Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon-Hydrogen Bonds

Abstract

Oxidation of the iron(II) precursor [(L¹ )Fe^II Cl₂ ], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^s PhIO) leads to the extremely reactive ferryl oxidant [(L¹ )(Cl)Fe^IV =O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non-heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C-H abstraction and the result of a rebound of the ensuing radical to an iron-bound Cl^- . The time-resolved formation of the halogenation product indicates that this primarily results from ^s PhIO oxidation of an initially formed oxido-bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹ )Fe^II Cl₂ ] indicate a high complex stability of the bispidine-iron complexes. DFT analysis shows that, due to a large driving force and small triplet-quintet gap, [(L¹ )(Cl)Fe^IV =O]⁺ is the most reactive small-molecule halogenase model, that the Fe^III /radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.

Keywords: C−H activation; DFT calculations; biomimetic coordination chemistry; non-heme-iron; reaction mechanism.