a hexadentate mode and (ii) bpzctb2- binds more strongly than bpctb(2-), affording distorted octahedral MIIN2(pyridine/pyrazine)N′2(amide)S2(thioether) coordination. To the best of our knowledge, 1 and 4 are the first examples of six-coordinate low-spin Fe(II) complexes of deprotonated pyridine/ pyrazine amide ligands having appended thioether functionality. The Fe(III) complexes display rhombic EPR spectra. Each complex exhibits in CH2Cl2/MeCN a reversible to quasireversible cyclic voltammetric response, corresponding to the FeIII-FeII redox process. The E½ value of 4 is more anodic by ∼0.2 V than that of 1, attesting that compared to pyridine, pyrazine is a better stabilizer of iron(II). Moreover, the E½ value of 1 is significantly higher (∼1.5 V) than that reported for six-coordinate Fe(II)/Fe(III) complexes of the tridentate pyridine-2-carboxamide ligand incorporating thiolate donor site. [ABSTRACT FROM AUTHOR]

A group of a diverse family of dinuclear copper(II) complexes [Cu2(L1&z.sbnd;O)(μ-OH)][ClO4]2 (1), [Cu2(L2&z.sbnd;O)(μ-OH)][ClO4]2 (2), [Cu2(L3&z.sbnd;O)(μ-C3H3N2)(OClO3)(H2O)][ClO4]·H2O (3), [Cu2(L4)(μ-OH)2][ClO4]2·H2O (4), [Cu2(L5&z.sbnd;O)2(OClO3)2] (5) and [Cu2(bpy)2(μ-OH)(μ-OH2)(μ-O2CMe)][ClO4]2 (6) has been chosen to investigate their potential as catalysts for the aerial oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) (catecholase activity) [L1&z.sbnd;OH, 2,6-[N-methyl-N-(2-pyridylethyl)aminomethyl]phenol; L2&z.sbnd;OH, 2,6-bis[N-(2-pyridylethyl)iminomethyl]phenol; L3&z.sbnd;OH, 1,3-bis[N-methyl-N-(2-pyridylethyl)amino]propan&z.sbnd;2-ol; L4, α,α′-bis[N-(6-methyl-2-pyridylmethyl)-N-(pyrazol-1-ylmethyl)amino]&z.sbnd;m-xylene; L5&z.sbnd;OH, 4-methyl-2,6-bis(pyrazol-1-ylmethy)phenol; bpy, 2,2′-bipyridine; C3H3N2, pyrazolate ion; H, dissociable hydrogen atom]. The synthesis of a new m-xylyl-based dinucleating ligand L4 is described. Using this ligand a new dihydroxy-bridged copper(II) complex 4 has been synthesised and characterised by elemental analysis and physicochemical measurements. The types of bridges present are endogenous phenoxo- and exogenous hydroxo- (1 and 2), endogenous alkoxo- and exogenous pyrazolate- (3), exogenous hydroxo- (4), endogenous diphenoxo-bridge (5) and exogenous hydroxo-, aqua- and acetate- (6). Barring complex 6, which is ferromagnetically coupled, all other complexes are antiferromagnetically coupled and all reported complexes are structurally characterised. The reaction between the dicopper(II) complexes and tetrachlorocatechol was investigated spectrophotometrically to investigate whether or not, during catalysis, binding of 3,5-DTBC is occurring to the copper centre(s). Kinetic experiments have been performed to determine initial rate of reactions and the activity order follows: 1>2≫3>4≫6 (for 5 initial rate could not be determined). For a fixed 3,5-DTBC concentration, a linear relationship for the initial rates and the complex concentration is obtained, suggestive of a first-order dependence on the catalyst concentration. Analysis of Michaelis–Menten kinetic treatment has been done for complexes 1, 2, 3 and 4. Complex 1 is the most active catalyst to show catecholase activity. [Copyright &y& Elsevier]

Studies the synthesis and characterization of bivalent, trivalent and tetravalent nickel complexes with a common tridentate deprotonated pyridine bis-amide ligand. Molecular structures of nickel(II) and nickel(IV); Redox properties of the nickel complexes; Solvent dependency of the redox processes.

Presents information on a study which focused on the reactivity aspects of tyrosinase and hemocyanin (Hc), a dinuclear copper-containing protein that bind or activate dioxygen. Experimental section; Results and discussion; Spectroscopic and magnetic investigations of deoxy and oxy forms of Hc using x-ray crystallography.

Aerobic reaction between the pyridine-2-carboxamide-2-aminophenol N3O ligand (H3L1) and Zn(ClO4)2·6H2O in CH3CN affords an N3 phenoxazinylate coordinated Zn(II) complex; its diradical electronic structure [ZnII{(L1*)˙−}2] has been elucidated from redox, spectroscopic (UV-VIS and EPR), and magnetic measurements and DFT calculations. Isolation and characterization of the metal-assisted redox-driven modified N3 ligand as a radical cation (H2L1*)˙+ and its Ni(II)-diradical complex [NiII{(L1*)˙−}2] have also been achieved. [ABSTRACT FROM AUTHOR]

A reaction between CoCl2 and L3-(CO2−)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [Co II4- {L3-(CO2−)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2−)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2−)2}Cl units connected by a dimeric Co II2 (μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm−1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = −2.5 cm−1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm−1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin–orbit coupling (λ = −158 cm−1) and axial distortion (energy gap Δ = −975 cm−1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations. [ABSTRACT FROM AUTHOR]

