Some physical and chemical aspects of spin trapping of organometallic radicals by quinones: optically active radical complexes

Abstract

. Can. J. Chem. 60, 1504Chem. 60, (1982.A brief account is presented on the use of substituted quinones as spin traps for organometallic radicals and on the applications of CIDEP and esr-HPLC techniques to the study of quinone -organometallic radical complexes. Many of these radical adducts are thermally very stable and can be isolated by the HPLC technique for detailed spectroscopic and chemical studies. This in turn allows us to present a different approach to the "spin trapping" chemistry of unstable organometallic radicals by generating initially a stable quinone -metal carbonyl radical complex, separating the parent radicals by esr-HPLC and using them to react with other organometals via ligand exchange between the CO group(s) and the organometals. It then follows that if the entering organometal ligand is optically active, the substituted radical complex would also be chiral. This simple concept has led to the isolation of the first pure optically active radical complex with well-defined physical properties, including optical rotation. For many years the popular nitroxide spin trapping technique has provided evidence for numerous transient metal centered radicals (1). The ability of quinones to form radical complexes and spin adducts with a variety of organometals was recently emphasized (2, 3) and added another dimension to the spin trapping chemistry. The quinone -metal radical complexes are interesting generally because both the metal centre and the quinone ligands can be redox active. Since the first major study on quinone-metal complexes was carried out by Eaton (4), many other interesting complexes have been synthesized directly by the reactions of metal carbonyls with quinones (5, 6). The use of quinones in spin trapping photochemistry has yet another advantage. There is a large pool of information available for the photochemical and photophysical properties of carbonyl compounds but the quenching of the excited states of carbonyl compounds by various electron donors, including some of the organometals, has continued to attract attention (7). In this laboratory, the photoreduction of quinones by alcohols and phenols (8,9) and by vitamins such as C and K l (10) have been extensively used as model systems for chemically induced magnetic polarization studies. They in turn provide some valuable information about the relative efficiencies of the triplet quenching reaction, the mechanisms of the secondary radical reactions, and the dynamics of the intersystem crossing processes which are responsible for the difference in the spin populations of the triplet sub-levels. The CIDEP technique was further applied to study the quenching of quinone triplets by organometals (11) and the results indicate the nature of electron transfer between the organotin compounds and the triplet quinone in the photochemical primary process. Unequivocal evidence of the electron transfer mechanism was subsequently obtained by the direct esr characterization of the triplet state of the primary radical ion pair formed and tra...

Ligand exchange in the rhenium–quinone radical complexes: kinetics and mechanism

“…From the examination of the ir carbonyl bands, it can be concluded that the C,,, structure is the correct one for all of the monosubstituted radical complexes which were studied. This is in agreement with the rather large (-25 G) phosphorus coupling constants which were observed (21,22).…”

“…However, a similar experiment with manganese carbonyls produced the radical complexes which decomposed while being eluted through a HPLC column. Since the rhenium carbonyl -quinone radicaI complexes undergo slow substitution reactions with a number of organolnetal donors (20)(21)(22), it follows that one could design kinetic experiments using the pure radical complexes. Furthermore, we have demonstrated (20,21) that the ligand substitution reactions involving optically active organometals and/or optically active quinones led to the isolation of some interesting optically active radical complexes.…”

“…Since the rhenium carbonyl -quinone radicaI complexes undergo slow substitution reactions with a number of organolnetal donors (20)(21)(22), it follows that one could design kinetic experiments using the pure radical complexes. Furthermore, we have demonstrated (20,21) that the ligand substitution reactions involving optically active organometals and/or optically active quinones led to the isolation of some interesting optically active radical complexes. These reactions are potentially highly stereo-specific and enantio-selective, it is important that some kinetics and mechanistic information of these rhenium carbonyl -quinone radical complexes involved in the substitution reactions be delineated.…”

Free radical and radical ion intermediates in the photochemical reactions of carbonyl compounds with organometals

“…Several nitro compounds have also been employed to trap alkyl-silyl radicals [51]. Recent reports indicate that nitro compounds can also complex manganese [52,53] and rhenium [53] centered radicals via both nitrogen and oxTgen atoms simultaneously, as shown below.…”

An esr and CIDEP study of the photoreduction of chromone and chromanone and their organometallic radical adducts

“…thiobenzophenones (19). Thus, tin-, silicon-, and germaniumcentered radicals are systematically added to chromones, chromanone, and thiachromanones and the resulting radical adducts are reported in Table 2.…”