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

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Synthesis, Structures, and Redox Properties of Tetracyano-Bridged Diferrocene Donor−Acceptor−Donor Systems Rajneesh Misra,*,† Thaksen Jadhav,† Dustin Nevonen,‡ Ellen M. Monzo,§ Shaikh M. Mobin,† and Victor N. Nemykin*,‡ †

Discipline of Chemistry, Indian Institute of Technology, Indore, 453 552, India Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada § Department of Chemistry & Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, United States ‡

S Supporting Information *

ABSTRACT: A set of tetracyanobutadiene (TCBD)- and dicyanoquinodimethane (DCNQ)bridged ferrocenyl dimers 5−8 were designed and synthesized by the [2 + 2] cycloaddition− retroelectrocyclization reaction of diferrocenyl complexes 3 and 4 with tetracyanoethylene (TCNE) and 7,7,8,8 tetracyanoquinodimethane (TCNQ), respectively. The effect of constitutional isomers (para vs meta) and different acceptors on their donor−acceptor interactions and photophysical and redox properties as well as electronic structures was evaluated using a variety of experimental and theoretical methods. The single-crystal X-ray structures of TCBD- and DCNQ-bridged ferrocenyl dimers 6 and 7 are reported. The DCNQ-bridged ferrocenyl dimers 7 and 8 have lower HOMO−LUMO gap values with red-shifted absorption bands in comparison to those of TCBD-bridged ferrocenyl dimers 5 and 6. Mössbauer spectra of 3−8 are suggestive of very similar isomer shifts and quadrupole splittings in all diferrocene complexes despite their different proximities to the electron-withdrawing fragment. Spectroelectrochemical data on 5−8 are suggestive of the presence of ferrocene-centered HOMOs in these compounds as well as a lack of electronic coupling between ferrocene groups.

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applications.1 Ferrocene derivatives have been studied in multiredox systems due to their well-defined redox properties, exhibiting low oxidation potential with formation of a stabilized ferrocenium ion.2 The studies on multiferrocenyl systems are of topical interest, as these systems are potential candidates for optoelectronic applications. The literature reveals that a wide range of ferrocene dimers have been designed and synthesized to evaluate the electronic communication between them.3 Tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) are powerful electron acceptors, and their reaction with π-conjugated electron-rich alkynes results in molecular systems with low HOMO−LUMO energy gaps. TCNE and TCNQ can be incorporated into an electron-rich acetylene bridge by a [2 + 2] cycloaddition−retroelectrocyclization reaction.4 TCBD and DCNQ are electroactive units which exhibit two stable and reversible single-electron reduction waves.5 The incorporation of TCBD and DCNQ units into the electron-rich acetylene results in strong donor− acceptor molecular systems with red-shifted absorptions which have been explored in organic photovoltaics.6 Our group has explored a wide variety of donor-functionalized TCBD- and DCNQ-based molecular systems for optoelectronic applications.7 The ferrocenyl-functionalized TCBD- and DCNQ-based molecular systems are expected to

INTRODUCTION

Ferrocene derivatives are widely studied organometallic compounds for developing anticancer drugs, catalytic processes, nonlinear optical (NLO) materials, molecular electronics, redox-switchable fluorescence markers, and light-harvesting Chart 1. Molecular Structures of TCBD- and DCNQBridged Ferrocenyl Dimers 5−8

Received: September 27, 2017

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.7b00728 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of TCBD- and DCNQ-Bridged Ferrocenyl Dimers 5−8

Figure 1. Crystal structures of diferrocene complexes (A) 6 and (B) 7.

catalyzed Sonogashira cross-coupling reaction of 4-/3-ferrocenyliodobenzene with ethynylferrocene in 45% and 33% yields, respectively (Scheme 1).8 The [2 + 2] cycloaddition− retroelectrocyclization reaction of ferrocenyl dimers 3 and 4 with TCNE at room temperature for 4 h resulted in tetracyanobutadiene (TCBD)-bridged ferrocenyl dimers 5 and 6 in 87% and 82% yields, respectively (Scheme 1). Similarly, the [2 + 2] cycloaddition−retroelectrocyclization reaction of 3 and 4 with excess amounts of TCNQ at 80 °C for 6 h resulted in DCNQ-functionalized ferrocenyl dimers 7 and 8 in 78% and 71% yields, respectively. The TCBD- and DCNQbridged ferrocenyl dimers 5−8 were purified by silica gel column chromatography. The ferrocenyl dimers 5−8 are readily soluble in common organic solvents and were well characterized by 1H NMR, 13C NMR, and HRMS techniques. The TCBD (6)- and DCNQ (7)-functionalized ferrocenyl dimers were also characterized by single-crystal X-ray analysis.

