New Na-bridged phosphomolybdates (H₂en)₁₂[{Na_0.8K_0.2(H₂O)}₂{Na[Mo₆O₁₂(OH)₃(HPO₄)₂(PO₄)₂]₂}₂]·7H₂O (1), (H₂bpp)₂[{Sr₂(H₂O)₄}{Na₃(H₂O)₁₀}{Na[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}]·2H₂O (2), and (H₂bpp)₆ [{Ca₂(H₂O)₄}{Na₂(H₂O)₄}{Na[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₂}₂] (3) (en = ethylenediamine, bpp = 1,3-bi(4-pyridyl)propane), were synthesized by a hydrothermal method and characterized. Single-crystal X-ray diffraction analysis revealed that the three hybrids were composed of fully reduced ‘hourglass-shaped’ {Na[P^V₄Mo^V₆X₃₁]₂}ⁿ⁻ (X = O/OH) clusters and N-containing organic cations. The electrochemical measurements showed that this kind of highly negatively charged metal–oxygen cluster in crystals 1–3 has the ideal behaviour of reversible electron gain or loss, which is related to the function of electron transfer catalysts in oxidation reduction reactions. Experimental results indicated that they were active as heterogeneous crystal catalysts in the catalytic reduction of inorganic hexavalent chromium with formic acid as reductant, and also had a significant catalytic effect on the photodegradation of the organic dye methylene blue under mild conditions.