Substitution and Redox Properties of Some Organoisocyanide Cobalt(II) Complexes

Abstract

The reactions of four tetrakis(arylisocyanide)cobalt(II) complexes, [Co(CNR)4(ClO4)2] {R = 2,6-Me2C6H3 (A), 2,4,6-Me3C6H2 (B), 2,6-Et2C6H3 (C), and 2,6-iPr2C6H3 (D)}, with two pyridines, 4-CNpy and 4-Mepy, have been kinetically studied in trifluoroethanol medium. Each of the reactions, which was monitored over a temperature range of 293 to 318 K, exhibited two distinct processes proposed to be an initial fast substitution process followed by a slow reduction process. For each pyridine, steric hindrance was observed to play a significant role in the rates of the reactions, which decrease with increasing size of the arylisocyanide ligand in the order k(A) > k(B) > k(C) > k(D). Addition of each of three triarylphosphines, PR3 (R = Ph, C6H4Me-p, C6H4OMe-p), to solutions of pentakis(t-octylisocyanide)cobalt(II), [Co(CNC8H17-t)5](ClO4)2, resulted in a shift in the λmax of the electronic spectrum accompanied by a change in color of the solutions. The shift is attributed to ligand substitution. The reactions of the cobalt(II) complex [Co(CNC8H17-t)5]2+ with the triarylphosphines are proposed to proceed via a combination of substitution, reduction, and disproportionation mechanisms with final formation of disubstituted Co(I) complexes. The order of reactivity of the complex with the triarylphosphines was found to be P(C6H4OMe-p)3 > P(C6H4Me-p)3 > PPh3. This order is explained in terms of the electron donating/π-acceptor properties of the phosphines.