title: Structural and spectroscopic characterization of iron(II), cobalt(II), and nickel(II) ortho-dihalophenolate complexes: insights into metal-halogen secondary bonding.
abstract: Metal complexes incorporating the tris(3,5-diphenylpyrazolyl)borate ligand (Tp(Ph2)) and ortho-dihalophenolates were synthesized and characterized in order to explore metal-halogen secondary bonding in biorelevant model complexes. The complexes Tp(Ph2)ML 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 Tp(Ph2)Ni(2,6-dichlorophenolate) complex to 2.88 Å for the Tp(Ph2)Fe(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp(Ph2)Co(2,6-dichlorophenolate) and Tp(Ph2)Ni(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.
authors.affiliation: Department of Chemistry, Whitman College , Walla Walla, Washington 99362, United States.
authors.forename: Timothy E
authors.initials: TE
authors.lastname: Machonkin
authors.forename: Monica D
authors.initials: MD
authors.lastname: Boshart
authors.forename: Jeremy A
authors.initials: JA
authors.lastname: Schofield
authors.forename: Meghan M
authors.initials: MM
authors.lastname: Rodriguez
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Grubel
authors.forename: Dalia
authors.initials: D
authors.lastname: Rokhsana
authors.forename: William W
authors.initials: WW
authors.lastname: Brennessel
authors.forename: Patrick L
authors.initials: PL
authors.lastname: Holland
mesh_terms: D001881:Borates / Q000737:chemistry; D003035:Cobalt / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D054879:Halogenation; D006219:Halogens / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D009532:Nickel / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*
chemical_list: D001881:Borates; D056831:Coordination Complexes; D006219:Halogens; D010636:Phenols; D003035:Cobalt; D009532:Nickel; D007501:Iron