Development of Axially Chiral Pyridylidene Amine Ligands and their Application in Pd-Catalyzed Enantioselective Allylic Substitution

A series of novel axially chiral pyridylidene amine (PYE) ligands has been developed, and their catalytic capability has been demonstrated in various highly efficient and enantioselective Pd-catalyzed asymmetric allylic substitutions. A density-functional theory (DFT) study explains the preferential enantiocontrol in the key transition states of the axially chiral PYE ligand-promoted Pd-catalyzed allylic alkylation.

Manganese(II) and Zinc(II) metal complexes of novel bidentate formamide-based Schiff base ligand: synthesis, structural characterization, antioxidant, antibacterial, and in-silico molecular docking study

A new bidentate Schiff base ligand (C16H16Cl2N4), condensation product of ethylene diamine and 4-chloro N-phenyl formamide, and its metal complexes [M(C16H16Cl2N4)2(OAc)2] (where M = Mn(II) and Zn(II)) were synthesized and characterized using various analytical and spectral techniques, including high-resolution mass spectrometry (HRMS), elemental analysis, ultraviolet-visible (UV-vis), Fourier-transform infrared (FTIR) spectroscopy, AAS, molar conductance, 1H NMR, and powder XRD. All the compounds were non-electrolytes and nanocrystalline. The synthesized compounds were assessed for antioxidant potential by DPPH radical scavenging and FRAP assay, with BHT serving as the positive control. Inhibitory concentration at 50% inhibition (IC50) values were calculated and used for comparative analysis. Furthermore, the prepared compounds were screened for antibacterial activity against two Gram-negative bacteria (Staphylococcus aureus and Bacillus subtilis) and two Gram-positive bacteria (Escherichia coli and Salmonella typhi) using disk-diffusion methods, with amikacin employed as the standard reference. The comparison of inhibition zones revealed that the complexes showed better antibacterial activity than the ligand. To gain insights into the molecular interactions underlying the antibacterial activity, the ligand and complexes were analyzed for their binding affinity with S. aureus tyrosyl-tRNA synthetase (PDB ID: 1JIL) and S. typhi cell membrane protein OmpF complex (PDB ID: 4KR4). These analyses revealed robust interactions, validating the observed antibacterial effects against the tested bacterial strains.

Temperature Dependence of the Ring Opening of Cyclopropene Imines on Thorium Metallocenes

The reactions of two highly strained cyclopropenimine ligands L1H and L2H (L1H = N1,N1,N2,N2-tetraisopropyl-3-iminocycloprop-1-ene-1,2-diamine, L2H = N1,N1,N2,N2-tetracyclohexyl-3-iminocycloprop-1-ene-1,2-diamine) with three thorium precursors Cp*2ThCl2, Cp*2Th(Cl)(CH3), and Cp*2Th(CH3)2 were studied. At -20 °C, L1H and L2H react with Cp*2ThCl2 to form Th1 (Th1 = Cp*2ThCl2(L1H)) and Th2 (Th2 = Cp*2ThCl2(L2H)), respectively, where the neutral ligand coordinates to the thorium metal center. Coordination of the ligand to the thorium metal center introduces aromaticity at the cyclopropene ring of the ligand. Reaction at room temperature results in the ring opening of the ligand to form Th3 (Th3 = Cp*2ThCl2((Z)-2,3-bis(diisopropylamino)acrylonitrile) and Th4 (Th4 = Cp*2ThCl2((Z)-2,3-bis(dicyclohexylamino)acrylonitrile), where the cyclopropenimine converts into a nitrile and coordinates to the thorium metal center. Reaction of L1H and L2H with Cp*2Th(Cl)(CH3) and/or Cp*2Th(CH3)2 at -20 °C results in a rapid methanolysis reaction and forms Cp*2Th(L1/L2)(CH3/Cl)-type complexes Th5 (Th5 = Cp*2Th(L1)(CH3)), Th6 (Th6 = Cp*2Th(L2)(CH3), Th7 (Th7 = Cp*2Th(L1)(Cl), and Th8 (Th8 = Cp*2Th(L2)(Cl). On the other hand, at room temperature, these reactions result in a ring opening of the ligand. Room-temperature reaction of L1H and L2H with Cp*2Th(CH3)2 results in Th9 (Th9 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th10 (Th10 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). Similarly, at room temperature, L1H and L2H react with Cp*2Th(Cl)(CH3) to form Th11 (Th11 = Cp*2Th(Cl)((Z)-3-imino- N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th12 (Th12 = Cp*2Th(Cl)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). The ring-opening reaction is assisted by the nucleophilic attack of the thorium-coordinated methyl group to the highly strained cyclopropene imine carbon.

