Structures and Syntheses of Carboranes

Following a discussion of bonding, the structures of the known carboranes of the clovo type and of the incompletely condensed types are described; the recently discovered carboranes with high carbon contents, such as tetracarbahexaborane (“boracarbane”) are also discussed. The preparation of carboranes from polyboranes and the possibility of obtaining organo carboranes from organoboranes are then described. Some rearrangements which take place within the carborane skeleton are also mentioned.     

8-Dioxane ferra(III) bis(dicarbollide): A paramagnetic functional molecule as versatile building block for introduction of a Fe(III) centre into organic molecules

The synthesis of a new, paramagnetic closo-[(8-(-CH₂CH₂O)₂-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Fe]⁰ (3) is reported. This compound can serve as a versatile building block for construction of both anionic and zwitterionic derivatives, as exemplified by the synthesis of a series of compounds of general formula closo-[(8-X-(CH₂CH₂O)₂-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Fe], bearing organic end groups (X = NC₅H₅ (4), (C₆H₅)₃P (5), OH (6), and 2-O(1-CH₃O-C₆H₄) (7)) attached to the cluster by a diethyleneglycol spacer. Molecular structures of 3, 4, 5 and 7 were determined by single-crystal X-ray diffraction analysis and by the long-time neglected method of paramagnetic, high field NMR (¹H, ¹³C and ¹¹B) spectroscopy.     

New approaches to cluster modification in the 12-vertex metallatricarbollide series

Cluster opening of [2-Cp-9-^tBuNH-closo-2,1,7,9-FeC₃B₈H₁₀] (1) , followed by oxidation, generates complexes [2-Cp-8-^tBuNH-closo-2,1,8,10-FeC₃B₈H₁₀] (2), [2-Cp-4-^tBuNH-closo-2,1,4,12-FeC₃B₈H₁₀] (3), [2-Cp-1-^tBuNH-closo-2,1,7,10-FeC₃B₈H₁₀] (4), and [1-Cp-10-^tBuNH-closo-1,2,3,10-FeC₃B₇H₉] (5). Another variation of the syntheses led to compounds [2-Cp-closo-2,1,8,10-FeC₃B₈H₁₁] (6), [4-Cp-1-^tBuNH-closo-4,1,6,8,-FeC₃B₉H₁₁] (7) and to two isomeric, not yet fully characterized, 13-vertex compounds of general nido structure [^tBuNH-Cp-FeC₃B₉H₁₂] (8 and 9).     

Reaction of a ruthenium B-carboranyl hydride complex and BH3(SMe2): Selective formation of a pincer-supported metallaborane LRu(B3H8)

Reaction of a boryl hydride pincer complex (POBOP)Ru(H)(PPh₃) (POBOP?=?1,7-OP(i-Pr)₂-m-2-carboranyl) and BH₃(SMe₂) at 70?°C led to the selective formation of a pincer-supported metallaborane (POBOP)Ru(B₃H₈). Single crystal structure of (POBOP)Ru(B₃H₈) was determined. This complex features coordination of the carborane cluster through adjacent boryl and borane groups that impose significantly different trans-influence on the coordinated B₃H₈ fragment.     

Synthesis of distorted molecules based on spatial control with icosahedral carboranes

In the crystal structure of 1,2-bis(o-carboranyl)benzene (2), the benzene ring is remarkably twisted out of planarity, owing to the steric bulkiness of two adjacent carboranyl groups. In the carboracycle (3), the two benzene rings are slantingly stacked owing to spatial control by the carborane cages.     

Chemistry related to cluster-borane analogues of the cyclopentadienide anion and ferrocene: New developments

A review on cluster-borane analogues of the cyclopentadienide anion (Cp) and ferrocene is presented. Analogues of Cp that have been so far isolated and characterised are the 11-vertex triheteroboranes of general structure [nido-E₃B₈H₈]⁻ (where E = CH or P and their combinations), the molecules of which contain an open pentagonal face. These anions were used as effective ligands for the preparation of “half- and full-sandwich” complexes [CpFeE₃B₈H₈] and [Fe(E₃B₈H₈)₂], respectively - analogues of ferrocene. Developments in this area of cluster-borane chemistry that include recent results in the synthesis and Fe-complexation reactions of 11-vertex tricarbaboranes (tricarbollides), phosphadicarbollides, and diphosphacarbollides are the subject of this work.     

