Dioxygen Reactivity of Copper(I) Complexes with Tetradentate Tripodal Ligands Having Aliphatic Nitrogen Donors: Synthesis, Structures, and Properties of Peroxo and Superoxo Complexes

Oxygenation of copper (I) with tetradentate tripodal ligands (L) comprised of a tris (aminoethyl) amine (tren) skeleton having sterically bulky substituent (s) on the terminal nitrogens has been investigated, where L = tris (N-benzylaminoethyl) amine (LH, Bn), tris (N-benzyl-N-methylaminoethyl) amine (LMe, Bn), or tris (N, N-dimethylaminoethyl) amine (LMe, Me). All the copper (I) complexes reacted with dioxygen at low temperatures to produce superoxocopper (II) and/or trans- (μ-1, 2-peroxo) -dicopper (II) complexes depending on the steric bulkiness of the terminal nitrogens and the reaction conditions. The reaction of a copper (I) complex Cu (LH, Bn) + at −90 °C in acetone resulted in the formation of a superoxo complex Cu (LH, Bn) (O2) + as a less stable species and a peroxo complex Cu (LH, Bn) 2 (O2) 2+ as a stable species. The structures of Cu (LH, Bn) ClO4 and Cu (LH, Bn) 2 (O2) (BPh4) 2·8 (CH3) 2CO were determined by X-ray crystallography. Cu (LH, Bn) 2 (O2) 2+ has a trans- (μ-1, 2-peroxo) -dicopper (II) core with a trigonal bipyramidal structure. The O–O bond distance is 1. 450 (5) Å with an intermetallic Cu···Cu separation of 4. 476 (2) Å. The resonance Raman spectrum of Cu (LH, Bn) 2 (O2) 2+ measured at −90 °C in acetone-d6 showed a broad ν (O–O) band at 837–834 cm−1 (788 cm−1 for an 18O labeled sample) and two ν (Cu–O) bands at 556 and 539 cm−1, suggesting the presence of two peroxo species in solution. Cu (LMe, Bn) + also produced both superoxo and trans-μ-1, 2-peroxo species, Cu (LMe, Bn) (O2) + and Cu (LMe, Bn) 2 (O2) 2+. At a lower concentration of Cu (LMe, Bn) + (∼0. 24 mM) and higher dioxygen concentration (P (O2) = ∼1 atm), the superoxo species is predominantly formed, whereas at a higher concentration of Cu (LMe, Bn) + (∼1 mM) and lower dioxygen concentration (P (O2) = ∼0. 02 atm) the formation of the peroxo species is observed. The resonance Raman spectrum of Cu (LMe, Bn) (O2) + (∼1 mM) in acetone-d6 at ∼−95 °C exhibited a ν (O–O) band at 1120 cm−1 (1059 cm−1 for an 18O labeled sample) and that of Cu (LMe, Bn) 2 (O2) 2+ (∼3 mM) in acetone-d6 at ∼−90 °C showed two ν (O–O) bands at 812 and 797 cm−1 (767 and 753 cm−1 for an 18O labeled sample), respectively. A similar observation was also made for Cu (LMe, Me) 2 (O2) 2+. Relationships between the energies of the LMCT and d–d transitions and those of the ν (O–O) and ν (Cu–O) stretching vibrations and the steric constraints in the Cu (II) – (O22−) –Cu (II) core are discussed.