Borane Cage Research Articles

Dihydrogen Bonds in Salts of Boron Cluster Anions [BnHn]2− with Protonated Heterocyclic Organic Bases

Dihydrogen bonds attract much attention as unconventional hydrogen bonds between strong donors of H-bonding and polyhedral (car)borane cages with delocalized charge density. Salts of closo-borate anions [B10H10]2− and [B12H12]2− with protonated organic ligands 2,2’-dipyridylamine (BPA), 1,10-phenanthroline (Phen), and rhodamine 6G (Rh6G) were selectively synthesized to investigate N−H...H−B intermolecular bonding. It was found that the salts contain monoprotonated and/or diprotonated N-containing cations at different ratios. Protonation of the ligands can be implemented in an acidic medium or in water because of hydrolysis of metal cations resulting in the release of H3O+ cations into the reaction solution. Six novel compounds were characterized by X-ray diffraction and FT-IR spectroscopy. It was found that strong dihydrogen bonds manifest themselves in FT-IR spectra that allows one to use this technique even in the absence of crystallographic data.

Chalcogens act as inner and outer heteroatoms in borane cages with possible consequences for σ-hole interactions

Chalcogen bonds are a subset of σ-hole interactions and the number of their applications has been increasing rapidly. While chalcogen bonding in organic compounds has been intensively studied in the past decade, it is entirely new in boron cluster chemistry. In this paper, we review the remarkable ability of chalcogenated boron clusters to form strong chalcogen bonds. Chalcogen atoms have a unique property to be incorporated in a boron cluster both innerly and outerly. Various molecular shapes (e.g. icosahedral, bicapped-square antiprismatical) have been computationally analyzed and discussed. No general physical trend has been observed and, therefore, it is recommended to tackle each cluster individually.

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 closo-heteroboranes is performed. Chalcogens and pnicogens are incorporated in the borane cage, whereas halogens are considered as exo-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.

A computational analysis of the apparent nido vs. hypho conflict: are we dealing with six- or eight-vertex open-face diheteroboranes?

A series of computational studies have been undertaken to investigate the electronic structures and bonding schemes for six hetero-substituted borane cages, all of which have been presented in the literature as potential hypho structures. The six species are hypho-7,8-[C2B6H13](-) (1a), hypho-7,8-[CSB6H11](-) (1b), hypho-7,8-[S2B6H9](-) (1c), hypho-7,8-[NSB6H11] (1d), exo-7-Me-hypho-7,8-[NCB6H12] (1e), and endo-7-Me-hypho-7,8-[NCB6H12] (1f) and the so-called mno rule has been applied to each of them. As no structural data are known for the carbathia-, azathia-, and dithiahexaboranes, we have also applied the ab initio/GIAO/NMR structural tool for 1b-1d, with 1c having been prepared for this purpose. We conclude that an mno count of 10 means that 1a, 1b, 1d, 1e, and 1f should be termed pseudo-nido or pseudo-hypho. Only 1c can be considered to be correctly termed hypho-7,8-[S2B6H9](-).

Charge Transfer via the Dative N−B Bond and Dihydrogen Contacts. Experimental and Theoretical Electron Density Studies of Four Deltahedral Boranes

In an approach combining high resolution X-ray diffraction at low temperatures with density functional calculations, two closo-borates, B12H12(2-) (1) and B10H10(2-) (2), and two arachno-boranes, B10H12L2 (L = amine (3) or acetonitrile (4)), are studied by means of Atoms In Molecules (AIM) theory and Electron Localizability Indicator (ELI-D). The charge transfer via the dative N-B bonds in the arachno-boranes and via dihydrogen contacts in the closo-borates is quantified. The dative N-B bond in 4 is significantly shorter and stronger than that in 3 and in small N-B Lewis acid base adducts from the literature. It is even shorter in the gas phase than in the crystal environment in contrast to the bond shortening in the crystal generally found for N-B Lewis acid-base adducts. Furthermore, the calculated charge transfer in terms of AIM charges is opposite to the expected N → B direction but still weak as found for all other N-B bonds. The intramolecular charge redistributions due to intermolecular interactions are quantified by the AIM and ELI-D analysis of contact ion pairs. The latter method gives a deeper understanding of delocalization effects in the borane cages as well as in the counterions. Since dihydrogen bonds are rarely found in crystal structures, one focus was directed to the topologies of the large number of 58 experimentally found contacts of this type. The analysis reveals that the electron density at the bond critical point, the corresponding Laplace function, and the curvature along the bond path (λ3) show a behavior that clearly discriminates these interactions from classical hydrogen bonds, confirming earlier theoretical findings.

