Ligand-Bridged Cu-M Dimers (M = Cu, Ni, or Mn) and Their Magnetic Properties Related to Water Proton Relaxation

<p>The high symmetry structure [Mn4O6(tacn)4]4+, where tacn is 1,4,7-Triazacyclononane, was successfully synthesized, characterized by UV-Vis and . The pH dependence of the relaxivity for this compound was studied. Such a compound has a symmetrically enforced ferromagnetic coupling between the four Mn(IV) ions and has a ground spin state of S=12. However, the protonation of this compound results in [Mn4O5(OH)(tacn4)] 5+ where the four Mn(IV) ions are antiferromagnetically coupled and the ground spin state is S=0. Since spin and relativity are related ( i.e. higher spin generally results in high relaxivities) changing the nature of the coupling in this species was expected to have an effect on relaxivity. The relaxivity for this tetra nuclear species was thus postulated to be pH dependent. Contrary to expectations, [Mn4O6(tacn)4]4+ has a relaxivity value that is almost ten times as low as the relaxivity for the currently used contrast agents. The low relaxivity is due to the lack of free coordination sites for water exchange to occur in this compound. Moreover, the relaxivity for [Mn4O5(OH)(tacn4)] 5+ was lower than that of [Mn4O6(tacn)4]4+ and this suggests a slow rate of prototropic exchange on [Mn4O5(OH)(tacn4)] 5+. This relaxivity was also pH independent.</p> <p>In order to take advantage of both spin and water exchange three dimers: [Cu (oxpn)Cu(bpy)](Cl)2, [Cu(oxpn)Ni(bpy)2](Cl)2·2H2O and [Cu(oxpn)Mn(bpy)2(ClO4)2·2H2O were synthesized . These were characterized by solution and solid states IR, UV-Vis and elemental analysis. The 1H longitudinal and transverse relaxation rates as well as 17O transverse relaxation rates were studied for these dimers, their respective monomers and their respective metal hexaaqua complexes. [Cu(oxpn)Mn(bpy)2(ClO4)2·2H2O had the highest relaxivity values and the highest water exchange rates, while [Cu(oxpn)Ni(bpy)2](Cl)2·2H2O had the lowest. [Mn(H2O6)]2+ also had the highest water exchange rate, followed by [Cu(H2O5)]2+ with [Ni(H2O6)]2+ having the lowest. These results demonstrate the importance of both spin and high water exchange in enhancing relaxivity and that each component alone may not be effective in enhancing relaxation rates.</p>