Infrared Matrix-Isolation and Theoretical Studies of the Reactions of Ferrocene with Ozone

Infrared Matrix-Isolation and Theoretical Studies of the Reactions of Bis(benzene)chromium with Ozone

Reactions of bis(benzene)chromium (Bz2Cr) and ozone (O3) were studied using low-temperature argon matrix-isolation infrared spectroscopy with supporting DFT calculations. When Bz2Cr and O3 were co-deposited, they reacted upon matrix deposition to produce two new prominent peaks in the infrared spectrum at 431 cm-1 and 792 cm-1. These peaks increased upon annealing the matrix to 35 K and decreased upon UV irradiation at λ = 254 nm. The oxygen-18 and mixed oxygen-16,18 isotopic shift pattern of the peak at 792 cm-1 is consistent with the antisymmetric stretch of a symmetric ozonide species. DFT calculations of many possible ozonide products of this reaction were made. The formation of a hydrogen ozonide (H2O3) best fits the original peaks and the oxygen-18 isotope shift pattern. Energy considerations lead to the conclusion that the chromium-containing product of this reaction is the coupled product benzene-chromium-biphenyl-chromium-benzene (BzCrBPCrBz). 2Bz2Cr+O3→H2O3+BzCrBPCrBz, ∆Ecalc=-52.13kcal/mol.

Probing the Structure of the 1:1 Hydrogen-Bonded Complexes of Metallocenes with HCl

Matrix isolation infrared spectroscopy combined with density functional calculations has been used to form, isolate, and characterize the 1:1 hydrogen-bonded complexes of HCl with ferrocene and ruthenocene. Two unique structures were calculated for each complex, analogous to the two binding sites proposed for the attachment of proton to these metallocenes. The spectra, combined with calculated shifts of the H-Cl stretching mode, support the formation of a complex with the HCl hydrogen bonding to one of the cyclopentadienyl rings, exo to the plane of the ring. Evidence also was obtained for a second structure with the hydrogen of the HCl subunit directed toward the Fe or Ru center between the two cyclopentadienyl rings. This structure is similar to the proposed metal-bound proton structure based on earlier mass spectrometric and computational studies. Importantly, these results were supported by parallel experiments with DCl and the two metallocenes.

PdRh-Sensitized Iron Oxide Ultrathin Film Sensors and Mechanistic Investigation by Operando TEM and DFT Calculation

Metal oxide semiconductor (MOS) thin films are of critical importance to both fundamental research and practical applications of gas sensors. Herein, a high-performance H2 sensor based on palladium (Pd) and rhodium (Rh) co-functionalized Fe2 O3 films with an ultrathin thickness of 8.9 nm deposited by using atomic layer deposition is reported. The sensor delivers an exceptional response of 105.9 toward 10 ppm H2 at 230 °C, as well as high selectivity, immunity to humidity, and low detection limit (43 ppb), which are superior to the reported MOS sensors. Importantly, the Fe2 O3 film sensor under dynamic H2 detection is for the first time observed by operando transmission electron microscopy, which provides deterministic evidence for structure evolution of MOS during sensing reactions. To further reveal the sensing mechanism, density functional theory calculations are performed to elucidate the sensitization effect of PdRh catalysts. Mechanistic studies suggest that Pd promotes the adsorption and dissociation of H2 to generate PdHx , while Rh promotes the dissociation of oxygen adsorbed on the surface, thereby jointly promoting the redox reactions on the films. A wireless H2 detection system is also successfully demonstrated using the thin film sensors, certifying a great potential of the strategy to practical sensors.

