Modeling catalytic steps on extra-framework metal centers in zeolites. A case study on ethylene dimerization

In a case study of organometallic catalytic reactions, this work benchmarks density functional theory calculations on zeolite-supported transition metal complexes. Elementary steps of ethylene dimerization and hydrogenation reactions involving the complex [Rh(C₂H₄)₂(H₂)]⁺, supported on faujasite, were examined by comparing explicit QM (quantum mechanics) cluster models as well as QM/MM (molecular mechanics) embedded models to plane-wave periodic models as reference. Two QM cluster models, 1T and 5T where T refers to tetrahedral units of zeolite, as well as four QM/MM cluster models were explored. For the MM region, the UFF force field was found preferable to the semiempirical method PM6. The embedded cluster models reproduce barriers of C-C and C-H bond formation with deviations from the reference of at most 10 kJ mol^-1. With variations of similar size, the effect of embedding on the energetics of the reactions under study is moderate, likely because of the small nonpolar reactants. For elucidating such catalytic reactions at transition metal species in zeolites, cluster models appear equally well-suited as periodic models but computationally advantageous.