Intramolecular Pericyclic Re-hydroboration via Six-membered Transition State: Synthesis of Fused Borolo[3,2-b]siloles.

We experimentally demonstrate a new type of the intramolecular reaction between non-activated alkyne units and the dialkylboryl group (9-BBN), which was previously only hypothesized and studied on a calculational level. The reaction described here can formally be classified as a 1,2-hydroboration reaction, but, in contrast to the classical mechanism via a cyclic four-membered transition state, the reaction proceeds by a pericyclic mechanism involving a six-membered transition state. In practice, the reaction allows the synthesis of a new class of the borolenes fused with silole or dihydrosilole units. Thus, the heating (125°C) of 2,3-dihydrosiloles or 2,3-dihydrosilolo[2,3-b]silole bearing 9-BBN and propargyl or dimethylpropargyl groups in positions 4 and 5, respectively, affords, in high to moderate yields, di- and tricyclic compounds containing a borolo[3,2-b]silole unit. In all cases, the reactions are accompanied by an unprecedented dehydroboration of the 9-BBN fragment, leading, in the final products, to a cyclooct-4-enyl group on the boron atom. The effects of the substituents on the rate and the selectivity of the reaction are discussed. The structures of all the new compounds have been determined from HRMS together with multinuclear 1D and 2D NMR spectroscopy, the results of which are in excellent agreement with DFT calculated chemical shifts.

Experimental and Computational 77Se NMR Spectroscopic Study on Selenaborane Cluster Compounds.

Calculated and measured 77Se nuclear magnetic resonance (NMR) chemical shift data on a diverse collection of 13 selenaborane cluster compounds, containing a total of 19 selenium centers, reveals a correlation between chemical shifts and the intracluster coordination of selenium atoms within their borane frameworks. A plot of the measured against calculated 77Se NMR chemical shifts shows an approximately linear relationship that can serve as a predictive tool in assessing the chemical shift range in which a selenium vertex from a particular compound might be expected to be found, thereby reducing expensive experimental time. Furthermore, the relative chemical shifts between selenium vertices in clusters containing more than one selenium atom are consistent across the range, thus allowing the assignment of the selenium resonances with a high degree of confidence even in relatively low-level density functional theory calculations. A new macropolyhedral 20-vertex selenaborane Se2B18H20 (A) is also reported.

Expanding Luminescence Horizons in Macropolyhedral Heteroboranes.

Luminescence is observed in three novel macropolyhedral nineteen- and eighteen-vertex chalcogenaboranes: Se2B17H17 (1), SeB17H19 (3) and SeB18H20 (4). This led us to the recognition that previously published macropolyhedral heteroborane species might also exhibit luminescence. Thus, the known nineteen- and eighteen-vertex dithiaboranes S2B17H17 (2), n-S2B16H16 (5) and i-S2B16H16 (6) were synthesised and also found to exhibit a range of luminescent properties. These macropolyhedral species are very different from the previously unique fluorescent binary borane B18H22 in terms of their structural architectures, by the presence of borane cluster hetero atoms, and, as in the cases of 5 and 6, that their synthetic origins are not derived simply through the modification of B18H22 itself. They consequently greatly expand the possibilities of finding new luminescent inorganic borane species.

Simple Route to [PSH][B9H14] and a Contemporary Study of Its Solid-State Dynamic Behavior.

The 1,8-bis(dimethylamino)naphthalenium ([PSH]+) decaborane salt, [PSH][B10H13], has been found to react in ethanol to form [PSH][B9H14] (1), affording a simple route to the synthesis of the arachno-nonaborate anion. This new polyhedral salt is characterized by NMR spectroscopy and X-ray diffraction. The measurement of diffusion coefficients by NMR methods demonstrates that the [PSH]+ cation and the [B9H14]- anion form ion pairs in a non-coordinating solvent such as CH2Cl2, whereas in CD3CN the formation of ion pairs was not observed. Insights into the long-known low-energy dynamic behavior, which involves the bridging and endo-terminal hydrogen atoms, are elucidated using DFT calculations. Salts [PSH][B9H14] (1) and [PSH][B9H14]·0.5CHCl3 (solvated, 1·0.5CHCl3) have also been studied by X-ray diffraction analysis. A solid-state NMR study has demonstrated that K[B9H14] and [PSH][B9H14] (1) undergo significantly different motion regimes, being a low-energy, weakly temperature-dependent process for 1, which may be ascribed to some type of low-amplitude reorientation of the whole boron cages. This process may be the mechanism for the low- to-room-temperature order-disorder hidden transition found by X-ray analysis.