Infrared Matrix-Isolation and Theoretical Study of the Reactions of Ruthenocene with Ozone

The thermal and photochemical reactions of ozone with ruthenocene were studied in argon matrices at 10-15 K by infrared spectroscopy. Irradiation of freshly deposited matrices with near-infrared light (λ = 880 nm) from an LED resulted in new peaks in their infrared spectra that were assigned to three new ruthenocene oxide structures (4, 5, and 6) calculated by the density functional theory. It is proposed that the near-infrared light caused photodissociation of some ozone molecules and subsequent reactions of the atomic oxygen produced with adjacent ruthenocene molecules in the matrix. Structures 4 and 5 contain a Ru=O oxo group resulting from the attack of atomic oxygen on the ruthenium atom, and structure 6 contains a C=O aldehyde group resulting from the attack of atomic oxygen on a ring carbon atom. Subsequent irradiation of the matrix with red light (λ = 625 nm) from an LED resulted in a fourth new structure (7), and it also initiated a reversible photochemical conversion 4 ⇄ 5 + O₂, with the forward direction promoted by red light (λ = 625 nm) and the reverse direction promoted by near-infrared light (λ = 880 nm). Structure 7, which contains ruthenium-coordinated cyclopentadienyl, cyclopentadienone, and hydride ion, is the most stable of the four new structures as shown by the calculated energies relative to ruthenocene plus O(³P). Structure 7 is proposed as an intermediate in the chemical vapor deposition and atomic layer deposition of the Ru/RuO₂ film forming reactions on substrates at elevated temperatures reported in the literature.

Multichromophoric hybrid species made of perylene bisimide derivatives and Ru(ii) and Os(ii) polypyridine subunits

Herein, the synthesis and the photophysical and redox properties of a new perylene bisimide (PBI) species (L), bearing two 1,10-phenanthroline (phen) ligands at the two imide positions of the PBI, and its dinuclear Ru(ii) and Os(ii) complexes, [(bpy)₂Ru(μ-L)Ru(bpy)₂](PF₆)₄ (Ru2; bpy = 2,2'-bipyridine) and [(Me₂-bpy)₂Os(μ-L)Os(Me₂-bpy)₂](PF₆)₄ (Os2; Me₂-bpy = (4,4'-dimethyl)-2,2'-bipyridine), are reported. The absorption spectra of the compounds are dominated by the structured bands of the PBI subunit due to the lowest-energy spin-allowed π-π* transition. The spin-allowed MLCT transitions in Ru2 and Os2 are inferred by the absorption at 350-470 nm, where the PBI absorption is negligible. The absorption band extends towards the red region for Os2 due to the spin-forbidden MLCT transitions, intensified by the heavy osmium center. The reduction processes of the compounds are dominated by two successive mono-electronic PBI-based processes, which in the metal complexes are slightly shifted compared to the free ligand. On oxidation, both metal complexes undergo an apparent bi-electronic process (at 1.31 V vs. SCE for Ru2 and 0.77 V for Os2), attributed to the simultaneous one-electron oxidation of the two weakly-interacting metal centers. In Ru2 and Os2, the intense fluorescence of L subunit (λ_max, 535 nm; τ, 4.3 ns; Φ, 0.91) is fully quenched, mainly by photoinduced electron transfer from the metal centers, on the ps timescale (time constant, 11 ps in Ru2 and 3 ps in Os2). Such photoinduced electron transfer leads to the formation of a charge-separated state, which directly decays to the ground state in about 70 ps in Os2, but produces the triplet π-π* state of the PBI subunit in 35 ps in Ru2. The results provide information on the excited-state processes of the hybrid species combining two dominant classes of chromophore/luminophore species, the PBI and the metal polypyridine complexes, and can be used for future design on new hybrid species with made-to-order properties.

