Grafting ligands to direct the self-assembly of Co/Ni2+ substituted polyoxometalate clusters

Abstract

Four transition metal incorporated phosphotungstates have been synthesized through the reaction of Na₂WO₄, H₃PO₄, Co/Ni(NO₃)₂, and the multifunctional amine N,N′-Bis(2-hydroxyethyl) piperazine (bhep). Na₂{MHbhep(H₂O)₄}[PW₁₀M₂O₃₈{Hbhep}₂] M = Co²⁺ (1), and Ni²⁺(2) represent rare examples of complete transition metal chelate encapsulation within a Keggin polyanion. Slight modification of the reaction procedure yielding 1 also yields the two dimensional material (H₂bhep)₃Na₄[Co₄(H₂O)₂(PW₉O₃₄)₂].15H₂O (3). Finally, the isolation of a phosphotungstate cluster containing 18 Co²⁺ ions is achieved through the bridging of two [Co₉(OH)₃(H₂O)₆(HPO₄)₂(B-α-PW₉O₃₄)₃]¹⁶⁻clusters by the bhep ligand to form the [{Co₉(OH)₃(H₂O)₃(HPO₄)₂(B-α-PW₉O₃₄)₃}₂ {C₈H₁₈N₂O₂}₃]³²⁻ dimer (4). The dimerisation process is achieved via the monodentate coordination of the hydroxyl groups from three bhep ligands to each polyanion by displacing the water ligands typically found coordinated to the {Co₉} core forming a nanoscale cylinder-like capsule capped by two {Co₉P₅W₂₇} with three BHEP-based ligand spacers.