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

Abstract

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]ClO4}(+), 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.

Keywords: epoxidation; gas-phase chemistry; halogen-bonding; iodosylbenzene; iron; sulfoxidation.