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<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O and <i>N</i>,<i>N</i>‘-bis(3-pyridyl)urea (<b>3bpu</b>)/<i>N</i>,<i>N</i>‘-bis(4-pyridyl)urea (<b>4bpu</b>) with a 1:2 metal:ligand ratio under various solvent systems (polar/nonpolar) afforded four discrete coordination complexes, namely, [{(H<sub>2</sub>O)<sub>2</sub>Cd(<b>3bpu</b>)<sub>4</sub>}·(NO<sub>3</sub>)<sub>2</sub>·2H<sub>2</sub>O·X] <b>1</b>, [{(H<sub>2</sub>O)<sub>2</sub>Cd(<b>3bpu</b>)<sub>4</sub>}·(NO<sub>3</sub>)<sub>2</sub>] <b>3</b>, [{(H<sub>2</sub>O)<sub>2</sub>Cd(<b>3bpu</b>)<sub>4</sub>}·(NO<sub>3</sub>)<sub>2</sub>·2H<sub>2</sub>O·<i>p</i>-xylene] <b>4</b><b><i>,</i></b> [{(H<sub>2</sub>O)<sub>2</sub> (NO<sub>3</sub>)<sub>2</sub>Cd(μ-4<b>bpu</b>)<sub>2</sub>}] <b>8</b>, and four coordination polymers, namely, [{(H<sub>2</sub>O)<sub>3</sub>(NO<sub>3</sub>)Cd(μ-<b>3bpu</b>)}·(NO<sub>3</sub>)·H<sub>2</sub>O·1/2<i>o</i>-xylene]<i><sub>n</sub></i> <b>2</b>, [{(H<sub>2</sub>O)<sub>3</sub>(NO<sub>3</sub>)Cd(μ-<b>3bpu</b>)}·(NO<sub>3</sub>)·X]<i><sub>n</sub></i> <b>5</b>, [{(NO<sub>3</sub>)<sub>2</sub>Cd(μ-<b>3bpu</b>)<sub>2</sub>}]<i><sub>n</sub></i> <b>6</b>, [{(H<sub>2</sub>O)(NO<sub>3</sub>)Cd(μ-<b>4bpu</b>)<sub>2</sub>}·(NO<sub>3</sub>)·EtOH·nitrobenzene]<i><sub>n</sub></i> <b>7</b>, 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.