Synthesis, Spectroscopic and Electrochemical Characterization, and DFT Study of Seventeen C70(CF3)nDerivatives (n=2, 4, 6, 8, 10, 12)

Electrophilic Trifluoromethylation of Dimetallofullerene Anions en Route to Air-Stable Single-Molecule Magnets with High Blocking Temperature of Magnetization

Lanthanide dimetallofullerenes with single-electron M-M bonds are an important class of single molecular magnets and qubit candidates, but stabilization of their unique electronic and spin structure in the form of a neutral molecule requires functionalization of the fullerene cage with a single radical group. The lack of selectivity of the currently available procedure results in a complicated and tedious separation process. Here we demonstrate that electrophilic trifluoromethylation of a mixture of metallofullerene anions with Umemoto reagent II is highly selective toward M₂@C₈₀^- (M = Tb, Y) anions, yielding M₂@C₈₀(CF₃) monoadducts as the main reaction product. Single-crystal X-ray diffraction study proved attachment of the CF₃ group to the pentagon/hexagon/hexagon junction and revealed that positions of metal atoms inside the fullerene cage in the cocrystal with NiOEP are strongly related to the position of the porphyrin moieties. Magnetic characterization of Tb₂@C₈₀(CF₃) showed that it is a robust single-molecule magnet with broad magnetic hysteresis, 100 s blocking temperature of 25 K, and the relaxation barrier of 801(4) K, corresponding to the flipping of the Tb magnetic moment in the strongly ferromagnetically coupled [Tb³⁺-e-Tb³⁺] spin system.

Electrochemically Induced Dimerization of p9mp-C70(CF3)12 Trifluoromethylated Fullerene

We report a comprehensive study of a novel isomer of C₇₀(CF₃)₁₂, p⁹mp-C₇₀(CF₃)₁₂, whose electrochemical behavior differs from most of the other trifluoromethylated fullerenes. The addition pattern of p⁹mp-C₇₀(CF₃)₁₂ is established by means of ¹⁹F-¹⁹F COSY NMR spectroscopy and DFT calculations. Like p⁷mp-C₇₀(CF₃)₁₀, the new isomer p⁹mp-C₇₀(CF₃)₁₂ undergoes dimerization to the [C₇₀(CF₃)₁₂]₂^2- upon single-electron reduction. The electrochemical observations are supported by the DFT calculations of dimerization energy and the temperature dependence of the CW X-band EPR spectroscopy data. Experimentally determined dimerization energies of p⁹mp-C₇₀(CF₃)₁₂^-• and p⁷mp-C₇₀(CF₃)₁₀^-• in solution are ca -8 and -26 kJ mol^-1, respectively, in good correspondence with DFT data.

Fused-Pentagon C70Cl6 and C70Cl8 Obtained via Chlorination-Promoted Skeletal Transformation of IPR C70

The isolated-pentagon-rule (IPR) D_5h-C₇₀ fullerene is least susceptible to skeletal transformations in comparison with higher fullerenes and even C₆₀. A cage transformation in IPR C₇₀ via a one-step Stone-Wales rearrangement was accomplished by high-temperature (440 °C) ampule chlorination with SbCl₅. Subsequent dechlorination at 450 °C, followed by high-performance liquid chromatography separation, allowed the isolation of non-IPR C₇₀Cl₆ and C₇₀Cl₈. X-ray diffraction study revealed the presence of an unprecedented C₇₀ carbon cage, possessing two pairs of fused pentagons and the chlorination patterns located on one cage hemisphere. A high energetic and thermal stability of both non-IPR chlorides was also confirmed by theoretical calculations of formation energies. Pathways of skeletal transformations of IPR C₇₀ in comparison with those in C₆₀ are discussed.

Highly soluble C2v-symmetrical fullerene derivatives: efficient synthesis, characterization, and electrochemical study

Through an efficient octa-substitution reaction, octabromofullerene (C₆₀Br₈) was reacted with alcohols or anisole in the presence of silver triflate to produce octaalkoxy and octaaryl fullerenes, respectively, in up to 79% yield with retention of C_2v-symmetry.

Negatively charged singly-bonded dimers of C1-[C70(CF3)10] and bare C70 fullerene

Singly-bonded {C₇₀(CF₃)₁₀⁻}₂ and (C₇₀⁻)₂ dimers are obtained as crystalline salts allowing their molecular structure, and optical and magnetic properties to be studied.

Facile Separation, Spectroscopic Identification, and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene

We survey the structure and electronic properties of the family of higher trifluoromethylated C70 (CF3 )n molecules with n=14, 16, 18, and 20. Twenty-two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π-system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed-shell cyclopentadienyl anions C70 (CF3 )n-1- through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP-C70 (CF3 )16 where potentiostatic electrolysis at the second reduction potential gives C70 (CF3 )15- oxidizable to a persistent C70 (CF3 )15. radical. Together with the literature data for the lower C70 (CF3 )n compounds with n=2-12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO-LUMO gaps within the ranges of -4.3 to -3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of -5.6 to -6.9 eV and show linear correlation with the number of addends.

