π-Charge-Transfer Complexes of Hexamethylborazole

Abstract

BORAZOLE has been described as inorganic benzene. It might be expected¹ that appropriately substituted borazoles would form complexes similar to the well-known π-molecular complexes of benzene and its derivatives². Recently it has been observed³ that mixtures of hexamethylborazole and tetracyanoethylene in chloroform show a broad featureless absorption band (λ_max = 561 mµ) not possessed by either component alone. It was suggested that hexamethylborazole, acting as an electron donor, forms a π-π-charge-transfer complex with the electron acceptor tetracyanoethylene. Other electron acceptors should also form such complexes with hexamethylborazole. Unfortunately complexes with hexamethylborazole are relatively weak: the association constant (K) for the tetracyanoethylene complex is 0.7 l.mole⁻¹ in chloroform at 21°, compared with K = 27.6 l.mole⁻¹ for the corresponding hexamethylbenzene complex. The only other similar interaction which was reported was with the electron acceptor chloranil. In this case, instability of the complex, and the position of the charge-transfer band which overlapped a transition of ehloranil itself, made the definite determination of the charge-transfer band impossible.