, 3 , 5-Triferrocenyl-2 , 4 , 6-tris ( ethynylferrocenyl )-benzene – a new member of the family of multiferrocenyl-functionalized cyclic systems † ‡

The consecutive synthesis of 1,3,5-triferrocenyl-2,4,6-tris(ethynylferrocenyl)benzene (6c) is described using 1,3,5-Cl3-2,4,6-I3-C6 (2) as starting compound. Subsequent Sonogashira C,C cross-coupling of 2 with FcCuCH (3) in the molar ratio of 1 : 4 afforded solely 1,3,5-Cl3-2,4,6-(FcCuC)3-C6 (4c) (Fc = Fe(η-C5H4)(η-C5H5)). However, when 2 is reacted with 3 in a 1 : 3 ratio a mixture of 1,3,5-Cl3-2-(FcCuC)-4,6-I2-C6 (4a) and 1,3,5-Cl3-2,4-(FcCuC)2-6-I-C6 (4b) is obtained. Negishi C,C cross-coupling of 4c with FcZnCl (5) in the presence of catalytic amounts of [Pd(CH2C(CH3)2P( C4H9)2)(μ-Cl)]2 gave 1,3-Cl2-5-Fc-2,4,6(FcCuC)3-C6 (6a), 1-Cl-3,5-Fc2-2,4,6-(FcCuC)3-C6 (6b) and 1,3,5-Fc3-2,4,6-(FcCuC)3-C6 (6c) of which 6b is the main product. Column chromatography allowed the separation of these organometallic species. The structures of 4a,b and 6a in the solid state were determined by single crystal X-ray diffractometry showing a π–π interacting dimer (4b) and a complex π–π pattern for 6a. The electrochemical properties of 4a–c and 6a–c were studied by cyclic voltammetry (=CV) and square wave voltammetry (=SWV). It was found that the FcCuC-substituted benzenes 4a–c show only one reversible redox event, indicating a simultaneous oxidation of all ferrocenyl units, whereby 4c is most difficult to oxidise (4a, E°’1 = 190, ΔEp = 71; 4b, E°’1 = 195, ΔEp = 59; 4c, E°’1 = 390, ΔEp = 59 mV). In case of 4c, the oxidation states 4c (n = 2, 3) are destabilised by the partial negative charge of the electronegative chlorine atoms, which compensates the repulsive electrostatic Fc–Fc interactions with attractive electrostatic Fc–Cl interactions. When ferrocenyl units are directly attached to the benzene C6 core, organometallic 6a shows three, 6b five and 6c six separated reversible waves highlighting that the Fc units can separately be oxidised. UV-Vis/NIR spectroscopy allowed to determine IVCT absorptions (=Inter Valence Charge Transfer) for 6c (n = 1, 2) (n = 1: νmax = 7860 cm , εmax = 405 L mol −1 cm, Δν1/2 = 7070 cm; n = 2: νmax = 9070 cm, εmax = 620 L mol −1 cm, Δν1/2 = 8010 cm) classifying these mixed-valent species as weakly coupled class II systems according to Robin and Day, while for 6a,b only LMCT transitions (=ligand to metal charge transfer) could be detected.


