A new heterometallic coordination compound containing pseudo-hexagonal columnar Cu6S6 and linear Cd3-oxo clusters: Multicomponent self-assembly, luminescent responses to nitroaromatic compounds in water

The stepwise crystallization strategy is the most commonly used method to construct Cu₆S₆ cluster-based heterometallic coordination compounds. In this work, a new heterometallic coordination compound containing pseudo-hexagonal columnar Cu₆S₆ and linear Cd₃-oxo clusters, {[Cu₆(mna)₆Cd₃(ip)(en)₄(H₂O)₂]·(solvents)_x}_n (1) (H₂mna = 2-mercaptoisonicotinic acid, H₂ip = isophthalic acid, en = ethylenediamine)], was obtained by a one-pot strategy, in which multiple components are self-assembled in an ordered manner through soft and hard acid-base recognitions, providing a new strategy for constructing Cu₆S₆-based heterometallic coordination compounds. Compound 1 is a rare three-dimensional framework containing microporous channels with a diameter of 9.53 Å, constructed from [Cu₆(mna)₆]⁶⁻ metalloligands, linear Cd₃-oxo clusters, single Cd²⁺ ions and (ip)²⁻ anions. Compound 1 in the solid state shows red light emission with a maximum emission wavelength of 650 nm. The fluorescence sensing experiments in water indicate that compound 1 presents sensitive luminescence quenching responses to nitroaromatic compounds (NACs) such as nitrobenzene (NB), 4-nitrotoluene (4-NT), 4-nitrophenol (4-NP), and 1,3-dinitronitrobenzene (1,3-DNB), and the corresponding quenching constants (K_SV) decrease in the following order: 4-NP > 1,3-DNB > NB > 4-NT. The calculation of HOMO and LUMO of fluorescence moiety in compound 1 and four NACs show photoinduced electron transfer plays an important role in fluorescence quenching, while the synergistic interactions of competitive absorption and hydrogen bonds between 4-NP and fluorescence moieties lead to the highest quenching efficiency of 4-NP to compound 1.