Fully Oxidized and Mixed-Valent Polyoxomolybdates Structured by Bisphosphonates with Pendant Pyridine Groups: Synthesis, Structure and Photochromic Properties

Hybrid organic-inorganic polyoxometalates (POMs) were synthesized in water by the reaction of a Mo-VI precursor with bisphosphonate ligands functionalized by pyridine groups. The fully oxidized POM [((Mo3O8)-O-VI)(2)(O)(O3PC(O)(C3H6NH2CH2C5H4NH) PO3) 2] 4-has been isolated as water insoluble pure Na salt (NaMo6(Ale-Py-4)(2)) or mixed Na/K salt (NaKMo6(Ale-4Py) 2) and their structure solved using single-crystal X-ray diffraction. The mixed-valent complex [((Mo2O4)-O-V)((Mo2O6)-O-VI)(2){O3PC(O)(C3H6N(CH2C5H4N)(2)((MoO3)-O-VI)) PO3}(2)](8-)was obtained as an ammonium salt (NH4Mo6(AlePy(2)Mo)(2)), in the presence of a reducing agent (hydrazine). 31P NMR spectroscopic studies in aqueous media have allowed determining the pH stability domain of NH4Mo6(AlePy(2)Mo)(2). NaMo6(Ale-Py-4)(2) and NaKMo6(Ale-Py-4)(2) exhibit remarkable solid-state photochromic properties in ambient conditions. Under UV excitation, they develop a very fast color-change from white to deep purple and proved to be the fastest photochromic organoammonium/POM systems. The coloration kinetics has been fully quantified for both salts and is discussed in light of the hydrogen-bonding networks.