Introduction
Multiferrocenyl-functionalized aromatics and heteroaromatics are fascinating molecules.Besides their uncommon molecular structures, such sterically crowded compounds possess, for example, interesting electronic properties. 1 Hence, they can be considered as model systems to study intramolecular electron transfer through π-conjugated carbon-rich organic linking units via the mixed-valence states derived from these multi-metallic compounds.In this respect, the ferrocenyl group is beneficial since the [Fe(II)/Fe(III)] redox couple shows an excellent electrochemical reversibility and high thermal stability. 2The degree of electronic communication among the appropriate metal centers has mostly been explored by electrochemical studies such as cyclic voltammetry (=CV), square wave voltammetry (=SWV) and spectroelectrochemistry (e.g., in situ UV-Vis/NIR spectroscopy).Other relevant applications for reversible multi-step redox systems include their use in the field of catalysis, 3 in biological studies 4 or as novel molecular electro-active materials. 5uper-crowded ferrocenyl-based organometallic compounds are moreover remarkable species because the expected steric encumbrance may hinder chemical conjugation between the aromatic core and the ferrocenyl substituents.Representatives of this class of compounds are, for example, ferrocenyl-endgrafted dendrimers 6,7 in which the intramolecular distance between the ferrocenyls is enlarged by various units such as ethynyl, 6a-c ethynyl benzene, 6d-e and ethynyl thiophene 1f or amidoamine-based dendrimers. 7Further examples of multiferrocenyl organometallic compounds are benzenes, 6b,c,8,9 5membered heterocycles 10,11 or even cobalt 12 and manganese 13 half-sandwich species with up to six terminal ferrocenyl or ethynyl ferrocenyl entities, i.e. (FcCuC) 6 C 6 , Fc 6 C 6 , 2,3,4,5-Fc 4 -c C 4 E (E = O, S, NPh, NMe), Co(η 4 -Fc 4 C 4 )(η 5 -C 5 H 5 ), and Mn-(η 5 -Fc 5 C 5 )(CO) 3 .Electrochemical studies revealed that for the respective super-crowded ferrocenyl thiophene significant electrostatic interaction among the four ferrocenyl groups occurs as oxidation progresses.The spectroelectrochemical results showed several UV-Vis and NIR peaks appearing or disappearing between 280 and 3000 nm as this compound is stepwisely oxidised to ultimately generate [2,3,4,5-Fc 4 -c C 4 S] 4+ .For the respective pyrrole compounds electronic interaction between the ferrocenyl/ferrocenium units is evidenced by in situ UV-Vis/ NIR spectroscopy.10b In contrast, Vollhardt's hexaferrocenyl benzene 9 and Astruc's hexa-ethynylferrocenyl benzene 6b,c show three separated redox events.
We here enrich this family of perferrocenylated benzenes and describe for the first time the synthesis of multiferrocenyl-substituted benzenes featuring alternating ferrocenyl and ethynyl ferrocenyl functionalities, which represent a combination of the structural motifs of Vollhardt's 9  as well as their electrochemical properties will be highlighted.
The Fc and FcCuC multi-substituted benzenes 4a-c and 6a-c (Schemes 1 and 2) were obtained as red (4b, 6b) or orange (4a,c and 6a,c) solids, which dissolve in almost all common organic solvents, including toluene, dichloromethane and tetrahydrofuran.They are stable towards air and moisture in the solid state and in solution.
The 1 H and 13 C{ 1 H} NMR spectroscopic properties of 4a-c and 6a-c correlate with their formulations as Fc and FcCuC multi-functionalised benzenes showing the respective signal patterns for the Fc, CuC and C 6 core building blocks.Most distinctive for the formation of these molecules is the appearance of the expected AA′XX′ signal pattern 19 for the C 5 H 4 units ( J HH = 1.9 Hz) and the singlet for the C 5 H 5 moieties (Experimental section).Further characteristic in the 13 C{ 1 H} NMR spectra of all complexes are the signals for the ethynyl units, which resonate at ca. 65 ppm (CuC-C 6 ) and ca. 100 ppm (CuC-Fc), respectively (Experimental section).2D experiments such as COSY, HSQC and HMBC were applied to assign the carbon signals in 4a-c and 6a-c unequivocally.Most characteristic in the IR spectrum of all newly synthesised compounds is the appearance of one sharp CuC stretching vibration between 2200 and 2220 cm -1 , specific for this distinctive unit. 20he formation of 4a-c and 6a-c was additionally evidenced from ESI-TOF mass spectrometric investigations.All organometallic compounds show the molecular ion peak [M] + (Experimental section).Moreover, comparison of the measured isotope patterns (Cl, I) of 4a-c and 6a,b with the calculated ones confirm the elemental composition and charge state.
