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Recent Developments in Chalcogen Chemistry

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RECENT DEVELOPMENTS IN CHALCOGEN CHEMISTRY Tristram Chivers Department of Chemistry, University of Calgary, Calgary, Alberta, Canada WHERE IS CALGARY? Lecture 1: Background / Introduction Outline • Chalcogens (O, S, Se, Te, Po) • Elemental Forms: Allotropes • Uses • Trends in Atomic Properties • Spin-active Nuclei; NMR Spectra • Halides as Reagents • Cation Formation and Stabilisation • Anions: Structures • Solutions of Chalcogens in Ionic Liquids • Oxides and Imides: Multiple Bonding 3 Elemental Forms: Sulfur Allotropes Sulfur S6 S7 S8 S10 S12 S20 4 Elemental Forms: Selenium and Tellurium Allotropes Selenium • Grey form - thermodynamically stable: helical structure cf. plastic sulfur. R. Keller, et al., Phys. Rev. B. 1977, 4404. • Red form - cyclic Cyclo-Se8 (cyclo-Se7 and -Se6 also known). Tellurium • Silvery-white, metallic lustre; helical structure, cf. grey Se. • Cyclic allotropes only known entrapped in solid-state structures e.g. Ru(Ten)Cl3 (n = 6, 8, 9) M. Ruck, Chem. Eur. J. 2011, 17, 6382 5 Uses – Sulfur Sulfur : Occurs naturally in underground deposits. • Recovered by Frasch process (superheated water). • H2S in sour gas (> 70%): Recovered by Klaus process: Klaus Process: 2 H S + SO 3/8 S + 2 H O 2 2 8 2 • Primary industrial use (70 %): H2SO4 in phosphate fertilizers 6 Uses – Selenium and Tellurium Selenium and Tellurium : Recovered during the refining of copper sulfide ores Selenium: • Photoreceptive properties – used in photocopiers (As2Se3) • Imparts red color in glasses Tellurium: • As an alloy with Cu, Fe, Pb and to harden steel • Toxicity! Formation of Me2Te 7 Specialised Uses of Metal Tellurides Solar energy • Silicon and GaAs are widely used • Other promising materials: CdTe – Band gap 1.49 eV • Major manufacturer of CdTe solar cells: First Solar (Phoenix, USA) • CdTe technology expected to increase tenfold over the next 10 years • Limitation: Very low natural abundance of Te (1-5 ppb) Thermoelectric Generators • Sb2Te 3, PbTe exhibit thermoelectric properties – band gaps ~ 0.21 eV • Low efficiency – use limited to solid-state refrigeration: e.g. Bi2Te 3 in portable food coolers • Improved efficiency could lead to applications in the conversion of waste heat from nuclear reactors and industrial equipment Ibers, Nature Chemistry, 2009, 1, 508. 8 Trends in Atomic Properties S Se Te Covalent Radius (Å) 1.03 1.17 1.35 van der Waals Radius 1.80 1.90 2.06 Electronegativity 2.6 2.6 2.1 First Ionization Potential 999.6 941.0 869.3 (kJ mol-1) D(E-E) (kJ mol-1) 266 192 137 • Steady increase in size S • Electronegativity decrease only between Se and Te • Heavy chalcogens more easily oxidized • Heavy chalcogens form much weaker homoatomic bonds 9 Binary Chalcogen Halides Monohalides, E2Cl2 • E = S, Se: Commercially available • E = Te: Dark brown, thermally unstable liquid Cp2TiSe5 Li2Te + TeCl4 Te 2Cl2 1,2-Te 2Se5 Laitinen, Chem. Commun, 1998, 2381. Dihalides, ECl2 • E = S: Readily disproportionates to S2Cl2 • E = Se: Disproportionates to Se2Cl2 and SeCl4 • E = Te: Unstable, but forms stable adducts e.g. TeCl2·tmtu (tmtu = tetramethylthiourea) Tetrahalides, ECl4 • E = S: Thermally unstable • E = Se, Te: Commercially available white solids 10 Spin Active Nuclei - NMR Spectra Nucleus Spin (I) Abundance (%) 33S 3/2 0.76 77Se 1/2 7.58 (123Te) 1/2 0.87 125Te 1/2 6.99 Et3P=Te 31P NMR 125Te NMR † † * * † = 125Te satellites * = 123Te satellites 20 0 -20 -850 -900 -950 11 Reactions of Selenium Dichloride Synthesis: Se + SO2Cl2 THF SeCl2 O 77 • Stable for 1 day at 23 C ( Se NMR: Disproportionates to Se2Cl2 + SeCl4) Chivers, Laitinen, Inorg. Chem. 1999, 38, 4093. MeO OMe MeO Cl Se Cl R SeCl 2 Se (1) (3) R R MeO (2) NH2 SeCl Se N N + N N + Se Se SeCl Se Se Cl Cl N N Se (1) Potapov, et al., Tetrahedron Letters, 2010, 51, 89; 2009, 50, 306. (2) Chivers, Laitinen, Chem Commun. 