Transmission of Electronic Substituent Effects across the 1,12-Dicarba-<i>closo</i>-dodecaborane Cage: A Computational Study Based on Structural Variation, Atomic Charges, and ¹³C NMR Chemical Shifts

The ability of the 1,12-dicarba-<i>closo</i>-dodecaborane cage to transmit long-range substituent effects has been investigated by analyzing the structural variation of a phenyl probe bonded to C1, as caused by a remote substituent X at C12. The geometries of 41 Ph–CB₁₀H₁₀C–X molecules, including 11 charged species, have been determined by MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed <i>S</i>_F^CARB. Multiple regression analysis of <i>S</i>_F^CARB, using appropriate explanatory variables, reveals the presence of resonance effects, superimposed onto the field effect of the remote substituent. The ability of the <i>para</i>-carborane cage to transmit resonance effects is, on average, about one-half of that of the <i>para</i>-phenylene frame in coplanar para-substituted biphenyls. Analysis of the π-charge variation of the phenyl probe confirms that the <i>para</i>-carborane frame is less capable than the coplanar <i>para</i>-phenylene frame of transmitting π-electrons from the remote substituent to the phenyl probe, or vice versa. The <i>para</i>-carborane cage is a better π-acceptor than π-donor; this makes π-donor substituents less effective than π-acceptors in exchanging π-electrons with the phenyl probe across the cage. When the remote substituent is an uncharged group, the <i>para</i>-carborane cage acts as a very weak π-acceptor toward the phenyl probe. The structural variation of the <i>para</i>-carborane cage has also been investigated. It consists primarily of a change of the C1···C12 nonbonded separation, coupled with a change of the five B–C–B angles at C12. This concerted geometrical change is controlled by the electronegativity of the substituent and the resonance interactions occurring between substituent and cage. These, however, appear to be important only when π-donor substituents are involved. The ¹³C NMR chemical shifts of the <i>para</i>-carbon of the phenyl probe correlate nicely with <i>S</i>_F^CARB, pointing to the reliability of these quantities as measures of long-range substituent effects. On the contrary, the ¹¹B and ¹³C chemical shifts of the cage atoms do not convey information on electronic substituent effects

Chalcogen and Pnicogen Bonds in Complexes of Neutral Icosahedral and Bicapped Square-Antiprismatic Heteroboranes

A systematic quantum mechanical study of σ-hole (chalcogen, pnicogen, and halogen) bonding in neutral experimentally known <i>closo</i>-heteroboranes is performed. Chalcogens and pnicogens are incorporated in the borane cage, whereas halogens are considered as <i>exo</i>-substituents of dicarbaboranes. The chalcogen and pnicogen atoms in the heteroborane cages have partial positive charge and thus more positive σ-holes. Consequently, these heteroboranes form very strong chalcogen and pnicogen bonds. Halogen atoms in dicarbaboranes also have a highly positive σ-hole, but only in the case of C-bonded halogen atoms. In such cases, the halogen bond of heteroboranes is also strong and comparable to halogen bonds in organic compounds with several electron-withdrawing groups being close to the halogen atom involved in the halogen bond

The Synthesis and Structural Characterization of Polycyclic Derivatives of Cobalt Bis(dicarbollide)(1^–)

