X-and Q-band EPR and ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper bis(oxazoline) complexes, based on the (−)-2,2′-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] ligand and bearing different counterions (chloride versus triflate); labelled [Cu II (1a-c)]. The geometry of the two heteroleptic complexes, [Cu II (1a)] and [Cu II (1c)], depended on the choice of counterion. Formation of the homoleptic complex was only evident when the Cu II (OTf

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... 2 salt was used (Cu II (Cl) 2 inhibited the transformation from heteroleptic to homoleptic complexes). The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved 19 F and 1 H couplings confirmed the presence of both coordinated water and TfO − counterions in [Cu (1a) ].