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)O2COZnCl3], [Co(pmea)O2COZnCl3].H2O [Co(trpyn)O2COZn(OH2)4OCO2Co(trpyn)](ZnCl4)2.H2O, [Co(trpyn)(O2COH)]ZnCl(4).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)]ZnCl(4).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 C=Oexo bond and shortening of the C-Oendo 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 Zn2+ to the exo O atom. They also show that such coordination of Zn2+ decreases the C-Oexo bond order.