Synthesis and Properties of Stable 1,2-Bis(metallocenyl)disilenes: Novel d–π Conjugated Systems with a Si=Si Double Bond

Abstract

Stable 1,2-bis(metallocenyl)disilenes were synthesized for the first time and were characterized by spectroscopic and X-ray crystallographic analyses. On the basis of cyclic voltammograms, (E)-1,2-bis(ferrocenyl)-1,2-bis(2,4,6-triisopropylphenyl)disilene (1a) was found to be a stable five-electron redox system with four steps, while (E)-1,2-bis(ruthenocenyl)-1,2-bis(2,4,6-triisopropylphenyl)disilene (1b) showed four-step redox couples with four electrons. The UVvis spectra and theoretical calculations for these disilenes suggested that they should be novel d³ conjugated systems containing a disilene unit. In addition, chalcogenation reactions of 1a resulted in the formation of new heterocyclic compounds bearing ferrocenyl units. For example, the first stable 1,2,3,4-dithiadisiletane derivative was obtained by the sulfurization reaction of 1a.Organic compounds containing ³-electron systems such as ethylene, benzene, and ketones are of great importance. Especially, extended ³-electron-conjugated systems of organic compounds exhibit lower HOMOLUMO gaps than those of olefins, showing unique electronic character and possibly applicable to semiconductors, organic electroluminescence, and so on.1 On the other hand, ferrocene is known to show unique electrochemical properties as a stable redox system. 2 d³ Electron-conjugated systems, e.g., multinuclear transitionmetal complexes bridged by organic ³-conjugated systems such as phenylene and C=C units, have attracted considerable attention from the viewpoint of their electrochemical properties.3 Particularly, stable multistep redox systems should be good model systems for the elucidation of the properties of mixed-valence states.4 Therefore, there has been much interest in the chemistry of ferrocene oligomers bearing a ³-electron spacer, e.g., Fc(Ph)C=C(Ph)Fc, 5 FcN=NFc, 6 and TbtP=PFc (Fc = ferrocenyl), 7 as models of mixed-valence states (Figure 1).The first stable disilene as a novel ³-bond system between two silicon atoms was synthesized and isolated by West and his co-workers in 1981. 8 Since disilenes are highly reactive due to the weak ³ bond between silicon atoms, kinetic stabilization should be necessary to isolate and handle disilene derivatives.

The Synthesis and Properties of Bimetallic d-^|^pi; Electron Systems Containing Metallocenyl Substituents of Fe or Ru, and ^|^pi;-Electron Spacers of Heavier Main Group Elements from Group 14 or 15

“…It should be noted that the oxidation processes for both Fc moieties were observed as two separated steps (ΔE E 1 E 2 ) with ΔE 0.55 (3b), 0.42 (4), and 0.48 V (5), respectively. These ΔE values are considerably larger than those observed for Fc N N Fc (ΔE 0.21 V), 1a (E) Fc(Ph)-C C(Ph)Fc (ΔE 0.18 V), 24 (E) Tip(Fc)Si Si(Fc)Tip (ΔE 0.19 V), 15,16 and (E) Tip(Fc)Ge Ge(Fc)Tip (ΔE 0.38 V), 19 suggesting a signi cantly increased and effective ability for conjugation of the heavier group 15 element π spacers relative to those of second row elements, and even heavier group 14 elements. Notably, the ΔE of diphosphene 3b was found to be a the largest of the 1,2 bis(ferrocenyl)dipnictenes.…”

“…16 Structural and spectral features of 1 were found to be similar to those of 1, except for the color of the compounds. Whereas 1 is orange red in the solid state and in solution of organic solvents, 1 is yellow.…”

“…θ 12.3 in Tip 2 Ge GeTip 2 ) 20 and 1,2 bis(Fc)disilene 1 (θ 27.9 ). 15,16 These structural parameters indicate a relatively weak Ge Ge double bond in 2, and suggest the possibility of thermally controlled dissociation of 2 in solution, yielding two equivalents of the corresponding germylene monomer Tip(Fc)Ge: (9).…”

“…The separation of the half wave oxidation potentials (ΔE 1/2 ) is indicative of the magnitude of electronic interaction between two redox centers. 26 The separation issigni cantly larger in 2 (ΔE 1/2 0.44 V) than in disilene 1 (ΔE 1/2 0.19 V) 15,16 and (E) Ph(Fc)C C(Fc)Ph (21, ΔE 1/2 0.18 V), 24 re ecting the increased degree of d π electron interaction in 2 relative to its silicon (1) and carbon (21) analogues. With respect to objective (i), the electrochemical analysis indicated that digermene 2 is a very effective d π electron system, exhibiting in comparison to disilene 1 and ole n 21 a stable, multi step redox behavior.…”

“…The synthetic route for the generation of 1,2 bis(Fc)disilene 1 is outlined in Scheme 3. 15,16 Dichlorosilane 6, containing a Tip group, was prepared in 49% isolated yield by the reaction between FcLi and TipSiCl 3 in THF at 78 . Dichlorosilane 6 is remarkably stable, probably due to the steric bulk of the Tip and Fc groups, and can even be manipulated in open air.…”