Metal-ion-controlled transamination in the synthesis of macrocyclic Schiff-base ligands. Part 2. Stepwise synthesis, ring expansion/contraction, and the crystal and molecular structure of a ten-co-ordinate barium(II) complex
Reaction of 2,6-diacetylpyridine with diethylenetriamine in the presence of salts of the alkaline-earth metal ions Mg2+–-Ba2+ in methanol at room temperature yields complexes of the open-chain Schiff-base ligand L1 derived from the condensation of one molecule of 2,6-diacetylpyridine with two molecules of diethylenetriamine. The use of higher reaction temperatures affords complexes (MgII excepted) of a macrocyclic ligand L2 derived from the [2 + 2] condensation of two molecules of diketone with two molecules of diprimary amine, provided the amine is not present in excess. The intermediacy of the L1 complexes in the L2 macrocycle formation is demonstrated by their subsequent ring-closure reactions both in the presence and absence of added diketone. A transamination mechanism is suggested for the ring closures in the absence of diketone. Crystals of [BaL2(ClO4)2] aremonoclinic with a= 19.427(12), b= 9.320(8), c= 9.768(11)Å, β= 116.1(1)°, Z= 2, and space group P21/a. 4 420 Reflections above background have been measured by diffractometer of which 2 312 were used in refinement to R 0.075. The macrocycle L2 contains an 18-membered inner large ring and not the expected 24-membered ring L3. The smaller (di-imine) ring may be considered as being derived from the larger (tetraimine) ring by nucleophilic addition of the two secondary amine groups across adjacent azomethine bonds with accompanying expulsion of two imidazolidine rings from the inner large ring. The Ba2+ ion is ten-co-ordinate being bonded to six nitrogens of the inner large ring in approximately planar array [Ba–N 2.841 (7)–-2.891 (8)Å] and to four oxygen atoms of two perchlorate groups [Ba–O 2.923(10), 3.060(11)Å]. The single co-ordinated alkaline-earth metal ion in the L2 complexes may be replaced by two copper(II) or two silver(I) ions with accompanying expansion of the macrocycle to the 24-membered (tetraimine) form L3. The reversibility of the L2⇌ L3 ring expansion/contraction has been established.