Copper ionic liquids: Examining the role of the anion in determining physical and electrochemical properties

Five new copper(II)-based ionic liquids and three crystalline compounds have been synthesized in order to further understand the role of the anion in determining their physicochemical properties. Materials were prepared with combinations of three different anions (2-ethylhexanoate (EHN), tetrafluoroborate (BF4), and triflate (OTf)) and six-coordinate Cu(II) cations. The complexes that contain at least one BF4 anion consistently displayed both the highest specific conductivity and electrochemical reversibility. The presence of one OTf anion (in combination with one EHN or BF4) facilitates the formation of crystalline materials. Single crystal X-ray diffraction studies were completed on two of the mixed anion compounds. The results show that both ethanolamine (EA) and diethanolamine (DEA) chelate to the Cu(II) centers in a highly distorted tetragonal geometry. The properties of the ionic liquid with the overall lowest viscosity and highest conductivity and electrochemical reversibility (Cu{NH2CH2CH2OH}6(BF4)2, 4) were studied as a function of temperature. The viscosity decreases with increasing temperature, but at 45 °C (over three hours) there is a loss of two EA ligands to form a more viscous four-coordinate complex. Cyclic voltammetry of 4 reveals a quasi-reversible Cu(II)/Cu(I) reduction wave that shifts to more positive potentials with increasing temperature.