Three-Coordinate Nickel(I) Complexes Stabilised by Six-, Seven- and Eight-Membered Ring N-Heterocyclic Carbenes: Synthesis, EPR/DFT Studies and Catalytic Activity

Ambiphilicity of ring-expanded N-heterocyclic carbenes

N-heterocyclic carbenes, such as imidazole-2-ylidenes and imidazolin-2-ylidenes, the popular class of singlet carbenes introduced by Arduengo in 1991 have not been shown to be ambiphilic owing to the two σ-withdrawing, π-donating amino groups flanking the carbene centre. However, our experimental data suggest that ring-expanded N-heterocyclic carbenes (RE-NHCs), especially the seven and eight membered rings, are significantly ambiphilic. Our results also show that the steric environment in RE-NHCs can become a determining factor for controlling the E-H bond activation.

Remote Steric and Electronic Effects of N-Heterocyclic Carbene Ligands on Alkene Reactivity and Regioselectivity toward Hydrocupration Reactions: The Role of Expanded-Ring N-Heterocyclic Carbenes

The remote groups in N-heterocyclic carbene (NHC) ligands have a significant influence on metal-catalyzed reactions. We examine how remote bulkiness, electronic groups, and expanded-ring NHCs (ER-NHCs) influence alkene reactivity and regioselectivity toward hydrocupration using density functional theory calculations. The impact of remote steric bulkiness on the Cu-H insertion rate is analyzed, revealing a strong correlation between the steric substituent constant and rate ratio, where a bulky group increases the rate due to reduced steric effects in the transition state (TS). The steric properties of the examined catalysts (with a remote group R2 = CPh3, CHPh2, CH2Ph, CH3, and H) and their corresponding TSs are found to be modulated greatly by the remote steric substitution group and the ring size of the NHC ligand. Enhanced bulkiness enhances the nucleophilic Cu-H moiety. The remote electronic groups have a smaller impact on insertion barrier compared to that of steric hindrance. Furthermore, ER-NHC exploration indicates that NHCs with over five-membered rings have a significantly negative influence on the reaction rate. Finally, with a highly bulky group (R2 = CPh3), anti-Markovnikov insertion preference is attributed to high interaction energy and improved steric properties. Overall, our findings here provide valuable insights for the development of a more effective catalyst in metal-catalyzed reactions.

Exploring the Gas-Phase Formation and Chemical Reactivity of Highly Reduced M8 L6 Coordination Cages

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16⁺ oxidation state to the 2⁺ state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C₆₀ -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species.

A mechanistic study on the gold(i)-catalyzed cyclization of propargylic amide: revealing the impact of expanded-ring N-heterocyclic carbenes

Density functional theory (DFT) was applied to understand the mechanistic pathway of the gold(i)-catalyzed cyclization of propargylic amide, and to reveal the impact of expanded-ring N-heterocyclic carbenes.

Well-Defined NHC-Ni Complexes as Catalysts: Preparation, Structures and Mechanistic Studies in Cross-Coupling Reactions

Developmental studies are ongoing to discover a way to utilise new N-heterocyclic carbene (NHC)-Ni complexes as catalysts. Using a bulky NHC ligand, it is possible to synthesise an NHC/phosphine-mixed heteroleptic Ni(II) complex, which can serve as an excellent catalyst for various cross-coupling reactions. During the study of the reaction mechanisms using these Ni complexes, NHC-Ni(I) complexes were accidentally discovered, and it was observed that they exhibit excellent catalytic activity for cross-coupling reactions. The possibility of the presence of NHC-Ni(I) intermediates in these catalytic reaction pathways has been experimentally demonstrated. Depending on the type of reaction, dinuclear Ni(I) and mononuclear Ni(I) complexes can function as intermediates. The results of the investigation of each reaction mechanism are summarised, and the prospects are described.

