Journal of Organometallic Chemistry 578 (1999) 31–42 Review Ferrocenophanes with all carbon bridges Richard W. Heo, T. Randall Lee * Department of Chemistry, Uni6ersity of Houston, Houston, TX 77204-5641, USA Received 10 July 1998 Abstract The most prevalent organometallic analogs of cyclophanes are the ferrocenophanes, in which the two cyclopentadienyl (Cp) groups of ferrocene are joined by an atomic or a molecular bridge. This review describes the synthesis and study of ferrocenophanes that possess exclusively carbon-based bridges. The use of this class of ferrocenophane in the design and development of new organometallic materials is emphasized. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Ferrocene; Ferrocenophane; Organometallic; Polymers 1. Introduction Soon after the discovery that ferrocene could easily be functionalized [13–15], interest in the preparation of Since its discovery in the early 1950s, ferrocene has bridged ferrocenes (ferrocenophanes) began. Ferro- attracted the attention of scientists worldwide [1]. Re- cenophanes are ring systems in which the two Cp groups markably, this ‘new type of organo-iron compound’ was of ferrocene are joined by an atomic or a molecular found to be aromatic, highly stable, and soluble in most bridge. Scientists have synthesized a wide variety of common organic solvents. The X-ray crystal structure of ferrocenophanes with numerous modes of ring attach- ferrocene revealed an iron metal sandwiched by two ment [16,17]. Some bridges contain from two to more cyclopentadienyl (Cp) ligands, which lie in parallel than ten contiguous carbon atoms, while others contain planes [2,3]. The Cp ligands are bonded covalently to the heteroatomic atoms, such as phosphorus, sulfur, silicon, iron center; however, they rotate freely with respect to and germanium [18–25]. In addition to mono-bridged each other and slightly prefer the staggered conforma- systems, multi-bridged ferrocenophanes containing as tion (ring twist angle=36°) [4]. While ferrocene is many as five bridges on one ferrocene have been pre- diamagnetic, the lone pairs on the metal center permit it pared [26–29]. Macromolecules containing more than to act as a weak base [5–10]. In the presence of strong one bridging ferrocene nucleus have also been synthe- non-oxidizing acids, for example, the metal center under- sized [30–46]. The incorporation of various types of goes protonation and subsequent rapid isotopic ex- bridges has given rise to questions regarding the struc- change with the ten protons of the Cp rings. Other tural integrity of ferrocenophanes and the influence that reviews provide comprehensive descriptions of the prop- the bridges can have on the chemistry of these molecules. erties and reactivities of ferrocene and its derivatives Depending on the size and chemical makeup, for exam- [11,12]. ple, bridging ultimately limits the relative motion of the rings. If the bridge cannot accommodate the natural CpCp% separation of ferrocene, then the rings are forced * Corresponding author. Tel.: +1-713-743-2724; fax: +1-713-743- to tilt away from planarity, influencing both the confor- 2726. mation as well as the chemistry of the ferrocenophanes. 0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0022-328X(98)01126-7 32 R.W. Heo, T.R. Lee / Journal of Organometallic Chemistry 578 (1999) 31–42 The nomenclature of ferrocenophanes is derived conjugated organometallic system that can be used as from the system proposed by Smith [47] and Vo¨gtle et the basis for the construction of new types of stable al. [48] for naming bridged organic aromatic cy- optoelectronic materials [53–55]. clophanes and metallocenophanes. All bridged fer- Due to these attractive features, ferrocenes and ferro- rocenes are referred to as ‘ferrocenophanes’. cenophanes have found use as building blocks for Ferrocenophanes can then be divided into two different metal-based polymers [49,56–60]. Polymers with metals classes. The first class is comprised of derivatives in incorporated in the backbone are of particular interest which one ferrocene nucleus is connected by one or due to potential interactions between the metal centers. more bridges between the Cp rings. These ferrocenes This type of ‘electronic communication’ might manifest are denoted as [m][n][o][p][q]ferrocenophanes, where itself either as through-bond effects, through-space ef- the number in the square brackets represents the length fects, or perhaps a combination of both. The coopera- of the bridge. Using the above convention, tivity can be used to influence redox processes and [3]ferrocenophane 1 contains a single three-carbon other reactions at the metal centers. To this end, bridge, and [3][3]ferrocenophane 2 contains two three- strained [1]ferrocenophanes