Syntheses, crystal structures and luminescence behaviors of four neutral penta-/hexacoordinate cadmium(II) compounds containing a tridentate Schiff base: Variation in coordination numbers, nuclearities and dimensionalities by changing halides/pseudohalides

Abstract

Two neutral pentacoordinate mononuclear cadmium(II) complexes of the type [Cd(L)X2] [L = N-(pyridin-2′-yl)benzylidene)-2-(piperidin-1-yl)ethane-1,2-diamine; X = Cl, (1) and X = Br, (2)], one neutral hexacoordinate dinuclear compound [Cd(L)(µ-NCS)NCS]2 (3) and one neutral hexacoordinate coordination polymer [Cd(L)(µ-N3)N3]n (4) have been prepared and characterized using microanalytical, spectroscopic, thermal and X-ray crystallographic results. Structural analyses show that each cadmium(II) center in the isostructural compounds 1 and 2 adopts a distorted square pyramidal geometry with a CdN3X2 [X = Cl in 1 and X = Br in 2] chromophore through coordination of three N atoms of L and two terminal halides. In 3 and 4, each metal(II) center affords a distorted octahedral geometry with MN5S (in 3) and MN6 (in 4) chromophores. Each metal(II) center in dinuclear compound 3 is attached to three N-atoms of L, one N-atom of terminal thiocyanate, and one N-atom and one S-atom of two μ1,3-bridging thiocyanate units, whereas three N-atoms of L and three N-atoms of one terminal and two μ1,3-bridging azide units are bound to each metal(II) center in the coordination polymer 4. Each metal(II) center in 4 is connected to two other metal(II) centers through two different μ1,3-azide bridges affording a zigzag one-dimensional (1D) chain. In the crystalline state, mononuclear units in both 1 and 2 are stabilized through intermolecular CH∙∙∙X (X = Cl in 1 and X = Br in 2) hydrogen bonds promoting to 2D sheet structures. The dinuclear units in 3 pack through cooperative intermolecular CH∙∙∙S and CH∙∙∙N hydrogen bonds giving rise to a 3D network structure. 1D chains in 4 are stabilized through cooperative intermolecular CH∙∙∙N hydrogen bonds promoting to a 2D sheet structure. These 2D sheets, in turn, self-assemble through weak non-covalent π∙∙∙π interactions to afford a 3D network structure. The thermally stable compounds (1–4) exhibit intraligand 1(π-π∗) fluorescence in DMF solutions at room temperature with lifetimes in range 0.83–2.94 ns.