Mechanochemical hydroquinone regeneration promotes gold salt reduction in sub-stoichiometric conditions of the reducing agent.

Bottom-up mechanochemical synthesis (BUMS) has been demonstrated to be an efficient approach for the preparation of metal nanoparticles (NPs), protected by surface agents or anchored on solid supports. However, there are limitations, such as precise size and morphological control, due to a lack of knowledge about the mechanically induced processes of NP formation under milling. In this article, we further investigate the BUMS of AuNPs. Using SiO2 as a solid support, we studied the effect of typical reducing agents, namely NaBH4, L-ascorbic acid, and hydroquinone (HQ), on the conversion of a AuIII source. XANES showed that HQ is the strongest reducing agent under our experimental conditions, leading to the quantitative conversion of gold salt in a few minutes. Interestingly, even when HQ was used in sub-stoichiometric amounts, AuIII could be reduced to ratios higher than 85% after two minutes of milling. Investigations into the byproducts by 1H NMR and GC-FID/MS enabled the identification HQ regeneration and the formation of its derivatives. We mainly focused on benzoquinone (BQ), which is the product of the oxidation of HQ as it reduces the gold salt. We could demonstrate that HQ is regenerated from BQ exclusively under milling and acidic conditions. The regenerated HQ and other HQ-chlorinated molecules could then reduce gold-oxidized species, leading to higher conversions and economy of reactants. Our study highlights the intriguing and complex mechanisms of mechanochemical systems, in addition to fostering the atom and energy economy side of mechanochemical means to produce metal nanoparticles.

Visible light-driven CO2 photoreduction by a Re(I) complex immobilized onto CuO/Nb2O5 heterojunctions.

The immobilization of Re(I) complexes onto metal oxide surfaces presents an elegant strategy to enhance their stability and reusability toward photocatalytic CO2 reduction. In this study, the photocatalytic performance of fac-[ClRe(CO)3(dcbH2)], where dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine, anchored onto the surface of 1%m/m CuO/Nb2O5 was investigated. Following adsorption, the turnover number for CO production (TONCO) in DMF/TEOA increased significantly, from ten in solution to 370 under visible light irradiation, surpassing the TONCO observed for the complex onto pristine Nb2O5 or CuO surfaces. The CuO/Nb2O5 heterostructure allows for efficient electron injection by the Re(I) center, promoting efficient charge separation. At same time CuO clusters introduce a new absorption band above 550 nm that contributes for the photoreduction of the reaction intermediates, leading to a more efficient CO evolution and minimization of side reactions.

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction.

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO2 reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc+, attributed to consecutive 1 e- reduction. Under CO2 atmosphere, a catalytic wave is observed (Eonset = 2.1 V vs. Fc/Fc+), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (kobs = 1.5 s-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TONCO = 21 and kobs = 2.0 s-1) and, additionally, formate was also detected (TONHCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Zinc(II) complexes bearing N,N,S ligands: Synthesis, crystal structure, spectroscopic analysis, molecular docking and biological investigations about its antifungal activity.

In the present work, the synthesis, characterization, antifungal activity, molecular docking study and in silico approach of five thiosemicarbazone derivatives and their corresponding zinc(II) complexes are described. The compounds were characterized by elemental analysis, IR, UV-Vis and NMR spectroscopic measurements, molar conductivity measurements, emission spectra, high-resolution mass spectrometry and X ray study. The antifungal activity of the free ligands and synthesized compounds was preliminarily evaluated against Candida albicans (ATCC 90028), Candida tropicalis (ATCC 13803) and Candida glabrata (ATCC 2001), by the minimum inhibitory concentration (MIC) assay. Two complexes, 4 (MIC = 3.18 to 6.37 µM) and 5 (MIC = 25.95 µM for all) showed promising results, being highly active against all strains evaluated. The X-ray analyses shown that the complex 2 crystallizes in the centrosymmetric space group P21/c of the monoclinic system and the coordination sphere around zinc(II) atom is better described as slightly distorted octahedral. The Hirshfeld surface (HS) analysis showed that non-classical H···H and C···H/H···C contacts contribute with 65.9% while the S···H and N···H (21%) and Cl···H and O···H interactions (12%) complete the HS area. The molecular docking results, performed against CYP51 enzyme (sterol 14α-demethylase) of C. albicans and C. glabrata shows that the complexes 4 (ΔG = -10.75 and - 12.90 kcal/ mol) and 5 (ΔG = -11.12 and - 14.53 kcal/ mol) showed the highest binding free energies of all compounds. The ADME-Tox (absorption, distribution, metabolism, excretion and toxicity) in silico parameters evaluated showed promising results for all compounds.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex.

