Chalcogen Bonding with Diaryl Ditellurides: Evidence from Solid State and Solution Studies

[Tc(NO)Cl2(PPh3)2(CH3CN)] and Its Reactions with 2,2'-Dipyridyl Dichalcogenides

The sparingly soluble technetium(I) complex [TcI(NO)Cl2(PPh3)2(CH3CN)] (1) slowly dissolves during reactions with 2,2'-dipyridyl ditelluride, (2-pyTe)2, 2,2'-dipyridyl diselenide, (2-pySe)2, or 2,2'-dipyridyl disulfide, (2-pyS)2, under formation of deeply colored solutions. Blue (Te compound) or red solids (Se compound) of the composition [{TcI(NO)Cl2(PPh3)2}2{µ2-(2-pyE)2}], E = Te (3), Se (4), precipitate from the reaction solutions upon addition of toluene. They represent the first technetium complexes with dichalcogenides. While [{TcI(NO)Cl2(PPh3)}2{µ2-(2-pyTe)2}] (3) is the sole product, a small amount of a second product, [TcII(NO)Cl2(PPh3)(2-pySe)] (5), was obtained from the respective mother solution of the reaction with the diselenide. From the corresponding reaction between 1 and (2-pyS)2, the technetium(II) compound, [TcII(NO)Cl2(PPh3)(2-pyS)] (6), could be isolated exclusively. The products were studied by single-crystal X-ray diffraction and spectroscopic methods including 99Tc NMR for the technetium(I) products and EPR spectroscopy for the Tc(II) complexes. The experimental results are accompanied by DFT considerations, which help to rationalize the experimental observations.

Telluronium-Catalyzed Halogenation Reactions: Chalcogen-Bond Activation of N-Halosuccinimides and Catalysis

The ability of triaryltelluronium salts to interact with N-halosuccinimides (NXS) through chalcogen bonding (ChB) in the solid state and in solution is demonstrated herein. Cocrystals of the triaryltelluronium bearing two CF3 electron-withdrawing groups per aryl ring with N-chloro-, N-bromo- and N-iodosuccinimide (respectively NCS, NBS and NIS) were analyzed by X-ray diffraction, evidencing a ChB between tellurium and the carbonyl group of NXS. This ChB was confirmed in solution by NMR spectroscopy, especially by 125Te NMR titration experiment, which allowed the determination of the association constant (Ka) between the telluronium and NBS. The so-obtained Ka value of 17.3±0.6 M-1 indicated a moderate interaction in solution because of the competitive role of the solvent. The strength of the Te⋅⋅⋅O ChB was however sufficient enough to promote the catalytic halofunctionalization of aromatics and of alkenes such as the intra- and intermolecular haloalkoxylation and haloesterification of alkenes.

Affinity of Telluronium Chalcogen Bond Donors for Lewis Bases in Solution: A Critical Experimental-Theoretical Joint Study

Telluronium salts [Ar2 MeTe]X were synthesized, and their Lewis acidic properties towards a number of Lewis bases were addressed in solution by physical and theoretical means. Structural X-ray diffraction analysis of 21 different salts revealed the electrophilicity of the Te centers in their interactions with anions. Telluroniums' propensity to form Lewis pairs was investigated with OPPh3 . Diffusion-ordered NMR spectroscopy suggested that telluroniums can bind up to three OPPh3 molecules. Isotherm titration calorimetry showed that the related heats of association in 1,2-dichloroethane depend on the electronic properties of the substituents of the aryl moiety and on the nature of the counterion. The enthalpies of first association of OPPh3 span -0.5 to -5 kcal mol-1 . Study of the affinity of telluroniums for OPPh3 by state-of-the-art DFT and ab-initio methods revealed the dominant Coulombic and dispersion interactions as well as an entropic effect favoring association in solution. Intermolecular orbital interactions between [Ar2 MeTe]+ cations and OPPh3 are deemed insufficient on their own to ensure the cohesion of [Ar2 MeTe ⋅ Bn ]+ complexes in solution (B=Lewis base). Comparison of Grimme's and Tkatchenko's DFT-D4/MBD-vdW thermodynamics of formation of higher [Ar2 MeTe ⋅ Bn ]+ complexes revealed significant molecular size-dependent divergence of the two methodologies, with MBD yielding better agreement with experiment.

Bidentate Ligand Driven Intramolecularly Te…O Bonded Organotellurium Cations from Synthesis, Stability to Catalysis

A new series of unsymmetrical phenyl tellurides derived from 2-N-(quinolin-8-yl) benzamide ligand has been synthesized in a practical manner by the copper-catalyzed method by using diaryl ditelluride and Mg as a reductant at room temperature. In order to augment the Lewis acidity of these newly formed unsymmetrical monotellurides, these have been transformed into corresponding unsymmetrical 2-N-(quinolin-8-yl)benzamide tellurium cations. Subsequently, these Lewis acidic tellurium cations were used as chalcogen bonding catalysts, enabling the synthesis of various substituted 1,2-dihydroquinolines by activating ketones with anilines under mild conditions. Moreover, the synthesized 2-N-(quinolin-8-yl)benzamide phenyl tellurium cation has also catalyzed the formation of β-amino alcohols in high regioselectivity by effectively activating epoxides at room temperature. Mechanistic insight by 1 H and 19 F NMR study, electrostatic surface potential (ESP map), control reaction in which tellurium cation reacted explosively with epoxide, suggested that the enhanced Lewis acidity of tellurium center seems responsible for efficient catalytic activities under mild conditions enabling β-amino alcohols with excellent regioselectivity and 1,2-dihydroquinolines with trifluoromethyl, nitro, and pyridylsubstitution, which were difficult to access.

Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?

Chalcogen bonding (ChB) is the non-covalent interaction occurring between chalcogen atoms as Lewis acid sites and atoms or groups of atoms able to behave as Lewis bases through their lone pair or π electrons. Analogously to its sister halogen bonding, the high directionality of this interaction was implemented for precise structural organizations in the solid state and in solution. Regarding catalysis, ChB is now accepted as a new mode of activation as demonstrated by the increased number of examples in the last five years. In the family of ChB catalysts, those based on tellurium rapidly appeared to overcome their lighter sulfur and selenium counterparts. In this review, we highlight the Lewis acid properties of tellurium-based derivatives in solution and summarize the start-of-the-art of their applications in catalysis.

2-Benzamide Tellurenyl Iodides: Synthesis and Their Catalytic Role in CO2 Mitigation

Benzamide-derived organochalcogens (chalcogen=S, Se, and Te) have shown promising interest in biological and synthetic chemistry. Ebselen molecule derived from benzamide moiety is the most studied organoselenium. However, its heavier congener organotellurium is under-explored. Here, an efficient copper-catalyzed atom economical synthetic method has been developed to synthesize 2-phenyl-benzamide tellurenyl iodides by inserting a tellurium atom into carbon-iodine bond of 2-iodobenzamides in one pot with 78-95 % yields. Further, the Lewis acidic nature of Te center and Lewis basic nature of nitrogen of the synthesized 2-Iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides enabled them as pre-catalyst for the activation of epoxide with CO2 at 1 atm for the preparation of cyclic carbonates with TOF and TON values of 1447 h-1 and 4343, respectively, under solvent-free conditions. In addition, 2-iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides have also been used as pre-catalyst for activating anilines and CO2 to form a variety of 1,3-diaryl ureas up to 95 % yield. The mechanistic investigation for CO2 mitigation is done by 125 Te NMR and HRMS studies. It seems that the reaction proceeds via formation of catalytically active Te-N heterocycle, an ebtellur intermediate which is isolated and structurally characterized.