Potent Anticancer Activity of a Dinuclear Gold(I) bis-N-Heterocyclic Imine Complex Related to Thioredoxin Reductase Inhibition in Vitro

A dinuclear gold(I) complex featuring a strongly donating bis-N-heterocyclic imine ligand was synthesised and characterised by different methods, including single crystal X-ray diffraction (SC-XRD) analysis. The compound has been tested for its antiproliferative effects in a panel of human cancer cell lines in vitro, showing highly selective anticancer effects, particularly against human A549 non-small cell lung cancer cells (NSCLC), with respect to non-tumorigenic cells (VERO). The accumulation of the compound in A549 and VERO cells was studied by high-resolution continuum source atomic absorption spectrometry (HRCS-AAS), revealing that the anticancer effects are not particularly related to the different amounts of gold taken up by the cells over 72 h. Enzyme inhibition studies to evaluate the activity of the seleno-enzyme thioredoxin reductase (TrxR) in cancer cell extracts show that the gold(I) compound is a potent inhibitor (IC50=0.567±0.208 μM), while the free ligand is ineffective. This result correlates with the observed compound's selectivity towards A549 cells overexpressing the enzyme.

Copper-Catalyzed C-C Cross-Couplings of Tertiary Alkyl Halides with Anilines Enabled by Cyclopropenimine-Based Ligands

Catalytic cross-couplings of tertiary alkyl electrophiles with carbon nucleophiles offer a powerful platform for constructing quaternary carbon centers, which are prevalent in bioactive molecules. However, these reactions remain underdeveloped primarily because of steric challenges that impede efficient bond formation. Herein, we describe the copper-catalyzed synthesis of such centers through the C(sp3)-C(sp2) bond-forming reaction between tertiary alkyl halides and arene rings of aniline derivatives, enabled by the strategic implementation of bidentate bis(cyclopropenimine) ligands. The copper catalyst bound by two imino-nitrogen atoms of these ligands, which have never been employed in metal catalysis previously, is highly effective in rapidly activating tertiary halides to generate alkyl radicals, allowing them to react with aryl nucleophiles under mild conditions with remarkably short reaction times (1-2 h). Various tertiary halides bearing carbonyl functional groups can be coupled with secondary or primary anilines, furnishing a range of quaternary carbon centers in good yields. Several mechanistic observations support the generation of copper(II) species and alkyl radicals which as a result elucidate the steps in the proposed catalytic cycle.

Exploring the supramolecular chemistry of cyclopropeniums: halogen-bonding-induced electrostatic assembly of polymers

Exploring new noncovalent synthons for supramolecular assembly is essential for material innovation. Accordingly, we herein report a unique type of cyclopropenium-based supramolecular motif and demonstrate its applications to polymer self-assembly. Because of the "ion pair strain" effect, trisaminocyclopropenium iodides complex strongly with fluoroiodobenzene derivatives, forming stable adducts. Crystal structure analysis reveals that halogen-bonding between the iodide anion and the iodo substituent of the fluoroiodobenzene is the driving force for the formation of these electrostatically complexed adducts. Such halogen-bonding-induced electrostatic interactions were further successfully applied to drive the assembly of polymers in solution, on surfaces, and in bulk, demonstrating their potential for constructing supramolecular polymeric materials.

