Step-stool conformation of a cyclic Schiff base derived from ethylenediamine and heptane-2,4,6-trione: evidence for partial hydrolysis in metal coordination

A Schiff base derived from ethylenediamine and heptane-2,4,6-trione, namely, 5,9,14,18-tetramethyl-1,4,10,13-tetraazacyclooctadeca-5,8,14,17-tetraene-7,16-dione (C18H28N4O2), abbreviated H4daaden, was prepared and characterized for the first time by single-crystal X-ray diffraction. The atoms of the Schiff base occupy two different planes and thus the molecule is essentially nonplanar. An axis running through the C—C atoms of the ethylenediamine groups separate the two planes and these two planes are connected by bridging ethylene groups showing an angle of 117.34 (8)°. As a result, the side view of the molecule shows a `step-stool' conformation. The nonplanar nature of the Schiff base plays an important role in metal coordination, which leads to partial hydrolysis of the ring structure.