Gd–Sc-Based Mixed-Metal Nitride Cluster Fullerenes: Mutual Influence of the Cage and Cluster Size and the Role of Scandium in the Electronic Structure

The influence of the cage as well as of the cluster size has been studied in Gd–Sc nitride cluster fullerenes, which have been synthesized and isolated for these studies. A series of carbon cages ranging from C₇₈ to C₈₈ have been synthesized, isolated, and characterized in detail using absorption and vibrational spectroscopy as well as electrochemistry and density functional theory calculations. Gd–Sc mixed-metal cluster fullerenes in carbon cages different from C₈₀ were described for the first time. A review of their structures, properties, and stability is given. The synthesis was performed with melamine as an effective solid source of nitrogen, providing high fullerene yield and suppressing empty fullerene formation. Substitution of gadolinium by scandium imposes a noticeable influence on the electronic structure of nitride cluster fullerenes as revealed by electrochemical, spectroscopic, and computational methods

Strain-Driven Endohedral Redox Couple Ce^IV/Ce^III in Nitride Clusterfullerenes CeM₂N@C₈₀ (M = Sc, Y, Lu)

Electrochemical and NMR spectroscopic studies prove the endohedral oxidation of Ce^III in nitride clusterfullerenes CeM₂N@C₈₀ (M = Sc, Y, Lu). The redox potential of the endohedral Ce^IV/Ce^III couple systematically varies with the ionic radius of the second cluster metal. DFT computations show that this metal dependence is caused by the release of strain when Ce^IV with a small ionic radius is formed. In particular, after endohedral oxidation the pyramidal CeY₂N cluster becomes planar in the [CeM₂N@C₈₀]⁺ cation

Dimerization of Radical-Anions: Nitride Clusterfullerenes versus Empty Fullerenes

In contrast with empty fullerenes, nitride clusterfullerenes usually exhibit irreversible reduction steps at moderate electrochemical scan rates. However, these reduction steps are chemically reversible, indicating that reversible follow-up reaction takes place. To explain this phenomenon, we analyze in this work if anion-radicals of nitride clusterfullerenes are more prone to dimerization than anion-radicals of empty fullerenes. Extensive DFT computations are performed to find the most stable dianionic dimeric structures of Sc₃N@C₆₈, Sc₃N@C₈₀, Sc₃N@C₈₀(CF₃)₂, [5,6] and [6,6] pyrrolidine adducts of Sc₃N@C₈₀ and Y₃N@C₈₀, a series of Y₃N@C_2<i>n</i> (2<i>n</i> = 78, 80, 84, 86, 88), as well as those of empty fullerenes C₆₀, C₇₀, and C₈₄. Dimerization energies of the most stable isomers are computed in the gas phase, with the use of van der Waals corrections, and in solution. It is found that dianionic dimers of nonderivatized nitride clusterfullerenes are substantially more stable than those of empty fullerenes, which can be an explanation of the electrochemical irreversibility of the former

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B–H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)­Ru­(CO)₂ (POBBOP = 1,7-OP­(<i>i</i>-Pr)₂-2,6-dehydro-<i>m</i>-carborane) was synthesized by double B–H activation within the strained <i>m</i>-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B–Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne–carboranyl transformations reported herein represent a new mode of cooperative metal–ligand reactivity of boron-based complexes

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B–H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)­Ru­(CO)₂ (POBBOP = 1,7-OP­(<i>i</i>-Pr)₂-2,6-dehydro-<i>m</i>-carborane) was synthesized by double B–H activation within the strained <i>m</i>-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B–Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne–carboranyl transformations reported herein represent a new mode of cooperative metal–ligand reactivity of boron-based complexes

Anion Radicals of Isomeric [5,6] and [6,6] Benzoadducts of Sc₃N@C₈₀: Remarkable Differences in Endohedral Cluster Spin Density and Dynamics

The anion radicals of isomeric [5,6] and [6,6] Sc₃N@C₈₀ benzoadducts were studied by electron spin resonance spectroscopy, density functional theory computations, and molecular dynamics. In both compounds the rotation of the Sc₃N cluster is frozen and the spin density distribution of the cluster is highly anisotropic, with hyperfine coupling constants of 9.1 and 2 × 33.3 G for the [5,6] adduct and ∼0.6 and 2 × 47.9 G for the [6,6] adduct. Remarkably, the subtle variation of the position of the exohedral group on the surface of the cage results in very pronounced changes in the spin density distribution and the dynamics of the encapsulated Sc₃N cluster

Redox-Active Scandium Oxide Cluster inside a Fullerene Cage: Spectroscopic, Voltammetric, Electron Spin Resonance Spectroelectrochemical, and Extended Density Functional Theory Study of Sc₄O₂@C₈₀ and Its Ion Radicals

