Borane Anions Research Articles

Investigation of the Interactions of Polyhedral Borane Anions with Serum Albumins

Boron neutron capture therapy (BNCT) is a type of radiation therapy used in the treatment of cancerous tumors such as gliboblastoma multiforme and metastatic melanoma. BNCT is dependent on the site specific delivery and retention of boron‐10 atoms within the tumor. The ability of polyhedral borane anions to form covalent bonds with biochemical macromolecules appears to be critical for boron retention within the tumor cell. The polyhedral borane anions [B20H17OH]4−, [B20H18]2−, and [B20H17SH]4− were allowed to react with serum albumins, and the reaction products were investigated using polyacrylamide gel electrophoresis (Native‐PAGE), matrix assisted laser desorption ionization time of fight (MALDI‐TOF), electrospray ionization and nuclear magnetic resonance (NMR). The formation of disulfide linkage between [B20H17SH]4− and the free cysteine residue in the albumin was observed while [B20H17OH]4− and [B20H18]2− showed no evidence of covalent bonding. The understanding of covalent binding of polyhedral borane anions will aid in the synthesis and design of agents used in BNCT.

Investigation of the interactions of polyhedral borane anions with serum albumins

The retention of polyhedral borane anions within tumor cells has been attributed to the possible formation of covalent bonds with nucleophilic protein substituents. In an effort to identify the nature of possible interactions between polyhedral borane anions and proteins, three polyhedral borane anions, [B20H18]2−, [B20H17OH]4−, and [B20H17SH]4−, were allowed to react with either bovine or human serum albumin. Reaction products were analyzed with matrix assisted laser desorption ionization (MALDI) mass spectrometry and gel electrophoresis. Evidence of disulfide bond formation was observed with the [B20H17SH]4− anion, whereas no evidence of covalent binding was observed with the [B20H18]2− and [B20H17OH]4− ions. The potential for disulfide bond formation was confirmed by examining the reactions of the [B20H17SH]4− ion with both DTNB and reduced glutathione. An understanding of the nature of the binding will provide a basis for the design and synthesis of boron-containing compounds for application in boron neutron capture therapy.

Synthesis and Comparative Toxicology of a Series of Polyhedral Borane Anion-Substituted Tetraphenyl Porphyrins

Three structurally similar tetraphenylporphyrins bearing polyhedral borane anions have been synthesized and their toxicological profiles obtained in rats. These conjugates were found to have quite different acute toxicities as manifested at the maximum tolerated dose (MTD). When given at the MTD and observed over 28 days, the most acutely toxic porphyrin was found to be devoid of toxicity, as measured by blood chemistry panels. The remaining two less acutely toxic compounds both elicited significant changes, characterized by moderate to severe thrombocytopenia, failure to gain weight normally and changes in liver enzymes indicative of mild hepatotoxicity. All toxic effects were transient, with platelets rebounding to above normal levels at day 28. We conclude that thrombocytopenia is the dose limiting toxicity for boronated porphyrins in mammals and suggest that these effects may be due to the porphyrin, not the borane or carborane.

Preconditions for reasonable detection sensitivity and for zone symmetry in electrophoretic separations of cluster borane anions

Preconditions for reasonable detection sensitivity and for zone symmetry in electrophoretic separations of cluster borane anions

Preconditions for reasonable detection sensitivity and for zone symmetry in electrophoretic separations of cluster borane anions

Single-cage boron cluster anions with at least 11 cluster atoms, free of bonded functional groups that strongly absorb UV light, and their cobalt complexes have been the investigated compounds. Their UV-absorption spectra have absolute maxima between 200 and 215 nm. Corresponding molar extinction coefficients that are of the order of 10 3–10 4 L mol −1 cm −1 indicate medium detection sensitivity. Its reaching requires elimination of background electrolytes that weaken the UV-light beam in any way. Frequently used carboxylic acids and zwitterionic Good's buffers cannot be used as buffering compounds from this reason. Freshly prepared 1 mM solutions of boron cluster compounds in aqueous sodium chloride, chosen as indifferent electrolyte, which contain 20–30% (v/v) of methanol or acetonitrile, give zones free of tailing. After storing in the fridge, zones of the compounds became pronouncedly tailed even if their solutions remain clear and free of precipitation, turbidity or opalescence. The tailing usually disappeared if the acetonitrile or the methanol concentration in samples was 40–60% (v/v) depending on the dissolved compound hydrophobicity. Solutions of extremely hydrophobic compounds, stored in the fridge, require mild heating to 30–40 °C for half an hour for the avoiding of the tailing. Permanent slow decrease in effective mobilites of boron cluster anions was found if background electrolytes contained acetonitrile and β-cyclodextrin. Analogous decrease was not observed with organic anions. Constant mobilities of boron cluster anions have been reached if acetonitrile was replaced with methanol. Analyte zones were more symmetrical in background electrolytes buffered with sodium borate of pH 9 than in background electrolytes buffered with sodium phosphate of pH 7.

