Influence of counterions on the structure of bis(oxazoline)copper(ii) complexes; an EPR and ENDOR investigation

Abstract

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)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 ¹⁹F and ¹H couplings confirmed the presence of both coordinated water and TfO⁻ counterions in [Cu(1a)].