A new potentially tetradentate redox-active o-aminophenol-based ligand, H2L = 2-(2-ethylthio)pyridine-anilino-4,6-di-tert-butylphenol, reacts with PdII(O2CCH3)2 in CH3OH in the presence of air and Et3N affording isolation of a green crystalline solid of composition [Pd(L)] 1. When examined by cyclic voltammetry (CV), 1 exhibits two quasireversible oxidative responses at E1/2 = 0.16 (peak-to-peak separation, ΔEp = 100 mV) and 0.89 V (ΔEp = 90 mV) vs SCE (saturated calomel electrode). Chemical oxidation of 1 by [FeIII(η5-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of two crystalline solids, red [Pd(L)][PF6]·CH2Cl2 2 and dark green [Pd(L)][BF4]2·2CH2Cl2 3, respectively. Single-crystal X-ray crystallography at 100(2) K unambiguously established that the O,N,S,N-coordinated ligand is present in the square-planar complexes [PdII{(LAP)2-}] 1, [PdII{(LISQ)·-}][PF6]·CH2Cl2 2, and [PdII{(LIBQ)0}][BF4]2·2CH2Cl2 3, as dianionic (LAP)2-, monoanionic o-iminobenzosemiquinonate(1-) p-radical (Srad = ¹/2) (LISQ)·-, and neutral o-iminobenzoquinone (LIBQ)0 redox level. Reaction of 1 and 2 with PPh3 afforded isolation of two crystalline complexes: dark green [PdII(L)(PPh3)] 4 and red [PdII{(LISQ)·-}(PPh3)][PF6]·CH2Cl2 5. X-ray structure determination of 5 at 100(2) K revealed PdIION2P coordination environment. The square-planar complexes 1-5 possess an S = 0, ¹/2, 0, 0, and ¹/2 ground-state, respectively, as was established by ¹H NMR and EPR spectroscopy, and room-temperature magnetic moment data. All redox processes are thus shown to be ligand-based. Absorption spectral measurements were done for all complexes. DFT calculations at B3LYP-level of theory adequately describe the electronic structures of 1-3, and 5, containing a spin-paired d8 PdII ion. Time-dependent-DFT calculations on 1-3 and 5 shed light on the origin of UV-vis-NIR spectral absorptions. [ABSTRACT FROM AUTHOR]

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H3L2 in its deprotonated form, at three redox levels, open-shell semiquinonate(1−) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N′ and N′′ (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L2)] 1 in CH2Cl2 established a three-membered electron-transfer series (oxidative response at E1/2 = 0.57 V and reductive response at −0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L2)]z (z = 0, 1+ and 1−). Oxidation of 1 with AgSbF6 affords [Ni(L2)](SbF6) (2) and reduction of 1 with cobaltocene yields [Co(η5-C5H5)2][Ni(L2)] (3). The molecular structures of 1·CH3CN, 2·0.5CH2Cl2 and 3·C6H6 have been determined by X-ray crystallography at 100 K. Characterization by 1H NMR, X-band EPR (gav = 2.006 (solid); 2.008 (CH2Cl2–C6H5CH3 glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [NiII{(L2)˙2−}], [NiII{(L2)−}]+/1+ and [NiII{(L2)3−}]−/1− electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic St (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state St = 0. Interestingly, the variable-temperature (2–300 K) magnetic measurement reveals that 1 with the St = 1/2 ground state attains the antiferromagnetic St = 0 state at a very low temperature, due to weak noncovalent interactions via π–π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800–1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions. [ABSTRACT FROM AUTHOR]

The synthesis, structural characterization, and magnetic properties of three copper(II) complexes, a mono-chloro-bridged [Cu2II(L6)2(μ-Cl)(Cl)(CuIICl4)] ( 1) and a dichloro-bridged [CuII2(L7)2(μ-Cl)2][ClO4]2 ( 3) discrete trimers and dimers, and a mono-chloro-bridged 1D-coordination polymer [CuII(L6)(μ-Cl)][ClO4] ·CH3CN ( 2), are reported. Molecular structures were authenticated by X-ray crystallography. Temperature-dependent magnetic measurements carried out on powdered samples of 1- 3 indicated a very weak antiferromagnetic exchange-coupling within the {CuII(μ-Cl)CuII(μ-Cl)CuIICl3}+ units in 1 [Curie-Weiss plot: θ = -0.19(1) K and g = 2.07(1)], a ferromagnetic behavior within {CuII(μ-Cl)2CuII}2+ units in 3 [ Ĥ = - JS1 · S2: J = +6.0(1) cm-1, D (zero-field splitting parameter) = 4.5(2) cm-1, and g = 2.11], and a weak intrachain antiferromagnetic behavior due to {CuII(μ-Cl)CuII}3+ units in 2 [single-chain polymer: J = -0.20(1) cm-1 and g = 2.11]. The square-pyramidal stereochemistry of the central copper(II) atoms in 1- 3 was identified by their absorption spectral properties in acetontrile. An attempt was made to rationalize the observed magneto-structural behavior. [ABSTRACT FROM AUTHOR]

A controlled reaction in DMF/CH3CN between a facially disposed tricyanide-containing building block (nBu4N)[FeIII(Tp)(CN)3] (Tp(--) = tris(pyrazol-1-yl)borate anion) and tailor-made Mn(II) and Cu(II) complexes of two tripodal tetradentate ligands providing two 1-methylbenzimidazole, a pyridyl, and a tertiary amine as donor sites, differing only in the (2-pyridyl)alkylamine chain length, has been employed to assemble cyanide-bridged complexes. The heterobimetallic complexes [FeIII(Tp)(CN)2(μ-CN)MnIICl(L¹)]·3DMF·3H2O (1), [FeIII(Tp)(CN)2(μ-CN)MnIICl(L²)]·DMF (2), [FeIII(Tp)(CN)2(μ-CN)CuII(L¹)](ClO4)·2CH3OH (3), and [FeIII(Tp)(CN)2(μ-CN)CuII(L²)](ClO4)·2CH3CN (4) [L¹ = bis{(N-methyl-benzimidazol-2-yl)methyl}(2-pyridylmethyl)amine; L² = bis{(N-methyl-benzimidazol-2-yl)methyl}(2-pyridylethyl)amine (L²)] have been structurally authenticated, revealing single cyanide-bridged discrete dimers with Fe(III)(μ-CN)Mn(II) and Fe(III)(μ-CN)Cu(II) structural motifs. In 1 and 2, both the metal ions have a distorted octahedral geometry. In 3 and 4, the Fe(III) centres are six-coordinate, but the two five-coordinate Cu(II) centres assume two different geometries: distorted trigonal bipyramidal in 3 and almost ideal square pyramidal in 4. Investigation of the magnetic properties reveals an antiferromagnetic interaction through the cyanide bridge between the six-coordinate low-spin Fe(III) (S = 1/2) ions and high-spin six-coordinate Mn(II) (S = 5/2) ions in 1 and 2 with the total spin St = 2 ground state; while 3 exhibits a ferromagnetic interaction between the cyanide-bridged Fe(III) (S = 1/2) and Cu(II) (S = 1/2) ions with the St = 1 state, 4 displays an antiferromagnetic interaction leading to the St = 0 state. [ABSTRACT FROM AUTHOR]

