Cobalt-mediated selective C-H bond activation.: Direct aromatic hydroxylation in the complexes [CoIII{o-OC6H3(R)N=NC5H4N}2]ClO4•H2O (R = H, o-Me/Cl, m-Me/Cl or p-Me/Cl).: Synthesis, spectroscopic and redox properties

The reactions of low-spin complexes [(CoL3)-L-II][ClO4](2). H2O 1 [L = 2-(arylazo)pyridine, (R)C6H4N=NC5H4N (R = H, o-Me/Cl, m-Me/Cl or p-Me/Cl] with m-chloroperbenzoic acid (m-ClC6H4CO3H) in acetonitrile solvent at room temperature resulted in low-spin [(CoL)-L-III'(2)]ClO4. H2O 2 [L' = o-OC6H3(R)N=NC5H4N]. In complexes 2 the o-carbon-hydrogen bond of the pendant phenyl ring of both parent ligands L has been selectively and spontaneously hydroxylated. During the transformation of 1 to 2 the metal ion is oxidised from the starting Co-II to Co-III. The meridional configuration (cis-trans-cis with respect to the oxygen, azo and pyridine nitrogens respectively) of complexes 2 has been established by H-1 and C-13 NMR spectroscopy. When one methyl or chloro group was present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. In acetonitrile solvent, complexes 2 systematically display one d-d transition ((1)A(1g) --> T-1(1g) near 850 nm, two metal to ligand charge-transfer transitions in the visible region and intraligand transitions in the UV region. In acetonitrile solution all complexes 2 exhibit irreversible Co-III --> Co-IV oxidation near 2 V and reversible Co-III reversible arrow Co-II reduction near 0.0 V versus Ag-AgCl. The ligand-based expected four azo (N=N) reductions are observed sequentially for all the complexes at the negative side of the reference Ag-AgCl. Complexes 2 can be quantitatively and stereoretentively reduced to the low-spin cobalt(II) congeners, [(CoL)-L-II'(2)] 2(-) electrochemically as well as chemically by using hydrazine hydrate. These complexes display eight-line EPR spectra in acetonitrile solution at 77 K. Complex 2a(-) exhibits a ligand to metal charge-transfer transition at 534 nm and intraligand transition at 345 nm. Two possible d-d transitions, E-2 --> T-2(1) and E-2 --> T-2(2) are observed at 700 and 800 nm respectively.