Substitution of Tetracyanoethene by Ethynyl–Metal Complexes Gives Tricyanovinylethynyl (Tricyanobutenynyl) Derivatives: Syntheses, Protonation, and Addition of Metal–Ligand Fragments

Abstract

A series of complexes containing the new tricyanovinylethynyl (3,4,4-tricyanobut-3-en-1-ynyl) ligand have been obtained by substitution of a CN group in tetracyanoethene upon reaction with the ethynyl complexes M(C≡CH)(PP)Cp′ (M = Ru, Os, (PP)Cp′ = (PPh3)2Cp; M = Ru, PP = dppe, Cp′ = Cp, Cp*). The reactions proceed in higher yield as the metal environment becomes more sterically hindered, the normal [2 + 2]-cycloaddition/ring-opened product M{C[═C(CN)₂]CH═C(CN)₂}(PP)Cp′ also being formed in some cases. The diynyl complex Ru(C≡CC≡CH)(dppe)Cp* reacts with tcne to give only the ring-opened adduct Ru{C≡CC[═C(CN)₂]CH═C(CN)₂}(dppe)Cp*. Protonation (HBF₄ or HPF₆) of Ru{C≡CC(CN)═C(CN)₂}(dppe)Cp* afforded the vinylidene cation [Ru{═C═CHC(CN)═C(CN)₂}(dppe)Cp*]⁺. A second transition-metal fragment MLn (MLn = Ru(PPh₃)₂Cp, M′(dppe)Cp* (M′ = Ru, Os), RuCl(dppe)₂) can be added to the CN group trans to the metal center; electrochemical, spectroscopic, and computational studies indicate that there is little ground-state delocalization between the metal centers. In the case of the tricyanovinylethynyl derivatives, an intense MLCT (or ML—LCT) transition can be identified in the visible region, which is responsible for the intense blue to purple color of these species; the analogous transition in the vinylidene-based complexes is significantly blue-shifted. The X-ray crystallographically determined structures of several of these complexes are reported. The cations [{Cp*(dppe)Ru}{μ-(C/N)≡CC(CN)═C(CN)(≡C/N)}{M(dppe)Cp*}]⁺ (M = Ru, Os) show some C≡C/C≡N disorder (and associated Ru/Os disorder in the case of the heterometallic example) in the crystals.