A full report on the synthesis, crystal structure, spectroscopic characterization, and magnetic properties of two new trinuclear complexes (one homo- and another heterotrinuclear) [(L)2CoII3(OAc)4(MeOH)2]·6MeOH (1) and [(L)2CoII2MnII(OAc)4(MeOH)2]·6MeOH (2) [HL = N-methyl-N-2-hydroxybenzyl-2-aminoethyl-2-pyridine] is presented. The properties of 1 and 2 are compared to that of three previously communicated complexes [(L)2Nin3(OAc)4(MeOH)2]-6MeOH (3), [(L)2Nin2Mnu(0Ac)3(H20)2] (4), and [(L)2Niu2Co,1(OAc)4(MeOH)2]-6MeOH (5) (Inorg. Chem.2007, 46, 5128-5130). All are centrosymmetric trimers with the central metal ion situated on an inversion center. Adjacent metal ions are triply bridged by a µ2-phenolate and two acetate (one in µ2-1,1 mode and another in µ2-1,3 mode) groups. Magnetic investigations reveal that the complexes are ferromagnetically coupled (J in cm-1): +1.20 (1) and +0.71 (2). The reported J values are +1.10 (3), -0.30 (4), and +1.06 (5). It reveals that total spins are as follows: S = 9/2 (1), 11/2 (2), 3 (3), 1/2 (4), 7/2 (5). The role of M-O(µ2-phenoxide)-M', M-O(µ2-1,1-acetate)-M', and M-O(µ2-1,3-acetate)-M' (M = M' = CoII, NiII; M = CoII, M' = MnII; M = NiII, M' = MnII, CoII) bond angles on the observed magnetic behavior has been noted. [ABSTRACT FROM AUTHOR]

The synthesis of four discrete tetranuclear complexes {[NiII(L2)][ClO4]}4·MeCN (1), {[CuII(L1)(O3SCF3)]}4·H2O (2), {[CuII(L3)(OClO3)]}4·MeCN (3) and {[CuII(L4)][ClO4]}4·3MeCN·4H2O (4), supported by a closely similar group of carboxylate–appended (2–pyridyl)alkylamine ligands [L1(−): 3-[N-methyl-{2-(pyridin-2-yl)ethyl}amino]propionate; L2(−): 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl)methyl}amino]propionate; L3(−): 3-[N-isopropyl-{2-(pyridin-2-yl)methyl}amino]propionate and L4(2−): 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)] is described. Structural characterization reveals that each NiIIcentre in 1has square-pyramidal NiIIN3O2coordination and each CuIIcentre in 2–4has distorted square-pyramidal CuIIN2O3coordination, utilizing three N and one carboxylate O in 1and two N and carboxylate O (one in 2and 3; two in 4) of the ligand and the fifth/fourth coordination is provided by an oxygen atom belonging to the carboxylate group of an adjacent molecule. A CF3SO3−or a ClO4−ion provides an O coordination in 2and 3, respectively. Temperature-dependent magnetic studies reveal the existence of ferromagnetic exchange interaction in each case, due to the presence of equatorial–equatorial syn–anticarboxylate bridge between MIIcentres. [ABSTRACT FROM AUTHOR]

Abstract: From the standpoint of supramolecular chemistry major advances have been made concerning the functional properties of metal–ligand-based coordination complexes/extended solids/polymers. This paper reviews recent results from our laboratories on two contemporary areas of research: (i) spin-crossover FeIIN6 complexes which are representative examples of molecular bistability and (ii) synthesis of tailor-made transition-metal-based 1D coordination polymers and investigation of their magnetic exchange phenomena from the standpoint of molecule-based magnetic materials. Our primary focus is on the magnetic properties which seem to hold promise for future applications. It has been shown that covalent bond-driven discrete complexes/coordination polymers are further stabilized by association due to multiple weak, noncovalent interactions. In the design of FeIIN6 complexes our laboratories have so far relied on synthesis of discrete molecules. However, they are connected by noncovalent interactions, imparting cooperativity. [Copyright &y& Elsevier]

Structural characterization of the newly synthesized complexes [MII(L1OO)(xH2O)][ClO4] · 2H2O [M = Co, x = 1 (1); M = Cu, x = 0 (2); L1OO- = 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl)methyl}amino]propionate] reveals that 1 and 2 are 1 D chainlike coordination polymers. A tridentate variety of this ligand afforded a discrete tetranuclear complex ([CuII(L2OO)(OClO3)]}4. MeCN (3) [L2OO- = 3-[N-methyl-{2-(pyridine-2-yl)ethyl}amino]propionate]. Analysis of the crystal packing diagrams reveals extensive π-π stacking in 1 and C-H…O hydrogen bonding interactions in 3, leading to the formation of network structures. For these complexes, absorption spectral properties have been investigated. All three complexes exhibit exchange interaction between the MII ions through a syn-anti bridging carboxylate pathway. Magnetic studies on 1 show spontaneous magnetization below 5 K, which corresponds to the presence of spin-canted antiferromagnetism. At T 2 K, the values of coercive field (Hc) and remnant magnetization (Mr) are 200 G and 0.019 μB, respectively. Analysis of the magnetic data through spin Hamiltonians in the form H = Σnkj -JijSSj (J is positive for a ferromagnetic interaction and negative for an antiferromagnetic interaction) leads to the following set of best-fit parameters: J = -2.65(2), -0.66(1), and +12.2(2) cm-1 for 1, 2, and 3, respectively. An attempt has been made to rationalize the observed magnetic behavior. [ABSTRACT FROM AUTHOR]

