Electron-Deficient Iron Alkyl Complexes Supported by Diimine Ligand (Ph<sub>2</sub>CN)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>: Evidence for Reversible Ethylene Binding
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Reaction of diimine ligands (Ph2CN)2CnH2n (n = 2, a; n = 3, b) with FeCl2 or FeBr2 results in formation of the corresponding high-spin ferrous complexes {(Ph2CN)2C2H4}FeX2 (1a, X = Cl; 2a, X = Br) and {(Ph2CN)2C3H6}FeCl2 (1b). Dialkyl {(Ph2CN)2C2H4}Fe(CH2SiMe3)2 (3a) was prepared by treatment of (py)2Fe(CH2SiMe3)2 with diimine ligand a. Addition of B(C6F5)3 to 3a at −30 °C resulted in Me3SiCH2 abstraction, affording [{(Ph2CN)2C2H4}Fe(CH2SiMe3)][Me3SiCH2B(C6F5)3] (5a). 19F NMR spectroscopy revealed that this compound exists as a contact ion-pair in toluene solution. Compound 5a decomposes at room temperature in bromobenzene-d5 or toluene-d8, affording dication [{(Ph2CN)2C2H4}2Fe]2+ (6a); at elevated temperatures in toluene-d8 the formation of [{(Ph2CN)2C2H4}Fe(CH2SiMe3)C6F5] (7a) was observed as well. Neither ferrous chloride {(Ph2CN)2C2H4}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.
