C−C Bond Cleavage of Acetonitrile by a Carbonyl Iron Complex with a Silyl Ligand

The photoreaction of a silyl iron complex Cp(CO)₂Fe(SiMe₃) (1) in acetonitrile in the presence of P(NMeCH₂)₂(OMe) (L) yielded Cp(CO)LFeMe (2), CpL₂FeMe (3), and CpL₂Fe(CN) (4), showing that carbon−carbon bond cleavage of acetonitrile was achieved. These C−C bond cleavage products were also obtained in the photoreaction of 1 with 1 equiv of MeCN in THF in the presence of L. The reaction with CD₃CN showed that the methyl group on the iron in the products is derived from acetonitrile. The corresponding reaction of Cp(CO)₂Fe(ER₃) (ER₃ = CH₃, GeMe₃, SnMe₃) generated a CO/L exchange complex, Cp(CO)LFe(ER₃), showing that a silyl ligand on the iron is indispensable for the C−C bond cleavage of acetonitrile. Theoretical studies on the C−C bond cleavage were performed using the hybrid DFT-B3LYP method. The direct C−C bond oxidative addition of acetonitrile to the 16e species Cp(CO)Fe(SiMe₃) expected to readily form from 1 in the photoreaction conditions has a very high activation barrier of 52.7 kcal/mol, suggesting that the oxidative addition is not an appropriate reaction pathway. A more feasible pathway was proposed. The end-on coordination of acetonitrile nitrogen to Cp(CO)Fe(SiMe₃), followed by the rearrangement to a CN side-on complex, with the activation energy of 14.8 kcal/mol occurs, and then the insertion of the CN bond into the Fe−Si bond with a small activation energy of 4.0 kcal/mol and the successive C−C bond cleavage of acetonitrile on the Fe coordination sphere with the activation energy of 15.0 kcal/mol take place to give Cp(CO)MeFe(CNSiMe₃). The isolation of an iron complex with a methyl group derived from acetonitrile and a silylisocyanide ligand was attained in the photoreaction of Cp(CO)₂Fe(SiPh₃) in MeCN in the presence of PPh₃. The product Cp (PPh₃)MeFe(CNSiPh₃) was confirmed by the X-ray structure analysis. The reaction mechanism leading to the iron cyanide complex has also been discussed.