Three different complexes of the (dppbe)Pt(η²-tolane) [dppbe = 1,2-bis(diphenylphosphino)benzene] type bearing CF₃ groups at the 2,2′- (4a), 3,3′- (4b), and 4,4′-positions (4c) of the tolane ligand were synthesized. A one-pot procedure for the synthesis of the tolane ligands from 2-methyl-3-butyn-2-ol as the acetylene derivative was developed. To investigate their selective photochemical carbon–carbon bond cleavage, 4a–4c were exposed to UV light (1 W, 356 nm) in the solid state and in solution. The oxidative addition of the C(sp)–C(sp²) bond to the (dppbe)Pt⁰ complex fragment to form the corresponding CF₃-substituted (dppbe)Pt(ethynylphenyl)(phenyl) complex was followed by ³¹P{¹H} NMR spectroscopy. The reactivity of 4a–4c relative to the position of the CF₃ substituent and the torsion angle between the substituted phenyl rings is discussed. All of the complexes were isolated and characterized by various spectroscopic methods and additionally by single-crystal X-ray diffractometry.

To learn from Nature how to create an efficient hydrogen-producing catalyst, much attention has been paid to the investigation of structural and functional biomimics of the active site of [FeFe]-hydrogenase. To understand their catalytic activities, the μ-S atoms of the dithiolate bridge have been considered as possible basic sites during the catalytic processes. For this reason, a series of [FeFe]-H₂ase mimics have been synthesized and characterized. Different [FeFe]-hydrogenase model complexes containing bulky Si–heteroaromatic systems or fluorene directly attached to the dithiolate moiety as well as their mono-PPh₃-substituted derivatives have been prepared and investigated in detail by spectroscopic, electrochemical, X-ray diffraction, and computational methods. The assembly of the herein reported series of complexes shows that the μ-S atoms can be a favored basic site in the catalytic process. Small changes in the (hetero)-aromatic system of the dithiolate moiety are responsible for large differences in their structures. This was elucidated in detail by DFT calculations, which were consistent with the experimental results.

The utilization of light and inexpensive catalysts to afford hydrogen represents a huge challenge. Following our interest in silicon-containing [FeFe]-hydrogenase ([FeFe]-H₂ase) mimics, we report a new model approach for a photocatalytic [FeFe]-H₂ase mimic 1, which contains a 1-silafluorene unit as a photosensitizer. Thereby, the photoactive ligand is linked to the [2Fe2S] cluster through S–CH₂–Si bridges. Photochemical H₂ evolution experiments were performed and revealed a turnover number (TON) of 29. This is the highest reported photocatalytic efficiency for an [FeFe]-H₂ase model complex in which the photosensitizer is covalently linked to the catalytic center.

Within this study, we report on the first controlled radical polymerization of styrene-based models of the active site of the [FeFe]-hydrogenase. Three different model complexes based on styrene were prepared including propanedithiolato-bridged, 2-azapropanedithiolato-bridged, and bifunctional styrene iron complex. These model complexes were copolymerized with styrene using free radical and the reversible addition-fragmentation chain transfer polymerization method. The polymerization behavior of the hydrogenase models is discussed and analyzed in detail. It could be shown that the model complex can be incorporated into copolymers. The obtained copolymers exhibit narrow molar mass distributions. The presence of the [FeFe]-hydrogenase models were proven by atomic absorption spectrometry, NMR and IR spectroscopy as well as cyclovoltammetric measurements. It could be shown that the [FeFe]-hydrogenase mimic copolymers, as well as the monomeric originating complexes exhibit electrocatalytic proton reduction at a low potential of –2.2 V. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

The reactions of triiron dodecacarbonyl with thiobenzophenone (2a) and 9H-thioxanthene-9-thione (2b) were investigated under different conditions. In the case of a 1:1 molar ratio of triiron dodecacarbonyl and 2a or 2b, the ortho-metallated complexes [Fe₂(CO)₆{μ,κ,S,SCH(C₆H₅)C₆H₄-η²}] (3a) and [Fe₂(CO)₆{μ,κ,S,SCH(C₆H₄)–S–C₆H₃-η²}] (4a) were obtained as the major products, respectively. In contrast, the treatment of triiron dodecacarbonyl with an excess of 2a or 2b afforded [Fe₂(CO)₆{μ-SCH(C₆H₅)C₆H₄S-μ}] (3b) and [Fe₂(CO)₆{μ-SCH(C₆H₄)–S–C₆H₃S-μ}] (4b), respectively, which are both bioinspired models for the active site of [FeFe]-hydrogenase. In addition to these complexes, the two reactions afforded [Fe₂(CO)₆{μ-SC(C₆H₅)₂S-μ}] (3c) and [Fe₂(CO)₆{μ-SC(C₆H₄–S–C₆H₄)S-μ}] (4c). Furthermore, [{Fe₂(CO)₆{μ-SCH(C₆H₅)₂}}₂(μ⁴-S)] (3d) was isolated from the reaction of Fe₃(CO)₁₂ with 2a. The molecular structures of all of the new complexes were determined from the spectroscopic and analytical data and the crystal structures for 3c, 3d, 4b, and 4c were obtained. A plausible mechanism for the formation of the isolated complexes that involves dithiirane derivatives as the key intermediates is proposed. Herein, thioketones 2a and 2b act as sulfur transfer reagents. The electrochemical experiments showed that complex 3b behaves as a catalyst for the electrochemical reduction of protons from acetic acid.

