Early-Lanthanide Complexes with All-Inorganic Macrocyclic Polyoxovanadate Ligands

Three discrete early-lanthanide complexes with cyclic polyoxovanadate ligands, [LaV10O30]7 (1), [CeV9O27]6 (2), and [PrV9O27]6 (3), were synthesized by reaction of [VO3] with early-lanthanide(III) salts. Complexes 13 were composed of a LnIII center in square-antiprism coordination mode with coordinated all-inorganic ligands [VnO3n]n (n = 9 or 10), which were constructed from a cyclic linkage of nine or ten VO4 tetrahedra through vertex sharing. Structural investigation of lanthanum complex 1 revealed a ten-membered ring of VO4 units in which eight VO4 units were coordinated to the lanthanum atom through their oxido groups and two VO4 units at opposite sides of the ring were not coordinated. Cerium complex 2 and praseodymium complex 3 both contained a nine-membered cyclic ligand with eight VO4 coordinated units and one uncoordinated unit among nine tetrahedral VO4 units. 51V NMR spectra of 13 in acetonitrile showed reversible temperature dependences at 4070 degrees C, thus indicating fluxional behavior of the macrocyclic polyoxovanadate ligands in solution. Exchange of the coordinated and uncoordinated VO4 sites in the ring was responsible for these reversible processes.