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Tridentate N^C^N Ligands in Cyclometalated Organo-nickel(II) Complexes for C-C Cross Coupling Catalysis
von Lukas Daniel KletschNew cyclometalated nickel(II) complexes [Ni(N^C^N)X] containing the tridentate N^C^N binding ligands and coligands were synthesized, characterized and studied in Negishi type C-C cross coupling reactions.
For more than 30 years palladium had been the choice for these important organic transformation reactions. In recent years efforts to replace the expensive and rare Pd through abundant metals has put a focus on nickel due to its chemical similarity. Although a big number of Ni catalysts are meanwhile reported, many mechanistic questions remained open and reactive or stable catalysts are still sought for. This project aimed to introduce these new cyclometalated complexes with an anionic N^C^N binding motive, representing a very stable ligand scaffold and donating electron density to the nickel atom, thus inducing a strong ligand field and leading to a rather nucleophilic reaction center. Also, these ligands allow subtle finetuning of the reactivity through substitution and replacement of individual groups. The biggest challenge of the project is the development of synthesis procedures to access a very broad variety of ligands and complex structures with the [Ni(N^C^N)X] motive.
Starting from the N^HC^N protoligand, a variety of substituted N^HC^N molecules was synthesized and studied, in which the electronic properties of the central anionic phenide ring were varied using EWGs or EDGs and the peripheric pyridines replaced by other N-containing aromats. Several attempts to vary the coligands were successful for all halides and NCS. In addition, a variety of carboxylates, alkoxides, perfluorinated alkylides, and the literary-known carbazolate were introduced as coligands. A new route of using the nitrato complex as a precursor for coligand exchange reactions was a breakthrough for more possible coligands.
First tests of these Ni(II) catalysts in C-C cross coupling reactions under Negishi conditions gave the target products selectively in good yields.
For more than 30 years palladium had been the choice for these important organic transformation reactions. In recent years efforts to replace the expensive and rare Pd through abundant metals has put a focus on nickel due to its chemical similarity. Although a big number of Ni catalysts are meanwhile reported, many mechanistic questions remained open and reactive or stable catalysts are still sought for. This project aimed to introduce these new cyclometalated complexes with an anionic N^C^N binding motive, representing a very stable ligand scaffold and donating electron density to the nickel atom, thus inducing a strong ligand field and leading to a rather nucleophilic reaction center. Also, these ligands allow subtle finetuning of the reactivity through substitution and replacement of individual groups. The biggest challenge of the project is the development of synthesis procedures to access a very broad variety of ligands and complex structures with the [Ni(N^C^N)X] motive.
Starting from the N^HC^N protoligand, a variety of substituted N^HC^N molecules was synthesized and studied, in which the electronic properties of the central anionic phenide ring were varied using EWGs or EDGs and the peripheric pyridines replaced by other N-containing aromats. Several attempts to vary the coligands were successful for all halides and NCS. In addition, a variety of carboxylates, alkoxides, perfluorinated alkylides, and the literary-known carbazolate were introduced as coligands. A new route of using the nitrato complex as a precursor for coligand exchange reactions was a breakthrough for more possible coligands.
First tests of these Ni(II) catalysts in C-C cross coupling reactions under Negishi conditions gave the target products selectively in good yields.