Metallacarborane chemistry of the hypho-[6,7-C2B6H13] anion: Reaction with nickelocene and the formation of three multimetallic nickel-boron clusters

The reaction of the hypho-[6,7-C₂B₆H₁₃]⁻ anion (1) with nickelocene and an excess of ‘proton sponge’ (1,8-bis-(dimethylamino-naphthalene)) in boiling acetonitrile leads to the formation of a pair of isomeric trimetallic nickel-boron clusters, [6,7,8-(CpNi)₃-1-CB₅H₆] (2) and [6,7,8-(CpNi)₃-2-CB₅H₆] (3), in a combined yield of 55%. Isomer (2) had been previously prepared from nido-2-CB₅H₉ but in much lower yield. Isomer (3) is without precedent and has been characterized using multi-nuclear NMR spectroscopy and mass spectrometry. Isomer (3) undergoes conversion to (2) via heating in boiling toluene. In addition to this isomeric pair, an interesting nido dimetallacarborane of constitution [6,6′-(CpNi)₂-7,7′-C₂B₆H₈] (4) has been isolated from the same reaction in 5% yield and characterized by single-crystal X-ray diffraction analysis.     

The enumeration of electron-rich and electron-poor polyhedral clusters

We present as dual processes the capping of closed triangulated polyhedra with apical atoms and the making of holes in such polyhedra either by puncture of the surface or by excision of atoms and their edges. These processes are shown to generate stable chemical species containing respectively less or more than 2n + 2 skeletal electrons. The former species are designated as electron-poor whereas the latter are called electron-rich. Pólya's enumeration method is used to enumerate the distinct ways of capping and excising the closed, triangulated polyhedra to yield systems containing from four to twelve vertices. For the enumeration of cappings the appropriate cycle index is that of the dual of the polyhedron being capped, whilst for the enumeration of the excisions the cycle index is that of the polyhedron being excised.     

Two-dimensional scaffolds for the parallel alignment of rod-shaped conjugated molecules

Described is a method for creating two-dimensional, ordered arrays of linear, conjugated organic molecules on polyimide scaffolds. Key to the design was the enforcement of consecutive 90 degrees twist angles along the polyimide backbone. Rod molecules that are templated by such scaffolds are held parallel and in the same plane and have optical properties that are similar to those of monomeric analogues. As supporting evidence for the structures of the polymers, a series of model compounds were synthesized and characterized by X-ray crystallography. The polymeric materials are soluble and have been characterized by 1H NMR, 13C NMR, MALDI-TOF, and UV-vis spectroscopy.     

Polycyclic rod-shaped azo dyes based on aminophenylpyrimidines

We have synthesized rod-shaped tricyclic monoazo and bisazo dyes based on amino derivatives of arylpyrimidines and N,N-dialkylanilines orp-nitroso-N,N-dialkylanilines. We have shown that the process of azo coupling of the diazonium salt obtained from 2,5-bis(p-arninophenyl)pyritnidine is accompanied by dediazoniation, with substitution of the diazonium group by hydrogen or an aryl group. The monoazo dyes obtained display mesomorphic properties which are absent in the bisazo dyes.Novosibirsk Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences, Novosibirsk 630090. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 693–702, May, 1996. Original article submitted January 25, 1996.     

Future prospects in organometallic chemisty of the Main Group metals: small molecules — supramolecular chemistry

Bulky alkyl ligands functionalised with N-donor groups of the type 2-C(SiMe₃)₂(6-R-2-pyridyl), R = H or Me, stabilise unusual bonding configurations, and lead into geminal bimetallic complexes, including the formation of a tetrametallic zinc cluster. This chemistry is poised for major advances in mixed metal complexes of the Main Group metals and beyond. Magnesium(anthracene)(THF)₃ and related complexes solve the ‘missing link’ in Grignard-reagent formation with the ability to routinely prepare Grignard reagents of benzylic halides. Future directions on the use of reactive metals in synthesis will adopt the principles of green chemistry, for example in the indium(0)-promoted reactions of organic halides with various functional groups in water. Supramolecular chemistry of o-carboranes in association with ‘rigid’ container and saddle-shape molecules has led to new classes of inclusion complexes of varying stoichiometry, and extension of this work into materials of higher complexity, with other carboranes and larger globular molecules, and with functionalised container molecules is a realisation.     

