Strong Σ-donor Research Articles

UiO-67 MOF-Encapsulated NHC-Based Single-Site Copper Catalyst and Its Application in Regioselective Borylation of Terminal Alkynes.

N-Heterocyclic carbenes (NHCs) act as versatile ligand backbones due to their strong σ-donation and π-acceptance properties. However, the encapsulation of NHC-coinage metal complexes in a metal-organic framework (MOF) to utilize them in organic catalysis is rare. In this work, an NHC-coordinated CuBr (NHC = Bn2Im; 1,3-dibenzyl-imidazol-2-ylidene) complex was encapsulated in UiO-67 MOF ((Bn2Im)2CuBr@UiO-67) and further utilized toward the regioselective protoboration of terminal alkynes. (Bn2Im)2CuBr@UiO-67 was found to show superior catalytic performance in aiding the protoboration of terminal alkynes, with a very high turnover frequency (TOF) of 14333.3 h-1, much higher than those of many other reported copper-based heterogeneous catalysts. Our catalyst also retained excellent catalytic efficiency for up to five cycles for the above-mentioned process. The newly synthesized (Bn2Im)2CuBr@UiO-67 and the recovered catalyst post-catalysis were characterized using various analytical techniques, including powder X-ray diffraction (PXRD), IR spectroscopy, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).

Dinuclear Cyclometalated Pincer Nickel(II) Complexes with Metal-Metal-to-Ligand Charge Transfer Excited States and Near-Infrared Emission.

Facile non-radiative decay of low-lying metal-centered (MC) d-d excited states has been well documented to pose a significant obstacle to the development of phosphorescent NiII complexes due to substantial structural distortions between the d-d excited state and the ground state. Herein, we prepared a series of dinuclear Ni2II,II complexes by using strong σ-donors, carbene-phenyl-carbene (CNHC^Cphenyl^CNHC) pincer ligands, and prepared their dinuclear Pt2II,II and Pd2II,II analogues. Dinuclear Ni2II,II complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni-Ni distances of 2.947-3.054 Å and singlet metal-metal-to-ligand charge transfer (1MMLCT) transitions at 500-550 nm. Their 1MMLCT absorption energies are red-shifted relative to the Pt2II,II and Pd2II,II analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni2II,II complexes produces valence-trapped dinuclear Ni2II,III species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni2II,II complexes display phosphorescence (τ=2.6-8.6 μs) in the NIR (800-1400nm)spectral region in 2-MeTHF and in solid state at 77 K, which is insensitive to π-conjugation of the coordinated [CNHC^Cphenyl^CNHC] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the 3dd excited state. Studies have found that the dinuclear Ni2II,II complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.

Combined Nucleophilic and Electrophilic Functionalization to Optimize Blue Phosphorescence in Cyclometalated Platinum Complexes.

Ligand-based functionalization strategies have emerged as powerful approaches to tune and optimize blue phosphorescence, which can involve nucleophilic addition to coordinated ligands or electrophilic functionalization via the coordination of exogenous Lewis acids. Whereas both have been used separately to enhance the photophysical properties of organometallic compounds with high-energy triplet states, in this work, we show that these two strategies can be used together on the same platform. Isocyanide-supported cyclometalated platinum compounds undergo nucleophilic addition with diethylamine to form a strong σ-donor acyclic diaminocarbene-supporting ligand. In a subsequent step, a cyanide ancillary ligand is converted into a more strongly π-acidic isocyanoborate via the coordination of a borane Lewis acid. Importantly, both of these ligand-based functionalization steps improve the quantum yields and lifetimes of the blue-phosphorescent complexes. This synergy results in complexes with photoluminescence quantum yields up to 0.40 for deep blue and 0.75 for sky blue regions and PL lifetimes on the order of 10-5 s.

Iron-Sensitized Solar Cells (FeSSCs).

