Phosphorescent Platinum(II) Complexes Featuring Chelated Acetoxime Pyrazoles: Synthetic, Structural, and Photophysical Study

Treatment of cis-[PtCl2(Me2SO)(2)] with the pyrazole acetoximes p-RC6H4{CN(H) NCCH}CH2C(Me)=NOH (R=OMe, H, F, Cl, CN, NO2), prepared by a cascade reaction from the vinyl isoxazoles p-RC6 H4C(H)= C(H){CONCC(NO2)} Me, leads to the complexes [PtCl2 (Ar{CN(H) NCCH} CH2C(Me)= NOH)] (71-88 %). The complexes exhibit solid state room temperature phosphorescence with emission maxima in the region of 644-691 nm and biexponential relaxation with t1 approximate to 2 ms and t2 approximate to 0.32 ms. In the platinum compounds, substituents R affect phosphorescence quantum yields, which increase by one order of magnitude on going from electron acceptor (R= NO2; 0.2%) to electron-donor groups (R= OMe; 2.0%). Based upon theoretical TD-DFT calculations using the CAM-B3LYP functional, the absorption spectra were attributed to metal-to-ligand charge transfer ((MLCT)-M-1) and ligand-to-ligand charge transfer ((LLCT)-L-1) processes. The pyrazole acetoximes and their complexes were characterized by elemental analyses (C, H, N), HRESI-MS, FTIR, H-1 NMR (for metalfree species), and CP-MAS TOSS C-13{1H} NMR (for the platinum complexes). In addition, two complexes were studied by single-crystal X-ray diffraction.