Thermodynamic Acidity Studies of 6,6'-Dihydroxy-2,2'-bipyridine: A Combined Experimental and Computational Approach

The organic ligand 6,6'-dihydroxy-2,2'-bipyridine (6,6'-dhbp) is frequently used to bind transition metals in order to form catalysts for many organic and inorganic transformations. 6,6'-dhbp exists in two tautomeric forms, the pyridinol tautomer and the amide (lactam) tautomer with each tautomer having two rotational conformers: cis or trans. Only the cis-pyridinol tautomer has the proper configuration to bind transition metals. The pendant OH (or O^- groups when deprotonated) in 6,6'-dhbp typically do not bind the metal when forming the metal catalyst but can facilitate the proton transfer steps in the catalysis process. Electronic structure calculations were used to predict the stability of all possible isomers (including conformers and protonation states) in the gas phase and aqueous solution. These results have been compared to experimental data including UV-vis and NMR spectra as a function of pH. The pK_a values for the 6,6'-dhbp ligand in the -2 to +2 structures were predicted, and these ligands show different behavior in the gas phase versus in aqueous solution.