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-t-butyl-phosphinophenyl)phosphine ((PPP)-P-tBu-P-H), 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)-P-tBu) complex. Stoichiometric reactions of the ((PPP)-P-tBu) 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)-P-tBu)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 ((PCP)-P-R)Ir catalysts. Accordingly, DFT calculations predict that ((PPP)-P-tBu)Ir and ((PCP)-P-tBu)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by ((PCP)-P-R)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), ((PPP)-P-tBu)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. ((PPP)-P-tBu)Ir and ((PCP)-P-tBu)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)-P-tBu)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

Automated summary

A new bulky triphosphine ligand, ((PPP)-P-tBu)Ir, has been reported to efficiently catalyze the dehydrogenation of alkanes. Compared to traditional ligands, this ligand results in a more crowded spatial arrangement around the metal center, leading to enhanced catalytic activity.