Improved Synthesis of (TBA)2[W6Br14] Paving the Way to Further Study of Bromide Cluster Complexes

Octahedral cluster complexes of molybdenum and tungsten, [M₆X₈Y₆]^n- (M = Mo, W; X, Y = Cl, Br, I), are promising active components in various fields, including biomedicine and solar energy. Cluster complexes draw considerable attention due to their X-ray opacity, red/near-IR luminescence, and ability to convert triplet molecular oxygen to active singlet oxygen under UV and visible irradiation. Among the octahedral cluster complexes of molybdenum and tungsten, compounds with a {W₆Br₈}⁴⁺ core are the least studied. There are only a few examples of compounds with substituted terminal ligands, and their properties are not well understood. Among other things, this is due to more labor-intensive and expensive methods for obtaining the starting compounds in comparison with molybdenum counterparts. In this paper, we describe the synthesis of an octahedral cluster complex, (TBA)₂[W₆Br₁₄] (TBA⁺ = tetrabutylammonium), in gram quantities, starting from simple substances─W, Br₂, and Bi─in 70% yield. The formation of pentanuclear tungsten cluster complexes was recorded as a byproduct. Compounds with substituted terminal ligands (TBA)₂[W₆Br₈Y₆] (Y = NO₃, Cl, I) were obtained. We also discuss the instability of (TBA)₂[W₆Br₈(NO₃)₆] under light exposure, the optical properties of a series of compounds (TBA)₂[W₆Br₈Y₆] (Y = Cl, Br, I), and the effect of terminal ligands on the chemical shifts in ¹⁸³W NMR spectra in dimethyl sulfoxide-d₆. The presented approach to the synthesis of one of the main precursors of various bromide cluster complexes on a gram scale can stimulate the study of their properties and development of new functional materials based on them.

π-Self-Assembly of a Coronene on Carbon Nanomaterial-Modified Electrode and Its Symmetrical Redox and H2O2 Electrocatalytic Reduction Functionalities

The structure-electroactivity relationship of graphene has been studied using coronene (Cor), polyaromatic hydrocarbon (PAH), and a subunit of graphene as a model system by chemically modified electrode approach. In general, graphene and PAH do not show any redox activity in their native form. Herein, we report a simple electrochemical approach for the conversion of electro-inactive coronene to a highly redox-active molecule (Cor-Redox; E°' = 0.235 ± 0.005 V vs Ag/AgCl) after being adsorbed on graphitic carbon nanomaterial and preconditioned at an applied potential, 1.2 V vs Ag/AgCl, wherein, the water molecule oxidizes to dioxygen via hydroxyl radical (^•OH) intermediate, in acidic solution (pH 2 KCl-HCl). When the same coronene electrochemical experiment was carried out on an unmodified glassy carbon electrode, there was no sign of faradic signal, revealing the unique electrochemical behavior of the coronene molecule on graphitic nanomaterial. The Cor-Redox peak is found to be highly symmetrical (peak-to-peak potential separation of ∼0 V tested by cyclic voltammetry (CV)) and surface-confined (Γ_Cor-Redox = 10.1 × 10^-9 mol cm^-2) and has proton-coupled electron-transfer (∂E°'/∂pH = -56 mV pH^-1) character. Initially, it was speculated that Cor is converted to a hydroxy group-functionalized Cor molecule (dihydroxy benzene derivative) on the graphitic surface and showed the electrochemical redox activity. However, physicochemical characterization studies including Raman, IR, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), redox-site selective oxidation probe, cysteine (for dihydroxy benzene), radical scavenger ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO), and scanning electrochemical microscopy (SECM) using ferricyanide redox couple have revealed that coronene cationic radical species like electroactive molecule is formed on graphitic material upon the electrochemical oxidation reaction at a high anodic potential. It has been proposed that ^•OH generated as an intermediate species from the water oxidation reaction is involved in the coronene cationic radical species. Studies on coronene electrochemical reaction at various carbon nanomaterials like multiwalled carbon, single-walled carbon, graphite, graphene oxide, and carbon nanofiber revealed that graphitic structure (without any oxygen functional groups) and its π-π bonding are key factors for the success of the electrochemical reaction. The coronene molecular redox peak showed an unusual electrocatalytic reduction of hydrogen peroxide similar to the peroxidase enzyme-biocatalyzed reduction reaction in physiological solution.

Coronene-based charge-transfer complexes

Recent developments in the arena of charge-transfer complexes composed of the D 6h-symmetric polycyclic aromatic hydrocarbon, coronene, are highlighted with emphasis on the structural and physical properties of these complexes. Because of the dual electron-donating and -accepting abilities of coronene, this group involves structurally-defined four cation salts and three anion salts. The Jahn-Teller distortions and in-plane motion of coronene molecules in the solids, both of which are closely associated with the high symmetry of coronene molecules, and syntheses of clathrate-type complexes are also presented.

Conducting π Columns of Highly Symmetric Coronene, The Smallest Fragment of Graphene

Coronene, which is the smallest D6h -symmetric polycyclic aromatic hydrocarbon, attracts particular attention as a basic component of electronic materials because it is the smallest fragment of graphene. However, carrier generation by physical methods, such as photo- or electric field-effect, has barely been studied, primarily because of the poor π-conduction pathway in pristine coronene solid. In this work we have developed unprecedented π-stacking columns of cationic coronene molecules by electrochemical hole-doping with polyoxometallate dianions. The face-to-face π-π interactions as well as the partially charged state lead to electrical conductivity at room temperature of up to 3 S cm(-1) , which is more than 10 orders of magnitude higher than that of pristine coronene solid. Additionally, the robust π-π interactions strongly suppress the in-plane rotation of the coronene molecules, which has allowed the first direct observation of the static Jahn-Teller distortion of cationic coronene molecules.