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An Alpha Shift Correlation (ASC) method is used to assess quantitatively NMR shielding effects in a series of halogen derivatives of closo-borane dianions, [Xm-BnHn−m]2−. There is a simple linear correlation between the universal αref shifts and those induced by the substituent X in all positions of the closo borane cluster: Δδ(11B) = k·αref (where Δδ = δs−δp and subscripts s and p denote the substituted and parent derivatives, respectively; k = correlation slope). The sensitivity of the chemical shift in a given cage position to X-changes is defined by the g factor (g = gradient = k× 102). The ASC approach revealed that the α-deshielding effect in the monosubstituted anionic [X-BnHn−1]2− series decreases in the order of n: 6 ＞ 12 ＞ 10 ＞ 9, and is dependent on the cage type and electron density at the substituted vertex. The sensitivity (g) in individual cluster positions to substituent changes decreases in the order α>A ＞ β ＞ γ and is of approximately additive character. The most important feature of the unsubstituted sphere is the antipodal (A) effect showing a g-trend inversed to that observed at the substituted α-sites. Of interest are also the remarkable pseudo-A and trans-γ shielding effects in the 9- and 10-vertex anionic series. [ABSTRACT FROM AUTHOR]

Developed was a systematic method for the incorporation of the ((η6-arene)Fe) fragment into a carborane cage. Reactions between [(η6-arene)2Fe](PF6)2, salts and Tl2[nido-7,8-C2B9H11] in refluxing (CH2)2C12 generated a series of [3-(η6-arene)-closo-3,1,2-FeC2B9H11] neutral complexes with variable types of polymethylated arene ligands in the structure. The structures of durene and hexamethylbenzene complexes were established by X-ray diffraction analyses. The work also shows that the original hi%htemperature method for the preparation of the [(η6-arene)2Fe]2+ dications fails for mesitylene and durene compounds, which was overcome by a reliable room-temperature modification. [ABSTRACT FROM AUTHOR]

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic closo -C 2 B 8 H 10 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-C 2 B 8 H 10 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-C 2 B 8 H 10 , closo -1-CB 9 H 10 - -B closo -B 10 H 10 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.

The paper reports specific syntheses of methylated decaborane(14), nido -B 10 H 14 ( 1 ), derivatives. The reaction of 1 with an excess of neat MeI and AlCl 3 yields 1,2,3,4-Me 4 - nido -B 10 H 10 ( 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-Me 7 - nido -B 10 H 6 ( 3a ). The formation of analogous 1-CF 3 SO 2 O-2,3,4,5,7,8,10-Me 7 - nido -B 10 H 6 ( 3b ) is achieved by heating 1 or 2 with an excess of MeSO 3 CF 3 in the presence of a catalytic amount of HOSO 2 CF 3 to 120 °C. Compounds 2 and 3 can be deprotonated to yield the corresponding anions [1,2,3,4-Me 4 - nido -B 10 H 9 ] - ( 2 - ), [1-I-2,3,4,5,7,8,10-Me 7 -B nido -B 10 H 5 ] - ( 3a - ), and [1-CF 3 SO 2 -B 7 - nido -B 10 H 5 ] - B and 3b - ). The structure of all the compounds isolated has been unambiguously confirmed by multinuclear ( 11 B and 1 H) 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.

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H 2 C 2 B 8 Me 8 have been prepared in high yields either from nido-5,6-H 2 C 2 B 8 H 10 or closo-1,2-H 2 C 2 B 8 H 8 via electrophilic methylation reactions and cluster-rearrangement methods. Prepared were also monosubstituted derivatives of general formulation closo-H 2 C 2 B 8 Me 7 -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 C 2 B 8 carborane scaffolding. The structures of all compounds isolated were confirmed unambiguously by multinuclear ( 11 B, 1 H, 13 C, and 19 F) NMR measurements, supported by X-ray diffraction analyses and geometry optimization methods on several compounds.

