Theoretical study on the mechanism of H₂activation mediated by two transition metal thiolate complexes: Homolytic for Ir, heterolytic for Rh

Abstract

The molecular mechanism of H(2) activation by two transition metal thiolate complexes [Cp*M(PMe(3))(SDmp)](BAr(F)(4)) (M = Ir, Rh) (Ohki, Y; Sakamoto, M; Tatsumi, K. J. Am. Chem. Soc., 2008, 130, 11610-11611) has been investigated using density functional theory calculations. According to our calculations, the reaction of the iridium thiolate complex with H(2) is likely to proceed through the following steps: (1) the oxidative addition of H(2) to the iridium center to generate a dihydride intermediate; (2) the reductive elimination of one Ir-bound hydrogen to produce the hydride thiol product. For the rhodium thiolate complex, its reaction with H(2) is to form the dihydrogen intermediate first, and then the H-H bond is heterolytically cleaved at the Rh-S bond via a four-center transition state to yield the hydride thiol product. The rate-determining step is the oxidative addition step (with a barrier of 18.0 kcal/mol in the solvent) for the iridium complex, and the formation of the dihydrogen complex (with a barrier of 13.9 kcal/mol in the solvent) for the rhodium complex. The calculated free energy profiles for both metal thiolate complexes can reasonably account for the observed reversible H(2) activation by two metal thiolate complexes under mild conditions.

Mechanism of the cooperative Si–H bond activation at Ru–S bonds

“…From our previous studies, we already knew that Si–H bond activation with cationic ruthenium( ii ) thiolate complexes 1 is remarkable facile, proceeding instantly at room temperature. 14 The tethered coordination mode of the SDmp ligand proved to be crucial: while the lability of a monodentate thiolate ligand in related rhodium( iii ) and iridium( iii ) complexes 16 resulted in decomposition and formation of various metal hydride species in the presence of hydrosilanes, the two-point binding mode in 1 imparts increased stability to the Ru–S bond. In addition, this motif leads to structural rigidity and improved steric accessibility of the Ru–S bond, as seen in the molecular structure of these complexes.…”

Metal–Ligand Cooperation

“…Based on a computational study of this reactivity, Li et al. suggested that H 2 activation by the Ir complex 54 occurs through initial oxidative addition of H 2 to give an Ir V dihydride followed by SH reductive elimination 62. At the same time, DFT studies suggest that the analogous Rh complex operates through metal–ligand cooperative heterolytic splitting of H 2 62…”

Frustration across the periodic table: heterolytic cleavage of dihydrogen by metal complexes

“…Upon warming to +23°C under H 2 , the substituted thiophenol dissociates, and the iridium oxidatively adds H 2 to give an iridium trihydride. DFT calculations suggest that H 2 binds to the iridium centre prior to heterolytic cleavage [27].…”