Homoleptic and heteroleptic bis-NHC Cu(I) complexes as carbene transfer reagents

Exploring the reductive CO2 fixation with amines and hydrosilanes using readily available Cu(II) NHC-phenolate catalyst precursors

N-Methylation of amines is of great interest in the synthesis of pharmaceuticals and valuable compounds, and the possibility to perform this reaction with an inexpensive and non-toxic substrate like CO2 and its derivatives is quite appealing. Herein, the synthesis of four novel homoleptic Cu(II) complexes with hybrid NHC-phenolate (NHC = N-Heterocyclic Carbene) ligands is reported, and their use in the catalytic N-methylation of amines with CO2 in the presence of hydrosilanes is explored. Both bidentate or tetradentate ligands can be used in the preparation of the complexes provided that the structural requirement that the two NHC and the two phenolate donors in the metal coordination sphere are mutually in trans is fulfilled. A new reaction protocol to perform the N-methylation of secondary aromatic amines and dibenzylamine in high yield under mild reaction conditions is developed, using the ionic liquid [BMMIM][NTf2] (1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide) as solvent and the catalyst precursor [Cu(L2)2]. Reactivity studies indicate that the reaction follows two different pathways with different hydrosilanes, and that the starting Cu(II) complexes are reduced under the catalytic conditions.

[Pd(NHC)(μ-Cl)Cl]2: The Highly Reactive Air- and Moisture-Stable, Well-Defined Pd(II)-N-Heterocyclic Carbene (NHC) Complexes for Cross-Coupling Reactions

ConspectusPalladium-catalyzed cross-coupling reactions owing to their high specificity and superb chemoselectivity represent a powerful tool for the rapid construction of C-C and C-X bonds across various areas of chemical research, including pharmaceutical development, polymer and agrochemical industries, bioactive natural products, and advanced functional materials, rendering them indispensable for modern synthetic chemists. The major driving force for the advances in this critical field is the design of increasingly more reactive and more selective ligands and precatalysts that aim not only to address challenging cross-coupling processes but also to achieve optimal reactivity, selectivity, and functional group compatibility under mild, user-friendly, operationally simple, and broadly applicable conditions. In this context, Pd(II)-N-heterocyclic carbene complexes (NHC = N-heterocyclic carbene) have garnered prevalent attention among practitioners of organic synthesis due to their unique electronic and steric characteristics that are unmatched among other ligands. In particular, the superior σ-donating ability of NHC ligands in conjunction with conformational flexibility as well as the ease of steric and electronic modification and high stability to air and moisture enable highly effective fundamental elementary steps in catalytic cycles and facile formation of well-defined complexes.The key factor in the design of well-defined, air- and moisture-stable Pd(II) precatalysts involves the incorporation of supporting ligands, which are essential for ensuring the stability of Pd(II)-NHC complexes and facile activation of Pd(II)-NHC precatalysts to catalytically active monoligated Pd(0)-NHC species under the reaction conditions. Notably, [Pd(NHC)(μ-Cl)Cl]2 chloro dimers, which can be readily synthesized via a one-pot, atom-economic process, are the most reactive Pd(II)-NHC complexes synthesized to date. These well-defined, air- and moisture-stable dimers readily dissociate to monomers and are activated to Pd(0)-NHC catalysts under both mild and strong base conditions, showcasing enhanced reactivity and selectivity among their Pd(II)-NHC counterparts. This balance between high, operationally simple stability, which is characteristic of Pd(II) complexes together with the ease of activation to the strongly nucleophilic Pd(0)-NHC catalysts, renders [Pd(NHC)(μ-Cl)Cl]2 the most reactive Pd(II)-NHC precatalysts developed to date for a broad range of general cross-coupling processes, including C-X, C-O, C-N, and C-S activation and enabling the direct late-stage functionalization of complex compounds decorated with a wide range of sensitive functional groups.In this Account, we outline [Pd(NHC)(μ-Cl)Cl]2 as a highly reactive Pd(II)-NHC precatalyst that should be routinely used as the first choice Pd complexes for a wide range of challenging cross-coupling reactions. The advancements in this field over the past 20 years emphasize the critical role of catalyst design to achieve optimal reactivity. Consequently, [Pd(NHC)(μ-Cl)Cl]2 chloro dimers should be recommended as the go-to complexes in the powerful toolbox of Pd-catalyzed cross-coupling reactions. These now commercially available Pd(II)-NHC complexes see widespread use across the synthetic chemistry community and enable the accelerated application of challenging cross-couplings in the synthesis of new molecules.

Detailed structural and spectroscopic elucidation of ferrocenium coupled N-heterocyclic carbene gold(I) complexes

Unambiguous assignment of redox sites on ferrocene coupled N-heterocyclic carbene gold(I) complexes [(Fc-NHC)2Au(I)]+ is critical to gain a greater mechanistic understanding of their activity in a cellular environment. Such information can be garnered with isolation and detailed characterization of the oxidized version of [(Fc-NHC)2Au(I)]+. Herein we disclose a study that unambiguously illustrates redox events pertaining to [(Fc-NHC)2Au(I)]+ that stem exclusively from ferrocene sites. This work also describes novel synthetic methodologies for isolating ferrocenium coupled N-heterocyclic carbene gold(I) complexes.

Simple synthesis of [Ru(CO3)(NHC)(p-cymene)] complexes and their use in transfer hydrogenation catalysis

A novel, efficient and facile protocol for the synthesis of a series of [Ru(NHC)(CO₃)(p-cymene)] complexes is reported. This family of Ru-NHC complexes was obtained from imidazol(in)ium tetrafluoroborate or imidazolium hydrogen carbonate salts in moderate to excellent yields, employing sustainable weak base. The ruthenium complexes were successfully utilized in the transfer hydrogenation of ketones as highly active multifunctional catalysts.

