N,N'-Bis-(2-methoxybenzylidene) adducts of 1,2-ethanediamine, 1,3-propanediamine and 1,4-butanediamine

We have isolated and crystallographically characterized the three homologous compounds N,N'-bis(2-methoxy­benzyl­idene)­ethane-1,2-di­amine (MeSalen), C18H20N2O2, N,N'-bis(2-methoxy­benzyl­idene)­propane-1,3-di­amine (MeSalpr), C19H22N2O2, and N,N'-bis(2-methoxy­benzyl­idene)­butane-1,4-di­amine (MeSalbu), C20H24N2O2. In contrast with MeSalpr, the mol­ecules of MeSalen and MeSalbu, which have an even number of methyl­ene units, have crystallographic symmetry. Comparing these methoxy-substituted species with their hydroxy equivalents shows that the aryl rings rotate upon removal of the O - HN hydrogen bonds. The packing of MeSalen and MeSalpr is controlled by C - Hπ interactions, whereas that of MeSalbu has only van der Waals contacts.