Ocansey Edward, Darkwa James, Makhubela Banothile C E
Research Centre for Synthesis and Catalysis, Department of Chemical Science, University of Johannesburg, Auckland Park, South Africa.
Front Chem. 2020 Nov 13;8:591353. doi: 10.3389/fchem.2020.591353. eCollection 2020.
A rise in atmospheric CO levels, following years of burning fossil fuels, has brought about increase in global temperatures and climate change due to the greenhouse effect. As such, recent efforts in addressing this problem have been directed to the use of CO as a non-expensive and non-toxic single carbon, C, source for making chemical products. Herein, we report on the use of tetrazolyl complexes as catalyst precursors for hydrogenation of CO. Specifically, tetrazolyl compounds bearing P-S bonds have been synthesized with the view of using these as PN bidentate tetrazolyl ligands (-) that can coordinate to iridium(III), thereby forming heteroatomic five-member complexes. Interestingly, reacting the ,'-bidentate tetrazolyl ligands with [Ir( led to serendipitous isolation of chiral-at-metal iridium(III) half-sandwich complexes (-) instead. Complexes - were obtained via prior formation of non-chiral iridium(III) half-sandwich complexes (-). The complexes undergo prior P-S bond heterolysis of the precursor ligands, which then ultimately results in new half-sandwich iridium(III) complexes featuring monodentate phosphine co-ligands with proton-responsive P-OH groups. Conditions necessary to significantly affect the rate of P-S bond heterolysis in the precursor ligand and the subsequent coordination to iridium have been reported. The complexes served as catalyst precursors and exhibited activity in CO and bicarbonate hydrogenation in excellent catalytic activity, at low catalyst loadings (1 μmol or 0.07 mol% with respect to base), producing concentrated formate solutions ( 180 mM) exclusively. Catalyst precursors with proton-responsive P-OH groups were found to influence catalytic activity when present as racemates, while ease of dissociation of the ligand from the iridium center was observed to influence activity in spite of the presence of electron-donating ligands. A test for homogeneity indicated that hydrogenation of CO proceeded by homogeneous means. Subsequently, the mechanism of the reaction by the iridium(III) catalyst precursors was studied using H NMR techniques. This revealed that a chiral-at-metal iridium hydride species generated served as the active catalyst.
经过多年燃烧化石燃料，大气中一氧化碳（CO）水平的上升由于温室效应导致了全球气温升高和气候变化。因此，近期解决这一问题的努力方向是将CO用作制造化学产品的廉价且无毒的单碳（C）源。在此，我们报道了使用四唑基配合物作为CO氢化反应的催化剂前体。具体而言，已合成了带有P-S键的四唑基化合物，旨在将其用作可以与铱（III）配位的PN双齿四唑基配体（-），从而形成杂原子五元配合物。有趣的是，将,'-双齿四唑基配体与[Ir(反应时，意外地分离得到了金属手性铱（III）半夹心配合物（-）。配合物-是通过预先形成非手性铱（III）半夹心配合物（-）得到的。这些配合物经历了前体配体的P-S键异裂，最终形成了具有质子响应性P-OH基团的单齿膦共配体的新型半夹心铱（III）配合物。已经报道了显著影响前体配体中P-S键异裂速率以及随后与铱配位的必要条件。这些配合物作为催化剂前体，在低催化剂负载量（相对于碱为1 μmol或0.07 mol%）下，在CO和碳酸氢盐氢化反应中表现出优异的催化活性，仅产生浓缩的甲酸盐溶液（180 mM）。发现具有质子响应性P-OH基团的催化剂前体以外消旋体形式存在时会影响催化活性，尽管存在供电子配体，但观察到配体从铱中心的解离容易程度会影响活性。均相性测试表明CO的氢化反应是通过均相方式进行的。随后，使用1H NMR技术研究了铱（III）催化剂前体的反应机理。这表明生成的金属手性铱氢化物物种作为活性催化剂。
