Diferrocenes Bridged by a Geminal Diethynylethene Scaffold with

Article Cite This: Organometallics XXXX, XXX, XXX-XXX

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Diferrocenes Bridged by a Geminal Diethynylethene Scaffold with Varying Pendant Substituents: Electronic Interactions in CrossConjugated System Yang Fan,*,† Hua-Min Li,† Guo-Dong Zou,† Xu Zhang,‡ Ying-Le Pan,† Ke-Ke Cao,† Meng-Li Zhang,† Pei-Lin Ma,† and Hai-Ting Lu† †

College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, People’s Republic of China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A series of geminal diethynylethene (gem-DEE) bridged diferrocenyl compounds with varying pendant substituents at the side chain, including xanthene, thioxanthene, dibenzo[a,d]cycloheptene, and fluorene, is reported with the aim of exploring electronic interactions in the cross-conjugated system. The compounds 9-[bis(ferrocenylethynyl)methylene]-9Hxanthene (1), 9-[bis(ferrocenylethynyl)methylene]-9H-thioxanthene (2), 5-[bis(ferrocenylethynyl)methylene]-5H-dibenzo[a,d]cycloheptene (3), and 9-[bis(ferrocenylethynyl)methylene]-9H-fluorene (4) were synthesized and characterized by using spectroscopic methods and single-crystal X-ray diffraction analyses. Electrochemical measurements using the weakly coordinating electrolyte Na[B{C6H3(3,5-CF3)2}4] reveal the moderate electronic coupling between the ferrocenyl (Fc) termini. Hush analysis of the intervalence charge-transfer (IVCT) bands for the monocation species is suggestive of the weakly coupled Robin−Day class II mixed-valence compounds. DFT and TDDFT calculations suggest that the low-energy transitions of these compounds mainly involve a metal to ligand charge transfer (MLCT) transition from the iron center of ferrocenyl termini to the pendant substituent.

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INTRODUCTION

could provide fundamental information on the electron transfer properties of the bridging ligand, as well as the intramolecular electronic interaction between the redox centers.4 In particular, studies on mixed-valence diferrocenyl-containing compounds gave an in-depth understanding of the long-range metal−metal electronic coupling.5 However, in comparison to molecular wire type linearly conjugated binuclear complexes, electron transfer over cross-conjugated bridging ligands has been rarely explored.6 The first two diferrocenyl-capped geminal diethynylethene (gem-DEE) compounds, namely, 1,5-bis(ferrocenyl)penta-1,4-diyn-3-one and 3-(dibromomethylidene)-1,5-bis(ferrocenyl)penta-1,4-diyne, were reported in pioneering works by Ren and co-workers.7a Weak electronic coupling between the two ferrocenyl centers was observed for gem-DEE bridged diferrocenyl compounds.7a Their computational studies on [M]−gem-DEE−[M] type binuclear complexes further

Due to their different conjugation pathway relative to that of the common linear π conjugation, cross-conjugated molecules exhibit unique physicochemical behaviors that have recently attracted growing interest.1,2 According to the definition for the term cross-conjugation, a cross-conjugated molecule consists of three unsaturated groups in which the two unsaturated groups are both conjugated to a third unsaturated center but are not conjugated to each other.3 In pioneering works, crossconjugated systems based on central building blocks including 1,3-butadiene, dendralenes, iso-polydiacetylenes, etc. have been synthesized which offer solid foundations for potential applications as optoelectronic functional materials.1 However, in comparison to the extensively investigated linearly conjugated materials, the intriguing properties of crossconjugated materials such as quantum interference have only begun to be explored in recent decades.2 Binuclear redox-active complexes with π-conjugated carbonrich bridges are an important class of model molecules that © XXXX American Chemical Society

