Room-temperature-persistent magnetic interaction between coordination complexes and nanoparticles in maghemite-based nanohybrids.

Maghemite nanoparticles functionalised with Co(II) coordination complexes at their surface show a significant increase of their magnetic anisotropy, leading to a doubling of the blocking temperature and a sixfold increase of the coercive field. Magnetometric studies suggest an enhancement that is not related to surface disordering, and point to a molecular effect involving magnetic exchange interactions mediated by the oxygen atoms at the interface as its source. Field- and temperature-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies show that the magnetic anisotropy enhancement is not limited to surface atoms and involves the core of the nanoparticle. These studies also point to a mechanism driven by anisotropic exchange and confirm the strength of the magnetic exchange interactions. The coupling between the complex and the nanoparticle persists at room temperature. Simulations based on the XMCD data give an effective exchange field value through the oxido coordination bridge between the Co(II) complex and the nanoparticle that is comparable to the exchange field between iron ions in bulk maghemite. Further evidence of the effectiveness of the oxido coordination bridge in mediating the magnetic interaction at the interface is given with the Ni(II) analog to the Co(II) surface-functionalised nanoparticles. A substrate-induced magnetic response is observed for the Ni(II) complexes, up to room temperature.

Orbital Magnetic Moment and Single-Ion Magnetic Anisotropy of the S = 1/2 K3[Fe(CN)6] Compound: A Case Where the Orbital Magnetic Moment Dominates the Spin Magnetic Moment.

The potassium hexacyanoferrate(III), K3[FeIII(CN)6], is known for its exceptional magnetic anisotropy among the 3d transition metal series. The Fe(III) ions are in the S = 1/2 low spin state imposed by the strong crystal field of the cyanido ligands. A large orbital magnetic moment is expected from previous publications. In the present work, X-ray magnetic circular dichroism was recorded for a powder sample, allowing direct measurement of the Fe(III) orbital magnetic moment. A combination of molecular multiconfigurational ab initio and atomic ligand field multiplets calculations provides the spin and orbital magnetic moments for the [FeIII(CN)6]3- isolated cluster, the crystallographic unit cell, and the powder sample. The calculations of the angular dependencies of the spin and orbital magnetic moments with the external magnetic induction direction reveal easy magnetization axes for each S = 1/2 molecular entity and the crystal. It also shows that the orbital magnetic moment dominates the spin magnetic moment for all directions. Our measurements confirm that the orbital magnetic moment contributes to 60% of the total magnetization for the powder, which is in excellent agreement with our theoretical predictions. An orbital magnetic moment greater than the spin magnetic moment is exceptional for 3d transition metal ions. The impact of crystal field strength and distortion, π back-bonding, spin-orbit coupling, and external magnetic induction was analyzed, leading to a deeper understanding of the spin and orbital magnetic anisotropies.

Molecular Magnetic Materials Based on {CoIII (Tp*)(CN)3 }- Cyanidometallate: Combined Magnetic, Structural and 59 Co NMR Study.

