Fast Carbon Dioxide Fixation by 2,6-Pyridinedicarboxamidato-nickel(II)-hydroxide Complexes: Influence of Changes in Reactive Site Environment on Reaction Rates
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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DOI：
10.1021/ic200942u
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发表时间：
2011-10-17
影响因子：
4.6
通讯作者：
Holm, R. H.
中科院分区：
文献类型：
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作者：
Huang, Deguang;Makhlynets, Olga V.;Holm, R. H.
含有末端羟基的平面配合物[Ni-II(pyN(2)(R2))(OH)](-)很容易由N,N'-(2,6-C6H3R2)-2,6-吡啶二甲酰胺(2-)三齿钳配体(R4N)(OH)和Ni(OTf)(2)制备。这些配合物在 CO2 固定过程中与 DMF 溶液中的二氧化碳发生干净、完全的反应，形成具有 eta(1)-OCO2H 连接的碳酸氢盐产物配合物 [Ni-II(pyN(2)(R2))(HCO3)](-)。固定反应遵循二级动力学（速率 = k(2)'[Ni-II-OH] [CO2]），具有负活化熵（-17 至 -28 eu）。通过生长来监测反应。以及 350-450 nm 处金属到配体电荷转移 (MLCT) 谱带的衰减。 298 K 时的速率顺序 R = Me > Macro > Et > Pr-i > Bu-i > Ph（macro = 大环钳配体）反映了反应位点处空间位阻的增加。这些络合物固有的高反应性本质上是 R = Me 系统的 k(2)' 近似为 10(6) M-1 s(-1)，而在 R = Ph 络合物中仅衰减了 100 倍。基于通过 DFT 方法计算 R = Pr-i 系统的焓反应曲线，提出了反应机理。 R = Et、Pr-i 和 Bu-i 系统显示双相动力学，其中初始快速过程之后是当前来源不确定的较慢一阶过程。
The planar complexes [Ni-II(pyN(2)(R2))(OH)](-), containing a terminal hydroxo group, are readily prepared from N,N'-(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 [Ni-II(pyN(2)(R2))(HCO3)](-) having eta(1)-OCO2H ligation. Fixation reactions follow second-order kinetics (rate = k(2)'[Ni-II-OH] [CO2]) with negative activation entropies (-17 to -28 eu). Reactions were monitored by growth. and decay of metal-to-ligand charge-transfer (MLCT) bands at 350-450 nm. The rate order R = Me > macro > Et > Pr-i > Bu-i > 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 k(2)' approximate to 10(6) M-1 s(-1) 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 = Pr-i system by DFT methods. The R = Et, Pr-i, and Bu-i systems display biphasic kinetics in which the initial fast process is followed by a slower first order process currently of uncertain origin.