Anion borane

Safety considerations are paramount in any boron hydride synthesis. The energy yield from the oxidations of boron hydrides is too high for any cavalier treatment of boron hydrides. Exclusion of air is the critical consideration in diborane reactions. Decaborane(14) is less reactive, generally, in a kinetic sense, but the thermodynamic potential is comparable. In addition, all volatile boron hydrides are toxic. The procedures described in the latter two preparations are within our experience non-hazardous. These procedures should be followed in every detail improvisation is not recommended. 

The BsHs anion has been shown by x-ray analysis to have the following structure  

The hydrolytic stability of this anion is comparable to that of BH4. Acid completely destroys the BgHs ion to give hydrogen and boric acid  

Nonsolvated salts such as CsBgHs and (CH3)4NBgH8 are the most stable. Closely related to BgHg are the neutral base derivatives of B3H7, e.g., (CHg)3N-B3H7, which can be prepared from the B3Hg ion by displacement of the hydride ion  

The structure of the BuHi4 ion has not been established. A discussion of the spectral properties and the structural implications may be found in the original article by Aftandilian et al. Hydrolytically, BnHi4 is more stable than BH4. Salts of BnHi4 slowly oxidize in air, particularly if they are solvated (water or ether). 

Checked by J. L. BOONEf (BjHr and BiaHii2-), P. GARRETT,t and M. F. IIAWTHORNEJ (Bull,4-) 

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Other expansion reactions between diborane and borane anions with a B—B edge bond have been reported (79), for example... 

Closo Borane Anions. This group contains a homologous series of very stable polyhedral anions, [doso —, n = 6-12. Just as the... 

In aqueous solution, closo borane anions are very stable as their conjugate acids, which possess acidity similar to sulfuric acid, yet their chemistry is remarkably different. Large unipositive cations, such as Tl", Cs", Rb", [(CH2)4N]", and [(CH2)3S]", yield water-iasoluble salts of ... 

N HCl) are necessary for the hydrolysis of The [B22H22] anion is the most hydrolytically stable borane anion, withstanding even 3 NUCl... 

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). 

B3Hg prepared in this way was the first poly-borane anion (1955) it is now more conveniently made by the reaction... 

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 ... 

Over 50 neutral boranes, B H , and an even larger number of borane anions have... 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arac/tno-boranes tend to be more reactive (and less stable to thermal decomposition) than niiio-boranes and reactivity also diminishes with increasing mol wt. C/oio-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

If (2 == 0 the compound is a borane or borane anion rather than a carborane. If = 0 there are no H or Hendo this is the case for all c/( 5( -carboranes except for the unique octahedral... 

Three kinds of B containing species may be used to react with metal halides cyclic compounds, carboranes and borane anions. The carboranes especially form an enormous number of complexes. 

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. 

Metal atoms have fewer valence electrons than orbitals available for bonding and in this they resemble boron. The consequences of this idea are examined and it is shown that many metals with electronegativities in the range 1.6-2.4 (B = 2.0) can subrogate boron atoms as vertices in polyhedral clusters. Such metalloboranes are often much more stable than the parent boranes or borane anions. Not only can metals mimic boron in known cluster geometries but the flexibility thus introduced can lead to novel and previously unsuspected cluster geometries. The construction of macropolyhedral clusters containing 17-20 vertices is also described. 

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,... 

Fig. 1. Polyhedra that form the bases for the structures of clo o-borane anions BbH 2- and carboranes C2B -2H .

There are three main structural types, closo, nido, and arachno. Closo structures are adopted by borane anions ( = 6 -> 12),... 

The positions of the hydrogen atoms in boranes and carboranes are worth noting. In doso-borane anions the n BH units are aligned... 

For the higher borane anions although the sets of bonding... 

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