Facile Synthesis and Functionality-Dependent Electrochemistry of Fe-Only Hydrogenase Mimics

A series of azadithiolate (adt)-bridged Fe-only hydrogenase model complexes, Fe2(CO)6(μ-adt)C6H4I-4 (1), Fe2(CO)6(μ-adt)C6H4CCR [R = C6H4NO2-4 (2), C6H4CHO-4 (3), C6H4NH2-4 (4), C6H4COOH-4 (5), C6H4COOCH2CH3-4 (6), C6H4F-4 (7), C6H5 (8), C6H4OCH3-4 (9), C6H4N(CH3)2-4 (10)], [Fe2(CO)5(PPh3)(μ-adt)C6H4I-4 (11), and Fe2(CO)5(PPh3)(μ-adt)C6H4CCC6H4NO2-4 (12), have been synthesized in high yields under mild conditions. The linear geometry and rigidity of a triple bond act as an effective bridge to anchor a functionality ranging from electron-donating to electron-accepting, even coordinative groups in the adt model complexes. X-ray crystal analysis of 2, 3, and 6−12 reveals that the model complexes retain the butterfly structure of Fe2S2 model analogues. A rigid phenylacetylene offers excellent control over the distance between the functional group and the active site of Fe2S2 model complexes. The unusual Fe−Fe distance and the angles found in the molecular packing of 6 are originated from the intriguing intermolecular C−H···O and C−H···S interactions. More importantly, electrochemical studies reveal that all of the complexes can catalyze electrochemical reduction of protons to molecular hydrogen, but the reduction potential for the electron-transfer step can be remarkably altered by the functionality R. The electroreductively active nitro group in 2 and 12 displays the enhanced current at a potential substantially less negative than the reduction of [FeIFeI] + e− → [FeIFe0], which is most accessible and becomes the initial step. For complex 3, the second reduction peak for the electron-transfer step involves the contribution from the aldehyde functionality. As the electroreductively inactive groups are incorporated, the reduction process of [FeIFeI] + e− → [FeIFe0] appears first and the second reduction peak for the electron-transfer step from the [FeIFe0] + e− → [Fe0Fe0] process for 4−10 is clearly observed. Therefore, the order of electron and proton uptake is closely related to the electroreductively active functionality, R. Varying the nature of the functionality R leads to the electron-transfer step changes from the reduction of the electroreductively active R group to the active site of Fe2S2 model complexes subsequently. Accordingly, notwithstanding, acetic acid is too weak to protonate the series of 2−12, different reduction pathways can be followed, and the electrochemically catalyzed behavior may occur at different reduction levels

Facile Synthesis and Functionality-Dependent Electrochemistry of Fe-Only Hydrogenase Mimics

A series of azadithiolate (adt)-bridged Fe-only hydrogenase model complexes, Fe2(CO)6(μ-adt)C6H4I-4 (1), Fe2(CO)6(μ-adt)C6H4CCR [R = C6H4NO2-4 (2), C6H4CHO-4 (3), C6H4NH2-4 (4), C6H4COOH-4 (5), C6H4COOCH2CH3-4 (6), C6H4F-4 (7), C6H5 (8), C6H4OCH3-4 (9), C6H4N(CH3)2-4 (10)], [Fe2(CO)5(PPh3)(μ-adt)C6H4I-4 (11), and Fe2(CO)5(PPh3)(μ-adt)C6H4CCC6H4NO2-4 (12), have been synthesized in high yields under mild conditions. The linear geometry and rigidity of a triple bond act as an effective bridge to anchor a functionality ranging from electron-donating to electron-accepting, even coordinative groups in the adt model complexes. X-ray crystal analysis of 2, 3, and 6−12 reveals that the model complexes retain the butterfly structure of Fe2S2 model analogues. A rigid phenylacetylene offers excellent control over the distance between the functional group and the active site of Fe2S2 model complexes. The unusual Fe−Fe distance and the angles found in the molecular packing of 6 are originated from the intriguing intermolecular C−H···O and C−H···S interactions. More importantly, electrochemical studies reveal that all of the complexes can catalyze electrochemical reduction of protons to molecular hydrogen, but the reduction potential for the electron-transfer step can be remarkably altered by the functionality R. The electroreductively active nitro group in 2 and 12 displays the enhanced current at a potential substantially less negative than the reduction of [FeIFeI] + e− → [FeIFe0], which is most accessible and becomes the initial step. For complex 3, the second reduction peak for the electron-transfer step involves the contribution from the aldehyde functionality. As the electroreductively inactive groups are incorporated, the reduction process of [FeIFeI] + e− → [FeIFe0] appears first and the second reduction peak for the electron-transfer step from the [FeIFe0] + e− → [Fe0Fe0] process for 4−10 is clearly observed. Therefore, the order of electron and proton uptake is closely related to the electroreductively active functionality, R. Varying the nature of the functionality R leads to the electron-transfer step changes from the reduction of the electroreductively active R group to the active site of Fe2S2 model complexes subsequently. Accordingly, notwithstanding, acetic acid is too weak to protonate the series of 2−12, different reduction pathways can be followed, and the electrochemically catalyzed behavior may occur at different reduction levels

