Macropolyhedral syn-B18H22, the "Forgotten" Isomer.

The chemistry and physics of macropolyhedral B18H22 clusters have attracted significant attention due to the interesting photophysical properties of anti-B18H22 (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn-B18H22 isomer, which has received very little attention since its discovery compared to its anti-B18H22 isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn-B18H22 exhibits blue fluorescence and becomes phosphorescent when substituted at various positions on the cluster, associated with peculiar microstructural-dependent effects. This work is a combined theoretical and experimental investigation that includes the synthesis, separation, structural characterization, and first elucidation of the photophysical properties of three different monothiol-substituted cluster isomers, [1-HS-syn-B18H21] 1, [3-HS-syn-B18H21] 3, and [4-HS-syn-B18H21] 4, of which isomers 1 and 4 have been proved to exist in two different polymorphic forms. All of these newly substituted macropolyhedral cluster derivatives (1, 3, and 4) have been fully characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, IR spectroscopy, and luminescence spectroscopy. This study also presents the first report on the mechanochromic shift in the luminescence of a borane cluster and generally enriches the area of rather rare boron-based luminescent materials. In addition, we present the first results proving that they are useful constituents of carbon-free self-assembled monolayers.

Carborane-thiol protected copper nanoclusters: stimuli-responsive materials with tunable phosphorescence.

Atomically precise nanomaterials with tunable solid-state luminescence attract global interest. In this work, we present a new class of thermally stable isostructural tetranuclear copper nanoclusters (NCs), shortly Cu4@oCBT, Cu4@mCBT and Cu4@ICBT, protected by nearly isomeric carborane thiols: ortho-carborane-9-thiol, meta-carborane-9-thiol and ortho-carborane 12-iodo 9-thiol, respectively. They have a square planar Cu4 core and a butterfly-shaped Cu4S4 staple, which is appended with four respective carboranes. For Cu4@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu4S4 staple flatter in comparison to other clusters. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) and collision energy-dependent fragmentation, along with other spectroscopic and microscopic studies, confirm their molecular structure. Although none of these clusters show any visible luminescence in solution, bright µs-long phosphorescence is observed in their crystalline forms. The Cu4@oCBT and Cu4@mCBT NCs are green emitting with quantum yields (Φ) of 81 and 59%, respectively, whereas Cu4@ICBT is orange emitting with a Φ of 18%. Density functional theory (DFT) calculations reveal the nature of their respective electronic transitions. The green luminescence of Cu4@oCBT and Cu4@mCBT clusters gets shifted to yellow after mechanical grinding, but it is regenerated after exposure to solvent vapour, whereas the orange emission of Cu4@ICBT is not affected by mechanical grinding. Structurally flattened Cu4@ICBT didn't show mechanoresponsive luminescence in contrast to other clusters, having bent Cu4S4 structures. Cu4@oCBT and Cu4@mCBT are thermally stable up to 400 °C. Cu4@oCBT retained green emission even upon heating to 200 °C under ambient conditions, while Cu4@mCBT changed from green to yellow in the same window. This is the first report on structurally flexible carborane thiol appended Cu4 NCs having stimuli-responsive tunable solid-state phosphorescence.

The Livelihood of Artisanal and Small-Scale Miners and Awareness of the Use of 3T Minerals in Rwanda-A Case Study in the Rutsiro District: A Qualitative Assessment.

This article examines the impact of artisanal and small-scale mining (ASM) on livelihood in mining communities in Rwanda (Rutsiro) where wolframite and coltan are mined. The paper discusses the development of ASM and other entrepreneur activities, in particular agriculture. With ASM activities, there is environmental degradation on the one hand but also an improvement in the well-being of the local population on the other. The 3T (tin, tungsten, tantalum) minerals extracted by ASM are used in the electronics industry for products such as smartphones, tablets, and laptops, which are mainly consumed in the developed world. Based on questionnaires and structured research with miners, it was determined how ASM affects their lives, or whether there is a deterioration or improvement in their well-being. The research builds on previous field research in Rwanda. Because of mining, communities in the mining areas have access to health care, they can pay tuition fees, insurance, etc. On the other hand, the lives of miners are endangered by respiratory diseases, accidents in mines, landslides in mining areas, and other negative environmental impacts. The extraction of these minerals, however, may lead to a worse quality of life for the miners responsible for the extraction in developing countries. This different view is also illustrated by the fact that miners themselves often do not know what 3T minerals are used for. ASM benefits miners from an economic perspective but may worsen their quality of life due to unsuitable working conditions. This study covers a broader understanding of socioeconomic impacts of ASM and tries to point out the lack of awareness about the mining of minerals important for the daily use of modern technologies. This article would like to contribute to the larger debate about the lack of awareness of the origin of 3T minerals.

