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

Abstract

The syntheses and X-ray structures of [Co(Me-tpa)O₂COZnCl₃], [Co(pmea)O₂COZnCl₃]·H₂O [Co(trpyn)O₂COZn(OH₂)₄OCO₂Co(trpyn)](ZnCl₄)₂·H₂O, [Co(trpyn)(O₂COH)]ZnCl₄·3H₂O and [Co(trpyn)(O₂CO)]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 CO_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 exoO 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 endoO atoms of the carbonate ligand remain essentially unaffected by coordination of Zn²⁺ to the exoO atom. They also show that such coordination of Zn²⁺ decreases the C–O_exo bond order.