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

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- -butyl-phosphinophenyl)phosphine ( P PP), 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 ( PPP) complex. Stoichiometric reactions of the ( PPP)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 ( PPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of -alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous ( PCP)Ir catalysts. Accordingly, DFT calculations predict that ( PPP)Ir and ( PCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by ( PCP)Ir proceeds through an intermediate -(PCP)IrH (alkene), ( PPP)Ir follows a pathway proceeding via -(PPP)IrH (alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. ( PPP)Ir and ( PCP)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 ( PPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.