Secondary Coordination Sphere Interactions within the Biomimetic Iron Azadithiolate Complexes Related to Fe-Only Hydrogenase: Dynamic Measure of Electron Density about the Fe Sites

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• 作者：Yu-Chiao Liu ; Ling-Kuang Tu ; Tao-Hung Yen ; Gene-Hsiang Lee ; Shu-Ting Yang ; Ming-Hsi Chiang
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：July 19, 2010
• 年：2010
• 卷：49
• 期：14
• 页码：6409-6420
• 全文大小：1192K
• 年卷期：v.49,no.14(July 19, 2010)
• ISSN：1520-510X

文摘

A series of iron azadithiolate complexes possessing an intramolecular secondary coordination sphere interaction and an ability to reduce HOAc at the potential near the first electron-transfer process are reported. A unique structural feature in which the aza nitrogen has its lone pair point toward the apical carbonyl carbon is observed in [Fe₂(μ-S(CH₂)₂NR(CH₂)₂S)(CO)_6−xL_x]₂ (R = ⁿPr, x = 0, 1a; R = ⁱPr, x = 0, 1b; R = ⁿPr, L = PPh₃, x = 1, 2; R = ⁿPr, L = PⁿBu₃, x = 1, 3) as biomimetic models of the active site of Fe-only hydrogenase. The presence of this weak N···C(CO_ap) interaction provides electronic perturbation at the Fe center. The distance of the N···C(CO_ap) contact is 3.497 Å in 1a. It increases by 0.455 Å in 2 when electronic density of the Fe site is slightly enriched by a weak σ-donating ligand, PPh₃. A longer distance (4.040 Å) is observed for the PⁿBu₃ derivative, 3. This N···C(CO_ap) distance is thus a dynamic measure of electronic nature of the Fe₂ core. Variation of electronic richness within the Fe₂ moiety among the complexes reflects on their electrochemical response. Reduction of 2 is recorded at the potential of −2.17 V, which is 270 mV more negative than that of 1. Complex 3 requires additional 150 mV for the same reduction. Such cathodic shift results from CO substitution by phosphines. Electrocatalytic hydrogen production from HOAc by both kinds of complexes (all-CO and phosphine-substituted species) requires the potential close to that for reduction of the parent molecules in the absence of acids. The catalytic mechanism of 1a is proposed to involve proton uptake at the Fe⁰Fe^I redox level instead of the Fe⁰Fe⁰ level. This result is the first observation among the all-CO complexes with respect to electrocatalysis of HOAc.