C C* Cyclometalated Platinum(II) NHC Complexes with β-Ketoimine Ligands

C<^>C* cyclometalated platinum(II) NHC complexes with chelating acetylacetonate ligands have been recently shown to be an interesting class of phosphorescent emitters. We sought to clarify the role of the acetylacetonate ligand by replacing one of the coordinating oxygen atoms in the auxiliary ligand with an NH group. This allowed us to study the effect on the emission, and we found that the nature of the emission process changes significantly. We herein report the synthesis of novel cyclometalated platinum(II) NHC complexes with the chelating beta-ketoimine ligands (3Z)-4-amino-3-penten-2-one and (2Z)-3-amino-1-phenyl-2-buten-1-one. Due to the unsymmetrical nature of the auxiliary ligands, two isomers were always formed, which could be separated and characterized. Their photophysical properties as well as solid-state structures of representative members of this new class of compounds are given. All new complexes emit in the blue to green region of the visible spectrum with quantum yields as high as 74% at room temperature. The photophysical data of the isomers show significant differences. The new compounds have been fully characterized by H-1 and C-13 as well as 2D NMR (COSY, HSQC, HMBC, NOESY) spectroscopy. DFT calculations were used to determine differences between the respective isomers and to predict the emission wavelengths of the synthesized complexes. We found the reason for the different properties in comparison to the acetylacetonate complexes to be the smaller HOMO - LUMO gap.