Structural diversity and luminescence properties of M(II) complexes based on 4-carboxyphenoxybutanoic acid and N-containing ligands

Abstract

4-Carboxyphenoxybutanoic acid (H2cpob) and its monosodium salt (NaHcpob) are synthesized and characterized. The crystal structure of NaHcpob shows that the sodium atoms are connected by the carboxyl group of Hcpob−, forming a three-dimensional coordination polymer network. NaHcpob reacts with metal salts and N-containing ligands produced following complexes: [Co(cpob)(H2O)2]n1, [Zn(cpob)(phen)(H2O)]22, [Cd(cpob)(phen)]n3, [Mn(cpob)(phen)·4H2O]n4, and [Cd2(cpob)2(bimb)]n5 (phen ​= ​1,10-phenanthroline, bimb ​= ​1,4-bis(imidazole-1-ylmethyl)benzene). Single-crystal X-ray diffraction analysis showed that these complexes have different metal-organic frameworks, ranging from binuclear (2) to one-dimensional (3 and 4), two-dimensional (1), and three-dimensional (5) arrangements. When only H2cpob forms a complex with metal ions, the ligand connects the metal ions into a higher-dimensional structure. In contrast, the terminal ligand (phen) added as an auxiliary ligand forms a low-dimensional complex. And when the bridging auxiliary ligand (bimb) is added, the high-dimensional complex can still be formed. The flexible conformation of H2cpob and the different N-containing ligands may be responsible for the structural diversity of binary H2cpob metal complexes. These metal complexes also displayed many differences in their fluorescence properties in the solid state.