Isolation and characterization of bis(silylene)-stabilized antimony(I) and bismuth(I) cations

Monovalent group 15 cations L₂Pn + (L = σ-donor ligands, Pn = N, P, As, Sb, Bi) have attracted significant experimental and theoretical interest because of their unusual electronic structures and growing synthetic potential. Herein, we describe the synthesis of a family of antimony(I) and bismuth(I) cations supported by a bis(silylene) ligand [(TBDSi₂)Pn][BAr^F₄] (TBD = 1, 8, 10, 9-triazaboradecalin; Ar^F = 3,5-CF₃-C₆H₃; Pn = Sb, (2); Bi, (3)). The structures of 2 and 3 have been unambiguously characterized spectroscopically and by X-ray diffraction analysis and DFT calculations. They feature bis-coordinated Sb and Bi atoms which exhibit two lone pairs of electrons. The reactions of 2 and 3 with methyl trifluoromethane sulfonate provide a approach for the preparation of dicationic antimony(III) and bismuth(III) methyl complexes. Compounds 2 and 3 serve as 2e donors to group 6 metals (Cr, Mo), giving rise to ionic antimony and bismuth metal carbonyl complexes 6-9.

Mesoionic N-Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

An extended class of stable mesoionic N-heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO₂ as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero-couplings between amide and amine using CO₂ . These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal-free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

Crystal structure of N-butyl-2,3-bis-(di-cyclo-hexyl-amino)-cyclo-propeniminium chloride benzene monosolvate

N-Butyl-2,3-bis-(di-cyclo-hexyl-amino)-cyclo-propenimine (1) crystallizes from benzene and hexa-nes in the presence of HCl as a mono-benzene solvate of the hydro-chloride salt, [1H]Cl·C6H6 or C31H54N3 +·Cl-·C6H6, in the P21/n space group. The protonation of 1 results in the generation of an aromatic structure based upon the delocalization of the cyclo-propene double bond around the cyclo-propene ring, giving three inter-mediate C-C bond lengths of ∼1.41 Å, and the delocalization of the imine-type C-N double bond, giving three inter-mediate C-N bond lengths of ∼1.32 Å. Ion-ion and ion-benzene packing inter-actions are described and illustrated.

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates

We report the synthesis and the reactivity of 1,2,3-triazolin-5-imine type mesoionic imines (MIIs). The MIIs are accessible by a base-mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O-stretching frequencies in MII-CO2 and -Rh(CO)2 Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N-heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent-induced fluorine-specific interactions and undergo C-H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO-LUMO gap compared to other related ligands. MIIs are potentially able to act as both π-donor and π-acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.

A cyclic divalent N(I) species isoelectronic to carbodiphosphoranes

The formation of a rare type of diphosphazenium cation is described. Its synthesis features a unique oxidative dealkylation of an iminophosphorane-phosphole by a silver(I) salt. DFT study of this compound reveals the low valent character of the N(I) center.

Recent Progress in Luminescent Cu(I) Halide Complexes: A Mini-Review

Copper(I) halide complexes are well sought-after materials due to their rich structural diversities and photophysical properties. Profoundly, there is a direct relationship between each structural variation and luminescence of these complexes, for a purported use. In this review, recent publications within the last 2 years about copper(I) halide complexes, centering on their structural dimensionalities with derivatives of nitrogen, phosphorus, and sulfur ligands, have been considered alongside their effects on luminescence.

Promoting halogen-bonding catalyzed living radical polymerization through ion-pair strain

Discovering efficient catalysts is highly desired in expanding the application of halogen-bonding catalysis. We herein report our findings on applying triaminocyclopropenium (TAC) iodides as highly potent catalysts for halogen-bonding catalyzed living radical polymerization. Promoted by the unique effect of ion-pair strain between the TAC cation and the iodide anion, the TAC iodides showed high catalytic efficiency in the halogen-bonding catalysis toward radical generation, and surpassed the previously reported organic iodide catalysts. With the TAC iodide as catalyst, radical polymerization with a living feature was successfully realized, which shows general applicability with a variety of monomers and produced block copolymers. In addition, the TAC-iodides also showed promising feasibility in catalyzing the radical depolymerization of iodo-terminated polymethacrylates. Noteworthily, the catalytic capacity of the TAC iodides is demonstrated to be closely related to the electronic properties of the TAC cation, which offers a molecular platform for further catalyst screening and optimization.

Promoted by the unique effect of ion-pair strain between the triaminocyclopropenium (TAC) cation and its iodide counter-anion, the TAC iodides showed high catalytic efficiency in the halogen-bonding catalysis toward radical polymerization.

