Mechanistic Investigations of Metal-Metal Cooperativity in Dinickel Complexes and Iron/Cobalt Prussian Blue Analogues

by Hendrik Stevens
Doctoral thesis
Date of Examination:2020-05-15
Date of issue:2020-06-12
Advisor:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Sven Schneider
Persistent Address: http://dx.doi.org/10.53846/goediss-8031

 

 

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Abstract

English

This dissertation focuses on two polymetallic systems exhibiting two different phenomena of Metal-Metal Cooperativity in the form of intramolecular C-H(sp2) activation and Charge-Transfer-Induced Spin Transition (CTIST). The first system, a dinickel complex, utilizes a preorganized ligand scaffold of two β-diketiminato units bridged by pyrazole. The proximity of the two metal centers results in an intramolecular C-H activation of the ligand backbone and a hydride- and phenyl-ligation. This C-H activation was also found to be reversible by the addition of dihydrogen resulting in a nickel dihydride complex. Here, the mechanism of the C-H / H2 activation was investigated, and a Ni(I)Ni(I) intermediate undergoing oxidative addition was hypothesized supported by trapping experiments with deuterium gas and benzaldehyde. Furthermore, an effect of the alkali metal towards reactivity was investigated, wherein a second C-H activation going through a σ-bond metathesis was observed. Based on the knowledge about the system the reactivities towards other small molecules, such as N2, O2, CO and CO2 were investigated. Molecular Iron/Cobalt Prussian Analogues (Fe/Co PBAs) can exhibit an electron-transfer-coupled spin transition between a diamagnetic and paramagnetic state upon irradiation of light. Therefore, the second part of this dissertation focuses on the development of new dinuclear Fe/Co PBAs bearing tetrapyrazolyl lutidine ligands. A thermal-induced CTIST was not observed for the synthesized complexes and the paramagnetic state prevailed over the whole temperature range as concluded by magnetic susceptibility measurements. To investigate the mechanism of a photo-induced CTIST one of the dinuclear system and a literature-known tetranuclear benchmark system was investigated by time-resolved spectroscopy. Through time-trace analysis, two different mechanisms dependent on the magnetic ground state were hypothesized, with only one of them revolving around a spin-transition.
Keywords: Metal-Metal Cooperativity; Nickel Hydride Complexes; C-H Activation; Low-Valent Metal Centers; Preorganized Ligand Scaffold; Iron/Cobalt Prussian Blue Analogues; Charge-Transfer-Induced Spin Transition; Transient Absorption Spectroscopy
 
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