Fluxionality of [(Ph_3P)_3M(X)] (M = Rh, Ir). The Red and Orange Forms of [(Ph_3P)_3lr(Cl)]. Which Phosphine Dissociates Faster from Wilkinson's Catalyst?

摘要

NMR studies of intramolecular exchange in [(Ph_3P)_3Rh(X)] (X = CF_3, CH_3, H, Ph, Cl) have produced full sets of activation parameters for X = CH_3 (E_a = 16.4 ± 0.6 kcal mol~(-1) △H~? = 16.0 ± 0.6 kcal mol~(-1), and △S~? = 12.7 ± 2.5 eu), H (E_a = 10.7 ± 0.2 kcal mol~(-1), AH~? = 10.3 ± 0.2 kcal mol~(-1), and △S~? = -7.2 ± 0.8 eu), and Cl (E_a = 16.3 ± 0.2 kcal mol~(-1) △H~? = 15.7 ± 0.2 kcal mol~(-1), and △S~? = -0.8 ± 0.8 eu). Computational studies have shown that for strong trans influence ligands (X = H, Me, Ph, CF_3), the rearrangement occurs via a near-trigonal transition state that is made more accessible by bulkier ligands and strongly donating X. The exceedingly fast exchange in novel [(Ph_3P)_3Rh(CF_3)] (12.1 s~(-1) at -100 ℃) is due to strong electron donation from the CF_3 ligand to Rh, as demonstrated by computed charge distributions. For weaker donors X, this transition state is insufficiently stabilized, and hence intramolecular exchange in [(Ph_3P)_3Rh(CI)] proceeds via a different, spin-crossover mechanism involving triplet, distorted-tetrahedral [(Ph_3P)_3Rh(CI)] as an intermediate. Simultaneous intermolecular exchange of [(Ph_3P)_3Rh(CI)] with free PPh_3 (THF) via a dissociative mechanism occurs exclusively from the sites cis to Cl (E_a = 19.0 ± 0.3 kcal mol~(-1), △H~? = 18.5 ± 0.3 kcal mol~(-1), and △S~? = 4.4 ± 0.9 eu). Similar exchange processes are much slower for [(Ph_3P)_3lr(Cl)] which has been found to exist in orange and red crystallographic forms isostructural with those of [(Ph_3P)_3Rh(Cl)].