Preparation,characterization and electrochemistry of an iron-only hydrogenase active site model covalently linked to a ruthenium tris(bipyridine) photosensitizer

An NH₂-functionlized [Fe₂S₂] model complex of the iron-only hydrogenase active site was covalently linked to the tris(bipyridine)ruthenium photosensitizer. The [RuFeFe] trinuclear complex 1 was characterized by MS, IR, UV-vis, ¹H & ¹³C NMR spectra. A quasi-reversible reduction peak at ?1.41 V versus Ag/Ag⁺ for the Fe^IFe^I/Fe^IFe⁰ process is observed in the cyclic voltammogram of 1.     

Bio-inspired, side-on attachment of a ruthenium photosensitizer to an iron hydrogenase active site model

The first ruthenium-diiron complex [(mu-pdt)Fe2(CO)5{PPh2(C6H4CCbpy)}Ru(bpy)2]2+ 1 (pdt = propyldithiolate, bpy = 2,2'-bipyridine) is described in which the photoactive ruthenium trisbipyridyl unit is linked to a model of the iron hydrogenase active site by a ligand directly attached to one of the iron centers. Electrochemical and photophysical studies show that the light-induced MLCT excited state of the title complex is localized towards the potential diiron acceptor unit. However, the relatively mild potential required for the reduction of the acetylenic bipyridine together with the easily oxidized diiron portion leads to a reductive quenching of the excited state, instead. This process results in a transiently oxidized diiron unit which may explain the surprisingly high light sensitivity of complex 1.     

A photosensitizer dinuclear ruthenium complex: intramolecular energy transfer to a covalently linked fullerene acceptor

A fullerene derivative (5) in which a dinuclear ruthenium complex is covalently linked to a fulleropyrrolidine (FP) through a rigid spacer has been prepared through azomethine ylide cycloaddition to C60. Electrochemical and photophysical studies revealed that ground-state electronic interactions between the bimetallic ruthenium chromophore and the FP moiety are small. The absorption spectrum of 5 displays a metal-to-ligand charge transfer (MLCT) transition at about 620 nm in CH2Cl2 which is shifted by nearly 160 nm relative to that of a previously reported mononuclear dyad (8). The photophysical investigations have also shown that both in dichloromethane and acetonitrile the photoexcited MLCT state of dyad 5 transforms into the fullerene triplet excited state with a quantum yield of 0.19 and that, contrary to mononuclear dyad 8, electron transfer, if any under the applied conditions, is negligible relative to energy transfer.     

Elaboration of covalently linked polyoxometalates with ruthenium and pyrene chromophores and characteriation of their photophysical properties

Keggin and Dawson-type polyoxometalates (POMs) decorated by organometallic [cyclometalated ruthenium(II) polypyridine complex] or organic (pyrene) chromophores were prepared by postfunctionalization of hybrid disilylated POM platforms. The connection is made in a very efficient and modular way via Sonogashira coupling reactions, which provide a rigid linkage between the POM and the photoactive centers. Electronic properties have been inferred from electrochemical and photophysical studies and reflect poor electronic interactions between both partners. The presence of the POM leads to luminescence quenching of the chromophores, which was attributed to an intramolecular electron transfer from the chromophore to the POM. The rate of this process is much faster in the POM-pyrene than in the POM-Ru system. It depends on the driving force dictated by the redox potentials of both partners but also in the case of the POM-Ru system on the presence of the metallacycle, which acts as a molecular insulator and delays the intramolecular electron transfer. In the POM-Ru system, a comparative study of the luminescence quenching showed that the electron transfer is still more important in the covalently bonded hybrids than in systems where the POM and the ruthenium complexes are assembled via electrostatic interactions.     

