Studies on the properties of organoselenium(IV) fluorides and azides

Catalytic 1,1-diazidation of alkenes

Compared to well-developed catalytic 1,2-diazidation of alkenes to produce vicinal diazides, the corresponding catalytic 1,1-diazidation of alkenes to yield geminal diazides has not been realized. Here we report an efficient approach for catalytic 1,1-diazidation of alkenes by redox-active selenium catalysis. Under mild conditions, electron-rich aryl alkenes with Z or E or Z/E mixed configuration can undergo migratory 1,1-diazidation to give a series of functionalized monoalkyl or dialkyl geminal diazides that are difficult to access by other methods. The method is also effective for the construction of polydiazides. The formed diazides are relatively safe by TGA-DSC analysis and impact sensitivity tests, and can be easily converted into various valuable molecules. In addition, interesting reactivity that geminal diazides give valuable molecules via the geminal diazidomethyl moiety as a formal leaving group in the presence of Lewis acid is disclosed. Mechanistic studies revealed that a selenenylation-deselenenylation followed by 1,2-aryl migration process is involved in the reactions, which provides a basis for the design of new reactions.

A Synthetic, Structural, Spectroscopic, and Computational Study of Alkali Metal-Thioether, -Selenoether, and -Telluroether Interactions

The rigid thioether- and selenoether-containing pro-ligands, 4,5-bis(phenylsulfido)-2,7,9,9-tetramethylacridan (H[AS2Ph2] (1)) and 4,5-bis(phenylselenido)-2,7,9,9-tetramethylacridan (H[ASe2Ph2] (2)), were deprotonated with one equiv of nBuLi to afford dimeric lithium complexes [Li(AE2Ph2)]2 (E = S (3), Se (4)) or with one equiv of KCH2Ph to afford the previously reported potassium complexes [K(AS2Ph2)(dme)]x (5) and [K(ASe2Ph2)(dme)2] (6). Attempts to prepare a direct telluroether analogue of compounds 1-2 were unsuccessful. However, the bulky selenoether- and telluroether-containing pro-ligands 4,5-bis(2,4,6-triisopropylphenylselenido)-2,7,9,9-tetramethylacridan (H[ASe2Tripp2] (7)) and 4,5-bis(2,4,6-triisopropylphenyltellurido)-2,7,9,9-tetramethylacridan (H[ATe2Tripp2] (8)) were accessed via the reaction of 4,5-dibromo-2,7,9,9-tetramethylacridan with three equiv of nBuLi, followed by the addition of two equiv of the corresponding diaryl dichalcogenide and quenching with dilute HCl(aq). The new selenoether- and telluroether-containing pro-ligands were subsequently deprotonated using KCH2Ph to afford [K(AE2Tripp2)(dme)2] (E = Se (9), Te (10)). Compounds 1-10 were characterized by 1H, 13C{1H}, 77Se{1H}, 125Te{1H}, and 7Li NMR spectroscopy, where applicable, and single-crystal X-ray structures were obtained for all lithium and potassium complexes (3-6 and 9-10). DFT calculations were also performed to assess the nature of bonding between the hard group 1 cations and the soft chalcogenoethers.

Synthesis and Single-Electron Oxidation of Bulky Bis(m-terphenyl)chalcogenides: The Quest for Kinetically Stabilized Radical Cations

Sterically encumbered bis(m-terphenyl)chalcogenides, (2,6-Mes₂ C₆ H₃ )₂ E (E=S, Se, Te) were obtained by the reaction of the chalcogen tetrafluorides, EF₄ , with three equivalents of m-terphenyl lithium, 2,6-Mes₂ C₆ H₃ Li. The single-electron oxidation of (2,6-Mes₂ C₆ H₃ )₂ Te using XeF₂ /K[B(C₆ F₅ )₄ ] afforded the radical cation [(2,6-Mes₂ C₆ H₃ )₂ Te][B(C₆ F₅ )₄ ] that was isolated and fully characterized. The electrochemical oxidation of the lighter homologs (2,6-Mes₂ C₆ H₃ )₂ E (E=S, Se) was irreversible and impaired by rapid decomposition.

