Molecular Pd(II) and Pt(II) triangles containing 4,5-dicyanoimidazolate and diimine ligands: Self-assembly, spectroscopic, and photophysical properties

Two isostructural molecular triangles, M(d-t-bpy)(im-CN₂)]₃(ClO₄)₃ (M ≡ Pd 1, Pt 2; d-t-bpy ≡ 4,4′-di-tert-butyl-2,2′-bipyridine, im-CN₂ ≡ 4,5-dicyanoimidazolate), were synthesized and characterized by the X-ray diffraction study. Both 1 and 2 show molecular capsule-like structures in the solid state, which were built from two triangles in a head-to-tail manner and one anion entrapped inside the cavity. It is noted that there are some anion?π and -CN?π interactions as an origin of electrostatic forces found in both 1 and 2, and these weak interactions with non-classical hydrogen bonding are most likely responsible for the formation of molecular capsules. In addition, 2 shows a vibronic-structured emission at ca. 457, 486, and 520 nm in solution, which is absent in 1. The observed vibronic spacing of 1200-1400 cm⁻¹ is within the expected stretching frequencies of aromatic diimines in the excited state, and the vibronic emission is thus suggested due to an intraligand transition of d-t-bpy possibly mixing with a metal-to-ligand charge-transfer transition.