From Prochiral N-Heterocyclic Carbenes to Optically Pure Metal Complexes: New Opportunities in Asymmetric Catalysis

Well-defined optically pure transition metal (TM) complexes bearing C₁- and C₂-symmetric N-heterocyclic carbene (NHC) ligands were prepared from prochiral NHC precursors. As predicted by DFT calculations, our strategy capitalizes on the formation of a metal-carbene bond which induces an axis of chirality. Configurationally stable atropisomers of various NHC-containing TM complexes were isolated by preparative HPLC on a chiral stationary phase in good yields and excellent optical purities (up to 99.5% ee). The carbene transfer from an optically pure Cu complex to a gold or palladium center reveals, for the first time, a full stereoretentivity, supporting the hypothesis of an associative mechanism for the transmetalation. The potential of these new chiral TM complexes was illustrated in asymmetric catalysis with up to 98% ee.

Bimetallic d10 -Metal Complexes of a Bipyridine Substituted N-Heterocyclic Carbene

The hybrid ligand 3-(2,2'-bipyridine-6-ylmethyl)-1-mesityl-1H-imidazolylidene (NHC^Bipy ) featuring both carbene and N-donor sites, was selectively complexed with various d¹⁰ metal cations in order to examine its coordination behavior with regard to homo and heterometallic structures. Respective silver complexes can be obtained by the silver oxide route and are suitable transmetallation reagents for the synthesis of gold(I) compounds. Starting from the mononuclear complexes [(NHC^Bipy )AuCl], [(NHC^Bipy )Au(C₆ F₅ )] and [(NHC^Bipy )₂ Au][ClO₄ ], open-chain as well as cyclic heterobimetallic complexes containing Cu⁺ , Ag⁺ , Zn²⁺ , Cd²⁺ , and Hg²⁺ were synthesized. Furthermore, the homobimetallic species [(NHC^Bipy )₂ M₂ ][ClO₄ ]₂ (M=Cu, Ag) were obtained. All bimetallic compounds were fully characterized including single-crystal X-ray analysis. Their photoluminescence (PL) properties were investigated in the solid state at temperatures between 15 and 295 K and compared with those of the mononuclear species. There is a clear difference in PL properties between the open chain and the cyclic heterobimetallic complexes. The latter species show different PL properties, depending on the metals involved. In addition, collision-induced dissociation (CID) experiments were performed on electrosprayed cations of the cyclic heterobimetallic compounds, to compare the metal binding at the carbene and N-donor sites.

Copper(i)-NHC complexes as NHC transfer agents

This report presents the recent advances in the use of copper(i)-NHC (NHC = N-heterocyclic carbene) complexes as transmetallating agents enabling the transfer of NHCs to a wide range of transition metals. The synthesis of these carbene transfer reagents is also discussed.

Dual targeting of the cancer antioxidant network with 1,4-naphthoquinone fused Gold(i) N-heterocyclic carbene complexes

To achieve a systems-based approach to targeting the antioxidant pathway, 1,4-naphthoquinone annulated N-heterocyclic carbene (NHC) [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] [silver(i) dichloride] (1), [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] chloride (2), and 1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i) chloride (3)) were designed, synthesized, and tested for biological activity in a series of human cancer cell lines. The solution phase of complexes 1-3 were assigned using several spectroscopy techniques, including NMR spectroscopic analysis. Complexes 1 and 3 were further characterized by single crystal X-ray diffraction analysis. Electrochemical and spectroelectrochemical studies revealed that quinone reductions are reversible and that the electrochemically generated semiquinone and quinone dianions are stable under these conditions. Complex 1, containing two NHC-quinone moieties (to accentuate exogenous ROS via redox cycling) centered around a Au(i) center (to inactivate thioredoxin reductase (TrxR) irreversibly), was found to inhibit cancer cell proliferation to a much greater extent than the individual components (i.e., Au(i)-NHC alone or naphthoquinone alone). Treatment of A549 lung cancer cells with 1 produced a 27-fold increase in exogenous reactive oxygen species (ROS) which was found to localize to the mitochondria. The inhibition of TrxR, an essential mediator of ROS homeostasis, was achieved in the same cell line at low administrated concentrations of 1. TrxR inhibition by 1 was similar to that of auranofin, a gold(i) containing complex known to inhibit TrxR irreversibly. Complex 1 was found to induce cell death via an apoptotic mechanism as confirmed by annexin-V staining. Complex 1 was demonstrated to be efficacious in zebrafish bearing A549 xenografts. These results provide support for the suggestion that a dual targeting approach that involves reducing ROS tolerance while concurrently increasing ROS production can perturb antioxidant homeostasis, enhance cancer cell death in vitro, and reduce tumor burden in vivo, as inferred from preliminary zebra fish model studies.

From metallic gold to [Au(NHC)2]+ complexes: an easy, one-pot method

A simple and direct method is described to prepare cationic bis(NHC)–Au(i) complexes using metallic gold as the starting material.

Facile routes to abnormal-NHC-cobalt(ii) complexes

Deprotonation of 1 with Co{N(SiMe₃)₂}₂ affords aNHC-Co(ii) complex 2, whereas carbene transfer from 3 to Co{N(SiMe₃)₂}₂ enables access to complex 4.

Copper complexes bearing an NHC–calixarene unit: synthesis and application in click chemistry

Novel N-heterocyclic carbene (NHC) copper complexes supported by calix[4]arene were synthesized and showed good catalytic activity when applied in click chemistry.

Ruthenium(ii) complexes of hemilabile pincer ligands: synthesis and catalysing the transfer hydrogenation of ketones

A series of hemilabile NHC Ru(ii) complexes were synthesised and assessed for their catalytic activity for transfer hydrogenation reactions.