Abstract: A full account of half-sandwich complexes of ruthenium(II) having three-legged “piano-stool” geometry supported by tridentate (2-pyridyl)alkylamine ligands is presented. Reaction of the dimer [{(η6-C6H6)RuCl(μ-Cl)}2] with N-methyl-N,N-bis(2-pyridylmethyl)amine (MeL∗) in CH3OH in the presence of NH4PF6 affords the complex [(η6-C6H6)Ru(MeL∗)][PF6]2 (1). A similar reaction with N-methyl-N,N-bis(2-pyridylethyl)amine (MeL∗∗), however, affords a non-organometallic Ru(III)-dimeric complex (5) (the composition of this complex has been established by physicochemical method). Nucleophilic addition reaction on 1 with NaBH4 leads to the isolation of a cyclohexadienyl complex [(η5-C6H7)Ru(MeL∗)][PF6] (3). The molecular structure of 1 ·2CH3CN, 3, and previously reported cyclohexadienyl complex [(η5-C6H7)Ru(MeL)][PF6] (4) [MeL=N-methyl-[(2-pyridyl)ethyl(2-pyridyl)-methyl]amine], obtained from the reaction between NaBH4 and previously reported “piano-stool” complex [(η6-C6H6)Ru(MeL)][PF6]2 (2), has been confirmed by X-ray crystallography. Solution-state structure of new complexes 1 and 3 has been elucidated by their 1H NMR spectra in CD3CN. The behavior of complex 3 has been investigated with the aid of two-dimensional 1H NMR spectroscopy, as well. An attempt has been made to provide a rationale for the effect of supporting tridentate N-donor ligand [MeL, MeL∗, and MeL∗∗], varying in the chelate ring-size on (i) the relative stability of half-sandwich η6-benzene Ru(II) complexes and (ii) the electrophilicity of Ru(II)-coordinated benzene ring on the nucleophilic addition reactions. [Copyright &y& Elsevier]

Abstract: The synthesis and characterization of two 1D coordination polymers [Cu2(MHL)Cl2][ClO4]2 ·CH3CN·THF (2 ·CH3CN·THF) and [Cu2(MPyPz)Cl2][ClO4]2 ·CH3CN (3 ·CH3CN), having repetitive units, of m-xylyl-based ligands with terminal tridentate (2-pyridyl)alkylamine (MHL=α,α′-bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-m-xylene) and (2-pyridyl)alkylamine/pyrazole (MPyPz=α,α′-bis[N-(2-pyridylethyl)-N-(pyrazol-1-ylmethyl)amino]-m-xylene) coordination have been accomplished. X-ray crystallographic studies reveal that the copper(II) centers in the recently reported dichloro-bridged discrete complex [{Cu(MeL)Cl}2][ClO4]2 (1) of a tridentate (2-pyridyl)alkylamine ligand [MeL=methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine], 2 ·CH3CN·THF, and 3 ·CH3CN have distorted square-pyramidal geometry, sharing a base-to-apex edge with parallel basal planes. Variable-temperature susceptibility measurements in the range of 2–300K reveal antiferromagnetic for 1 [J (singlet–triplet energy gap)=−3.89cm−1] and 2 ·CH3CN·THF (J =−1.84cm−1), and ferromagnetic for 3 ·CH3CN (J =+6.27cm−1) coupling. The complexes provide useful information for the magneto-structural correlations. [Copyright &y& Elsevier]

Four dinuclear complexes of composition [MII2(L)2] · xS [M = Co, x = 0.5, S = 1 ,4-dioxane (1 · 0.5 1 ,4-dioxane); Ni, x = 0 (2) [single crystals have x 2, S = diethyl ether (2 · 2 diethyl ether)]; Cu, x = 0 (3); Zn, x = 0.5, S = 1,4-dioxane (4 · 0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-ditert-butyl-6-{[(2-pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H2L)], in its deprotonated form, providing a N2O2 donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-ditert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes MIIN2O3 coordination. Whereas the geometry around the metal ion in 1 · 0.5 1 ,4-dioxane, 2 · 2 diethyl ether and, 4 · 0.5 1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment. Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange coupling for these compounds (-J = 6.1, 28.6, and 359 cm-1 for 1 · 0.5 1,4-dioxane, 2, and 3, respectively). Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry (CV), all four complexes undergo in CH2Cl2 two reversible ligand-based (2,4-ditert-butylphenolate unit) one-electron oxidations [E1/2¹ = 0.50-0.58 and E1/2² = 0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/ coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous abstraction of the hydrogen atom, and for 4 · 0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR spectrum characteristic of the presence of ZnII-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the B3LYP level of theory. [ABSTRACT FROM AUTHOR]

Abstract: Synthetic modeling of tyrosinase (o-phenol ring hydroxylation) has emerged as a novel class of successful biomimetic studies. It is a well-established fact that the reaction of dioxygen with copper(I) complexes of m-xylyl-based ligands generate putative copper–oxygen intermediate species such as side-on peroxo {CuII 2(μ-O2)}2+ [in some cases bis-oxo in equilibrium with isomeric side-on peroxo], due to oxygen activation. Electrophilic attack of such species brings about monooxygenase activity by incorporating one of the oxygens to m-xylyl ring of the ligand and the other oxygen is reduced to hydroxide ion. The goal of this review is to provide a concise overview of the present day knowledge in this field of research to emphasize the important role the designed ligands play in eliciting the desired tyrosinase-like chemistry. [Copyright &y& Elsevier]

Abstract: The ligand 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane (L8) has afforded six-coordinate monomeric and dimeric complexes [(L8)CoII(H2O)2][ClO4]2 (1), [(L8)NiII(MeCN)2][BPh4]2 (2), [(L8)NiII(O2CMe)][BPh4] (3), and . The crystal structures of 1, 2 ·MeCN, 3, and 4 revealed that the ligand L8 is flexible enough to expand its coordinating ability by fine-tuning the angle between the chelating fragments and hence folds around cobalt(II)/nickel(II) centers to act as a tetradentate chelate, allowing additional coordination by two trans-H2O, cis-MeCN, and a bidentate acetate affording examples of distorted octahedral , , and coordination. The angles between the two CoN2/NiN2 planes span a wide range 23.539(1)° (1), 76.934(8)° (2), and 69.874(14)° (3). In contrast, complex 4 is a bis-μ-1,3-acetato-bridged (syn–anti coordination mode) dicobalt(II) complex [Co⋯Co separation: 4.797(8)Å] in which L8 provides terminal bidentate pyridylpyrazole coordination to each cobalt(II) center. To our knowledge, this report provides first examples of such a coordination versatility of L8. Absorption spectral studies (MeCN solution) have been done for all the complexes. Complexes 1–3 are uniformly high-spin. Temperature-dependent (2–300K) magnetic studies on 4 reveal weak ferromagnetic exchange coupling between two cobalt(II) (S =3/2) ions. The best-fit parameters obtained are: Δ (axial splitting parameter)=−765(5)cm−1, λ (spin-orbit coupling)=−120(3)cm−1, k (orbital reduction factor)=0.93, and J (magnetic exchange coupling constant)=+1.60(2)m−1. [Copyright &y& Elsevier]

