Date of Award

Spring 2019

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Dmitry V. Peryshkov

Second Advisor

Richard D. Adams

Abstract

Herein, we report the activation of relatively strong C–H and C–O bonds under mild conditions by a cooperative interaction of TaCl5 and PPh3. An unprecedented activation of nitrile adducts of high-valent transition metals with the formation of novel zwitterionic imido complexes was observed. The scope of the reaction includes a range of substituted unactivated nitriles including the C–H bond activation of both aliphatic and aromatic nitriles.

The cooperative action of a Lewis acidic high-valent transition metal halide (TaCl5 or NbCl5) and a weakly nucleophilic base (NEt3 or PPh3) leads to the deprotonation of a coordinated nitrile and subsequent rearrangement to an azaallene/carbocation intermediate. The nucleophilic attack of the base on the intermediate results in the formation of the cationic vinyl imido group. The presence of sterically hindered nitriles and weakly nucleophilic bulky organic base resulted in dimerization of the imido complexes through C–C bond coupling. The C–O bond activation was observed when unactivated ethers (anisole and diethyl ether), ketone (acetophenone) and aldehyde (4-phenylbenzaldehyde) were employed as substrates. For example, the reaction of the TaCl5–PPh3 pair and anisole at room temperature led to the selective cleavage of the sp3-C–O bond.

Remarkably, all the reported activation processes proceed through cooperative Lewis acid-Lewis base interactions and do not involve redox events at the metal center. These transformations highlight the special role of the high-valent transition metal halide in the activation process and distinguish the reactivity of the TaCl5–PPh3 system from both non-metal-based frustrated Lewis pairs and late transition metal-based FLP-like systems. Further study will demonstrate that this conceptually novel cooperative Lewis pair has a potential to activate a variety of strong bonds in small molecules. Furthermore, during the exploration of chemistry of high-valent early metal halides, a series of novel molecular multimetallic nitride complexes of Ta(V) and Ti(IV) were synthesized.

Rights

© 2019, Md. Mamdudur Rahman

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