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

Photocatalytic Synthesis of Quinolines via Povarov Reaction under Oxidant-Free Conditions

We describe here an approach for synthesizing quinolines either from N-alkyl anilines or from anilines and aldehydes. A dual-catalyst system consisting of a photocatalyst and a proton reduction cocatalyst is employed. Without the use of any sacrificial oxidant and under extremely mild conditions, the reactions afford quinolines in excellent yields and produce H2 as a byproduct

Space Craft-like Octanuclear Co(II)-Silsesquioxane Nanocages: Synthesis, Structure, Magnetic Properties, Solution Behavior, and Catalytic Activity for Hydroboration of Ketones

Two novel space craft-like octanuclear Co­(II)-silsesquioxane nanocages, {Co8[(MeSiO2)4]2­(dmpz)8} (SD/Co8a) and {Co8[(PhSiO2)4]2­(dmpz)8} (SD/Co8b) (SD = SunDi; Hdmpz = 3,5-dimethylpyrazole), have been constructed from two similar multidentate silsesquioxane ligands assisted with a pyrazole ligand. The Co8 skeleton consists of eight tetrahedral Co­(II) ions arranged in a ring and is further capped by two (MeSiO2)4 ligands up and down. The auxiliary dmpz– ligands seal the ring finally. Electrospray ionization mass spectrometry revealed SD/Co8a and SD/Co8b are highly stable in CH2Cl2. Magnetic analysis implies that SD/Co8a announces antiferromagnetic interactions between Co­(II) ions. Moreover, both of them display good homogeneous catalytic activity for hydroboration of ketones in the presence of pinacolborane under mild conditions

An Octanuclear Cobalt Cluster Protected by Macrocyclic Ligand: In Situ Ligand-Transformation-Assisted Assembly and Single-Molecule Magnet Behavior

Macrocyclic molecules with multiple coordination sites have been widely used as promising ligands to build polynuclear metal clusters; however, cyclic silsesquioxane-based metal clusters are still rare. Herein, we report a new octanuclear Co-silsesquioxane cluster [Co8(OH)2{(MeSiO2)6}2­(bpy)2(Obpy)2] (SD/Co8c; SD = SunDi), wherein the Co8 disc-like core is sandwiched by two hexamethylcyclohexasiloxanolate ligands (MeSiO2)6 at two poles and finally encircled by two bpy (bpy = 2,2′-bipyridine) and two Obpy (HObpy = 6-hydroxy-2,2′-bipyridine) ligands at the equatorial region. Interestingly, both MeSi­(OMe)3 and bpy undergo in situ transformations to generate hexameric cyclic (MeSiO2)6 and Obpy, respectively. The unusual hydroxylation of bpy and the OH– anion in the center of Co8 core provide additional binding sites to induce the formation of the larger cluster instead of the traditional hexanuclear cluster. The solution stability and fragmentation route in the gas phase were studied by cold-spray ionization and collision-induced dissociation mass spectrometry, respectively. Both results reveal that the Co8 core is quite stable in solution as well as in the gas phase, even with increased collision voltage. Magnetic susceptibility studies of SD/Co8c show the slow magnetization relaxation indicative of single-molecule magnet (SMM) behavior. This work not only presents the multiple in situ ligand-transformation-assisted assembly of polynuclear cobalt cluster but also provides some new insights into the magnetism–structure relationship for SMMs

Heptanuclear Co^II₅Co^III₂ Cluster as Efficient Water Oxidation Catalyst

Inspired by the transition-metal-oxo cubical Mn₄CaO₅ in photosystem II, we herein report a disc-like heptanuclear mixed-valent cobalt cluster, [Co^II₅Co^III₂(mdea)₄­(N₃)₂(CH₃CN)₆(OH)₂­(H₂O)₂·4ClO₄] (<b>1</b>, H₂mdea = <i>N</i>-methyldiethanolamine), for photocatalytic oxygen evolution. The topology of the Co₇ core resembles a small piece of cobaltate protected by terminal H₂O, N₃^–, CH₃CN, and multidentate <i>N</i>-methyldiethanolamine at the periphery. Under the optimal photocatalytic conditions, <b>1</b> exhibits water oxidation activity with a turnover number (TON) of 210 and a turnover frequency (TOF_initial) of 0.23 s^–1. Importantly, electrospray mass spectrometry (ESI-MS) was used to not only identify the possible main active species in the water oxidation reaction but also monitor the evolutions of oxidation states of cobalt during the photocatalytic reactions. These results shed light on the design concept of new water oxidation catalysts and mechanism-related issues such as the key active intermediate and oxidation state evolution in the oxygen evolution process. The magnetic properties of <b>1</b> were also discussed in detail