Carboranethiol-Protected Propeller-Shaped Photoresponsive Silver Nanomolecule.

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag21 nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag13 central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag-S-Ag bridging motifs. While 12 MCT ligands protect the core through metal-thiolate bonds in a 3-6-3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV-vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag21 nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120-160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 µs for the carriers, spread over a broad wavelength region of 520-650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.

Light-Activated Intercluster Conversion of an Atomically Precise Silver Nanocluster.

Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron-carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag42(CBDT)15(TPP)4]2- (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 was confirmed using UV-vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV-vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8-Ag6 core-shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag37(CBDT)12(TPP)4]3- and [Ag35(CBDT)8(TPP)4]2- were formed after 8 h of light irradiation, and the second set comprised of [Ag30(CBDT)8(TPP)4]2-, [Ag26(CBDT)11(TPP)4]2-, and [Ag26(CBDT)7(TPP)7]2- were formed after 16 h of irradiation. After 24 h, the conversion to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 µs, indicating phosphorescence, while Ag14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag42 is shown to be responsible for the core transformation.

Lewis Superacidic Tellurenyl Cation-Induced Electrophilic Activation of an Inert Carborane.

The aryltellurenyl cation [2-(tBuNCH)C6 H4 Te]+ , a Lewis super acid, and the weakly coordinating carborane anion [CB11 H12 ]- , an extremely weak Brønsted acid (pKa =131.0 in MeCN), form an isolable ion pair complex [2-(tBuNCH)C6 H4 Te][CB11 H12 ], in which the Brønsted acidity (pKa 7.4 in MeCN) of the formally hydridic B-H bonds is dramatically increased by more than 120 orders of magnitude. The electrophilic activation of B-H bonds in the carborane moiety gives rise to a proton transfer from boron to nitrogen at slightly elevated temperatures, as rationalized by the isolation of a mixture of the zwitterionic isomers 12- and 7-[2-(tBuN{H}CH)C6 H4 Te(CB11 H11 )] in ratios ranging from 62 : 38 to 80 : 20.

Navigational Needs and Preferences of Hospital Patients and Visitors: What Prospects for Smart Technologies?

In this paper, we map navigational needs and preferences of patients and visitors to evaluate the appropriateness of a smartphone navigation application in the hospital in contrast to other, more traditional navigational cues. We test the effects of sociodemographic variables (age, gender, education) on wayfinding strategies and preferences of respondents (using chi2 tests). Empirical research is based on the survey among 928 patients/visitors of the Vítkovice Hospital in Ostrava, Czechia. We found a relatively weak association between gender and wayfinding-no major differences between men and women in navigational preferences were found. Age was the most important predictor of wayfinding. Respondents in the over-60-year age group were characteristic of a lower interest in changes of the navigational system and low willingness to use mobile applications for navigation-people between 41 years and 60 years were the biggest supporters of changes. Correspondingly, demand for improvement of navigation (including a mobile application) was positively correlated with educational level.

A theoretical analysis of the structure and properties of B26H30 isomers. Consequences to the laser and semiconductor doping capabilities of large borane clusters.

Decaborane(14), nido-B10H14, is the major commercially available molecular building block in boron cluster chemistry. The condensation of two such {nido-B10} blocks gives the known isomers of B18H22- a molecule used in the fabrication of p-type semiconductors and capable of blue laser emission. Here, we computationally determine the structures and thermodynamic stabilities of 20 possible B26H30 regioisomers constructed from the fusion of three {nido-B10} blocks with the three subclusters conjoined by two-boron atom shared edges. In addition, density functional theory, time-dependent (TD)-DFT and multiconfigurational CASPT2 methods have been used to model and investigate the physical and photophysical properties of the three most stable of these isomers. Our findings predict these isomers to be potentially useful materials for the semiconductor industry, as high boron-content doping agents, and in the fabrication of new optical materials.

