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 (tBuPHPP), 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.

Homogeneous catalysis with polyhydride complexes

“…High yields, 87-97%, of the corresponding alcohols are formed in short times, with turnover frequency values at 50% conversion between 1011 and 1960 h À1 , with 0.2 mol% of catalyst, at 80 1C (Scheme 25). 141 The bifunctional systems formed by the osmium(IV)polyhydrides 18 and OsH 3 {k 3 -P,N,P-[N(CH 2 CH 2 P i Pr 2 )]} (61) and the iridium(III) complexes 48 and 49 catalyze at room temperature the hydrogen transfer from primary and secondary alcohols to acetophenone and cyclohexanone, in principle, via a b-type outer sphere mechanism. Both osmium and iridium systems display comparable activities, whereas the turnover frequencies in ethanol are lower than in 2-propanol.…”

“…60 Goldman and coworkers have recently reported an IrH 3 ( P , P , P ) system, which is two orders of magnitude more active than the IrH x ( P , C , P ) ( x = 2, 4) complexes for the alkane dehydrogenation in the presence of a hydrogen acceptor. 61 The introduction of a heteroatom into the ring system significantly decreases the enthalpy of dehydrogenation. 62–64 As a consequence, the metal-promoted acceptorless dehydrogenation of cyclic amines are receiving noticeable attention.…”

High Activity and Selectivity for Catalytic Alkane–Alkene Transfer (De)hydrogenation by (tBuPPP)Ir and the Importance of Choice of a Sacrificial Hydrogen Acceptor

“…( tBu P H PP)IrH 3 ( tBu P H PP = bis(2-di-i-butyl-phosphinophenyl)phosphine; Scheme ) was recently reported to act as a precursor of the catalytically active fragment ( tBu PPP)Ir, which dehydrogenated n -alkanes with the use of sterically unhindered terminal alkenes as hydrogen acceptors at temperatures as low as 80 °C . This exceptionally high activity, however, was not apparent when using the COA/TBE benchmark transfer dehydrogenation couple.…”

“…With the use of 1-hexene as the hydrogen acceptor, for example, 60 mM octenes was obtained after 6 min at 100 °C under the same conditions . When propene was used as the acceptor, we typically conducted the reaction at 80 °C; even at that low temperature, for example, 78 mM octenes was obtained after 30 min …”

“…This explanation would clearly follow if hydrogenation of the acceptor is the rate-determining step and the catalyst resting state is a hydrogenated form which reacts with the acceptor, e.g., ( tBu PPP)IrH 2 . However, NMR observations indicate that this is not the case; instead, the resting state appears to be a dehydrogenated form, generally ( tBu PPP)Ir(olefin) . Further evidence that the reaction rate is not simply equal to the rate of the reaction of a hydrogenated complex (e.g., ( tBu PPP)IrH 2 ) with the acceptor is obtained from experiments with COA.…”