Hydrocarbon-soluble, hexaanionic fulleride complexes of magnesium

The reaction of the magnesium(i) complexes [{(Arnacnac)Mg}2], (Arnacnac = HC(MeCNAr)2, Ar = Dip (2,6-iPr2C6H3), Dep (2,6-Et2C6H3), Mes (2,4,6-Me3C6H2), Xyl (2,6-Me2C6H3)) with fullerene C60 afforded a series of hydrocarbon-soluble fulleride complexes [{(Arnacnac)Mg} n C60], predominantly with n = 6, 4 and 2. 13C{1H} NMR spectroscopic studies show both similarities (n = 6) and differences (n = 4, 2) to previously characterised examples of fulleride complexes and materials with electropositive metal ions. The molecular structures of [{(Arnacnac)Mg} n C60] with n = 6, 4 and 2 can be described as inverse coordination complexes of n [(Arnacnac)Mg]+ ions with C60 n- anions showing predominantly ionic metal-ligand interactions, and include the first well-defined and soluble complexes of the C60 6- ion. Experimental studies show the flexible ionic nature of the {(Arnacnac)Mg}+···C60 6- coordination bonds. DFT calculations on the model complex [{(Menacnac)Mg}6C60] (Menacnac = HC(MeCNMe)2) support the formulation as an ionic complex with a central C60 6- anion and comparable frontier orbitals to C60 6- with a small HOMO-LUMO gap. The reduction of C60 to its hexaanion gives an indication about the reducing strength of dimagnesium(i) complexes.

Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids

Alkali metal intercalation into polyaromatic hydrocarbons (PAHs) has been studied intensely after reports of superconductivity in a number of potassium- and rubidium-intercalated materials. There are, however, no reported crystal structures to inform our understanding of the chemistry and physics because of the complex reactivity of PAHs with strong reducing agents at high temperature. Here we present the synthesis of crystalline K₂Pentacene and K₂Picene by a solid-solid insertion protocol that uses potassium hydride as a redox-controlled reducing agent to access the PAH dianions, and so enables the determination of their crystal structures. In both cases, the inserted cations expand the parent herringbone packings by reorienting the molecular anions to create multiple potassium sites within initially dense molecular layers, and thus interact with the PAH anion π systems. The synthetic and crystal chemistry of alkali metal intercalation into PAHs differs from that into fullerenes and graphite, in which the cation sites are pre-defined by the host structure.

Polarizability as a tool to determine the electrostatic shielding effect of nanocarbon cages: a polarizability distribution study on noble gas endohedral fullerenes

A noble gas atom can be treated as a sensor to determine the dielectric properties of nanocages including fullerenes.

Nanoscale orbital excitations and the infrared spectrum of a molecular Mott insulator: A15-Cs3C60

The quantum physics of ions and electrons behind low-energy spectra of strongly correlated molecular conductors, superconductors and Mott insulators is poorly known, yet fascinating especially in orbitally degenerate cases. The fulleride insulator Cs₃C₆₀ (A15), one such system, exhibits infrared (IR) spectra with low temperature peak features and splittings suggestive of static Jahn-Teller distortions with a breakdown of orbital symmetry in the molecular site. That is puzzling, since there is no detectable static distortion, and because the features and splittings disappear upon modest heating, which they should not. Taking advantage of the Mott-induced collapse of electronic wavefunctions from lattice-extended to nanoscale localized inside a caged molecular site, we show that the unbroken spin and orbital symmetry of the ion multiplets explains the IR spectrum without adjustable parameters. This demonstrates the importance of a fully quantum treatment of nuclear positions and orbital momenta in the Mott insulator sites, dynamically but not statically distorted. The observed demise of these features with temperature is explained by the thermal population of a multiplet term whose nuclear positions are essentially undistorted, but whose energy is very low-lying. That term is in fact a scaled-down orbital excitation analogous to that of other Mott insulators, with the same spin 1/2 as the ground state, but with a larger orbital momentum of two instead of one.

Optimized unconventional superconductivity in a molecular Jahn-Teller metal

Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above T c is a major challenge for all unconventional superconductors. The molecular A3C60 fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C60 (3-) electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller-active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of T c with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.