Electronic structure and bonding factors of transition metal-phosphine and -carbene molecules

Abstract

The bonding interactions of phosphines and carbenes with a transition metal center have been explored with gas-phase photoelectron spectroscopy and computational methods. The interactions governing the electronic differences between these two species are probed in order to comment on differences in reactivity that are seen in transition-metal catalytic species. The principles governing the nature of sigma and pi bonding for phosphines and carbenes have been explored and quantified. The electronic bonding factors of the ligand L in the (L)₂(CL)₂Ru=CHPh have been probed in order to explain the catalytic reactivity differences in Grubb's first generation bisphosphine species where L = tricyclohexylphosphine (PCy₃) to the second generation ruthenium catalyst where an N-heteocyclic carbene (NHC) 1,3-dimesityl-imidazolidine-2-ylidene (H₂IMes) replaces one of the phosphines in the catalyst. To directly compare the bonding modes of PCy₃ and NHCs, the (CO)₅MoL system is utilized in order to take advantage of its high symmetry. Results indicate that the NHC ligands are stronger σ donors than phosphines, and essentially have no π-acceptor ability. These electronic differences have key implications to the differences these catalyst exhibit in terms of initiation and propagation. Next, the bonding in the Cp*Ru(Cl)L molecules, where L = PCy₃, PⁱPr₃, H₂IMes, IMes and Prⁱ₂Me₂Im, is explored by photoelectron spectroscopic investigations and supporting electronic structure calculations. The Cp*Ru(Cl)L system is a coordinatively unsaturated 16 electron system which can electronically and satirically bind small molecules. This system has been found to have electronic structure interactions similar to half-sandwich complexes. In addition, the ionization energies measured from the photoelectron spectra of Cp*Ru(C)L molecules correlate well to solution calorimetry measurements of bond energies by Nolan and co-workers. Finally, the nature of a rare "internal" transition metal iridium carbene is probed via gas-phase photoelectron spectroscopy and density functional calculations. Ionizations measured for the [IrCl(ᵗBu₂PCH₂CH₂CCH₂CH₂PᵗBu₂)] complex with the support of theoretical calculations serve to establish the valency of the iridium metal center. This internal pincer has been found to have a "covalent carbene-metal" interaction.