Guillaume Rousseau

A stable hybrid bisphosphonate polyoxometalate single-molecule magnet.

The long history of polyoxometalate (POM) chemistry initially featured the characterization of diamagnetic species. However, the last few decades have seen the development of several families of paramagnetic POM molecules that display a diverse range of structures and properties. These entities can be of nanosize scale, as exemplified by the discovery of the spin-frustrated Keplerate clusters of general formula {(Mo)Mo5}12M30 (M=Fe , Cr, V). Although of smaller size than these molybdenum compounds, high-nuclearity polyoxovanadate mixed-valent species have also been characterized. The unexpectedly strong magnetic couplings observed in a number of these systems have been studied in detail both experimentally and theoretically. The third— and most extended—magnetic POM family is that of polyoxotungstates. Large clusters in which vacant polyoxotungstate ligands encapsulate 3d or 4f metals have been obtained, and most of these systems are purely inorganic. In particular, very few hybrid polynuclear 3d magnetic polyoxotungstates synthesized under mild conditions have been characterized to date. Moreover, to the best of our knowledge, no general method for the preparation of hybrid, soluble, and stable polyoxotungstates encapsulating magnetic clusters has been proposed. This contrasts with the case of the diamagnetic polyoxotungstates, for which numerous triol-functionalized, organosilyl and organostannic derivatives have been reported. Hybrid molybdenum-based POM chemistry has also been widely developed, but mainly {Mo6} Lindqvist and Anderson-type systems encapsulating a maximum of one 3d cation have been used as a platform for connecting various organic substrates. The establishment of a method allowing the rational synthesis of stable hybrid POMs incorporating several divalent or trivalent 3d metal centers thus remains a challenge. Such materials would be of interest in various fields as they would permit the covalent grafting of POMs containing 3d clusters to a range of organic substrates or surfaces, and thus the elaboration of new functional materials. For example, the covalent connection of photoactive organic groups to multi-electrochromic 3d-substituted inorganic entities may afford optical materials with new properties. Alternatively, Co-substituted polyoxotungstates have been shown to be efficient photocatalysts, which may in itself motivate the search for related hybrid materials. Clearly, hybrid 3d magnetic polyoxotungstates are also interesting in the molecular magnetism field. Since 2008, it is known that 3dor 4f-substituted polyoxotungstates can behave as single molecule magnets (SMMs). Moreover, deposition of POM SMMs on single-wall carbon nanotubes (SWCNTs) has provided evidence that individual POM molecules present a slow relaxation of the magnetization, a crucial point with respect to the potential use of such nanocomposites as information storage devices. However, in such a system, no control of the SWCNT/POM interface is possible. This is due to the purely inorganic nature of the deposited POM—a characteristic of all the 3d POM SMMs species reported to date. Indeed, the reinforcement of the interactions between the two components via covalent grafting or p–p stacking together with the control of the POM/substrate distance obviously implies the elaboration of hybrid POM SMM systems. Herein, we report the characterization of the polyanion [{(B-a-PW9O34)Co3(OH) ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(O3PC(O) ACHTUNGTRENNUNG(C3H6NH3)PO3)}2Co] 14 (1), which is built of a heptanuclear Co core sandwiched by two {PW9} units and connected to two bisphosphonato ligands, each possessing a functionalizable alkyl ammonium arm. The stability of this hybrid unit has been unambiguously assessed by using P and H NMR spectroscopy and electrochemical techniques. Single-crystal studies performed on an array of micro-SQUIDs have revealed that the ferromagnetic species 1 exhibits SMM behavior. [a] Dr. H. El Moll, Dr. A. Dolbecq, Dr. J. Marrot, G. Rousseau, Dr. M. Haouas, Dr. F. Taulelle, Prof. P. Mialane Institut Lavoisier de Versailles UMR 8180, Universit de Versailles Saint-Quentin 45 Avenue des Etats-Unis, 78035 Versailles Cedex (France) Fax: (+33)1-39-25-43-81 E-mail : mialane@chimie.uvsq.fr [b] Dr. G. Rogez Institut de Physique et de Chimie des Mat riaux de Strasbourg (IPCMS) UMR 7504, CNRS Universit de Strasbourg 23 rue du Lœss, BP 43, 67034 Strasbourg Cedex 2 (France) [c] Dr. W. Wernsdorfer Institut N el, CNRS et Universit Joseph Fourier BP 166, F-38042 Grenoble Cedex 9 (France) [d] Dr. B. Keita Laboratoire de Chimie Physique Groupe d’Electrochimie et de Photo lectrochimie, UMR 8000, CNRS Universit Paris-Sud, B timent 350, 91405 Orsay cedex (France) E-mail : bineta.keita@lcp.u-psud.fr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200140.

Route for the elaboration of functionalized hybrid 3d-substituted trivacant Keggin anions.