Structural analysis of four-coordinate (S = 0) complex [Ni(L1)2] 1 (H2L1 = 4,6-di-tert-butyl-2-(2-benzylthiophenyl)aminophenol) revealed that it has approximately planar NiN2O2 coordination with dangling thioether arms (Mukherjee A and Mukherjee R 2005 Indian J Chem50A 484–490). The two coordinated ligands are in 2-iminobenzosemiquinonate(1−) π radical (LISQ)·− redox level. Chemical oxidation of 1 by [Fe(η5-C5H5)2](PF6) in CH2Cl2 in the air led to oxidation of one of the (LISQ)·− ligands affording a six-coordinate Ni(II) complex [Ni(L1)2](PF6) 2 in mer configuration. The two tridentate ligands are in (LISQ)·− and 2-iminobenzoquinone (LIBQ)0 redox level (ligand mixed valency). The Ni–N/O/S distances and ligand backbone metrical parameters led us to assign unambiguously the electronic structure of 11+/2 as [NiII{(LISQ)·−}{(LIBQ)0}]1+. The EPR spectral signal of 2 at 120 K exhibits a very large g anisotropy and the magnetic measurement indicates an S = 3/2 ground state. The potential shift observed in going from 1 to coulometrically-generated 12+ is marginal. DFT calculations at the CAM-B3LYP level of theory rationalizes the electronic structure of 1, 11+ and 12+. Time-Dependent (TD)-DFT calculations throw light on the nature of observed absorptions of 1, 11+ and 12+. One-electron oxidation of four-coordinate [NiII(L1)2] 1 (S = 0) Causes additional coordination by two dangling thioethers affording six-coordinate [NiII(L1)2](PF6) 2 (S = 3/2); characterization of 2 has been done by X-ray structural analysis, MOS values, magnetism (ferromagnetic coupling), absorption and EPR spectra, and DFT and TD-DFT calculations. [ABSTRACT FROM AUTHOR]

Using a tridentate bis-amide ligand 2,6-bis(N-phenylcarbamoyl)pyridine (H[sub 2]L), in its deprotonated form, a new mononuclear ruthenium(III) complex [Et[sub 4]N][RuL[sub 2]]·H[sub 2]O (1) has been synthesized. Structural analysis reveals that the RuN[sub 6] coordination comprises four deprotonated amide-N species in the equatorial plane and two pyridine-N donors in the axial positions, imparting a tetragonally compressed octahedron around Ru. To the best of our knowledge, this is the first time that a ruthenium(III) complex coordinated solely by two tridentate deprotonated peptide ligands has been synthesized and structurally characterized. When examined by cyclic voltammetry, complex 1 displays in MeCN/CH[sub 2]Cl[sub 2] solution three chemically/electrochemically reversible redox processes: a metal-centered reductive Ru[sup III]-Ru[sup II] couple (E[sub ½] = -0.84/-0.89 V vs SCE) and two ligand-centered oxidative responses (E[sub ½] = 0.59/0.60 and 1.05/1.05 V vs SCE). Isolation of a dark blue one-electron oxidized counterpart of 1, [RuL[sub 2]]·H[sub 2]O (2), has also been readily achieved. The complexes have been characterized by analytical, solution electrical conductivity, IR, electronic absorption and EPR spectroscopy, and temperature-dependent magnetic susceptibility measurements. For complex 1, a weak and broad transition within the t[sub 2g] level has been identified at ∼1400 nm and supported by EPR spectral analysis (S = ½). Temperature-dependent magnetic susceptibility data provide unambiguous evidence that in 2 strong antiferromagnetic coupling of the S = ½ ruthenium atom with the S = ½ ligand π-cation radical leads to an effectively S = 0 ground state (¹H NMR spectra in CDCl[sub 3] solution). [ABSTRACT FROM AUTHOR]

Using a bidentate heterocyclic ligand with mixed hard–soft nitrogen donor set [L5=2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine] two types of copper(II) complexes have been synthesized and characterized: (i) [Cu(L5)2][ClO4]2·H2O (1) and (ii) [Cu(L5)2X][ClO4] (X=Cl− (2), NCS− (3) and N3− (4)). Structural analysis of 1 reveals that its unit cell consists of a 1:1 mixture of square planar [Cu(L5)2]2+ (coordination by two pyrazoles and two pyridines) and tetragonal [Cu(L5)2(H2O)2]2+, having additional weak interaction with the water molecules in the axial positions. Preliminary X-ray structural analysis of 3 with a poor data set, which is not reported here, revealed that the thiocyanate group is S-coordinated. For each complex the stereochemistry of the copper(II) centre has been investigated in the solid state as well as in MeCN solution, by absorption and EPR spectroscopy. In the polycrystalline state as well as in MeCN solution complex 1 displays axial EPR spectra having a dx2−y2 ground state. However, solid samples of 2–4 exhibit rhombic EPR spectral features, characteristic of a compressed dz2 ground state. In the solution phase (80 K) 2 exhibits an isotropic signal; 3 and 4 exhibit axial spectra, characteristic of a dx2−y2 ground state. The observed E1/2 values for the CuII–CuI redox couple [cyclic voltammetry: MeCN solution; V vs. saturated calomel electrode (SCE)] are fairly positive: 0.41 (1), 0.29 (2), 0.30 (3) and 0.19 (4). [Copyright &y& Elsevier]

Four new complexes of composition [M II2 (L3)2(CH3OH)2](ClO4)2 (M = Mn, 1; Co, 2; Ni, 3) and [Cu II2 (L3)2](ClO4)2 (4) (L3(−) = 3-[2-(((6-methyl)pyridin-2-yl)methyl){(dimethylamino)ethyl}-amino]propionate) have been synthesized and characterized. Structural analysis reveals that 1–4 are discrete syn–anti carboxylate-bridged binuclear coordination complexes with a {M II2 (–O–C–O–)2}2+ structural motif. In 1–3 distorted octahedral MN3O3 and in 4 distorted square pyramidal CuN3O2 coordination is satisfied at each M(II) center by three N (a pyridyl and two tertiary aliphatic amines) and a carboxylate O of the ligand, and an O of the carboxylate group. In 1–3 the M(II) center is also coordinated by a CH3OH molecule. Variable-temperature magnetic measurements reveal the presence of weak antiferromagnetic interactions in 1 and weak ferromagnetic interactions in 2–4. The electronic structure of 1–4 has been probed by density functional theory (DFT) calculations at the B3LYP level of theory. In order to gain insight into the origin of the observed electronic transitions, time-dependent (TD)-DFT calculations have been done. [ABSTRACT FROM AUTHOR]

