Reactivity of Coordinatively Unsaturated Bis(N-heterocyclic carbene) Pt(II) Complexes toward H2. Crystal Structure of a 14-Electron Pt(II) Hydride Complex

The reactivity toward H2 of coordinatively unsaturated Pt­(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt­(NHC′)­(NHC)]+ (where NHC′ stands for a cyclometalated NHC ligand) react very fast with H2 at room temperature, leading to hydrogenolysis of the Pt–CH2 bond and concomitant formation of hydride derivatives [PtH­(NHC)2]+ or hydrido–dihydrogen complexes [PtH­(H2)­(NHC)2]+. The latter species release H2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH­(IPr)2]­[SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H2 in complexes [PtH­(H2)­(NHC)2]+ is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (ItBu). This outcome is related to the higher close-range steric effects of the ItBu ligands. Accordingly, H/D exchange reactions between hydrides [PtH­(NHC)2]+ and D2 take place considerably faster for IPr and IMes* derivatives than for ItBu ones. The reaction mechanisms for both H2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a σ-complex assisted-metathesis mechanism in the case of ItBu.