Telechelic or side-functionalized diorganosiloxanes with ferrocenylimine groups

Abstract

Formyl-ferrocene, FFc, was prepared and used as agent to attach ferrocenyl moiety to siloxanes through imine group. Thus, 1,3-bis(3-aminopropyl)tetramethyldisiloxane, L1, α,ω-bis(3-aminopropyldimethylsiloxy)oligodiphenylsiloxane, L2, and poly[dimethyl-methyl(3-aminopropyl)siloxane], L3 (the last two prepared by us) were converted in the corresponding azomethine derivatives: 1,3-bis(3-ferrocenyliminepropyl)tetramethyldisiloxane, L1Fc, α,ω-bis(ferrocenyliminepropyldimethylsiloxy)oligodiphenylsiloxane, L2Fc, and poly[dimethyl-methyl(3-ferrocenyliminopropyl)siloxane], L3Fc. The iron spin state was determined by Mossbauer spectroscopy, emphasizing the presence of mixed-valence iron of interest for molecular electronics. Cyclic voltammograms of the compounds, both in solution and as films deposited on glassy carbon electrodes, showed quasi-reversible oxidation/reduction waves making them suitable for sensing applications. The solvent effect was studied by UV–Vis spectra.

Introduction

It is believed that the discovery of ferrocene has contributed greatly to the rapid development of modern organo-transition metal chemistry. A variety of metals were used as central atoms in such sandwich compounds, including titanium, cobalt, zirconium, platinum, chromium, osmium, and uranium, resulting in the new compounds class, metallocenes. However, ferrocene has been most studied due to their interesting properties (thermal, redox, electrical, optical, magnetic, catalytical, etc.) [1] that have found application in various fields such as asymmetric catalysis, electrochemistry and development of new pharmaceuticals [1], [2]. Ferrocene is relatively stable towards acids or bases but it is sensitive to oxidizing agents [1]. When is oxidized to the ferricinium species, the ferrocene moieties may act as oxidizing agents [3]. The most reversible Fc/Fc⁺⁺ system is useful in electron transfer and redox catalysis [2]. Due to the redox properties, ferrocene-containing materials can be used in sensing applications [4]. The redox potential of ferrocene derivatives may be tuned in some extent by the proper functionalization [5]. Co-existence in the same compounds of Fe(II) and Fe(III) species results in mixed valence compounds. Such compounds are suitable for the study of intramolecular electron and energy transfer. The electronic properties of mixed valence compounds strongly depend on the extent of the electronic interaction between the redox centers [6]. Functionalized ferrocene is a useful tool for the incorporation of metal into polymeric or high organic structure [7]. In principle, one could prepare a whole series of functionalized redox-active polymers by combining the ferrocene-containing monomer with a variety of other monomers. Thus, this sandwich complex was incorporated into a variety of polymeric materials, including ferrocene-containing polyesters and amides, polyvinylferrocenes, ferrocene-containing polyurethanes, polysilanes and polysiloxanes [3], ferrocenyl-based polymers and copolymers being important due to their redox, electrical, optical, magnetic, catalytic, preceramic, and elastomeric properties. The ferrocene presence also induces high photochemical and thermal stability. The polymers containing ferrocene are useful for the manufacture of electronic devices such as microelectrochemical diodes, in the formation of redox gels, which show charge transfer properties, in the modification of electrodes, in the construction of amperometric biosensors, as precursors to ferromagnetic ceramics, in the area of non-linear optical (NLO) materials, etc. [2], [5], [8], [9].

The widespread utility of organosiloxane polymers has led to an increasing interest in modification of their properties by incorporation of various moieties in the polymer chain. The chemical insertion of the metals in siloxane-based polymeric structures constitutes a challenge for obtaining materials with new properties, taking into account the polysiloxanes uniqueness [10].

There are many attempts in literature to prepare ferrocenyl-siloxane structures [11], [12], [13], [14]. Thus, a ferrocene-derivatized monomer was prepared in which ferrocene is appended to an oligosiloxane chain that is terminated in Si–H groups. Ferrocene grafted siloxanes were prepared in high yields via Rh-catalyzed dehydrogenative coupling of a series of monomeric, polymeric, and cyclic hydrosiloxanes with ferrocenemethanol [14]. It has been shown that the high conformational flexibility of the siloxane backbones allows close contact between ferrocene centers [10]. The fact that the ferrocenyl moieties are either an ending part of the main polymer chain or pendant groups attached to the flexible siloxane backbones is of significance in providing information about the influence of the polymer structure and ferrocene substituent mobility on the physical, chemical and electrochemical properties of these redox-active polymers [10].

In this paper we report the synthesis and characterization of some diorganosiloxanes possessing ferrocenyl units side or ending attached. The compounds were investigated by Mossbauer spectroscopy to identify the oxidation state of the iron ions. Due to the ideal redox properties of the ferrocenyl group, the compounds containing it have received considerable attention having high potential applications as electrochemical labels or redox mediators for the development of electrochemical biosensors. Electrochemical behavior was studied by cyclic voltametry. The solvent effect was investigated by UV–Vis absorption spectroscopy in solution. The ferrocene derivatives containing atoms with donor abilities (imine nitrogen in our case) attracted great interest since the coordination of a metal to this heteroatom produces multicentre molecules [15]. The introduction of additional metal centers is a pathway to build a large range of coordination architectures of high potential for the development of functional materials [16].