Speciation atlas of polyoxometalates in aqueous solution (Part II): Molybdenum browns

We present a validated, openly accessible workflow and spectral database charting the solution speciation of molybdenum browns. These redox-active polyoxometalates are central to catalysis and increasingly explored for bioinspired transformations and nanomedicine, making their solution behavior critical for both mechanistic understanding and practical use. Two archetypal Keplerate-type polyoxometalates, {Mo₇₂V₃₀} and {W₇₂V₃₀}, were investigated under 69 systematically varied aqueous conditions. Complementary ⁵¹V/³¹P nuclear magnetic resonance, resonance Raman, electrospray ionization mass spectrometry, and ultraviolet-visible spectroscopy interrogate the effects of pH, buffer identity, temperature, and incubation time on Keplerate integrity. Substituting Mo with W changes redox potential and hard/soft metal balance, yielding distinct stability patterns: {Mo₇₂V₃₀} breaks into fragments under all tested conditions whereas {W₇₂V₃₀} remains intact in fresh solutions and retains structural integrity up to pH 4 after 24 hours. All raw spectra, processed data, and stepwise protocols are deposited in a FAIR (findable, accessible, interoperable, reusable) repository, providing an open resource to accelerate catalytic and biological applications of polyoxometalates.