Electron-Deficient Iron Alkyl Complexes Supported by Diimine Ligand (Ph2CN)(2)C2H4:Evidence for Reversible Ethylene Binding

Reaction of diimine ligands (Ph2CN)(2)CnH2n (n = 2, a; n = 3, b) with FeCl2 or FeBr2 results in formation of the corresponding high-spin ferrous complexes {Ph2CN)(2)C2H4} FeX2 (1a, X = Cl; 2a, X = Br) and {(Ph2CN)(2)C3H3}FeCl2 (1b). Dialkyl {(Ph2CN)(2)C2H4) Fe(CH2SiMe3)(2) (3a) was prepared by treatment of (py)(2)Fe(CH2SiMe3)(2) with diimine ligand a. Addition of B(C6F5)(3) to 3a at -30 degrees C resulted in Me3SiCH2 abstraction, affording [{(Ph2CN)(2)C2H4} Fe(CH2SiMe3)][Me3SiCH2B(C6F5)(3)] (5a). F-19 NMR spectroscopy revealed that this compound exists as a contact ion-pair in toluene solution. Compound 5a decomposes at room temperature in bromobenzene-d(5) or toluene-d(8), affording dication [{(Ph2CN)(2)CH4)(2)Fe](2+) (6a); at elevated temperatures in toluene-d(8) the formation of [{(Ph2CN)(2)C2H4)Fe(CH2SiMe3)C6F5] (7a) was observed as well. Neither ferrous chloride {(Ph2CN)(2)C2H4} FeCl2 activated with methylaluminoxane nor contact ion-pair 5a is active in the polymerization of ethylene. Instead we were able, for the first time, to observe reversible ethylene binding to a cationic iron alkyl complex.