Co-pyrolysis reactions of B2Br4 with S2Br2 at 350 degrees C in vacuo yielded the brominated thiaboranes closo-SB5Br5 (1), closo-1-SB9Br9 (2) and closo-SB11Br11 (3), confirmed by high-resolution mass spectrometry, experimental and computational 11B NMR spectroscopy. The strong Br delta+(sigma-hole)& ctdot;Br delta-(ring) attraction has been the decisive energy contribution in the crystal of 1.

Six-vertex closo-SB5Cl5 (1) and ten-vertex closo-1-SB9Cl9 (2) thiaboranes have been prepared, besides the already known 12-vertex closo-SB11Cl11 (3), from the co-pyrolysis reaction of B2Cl4 with S2Cl2 at 280 °C in vacuo. The compounds are sublimable, off-white solids. Their elemental composition has been determined by high-resolution mass spectrometry. They were further characterized by one- and two-dimensional 11B NMR spectroscopy and X-ray structure determination for 1 and 3. Ab initio/GIAO/NMR computations support octahedral, bicapped square-antiprismatic, and icosahedral geometries for 1, 2 and 3, respectively, as expected based on their closo-electron counts. 1 is the first isolated example of a neutral polyhedral closo-thiaborane with a cluster size smaller than ten vertices. The solid-state structure of 3 is one of the rare examples of a single-crystal X-ray structure determination of an icosahedral heteroborane reported. The corresponding crystal-packing forces show the different role of chalcogen bonding in these octahedral and icosahedral crystals. In addition, there is a mass-spectrometry evidence for the recurrent formation of further thiaborane homologs of closo-SBnCln with n=4, 6, 10, and supra-icosahedral 12.

Octahedral closo-1,2-Pn2B4Br4 (Pn=P, As) molecules react with tetrahydrothiophene (THT), regarded as a soft base, much slower than with tetrahydrofuran (THF), considered a hard base. Namely, the reaction begins with only Pn=P at temperatures above 140 degrees C; 4-Br(CH2)4S-closo-1,2-P2B4Br3 has been detected as the first product resulting from the cleavage and insertion of a thiobutylate group into the B-Br bond. Unlike in the reaction with THF, the addition of a second THT moiety has not been observed. Conversely, also starting from 140 degrees C, conjuncto-3,3'-(1,2-Pn2B4Br3)2 has been found as the final product of the reaction, which indicates the presence of 3-Br(CH2)4S-closo-1,2-P2B4Br3 as another intermediate during the conversion. A computational examination has revealed that it occurs through a pentagonal pyramidal stationary point as an additional intermediate, the latter of which serves as the crucial structural assembly for the thermal conversion that produces the conjuncto motif. Computations of the energy balance and 11B and 31P NMR chemical shifts are in agreement with experimental observations. This reaction is made possible by the presence of sigma-holes on the bromine atoms. In addition, the presence of the sigma-holes on the pnictogens has enabled ESI-MS to detect various anions appearing during earlier and current syntheses. Octahedral closo-1,2-Pn2B4Br4 (Pn=P, As) react with tetrahydrotoluene (THT) to get conjuncto-3,3'-(1,2-P2B4Br3)2. 4-THT-closo-1,2-P2B4Br3 has also been detected which is isomerized through a pentagonal pyramidal intermediate to the corresponding 3-THT isomer to yield the conjuncto motif. This reaction is due to the presence of sigma-holes on Br's. The sigma-holes on Pn has enabled ESI-MS to detect various anions. image

Six-vertex closo-TeB5 Cl5 (1) and twelve-vertex closo-TeB11 Cl11 (2) telluraboranes have been prepared via co-pyrolysis of B2 Cl4 with TeCl4 in vacuo at temperatures between 360 °C and 400 °C. Both compounds are sublimable, off-white solids, and they have been characterized by one- and two-dimensional 11 B NMR and high-resolution mass spectroscopy. Both ab initio/GIAO/NMR and DFT/ZORA/NMR computations support octahedral and icosahedral geometries for 1 and 2, respectively, as expected due to their closo-electron counts. The octahedral structure of 1 has been confirmed by single-crystal X-ray diffraction on an incommensurately modulated crystal. The corresponding bonding properties have been analyzed in terms of the intrinsic bond orbital (IBO) approach. 1 is the first example of a polyhedral telluraborane with a cluster size smaller than 10 vertices.

