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Nickel bis(N-heterocyclic carbene) complexes: rational design strategies and new structural motifs


Nickel is an attractive metal for use in a wide variety of catalytic transformations that normally require precious metals. Of particular interest is the reduction of CO2 to formate, which is a potential hydrogen storage medium, fuel, and chemical building block. The nickel bis-diphosphine class of complexes has recently been studied for this application. However, it has been found that existing nickel bis-diphosphine complexes cannot produce hydrides that are capable of reducing CO2 to form formate.

In this dissertation, I first provide a detailed analysis of the thermodynamic relationships which are responsible for the inadequate reduction power of hydrides formed by known nickel bis-diphosphines. It is established that by designing complexes with a more negative reduction potential than that of a typical nickel bis-diphosphine complex, more powerfully reducing hydrides may be formed. N-heterocyclic carbene (NHC) ligands are chosen as a substitute for phosphine ligands in this system, as NHC ligands are generally more electron donating than typical phosphine ligands, and are thus expected to result in complexes with more negative reduction potentials.

By direct analogy to known nickel bis-diphosphine complexes, homoleptic nickel bis(NHC) complexes are synthesized. These are found to form sterically hindered, highly reducing Ni(0) compounds upon reduction. These Ni(0) compounds are the first group 10 M(0) (M = nickel, palladium, platinum) tetracarbene complexes reported. They display an unusual sawhorse geometry in the Ni(0) state.

In order to address the electrochemical irreversibility displayed the nickel tetracarbene complexes, a less sterically encumbering bis(NHC) ligand linked by a propyl group is chosen. This ligand is found to provide access to the [Ni(NHC2)]2+ fragment, which in turn provides access to a new Ni(II)(NHC2)¬L2 class of complexes. Four Ni(II)(NHC2)¬(bipyridine) complexes are synthesized and characterized. It is found that a rigid, twice-linked bis(NHC) ligand is capable of imparting electrochemical reversibility to these systems.

This new bis(NHC) ligand is used to synthesize the first Ni(II)(NHC2)(diphosphine) complex. This compound is isolated in both the Ni(II) and Ni(0) oxidation states. Strategies to further develop this system in order to be suitable for CO2 reduction are discussed. Future experiments involving the [Ni(NHC2)]2+ fragment are also suggested.

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