Magnetic properties of (m−MPYNN+)[Mn12O12(O2CPh)16(H2O)4]−: Enhancement of magnetic relaxation in the Mn12 cluster caused by the organic radical

Abstract

We report magnetic properties of the organic/inorganic hybrid salt, $(m-{\mathrm{MPYNN}}^{+})\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}\right({\mathrm{O}}_2\mathrm{CPh}{)}_{16}({\mathrm{H}}_2\mathrm{O}{)}_4{]}^{-}$ (2), where $m-{\mathrm{MPYNN}}^{+}$ $(=m-N-$methylpyridinium nitronylnitroxide) is a stable organic radical cation with $S=\frac{1}{2}$ and $\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}\right({\mathrm{O}}_2\mathrm{CPh}{)}_{16}({\mathrm{H}}_2\mathrm{O}{)}_4{]}^{-}$ is a 12-nuclei Mn cluster anion with $S=\frac{19}{2}$, together with the simple salt, [${\mathrm{PPh}}_4^{+}$(=tetraphenylphosphonium)] $\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}\right({\mathrm{O}}_2\mathrm{CPh}{)}_{16}({\mathrm{H}}_2\mathrm{O}{)}_4{]}^{-}\cdot 2{\mathrm{H}}_2\mathrm{O}$ ( 1). The EPR spectra of 2 are composed of the separate absorption signals of the ${\mathrm{Mn}}_{12}$ cluster and $m-{\mathrm{MPYNN}}^{+}$, indicating a negligible exchange interaction between them. Below 65 K, however, an overlap between the absorption tails is clearly observed, suggesting an energy transfer between the resonances. While the temperature dependences of the paramagnetic susceptibilities of 1 and 2 are similar above $\sim 4\hspace{0.5em}\mathrm{K}$, the two salts show quite different spin dynamics below it. The magnetization curve and ac susceptibility measurements reveal that the organic radical enhances the relaxation of the magnetization of the ${\mathrm{Mn}}_{12}$ cluster and prevents the magnetization freezing.