Real-Space Indicators for Chemical Bonding. Experimental and Theoretical Electron Density Studies of Four Deltahedral Boranes

In an approach combining high-resolution X-ray diffraction at low temperatures with density functional theory calculations, two closo-borates, B(12)H(12)(2-) (1) and B(10)H(10)(2-) (2), and two arachno-boranes, B(10)H(12)L(2) [L = amine (3) or acetonitrile (4)], were analyzed by means of the atoms-in-molecules (AIM) theory and electron localizability indicator (ELI-D). The two-electron three-center (2e3c) bonds of the borane cages are investigated with the focus on real-space indicators for chemical bonding and electron delocalization. In compound 2, only two of the three expected bond critical points (bcp's) are found. However, a weakly populated ELI-D basin is found for this pair of adjacent B atoms and the delocalization index and the Source contributions are on the same order of magnitude as those for the other pairs. The opposite situation is found in the arachno-boranes, where no ELI-D basins are found for two types of B-B pairs, which, in turn, exhibit a bcp. However, again the delocalization index is on the same order of magnitude for this bonding interaction. The results show that an unambiguous real-space criterion for chemical bonding is not given yet for this class of compounds. The arachno-boranes carry a special B-B bond, which is the edge of the crown-shaped molecule. This bond is very long and extremely curved inward the B-B-B ring. Nevertheless, the corresponding bond ellipticity is quite small and the ELI-D value at the attractor position of the disynaptic valence basin is remarkably larger than those for all other B-B valence basins. Furthermore, the value of the ED is large in relation to the B-B bond length, so that only this bond type does not follow a linear relationship of the ED value at the bcp versus B-B bond distances, which is found for all other B-B bcp's. The results indicate that both 2e2c and 2e3c bonding play a distinct role in borane chemistry.

Interactions of Boranes and Carboranes with Aromatic Systems: CCSD(T) Complete Basis Set Calculations and DFT-SAPT Analysis of Energy Components

The noncovalent interactions of heteroboranes with aromatic systems have only recently been acknowledged as a source of stabilization in supramolecular complexes. The physical basis of these interactions has been studied in several model complexes using advanced computational methods. The highly accurate CCSD(T)/complete basis set (CBS) value of the interaction energy for the model diborane···benzene complex in a stacking geometry exhibiting a B(2)H···π hydrogen bond was calculated to be -4.0 kcal·mol(-1). The DFT-SAPT/CBS approach, which is shown to reproduce the CCSD(T)/CBS data reliably asserted that the major stabilizing component was dispersion, followed by electrostatics. Furthermore, the effect of the benzene heteroatom- and exosubstitutions was studied and found to be small. Next, when aromatic molecules were changed to cyclic aliphatic ones, van der Waals complexes stabilized by the dispersion term only were formed. As the last step, interactions of two larger icosahedral borane cages with benzene were explored. The complex of the monoanionic CB(11)H(12)(-) exhibited two minima: the first stacked above the plane of the benzene ring with a C-H···π hydrogen bond and the second planar, in which the carborane cage bound to benzene via five B-H···H-C dihydrogen bonds. The DFT-SAPT/CBS calculations revealed that both of these binding motifs were stabilized by dispersion followed by electrostatic terms, with the planar complex being 1.4 kcal·mol(-1) more stable than the stacked one. The dianionic B(12)H(12)(2-) interacted with benzene only in the planar geometry, similarly as smaller anions do. The large stabilization energy of 11.0 kcal·mol(-1) was composed of dominant attractive dispersion and slightly smaller electrostatic and induction terms. In summary, the borane/carborane···aromatic interaction is varied both in the complex geometries and in the stabilizing energy components. The detailed insight derived from high-level quantum chemical computations can help us understand such important processes as host-guest complexation or carborane···biomolecule interactions.

