Reactions of 7,7,8,8-Tetracyanoquinodimethane with Poly-ynyl Ruthenium and Iron Complexes

The reaction of tetracyanoquinodimethane (TCNQ) with Ru(C CC CH)(dppe)Cp* (5) at the outer (from Ru) C C triple bond gives eta(1)-(butadienyl)ethynyl Ru{C CC[CH=C(CN)(2)]=C6H4=C(CN)(2)}(dppe)Cp (8), which reacts with a second equivalent of diynyl-Ru complex to give {Ru(dppe)Cp*}{C CC[=C6H4=C(CN)(2)]CH=CHC[=C(CN)(2)]C C}{Ru(dppe)Cp*} (9). The Ph-substituted complexes M{C CC CPh}(dppe)Cp* (M = Fe 6-Fe, Ru 6-Ru) and Ru{(C C)(3)Ph}(PPh3)(2)Cp (7) react with TCNQ to give the eta(1)-(butadienyl)ethynyls M{C CC[CPh=C(CN)(2)]=C6H4=C(CN)(2)}(dppe)Cp (10-Fe, 10-Ru) and Ru{C CC[C(C CPh)=C(CN)(2)]= C6F4=C(CN)(2)}(PPh3)(2)Cp (11), respectively. Single-crystal X-ray diffraction molecular structure determinations for 8-11 have been carried out. In the Fe series, we suggest that the initial step of the mechanism involves electron transfer to form the [TCNQ](-center dot) salt of the diynyl-iron cation, followed by C-C bond formation to give a zwitterionic intermediate. Isolated products can be rationalized by further reaction involving [2 + 2]-cycloaddition of one of the C=C(CN)(2) groups of TCNQ to a C C triple bond of the metal poly-ynyl complex and a subsequent ring-opening reaction of the resulting (unobserved) cyclobutenyl intermediate. On the basis of X-ray diffraction data, redox potential determinations, and Fe-57 Mossbauer and UV-vis spectroscopies, the electronic structures of the new compounds contain significant contributions from polarized mesomers involving charge transfer from the electron-rich metal ligand fragment to the cyanocarbon ligand via the conjugated unsaturated carbon linker.