Studies of 2,5;6,10;8,10-tri-µ-hydro-nonahydro-nido-nonaborate(1–), [B9H12]–: preparation, crystal and molecular structure, nuclear magnetic resonance spectra, electrochemistry, and reactions

Abstract

Studies of 2,5;6,10;8,10-tri-µ-hydro-nonahydro-nido-nonaborate(1–)[B₉H₁₂]^– have involved new syntheses, which have led to crystals suitable for the determination of the X-ray crystal and molecular structure, through reactions of B₉H₁₃(SMe₂) with [OCN]^– or other bases. Crystallographic study of [N(PPh₃)₂][B₉H₁₂] reveals the structure of the anion to be that of a nido-nine-vertex cage, based on the parent bicapped square antiprism with one 5-connected vertex removed. The pentagonal open face is symmetrically bridged by three (µ-H) atoms (two involving the lowest connected boron), conferring effective C_s symmetry upon the polyhedron. Crystals are monoclinic, space group P2₁/n, with a= 26.759(10), b= 10.340(4), c= 27.044(5)Å, β= 103.47(2)°, and Z= 8. Using 6 777 diffracted intensities recorded at 185 K on an Enraf-Nonius CAD4 diffractometer, the structure has been refined to R 0.0847, R′ 0.1216. The ¹¹B, two-dimensional ¹¹B(COSY), and ¹H n.m.r. parameters all confirm the structural assignment and the n.m.r. spectra have been assigned unambiguously. Cyclic and a.c. voltammetry and coulometry of [B₉H₁₂]^– in several solvents at Pt led to the electrochemical parameters and to an isomer-specific electrochemical synthesis of anti-[B₁₈H₂₁]^–. Chemically, the [N(PPh₃)₂]⁺ salt of [B₉H₁₂]^– reacted with HCl to give [B₉H₁₃Cl]^–, whereas the [NBuⁿ₄]⁺ salt yielded anti- B₁₈H₂₂; reactions with CF₃CO₂H gave primarily anti-[B₁₈H₂₁]^–.