be potential redox systems in the study of electron transfer and magnetic exchange interactions. In this contribution, we extend our research on ferrocenyl-based donor−acceptor systems. We were interested to see the effect of a phenyl spacer and a tetracyano acceptor on the electronic communication of the two ferrocenyl units (Chart1). The electronic communication between two ferrocene moieties across TCBD and DCNQ bridges is the focus of this study. The effects of TCBD and DCNQ bridges on the electronic interchange between the ferrocene moieties were studied by synthesizing four constitutional dimers of ferrocene (Chart 1). Their crystal structures and spectroscopic and electrochemical properties are discussed and further supported by DFT calculations.

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RESULTS AND DISCUSSION The ferrocenyl dimers linked through an ethynyl phenyl linkage at para (3) and meta (4) positions were synthesized by the PdB


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Organometallics

Figure 2. Packing diagram of the TCBD-bridged ferrocenyl dimer 6.

Figure 3. Packing diagram of the dicyanoquinodimethane (DCNQ)-bridged ferrocenyl dimer 7.

monoclinic P21/c and triclinic P1̅ space groups, respectively. The important crystallographic parameters are given in Table S1 in the Supporting Information. The TCNE (6)- and TCNQ (7)-bridged ferrocenyl dimers exhibit an s-cis conformation of the butadiene-2,3-diyl moiety (Figure 1). The highly crowded structure and strong repulsion of cyano groups results in highly distorted structures with dihedral angles of 72.28° (C(5)− C(6)−C(7)−C(8)) and 82.59° (C(1)−C(4)−C(5)−C(6)) for 6 and 7, respectively. The cyclopentadienyl rings of the ferrocene exhibits skew-eclipsed conformation for 6 and 7. The selected bond lengths and bond angles of the single-crystal structures of 6 and 7 are given in Tables S2 and S3 in the Supporting Information. The packing diagram of 6 reveals the presence of intermolecular H-bonding and C−H···π interactions. The C− H···N hydrogen-bonding interaction involves hydrogen atom H(23) of the cyclopentadienyl ring and nitrogen atom N(1) of the adjacent molecule, C(33)−H(33)···N(1) = 2.658(8) Å; moreover, the H atom H(23) of the cyclopentadienyl ring forms a C−H···π interaction with the adjacent cyclopentadienyl ring, leading to the formation of a 1D network. These 1D chains are further connected via C(29)−H(29) with the π-electron cloud of the cyclopentadienyl ring of

Figure 4. Electronic absorption spectra of 5−8 in dichloromethane.

X-ray Crystal Structures of 6 and 7. Single crystals of 6 and 7 suitable for X-ray crystallographic analysis were obtained by slow diffusion of hexane into dichloromethane solution of the compounds. The single crystals of 6 and 7 crystallizes in C


Article

Organometallics Table 1. Spectroscopic and Redox Properties of TCNE- and TCNQ-Bridged Ferrocenyl Dimers 5−8 redox potential, Vc UV−vis spectra λmax, nm (ε, 10 M a

5 6 7 8 a

237 239 232 231

(2.36), (2.59), (2.96), (2.77),

359 285 364 286

(2.16), (2.24), (2.73), (2.34),

4

607 326 439 445

−1

−1

cm ))

Mössbauer data δ/ΔEQ/Γ, mm/s

Red2

Red1

Ox1

Ox2

0.44/2.19/0.28 0.43/2.20/0.27 0.44/2.18/0.27 0.44/2.02/0.27 0.44/2.34/0.27

−1.32 −1.33

−0.96 −0.94

0.26 0.21 0.24 0.21

0.70 0.71 0.54 0.54

b

(0.65) (1.84), 627 (0.24) (1.76), 506 (1.73), 765 (0.36) (1.54), 778 (0.29)

−0.78d −0.81d

Measured in dichloromethane. bAt 293 K, referenced to α-Fe. cVersus FcH/FcH+ couple; dBroad, two-electron-reduction wave.

