Wendy R. Gill

C-arylation and C-heteroarylation of icosahedral carboranes via their copper(I) derivatives

Abstract Reaction between C-mono- or C,C′-di-copper(1) derivatives of 1,2-, 1,7-, or 1,12-dicarba-closo-dodecaborane(12) and aryl iodides in the presence of pyridine gives the corresponding C-mono- or C,C′-diaryl derivatives of 1,7- and 1,12-dicarba-closo-dodecaboranes(12); 1,2-dicarba-closo-dodecaborane(12) gives only the C-monoaryl product. Cyclic or linear arylene coupled systems are obtained when di-iodoarenes are used. Copper(I) derivatives may be generated from C-unsubstituted or C-monosubstituted carboranes using copper(I) t-butoxide when substituents incompatible with the use of C-lithio-intermediates are involved. The C-copper(I) derivative of 1,2-dicarba-closo-dodecaborane(12) gives 1,2-di-2′-pyridyl-1,2-dicarba-closo-dodecaborane(12) specifically with 2-bromopyridine. The (inferred) intermediate mono-2-pyridyl-derivative, obtained independently from 2-ethynylpyridine and the dimethylsulphide complex of decaborane, gives 1-phenyl-2,2′-pyridyl-1,2-dicarba-closo-dodecaborane(12) upon conversion into its copper(I) derivative and treatment with iododobenzene. However, the copper(I) derivative of 1-phenyl-1,2-dicarba-closo-dodecaborane(12) does not react to a significant extent with 2-bromopyridine.

Deboronation of C-substituted ortho- and meta-closo-carboranes using “wet” fluoride ion solutions

Abstract Hydrated tetrabutylammonium fluoride has been found to be more effective than the anhydrous salt as a reagent for the conversion of ortho and metal closo -carborane derivatives, R′R″C 2 B 10 H 10 , into nido -carborane anions, [R′R″C 2 B 9 H 10 ] − , allowing access to a number of new nido -carborane derivatives difficult to prepare, or inaccessible, using other deboronating reagents, notably the meta -carborane derivatives, [7,9-R′R″-7,9-C 2 B 9 H 10 ] − [Bu 4 N] + (R′ = 4-HOC 6 H 4 , 4-O 2 NC 6 H 4 ; R″ = H and R′ = R″ = 4-PhOC 6 H 4 , 4-O 2 NC 6 H 4 , 2-C 5 H 4 N, 4-H 2 NC 6 H 4 ). Experiments with the bis( p -bromophenyl- ortho -carboranyl)benzene, 1,4-[4-BrC 6 H 4 CB 10 H 10 C] 2 C 6 H 4 , carried out to determine the quantity of tetrabutylammonium fluoride needed to deboronate both cages, afforded the salt 1,4-[7-(4-BrC 6 H 4 )C 2 B 9 H 10 ] 2 C 6 H 4 2− [Bu 4 N + ] 2

Preparation of C-2-pyridyl derivatives of icosahedral carboranes via copper(I) intermediates

Abstract The C-mono- or C,C′-di-2-pyridyl derivatives of 1,7- and 1,12-dicarba-closododecaborane( 12 ) are obtained in good yield by treatment of the corresponding dicarba-closo-dodecaborane( 12 ) copper(I) derivatives with 2-bromopyridine in the presence of pyridine. (In contrast, earlier work showed that only the 1,2-di-2′-pyridyl derivative is formed from 1,2-dicarba-closo-dodecaborane( 12 ) under the same conditions.) With 2,6-dibromopyridine the dicopper(I) derivative of 1,7-dicarba-closo-dodecaborane( 12 ) gives the tripyridylene macrocycle ( 14 ) in up to 10% yield. The pyridyl compounds are considerably weaker bases than pyridine, but 1,12-di-2′-pyridyl-1,12-dicarba-closo-dodecaborane( 12 ), for example, slowly gives the N , N ′dimethylpyridinium salt with trimethyloxonium tetrafluoroborate.

A pentuply-bridging thiocarbonyl group: x-ray crystal structure of a salt of the 1-thio-2-phenyl-1,2-dicarbadodecaborate (12) anion, [LH]+[S(Ph)C2B10H10]− (L = 1,8-N,N,N1,N1-tetramethylnaphthalene diamine)

Abstract The carboranyl sulphide anion [1,2-S(Ph)C2B10H10]−, characterized as its salt [C10H6(NMe2)2H]+[S(Ph)C2B10H10]− (1), has been prepared from the carboranyl thiol and proton sponge and characterized by X-ray crystallography. The anion has a nido-shaped [PhCB10H10] residue in which the CB4 pentagonal face is capped by a μ5-thiocarbonyl unit [CS distance 1.729(4) A], attached by bonds of length CC 1.835(5) A [C(1) to C(2)], CB 1.742(6), 1.695(5), 1.704(6) and 1.749(6) A [C(1) to B(3), (4), (5) and (6)]. This slipped structure reflects delocalization of the anionic charge from the sulphur atom into the polyhedron, a delocalization supported by AM1 calculations and also reflected in 11B chemical shift data which show a pronounced antipodal effect.

