Dimer-formation in the bis(arene)chromium fulleride Cr(C7H8)(2) C-60

Bis(toluene)chromium fulleride Cr(C7H8)(2) C-60 has been synthesized as a m icrocrystalline powder from C-60 and Cr(C7H8)(2) in toluene. One electron i s transferred from the chromium complex to the fullerene resulting in a mag netic moment of about 2.5 +/-0.2 mu (B) at room temperature, which correspo nds to two electron spins per formula unit. At 250 K a reversible phase tra nsition from a primitive cubic high temperature paramagnetic phase to a tri clinic low temperature dimer phase occurs. The high temperature phase (Pm ( 3) over barm, a=9.986 Angstrom) is constituted of dynamically disordered fu lleride anions and bis(toluene)chromium (I) cations in a CsCl-type arrangem ent. The low temperature modification is triclinic (P (1) over bar, a=13.54 1 Angstrom, b=13.864 Angstrom, c=14.122 Angstrom, alpha =120.59, beta =91.7 8, gamma =116.88) and carries one unpaired electron per formula unit (mu =1 .72 +/-0.01 mu (B)), which according to ESR measurements is localized at th e chromium site (g(iso)=1.9870). The phase transition has been monitored by x-ray diffraction, ESR and solid state NMR spectroscopy as well as by SQUI D measurements. All experimental results can consistently be described with in the scenario of dimer formation of the C-60 monoanions during the transf ormation. (C) 2001 American Institute of Physics.