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.

Regioisomer-specific electron affinities and electronic structures of C70para-adducts at polar and equatorial positions with (bromo)benzyl radicals: photoelectron spectroscopy and theoretical study

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5-150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

Crystallographic Evidence for Direct Metal-Metal Bonding in a Stable Open-Shell La2 @Ih -C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed-shell adducts by attaching an odd number of addends to open-shell EMFs (such as Sc3 C2 @Ih -C80 ) whereas an even number of groups are added to closed-shell EMFs (for example Sc3 N@Ih -C80 ). Herein we report that benzyl radical addition to the closed-shell La2 @Ih -C80 forms a stable, open-shell monoadduct instead of the anticipated closed-shell bisadduct. Single-crystal X-ray diffraction results show the formation of a stable radical species. In this species, the La-La distance is comparable to the theoretical value of a La-La covalent bond and is shorter than reported values for other La2 @Ih -C80 derivatives, providing unambiguous evidence for the formation of direct La-La bond.

(19)F NMR-, ESR-, and vis-NIR-spectroelectrochemical study of the unconventional reduction behaviour of a perfluoroalkylated fullerene: dimerization of the C70(CF3)10(-) radical anion

¹⁹F NMR spectroelectrochemistry is applied to study the reduction mechanism of perfluoroalkylated fullerene C₇₀(CF₃)₁₀.

The most abundant isomer of C₇₀(CF₃)₁₀ (70-10-1) is a rare example of a perfluoroalkylated fullerene exhibiting electrochemically irreversible reduction. We show that electrochemical reversibility at the first reduction step is achieved at scan rates higher than 500 V s^–1. Applying ESR-, vis-NIR-, and ¹⁹F NMR-spectroelectrochemistry, as well as mass spectrometry and DFT calculations, we show that the (70-10-1)^– radical monoanion is in equilibrium with a singly-bonded diamagnetic dimeric dianion. This study is the first example of ¹⁹F NMR spectroelectrochemistry, which promises to be an important method for the elucidation of redox mechanisms of fluoroorganic compounds. Additionally, we demonstrate the importance of combining different spectroelectrochemical methods and quantitative analysis of the transferred charge and spin numbers in the determination of the redox mechanism.

Fullerene cyanation does not always increase electron affinity: an experimental and theoretical study

The electron affinities of C70 derivatives with trifluoromethyl, methyl and cyano groups were studied experimentally and theoretically using low-temperature photoelectron spectroscopy (LT PES) and density functional theory (DFT). The electronic effects of these functional groups were determined and found to be highly dependent on the addition patterns. Substitution of CF3 for CN for the same addition pattern increases the experimental electron affinity by 70 meV per substitution. The synthesis of a new fullerene derivative, C70(CF3)10(CN)2, is reported for the first time.

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.

Electronic vs steric effects on the stability of anionic species: a case study on the ortho and para regioisomers of organofullerenes

The stability of the anionic species of the ortho and para regioisomers of (MeO)BnC(2n) (Me = methyl, Bn = benzyl, n = 30 or 35) has been examined. The results show that the ortho adducts (electronically favored regioisomers) are stable upon receiving one or two electrons, while the para ones (sterically favored adducts) decompose by removing the methoxy group under similar conditions. Computational calculations indicate that the stability of the anionic species is significantly affected by the electronic structure, where the [5,6]-double bond is responsible for the instability of the reduced species of the para adducts. Further study with 1,15-(MeO)2-2,4-Bn2C60, an adduct with both the ortho and para positioned methoxy, shows that the reduced species is stable, indicating that the 1,2,4,15-configuration is an electronically preferential structure even though it has a [5,6]-double bond.

Transalkylation of higher trifluoromethylated fullerenes with C70: a pathway to new addition patterns of C70(CF3)8

We report three new isomers of C70(CF3)8, structurally related to p(7)mp-C70(CF3)10, that are inaccessible by direct trifluoromethylation, but can be easily identified among the products of the transalkylation of higher trifluoromethylfullerenes with C70. The reported compounds are characterized by UV/Vis, 1 D and 2 D COSY (19)F NMR spectroscopy, and DFT calculations. A rather unusual addition pattern is observed in p(6),i-C70(CF3)8 in which one addend is attached remotely from the others; polarization of the adjacent unsaturated bonds by the addend makes the molecule readily oxidizable.

