Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times.

Three structural isomers of tris{bis(trifluoromethyl)phenyl}borane have been studied as the acidic component of frustrated Lewis pairs. While the 3,5-substituted isomer is already known to heterolytically cleave H2 to generate a bridging-hydride; ortho-substituents in the 2,4- and 2,5-isomers quench such reactivity through electron donation into the vacant boron pz orbital and steric blocking of the boron centre; as shown by electrochemical, structural and computational studies. Electrochemical studies of the corresponding borohydrides identify that the two-electron oxidation of terminal-hydrides occurs at more positive potentials than observed for [HB(C6F5)3]−, while the bridging-hydride oxidizes at a higher potential still, comparable to that of free H2.

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

The novel 1:1:1 hetero-tri(aryl)borane (pentafluorophenyl){3,5-bis(trifluoromethyl)phenyl}(pentachlorophenyl)borane has been synthesised and structurally characterised. This has been show to act as the Lewis acidic component in FLPs for the heterolytic cleavage of H2 with three Lewis bases.

We herein report the synthesis of novel “Janus” calix[4]arenes bearing four “molecular tethering” functional groups on either the upper or lower rims of the calixarene. These enable facile multipoint covalent attachment to electrode surfaces with monolayer coverage. The other rim of the calixarenes bear either four azide or four ethynyl functional groups, which are easily modified by the copper(I)-catalyzed azide–alkyne cycloaddition reaction (CuAAC), either pre- or postsurface modification, enabling these conical, nanocavity reactor sites to be decorated with a wide range of substrates to impart desired chemical properties. Redox active species decorating the peripheral rim are shown to be electrically connected by the calixarene to the electrode surface in either “up” or “down” orientations of the calixarene.

Reaction of [Au(C6F5)(tht)] (tht = tetrahydrothiophene) with 2,2′:6′,2″-terpyridine (terpy) leads to complex [Au(C6F5)(η1-terpy)] (1). The chemical oxidation of complex (1) with 2 equiv of [N(C6H4Br-4)3](PF6) or using electrosynthetic techniques affords the Au(III) complex [Au(C6F5)(η3-terpy)](PF6)2 (2). The X-ray diffraction study of complex 2 reveals that the terpyridine acts as tridentate chelate ligand, which leads to a slightly distorted square-planar geometry. Complex 1 displays fluorescence in the solid state at 77 K due to a metal (gold) to ligand (terpy) charge transfer transition, whereas complex 2 displays fluorescence in acetonitrile due to excimer or exciplex formation. Time-dependent density functional theory calculations match the experimental absorption spectra of the synthesized complexes. In order to further probe the frontier orbitals of both complexes and study their redox behavior, each compound was separately characterized using cyclic voltammetry. The bulk electrolysis of a solution of complex 1 was analyzed by spectroscopic methods confirming the electrochemical synthesis of complex 2.

Frustrated Lewis pairs have found many applications in the heterolytic activation of H2 and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H2 can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H2 oxidation by 610 mV (117.7 kJ mol–1). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology.

We report the first known examples of triazole-derivatized cymantrene complexes (η5-[4-substituted triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I), obtained via a “click” chemical synthesis, bearing a phenyl, 3-aminophenyl, or 4-aminophenyl moiety at the 4-position of the triazole ring. Structural characterization data using multinuclear NMR, UV–vis, ATR-IR, and mass spectrometric methods are provided, as well as crystallographic data for (η5-[4-phenyltriazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I) and (η5-[4-(3-aminophenyl)triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I). Cyclic voltammetric characterization of the redox behavior of each of the three cymantrene–triazole complexes is presented together with digital simulations, in situ infrared spectroelectrochemistry, and DFT calculations to extract the associated kinetic and thermodynamic parameters. The trypanocidal activity of each cymantrene–triazole complex is also examined, and these complexes are found to be more active than cymantrene alone.