Thiocyanation of closo-Dodecaborate B12H122−. A Novel Synthetic Route and Theoretical Elucidation of the Reaction Mechanism

Although vast experimental experience has been accumulated about the reactions of icosahedral B(12)H(12)(2-) borane cages, little is known about the mechanisms by which these reactions proceed. To address this issue, we have chosen the thiocyanation of B(12)H(12)(2-) and have studied this reaction using both experimental and theoretical methods. First, we present a novel and more convenient synthetic route using in situ generated thiocyanogen, (SCN)(2). The synthesized disubstituted product B(12)H(10)(SCN)(2)(2-) is exclusively the meta positional isomer, as confirmed by the X-ray crystallographic analysis. The quantum chemical calculations at the B3LYP/def2-TZVP//RI-PBE/def2-SVP level show that the free energy differences between the ortho, meta, and para disubstituted species are very small and as such cannot explain the observed positional preferences. The calculations of the kinetic aspects reveal that the reaction is best described as an electrophilic substitution. The calculated isomer preferences for the second SCN substituent are meta > para > ortho. The major outcome of this work is a clear and consistent picture of the electrophilic substitution reaction mechanism of the thiocyanation of B(12)H(12)(2-), thus contributing to our understanding of the general features of boron hydride reactivity.

Effects of halogen substitution on the properties of eight- and nine-vertex closo-boranes

The symmetry constrained geometries of the eight- and nine-vertex polyhedral boranes and haloboranes BnXnz (n = 8 and 9; X = H, F and Cl; z = -2, -1 and 0) were optimized at the B3LYP/6-311+G(d) level and their nucleus-independent chemical shifts (NICS) were calculated using the GIAO method with Kohn-Sham orbitals. Substitution of halogens on borane cages was found to significantly impact not only the geometric but also magnetic properties. Multiple fluorine substituents cause a deviation from the Wade-Mingos skeletal electron rules in B8F8(2-), resulting in a distortion from the expected D2d bisdisphenoid to a C2v nido type bicapped trigonal prism. However, all of the nine-vertex cages B9X9z retain the D3h tricapped trigonal prismatic structure of B9H9(2-). The presence of halogen substituents was found to enhance the three-dimensional diatropic ring currents within the dianionic borane cages B8X8(2-) and B9X9(2-). For the neutral structures the NICS values indicate BnFn to be aromatic, BnCln to be essentially non-aromatic, and BnHn to be antiaromatic (n = 8, 9).

Streptavidin in antibody pretargeting. 5. chemical modification of recombinant streptavidin for labeling with the alpha-particle-emitting radionuclides 213Bi and 211At.

