Calculating accurate barriers for olefin insertion and related reactions

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 pi-complex stage on. No functional performs entirely satisfactorily (within similar to 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 pi-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. (C) 2014 Elsevier B.V. All rights reserved.