Received: September 8, 2017

A

DOI: 10.1021/acs.organomet.7b00686 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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revealed that both the in-plane and out-of-plane π orbitals of the gem-DEE ligand are capable of mediating electronic coupling between the two metal centers.7b In addition, previous theoretical studies by Lüthi and co-workers also demonstrated vertical π conjugation and in-plane σ hyperconjugation in crossconjugated diethynylethenes.8 Diruthenium compounds bearing a cross-conjugated σ-gem-DEE ligand also exhibit moderate electronic coupling between the two Ru2(II,III) termini. A comparison with a buta-1,3-diyne-bridged diruthenium compound indicates attenuated electronic coupling over the crossconjugated gem-DEE bridge.7c Recently, the electronic coupling between two Fc termini across the cross-conjugated perphenylated iso-polydiacetylene bridge was also reported by Low and Hartl and co-workers, in which the weakly coordinating electrolyte n-Bu4N[B{C6H3(3,5-CF3)2}4] shows an important role in resolving the closely overlapped ferrocene-based oneelectron oxidation processes.6c In comparison to the single π-conjugated channel for a linearly conjugated bridging ligand, a cross-conjugated bridge offers an additional channel by the side chain to modulate electronic communication between the redox-active termini. In the recently proposed single-molecule quantum-interferencebased transistor, the numerical simulations indicate that the charge transfer efficiency from the donor (source) to the acceptor (drain) can be controlled by the gate group on the side chain.9 Systematic studies on two-dimensional (2D) crossconjugated cruciform fluorophores were reported by Bunz and co-workers. The results demonstrated that the frontier orbital distribution and the resulting optical properties can be finetuned by varying substitution on the cruciform framework.10 In this work, a series of diferrocenyl compounds with two Fc termini bridged by a cross-conjugated gem-DEE scaffold were synthesized (Figure 1). Pendant substituents including

RESULTS AND DISCUSSION In this synthesis, 1,1-dibromoalkenes were used as the essential building blocks; these were obtained by Corey−Fuchs dibromoolefination of the different starting ketones.11 Pdcatalyzed Sonogashira coupling of ethynylferrocene with the respective 1,1-dibromoalkenes, including 9-(dibromomethylene)-9H-xanthene, 9-(dibromomethylene)-9H-thioxanthene, 5-(dibromomethylene)-5H-dibenzo[a,d]cycloheptene, and 9(dibromomethylene)-9H-fluorene, readily gave the gem-DEEbridged diferrocenyl compounds 1−4 (Scheme 1). Moreover, Scheme 1. Synthetic Route for Compounds 1−4a

a For the products after step ii, X = O (1), S (2), CHCH (3), none (4). Reagents and conditions: (i) CBr4 (2.0 equiv), PPh3 (4.0 equiv), heptane, reflux 48 h; (ii) FcCCH (3.0 equiv), Pd(PPh3)2 (0.13 equiv), CuI (0.33 equiv), Et3N, 95 °C, 24 h.

pendant substituents were introduced into the cross-conjugated frameworks by the 1,1-dibromoalkene building blocks. These compounds were fully characterized by NMR spectroscopy (Figures S1−S5 in the Supporting Information), mass spectra, and elemental analysis. They are all stable under ambient conditions in solution or in the solid state. It should be noted that compound 4 was previously reported in the literature, which was introduced as a reference compound in this work.6f Molecular structures of the compounds were further determined by single-crystal X-ray diffraction analysis. Suitable crystals of compounds 1−4 were obtained by diffusion of nhexane into a CH2Cl2 solution containing the respective compounds at ambient temperature. The title compounds crystallize in the monoclinic space group P21/n (1−3) and C2/ c (4), with one (1−3) or a half (4) crystallographically independent molecule in the asymmetric unit (Table S1 in the Supporting Information). As shown in Figure 2, all of these compounds possess a central Y-shaped gem-DEE sacffold connecting the two terminal Fc units and the pendant substituent. The gem-DEE scaffold in these compounds exhibits typical bond lengths and bond angles for the cross-conjugated structure, with CC bond lengths of 1.185(3)−1.200(3) Å, CC bond lengths of 1.352(3)−1.369(5) Å, and bond angles of about 111−125°.6d The Fe...Fe distances in the crystal structures of these diferrocenyl compounds are about 6.99 Å (1), 6.99 Å (2), 7.18 (3), and 6.74 Å (4). For compound 4, due to the rigid structure of fluorene, the pendant substituent is almost coplanar with the gem-DEE plane. In contrast, for compounds 1 and 2, this coplanarity was not well retained due to the flexibility of the xanthene and thioxanthene ligands. In particular, for compound 3, the pendant dibenzo[a,d]cycloheptene substituent exhibits a severely distorted structure that is almost perpendicular to the gem-DEE plane. The redox properties of these compounds were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. Na[B{C6H3(3,5-CF3)2}4] was used as the supporting electrolyte to minimize the ion-pairing effect and improve resolution between coupled oxidation processes.12 As shown in Figure 3, the CV curves of these compounds exhibit a pair of closely spaced but clearly resolved one-electronoxidation processes, which can be assigned to the stepwise

Figure 1. gem-DEE-bridged diferrocenyl compounds 1−4.

xanthene, thioxanthene, dibenzo[a,d]cycloheptene, and fluorene were introduced into the side chain of the crossconjugated scaffold. An electrochemistry and IVCT band analysis indicates moderate electronic communication between the Fc termini across the gem-DEE bridge. UV−vis spectroscopy and DFT/TDDFT calculations suggest electronic interactions between the Fc termini and the pendant substituent over the cross-conjugated scaffold. The results in this work provide valuable examples for understanding electronic interactions in cross-conjugated systems. B


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Organometallics

Figure 2. X-ray crystal structures of compounds 1−4. The ellipsoids are drawn at the 30% probability level. H atoms are omitted for clarity. Symmetry code A (compound 4): −x + 1, y, −z + 1/2.

redox splitting. In contrast, as demonstrated by previous research, a weakly coordinating electrolyte such as n-Bu4NB(C6F5)4, n-Bu4N[B{C6H3(3,5-CF3)2}4], or Na[B{C6H3(3,5CF3)2}4] could provide more reliable ΔE values due to the greatly reduced ion-pairing effect of the weakly coordinating borate electrolyte anion.12,14,15 Moreover, the small Na+ countercation could further induce competitive ion pairing with the electrolyte anion that would benefit the resolution of coupled redox processes.12 The different electrochemical behaviors of the diferrocenyl compounds recorded with the common electrolyte and the weakly coordinating electrolyte suggest that redox splitting is mainly contributed by the electrostatic interaction between the two Fc termini, as demonstrated in previous reports by Lang and co-workers.14 Therefore, although through-bond electronic coupling between the Fc termini across the gem-DEE bridge cannot be ruled out, it is more likely that a through-space interaction is operative, considering the short Fe...Fe distance of ∼7 Å. It is noteworthy that compound 4 exhibits the highest first oxidation potential at 90 mV, while the other compounds are easier to oxidize at lower oxidation potentials of 33 (1), 31 (2), and 13 mV (3), respectively (Figure 4). This result suggests that the electronic effects of pendant substituents have considerable influence on terminal Fc units. The lower oxidation potentials of the ferrocene center in compounds 1−3 can be attributed to the electron-donating ability of the ether oxygen atom, ether sulfur atom, and internal alkenyl group in the respective pendant substituents. This observation

oxidations of two Fc centers. In agreement with the CV results, the DPV curves show a pair of overlapping but separated peaks with nearly the same intensity. The ferrocene-centered stepwise one-electron-oxidation process is suggestive of the electronic coupling between the Fc termini across the gem-DEE bridge. The electrochemical data for these compounds are given in Table 1. As estimated from the DPV measurement, the potential differences (ΔEs) between the ferrocene-centered first and second oxidation processes are about 160, 140, 148, and 156 mV for compounds 1−4, respectively. The thermodynamic stability of the mixed-valence monocation 1+ can be estimated from the comproportionation constant (Kc) for the process 1 + 12+ ⇆ 2(1+), according to the equation Kc = exp(FΔE1/2/RT).13 The Kc values indicate the modest stability of these mixed-valence species (Table 1). Figure S6 in the Supporting Information shows the CV and DPV curves for these compounds recorded with n-Bu4NPF6 as the supporting electrolyte under the same conditions. All of these compounds display a single reversible redox process without any potential splitting, indicating simultaneous oxidation of the two Fc termini. This observation is in agreement with the previously reported electrochemistry of compound 4 recorded with Et4NClO4 as the supporting electrolyte.6f As is known, due to the strong ion-pairing interaction with the small [PF6]¯ electrolyte anion, the electrostatic interaction between the ferrocenium units would be greatly shielded.14 This results in poor resolution between coupled redox processes and thus a lower ΔE value for the C


