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.