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43703-AC1
C-C-C N-Heterocyclic Carbene Pincer Complexes: A General Method of Preparation Critical for Applications in Catalysis

T. Keith Hollis, The University of Mississippi

A general solution to the preparation of pincer complexes that require the formation of two N-heterocyclic carbenes (NHC's) and the activation of an aryl C-H bond has been developed.  Continued work on this methodology has been carried out during the project period with new coworkers learning the method and developing large scale syntheses.  The reaction of a phenylene-bridged bis(imidazolium) salt with Zr(NMe₂)₄ generated the requisite CCC-NHC Zr pincer complex 2 (Scheme 1), which has been characterized by X-ray crystallography (Figure 1). 

Scheme 1.  Synthesis of CCC-NHC Zr pincer complex of 2.  Complex 3 was identified in the X-ray crystal structure and occupies 9% of the crystal sites.

Figure 1.  ORTEP diagram (50% probability ellipsoids, hydrogens omitted for clarity) of complex 2 and 3.  A solid solution was observed in the crystal with 91% of the crystal showing two amido groups and one iodide group and 9% of the crystal showing one amido group and two iodides.

Importantly, it has been found that CCC-NHC Zr pincer complexes can be generated in situ and transmetallated with an appropriate Rh source to generate CCC-NHC Rh pincer structures (4, 5, Scheme 2), which have also been characterized by X-ray crystallography (Figure 2).

Scheme 2.  Transmetallation from Zr to Rh in the preparation of late transition metal CCC-NHC Rh pincer complexes 4 and 5.

Figure 2. a) ORTEP plot (50% probability, hydrogens omitted for clarity) of the asymmetric unit of CCC-NHC Rh pincer complex 5.  b) ORTEP plot (50% probability, hydrogens and carbons of the butyl groups omitted for clarity) of the edge-on view of the iodo-bridged Rh dimer structure that completes the octahedron of each molecule.  Rh – Rh = 4.01 Å.

            These two methodologies, metallation of CCC-NHC precursors with transition metal amido complexes combined with transmetallation, hold great promise for opening general synthetic pathways to a wide variety of transition metal CCC-NHC pincer complexes.

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