In all metallofullerenes known before this work, metal atoms form single highly polar bonds with non-metal atoms in endohedral cluster. This is rather surprising for titanium taking into account the diversity of organotitanium compounds. Here we show that the arc-discharge synthesis of mixed titanium-lutetium metallofullerenes in the presence of ammonia, melamine or methane unexpectedly results in the formation of TiLu 2 C@I h -C 80 with an icosahedral I h (7) carbon cage. Single-crystal X-ray diffraction and spectroscopic studies of the compound reveal an unprecedented endohedral cluster with a m 3 -carbido ligand and Ti-C double bond. The Ti(IV) in TiLu 2 C@I h -C 80 can be reversibly reduced to the Ti(III) state. The Ti ¼ C bonding and Ti-localized lowest unoccupied molecular orbital in TiLu 2 C@I h -C 80 bear a certain resemblance to titanium alkylidenes. TiLu 2 C@I h -C 80 is the first metallofullerene with a multiple bond between a metal and the central, non-metal atom of the endohedral cluster.

The use of methane as ar eactive gas dramatically increases the selectivity of the arc-discharge synthesis of M-Ticarbide clusterfullerenes (M = Y, Nd, Gd, Dy,E r, Lu). Optimization of the process parameters allows the synthesis of Dy 2 TiC@C 80 -I and its facile isolation in asingle chromatographic step.An ew type of cluster with an endohedral acetylide unit, M 2 TiC 2 @C 80 ,i sd iscovered along with the second isomer of M 2 TiC@C 80 .D y 2 TiC@C 80 -(I,II) and Dy 2 TiC 2 @C 80 -I are shown to be single-molecule magnets (SMM), but the presence of the second carbon atom in the cluster Dy 2 TiC 2 @C 80 leads to substantially poorer SMM properties.The field of endohedral metallofullerene (EMF) research was revolutionized in 1999, when it was discovered that the presence of small amounts of nitrogen gas in the arc-discharge generator afforded Sc 3 N@C 80 ,anew type of EMF with at rimetalnitride cluster inside the carbon cage.[1] Theu se of NH 3 as areactive gas instead of molecular nitrogen resulted in much higher selectivity in the synthesis of nitride clusterfullerenes as the yield of empty fullerenes in such conditions decreased dramatically.[2] Discovery of nitride clusterfullerenes triggered exhaustive studies of other clusterfullerenes, resulting in av ariety of EMF families with endohedral S, [3] O,[4] C 2 , [5] CH, [6] CN, [7] and other nonmetal units.[8]One of the advantages of the trimetallic cluster in nitride clusterfullerenes is the possibility of combining two or even three different metals within one EMF molecule.M ixedmetal nitride clusterfullerenes may exhibit new properties not present in homometallic nitride clusterfullerenes.E xamples include unusual redox behavior, [9] stabilization of unconventional carbon cages, [10] and strong variation of chemical reactivity [11] and magnetization behavior [12] depending on the number of lanthanide ions in the cluster.H owever, ad isadvantage of the mixed-metal EMFs is the increased complexity of their chromatographic separation.Whereas nitride clusterfullerenes are usually formed with Group III metals,s uch as Sc, Y, and trivalent lanthanides, [13] Yang et al. demonstrated that as ingle Ti ion can be introduced into the mixed-metal nitride cluster together with Sc or Y. [14] Due to the trivalent Ti,M 2 TiN@C 80 clusterfullerenes have unusual electronic and chemical properties. [15] Recently,i na na ttempt to obtain Ti-based nitride clusterfullerenes with Lu using NH 3 as ar eactive gas or melamine as as olid organic nitrogen source,w eh ave discovered an ew type of clusterfullerene,L u 2 TiC@C 80 , which has an endohedral m 3 -carbide ion and aT i ÀCd ouble bond.[16] Them olecule is an isostructural analogue of Lu 2 ScN@C 80 ,i nw hich the Sc-N fragment is replaced by the isoelectronic Ti = Cfragment. Unfortunately,inthe Lu/Ti/NH 3 and Lu/Ti/melamine syntheses,L u 2 TiC@C 80 is only am inor by-product;t he products are predominantly Lu 3 N@C 2n nitride clusterfullerenes,which precludes further exploration of this new type of clusterfullerenes.H ...

The formationo fe ndohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal ScTi system utilizing methane as ar eactive gas.C omparison of these resultsw ith those from the Sc/CH 4 andT i/CH 4 systems as well as syntheses without methane revealed as trong mutual influence of all key components on the product distribution. Whereas am ethane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH 4 system forms mainly empty cage fullerenes. In contrast,t he main fullerene products in the Sc/CH 4 system are Sc 4 C 2 @C 80 (the most abundant EMF from this synthesis), Sc 3 C 2 @C 80 ,i somers of Sc 2 C 2 @C 82 ,a nd the family Sc 2 C 2 n (2 n = 74, 76, 82, 86, 90, etc.), as well as Sc 3 CH@C 80 .T he Sc-Ti/CH 4 system produces the mixed-metal Sc 2 TiC@C 2 n (2 n = 68, 78, 80) and Sc 2 TiC 2 @C 2 n (2 n = 80) clusterfullerene families. The molecular structures of the new,t ransition-metal-containing endohedral fullerenes, Sc 2 TiC@I h -C 80 ,S c 2 TiC@D 5h -C 80 ,a nd Sc 2 TiC 2 @I h -C 80 ,w ere characterizedb yN MR spectroscopy.T he structure of Sc 2 TiC@I h -C 80 was also determined by single-crystalX -ray diffraction, which demonstrated the presence of ashort Ti=C double bond. Both Sc 2 TiC-and Sc 2 TiC 2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redoxactive Ti ion inside the fullerene cage enables analysiso ft he cluster-cage strain in the endohedralf ullerenes through electrochemical measurements.

Sc3CH@C80 is synthesized and characterized by 1H, 13C, and 45Sc NMR. A large negative chemical shift of the proton, −11.73 ppm in the Ih and −8.79 ppm in the D5h C80 cage isomers, is found. 13C satellites in the 1H NMR spectrum enabled indirect determination of the 13C chemical shift for the central carbon at 173 ± 1 ppm. Intensity of the satellites allowed determination of the 13C content for the central carbon atom. This unique possibility is applied to analyze the cluster/cage 13C distribution in mechanistic studies employing either 13CH4 or 13C powder to enrich Sc3CH@C80 with 13C.