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, LMeH3, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, LBuH3, 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, LOCH 3 H3, and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, LPrH, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N3O3 donor set have been synthesized: [LMeVIII] (1), [LMeVIV]PF6 (2), [(LMeH)VV(O)]PF6 (3), [LBuVIV]PF6 (4), [LOCH 3 VIV]PF6 (5), [LMeCoIII] (6), [LBuCoIII] (7), [LOCH 3 CoIII] (8). In addition, two complexes containing the LPrCoIII fragment have been prepared: [LPrCoIII(acac)](ClO4) (9) and [LPrCoIII(Cl4cat)]·CH3CN (10), where acac- represents the ligand pentane-2,4-dionate and Cl4cat2- 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) Å3, 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) Å3, 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 [LMeHVIV(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.