Endofullerenes with 3d- and 4f-metals: synthesis, physicochemical and structural properties


Lebedev V.T., Szhogina A.A., Suyasova M.V.


B.P. Konstantinov Petersburg Nuclear Physics Institute, NRC Kurchatov Institute, 188300 Gatchina, Leningrad distr., Russia



Abstract:

Review of recent results in the synthesis and studies of physicochemical and structural properties of the series of endofullerenes with 3d and 4f-metals has been presented. The specific features of electric arc process of the evaporation carbon rods with embedded metal compounds are considered as connected with the regulation of formation of desirable endohedral structures. The mechanisms of obtaining water-soluble derivatives of endofullerenes are analyzed regarding to their biomedical applications in Magneto-Resonance Imaging diagnostics and the NMR-relaxivity of their aqueous solutions are discussed. The models of the coordination of metal atoms in the molecules of endofullerenes with 3d and 4f-metals are proposed as connected with the endohedral complexes formation and charge transfer from the encapsulated atoms to the carbon cage. Neutron and X-Ray small-angle scattering data have been treated and the geometry of the fractal structures of endofullerenes in aqueous solutions was characterized in terms of the molecular self-assembly models. The small-angle neutron scattering studies has shown a self-assembly of water soluble derivatives of endofullerenes Gd@C82 in aqueous solutions as dependent on their concentration and the addition of salts to regulate pH-factor. The original endofullerenes Gd@C82 were transformed into water soluble form Gd@C82(OH)X (X ~ 20-30) and these hydroxylated fullerenes (fullerenols) with Gadolinium atoms possessing magnetic moment are considered as very perspective substances to be used as highly effective and safety (non toxic) contrasting agents for Magneto-Resonance-Imaging to improve the resolution of this method by an order in magnitude. These functional properties are strongly dependent on the processes of self-organization of fullerenols having both magnetic and electric dipole moments the interactions of which stimulates molecular ordering in various forms, e.g. globular and low-dimensional (chain-like) aggregates with a characteristic size of few tens of nanometers. The dimensions, geometry and masses of fullerenolsí aggregates have been analyzed by small-angle neutron scattering and the mechanisms as well as some peculiarities of fullerenolsí ordering were established using the formalism of molecular correlation functions to understand subtle features of effects of enhancement of relaxivity of surrounding protons in biological media. The related contrasting abilities of these substances can be used in tomography also in mixtures with empty fullerenols when in magnetic fields the paramagnetic and induced diamagnetic moments of fullerenols Gd@C82(OH)X and C82(OH)X may provide a collective strong action on the relaxation rate of protons in tissues.