The effects of point of substitution on the electrochemical behavior of new manganese phthalocyanines, tetra-substituted with diethylaminoethanethiol

The syntheses and comparative studies of the spectral, voltammetry and spectroelectrochemical properties of new manganese phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the peripheral (complex 3a) and non-peripheral positions (complex 3b) are reported. Solution electrochemistry of complex 3a showed quasi-reversible metal-based (Mn^IIIPc⁻²/Mn^IIPc⁻², E_1/2 = −0.07 V vs. Ag|AgCl) and ring-based (Mn^IIPc⁻²/Mn^IIPc⁻³, E_1/2 = −0.78 V vs. Ag|AgCl) reductions, but no ring-based oxidation. However, complex 3b showed weak irreversible ring-oxidation signal (E_p = +0.86 vs. Ag|AgCl). Reversible metal-based (Mn^IIIPc⁻²/Mn^IIPc⁻², E_1/2 = −0.04 V vs. Ag|AgCl) and ring-based (Mn^IIPc⁻²/Mn^IIPc⁻³, E_1/2 = −0.68 V vs. Ag|AgCl) reductions were also observed for complex 3b. Spectroelectrochemistry was used to confirm these processes. Reduction process involving the metal (Mn^IIIPc⁻²/Mn^IIPc⁻²) was associated with the formation of manganese μ-oxo complex in complex 3a.     

Syntheses, structures and antimicrobial activities of various metal complexes of hinokitiol

Metal-oxygen bonding complexes (M = Mg^II, Mn^II, Ni^II, Mo^VI, W^VI, Pd^II, Sb^III, Bi^III, Fe^III, Ti^IV, K^I, Ba^II, Zr^IV and Hf^IV) with a hinokitiol (Hhino; 2-hydroxy-4-isopropylcyclohepta-2,4,6-trienone or β-thujaplicin) ligand, which has two unequivalent oxygen donor atoms, were synthesized and characterized by elemental analysis, TG/DTA, FT-IR and solution (¹H and ¹³C) NMR spectroscopy. Single-crystal X-ray structure analysis revealed various molecular structures for the complexes, which were classified into several families of family, i.e. type A [M^II(hino)₂(L)]₂ (M = Mg^II, Mn^II, Ni^II; L = EtOH or MeOH), with a dimeric structure consisting of one bridging hino⁻ anion, one chelating hino⁻ anion and one alcohol or water molecule, type B, with the octahedral, cis-dioxo, bis-chelate complexes cis-[M^VIO₂(hino)₂] (M = Mo^VI, W^VI), type C, with square planar complex [M^II(hino)₂] (M = Pd^II), type D, with tris-chelate, 7-coordinate complexes with one inert electron pair [M^III(hino)₃] (M = Sb^III, Bi^III), type D′, with the bis-chelate, pseudo-6-coordinate complexes with one inert electron pair [M^III(hino)₂X] (M = Sb^III, X = Br), type E, with tris-chelate, 6-coordinate complexes with Δ and Λ isomers [M^III(hino)₃] (M = Fe^III), type E′ of bis-chelate, 6-coordinate complex [M^IV(hino)₂X₂] (M = Ti^IV, X = Cl), type F, with water-soluble alkali metal salts [M^I(hino)] (M = K^I), and type H, with tetrakis-chelate, 8-coordinate complexes [M^IV(hino)₄](M = Zr^IV, Hf^IV). These structural features were compared with those of metal complexes with a related ligand, tropolone (Htrop). The antimicrobial activities of these complexes, evaluated in terms of minimum inhibitory concentration (MIC; μg mL⁻¹) in two systems, were compared to elucidate the relationship between structure and antimicrobial activity.     

