A new route to isomers of icosaborane(26), B20H26. The use of 115.5-MHz 11B and 11B-{1H} nuclear magnetic resonance spectroscopy for the comparison and characterisation of separated isomers and the identification of three further icosaboranes as 1,2′-, 2,5′-, and 5,5′(or 5,7′)-(B10H13)2

Abstract

Irradiation of crystalline B₁₀H₁₄ in the beam of a 3-MeV van der Graaf generator, followed by various chromatographic separations, has been developed as a new route to several individual isomers of the conjuncto-borane, (B₁₀H₁₃)₂. The high-field (115.5-MHz)¹¹B and ¹¹B-{¹H(broad-band noise)} n.m.r. spectra of the previously characterised icosaboranes 2,2′-, 2,6′-, 6,6′-, and 1,5′-(B₁₀H₁₃)₂ have been recorded and assigned. Comparison with the spectra of three new isomers isolated from the u.v. photolysis of B₁₀H₁₄ in cyclohexane solution and from the high-energy electron bombardment of solid B₁₀H₁₄ has led to further structural characterisations: two of the new isomers are identified as 1,2′- and 2,5′-(B₁₀H₁₃)₂ and the third is 5,5′-(B₁₀H₁₃)₂ or its meso-diastereoisomer 5,7′-(B₁₀H₁₃)₂. Analytical high-pressure liquid chromatography has been used to compare the isomeric composition of mixtures of icosaboranes produced by the above routes and by the catalytic thermolysis of B₁₀H₁₄, and the results permit further discussion on possible mechanisms of formation of bi(nido-decaboranyl). The melting points of individual isomers have been determined as follows: 1,2′ 139–142; 1,5′ 114–115; 2,2′ 178–179; 2,5′ 109–111; 2,6′ 154–155; 5,5′(or 5,7′) 97–98; and 6,6′-(B₁₀H₁₃)₂ 198–199 °C.