We report a kinetic and mechanistic study into the one-electron reduction of the archetypal Lewis acid tris(pentafluorophenyl)borane, B(C6F5)3, in dichloromethane and 1,2-difluorobenzene. Electrochemical experiments, combined with digital simulations, DFT computational studies and multinuclear NMR analysis allow us to obtain thermodynamic, kinetic and mechanistic information relating to the redox activity of B(C6F5)3. We show that tris(pentafluorophenyl)borane undergoes a quasi-reversible one-electron reduction followed by rapid chemical decomposition of the B(C6F5)3˙− radical anion intermediate via a solvolytic radical pathway. The reaction products form various four-coordinate borates of which [B(C6F5)4]− is a very minor product. The rate of the follow-up chemical step has a pseudo-first order rate constant of the order of 6 s−1. This value is three orders of magnitude larger than that found in previous studies performed in the donor solvent, tetrahydrofuran. The standard reduction potential of B(C6F5)3 is reported for the first time as −1.79 ± 0.1 V and −1.65 ± 0.1 V vs. ferrocene/ferrocenium in dichloromethane and 1,2-difluorobenzene respectively.

Herein, we report the synthesis of three covalently linked superparamagnetic nanocrystal-multi-walled carbon nanotube (MWCNT) composites. A generic strategy for amphiphilic polymer coating of nanocrystals and further functionalization for amide bond formation with the MWCNTs is discussed. This approach can in principle allow attachment of any colloidal nanocrystal to the MWCNTs. The materials were characterized at each stage of the syntheses using DLS, zeta-potential measurements, FT-IR, TEM, and XPS techniques. The practicality of this linkage is demonstrated by the reversible magnetic immobilization of these materials on an electrode during non-aqueous electrochemistry.Graphical abstractHighlights► Preparation of superparamagnetic nanoparticles-carbon nanotube composites. ► Polymer coating the nanoparticles passivates them and allows regioselective attachment to CNTs. ► The majority of CNT surface remains available for further modification. ► “Magnetic nanotube” composites are characterized using a variety of techniques.

3-Aryl-3-(trifluoromethyl)diazirines are shown to be synthetically useful photoactivated carbene precursors that can be used as molecular “tethers” to facilitate the improved covalent surface modification of graphitic carbon and carbon nanotubes with a potentially large variety of chemical species. Proof-of-concept is demonstrated by the synthesis, as well as spectroscopic and electrochemical characterization, followed by photoactivated attachment of the organometallic diazirine derivative, 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate, to the surface of vitreous carbon, and also to two different morphologies of multiwalled carbon nanotubes (“bamboo-like” and “hollow-tube”, denoted as b-MWCNTs and h-MWCNTs, respectively). The latter differ only in the relative amounts of “edge-plane-like” defect sites (at the termini of the nanotubes) and “basal-plane-like” pristine sidewall regions. The facile covalent coupling of the ferrocenyl “probe” moiety to the diazirine “linker” was confirmed by UV–vis, 1H and 19F NMR spectroscopy, and cyclic voltammetry (CV). Upon exposure to UV irradiation in the presence of graphitic materials, the resulting covalent surface attachment of the ferrocenyl groups via the diazirine “linker” was characterized by Raman and X-ray photoelectron spectroscopy (XPS) and by CV experiments performed in nonaqueous electrolyte. The surface coverage of 3-[3-(trifluoromethyl)diazirin-3-yl]phenyl ferrocene monocarboxylate, analyzed from both CV and XPS experiments was found to be 7%–11% of that estimated for a complete monolayer, and was 20-fold greater than that achieved in control experiments that employed conventional covalent modification strategies to form esters between ferrocene methanol and surface carboxylate groups on the graphitic materials. The surface loading of ferrocene groups on the b-MWCNTs was found to be only ca. 60%–70% that achieved on h-MWCNTs, reflecting the ability of the functionalized carbene intermediate formed upon photolysis of the parent diazirine to insert into C═C bonds in the otherwise relatively inert sidewalls of the nanotubes. This was further confirmed by Raman spectroscopic characterization, which revealed that the h-MWCNTs experienced significantly more sidewall functionalization than the b-MWCNTs, yet still retained good electronic conduction in electrochemical experiments. The relative chemical stability of 3-aryl-3-(trifluoromethyl)diazirines, the ease with which they can be potentially be coupled to a large range of different organic, inorganic, and biological species, and the enhanced surface loading that can be achieved as a result of the reactive carbene intermediate formed during their photolysis, render diazirines highly versatile and potent “linker” molecules for the development of chemically modified materials.Keywords: carbene; carbon nanotubes; chemical modification; diazirine; ferrocene-functionalized carbene; photolysis; sidewall functionalization; surface modification; voltammetry;

