Luminescent tetranuclear silver(I) arylacetylide complexes bearing tricyclohexylphosphine ligands: Synthesis, molecular structures, and spectroscopic comparison with gold(I) and copper(I) arylacetylides

Abstract

A series of luminescent tetranuclear silver(I) arylacetylides [Ag4[μ(C≡C)xC6H4R-p)4 }-(PCy3)y (x = 1, y = 2, R = H (1), CH3 (2), OCH3 (3), Cx≡CPh (5); x = 2, y = 4, R = H (4)) were synthesized by treatment of tricyclohexylphosphine (PCy3) with [Ag{(C≡C)xC6H4R-p}]∞ in dichloromethane in the absence of light, and their spectroscopic and photophysical properties have been investigated. The X-ray structure of 1 exhibits a planar parallelogram-like Ag4core and consists of [Ag(C≡CPh)2] and [Ag(PCy3)] fragments with phenylacetylide σ-coordinated to silver atoms in a μ2-bonding mode (mean asymmetric Ag-C(α) distances are 2.05 and 2.33 Å). For 2, 3, and 5, with different para substituents in the arylacetylide units, similar tetranuclear structures are observed. In contrast, the structure of 4 can be described as two interpenetrating tetrahedra of four silver atoms and four phenylbutadiynyl moieties forming a twisted cubane structure, with each silver atom further coordinated to a PCy3 ligand. The electronic absorption spectra of 1-5 in CH2Cl2 are dominated by strong, vibronically structured absorptions at 250-332 nm (ε 104-105 dm3 mol-1 cm-1) with vibrational spacings of 1420-2160 cm-1 which are attributed to v(C≡C) or admixtures of acetylenic and aryl stretching frequencies. The solid-state emission spectra of 1-3 at 298 and 77 K show intense vibronically structured bands at 422-607 nm which are attributed to 3(ππ*) excited states of the arylacetylide ligands. For 4, the solid-state emission is slightly red-shifted in energy from the 3(ππ*) emission of the corresponding mononuclear [Au(PCy3)-(C≡C-C≡CPh)] congener, whereas for 5, a red-shift in its emission is observed from that of 1-4. A comparison between the emission of Cu(I), Ag(I), and Au(I) arylacetylides is made.