Reexamination of the co-pyrolysis reactions of B2Cl4 with C2Cl4 at 350 & DEG;C and of B2Br4 with CBr4 at 300 & DEG;C in vacuo confirmed the carboranes C2B5Cl7 (1), C2B7Cl9 (2), and C2B5Br7 (3) as low-yield products. While 1 only could be concentrated by repeated vacuum fractionation, 2 and 3 could now be isolated from the conglomerate mixtures for a full spectroscopic characterization and the compounds were verified in their geometries by detailed DFT computations. Surprisingly, the perhalogenated carboranes do not adopt the expected "all-endo"-geometries with cluster sizes derived by the sum of the n boron and two carbon atoms (n+2) as known from the syntheses of the parent closo-carboranes C2BnHn+2. Instead, DFT/GIAO(ZORA)/NMR (GIAO for X=Cl, ZORA for X=Br) computations revealed that the perhalogenated carboranes favor structures with BX2 groups as exo-skeletal ligands attached to both cage-carbon atoms yielding the five-vertex closo-1,5-(CBX2)(2)B3X3 (1: X=Cl; 3: X=Br) and the seven-vertex closo-2,4-(CBCl2)(2)B5Cl5 for 2. In contrast to these perhalogenated carboranes, analogous computations on the hydrogen substituted carboranes C2BnHn+2, silaboranes Si2BnHn+2 and Si2BnXn+2 (n=5, 7) show in all cases a thermodynamic favorization of structures where all boron atoms of the formula are endo-skeletally incorporated into the cluster frameworks.

The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B2Cl4 with Se2Cl2 at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB5Cl5 (1a) and closo-SeB11Cl11 (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB9Cl9 (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D 11B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution 11B NMR and 77Se NMR experiments have resolved both coupling constants 1J(11B11B) and 1J(77Se11B) as well as the 77Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed 11B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield 11B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n - 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeBnCln family (n = 4-12).

Borane and heteroborane clusters have been known as neutral or anionic species. In contrast to them, several ten-vertex monocationic nido and closo dicarbaborane-based systems have recently emerged from the reaction of the parent bicapped-square antiprismatic dicarbaboranes with N-heterocyclic carbenes followed by the protonization of the corresponding nido intermediates. The expansion of these efforts has afforded the very first closo-dicationic octahedral phosphahexaborane along with new closo-monocationic pnictogenahexaboranes of the same shapes. All are the products of the one-pot procedure that consists in the reaction of the same carbenes with the parent closo-1,2-Pn2B4Br4 (Pn = As, P). Whereas in the case of phosphorus such a monocation appears to be a mixture of stable intermediates, and arsenahexaboranyl monocation has occurred as the final product, all of them without using any subsequent reaction. The well-established DFT/ZORA/NMR approach has unambiguously confirmed the existence of these species in solution, and computed electrostatic potentials have revealed the delocalization of the positive charge in these monocations and in the very first dication, namely within the octahedral shapes in both cases.

