Synthesis, Structure, and Photophysical Properties of Platinum(II) (,,') Pincer Complexes Derived from Purine Nucleobases†.

The synthesis of a series of Pt{κ-,,'-[L]}X (X = Cl, RC≡C) pincer complexes derived from purine and purine nucleosides is reported. In these complexes, the 6-phenylpurine skeleton provides the ,-cyclometalated fragment, whereas an amine, imine, or pyridine substituent of the phenyl ring supplies the additional '-coordination point to the pincer complex. The purine ,-fragment has two coordination positions with the metal (1 and 7), but the formation of the platinum complexes is totally regioselective. Coordination through the 7 position leads to the thermodynamically favored [6.5]-Pt{κ-7,,'-[L]}X complexes. However, the coordination through the 1 position is preferred by the amino derivatives, leading to the isomeric kinetic [5.5]-Pt{κ-1,,'-[L]}X complexes. Extension of the reported methodology to complexes having both pincer and acetylide ligands derived from nucleosides allows the preparation of novel heteroleptic bis-nucleoside compounds that could be regarded as organometallic models of Pt-induced interstrand cross-link. Complexes having amine or pyridine arms are green phosphorescence emitters upon photoexcitation at low concentrations in CHCl solution and in poly(methyl methacrylate) (PMMA) films. They undergo self-quenching at high concentrations due to molecular aggregation. The presence of intermolecular π-π stacking and weak Pt···Pt interactions was also observed in the solid state by X-ray diffraction analysis.