The catalytic role of triphenyl bismuth in curing reactions: a theoretical study

Triphenyl bismuth (TPB), a bismuth-based organometallic compound, has been extensively utilized as a curing catalyst. However, the relevant catalytic mechanism is unclear. In this work, we employ density functional theory to elucidate the reaction mechanism of TPB-catalyzed curing reactions with methyl isocyanate and methanol as representative reactants. In contrast to simplified conceptual models in the literature, which suggested a catalytic mechanism (4C mechanism) involving a high-energy four-membered cyclic transition state, our calculations demonstrate that TPB only exhibits a catalytic ability when following a reaction mechanism (6C mechanism) with a six-membered cyclic transition state structure, in which an additional methanol molecule assists in the breaking of the OH bond and the subsequent proton transfer. In the optimal reaction pathway, which follows the 6C mechanism, a reduction in the energy barrier of 3.5 kcal mol^-1 in a vacuum is expected to be achieved by including the TPB catalyst, which significantly accelerates the curing process.