A tale of two ligands. Subtle differences in sterics exert major differences in the energetics of alkane dehydrogenation catalyzed by pincer complexes (tBuPCP)Ir vs. (tBuPOCOP)Ir

Complexes of pincer-ligated iridium fragments (^tBuPCP)Ir and (^tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (^tBuPCP)Ir has been reported to effect metathesis of C_n alkanes with the desirable selectivity for the C_2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (^tBuPOCOP)Ir(C₂H₄) is approximately 6 times faster than (^tBuPCP)IrH₂ for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (^tBuPCP)IrH₂ is > ca. 100 times faster than (^tBuPOCOP)Ir(C₂H₄). Preliminary results indicate that the rate-determining step in (^tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (^tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH₂ in PCP) "holds back" the PR₂ groups to significantly "open up" the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (^tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.