Dimer-formation in the bis(arene)chromium fulleride Cr(C7H8)2 C60

Bis(toluene)chromium fulleride ${\mathrm{Cr(C}}_{\mathrm{7}}{\mathrm{H}}_8{)}_2$ ${\mathrm{C}}_{60}$ has been synthesized as a microcrystalline powder from ${\mathrm{C}}_{60}$ and ${\mathrm{Cr(C}}_{\mathrm{7}}{\mathrm{H}}_8{)}_2$ in toluene. One electron is transferred from the chromium complex to the fullerene resulting in a magnetic moment of about ${\text{2.5±0.2\hspace{0.05em}μ}}_B$ at room temperature, which corresponds to two electron spins per formula unit. At 250 K a reversible phase transition from a primitive cubic high temperature paramagnetic phase to a triclinic low temperature dimer phase occurs. The high temperature phase $\text{(Pm3̄m,}$ $\text{a=9.986\hspace{0.05em}}\mathrm{\AA })$ is constituted of dynamically disordered fulleride anions and bis(toluene)chromium (I) cations in a CsCl-type arrangement. The low temperature modification is triclinic $\text{(P1̄,}$ $\text{a=13.541\hspace{0.05em}}\mathrm{\AA },$ $\text{b=13.864\hspace{0.05em}}\mathrm{\AA },$ $\text{c=14.122\hspace{0.05em}}\mathrm{\AA },$ α=120.59, β=91.78, γ=116.88) and carries one unpaired electron per formula unit ${\text{(μ=1.72±0.01\hspace{0.05em}μ}}_B\mathrm{),}$ which according to ESR measurements is localized at the chromium site ${\mathrm{(g}}_{\mathrm{iso}}\text{=1.9870).}$ The phase transition has been monitored by x-ray diffraction, ESR and solid state NMR spectroscopy as well as by SQUID measurements. All experimental results can consistently be described within the scenario of dimer formation of the ${\mathrm{C}}_{60}$ monoanions during the transformation.