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(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.