Controlled translocation of palladium(ii) within a 22 ring atom macrocyclic ligand†
Double aza-Michael addition of n-butylamine to the two acrylamide groups of acyclic N2,N6-bis(6-acrylamidopyridin-2-yl)pyridine-2,6-dicarboxamide gives the corresponding macrocycle, H4L. H4L has potential coordination pockets associated with the 2,6-dicarboxamide (head) and the butylamine (tail) regions of the macrocycle. Depending on the conditions employed, macrocyclic complexes with palladium(II) coordinated to either the tail or the head of the macrocycle can be isolated. Thus, treatment of H4L with [PdCl2(NCPh)2] and sodium acetate, or [Pd(OAc)2] gives the closely related “tail-coordinated” complexes [PdCl(H3L)] (3a) or [Pd(OAc)(H3L)] (3b), respectively. However, employment of the bases 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or pyridine during the treatment of H4L with [Pd(OAc)2] results in the “head-coordinated” complexes [Pd(NH2R)(H2L)] (NH2R = N-(3-aminopropyl)caprolactam, which is formed by hydrolysis of DBU) (5) or [Pd(OH2)(H2L)] (6), respectively. Translocation of the palladium ion from the macrocycle tail in 3b to the head occurs on treatment with either DBU or N-(3-aminopropyl)caprolactam. In both cases the product 5 is formed. The aqua ligand in 6 is labile and easily displaced by the N-donor ligands n-butylamine, N-(3-aminopropyl)caprolactam or DBU to give the corresponding complexes [Pd(NH2nBu)(H2L)] (4), (5), or [Pd(DBU)(H2L)] (7). The data suggest that hydrolysis of DBU to produce the N-(3-aminopropyl)caprolactam ligand in 5 is catalysed by the acetic acid formed during ligand metallation rather than by coordination to palladium. The X-ray crystal structures of H4L, 3a, 5 and 6 are reported.