The photochemical reaction of 1,2-digermacyclobutane with C60: possible example of a closed [6,5]-bridged fullerene derivative of germacyclopropane

The photolysis of 1,2-digermacyclobutane derivatives (1) with bulky bistrimethylsilylmethyl (disyl) groups in a toluene solution of C60 provided the germylene (2) and germacyclopropane (3) adducts of C60, respectively. The structures of the two products were characterized using 1H- and 13C-NMR spectra. The signals showed a dramatic change in line shape with temperature. The non-equivalence of the two disyl groups at high temperature, as well as the presence of quaternary aliphatic carbon, suggested that the germylene adduct (2) may be a closed [6,5] adduct: a possible first example of a closed [6,5]-bridged fullerene adduct. PM3 and ab initio calculations also support an energetically preferred closed [6,5] germacyclopropane adduct over an open [6,5] germa-annulene. The non-equivalence of the two disyl groups at low temperatures, as well as the presence of two methylene groups, indicated a germacyclopropane adduct (3), having a germacyclopentane structure that includes a closed [6,6] bond on the C60 surface. To take into account the regio-selective formation of the [6,5] adducts, a fragmentation mechanism of the digermacyclobutane on the C60 surface is proposed.