Hansen Colin, Docherty Scott R, Cao Weicheng, Yakimov Alexander V, Copéret Christophe
ETH Zurich, Department of Chemistry and Applied Biosciences Vladimir Prelog Weg 1-5 CH-8093 Zurich Switzerland
Chem Sci. 2024 Jan 17;15(8):3028-3032. doi: 10.1039/d3sc04067d. eCollection 2024 Feb 22.
Molecular-level understanding of the acid/base properties of heterogeneous catalysts requires the development of selective spectroscopic probes to establish structure-activity relationships. In this work we show that substituting the surface protons in oxide supports by isolobal N-heterocyclic carbene (NHC) Ag cations and measuring their Ag nuclear magnetic resonance (NMR) signatures enables to probe the speciation and to evaluate the corresponding Brønsted acidity of the substituted OH surface sites. Specifically, a series of silver N-heterocyclic carbene (NHC) Ag(i) complexes of general formula [(NHC)AgX] are synthesized and characterized, showing that the Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X (, HX), thus establishing an acidity scale based on Ag NMR chemical shift. The methodology is then used to evaluate the Brønsted acidity of the OH sites of representative oxide materials using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy.
要从分子层面理解多相催化剂的酸碱性质，需要开发选择性光谱探针来建立结构-活性关系。在这项工作中，我们表明，用等瓣N-杂环卡宾（NHC）银阳离子取代氧化物载体表面的质子，并测量其银核磁共振（NMR）信号，能够探测物种形成情况，并评估取代的OH表面位点相应的布朗斯特酸度。具体而言，合成并表征了一系列通式为[(NHC)AgX]的银N-杂环卡宾（NHC）Ag(i)配合物，结果表明该系列配合物的Ag NMR化学位移与X的共轭酸（即HX）的布朗斯特酸度相关，从而基于Ag NMR化学位移建立了酸度标度。然后，利用动态核极化（DNP-）增强固态NMR光谱法，该方法被用于评估代表性氧化物材料OH位点的布朗斯特酸度。
