Structural and Spectroscopic Characterization of Iron(II),
Cobalt(II), and Nickel(II) <i>ortho</i>-Dihalophenolate
Complexes: Insights into Metal–Halogen Secondary Bonding
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Abstract
Metal complexes incorporating the
tris(3,5-diphenylpyrazolyl)borate ligand (Tp<sup>Ph2</sup>) and <i>ortho</i>-dihalophenolates were synthesized and characterized
in order to explore metal–halogen secondary bonding in biorelevant
model complexes. The complexes Tp<sup>Ph2</sup>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<sup>Ph2</sup>Ni(2,6-dichlorophenolate)
complex to 2.88 Å for the Tp<sup>Ph2</sup>Fe(2,6-dibromophenolate)
complex. Variable temperature NMR spectra of the Tp<sup>Ph2</sup>Co(2,6-dichlorophenolate)
and Tp<sup>Ph2</sup>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 <i>ortho</i>-dihalohydroquinone substrates and phenol inhibitors