Fullerene C60 as an endohedral molecule within an inorganic supramolecule.

The synthesis and properties of large spherical molecules and giant clusters is a fascinating field in contemporary chemistry, unifying all areas of organic/supramolecular, inorganic, and coordination chemistry as well as organometallic chemistry. Recent developments of low-valent group 13 metalloid clusters,1 chalcogeno-bridged copper, and silver clusters2 as well as giant polyoxomolybdates3 represent the largest structurally characterized clusters reported so far. Owing to its unique chemical and physical properties, buckminsterfullerene C60 has found wide applications ranging from superconductivity to medicine and biology.4 Three types of organometallic fullerenes are presently known: (i) fullerides,5 (ii) endohedral metal-fullerene complexes in which metal atoms or metal-containing clusters are encapsulated inside the fullerene,6 and (iii) exohedral metallofullerenes,7 in which metal moieties are attached to the fullerene exterior. Furthermore, fullerenes have had a significant impact in supramolecular chemistry,8 in that weak donor-acceptor associates have been formed with receptors such as calixarenes, porphyrins, crown ethers, and other electron-donating π-systems.9 We report herein the first example of a complete encapsulation of C60 itself by an inorganic fullerene-like system via molecular recognition of the fiveand sixfold-symmetric structural motif of C60. The product C60⊂[Cu26Cl26L2{Cp*Fe(η-P5)}13(CH3CN)9] (L ) H2O) exhibits an inorganic cover consisting of 99 inorganic core atoms from pentaphosphaferrocene and Cu(I) chloride building blocks. The overall spherical molecule possesses an outer diameter of 2.3 nm and shows a remarkable electronic interaction between C60 and the encapsulating giant molecule. We recently reported the use of [CpFe(η-P5)] in the formation of the soluble, fullerene-like cage compounds [{CpxFe(η5:η1:η1: η:η:η-P5)}12{CuX}10{Cu2X3}5{Cu(CH3CN)2}5] (1: Cpx ) Cp*, X ) Cl (a),10 Cpx ) η-C5Me4Et, X ) Br (b),11 which possess a 90-atom core consisting entirely of non-carbon atoms. The outer diameter of 1a of about 2.14 nm is three times larger than that of C60 (0.7 nm), while the inner cavity has a diameter of 1.25 nm. Each of these spherical moieties encapsulates a molecule of [CpxFe(η-P5)], but no solvent molecules, although numerous were found in the crystal lattice of the compounds. The long reaction times in the formation of 1a,b (2 weeks by diffusion techniques) raise the question of whether the pentaphosphaferrocene molecules are incidentally incorporated in the spherical molecule or whether a molecule of fivefold symmetry is required in order to form a spherical cluster via molecular recognition. Short reaction times and concentrated solutions lead to the formation of polymeric compounds in which not all of the P atoms of the cyclo-P5 rings are coordinated to Cu centers.12 Since C60 has the proper size and symmetry to be incorporated into a spherical molecule such as 1, we sought to verify the abovementioned hypothesis by carrying out the reaction of [Cp*Fe(η5P5)] with CuCl under dilute conditions in the presence of C60. In this way, the exclusive formation of black crystals of a soluble, supramolecular assembly of the composition C60⊂[Cu26Cl26(H2O)2{Cp*Fe(η-P5)}13(CH3CN)9] (2) was found in about 45% isolated yield, with no evidence of 1a as a byproduct.13 Compound 2 crystallizes in the orthorhombic space group Pnma with 24 o-dichlorobenzene and four acetonitrile solvent molecules in the unit cell. The pseudo-spherical molecules of 2 form a distorted body-centered cubic packing motif in the crystal lattice.