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(Me)H(3), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(Bu)H(3), 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(OCH)()3H(3), and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(Pr)H, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N(3)O(3) donor set have been synthesized: [L(Me)V(III)] (1), [L(Me)V(IV)]PF(6) (2), [(L(Me)H)V(V)(O)]PF(6) (3), [L(Bu)V(IV)]PF(6) (4), [L(OCH)()3V(IV)]PF(6) (5), [L(Me)Co(III)] (6), [L(Bu)Co(III)] (7), [L(OCH)()3Co(III)] (8). In addition, two complexes containing the L(Pr)Co(III) fragment have been prepared: [L(Pr)Co(III)(acac)](ClO(4)) (9) and [L(Pr)Co(III)(Cl(4)cat)].CH(3)CN (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&onemacr; with a = 9.493(1) Å, b = 9.760(1) Å, c = 18.979(2) Å, alpha = 88.57(1) degrees, beta = 78.60(1) degrees, gamma = 79.24(1) degrees, V = 1693.3(3) Å(3), and Z = 2; 10 crystallizes in the monoclinic space group P2(1)/n with a = 10.184(2) Å, b = 24.860(5) Å, c = 14.872(3) Å, beta = 97.95(3) degrees, V = 3729(1) Å(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 [L(Me)HV(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.
Sokolowski
Achim
A
Max-Planck-Institut für Strahlenchemie, D-45470 Mülheim an der Ruhr, Germany.
Adam
Britta
B
Weyhermüller
Thomas
T
Kikuchi
Akihiro
A
Hildenbrand
Knut
K
Schnepf
Robert
R
Hildebrandt
Peter
P
Bill
Eckhard
E
Wieghardt
Karl
K