title: Conformations of N-heterocyclic carbene ligands in ruthenium complexes relevant to olefin metathesis.
abstract: The structure of ruthenium-based olefin metathesis catalyst 3 and model pi-complex 5 in solution and in the solid state are reported. The N-tolyl ligands, due to their lower symmetry than the traditional N-mesityl substituents, complicate this analysis, but ultimately provide explanation for the enhanced reactivity of 3 relative to standard catalyst 2. The tilt of the N-tolyl ring provides additional space near the ruthenium center, which is consistent with the enhanced reactivity of 3 toward sterically demanding substrates. Due to this tilt, the more sterically accessible face bears the two methyl substituents of the N-aryl rings. These experimental studies are supported by computational studies of these complexes by DFT. The experimental data provides a means to validate the accuracy of the B3LYP and M06 functionals. B3LYP provides geometries that match X-ray crystal structural data more closely, though it leads to slightly less (approximately 0.5 kcal mol-1) accuracy than M06 most likely because it underestimates attractive noncovalent interactions.
authors.affiliation: The Arnold and Mabel Beckman Laboratory of Chemical Synthesis and the Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Ian C
authors.initials: IC
authors.lastname: Stewart
authors.forename: Diego
authors.initials: D
authors.lastname: Benitez
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: O'Leary
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Tkatchouk
authors.forename: Michael W
authors.initials: MW
authors.lastname: Day
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: Robert H
authors.initials: RH
authors.lastname: Grubbs
mesh_terms: D001597:Benzylidene Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008697:Methane / Q000031:analogs & derivatives / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D001597:Benzylidene Compounds; D006571:Heterocyclic Compounds; D008024:Ligands; C030011:carbene; D012428:Ruthenium; D008697:Methane