Metal- versus Ligand-Centered Oxidations in Phenolato−Vanadium and −Cobalt Complexes:  Characterization of Phenoxyl−Cobalt(III) Species

The coordination chemistry of the pendent-arm macrocycles 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^MeH₃, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^BuH₃, 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^OCH₃H₃, and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^PrH, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N₃O₃ donor set have been synthesized:  [L^MeV^III] (1), [L^MeV^IV]PF₆ (2), [(L^MeH)V^V(O)]PF₆ (3), [L^BuV^IV]PF₆ (4), [L^OCH₃V^IV]PF₆ (5), [L^MeCo^III] (6), [L^BuCo^III] (7), [L^OCH₃Co^III] (8). In addition, two complexes containing the L^PrCo^III fragment have been prepared:  [L^PrCo^III(acac)](ClO₄) (9) and [L^PrCo^III(Cl₄cat)]·CH₃CN (10), where acac^- represents the ligand pentane-2,4-dionate and Cl₄cat^2- is tetrachlorocatecholate. Complexes 9 and 10 have been characterized by single-crystal X-ray crystallography:  9 crystallizes in the triclinic space group P1̄ with a = 9.493(1) Å, b = 9.760(1) Å, c = 18.979(2) Å, α = 88.57(1)°, β = 78.60(1)°, γ = 79.24(1)°, V = 1693.3(3) ų, and Z = 2; 10 crystallizes in the monoclinic space group P2₁/n with a = 10.184(2) Å, b = 24.860(5) Å, c = 14.872(3) Å, β = 97.95(3)°, V = 3729(1) ų, and Z = 4. Electrochemically, complexes 2, 4, and 5 can be reversibly oxidized by one electron, yielding vanadium(V), and one-electron-reduced, affording vanadium(III) species; 3 can be reduced to [L^MeHV^IV(O)]. These redox processes are shown to be metal-centered. In contrast, the cyclic voltammograms of 7 and 8 display three reversible one-electron oxidations. For the monocations [7]^•+ and [8]^•+, EPR and UV−vis spectroscopies reveal that these are phenoxyl−cobalt(III) species. Thus, the redox processes are ligand-centered. Similarly, [9]⁺ is reversibly oxidized to the phenoxyl−cobalt(III) complex [9]^•2+ (S = ¹/₂). For 10, two reversible one-electron oxidation steps have been identified generating [10]^•+ (S = ¹/₂) and [10]^2•2+ (S = 1). It is unambiguously shown by EPR and resonance Raman spectroscopies that [10]^•+ contains a (phenolato)(semiquinonato)cobalt(III) unit whereas in [10]^2•2+ a phenoxyl(semiquinonato)cobalt(III) unit prevails.