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, (LH3)-H-Me, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyolononane, (LH3)-H-BU, 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, (LH3)-H-OCH3, and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, (LH)-H-Pr, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N3O3 donor set have been synthesized: [(LVIII)-V-Me] (1), [(LVIV)-V-Me]PF6 (2), [((LH)-H-Me)V-V(O)]PF6 (3), [(LVIV)-V-Bu]PF6 (4), [(LVIV)-V-OCH3]PF6 (5), [(LCOIII)-C-Me] (6), [(LCoIII)-Co-Bu] (7), [(LCoIII)-Co-OCH3] (8). In addition, two complexes containing the (LCoIII)-Co-Pr fragment have been prepared: [L-Pr-Co-III(acac)](ClO4) (9) and [(LCoIII)-Co-Pr(Cl(4)cat)]. CH3CN (10), where acac(-) represents the ligand pentane-2,4-dionate and Cl(4)cat(2-) is tetrachlorocatecholate. Complexes 9 and 10 have been characterized by single-crystal X-ray crystallography: 9 crystallizes in the triclinic space group <P(1)over bar> with a = 9.493(1) Angstrom, b = 9.760(1) Angstrom, c = 18.979(2) Angstrom, alpha = 88.57(1)degrees, beta = 78.60(1)degrees, gamma = 79.24(1)degrees, V = 1693.3(3) Angstrom(3), and Z = 2; 10 crystallizes in the monoclinic space group P2(1)/n with a = 10.184(2) Angstrom, b = 24.860(5) Angstrom, c = 14.872(3) Angstrom, beta = 97.95(3)degrees, V = 3729(1) Angstrom(3), 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 [(LHVIV)-H-Me(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 W-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(LII) unit prevails.