Structural and Electronic Properties 1121 Borane Clusters

Borane clusters have been classified according to the topology of their molecular frameworks. Accordingly, closo boranes (from the Greek clovos, meaning cage) are closed polyhedra, nido (from the Latin nidus, meaning nest) boranes are semi-closed polyhedra, and arachno (from the Greek arachne, meaning spider's web) boranes possess open structures [6]. Both nido and arachno boranes are neutral specie whereas closo boranes are stable as dianions of chemical formula [Bn Hn ]2-, with n = 5-12. Figure 11.1 displays the crystal structures of nido-B5H9 (GAC-WOW), nido-B10H14 (FUYYIH02), arachnoBHn (FUYYUT) closo-[B6H6]2-(GIGRIX, Oh), and closo-tB^H^]2- (GAZLEY, Ih). The polyhedral clusters are often called deltahedra for their faces are made of triangular B3 units. The experimentally determined B-B bond distances in both nido and arachno boranes are in the range from 1.77 to 1.98 A. That of closo-[B6H6]2- is at ~1.77 A and the B-B distance in closo-[B12H12]2- is at ~1.8 A.

In comparison to carbon, boron is said to be electron-deficient inasmuch as its valence shell is made of four atomic orbitals (one s and three p) occupied by three electrons [6]. It follows that chemical bonding in boranes cannot be described only with the use of two-center-two-electron (2c-2e) interactions but

GACWOW

FUYYUT

GACWOW

FUYYIH02

FUYYUT

FUYYIH02

GIGRIX

GAZLEY

Figure 11.1. Crystal structures of some borane clusters.

GIGRIX

GAZLEY

Figure 11.1. Crystal structures of some borane clusters.

one must resort on three-center-two-electron (3c-2e) interactions as well. The latter were introduced by Christopher Longuet-Higgins in 1949 and Lipscomb further developed a qualitative theory of localized bonding interactions in boranes [6]. This theory employs the 2c-2e interactions to describe both B-H and B-B bonds whereas the 3c-2e interactions are necessary for describing the chemical bonding within both B-H-B and B-B-B bridges as well that in the B3 units of the deltahedron. Theoretical arguments based on molecular orbital (MO) theory are more powerful in describing the electronic structures of closo boranes of high symmetry such as closo-[B12H12]2-. With the advent of fast computers and efficient algorithms, structurally complex boranes and their derivatives are amenable to quantitative theoretical treatment with the aid of computational quantum chemistry methods [14].

Functionalization of borane clusters has been successfully achieved following a variety of synthetic routes [6]. Particularly interesting are the derivatives of closo-[B12H12]2- some of which are shown in Figure 11.2. Both mono- and disubstituted derivatives have been prepared. An interesting compound is PAVWUE which contains a 1,2-oxalate dianion coordinated via two of its oxygen atoms to one edge of the polyhedron. The existence of many crystal structures, several of which are polymeric, where transition metals are coordinated to either side of the 1,2-oxalate dianion suggests the possibility of using this building block in the preparation of novel nanostructured materials.

The poly-functionalization of closo-[B12H12]2- has been achieved in closo-[B12F12]2- (GUZFOW) and closo-[B12Me12]2-(XETBON), whose crystal structures are shown in Figure 11.3. Hawthorne's group has recently discovered that the oxidation of closo-[B12H12]2- with hydrogen peroxide yields closo-[B12(OH)12]2-(JOQVAM) in which the OH groups have fully replaced the H atoms of the starting compound [15].

R1KRIJY (X-NH3)

RILQEI (X=Phenyl) HIZYUK (Y=CO) PAVWUE

SEPB1Y (X=CH3) HI/ZAR (Y=C02H2) ■I'AKRA/ (X=SCN) FADMUT(X=OH)

R1KRIJY (X-NH3)

RILQEI (X=Phenyl) HIZYUK (Y=CO) PAVWUE

SEPB1Y (X=CH3) HI/ZAR (Y=C02H2) ■I'AKRA/ (X=SCN) FADMUT(X=OH)

Figure 11.2. Some derivatives of closo-[B12H12]2-. Each vertex of the polyhedron represents a BH moiety.

This novel reaction paves the way towards the synthesis of polyfunctionalized borane clusters (closomers) bearing large organic groups attached to the B12 core. With diameters greater than 1 nm, these novel compounds can be considered true molecular nanoparticles. We discuss them in detail later on in this chapter. Further information on the preparation and reactivity of borane compounds can be found in the more specialized chemistry literature [6].

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  • semhar
    How is the structure of closo borane (B12H12)?
    9 months ago

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