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

Experimental and Computational Study on Photophysical Properties of Mesoionic Chalcogenones

N-Heterocyclic carbene adducts with main group elements (NHC=E) have aroused great interest and have been widely investigated in coordination chemistry. Among them, N-heterocyclic carbene adducts with chalcogens (NHC=Ch) have been known for a long time. Their investigations mostly focused on synthesis, coordination chemistry and electrochemistry. Their photophysical properties still remain unexplored. In this work, the photophysical properties of mesoionic carbene adducts with sulfur and selenium have been investigated both in solution and solid state. These compounds showed blue fluorescence in dichloromethane. While in solid state, orange to red room-temperature phosphorescence can be observed, and dual emission was found in mesoionic thiones. Furthermore, time-dependent density functional theory (TD-DFT) calculations were used to obtain insights into the luminescent mechanism.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-μ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Trisaminocyclopropenium Cations as Small-Molecule Organic Fluorophores: Design Guidelines and Bioimaging Applications

The discovery of fluorescence two centuries ago ushered in, what is today, an illuminating field of science rooted in the rational design of photochromic molecules for task-specific bio-, material-, and medical-driven applications. Today, this includes applications in bioimaging and diagnosis, photodynamic therapy regimes, in addition to photovoltaic devices and solar cells, among a vast multitude of other usages. In furthering this indispensable area of daily life and modern-day scientific research, we report herein the synthesis of a class of trisaminocyclopropenium fluorophores along with a systematic investigation of their unique molecular and electronic dependent photophysical properties. Among these fluorophores, tris[N(naphthalen-2-ylmethyl)phenylamino] cyclopropenium chloride (TNTPC) displayed a strong photophysical profile including a 0.92 quantum yield ascribed to intramolecular charge transfer and intramolecular through-space conjugation. Moreover, this cyclopropenium-based fluorophore functions as a competent imaging agent for DNA visualization and nuclear counterstaining in cell culture. To facilitate the broader use of these compounds, design principles supported by density functional theory calculations for engineering analogs of this class of fluorophores are offered. Collectively, this study adds to the burgeoning interest in cyclopropenium compounds and their unique properties as fluorophores with uses in bioimaging applications.

Highly Efficient Transfer Hydrogenation Catalysis with Tailored Pyridylidene Amide Pincer Ruthenium Complexes

The rational optimization of homogeneous catalysts requires ligand platforms that are easily tailored to improve catalytic performance. Here, it is demonstrated that pyridylidene amides (PYAs) provide such a platform to custom-shape transfer hydrogenation catalysts with exceptional activity. Specifically, a series of meta-PYA pincer ligands with differently substituted PYA units has been synthezised and coordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)(MeCN)₃ ](PF₆ )₂ (R=OMe, Me, H, Cl, CF₃ ). Analytic studies including ¹ H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography reveal a direct influence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones, with activities directly encoded by the PYA substitution pattern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh₃ . While complexes [Ru(R-PYA-pincer)(PPh₃ )(MeCN)₂ ](PF₆ )₂ were only isolated for R=H, Me, an in situ protocol was developed to generate these complexes in situ for R=OMe, Cl, CF₃ by using a 1:2 ratio of the complexes and PPh₃ . This in situ protocol together with a short catalyst pre-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYA ligand and reached turnover frenquencies of 210 000 h^-1 under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the most active transfer hydrogenation systems known to date.

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent NI compounds and can be represented with the general formula L → N+ ← L, where L is an electron donating ligand. In the recent past, several divalent NI compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N+ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent NI compounds (nitreones) which carry coordination bonds between ligands and N+ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent NI compounds have been explored.

Utilizing the Azaazulene Scaffolds in the Design of New Organic Superbases

New neutral organic superbases with 1-azaazulene(s) as a molecular backbone are computationally designed, employing two basic substituents: dimethylaminocyclopropen-imines (CPI) and dimethylaminocyclopropeniminophosphazenes (CPI-P). Their proton affinities, gas basicities, and pK _a values in acetonitrile are obtained using density functional theory. Azaazulenes substituted with CPI have a computed PA in the gas phase ranging between 272.9 and 306.8 kcal mol^-1, with pK _a values in acetonitrile between 28.8 and 36 units. The substitution with the CPI-P group resulted in even stronger superbases, with a PA from 296.5 to 335.2 kcal mol^-1 and corresponding pK _a values from 33.9 to 50 units. This exceptionally strong thermodynamic basicity is accompanied by very high kinetic basicity as well; contrary to typical proton sponges, the release of a proton from the conjugate superbase does not demand high activation energy. Because synthetic routes for both substituents and azaazulenes are already known, newly designed superbases represent suitable targets for synthesis and application.

