Carbon-Rich Trinuclear Octamethylferrocenophanes - Inorganic

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Carbon-Rich Trinuclear Octamethylferrocenophanes Max Roemer,*,†,§ Duncan A. Wild,† Alexandre N. Sobolev,‡ Brian W. Skelton,‡ Gareth L. Nealon,‡ Matthew J. Piggott,† and George A. Koutsantonis*,† †

Chemistry, M310, School of Molecular Sciences and ‡Center for Microscopy, Characterization, and Analysis, The University of Western Australia, 35 Stirling Hwy, Crawley, Western Australia 6009, Australia

Inorg. Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 03/05/19. For personal use only.

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ABSTRACT: Several trinuclear ferrocenes are obtained by Friedel− Crafts reaction of octamethylferrocene with ferrocenoyl chloride and subsequent modifications. 1,1′-Diferrocenoyloctamethylferrocene (3) is transformed to the divinyl derivative (4a) by reaction with MeLi and AlCl3. The reactive 4a cyclizes spontaneously to a [4]ferrocenophane with buta-1,3-diene handle (5) or in the presence of AlCl3 to a [3]ferrocenophane with propene handle (6). Structure assignments are supported by X-ray crystallography and NMR spectroscopy, and mechanisms are proposed. Electrochemical behavior of the compounds was investigated with cyclic voltammetry, and assignments of the redox processes were carried out with the aid of density functional theory calculations. The synthesized compounds and demonstrated transformations represent useful tools for preparation of materials for charge-transport studies in metal−molecule−metal junctions.

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realized by oxidation of decamethylferrocene.39 Thus, 1 represents an attractive building block for compounds used in molecular electronics applications. We recently reported the diacetylation of octamethylferrocene40 and formation of ferrocenophanes from the 1,1′-bis(1-chlorovinyl)-derivative.40,41 1,1′-Divinyl- and -ethynylferrocenes are known to undergo cyclizations to [4]ferrocenophanes, as demonstrated for mononuclear derivatives bearing trifluorovinyl,42−44 1chlorovinyl,41 vinyl,45 and ethynyl substituents.41,46,47 Herein, we report an efficient synthetic protocol for the preparation of trinuclear octamethylferrocenes and cyclization reactions of 1,1′-bis(1-ferrocenylvinyl)-octamethylferrocene affording [3]and [4]ferrocenophanes.

INTRODUCTION The interest in ferrocenes shows no signs of abating. Since the discovery of the archetypal organometallic complex,1,2 thousands of derivatives have been prepared with the number of publications involving ferrocenes steadily rising to over 900/ year in the last five years.3 Increasingly, ferrocenes4 are finding application in a number of fields such as sensing,5−9 cancer treatment,10−12 materials applications,13−20 and in synthetic methodology.21−25 Their excellent redox and chemical stability and synthetic accessibility4 make ferrocenes compounds of choice in metal−molecule−metal junctions.26−31 Chargetransport studies using large-area self-assembled monolayer devices revealed diode-type behavior for a number of ferrocenes, with binuclear ferrocenes giving high molecular rectification.32,33 Particularly, binuclear alkynyl-bridged conjugated ferrocenes immobilized on platinum surfaces led to giant rectification of electric current.34 Macrocyclic polynuclear ferrocenes have also been proposed for molecular electronics applications.35,36 In this regard, the role of the synthetic chemist is to provide access to molecular architectures that will allow functional properties to be investigated. While syntheses of substituted ferrocenes starting from the parent compound are relatively straightforward,37,38 the syntheses of octamethylferrocene (1) (Scheme 1) derivatives are much less established. The methyl groups of the tetramethylcyclopentadienyl (Me4Cp) ligand impart higher solubility compared to Cp, which is an important practical consideration for the preparation of larger molecules. Derivatives of 1 are significantly more electron rich than ferrocene, which results in lowered HOMO/LUMO energy levels and redox potentials. This in turn alters charge-transport behavior and chemical reactivity. For instance, Fe(IV) oxidation states have been © XXXX American Chemical Society

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RESULTS AND DISCUSSION Trinuclear ferrocene (3) was prepared by the Friedel−Crafts reaction of octamethylferrocene (1) and ferrocenoyl chloride (2) (Scheme 1). Diketone 3 was converted into the 1,1′divinyl derivative (4a) by addition of MeLi in THF, followed by elimination induced by AlCl3. The optimized reaction in solution affords 4a in near quantitative yield with a purity of ∼95%, which we used as method for preparation of 4a throughout this study. If the reaction is quenched prior to the addition of AlCl3, then the diol 4c is generated in excellent yield, and incomplete reaction resulted in the isolation of the monovinyl tertiary alcohol 4b. Molecular structures of 3 and 4a−c derived from single crystal X-ray diffraction experiments are shown in Figure 1. The Received: December 5, 2018

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DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Friedel−Crafts Reaction of Octamethylferrocene (1) and Ferrocenylcarbonyl Chloride (2) and Subsequent Transformations Provide Access to a Variety of Trinuclear Complexes with a Central Octamethylferrocene Moiety

Figure 1. Molecular structures as determined by X-ray single crystal diffraction for compounds 3 (a), 4a (b), 4b (c), and 4c·THF (d). H-atoms and disorder are not shown for clarity. Atomic displacement ellipsoids are shown at a probability level of 50%.

the adjacent, sterically encumbered carbonyl group. Both 4b and 4c can be converted almost quantitatively to 4a by dehydration, and very minor amounts of 5 and 6 are also

crystal structure of 4c revealed it to be the racemic chiral diasteromer. Presumably, the intermediate monolithiumalkoxide directs the second equivalent of MeLi stereoselectively to B

