Unusual ligand isomerization dictated by coordination requirements of the metal ion: A double-bond shift in Cu(II)-assisted template condensation between diacetylpyridine and a tripodal tetramine

This paper reports a new effect in template-directed Schiff base condensati on, a "fine-tuning" of the macrocyclic structure in accordance with the pre cise coordination requirements of the metal ion. The nature of the metal te mplate determines the outcome of Schiff base condensation between a difunct ional carbonyl component (2,6-diacetylpyridine) and a trifunctional amine c omponent (tris(3-aminopropyl)amine, trpn). Both Cu(II) and Ni(II) facilitat e [1 + 1] condensation with the formation of macrocyclic products in 50-60% yield, hut single-crystal X-ray diffraction reveals that the chemical stru cture of the major product is metal-ion dependent. In the nickel(II) comple x [Ni(L-sym)](2+), the C=N double bonds are conjugated with the pyridine ri ng, as expected from the structure of the starting diacetylpyridine. In con trast, the copper(TI) ion caused isomerization of the macrocyclic ligand, w ith one of the C=N double bonds migrating into the initially saturated six- membered chelate ring forming [Cu(L-asym)](2+). The second product isolated , in small quantities, from the copper(II)-templated condensation has been characterized as an isomeric symmetric macrocycle [Cu(L-sym)](2+) In the ca se of the zinc(II) template, no macrocyclic product has been isolated; inst ead, a zinc(II) complex of the starting tetramine, Zn(trpn)Cl-2. 2H(2)O, wa s recovered and structurally characterized. An unusual double-bond migratio n in the course of Cu(II)-directed template condensation is governed by the coordination requirements of the central metal ion. The asymmetric ligand isomer is more flexible than its symmetric counterpart and can therefore be tter accommodate the five-coordinate central metal ion. This effect is more pronounced for the copper(II) complex than for the nickel(II) complex, bec ause of shorter in-plane M-N distances in the former, as follows from the X -ray structure determination. The position of the double bond in the macroc yclic ring influences the spectral properties of the copper(II) complexes a nd the ionization constants of the amino group in the side arm.