Azolium Rings Research Articles

Chloro(η2,η2-cycloocta-1,5-diene){1-[(2-[(S)-1-(hydroxymethyl)-3-methylbutyl]amino)-2-oxoethyl]-3-(1-naphthalenylmethyl)benzimidazol-2-ylidene}rhodium(I)

Commercially available and air- and moisture-stable rhodium complex [Rh(OH)(cod)]2 (2) was utilized in the synthesis of [RhX(cod)(NHC)] (3). The presence of an OH group in complex 2 serves as an internal base, facilitating the deprotonation of the C–H bond of the azolium ring in the hydroxyamide-substituted benzimidazolium salt 1. This reaction between 1 and 2 proceeded in THF at room temperature without temperature control, affording the desired NHC/Rh complex 3 in excellent yield. The characterization of complex 3 was accomplished through NMR and HRMS analyses, revealing its existence as a diastereomeric mixture of two NHC/Rh complexes. Furthermore, its catalytic performance was briefly evaluated in the reaction between 2-naphthaldehyde (5) and phenylboronic acid (6).

Imidazol(in)ium-2-Thiocarboxylate Zwitterion Ligands: Structural Aspects in Coordination Complexes

Azolium-2-thiocarboxylate zwitterion ligands have emerged as a promising class of compounds in the field of coordination chemistry due to their unique structural features and versatile applications. These ligands are characterized by a positively charged azolium ring and a negatively charged thiocarboxylate moiety, making them capable of forming stable coordination complexes with various metal ions. One of the key structural aspects that make these ligands attractive for coordination chemistry is their ability to adopt diverse coordination modes with metal centers. The nature of these ligands enables them to engage in both monodentate and bidentate coordination, resulting in the formation of chelated complexes with enhanced stability and controlled geometry or the formation of polynuclear structures. This versatility in coordination behavior allows for the design of tailored ligands with specific metal-binding preferences, enabling the creation of unique and finely tuned coordination architectures. The azolium-2-thiocarboxylate zwitterionic ligands offer a promising platform for the design of coordination complexes with diverse structural architectures.

Synthesis, Characterization, and Biological Activity of Water-Soluble, Dual Anionic and Cationic Ruthenium–Arene Complexes Bearing Imidazol(in)ium-2-dithiocarboxylate Ligands

An efficient synthetic protocol was devised for the preparation of five cationic ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands from the [RuCl2(p-cymene)]2 dimer and 2 equiv of an NHC·CS2 zwitterion. The reactions proceeded cleanly and swiftly in dichloromethane at room temperature to afford the expected [RuCl(p-cymene)(S2C·NHC)]Cl products in quantitative yields. When the [RuCl2(p-cymene)]2 dimer was reacted with only 1 equiv of a dithiolate betaine under the same experimental conditions, a set of five bimetallic compounds with the generic formula [RuCl(p-cymene)(S2C·NHC)][RuCl3(p-cymene)] was obtained in quantitative yields. These novel, dual anionic and cationic ruthenium-arene complexes were fully characterized by various analytical techniques. NMR titrations showed that the chelation of the dithiocarboxylate ligands to afford [RuCl(p-cymene)(S2C·NHC)]+ cations was quantitative and irreversible. Conversely, the formation of the [RuCl3(p-cymene)]- anion was limited by an equilibrium, and this species readily dissociated into Cl- anions and the [RuCl2(p-cymene)]2 dimer. The position of the equilibrium was strongly influenced by the nature of the solvent and was rather insensitive to the temperature. Two monometallic and two bimetallic complexes cocrystallized with water, and their molecular structures were solved by X-ray diffraction analysis. Crystallography revealed the existence of strong interactions between the azolium ring protons of the cationic complexes and neighboring donor groups from the anions or the solvent. The various compounds under investigation were highly soluble in water. They were all strongly cytotoxic against K562 cancer cells. Furthermore, with a selectivity index of 32.1, the [RuCl(p-cymene)(S2C·SIDip)]Cl complex remarkably targeted the erythroleukemic cells vs mouse splenocytes.