2000, 759. (3) Bendokov, et al., J. Am. Chem. Soc., 2008, 130, 6734. 12 Stabilization of Heavy Chalcogen Dihalides Tetramethylthiourea (tmtu) complexes TeO2 + HClaq + tmtu TeCl2·(tmtu)n (n = 1,2) L . Cl N Te. L = S Cl L N Foss, Acta Chem. Scand., 1986, A40, 675. Bipyridyl complexes • Thermally stable • Metathesis with RMgX (R = Ph, Bz) gives R2E (E = Se, Te) E = Se, X = Cl, Br; E = Te, X = Cl Ragogna, Chem. Eur. J. 2009, 39, 10263. 13 Trialkylphosphine Adducts of TeCl2 Et3P=Te + SO2Cl2 (or I2) Et3PTeX2 (X = Cl, I) + SO2 Et3PTeCl2 + Me3SiBr Et3PTeBr2 Cl Te X d(Te-P) (Å) δ (125Te) (ppm) 1J(P-Te) (Hz) Cl 2.466(1) 766 1395 Br 2.473(1) 627 1312 I 2.490(1) 331 1248 J. Konu and T. Chivers, Dalton Trans., 2006, 3941. 14 Ligand-Stabilized Chalcogen Dications Electron-donor ligands, e.g. diazabutadiene (DAB) (26a,b) or N-heterocyclic carbenes (NHC) (26c) stabilize highly electrophilic chalcogen dications E = S, Se S, Se: Ragogna, ACIE, 2009, 48, 2210. Te: Ragogna, ACIE, 2009, 48, 4409. 15 2+ DAB Complexes as Se Transfer Agents Se2+ complexes: - - Preparation and ligand exchange reactions (NB: CF3SO3 (OTf ) anions) Cy = cyclohexyl Activation of small molecules by Se2+? Ragogna, Chem Commun., 2010, 46, 1041. 16 Anions: Polysulfides Dianions 2- • Unbranched chains Sx (x = 2-8) + + + • Stabilized by large cations (Cs , Na(15-crown-5)] , [PPh4] ) 2- e.g. two distinct geometries of S7 : all-trans (++++) trans-cis-trans (++--) M. G. Kanatzidis, et al. Inorg. Chem. 1983, 22, 290. C. Müller, P. Böttcher, Z. Naturforsch. B, 1995, 50, 1623. Radical Anions –• • Formation of the blue trisulfur radical anion S3 (λmax ~ 620 nm) is a common feature of solutions of polysulfides . Chivers, Inorg. Chem. 1972, 11, 2515. “Ubiquitous Trisulfur Radical Anion” Chivers, Nature, 1974, 252, 32. 17 –• The Stable Radical Anion Cyclo-S6 + • Isolated as [PPh4] salt • Two long S···S bonds (2.633 Å) • MO analysis and EPR spectra •• –• indicate 2 fragments, S3 and S3 • Electron-pair bond between 2b1 SOMOs of both fragments • 3-electron bond between 1a2 of the biradical and 1a2 of the radical Cyclo-S – • Cyclo-S anion 6 6 K. Dehnicke, Angew. Chem. Int. Ed. 2000, 39, 4580 • Cyclo-S6 has one fewer electron 18 Anions: Polyselenides and Polytellurides Polyselenides 2- • Chain structures Sex (x = 2-8) 2- • Se also forms bicyclic and spirocyclic dianions Sex (x = 10, 11) • 3- and 4-coordinate Se atoms participate in 3-centre 2e- bonding 2- 2- Se10 Se11 D. Fenske, Angew. Chem. 1990, 29, 390. B. Krebs, Z. Anorg. Allg. Chem., 1991, 592, 17. Polytellurides • Charges either less or greater than 2- may be observed, 3- e.g. [Te6 ] in Cs3Te22 • Hypervalent Bonding σ* • Intra- and inter-molecular np2 σ* bonding 2 np 19 Solutions of Chalcogens in Ionic Liquids The Chalcogens as Reagents Sulfur: Poor solubility in CCl4, pyridine and toluene; dissolves well in CS2 Selenium: Slightly soluble in CS2 Tellurium: In ethylenediamine → Nanotubes of Te and Se J. Lu, et al., J. Mat. Chem. 2002, 12, 2755. Carbon Disulfide: Neurotoxic, highly flammable, reactive solvent Ionic Liquids • Safe alternative to CS2 for dissolving sulfur O • At 100 -155 C S8 has very high solubility i • In [P Bu3Me][OTs] sulfur forms bright blue solutions → carmine red at higher concentrations Identity of sulfur species in these solutions? Seddon, Chem. Commun. 2010, 46, 716. 