Stereoelectronic Effects in C–H Bond Oxidation Reactions of Ni(I) N-Heterocyclic Carbene Complexes

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Abnormal N-heterocyclic Carbene Based Ni(II) π-allyl Complex towards Molecular Oxygen Activation

Abnormal N-heterocyclic carbene (aNHC) based Ni(II) π-allyl complexes (3 and 4) were synthesized starting from a Ni(0) precursor. These complexes were characterized by NMR spectroscopy, single-crystal X-ray crystallography (4) and elemental analysis data. The underlying mechanism for the formation of Ni(II) η³ -allyl complexes from a Ni(0) precursor on treatment with a free abnormal N-heterocyclic carbene in absence of any external additive or oxidant was unraveled. Later, complex 3 was exposed to O₂ gas under ambient pressure resulting in molecular oxygen activation to form a μ-hydroxo bridged Ni(II) dimer.

1-Butyl-3-methylimidazolium tribromido(triphenylphosphane-κP)nickelate(II) butan-1-ol hemisolvate

The solvated title salt, (C8H15N2)[NiBr3(P(C6H5)3)]·0.5C4H10O, was obtained in the form of single crystals directly from the reaction mixture. The molecular structure consists of separated 1-butyl-3-methylimidazolium cations, tribromido(triphenylphosphane)nickelate(II) anions and half a solvent molecule of 1-butanol, all connected via multiple hydrogen contacts to form a three-dimensional network. The co-crystallized 1-butanol molecule is disordered and adopts two orientations. The central C—C bonds of both orientations are located on an inversion centre (Wyckoff site 2b of space group P21/n). Thereby, each orientation has again two orientations with the OH group being located either on one or the other side of the C4 alkyl chain. The dried solvent-free compound exhibits a relatively low melting point (m.p. = 412 K).

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Metal complexes with oxygen-functionalized NHC ligands: synthesis and applications

Ligand design has met with considerable success with both categories of hybrid ligands, which are characterized by chemically different donor groups, and of N-heterocyclic carbenes (NHCs). Their spectacular development and diversity are attracting worldwide interest and offers almost unlimited diversity and potential in e.g. coordination/organometallic main group and transition metal chemistry, catalysis, medicinal chemistry and materials science. This review aims at providing a comprehensive update on a specific class of ligands that has enjoyed much attention in the past few years, at the intersection between the two categories mentioned above, that of hybrid NHC ligands in which the functionality associated with the carbene donor is of the oxygen-donor type. For each type of oxygen-donor present in such chelating (Section 1) or bridging (Section 2) hybrid ligands, we will examine the synthesis, structures and reactivity of their metal complexes and their applications, with a special focus on homogeneous catalysis (Section 3). Thus, hydrogenation, C-H bond activation, C-C, C-N, C-O bond formation, hydrolysis of silanes, oligomerization, polymerization, metathesis, hydrosilylation, C-C bond cleavage, acceptorless dehydrogenation, dehalogenation/hydrogen transfer, oxidation and reduction reactions will be successively presented in a tabular manner, to facilitate an overview and a rapid identification of the relevant publications describing which metals associated with a given oxygen functionality are most suitable. The literature coverage includes the year 2015.

Quantifying and understanding the steric properties of N-heterocyclic carbenes

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include (a) the percent buried volume (%V_bur), which provides a convenient single number to measure steric impact and (b) steric maps, which provide a graphical representation of the steric profile of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs, from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare their steric properties.

Unsymmetrical NCN-pincer mononuclear and dinuclear nickel(ii) complexes of N-heterocyclic carbene (NHC): synthesis, structure and catalysis for Suzuki-Miyaura cross-coupling

