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Stereoelectronic modification of organochromium complexes

University of British Columbia

This thesis explores the intimate relationship between structure and reactivity between two classes of organochromium complexes. The stereoelectronic environment of a metal center can have profound impacts on the observed reactivity of the complex. By manipulating the coordination environment and observing how the reactivity and properties of the complex change, patterns can be teased out that guide future design of complexes tailored for specific reactivity. In Chapter 2, cyclopentadienyl complexes of chromium featuring guanidinate complexes are synthesized and compared to cyclopentadienyl β-diketiminate organochromium complexes which have been previously investigated in the Smith research group. It was found that the reduced steric pressure of the guanidinate ligands compared to the β-diketiminate ligands results in increased reductive power of the complex as demonstrated by the ability to activate tertiary alkyl halides as well as subsequent trapping of the tertiary alkyl radical. Furthermore, increased rates of oxidative addition of primary alkyl halides were concluded by comparing activation of iodomethane using the β-diketiminate complexes an d phenethyl chloride using the guanidinate complexes. The reduced steric pressure of the guanidinate complexes results in stronger CrIII-X (X = alkyl, halide) bonds which leads to difficulties in reduction of the complex to regenerate the active chromium(II) complex. In the second part of Chapter 2 it was found that the use of a hypervalent silane is an effective homogeneous reductant for the reduction of both the chromium(III) β-diketiminate and guanidinate complexes. The chapter concludes with the implementation of this reductant in a simple radical homocoupling reaction using a series of CpCr(LX) catalysts. Chapter 3 of this thesis explores unexpected photoactivity observed when exposing cationic [Cr(bpy) ₂(Ar) ₂]⁺ complexes. It was found that exposure to light induces reductive elimination of the aryl groups forming biaryl compounds. Experiments previously conducted by Jesse Crescenzo as well as subsequent trapping and mass spectrometry experiments elucidate the mechanism of this reductive elimination event. It was found that replacement of one of the bipyridine ligands with LX ligands affords neutral compounds that exhibit similar photoactivity observed by ¹H NMR. Future experiments are outlined at the conclusion of this chapter to exploit this unique reactivity mode.

Text

2020-09-16

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76050

Chemistry

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/