Selenoxides as Excellent Chalcogen Bond Donors: Effect of Metal Coordination

The chalcogen bond has been recently defined by the IUPAC as the attractive noncovalent interaction between any element of group 16 acting as an electrophile and any atom (or group of atoms) acting as a nucleophile. Commonly used chalcogen bond donor molecules are divalent selenium and tellurium derivatives that exhibit two σ-holes. In fact, the presence of two σ-hole confers to the chalcogen bonding additional possibilities with respect to the halogen bond, the most abundant σ-hole interaction. In this manuscript, we demonstrate that selenoxides are good candidates to be used as σ-hole donor molecules. Such molecules have not been analyzed before as chalcogen bond donors, as far as our knowledge extends. The σ-hole opposite to the Se=O bond is adequate for establishing strong and directional ChBs, as demonstrated herein using the Cambridge structural database (CSD) and density functional theory (DFT) calculations. Moreover, the effect of the metal coordination of the selenoxide to transition metals on the strength of the ChB interaction has been analyzed theoretically. The existence of the ChBs has been further supported by the quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction plot (NCIPlot).

Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5 - Salt Accessible from SF6

Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF5 - anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF5 - and TeF5 - anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)-λ5 -selenate PhSeF4 - and -tellurate PhTeF4 - from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF4 ]- by oxidation to trans-PhTeF4 O- and subsequent alkylation gives access to a new class of trans-(alkoxy)(phenyl)tetrafluoro-λ6 -tellanes (trans-PhTeF4 OR), thus providing an approach to introduce the functional group into organic molecules.

Can modified DNA base pairs with chalcogen bonding expand the genetic alphabet? A combined quantum chemical and molecular dynamics simulation study

A comprehensive (DFT and MD) computational study is presented with the goal to design and analyze model chalcogen-bonded modified nucleobase pairs that replace one (i.e., A_XY:T, G:C_XY, G_XY:C) or two (G_XY:C_X'Y', X/X' = S, Se and Y/Y' = F, Cl, Br) Watson-Crick (WC) hydrogen bonds of the canonical A:T or G:C pair with chalcogen bond(s). DFT calculations on 18 base pair combinations that replace one WC hydrogen bond with a chalcogen bond reveal that the bases favorably interact in the gas phase (binding strengths up to -140 kJ mol^-1) and water (up to -85 kJ mol^-1). Although the remaining hydrogen bond(s) exhibits similar characteristics to those in the canonical base pairs, the structural features of the (Y-XO) chalcogen bond(s) change significantly with the identity of X and Y. The 36 doubly-substituted (G_XY:C_X'Y') base pairs have structural deviations from canonical G:C similar to those of the singly-substituted modifications (G:C_XY or G_XY:C). Furthermore, despite the replacement of two strong hydrogen bonds with chalcogen bonds, some G_XY:C_X'Y' pairs possess comparable binding energies (up to -132 kJ mol^-1 in the gas phase and up to -92 kJ mol^-1 in water) to the most stable G:C_XY or G_XY:C pairs, as well as canonical G:C. More importantly, G:C-modified pairs containing X = Se (high polarizability) and Y = F (high electronegativity) are the most stable, with comparable or slightly larger (by up to 13 kJ mol^-1) binding energies than G:C. Further characterization of the chalcogen bonding in all modified base pairs (AIM, NBO and NCI analyses) reveals that the differences in the binding energies of modified base pairs are mainly dictated by the differences in the strengths of their chalcogen bonds. Finally, MD simulations on DNA oligonucleotides containing the most stable chalcogen-bonded base pair from each of the four classifications (A_XY:T, G:C_XY, G_XY:C and G_XY:C_X'Y') reveal that the singly-modified G:C pairs best retain the local helical structure and pairing stability to a greater extent than the modified A:T pair. Overall, our study identifies two (G:C_SeF and G_SeF:C) promising pairs that retain chalcogen bonding in DNA and should be synthesized and further explored in terms of their potential to expand the genetic alphabet.

