Theoretical Study on the Two-State Reaction Mechanism for the Formation of a Pyridin-2-one Cobalt Complex from Cobaltacyclopentadiene and Isocyanate

The two-state reaction (TSR) mechanism of CpCo(C4H4) with isocyanate on the triplet and singlet potential energy surfaces has been investigated at the B3LYP level. The minimal energy crossing point (CPs) in the crossing seam between the potential energy surfaces are located using the method of Harvey et al., and possible spin inversion processes are discussed by means of spin-orbit coupling (SOC) calculations. As a result, the two distinct reaction pathways for the formation of a pyridin-2-one cobalt complex have been found. For the first pathway, there are two key crossing points along the reaction pathway. The first crossing point, CP2, exists near B-1. The reacting system will change its spin multiplicity from the triplet state to the singlet state near this crossing region because the magnitude of the spin-multiplicity mixing (SOC1-e,1-center = 393.37 cm(-1)) increases in a small energy gap between high- and low-spin states will greatly enhance the probability of intersystem crossing. The single (P-1(ISC)) and double (p(2)(ISC)) passes estimated at CP2 are approximately 0.28 and 0.48, respectively. The second crossing point, CP3, will again change its spin multiplicity from the singlet state to the triplet state in the Co-C-gamma bond activation pathway, leading to a decrease in the barrier height of (TS)-T-1(CF) from 19.5 to 9.5 kcal/mol. Thus, the reaction system will access a lower energy pathway and then move on the triplet potential energy surface as the reaction proceeds. As for the second pathway, the formation of the initial transition is a very unfavorable process kinetically. However, from the beginning of H-1, TSR is a very favorable process kinetically and thermodynamically. After passing point CP5, the triplet potential energy surface can provide a low-cost reaction pathway toward the product complex.