| Abstract
| - Weakly coupled mixed-valence E- and Z-diferrocenylethylene complexes of 1, 2, and 3 were synthesized and structurally characterized. The analysis based on the Hush formalism suggests that Coulomb repulsion energy dominates the overall stability of their mixed-valence states. IVCT spectra and electrochemical data of 1−3 were compared with analogous diferrocenyl complexes. The paradoxical phenomena between experimental results of mixed-valence complexes of E- and Z-1,2-bis(1‘-ethyl-1-ferrocenyl)-1,2-dimethylethylene and theories were also discussed.
- To study metal-to-metal interactions in mixed-valence states of two weakly coupling ferrocenyl groups assembledin E or Z conformation on an ethylenic double bond, E-1,2-dimethyldiferrocenylethylene (1), Z-1,2-dimethyldiferrocenylethylene (2), and 1,2-diferrocenylcyclohexene (3) were synthesized and structurally characterized. Crystalsof 1 are triclinic, P1̄, with a = 7.494(9) Å, b = 10.801(3) Å, c = 11.971(2) Å, α = 102.17(2)°, β = 106.12(9)°,γ = 90.42(2)°, V = 907.8 Å3, and Z = 2. Crystals of 2 are monoclinic, P21/c, with a = 13.601(8) Å, b =11.104(4) Å, c = 13.732(1) Å, β = 114.26(7)°, V = 1890.8(3) Å3, and Z = 4. Crystals of 3 are orthorhombic,P212121, with a = 5.766(2) Å, b = 13.090(1) Å, c = 26.695(2) Å, V = 2014.9(3) Å3, and Z = 4. Intervalencetransition spectra (IT) and electrochemical data have been determined and compared with those of diferrocenylbenzene (para, ortho, and meta). The comproportionation constants in nitrobenzene at 25 °C were found to be490 and 813 for 1 and 3, respectively. That of 2 was not measured because of the fact that 2+ isomerizes rapidlyin all solvents tested, yielding nearly a racemic mixture of E and Z conformers. This finding helps to clear theparadoxical phenomenon between experimental results of mixed-valence complexes of E- and Z-1,2-bis(1‘-ethyl-1-ferrocenyl)-1,2-dimethylethylene and theories. The stability of the mixed-valence species was discussed in termsof resonance delocalization, Coulomb repulsion energy, inductive effect, magnetic interaction, structural factors,and statistical factor. According to our analysis based on the Hush formalism, the contribution due to Coulombrepulsion energy dominates the overall stability of the mixed-valence state in 1+, 2+, and 3+. Stabilization thatarises from resonance delocalization is only minor and contributes less than 4% to the overall stability, even in3+ where linked Cp rings and the ethylenic plane are coplanar. In calculating the resonance contribution,crystallographic Fe−Fe distances of 7.44 Å (1) and 6.68 Å (3) were used for 1+, and 3+, respectively.
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