Ciclometalación fotoquímica de compuestos heteroaromáticos en complejos de platino (II) y luminiscencia de derivados con ligandos N,C,C

Abstract

The cyclometalation reaction is one of the most important in the field of organometallic chemistry and has been widely exploited for the development of catalytic processes for the activation and functionalization of C-H bonds, as well as for the synthesis of luminescent and photofunctional organometallic complexes with manifold applications in technologies based on light-matter interactions. In particular, Pt(II) complexes with cyclometalated 2-arylpyridines or related heteroaromatic ligands constitute one of the most studied classes of luminescent complexes and have found application as dopants for electroluminescent devices, probes for bioimaging, chemosensors and photocatalysts. Although photochemistry has been previously applied for the intermolecular activation of hydrocarbons by transition metals, in very few cases has it been employed for the synthesis of cyclometalated complexes, and the systematic development of photochemical cyclometalation procedures has not been addressed. The objectives of this thesis are articulated around the design of photochemical strategies for the activation of C-H bonds in Pt(II) systems as a tool for the synthesis of phosphorescent cyclometalated complexes or with potential application for the development of catalytic cycles for the light-induced functionalization of C-H bonds. New platforms have been successfully developed for the photochemical cyclometalation of 2-arylpyridines by photooxidative addition of aromatic C-H bonds, based on precursors [Pt2(-Cl)2(C^N)2], where C^N is a cyclometalated 2-arylpyridine, which react with different 2-arylpyridines (N'^C'H) to give complexes cis-N,N-[PtCl(C^N)(N'^C'H)]. Visible light irradiation of these complexes produces bis-cyclometalated Pt(IV) hydrides, which could be isolated and characterized. These are the first Pt(IV) hydrides isolated as a result of a cyclometalation reaction. When the C^N ligand in the precursors [Pt2(-Cl)2(C^N)2] is a monocyclometalated 2,6-diarylpyridine, irradiation in the presence of N'^C'H ligands leads to a transcyclometalation process involving three photochemical steps. The insertion of alkynes into the Pt-H bond of one of the isolated hydrides, leading to alkenyl-complexes, has been studied, demonstrating their ability to participate in subsequent organometallic reactions. These results could serve as a basis for the design of photocatalytic C-H functionalization cycles. Room temperature photochemical cyclometalation procedures involving the irradiation of simple Pt(II) precursors in the presence of 2-arylpyridines or precursors of N^N^C, N^C^N or N^C^C terdentate ligands have been developed. Depending on the irradiation wavelength and the metal precursor, mono- or bis-cyclometalated complexes with C^N ligands can be obtained. Monitoring experiments have shown that both the ligand coordination through the N atom and the C-H bond activation are induced by light. Photochemical procedures for the cycloplatination of terdentate N^N^C and N^C^N ligands are clearly advantageous over thermal methods, which involve long heating times. This methodology has allowed the preparation of the first Pt(II) complexes with N^C^C ligands, of general formula [Pt(N^C^C)(L)], of which different derivatives have been prepared by varying both the N^C^C ligand and the ancillary ligand L. These complexes constitute a new class of luminescent Pt(II) materials, which show emissions from mixed intraligand charge-transfer and metal-to-ligand charge-transfer triplet states (3ILCT/MLCT). Depending on the ligand combination, some of these complexes can form molecular aggregates in the solid state and in solution via metallophilic Pt‧‧‧‧Pt contacts, π interactions between aromatic systems or non-classical hydrogen bonds, which have been corroborated by X-ray diffraction studies, and lead to red-shifted phosphorescent emissions, providing a way for the modulation of their properties.