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

We report an easily prepared bis(thioether) amine ligand, SNS, along with the synthesis, characterization, and reactivity of the paramagnetic iron(II) bis(amido) complex, [Fe(κ-SNS)] (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-SNS)(κ-SNS)] (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 [LFe(κ-SNS)] (3-5). Addition of a Brønsted acid, HNTf, to 1 produces the paramagnetic, iron(II) amine-amido cation, [Fe(κ-SNS)(κ-SNS)](NTf) (6; Tf = SOCF). Cation 6 readily undergoes amine ligand substitution by triphos, affording the 16e complex [Fe(κ-SNS)(κ-triphos)](NTf) (7; triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; 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.