title: Mixed-Metal Hybrid Polyoxometalates with Amino Acid Ligands: Electronic Versatility and Solution Properties.
abstract: Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal-oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal-oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled MoVcenters in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal-oxo cores.
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Fateme Akhlaghi
authors.initials: FA
authors.lastname: Bagherjeri
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Michele
authors.initials: M
authors.lastname: Vonci
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Edward A
authors.initials: EA
authors.lastname: Nagul
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Chris
authors.initials: C
authors.lastname: Ritchie
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Gable
authors.affiliation: School of Science, RMIT University , Melbourne, Victoria 3001, Australia.
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Taylor
authors.affiliation: School of Science, RMIT University , Melbourne, Victoria 3001, Australia.
authors.forename: Gary
authors.initials: G
authors.lastname: Bryant
authors.affiliation: School of Chemistry, Monash University , Clayton, Victoria 3800, Australia.
authors.forename: Si-Xuan
authors.initials: SX
authors.lastname: Guo
authors.affiliation: School of Chemistry, Monash University , Clayton, Victoria 3800, Australia.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Departament de Quı́mica Fı́sica i Inorgànica, Universitat Rovira i Virgili , Marcel·lí Domingo 1, 43007 Tarragona, Spain.
authors.forename: Pablo A
authors.initials: PA
authors.lastname: Aparicio
authors.affiliation: Departament de Quı́mica Fı́sica i Inorgànica, Universitat Rovira i Virgili , Marcel·lí Domingo 1, 43007 Tarragona, Spain.
authors.forename: Xavier
authors.initials: X
authors.lastname: López
authors.affiliation: Departament de Quı́mica Fı́sica i Inorgànica, Universitat Rovira i Virgili , Marcel·lí Domingo 1, 43007 Tarragona, Spain.
authors.forename: Josep M
authors.initials: JM
authors.lastname: Poblet
authors.affiliation: School of Chemistry, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Colette
authors.initials: C
authors.lastname: Boskovic