Zinc and cadmium metal-directed coordination polymers: in situ flexible tetrazole ligand synthesis, structures, and properties

Abstract

Seven zinc and cadmium coordination frameworks, {[Zn₂(TE)₂]·H₂O} (1a); {[Cd₂(TE)(SO₄)(OH)(H₂O)]} (1b); TE = 2-(1H-tetrazol-5-yl)-ethanol, {[Zn_1.5(TA)(INA)]·4H₂O} (2a); {[Cd_4.5(TA)₃(N₃)(OH)₂(H₂O)₂]·H₂O} (2b); {[Zn(MT)(INA)]·H₂O} (2c); TA = (1H-tetrazol-5-yl)-acetic acid, MT = 5-methyl-tetrazole, INA = isonicotinic acid, {[Cd(BTMA)]·H₂O} (3a); [Cd₂(TTMA)(OH)] (3b); BTMA = bis-(1H-tetrazol-5-ylmethyl)-amine; TTMA = tris-(1H-tetrazol-5-ylmethyl)-amine, were obtained through in situ tetrazole synthesis and have been structurally characterized by single crystal and powder X-ray diffraction, as well as elemental analyses, FT-IR spectroscopy and thermal studies. Interestingly, by varying the reaction conditions of hydrothermal synthesis, the unprecedented in situ generation of TA and MT for 2 based on cyanoacetamide or BTMA and TTMA for 3 based on iminodiacetonitrile was observed, respectively. Compound 1a represents a very interesting example of three-dimensional (3D) frameworks containing hexagonal channels and meso-helical chains (P + M) alternately trapped by Zn–TE coordination interactions; compound 1b possesses a two-dimensional (2D) layered structure constructed from infinite organic chains and sulfate anions. Compounds 2a and 2b both crystallize in 3D frameworks containing infinite rod-shaped SBUs; compound 2c displays a two-fold diamondoid network constructed by the interconnection of Zn–MT–INA honeycomb layers and INA ligands. Compound 3a shows a 3D framework consisting of cyclic [Cd₂(BTMA)₂] dimer subunits, possessing an 8-connected network topology. Compound 3b shows a 3D binodal (4,8)-connected net based on tetranuclear [Cd₄O₂] polyoxometalates (POMs) as nodes. Furthermore, the luminescence properties of these compounds were investigated. Notably, compound 1b crystallizes in an acentric space group representing significant second harmonic generation (SHG) efficiencies.