Synthesis of trispyrazolylborate-supported nickel complexes for small molecule activation
Date
2015
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The structure and function of enzymes has been of great interest to the scientific community for many years. In particular, interest lies in the ability ofmetalloenzymes to catalyze the activation of small molecules. In 1996, a new metal variant of superoxide dismutase was discovered, which contained nickel in the active site. Additionally, of the nine known nickel-containing enzymes, five contain nickel-sulfur bonds including nickel superoxide dismutase (NiSOD), acetyl-coenzyme A synthase/carbon monoxide dehydrogenase (ACS/COdH), [NiFe] hydrogenase, and methyl-coenzyme M reductase (MCR). The discovery of NiSOD and a nickel form of the acireductone dioxygenase sparked interest in nickel-dioxygen chemistry. While numerous synthetic complexes have been prepared in an effort to model the structure and function of these nickel-containing enzymes, very few examples exist that utilize nickel complexes supported by the trispyrazolylborate (Tp) ligand system. This thesis addresses synthetic model systems by examining the synthesis, characterization, and reactivity profiles of several new nickel-dioxygen, nickel-sulfur, and nickel-selenium species supported by the Tp ligandclass. The synthesis and reactivity of [Tp Ph,Me ]Ni(O2 ) is described and compared to previously reported [TptBu,Me ]Ni(O 2 ). [TpPh,Me ]Ni(O2 )was found to be more reactive than [TptBu,Me ]Ni(O2 ) with respect to phosphine oxidation. While[TptBu,Me ]Ni(O2 )oxidizes only alkyl phosphines, [TpPh,Me ]Ni(O2 )readily oxidizes both aryl and alkyl phosphines. However, [TpPh,Me ]Ni(O2 ) is not capable of activating weak C-H bonds as readily as [TptBu,Me ]Ni(O2 ), converting only 9, 10-dihydroanthracene to anthraquinone in low yields. Interestingly, [TpPh,Me ]Ni(O2 )deformylates aldehydes in quantitative yields. The nickel(I) and nickel(II) complexes formed from the synthesis of [Tp Ph,Me ]Ni(O2 ) were used in an attempt to prepare {[Tp Ph,Me ]Ni}2 (μ-S2 ) and {[TpPh,Me ]Ni} 2 (μ-Se2 ). [TpPh,Me ]Ni(S(CH2 ) 2 ) was formed from the reaction of [TpPh,Me ]Ni(PPh 3 ) with ethylene sulfide. This nickel complex possesses a magnetic moment consistent with the expected doublet ground state, S = 1⁄2. In addition, [TpPh,Me ]Ni(S(CH2 )2 ) is inert towards reaction with CO, O2 , and PEt3 . Reaction with PMe3 yielded [TpPh,Me ]2 Ni and [PMe 3 Et]+ . Reactions with methyl iodide and n -butyl lithium did not yield the desired nucleophilic reaction products. Attempts to make the analogous complex, [TptBu,Me ]Ni(S(CH2 ) 2 ), were unsuccessful resulting in rapid decomposition of the product to pyrazole and nickel sulfide. The instability of this complex as compared to [TpPh,Me ]Ni(S(CH2 )2 ) indicates that the latter is a uniquely stable nickel(III) complex. As such, the rapid decomposition of [TptBu,Me ]Ni(S(CH2 )2 ) supports the proposed nickel(III) mechanistic intermediate in catalytic cross-coupling reactions while also identifying a stable nickel(III) complex whose geometric and electronic properties can be well studied. In addition, efforts were pursued to synthesize two new Tp-supported nickel-dioxygen species, [TpFc,Me ]Ni(O2 ) and [Tp Ind,Me ]Ni(O2 ). The series of halide complexes were made for each Tp ligand. However, conventional nickel(I) and nickel(II) routes for synthesizing the dioxygen complexes were unsuccessful. Two new sulfur- and selenium-containing complexes were synthesized using the [Tp Ind,Me ]Ni halide complexes, namely [TpInd,Me ]Ni(SH) and ([Tp Ind,Me ]Ni)2 (μ-Se). Overall, this thesis describesa stable Tp-supported nickel(III) (or Ni(II)L*) complex as well as several new Tp-supported nickel-sulfur and nickel-selenium complexes. A new Tp-supported superoxide complex, [TpPh,Me ]Ni(O 2 ), is also described which exhibits a much different reactivity scope than the previously prepared [TptBu,Me ]Ni(O2 ). The difference in reactivity is largely attributed to the more open coordination sphere in [TpPh,Me ]Ni(O2 ) versus [TptBu,Me ]Ni(O2 ). Continued work with TpInd,Me and TpFc,Meligated nickel complexes to synthesize the corresponding dioxygen complexes could provide for further comparison to [TpPh,Me ]Ni(O2 ) and [TptBu,Me ]Ni(O2 ). These complexes have similar open coordination spheres but will likely exhibit much different reactivity due to the electronic differences of the Tp substitutents as well as the introduction of a redox-active moiety on the ligand.