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

The planar complexes [NiII(pyN2R2)(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 [NiII(pyN2R2)(HCO3)]− having η1-OCO2H ligation. Fixation reactions follow second-order kinetics (rate = k2′[NiII–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 > 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′ ≈ 106 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 = 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.端基含羟基的平面二价镍配合物[NiII(pyN2R2)(OH)]−，可由N,N′-(2,6-二取代苯基)-2,6-吡啶二酰胺根(2-)型三齿钳形配体(R4N)(OH)与三氟甲磺酸镍(II)[Ni(OTf)2]简便制备。该类配合物在N,N-二甲基甲酰胺(DMF)溶液中可与二氧化碳发生完全且无副反应的反应，即二氧化碳固定过程，生成具有η1-O配位模式的碳酸氢根产物配合物[NiII(pyN2R2)(HCO3)]−。固定反应遵循二级动力学规律，速率方程为v = k2′[NiII–OH][CO2]，且活化熵为负值（-17~-28 熵单位(eu)）。反应通过监测350~450 nm波段处金属到配体电荷转移（metal-to-ligand charge-transfer, MLCT）吸收带的增强与衰减得以追踪。在298 K下，反应速率顺序为R=甲基(macro) > 大环 > 乙基 > 异丙基 > 异丁基 > 苯基，其中macro指代大环钳形配体，该顺序反映了反应位点处空间位阻的逐渐增大。以R=甲基体系为例，其二阶速率常数k2′≈10^6 M–1 s–1，展现出极高的本征反应活性；而R=苯基体系的反应活性仅被削弱了100倍。基于对R=异丙基体系的焓反应历程进行密度泛函理论(Density Functional Theory, DFT)计算，我们提出了该反应的机理。R=乙基、异丙基与异丁基的体系表现出双相动力学特征：先经历一个快速过程，随后是一个当前起源尚不明确的慢一级过程。