Bi-Oxazoline (biOx) has emerged as an effective ligand framework for promoting nickel-catalyzed cross-coupling, cross-electrophile coupling, and photoredox-nickel dual catalytic reactions. This report fills the knowledge gap of the organometallic reactivity of (biOx)Ni complexes, including catalyst reduction, oxidative electrophile activation, radical capture, and reductive elimination. The biOx ligand displays no redox activity in (biOx)Ni(I) complexes, in contrast to other chelating imine and oxazoline ligands. The lack of ligand redox activity results in more negative reduction potentials of (biOx)Ni(II) complexes and accounts for the inability of zinc and manganese to reduce (biOx)Ni(II) species. On the basis of these results, we revise the formerly proposed "sequential reduction" mechanism of a (biOx)Ni-catalyzed cross-electrophile coupling reaction by excluding catalyst reduction steps.
Low-valent
organonickel radical complexes are common intermediates
in cross-coupling reactions and metalloenzyme-mediated processes.
The electronic structures of N-ligand supported nickel
complexes appear to vary depending on the actor ligands and the coordination
number. The reduction products of a series of divalent (pyrox)Ni complexes
establish the redox activity of pyrox in stabilizing electron-rich
Ni(II)–alkyl and −aryl complexes by adopting a ligand-centered
radical configuration. The reduced pyrox imparts an enhanced trans-influence. In contrast, such redox activity was not
observed in a (pyrox)Ni(I)–bromide species. The excellent capability
of pyrox in stabilizing electron-rich Ni species resonates with its
proclivity in promoting the reductive activation of C(sp3) electrophiles in cross-coupling reactions.
During
the preparation of V{N(SiMe3)2}3 (1), a discrepancy between the violet color that we observed
and the brown color previously reported prompted further investigation
of this compound. As a result, a new spectroscopic study and a full
structural characterization are presented. The synthesis, spectroscopy,
and structural characteristics of its reduced salt, [K(18-crown-6)(Et2O)2][V{N(SiMe3)2}3] (2), and its chromium congener, [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (3), are also described. The 1H NMR spectra
for 1–3 and Cr{N(SiMe3)2}3 as well as their cyclic voltammograms
are also reported.
The synthesis of the first linear coordinated Cu(II) complex Cu{N(SiMe3 )Dipp}2 (1 Dipp=C6 H5 -2,6Pr(i) 2 ) and its Cu(I) counterpart [Cu{N(SiMe3 )Dipp}2 ](-) (2) is described. The formation of 1 proceeds through a dispersion force-driven disproportionation, and is the reaction product of a Cu(I) halide and LiN(SiMe3 )Dipp in a non-donor solvent. The synthesis of 2 is accomplished by preventing the disproportionation into 1 by using the complexing agent 15-crown-5. EPR spectroscopy of 1 provides the first detailed study of a two-coordinate transition-metal complex indicating strong covalency in the Cu-N bonds.
The synthesis and
spectroscopic, structural, and magnetic characterization of the quasi-linear
metal(II) bis(amides) M{N(SiPr
i
3)Dipp}2 [Dipp = C6H3-2,6-Pr
i
3; M = Fe (1), Co
(2), or Zn (3)] are described. The magnetic
data demonstrate the impact of metal ligand π-interactions on
the magnetic properties of these two-coordinate transition metal amides.
Disproportionation of the copper(I) amide species featuring the ligand
-N(SiPr
i
3)Dipp resulted in
the decomposition product [(Pr
i
3Si)N(c-C6H2-2,6-Pr
i
2)]2 (4). The
electron paramagnetic resonance spectrum of the unstable two-coordinate
Cu{N(SiPr
i
3)Dipp}2 displays significantly less Cu–N bond covalency than the
stable two-coordinate copper(II) species Cu{N(SiMe3)Dipp}2. The testing of -N(SiPr
i
3)Dipp and a range of other, related bulky amide ligands with
their copper derivatives highlights the peculiar combination of steric
and electronic properties of the Wigley ligand -N(SiMe3)Dipp that enable it to stabilize the unique two-coordinate copper(II)
complex Cu{N(SiMe3)Dipp}2.
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