Cobalt-mediated selective C-H bond activation. Direct aromatic hydroxylation in the complexes [Co^III{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

Abstract

The reactions of low-spin complexes [Co^IIL₃][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 [Co^IIIL'2]ClO₄.H₂O 2 [L' =o-OC₆H₃(R)NNC₅H₄N]. 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 ¹H and ¹³C 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 (¹A_1g →¹ T_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 ⇋ Co^II reduction near 0.0 V versus Ag-AgCl. The ligand-based expected four azo (NN) 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, [Co^IIL'₂] 2- electrochemically as well as chemically by using hydrazine hydrate. These complexes display eight-line EPR spectra in acetonitrile solution at 77κ. Complex 2a^- exhibits a ligand to metal charge-transfer transition at 534 nm and intraligand transition at 345 nm. Two possible d-d transitions, ²E → ²T₁ and ²E → ²T₂ are observed at 700 and 800 nm respectively.