Long-range metal–ligand bifunctional catalysis: cyclometallated iridium catalysts for the mild and rapid dehydrogenation of formic acid
作者:Jonathan H. Barnard、Chao Wang、Neil G. Berry、Jianliang Xiao
DOI:10.1039/c2sc21923a
日期:——
Formic acid (HCO2H) is an important potential hydrogen storage material, which, in the presence of appropriate catalysts can be selectively dehydrogenated to give H2 and CO2. In this work, well defined N^C cyclometallated iridium(III) complexes based on 2-aryl imidazoline ligands are found to be excellent catalysts for the decomposition of HCO2HâNEt3 mixtures to give H2 and CO2 under mild conditions with high turnover frequencies (up to 147â000 hâ1 at 40 °C) and essentially no CO formation. The modular structures of these catalysts have allowed for the construction of structureâactivity relationships for the complexes, leading to the rational optimisation of the catalyst structure with respect to both the rate of H2 production and catalyst lifetime. In particular, the presence of the remote γ-NH unit in the ligand is shown to be essential for catalytic activity, without which no reaction occurs. Mechanistic studies suggest that the dehydrogenation is rate-limited by the step of hydride protonation, which is made feasible by the γ-NH unit via an unusual form of long-range metalâligand bifunctional catalysis involving formic acid-assisted proton hopping.
甲酸 (HCO2H) 是一种重要的潜在氢储存材料,在适当的催化剂存在下,可以选择性地脱氢生成氢气 (H2) 和二氧化碳 (CO2)。本研究发现,基于 2-芳基咪唑啉配体的明确 N^C 环金属化铱(III) 配合物是 HCO2H–NEt3 混合物分解的优良催化剂,在温和条件下以高周转频率(在 40°C 下可达 147,000 h–1)产生 H2 和 CO2,并几乎没有 CO 的生成。这些催化剂的模块化结构使得能够建立配合物的结构–活性关系,从而合理优化催化剂结构以提高 H2 生产速率和催化剂寿命。特别是,配体中远程的 γ-NH 单元被证明对催化活性至关重要,没有该单元时不发生反应。机理研究表明,脱氢反应的速率受限于氢化物质子化步骤,而这一过程通过 γ-NH 单元以一种不寻常的长程金属–配体双功能催化形式实现,这涉及到甲酸辅助的质子跃迁。