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

Abstract

A comparison between different M–C bonds (M = Cu(I), Ni(II), Co(I), Rh(I) and Ir(I)) has been reported by using density functional theory (DFT) calculations to explore the role of the metal in the fixation or incorporation of CO₂ into such complexes. The systems investigated are various metal based congeners of the Ir-complex 8 [(cod)(IⁱPr)Ir-CCPh], with a ligand scaffold based on cod and IⁱPr ligands (cod = 1,5-cyclooctadiene; IⁱPr = 1,3-bis(isopropyl)imidazol-2-ylidene). The results of this study show that the calculated CO₂ insertion barriers follow the trend: Cu(I) (20.8 kcal mol⁻¹) < Rh(I) (30.0 kcal mol⁻¹) < Co(I) (31.3 kcal mol⁻¹) < Ir(I) (37.5 kcal mol⁻¹) < Ni(II) (45.4 kcal mol⁻¹), indicating that the Cu(I) based analogue is the best CO₂ fixer, while Ni(II) is the worst in the studied series.

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The development of catalysts capable of utilising CO₂ as feedstock,^1–4 possibly in large volume transformations,^5–7 is considered to be one of the key challenges in modern chemistry. However, the wide spread use of CO₂ in chemical transformations is limited due to its chemical inertness.⁸ In the recent past, despite some attempts with metal-free processes,^9,10 even organocatalyzed trapping of CO₂ through C–C and C–O bond formation,¹¹ progress has been made in exploring the CO₂ reactivity with a large number of its reactions being insertion into a transition metal–X bond (where X = C, O, N) supported with appropriate ligands.^12–17 These reactions are believed to be one of the most challenging organometallic reactions as CO₂ is thermodynamically stable and kinetically inert.^18–24 To this end, a novel organometallic complex as well as a chemical mechanism able to perform or explain the capture of CO₂ would be extremely interesting and valuable in directing catalyst design efforts.^25–46 Therefore, a thorough understanding of the mechanism of CO₂ insertion into the M–X bonds is of fundamental importance and could guide advances in the development of this important reaction.

Despite the very recent large number of experimental^47–52 and theoretical^53–59 studies focusing on CO₂ insertion, reports on direct comparison between theoretical and experimental results remain very limited.^53,60,61 To partially fill this gap, we have recently reported density functional theory based investigations of the CO₂ fixation mechanism as proposed by Nolan et al. for a new Ir(I) complex,^61–63 [Ir(cod)IⁱPr)(OH)] (1) (cod = 1,5-cyclooctadiene; IⁱPr = 1,3-bis(isopropyl)imidazol-2-ylidene) and its derivatives containing Ir–O (complexes 3 and 4), Ir–N (complex 5), and Ir–C bonds (complexes 6–9; see Scheme 1).⁶⁴ According to the experiments, complexes containing Ir–heteroatom bonds (1, 3–5) display high reactivity towards CO₂, while complexes with Ir–C bonds (6–9) are completely unreactive towards the greenhouse gas.

Scheme 1

Based on the experimental findings, a plausible mechanism for CO₂ 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 CO₂ does not proceed via the oxygen atoms but through the carbon atom, involving a four-membered cyclic center, hypothetically structure B, i.e. most probably B is the transition state itself. The particular structural nature of 1 allows for a second molecule of 1 to react yielding a dimeric structural product 2; this occurs once the water molecule has been released from intermediate C.

Scheme 2

Our detailed investigation of the proposed mechanism in Scheme 2 for complex 1 revealed that the CO₂ insertion step is the rate determining step rather than the subsequent dimerization step, C → 2 (15.8 kcal mol⁻¹vs. 9.4 kcal mol⁻¹, respectively).⁶² This finding contradicts the recent study by Truscott et al.⁶¹ as they argued that dimerization is probably the rate-limiting step for complex 1. In fact, we pointed out that the facile dimerization step is due to the cooperative nature of the iridium centers. Additionally, in agreement with experiments, the calculated low CO₂ insertion barriers for complex 1 and its derivatives 3–5 (3 (11.8 kcal mol⁻¹) < 4 (22.5 kcal mol⁻¹) < 5 (17.7 kcal mol⁻¹)) indicated that the CO₂ insertion into the Ir–O and Ir–N bonds is kinetically facile. On the other hand, significantly higher barrier values were estimated for complexes 6–9 (ranges from 35.0 to 50.0 kcal mol⁻¹), suggesting that CO₂ insertion into the Ir–C bonds is kinetically more challenging and supports the experimental observation that the Ir–C bonds are unreactive towards CO₂.

