Oxidation-reduction Reactions of Complexes with Macrocyclic Ligands. Kinetic and Electrochemical Studies of Metal-ligand Synergism
Two reaction pathways have been found for oxidations of macrocyclic β-diimine cobalt, nickel, and copper complexes with Fe(phen) 3 3+: one pathway apparently involves electron transfer from the metal, the other electron transfer from the ligand. In solutions of moderate acidity, 1.0 M ≥ [H +] < 0.01 M, rates are first order in each reactant and f obsd = k M + k L[H +] -l. For Co(N 4)Cl 2 2+ k M ∼ 0 M -1 s -1 while for Co(N 4)(NCS) 2 +, Ni(N 4) 2+, and Cu(N 4) 2+ finite values of k M are found. For Co(N 4)(NCS) 2 + k M = 155 ± 2 M -1 s -1 and oxidation of NCS - is implicated. For the nickel complexes: k M = (3.4 ± 0.3) × 10 5 M -1 s -1, N 4 = Me 24,7-dieneN 4; k M = (7.7 ± 0.7) × 10 3 M -1 s -1, N 4 = Me 24,6-dieneN 4). For Cu(Me 2-4,7-dieneN 4) 2+, k M = (9.3 ± 1.0) × 10 3 M -1 s -1. The k M pathway for the nickel and copper complexes must involve oxidation of the metal center, while the acid-dependent pathway is associated with oxidation of the β-diiminato ligand. Values of pK a for these coordinated ligands vary between 6 and 9, and Fe(phen) 3 3+ oxidation of the β-diiminato ligand is inferred to be approximately diffusion limited. Thus, the intrinsic barrier for oxidation of β-diiminato moieties must be very small, and ligand oxidation in these and analogous systems can be a very important electron-transfer pathway. The larger intrinsic barrier associated with electron transfer from the metal centers is attributed to Franck-Condon factors. © 1983 American Chemical Society.
J. A. Switzer et al., "Oxidation-reduction Reactions of Complexes with Macrocyclic Ligands. Kinetic and Electrochemical Studies of Metal-ligand Synergism," Journal of the American Chemical Society, American Chemical Society (ACS), Jan 1983.
The definitive version is available at http://dx.doi.org/10.1021/ja00339a011
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© 1983 American Chemical Society (ACS), All rights reserved.