The cyanidocobaltate of formula fac-PPh4 [CoIII (Me2 Tp)(CN)3 ] ⋅ CH3 CN (1) has been used as a metalloligand to prepare polynuclear magnetic complexes (Me2 Tp=hydrotris(3,5-dimethylpyrazol-1-yl)borate). The association of 1 with in situ prepared [FeII (bik)2 (MeCN)2 ](OTf)2 (bik=bis(1-methylimidazol-2-yl)ketone) leads to a molecular square of formula {[CoIII {(Me2 Tp)}(CN)3 ]2 [FeII (bik)2 ]2 }(OTf)2 ⋅ 4MeCN ⋅ 2H2 O (2), whereas the self-assembly of 1 with preformed cluster [CoII 2 (OH2 )(piv)4 (Hpiv)4 ] in MeCN leads to the two-dimensional network of formula {[CoII 2 (piv)3 ]2 [CoIII (Me2 Tp)(CN)3 ]2 ⋅ 2CH3 CN}∞ (3). These compounds were structurally characterized via single crystal X-ray analysis and their spectroscopic (FTIR, UV-Vis and 59 Co NMR) properties and magnetic behaviours were also investigated. Bulk magnetic susceptibility measurements reveal that 1 is diamagnetic and 3 is paramagnetic throughout the explored temperature range, whereas 2 exhibits sharp spin transition centered at ca. 292 K. Compound 2 also exhibits photomagnetic effects at low temperature, selective light irradiations allowing to promote reversibly and repeatedly low-spin⇔high-spin conversion. Besides, the diamagnetic nature of the Co(III) building block allows us studying these compounds by means of 59 Co NMR spectroscopy. Herein, a 59 Co chemical shift has been used as a magnetic probe to corroborate experimental magnetic data obtained from bulk magnetic susceptibility measurements. An influence of the magnetic state of the neighbouring atoms is observed on the 59 Co NMR signals. Moreover, for the very first time, 59 Co NMR technique has been successfully introduced to investigate molecular materials with distinct magnetic properties.

Magnetic Hysteresis in a Monolayer of Oriented 6 nm CsNiCr Prussian Blue Analogue Nanocrystals.

Prussian blue analogue nanocrystals of the CsINiII[CrIII(CN)6] cubic network with 6 nm size were assembled as a single monolayer on highly organized pyrolytic graphite (HOPG). X-ray magnetic circular dichroism (XMCD) studies, at the Ni and Cr L2,3 edges, reveal the presence of an easy plane of magnetization evidenced by an opening of the magnetic hysteresis loop (coercive field of ≈200 Oe) when the magnetic field, B, is at 60° relative to the normal to the substrate. The angular dependence of the X-ray natural linear dichroism (XNLD) reveals both an orientation of the nanocrystals on the substrate and an anisotropy of the electronic cloud of the NiII and CrIII coordination sphere species belonging to the nanocrystals' surface. Ligand field multiplet (LFM) calculations that reproduce the experimental data are consistent with an elongated tetragonal distortion of surface NiII coordination sphere responsible for the magnetic behavior of monolayer.

Solvothermal synthesis, structure and magnetic properties of heterometallic coordination polymers based on a phenolato-oxamato co-bidentate-tridentate ligand.

The use of solvothermal conditions has succesfully led to the preparation of heterometallic 1D coordination polymers from a co-bidentate-tridentate phenolato-oxamato ligand. The reaction of the N-(2-hydoxyphenyl)oxamic acid (ohpma) with acetate salts of transition metal ions at 80 °C has yielded the heterobimetallic [Cu(ohpma)M(OAc)(DMF)2] (M = Co (1); Mn (2)) and the heterotrimetallic [Cu(ohpma)Co0.57Mn0.43(OAc)(DMF)2] (3) chain compounds. Single-crystal and powder diffraction studies show that the polymers are isostructural. Magnetic studies suggest the existence of an inter-chain two-dimensional antiferromagnetic interaction taking place in compounds 1-3.

A [FeIII(Tp)(CN)3]- scorpionate-based complex as a building block for designing ion storage hosts (Tp: hydrotrispyrazolylborate).

Using a scorpionate-based complex, [FeIII(Tp)(CN)3]-, as a building block, a new cyanide-based molecular material [{FeIII(Tp)(CN)3}2NiII(H2O)2]·4H2O (1), which can be viewed as a lower dimensional model of Prussian blue analogues, was investigated as a lithium-ion storage host.

Link between Affinity and Cu(II) Binding Sites to Amyloid-ß Peptides Evaluated by a New Water-Soluble UV-Visible Ratiometric Dye with a Moderate Cu(II) Affinity.