This report describes the synthesis and characterization of a family of unsymmetrical NCN pincer Ni(ii) complexes 2-8 with NHC-triazole arms. All of these complexes have been fully characterized by X-ray single crystal analysis, NMR spectroscopy, and elemental analysis. Complexes 2 and 4-6 were square planar with a chloride trans to the carbene carbon atoms. Complex 3 was a paramagnetic octahedral complex with its central metal surrounded by two NCN pincer ligands. Complexes 7 and 8 contain [(NHC)₂Ni₂-OH] moieties bearing a OH bridge. Both the [(NHC)₂Ni₂-OH] complexes 7 and 8 and [(NC_NHCN)Ni-Cl] complexes 2 and 4-6 were synthesized similarly via the reactions of the in situ formed Ag-NHCs from the corresponding imidazolium salts with [NiCl₂(PPh₃)₂]. The catalytic activities of all complexes for Suzuki-Miyaura cross-coupling were examined. Under the optimized conditions, complex 4 was active in the Suzuki-Miyaura cross-coupling reactions of aryl iodides and aryl bromides at 110 °C. Aryl chlorides were successfully coupled in the presence of triphenylphosphine as an additive.

Mono- and dinuclear Ni(i) products formed upon bromide abstraction from the Ni(i) ring-expanded NHC complex [Ni(6-Mes)(PPh3)Br]

New T- and Y-shaped Ni(i) complexes are reported and analysed by DFT and EPR.

Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Potassium graphite reduction of the half-sandwich Ni(II) ring-expanded diamino/diamidocarbene complexes CpNi(RE-NHC)Br gave the Ni(I) derivatives CpNi(RE-NHC) (where RE-NHC = 6-Mes (1), 7-Mes (2), 6-MesDAC (3)) in yields of 40%-50%. The electronic structures of paramagnetic 1-3 were investigated by CW X-/Q-band electron paramagnetic resonance (EPR) and Q-band ¹H electron nuclear double resonance (ENDOR) spectroscopy. While small variations in the g-values were observed between the diaminocarbene complexes 1 and 2, pronounced changes in the g-values were detected between the almost isostructural species (1) and diamidocarbene species (3). These results highlight the sensitivity of the EPR g-tensor to changes in the electronic structure of the Ni(I) centers generated by incorporation of heteroatom substituents onto the backbone ring positions. Variable-temperature EPR analysis also revealed the presence of a second Ni(I) site in 3. The experimental g-values for these two Ni(I) sites detected by EPR in frozen solutions of 3 are consistent with resolution on the EPR time scale of the disordered components evident in the X-ray crystallographically determined structure and the corresponding density functional theory (DFT)-calculated g-tensor. Q-band ¹H ENDOR measurements revealed a small amount of unpaired electron spin density on the Cp rings, consistent with the calculated SOMO of complexes 1-3. The magnitude of the ¹H A values for 3 were also notably larger, compared to 1 and 2, again highlighting the influence of the diamidocarbene on the electronic properties of 3.

N,N'-Diamidocarbenes: Isolable Divalent Carbons with Bona Fide Carbene Reactivity

Since the first reported isolation of a carbene just over a quarter century ago, the study of such compounds-including stable derivatives-has flourished. Indeed, N-heterocyclic carbenes (NHCs), of which imidazolylidenes and their derivatives are the most pervasive subclass, feature prominently in organocatalysis, as ligands for transition metal catalysts, and as stabilizers of reactive species. However, imidazolylidenes (and many other NHCs) typically lack the reactivity characteristic of electrophilic carbenes, including insertion into unactivated C-H bonds, participation in [2 + 1] cycloadditions, and reaction with carbon monoxide. This has led to debates over whether NHCs are truly carbenic in nature or perhaps better regarded as ylides. The fundamental and synthetic utility of transformations that involve electrophilic carbenes has motivated our group and others to expand the reactivity of NHCs and other stable carbenes to encompass electrophilic carbene chemistry. These efforts have led to the development of the diamidocarbenes (DACs), a stable and unique subset of the NHCs that feature carbonyl groups inserted into the N-heterocyclic scaffold. To date, crystalline five-, six-, and seven-membered DACs have been prepared and studied. Unlike imidazolylidenes, which are often designated as prototypical NHCs, the DACs exhibit a reactivity profile similar to that of bona fide carbenes, reactive species that are less "tamed" by heteroatom π conjugation. The DACs engage in [2 + 1] cycloadditions with electron-rich or -poor alkenes, aldehydes, alkynes, and nitriles, and doing so in a reversible manner in some cases. They also react with isonitriles, reversibly couple to CO, and mediate the dehydrogenation of hydrocarbons. Such rich chemistry may be rationalized in terms of their ambiphilicity: DACs are nucleophilic, as required for some of the reactions above, yet also have electrophilic character, as evidenced by their insertions into unactivated N-H and C-H bonds, including nonacidic derivatives. As will become clear, such reactivity is unique among isolable carbenes. DAC chemistry is expected to find applications in synthesis, dynamic covalent chemistry, and catalysis. For example, the hydrolysis of DAC-derived diamidocyclopropanes and -propenes affords carboxylic acids and cyclopropenones, respectively. These new hydrocarboxylation and carbonylation methodologies are significant in that they represent alternatives to processes that typically involve precious metals and gaseous carbon monoxide. Future efforts in this area may involve modifications that transform the stoichiometric conversions facilitated by DACs into catalytic variants. In this context, the reversible binding of CO to DACs is an indication that the latter may serve as a blueprint for the development of more electrophilic, stable carbenes with the capacity to activate other challenging small molecules.

