From the reactivity of Pincer CCC nickel complexes to catalysis with simple nickel salts

nickel;pincer;CCC;Ni(IV);NiIV;bis(carbene)

Espinosa Martinez, Gabriel Antonio

关键词: nickel;    pincer;    CCC;    Ni(IV);    NiIV;    bis(carbene);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101171/ESPINOSAMARTINEZ-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

N-heterocyclic carbenes (NHCs) have provided significant improvements in the field of transition metal homogenous catalysis. Additionally, in the field of synthetic inorganic chemistry, chelating NHC-based ligands have aided in the observation and isolation of metal complexes featuring higher oxidation states metal centers (M = Mn, Fe, Co), as well as support for complexes featuring metal-ligand multiple bonds.Anionic carbon-based ligands provided a ligand field suitable to stabilize high-valent NiIV oxidation states. Interested in employing a carbon-based ligand that enforced pincer coordination and featured an anionic carbon-metal linkage and strongly donating NHCs to access metal-based multielectron reactivity, the use of the DIPPCCC (DIPPCCC = bis(diisopropylphenyl-benzimidazol- 2-ylidene)phenyl) ligand platform was explored.Chapter 2 describes the installation of nickel onto the DIPPCCC ligand platform. This was achieved via C–H activation by NiII or via oxidative addition by Ni0 to form a NiII-hydride. (DIPPCCC)NiCl was accessed in great yields in one step by reacting the ligand precursor [H3(DIPPCCC)]Cl2, NiCl2py4, and LiN(SiMe3)2. Treatment of (DIPPCCC)NiCl with alkyllithium reagents generated Ni-alkyl complexes. The series of nickel compounds was characterized by NMR spectroscopy, X-ray crystallography, and electronic absorption spectroscopy.Motivated by our goal of accessing a high-valent nickel center and promoting metal- centered multielectron reactivity with the aid of the strongly-donating ligand platform, we explored the oxidation chemistry of these nickel complexes. Chapter 3 describes the oxidation of (DIPPCCC)NiX (X = Cl, Br) complexes with halogen surrogates, as well as elemental bromine, resulting in the isolation of a family of NiIV complexes. Crystallographic characterization revealed no significant changes in the bond metrics of the ligand scaffold compared to the NiII complexes, discounting the possibility of a ligand-centered radical and confirming a formal +4 oxidation state of the Ni center.Initial exploration into the catalytic activity of (DIPPCCC)NiCl demonstrated its competency toward the cross-coupling of aryl halides with LiN(SiMe3)2. Interested in having access to a platform to compare reactivity differences of the analogous palladium compound in this reaction and other catalytic transformations, we set out to synthesize the palladium complex, (DIPPCCC)PdCl. Chapter 4 describes the preparation and oxidation chemistry of the palladium complexes featuring the DIPPCCC ligand scaffold. Under the studied conditions, (DIPPCCC)PdCl was not nearly as effective for the described C–N bond forming transformation.Over the course of this study, we discovered that simple nickel salts, such as NiX2 (X = Cl, Br) and (PPh3)2NiX2 (X = Cl, Br), catalyzed the coupling reaction without the use of privileged ligands. Chapter 5 the development of a method employing (PPh3)2NiCl2 for the amination of (hetero)aryl bromides with LiN(SiMe3)2. Our protocol showcased a large functional group tolerance and required low loading of the Ni source. During the course of this study, the involvement of NiI was probed, and under the reaction conditions it could be the catalyst responsible for the transformation; however, the participation of NiII during the catalytic transformation could not be ruled out.