The nature of the phosphorus-tellurium bond in tertiary phosphine tellurides is not well understood. There is also controversy over the nature of multiple bonding in the lighter chalcogenides and the related ylides and imides. Density functional theory (DFT) was used to investigate the interactions in the molecule, Me3PE (E = 0 , S, Se, Te, BH,, CH,, NH). The calculated PE bond energies and orbital populations reveal contributions from both u donation from the phosphine and T back-donation to the phosphine in all of the above cases. Down the group from oxygen to tellurium, the PE bond weakens from 544 kJ mol-' to 184 kJ mol-I, but multiple bonding becomes more significant with respect to the single bond. For E = BH,, the PB bond energy is 166 kJ mol-'. Trimethylphosphine ylide was found to have a T-bond order of 0.5, while that of trimethylphosphine imine is 0.6. For comparison, the oxides of trimethylamine and trimethylarsine were also calculated to examine the pnictogen4xygen bond; Me,N does not participate in multiple bonding with oxygen, while the T-bond orders for Me3P0 and Me3As0 were calculated as 0.7 and 0.6, respectively.Key words: phosphine chalcogenides, phosphine ylides, phosphine imides, DFT calculations Resume : La nature de la liaison phosphore-tellure dans les tellurures de phosphine tertiaires n'est pas trks bien comprise. I1 ya a Cgalement une controverse relativement la nature de la liaison multiple dans les chalcogtnures plus lCgbres et dans les ylures et imides apparentts. On a fait appel la thCorie de la densitt fonctionnelle (TDF) pour ttudier les interactions dans la molCcule de Me,PE (E = 0 , S, Se, BH,, CH,, NH). Les Cnergies de liaison PE calculCes et les populations des orbitales rCvklent des contributions a la fois du transfert u a partir de la phosphine et du transfert en retour T vers la phosphine dans tous les cas cidessus. La liaison PE s'affaiblit lorsqu'on descend dans la famille de l'oxygbne vers le tellure, elle passe de 544 kJ mol-I a 184 kJ mol-I, mais la liaison multiple devient plus significative par rapport a la liaison simple. Pour E = BH,, 1'Cnergie de la liaison PB est de 166kJ mol-I. On a trouvt que l'ylure de la trimCthylphosphine a une liaison T de l'ordre de 0,5, tandis que pour l'imine de la trimkthylphosphine l'ordre est de 0,6. Pour fins de comparaison, on a tgalement calculC les oxydes de la trimCthylamine et de la trimtthylarsine, pour examiner la liaison oxygbne-azote; Le Me,N ne participe pas a la liaison multiple avec l'oxygbne, alors que calcul des ordres de la liaison T pour le Me,PO et le Me3As0 donnent 0,7 e 0,6 respectivement.Mots clks : chalcogCnures de phosphine, ylure de phosphore, imides de phosphine, calculs de TDF.[Traduit par la redaction]

Density functional theory (DFT) calculations have been used to investigate the process of dimerization for the three chalcogen diimides MeNdEdNMe (E ) S, Se, Te). DFT calculations for these monomers reveal that the energies of the syn,syn and syn, anti isomers differ by <3.5 kJ mol -1 for all three chalcogens while the anti,anti isomers are substantially higher in energy (by 25-39 kJ mol -1 ). A qualitative understanding of this difference can be derived from consideration of the electronic structures of the chalcogen diimides. In particular, the antibonding interaction between the in-plane nitrogen lone pairs and the p z orbital on sulfur destabilizes the most sterically favorable anti,anti isomer. The calculated dimerization energies for MeNdEdNMe show that the process is endothermic (∆E ) 34.9 kJ mol -1 ) for E ) S, approximately thermoneutral (∆E ) -2.8 kJ mol -1 ) for E ) Se, and strongly exothermic (∆E ) -82.9 kJ mol -1 ) for E ) Te. A qualitative analysis of the orbital interactions involved in the dimerization process reveals that the LUMOs of the diimide monomers are populated in a stabilized bonding LUMO-LUMO interaction that is lower in energy than the antibonding HOMO-HOMO interaction. The most significant contribution to the energy of dimerization is the energy required to distort the planar diimide monomer into half of the butterfly dimer.

