Comparative NMR Relaxivity Study of Polyoxometalate-Based Clusters [Mn$_{4}$(H$_{2}$O)$_{2}$(P$_{2}$W$_{1S}$O$_{56}$)$_{2}$]$^{16-}$ and [{Dy(H$_{2}$O)$_{6}$}$_{2}$Mn$_{4}$(H$_{2}$O)$_{2}$(P$_{2}$W$_{15}$O$_{56}$)$_{2}$]$^{10-}$ from 20 MHz to 1.2 GHz

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) [Mn$_{4}$(H$_{2}$O)$_{2}$(P$_{2}$W$_{15}$O$_{56}$)$_{2}$]$^{16-}$ (Mn$_{4-}$P$_{2}$W$_{15}$) and [{Dy(H$_{2}$O)$_{6}$}$_{2}$Mn$_{4}$(H$_{2}$O)$_{2}$(P$_{2}$W$_{15}$O$_{56}$)$_{2}$]$^{10-}$ (Dy$_{2}$Mn$_{4}$-P$_{2}$W$_{15}$) were selected as models to be studied at $^{1}$H Larmor frequencies from 20 MHz to 1.2 GHz. Structurally, the POM Dy$_{2}$Mn$_{4}$-P$_{2}$W$_{15}$ is similar to the tetra-nuclear manganese(II)-substituted sandwich-type POM Mn$_{4}$-P$_{2}$W$_{15}$, with the two coordinated Dy$^{III}$ cations acting as linkers connecting Mn$_{4}$-P$_{2}$W$_{15}$ 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 (Dy$_{2}$Mn$_{4}$-P$_{2}$W$_{15}$) 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.