Photoswitching a molecular catalyst to regulate CO2 hydrogenation

Abstract

Inspired by nature's ability to regulate catalysis using physiological stimuli, azobenzene was incorporated into Rh(bis)diphosphine CO₂ hydrogenation catalysts to photoinitiate structural changes to modulate the resulting catalytic activity. The rhodium bound diphosphine ligands (P(Ph₂)-CH₂-N(R)-CH₂-P(Ph₂)) contain the terminal amine of a non-natural amino acid, with the R-group being either β-alanine (β-Ala) or γ-aminobutyric acid (GABA). For both β-Ala and GABA containing complexes, the carboxylic acids of the amino acids were coupled to the amines of diaminoazobenzene, creating a complex consisting of a rhodium bound to a photo-responsive tetradentate ligand. The photo-induced cis–trans isomerization of the azobenzene-containing complexes imposes structural changes on these complexes, as evidenced by NMR studies. We found that the CO₂ hydrogenation activity for the β-Ala bound rhodium complex is 40% faster at 27 °C with the light on, i.e. azobenzene in the cis-conformation (TOF = 16 s⁻¹) than when the complex was in the dark and the azobenzene in the trans-conformation (TOF = 11 s⁻¹). In contrast the γ-aminobutyric acid containing rhodium complex has the same rate (TOF ∼17 s⁻¹) with the azobenzene in either the cis or the trans-conformation at 27 °C. The corresponding (bis)diphosphine complexes without the attached azobenzene were also prepared, characterized, and catalytically tested for comparison, and have TOF's of 30 s⁻¹. Computational studies were undertaken to evaluate if the difference in rate between the cis- and trans-azobenzene isomers for the β-Ala bound rhodium complex were due to structural differences. These computational investigations revealed major structural changes between all cis- and trans-azobenzene structures, but only minor structural changes that would be unique to the β-Ala bound rhodium complex. We postulate that the different rates between the cis- and trans-azobenzene β-Ala bound containing rhodium complexes are due to subtle changes in the bite angle arising from steric strain due to the azobenzene-containing tetradentate ligand. This strain alters the hydricity of the subsequent rhodium hydride and consequently the rate.