NMR spectroscopic investigations into the mechanism of absorption and desorption of CO2 by (tris-pyridyl)amine Zn complexes

The Zn complex [(NN3)Zn(OH)]2(NO3)2 (1(NO3)2, NN3 = tris(2-pyridylmethyl)amine) reacts with atmospheric CO2 to form a zinc carbonate species {[(NN3)Zn]3CO3}(NO3)4 (2(NO3)4), isolable as a crystalline product from organic solvents. The aqueous chemistry of the CO2 absorption and desorption processes for 1(NO3)2 and the presumed end-point of the reaction, 2(NO3)4, was unknown and hence investigated by NMR spectroscopy. Carboxylation of aqueous solutions of both 1(NO3)2 and 2(NO3)4 form products that can best be described as mixtures of monomeric [(NN3)ZnCO3H]+ and dimeric {[(NN3) Zn]2CO3}2+, which are in a dynamic equilibrium on the NMR time-scale. No evidence for the involvement of 2(NO3)4 in the carboxylation-decarboxylation processes is observed. Rather, the data suggest that 2 (NO3)4 provides [(NN3)Zn(OH2)]2+ that does not participate in the CO2 chemistry upon warming. A mechanism that is supported by NMR experiments and that accounts for the formation of [(NN3) ZnCO3H]+ and {[(NN3)Zn]2CO3}2+ from both ends of the reaction manifold is proposed.