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(mu-adt)C6H4I-4 (1), Fe2(CO)6(mu-adt)C6H4C[triple bond]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)(mu-adt)C6H4I-4 (11), and Fe2(CO)5(PPh3)(mu-adt)C6H4C[triple bond]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 [Fe(I)Fe(I)] + e(-) --> [Fe(I)Fe(0)], 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 [Fe(I)Fe(I)] + e(-) --> [Fe(I)Fe(0)] appears first and the second reduction peak for the electron-transfer step from the [Fe(I)Fe(0)] + e(-) --> [Fe(0)Fe(0)] 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.
Si
Gang
G
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & Graduate University, The Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
Wang
Wen-Guang
WG
Wang
Hong-Yan
HY
Tung
Chen-Ho
CH
Wu
Li-Zhu
LZ
D000814:Aniline Compounds / Q000737:chemistry; D001547:Benzaldehydes / Q000737:chemistry; D040761:Biomimetic Materials / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism; D002264:Carboxylic Acids / Q000737:chemistry; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004579:Electron Transport; D006864:Hydrogenase / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism
D000814:Aniline Compounds; D001547:Benzaldehydes; D002264:Carboxylic Acids; D007506:Iron-Sulfur Proteins; D009942:Organometallic Compounds; D007501:Iron; C118830:iron hydrogenase; D006864:Hydrogenase; C023650:aniline; C032175:benzaldehyde