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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When a solution of 4a in THF-d8 was treated with a drop of deuterochloric acid in an NMR experiment, the spectra indicated formation of the product but immediate broadening of the signals was also observed suggesting the presence of paramagnetic material. Treatment of complex 4a with a catalytic amount of the oxidant DDQ in DCM led to an immediate color change from yellow-orange to deep purple. 1H NMR spectroscopy revealed an instantaneous and complete consumption of 4a with formation of 5 as the major product, along with minor amounts of the [3]ferrocenophane 6. Based on the described observations, we propose a potential mechanism for cyclization of 4a to 5, shown in Scheme 3. Cyclization is initiated by oxidation of the core electron-rich octamethylferrocene moiety in 4a to the corresponding ferrocenium ion 4a+. It is worth noting that protonation facilitates such oxidation,48,49 potentially explaining the observed rate acceleration of the cyclization reaction in the presence of acids.50−52 Single electron transfer (SET) from the vinylic π-orbital into the Fe-centered SOMO initiates or facilitates (in a concerted fashion as shown) a radical rearrangement in which the carbon bridge is formed. Deprotonation of the resultant radical cation (I1) gives rise to a radical (I2), which can undergo a second single electron oxidation, either directly or via iron oxidation and internal SET, to give carbocation I3. A final deprotonation gives the product 5. Intriguingly, increasing the amount of AlCl3 from 3 to 6 equiv after treating 3 with MeLi resulted in the formation of significant quantities of [3]ferrocenophane 6, an isomer of 4a. Indeed, 4a was converted to 6 when treated with AlCl3 in THF, which shows that MeLi is not required for the reaction. A similar reaction has been reported when 1,1′-bis(ferrocenoyl)ferrocene was treated with a large excess of both MeLi and AlCl3 (6b, Scheme 2).53 In that case, the isolation of a 1,1′-divinyl species was not reported, and a mechanism involving nucleophilic addition of a methyl group and subsequent deprotonation followed by rearrangement was proposed. In light of the results presented here, it seems likely that 1,1′-bis(1-ferrocenylvinyl)ferrocene is intermediate in the formation of 6b. We propose the mechanism depicted in Scheme 4 for the formation of 6, supported by the abovementioned observations. Intramolecular attack of a vinyl group on the α-carbon is enabled by activation of the electrophilic πbond by aluminum coordination. The resulting zwitterionic intermediate is stabilized by the electron-donating octamethylferrocene core. Deprotonation and proto-demetalation results in generation of 6 bearing a methyl group in the αposition. This transformation proceeds even in the presence of small amounts of AlCl3, and as consequence 6 is always formed in the process of generating 4a from 3 in solution. This sidereaction could not be avoided, but controlling the quantity of AlCl3 used and adding it slowly suppressed its formation, affording 4a in a purity of ∼95%. In support of the mechanism presented in Scheme 4, orthodi(1-phenylvinyl)benzene and related compounds undergo FeCl3-mediated cyclization to give indenes,54,55 structurally analogous to 6. That reaction is also acid catalyzed.56 It seems that in the case of the ferrocene 4a, acid-mediated oxidation to the ferrocenium ion is faster than acid-catalyzed cyclization to 6. Upon attempted chromatography of compound 4a on silica, another ferrocenophane, the diol 7 (Scheme 2), was formed and isolated in small amounts after selective crystallization

formed, achieved simply by heating in air above the melting point. This can be observed visually when heating a crystal of 4b using a melting point microscope, which initially results in gas evolution (presumably H2O) followed by melting and subsequent formation of crystalline 4a above the initial melting point of 183−185 °C. Small-scale dehydration experiments for 4b and 4c revealed the formation of 4a after heating neat samples to 200 °C for 5 min, indicating that the divinyl derivative 4a is temperature and air stable as we did not observe significant decomposition. In solution, however, 4a is very reactive and undergoes an unexpected, spontaneous cyclization to the [4]-octamethylferrocenophane (5), bearing a buta-1,3-diene handle with ferrocenyl groups in the two α-positions (Scheme 2). Scheme 2. Cyclization of 4a to a [4]Ferrocenophane with Butadiene Handle (5) and to a [3]Ferrocenophane with Propene Handle (6) Bearing Two Ferrocenesa

a

Compound 4a reacts on silica to the diol 7. Compound 6b represents the non-methylated analogue of 6.

Ferrocenophane formation from 1,1′-alkenylferrocenes is well-known, as described in the Introduction; however, most reported reactions proceed by addition of a nucleophile to the α-position of a vinyl or alkynyl group, which initiates ringclosure and rearrangement steps.42−47 In the present case, it is noteworthy that the transformation proceeds under formal loss of one equivalent of H2 per cyclized molecule and thus is oxidative. The cyclization proceeds near quantitatively in polar solvents like CHCl3, DCM or THF, even with the exclusion of air and at low temperatures. Attempted recrystallization of 4a from either DCM or THF, under Schlenk conditions, resulted in crystallization of 5 and 5·THF, respectively. We monitored the cyclization by 1H NMR spectroscopy in chloroform-d and benzene-d6. While near quantitative (95%) conversion occurs in CDCl3 over 16 h (Figure 2) and is complete in 1.5 days, the reaction in C6D6 is significantly slower with only 6% conversion after 16 h and 21% conversion after 3 days. This suggested that the greater polarity of this solvent, relative to C6D6, and/or adventitious acid in the CDCl3, accelerates the reaction rate. Indeed, while 4a is stable upon heating for at least 1 h in THF/water, addition of a few drops of dilute HCl resulted in an immediate color change from orange to red and quantitative formation of 5. C

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Selected ranges of the 1H NMR spectra (500 MHz) in CDCl3 upon sample preparation (bottom) and standing for 16 h at room temperature. Initially, the sample contains mostly 4a and minor amounts of 5 (bottom). After standing, nearly all 4a is consumed with 5 being the only product.