Photo-Induced Assembly of a Luminescent Tetraruthenium Square

Self-assembly is a powerful synthetic tool that has led to the development of one-, two- and three-dimensional architectures. From MOFs to molecular flasks, self-assembled materials have proven to be of great interest to the scientific community. Here we describe a strategy for the construction and de-construction of a supramolecular structure through unprecedented photo-induced assembly and dis-assembly. The combination of two approaches, a [n×1]-directional bonding strategy and a ligand photo-dissociation strategy, allows the photo-induced assembly of a polypyridyl Ru^II precursor into a discrete molecular square. Diffusion-ordered NMR spectroscopy confirmed the synthesis of a higher volume species, while the identity of the species was established by high-resolution mass spectrometry and single-crystal X-ray diffraction studies. The self-assembled square is not obtained by classical thermal techniques in similar conditions, but is obtained only by light-irradiation. The tetraruthenium square has an excited-state lifetime (135 ns), 40 times that of its mononuclear precursor and its luminescence quantum yield (1.0 %) is three orders of magnitude higher. These remarkable luminescence properties are closely related to the relatively rigid square structure of the tetraruthenium assembly, as suggested by slow radiationless decay and transient absorption spectroscopy. The results described herein are a rare example of photo-induced assembly and dis-assembly processes, and can open the way to a new avenue in supramolecular chemistry, leading to the preparation of structurally organized supermolecules by photochemical techniques.

Reinterpretation of Dynamic Vibrational Spectroscopy to Determine the Molecular Structure and Dynamics of Ferrocene

Molecular distortion of dynamic molecules gives a clear signature in the vibrational spectra, which can be modeled to give estimates of the energy barrier and the sensitivity of the frequencies of the vibrational modes to the reaction coordinate. The reaction coordinate method (RCM) utilizes ab initio-calculated spectra of the molecule in its ground and transition states together with their relative energies to predict the temperature dependence of the vibrational spectra. DFT-calculated spectra of the eclipsed (D_5h ) and staggered (D_5d ) forms of ferrocene (Fc), and its deuterated analogue, within RCM explain the IR spectra of Fc in gas (350 K), solution (300 K), solid solution (7-300 K), and solid (7-300 K) states. In each case the D_5h rotamer is lowest in energy but with the barrier to interconversion between rotamers higher for solution-phase samples (ca. 6 kJ mol^-1 ) than for the gas-phase species (1-3 kJ mol^-1 ). The generality of the approach is demonstrated with application to tricarbonyl(η⁴ -norbornadiene)iron(0), Fe(NBD)(CO)₃ . The temperature-dependent coalescence of the ν(CO) bands of Fe(NBD)(CO)₃ is well explained by the RCM without recourse to NMR-like rapid exchange. The RCM establishes a clear link between the calculated ground and transition states of dynamic molecules and the temperature-dependence of their vibrational spectra.

Charge injection into nanostructured TiO2 electrodes from the photogenerated reduced form of a new Ru(ii) polypyridine compound: the “anti-biomimetic” mechanism at work

Two charge injection mechanisms are active in a new dye-TiO₂ assembly, varying the sacrificial donor.

Iron-Promoted C-C Bond Formation in the Gas Phase

An unusual iron transfer and carbon-carbon coupling take place in gas-phase ionized mixtures containing ferrocene and dichloromethane. Ferrous chloride and the protonated benzenium ion are eventually formed by a thermal and efficient reaction, through stable intermediates that undergo a remarkable reorganization. The mechanism of the concerted iron extrusion, carbon-chlorine bond activation and carbon-carbon bond formation is elucidated by electronic structure calculations that show the crucial role of iron.

Water oxidation catalysis upon evolution of molecular Co(iii) cubanes in aqueous media

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(iii) cubanes, stabilised by a pyridine/acetate ligand pool, to active species that perform water oxidation to oxygen. Primary evolution of the cubane in aqueous solution is likely initiated by removal of an acetate bridge, opening the coordination sphere of the cobalt centres. This cobalt derivative, where the pristine ligands still impact on the reactivity, shows enhanced electron transfer rates to Ru(bpy)₃³⁺(hole scavenging) within a photocatalytic cycle with Ru(bpy)₃²⁺as the photosensitiser and S₂O₈²⁻as the electron sink. A more accentuated evolution occurs under continuous irradiation, where Electron Paramagnetic Resonance (EPR) spectroscopy reveals the formation of Co(ii) intermediates, likely contributing to the catalytic process that evolves oxygen. All together, these results confirm the relevant effect of molecular species, in particular in fostering the rate of the electron transfer processes involved in light activated cycles, pivotal in the design of a photoactive device.