ConspectusThe harvesting and conversion of solar energy have become a burning issue for our modern societies seeking to move away from the exploitation of fossil fuels. In this context, dye-sensitized solar cells (DSSCs) have proven to be trustworthy alternatives to silicon-based cells with advantages in terms of transparency and efficiency under low illumination conditions. These devices are highly dependent on the ability of the sensitizer that they contain to collect sunlight and transfer an electron to a semiconductor after excitation. Ruthenium and polypyridine complexes are benchmarks in this field as they exhibit ideal characteristics such as long-lasting metal-ligand charge transfer (MLCT) states and efficient separation between electrons and holes, limiting recombination at the dye-semiconductor interface. Despite all of these advantages, ruthenium is a noble metal, and the development of more sustainable energy devices based on earth-abundant metals is now a must. A quick glance at the periodic table reveals iron as a potential good candidate, since it belongs to the same group of ruthenium, which suggests similar electronic properties. However, striking photophysical differences exist between ruthenium(II) polypyridyl complexes and their Fe(II) analogues, the latter suffering from short-lived MLCT states resulting of their ultrafast relaxation into metal-centered (MC) states. Pyridyl-N-heterocyclic carbenes (pyridylNHC) brought a strong σ-donor character required to promote a higher ligand field splitting of the iron d orbitals. This induces destabilization of the MC states over the MLCT manifold and a consequent slowdown of the excited states deactivation providing iron(II) complexes with tens of picoseconds lifetimes, making them more promising for applications in DSSCs. This Account highlights our recent advances in the development and characterization of iron-sensitized solar cells (FeSSCs) with a focus on the design of efficient sensitizers going from homoleptic to heteroleptic complexes (bearing different anchoring groups) and the tuning of electrolyte composition. Our rational approach led to the best photocurrent and efficiency ever reported for an iron sensitized solar cell (2% PCE and 9 mA/cm2) using a cosensitization process. This work clearly evidences that the solar energy conversion based on iron complex sensitization is now an opened and fruitful route.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs).

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Molar excess of coordinating N-heterocyclic carbene ligands triggers kinetic digestion of gold nanocrystals.

There has been much interest in evaluating the strength of the coordination interactions between N-heterocyclic carbene (NHC) molecules and transition metal ions, nanocolloids and surfaces. We implement a top-down core digestion test of Au nanoparticles (AuNPs) triggered by incubation with a large molar excess of poly(ethylene glycol)-appended NHC molecules, where kinetic dislodging of surface atoms and formation of NHC-Au complexes progressively take place. We characterize the structure and chemical nature of the generated PEG-NHC-Au complexes using 1D and 2D 1H-13C NMR spectroscopy, supplemented with matrix assisted laser desorption/ionization mass spectrometry, and transmission electron microscopy. We further apply the same test using thiol-modified molecules and find that though etching can be measured the kinetics are substantially slower. We discuss our findings within the classic digestion of transition metal ores and colloids induced by interactions with sodium cyanide, which provides an insight into the strength of coordination between the strong σ-donating (soft Lewis base) NHC and Au surfaces (having a soft Lewis acid character), as compared to gold-to-gold covalent binding.

Reactivities of phosphaalkynes towards diverse bis-silylenes.

Bis-silylenes do not only act as strong chelating σ-donor ligands, but also exhibit cooperative behaviour in the activation of small molecules. Three different P-Si containing molecules were prepared from the reaction between tBuCP and different bis-silylenes, which are bridged by ferrocenediyl, diaminobenzene, or o-carborane.

Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride.

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]2 to give the corresponding [Pd(η3-allyl)Cl(mNHO)] coordination complexes. 13C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH3-mNHO adduct by reacting the triazolium salts with AuCl(SMe2) and BH3·THF, respectively. The BH3-mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.

The coordination of alkali-metal nickelates to organic π-systems: synthetic, structural and spectroscopic insights.

