Synthesis, structural studies and computational evaluation of cyclophanes incorporating imidazole-2-selones

We report new cyclophanes containing imidazole-2-selone groups linked by xylylene rings. A set of imidazole-2-selone cyclophanes is synthesized by reaction corresponding to imidazolium cyclophanes with selenium in the presence of K₂CO₃. The structural behavior of the new imidazole-2-selone cyclophanes was determined by ¹H and ¹³C NMR spectra and X-ray diffraction studies. Cyclophanes incorporating o-xylylene or mesitylene-m-cyclophane linked by selone groups were mutually syn in both the solid state and solution, and the cyclophanes showed a conformation similar to the cone conformation of calix[4]arenes. Cyclophanes incorporating p-xylylene or m-xylylene linked by selone groups showed two conformations in the solution: one mutually syn and the other mutually anti. There was no interconversion for both conformations observed on the NMR timescale. In the solid state, three conformations were detected for the p-xylylene-linked cyclophane: one is mutually syn and the other two are mutually anti and partial cone conformations. In the m-xylylene-linked case, only anti-conformation was characterized in the solid state. A density functional analysis was conducted to interpret the stability of the studied compounds and shed light on their origin. The energy preference analysis is in consistent agreement with the observed geometries and their co-existence.

Efficient Suzuki coupling over novel magnetic nanoparticle: Fe3O4/L-(+)-tartaric acid/Pd(0)

A new eco-friendly catalytic system is devised for C-C bond formation through Suzuki coupling, using an impressive nanocatalyst (Fe₃O₄/L-(+)-tartaric acid/Pd-NPs). It contains immobilized palladium (0) onto magnetite nanoparticles, stabilized by tartaric acid, and is characterized by FT-IR, XRD, EDS, SEM, TEM, TGA, and VSM. The catalyst is used in an efficient synthesis of biaryls in EtOH/H₂O (1:1), in the presence of K₂CO₃. Our Fe₃O₄/tartaric acid/Pd-NPs exhibit magnetic recoverability and reusability for five cycles without measurable loss of its activity.

Organoselenium ligands for heterogeneous and nanocatalytic systems: development and applications

Organoselenium ligands have attracted great attention among researchers during the past two decades. Various homogeneous, heterogeneous and nanocatalytic systems have been designed using such ligands. Although reports on selenium ligated homogeneous catalysts are quite high in number, significant work has also been done on the development of heterogeneous and nanocatalytic systems using organoselenium ligands. A review article, focusing on the utility of organoselenium compounds in the development of catalytic systems, was published in 2012 (A. Kumar, G. K. Rao, F. Saleem and A. K. Singh, Dalton Trans., 2012, 41, 11949). Moreover, it mainly covered the homogeneous catalysts. There are no review articles in the literature on heterogeneous and nanocatalytic systems designed using organoselenium compounds and their applications. Hence, this perspective aims to cover the developments pertaining to the synthetic aspects of such catalytic systems (using organoselenium compounds) and their applications in catalysis of a variety of chemical transformations. Salient features and advantages of organoselenium compounds have also been highlighted to justify the rationale behind their use in catalyst development. Their performance in various chemical transformations [viz. Suzuki-Miyaura coupling, Heck coupling, Sonogashira coupling, O-arylation of phenol, transfer hydrogenation of aldehydes and ketones, aldehyde-alkyne-amine (A3) coupling, hydration of nitriles, conversion of aldehydes to amides, cross-dehydrogenative coupling (CDC), photodegradation of substrates (formic acid, methylene blue), reduction of nitrophenols, electrolysis (hydrogen evolution reaction and oxygen reduction reactions), organocatalysis and dye sensitized solar cells] and relevant aspects of catalytic processes (such as recyclability, substrate scope and green aspects) have been critically analyzed. Future perspectives have also been discussed.

Organosulfur, organoselenium, and organotellurium ligands in the development of palladium, nickel, and copper-based catalytic systems for Heck coupling

Organosulfur, organoselenium, and organotellurium ligands in designing Pd, Ni, and Cu-based homogeneous, heterogeneous, and nanocatalytic systems for Heck coupling.

CO-free, aqueous mediated, instant and selective reduction of nitrobenzene via robustly stable chalcogen stabilised iron carbonyl clusters (Fe3E2(CO)9, E = S, Se, Te)

Highly stable and thermally robust iron chalcogenide carbonyl clusters Fe₃E₂(CO)₉ (E = S, Se or Te) have been explored for the reduction of nitrobenzene. A 15 min thermal heating of an aqueous solution of nitrobenzene and hydrazine hydrate in the catalytic presence of Fe₃E₂(CO)₉ (E = S, Se or Te) clusters yield average to excellent aniline transformations. Among the S, Se and Te based iron chalcogenised carbonyl clusters, the diselenide cluster was found to be most efficient and produce almost 90% yield of the desired amino product, the disulfide cluster was also found to be significantly active, produce the 85% yield of amino product, while the ditelluride cluster was not found to be active and produced only 49% yield of the desired product. The catalyst can be reused up to three catalytic cycles and it needs to be dried in an oven for one hour prior to reuse for the best results. The developed method is inexpensive, environmentally benign, does not require any precious metal or a high pressure of toxic CO gas and exclusively brings the selective reduction of the nitro group under feasible and inert free conditions.

In this study, a strongly feasible method for the reduction of nitrobenzene has been developed through highly stable and thermally robust iron chalcogenide carbonyl clusters Fe₃E₂(CO)₉ (E = S, Se or Te).

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Half-sandwich (η5-Cp*)Rh(iii) complexes of pyrazolated organo-sulfur/selenium/tellurium ligands: efficient catalysts for base/solvent free C-N coupling of chloroarenes under aerobic conditions

Three new pyrazolated chalcogenoether ligated Rh(iii) half-sandwich complexes (1-3) were synthesised by the thermal reaction of chalcogenoether (S, Se and Te) substituted 1H-pyrazole ligands (L1-L3) and [(η5-C5Me5)RhCl]2 in methanol. The complexes were fully characterised by various spectroscopic techniques, and the molecular structures of complexes 1 and2 were also established through single crystal X-ray crystallographic analysis, which indicates a pseudo-octahedral half-sandwich piano-stool geometry around the rhodium metal. All three complexes were found to be thermally stable and insensitive towards air and moisture. One mol% of Rh(iii) complexes (1-3) along with 10 mol% of Cu(OAc)2 were explored for the Buchwald-Hartwig type C-N coupling reactions of amine and aryl chloride. Good to excellent yields (89-92%) of the coupling products were obtained with seleno- and thio-ether functionalised pyrazolated Rh(iii) complexes (1 and 2), while an average yield (39%) was obtained with the telluro-ether functionalised complex (3). In contrast to the previously reported C-N coupling reactions the present reaction works under solvent- and base-free conditions, and the coupling reaction is accomplished in just 6 h with a high yield of the coupling product. The present methodology was also found to be efficient for a wide variety of functionalised aryl halides, and aliphatic or aromatic amines (1° and 2°). Moreover, the reaction also enables the C-N coupling of electron-withdrawing substrates and base-sensitive functionalities.

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.