Furthermore, single crystal X-ray diffraction studies have been carried out to determine the molecular structures of 4a (Fig. 1), 4b (Fig. 2) and 6a (Fig. 3) in the solid state.Suitable single crystals of 4a,b and 6a could be obtained either by crystallisation of 4a and 6a from dichloromethane solutions, or by slow diffusion of n-hexane into a dichloromethane solution containing 4b at ambient temperature (Experimental section).Important bond distances (Å), bond angles (°) and torsion angles (°) are summarised in the captions of Fig. 1-3.For crystal and structure refinement data see ESI. ‡ Compound 4a crystallises in the triclinic space group P1 ˉ, 4b in the monoclinic space group C2/c and 6a in the orthorhombic space group Pccn.
Compound 4b can best be transcribed by the symmetry operation −x, 1 − y, −z, which results in a rectangular shaped dimer (Fig. SI3 ‡) with parallel displaced π-π interactions between both C 6 cycles of 3.615(13) Å. 22 Furthermore, 4b is strongly disordered over two positions (0.6 : 0.4) in which the ferrocenes of the disordered part correspond to the corners of the rectangle formed by the initial dimer.However, the C 6 core is rotated by 45 °providing interaction with a third ferrocenyl corner (Fig. SI2 and SI3, ESI ‡).

Electrochemistry
The redox properties of 4a-c and 6a-c have been determined by cyclic voltammetry (=CV) and square-wave voltammetry (=SWV) (Fig. 5).Dichloromethane solutions containing the respective analyte (1.0 mmol L −1 ) and [ n Bu 4 N][B(C 6 F 5 ) 4 ] (0.1 mol L −1 ) 10,11,23,24 as supporting electrolyte were used for the measurements.The CV studies have been performed at a  scan rate of 100 mV s −1 and the results are summarised in Fig. 5.The appropriate potential values are given in Table 1.
All redox potentials are referenced to the FcH/FcH + redox couple (E°′ = 0 mV, FcH = Fe(η 5 -C 5 H 5 ) 2 ). 25 From Fig. 5 it can be seen that the cyclic and square wave voltammograms of 4a-c show only one reversible redox event irrespective of the number of FcCuC units present, evincing the simultaneous oxidation of the Fc groups.Furthermore, it is found that an increasing number of redox-active Fc groups at the benzene core results in a shift of the E°′ 1 values to higher potentials (4a, E°′ 1 = 190 mV; 4b, E°′ 1 = 195 mV; 4c, E°′ 1 = 390 mV) (Table 1).This indicates that the more FcCuC moieties are present, the more difficult is the oxidation of the Fe(II) centres, which is in agreement with the electron withdrawing character of the ferrocenyl ethynyl building blocks.In contrast to 4c, 1,3,5-tris(ethynylferrocenyl) benzene (dichloromethane, possess quite similar ion pairing capabilities in dichloromethane and both these fluorinated borates act as very weak coordinating counter ions, thus it is expected that the appropriate ΔE°′ values are similar for both electrolytes. 23Against this background the different redox behaviour of 4c and 1,3,5tris(ethynylferrocenyl) benzene is surprising.On the one hand it could be shown that the electronic communication between the terminal ferrocenyl units is suppressed, when electron poor aromatics are used as bridging systems.1c,10b, 26 On the other hand, the electron withdrawing effect of the chlorine atom leads to a partially negative charge, which enables attractive interactions with the neighbouring Fc + CuC units,   d Potential difference between the two redox processes determined by the application of the Richardson and Taube method. 31When using the deconvolution of the redox separation of the oxidation potentials in SWV (Fig. SI4), ΔE°′ = 60 mV.e Values determined using Square Wave Voltammetry.
