Synthesis and reactivity studies of chromium complexes supported by β–diketiminate ligands

Abstract

This dissertation describes research on paramagnetic chromium complexes coordinated by β-diketiminato ligands (this ligand is often referred to as nacnac). The motivation of this research is to explain and understand the thermally stable alkyl hydride complex, (LMeCr)2(μ-CH2SiMe3)(μ-H) (1), that the Theopold research group reported earlier. The thesis also presents the synthesis of a nitride complex that is an isomer of previously reported chromium dinitrogen complex, (LiPrCr)2(μ2-η2:η2-N2) (23). ☐ Chapter 1 describes the reaction of (LMeCr)2(μ-H)2 (2) with 1.0 equiv. of trimethylsilyl-diazomethane, which resulted in a set of unexpected molecules. The structures of dinitrogen complexes [LMeCr(THF)]3(μ-N2)3 (3), (LMeCr)4(μ-N2)4 (4), and (LMeCr)3(μ-N2)3(μ-N)CrLMe (6) were determined and these complexes were discussed. Complexes (LMeCr)2(μ-NSiMe3)(μ-H) (7) and (LMeCr)2(μ-NCH2)(μ-NH) (8) were also found in the reaction. These complexes were separated by fractional crystallization. A proposed mechanism for the formation of this variety of products is shown in Appendix A. ☐ Chapter 2 starts with the synthesis of chromium(II) alkyls; an investigation of their thermal stability was conducted. Hydrogenolysis of chromium(II) alkyls led to the formation of alkyl hydride derivatives, specifically (LMeCr)2(μ-CH2SiMe3)(μ-H) (1), (LMeCr)2(μ-CH2CMe3)(μ-H) (16), and (LMeCr)2(μ-CH2C6H5)(μ-H) (17). Their structures as well as the characterizations are described. Their reactivities were evaluated and (LMeCr)2(μ-CH2CMe3)(μ-H) (16) was found to undergo C-H bond activation in the presence of a number of organic substrates, such as benzene, toluene and tetramethylsilane. These activations led to the transformation into (LMeCr)2(μ- C6H5)(μ-H) (18), (LMeCr)2(μ-m-CH3C6H4)(μ-H) (19), and (LMeCr)2(μ-CH2SiMe3)(μ- H) (1), respectively. Labeling experiments of 16 with labeled benzene (C6D6) and 16- d1 with benzene (C6H6), were carried out and the outcomes were examined by mass spectrometry. The results suggested that the bridging hydride or deuteride of the reactants were preserved in the products. Finally, a mechanism for the transformation of alkyl hydrides was proposed based on all the experimental observations. ☐ Chapter 3 reports on the studies of isomeric chromium complexes coordinated only by nitrogen atoms. Side-on bridged dinitrogen complex (LiPrCr)2(μ2-η2:η2-N2) (23) was previously reported. This chapter describes the discovery of an isomer of 23, namely (LiPrCr)2(μ-N)2 (25). 25 was originally found in the irradiation reaction of a chromium azido complex. The isolation of 25 was difficult due to its subsequent transformation into the final product, 23. In this vein, chromium azido complexes supported by variously substituted nacnac ligands were prepared and the irradiation reactions were studied. The bis(μ-nitrido) complexes (LMeCr)2(μ-N)2 (30), (LEtCr)2(μ- N)2 (32), and (*LiPrCr)2(μ-N)2 (34) were structurally characterized. Another side-on bridged dinitrogen complex, (LEtCr)2(μ2-η2:η2-N2) (33), was found along the way. A number of attempts at clean formation of 25 were carried out, but failed to produce 25. Finally, a successful synthesis for 25 was developed, which involved the irradiation of (LiPrCrCl)2(THF)(μ-N3)2 (37), followed by reductive dehalogenation of (LiPrCrN)2(μ- Cl)2 (38). The conversion of bis(μ-nitrido) 25 into its isomeric dinitrogen complex 23 was successfully achieved by photolysis.