Solution and Structural Characterization of Iron(II) Complexes with <i>Ortho</i>-Halogenated Phenolates: Insights Into Potential Substrate Binding Modes in Hydroquinone Dioxygenases

The new ligand cis,cis-1,3,5-tris-(E)-(tolylideneimino)cyclohexane (TACH-o-tolyl) forms a 1:1 complex with iron(II). Addition of substituted phenolates forms 1:1:1 ligand:iron:phenolate complexes, which have been characterized both in the solid state and in solution. There is complete binding of the phenolate to the complex only when there are ortho-halogens on the phenolate. The tertiary complexes with ortho-halo-substituted phenolates exhibit short Fe−halogen distances, and the complex containing a non-coordinating but similarly sized ortho-methyl phenolate has a significantly different conformation and coordination geometry. Therefore, it is likely that the metal−halogen interaction stabilizes the complexes. The iron(II)−halogen interaction in these complexes may explain the substrate specificity of PcpA and LinE, enzymes that preferentially bind phenols and hydroquinones containing halogen substituents in ortho positions.