Oxidation-state sensitive light-induced dynamics of Ruthenium-4H-Imidazole complexes

Oxidized molecular states are key intermediates in photo-induced redox reactions, e. g., intermolecular charge transfer between photosensitizer and catalyst in photoredox catalysis. The stability and longevity of the oxidized photosensitizer is an important factor in optimizing the respective light-driven reaction pathways. In this work the oxidized states of ruthenium(II)-4H-imidazole dyes are studied. The ruthenium complexes constitute benchmark photosensitizers in solar energy interconversion processes with exceptional chemical stability, strong visible light absorption, and favourable redox properties. To rationalize the light-induced reaction in the oxidized ruthenium(III) systems, we combine UV-vis absorption, resonance Raman, and transient absorption spectroelectrochemistry (SEC) with time-dependent density functional theory (TDDFT) calculations. Three complexes are compared, which vary with respect to their coordination environment, i. e., combining an 4H-imidazole with either 2,2'-bipyridine (bpy) or 2,2';6'2"-terpyridine (tpy) coligands, and chloride or isothiocyanate ligands. While all oxidized complexes have similar steady state absorption properties, their excited state kinetics differ significantly; the study thus opens the doorway to study the light-driven reactivity of oxidized molecular intermediates in intermolecular charge transfer cascades.

Diiodo-BODIPY Sensitizing of the [Mo3S13]2- Cluster for Noble-Metal-Free Visible-Light-Driven Hydrogen Evolution within a Polyampholytic Matrix

We report on a photocatalytic setup that utilizes the organic photosensitizer (PS) diiodo-BODIPY and the non-precious-metal-based hydrogen evolution reaction (HER) catalyst (NH₄)₂[Mo₃S₁₃] together with a polyampholytic unimolecular matrix poly(dehydroalanine)-graft-poly(ethylene glycol) (PDha-g-PEG) in aqueous media. The system shows exceptionally high performance with turnover numbers (TON > 7300) and turnover frequencies (TOF > 450 h^-1) that are typical for noble-metal-containing systems. Excited-state absorption spectra reveal the formation of a long-lived triplet state of the PS in both aqueous and organic media. The system is a blueprint for developing noble-metal-free HER in water. Component optimization, e.g., by modification of the meso substituent of the PS and the composition of the HER catalyst, is further possible.

Novel [FeFe]-Hydrogenase Mimics: Unexpected Course of the Reaction of Ferrocenyl α-Thienyl Thioketone with Fe3(CO)12

The influence of the substitution pattern in ferrocenyl α-thienyl thioketone used as a proligand in complexation reactions with Fe₃(CO)₁₂ was investigated. As a result, two new sulfur–iron complexes, considered [FeFe]-hydrogenase mimics, were obtained and characterized by spectroscopic techniques (¹H, ¹³C{¹H} NMR, IR, MS), as well as by elemental analysis and X-ray single crystal diffraction methods. The electrochemical properties of both complexes were studied and compared using cyclic voltammetry in the absence and in presence of acetic acid as a proton source. The performed measurements demonstrated that both complexes can catalyze the reduction of protons to molecular hydrogen H₂. Moreover, the obtained results showed that the presence of the ferrocene moiety at the backbone of the linker of both complexes improved the stability of the reduced species.

Di-, tri- and tetraphosphine-substituted Fe/Se carbonyls: Synthesis, Characterization and electrochemical properties

The active sites of [FeFe]-hydrogenase promoted by Fe/E (E=S, Se) clusters have attracted considerable interest due to their significance for understanding the interconversion of hydrogen with protons and electrons. As...

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C–H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe–C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H₂ activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe–carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H₂/hydride-induced loss of the organometallic acyl unit (i.e. pyCH₂–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O → pyCH₃+C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O). The known reduced hydride species [HFe(CO)₄]⁻ and [HFe₃(CO)₁₁]⁻ have been observed as products by ¹H/²H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)₄]. The former species (i.e. [HFe(CO)₄]⁻) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([^TolIm](BAr^F) = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor.

Synthesis of Novel Heteroleptic Oxothiolate Ni(II) Complexes and Evaluation of Their Catalytic Activity for Hydrogen Evolution

Two heteroleptic nickel oxothiolate complexes, namely [Ni(bpy)(mp)] (1) and [Ni(dmbpy)(mp)] (2), where mp = 2-hydroxythiophenol, bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine were synthesized and characterized with various physical and spectroscopic methods. Complex 2 was further characterized by single crystal X-ray diffraction data. The complex crystallizes in the monoclinic P 21/c system and in its neutral form. The catalytic properties of both complexes for proton reduction were evaluated with photochemical and electrochemical studies. Two different in their nature photosensitizers, namely fluorescein and CdTe-TGA-coated quantum dots, were tested under various conditions. The role of the electron donating character of the methyl substituents was revealed in the light of the studies. Thus, catalyst 2 performs better than 1, reaching 39.1 TONs vs. 4.63 TONs in 3 h, respectively, in electrochemical experiments. In contrast, complex 1 is more photocatalytically active than 2, achieving a TON of over 6700 in 120 h of irradiation. This observed reverse catalytic activity suggests that HER mechanism follows different pathways in electrocatalysis and photocatalysis.

