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Theses Doctoral

Main Group Metal Hydride, Alkyl and Fluoride Complexes: Towards CO2 Functionalization with Earth Abundant Metals

Rauch, Michael S.

As levels of carbon dioxide continue to increase in the atmosphere, it is appealing to consider the prospect of utilizing CO2 as a C1 building block for the synthesis of value-added organic chemicals. Such transformations offer potential to directly counteract environmental concerns, and could also enhance the recyclability of current materials. To meet this challenge, the development of metal catalysts capable of promoting the functionalization of carbon dioxide is necessary. Furthermore, there is great interest in employing main group metals for these transformations, particularly those metals that are earth-abundant, non-toxic and affordable. To address these needs and others, the research herein has been driven by the synthesis and characterization of main group metal hydride, alkyl and fluoride complexes with the ultimate aim of developing catalysts for CO2 functionalization.
Chapter 1 investigates the synthesis of magnesium, zinc and calcium complexes supported by the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [TismPriBenz]. Most significantly, the magnesium carbatrane compound, [TismPriBenz]MgH, which possesses a terminal hydride ligand, has been synthesized and structurally characterized. The corresponding magnesium methyl derivative, [TismPriBenz]MgMe, was also prepared, and the reactivity of these compounds with respect to both metathesis and insertion is explored in great detail. The synthesis and characterization of the corresponding zinc hydride complex, [κ3 TismPriBenz]ZnH, is also described, as well as the preparation of a rare example of a monomeric calcium benzyl compound, [TismPriBenz]CaCH2Ph. Some reactivity of the zinc and calcium derivatives is also described.
In Chapter 2, the aforementioned magnesium and zinc compounds and their reactivity towards CO2 is described in detail. Systems comprised of [TismPriBenz]MH (M = Mg, Zn) and tris(pentafluorophenyl)borane are highly effective for the room temperature reduction of CO2 with R3SiH to afford sequentially the bis(silyl)acetal, H2C(OSiR3)2, and CH4 (R3SiH = PhSiH3, Et3SiH and Ph3SiH). Notably, the selectivity of the catalytic system may be controlled by the nature of the silane. Catalytic intermediates were isolated and structurally characterized, including an interesting magnesium formatoborate complex, which has helped elucidate an understanding of the mechanism of the catalysis. Most significantly, it was found that H2C(OSiPh3)2 can be prepared on a multi-gram scale as a crystalline solid and can be converted directly into formaldehyde (CH2O), which is an important industrial chemical. Thus, H2C(OSiPh3)2 can serve as a formaldehyde surrogate and its ability to provide a means to incorporate CH and CH2 moieties into organic molecules is described. Isotopologues of H2C(OSiPh3)2, namely D2C(OSiPh3)2, H213C(OSiPh3)2, and D213C(OSiPh3)2, may be synthesized from the appropriate combinations of (12C/13C)O2 and Ph3Si(H/D), thereby providing a direct and convenient means to use carbon dioxide as a source of isotopic labels in complex organic molecules.
In Chapter 3, details pertaining to other transformations catalyzed by [TismPriBenz]MgR (R = H, Me) are provided and their mechanisms are discussed. Notably, [TismPriBenz]MgR is a catalyst for hydrosilylation and hydroboration of styrene to afford exclusively the Markovnikov products, Ph(Me)C(H)SiH2Ph and Ph(Me)C(H)Bpin; the magnesium alkyl intermediate in the catalytic process, [TismPriBenz]MgCH(Me)Ph, has been isolated and structurally characterized, providing the first structural evidence for the insertion of an olefin into a magnesium hydride bond. Other catalytic transformations are described, including hydroboration of carbodiimides to form N-boryl formamidines and hydroboration of pyridine to provide N-boryl 1,4- and 1,2-dihydropyridines. Additionally, the ability for the magnesium hydride and methyl complexes to catalyze dehydrocoupling reactions is discussed. Finally, the ability for [TismPriBenz]MgMe to catalyze the isomerization of a terminal alkyne is reported.
Chapter 4 outlines the chemistry of magnesium and zinc compounds supported by a different scaffold, namely, the tris(3-tert-butyl-5-methylpyrazolyl)hydroborato, [TpBut,Me], ligand. The magnesium methyl compound, [TpBut,Me]MgMe, was used as a precursor to prepare [TpBut,Me]MgF via metathesis with Me3SnF, and is the first example of a structurally characterized monomeric magnesium fluoride complex. The reactivity of [TpBut,Me]MgF is described, including its ability to serve as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. Corresponding zinc chemistry was studied, including interesting reactivity of [TpBut,Me]ZnH and [TpBut,Me]ZnF. Finally, new heterobimetallic compounds containing magnesium or zinc supported by the [TpBut,Me] ligand and tungsten were synthesized and structurally characterized.


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More About This Work

Academic Units
Thesis Advisors
Parkin, Gerard F.
Ph.D., Columbia University
Published Here
August 30, 2019
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