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42822-B3
Investigation of the Lewis Acidic Properties of N-Heterocyclic Carbene Ligands and Their Implications for Catalysis

Colin D. Abernethy, Keene State College

Our research in the in the last 12 months has focused on the preparation of several cyclopentadienyl complexes of group 5 metal chlorides, which also contain N-heterocyclic carbene (NHC) ligands.  By comparing the interligand Cl···C_carbene distances in these complexes we sought to determine whether the oxidation state of the metal center influences the degree of Lewis acidity displayed by the carbenic carbon of the NHC ligand.  Accordingly, complexes featuring vanadium(II) (1), vanadium(III) (2), and vanadium(V) (3) have been prepared by the reactions as shown:  

In the vanadium(II) and (III) complexes (1 and 2 respectively) no Cl···C_carbene interactions  are observed as the distances between these atoms are greater than the sum of their van der Waals' radii.  However, in the vanadium(V) complex (3) the Cl···C_carbene distance is significantly shorter than the sum of their van der Waals radii, indicating that the C_carbene atom in this complex is Lewis acidic.  These results suggest that coordination to a high-oxidation-state metal center is required to induce Lewis acidic character in NHC ligands.  We are currently performing DFT calculations on model compounds of 1, 2, and 3 to gain further insights into these systems.

The three undergraduate students participating in this research have presented aspects of this work as posters at three ACS meetings: NERM (Binghamton, NY), October 2006; the Chicago National Meeting, March 2007; and the Boston National Meeting, August 2007.      

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