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 (kappa(N),kappa(C),kappa(N)'-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)N-pz, which were refluxed in xylene with (i-PrCN)NiBr2 and NEt3 to give the complexes (NCNpz)NiBr (1) and (MeO-NCNpz)NiBr (2) via CH nickelation. The aryloxide derivative (NCNpz)Ni(OAr) (3; Ar = 2,6-t-Bu-2-4-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)N-pz (major) and NC(OEt)N-pz (minor). The new complexes 13 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 I-2 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 Br-2 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)N-pz and NC(OH)N-pz (from 1) or MeO-NC(OH)N-pz (from 2), whereas 1 reacted with I-2 to generate (NCNpz)NiBr.I-2, an iodine adduct displaying weak BrI interactions. Heating 1 in EtOH in air generated NC(OEt)N-pz. 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 C-ipso-X bond formation reactions are discussed in the context of relevant literature precedents.