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 (RH2Pm, R = phenyl, cyclohexyl) react with [RhCl(CE)(PPh3)2] (E = O, S) to afford the bimetallic complexes [RhCl(CE)(mu-RH2Pm)](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)(mu-PhH2Pm)](2) with KC8 results in the bimetallic rhodium(0) complex, [Rh(mu-CO)(PhH2Pm)](2), with a formal Rh-Rh bond and a hydrogen-bonding interaction between rhodium and the central methylene group (C-H?Rh = 2.802 & Aring;). 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)]BF4. Treatment of [RhCl(CO) (PhH2Pm)]2 with lithium (trimethylsilyl)acetylide provides another bimetallic species, [Rh(CuCSiMe3)(CO) (PhH2Pm)](2), however heating this species does not proceed cleanly to the monomeric NHC complex, [Rh (CuCSiMe3)(CO)(PhPm)] which may however be obtained from [RhCl(RPm)] and LiCuCSiMe3.