Department of Chemistry, Indiana University , 800 E. Kirkwood Ave., Bloomington, Indiana 47405, United States.
J Am Chem Soc. 2014 Jul 16;136(28):9862-5. doi: 10.1021/ja504850f. Epub 2014 Jul 1.
The formation and stabilization of well-defined transition-metal single sites at surfaces may open new routes to achieve higher selectivity in heterogeneous catalysts. Organic ligand coordination to produce a well-defined oxidation state in weakly reducing metal sites at surfaces, desirable for selective catalysis, has not been achieved. Here, we address this using metallic platinum interacting with a dipyridyl tetrazine ligand on a single crystal gold surface. X-ray photoelectron spectroscopy measurements demonstrate the metal-ligand redox activity and are paired with molecular-resolution scanning probe microscopy to elucidate the structure of the metal-organic network. Comparison to the redox-inactive diphenyl tetrazine ligand as a control experiment illustrates that the redox activity and molecular-level ordering at the surface rely on two key elements of the metal complexes: (i) bidentate binding sites providing a suitable square-planar coordination geometry when paired around each Pt, and (ii) redox-active functional groups to enable charge transfer to a well-defined Pt(II) oxidation state. Ligand-mediated control over the oxidation state and structure of single-site metal centers that are in contact with a metal surface may enable advances in higher selectivity for next generation heterogeneous catalysts.
在表面上形成和稳定定义明确的过渡金属单原子位可能为实现异相催化剂更高的选择性开辟新途径。在表面上的还原能力较弱的金属位中，通过有机配体配位产生所需的明确氧化态，以实现选择性催化，但目前尚未实现这一点。在这里，我们使用与单晶金表面上的二吡啶四嗪配体相互作用的金属铂来解决此问题。X 射线光电子能谱测量证明了金属-配体的氧化还原活性，并与分子分辨率扫描探针显微镜相结合，以阐明金属-有机网络的结构。与作为对照实验的无氧化还原活性的二苯基四嗪配体进行比较表明，表面上的氧化还原活性和分子级有序性依赖于金属配合物的两个关键要素：（i）当与每个 Pt 配对时，提供合适的平面正方形配位几何形状的双齿结合位点，以及（ii）氧化还原活性官能团，以使电荷转移到明确的 Pt（II）氧化态。配体介导的对与金属表面接触的单原子中心的氧化态和结构的控制，可能会推动下一代异相催化剂的更高选择性的发展。
