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46982-AC3
New Robust Tridentate N-Heterocyclic Carbene Ligands for C-H Activation

Michael D. Fryzuk, University of British Columbia

            During the past 12 months, much progress has been made.  The synthesis of the original PCP ligand has been improved and made more efficient.  While it is still a multi-step procedure, each of the steps has been optimized to give reproducibly high yields.  We have also examined the synthesis the proposed bis(pyrrole) ligand system.  While the organic framework could be assembled, the introduction of the flanking phosphine units could not be achieved under any circumstances.

For this reason, we have abandoned this alternative ligand system to focus entirely on the original PCP ligand.

            We have discovered that the ligand system as the imidazolidinium salt, [(PCP)H]PF₆ can easily be installed on the group 10 metals to generate the corresponding hydride complexes, [(PCP)MH]PF₆, where M = Ni, Pd and Pt. Equation 1 shows the preparation of the Ni-hydride:

All of the group 10 complexes have been characterized fully by NMR spectroscopy, elemental analyses and by X-ray crystallography. The nickel hydride undergoes a fascinating reaction with ethylene to generate a new carbon-carbon bond at the carbene carbon.  By following this reaction using deuterium labeling, we were able to propose that the process first involves insertion of ethylene into the Ni-H bond followed by a reductive elimination of the Ni-ethyl and the Ni-carbene.  In an effort to shed light on the specifics of this process, we initiated a collaboration with Professor Jennifer C. Green at the Inorganic Chemistry Laboratory at Oxford University.  Using DFT calculations, she was able to provide us with a reasonable clue as to what the transition state looks like.  In addition, her calculations showed that the isopropyl groups on the phosphine flanks of the tridentate ligand exert a profound steric effect on this process.  The significance of this study is that is shows another way that these N-heterocyclic carbene ligands can act non-innocently in a chemical transformation.

            We have also examined the introduction of this PCP ligand system onto rhodium and iridium, which are central to the proposal in examining C-H activation processes.  This has been incredibly successful as we have been able to prepare a variety of useful precursors, which will serve us well in the following year.  Our initial attempts to use the same imidazolidinium salt that worked so well with the group 10 metals failed for the heavier members of  group 9 (Rh(I) and Ir(I)). However, deprotonation of the salt with KN(SiMe₃)₂ generates the bisphosphinecarbene that can be directly added the 1,5-cyclooctadiene dimers, [M(COD)Cl]₂, to generate the square planar derivatives (PCP)MCl, where M= Rh, and Ir.  Subsequent reaction of the rhodium complex (PCP)RhCl with KBEt₃H generates the corresponding hydride complex (PCP)RhH, which we have characterized fully; this is shown in equation 2.

This complex reacts with H₂ to generate a trihydride, which can be characterized by NMR spectroscopy.  We are now looking at the thermal reactions of theses systems with a variety of alkanes to examine their ability to activate and functionalize hydrocarbons, which will be of interest as a new way to functionalize petroleum products.  Future work will focus on group 9 as well as group 8 metals, in particular Ru and Os.

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