The donor ability of the chelated carbonate ligand: Protonation and metallation of [(L)Co(O₂CO)]⁺ complexes in aqueous solution

The syntheses and X-ray structures of [Co(Me-tpa)O₂COZnCl ₃], [Co(pmea)O₂COZnCl₃]•H₂O [Co(trpyn)O₂COZn(OH₂)₄OCO₂Co(trpyn)] (ZnCl₄)₂•H₂O, [Co(trpyn)(O ₂COH)]ZnCl₄•3H₂O and [Co(trpyn)(O ₂CO)]ClO₄ are reported (Me-tpa = [(6-methyl-2-pyridyl) methyl]bis(2-pyridylmethyl)amine, pmea = bis(2-pyridylmethyl)-2-(2-pyridylethyl) amine, trpyn = tris(2-(1-pyrazolyl)ethyl)amine). The chelated bicarbonate complex [Co(trpyn)(O₂COH)]ZnCl₄•3H₂O is isolated as a crystalline solid from an acidic solution of the parent carbonate [Co(trpyn)(O₂CO)]ClO₄, and X-ray structural analysis shows that lengthening of the O_exo bond and shortening of the C-O _endo bond accompanies protonation. The bimetallic complex [Co(Me-tpa)O₂COZnCl₃] results from the unexpected coordination of ZnCl₃^- to the exo O atom of a chelated carbonate ligand. This complex is obtained from both acidic and neutral solutions in which [Zn²⁺] = 1.0 M, while the structurally similar complex [Co(pmea)O₂COZnCl₃]•H₂O is isolated from an analogous neutral solution. The trimetallic complex [Co(trpyn)O₂COZn(OH₂)₄OCO₂Co(trpyn)] (ZnCl₄)₂•H₂O crystallises on prolonged standing of [Co(trpyn)(O₂CO)]ClO₄ in a neutral solution having [Zn²⁺] = 1.0 M. The Zn-O bond lengths in all three complexes are indicative of bonds of significant strength. DFT calculations show that the nature of the bonding interaction between the Co(iii) ion and the endo O atoms of the carbonate ligand remain essentially unaffected by coordination of Zn ²⁺ to the exo O atom. They also show that such coordination of Zn²⁺ decreases the C-O_exo bond order.