- Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes
-
o-C2B10H12 isomerizes to m-C2B10H12 upon heating at 400 °C. Deboronation in o-C2B10H12 is a relatively easy process, whereas it is more difficult in m-C2B10H12. These two experimental facts indicate that m-C2B10H12 is thermodynamically more stable than o-C2B10H12. On the other hand, it is widely accepted that closo-boranes and -carboranes are aromatic compounds. In this work, we relate the difficulty in the deboronation of the carboranes with their stability and aromaticity. We do this by combining lab work and DFT calculations. Computationally, our results show that the higher thermodynamic stability of m-C2B10H12 is not related to aromaticity differences but to the location of the C atoms in the carborane structure. It is also demonstrated that the aromaticity observed in closo-boranes and -carboranes is also present in their nido counterparts, and consequently, we conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g., ferrocene) and sandwich metallabis(dicarbollides) (e.g., [Co(C2B9H11)2]-) have traditionally been considered to be similar. Here it is shown that they are not. Metallabis(dicarbollides) display global aromaticity, whereas metallocenes present local aromaticity in the ligands. Remarkable and unique is the double probe given by 1H and 11B NMR tracing the reciprocally antipodal endocyclic open face Hec and B1. These magnetic studies have permitted one to correlate both nuclei and relate them to a diatropic current in the plane at the middle of the nido-[C2B9H12]-. This observation is the first unique evidence that proves experimentally the existence of diatropic currents, and thence aromaticity, in clusters and is comparable to the existence of diatropic currents in planar aromatic compounds. Additionally, heteroboranes with two carbon atoms have been compared to heterocycles with two nitrogen or boron atoms, e.g., C2B10H12 carboranes versus planar N2C4H4 diazines or [B2C4H4]2- diboratabenzenes, thereby proving the higher persistence of the aromaticity of the tridimensional compounds in heteroatom-substituted species. This research accounts very well for the paradigm for the electron requirements of clusters , in which a closo-cluster that is aromatic upon addition of 2e- becomes also an aromatic nido-species, and explains the informative schemes by Rudolph and Williams.
- Bennour, Ines,Escayola, Sílvia,Poater, Jordi,Solà, Miquel,Teixidor, Francesc,Vi?as, Clara
-
-
Read Online
- Photoluminescence in Carborane–Stilbene Triads: A Structural, Spectroscopic, and Computational Study
-
A set of triads in which o- and m-carborane clusters are bonded to two stilbene units through Ccluster?CH2bonds was synthesized, and their structures were confirmed by X-ray diffraction. A study on the influence of the o- and m- isomers on the absorption and photoluminescence properties of the stilbene units in solution revealed no charge-transfer contributions in the lowest excited state, as confirmed by (TD)DFT calculations. The presence of one or two B?I groups in m-carborane derivatives does not affect the emission properties of the stilbenes in solution, probably due to the rather large distance between the iodo substituents and the fluorophore. Nevertheless, a significant redshift of the photoluminescence (PL) emission maximum in the solid state (thin films and powder samples) compared to solution was observed; this can be traced back to PL sensitization, most probably due to more densely packed stilbene moieties. Remarkably, the PL absolute quantum yields of powder samples are significantly higher than those in solution, and this was attributed to the restricted environment and the aforementioned sensitization. Thus, the bonding of the carborane clusters to two stilbene units preserves their PL behavior in solution, but produces significant changes in the solid state. Furthermore, iodinated species can be considered to be promising precursors for theranostic agents in which both imaging and therapeutic functions could possibly be combined.
- Cabrera-González, Justo,Vi?as, Clara,Haukka, Matti,Bhattacharyya, Santanu,Gierschner, Johannes,Nú?ez, Rosario
-
-
Read Online
- Thermal isomerizations of monothiolated carboranes (HS)C2B10H11 and the solid-state investigation of 9-(HS)-1,2-C2B10H11 and 9-(HS)-1,7-C2B10H11
-
At 300-500 °C, three C-thiolated closo-dicarbadodecaborane isomers 1-(HS)-1,2-C2B10H11 (1-o), 1-(HS)-1,7-C2B10H11 (1-m), and 1-(HS)-1,12-C2B10H11 (1-p), and two B-thiolated isomers 9-(HS)-1,7-C2B10H11 (9-m) and 9-(HS)-1,2-C2B10H11 (9-o) show two types of reaction: first, removal of an SH group from the closo-dicarbadodecaborane skeleton, and second, skeletal isomerizations from ortho to meta to para that lead to new isomers. A previously unreported SH skip from carbon-to-boron is also observed. The effect of the thiol group on the skeletal rearrangement is discussed. The isomerisation products are assigned on the basis of correlation of their computationally obtained dipole moments with their gas-chromatographic retention times. Computational results on molecular energies for the mono-thiolated species show good agreement between the calculated relative stabilities and the incidence and relative quantities of the isomerization products. Two of the starting B-thiolated isomers, 9-o and 9-m, were characterized using single-crystal X-ray diffraction analyses and their crystallographic packings as well as some selected structural parameters are discussed. All starting compounds were characterized using multinuclear NMR spectroscopy.
