Structural diversity and optical properties in the M–X-isonicotinic (M = Zn, Cd; X = Cl, Br, I) system: New zero-, one-, two- and three-dimensional inorganic–organic hybrids

Assembly of isonicotinic acid ligand (HL) with metal halide, five new hybrid complexes [CdI2(C5H4NCOOH)(C5H4NHCOO)] · H2O (1), Nan[ZnCl2(C5H4NCOO)]n · 2n H2O (2), [CdX(C5H4NCOO)]n (X = Br (3), I (4)) and [Cd3Cl2(OH)2(C5H4NCOO)2]n (5) were obtained, which display a variety of structural motifs, ranging from zero-dimensional to complicated three-dimensional networks. Complex 1 possesses an isolated unit MX2 that is further connected into 3D networks through hydrogen bonding and π–π stacking interactions. Complex 2 is characterized by an infinite one-dimensional chain of zinc atoms bridged by L− ligands. While complexes 3 and 4 possess X-bridging ∞1[CdX2/2]∞1[CdX2/2] inorganic chains connected by L− ligands to form a 2D hybrid network structure. In the case of 5, the cadmium(II) cation is bridged by μ3-Cl atom and μ3-OH− group to form a 2-D ∞2[Cd6/2Cl6/3(μ3-OH)2]∞2[Cd6/2Cl6/3(μ3-OH)2] inorganic layer which is further extended into 3-D framework by bridging L− ligand via Cd–N and Cd–O bonds. The optical properties of 1, 4, and 5 in the solid state are investigated at room temperature and time-dependent DFT (TDDFT) calculation using the B3LYP functional has been performed on 1. The result indicated that the emission band of 1 is attributed to an admixture of MLCT (metal-to-ligand charge–transfer) and LLCT (ligand-to-ligand charge-transfer).