gel did not trigger a reaction, hence ruling out hydration of 5 as the source of 7. Similarly, 4a was stable in t-BuOH/water. Ferrocenophanes bearing OH-groups in the α-positions were proposed as transient intermediates arising from nucleophilic addition of HO− to a trifluorovinyl group and subsequent cyclizations of fluorovinylferrocenes.42,44 In the current case, the α-position is sterically crowded by the ferrocenyl and methyl groups, which likely prevents such a mechanism. The most likely source of 7 is interception of the carbocation I1 (Scheme 3) by water, followed by oxidation and a second similar reaction at the α′-position. While we obtained crystals suitable for an X-ray single crystal diffraction experiment, by selection under a microscope, we did not have a quantity sufficient to provide a 13C NMR spectrum, and the sample contained impurities visible in the 1H NMR spectrum (SI, Figure S66). Molecular structures of all trinuclear ferrocenes herein reported were elucidated by X-ray single crystal diffraction. Molecular structures are shown in Figures 1 and 3. Details of the X-ray crystal structure determinations are reported in the SI (Tables S1 and S7). The Cp rings of the octamethylferrocene core of 5 and 4c are essentially nontilted (α < 1°, Table 1, see Figure 4a for definition of α), whereas 6 shows a small tilt of ∼6° for both molecules in the asymmetric unit due to the shorter C3 handle. As expected, none of the terminal ferrocenes exhibit a noteworthy tilt. The central ferrocene of 6b is reported to have a tilt angle of 8.5°.53 The slightly smaller tilt regarding of 6 and the essentially non-tilted structures of 5 and 7 are in line with our previous observation that octamethylferrocenophanes are less tilted compared to non-methylated ferrocenophanes of similar structure,41 likely due to steric repulsion between the methyl groups on opposing rings. The diol 4c crystallizes with two different enantiomers in the asymmetric unit as a THF tetrasolvate. H-bonds, intra-

Scheme 3. Proposed Mechanism for Spontaneous Cyclization of 4a to a [4]Ferrocenophane in Solution

Scheme 4. Proposed Mechanism for AlCl3 Catalyzed Cyclization of 4a to the [3]Ferrocenophane 6

from a mixture containing 4a and 5. Treatment of 5 with water/THF and t-BuOH/water mixtures and exposure to silica D

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Representations of the molecular structures of ferrocenophanes 5 (a), 6 (b), and 7 (c) as determined by X-ray single crystal diffraction. For 6, only one of two molecules in the asymmetric unit is shown. H-atoms are omitted for clarity. Atomic displacement ellipsoids are shown at a probability level of 50%.

the 1H NMR spectra display an individual multiplet for each H-atom of the two substituted Cp rings, with evidence for appreciable coupling between all hydrogens of a given ring displayed in the COSY spectrum of 6. The methyl groups of the octamethylferrocene moieties and each ipso-C give rise to an individual resonance in the 13C NMR spectra, again consistent with the low overall symmetry of 4b and 6. The vinyl protons of 4a and 4b give rise to two doublets with geminal 1H−1H coupling constants of 2.5 Hz. The two vinylic protons of butadiene 5 are chemically equivalent and consequently appear as a singlet. Chemical shifts for the vinylic protons of 4a/b appear in the expected region, around 6 ppm, and are shifted further downfield to 6.77 and 7.06 ppm for 5 and 6, respectively. We investigated the redox behavior of the novel compounds by cyclic voltammetry (CV). CV data were acquired in two different supporting electrolytes: tetrabutylammonium hexafluorophosphate (THFP) and the weakly coordinating tetrabutylammonium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (TBArF) in DCM. Weakly coordinating anions have been used successfully in measurements of polynuclear ferrocenes to suppress ion-pairing processes making the observation of multiple redox processes efficacious.35,42,57 CVs are shown in Figure 5a (THFP) and 5b (TBArF), respectively. Determined oxidation potentials are listed in Table 2. CVs show a clean redox wave with good reversibility for all compounds for the first redox process (Fe(II) → Fe(III); E1/2(1)) for both supporting electrolytes. This oxidation process emanates from the octamethylferrocene cores. The electrondonating methyl groups lower the oxidation potential of the Fe center significantly compared to the non-methylated ferrocenes, thus the first oxidation occurs here. Density functional theory (DFT) calculations support this assignment as all HOMOs of the neutral species are centered here (Figure 6). The HOMO of 5 is further distributed along the butadiene handle toward the two terminal ferrocenes. The second and third oxidation processes, E1/2(2) and E1/2(3), are assigned to the metals at the terminal ferrocenes and appear at significantly higher potentials. This in turn is further supported by the DFT calculations, as the HOMOs of the cations of 3+, 4a+, and 5+ are centered at both outer ferrocenes (Figure 6) with distribution along the handle for 5+. E1/2(1) occurs for 3 at −151/−178 mV (THFP/TBArF), which is significantly higher compared to all other compounds (−336 to −524 mV) in the

Table 1. Tilt and Twist Angles of the Central Ferrocenes of Compounds 4c−7 compound 4ca 5 6a 6ba (ref 53) 7

tilt angle α (°)

twist angle β (°)

O−H distanceb (Å)

5.97(6) 4.79(6) 0.93(4) 6.22(12) 6.12(13) 8.5 8.4 0.57(14)

25.0 16.2 15.6 7.2 6.2 6.4 5.9 10.3

1.934(2) 1.974(2) − − − 1.876(4)

a

The asymmetric unit contains two molecules. bIntramolecular Hbond.

