Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

Abstract

The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (^tBuP^HPP), has been synthesized. When complexed to iridium, it adopts a meridional ("pincer") configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based-pincer (^tBuPPP) complex. Stoichiometric reactions of the (^tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; and the remaining site is particularly open. The (^tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (^RPCP)Ir catalysts. Accordingly, DFT calculations predict that (^tBuPPP)Ir and (^tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (^RPCP)Ir proceeds through an intermediate trans-(PCP)IrH₂(alkene), (^tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH₂(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (^tBuPPP)Ir and (^tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus, the resting state of the (^tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.