This communication describes the synthesis and reactivity of Ni(IV)(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4'-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni(IV) complexes can be accessed under mild conditions via the oxidation of Ni(II) precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni(IV) intermediates undergo high yielding aryl-CF3 bond-forming reductive elimination. These studies support the potential viability of Ni(IV) intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.
This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic Ni complexes of general structure TpNi(R)(R) (Tp = tris(pyrazolyl)borate). With appropriate selection of the R and R ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these Ni compounds undergo C(sp)-C(sp) or C(sp)-C(sp) bond-forming reactions that are challenging at lower oxidation states of nickel.
This paper describes
the one-electron interconversions of isolable
NiIII and NiIV complexes through their reactions
with carbon-centered radicals (R•). First, model NiIII complexes are shown to react with alkyl and aryl radicals to afford
NiIV products. Preliminary mechanistic studies implicate
a pathway involving direct addition of a carbon-centered radical to
the NiIII center. This is directly analogous to the known
reactivity of NiII complexes with R•, a step that
is commonly implicated in catalysis. Second, a NiIV–CH3 complex is shown to react with aryl and alkyl radicals to
afford C–C bonds via a proposed SH2-type mechanism.
This pathway is leveraged to enable challenging H3C–CF3 bond formation under mild conditions. Overall, these investigations
suggest that NiII/III/IV sequences may be viable redox
pathways in high-oxidation-state nickel catalysis.
This perspective describes recent developments and future directions in bioinorganic chemistry and biomimetic catalysis centered at metal–organic framework secondary building units.
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