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The Synthesis and Structural Characterization of Main Group and Lanthanide Metal Compounds Supported by the Multidentate [N₃C] Donor Ligand tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl, [TismPriBenz]M

The Parkin group has recently synthesized tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methane, [TismPriBenz]H, a bulky tetradentate tripodal ligand, which upon deprotonation can coordinated to form a variety of carbatrane metal complexes.The [TismPriBenz] ligand has been previously shown to stabilize metal hydride complexes, for example [TismPriBenz]MgH and [TismPriBenz]ZnH, and the ligand also has been incorporated in complexes featuring all of the non-radioactive Group 12 and Group 13 metals, as well as a large range of transition metals. However, the reactivity of these complexes towards carbonyl compounds is largely unexplored. Additionally, beyond [TismPriBenz]Li, there has been no attempt to introduce the heavier alkali metals into the [TismPriBenz] framework, which could potentially provide more reactive starting materials to generate other previously inaccessible metal complexes of the ligand; for instance, prior to this report, there has been no example of a lanthanide complex of [TismPriBenz].

In Chapter 1, the reactivity of [TismPriBenz]MgH and [TismPriBenz]MgMe towards ketones, aldehydes, and esters is explored. Generally, these magnesium complexes are able to insert a C=O double bond into a Mg–Me or Mg–H bond respectively, providing access to a large class of magnesium alkoxides. Specifically, [TismPriBenz]MgR (R = H, Me) can insert benzaldehyde and benzophenone to give [TismPriBenz]MgOCRHPh or[TismPriBenz]MgOCRPh₂ respectively. Additionally, [TismPriBenz]MgMe has shown rare reactivity towards methyl ketones, in that it forms the magnesium enolate compounds [TismPriBenz]MgC(Me)=CH₂ and [TismPriBenz]MgC(Ph)=CH2 upon treatment with acetone or acetophenone. In fact, [TismPriBenz]MgC(Me)=CH₂ is only the fourth acetone enolate complex to be structurally characterized, and the first such magnesium example.

In the presence of the ester compounds methyl formate and ethyl acetate, [TismPriBenz]MgH and [TismPriBenz]MgMe are able to follow the insertion of the carbonyl with immediate elimination of either an aldehyde or ketone to yield the simple alkoxides [TismPriBenz]MgOMe and [TismPriBenz]MgOEt, a reaction with little precedence in the literature. [TismPriBenz]MgMe is also able to prompt the Claisen condensation of ethyl acetate, forming the first [TismPriBenz] complex with a 6-member chelating ring, [TismPriBenz]Mg(κ²-OC(Me)HC(O)OEt). These various alkoxides have demonstrated the ability to catalyze the Tishchenko reaction, the dimerization of an aldehyde to make an ester, and have also shown promise as catalysts for hydroboration and retro-aldol reactions. Lastly, the [TismPriBenz]Mg compounds have shown interesting reactivity towards O₂, leading to the isolation of both the rare peroxide dimer {[TismPriBenz]Mg}₂(μ-O₂) and the alkyl peroxide [TismPriBenz]MgOOMe.

In Chapter 2, the reactivity of the complex [TismPriBenz]Tl is further developed, providing access to previously known methyl and iodide compounds of magnesium, zinc, and cadmium. Additionally, [TismPriBenz]Tl has been shown to react directly with the alkali metals sodium, potassium, and rubidium to form the novel alkali metal complexes [TismPriBenz]M (M = Na, K, Rb). Furthermore, [TismPriBenz]Li can react with CsF to afford [TismPriBenz]Cs, completing the non-radioactive Group 1 [TismPriBenz]M series. This makes [TismPriBenz] one of only a handful of organic ligands to have structurally characterized compounds with all of the alkali metals from Li to Cs, and the only ligand that formsmonomeric complexes in each case. [TismPriBenz]K was also used as a starting material to synthesize the first [TismPriBenz] lanthanide complexes, [TismPriBenz]YbI and [TismPriBenz]YbCl₂. [TismPriBenz]YbI itself can further react with KN(SiMe₃)₂ and NaCp to give [TismPriBenz]YbN(SiMe₃)₂ and [TismPriBenz]YbCp respectively.

Lastly, the ability of the [TismPriBenz]Zn halide series to form ion pair complexes was investigated. [TismPriBenz]ZnI can react with ZnI₂ to afford {[TismPriBenz]Zn}₂[Zn₃I₈], which contains the novel zinc halide species [Zn₃I₈]²⁻. Additionally, all of the [TismPriBenz]ZnX (X = Cl, Br, I) complexes are able to react with excess ZnX₂ in THF to give the series {[TismPriBenz]Zn}[Zn(THF)X₃].
Date January 2023
CreatorsVaccaro, David Alexander
Source SetsColumbia University
Detected LanguageEnglish

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