Impact of hydrogen-bonding interactions on the properties of biomimetic Co₂(μ-OH)₂ complexes

Metalloenzyme active sites depend critically on the presence of hydrogen-bonding (H-bonding) interactions for their function. To explore this dependence, we examined a homologous series of biomimetic Co₂(μ-OH)₂ complexes containing zero, one, or two κ¹-carboxylate ligands as intramolecular H-bond acceptors to bridging μ-OH ligands. From single crystal X-ray diffraction (XRD), we show that these complexes are structurally similar across the series in the solid state. We also do not observe significant changes in the UV–vis absorption or FTIR spectra across the series, with the exception of the magnitude of μ-OH stretching frequencies observed for the latter. Employing TDDFT computations and natural transition orbital (NTO) analyses, we examined electronic structure, revealing that an inverted ligand field is operative in these complexes. Despite the minimal structural and electronic variation across the series, assessing the pK_as of these complexes experimentally revealed > 10⁹ range in proton affinities. This study highlights that even with all other factors held constant, intramolecular H-bonding networks can critically modulate cofactor pK_as within metalloenzyme active sites.