Metal-metal interactions in weakly coupled mixed-valence E- and Z-diferrocenylethylene complexes

To study metal-to-metal interactions in mixed-valence states of two weakly coupling ferrocenyl groups assembled in E or Z conformation on an ethylenic double bond, E-1,2-dimethyldiferrocenylethylene (1), Z-1,2-dimethyldi-ferrocenylethylene (2), and 1,2-diferrocenylcyclohexene (3) were synthesized and structurally characterized. Crystals of 1 are triclinic, P1, with a = 7.494(9) A, b = 10.801(3) A, c = 11.971(2) A, alpha = 102.17(2) degrees, beta = 106.12(9) degrees, gamma = 90.42(2) degrees, V = 907.8 A3, and Z = 2. Crystals of 2 are monoclinic, P2(1)/c, with a = 13.601(8) A, b = 11.104(4) A, c = 13.732(1) A, beta = 114.26(7) degrees, V = 1890.8(3) A3, and Z = 4. Crystals of 3 are orthorhombic, P2(1)2(1)2(1), with a = 5.766(2) A, b = 13.090(1) A, c = 26.695(2) A, V = 2014.9(3) A3, and Z = 4. Intervalence transition spectra (IT) and electrochemical data have been determined and compared with those of diferrocenyl-benzene (para, ortho, and meta). The comproportionation constants in nitrobenzene at 25 degrees C were found to be 490 and 813 for 1 and 3, respectively. That of 2 was not measured because of the fact that 2+ isomerizes rapidly in all solvents tested, yielding nearly a racemic mixture of E and Z conformers. This finding helps to clear the paradoxical 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 terms of 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 Coulomb repulsion energy dominates the overall stability of the mixed-valence state in 1+, 2+, and 3+. Stabilization that arises from resonance delocalization is only minor and contributes less than 4% to the overall stability, even in 3+ where linked Cp rings and the ethylenic plane are coplanar. In calculating the resonance contribution, crystallographic Fe-Fe distances of 7.44 A (1) and 6.68 A (3) were used for 1+, and 3+, respectively.