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 (L-H,L-Bn), tris(N-benzyl-N-methylaminoethyl)amine (L-Me,L-Bn), or tris(N,N-dimethylaminoethyl)amine (L-Me,L-Me). All the copper(I) complexes reacted with dioxygen at low temperatures to produce superoxocopper(II) and/ or trans-(mu-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(L-H,L-Bn)](+) at -90 degreesC in acetone resulted in the formation of a superoxo complex [Cu(L-H,L-Bn)(O-2)](+) as a less stable species and a peroxo complex [{Cu(L-H,L-Bn))(2)(O-2)](2+) as a stable species. The structures of [Cu(L-H,L-Bn)]ClO4 and [(Cu(L-H,L-Bn)}(2)(O-2)](BPh4)(2).8(CH3)(2)CO were determined by X-ray crystallography. [[Cu(L-H,L-Bn))(2)(O-2)](2+) has a trans-(mu-1,2-peroxo)-dicopper(II) core with a trigonal bipyramidal structure. The O-O bond distance is 1.450(5) Angstrom with an intermetallic Cu...Cu separation of 4.476(2) Angstrom. The resonance Raman spectrum of [{Cu(L-H,L-Bn)](2)(O-2)](2+) measured at -90 degreesC in acetone-d(6) showed a broad v(O-O) band at 837-834 cm(-1) (788 cm(-1) for an O-18 labeled sample) and two v(Cu-O) bands at 556 and 539 cm(-1), suggesting the presence of two peroxo species in solution. [Cu(L-Me,L-Bn)](+) also produced both superoxo and trans-mu-1,2-peroxo species, [Cu(L-Me,L-Bn)(O-2)](+) and [{Cu(L-Me,L-Bn))(2)(O-2)](2+). At a lower concentration of [Cu(L-Me,L-Bn)](+) (similar to0.24 mM) and higher dioxygen concentration (P(O-2) = similar to1 atm), the superoxo species is predominantly formed, whereas at a higher concentration of [Cu(L-Me,L-Bn)](+) (similar to1 mM) and lower dioxygen concentration (P(O-2) = similar to0.02 atm) the formation of the peroxo species is observed. The resonance Raman spectrum of [Cu(L-Me,L-Bn)(O-2)](+) (similar to1 mM) in acetone-d(6) at similar to-95 degreesC exhibited a v(O-O) band at 1120 cm(-1) (1059 cm(-1) for an O-18 labeled sample) and that of [(Cu(L-Me,L-Bn)](2)(O-2)](2+) (similar to3 mM) in acetone-d(6) at similar to-90 degreesC showed two v(O-O) bands at 812 and 797 cm(-1) (767 and 753 cm(-1) for an O-18 labeled sample), respectively. A similar observation was also made for [[Cu(L-Me,L-Me)}(2)(O-2)](2+). Relationships between the energies of the LMCT and d-d transitions and those of the v(O-O) and v(Cu-O) stretching vibrations and the steric constraints in the Cu(II)(O-2(2-))-Cu(II) core are discussed.