Preorganized Bimetallic Nickel Complexes of Pyrazolate-Bridged Ligands for Cooperative Substrate Transformation
von Dennis-Helmut Manz
Datum der mündl. Prüfung:2016-10-19
Betreuer:Prof. Dr. Franc Meyer
Gutachter:Prof. Dr. Franc Meyer
Gutachter:Prof. Dr. Sven Schneider
EnglischThis thesis focuses on square-planar, pyrazolate-bridged dinuclear platforms to investigate their reactivity towards small molecules. In the scope of this work two ligands were prepared: a pyrrole-pyrazole based (H3Lpyr) and a bis-β-diketiminate-pyrazole based system (H3Lbisnac). Both of these hexadentate, trianionic ligands provide two binding pockets to accommodate transition metals. Use of nickel-salts gave rise to a variety of preorganized dinuclear nickel(II) complexes containing a small exogenous ligand serving as a bridging unit between the two metal cations. Each of these complexes showed remarkable properties which were investigated in depth: 1) An extremely slow proton and water exchange of a metal-bridging hydroxide. The unusual exchange was elucidated via NMR spectroscopic studies using isotopic labeling (D2O, H217O). With the obtained kinetic and thermodynamic parameters a mechanistic picture could be derived, which was further supported by density functional theory (DFT) calculations. 2) A dinuclear nickel dihydride complex which showed an unusual H-D exchange behavior in a D2 atmosphere. In the process the deuterated dihydride complex was formed. However, it was shown that exclusively H2 was released during the exchange, indicating that the initial D2 was not scrambled to HD. In addition to NMR spectroscopic studies, this was further supported by DFT calculations which provided insight into the mechanistic nature of this unusual reaction. Another feature of the dihydride compound is its reactivity towards water which gave rise to a hydrido-hydroxo complex. This complex eventually transforms to a hydroxo-bridged dinickel complex via release of H2 and further reaction with water. 3) A phenylvinyl-bridged complex which was derived from a conversion of the dihydride complex with phenylacetylene. Isotopic labeling and NMR spectroscopic investigations revealed the mechanism to be similar to the reaction of the dihydride with D2 in which the initial hydrides are released as H2. It was shown that the phenylvinyl-bridged complex acts as a catalyst in the semihydrogenation of phenylacetylene to styrene under a H2 atmosphere. The process exclusively led to the cis-hydrogenated product which was proven by NMR spectroscopic experiments including isotopic labeling and parahydrogen induced polarization (PHIP). 4) A series of model complexes of intermediates in the process of nitrogen fixation including the binding of N2 and the first step of its reduction. In particular, dinickel complexes with the bridging ligands N2-, N2H-, N2H22-, N2H3-, N2H4 and NH2- were prepared and fully characterized. Inspired by the proposed binding process of N2 to FeMo-co in nature, the coordination of N2 was accomplished from the dihydride complex concomitant with the release of H2. X-ray crystallography, 15N isotopic labeling and IR- and Raman spectroscopic studies revealed the N2- to be bridged between the two nickel atoms in a µ-η1:η1-fashion.
Keywords: Nitrogen binding; Nitrogen activation; Nitrogen fixation; Water exchange; Nacnac; Pyrazole; Nickel; Dinuclear complex; Exchange kinetics; PHIP; Parahydrogen; Phenylacetylene; Semihydrogenation; Styrene; H-D exchange; Hydroxo exchange; Slow exchange; Dihydride; Hydroxo-bridged; Nitrogen-bridged