Thomas Strassner

Blue phosphorescent emitters: new N-heterocyclic platinum(II) tetracarbene complexes

Photoluminescence measurements show that platinum(II) tetracarbene complexes, which could be obtained via different synthetic routes, are interesting lead structures for the development of blue emitters for PhOLEDs with good quantum yields and high photostability.

1,2,4-Triazole-Based Tunable Aryl/Alkyl Ionic Liquids

Ionic liquids based on aryl-/alkyl-substituted imidazolium salts constitute a new generation of ionic liquids with a high degree of flexibility. The new concept could now also be extended to aryl-/alkyl-substituted 1,2,4-triazolium salts. The two different phenyl-substituted 1H- and 4H-1,2,4-triazoles have been synthesized by a coupling reaction or a one-pot synthesis with diformylhydrazine, respectively. Reaction with alkyl bromides provided the 1,2,4-triazolium bromides, and an anion-exchange reaction led to the corresponding bis(trifluoromethylsulfonyl)imide salts with melting points well below 100 °C.

Synthesis and structural characterization of novel bridged platinum(II) biscarbene complexes

Abstract Novel bridged platinum(II) biscarbene complexes are reported: 1,1′-dimethyl-3,3′-methylene-4-diimidazolin-2,2′-diylidene platinum(II) ( 3 ) and 1,1′-dimethyl-3,3′-ethylene-4-diimidazolin-2,2′-diylidene platinum(II) complexes 4 are directly accessible in high yields starting from platinum halides. The one-pot synthesis obviates the need for multi-step reactions via metal precursors or free carbenes. An X-ray crystal structure of 1,1′-dimethyl-3,3′-methylene-4-diimidazolin-2,2′-diylidene platinum(II) dibromide ( 3b ) confirmed the structural similarity to the known corresponding palladium complexes. Since free 1,1′-di- R -3,3′-methylene-4-diimidazolin-2,2′-diylidenes are only available in low yields this synthetic route provides an easy access to the corresponding carbene complexes.

Bi(OTf)3-Catalyzed Diastereoselective SN1-Type Reactions of Chiral Propargylic Acetates

The facial diastereoselectivity of intermolecular SN1 reactions in which chiral carbocations are putative intermediates has not been studied extensively. Recently, we started to investigate more closely the reactions of benzylic carbocations, which often exhibit a very high degree of facial diastereoselectivity. We prepared the chiral cation 1 (R = H) and established its preferred conformation spectroscopically. The results led us to address the question, whether propargylic carbocations with the general structure 2 react in a similar fashion, and to investigate which parameters control their selectivity. We now report a highly diastereoselective, Bi(OTf)3-catalyzed [3, 4] reaction of chiral propargylic acetates with various weak carbon nucleophiles, 6] in which chiral carbocations are very likely to be involved as intermediates. The diastereoselectivity of the reaction cannot be immediately accounted for by the preferred conformation of the cation 2.

Propane Activation by Palladium Complexes with Chelating Bis(NHC) Ligands and Aerobic Cooxidation

The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of C-H bonds in alkanes. Herein we report the development of a C-H functionalization procedure for propane by using a palladium catalyst with chelating bis(N-heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co-catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving C-H activation by palladium(II), followed by oxidation to palladium(IV) by bromine.

Enlarging the π System of Phosphorescent (C^C*) Cyclometalated Platinum(II) NHC Complexes

Cyclometalated (C^C*) platinum(II) N-heterocyclic carbene (NHC) complexes are emerging as a new class of phosphorescent emitters for the application in organic light-emitting devices (OLEDs). We present the synthesis of six new complexes of this class to investigate the influence of extended π systems. Therefore, six different NHC ligands with a varying number of additional phenyl substituents were used in combination with the monoanionic acetylacetonate (acac) ligand to obtain complexes of the general formula [(NHC)Pt(II)(acac)]. The complexes were fully characterized by standard techniques and advanced spectroscopic methods ((195)Pt NMR). For all complexes the solid-state structure determination revealed a square-planar coordination of the platinum atom. Absorption and emission spectra were measured in thin amorphous poly(methyl methacrylate) films at room temperature. Four compounds emit in the blue-green region of the visible spectrum with quantum yields of up to 81%.

