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

Abstract

The first chalcogen-bridged [1]ferrocenophanes Fe(η-C5H3R)2E (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)° and 26.4(2)°, 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 Hückel 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(η-C5H4)2S] n 8 and poly(ferrocenyl selenide) [Fe(η-C5H4)2Se] 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)°. Thermal and anionic ROP of the isomer mixture 12 afforded the first soluble poly(ferrocenyl sulfide) [Fe(η-C5H3Me)2S] n 13 which was characterized by 1H and 13C 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 ΔE = 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.

Strained Metallocenophanes and Related Organometallic Rings Containing π‐Hydrocarbon Ligands and Transition‐Metal Centers

“…Tilting affects the lowest-energy transition in two ways. [24] As the tilt angle increases, the a 1 ' HOMO is raised in energy while the LUMO energy is lowered, which leads to the red shift to lower energy (see Figure 8). Additionally, as tilting reduces the symmetry of the complex (ferrocene is of D 5d (staggered) or D 5h (eclipsed) symmetry while an ansa metallocene can at most have C 2v symmetry) and ligand contributions to the LUMO appear to increase, [15b] the Laporte selection rule is relaxed resulting in a more intense absorption.…”

“…[124] Sulfur-and selenium-bridged [1]ferrocenophanes 33 and 34 can be made by typical salt metathesis methods by using dilithiofer- rocene, given careful workup at low temperatures. [24,124] These tilted, strained species should exhibit interesting reactivity but, to date, this area has not been explored in detail.…”

“…This red shift is accompanied by an increase in the intensity of the band and is understood in terms of a decreasing HOMO-LUMO gap with progressively tilted cyclopentadienyl ligands. [24] Figure 9 depicts an EHMO comparison of the frontier orbitals of ferrocene with two [1]ferrocenophanes with moderately tilted ([fcSiH 2 ], a = 19.18) and highly tilted ([fcS], a = 31.058) cyclopentadienyl ligands and the hypothetical highly tilted, but unbridged, ferrocene (a = 31.058). The two weakest absorptions (440 and 325 nm in ferrocene) have been attributed to transitions between filled e' and a 1 ' molecular orbitals and the e'' LUMO.…”

Polyferrocenylsilane‐Based Polymer Systems

“…[60] [Fe(h 5 -C 5 H 4 ) 2 P(p-C 6 H 4 -tBu)] 26.9(3) [61] [Fe(h 5 -C 5 H 4 ) 2 P(Ph)·BH 3 ] 24.38(10) [62] [Fe(h 5 -C 5 H 4 ) 2 P(Ph)·BCl 3 ] 25.01(4) [62] [a] Attachment of the sterically demanding SiMe 3 substituents to the Cp rings introduces further strain, as in the case of the SiMe 2 SiMe 2 bridge. [31] [Fe(h 5 -C 5 H 4 ) 2 Se] 26.4(2) [32] [Fe(h 5 -C 5 H 4 ) 2 S] 31.05(10) [33] [ [a] [35] [Fe(h 5 -C 5 H 3 tBu) 2 SiPh 2 ] 18.81 [36] [(h 5 -C 5 H 4 )Fe(h 5 -C 5 Me 4 )CH 2 S] 18.5(1) [35] [a] Two independent molecules in the unit cell. [b] The largest value in the literature.…”

Polymer science with transition metals and main group elements: Towards functional, supramolecular inorganic polymeric materials

“…However, polyferrocenylethylenes 9 (M ϭ Fe, E ϭ EЈ ϭ CH 2 , RЈ ϭ Me) with a CH 2 OCH 2 spacer (and other polymers derived from the ROP of [2]ferrocenophanes) possess much weaker metal-metal interactions than PFSs, whereas for polyferrocenylsulfides 7 (E ϭ S) the interaction is stronger. 27,37 Polyferrocenylphosphines (PFPs) 7 (E ϭ PR) offer additional possibilities because facile coordination of transition metals to phosphorus is possible. distribution is broad (PDI ϭ ca.…”