Photochemistry of fac‐[Re(bpy)(CO)3Cl

Shunsuke Sato,Hiroshi Miyasaka,Noboru Mataga,Seiji Taniguchi,Magnus Falkenström,Yasuo Matubara,Yukihide Ishibashi,Haik Chosrowjan,Shinya Koshihara,Naoto Fukazawa,Tetsuro Katayama,Ken Onda,Osamu Ishitani,Kazuhide Koike

doi:10.1002/chem.201202734

Abstract

The photochemistry of fac-[Re(bpy)(CO)3Cl] (1 a; bpy=2,2′-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)2Cl(MeCN)] (2 c; bpy=2,2′-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (3HES(1)). In THF, 3HES(1) was competitively converted to both the triplet ligand field (3LF) and metal-to-ligand charge transfer (3mLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)2Cl(thf)]+ (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.

Highlights

The photochemistry and photophysics of the rhenium(I) complex fac-[ReACHTUNGRE(bpy)(CO)3Cl] (1 a; bpy = 2,2’-bipyridine) have generated substantial interest because this complex emits from the triplet metal-to-ligand charge-transfer (MLCT) excited state even in solution at ambient temperature[1] and can function as a photocatalyst for CO2 reduction.[2]
Only limited information on the excited states of 1 a has been reported except for the lowest 3MLCT excited state from which energy- and electron-transfer reactions proceed.[1f,h,2d,6,7] Recently, ultrafast emission spectroscopy was applied to 1 a, and the kinetics of intersystem crossing from the singlet MLCT excited state and vibrational relaxation processes of the hot triplet states have been reported.[8a,e]
Photochemistry in mixed solvents of THF and MeCN: As we have already reported, irradiation of solution of 1 a in THF caused the isomerization of 1 a to the corresponding meridional isomer (1 b).[9b] the photochemical isomerization of 1 a was drastically suppressed by the addition of a small amount of MeCN into the reaction solution

Summary

Introduction

2 a should form by means of the associative mechanism initiated by the attack of an MeCN molecule on the excited state of 1 a (process A in Scheme 1) because the yield of 2 a was strongly dependent on the concentration of MeCN in the reaction solution, but the rate of formation of 2 a was almost the same under a CO atmosphere as that under Ar. To gain a deeper insight into the mechanism, we investigated the effects of the solvent viscosity on the photochemical reaction of 1 a using a 1:1 mixture of N,N-dimethylformamide (the absolute viscosity coefficient at 298 K is 0.794 MPa s)–MeCN (0.369 MPa s) and N,N-dimethylacetamide (1.956 MPa s)–MeCN, which have similar relative permittivity values (eDMF = 36.7; eDMA = 37.8).

Results

Conclusion