Ion-pair complexes with strong near infrared absorbance: Syntheses, crystal structures and spectroscopic properties

Three ion-pair complexes, [4-NH2-Py]2[M(mnt)2] (4-NH2-Py1+ = 4-amino-pyridinium; mnt2− = maleonitriledithiolate; M = Pt (1), Pd (2) or Ni (3)), have been synthesized and characterized. In the crystal of 1, the strong H-bonding interaction was found from the protonated N-atom of pyridinium to the CN group of [Pt(mnt)2]2− together with a weak Pt…H interaction between the anion and the cation. The crystals of 2 and 3 are isostructural with very similar lattice parameters and packing structures, which are distinct from the crystal of 1. Two kinds of strong H-bonding interactions are observed in the crystals of 2 and 3 between the CN groups of [M(mnt)2]2− anion and the protonated N-atom of 4-NH2-Py1+ cation as well as the CN groups of [M(mnt)2]2− anion and the amino group of 4-NH2-Py1+ cation. Complex 1 shows an intense near-IR absorbance in acetonitrile and solid state, such an absorption band is probably assigned to IPCT transition as well as a trace amount of [Pt(mnt)2]1− species; complex 3 possesses a weak near-IR absorption band which can be attributed to the mixture of d–d transition in [Ni(mnt)2]2− and IPCT transition as well as a trace amount of [Ni(mnt)2]1− species.