Mechanistic study of CO/CO₂ conversion catalyzed by a biomimetic Ni(II)‐iminothiolate complex

Abstract

We investigate the CO oxidation into CO 2 catalyzed by a biomimetic Ni(II)-iminothiolate complex in the presence of the sacrificial oxidizing agent methylviologen. We propose a catalytic mechanism supported by the density functional theory analysis of reaction intermediates that agrees with available experimental observations and kinetic data. We rule out a fivecoordinate Ni(II) species as well as a Ni(III) intermediate which was previously proposed and instead identify a key four-coordinate Ni(II) carbonyl species. We find that the turnoverlimiting step is likely the formation of the Ni(II) carboxylic acid species, although if a worse oxidant were used or if the concentration was less, then the oxidation of the Ni(I) species would be the turnover-limiting step. The reported findings should enable the design of better catalysts to favor one of the two competing pathways for CO to CO 2 conversion as catalyzed by this particular complex.catalytic mechanism, CO oxidation, CO 2 conversion, DFT, nickel catalyst 1 | I N TR ODU C TI ON CO/CO 2 conversion is central to a wide range of reaction mechanisms from carbon metabolism in biology to H 2 production in industrial applications based on the water-gas shift reaction. [1] The conversion mechanism could also provide meaningful insights into the reverse reaction that converts the greenhouse gas CO 2 into CO, a key intermediate on the path to liquid fuels. [2][3][4] In Nature, reversible CO/CO 2 conversion is catalyzed by the natural enzyme carbon monoxide dehydrogenase (CODH) which has a catalytically active Ni(II) center, [5][6][7][8] We focus on a Ni(II)-iminothiolate catalyst, proposed and synthesized by Crabtree and coworkers as a functional biomimetic CODH model system (Scheme 1). The Ni(II)-iminothiolate complex converts CO to CO 2 at room temperature in the water/methanol mixed solvent (Scheme 2) in the presence of the oxidizing agent methylviologen (mv 21 ) and acetate (AcO -) as a weak base. [9,10] A five-step catalytic cycle has been suggested for the mechanism of CO to CO 2 conversion, [9,10] in which the decarboxylation of LNi I -CO 2 is the turnover limiting step, as supported by the overall reaction rate dependence on P CO , [mv 21 ], [H 2 O], [Ni], and solution pH. However, the proposed intermediates have yet to be supported (or, otherwise ruled out) by spectroscopic and theoretical studies.Herein, we carry out a theoretical analysis of the catalytic mechanism at the density functional theory (DFT) level. The reported findings provide valuable insights into the reaction intermediates as well as guidelines for computational design of molecular catalysts for CO 2 /CO conversion in this class of compounds, using inverse design methodologies demonstrated for other systems. [11] Yueshen Wu and Benjamin Rudshteyn contributed equally to this study.Int J Quantum Chem. 2018;118:e25555.We have analyzed for the first time the catalytic mechanism of CO/CO 2 conversion by the Ni(II)-iminothiolate catalyst at the DFT level. Our results and direct comparisons to experime...

Inverse Design of a Catalyst for Aqueous CO/CO2 Conversion Informed by the NiII–Iminothiolate Complex

“…The postulated catalytic mechanism is consistent with available kinetic data, similar to the well-known water–gas shift reaction …”

“…The iminothiolate ligand has been shown to reduce the tendency for dimerization in polar aqueous environments while maintaining the electron-donating properties of the thiolate ligand. 25 The postulated catalytic mechanism 26 is consistent with available kinetic data, similar to the well-known water−gas shift reaction 19…”

“…The energies of the next highest spin states of steps 1–6 of J were calculated via DFT and compared to those of their corresponding lower spin states. It was found that the higher spin state was always higher in energy and, thus, inaccessible, as displayed in the Supporting Information, just as for the original catalyst …”

“…We employ DFT − calculations to identify and characterize the lowest energy intermediates for each step of the catalytic cycle using the same methods as in our previous paper . Briefly, we used the well-established B3LYP hybrid density functional, including Becke’s three parameter exchange and the Lee, Yang, and Parr correlation, as implemented in Gaussian 09, Revision D.01 .…”

“…23 We have reported a detailed ab initio mechanistic study to analyze the catalytic reaction pathway, including a complete analysis of the reaction energy profile and energy barrier for the rate-limiting step. 26 We found that the mechanism is as follows: (i) the dimer is cleaved by water to form the active monomer species; (ii) the water ligand is exchanged by CO; (iii) a water nucleophilic attack and subsequent deprotonation forms a Ni II carboxylic acid species during the rate-limiting step; (iv) proton transfer followed by electron transfer forms a Ni I carboxylate; (v) a water nucleophilic attack forms a fivecoordinate Ni I species; (vi) electron transfer and loss of CO 2 reforms the active species.…”