Our laboratory has reported that (CX3Phebox)Ir(H)(OAc) (X = H, F) catalysts are highly active for the acceptorless dehydrogenation of n-alkanes1, particularly in the presence of Lewis acids. In this work we report the synthesis of isoelectronic (Pybox)Os(H)(OAc) and (Pybox)Ru(H)(OAc), and investigation of these complexes for alkane dehydrogenation. DFT calculations predict (Pybox)Ru(H)(OAc) to catalyze acceptorless alkane dehydrogenation with a barrier lower than that for (CH3Phebox)Ir(H)(OAc), while the barrier calculated for (Pybox)Os(H)(OAc) is even lower. The rate-limiting step chemical for the catalytic cycle is calculated to be a net M-H/C-H σ-bond metathesis reaction, although expulsion of H2 from the reaction mixture was found to be rate-determining under typical conditions for acceptorless n-alkane dehydrogenation catalyzed by (CF3Phebox)Ir(H)(OAc). H/D exchange experiments were used to probe the kinetics of C-H activation yielding the order of activity: (Pybox)Os(H)(OAc) &gt; (Pybox)Ru(H)(OAc) &gt; (CF3Phebox)Ir(H)(OAc). Experimental investigation of catalysis by (Pybox)Ru(H)(OAc) and (Pybox)Os(H)(OAc) is still in progress but the Ru complex, unfortunately, does not appear to be stable at the high temperatures required for acceptorless alkane dehydrogenation. We have also reported that (CH3Phebox)Ir(C2H4)2 catalyzes selective dehydrogenative coupling of ethylene to butadiene via an iridacyclopentane complex.2 In this work we used the precursor (Pybox)OsH4 to investigate the same catalytic reaction and appears to result in and analogous dehydrogenative coupling of ethylene to form butadiene via an osmacyclopentane.

Synthesis and reactivity of (Pybox)Os and (Pybox)Ru complexes: Comparison with isoelectronic (Phebox)Ir complexes

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.