A new ruthenium polypyridyl complex, [Ru(bpy)2(acpy)]+ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)2(pic)]+ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the RuII center. The reaction is one of the few examples of C-H activation at mild conditions using O2 as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10-2 h-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O2 binding to [Ru(bpy)2(acpy)]+ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting RuIII-OO˙- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG‡ = 41.1 kcal mol-1), thus the formation of a diruthenium µ-peroxo intermediate, [(RuIII(bpy)2(O-acpy))2O2]2+via interaction of a second [Ru(bpy)2(acpy)]+ was examined as an alternative pathway. The dimer yields two RuIVO centers with a low ΔG‡ of 2.3 kcal mol-1. The resulting RuIVO species can activate C-H bonds in acpy (ΔG‡ = 23.1 kcal mol-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O2 as the sole oxygen source.

Spectroscopic characterization of a new Re(I) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications.

This work describes the preparation of a new thiosemicarbazone derivative, (Z)-N-ethyl-2-(6-oxo-1,10-phenanthrolin-5(6H)-ylidene)hydrazinecarbothioamide (phet) and its respective Re(i) tricarbonyl chloro complex, fac-[ReCl(CO)3(phet)]. The spectroscopic, photophysical and electrochemical properties of the new complex were fully investigated through steady state and time-resolved techniques along with computational calculations. In fac-[ReCl(CO)3(phet)], the new ligand is coordinated to the metal center through the pyridyl rings of the phenanthroline moiety. The unbound electron pairs in the S atom of the bending thiosemicarbazone group induce new low energy lying electronic transitions. Consequently, enhanced visible light absorption up to 550 nm is observed in acetonitrile due to the overlap between MLCTRe→phet and ILphet(n→π*) transitions. The absorption bands and emission quantum yields of fac-[ReCl(CO)3(phet)] are sensitive to proton concentration due to an acid-basic equilibrium in the N atoms of the thiosemicarbazone. Proton dissociation constants of 10.0 ± 0.1 and 11.4 ± 0.2 were determined respectively for the ground and excited states of the new complex. Spectral changes could also be observed in the presence of Zn2+ cations which can be further explored for sensing applications. The electrochemical behavior of the new complex was studied in detail, revealing up to four one electron reduction processes in the range from 0 to -2.4 V vs. Fc+/Fc. With support of DFT calculations, the first three processes are ascribed to the reduction of the coordinated phet ligand followed by the ReI/0 reduction and consequent Cl- release. The new complex was able to act as an electrocatalyst for CO2 reduction into CO (Eonset = -1.92 V vs. Fc+/Fc), with a turnover frequency of 2.81 s-1 and turnover number of 24 ± 1 in anhydrous acetonitrile, being the first Re(i) tricarbonyl complex with a thiosemicarbazone derivative described for this goal. The detailed characterization carried out here can drive the development of new Re(i)-thiosemicarbazone derivatives for different applications.

Photochemistry of fac-[Re(CO)3(dcbH2)( trans-stpy)]+: New Insights on the Isomerization Mechanism of Coordinated Stilbene-like Ligands.

In this work, a novel complex fac-[Re(CO)3(dcbH2)( trans-stpy)]+, (dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine; trans-stpy = trans-4-styrylpyridine) was synthesized and characterized toward its spectroscopic, photochemical, and photophysical properties. The experimental data provide new insights on the mechanism of photochemical trans-to- cis isomerization of the stilbene-like ligand coordinated to Re(I) polypyridyl complexes. The new complex exhibits an unusual and strong dependence of the isomerization quantum yield (Φt →c) on the irradiation wavelength. Φt →c was 0.81 ± 0.08 for irradiation at 365 nm and continuously decreased as the irradiation wavelength is shifted to the visible. At 405 nm irradiation Φt →c is almost 2 orders of magnitude lower (0.010 ± 0.005) than that observed at 365 nm excitation. This behavior can be explained by the low-lying triplet metal-to-ligand charge-transfer excited state (3MLCT) that hinders the triplet photoreaction mechanism under visible light absorption. Under UV irradiation, direct population of styrylpyridine-centered excited state (1IL) leads to the occurrence of the photoisomerization via a singlet mechanism. Further experiments were performed with the complex immobilized on the surface of TiO2 and Al2O3 films. The nonoccurrence of isomerization at the oxide surfaces even under UV excitation evidences the role of energy gap between the 1IL/1MLCT states on the photochemical/photophysical processes. The results establish important relationships between the molecular structure and the photoelectrochemical behavior, which can further contribute to the development of solid-state molecular switches based on Re(I) polypyridyl complexes.