Electronic and Structural Comparisons between Iron(II/III) and Ruthenium(II/III) Imide Analogs

To examine structural and electronic differences between iron and ruthenium imido complexes, a series of compounds was prepared with different phosphine basal sets. The starting material for the ruthenium complexes was Ru(NAr/Ar*)(PMe₃)₃ (Ru1/Ru1*), where Ar = 2,6-(ⁱPr)₂C₆H₃ and Ar* = 2,4,6-(ⁱPr)₃C₆H₂, which were prepared from cis-RuCl₂(PMe₃)₄ and 2 equiv of LiNHAr/Ar*. The starting materials for the iron complexes were the analogous Fe(NAr/Ar*)(PMe₃)₃ species (Fe1/Fe1*), which were not isolated but could be generated in situ from FeCl₂, PMe₃, and LiNHAr/Ar*. With both iron and ruthenium, the PMe₃ starting materials underwent phosphine replacement with chelating ligands to give new group 8 imido complexes in the +2 oxidation state. Addition of 1,2-bis(diphenylphosphino)ethane (dppe) to M1/M1* gave Ru(NAr/Ar*)(PMe₃)(dppe) and Fe(NAr/Ar*)(PMe₃)(dppe). Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) provided Ru(NAr/Ar*)(dmpe)₂. A triphos ligand, {P(Me)₂CH₂}₃Si^tBu (^tP₃), was also examined. Addition of ^tP₃ to Fe1 provided Fe(NAr)(^tP₃) (Fe4), but a similar reaction with Ru1 only gave intractable materials. Oxidation of Fe4 with AgSbF₆ gave {Fe(NAr)(^tP₃)}⁺SbF₆^- (Fe4a). Oxidation of Ru2 with AgSbF₆ gave the unstable cation {Ru(NAr)(PMe₃)(dppe)}⁺, which dimerized in the presence of acetonitrile via C-C bond formation at the aryl group C4 positions, affording {Ru(NAr)(PMe₃)(NCMe)(dppe)}₂⁺. This suggested that there was substantial radical character in the imide π system on oxidation and that an aromatic group substituted at the 4-position might provide greater stability. The cations {Fe(NAr*)(PMe₃)(dppe)}⁺ (Fe2a*), {Ru(NAr*)(PMe₃)(dppe)}⁺ (Ru2a*), and Fe4a were examined by EPR spectroscopy, which suggested differences in electronic structure depending on the metal and ligand set. CASPT2 calculations on model systems for Ru2a* and Fe2a* suggested that the large differences in electronic structure are related to the energy gap between the π-antibonding HOMO and the π-bonding HOMO-1. Both the geometry of the phosphines, which is slightly different between the iron and ruthenium analogs, and the metal center seem to contribute to this energetic difference.