Iron(II) Complexes of a Hemilabile SNS Amido Ligand: Synthesis, Characterization, and Reactivity

We report an easily prepared bis­(thioether) amine ligand, S<sup><i>Me</i></sup>N<sup><i>H</i></sup>S<sup><i>Me</i></sup>, along with the synthesis, characterization, and reactivity of the paramagnetic iron­(II) bis­(amido) complex, [Fe­(κ<sup>3</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)<sub>2</sub>] (<b>1</b>). Binding of the two different thioethers to Fe generates both five- and six-membered rings with Fe–S bonds in the five-membered rings (av 2.54 Å) being significantly shorter than those in the six-membered rings (av 2.71 Å), suggesting hemilability of the latter thioethers. Consistent with this hypothesis, magnetic circular dichroism (MCD) and computational (TD-DFT) studies indicate that <b>1</b> in solution contains a five-coordinate component [Fe­(κ<sup>3</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)­(κ<sup>2</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)] (<b>2</b>). This ligand hemilability was demonstrated further by reactivity studies of <b>1</b> with 2,2′-bipyridine, 1,2-bis­(dimethylphosphino)­ethane, and 2,6-dimethylphenyl isonitrile to afford iron­(II) complexes [L<sub>2</sub>Fe­(κ<sup>2</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)<sub>2</sub>] (<b>3</b>–<b>5</b>). Addition of a Brønsted acid, HNTf<sub>2</sub>, to <b>1</b> produces the paramagnetic, iron­(II) amine–amido cation, [Fe­(κ<sup>3</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)­(κ<sup>3</sup>-S<sup><i>Me</i></sup>N<sup><i>H</i></sup>S<sup><i>Me</i></sup>)]­(NTf<sub>2</sub>) (<b>6</b>; Tf = SO<sub>2</sub>CF<sub>3</sub>). Cation <b>6</b> readily undergoes amine ligand substitution by triphos, affording the 16e<sup>–</sup> complex [Fe­(κ<sup>2</sup>-S<sup><i>Me</i></sup>NS<sup><i>Me</i></sup>)­(κ<sup>3</sup>-triphos)]­(NTf<sub>2</sub>) (<b>7</b>; triphos = bis­(2-diphenylphosphinoethyl)­phenylphosphine). These complexes are characterized by elemental analysis; <sup>1</sup>H NMR, Mössbauer, IR, and UV–vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine–borane dehydrogenation catalysis show complex <b>7</b> to be a selective and particularly robust precatalyst.