Characterization of Group 10-Metal-p-Substituted Phenoxyl Radical Complexes with Schiff Base Ligands.

Abstract

Methylthiophenoxyl radical plays an important role as the active form of galactose oxidase (GO), which catalyzes oxidation of a primary alcohol to the corresponding aldehyde. Although many metal(II)‐phenoxyl radical species have been reported, only a few studies have been reported on the properties of methylthiophenoxyl radical‐metal complexes. We have prepared the group 10 metal (Ni, Pd and Pt) complexes of a salen‐type ligand with a methylthio group at para‐position of the two phenolate moieties and characterized them by X‐ray crystal structure analyses and various spectroscopic methods in order to understand the role of the methylthio moiety in phenoxyl radical metal complexes. The corresponding p‐methoxy substituted derivatives have been also characterized for comparison. All the one‐electron oxidized group 10 metal methylthiophenolate complexes have a relatively localized radical site on one of the two phenolate moieties in comparison to the one‐electron oxidized complexes of p‐methoxy derivatives and exhibit different properties dependent on the central metal ions.

π–π Stacking Interaction in an Oxidized CuII–Salen Complex with a Side‐Chain Indole Ring: An Approach to the Function of the Tryptophan in the Active Site of Galactose Oxidase

“…The cyclic voltammograms of all the precursor complexes exhibited reversible redox waves (Figure ). The first redox potential of 1 (Table ) is similar to the values of 2 and Cu(MeO‐salen) and Ni(MeO‐salen) complexes, which have been assigned to the oxidation of the MeO‐substituted phenolate . We may therefore assign the first redox potential to a ligand‐based oxidation process.…”

“…It is noticed that the position of the counter anion in both oxidized complexes of [ 1 ]SbF 6 is different from the positions observed for [ 2 ]PF 6 and the other oxidized salen‐type complexes without the side‐chain indole ring . The counter anion in [ 1 ]SbF 6 is moved from the proximal position of one of the phenolate moieties by the π–π stacking interaction of the pendent indole moiety.…”

“…The first redox potentialo f1 ( Table 2) is similar to the values of 2 and Cu(MeOsalen) and Ni(MeO-salen) complexes,w hich have been assignedt ot he oxidation of the MeO-substituted phenolate. [39][40][41][42][43] We may therefore assign the first redox potential to al igandbased oxidation process. Comparison of the voltammograms of 1 and 2 shows that the differences between the first and the second redox potential( DE)a re slightly but significantly different;c omplex 1 (DE = 0.20 V) exhibited al arger value than complex 2 (DE = 0.17 V).…”

“…Comparison of the voltammograms of 1 and 2 shows that the differences between the first and the second redox potential( DE)a re slightly but significantly different;c omplex 1 (DE = 0.20 V) exhibited al arger value than complex 2 (DE = 0.17 V). The differenceo fthet wo redox potentials indicatest he stabilization of the one-electron-oxidized form of salen-type complexes, [36,40,[42][43][44][45] al arger separation of the potentials indicating ah ighers tability of the one-electronoxidizeds pecies. The result indicates that complex 1 gives a more stable one-electron-oxidized species compared with 2.…”

“…It is noticedt hat the position of the counter anion in both oxidizedc omplexes of [1]SbF 6 is different from the positions observed for [2]PF 6 and the other oxidized salen-type complexesw ithoutt he side-chain indole ring. [39,43,44,[46][47][48][49][50][51][52] The counter anion in [1]SbF 6 is movedf rom the proximal position of one of the phenolate moieties by the p-p stacking interaction of the pendent indolem oiety.T he result suggests that the electrostatic interaction of the counter anion with the phenoxyl radical moiety is less effective than the p-p stacking interaction of the pendent indole moiety.…”