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Organometallics

Figure 3. CV (top) and DPV (bottom) of compounds 1 (a), 2 (b), 3 (c), and 4 (d) recorded in CH2Cl2 solution containing 0.01 M Na[B{C6H3(3,5-CF3)2}4] at room temperature. Scan rate: 50 mV s−1.

Table 1. Electrochemical Data from DPV Measurements (versus Ag/AgCl) and Comproportionation Constants for the Monocation compd

Ox1, mV

Ox2, mV

ΔE, mV

Kc

1 2 3 4

33 31 13 90

193 171 161 246

160 140 148 156

508 233 318 435

is in agreement with the cross-conjugation character that oxidation potential of the Fc termini could be affected through π conjugation with the pendant substituents. However, on the other hand, these compounds exhibit relatively similar potential splittings (ΔEs) between the oxidative pair as discussed above. These results imply that the pendant substituents on the gemDEE scaffold have little influence on the strength of electronic coupling between Fc termini. This is in accordance with the proposed through-space electronic coupling due to the reduced direct electron delocalization across the gem-DEE bridge. Figure 5 shows UV−vis spectra of these compounds in CH2Cl2 solution. The low-energy absorption band in the region of 450−600 nm is characteristic of ferrocene-containing compounds involving d−d transitions.16 It is noteworthy that the low-energy absorption band of 4 (λmax 502 nm, ε = 6122 M−1 cm−1) was significantly enhanced and red-shifted in

Figure 4. DPV curves of compounds 1−4.

comparison with those of 1 (λmax = 479 nm, ε = 5373 M−1 cm−1), 2 (λmax = 472 nm, ε = 4588 M−1 cm−1), and 3 (λmax = 450 nm, ε = 2540 M−1 cm−1). This result suggests that the lowenergy absorption of 1 involves appreciable charge-transfer character. As revealed by the crystal structure of 4, the pendant fluorene substituent keeps a good planarity with the gem-DEE scaffold that allows better π-electron delocalization in the crossD


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Organometallics

possible ligand to metal charge-transfer (LMCT; ligand to FeIII) band (Figure 7 and Figure S8 in the Supporting

Figure 5. UV−vis spectra of compounds 1−4 in CH2Cl2 solution.

conjugated framework. This would further enhance the chargetransfer interaction between the Fc termini and the pendant ligands. Moreover, the mixing of charge-transfer character would relax the Laporte-forbidden ligand-field d−d transition and increase the intensity of the low-energy band.17 In comparison, due to the distorted planarity of the pendant substituent in 1 and 2, and particularly for 3, the conjugation and the charge transfer interaction between the Fc termini and pendant substituent should be greatly reduced, resulting in weak low-energy transitions. UV−vis−NIR spectra for the chemical oxidation of these diferrocenyl compounds by NOBF4 were investigated (Figure 6

Figure 7. Gaussian deconvolution of NIR absorptions for the monocation 1+.

