Solvent-stabilized alkylrhodium(III) hydride complexes: A special mode of reversible C-H bond elimination involving an agostic intermediate

Reaction of the complex [Rh(coe)(2)(solv)(n)]BF4 (coe = cyclooctene) with t he phosphane 1-di-tert-butylphosphinomethyl-2,4,6-trimethylbenzene (1) resu lts in selective C-H bond activation, yielding the spectroscopically charac terized solvento complexes [(solv)(n)RhH{CH2C6H2(CH3)(2)[CH2P(tBu)(2)]}]BF4 (solv = acetone, 2a; THF, 2b; methanol, 2c), The stability of these comple xes is solvent dependent, alcohols providing significant stabilization. Alt hough cis-alkylrhodium hydride complexes containing labile ligands are gene rally unstable, 2a-c an stable at room temperature. Complex [(acetone) (ket ol)RhH{CH2C6H2(CH3)(2)[CH2P(t-Bu)(2)]}]BF4 (2d, ketol=4-hydroxy-4-methyl 2- pentanone, the product of acetone aldol condensation), crystallized from a solution of 2a in acetone and was structurally characterized. Unusual solve nt- and temperature-dependent selectivity in reversible C-H bond eliminatio n of these complexes, most probably controlled by a special mode of strong agostic interactions, is observed by spin saturation transfer experiments.