Synthesis and characterization of ionic liquids containing copper, manganese, or zinc coordination cations

Abstract

Copper-, manganese-, and zinc-based ionic liquids (Cu{NH₂CH₂CH₂OH}₆[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂ (2), Cu{NH(CH₂CH₂OH)₂}₆[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂ (3A), Cu{NH(CH₂CH₂OH)₂}₆[CF₃SO₃]₂ (3B), Cu{NH(CH₂CH₂OH)₂}₆[(CF₃SO₂)₂N]₂ (3C), Mn{NH(CH₂CH₂OH)₂}₆[CF₃SO₃]₂ (4), and Zn{NH₂CH₂CH₂OH}₆[CF₃SO₃]₂ (5)) are synthesized in a single-step reaction. Infrared data suggest that ethanolamine preferentially coordinates to the metal center through the amine group in 2 and the hydroxyl group in 5. In addition, diethanolamine coordinates through the amine group in 3A, 3C, and 4 and the hydroxyl group in 3B. The compounds are viscous (>1000 cP) at room temperature, but two (3C and 4) display specific conductivities that are reasonably high for ionic liquids (>20 mS cm⁻¹). All of the compounds display a glass transition (T_g) below −50 °C. The cyclic voltammograms (CVs) of 2, 3A, 3B, and 3C display a single quasi-reversible wave associated with Cu(II)/Cu(I) reduction and re-oxidation while 5 shows a wave attributed to Zn(II)/Zn(0) reduction and stripping (re-oxidation). Compound 4 is the first in this new family of transition metal-based ionic liquids (MetILs) to display reversible Mn(II)/Mn(III) oxidation and re-reduction at 50 mV s⁻¹ using a glassy carbon working electrode.