Determination of the Solution Supramolecular Structure of Organometallic Catalysts through Nmr Spectroscopy

In our laboratory, we have been interested in solution supramolecular structure of transition-metal organometallic catalysts for last decade1,2. In particular, we are trying to correlate the effect of intermolecular structure on the chemical reactivity. For intermolecular structure, we mean both the relative position of the interacting moieties, determined by NOE (Nuclear Overhauser Effect) NMR experiments1, and the level of aggregation, estimated by PGSE (Pulsed Field Gradient Spin-Echo) NMR technique2. In this contribution, after having recalled the essential aspects of the NMR techniques and the methodological procedures that can be used to obtain accurate information3, recent findings regarding ruthenium(II) catalyst for transfer hydrogenation reactions [I]4, Zirconaaziridinium salts bearing long alkyl chains [II]5 and gold(I) catalysts for the activation of carbon-carbon multiple bonds toward nucleophilic attach [III]6 will be presented. [I] PGSE (pulsed field gradient spin-echo) NMR measurements and ONIOM (B3PW91:UFF) calculations indicate that amino-amidate half-sandwich ruthenium catalysts have a marked tendency to form dimers and higher aggregates in several solvents including 2-propanol, the solvent used in catalysis. [II] Homogeneous olefin polymerizations are usually carried out in very low polar solvents, the presence of the aliphatic chain allows studying the tendency to self-aggregate of zirconocenium ion pairs in cyclohexane where the tendency to self-aggregates results enormously enhanced to the point of rendering not negligible the percentage of ion quadruples also at the lowest concentration level used in the olefin polymerizations. [III] The relative anion-cation orientation in [(L-Au-S)+X-] [S= alkenes and alkynes, L= phosphine and NHC: 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene], investigated by NMR spectroscopy and DFT calculations, shows that the exact position of the counterion is critically determined by the nature of the ancillary ligand and substrate; this opens the way to a greater control over the properties and activity of these catalysts. References 1. Macchioni, A. Eur. J. Inorg. Chem. 2003, 195. 2. Binotti, B., et al. Comments Inorg. Chem. 2002, 23, 417. 3. Zuccaccia, D., et al. Organometallics 2005, 24, 3476. Macchioni, A., et al. Chem. Soc. Rev., 2008, 37, 479. 4. Zuccaccia, D., et al. New J. Chem. 2005, 29, 430. Ciancaleoni, G., et al. Organometallics 2009, 28, 960. 5. Rocchigiani, L. et al. Chem. Eur. J. 2008, 14, 6589. Rocchigiani, L. et al. Organometallics 2011, 30, 100. 6. Zuccaccia D. et al. J. Am. Chem Soc. 2009,131, 3170. Zuccaccia, D. et al. Inorg. Chem. 2010, 49, 3080. Salvi, N., et al. J. of Organomet. Chem. 2010, 695, 2679. Acknowledgements: This work was funded by PRIN and FIRB programs