Endo- and exohedral chloro-fulleride as η⁵ ligands: a DFT study on the first-row transition metal complexes

C₆₀ fullerene coordinates to transition metals in η²-fashion through its C-C bond at the 6-6 ring fusion site, whereas other coordination modes η³, η⁴, η⁵ and η⁶ are rarely observed. The coordination power of C₆₀ to transition metals is weak owing to the inherent π-electron deficiency on each C-C bond as 60 electrons get delocalized over 90 bonds. The encapsulation of Cl^- by C₆₀ describes a highly exothermic reaction and the resulting Cl^-@C₆₀ behaves as a large anion. Similarly, the exohedral chloro-fulleride Cl^-C₆₀ acts as an electron-rich ligand towards metal coordination. A comparison of the coordinating ability of Cl^-@C₆₀ and Cl^-C₆₀ with that of the Cp^- ligand is done for early to late transition metals of the first row using the M06L/6-31G** level of density functional theory. The binding energy (E_b) for the formation of endohedral (Cl^-@C₆₀)(ML_n)⁺ and exohedral (Cl^-C₆₀)(ML_n)⁺ complexes by the chloro-fulleride ligands ranges from -116 to -170 kcal mol^-1 and from -111 to -173 kcal mol^-1, respectively. Variation in E_b is also assessed for the effect of solvation by o-dichlorobenzene using a self-consistent reaction field method which showed 69-88% reduction in the binding affinity owing to more stabilization of the cationic and anionic fragments in the solvent compared to the neutral product complex. For each (Cl^-@C₆₀)(ML_n)⁺ and (Cl^-C₆₀)(ML_n)⁺ complex, the energetics for the transformation to C₆₀ and ML_nCl is evaluated which showed exothermic character for all endohedral and exohedral Co(i) and Ni(ii) complexes. The rest of the exohedral complexes, viz. Sc(i), Ti(ii), Ti(iv), V(i), Cr(ii), Mn(i), Fe(ii) and Cu(i) systems showed endothermic values in the range 2-35 kcal mol^-1. The anionic modification makes the C₆₀ unit a strong η⁵ ligand similar to Cp^- for cationic transition metal fragments. The bulky anionic nature and strong coordination ability of chloro-fulleride ligands suggest new design strategies for organometallic catalysts.