A Direct Route to closo-SB n Cl n Thiaboranes from Simple Electron-Precise Synthons: The Different Role of Chalcogen Bonding in SB5Cl5 and SB11Cl11 Crystals.

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 ºC 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-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-spectroscopy evidence for the recurrent formation of further thiaborane homologs of closo-SBnCln with n = 4, 6, 10, and supra-icosahedral 12.

Rearrangement of dicarboranyl methyl cation to icosahedral C3 B9 H12 + : An ab initio dynamics view.

Closo-carborane anions are prominent, whereas the cations of the same are less abundant in the literature. As these ions have similar size and are weakly coordinating, the ionic liquids of these two ions could have important applications in many areas of chemistry. In view of limited number of polyhedral carborane cations available, we revisited the rearrangement of dicarboranyl methyl cation (7-CH2 7,9-nido-C2 B9 H10 + ) using ab initio molecular dynamics calculations with metadynamics. Our simulations confirmed the concerted mechanism of the rearrangement. We believe this work will resume the interest in its synthesis and carborane cations in general.

B-H⋯π and C-H⋯π interactions in protein-ligand complexes: carbonic anhydrase II inhibition by carborane sulfonamides.

Among non-covalent interactions, B-H⋯π and C-H⋯π hydrogen bonding is rather weak and less studied. Nevertheless, since both can affect the energetics of protein-ligand binding, their understanding is an important prerequisite for reliable predictions of affinities. Through a combination of high-resolution X-ray crystallography and quantum-chemical calculations on carbonic anhydrase II/carborane-based inhibitor systems, this paper provides the first example of B-H⋯π hydrogen bonding in a protein-ligand complex. It shows that the B-H⋯π interaction is stabilized by dispersion, followed by electrostatics. Furthermore, it demonstrates that the similar C-H⋯π interaction is twice as strong, with a slightly smaller contribution of dispersion and a slightly higher contribution of electrostatics. Such a detailed insight will facilitate the rational design of future protein ligands, controlling these types of non-covalent interactions.

DFT Surface Infers Ten-Vertex Cationic Carboranes from the Corresponding Neutral closo Ten-Vertex Family: The Computed Background Confirming Their Experimental Availability.

Modern computational protocols based on the density functional theory (DFT) infer that polyhedral closo ten-vertex carboranes are key starting stationary states in obtaining ten-vertex cationic carboranes. The rearrangement of the bicapped square polyhedra into decaborane-like shapes with open hexagons in boat conformations is caused by attacks of N-heterocyclic carbenes (NHCs) on the closo motifs. Single-point computations on the stationary points found during computational examinations of the reaction pathways have clearly shown that taking the "experimental" NHCs into account requires the use of dispersion correction. Further examination has revealed that for the purposes of the description of reaction pathways in their entirety, i.e., together with all transition states and intermediates, a simplified model of NHCs is sufficient. Many of such transition states resemble in their shapes those that dictate Z-rearrangement among various isomers of closo ten-vertex carboranes. Computational results are in very good agreement with the experimental findings obtained earlier.

The Telluraboranes closo-TeB5 Cl5 and closo-TeB11 Cl11 with Exceptionally Long Body Diagonals: Synthetic and Bonding Motifs for Innovative Octahedral and Icosahedral Geometries.

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.

Reactivity of Perhalogenated Octahedral Phospha- and Arsaboranes toward THF: A Joint Experimental/Computational Study.

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.

Thiaborane Icosahedral Barrier Increased by the Functionalization of all Terminal Hydrogens in closo-1-SB11H11.

The electrophilic substitution of icosahedral closo-1-SB11H11 with methyl iodide has resulted in two B-functionalized thiaboranes, 7,12-I2-2,3,4,5,6,8,9,10,11-(CH3)9-1-closo-SB11 and 7,8,12-I3-2,3,4,5,6,9,10,11-(CH3)8-closo-1-SB11, with the former being significantly predominant. These two icosahedral thiaboranes are the first cases of polysubstituted polyhedral boron clusters with another vertex that differs from B and C. Such polyfunctionalizations have increased the earlier observed thiaborane icosahedral barrier, not exhibiting any reactivity toward bases, unlike the parent thiaborane. The search for methylation pathways has revealed that the complete B11-methylation is impossible, like in the case of decaborane(14), where this seems to be a result of the positively charged upper parts of these two molecules.

Benchmark Data Sets of Boron Cluster Dihydrogen Bonding for the Validation of Approximate Computational Methods.

The success of approximate computational methods, such as molecular mechanics, or dispersion-corrected density functional theory, in the description of non-covalent interactions relies on accurate parameterizations. Benchmark data sets are thus required. This area is well developed for organic molecules and biomolecules but practically non-existent for boron clusters, which have been gaining in importance in modern drug as well as material design. To fill this gap, we have introduced two data sets featuring the most common non-covalent interaction of boron clusters, the dihydrogen bond, and calculated reference interaction energies at the "golden standard" CCSD(T)/CBS level. The boron clusters studied interact with formamide, methanol, water and methane at various distances and in two geometrical arrangements. The performance of the tested approximate methods is variable and recommendations for further use are given.

Face-Fusion of Icosahedral Boron Hydride Increases Affinity to γ-Cyclodextrin: closo,closo-[B21 H18 ]- as an Anion with Very Low Free Energy of Dehydration.

The supramolecular recognition of closo,closo-[B21 H18 ]- by cyclodextrins (CDs) has been studied in aqueous solution by isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. These solution studies follow up on previous mass-spectrometric measurements and computations, which indicated the formation and stability of CD ⋅ B21 H18- complexes in the gas phase. The thermodynamic signature of solution-phase binding is exceptional, the association constant for the γ-CD complex with B21 H18- reaches 1.8×106  M-1 , which is on the same order of magnitude as the so far highest observed value for the complex between γ-CD and a metallacarborane. The nature of the intermolecular interaction is also examined by quantum-mechanical computational protocols. These suggest that the desolvation penalty, which is particularly low for the B21 H18- anion, is the decisive factor for its high binding strength. The results further suggest that the elliptical macropolyhedral boron hydride is another example of a CD binder, whose extraordinary binding affinity is driven by the chaotropic effect, which describes the intrinsic affinity of large polarizable and weakly solvated chaotropic anions to hydrophobic cavities and surfaces in aqueous solution.

Electrophilic Methylation of Decaborane(14): Selective Synthesis of Tetramethylated and Heptamethylated Decaboranes and Their Conjugated Bases.

The paper reports specific syntheses of methylated decaborane(14), nido-B10H14 (1), derivatives. The reaction of 1 with an excess of neat MeI and AlCl3 yields 1,2,3,4-Me4-nido-B10H10 (2) essentially quantitatively when performed at room temperature. Heating the same mixture to 120 °C provides 1-I-2,3,4,5,7,8,10-Me7-nido-B10H6 (3a). The formation of analogous 1-CF3SO2O-2,3,4,5,7,8,10-Me7-nido-B10H6 (3b) is achieved by heating 1 or 2 with an excess of MeSO3CF3 in the presence of a catalytic amount of HOSO2CF3 to 120 °C. Compounds 2 and 3 can be deprotonated to yield the corresponding anions [1,2,3,4-Me4-nido-B10H9]- (2-), [1-I-2,3,4,5,7,8,10-Me7-nido-B10H5]- (3a-), and [1-CF3SO2O-2,3,4,5,7,8,10-Me7-nido-B10H5]- (3b-). The structure of all the compounds isolated has been unambiguously confirmed by multinuclear (11B and 1H) NMR measurements, and the structures of 2-, 3a, 3a-, and 3b have been established by X-ray diffraction analyses. The very high volatility of 2 has made it impossible to apply X-ray diffraction in this case; therefore, its structure has been derived computationally using the ab initio/GIAO/NMR tool. DFT-based computational protocols have also outlined the reason why it is impossible to obtain an octamethyl derivative of 1 experimentally.

The Influence of Halogenated Hypercarbon on Crystal Packing in the Series of 1-Ph-2-X-1,2-dicarba-closo-dodecaboranes (X = F, Cl, Br, I).

