Orienting Effect of the Cage Addends: The Case of Nucleophilic Cyclopropanation of C2‐C70(CF3)8

C₂‐C₇₀(CF₃)₈ was found to be a very promising substrate in the Bingel and the Bingel–Hirsch reactions combining perfect regioselectivity with much higher reactivity compared to its analogs. The reactions with diethyl malonate yield a single isomer of the monoadduct C₇₀(CF₃)₈[C(CO₂Et)₂] and a single C₂‐symmetrical bisadduct C₇₀(CF₃)₈[C(CO₂Et)₂]₂. The Bingel–Hirsch variation is particularly interesting in that it additionally affords, in a similar regioselective manner, the unexpected alkylated derivatives C₇₀(CF₃)₈[CH(CO₂Et)₂]H and C₇₀(CF₃)₈[C(CO₂Et)₂][CH(CO₂Et)₂]H. The novel compounds have been isolated and structurally characterized by means of ¹H and ¹⁹F NMR spectroscopy as well as single‐crystal X‐ray diffraction. The mechanistic and regiochemical aspects of the reaction are explained with the aid of DFT calculations.     

Synthesis, spectroscopic and electrochemical characterization, and DFT study of seventeen C70(CF3)n derivatives (n=2, 4, 6, 8, 10, 12)

Eight new C70(CF3)n derivatives (n=2, 6, 10, 12) have been synthesized and characterized by UV/Vis and 19F NMR spectroscopy, cyclic voltammetry, and quantum chemical calculations at the DFT level of theory. Nine previously known derivatives of C70(CF3)n with n=2-12 were also studied by cyclic voltammetry (and seven of them by UV/Vis spectroscopy for the first time). Most of the 17 compounds exhibited two or three reversible reductions at scan rates from 20 mV s(-1) up to 5.0 V s(-1). In general, reduction potentials for the 0/- couple are shifted anodically relative to the C70 0/-) couple. However, the 0/- E1/2 values for a given composition are strongly dependent on the addition pattern of the CF3 groups. The data show that the addition pattern is as important, if not more important in some cases, than the number of substituents, n, in determining E1/2 values. An analysis of the DFT-predicted LUMOs indicates that addition patterns that have non-terminal double bonds in pentagons result in derivatives that are strong electron acceptors.     

Bis(trifluoromethyl)phosphinous acid (CF3)2P-O-H: an example of a thermally stable phosphinous acid--synthesis, gas-phase structure, and rotational isomers

The bis(trifluoromethyl)phosphinous acid, (CF3)2P-O-H, is the only known example of a thermally stable phosphinous acid. Although this compound has been known since 1960, little is known about the chemistry of this extraordinary compound; this might be due to the tedious, and in some part risky, synthesis that was originally published. An improved, simple, and safe synthesis that is based on the treatment of the easily accessible (CF3)2PNEt2, with at least three equivalents of p-toluene sulfonic acid, is presented. The reaction results in a complete conversion to the phosphinous acid, which is isolated in almost 90 % yield. The compound exists in an equilibrium of two P--OH rotational isomers, a fact which is supported by quantum chemical calculations. The relative enthalpy difference of 6.4 kJ mol(-1), calculated at the B3PW91/6-311G(3d,p) level, is in excellent agreement with the experimental value of 5.9 kJ mol(-1), which was determined from the temperature dependence of the nu(OH) bands of the two rotational isomers. The complete experimental vibrational spectra of both rotamers, their predicted vibrational spectra obtained by using quantum chemical calculations, and an attempt at photoinduced isomerization of matrix-isolated (CF3)2POH is presented. The experimental structure, obtained from an electron-diffraction study in the gas phase, is reproduced very well by ab initio and density functional theory (DFT) methods.     

