Syntheses and crystal structures of four metal–organic co-ordination networks constructed from cadmium(II) thiocyanate and nicotinic acid derivatives with hydrogen bonds

Abstract

Four novel non-interpenetrating metal–organic co-ordination networks of the Cd^II–L–NCS⁻ system have been prepared using potentially bridging nicotinic acid derivatives L [nicotinic acid (HL¹), nicotinamide (L²), isonicotinamide (L³), or isonicotinate (L⁴)]. Their crystal structures were determined by X-ray diffraction. In [Cd(SCN)₂(HL¹)₂]·HL¹1 each pair of cadmium(II) ions is bridged by two inversely related μ-NCS⁻-N,S ligands to form infinite chains with the remaining two trans positions of six-co-ordinated Cd atoms being occupied by two HL¹ ligands, which form head-to-head double hydrogen bonds using the unco-ordinated carboxyl groups between adjacent chains to form two-dimensional layers. Weak S ⋯ S interactions between the NCS⁻ ligands extend the layers into a three-dimensional framework with the channels enclosing HL¹ guest molecules, which are interlinked into chains through O–H ⋯ N hydrogen bonds. [Cd(SCN)₂(L²)₂]·H₂O 2 and [Cd(SCN)₂(L³)₂] 3 are interesting in that they contain unprecedented 16-membered [Cd₄(μ-SCN-N,S)₄] rings in the two-dimensional sheets. Between the sheets N–H ⋯ O amide–amide hydrogen bonds extend the two-dimensional layers to three-dimensional structures. [Cd(SCN)(L⁴)(H₂O)] 4 exhibits two-dimensional wave-like networks with bridging NCS⁻ and L⁴ ligands using both ends in co-ordination. The results demonstrate that the structures of the Cd^II–L–NCS system deeply depend on the nature of the nicotinic acid derivatives L.