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Pure Appl. Chem., 2010, Vol. 82, No. 8, pp. 1663-1671

Published online 2010-05-18

Comparison of donor properties of N‑heterocyclic carbenes and N-donors containing the 1H-pyridin-(2E)-ylidene motif

John Slattery, Robert J. Thatcher, Qi Shi and Richard E. Douthwaite*

Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

Abstract: IR spectroscopic and X-ray structural data of rhodium and palladium complexes of N-heterocyclic carbene (NHC) and 1H-pyridin-(2E)-ylidene (PYE) ligands indicate that both ligand classes exhibit similar electron-donating properties. However, catalytic application of palladium PYE complexes appears to be limited by PYE ligand loss. Density functional theory (DFT) calculations show that the Pd–CNHC σ-bond is very low-lying in energy (HOMO-14 and 15, ca. –11 eV) and a π-backbonding contribution is also present, whereas the Pd-NPYE σ-bond is comparatively high-lying (HOMO-9 and 10, ca. –8 eV) and the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap is also significantly less (4.0 vs. 5.6 eV). Essentially, electronegativity differences between Pd, C, and N render the Pd–N bond much more polarized and susceptible to electrophilic and nucleophilic attack and hence ligand substitution.