Strained Ferrocenophanes - American Chemical Society

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Strained Ferrocenophanes Rebecca A. Musgrave, Andrew D. Russell, and Ian Manners* School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. ABSTRACT: The linking of the cyclopentadienyl rings of ferrocene by a short ansa [n] bridge gives rise to a broad class of strained organometallic rings, which incorporate the ferrocenyl fragment and a large number of possible bridging components. The resulting [n]ferrocenophanes exhibit fascinating structures and reactivity, and interesting comparisons can be drawn between these species and the familiar strained cyclic organic molecules. Providing the bridging moiety is sufficiently short as to induce strain, [n]ferrocenophanes have the propensity to undergo ring-opening reactions, in many cases to yield high-molecular-weight polyferrocenes with a range of interesting properties. This review aims to summarize the recent advances in the field, focusing on the preparation, structural characterization, electronic structure, and unusual reactivity of strained ferrocenophanes.

1. INTRODUCTION Prior to the discovery of ferrocene (1) and the elucidation of the sandwich structure of this remarkable species in the early 1950s,1−3 the field of transition-metal organometallic chemistry was relatively unexplored. The stability and reactivity of this prototypical metallocene and various analogues with sandwich structures such as bis(benzene)chromium4 played a profound role in terms of inspiring the subsequent development of organotransition metal chemistry. As a result, the 1973 Nobel Prize in Chemistry was awarded to Fischer and Wilkinson for their pioneering work in the field of sandwich compounds. Strained ferrocenophanes, together with related metallocenophanes and species with other π-hydrocarbon ligands, make up by far the broadest and most well-studied class of strained metal-containing rings and have attracted substantial attention over the last 50 years. Their unusual molecular and electronic structures, in which the favored parallel disposition of cyclopentadienyl ligands found for d6−d8 metallocenes is replaced by a distorted, higher energy ring-tilted arrangement, have attracted much interest. Furthermore, their enhanced reactivity, including stoichiometric ring-opening reactions on surfaces,5 and ring-opening polymerization (ROP) processes in the melt or solution have made these species attractive targets for research.5−7 Strained [n]metallocenophanes (2) possess a short ansa [n] bridge (an inter-ring bridge composed of n bridging atoms) which links the cyclopentadienyl (Cp) rings and tilts them about the metal center (Figure 1). Rinehart, Jr. et al. reported the first example, 3, in 1960,8 with the first [1]ferrocenophane featuring silicon as a bridging element, 2 (ERx)y = SiPh2, described by Osborne et al. 15 years later (Figure 1).9 Related [n.n]ferrocenophanes are species containing two ferrocene moieties where each Cp ring is linked by n atoms to the corresponding Cp ring on the second ferrocene unit, and these will be mentioned on several occasions in the review. © XXXX American Chemical Society

Figure 1. Ferrocene (1) and [n]ferrocenophanes (2−4).

The area of strained [n]ferrocenophane chemistry is now a relatively mature field. However, given the interesting new reactivity modes of these species, coupled with their employment as precursors to organometallic polymers, this research area is still expanding at a substantial rate. This review is not intended as an exhaustive overview of strained [n]metallocenophane chemistry but aims to highlight selected recent and particularly interesting contributions to the field of strained ferrocenophanes. For earlier overviews of strained metallocenophanes and related species containing π-hydrocarbon ligands, the reader is referred to several reviews.7,10−13 We also note that [n]ferrocenophanes with n ≥ 3 also represent a fascinating class of molecules but, in general, these compounds possess undistorted structures with low tilt angles and, in most cases, the degree of ring strain is correspondingly negligible.14−21 Consequently, these cyclic species will not be discussed in detail in this review.

2. OVERVIEW OF BASIC FEATURES OF STRAINED FERROCENOPHANES (a). Molecular and Electronic Structures. The degree of ring tilt can be expressed quantitatively by α, the dihedral angle between the two Cp rings. Other angles commonly used to express the geometry of [1]ferrocenophanes include β, the Cpcentroid−Cipso−E angle, δ, the Cpcentroid−Fe− Cp′centroid angle, Special Issue: Ferrocene - Beauty and Function Received: May 29, 2013

A

dx.doi.org/10.1021/om400488k | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Figure 2. Geometric parameters that characterize the structural distortions in the molecular structures of [1]- and [2]ferrocenophanes (2).

and large α angles (α > 12°), these complexes are strongly susceptible to ring-opening reactions, as shown by stoichiometric ring-opening of [1]ferrocenophanes such as 4 for the derivatization of surfaces containing Si−OH groups.5 However, it was not until the early 1990s that [n]ferrocenophanes were shown to undergo ring-opening polymerization (ROP) to yield high-molecular-weight polyferrocenes. The first examples involved the thermal ROP of sila[1]ferrocenophanes (5), yielding polyferrocenylsilanes (PFSs; 6) (Scheme 1).23−26

and θ, the Cipso−E−C′ipso angle (Figure 2). An additional angle, τ, is used to describe the projected dihedral angle between the Cpcentroid−Fe−Cp′centroid plane and the bond vector between the two bridging atoms in [2]ferrocenophanes (Figure 2, right side). The parallel ring structure of ferrocene is a consequence of the overlap of the Cp ligand π orbitals with the s, p, and d orbitals of the d6 iron(II) center. The introduction of an ansa [n] bridge (n = 1−3) alters the normal geometry of the sandwich complex, tilting the two Cp rings from their preferred parallel dispositions. The iron center tends to lie closer to the ipso carbon atoms, leading to the shortening of the carbon−carbon bond within the Cp ring opposite to this carbon, and the angle θ at the bridging element is generally smaller than that for an ideally hybridized atom. The deformation from planarity of the Cp ligands with respect to one another increases antibonding interactions and electron−electron repulsion within the molecule and, for metals with more than four d electrons, this results in an increase in the total energy (strain) and weakening of the Cipso−E and Fe−Cp (and in [2]ferrocenophanes potentially the E−E) bonds (Figure 3).22 As the tilt angle increases, the HOMO−LUMO gap

Scheme 1. Ring-Opening Polymerization of Sila[1]ferrocenophanes (5) Yielding Polyferrocenylsilanes (6)

