Valdemiro Pereira Carvalho Júnior

The PN acetate ruthenium(II) complex [Ru(κ ² ‐OAc)(PPye) ₂ ]OAc ( 1 ) was synthesized from [Ru(κ ² ‐OAc) ₂ (PPh ₃ ) ₂ ] and 1‐(diphenylphosphino)‐2‐(2‐pyridyl)ethane (PPye) in methanol and isolated in 70% yield. Treatment of [Ru(κ ² ‐OAc) ₂ (PPh ₃ ) ₂ ] with 2‐(diphenylphosphino)pyridine (PPy) afforded [Ru(κ ¹ OAc)(PPh ₃ )(PPy) ₂ ]OAc ( 2 ) in high yield. The complexes 1 and 2 display photocatalytic activity in the TH of ketones using 2‐propanol. Complex 1 exhibits superior activity, affording quantitative conversion of aryl ketones under mild conditions (40°C), whereas no activity was observed in dark. On–off experiments confirmed a photoresponsive nature of the catalytic process. Mechanistic studies revealed the formation of different ruthenium monohydride species under thermal and irradiated conditions. Luminescence quenching studies in the presence of NaO i Pr support the involvement of excited‐state processes associated with alkoxide activation and hydride generation.

Consumption of formic acid (FA) is observed in neat triethylamine (TEA) in the presence of 2-, 3-, or 4-methylpiperidine-coordinated Ru-(η6-p-cymene) complexes, with the formation of molecular hydrogen and carbon dioxide as the only gaseous products, as confirmed by CG-TCD analysis. The reactions were kinetically monitored by measuring the gas volume, which increased with increasing complex concentration (4–13 μmol) and temperature (70–100 °C). No significant difference in the initial reaction rates was observed upon changing the amine ancillary ligand. All reactions reached the same saturation yields, which decreased in the presence of water compared with reactions carried out in neat TEA. A reaction mechanism is proposed based on in situ 1H NMR measurements. The chloride ion in the precursor complex is replaced by a formate ion through reaction with an FA/TEA adduct, followed by CO2 elimination to form a metal hydride species. Subsequently, the hydride ion is protonated by another FA/TEA adduct, leading to H2 evolution and regeneration of the cyclic formate complex. The most novel finding is that, in the latter step, the η6-p-cymene ligand undergoes a hapticity shift to η2-p-cymene, which may facilitate its displacement. Nevertheless, the resulting lower-coordinate amine complex remains catalytically active through a parallel catalytic cycle.

This study evaluated the use of genipin in monoamino-N-aminoethyl (MANAE)-agarose in the immobilization of fructosyltransferase (FTase) from Aspergillus aculeatus OI4C2, focusing on the stability and catalytic efficiency for the production of fructooligosaccharides (FOS) from sucrose. The immobilization of FTase on genipin-cross-linked MANAE-agarose achieved a 98.53% yield, 76.71% activity recovery, and 77.86% efficiency, and it resulted in the synthesis of predominantly short-chain fructooligosaccharides. Comparatively, the maximum amounts of FOS obtained were 73.44 g L^–1 for the free enzyme and 97.37 g L^–1 for the immobilized enzyme. Immobilization improved the thermal stability at 40–60 °C and increased the enzyme stability under acidic pH conditions (4.5–6.5) for up to 72 h. The operational stability of the immobilized enzyme retained 50% of its initial activity after 12 reuse cycles. These results indicate that A. aculeatus FTase immobilized on MANAE-agarose–genipin is a promising biocatalyst for large-scale FOS production, with prospects in the food, pharmaceutical, and nutraceutical industries.

A series of new monocationic Ru complexes containing phenanthroline derivatives were developed. The monometallic complexes [Ru­(κ²-OAc)­(dppb)­(N,N)]­OAc derivatives were synthesized in high yield via the reaction between [Ru­(κ²-OAc)₂dppb] and the corresponding N,N ligand. Additionally, dinuclear [(dppb)­(κ²-OAc)­(Ru­(μ-N,N--C,N)­Ru­(κ²-OAc)­(dppb)]­OAc complexes were synthesized from equimolar amounts of the appropriate monometallic complex and [Ru­(κ²-OAc)₂dppb]. All complexes were characterized by NMR, FTIR, UV–vis spectroscopy, and cyclic voltammetry. These precatalysts display selective catalytic activity toward dehydrogenation of formic acid for H₂ production, with the dinuclear systems demonstrating superior performance, achieving up to 100% conversion under optimized conditions. The dinuclear system maintained consistent TOF₅₀ values through several catalytic cycles, demonstrating excellent stability. Mechanism investigations revealed the formation of two Ru-monohydride species, showing a fac-RuHP₂ and a mer-RuHP₂ arrangement, respectively, formed via substitution of a κ²-OAc by a κ²-O₂CH followed by a β-elimination, where both are involved in the mechanisms. DFT calculations of the species involved in the mechanism showed that fac-RuHP₂ is lower in energy than mer-RuHP₂. The complexes were additionally applied in the transfer hydrogenation of CO₂ to produce formic acid with 2-propanol.