Electrophilic methylation of the closo -1,10-R 2 C 2 B 8 H 8 (1) (R = H or Me) dicarbaboranes at higher temperatures or thermal rearrangement of the 1,6-R 2 C 2 B 8 Me 8 (3) compounds at 400-500 °C generated the B-permethylated derivatives closo -1,10-R 2 C 2 B 8 Me 8 (2) in quantitative (>95%) yields. The compounds exhibit extreme air stability as a consequence of a rigid, egg shaped hydrocarbon structures incorporating inner 1,10-C 2 B 8 carborane core., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Reported are procedures leading to the first types of methyl camouflaged dicarbadecaboranes with fewer than eleven vertices. The compounds contain the closo-1,6-C2B8 scaffolding inside the egg-shaped hepta - decamethyl sheath, which imparts unusually high air and solvolytic stability to all of these compounds.

Alkylation of the [nido-5,6-R 1 2 B 2 B 8 H 9 ] - anions (where R 1 = H and Me) with alkyl halides (RX, where R = primary and secondary alkyls) in boiling tetrahydrofuran (THF) proceeds via unusual H 2 elimination, followed by cage closure to give a series of the neutral closo-1,2-R 1 B 2 C 2 B 8 B 7 -3-R derivatives in ∼70-80% yields. In contrast, treatment of the unsubstituted [nido-5,6-C 2 B 8 H 11 B - anion with tert-butyl bromide (t-BuBr) led to the formation of the parent closo-1,2-C 2 B 8 B, 10 in >85% yield. The constitution of all compounds isolated has been confirmed unambiguously by multinuclear ( 11 B, 1 H, and 13 C) nuclear magnetic resonance measurements and α-shift correlation assessments.

The principles of a new α-shift correlation (ASC) NMR method are demonstrated on a model series of substituted derivatives of o-carborane for which reliable NMR data are available. This graphical method revealed an acceptable linear correlation between α( 11 B) or α( 13 C) shifts and those induced by substituents in unsubstituted (u) positions of the carborane cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N) u = g × α (where N = 11 B, 13 C, and 1 H). The factor g (slope of the correlation line × 10 2 ) becomes an important measure of sensitivity of a given cage position to substituent changes. The β, γ, and δ = A (= antipodal) shifts can be therefore derived from the α-shift, are linearly proportional, and reflect additive character in double substitution. The ASC method appears to be an important tool for quantitative assessment of substituent NMR effects in all exo-substituted boron-cluster systems.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C 2 B 8 H 10 (1) have been carried out to reveal that the substitution takes place at B7 and B10 vertexes, which are the most removed from the CH positions. The course of the halogenation is strongly dependent on the nature of the halogenation agent and reaction conditions. Individual reactions led to the isolation of the monosubstituted compounds 1,2-C 2 B 8 H 9 -10-X (2) (where X = F, I) and 1,2-C 2 B 8 H 9 -7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C 2 B 8 H 8 -7,10-X 2 (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR ( 11 B, 1 H, 13 C) spectroscopy combined with two-dimensional [ 11 B- 11 B]-COSY, 1 H{ 11 B(selective)}, and [ 11 B- 1 H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal 13 C and 1 H NMR shielding at the CH1 vertex, increasing in the order H < I < Br < Cl < F. The structures of 1,2-C 2 B 8 H 8 -7,10-X 2 derivatives (where X = Cl, I, 4b,d) were established by X-ray diffraction analyses.

The reaction between selected X-nido-5,6-C 2 B 8 H 11 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-C 2 B 8 H 11 compounds with one equivalent of PS in hexane or CH 2 Cl 2 at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH + [10-X-5,6-C 2 B 8 H 10 ] - salts. Reprotonation using concentrated H 2 SO 4 in CH 2 Cl 2 generates a series of neutral carbaboranes 10-X-5,6-C 2 B 8 H 11 , 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-C 2 B 8 H 11 gave ∼66% conversion to 3-Cl-5,6-C 2 B 8 H 11 . 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 11 B NMR chemical shifts are well reproduced by the computations; as expected δ( 11 B) for B(10) atoms in derivatives with X = Br and I are heavily affected by spin-orbit coupling.

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

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