Unlike other synthetic routes to organometallic polymers, ROP of 5 represented one of the first examples of a polymerization via a chain growth rather than a step growth/ polycondensation mechanism,27 which has several advantages over other methods of polymerization, including the formation of high-molecular-weight polymer chains even at low levels of monomer conversion. Several other methods for the ROP of silicon-bridged [n]ferrocenophanes have since been developed, including anionic,28 cationic,29 transition-metal-catalyzed,30,31 and photolytic-anionic methods.32 Both thermal and anionic ROP yield high-molecular-weight PFS and appear to proceed through the cleavage of Si−Cp bonds.28,33 Transition-metal complexes can catalyze the ROP of ferrocenophanes at room temperature, and unlike the carbanionic method, this procedure does not require exceptional monomer purity.30,31 In contrast to previously reported methods, photolytic ROP of ferrocenophanes involves the photoinduced cleavage of the Fe−Cp bond and nucleophilic attack at the iron center.32,34,35 Following the discovery of the ROP of sila[1]ferrocenophanes, subsequent work led to the ROP of ferrocenophanes bridged with a range of elements other than silicon. Thus soluble, high-molecular-weight polyferrocenes which incorporate germanium,36,37 tin,38,39 carbon,40−42 and phosphorus43,44 spacers have been prepared. (c). Synthetic Routes. Ferrocenophanes are prepared by several different synthetic routes (Scheme 2). A salt metathesis approach involves the reaction of dilithioferrocene·tmeda (tmeda = N,N,N′,N′-tetramethylethylenediamine) and an element dihalide, and has been employed in the synthesis of most [1]ferrocenophanes, including those bridged by group 13− 16 main-group elements and group 4 and 10 transition metals.9,45,46 The “fly-trap” approach involves the synthesis of a bridged (C5H4)2(ERx)y dianionic ligand before complexation with an FeII center, and is more common for [2]- and

Figure 3. Calculated variation of the energy (E) of FeCp2 (1) as a function of α (adapted with permission from ref 22).

decreases, and a bathochromic shift of the lowest energy absorbance of the [1]ferrocenophane is generally observed. A steady gradient in color of [1]ferrocenophanes is observed as the bridging element decreases in size and the angle α increases, from amber (unbridged ferrocene) to increasingly dark red (Si- and Pbridged [1]ferrocenophanes) to purple (S-bridged [1]ferrocenophanes). This trend is accompanied by an increase in intensity of the color as the centrosymmetry is lost on bending ferrocene to form a [1]ferrocenophane, and the Laporte rule is gradually relaxed. As α increases, d−d transitions become more allowed and their intensity is therefore enhanced. (b). Ring-Opening Reactions. As a result of the strain present in [n]ferrocenophanes with short ansa bridges (n ≤ 2) B


Organometallics

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Scheme 2. Common Synthetic Routes to Ferrocenophanes (2)

[3]ferrocenophanes. Incidentally, this method was used to synthesize the first ferrocenophane, 3, in 1960.8

3. RECENT WORK ON STRAINED [1]FERROCENOPHANES (a). [1]Ferrocenophanes Bridged by Group 13 Elements. To date, the largest α angle of any [1]ferrocenophane (32.4°) was reported for a boron-bridged species, 7, with a B(N(SiMe3)2) bridging moiety in 1997 (Figure 5).47 This highly strained complex, stabilized by a sterically demanding π-donating aminoborane fragment, undergoes differing reactivity pathways in comparison to less strained [1]ferrocenophanes. These pathways are facilitated by a weakening of the Fe−Cp bonds and include the insertion of metal carbonyl fragments (see section 5).48 [1]Ferrocenophanes bridged by the heavier group 13 elements aluminum and gallium have been known since 2005, although their α angles are significantly smaller than those found in the boron analogue, due to an increase in covalent radius of the bridging atom. Their synthesis often employs bulky trisyl-derived ligands to provide both steric shielding and intramolecular stabilization via a pendant N donor: for example, in the galliumbridged [1]ferrocenophane 8 (Figure 4).49−51

Figure 5. [1]Ferrocenophanes 7 and 8, and 9 and 10 (E = Al, Ga and −N = 2-C5H4N) and [1.1]ferrocenophanes 11 and 12 (E = Al, Ga).

Figure 6. Intramolecularly coordinating ligands.

(Figure 6), but the strained [1]ferrocenophane intermediate was not isolated.55,56 Steric interactions between the o-tBu group of the Mamx ligand and the ferrocene moiety have been shown to induce additional strain in these aluminum- and gallium-bridged [1]ferrocenophanes.56 Recent work in this field has focused on the design of ligands with sufficient steric protection to facilitate the formation of [1]ferrocenophanes, yet small enough to allow polymerization to proceed.55,57 To this end, [1]ferrocenophanes have since been reported containing the bulky Pytsi, Me2Ntsi, and Mamx ligands, where the bridging element is part of a fouror five-membered ring (Figure 6). [1.1]Ferrocenophanes have also been reported employing the smaller Ar′ and p-Me3SiAr′ ligands.39,58 The successful synthesis of aluminum- and gallium-bridged [1]ferrocenophanes was achieved using the Me2Ntsi ligand (see, for example 8 in Figure 4), with α angles of 14.33(14) and 15.83(19)°, respectively, and some evidence of thermal ROP was observed above 210 °C.51 (b). Silicon-Bridged [1]Ferrocenophanes. Ferrocenophane chemistry has been dominated since its inception by silicon-bridged species, primarily due to the ease of synthesis of strained sila[1]ferrocenophanes via the salt metathesis method and also their ability to polymerize to yield high-molecular-

Figure 4. Molecular structure of the gallium-bridged [1]ferrocenophane 8 with Me2Ntsi ligand (reproduced with permission from ref 51).

The use of smaller intramolecularly stabilizing “one armed” ligands such as Ar′ and p-tBuAr′ (Figure 5) does not give the desired [1]ferrocenophane product and instead leads to [1.1]ferrocenophanes,52,53 a result attributed to the diminished steric protection due to the lack of trimethylsilyl groups. However, the bulky trisyl-based ligands that make these [1]ferrocenophanes accessible have been shown to either hinder or result in sluggish ROP.54 A gallium-bridged polyferrocene with observable tacticity was prepared in 2009 via the reaction of dilithioferrocene with a Mamx-substituted gallium dichloride C


Organometallics

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weight polyferrocenylsilanes. Many silicon-bridged ferrocenophanes have thus been reported, including those substituted with alkyl, aryl, amino, and halo substituents.11 Heavier elements from group 14 can also be used to bridge the two Cp rings of [1]ferrocenophanes. Germanium- and tin-bridged [1]ferrocenophanes have been prepared and shown to undergo ROP to yield polyferrocenylgermanes and polyferrocenylstannanes, respectively.37,59,60 Before the reported synthesis of the first hypercoordinate silicon-bridged [1]ferrocenophane in 2000,61 all known Sibridged species were tetracoordinate with a tetrahedral Si center. Addition of alkyl- or aryllithium reagents to dialkylsilyl-bridged [1]ferrocenophanes results in living anionic polymerization.62 However, there is literature precedent for substitution reactions on addition of these same initiators to chlorosilyl-bridged [1]ferrocenophanes, without cleavage of the Cipso−Si bond occurring.63 Making use of this approach, the pentacoordinate silicon-bridged [1]ferrocenophane 14 was synthesized by reaction of chloromethylsila[1]ferrocenophane (13) and Li[(σC6H4)CH2NMe2] (Scheme 3).

