Letter Cite This: Org. Lett. XXXX, XXX, XXX-XXX
pubs.acs.org/OrgLett
Electronic Interplay between TTF and Extended-TCNQ Electrophores along a Ruthenium Bis(acetylide) Linker Antoine Vacher, Morgan Auffray, Frédéric Barrière, Thierry Roisnel, and Dominique Lorcy* Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Matière Condensée et Systèmes Electroactifs (MaCSE), Campus de Beaulieu, Bât 10A, Rennes 35042 Cedex, France S Supporting Information *
ABSTRACT: A bis(TTF-butadiynyl) ruthenium D−D′−D complex, with intramolecular electronic interplay between the three electron-donating electrophores, was easily converted through a cycloaddition−retroelectrocyclization with TCNQ into a D−A−D′−A−D pentad complex, which exhibits an intense intramolecular charge transfer together with an electronic interplay between the two acceptors along the conjugated organometallic bridge.
E
The synthetic route toward the target complexes is outlined in Scheme 1. Complex 2 is prepared by reacting TTF-butadiyne
lectronic interactions between organic and/or organometallic electrophores along a π-conjugated carbon bridge have attracted a great deal of interest for the elaboration of molecular materials.1,2 In this respect, insertion within the conjugated linker of an organometallic fragment, such as a trans-ruthenium bis(acetylide), is known to allow and possibly amplify the electronic communication between electrophores.3 Tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) are among the most studied organic electrophores, particularly as precursors of conducting materials.4 Both derivatives can be reversibly oxidized (TTF) or reduced (TCNQ) in two sequential steps at easily accessible potentials. We recently demonstrated that the connection of a TTF to a ruthenium center through an acetylide bridge leads to a strong electronic coupling between those two electrophores.5 Moreover, we also established that the introduction of a transruthenium−bis(acetylide) linker efficiently mediates electronic communication between the two TTFs cores in a redox active triad DTTF−D′Ru−DTTF, namely trans-[Ru(CCMe3TTF)2(dppe)2].6 The use of a longer conjugated linker, the buta-1,3diyne-1,4-diyl, has also been shown to allow electronic interplay between two different electroactive metals M in [trans-Ru(C C−CC−MLx)2] (M = Ru, Fe).3 Thus, it is of interest to analyze the effect of the length of the conjugated linker between the TTF cores and the metallic center on their electronic interplay within the extended triad. This can be adequately performed by replacing the ethyne linker by a butadiyne. Herein, we report the syntheses and the structures of a triadic Ru bis(buta-1,3-diynyl-TTF) complex, DTTF−π−D′Ru−π−DTTF and its conversion via a [2 + 2] cycloaddition−retroelectrocyclization (CA-RE)7 reaction with TCNQ to a redoxactive pentad, D−A−D′−A−D. Electrochemical and spectroelectrochemical investigations carried out on both complexes indicate a strong electronic communication between the metal center and the organic electrophores as well as between the two similar electrophores, either TTF or extended-TCNQ, along the organometallic linker. © XXXX American Chemical Society
Scheme 1. Synthesis of the Molecular Triad 2 (D−D′−D) and Pentad 3 (D−A−D′−A−D)
18 with cis-[RuCl2(dppe)2] (dppe = Ph2PCH2CH2PPh2) in the presence of NaPF6 and triethylamine in dichloromethane at room temperature under inert atmosphere. During the course of the reaction monitored by 31P NMR spectroscopy, the decrease of the set of signals due to the starting complex cis-[RuCl2(dppe)2] at δp = 37.8 ppm and δp = Received: September 8, 2017
A
DOI: 10.1021/acs.orglett.7b02818 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters 45.1 ppm is concomitant with the growth of a novel signal at δP = 51.05 ppm. The presence of one single peak indicates an equivalent environment for the phosphorus atoms of the dppe ligands due to a trans configuration of the TTF ligands on the metallic center. A CA-RE cascade reaction yields 3 by mixing a tetrahydrofuran solution of 2 with an acetonitrile solution of TCNQ. In principle, the two triple bonds on both sides of the Ru atom can react with TCNQ for the [2 + 2] cycloaddition leading to a cyclobutenyl intermediate which undergo a ring opening (see the Supporting Information).9−11 Due to the steric hindrance generated by the dppe ligands, the CC bonds closest to the TTFs are involved in the CA-RE reaction as demonstrated by X-ray crystallographic analysis (vide infra). This reactivity at the outer CC bond of a diynyl ligand has been observed in trans-bis(buta-1,3-diynyl)Pt complexes as well as in M(−CC−CC−R)(dppe)Cp* (R = Ph, H and M = Ru, Fe).9,11 It is worth mentioning that it is the first time that an alkyne bond connected to a TTF undergoes such a reaction with TCNQ.10 Moreover, the regioselectivity