Multiwalled carbon nanotubes (MWCNTs) were covalently modified with polymer-coated superparamagnetic Fe3O4 nanoparticles via amide bond formation to surface oxo-groups located predominantly at the ends of the nanotubes to form “magnetic MWCNTs”. The sidewalls of the magnetic MWCNTs were then selectively covalently modified with ferrocenyl groups via the photolysis of 3-[3-(trifluoromethyl) diazirin-3-yl] phenyl ferrocene monocarboxylate, which uses an aryldiazirine moiety as a molecular “tether”. We demonstrate that the assembly of the chemically-modified magnetic MWCNTs onto the surface of a magnetic carbon electrode enables one to obtain stable voltammetric signals of chemically-modified carbon nanotubes in non-aqueous electrolytes even under vigorous hydrodynamic conditions of stirring at 3000 rpm for up to twenty minutes. In contrast, non-magnetic chemically modified MWCNTs are removed from the electrode surface within the first two minutes of stirring.Highlights► Stable non-aqueous voltammetry of chemically modified CNTs achieved. ► CNTs co-modified with superparamagnetic nanoparticles and ferrocenyl groups. ► Selective end and sidewall co-modification.

We report a kinetic and mechanistic study into the one-electron reduction of the archetypal Lewis acid tris(pentafluorophenyl)borane, B(C6F5)3, in dichloromethane and 1,2-difluorobenzene. Electrochemical experiments, combined with digital simulations, DFT computational studies and multinuclear NMR analysis allow us to obtain thermodynamic, kinetic and mechanistic information relating to the redox activity of B(C6F5)3. We show that tris(pentafluorophenyl)borane undergoes a quasi-reversible one-electron reduction followed by rapid chemical decomposition of the B(C6F5)3˙− radical anion intermediate via a solvolytic radical pathway. The reaction products form various four-coordinate borates of which [B(C6F5)4]− is a very minor product. The rate of the follow-up chemical step has a pseudo-first order rate constant of the order of 6 s−1. This value is three orders of magnitude larger than that found in previous studies performed in the donor solvent, tetrahydrofuran. The standard reduction potential of B(C6F5)3 is reported for the first time as −1.79 ± 0.1 V and −1.65 ± 0.1 V vs. ferrocene/ferrocenium in dichloromethane and 1,2-difluorobenzene respectively.

Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times.

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

Three structural isomers of tris{bis(trifluoromethyl)phenyl}borane have been studied as the acidic component of frustrated Lewis pairs. While the 3,5-substituted isomer is already known to heterolytically cleave H2 to generate a bridging-hydride; ortho-substituents in the 2,4- and 2,5-isomers quench such reactivity through electron donation into the vacant boron pz orbital and steric blocking of the boron centre; as shown by electrochemical, structural and computational studies. Electrochemical studies of the corresponding borohydrides identify that the two-electron oxidation of terminal-hydrides occurs at more positive potentials than observed for [HB(C6F5)3]−, while the bridging-hydride oxidizes at a higher potential still, comparable to that of free H2.