Athanasios Zavras

Metalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg₂(H)]⁺by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO₂CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag₂(H)]⁺and [Ph₃PAg₂(H)]⁺react with formic acid yielding Lewis adducts, while [(Ph₃P)₂Ag₂(H)]⁺is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag₂(H)⁺scaffold, triggering reactivity towards formic acid, to produce [dppmAg₂(O₂CH)]⁺and H₂. Decarboxylation of [dppmAg₂(O₂CH)]⁺via CID regenerates [dppmAg₂(H)]⁺. These gas-phase insights inspired variable temperature NMR studies that show CO₂and H₂production at 70 °C from solutions containing dppm, AgBF₄, NaO₂CH and HO₂CH.

Two novel cell-permeable and bacteria-compatible ‘turn-on’ fluorescent probes, designed to be compatible in a multi-dye system and to fluoresce in the blue region exhibiting emission maxima of 440–490 nm, were synthesized. The profluorescent nitroxide probe (11) was developed to visualize and quantify free radical and redox processes, and the Cu(II)-complexed coumarin-based probe (16a) was developed for NO detection. Confocal laser-scanning microscopy and subsequent digital analysis of Pseudomonas aeruginosa biofilms stained with 11 or 16a determined that free radical and redox processes and NO generation occur predominantly in live cells during normal biofilm growth.

Borohydrides react with silver salts to give products that span multiple scales ranging from discrete mononuclear compounds through to silver nanoparticles and colloids. The cluster cations [Ag₃(H)(BH₄)L₃]⁺ are observed upon electrospray ionization mass spectrometry of solutions containing sodium borohydride, silver(I) tetrafluoroborate and bis(dimethylphosphino)methane (L^Me) or bis(diphenylphosphino)methane (L^Ph). By adding NaBH₄ to an acetonitrile solution of AgBF₄ and L^Ph, cooled to ca. −10 °C, we have been able to isolate the first mixed silver hydride borohydride nanocluster, [Ag₃(μ₃-H)(μ₃-BH₄)L^Ph ₃]BF₄, and structurally characterise it via X-ray crystallography. Combined gas-phase experiments (L^Me and L^Ph) and DFT calculations (L^Me) reveal how loss of a ligand from the cationic complexes [Ag₃(H)(BH₄)L₃]⁺ provides a change in geometry that facilitates subsequent loss of BH₃ to produce the dihydride clusters, [Ag₃(H)₂L_n]⁺ (n = 1 and 2). Together with the results of previous studies (Girod et al., Chem. – Eur. J., 2014, 20, 16626), this provides a direct link between mixed silver hydride/borohydride nanoclusters, silver hydride nanoclusters, and silver nanoclusters.

The ion–molecule reactions of the rhenium oxide anions, [ReO _x]⁻ ( x = 2–4) with the organic substrates methane, ethene, methanol, and acetic acid have been examined in a linear ion trap mass spectrometer. The only reactivity observed was between [ReO₂]⁻ and acetic acid. Isotope-labelled experiments and high-resolution mass spectrometry measurements were used to assign the formulas of the ionic products. Collision-induced dissociation and ion–molecule reactions with acetic acid were used to probe the structures of the mass-selected primary product ions. Density functional theory calculations [PBE0/LanL2DZ6-311+G(d)] were used to suggest possible structures. The three primary product channels observed are likely to arise from the formation of the metallalactone [ReO₂(CH₂CO₂)]⁻ ( m/z 277) and H₂, [CH₃ReO₂(OH)]⁻ ( m/z 251) and CO, and [ReO₃]⁻ ( m/z 235), H₂ and CH₂CO.

The decomposition of nitroimidazoles involves the nitro functional group −NO ₂ irrespective of the initial charge state.

The bis(diphenylphosphino)methane (L = Ph₂PCH₂PPh₂) ligated silver deuteride nanocluster dication, [Ag₁₀D₈L₆]²⁺, has been synthesised in the condensed phase via the reaction of bis(diphenylphosphino)methane, silver nitrate and sodium borodeuteride in the methanol : chloroform (1 : 1) mixed solvent system. The photoionisation and photofragmentation of this mass-selected cluster were studied using a linear ion trap coupled to the DESIRS VUV beamline of the SOLEIL Synchrotron. At 15.5 eV the main ionic products observed are [Ag₁₀D₈L₅]²⁺, [Ag₁₀D₈L₄]²⁺, [Ag₁₀D₈L₆]³⁺˙, [Ag₉D₈L₄]²⁺˙, and [AgL₂]⁺. The later two products arise from fragmentation of [Ag₁₀D₈L₆]³⁺˙. An analysis of the yields of these product ions as a function of the photon energy reveals the onset for the formation of [AgL₂]⁺ and [Ag₉D₈L₄]²⁺˙ is around 2 eV higher than that for ionisation to produce [Ag₁₀D₈L₅]³⁺˙. The onset of ionisation energy of [Ag₁₀D₈L₆]²⁺ was determined to be 9.3 ± 0.3 eV from a fit of the yield of the product ion, [Ag₁₀D₈L₆]³⁺˙, as a function of the VUV photon energy. DFT calculations at the RI-PBE/RECP-def2-SVP level of theory were carried out to search for a possible structure of the cluster and to estimate its vertical and adiabatic ionisation energies. The calculated lowest energy structure of the [Ag₁₀D₈L₆]²⁺ nanocluster contains a symmetrical bicapped square antiprism as a silver core in which hydrides are located as a mix of triangular faces and edges. Four of the bisphosphines bind to the edges of the cluster core as bidentate ligands, the remaining two bisphosphines bind via a single phosphorus donor atom to each of the apical silver atoms. The DFT calculated adiabatic ionisation energy for this structure is 8.54 eV, in satisfactory agreement with experiment.

Multistage mass spectrometry and density functional theory (DFT) were used to characterise the small silver hydride nanocluster, [Ag₃H₂L]⁺ (where L=(Ph₂P)₂CH₂) and its gas‐phase unimolecular chemistry. Collision‐induced dissociation (CID) yields [Ag₂HL]⁺ as the major product while laser‐induced dissociation (LID) proceeds via H₂ formation and subsequent release from [Ag₃H₂L]⁺, giving rise to [Ag₃L]⁺ as the major product. Deuterium labelling studies on [Ag₃D₂L]⁺ prove that the source of H₂ is from the hydrides and not from the ligand. Comparison of TD‐DFT absorption patterns obtained for the optimised structures with action spectroscopy results, allows assignment of the measured features to structures of precursors and products. Molecular dynamics “on the fly” reveal that AgH loss is favoured in the ground state, but H₂ formation and loss is preferred in the first excited state S₁, in agreement with CID and LID experimental findings. This indicates favourable photo‐induced formation of H₂ and subsequent release from [Ag₃H₂L]⁺, an important finding in context of metal hydrides as a hydrogen storage medium, which can subsequently be released by heating or irradiation with light.