Being able to easily determine the Cu(II) affinity for biomolecules of moderate affinity is important. Such biomolecules include amyloidogenic peptides, such as the well-known amyloid-ß peptide involved in Alzheimer's disease. Here, we report the synthesis of a new water-soluble ratiometric Cu(II) dye with a moderate affinity (109 M-1 at pH 7.1) and the characterizations of the Cu(II) corresponding complex by X-ray crystallography, EPR, and XAS spectroscopic methods. UV-vis competition was performed on the Aß peptide as well as on a wide series of modified peptides, leading to an affinity value of 1.6 × 109 M-1 at pH 7.1 for the Aß peptide and to a coordination model for the Cu(II) site within the Aß peptide that agrees with the one mostly accepted currently.

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes.

Superparamagnetic nanoparticles are promising objects for data storage or medical applications. In the smallest--and more attractive--systems, the properties are governed by the magnetic anisotropy. Here we report a molecule-based synthetic strategy to enhance this anisotropy in sub-10-nm nanoparticles. It consists of the fabrication of composite materials where anisotropic molecular complexes are coordinated to the surface of the nanoparticles. Reacting 5 nm γ-Fe2O3 nanoparticles with the [Co(II)(TPMA)Cl2] complex (TPMA: tris(2-pyridylmethyl)amine) leads to the desired composite materials and the characterization of the functionalized nanoparticles evidences the successful coordination--without nanoparticle aggregation and without complex dissociation--of the molecular complexes to the nanoparticles surface. Magnetic measurements indicate the significant enhancement of the anisotropy in the final objects. Indeed, the functionalized nanoparticles show a threefold increase of the blocking temperature and a coercive field increased by one order of magnitude.

Synthesis, structure and magnetic properties of phenylhydroxamate-based coordination clusters.

The strategic recombination of preformed coordination clusters in the presence of polymodal bridging ligands has successfully led to the characterisation of five new compounds of structural and magnetic interest. Indeed using the dinuclear complex [M2(H2O)(piv)4(Hpiv)4] (M = Co, Ni; Hpiv = pivalic acid) as starting material and reacting it with phenylhydroxamic acid (H2pha) has yielded the four tetrametallic coordination clusters [Co4(Hpha)2(piv)6(Hpiv)4] (1), [Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2] (2), [Co4(Hpha)2(piv)6(EtOH)2(H2O)2] (3), [Ni4(Hpha)2(piv)6(EtOH)2(H2O)2] (4) and the hexanuclear complex [Co6(Hpha)4(piv)8(EtOH)2]·EtOH (5). All the compounds have been structurally characterised revealing a particular binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed and the modelling of these properties has been done using the appropriate hamiltonians for each compound. The experimental data and their modelling show non-zero spin ground states for compounds 4 and 5.

Synergy in photomagnetic/ferromagnetic sub-50 nm core-multishell nanoparticles.

Based on nickel hexacyanidochromate and cobalt hexacyanidoferrate Prussian blue analogues, two series of photomagnetic/ferromagnetic sub-50 nm core multishell coordination nanoparticles have been synthesized in a surfactant-free one-pot multistep procedure with good control over the dispersity (10% standard deviation) and good agreement with the targeted size at each step. The composition and the valence state of each shell have been probed by different techniques that have revealed the predominance of Co(II)-NC-Fe(III) pairs in a series synthesized without alkali while Co(III)-NC-Fe(II) photoswitchable pairs have been successfully obtained in the photoactive coordination nanoparticles by control of Cs(+) insertion. When compared, the photoinduced behavior of the latter compound is in good agreement with that of the model one. Exchange coupling favors a uniform reversal of the magnetization of the heterostructured nanoparticles, with a large magnetization brought by a soft ferromagnetic shell and a large coercivity due to a harder photomagnetic shell. Moreover, a persistent increase of the photoinduced magnetization is observed for the first time up to the ordering temperature (60 K) of the ferromagnetic component because of a unique synergy.