An Addition to the Oxoacid Family: H2B12(OH)12.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Aggregation and other intermolecular interactions of biological buffers observed by capillary electrophoresis and UV photometry

Electrophoretic and photometric experiments strongly indicate that monovalent anions, which arise by deprotonation of the nitrogen atom in zwitterionic Good’s buffers 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO) and 3-morpholinopropanesulfonic acid (MOPS), spontaneously aggregate. Cationic migration of sanguinarine (SA) and chelerythrine (CHE) in highly alkaline 1,3-bis[tris(hydroxymethyl)methylamino]propane (Bis–Tris–propane), in which the concentration of cations of both alkaloids is negligible, may be explained by the existence of an aggregate, which contains uncharged sanguinarine or chelerythrine and one monovalent cation of Bis–Tris–propane at least. Tendency of tris(hydroxymethyl)aminomethane (Tris), bis (2-hydroxyethyl)iminotris(hydroxymethyl)methane (Bis–Tris) and Bis–Tris–propane cations to ion pairing with synthetic cluster borane anions and with fused silica markedly rises up with the size and charge of these cations. The drop in mobility of cluster borane compounds sometimes exceeds 50% of their mobility found at identical pH and ionic strength in buffers with sodium cation. The electroosmosis drop approached 70% if background electrolyte contained Bis–Tris–propane cations instead of sodium cations. Nitrate, taken as a model inorganic ion, and four randomly chosen organic anions interacted markedly less with Tris, Bis–Tris and Bis–Tris–propane cations than cluster borane anions. 2-( N-morpholino)ethanesulfonic (MES) acid anions present in background electrolyte affect the ion pairing of Tris, Bis–Tris and Bis–Tris–propane cations with anionic analytes and, in this way influence also mobilites of these anionic analytes. Limited hydrophilicity at least one of interacting species appears to be the most probable cause of observed intermolecular interactions of biological buffers.

Cyclopentadienyl ruthenium, rhodium, and iridium vertices in metallaboranes: geometry and chemical bonding.

Most cyclopentadienylmetallaboranes containing the vertex units CpM (M = Co, Rh, Ir; Cp = eta(5)-cyclopentadienyl ring, mainly eta(5)-Me(5)C(5)) and CpRu donating two and one skeletal electrons, respectively, have structures closely related to binary boranes or borane anions. Smaller clusters of this type, such as metallaborane analogues of arachno-B(4)H(10) (e.g., (CpIr)(2)B(2)H(8)), nido-B(5)H(9) (e.g., (CpRh)(2)B(3)H(7) and (CpRu)(2)B(3)H(9)), arachno-B(5)H(11) (e.g., CpIrB(4)H(10)), B(6)H(6)(2)(-) (e.g., (CpCo)(4)B(2)H(4)), nido-B(6)H(10) (e.g., CpIrB(5)H(9) and (CpRu)(2)B(4)H(10)), and arachno-B(6)H(12) (e.g., (CpIr)(2)B(4)H(10)), have the same skeletal electron counts as those of the corresponding boranes. However, such clusters with eight or more vertices, such as metallaborane analogues of B(8)H(8)(2)(-) (e.g., (CpCo)(4)B(4)H(4)), arachno-B(8)H(14) (e.g., (CpRu)(2)B(6)H(12)), and nido-B(10)H(14) (e.g., (CpRu)(2)B(8)H(12)), have two skeletal electrons less than those of the corresponding metal-free boranes, analogous to the skeletal electron counts of isocloso boranes relative to those of metal-free deltahedral boranes. Some metallaboranes have structures not analogous to metal-free boranes but instead analogous to metal carbonyl clusters such as 3-capped square pyramidal (CpRu)(2)B(4)H(8) and (CpRu)(3)B(3)H(8) analogous to H(2)Os(6)(CO)(16) and capped octahedral (CpRh)(3)B(4)H(4) analogous to Os(7)(CO)(21). In the metallaborane structures closely related to metal-free boranes, the favored degrees of BH and CpM vertices appear to be 5 and 6, respectively.