Scheme 4. Synthesis of Pentacoordinate Silicon-Bridged [1]Ferrocenophane 16

oxygen−silicon donor interaction is absent, which is unreactive under similar conditions. Pentacoordinate silicon-bridged [1]ferrocenophanes such as 14 are useful models for potential intermediates in thermal ROP processes. For example, their structures provide evidence that trace amounts of neutral nucleophilic species may function as initiators as the coordination lengthens and thereby weakens the trans bond and should therefore facilitate ring-opening. Significantly, studies of the ambient-temperature ROP mechanism for stanna[1]ferrocenophanes indicated a process catalyzed by neutral nucleophilic species such as amines as impurities.65 Other interesting developments in silicon-bridged ferrocenophane chemistry include the preparation of the first [1]ferrocenophanium ion, a species related to neutral [1]ferrocenophanes by loss of an electron. Ferrocene undergoes a reversible one-electron oxidation to yield the 17-electron, dark blue FeIII ferrocenium ion 17 (Figure 8), which can be isolated as

Scheme 3. Synthesis of Pentacoordinate Silicon-Bridged [1]Ferrocenophane 14

Figure 8. The ferrocenium ion (17) and examples of [2]ferrocenophanium ions (18, 19).

stable salts via chemical oxidation. The reversibility of this oxidation has led to the widespread use of ferrocene as a standard in electrochemical studies.66 The electrochemistry of other ferrocenophanes has also been explored. For example, species with two or more atoms in the bridge have previously been oxidized to form 17-electron ferrocenophanium ions and some have been isolated as salts (Figure 8).67,68 Although many electrochemical studies on [1]ferrocenophanes have been reported, and the one-electron oxidation is reversible on the time scale of cyclic voltammetry experiments, chemical oxidation to give a stable product was not achieved until 2009.69 Sila[1]ferrocenophane 20 was oxidized by [N(C 6H 4 -4Br)3]+[SbF6]− (“magic blue”), a reagent less reactive toward the Fe−Cp and Si−Cp bonds than other common oxidizing agents (Scheme 5). The presence of electron-donating tBu groups on the silicon facilitates oxidation and stabilization of the cationic product. The resulting [1]ferrocenophanium ion 21 (Figure 9) possessed an increased tilt angle in comparison to the 18electron precursor (20, α = 18.69(9)°; 21, α = 28.9(13)°).69 This increased strain enhances the reactivity of the complex and, unlike the neutral precursor, the cation undergoes hydrolysis and methanolysis at room temperature to afford the corresponding

Figure 7. Molecular structure of pentacoordinate silicon-bridged [1]ferrocenophane 14 (reproduced with permission from ref 61).

Characterization of 14 was achieved in the solid state by singlecrystal X-ray diffraction, which confirmed the coordination of the 2-C6H4CH2NMe2 substituent without cleavage of the Cipso−Si bond (Figure 7). The Si−N distance (2.776(2) Å) is the shortest reported for a tetraorganosilane, attributed to the incorporation of the silicon atom into a strained cyclic system and the unusual electronic structure of ferrocenophanes. Although both Cipso−Si bonds are longer than those reported for tetracoordinate sila[1]ferrocenophanes, the Cipso−Si bond trans to the amino group is significantly longer than the other. Dynamic behavior of the pentacoordinate complex was observed by variable-temperature solution 1H and 29Si NMR studies, revealing a fast equilibrium between the coordinated and uncoordinated species. Five years later, another pentacoordinate silicon-bridged ferrocenophane, 16, was reported (Scheme 4).64 In this case, the weaker Cipso−Si bond is easily cleaved to initiate ROP in the presence of a cationic catalyst, in contrast to the case for the analogous tetracoordinate sila[1]ferrocenophane, in which the D


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Scheme 5. Synthesis of the Silicon-Bridged [1]Ferrocenophanium Ion 21

Scheme 6. Synthesis of Group 4 Bridged [1]Ferrocenophanes 22−24

dichloride. The covalent radii of the group 4 metals range from 1.40 to 1.54 Å, larger than that of silicon in the well-known silicon-bridged [1]ferrocenophanes. As a result, the reported zirconium-bridged structure 23 is only modestly strained (α = 6°), but interestingly the β-angle is substantial (40.1°) and the short Zr−Fe distance (2.9621(5) Å) observed was suggested to implicate the presence of a weak dative bond between the electron-rich d6 iron center and electron-poor d0 tetrahedral zirconium center. More recently, [1]ferrocenophanes bridged by group 10 metals bearing supporting phosphine ligands were prepared (Scheme 7 and Figure 10).73,74 It is somewhat unexpected that Scheme 7. Synthesis of Group 10 Bridged [1]Ferrocenophanes 25−27 Figure 9. Molecular structure of the ferrocenophanium ion 21 (reproduced with permission from ref 69).

ring-opened species. In addition, the temperature required for ring-opening (65 °C) is significantly lower than for the neutral species, but a polymer is not generated in this instance, presumably due to a competitive fluoride transfer reaction involving the [SbF6]− counteranion.69 Species 21 possesses an unexpected red-brown color, unlike the dark blue ferrocenium ion, which undergoes a red shift in the λmax value from 620 to 650 nm when the Cp ligands are substituted with an electron-donating group such as tert-butyl. In contrast, λmax for species 21 was observed at 435 nm in CH2Cl2, blue-shifted relative to the unstrained ferrocenium ion. This unexpected color is explained by changes in the electronic structure of oxidized ferrocene-containing compounds upon tilting of the Cp ligands away from planarity.69−71 Interestingly, the most intense band for species 21 is at relatively high energy and has considerable d−d character. This is explained by the ringtilting and the lower symmetry of this species and the relaxation of the Laporte selection rule.69 (c). Transition-Metal-Bridged [1]Ferrocenophanes. A large number of [n]ferrocenophanes bridged by various p-block elements are known. Perhaps surprisingly, given that the main synthetic route appears applicable to any element where the dichloride is readily available through reaction with dilithioferrocene·tmeda, those bridged by metal atoms are far less common. However, the reaction of many metal dihalides with dilithioferrocene results only in reduction of the metal center, and only a few examples of metal-bridged ferrocenophanes are known. The first successful synthesis of [1]ferrocenophanes bridged by a transition metal (22−24) was achieved using the salt metathesis approach in 1990 (Scheme 6).72 Group 4 bridges were employed so as to afford structures with both one electron-rich and one electron-poor metal center. Predictably, the metalla[1]ferrocenophanes react with HCl gas to give quantitative amounts of ferrocene and the metallocene