Synthesis, structure, and theoretical study of trifluoromethyl derivatives of C84(22) fullerene

Trifluoromethylation of higher fullerene mixtures with CF(3)I was performed in ampoules at 400 to 420 and 550 to 560 °C. HPLC separation followed by crystal growth and X-ray diffraction studies allowed the structure elucidation of nine CF(3) derivatives of D(2)-C(84) (isomer 22). Molecular structures of two isomers of C(84)(22)(CF(3))(12), two isomers of C(84)(22)(CF(3))(14), four isomers of C(84)(22)(CF(3))(16), and one isomer of C(84)(22)(CF(3))(20) were discussed in terms of their addition patterns and relative formation energies. DFT calculations were also used to predict the most stable molecular structures of lower CF(3) derivatives, C(84)(22)(CF(3))(2-10). It was found that the addition of CF(3) groups to C(84)(22) is governed by two rules: additions can only occur at para positions of C(6)(CF(3))(2) hexagons and no additions can occur at triple-hexagon-junction positions on the fullerene cage.

Solution-Phase Perfluoroalkylation of C60 Leads to Efficient and Selective Synthesis of Bis-Perfluoroalkylated Fullerenes

A solution-phase perfluoroalkylation of C₆₀ with a series of R_FI reagents was studied. The effects of molar ratio of the reagents, reaction time, and presence of copper metal promoter on fullerene conversion and product composition were evaluated. Ten aliphatic and aromatic R_FI reagents were investigated (CF₃I, C₂F₅I, n-C₃F₇I, i-C₃F₇I, n-C₄F₉I, (CF₃)(C₂F₅)CFI, n-C₈F₁₇I, C₆F₅CF₂I, C₆F₅I, and 1,3-(CF₃)₂C₆F₃I) and eight of them (except for C₆F₅I and 1,3-(CF₃)₂C₆F₃I) were found to add the respective R_F groups to C₆₀ in solution. Efficient and selective synthesis of C₆₀(R_F)₂ derivatives was developed.

Poly(perfluoroalkylation) of metallic nitride fullerenes reveals addition-pattern guidelines: synthesis and characterization of a family of Sc3N@C80(CF3)n (n = 2-16) and their radical anions

A family of highly stable (poly)perfluoroalkylated metallic nitride cluster fullerenes was prepared in high-temperature reactions and characterized by spectroscopic (MS, (19)F NMR, UV-vis/NIR, ESR), structural and electrochemical methods. For two new compounds, Sc(3)N@C(80)(CF(3))(10) and Sc(3)N@C(80)(CF(3))(12,) single crystal X-ray structures are determined. Addition pattern guidelines for endohedral fullerene derivatives with bulky functional groups are formulated as a result of experimental ((19)F NMR spectroscopy and single crystal X-ray diffraction) studies and exhaustive quantum chemical calculations of the structures of Sc(3)N@C(80)(CF(3))(n) (n = 2-16). Electrochemical studies revealed that Sc(3)N@C(80)(CF(3))(n) derivatives are easier to reduce than Sc(3)N@C(80), the shift of E(1/2) potentials ranging from +0.11 V (n = 2) to +0.42 V (n = 10). Stable radical anions of Sc(3)N@C(80)(CF(3))(n) were generated in solution and characterized by ESR spectroscopy, revealing their (45)Sc hyperfine structure. Facile further functionalizations via cycloadditions or radical additions were achieved for trifluoromethylated Sc(3)N@C(80) making them attractive versatile platforms for the design of molecular and supramolecular materials of fundamental and practical importance.

Isolation and structural X-ray investigation of perfluoroalkyl derivatives of six cage isomers of C84

Perfluoroalkylation of a higher fullerene mixture with CF(3)I or C(2)F(5)I, followed by HPLC separation of CF(3) and C(2)F(5) derivatives, resulted in the isolation of several C(84)(R(F))(n) (n=12, 16) compounds. Single-crystal X-ray crystallography with the use of synchrotron radiation allowed structure elucidation of eight C(84)(R(F))(n) compounds containing six different C(84) cages (the number of the C(84) isomer is given in parentheses): C(84) (23)(C(2)F(5))(12) (I), C(84) (22)(CF(3))(16) (II), C(84) (22)(C(2)F(5))(12) (III), C(84) (11)(C(2)F(5))(12) (IV), C(84) (16)(C(2)F(5))(12) (V), C(84) (4)(CF(3))(12) (VI with toluene and VII with hexane as solvate molecules), and C(84) (18)(C(2)F(5))(12) (VIII). Whereas some connectivity patterns of C(84) isomers (22, 23, 11) had previously been unambiguously confirmed by different methods, derivatives of C(84) isomers numbers 4, 16, and 18 have been investigated crystallographically for the first time, thus providing direct proof of the connectivity patterns of rare C(84) isomers. General aspects of the addition of R(F) groups to C(84) cages are discussed in terms of the preferred positions in the pentagons under the formation of chains, pairs, and isolated R(F) groups.