Mechanochemistry of fullerenes and related materials

Mechanochemistry as an emerging tool for molecular synthesis: what can it offer?

Mechanochemistry is becoming more widespread as a technique for molecular synthesis with new mechanochemical reactions being discovered at increasing frequency. Whilst mechanochemical methods are solvent free and can therefore lead to improved sustainability metrics, it is more likely that the significant differences between reaction outcomes, reaction selectivities and reduced reaction times will make it a technique of interest to synthetic chemists. Herein, we provide an overview of mechanochemistry reaction examples, with 'direct' comparators to solvent based reactions, which collectively seemingly show that solid state grinding can lead to reduced reaction times, different reaction outcomes in product selectivity and in some instances different reaction products, including products not accessible in solution.

Multicomponent mechanochemical synthesis

Historically, the use of mechanochemical methods in synthesis has been almost negligible, but their perception by the synthetic community has changed in recent years and they are on their way to becoming mainstream. However, the hybridization of mechanochemical synthesis with methodologies designed to increase synthetic efficiency by allowing the generation of several bonds in a single operation has taken off only recently, but it already constitutes a very promising approach to sustainable chemistry. In this context, we provide in this Perspective a critical summary and discussion of the main known synthetic methods based on mechanochemical multicomponent reactions.

Palladium-catalyzed ortho-halogenations of acetanilides with N-halosuccinimides via direct sp2 C-H bond activation in ball mills

A solvent-free palladium-catalyzed ortho-iodination of acetanilides using N-iodosuccinimide as the iodine source has been developed under ball-milling conditions. This present method avoids the use of hazardous organic solvents, high reaction temperature, and long reaction time and provides a highly efficient methodology to realize the regioselective functionalization of acetanilides in yields up to 94% in a ball mill. Furthermore, the current methodology can be extended to the synthesis of ortho-brominated and ortho-chlorinated products in good yields by using the corresponding N-halosuccinimides.

Mechanochemical Preparation of Stable Sub-100 nm γ-Cyclodextrin:Buckminsterfullerene (C60) Nanoparticles by Electrostatic or Steric Stabilization

Buckminster fullerene (C₆₀ )'s main hurdle to enter the field of biomedicine is its low bioavailability, which results from its extremely low water solubility. A well-known approach to increase the water solubility of C₆₀ is by complexation with γ-cyclodextrins. However, the formed complexes are not stable in time as they rapidly aggregate and eventually precipitate due to attractive intermolecular forces, a common problem in inclusion complexes of cyclodextrins. In this study we attempt to overcome the attractive intermolecular forces between the complexes by designing custom γ-cyclodextrin (γCD)-based supramolecular hosts for C₆₀ that inhibit the aggregation found in native γCD-C₆₀ complexes. The approach entails the introduction of either repulsive electrostatic forces or increased steric hindrance to prevent aggregation, thus enhancing the biomedical application potential of C₆₀ . These modifications have led to new sub-100 nm nanostructures that show long-term stability in solution.

TEMPO-Mediated Synthesis of Tetrahydropyridinofullerenes: Reaction of [60]Fullerene with α-Methyl-Substituted Arylmethanamines and Aldehydes in the Presence of 4-Dimethylaminopyridine

A series of scarce tetrahydropyridinofullerenes were synthesized by the metal-free-mediated reaction of [60]fullerene with cheap and easily available α-methyl-substituted arylmethanamines and aldehydes in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-dimethylaminopyridine (DMAP) in moderate to good yields comparable to the previously reported data for most monoadducts. The in situ generation of azadienes played a crucial role in the successful synthesis of tetrahydropyridinofullerenes. A plausible reaction mechanism was proposed to elucidate the reaction process.

Stereoselective synthesis of cyclopentafullerenes: the reaction of [60]fullerene with aldehydes and triethylamine promoted by magnesium perchlorate

The magnesium perchlorate-mediated reaction of [60]fullerene with aldehydes and triethylamine generated a series of rare amino-substituted cyclopentafullerenes with high stereoselectivity.

Metal-free synthesis of fulleropyrrolidin-2-ols: a novel reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes

The reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes in the absence of metal salts afforded a series of novel fulleropyrrolidin-2-ols in moderate yields.

Solvent-free N-iodosuccinimide-promoted synthesis of spiroimidazolines from alkenes and amidines under ball-milling conditions

An environmentally benign and N-iodosuccinimide-promoted intermolecular cyclization between alkenes and amidines has been demonstrated under solvent-free ball-milling conditions, affording a variety of spiroimidazolines with remarkable functional group tolerance and good to excellent yields.

Functionalization of [60]fullerene through fullerene cation intermediates

Fullerene cations, namely [60]fullerene radical cation (C₆₀˙⁺) and organo[60]fullerenyl cation (RC₆₀⁺), open paths for the efficient derivatization of a great variety of fullerenes.

