Selbstorganisation eines riesigen tetraedrischen 3d-4f-Einzelmolekülmagneten innerhalb eines Polyoxometallatsystems

NMR Relaxivities of Paramagnetic, Ultra-High Spin Heterometallic Clusters within Polyoxometalate Matrix as a Function of Solvent and Metal Ion

Selectivity and image contrast are always challenging in magnetic resonance imaging (MRI), which are - inter alia - addressed by contrast agents. These compounds still need to be improved, and their relaxation properties, i. e., their paramagnetic relaxation enhancement (PRE), needs to be understood. The main goal is to improve specificity and relaxivities, especially at the high magnetic fields currently exploited not only in material science but also in the medical environment. Longitudinal and transverse relaxivities, r₁ and r₂ , which correspond to the longitudinal and transverse relaxation rates R₁ and R_2, normalized to the concentration of the paramagnetic moieties, need to be considered because both contribute to the image contrast. ¹ H-relaxivities r₁ and r₂ of high-spin heterometallic clusters were studied containing lanthanide and transition-metal ions within a polyoxometalate matrix. A wide range of magnetic fields from 0.5 T/20 MHz to 33 T/1.4 GHz was applied. The questions addressed here concern the rotational and diffusion correlation times which determine the relaxivities and are affected by the solvent's viscosity. Moreover, the variation of the lanthanide and transition-metal ions of the clusters provided insights into the sensitivity of PRE with respect to the electron spin properties of the paramagnetic centers as well as cooperative effects between lanthanides and transition metal ions.

NiII 36 -Containing 54-Tungsto-6-Silicate: Synthesis, Structure, Magnetic and Electrochemical Studies

The 36-NiII -containing 54-tungsto-6-silicate, [Ni36 (OH)18 (H2 O)36 (SiW9 O34 )6 ]6- (Ni36 ) was synthesized by a simple one-pot reaction of the Ni2 -pivalate complex [Ni2 (μ-OH2 )(O2 CCMe3 )4 (HO2 CCMe3 )4 ] with the trilacunary [SiW9 O34 ]10- polyanion precursor in water and structurally characterized by a multitude of physicochemical techniques including single-crystal XRD, FTIR, TGA, elemental analysis, magnetic and electrochemical studies. Polyanion Ni36 comprises six equivalent {NiII 6 SiW9 } units which are linked by Ni-O-W bridges forming a macrocyclic assembly. Magnetic studies demonstrate that the {Ni6 } building blocks in Ni36 remain magnetically intact while forming a hexagonal ring with antiferromagnetic exchange interactions between adjacent {Ni6 } units. Electrochemical studies indicate that the first reduction is reversible and associated with the WVI/V couple, whereas the second reduction is irreversible attributed to the NiII/0 couple.

Trigonal Prismatic Coordination of Discrete Rare Earth Ions, Enforced by the Polyoxotungstate [P4 W27 O99 (H2 O)]16

A family of solution-stable polyanions [Na⊂{LnIII (H2 O)}{WVI O(H2 O)}PV 4 WVI 26 O98 ]12- (Ln=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) represent the first examples of polyoxometalates comprising a single lanthanide(III) or yttrium(III) ion in a rare trigonal prismatic O6 environment. Their synthesis exploits the reactivity of the organophosphonate-functionalized precursor [P4 W24 O92 (C6 H5 PV O)2 ]16- with heterometal ions and yields hydrated potassium or mixed lithium/potassium salts of composition Kx Lny H12-x-y [Na⊂{Ln(H2 O)}{WO(H2 O)}P4 W26 O98 ]⋅nH2 O⋅mLiCl (x=8.5-11; y=0-2; n=24-34; m=0-1.5). The Dy, Ho, Er and Yb derivatives are characterized by slow magnetization relaxation.