The complexes [(L2) 2 CuII(X)]z+(ClO 4) n (X = H 2 O, n = z = 2; X = N 3 −, SCN−, NO 2 −, MeCO 2 −, Cl−, Br− n = z = 1) have been synthesized and structurally characterized. For anion-bound five-coordinate complexes, useful linear correlations have been established: (i) E pc (cathodic peak potential of CuII–CuI redox process) vs. structural index parameter τ , (ii) g || vs. τ , (iii) E pc vs. g || and (iv) A ‖ vs. τ. • The complexes [(L2) 2 CuII(X)]z+(ClO 4) n (X = H 2 O, z = 2; X = N 3 −, SCN−, NO 2 −, MeCO 2 −, Cl−, Br− z = 1). • Variation of Structural index parameter τ. • Linear correlations: E pc vs. τ , g ‖ vs. τ, A ‖ vs. τ and E pc vs. g ‖. • E pc vs. τ plot: first report for five-coordinate anion-bound copper(II) complexes. Mononuclear five-coordinate complexes [(L2) 2 CuII(X)]z+(ClO 4) n (X = H 2 O 1 , n = z = 2; X = N 3 − 2 , SCN− 3 , NO 2 − 4 , MeCO 2 − 5 , Cl− 6 , Br− 7 , n = z = 1; L2 = 1-benzyl-[3-(2-pyridyl)]pyrazole) have been synthesized and structurally characterized with crystal composition 1 , 2 , 3 ·2MeCN, 4 ·Et 2 O, 5 ·3H 2 O, 6 ·MeCN and 7 ·MeCN. The bidentate ligand L2 provides a pyridyl and a pyrazole for coordination. Structural index parameter (τ ; it assumes a value of 0 and 1 for ideal square pyramidal and trigonal bipyramidal geometry, respectively) for 1 – 3 , 5 – 7 are 0.60, 0.01, 0.33, 0.26, 0.62, 0.57, respectively. Absorption and EPR spectroscopic, and redox (except for 1) properties of 1 – 7 have been investigated. Cathodic peak potential (E pc , V vs. saturated calomel electrode, SCE) values in MeCN of 2 – 7 are −0.05, 0.07, 0.05, 0.05, 0.23 and 0.14, respectively. For anion-bound complexes 2 , 3 , 5 – 7 useful correlations have been established: (i) E pc (CuII–CuI redox process) vs. structural index parameter τ , (ii) g ‖ vs. τ , (iii) A ‖ vs. τ and (iv) E pc vs. g ‖. The observed trends have been rationalized. [ABSTRACT FROM AUTHOR]

Aerobic reaction between [RuII(DMSO)4Cl2], a redox-active 2-aminophenol-based ligand (H2L1: 2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) and Et3N in MeOH under refluxing conditions afforded a purple complex [Ru(L1)2] (S = 0). Structural analysis reveals that the tridentate ligand coordinates in a mer conformation providing a distorted octahedral RuN2O2S2 coordination. Cyclic voltammetry on 1 in CH2Cl2 reveals the accessability of the monocation, dication and monoanion forms. Reddish purple monocation [Ru(L1)2](PF6)·CH2Cl2 ([1OX1](PF6)·CH2Cl2; S = 1/2) and green dication [Ru(L1)2](BF4)2·H2O ([1OX2](BF4)2·H2O; S = 0) have been isolated through the chemical oxidation of 1 in CH2Cl2 by [FeIII(η5-C5H5)2](PF6) and AgBF4, respectively. A structural analysis of the single crystals of the monocation and the dication with the compositions [1OX1](PF6)·CH2Cl2·H2O (2) and [1OX2](BF4)2·1.7H2O (3), respectively, has been done. Metrical (metal–ligand and ligand backbone) parameters, values of metrical oxidation states of coordinated ligands, 1H NMR spectra of 1 and [1OX2](BF4)2·H2O, EPR spectra of [1OX1](PF6)·CH2Cl2, X-ray photoelectron and UV-VIS-NIR spectra of 1–3, spin population analysis from broken-symmetry (BS) density functional theory (DFT) calculations and quasi-restricted orbital (QRO) analysis have allowed us to assign the electronic structure of the complexes. The complexes exhibit highly covalent metal–ligand interactions. The electronic states of 1, [1OX1]1+ and [1OX2]2+ are best described as [RuII{(LISQ)˙−}2] ↔ [RuIII{(LAP)2−}{(LISQ)˙−}] (S = 0), [RuIII{(LISQ)˙−}2]1+ (S = 1/2) and [RuII{(LIBQ)0}2]2+ ↔ [RuIII{(LISQ)˙−}{(LIBQ)0}]2+ (S = 0), respectively. Notably, all redox processes are ligand-centred. Absorption spectral properties have been rationalized based on time-dependent (TD)-DFT calculations. For 1, the appearance of an IVCT band at 1100 nm supports its Class II–III (borderline) ligand-based mixed-valence character. [ABSTRACT FROM AUTHOR]

The complexes [(L)2NiII2MII(μ2-1 ,3-OAc)2(μ2-1,1-OAc)2(S)2]·xMeOH [HL = N-methyl-N-(2-hydroxybenzyl)-2-aminoethyl-2-pyridine; M = Ni, S = MeOH, x = 6 (1); M = Mn, S = H2O, x = 0 (2); M = Co, S = MeOH, x = 6 (3)] have been synthesized. Crystal structures reveal that three octahedral MII ions form a linear array with two terminal moieties {(L)NiII(μ2-1,3-OAc)(μ2-1,1-OAc)(MeOH/H2O)}- in a facial donor set and a central MII ion which is connected to the terminal ions via bridging phenolate and two types of bridging acetates. Magnetic measurements reveal that the NiII3 and NiII2CoII centers are ferromagnetically and NiII2MnII center is antiferromagnetically coupled. An attempt has been made to rationalize the observed magneto-structural behavior. [ABSTRACT FROM AUTHOR]

