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Complexes [M(LN,O,S)2] [M = Ru, Os; (LN,O,S)2– = 4,6-di-tert-butyl(2-methylthiophenylamido)-o-phenolate] were obtained and structurally characterized as metal(IV) complexes with mer-configured tridentate ligands. Two reversible oxidations and one (Os) or two (Ru) reversible reductions were investigated by EPR and UV/Vis/NIR spectroelectrochemistry. The first reduction leads to EPR-silent MIII species, whereas the oxidation produces iminosemiquinone complexes with ligand-centered spin and small-metal participation at the singly occupied MO. Absorptions in the visible and near-IR region are assigned with the help of time-dependent (TD)-DFT calculations.

The compounds [(dppf)Cu(L)](BF4) (1[BF4]) [dppf = 1,1′-bis(diphenylphosphanylferrocene), L = N-(2-methyl-5,8-dioxo-5,8-dihydroquinolin-7-yl)acetamide] and [(acac)2Ru(L)] (2) (acac = acetylacetonato) were prepared from the reactions of [Cu(dppf)(CH3CN)2](BF4) or [Ru(acac)2(CH3CN)2], respectively with L. Structural characterization of 1[BF4] shows a distorted tetrahedral coordinated copper center. Both complexes show one-electron reversible oxidation as well as reduction processes in their cyclic voltammogram. The first oxidation process of 1[BF4] leads to absorptions at around 900 nm in the UV/Vis/NIR spectrum and results in an EPR silent species up to 110 K thus indicating a dppf-centered oxidation. The reduction processes of 1[BF4] show features in the UV/Vis/NIR, IR as well as EPR spectrum that are compatible with L-centered redox processes. In contrast to this, the ruthenium complex 2 shows metal-centered spin in both the one-electron oxidized as well as reduced forms. These results show that the copper center does not participate in the redox processes in 1[BF4] whereas the ruthenium center participates strongly in the electron-transfer processes in 2. Results obtained from electrochemistry as well as from UV/Vis/NIR, IR, and EPR spectroelectrochemistry are invoked to show the noninnocent nature of L in these complexes as well as the more relative importance of metal–L covalency in 2 as compared to 1[BF4].

The new copper(I) complexes [Cu(L1)2](BF4), L1 = 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine, and [Cu(L2)2](ClO4), L2 = 2-pyridyl-N-(2′-benzylthiophenyl)methyleneimine, have been prepared and structurally characterized. In contrast to the known [Cu(L1)2](ClO4)2, which exhibits partial thioether S binding to effect five-coordinate Cu2+, the copper(I) compounds reported here contain four-coordinate metal ions with exclusively N-donor binding. Cyclic voltammetry reveals a fully reversible oxidation of the CuI species, which suggests a small barrier for reorganization. The reduction at negative potentials is irreversible for compounds [Cu(L)2](X) and for the structurally characterized new compound [Cu(L1)(PPh3)2](ClO4). UV/Vis spectroelectrochemistry shows the typical low-energy absorption bands of copper(I) (MLCT transition) and copper(II) (ligand-field transition) in the visible region; the CuII form develops an intense band at 350 nm attributed to a S-to-Cu ligand-to-metal charge transfer (LMCT).

The redox properties of organometallic nickel complexes of the type [(α-diimine)Ni(Mes)Br], [(α-diimine)Ni(Mes)2] and the complexes trans-[(PPh3)2Ni(Mes)Br] and trans-[(PPh3)2Ni(Fmes)Br] [Mes = mesityl (2,4,6-trimethylphenyl); Fmes = tris(2,4,6-trifluoromethyl)phenyl] have been studied in detail by various electrochemical and spectroelectrochemical (UV/Vis/NIR and EPR) methods. Upon electrochemical reduction, the bromido mesityl derivatives undergo cleavage of the bromide ligand. The resulting reactive species and their products from follow-up reactions are investigated. Electrochemical oxidation leads to formally trivalent nickel species. The metal contribution to the unpaired electron for the formally monovalent and trivalent nickel species can be estimated from EPR spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Chemical reduction of the tetracobalt cluster complex [Co4(CO)3(μ3-CO)3(μ3-C7H7)(η5-C7H9)] (3), followed by addition of [PPh4]Br, gives the complex [{Co4(CO)3(μ3-CO)3(μ3-C7H7)}2{μ-η4:η4-(C7H9)2}]2− as a mixture of two diastereomers [4A]2− and [4B]2− in high yield. The crystal structure of [4A]2−[PPh4]2·1.5C7H8 has been determined and confirms the reductive coupling of two Co4 cluster coordinated apical cycloheptadienyl rings to form a bridging bicycloheptyl-3,5,3′,5′-tetraene ligand. Reduction of 3 with Li[HBEt3] and subsequent treatment with aqueous [NnBu4]Cl results in the formation of [Co4(CO)3(μ3-CO)3(μ3-C7H7)(C7H10)]− [5]−. Addition of [(η-C6H6)Ru(NCMe)3][BF4]2 after the borohydride reduction gives [Ru(η-C6H6)Co3(CO)3(μ3-CO)3(μ3-C7H7)] (6), a product derived from reductive Co4 cluster degradation. A detailed electrochemical and spectro-electrochemical study of the redox behaviour of 3 and [4]2− has been carried out. The complex potential current response of 3 is rationalized in terms of the formation of the radical anion [3]− as the primary intermediate, which may be reversibly reduced further to give the much more stable [3]2− and then [3]3−. Dimerization of [3]− to give [4]2− occurs by formation of a new carbon−carbon bond between the apical C7H9 ligands. The two redox-active moieties in [4]2− behave as independent, non-interacting redox centres. The oxidized form 4 is unstable and dissociates back to 3 almost quantitatively, thus completing a redox cycle characteristic of a “molecular battery”. The homogeneous rate constant for dimerization has been evaluated as kDIM (2 [3]− [4]2−) = 0.30 ± 0.05 mM−1 s−1.