Although 1-Ph-2-X-closo-1,2-C2B10H10 (X = F, Cl, Br, I) derivatives had been computed to have positive values of the heat of formation, it was possible to prepare them. The corresponding solid-state structures were computationally analyzed. Electrostatic potential computations indicated the presence of highly positive σ-holes in the case of heavy halogens. Surprisingly, the halogen•••π interaction formed by the Br atom was found to be more favorable than that of I.

Icosahedral Carbaboranes with Peripheral Hydrogen-Chalcogenide Groups: Structures from Gas Electron Diffraction and Chemical Shielding in Solution.

Carbaboranes 1,2-(EH)2 -closo-1,2-C2 B10 H10 (E=S, Se) were prepared, in the case of E=Se for the first time. Their semi-experimental equilibrium molecular structures were established by the concerted use of quantum-chemical calculations and gas electron diffraction. A method was developed and implemented to quantify the contribution of experimental data to each refined structural parameter. The accuracy of the experimental structures and those calculated at the MP2 level of theory were gauged by comparison of experimental 11 B NMR chemical shifts with quantum-chemically computed values; the inclusion of electron correlation (GIAO-MP2) provided superior results. For the purpose of geometrical prediction, the remaining group 16 elements were considered, and the icosahedral structures for E=O and Te were also computed; for E=O the same theoretical approach was used as for E=S, and for E=Te a description similar to that for E=Se was employed.

Investigation of Thiaborane closo- nido Conversion Pathways Promoted by N-Heterocyclic Carbenes.

The 12-X- closo-SB11H10 (X = H or I) thiaboranes react with one or two molar equivalents of various N-heterocyclic carbenes (NHCs) to give the deprotonated 12-vertex species of [12-X-SB11H9·NHC]-[NHC-H]+composition as kinetic products. The use of one molar equivalent of a sterically more hindered NHC reactant leads to the formation of 12-X-SB11H10·NHC adducts with a heavily distorted cage and the nido electron count. Further reaction of 12-I-SB11H10·NHC to deboronated 12-X-SB10H9·NHC proceeds in acetone to complete the closo- nido reaction pathway under the thermodynamic control. The structures of all compounds have been investigated by NMR spectroscopy and diffraction techniques. The results are supported by theoretical methods.

Synthesis of closo-1,2-H2C2B8Me8 and 1,2-H2C2B8Me7X (X = I and OTf) Dicarbaboranes and Their Rearrangement Reactions.

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H2C2B8Me8 have been prepared in high yields either from nido-5,6-H2C2B8H10 or closo-1,2-H2C2B8H8 via electrophilic methylation reactions and cluster-rearrangement methods. Prepared were also monosubstituted derivatives of general formulation closo-H2C2B8Me7-X (X = I or OTf). The permethylated compounds exhibit extreme air stability in comparison to unprotected counterparts as a consequence of rigid, egg-shaped hydrocarbon structures incorporating inner C2B8 carborane scaffolding. The structures of all compounds isolated were confirmed unambiguously by multinuclear (11B, 1H, 13C, and 19F) NMR measurements, supported by X-ray diffraction analyses and geometry optimization methods on several compounds.

Surface termination of MgB2 unveiled by a combination of adsorption experiments and theoretical calculations.

Superconductivity in polycrystalline and thin-film MgB2 is strongly affected by the termination of its surface, but a reliable determination of the surface termination is still a challenging task of surface chemistry. Here, the surface properties of superconducting MgB2 were investigated using a combination of inverse gas chromatography and van der Waals corrected density functional theory calculations. The dispersive surface energy was measured as a function of the surface coverage and its value (58 mJ m-2 to 48 mJ m-2) was verified by high-level non-local EXX + RPA calculations, which predicted that the dispersive contribution to the cleavage energy was 56 mJ m-2. The isosteric adsorption enthalpies of cyclohexane, dioxane, acetone and acetonitrile molecules were measured on an MgB2 sample and compared to the DFT calculated enthalpies for the Mg-terminated MgB2, B-terminated MgB2 and MgO(001) surfaces. The close agreement between theory and experiment for the Mg-terminated surface suggested that the magnesium termination is the dominant surface phase of MgB2. Thus, combining inverse gas chromatography experiments with theoretical calculations may provide information about the surface termination.

Chalcogen Bonding due to the Exo-Substitution of Icosahedral Dicarbaborane.

