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.