Synthesis, Structure, and Reactivity of Paramagnetic Iron(II) and Iron(III) Amidodiphosphine Complexes

The preparation and characterization of a series of divalent and trivalent iron complexes that incorporate the amidodiphosphine ligand N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub> are reported. Mononuclear, tetrahedral complexes of the type FeX[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>], where X = Cl, Br, have been prepared; magnetic susceptibility studies indicate that these are high-spin d<sup>6</sup> systems. The preparation of Fe(II) alkyls of the formula FeR[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>] was examined; small alkyl substituents such as R = Me or CH<sub>2</sub>Ph were found to be thermally unstable and, thus, not isolable; however, for R = CH<sub>2</sub>SiMe<sub>3</sub> and CH(SiMe<sub>3</sub>)<sub>2</sub>, room-temperature stable, yellow, paramagnetic solids could be obtained. The reaction of FeCl[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>] with NaCp·DME generates the thermally stable, diamagnetic complex Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>]. The corresponding Fe(III) derivative FeX<sub>2</sub>[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>] could be prepared in high yield by direct reaction of the LiN(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub> ligand precursor with either FeCl<sub>3</sub> or FeBr<sub>3</sub>. Variable-temperature magnetic susceptibility studies indicate that FeBr<sub>2</sub>[N(SiMe<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>] is an example of a spin-admixed system (<i>S</i> = <sup>3</sup>/<sub>2</sub> and <sup>5</sup>/<sub>2</sub> spin states).