Transition metal carbonyl complexes, such as [Ru(bpy)₂(CO)₂]²⁺ (bpy = 2,2'-bipyridyl), are known catalysts for electrochemical reduction of CO₂ to solar fuels and industrial feedstocks, such as methanol. This catalysis proceeds via a formyl intermediate, [Ru(bpy)₂(CO)(CHO)]⁺. Currently, we only have limited information on the reactivity of these formyl intermediates due to the severe instability of the complex in solution.
In this project, we use a combined approach of multinuclear NMR and ultrafast two-dimensional infrared (2DIR) spectroscopy to demonstrate that the stability of this important formyl intermediate is greatly enhanced by the addition of a Lewis acid (LA), such as Li⁺ or Mg²⁺, without adversely affecting its electrochemical reduction potentials. Through comparison of different spectroscopic observables that report on the electron density within the complex, such as chemical shifts and vibrational frequencies, we propose that the stabilization results from the formation of a ‘carbene-like’ structure for the Ru-CHO--LA adduct, with an increased Ru-C bond order compared to the free formyl complex.
From the 2DIR spectra, we observe evidence of a chemical exchange as the appearance and growth of cross peaks that are associated with the rotational isomerization of the formyl ligand in the absence of Lewis acids that occurs on picosecond timescales. Upon complexation to a Lewis acid, the chemical exchange cross-peaks are no longer detectable by 2DIR. However, we then observe evidence of restricted rotation on much slower NMR timescales, further supporting the formation of a carbene-like structure (Ru=C).
Altogether, this work demonstrates a tunable method of stabilizing formyl ligands in solution phase, which may enable additional reactivities and open new pathways to liquid solar fuels through optimization of CO₂ reduction catalysis.

James Shipp

University of Pittsburgh

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