The clusterfullerene Sc₄O₂@C₈₀ with a mixed redox state of scandium was found to be an exciting molecule for endohedral electrochemistry as demonstrated by means of an in situ electron spin resonance (ESR) spectroelectrochemical study of the spin density distribution in its electrochemically generated cation and anion radicals. The compound exhibits two reversible reduction and oxidation steps with a relatively small electrochemical gap of 1.10 V. The ESR spectra of the ion radicals have a rich hyperfine structure caused by two pairs of equivalent Sc atoms. The Sc-based hyperfine structure with large hyperfine coupling constants shows that both oxidation and reduction of Sc₄O₂@C₈₀ are in cavea redox processes, which is the subject of endohedral electrochemistry. The assignment of the experimentally determined <i>a</i>(⁴⁵Sc) values to the two types of Sc atoms in the Sc₄O₂ cluster was accomplished by extended density functional theory and molecular dynamics simulations. Sc atoms adopting a divalent state in the neutral Sc₄O₂@C₈₀ exhibited an especially large coupling constant of 150.4 G in the cation radical, which is the record high <i>a</i>(⁴⁵Sc) value for Sc-based endohedral metallofullerenes. Such a high value is explained by the nature of the highest occupied molecular orbital (HOMO) localized on the six-atom Sc₄O₂ cluster. This HOMO is a Sc–Sc bonding MO and hence has large contributions from the 4s atomic orbitals of Sc^II. We claim that ESR spectroelectrochemistry is an invaluable experimental tool in the studies of metal–metal bonding in endohedral metallofullerenes and in endohedral electrochemistry

Perfluoroalkyl [70]-Fullerenes as Robust Highly-Luminescent Fluorocarbons, or Position of One CF₃ Group Matters

The photophysical properties of two C₇₀(CF₃)₈ and three C₇₀(CF₃)₁₀ isomers were studied using steady-state and time-resolved absorption and fluorescence spectroscopy. Four of the compounds exhibited quantum yields (Φ_F) higher than for any C₇₀ derivative reported to date, and three exceeded 0.24, the highest Φ_F reported for any fullerene or fullerene derivative. A difference in the location of only one CF₃ group in C₇₀(CF₃)₈ and C₇₀(CF₃)₁₀ isomers resulted in 200-fold and 14-fold increases in Φ_F, respectively. The isomer of C₇₀(CF₃)₁₀ with the highest Φ_F (0.68 in toluene) also exhibited the longest fluorescence lifetime, 51 ns, thus competing favorably in its luminescent properties with the most luminescent carbon materials studied to date. Formation of the S₁ state in one of the C₇₀(CF₃)₁₀ isomers occurred within 0.6 ps and its nanosecond-long decay was monitored by ultrafast transient absorption spectroscopy. Time-dependent density functional theory calculations were performed to provide a physically meaningful understanding of the photophysical properties of C₇₀(CF₃)_<i>n</i> derivatives

Spin Density and Cluster Dynamics in Sc₃N@C₈₀^– upon [5,6] Exohedral Functionalization: An ESR and DFT Study

A radical-anion of [5,6]-pyrrolidine-Sc₃N@C₈₀ is generated both chemically and electrochemically and studied by ESR spectroscopy. The rotation of the Sc₃N cluster is shown to be frozen on the ESR time scale resulting in nonequivalent Sc atoms with hyperfine coupling constants noticeably smaller than in the radical anion of the pristine Sc₃N@C₈₀ but larger than in any other derivatives of Sc₃N@C₈₀. Experimental ESR studies are supported by extended DFT calculations of the cluster rotational pathways, spin density distribution, and hyperfine coupling constants

Thermal [6,6] → [6,6] Isomerization and Decomposition of PCBM (Phenyl‑C₆₁-butyric Acid Methyl Ester)

For the first time, the thermal stability limits of one of the most highly cited and well-studied fullerene derivative electron acceptors, phenyl-C₆₁-butyric acid methyl ester (PCBM), have been investigated under thermal annealing and vapor deposition conditions. Significant decomposition is observed when PCBM is heated, even briefly, to and beyond its melting temperature in an inert atmosphere, as evidenced and quantified here by proton nuclear magnetic resonance, atmospheric-pressure chemical ionization mass spectrometry, and UV–vis spectroscopy, as well as high-performance liquid chromatography. The major thermally induced decomposition product of PCBM has been isolated, characterized, and identified as a new pentacyclic [6,6]-addition motif isomer of PCBM (iso-PCBM). Cyclic voltammetry studies show no difference in electrochemical properties between PCBM and iso-PCBM, and our quantum chemical calculations predict the new isomer to be ∼43 kJ/mol more thermodynamically stable than PCBM