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

The synthesis and full characterization of new air-stable AgI and CuI complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag2 O afforded a collection of AgI complexes, and their first use as ligand transfer reagents led to novel isostructural CuI or AuI complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new CuI complexes. The solid-state structures feature large N-CNHC -N angles (118-128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%Vbur ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%Vbur =57.5) overcomes to other known erNHC-metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76-93 % product at the 0.5-2.5 mol % catalyst loading, proving the stability of CuI erNHC complexes at elevated temperatures (100 °C).

The first ring-expanded NHC–copper(i) phosphides as catalysts in the highly selective hydrophosphination of isocyanates

The first copper(i) phosphides supported by ring-expanded N-heterocyclic carbenes have been synthesised and react readily with heterocumulenes. These copper(i) phosphides are highly active and selective in the hydrophosphination of isocyanates.

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a-h were prepared by the reaction of readily available N,N'-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a-h with tBuOK in the presence of AuCl(SMe2 ) provided fast access to NHC-gold(I) complexes 3-10. These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%Vbur up to 53.0) which is caused by their expanded-ring architecture.

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.

Ni(i)–Ni(iii) cycle in Buchwald–Hartwig amination of aryl bromide mediated by NHC-ligated Ni(i) complexes

NHC-ligated Ni(i) intermediates in Buchwald–Hartwig amination of aryl halides were isolated and determined. The presence of a Ni(iii) intermediate was also indicated at low temperature.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Ring-Expanded N-Heterocyclic Carbenes for Copper-Mediated Azide-Alkyne Click Cycloaddition Reactions

A series of well-defined copper(I) complexes bearing ring-expanded N-heterocyclic carbene (NHC) ligands has been applied to the azide-alkyne cycloaddition reaction. The obtained results notably showed that the six-membered NHC ligands outperform well-established five-membered ones. [CuI(Mes-6)] displayed a remarkable catalytic activity while respecting the strict criteria for click reactions.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)Ni^IX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni^I(Ar) (Ar = Ph, o-tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-ⁱPr₃C₆H₂). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand. The most unstable Ni(I) aryl species are the most active precatalysts for Suzuki-Miyaura reactions because they rapidly decompose to generate the active Ni(0) catalyst. This study shows that Ni(I) aryl species are initially formed in the activation of Ni(I) halide precatalysts for Suzuki-Miyaura reactions and establishes their stoichiometric and catalytic reactivity profile.

Eight-membered-ring diaminocarbenes bearing naphthalene moiety in the backbone: DFT studies, synthesis of amidinium salts, generation of free carbene, metal complexes, and solvent-free copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction

A new type of eight-membered ring N-heterocyclic carbene (NHC) bearing a rigid naphthalene moiety in the backbone is reported for the first time. Stereoelectronic properties of 4,5-dihydro-1H-naphtho[1,8-ef][1,3]diazocin-3(2H)-ylidene (NaphtDHD) and smaller ring NHCs were theoretically studied at the DFT level. Amidinium salts were prepared from corresponding amidines and dibromides. Free carbene NaphtDHD-Dipp (Dipp = 2,6-diisopropylphenyl) was generated in solution by treatment of the corresponding salt with LiHMDS. It is stable in solution at low temperatures, while decomposing rapidly at room temperature. Silver(i) and copper(i) complexes were synthesized and structurally characterized in the solid state. The copper(i) complex [(NaphtDHD-Mes)CuBr] (Mes = mesityl, 2,4,6-trimethylphenyl) exhibits high catalytic activity in alkyne-azide cycloaddition (CuAAC) reaction under solvent-free conditions.

[Ru3(6-NHC)(CO)10]: synthesis, characterisation and reactivity of rare 46-electron tri-ruthenium clusters

The room temperature reaction of 6-membered ring N-heterocyclic carbenes with [Ru₃(CO)₁₂] affords [Ru₃(6-NHC)(CO)₁₀], rare examples of coordinatively unsaturated, 46-electron tri-ruthenium clusters. Upon mild heating in the presence of C₅H₅N, H₂ or PPh₃, these compounds lose carbene.

