H-X bond activation via hydrogen transfer to hydride in ruthenium N-heterocyclic carbene complexes: density functional and synthetic studies

Ruthenium imidazolylidene hydrido (hydrogen sulfide) complexes undergo H-transfer from H2S ligand to ruthenium, affording (h2-dihydrogen)hydride; the potential energy surface of the hydrogen transfer reaction was explored at QM/MM level. The reactions of trans,cis,cis-Ru(IMes)2(AsPh3)(CO)H2 (1, IMes = 1,3-dimesitylimidazol-2-ylidene) with HX substrates (X = OH, OEt, SH, SnPr) was reinvestigated and shown to lead directly to the formation of the 16-electron species [Ru(IMes)2(CO)(X)H] (4-X). The fluoro analog Ru(IMes)2(CO)(F)H (4-F) was also prepd., and x-ray and neutron diffraction studies show that complex 4-F exhibits a square-pyramidal geometry with hydride in the axial site. D. functional calcns. were performed on one possible mechanism for the formation of 4-X from 1 with various HX (X = F, Cl, OH, SH, NH2, PH2, CH3, and SiH3), involving initial AsPh3/HX substitution followed by H-transfer to hydride and H2 loss. With X = SH, H-transfer in both trans,cis,cis-[Ru(IMes)2(CO)(H2S)(H)2] and trans,trans,trans-[Ru(IMes)2(CO)(H2S)(H)2] was considered and shown to be kinetically accessible and thermodynamically favored, suggesting that such dihydrides should not be stable with respect to this step. The calcns. indicate that the ease of formation of 4-X becomes more kinetically and thermodynamically favored according to the trends F > OH > NH2 > CH3 and Cl > SH > PH2 < SiH3, with the reactions of second-row HX substrates being more favored than the first-row analogs. Calcd. reaction exothermicities allow the derivation of relative Ru-X bond strengths in 4-X, and comparison with exptl. detd. M-X relative bond strengths in the literature highlights the importance of X -> M pi-donation in detg. trends in M-X bond dissocn. energies in unsatd. systems.