Three-Coordinate Copper(II)−Phenolate Complexes

The reactions of LCuCl (L = 2,4-bis((2,6-diisopropylphenyl)imido)pentane (LiPr), 2,4-bis((2,6-diisopropylphenyl)imido)-3-chloropentane (LCliPr)) with the phenolates TlOAr (Ar = C6H3Me2, C6H4OMe, C6H4tBu) and NaOC6H3(tBu)2 were explored. Novel three-coordinate Cu(II)−phenolates, LCuOAr, were isolated from the reactions with
the thallium phenolates and were characterized by X-ray crystallography and spectroscopy (UV−vis, EPR). The
complexes feature short Cu−O(phenolate) distances (average Cu−O = 1.81 Å) and, with one exception, irregular
N−Cu−O(phenolate) angles that differ within each compound (15° < Δ < 28°, where Δ = ∠N1−Cu−O −
∠N2−Cu−O). The exception is LiPrCu(OC6H4tBu), for which X-ray structures at −100 and 25 °C differed due
to an unusual reversible phase change with nonmerohedral twinning (2:1 ratio) in the low-temperature form. The
high-temperature form has local C2v symmetry (Δ = 0°), and upon cooling below the phase transition temperature
(−8 ± 5 °C) lateral movement of the phenolate ligand (Δ = 17.6°) and rotation of the phenolate plane by 10.7°
occurs. Resonance Raman spectroscopic data acquired for LiPrCu(OC6H4tBu) corroborated assignment of phenolate
→ Cu(II) LMCT character in the UV−vis spectra. Cyclic voltammetry experiments (THF, 0.5 M NBu4PF6) revealed
negative E1/2 values for the Cu(II)/Cu(I) couples relative to NHE, consistent with enhanced stabilization of the
Cu(II) state by both the strongly electron donating β-diketiminate ligand and the phenolates. Although thermally
stable, the Cu(II)−phenolates are unusually reactive with dioxygen, albeit to give product(s) that have yet to be
identified. In the reaction of LiPrCuCl with NaOC6H3(tBu)2 no Cu(II)−phenolate was observed. Instead, a Cu(I)
complex was generated quantitatively by trapping with added isocyanide, [LiPrCuNC(C6H3Me2)], along with
3,3‘,5,5‘-tetra-tert-butyl-4,4‘-dibenzoquinone and 2,6-di-tert-butylphenol in 27 ± 3% and 46 ± 6% yields,
respectively, corresponding to the overall reaction 4LiPrCuIICl + 4NaOAr → 4LiPrCu(I) + 4NaCl + dibenzoquinone
+ 2(phenol).