An addition to the oxoacid family: H2B12(OH)12.

The acid H(2)B(12)(OH)(12) can be isolated as a crystalline solid by protonation of the hydroxylated borane anion, B(12)(OH)(12)(2)(-). This acidic compound has low solubility in water, conducts protons in the solid state, and has thermal stability to a temperature of 400 degrees C. The conductivity mechanism is a Grotthuss mechanism with a low activation enthalpy (9-13 kcal/mol). This new acid represents an addition to the class of oxoacids, of which sulfuric and phosphoric acid are the most prominent examples.

An Improved Method for the Synthesis of [closo‐B12(OH)12]‐2.

The dicesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2−), closo-Cs2B12(OH)12, Cs24, was prepared using an improved synthetic pathway. Heating cesium dodecahydro-closo-dodecaborate, closo-Cs2B12H12, Cs21, with 30% hydrogen peroxide added in successive increments at 105−110 °C provided Cs24 in 95% yield. Reaction progress was monitored using 1H-decoupled 11B NMR while 17O NMR provides the most reliable way to detect the presence of peroxides in the reaction solution. The reaction may be safely increased in scale to afford Cs24 in multigram quantities.

An improved method for the synthesis of [closo-B12(OH)12]-2.

The dicesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2-), closo-Cs(2)B(12)(OH)(12), Cs(2)4, was prepared using an improved synthetic pathway. Heating cesium dodecahydro-closo-dodecaborate, closo-Cs(2)B(12)H(12), Cs(2)1, with 30% hydrogen peroxide added in successive increments at 105-110 degrees C provided Cs(2)4 in 95% yield. Reaction progress was monitored using (1)H-decoupled (11)B NMR while (17)O NMR provides the most reliable way to detect the presence of peroxides in the reaction solution. The reaction may be safely increased in scale to afford Cs(2)4 in multigram quantities.

Defective vertices in arachno borane networks.

Triangulated boron networks can be described in terms of the deviation of their local vertex environments from the degree 5 vertices found in ideal icosahedra. Vertices of degrees other than 5 or equivalent are considered to be defective vertices. This method, which was previously applied to deltahedral borane anions B(n)H(n)(2-) and nido-B(n)H(n+4) boranes, has now been applied to arachno boranes of the types B(n)H(n+6) and B(n)H(n+5)(-) (4 < or = n < or = 10). The known structures of the neutral arachno boranes B(4)H(10), B(8)H(14), and n-B(9)H(15) consist of triangulated boron networks with no defective vertices in accord with their higher stabilities relative to other neutral arachno boranes. In other structures of known arachno boranes, there are relatively small numbers of defective vertices, and these are isolated as far as possible from each other.

Original packing and unusual molecular conformation of the bis(ethylenedithio)tetrathiafulvalene (ET) donors induced by cation–anion interactions in the radical salt ET5[B10I10]·(CH2Cl2)0.8

The crystal structure of the radical salt ET5[B10I10]·0.8CH2Cl2 has been determined and the packing and molecular conformation of the organic donors are discussed in terms of the C–H⋯I contacts between the ethylene groups of the ET molecules and the iodine atoms of the borane anion [B10I10]2−.

Carbonium ion salts. Part 16. Molecular orbital study of tropylium ions with borane anion and carborane substituents (hemiousene compounds)

We have extended our ab initio study of substituted tropylium ions to examine several known and postulated hemiousene compounds where the ring has a substituent derived from a polyhedral borane anion, carborane, or carborane anion. Tropyliumyl derivatives of the borane anions closo-B 10H 10 2−, closo-B 12H 12 2−, and closo-CB 9H 10 − are Type III substituted tropylium ions with substituent to ring charge-transfer in the HOMO–LUMO (HL) transition. An additional boron electron pair of the closo-cage in 2-C 7H 6–B 10H 9 − and C 7H 6–B 12H 11 − is completely delocalized over cage and ring. Tropyliumyl derivates of the carborane and carborane anions closo-C 2B 10H 12 and closo-CB 11H 12 − are Type II substituted tropylium ions with polarization of ring π-system by substituent and little if any charge-transfer HL interaction. The tropyliumyl derivative of nido-C 2B 9H 12 − is in a class by itself; however both theory and experiment demonstrate that it has strong HL charge-transfer like a Type III ion.

