Synthesis, photophysical properties and photochemical reactivity of cyclometalated platinum(IV) complexes

Abstract

The interest in complexes with chelating heteroaromatic ligands, such as 2,2'-bipyridines and cyclometalated 2-arylpyridines, is due to their luminescence and other excited-state properties, which allow their use in applications such as the development of electroluminescent materials, probes for bioimaging, sensors, photosensitizers for singlet oxygen generation and photodynamic therapy, and photocatalysts in organic synthesis. Metal ions with a d6 electronic configuration, such as Re(I), Ru(II), Os(II) and Ir(III), or d8, such as Pt(II) and Au(III), have been the most widely employed for the development of this type of complexes. However, before the start of this thesis, only a few series of luminescent Pt(IV) complexes with chelating heteroaromatic ligands had been described and there were important challenges to improve the synthetic methods and the possibilities to modulate their luminescence. This thesis addresses the development of alternative methods for obtaining strongly luminescent Pt(IV) complexes that allow the introduction of cyclometalated and ancillary ligands of different electronic properties. In addition, the synthesis and study of the photochemical reactivity of Pt(IV) complexes that can give rise to reductive couplings with potential interest for the development of photochemical catalytic processes is addressed. For this purpose, synthetic methods based on oxidative addition reactions to bis-cyclometalated Pt(II) complexes have been investigated. A method has been developed for the direct synthesis of homoleptic and heteroleptic tris-cyclometalated Pt(IV) complexes with a facial geometry, which allows the introduction of 2-arylpyridine ligands of different electronic properties. This method is based on oxidative addition reactions of 2-(2-pyridyl)benzenediazonium salts to bis-cyclometalated Pt(II) complexes, which give rise to tris-chelate intermediates containing a diazenide ligand, which undergoes thermal or photochemical nitrogen elimination. The new tris-cyclometalated complexes can produce efficient luminescence and, in some cases, dual phosphorescence from two excited states centered on different ligands, which could allow their use for the development of white light emitters. Oxidative addition reactions of iodobenzene and diaryliodonium salts to Pt(II) bis-cyclometalated complexes have been studied, yielding bis-cyclometalated Pt(IV) complexes with an halide and an aryl ligand. The first intermolecular oxidative addition reactions of an aryl halide to Pt(II) complexes have been described, which were achieved by a photochemical procedure. The luminescence of two analogous series of these complexes, containing fluoride or chloride and different cyclometalated ligands, has been studied and it has been shown that high emission quantum yields can be achieved. In addition, the introduction of the fluoride ligand allows the modulation of lifetimes. The synthesis of tris-chelate Pt(IV) complexes containing two cyclometalated 2-arylpyridine ligands and a dimetalalated biaryl has been achieved through the oxidative addition of dibenzoiodolium salts to bis-cyclometalated Pt(II) complexes, and the reaction mechanism has been elucidated. These are the first neutral tris-chelate Pt(IV) complexes capable of producing efficient luminescence. These derivatives are particularly suitable for the development of low-energy emitters. The photochemical reactivity of bis-cyclometalated Pt(IV) complexes containing an alkynyl ligand and a carboxylate has been studied, showing that reductive coupling and annulation reactions between the 2-arylpyridine ligand and the alkynyl, photoisomerizations and coupling reactions between the carboxylate and a 2-arylpyridine ligand can be achieved. Based on the obtained ratios of photoproducts, the electronic properties of the carboxylate and alkynyl ligands, and TDDF calculations, the observed reactivity is associated with the population of LMCT or LLCT triplet states. These photoinduced reactions provide new knowledge for the development of photochemical processes with potential application in synthesis.