A cyanide and hydroxo-bridged nanocage: a new generation of coordination clusters.

Combining serendipitously-formed hydroxo-clusters, [Co(II)(3)(OH)(piv)(4)(L)](+) (where L = MeCN or Hpiv), with assembling cyanide building block, [Fe(III)(Tp)(CN)(3)](-), has led to an unprecedented architecture where polymetallic cobalt clusters and blocked tris-cyanide iron complexes define the apexes of a unique magnetic cubic nanocage.

Tuning the magnetic anisotropy in coordination nanoparticles: random distribution versus core-shell architecture.

Core-shell magnetic coordination nanoparticles made of a soft core and a hard magnetic shell, containing anisotropic Co(II) ions, display a dramatic increase in their average blocking temperature with a coercive field value 25 times larger than that of the soft core, due to a large enhancement of the magnetic anisotropy.

Investigation of the photoinduced magnetization of copper octacyanomolybdates nanoparticles by X-ray magnetic circular dichroism.

Through an extensive set of SQUID magnetic measurements, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism, we have determined the nature of the metastable photomagnetic phase in the cyano-bridged 3D network Cs(2)Cu(7)[Mo(CN)(8)](4). The photomagnetic effect is induced by the photoconversion of Mo(IV) ions in low spin (LS) configuration (S = 0) into Mo(IV) ions in high spin (HS) configuration (S = 1). The magnetic and spectroscopic measurements fully support the LS to HS conversion, whereas the previously invoked charge transfer mechanism Mo(IV) + Cu(II) ⇒ Mo(V) + Cu(I) can be completely ruled out.

Tailored coordination nanoparticles: assessing the magnetic single-domain critical size.

Negatively charged nanocrystals of the magnetic coordination network CsNiCr(CN)(6) were prepared in water through a seed-mediated growth with a few atomic layers accuracy and final sizes tailored from 6 to 30 nm. A lower limit of the magnetic single-domain critical size was determined to be around 15 nm possessing a blocking temperature above 20 K.

Effect of cyanato, azido, carboxylato, and carbonato ligands on the formation of cobalt(II) polyoxometalates: characterization, magnetic, and electrochemical studies of multinuclear cobalt clusters.

Five Co(II) silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K(20)[{(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)}(2)Co(H(2)O)(2)]47 H(2)O (1) consists of two {(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)} units connected by a {CoO(4)(H(2)O)(2)} group. In the chiral species K(7)[Co(1.5)(H(2)O)(7))][(gamma-SiW(10)O(36))(beta-SiW(8)O(30)(OH))Co(4)(OH)(H(2)O)(7)]36 H(2)O (2), a {gamma-SiW(10)O(36)} and a {beta-SiW(8)O(30)(OH)} unit enclose a mononuclear {CoO(4)(H(2)O)(2)} group and a {Co(3)O(7)(OH)(H(2)O)(5)} fragment. The two trinuclear Co(II) clusters present in 1 enclose a mu(4)-O atom, while in 2 a mu(3)-OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co-L-Co (L=mu(4)-O (1), mu(3)-OH (2)) bridging angles in 2 (theta(av(Co-L-Co))=99.1 degrees ) than in 1 (theta(av(Co-L-Co))=92.8 degrees ). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co-L-Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of Co(II) centers, and indicated that two types of Co(II) centers in the structure were oxidized in separate waves. Redox activity of the seventh Co(II) center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(3)(CH(3)COO)(3)]18 H(2)O (3) and K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(N(3))(2)(CH(3)COO)(3)]18 H(2)O (4) were characterized: in both, a tetrahedral {Co(4)(L(1))(L(2))(2)(CH(3)COO)(3)} (3: L(1)=L(2)=OH; 4: L(1)=OH, L(2)=N(3)) unit capped the [A-alpha-SiW(9)O(34)](10-) trivacant polyanion. The octanuclear complex K(8)Na(8)[(A-alpha-SiW(9)O(34))(2)Co(8)(OH)(6)(H(2)O)(2)(CO(3))(3)]52 H(2)O (5), containing two {Co(4)O(9)(OH)(3)(H(2)O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed.