Three-dimensional aromaticity in polyhedral boranes and related molecules.

Chemical bonding models based on graph theory or tensor surface harmonic theory demonstrate the analogy between the aromaticity in two-dimensional planar polygonal hydrocarbons such as benzene and that in three-dimensional deltahedral borane anions of the type BnHn2- (6 < or = n < or = 12). Such models are supported both by diverse computational studies and experimental determinations of electron density distribution. Related methods can be used to study the chemical bonding in the boron polyhedra found in other structures including neutral binary boron hydrides, metallaboranes, various allotropes of elemental boron, and boron-rich solid-state metal borides.

Dodecahydroxy-closo-dodecaborate(2-).

The cesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2-), Cs(2)[closo-B(12)(OH)(12)], Cs(2)1, was prepared by heating cesium dodecahydro-closo-dodecaborate(2-), Cs(2)[closo-B(12)H(12)], Cs(2)2, with 30% hydrogen peroxide. The other alkali metal salts A(2)1 (A = Li, Na, K, Rb) precipitated upon addition of ACl to warm aqueous solutions of Cs(2)1. The ammonium salt, [NH(4)](2)1, and the (mu-nitrido)bis(triphenylphosphonium) salt, [PPN](2)1, were obtained similarly. The [H(3)O](2)1 salt precipitated upon acidification of aqueous solutions of Cs(2)1 with hydrochloric acid. The solubility of these salts in water was determined by measuring the boron content of saturated aqueous solutions of A(2)1 (A = Li, Na, K, Rb, Cs), [H(3)O](2)1, and [NH(4)](2)1 using ICP-AES. Although these salts are derived from a dianion with twelve pendant hydroxyl groups, the alkali metal salts surprisingly displayed low water solubilities. Water solubility decreases with a decrease in the radius of A(+), except for the lithium salt, which is slightly more soluble than the potassium salt. The [H(3)O](2)1 and the [NH(4)](2)1 salts provide rare examples of water-insoluble hydronium and ammonium salts. The low water solubility of the A(2)1 salts is attributed to the dianion's pendant hydroxyl groups, which appear to function as cross-linking ligands. Four alkali metal salts, A(2)1 (A = Na, K, Rb, Cs), were characterized in the solid state by single-crystal X-ray crystallography. These data revealed intricate networks in which several anions are complexed through their hydroxyl groups to each alkali metal cation. In addition, the anions are engaged in hydrogen bonding with each other and, if present, with water of hydration. This cross-linking results in the precipitation of aggregated salts. Cation coordination numbers decrease with cation radius. Thus, cesium and rubidium are ten-coordinate, whereas potassium is seven-coordinate and sodium is six-coordinate. The geometry of anion 1(2)(-) is independent of cation identity; the B-B and B-O bond lengths of the various A(2)1 salts (A = Na, K, Rb, Cs) are identical.

New approaches to the design of polymer and liquid electrolytes for lithium batteries

All non-aqueous lithium battery electrolytes are Lewis bases that interact with cations. Unlike water, they do not interact with anions. The result is a high degree of ion pairing and the formation of triplets and higher aggregates. This decreases the conductivity and the lithium ion transference, and results in polarization losses in batteries. Approaches that have been used to increase ion dissociation in poly(ethylene oxide) (PEO)-based electrolytes are the use of salts with low lattice energy, the addition of polar plasticizers to the polymer, and the addition of cation complexing agents such as crown ethers or cryptands. Complexing of the anions is a more promising approach, since it should increase both ion dissociation and the lithium transference. At Brookhaven National Laboratory (BNL) we have synthesized two new families of neutral anion complexing agents, each based on Lewis acid centers. One is based on electron deficient nitrogen sites on substituted aza-ethers, wherein the hydrogen on the nitrogen is replaced by electron withdrawing groups such as CF 3SO 3–. The other is based on electron deficient boron sites on borane or borate compounds with various fluorinated aryl or alkyl groups. Some of the borane-based anion receptors can promote the dissolution of LiF in several solvents. Several of these compounds, when added in equivalent amounts, produce 1.2 M LiF solutions in DME, an increase in solubility of LiF by six orders of magnitude. Some of these LiF electrolytes have conductivities as high as 6×10 −3 S cm −1. The LiF electrolytes with borane anion acceptors in PC:EC:DEC solvents have excellent electrochemical stability. This has been demonstrated in small Li/LiMn 2O 4 cells.