Ligand effects on structural, protophilic and reductive features of stannylated dinuclear iron dithiolato complexes

The synthesis and characterization of Fe₂(CO)₅(L){μ-(SCH₂)₂SnMe₂} (L = PPh₃ (2) and P(OMe)₃ (3)) derived from the parent hexacarbonyl complex Fe₂(CO)₆{μ-(SCH₂)₂}SnMe₂ (1) are reported.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Phosphine-substituted Fe–Te clusters related to the active site of [FeFe]-H2ases

Mono-, di-, and tetranuclear phosphine-substituted Fe/Te clusters 1–6 were described.

Electrochemical and Computational Insights into the Reduction of [Fe2(CO)6{µ-(SCH2)2GeMe2}] Hydrogenase H-Cluster Mimic

The electrochemical reduction of the complex [Fe2(CO)6{µ-(SCH2)2GeMe2}] (1) under N2 and CO is reported applying cyclic voltammetry. Reduction of complex 1 in CO saturated solutions prevents the possible release of CO from the dianion 12−, while the latter reacts with additional CO forming a spectroscopically uncharacterized product P1. This product undergoes a reversible redox process at E1/2 = −0.70 V (0.2 V∙s−1). In this report, the structure of the neutral complex 1, isomers of dianionic form of 1, and P1 are described applying DFT computations. Furthermore, we propose reaction pathways for H2 production on the basis of the cyclic voltammetry of complex 1 in presence of the strong acid CF3SO3H.

Electrocatalytic hydrogen production using [FeFe]-hydrogenase mimics based on tetracene derivatives

The synthesis, structure, physical properties, and electrocatalytic hydrogen production of tetrathiatetracene ligand based [FeFe]-hydrogenase mimic molecules were investigated.

Sulphur–sulphur, sulphur–selenium, selenium–selenium and selenium–carbon bond activation using Fe3(CO)12: an unexpected formation of an Fe2(CO)6 complex containing a μ2,κ3-C,O,Se-ligand

Four diiron hexacarbonyl-complexes containing dithiolato (5), diselenolato (6), selenolato-thiolato (7) and μ²,κ³-C,O,Se-ligands (8), respectively have been prepared as [FeFe]-hydrogenase mimics.

Hydrogel-Embedded Model Photocatalytic System Investigated by Raman and IR Spectroscopy Assisted by Density Functional Theory Calculations and Two-Dimensional Correlation Analysis

The presented study reports the synthesis and the vibrational spectroscopic characterization of different matrix-embedded model photocatalysts. The goal of the study is to investigate the interaction of a polymer matrix with photosensitizing dyes and metal complexes for potential future photocatalytic applications. The synthesis focuses on a new rhodamine B derivate and a Pt(II) terpyridine complex, which both contain a polymerizable methacrylate moiety and an acid labile acylhydrazone group. The methacrylate moieties are afterward utilized to synthesize functional model hydrogels mainly consisting of poly(ethylene glycol) methacrylate units. The pH-dependent and temperature-dependent behavior of the hydrogels is investigated by means of Raman and IR spectroscopy assisted by density functional theory calculations and two-dimensional correlation spectroscopy. The spectroscopic results reveal that the Pt(II) terpyridine complex can be released from the polymer matrix by cleaving the C═N bond in an acid environment. The same behavior could not be observed in the case of the rhodamine B dye although it features a comparable C═N bond. The temperature-dependent study shows that the water evaporation has a significant influence neither on the molecular structure of the hydrogel nor on the model photocatalytic moieties.

[FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2-8): A Systematic Study

The effect of the nature of the dithiolato ligand on the physical and electrochemical properties of synthetic H-cluster mimics of the [FeFe]-hydrogenase is still of significant concern. In this report we describe the cyclization of various alkanedithiols to afford cyclic disulfide, tetrasulfide, and hexasulfide compounds. The latter compounds were used as proligands for the synthesis of a series of [FeFe]-hydrogenase H-cluster mimics having the general formulas [Fe₂(CO)₆{μ-S(CH₂)_nS}] (n = 4-8), [Fe₂(CO)₆{μ-S(CH₂)_nS}]₂ (n = 6-8), and [Fe₂(CO)₆{(μ-S(CH₂)_nS)₂}] (n = 6-8). The resulting complexes were characterized by ¹H and ¹³C{¹H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis. The purpose of this research was to study the influence of the systematic increase of n from 2 to 7 on the redox potentials of the models and the catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry.