Quinones and bimetallic complexes: Towards new carriers for electrochemically modulated complexation of carbon dioxide.

摘要

Properly designed redox active molecules can be used to concentrate carbon dioxide by binding CO2 from a dilute gas mixture to the carrier molecule in one oxidation state. When the oxidation state of the carrier is changed, the binding affinity for CO2 is decreased, and the gas is released. Several quinone dianions were investigated to determine their air stability and their binding constants with carbon dioxide. Quinones with more negative redox potentials showed higher carbon dioxide binding constants. However they also were more sensitive to oxygen. Therefore, a quinone with an intermediate redox potential, 2,3 dicyanobenzoquinone, was chosen for study. This quinone was found to be stable to a 5% O2/10% CO2 gas mixture in nitrogen. In a batch experiment, carbon dioxide was concentrated from 10% to 91% from a mixture with nitrogen by cycling the 2,3 dicyanobenzoquinone between its reduced high-affinity form and its oxidized low-affinity form. However, this quinone decomposes upon oxidation, so the cycle cannot be repeated.; Macrocyclic nickel and copper complexes of the ligands DMC (5,7-dimethyl-1,4,8,11-tetraazacyclotetradecane) and o-xyl-DMC, (alpha, alpha'-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)- o-xylene) were also investigated for their ability to concentrate carbon dioxide. The oxidized (Ni(II) or Cu(II)) complexes react with the carbonate or bicarbonate ions that form when carbon dioxide dissolves in water. Therefore, these complexes have the advantage that the high-affinity form is air stable. Binding constants for carbonate ion were determined for [Ni2( o-xyl-DMC2)]4+. However, the reduction potential for [Ni2(o-xyl-DMC2)(mu-CO 3)]2+ is so negative that it cannot be reduced by controlled potential electrolysis. A chemical reduction with sodium amalgam did indicate that carbonate is released upon reduction of Ni(DMC)2(CO3 )2+ to the Ni(I) complex.; In an unrelated project, the dinuclear molybdenum complex, (MeCpMomu-S) 2S2CH2 was reacted with 2,4 hexadiyne or 1,4 diphenylbutadiyne. The alkynes reacted with the bridging sulfido ligands to form tetranuclear complexes linked by bis(alkenedithiolate) ligands. The cyclic voltammograms of these complexes showed splitting of the MoIII-MoIII/MoIII-IV oxidation wave, indicating electronic communication between the two dinuclear units in the tetranuclear complexes. This is the first example of such communication associated with a bis-dithiolene ligand.