Dioxygen activation by trispyrazolylborate-supported, monovalent nickel complexes and associated reactivity
Date
2016
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The activation of dioxygen by transition metal complexes is an area of intense
study due to the potentially invaluable synthetic utility such systems may provide.
While a number of 1st row transition metals have been extensively investigated in this
regard, the use of nickel to activate O2 has seen considerably less development. The
knowledge gap in this area is due in large part to the relatively small number of nickel
systems known to display this type of reactivity as well as the even smaller number of
stable, nickel-dioxygen adducts available for study.
This dissertation describes efforts towards the preparation of a new class of
monovalent nickel complexes, supported by the trispyrazolylborate (Tp) ligand
system, and the use of these complexes to facilitate dioxygen activation. The pursuit of
this goal has led to the successful isolation and characterization of the first Tp
supported monovalent nickel compounds. Furthermore, the study of these complexes
has resulted in the discovery of numerous interesting derivative species including two
superoxo-nickel complexes, [TptBu,Me]Ni(O2) and [TpPh,Me]Ni(O2). These new
dioxygen adducts have been fully characterized and, in the case of [TptBu,Me]Ni(O2),
have been extensively investigated with regard to potential avenues of reactivity.
Two trispyrazolylborate ligands with different steric and electronic
environments were selected for study. The ligands chosen, TptBu,Me and TpPh,Me, were
used to prepare divalent nickel complexes, which were then reduced in the presence of
suitable trapping agents. These experiments resulted in the preparation of stable
monovalent species including [TptBu,Me]Ni(CNtBu), [TptBu,Me]Ni(CNCy),
[TptBu,Me]Ni(CO), [TpPh,Me]Ni(PPh3) and [TpPh,Me]Ni(CO). All of these complexes
have demonstrated the ability to activate dioxygen and, with the exception of
[TpPh,Me]Ni(PPh3), to yield dioxygen adducts amenable to structural characterization.
In addition, the isonitrile trapped complexes were found to react with iodosylbenzene
to generate nickel(II) carbamates. The putative mechanism of this transformation
suggests the involvement of an oxo-nickel intermediate. Related studies also led to
preparation of a monomeric nickel hydroxide complex, [TptBu,Me]Ni(OH).
Treatment of [TptBu,Me]Ni(CNtBu) with dioxygen resulted in the isolation of a
nickel-O2 adduct, [TptBu,Me]Ni(O2). This complex was characterized by 1H NMR, FTIR,
elemental analysis, L-edge X-ray absorption spectroscopy and X-ray
crystallography. Characterization led to the unequivocal assignment of the complex as
nickel(II)-superoxide with the O2 unit bound to nickel in a “side-on” fashion. The
magnetic moment of the complex was determined to be 2.3(1) μB, indicating an S =
1/2 spin state. [TptBu,Me]Ni(O2) is stable under ambient conditions, both in the solid
state and in solution. In addition, the complex exhibits considerable resistance to
thermally induced decomposition. A second superoxo-nickel complex,
[TpPh,Me]Ni(O2), was prepared from [TpPh,Me]Ni(CO) under conditions similar to those
used to generate [TptBu,Me]Ni(O2). This complex, which has also been fully
characterized, is stable under ambient conditions in the solid state but is unstable in
solution. The divergence in stability between the two dioxygen adducts has been
attributed primarily to differences in steric environment.
The reactivity of [TptBu,Me]Ni(O2) was explored with a major focus on oxygen
atom transfer, C-H activation and aldehyde deformylation. [TptBu,Me]Ni(O2) facilitates
O-atom transfer to both nitric oxide and alkylphospines. In the latter case, the transfer
was shown to be catalytic in nature with strong implications for the involvement of an
oxo-nickel intermediate. The complex was also competent in the C-H activation of
multiple substrates including 1,4-cyclohexadiene (CHD), 9,10-dihydroanthracene
(DHA) and xanthene. In the case of CHD, reaction with [TptBu,Me]Ni(O2) resulted in
the production of benzene. Investigations into the mechanism of this reaction implied
the existence of a hydroperoxo-nickel species; a result supported by independent lines
of reactivity. The reaction of [TptBu,Me]Ni(O2) with DHA resulted in the unexpected
generation of 9,10-anthraquinone. Mechanistic interrogation of the reaction suggests a
complex, multistep process involving H-atom abstraction, radical rebound and the
agency of both oxo-nickel and hydroperoxo-nickel intermediates. A kinetic analysis of
the reaction reveals a KIE (kH/kD) of at least 12. Activation parameters, ΔH‡ = 14(1)
kcal/mol and ΔS‡ = -117(10) J mol-1K-1, were determined from an Eyring analysis. A
subsequent study of the reaction between [TptBu,Me]Ni(O2) and xanthene show very
strong similarities to the reaction with DHA and argues for the involvement of an
analogous mechanistic pathway. Lastly, [TptBu,Me]Ni(O2) was found to perform
aldehyde deformylation when reacted with 2-phenylpropionaldehyde (2-PPA). In
addition to the organic product, acetophenone, the reaction resulted in the formation of
several divalent nickel derivatives. With regard to the nickel containing products, 1H
NMR and LIFDI analysis has resulted in the positive identification of metallacycle,
hydroxo and formate complexes. Additional nickel derivatives remain unidentified.