Copper(II) complexes of N3O ligands as models for galactose oxidase: Effect of variation of steric bulk of coordinated phenoxyl moiety on the radical stability and spectroscopy

Three N3O tripodal ligands (two pyridines, one phenol) HLMes, HLTRIP and HLNap were synthesized. The phenol ortho substituent is a mesityl, 2,4,6-tri-isopropylphenyl or 2-naphthyl group, respectively. The dinuclear copper(II)-phenolate complexes [ÎŒOH-Cu2II(LMes)(HLMes)]2+ and [Cu2II(LNap)2]2+ were isolated as single crystals. In [ÎŒOH-Cu2II(LMes)(HLMes)]2+ the two copper centers are bridged by a hydroxo unit. In [Cu2II(LNap)2]2+ two phenolate groups bridge the copper atoms. In the absence of base the mononuclear copper(II)-phenol complexes are formed. The complexes [CuII(HLMes)(CH3CN)]2+ and [CuII(HLNap)(CH3CN)]2+ were structurally characterized, showing a distorted octahedral copper ion. The phenol remains protonated and occupies one axial position, while a counter ion is weakly bound on the opposite position and an acetonitrile molecule completes the coordination sphere. In the presence of pyridine mononuclear copper(II)-phenolate complexes of general formula [CuII(LNap)(Py)]+, [CuII(LMes)(Py)]+ and [CuII(LTRIP)(Py)]+ are obtained. The copper(II)-phenol complexes exhibit a reversible reduction wave in the range −0.27 to −0.29V vs. Fc+/Fc in their CV curve, which corresponds to the CuII/CuI redox couple, as well as an irreversible oxidation wave. In the presence of pyridine a reversible oxidation wave is observed at 0.09–0.13V. It is assigned to the phenoxyl/phenolate redox couple. The one-electron oxidized copper(II)-phenoxyl complexes are mononuclear triplet species, as judged by the ΔMS=2 resonance in their EPR spectra. The |D| value is estimated at 1700MHz (axial ZFS). All these radical species exhibit an intense band at 427nm, which combines the features of both phenoxyl π–π∗ and CT transitions. The stability of the radicals follows the order [CuII(LNap)(Py)]2+>[CuII(LMes)(Py)]2+>[CuII(LTRIP)(Py)]2+. By DFT calculations we show that all the phenoxyl radical species adopt a common trigonal bipyramidal geometry, wherein the phenoxyl moiety coordinates in the trigonal plane. They possess a triplet ground spin state due to the orthogonality between the two SOMOs, which mainly correspond to dz2 and phenoxyl π orbitals.

Références

Titre
Copper(II) complexes of N3O ligands as models for galactose oxidase: Effect of variation of steric bulk of coordinated phenoxyl moiety on the radical stability and spectroscopy
Type de publication
Article de revue
Année de publication
2018
Revue
Inorganica Chimica Acta
Volume
481
Pagination
129 - 142
ISSN
0020-1693
Soumis le 19 février 2019