Reactions of the perhalogenated polyhedral pnictogenaboranes closo-1,2-Pn2B4Hal4 (Pn = P, As; Hal = Cl, Br) with Lewis bases are presently being studied with a focus on rationalizing the sites of nucleophilic attacks on clusters bearing σ-holes. These σ-holes are localized both on pnictogens and, for Hal = Br, on bromine atoms, as revealed by electrostatic potential (ESP) and intrinsic bond orbital (IBO) analyses. Surprisingly, the attack of the cyclic ether THF on closo-1,2-Pn2B4Br4 does not occur on the site with the largest positive partial charge, centered in the middle of the pnictogen-pnictogen vector. Instead, presumably promoted by the positivated bromine substituents, THF inserts into the boron-bromine bonds of the negatively charged boron atoms opposite to the pnictogen atoms to form 4-(4-bromobut-1-oxy)-closo-1,2-Pn2B4Br3 (1-PB and 1-AsB) and 4,6-(4-bromobut-1-oxy)2-closo-1,2-Pn2B4Br2 (2-PB and 2-AsB). 11B and 31P chemical shift computations at various levels support the assignments of the signals, which reflect the correctness of the molecular geometries in solutions. The Lewis-acidic perchlorinated analogues closo-1,2-P2B4Cl4, closo-1,2-As2B4Cl4, and the mixed closo-1,2-AsPB4Cl4 bear negative charges. These negative charges are revealed by the Vs,max values when computing the electrostatic potentials both on the boron and the chlorine atoms. Due to this negative charge, the analogues do not react with THF unless they are heated above 66 °C, where they slowly decompose to borate esters B(OR)3 without the formation of concrete intermediates. The evaluation of 31P NMR data of 1-PB has allowed the experimental determination of the coupling constant 1J(31P(1), 31P(2)) = |143| Hz in a closo-diphosphaborane for the first time, which agrees well with the computed value of -178 Hz. The pioneering joint experimental vs computational interpretation of 31P NMR spectra in the area of boron cluster chemistry was decisive for the structural characterization of 1-PB and 2-PB.

The synthesis, spectroscopic data, redox properties of B8Cl8, [B8Cl8](.-), and [B8Cl8](2-), and the single-crystal X-ray structure analysis of [Ph4P](+)(2) [B8Cl8](2-) CH3CN are described. Temperature-dependent B-11 NMR measurements and theoretical studies (DFT) concerning the fluxional behavior of [B8Cl8](2-) will be discussed. - B8Cl8 reacts with reductants like iodide to the intense purple colored radical anion [B8Cl8](.-) which is formed as well from B8Cl8 by traces of water. The colorless dianion closo-[B8Cl8](2-) is obtained by the reaction of B8Cl8 or [B8Cl8](.-) with iodide. The single-crystal X-ray structure analysis of [Ph4P](+)(2)[B8Cl8](2-) confirms the dodecahedral D-2d structure of [B8Cl8](2-). The B-11 NMR spectra of [B8Cl8](2-) solutions show only one broad signal even at low temperatures due to the fast fluctuation of the [B8Cl8](2-) polyhedron. The energy of the C-2v structure isomer of [B8Cl8](2-) was calculated by DFT methods to be only 0.6 kcal/mol higher in energy than the D-2d form, and the activation barrier for the D-2d to C-2v transformation is predicted to be about 0.9 kcal/mol.

The Front Cover shows the redox scheme of the octaborane cluster series hypercloso-B8Cl8, hypercloso-[B8Cl8]−, and closo-[B8Cl8]2−. Neutral B8Cl8 – a dark green solid – has in its polyhedrons only 2n = 16 framework electrons instead of 2n + 2 = 18, as the closo-clusters should have according to the rules of Wade. Consequently, B8Cl8 is a strong oxidant and easily adopts an electron, giving the intense purple colored radical anion [B8Cl8]−, which can also be easily reduced to the more stable dianion closo-[B8Cl8]2−. This closo-cluster has a D2d dodecahedral molecular structure in the solid state, but in solution it shows a highly fluxional mobility. More information can be found in the Full Paper by W. Einholz and co-workers.

