Syntheses and Crystal Structures of Ferrocenoindenes

Ferrocenoindenes display planar chirality and thus represent valuable ligands for asymmetric catalysis. Here, we report on the synthesis of novel 3-(1,1-dibromomethylene)ferroceno[1,2-a]indene, (Z)-3-(1-bromomethylene)-6-iodoferroceno[1,2-a]indene, and benzo[5,6-f]ferroceno[2,3,a]inden-1-one. Any application-oriented design of chiral catalysts requires fundamental knowledge about the ligands involved, not only in terms of atom-connectivity, but also in terms of their three-dimensional structure and steric demand. Therefore, the crystal structures of 2-ferrocenylbenzoic acid, ferroceno[1,2-a]indene, and (Z)-3-(1-bromomethylene)-6-iodoferroceno[1,2-a]indene have been determined. The bond-lengths that can be retrieved therefrom also allow for an estimation of the reactivity of the aryl-iodo, bromo-methylidene and dibromomethylidene moieties.


Introduction
Metallocene-based ligands, and here particularly ferrocene-, ruthenocene-and cobaltocene-based systems are of interest due to their unique and intriguing properties such as reversibility of oxidation state, Lewis-base/Lewis-acid behavior, and steric design [1].Their interactive and cooperative effects are uncovered by probing spectroscopic, photonic, magnetic, electronic and Moessbauer behavior of these substances [2].The resulting design as well as manufacture of new materials represent two key steps in the advancement of technology, as it depends almost completely on the rate at which useful new materials can be devised and synthesized [3].Due to its stability and availability and the vast repertoire of elaborated derivation sequences, ferrocene is again favored over other metallocenes [4] or other organometallic fragments [5].In particular, planar chiral ferrocenes are important ligands in the area of homogeneous asymmetric catalysis [6].In order to enforce coplanarity, the pendant ferrocene has to be attached by anellation instead of simple substitution based on one single bond.Such systems can be provided by a straightforward, optimized route based on ferroceno [2,3-a]indenone and its derivatives [7].While chiral diphosphinoferrocenes such as Josiphos ® , Taniaphos ® , MandyPhos ® and BoPhoz ® have successfully been used in enantioselective reactions [8][9][10][11], planar chiral ferroceno [2,3-a]indenes have found application as ligands in stereoselective metallocene-catalyzed olefin polymerization [7] as well as redox active incorporates for macromolecular arrays [12].In this contribution we describe the synthesis of ferroceno [2,3-a]indenes relevant to the above mentioned applications.The crystal structures of three selected compounds are presented.
The asymmetric units of 1, 3, and 7 contain two nearly identical molecules.In all crystals the axes of the centroids of the five-membered rings through the iron atoms Fe1 and Fe2 are approximately perpendicular to each other.The ferrocene moieties adopt eclipsed conformations.Between the molecules are no strong interactions, except for 1, where two molecules form an eight-membered ring via hydrogen bonding of the two carboxylic acid groups (Figure 1 4)°).However, the two independent molecules in the structures of 3 and 7 represent enantiomers due to their planar chirality, as can be seen from the overlays in Figures 2 and 3.These are therefore racemic crystals.Crystal data and structure refinement details are summarized in Table 1.

3-(1,1-Dibromomethylene)ferroceno[1,2-a]indene (4)
Triphenylphosphine (1365 mg, 5.206 mmol) and CBr 4 (863 mg, 2.603 mmol) were dissolved in THF (10 mL), magnesium (63 mg, 2.603 mmol, activated with mercury (II) chloride) was added, and the mixture was stirred for 30 min. 2 (500 mg, 1.74 mmol) was added, the mixture was stirred at room temperature for 20 h and finally refluxed for another 48 h.The product was obtained by flash chromatography over silica gel (G60, hexane).Intensity data were collected on a Bruker P4 diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å).The unit cell parameters were determined and refined from around 25 randomly selected reflections, obtained by P4 automatic routines.Data were measured via ω scans and corrected for Lorentz and polarization effects.For 3 and 7 an empirical absorption correction (ψ scan) was applied.The structures were solved by direct methods (SHELXS-86) [19] and refined by a full matrix least-squares procedure using F 2 (SHELXL-97) [20].All non-hydrogen atoms were refined with anisotropic displacement parameters.All hydrogen atoms were located by difference Fourier methods, but in the refinement they were included in calculated positions and refined with isotropic displacement parameters U iso = 1.2 U eq of the attached carbon atoms.Only the hydrogen atoms at O(2) and O(4) of 1 were refined with isotropic displacement parameters.The structure of 3 was solved using a pseudo-merohedral twinning.This twinning can be described by a rotation around the a*-or c*-axis or by mirroring through the a*b*-or c*b*-planes, respectively.The twin ratio was around 2.7:1.By using the twin matrix the R 1 value changed from 0.141 to 0.034.CCDC reference numbers: 904160-904162.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Conclusions
In summary, the synthetic routes to a series of substituted ferrocenoindenes and ferrocenobenzoindenes are presented.They are accessible via intramolecular Friedel-Crafts reactions.It should be stated that all indene-type compounds are produced as racemates; however, these can be separated via chiral HPLC.Therefore, the concept is valid for the synthesis of enantiomerically pure ferrocenoindenes and ferrocenobenzoindenes which may in due course be utilized as ligands in catalysis and asymmetric synthesis, respectively.

Figure 1 .
Figure 1.Hydrogen bonding between two independent molecules of 1.

Figure 2 .Figure 3 .
Figure 2. (a) Two independent molecules in the crystal structure of 3; (b) Overlay of the two enantiomers.

Table 1 .
Crystal data and structure refinement details.