Abstract
ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)-copper(ii) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and (Cu)A values were affected by traces of water present in the solvent. The ligand (H)Ai hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons ((H)Ai = 1.35, -1.62, -2.12 MHz; a(iso) = -0.80 MHz) and smaller couplings to the fully averaged methyl group protons ((H)Ai = -0.65, 1.658, -0.9 MHz; a(iso) = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol(-1) by DFT, rapid rotation of these -CH3 groups, even at 10 K, leads to an averaged value of (H)Ai. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters (H)Ai = 3.45, 2.9, 2.62 MHz, a(iso) = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large A(dip) and a(iso) component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.
Highlights
The bis(acetylacetonato)-copper(II) complex [Cu(acac)2] is one of the most extensively studied d-transition metal compounds by Electron Paramagnetic Resonance (EPR) spectroscopy (Scheme 1).[1,2,3,4,5,6,7,8,9,10,11,12,13,14] This can largely be attributed to its relative simplicity, ease of preparation and favourable stability constant, which collectively facilitate the investigation of this complex in single crystal,[1,2,3,4] frozen solution[5,6,7,8,9,10,11,12] and powder (doped Cu–Pd solid solution)[13] forms
The complex has been characterised in detail by continuous wave (CW) Electron Paramagnetic Resonance (EPR), so that both the local geometry and electronic structure of the paramagnetic Cu(II) centre are well defined
Using complementary density functional theory (DFT) calculations, we have investigated the anisotropic hyperfine couplings to the ligand protons, and used this information to explore in detail the influence of both freely and hindered rotating methyl groups on the resulting frozen solution ENDOR spectrum
Summary
The bis(acetylacetonato)-copper(II) complex [Cu(acac)2] is one of the most extensively studied d-transition metal compounds by Electron Paramagnetic Resonance (EPR) spectroscopy (Scheme 1).[1,2,3,4,5,6,7,8,9,10,11,12,13,14] This can largely be attributed to its relative simplicity, ease of preparation and favourable stability constant, which collectively facilitate the investigation of this complex in single crystal,[1,2,3,4] frozen solution[5,6,7,8,9,10,11,12] and powder (doped Cu–Pd solid solution)[13] forms. Using complementary DFT calculations, we have investigated the anisotropic hyperfine couplings to the ligand protons, and used this information to explore in detail the influence of both freely and hindered rotating methyl groups on the resulting frozen solution ENDOR spectrum.
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