Javier Alnald, Li Ding, Cruz Juan, Binamira-Soriaga Elizabeth, Balbuena Perla B, Soriaga Manuel P
Joint Center for Artificial Photosynthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
Dalton Trans. 2014 Oct 21;43(39):14798-805. doi: 10.1039/c4dt02137a.
Previous studies, based on thin-layer electrochemistry (TLE), in situ scanning tunneling microscopy (EC-STM), high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) computations, on the chemical adsorption of hydroquinone from aqueous solutions onto atomically smooth Pd (and Pt) electrode surfaces indicated two modes of attachment that depended upon the solution concentration. At low activities, the diphenol was oxidatively chemisorbed as benzoquinone in a flat orientation, suggestive of a Pd(2,3,5,6-η-C6H4O2) surface complex; at higher concentrations, vertical chemisorption was effected via two C-H bond activations (or metalations) at the 2 and 3 ring positions, evocative of an o-phenylene organopalladium compound. We have extended the work to 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) electrodes to probe the effect of two methyl substituents on only one side of the diphenol ring. Surface coverage and adsorbed-molecule cross section data from TLE and HREELS measurements revealed non-random concentration-dependent adsorbate orientations similar to the oxidative chemisorption of hydroquinone: flat at low concentrations and edgewise at elevated concentrations. The DFT results suggested that, for the flat structure, surface coordination is via the two double bonds of the quinone ring as in [Pd(2,3,5,6-η)-2,3-dimethyl-p-quinone]. For the edge-vertical orientation, a structure analogous to an o-phenylene compound is generated in which C-H bonds at the 5 and 6 ring positions are activated and then metalated. DFT-simulated HREELS spectra helped identify the observed peaks that distinguish the surface-coordinated quinone from the surface-metalated diphenol.
以往基于薄层电化学（TLE）、原位扫描隧道显微镜（EC - STM）、高分辨率电子能量损失谱（HREELS）以及密度泛函理论（DFT）计算的研究，针对对苯二酚从水溶液中化学吸附到原子级光滑的钯（和铂）电极表面的情况表明，存在两种吸附模式，这取决于溶液浓度。在低活性时，二酚以苯醌的形式氧化化学吸附，呈平面取向，这表明形成了一种Pd(2,3,5,6 - η - C6H4O2)表面络合物；在较高浓度时，通过在苯环的2和3位进行两个C - H键的活化（或金属化）实现垂直化学吸附，这让人联想到邻亚苯基有机钯化合物。我们已将该研究扩展至2,3 - 二甲基 - 1,4 - 二羟基苯在Pd(pc)和Pd(111)电极上的吸附，以探究二酚环一侧的两个甲基取代基的影响。来自TLE和HREELS测量的表面覆盖度和吸附分子横截面数据显示，吸附质的取向与浓度有关且呈非随机分布，类似于对苯二酚的氧化化学吸附：低浓度时为平面取向，高浓度时为边缘取向。DFT结果表明，对于平面结构，表面配位是通过醌环的两个双键进行的，就如同[Pd(2,3,5,6 - η) - 2,3 - 二甲基 - 对苯醌]。对于边缘垂直取向，会生成一种类似于邻亚苯基化合物的结构，其中苯环5和6位的C - H键被活化然后金属化。DFT模拟的HREELS光谱有助于识别区分表面配位醌和表面金属化二酚的观测峰。