compensating the repulsive electrostatic destabilisation (Fig. 6).Thus, the thermodynamic stability of mixed-valent oxidation states 4c n+ (n = 1, 2) is reduced and no redox splitting could be observed.The importance of electrostatic effects on the ΔE°′ values especially in case of weakly coupled systems has recently been pointed out by Winter, who strongly emphasizes that ΔE°′ is not a sufficient measure for the electron delocalisation within mixed-valent species. 27However, attempts to accurately model such electrostatic interactions and their effect on ΔE°′ may in future help for a better understanding of the electrochemical properties of mixed-valent systems. 28hen the chlorine substituents of 4c were stepwisely replaced with ferrocenyl units in 6a-c, a more resolved redox behaviour with a separate oxidation of the individual ferrocenyls could be observed.A comparison of the formal oxidation potentials of ferrocenyl benzene (E°′ = 40 mV) 29 and ethynylferrocenyl benzene (E°′ = 115 mV) 30 allows to estimate that the ferrocenyls are oxidised prior to the FcCuC units in 6a-c.The oxidation potential of the 1 st Fc oxidation decreases from 6a (E°′ = 40 mV) to 6c (E°′ = −80 mV) as the electron withdrawing chlorine substituents are replaced by electron-rich ferrocenyl termini.The redox splitting between the directly bonded ferrocenyl groups for 6c (ΔE°′ 1 = ΔE°′ 2 = 150 mV) resembles those of triferrocenyl benzene (ΔE°′ 1 = 140 mV; ΔE°′ 2 = 145 mV).In contrast to 4c, the ethynylferrocenyl units of 6c are oxidised separately.For 6c 3+ the directly bonded Fc units are oxidised to ferrocenium termini which possess an equal or ever stronger electron withdrawing character as the chlorine substituents in 4c, nevertheless, those groups are positively charged and therefore, add further repulsive electrostatic interactions in 6c (Fig. 6).
Noteworthy is the high ΔE p value of 108 mV for the second redox wave of 6a, suggesting that two individual reversible oneelectron processes take place in a close potential range.Hence, the square wave voltammogram gives an integrated peak area of 1 : 2 : 1, which verifies the presence of two closely spaced one-electron processes (Fig. 5).Deconvolution of the SWV of 6a using four Gaussian-shaped functions resulted in ΔE°′ 2 = 60 mV (ESI ‡ Fig. SI4).The calculation of the signal width at half of the maximum current 31 to estimate the redox separation gave a similar value of ΔE°′ 2 = 68 mV.This clearly confirms that the second oxidation process consists of two superimposed redox waves.
24e Due to the use of different electrolytes in the electrochemical measurements a comparison with related work is difficult.Vollhart's hexaferrocenyl benzene gave only three redox processes consistent of a one (E°′ 1 = −163 mV), a two (E°′ 2 = −32 mV) and a three (E°′ 3 = 222 mV) electron process (dichloromethane, [ n Bu 4 N][PF 6 ] as supporting electrolyte). 9owever, the use of the classical [PF 6 ] − counter ion compensates most of the electrostatic repulsion by ion-pairing with the analyte.Hence, it is expected that the use of a weakly coordinating anion (= WCA, i.e.For a further investigation of the electronic properties of 6a-c in situ spectroelectrochemical UV-Vis/NIR measurements have been carried out to prove, if the interactions between the Fc/Fc + groups are solely caused by electrostatic contributions or if an intramolecular electron transfer between the redoxactive ferrocenyl moieties via the carbon-rich connectivities occurs.
The electron poor character of the benzene core of 6b, caused by the electron-withdrawing effect of the chlorine in position 1, is not capable of facilitating the charge transfer between Fc/Fc + in 3,5-positions.
In addition, in situ UV-Vis/NIR studies revealed IVCT excitations in the mixed-valent oxidation states of 6c + and 6c 2+ attributed to the Fe(II)/Fe(III) metal centres of the directly bonded ferrocenyl groups.Therefore, the mixed-valent species 6c n+ (n = 1, 2) can be classified as weakly coupled class II systems according to Robin and Day. 35The spectroscopic characteristics of 6c n+ (n = 1, 2) resemble those of 1,3,5-Fc 3 C 6 H 3 and 2,4,6-Fc 3 C 5 H 2 N 1c demonstrating that the electron transfer occurs between the Fc/Fc + groups, while the pathway through the ortho-substituted Fc + /FcCuC units is unsuited for electronic interactions.This was confirmed by in situ UV-Vis/ NIR investigations of 1-ethynylferrocenyl-2-ferrocenyl benzene (9) showing no IVCT absorptions in the mixed-valent oxidation state.Class I systems 6a,b showed only LMCT transitions during these measurements.