Figure 4. Tilt angles α (a) and twist angles β (b) of the octamethylferrocene core of the trinuclear ferrocenophanes.

molecular between both OH groups (1.974(2) and 1.934(2) Å), and intermolecular between OH and a THF molecule (2.020(3) and 2.000(3)Å), are present. Due to the intramolecular H-bonds in the solid state, the molecule resembles a [5]-ferrocenophane. The octamethylferrocene core is slightly tilted (α = 5.97°(6), 4.79°(6)); however, the tilt is not toward the handle as observed for 6, but away from it. This is presumably due to repulsion between the two terminal ferrocenyl groups. None of the central Cps are in ideally eclipsed positions with twist angles β varying from 5.9 to 25.0° (see Figure 4b for definition of β). All of the new compounds displayed NMR spectra consistent with their molecular structures. Compounds 3 (Figures S13−S16, SI), 4a (Figures S23 and S24, SI), and 5 (Figures S45 and S46, SI) provide relatively simple 1H- and 13 C NMR spectra due to the symmetry of the molecules. Cp protons of the substituted terminal Cp rings give rise to two pseudotriplets; unsubstituted Cp rings give rise to one singlet, and the methyl groups of the central octamethylferrocene are pairwise equivalent giving two singlets. In contrast, the NMR spectra of compounds 4b (Figures S25−S29, SI) and 6 (Figures S52−S55, SI) are complicated. Due to the asymmetry, E

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) CVs of trinuclear ferrocenes 3, 4a, 4c, 5, and 6 recorded in 0.1 M THFP in DCM. The analyte concentration was 3 mM (3) and 1 mM (4a, 4c, 5, and 6). (b) CVs of 3, 4a, 4c, 5, and 6 recorded in 0.06 M TBArF in DCM. The analyte concentration was 1 mM. All shown CVs were recorded at a scan rate of 100 mV/s.

Table 2. Summary of Oxidation Potentials As Determined by CV for Compounds 3−6 using THFP and TBArF Supporting Electrolytesa compound 3 4a 4c 5 6 1,1′-Fc(FcCCH2)2 (ref 58) 1,1′-Fc(CH2OMFc)2 (ref 58)

SE

E1/2(1)

THFP TBArF THFP TBArF THFP TBArF THFP TBArF THFP TBArF THFP THFP

−178 −151 −482 −524 −480 −500 −336 −343 − 415 − 506 −110 −470b

E1/2(2)

E1/2(3)

b

397 648c 56d 100c 66b 161 162b 293 119b 227 150

970c 266 987c 528c 617c 421 270 −70

ΔE1/2(1−2) 575 799 538 624 546 661 498 636 534 733 260 400

ΔE

(2−3) 1/2

− 322 210 887 − 367 − 324 − 194 120 −

a

All potentials are reported in mV and vs the Fc/Fc+ couple. SE: supporting electrolyte, OMFc: octamethylferrocene. bThe two processes are not resolved. cRedox waves not well resolved. Reported values are estimates. dIrreversible oxidation; reported value is peak potential of oxidation wave.

moiety and two outer octamethylferrocenes connected via methylene bridges was reported to show two processes, at −470 mV and −70 mV, respectively. The first oxidation was assigned to the simultaneous oxidation of both outer octamethylferrocenes and the second to the central ferrocene.58 The processes E 1/2 (2) and E1/2 (3) show good reversibility for 3, 4c, 5, and 6 using the HFP-based electrolyte, but the two processes ( E1/2(2,3)) are not well resolved. The CVs of 3 and 6 show only one wave for E1/2(2) and E1/2(3), while in the case of 4c and 5 we observed shoulders, which indicates two processes in close proximity. Using the TBArF supporting electrolyte resulted in decoupling of E1/2(2) and E1/2(3) for 3, 4c, and 5, giving a clean redox wave for E1/2(2) for 4c and 5, while the wave attributed to E1/2(3) is broadened. Regarding 3, both waves attributed to E1/2(2) and E1/2(3) are broadened. The reactive divinyl derivative 4a exhibits irreversible oxidations for E1/2(2)/E1/2(3). Repeated scans led to visible darkening of the solution and change of shape of the

series and attributed to the presence of the two electronwithdrawing carbonyl groups. Notably, the first oxidation of ferrocenophanes 5 and 6 E1/2(1) occurs at elevated potentials, +146/181 mV and +67/18 mV (THFP/TBArF), respectively, compared to the noncyclized 4a, while 5 is of nearly identical formula with two H-atoms less and 6 is an isomer of 4a. A nonmethylated counterpart of 4a was reported to exhibit three redox processes at −110 mV, 150 mV, and 270 mV, and the central ferrocene moeity was assigned to the first oxidation.58 The redox splitting between E1/2(1) and E1/2(2) for all compounds is high (528−799 mV), compared to a bisoctamethylferrocenophane (202 mV) we recently reported,41 and a binuclear 3,3′-di-tert-butyl-ferrocenophane bearing a partially fluorinated n-butylene handle with a ferrocene in the α-position (296 mV)44 and other polynuclear ferrocenes.59,60 This originates in the different chemical environment of the iron centers (Cp vs tetramethyl Cp ligands). In line with our observation, a trinuclear complex bearing a central ferrocene F

