Boranes, anions from

It has been estimated that using available neutron intensities such as 10 neutrons/(cm -s) concentrations of B from 10—30 lg/g of tumor with a tumor cell to normal cell selectivity of at least five are necessary for BNCT to be practical. Hence the challenge of BNCT ties in the development of practical means for the selective deUvery of approximately 10 B atoms to each tumor cell for effective therapy using short neutron irradiation times. Derivatives of B-enriched /oj o-borane anions and carboranes appear to be especially suitable for BNCT because of their high concentration of B and favorable hydrolytic stabiUties under physiological conditions. 

Alternative high-yield syntheses of these various boranes via hydride-ion abstraction from borane anions by BBra and other Lewis acids have recently been devised l (see p. 162). 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. 

Scheme 3.2-9. Synthesis of the simplest monocarba-closo-borane anion 15a starting from triborabicyclo[l. 1. OJbutane 13a, and the one-electron oxidation product of 15a.

In the above preparations of BioHm, B5H11, and Bi+Hio, abstraction of H from a borane anion gives an unstable neutral fragment which is believed to obtain a BH3 unit from another like fragment to form the desired borane product. One hundred per cent conversion of the starting material to the desired boron hydride cannot be achieved. On the other hand if BH3 could be "funneled" into the reaction mixture to react with the unstable intermediate, then 100% conversion could be achieved in principle. 

From the early structural studies carried out on boranes, it appeared that most of these compounds adopted structures based on arrangements of their boron atoms that defined fragments of icosahedra. That other triangular-faced polyhedra were important became apparent during the 1960s 141, 145, 164, 166, 212), as the structures of key materials such as the borane anions and carboranes C2B 2H were determined,... 

All these molecules contain 86 electrons associated with the valence shells of the 6 metal atoms (the core carbon atoms of the carbides are considered to contribute all their valence shell electrons to these clusters). In the hydride H2Rufl(CO)i8, for example, the metal atoms supply 48 electrons, the carbonyl ligands 36, and the hydrogen atoms 2. Formally, this hydride maj be regarded as derived from the anion [Ru(CO)3]g , which, in turn, can be shown to be formally related to the c(oso-borane anion RgHe " as follows (199, 200). 

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. 

Copper and Ag complexes of the higher boranes are prepared by reaction of the metal-halide complex with a borane anion, including " BjHg, B H, B H S, B,H(, BjqH(3 and Bj,Hj. Pentaborane(9) and hexaborane(lO) form the Cu complexes (PhjDjCuBjHg and (Ph3P)jCuBgH5 both are white, air-stable solids, formed in THF-CHjf Jlj from which they are precipitated by ether . ... 

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. 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

Borane Anions and Metallo-derivatiyes.—The derivatives (R4N)BH4 have been prepared conveniently and in good yield (R = Me, 71% R = Et, 81.5%) from KBH4 by ion exchange on cationic resins. ... 

The optically pure secondary phosphine boranes are easily deprotonated, giving extremely nucleophilic phosphide borane anions, which preserve the absolute configuration at the P atom at low temperatures. This fact prompted Danjo, Imamoto and co-workers to couple the phosphide boranes derived from 9 with aryl cation equivalents. In particular, they found that fluoroarene tricarbonylchromium complexes such as 17 were excellent substrates for the SNAr process due to the high electron deficiency of the arene ring and significant electrophilicity of the ipso carbon (Scheme 5.9). 

---