The cobalt bis­(dicarbollide) anion [(1,2-C₂B₉H₁₁)₂-3,3′-Co]⁻ (<b>1</b>^<b>–</b>) is an increasingly important building block for the design of new biologically active compounds. Here we report the reactions of lithiated <b>1</b>^<b>–</b> with <i>N</i>-(ω-bromoalkyl)­phthalimides Br-(CH₂)_<i>n</i>-N­(CO)₂NC₆H₄ (where <i>n</i> = 1 to 3) that give a number of new compounds substituted at dicarbollide carbon atom positions. For <i>n</i> = 2 and 3, substitution of the cobalt bis­(dicarbollide) anion is accompanied by cyclocondensation of the organic moieties to give polycyclic ring structures attached to the cage. Predominant products correspond to oxazolo­[2,3-<i>a</i>]­isoindol-5­(9b<i>H</i>)-1,2,3-dihydro-9<i>b-</i>yl)-(1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>2</b>^<b>–</b>) and 1-(2<i>H</i>-[1,3]-oxazino­[2,3-<i>a</i>]­isoindol-6­(10b<i>H</i>)-1,3,4-dihydro-10<i>b</i>-yl)-(1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>4</b>^<b>–</b>) ions with isoindolone functions containing either five- or six-membered lateral oxazine rings. Additionally, products (tetrahydro-2-benzo­[4,5]­furan-1­(3<i>H</i>)-1-[3,3]-yl-)-1,1′-μ-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>3</b>^<b>–</b>) and (2-(azetidin-yl-carbonyl)­benzoyl-)-1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>5</b>^<b>–</b>) were isolated, which display unusual cyclic structures featuring a bicyclic benzofuranone ring attached in a bridging manner by a quarternary carbon to two skeletal carbon atoms and a ketobenzoic acid amide substituent with a side azetidine ring. However, in the case of <i>n</i> = 1, only the anticipated methylene amine derivative [(1-NH₂CH₂-1,2-C₂B₉H₁₁)­(1′,2′-C₂B₉H₁₁)₂-3,3′-Co]⁻ (<b>6</b>^<b>–</b>) was isolated in low yield after cleavage of the phthalimide intermediate species. The molecular structures of all isolated cyclic products <b>2</b>^<b>–</b> to <b>5</b>^<b>–</b> were confirmed by single-crystal X-ray diffraction studies, and the structure of cobalt bis­(dicarbollide)-1-CH₂NH₂ <b>6</b>^<b>–</b> was delineated using density functional theory applied at BP86/AE1 level in combination with NMR spectroscopy. Thus, the synthetic method described herein presents a facile route to new cobalt bis­(dicarbollide) derivatives substituted by polycyclic structural motifs with potential biological activity

The Synthesis and Structural Characterization of Polycyclic Derivatives of Cobalt Bis(dicarbollide)(1^–)

The cobalt bis­(dicarbollide) anion [(1,2-C₂B₉H₁₁)₂-3,3′-Co]⁻ (<b>1</b>^<b>–</b>) is an increasingly important building block for the design of new biologically active compounds. Here we report the reactions of lithiated <b>1</b>^<b>–</b> with <i>N</i>-(ω-bromoalkyl)­phthalimides Br-(CH₂)_<i>n</i>-N­(CO)₂NC₆H₄ (where <i>n</i> = 1 to 3) that give a number of new compounds substituted at dicarbollide carbon atom positions. For <i>n</i> = 2 and 3, substitution of the cobalt bis­(dicarbollide) anion is accompanied by cyclocondensation of the organic moieties to give polycyclic ring structures attached to the cage. Predominant products correspond to oxazolo­[2,3-<i>a</i>]­isoindol-5­(9b<i>H</i>)-1,2,3-dihydro-9<i>b-</i>yl)-(1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>2</b>^<b>–</b>) and 1-(2<i>H</i>-[1,3]-oxazino­[2,3-<i>a</i>]­isoindol-6­(10b<i>H</i>)-1,3,4-dihydro-10<i>b</i>-yl)-(1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>4</b>^<b>–</b>) ions with isoindolone functions containing either five- or six-membered lateral oxazine rings. Additionally, products (tetrahydro-2-benzo­[4,5]­furan-1­(3<i>H</i>)-1-[3,3]-yl-)-1,1′-μ-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>3</b>^<b>–</b>) and (2-(azetidin-yl-carbonyl)­benzoyl-)-1-cobalt­(III) bis­(1,2-dicarbollide)­(1^–) (<b>5</b>^<b>–</b>) were isolated, which display unusual cyclic structures featuring a bicyclic benzofuranone ring attached in a bridging manner by a quarternary carbon to two skeletal carbon atoms and a ketobenzoic acid amide substituent with a side azetidine ring. However, in the case of <i>n</i> = 1, only the anticipated methylene amine derivative [(1-NH₂CH₂-1,2-C₂B₉H₁₁)­(1′,2′-C₂B₉H₁₁)₂-3,3′-Co]⁻ (<b>6</b>^<b>–</b>) was isolated in low yield after cleavage of the phthalimide intermediate species. The molecular structures of all isolated cyclic products <b>2</b>^<b>–</b> to <b>5</b>^<b>–</b> were confirmed by single-crystal X-ray diffraction studies, and the structure of cobalt bis­(dicarbollide)-1-CH₂NH₂ <b>6</b>^<b>–</b> was delineated using density functional theory applied at BP86/AE1 level in combination with NMR spectroscopy. Thus, the synthetic method described herein presents a facile route to new cobalt bis­(dicarbollide) derivatives substituted by polycyclic structural motifs with potential biological activity

