Cobalt(III) Carbonate and Bicarbonate Chelate Complexes of Tripodal Tetraamine Ligands Containing Pyridyl Donors: The Steric Basis for the Stability of Chelated Bicarbonate Complexes

The synthesis and characterization (X-ray crystallography, UV/vis spectroscopy, electrochemistry, ESI-MS, and 1H, 13C, and 59Co NMR) of the complexes [Co(L)(O2CO)]ClO4·xH2O (L = tpa (tpa = tris(2-pyridylmethyl)amine) (x = 1), pmea (pmea = bis((2-pyridyl)methyl)-2-((2-pyridyl)ethyl)amine) (x = 0), pmap (pmap = bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine) (x = 0), tepa (tepa = tris(2-(2-pyridyl)ethyl)amine) (x = 0)) which contain tripodal tetradentate pyridyl ligands and chelated carbonate ligands are reported. The complexes display different colors in both the solid state and solution, which can be rationalized in terms of the different ligand fields exerted by the tripodal ligands. Electrochemical data show that [Co(tepa)(O2CO)]+ is the easiest of the four complexes to reduce, and the variation in Ered. values across the series of complexes can also be explained in terms of the different ligand fields exerted by the tripodal ligands, as can the 59Co NMR data which show a chemical shift range of over 2000 ppm for the four complexes. [Co(pmea)(O2CO)]+ is fluxional in aqueous solution, and VT NMR spectroscopy (1H and 13C) in DMF-d7 (DMF = dimethylformamide) over the temperature range -25.0 to 75.0 °C are consistent with inversion of the unique six-membered chelate ring. This process shows a substantial activation barrier ((delta)G++ = 58 kJ mol-1). The crystal structures of [Co(tpa)(O2CO)]ClO4·H2O, [Co(pmea)(O2CO)]ClO4·3H2O, [Co(pmap)(O2CO)]ClO4, and [Co(tepa)(O2CO)]ClO4 are reported, and the complexes containing the asymmetric tripodal ligands pmea and pmap both crystallize as the 6-isomer. The carbonate complexes all show remarkable stability in 6 M HCl solution, with [Co(pmap)(O2CO)]+ showing essentially no change in its UV/vis spectrum over 4 h in this medium. The chelated bicarbonate complexes [Co(pmea) (O2COH)]ZnCl4, [Co(pmap)(O2COH)][Co(pmap)(O2CO)](ClO4)3, [Co(pmap)(O2COH)]ZnCl4·H2O, and [Co(pmap(O2COH)]ZnBr4·2H2O can be isolated from acidic aqueous solution, and the crystal structure of [Co(pmap)(O2COH)]ZnCl4·3H2O is reported. The stability of the carbonate complexes in acid is explained by analysis of the crystallographic data for these, and other slow to hydrolyze chelated carbonate complexes, which show that the endo (coordinated) oxygen atoms are significantly hindered by atoms on the ancillary ligands, in contrast to complexes such as [Co(L)(O2CO)]+ (L = (NH3)4, (en)2, tren, and nta), which undergo rapid acid hydrolysis and which show no such steric hindrance.