Spectroscopic and computational characterization of CuII-OOR (R = H or cumyl) complexes bearing a Me6-tren ligand

Abstract

A copper(II)-hydroperoxo complex, [Cu(Me(6)-tren)(OOH)](+) (2), and a copper(ii)-cumylperoxo complex, [Cu(Me(6)-tren)(OOC(CH(3))(2)Ph)](+) (3), were synthesized by reacting [Cu(Me(6)-tren)(CH(3)CN)](2+) (1) with H(2)O(2) and cumyl-OOH, respectively, in the presence of triethylamine. These intermediates, 2 and 3, were successfully characterized by various physicochemical methods such as UV-vis, ESI-MS, resonance Raman and EPR spectroscopies, leading us to propose structures of the Cu(II)-OOR species with a trigonal-bipyramidal geometry. Density functional theory (DFT) calculations provided geometric and electronic configurations of 2 and 3, showing trigonal bipyramidal copper(II)-OOR geometries. These copper(II)-hydroperoxo and -cumylperoxo complexes were inactive in electrophilic and nucleophilic oxidation reactions.

Fig. 1

X-ray structure of the [Cu(Me₆ -tren)(CH₃CN)]²⁺ cation in 1-(BPh₄)₂ showing 30% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Fig. 2

(a) UV-vis spectra of 1 (0.4 mM) (black line) and 2 (0.4 mM) (red line) in CH₃CN/CH₃OH (1:1) at 0 °C. (b) ESI-MS of 2-¹⁸O. Inset shows isotope distribution patterns for 2-¹⁸O (blue line) and 2-¹⁶O (red line). (c) Resonance Raman spectra of 2 (4 mM) obtained upon excitation at 407 nm in CH₃CN/CH₃OH (1:1) at −20 °C. Red/blue lines: samples prepared with H₂¹⁶O₂/H₂¹⁸O₂; green line: difference spectrum, (¹⁶O–¹⁸O). The peak marked with “s” is ascribed to the solvents. (d) X-band EPR spectrum of 2 recorded in CH₃CN/CH₃OH (1:1) at 4.3 K. Spectroscopic settings: frequency = 9.10 GHz, microwave power = 1 mW, modulation frequency = 100 kHz.

Fig. 3

(a) UV-vis spectrum of 3 (2 mM) in CH₃CN at 25 °C. (b) ESI-MS of 3-¹⁶O. Inset shows isotope distribution patterns for 3-¹⁶O (red line) and 3-¹⁸O (blue line). Isotopically labeled cumyl-¹⁸O¹⁸OH (70% ¹⁸O-enriched) was used for 3-¹⁸O. (c) Resonance Raman spectra of 3 (16 mM) obtained upon excitation at 442 nm in CH₃CN at −20 °C. Red/blue lines were obtained with samples prepared with cumyl-¹⁶O¹⁶OH/Cumyl-¹⁸O¹⁸OH (90% ¹⁸O-enriched); green line: difference spectrum, (¹⁶O – ¹⁸O). The peaks marked with “s” are ascribed to the solvent. (d) X-band EPR spectrum of 3 recorded in CH₃CN at 4.3 K. Spectroscopic settings: frequency = 9.10 GHz, microwave power = 1 mW, modulation frequency = 100 kHz.

Fig. 4

DFT calculated structures for 2 (top) and one of the isomers of 3a (bottom) with the bond distances shown for the Cu–O, O–O, Cu–N_ax and (largest) Cu–N_eq bonds (gray, C; white, H; red, O; blue, N; yellow, Cu).

Fig. 5

The singly occupied molecular orbital (SOMO) for 2 (and also for 3) is a σ*_z² orbital.

Scheme 1

Mononuclear copper–O₂ adducts and derived species of interest.

Scheme 2