The Chemistry of Borabenzenes

In the 16 years since the discovery of the first borabenzene derivative, the chemistry of borabenzene metal complexes has lost its pioneering... 

The chemistry of borabenzene metal complexes has largely remained the domain of a single research group. We hope that this review will provide stimuli and orientation, especially to those who intend to contribute to the further development of this area. 

Boratabenzenes represent a second important class of unsaturated boracycles. " While the dicoordinate borabenzene itself has not yet been reported, many Lewis base adducts, boratabenzene salts, and metal complexes are known. Boratabenzenes are six r-electron aromatic anions that resemble cyclopentadienides. For example, (V, A-diisopropyl-1-amino-boracyclohexadiene shows an acidity in DMSO which is similar to that of cyclopentadiene. The chemistry of boratabenzene ligands dates back to work by Herberich and Ashe in the early 1970s, which has been reviewed. However, many new achievements have been reported over the last decade and will be discussed in the following sections. 

Reaction of the same neutral borabenzene-ligand adduct, C5H5B-PMe3, with a transition, rather than an alkali, metal alkyl or amide can furnish r 6-boratabenzene complexes in a single step (Scheme 8).17 This efficient transformation presumably proceeds through initial ir-coordination of CsHsB-PMes to the transition metal, followedby an intramolecular substitution reaction. In contrast to other approaches to the synthesis of T 6-boratabenzene complexes, this synthetic route does not have a parallel in if-cyclopentadienyl chemistry. 

The discovery of 1 (1), in 1970, opened a new and fascinating chapter of organometallic chemistry. This cation was the first compound derived from the hypothetical borabenzene 2 and the first complex of a classical boron-carbon ligand. Since then approximately 100 borabenzene derivatives, mainly complexes of 3d metals, have been characterized. Other unsaturated boron-carbon systems have been shown to act as ligands to metals (2). This development has also strongly stimulated the challenging quest for the simple species 2-5. 

The characterized borabenzene metal compounds are listed in Table V. In this section the various entries into borabenzene-metal chemistry are described Sections VI and VII treat the reactions of these compounds. 

Borabenzene metal complexes, like their cyclopentadienyl counterparts, are not readily amenable to breaking of the metal-ligand bond. Such bond breaking will occur more easily in complexes with antibonding electrons. By fortuitous chance the cobaltocene route to borabenzene chemistry (Section V,A) provided 19-e complexes with the useful property of inherently weakened metal-ligand bonds. Thus most of this section centers on the reactivity of Co(C5H5BMe)2 (7) and Co(CsH5BPh)2 (13). 

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