Transfer-dehydrogenation of alkanes catalyzed by rhodium(I) phosphine complexes

Complexes of the form Rh(PMe₃)₂ClL' (L' = CO or trisubstituted phosphine) and [Rh(PMe₃)₂Cl]₂ have previously been reported to catalyze the transfer-dehydrogenation of alkanes, using olefinic hydrogen acceptors under a dihydrogen atmosphere. Such complexes are herein reported to effect transfer-dehydrogenation in the absence of H₂ but with much lower rates and total catalytic turnovers, even at much greater temperatures. Analogs with halides other than chloride (Br, I), or with pseudo-halides (OCN, N₃), are found to exhibit generally similar behavior: high catalytic activity under H₂ and measurable but much lower activity in the absence of H₂. Thermolysis (under argon) of complexes [RhL₂Cl]n (n = 1, 2; L is a phosphine bulkier than PMe₃) in cyclooctane in the absence of hydrogen acceptors yielded cyclooctene. However, transfer-dehydrogenation was plagued by ligand decomposition. Under a hydrogen atmosphere complexes containing ligands much bulkier than PMe₃ do not effect dehydrogenation. Complexes with tridentate ligands, η³-PXP)RhL' (PXP = (Me₂PCH₂Me₂Si)₂N, Me₂PCH₂(2,6-C₆H₃)CH₂PMe₂; L' = CO, C₂H₄), were also found to catalyze thermal or photochemical dehydrogenation of cyclooctane with limited reactivity. The structure of [Rh(PMe₃)₂Cl]₂ was determined by single-crystal diffraction. The Rh(μ-Cl)₂Rh bridge of 1 is folded like that of [Rh(CO)₂Cl]₂, unlike that of the planar PPh₃ and PⁱPr₃ analogs.