Rhenium and Osmium PNP Pincer Complexes for Nitrogen Fixation and Nitride Transfer

by Florian Wätjen
Doctoral thesis
Date of Examination:2019-09-27
Date of issue:2019-12-19
Advisor:Prof. Dr Sven Schneider
Referee:Prof. Dr. Sven Schneider
Referee:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Marinella Mazzanti
Persistent Address: http://dx.doi.org/10.53846/goediss-7777

 

 

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Abstract

English

Activation, splitting and fixation of dinitrogen remains one of the most challenging reactions in coordination chemistry. In this thesis, osmium and rhenium complexes based on a PNP pincer ligand are synthesized and investigated in the overall context of dinitrogen fixation. The synthesis of a rare, low-valent Os(IV) nitrido complex in a square-planar coordination environment is described. It is shown to exhibit nitride centered ambiphilic reactivity which is further elucidated by DFT computations. Moreover, the potential of the osmium PNP pincer platform towards dinitrogen fixation is investigated. In the case of rhenium, the previously reported splitting of dinitrogen into terminal nitride complexes is investigated and a key intermediate, i.e. a temperature labile N2-bridged dimer is characterized by NMR and X-ray diffraction. Furthermore, the influence of ligand modifications is investigated. Especially the change of the pincer substituents from tert-butyl to iso-propyl is shown to have a tremendous effect on the platform's reactivity. A starting Re(III) platform with an unique electronic structure is obtained, which is analyzed by means of multireference calculations. Reduction under dinitrogen atmosphere yields a temperature stable N2 bridged dimer. In a rare example of N2 photolysis, visible light irradiation results in almost quantitative conversion into terminal Re(V) nitrido complexes. Upon reaction with benzoylchloride, the nitrogen atom can be incorporated into benzonitrile and benzamide in a metal-ligand cooperative proton-coupled electron transfer step. Finally, upon electrochemical reduction in the presence of a weak acid, the N2 bridged dimer can be regenerated. Thus, a synthetic cycle for N2 fixation is achieved utilizing visible light photolysis as well as electrolysis.
Keywords: Coordination Chemistry; Dinitrogen Fixation; Photochemistry; Low Valent Nitrido Complexes; Osmium Pincer Complexes; Rhenium Pincer Complexes; Proton Coupled Electron Transfer
 
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