Chalcogen atoms are a class of substituents capable of generating inner and outer derivatives of boron clusters. It is well known that chalcogenated boron clusters can form strong σ-hole interactions when a chalcogen atom is a part of an icosahedron. This paper studies σ-hole interactions of dicarbaboranes with two exopolyhedral chalcogen atoms bonded to carbon vertices. Specifically, a computational investigation has been carried out on the co-crystal of (1,2-C2B10H10)2Se4•toluene and a single crystal of (1,2-C2B10H10)2Te4.

A systematic examination of classical and multi-center bonding in heteroborane clusters.

This paper presents a systematic study of multicenter and classical bonding on a broad series of experimentally known heteroboranes covering closo, nido, arachno and hypho types of cages with incorporated tetrel, pnictogen or chalcogen heterovertices up to the third-row elements. The nature of bonding is studied using a novel quantum-chemical tool, the intrinsic atomic/bond orbital (IAO/IBO) approach, which provides a direct connection between quantum chemistry and chemical concepts. We also discuss how the computed IBO properties are related to molecular observables such as interatomic distances, molecular electrostatic potential surfaces and dipole moments.

Unusual Cage Rearrangements in 10-Vertex nido-5,6-Dicarbaborane Derivatives: An Interplay between Theory and Experiment.

The reaction between selected X-nido-5,6-C2B8H11 compounds (where X = Cl, Br, I) and "Proton Sponge" [PS; 1,8-bis(dimethylamino)naphthalene], followed by acidification, results in extensive rearrangement of all cage vertices. Specifically, deprotonation of 7-X-5,6-C2B8H11 compounds with one equivalent of PS in hexane or CH2Cl2 at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH+[10-X-5,6-C2B8H10]- salts. Reprotonation using concentrated H2SO4 in CH2Cl2 generates a series of neutral carbaboranes 10-X-5,6-C2B8H11, with the overall 7 → 10 conversion being 75%, 95%, and 100% for X = Cl, Br, and I, respectively. Under similar conditions, 4-Cl-5,6-C2B8H11 gave ∼66% conversion to 3-Cl-5,6-C2B8H11. Since these rearrangements could not be rationalized using the B-vertex swing mechanism, new cage rearrangement mechanisms, which are substantiated using DFT calculations, have been proposed. Experimental 11B NMR chemical shifts are well reproduced by the computations; as expected δ(11B) for B(10) atoms in derivatives with X = Br and I are heavily affected by spin-orbit coupling.

The behavior of a paramagnetic system in electric and magnetic fields as exemplified by revisiting Li@B10H14.

The electric and magnetic properties of the Li@B10H14 complex, considered as a novel inorganic electride-type system with potential for second-order non-linear optical (NLO) applications, have already been studied. However, the reported C2v structure is not the global energy minimum and therefore its electronic and magnetic properties need to be revisited. Moreover, by applying more accurate computational protocols (ROHF-CCSD/CCSD(T) and larger basis sets) we show that the model chemistry used earlier (UMP2/6-31+G(d)) is not sufficient for reliable description of the NLO responses of this open-shell doublet complex. The global minimum based on the Cs symmetry is significantly (by ca. 25 kcal mol-1) more stable than the C2v structure and it should be viewed as a system with moderate NLO responses. An excess of unpaired electron density is also responsible for the contact and pseudo-contact contributions to the magnetic properties, which was not considered in the earlier work.

B-Hπ: a nonclassical hydrogen bond or dispersion contact?

Close B-Hπ contacts have recently been observed in crystallographic structures of Ir-dithiolene-phosphine complexes containing boron hydride cluster. This finding was interpreted using quantum chemical calculations as a new type of electrostatically driven nonclassical hydrogen bonding. However, such an explanation contradicts the wealth of evidence for unique noncovalent interactions of boron hydrides. Moreover, care must be exercised when computational methods are used to interpret new phenomena. Therefore, here, we cautiously examine the B-Hπ interaction by means of advanced quantum chemistry and disprove the claimed attractive electrostatic nature and rather define it as a nonspecific dispersion-driven contact. In summary, we present evidence that the crystallographically observed B-Hπ contacts do not constitute a novel type of hydrogen bonding of boron hydride clusters.