Synthesis and characterization of chromium(V) nitrido complexes and the study of chromium-to-nitrido migration of alkyl/aryl ligands
Date
2022
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Publisher
University of Delaware
Abstract
A rare example of C-N bond formation by apparent nitrido migratory insertion into a Cr-C bond was discovered in the Theopold group recently. Thus, the reaction of LiPrCr(N)OTf (1) with MeLi yielded the unexpected dinuclear complex LiPrCr(Me)(μ-N)(μ-NMe)CrLiPr (2) (LiPr = [ArNC(Me)]2CH, Ar = 2,6-iPr2C6H3). For this reaction, we proposed a three-step mechanism. 1) the methyl group substitution for the triflate ligand produces an intermediate LiPrCr(N)Me (3a), 2) nitrido migratory insertion into the Cr-C bond generates LiPrCr(NMe) (3a’), 3) the coupling of 3a and 3a’ yields the final product LiPrCr(Me)(μ-N)(μ-NMe)CrLiPr (2). ☐ Chapter 2 To explore the nitride migratory insertion mechanism, we used the Lewis bases pyridine, THF and PMe3, to trap the intermediate generated in the MeLi reaction. The intermediate LiPrCr(N)(Me) (3a) was stabilized as LiPrCr(N)(Me)(py) (5a), LiPrCr(N)(Me)(THF) (6) and LiPrCr(N)(Me)(PMe3) (9) On the other hand, analogous alkylation/arylations were studied where benzyl, (trimethylsilyl)methyl, allyl, and cyclopropyl lithium, or Grignard reagents were used in the place of MeLi, yielding LiPrCr(N)(Bn) (3b), LiPrCr(N)(CH2TMS) (3c) and LiPrCr(N)(η3-C3H5) (3e). ☐ Chapter 3 Having synthesized LiPrCr(N)(R) in Chapter 2, we explored the oxidation, nucleophilic attack and reduction reactions to study nitride migratory insertion mechanism. Oxidation led to the loss of alkyl ligands, reduction of LiPrCr(N)(CH2TMS) (3c) resulted in two 3c molecules bridged by two K atoms, and nucleophilic attack on the nitrido ligand did not happen. ☐ On the other hand, with LiPrCr(Me)(μ-NMe)(μ-N)CrLiPr (2) and LiPrCr(N)(R) (R = Bn (3b), η3-C3H5 (3e)) in hand, photolyses and thermal decomposition reactions were studied. When exposed to blue light (Kessil LED Light, A160WE, Tuna Blue, 40W) or elevated temperature (30 - 70̊ C), LiPrCr(Me)(μ-NMe)(μ-N)CrLiPr (2) lost one methyl group from Cr, forming LiPrCr(μ-NMe)(μ-N)CrLiPr(15). Similarly, LiPrCr(μ-NBn)(μ-N)CrLiPr (18) and LiPrCr(μ-NC3H5)(μ-N)CrLiPr (19) were produced when LiPrCr(N)(R) (R = Bn, C3H5) was heated or exposed to light.
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Keywords
Migratory insertion, Cr(V) nitrido complex