DFT/ECP Study of C−H Activation by (PCP)Ir and (PCP)Ir(H)₂(PCP = η³-1,3-C₆H₃(CH₂PR₂)₂). Enthalpies and Free Energies of Associative and Dissociative Pathways

Abstract

(PCP)Ir(H)2 (PCP = eta3-1,3-C6H3(CH2PR2)2) complexes are highly effective catalysts for the dehydrogenation of alkanes; in particular, they are the first efficient molecular catalysts for alkane dehydrogenation that do not require a sacrificial hydrogen acceptor. Using density functional theory/effective core potential methods, we have examined C-H bond cleavage in alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)2. C-H addition to the dihydride is accompanied by loss of H2; both associative and dissociative pathways for this exchange reaction have been examined. The energetic barrier (deltaE(is not equal)) for associative displacement of H2 by benzene is much lower than the barrier for a dissociative pathway involving initial loss of H2; however, the pathways have very comparable free energy barriers (deltaG(is not equal)). Extrapolation to the higher temperatures, bulkier phosphine ligands, and the alkane substrates used experimentally leads to the conclusion that the pathway for the "acceptorless" dehydrogenation of alkanes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are required for transfer-dehydrogenation, dissociative pathways are calculated to be much more favorable than associative pathways. We emphasize that it is the free energy, not just the internal energy or enthalpy, that must be considered for elementary steps that show changes in molecularity.

Catalytic Methods for CH Bond Functionalization: Application in Organic Synthesis

“…[130] On the other hand, Goldman concluded that the dehydrogenation reaction takes place via the dissociation of dihydrogen from ( R PCP)IrH 2 , leading to the ( R PCP)Ir (Figure 4, left panel). [131] Goldman×s results satisfactorily explain the experimental results such as no H/D exchange between the hydrogen in ( R PCP)IrH 2 and cyclohexane-d 12 . Although the actual reaction mechanism is still controversial, both studies provide an important consideration in future efforts with respect to the development of related acceptor-less thermal dehydrogenation reactions of alkanes.…”

“…[130,131] Hall predicted that the acceptor-less dehydrogenation of alkanes using [h Figure 4, right panel). [130] On the other hand, Goldman concluded that the dehydrogenation reaction takes place via the dissociation of dihydrogen from ( R PCP)IrH 2 , leading to the ( R PCP)Ir (Figure 4, left panel).…”

Thermally Stable Homogeneous Catalysts for Alkane Dehydrogenation

“…However, the details of the formation of Ir III (R)(H) from Ir III (H) 2 For both model catalysts, the anthraphos Ir(H) 2 8 studied here and the previously investigated pincer Ir(H) 2 5, the route with the lowest internal energy (lowest barriers and most stable intermediates) passes through the associative path (A). [27] As we began our investigation of the anthraphos model catalyst 8 and examined the relative energies of the A and D paths, we discovered an unexpected TS for an interchange mechanism (I). [27] As we began our investigation of the anthraphos model catalyst 8 and examined the relative energies of the A and D paths, we discovered an unexpected TS for an interchange mechanism (I).…”

Activation of C–O and C–F Bonds by Pincer–Iridium Complexes

“…The first pincer-iridium-based alkane dehydrogenation catalyst, (PCP)IrH 2 (PCP = κ 3 -C 6 H 3 -2, 6-[CH 2 P(t-Bu) 2 ] 2 ) (1 -H 2 ), was reported by Kaska and Jensen [11] in the mid-1990s, and is active for the transfer dehydrogenation of nalkanes and cycloalkanes, a reaction that we have developed and studied mechanistically [12][13][14][15][16][17]. The initial success of (PCP)Ir in transfer dehydrogenation was followed by the development of a series of (pincer)Ir catalysts with varied steric and electronic properties [18][19][20].…”

“…These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp 2 -and sp-hybridized carbon [8][9][10]. Most notable, however, has been the activation of C(sp 3 )-H bonds, leading to alkane dehydrogenation [6,7].The first pincer-iridium-based alkane dehydrogenation catalyst, (PCP)IrH 2 (PCP = κ 3 -C 6 H 3 -2, 6-[CH 2 P(t-Bu) 2 ] 2 ) (1 -H 2 ), was reported by Kaska and Jensen [11] in the mid-1990s, and is active for the transfer dehydrogenation of nalkanes and cycloalkanes, a reaction that we have developed and studied mechanistically [12][13][14][15][16][17]. The initial success of (PCP)Ir in transfer dehydrogenation was followed by the development of a series of (pincer)Ir catalysts with varied steric and electronic properties [18][19][20].…”