We are investigating the use of recombinant streptavidin (rSAv) as a carrier molecule for the short-lived alpha-particle-emitting radionuclides 213Bi ( t 1/2 = 45.6 min) and 211At ( t 1/2 = 7.21 h) in cancer therapy. To utilize rSAv as a carrier, it must be modified in a manner that permits rapid chelation or bonding with these short-lived radionuclides and also modified in a manner that diminishes its natural propensity for localization in the kidney. Modification for labeling with (213)Bi was accomplished by conjugation of rSAv with the DTPA derivative p-isothiocyanato-benzyl-CHX-A'' (CHX-A''), 3a. Modification for direct labeling with 211At was accomplished by conjugation of rSAv with an isothiocyanatophenyl derivative of a nido-carborane (nCB), 3b, or an isothiocyanatophenyl-dPEG/decaborate(2-) derivative, 3c. After conjugation of the chelating or bonding moiety, rSAv was further modified by reaction with an excess (50-100 equivalents) of succinic anhydride. Succinylation of the lysine amines has previously been shown to greatly diminish kidney localization. rSAv modified by conjugation with 3a and succinylated rapidly radiolabeled with 213Bi (<5 min), providing a 72% isolated yield. 211At labeling of modified rSAv was accomplished in aqueous solution using chloramine-T as the oxidant. Astatination of rSAv conjugated with 3b and succinylated occurred very rapidly (<1 min), providing a 50% isolated radiochemical yield. Astatination of rSAv conjugated with 3c and succinylated was also very rapid (<1 min) providing 66-71% isolated radiochemical yields. Astatination of succinylated rSAv, 2a, which did not have conjugated borane cage moieties, resulted in a much lower radiolabeling yield (18%). The 213Bi or 211At-labeled modified rSAv preparations were mixed with the corresponding 125 I-labeled rSAv, and dual-label in vivo distributions were obtained in athymic mice. The in vivo data show that 213Bi-labeled succinylated rSAv [ 213Bi] 6a has tissue concentrations similar to those of 125 I-labeled modified rSAv [ 125 I] 6b, suggesting that (213)Bi is quite stable toward release from the chelate in vivo. In vivo data also indicate that the (211)At-labeled rSAv conjugated with 3b or 3c and succinylated are stable to in vivo deastatination, whereas succinylated rSAv lacking a boron cage moiety is subject to some deastatination. The modified rSAv conjugated with nido-carborane derivative 3b has a higher retention in many tissues than rSAv without the carborane conjugated. Interestingly, the rSAv conjugated with 3c, which also contains an m-dPEG 12 moiety, has significantly decreased concentrations in blood and other tissues when compared with those of direct-labeled rSAv, suggesting that it may be a good candidate for further study. In conclusion, rSAv that has been modified with CHX-A'' and succinylated (i.e., 5a) may be useful as a carrier of 213Bi. The encouraging results obtained with the PEGylated decaborate(2-) derivative 3c and succinylated (i.e., 5c) suggests that its further study as a carrier of 211At in pretargeting protocols is warranted.

Reactions of Boranes and Metallaboranes with Phosphines

AbstractFor Abstract see ChemInform Abstract in Full Text.

The molecular structures of pentaborane(9) with halogen substituents in apical and basal positions, determined by electron diffraction and theoretical calculations.

The molecular structures of 1-bromo-pentaborane(9) and 2-bromo-pentaborane(9) in the gas phase have been determined by electron diffraction and ab initio and DFT computational methods. Computational methods have also been applied to the fluoro and chloro analogues, to 1,2-dibromo-pentaborane(9), and to the parent unsubstituted borane. The electronic effects of halogen substitution on the borane cage are remarkably small, particularly for chlorine and bromine substituents, and steric effects are also minimal, even in the compound with two bromine atoms. The largest effects are (a) lengthening of B((base))-B((apex)) bonds adjacent to the halogen in the 2-isomers, with an associated shortening of the opposite base-apex bond, (b) shortening of the B((base))-B((apex)) bond in the 1-fluoro compound, and (c) increase of the B((base))-B((apex))-F angle in 1-F-B(5)H(8), but a decrease in this angle in the 2-bromo compounds.

