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 π···π
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.