Having rationalized the behaviour of the systems proposed in the literature (1, 3–9), in the present study we turn our attention to the effect of changing the metal (but keeping the same ligand scaffold; cod and IⁱPr ligands) with the aim of finding one metal that can induce CO₂ insertion into the M–C bonds. It is well documented in the literature that CO₂ can be inserted into the M–C bonds of Rh and other transition metals such as Cu and Ni.^65–67 To this end, we studied the CO₂ insertion step for different metal (Cu(I), Ni(II)), Co(I) and Rh(I)⁶⁸ based congeners of 8 [(cod)(IⁱPr)Ir-CCPh], which presented the lowest CO₂ insertion barrier (37.5 kcal mol⁻¹) among the investigated Ir–C bond containing complex in the series 6–9.

To clarify the role of the metal in the CO₂ insertion DFT calculations were carried out. All DFT static calculations have been performed at the GGA level with the Gaussian09 set of programs,⁶⁹ using the BP86 functional of Becke and Perdew.^70,71 The electronic configuration of the molecular systems was described with the standard split-valence basis set with a polarization function of Ahlrichs and co-workers for H, C, N, and O (TZVP keyword in Gaussian).⁷² For Co, Cu, Ni, Rh and Ir, the quasi-relativistic Stuttgart/Dresden effective core potential was used, with the associated valence basis set (standard SDD keywords in Gaussian09).^73–75 The geometry optimizations were carried out without symmetry constraints, and the characterization of the stationary points was performed by analytical frequency calculations. Single point calculations on the BP86 optimized geometries were performed using the M06 functional⁷⁶ with the triple-ζ basis set of Ahlrichs for main-group atoms (TZVP keyword in Gaussian),⁷⁷ while for Ir the SDD basis set has been employed. These single point calculations also used the polarizable continuous model PCM to address solvent effects, using benzene as a solvent.^78,79 The reported free energies in this work include energies obtained at the M06/TZVP level corrected with zero-point energies, thermal corrections and entropy effects evaluated at 298 K by the BP86/SVP method in the gas phase.^80,81

Fig. 1 presents the computed energy profile for the CO₂ insertion into the M–C bond of different metal analogues of 8. The reference point is the energy of complex 8 analogues plus free CO₂. For the selected metals, the calculated CO₂ insertion barriers (for simplicity named as TS₈, in analogy with our Ir(I)-complexes of Scheme 1) follows the trend: Cu(I) (20.8 kcal mol⁻¹) < Rh(I) (30.0 kcal mol⁻¹) < Co(I) (31.3 kcal mol⁻¹) < Ir(I) (37.5 kcal mol⁻¹) < Ni(II) (45.4 kcal mol⁻¹), indicating that the Cu(I) based analogue is the best CO₂ fixer, while Ni(II) is the worst among the studied metal series. However, from a thermodynamic stand point, the overall CO₂ insertion reaction is endergonic in nature for all the examined metals (see the energy values for II₈ in Fig. 1), being less endergonic in the case of Cu(I) (5.0 kcal mol⁻¹) than Ni(II) (22.3 kcal mol⁻¹). It is worth mentioning here that we calculated the CO₂ insertion barrier for high spin Co(II) complex and is predicted to be 32.9 kcal mol⁻¹, which is slightly higher in energy (by 1.6 kcal mol⁻¹) than the calculated barrier for low spin Co complex (31.3 kcal mol⁻¹).

