Main Group Metal Hydride, Alkyl and Fluoride Complexes: Towards CO2 Functionalization with Earth Abundant Metals
Creators: Rauch, Michael S.
Publisher: name: Columbia University
Year: 2019
Subjects: Chemistry
Carbon dioxide
Metal catalysts

description: As levels of carbon dioxide continue to increase in the atmosphere, it is appealing to consider the prospect of utilizing CO₂ as a C1₁ 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, [Tismᴾʳ¹ᴮᵉⁿᶻ]. Most significantly, the magnesium carbatrane compound, [Tismᴾʳ¹ᴮᵉⁿᶻ]MgH, which possesses a terminal hydride ligand, has been synthesized and structurally characterized. The corresponding magnesium methyl derivative, [Tismᴾʳ¹ᴮᵉⁿᶻ]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 Tismᴾʳ¹ᴮᵉⁿᶻ]ZnH, is also described, as well as the preparation of a rare example of a monomeric calcium benzyl compound, [Tismᴾʳ¹ᴮᵉⁿᶻ]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 [Tismᴾʳ¹ᴮᵉⁿᶻ]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 [Tismᴾʳ¹ᴮᵉⁿᶻ]MgR (R = H, Me) are provided and their mechanisms are discussed. Notably, [Tismᴾʳ¹ᴮᵉⁿᶻ]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, [Tismᴾʳ¹ᴮᵉⁿᶻ]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 [Tismᴾʳ¹ᴮᵉⁿᶻ]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.