Synthesis, structural diversity and fluorescent characterisation of a series of d10 metal–organic frameworks (MOFs): reaction conditions, secondary ligand and metal effects

Abstract

Along with our recent investigation on the flexible ligand of H₂ADA (1,3-adamantanediacetic acid), a series of Zn^II and Cd^II metal–organic frameworks, namely, [Zn₃(ADA)₃(H₂O)₂]_n·5nH₂O (1), [Zn(ADA)(4,4′-bipy)_0.5]_n (2), [Zn₂(ADA)₂(bpa)]_n (3), [Zn₂(ADA)₂(bpa)]_n (4), [Zn₂(ADA)₂(bpp)]_n (5), [Cd(HADA)₂((4,4′-bipy)]_n (6), [Cd₃(ADA)₃(bpa)₂(CH₃OH)(H₂O)]_n (7), and [Cd₂(ADA)₂(bpp)₂]_n·7nH₂O (8) have been synthesized and structurally characterized (where 4,4′-bipy = 4,4′-dipyridine, bpa = 1,2-bis(4-pyridyl)ethane and bpp = 1,3-bis(4-pyridyl)propane). Due to various coordination modes and conformations of the flexible dicarboxylate ligand and the different pyridyl-containing coligands, these complexes exhibit structural and dimensional diversity. Complex 1 exhibits a three-dimensional (3D) framework containing one-dimensional (1D) Zn^II–O–C–O–Zn^II clusters. Complex 2 exhibits a 2D structure constructed by 1D double chains based on [Zn₂ADA₂] units and a 4,4′-bipy pillar. Complexes 3 and 4 possess isomorphic 2D layer structures, resulting from the different coordination modes of carboxylate group of ADA ligands. Complex 5 features a 2D 4⁴ layer in which ADA ligands and Zn^II atoms construct a 1D looped chain and the chains are further connected by bpp ligands. Complex 6 is composed of 1D zig-zag chains that are entangled through hydrogen-bonding interactions to generate a 2D network. Complex 7 is a rare (3,5)-connected network. Complex 8 possesses a 3D microporous framework with lots of water molecules encapsulated in the channels. The structural diversity of the complexes perhaps mainly results from using diverse secondary ligands and different metal centre ions, and means the assistant ligand and metal centre play important roles in the design and synthesis of target metal–organic frameworks. This finding revealed that ADA could be used as an effective bridging ligand to construct MOFs and change coordination modes and conformational geometries in these complexes. The thermogravimetric analyses, X-ray powder diffraction and solid-state luminescent properties of the complexes have also been investigated.