The use of polyoxometalates for the design of high-nuclearity magnetic clusters, single-ion magnets and photo-responsive materials

Resumen

A brief summary including the principal aspects of the work developed in each chapter is provided below: 1. Chapter 1 constitutes a historic overview of POMs, a summary of synthetic strategies and structural description of the relevant POMs of this thesis and a short description of the general properties and the applications of POMs. 2. Chapter 2 discusses the synthesis and characterisation of several novel multinuclear cobalt-substituted POMs 2.1-2.9. This chapter provides a deep insight into the experimental details of the synthesis and characterisation of cobalt-containing POMs that have been obtained during the course of this doctoral work. This chapter is divided into the following three main parts: (1) In the first part, the synthesis and full characterisation of four novel tetra-cobalt(II) or hepta-cobalt(II,III)-substituted POMs, having cobalt cores exhibiting cubane or di-cubane topologies, is described. The tetra-cobalt(II)-substituted POM [Co4(OH)3(H2O)6(PW9O34)]4– (2.1) consists of a trilacunary B-α-[PW9O34]9– unit which accommodates a cubane-like [CoII4O4] core. The three hepta-cobalt(II,III)-containing POMs [Co7(OH)6(H2O)6(PW9O34)2]9– (2.2), [Co7(OH)6(H2O)4(PW9O34)2]n9n– (2.3) and [Co7(OH)6(H2O)6(P2W15O56)2]15– (2.4) encapsulate a dicubane-like [CoII6CoIIIO8] core between two trilacunary B-α-[PW9O34]9– units (in 2.2 and 2.3) or two trilacunary -[P2W15O56]12– (in 2.4) units. The chemical structure, stability studies in solution by UV-vis spectroscopy and cyclic voltammetry, and the magnetic properties of the resulting clusters are discussed. (2) In the second part, a synthetic procedure is presented to construct new magnetic POMs containing one or two subunits of '[Co4(OH)3(H2O)6-n(PW9O34)]3-' (n = 3 or 5). The substitution of the water ligands present in these subunits by oxo or hydroxo ligands belonging to other POM fragments, gives rise to the following larger magnetic POM anions: [Co7(OH)6(H2O)6(PW9O34)2]9– (2.2), [Co7(OH)6(H2O)4(PW9O34)2]n9n- (2.3), [Co8(OH)6(H2O)6{W8O28(OH)2}(PW9O34)2]16- (2.5), [Co11(OH)5(H2O)5(W6O24)(PW9O34)3]22– (2.6) and [{Co4(OH)3(H2O)(PW9O34)}2{K(H2W12O41)2}{Co(H2O)4}2]17- (2.7). The crystal structures, magnetic characterisation and stabilities in aqueous solutions of these POM derivatives are also presented. (3) In the third part, two structurally intriguing cobalt-substituted POMs are described, formulated as [CoII4(H2O)2(CoIIIW9O34)(PW9O34)]12– (2.8) and [Co2{Co3(H2O)(Co(OH)2W7O26)(PW9O34)}2]22– (2.9). 2.8 contains a pentacobalt cluster {CoII4CoIII} and represents the first asymmetric sandwich exhibiting Weakley’s topology, while POM 2.9 is made up of two capping B-α-[PW9O34]9– units and two bridging [W7O26]10– units that assemble to encapsulate a novel deca-Co(II) cluster core comprising octahedral and tetrahedral Co(II) ions. An original reaction based on the partial reductive hydrolysis of the penta-Co(II/III) cluster 2.8 leads to the formation of a higher nuclearity cluster 2.9. The chemical nature and physical properties of both compounds are studied by XPS, mass spectrometry, 31P-NMR spectroscopy, UV-vis spectroscopy, cyclic voltammetry and magnetic measurements. 3. Chapter 3 deals with a family of heteropolymolybdates-based lanthanoid single molecular magnets (SMMs), with the general formula [Ln(beta-Mo8O26)2]5- (Ln3+ = Tb (3.1), Dy (3.2), Ho (3.3), Er (3.4), Tm (3.5), and Yb (3.6)) that are soluble in organic solvents (such as acetone, acetonitrile, N,N-dimethylformamide). The magnetic properties of this series have been correlated with the nature and structure of the coordination site around the lanthanoid cations. The static and dynamic magnetic measurements of this family of complexes have been well studied. For the Ho (3.3) and Er (3.4) derivatives, slow relaxations of the magnetizations have been observed. A magneto-structural analysis of this family of compounds has been carried out, which is based on an effective crystal field model and compared with the results reported in analogous lanthanoid single ion magnets (SIMs) based on polyoxotungstates. 