Prototropic μ‑H^8,9 and μ‑H^9,10 Tautomers Derived from the [<i>nido</i>-5,6‑C₂B₈H₁₁]⁻ Anion

Reported is an unusual tautomeric behavior within the [<i>nido</i>-5,6-C₂B₈H₁₁]⁻ (<b>1a^–</b>) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-<i>nido</i>-5,6-C₂B₈H₁₀-μ^8,9]⁻ (<b>2d^–</b>) and [5,6-Me₂-<i>nido</i>-5,6-C₂B₈H₉-μ^9,10]⁻ (<b>3b^–</b>), which differ in the positioning of the open-face hydrogen bridge. Their structures have been determined by X-ray diffraction analyses. The <b>3b^–</b>structure is stabilized by intermolecular interaction involving Et₃NH⁺ and B8–B9 and H8 atoms in the solid phase; however, its dissolution in CD₃CN causes instant conversion to the more stable [5,6-Me₂-<i>nido</i>-5,6-C₂B₈H₉-μ^8,9]⁻ (<b>2b^–</b>) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [<i>nido</i>-5,6-C₂B₈H₁₁-μ^8,9]⁻ (<b>2a^–</b>) tautomer is 3.9 kcal·mol^–1 more stable than the [<i>nido</i>-5,6-C₂B₈H₁₁-μ^9,10]⁻ (<b>3a^–</b>) counterpart and the μ^8,9 structure <b>2^–</b> is therefore the most stable tautomeric form in the solution, as was also demonstrated by multinuclear (¹H, ¹¹B, and ¹³C) NMR measurements on the whole series of C-substituted compounds

Prototropic μ‑H^8,9 and μ‑H^9,10 Tautomers Derived from the [<i>nido</i>-5,6‑C₂B₈H₁₁]⁻ Anion

Reported is an unusual tautomeric behavior within the [<i>nido</i>-5,6-C₂B₈H₁₁]⁻ (<b>1a^–</b>) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-<i>nido</i>-5,6-C₂B₈H₁₀-μ^8,9]⁻ (<b>2d^–</b>) and [5,6-Me₂-<i>nido</i>-5,6-C₂B₈H₉-μ^9,10]⁻ (<b>3b^–</b>), which differ in the positioning of the open-face hydrogen bridge. Their structures have been determined by X-ray diffraction analyses. The <b>3b^–</b>structure is stabilized by intermolecular interaction involving Et₃NH⁺ and B8–B9 and H8 atoms in the solid phase; however, its dissolution in CD₃CN causes instant conversion to the more stable [5,6-Me₂-<i>nido</i>-5,6-C₂B₈H₉-μ^8,9]⁻ (<b>2b^–</b>) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [<i>nido</i>-5,6-C₂B₈H₁₁-μ^8,9]⁻ (<b>2a^–</b>) tautomer is 3.9 kcal·mol^–1 more stable than the [<i>nido</i>-5,6-C₂B₈H₁₁-μ^9,10]⁻ (<b>3a^–</b>) counterpart and the μ^8,9 structure <b>2^–</b> is therefore the most stable tautomeric form in the solution, as was also demonstrated by multinuclear (¹H, ¹¹B, and ¹³C) NMR measurements on the whole series of C-substituted compounds

Structures of, and Related Consequences of Deprotonation on, Two <i>C</i>_<i>s</i>‑Symmetric <i>Arachno</i> Nine-Vertex Heteroboranes, 4,6‑X₂B₇H₉ (X = CH₂; S) Studied by Gas Electron Diffraction/Quantum Chemical Calculations and GIAO/NMR

