Binuclear C^C* cyclometalated NHC platinum(II) compounds with bridging amidinate ligands were synthesized to evaluate their photophysical properties. Their three-dimensional structures were determined by a combination of 2D NMR experiments, mass spectrometry, DFT calculations, and solid-state structure analysis. The bridging amidinate ligands enforce short distances between the platinum centers of the two cyclometalated structures, which gives rise to extraordinary photophysical properties.
The synthesis and characterization of eight unprecedented phosphorescent C^C* cyclometalated mesoionic aryl-1,2,3-triazolylidene platinum(II) complexes with different β-diketonate ligands are reported. All compounds proved to be strongly emissive at room temperature in poly(methyl methacrylate) films with an emitter concentration of 2 wt %. The observed photoluminescence properties were strongly dependent on the substitution on the aryl system and the β-diketonate ligand. Compared to acetylacetonate, the β-diketonates with aromatic substituents (mesityl and duryl) were found to significantly enhance the quantum yield while simultaneously reducing the emission lifetimes. Characterization was carried out by standard techniques, as well as solid-state structure determination, which confirmed the binding mode of the carbene ligand. DFT calculations, carried out to predict the emission wavelength with maximum intensity, were in excellent agreement with the (later) obtained experimental data.
Two series of heteroleptic platinum(II) carbene compexes of the type [Pt(C ∧ C*)(O ∧ O)] (O ∧ O = acetylacetonate)with C ∧ C* cyclometalated 4-phenyl-1,2,4-triazol-5-ylidene as well as 1phenyl-1,2,4-triazol-5-ylidene ligands were prepared. The effect of various substituents in the 4-position of the phenyl ring (4phenyltriazole: H, Cl, OCH 3 , NO 2 ; 1-phenyltriazole: H, Cl, OCH 3 , CH 3 ) was studied. The ligand precursors were obtained by quaternization of the corresponding aryl-1,2,4-triazoles with iodomethane. The complexes were synthesized by a reaction sequence starting from the triazolium precursors via the silver(I) carbenes, transmetalation with dichloro(1,5-cyclooctadiene)platinum(II), cyclometalation reaction, and finally addition of acetylacetonate. All complexes have been characterized by standard techniques such as 1 H NMR, 13 C NMR, and elemental analysis. Additionally, solid-state structures of seven complexes could be obtained by singlecrystal X-ray diffraction. According to absorption and emission spectra, the complexes reveal interesting photophysical properties with strong emissions in the blue region of the visible spectrum at room temperature, which are sensitive to the donor and acceptor properties of the substituents at the cyclometalating ligand.
We present the syntheses of 12 cyclometalated palladium C ∧ N 2-phenylimidazole carbene complexes with different N-1 groups as well as different substituents at the C-2 phenyl group of the cyclometalating imidazole. We investigated the influence of these substituents by comparing the catalytic performance of the complexes in the Suzuki−Miyaura cross-coupling reaction of aryl chlorides. We can show a strong dependence between the steric demand of the N-1 substituent of the cyclometalating imidazole and the catalytic activity in the cross-coupling reaction. The most active complex shows a wide substrate scope, where several aryl as well as benzyl chlorides could be coupled with different boronic acids in excellent yields using very low catalyst concentrations of 0.05 mol %.
Neutral cyclometalated platinum(ii) N-heterocyclic carbene complexes [Pt(C^C*)(O^O)] with C^C* ligands based on 1-phenyl-1,2,4-triazol-5-ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato (O^O = mesacac) ancillary ligands were synthesized and characterized. All complexes are emissive at room temperature in a poly(methyl methacrylate) (PMMA) matrix with emission maxima in the blue region of the spectrum. High quantum efficiencies and short decay times were observed for all complexes with mesacac ancillary ligands. The sterically demanding mesityl groups of the mesacac ligand effectively prevent molecular stacking. The emission behavior of these emitters is in general independent of the position of the nitrogen in the backbone of the N-heterocyclic carbene (NHC) unit and a variety of substituents in 4-position of the phenyl unit, meta to the cyclometalating bond.
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