Calculating accurate barriers for olefin insertion and related reactions

• Extensive benchmarking of functionals and basis sets against CCSD(T,FULL) results.
• TS, π-coord., reaction energies and agostic interactions for early and late TM.
• M06-2X for ETM (MAD 1.0).
• TPSSh-D0 for LTM (MAD 1.3) and acceptable “across-the-board functional”.
• Ethene insertion barriers:∼0 for titanocenes, few kcal/mol for zirconocenes and CGC.

Highly accurate (extrapolated CCSD(T)/aug-cc-pwCVQZ) reference energies have been calculated for olefin insertion in a set of simple models for olefin polymerization catalysts, and also for related reactions (hydrogen transfer to metal and to monomer, allyl formation). The model systems cover early (groups 3 and 4, 10 basic systems, 216 geometries) and late (group 10, 3 basic systems, 51 geometries) transition metals. The reference energies were then used to evaluate the performance of 22 commonly used density functionals, as well as several ab-initio methods. Dispersion corrections (either implicit or explicit) are essential in getting the olefin complexation energy right, but are less important for describing further reactions from the olefin π-complex stage on. No functional performs entirely satisfactorily (within ∼1 kcal/mol) for both early and late transition metals. Of the functionals tested, TPSSh-D0 shows the best “across-the-board” performance (important if chemistry involving more than a single metal needs to be described e.g. chain shuttling, bimetallic complexes, bifunctional systems), but M06-2X performs somewhat better specifically for early transition metals. A subsequent benchmark on 21 experimentally known π-coordination enthalpies and barrier heights for early, middle and late TM systems yielded an MAD of 0.60 kcal/mol using our recommended protocol. Finally, we revisited prototypical catalysts systems studied in early computational work, and conclude that insertion of ethene is nearly barrierless for titanocenes, and has a slightly higher barrier for constrained-geometry catalysts and for Zr analogs.

TPSSh-D0 was identified as an acceptable “across-the-board functional” for transition states energies, agostic interactions, reaction energies and π-coordination energies for early and late transition metals. Our recommended protocol should produce reaction energies and barrier heights with reasonable error margins (1–2 kcal/mol) for a wide range of reactions.Figure optionsDownload as PowerPoint slide