Synthesis and properties of trinuclear polypyridyl complexes Ru(II)-Co(II)-Ru(II) and Ru(II)-Co(III)-Ru(II): Their photoinduced interconversion

The trinuclear [{Ru^II(bpy)₂(bpy-terpy)}₂Co^II]⁶⁺ complex (1⁶⁺) in which a Co(II)-bis-terpyridine-like centre is covalently linked to two Ru(II)-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised. Its electrochemical, photophysical and photochemical properties have been investigated in CH₃CN. The cyclic voltammetry exhibits two successive reversible oxidation processes, corresponding to the Co^III/Co^II and Ru^III/Ru^II redox couples at E_1/2 = −0.06 and 0.91 V vs Ag/Ag⁺ 10 mM, respectively. The one-electron oxidized form of the complex, [{Ru^II(bpy)₂(bpy-terpy)}₂Co^III]⁷⁺ (1⁷⁺) obtained after exhaustive electrolysis carried out at 0.2 V is fully stable. 1⁶⁺ and 1⁷⁺ are only poorly luminescent, indicating that the covalent linkage of the Ru(II)-tris-bipyridine centre to the cobalt subunit leads to a strong quenching of the Ru^II excited state by an intramolecular process. Luminescence lifetime experiments carried out at different temperatures indicate that the transfer is more efficient for 1⁷⁺ compare to 1⁶⁺ due to lower activation energy. Continuous irradiation of 1⁷⁺ performed at 405 nm in the presence of P(Ph)₃ acting as sacrificial electron donor leads to its quantitative reduction into 1⁶⁺, whereas similar experiment starting from 1⁶⁺ with a sulfonium salt as sacrificial electron acceptor converts 1⁶⁺ into 1⁷⁺ with a slower rate and a maximum yield of 80%. These photoinduced electron transfers were followed by UV-Visible spectroscopy and compared with those obtained with a simple mixture of both mononuclear parent complexes i.e. [Ru^II(bpy)₃]²⁺ and [Co^II(tolyl-terpy)₂]²⁺ or [Co^III(tolyl-terpy)₂]³⁺ (tolyl-terpy = 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine).     

Reactivity of a cobalt(III)-peroxo complex in oxidative nucleophilic reactions

A mononuclear cobalt(III)-peroxo complex bearing a macrocyclic tetradentate N₄ ligand, [Co^III(TMC)(O₂)]⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was generated in the reaction of [Co^II(TMC)]²⁺ and H₂O₂ in the presence of triethylamine in CH₃CN. The reactivity of the cobalt(III)-peroxo complex was investigated in aldehyde deformylation with various aldehydes and compared with that of iron(III)- and manganese(III)-peroxo complexes, such as [Fe^III(TMC)(O₂)]⁺ and [Mn^III(TMC)(O₂)]⁺. In this reactivity comparison, the reactivities of metal-peroxo species were found to be in the order of [Mn^III(TMC)(O₂)]⁺ > [Co^III(TMC)(O₂)]⁺ > [Fe^III(TMC)(O₂)]⁺. A positive Hammett ρ value of 1.8, obtained in the reactions of [Co^III(TMC)(O₂)]⁺ and para-substituted benzaldehydes, demonstrates that the aldehyde deformylation by the cobalt(III)-peroxo species occurs via a nucleophilic reaction.     

Seven-coordinate Mn(II) and Co(II) complexes of the pentadentate ligand 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone): Synthesis, crystal structure and magnetic properties

The metal complexation properties of a functionalized N₃O₂ donor ligand H₂L², where H₂L² stands for 2,6-diacetyl-4-carboxymethyl-pyridine bis(benzoylhydrazone), are investigated by structural and spectroscopic (IR, ESI-MS and EPR) characterization of its Mn(II) and Co(II) complexes. The ligand H₂L² is observed to react essentially in the same fashion as its unmodified parent H₂L¹ producing mixed-ligand [M(H₂L²)(Cl₂)] complexes (M = Mn^II (1), Co^II (3)) upon treatment with MCl₂. Complexes [M(HL²)(H₂O)(EtOH)]BPh₄ (M = Mn 2, M = Co 4), incorporating the supporting ligand in the partially deprotonated form (HL²)⁻, are formed by salt elimination of the [M(H₂L²)(Cl₂)] compounds with NaBPh₄. Compounds 2 and 4 are isostructural featuring distorted pentagonal-bipyramidal coordinated Mn^II and Co^II ions, with the H₂O and EtOH ligands bound in axial positions. Intermolecular hydrogen bonding interactions of the type M-OH₂?O-M involving the H₂O ligands and the carbonyl functions of the supporting ligand assembles the complexes into dimers. Temperature-dependent magnetic susceptibility measurements (2-300 K) show a substantially paramagnetic Curie behavior for the Mn²⁺ compound (2) influenced by zero-field splitting and significant orbital angular momentum contribution for 4 (high-spin Co^II). The exchange coupling across the Mn^II-OH₂?O-Mn^II bridges in 2 was found to be less than 0.1 cm⁻¹, suggesting that no significant intradimer exchange coupling occurs via this path.     

