An ENDOR and DFT analysis of hindered methyl group rotations in frozen solutions of bis(acetylacetonato)-copper(ii)

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 ^CuA values were affected by traces of water present in the solvent. The ligand ^HA_i hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons (^HA_i = 1.35, −1.62, −2.12 MHz; a_iso = −0.80 MHz) and smaller couplings to the fully averaged methyl group protons (^HA_i = −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 ^HA_i. 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 ^HA_i = 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.