Mechanism of Formation of [(PMe₃)₃Rh(−C⋮C−R)₂(H)] via C−H Oxidative Addition: Isomerization, Alkyne Exchange, and Hydride Replacement

Abstract

The mechanism of formation of mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] from [(PMe3)4Rh(Me)] and terminal alkyne has been studied. The initial step of the reaction is the elimination of methane and the formation of the trigonal bipyramidal complex [(PMe3)4Rh(−C⋮C−R)], a reaction that is complete in time of mixing at −78 °C. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe3)3Rh(−C⋮C−R)2H] as the kinetic product. This fac isomer is not stable above −20 °C and isomerizes to the thermodynamic product mer,trans-[(PMe3)3Rh(−C⋮C−R)2H]. fac-[(PMe3)3Rh(−C⋮C−R)2H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic barrier than the isomerization to mer,trans-[(PMe3)3Rh(−C⋮C−R)2H]. Density functional theory studies on all stages of the formation of mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] have been carried out and give ground state energies in line with those experimentally observed. Once formed, mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] is configurationally stable and not prone to scrambling, although it will react with chloroform, whereupon the hydride is replaced by chloride. The initial product of this reaction is mer,trans-[(PMe3)3Rh(−C⋮C−R)2Cl], and this compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe3)3Rh(−C⋮C−R)2Cl] isomerizes slowly to mer,cis-[(PMe3)3Rh(−C⋮C−R)2Cl].