Borane clusters metallacarboranes

The structures of a representative cross-section of known carboranes are shown in Figures 3.2-3.6. Though many new carboranes have been synthesized and characterized since the borane/carborane structural pattern was hrst described,these have tended to consolidate rather than greatly extend the picture. A little progress has been made with the synthesis of macropoly-hedral borane clusters in which fused smaller polyhedra share edges or faces.This area need not concern us here, as it almost exclusively involves boranes and metallaboranes rather than carboranes and metallacarboranes. 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

In Chapter 15 we observed that the 18-eleciron rule was adequate for predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us to make predictions about borune structures based on the number of framework electrons. These rules al.so are adequate for most carboranes, metallacarboranes, and other heteroboranes.Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters. 9... 

In the area of synthesis, the structural and theoretical framework contributed by Lipscomb provided a foundation for the monumental synthetic work of M. F. Hawthorne and his students, who transformed boron cluster chemistry first by their synthesis of the polyhedral borane anions BioHio and Bi2Hi2 (30) and later by the synthesis of metallacarboranes (31). In our own laboratory, considerations of charge distribution in borane anions (14) prompted us to explore reactions of transition metal ions with small borane and carborane anions (32). This, in turn, led to a number of advances including the first closed polyhedral metallaboranes (33), the discovery of metal-induced oxidative cage fusion (34), and the synthesis of B12H16, the only neutral Bi2 hydride, via metal-promoted fusion of BeHQ (35)... 

Boranes and carboranes are comprised of a-aromatic cage-like structures with boron and carbon vertices ranging from small tetrahedral to supra-icosahedral clusters. Despite more than four decades of work with this class of inorganic compound, there have been few reports of their photophysical properties, with a distinct paucity of information regarding luminescence. Carboranes have nevertheless been incorporated into photophysically active metal complexes, usually as ancillary components. But what of boranes, carboranes, and metallacarboranes (where a transition metal atom can either be an integral vertex in the polyhedral skeletal framewoik or an exterior component thereof) as chromophores themselves and their resultant optoelectronic behavior In this arena, there are very few contributors. 

In this chapter, we report herein on large high-boron-content molecules that contain several icosahedral borane, carborane, and metallacarborane clusters either as part of the scaffolds or as decorating the periphery of platforms. On purpose, we will not deal with in this chapter on polymers containing boranes, carboranes, and metallacarboranes. 

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