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 η 1-(butadienyl)ethynyl Ru{C≡CC[CH=C(CN) 2]=C 6H 4=C(CN) 2}(dppe)Cp (8), which reacts with a second equivalent of diynyl-Ru complex to give {Ru(dppe)Cp*}{C≡CC[= C 6H 4=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}(PPh 3) 2Cp (7) react with TCNQ to give the η 1-(butadienyl)ethynyls M{C≡CC[CPh=C(CN) 2]=C 6H 4=C(CN) 2}(dppe)Cp (10-Fe, 10-Ru) and Ru{C≡CC[C(C≡CPh)=C(CN) 2]=C 6H 4=C(CN) 2}(PPh 3) 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. © 2012 American Chemical Society.