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. HOMOs of compounds 3, 3+, 4a, 4a+, 4c, 5, 5+, and 6 obtained from DFT calculations. The 6-31G* basis sets were used for lighter atoms (H, C, O), while the LANL2DZ basis set incorporating an effective core potential was used for iron. All surfaces depicted were constructed using an isosurface value of 0.02. All HOMOs in the neutral state are centered at the octamethylferrocene core, while the HOMO of 5 is distributed along the handle toward the outer ferrocenes. The HOMOs of cations 3+, 4a+, and 5+ are located at both outer ferrocenes with distribution along the handle for 5+. solvents were distilled under reduced pressure prior to use. Octamethylferrocene was prepared as reported previously.40 Other compounds were purchased from Sigma-Aldrich and used as received. NMR spectra were recorded on Bruker AVIII HD 500 and AVIII HD 600 spectrometers. Chemical shifts of residual solvent signals were used as internal references.62 Mass spectra were recorded on a Waters LCT Premier mass spectrometer in ESI+ or APCI+ modes. Elemental analyses were obtained from the Elemental Analysis Service of the London Metropolitan University. Infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer equipped with an ATR sampling accessory. Melting points were determined using a Reichert melting point microscope. Syntheses. FcCOOH. A Schlenk flask was charged with 2.0 g (10.8 mmol) ferrocene, 121 mg (1.1 mmol) t-BuOK, and 100 mL THF and cooled to −78 °C. t-BuLi (15.8 mL, 26.9 mmol; 1.7 M in pentane) was added dropwise over 30 min and stirring at −78 °C was continued for 1.5 h, affording an orange precipitate. An excess of dry ice (15 g) was added, and the mixture was allowed to warm up to rt. The mixture was treated with 150 mL aqueous HCl (5%), and the organic phase was collected concentrated to dryness. The solids were suspended in 30 mL hexanes, collected using a Büchner funnel, and washed with 30 mL hexanes. Drying under high vacuum afforded 2.244 g (9.8 mmol, 91%) of ferrocenecarboxylic acid (FcCOOH) as pale orange solid. Analytical data were in agreement with the literature.63 3. FcCOOH (848 mg, 3.7 mmol) was suspended in 60 mL dichloromethane in a Schlenk flask. Oxalyldichloride (0.94 mL, 3 equiv) and several drops of DMF were added under stirring. The solids dissolved within a few minutes, resulting in a deep red solution. The solution was stirred for 30 min, and the solvents were removed under vacuum to afford ferrocenecarboxylic acid chloride (2) as deep red solid, which was redissolved in 60 mL dichloromethane. Octamethylferrocene (500 mg, 1.7 mmol) was added, and the mixture was cooled to −50 °C. Aluminum(III) chloride (541 mg, 4.1 mmol) was added slowly over several portions. The temperature was allowed to raise to −30 °C, and stirring was continued for 2 h before

curve, indicating electrochemically induced decomposition and demonstrating the sensitivity of the compound toward oxidation. Scan rate-dependent measurements of 3 in TBArF showed significantly less well resolved redox-waves for E1/2(2)/ E1/2(3) that were at faster scan rates (Figure S1, SI), while repeated scans using the THFP electrolyte likely led to poisoning of the working electrode. On the contrary, repeated scan rate-dependent measurements indicate good reversibility61 (ipa/ipc ≈ 1) for compounds 4c, 5, and 6 using both electrolytes (Figure S2−S7, SI). To summarize, the use of the BArF supporting electrolyte allows decoupling from E1/2(2) and E1/2(3), but at the expense of broadening of E1/2(3), and for the case of 3 both E1/2(2) and E1/2(3).

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CONCLUSIONS In summary, we developed viable synthetic methods for preparation of several trinuclear ferrocenes with a central octamethylferrocene moiety. The divinyl derivative 4a can be obtained in quantitative yield by different pathways. It is a highly reactive precursor, readily cyclizing to two different types of ferrocenophanes according to the reaction conditions. Oxidative formation of a ferrocenophane with a buta-1,3-diene handle proceeds by a new cyclization mechanism. The ferrocenophanes show excellent redox and chemical stability, which makes them attractive building blocks for metalmolecule-metal junctions in molecular electronics applications.

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EXPERIMENTAL SECTION

General. Reactions were performed under Ar atmosphere using standard Schlenk techniques unless otherwise stated. THF was distilled from sodium/benzophenone. Dichloromethane was purified using an Innovative Technologies solvent purification system. Other G