(Bis-selenolato) and (bis-tellurolato)diiron complexes [2Fe2E(Si)] were prepared and compared with the known (bis-thiolato)diiron complex A to assess their ability to produce hydrogen from protons. Treatment of [Fe₃(CO)₁₂] with 4,4-dimethyl-1,2,4-diselenasilolane (1) in boiling toluene afforded hexacarbonyl{μ-{[1,1′-(dimethylsilylene)bis[methaneselenolato-κSe : κSe]](2 −)}}diiron(FeFe) (2). The analog bis-tellurolato complex hexacarbonyl{μ-{[1,1′-(dimethylsilylene)bis[methanetellurolato-κTe : κTe]](2 −)}}diiron(FeFe) (3) was obtained by treatment of [Fe₃(CO)₁₂] with dimethylbis(tellurocyanatomethyl)dimethylsilane, which was prepared in situ. All compounds were characterized by NMR, IR spectroscopy, mass spectrometry, elemental analysis and single-crystal X-ray analysis. The electrocatalytic properties of the [2Fe2X(Si)] (X=S, Se, Te) model complexes A, 1, and 2 towards hydrogen formation were evaluated.

The organometallic active sites in [NiFe]- and [FeFe]H₂ases are sensitive to oxygen in varying degrees. The microorganisms that utilize these enzymes for their hydrogen metabolism, and the enzymes themselves, have evolved from a reducing to an oxidizing environment in ways to avoid competition with oxygen, primarily by burying the active site machinery deeply within the protein matrix. In the case of [NiFe]H₂ase, biological studies indicate that repair mechanisms exist for reversible O₂-inhibition processes. This Microreview explores the possibility that S-oxygenation may represent reparable O-damaged enzyme active sites. Such S-oxygenation has precedent in chemical models for the terminal thiolate sulfur atoms of the nickel site in [NiFe]H₂ase as well as the bridging thiolate sulfur in the [FeFe]H₂ase active site. A discussion of the processes of O₂ damage leading to both reversible and irreversible enzyme inhibition, and reclamation of activity in the H₂ases, as explored by various biochemical assays and spectroscopic methods, is also presented.

Cyanido- and phosphane-substituted [FeFe] hydrogenase models are known to show increased basicity of the [2Fe2S] cluster. Following our continued interest in [2Fe2S(Si)] compounds, substitution reactions of such complexes with tetraethylammonium cyanide as well as triphenylphosphane were investigated to yield the respective monosubstituted triphenylphosphane complexes 2a–c and cyanido complexes 3a–c. Cyclic voltammetry was used to investigate the electrocatalytic H₂ formation with acetic acid.

A short overview of diiron dichalcogenolato (Se and Te) model complexes related to the chemistry of the diiron subsite of [FeFe] hydrogenase is presented. These model complexes allow direct comparison with the diiron dithiolato compound analogues for their ability to catalyze the formation of H₂ from weak acids. Few detailed photoelectron spectroscopy (PES) investigations and density functional theory (DFT) calculations have been reported until now, and the results will be summarized here. On the basis of preliminary results, we propose targets for the synthesis of more efficient biomimetic diiron dichalcogenolato catalysts.

Dodecacarbonyltriiron reacts with 3,3,5,5-tetraphenyl-1,2,4-trithiolanes (1e) to give the ortho-metalated complex Fe₂(CO)₆[κ,μ-S,η²-(C₁₃H₁₀S)] (9a), complexes of the type (Ph₂C)S₂Fe₂(CO)₆ and the well known trinuclear complex Fe₃S₂ (CO)₉ as by-products. Complex 9a can also be obtained from the reaction of Fe₃(CO)₁₂ with thiobenzophenone (2a). In a similar way, 4,4′-bis(dimethylamino)thiobenzophenone (2b) reacts with Fe₃(CO)₁₂ to give Fe₂(CO)₆[κ,μ-S,η²-(C₁₇H₂₀N₂S)] (9b). The cyclic aromatic thioketones such as dibenzosuberenethione (2c) and xanthione (2d) react with Fe₃(CO)₁₂ to give the cyclometalated products Fe₂(CO)₆[κ,μ-S,η²-(C₁₅H₁₂S)] (9c) and Fe₂(CO)₆[κ,μ-S,η²-(C₁₃H₈OS)] (9d), respectively, and a small amount of Fe₃S₂(CO)₉. Complexes 9a–d have been characterized by IR and NMR spectroscopies, elemental analyses, and X-ray single crystal structure analyses.

