Intramolecular Nonbonded Interactions Between Divalent Selenium Centers with Donor and Acceptor Substituents

From Noncovalent Chalcogen-Chalcogen Interactions to Supramolecular Aggregates: Experiments and Calculations

This review considers noncovalent bonds between divalent chalcogen centers. In the first part we present X-ray data taken from the solid state structures of dimethyl- and diphenyl-dichalcogenides as well as oligoalkynes kept by alkyl-sulfur, -selenium, and -tellurium groups. Furthermore, we analyzed the solid state structures of medium sized (12-24 ring size) selenium coronands and medium to large rings with alkyne and alkene units between two chalcogen centers. The crystal structures of the cyclic structures revealed columnar stacks with close contacts between neighboring rings via noncovalent interactions between the chalcogen centers. To get larger space within the cavities, rings with diyne units between the chalcogen centers were used. These molecules showed channel-like structures in the solid state. The flexibility of the rings permits inclusion of guest molecules such as five-membered heterocycles and aromatic six-membered rings. In the second part we discuss the results of quantum chemical calculations. To treat properly the noncovalent bonding between chalcogens, we use diffuse augmented split valence basis sets in combination with electron correlation methods. Our model substances were 16 dimers consisting of two Me-X-Me (X = O, S, Se, Te) pairs and dimers of Me-X-Me/Me-X-CN (X = O, S, Se, Te) pairs. The calculations show the anticipated increase of the interaction energy from (Me-O-Me)₂ (-2.15 kcal/mol) to (Me-O-Me/Me-Te-CN) (-6.59 kcal/mol). An analysis by the NBO method reveals that in the case of the chalcogen centers O and S the hydrogen bridges between the molecules dominate. However, in the case of Se and Te the major bonding between the pairs originates from dispersion forces between the chalcogen centers. It varies between -1.7 and -4.0 kcal/mol.

Chalcogen bonding interactions in organic selenocyanates: from cooperativity to chelation

Organic selenocyanates form recurrent chain-like motifs ⋯Se(R)–CN⋯Se(R)–CN⋯ through short and linear chalcogen bonding Se⋯NC interactions. A chelating motif is also observed in a DMF solvate with two neighboring CH₂–SeCN groups linked to the DMF oxygen atom.

Synthesis of Terminal Ethynyl Aryl Selenides and Sulfides Based on the Retro-Favorskii Reaction of Hydroxypropargyl Precursors

The retro-Favorskii reaction is an excellent way to achieve terminal alkynes. Methodologies that connect the synthesis of terminal alkynes and organochalcogen motifs are important for the construction of novel compounds. Fourteen new terminal alkynes containing either C_sp -S or C_sp -Se bonds were selectively prepared through the retro-Favorskii reaction from the respective carbinol precursors. It was discovered that terminal chalcogen alkynes were stable for weeks if stored as a solution in hexanes.

5-Selenocyanatouracil: A Potential Hypoxic Radiosensitizer. Electron Attachment Induced Formation of Selenium Centered Radical

The propensity of 5-selenocyanatouracil (SeCNU) to decomposition induced by attachment of electron was scrutinized with the G3B3 composite quantum-chemical method and radiolytic studies. Favorable thermodynamic (Gibbs free reaction energy of -13.65 kcal/mol) and kinetic (Gibbs free activation energy of 1.22 kcal/mol) characteristics revealed by the G3B3 free energy profile suggest SeCNU to be sensitive to electron attachment. The title compound was synthesized in the reaction between uracil and selenocyanogen chloride in acetic acid. Then, an aqueous and deoxygenated solution of the HPLC purified compound containing tert-butanol as a hydroxyl radical scavenger was irradiated with X-rays. SeCNU radio-degradation results in two major products: the U-Se-Se-U dimer and the adduct of the ^●OtBu radical to the U-Se^● radical, U-Se-OtBu. The effects of radiolysis as well as the results of G3B3 calculations point to U-Se^● as the primary product of dissociative electron attachment to SeCNU. The MTT test shows that SeCNU is nontoxic in vitro in concentrations equal to or lower than 10^-6 M. Ionizing radiation will probably induce cytotoxic intra- and interstrand DNA cross-links as well as protein-DNA cross-links in the genomic DNA labeled with SeCNU.

Intra- and intermolecular Se...X (X = Se, O) interactions in selenium-containing heterocycles: 3-benzoylimino-5-(morpholin-4-yl)-1,2,4-diselenazole

In the selenium-containing heterocyclic title compound {systematic name: N-[5-(morpholin-4-yl)-3H-1,2,4-diselenazol-3-ylidene]benzamide}, C13H13N3O2Se2, the five-membered 1,2,4-diselenazole ring and the amide group form a planar unit, but the phenyl ring plane is twisted by 22.12 (19)° relative to this plane. The five consecutive N—C bond lengths are all of similar lengths [1.316 (6)–1.358 (6) Å], indicating substantial delocalization along these bonds. The Se...O distance of 2.302 (3) Å, combined with a longer than usual amide C=O bond of 2.252 (5) Å, suggest a significant interaction between the amide O atom and its adjacent Se atom. An analysis of related structures containing an Se—Se...X unit (X = Se, S, O) shows a strong correlation between the Se—Se bond length and the strength of the Se...X interaction. When X = O, the strength of the Se...O interaction also correlates with the carbonyl C=O bond length. Weak intermolecular Se...Se, Se...O, C—H...O, C—H...π and π–π interactions each serve to link the molecules into ribbons or chains, with the C—H...O motif being a double helix, while the combination of all interactions generates the overall three-dimensional supramolecular framework.