Synthesis of palladium(II) 3d-metal(II) paddlewheel acetate-bridged heterodimetallic complexes: Unexpected catalysis by water molecules

A study of the reaction kinetics between the trinuclear palladium(II) acetate Pd₃(μ-OOCMe)₆ (1) and the mononuclear 3d-metal (Ni^II, Co^II, Cu^II) acetates in acetic acid under water-specified conditions revealed a fairly complicated reaction mechanism triggered by the primary hydrolytic cleavage of an acetate bridge in molecule 1. The isolated reaction products, as established by X-ray diffraction study, are 1D polymeric complexes {Pd(μ-OOCMe)₄M(OH₂)(HOOCMe)₂}_n (M = Ni^II, Co^II, Cu^II, Mn^II, Zn^II) built of the Pd^II-based paddlewheel units [Pd(μ-OOCMe)₄M] and linked trough the H-bonded H₂O and MeCOOH molecules.     

Nature of the Ln-Co magnetic interactions in compounds [Ln2Co3(C5H4NPO3)6] · 4H2O with open-framework structures

The reactions of Ln(NO₃)₃ · xH₂O, CoSO₄ · 7H₂O or ZnSO₄ · 6H₂O and 2-pyridylphosphonic acid under hydrothermal conditions result in heterometallic phosphonate compounds with formula [Ln₂M₃(C₅H₄NPO₃)₆] · 4H₂O (Ln₂M₃; M = Co^II or Zn^II; Ln = La^III, Ce^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Tb^III, Dy^III). These compounds are isostructural and crystallize in a chiral cubic space group I2₁3. Each structure contains the {LnO₉} polyhedra and {MN₂O₄} octahedra which are connected by edge-sharing to form an inorganic open-framework structure with a 3-connected 10-gon (10, 3) topology. The nature of Ln^III-Co^II magnetic interactions in Ln₂Co₃ is investigated by a comparison with their Ln^III-Zn^II analogues. It is found that the Ln^III-Co^II interaction is weak antiferromagnetic for Ln = Ce and ferromagnetic for Ln = Sm, Gd, Tb and Dy. In the cases of Ln = Pr, Nd and Eu, no significant magnetic interaction is observed.     

Vibrational and structural mapping of [Os(bpy)3] and [Os(phen)3]