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the mixture was allowed to warm up to room temperature. Water was added, and the phases were separated. The organic layer was dried over sodium sulfate and concentrated to dryness. The crude red solids were subject to column chromatography on silica using initially dichloromethane/ethyl acetate (99:1) and slowly increasing the amount of ethyl acetate to a ratio of 10:1. The deep red main fraction was concentrated to dryness and dried under high vacuum, affording 731 mg (0.10 mmol, 60%) of the product as deep red solid. 1H NMR (600 MHz, CD2Cl2): δ = 4.58 (m, 4H, Cp, H6), 4.37 (m, 4H, Cp, H7), 4.09 (s, 10H, Cp, H8), 1.83 (s, 12H, CH3, H1a), 1.80 (s, 12H, CH3, H2a) ppm. 13C NMR (150 MHz, CD2Cl2): δ = 202.8 (C4), 84.8 (C3), 83.5 (C5), 83.5 (C2), 81.9 (C1), 71.3 (C7), 71.2 (C6), 70.0 (C8), 11.4 (C1a), 9.5 (C2a) ppm. IR (ATR): 3102 (w), 3088 (w), 2972 (w), 2952 (w), 2897 (m), 1779 (w), 1739 (w), 1714 (w), 1651 (m), 1631 (s), 1439 (s), 1379 (s), 1387 (s), 1371 (m), 1335 (m), 1254 (s), 1207 (w), 1150 (w), 1108 (m), 1083 (m), 1050 (w), 1028 (s), 1007 (m), 956 (w), 935 (w), 887 (w), 877 (w), 850 (m), 839 (m), 819 (s), 755 (m), 726 (m), 659 (m), 615 (w), 596 (w), 561 (m), 542 (w), 533 (w). Elemental analysis: calcd for C40H42Fe3O2: C 66.51; H 5.86; found C 66.37; H 5.83. MS (ESI+): 723 (M+ + 1, 100%), 470 (20%), 242 (40%). HRMS: calcd for C40H43O2Fe3: 723.1311; found: 723.1318. Mp: 235−238 °C. 4a. A Schlenk tube was charged with 50 mg (0.07 mmol) of compound 3 and 4 mL of THF. The solution was cooled to 0 °C and 0.11 mL (0.17 mmol, 2.5 equiv) of methyllithium (1.6 M in diethyl ether) were added dropwise over two min. Stirring at 0 °C was continued for 0.5 h before 28 mg (0.21 mmol) of aluminum(III) chloride, dissolved in 2 mL THF, was added slowly over 1 h using a syringe pump. The mixture was allowed to warm up to room temperature, and stirring was continued for another hour. Degassed water (5 mL) was added, and the product was extracted with 10 mL dichloromethane, affording a deep orange extract. The solution was dried over sodium sulfate, filtered through a PTFE-syringe filter, pore size 0.45 μm, and concentrated to dryness under high vacuum, affording 50 mg (0.07 mmol, ∼98%) of an orange crystalline solid with a purity of >95%. The only impurities present were found to be trace amounts of ferrocenophanes 5 and 6. Attempted recrystallization from dichloromethane and THF at −10 °C resulted both in conversion to ferrocenophane 5 and crystallization of it. Sublimation under vacuum (160 °C, 0.05 * 10−3 mbar) resulted in sublimation of both compound 4a and the minor amounts of 5/6. Attempted purification column chromatography on silica did not result in separation of the compounds, but significant oxidative decomposition, manifested in a color change to deep green, occurred. Alternatively, we obtained 4a by heating 4b or 4c for 5 min to 200 °C. The reported analytical data were obtained from a sample prepared in solution as described above. Samples prepared by heating 4b and 4c were characterized by 1H NMR spectroscopy, which matched the data reported below. 1H NMR (C6D6): δ = 6.05 (d, J = 2.5 Hz, 2H, C CH2), 6.00 (d, J = 2.5 Hz, 2H, CCH2), 4.12 (pst, 4H, Cp), 3.98 (s, 10H, Cp), 3.95 (pst, 4H, Cp), 1.81 (s, 12H, CH3), 1.75 (s, 12H, CH3) ppm. 13C NMR (C6D6): δ = 141.6 (CCH2), 116.3 (C CH2), 89.9, 87.8, 79.1, 70.0, 68.5, 68.1, 11.3 (CH3), 9.8 (CH3) ppm. IR (ATR): 3094 (w), 3080(w), 2921(s), 2853(m), 2717(w), 1757(w), 1660(w), 1621(m), 1588(w), 1572(w), 1462(m), 1446(m), 1429(m), 1411(m), 1377(m), 1361(m), 1336(w), 1317(w), 1262(m), 1212(w), 1192(m), 1183(w), 1153(w), 1105(s), 1023(s), 1000(s), 914(m), 886(m), 849(m), 808(s), 770(m), 739(m), 719(w), 696(m), 674(w), 645(w), 620(w), 613(w), 586(w), 567(w), 552(m), 543(m), 531(w), 518(w). Elemental analysis: calcd for C42H46Fe3: C 70.22, H 6.45; found: C 70.11, H 6.45. MS (ESI+): 718 (M+ + H, 100%). HRMS: calcd for C42H46Fe3: 718.1648; found: 718.1649. Mp: 225−233 °C. 4b. A Schlenk tube was charged with 100 mg (0.14 mmol) of compound 3 and 8 mL of THF. The solution was cooled to 0 °C, and 0.22 mL (0.34 mmol, 2.5 equiv) of methyllithium (1.6 M in diethyl ether) was added dropwise over 2 min. Stirring at 0 °C was continued for 0.5 h before 28 mg (0.21 mmol) of aluminum(III) chloride, dissolved in 2 mL THF, was added over 1 h using a syringe pump. Workup analogously to preparation of 4a, followed by flash

