Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study

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• 作者：Lei Dong ; Yujuan Wang ; Yanzong Lv ; Zhuqi Chen ; Fuming Mei ; Hui Xiong ; Guochuan Yin
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：May 6, 2013
• 年：2013
• 卷：52
• 期：9
• 页码：5418-5427
• 全文大小：346K
• 年卷期：v.52,no.9(May 6, 2013)
• ISSN：1520-510X

文摘

Redox-inactive metal ions have been recognized to be able to participate in redox metal-ion-mediated biological and chemical oxidative events; however, their roles are still elusive. This work presents how the redox-inactive metal ions affect the oxidative reactivity of a well-investigated manganese(II) with its corresponding manganese(IV) complexes having cross-bridged cyclam ligand. In dry acetone, the presence of these metal ions can greatly accelerate stoichiometric oxidations of triphenylphosphine and sulfides by the manganese(IV) complexes through electron transfer or catalytic sulfoxidations by the corresponding manganese(II) complexes with PhIO. Significantly, the rate enhancements are highly Lewis-acid strength dependent on added metal ions. These metal ions like Al³⁺ can also promote the thermodynamic driving force of the Mn^IV鈥揙H moiety to facilitate its hydrogen abstraction from ethylbenzene having a BDE_CH value of 85 kcal/mol, while it is experimentally limited to 80 kcal/mol for Mn^IV鈥揙H alone. Adding Al³⁺ may also improve the manganese(II)-catalyzed olefin epoxidation with PhIO. However, compared with those in electron transfer, improvements in hydrogen abstraction and electron transfer are minor. The existence of the interaction between Lewis acid and the manganese(IV) species was evidenced by the blue shift of the characteristic absorbance of the manganese(IV) species from 554 to 537 nm and by converting its EPR signal at g = 2.01 into a hyperfine 6-line signal upon adding Al³⁺ (I = 5/2). Cyclic voltammograms of the manganese(IV) complexes reveal that adding Lewis acid would substantially shift its potential to the positive direction, thus enhancing its oxidizing capability.