Boratabenzenes Structure and Bonding

Boratastilbene has recently been synthesized and structurally characterized.26 A study of the consequence of the isoelectronic B for C substitution on the pho-tophysics/photochemistry led to the conclusion that, due to intramolecular charge transfer arising from the inequivalent charge density of the two aromatic rings, nonaggregated boratastilbene is highly emissive relative to stilbene. 

The structure of MeIn(C5H5B-Me)2 has been explored, and it has been determined to be fluxional in solution on the NMR time scale, with average C2V symmetry.29 An X-ray crystallographic investigation has revealed soft structural parameters of the indium-ring coordination, consistent with the solution studies. 

In contrast to these adducts in which the boratabenzene ring is bound ti to the main-group metal, reaction of [C5H5B-Me]Li with PbCl2 affords a bent-sandwich complex, Pbfi/ -CsI LBMeh.31 This report provided the first structural characterization of an r 6-bonding mode to a p-block metal. Reaction of Pb(Ti6-C5H5BMe)2 with a Lewis base such as bipyridine leads to a complex wherein the bipyridine is bound in the pseudoequatorial plane. 

A very different mode of r 3 complexation has been observed for the Sc(III)-(bis)boratabenzene complex illustratedbelow.36 In this instance, a crystallographic study reveals that r 3 coordination to one of the boratabenzene rings occurs through the exo nitrogen, the boron, and the 2-carbon. In solution, the complex is fluxional. 

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