Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C₆₀ and Ng₂@C₆₀ (Ng=He–Xe)

Abstract

Recently, it was shown that genuine Ng-Ng chemical bonds are present in the endohedral fullerenes Ng(2)@C(60) in the case of Ng=Xe, while it is more debatable whether a chemical bond exist for Ng=Ar and Kr. The lighter homologues with helium and neon are weakly bonded van der Waals complexes. The presence of a noble gas dimer inside the cage is expected to modify the exohedral reactivity of the C(60) cage with respect to that of free C(60). To investigate the impact of encapsulated diatomic noble gas molecules on the chemical reactivity of C(60), we analyzed the thermodynamics and the kinetics of [4+2] Diels-Alder cycloaddition of 1,3-cis-butadiene at all nonequivalent bonds in free C(60), Ng@C(60), and Ng(2)@C(60) (Ng=He, Ne, Ar, Kr, and Xe). Our BP86/TZP calculations reveal that introduction of single noble gas atoms in Ng@C(60) and noble gas dimers He(2) and Ne(2) in Ng(2)@C(60) has almost no effect on the exohedral reactivity compared to free C(60), in agreement with experimental results. In all these cases cycloaddition is clearly favored at the [6,6] bonds in the fullerene cage. For the endohedral compounds He(2)@C(60) and Ne(2)@C(60) a slight preference (by less than 2 kcal mol(-1)) for bonds closer to the C(5) symmetry axis is found. This picture changes dramatically for the endohedral compounds with heavier noble gas dimers. Encapsulation of these noble gas dimers clearly enhances the reaction, both under thermodynamic and kinetic control. Moreover, in the case of Xe(2)@C(60), addition to [6,6] and [5,6] bonds becomes equally viable. These reactivity changes in endohedral fullerenes are attributed to stabilization of the LUMO, increased fullerene strain energy, and greater compression of the encapsulated Ng(2) unit along the He to Xe series.

Synthesis and Properties of Open Fullerenes Encapsulating Ammonia and Methane

“…Heating the open-cage fullerene 1 [12] at 200 8Cf or 68 h, under 153 atm of methaneg ave CH 4 @1 with 65 %e ncapsulationo f the endohedral molecule in quantitative yield (Scheme 1). The filling factor of CH 4 @1 was established by comparison of integrated resonances for CH 4 and an exohedral alkene proton in the 1 HNMR spectrum.F or comparison, CH 4 @3 wasp repared by Iwamatsu and co-workers in 20 %y ield and 39 %i ncorporation, after 20 hat200 8Cunder 190 atm of CH 4 . [17] The rate of loss of CH 4 from CH 4 @1 was measured at several temperatures between 428 and 448 Ka nd displayed the expected 1 st orderk inetics.…”

“…The 13 Cr esonance of endohedralC H 4 appearsa sapentet, centred at d = À19.47 ppm with 1 J CH = 124.8 Hz, in the protoncoupled 13 CNMR spectrum( 125.7 MHz, 1,2-dichlorobenzened 4 )c ompared with d = À8.65 ppm [22] and 1 J CH = 125.3 Hz [23] for gaseous CH 4 .…”

“…[1] The inert and highly symmetric, three-dimensionale nvironmentoft he cavity,means that enclosed (endohedral) species are expectedt ob ehave much as they would in the very low pressure gas state, with preservation of free rotationd own to cryogenic temperatures. [2] Although direct synthesis of endohedral metallofullerenes, [3] and fullerenes containing individual atoms (nobleg as@C 60 [4] and the remarkable N@C 60 [5] )i s possible in very low yield, currently the only high-yielding route to small molecule endofullerenes is via the process of "moleculars urgery" [6] wherebyc hemical transformationsa re used to open ah ole in the fullerene, am olecule is inserted, and af urthers eries of reactions is then used to suture the openinga nd reform the pristine fullerenes hell. To date, this has only been achieved for the incorporation of H 2 , [7] H 2 O [8] and HF, [9] as well as their related isotopologues.…”

“…Althought he high symmetry of the closed cage is lost in these open derivatives, many other properties including isolation are retained. [21] Encapsulated NH 3 and CH 4 are of particulari nterest because (as with H 2 Oa nd H 2 )t heir symmetry leads to interaction between the rotational( J)a nd nuclear spin (I)q uantum states. Methane exists as three nuclear spin isomers (with overall spin We describe the synthesis and characterisation of open fullerene (1)a nd its reducedf orm (2)i nw hich CH 4 and NH 3 are encapsulated, respectively.T he 1 HNMR resonance of endohedral NH 3 is broadened by scalar couplingt ot he quadrupolar 14 N nucleus, which relaxesr apidly.T his broadening is absent for small satellite peaks, whicha re attributed to natural abundance 15 N. Thei nfluence of the scalar relaxation mechanism on the linewidth of the 1 Ha mmonia resonancei sp robed by variable temperature NMR.…”

“…Variable temperature 1 HNMR spectra indicate that endohedral CH 4 is also able to rotate freely at 223 K, on the NMR timescale. Inelastic neutron scattering (INS) spectra of CH 4 @1 show both rotational and translational modes of CH 4 .E nergy of the first excited rotational state (J = 1) of CH 4 @1 is significantly lower than that of free CH 4 .…”