4. Chapter 4 describes the synthesis and full characterisation of two new POMs: a photo-responsive molecular polyanion with the formula {K[(A-alfa-PW9O34)2Fe2(C2O4)2]}15- (4.1) in which two Fe(III) ions are simultaneously coordinated by two A-α-[PW9O34]9− units and two oxalato ligands; and the analogous derivative with formula {K[(A-alfa-PW9O34)2Fe2(C3H2O4)2]}5- (4.2) containing malonato ligands. When irradiated with UV light, the potassium salt of 4.1, K15{K[(A-alfa-PW9O34)2Fe2(C2O4)2]}·29H2O, exhibits a remarkable photocolouration effect attributable to the partial reduction of the POM, giving rise to a mixed-valence species. The photo-induced colouration process is confirmed to be intramolecular and involves electron transfer from the oxalato ligands, which partially decompose releasing CO2, towards the Fe(III) ions and the POM units. The mechanism of this photocolouration process is confirmed by DRS, IR, XPS, Mössbauer spectroscopies, magnetism and elemental analysis. 5. General Conclusions. Although specific conclusions have been included at the end of each chapter, the most relevant overall conclusions are summarized in this part of the thesis. 6. Resumen. This section contains a resume of the thesis in Spanish. Most of the results reported in this thesis have already been published. A list of publications is given below: [1] Yan Duan, Juan M. Clemente-Juan, Carlos Giménez-Saiz and Eugenio Coronado: Cobalt clusters with cubane-type topologies based on trivacant polyoxometalate ligands. Inorganic Chemistry 2016, 55, 925 – 938; DOI: 10.1021/acs.inorgchem.5b02532. [2] Yan Duan, Juan Modesto Clemente Juan, J L G Fierro, Carlos Giménez-Saiz and Eugenio Coronado: A decacobalt(II) cluster with triple-sandwich structure obtained by partial reductive hydrolysis of a pentacobalt(II/III), Weakley-type, polyoxometalate. Chemical Communications 2016, 52, 13245 – 13248; DOI: 10.1039/C6CC05485D. [3] Yan Duan, Juan M. Clemente-Juan, Eugenio Coronado, Carlos Giménez-Saiz: Construction of larger polyoxometalates containing '[Co4(OH)3(H2O)6-n(PW9O34)]3-' (n = 3 or 5) as a common subunit. Crystal Growth & Design, Submitted. [4] Yan Duan, José J. Baldoví, Carlos Bustos, Salvador Cardona-Serra, Pierre Gouzerh, Richard Villanneau, Geoffrey Gontard, Juan Modesto Clemente Juan, Alejandro Gaita-Ariño, Carlos Giménez-Saiz, Anna Proust, Eugenio Coronado: Single ion magnets based on lanthanoid polyoxomolybdate complexes. Dalton Transactions 2016, 45, 16653 – 16660; DOI: 10.1039/C6DT02258H. [5] Yan Duan, João Carlos Waerenborgh, Juan Modesto Clemente Juan, Carlos Giménez-Saiz, Eugenio Coronado: Light-induced decarboxylation in a photo-responsive iron-containing complex based on polyoxometalate and oxalato ligands. Chemical Science 2017, 8, 305 – 315; DOI: 10.1039/C6SC01919F. In addition, in the course of this thesis I have been involved in the research dealing with the use of magnetic POMs in the theoretical modelling of SIMs and in quantum computing. This physics-oriented research has given rise to the following publications: [6] José J Baldoví, Juan M Clemente-Juan, Eugenio Coronado, Yan Duan, Alejandro Gaita-Ariño, Carlos Giménez-Saiz: Construction of a General Library for the Rational Design of Nanomagnets and Spin Qubits Based on Mononuclear f-Block Complexes. The Polyoxometalate Case. Inorganic Chemistry 2014, 53, 9976 − 9980; DOI: 10.1021/ic501867d. [7] Muhandis Shiddiq, Dorsa Komijani, Yan Duan, Alejandro Gaita-Ariño, Eugenio Coronado and Stephen Hill: Enhancing coherence in molecular spin qubits via atomic clock transitions. Nature 2016, 531, 348 – 351; DOI: 10.1038/nature16984. [8] José J Baldoví, Yan Duan, Roser Morales, Alejandro Gaita-Ariño, Eliseo Ruiz, Eugenio Coronado: Rational Design of Lanthanoid Single-Ion Magnets: Predictive Power of the Theoretical Models. Chemistry-A European Journal 2016, 22, 13532 – 13539; DOI: 10.1002/chem.201601741. [9] M. D. Jenkins, Y. Duan, B. Diosdado, J. J. García-Ripoll, A. Gaita-Ariño, C. Giménez-Saiz, P. J. Alonso, E. Coronado, F. Luis: Coherent manipulation of three-qubit states in a molecular single-ion magnet. Physical Review B 00, 004400 (2017); DOI: 10.1103/PhysRevB.00.004400.