Neutral and Cationic NCN Pincer Platinum(II) Complexes with 1,3-Bis(benzimidazol-2′-yl)benzene Ligands: Synthesis, Structures, and Their Photophysical Properties

A series of neutral pincer Pt(II) chloride complexes 3-7 were first synthesized via the reaction of 1,3-bis(N-substituted benzimidazol-2'-yl) benzene (bzimb) ligands 2a-e with K2PtCl4 in refluxing HOAc. By treatment of complex 3 with KI and aromatic terminal alkynes in methanolic NaOH, the Pt(II) iodide complex 8 and Pt(II) acetylide complexes 9-11 were then obtained. Cationic Pt(II) complexes 12-15 were finally produced by the chloro-substitution reaction of complex 3 with AgOTf and several neutral N-donor ligands. With these new compounds in hand, various characterizations including elemental analysis, NMR, and IR were carried out to investigate their properties. On the basis of the X-ray single-crystal structure analysis of complexes 3-5, 8, 9, and 12, weak intermolecular interactions including pi center dot center dot center dot pi stacking and/or hydrogen bonds were found to be responsible for their assembly. After full investigation of the electronic absorption and photoluminescent properties of the Pt(II) complexes 3-15, time-dependent density functional theory (TD-DFT) calculations were carried out to elucidate the origins of the UV-vis absorptions. We were delighted to find that the luminescence quantum yields in degassed CH2Cl2 solution were around 0.13 in most cases and the emission lifetimes were in the microsecond range, indicating that these complexes could be used as bright phosphorescent materials with various potential applications in the future.