Kawanami Hajime, Iguchi Masayuki, Himeda Yuichiro
Research Institute for Chemical Process Technology, Department of Material and Chemistry, National Institute of Advanced Industrial Science and Technology, Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi 983-8551, Japan.
Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan.
Inorg Chem. 2020 Apr 6;59(7):4191-4199. doi: 10.1021/acs.inorgchem.9b01624. Epub 2020 Feb 17.
A series of CpIr (Cp = pentamethylcyclopentadienyl anion) complexes with amino-functionalized ligands were developed for the production of high-pressure H via catalytic dehydrogenation of formic acid (DFA) in water under base-free conditions. The Ir complexes with 2,2'-bipyridine (bpy) ligands bearing amino or alkylamino groups at the positions exhibited high activity and stability for DFA compared with complexes containing bpy ligands bearing -hydroxyl groups. In addition, -amino groups afforded superior catalytic stability under high-pressure conditions compared with -amino groups. By exploiting these amino-functionalized Cp*Ir complexes, it was possible to continuously produce high-pressure CO-free H via selective DFA in water upon the addition of concentrated FA (>99.5 wt %) to the base-free solution. Systematic investigation of the ligand effects on DFA revealed that the presence of alkylamino groups on the bpy ligand enhanced the catalytic activity (initial turnover frequency, TOF), although the stability decreased with increasing alkyl chain length on the amino groups. According to a Hammett plot, the increased catalytic activity of the Ir complexes after the introduction of amino-functionalized ligands may be attributable to the electron-donating effect of -amino groups on the bpy ligand. Based on the experimental results, a reaction mechanism is proposed that involves a hydride intermediate whose stability is affected by the position of the amino groups on the bpy ligand, as confirmed through NMR studies.
开发了一系列带有氨基官能化配体的CpIr（Cp = 五甲基环戊二烯基阴离子）配合物，用于在无碱条件下通过水中甲酸的催化脱氢（DFA）生产高压氢气。与含有在β位带有羟基的联吡啶（bpy）配体的配合物相比，在α位带有氨基或烷基氨基的bpy配体的Ir配合物对DFA表现出高活性和稳定性。此外，与β-氨基相比，α-氨基在高压条件下提供了更好的催化稳定性。通过利用这些氨基官能化的Cp*Ir配合物，在向无碱溶液中加入浓FA（>99.5 wt%）后，有可能通过水中的选择性DFA连续生产无高压CO的氢气。对配体对DFA影响的系统研究表明，bpy配体上烷基氨基的存在增强了催化活性（初始周转频率，TOF），尽管稳定性随着氨基上烷基链长度的增加而降低。根据哈米特图，引入氨基官能化配体后Ir配合物催化活性的提高可能归因于bpy配体上α-氨基的给电子效应。基于实验结果，提出了一种反应机理，该机理涉及一种氢化物中间体，其稳定性受bpy配体上氨基位置的影响，这通过核磁共振研究得到证实。
