Bis(μ-oxo) dicopper(III) species of the simplest peralkylated diamine: enhanced reactivity toward exogenous substrates

Abstract

N,N,N',N'-tetramethylethylenediamine (TMED), the simplest and most extensively used peralkylated diamine ligand, is conspicuously absent from those known to form a bis(μ-oxo)dicopper(III) (O) species, [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+), upon oxygenation of its Cu(I) complex. Presented here is the characterization of this O species and its reactivity toward exogenous substrates. Its formation is complicated both by the instability of the [(TMED)Cu(I)](1+) precursor and by competitive formation of a presumed mixed-valent trinuclear [(TMED)(3)Cu(III)Cu(II)(2)(μ(3)-O)(2)](3+) (T) species. Under most reaction conditions, the T species dominates, yet, the O species can be formed preferentially (>80%) upon oxygenation of acetone solutions, if the copper concentration is low (<2 mM) and [(TMED)Cu(I)](1+) is prepared immediately before use. The experimental data of this simplest O species provide a benchmark by which to evaluate density functional theory (DFT) computational methods for geometry optimization and spectroscopic predictions. The enhanced thermal stability of [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+) and its limited steric demands compared to other O species allows more efficient oxidation of exogenous substrates, including benzyl alcohol to benzaldehyde (80% yield), highlighting the importance of ligand structure to not only enhance the oxidant stability but also maintain accessibility to the nascent metal/O(2) oxidant.

Figure 1

UV-Vis spectra of the TMED O species (solid, [Cu] = 1 mM, CF₃SO₃⁻, acetone) and proposed TMED T species (dash, [Cu] = 1 mM, CF₃SO₃⁻, THF) at 183 K.

Figure 2

Resonance Raman spectra of [(TMED)₂Cu(III)₂O₂](CF₃SO₃)₂ species (λ_ex=413.1 nm, 10% CH₂Cl₂/90% acetone, 77 K, [Cu] = 2 mM) oxygenated with ¹⁶O₂ (solid line) or ¹⁸O₂ (dash line). Solvent peaks are labeled with an * mark, plasma line is labeled with a “p”.

Figure 3

Normalized Cu K-edge absorption spectrum of [(TMED)₂Cu(III)₂O₂](SbF₆)₂ in acetone (5% CH₂Cl₂, [Cu] = 1 mM, 10 K). The inset shows the magnified pre-edge region (solid) and its smoothed second derivative (dashed).

Figure 4

(a) Cu K-edge k³-weighted EXAFS data for [(TMED)₂Cu(III)₂O₂](SbF₆)₂ (top) with offset fit residual (bottom) and (b) corresponding non-phase shift corrected Fourier transforms. Data (·····); fit (—).

Figure 5

(a) DFT optimized D₂ geometry of [(TMED)₂Cu(III)₂O₂]²⁺ with B3LYP/6-31G(d) method (Hydrogen atoms are omitted for clarity): Cu-O = 1.79 Å, Cu-N = 1.95 Å, Cu(···Cu = 2.72 Å, O(···O = 2.31 Å. (b) Scheme of two limiting high-symmetry conformations.

Figure 6

Oscillator strength and simulated electronic spectra of [(TMED)₂Cu(III)₂O₂]²⁺ from DFT calculation with PCM-TD-B3LYP/BS1//B3LYP/6-31G(d) method (half-bandwidth = 2500 cm⁻¹).

Scheme 1

Peralkylated diamine ligands (PDLs).

Scheme 2

Proposed mechanism for formation of O and T species (L = ligand).

Scheme 3

Proposed N–dealkylation decomposition mechanism of [(TMED)₂Cu(III)₂O₂]²⁺ species.