Figure 10. Molecular structure of 27 (reproduced with permission from ref 74).

such electron-rich species can be readily synthesized, as they contain both a d6 iron center and a d8 distorted-square-planar group 10 center in the bridge. The more constraining geometry, in addition to the smaller covalent radii of the group 10 metals, accounts for the larger α angles reported for these complexes (25, 28.4°; 26, 24.5°; 27, 25.2°) than for the Zr analogue 23. However, there is no evidence of significant bonding between the bridging metal and the iron center, as the spatial separation is significantly longer than the sum of the covalent radii of both metals. Although the group 10 bridged complexes appear to be sufficiently strained, all polymerization attempts have been unsuccessful to date. However, further reactions of 25 and 26 with stoichiometric amounts of anionic initiators (addition of 1 E


Organometallics

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Scheme 8. Anionic Ring-Opening, Insertion of CO, and Oxidation using Elemental Sulfur

Table 1. Structural Information for Selected [2]Ferrocenophanes compd d

34 35 36 37a 38 39a 40b 41 42d 43 44 45 46 47 48 49d 50 51 52

ERx

ERx′

α, deg

β1, deg

β2, deg

δ, deg

τ, deg

E−E bond, Å

ref

CH2 CMe2 CH CHPh CHiPr CHtBu CHCy CHPh CHMe CH2 CH2 CH2 BMes BNMe2 P3N3Cl4 P(O)-P(H)Tbtc P(tBu) SnMe2 GeMe2

CH2 CMe2 CH CHPh CHiPr CHtBu CHCy CH2 CH2 SiMe2 PPh S BMes BNMe2 P3N3Cl4 P-P(H)Tbtc P(tBu) SnMe2 GeMe2

21.6(4) 23.3 22.6 22.7(3) 23.8(1) 25.5(2) 23.80(1) 23.18(9) 23.8(2) 11.8(1) 14.8(3) 18.5(1) 10.5 12.8 7.7 11.5 13.6 0.7 3.9

11.2 10.6 15.7 15.1(6) 13.9(2) 9.8 13.2 13.7 14.2(2) 7.3(3) 9.8(4) 14.0(3) 25.3 21.2 13.4 15.2 10.8 10.5 10.1

20.1 8.9 15.9 n/a n/a 14.4 10.6 13.4 29.2(4) 17.9(3) 18.0(4) 9.4(3) n/a n/a 12.8 14.5 n/a n/a n/a

164.2 156.8 162.5 163.5(12) 162.6(9) 162.9 163.1 162.9 162.3(1) 170.9(14) 169.6(7) 167.2(1) 173.4 170.1 176.2 172.5 170.9 179.4 176.5

35.2 25.4 8.9 30.8(1) 29.5(2) 34.9 27.9 36.1 24.9(5) 0.5(2) 21.7(3) 17.9(3) 38.3 27.6 27.5 45.9 32.8 2.9 5

1.539(12) 1.584(14) 1.315(5) 1.572(3) 1.559(3) 1.565(7) 1.591(2) 1.545(3) 1.565(7) 1.914(3) 1.892(5) 1.844(2) 1.698(4) 1.724(3) 2.219(1) 2.218 2.2502(8) 2.7622(5) 2.418(1)

80 81 82,83 41 41 84 40 40 41 85 85 85 86 87 88 89 90 79 75

a

Data given for rac isomer. bData given for meso isomer. cTbt = 2,4,6-tBu3C6H2. dBridging atoms disordered over two sites, data for site with highest relative occupancy given.

4. RECENT WORK ON STRAINED [2]FERROCENOPHANES

equiv of phenyllithium) afforded the ring-opened, acyclic, phenylated complexes (Scheme 8). Both the nickel- and palladium-bridged [1]ferrocenophanes 25 and 26 were shown to react with carbon monoxide. With the nickel species 25, reductive carbonylation occurs at the Ni center, forming [Ni(CO)2(PnBu3)2]. However, in the case of 26, migratory insertion occurs, with CO inserting into one of the Pd−C bonds (to give 28). All three complexes 25−27 react with elemental sulfur. The products contain a ligated pentafulvalene unit, formed by fusion of the cyclopentadienyl ligands. The C10 ligand in the Pd and Pt cases is η2-coordinated (products 31 and 32, respectively), whereas in the Ni case it is η4:η0-coordinated (product 33). The free, uncoordinated pentafulvalene molecule is unstable above −30 °C with respect to [2 + 2] cycloaddition, and this is the first example of its stabilization by η2 or η4 coordination to a metal center.

When the number of atoms in the ferrocenophane bridging moiety is increased to two, the family of [2]ferrocenophanes is realized. To date, a very large number of these species have been synthesized through variation of the bridge, including a variety of homo- and heteroatomic linkers of main-group elements. As expected, the introduction of an additional bond into the bridge affords [2]ferrocenophanes which are less tilted than their singleatom-bridged analogues. The degree of tilt is dictated by the atomic radii of the bridging elements and, accordingly, the length of the bridging E−E bond between them (Table 1). Thus, for bridge bonds between heavier main-group elements (e.g., Si, Ge, and Sn)75−79 the bridge E−E bond length is sufficiently large (2.362(3)−2.7622(5) Å) to render the [2]ferrocenophanes (e.g. 51 and 52) unstrained (α = 0.7−3.9°). However, in the cases of smaller main group-elements (C and B)11 this bond is shorter (1.315−1.724 Å) and thus these species possess sufficient ring F


