Werner Complexes Can Still Surprise: The Chelated O₂BOH^2- Ligand

The synthesis and characterization of the Co(III) complex [Co(pmap)(O₂BOH)]ClO₄·4H₂O (pmap = bis(2-(2-pyridyl)ethyl)-(2-pyridylmethyl)amine) are reported. This is an unprecedented example of a mononuclear transition metal complex containing an oxoanion ligand derived from the deprotonation of boric acid. The complex was synthesized by air oxidation of a solution containing [Co(OH₂)₆](ClO₄)₂, pmap, and borax (Na₂B₄O₇·10H₂O), with purification effected by cation exchange chromatography. Bond lengths within the chelated hydrogen borate (O₂BOH^2-) ligand confirm its identity, clearly distinguishing it from carbonate, and mass spectrometry is also unequivocal in this respect. The complex reacts rapidly with both MeOH and EtOH to form the respective chelated borate monoesters, while ¹⁸O exchange with the complex is also rapid. Computational results are consistent with the boron atom being electron-deficient, consistent with this reactivity.