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-NCN pz (κ N,κ C,κ N ′ -1,3-bis(pyrazole),5-R-C 6 H 2; 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)NiBr 2 and NEt 3 to give the complexes (NCN pz )NiBr ( 1 ) and (MeO-NCN pz )NiBr ( 2 ) via C–H nickelation. The aryloxide derivative (NCN pz )Ni(OAr) ( 3; Ar = 2,6- t -Bu 2 -4-Me-OC 6 H 2 ) 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 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 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 H 2 O 2 (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 (NCN pz )NiBr·I 2, an iodine adduct displaying weak Br–I interactions. Heating 1 in EtOH in air generated NC(OEt)N pz . This ligand derivatization could be extended to other alcohols (MeOH, i -PrOH, CF 3 CH 2 OH) 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.