Half-sandwich nickel complexes with ring-expanded NHC ligands – synthesis, structure and catalytic activity in Kumada–Tamao–Corriu coupling

We report the synthesis and catalytic properties of novel [Ni(Cp)(X)(NHC)] complexes with ring-expanded NHC ligands of six-, seven-, and eight-membered heterocycles.

Crystal structures of di-iodido-bis-[(1S,5S)-4-mesityl-1,2,8,8-tetra-methyl-2,4-di-aza-bicyclo-[3.2.1]octan-3-yl-idene-κC (3)]palladium(IV) and di-chlorido-[(1S,5S)-4-mesityl-1,2,8,8-tetra-methyl-2,4-di-aza-bicyclo[3.2.1]octan-3-yl-idene-κC (3)](tri-phenyl-phosphane-κP)palladium(IV)

The mol-ecular structures of the chiral title compounds, [Pd(C19H28N2)2I2], (I), and [Pd(C19H28N2)Cl2(C18H15P)], (II), show a distorted square-planar coordination around the Pd(II) atoms with two halogenide (Hal) ligands each and two N-heterocyclic carbene (NHC) ligands in (I) or one NHC and one tri-phenyl-phosphane ligand in (II). The deviations of the Pd(II) atoms from the L 2Hal2 best plane (L = NHC or tri-phenyl-phosphane ligand) are 0.206 (1) Å for (I) and 0.052 (1) Å for (II). The crystal packings exhibit inter-molecular C-H⋯Hal hydrogen bonds.

A Stable Planar-Chiral N-Heterocyclic Carbene with a 1,1'-Ferrocenediyl Backbone

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-t-BuC5H3)NpN}2C:] (A'-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3'-positions. A'-Np was synthesized in nine steps starting from 1,1'-di-tert-butylferrocene (1), the first step being its 3,3'-dilithiation to afford rac-[Fe(η(5)-t-BuC5H3Li)2] (rac-fc'Li2, 2). The structures of rac-fc'(SiMe3)2 (3), rac-fc'Br2 (4), rac-fc'(N3)2 (5), and the immediate carbene precursor [A'-NpH]BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A'-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. -4.0 eV) and low singlet-triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm(-1) was derived from IR data of cis-[RhCl(A'-Np)(CO)2], indicating the high donicity of A'-Np as a ligand. Consistent with its ambiphilic nature, A'-Np was found to react readily with carbon monoxide, affording the betainic enolate (A'-Np)2CO as four stereoisomers, viz. (RpRp-A'-Np)═C(O(-))(RpRp-A'-Np(+)), (SpSp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), (RpRp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), and (SpSp-A'-Np)═C(O(-))(RpRp-A'-Np(+)). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A'-Np was found to react swiftly with dichloromethane, affording the addition product A'-NpH-CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH-CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A'-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of -0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A'-Np were found to be reversibly oxidized at half-wave potentials of -0.418 and -0.437 V, respectively.