Weak Te,Te interactions through the looking glass of NMR spin-spin coupling

Phosphine and Selenoether peri-Substituted Acenaphthenes and Their Transition-Metal Complexes: Structural and NMR Investigations

A series of peri-substituted acenaphthene-based phosphine selenoether bidentate ligands Acenap(iPr2P)(SeAr) (L1-L4, Acenap = acenaphthene-5,6-diyl, Ar = Ph, mesityl, 2,4,6-trisopropylphenyl and supermesityl) were prepared. The rigid acenaphthene framework induces a forced overlap of the phosphine and selenoether lone pairs, resulting in a large magnitude of through-space 4JPSe coupling, ranging from 452 to 545 Hz. These rigid ligands L1-L4 were used to prepare a series of selected late d-block metals, mercury, and borane complexes, which were characterized, including by multinuclear NMR and single-crystal X-ray diffraction. The Lewis acidic motifs (BH3, Mo(CO)4, Ag+, PdCl2, PtCl2, and HgCl2) bridge the two donor atoms (P and Se) in all but one case in the solid-state structures. Where the bridging motif contained NMR-active nuclei (11B, 107Ag, 109Ag, 195Pt, and 199Hg), JPM and JSeM couplings are observed directly, in addition to the altered JPSe in the respective NMR spectra. The solution NMR data are correlated with single-crystal diffraction data, and in the case of mercury(II) complexes, they are also correlated with the solid-state NMR data and coupling deformation density calculations. The latter indicate that the through-space interaction dominates in free L1, while in the L1HgCl2 complex, the main coupling pathway is via the metal atom and not through the carbon framework of the acenaphthene ring system.

Evidence for and evaluation of fluorine-tellurium chalcogen bonding

In the field of noncovalent interactions, chalcogen bonding (ChB) involving the tellurium atom is currently attracting much attention in supramolecular chemistry and in catalysis. However, as a prerequisite for its application, the ChB should be studied in solution to assess its formation and, if possible, to evaluate its strength. In this context, new tellurium derivatives bearing CH2F and CF3 groups were designed to exhibit Te⋯F ChB and were synthesized in good to high yields. In both types of compounds, Te⋯F interactions were characterized in solution by combining 19F, 125Te and HOESY NMR techniques. These Te⋯F ChBs were shown to contribute to the overall JTe-F coupling constants (94-170 Hz) measured in the CH2F- and CF3-based tellurium derivatives. Finally, a variable temperature NMR study allowed us to approximate the energy of the Te⋯F ChB, from 3 kJ mol-1 for the compounds with weak Te σ-holes to 11 kJ mol-1 for Te σ-holes activated by the presence of strong electron withdrawing substituents.

Speciation of Tellurium(VI) in Aqueous Solutions: Identification of Trinuclear Tellurates by 17O, 123Te, and 125Te NMR Spectroscopy

Tellurates have attracted the attention of researchers over the past decade due to their properties and as less toxic forms of tellurium derivatives. However, the speciation of Te(VI) in aqueous solutions has not been comprehensively studied. We present a study of the equilibrium speciation of tellurates in aqueous solutions at a wide pH range, 2.5-15 by ¹⁷O, ¹²³Te, and ¹²⁵Te NMR spectroscopy. The coexistence of monomeric, dimeric, and trimeric oxidotellurate species in chemical equilibrium at a wide pH range has been shown. NMR spectroscopy, DFT computations, and single-crystal X-ray diffraction studies confirmed the formation and coexistence of trimeric tellurate anions with linear and triangular structures. Two cesium tellurates, Cs₂[Te₄O₈(OH)₁₀] and Cs₂[Te₂O₄(OH)₆], were isolated from the solution at pH 5.5 and 9.2, respectively, and studied by single-crystal X-ray diffractometry, revealing dimeric and tetrameric tellurate anions in corresponding crystal structures.

Tellurium-Modified Nucleosides, Nucleotides, and Nucleic Acids with Potential Applications

Tellurium was successfully incorporated into proteins and applied to protein structure determination through X-ray crystallography. However, studies on tellurium modification of DNA and RNA are limited. This review highlights the recent development of Te-modified nucleosides, nucleotides, and nucleic acids, and summarizes the main synthetic approaches for the preparation of 5-PhTe, 2'-MeTe, and 2'-PhTe modifications. Those modifications are compatible with solid-phase synthesis and stable during Te-oligonucleotide purification. Moreover, the ideal electronic and atomic properties of tellurium for generating clear isomorphous signals give Te-modified DNA and RNA great potential applications in 3D crystal structure determination through X-ray diffraction. STM study also shows that Te-modified DNA has strong topographic and current peaks, which immediately suggests potential applications in nucleic acid direct imaging, nanomaterials, molecular electronics, and diagnostics. Theoretical studies indicate the potential application of Te-modified nucleosides in cancer therapy.

