Paramagnetic {[Fe(CO)2]2-μ2-η2:η2-η2:η2-(C60)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.

Abstract

The reaction between an excess of Fe(CO)5 with {Cryptand(K+)}(C60•-) produced the salt {Cryptand(K+)}2{[Fe(CO)2]2-μ2-η2:η2-η2:η2-(C60)2}2-·4C6H4Cl2 (1) containing negatively charged iron-bridged fullerene dimers. In these dimers, the C60 cages are linked via two Fe(CO)2 fragments, forming short Fe-C(C60) 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 C60 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+)}(C60•-) and Fe3(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](C60•-) intermediate, but the subsequent interaction of this intermediate with Fe3(CO)12 or its dimerization yields different dimers. Therefore, the starting carbonyls can control the structures and properties of the resulting dimers.