Reaction of hydroxyamide-functionalized azolium salt with Ag2O: Three different transformations

We report three different transformations in the reaction of the hydroxyamide-functionalized azolium chloride 1 with Ag2O. These transformations were controlled by the N-substituent at the azolium ring of 1. Treatment of 1c (R = Me, alkyl) with Ag2O afforded the corresponding monodentate NHC-Ag (NHC represents N-heterocyclic carbene) complex, 6c, in 95% yield. In contrast, the reaction of 1b (R = Ph, aryl) with Ag2O yielded the corresponding 2-hydroxyoxazolidine-substituted azolium hydroxide, 7b, in 90% yield. In this transformation, perhaps the cyclization of the hydroxyamide moiety of 1b to 2-hydroxyoxazolidine occurred under the influence of the hydroxide counter anion. Several aryl-substituted azolium chlorides, 1d-f, were also converted into the corresponding cyclization products, 7d-f. However, the reaction of 1h (R = 2,4,6-Me3C6H2, aryl) with Ag2O afforded the corresponding NHC-Ag complex, 6h, in 93% yield, whereas the reaction of 1i (R = 4-MeOC6H4, aryl) with Ag2O provided a mixture of 6i and 7i in 56% and 32% yields, respectively. Moreover, the reaction of azolium chloride, 1j (R = 2,6-iPr2C6H3, aryl), which has a more sterically hindered substituent at the ortho-position of the aryl side-arm, was investigated. Treatment of 1j with Ag2O gave unexpected urea derivative, 8j, in 99% yield.

Functionalized N‐Heterocyclic Carbene Ligands for Dual Enantioselective Control in the Cu‐Catalyzed Conjugate Addition of Dialkylzinc Compounds to Acyclic Enones

AbstractA series of highly tunable, functionalized azolium compounds have been synthesized from chiral α‐amino acid derivatives such as β‐amino alcohols or α‐amino esters. The combination of a Cu salt and a chiral azolium efficiently facilitated the asymmetric conjugate addition (ACA) reactions of acyclic enones with dialkylzinc under ambient conditions without temperature control. Of the (hydroxy amide)‐functionalized azolium ligand precursors, which were derived from β‐amino alcohols, an azolium salt containing a tert‐butyl group as the stereodirecting group was found to be the best choice of ligand. Ligand screening revealed that the use of an azolium salt containing a sterically bulky alkyl substituent such as N‐CHRR′ on the azolium ring led to a marked increase in the enantioselectivity of the ACA reaction. Thus, a new efficient (hydroxy amide)‐functionalized azolium ligand precursor such as 26, which was prepared from 1‐(diphenylmethyl)benzimidazole and (S)‐tert‐leucinol, has been developed. The treatment of benzalacetone (30) with Et2Zn in the presence of catalytic amounts of [Cu(OTf)]2(C6H6) and 26 at room temperature gave the corresponding 1,4‐adduct 31 with excellent enantioselectivity (92 % ee). On the other hand, the enantioselectivity of the ACA reactions of acyclic enones with dialkylzinc was successfully reversed by the use of (ester amide)‐functionalized azolium salts, which were obtained from α‐amino esters. Several functionalized azolium ligand precursors from readily available serine esters have been designed and synthesized. Finally, it was found that 30 could react with Et2Zn under the influence of a bis(hexafluoroacetylacetonato)copper(II) salt and C2‐symmetric azolium salt 36 to afford ent‐31 in almost quantitative yield with 89 % ee. A plausible reaction mechanism for the switching of the stereoselectivity in the ACA reaction by these two functionalized azolium compounds is described.

Two Azolium Rings Are Better Than One: A Strategy for Controlling Catenation and Morphology in Zn and Cu Metal–Organic Frameworks

Four new azolium-containing metal–organic frameworks (MOFs) have been synthesized, where the azoliums are potential organocatalyst precursors. Modifying the number of azoliums on a standard biphenyl dicarboxylate strut affects the morphology or the degree of interpenetration in two representative types of MOFs: NbO-type Cu-paddlewheel 2D sheets and cubic IRMOFs.

Assembled Structures and Cation−Anion Interactions in Crystals of Alkylimidazolium and Alkyltriazolium Iodides with Ferrocenyl Substituents

To investigate cation−anion interactions in imidazolium salts with ferrocenyl substituents, 1-ferrocenyl-3-alkyl-imidazolium iodides and their 1-ferrocenylphenyl derivatives were prepared and crystallographically characterized. In addition, triazole and pyrazole derivatives were also prepared. The triazole derivatives exhibited higher melting points than the corresponding imidazole derivatives. The iodide anions in the crystals are generally in contact with the azolium rings and are surrounded by azolium hydrogens of the neighboring cations via short CH···I contacts. These interactions between ionic moieties form two-dimensional networks leading to layer-like assembled structures. The imidazolium salts with ferrocenylphenyl substituents exhibited π−π interactions between the cations, whereas no such interactions were seen in other salts whose molecules are arranged orthogonal to each other. The crystal structures of ferrocenylimidazole and ferrocenylphenylimidazole were also determined and compared with t...