One-Pot Synthesis of 2,5-Dihydrosiloles and Their Silole-Annulated Analogs Starting from Alkynylsilanes with a Terminal Alkynyl Group.

In contrast to the reaction of vinyl(alkynyl)silanes with 9-BBN-H, leading to the quantitative formation of 5-R-4-(9-BBN)-2,3-dihydro-1H-siloles, treatment of bis(alkynyl)silanes bearing one terminal alkynyl group with 2 equiv of 9-BBN-H followed by methanolysis afforded 5-R-4-(9-BBN)-2,5-dihydro-1H-siloles with yields of 85-90% (by NMR integration). The reaction proceeds via a double 1,2-hydroboration of the terminal triple bond with the formation of the geminal diborane followed by ring closure via intramolecular 1,1-carboboration of the remaining alkynyl fragment. Depending on the nature of the substituent R in position 5, the allylic BBN group locates in position 3 (R = Ph) or position 5 (R = SiHMe2, SiMe3) to give 2,3- or 2,5-dihydrosiloles, respectively. The protodeborylation of the allylic BBN group with MeOH of both 3,4-(9-BBN)2-2,3-dihydro- and 4,5-(9-BBN)2-2,5-dihydrosiloles results in the exclusive formation of 4-(9-BBN)-2,5-dihydrosiloles. In all cases, the formation of 10-12% of 2-R-2,4-(9-BBN)2-2,3-dihydrosilole minor isomers has been observed, which occurs from vicinal diboranes formed as side products by a second hydroboration of the terminal triple bond. Similarly, treatment of the tri- and tetraalkynes containing a terminal triple bond with 2 equiv of 9-BBN-H followed by treatment with methanol resulted in the high-yield formation of 1,2,6,6a-tetrahydro-1,6-disilapentalenes and 2,6,7,7a-tetrahydro-1,6,7-trisila-1H-cyclopenta[a]pentalenes, respectively.

Ligand Lability Driven by Metal-to-Borane Pseudorotation: A Mechanism for Ligand Exchange.

The discovery of systems that interact with small molecules plays a vital facilitating role in the development of devices that show sensitivity to their surroundings and an ability to quickly relay chemical and physical information. Herein, we report on the reaction of [NiCl2(dppe)] with decaborane that produces in usable yield a new 11-vertex nickelaborane, [7,7-(dppe)-nido-7-NiB10H12] (1), which shows interesting reactivity and functionality toward carbon monoxide and ethylisonitrile. This contribution describes the synthesis and full structural characterization of 1 and its small-molecule EtNC and CO adducts, 2 and 3, and delineates the dynamic molecular behavior of all of these species in solution. This information sets a foundation from which more advanced work on this and related metallaborane systems can be conceived and provides a more general reference to how NMR spectroscopy, combined with DFT calculations, can be used to analyze the precise locomotion of labile ligands around a metal center held within a borane cluster.

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.

Helicenes Built from Silacyclopentadienes via Ring-by-Ring Knitting of the Helical Framework.

Despite the apparent diversity of the protocols developed for the synthesis of helicenes, they essentially follow the same strategy: the closure of one, or several, internal rings in a key step. Herein, we report the synthesis of a new family of the heterohelicenes consisting of fused silacyclopentadiene rings formed via a facile and novel process. The treatment of oligo(alkynilydenesilylene) precursors of type H2 C=CH-(SiMe2 -C≡C)n -R (n=3-7), bearing a vinyl group on the terminal silicon atom, with 9-borabicyclononane leads first to 1,2-hydroboration of the terminal double bond which then continues with a cascade of intramolecular 1,1-carboboration reactions accompanied with the closure of a new silole ring after each step affording the target silahelicenes with, currently, up to seven condensed silole rings and with excellent yields. According XRD analysis, the seven fused silole rings of the heptacyclic compound 11 b form an almost complete turn of a helix. The presented one-pot sequence of reactions is the first example of ring-by-ring knitting of a helical framework starting from easily available linear precursors.

Methyl camouflage in the ten-vertex closo-dicarbaborane(10) series. Isolation of closo-1,6-R2C2B8Me8 (R = H and Me) and their monosubstituted analogues.

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.

Unusual Closure of the Ten-Vertex Nido Cage via Alkylation: Regiospecific Synthesis of 3-Alkyl Derivatives of closo-1,2-C2B8H10.

Alkylation of the [nido-5,6-R12C2B8H9]- anions (where R1 = H and Me) with alkyl halides (RX, where R = primary and secondary alkyls) in boiling tetrahydrofuran (THF) proceeds via unusual H2 elimination, followed by cage closure to give a series of the neutral closo-1,2-R12C2B8H7-3-R derivatives in ∼70-80% yields. In contrast, treatment of the unsubstituted [nido-5,6-C2B8H11]- anion with tert-butyl bromide (t-BuBr) led to the formation of the parent closo-1,2-C2B8H10 in >85% yield. The constitution of all compounds isolated has been confirmed unambiguously by multinuclear (11B, 1H, and 13C) nuclear magnetic resonance measurements and α-shift correlation assessments.

Quantitative syntheses of permethylated closo-1,10-R2C2B8Me8 (R = H, Me) carboranes. Egg-shaped hydrocarbons on the Frontier between inorganic and organic chemistry.

Electrophilic methylation of the closo-1,10-R2C2B8H8 (1) (R = H or Me) dicarbaboranes at higher temperatures or thermal rearrangement of the 1,6-R2C2B8Me8 (3) compounds at 400-500 °C generated the B-permethylated derivatives closo-1,10-R2C2B8Me8 (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-C2B8 carborane core.

Quantitative Assessment of Substitution NMR Effects in the Model Series of o-Carborane Derivatives: α-Shift Correlation Method.

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(11B) or α(13C) 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 = 11B, 13C, and 1H). The factor g (slope of the correlation line × 102) becomes an important measure of sensitivity of a given cage position to substituent changes. The ß, 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.

Electrophilic Halogenation of closo-1,2-C2B8H10.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C2B8H10 (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-C2B8H9-10-X (2) (where X = F, I) and 1,2-C2B8H9-7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C2B8H8-7,10-X2 (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR (11B, 1H,13C) spectroscopy combined with two-dimensional [11B-11B]-COSY, 1H{11B(selective)}, and [11B-1H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal 13C and 1H NMR shielding at the CH1 vertex, increasing in the order H < I < Br < Cl < F. The structures of 1,2-C2B8H8-7,10-X2 derivatives (where X = Cl, I, 4b,d) were established by X-ray diffraction analyses.

Prototropic µ-H8,9 and µ-H9,10 Tautomers Derived from the [nido-5,6-C2B8H11]- Anion.

Reported is an unusual tautomeric behavior within the [nido-5,6-C2B8H11]- (1a-) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido-5,6-C2B8H10-µ8,9]- (2d-) and [5,6-Me2-nido-5,6-C2B8H9-µ9,10]- (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 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-nido-5,6-C2B8H9-µ8,9]- (2b-) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [nido-5,6-C2B8H11-µ8,9]- (2a-) tautomer is 3.9 kcal·mol-1 more stable than the [nido-5,6-C2B8H11-µ9,10]- (3a-) counterpart and the µ8,9 structure 2- 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.

Click Dehydrogenation of Carbon-Substituted nido-5,6-C2B8H12 Carboranes: A General Route to closo-1,2-C2B8H10 Derivatives.

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.

Sequential Camouflage of the arachno-6,9-C2B8H14 Cage by Substituents.

Sequential methylation of arachno-6,9-C2B8H14 (1) led to a series of methyl derivatives and finally to the camouflaging of all boron positions by mixed persubstitution. Thus, deprotonation of 1 produced the [arachno-6,9-C2B8H13] anion (1(-)), the methylation of which with MeI in tetrahydrofuran proceeded on the open-face boron vertexes with the formation of 5-Me-arachno-6,9-C2B8H13 (2; yield 28%) and 5,8-Me2-arachno-6,9-C2B8H12 (3; yield 36%). Observed in this reaction was also a side formation of 2-Me-closo-1,6-C2B8H9 (4; yield 6%).The electrophilic AlCl3-catalyzed CH3(+) attack of the neutral 1 in neat MeI at ambient temperature afforded 1,3-Me2-arachno-6,9-C2B8H12 (5), while a 76-h heating at 120 °C generated a mixture of the di- and triiodo derivatives 1,2,3,4,8,10-Me6-5,7-I2-arachno-6,9-C2B8H6 (6) and 1,2,3,4,7-Me5-5,7,10-I3-arachno-6,9-C2B8H6 (7). On the other hand, a HOTf-catalyzed reaction between 1 and MeOTf at reflux resulted in the isolation of 2-TfO-1,3.4,5,7,8,10-Me7-arachno-6,9-C2B8H6 (8; Tf = CF3SO2; yield 65%). The compounds were characterized by multinuclear ((11)B, (1)H, (13)C, and (19)F) NMR spectroscopy, mass spectrometry, and elemental analysis, and the structures of compounds 1, 1(-), 5, and 6 were established by X-ray diffraction analysis.