Acid-Base Control of Valency within Carboranedithiol Self-Assembled Monolayers: Molecules Do the Can-Can.

We use simple acid-base chemistry to control the valency in self-assembled monolayers of two different carboranedithiol isomers on Au{111}. Monolayer formation proceeds via Au-S bonding, where manipulation of pH prior to or during deposition enables the assembly of dithiolate species, monothiol/monothiolate species, or combination. Scanning tunneling microscopy (STM) images identify two distinct binding modes in each unmodified monolayer, where simultaneous spectroscopic imaging confirms different dipole offsets for each binding mode. Density functional theory calculations and STM image simulations yield detailed understanding of molecular chemisorption modes and their relation with the STM images, including inverted contrast with respect to the geometric differences found for one isomer. Deposition conditions are modified with controlled equivalents of either acid or base, where the coordination of the molecules in the monolayers is controlled by protonating or deprotonating the second thiol/thiolate on each molecule. This control can be exercised during deposition to change the valency of the molecules in the monolayers, a process that we affectionately refer to as the "can-can." This control enables us to vary the density of molecule-substrate bonds by a factor of 2 without changing the molecular density of the monolayer.

Nuclear Magnetic Shielding of Monoboranes: Calculation and Assessment of 11B NMR Chemical Shifts in Planar BX3 and in Tetrahedral [BX4]- Systems.

11B NMR chemical shifts of tricoordinated BX3 and tetracoordinated BX4- compounds (X = H, CH3, F, Cl, Br, I, OH, SH, NH2, and CH═CH2) were computed, and the shielding tensors were explored not only within the nonrelativistic GIAO approach but also by application of both relativistic ZORA computations including spin-orbit coupling as well as scalar nonrelativistic ZORA computations (BP86 level of density functional theory). The contributions of the spin-orbit coupling to the overall shieldings are decisive for X = Br and I in both series. No relationship was found between the 2p orbital occupancies or 1/ΔE (difference between LUMO and suitably occupied MO that can be coupled with LUMO) with the shielding tensors (or their principal values) in the BX3 series. However, a multidimensional statistical approach known as factor analysis (frequently used in chemometrics) revealed that three factors account for 92% of the cumulative proportion of total variance. The main components of the first factor are occupancies in the 2px and 2py orbitals and 1/ΔE; the second factor is mainly the occupancy in the 2pz orbital and the inductive substituent parameters by Taft. Finally, the third factor consists exclusively (98.4%) of the electrostatic potential (Vmax), which is directly related to the so-called π-hole magnitudes.

Unique stereocontrol in carborane chemistry: skeletal alkylcarbonation (SAC) versus exoskeletal alkylmethylation (EAM) reactions.

Reactions between the arachno-6,9-C2B8H14 (1) dicarbaborane and acyl chlorides, RCOCl (2), are subject to stereocontrol that completely changes the nature of the reaction products. While most chlorides produce the 8-R-nido-7,8,9-C3B8H11 (3) tricarbollides (by skeletal alkylcarbonation=SAC), bulky RCOCls (2; where R=1-adamantyl, 2 a; 1-mesityl, 2 b; 9-anthranyl, 2 c; 1-naphthyl, 2 d) in 1,2-dichloroethane (DCE) in the presence of triethylamine at 40-60 °C gave a series of entirely different 1-R-2-CH3-closo-1,6-C2B8H8 (4) dicarbaboranes upon acidification with conc. H2SO4 (by exosleletal alkylmehylation=EAM). Both types of reactions seem to proceed via a common [8-R-nido-7,8,9-C3B8H10](-) (3(-)) anion which in the EAM case is unstable because of steric crowd and undergoes rearrangement via the isomeric [R-nido-7,8,10-C3B8H10](-) tricarbollide structures which, on protonation, undergo reductive extraction of one CH vertex to generate the 2-CH3 substituent in structure 4.

The dominant role of chalcogen bonding in the crystal packing of 2D/3D aromatics.

The chalcogen bond is a nonclassical σ-hole-based noncovalent interaction with emerging applications in medicinal chemistry and material science. It is found in organic compounds, including 2D aromatics, but has so far never been observed in 3D aromatic inorganic boron hydrides. Thiaboranes, harboring a sulfur heteroatom in the icosahedral cage, are candidates for the formation of chalcogen bonds. The phenyl-substituted thiaborane, synthesized and crystalized in this study, forms sulfur⋅⋅⋅π type chalcogen bonds. Quantum chemical analysis revealed that these interactions are considerably stronger than both in their organic counterparts and in the known halogen bond. The reason is the existence of a highly positive σ-hole on the positively charged sulfur atom. This discovery expands the possibilities of applying substituted boron clusters in crystal engineering and drug design.

