Photochemistry of fac‐[Re(bpy)(CO)₃Cl]

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.

Luminescent Re(I) Carbonyl Complexes as Trackable PhotoCORMs for CO delivery to Cellular Targets

“…44 These authors have shown that complexes like [ReCl(CO) 3 (bpy)] + and [Re(CO) 3 (bpy)py] + containing either σ -donating or moderately π -accepting ancillary ligands require higher energy UVB light ( λ < 313 nm) for CO photorelease. 45 However, complexes of type [Re(CO) 3 (bpy) (PR 3 )] + with a strong π -accepting coligand release CO upon exposure to UVA light of relatively lower energy ( λ = 360 nm). 44,46 This difference in sensitivity has been attributed to the weaker trans -labilizing ability of Cl − or py (pyridine) ligand compared to a phosphorus donor (as in complex 4 ).…”

The photochemistry of rhenium(i) tricarbonyl N-heterocyclic carbene complexes

“…The photochemical CO substitution was shown to occur via a dissociative mechanism from a reactive ligand field excited state of triplet multiplicity, 3 LF, 23 which can be thermally accessible from the lower-lying emissive 3 MLCT state. 24,25 This complex lacks the coordination of a labilizing ligand such as phosphine, phosphite or isonitrile and it is photostable when its lowest-lying triplet manifold is populated. 23 It was also shown that, after the dissociation of the CO ligand, no scrambling of the other two CO ligands occurs; hence the new entering ligand always coordinates trans to the labilizing ligand, yielding only one of the three possible geometrical isomers (or two isomers in the case of C 2 -symmetrical diim ligands).…”

“…The Re-CO dissociation occurs as a consequence of the distortion caused by the population of the 3 LF state, coupled with the labilization of the CO opposite to the ligand displaying a strong trans effect. 25 We have recently reported the synthesis and photophysical characterization of the tricarbonyl rhenium(I) N-heterocyclic carbene (NHC) complexes 1Cl and 1Br, whose structures are shown in Fig. 23 More recently, Ishitani reported the photochemical CO substitution of fac-[Re(CO) 3 (bipy)Cl], where bipy = 2,2′-bipyridine.…”

CO-Induced apoptotic death of colorectal cancer cells by a luminescent photoCORM grafted on biocompatible carboxymethyl chitosan

“…Ishitani and co-workers clearly delineated the requirement of UV-B radiation to trigger CO release from rhenium carbonyl complexes of type [Re(CO) 3 (bpy)(X)] (where the X is either σ-donating or moderately π-accepting ancillary ligands). 19 This requirement presumably arises from the weaker trans labilizing capacity of chloride and pyridine as coligand. The photo behaviour of 1 is consistent with such observation.…”