Structural changes between [Os^IIL₃]²⁺ and [Os^IIIL₃]³⁺ (L: 2,2′-bipyridine; 1,10-phenanthroline) and molecular and electronic structures of the Os^III complexes [Os^III(bpy)₃]³⁺ and [Os^III(phen)₃]³⁺ are discussed in this paper. Mid-infrared spectra in the ν(bpy) and ν(phen) ring stretching region for [Os^II(bpy)₃](PF₆)₂, [Os^III(bpy)₃](PF₆)₃, [Os^II(phen)₃](PF₆)₂, and [Os^III(phen)₃](PF₆)₃ are compared, as are X-ray crystal structures. Absorption spectra in the UV region for [Os^III(bpy)₃](PF₆)₃ and [Os^III(phen)₃](PF₆)₃ are dominated by very intense absorptions (ε = 40 000-50 000 M⁻¹ cm⁻¹) due to bpy and phen intra-ligand π → π^∗ transitions. In the visible region, relatively narrow bands with vibronic progressions of ∼1500 cm⁻¹ appear, and have been assigned to bpy or phen-based, spin-orbit coupling enhanced, ¹π → ³π^∗ electronic transitions. Also present in the visible region are ligand-to-metal charge transfer bands (LMCT) arising from π(bpy) → t_2g(Os^III) or π(phen) → t_2g(Os^III) transitions. In the near infrared, two broad absorption features appear for oxidized forms [Os^III(bpy)₃](PF₆)₃ and [Os^III(phen)₃](PF₆)₃ arising from dπ-dπ interconfigurational bands characteristic of dπ⁵Os^III. They are observed at 4580 and 5090 cm⁻¹ for [Os^III(bpy)₃](PF₆)₃ and at 4400 and 4990 cm⁻¹ for [Os^III(phen)₃](PF₆)₃. The bpy and phen infrared vibrational bands shift to higher energy upon oxidation of Os(II) to Os(III). In the cation structure in [Os^III(bpy)₃](PF₆)₃, the Os^III atom resides at a distorted octahedral site, as judged by ∠N-Os-N, which varies from 78.78(22)° to 96.61(22)°. Os-N bond lengths are also in general longer for [Os^III(bpy)₃](PF₆)₃ compared to [Os^II(bpy)₃](PF₆)₂ (0.010 Å), and for [Os^III(phen)₃](PF₆)₃ compared to [Os^II(phen)₃](PF₆)₂ (0.014 Å). Structural changes in the ligands between oxidation states are discussed as originating from a combination of dπ(Os^II) → π^∗ (bpy or phen) backbonding and charge redistribution on the ligands as calculated by natural population analysis.     

Photochemical and photophysical properties of ruthenium(II) bis-bipyridine bis-nitrile complexes: Photolability

The electrochemical and photophysical properties of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy)₂(L)₂](PF₆)₂, where bpy is 2,2′-bipyridine and L is AN = CH₃CN and sn = NC-CH₂CH₂-CN, have been investigated. Electrochemical data are typical of Ru-bpy complexes with two reversible reduction peaks located near −1.3 and −1.6 V assigned to each bipyridine ligand and one Ru^II/Ru^III oxidation wave centered at approximately +1.5 V. The sn derivative is both IR and Raman active with its coordinated CN stretch appearing at 2277 cm⁻¹ and 2273 cm⁻¹, respectively. The UV/Vis absorption spectrum of the sn derivative is dominated by an intense (ε_max ∼ 58700 M⁻¹ cm⁻¹) absorption band at 287 nm assigned as a LC (π → π∗) transition. The peak observed at 418 nm (ε ∼ 10 400 M⁻¹ cm⁻¹) is an MLCT band while the one at 244 nm (ε ∼ 23 600 M⁻¹ cm⁻¹) is of LMLCT character. The AN derivative behaves similarly. Both complexes show low-temperature emission at around 537 nm with a lifetime near 10.0 μs. ¹H and ¹³C assignments are consistent with the formulation of the complexes. The complexes undergo photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental results support replacement of the nitrile ligands by solvent. Based on DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron density of the LUMO resides primarily on the bipyridine ligands. The electronic spectra obtained from TDDFT calculations closely match the experimental ones.     

Synthesis, crystal structures, and molecular hyperpolarizabilities of a new Schiff base ligand, and its copper(II), nickel(II), and cobalt(II) metal complexes

A new ligand (HL) obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde with 4-nitroaniline is reported, with its nickel(II), copper(II), and cobalt(II) complexes. The crystal structures are reported for the four derivatives. While, Ni^IIL₂ and Cu^IIL₂ are centrosymmetric molecules, Co^IIL₂ exhibits a pseudo-tetrahedral molecular structure. The quadratic hyperpolarizabilities (β) of HL and Co^IIL₂, measured by electric field induced second harmonic (EFISH) technique, are equal to 66 and 110 × 10⁻³⁰ cm⁵ esu⁻¹, respectively. Beside a geometric effect (pseudo-T_d symmetry), the coordination of the metal center provides an intrinsic enhancement of the NLO response. In addition, an enhancement of the thermal stability of about 60° is found upon metal complexation.     

