Two three-dimensional Fe(ii) frameworks based on {P4Mo6} tetrameric clusters exhibiting efficient visible-light photocatalytic properties for the degradation of Cr(vi) and methylene blue

Abstract

Two three-dimensional frameworks based on the {P₄Mo₆} unit, H(4,4′-bipy)₂[Fe₄(PO₄)(H₂O)₄Na₆][Fe₆(H₂O)₄][(Mo₆O₁₂)(HPO₄)₃(PO₄)(OH)₃]₄·5H₂O (4,4′-bipy = 4,4′-bipyridine) (1) and H₃(C₁₂H₁₄N₂)₄[Fe₄(PO₄)(H₂O)₄Na₄][Fe₂(Mo₆O₁₂(HPO₄)₃(PO₄)(OH)₃)₄]·6H₂O (2) were successfully synthesized by varying the solvent. The extended structures of the two compounds were formed by transition metal Fe(II) ions bridging the {P₄Mo₆}-based tetrameric clusters around [Na_XFe₄(PO₄)] (X = 6 (1), or X = 4 (2)) core. The 4,4′-bipy molecules and in situ generated methyl viologen cations as templates induce the formation of two three-dimensional structures, an 8-connected bcu topology framework for 1 and a 4-connected 2-fold interpenetrating diamond-like topological network for 2, respectively. Additionally, multiform hydrogen bonds are found in the framework and also play an important role in stabilizing the structure. The proton conduction mechanism of the two compounds can be mainly classified as the Grotthuss mechanism; the proton conductivity values are 1.06 × 10⁻³ S cm⁻¹ for 1 and 3.13 × 10⁻³ S cm⁻¹ for 2 at 75 °C under 98% relative humidity. The visible-light photocatalytic activity was evaluated by photocatalytic decomposition of Cr(VI) and MB dye, and the removal ratios can reach 95.6% (1) and 82% (2) for Cr(VI), and 98% (1) and 99% (2) for MB.