Theoretical study of formation of pyridines by interaction of a cobaltacyclopentadiene with model nitriles (hydrogen cyanide or trifluoroacetonitrile): Electronic effects of nitriles on the reaction mechanism

摘要

Theoretical calculations on the reaction of HCN with (η~5- cyclopentadienyl)cobaltacyclopentadiene (1) were made by using B3LYP and CCSD(T) methods. Since it is coordinatively unsaturated, 1 is more stable in the triplet state than in the singlet state. However, when HCN interacts with 1, the singlet state becomes more stable, and the CN bond inserts into the Co-C bond in the singlet state to form an azacobaltacycloheptatriene intermediate (4aS). The reaction can follow two courses from 4aS. One is reductive elimination to give an η~4-pyridine complex (5aS) that retains a singlet spin state. The other involves a change in spin state to the triplet state to form the more stable triplet state azacobaltacycloheptatriene (4T), from which reductive elimination takes place to give a triplet η~2-pyridine complex (5bT). The η~4-pyridine complex in the singlet state (5bS) is the most stable pyridine complex and contains four carbon atoms of the pyridine ring coordinated to the Co atom. The rearrangement reactions of 5aS or 5bT to give 5bS involve a change in the spin state. The mechanism therefore shows two-state reactivity. This mechanism is different from the reaction of acetonitrile, in which [4 + 2] cycloaddition of MeCN to cobaltacyclopentadiene 1 takes place in the singlet state instead of insertion into the Co-C bond and reductive elimination. This difference can be rationalized in terms of the difference in the energies of the frontier orbitals, so that an electron-donating group favors [4 + 2] cycloaddition and an electron-withdrawing group favors insertion of the CN bond into the Co-C bond. This was confirmed by calculations on the reactions of CF_3CN.