Low-valent nickelates have recently been shown to be key intermediates in challenging cross-coupling reactions using aryl ethers as electrophiles. Key for the success of these transformations is the activation of the substrate through π-coordination to the nickelate intermediate, however there is still limited knowledge about the fundamental structure and coordination chemistry of these heterobimetallic complexes. Herein, we report the synthesis, structures, and spectroscopic analysis of a diverse family of alkali-metal nickelates derived from phenyl-alkali-metal reagents and Ni(ttt-CDT), where ttt-CDT = trans,trans,trans-1,5,9-cyclododecatriene. The co-complexation of PhLi with Ni(ttt-CDT) was found to yield 1 : 1, 2 : 1 or 4 : 2 lithium nickelates depending on the stoichiometry and reaction conditions employed. The high lability of the ttt-CDT ligand enables facile ligand exchange with an assorted series of organic π-acceptors, ranging from polyaromatic hydrocarbons to ketones, imines and nitriles. For anthracene and phenanthrene, a homologous series of Li, Na and K nickelates could be obtained, which lead to different structural motifs or degrees of aggregation in the solid-state spanning solvated monomers to complex polymeric arrangements. For π-extended systems such as perylene or coronene, competing single-electron-transfer to give the corresponding radical anions was observed, illustrating the highly reducing nature of the alkali-metal nickelates. X-ray crystallographic analysis and NMR spectroscopy of the phenyl-alkali-metal nickelates reveal extreme back-bonding from Ni(0) to the organic π-acceptors due to strong σ-donation from the carbanionic ligands.

Uncovering diverse reactivity of NHCs with diazoalkane: C-H activation, C=C bond formation, and access to N-heterocyclic methylenehydrazine

N-heterocyclic carbenes (NHCs) have attracted significant attention due to their strong σ-donating capabilities, as well as their transition-metal-like reactivity towards small molecules. However, their interaction with diazoalkanes remains understudied. In...

The Effect of Monodentate Co-Ligands on the Properties of Pt(II) Complexes Bearing a Tridentate C^N*N-Luminophore

In this study, the insertion of different monodentate co-ligands on Pt(II) complexes bearing a monoanionic C^N*N luminophore as a tridentate chelator was achieved beyond the previously reported chlorido- ([PtCl(L)]) and cyanido-decorated ([PtCN(L)]) analogues. To investigate the impact of the auxiliary ligand on the photophysical properties, we introduced a neutral carbonyl-ligand and observed a lower photoluminescence quantum yield (ΦL) than with a cyanido moiety. However, the direct substitution of the chlorido co-ligand by a NO-related derivative was not successful. Interestingly, the attempted reduction of the successfully inserted nitrito-N-ligand in [PtNO2(L)] resulted in the oxidation of the Pt(II)-center to Pt(IV), as demonstrated by X-ray diffractometry. For comparison, the trifluoroacetato Pt(II) and chlorido Pt(IV) complexes ([PtTFA(L)] and [PtCl3(L)], respectively) were also synthesized. The photophysical characterization revealed similar photoluminescence profiles for all complexes, indicating a weak effect of the co-ligand on the excited state; in fact, all complexes display emission from metal-perturbed ligand-centered states (even the Pt(IV) species). Nonetheless, longer excited state lifetimes (τav) suggest a reduced thermally-activated radiationless deactivation via metal-centered states upon exchange of the chlorido units for other monodentate entities, yet without significantly improving the overall ΦL at room temperature. The irreversible oxidation waves (measured via cyclic voltammetry) mostly stem from the Pt(II)-center; the co-ligand-related drop of these potentials correlates with the increasing σ-donating capacities of the ancillary ligand. In summary, an enhanced π-acceptor capacity does not necessarily improve the ΦL and can even impair radiative rates by compromising the perturbative participation of the metal center on the emissive triplet state; in addition, strong σ-donor abilities improve the phosphorescence efficiencies by hampering the thermal population of dissociative electronic configurations related to the participation of antibonding d*-orbitals at the metal center.

Nickel-Catalyzed Selective C(sp2 )-F Bond Alkylation of Industrially Relevant Hydrofluoroolefin HFO-1234yf.

The selective transition-metal catalyzed C-F bond functionalization of inexpensive industrial fluorochemicals represents one of the most attractive approaches to valuable fluorinated compounds. However, the selective C(sp2 )-F bond carbofunctionalization of refrigerant hydrofluoroolefins (HFOs) remains challenging. Here, we report a nickel-catalyzed selective C(sp2 )-F bond alkylation of HFO-1234yf with alkylzinc reagents. The resulting 2-trifluoromethylalkenes can serve as a versatile synthon for diversified transformations, including the anti-Markovnikov type hydroalkylation and the synthesis of bioactive molecule analogues. Mechanistic studies reveal that lithium salt is essential to promote the oxidative addition of Ni0 (Ln ) to the C-F bond; the less electron-rich N-based ligands, such as bipyridine and pyridine-oxazoline, feature comparable or even higher oxidative addition rates than the electron-rich phosphine ligands; the strong σ-donating phosphine ligands, such as PMe3 , are detrimental to transmetallation, but the less electron-rich and bulky N-based ligands, such as pyridine-oxazoline, facilitate transmetallation and reductive elimination to form the final product.

