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

Complexes of the form Rh(PMe(3))(2)ClL' (L' = CO or trisubstituted phosphine) and [Rh(PMe(3))(2)Cl](2) have previously been reported to catalyze the transfer-dehydrogenation of alkanes, using olefinic hydrogen accepters under a dihydrogen atmosphere. Such complexes are herein reported to effect transfer-dehydrogenation in the absence of H-2 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, N-3), are found to exhibit generally similar behavior: high catalytic activity under H-2 and measurable but much lower activity in the absence of H-2. Thermolysis (under argon) of complexes [RhL(2)Cl](n) (n = 1, 2; L is a phosphine bulkier than PMe(3)) in cyclooctane in the absence of hydrogen accepters yielded cyclooctene. However, transfer-dehydrogenation was plagued by ligand decomposition. Under a hydrogen atmosphere complexes containing ligands much bulkier than PMe(3) do not effect dehydrogenation. Complexes with tridentate ligands, (eta(3)-PXP)RhL' (PXP = (Me(2)PCH(2)Me(2)Si)(2)N, Me(2)PCH(2)(2,6-C6H3)CH(2)PMe(2); L'= CO, C2H4), were also found to catalyze thermal or photochemical dehydrogenation of cyclooctane with limited reactivity. The structure of [Rh(PMe(3))(2)Cl](2) was determined by single-crystal diffraction. The Rh(mu-Cl)(2)Rh bridge of 1 is folded like that of [Rh(CO)(2)Cl](2), unlike that of the planar PPh(3) and (PPr3)-Pr-i analogs.