Donor→acceptor coordination interactions in 1,3-bis(NHC)triazenyl Cations: An electronic structure analysis

Donor→acceptor coordination interactions (L → N) between ligands and nitrogen center as in L → N^⊕  ← L were reported in the recent past. This article describes the possibility of L → N coordination interactions in triazenyl cation species L → N₃ ^⊕  ← L. A few 1,3-bis(NHC)triazenyl cation species were experimentally known, the electronic structure analysis reported in this work reveals the presence of L → N (donor→acceptor) interactions in these species. Molecular orbital analysis, NBO charge analysis, energy decomposition analysis, and so forth, confirm the possibility of L → N coordination bond character. Ten molecules with the general formula L → N₃ ^⊕  ← L have been designed carrying L → N₃ ^⊕  ← L interactions. © 2019 Wiley Periodicals, Inc.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Azines: synthesis, structure, electronic structure and their applications

Azines (2,3-diaza-1,3-butadienes): structure, electronic structure, tautomerism, and their applications in organic synthesis, medicinal chemistry and materials chemistry.

Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters

Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm^R)₃ that can bind [Fe₄S₄]²⁺ or [Fe₆S₆]³⁺ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm^R)₃ ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm^Me)₃ (tris(1,3,5-(3-( N, N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] and 3 equiv of NaBPh₄ furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] provides (L(NIm^Me)₃)Fe₆S₆Cl₆ in high yield. Formation of an [Fe₆S₆]³⁺ cluster using L(NIm^Tol)₃ (tris(1,3,5-(3-( N, N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe₄S₄]²⁺ cluster [(L(NIm^Tol)₃)(Fe₄S₄Cl)][BPh₄] is cleanly generated when 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe₆S₆]³⁺ clusters but not [Fe₄S₄]²⁺ clusters. Thus, the structure and nuclearity of L(NIm^R)₃-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.

NL2+ Systems as New-Generation Phase-Transfer Catalysts

Phase-transfer catalysts (PTCs), currently, are one of the most important tools of chemists for performing organic reactions. PTCs accelerate several types of reactions in biphasic systems, giving excellent yields of the desired product. Most of the PTCs belong to the general formula NR₄⁺X^-. In the recent past, several compounds possessing a novel scaffold with the general formula NL₂⁺X^- have been reported as PTCs. In the NL₂⁺ species, a nitrogen atom with a formal positive charge accepts electron density from electron-donating ligands. Electronic structure studies reported in the literature confirmed the possibility of L → N coordination (donor-acceptor) interactions in these species, and thus, this class of compounds are known as divalent N^I compounds. These species are reported to exhibit better catalytic potential in comparison to the traditional NR₄⁺ systems. Some of the NL₂⁺ systems are found to be useful in asymmetric phase-transfer catalysis. Thus, these systems offer extensive opportunities for exploring the catalytic properties and novel mechanistic aspects associated with their unique electronic structure. In this paper, the synthesis, electronic, and structural properties and the applications in catalysis of the NL₂⁺-based PTCs are reviewed with their bright future scope in catalytic organic chemistry.

Synthesis and Characterization of Segmented Polyurethanes Containing Trisaminocyclopropenium Carbocations

Diol-functionalized trisaminocyclopropenium (TACP) carbocations were used as chain extenders in a two-step synthesis of a segmented polyurethane. Differential scanning calorimetry demonstrated significant differences in the crystallization behavior of the poly(tetramethylene oxide) soft segment when minor changes were made to the TACP structure and when compared to a control that was chain extended with butane diol. Fourier transform infrared spectroscopy was used to characterize the different level of hydrogen bonding in the polymers and showed that the bulky, charged TACP chain extender limited hydrogen bonding interactions when compared to the control. Dynamic mechanical analysis was used to probe the thermomechanical behavior of polymers that showed that the TACP-containing polymers were much more resistant to flow at high temperatures when compared to the control. Small-angle X-ray scattering showed a phase separated morphology for all the polymers tested. Tensile testing of the TACP polyurethanes demonstrated an elastic response over a wide range of strain, followed by a significant strain hardening. These results suggest a morphology of ionic aggregates rather than hard segment physical cross-links.

