First synthesis and spectroscopic characterization of isolated butatrienylidene complexes of transition metals

The complex Cp*Fe(dippe)Fe(C=CC=C)Fe(CO)(2)Cp* (3b, dippe = 1,2-bis(diisopropylphosphino)ethane, Cp* = pentamethylcyclopentadienyl) was prepared by activation of the terminal butadiyne Cp*(CO)(2)FeC=CC=CH (2) with the chloro iron complex Cp*(dippe)FeCl (1b) in the presence of KPF6 and KOBut. Treatment of Cp*(P-2)Fe((C=CC=C)Fe(CO)(2)Cp* (3a, P-2 = dppe, dppe = 1,2-bis(diphenylphosphinoethane); 3b, P-2 = dippe) with HBF4. Et2O produced the secondary iron butatrienylidene complexes [Cp*(P-2)Fe{=C=C=C=C(H)Fe(CO)(2)Cp*}][BF4] (4a. P-2 = dppe, 75%; 4b, P-2 = dippe, 93%). The slightly more stable tertiary butatrienylidene iron derivatives [Cp*(P-2)Fe{=C=C=C=C(CH3)Fe(CO)(2)Cp*}][OSO2CF3] (5a, P-2 = dppe. 22%; 5b, P-2 = dippe, 75%) were made by reacting the precursor complexes 3a-b with methyl triflate under similar conditions. All the compounds 4a-b and 5a-b are almost stable in solution at 20 degrees C. They are light and air sensitive, even in solid state. The solid samples can be stored under argon for few days in the dark at 5 degrees C. The complexes 4a-b and 5a-b were characterized by multinuclear NMR, IR, UV-vis, and Mossbauer spectroscopies, mass spectrometry and cyclic voltammetry. Their electronic structures are discussed in connection with the spectroscopic data. (C) 1999 Elsevier Science S.A. All rights reserved.