Synthesis and characterizations of cyclic octanuclear mixed-valence vanadium(IV,V) clusters with polyoxometalate counterions.

Hydrothermal reaction of Na2WO4, VOSO4, 2,2'-bpy and H3PO4 has afforded in high yield the compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PW11V(V)O40](bpy).12H2O (1). Compound 1 contains a novel octanuclear mixed valence V(IV,V) cluster, [V(IV)2V(V)6O14(bpy)8(PO4)2]4+, with [PW11V(V)O40]4- as counterion. In the vanadium cluster, four V(V) centers are localized and the remaining two V(IV) and two V(V) ions are disordered over four crystallographically equivalent positions. The isostructural compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PMo11V(V)O40](bpy).3H2O (2) has also been synthesized. Thermodiffractometry experiments indicate that 2 is stable up to 360 degrees C. Redox activities for both the vanadium and molybdenum centers have been observed by solid-state electrochemical measurements performed on mechanically attached microparticles of 2. Magnetic measurements performed on have shown the occurrence of weak ferromagnetic interactions between the V(IV) centres (J = +0.34 cm(-1), H(ex) = -JS1 x S2), and combined with DFT calculations, have allowed to propose a localization of the two V(IV) centers on two of the four equivalent crystallographic sites. Finally high field electron paramagnetic resonance has evidenced the magnetic axial anisotropy of the paramagnetic centers (g(x) = g(y) = 1.975(3); g(z) = 1.939(4)).

Synthesis and characterization of octa- and hexanuclear polyoxomolybdate wheels: role of the inorganic template and of the counterion.

Eight new compounds based on [O3PCH2PO3]4- ligands and {MoV2O4} dimeric units have been synthesized and structurally characterized. Octanuclear wheels encapsulating various guests have been isolated with different counterions. With NH4+, a single wheel was obtained, as expected, with the planar CO32- guest, (NH4)12[(MoV2O4)4(O3PCH2PO3)4(CO3)2].24H2O (1a), while with the pyramidal SO32- guest, only the syn isomer (NH4)12[(MoV2O4)4(O3PCH2PO3)4(SO3)2].26H2O (2a) was characterized. The corresponding anti isomer was obtained with Na+ as counterions, Na12[(MoV2O4)4(O3PCH2PO3)4(SO3)2]39H2O (2b), and with mixed Na+ and NH4(+) counterions, Na+(NH4)11[(MoV2O4)4(O3PCH2PO3)4(SO3)2].13H2O (2d). With [O3PCH2PO3]4- extra ligands, the octanuclear wheel Li12(NH4)2[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2].31H2O (4a) was isolated with Li+ and NH4+ counterions and Li14[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2].34H2O (4c) as a pure Li+ salt. A new rectangular anion, formed by connecting two MoV dimers and two MoVI octahedra via methylenediphosphonato ligands with NH4+ as counterions, (NH4)10[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2(HO3PCH2PO3)2].15H2)O (3a), and Li9(NH4)2Cl[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2]. 22H2O (3d) as a mixed NH4+ and Li+ salt have also been synthesized. The structural characterization of the compounds, combined with a study of their behavior in solution, investigated by 31P NMR, has allowed a discussion on the influence of the counterions on the structure of the anions and their stability. Density functional theory calculations carried out on both isomers of the [(MoV2O4)4(O3PCH2PO3)4(SO3)2]12- anion (2), either assumed isolated or embedded in a continuum solvent model, suggest that the anti form is favored by approximately 2 kcal mol(-1). Explicit insertion of two solvated counterions in the molecular cavity reverses this energy difference and reduces it to less than 1 kcal mol(-1), therefore accounting for the observed structural versatility.

Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups.

Five new materials built from polyoxotungstates and Cu(ii) ions as linkers have been synthesized by hydrothermal reactions from a mixture of sodium tungstate, copper chloride and bipyridine. The value of the initial pH, the nature of the heteroelement (P or Si) and of the ligand (2,2'- and/or 4,4'-bipyridine) permit the control of the nature of the polyoxotungstate clusters and their connectivity via the copper ions, and hence the dimensionality of the framework. A single phase has been obtained with silicon as heteroelement at an initial pH of 5, namely the 2D material [SiW(12)O(40)][Cu(2,2'-bpy)(2)](2).10H(2)O (1) with saturated Keggin polyoxotungstates linked by {Cu(2,2'-bpy)(2)}(2+) groups. With phosphorous as heteroelement and at the same initial pH, three different structures have been isolated according to the nature of the ligand. Indeed, the two 1D materials [{Cu(5)(2,2'-bpy)(5)(H(2)O)(HPO(4))(PO(4))}PW(11)CuO(39)].6H(2)O (2) with 2,2'-bpy and [4,4'-Hbpy][{Cu(2)(2,2'-bpy)(2)(4,4'-bpy)(2.5)}PW(11)CuO(39)].16H(2)O (3) with a mixture of 2,2'- and 4,4'-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na(3)[4,4'-Hbpy]{Cu(4)(4,4'-bpy)(8)(H(2)O)(8)}[PW(11)CuO(39)(H(2)O)][PW(10)Cu(2)O(38)(H(2)O)(2)].38H(2)O (4) with 4,4'-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Na(2)[{Cu(8)(2,2'-bpy)(8)}(PW(8)O(31))(2)].15H(2)O (5) has been evidenced. Magnetic studies of compound 2 revealed that the predominant interactions involve only 4 paramagnetic centers, which are interacting within pairs, among the 6 Cu(ii) centers. The chi(M)T=f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H=-JS(1)xS(2), with S(1)=S(2)=(1/2) and J=-105.4 cm(-1), showing strong antiferromagnetic interactions within the two Cu(ii) pairs.

Solid-State and solution studies of [Ln(n)(SiW11O39)] polyoxoanions: an example of building block condensation dependent on the nature of the rare earth.

The reactivity of the [alpha-SiW(11)O(39)](8-) monovacant polyoxometalate with lanthanide has been investigated for four different trivalent rare-earth cations (Ln = Nd(III), Eu(III), Gd(III), Yb(III)). The crystal structures of KCs(4)[Yb(alpha-SiW(11)O(39))(H(2)O)(2)] x 24H(2)O (1), K(0.5)Nd(0.5)[Nd(2)(alpha-SiW(11)O(39))(H(2)O)(11)] x 17H(2)O (2a), and Na(0.5)Cs(4.5)[Eu(alpha-SiW(11)O(39))(H(2)O)(2)] x 23H(2)O (3a) are reported. The solid-state structure of compound 1 consists of linear wires built up of [alpha-SiW(11)O(39)](8-) anions connected by Yb(3+) cations, while the linkage of the building blocks by Eu(3+) centers in 3a leads to the formation of zigzag chains. In 2a, dimeric [Nd(2)(alpha-SiW(11)O(39))(2)(H(2)O)(8)](10-) entities are linked by four Nd(3+) cations. The resulting chains are connected by lanthanide ions, leading to a bidimensional arrangement. Thus, the dimensionality, the organization of the polyoxometalate building units, and the Ln/[alpha-SiW(11)O(39)](8-) ratio in the solid state can be tuned by choosing the appropriate lanthanide. The luminescent properties of compound 3a have been studied, showing that, in solution, the polymer decomposes to give the monomeric complex [Eu(alpha-SiW(11)O(39))(H(2)O)(4)](5-). The lability of the four exogenous ligands connected to the rare earth must allow the functionalization of this lanthanide polyanion.