Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon–Carbon Bond Formation Reactions

Abstract

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic Ni intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated Ni-dialkyl complex and show that this species is involved in stoichiometric and catalytic C-C bond formation reactions. Interestingly, the rate of C-C bond formation from a Ni center is enhanced in the presence of an oxidant, suggesting the involvement of transient Ni species. Indeed, such a Ni species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both Ni and Ni species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.

Aerobic C–C and C–O bond formation reactions mediated by high-valent nickel species

“…45,46 With these ligands a series of cycloneophyl complexes were synthesized through a ligand exchange of (py) 2 Ni II (cycloneophyl). 47,48 The orange Ni II complex ( Me N4)Ni II (cycloneophyl), 1, was prepared in a 72% yield (Scheme 1), 49 while the yellow Ni II complex ( TsMe N4)Ni II (cycloneophyl), 2, was prepared in 81% yield and fully characterized. 50 The cyclic voltammetry (CV) characterization of 2 reveals two main oxidation events at À990 mV and 750 mV vs. Fc + /Fc, along with a smaller oxidation wave at 70 mV (Table 1).…”

Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a Novel Diphosphonite Pyridinophane Ligand

“…In the past several years we have reported the isolation and characterization of mononuclear organometallic Ni III complexes stabilized by tetradentate R N4 ligands ( R N4 = N,N'-dialkyl-2,11-diaza [3.3](2,6)pyridinophane, R = Me, i Pr, or t Bu) as well as its C-donor derivative R N3C -ligands (R = t Bu, tert-butyl or Np, neopentyl) that can undergo C-C and C-heteroatom bond formation reactions. [26][27][28][29][30][31][32][33][34][35][36] These results suggest that Ni III complexes are more common than previously anticipated, and thus it prompted us to now target the low-valent Ni I species using a slightly modified ligand system. Inspired by the many examples of PONOP-type pincer metal complexes, [37][38][39][40][41] we have developed the N-and P-donor tetradentate ligand N2P2 (N2P2 = P,P'ditertbutyl-2,11-diphosphonito [3.3](2,6)pyridinophane) and investigated the redox properties and reactivity of its Ni complexes.…”

Catalytic Applications of Pyridine‐Containing Macrocyclic Complexes

“…When less bulky ligand 2b was used instead, a square‐planar dialkyl complex of formula [Ni II (L‐N 4 Me 2 )Me 2 ] was isolated; it can in turn be oxidised to a stable octahedral [Ni III (L‐N 4 Me 2 )Me 2 ] + cationic complex. This last was a competent catalyst for Kumada cross‐coupling …”