Molybdenum Bisphosphonates with Cr(III) or Mn(III) Ions

The synthesis of MoVI bisphosphonates (BPs) complexes in the presence of a heterometallic element has been studied. Two different BPs have been used, the alendronate ligand, [O3PC(C3H6NH3)(O)PO3]4− (Ale) and a new BP derivative with a pyridine ring linked to the amino group, [O3PC(C3H6NH2CH2C5H4N)(O)PO3]4− (AlePy). Three compounds have been isolated, a tetranuclear MoVI complex with CrIII ions, (NH4)5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Cr]·11H2O (Mo4(Ale)2Cr), its MnIII analogue, (NH4)4.5Na0.5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Mn]·9H2O (Mo4(Ale)2Mn), and a cocrystal of two polyoxomolybdates, (NH4)10Na3[(Mo2O6)2(O3PC(C3H6NH2CH2C5H4N)(O)PO3)2Cr]2[CrMo6(OH)6O18]·37H2O ([Mo4(AlePy)2Cr]2[CrMo6]). In this latter compound an Anderson-type POM [CrMo6(OH)6O18]3− is sandwiched between two tetranuclear MoVI complexes with AlePy ligands. The protonated triply bridging oxygen atoms bound to the central CrIII ion of the Anderson anion develop strong hydrogen bonding interactions with the oxygen atoms of the bisphosphonate complexes. The UV–Vis spectra confirm the coexistence in solution of both POMs. Cyclic voltammetry experiments have been performed, showing the reduction of the Mo centers. In strong contrast with the reported MoVI BP systems, the presence of trivalent cations in close proximity to the MoVI centers dramatically impact the potential solid-state photochromic properties of these compounds.