Currently, 3 d metal complexes have attracted considerable attention as photocatalysts in photopolymerization reactions, representing a viable alternative to noble metals. In this work, two Ni(II) thiosemicarbazone (TSC) complexes bearing different substituents, methyl and phenyl, [Ni(N,N,S‐TSC) (4‐NH ₂ Py)] (Ni‐TSC ^Me and Ni‐TSC ^Ph ), were synthesized via the reaction of [NiCl ₂ (PPh ₃ ) ₂ ] with the corresponding TSC ligand, followed by coordination of 4‐aminopyridine (4‐NH ₂ Py). The resulting complexes were fully characterized by FTIR, UV–Vis, NMR, elemental analysis, and ESI mass spectrometry. Their performance as photocatalysts was investigated in photoinitiating systems containing diphenyliodonium hexafluorophosphate (Iod) and ethyl 4‐(dimethylamino)benzoate (EDB) amine for the free radical photopolymerization of trimethylolpropane ethoxylate triacrylate (TMPETA). The absorption, fluorescence, and electrochemical properties were evaluated to provide deeper insight into their photocatalytic activity, reactivity, and the proposed reaction mechanism. Both complexes exhibited enhanced performance under UV irradiation, while Ni‐TSC ^Me maintained remarkable photocatalytic activity under both UV and violet light. Furthermore, the interaction between the Ni(II) complexes and the additives was investigated through photolysis experiments under UV light and free energy change calculations, which served as key parameters in proposing a reaction mechanism.

A series of half-sandwich ruthenium­(II) complexes bearing substituted α-diimine ligands with the general formula [RuCl­(p-cym)­(N–N ⁿ )]­(PF₆) was synthesized (where n = 1–7; N–N ¹ = N1,N2-bis(2,6-dimethylphenyl)­ethane-1,2-diimine, N–N ² = N1,N2-bis(2,4-dimethylphenyl)­ethane-1,2-diimine, N–N ³ = N1,N2-bis(2,4,6-trimethylphenyl)­ethane-1,2-diimine, N–N ⁴ = N1,N2-bis[2,6-bis­(propan-2-yl)­phenyl]­ethane-1,2-diimine, N–N ⁵ = N1,N2-bis(4-fluorophenyl)­ethane-1,2-diimine, N–N ⁶ = N1,N2-bis(4-chlorophenyl)­ethane-1,2-diimine and N–N ⁷ = N1,N2-dicyclohexylethane-1,2-diimine). Ligands and complexes were fully characterized by elemental analysis, NMR, FTIR, UV–vis spectroscopy, and single-crystal X-ray diffraction for 2, 5, and 6.CH ₂ Cl ₂ . The complexes displayed a distorted pseudo-octahedral “piano-stool” geometry, with the α-diimine ligands coordinating in a bidentate manner. The p-cymene ring was observed to rotate around its bond to the metal, as evidenced by variable-temperature ¹H NMR spectra and NOE measurements. DFT calculations were used to investigate the electronic structures of complexes 1–4, revealing how different substituents affect their stability and HOMO–LUMO energy gaps. Additionally, the most nucleophilic and electrophilic regions in the optimized structures were identified using the Hirshfeld charge method applied to the Fukui function. All complexes were evaluated as precatalysts in the solvent-free dehydrogenation of formic acid, in the presence of a Bro̷nsted–Lowry base, achieving up to 94.8% conversion in a first run and a maximum turnover frequency (TOF) of 627 h^–1 under mild conditions (60 °C, 1:1204:843 molar ratio of Ru/FA/base). Total conversion and improvement in TOF values were observed in a subsequent run. A detailed mechanistic study combining kinetic data and DFT modeling supports a chloride displacement–initiated cycle involving a β-hydride elimination pathway for H₂ and CO₂ release. This methodology is consistent with the observed induction period and activation parameters (ΔG ^‡ = 24.5 kcal mol^–1; ΔS ^‡ = +137 cal mol^–1 K^–1), and ΔH ^‡ = 70.3 kcal mol^–1, which are in excellent agreement with E _a = 70.9 kcal mol^–1). The catalytic activity was strongly influenced by both the electronic nature and steric hindrance of the α-diimine ligands, as well as the Bro̷nsted–Lowry character of the base.