In the search for novel [FeFe] hydrogenase model systems several di- and tripeptides containing an 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) moiety or N-phenylsulfonyl-Adt-OMe were treated with Fe₃(CO)₁₂. The resulting [FeFe] complexes were characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry and elemental analysis. All complexes were investigated by cyclic voltammetry, and their ability to catalyze the reduction of protons to dihydrogen, as a function of acid concentration, was studied in different solvents.

Reactions of 1,2,4-trithiolanes with Pt⁰ complexes often proceed via an oxidative addition of the Pt⁰ complex fragment into the S–S bond and subsequent extrusion of a thioketone molecule by ring contraction. The six-membered intermediate 5a, formed in the course of the reaction of the parent 1,2,4-trithiolane (1a) and Pt⁰ complex 4, was detected bymeans of low-temperature NMR spectroscopy. Stable derivatives of this type (compounds 9, 11 and 12) were isolated either by using (dppe)Pt⁰ complex 8 or 1,2,4-trithiolane 4-S-oxide (10). The molecular structures of platinum complexes 9, 11 and 12, as well as their unexpected stability, are discussed.

In search of an applicable parameter to express the steric requirement of bidentate ligands, the generalised equivalent cone angle Θ–_b is established. It is designed in the style of the solid-angle concept for monodentate ligands, utilising crystal structure data. Its reliability is tested with bidentate phosphanes coordinated to a central platinum atom. Therefore, a detailed evaluation of the Cambridge Database for platinum complexes with bridging phosphanes is performed. Herein, we present the results of the analysis of more than 900 molecular structures and over 280 different bidentate phosphanes. We intend to establish diselenolatoplatinum(II) complexes as model compounds for the fast prediction of Θ–_b instead of calculating a multiplicity of molecular structures that is often not available. The complexes reported in Part 1 of this publication turn out to be rather acceptable for rough estimates of the generalised equivalent cone angle. On the other hand, the compounds are used to demonstrate the practicability of the concept, which is easily extended to other bidentate ligands, as is shown for the 2,2-bis(hydroxymethyl)-1,3-diselenolato moiety. In conclusion, the generalised equivalent cone angle Θ–_b proves to be a feasible parameter for the steric demand of bidentate ligands.

An improved synthesis of 4,4-bis(hydroxymethyl)-1,2-diselenolane and the complexation properties of the corresponding diselenolato dianion to group-10 metals are reported. We describe an efficient and straightforward procedure that bypasses the isolation of the malodorous and air-sensitive diselenol and starts with the diselenide and an appropriate group-10 metal complex bearing phosphane and chlorido ligands. A series of complexes with various mono- and bidentate phosphanes is prepared and characterised by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, the structure of most complexes is studied by single-crystal X-ray diffraction to establish their supramolecular arrangement in the solid state. Consequently, several group-10 metal complexes with P–M–P angles (bite angles) in the range from 71–108° are investigated. The use of the sterically demanding bridging phosphane 4,5-bis(diphenylphosphanyl)-9,9-dimethylxanthene, which exhibits a large bite angle yields a mixture of a di- and trinuclear complex. While the platinum-containing complexes are proven to be rather stable, the palladium and nickel analogues tend to decompose. Especially, the nickel complexes were found to be sensitive against oxidation. This circumstance leads to the formation of the so far unknown 1,8-bis(diphenylphosphanyl)naphthalene monooxide, the formation and structure of which could be confirmed from NMR spectroscopic data and single-crystal X-ray diffraction.

A series of Pt⁰(η²-nbe) complexes [nbe = norbornene (bicyclo[2.2.1]hept-2-ene)] bearing bridged bisphosphane ligands with various bite angles (5a–c) was treated with 3,3,5,5-tetraphenyl-1,2,4-trithiolane (1) at 50 °C in a toluene solution. These reactions resulted in the formation of the appropriate dithiolato complexes 6a–c as well as the η²-thioketone complexes 7a–c with respect to the ³¹P{¹H} NMR spectroscopic data. All isolated complexes were fully characterized. Kinetic investigations using ³¹P{¹H} NMR spectroscopy revealed first-order kinetics in 1, pointing to the known cycloreversion of 1 as the initial and rate-determining step for these reactions. The formation of dithiolato complexes 6a–c suggests 1,3-dipolar electrocyclization of diphenylthiosulfine (3) to diphenyldithiirane (4) in solution. On the basis of these results a plausible mechanism for the overall reaction is developed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture shows the mechanism of the treatment of (bisphosphane)platinum(0) complex fragments bearing various bridged bisphosphane ligands with the 3,3,5,5-tetraphenyl-1,2,4-trithiolane. As a result of these reactions dithiolato complexes and the appropriate η²-thioketone complexes could be isolated. This, as well as the determined first-order reaction kinetics in the trithiolane, approves the depicted mechanism starting with its decomposition into thioketone and thiosulfine, which is in equilibrium with the corresponding tautomeric dithiirane. The background shows sublimed sulfur (photo by Thomas Weisheit) referring to the investigated sulfur-rich heterocycle. Details are discussed in the article by W. Weigand et al. on p. 3545 ff.