Recently, we reported an efficient strategy for the conversion of CO into methanol mediated by organometallic compounds and recyclable benzimidazole organic hydrides (BIHs). The first step of this transformation is the conversion of cis-[Ru(bpy)₂(CO)₂]²⁺, where bpy = 2,2’-bipyridine, and BIH to the corresponding formyl complex, cis-[Ru(bpy)₂(CO)(CHO)]⁺, and the oxidized benzimidazolium, BI⁺. Although this process corresponds to a formal hydride transfer reaction, detailed mechanistic investigations and theoretical calculations suggest that the reaction proceeds via an electron transfer (ET) from BIH to cis-[Ru(bpy)₂(CO)₂]²⁺, yielding first the radical cation BI^●+ and the one-electron reduced cis-[Ru(bpy)₂(CO)₂]⁺, followed by a consecutive proton-coupled electron transfer (PCET). Under normal reaction conditions, it was not possible to separate these two processes to study them individually. Herein, we developed a strategy for distinguishing the two reactions with the aid of a photoredox sensitizer, [Ru(dmb)₃]²⁺, where dmb = 4,4’-dimethyl-2,2’-bipyridine, and a BIH that is not hydridic enough to drive the reaction thermally, in the dark. Upon visible light illumination, the excited [Ru(dmb)₃]^2+* was reductively quenched by the BIH, and the resulting [Ru(dmb)₃]⁺ transferred an electron to cis-[Ru(bpy)₂(CO)₂]²⁺. The rapid (ns – μs) formation of the reduced cis-[Ru(bpy)₂(CO)₂]⁺ was confirmed by the characteristic CO vibrations at 2070 and 2012 cm^-1. At longer timescales (s), formation of cis-[Ru(bpy)₂(CO)(CHO)]⁺ through the PCET reaction was observed by IR and NMR spectroscopies. The separation of the ET and PCET processes not only allowed a better mechanistic understanding of the reactions directing the formation of liquid fuels, but also allowed us to capitalize on visible light to provide energy to drive the transformation using BIHs that are easier to recycle.

Andressa Müller

Brookhaven National Laboratory Chemistry Division

Appearances