Structural and magnetic studies have been done on a dicopper(II) and a tetracopper(II) molecular square; the two Cu(II) centres and four Cu(II) centres, respectively, are anti- and ferromagnetically coupled. [Display omitted] • Carboxylate-appended (2-pyridyl)alkylamine-based ligands are used. • During the synthesis L3/L4-(CO 2 Me) 2 are fully or partially hydrolyzed. • Discrete copper(II) complexes have been structurally characterized. • Cu(II) dimer and tetramer have μ 2 -1,1-CO 2 – and μ 2 -1,3-CO 2 – bridges, respectively. • Dimer is anti- (J = ‒8.37(6) cm–1) and tetramer is ferro-magnet (J = +13.1(1)). Two discrete complexes [CuII 2 {L3-(CO 2 –) 2 } 2 ]•MeOH•H 2 O (1) and [CuII 4 {L4-(CO 2 –)(CO 2 Me)} 4 ](ClO 4) 4 •unknown solvent (2) have been synthesized from the reaction between copper(II) salts and L3-(CO 2 Me) 2 / L4-(CO 2 Me) 2 [3,3′-[(pyridin-2-ylmethyl)imino]dipropionate dimethylester / 3,3′-[(pyridin-2-ylethyl)imino]dipropionate dimethylester]. Structural analysis reveals that 1 is a monatomic bridging carboxylate dinuclear complex with CuII 2 (μ 2 -1,1-OCO–) 2 2+ structural unit. Each Cu(II) centre has distorted square-pyramidal CuN 2 O 3 coordination, provided by two N (pyridyl and amine) and two carboxylate O from L3-(CO 2 –) 2 and the fifth coordination is provided by an O atom belonging to the carboxylate group of an adjacent molecule. Complex 2 is a syn - anti carboxylate-bridged tetranuclear coordination cluster with a {CuII 4 {(μ 2 -1,3-OCO–)(μ 2 -1,3-OCOMe)} 4 4+ structural motif. Here also, each Cu(II) centre has distorted square-pyramidal CuN 2 O 3 coordination, and an O atom belonging to the carboxylate group of an adjacent molecule completes the fifth coordination. Variable temperature magnetic studies reveal antiferromagnetic coupling in 1 (J = –8.37(6) cm−1), while 2 exhibits ferromagnetic behaviour (J = +13.1(1) cm−1). [ABSTRACT FROM AUTHOR]

Synthesis of three multi-nuclear copper(II) complexes without (di) and with (tetra and hexa) benzoate bridges have been accomplished; core interconversion and magnetic studies have also been done. Reactions of stoichiometric amounts of L1(−) (HL1 = 3-(2-pyridyl)pyrazole) with [Cu(H 2 O) 6 ](ClO 4) 2 , with or without PhCO 2 −, in MeOH or N , N ′-dimethylformamide (dmf), led to the isolation of three copper(II) complexes of varying nuclearity, [CuII 2 (L1) 2 (ClO 4) 2 (MeOH) 2 ] (1), [CuII 4 (L1) 4 (O 2 CPh) 2 (MeOH) 4 ](ClO 4) 2 ·2H 2 O (2) and [CuII 6 (L1) 6 (O 2 CPh) 2 (ClO 4) 2 (dmf) 4 ](ClO 4) 2 ·2dmf·2H 2 O (3). Structural analysis reveals two centrosymmetric four-coordinate {CuII(L1)(ClO 4)(MeOH)} units are dipyrazolate-bridged in 1 , giving rise to a square-pyramidal (SP; τ = 0.13) coordination to the CuII ion. In 2 , two centrosymmetric four-coordinate dipyrazolate-bridged {CuII 2 (μ -L1) 2 (MeOH) 2 }2+ units in two layers are held by two syn – anti PhCO 2 − bridges, giving rise to CuII centres with a distorted SP geometry (τ = 0.28) geometry. In 3 , a three-coordinate dipyrazolate-bridged {CuII 2 (μ -L1) 2 }2+ unit is held between two {CuII 2 (L1) 2 (ClO 4)(dmf) 2 }}2+ units in two different layers by two syn – anti bridging PhCO 2 − ligands in a centrosymmetric manner, generating two SP (τ = 0.03 and 0.28) and a square-planar geometry. In 2 , two successive layers are held by two and in 3 by one PhCO 2 − bridge(s). Systematic core-interconversion studies as a function of PhCO 2 −, solvent (MeOH, dmf) and concentration have been done and the observed results have been rationalized. Variable-temperature magnetic studies (1.9–300 K) for powdered samples of 1 – 3 revealed strong antiferromagnetic coupling between the two copper(II) centres in the {CuII 2 (μ -pyrazolato) 2 }2+ units. For 2 and 3 , additional magnetic interactions through PhCO 2 − bridge(s) between two layers have also been realized. The magnitude of the coupling is discussed in terms of the structural parameters. [ABSTRACT FROM AUTHOR]

Using a potentially tridentate o-aminophenol-based redox-active ligand H2L1 (2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) in its deprotonated form, [Cu(L1)2] has been synthesized and crystallized as [CuII(L1)2]·CH2Cl2 (1·CH2Cl2). A cyclic voltammetry experiment (in CH2Cl2; V vs. SCE (saturated calomel electrode)) on 1·CH2Cl2 exhibits two oxidative (E11/2 = 0.20 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 0.90 V (ΔEp = 140 mV)) and two reductive (E11/2 = −0.52 V (ΔEp = 110 mV) and E21/2 = −0.92 V (ΔEp = 120 mV)) responses. Upon oxidation using a stoichiometric amount of [FeIII(η5-C5H5)2](PF6), 1·CH2Cl2 yielded [Cu(L1)2](PF6) (2). Structural analysis (100 K) reveals that 1·CH2Cl2 is a four-coordinate bis(iminosemiquinonato)copper(ii) complex (CuN2O2 coordination), and that the thioethers remain uncoordinated. The twisted geometry of 1 (distorted tetrahedral) results in considerable changes in the electronic structure, compared to well-known square-planar analogues. Crystallographic analysis of 2 both at 100 K and at 293 K reveals that it is effectively a four-coordinate complex with a CuN2OS coordination; however, a substantial interaction with the other phenolate O is observed. The metal–ligand bond distances and metric parameters associated with the o-aminophenolate rings indicate a valence-tautomeric (VT) equilibrium involving monocationic (iminosemiquinonato)(iminoquinone)copper(ii) and bis(iminoquinone)copper(i). Complex 1·CH2Cl2 is a three-spin system and a magnetic study (4–300 K) established that it has a S = 1/2 ground-state, owing to the strong antiferromagnetic coupling between the unpaired spin of the copper(ii) and the iminosemiquinonate(1−) π-radical anion. Electron paramagnetic resonance (EPR) spectral studies corroborate this result. Complex 2 is diamagnetic and the existence of VT in 2 was probed using variable-temperature (248–328 K) 1H NMR and EPR (100–298 K) spectral measurements and X-ray photoelectron spectroscopic studies at 298 K. Remarkably, modification of the well-studied 2-anilino-4,6-di-tert-butylphenol by incorporation of a benzylthioether arm leads to the occurrence of VT in 2. The electronic structure of 1·CH2Cl2 and 2 has been assigned using density functional theory (DFT) calculations at the B3LYP-D3 level of theory. Time-dependent (TD)-DFT calculations have been performed to elucidate the origin of the observed UV-VIS-NIR absorptions. [ABSTRACT FROM AUTHOR]

