Structural and Spectroscopic Characterization of Iron(II), Cobalt(II), and Nickel(II) ortho-Dihalophenolate Complexes: Insights into Metal–Halogen Secondary Bonding

Metal complexes incorporating the tris(3,5-diphenylpyrazolyl)borate ligand (TpPh2) and ortho-dihalophenolates were synthesized and characterized in order to explore metal–halogen secondary bonding in biorelevant model complexes. The complexes TpPh2ML were synthesized and structurally characterized, where M was Fe(II), Co(II), or Ni(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal–halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the TpPh2Ni(2,6-dichlorophenolate) complex to 2.88 Å for the TpPh2Fe(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the TpPh2Co(2,6-dichlorophenolate) and TpPh2Ni(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ∼30 and ∼37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal–halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal–halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for ortho-dihalohydroquinone substrates and phenol inhibitors.