The cover picture shows the formation of conjuncto-3,3'-(1,2-P2B4Br3)2 (1) in a two-component, one-pot thermal reaction of diboron tetrabromide and phosphorus tribromide. The co-pyrolysis also generates the icosahedral closo-1,7-P2B10Br10 (2) and octahedral closo-1,2-P2B4Br4. 1 represents the first boron-boron coupled polyhedral phosphaborane proven in its structure by a single-crystal X-ray study. The assembly of electron-deficient polyhedral frameworks from small, electron-precise precursory molecules provides an unique entry into the field of phosphorus- and other heteroatom-containing boranes. More details can be found in the article by Willi Keller on page 517 ff.

The synthesis of the perbrominated arsaboranes closo-1,2-As2B4Br4 (1) and closo-1,2-As2B10Br10 (2) occurs by co-pyrolysis of B2Br4 and AsBr3 at 500 degrees C. Repeated fractionation of the sublimable products in vacuo yields both compounds in pure form. The X-ray structure determination for orthorhombic closo-1,2-As2B4Br4 (1) [space group: Pbcn, a = 2345.48(17) pm, b = 627.31(4) pm, c = 1294.02(9) pm for Z = 8] and the corresponding phosphorus compound, monoclinic closo-1,2-P2B4Br4 (3) [space group: P2(1)/n, a = 806.84(6) pm, b = 1247.96(9) pm, c = 974.91(7) pm, = 90.493(3)degrees and Z = 4] confirmed that both 1 and 3, consistent with their 14-skeletal electron counts, adopt octahedral structures distorted from regular by two arsenic or phosphorus atoms in the 1,2-positions. The shortest boron-boron bonds within the cluster frameworks are located between the boron atoms antipodal to the heteroatoms.

Co‐pyrolysis of B2Br4 with PBr3 at 480 °C gave, in addition to the main product closo‐1,2‐P2B4Br4, conjuncto‐3,3′‐(1,2‐P2B4Br3)2 (1) and the twelve‐vertex closo‐1,7‐P2B10Br10 (2), both in low yields. X‐ray structure determination for 1 [triclinic, space‐group P1 with a = 7.220(2) Å, b = 7.232(2) Å, c = 8.5839(15) Å, α = 97.213(15)°, β = 96.81(2)°, γ = 94.07(2)° and Z = 1] confirmed that 1 adopts a structure consisting of two symmetrically boron–boron linked distorted octahedra with the bridging boron atoms in the 3,3′‐positions and the phosphorus atoms in the 1,2‐positions. The intercluster 2e/2c B–B bond length is 1.61(3) Å. The shortest boron–boron bond within the cluster framework is 1.68(2) Å located between the boron atoms antipodal to the phosphorus atoms. The icosahedral phosphaborane 2 was characterized by 11B‐11B COSY NMR spectroscopy showing cross peaks indicative for the isomer with the phosphorus atoms in 1,7‐positions. Both the X‐ray data of 1 and the NMR spectroscopic data of 1 and 2 give further evidence for the influence of an antipodal effect of heteroatoms to cross‐cage boron atoms and, vice versa, of an additional shielding of the phosphorus atoms caused by B‐Hal substitution at the boron positions trans to phosphorus.

The six-vertex closo-1-SeB5Cl5 (1) and the 12-vertex closo-1-SeB11Cl11 (2) selenaboranes were prepared in low yields from the co-pyrolysis reaction of B2Cl4 with Se2Cl2 at 330 °C. Both compounds are sublimable, colorless solids and were tentatively characterized by 11B NMR and high-resolution mass spectroscopy. DFT/GIAO/NMR calculations support octahedral and icosahedral arrangements for 1 and 2, respectively, as expected due to their closo electron counts. Compound 1 represents the first example of a polyhedral selenaborane with a cluster size smaller than 10 vertices.

