Bisoxazaborolidines and boron complexes derived from tetradentate ligands: synthesis and spectroscopic studies

A series of tetradentate ligands were synthesized and their reaction with arylboronic acids was studied. Two classes of tetradentate ligands are discussed, which are mainly based on ONNO and ONOO donor sets and involve 2-aminophenol derivatives and Schiff bases. The reaction of phenylboronic acid with tetradentate ligands derived from 2-aminophenol leads to bisoxazaborolidines containing five-membered rings formed by CCNBO atoms, where the boron atoms have a trigonal geometry. In contrast, tetradentate ligands derived from Schiff bases lead to monomeric and dimeric boron complexes, in which the boron atoms have a tetrahedral geometry stabilized by an intramolecular N–B dative bond. The structures of two boron compounds were established by X-ray diffraction analysis.