Characterization of one-electron oxidized copper(ii)-salophen-type complexes; effects of electronic and geometrical structures on reactivities

Abstract

One-electron oxidized salophen-type complexes, [Cu(salophen)](+) (H2salophen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminobenzene), and its methoxy derivatives, [Cu(MeO-salophen)](+) and [Cu(salophen-(MeO)2)](+) (H2MeO-salophen = N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-diaminobenzene, H2salophen-(MeO)2 = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diamino-4,5-dimethoxybenzene), have been synthesized and structurally characterized, and their reactivities have been investigated. The solid state structures of the one-electron oxidized forms of these complexes suggested that [Cu(salophen)](+) and [Cu(MeO-salophen)](+) can be assigned to relatively localized Cu(ii)-phenoxyl radical complexes, while [Cu(salophen-(MeO)2)](+) is the diiminobenzene radical complex. On the other hand, [Cu(salophen)](+) in solution showed a different electronic structure from that of the solid sample, the radical electron being delocalized over the whole π-conjugated ligand. The reaction of these oxidized complexes with benzyl alcohol has been investigated in the presence of a large excess of substrate, which revealed the difference in the kinetic behavior between the complexes. The mechanisms of the oxidation have been discussed on the basis of the electronic and geometrical structures and the reaction kinetics.

Properties of the indole ring in metal complexes. A comparison with the phenol ring

“…Complex 1,2-salcn + , which is a Cu(III)-phenolate complex in the solid state, is in equilibrium between the Cu(III)-phenolate ground state and the Cu(II)-phenoxyl radical in CH 2 Cl 2 [86]. A similar equilibrium was observed for saloph + in CH 2 Cl 2 , which was found to be in equilibrium between the localized phenoxyl radical ground state and the delocalized ligand radical [92]. The electronic structures of the other three oxidized complexes are maintained in solution.…”

“…Complexes 1,2-salcn + , 1,3-salcn + , and saloph + showed similar reaction kinetics, suggesting that the reaction mechanisms are similar [87,92]. The kinetic isotope effect (KIE) with the use of α-d 2 -benzyl alcohol was estimated to be very large (k D /k H N 10), which indicates that hydrogen abstraction from α-position of the benzyl alcohol is involved in the rate determining step of the oxidation (Scheme 3) [95,96].…”

“…1, suggesting that the reaction of 4,5-diMeO-saloph + does not involve the hydrogen abstraction from benzyl alcohol. Therefore, a possible reaction mechanism will be that the oxidant-substrate complex is formed in the rate-determining step followed by the substrate oxidation by electron transfer (Scheme 4) [92]. This electron transfer mechanism for 4,5-diMeO-saloph + may be reasonable, since the o-diiminobenzene radical can be considered to be less reactive compared with the localized phenoxyl radical.…”

“…This fact may suggest a possibility that the indole radical is involved only in the electron transfer pathway in biological systems. The relatively localized phenoxyl radical complex, p-MeO-saloph + , shows a very slow reaction rate with no significantly different KIE value, and therefore a different reaction mechanism from that of 1,3-salcn + , an electron transfer mechanism, is suggested [92].…”

Kinetic study of the oxovanadium(IV)–salen-catalyzed H2O2 oxidation of phenols

“…The generation of phenoxyl radicals may take place in a single step through hydrogen atom abstraction, or in two steps via electron transfer followed by proton transfer (ETPT), or by proton transfer followed by electron transfer (PTET) [3]. Furthermore, phenols are building blocks for many natural products such as alkaloids, and the biosynthesis of these compounds (by oxidative coupling) seems to proceed via phenoxyl radicals [4].…”