Reversal of Stereoselectivity in the Cu-Catalyzed Conjugate Addition Reaction of Dialkylzinc to Cyclic Enone in the Presence of a Chiral Azolium Compound

Reversal of enantioselectivity in a Cu-catalyzed asymmetric conjugate addition reaction of dialkylzinc to cyclic enone with use of the same chiral ligand was successfully achieved. The reaction of 2-cyclohexen-1-one (30) with Et(2)Zn catalyzed by Cu(OTf)(2) in the presence of an azolium salt derived from a chiral beta-amino alcohol gave (S)-3-ethylcyclohexanone (31) in good enantioselectivity. Among a series of chiral azolium compounds examined, the benzimidazolium salt (10) having both a tert-butyl group at the stereogenic center and a benzyl substituent at the azolium ring was found to be the best choice of ligand in the Cu(OTf)(2)-catalyzed reaction. Good enantioselectivity was observed when the reaction was conducted by employing a benzimidazolium derivative rather than an imidazolium derivative. The influence of the substituent at the azolium ring on the stereoselectivity of the reaction was also examined. In addition, from the results of the reaction catalyzed by Cu(OTf)(2) combined with an azolium compound derived from (S)-leucine methyl ester, it was found that the hydroxy side chain in the chiral ligand is probably crucial for the enantiocontrol of the conjugate addition reaction. On the other hand, it was discovered from a screening test of copper species that the reversal of enantioselectivity was realized by allowing 30 to react with Et(2)Zn in the presence of Cu(acac)(2) combined with the same ligand precursor to afford (R)-31 as a major product. The influence of the stereodirecting group at the chiral ligand on the stereoselectivity in the Cu(acac)(2)-catalyzed reaction differed completely from that observed in the Cu(OTf)(2)-catalyzed reaction. Reaction with a cyclic enone consisting of a seven-membered ring such as 2-cyclohepten-1-one (40) resulted in increasing the enantioselectivity of the reaction. Thus, treatment of 40 with Et(2)Zn catalyzed by Cu(OTf)(2) combined with a benzimidazolium salt produced the corresponding (S)-conjugate adduct in a 92:8 enantiomer ratio (er), while the Cu(acac)(2)-catalyzed reaction with the same ligand afforded (R)-product in a 9:91 er.

Effects of gamma radiation on electrochemical properties of ionic liquids

The electrochemical properties of ionic liquids (ILs) make them attractive for possible replacement of inorganic salts in high temperature molten salt electrochemical processing of nuclear fuel. To be a feasible replacement solvent, ILs need to be stable in moderate and high doses of radiation without adverse chemical and physical effects. Here, we exposed seven different ILs to a 1.2 MGy dose of gamma radiation to investigate their physical and chemical properties as they related to radiological stability. The azolium-based ILs experienced the greatest change in appearance, but these ILs were chemically more stable to gamma radiation than some of the other classes of ILs tested, due to the presence of aromatic electrons in the azolium ring. All the ILs exhibited a decrease in their conductivity and electrochemical window (at least 1.1 V), both of which could affect the utility of ILs in electrochemical processing. The concentration of the irradiation decomposition products was less than 3 mole %, with no impurities detectable using NMR techniques.

Dinuclear gold(i) and gold(iii) complexes of bridging functionalized bis(N-heterocyclic carbene) ligands: synthesis, structural, spectroscopic and electrochemical characterizations

We report the synthesis of Au(I) and Au(III) complexes, involving alcohol functionalized bis(N-heterocyclic carbene) ligands. Two short reaction pathways lead to the diimidazolium precursors, namely 1,1'-(2,6-pyridinediyl)bis[3-(2-hydroxyethyl)-1H-imidazol-3-ium]diiodide (), 3,3'(methanediyl)bis[1-(2-hydroxy-2-methylpropyl)-1H-imidazol-3-ium]dibromide () and 3,3'-(1,3-propanediyl)bis[1-(2-hydroxy-2-methylpropyl)-1H-imidazol-3-ium]bis(4-methylbenzenesulfonate) (), in which the two azolium rings are bridged by a rigid pyridine unit or an aliphatic chain (C1 or C3). The Au(I) complexes [AuI(Lpy)]2(2+)[PF6-]2 () and [AuI(LC1)]2(2+)[PF6-]2 () were obtained by direct metallation of the salts and , respectively, in the presence of sodium acetate with Au(SMe(2))Cl, followed by an anionic metathesis in the presence of KPF6. The trimethylene compound [AuI(LC3)]2(2+)[PF6-]2 () was prepared by transmetallation between the related precursor [AgI(LC3)]2(2+)[PF6-]2 () and Au(SMe2)Cl. The Au(III) complexes, [AuIII(Lpy)Br2]2(2+)[PF6-]2 (), [AuIII(LC1)Br2]2(2+)[PF6-]2 () and [AuIII(LC3)Br2]2(2+)[PF6-]2 () were generated by oxidation of the corresponding Au(I) species with an excess of elemental bromine. Complexes , [AuI(LC1)]2(2+)[Br-]2 () and have been characterized by single-crystal X-ray diffraction analyses. The electrochemical and luminescence properties of the Au(I) and Au(III) compounds have been studied.

