Structural aspects of two-stage dimerization in an ionic C60 complex with bis(benzene)chromium:: Cr(C6H6)2•C60•C6H4Cl2

Single crystals of the ionic C-60 complex with bis( benzene) chromium: {Cr-I(C6H6)(2)(center dot+)} center dot(C-60(center dot-)) center dot C6H4Cl2 ( 1) were obtained. The crystal structure of 1 shows the presence of monomeric C-60(center dot-) radical anions at 250 K and the formation of single-bonded (C-60(-))(2) dimers at 90 K. The dimerization is realized in two types of the C-60(center dot-) pairs with different interfullerene center-to-center distances of 10.052 and 10.279 angstrom arranged in zigzag chains along the a-direction. As a result, two symmetrically independent (C-60(-))(2) dimers found in 1 at 90 K have different environments, intercage C - C bond lengths and C-60(-) center- to-center distances. Such differences should provide different thermal stability of these dimers and result in the appearance of two stages at the dimerization. Indeed, according to SQUID measurements, the magnetic moment of 1 decreases stepwise at the dimerization in two temperature ranges at 240 - 200 and 200 - 160 K.