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

Abstract

The reaction of tetracyanoquinodimethane (TCNQ) with Ru(C≡CC≡CH)(dppe)Cp* (5) at the outer (from Ru) C≡C triple bond gives η1-(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)3Ph}(PPh3)2Cp (7) react with TCNQ to give the η1-(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]═C6H4═C(CN)2}(PPh3)2Cp (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]−• 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 57Fe Mössbauer 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.