(POP)Rh pincer hydride complexes: unusual reactivity and selectivity in oxidative addition and olefin insertion reactions

Abstract

We report on the synthesis and reactivity of rhodium complexes featuring bulky, neutral pincer ligands with a “POP” coordinating motif, ^tBuxanPOP, ^iPrxanPOP, and ^tBufurPOP (^tBuxanPOP = 4,5-bis(di-tert-butylphosphino)-9,9-dimethyl-9H-xanthene; ^iPrxanPOP = 4,5-bis(diisopropylphosphino)-9,9-dimethyl-9H-xanthene; ^tBufurPOP = 2,5-bis((di-tert-butylphosphino)methyl)furan). The (POP)Rh complexes described in this work are, in general, more reactive than their (PNP)Rh and (PCP)Rh analogues, which allows for the generation of several new species under relatively mild conditions. Thus, monomeric (POP)RhCl complexes oxidatively add H₂ to form (POP)Rh(H)₂Cl, from which the coordinatively unsaturated hydride complexes (POP)Rh(H)₂⁺ and (^tBuxanPOP)Rh(H) can be obtained. In the case of the new ligand ^tBufurPOP, a major kinetic product of the reaction with H₂ is, surprisingly, the trans dihydride, i.e. trans-(^tBufurPOP)Rh(H)₂Cl; this is most likely attributable to reversible decoordination of one of the pincer coordinating groups, followed by addition of H₂ to a highly reactive three-coordinate species. Ethylene is hydrogenated by (^tBuxanPOP)Rh(H)₂⁺ at 25 °C, but propylene is not, even at elevated temperatures. Ethylene undergoes insertion into the Rh–H bond of (^tBuxanPOP)RhH; this reaction is reversible, allowing for an experimental determination of the equilibrium constant for this hydrometalation. The less bulky ^iPrxanPOP ligand affords a dihydride complex which functions as a modestly active alkane dehydrogenation catalyst, the first such example for a cationic pincer complex of any metal.