Anion-dependent structural diversity of cadmium(II) complexes: synthesis, crystal structures, luminescence properties, and unusual C-H/σ supramolecular interactions involving σ-aromatic M<sub>2</sub>X<sub>2</sub> cores

<p>Four complexes based on N,N′-bis(2-pyridylmethylene)-1,3-propanediamine (L) and different Cd(II) salts have been synthesized and characterized by single-crystal X-ray diffraction analysis. The complexes are [Cd<sub>2</sub>(L)<sub>2</sub>(μ-Cl)<sub>2</sub>](ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>), [Cd<sub>2</sub>(L)<sub>2</sub>(μ-Br)<sub>2</sub>](ClO<sub>4</sub>)<sub>2</sub> (<b>2</b>), [Cd(L)I](ClO<sub>4</sub>) (<b>3</b>), and [Cd(L)(NCS)<sub>2</sub>] (<b>4</b>). L exhibits the same coordination mode in <b>1</b>–<b>4</b>. The radius of each anion plays a role in affecting the structures and luminescent intensities of the final products. CdCl<sub>2</sub>, CdBr<sub>2</sub>, and CdI<sub>2</sub> react with L to produce chloride or bromido-bridged binuclear complexes and mononuclear iodido complex, respectively, whereas an unusual mononuclear trigonal prismatic (TP) <b>4</b> is obtained when thiocyanate was used as a coligand. Photoluminescence properties of all the complexes show that the trend of fluorescence intensity is <b>1</b> > <b>4</b> > <b>2</b> > <b>3</b>. All four complexes exhibit different supramolecular interactions such as C–H/σ, <i>π</i>–<i>π</i>, and C–H/π and hydrogen bonding interactions. The experimental findings are complemented by density functional theory (DFT) calculations.</p>