Synthesis of fullerotetrahydropyridazines via the copper-catalyzed heteroannulation of [60]fullerene with hydrazides

Fullerotetrahydropyridazines have been efficiently synthesized via the copper-catalyzed reaction of [60]fullerene with hydrazides.

Stereoselective synthesis of N-ethyl-2-arylvinyl-5-methyl fulleropyrrolidines: reaction of [60]fullerene with aromatic aldehydes and triethylamine/diethylamine in the absence or presence of manganese(iii) acetate

Reaction of [60]fullerene with aromatic aldehydes and triethylamine/diethylamine afforded a series of novel fulleropyrrolidines.

Main group mechanochemistry

Over the past decade, mechanochemistry has emerged as a powerful methodology in the search for sustainable alternatives to conventional solvent-based synthetic routes. Mechanochemistry has already been successfully applied to the synthesis of active pharmaceutical ingredients (APIs), organic compounds, metal oxides, coordination compounds and organometallic complexes. In the main group arena, examples of synthetic mechanochemical methodologies, whilst still relatively sporadic, are on the rise. This short review provides an overview of recent advances and achievements in this area that further validate mechanochemistry as a credible alternative to solution-based methods for the synthesis of main group compounds and frameworks.

Highly efficient synthesis of [60]fullerene oxides by plasma jet

Atmospheric pressure nonequilibrium plasma jet has been applied to the synthesis of [60]fullerene oxides (C₆₀O_n) for the first time. C₆₀O and C₆₀O₂ were produced and isolated in high yields up to 44% and 21%, respectively. The structural assignment of C₆₀O was confirmed by comparison with the reported spectroscopic data. Theoretical calculations of ¹³C NMR chemical shifts for eight isomers of C₆₀O₂ were performed and compared with the experimental data to assign the most possible structure for the obtained C₆₀O₂ dominantly as an e isomer.

3D-printed jars for ball-milling experiments monitoredin situby X-ray powder diffraction

Mechanochemistry is flourishing in materials science, but a characterization of the related processes is difficult to achieve. Recently, the use of plastic jars in shaker mills has enabledin situX-ray powder diffraction studies at high-energy beamlines. This paper describes an easy way to design and manufacture these jars by three-dimensional (3D) printing. A modified wall thickness and the use of a thin-walled sampling groove and a two-chamber design, where the milling and diffraction take place in two communicating volumes, allow for a reduced background/absorption and higher angular resolution, with the prospect for use at lower-energy beamlines. 3D-printed polylactic acid jars show good mechanical strength and they are also more resistant to solvents than jars made of polymethyl methacrylate. The source files for printing the jars are available as supporting information.

Solvent-Free Mechanochemical Synthesis of Nitrogen-Doped Nanoporous Carbon for Electrochemical Energy Storage

Nitrogen-doped nanoporous carbons were synthesized by a solvent-free mechanochemically induced one-pot synthesis. This facile approach involves the mechanochemical treatment and carbonization of three solid materials: potassium carbonate, urea, and lignin, which is a waste product from pulp industry. The resulting nitrogen-doped porous carbons offer a very high specific surface area up to 3000 m²  g^-1 and large pore volume up to 2 cm³  g^-1 . The mechanochemical reaction and the impact of activation and functionalization are investigated by nitrogen and water physisorption and high-resolution X-ray photoelectron spectroscopy (XPS). Our N-doped carbons are highly suitable for electrochemical energy storage as supercapacitor electrodes, showing high specific capacitances in aqueous 1 m Li₂ SO₄ electrolyte (177 F g^-1 ), organic 1 m tetraethylammonium tetrafluoroborate in acetonitrile (147 F g^-1 ), and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate; 192 F g^-1 ). This new mechanochemical pathway synergistically combines attractive energy-storage ratings with a scalable, time-efficient, cost-effective, and environmentally favorable synthesis.

Micellization and Single-Particle Encapsulation with Dimethylammoniopropyl Sulfobetaines

Sulfobetaines (SBs) are a class of zwitterionic surfactants with a reputation for enhancing colloidal stability at high salt concentrations. Here, we present a systematic study on the self-assembly of SB amphiphiles (sultaines or hydroxysultaines) in aqueous solutions, as a function of chain length and composition, ionic strength, and in the presence of alkanethiol-coated Au nanoparticles (GNPs). The diameters of the micelles assembled from SB and amidosulfobetaine (ASB) generally increase monotonically with chain length, although ASB micelles are smaller relative to alkyl SB micelles with similarly sized tailgroups, and oleyl sulfobetaine (OSB) micelles are slightly larger. SB amphiphiles can stabilize alkanethiol-coated GNPs in physiologically relevant buffers at concentrations well below their CMC, with size increases corresponding to single-particle encapsulation. SB-encapsulated GNPs were prepared by three different methods with SB:GNP weight ratios of 10:1, followed by dispersion in water or 1 M NaCl. The low hydrodynamic size of the SB micelles and SB-coated NPs is within the range needed for efficient renal clearance.