Reactions of boranes and metallaboranes with phosphines

Abstract This paper reports extensions of the well-established field of phosphine–borane chemistry. Linked clusters, for example, {[(PPh3)2(CO)OsB4H7-3-BH2-PPh2]2 [(Fe(C5H4)2]}, are formed in reactions of rigid backboned bidentate phosphines with {2,2,2-(PPh3)2(CO)-nido-2-OsB5H9]. Reaction of bidentate phosphines with the unsaturated clusters [8,8-PPh3)2-nido-8,7-RhSB9H10] and [9,9-(PPh3)2-nido-9,7,8-RhC2B8 H11] leads to the isolation of novel species such as {1-(PPh3)[1,3-(µ-dppm)]-closo -1,2-RhSB9H8},with a dppm molecule bridging adjacent metal and boron vertices in the cage, [1,1-(ç2-dppe)-3-(ç1-dppm)-closo-1,2-RhSB 9H8], a mixed ligand system, and {9,9-ç2-[(ç2-BH3)PPh2PCH2 PPh2]-nido- 9,7,8-RhC2B8H11} which contains a bidentate dppm BH3 moiety. The formation of bidentate phosphine-linked main group transition-metal moieties such as [BH3PPh2(CH)nPPh2IrCp*Cl2] is also described as is an example of a bidentate phosphine-polyhedral borane adduct [NR4][B11H12(dppm)] in which the borane cage is isoelectronic with [B11H14]–. Finally, a novel oxidative coupling reaction promoted by Cp2ZrCl2/n-BuLi to form the new nonaborane cage system, arachno -B9H11(PPh3)2, an unusual homolog of the well-known species B10H12(PPh3)2, is reported.

Heteroboranes of the p-block elements

The syntheses, structures and reactivities of the heteroboranes in which p-block elements have replaced one or more vertices in the parent borane cages have been reviewed. The purpose is to provide an overview of this field, rather than give a detailed history of its development.

The Relation Between Polyhedral Borane Sandwiches and Endohedral Complexes; the Electronic Structure and Stability of X@YmBnHn+mq (X = He, Ne, Li, Be; Y = B, C, Si; m = 0-3; n = 12-9; q = -2 to +2), (C2B4H6)2Xq (X = Li, Al, Si; q = -3, -1, 0) and X2@B17H17q (X = He, Li; q = -2, 0)

An electronic structural connection is established for sandwich complexes and polyhedral boranes containing encapsulated atoms. The charge requirements of these extreme geometrical patterns, examples 3 and 9, depend on the size of the central atom or on the distance between the adjacent rings. While going from the endohedral to the corresponding sandwich complexes the unoccupied a2u and eg molecular orbitals are stabilized considerably requiring additional 6 electrons for stability. The two endohedral atoms in the doped structures 10 resulting from the multidecker sandwich complexes 4 are found to stabilize the large borane skeleton. The energetics and geometries of the relatively less explored endohedral boranes show that endohedral silaboranes are more stable than the endohedral carbaboranes. In general, when an atom is encapsulated in a borane cage, its skeletal bonds are elongated. The exo bonds are shortened due to the possible reduction in the torsional strain between the adjacent vertices. A comparison of the endohedral complexes with the corresponding exo isomers shows that encapsulation makes the system more strained.

Molecular Models of Solid State Metal Boride Structures

Based on existing connections between borane cages and boron-rich metal borides on the one hand and metal clusters containing boron atoms and metal-rich borides on the other, selected metallaboranes of intermediate stoichiometry are investigated as models for transition metal borides with similar M/B ratios. This study is possible due to the recent development of a general route to metallaboranes for group 5–9 metals that permits the distinct role of the transition metal in metallaborane cluster bonding to be revealed. The comparison shows that it is MM and MB covalent bonding rather than M and B electronic charges that drives change in structure with metal identity.

Molecular mechanics calculations of 10-vertex boron cage compounds.

The model proposed earlier for molecular mechanics calculations of 7- and 12-vertex boranes, carboranes, and metallocarboranes has been extended to the case of 10-vertex borane cage compounds. To use the MM3 program with the standard connectivity file, and to avoid program alterations, the 10-vertex cages of the molecules were presented as a superposition of four formally independent fragments. Interactions between the fragments were described with a Hill-like potential, with the parameters adjusted for valence interactions. Standard values for the bond lengths and bond angles in the 10-vertex boron cage have been found by statistical analysis of X-ray data on borane cage compounds stored in the Cambridge Structural Database. Several substituted neutral molecules and anions have been considered, and good agreement of the calculated and experimental data has been obtained. Using the approach developed, the unknown structure of the [mu-B20H16O(CH2)4O(CH2)2CH(CH3)2]3- ion has been calculated.