Information).18a According to the Hush model, the electronic coupling metric element (Hab) can be estimated from the spectral parameters of the IVCT band by using the equation Hab = (2.05 × 10−2)[(νmaxεmaxΔν1/2)1/2/rab].19 In this equation, νmax is the frequency in wavenumbers of IVCT band maxima, Δν1/2 is the band half-width in wavenumbers, and εmax is the molar absorptivity at the maximum of the IVCT band. rab (Å) is the distance between two iron centers of the Fc termini, which was derived from the crystal data. The corresponding NIR data summarized in Table 2 suggest that the monocations of these compounds belong in the weakly coupled class II mixed-valence system according to the Robin−Day classification.20 DFT and TDDFT calculations at the B3LYP/6-31G** level were performed to unravel the electronic structure and electronic transition nature of these compounds. As shown in Figure 8, for compounds 1−4, the frontier molecular orbitals on the central gem-DEE skeleton exhibit a similar feature with π orbitals localized on two acetylene units and one ethene uint. The HOMO on the gem-DEE skeleton is the out-of-phase combination of three π orbitals, and the LUMO is the in-phase combination of three π* orbitals. This result agrees well with a computational study on the free gem-DEE ligand by Ren and co-workers, indicating π-electron delocalization through the cross-conjugated bridge.7b The HOMO on the Fc units has a dominant Fe 3d orbital contribution, along with some coefficients of π orbitals on cyclopentadienyl rings. The densities of the HOMO orbital also spread over the pendant substituent, indicating their conjugation with the gem-DEE center. The structure of the LUMO shows, in general, a decrease in the electron density on Fc units and a concomitant increase in the pendant ligands, which is suggestive of a chargetransfer interaction from Fc units to the pendant ligands. DFT calculations (B3LYP/6-31G** level, in the gas phase) on the monocation 1+ were further carried out to gain insights into the mixed-valence states of the cross-conjugated system. As shown in Figure 9, the spin densities of 1+ are mainly localized on one ferrocenyl unit with some contribution from the gem-DEE skeleton. This result confirms the ferrocene-centered oxidation in the monocation 1+ and is in agreement with the localized

Figure 6. UV−vis−NIR spectra recorded during stepwise chemical oxidation of compound 1 by NOBF4 in CH2Cl2.

and Figure S7 in the Supporting Information).18 For compound 1, under stepwise chemical oxidation, the initial bands at 392 nm gradually decreased. Meanwhile, a very broad NIR band at about 1000 nm appeared and gradually increased. Since the initial spectra of 1 exhibit no absorption above 650 nm, the NIR absorption band indicates the generation of an intervalence charge transfer (IVCT) transition of the mixedvalence monocation 1+.18 The other compounds also exhibit characteristic IVCT bands at about 1000 nm during the stepwise chemical oxidation by NOBF4. Gaussian deconvolution of the NIR spectra yield the IVCT band accompanied by a E


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Organometallics Table 2. NIR Data for Mixed-Valence States of Compounds 1−4 Generated under Chemical Oxidation compd +

1 2+ 3+ 4+

νmax (cm−1)

ε (L mol−1 cm−1)

Δν1/2 (cm−1)

rab (Å)

Δν1/2(theor) (cm−1)

Hab (cm−1)

10020 10400 11300 11200

723 635 450 669

3900 3400 4800 3300

6.987 6.987 7.179 6.739

4811 4901 5109 5086

495 442 448 481

Figure 8. HOMOs and LUMOs for compounds 1−4.

calculated lowest energy transition at 542 nm consists of several overlapping transitions mainly involving the HOMO-3 → LUMO+6 and HOMO-4 → LUMO+3 transitions (Table S2 in the Supporting Information). These transitions can be assigned as the Fe(II) → xanthene metal to ligand charge-transfer (MLCT) transition along with the ferrocene-centered d−d transitions (Figure S10 in the Supporting Information). The predicted low-energy transition at 475 nm originating from the HOMO-3 → LUMO+6 transition also has dominant MLCT character. The TDDFT calculated most intense transition at 428 nm has predominant contribution from the HOMO → LUMO transition. In agreement with the absorption band at 390 nm, the predicted transition at this wavelength corresponds to the HOMO-4 → LUMO transition. The calculated transitions at 428 and 390 nm mainly involve the intraligand (IL) π−π* transition of the gem-DEE and xanthene ligands, with some mixing of MLCT character. The low-energy transitions of the other compounds, as revealed by TDDFT calculations (Figure S9 in the Supporting Information), have similar features that involve the MLCT transition, intraligand π−π* transition, and d−d transition (Figures S11−S13 in the Supporting Information). For 4, TDDFT calculations indicate that the low-energy absorption has a predominant contribution from the HOMO → LUMO transition with high oscillator strength. This is in agreement with the experimental results on its much enhanced low-energy absorption band.

Figure 9. Spin-density isosurface plot (left) and SOMO (right) of the monocation 1+.

class II character of mixed-valence compounds as revealed by previous computational studies.21 The TDDFT calculated UV−vis spectra of compound 1 as well as the experimental data are shown in Figure 10. The

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CONCLUSION The cross-conjugated gem-DEE-bridged diferrocenyl compounds, including 9-[bis(ferrocenylethynyl)methylene]-9Hxanthene (1), 9-[bis(ferrocenylethynyl)methylene]-9H-thioxanthene (2), 5-[bis(ferrocenylethynyl)methylene]-5Hdibenzo[a,d]cycloheptene (3), and 9-[bis(ferrocenylethynyl)methylene]-9H-fluorene (4), were synthesized and structurally

Figure 10. Experimental (black line) and TDDFT calculated (blue vertical line) UV−vis spectra of 1 in CH2Cl2. F


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Organometallics

mmol), and CuI (20 mg, 0.1 mmol) in triethylamine (10 mL) was sealed in a 120 mL Teflon screw-capped tube under an N 2 atmosphere. The reaction mixture was heated at 95 °C with stirring for 24 h. After it was cooled, the reaction mixture was diluted with CH2Cl2 (100 mL) and washed with saturated aqueous NH4Cl solution (2 × 50 mL) and distilled water (2 × 50 mL). The organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (CH2Cl2/petroleum ether 1/6) to yield the title compound 1 as a red solid (122 mg, 67% yield). 1H NMR (600 MHz, CDCl3): δ 8.62 (d, J = 7.7 Hz, 2H), 7.38 (t, J = 7.3 Hz, 2H), 7.25−7.22 (m, 4H), 4.52 (s, 4H), 4.25 (s, 14H). 13C NMR (150 MHz, CDCl3): δ 152.29, 135.49, 129.72, 127.53, 123.26, 122.30, 116.39, 98.50, 92.58, 86.04, 71.26, 69.95, 69.05, 65.37. Anal. Calcd for C38H26OFe2: C, 74.78; H, 4.29. Found: C, 74.93; H, 4.31. HR-MS (ESI): m/z calcd for C38H26OFe2 610.0682 [M]+, found 610.0673. 9-[Bis(ferrocenylethynyl)methylene]-9H-thioxanthene (2). This compound was prepared from 9-(dibromomethylene)-9Hthioxanthene by the same procedure as that for 1. Chromatography on silica gel (CH2Cl2/petroleum ether 1/6) gave the desired product as a red solid (103 mg, 55% yield). 1H NMR (600 MHz, CDCl3): δ 8.20 (d, J = 7.7 Hz, 2H), 7.50 (d, J = 7.7 Hz, 2H), 7.37 (t, J = 7.4 Hz, 2H), 7.30 (t, J = 7.4 Hz, 2H), 4.45 (s, 4H), 4.22 (s, 4H), 4.18 (s, 10H). 13 C NMR (150 MHz, CDCl3): δ 144.01, 134.35, 133.30, 129.11, 127.72, 126.22, 125.31, 103.75, 92.06, 84.85, 71.25, 69.92, 68.98, 65.01. Anal. Calcd for C38H26SFe2: C, 72.86; H, 4.18. Found: C, 72.97; H, 4.19. HR-MS (ESI): m/z calcd for C38H26SFe2 626.0454 [M]+, found 626.0850. 5-(Dibromomethylene)-5H-dibenzo[a,d]cycloheptene (3a). In an oven-dried three-necked flask, a mixture of 5H-dibenzo[a,d]cyclohepten-5-one (2.06 g, 10 mmol), CBr4 (6.63 g, 20.0 mmol), and PPh3 (10.5 g, 40.0 mmol) in heptane (100 mL) was refluxed under an N2 atmosphere for 48 h. After it was cooled to room temperature, the reaction mixture was filtered through Celite, and the solid residue was washed with petroleum ether. The solvent was removed under reduced pressure to give the crude product. Chromatography on silica gel using CH2Cl2 as the eluent afforded the product as a white solid (1.26 g, 35% yield). 