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

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 HAi hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons (HAi = 1.35, −1.62, −2.12 MHz; aiso = −0.80 MHz) and smaller couplings to the fully averaged methyl group protons (HAi = −0.65, 1.658, −0.9 MHz; aiso = 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 HAi. 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 HAi = 3.45, 2.9, 2.62 MHz, aiso = 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 Adip and aiso 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.