Heptanuclear Co^II₅Co^III₂ Cluster as Efficient Water Oxidation Catalyst

Inspired by the transition-metal-oxo cubical Mn₄CaO₅ in photosystem II, we herein report a disc-like heptanuclear mixed-valent cobalt cluster, [Co^II₅Co^III₂(mdea)₄­(N₃)₂(CH₃CN)₆(OH)₂­(H₂O)₂·4ClO₄] (<b>1</b>, H₂mdea = <i>N</i>-methyldiethanolamine), for photocatalytic oxygen evolution. The topology of the Co₇ core resembles a small piece of cobaltate protected by terminal H₂O, N₃^–, CH₃CN, and multidentate <i>N</i>-methyldiethanolamine at the periphery. Under the optimal photocatalytic conditions, <b>1</b> exhibits water oxidation activity with a turnover number (TON) of 210 and a turnover frequency (TOF_initial) of 0.23 s^–1. Importantly, electrospray mass spectrometry (ESI-MS) was used to not only identify the possible main active species in the water oxidation reaction but also monitor the evolutions of oxidation states of cobalt during the photocatalytic reactions. These results shed light on the design concept of new water oxidation catalysts and mechanism-related issues such as the key active intermediate and oxidation state evolution in the oxygen evolution process. The magnetic properties of <b>1</b> were also discussed in detail

Hierarchical Assembly of a {Mn^II₁₅Mn^III₄} Brucite Disc: Step-by-Step Formation and Ferrimagnetism

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn₇) to nonadecamer (Mn₁₉) satisfying the relation 1 + Σ_<i>n</i>6<i>n</i>, where <i>n</i> is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three intermediate clusters, Mn₁₀, Mn₁₂, and Mn₁₄, were identified. Furthermore, the Mn₁₉ disc remains intact when dissolved in acetonitrile with a well-resolved general formula of [Mn₁₉­(<i>L</i>)_<i>x</i>­(OH)_<i>y</i>­(N₃)_{36–<i>x</i>−<i>y</i>}]²⁺ (<i>x</i> = 18, 17, 16; <i>y</i> = 8, 7, 6; H<i>L</i> = 1-(hydroxy­methyl)-3,5-dimethylpyrazole) indicating progressive exchange of N₃^– for OH^–. The high symmetry (<i>R</i>-3) Mn₁₉ crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums, the charge state is mixed-valent, [Mn^II₁₅Mn^III₄]. Its magnetic properties and electrochemistry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as confirmed by the lack of ordering in frozen CH₃CN. The moment of nearly 50 Nμ_B suggests Mn^II–Mn^II and Mn^III–Mn^III are ferromagnetically coupled while Mn^II–Mn^III is antiferromagnetic which is likely if the Mn^III are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from nonconnected high-spin molecules

Hierarchical Assembly of a {Mn^II₁₅Mn^III₄} Brucite Disc: Step-by-Step Formation and Ferrimagnetism

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn₇) to nonadecamer (Mn₁₉) satisfying the relation 1 + Σ_<i>n</i>6<i>n</i>, where <i>n</i> is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three intermediate clusters, Mn₁₀, Mn₁₂, and Mn₁₄, were identified. Furthermore, the Mn₁₉ disc remains intact when dissolved in acetonitrile with a well-resolved general formula of [Mn₁₉­(<i>L</i>)_<i>x</i>­(OH)_<i>y</i>­(N₃)_{36–<i>x</i>−<i>y</i>}]²⁺ (<i>x</i> = 18, 17, 16; <i>y</i> = 8, 7, 6; H<i>L</i> = 1-(hydroxy­methyl)-3,5-dimethylpyrazole) indicating progressive exchange of N₃^– for OH^–. The high symmetry (<i>R</i>-3) Mn₁₉ crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums, the charge state is mixed-valent, [Mn^II₁₅Mn^III₄]. Its magnetic properties and electrochemistry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as confirmed by the lack of ordering in frozen CH₃CN. The moment of nearly 50 Nμ_B suggests Mn^II–Mn^II and Mn^III–Mn^III are ferromagnetically coupled while Mn^II–Mn^III is antiferromagnetic which is likely if the Mn^III are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from nonconnected high-spin molecules