Pincer-porphyrin hybrids : Synthesis, self-assembly, and catalysis

Abstract

Metal complexes play an important role in established research areas such as catalysis and materials chemistry as well as in emerging fields of chemical exploration such as bioinorganic chemistry. Changes in the metal center's ligand environment, i.e., the nature and number of the Lewis basic atoms involved in coordination, often have profound effects on the properties of the complex. In particular, these factors affect the electron density on the metal center, which largely governs the reactivity of the complex as a whole. The electronic properties of any given set of ligands can be fine-tuned by the modulation of their features through functional group substitution. Electron-donating substituents, for instance, render the attached ligand electron-rich, which in turn increases the electron density on the metal center. The modification of ligands, however, in most cases requires a number of organic reactions. Ideally, these reactions are performed on the metal-ligand complex, but the metal-ligand bond often does not tolerate the required reaction conditions. Therefore the ligand is normally modified prior to complexation to the metal, which can be an iterative and time-consuming process, especially when optimization of the electronic properties of the metal-ligand complex is desired. The research described in this thesis focuses on the development of a novel, modular strategy to fine-tune the electron density on the metal center of a metal complex by varying the electronic properties of its ligand. The prerequisites for such a strategy to succes are twofold: (a) the metal complex should be amenable to electronic tuning through its ligand(s) and (b) the electronic properties of the ligand should be readily variable in a mild way using a minimal number of steps. As the metal complex, the organometallic ECE-metallopincer was selected owing to the tunability of the metal-centered electron density through its para-substituent. At the pincers' para-position a porphyrin was attached, whose electronic properties can be readily varied by virtue of metalation in a single step. This thesis describes the synthesis of pincer-porphyrin hybrid ligand compounds and it details several strategies to metalate the ligand sites independently in a stepwise manner. The resulting hetero-multimetallic complexes exhibit intramolecular electronic communication between the metal complexes which is also corroborated by catalysis, which shows different catalytic activities for several pincer-porphyrin hybrids in which only the porphyrin metal has been changed. The tuning of the catalytic activity of the complexes has not only been accomplished through intramolecular interactions, but also by virtue of supramolecular, intermolecular interactions. It is furthermore shown that, when only the porphyrin core is metalated, interesting supramolecular aggregates can be formed.