Kinetic-Mechanistic and Solid-State Study of the Oxidative Addition and Migratory Insertion of Iodomethane to [Rhodium(S,O-BdiPT or N,O-ox)(CO)(PR1R2R3)] Complexes

Rhodium(I) carbonyl complexes containing bidentate X,O-Bid [S,O-BdiPT or N,O-ox; S,O-BdiPTH = N-benzoyl-N,N-(diphenyl)thiourea; N,O-oxH = 8-hydroxyquinoline] ligands of the form [Rh(X,O-Bid)(CO)((PRRR3)-R-1-R-2)] (R-1, R-2, R-3 = Ph or Cy) bearing different phosphine ligands, were investigated, the structural characterization of four example complexes is described and an extensive spectroscopic kinetic-mechanistic study of the oxidative addition of iodomethane thereto is discussed. Reaction with iodomethane led to Rh-III-acyl species as secondary (final) products, whereas the primary Rh-III-alkyl complexes, although rapidly formed, were only observed as intermediates, in small quantities for S,O-BdiPT (large S-Rh-O bite angle of 90-91 degrees) but in significant amounts for N,O-ox complexes (less steric with a smaller N-Rh-O bite angle of 79-80 degrees). Overall, almost an order-of-magnitude difference in rate constants was observed for the S,O-BdiPT complexes, with the PPh2Cy- and PPhCy2-bearing complexes showing the largest variation. In both the S,O-BdiPT and N,O-ox ligand systems an associative activation is inferred from the large negative S values. The relative reactivity of Rh-I-X,O-Bid complexes, where X = O, S or N, follows a surprising similar reactivity relationship when stepwise varying the PPh3, PPh2Cy, PPhCy2 and PCy3 tertiary phosphine ligands, suggesting a systematic behavior by the PR3 ligands, independent of the X,O-Bid ligand at the Rh-I metal center.