Clusters carborane

A particularly elegant route to metallacarbor-anes is the direct oxidative insertion of a metal centre into a c/oso-carborane cluster the reaction uses zero-valent derivatives of Ni, Pd and Pt in a concerted process which involves a nett transfer of electrons from the nucleophilic metal centre to the cage ... 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Several reports of carboranylated porphyrins have appeared for utilization in the BNCT of cancer.150 The examples include both nido- (126) (Fig. 73) and closo-carborane cluster-containing (124 and 125) (Fig. 73) materials. Metal-free nido-carboranylporphyrins were able to deliver a higher amount of boron to cells than the corresponding zinc complexes. 

Figure 73 The closo- (124 and 125) and nido- (126) carborane cluster-containing porphyrins for BNCT. (Adapted from ref. 150.)...

Figure 80 A first-generation organometallic dendrimer (145) containing grafted carborane clusters on tetra(terpyridine) cores. (Adapted from ref. 159.)... 

Although most known metallocarboranes have only 1 metal atom per polyhedron, the existence of an increasing number of metallocarboranes with 2 or more metal atoms per cluster emphasizes the close relationship between metal clusters and borane clusters. Since they can be synthesized from cZoao-carboranes by replacing BH units by metal-carbonyl or metal-cyclopentadienyl residues, carborane clusters can effectively be used as disposable templates on which to fabricate... 

Boron Hydrides Boron Inorganic Chemistry Boron Organoboranes Boron Polyhedral Carboranes Cluster Compounds Inorganometalhc Compounds Containing Transition Metal Main Group Elements Electronic Structure of Main-group Compounds. 

Although recent reviews of metal-carborane electrochemistry (37, 38) obviate the need to discuss multimetallic carborane clusters here, it seems appropriate to mention the results of studies on triple-decker sandwich compounds. The only structurally confirmed (39) all-hydrocarbon tripledecker, [Ni2Cp3] (18), has apparently not been investigated electro-chemically, but carborane analogues containing two or three boron atoms... 

Through this study we suggest a useful synthetic procedure for good candidate compounds of a new cage-cluster. The ort/io-carboranes are very stable chemicals in this research. Carboranyl acetic ester can be easily introduced to the ortho- carborane skeleton and then many types of cyclisation of carboranyl cluster can be made to occur. The reactivity and reaction mechanism for cyclisation of carborane cluster have been discussed. The mechanisms for boron cluster expansion reaction and cyclisation at carboranyl edge are summarized as follows. 

The solvent effect is high in the case of C-allylation to ortho-carborane cluster. In non-polar solvents the C-allylation is accelerated, while in polar solvents the C-allylation is decelerated. 

The reaction mechanism for the cyclisation at carboranyl edge is so suggested that the ft -carbon site in ort/io-carborane cluster reacts to give carbene intermediate in the first step, and in the second step the carbenoid intermediate is produced, which generates the final product at the end as shown in Scheme 2. 

In the presence of Pd (OAc >2, the cyclisation at carboranyl edge occurs effectively through the reaction of ortho-carborane cluster with diazomethyl ethyl ketone as shown in Scheme 4. 

However, totally different reaction products are formed when ortho-carborane cluster reacts with phenyl diazomethane in the presence of Pd(OAc>2 and without any other irradiation as shown in Scheme 5. 

As an alternative to the reductive generation of a nido borane or carborane cluster from a closo parent, one can regard a nido species as the product of decapitation or deboronation of a closo parent. Formal removal of a B kf unit, normally from a high connectivity site in a closo borane or carborane, followed by protonation of the anionic residue around the open face generated, leaves a neutral nido residue. 

The neutral chloride B4CLt, with four skeletal bond pairs, and tetrahedrane C4H4 or tetraphosphorus P4 and related molecules, with six skeletal bond pairs, are systems with regular tetrahedral structures. Tetrahedral molecules held together by six skeletal bond pairs can of course be accommodated in the carborane cluster systematics as nido clusters with n atoms and n -1- 2 skeletal bond pairs, if a low connectivity (axial) vertex of the parent trigonal bipyramid is left vacant, instead of the expected high-connectivity (equatorial) vertex. 

Figure 3.20. Incorporation of an iron atom in an icosahedral carborane cluster.

At the beginning of the 1970 s, although many molecular orbital calculations had been performed on individual borane and carborane clusters, no generalisations had been made which would result in a theoretical unification of the field. Initially most boranes were thought to adopt structures based on fragments of an icosahedron, but it later became apparent, as more borane clusters were structurally characterised, that other deltahedrally based structures were quite common70). Williams first elucidated a struc-... 

Mixtures of alkynes, cyclopentadiene and boranes with Co atoms yield metallo-carborane clusters in small amounts e.g., with 2-butyne and BjHg ... 

FIGURE 43 Crystal structure of tetranuclear carborane clusters [7j -l-Tb(THF) -2,4-(SiMe3)2-2,4-C2B4H4]4(/i-Cl)2(/i4-0) (redrawn after Wang et al., 2003, 2004). 

---