Co−MOFs with 1,1′-(5-methyl-1,3-phenylene)bis(1H-imidazole) and aromatic carboxylates as coligands: synthesis, structure, and spectroscopic and thermal characterizations

Abstract

<p>Four cobalt(II) compounds, [Co(Bim)(IA)(H₂O)₂]_<i>n</i>·0.5nH₂O (<b>1</b>), [Co(Bim)(MA)(H₂O)₂]_<i>n</i> (<b>2</b>), [Co₂(Bim)₂(MA)₂]_<i>n</i>·nH₂O (<b>3</b>), and [Co₃(Bim)₄(TA)₂(H₂O)₂]_<i>n</i>·2.5nH₂O (<b>4</b>), have been synthesized by solvothermal reactions of cobalt(II) salts with 1,1′-(5-methyl-1,3-phenylene)bis(1H-imidazole) [Bim] and aromatic polycarboxylic acids (H₂IA = isophthalic acid, H₂MA = 5-methylisophthalic acid, and H₃TA = trimesic acid) as coligands. The four complexes were characterized by IR and UV−vis spectra, elemental analyses, X-ray powder and single-crystal diffractions, and thermogravimetric analyses (TGAs). <b>1</b> features a zigzag polymeric macrocycle chain containing a nanotubular channel, which is constructed by bridging the folded 20-membered macrocyclic [Co₂(Bim)₂] subunits with IA ligands. <b>2</b> represents a double-chain structure containing 18-membered macrocyclic [Co₂(Bim)(MA)] subunits. Both <b>3</b> and <b>4</b> are 2-D porous coordination polymers but have different architectures. In <b>3</b>, cage-like [Co₄(Bim)₂(MA)₄] subunits are 4-connected nodes that are further bridged by another half-set of Bim ligands to form a 2-D helical structure containing one-dimensional achiral channels and alternately arranged left- and right-handed helical tubular channels. In <b>4</b>, Bim ligands bridge three crystallographically independent Co centers into sharply distorted left- and right-handed helices which are further connected by TA ligands to form a meso layer about 3.0 nm monolayer thickness with a unique (3,4)-connected topology. The structural diversities of coordination polymers <b>1</b>–<b>4</b> are tuned by the flexible coordination number of Co and coligand polycarboxylates. Thermal analyses show that the main frameworks of all compounds remain stable to 352 °C. Moreover, the interesting color changes of crystals <b>1</b>–<b>4</b>, varying from pink to purple and dark blue, result from the d → d* transitions of chromophoric Co²⁺ in different coordination geometries as determined by the UV–vis spectra in combination with crystal structure analyses.</p