New boronates prepared from 2,4-pentanedione derived ligands of the NO2 and N2O2 type – comparison to the complexes obtained from the corresponding salicylaldehyde derivatives

2,4-Pentanedione (=acetylacetone) has been reacted with 2-aminoethanol, 1,2-diaminoethane and 1,3-diaminopropane to give the NO2 and N2O2 type ligands named acacaminolH2, acacenH2 and acpenH2, which are structurally and electronically related to the corresponding ligands derived from salicylaldehyde (salaminolH2 and salenH2). On reaction of acacaminolH2 with phenylboronic acid a dinuclear monomeric complex has been obtained containing one three- and one four-coordinate boron atoms as well as one six-membered and one seven-membered heterocyclic ring. Since with salaminolH2 a dimeric complex with a central 10-membered heterocycle had been reported, it becomes apparent that there may be differences in reactivity when comparing 2,4-pentanedione and salicylaldehyde derived ligands. The molecular compositions of the boron complexes prepared from acacenH2 and acacpenH2 are analogous to the corresponding salen and salpen derivatives, however, the presence of two methyl groups in the six-membered chelate rings generates some structural changes, the most important being the distortion of the boat conformation of the central heterocyclic ring. This was predicted by computational methods and confirmed experimentally for one of the complexes. A further important observation was that the products described in here are much more soluble than the salicylaldehyde derivatives. As lateral product the adduct of acacenH2 with 1,3,5-triphenylboroxine was crystallized. Elemental analysis, IR and NMR (1H, 13C, 11B) spectroscopy, mass spectrometry, ab intio theoretical calculations and X-ray crystallography have been applied to carry out this study.