Polyhedral borane anions

Another class of molecules that will be discussed contains cluster compounds such as the polyhedral borane anions, B H -", and some metal containing species such as the metal carbonyl clusters. 

Much of the current literature on metal atom cluster species employs bonding concepts that are derived from MO treatment of the polyhedral borane anions, We thus begin by discussing these species, of which the most important examples are shown in Figure 8.15. We shall deal with the BaHg" ion in detail to illustrate the general approach to these systems. 

Boranes are recognized as clusters of boron atoms which represent triangularfaced polyhedra. It was Williams62 in 1970 who pointed out in a classic paper that the structures of all boranes are derived from those of the closed polyhedral borane anions, or carboranes, with vertices ranging from 5 to 12. These regular polyhedra are shown in Fig. 5 and represent the structures of the closo-boranes (or carboranes) from which all other borane structures are derived. 

Subsequent to Stock s work, many additional hydrides have been prepared and characterized, including a number by R. Schaeffer and associates, and today about 35 neutral binary boranes (including isomers) are known (Table 5-2). Many derivatives containing organic or halo substituents have been synthesized, and thousands of other species have been generated via insertion of metal or nonmetal heteroatoms into borane frameworks as described in Section 5-6. The polyhedral borane anions (see below) add yet another dimension to the remarkably rich area of polybo-rane chemistry. 

CAMOUFLAGED AND TOTALLY FUNCTIONALIZED CARBORANES AND POLYHEDRAL BORANE ANIONS... 

Polyhedral hydroborate anions (commonly referred to as polyhedral borane anions in the current literature) of generic formula have assumed an unusual status in inorganic chemistry within recent years, not only because of their high thermal and hydrolytic stability, but also because of the wide range of geometries which the boron atom skeletons in compounds of the series (n = 6-12) can adoptd Convenient syntheses... 

Octahydrotriborate (1 — )ion, [B3H8], is an important intermediate in the synthesis of higher boranes, polyhedral borane anions, and transition-metal complexes. Salts containing this ion have usually been prepared1 from diborane, which is toxic, spontaneously flammable, and expensive. In addition, pressure facilities are often required for the synthesis. 

The semitopological scheme does not always provide the best description of bonding in the boranes and related species such as the polyhedral borane anions and carboranes to be discussed below. Where there is symmetry of a high order it is often more convenient and conceptually simpler to think in terms of a highly delocalized molecular-orbital description of the bonding. For instance, in B5H9 where the four basal boron atoms are equivalently related to the apical boron atom, it is possible to depict a resonance hybrid involving the localized B""ti B and B—B elements, viz. ... 

Besides the polyhedral borane anions and carboranes, the closo species, there are others of importance that lack the closure. There are several nest-like or m tfocarboranes, such as 4,5-B4C2H8 which has been mentioned and there are two important small borane anions, BH4 and B3H8. 

Another important non-polyhedral borane anion is BjHg, most easily prepared by the reaction... 

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)... 

While visiting Harvard in the spring of 1959,1 talked with Roald Hoffmann about programming extended Hiickel theory for the new IBM computer which I influenced Harvard to obtain. It was only after he spent a year in Moscow that he returned to start the molecular orbital work which was first successfully used on the polyhedral borane anions, and now has been applied to many parts of the periodical table, including (years later) tests during the development of the... 

XXVlll. New polyhedral borane anions, BgHg , BgHg , and Inorg Chem 6 ... 

E. M. Georgiev, K. Shelly, D. A. Feakes, J. Kuniyoshi, S. Romano, M. Hawthorne, Synthesis of amine derivatives of the polyhedral borane anion [BjoHig] ", Inorg. Chem., 1996, 35, 5412-5416. 

Design and Development of Polyhedral Borane Anions for Liposomal Delivery... 

Hawthorne and coworkers developed a working hypothesis based on the biodistributions of the [BioHio] anion and the isomers of the [B2oH,g] ion polyhedral borane anions with the potential to form covalent bonds to intracellular protein moieties would be retained within the tumor cells after liposomal delivery, while polyhedral borane anions which are not capable of reacting with intracellular protein moieties would be cleared from all tissues (Shelly et al., 1992). This hypothesis was supported by the biodistribution of liposomally encapsulated [B2oHi9] and, in a separate experiment, the [B2oHi70H] ion. 

Unencapsulated [fle-B2oHi7NH3] - with a dose of 175 J,g B ( 10 mg/kg body weight), was investigated in a biodistribution experiment. Althongh the unencapsulated compound exhibited a higher propensity for tumor retention than other unencapsnlated polyhedral borane anions, the high tumor boron concentrations observed with liposomal encapsulation are clearly a result of the specificity of the liposomal delivery. 

Sulfhydryl derivatives of polyhedral borane anions have been of interest to BNCT researchers for many years because of the mercaptoundecahydro-c/o5t -dodecaborate dianion ([B,2H SH] , abbreviated borocaptate sodium (BSH) that has been used in a variety of preclinical and clinical BNCT trials (Hawthorne, 1993 Soloway et al., 1998 Hawthorne and Lee, 2003). Although only the free sodium salt of the [B,2H SHp has been used in the trials researchers have investigated the biodistribution of BSH encapsulated in liposomes (Shelly et al., 1992 Maruyama et al., 2004 Doi et al.. 

The [B2oHj7SH] - ion has the potential to react with intracellular protein substituents through a variety of mechanisms. The ion has the potential to form disulfide bonds in a manner similar to that proposed, but not observed, for the BSH ion (Figure 12.10). Formation of the disulfide bonds would covalently bond the polyhedral borane anion within the tumor cells. 

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