Organometallics

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Scheme 10. Thermally Induced Isomerization of meso-/rac-39

strain to induce bond cleavage (α = 10.5−23.8°), and we will focus on these examples in this review. (a). Dicarba[2]ferrocenophanes. The dicarba[2]ferrocenophanes contain a CR2CR2 bridging moiety, and the first example was reported in the 1960s.8 It is not surprising to note that these species possess the greatest degree of structural distortion of all [2]ferrocenophanes (α = 21.6−23.8°), a result of the smaller relative atomic radius of carbon in comparison to that of the other main-group elements incorporated into a diatomic linker in these species. Since their inception, isolation of a range of different dicarba[2]ferrocenophanes has been achieved through variation of the substituents on the bridge, to include a variety of alkyl, cycloalkyl, or aryl substituents on one or both of the carbon atoms. The number of species now known is testament to the broad applicability of the synthetic approaches employed in the preparation of dicarba[2]ferrocenophanes. As such, most dicarba[2]ferrocenophanes are prepared through utilization of synthetic methodologies91 involving “fly-trap” ligands of the type Li2[(C5H4)2(C1R1R2)(C2R3R4)] in lowtemperature reactions with FeCl2 in thf (Scheme 1).40,41,84 Alternatively, “fly-trap” ligands of the type Na2[(C5H4)2(CR2)2] and Ca[(C5H4)2(CHR)2]·2thf can be prepared through reduction of the parent substituted-fulvene species.8,92 Reaction of these dicyclopentadienide salts with FeCl2 affords the corresponding dicarba[2]ferrocenophane products.41,93 In addition, a direct synthesis of 35 involving condensation of 6,6-dimethylfulvene with iron vapor has also been reported.94 Early reactivity studies offered insight into the effects of the ring-strain inherent to these species. Due to the lack of polarity of the Cpipso−Cbridge bond, treatment with strongly nucleophilic initiators does not result in anionic ROP.95 However, species 34 (Table 1) can be polymerized thermally to produce moderatemolecular-weight poly(ferrocenylethylene).42,80 It was noted that Cp methylation is necessary to produce soluble polymeric products, a property reflected in the poly(ferrocenylenevinylene) products produced from the ring-opening metathesis polymerization (ROMP) of dicarba[2]ferrocenophanes such as 36,83 bearing an unsaturated backbone.96,97 Recent studies of the thermal reactivity of dicarba[2]ferrocenophanes have been extended to species bearing nonhydrogen substituents at the bridge, and these species display more complex thermal behavior. Thus, thermolysis of dicarba[2]ferrocenophanes bearing equal numbers of nonhydrogen substituents on each carbon of the dicarba bridge do not undergo thermal ROP, a process characterized by immobilization of the red melt formed during the thermolysis. Instead, thermal treatment of methyl-substituted species 35 (bridge C−C = 1.584(14) Å)98 at 240 °C yields a red melt which remains free flowing for the duration of the thermolysis and instead undergoes homolytic cleavage of the dicarba bridge followed by a radical rearrangement to afford acyclic ferrocene derivative 53 (Scheme 9).84

The thermolysis of species meso-/rac-39, bearing tBu substituents on both carbons of the bridge, also proceeds via homolytic cleavage of the C−C bond; however, in this instance isomerization occurs to form the rac-39 isomer (Scheme 10). Computational studies on this system have confirmed the relative thermodynamic preference for the rac isomer over the meso form, a property reflected in the experimentally determined relative C−C bond lengths of the meso (1.612(3) Å) and rac isomers (1.565(7) Å).84 The differences in observed reactivity for these species were rationalized through the presence or absence of hydrogen at the α position relative to the carbon atoms in the bridge. Thus, in species 35, an α-hydrogen facilitates H• abstraction and radical disproportionation to afford the ferrocene derivative 53. For species meso-/rac-39, the absence of an α-hydrogen inhibits the radical disproportionation pathway, and the dicarba bridge is re-formed, following bond rotation, to afford the thermodynamically more stable rac isomer. The reactivity pathways appear general for other dicarba[2]ferrocenophanes bearing an equal number of non-hydrogen substituents at each carbon on the dicarba bridge. Consequently, thermolysis of species 38 and 40 affords the corresponding disubstituted ferrocene derivatives analogous to 53 (Scheme 9), and thermolysis of meso-/rac-37 affords the favored rac-37 isomer.40 Differing thermal behavior was observed for dicarba[2]ferrocenophanes bearing either a single or no non-hydrogen substituent on each carbon of the bridge. Upon thermolysis these species undergo thermal ROP to afford cyclic, moderatemolecular-weight polymeric products (54) (Scheme 11). Scheme 11. Thermal ROP of Dicarba[2]ferrocenophanes 34 and 41

Investigations into the mechanism of thermal ROP of these species, involving trapping of ring-opened monomeric species, has indicated that propagation most likely proceeds via a heterolytic Fe−Cp bond cleavage mechanism.40 A rationale for the differences in thermal reactivity based on the structural changes that result from differing bridge substitution has been proposed.40 Species 37−40, with an equal number of non-hydrogen substituents on each carbon of the bridge, possess longer C−C bridge bonds relative to species 34 and 41, which have one or zero non-hydrogen substituent, respectively. Thus, for species 35 and 37−39, weakening of the C−C bridge bond, in combination with the ability of bridge substituents to stabilize the radical intermediates formed from

Scheme 9. Thermally Promoted Radical Rearrangement of 35

G


Organometallics

Review

Scheme 12. Synthesis of Dibora[2]ferrocenophane 47

homolytic cleavage, facilitates reactivity pathways involving homolytic C−C bridge cleavage. Alternatively for 34 and 41, where one or zero non-hydrogen substituent is present, the C−C bond is relatively short, hypothetical radical intermediates formed from homolytic C−C cleavage are not stabilized, and thermal ROP occurs via a heterolytic Fe−Cp bond cleavage mechanism to afford polyferrocenylethylene products. (b). Dibora[2]ferrocenophanes. Dibora[2]ferrocenophanes are inherently less tilted than their carbon analogues due to the larger atomic radius of boron and the corresponding increase in the length of the B−B bond (α = 10.5−12.8°; for known examples see Table 1). However, the trigonal-planar environment of the bridging boron atoms, with preferred angles approaching 120°, in comparison to ca. 109° for both carbon and silicon analogues, contributes to the overall strain in these species and explains the extent of the distortion at the Cp ipso carbon (β = 21.2−25.2°) relative to other homoatomic-bridged species (cf. disila-bridged Fe(ηC5H4)2(SiMe2)2 (55) with α = 4.19° and β = 10.8°78 and dicarba-bridged 37 with α = 22.7° and β = 15.1°). Initial reports of the first dibora[2]ferrocenophane describe a synthesis employing the salt metathesis reaction of dilithioferrocene·tmeda with 1 equiv of the dihalide B2Cl2(NMe2)2 in hexane to afford the orange product 47.99 Despite extensive solution characterization, no structural characterization was provided in this instance. An alternative synthesis has since been utilized, involving the first use of a diborane “fly-trap” ligand with an iron(II) source. Accordingly, deprotonation of 1,2-bis(dimethylamino)-1,2-bis(η1-cyclopentadienyl)diborane with nBuLi and subsequent reaction with FeCl2 also yielded dibora[2]ferrocenophane 47, in a slightly improved yield (Scheme 12).100 X-ray crystallographic characterization confirmed that the B−N bond lengths in 47 (1.389(3) and 1.390(3) Å) are within the range of typical BN double bonds. More recently, the successful isolation of the first dibora[2]ferrocenophane was reported where no stabilization of the electron-deficient bridging boron atoms by π-donation from a nitrogen donor is present. Utilizing a methodology similar to that previously reported for the preparation of the first example of these species, a salt metathesis reaction of dilithioferrocene· tmeda with the associated dihalide B2Mes2Cl2 yielded product 46 in a moderate yield, which was structurally characterized by X-ray diffraction (Figure 11).86 Slight structural differences for species 46 relative to the πstabilized species 47 were observed. Specifically, the B−B bond is shortened (1.698(4) Å) in 46, a feature potentially attributable to the reduced π bonding at boron in this species due to the absence of amine substituents.86 Despite the reduction in E−E bond length in 46, the degree of tilt is reduced for this species relative to that in 47 with a reported α angle of 10.5(2)° (47, α = 12.8(2)°). This can be attributed to an increase in distortion at the Cpipso carbon in 46, with an increased β angle reported of 25.3° (47, β = 21.2°). The reduced structural distortion and degree of strain present in dibora[2]ferrocenophanes, relative to their carbon analogues,

