Calcium complexes of macrocyclic lactams: their structure and calcium induced conformational changes

We have examined the coordination sites of calcium complexes of macrocyclic bislactams and found that macrocyclic lactams are able to form solid complexes with Ca2+ ions located inside or outside the cavity depending on the ring's size. The X-ray structures for the complex [Ca(1)(H2O)(4)](ClO4)(2) (1-Ca2+) with 20-membered bisamide 1, where a calcium ion is located inside the cavity, and the complex [Ca(5)(2)(ClO4)(2)](n) (5-Ca2+) with 15-membered bisamide 5, where calcium ions are outside the cavity, are reported. Formation of the complex 1-Ca2+ causes conformational change in the ligand structure. The carbonyl oxygen donor groups change their orientation and rotate, becoming directed inside the cavity. Such a conformational change breaks hydrogen bonds within the ligand structure. An X-ray structure is presented as evidence for such a conformational flip. This confirms an earlier postulation for the structure of a 1-Pb2+ complex.