20 The Trisulfur Radical Ion in Ionic Liquids • UV-Vis spectrum shows an isosbestic point • Dilute solutions are blue (617 nm) and concentrated solutions are red • Equilibrium between hexasulfide and trisulfur radical anion 2- -• S6 2 S3 •- S3 is the chromophore in lapis lazuli and ultramarine blue 21 •- Sensor Material Based on Occluded S3 Univalent Zinc, Zn+ Paramagnetic Zn+ incorporated into zeolite by reaction of Zn vapor (at 450 OC) with protons of two Brønsted acid sites → Zn@SAPO (Si Al Phosphate) o Sulfur vapour at 280 C introduced into Zn@SAPO cage and S3 is trapped in the cavity and then reacts with Zn+ to produce blue S •- 3 Li and Chen, JACS, 2003, 125, 6622; J. Mater. Chem., 2010, 20, 3307. 22 •- Occluded S3 as a Sensor for Water Sensoring Mechanism •- • Occluded S3 is a sensitive detector for H2O in air or organic solvents •- •- S3 + H2O S3 + H2O • Monitored by visible and EPR spectroscopy Colorimetric cards estimate ppm water based on amount of sensor material used (3.0, 6.3, 9.0 mg) 23 Solutions of Se and Te in Ionic Liquids • Se and Te also dissolve in ionic liquids at elevated temps to give orange (Se at 50 oC) and purple (Te at 170 OC) solutions •- • Orange colour may be Se3 ; identity of purple species unknown • Reactivity of chalcogen solutions in ionic liquids demonstrated by reactions with PPh3 to give Ph3PE ( E = Se, Te) 24 Binary Chalcogen Dioxides: Multiple Bonding Sulfur Dioxide • A monomeric gaseous molecule • S=O double bonds – bent structure C2v Selenium and Tellurium Oxides • White solids with polymeric structures • (SeO2)n : 2-D polymer – Both Se–O and Se=O bonds • (TeO2)n : 3-D polymer - Only Te–O single bonds O S O O O O Se O Te n O O 25 Chalcogen Diimides - Structures Sulfur and Selenium Diimides • Monomeric: Cis, trans isomer usually preferred R E E N N N N R E R N N R R R (cis, cis) (cis, trans) (trans, trans) E = Se, R = Ad: T. Maaninen, R. Laitinen, T. Chivers, Chem Commun. 2002, 1812. Tellurium Diimides • Dimeric: Two known conformations R R R R R N R R N N N N Te Te Te Te N N R N t n cis, endo, endo (R = Bu) trans, exo, exo (R = Oct, R’ = PPh2NSiMe3) Chivers, et al. JACS, 1995, 117, 2519; Inorg. Chem., 1996, 35, 9. 26 Chalcogen Diimides – Dimerization Energies (R = Me) E = S, Endothermic; E = Se, ~ Thermoneutral; E = Te, Exothermic Tuononen, Laitinen, Inorg.
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  • ELECTRONEGATIVITY D Qkq F=

    ELECTRONEGATIVITY D Qkq F=

    ELECTRONEGATIVITY The electronegativity of an atom is the attracting power that the nucleus has for it’s own outer electrons and those of it’s neighbours i.e. how badly it wants electrons. An atom’s electronegativity is determined by Coulomb’s Law, which states, “ the size of the force is proportional to the size of the charges and inversely proportional to the square of the distance between them”. In symbols it is represented as: kq q F = 1 2 d 2 where: F = force (N) k = constant (dependent on the medium through which the force is acting) e.g. air q1 = charge on an electron (C) q2 = core charge (C) = no. of protons an outer electron sees = no. of protons – no. of inner shell electrons = main Group Number d = distance of electron from the nucleus (m) Examples of how to calculate the core charge: Sodium – Atomic number 11 Electron configuration 2.8.1 Core charge = 11 (no. of p+s) – 10 (no. of inner e-s) +1 (Group i) Chlorine – Atomic number 17 Electron configuration 2.8.7 Core charge = 17 (no. of p+s) – 10 (no. of inner e-s) +7 (Group vii) J:\Sciclunm\Resources\Year 11 Chemistry\Semester1\Notes\Atoms & The Periodic Table\Electronegativity.doc Neon holds onto its own electrons with a core charge of +8, but it can’t hold any more electrons in that shell. If it was to form bonds, the electron must go into the next shell where the core charge is zero. Group viii elements do not form any compounds under normal conditions and are therefore given no electronegativity values.