Paramagnetic {[Fe(CO) 2 ] 2 -μ 2 -η 2 :η 2 -η 2 :η 2 -(C 60 ) 2 } 2- Dimer Bridged by Iron Atoms and C-C Bonds: Effect of Starting Iron Carbonyls on Structures and Properties of Negatively Charged Iron-Bridged Fullerene Dimers.

The reaction between an excess of Fe(CO) ₅ with {Cryptand(K ⁺ )}(C ₆₀ ^•- ) produced the salt {Cryptand(K ⁺ )} ₂ {[Fe(CO) ₂ ] ₂ -μ ₂ -η ² :η ² -η ² :η ² -(C ₆₀ ) ₂ } ^2- ·4C ₆ H ₄ Cl ₂ ( 1 ) containing negatively charged iron-bridged fullerene dimers. In these dimers, the C ₆₀ cages are linked via two Fe(CO) ₂ fragments, forming short Fe-C(C ₆₀ ) bonds with a length of 2.070(3) Å and via two intercage C-C bonds with a length of 1.566(3) Å. Interfullerene center-to-center distance is short, being 9.02 Å. Thus, the coordination-induced dimerization of fullerenes is observed in 1 . The dimer is negatively charged, with additional negative electron density mainly localized on iron atoms and, to a lesser extent, on the C ₆₀ cages, as revealed by optical and electron paramagnetic resonance spectra. These dimers have a diamagnetic singlet ground state with a small singlet-triplet gap of 25 K; consequently, they transfer to a paramagnetic state with two S = 1/2 spins per dimer above 50 K. Previously, different dimers with isomeric structures were obtained starting from {Cryptand(K ⁺ )}(C ₆₀ ^•- ) and Fe ₃ (CO) ₁₂ . However, these dimers exhibit diamagnetic properties, owing to the formation of a Fe-Fe bond. In contrast, in dimer 1 , the Fe atoms are positioned too far apart to form such a bond, preserving the spin on Fe. We assume that both dimers are formed through the same [Fe(CO) ₃ ](C ₆₀ ^•- ) intermediate, but the subsequent interaction of this intermediate with Fe ₃ (CO) ₁₂ or its dimerization yields different dimers. Therefore, the starting carbonyls can control the structures and properties of the resulting dimers.