In Search of Fullerene-Based Superacids: Synthesis, X-ray Structure, and DFT Study of C60(C2F5)5H

Synthesis of Cs-symmetrical C60 tetra-adducts via reactions of C60Cl6 with CH-acids and enol silyl ester

Here we report the synthesis of novel fullerene derivatives with attached aliphatic residues based on the previously unknown reactions of chlorofullerene C60Cl6 with CH-acids and silyl enol ether.

Crystalline functionalized endohedral C60 metallofullerides

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based monomeric metallofullerenes, except for [Li@C60]+(SbCl6)- salt. Pure (metal)@C60 compounds have not been obtained because of their extremely high chemical reactivity. One route to their stabilization is through chemical functionalization. Here we report the isolation, structural determination and electromagnetic properties of functionalized crystalline C60-based metallofullerenes Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3 groups. Magnetic measurements show the presence of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures.

Reductive Hydrogenation of Cs -C70 (CF3 )8 and C1 -C70 (CF3 )10

CF3 -derivatized fullerenes prove once again to be promising scaffolds for regioselective fullerene functionalization: now with the smallest possible addends-hydrogen atoms. Hydrogenation of Cs -C70 (CF3 )8 and C1 -C70 (CF3 )10 by means of reduction with Zn/Cu couple in the presence of water proceeds regioselectively, yielding only one major isomer of C70 (CF3 )8 H2 and only two for C70 (CF3 )10 H2 , whose addition patterns are combined in the only abundant isomer of C70 (CF3 )10 H4 . The observed selectivity is governed by the electronic structure of trifluoromethylated substrates. Interestingly, we discovered that Clar's theory can be utilized to predict the regiochemistry of functionalization, and we look forward to testing it on forthcoming cases of derivatization of pre-functionalized fullerene building blocks.

A faux hawk fullerene with PCBM-like properties

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)- can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F-. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C-F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/-) values of 2 and C60, -0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/-) of PCBM is -0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene-fullerene interactions with centroid···centroid (⊙···⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙···⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Unprecedented chemical reactivity of a paramagnetic endohedral metallofullerene La@C(s)-C82 that leads hydrogen addition in the 1,3-dipolar cycloaddition reaction

Synthesizing unprecedented diamagnetic adducts of an endohedral metallofullerene was achieved by using 1,3-dipolar cycloaddition reaction of paramagnetic La@C(s)-C82 with a simultaneous hydrogen addition. The selective formation of two main products, La@C(s)-C82HCMe2NMeCHPh (2a and 2b), was first detected by HPLC analysis and MALDI-TOF mass spectrometry. 2a and 2b-O, which was readily formed by the oxidation of 2b, were isolated by multistep HPLC separation and were fully characterized by spectroscopic methods, including 1D and 2D-NMR, UV-vis-NIR measurements and electrochemistry. The hydrogen atom was found to be connected to the fullerene cage directly in the case of 2a, and the redox behavior indicated that the C-H bond can still be readily oxidized. The reaction mechanism and the molecular structures of 2a and 2b were reasonably proposed by the interplay between experimental observations and DFT calculations. The feasible order of the reaction process would involve a 1,3-dipolar cycloaddition followed by the hydrogen addition through a radical pathway. It is concluded that the characteristic electronic properties and molecular structure of La@C(s)-C82 resulted in a site-selective reaction, which afforded a unique chemical derivative of an endohedral metallofullerene in high yields. Derivative 2a constitutes the first endohedral metallofullerene where the direct linking of a hydrogen atom has been structurally proven.

Ion formation pathways of crown ether–fullerene conjugates in the gas phase

Crown ether–fullerene conjugates are studied by ESI and MALDI-MS covering ionisation mechanisms, charge localisation and relative affinity to alkali metal ions.

Fullerene "superhalogen" radicals: the substituent effect on electronic properties of 1,7,11,24,27-C60X5

Hexasubstituted fullerenes with the skew pentagonal pyramid (SPP) addition pattern are predominantly formed in many types of reactions and represent important and versatile building blocks for supramolecular chemistry, biomedical and optoelectronic applications. Regioselective synthesis and characterization of the new SPP derivative, C60(CF3)4(CN)H, in this work led to the experimental identification of the new family of "superhalogen fullerene radicals", species with the gas-phase electron affinity higher than that of the most electronegative halogens, F and Cl. Low-temperature photoelectron spectroscopy and DFT studies of different C60X5 radicals reveal a profound effect of X groups on their electron affinities (EA), which vary from 2.76 eV (X=CH3) to 4.47 eV (X=CN). The measured gas-phase EA of the newly synthesized C60(CF3)4CN equals 4.28(1) eV, which is about 1 eV higher than the EA of Cl atom. An observed remarkable stability of C60(CF3)4CN(-) in solution under ambient conditions opens new venues for design of air-stable molecular complexes and salts for supramolecular structures of electroactive functional materials.