Photochemical Processes in a Rhenium(I) Tricarbonyl N-Heterocyclic Carbene Complex Studied by Time-Resolved Measurements

Abstract

We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)(PyImPh)(MeCN)], is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)–Re(i) supramolecular photocatalysts for CO2reduction using time-resolved IR measurements

“…TRIR measurements with picosecond temporal resolution were performed by the pump-probe method using two electronically synchronized pulsed lasers: a femtosecond Ti : sapphire amplifier (Spectra Physics Spitfire Ace) and a picosecond Nd : YVO 4 laser (InnoLas picolo AOT-YVO-25). 59 A tunable mid-infrared pulse (bandwidth = 150 cm –1 , tunable range = 1000–4000 cm –1 ) was generated by an optical parametric amplifier equipped with a difference frequency generation crystal (Light Conversion TOPAS Prime) from the output of the amplifier (central wavelength = 800 nm, pulse duration = 120 fs). The 532 nm pump pulse was generated by frequency doubling the 1064 nm fundamental output of the Nd : YVO 4 laser (pulse duration = 600 ps).…”

Properties and prospects for rhenium(i) tricarbonyl N-heterocyclic carbene complexes

“…The overall mechanism was elucidated by means of temperature dependent measurements of the excited state lifetime decays as well as by time-resolved infrared spectroscopy (TR-IR). 37 The measurements for both techniques were obtained in nonreactive conditions (dichloromethane) and reactive conditions (acetonitrile). The complex 2(phenyl)Br was chosen for this investigation.…”

A Robust Pyridyl-NHC-Ligated Rhenium Photocatalyst for CO2 Reduction in the Presence of Water and Oxygen

“…Here, we reasoned that this group allows better delocalization of the added electron on the singly reduced catalyst across the ligand framework since the lowest energy excitation is thought to be a metal-to-ligand charge transfer (MLCT) followed by addition of an electron [24][25][26]. To analyze the role of the phenyl-CF3 substituent, the geometry of Re(pyNHCPhCF3)(CO)3Br was first optimized with DFT at the M06-L/6-31++G(d,p) level.…”

“…Among known NHC-ligated photocatalysts, the phenyl-CF 3 NHC substituent was previously found to give the most reactive catalyst when compared with phenyl, p-hexyloxyphenyl, or methyl substituents on the NHC ring [10]. Here, we reasoned that this group allows better delocalization of the added electron on the singly reduced catalyst across the ligand framework since the lowest energy excitation is thought to be a metal-to-ligand charge transfer (MLCT) followed by addition of an electron [25][26][27]. To analyze the role of the phenyl-CF 3 substituent, the geometry of Re(pyNHC-PhCF 3 )(CO) 3 Br was first optimized with DFT at the M06-L/6-31++G(d,p) level.…”