Dinuclear iron isonitrile complexes: models for the iron hydrogenase active site

Reaction of Fe(2)(mu-S(2)C(3)H(6))(CO)(6) (1) with 2 equiv of t-BuNC affords a disubstitued species Fe(2)(mu-S(2)C(3)H(6))(CN-t-Bu)(2)(CO)(4) (2). The structure of 2 has been determined by X-ray crystallography, which shows that in the solid state both isonitrile ligands are cis to sulfur. In solution, NMR and IR spectroscopy suggest that multiple isomers are present. Protonation of 2 occurs at the Fe-Fe bond to give a cationic complex 3 as four different isomeric species. Complex 3 does not react with deuterium gas (98 psi) in the absence of light. Irradiation of solutions of 3 with visible light under D(2) gas leads to formation of HD.     

Synthesis and photophysical properties of novel organic–inorganic hybrid materials covalently linked to a europium complex

A novel sol–gel derived hybrid material (classed as Eu-DBM-Si) covalently grafted with Eu(DBM-OH)₃·2H₂O (where DBM-OH = o-hydroxydibenzoylmethane) was prepared through the primary β-diketone ligand DBM-OH. All the synthesized ligands were characterized by ¹H NMR, elemental analyses and Fourier transform infrared spectra (FTIR). The resultant Eu-DBM-Si material exhibited good transparent and homogenous property. Compared to the Eu-DBM hybrid prepared by physically doped silicon dioxide with Eu(DBM-OH)₃·2H₂O, the Eu-DBM-Si hybrid presented more efficient ligand-to-Eu³⁺ energy transfer and a significant improvement in the measured emission quantum yield. Furthermore, the photophysical properties of these hybrid materials, such as the photoluminescence (PL) spectra, PL intensities, symmetry properties, lifetime decays, and Judd-Ofelt parameters were also investigated in detail.     

Dynamic ligation at the first amine-coordinated iron hydrogenase active site mimic

The first model of the iron hydrogenase active site has been prepared in which an amine ligand is loosely coordinated to an Fe(i) centre, and can be replaced by a solvent molecule; irrespective of the ligand set, the one electron reduction of both complexes is chemically reversible and is shown to proceed through the same species which features a bridging CO ligand.     

单铁氢化酶活性中心的仿生模拟研究进展

单铁氢化酶的活性中心能在自然环境条件下催化异裂氢分子并且选择性还原特定底物。自从20世纪90年代,其第一次被分离出来后,科学家一直在努力模拟单铁氢化酶活性中心的结构及功能,期望通过仿生手段,实现类似自然界温和利用氢气的功能,来解决当今氢能在使用中贵金属催化剂等问题。仿生单铁氢化酶活性中心模型化合物被不断合成研究,促进了对酶活性中心几何结构和电子特性的认知。红外光谱特征、催化禁阻特性、质子化特性、密度泛函分析(DFT)以及催化机理探索等为未来研究提供了理论基础。本篇综述主要总结了近些年单铁氢化酶的分离表征、晶体结构、活性中心的仿生模拟、催化机理探索方面研究进展。     

Ligand versus metal protonation of an iron hydrogenase active site mimic

The protonation behavior of the iron hydrogenase active-site mimic [Fe2(mu-adt)(CO)4(PMe3)2] (1; adt=N-benzyl-azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron-donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe2(mu-Hadt)(CO)4(PMe3)2]+ ([1 H]+), the Fe-Fe bond to give [Fe2(mu-adt)(mu-H)(CO)4(PMe3)2]+ ([1 Hy]+), or both sites simultaneously to give [Fe2(mu-Hadt)(mu-H)(CO)4(PMe3)2]2+ ([1 HHy]2 +). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, (1)H, and (31)P NMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of 1 in the solid state. Density functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1 H]+ (pKa=12) is the initial, metastable protonation product while the hydride [1 Hy]+ (pKa=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1 H]+ to [1 Hy]+ does not occur spontaneously. However, it can be catalyzed by HCl (k=2.2 m(-1) s(-1)), which results in the selective formation of cation [1 Hy]+. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pK(a)=8) and the ammonium proton (pK(a)=5) of the doubly protonated cationic complex [1 HHy]2+ are considerably more acidic than in the singly protonated analogues. The formation of dication [1 HHy]2+ from cation [1 H]+ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k=0.15 m(-1) s(-1)), while the dication is formed substantially faster (k>10(2) m(-1) s(-1)) with hydrobromic acid. Electrochemically, 1 undergoes irreversible reduction at -2.2 V versus ferrocene, and this potential shifts to -1.6, -1.1, and -1.0 V for the cationic complexes [1 H]+, [1 Hy]+, and [1 HHy]2+, respectively, upon protonation. The doubly protonated form [1 HHy]2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.     