Azolium-Linked Cyclophanes: Effects of Structure, Solvent, and Counteranions on Solution Conformation Behavior

This paper describes the synthesis, structural characterization, and solution behavior of some xylyl-linked imidazolium and benzimidazolium cyclophanes decorated with alkyl or alkoxy groups. The addition of alkyl/alkoxy chains to the cyclophanes allows for studies in chlorinated solvents, whereas previous solution studies of azolium cyclophanes have generally required highly polar solvents. The azolium cyclophanes may exist in a syn/syn conformation (azolium rings mutually syn, arene rings mutually syn) or a syn/anti conformation (azolium rings mutually syn, arene rings mutually anti). The preferred conformation is significantly affected by (i) binding of bromide (ion pairing) to the protons on the imidazolium or benzimidazolium rings, which occurs in solutions of bromide salts of the cyclophanes in chlorinated solvents, and (ii) the addition of alkoxy groups to the benzimidazolium cyclophanes. These structural modifications have also led to cyclophanes that adopt conformations not previously identified for similar azolium cyclophane analogues. Detailed (1)H NMR studies for one cyclophane identified binding of bromide at two independent sites within the cyclophane.

Aliphatic versus Aromatic C−H Activation in the Formation of Abnormal Carbenes with Iridium: A Combined Experimental and Theoretical Study

The metalation of a series of C2-Me-substituted monoimidazolium and bisimidazolium salts to [Cp*IrCl2]2 is described. The reaction of the monoimidazolium salt provides the species Cp*Ir(aNHC)Cl2, in which the NHC shows an abnormal coordination mode. The use of the bisimidazolium salt provides different reaction patterns depending on the linker length between the two azolium rings. For the methylene-linked bisimidazolium salt, the only compound obtained shows an unusual type of coordination in which the chelating ligand is coordinated through an abnormal NHC and a methylene group resulting from the CH activation of the C2-Me group. For the ethylene-linked bisimidazolium salt, a similar product is obtained, together with the chelating bis-abnormal-NHC species. All compounds have been fully characterized by usual spectroscopic techniques, and X-ray molecular structures are described. The formation of the reaction products, in the case of the methylene linker, has been rationalized by means of DFT calculations with inclusion of solvent effects (PCM). The calculations could not discriminate the nature of the first metalation between direct deprotonation of the ligand by the base and metalation through C−H activation at Ir. However both cases point to a kinetic preference for first metalation at the C2-Me group. The second metalation process is the result of kinetically preferred C−H activation at the C5 position.

Use of azolium ylides in the synthesis of con-densed heterocyclic systems from 2-quinox-alylacetonitriles

The reaction of α-substituted 2-quinoxalylacetonitriles with 1-alkyl(aryl)imidazoles, -benzimidazoles, -1,2,4-triazoles and 5,6-dihydroimidazo[i,j]quinoline was studied. It was found that during the course of the reaction an unusually easy dealkylation of the azole ring takes place, while the aryl substituent is not split off. A reaction mechanism has been proposed including the formation of an ylide intermediate, followed by subsequent electrophilic attack on the C(2) position of the azolium ring. The applicability boundaries of the reaction studied and the spectral characteristics of the synthesized compounds were investigated.

Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds, part II: Polarographic and voltammetric reduction of some nitramines, pyrazolium, and imidazolium nitro derivatives

Reduction of 1-nitropyrazole (I) results in acidic media in a cleavage of the NN bond and formation of nitrous acid which is further reduced at more negative potentials. At pH > 4 a competitive reduction of the nitramine to nitrosamine occurs. Four-electron reduction of nitro derivatives of pyrazolium (II) and (III) and imidazolium (IV) and (V) salts results in a formation of a hydroxylamine derivative. The reduction follows the scheme: H+, e, e, H+, 2e, 2H+. In the reduction product the azolium ring is reduced at more negative potentials. Whereas the effect of pyrazolium and imidazolium rings on the reducibility of the nitro group is comparable, the interaction between the nitro group and the azolium ring markedly depends on the position of the nitro group on the ring.

Torsional angles and substituent effects in phenyl-substituted azoles by carbon-13 N.M.R. chemical shifts

The torsional angles in 31 N-phenyl-substituted azole derivatives have been estimated by use of the 13C substituent chemical shifts of the meta and para carbon atoms. Steric interaction between the C 5 substituent on the azole rings and the ortho hydrogen atoms of the benzene ring is the major cause of non-planarity in these compounds. The azole rings undergo structural deformation with varying substituents on the azole ring. The inductive effect of a number of azole and azolium rings is discussed.