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

A series of luminescent tetranuclear silver(I) arylacetylides [Ag-4{mu-(CdropC)(x)C6H4R-p(4)}-(PCy3)(y)] (x = 1, y = 2, R = H (1), CH3 (2); OCH3 (3), CdropCPh (5); x = 2, y = 4, R = H (4)) were synthesized by treatment of tricyclohexylphosphine (PCy3) with [Ag{(CdropO)(x)C6H4R-p}](infinity) 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 Ago core and consists of [Ag(CdropCPh)(2)] and [Ag(PCy3)] fragments with phenylacetylide sigma-coordinated to silver atoms in a mu(2)-bonding mode (mean asymmetric Ag-C(alpha) distances are 2.05 and 2.33 Angstrom). 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 (epsilon 10(4)-10(5) dm(3) mol(-1) cm(-1)) with vibrational spacings of 1420-2160 cm(-1) which are attributed to nu(CdropC) 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)(pipi*) excited states of the arylacetylide ligands. For 4, the solid-state emission is slightly red-shifted in energy from the (3)(pipi*) emission of the corresponding mononuclear [Au(PCy3)-(CdropC-CdropCPh)] 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.