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 [Ni^II(pyN₂^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(pyN₂ ^R2)(HCO₃)]^- having η¹-OCO ₂H ligation. Fixation reactions follow second-order kinetics (rate = k₂′[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 > 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.