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

Abstract

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.

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

“…An innersphere mechanism has been proposed for the vast majority of carbon-carbon and C(sp 2 )-heteroatom coupling reactions at Pt IV , [19] Pd IV , [4] Pd III , [20] Ni IV , [21] and Ni III centers. [3,22] In contrast, the Ni III analogue 1-Ni III+ reacts to selectively form the C À C coupled product 3 in either the presence or absence of the acetate nucleophile.T hese results demonstrate that the oxidation state of high-valent Ni can play ak ey role in dictating both the mechanism and selectivity of the favored reductive elimination process in these systems.W hile this study focuses on model complexes,o ngoing investigations in our lab are probing the generality of obtaining complementary bond-forming reactions by manipulating the oxidation state of high-valent Ni intermediates.I ft hese observations prove general, they are likely to find broader applications in catalysis.…”

A Novel Family of Polyiodo‐Bromoantimonate(III) Complexes: Cation‐Driven Self‐Assembly of Photoconductive Metal‐Polyhalide Frameworks

“…For the all‐polyhalide complexes, single examples are known for polyiodides and polybromides . Coordination of mixed polyhalides ((BrI 2 ) − , (ClBr 2 ) − ) was reported mostly for heteroligand complexes …”

“…[12][13][14][15][16][17] Coordination of mixed polyhalides( (BrI 2 ) À , (ClBr 2 ) À )w as reported mostly for heteroligand complexes. [18][19][20][21] Very recently, we discovered as eries of Bi III polybromide complexes,f eaturing ar emarkable structurald iversity and con-taining{ Br x }u nits of variable nuclearity (upt o9 ). [22] In all cases, the compounds were obtained by as traightforward method: "[BiBr 6 ] 3À + Br 2 + (cation)Br x + HBr".…”

Design, synthesis, and carbon-heteroatom coupling reactions of organometallic nickel(IV) complexes

“…Notably, C(sp 3 )-heteroatom coupling reactions of this type are rare in organometallic chemistry (20)(21)(22)(30)(31)(32)(33)(34), and most previously reported examples involve second or third row metal centers. In addition, the observation of selective C(sp 3 )-heteroatom coupling is complementary to the reactivity of other oxidation states of Ni, where C(sp 2 )-heteroatom bond-formation has significant precedent (35)(36)(37).…”