Cage expansion reactions

Scheme 9 Proposed cage expansion reactions leading to a ferrapentacarbaborane. Reproduced by permission of the American Chemical Society from Organometallics 1995, 14, 4463.

The syntheses of the heteroboranes often involves the addition of heteromolecules to nido-boranes, in so called cage expansion reactions. Two of the most used borane precursors are pentaborane(9) (nido-BsHg) and decab-orane(14) (niT/o-BioHn) both of which are shown in Figure 2. 

In the reactions of 10.13a with alkali metal terr-butoxides cage expansion occurs to give the sixteen-atom cluster 10.15, in which two molecules of MO Bu (M = Na, K) are inserted into the dimeric structure. The cluster 10.13a also undergoes transmetallation reactions with coinage metals. For example, the reactions with silver(I) or copper(I) halides produces complexes in which three of the ions are replaced by Ag" or Cu" ions and a molecule of lithium halide is incorporated in the cluster. ... 

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. 

Cage closure from nido or arachno to closo commonly occurs on heating, though disproportionation may occur simultaneously [Eq. (3.4)]. Reaction with diborane can be used to effect cage expansion [Eq. (3.5)] ... 

Cage expansion of the carborane nido-2,3- C2H )2C2B4Hq results from reaction with BH3-N(C2Hs)3 at 140 C to form ctoso-2MO2 5)202BsHs [17]. 

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. 

Pentametal boride clusters feature far less frequently in the Uterature than do tetrametal or hexametal species, and systematic expansion of an M4B to M5B or M4M B cage is difficult reaction pathways tend to continue through to the species in which the boron atom is fully encapsulated.During the formation of [Fe4Rh2(CO)i6B], Fehlner had observed the formation of an intermediate species proposed as an Fe4RhB cluster, but it eluded isolation. 

Hydrido-clusters are usually prepared by protonation of anions. For example, the reaction of [Ru3(CO)i2] with aqueous KOH in THF solution results in cluster expansion to the octahedral dianion [Ru6(CO)ig] , protonation of which gives the monoanion [Ru6H(CO)i8] (in which the single H atom resides inside the Ru6 cavity), or the neutral dihydrido-cluster [H2Ru6(CO)ig] (in which the hydride ligands cap opposite faces of the octahedral metal cage), depending on the reaction conditions. 

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