[{[(η7-C2B10H12)(η6-C2B10H12)U][K2(thf)5]}2]: A Metallacarborane Containing the Novel η7-C2B10H124− Ligand

The tetranion ligand η⁷-C₂B₁₀H₁₂⁴⁻ has been observed for the first time in 1 , which was obtained from the reaction of o-C₂B₁₀H₁₂ with excess K metal followed by treatment with UCl₄. As shown in the picture (without K cations and coordinated THF molecules), 1 is a centrosymmetric dimer with a bent sandwich structure.     

Towards an order-N DFT method

One of the most important steps in a Kohn-Sham (KS) type density functional theory calculation is the construction of the matrix of the KS operator (the “Fock” matrix). It is desirable to develop an algorithm for this step that scales linearly with system size. We discuss attempts to achieve linear scaling for the calculation of the matrix elements of the exchange-correlation and Coulomb potentials within a particular implementation (the Amsterdam density functional, ADF, code) of the KS method. In the ADF scheme the matrix elements are completely determined by 3D numerical integration, the value of the potentials in each grid point being determined with the help of an auxiliary function representation of the electronic density. Nearly linear scaling for building the total Fock matrix is demonstrated for systems of intermediate size (in the order of 1000 atoms). For larger systems further development is desirable for the treatment of the Coulomb potential. Received: 30 March 1998 / Accepted: 6 July 1998 / Published online: 15 September 1998     

Hyperbranched molecules based on calixarenes

Synthesis of the diamide 2 derived from tris(2-aminoethyl)amine and monocarboxymethylcalix[4]arene provides a starting material useful for the preparation of a variety of hyperbranched molecules. Metal ion binding is one of the potentially useful properties of these new materials.     

Towards biosensors based on conducting polymer nanowires

We report the electrochemical deposition of poly(pyrrolepropylic acid) nanowires, their covalent modification with antibodies and their conversion into potential functional sensor devices. The nanowires and the devices were characterised by optical microscopy, fluorescence microscopy, electron microscopy and electrical measurements. Fluorescence images, current–voltage (I–V) profiles and real-time sensing measurements demonstrated a rapid and highly sensitive and selective detection of human serum albumin (HSA), a substance that has been used to diagnose incipient renal disease. The detection is based on the selective binding of HSA onto anti-HSA that is covalently attached to the nanowires. The binding changes the electrical properties of the nanowires thus enabling the real-time detection. Whilst the utility of the research was demonstrated for protein binding/detection, the technology could easily be designed for the detection of other analytes by the modification of polymer nanowires with other analyte-specific molecules/biomolecules. Therefore, the technology has the potential to positively impact broad analytical applications in the biomedical, environmental and other sectors. Figure Real-time dynamic current response on sequential exposure of buffer, bovine serum albumin (BSA) and human serum albumin (HSA) onto anti-HSA modified poly (pyrrolepropylic acid) nanowires. Fluorescence images of poly(pyrrolepropylic acid) nanowire (top right) and polypyrrole nanowire control (bottom right) after sequential treatment with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), anti HSA and fluorophore-labeled HSA.     

The Critical Importance of Water in the Asymmetric Allylboration of N-Trimethylsilylbenzaldimines with B-Allyldiisopinocampheylborane

The free aldimine is probably the intermediate in the asymmetric allylboration of N-trimethylsilylaldimines in the presence of water (see scheme), which is critical for the reaction. The aldimine is rapidly captured by the allylborating agent. Ipc₂BAll=B-allyldiisopinocampheylborane.     

Adaptation of the Barton Reaction to Carborane Chemistry: The Synthesis and Reactivity of 2-Hydroxyimino-1-hydroxymethylnona-B-methyl-1,12-dicarba-closo-dodecaborane(12)

Just one of the ten methyl groups of deca-B-methyl-1,12-dicarba-closo-dodecarborane(12) ( 1 ) is selectively functionalized in a reaction sequence in which photolysis of a nitrite of 1 is the key step. Reduction of the resulting aldoxime with LiAlH₄ generates, by way of the first Beckmann rearrangement of a boron-substituted oxime, the methylamino alcohol 2 .