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−)

Abstract

The binuclear μ-bis(ethynyl)thiophene complex [Cp*(dppe)Fe−C⋮C-2,5-C4H2S−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 1H, 13C, and 31P 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 (ipa/ipc) current ratio of unity. A third almost reversible process was observed at a more positive potential (ipa/ipc = 0.83, 0.100 V s-1 scan rate). The redox potentials E0 are −0.39, −0.05, and 1.04 V vs SCE. The wave separation (|ΔE0| = 0.34 V) between the two reversible oxidation processes leads to a big comproportionation constant (Kc = 5.8 × 105), establishing the stability of the mixed-valence complex (MV). Addition of 0.95 equiv of [Cp2Fe][PF6] to a CH2Cl2 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*][PF6] (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 Vab determined in CH2Cl2 is 2515 and 2520 cm-1 for the complexes 4 and 5, respectively.