Figure 11. Molecular structure of dibora[2]ferrocenophane 46 (reproduced with permission from ref 86).

is reflected in the electronic spectra of these species. UV/vis spectroscopic analysis of 46 displays an absorbance at λmax 453 nm in benzene, red-shifted relative to that of unstrained ferrocene (λmax 440 nm in hexanes)101 but blue-shifted in comparison to the more highly strained dicarba[2]ferrocenophanes (e.g. 37, α = 22.7°, λmax 465 nm in thf)40 for reasons previously discussed (see section 2a). Despite the larger deformation from planarity observed for the Cpipso carbon in these species (β angle), the reactivity of dibora[2]ferrocenophanes is largely confined to cleavage of the bridging B−B bond. The majority of the reactions of these species involve the transition-metal-mediated diboration of alkynes and dialkynes. Although there is literature precedent for this synthetic pathway utilizing unstrained, yet sterically crowded diboranes,102−104 the presence of ring strain in dibora[2]ferrocenophanes has been shown to enhance the reactivity of the B−B bond, facilitating previously unrealized reactivity modes. The reactivity of 47 in the first heterogeneous catalytic diboration of an alkyne represents an example of B−B bond reactivity enhancement as a result of ring strain.105 The reaction proceeds via an oxidative-addition of Pt across the B−B bond (to give 56), followed by reaction with an alkyne to afford the associated [4]ferrocenophanes (57) (Scheme 13), and has since been extended to a range of unsaturated organic substrates.106 Additional examples of B−B bond reactivity in dibora[2]ferrocenophanes include insertion of elemental S and Se in the B−B bond of 47 to afford the corresponding [3]ferrocenophanes107 and the 1,1-diboration of isocyanide via reaction with either 47 or 56 to afford an dibora[3]ferrocenophane with a diborated isocyanide unit in the bridge.108 Furthermore, diboration of the NN bond in diimines, which poses a significant synthetic challenge and has previously been limited to highly reactive B−B bonds,109,110 has been reported.111 The transition-metal-mediated mechanism, which involves insertion of the NN bond of azobenzene into the B− B bridge of 47 to afford the corresponding [4]ferrocenophane, is believed to be facilitated by the inherent ring strain in this species. Finally, cleavage of the Cpipso−B bond has been achieved through H


Organometallics

Review

Scheme 13. Transition-Metal-Mediated Insertion of an Alkyne into the B−B Bond of 47

a boron−boron exchange reaction of 47 with the diboranes B2Pin2 and B2Cat2 (Pin = pinacol, Cat = catechol), to afford disubstituted ferrocenes, with decomposition of the (NMe2)B− B(NMe2) backbone reported.112 (c). Diphospha[2]ferrocenophanes. Despite the relatively long P−P bond length in diphospha[2]ferrocenophanes113 (2.2187(5)−2.3100(7) Å), these species can still be regarded as being marginally strained due to the relative tilt of their cyclopentadienyl ligands (α = 7.7−13.6°). The first isolation of a diphospha[2]ferrocenophane was as a side product of a reaction of dilithioferrocene with the cyclic trimer hexachlorotriphosphazene.88 Recent studies have expanded the scope of known diphospha[2]ferrocenophanes, with the isolation and structural characterization of species 49 (Table 1), bearing both trivalent and pentavalent phosphorus atoms in the bridge.89 Finally, diphospha[2]ferrocenophane 50 was isolated and structurally characterized (Figure 12), in addition to two similar species

Scheme 14. Block Copolymer Synthesis of PFE-b-PFS (58) via Sequential Photocontrolled ROP

Figure 12. Molecular structure of diphospha[2]ferrocenophane 50 (reproduced with permission from ref 114).

Figure 13. Molecular structure of cyclic hexamer [Fe(η5-C5H4)2SiMe2]6 (59), a [1.1.1.1.1.1]ferrocenophane (reproduced with permission from ref 118).

bearing either nBu of Ph substituents, which represent the first examples of [2]ferrocenophanes bearing two trivalent phosphorus atoms in the bridge.114 Utilizing a general methodology similar to that employed in the synthesis of 49, species 50 was prepared via ring closure of a 1,1′-disubstituted ferrocene and formation of the E−E bridge, a synthetic methodology not commonly employed in strained [n]ferrocenophane synthesis. Although, to date, no reactivity pathways that could be attributed to the release of ring-strain have been reported, the potential application of diphospha[2]ferrocenophanes as diphosphine ligands has been assessed, through complexation of 50, and analogous species, with [W(CO)5] and [Cr(CO)5] metal fragments.114

5. PHOTOINDUCED REACTIVITY OF STRAINED FERROCENOPHANES AND CONTROLLED CLEAVAGE OF Fe−Cp BONDS The reactivity of [n]metallocenophanes based on photoexcitation of the metallocene unit, with Pyrex-filtered UV light or bright sunlight, has markedly increased the known reactivity modes of these strained species. Unlike ferrocene, which is photoinert under these conditions and where the lowest energy band corresponds to a metal-localized Laporte-forbidden HOMO−LUMO d-d transition, irradiation of [n]ferrocenophanes results in a weakening of the Fe−Cp bond and facilitates Cp ligand displacement by nucleophilic attack at iron. Through both comparison with benzoylferrocene,115 which undergoes photoinduced reactivity similar to that of [n]ferrocenophanes, and theoretical calculations on these tilted I


