Four new Cu₆S₆ cluster-based coordination compounds: synthesis, crystal structures and fluorescence properties

A hexagonal prismatic Cu₆S₆ cluster exhibits excellent near-infrared fluorescence properties due to its short Cu-Cu bonds, however, the construction of Cu₆S₆ cluster-based compounds with extended structures is still a challenge. Here, four new Cu₆S₆ cluster-based coordination compounds, formulated as Cu₃(pymt)₃ (1), {(CuCN)₂[Cu₃(mpymt)₃]}_n (2), {(CuSCN)[Cu₃(mpymt)₃]}_n (3) and {(CuCN)₂[Cu₃(dmpymt)₃]·CH₃CN}_n (4) (Hpymt = pyrimidine-2-thiolate, Hmpymt = 4-methyl-pyrimidine-2-thione and Hdmpymt = 4,6-dimethylpyrimidine-2-thione), have been synthesized through the reactions of mercaptopyrimidine derivatives and CuCN or CuSCN under solvo-thermal conditions and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, IR spectroscopy, elemental analysis, and thermal gravimetric analysis. Single-crystal X-ray diffraction analysis reveals that compound 1 is a zero-dimensional Cu₆(pymt)₆ molecule containing a distorted pseudo-hexagonal prismatic Cu₆S₆ core. Compounds 2 and 4 with isomorphic frameworks but different organic linkers show a rare three-dimensional framework with nor topology constructed from Cu₆(mpymt)₆ units and one-dimensional chiral [Cu(CN)]_n chains; compared with compound 2, a more hydrophobic one-dimensional channel in compound 4 is observed due to the increase of the methyl groups on the pyrimidine ligand, in which acetonitrile molecules are filled in the channels of compound 4. Compound 3 shows a rare two-dimensional layer constructed from Cu₆(mpymt)₆ units and one-dimensional puckered (CuSCN)_n chains. For the first time, Cu₆S₆ clusters are connected to one-dimensional inorganic CuCN (or CuSCN) chains through mercaptopyrimidine derivatives to obtain extended arrays in compounds 2-4. The crystals of compounds 1-4 in the solid state all show apparent red light emission. Compound 4 shows sensitive luminescence quenching response to nitrobenzene (NB), and the corresponding quenching constant (K_sv) and detection limit are 2.06 × 10³ M^-1 and 9.27 ppm, respectively. This study provides a new strategy to construct Cu₆S₆ cluster-based coordination polymers that have great potential in various applications such as luminescence sensing.