Comparative NMR Relaxivity Study of Polyoxometalate-Based Clusters [Mn4(H2O)2(P2W1SO56)2]16− and [{Dy(H2O)6}2Mn4(H2O)2(P2W15O56)2]10− from 20 MHz to 1.2 GHz

Abstract

Nuclear Magnetic Resonance relaxivities are a measure for the sensitivity of a contrast agent (CA), i.e. the potential of a paramagnetic moiety to enhance longitudinal and transverse relaxation of molecules in its near neighbourhood. The underlying mechanism is called Paramagnetic Relaxation Enhancement (PRE). The relaxivity, characterizing PRE, depends not only on the external applied magnetic field but also depends on numerous factors, such as number of coordinated water molecules, water exchange rate, rotational diffusion, first and second coordination hydration sphere, electronic and magnetic properties of paramagnetic centers and the molecular shape/size of the CA. Relaxation rates are usually normalized to the concentration of the contrast agent to provide the relaxivities. To investigate the influence of these factors on PRE of newly synthesized potential CA, two paramagnetic metals containing polyoxometalates (POMs) [Mn4(H2O)2(P2W15O56)2]16− (Mn4-P2W15) and [{Dy(H2O)6}2Mn4(H2O)2(P2W15O56)2]10− (Dy2Mn4-P2W15) were selected as models to be studied at 1H Larmor frequencies from 20 MHz to 1.2 GHz. Structurally, the POM Dy2Mn4-P2W15 is similar to the tetra-nuclear manganese(II)-substituted sandwich-type POM Mn4-P2W15, with the two coordinated DyIII cations acting as linkers connecting Mn4-P2W15 units, thus forming a 1D ladder-like chain structure based on sandwich-type rungs strung together by the dysprosium cations. This study shows that POM (Dy2Mn4-P2W15) is a promising CA at high magnetic fields and proves that the use of heterometallic clusters is an effective strategy to increase PRE due to the synergistic effects from different metal ions.

NMR Relaxivities of Paramagnetic Lanthanide-Containing Polyoxometalates

“…To investigate the paramagnetic relaxation enhancement of the POMs [Dy-III (H2O)4GeW11O39] 5-(Dy-W11), [Er III (H2O)3GeW11O39] 5− (Er-W11) and [{Er III (H2O)(CH3COO)(P2W17O61)}2] 16− (Er2-W34), longitudinal and transverse relaxivity measurements were performed at NMR Larmor frequencies from 20 MHz to 1.4 GHz. The detailed experimental procedures are explained in the previous publications [20][21][22]40]. For the current studies, pure crystals of POMs were dissolved in 9:1 D2O/H2O, and sets of five dilutions were prepared: 0-, 0.2-, 0.4-, 0.6-, 0.8-and 1-mM of complex.…”

“…This indicates that the solution of the clusters is stable and no phase separation occurs. Both longitudinal and transverse relaxation rates of these POM clusters were determined [20][21][22]40]. For the current studies, pure crystals of POMs were dissolved in 9:1 D 2 O/H 2 O, and sets of five dilutions were prepared: 0-, 0.2-, 0.4-, 0.6-, 0.8-and 1-mM of complex.…”

NMR-Relaxometric Investigation of Mn(II)-Doped Polyoxometalates in Aqueous Solutions

“…One of the dominant research directions in the chemistry of nanomaterials is the development of new nanoprobes for medical imaging [ 1 ]. In this respect, polyoxometalates (POM) constitute promising candidates for development of such nanoprobes due to the following advantages: (i) biological activity (e.g., anti-RNA [ 2 ], anti-influenza [ 3 ], anti-COVID-19 [ 4 ]), which makes possible teranostic applications [ 5 ]; (ii) tendency for self-organization into hierarchically ordered nanostructures [ 6 , 7 , 8 ]; and (iii) suitable magnetic-relaxation characteristics of POM-based complexes [ 9 , 10 ]. From a structural point of view, spherical or nearly spherical nano-sized polyoxometalates have an internal cavity and pores on the surface, allowing an exchange between the mass of the solution and the internal cavity of the clusters [ 11 ].…”