Revisiting the Hitherto Elusive Cyclohexanehexone Molecule: Bulk Synthesis, Mass Spectrometry, and Theoretical Studies

Designer "Quasi-Benzyne": The Spontaneous Reduction of Ortho-Diiodotetrafluorobenzene on Water Microdroplets

It has been widely shown that water microdroplets have a plethora of unique properties that are highly distinct from those of bulk water, among which an especially intriguing one is the strong reducing power as a result of the electrons spontaneously generated at the air-water interface. In this study, we take advantage of the reducing power of water microdroplets to reduce ortho-diiodotetrafluorobenzene (o-C6F4I2) into a C6F4I2•- radical anion. Photoelectron spectroscopy and density functional theory computations reveal that the excess electron in C6F4I2•- occupies the I-C1-C2-I linkage, which elongates the C-I bonds but surprisingly shortens the C1-C2 bond, making the bond order higher than a double bond, similar to the benzyne molecule, so we named it "quasi-benzyne". The C6F4I2•- anion was further successfully utilized in a Diels-Alder reaction, a typical reaction for benzyne. This study provides a good example of strategically utilizing the spontaneous properties of water microdroplets and generating an especially exotic anion, and we anticipate that microdroplet chemistry can be an avenue rich in opportunities for new catalyst-free organic reactions.

The Spontaneous Electron-Mediated Redox Processes on Sprayed Water Microdroplets

Water is considered as an inert environment for the dispersion of many chemical systems. However, by simply spraying bulk water into microsized droplets, the water microdroplets have been shown to possess a large plethora of unique properties, including the ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and/or to trigger spontaneous reactions that cannot occur in bulk water. A high electric field (∼10⁹ V/m) at the air-water interface of microdroplets has been postulated to be the probable cause of the unique chemistries. This high field can even oxidize electrons out of hydroxide ions or other closed-shell molecules dissolved in water, forming radicals and electrons. Subsequently, the electrons can trigger further reduction processes. In this Perspective, by showing a large number of such electron-mediated redox reactions, and by studying the kinetics of these reactions, we opine that the redox reactions on sprayed water microdroplets are essentially processes using electrons as the charge carriers. The potential impacts of the redox capability of microdroplets are also discussed in a larger context of synthetic chemistry and atmospheric chemistry.

Spontaneous Reduction of Transition Metal Ions by One Electron in Water Microdroplets and the Atmospheric Implications

Freshman chemistry teaches that Fe³⁺ and Cu²⁺ ions are stable in water solutions, but their reduced forms, Fe²⁺ and Cu⁺, cannot exist in water as the major oxidation state due to the fast oxidation by O₂ and/or disproportionation. Contrary to these well-known facts, significant fractions of dissolved Fe and Cu species exist in their reduced oxidation states in atmospheric water such as deliquesced aerosols, clouds, and fog droplets. Current knowledge attributes these phenomena to the stabilization of the lower oxidation states by the complexation of ligands and the various photochemical or thermal pathways that can reduce the higher oxidation states. In this study, by spraying the water solutions of transition metal ions into microdroplets, we show the results of the spontaneous reduction of ligated Fe(III) and Cu(II) species into Fe(II) and Cu(I) species, presenting a previously unknown source of reduced transition metal ions in atmospheric water. It is the spontaneously generated electrons in water microdroplets that are responsible for the reduction. Control experiments in the atmosphere and in a glove box filled with precisely controlled gaseous contents reveal that O₂, CO₂, and NO₂ are the major competitors for the electrons, forming O₂^-, HCO₂^-, and NO₂^-, respectively. Taking these findings together, we opine that microdroplet chemistry might play significant but previously underestimated roles in atmospheric redox chemistry.

Spontaneous Reduction by One Electron on Water Microdroplets Facilitates Direct Carboxylation with CO2

Recent advances in microdroplet chemistry have shown that chemical reactions in water microdroplets can be accelerated by several orders of magnitude compared to the same reactions in bulk water. Among the large plethora of unique properties of microdroplets, an especially intriguing one is the strong reducing power that can be sometimes as high as alkali metals as a result of the spontaneously generated electrons. In this study, we design a catalyst-free strategy that takes advantage of the reducing ability of water microdroplets to reduce a certain molecule, and the reduced form of that molecule can convert CO₂ into value-added products. By spraying the water solution of C₆F₅I into microdroplets, an exotic and fragile radical anion, C₆F₅I^•-, is observed, where the excess electron counter-intuitively locates on the σ* antibonding orbital of the C-I bond as evidenced by anion photoelectron spectroscopy. This electron weakens the C-I bond and causes the formation of C₆F₅^-, and the latter attacks the carbon atom on CO₂, forming the pentafluorobenzoate product, C₆F₅CO₂^-. This study provides a good example of strategically making use of the spontaneous properties of water microdroplets, and we anticipate that microdroplet chemistry will be a green avenue rich in new opportunities in CO₂ utilization.

