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. We describe novel metal-ligand cooperativity of the iridium-phosphido unit. 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, while 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 two orders of magnitude. The electronic properties of the iridium center are very 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 unusual architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

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

“…10 The monosubstituted olefins 1-hexene, IPP, and TBP are all calculated to bind equally strongly to ( tBu PPP)Ir; the lack of any appreciable difference is attributable to the positioning of the olefin alkyl substituent in the very open sector of the coordination sphere at the exo face of the pyramidal PPP pincer ligand. 10 In contrast, 1,2-disubstituted olefins, either cis or trans, require that a substituent is placed in the extremely crowded sector at the endo face of the pincer; thus, even cis-or trans-2-hexene (Figure 4) binds more weakly than the unsymmetrically bulky olefin TBE. 10 This contrasts with the more symmetrical ( tBu PCP)Ir fragment for which coordination of TBE is less favorable than coordination of cis-or trans-2hexene.…”

“…10 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. 10 We decided to explore various acceptors, partly in the hope of finding one more effective than TBE but more convenient to use than gas-phase propene at the bench scale, particularly for catalyst screening. As noted, initial rates of n-octane/1-hexene transfer dehydrogenation are high.…”

“…However, 1-hexene was quickly isomerized to trans-and cis-2-hexene, and the reaction thereafter proceeded very slowly; the internal olefins are apparently very ineffective as acceptors. 10 We considered that 4,4-dimethylpent-1-ene, or "tertbutylpropene" (TBP; Scheme 3), would have activity comparable to that of 1-alkenes but would be resistant to isomerization. In principle, 1,2-double-bond migration would be possible with TBP as with 1-hexene, but we anticipated that isomerization might be disfavored by the presence of the bulky tert-butyl group.…”

“…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). 10 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. In contrast with the appreciable rate of n-octane/TBP transfer dehydrogenation at 50 °C, COA/TBP transfer dehydrogenation proceeds at a negligible rate at 100 °C.…”

“…10 In contrast, 1,2-disubstituted olefins, either cis or trans, require that a substituent is placed in the extremely crowded sector at the endo face of the pincer; thus, even cis-or trans-2-hexene (Figure 4) binds more weakly than the unsymmetrically bulky olefin TBE. 10 This contrasts with the more symmetrical ( tBu PCP)Ir fragment for which coordination of TBE is less favorable than coordination of cis-or trans-2hexene.…”