Synthesis of Diiron m-Allenyl Complexes by Electrophilic Addition to Propen-2-yl-Dimetallacyclopentenone Species: a Joint Experimental and DFT Study

The complex [Fe2Cp2(CO)(µ-CO){µ-η1:η3-C(H)=C(C(CH3)CH2)C(=O)}] (2) underwent addition of a series of electrophiles to afford the -allenyl [Fe2Cp2(CO)2(-CO)-1:2,-CH=C=C(CH3)(CH2E)}][X] (E = CPh3, X = BF4, [3][BF4]; E = SiMe3, X = Cl, [4][Cl]; E = H, X = BF4, [1][BF4]), in high yields. The new compounds [3][BF4] and [4][Cl] were characterized by analytical, spectroscopic and electrochemical methods, moreover the molecular structure of [3][BF4] was elucidated by X-ray diffraction. DFT calculations were carried out on [3]+ and [4]+: in both cases, the structure bearing the C-methyl substituent pointing to the FeFe axis is more stable than the structure with opposite configuration of the C-substituents (U = 7 Kcal mol1 for [3]+ and U = 1 Kcal mol1 for [4]+). These computer outcomes are in agreement with the X-ray structure of [3][BF4] and with NMR evidences. The protonation of 2 was investigated by DFT calculations, thus the C carbon resulted the most favorable site of H+ attack in 2 on theoretical grounds: the formation of the allenyl complex [1]+ rather than the vinyl isomer [Fe2Cp2(CO)2(µ-CO){µ-η1:η2-CH=CHC(Me)=CH2}]+ ([7]+) is thermodinamically more favoured by … Kcal mol1. Attempts to convert [1]+ into ([7]+) by high temperature reaction resulted in fragmentation of the diiron frame. The reaction of [1]+ with NHEt2 in the presence of PhSSPh resulted in prevalent formation of [FeCp(CO)2SPh] (8).