Reactions of (Butadiene)tantalocene Cation with Alkyl Isocyanides

Treatment of (η4-s-trans-butadiene)tantalocene cation (3, with methyltris(pentafluorophenyl)borane anion) with tert-butyl isocyanide (4 h, at 45 °C in bromobenzene) yields the [(η2-butadiene)(C⋮NCMe3)TaCp2+] complex 4a (70% isolated, ν̃(C⋮NR) 2164 cm-1). Likewise treatment with n-butyl or cyclohexyl isocyanide gives the analogous products 4b and 4c, respectively. The cyclohexyl isocyanide adduct 4c was characterized by X-ray diffraction. It shows the η2-butadiene ligand oriented at the front of the Cp2Ta+ bent metallocene wedge with the noncoordinated butadiene vinyl group located in the “vinyl-inside” position. The Ta−C⋮N−R unit is almost linear (angles Ta−C−N = 177.2(5)°, C⋮N−R = 174.0(8)°; d(C⋮N) = 1.152(6) Å). Photolysis of [(butadiene)TaCp2+] (3) with excess cyclohexyl isocyanide followed by thermal treatment gave rise to the formation of [bis(κC-cyclohexyl isocyanide)TaCp2+] (8, with [CH3B(C6F5)3-] counteranion; >50% isolated, ν̃(C⋮NR) 2134 and 2088 cm-1). Complex 8 was characterized by an X-ray crystal structure analysis. It shows a pseudotetrahedral [Cp2Ta(C⋮NR)2+] cation (R = C6H11). The Ta−C⋮N−R units are both slightly bent at nitrogen (angles C⋮N−R = 169.0(13) and 163.7(8)°; the corresponding Ta−C−N angles are 179.4(9) and 177.4(7)°). The bonding situations of complexes 4 and 8 were analyzed by DFT calculations. The computational study shows that metal to isonitrile back-bonding does not contribute significantly to the Cp2Ta+−C⋮NR bonding interaction in these complexes. The bonding between Cp2Ta+ and the C⋮NR ligands is dominated by electrostatic effects.

The ubiquitous icosahedral B12 in boron chemistry

Though boranes exhibit a wide variety of polyhedral structures, all the three polymorphs of elemental boron essentially contain icosahedral B12 units as the predominant building block in their unit cell. Theoretical and experimental studies on boranes show that the icosahedral arrangement leads to most stable boranes and borane anions. This paper attempts to explain the phenomenal stability associated with the icosahedral B12 structure. Using fragment molecular orbital theory, the remarkable stability of B12H 2−12 amongcloso boranes are explained. The preferential selection icosahedral B12 unit by elemental boron is explained by improvising a contrived B84 sub-unit of the β-rhombohedron, the most stable polymorph. This also leads to a novel covalent way of stuffing fullerenes with icosahedral symmetry.

Mechanistic aspects of boron hydride reactions

The information regarding the mechanistic details of selected systems of borane and borane anion reactions has been surveyed. For each family of reactions, the data from different investigations over a substantial time period were gathered together to make these data more accessible. The reaction systems studied were: preparation of B 3H 8 − , the preparations, NMR spectra and structures of the five known B 5 anions, the preparation of bridge-substituted B 5H 9 derivatives, the equilibrium between the isomers of Me–B 5H 10, conversions between nido and arachno structures in B 6 compounds and a variety of cluster growth reactions. Information and proposals regarding mechanisms are given where they are available. In some cases we have advanced proposals of our own. An important aim of this review is to stimulate additional experiments and calculations that will advance understanding of the mechanisms of reactions of boron compounds.