Organometallics

Review

Scheme 15. Photoinduced η5 to η1 Haptotropic Shifts of Cp Rings in [n]Ferrocenophanes

Scheme 16. Photoirradiation of [1]Ferrocenophanes 64 and 65 in the Presence of [Fe(CO)5] or [Co2(CO)8]

Scheme 17. Irradiation of [1]Ferrocenophanes 68 and 69 in the Presence and Absence of [Fe(CO)5]

species,101,116 a mechanism of Fe−Cp bond weakening has been suggested.32 The emergence of low-lying ligand-based states on tilting the Cp ligands in ferrocenophanes has been proposed to both weaken the Fe−Cp bond and enhance the electrophilicity of the Fe center upon irradiation, facilitating novel reactivity pathways. Most notable of these reactivity pathways is photoinduced anionic ROP.32,117 The “living” nature of this polymerization provides the same distinct advantages as anionic ROP initiated by strong nucleophilic bases such as nBuLi;62 notably by providing molecular weight control, narrow molecular weight distributions, and enabling block copolymer synthesis.7 Significantly, as a result of the need for only weakly nucleophilic initiators such as Na[C5H5], photolytic ROP is considerably more functional group tolerant; for example, this method has been employed in polymerizations of sila[n]ferrocenophanes bearing reactive alkyne substituents.32 In addition, further studies of the photolytic ROP of [1]ferrocenophanes with phosphorus as the bridging moiety have been performed, allowing access to welldefined polyferrocenylphosphine (PFP) homopolymers and PFP-b-PFS block copolymers.43 Recent advances in photocontrolled reactivity of [n]ferrocenophanes have led to the technique being extended to monomers bearing alternative bridging moieties. For example, photolytic ROP of a range of dicarba[2]ferrocenophanes has been reported.41 Dicarba[2]ferrocenophanes 37 and 42 (Table 1) undergo photolytic ROP to afford soluble polyferrocenylethylene products with narrow polydispersities. Molecular weight control was effectively demonstrated through variation of the monomer to initiator ratios. Furthermore, the ability to utilize this technique in the synthesis of block copolymers was demonstrated through the addition of dimethylsila[1]ferrocenophane (4) to a “living” polyferrocenylethylene macro

initiator and, following additional irradiation, polyferrocenylethylene-b-polyferrocenylsilane (PFE-b-PFS) (58) was afforded (Scheme 14). Photolytic ROP of dimethylsila[1]ferrocenophane 4 in the presence of substitutionally labile Lewis bases has also been employed in the synthesis of cyclic poly- and oligoferrocenylsilanes. As shown in 2009, use of bipyridine as the initiator led to ring opening of 4, chain propagation, and chain termination by cyclization reactions.118 In contrast to previous studies on linear oligomeric species based on the same repeat unit,28 the formation of a distribution of cyclic polyferrocenylsilanes and a series of cyclic oligomers containing two to seven repeat units was detected. These oligomers were successfully separated via column chromatography, allowing comparison of the different electrochemical properties and structures to be conducted (for the molecular structure of the cyclic hexamer [Fe(η 5 C5H4)2SiMe2]6 (59) determined by X-ray diffraction see Figure 13). Further reactivity modes of [1]ferrocenophanes facilitated by photoirradiation include the coordination of mono- and diphosphine ligands to the metal center, which has previously been shown to result in ring slippage of the Cp ligand from the η5 to η1 coordination mode.34 For example, species 4 affords the ring-slipped species 61 under these conditions (Scheme 15). The reversibility of this transformation was demonstrated through heating the ring-slipped species and was found to be dependent on the degree of the strain present in the precursor.119 Additional studies in this area have extended this reactivity mode to include dicarba[2]ferrocenophane 34, affording ring-slipped species 60.120 The ability to synthesize ring-opened species 62 and 63 through the addition of NaCp to ring-slipped species 60 and 61 was also demonstrated (Scheme 15). J


Organometallics

Review

Scheme 18. Irradiation of [1]Ferrocenophanes 68 and 69 in the Presence of [Co2(CO)8]

substantial motivation for the further exploration of this area and that of analogous species.6,132

The increased lability of the Fe−Cp bond under photoirradiation has also been utilized in the insertion of various metal carbonyl fragments to afford unusual products. This reactivity mode is in stark contrast to the previously observed reactivity between strained [1]ferrocenophanes with metal fragments in the absence of photoirradiation, where insertion occurs into the Cpipso−bridge bond.121,122 Studies on the photoinduced reactivity of boron-bridged [1]ferrocenophanes 64 and 65 in the presence of [Fe(CO)5] (or [Fe2(CO)9]) and [Co2(CO)8] have led to the isolation of Fe− Cp insertion products 66 and 67, respectively (Scheme 16),48 and have since been extended to other strained [1]ferrocenophanes. As such, irradiation of [1]ferrocenophanes 68 and 69 in the presence of [Fe(CO)5] also yielded the associated Fe−Cp insertion products 70 and 71.123 In the absence of metal carbonyl complexes, irradiation of 69 yielded the first group 16 bridged [1.1]ferrocenophane 72 in addition to small oligomeric species, a pathway not observed during the photolytic reactivity studies of species 68 (Scheme 17). The differences in reactivity of the two species were attributed to the increased ring strain in 69 (α = 31.1°)101 relative to 68 (α = 19.1°)124 Interestingly, irradiation of the same two species in the presence of [Co2(CO)8] also results in contrasting reactivity (Scheme 18). For [1]ferrocenophane 68, the aforementioned reaction yielded the tetrametallic dimer 75. A postulated mechanism of formation of this species involves photoactivation with insertion of the [Co(CO)4] fragment into the Fe−Cp bond, followed by CO redistribution to afford intermediate species 73, with dimerization and loss of [Co2(CO)8] to afford dimer 75. In contrast, the reaction of species 69 with [Co2(CO)8] yielded the Fe−Cp insertion product 74, the analog of the proposed intermediate (73) in the corresponding reaction of 68.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biographies

Rebecca A. Musgrave was born in 1990 in London, England. She studied at the University of Oxford and graduated in 2012 with an MChem honors degree, having spent one year working with Dr. J. Goicoechea on low-valent, low-oxidation-state transition-metal complexes bearing Nheterocyclic dicarbene ligands. In the same year she started doctoral studies in Ian Manners’ group in Bristol and is now working on the synthesis and reactivity of strained metal-containing rings and lowvalent main-group compounds.

