C-H activation in bimetallic rhodium complexes to afford N-heterocyclic carbene pincer complexes.

The pro-ligands 1,8-bis(di-R-phosphinomethyl)-2,3-dihydroperimidine (RH 2 Pm, R = phenyl, cyclohexyl) react with [RhCl(CE)(PPh 3 ) 2 ] (E = O, S) to afford the bimetallic complexes [RhCl(CE)(μ-RH 2 Pm)] 2 (E = O, S). Upon heating, these species undergo double C-H activation to afford the N-heterocyclic carbene (NHC) pincer complexes [RhCl(RPm)]. Reduction of [RhCl(CO)(μ-PhH 2 Pm)] 2 with KC 8 results in the bimetallic rhodium(0) complex, [Rh(μ-CO)(PhH 2 Pm)] 2 , with a formal Rh-Rh bond and a hydrogen-bonding interaction between rhodium and the central methylene group (C-H⋯Rh = 2.802 Å). Upon treatment with tritylium, ferrocenium or triphenylcyclopropenium tetrafluoroborates this species undergoes double C-H activation to afford a mononuclear NHC pincer complex salt, [Rh(CO)(PhPm)]BF 4 . Treatment of [RhCl(CO)(PhH 2 Pm)] 2 with lithium (trimethylsilyl)acetylide provides another bimetallic species, [Rh(CCSiMe 3 )(CO)(PhH 2 Pm)] 2 , however heating this species does not proceed cleanly to the monomeric NHC complex, [Rh(CCSiMe 3 )(CO)(PhPm)] which may however be obtained from [RhCl(RPm)] and LiCCSiMe 3 .