Cyanide-alkene competition in a diiron complex and isolation of a multisite (cyano)alkylidene-alkene species

The μ-(amino)alkylidyne complex [Fe2Cp2(CO)2(μ-CO){μ-CNMe(CH2CHCH2)}]CF3SO3, [1]CF3SO3, reacted with NBu4CN in dichloromethane affording the μ-(cyano)(amino)alkylidene [Fe2Cp2(CO)2(μ-CO){μ-C(CN)N(Me)(CH2CHCH2)}], 2, in 91% yield. Decarbonylation of 2 by using Me3NO in acetone at room temperature yielded [Fe2Cp2(CO)(μ-CO){μ-κ3C-C(CN)N(Me)(CH2CHCH2)}], 3, containing a multidentate alkylidene-alkene ligand occupying both a bridging site and a terminal site, in admixture with the μ-(amino)alkylidyne cyanide product [Fe2Cp2(CN)(CO)(μ-CO){μ-CN(Me)(CH2CHCH2)}], 4. The reaction of the μ-(amino)alkylidyne imine complex [Fe2Cp2(CO)(μ-CO)(NHCPh2){μ-CN(Me)(CH2CHCH2)}]CF3SO3, [7]CF3SO3, with NBu4CN gave 3 with an optimized yield of 75% via imine elimination. According to DFT calculations, 3 is less stable than its geometric isomer 4 by 13.4 kcal mol-1 and quantitative conversion to 4 was achieved by refluxing a THF solution of 3 for 2 hours. No replacement of alkene coordination occurred upon treating 3 with CO or PPh3. The previously unknown compounds 2, 3, 4 and [7]CF3SO3 were fully characterized by analytical and spectroscopic techniques and the structure of 3 was elucidated by single crystal X-ray diffraction.