Synthesis and Supercapacitor Application of Alkynyl Carbon Materials Derived from CaC2 and Polyhalogenated Hydrocarbons by Interfacial Mechanochemical Reactions

The discovery of new carbon materials and the reactive activation of CaC₂ are challenging subjects. In this study, a series of alkynyl carbon materials (ACMs) were synthesized by the interfacial mechanochemical reaction of CaC₂ with four typical polyhalogenated hydrocarbons. Their properties and structures were characterized, and their electrochemical performances were examined. The reaction was rapid and efficient arising from the intense mechanical activation of CaC₂. The ACMs are micro-mesoporous materials with distinct layered structure, specific graphitization degree, and clear existence of sp-C. In addition, the ACMs exhibit high specific capacitance in the range of 57-133 F g^-1 and thus can be ideal candidates for active materials used in supercapacitors. The results may imply an alternative synthesis of carbon allotropes, as well as an efficient approach for the activation of CaC₂.

Hydration or hydroxylation: direct synthesis of fullerenol from pristine fullerene [C60] via acoustic cavitation in the presence of hydrogen peroxide

A green low energy approach avoiding the use of toxic or corrosive reagents/solvents is proposed for the synthesis of potential fullerenol moieties [C₆₀(OH)_n·mH₂O], in which C₆₀ in dil. H₂O₂ (30%) aqueous media was ultrasonicated (20 kHz, 200 W) at 30% amplitude for 1 h.

Mechanochemistry for facilitated access to N,N-diaryl NHC metal complexes

A solvent-free synthesis of NHC–silver, gold, copper and palladium complexes in a ball-mill was achieved.

Which fullerenols are water soluble? Systematic atomistic investigation

Insights to guide a choice of most soluble fullerenols.

Boosting the photocatalytic H2 evolution activity of Fe2O3 polymorphs (α-, γ- and β-Fe2O3) by fullerene [C60]-modification and dye-sensitization under visible light irradiation

Different morphologies of Fe₂O₃ polymorphs were constructed and modified by fluorescein sensitization and C₆₀ cocatalyst to prepare highly active photocatalysts.

Regioselective acylation and carboxylation of [60]fulleroindoline via electrochemical synthesis

A regioselective and highly efficient electrochemical method for direct acylation and carboxylation of a [60]fulleroindoline, affording 1,2,3,16-functionalized [60]fullerene derivatives, regioselectively, has been developed.

Solvent-free iodine-promoted synthesis of 3,2′-pyrrolinyl spirooxindoles from alkylidene oxindoles and enamino esters under ball-milling conditions

A solvent-free mechanochemical reaction of alkylidene oxindoles with enamino esters via an iodine-promoted tandem Michael/cyclization sequence has been developed to provide a variety of spirocyclic oxindoles in moderate to good yields with excellent diastereoselectivities.

The cyclopropanation of [60]fullerobenzofurans via electrosynthesis

The regioselective cyclopropanation of dianionic [60]fullerobenzofurans by diethyl dibromomalonate is dominantly controlled by steric factor.

Metal-free-mediated synthesis of fulleropyrrolines by the reaction of [60]fullerene with β-substituted ethylamines

Reaction of [60]fullerene with β-substituted ethylamines under air conditions generated a series of scarce fulleropyrrolines with trisubstituted alkene functional groups in moderate yields.

Cu(0), O2 and mechanical forces: a saving combination for efficient production of Cu-NHC complexes

Mechanical forces induced by ball-milling agitation enabled the highly efficient and widely applicable synthesis of Cu-carbene complexes from N,N-diaryl-imidazolium salts and metallic copper. The required amount of gaseous dioxygen and insoluble copper could be reduced down to stoichiometric quantities, while reaction rates clearly outperformed those obtained in solution. Utilisation of Cu(0) as the copper source enabled the application of this approach to a wide array of N,N-diaryl-imidazolium salts (Cl^-, BF₄^- and PF₆^-) that transferred their counter anion directly to the organometallic complexes. Cu-NHC complexes could be produced in excellent yields, including utilisation of highly challenging substrates. In addition, five unprecedented organometallic complexes are reported.

The First Synthesis of the Sterically Encumbered Adamantoid Phosphazane P4 (N(t) Bu)6 : Enabled by Mechanochemistry

All reported attempts to synthesize the tert-butyl-substituted adamantoid phosph(III)azane P4 (N(t) Bu)6 have failed, leading to the classification of this molecule as inaccessible and a literature example of steric control in chemistry of phosphorus-nitrogen compounds. We now demonstrate that this structure is readily accessible by a solvent-free mechanochemical milling approach, highlighting the importance of mechanochemical reaction environments in evaluating chemical reactivity.