Photodelivery of CO by Designed PhotoCORMs: Correlation between Absorption in the Visible Region and Metal–CO Bond Labilization in Carbonyl Complexes

Abstract

The therapeutic potential of photoactive CO-releasing molecules (photoCORMs) have called for close examination of the roles of the ligand(s) and the central metal atoms on the overall photochemical labilization of the metal-CO bonds. Along this line, we have synthesized four metal complexes, namely, [MnBr(azpy)(CO)3 ] (1), [Mn(azpy)(CO)3 (PPh3 )]ClO4 (2), [ReBr(azpy)(CO)3 ] (3), and [Re(azpy)(CO)3 (PPh3 )]ClO4 (4), derived from 2-phenylazopyridine. These complexes were characterized by spectroscopic and crystallographic studies. Although both 1 and 3 exhibit strong metal-to-ligand charge-transfer bands in the 500-600 nm region, only 1 photoreleases CO upon illumination with visible light. Results of theoretical studies were used to gain insight into this surprising difference. Strong spin-orbit coupling (prominent in heavy metals) appears to promote intersystem crossing to a triplet state in 3, a step that discourages CO release upon illumination with visible light. Slow release of CO from 2 and 4 also indicates that strong σ-donating ligands, such as Br(-) , accelerate the rate of CO photorelease relative to π-acid ligands, such as PPh3 .

Synthesis and Characterization of a “Turn-On” photoCORM for Trackable CO Delivery to Biological Targets

“…The Mn− Br distance in 1 (2.5254(15) Å) is comparable to that in the corresponding bpy complex [MnBr(bpy)(CO) 3 ] (2.538(10) Å). 25 The Mn−CO bond lengths of 1 are also comparable to those observed with the bpy complex [MnBr(bpy)(CO) 3 ].…”

Re(I) carbonyl complexes: Multimodal platforms for inorganic chemical biology

“…Although very interesting, the main limitation of this complex relies on the fact that high-energy light is required to promote CO release. In an attempt to increase the excitation wavelength of Re-based photo-CORMS, Mascharak et al have developed Re complexes exhibiting strong MLCT bands in the visible region, compound 15, Figure 6 [66]. However, CO release was not observed upon illumination in the visible region, the absorbed energy being dissipated through pathways that do not initiate CO release.…”

PhotoCORMs: CO release moves into the visible

“…25). 74,75 These systems offer good CO release behaviour in the visible, with fac-[Mn (38)(CO) 3 Br] showing an absorption maximum at 586 nm, and a rate for CO release of 5 min −1 when irradiated with broadband light (cutoff 520 nm). Carbon monoxide release from fac-[Mn(39)(CO) 3 Br] can be traced in vitro by fluorescence measurement and was shown to lead to cell apoptosis.…”

“…65,67,75,80 Whilst these studies show that the compounds tests are not toxic, the potential of photoCORMs in active toxicity toward cancer cells has also been considered. 46,74 The latter studies focus on the possible 'turn on' of activity when irradiated.…”