Hexa- and Dodeca-nuclear Poly-oxo-molybdate Cyclic Compounds: Application toward the Facile Synthesis of Nano-particles and Film Electrodeposition

Two new compounds based on O3PCH2PO34- ligands and (Mo(V)2O4) dimeric units have been synthesized and structurally characterized. The dodeca-nuclear Mo(V) poly-oxo-molybdate species in (NH4)18[(Mo(V)2O4)6(OH)6(O3PCH2PO3)6)].33 H2O (1) is a cyclohexane-like ring in a chair conformation with pseudo S, symmetry. In the solid state, the wheels align side by side, thus delimiting large rectangular voids. The hexa-nuclear anion in Na8[(Mo(V)2O4)3(O3PCH2PO3)3(CH3AsO3)].19H2O (2) has a triangular framework and encapsulates a methylarsenato ligand. 31P NMR spectroscopic analysis revealed the stability of 2 in various aqueous media, whereas the stability of 1 depends on the nature of the cations present in solution. It has been evidenced that the transformation of 1 into 2 occurs in the presence of CH3AsO32- ions. This behavior shows that 1 call be used as a new precursor for the synthesis of Mo(V)/diphosphonate systems. The two complexes were very efficient both as reductants of Pt and Pd metallic salts and as capping agents for the resulting Pt0 and Pd0 nano-particles. The size of the obtained nano-particles depends both on the nature of the poly-oxo-metalate (POM i.e., 1 or 2) and on the [metallic salt]/[POM] ratio. In all cases, X-ray photoelectron spectroscopy (XPS) measurements have revealed the presence of Mo(VI) species that stabilize the nano-particles and the absence of Mo(V) moieties. Diffuse-reflectance FTIR spectra of the Pt nano-particles show that the capping Mo(VI) POMs are identical for both systems and contain the di-phosphonato ligand. The colloidal solutions do not show any precipitate and the nano-particles remain well-dispersed for several months. The electrochemical reduction of Mo(V) species was studied for 2. Cyclic voltammetry alone and electrochemical quartz crystal microbalance coupled with cyclic voltammetry show the deposition of a film on the electrode surface during this reduction. (authors)