of the TCNQ addition to 2 leads to the dicyanoquinodimethane (DCNQ) moiety at the β-carbon atom of the linkers relative to the TTF core. This is evidenced by the X-ray crystal structure analysis of 3. This regioselectivity is not common as for the bis(4NMe2phenylbuta-1,3-diynyl)Pt complexes, where the DCNQ moieties were located at the α-carbon atom relative to the NMe2phenyl ring.9 The extent of the conjugation of the linker between the TTF and the Ru atom in 2 and 3 can be characterized by IR spectroscopy. For each CC bond of the diyne bridges, the IR spectrum of 2 exhibits two stretching vibration bands at 2129 and 1993 cm−1, while for 3 the stretching vibration of the C N, CC, and CC bonds are observed at 2202, 1932, and 1590 cm−1, respectively.11 The CC bond IR vibrations in 2 and 3 are at a lower energy than in the starting TTF 1 (2201 cm−1), which indicates a high degree of conjugation between the TTF and the Ru atom in 2 as well as between the DCNQ moieties and the Ru in 3. Single crystals of complexes 2 and 3 were successfully obtained by slow diffusion of pentane into a concentrated solution of 2 in THF under inert atmosphere and by slow concentration of a solution of 3 under air. Complex 3 cocrystallizes with one molecule of neutral TCNQ.12 The molecular structures of 2 and 3 are shown in Figure 1. In both cases, the Ru(II) center is chelated by two dppe units and the octahedral coordination geometry is completed by two acetylide ligands, coordinated in a trans arrangement. The neutral TTFs are not planar in 2 and exhibit a boat conformation (asymmetric folding angles along the S···S axis of 13 and 17°). The −Ru−C1C2−C3C4−TTF linker is slightly bent with the angles C1C2−C3, C2−C3C4, and C3C4−CTTF of 169.7(2)°, 176.8 (2)°, and 175.5(2)°, respectively. The bond lengths of C1C2, C2−C3, and C3C4 are, respectively, 1.218, 1.370, and 1.215 Å. In 3, the TTFs are located in planes perpendicular to those of the DCNQ moieties. This organization does not seem to be imposed by the isolated TCNQ molecule present in the cocrystal 3−TCNQ since a similar conformation is obtained after DFT geometry optimization of the D−A−D′−A−D 3 alone. The central CC bond length of the TTF is characteristic of a neutral TTF derivative and is similar to that observed for 2 (1.349(4) Å for 3 and 1.343(3) Å for 2). If we now concentrate on the linker between the DCNQ and the Ru atom, the Ru−C, CC, and C−C bond lengths are only
Figure 1. Molecular structure of 2 (top) and 3 (bottom). Phenyl substituents of the dppe ligands and hydrogen atoms are omitted for clarity.
slightly modified compared to 2 (Ru−C 2.032(2)/2.051(2), CC 1.229(3)/1.218(3), and C−C 1.399(3)/1.370(3) Å for 3/2), indicating that the two DCNQ moieties are connected through a conjugated Ru-bis(acetylide) linker. Investigation of the bond lengths of the extended-TCNQ moieties compared with those of the TCNQ itself 13 demonstrates that the quinoid character, δr, is slightly decreased in the extended TCNQ compared to the TCNQ (δr is 0.102 Å for TCNQ and 0.08 Å for extended-TCNQ, while for benzene δr is 0 Å; see the SI). Cyclic voltammetry (CV) of 2 shows three main reversible oxidation waves. A closer look at the CV in Figure 2 reveals that the second redox
Figure 2. CV of 2 in CH2Cl2-[nBu4N][PF6] 0.1 M, v = 100 mV·s−1 (insert DPV experimental (red line), respective Gaussian decompositions (green dashed line) and best fit (black dashed line)).
process corresponds to two closely spaced reversible oxidation waves (Figure 2). While the first oxidation of the two TTF moieties is unresolved at E1 = 0.24 V, the second oxidation process occurs stepwise at E2 = 0.69 V and E3 = 0.76 V vs SCE (saturated calomel electrode). The splitting of the second process was also confirmed by differential pulse voltammetry (DPV, inset of Figure 2). The last redox system at E4 = 1.13 V is logically assigned to the RuII/RuIII couple. The introduction in 2 of a diacetylenic rod as opposed to a single acetylenic bridge between the TTF and the Ru electrophores, as in [Ru(CC−Me3TTF)2(dppe)2] (E1 = B
DOI: 10.1021/acs.orglett.7b02818 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters 0.07 V), induces an anodic shift of the first oxidation potential due to the electron-withdrawing character of the additional acetylenic unit. The first oxidation process of 2 (0.24 V) occurs, however, at a lower potential than that of TTF-butadiyne 18 (0.40 V). This can be rationalized by an increase of the electron density on the TTF core due to the donating organometallic fragment and efficiently transmitted by the butadiyne linker. Concerning the CV of complex 3 (Figure 3), three oxidation
Upon gradual oxidation to the bis radical cation state 32+ (Figure 4 top), only a slight decrease of the CT band is
Figure 3. CV of 3 in CH3CN-[nBu4N][PF6] 0.1 M, v = 0.1 V·s−1.