Figure 5. Room-temperature Mössbauer spectra of complexes 5−8. Individual doublets are shown in red and blue, while the cumulative simulated spectrum is shown in green.

Figure 6. Cyclic and differential pulse voltammograms for compounds 5−8.

D


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Figure 7. Stepwise oxidation of the complexes 5 and 6 under spectroelectrochemical conditions in a DCM/0.15 M TFAB system.

Figure 8. Stepwise oxidation of the complexes 7 and 8 under spectroelectrochemical conditions in a DCM/0.15 M TFAB system.

250 and 600 nm corresponding to a π → π* transitions and a broad charge transfer (CT) band from ferrocene to the TCNQ unit between 700 and 900 nm. This reveals that DCNQ is a stronger acceptor than TCBD and results in a strong D−A interaction. Solvatochromic studies on ferrocenyl dimers 5−8 were performed, which exhibit nonsolvatochromic behavior. The TCNE- and DCNQ-bridged ferrocenyl dimers 5−8 are nonluminescent in nature. Mössbauer Spectroscopy. Mössbauer spectra of compounds 5−8 are shown in Figure 5, with parameters being given in Table 1. In all cases, a characteristic for the low-spin Fe(II) ferrocene derivative doublet was observed.8 Similar to the other ferrocene derivatives with electron-withdrawing groups, the isomer shifts in complexes 5−8 are very close to those in the starting materials 3 (δ = 0.44, ΔEQ = 2.29 mm/s) and 4 (δ = 0.44, ΔEQ = 2.31 mm/s), while quadrupole splitting in 5−8 is smaller than in 3 and 4. The line width in the Mössbauer spectra of compounds 3−7 (0.26−0.28 mm/s) is indicative of only one Mössbauer doublet. Although this is quite unusual for diferrocene systems with two ferrocene fragments located in rather different environments, our DFT calculations discussed below suggest that the quadrupole

another chain, leading to the formation of a 2D network (Figure 2). The packing diagram of 7 reveals the presence of intermolecular H-bonding and π−π stacking interactions. The C−H···N H-bonding interactions are between the hydrogen atom H(40) of the cyclopentadienyl ring and nitrogen atom N(4) of an adjacent molecule, C(40)−H(40)···N(4) = 2.527 Ȧ . Moreover, π−π stacking interactions occur between π-electron clouds of the cyclopentadienyl ring and the π-electron clouds of an adjacent phenyl ring, leading to the formation of a 3D network (Figure 3). Photophysical Properties. The electronic absorption spectra of TCNE- and TCNQ-bridged ferrocenyl dimers 5−8 were recorded in dichloromethane at room temperature (Figure 4), and data are given in Table 1. The TCBD-bridged ferrocenyl dimers 5 and 6 exhibit a strong absorption band between 250 and 400 nm corresponding to a π → π* transition and a broad charge transfer (CT) band from ferrocene to the TCBD unit between 500 and 800 nm. On the other hand, the DCNQ-bridged ferrocenyl dimers 7 and 8 show a red-shifted absorption band in comparison to TCBD-bridged ferrocenyl dimers 5 and 6 and exhibit a strong absorption band between E


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Organometallics

Figure 9. Energy level diagram of frontier orbitals of 5−8.

Figure 10. Frontier π-molecular orbitals for 5 and 6.

splitting values in 5−7 should be close to each other. The line width in the Mössbauer spectra of complex 8 is significantly larger than that in 3−7 (Γ = 0.39 mm/s), and thus, the spectrum was modeled with two doublets representing two iron sites. Again, this correlates well with our DFT calculations discussed below. Redox Properties. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to investigate the

electrochemical behavior of compounds 3−8 using a DCM/ 0.05 M TFAB system (Figure 6 and Figure S3 in the Supporting Information). The precursors 3 and 4 show two oxidation peaks, and no reduction was observed. For compounds 5 and 6, two reversible single-electron oxidations, one reversible single-electron reduction, and one quasireversible single-electron reduction were observed. The voltammograms of 7 and 8 both show two reversible oxidations F