Stabilities,electronic properties of exohedral fluorine and trifluoromethyl derivatives for Td C28 fullerene C28F4–n(CF3)n (n = 0,1,2,3,4)

ABSTRACT: Stability and electronic property calculations are performed systematically based on density functional theory at the B3LYP/6‐31G(d) level for T_d C₂₈ fullerene and exohedral fluorine and trifluoromethyl derivatives C₂₈F_4–n(CF₃)_n (n = 0,1,2,3,4). All the exohedral derivatives that are on the potential energy surfaces are kinetically stable with large HOMO‐LUMO gaps. Further investigations show that binding energies of C₂₈F_4–n(CF₃)_n (n = 0,1,2,3,4) molecules are positive, suggesting they are thermodynamically stable. An analysis of the π‐orbital axis vector indicates the high strain in T_d C₂₈ cage could be greatly released by fluorine and trifluoromethyl decorations. Mulliken charge analysis reveals that adding different electron groups to the T_d C₂₈ cage can cause remarkably different charge populations. In addition, from the ionization potential and electron affinity investigations, the C₂₈F_4–n(CF₃)_n (n = 0,1,2,3,4) molecules manifest weak redox properties. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

ESR‐Vis/NIR Spectroelectrochemical Study of C70(CF3)2−. and C70(C2F5)2−. Radical Anions

A spectroelectrochemical study of the two isostructural asymmetric perfluoroalkyl derivatives C₁‐7,24‐C₇₀(CF₃)₂ and C₁‐7,24‐C₇₀(C₂F₅)₂ is presented. Reversible formation of their stable monoanion radicals is monitored by cyclic voltammetry and by in situ ESR‐Vis‐NIR spectroelectrochemistry. The ESR spectrum of the C₇₀(CF₃)₂^?. radical is a 1:3:3:1 quartet with a ¹⁹F hyperfine coupling constant (a(F)) of 0.323(4) G, demonstrating that the unpaired spin is coupled to only one of the two CF₃ groups. The ¹³C satellites are assigned to specific carbon atoms. The ESR spectrum of the C₇₀(C₂F₅)₂^?. radical is an apparent octet with an apparent a(F) value of 0.83(2) G. DFT calculations suggest that this pattern is due to the superposition of spectra for four nearly isoenergetic C₇₀(C₂F₅)₂^?. conformers. Time‐dependent DFT calculations suggest that the NIR band at 1090 nm exhibited by both C₇₀(R_f)₂^?. radical anions is assigned to the SOMO→LUMO+3 transition. The analogous NIR band exhibited by the closed‐shell C₇₀(CF₃)₂^2? dianion was blue‐shifted to 1000 nm.     

A close look at short C-CH3...potassium contacts: synthetic and theoretical investigations of [M2Co2(mu3-OtBu)2(mu2-OtBu)4(thf)n] (M = Na, K, Rb, thf = tetrahydrofuran)

Agostic interactions of the type Si-CH3M+ (M = alkali metal) are frequently mentioned in discussions of solid-state structures of trimethylsilyl compounds and the purpose of this work was to elucidate if they also exist in the related tert-butyl species by using density functional theory. The compounds [M2Co2(mu3-OtBu)2(mu2-OtBu)4(thf)n] (M = Na, n = 2; M = K, n = 0; M = Rb, n = 1) have been synthesised and their crystal structures determined. Close contacts of methyl groups with K atoms are observed in the solid-state structure of [K2Co2(mu3-OtBu)2(mu2-OtBu)4], and calculations of the rotational barrier of a tert-butoxy group about the axis through the C-O bond were performed. It was shown that apparent short C-CH3K distances are in this case a consequence of the packing in the extended solid-state structure.     

[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.     

Preparation, crystallographic characterization and theoretical study of two isomers of C70(CF3)12

Two isomers of C70(CF3)12 have been isolated from a mixture obtained by trifluoromethylation of C70 with CF3I; their molecular structures determined by X-ray crystallography are in good agreement with the results of theoretical DFT calculations for the most stable C70(CF3)12 isomers.     

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 C_s‐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)₈H₂ and only two for C₇₀(CF₃)₁₀H₂, whose addition patterns are combined in the only abundant isomer of C₇₀(CF₃)₁₀H₄. 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.     