High Electric Field on Water Microdroplets Catalyzes Spontaneous and Ultrafast Oxidative C-H/N-H Cross-Coupling

Oxidative C-H/N-H cross-coupling has emerged as an atom-economical method for the construction of C-N bonds. Conventional oxidative C-H/N-H coupling requires at least one of the following: high temperatures, strong oxidizers, transition metal catalysts, organic solvents, light, and electrochemical cells. In this study, by merely spraying the water solutions of the substrates into microdroplets at room temperature, we show a series of oxidative C-H/N-H coupling products that are strikingly produced in a spontaneous and ultrafast manner. The reactions are accelerated by six orders of magnitude compared to the same reactions in the bulk. It has been previously proposed by fluorescence microscopy and theory that the spontaneously generated electric field at the microdroplets peripheries can be in the ∼10⁹ V/m range. Based on mass spectrometric analysis of key radical intermediates, we opine that the ultrahigh electric field catalytically oxidizes the substrates by removing an electron, which further promotes C/N coupling. Taken together, we anticipate that microdroplet chemistry will be an avenue rich in green opportunities of constructing C-heteroatom bonds.

Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions

Water microdroplets can accelerate chemical reactions by orders of magnitude compared to the same reactions in bulk water and/or trigger spontaneous reactions that do not occur in bulk solution. Among the properties of water microdroplets, the unique redox ability resulting from the spontaneous dissociation of OH⁻ into a released electron and •OH at the air–water interfaces is especially intriguing. At the air–water interface, OH⁻ exhibits a strong reducing potential, and the resulting •OH is highly oxidative, making water microdroplets a unity of opposites. We report the reduction of pyridine into pyridyl anions (C₅H₅N⁻) and the oxidation of pyridine into hydroxypyridine, which extends what we know about the redox power of water microdroplets.

The anion of pyridine, C₅H₅N⁻, has been thought to be short lived in the gas phase and was only previously observed indirectly. In the condensed phase, C₅H₅N⁻ is known to be stabilized by solvation with other molecules. We provide in this study striking results for the formation of isolated C₅H₅N⁻ from microdroplets of water containing dissolved pyridine observed in the negative ion mass spectrum. The gas-phase lifetime of C₅H₅N⁻ is estimated to be at least 50 ms, which is much longer than previously thought. The generated C₅H₅N⁻ captured CO₂ molecules to form a stable (Py-CO₂)⁻ complex, further confirming the existence of C₅H₅N⁻. We propose that the high electric field at the air–water interface of a microdroplet helps OH⁻ to transfer an electron to pyridine to form C₅H₅N⁻ and the hydroxyl radical •OH. Oxidation products of the Py reacting with •OH are also observed in the mass spectrum recorded in positive mode, which further supports this mechanism. The present study pushes the limits of the reducing and oxidizing power of water microdroplets to a new level, emphasizing how different the behavior of microdroplets can be from bulk water. We also note that the easy formation of C₅H₅N⁻ in water microdroplets presents a green chemistry way to synthesize value-added chemicals.

Spontaneous Reduction-Induced Degradation of Viologen Compounds in Water Microdroplets and Its Inhibition by Host-Guest Complexation

Water serves as an inert environment for the dispersion and application of many kinds of herbicides. Viologen compounds, a type of widely used but highly toxic herbicide, are stable in bulk water, whose half-life can be up to 23 weeks in natural water, imposing a severe health risk to mammals. In this study, we present the striking results of the spontaneous and ultrafast reduction-induced degradation of three viologen compounds in water microdroplets and provide the concentration, time, temperature dependence, mechanism, and scale-up of the reactions. We postulate that the electrons existing at the air-water interface of the microdroplets due to the unique redox potential therein initiate the reduction, from which further degradation occurs. The host-guest complexation between cucurbit[7]uril and viologens only slightly changes the redox potential of viologens in the bulk but completely inhibits the reactions in microdroplets, adding to the uniqueness of the redox potentials at the air-water interfaces of microdroplets. Taken together, microdroplets might have been functioning as naturally occurring ubiquitous tiny electrochemical cells for a plethora of unique redox reactions that were thought to be impossible in the bulk water.