Visible light absorption of binuclear TiOCo^II charge-transfer unit assembled in mesoporous silica

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• 作者：Hongxian Han ; Heinz Frei
• 关键词：Metal to metal charge transfer ; Mesoporous silica ; Visible light chromophore ; Binuclear unit ; Cobalt ; Titanium
• 刊名：Microporous and Mesoporous Materials
• 出版年：2007
• 期刊代码：78_13871811
• 类别：ch
• 出版时间：20 June 2007
• 卷：103
• 期：1-3
• 页码：265-272
• 文件大小：408 K

摘要

Grafting of Co^II(NCCH₃)₂Cl₂ onto mesoporous Ti–MCM-41 silica in acetonitrile solution affords binuclear Ti–O–Co^II sites on the pore surface under complete replacement of the precursor ligands by interactions with anchored Ti centers and the silica surface. The Co^II ligand field spectrum signals that the Co centers are anchored on the pore surface in tetrahedral coordination. FT-infrared action spectroscopy using ammonia gas adsorption reveals Co–O–Si bond modes at 831 and 762 cm⁻¹. No Co oxide clusters are observed in the as-synthesized material. The bimetallic moieties feature an absorption extending from the UV into the visible to about 600 nm which is attributed to the Ti^IV–O–Co^II → Ti^III–O–Co^III metal-to-metal charge-transfer (MMCT) transition. The chromophore is absent in MCM-41 containing Ti and Co centers isolated from each other; this material was synthesized by grafting Co^II onto a Ti–MCM-41 sample with the Ti centers protected by a cyclopentadienyl ligand. The result indicates that the appearance of the charge-transfer absorption requires that the metal centers are linked by an oxo bridge, which is additionally supported by XANES spectroscopy. The MMCT chromophore of Ti–O–Co^II units has sufficient oxidation power to serve as visible light electron pump for driving multi-electron transfer catalysts of demanding uphill reactions such as water oxidation.