Diisocyanide-Containing Self-Assembled Monolayers as Dual Vibrational Probes and Tethering Linkers for Transition Metal Complexes Characterized via Sum Frequency Generation

Abstract

The study and use of non-Faradaic electric field effects on catalytic species immobilized on electrodes are of recent interest. Present complications limiting wider deployment of electric fields in catalysis include uncertainty around the precise mechanistic connection between the structure, orientation, and order of interfacial species and the resulting electric field magnitude at the active site. This uncertainty is due in part to experimental challenges relating to characterizing interfacial structure and associated electric field strength. Here, we investigate the structure of alkyl- and aryl-diisocyanide self-assembled monolayers (SAMs) and the utility of these SAMs as catalyst binding linkers. Vibrational sum frequency generation (SFG) spectra in the CN and CH stretching regions of SAMs prepared under a variety of conditions showed that use of tetrahydrofuran rather than dichloromethane was essential to produce well-ordered diisocyanide monolayers with abundant free CN groups capable of binding molecular catalysts with the greatest effect observed in the case of the alkyl-based monolayers. Next, the catalyst binding ability of these monolayers was explored using Ru(TPP)CO as a model system. Following functionalization with Ru(TPP)CO, electrochemical characterization showed that for the alkyl-diisocyanide SAMs, the more well-ordered the monolayer, the greater amount surface-bound Ru was observed. Taken together, the spectroscopic and electrochemical data demonstrate the importance of preparation conditions when forming dual-functional, well-ordered monolayers on a surface to maximize catalyst binding properties. These results also highlight the utility of SFG as an experimental approach for characterizing aspects of SAM structure and order that are expected to exert a critical influence on the magnitude of the electric field that is felt at bound molecular catalysts.