Syntheses, Structural Characterization, and First Electroluminescent Properties of Mono-cyclometalated Platinum(II) Complexes with Greater than Classical π−π Stacking and Pt−Pt Distances^†

A series of mono-cyclometalated Pt(II) complexes (9, 10, 13, 15, and 17) bearing 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (1) as the same chromophoric N∧C cyclometalated ligand and the second nonchromophoric bidenate ligands with increasing bulkiness were synthesized by the following steps. The palladium-catalyzed cross-coupling reaction between 2-chloro-4-methylpyridine (2) and 3,5-bis(trifluoromethyl)phenylboronic acid (3) was used to prepare 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (1). Di-tert-butyl(trimethylsilyl)anylmethylphosphine, tert-Bu2P-CH2-SiMe3 (5), was used as a ligand for the Suzuki coupling. The cyclometalation of 1 by K2PtCl4 (7) was carried out in (MeO)3PO (8) with the formation of a chloro-bridged dimer N∧CPt(μ-Cl)2PtC∧N (9). The reactions of 9 with 2,2,6,6-tetramethylheptane-3,5-dionate (11), 1,1,1,3,3,3-hexafluoro-2-pyrazol-1-ylmethylpropan-2-ol (12), 2-[(diphenylphosphanyl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (14), and 2-[(di-tert-butylphosphanyl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (16) afforded square-planar Pt(II) complexes 10, 13, 15, and 17, correspondingly. According to X-ray analyses, the plane-to-plane distances of the π−π stacked complexes (10, 13, 15, and 17) increase from 3.478 Å for 10 to 4.203 Å for 15. The value of 4.203 Å for aromatic−aromatic π−π interaction is greater than the usual range of 3.3−3.8 Å for these phenomena. Pt−Pt distances increase from 5.076 Å for 13 to 8.123 Å for 17. The geometry of complex 17 deviates greatly from square planar and is best described as “bowl-shaped”. The effect of an increased π−π stacking distance on the electroluminescent properties of these Pt compounds is studied. The typical yellow luminescence band associated with the excimers of Pt compounds is absent for complex 15; however, it shows a green 540 nm electroluminesce band instead. An efficient LED constructed using compound 15 shows green electroluminescence with a peak efficiency of 6 cd/A.