Carboranedithiols: building blocks for self-assembled monolayers on copper surfaces.

Two different positional isomers of 1,2-dicarba-closo-dodecaboranedithiols, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (1) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.

Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces.

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B(10)H(13)] 1, [2-(HS)-nido-B(10)H(13)] 2, and [1,2-(HS)(2)-nido-B(10)H(12)] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B(18)H(22)·C(6)H(6) solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B(10)H(13)(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol (1)H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe(2)(PMe(2)Ph)(2)] reacts with the exposed and acidic B-H-B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe(2)Ph)(2)-nido-7-PtB(10)H(11)] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.

Tuning the surface potential of Ag surfaces by chemisorption of oppositely-oriented thiolated carborane dipoles.

Two selected carboranethiol isomers were used to modify flat silver surfaces. Both isomers, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (a) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (b), are relatively strong dipoles with two SH groups per molecule. They are both anchored to the surface via two SH groups per molecule. Topography and surface potential changes of the modified silver surfaces were studied using Scanning Kelvin Probe Force Microscopy (SKPFM). These measurements proved that both isomers are oppositely oriented on the surface. The former isomer increases, and the latter one decreases the surface potential of a modified silver film. The relative changes of the surface potential correlate well with the dipole moments of the isomers. Competitive chemisorption from a 1:1 mixture of both isomers shows that the isomer (a) is found in a significantly higher concentration on the surface than the isomer (b). This has been proved by both SKPFM and X-ray photoelectron spectroscopy (XPS) techniques. Additionally, contact angle measurements were carried out to characterise the modified surfaces, and these and XPS results show the presence of hydrophobic hydrocarbon contaminants.

Ferrocene-like iron bis(dicarbollide), [3-Fe(III)-(1,2-C(2)B(9)H(11))(2)](-). The first experimental and theoretical refinement of a paramagnetic (11)B NMR spectrum.

Nuclear magnetic resonance (NMR) of paramagnetic molecules (pNMR) provides detailed information on the structure and bonding of metallo-organic systems. The physical mechanisms underlying chemical shifts are considerably more complicated in the presence of unpaired electrons than in the case of diamagnetic compounds. We report for the first time a combined first-principles theoretical as well as experimental liquid-state (11)B NMR study of a paramagnetic compound, applied on the [3-Fe(III)-(1,2-C(2)B(9)H(11))(2)](-) metallaborane, which is an electronically open-shell structure where the iron centre binds two hemispherical boron-carbon cages. We show that this combined theoretical and experimental analysis constitutes a firm basis for the assignment of experimental (11)B NMR chemical shifts in paramagnetic metallaboranes. In the calculations, the roles of the different physical contributions to the pNMR chemical shift are elaborated, and the performance of different popular exchange-correlation functionals of density-functional theory as well as basis sets, are evaluated. A dynamic correction to the calculated shifts via first-principles molecular dynamics simulations is found to be important. Solvent effects on the chemical shifts were computed and found to be of minor significance.

Computational studies of structures and properties of metallaboranes. Part 3: protonated iron bis(dicarbollide), [3-Fe-(1,2-C2B9H11)2H]-.

On the basis of the energies and 11B NMR chemical shifts computed at the BP86/AE1(*) and GIAO-B3LYP/II' levels of density functional theory, respectively, the structure of the long-known protonated iron(II) bis(dicarbollide) can be assigned to a staggered isomer with a cisoid conformation of the carborane ligands. In the unprotonated species, in contrast, these ligands adopt the usual trans orientation, suggesting that suitable control of protonation/deprotonation equilibria could induce rotary motion at the molecular level.

The first member of the eleven-vertex azadicarbaborane series, 1,6,9-NC2B8H13, and its N-alkyl derivatives.

