Asymmetric Coordination Mode of Phenanthroline-like Ligands in Gold(I) Complexes: A Case of the Antichelate Effect

Abstract

A series of new cationic complexes [(PPh3)Au(L)](OTf) (L = 1,10-phenanthroline (phen) (1); 5,6-dimethyl-1,10-phenanthroline (dmphen) (2); 5,6-epoxy-5,6-dihydro-1,10-phenanthroline (ephen) (3); dimethyl 2,2’-biquinoline-4,4’-dicarboxylate (dmcbiq) (4); 2,3,4,5,6,7,8,9-octachloro-1,10-phenantroline (ocphen) (5)) were obtained in high yields by the standard ligand exchange reactions from [(PPh3)AuCl]. The compounds were characterized by analytical and spectroscopic methods. For all compounds, the crystal structure was established using X-ray diffraction analysis. In all structures, an atypical asymmetric mode of coordination of the diimine ligand (one shortened and the second elongated Au–N bond) was found, which is a consequence of the so-called antichelate effect. Structures 1–3 with electron-donating substituents (H, methyl, epoxide) are the most asymmetric, while structures 4 and 5 with electron-withdrawing groups (CO2Me and Cl) are the least asymmetric. According to the results of the density functional theory (DFT) calculations, both Au–N bonds are covalent, and the geometry around gold(I) can be considered as a distorted triangular one. Cationic complexes [(PPh3)Au(L)]+ form dimers in the crystal structure due to π–π interactions, the calculated energy of which reaches 4.2 kcal·mol–1 in the case of structure 5. These dimers in 5 additionally interact with OTf– anions through different kinds of contacts with a calculated total energy of 8.8 kcal·mol–1. Inside the {[(PPh3)Au(ocphen)]2}2+ associates, intermolecular Au···Cl noncovalent interactions were also found. Remarkably, the dimers in 5 are built through nonclassical π–π interactions between geometrically different parts of the ocphen ligand, which is a consequence of its asymmetric coordination to the gold(I) center. All complexes show multiple photoluminescence in the solid state at room temperature. The lifetimes of the excited state are in the microsecond range, which is characteristic of phosphorescence. Time-dependent DFT (TD-DFT) calculations reveal that electronic transitions of different nature are responsible for the photoluminescence of complexes 1–5.