Photocatalytic Hydrogen Evolution from Rhenium(I) Complexes to [FeFe] Hydrogenase Mimics in Aqueous SDS Micellar Systems: A Biomimetic Pathway

To offer an intriguing access to photocatalytic H2 generation in an aqueous solution, the hydrophobic photosensitizer, Re(I)(4,4′-dimethylbpy)(CO)3Br (1) or Re(I)(1,10-phenanthroline)(CO)3Br (2), and [FeFe] H2ases mimics, [Fe2(CO)6(μ-adt)CH2C6H5] (3) or [Fe2(CO)6(μ-adt)C6H5] (4) [μ-adt = N(CH2S)2], have been successfully incorporated into an aqueous sodium dodecyl sulfate (SDS) micelle solution, in which ascorbic acid (H2A) was used as a sacrificial electron donor and proton source. Studies on the reaction efficiency for H2 generation reveal that both the close contact and the driving force for electron transfer from the excited Re(I) complexes and [FeFe] H2ases mimics are crucial for efficient H2 generation with visible light irradiation. Steady-state and time-resolved investigations demonstrate that the electron transfer takes place from the excited Re(I) complex 1 or 2 to [FeFe] H2ases mimic catalyst 3, leading to the formation of the long-lived Fe(I)Fe(0) charge-separated state that can react with a proton to generate Fe(I)Fe(II)·H, an intermediate for H2 production. As a result, a reaction vessel for the photocatalytic H2 production in an aqueous solution is established

Synthesis, Spectroscopic, Electrochemical and Pb²⁺-Binding Studies of Tetrathiafulvalene Acetylene Derivatives

A series of tetrathiafulvalene acetylene derivatives, [TTF−C⋮C−A] [A = C6H4N(CH3)2-4 (1), C6H4OCH3-4 (2), C6H5 (3), C6H4F-4 (4), C6H4NO2-4 (5), C5H4N-2 (6), C5H4N-3 (7), and C5H4N-4 (8)], have been designed and synthesized to provide insight into the nature of the donor−acceptor interaction via a π-conjugated triple bond. The X-ray crystal structure of [TTF−(C⋮C)−C6H4OCH3-4] (2) reveals that the phenyl ring linked by acetylene is almost coplanar to the plane of TTF with a dihedral angle of 3.6°. The strong intermolecular C−H···O hydrogen bonding was found to direct the molecular helical assemblies with a screw pitch of 5.148 Å when viewed along the a-axis. Spectroscopic and electrochemical behaviors of the tetrathiafulvalene acetylene derivatives demonstrate that the TTF unit interacts with the electron-accepting group through the triple bond, thus leading to the intramolecular charge transfer. The pyridine-substituted TTF compounds 6−8 show remarkable sensing and coordinating properties toward Pb2+. Comparison of the spectroscopic and electrochemical properties and the calculation at the B3LYP/6-31G* level available in Gaussian 03 reveals that varying the bridged unit of the TTF-π-A system from a double bond to a triple bond leads to positive shifts for the first and second oxidation potentials of the TTF moieties, while the extent of intramolecular charge transfer interactions through the π-conjugated triple bond is smaller than that through the double bond

Synthesis, Spectroscopic, Electrochemical and Pb²⁺-Binding Studies of Tetrathiafulvalene Acetylene Derivatives

A series of tetrathiafulvalene acetylene derivatives, [TTF−C⋮C−A] [A = C6H4N(CH3)2-4 (1), C6H4OCH3-4 (2), C6H5 (3), C6H4F-4 (4), C6H4NO2-4 (5), C5H4N-2 (6), C5H4N-3 (7), and C5H4N-4 (8)], have been designed and synthesized to provide insight into the nature of the donor−acceptor interaction via a π-conjugated triple bond. The X-ray crystal structure of [TTF−(C⋮C)−C6H4OCH3-4] (2) reveals that the phenyl ring linked by acetylene is almost coplanar to the plane of TTF with a dihedral angle of 3.6°. The strong intermolecular C−H···O hydrogen bonding was found to direct the molecular helical assemblies with a screw pitch of 5.148 Å when viewed along the a-axis. Spectroscopic and electrochemical behaviors of the tetrathiafulvalene acetylene derivatives demonstrate that the TTF unit interacts with the electron-accepting group through the triple bond, thus leading to the intramolecular charge transfer. The pyridine-substituted TTF compounds 6−8 show remarkable sensing and coordinating properties toward Pb2+. Comparison of the spectroscopic and electrochemical properties and the calculation at the B3LYP/6-31G* level available in Gaussian 03 reveals that varying the bridged unit of the TTF-π-A system from a double bond to a triple bond leads to positive shifts for the first and second oxidation potentials of the TTF moieties, while the extent of intramolecular charge transfer interactions through the π-conjugated triple bond is smaller than that through the double bond