Oxidative Dimerization of Aryldiynyl–Ruthenium Complexes

Chemical oxidation of Ru­(CCCCR)­(dppe)­Cp
with [FeCp2]­PF6 affords the binuclear cations
[{Ru­(dppe)­Cp}2C8R2]2+(PF6–)2 (R = Ph, 72+(PF6–)2; R = Fc, 82+(PF6–)2) by radical coupling at electron-rich
sites involving
the inner and outer CC triple bonds, to give cyclobutenediylidene
derivatives. In each case, mixtures of symmetrical and asymmetrical
isomers were obtained (2:1 for 72+(PF6–)2, 1:1 for 82+(PF6–)2), shown by single-crystal X-ray diffraction structure determinations
of 72+-asym(PF6–)2 and 82+-sym(AsF6–)2. 82+-sym consists of a central C4 ring
with two CCRu­(dppe)­Cp groups in the 1,2-positions
and two Fc substituents in the 3,4-positions, whereas for 72+-asym the substituent Ru­(dppe)­Cp is in
the 1-position, CCRu­(dppe)­Cp is in the 2-position,
Ph is in the 3-position, and −CCPh is in the 4-position.
DFT calculations reveal that the precursor cationic diynyl complexes
show important electron density on Cβ and Cδ, suggesting that radical coupling (Cδ + Cδ) or (Cβ + Cδ) affords the sym and asym isomers, respectively.