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.

Synthesis, Characterisation and Reactions of Truly Cationic NiI -Phosphine Complexes

The recently published purely metallo-organic Ni^I salt [Ni(cod)₂ ][Al(OR^F )₄ ] (1, cod=1,5-cyclooctadiene, R^F =C(CF₃ )₃ ) provides a starting point for a new synthesis strategy leading to Ni^I phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)₂ ][Al(OR^F )₄ ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)₂ ][Al(OR^F )₄ ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh₃ )₃ ][Al(OR^F )₄ ] (4) and a remarkable two-coordinate Ni^I phosphine complex [Ni(PtBu₃ )₂ ][Al(OR^F )₄ ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d⁹ -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these Ni^I compounds between 0.7 and 1.7 V versus Fc/Fc⁺ . Screening reactions with O₂ and CO gave first insights on the reaction behaviour of the Ni^I phosphine complexes towards small molecules with formation of mixed phosphine-CO-Ni^I complexes and oxidation processes yielding new Ni^I and/or Ni^II derivatives. Moreover, 4 reacted with CH₂ Cl₂ at RT to give a dimeric Ni^II ylide complex (4 c). As CH₂ Cl₂ is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.

A T-Shaped Nickel(I) Metalloradical Species

A T-shaped Ni^I complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni^I species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

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

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

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.

Geometric and Electronic Structures of Nickel(II) Complexes of Redox Noninnocent Tetradentate Phenylenediamine Ligands

Five tetradentate ligands based on the N,N'-bis(2-amino-3,5-di-tert-butylphenyl)-o-phenylenediamine backbone were prepared, with different substituents at positions 4 and 5 (CH3 (3a), p-CH3O-C6H4 (3b), H (3c), Cl (3d), F (3e)). Their reaction with a nickel(II) salt in air affords the neutral species 4(a-e), which were isolated as single crystals. 4(a-e) feature two antiferromagnetically exchange-coupled diiminosemiquinonate moieties, both located on peripheral rings, and a diamidobenzene bridging unit. Oxidation of 4(a-e) with 1 equiv of AgSbF6 yields the cations 4(a-e)(+), which harbor a single diiminosemiquinonate radical. Significant structural differences were observed within the series. 4b(+) is mononuclear and contains a localized diiminosemiquinonate moiety. In contrast, 4c(+) is a dimer wherein the diiminosemiquinonate radical is rather delocalized over both peripheral rings. 4d(+) represents an intermediate case where the complex is mononuclear, but the radical is fully delocalized. Oxidation of 4(a-e) with 2 equiv of AgSbF6 produces the corresponding mononuclear dications. X-ray diffraction data on 4(b-d)(2+) reveals that the bridging ring retains its diamidobenzene character, whereas both peripheral rings have been oxidized into diiminobenzoquinone moieties. All the complexes were characterized by electrochemistry, EPR, and UV-vis-NIR spectroscopy. Remarkably, the electronic structures of the complexes differ from those reported by Wieghardt et al. for copper and zinc complexes of a related ligand involving a mixed N2O2 donor set (J. Am. Chem. Soc. 1999, 121, 9599). The easier oxidation of phenylenediamine moieties in comparison to aminophenols is proposed to account for the difference.

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.

[Ni(cod)2][Al(OR(F))4], a Source for Naked Nickel(I) Chemistry

The straightforward synthesis of the cationic, purely organometallic Ni(I) salt [Ni(cod)2](+)[Al(OR(F))4](-) was realized through a reaction between [Ni(cod)2] and Ag[Al(OR(F))4] (cod = 1,5-cyclooctadiene). Crystal-structure analysis and EPR, XANES, and cyclic voltammetry studies confirmed the presence of a homoleptic Ni(I) olefin complex. Weak interactions between the metal center, the ligands, and the anion provide a good starting material for further cationic Ni(I) complexes.