1H NMR (600 MHz, CDCl3): δ 7.45 (d, J = 7.4, 2H), 7.40−7.37 (m, 4H), 7.33 (t, J = 7.0, 2H), 6.96 (s, 2H). 13C NMR (150 MHz, CDCl3): δ 145.23, 137.96, 133.22, 130.42, 128.49, 128.20, 127.93, 127.77, 92.64. Anal. Calcd for C16H10Br2: C, 53.08; H, 2.78. Found: C, 53.18; H, 2.79. HR-MS (ESI): m/z calcd for C16H10Br2 359.9149, found 359.9141. 5-[Bis(ferrocenylethynyl)methylene]-5H-dibenzo[a,d]cycloheptene (3). This compound was prepared from 5-(dibromomethylene)-5H-dibenzo[a,d]cycloheptene by the same procedure as that for 1. Chromatography on silica gel (CH2Cl2/petroleum ether 1/ 5) gave the desired product as a red solid (91 mg, 49% yield). 1H NMR (600 MHz, CDCl3): δ 7.74 (d, J = 7.5 Hz, 2H), 7.46 (t, J = 7.1 Hz, 2H), 7.40−7.36 (m, 4H), 6.95 (s, 2H), 4.33 (s, 2H), 4.25 (s, 2H), 4.15 (s, 2H), 4.12 (s, 2H), 4.03 (s, 10H). 13C NMR (150 MHz, CDCl3): δ 152.32, 138.09, 134.01, 131.08, 129.12, 128.58, 127.71, 127.68, 106.67, 90.47, 83.48, 71.36, 71.10, 69.95, 68.77, 64.95. Anal. Calcd for C40H28Fe2: C, 77.44; H, 4.55. Found: C, 77.61; H, 4.59. HRMS (ESI): m/z calcd for C40H28Fe2 620.0890 [M]+, found 620.0903. 9-[Bis(ferrocenylethynyl)methylene]-9H-fluorene (4). This compound was synthesized from 9-(dibromomethylene)-9H-fluorene by the literature method.6f The title compound 4 was obtained as a red solid (133 mg, 75% yield). 1H NMR (600 MHz, CDCl3): δ 8.77 (d, J = 7.4 Hz, 2H), 7.72 (d, J = 7.1 Hz, 2H), 7.42−7.36 (m, 4H), 4.66 (s, 4H), 4.36 (s, 4H), 4.33 (s, 10H). 13C NMR (150 MHz, CDCl3): δ 142.69, 139.77, 137.88, 128.96, 127.17, 124.85, 119.62, 102.86, 98.28, 85.92, 71.72, 70.26, 69.60, 64.59. Anal. Calcd for C38H26Fe2: C, 76.80; H, 4.41. Found: C, 76.96; H, 4.45. HR-MS (ESI): m/z calcd for C38H26Fe2 594.0733 [M]+, found 594.0728.

characterized. The CV and DPV measurements using the weakly coordinating electrolyte Na[B{C6H3(3,5-CF3)2}4] reveal moderate electronic coupling between the Fc termini over the gem-DEE bridge. The spectroscopic signatures for the mixed-valence monocation were obtained via stepwise chemical oxidation of these compounds, which are characterized by NIR bands observed at ∼1000 nm. Hush analysis of the IVCT bands is suggestive of the Robin−Day class II mixed-valence system. UV−vis spectroscopy and DFT/TDDFT calculations suggest that the low-energy transitions of these compounds involve a metal to ligand charge-transfer (MLCT) transition from the iron center of Fc termini to the pendant substituent. These results indicate that the gem-DEE centered cross-conjugated system possesses multiple π-conjugated channels in which the electronic interaction can be potentially modulated.