The 12-vertex closo-phosphaborane 1,7-P2B10Cl10 (1) has been prepared in low yield from the pyrolysis reaction of B2Cl4 with PCl3 at temperatures above 400 degrees C. A single-crystal X-ray structure determination of 1 (monoclinic space group P2(1)/n with a = 9.239(2) A, b = 16.786(3) A, c = 15.739(3) a, beta = 93.25(3) degrees, and Z = 4) confirmed that, consistent with its 26 skeletal electron count, the phosphaborane adopts a distorted icosahedral structure with the phosphorus atoms in the 1,7-positions. Crystals of 1 contain toluene in a 1:1 molar ratio embedded between each P atom of neighboring cluster molecules. Alteration of the pyrolytic conditions resulted in the formation of the phosphaboranes P4B8Cl6 (2) and P2B8Cl8 (3), which were characterized spectroscopically. Copyrolysis of B2Cl4 with a mixture of PCl3 and AsCl3 at 450 degrees C generated the six-vertex arsaphosphaborane AsPB4Cl4 (4) and traces of the icosahedral arsaphosphaborane AsPB10Cl10. These compounds are examples of heteroboranes which contain two different group-15 atoms within a single molecule.

The perchlorinated arsaboranes closo-1,2-As₂B₁₀Cl₁₀ and As₄B₈C₁₆ are formed in the co­ pyrolysis of B₂C₁₄ and AsCl₃ at temperatures of 330-450°C. The icosahedral structure of closo-1,2-As₂B₁₀Cl₁₀ is confirmed by ¹¹B-¹¹B-COSY-NMR spectroscopy. As₄B₈Cl₆ is suppo­ sed to be a conjuncto-borane (As₂B₄Cl₃)₂ with two boron-boron linked As₂B₄Cl₃ octahedra. The copyrolysis of B₂Cl₄ andSbCl₃ or SbCl₅ yields Sb₂B₁₀Cl₁₀, and the corresponding reaction of B₂Cl₄ with S₂Cl₂ gives the icosahedral thiaborane cluster SB₁₁Cl₁₁ .

Abstract The perchlorinated arsaboranes closo-1,2-As2B10Cl10 and As4B8C16 are formed in the co­ pyrolysis of B2C14 and AsCl3 at temperatures of 330-450°C. The icosahedral structure of closo-1,2-As2B10Cl10 is confirmed by 11B-11B-COSY-NMR spectroscopy. As4B8Cl6 is suppo­ sed to be a conjuncto-borane (As2B4Cl3)2 with two boron-boron linked As2B4Cl3 octahedra. The copyrolysis of B2Cl4 andSbCl3 or SbCl5 yields Sb2B10Cl10, and the corresponding reaction of B2Cl4 with S2Cl2 gives the icosahedral thiaborane cluster SB11Cl11 .

The reactions of Na+Li+[nido-R2C2B4H4]2-(R = Et, Bz) with R'PCl2(R'= Ph,t-Bu,Me) have been found to yield new phosphacarboranes 1-6 of the general formula R'R2PC2B4H4. Their observed spectroscopic data and the results of an ab initio/IGLO/NMR study indicate that these compounds have open-cage 7-vertex nido-6-R'-3,4-R2-6,3,4-PC2B4H4 geometries, based on a dodecahedron missing one rive-connected vertex, with the carbon and phosphorus atoms occupying positions on the open face. The reaction of nido-6-Ph-3,4-Et2-6,3,4-PC2B4H4, 1, with lithium metal or sodium naphthalenide in THF gives quantitative reduction of the phosphacarborane to form arachno-PhEt2PC2B4H42-, 1(2-). Subsequent reaction of 1(2-) with PhPCl2 gives a product that the NMR and mass spectral evidence suggest is the diphosphacarborane arachno-Ph2Et2P2C2B4H4. Comparison of the spectral data obtained for arachno-PhEt2PC2B4H4(2-) and arachno-Ph2Et2P2C2B4H4 with the results of ab initio/IGLO/NMR calculations support 7-vertex and 8-vertex arachno geometries, respectively, for these cage systems.