Alkane C–H Functionalization and Oxidation with Molecular Oxygen

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C-H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C-H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

The Role of NHC Ligands in Oxidation Catalysis

During the last decade N-heterocyclic carbenes (NHC) have become a very important class of ligands for catalytic applications. They have been employed in reactions like C-C/C-N coupling, olefin metathesis, hydrosilylation and hydrogenation. Recently, also in the field of oxidation catalysis significant progress has been achieved. NHC ligands are known to efficiently activate metal centers and they are surprisingly resistant, even in acidic media. In contrast to phosphine ligands, the metal-NHC bond is also stable under oxidizing conditions. This review covers the oxidation of organic substrates by NHC catalysts as well as the fixation and activation of O2 and CO2 by NHC complexes. A variety of oxidation reactions have been studied using NHC catalysts, but the oxidation of alcohols and olefins was of particular interest. Most of the work in this area was conducted with palladium catalysts, which even allow the efficient oxidation of alcohols at ambient temperature using molecular oxygen. Bi- and tridentate NHC ligands are described as well as a variety of complexes with different metals other than palladium. Reactive intermediates like peroxo complexes have been characterized experimentally, while DFT calculations have been used to explain the mechanisms of the oxidation chemistry of transition metal NHC complexes.

Heteroleptic platinum(II) NHC complexes with a C^C* cyclometalated ligand – synthesis, structure and photophysics

Platinum(II) complexes [(NHC)Pt(L)] with various β-diketonate based auxiliary ligands (L: 3-meacac = 3-methylacetylacetonato, dpm = dipivaloylmethanato, dbm = dibenzoylmethanato, mesacac = dimesitoylmethanato, duratron = bis(2,3,5,6-tetramethylbenzoyl)methanato) and a C^C* cyclometalated N-heterocyclic carbene ligand (NHC: dpbic = 1,3-diphenylbenzo[d]imidazol-2-ylidene, dpnac = 1,3-diphenylnaphtho[2,3-d]imidazol-2-ylidene or bnbic = 1-phenyl-3-benzylbenzo[d]imidazol-2-ylidene) were found to show different aggregation and photophysical properties depending on the auxiliary ligand. Eight complexes were prepared from a silver(I)–NHC intermediate by transmetalation, cyclometalation and subsequent treatment with potassium-tert-butanolate and β-diketone. They were fully characterized by standard techniques including 195Pt NMR. Five complexes were additionally characterized by 2D NMR spectroscopy (COSY, HSQC, HMBC and NOESY). Solid-state structures of five complexes could be obtained and show the tendency of the square-planar compounds to form pairs with different Pt–Pt distances depending on the bulkiness of the substituents at the auxiliary ligand. The result of the photophysical measurements in amorphous PMMA films reveals quantum yields of up to 85% with an emission maximum in the blue region and comparatively short decay lifetimes (3.6 μs). Density functional theory (DFT/TD-DFT) calculations were performed to elucidate the emission process and revealed a predominant 3ILCT/3MLCT character. Organic light-emitting devices (OLEDs) comprising one of the complexes achieved 12.6% EQE, 11.9 lm W−1 luminous efficacy and 25.2 cd A−1 current efficiency with a blue emission maximum at 300 cd m−2. The influence of an additional hole-transporter in the emissive layer was investigated and found to improve the device lifetime by a factor of seven.

ortho-Phenylene bridged palladium bis-N-heterocyclic carbene complexes: synthesis, structure and catalysis

A series of ortho-phenylene bridged palladium bis-NHC complexes has been synthesized. Complexes with imidazolium and benzimidazolium derived NHCs and methyl-/benzyl-wingtips are reported. Bis(benz)imidazoles with a doubly brominated ortho-phenylene bridge could be obtained by an electrophilic substitution reaction. The structure of the complexes could be confirmed by three solid-state structures. All catalysts have been tested in the catalytic functionalisation of propane. The catalytic activity is highly dependent on the ligand, whereas ligand effects on the regioselectivity (n/iso) are much smaller.