The Mechanism of CO₂ Insertion into Iridium(I) Hydroxide and Alkoxide Bonds: A Kinetics and Computational Study

Abstract

Abstract:We recently reported the facile insertion of CO 2 into Ir

Mechanism of CO2 Fixation by IrI–X Bonds (X = OH, OR, N, C)

“…The concerted TS2 barrier is predicted to be 6.4 kcal/mol lower in energy than TS1 (9.4 vs. 15.8 kcal/mol, respectively), which suggests that the CO 2 insertion is the rate-limiting step for the studied reaction mechanism. Furthermore, the calculated activation freeenergy barrier of 15.8 kcal/mol is not only comparable to the experimental activation free energy [ΔG(298 K) = 18.9 Ϯ 0.4 kcal/mol] [14] but also quite reasonable as the experimental insertion of CO 2 into 1 and the subsequent dimerization to give 2 is facile under ambient conditions. This observation contradicts the recent DFT work by Truscott et al in which the authors state that the dimerization step is the rate-limiting step.…”

“…From a structural point of view, TS2 enjoys the favorable interaction of the Ir2 atom with the O2 atom and also the stronger coordination of the O3 atom to the carbon atom of the fixed CO 2 ; thus, TS2 is more stable than the concerted transition state previously reported by Truscott et al (see Figure S1 for a more detailed comparison between the geometries of TS2 and the similar concerted transition state TS1.3-2 reported by Truscott et al). [14] Overall, the studied reaction is exothermic, as the final product 2 lies 3.7 kcal/mol below the starting complex 1. The concerted TS2 barrier is predicted to be 6.4 kcal/mol lower in energy than TS1 (9.4 vs. 15.8 kcal/mol, respectively), which suggests that the CO 2 insertion is the rate-limiting step for the studied reaction mechanism.…”

“…A recent combined experimental and DFT study by Truscott et al describes in detail the CO 2 insertion mechanism for 1. [14] To summarize, solidstate kinetic experiments indicated that the formation of the dimeric product 2 from 1 and CO 2 is a pseudo-first-order transformation with a small enthalpy barrier and is mainly governed by entropy [ΔH = 3.6 Ϯ 0.3 kcal/mol, ΔS = -51.5 Ϯ 0.9 kcal/mol, ΔG(298 K) = 18.9 Ϯ 0.4 kcal/mol]. Additionally, DFT-based mechanistic investigations for 1 show that the rate-limiting step is the dimerization step, which involves an autocatalytic mechanism mediated by an H 2 O molecule with an estimated free-energy barrier (ΔG = 28.0-30.0 kcal/mol) that is significantly higher than the experimental value.…”

“…Additionally, DFT-based mechanistic investigations for 1 show that the rate-limiting step is the dimerization step, which involves an autocatalytic mechanism mediated by an H 2 O molecule with an estimated free-energy barrier (ΔG = 28.0-30.0 kcal/mol) that is significantly higher than the experimental value. [14] Furthermore, the computed barrier for the direct formation of 2 from C (C Ǟ 2) is ca. 39.0 kcal/mol, which suggests that this pathway is not viable under experimental conditions.…”

How easy is CO2 fixation by M–C bond containing complexes (M = Cu, Ni, Co, Rh, Ir)?

“…Despite the very recent large number of experimental [47][48][49][50][51][52] and theoretical [53][54][55][56][57][58][59] studies focusing on CO 2 insertion, reports on direct comparison between theoretical and experimental results remain very limited. 53,60,61 Based on the experimental findings, a plausible mechanism for CO 2 insertion into the M-X bonds (X = O or N) proposed by Nolan et al is outlined in Scheme 2. Considering complex A, the nucleophilic attack of the heteroatom X onto CO 2 does not proceed via the oxygen atoms but through the carbon atom, involving a four-membered cyclic center, hypothetically structure B, i.e.…”