New Entangled Coordination Networks Based on Charge-Tunable Keggin-Type Polyoxometalates

Investigation into a hydrothermal reaction system with transition-metal (TM) ions, 1,4-bis(1,2,4-triazol-1-lmethyl)benzene (BBTZ) and various charge-tunable Keggin-type polyoxometalates (POMs) led to the preparation of four new entangled coordination networks, [Co-II(HBBTZ)(BBTZ)(2.5)][PMo12O40] (1), [Cu-I(BBTZ)](5)[BW12O40]H2O (2), [Cu-II(BBTZ)](3)[(AsW3W9O40)-W-V-O-VI]10H(2)O (3), and [Cu-5(II)(BBTZ)(7)(H2O)(6)][P2W22Cu2O77(OH)(2)]6H(2)O (4). All compounds were characterized by using elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The mixed valence of W centers in compound 3 was further confirmed by using XPS spectroscopy and bond-valence sum calculations. In the structural analysis, the entangled networks of 1-4 demonstrate zipper-closing packing, 3D polythreading, 3D polycatenation, and 3D self-penetration, respectively. Moreover, with the enhancement of POM negative charges and the use of different TM types, the number of nodes in the coordination networks of 1-4 increased and the basic metal-organic building motifs changed from a 1D zipper-type chain (in 1) to a 2D pseudorotaxane layer (in 2) to a 3D diamond-like framework (in 3) and finally to a 3D self-penetrating framework (in 4). The photocatalytic properties of compounds 1-4 for the degradation of methylene blue under UV light were also investigated; all compounds showed good catalytic activity and the photocatalytic activity order of Keggin-type species was initially found to be {XMo12O40}>{XW12O40}>{XW12-nTMnO40}.