chromatography on silica using hexane/EtOAc (9:1), afforded 42 mg (0.06 mg, 41%) of the title compound as yellow-orange crystalline solid after solvent evaporation and drying under high vacuum. 1H NMR (600 MHz, C6D6): δ = 6.18 (d, J = 2.5 Hz, 2H, CCH2), 6.14 (d, J = 2.5 Hz, 2H, CCH2), 4.45 (m, 1H, Cp), 4.22 (m, 1H, Cp), 4.08 (s, 5H Cp; one Cp-H concealed), 3.97 (s, 5H Cp), 3.96 (m, 1H, Cp), 3.95 (m, 1H, Cp), 3.90 (m, 1H, Cp), 3.84 (m, 1H, Cp), 3.82 (m, 1H, Cp), 2.68 (s, 1H, OH), 2.12 (s, 3H, CH3, H7), 2.11 (s, 3H, CH3), 1.87 (s, 3H, CH3), 1.82 (s, 3H, CH3), 1.81 (brd, 6H, CH3), 1.75 (s, 3H, CH3), 1.55 (s, 3H, CH3), 1.54 (s, 3H, CH3) ppm. 13C NMR (150 MHz, CDCl3): 143.1 (C20), 115.9 (C19), 102.3 (C8), 92.9 (C5), 88.8 (C18), 88.0 (C21), 80.7, 80.5, 79.7, 79.6, 79.5, 79.0, 78.1, 76.5, 72.8 (C6), 70.1, 68.84, 68.82, 68.77, 68.1, 68.0, 67.4, 67.23, 67.20, 66.4, 32.6 (C7), 13.2, 12.7, 12.3, 11.7, 10.3, 10.1, 9.9, 9.8 ppm. IR (ATR): 3577 (w), 3092(w), 2965(m), 2939(w), 2902(m), 1759(w), 1738(w), 1687(w) 1614(w), 1472(w), 1455(m), 1426(m), 1411(m), 1374(m), 1332(w), 1290(w), 1262(m), 1209(w), 1182(w), 1160(w), 1104(s), 1080(m), 1060(m), 1023(s), 1000(s), 925(w), 913(w), 891(m), 880(m), 862(w), 811(s), 731(m), 697(m), 674(w), 640(w), 613(w), 584(w), 572(w), 555(w), 541(w, 532(w), 522(w). MS (ESI): 736 (M+, 100%). Elemental analysis: calcd for C42H48Fe3O: C 68.51, H 6.57; found: 68.43, H 6.71. HRMS: calcd for C42H49Fe3O: 737.1831; found: 737.1832. Mp: 183−185 °C, melting with conversion to 4a and subsequent crystallization. 4c. Compound 3 (25 mg, 0.035 mmol) was dissolved in 2 mL THF in a Schlenk tube. Methyllithium (0.05 mL, 0.087 mmol; 2.5 equiv) was added dropwise at 0 °C. After 20 min, the mixture was allowed to warm up to room temperature, and 5 mL of water was added. The mixture was extracted with 10 mL ethyl acetate, and the organic layer was collected and dried over sodium sulfate. The yellow solution was filtered through a PTFE syringe filter (pore size: 0.45 μm), and the solvents were removed using a rotary evaporator, affording a yelloworange oil which immediately crystallized. Drying under high vacuum afforded 24 mg (0.032 mmol, 92%) of the title compound as yelloworange crystalline solid. 1H NMR (600 MHz, C6D6): δ = 4.73 (m, 2H, Cp), 4.35 (m, 2H, OH), 4.10 (s, 10H, H13), 3.90 (m, 2H, Cp), 3.84 (m, 2H, Cp), 3.80 (m, 2H, Cp), 2.25 (s, 6H, H7), 2.20 (s, 6H, CH3), 1.87 (s, 6H, CH3), 1.47 (s, 12H, CH3) ppm. 13C NMR (150 MHz, CDCl3): δ = 101.7 (C8), 92.2 (C5), 80.9 (ipso-C), 80.6 (ipso-C), 78.4 (ipso-C), 75.9 (ipso-C), 72.9 (C6), 68.9 (C13), 67.9, 67.2, 66.8, 65.6, 32.8 (C7), 13.8, 12.7, 9.7, 9.5 ppm. IR (ATR): 3617(w), 3566(w), 3420(w), 3085(w), 2998(w), 2956(m), 2906(m), 2187(w), 2168(w), 2050(w), 1981(w), 1769(w), 1716(w), 1634(w), 1479(m), 1464(m), 1409(m), 1377(s), 1360(m), 1341(m), 1310(m), 1262(w), 1236(w), 1225(w), 1161(w), 1105(s), 1085(m), 1052(m), 1023(s), 998(s), 950(w), 926(m), 890(m), 859(w), 817(s), 732(w), 708(w), 697(w), 666(w), 623(w), 615(w), 597(w), 589(w), 579(w), 568(w), 551(w), 539(w), 532(w), 524(w), 518(w). Elemental analysis: calcd for C42H50Fe3O2·1 EtOAc: C 65.58, H 6.94; found: 65.34, H 6.66. MS (ESI): 754 (M++H, 100%), 737 (83%), 719 (26%). HRMS: calcd for C42H50Fe3O2: 754.1859; found: 754.1860. Mp: 160−162 °C. 5. A round-bottom flask was charged with 50 mg (0.07 mmol) of 4 and 10 mL of degassed chloroform. The solution was stirred for 16 h at room temperature. Solvent evaporation and drying under high vacuum gave a red solid which was flash filtered through a plug of silica using dichloromethane as eluent. Solvent evaporation and drying under high vacuum afforded 37 mg (0.05 mmol, 74%) of the title compound as red crystalline material. 1H NMR (600 MHz, C6D6): δ = 6.77 (s, 2H, H5), 4.22 (pst, 6H, H8), 4.04 (s, 10H, H9), 4.01 (pst, 6H, H7), 1.87 (s, 12H, CH3), 1.69 (s, 12H, CH3) ppm. 13C NMR (150 MHz, C6D6): δ = 138.7 (C4), 122.8 (C5), 91.1 (C6), 81.5 (C3), 81.3 (C1), 77.7 (C2), 70.0 (C9), 68.7 (C7), 67.7 (C8), 11.1 (CH3), 9.7 (CH3) ppm. IR (6 × THF, ATR): 3088 (w), 2967(m), 2938(m), 2904(m), 2885(m), 1721(w), 1645(w), 1609(w), 1565(w), 1472(w), 1455(m), 1436(m), 1413(m), 1385(w), 1375(m), 1315(w), 1289(w), 1252(2, 1243(w), 1207(w), 1174(w), 1158(w), 1107(m), 1067(s), 1029(s), 1002(s), 966(w), 907(m), 870(m), 852(m), 812(s), 772(w), 730(w), 702(w), 684(w), 666(w), 658(w), 639(w), 614(w), 598(w), 589(w), 574(w), 561(w), 547(w), 529(w), 518(w). Elemental analysis: calcd for C42H44Fe3·2/3 DCM: H

DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX

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C 66.30, H 5.91; 66.24, H 5.58. MS (APCI+): 717 (M+ + H, 17%), 676 (5%), 426 (43%), 379 (9%), 338 (100%). HRMS: calcd for C42H45Fe3: 717.1569 found: 717.1570. Mp: 272−290 °C. 6. A Schlenk tube was charged with 20 mg (0.03 mmol) of 4a and 2 mL THF. Aluminum(III) chloride 7 mg (0.06 mmol), dissolved in 1 mL THF, was added under stirring at 0 °C. After 0.5 h the mixture was allowed to warm up to room temperature, and stirring was continued for another hour. Water was added, mixture was extracted with dichloromethane, and the organic phase was dried over sodium sulfate. Solvent evaporation afforded a yellow-orange solid. The solvent was evaporated to dryness, and the solid was washed with several small portions of pentane. Drying under high vacuum afforded 11 mg (0.02 mmol, 55%) of the title compound as pale orange solid. 1 H NMR (600 MHz, C6D6): δ = 7.06 (s, 1H, H7), 4.55 (m, 1H, Cp), 4.50 (m, 1H, Cp), 4.34 (m, 1H, Cp), 4.222 (s, 5H, Cp), 4.220 (s, 5H, Cp), 4.11 (m, 1H, Cp), 4.06 (m, 1H, Cp), 4.03 (m, 1H, Cp), 4.01(m, 1H, Cp), 3.96 (m, 1H, Cp), 2.15 (s, 3H, H9), 1.94 (s, 3H, CH3), 1.81 (s, 3H, CH3), 1.67 (s, 3H, CH3), 1.66 (s, 3H, CH3), 1.61 (s, 3H, CH3), 1.59 (s, 3H, CH3), 1.47 (s, 3H, CH3), 1.21 (s, 3H, CH3) ppm. 13 C NMR (150 MHz, C6D6): δ = 144.9 (C7), 129.7 (C6), 103.6 (C15), 89.3 (C21), 86.6 (C14), 84.0 (C5), 82.1, 82.1, 81.8, 81.4, 80.4, 79.8, 79.8, 70.2, 69.7, 69.2, 68.9, 68.2, 68.2, 68.01, 68.00, 67.7, 65.1, 40.1 (C8), 33.2 (C9), 14.5, 13.1, 11.17, 11.16, 10.3, 10.1, 10.0, 9.9 ppm. MS (ESI+): 718 (M+ + H, 100%). HRMS: calcd for C42H46Fe3: 718.1648; found: 718.1649. Elemental analysis: calcd for C42H46Fe3: C 70.22, H 6.45; found: 70.15, H 6.34. IR (ATR): 3359 (w), 3087(w) 2953(m), 2904(m), 2855(m), 2725(w), 1714(w), 1653(w), 1633(w), 1455(m), 1444(m), 1427(m), 1411(m), 1378(s), 1308(w), 1261(w), 1250(w), 1223(w), 1185(w), 1158(w), 1106(s), 1085(m), 1049(m), 1027(s), 1018(s), 1000(s), 958(w), 927(w), 907(w), 876(m), 863(w), 813(s), 758(w), 737(w), 706(w), 696(w), 682(w), 670(w), 621(w), 601(w), 589(w), 582(w), 564(w), 537(w), 521(w). Mp: decomposition ∼290 °C. 7. Preparation of compound 5 as described above and attempted column chromatography resulted in collection of a mixture of 5 and 7 from the deep red main fraction. Crystals of both compounds were obtained by slow evaporation of a mixture of pentane/dichloromethane at room temperature. Diol 7 crystallized in form of yellow platelets, while crystals of 5 are red. Yellow platelets were collected under the microscope and used for X-ray structure analysis and characterization. 1H NMR (C6D6): δ = 4.37 (m, 2H, Cp), 4.22 (m, 2H, Cp), 4.05 (s, 10H, Cp), 3.95 (m, 2H, Cp), 3.86 (m, 2H, Cp), 3.44 (s, 2H, OH), 3.12 (m, 2H, CH2), 2.70 (m, 2H, CH2), 2.49 (s, 6H, CH3), 1.57 (s, 6H, CH3), 1.40 (s, 6H, CH3), 1.31 (s, 6H, CH3) ppm. MS (ESI): 752 (M+ + H, 100%). HRMS: calcd for C42H49Fe3O2: 753.1781; found: 753.1805. Mp: decomposition ∼275 °C. Electrochemistry. Cyclic voltammetry measurements were performed on a Princeton Applied Research VersaSTAT 3 potentiostat using a three-electrode setup with Pt working electrode and platinum-coated titanium rods as counter and pseudoreference electrodes. We used tetrabutylammonium hexafluorophosphate (THFP) and tetrabutylammonium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (TBArF) as supporting electrolytes in concentration of 0.1 and 0.06 M in dichloromethane, respectively. We used either ferrocene or decamethylferrocene (DMFc) as internal standards and converted all potentials to the ferrocene/ferrocenium couple (E1/2(DMFc) = E1/2(Fc) − 480 mV).57 DFT Calculations. DFT calculations were undertaken using the familiar B3LYP functional, with the incorporation of the GD3 empirical dispersion correction of Grimme and co-workers.64 The 631G* basis sets were used for lighter atoms (hydrogen, carbon, oxygen), while the LANL2DZ basis set incorporating an effective core potential was used for iron to reduce the computational burden. All calculations were undertaken using the Gaussian 09 program suite.65

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03389. Crystallographic information files in cif-format for compounds 3−7 (CCDC 1837183−1837188, 1848108, and 1848109), details from the X-ray structure determinations, NMR numbering schemes and spectral data. Molecular graphics: OLEX2 (ref 66) (PDF) Accession Codes

CCDC 1837183−1837188 and 1848108−1848109 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Matthew J. Piggott: 0000-0002-5857-7051 George A. Koutsantonis: 0000-0001-8755-3596 Present Address §

Department of Molecular Sciences, Macquarie University, Sydney, New South Wales 2109, Australia Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under Australian Research Council’s Discovery Projects funding scheme (project number DP 150104117). We acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Center for Microscopy, Characterization and Analysis, The University of Western Australia, a facility funded by the University, State, and Commonwealth Governments.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03389 Inorg. Chem. XXXX, XXX, XXX−XXX