Activity studies of pentamethylcyclopentadienyl chromium complexes in N2 activation
Date
2021
Authors
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Publisher
University of Delaware
Abstract
The discovery of mild methods for metal mediated cleavage of the dinitrogen triple bond to produce metal nitride derivatives is a fundamental aspect of nitrogen fixation. An important step in this process is to understand and study the factors that affect the ligated dinitrogen triple bond distance. Designing transition metal complexes that can coordinate to the free dinitrogen ligand, elongate the dinitrogen bond length and finally cleave its triple bonds with a low barrier, is of great importance. ☐ Our group has previously worked on this project, using chromium complexes with two different ligand platforms, i.e. 2,4-Pentane-N,N’-bis(aryl or alkyl) ketiminato and (3-R,5-R’pyrazolyl)borate. In an effort to continue that work, this dissertation will describe the synthesis and structural characterization of a new chromium dinitrogen complex, namely [Cp*Cr(dmpe)]2(µ-N)2 (10), and it’s desired cleavage product, i.e. [Cp*(dmpe)Cr≡N] (13), which was prepared by an independent method. 10 can be reduced and oxidized and the effect of these transformations on the N2 ligand was monitored by structural studies. ☐ Upon reduction of compound 10, another dinitrogen compound with the formal Cr oxidation state of 0 was produced, namely [Cp*(dmpe)Cr(µ-N2)Na(TMEDA)2] (21). Unexpectedly, formal reduction of chromium decreased the N-N bond distance from 1.195(4) A° in 10 to 1.175(4) A° in 21. Upon oxidation of 10, a mixed valent (CrI/CrII) dinitrogen compound was formed, namely [(Cp*(dmpe)Cr)2(µ-N2)]BArF4 (25). The N-N bond distance in compound 25 showed an increase to 1.211(4) and 1.200(4) A°. Further oxidation of 10 resulted in a dicationic dinitrogen complex, namely [(Cp*(dmpe)Cr)2(µ-N2)](BArF4)2 (26), which showed an increase in the N-N bond distance compared to the neutral dinitrogen complex 10, similar to the monocationic complex. ☐ The oxidation of 13, yielded the cationic nitride complex, i.e. [Cp*(dmpe)Cr≡N]BArF4 (29). The neutral and cationic dinitrogen and nitride complexes were studied in different experiments including reduction, oxidation, heating and reacting with LA, to see if the interconversion of dinitrogen and nitride complexes can happen. From heating the neutral, monocationic and dicationic dinitrogen complexes, the neutral or the monocationic nitride complex were not produced. Heating the neutral nitride complex, produced a dinuclear chromium complex. In this complex the nitride does an insertion into the Cr-P bond to make a phosphiniminato ligand on the chromium complex, i.e. [Cp*2Cr2(µ,µ-{NP(CH3)2}C2H4] (19). Upon heating the neutral, monocationic and dicationic dinitrogen complexes, the neutral or the cationic nitride complex were not produced and the neutral dinitrogen complex was decomposed to an unidentified mixture at high temperatures. ☐ Due to the lack of experimental evidence for the interconversion between the dinitrogen and nitride isomers, it was decided to do DFT calculations to find out which way the reaction should go. DFT calculations were performed on the neutral and cationic complexes of dinitrogen and nitride to find the thermodynamic product of the interconversion reaction. The same calculations were done for the Cp analog of these complexes to get more information about the effect of Cp* ligand on the interconversion process. Based on the calculations, the dinitrogen complex is the thermodynamic product of the interconversion reaction and the ∆G‡ for the interconversion was calculated to be 56.4 and 47.0 kcal/mol for the Cp*and Cp analog, respectively. These numbers are very large, suggesting an insurmountable intrinsic activation barrier for the formation of the N2 molecule.
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Keywords
Pentamethylcyclopentadienyl chromium, Nitrogen fixation, Ligated dinitrogen triple bond, Transition metal complexes, Chromium dinitrogen complex