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

We report an easily prepared bis(thioether) amine ligand, S Me N H S Me, along with the synthesis, characterization, and reactivity of the paramagnetic iron(II) bis(amido) complex, [Fe(κ 3 -S Me NS Me ) 2 ] ( 1 ). 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 1 in solution contains a five-coordinate component [Fe(κ 3 -S Me NS Me )(κ 2 -S Me NS Me )] ( 2 ). This ligand hemilability was demonstrated further by reactivity studies of 1 with 2,2′-bipyridine, 1,2-bis(dimethylphosphino)ethane, and 2,6-dimethylphenyl isonitrile to afford iron(II) complexes [L 2 Fe(κ 2 -S Me NS Me ) 2 ] ( 3 – 5 ). Addition of a Brønsted acid, HNTf 2, to 1 produces the paramagnetic, iron(II) amine–amido cation, [Fe(κ 3 -S Me NS Me )(κ 3 -S Me N H S Me )](NTf 2 ) ( 6; Tf = SO 2 CF 3 ). Cation 6 readily undergoes amine ligand substitution by triphos, affording the 16e – complex [Fe(κ 2 -S Me NS Me )(κ 3 -triphos)](NTf 2 ) ( 7; triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; 1 H NMR, Mössbauer, IR, and UV–vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine–borane dehydrogenation catalysis show complex 7 to be a selective and particularly robust precatalyst.