Molecular wires:: Synthesis and properties of the new mixed-valence complex [Cp*(dppe)Fe-CC-X-CC-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 mu-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 H-1, C-13, and P-31 NMR, FTIR, UV-vis, and Mossbauer 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 (\Delta E-0\ = 0.34 V) between the two reversible oxidation processes leads to a big comproportionation constant (K-c = 5.8 x 10(5)), 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, Mossbauer, 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 Mossbauer spectrum of 4 shows the presence of a sharp unique doublet, indicating that the odd electron is delocalized on both metal centers on the Mossbauer time scale in the range 300 > T > 4 K. In contrast, the Mossbauer 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 V-ab determined in CH2Cl2 is 2515 and 2520 cm(-1) for the complexes 4 and 5, respectively.