Fullerene polypyridine ligands: synthesis, ruthenium complexes, and electrochemical and photophysical properties

Fullerene coordination ligands bearing one bipyridine or terpyridine unit were synthesized, and their coordination to ruthenium(II) formed linear rod-like donor-acceptor systems. Steady-state fluorescence of [Ru(bpy)(2)(bpy-C(60))](2+) showed a rapid solvent-dependent, intramolecular quenching of the ruthenium(II) MLCT excited state. Time-resolved flash photolysis in CH(3)CN revealed characteristic transient absorption changes that have been ascribed to the formation of the C(60) triplet state, suggesting that photoexcitation of [Ru(bpy)(2)(bpy-C(60))](2+) results in a rapid intramolecular transduction of triplet excited state energy. The electrochemical studies on both [Ru(bpy)(2)(bpy-C(60))](2+) and [Ru(tpy)(tpy-C(60))](2+) indicated electronic coupling between the metal center and the fullerene core.     

Iron hydrogenase active site mimic holding a proton and a hydride

The first model of the iron hydrogenase active site has been prepared which concomitantly carries a proton and a hydride; the title species was characterized by IR and NMR spectroscopy and is reduced at more positive potential than any other mimic of this kind.     

Diiron chelate complexes relevant to the active site of the iron-only hydrogenase

A novel unsymmetrically disubstituted propanedithiolate compound [Fe₂(CO)₄(κ²-dmpe)(μ-pdt)] (1) (pdt = SCH₂CH₂CH₂S, dmpe = Me₂PCH₂CH₂PMe₂) was synthesized by treatment of [Fe₂(CO)₆(μ-pdt)] with dmpe in refluxing THF. Compound 1 was characterized by single-crystal X-ray diffraction analysis. Protonation of 1 with HBF₄·Et₂O in CH₂Cl₂ gave at room temperature the μ-hydrido derivative [Fe₂(CO)₄(κ²-dmpe)(μ-pdt)(μ-H)](BF₄)] (2). At low temperature, ¹H and ³¹P–{¹H} NMR monitoring revealed the formation of a terminal hydride intermediate 3. Comparison of these results with those of a VT NMR study of the protonation of symmetrical compounds [Fe₂(CO)₄L₂(μ-pdt)] [L = PMe₃, P(OMe)₃] suggests that in disubstituted bimetallic complexes [Fe₂(CO)₄L₂(μ-pdt)], dissymmetry of the complex is required to observe terminal hydride species. Attempts to extend the series of chelate compounds [Fe₂(CO)₄(κ²-L₂)(μ-pdt)] by using arphos (arphos = Ph₂AsCH₂CH₂PPh₂) were unsuccessful. Only mono- and disubstituted derivatives [Fe₂(CO)_6−n(Ph₂AsCH₂CH₂PPh₂)_n(μ-pdt)] (n = 1, 4a; n = 2, 4b), featuring dangling arphos, were isolated under the same reaction conditions of formation of 1. Compound 4b was structurally characterized.     

Synthesis and properties of covalently linked thiaporphyrin-ferrocene conjugates

Four examples of ferrocene-thiaporphyrin conjugates in which the ferrocenyl group was covalently connected either directly at meso-position of thiaporphyrin or to meso-phenyl group of thiaporphyrin via ethyne bridge were prepared by coupling bromo- or iodo thiaporphyrin with α-ethynylferrocene under mild Pd(0) coupling conditions. NMR, absorption and electrochemical studies indicated that the thiaporphyrin and ferrocenyl units interact strongly in ethyne bridged porphyrin-ferrocene conjugates but the interaction is very weak in phenyl ethyne bridged porphyrin-ferrocene conjugates. The steady state fluorescence studies indicated that the fluorescence yields are reduced to 50% in phenyl ethyne conjugates but the fluorescence is completely quenched in ethyne bridged conjugates. The partial or complete quenching of porphyrin fluorescence in these conjugates is due to electron transfer from ferrocene unit to excited state of porphyrin sub-unit. Oxidation of ferrocene to ferrocenium ion with an oxidizing agent in ethyne bridged conjugates resulted in a recovery of porphyrin fluorescence.     