- Ba?e, Tomá?,Machá?ek, Jan,Hájková, Zuzana,Langecker, Jens,Kennedy, John D.,Carr, Michael J.
-
p. 132 - 140
(2015/12/18)
-
- Mechanistic features of boron-iodine bond activation of B-iodocarboranes
-
Oxidative addition of the B-I bond of 9-iodo-m-carborane to [(Ph3P)nPd] (n = 3, 4) is reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence of [(Ph3P)4Pd] and [Bu4N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydrido Pd(II) species generated upon thermal decomposition of [(Ph3P)2Pd(Ph)(O2CH)] and [(Ph3P)2Pd2(Ph)2 (μ-O2CH)2] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal decomposition of [(Ph3P)2Pd2(Ph)2 (μ-O2CH)2] in the presence of excess 9-iodo-m-carborane and PhI (1:1) results in the formation of m-carborane (3%) and [(Ph3P)2Pd2(Ph)2 (μ-I)2] (97%), whose structure was confirmed by single-crystal X-ray diffraction. X-ray analysis of 9-iodo-m-carborane and m-carboran-9-yl(phenyl)iodonium tetrafluoroborate shows that in the iodonium salt the B-I bond is longer by ca. 0.03 A? than in the iodocarborane. In contrast, the C-I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) A?) and in iodobenzene (2.098(4) A?) are only marginally different. The elongation of the B-I bond, not the C-I bond, likely contributes to (i) the enhanced reactivity of B-carboranyl(phenyl)-iodonium cations toward nucleophiles and (ii) the remarkably high selectivity of these SN reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10-diiodo-m-carborane is reported.
- Marshall, William J.,Young Jr., Robert J.,Grushin, Vladimir V.
-
p. 523 - 533
(2008/10/08)
-
- Suzuki Cross Coupling in the Carborane Series
-
Products of the Suzuki cross coupling of 9-iodo-m-carborane and 9-iodo-o-carborane with phenylboric acid and with dibutyl vinylboronic acid, catalyzed by various palladium compounds, were studied. It was found that the reaction with carboranes occurs in a much different way than with organohalogen compounds.
- Zakharkin,Balagurova,Lebedev
-
p. 922 - 924
(2007/10/03)
-
- Synthesis and structural studies of subicosahedral adjacent-carbon carboranes
-
Synthesis and structural studies, employing combined NMR, X-ray crystallographic, and ab initio/IGLO/NMR methods, of a variety of new subicosahedral carboranes with adjacent cage carbons are reported. Acetonitrile-induced cage degradation of arachno-4,5-C2B7H12- gave nido-4,5-C2B6H9-(1-) in nearly quantitative yield, which can then be protonated to give the neutral carborane nido-4,5-C2B6H10 (1) in good yield. Both of these nido electroncount clusters are shown to have an arachno-type geometry, i.e. a six-membered open face. The nido-4,5-C2B6H10 (1) hydroborated alkenes or alkynes which following deprotonation gave nido-7-R-4,5-C2B6H8- (2a--c-) ions. Both nido-4,5-C2B6H9- (1-) and nido-4,5-C2B6H10 (1) serve as useful precursors to other adjacent cage-carbon clusters. Thus, nido-4,5-C2B6H9- (1-) reacted with BH3·THF to give arachno-5,6-C2B7H12- (3-) which a single-crystal X-ray diffraction study showed is the first carborane to adopt the n-B9H15 cage geometry. Thermal or chemical degradation of nido-4,5-C2B6H10 (1) gave closo-2,3-C2B5H7 (5) in good to moderate yields. The nido-4,5-C2B6H9- (1-) was also found prone to lose a cage boron as evidenced by its reactions with (η-C5H5)Co(CO)12 and (η6-C6Me6)2Ru2Cl4 which gave closo-3,1,2-(η-C5H5)CoC2B5H7 (6) and closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7), respectively. NMR studies showed the nido-4,5-C2B6H10 (1) was converted to arachno-4,5-C2B6H11- by reaction with LiEt3BH, and an alkyl derivative, arachno-7-CH3-4,5-C2B6H10- (4-), was formed by reacting MeLi with nido-4,5-C2B6H9- (1-) followed by protonation. The closo-2,3-C2B5H7 (5) was also converted in high yields to the smaller nido carborane, nido-2,3-C2B4H8, via reaction with TMEDA/H2O, and to nido-3,4-C2B5H8- (8-) by reaction with LiEt3BH.
- Bausch, Joseph W.,Matoka, Derek J.,Carroll, Patrick J.,Sneddon, Larry G.