General conditions
All reactions were carried out under an atmosphere of argon using standard Schlenk techniques.Drying of n-hexane, diethyl ether and dichloromethane was performed with a MBraun MB SPS-800 system (double column solvent filtration, working pressure 0.5 bar).Tetrahydrofuran was purified by distillation from sodium/benzophenone ketyl, and methanol was purified by distillation from magnesium.Diisopropylamine was purified by distillation from calcium hydride.

Reagents
Periodic acid, potassium iodide, 1,3,5-trichlorobenzene (1), triphenylphosphane, copper(I)iodide, t BuLi (1.9 M solution in n-pentane), ferrocene, 1-bromo-2-iodo-benzene (7) and KO t Bu were purchased from commercial suppliers and were used without further purification.FcCuCH (3), 36   were recorded with a Bruker Avance III 500 spectrometer operating at 298 K in the Fourier transform mode.Chemical shifts are reported in δ units ( parts per million) using undeuterated solvent residues as internal standard (CDCl 3 : 1 H at 7.26 ppm and 13 C{ 1 H} at 77.16 ppm).Infrared spectra were recorded using a FT-Nicolet IR 200 equipment.The melting points of analytical pure samples (sealed off in nitrogen-purged capillaries) were determined with a Gallenkamp MFB 595 010 M melting point apparatus.Microanalyses were performed using a Thermo FLASHEA 1112 Series instrument.High-resolution mass spectra were performed with a micrOTOF QII Bruker Daltonite workstation.radiation (λ = 0.71073 Å).The molecular structures were solved by direct methods using SHELXS-97 39 and refined by fullmatrix least-squares procedures on F 2 using SHELXL-97. 40All non-hydrogen atoms were refined anisotropically and a riding model was employed in the treatment of the hydrogen atom positions.

Electrochemistry
Measurements on 1.0 mmol L −1 solutions of the analytes in dry air free dichloromethane containing 0. . 24Successive experiments under the same experimental conditions showed that all formal reduction and oxidation potentials were reproducible within ±5 mV.Experimentally potentials were referenced against an Ag/Ag + reference electrode but results are presented referenced against ferrocene 41 (FcH/FcH + couple = 220 mV vs. Ag/Ag + , ΔE p = 61 mV) as an internal standard as required by IUPAC. 25When decamethylferrocene was used as an internal standard, the experimentally measured potential was converted into E vs. FcH/FcH + (under our conditions the Fc*/Fc* + couple was at −614 mV vs. FcH/FcH + , ΔE p = 60 mV). 42ata were then manipulated on a Microsoft Excel worksheet to set the formal redox potentials of the FcH/FcH + couple to E°′ = 0.000 V.The cyclic voltammograms were taken after typical two scans and are considered to be steady state cyclic voltammograms in which the signal pattern differs not from the initial sweep.The preparation of compound 2 was carried out using a modified procedure from the literature. 14 General proceduresynthesis of 4a,b

Spectroelectrochemistry
In a Schlenk flask, 50 mL of degassed diisopropylamine, 6.00 mol% of CuI (65.3 mg, 0.34 mmol) and 0.50 mol% of [PdCl 2 (PPh 3 ) 2 ] (20 mg, 0.03 mmol) were added and the solution was stirred for 5 min.The reaction mixture was treated with 1.00 g (1.79 mmol) of 2, 3.2 eq. of ethynylferrocene (3) (1.20 g, 5.7 mmol) and 6.00 mol% of PPh 3 (90.0mg, 0.34 mmol) and was then heated to reflux for 24 h, whereby the crimson solution turned orange.After cooling it to room temperature and evaporation of all volatiles, the orange residue was worked-up by column chromatography (column size: 3 × 10 cm, alumina, n-hexane).As eluent a n-hexane-diethyl ether mixture of ratio 20 : 1 (v/v) was used.The 1 st fraction contained ethynylferrocene (3), while from the 2 nd fraction 4a and from the 3 rd fraction 4b could be isolated.All volatiles were removed under reduced pressure.