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.

Starting from 2-ferrocenylbenzoic acid (1), 2-ferrocenylbenzoyl chloride was prepared using PCl₅ and cyclized by intramolecular Friedel-Crafts acylation using AlCl₃ to give ferroceno[2,3-a]inden-1-one (2), a planar chiral compound, as a racemate (Scheme 1) [7,13]. Separation of the two enantiomers by HPLC on a β-cyclodextrin-derived chiral column has been reported [13]. Several synthetic routes to ferroceno[2,3a]indene (3) have been disclosed: (1) reduction of 2 with LiAlH₄ to diastereomeric ferroceno[2,3-a]inden-1-ols, subsequent formation of a cationic complex, and reduction with Na/NH₃ [7]; or (2) direct hydrogenation of 2 [7]. Alternatively, intramolecular Friedel-Crafts alkylation of 2-ferrocenylbenzylic chloride using AlCl₃ [14] also gave the indene 3 in low yield. Single crystals of the starting material 1 were obtained from CH₂Cl₂. The radical cation of indene 3 yielded an interesting dimer, the crystal structure of which has been determined [15].

The new dibromo-compound, 3-(1,1-dibromomethylene)ferroceno[1,2-a]indene (4), was obtained by Wittig-type reaction of 2 with CBr₄/PPh₃ in good yield and gave X-ray quality crystals from CH₂Cl₂. Reaction of 3 with n-butyllithium followed by the addition of Cl₂SiMe₂ gave the dimethylsilylene-bridged compound rac-bis(4H-indeno[2,3-a]ferrocen-4-yl)dimethylsilane, the crystal structure of which has been reported [16]. This compound has been used for the manufacture of Zr-based metallocenes for 1-olefin polymerization and copolymerization with norborn-2-ene [7].

By analogy, 6-iodoferroceno[2,3-a]inden-1-one (6) was accessible from 5-iodo-2-ferrocenylbenzoic acid (5) [13]. This compound was used for a Sonogashira-Hagihara coupling [13]. A Wittig bromomethylenation gave the new 3-(bromomethylene)-6-iodoferroceno-[1,2-a]indene (7) in acceptable yield. The E and Z isomers were separated, and the Z isomer crystallized from CH₂Cl₂. This ferrocene-annellate offers all the potential for Pd-mediated coupling reactions with two sites of different reactivity (Br < I). By analogy again, ferroceno[2,3-a]benzo[5,6]inden-1-one (9) was prepared from 3-ferrocenyl-2-naphthoic acid (8).

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) (O₂-H...O₃: H...O₃ 1.75(4) Å and O₂...O₃ 2.620(3) Å, O₂-H-O₃ 170(4)°; O₄-H...O₁: H...O₄ 1.86(5) Å and O₄...O₁ 2.713(3) Å, O₄-H-O₁ 169(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 Figure 2, Figure 3. These are therefore racemic crystals. Crystal data and structure refinement details are summarized in Table 1.

All reactions were carried out in the absence of air using standard Schlenk techniques. Solvents were deoxygenated, purified and dried using standard procedures. Instrumentation: Bruker AC 200 and Varian Gemini 200 (¹H and ¹³C NMR); Nicolet 510 FT-IR (IR); Varian CH-7 (MS); Siemens P4 (X-ray). Melting points were determined on a Kofler hot-plate microscope and are uncorrected. The synthesis and the spectroscopic data of 2-ferrocenylbenzoic acid (1) [7,17], ferroceno[2,3-a]inden-1-one (2) [7,17], ferroceno[1,2-a]indene (3) [7], 2-ferrocenyl-5-iodobenzoic acid (5) [13], 6-iodoferroceno[2,3-a]inden-1-one (6) [13], and 3-ferrocenyl-2-naphthoic acid (8) [18] have been reported elsewhere.

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.