The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to “traditional” precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal–ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

The synergistic combination of NiH-catalyzed alkene isomerization with nickel-catalyzed cross-coupling has yielded a general protocol for the synthesis of a wide range of structurally diverse 1,1-diarylalkanes in excellent yields and high regioselectivities from readily accessible olefin starting materials. Furthermore, the practicality and synthetic flexibility of this approach is highlighted by the successful employment of isomeric mixtures of olefins for regioconvergent arylation.

Mild and Regioselective Benzylic C–H Functionalization: Ni-Catalyzed Reductive Arylation of Remote and Proximal Olefins

A highly efficient strategy for remote reductive cross-electrophile coupling has been developed through the ligand-controlled nickel migration/arylation. This general protocol allows the use of abundant and bench-stable alkyl bromides and aryl bromides for the synthesis of a wide range of structurally diverse 1,1-diarylalkanes in excellent yields and high regioselectivities under mild conditions. We also demonstrated that alkyl bromide could be replaced by the proposed olefin intermediate while using n-propyl bromide/Mn0 as a potential hydride source.

Remote Migratory Cross-Electrophile Coupling and Olefin Hydroarylation Reactions Enabled by in Situ Generation of NiH.

A direct hydroalkylation of disubstituted alkynes with unfunctionalized ethers and amides was achieved in an atom-efficient and additive-free manner through the synergistic combination of photoredox and nickel catalysis. The protocol was effective with a wide range of internal alkynes, providing products in a highly selective fashion. Notably, the observed regioselectivity is complementary to conventional radical addition processes. Mechanistic investigations suggest that the photoexcited iridium catalyst facilitated the nickel activation via single-electron transfer.

Photoinduced Nickel-Catalyzed Chemo- and Regioselective Hydroalkylation of Internal Alkynes with Ether and Amide α-Hetero C(sp3)–H Bonds

Nickel plays an important role in areas as diverse as metallurgy, magnetism and biology as well as in chemical applications such as the catalytic transformation of organic substrates. Despite nickel's importance, the investigation of its compounds in various oxidation states remains uneven and those in the +1 oxidation are less common than those in the neighboring 0 and +2 oxidation states. Nonetheless, in recent years, the volume of work on Ni(i) complexes has increased to the extent that they can be no longer regarded as rare. This review focuses on the syntheses and structures of Ni(i) complexes and shows that they display a range of structures, reactivity and magnetic behavior that places them in the forefront of current nickel chemistry research.

Complexes of Ni(I): a “rare” oxidation state of growing importance

Nickel hydride complexes, defined herein as any molecules bearing a nickel hydrogen bond, are crucial intermediates in numerous nickel-catalyzed reactions. Some of them are also synthetic models of nickel-containing enzymes such as [NiFe]-hydrogenase. The overall objective of this review is to provide a comprehensive overview of this specific type of hydride complexes, which has been studied extensively in recent years. This review begins with the significance and a very brief history of nickel hydride complexes, followed by various methods and spectroscopic or crystallographic tools used to synthesize and characterize these complexes. Also discussed are stoichiometric reactions involving nickel hydride complexes and how some of these reactions are developed into catalytic processes.

Nickel Hydride Complexes

Metathesis reactivity of bis(phosphinite) pincer ligated nickel chloride, isothiocyanate and azide complexes

Dehydrogenative Coupling of Aldehydes with Alcohols Catalyzed by a Nickel Hydride Complex

Reduction of CO2 to CO, formic acid or its salts, methanol, methane, or carboxylic acids is of high importance to carbon management, energy storage, and sustainable synthesis. The evolution of the chemistry of pincer compounds has provided organometallic chemists with powerful tools to develop efficient catalysts for transforming CO2 to value-added products. This chapter focuses specifically on the utilization of various pincer complexes as catalysts for the hydrogenation, hydrosilylation, hydroboration, alkylation, allylation, or electrochemical reduction of CO2. Because of the uniqueness of pincer ligands, many of these complexes have catalytic performance superior to the nonpincer systems. Also described here are examples of stoichiometric reduction of CO2 by pincer complexes, especially for those metal systems that catalytic processes have yet to be established.

Nathan A. Eberhardt

Papers

Nickel Hydride Complexes

Metathesis reactivity of bis(phosphinite) pincer ligated nickel chloride, isothiocyanate and azide complexes

Dehydrogenative Coupling of Aldehydes with Alcohols Catalyzed by a Nickel Hydride Complex

Reduction of CO2 Mediated or Catalyzed by Pincer Complexes

Nickel Hydride Complexes