X-ray structure and bonding of 1-phenylethynyl-2-phenyl-1,2-dicarbadodecaborane(12), [1-(PhCC)-2-Ph-1,2-C2B10H10], a model alkyne complex containing a rich variety of carbon-carbon bond types

Abstract The crystal and molecular structure of 1-phenylethynyl-2-phenyl-1,2-dicarbadodecaborane(12) (1), prepared by the reaction between 1,4-diphenylbutadi-yne and the decaborane adduct B10H12(NCMe)2 in boiling toluene, have been established by an X-ray crystallographic study. Evidence of some delocalization of pi-electronic charge in the phenylethynyl ligand towards the carborane cage is provided by the carboncarbon bond distances [1.431(2), 1.194(2) and 1.433(2) A, respectively from benzene ring to carborane isocahedron] and Molecular Orbital Bond Index (MOBI) calculations, which also reveal subtle differences in the bonding of the two skeletal carbon atoms to their boron neighbours in the icosahedron. Consideration of (1) as an adduct in which one alkyne function of PhCCCCPh coordinates to an arachno-shaped B10H10 residue reveals the strong electron-withdrawing capacity of the latter, which is compared with other borane and metal cluster residues.

X-Ray crystal structures of some lithium aryl thiolates: monomeric [o-MeC6H4SLi·(NC5H5)3], chain [PhSLi·(NC5H5)2]∞, and the unique folded ladder of [PhCH2SLi·NC5H5]∞ with Li–S rungs

The X-ray crystallographic study of the title compounds has shown a remarkable structural variation induced by minor changes in the anion, [PhCH2SLi·NC5H5]∞ forming an infinite folded ladder polymer with Li–S rungs, removal of the CH2 group to give [PhSLi·(NC5H5)2]∞ resulting in a polymer containing an infinite Li–S chain, while the introduction of an Me group into the ortho-position in [o-MeC6H4SLi·(NC5H5)3] prevents association; all three structures have tetrahedral lithium whereas the co-ordination around sulphur varies from 2 to 4.

Crystallographic evidence for the diene character of C2B10H10C4H4(‘benzocarbonae’) and a Diels–Alder reaction of its anionic nido-analogue, [C2B9H10C4H4]–: crystal structures of C2B10H10C4H4 and C2B10H10C4H6

Single-crystal X-ray diffraction studies on [C2B10H10C4H4](‘benezocarborane’) and C2B10H10C4H6(‘dihydrobenzocarborane’) show that the C6 rings of each posses localised double bonds and this has been confirmed by bond-order calculations: the nido-anion. [C2B9H10C4H4]– synthesised from C2B10H10C4H4, with maleic anhydride forms a Diels-Alder adduct.

SOME BORON-CONTAINING RING SYSTEMS

The synthesis and characterisation of series of new types of boron-containing ring systems are described. They include: (a) carborane systems containing C-H--X (X = O or N) hydrogen bonds; (b) systems in which Cn rings share C-C links with o-carborane units; (c) macrocycles containing 2,3 or 4 carborane icosahedra linked through benzene or pyridine rings, and (d) ‘new types of ‘carborazacycles’ containing one carbon, two boron and three nitrogen atoms in a single 6-membered ring system.

The first crystal structure characterization of an alkali metal monothiocarboxylate, (PhCOSLi·TMEDA)2(TMEDA = tetramethylethylenediamine): a chair-shaped eight-membered (COSLi)2 ring system

An X-ray diffraction study of the title compound, a TMEDA complexed lithium monothiobenzoate, has shown that it is a centrosymmetric dimer with an eight membered ring composed of coplanar C, O, and S atoms and tetrahedrally co-ordinated Li atoms above and below this plane; ab initio optimisation calculations on (HCOSLi)2, which is all-planar, imply that displacement of the atoms in the complex occurs in order to reduce excessively large ring angles at O and, most crucially, at Li, in order to accomodate the TMEDA donor.

Stability patterns in borane cluster chemistry rationalised by extended Hückel molecular orbital studies

Abstract Series of extended Huckel molecular orbital calculations have been used to probe stability patterns among borane clusters which provide models for many other types of cluster systems, including polyhedral metal carbonyls, organometallics and pyramidal carbocations. Questions addressed include: the way that the stabilities of closo systems Bn Hn2– vary with n; the existence of some neutral closo halides BnXn; the preference of carbon atoms for sites of low connectivity in carboranes; the preferred shapes of nido systems BnHn+4 etc. that leave vacant a high connectivity site of the parent polyhedron; the preferred shapes of arachno systems BnHn+6 etc. that leave vacant a second adjacent site; and the sites around the open face preferred by the endo (BHB bridging or extra BH terminal) hydrogen atoms in the nido and arachno systems. The relative energies, degeneracies and electron distributions of the frontier orbitals of these cluster systems (in which the more electronegative skeletal units —such as the CH units of carboranes—or endo hydrogen atoms in nido or arachno systems serve as guides to the regions of greater electron density) are seen to be of paramount importance throughout. Selected examples are used to illustrate the patterns discussed, including the families of 5- and 6-skeletal atom nido carboranes formally related to B5H9 and B6H10, in which the competing requirements of carbon and endo hydrogen atoms for sites of high electron density lead to interesting implications for the relative stabilities of possible isomers.