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = −0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50–300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1–3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233–313 K) 1H NMR spectral features of 1–3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2−}{(LISQ)}Ṗ−] or [CoIII{(L1)2}Ṗ3−] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)Ṗ−}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2−}2]·MeCN (3) [(LAP)2− and (LISQ)Ṗ− represent the redox-level of coordinated ligands o-amidophenolate(2−) ion and o-iminobenzosemiquinonate(1−) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1–3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1–3 from experimental data. The origins of the observed UV–VIS–NIR absorptions for 1–3 have been assigned, based on time-dependent (TD)-DFT calculations. [ABSTRACT FROM AUTHOR]

Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamide 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of FeII/CoII and FeIII/CoIII have been synthesized. Molecular structures of six of them reveal that these six-coordinate complexes utilize all available donor sites of the ligands and assume MII/IIIN2(pyridine/pyrazine)N′2(amide)N′′2(azo) coordination. Complexes of FeII and CoIII are diamagnetic and those of FeIII and CoII are paramagnetic (S = 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CH2Cl2 reveal facile metal-centred FeIII/FeII and CoIII/CoII redox responses, and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E1/2 values of MIII/MII redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with those for six-coordinate [MIII(bpy)3]3+/[MII(bpy)3]2+, [MIII(terpy)2]3+/[MII(terpy)2]2+, [MIII(L)]+/[MII(L)]0 or [MIII(L′)2]+/[MII(L′)2]0 (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2′′-terpyridine, hexadentate L(2−) = 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane and tridentate L′(−) = {2-[2-(arylimino)phenylazo]-pyridine}) couples. Density functional theory (DFT) at the B3LYP level and time-dependent (TD)-DFT calculations rationalize the electronic structure of the present complexes and throw light on the origin of observed electronic transitions. [ABSTRACT FROM AUTHOR]

A μ-phenoxo-bis(μ2-1,3-acetato)-bridged dicopper(ii) complex [CuII2(L1)(μ-O2CMe)2][NO3] (1) has been synthesized from the perspective of modeling phosphodiesterase activity. Structural characterization was done initially with 1·3Et2O (vapour diffusion of Et2O into MeOH solution of 1; poor crystal quality) and finally with its perchlorate salt [CuII2(L1)(μ-O2CMe)2][ClO4]·1.375MeCN·0.25H2O, crystallized from vapour diffusion of n-pentane into a MeCN–MeOH mixture (comparatively better crystal quality). An asymmetric unit of such a crystal contains two independent molecules of compositions [CuII2(L1)(μ-O2CMe)2][ClO4] and [CuII2(L1)(μ-O2CMe)2(MeCN)][ClO4] (coordinated MeCN with 0.75 occupancy), and two molecules of MeCN and H2O (each H2O molecule with 0.25 occupancy) as the solvent of crystallization. These two cations, each having five-coordinate (μ-phenoxo)bis(μ-acetato)-bridged CuII ions, differ by only the coordination environment of only one CuII ion, which has a weakly coordinated acetonitrile molecule in its sixth position. Temperature-dependent magnetic studies on 1 reveal that the copper(ii) centres are antiferromagnetically coupled with the exchange-coupling constant J = −124(1) cm−1. Theoretically calculated J = −126.51 cm−1, employing a broken-symmetry DFT approach, is in excellent agreement with the experimental value. The dicopper(ii) complex has been found to be catalytically efficient in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Detailed kinetic experiments and solution studies (potentiometry, species distribution and ESI-MS) were performed to elucidate the reaction mechanism. DFT calculations were performed to discriminate between different possible mechanistic pathways. The free-energy barrier for HPNP hydrolysis catalyzed by 1 is comparable to that obtained from the experimentally-determined value. The involvement of non-covalent (hydrogen-bonding) interaction has also been probed by DFT calculations. The activity of 1 is found to be the highest, compared to the structurally-characterized MnII2, CoII2, NiII2 and ZnII2 complexes of L1(−) reported earlier, under identical experimental conditions, in which each metal centre is six-coordinate. [ABSTRACT FROM AUTHOR]

Using a dicarboxylate ligand appended with (2-pyridyl)ethylamine unit, a new cluster [NiII8(L4)6(DMF)2(CH3OH)2(H2O)6][ClO4]4·2CH3OH·2CH3CO2C2H5 (1) [L4(2−): 3-[N-{2-(pyridin-2-yl)ethyl}amino]-bis(propionate)] has been synthesized, through ‘coordination-driven self-assembly’. The crystal structure of 1 reveals a centrosymmetric octanuclear carboxylate-bridged nickel(ii) tetracation, with a distorted cubane topology. The four crystallographically independent nickel(ii) centres differ markedly in their coordination environment. Magnetic studies (2–300 K) reveal that in 1 the net magnetic-exchange is antiferromagnetic. Based on geometric parameters associated with two interacting nickel(ii) centres, six magnetic-exchange coupling constants (J values) were considered for magnetic data analysis. Notably, 1 provides the first example of a NiII8 cluster (i) bridged solely by carboxylates in three bridging modes (monatomic, syn–anti and anti–anti), (ii) exhibiting four ferromagnetic and two antiferromagnetic magnetic-interactions and (iii) demonstrating a good agreement between six J values (obtained from experimental data analysis) and those obtained from DFT calculations, at the B3LYP-level of theory. [ABSTRACT FROM AUTHOR]