Isomerisation of nido-[C2B10H12]2- dianions: unprecedented rearrangements and new structural motifs in carborane cluster chemistry

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent

In this work, we report the first example of the PCET reactivity for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo-carboranyl diphosphine, 1-PtBu2-2-PiPr2-closo-C2B10H10. As we have previously reported the conversion of this closo-carboranyl diphosphine into the zwitterionic nido- derivative upon reaction with main group hydrides, the transformation reported herein represents a complete synthetic cycle for the metal-free reduction of quinones, with the redox-active carboranyl diphosphine scaffold acting as a mediator. The proposed mechanism of this reduction, based on pKa determination, electrochemical studies, and kinetic isotope effect determination, involves the electron transfer from the nido- cluster to the quinone coupled with the delivery of protons.

Full Electron Delocalization across the Cluster in 1,12-bisBMes2-p-carborane Radical Anion

Conjugation between three-dimensional (3D) carboranes and the attached substituents is commonly believed to be very weak. In this paper, we report that reducing 1,12-bis(BMes2)-p-carborane (B2pCab) with one electron gives a radical anion with a centrosymmetric semiquinoidal structure. This radical anion shows extensive electron delocalization between the two boron centers over the p-carborane bridge due to the overlap of carborane lowest unoccupied molecular orbital (LUMO) and the BMes2 LUMO. Unlike dianions of other C2B10H12 carboranes, which rearrange to a nido-form, two-electron reduction of B2pCab leads to a rearrangement into a basket-shaped intermediate.

Redox-active carborane clusters in bond activation chemistry and ligand design

Icosahedral closo-dodecaboranes have the ability to accept two electrons, opening into a dianionic nido-cluster. This transformation can be utilized to store electrons, drive bond activation, or alter coordination to metal cations. In this feature article, we present cases for each of these applications, wherein the redox activity of carborane facilitates the generation of unique products. We highlight the effects of exohedral substituents on reactivity and the stability of the products through conjugation between the cluster and exohedral substituents. Futher, the utilization of the redox properties and geometry of carborane clusters in the ligand design is detailed, both in the stabilization of low-valent complexes and in the tuning of ligand geometry.

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.

Transformation of various multicenter bondings within bicapped-square antiprismatic motifs: Z-rearrangement

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic closo-C₂B₈H₁₀ by means of high-quality computations that disprove the earlier postulated dsd (diamond-square-diamond) scheme for these isomerizations. The experimentally existing closo-1,2-C₂B₈H₁₀ was able to be converted to 1,6-, and 1,10-isomers by pyrolysis, and the dsd (diamond-square-diamond) mechanism was offered as an explanation of these processes. However, these computations disprove the postulated dsd scheme for these isomerizations that take place in the ten-vertex closo series. Experimentally observed thermal rearrangements, both in the parent and substituted closo-1,2-C₂B₈H₁₀, closo-1-CB₉H₁₀^-, and closo-B₁₀H₁₀^2-, indirectly support these refined computations. All these processes are based on the new concept of the so-called Z-mechanism, being consistent with a transition state of a boat shape with an open hexagonal belt that results from the initial breakage of three bonds. Such bond breakings and the consequent bond formations bring to mind the shape of the letter Z. In effect, the pattern of multicenter bonding shifts from reactant through a transition state to product. The molecular rearrangements that are available experimentally favour either the axial or equatorial isomers, and this ratio depends on temperature and the type of cluster and its substitution.

Metal-Free Bond Activation by Carboranyl Diphosphines

We report metal-free bond activation by the carboranyl diphosphine 1-P^tBu₂-2-PⁱPr₂-C₂B₁₀H₁₀. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of "regular" phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.

Pathway bifurcations in the cage rearrangement of metallacarboranes: experimental and computational evidence

Thirteen new metallacarborane complexes of rhodium and iridium with covalently bound cage carbon atoms were synthesized and their thermal stability was investigated. Two iridium complexes undergo a polyhedral rearrangement with the formation of more than one isomer. The structures of the new isomers were determined by a single crystal X-ray diffraction analysis and ¹¹B{¹H}-¹¹B{¹H} COSY NMR. A full isomerization scheme of the less thermally stable complex was proposed based on DFT calculations. According to this mechanism sequential downhill and uphill bifurcations arise in the reaction pathway. Each bifurcation is responsible for a new product formation.

Reactions of Experimentally Known Closo-C2B8H10 with Bases. A Computational Study

On the basis of the direct transformations of closo-1,2-C2B8H10 with OH(−) and NH3 to arachno-1,6,9-OC2B8H13(−) and arachno-1,6,9-NC2B8H13, respectively, which were experimentally observed, the DFT computational protocol was used to examine the corresponding reaction pathways. This work is thus a computational attempt to describe the formations of 11-vertex arachno clusters that are formally derived from the hypothetical closo-B13H13(2−). Moreover, such a protocol successfully described the formation of arachno-4,5-C2B6H11(−) as the very final product of the first reaction. Analogous experimental transformations of closo-1,6-C2B8H10 and closo-1,10-C2B8H10, although attempted, were not successful. However, their transformations were explored through computations.

Thiaboranes on Both Sides of the Icosahedral Barrier: Retaining and Breaking the Barrier with Carbon Functionalities

The concept of icosahedral barrier has been expanded from the chemistry of carbaboranes to the area of thiaboranes. Both representatives of this barrier, i. e., closo-1,2-C₂ B₁₀ H₁₂ and closo-1-SB₁₁ H₁₁ , are similar in their electron distribution, which is dominated by positive charge in the midpoint of the C-C vector and on the sulfur atom with experimentally determined dipole moments of 4.50 D and 3.64 D, respectively. This is a driving force for their reactivity as exemplified by their reactions with different carbon functionalities. Icosahedral closo-1-SB₁₁ H₁₁ reacts both with an electron sextet containing carbon (in the form of N-heterocyclic carbenes), reported earlier, and with methyl iodide with an electron octet on the carbon. The latter reaction provides hexamethylated thiaborane on the basis of methylation so far unknown in this area of heteroborane chemistry. The computations of the heat of formation (ΔH_f ²⁹⁸ ) make it possible to estimate the height of the barrier as well as to propose closo-thiaboranes beyond the barrier. Eleven and twelve vertex thiaboranes with nido electron count are known experimentally for breaking the barrier. These computations also suggest that the larger nido-thiaboranes are promising candidates for the corresponding experimental availability, i. e., the ΔH_f ²⁹⁸ of a 13-vertex nido-thiaborane cluster has been computed to be more negative than that of the well-known nido-SB₁₀ H₁₁ ^- cluster (-6.7 and -5.6 kcal mol^-1 per vertex, respectively).

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.

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.

Observation of a metal-centered B2-Ta@B18−tubular molecular rotor and a perfect Ta@B20−boron drum with the record coordination number of twenty

A B₂-Ta@B₁₈⁻tubular molecular rotor and a Ta@B₂₀⁻boron drum with the record coordination number of twenty were observedviaa joint experimental and theoretical investigation.

Steric versus electronic factors in metallacarborane isomerisation: nickelacarboranes with 3,1,2-, 4,1,2- and 2,1,8-NiC2B9 architectures and pendant carborane groups, derived from 1,1′-bis(o-carborane)

Nickelacarborane derivatives of 1,1′-bis(o-carborane) allow comment on the factors important in the isomerisation of metallacarboranes.