Isomerism in Coordination Complexes and Polymers Derived from Bispyridylurea Ligands:  Effect of Solvents, Conformational Flexibility, and Positional Isomerism of the Ligands

Reactions between Cd(NO3)2·6H2O and N,N‘-bis(3-pyridyl)urea (3bpu)/N,N‘-bis(4-pyridyl)urea (4bpu) with a 1:2 metal:ligand ratio under various solvent systems (polar/nonpolar) afforded four discrete coordination complexes, namely, [{(H2O)2Cd(3bpu)4}·(NO3)2·2H2O·X] 1, [{(H2O)2Cd(3bpu)4}·(NO3)2] 3, [{(H2O)2Cd(3bpu)4}·(NO3)2·2H2p-xylene] 4, [{(H2O)2 (NO3)2Cd(μ-4bpu)2}] 8, and four coordination polymers, namely, [{(H2O)3(NO3)Cd(μ-3bpu)}·(NO3)·H2O·1/2o-xylene]n 2, [{(H2O)3(NO3)Cd(μ-3bpu)}·(NO3)·X]n 5, [{(NO3)2Cd(μ-3bpu)2}]n 6, [{(H2O)(NO3)Cd(μ-4bpu)2}·(NO3)·EtOH·nitrobenzene]n 7, which were mainly characterized by single-crystal X-ray diffractometry. The effect of solvents, conformational flexibility, and positional isomerism of the ligands on the various isomerisms (supramolecular isomerism and polymorphism) is discussed.