Regioselectivity of Sc2C2@C3v(8)-C82: Role of the Sumanene-Type Hexagon in Diels–Alder Reaction

Computational insights into Diels-Alder reactions of paramagnetic endohedral metallofullerenes: M@C82 (M = Sc, Y, La) and La@C72

In fullerene chemistry, Diels-Alder cycloaddition is an essential reaction for exohedral modification of carbon cages. M@C2v(9)-C82 (M = Sc, Y, and La), incorporating one metal atom within the fullerene cage, are key compounds for understanding the impact of both endohedral and exohedral modifications on their electronic structures. In this work, the Diels-Alder (DA) cycloaddition of cyclopentadiene (Cp) to M@C2v(9)-C82 (M = Sc, Y, and La) and La@C2(10612)-C72 was systematically studied using density functional theory. The most reactive bonds were initially chosen for detailed mechanistic exploration, considering both concerted and stepwise mechanisms. Our findings revealed that DA cycloadditions for the three metals (Sc, Y, and La) consistently exhibit the same regioselectivity, favoring the concerted attack on the [5,6] bond. This observation is in agreement with previous experimental and theoretical studies on the regioselectivity of the Diels-Alder reaction between La@C2v(9)-C82 and Cp. In the case of La@C2(10612)-C72, the most favored pathway is the concerted attack on the [6,6] bond both kinetically and thermodynamically. In toluene and ortho-dichlorobenzene, while the energy barriers and the reaction free energies increased to different extents for most pathways, the regioselectivity largely mirrored that observed in the gas phase.

Reversible Diels–Alder Addition to Fullerenes: A Study of Dimethylanthracene with H2@C60

The study of isolated atoms or molecules inside a fullerene cavity provides a unique environment. It is likely to control the outer carbon cage and study the isolated species when molecules or atoms are trapped inside a fullerene. We report the Diels–Alder addition reaction of 9,10-dimethyl anthracene (DMA) to H2@C60 while 1H NMR spectroscopy is utilized to characterize the Diels–Alder reaction of the DMA with the fullerene. Through 1H NMR spectroscopy, a series of isomeric adducts are identified. The obtained peaks are sharp, precise, and straightforward. Moreover, in this paper, H2@C60 and its isomers are described for the first time.

How Can the η1-Type Fullerene-Metal Bond Survive? A Systematic Survey of Reactions between Mono-EMFs and (M'Ln)2 Dimers

Recently, one η¹-coordinated complex of endohedral metallofullerene (EMF) Y@C_2v(9)-C₈₂[Re(CO)₅] has been synthesized and characterized with a highly efficient radical-coupling methodology by performing a photochemical reaction between Y@C_2v(9)-C₈₂ and [Re(CO)₅]₂ complexes. Theoretical investigations with the density functional theory reveal that this complex is stabilized by an ionic C-Re bond. The reactions of M@C_2v(9)-C₈₂ (M = Sc, Y, La) with [Re(CO)₅]₂ suggest that the reaction energies differ little because of similar single occupied molecular orbitals (SOMOs) of M@C_2v(9)-C₈₂. In the reactions of Y@C_2v(9)-C₈₂ with various transition-metal complexes [M'L_n]₂ (M' = Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir), the C-M' bonds with Mn, Tc, Re, Fe, Ru, and Os can stably exist, whereas those with Co, Rh, and Ir are unstable. Further analyses disclose that, in each element group, the stability of the C-M' bond is mainly determined by the bond energy of the M'-M' bond, which is related to the d_σ orbital of the M'L_n species. Moreover, the very-low-energy d_σ orbitals and large geometrical distortions of M'(CO)₄ (M' = Co, Rh, Ir) lead to poor stabilities of the C-M' (M' = Co, Rh, Ir) bonds. As comparison, the reactions of Y@C_s(6)-C₈₂ and La@C₇₂ have been investigated. The Y@C_s(6)-C₈₂ structure is more reactive toward the [M'L_n]₂ complexes than Y@C_2v(9)-C₈₂ thanks to a lower SOMO of Y@C_s(6)-C₈₂ than that of Y@C_2v(9)-C₈₂, which derives from position change of the Y atom in C_s(6)-C₈₂ during the reactions. However, the formation of [Y@C_s(6)-C₈₂]₂ suppresses the formation of several C-M' bonds. The reactivity of La@C₇₂ is weak due to a high LUMO+1 of C₇₂, which leads to a high SOMO of La@C₇₂. We believe that this theoretical study provides primary principles of radical-coupling reactions of EMFs and will be valuable for future research of organometallic complexes of fullerene.

Diels-Alder Cycloaddition on Nonisolated-Pentagon-Rule C2v(19 138)-C76 and YNC@C2v(19 138)-C76: The Difference in Regioselectivity Caused by the Inner Metallic Cluster

Diels-Alder reactions of cyclopentadiene to C_2v(19 138)-C₇₆ and YNC@C_2v(19 138)-C₇₆ violating the isolated pentagon rule have been systematically studied by means of density functional theory calculations. As for the free fullerene, the pentalene-type [5,5]-bond in the adjacent pentagon pair is the most favorable from thermodynamic and kinetic viewpoints, which is attributed to the highly strained carbon atoms accompanied by the suitable lowest unoccupied molecular orbital shape with a large distribution to interact with cyclopentadiene. Upon encapsulating the YNC cluster, a corannulene-type [5,6]-bond and a pyracylene-type [6,6]-bond become the two most reactive addition sites under thermodynamic and kinetic conditions, which possess similar reaction energies and energy barriers. Especially, the [5,6]-bond exhibits a larger reaction energy and a lower energy barrier than that on the free fullerene, which should be ascribed to its shorter bond length and larger π-orbital axis vector value after trapping the metallic cluster. The suitable unoccupied molecular orbital lobes with large distributions on the [5,6]- and [6,6]-bonds are also an advantage of cycloadditions. This work presents the first example that the most favorable addition site is remote from the adjacent pentagon pair in the fullerene cage after encapsulating a metallic cluster.

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.