Monomeric, Dimeric Helical, and 1D Nickel Polyoxotungstates Structured by Carboxylate Derivatives


  • Dedicated to Professor Michael T. Pope at the occasion of his 80th birthday


Three new nickel-substituted Keggin-type polyoxometalates functionalized by carboxylate derivatives were characterized. In the monomeric tetranuclear complex Na1.5K2.5[Ni(H2O)6]0.5[(SiW9O34)(OH)3Ni4(C6H13NO2)3]·17H2O (1), three dimethylaminobutyrate amino acids are connected to a {SiW9Ni4} subunit through the carboxylato groups, and the pendant amino functions are protonated. In the dimeric, octanuclear species Na2K12[Ni(H2O)6][(SiW9O34)2(OH)6Ni8(C6H8O4)3]·40H2O (2), three saturated adipato (C6, dicarboxylate) ligands connect two {SiW9Ni4} fragments. This unique molecular-helical polyoxometalate is obtained as a racemic mixture of the M and P enantiomers. When the aromatic dicarboxylate p-phenylenediacetate is used as a bridging ligand, the compound Na6K8[(SiW9O34)2(OH)6Ni8(C10H8O4)3]·45H2O (3) is obtained. In this species, two equivalent p-phenylenediacetato linkers connect two {SiW9Ni4} subunits to form a {Si2W18Ni8} assembly, and a third dicarboxylato ligand connects these fragments, which affords a 1D polyoxometalate with a zigzag-chain topology. Solution 1H NMR studies and IR experiments show that redissolving all these species in water affords carboxylato-free compounds after few hours. The magnetic properties of complexes 1 and 2 were also studied. For these two compounds, a quantification of the magnetic exchange interactions revealed the presence of competitive antiferromagnetic and ferromagnetic interactions.