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EXPERIMENTAL SECTION

Reagents. Xanthone (98.0%), thioxanthen-9-one (99%), 9fluorenone (99%), tetrabromomethane (98%), triphenylphosphine (99.0%), heptane (98%), and triethylamine (99.0%) were purchased from Shanghai Macklin Biochemical Co., Ltd. 5H-Dibenzo[a,d]cyclohepten-5-one (98%) was purchased form Adamas Reagent, Ltd. These reagents were used as received without further purification. Na[B{C6H3(3,5-CF3)2}4],22a ethynylferrocene,22b 9-(dibromomethylene)-9H-fluorene,11 9-(dibromomethylene)-9H-xanthene,11 and 9(dibromomethylene)-9H-thioxanthene11 were synthesized by literature methods. Instruments. 1H NMR and 13C NMR spectra were recorded on a Bruker BioSpin GmbH 600 MHz NMR spectrometer with chemical shifts reported in ppm (in CDCl3; TMS as internal standard). Highresolution electrospray ionization mass spectra were obtained on a Waters Xevo G2-Xs QTof instrument. Elemental analyses (C and H) were performed using a Vario MICRO elemental analyzer. UV−vis− NIR spectra of the samples in CH2Cl2 solution were recorded on a PerkinElmer Lambda-950 spectrophotometer. Electrochemistry. The electrochemical experiments were carried out with a CHI 660E electrochemistry analyzer (CH Instruments). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed using a three-electrode setup with a glassy-carbon working electrode, a Pt-wire counter electrode, and a Ag/AgCl (3 M KCl) reference electrode. Na[B{C6H3(3,5-CF3)2}4] in CH2Cl2 (0.01 M, N2-degassed) was used as the supporting electrolyte. X-ray Crystallography. The X-ray diffraction data for compounds 1−4 were collected at room temperature on a Xcalibur Eos Gemini CCD diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. The structures were solved by direct methods using SHELXS-2013 and refined by full-matrix least squares on F2 using SHELXL-2016.23 The non-H atoms were refined with anisotropic thermal parameters. All H atoms were in calculated positions and refined as riding on their parent atoms. The crystal data and refinement details for compounds 1−4 are given in Table S1 in the Supporting Information. CCDC 1573055 (1), 1573056 (2), 1573057 (3), and 1573058 (4) contain supplementary crystallographic data for this paper. Computations. DFT and TDDFT calculations at the B3LYP/631G** level of theory were performed by using the Gaussian 09 package. The initial geometries of the compounds were taken from the corresponding crystal structure data. No symmetry constraints were applied in the optimization. The optimized geometries were fully characterized as true minima via analytical frequency calculations. TDDFT calculations were performed on the basis of the optimized structures. The first 50 excited states were considered in TDDFT calculations. The solvation effects of CH2Cl2 were considered in DFT and TDDFT calculations using the polarized continuum model (PCM). 9-[Bis(ferrocenylethynyl)methylene]-9H-xanthene (1). A mixture of 9-(dibromomethylene)-9H-xanthene (105 mg, 0.3 mmol), ethynylferrocene (189 mg, 0.9 mmol), Pd(PPh3)2Cl2 (28 mg, 0.04 G


Article

Organometallics

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00686. Crystal data and structure refinement summary, TDDFT calculated main orbital contributions, 1H and 13C NMR spectra, CV and DPV curves, UV−vis−NIR spectra, TDDFT calculated absorption spectra, and selected frontier orbital plots (PDF) Accession Codes

CCDC 1573055−1573058 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], 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 Author

*E-mail for Y.F.: [email protected]. ORCID

Yang Fan: 0000-0003-4858-4036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support by the National Natural Science Foundation of China (No. 21002082) and the Nanhu Scholars Program for Young Scholars of XYNU.

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REFERENCES

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Diferrocenes Bridged by a Geminal Diethynylethene Scaffold with

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