Fig. 1 Computed free energy profile for the CO₂ insertion into the M–C bond of different metal analogues of Ir(I)-complex 8 (where R¹ = CCPh). Free energies are given in kcal mol⁻¹. Ni(II) is a cationic complex (overall charge +1).

A plausible explanation for the remarkable metal effect on the calculated CO₂ insertion barriers can be drawn from the geometries of the TS structures (see Fig. 2) and by considering the Hammond postulate. In detail, the calculated high energy TS for Ni(II) can be considered as a textbook example of a “late TS” closer to the product (r(Ni–O) = 2.269 Å; see Fig. 2e). By changing the metal from Ni(II) to Cu(I), the transition state moves towards the reactants as the reaction becomes less endothermic. Thus, the calculated low energy TS for Cu(I) can be considered as an “early TS” closer to the reactants (r(Cu–O) = 3.053 Å; see Fig. 2a). Intermediate situations are reported for Rh and Ir (r(Rh–O) = 2.891 Å and r(Ir–O) = 2.716 Å; Fig. 2b and c, respectively).

Fig. 2 Calculated transition states of the CO₂ fixation mechanism promoted by different metals analogues of Ir(I)-complex 8: (a) Cu(I); (b) Rh(I); (c) Co(I); (d) Ir(I); and (e) Ni(II). The most notable distances are given in Å.

Having these results in hand, we then calculated the CO₂ insertion barrier for the experimentally known Cu(I) complex, [Cu(IPr)(CCPh)],⁸² and further, to be compared with other group 11 metals Ag(I) and Au(I). The calculated CO₂ insertion barrier follows the trend: Ag(I) (30.9 kcal mol⁻¹) < Cu(I) (34.6 kcal mol⁻¹) < Au(I) (37.0 kcal mol⁻¹), indicating that the Ag(I) based analogue is the best CO₂ fixer, followed by Cu(I) and Au(I) based analogues. However, the calculated CO₂ insertion barriers are still above 30.0 kcal mol⁻¹, suggesting that the CO₂ fixation is kinetically challenging in the considered group 11 metal series. Calculations attempting to screen a longer series of ligands to characterize in detail how the CO₂ insertion barrier is affected by the nature of the ligand are ongoing.^82,83

Last but not least, even though the energy data include the correction for dispersion implicitly, the geometry optimization is not included, as previously reported,^62,79,80 however new tests performed on the transition states for Ni and Cu displayed in Fig. 2 reveal an increase in the energy barrier of +0.4 kcal mol⁻¹ for Ni and a decrease of 0.8 kcal mol⁻¹ for Cu, which suggest that in absolute value the difference is small to negligible and that there is no obvious trend concerning the effect of dispersion on the CO₂ insertion barriers.

Conclusions

We have explored the role of metals in the CO₂ insertion into a M–C bond by considering various metal (M = Cu(I), Co(I), Rh(I) and Ni(II)) analogues of Ir(I)-complex 8. The results show that among the considered metals, Cu(I) was the best CO₂ fixer with an estimated barrier of 20.8 kcal mol⁻¹, while Ni(II) was found to be the least reactive with a barrier of 45.4 kcal mol⁻¹. As a final note, the large spread in the activation energies estimated by changing the metal but with the same ligand scaffold (cod and IⁱPr ligands) strongly suggests that there is a large space to explore for a suitable combination of ligands and metals to add important components to improve/affect the thermodynamics of this fascinating reaction.

Acknowledgements

A.P. thanks the Spanish MINECO for a project CTQ2014-59832-JIN, and European Commission for a Career Integration Grant (CIG09-GA-2011-293900). S.V.C.V., S.P.N. and L.C. thank King Abdullah University of Science and Technology (CCF-1974-3 project) for support. G.T. thanks the KAUST for the visiting contract KAUST-2015-C0546.

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Footnote

† Electronic supplementary information (ESI) available: Energies, Cartesian coordinates, and 3D view for all DFT optimized species. See DOI: 10.1039/c5qo00281h

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