Face-localized bonding models for borane cage ligands in transition metal coordination chemistry

Substitution of a borane vertex by an isolobal transition metal vertex in a deltahedron can change the shape and apparent electron count. More specifically, in many metallaborane deltahedra with 9, 10, and 11 vertices some transition metal units are found to occur at degree 6 vertices. In the case of 9- and 10-vertex deltahedra this requires a change in the shape of the deltahedron from the metal-free closo borane deltahedra having only degree 4 and 5 vertices to alternative so-called isocloso deltahedra having a single degree 6 vertex. The change in electron count from 2n+2 skeletal electrons for metal-free borane closo deltahedra having the maximum number of degree 4 and 5 vertices to 2n skeletal electrons for metallaborane isocloso deltahedra with the metal at a single degree 6 vertex can be rationalized by a major change in the skeletal bonding topology from globally delocalized in the closo deltahedra to face-localized in the isocloso deltahedra. In this model the skeletal bonding in closo deltahedra with n vertices can be constructed from n two-center two-electron surface bonds and a single n-center core bond whereas the entire skeletal bonding in isocloso deltahedra can arise from n three-center two-electron bonds in n of the 2n−4 faces. The isocloso metallaboranes with n vertices can be considered to be metal complexes of an open borane ligand having all triangular faces except for a single hexagonal face, which forms a hexahapto bond to the transition metal vertex similar to the hexahapto arene-metal bonds in arene metal complexes. The 10-vertex deltahedra (C5H5M)2C2B6H8 (M=Co, Fe) discovered by Hawthorne and co-workers in the 1970s provide an early example of a structurally characterized closo/isocloso deltahedral pair of the same stoichiometry in which the cobalt derivative has the 2n+2 skeletal electrons and geometry of the closo 10-vertex deltahedron (the bicapped square antiprism) whereas the iron derivative has only 2n skeletal electrons and the different geometry of the isocloso 10-vertex deltahedron with one of the iron atoms at the unique degree 6 vertex.

Application of the NOE experiment to the analysis of boron hydride derivatives: confirming the assignments of the pseudocloso-complex [1,2-Ph 2-3-{Cp*}-3,1,2-IrC 2B 9H 9] (Cp*=η 5-C 5Me 5) and the closo-compounds 1-Ph-1,2-C 2B 10H 11 and 1-Ph-2-Me-1,2-C 2B 10H 10

The NOE experiment is demonstrated here as a viable tool in the NMR analyses of substituted borane and heteroborane compounds. We demonstrate that the saturation of 1H NMR signals assigned to a substituent ligand can give rise to enhancements of signals arising from B–H’s within a borane cage. These enhancements, though small (ca. 1–2%), are clearly detectable and have been used to confirm assignments for the pseudocloso-complex [1,2-Ph 2-3-{Cp*}-3,1,2-IrC 2B 9H 9] (Cp*=η 5-C 5Me 5), the mono-substituted closo-carbaborane 1-Ph-1,2-C 2B 10H 11 and the di-substituted closo-carbaborane 1-Ph-2-Me-1,2-C 2B 10H 10. We compare the results of applying three alternative experimental NOE procedures to these problems, namely phase sensitive 2D NOESY, 1D truncated driven NOE and 1D DPFGSE-NOE. The differences between the quality and relevance of these procedures to the study of borane and heteroborane derivatives are discussed.

Sputter deposition of high resistivity boron carbide

We have succeeded in the rf magnetron sputter deposition of high resistivity boron carbide (B 1− x C x ). This has been accomplished by the sputter depositing the boron carbide from a methane saturated boron carbide target. We show that the composition and optical band gap of the sputter deposited material are functions of the applied rf power. Furthermore, boron carbide/silicon heterojunction diodes fabricated via sputtering compare favorably with those fabricated from borane cage molecule sources using plasma enhanced chemical vapor deposition (PECVD).