Reactions between closo-1,2-C(2)B(8)H(10) (1) and amines of general formulation R(1)R(2)NH (where R(1), R(2) = H, H; Me, H; t-Bu, H and Et, Et) resulted in a straightforward cluster expansion and formation of the 11-vertex arachno-azadicarbaboranes of the 1,1-R(1),R(2-)1,6,9-NC(2)B(8)H(11) (2) cluster constitution (where R(1), R(2) = H, H 2a; Me, H 2b; t-Bu, H 2c and Et, Et 2d) in yields 10-75%, depending on the nature of the amine used. The reactions are the first example of a direct closo to arachno transformation in the area of cluster-boron compounds. Compounds 2b and 2c were isolated in two isomeric forms anti- and syn- that differ in the positioning of the t-Bu substituent with respect to the bridging hydrogen site. Deprotonation of compounds 2 generally leads to removal of the bridging proton and formation of the [1,1-R(1),R(2-)1,6,9-NC(2)B(8)H(11)](-) (2-) anions that, in the case of the monoalkylated Me and t-Bu derivatives, adopt only an anti configuration. The structure of anti-2c was determined by X-ray diffraction analysis and the geometries of the parent compound and the corresponding syn and anti isomers were optimised at the RMP2/6-31G* level. The composition of all compounds is consistent with the results of mass spectrometry and multinuclear ((1)H and (11)B) spectroscopy complemented by two-dimensional [(11)B-(11)B]-COSY and (1)H{(11)B(selective)} NMR measurements. Experimental (11)B chemical shifts generally show acceptable agreement with theoretical values calculated by GIAO methods, in particular at GIAO-MP2/II, where possible.

New route to 1-thia-closo-dodecaborane(11), closo-1-SB11H11, and its halogenation reactions. The effect of the halogen on the dipole moments and the NMR spectra and the importance of spin-orbit coupling for the 11B chemical shifts.

Reaction between nido-B10H14 (1) and elemental sulfur in CHCl3 in the presence of Et3N at room temperature, followed by treatment with Et3N.BH3 at 170-190 degrees C, resulted in the isolation of closo-1-SB11H11 (2) in 50% yield. Selected electrophilic halogenation reactions of compound led to the isolation of a series of monohalogenated derivatives of general constitution 12-X-closo-1-SB11H10 (12-X-, where X = Cl, Br, and I). The structures of 12-Cl- and 12-I- were determined by an X-ray diffraction analysis and the structures of all compounds were geometry optimised at the RMP2(fc)/6-31G* level. The constitution of all compounds is consistent with the results of mass spectrometry and multinuclear (1H and 11B) spectroscopy complemented by two-dimensional [11B-11B]-COSY and 1H{11B(selective)} NMR measurements. Experimental 11B chemical shifts generally show acceptable agreement with theoretical values calculated by GIAO methods, but spin-orbit coupling must be included for nuclei bearing heavy-atom substituents such as Br or I. The dipole moments determined for the B12-X bonds show similarities to those of aliphatic C-X bonds and confirm unambiguously the B12 --> S dipole moment orientation in the SB11 cage.

Computational study of structures and properties of metallaboranes: cobalt bis(dicarbollide).

A density functional study at the BP86/AE1 level is presented for the cobalt bis(dicarbollide) ion [3-Co-(1,2-C2B9H11)2]- (1) and selected isomers and rotamers thereof. Rotation of the two dicarbollide moieties with respect to each other is facile, as judged by the small energetic separation of the three rotamers located (within 11 kJ mol(-1)) and by the low barriers for their interconversion (at most 41 kJ mol(-1)). Among the isomers differing in carbon atom positions that contain two equivalent dicarbollide ligands, the 1,7 ("carbon apart") form [2-Co-(1,7-C2B9H11)2]- is the most stable, 121 kJ mol(-1) below 1. The electronic structure of 1 is characterized in terms of molecular orbitals, population analysis, and excitation energies from time-dependent density functional theory, relevant to UV/Vis spectroscopy. Experimental 11B NMR chemical shifts of 1 are reproduced to better than 5 ppm at the GIAO-B3LYP/II' level, and the computed delta(11B) values are only little affected by rotational averaging or the presence of a polarizable continuum. Larger such effects are found for the as-yet unknown 59Co chemical shift, for which a value in the range between -1800 and -2400 ppm is predicted. Even though the accuracy achieved for the theoretical delta(11B) values is somewhat lower than that for heteroboranes at conventional ab initio levels, the level of density functional employed can afford qualitatively reliable chemical shifts, which can be useful in assignments and structural refinements of heteroboranes containing transition metal.