First Isomers of Pristine C104 Fullerene Structurally Confirmed as Chlorides,C104(258)Cl16 and C104(812)Cl24

Isolation and characterization of very large fullerenes is hampered by a drastic decrease of their content in fullerene soot with increasing fullerene size and a simultaneous increase of the number of possible IPR (Isolated Pentagon Rule) isomers. In the present work, fractions containing mixtures of C₁₀₂ and C₁₀₄ were isolated in very small quantities (several dozens of micrograms) by multi‐step recycling HPLC from an arc‐discharge fullerene soot. Two such fractions were used for chlorination with a VCl₄/SbCl₅ mixture in glass ampoules at 350–360 °C. The resulting chlorides were investigated by single‐crystal X‐ray diffraction using synchrotron radiation. By this means, two IPR isomers of C₁₀₄, numbers 258 and 812 (of 823 topologically possible isomers), have been confirmed for the first time as chlorides, C₁‐C₁₀₄(258)Cl₁₆ and D₂‐C₁₀₄(812)Cl₂₄, respectively, while an admixture of C₂‐C₁₀₄(811)Cl₂₄ was assumed to be present in the latter chloride. DFT calculations showed that pristine C₁₀₄(812) belongs to rather stable C₁₀₄ cages, whereas C₁₀₄(258) is much less stable.     

Structures and electronic properties of C(56)Cl(8) and C(56)Cl(10) fullerene compounds.

Stimulated by the recent observation of the first C(56)Cl(10) chlorofullerene (Science, 2004, 304, 699), we performed a systematic density functional study of the structures and properties of C(56)Cl(10) and related compounds. The fullerene derivatives C(56)Cl(8) and C(56)Cl(10) based on the parent fullerene C(56)(C(2v):011), rather than those from the most stable C(56) isomer with D(2) symmetry, are predicted to possess the lowest energies, and they are highly aromatic. Further investigations show that the heats of formation of the C(56)Cl(8) and C(56)Cl(10) fullerene derivatives are highly exothermic, that is, -48.59 and -48.89 kcal mol(-1) per Cl(2) (approaching that of C(50)Cl(10)), suggesting that adding eight (or ten) Cl atoms releases much of the strain of pure C(56)(C(2v):011) fullerene and leads to highly stable derivatives. In addition, C(56)Cl(8) and C(56)Cl(10) possess large vertical electron affinities, especially for C(56)Cl(8) with value of 3.20 eV, which is even larger than that (3.04 eV) of C(50)Cl(10), indicating that they are potential good electron acceptors with possible photonic/photovoltaic applications. Finally, the (13)C NMR chemical shifts and infrared spectra of C(56)Cl(8) and C(56)Cl(10) are simulated to facilitate future experimental identification.     

Preparation, crystallographic characterization and theoretical study of C70(CF3)16 and C70(CF3)18

C70(CF3)16 and C70(CF3)18, the first trifluoromethylated fullerene derivatives to comprise a pair of adjacent CF3 groups, have been isolated from a mixture obtained via reaction of C70 with CF3I, characterized in a single crystal XRD study and theoretically investigated at the DFT level of theory.     

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 p⁷mp‐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 ¹⁹F NMR spectroscopy, and DFT calculations. A rather unusual addition pattern is observed in p⁶,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.     

Bonding of multiple noble-gas atoms to CUO in solid neon: CUO(Ng)n (Ng=Ar, Kr, Xe; n=1, 2, 3, 4) complexes and the singlet-triplet crossover point

Laser-ablated U atoms co-deposited with CO in excess neon produce the novel CUO molecule, which forms distinct Ng complexes (Ng=Ar, Kr, Xe) with the heavier noble gases. The CUO(Ng) complexes are identified through CO isotopic and Ng reagent substitution and comparison to results of DFT frequency calculations. The U[bond]C and U[bond]O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from neon matrix (1)Sigma(+) CUO values, which indicates a (1)A' ground state for the CUO(Ng) complexes. The CUO(Ng)(2) complexes in excess neon are likewise singlet molecules. However, the CUO(Ng)(3) and CUO(Ng)(4) complexes exhibit very different stretching frequencies and isotopic behaviors that are similar to those of CUO(Ar)(n) in a pure argon matrix, which has a (3)A" ground state based on DFT vibrational frequency calculations. This work suggests a coordination sphere model in which CUO in solid neon is initially solvated by four or more Ne atoms. Up to four heavier Ng atoms successively displace the Ne atoms leading ultimately to CUO(Ng)(4) complexes. The major changes in the CUO stretching frequencies from CUO(Ng)(2) to CUO(Ng)(3) provides evidence for the crossover from a singlet ground state to a triplet ground state.