Redox potential, ligand and structural effects in rhodium(I) complexes

The electrochemical behaviour of the set of tetracoordinate rhodium(I) comp lexes [Rh(O<^>O)(CO)L] [O<^>O= MeC(O)CHC(O)Me (acac), L = CO (1), P(NC4H4)( 3) (2), PPh(NC4H4)(2) (3), PPh2(NC4H4) (4), PPh3 (5), PCy3 (6), P(OPh)(3) ( 7) or PPh2(C6H4OMe-4 (8). O<^>O = PhC(O)CHC(O)Me (bac), L = CO (9) or PPh3 (10); O<^>O = PhC(O)CHC(O)CF3(bta), L = CO (11) or PPh3 (12)] and of the pe ntacoordinate [RhH(CO)L-3] [L = P(NC4H4)(3) (13), PPh3 (14), P(OPh)(3) (15) or P(OC6H4Me-4), (16)] and [RhHL4] [L = PPh3 (17) or P(OC6H4Me-3)(3) (18)] was studied by cyclic voltammetry and controlled potential electrolysis, i n aprotic medium, at a Pt electrode. They present a single-electron oxidati on wave (1) (irreversible or quasi-reversible) that can be followed, at a h igher potential, by a second and irreversible one (II). The values of first oxidation potential for the tetracoordinate complexes fit the additive Lev er's electrochemical parameterisation, and the ligand electrochemical Lever E-L and Pickett P, parameters were estimated for the N-pyrrolyl phosphines PPhn(NC4H4)(3-n)(n = 0, 1 or 2) and for the organophosphines PCy3 and PPh2 (C6H4OMe-4), the former behaving as weaker net electron donors (the electro n donor ability decreases with the increase of the number of N-pyrrolyl gro ups) than the latter phosphines. The pentacoordinate hydride complexes 13-1 8 fit a distinct relationship which enabled the estimate of the E-L ligand parameter for the phosphites P(OC6H4Me-3)(3) and P(OC6H4Me-4)(3). Electroch emical metal site parameters were obtained for the square planar and the pe ntacoordinate Rh(I)/Rh(II) couples and, for the former, the redox potential is shown to present a much higher sensitivity to a change of a ligand than the octahedral redox couples investigated so far. Linear relationships wer e also observed between the oxidation potential and the P, ligand parameter (for the series [Rh(acac)(CO)L]) or the infrared v(CO) frequency, and a ge neralisation of the former type of correlation is proposed for series of sq uare-planar 16-electron complexes [M'L-s] with a common 14-electron T-shape d binding metal centre {M'(s)}. Oxidation of 5 by Ag[PF6] leads to the dime risation of the derived Rh(II) species. (C) 2001 Elsevier Science B.V. All rights reserved.