Effect of X Ligands on the Photocatalytic Reduction of CO2 to CO with Re(pyridylNHC-CF3)(CO)3X Complexes

A series of five Re(pyNHC-aryl)(CO)(3)X complexes varying the X ligand where pyNHC is a pyridyl N-heterocyclic carbene have been synthesized and characterized through NMR, UV/Vis absorption spectroscopy, IR, mass spectrometry, time-correlated single photon counting, computational analysis, and cyclic voltammetry. The photocatalytic reduction of CO2 to CO in the presence of a sacrificial electron donor with these complexes is evaluated using a simulated solar spectrum (AM 1.5G). Comparison of Br and CH3CN as the X ligand shows the same photocatalytic activity (both 32 TON). The use of Cl, NCS and P(OEt)(3) X ligands all led to diminished reactivity with as little as 2 TON for the P(OEt)(3) complex. These results were rationalized through computational analysis of X dissociation and excited-state lifetime measurements. These results highlight the importance of the X ligand selection on catalysis with Re-pyNHC complexes.