One particularly often used class of (pre)catalysts for olefin metathesis are so called Grubbs-II-type complexes (1-3), where the central Ruthenium atom is coordinated by a N-heterocyclic carbene (NHC), two chlorides, an easily dissociating phosphine ligand and a benzylidene ligand. Three different complexes were investigated here, varying in the substitution of the NHC with R and R' being either an electron-donating NEt2 group or an electronically neutral H.

The structure and electronic properties were readily characterized by X-ray cristallography and cyclic voltametry. However, to study the complexes in solution, NMR spectroscopy is the tool of choice. The dynamic processes (denoted by their rate constants k0-k4) were monitored at varying temperatures (223-238K) using jump-symmetrized ROESY spectra. We employed 1D pulsed field gradient spin echo experiments (PFGSE) to measure nuclear Overhauser enhancements and chemical exchange rates. From these exchange rates the rotational barriers were calculated and correlated with the substitution at R and R'.

From this data we were able to better explain the different dynamic behavior of the (at room temperature) atropisomers 2 and 3, when compared to the symmetrically substituted complex 1. Combining these results with cyclic voltammetry, we demonstrated the stronger influence of the R substituent located above the benzylidene moiety versus the R' substituent above the emtpy coordination site on the catalytic properites of such Ruthenium complexes.

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1 S. Leuthäußer, V. Schmidts, C. M. Thiele, H. Plenio, π-Face Donor Properties of N-Heterocyclic Carbenes in Grubbs II Complexes, Chem. Eur. J. 2008, 14, 5465-5481.