Carboxy-functionalized dithiolate di-iron complexes related to the active site of Fe-only hydrogenase

Reactions of the di-iron complex [Fe₂(μ-S)₂(CO)₆]²⁻ with carboxy-functionalized dihalide derivatives (XCH₂)₂R (X = Cl, R = NC₆H₄CH₂CO₂CH₃; X = Br, R = C₆H₃COOH, C₆H₃COON(COCH₂)₂) gave new functionalized dithiolate di-iron complexes [Fe₂(μ-SRS)(CO)₆] (R = (CH₂)₂NC₆H₄CH₂CO₂CH₃ (1), (CH₂)₂C₆H₃COOH (2), (CH₂)₂C₆H₃COON(COCH₂)₂ (3)) in low yields. The azadithiolate complex 1 has been characterized by a single crystal X-ray diffraction analysis and studied by electrochemical methods.     

Dihydrogen activation by a diruthenium analogue of the Fe-only hydrogenase active site

The photochemical reaction of Ru2(S2C3H6)(CO)4(PCy3)2 (1) and H2 gives the dihydride Ru2(S2C3H6)(mu-H)(H)(CO)3(PCy3)2 (2). NMR and crystallographic studies reveal mutually trans basal phosphine ligands and both bridging and terminal hydrides. Ru2(S2C2H4)(CO)4(PCy3)2 behaves similarly. Other HX substrates undergo photoaddition to 1, affording Ru2(S2C3H6)(mu-H)(X)(CO)3(PCy3)2 for X = OTs (3a), Cl (3b), and SPh (3c). Treatment of Ru2(S2C3H6)(mu-H)(H)(CO)3(PCy3)2 with [H(OEt2)]BArF4 (ArF = B(C6H3-3,5-(CF3)2) in CD2Cl2 gives [Ru2(S2C3H6)(mu-H)(CO)3(PCy3)2(H2)]+ (4), which catalyzes H2-D2 exchange. The reaction of 2 with [D(OEt2)]BArF4 gave [Ru2(S2C3H6)(mu-H)(CO)3(PCy3)2(HD)]+ (JH-D = 31 Hz). These studies provide the first models for the Fe-only hydrogenases that bear dihydrogen and terminal hydrido ligands.     

Preparation and photophysical and photoelectrochemical properties of a covalently fixed porphyrin-chemically converted graphene composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl-functionalized CCG and porphyrin boronic ester. The covalently linked CCG-porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady-state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin-linked CCG revealed a short-lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO(2) electrode coated with the porphyrin-linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π-conjugated systems in the excited and ground states.     

An azadithiolate bridged Fe2S2 complex as active site model of FeFe-hydrogenase covalently linked to a Re(CO)3(bpy)(py) photosensitizer aiming for light-driven hydrogen production

In order to create photoactive catalysts for hydrogen production, a novel trimetallic Re–Fe₂S₂ complex 4 was synthesized by the coordination of the free –PPh₂ group of the ligand of the rhenium photosensitizer 6 to an azadithiolate (ADT)-bridged diiron complex 8 with the assistance of the decarbonylation reagent Me₃NO. Complex 4 was characterized by ¹H, ¹³C, ³¹P NMR and HRMS spectra. The IR, UV–vis and electrochemical data indicate some interactions between Re and Fe₂S₂ moieties, and the photo-induced electron transfer from the excited state of the Re moiety to the Fe₂S₂ catalyst is thermodynamically feasible.