Coordination polymers of manganese(II) and cobalt(II) of a flexible tetradentate pyridine amide ligand: 1D zigzag network and non-covalent interactions

Synthesis and crystal structure of two coordination polymers of composition [Mn^II(H₂bpbn)_1.5][ClO₄]₂ · 2MeOH · 2H₂O (1) and [Co^II(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 Mn^II ion is surrounded by three pyridine amide units, providing three pyridine nitrogen and three amide oxygen donors. Each Mn^II center in 1 has distorted MnN₃O₃ coordination. In 2 each Co^II 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.     

Structural, spectroscopic and redox studies of a new ruthenium(III) complex with an imidazole-rich tripodal ligand

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[Ru^III(L)(Cl)₂]ClO₄ complex (1). The crystal structure of 1 shows Ru^III in a distorted octahedral geometry, in which two chloride ions, cis-positioned to each other, are coordinated besides the four nitrogen atoms from the tetradentate ligand L. The cyclic voltammogram of 1 exhibits three redox processes at −67, +73 and +200 mV versus SCE, which are attributed to the Ru^III/Ru^II couple in the cis-[Ru^III(L)(Cl)₂]⁺, cis-[Ru^II(L)(H₂O)(Cl)]⁺ and cis-[Ru^II(L)(H₂O)₂]²⁺, respectively. After chemical reduction (Zn(Hg) or Eu^II) only the cis-[Ru^II(L)(H₂O)₂]²⁺ species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (ε=1.4×10³ mol⁻¹ L cm⁻¹), 335 nm (ε=7.9×10³ mol⁻¹ L cm⁻¹), 301 nm (ε=6.7×10³ mol⁻¹ L cm⁻¹) and 264 nm (ε=9.9×10³ mol⁻¹ L cm⁻¹), in water solution (CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ with CF₃COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the Ru^III center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the Ru^II center to the pyridine ligand. The lability of the water ligands in the cis-[Ru^II(L)(H₂O)₂]²⁺ species has been investigated using the auxiliary ligand pyrazine. Reactions in the presence of stoichiometric and excess of pyrazine yield the same species, cis-[Ru^II(L)(H₂O)(pz)]²⁺, which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (ε=5.1×10³ mol⁻¹ L cm⁻¹) and 394 nm (ε=4.2×10³ mol⁻¹ L cm⁻¹). Experiments performed with a large excess of pyrazine gave a specific rate constant k₁=(2.8±0.5)×10⁻² M⁻¹ s⁻¹, at 25 °C, in CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ (with CF₃COONa).     

Electrolyte effects on the intervalence transition within discrete binuclear cyano-bridged complexes. An estimation of activation free energy from static, optical and electrochemical data

A study of the metal-to-metal charge-transfer (MMCT) transition within the binuclear cyano-bridged complexes cis-[L¹³Co^III(μ-NC)Fe^II(CN)₅]⁻ (L¹³ = 12-methyl-1,4,7,10-tetraazacyclotridecan-12-amine), trans-[L¹⁴Co^III(μ-NC)Fe^II(CN)₅]⁻ (L¹⁴ = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) and trans-[L¹⁵Co^III(μ-NC)Fe^II(CN)₅]⁻ (L¹⁵ = 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine) has been carried out in electrolyte solutions at varying concentrations. Using these data, as well as the reaction free energies obtained from electrochemical measurements, the reorganisation and activation free energies for the forward and reverse thermal electron-transfer processes have been estimated. The changes of these parameters with the electrolyte concentration, as well as those of the energy of the maximum MMCT band and the reaction free energy, are mainly due to ion-pairing effects.     

Heterodimetallic Pd-based carboxylate-bridged complexes: Synthesis and structure of single-crystalline Pd-M (M = Mn, Co, Ni, Cu, Zn, Nd, Eu, Ce) acetates

A series of crystalline Pd^II-based heterodimetallic acetate-bridged complexes containing the transition (Mn^II, Co^II, Ni^II, Cu^II), post-transition (Zn^II) and rare-earth (Ce^IV, Nd^III, Eu^III) metals were synthesized starting from Pd₃(OOCMe)₆ and the complementary metal(II, III) acetates. The crystal and molecular structures of the binuclear Pd^IIM^II(μ-OOCMe)₄L (M = Mn, Co, Ni, Zn; L = H₂O, MeCN), trinuclear and tetranuclear (M = Nd, Eu) and complexes were established by X-ray diffraction.     

The structures and magnetism of trinuclear Ni(II), Co(II) and Mn(II) complexes derived from unsymmetrical compartmental ligands

Trinuclear Ni(II), Co(II) and Mn(II) complexes have been prepared from the asymmetric compartmental proligands 2-alkyliminomethyl-4-methyl-6-{[methyl-(2-pyridin-2-yl-ethyl)-amino]-methyl}-phenol(alkyl=ethyl, n-propyl and n-butyl) and 2-[(2-methoxy-ethylimino)-methyl]-4-methyl-6-{[methyl-(2-pyridin-2-yl-ethyl)-amino]-methyl}-phenol, which provide adjacent tridentate N₂O and bidentate NO donor sets. The crystal structures of [Ni₃(L⁶)₂(OAc)₂(NCS)₂] · CH₃OH · H₂O, [Co₃(L⁴)₂(OAc)₂(NCS)₂], and [Mn₃(L⁷)₂(OAc)₂(NCS)₂] · CH₃OH · H₂O were determined and show that the metals provide an isosceles triangle with M¹-M²=M¹-M³ ≈ 3.2 Å and M²-M³ ≈ 5.0 Å. The cryomagnetic properties of the complexes have been studied and indicate a weak antiferromagnetic interaction between the adjacent, M¹M² and M¹M³ ions with little magnetic interaction between the terminal M²M³ ions.     

Synthesis and structure of iron (III) and iron (II) complexes in S4P2 environment created by diethyldithiocarbamate and 1,2-bis(diphenylphosphino)ethane chelation: Investigation of the electronic structure of the complexes by Mössbauer and magnetic studies

Iron (II) and iron (III) complexes, [Fe^II(DEDTC)₂(dppe)] · CH₂Cl₂ (1), [Fe^II(ETXANT)₂(dppe)] (2) (DEDTC = diethyldithiocarbamate, ETXANT = ethyl xanthate, dppe = 1,2-bis (diphenylphosphino) ethane), and [Fe^III(DEDTC)₂(dppe)] [Fe^IIICl₄] (3) have been synthesized and characterized. Since 3 contains two magnetic centers, an anion metathesis reaction has been conducted to replace the tetrahedral FeCl₄⁻ by a non-magnetic BPh₄⁻ ion producing [Fe^III(DEDTC)₂(dppe)]BPh₄ (4) for the sake of unequivocal understanding of the magnetic behavior of the cation of 3. With the similar end in view, the well-known FeCl₄⁻ ion, the counter anion of 3, is trapped as PPh₄[Fe^IIICl₄] (5) and its magnetic property from 298 to 2 K has been studied. Besides the spectroscopic (IR, UV-Vis, NMR, EPR, Mass and XPS) characterization of the appropriate compounds, especially 2, others viz. 1, 3 and 4 have been structurally characterized by X-ray crystallography. While Fe^II complexes, 1 and 2, are diamagnetic, the Fe^III systems, namely the cations of 3, and 4 behave as low-spin (S = 1/2) paramagnetic species from 298 to 50 K. Below 50 K 3 shows gradual increase of χ_MT up to 2 K suggesting ferromagnetic behavior while 4 exhibits gradual decrease of magnetic moment from 60 to 2 K, indicating the occurrence of weak antiferromagnetic interaction. These conclusions are supported by the Mössbauer studies of 3 and 4. The Mössbauer pattern of 1 exhibits a doublet site for diamagnetic (2-400 K) Fe^II. The compounds 1, 2 and 4 encompass interesting cyclic voltammetric responses involving Fe^II, Fe^III and Fe^IV.     

Synthesis, structure, and properties of monomeric Fe(II), Co(II), and Ni(II) complexes of neutral N-(aryl)-2-pyridinecarboxamides

We have prepared and structurally characterized six-coordinate Fe(II), Co(II), and Ni(II) complexes of types [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Fe, 1; Co, 3; and Ni, 5) and [M^II(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 [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Cu and Zn) and [Co^III(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.     

A new mixed-valence hexanuclear cobalt complex, [Co4Co2(dea)2(Hdea)4)(piv)4](ClO4)2·H2O: Synthesis, crystal structure and magnetic properties

A new Co^II/Co^III hexanuclear complex, [Co₄^IICo₂^III(dea)₂(Hdea)₄)(piv)₄](ClO₄)₂·H₂O 1, has been obtained by reacting cobalt(II) perchlorate, diethanolamine, and pivalic acid (H₂dea = diethanolamine and piv = pivalato anion). The cobalt ions are held together by four μ₃ and four μ₂ alkoxo bridges as well as by four syn-syn carboxylato groups. The hexanuclear motif contains four Co(II) and two Co(III) ions. The {Co^II₄Co^III₂(μ₂-O)₄(μ₃-O)₄} core can be described as a four face-sharing monovacant and bivacant distorted heterocubane units. The cobalt(III) ions are hexacoordinated. Two of the cobalt(II) are hexacoordinated, while the two others are pentacoordinated with a bipyramidal stereochemistry. The magnetic properties of 1 have been investigated in the temperature range 1.9-300 K. Compound 1 exhibits an overall antiferromagnetic behaviour with a ground singlet spin state.     

Binuclear manganese(III) complexes of an unsymmetric pyrazolate-based compartmental ligand with hard donor set

The synthesis, crystal structure and magnetic properties of manganese(III) binuclear complexes [Mn^III₂(L-3Н)₂(CH₃ОH)₄]·2CH₃ОH (1) and [Mn^III₂(L-3Н)₂(Py)₄]·2Py (2) (L = 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid) are reported. The ligand contains two distinct donor compartments formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing that both 1 and 2 consist of dinuclear units in which the two metal ions are linked by double pyrazolate bridges with a planar {Mn₂N₄} core. Cryomagnetic measurements show antiferromagnetic interaction with g = 1.99, J = −3.6 cm⁻¹, Θ = −2.02 K for 1 and g = 2.00, J = −3.7 cm⁻¹, Θ = 1.43 K for 2.     

Structures and magnetic properties of dicopper(II) and dinickel(II) complexes with end-on azido bridges

An asymmetric single EO azido bridged dinuclear copper(II) complex, [Cu₂(dmterpy)₂(μ-1,1-N₃)(N₃)₂] · NO₃ · (H₂O)₂1 [dmterpy = 5,5″-dimethyl-2,2′:6′,2″-terpyridine], and a double EO azido bridged dinuclear nickel(II) complex, [Ni₂(pbdiim)₄(μ-1,1-N₃)₂] · 2(N₃) · 6(H₂O) 2 [pbdiim = 2-(2′-pyridyl)benzo[1,2-d:4,5-d′]diimidazole], have been synthesized and characterized structurally and magnetically. Compound 1 consists of a single EO azido bridged Cu^II dimer in which each Cu^II ion is five-coordinated in the form of a distorted square-based pyramid. The N_(μ−1,1) atom holds on the apical position of one Cu^II pyramid with an elongated bond length of 2.305 Å and on the basal plane of another distorted Cu^II pyramid with a bond length of 1.991 Å. The Cu-N_(μ−1,1)-Cu angle is 117.4 (2)°. The copper(II) dimer forms a 1 D zig-zag chain via hydrogen bondings between azide ions, water molecules and the nitrate anion. Compound 2 consists of a double EO azido bridged Ni^II dimer with the Ni-N_(μ−1,1)-Ni bond angle of 102.96 (13)°. The coordination geometry of Ni^II is octahedral. Their magnetic properties have been measured in the range from 300 to 2 K and correlated with the molecular structures. Compound 1 shows weak ferromagnetic interactions within the copper(II) dimer (J = 2.88 cm⁻¹), despite the large EO azide bridge angle (117.4 (2)°). The intramolecular coupling between the Ni^II (S = 1) ions in compound 2 was found to be ferromagnetic (J = 27.87 cm⁻¹).