Extending the Phosphorus-donor Manifold from Symmetric to Unsymmetric in Pincer Metal Catalysts

Abstract

The ECE-pincer ligand framework contains several positions that are readily functionalized and by doing so offer a wealth of possibilities to modify and fine-tune the structural and electronic properties of the metal centre in ECE-pincer metal complexes. Many ECE-type pincer metal complexes show remarkable catalytic performances in a number of catalytic reactions. With the aim to achieve enantioselective transformations, the development of chiral pincer metal complexes has been an integral part of pincer chemistry. The development of ECE’-type (E≠=/=E’) pincer metal complexes is a relatively new aspect of pincer chemistry. In such ECE’-type pincer complexes, the combination and cooperation of two different types of donor atom groups, E and E’, could lead to very interesting electronic and structural properties of the metal centers, which in turn could lead to improved catalytic performances. This thesis reports about a novel series of PCN- and PCS-pincer Pd- and Pt-complexes, phosphite P’CP’-pincer Pd-complexes, C,P-cyclometalated Pd- and Pt-complexes, and P-chiral phosphoramidite-pincer Pd-complexes. It provides an overview of the fine-tuning of the geometrics and electronic properties of these phosphorus-based pincer metal complexes through appropriate organic transformations of the pincer ligand. The newly reported pincer complexes were found to be active catalysts in C-P cross coupling, homoallylation, tandem catalytic, and aza-Michael addition reactions. Several aspects of this study deserve special attention. First of all, the development of new P-chiral pincer complexes derived from readily available chiral amino alcohols provide access to a library of new P-chiral phosphoramidite pincer metal complexes with great potential in enantioselective catalysis. Moreover, the synthetic protocol toward a series of electronically and structurally fine-tuned PCE-pincer metal complexes (E = N and S) using isovanillin as the common intermediate holds a great promising for the synthesis of new tailor-made ECE’-type pincer metal complexes. The combination and possible cooperativity of different E-donors has proven potential to alter the catalytic activities of pincer complexes. The further development of PCE-pincer metal complexes could, therefore, provide a stepping stone to improve and extend their application in homogeneous catalysis.