Carborane adducts

Scheme 15.3 Preparation of novel NHC-carborane adducts wherein all boron atoms are retained [72].

Davidson, M. G. Fox, M. A. Hibbert, T.G. Howard, J.A.K. Mackinnon, A. Neretin, I.S. Wade, K. Deboronation of ortho-carborane by an iminophosphorane crystal structures of the novel carborane adduct <7o-C2BioHi2 HNP(NMe2)3 and the borenium salt [(Me2N)3PNHBNP(NMe2)3]20 +(C2B9H )2, Chem. Commun., 1999, 1649. 

A/,A/-Dimethyiimidazoiidine, 360 A/,A/ -Diaryiimidazoiidine, 360 2,3-Dihydro-1,3-benzothiazoie, 361 Diethylamine Adduct, 361 A/-Methoxy-A/-methyiamine Adduct, 362 o-Carborane, 362... 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

Halide abstraction reactions with these carborane derivatives gives sometimes intermediate products rather than metathesis, as in the reaction of Ag(6,7,8,9,10-Br5CB9H5)(r/2-C7I Ix) with [IrCl(CO)(PPh3)] that yields an Ir—Ag adduct (238),1350 or the reaction of [MoCpI(CO)3] with Ag(CBnH12) which gives (239).I357 135 ... 

Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl-... 

Two carborane cations, zV/ -2,3,4,5,6-McsC5BBr+ and the tentatively proposed arachno-N ci WX I71, were reported. The identity and pyramidal structure of the pentacarbaborane cation was determined by X-ray crystallography.112 The small arachno-carborane was detected on the basis of ab initio computations and 11B NMR data in the mixture from reaction of the 2-propyl cation with diborane.113 Two neutral adducts based on the pentacarbaborane cation, ///V/9-2,3,4,5,6-McsCsBBKCI2 (R = Cl or SiClj), were made from MesCsSiMe3 or (MesCs Si with B2CI4, respectively.114 The BCI3 adduct was structurally characterized by X-ray crystallography. 

The solid-state structure of ort o-carborane was determined for the first time in 1996 featuring HMPA as a weak hydrogen bond adduct in the solid state.569 The geometry of ort o-carborane, I T BjoH, is a distorted icosahedron with a bond of 1.62 A between the cage carbons (C1-C2). This bond is remarkably elastic being elongated up to 1.85 A... 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

HMPA (hexamethylphosphoramide) OP(NMe2)3 4 does not deboronate the C2B10H12 isomers at room temperature but acts as an hydrogen bond acceptor for the acidic hydrogens at the carbon atoms of these clusters to yield 1 1 carboranerHMPA adducts on crystallization.6 These C-H--0 hydrogen bonded supramolecular structures provide the first definitive X-ray structures of the unsubstituted carboranes. 

Crystals formed from the reaction mixture were identified by X-ray crystallography as a 1 1 polymeric adduct of [H2NP(NMe2)3+]2C032"-B(0H)3. Unlike HMPA 4, the iminophosphorane 5 absorbs carbon dioxide and water in the presence of air. Base 5 converts or/Ao-carborane into the nido-anion just as effectively in the absence of air indicating that water and C02 are not vital ingredients in the deboronation process. 

Under nitrogen, a 1 1 ratio mixture of carborane 2 and base 5 in toluene at -20°C gave crystals identified as a 1 1 polymeric adduct 11.12 The crystal structure of 11 revealed the presence of two C-H—N hydrogen bonds per molecule. A 2 1 crystalline adduct 12, where one C-H—N hydrogen bond per base molecule would be expected, could not be obtained. 

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