| The reactivities of a series of bimetallic copper(II) and nickel(II) complexes with carbonate ion have been studied in buffered aqueous solution, using the ligands taec (N, N', N″, N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane) and tpmc (N, N', N″, N‴ -tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane). The binding constants with carbonate and changes in reduction potentials resulting from carbonate binding were determined. X-ray diffraction studies were carried out for Cu2(mu-CO3)(taec)(BPh4)2 and Cu2(mu-CO3)(tpmc)(BPh4)2, and the bridging carbonate in each of these structures was found to have an eta 1-eta1 coordination mode. The stabilities of the copper(I) complexes were tested in aqueous solution, and the affinity for carbonate in the reduced, copper(I) oxidation state appeared to be very low. In addition to having a high binding constant for carbonate, Cu2(tpmc)(NO 3)4 was determined to be air stable in the copper(II) oxidation state, reducible in aqueous solution, and stable in aqueous solution in the reduced form. In a batch experiment to show proof of concept, carbon dioxide was concentrated from 10% to 75% by using a buffered aqueous solution of Cu 2(tpmc)(NO3)4 to bind and release carbonate by cycling between the high affinity, copper(II) oxidation state and the low affinity, copper(I) oxidation state. |