-
p. 11423 - 11433
(2007/10/03)
-
- Thermal isomerization of regiospecifically 10B-labeled icosahedral carboranes
-
Thermal treatment of regiospecifically 10B-enriched (96%) 3-(10B)-1-2-C2nB9H12 and 2-(10B)-1,-7-C2nB9H12 (nB = boron with normal isotopic abundances) followed by 10B and 11B NMR analysis reveal that (1) 3-(10B)-1,2-C2nB9H12 undergoes rearrangements that completely scramble the enriched boron atom, (2) these rearrangements occur at a rate faster than the conversion of 1,2-C2B10H12 to 1,7-C2B10H12, (3) the 1,7-C2B10H12 that is formed does not re-form 1,2-C2B10H12 at a detectable rate, and (4) at temperatures at which 1,2-C2B10H12 forms 1,7-C2B10H12 at a significant rate, the latter undergoes no further rearrangements. The movement of 10B at 350°C in 1,2-C2B10H12 is compared to the movement predicted by various isomerization mechanisms. The mechanism that appears to give the closest agreement involves a 12-vertex nido intermediate. It was found that a number of previously considered mechanisms, including simple exchange between two boron sites, triangular face rotation in an icosahedron, diamond-square-diamond twist, and rotation of pentagonal pyramids, are extensions of the nido intermediate mechanism. The synthesis and thermal rearrangement of 3-F-S-(10B)-1-2-C2nB9H11 demonstrate that independent intramolecular fluorine migration does not occur during rearrangement.
- Edvenson, Gary M.,Gaines, Donald F.
-
p. 1210 - 1216
(2008/10/08)
-
- ELECTROCHEMICAL SYMMETRIZATION OF CARBORANYLMERCURY SALTS
-
Preparative electrochemical symmetrization of C- and B-carboranylmercury salts was carried out.The mechanism of the electrochemical reaction is discussed.
- Usiatinsky, A. Ya.,Shreider, V. A.,Shcherbina, T. M.,Bregadze, V. I.,Godovikov, N. N.,Knuniants, I. L.
-
-
- o-Carboranylcarbene and m-Carboranylcarbene
-
Addition of o- and m-carboranylcarbene to olefins takes place in a largely stereospecific fashion.In the addition to cis olefins, it is the anti isomer that predominates.Insertion into carbon-hydrogen bonds occurs with a secondary/primary selectivity ratio of 3.0. (Methyl-o-carboranyl)carbene behaves like tolylcarbene in intramolecular reactions.It yields vinyl-o-carborane and the carborane analogue of benzocyclobutene. (Methyl-m-carboranyl)carbene does not give vinyl-m-carborane.
- Chari, Sarangan L.,Chiang, Sheau-Hwa,Jones, Maitland
-
p. 3138 - 3145
(2007/10/02)
-
- Direct synthesis of closo-carboranes
-
Pentaborane-9 and acetylene can react in a continuous-flow system to produce directly the smaller closo-carboranes 1,5-C2B3H5, 1,6-C2B4H6, and 2,4-C2B5H7 in combined yields approaching 65-70%. Reactions of decaborane-14 with acetylene under the same conditions, excepting a somewhat higher temperature, give predominantly 1,7-C2B10H12 in yields also on the order of 70%. Optimal conditions for both systems include short residence times (~0.5 sec), high temperatures (500-600°, depending on the system), a diluent gas, and a steel wool plug or diffuser. The reactions are highly exothermic, and drastic reduction of yields can occur if the heat is not dissipated, as was evident in a scaled-up unit where very careful diluent control had to be exercised to prevent sudden surges to very high temperatures.
- Ditter,Klusmann,Oakes,Williams, Robert E.
-
p. 889 - 892
(2008/10/08)
-
- The polyhedral B6C2H8, B7C2H9, and B8C2H10 carboranes and their C-monomethyl and C-monophenyl derivatives
-
The pyrolysis of 1,3-B7C2H13 at 215° in the absence of diborane yields the B6C2H8, B7C2H9, and 1,6-B8C2H10 carboranes, while in the presence of diborane the yield of 1,6-B8C2H10 is considerably enhanced. Isomerization of 1,6-B8C2H10 to the 1,10-B8C2H10 isomer occurs at 300°. Pyrolysis of 1,3-NaB7C2H12 at 200° yields 1,7-B7C2H9. The C-monomethyl and C-monophenyl derivatives of these carboranes are also described.
- Garrett, Philip M.,Smart, James C.,Ditta, Gary S.,Hawthorne, M. Frederick
-
p. 1907 - 1910
(2008/10/08)
-
- Neocarboranes, a new family of stable organoboranes isomeric with the carboranes
-
Heating of the recently described compound2 carborane, C2B10H12, at 465-500° for 24 hr. produced the first member of a new family of organoboranes. The name neocarborane has been assigned to this product, which is isomeric with the original carborane. The chemistry ot neocarborane has been explored and is compared to that of carborane. In general, neocarborane derivatives are less polar and more stable than their carborane isomers. As in thecarboranes, the C2B10H10 unit is unusually inert and many standard organic processes can be effected on organofunctional substituents attached to the neocarborane carbon atoms. Carborane is assigned the distorted icosahedral structure, A, with the two carbon atoms sharing a short, depressed edge of an otherwise regular icosahedron. A regular icosahedral structure, C or D, is suggested for neocarborane. Electronic structures and possible reaction paths are indicated.
- Grafstein, Daniel,Dvorak, Joseph
-
p. 1128 - 1133
(2008/10/08)
-