Zinc complexes of hydrogen bond accepting ester substituted trispyrazolylborates

As models for the labile water ligands so ubiquitous in zinc metalloprotein active sites, aquo and hydroxo complexes of zinc with the ester substituted trispyrazolylborate ligand, [(TpCO2Et,Me) have been isolated and crystallographically characterized. These complexes are stabilized by internal hydrogen bonding between the water or hydroxide and the ester carbonyls of the ligand. The dinuclear hydroxo complex, which maintains its structure in solution, appears to catalyze self-transesterification reactions in alcoholic solvents. However, the expansion of the coordination sphere from four- to five-coordinate in 2 or the dimerization that occurs with 3 suggest that a more sterically restrictive ester substituted ligand will be needed to enforce the desired mononuclear four-coordinate pseudotetetrahedral geometry most commonly seen in zinc metalloenzyme active sites.