Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes Defines Structural Diversity

The face-capping triphosphine, 1,1,1-tris(diphenylphosphino)methane (tppm), together with bridging alkynyl ligands and the counterions, facilitates the formation of a family of silver complexes, which adopt cluster frameworks of variable nuclearity. The hexanuclear compounds [Ag₆(C₂C₆H₄-4-X)₃(tppm)₂(An^-)₃] (X = H (1), CF₃ (2), OMe (3), An^- = CF₃SO₃ ^- X = OMe (4), An^- = CF₃COO^-) are produced for the electron-accepting to moderately electron-donating alkynes and the appropriate stoichiometry of the reagents. 1 and 3 undergo an expansion of the metal core when treated with 1 equiv of Ag⁺ to give the species [Ag₇(C₂C₆H₄-4-X)₃(tppm)₂(CF₃SO₃)₃](CF₃SO₃) (X = H (5), OMe (6)). The electron-donating substituent (X = NMe₂) particularly favors this Ag₇ arrangement (7) that undergoes geometry changes upon alkynylation, resulting in the capped prismatic cluster [Ag₇(C₂C₆H₄-4-NMe₂)₄(tppm)₂(CF₃SO₃)](CF₃SO₃)₂ (8). Alternatively, for the aliphatic ^tBu-alkyne, only the octanuclear complex [Ag₈(C₂Bu^t)₄{(PPh₂)₃CH}₂(CF₃SO₃)₂](CF₃SO₃)₂ (9) is observed. The structures of 1-4 and 6-9 were determined by X-ray diffraction analysis. In solution, all the studied compounds were found to be stereochemically nonrigid that prevented their investigation in the fluid medium. In the solid state, clusters 2, 3, 5-8 exhibit room temperature luminescence of triplet origin (maximum φ_em = 27%, λ_em = 485-725 nm). The observed emission is assigned mainly to [d(Ag) → π∗(alkyne)] electronic transitions on the basis of TD-DFT computational analysis.