Catalytic, Enantioselective syn-Diamination of Alkenes

The enantioselective, vicinal diamination of alkenes represents one of the stereocontrolled additions that remains an outstanding challenge in organic synthesis. A general solution to this problem would enable the efficient and selective preparation of widely useful, enantioenriched diamines for applications in medicinal chemistry and catalysis. In this article, we describe the first enantioselective, syn-diamination of simple alkenes mediated by a chiral, enantioenriched organoselenium catalyst together with a N,N'-bistosyl urea as the bifunctional nucleophile and N-fluorocollidinium tetrafluoroborate as the stoichiometric oxidant. Diaryl, aryl-alkyl, and alkyl-alkyl olefins bearing a variety of substituents are all diaminated in consistently high enantioselectivities but variable yields. The reaction likely proceeds through a Se(II)/Se(IV) redox catalytic cycle reminiscent of the syn-dichlorination reported previously. Furthermore, the syn-stereospecificity of the transformation shows promise for highly enantioselective diaminations of alkenes with no strong steric or electronic bias.

New Twists and Turns for Actinide Chemistry: Organometallic Infinite Coordination Polymers of Thorium Diazide

Two organometallic 1D infinite coordination polymers and two organometallic monometallic complexes of thorium diazide have been synthesized and characterized. Steric control of these self-assembled arrays, which are dense in thorium and nitrogen, has also been demonstrated: infinite chains can be circumvented by using steric bulk either at the metallocene or with a donor ligand in the wedge.

Structural isomerism in (p-XC6H4)SeCl3 and (p-XC6H4)SeBr3 (X = F, Cl) compounds. Co-crystallisation of cis- and trans-dimeric forms of (p-ClC6H4)SeCl2(μ-Cl)2(p-ClC6H4)SeCl2. A new structural modification for the "PhSeBr" reagent, Ph2Se2Br2, containing an elongated Se-Se bond

A series of di(para-halophenyl)diselenides, (p-XC(6)H(4))(2)Se(2) (X = F, Cl) have been reacted with three equivalents of SO(2)Cl(2) or Br(2), leading to the formation of selenium(iv) RSeX(3) compounds. The structures of (p-FC(6)H(4))SeX(3) (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX(2)(μ-X)(2)RSeX(2) structure consisting of two "saw-horse" (p-FC(6)H(4))SeX(3) units linked by two halide bridges, with an overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are planar, with both aryl rings located on the same side of the Se(2)X(6) plane (cis-isomer). The structure of (p-ClC(6)H(4))SeCl(3) also adopts a planar dimeric structure, however both cis- and trans-dimeric molecules are co-crystallised in the unit cell. In contrast, the structure of (p-ClC(6)H(4))SeBr(3) adopts a folded cis-dimeric structure due to steric constraints. Secondary Se···X interactions to the "vacant" sixth coordination site at selenium are a feature of most of these structures, but are most prominent in the folded structure of (p-ClC(6)H(4))SeBr(3). A re-examination of the PhSeBr/PhSeBr(3) system resulted in the isolation of crystals of a second structural form of "PhSeBr". The structure of Ph(2)Se(2)Br(2) consists of two PhSeBr units linked by an elongated Se-Se bond of 2.832(4) Å, and longer secondary Se···Br interactions of 3.333(4) Å to form a chain structure. Further weak Se···Br and Br···Br interactions are present, which form loosely linked rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form, Ph(4)Se(4)Br(4).

A new ortho-rhom-bic polymorph of 1,1'-seleno-bis-(N,N-diethyl-sulfanecarbothio-amide)

The title compound [systematic name: N,N-dieth­yl({[(diethyl­carbamothio­yl)sulfan­yl]selan­yl}sulfan­yl)carbothio­amide], C₁₀H₂₀N₂S₄Se, crystallizes in a new form in the space group Pca2₁: the previously reported polymorph crystallizes in the space group P2₁2₁2₁. The new phase contains two independent mol­ecules in the asymmetric unit. The Se atoms are tetra­coordinated, with a distorted square-planar geometry. The ligands coordinate asymmetrically to the Se atoms, with one strong Se—S bond [range 2.2833 (13)–2.3041 (15) Å] and one weaker bond [range 2.7318 (14)–2.7873 (12) Å].

Recognition and sensing of fluoride anion

Fluoride anion recognition is attracting a mounting interest in the scientific community due to its duplicitous nature. It is a useful chemical for many industrial applications, and it has been used in human diet, but, recently it has been accused for several human pathologies. Here we describe the ample panorama of different approaches the chemists world-wide have employed to face the challenge of fluoride binding, and we outline some of the research which in our view can contribute to the development of this field, especially when fluoride binding has to be achieved in highly competitive protic solvents and water.