57Fe NMR spectroscopy of ferrocenes derived from aminoferrocene and 1,1'-diaminoferrocene.

Three series of ferrocenes, derived from aminoferrocene Fc-NH2 and 1,1'-diaminoferrocene fc(NH2)2, were studied by 57Fe NMR spectroscopy. A marked decrease in 57Fe magnetic nuclear shielding with respect to ferrocene is observed if the nitrogen atom becomes part of a pi-acceptor linked to one or both cyclopentadienyl rings. In contrast, pi-donor properties of the amino group(s) affect delta57Fe to a much smaller extent. In the case of the fairly rigid structures of 1,3-diaza-2-element-[3]ferrocenophanes, a significant increase of 57Fe nuclear magnetic shielding is observed, in contrast to the corresponding [n]ferrocenophanes with n > 3. Structures of numerous of the ferrocene derivatives have been optimized for the gas phase by calculations (B3LYP/6-311 + G(d,p) level of theory), and 57Fe nuclear magnetic shieldings were calculated using these geometries. There is reasonable agreement in the trends for experimental and calculated data.

Stable exo-nido-metallacarboranes (M = Os(IV)) that incorporate meta-carborane-based dicarbollide ligands.

Reactions of the [K]+ salts of the [nido-7,9-C2B9H12]- anion (2) and its C-phenylated derivative [7-Ph-nido-7,9-C2B9H11]- (4) with [OsCl2(PPh3)3] (3) proceed in benzene at ambient temperature with the formation of 16-electron chlorohydrido-Os(IV) exo-nido complexes, [exo-nido-10,11-{(Ph3P)2OsHCl}-10,11-(mu-H)2-7-R-7,9-C2B9H8] (5: R = H; 6: R = Ph), along with the small amounts of the charge-compensated nido-carboranes [nido-7,9-C2B9H11PPh3] (7) and [7-Ph-nido-7,9-C2B9H10PPh3] (8) as byproducts. However, when carried out under mild heating in ethanol, the reaction of 2 with 3 selectively afforded a 16-electron dihydrido-Os(IV) exo-nido complex [exo-nido-10,11-{(Ph3P)2OsH2}-10,11-(mu-H)2-7,9-C2B9H9] (9). Structures of both complexes 5 and 9 have been confirmed by single-crystal X-ray diffraction studies, which revealed that nido-carboranes in these species function as a bidentate dicarbollide ligands [7-R-nido-7,9-C2B9H10]2- linked to the Os(IV) center via two B-H...Os bonds involving adjacent B-H vertices in the upper CBCBB belt of the carborane cage. Thus, compounds 5 and 9 represent the first structurally characterized exo-nido-metallacarboranes based on meta-dicarbollide-type ligands. Variable-temperature 1H and 31P{1H} NMR experiments indicate that complex 9 is fluxional in solution and shows an unusual exchange between terminal Os-(H)2 and bridging {B-H}2...Os hydrogen atoms. Upon heating in d8-THF at 65 degrees C, complex 9 converts irreversibly to its closo isomer [2,2-(PPh3)2-2,2-H2-closo-2,1,7-OsC2B9H11] (13), which could thus be obtained as a pure crystalline solid. The structure of 13 has been established on the basis of analytical and multinuclear NMR data and a single-crystal X-ray diffraction study.

Indirect nuclear 57Fe-13C and 57Fe-1H spin-spin coupling in ferrocenes and cyclopentadienyliron complexes: measurements and DFT calculations.

Several 57Fe-labeled ferrocene derivatives and other cyclopentadienyliron complexes were studied by 57Fe and 13C NMR with respect to isotope-induced chemical shifts 1Delta12/13C(57Fe) and the magnitude and sign of coupling constants 1J(57Fe,13C) and 2J(57Fe,1H). The geometries of the complexes were optimized by DFT methods [B3LYP/6-311+G(d,p)] and chemical shifts (GIAO) and coupling constants were calculated at the same level of theory. The trends in calculated 57Fe nuclear shielding agree well with the experimental data and, in the case of coupling constants 1J(57Fe,13C) and 2J(57Fe,1H), both sign and magnitude are correctly reproduced.