Theoretical Study on the Reactivity and Regioselectivity of the Diels-Alder Reaction of Fullerene: Effects of Charges and Encapsulated Lanthanum on the Bis-Functionalization of C70

Exohedral Functionalization of Fullerene by Substituents Controlling of Molecular Organization for Spontaneous C60 Dimerization in Liquid Crystal Solutions and in a Bulk Controlled by a Potential

A study was carried out on the possibility of orderly and spontaneous dimerization at room temperature of C₆₀ cages in fullerene liquid crystal fullerene dyads (R-C₆₀). For this purpose, dyads with a structural elements feature supporting π-stacking and Van der Waals interactions were tested, due to the presence of terthiophene donors linked through an α-position or dodecyloxy chains. In addition, this possibility was also tested and compared to dyads with shorter substituents and the pristine C₆₀. Research has shown that only in dyads with the features of liquid crystals, π-dimerization of C₆₀ units occurs, which was verified by electrochemical and spectroelectrochemical (ESR) measurements. Cyclic voltammetry and differential voltammetry studies reveal π-dimerization in liquid crystal dyad solution even without the possibility of previous polymerization (cathodic or anodic) under conditions in the absence of irradiation and without the availability of reaction initiators, and even with the use of preliminary homogenization. These dyads undergo six sequential, one-electron reductions of π-dimer (R-C₆₀···C₆₀-R), where two electrons are added successively to each of the two fullerene cages and first form two radical anion system (R-C₆₀)^•−(R-C₆₀)^•− without pairing with the characteristics of two doublets. Similarly, the second reductions of π-dimer occur at potentials that are close to the reduction potential for the conversion to a system of two triplet dianions (R-C₆₀)²⁻(R-C₆₀)²⁻. Electron paramagnetic resonance spectra indicate a significant interaction between C₆₀ cages. Interestingly, the strength of intermolecular bonds is so significant that it can overcome Coulombic repulsion, even with such highly charged particles as dianions and trianions. Such behavior has been revealed and studied so far only in covalently bonded C₆₀ dimers.

A Theoretical Study on the Stability of PtL2 Complexes of Endohedral Fullerenes: The Influence of Encapsulated Ions, Cage Sizes, and Ligands

The {η²-(X@C _n )}PtL₂ complexes possessing three kinds of encapsulated ions (X = F^-, Ø, Li⁺), three various ligands (L = CO, PPh₃, NHC^Me), and twelve cage sizes (C₆₀, C₇₀, C₇₂, C₇₄, C₇₆, C₇₈, C₈₀, C₈₄, C₈₆, C₉₀, C₉₆, C₁₀₀) are theoretically examined by using the density functional theory (M06/LANL2DZ). The present computational results demonstrate that the backward-bonding orbital interactions, rather than the forward-bonding orbital interactions, play a dominant role in the stability of {η²-(X@C _n )}PtL₂ complexes. Additionally, our theoretical study shows that the presence of the encapsulated Li⁺ ion can greatly improve the stability of {η²-(X@C _n )}PtL₂ complexes, whereas the existence of the encapsulated F^- ion can heavily reduce the stability of {η²-(X@C _n )}PtL₂ complexes. Moreover, the theoretical evidence strongly suggests that the backward-bonding orbital interactions as well as the stability increase in the order {η²-(X@C _n )}Pt(CO)₂ < {η²-(X@C _n )}Pt(PPh₃)₂ < {η²-(X@C _n )}Pt(NHC^Me)₂. As a result, these theoretical observations can provide experimental chemists a promising synthetic direction.

Benchmark study of popular density functionals for calculating binding energies of three-center two-electron bonds

Density functional theory (DFT) can be used to study the three-center two-electron (3c2e) bonding mode, which is universal in catalysts containing alkaline-earth (Ae) and boron-group (Bg) elements. However, because of the delocalization pattern of the 3c2e bond, the wavefunction cannot be accurately described by DFT methods. The calculated energies of Ae and Bg catalysts therefore fluctuate greatly when different functionals are used, largely because of inconsistent DFT-calculated binding energies of 3c2e bonds. Nevertheless, with the development of supercomputers and theoretical calculation software, the DFT method is becoming increasingly popular for studying Ae and Bg catalysts. In this study, we compared the performances of 21 functionals with the high-level composite G3B3 method in calculations for the binding energies of 3c2e bonds. Several frequently used post-Hartree-Fock methods were also tested. The calculation results indicate that the M06-2X, MN12-L, and MN15 functionals give consistent and reliable binding energies for common 3c2e bonds. © 2018 Wiley Periodicals, Inc.

Rationalizing the Regioselectivity of the Diels-Alder Biscycloaddition of Fullerenes

The physical factors governing the regioselectivity of the double functionalization of fullerenes have been explored by means of density functional theory calculations. To this end, the second Diels-Alder cycloaddition reactions involving 1,3-butadiene and the parent C₆₀ fullerene as well as the ion-encapsulated system Li⁺@C₆₀ have been selected. In agreement with previous experimental findings on related processes, it is found that the cycloaddition reaction, involving either C₆₀ or Li⁺@C₆₀, occurs selectively at specific [6,6]-bonds. The combination of the activation strain model of reactivity and the energy decomposition analysis methods has been applied to gain a quantitative understanding into the markedly different reactivity of the available [6,6]-bonds leading to the observed regioselectivity in the transformation.

Reactivity and regioselectivity in Diels-Alder reactions of anion encapsulated fullerenes

Encapsulation and surface chemical modification are methodologies to enhance the properties of fullerenes for various applications. Herein, density functional theory calculations are performed to study the Diels-Alder (DA) reactivity of anion encapsulated C₆₀, including [X@C₆₀]^- (X = F, Cl, Br, or I), [S@C₆₀]^2-, and [N@C₆₀]^3-. Computational results reveal that encapsulated Cl^-, Br^-, I^-, or S^2- anions are located close to the center of the C₆₀ molecule; however, encapsulated F^- is displaced from the center. Encapsulated N^3- bonds to the inner surface of the carbon cage, which leads to a negative charge transfer to the C₆₀. In [N@C₆₀]^3-, C-C bonds near to the encapsulated N atom are more reactive. Our calculations reveal that encapsulated halogen or S anions decrease the DA reactivity because of the stronger closed-shell repulsion of the encapsulated anion. However, encapsulated N^3- increases the DA reactivity. The higher distortion energy of the halogen- or S^2--anion encapsulated C₆₀ leads to lower reactivity of the 6-5 bond. Opposite regioselectivity of the DA reaction with [N@C₆₀]^3- is attributed to distortion energy of the cyclopentadiene (CPD) moiety. The asymmetrical transition state geometry leads to a lower distortion energy of the CPD moiety.

Simple bond patterns predict the stability of Diels–Alder adducts of empty fullerenes

A systematic study of Diels–Alder cycloadditions to empty fullerenes reveals that π effects control site preference. Simple Hückel calculation allows to obtain quantitative descriptors and to understand why addition occurs preferentially at certain types of bonds. A couple of simple rules are proposed as a visual guide for a rapid prediction of Diels–Alder reactivity.

Regioselective Diels–Alder reaction to open-cage ketolactam derivatives of C60

Open-cage ketolactam fullerenes reacted with dienes on the rim of the orifice both regio- and endo-selectively, which were confirmed by 2D INADEQUATE ¹³C NMR of ¹³C enriched material/HMBC spectra as well as the theoretical calculations.