Design, Identification, and Evolution of a Surface Ruthenium(II/III) Single Site for CO Activation

Abstract

Ru II compounds are widely used in catalysis, photocatalysis, and medical applications. They are usually obtained in a reductive environment as molecular O 2 can oxidize Ru II to Ru III and Ru IV . Here we report the design, identification and evolution of an air-stable surface [bipy-Ru II (CO) 2 Cl 2 ] site that is covalently mounted onto a polyphenylene framework. Such a Ru II site was obtained by reduction of [bipy-Ru III Cl 4 ] À with simultaneous ligand exchange from Cl À to CO. This structural evolution was witnessed by a combination of in situ X-ray and infrared spectroscopy studies. The [bipy-Ru II (CO) 2 Cl 2 ] site enables oxidation of CO with a turnover frequency of 0.73 10 À2 s À1 at 462 K, while the Ru III site is completely inert. This work contributes to the study of structure-activity relationship by demonstrating a practical control over both geometric and electronic structures of single-site catalysts at molecular level.

Formal Oxidation States and Coordination Environments in the Catalytic Reduction of CO to Methanol

“…4 Figure S1 shows the Ru K edges for the resting state of three key Ru complexes in the catalytic cycle. The deep absorption edges for all these Ru complexes were fully consistent with a formal oxidation state of II, 3,5,6 and differed significantly from the spectrum expected of Ru complexes with an oxidation state of III. 7 In addition, a similar geometry for the ligands around the Ru metal center led to pre-edges with a close position and intensity.…”

“…Oxidation states assignments are hence a particular challenge for the catalytic reduction of CO or CO 2 that typically includes both ligand-and metal-based reductions. 3 Herein, X-ray absorption near edge structure spectroscopy (XANES) is utilized to characterize the reduction of CO to methanol for the first time in a newly proposed catalytic cycle that utilizes renewable organic hydrides. 4 Figure 1a shows the proposed catalytic cycle for cis-[Ru(bpy) 2 (CO) 2 ] 2+ , where reduction by a dihydrobenzimidazole hydride results in quantitative generation of a formyl intermediate, cis-[Ru(bpy) 2 (CO)(CHO)] + that was suggested to occur by an electron transfer−proton-coupled electron transfer (ET-PCET) mechanism with a kinetic isotope effect (KIE) of 9.6.…”