Complexation of Boron and Aluminum with a Bidentate Hydroxy-BN-naphthalene Ligand

Abstract

The isoelectronic relationship of 1,2-azaborinine (B=N structural motif) and benzene (C=C) is well documented. Upon deprotonation of the former, the anionic 1,2-azaboratabenzene is obtained, which is isosteric with pyridine (C=N) and has a similar capability as an aromatic N-donor. We present the complexation of boron and aluminum precursors with a κ2-N,O-donating 8-hydroxy-BN-naphthalene ligand (H2(BQ), 1). Six chelate complexes with 1:1 and 2:1 stoichiometries were isolated and characterized by X-ray diffraction analysis and NMR spectroscopy. Comparing the isosteric dimethylaluminum complexes of H2(BQ) and an 8-hydroxyquinoline (HQ’, 2) as a reference allowed us to quantify the influence of a formal substitution of carbon by boron on the structure and the electronic properties: While the structural parameters of the ligands were similar, the electropositive boron atom affected the electron density distributions within the complexes substantially. As the consequence, the Al–N bond was significantly shortened, and the aluminum atom showed a different coordination geometry than in the quinoline analog. Moreover, strong hypsochromic shifts of both the absorption and the emission were observed. The results highlight that the differences between CN and BN polyaromatic complexes are more distinct than between equally charged BN and CC congeners.