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

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.