Synthesis, crystal structures, photoluminescence, electrochemistry and DFT study of aluminium(III) and gallium(III) complexes containing a novel tetradentate Schiff base ligand

Single crystals of the aluminium and gallium complexes of 6,6′-{(1E,1′E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenol), namely diaqua(6,6′-{(1E,1′E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ4 O 1,N,N′,O 1′)aluminium(III) nitrate ethanol monosolvate, [Al(C22H18N2O4)(H2O)2]NO3·C2H5OH, 1, and diaqua(6,6′-{(1E,1′E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ4 O 1,N,N′,O 1′)gallium(III) nitrate ethanol monosolvate, [Ga(C22H18N2O4)(H2O)2]NO3·C2H5OH, 2, were obtained after successful synthesis in ethanol. Both complexes crystallized in the triclinic space group P\overline{1}, with two molecules in the asymmetric unit. In both structures, in one of the independent molecules the tetradentate ligand is almost planar while in the other independent molecule the ligand shows significant distortions from planarity, as illustrated by the largest distance from the plane constructed through the central metal atom and the O,N,N′,O′-coordinating atoms of the ligand in 1 of 1.155 (3) Å and a distance of 1.1707 (3) Å in 2. The possible reason for this is that there are various strong π-interactions in the structures. This was confirmed by density functional theory (DFT) calculations, as were the other crystallographic data. DFT was also used to predict the outcome of cyclic voltammetry experiments. Ligand oxidation is more stabilized in the gallium complex. Solid-state photoluminescence gave an 80 nm red-shifted spectrum for the gallium complex, whereas the aluminium complex maintains the ligand curve with a smaller red shift of 40 nm.