Abstract
A series of hybrid nickel bisphosphonate (BP) polyoxometalates (POMs) has been isolated. The complexes NaK-Ni7-Ale2 (Ale=[H2O3PC(C3H6NH2)(OH)PO3H2]) and NaNH4-Ni7-Ale2 are both made of two {PW9O34} fragments enclosing a heptanuclear NiII core connected to two alendronate ligands. By pre-functionalising the alendronate moiety through the amino group, the naphthalene (napht) derivative Ni7-(AleNapht)2 and the compounds Ni7-(AlePy2Ni)2 (py=pyridyl) and Ni7-(AleAc2Ni)2 (Ac=acyl) have been obtained. Compared with the NaK-Ni7-Ale2 species, these last two complexes contain two additional NiII centres connected through two bis(2-pyridylmethyl)amine or two iminodiacetate groups, respectively. This results show that it is possible to functionalise the heptanuclear POM in a controlled manner. Quantification of the magnetic interactions in NaK-Ni7-Ale2 revealed that in the {Ni7} core, ferromagnetic interactions are predominant, with a S=5 ground state. Magnetisation versus dc field sweeps on a single crystal of NaK-Ni7-Ale2 exhibited hysteresis at low temperature. 1H and 31PNMR studies in aqueous solution performed on NaK-Ni7-Ale2 and Ni7-(AleNapht)2 evidenced that the PW9/Ni7/bisphosphonate assembly is stable in solution. This was completed by 31P magic angle spinning (MAS) investigations and confirmed by 1H DOSY experiments. The electrochemistry of these compounds proceeds through two well-defined four-electron chemically reversible waves in a medium at pH6. NaK-Ni7-Ale2 proved to be efficient for the electrocatalytic reduction of nitrate, nitrite and nitrous oxide. Remarkably, its electrocatalytic efficiency for nitrate reduction is approximately three times higher than those previously reported for POMs in a medium at pH >4 under the same potential. The catalytic properties of two representatives of the hybrid family were also examined. It is shown that these nickel bisphosphonate polyoxotungstates are pre-catalysts for the oxidation of alcohols into ketones or carboxylic acids, depending on the classes of alcohols considered, the stoichiometric oxidant used being H2O2. Noticeably, it has been found that an analogous cobalt bisphosphonate polyoxotungstate complex does not present any related activity, highlighting the crucial role of the 3d cations on the catalytic process.