Quantum chemical calculations of 77 Se and 125 Te nuclear magnetic resonance spectral parameters and their structural applications

An accurate quantum chemical (QC) modeling of ⁷⁷ Se and ¹²⁵ Te nuclear magnetic resonance (NMR) spectra is deeply involved in the NMR structural assignment for selenium and tellurium compounds that are of utmost importance both in organic and inorganic chemistry nowadays due to their huge application potential in many fields, like biology, medicine, and metallurgy. The main interest of this review is focused on the progress in QC computations of ⁷⁷ Se and ¹²⁵ Te NMR chemical shifts and indirect spin-spin coupling constants involving these nuclei. Different computational methodologies that have been used to simulate the NMR spectra of selenium and tellurium compounds since the middle of the 1990s are discussed with a strong emphasis on their accuracy. A special accent is placed on the calculations resorting to the relativistic methodologies, because taking into account the relativistic effects appreciably influences the precision of NMR calculations of selenium and, especially, tellurium compounds. Stereochemical applications of quantum chemical calculations of ⁷⁷ Se and ¹²⁵ Te NMR parameters are discussed so as to exemplify the importance of integrated approach of experimental and computational NMR techniques.

Chalcogen Bonding: An Overview

In the last few decades, "unusual" noncovalent interactions like anion-π and halogen bonding have emerged as interesting alternatives to the ubiquitous hydrogen bonding in many research areas. This is also true, to a somewhat lesser extent, for chalcogen bonding, the noncovalent interaction involving Lewis acidic chalcogen centers. Herein, we aim to provide an overview on the use of chalcogen bonding in crystal engineering and in solution, with a focus on the recent developments concerning intermolecular chalcogen bonding in solution-phase applications. In the solid phase, chalcogen bonding has been used for the construction of nano-sized structures and the self-assembly of sophisticated self-complementary arrays. In solution, until very recently applications mostly focused on intramolecular interactions which stabilized the conformation of intermediates or reagents. In the last few years, intermolecular chalcogen bonding has increasingly also been exploited in solution, most notably in anion recognition and transport as well as in organic synthesis and organocatalysis.

Visualization of Electron Paramagnetic Resonance Hyperfine Structure Coupling Pathways

The close relation between the EPR hyperfine coupling constant and NMR indirect spin-spin coupling constant is well-known. For example, the Karplus-type dependence of hyperfine constants on the dihedral angle, originally proposed for NMR spin-spin coupling, is widely used in pNMR studies. In the present work we propose a new tool for visualization of hyperfine coupling pathways based on our experience with visualization of NMR indirect spin-spin couplings. The plotted 3D-function is the difference between the total electron densities when the magnetic moment of the nucleus of interest changes its sign and as such is an observable from the physical point of view. It has been shown that it is proportional to the linear response of the spin density to the nuclear spin (i.e., magnetic moment). In contrast to the widely used visualization of spin density, our new approach depicts only the part of the electron cloud of a molecule that is affected by the interaction of the unpaired electron(s) with the desired nuclear magnetic moment. Because visualization of NMR spin-spin couplings and hyperfine interaction is based on the same ideas and done using similar techniques, it allows a direct comparison of the corresponding pathways for the two phenomena so as to analyze their resemblance and/or dissimilarity.

Conformational analysis of small molecules: NMR and quantum mechanics calculations

This review deals with conformational analysis in small organic molecules, and describes the stereoelectronic interactions responsible for conformational stability. Conformational analysis is usually performed using NMR spectroscopy through measurement of coupling constants at room or low temperature in different solvents to determine the populations of conformers in solution. Quantum mechanical calculations are used to address the interactions responsible for conformer stability. The conformational analysis of a large number of small molecules is described, using coupling constant measurements in different solvents and at low temperature, as well as recent applications of through-space and through-hydrogen bond coupling constants JFH as tools for the conformational analysis of fluorinated molecules. Besides NMR parameters, stereoelectronic interactions such as conjugative, hyperconjugative, steric and intramolecular hydrogen bond interactions involved in conformational preferences are discussed.

Tellurium: a maverick among the chalcogens

This tutorial review elucidates the fundamental concepts necessary for an understanding of the unique structures and reactivities of tellurium compounds.

Modular functionalized polyphosphines for supported materials: previously unobserved31P-NMR «through-space» ABCD spin systems and heterogeneous palladium-catalysed C–C and C–H arylation

Functionalized modular polyphosphines introduce a conceptual novelty in controlling both implantation and conformation of donors in immobilized catalysts.

Electrochemically informed synthesis: oxidation versus coordination of 5,6-bis(phenylchalcogeno)acenaphthenes

Chalcogen dications: Facile synthesis of E--E bonded dications can be readily achieved. Radical cations are identified as the intermediates.