期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 108, 期 4, 页码 1222-1227出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1017430108
关键词
nickel hydroxide; carbon dioxide-bicarbonate conversion; reaction mechanism
资金
- Harvard University by the National Institutes of Health [GM-28856]
- Tufts University by the National Science Foundatio [0750140, 0639138]
- University of Waterloo by the Natural Sciences and Engineering Research Council
- Canada Foundation for Innovation
- Ontario Research Fund
- Shared Hierarchical Academic Research Computer Network
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [0821508, 0750140] Funding Source: National Science Foundation
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 [Ni-II(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 less than or similar to 10(3) M-1 s(-1) in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 x 10(5) M-1 s(-1) in N, N'-dimethylformamide at 298 K, a value within the range of k(cat)/K-M approximate to 10(5)-10(8) 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(eta(2)-OCO2H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Nid.1-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 [.H 3.2d5_ kcal/ mol]. 3
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