C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.

3d Transition Metals for C-H Activation.

Iron Catalysis in Organic Synthesis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term “smart” is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Smart N-Heterocyclic Carbene Ligands in Catalysis

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

N-Heterocyclic Carbene Complexes in C–H Activation Reactions

This review covers the advance in the development of Fe, Co, and Ni catalysts for the alkene hydrosilylation reaction, as well as the related dehydrogenative silylation reaction. The hydrosilylation of alkene is an important reaction for the synthesis of alkylsilanes that has widespread applications in numerous silicon-based materials, and for decades, this transformation has been relying on the use of Pt catalysts. Recently, the high abundance and low cost, coupled with the environmentally benign nature of the base metals have stimulated enormous research on the development of first-row transition-metal catalysts as replacements for the precious Pt catalysts. Several base-metal catalysts which have emerged during the past 5 years offer high activity, broad substrate scope, and excellent regioselectivity. Both of the anti-Markovnikov and the unusual Markovnikov additions can be achieved in a high degree of regioselectivity. The reactions of acyclic internal olefins catalyzed by the base-metal catalysts re...

Advances in Base-Metal-Catalyzed Alkene Hydrosilylation

A three-coordinate cobalt(I) complex exhibits high catalytic efficiency and selectivity as well as good functional group compatibility in alkyne hydrosilylation. [Co(IAd)(PPh3)(CH2TMS)] (1) (IAd = 1,3-diadamantylimidazol-2-ylidene) facilitates regio- and stereoselective hydrosilylation of terminal, symmetrical internal, and trimethylsilyl-substituted unsymmetrical internal alkynes to produce single hydrosilylation products in the forms of β-(E)-silylalkenes, (E)-silylalkenes, and (Z)-α,α-disilylalkenes, respectively, in high yields. The comparable catalytic efficiency and selectivity of the Co(I) silyl complex [Co(IAd)(PPh3)(SiHPh2)] that was prepared from the reaction of 1 with H2SiPh2, and the isolation of an alkyne Co(I) complex [Co(IAd)(η2-PhC≡CPh)(CH2TMS)] from the reaction of 1 with the acetylene, point out a modified Chalk–Harrod catalytic cycle for these hydrosilylation reactions. The high selectivity is thought to be governed by steric factors.

Regio- and Stereoselective Hydrosilylation of Alkynes Catalyzed by Three-Coordinate Cobalt(I) Alkyl and Silyl Complexes

The slow magnetic relaxation typical for single-ion magnets has been known for certain low-coordinate 3d metal complexes with d6, d7, and d9 electronic configurations, but never for d8 complexes. Herein, we report a study on two-coordinate d8 cobalt(i)-N-heterocyclic carbene complexes, for which slow magnetic relaxation behavior was observed for [Co(IMes)2][BPh4] (IMes: 1,3-dimesitylimidazol-2-ylidene) under an applied dc field. The system represents the first d8 single-ion magnet, and features a fitted energy barrier of Ueff = 21.3 cm-1 and pre-exponential factor of τ0 = 6.6 × 10-6 s. The analog two-coordinate cobalt(i) complexes with different NHC ligands, [Co(sIMes)2][BPh4] (sIMes: 1,3-dimesitylimidazolin-2-ylidene) and [Co(IAd)2][BArF4] (IAd: 1,3-dimesitylimidazol-2-ylidene; BArF4: tetra(3,5-ditrifluoromethylphenyl)borate), do not show such single-ion magnet behaviour. Ab initio calculations imply that the dihedral angle between the two NHC planes and the degree of unsaturation of the NHC ligands can dramatically alter the D value of the two-coordinate cobalt(i)-NHC ions, possibly via changing of the Co-NHC π-interactions, and hence affect the spin-orbit coupling splitting.

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Observation of the single-ion magnet behavior of d8 ions on two-coordinate Co(I)–NHC complexes

Silyl-donor functionalized NHC-cobalt complexes are developed and used as catalyst for the hydrosilylation of 1-octene with PhSiH3 (best catalyst is shown).

Anchoring of Silyl Donors on a N-Heterocyclic Carbene through the Cobalt-Mediated Silylation of Benzylic C ? H Bonds

The synthesis and structures of the first SiII -donor supported manganese(II) complexes [L1]MnCl2 , [L2]MnCl2 , and [L3]2 MnCl2 are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1 mol % loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl2 shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98 %) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH2 , NO2 , and OH groups at the phenyl group of 1-phenyl substituted alkynes.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201800160

Zhenbo Mo

Papers

Regio- and Stereoselective Hydrosilylation of Alkynes Catalyzed by Three-Coordinate Cobalt(I) Alkyl and Silyl Complexes

Observation of the single-ion magnet behavior of d8 ions on two-coordinate Co(I)–NHC complexes

Anchoring of Silyl Donors on a N-Heterocyclic Carbene through the Cobalt-Mediated Silylation of Benzylic C ? H Bonds

Stereoselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins with N-Heterocyclic Silylene–Manganese Catalysts

Intramolecular C(sp3)–H Bond Activation Reactions of Low-Valent Cobalt Complexes with Coordination Unsaturation