On the basis of four examples involving various transition-metal cations and various carboxylic acid derivatives, we have shown that hybrid 3d metal-containing polyoxometalate systems can be easily built under mild conditions.

ε-Keggin-based coordination networks: Synthesis, structure and application toward green synthesis of polyoxometalate@graphene hybrids

Four coordination networks based on the {ε-PMo(V)(8)Mo(VI)(4)O(40)(OH)(4)Zn(4)} Keggin unit (εZn) have been synthesized under hydrothermal conditions. (TBA)(3){PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)}[C(6)H(4)(COO)(2)](2) (ε(isop)(2)) is a 2D material with monomeric εZn units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations lying between the planes. In (TPA)(3){PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)}[C(6)H(3)(COO)(3)] (TPA[ε(trim)](∞)), 1D inorganic chains formed by the connection of εZn POMs, via Zn-O bonds, are linked via 1,3,5 benzenetricarboxylate (trim) ligands into a 2D compound with tetrapropylammonium (TPA) cations as counter-cations. (TBA){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(C(7)H(4)N(2))(2)(C(7)H(5)N(2))(2)·12H(2)O (ε(bim)(4)) is a molecular material with monomeric εZn POMs bound to terminal benzimidazole (bim) ligands. Finally, (TBA)(C(10)H(10)N(4))(2)(HPO(3)){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(2)(C(10)H(9)N(4))(3)(C(10)H(8)N(4)) (ε(2)(pazo)(4)) is a 1D compound with dimeric (εZn)(2) POMs connected by HPO(3)(2-) ions and terminal para-azobipyridine (pazo) ligands. In this compound an unusual bond cleavage of the central N[double bond, length as m-dash]N bond of the pazo ligand is observed. We report also a green chemistry-type one-step synthesis method carried out in water at room temperature using ε(2)(pazo)(4) and ε(isop)(2) as reducing agent of graphite oxide (GO) to obtain graphene (G). The POM@G hybrids were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, powder X-ray diffraction, energy dispersive X-ray analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and cyclic voltammetry.

Properties of a Tunable Multinuclear Nickel Polyoxotungstate Platform

A series of hybrid nickel bisphosphonate (BP) polyoxometalates (POMs) has been isolated. The complexes NaK-Ni7-Ale2 (Ale = [H2O3PC(C3H6NH2)(OH)PO3H2]) and NaNH4-Ni7-Ale2 are both made of two {PW9O34} fragments enclosing a heptanuclear Ni(II) core connected to two alendronate ligands. By pre-functionalising the alendronate moiety through the amino group, the naphthalene (napht) derivative Ni7-(AleNapht)2 and the compounds Ni7-(AlePy2 Ni)2 (py = pyridyl) and Ni7-(AleAc2Ni)2 (Ac = acyl) have been obtained. Compared with the NaK-Ni7-Ale2 species, these last two complexes contain two additional Ni(II) centres connected through two bis(2-pyridylmethyl)amine or two iminodiacetate groups, respectively. This results show that it is possible to functionalise the heptanuclear POM in a controlled manner. Quantification of the magnetic interactions in NaK-Ni7-Ale2 revealed that in the {Ni7} core, ferromagnetic interactions are predominant, with a S = 5 ground state. Magnetisation versus dc field sweeps on a single crystal of NaK-Ni7-Ale2 exhibited hysteresis at low temperature. (1)H and (31)P NMR studies in aqueous solution performed on NaK-Ni7-Ale2 and Ni7-(AleNapht)2 evidenced that the PW9/Ni7/bisphosphonate assembly is stable in solution. This was completed by (31)P magic angle spinning (MAS) investigations and confirmed by (1)H DOSY experiments. The electrochemistry of these compounds proceeds through two well-defined four-electron chemically reversible waves in a medium at pH 6. NaK-Ni7-Ale2 proved to be efficient for the electrocatalytic reduction of nitrate, nitrite and nitrous oxide. Remarkably, its electrocatalytic efficiency for nitrate reduction is approximately three times higher than those previously reported for POMs in a medium at pH >4 under the same potential. The catalytic properties of two representatives of the hybrid family were also examined. It is shown that these nickel bisphosphonate polyoxotungstates are pre-catalysts for the oxidation of alcohols into ketones or carboxylic acids, depending on the classes of alcohols considered, the stoichiometric oxidant used being H2O2. Noticeably, it has been found that an analogous cobalt bisphosphonate polyoxotungstate complex does not present any related activity, highlighting the crucial role of the 3d cations on the catalytic process.