An improved synthesis of the tellurium diimide dimer tBuNTe(μ-NtBu)2TeNtBu (1) using THF as solvent is reported. The reactions of 1 with tellurium tetrahalides (1:2 molar ratio) in THF give high yields of tert-butylimidotellurium dihalides (tBuNTeX2) n (2a, X = Cl, n = 6; 2b, X = Br). The intermediate tBuNTe(μ-NtBu)2TeCl2 (3) is obtained when a 3:2 molar ratio is used. The dibromide 2b may also be prepared by halide exchange between 2a and 2 equiv of Me3SiBr. The crystal structure of 2a·2THF reveals a hexamer formed by linking three (tBuNTeCl2)2 dimers via chloride bridges. The Cl ligands in the central dimer occupy the axial positions of a distorted trigonal bipyramid on both Te atoms. In the two terminal dimers one of the Te atoms has both Cl ligands in axial positions, whereas the other Te is attached to one axial Cl substituent and one equatorial Cl substituent. There are weak Te···Cl interactions between hexamer units in the range 3.107(1)−3.704(1) Å. Crystals of 2a·2THF are monoclinic, space group C2/c, a = 35.2264(9) Å, b = 11.4548(3) Å, c = 14.3656(4) Å, β = 101.645(1)°, V = 5677.4(3) Å3, and Z = 8. The reaction of 2a with 2 equiv of KOtBu in THF produces (tBuO)2Te(μ-NtBu)Te(OtBu)2 (4). The crystal structure of 4 comprises a centrosymmetric dimer with a planar Te2N2 ring and significantly different Te−N bond lengths [1.943(4) and 2.217(4) Å]. Crystals of 4 are triclinic, space group P1̄, a = 9.646(5) Å, b = 10.899(5) Å, c = 9.439(4) Å, α = 94.12(4)°, β = 118.89(3)°, γ = 65.26(4)°, V = 778.5(7) Å3, and Z = 2.

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Diborane(4) compounds are key reagents in transition-metal-catalyzed diboration 1 and Suzuki-Miyaura coupling reactions. 2 Two of the most widely used compounds are the pinacolate derivative B 2 (pin) 2 (pin ¼ OCMe 2 CMe 2 O) 3 and the catecholate species B 2 (cat) 2 (cat ¼ 1,2-O 2 C 6 H 4 ), 4 both of which are prepared from tetrakis(dimethylamino) diborane(4), B 2 (NMe 2 ) 4 , described initially by Brotherton. 5 Detailed preparations for B 2 (pin) 2 and B 2 (NMe 2 ) 4 have been described in Ref. 3b. Here we present a slightly different preparation for B 2 (NMe 2 ) 4 together with details of the synthesis of 1,2-B 2 Cl 2 (NMe 2 ) 2 6 and B 2 (cat) 2 . Simple modifications of the B 2 (cat) 2 synthesis given here allow for the preparation of a host of diol-derived diborane(4) compounds.All solvents were freshly distilled under dinitrogen from an appropriate drying agent immediately prior to use. Glassware was either kept in an oven at 150 C overnight or flame-dried under vacuum. All manipulations were carried out under dinitrogen using standard Schlenk techniques. Procedure& Caution. Sodium can spontaneously ignite on exposure to water or air. BCl 3 fumes vigorously in air, producing HCl. These reagents should only be handled in a fume hood.To a two-necked 250-mL round-bottomed flask equipped with a sidearm and a magnetic stirring bar and mounted on a magnetic stirrer, B(NMe 2 ) 3 (Aldrich, 33 mL, 0.2 mol) is added under a constant stream of nitrogen and stirring is started. The flask is then cooled to about À78 C by means of an external dryice ethanol bath, and a solution of BCl 3 in heptane (Aldrich, 100 mL of a 1.0 M solution) is added. (Note: It is important that heptane rather than hexane be used, since yields are much lower when hexane is employed.) The reaction mixture is then stirred at low temperature for 1 h, after which time the cooling bath is removed, and the reaction flask and contents are allowed to warm to room temperature. Stirring is continued for a further 3 h. After this time the reaction mixture forms a pale yellow solution with small quantities of a white solid.[Note: Checkers state that at this stage the reaction can be assayed by 11 B NMR, although this is rarely necessary if freshly distilled or purchased BCl 3 and B(NMe 2 ) 3 are used. The solid can be removed by filtration, although its presence in subsequent steps seems not to affect the yield.] Clean metallic sodium (6.9 g, 0.3 mol) is then slowly added in small pieces, and afterward a Liebig condenser is attached to the reaction flask. The magnetic stirrer is replaced with a stirrer-heating mantle, and the reaction mixture is then refluxed for 16 h, resulting in a brown solution and a purple precipitate. After cooling to room temperature, the condenser is removed, and the reaction mixture is then filtered through a glass frit into a separate flask. The filter cake is washed with hexanes (2 Â 10 mL). All solvent is removed from the filtered reaction solution by vacuum pumping (standard vacuum pump) at room temperature, affording a bro...

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