Endohedral fullerene with μ3-carbido ligand and titanium-carbon double bond stabilized inside a carbon cage

Abstract

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.

Methane as a Selectivity Booster in the Arc‐Discharge Synthesis of Endohedral Fullerenes: Selective Synthesis of the Single‐Molecule Magnet Dy2TiC@C80 and Its Congener Dy2TiC2@C80

“…In the second one (9 kJ mol À1 higher in energy), the C 2 fragment is tilted out of the M 2 Ti plane,h as m 2 -coordination with Ti and one lanthanide,and m 1 -coordination with another lanthanide.C 2 is likely to exhibit fluxional motion at room temperature.T he structure and dynamics of the M 2 TiC 2 cluster are similar to those of M 3 C 2 clusters in Sc 3 C 2 @C 80 [22] and Lu 3 C 2 @C 88 . [23] M 2 TiC@C 80 -I has ac luster-localized LUMO with large contribution of Ti and af ullerene-based HOMO, [16] and similar spatial distribution of the frontier MOs is predicted for M 2 TiC@C 80 -II. In Lu 2 TiC 2 @C 80 -I, both HOMO and LUMO are predominantly localized on the endohedral cluster and have al arge contribution of the acetylide fragment.…”

“…[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 erein we demonstrate that 1) M 2 TiC@C 80 clusterfullerenes can be synthesized with high selectivity for many lanthanides using methane as areactive gas;2)this synthetic route also affords appreciable amounts of another cage isomer of M 2 TiC@C 80 as well as an ew type of clusterfullerenes,M 2 TiC 2 @C 80 ;3 )Dy 2 TiC@C 80 exhibits single-molecule magnet (SMM) behavior, whereas the SMM properties of Dy 2 TiC 2 @C 80 are are less pronounced owing to the presence of the second carbon atom in the central unit.…”

“…[16] TheLu 2 TiCcluster rotates inside the C 80 -I h (7) fullerene resulting in as imple two-line 13 CNMR spectrum of the cage carbons.T he use of 13 Cp owder for the synthesis of Lu 2 TiC@C 80 -I in this work allowed detection of the 13 Cr esonance of the central carbon atom at 340.98 ppm (Figure 2). This value is much more positive than the chemical shift of endohedral carbon atoms reported before:2 20-260 ppm in M 2 C 2 @C 2n clusterfullerenes, [8c] 292.4 ppm in YCN@C 82 , [17] and 328.3 ppm in the Sc 3 C 2 @C 80 À anion.…”

“…Owing to the higher selectivity in the synthesis with CH 4 as the reactive gas,o ther Ln-Tic arbide clusterfullerenes could be detected;this was not possible in the first report on Lu 2 TiC@C 80 -I. [16] Mass spectrometry studies proved the formation of M 2 TiC@C 2n with larger cages (C 82 ,C 84 ), albeit in rather small amounts making their isolation impractical at this moment (see SI). More importantly,weidentified the second isomer of M 2 TiC@C 80 and anew type of M-Ti-carbide cluster with one more carbon atom in the structure,M 2 TiC 2 @C 80 (the new clusterfullerene also has two isomers,s ee Figures S1 b and S2 a).…”

Endohedral Clusterfullerenes: Future Perspectives

“…Popov and co-workers found that a new type of μ 3 -C mixed-metal carbide clusterfullerene, Dy 2 TiC@I h -C 80, also exhibits SMM behavior. [30][31][32] However, the SMM properties of Dy 2 TiC 2 @I h -C 80 are less functional due to the presence of the second carbon atom in the central unit. In this regard, it is also anticipated that new carbide clusterfullerenes with M 2 C@C 2n compositions with unusual properties will be discovered in the near future.…”