Influence of Pb element on the catalytic properties of {P4Mo6}-polyoxometalate for redox reactions

Abstract

To introduce Pb element into the crystal framework of a polyoxoanion, a series of hybrid compounds were synthesized and characterized: (H₂bpp)₅[PbMn(H₂O)₂]₂H₂{Mn[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·8H₂O (1), (H₂bpp)₃[PbM(H₂O)₂]₂H₆{M[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·6H₂O (M = Ni (2), Co (3)) and (H₂bpp)₃[PbCd(H₂O)₂]₂H₆{Cd[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂·4H₂O (4) (bpp = 1,3-bis(4-pyridyl)propane). Crystal structures were determined and characterized by IR, TG, XPS and single-crystal X-ray diffraction. Hybrids 1–4 have similar compositions; the inorganic moiety has a high-dimensional polyanionic arrangement formed from bi-metallic {PbM} subunits covalently bridging negative {M(P₄Mo^V₆)₂} clusters; the organic moiety is made up of protonated bpp cations that surround anionic clusters through many complex supramolecular interactions. Experimental results showed that the introduction of Pb can modulate the electrochemical properties of crystal materials, leading to the more reversible redox process of {M(P₄Mo₆)₂}-based hybrids. Hybrids 1–4 were employed as heterogeneous catalysts for redox reactions to evaluate their activity involving the transfer of electrons. It was found that these kinds of hybrids were catalytically active in the reduction reaction of [Fe(CN)₆]³⁻ (+S₂O₃²⁻) → [Fe(CN)₆]⁴⁻, but were inactive for Cr(VI) (+HCOOH) → Cr(III) at 25–70 °C. The effect of temperature on the catalytic ability of hybrids was studied, and it was found that 50 °C was the optimal reaction condition for the four compounds to catalyze [Fe(CN)₆]³⁻ → [Fe(CN)₆]⁴⁻. The reduction percentages of Fe(III) in 270 min were about 95.7% (1), 76.95% (2), 89.39% (3) and 80.8% (4). The well-defined polyanionic structure with tunable structural features might be beneficial for exploring the mechanism of catalytic redox reactions at the molecular level.