Para-Functionalized NCN-Pincer Palladium and Platinum Complexes as Building Blocks in Organometallic Chemistry

Abstract

A rapidly evolving field in chemistry is the application of organometallic and coordination complexes as building blocks or active components for the construction of new materials exhibiting specific catalytic, redox, optical or sensor activities. A central theme in the construction of these inorganic building blocks is the targeted functionalization of ligands, either prior to or, less conventionally, after the metallation step. Ligand functionalization enables the immobilization of the transition-metal complexes on macromolecular or inorganic supports, the regulation of their solubility, the introduction of additional functional moieties, as well as the electronic tuning of the metal. Furthermore, the functionalized complexes can be applied in inorganic crystal engineering or for targeted (supramolecular) assembly in solution. The NCN-pincer ligand (NCN = 2,6-bis[(dimethylamino)methyl]-phenyl anion) is a versatile building block for these purposes. NCN-pincer palladium(II) and platinum(II) complexes (Chart 1) are air- and water-stable, and find widespread applications in the field of catalysis and as sensor materials. Para-functionalization of these complexes offers an anchoring point, while leaving the structural integrity of the metal center intact.

X M=Pd(II), Pt(II) ! X= counter ion and/or Me2N---M---Nme2 coordinating ligand ?? Z= functionalization site Z

Chart 1

The results described in this thesis show that NCN-pincer palladium and platinum complexes are versatile building blocks for the construction of new organometallic materials with applications in diverse fields such as catalysis, crystal engineering, and (macro)molecular visualization. The pathways presented for the synthesis of the new para-functionalized NCN-pincer complexes are of crucial importance for generating a suitable anchoring point for further functionalizations without affecting the M C bond. Important aspects concerning their synthesis include: i) the exceptional stability of the NCN-complexes, allowing ligand modifications after the metallation step, and ii) the availability of various metallation procedures for selective introduction of the palladium or platinum center in the NCN-ligand. Both features offer a high degree of flexibility in the synthesis of the para-functionalized complexes, making functionalization with virtually any (organic) group feasible. Noteworthy are the linear Hammett correlations found for the para-substituted NCN-platinum complexes. Extension of these correlations to NCN-pincer complexes of other metals, and eventually to PCP- or SCS-pincer complexes, allows subtle tuning of the electron density on their metal centers, and consequently theoretical predictions of their catalytic and/or optical properties. The application of NCN-pincer building blocks in the examples shown in this thesis illustrate the above-mentioned features, i.e. selective metallation of the ligand at various stages of the syntheses and modifications on the ligand after the metallation step. The methodology employed in the preparation of the pincer complexes can be used as a starting point for the construction of new organometallic materials based on the pincer ligand. These materials can be designed to exhibit bio- or solvent-compatibility and/or specific aggregation behavior. Finally, (non)-covalent assembly of catalytically active NCN-pincer complexes with other functional moieties, e.g. (co)catalysts or receptor sites, offers the opportunity to construct bifunctional or supramolecular catalysts.