Template Synthesis of a Tetraaza Macrocycle Which Involves Benzaldehyde Rather Than Formaldehyde as a Building Block. Isolation and Structure Determination of the Open-Chain Schiff Base Intermediate Complex

The classical formaldehyde building block has been replaced by the bulkier benzaldehyde in the CuII template synthesis of the cyclam-like tetraaza macrocycle of type 1, in which nitroethane operated as locking fragment. The synthetic pathway involves three distinct steps:  (i) Schiff base condensation of the metal-free open-chain tetramine; (ii) CuII coordination and preorientation of the Schiff base; (iii) nucleophilic attack by the deprotonated nitroethane fragment and formation of the macrocyclic complex. Both the Schiff base CuII complex and the CuII macrocyclic complex were isolated in a crystalline form and their molecular structures were determined:  {N-[2-((E)-benzylideneamino)ethyl]-N‘-[2-((Z)-benzylideneamino)ethyl]propane-1,3-diamine}copper(II) nitrate:  triclinic, space group P1̄, with a = 12.296(5) Å, b = 10.787(6) Å, c = 10.547(7) Å, V = 1161(1) Å3, and Z = 2 (R = 0.055, Rw = 0.061); [(5R,6S,7S)-6-methyl-6-nitro-5,7-diphenyl-1,4,8,11-tetraazacyclotetradecane]copper(II) perchlorate:  monoclinic, space group P21/n, with a = 15.246(5) Å, b = 23240(7) Å, c = 8.540(4) Å, V = 2980(2) Å3, and Z = 4 (R = 0.095, Rw = 0.095). This allowed us to define mechanistic details of the macrocyclization process. It is suggested that the same three-step pathway takes place in the much easier and faster one-pot template syntheses of cyclam-like macrocyles, which involve formaldehyde as a building block.