Moving from Fuel to Feedstock

Carbon-hydrogen bond activations and their subsequent functionalisation have long been an important target in chemistry because C–H bonds are ubiquitous throughout nature, making C–H derivatisation reactions highly desirable. The selective and efficient functionalisation of this bond into many more useful carbon-element bonds (for example, C–B, C–Si, C–O and C–S bonds) would have many uses in pharmaceutical and bulk chemical synthesis. Activation of the C–H bond is, however, challenging due to the high strength and low bond-polarity of this bond rendering its cleavage unfavourable. With the correct choice of reagents and systems, especially those utilising directing groups, kinetically and thermodynamically favourable catalytic processes have been developed. However, a key remaining challenge is the development of undirected, intermolecular reactions using catalysts that are both selective and active enough to make useful processes. In this review, the progress towards optimising Group 9 C–H activation catalysts is discussed, particularly focusing on undirected reactions that are kinetically more difficult, starting with a brief history of C–H activation, identifying the importance of auxiliary ligands including the nature of anionic ligand (for example, cyclopentadienyl, indenyl, fluorenyl and trispyrazolylborate) and neutral ligands (such as phosphines, carbonyl, alkenes and N-heterocyclic carbenes (NHCs)) that contribute towards the stability and reactivity of these metal complexes. The tethering of the anionic ligand to strong σ-donating ligands is also briefly discussed. The focus of this review is primarily on the Group 9 metals rhodium and iridium, however, C–H activation using Group 8 and 10 metals are compared where useful. The most recent advances in this field include the development of C–H borylation of many small hydrocarbon substrates such as arenes, heterocycles and n-alkanes as well as the more challenging substrate methane.

Highly efficient and stable thermally activated delayed fluorescent palladium(II) complexes for organic light-emitting diodes

Transition metal complexes exhibiting thermally activated delayed fluorescence (TADF) remain underdeveloped for organic light-emitting diodes (OLEDs). Here, we describe a design of TADF Pd(II) complexes featuring metal-perturbed intraligand charge-transfer excited states. Two orange- and red-emitting complexes with efficiencies of 82 and 89% and lifetimes of 2.19 and 0.97 μs have been developed. Combined transient spectroscopic and theoretical studies on one complex reveal a metal-perturbed fast intersystem crossing process. OLEDs using the Pd(II) complexes show maximum external quantum efficiencies of 27.5 to 31.4% and small roll-offs down to 1% at 1000 cd m −2 . Moreover, the Pd(II) complexes show exceptional operational stability with LT 95 values over 220 hours at 1000 cd m −2 , benefiting from the use of strong σ-donating ligands and the presence of multiple intramolecular noncovalent interactions beside their short emission lifetimes. This study demonstrates a promising approach for developing efficient and robust luminescent complexes without using the third-row transition metals.

Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials.

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.

Recent advances in sustainable N-heterocyclic carbene-Pd(II)-pyridine (PEPPSI) catalysts: A review

