Difluoromethylenation of fullerene C70 provides isomeric diversity and availability of equatorial [5,6]-homofullerene C70(CF2)

We report synthesis, isolation, and spectroscopic characterization of the novel [5,6]-open C70(CF2) isomer III along with the already known [6,6]-closed and [6,6]-open C70(CF2) isomers I and II. The compounds were...

Regioselective CF2 functionalization of Sc3N@D3h(5)-C78

We report the first synthesis and computational study of Sc₃N@C₇₈(CF₂) - an analog of the previously reported Sc₃N@C₈₀(CF₂) with a less common carbon cage whose chemical properties presently remain far less studied. Sc₃N@C₇₈ appears to be considerably more reactive toward CF₂ addition than Sc₃N@C₈₀ and somewhat more reactive than C₆₀. Even though the less symmetric D_3h(5)-C₇₈ carbon cage offers far broader opportunities for isomerism than I_h-C₈₀, CF₂ addition to Sc₃N@C₇₈ proceeds regioselectively, similarly to other common fullerene reactions. A DFT survey of the thermodynamic and kinetic aspects of CF₂ addition demonstrates that the regioselectivity is controlled kinetically.

Designing New Indene-Fullerene Derivatives as Electron-Transporting Materials for Flexible Perovskite Solar Cells

The synthesis and characterization of a family of indene-C₆₀ adducts obtained via Diels-Alder cycloaddition [4 + 2] are reported. The new C₆₀ derivatives include indenes with a variety of functional groups. These adducts show lowest unoccupied molecular orbital energy levels to be at the right position to consider these compounds as electron-transporting materials for planar heterojunction perovskite solar cells. Selected derivatives were applied into inverted (p-i-n configuration) perovskite device architectures, fabricated on flexible polymer substrates, with large active areas (1 cm²). The highest power conversion efficiency, reaching 13.61%, was obtained for the 6'-acetamido-1',4'-dihydro-naphtho[2',3':1,2][5,6]fullerene-C₆₀ (NHAc-ICMA). Spectroscopic characterization was applied to visualize possible passivation effects of the perovskite's surface induced by these adducts.

Addition of CF2 group to endohedral fullerene Sc3N@Ih-C80

We report the first successful synthesis of a CF₂ derivative of the stable endohedral fullerene Sc₃N@I_h-C₈₀. Reaction with CF₂ClCOONa yields a single C_s-symmetric Sc₃N@C₈₀(CF₂) adduct where the CF₂ group is inserted into a [6,6]-bond and opens it to 2.3 Å between the bridgehead carbon atoms. As evidenced by absorption and fluorescence spectroscopy as well as cyclic voltammetry, both the HOMO and the LUMO level of Sc₃N@C₈₀(CF₂) are slightly (ca. 0.1 eV) downshifted with respect to the parent Sc₃N@I_h-C₈₀, so the HOMO-LUMO gap remains essentially unchanged. The DFT calculations suggest that the reaction mechanism is not the previously assumed [2 + 1]-cycloaddition of :CF₂ carbene but rather nucleophilic addition of CF₂Cl^- anion followed by elimination of Cl^- and closing of the CF₂ bridge via intramolecular nucleophilic substitution. Selective formation of the [6,6]-Sc₃N@C₈₀(CF₂) turns out to be kinetically controlled and promoted by a particular orientation of the endohedral Sc₃N cluster with respect to the CF₂Cl^- addition site. In its turn, the CF₂ addend partly hampers the rotation of Sc₃N the endohedral cluster compared to its quasi-free reorientations in the parent Sc₃N@I_h-C₈₀.

Regioselective Mono- and Dialkylation of [6,6]-open C60 (CF2 ): Synthetic and Kinetic Aspects

Alkylation of homofullerene [6,6]-C₆₀ (CF₂ )^2- dianion with the set of alkyl halides, RX, was established to demonstrate an effect of RX nature on the conversion, product composition, and regioselectivity. The respective C₆₀ (CF₂ )RH, C₆₀ (CF₂ )R₂ and C₆₀ (CF₂ )R'R'' compounds were obtained in the reaction with sterically unhindered RX, isolated by HPLC and unequivocally characterized. The kinetic studies evidenced S_N 2 mechanism for both alkylation steps, yielding mono- and dialkylated C₆₀ (CF₂ ), respectively, and indicated the negative charge localization at the bridgehead carbon atoms as well as a steric hindrance of the CF₂ moiety likely to be a key factors for the S_N 2 reaction mechanism and observed regioselectivity. The significant difference in the rate constants of the first and the second steps is attributed to the different activation barriers predicted by DFT calculations which makes possible to develop synthetic methods for the regioselective preparation of monoalkylated C₆₀ (CF₂ )RH and heterodialkylated C₆₀ (CF₂ )R'R'' derivatives.

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.

Alkali-metal trichloroacetates for dichloromethylenation of fullerenes: nucleophilic addition-substitution route

The first experimental evidence that fullerenes react with alkali-metal trichloroacetates through a nucleophilic addition-substitution route, yielding dichloromethylenefullerenes as the final products, is reported. The intermediates, C60 (CCl3 )(-) and C70 (CCl3 )(-) anions, have been isolated in their protonated forms as ortho-C60 (CCl3 )H, as well as three ortho and one para isomer of C70 (CCl3 )H. The structures were unambiguously determined by means of (1) H, (13) C, and (1) H-(13) C HMBC NMR spectroscopy along with UV/Vis spectroscopy. The observed regiochemistry was analyzed with the aid of quantum chemical calculations. Conversion of the protonated compounds into the [6,6]-closed C60/70 (CCl2 ) cycloadducts under basic conditions can be effected only for the ortho isomers, whereas para-C70 (CCl3 )H decomposes back into pristine C70 .

[6,6]-Open and [6,6]-closed isomers of C70(CF2): synthesis, electrochemical and quantum chemical investigation

Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.

SEARCHING FOR NEW MEMBERS OF C70 HOMOFULLERENES BY FIRST-PRINCIPLES COMPUTATIONS: BENT'S RULE AT WORK ON C70 SURFACE

We carried out systematic density functional theory computations to search for new members of C70 homofullerenes inspired by the Bent's rule. By substituting CH2 by CF2 , we identified five experimentally accessible C70 homofullerenes, among which one member has different bonding character as its CH2 counterpart (where the closed three-membered ring is formed by CH2 addition). This unique C70 homofullerene is expected to be synthesized very smoothly using the standard solution phase chemistry, due to its high thermodynamic stability and the rather high local reactivity of the carbon bond attached to the CF2 addend.

Efficient regioselective [4+2] cycloaddition of o-quinodimethane to C(70)(CF(3))(10)

The Diels-Alder reaction of C(1)-C(70)(CF(3))(10) and 3,6-dimethoxy-1,2-quinodimethane leads regioselectively to the formation of a new cycloadduct that has been fully characterized by spectroscopic and electrochemical methods as well as by X-ray diffraction.