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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry
- Materials Chemistry