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-BrC 6 H 4 ) 3 B 3 O 3 (PBB) (3-NO 2 C 6 H 4 ) 3 - B 3 O 3 (MNB), and (3-NH 2 C 6 H 4 ) 3 B 3 O 3 (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 Et 2 O at room temperature. All 25 adducts have been characterised by elemental analysis, Mpt, IR and by 1 H and 13 C NMR. MNB and 4 of its adducts have been characterised by 1l B MAS NMR. In solution variable temperature 1 H 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/m 3 , and final R values [I > 2σ (I)] of R 1 = 0.0376, and ωR 2 = 0.0691 from 3786 independent reflections.