Orientation of Cyano-Substituted Bipyridine Re(I) fac-Tricarbonyl Electrocatalysts Bound to Conducting Au Surfaces

Sum frequency generation spectroscopy (SFG) and calculations of SFG spectra based on density functional theory are combined to elucidate the orientation of two Re(R2,2′-bipyridine)(CO)3Cl (R = 4-cyano or 4,4′-dicyano) electrocatalysts when adsorbed on conductive gold surfaces. We find that the electrocatalysts lean on the Au surface to orient the plane of the bipyridine ligand at 63° relative to the surface normal. While the weak binding of the complexes to the gold surface precluded the ability to perform surface immobilized catalysis, homogeneous electrochemical experiments show that the molecular catalysts are active toward the reduction of CO2 to CO and carbonate in the triply reduced state (TOF of 13.3 and 7.2 s−1 for the doubly and singly substituted complexes, respectively). These findings demonstrate the capabilities of the approach of including rigorous spectroscopic and theoretical methods for revealing the conformation and orientation of CO2 reduction catalysts bound to electrode surfaces, which are critical considerations for redox state transitions and catalytic turnover. ■ INTRODUCTION As global anthropogenic carbon dioxide (CO2) emissions continue to rise, there is a need not only to reduce production of CO2 but also to use it as a substrate for value-added products. One solution would be to utilize this CO2 to synthesize liquid fuels, which would help create a carbon neutral cycle. The first step of the cycle can be done by electrocatalytically reducing CO2, and both heterogeneous 2,3 and homogeneous systems have been employed for this purpose. While homogeneous systems are favored for their selectivity and tunability, heterogeneous catalysts have the advantages of stability, low catalyst loading, and straightforward product separation. Linking molecular catalysts to conductive surfaces provides an opportunity to combine the advantages of both systems. It is important to understand the nature of catalyst-to-surface binding as orientation can greatly affect electron transfer and substrate access to the active site. In recent years, infraredvisible sum frequency generation (SFG) spectroscopy has emerged as a useful tool for determining the average molecular orientation at surfaces and interfaces. As a second-order nonlinear technique, it is only allowed at interfaces that lack an inversion symmetry and is therefore a useful technique for probing complexes adsorbed to solid surfaces. Previously, we have investigated the molecular orientation and vibrational relaxation dynamics of a series of fac-Re(dcbpy)(CO)3Cl (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) CO2 reduction catalysts on single crystal TiO2 and gold surfaces. 15−17 These studies indicated that the catalytically active site of the rhenium favorably oriented outward from the surface and that longer alkyl anchoring chains increased the tilt angle of the catalyst toward the TiO2 surface. Considering the great potential and interest in attaching molecular catalysts to electrode surfaces, more attachment strategies need to be investigated. Rebpy-based catalysts are excellent model systems because of their high activity for CO2 reduction, the wealth of mechanistic information known, and the easy synthetic tunability of the bpy ligand. A noncovalent attachment of a Rebpy catalyst to graphitic carbon demonstrated that strong binding was not necessary for immobilizing an active catalyst, and this attachment strategy was successfully employed toward other molecular catalysts as well. However, little is known about how these types of noncovalent attachment strategies affect the orientation of the complexes. This can be investigated using Au electrodes, as Au is well-known for its ability to form air-stable self-assembled monolayers (SAMs) with multiple substituents of differing binding strengths. Au is furthermore ideal for our purposes as Received: November 6, 2015 Revised: December 22, 2015 Published: January 5, 2016 Article