and [2]ferrocenophanes carbon bridges. Ferrocenophanes can further be de- have been used as monomers in thermal ring opening scribed by denoting the location of these bridges on polymerizations to give polymers with transition metals each of the Cp rings, as in [3](1,1%)[3](3,3%)ferro- in the backbone [49,57–60]. While these polymers are cenophane 2. This nomenclature indicates that the stable toward oxygen and elevated temperatures [59], bridges are located on C-1 and C-3 of both Cp rings. they appear to lack the necessary conjugation required for electron delocalization through the backbone [49]. An alternative strategy utilized by our research group involves the ring opening metathesis polymerization (ROMP) of unsaturated ferrocenophanes [60]. The driving force for this method of polymerization is the relief of ring strain in the monomer [61,62]. This ap- proach is particularly attractive for generating conju- The second class of ferrocenophanes is composed of gated polymers because the unsaturation in the derivatives in which two or more ferrocene nuclei are monomer is retained in the resulting polymer. Conse- connected by one or more bridges. These structures are quently, the goal of achieving conjugation through the named [m.n.o.p. .]ferrocenophanes; the numbers in the backbone of the polymer chain can be realized. Fur- square brackets represent the length of the bridge join- thermore, since ROMP catalysts can tolerate a wide ing the two nuclei, and the number of ferrocene nuclei range of functional groups [63], it should be possible to can be derived from the number of different bridges. utilize specifically designed ferrocenophanes to yield Thus, using this convention, [1.1]ferrocenophane 3 is a conjugated organometallic polymers with tailored elec- binuclear structure with two bridges joining the two tronic properties. ferrocene moieties. These species can be further de- Indeed, there have been many different classes of scribed by denoting the placement of the bridges on the ferrocenophanes reported in the past 30 years, and %% % %%% ring, as in [1.1](1,1 ;1 ,1 )ferrocenophane 3. several reviews have documented these discoveries Ferrocenophanes have drawn interest for a variety of [16,17]. Due, however, to the recent developments out- reasons [49,50]. Many of the desirable characteristics of lined above, we feel that a review of carbon-rich ferro- the parent ferrocene are retained in the ferro- cenophanes is timely. Consequently, this review will be cenophanes, leading to useful applications for the restricted to the synthesis and structure of ferro- bridged structures. First, ferrocene and many of its cenophanes having all carbon bridges. derivatives are highly thermally stable, withstanding temperatures as high as 500°C [51]. Second, unlike many organometallic compounds, ferrocene is stable in 2. [1]Ferrocenophanes air. Consequently, ferrocene-based materials can be used in a wide variety of applications without fear of To our knowledge, no examples of [1]ferroceno- degradation. Third, ferrocene and its derivatives can phanes bridged by a single carbon atom have been reversibly undergo single electron oxidation to give reported. A single carbon bridge would substantially stable, deep green ferrocenium radical cations. This distort the Cp rings from planarity, forcing the ferro- feature makes ferrocenyl systems particularly attractive cenyl moiety into an unstable puckered conformation. electrochemical agents for use in chemical sensing and There are, however, numerous examples of other technologies [52]. Fourth, the p-electrons of both [1]ferrocenophanes bridged by single heteroatoms, such Cp rings of ferrocene are delocalized through the d- as oxygen, sulfur, silicon, and germanium [20–24]. Ap- electrons of the transition metal, thereby providing a parently, the relatively large atomic radii of these atoms R.W. Heo, T.R. Lee / Journal of Organometallic Chemistry 578 (1999) 31–42 33 Scheme 1. Synthesis of 1,1,2,2-tetramethyl[2]ferrocenophane 4 from 6,6-dimethylfulvene. affords less tilting of the Cp rings and thus greater Scheme 2 illustrates the concept that the incorpora- stability than in the case of a single carbon bridge. tion of unsaturated carbon bridges into ferro- cenophanes can be used as a strategy to produce fully conjugated organometallic polymers. As with other 3. [2]Ferrocenophanes macromolecules of this type [75,76], however, poor solubility has hindered the preparation and characteri- Due to the relatively short length of CC bonds, zation of high molecular weight species. Consequently, ferrocenophanes bridged by two carbon atoms, the problems associated with solubility must be solved [2]ferrocenophanes, would also be expected to experi- before useful, processable