Polyoxomolybdate Bisphosphonate Heterometallic Complexes: Synthesis, Structure, and Activity on a Breast Cancer Cell Line

Six polyoxometalates containing Mn(II) , Mn(III) , or Fe(III) as the heteroelement were synthesized in water by treating Mo(VI) precursors with biologically active bisphosphonates (alendronate (Ale), zoledronate (Zol), an n-alkyl bisphosphonate (BPC9 ), an aminoalkyl bisphosphonate (BPC8 NH2 )) in the presence of additional metal ions. The Pt complex was synthesized from a polyoxomolybdate bisphosphonate precursor with Mo(VI) ions linked by the 2-pyridyl analogue of alendronate (AlePy). The complexes Mo4 Ale2 Mn, Mo4 Zol2 Mn, Mo4 Ale2 Fe, Mo4 Zol2 Fe, Mo4 (BPC8 NH2 )2 Fe, and Mo4 (BPC9 )2 Fe contain two dinuclear Mo(VI) cores bound to a central heterometallic ion. The oxidation state of manganese was determined by magnetic measurements. Complexes Mo12 (AlePy)4 and Mo12 (AlePy)4 Pt4 were studied by solid-state NMR spectroscopy and the photochromic properties were investigated in the solid state; both methods confirmed the complexation of Pt. Activity against the human breast adenocarcinoma cell line MCF-7 was determined and the most potent compound was Mn(III) -containing Mo4 Zol2 Mn (IC50 ≈1.3 μM). Unlike results obtained with vanadium-containing polyoxometalate bisphosphonates, cell growth inhibition was rescued by the addition of geranylgeraniol, which reverses the effects of bisphosphonates on isoprenoid biosynthesis/protein prenylation. The results indicate an important role for both the heterometallic element and the bisphosphonate ligand in the mechanism of action of the most active compounds.

Sequential Synthesis of 3 d–3 d, 3 d–4 d, and 3 d–5 d Hybrid Polyoxometalates and Application to the Electrocatalytic Oxygen Reduction Reactions

The Co7 (AlePy)2 polyoxometalate, which encloses a {(PW9 )2 Co(II) 7 } core covalently bound to two free aminopyridine groups through bisphosphonate ligands (AlePy), has been isolated. It can be used as a precursor, allowing the synthesis of heterometallic hybrid compounds, as illustrated by the characterization of cobalt/zinc (Co7 (AlePyZn)2 ), cobalt/palladium (Co7 (AlePyPd)2 ), and cobalt/platinum (Co7 (AlePyPt)2 ) species. A composite based on the water-insoluble precious metal-free Co7 (AlePyZn)2 compound and the low-cost carbon material Vulcan XC-72 has been selected as a cathode material (Co7 Zn/C) for oxygen reduction reaction studies. The electrocatalytic performances of the Co7 Zn/C hybrids were assessed at neutral and basic pH, showing that Co7 Zn/C exhibits high selectivity for the four-electron reduction of O2 . Moreover, its durability is superior to that of a commercial Pt/C catalyst with 20 % loading. Also, comparative studies performed in the presence of methanol have indicated that Co7 Zn/C has a much better tolerance to the crossover effect than Pt/C. Altogether, these results indicate for the first time that, even in neutral media, polyoxometalate/carbon composites can represent low-cost oxygen reduction catalysts that can function stably, for a long time, and with high performance.

Molybdenum Bisphosphonates with Cr(III) or Mn(III) Ions

The synthesis of MoVI bisphosphonates (BPs) complexes in the presence of a heterometallic element has been studied. Two different BPs have been used, the alendronate ligand, [O3PC(C3H6NH3)(O)PO3]4− (Ale) and a new BP derivative with a pyridine ring linked to the amino group, [O3PC(C3H6NH2CH2C5H4N)(O)PO3]4− (AlePy). Three compounds have been isolated, a tetranuclear MoVI complex with CrIII ions, (NH4)5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Cr]·11H2O (Mo4(Ale)2Cr), its MnIII analogue, (NH4)4.5Na0.5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Mn]·9H2O (Mo4(Ale)2Mn), and a cocrystal of two polyoxomolybdates, (NH4)10Na3[(Mo2O6)2(O3PC(C3H6NH2CH2C5H4N)(O)PO3)2Cr]2[CrMo6(OH)6O18]·37H2O ([Mo4(AlePy)2Cr]2[CrMo6]). In this latter compound an Anderson-type POM [CrMo6(OH)6O18]3− is sandwiched between two tetranuclear MoVI complexes with AlePy ligands. The protonated triply bridging oxygen atoms bound to the central CrIII ion of the Anderson anion develop strong hydrogen bonding interactions with the oxygen atoms of the bisphosphonate complexes. The UV–Vis spectra confirm the coexistence in solution of both POMs. Cyclic voltammetry experiments have been performed, showing the reduction of the Mo centers. In strong contrast with the reported MoVI BP systems, the presence of trivalent cations in close proximity to the MoVI centers dramatically impact the potential solid-state photochromic properties of these compounds.