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  Molybdän- und Wolfram-Katalysatoren mit Tripodalen Liganden für die Alkin-Metathese & Isolierung eines homoleptischen Mo(V) Alkoxid Komplexes/Molybdenum and Tungsten Alkylidyne Complexes with a Tripodal Ligand Sphere for Alkyne Metathesis & Isolation of a Homoleptic Non-oxo Mo(V) Alkoxide Complex

Hillenbrand, J. (2021). Molybdän- und Wolfram-Katalysatoren mit Tripodalen Liganden für die Alkin-Metathese & Isolierung eines homoleptischen Mo(V) Alkoxid Komplexes/Molybdenum and Tungsten Alkylidyne Complexes with a Tripodal Ligand Sphere for Alkyne Metathesis & Isolation of a Homoleptic Non-oxo Mo(V) Alkoxide Complex. PhD Thesis, Technische Universität, Dortmund.

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 Creators:
Hillenbrand, Julius1, Author           
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1Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_1445584              

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 Abstract: A new generation of molybdenum alkylidyne catalysts for alkyne metathesis is presented. Previously, well-defined molybdenum alkylidyne complexes with a tripodal ligand framework have been elusive. Therefore, this work marks an important milestone in the field of alkyne metathesis. A newly developed tripodal silanolate ligand and an improved complexation method gave access to these catalysts on scale. Moreover, the modular ligand design allowed us to prepare a library of different catalysts and to investigate the critical parameter for catalysis. A systematic 95Mo NMR study provided valuable insights into the electronic structure of these complexes and turned out to be a very useful tool for catalyst design. Chemical shift tensor analysis of the alkylidyne carbon atom revealed an increased electrophilicity imparted by the tripodal ligand sphere. Benchmarking of the catalytic activity of a series of different catalysts showed that the catalyst with a methyl substituted silanolate ligand is by far the most active catalyst in the entire new series. Isolation of a molybdenacyclobutadiene complex endowed with triphenylsilanolates and extensive investigations on molybdenatetrahedranes with tripodal silanolate ligands proved that both intermediates are accessible and reversibly interconverting. The tripodal ligand sphere generates two distinct molybdenacyclobutadiene forms, which need to pass through a pseudorotation for productive alkyne metathesis. The newly developed catalysts revealed an unrivaled functional group tolerance, tolerate protic functional groups such as unprotected alcohols and even entailed a certain stability towards water. Furthermore, the excellent application profile has been demonstrated on challenging ring-closing alkyne metathesis reactions for polyfunctionalized (natural) products. Therefore, we termed the newly developed molybdenum alkylidyne complexes with a tripodal ligands sphere “canopy catalysts”. In the second part of this work, we extended the coverage to tripodal tungsten alkylidyne complexes and developed a new competent tungsten based catalyst for alkyne metathesis that outperformed the classical Schrock catalyst. Isolation of a mixed tungstenacyclobutadiene complex provided compelling evidence that tripodal silanolate ligands on tungsten result in poor catalytic performance. A combined spectroscopic, crystallographic, and computational study provided insights into the structure and electronic character of tungsten alkylidyne complexes. 183W NMR spectroscopy revealed that tripodal silanolate ligands upregulate the Lewis acidity of tungsten alkylidyne complexes, in that the tungstenacyclobutadiene intermediate gets over-stabilized and catalytic activity is lost. All tungsten alkylidyne complexes with tripodal silanolate ligands as well as several literature-known catalysts gave disappointing results in simple homo-metathesis reactions. Therefore, we developed a more strongly donating tripodal alkoxide ligand, which upon complexation gave the corresponding tungsten complex in quantitative yield on scale. The newly developed catalyst outperformed all other tungsten-based catalysts in terms of activity and selectivity. In the third chapter, we report the isolation of the first monomeric, homoleptic, five-coordinated alkoxide, non-oxo 4d1 Mo(V) complex. High-valent molybdenum complexes endowed with alkoxide ligands are prone to dimerize and/or form molybdenum-oxo-complexes. [Mo(OtBu)5] represents the first example of an entirely new class of high-valent molybdenum complexes. In addition, we isolated a molybdenum nitrido complex endowed with tert-butoxide ligands. Previously, low-valent Mo(III) alkoxide complexes have not been sufficiently shielded to suppress dimerization and their reactivity for small molecule activation is unknown. Intriguingly, the molybdenum nitrido complex is formed by activation of acetonitrile by the low-valent Mo(III) alkoxide complex.

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Language(s): eng - English
 Dates: 2021-04-142021-04-14
 Publication Status: Published in print
 Pages: 273
 Publishing info: Dortmund : Technische Universität
 Table of Contents: -
 Rev. Type: -
 Identifiers: -
 Degree: PhD

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