Abstract
Hybrid organic-inorganic polyoxometalates (POMs) were synthesized in water by the reaction of a MoVI precursor with bisphosphonate ligands functionalized by pyridine groups. The fully oxidized POM [(MoVI3O8)2(O)(O3PC(O)(C3H6NH2CH2C5H4NH)PO3)2]4− has been isolated as water insoluble pure Na salt (NaMo6(Ale-4Py)2) or mixed Na/K salt (NaKMo6(Ale-4Py)2) and their structure solved using single-crystal X-ray diffraction. The mixed-valent complex [(MoV2O4)(MoVI2O6)2{O3PC(O)(C3H6N(CH2C5H4N)2(MoVIO3))PO3}2]8− was obtained as an ammonium salt (NH4Mo6(AlePy2Mo)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(AlePy2Mo)2. NaMo6(Ale-4Py)2 and NaKMo6(Ale-4Py)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.
Highlights
Polyoxometalates (POMs) are discrete soluble metal oxide fragments of d-block transition metals in high oxidation states (WV,VI, MoV,VI, VIV,V) with a wide range of properties going from magnetism [1]to biology [2] and catalysis [3]
It has been demonstrated that the presence of covalently bonded organic ligands can bring new functionalities and allow the elaboration of unique materials and devices [8], as demonstrated by the characterization of surfaces patterned with POMs [9,10], the synthesis of fluorescent [11] and catalytic
We have developed for several years the functionalization of POMs by bisphosphonates (BPs) with the general formula H2O3PC(R1)(R2)PO3H2 (R1 = H, OH) [14]
Summary
Polyoxometalates (POMs) are discrete soluble metal oxide fragments of d-block transition metals in high oxidation states (WV,VI, MoV,VI, VIV,V) with a wide range of properties going from magnetism [1]. To biology [2] and catalysis [3] Among these properties, photochromism attracts increasing interest due to the potential applications of POM-based photoresponsive molecular materials as reversible memory photodevices [4]. The mechanism has been rationalized considering the transfer of an electron from an O atom of the POM unit to a MoVI center, coupled with the transfer of a hydrogen atom (see below) These materials are ionic crystals combining anionic POMs and cationic alkylammonium counter-ions. We decided to post-functionalize the Ale ligand with pyridine groups with the aim to explore its chelating properties towards additional metal ions and study their influence on the structure and properties of the molybdobisphosphonate complexes. Structure and abbreviations of the ligands used in this work
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