Synthesis and Reactivities of New NCN-Type Pincer Complexes of Nickel

This
report describes the preparation, characterization, and reactivities
of a new family of Ni­(II) complexes based on the pincer-type ligands
R-NCNpz (κN,κC,κN′-1,3-bis­(pyrazole),5-R-C6H2; R = H, OMe).
Ullman coupling of pyrazole with 1,3-diiodobenzene or 1,3-dibromo-5-methoxybenzene
gave the compounds R-NC­(H)­Npz, which were refluxed in xylene
with (i-PrCN)­NiBr2 and NEt3 to give the complexes (NCNpz)­NiBr (1) and
(MeO-NCNpz)­NiBr (2) via C–H nickelation.
The aryloxide derivative (NCNpz)­Ni­(OAr) (3; Ar = 2,6-t-Bu2-4-Me-OC6H2) was prepared by treating the bromo precursor 1 with NaOAr, whereas the analogous reaction with NaOEt gave instead
the protonated or ethoxy-functionalized ligand NC­(H)­Npz (major) and NC­(OEt)­Npz (minor). The new complexes 1–3 were fully characterized, including
solid-state structures. Attempted oxidation of these complexes failed
to give the target trivalent derivatives, leading instead to intractable,
deeply colored Ni-containing solids, various ligand-derived side products,
or an unusual I2 adduct. For example, treatment of 1 or 2 with N-bromosuccinimide
gave sparingly soluble dark solids that appear to be paramagnetic,
whereas 3 reacted with Br2 to give short-lived
dark intermediates that decomposed over seconds to give ArOH. Reaction
of the bromo complexes with aqueous H2O2 (30%)
gave the functionalized ligands Br-NC­(OH)­Npz and NC­(OH)­Npz (from 1) or MeO-NC­(OH)­Npz (from 2), whereas 1 reacted with I2 to generate
(NCNpz)­NiBr·I2, an iodine adduct displaying
weak Br–I interactions. Heating 1 in EtOH in air
generated NC­(OEt)­Npz. This ligand derivatization could
be extended to other alcohols (MeOH, i-PrOH, CF3CH2OH) and amines (morpholine, cyclohexylamine,
aniline). Possible mechanistic scenarios for the observed oxidative,
Ni-promoted Cipso–X bond formation reactions are
discussed in the context of relevant literature precedents.