Structural characterisation of Cu(II) complexes of biological relevance: an EPR and ENDOR investigation.

ENDOR spectroscopy and DFT calculations were used for the first time to thoroughly investigate the ligand hyperfine couplings for [Cu(acac)2] in frozen solution. 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, 1.66, -0.90 MHz; aiso = 0.04 MHz) were identified by simulation of the angular selective CW Q-band ENDOR spectra and confirmed by DFT. Variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. These additional couplings were characterised by HAi = 3.45, 2.90, 2.62 MHz, aiso = 2.99 MHz and assigned to a sub-set of methyl groups undergoing hindered rotation in frozen solution. From DFT calculations the hindered rotation is proposed to occur in 120° jumps via proton tunnelling. Changes in the spin Hamiltonian parameters (g/CuA) of [Cu(acac)2] upon addition of nitrogen bases evidenced the formation of adducts. Pyridine bases were found to form square pyramidal, mono-axial adducts, labelled [Cu(acac)2(X)], even in a large molar excess of base, whilst both mono-axial [Cu(acac)2Im] and bis-equatorial [Cu(acac)2Im2] adducts were formed by imidazole coordination depending on the molar equivalents of imidazole present in solution. Angular selective 1H ENDOR studies confirmed that the acetylacetonato ligand protons were largely unaltered upon adduct formation. The proton hyperfine tensors for the coordinated substrates were found to be informative on the mode of coordination, confirming the axial coordination of the pyridine bases. Furthermore, discrimination between endocyclic and exocyclic coordination of the 2-aminomethylpyridine bases was achieved. A cis-mixed plane geometry of the [Cu(acac)2Im2] adduct was observed in the 10K ENDOR spectrum and supported by DFT calculations. This was in contrast to the bis-equatorial configuration evident in the 140K EPR, implying a temperature dependant isomerisation. Finally, a series of square planar Casiopeina copper complexes of general formula [Cu(acac)(N-N)], consisting of an acetylacetonato ligand and a diimine ligand (N-N) of varying size, were prepared and characterised by EPR and ENDOR spectroscopy. Angular selective 1H ENDOR spectra revealed the magnitude of the largest imine hyperfine component, observed at g, to be sensitive to size of the diimine ligand. This is thought to be related to the extent of spin delocalisation in the diimine ligand