6. SUMMARY AND OUTLOOK The field of strained ferrocenophanes has been widely explored over the last 50 years, and interesting comparisons can be drawn between this new type of ring system and the many wellunderstood strained cyclic organic systems. Ring-opening reactions of strained organometallic rings represent a versatile route to polymers with varying functionality, determined by both the metal center and the spacers present. To date, these metallopolymers have been employed in a number of applications, including uses as catalytic and magnetic ceramic precursors,125,126 etch resists,127−129 self-assembled nanostructured materials,130 and the redox-active actuator for electroactive photonic crystal displays.131 The remarkable chemistry of strained ferrocenophanes and the potential applications of the ring-opened metal-containing polymeric materials provide

Andrew D. Russell was born in Norwich, England, in 1985. He graduated from the University of Bristol in 2008 with an MChem honors degree with industrial experience, spending one year working at AWE on polyphosphazene chemistry and his Masters’ project in the group of K


Organometallics

Review

(21) Finckh, W.; Tang, B. Z.; Lough, A.; Manners, I. Organometallics 1992, 11, 2904−2911. (22) Green, J. C. Chem. Soc. Rev. 1998, 27, 263−272. (23) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992, 114, 6246−6248. (24) Manners, I. Can. J. Chem. 1998, 76, 371−381. (25) Foucher, D. A.; Ziembinski, R.; Tang, B. Z.; Macdonald, P. M.; Massey, J.; Jaeger, C. R.; Vancso, G. J.; Manners, I. Macromolecules 1993, 26, 2878−2884. (26) Nguyen, M. T.; Diaz, A. F.; Dement’ev, V. V.; Pannell, K. H. Chem. Mater. 1993, 5, 1389−1394. (27) Manners, I. Adv. Organomet. Chem. 1995, 37, 131−168. (28) (a) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683−12695. (b) Rulkens, R.; Ni, Y.; Manners, I. J. Am. Chem. Soc. 1994, 116, 12121−12122. (29) Resendes, R.; Nguyen, P.; Lough, A. J.; Manners, I. Chem. Commun. 1998, 1001−1002. (30) Reddy, N. P.; Yamashita, H.; Tanaka, M. J. Chem. Soc., Chem. Commun. 1995, , 2263−2264. (31) Ni, Y.; Rulkens, R.; Pudelski, J. K.; Manners, I. Macromol. Rapid Commun. 1995, 16, 637−641. (32) Tanabe, M.; Vandermeulen, G. W. M.; Chan, W. Y.; Cyr, P. W.; Vanderark, L.; Rider, D. A.; Manners, I. Nature Mater. 2006, 5, 467−470. (33) Pudelski, J. K.; Manners, I. J. Am. Chem. Soc. 1995, 117, 7265− 7266. (34) Mizuta, T.; Imamura, Y.; Miyoshi, K. J. Am. Chem. Soc. 2003, 125, 2068−2069. (35) Mizuta, T.; Onishi, M.; Miyoshi, K. Organometallics 2000, 19, 5005−5009. (36) Peckham, T. J.; Massey, J. A.; Edwards, M.; Foucher, D. A.; Manners, I. Macromolecules 1996, 29, 2396−2403. (37) Foucher, D. A.; Edwards, M.; Burrow, R. A.; Lough, A. J.; Manners, I. Organometallics 1994, 13, 4959−4966. (38) Baumgartner, T.; Jäkle, F.; Rulkens, R.; Zech, G.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2002, 124, 10062−10070. (39) Bagh, B.; Breit, N. C.; Dey, S.; Gilroy, J. B.; Schatte, G.; Harms, K.; Müller, J. Chem. Eur. J. 2012, 18, 9722−9733. (40) Gilroy, J. B.; Russell, A. D.; Stonor, A. J.; Chabanne, L.; Baljak, S.; Haddow, M. F.; Manners, I. Chem. Sci. 2012, 3, 830−841. (41) Herbert, D. E.; Mayer, U. F. J.; Gilroy, J. B.; López-Gómez, M. J.; Lough, A. J.; Charmant, J. P. H.; Manners, I. Chem. Eur. J. 2009, 15, 12234−12246. (42) Nelson, J. M.; Rengel, H.; Manners, I. J. Am. Chem. Soc. 1993, 115, 7035−7036. (43) Patra, S. K.; Whittell, G. R.; Nagiah, S.; Ho, C.-L.; Wong, W.-Y.; Manners, I. Chem. Eur. J. 2010, 16, 3240−3250. (44) Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830−2837. (45) Osborne, A. G.; Whiteley, R. H. J. Organomet. Chem. 1980, 193, 345−357. (46) Seyferth, D.; Withers, H. P., Jr. J. Organomet. Chem. 1980, 185, C1−C5. (47) Braunschweig, H.; Dirk, R.; Müller, M.; Nguyen, P.; Resendes, R.; Gates, D. P.; Manners, I. Angew. Chem., Int. Ed. 1997, 36, 2338−2340. (48) Berenbaum, A.; Braunschweig, H.; Dirk, R.; Englert, U.; Green, J. C.; Jäkle, F.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2000, 122, 5765− 5774. (49) Schachner, J. A.; Lund, C. L.; Quail, J. W.; Müller, J. Organometallics 2005, 24, 4483−4488. (50) Schachner, J. A.; Lund, C. L.; Quail, J. W.; Müller, J. Organometallics 2005, 24, 785−787. (51) Lund, C. L.; Schachner, J. A.; Quail, J. W.; Müller, J. Organometallics 2006, 25, 5817−5823. (52) Braunschweig, H.; Burschka, C.; Clentsmith, G. K. B.; Kupfer, T.; Radacki, K. Inorg. Chem. 2005, 44, 4906−4908. (53) Schachner, J. A.; Orlowski, G. A.; Quail, J. W.; Kraatz, H.-B.; Müller, J. Inorg. Chem. 2006, 45, 454−459.

Chris Willis studying organic reaction mechanisms. In 2010 he began doctoral studies with Ian Manners, where his work focuses on the synthesis and novel reactivity modes of strained metallocenophanes and their application as precursors to new materials.

Ian Manners was born in London, England, in 1961. After receiving his Ph.D. from the University of Bristol in 1985 in the area of transitionmetal chemistry he conducted postdoctoral work in Germany in maingroup chemistry and in the USA on polymeric materials. He joined the University of Toronto, Canada, in 1990 and after 15 years returned to his Alma Mater to take up a Chair in Inorganic, Macromolecular and Materials Chemistry. His research interests focus on the development of new synthetic reactions and self-assembly protocols and their applications in molecular synthesis, polymer and materials science, and nanoscience.

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Organometallics

Review

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Organometallics

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Strained Ferrocenophanes - American Chemical Society

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