Gas-phase structure determinations have been performed for <i>arachno</i>-4,6-(CH₂)₂B₇H₉ and <i>arachno</i>-4,6-S₂B₇H₉ by combining quantum-chemical calculations and gas electron diffraction (GED) data. In addition, the monoanion derivatives of each of the aforementioned species have been studied using ab initio calculations. In all cases, comparison with experimental ¹¹B NMR chemical shifts have been achieved by calculating the appropriate NMR chemical shifts using GIAO-MP2 methods and the IGLO-II basis set for various geometries, both experimental and calculated. The NMR parameters calculated for the geometry obtained from the SARACEN GED refinement appeared to be quite reasonable, and in general, the fit between theoretical and experimental δ­(¹¹B) NMR was found to be consistently good for all four species investigated

Click Dehydrogenation of Carbon-Substituted <i>nido</i>-5,6‑C₂B₈H₁₂ Carboranes: A General Route to <i>closo</i>-1,2‑C₂B₈H₁₀ Derivatives

Triethylamine-catalyzed dehydrogenation of carbon-disubstituted dicarbaboranes 5,6-R₂-<i>nido</i>-5,6-C₂B₈H₁₀ [<b>1</b>, where R = H (<b>1a</b>), Me (<b>1b</b>), and Ph (<b>1c</b>)] in refluxing acetonitrile leads to a high-yield (up to 85–95%) formation of a series of dicarbaboranes 1,2-R₂-<i>closo</i>-1,2-C₂B₈H₈ (<b>2</b>). The monosubstituted 6-R-<i>nido</i>-5,6-C₂B₈H₁₁ (<b>3</b>) analogues [where R = Ph (<b>3a</b>), naph (1-naphthyl; <b>3b</b>), Bu (<b>3c</b>)] afforded 1-R-1,2-<i>closo</i> C₂B₈H₉ (<b>4</b>) isomers [where R = Ph (<b>4a</b>), naph (<b>4b</b>), <i>n</i>-Bu (<b>4c</b>)] as the main products; compounds <b>4a</b> and <b>4c</b> were accompanied by 2-R-1,2-C₂B₈H₉ (<b>5</b>) isomers (total yields up to 90%), with the <b>4</b>/<b>5</b> molar ratio being strongly dependent on the nature of R (4:1 and 1:1, respectively). All of these cage-closure reactions are supposed to proceed via the stage of the corresponding Et₃NH⁺ salts of <i>nido</i> anions [5,6-R₂-5,6-C₂B₈H₉]⁻ (<b>1</b>^–) and [6-R-5,6-C₂B₈H₁₀]⁻ (<b>3</b>^–), which lose H₂ and Et₃N upon heating (dehydrodeamination). The cage-closure mechanisms leading to <i>closo</i> isomers <b>2</b>, <b>4</b>, and <b>5</b> have been substantiated by B3LYP/6-31+G* calculations of the reaction profile for a simple <b>1a</b>^– → <b>2a</b> + H^– conversion. All of the compounds isolated have been characterized by multinuclear (¹¹B, ¹H, and ¹³C) NMR spectroscopy, mass spectrometry, and elemental analyses, and the structure of 1-Ph-<i>closo</i>-1,2-C₂B₈H₉ (<b>4a</b>) was established by an X-ray diffraction study

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

The reaction between selected X-<i>nido</i>-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

Acidities of <i>closo</i>-1-COOH-1,7‑C₂B₁₀H₁₁ and Amino Acids Based on Icosahedral Carbaboranes

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol^–1, computed GA = 1321 kJ·mol^–1) and liquid-phase acidity (measured p<i>K</i>_a = 2.00, computed p<i>K</i>_a = 1.88) of the carborane acid <i>closo</i>-1-COOH-1,7-C₂B₁₀H₁₁. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid–base context, to corresponding <i>ortho</i>, <i>meta</i>, and <i>para</i> amino acids derived from icosahedral carborane cages, 1-COOH-<i>n</i>-NH₂-1, <i>n</i>-R with {R = C₂B₁₀H₁₀, <i>n</i> = 2, 7, 12}, and from benzene {R = C₆H₄, <i>n</i> = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids