Solvent-controlled three families of Zn(ii) coordination compounds: synthesis, crystal structure, solvent-induced structural transformation, supramolecular isomerism and photoluminescence

Abstract

This work reports the assembly, topological structure, supramolecular isomerism and luminescence of three solvent-controlled families of coordination compounds, [Zn(bpz)₂(H₂O)₃·2Hpta] (1), [Zn(bpz)(pta)]_n (2), [Zn(bpz)(tpa)(H₂O)]_n (3), [Zn(bpz)(tpa)·4H₂O]_n (4), and [Zn(bpz)(npa)·H₂O]_n (5 and 6) (bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole, H₂pta = phthalic acid, H₂tpa = terephthalic acid, H₂npa = 4-nitrophthalic acid). The six transition metal compounds reported in this study were definitely characterized by X-ray crystallography to reveal how networks with different topologies are constructed around the same four-connected metal centers. Compound 1 is a 0D discrete molecule, in which Zn(II) is in a trigonal bipyramidal coordination geometry, whereas the guest Hpta⁻ as counteranion is hydrogen-bonded with the [Zn(bpz)₂(H₂O)₃]. In compound 2, the Zn(II) center is linked by two bpz and two pta; thus, a 4-connected dia network with the point symbol {6⁶} is formed. In 3, the Zn(II) center is six-coordinated, but in fact it is also a 4-connected node in the whole network due to the terminal aqua ligand and bidentate chelating carboxylate group thus, the resultant network has a 4-connected cds topology with the point symbol {6⁵·8}. Compound 4 exhibits a chiral two-fold interpenetrated 4-connected qtz network with the point symbol {6⁴·8²}. Compounds 5 and 6 are a pair of genuine supramolecular isomers with identical 4-connected dia topology. The three families of compounds, namely, 1/2, 3/4, and 5/6, are structurally controlled by the solvent systems H₂O/CH₃OH–H₂O, H₂O/DMF–CH₃OH, and CH₃OH–H₂O/CH₃CN–H₂O, respectively. Except for the discrete molecule 1, the other five compounds have the same 4-connected coordination networks, but with different topologies ranging from dia (2, 5, 6), cds (3) to qtz (4), suggesting the important influences that the linkage orientations of the ligand and different geometries of the 4-connected node exert in self-assembly. Interestingly, discrete 1 can be irreversibly transformed from a 0D discrete molecule to an infinite 3D structure (2) by heating it in CH₃OH–H₂O solvent, indicating a solvent-induced structural transformation. In addition, results about thermal stabilities and photoluminescence spectra are also discussed in detail.