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 1,3diiodobenzene 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.

This report describes the C−O, C−N, and C− halogen functionalization of the Ni III −Ar moiety stabilized within a pincer framework that serves as a model system for studying C−heteroatom coupling reactions promoted by highvalent Ni compounds. Treating van Koten's pincer complex (NCN)Ni III Br 2 under a nitrogen atmosphere with water, 1°or 2°alcohols, 1°amines, HCl, or HBr results in heterofunctionalization at the ipso-C of the pincer ligand's aryl moiety. The yields of these heterofunctionalizations are generally <50%, which has been attributed to the occurrence of a comproportionation reaction between the trivalent precursor and a Ni I species arising from the reductive elimination step in the functionalization process. Other side-reactions include a C−OH coupling with residual water and C−H coupling (net protonation) that is prevalent with mineral acids, some alcohols, and aqueous NH 3 . Kinetic measurements have established that the reaction with MeOH is first-order with respect to [(NCN)Ni III Br 2 ], and a kinetic isotope effect of 0.47 has been obtained for functionalization with CH 3 OH/CD 3 OD. These and other observations have allowed us to propose two different mechanistic postulates for the involvement of trivalent intermediates in the functionalization reactions under discussion. Tetravalent species such as [(NCN)Ni IV Br 2 ] + can be generated in situ under strongly oxidative conditions and they do promote C−Br coupling, but such species play no role in the C−heteroatom coupling reactions under nonoxidative conditions.

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The growing popularity of metal-promoted C-H functionalization methodologies has led to increased efforts aimed at improving our understanding of the mechanistic underpinnings of C-C or C-heteroatom bond forming steps. One...

This report presents the synthesis of new mono-and dicationic NCN-Ni III complexes and describes their reactivities with protic substrates. (NCN is the pincer-type ligand k N , k C , k N-2,6-(CH2NMe2)2-C6H3.