The stabilisation of low oxidation state transition metal complexes. Preparation and electrochemistry of cobalt(II) unsaturated macrocyclic complexes and the stabilisation of a cobalt(I) derivative. Crystal and molecular structures of [Co^II(L)(CH₃OH)₂][BF₄]₂ and [Co^I(L){P(OCH₃)₃}][BF₄]

A series of seven cobalt(II) complexes [Co^II(L)X₂]²⁺, based on the planar macrocyclic ligand L derived from 2,9-di(1-methylhydrazino)-1,10-phenanthroline and 2,6-diacetylpyridine have been prepared with varying axial ligands X[X = H₂O, pyridine(py), 4-aminopyridine (apy), 4-cyanopyridine (cpy), 4-dimethylaminopyridine (dap), imidazole (im), 1-methylimidazole (mim), quinoline (quin), pyrazine (pyz), or 1,4-diazabicyclo[2.2.2]octane (dabco)]. The single-crystal X-ray structure of the bis-methanol adduct [Co^II(C₂₃H₂₁N₇)(CH ₃OH)₂][BF₄]₂, which crystallises in space group P2₁/n with a = 12.110(8), b = 17.205(9), c = 15.411(8) Å, β = 112.62(3)°, and Z = 4, was solved by a combination of Patterson and Fourier techniques, and refined by blocked full-matrix least sauares to R = 0.088 for 1 754 observed diffractometer data. The structure confirms the seven-co-ordinate geometry for this complex with the cobalt ion in the plane of the macrocyclic ligand. Cyclic voltammetry of the complexes in acetonitrile at a platinum microsphere shows a reversible reduction wave near E_1/2 = -1.4 V which may be tentatively assigned to the initial formation of a highly unstable cobalt(II) ligand radical species. Addition of P(OCH₃)₃ to a solution of [Co(L)(H₂O)₂][BF₄]₂ in acetonitrile shifts the reversible reduction wave from E_1/2 = -1.43 to -1.199 V; electrochemical reduction of such a solution by controlled potential electrolysis yields the metal-reduced cobalt(I) complex [Co^I(C₂₃H₂₁N₇){P(OCH ₃)₃}][BF₄]. The single-crystal X-ray structure shows it to crystallise in space group P2₁/n with a = 10.745(4), b = 10.721(5), c = 24.722(8) Å, β = 92.57(2)°, and Z = 4. The structure was solved by a combination of Patterson and Fourier-difference techniques, and refined by blocked-cascade least squares to R = 0.069 for 2 519 observed diffractometer data. The cobalt ion is displaced out of the plane of the five donor nitrogen atoms by 0.36 Å towards the phosphorus atom. The macrocyclic ligand is not planar but has the shape of a shallow dome, the distortion being away from the axial ligand. The stabilisation of the metal-reduced cobalt(I) complex is attributed to the combined π-acceptor properties of the unsaturated macro-cyclic ligand and the axial P(OCH₃)₃ ligand.