Halogen-Bonding-Assisted Iodosylbenzene Activation by a Homogenous Iron Catalyst

The iron(III) complex of hexadentate N,N,N′-tris(2-pyridylmethyl)ethylendiamine-N′-acetate (tpena ^-) is a more effective homogenous catalyst for selective sulfoxidation and epoxidation with insoluble iodosylbenzene, [PhIO] _n, compared with soluble methyl-morpholine-N-oxide (NMO). We propose that two molecules of [Fe(tpena)] ²⁺ cooperate to solubilize PhIO, extracting two equivalents to form the halogen-bonded dimeric {[Fe(tpena)OIPh] ₂} ⁴⁺. The closest intradimeric I⋯O distance, 2.56 Å, is nearly 1 Å less than the sum of the van de Waals radii of these atoms. A correlation of the rates of the reaction of {[Fe(tpena)OIPh] ₂} ⁴⁺ with para-substituted thioanisoles indicate that this species is a direct metal-based oxidant rather than a derived ferryl or perferryl complex. A study of gas-phase reactions indicate that an ion at m/z=231.06100 originates from solution-state {[Fe(tpena)OIPh] ₂} ⁴⁺ and is ascribed to [Fe ^III(tpenaO)] ²⁺, derived from an intramolecular O atom insertion into an Fe-tpena donor bond. Proposed ion pairs, {[Fe(tpena)OIPh]Cl} ⁺ and {[Fe(tpena)OIPh]ClO ₄} ⁺, are more stable than native [Fe(tpena)OIPh] ²⁺ ions, suggesting that halogen-bonding, as for the solution and solid states, operates also in the gas phase.