Co­(II) complexes, despite their potential as cost-effective alternatives to noble-metal systems, remain underexplored as photocatalysts (PCs) in free radical photopolymerization (FRP). In this study, a series of Co­(II) complexes bearing symmetrical Schiff bases (Co–Ph, Co–EtO, Co–Cl, Co–Me, and Co–t Bu) was synthesized and characterized by FTIR, UV–vis, \ fluorescence spectroscopy, MALDI-TOF mass spectrometry, cyclic voltammetry, and advanced density functional theory (DFT/TD-DFT) calculations. Their photocatalytic performance was evaluated in three-component photoinitiating systems with ethyl 4-(dimethylamino)­benzoate (EDB) and diphenyliodonium hexafluorophosphate (Iod) for the FRP of trimethylolpropane ethoxylate triacrylate (TMPETA) under UV, violet, and blue LED irradiation. Co­(II) complexes enabled efficient polymerization under optimized conditions, reaching high conversions without an inhibition period under UV irradiation. Co–EtO demonstrated a superior photocatalytic efficiency across all tested wavelengths relative to that of the other Co­(II) complexes evaluated in this study. This enhanced performance is attributed to a synergistic combination of its unique structural, electronic, and electrochemical properties. The proposed mechanism was supported by photolysis experiments, literature data, and free energy calculations, indicating the involvement of both oxidative and reductive pathways.

Heterobimetallic complexes offer greater versatility for applications in homogeneous catalysis than their monometallic counterparts. To take advantage of this synergy, three [Pd( N,N,S ‐TSC)(4‐NH ₂ Py)] complexes ( mono‐Pd ^Me , mono‐Pd ^Me,Me and mono‐Pd ^Ph ) were synthesized from [PdCl ₂ (MeCN) ₂ ] and appropriate thiosemicarbazide (TSC) ligand precursors, followed by the addition of 4‐aminopyridine (4‐NH ₂ Py). The reaction of these mono‐Pd ^L complexes with [RuCl ₂ (η ⁶ ‐ p ‐cymene)] ₂ then afforded three heterobimetallic complexes with the generic formula [{RuCl ₂ ( p ‐cymene)}‐μ‐{(4‐NH ₂ Py)Pd (TSC)}] ( Ru‐Pd ^Me , Ru‐Pd ^Me,Me and Ru‐Pd ^Ph ). All these novel compounds were characterized by elemental analysis, spectroscopic techniques, cyclic voltammetry, and density functional theory (DFT) calculations. The molecular structure of mono‐Pd ^Me was determined by X‐ray diffraction analysis. The Ru‐Pd ^L complexes were evaluated as bifunctional catalysts in the reductive Heck coupling of norbornadiene (NBD) with iodobenzene and the ring‐opening metathesis polymerization (ROMP) of phenylnorbornene (Ph‐NBE). They were very active for the reductive Heck reaction, reaching yields higher than 90% when a [Pd]/[NBD]/[PhI] ratio of 1/418/1344 was employed at 120°C. Upon activation with ethyl diazoacetate (EDA), they were also able to polymerize Ph‐NBE with yields around 70% using a [Ph‐NBE]/[Ru‐Pd ^L ] ratio of 3000 at 50°C for 30 min. The Ru‐Pd ^Me,Me complex showed the highest activity in the ROMP of Ph‐NBE and was successfully applied for mediating the synthesis of poly (Ph‐NBE) from NBD via tandem catalysis.

Photocatalysts, particularly metal‐based complexes, have gained significant attention in photoredox catalysis, enabling controlled radical photopolymerization (CRP2) under mild irradiation conditions, such as sunlight or LED bulbs. Herein, three Fe(II) complexes—Fe^IISalen, Fe^IISaloex, and Fe^IISalophen—bearing nonsymmetrical Schiff base ligands are synthesized and designed for potential application in CRP2. The complexes were characterized using fourier transform infrared spectroscopy (FTIR), UV‐Vis, and fluorescence spectroscopy; matrix‐assisted laser desortion ionization time‐of‐flight (MALDI‐TOF) mass spectrometry; and elemental analysis. Additionally, density functional theory and time‐dependent density functional theory calculations are conducted to extract key structural, electronic, and excited‐state properties. The catalytic performance of these Fe(II) complexes in the CRP2 of methyl acrylate (MA) is evaluated under different conditions, using ethyl α‐bromophenylacetate as the initiator and triethylamine as the electron donor. The photopolymerizations proceed efficiently, yielding polymers with controlled molecular weights and narrow molecular weight distributions. Among the tested photocatalysts, Fe^IISaloex exhibits the best performance, achieving high conversion and low polydispersity under LED irradiation. Furthermore, a mechanism is proposed based on spectroscopic analyses and literature data.