Controlled CO release using photochemical, thermal and electrochemical approaches from the amino carbene complex [(CO)₅CrC(NC₄H₈)CH₃]

Abstract

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO) 5 CrC(NC 4 H 8 )CH 3 ] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state.CO releasing molecules (CORMs) are an area of current interest due to the physiological role of CO in the body. 1,2 CO is endogenously generated during oxidative heme degradation by the heme oxygenase (HO) enzymes. 3 The potential therapeutic applications of CO include vasodilation, anti-inflammatory and anti-proliferative effects. CORMs have been developed to deliver CO in a controlled manner. However, there is a major challenge in controlled CO release, be it photochemically, thermally or enzymatically induced. It is well known that chromium carbene complexes can release CO both thermally and photochemically, as the resulting tetracarbonyl reactive intermediates have been used as synthons in organic chemistry. 4 Recently, Lynam et al. reported the suitability of heteroatom (amino, methoxy and thio) stabilised chromium carbenes as thermal CORMs. 5 Rapid CO release was observed for the thio-and methoxy carbenes (t 1/2 = 313 s and 306 s, respectively, for a 60 mM solution of the complex to achieve an Mb-CO concentration of 30 mM), compared to slow release in the case of the amino analogue which required more than 2 h for a 60 mM solution of the complex to form 10 mM of carboxy-myoglobin (Mb-CO). In addition the chromium carbonyls investigated, demonstrated the fastest and most regulated CO release characteristics compared to the molybdenum or tungsten complexes studied. 5 In this communication we set out to compare the rate of CO release from [(CO) 5 CrC(NC 4 H 8 )CH 3 ] using photochemical, thermal and electrochemical stimulation. To the best of our knowledge this is first example of a CORM involving electrochemical stimulation. In the present study we report our findings on a ps-time resolved infrared (ps-TRIR) study on [(CO) 5 CrC(NC 4 H 8 )CH 3 ] in conjunction with DFT calculations to investigate the excited state responsible for the CO release. Chromium Fischer carbenes have received significant attention as photochemical synthons for a range of organic compounds. Photolysis of [(CO) 5 CrC(XR)R 0 ] in the presence of imines, olefins, aldehydes, or alcohols are known to yield b-lactams, cyclobutanones, b-lactones or amino esters respectively. 6 The proposed mechanism for these reactions involves a metal-toligand charge transfer photoexcitation resulting in the formation of a ketene intermediate which is formed by the insertion of a cis-CO ligand into the adjacent Cr-carbene bond (Fig. 1). Experimental and theoretical calculations reported by Sierra et al. support the proposal that this occurs through a MLCT triplet excited state. 7 Although a number of low temperature and time-resolved studies have investigated photo-induced CO release from chromium carbene...

An investigation into the photochemistry of, and the electrochemically induced CO-loss from, [(CO)5MC(OMe)Me](M = Cr or W) using low-temperature matrix isolation, picosecond infrared spectroscopy, cyclic voltammetry, and time-dependent density functional theory

“…Recently we have provided the first evidence that this can be achieved electrochemically. 22 In this paper we show that (1) and (4) release CO electrochemically.…”

“…The CO-loss product (5) at 2043 and 1896 cm −1 (k obs = 5.5(±0.9)× 10 10 s −1 ) was formed over the subsequent 50 ps and these band positions are consistent with those observed previously for the solvated [(CO) 4 CrC(C 4 H 8 N)-Me] species. 22 Scheme 1 The photophysical processes of (1) leading to the formation of the metallaketene via exclusively singlet excited states (diagnostic IR bands in parentheses) and the CO-loss species [(CO) 4 (S)CrC(OMe)Me] following 320 nm irradiation (S = n-heptane). psTRIR study of (4) using 400 nm excitation…”

“…Recently we reported a psTRIR study on the amino Fischer carbene complex [(CO) 5 CrC(NC 4 H 8 )(Me)] and showed that CO loss occurred, during the first 50 ps following excitation at 400 nm via an excited state which could be observed spectroscopically. 22 No evidence was obtained for the formation of metallaketene or metallacyclopropanone intermediates. This is, perhaps, not too surprising because many amino Fischer carbene complexes are poor reagents in the synthesis of β-lactams.…”

“…Over the timeframe of the experiments, the concentration of CO in the headspace indicates the loss of only one CO molecule per molecule of the complex. These experiments indicate that both (1) and the amino analogue [(CO) 5 CrC(C 4 H 8 N)Me] release similar quantities of CO. 22 …”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…The product identification was accomplished through DFT calculation of structures and comparison of the predicted and observed IR peaks. An experimental and computational study of photo, thermal, and electrochemicallymediated CO release from a Fischer aminocarbene chromium complex was also reported [936]. A c c e p t e d M a n u s c r i p t 83 Successful arylithium formation in the presence of aminocarbene complex functionality (e.g.…”