Ditopic hydridoborates and hydridoboranes: bridging ligands in coordination polymers and versatile hydroboration reagents

Abstract

Single crystals of the meta- and para-phenylene-bridged ditopic trihydridoborates (Li(THF)₂)₂[m-C₆H₄(BH₃)₂] and (Li(THF)₂)₂[p-C₆H₄(BH₃)₂] have been prepared and investigated by X-ray crystallography. The compounds turned out to be coordination polymers in which each trihydridoborate substituent is connected with one trihydridoborate substituent of a neighbouring monomer via two bridging Li(THF)₂⁺ ions. (Li(THF)₂)₂[m-C₆H₄(BH₃)₂] and (Li(THF)₂)₂[p-C₆H₄(BH₃)₂] suffer from poor solubility in all common non-protic solvents. Thus, a more soluble derivative of (Li(THF)₂)₂[p-C₆H₄(BH₃)₂], equipped with n-hexyl groups at the positions 2 and 5 of the phenylene ring, has been used for all further investigations (i.e., compound Li₂[6]). Treatment of Li₂[6] with Me₃SiCl in the presence of excess N(Me)₂Et leads to the abstraction of one hydride ion per boron atom under formation of the ditopic amine-borane adduct p-C₆H₂(n-hexyl)₂(BH₂–N(Me)₂Et)₂ (7). The compound turned out to be an efficient hydroboration reagent both for internal olefins (i.e., 1,5-cyclooctadiene) and terminal alkynes (i.e., tert-butyl acetylene) to give p-C₆H₂(n-hexyl)₂(9-BBN)₂ (8; 9-BBN = 9-borabicyclo[3.3.1]nonyl) and p-C₆H₂(n-hexyl)₂(B(C(H)C(H)tBu)₂)₂ (9), respectively.