Molecular Wires: Synthesis and Properties of the New Mixed-Valence Complex [Cp*(dppe)Fe−C⋮C−X−C⋮C−Fe(dppe)Cp*][PF₆] (X = 2,5 -C₄H₂S) and Comparison of Its Properties with Those of the Related All-Carbon-Bridged Complex (X = −C₄−)

The binuclear μ-bis(ethynyl)thiophene complex [Cp*(dppe)Fe−C⋮C-2,5-C 4 H 2 S−C⋮C−Fe(dppe)Cp*] ( 3, Cp* = pentamethylcyclopentadienyl, dppe = 1,2-bis(diphenylphosphino)ethane) was prepared in a three-step one-pot procedure from the 2,5-bis(trimethylsilylethynyl)thiophene and the chloro organoiron complex Cp*(dppe)FeCl ( 2 ). Complex 3, isolated as an orange thermally stable powder (72%), was characterized by 1 H, 13 C, and 31 P NMR, FTIR, UV−vis, and Mössbauer spectroscopies and elemental analysis. CV of 3 from −1.5 to 1.4 V vs SCE displayed two reversible one-electron waves with the ( i p a / i p c ) current ratio of unity. A third almost reversible process was observed at a more positive potential ( i p a / i p c = 0.83, 0.100 V s - 1 scan rate). The redox potentials E 0 are −0.39, −0.05, and 1.04 V vs SCE. The wave separation (|Δ E 0 | = 0.34 V) between the two reversible oxidation processes leads to a big comproportionation constant ( K c = 5.8 × 10 5 ), establishing the stability of the mixed-valence complex (MV). Addition of 0.95 equiv of [Cp 2 Fe][PF 6 ] to a CH 2 Cl 2 solution of 3 produces the MV Fe(II)−Fe(III) complex 4 (86%). Analytically pure samples in 4 were characterized by FTIR, Mössbauer, ESR, UV−vis, and NIR spectroscopies. It is established that the two metal sites in 4 are not equivalent on the very fast IR time scale (10 - 13 s). The Mössbauer spectrum of 4 shows the presence of a sharp unique doublet, indicating that the odd electron is delocalized on both metal centers on the Mössbauer time scale in the range 300 > T > 4 K. In contrast, the Mössbauer spectrum of [Cp*(dppe)Fe−C⋮C−C⋮C−C⋮C−C⋮C−Fe(dppe)Cp*][PF 6 ] ( 5 ) displayed three doublets corresponding to Fe(II), Fe(III), and Fe(averaged), corresponding to the simultaneous presence of the trapped and detrapped valence in the same sample. The electronic coupling V ab determined in CH 2 Cl 2 is 2515 and 2520 cm - 1 for the complexes 4 and 5, respectively.