Kinetics and Mechanistic Analysis of an Extremely Rapid Carbon Dioxide Fixation Reaction

Abstract

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [NiII(NNN)(OH)]1- containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH + CO2 → M-OCO2H; with variant M, previous rate constants are ≲103 M-1 s-1 in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 105 M-1 s-1 in N,N′-dimethylformamide at 298 K, a value within the range of kcat/KM≈105–108 M-1 s-1 for carbonic anhydrase, the most efficient catalyst of CO2 fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO2, followed by insertion of a CO2 oxygen atom into the Ni-OH bond to generate a four center Ni(η2-OCO2H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η1-OCO2H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH‡ = 3.2(5) kcal/mol].