Highly strained, ring-tilted [1]ferrocenophanes containing group 16 elements in the bridge: Synthesis, structures, and ring-opening oligomerization and polymerization of [1]thia- and [1]selenaferrocenophanes

The first chalcogen-bridged [1]ferrocenophanes Fe(eta-C5H3R)(2)E (6, E = S, R = H; 7, E = Se, R = H; 12, E = S, R = Me) have been synthesized and characterized both structurally and spectroscopically. Synthesis of sulfur- and selenium-bridged species 6 and 7 was achieved by the reaction of dilithioferrocene.TMEDA (TMEDA = tetramethylethylenediamine) with bis(phenylsulfonyl) sulfide S(O2SPh)(2) and selenium diethyldithiocarbamate Se(S2CNEt2)(2), respectively, in 20-30% yields. Structural characterization of both 6 and 7 revealed highly strained structures with tilt-angles between the cyclopentadienyl ligands of 31.05(10)degrees and 26.4(2)degrees, respectively. Compounds 6 and 7 are purple and red-purple, respectively; comparison of the structures of known [1]ferrocenophanes 1 showed that when the second period (from group 14-16) is traversed, there is a substantial increase in cyclopentadienyl ring-tilting in main group element bridged [1]ferrocenophanes, and the lowest energy UV/vis absorption peaks become increasingly red-shifted. Extended Huckel MO calculations were performed and, consistent with this observation, predicted a decrease in the HOMO-LUMO gap as the ring-tilt increases. Thermal ring-opening polymerization (ROP) of both 6 and 7 afforded the insoluble poly(ferrocenyl sulfide) [Fe(eta-C5H4)(2)S](n) 8 and poly(ferrocenyl selenide) [Fe(eta-C5H4)(2)Se](n) 9, respectively. Differential scanning calorimetry studies of the ROP process provided estimates of the strain energies of 6 and 7 which were ca. 130(+/-20) and 110(+/-20) kJ mol(-1), respectively. Anionic ROP of 6 also yielded the insoluble poly(ferrocenyl sulfide) 8, However, linear soluble dimeric and trimeric trimethylsilyl-capped oligo(ferrocenyl sulfides) 10b and 11b were synthesized by the reaction of 6 with dilithioferrocene.TMEDA followed by the addition of Me3SiCl and were characterized spectroscopically, electrochemically, and, for 11b, by X-ray diffraction, and provide useful models for the analogous high polymer. The dimethylated sulfur-bridged species 12 was prepared as a mixture of isomers from the reaction between dilithiodimethylferrocene.TMEDA and S(O2SPh)(2). and X-ray structural characterization of a single isomer 12a showed the presence of a large tilt-angle of 31.46(8)degrees. Thermal and anionic ROP of the isomer mixture 12 afforded the first soluble poly(ferrocenyl sulfide) [Fe(eta-C5H3Me)(2)S](n) 13 which was characterized by H-1 and C-13 NMR, elemental analysis, thermogravimetric analysis, and gel permeation chromatography. Cyclic voltammetric studies of 13 showed the presence of two reversible oxidation waves with a redox coupling Delta E = ca. 0.32 V, which is consistent with the presence of significantly stronger M...M interactions compared to those present in other ring-opened poly(ferrocenes) derived from [1]ferrocenophanes.