Various catalysts in homogeneous or heterogeneous catalysis deploy unconventional reaction pathways by lowering the activation energy (AE) barrier, controlling the selectivity, and creating environmental impact, thereby bringing about economic viability. Hence, the study of these methodologies is of immense interest. To develop a new chemistry, there is much scope for the invention of brilliant candidates that could effectively catalyze diverse reaction methodologies. The palladium-catalyzed reactions motivate interesting applications on various organic transformations under mild reaction conditions. Although phosphorous designed ligands or catalysts have been used, despite their expensiveness, sensitivity and other properties, there is the necessity of developing even better cross-coupling ligands or catalysts such as N-heterocyclic carbene (NHC)-based palladium complexes. These palladium-NHCs (Pd–NHC) are novel and universal nucleophilic entities that have come into light as the most successful class of catalysts in organometallic chemistry. In the same class, a specific category of palladium-NHCs such as palladium-pyridine enhanced pre-catalyst preparation by the stabilization initiation (palladium-PEPPSI) complexes, are emerging as versatile alternatives to phosphine containing palladium complexes for various cross-coupling reactions due to their excellent catalytic activity. Further to mention that NHCs are recently extensively used as ancillary ligands in organometallic chemistry, which includes industrial-related catalytic transformations due to strong σ-donors to metal centres. Apart from this, many NHC-metal complexes are the fascinating consideration in material science as probable metallo-pharmaceuticals. The current review offers a brief exploration of palladium-PEPPSI complexes over the past few years. Further, the synthesis of a variety of these types of catalysts, their applications in Suzuki-Miyaura, Buchwald-Hartwig, Sonogashira, Negishi couplings direct C2-arylation, O–C(O) cleavage, α-arylation/alkylation of carbonyl compounds and trans-amidation reactions via cross-coupling methodologies, which are covered. Additionally, reported recent developments on reusable heterogeneous PdPEPPSI complexes and their catalytic applications are being covered. Finally, the chiral Pd complexes and their asymmetric transformations are discussed.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis.

In the last decade, major advances have been made in homogeneous gold catalysis. However, AuI /AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI /AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Anionic N-heterocyclic carbenes featuring weakly coordinating perfluoroalkylphosphorane moieties.

The anionic 1-methyl-3-(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate 3 and the 1,3-bis(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate dianion 4 were obtained in high yield by deprotonation of {(C2F5)3PF2}-methylimidazole 1 and the {(C2F5)3PF2}2-imidazolate anion 2. Carbenes 3 and 4 are first examples for a novel class of NHCs carrying weakly coordinating anions (WCA-NHCs). First reactions of these new ligands with elemental selenium and chloro(phosphine)gold(I) complexes to result in an anionic selenium adduct (5) and WCA-NHC gold complexes (6 and 7) have been undertaken. The structural and spectroscopic properties of these NHC derivatives in conjunction with data from quantum chemical calculations provide an insight into the electronic and steric properties of the WCA-NHCs 3 and 4. Especially the combination of properties such as weakly coordinating periphery combined with the coordinative carbene center, negative charge, large buried volume (%Vbur), and strong σ-donor as well as efficient π-acceptor ability render NHCs 3 and 4 unique and promising new ligands.

Au(Np#)Cl]: Highly Reactive and Broadly Applicable Au(I)─NHC Catalysts for Alkyne π-Activation Reactions.

Cationic Au(I)─NHC (NHC = N-heterocyclic carbene) complexes have become an important class of catalysts for alkyne π-activation reactions in organic synthesis. In particular, these complexes are characterized by high stability of catalytic species engendered by strong σ-donation and metal backbonding. Herein, we report the synthesis and characterization of well-defined [Au(NHC)Cl] complexes featuring recently discovered IPr# family of ligands that hinge upon modular peralkylation of aniline. These ligands have been commercialized in collaboration with MilliporeSigma (IPr#: 915653; Np#: 915912; BIAN-IPr#: 916420). Evaluation of the [Au(NHC)Cl] complexes in a series of Au(I)─NHC-catalyzed π-functionalizations of alkynes, such as hydrocarboxylation, hydroamination and hydration, resulted in the identification of wingtip-flexible [Au(Np#)Cl] as a highly reactive and broadly applicable catalyst with the re-activity outperforming the classical [Au(IPr)Cl] and [Au(IPr*)Cl] complexes. The utility of this catalyst has been demonstrated in the direct late-stage derivatization of complex pharmaceuticals. Structural and computational studies were conducted to determine steric effects, frontier molecular orbitals and bond orders of this class of catalysts. Considering the attractive features of well-defined Au(I)─NHC complexes, we anticipate that this class of bulky and wingtip-flexible Au(I)─NHCs based on the modular peralkylated naphthylamine scaffold will find broad application in π-functionalization of alkynes in various areas of organic synthesis and catalysis.

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides.

Nickel(0) nanoparticles coordinated to NHC ligands bearing N-coordinated cinnamyl moieties were readily prepared by reduction of a [NiCpBr(NHC-cinnamyl)] complex with methyl magnesium bromide. The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticle aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.