Can Remote N-Heterocyclic Carbenes Coordinate with Main Group Elements? Synthesis, Structure, and Quantum Chemical Analysis of N+ -Centered Complexes

Remote N-heterocyclic carbenes (rNHCs), such as N-methyl-4-pyridylidene, are known to form coordination complexes with TMs. Herein, it is established that rNHCs can also coordinate to the N⁺ centre. Synthesis of some novel divalent N^I complexes with the general formula (rNHC)→N⁺ ←(NHC) and (rNHC)→N⁺ ←(rNHC) was achieved, and X-ray diffraction studies supported the coordination bond character between the rNHCs and the N⁺ centre. Quantum chemical analysis established the presence of divalent N^I character at the central nitrogen in these systems.

Synthesis and physical properties of tris(dialkylamino)cyclopropenium dicyanamide ionic liquids

The synthesis and properties of 16 tris(dialkylamino)cyclopropenium (TDAC) cations with the dicyanamide (DCA) anion, [N(CN)₂]⁻, are described. D_3h- and C_3h-symmetric cations ([C₃(NR₂)₃]DCA (R = Me, Et, Pr, Bu, Pent, Hex, Dec) and [C₃(NRMe)₃]DCA (R = Bu, St), respectively) were synthesised by reaction of C₃Cl₅H with the corresponding amine. Reaction of the alkoxydiaminocyclopropenium salt [C₃(NEt₂)₂(OMe)]⁺ with amines led to a series of C_2v-symmetric salts [C₃(NEt₂)₂(NR₂)]DCA (R = Me, Bu, Hex) and two C_s-symmetric salts and [C₃(NEt₂)₂(NRMe)]DCA (R = Me, Bu). Similarly, [C₃(NMe₂)₂(OMe)]⁺, was used to prepare the C_s-symmetric salts [C₃(NMe₂)₂(NRMe)]DCA (R = Pr, Bu). In addition to characterisation by NMR, mass spectrometry and microanalysis, the salts were characterised by DSC, TGA, density, viscosity, conductivity and miscibility/solubility studies. Comparisons have been made with similar series of bistriflimide (NTf₂⁻) salts that have been previously reported to see whether the same trends are observed with a different anion.

An investigation of symmetry (D_3h, C_3h, C_2v and C_s) and alkyl chain length (6–60 alkyl carbon atoms) effects on the physical properties of peralkylated triaminocyclopropenium dicyanamide salts.

Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.

Sulfonamide vs. sulfonimide: tautomerism and electronic structure analysis of N-heterocyclic arenesulfonamides

Experimental and computational studies suggest a preference toward the sulfonimide tautomer in N-heterocyclic arenesulfonamide.

Donor–acceptor bonding in novel low-coordinated compounds of boron and group-14 atoms C–Sn

Donor–acceptor complexes of one, two or three atoms E = B, Si–Sn which are stabilized by σ-donor ligands L are discussed.

Synthesis, characterization and properties of amino acid ionic liquids derived from the triaminocyclopropenium cation

Amino acid ionic liquids based on the triaminocyclopropenium cation form a variety of structural types depending on the amino acid. Dications were obtained with Lys, His, Arg, and Cys.

Design, Synthesis, and Structural Analysis of Divalent N(I) Compounds and Identification of a New Electron-Donating Ligand

The dative-bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L→N←L')(+) arrangement are of special interest because of their therapeutic importance. This work reports the design and synthesis of novel chemical species with the general structural formula (L→N←L')(+) carrying the unusual ligand cyclohexa-2,5-diene-4-(diaminomethynyl)-1-ylidene. Four species belonging to the (L→N←L')(+) class carrying this unconventional ligand were synthesized. Quantum chemical and X-ray diffraction analyses showed that the electronic and geometric parameters are consistent with those of already reported divalent N(I) compounds. The molecular orbital analysis, geometric parameters, and spectral data clearly support the L→N and N←L' interactions in these species. The newly identified ligand has the properties of a reactive carbene and high nucleophilicity.