N-Assisted CPh–H Activation in 3,8-Dinitro-6-phenylphenanthridine. New C,N-Cyclometalated Compounds of Platinum(II): Synthesis, Structure, and Luminescence Studies

The activation of a C Ph –H bond in the phenyl ring of 3,8-dinitro-6-phenylphenanthridine (HC ∧ N) can be achieved by refluxing the intermediate [PtCl(η 3 -2-Me-C 3 H 4 )(HC ∧ N-κ N )] ( 1 ) (η 3 -2-Me-C 3 H 4 = η 3 -2-methylallyl) in 2-methoxyethanol to render the new cyclometalated complex [{Pt(μ-Cl)(C ∧ N)} 2 ] ( 2 ). The cleavage of the bridging system in 2 by the neutral ligands L rendered the mononuclear complexes [PtCl(C ∧ N)L] (L = tht 3, PPh 3 4, CN t Bu 5 ) with the geometry ( trans C, Cl). The air- and temperature-stable cationic compound [Pt(C ∧ N)(CNXyl) 2 ]ClO 4 ( 6 ) could be prepared from 2 by addition of CNXyl (1:4 molar ratio) after the Cl abstraction with AgClO 4 . Compound [Pt(C ∧ N-κ C )(tht) 3 ]ClO 4 ( 7 ) was prepared similarly to 6 but using a significant excess of tht, which produces the N-dissociation of the C ∧ N ligand. The photophysical properties of compounds 3 – 6 have been studied with the help of time-dependent density functional theory (TD-DFT) calculations. In 2-Me-THF at low temperature (77 K) the green emission of the HC ∧ N ligand turns to red phosphorescence in compounds 3 – 6, which was assigned to a mixed metal-to-ligand charge transfer/intraligand/ligand-to-ligand charge transfer [ 3 MLCT/ 3 IL/ 3 L′LCT] excited state. In the solid state at low temperature (77 K) the emissive behaviors are quite similar to that observed in glassy solutions with some contribution of excimeric π–π* emissions in the neutral chloro derivatives. Compounds 4 – 6 are also emissive in the solid state at room temperature with photoluminescence quantum yields (Φ) between 0.032 and 0.05.