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

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.