From Boron to Nitrogen Based Pincer Complexes: Bonding and Reactivity Patterns

The aim of this work was the investigation of novel tridentate boron-based ligands and their complexes. The focus was laid on understanding their formation, bonding situation, intramolecular exchange processes, reactivity towards other ancillary ligands, acids and bases and potential as catalysts fo...

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Main Author: Vondung, Lisa
Contributors: Langer, Robert (Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2017
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Summary:The aim of this work was the investigation of novel tridentate boron-based ligands and their complexes. The focus was laid on understanding their formation, bonding situation, intramolecular exchange processes, reactivity towards other ancillary ligands, acids and bases and potential as catalysts for homogeneous dehydrogenation and C-H activation reactions. Furthermore, the effects of varying the metal, the central donor-group of the ligand and the ancillary ligands was to be evaluated. The second task of this dissertation was the investigation of the influence of relativistic eff ects on the proton affinities of CNC-pincer complexes of the coinage metals. In order to answer these questions, first, a novel iron PBP-pincer complex was investigated. It was shown that the novel formation sequence proceeds via a phosphine-borane complex, dehydrocoupling to a phoshpine-boryl complex and subsequent P-B bond formation and B-H activation to form the PBP-pincer complex. The iron-boron bond was characterized as a donor-acceptor bond from boron to iron, making the central donor group the first phosphine-stabilized borylene which has been characterized and reported. The exchange mechanism between the Fe-H and B-H was shown to proceed formally via a reversible reductive elimination/oxidative addition of the Fe-H and B-H. The (de)protonation of the amines in the ligand backbone infl uences the barrier and rate of this exchange process signifi cantly. A was also shown to be active in catalytic dehydrogenation of benzyl alcohol. Attempts to directly synthesize the PBP ligand led to the formation of a novel iron-hydride complex with a tridentate bis(phosphino)borate ligand. The central (R3P)2BH2-group exhibits ambireactive behavior, in that it undergoes different bond activation processes depending on the temperature: Below 4 °C, intra- and intermolecular C-H activation reactions were observed, while above 4 °C, B-H activation and P-B bond cleavage occur. These processes were characterized by temperature dependent 1D- and 2D NMR spectroscopy, deuteration experiments and quantum chemical calculations. Furthermore, this complex was also shown to be highly active in the catalytic H/D exchange of deuterated solvents. The second synthetic approach to the PBP-ligand proved successful and a novel palladium complex with the PBP pincer ligand was synthesized. Quantum chemical calculations confirmed that the boron moiety can also be described as phosphine-stabilized borylene. Comparison with palladium complexes that contain tricoordinate boron based X- and Z-type ligands showed, that a clear distinction between these three ligand classes is possible. Next, the variation of different aspects in the iron PBP-pincer complex were investigated. First, the influence of different ancillary ligands other than CO on the complex formation was tested. It was found that, for tert-butyl isonitrile and cyanide, in principle the same intermediates as for carbon monoxide can be observed. The stability of these intermediates, especially the hapto-1-phosphine-borane complex, however, is strongly influenced by the ancillary ligands. They also determine, where the first H2-elimination takes place and thus, if the PBP-pincer complex is formed, or if a different phosphine-borane complex and a hydrido complex are the products. After clarification of the influence of the ancillary ligand, the variation of the metal atom from iron to ruthenium, osmium, manganese and cobalt and at the same time variation of the central donor moiety from BH to AlH, CH and C was investigated. In order to ascertain if those complexes should be accessible and how the bonding situation can be described in each of these complexes, quantum chemical calculations were performed. Variation of the metal within group 8 did not lead to any changes in the bonding situation and all corresponding complexes with BH, CH and C should be stable. For manganese, the same applies, while the higher oxidation state of the corresponding d6 cobalt atom leads to an instable complex with the positively charged CH-based ligand. Within the donor groups, it was found that AlH is not suffi ciently stabilized by the phosphines and thus the corresponding complexes are rather unstable, which has also been observed experimentally. The complexes with a CDP-based donor groups on the other hand should be accessible and can be described as L-type ligands as expected. For CH as a donor group, a similar intramolecular exchange-pathway as observed for Fe-BH should be possible, while for the AlH based complexes no suitable transition states could be found. Isolation of the CDP iron complex might not be possible, as the CH-Fe(0) isomer resulting from reductive proton transfer lies lower in energy than the CDP-Fe(II) complex. In the second part of this work, the investigations on unusual reactivities in transition metal pincer complexes were extended to a series of copper, silver, and gold complexes with a carbene-/carbazole-based CNC-pincer ligand. Experimentally, a similar gold com plex had already been isolated by another group. This complex showed a rather unusual behavior, as it was protonated at the gold(I) atom and not at one of the electron-rich nitrogen atoms. High-level quantum chemical calculations with and without inclusion of relativistic eff ects showed that for the copper and silver complexes, protonation should occur at the nitrogen atom, as expected. In the gold complex, however, relativistic effects infl uence its charge, population and electron distribution enough to raise the proton affinity of the gold site to approximately the same value as the carbazole nitrogen. Experimentally, it had been observed that those two protonation sites are both accessible and through the reversibility of the reaction, eventually all complexes are protonated at the gold atom.
Physical Description:228 Pages