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

Reactions between Cd(NO 3 ) 2 ·6H 2 O 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, [{(H 2 O) 2 Cd( 3bpu ) 4 }·(NO 3 ) 2 ·2H 2 O·X] 1, [{(H 2 O) 2 Cd( 3bpu ) 4 }·(NO 3 ) 2 ] 3, [{(H 2 O) 2 Cd( 3bpu ) 4 }·(NO 3 ) 2 ·2H 2 O· p -xylene] 4, [{(H 2 O) 2 (NO 3 ) 2 Cd(μ-4 bpu ) 2 }] 8, and four coordination polymers, namely, [{(H 2 O) 3 (NO 3 )Cd(μ- 3bpu )}·(NO 3 )·H 2 O·1/2 o -xylene] n 2, [{(H 2 O) 3 (NO 3 )Cd(μ- 3bpu )}·(NO 3 )·X] n 5, [{(NO 3 ) 2 Cd(μ- 3bpu ) 2 }] n 6, [{(H 2 O)(NO 3 )Cd(μ- 4bpu ) 2 }·(NO 3 )·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.