X‐Ray Structures of Copper(II) and Nickel(II) Radical Salen Complexes: The Preference of Galactose Oxidase for Copper(II)

Abstract

Kupfer oder Nickel? Der gezeigte CuII‐Salen‐Komplex, ein Modell des aktiven Zentrums der Galactose‐Oxidase (GO), liegt im Festkörper als lokalisiertes Radikal vor, wie an der chinoiden Verteilung der Bindungslängen in einem der Ringe zu erkennen ist. Die Struktur des radikalischen Liganden hängt nicht vom Metall ab, die Zusammensetzung des SOMO dagegen schon. Dies könnte die viel geringere Reaktivität des Ni‐Komplexes erklären, ebenso wie die Tatsache, dass natürliche GO ein CuII‐Zentrum bevorzugt.

Oxidation Chemistry of Metal(II) Salen-Type Complexes

“…These characteristics are in good agreement with the Cu(III) valence state [12,13]. On the other hand, the MeO-substituted complex [Cu(MeO-Salcn)]SbF6 exhibits that the C-O bond of one-side of the phenolate moieties is shortened and that the Cu-O bond with the shortened phenolate moiety becomes longer [44]. The counterion is close to the phenolate moiety of the shortened C-O bond.…”

“…Structures of three one-electron oxidized Cu II salen-type complexes are shown in Figure 6 [44,45,47]. X-ray analyses of all these complexes established that their structures are similar to those before oxidation, indicating a simple one-electron transfer from these precursors.…”

“…Such a trend is also applied to the salen-type complexes, and most of the Cu(II) and group 10 metal(II) salen-type complexes exhibited two reversible redox waves in the range of 0 to 1.5 V vs NHE, due to having two phenolate moieties [29][30][31][39][40][41][42][43][44][45][46][47]. On the other hand, the metal centered one-and two-electron oxidized complexes may possibly be generated in the similar potential range.…”

“…The present argument is focused on recent advances in the chemistry related with the synthesis, characterization, and reactivity of some of the one-electron oxidized metal(II)-salen type complexes, especially the complexes of group 10 metal(II) ions, Ni(II), Pd(II), and Pt(II) [39][40][41][42][43], and Cu(II) complexes [44][45][46][47] (Figure 3). Various useful and interesting salen-type complexes have been synthesized and characterized until now [48], and detailed electronic structural studies have recently been reported for a dinuclear chelating salen-type ligand having a catechol and a tetra(amino)tri(hydroxy)phenyl moiety [49,50].…”

“…The small potential differences are due to the substitution of the phenolate moiety and the ligand structure. However, there are some different electronic structures among the one-electron oxidized forms of the complexes in Table 1, i.e., a M III -phenolate ground state complex, a M II -phenoxyl radical where the radical electron is fully delocalized on two phenolate moieties, a relatively localized M II (phenolate)(phenoxyl) complex, and a M II (dinitrogen ligand radical)(phenolate)2 species [39][40][41][42][43][44][45][46][47]. The similarity of the potentials indicates that formation of all of the oxidized species is due to a simple electron transfer without significant structural changes.…”