[{ReQ}(SO)] (Q = S or Se): Facile Synthesis and Properties of the Most Highly Charged Octahedral Cluster Complexes and High Magnetic Relaxivity of Their Colloids with Gd Ions.

New octahedral rhenium cluster complexes [{ReQ}(SO)] (Q = S or Se) were synthesized starting from [{ReQ}(HO)(OH)]·12HO. The complexes were crystallized as sodium salts and characterized by X-ray single-crystal diffraction and elemental analyses, IR, UV/vis and luminescence spectroscopies. Magnetic relaxation data demonstrate the complex formation of the cluster units with gadolinium ions. The analysis of the magnetic relaxation rates measured at various Gd:cluster ratios and different concentrations revealed the conversion of the aggregates (Gd[{ReSe}(SO)]) into a nanoparticulate form even at = 1 and ≥ 1. Thus, the self-assembly of the cluster units into the nanoparticles is greatly facilitated by counterion binding with sodium cations. The concentration conditions were optimized for the formation and hydrophilization of NaGd[{ReQ}(SO)]-based colloids with the magnetic relaxivity values of = 21.0(24.1) and = 25.9(29.8) mM s for the {ReS} and {ReSe} derivatives, respectively.