Expanding M(cyclam) chemistry; cobalt acetylide complexes for charge delocalization, and functionalized nickel complexes for CO2 reduction
Described in this work are the preparation and characterization of an extensive family of CoIII(cyclam)-oligoynyl compounds (cyclam = 1,4,8,11,tetraazacycloctetradecane), and elucidation of their electronic structures through DFT calculations. Monomeric Co compounds bearing one oligoynyl, namely [Co(cyclam)(C2nR)Cl] + with n = 1 - 3, and two butadiynyls [Co(cyclam)(C 4H)2]+ were prepared from the reactions between [Co(cyclam)Cl2]Cl, Et3N or Et2NH, and the corresponding alkynes. The oligoyndiyl-bridged (μ-C2 m) dimers of CoIII(cyclam)Cl with m = 2 and 3 were prepared via the same process by varying alkyne stoichiometry, while those with m = 4 and 6 were prepared using the Glaser coupling reaction. The complexes were prepared in moderate yield (with the exception of m = 6) under mild conditions without requiring anaerobic or anhydrous environment, and are generally stable towards ambient atmosphere. Bis-acetylides complexes, both dimeric and monomeric, were also synthesized through the use of lithium acetylide reagents to compare their characteristics with the analogous mono-acetylides. Voltammetric analysis of the dimeric complexes revealed weak Co-Co interaction through the bridge, which is attenuated by the length of oligoyne. The orbital origin of Co-Co interaction is rationalized through DFT analysis. Additionally, several nickel(II) complexes of cyclams bearing aryl groups on the carbon backbone were prepared and evaluated for their propensity to catalyze electrochemical reduction of CO2 to CO, and/or H+ to H2, representing the first catalytic analysis to be performed on an aryl-cyclam metal complex. Cyclic voltammetry revealed an attenuation of catalytic activity when the aryl group bears the strong electron-withdrawing trifluoromethyl substituent, whereas the phenyl, p-tolyl, and aryl-free derivatives displayed a range of catalytic activity. The gaseous-product distribution for the active complexes was determined by means of controlled-potential electrolysis, and revealed that the phenyl derivative is the most active as well as the most selective, and surpasses even Ni(cyclam) in its activity for CO production from CO2 despite operating at similar overpotential. Stark differences in activity of the complexes studied are rationalized through comparison of their X-ray structures, absorption spectra, and controlled-potential electrolysis profiles.
Ren, Purdue University.
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