Syntheses, structures and properties of group 12 element (Zn, Cd, Hg) coordination polymers with a mixed-functional phosphonate-biphenyl-carboxylate linker

Abstract

The new phosphonate-carboxylate ligand from 4-phosphono-biphenyl-4′-carboxylic acid (H₂O₃P–(C₆H₄)₂–CO₂H, H₃BPPA) is based on the rigid biphenyl system and is studied toward the coordination behavior of group 12 elements zinc, cadmium and mercury. The crystalline products from hydrothermal syntheses highlight the versatile and different coordination modes with the (partially) deprotonated H₃BPPA ligand to give coordination polymeric 3D-[Zn₅(μ₃-OH)₄(μ₄-O₃P–(C₆H₄)₂–CO₂-μ₂)₂]_n (5), 2D-[Zn(μ₆-O₃P–(C₆H₄)₂–CO₂H)]_n (6), 3D-[Cd₃(μ₅-O₃P–(C₆H₄)₂–CO₂-μ₂)(μ₆-O₃P–(C₆H₄)₂–CO₂-μ₃)]_n (7) and 2D-[Hg(μ₃-HO₃P–(C₆H₄)₂–CO₂H)]_n (8). The cobalt complex, 2D-[Co(μ₄-O₃P–(C₆H₄)₂–CO₂H)]_n (9) is isostructural to 6. Through additional classic strong carbonyl O–H⋯O hydrogen bonding the dimensionality of the 2D coordination networks increases to 3D supramolecular frameworks. The carboxy-phosphonate ligand shows five different coordination modes which can be described as μ₄-O₃P–CO₂-μ₂ (5), μ₆-O₃P– (6), μ₅-O₃P–CO₂-μ₂, μ₅-O₃P–CO₂-μ₃ (7), and μ₃-O₃P– (8), that is, the ligand bridges altogether between 3 to 8 metal atoms with the phosphonate group alone connecting already 3 to 6 metal atoms. Layers of metal–oxygen polyhedra are interconnected via the biphenyl linker, which either coordinates metal atoms with both donor groups or the –COOH end forms tail-to-tail hydrogen bonds to create 3D or 2D coordination networks, respectively. In the flat {MO_x} layers in 6 and 7 the Zn and Cd metal nodes represent a honeycomb and an mcm net, respectively. The coordination polyhedra of the Cd atoms in compound 7 were analyzed towards a trigonal-prismatic coordination environment. The complexes are hydrolytically very stable due to their hydrothermal preparation from aqueous solution at 180–200 °C. The compounds could be stored in water or air for months without apparent decomposition. Compounds 5 and 7, where the ligand is fully deprotonated, start to decompose at ∼400 °C. The fluorescence emission spectrum of the ligand, 4, shows an intense peak at 365 nm (λ_ex = 316 nm). The fluorescence emission of the metal complexes 5, 7 and 9 is shifted towards larger wavelengths with values of 417 nm, 415 nm and 410 nm, respectively (λ_ex = 354 nm for 5, λ_ex = 350 nm for 7, λ_ex = 400 nm for 8, λ_ex = 360 nm for 9). In addition, the crystal structures of the H₃BPPA ligand precursors 4-iodo-4′-biphenylcarboxylic acid methyl ester, and 4-diethylphosphono-4′-biphenylcarboxylic acid methyl ester are described here for the first time.