Synthesis and Characterisation of Amine Adducts of Tri(4-bromophenyl)boroxine,Tri(3-nitrophenyl)boroxine,and Tri(3-aminopheny1)boroxine;Molecular Structure of 3-Picoline·tri(4-bromopheny1)boroxine

Abstract

Twenty five 1:l adducts of the triarylboroxines (4-BrC₆H₄)₃B₃O₃(PBB) (3-NO₂C₆H₄)₃- B₃O₃ (MNB), and (3-NH₂C₆H₄)₃B₃O₃ (MAB) with N donor ligands (cyclohexylamine, 4- picoline, 3-picoline, piperidine, morpholine, isobutylamine, triethylamine, pyridine, isoquinoline, benzylamine) have been prepared by reaction of stoichiometric quantities of ligand and triarylboroxine in Et₂O at room temperature. All 25 adducts have been characterised by elemental analysis, Mpt, IR and by ¹H and ¹³C NMR. MNB and 4 of its adducts have been characterised by ^1lB MAS NMR. In solution variable temperature ¹H NMR of selected PBB derivatives and of MNB·cyclohexylamine indicate that a ligand dissociation-recombination process is occuring with ΔG⁺ of ca. 39–54 kJ·mol⁻¹. The crystal and molecular structure of 3-picoline·PBB has been determined by a single-crystal X-ray diffraction study. The colourless compound crystallises in the orthorhombic space group (Pbca) with a = 14.1710(10), b = 15.446(5), and c = 22.418(7) A, with Z = 8, calculated density = 1.737 Mg/m³, and final R values [I > 2σ (I)] of R₁ = 0.0376, and ωR₂ = 0.0691 from 3786 independent reflections.