Deliberate Construction of Polyoxoniobates Exploiting the Carbonate Ligand

Polyoxometalates (POMs, metals=V^V , Nb^V , Ta^V , Mo^VI , W^VI ) are molecular metal oxides that can be isolated without capping ligands. The high negative charge of polyoxoniobates (PONb) provides strong interactions with heterocations, advantageous for electrostatic assembly of modular materials. In four single-crystal X-ray structures, we demonstrate that carbonate combined with the very reactive decaniobate [Nb₁₀ O₂₈ ]^6- reassembles into a new decaniobate, [Nb₁₀ O₂₅ (CO₃ )₆ ]^12- , featuring three carbonate-ligated Nb-polyhedra. These Nb-sites can be replaced by heterometals (lanthanides), and the tridentate carbonate can serve as an anchor point to build niobate-frameworks. Small-angle X-ray scattering and two additional X-ray structures reveal that the reaction pathway proceeds through a Nb₂₄ -PONb intermediate, and the obtained PONb (with or without carbonate) is counterion, temperature, and solvent-dependent (water or mixed water-methanol). This provides an uncommon level of control for PONb chemistry.

Ion-Exchangeable Microporous Polyoxometalate Compounds with Off-Center Dopants Exhibiting Unconventional Luminescence

The synthesis of luminescent polyoxometalates (POMs) typically relies on the assembly of POM ligands with rare earth or transition metals, placing significant constraints on the composition, structure, and hence the luminescence properties of the resultant systems. Herein, we show that the ion-exchange strategy can be used for the synthesis of novel POM-based luminescent materials. We demonstrate that introducing bismuth ions into an ion-exchangeable, microporous POM compound yields an unconventional system luminescing in the near-infrared region. Experimental characterization, coupled with quantum chemical calculations, confirms that bismuth ions site-specifically occupy an off-center site in the lattice, and have an asymmetric coordination geometry unattainable by other means, thus giving rise to peculiar emission. Our findings offer an effective strategy for the synthesis of POM-based luminescent materials, and the design concept may potentially be adapted to the creation of POM-based systems with other functionalities.

A General Approach to Access Morphologies of Polyoxometalates in Solution by Using SAXS: An Ab Initio Modeling Protocol

Herein, we reported a general protocol for an ab initio modeling approach to deduce structure information of polyoxometalates (POMs) in solutions from scattering data collected by the small-angle X-ray scattering (SAXS) technique. To validate the protocol, the morphologies of a serious of known POMs in either aqueous or organic solvents were analyzed. The obtained particle morphologies were compared and confirmed with previous reported crystal structures. To extend the feasibility of the protocol to an unknown system of aqueous solutions of Na₂ MoO₄ with the pH ranging from -1 to 8.35, the formation of {Mo₃₆ } clusters was probed, identified, and confirmed by SAXS. The approach was further optimized with a multi-processing capability to achieve fast analysis of experimental data, thereby, facilitating in situ studies of formations of POMs in solutions. The advantage of this approach is to generate intuitive 3D models of POMs in solutions without confining information such as symmetries and possible sizes.

Lanthanoid Template Isolation of the α-1,5 Isomer of Dicobalt(II)-Substituted Keggin Type Phosphotungstates: Syntheses, Characterization, and Magnetic Properties

A new series of heterometallic 3d-4f sandwich type phosphotungstates, [Ln{PCo2W10O38(H2O)2}2](11-) (Ln = Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III), denoted 1a-10a, respectively), have been synthesized by a one-pot reaction procedure on reacting the dilacunary K14[P2W19O69(H2O)]·24H2O precursor with Ln(NO3)3·nH2O and Co(NO3)2·6H2O in an aqueous potassium chloride solution. All the compounds were isolated as potassium salts and further characterized with different analytical techniques such as single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution electrospray ionization mass spectrometry, elemental analysis by inductively coupled plasma atomic emission spectroscopy, magnetic measurement, and thermogravimetric analysis. Single-crystal X-ray diffraction analysis of the compounds reveals that all these compounds are isostructural and crystallized in the orthorhombic crystal system in space group Iba2. The polyanions contain the α-1,5 isomer of dicobalt-substituted α-Keggin phosphotungstate, which sandwiched lanthanoid cation and formed novel heterometallic dimer species. The temperature-dependent magnetic susceptibilities of 1a, 2a, 4a, and 7a-10a indicate the dominant contribution of the ferromagnetic interaction between Co(II) and Co(II) within the cluster, while the antiferromagnetic interaction between Co(II) and Ln(III) dominates for 3a, 5a, and 6a. The isothermal magnetizations of 1a-10a show a gradual increase in magnetization at low fields and do not reach saturation even at 50 kOe.