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Redox potential, ligand and structural effects in rhodium(I) complexes |
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| Authors: | M. F tima C. Guedes da Silva, Anna M. Trzeciak, J zef J. Zi lkowski,Armando J. L. Pombeiro |
| Affiliation: | a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal b Universidade Lusófona de Humanidades e Tecnologias, Avenida Campo Grande 376, 1749-024 Lisbon, Portugal c Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw Poland |
| Abstract: | The electrochemical behaviour of the set of tetracoordinate rhodium(I) complexes [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 pentacoordinate [RhH(CO)L3] [L=P(NC4H4)3 (13), PPh3 (14), P(OPh)3 (15) or P(OC6H4Me-4)3 (16)] and [RhHL4] [L=PPh3 (17) or P(OC6H4Me-3)3 (18)] was studied by cyclic voltammetry and controlled potential electrolysis, in aprotic medium, at a Pt electrode. They present a single-electron oxidation wave (I) (irreversible or quasi-reversible) that can be followed, at a higher potential, by a second and irreversible one (II). The values of first oxidation potential for the tetracoordinate complexes fit the additive Lever's electrochemical parameterisation, and the ligand electrochemical Lever EL and Pickett PL 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 electron donor ability decreases with the increase of the number of N-pyrrolyl groups) than the latter phosphines. The pentacoordinate hydride complexes 13–18 fit a distinct relationship which enabled the estimate of the EL ligand parameter for the phosphites P(OC6H4Me-3)3 and P(OC6H4Me-4)3. Electrochemical metal site parameters were obtained for the square planar and the pentacoordinate 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 were also observed between the oxidation potential and the PL ligand parameter (for the series [Rh(acac)(CO)L]) or the infrared ν(CO) frequency, and a generalisation of the former type of correlation is proposed for series of square-planar 16-electron complexes [M′SL] with a common 14-electron T-shaped binding metal centre {M′S}. Oxidation of 5 by Ag[PF6] leads to the dimerisation of the derived Rh(II) species. |
| Keywords: | Redox potential–structure relationships Electrochemistry Electrochemical parameters Ligand effects Rhodium complexes N-Pyrrolyl phosphines |
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