Fast Carbon Dioxide Fixation by 2,6-Pyridinedicarboxamidato-nickel(II)-hydroxide Complexes: Influence of Changes in Reactive Site Environment on Reaction Rates

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• 作者：Deguang Huang ; Olga V. Makhlynets ; Lay Ling Tan ; Sonny C. Lee ; Elena V. Rybak-Akimova ; R. H. Holm
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：October 17, 2011
• 年：2011
• 卷：50
• 期：20
• 页码：10070-10081
• 全文大小：1243K
• 年卷期：v.50,no.20(October 17, 2011)
• ISSN：1520-510X

文摘

The planar complexes [Ni^II(pyN₂^R2)(OH)]^{鈭?/sup>, containing a terminal hydroxo group, are readily prepared from N,N鈥?(2,6-C₆H₃R₂)-2,6-pyridinedicarboxamidate(2-) tridentate pincer ligands (R₄N)(OH), and Ni(OTf)₂. These complexes react cleanly and completely with carbon dioxide in DMF solution in a process of CO₂ fixation with formation of the bicarbonate product complexes [NiII(pyN₂R2)(HCO₃)]鈭?/sup> having 畏1-OCO₂H ligation. Fixation reactions follow second-order kinetics (rate = k₂鈥瞇NiII鈥揙H][CO₂]) with negative activation entropies (鈭?7 to 鈭?8 eu). Reactions were monitored by growth and decay of metal-to-ligand charge-transfer (MLCT) bands at 350鈥?50 nm. The rate order R = Me > macro > Et > Pri > Bui > Ph at 298 K (macro = macrocylic pincer ligand) reflects increasing steric hindrance at the reactive site. The inherent highly reactive nature of these complexes follows from k₂鈥?鈮?106 M鈥? s鈥? for the R = Me system that is attenuated by only 100-fold in the R = Ph complex. A reaction mechanism is proposed based on computation of the enthalpic reaction profile for the R = Pri system by DFT methods. The R = Et, Pri, and Bui systems display biphasic kinetics in which the initial fast process is followed by a slower first order process currently of uncertain origin.}