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Figure 11. Frontier π-molecular orbitals of 7 and 8.

centers in these complexes. Overall, the spectroelectrochemical experiments on complexes 5−8 are clearly indicative of a lack of long-range electronic coupling in these systems, which is not surprising, as two ferrocene groups in each complex are located in very different environments. DFT and TDDFT Calculations. The excitation energies and electronic structures of the multicyano-bridged ferrocene dimers 5−8 were analyzed using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations.9,10 The polarized continuum model (PCM) approach using dichloromethane as a solvent was used in all calculations to correlate redox properties and experimental UV−vis spectra with DFT and TDDFT data. The frontier molecular orbital energy diagram for 5−8 predicted by DFT-PCM calculations is shown in Figure 9, and images of the frontier π-molecular orbitals are depicted in Figures 10 and 11. In addition, the molecular orbital compositions for frontier MOs are given in Table S4 in the Supporting Information. In compounds 5−8, the DFT-predicted highest occupied molecular orbital (HOMO) is best represented as a MO with a large electron density contribution from the ferrocene and benzene ring that are directly bound together, which correlates well with electrochemical and spectroelectrochemical data on these compounds. As expected, the ferrocene-centered MOs for the ferrocene group connected to the electron-withdrawing TCNQ or TCNE fragments have lower energies in comparison to the ferrocene-centered MOs localized at the ferrocene group attached to the phenyl group. The ferrocene-centered MOs are closely spaced in energy in all complexes. The LUMOs in complexes 5−8 are predominantly centered at the electrondeficient bridge fragments and are well separated in energy from the LUMO+1. Such electronic structures are very characteristic for ferrocene-containing compounds with electron-deficient π systems directly attached to the ferrocene groups and are indicative of the potential low-energy MLCT bands.6c,d The DFT-predicted Mössbauer quadrupole splittings for complexes 5−8 are shown in Table S1 in the Supporting Information. In agreement with the previous discussion, calculations8b conducted with the CAM-B3LYP exchangecorrelation functional overestimate values for the quadrupole splittings in ferrocenes, while those conducted with the BP86

and one broad peak for reversible reduction. Close inspection of the CV data for 7 and 8 reveal that two overlapped singleelectron-reduction processes occurred in each case. This is best illustrated by the DPV data, which show a much broader, single reduction peak which was interpreted as two overlapping peaks. Both reversible oxidation processes were attributed to stepwise oxidation of ferrocene fragments. The first oxidation potential in 5−8 is significantly lower than the ferrocene-centered oxidation process in dicyanovinyl-6c and tricyanovinylferrocene6d and was assigned to the oxidation of the ferrocene group directly connected to the phenyl ring. The second ferrocenecentered process in 5−8 was attributed to the oxidation of the ferrocene group connected to the electron-withdrawing fragment in 5−8. Taking into consideration the reduction potential for ferrocene, UV−vis data, and results of DFT calculations, all reduction processes in 5−8 were assigned to the organic π system in these compounds. Spectroelectrochemistry was employed to investigate the spectroscopic signatures of the redox-active species obtained by stepwise oxidation of complexes 5−8 in order to get insight into possible long-range metal−metal coupling in these compounds (Figures 7 and 8). Compounds 5 and 6 show two clear oxidative transformation processes associated with two sequential oxidations of ferrocene groups. For instance, during the first oxidation of complex 5 under spectroelectochemical conditions, an initial broad and low-energy MLCT band centered at 611 nm decreased in intensity and transformed into a broad band centered at 620 nm, while the most intense band undergoes a higher-energy shift to 348 nm (Figure 7). During the second oxidation process under spectroelectrochemical conditions, the intensity of the 620 nm band decreased and a new band at 308 nm formed. More importantly, no indication of the intervalence charge-transfer (IVCT) band in the NIR region (up to 2600 nm) was detected for [5]+ and [6]+, which is suggestive of a lack of long-range metal−metal coupling in these compounds. Similarly, during stepwise oxidation of compounds 7 and 8, the low-energy MLCT band slowly degrades and undergoes a small energy shift (Figure 8), while the most intense bands undergo complex transformations. Again, no IVCT band was observed in complexes [7]+ and [8]+, which is indicative of the lack of electronic communication between two ferrocene G


