One to Find Them All: A General Route to Ni(I)–Phenolate Species

Abstract

The past 20 years have seen an extensive implementation of nickel in homogeneous catalysis through the development of unique reactivity not easily achievable by using noble transition metals. Many catalytic cycles propose Ni­(I) complexes as potential reactive intermediates, yet the scarcity of nickel­(I) precursors and the lack of a general, non-ligand-specific protocol for their synthesis have hampered progress in this field of research. This has in turn also limited the access to novel, well-defined Ni­(I) species for the development of new catalytic reactions. Herein, we report a simple, general route to access a wide variety of Ni­(I)–phenolate complexes via an unusual example of an olefinic Ni­(I) complex, [Ni­(COD)­(OPh*)] (COD = 1,5-cyclooctadiene, OPh* = O­( t Bu)3C6H2). This route has proven to be highly efficient for several coordination numbers and ligand classes enabling access to the following complexes: [Ni­(IPr)­(OPh*)] (IPr = 1,3-bis­(2,6-diisopropyl­phenyl)­imidazol-2-ylidene), [Ni­(dcype)­(OPh*)] (dcype = 1,2-bis­(dicyclohexyl­phosphino)­ethane), [Ni­(dppe)­(OPh*)] (dppe = 1,2-bis­(diphenyl­phosphino)­ethane), and [Ni­(terpy)­(OPh*)] (terpy = 2,2′:6′,2″-terpyridine). Moreover, reacting [Ni­(dcype)­(OPh*)] with trimethylsilyl triflate has led to the isolation of a unique example of a cationic binuclear Ni­(I)–arene complex. All these complexes have been characterized by single-crystal X-ray, DFT, and EPR analyses, thus providing crucial experimental and theoretical information about their coordination environment and confirming a d9 electronic structure for all complexes involved. Overall, this new synthetic approach offers exciting opportunities for the discovery of new stoichiometric and catalytic reactivity as well as the mechanistic elucidation of Ni-based catalytic cycles.

Mechanisms of the Nickel-Catalysed Hydrogenolysis and Cross-Coupling of Aryl Ethers

“…Despite experimental validation that the presence of excess base is necessary for improved yields, kinetic studies reveal a 0 th order dependence in NaO t Bu indicating it is not involved in the rate-limiting step of the catalytic cycle. Instead, a first order dependence in the concentration of both (SIPr)Ni(η 6 arene) and diaryl ether was determined and this is consistent with C-O bond cleavage and oxidative addition to Ni(0) being rate determining.…”

“…3 Mechanisms involving a Ni(I)/Ni(III) catalytic cycle have been identified in several coupling reactions, 4,5 and significant effort has been made to develop efficient routes to Ni(I) and Ni(III) species. 6,7 Furthermore, nickel catalysts can activate more challenging and inert substrates outside the scope of Pd allowing unconventional, yet readily available, electrophiles to serve as viable cross-coupling reaction partners. 8,9 As early as 1979, Wenkert discovered that aryl ethers can undergo C aryl -O bond cleavage and Kumada-Corriu type cross-coupling with Grignard reagents to furnish biaryls (Scheme 1).…”

Unmasking the constitution and bonding of the proposed lithium nickelate “Li3NiPh3(solv)3”: revealing the hidden C6H4ligand

“…43 The desired amine was obtained in rather lower yields than previously reported pre-catalysts (28% with 2, 22% with 5) or with Ni(COD) 2 alone (52%). 43,44 This is likely due to the nickelates lacking the robust NHC or bidentate phosphine ligands necessary for nickel intermediates to withstand the harsh reaction conditions. Compound 5 is also a competent pre-catalyst in the Ni-catalysed cross-coupling of 2-methoxynaphthalene with PhLi (46% yield of 2-phenylnaphthalene), albeit with a slight reduction in yield compared to 2 (Scheme 4c).…”

The Journey of Ni(I) Chemistry

“…Hillhouse , [34] Tilley , [35] Grützmacher , [36] and many others [25,37] have left an important imprint, which has been documented in detail by Power and Lin in a recent review [25] . We have recently also contributed to this field with a synthesis of differently coordinated Ni(I) species, thus providing a unified route to Ni(I)‐complexes, which is easily applicable to many ligand systems [38] …”

“…[25] We have recently also contributed to this field with a synthesis of differently coordinated Ni(I) species, thus providing a unified route to Ni(I)complexes, which is easily applicable to many ligand systems. [38] Aside from this, an immense contribution came from other researchers who focused on the development of Ni-catalyzed organic transformations, in particular within the realm of cross-coupling reactions. [19,21] Although a generic catalytic crosscoupling cycle can usually be divided into three steps: oxidative addition of an electrophile, transmetalation with a nucleophile, and reductive elimination of the product, [2] in the case of nickel this is complicated by the small energy gap in redox potentials that can easily trigger alternative paths such as disproportionation.…”