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states 1 and 3) are dominated by single-electron excitations from the ferrocene group connected to the phenyl ring to the π* MO predominantly located at the electron-withdrawing fragment. Taking into consideration the long distance between these groups, it is not surprising to see the low intensities for such excited states predicted by TDDFT calculations. The second excited state predicted by TDDFT calculations has much higher intensity and is dominated by the single-electron excitations from the occupied MOs centered at the ferrocene group connected to the electron-withdrawing fragment of 5 and 6 to the π* MO predominantly located at the electronwithdrawing fragment in these compounds. Similarly, in the case of the TCNQ-containing complexes 7 and 8, TDDFT predicts that the low-energy band observed in their UV−vis spectra should have predominant MLCT character and originates from charge transfer from the ferrocene group connected to the electron-withdrawing fragment.

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, we have designed and synthesized TCBD-/ DCNQ -bridged ferrocenyl dimers 5−8 by using a [2 + 2] cycloaddition−retroelectrocyclization reaction in good yields. The pronounced effect of isomeric positions (ortho/meta) and TCBD/DCNQ acceptors on the donor−acceptor interactions, photonic properties, electrochemical properties, and energy levels were observed. The single-crystal structures of TCBD (6)- and DCNQ (7)-bridged ferrocenyl dimers exhibit an s-cis conformation of the butadiene-2,3-diyl moiety. The electronic absorption study reveals red-shifted absorption bands of DCNQ-bridged ferrocenyl dimers 7 and 8 in comparison to those of TCBD-bridged ferrocenyl dimers 5 and 6. The TCBD-/DCNQ-bridged ferrocenyl dimers 5−8 exhibit two strong charge transfer bands (from two different ferrocenes to TCBD/DCNQ). Mössbauer spectral parameters for the two different ferrocene groups in 5−8 are quite close to each other, which was also supported by the DFT calculations. DFT also predicts that the HOMO in complexes 5−8 is dominated by the ferrocene group directly attached to the phenyl group, while the LUMO is delocalized over the electron-withdrawing bridging fragment. TDDFT calculations are supportive of the low-energy MLCT bands observed between 500 and 800 nm.

* Supporting Information

Figure 12. Experimental (upper) and TDDFT-PCM simulated (lower) UV−vis spectra of compounds 5−8.

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00728. General experimental methods, 1H and 13C NMR and HRMS spectra of all new compounds, crystallographic data for 6 and 7, DFT calculation data, and crystal structures (PDF)

GGA-based exchange-correlation functional are in good agreement with the experimental data on 5−8. For both exchange-correlation functionals DFT predicts very small differences in quadrupole splitting for the two iron centers in complexes 5−8. More importantly, the largest difference in predicted quadrupole splitting values for the iron centers was found in the case of complex 8, which explains the broadening of the Mössbauer doublet observed for this compound. TDDFT-predicted and experimental UV−vis spectra of complexes 5−8 are shown in Figure 12 and generally show good agreement. In the case of the TCNE-containing compounds 5 and 6 TDDFT predicts that the low-energy part of the UV−vis spectra should consist of three excited states with predominant MLCT character. Two of those (excited

Accession Codes

CCDC 1562190−1562191 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. H


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AUTHOR INFORMATION

Corresponding Authors

*R.M.: e-mail, [email protected]; fax, +91 731 2361 482; tel, +91 731 2438 754. *V.N.N.: e-mail, [email protected]. ORCID

Rajneesh Misra: 0000-0003-3225-2125 Shaikh M. Mobin: 0000-0003-1940-3822 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

R.M. acknowledges the INSA (Project No. SP/YSP/139/2017/ 2293), the Department of Science and Technology Project No. (Project No. EMR/2014/001257), and the Council of Scientific and Industrial Research (Project No. 01/(2795)/ 14/EMR-II), New Delhi, India, for financial support. We also thank the Sophisticated Instrumentation Centre (SIC), Indian Institute of Technology (IIT) Indore. Generous support from the Minnesota Supercomputing Institute, NSERC, CFI, University of Manitoba, and WestGrid Canada to V.N.N. is greatly appreciated.

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