waves are observed. The first one at 0.52 V in CH2Cl2 (0.61 V in CH3CN), being fully reversible, results from the unresolved oxidation of the two TTFs into the radical cations, while the second one at 0.9 V in CH2Cl2 (1.02 V in CH3CN) corresponds to their oxidation into the TTF dication. It is worth noting in this case that no splitting of the redox system could be observed. Compared with complex 2, the presence of the extended-TCNQ moieties and the rupture of conjugation in 3 decrease significantly the overall donating ability of the TTF as a potential anodic shift of about 280 mV is observed. The last oxidation process occurs at 1.24 V and is assigned to the RuII/III couple. On the cathodic scan, the CV displays three redox systems associated with the redox behavior of the two extended-TCNQ acceptors. The first redox process at −0.12 V in CH3CN involves the unresolved reduction of these acceptors into the radical anions while the reduction to the dianion occurs sequentially at −0.24 V and −0.34 V in CH3CN as confirmed by DPV (Figure S4). The presence of the two TTFs and the Ru center in close vicinity decreases the overall electron-accepting ability compared to a pristine TCNQ molecule (E1 = 0.17 V).14 The absorption spectra show that complex 2 absorbs in the UV−vis region but not beyond 450 nm, while complex 3 (Figure S5) exhibits an additional intense absorption band at λmax = 784 nm (1.58 eV, ε = 82700 M−1·cm−1). The optical band gap was determined from the low energy absorption (λcutoff = 970 nm, 1.27 eV). This absorption is characteristic of an intramolecular charge transfer (CT) interaction from a donor to an acceptor. The acceptor is clearly identified in 3 as the extended-TCNQ moiety, while for the donor, it could arise from either the TTFs and/or from the Ru center. The electrochemical band gap determined from the potentials would favor the TTF as the donor. Indeed ΔE = E1TTF − E1exTCNQ = 0.64 eV, while a larger gap is found for the Ru with ΔE = ERu − E1exTCNQ = 1.36 eV. To gain better insight into the origin of this CT band, we performed spectroelectrochemical investigations in the UV−vis−NIR range upon oxidation of both TTFs and upon reduction of both extended-TCNQs.
Figure 4. UV−vis−NIR monitoring of the electrochemical oxidation of 3 (top) and electrochemical reduction (bottom).
observed together with the growth of two new bands at 420 and 600 nm which correspond to the signature of the TTF radical cation species. Also upon oxidation to the tetracationic species 34+, the CT band and most of the spectrum are not significantly modified. Thus, this observation is in favor of a CT between the organometallic Ru fragment and the extendedTCNQs. Upon reduction to 34− the CT band gradually decreases (Figure 4 bottom) until it disappears totally from the spectrum (Figure S6) while a broad band observed at lower energy grows (1200 nm). The presence of this latter absorption band is in agreement with the CV of 3 and confirms the electronic coupling between the two extended-TCNQ moieties separated by 9.3 Å across the organometallic linker. The width at half-height amounts to 1760 cm−1 which is significantly less than the 4405 cm−1 obtained using the Hush formula for a class II system, Δν1/2 = [2310(νmax)]1/2, suggesting a fully delocalized organic mixed valence species. Such a long-range electronic interplay between organic redox units is not uncommon in the literature15 but is unprecedented between two TCNQ like acceptors. DFT (density functional theory) calculations were also performed on 2 and 3 at the B3LYP/LanL2DZ level in the gas phase. Full geometry optimizations led to the molecular structure depicted in Figure 5 (see also the SI). As expected, the quasi-degenerated HOMO and HOMO−1 of 2 are delocalized on the two TTFs with significant metal and butadiyne contributions, while the HOMO−2 has an organometallic character. Among the three quasi-degenerated HOMO in 3, two have a strong TTF character. The LUMO and LUMO +1 of 3 are essentially delocalized on the two dicyanoquinodiC
DOI: 10.1021/acs.orglett.7b02818 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was granted access to the high-performance computing resources of CINES (Montpellier, France) under allocation 2017-A0020805032 awarded by GENCI.
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Figure 5. HOMO and HOMO−1 for 2 (left) and LUMO and LUMO +1 for 3 (right) shown with a cutoff of 0.04 [e/bohr3]1/2.
methane moieties across the conjugated bridge with an additional metal contribution for the LUMO+1, Figure 5. In summary, a ruthenium bis(butadiynyl-TTF) complex 2 has been prepared that exhibits intramolecular electronic interactions between the three electron-rich moieties, namely the Ru(dppe)2 and the two TTFs centers. The DTTF−π− D′Ru−π−DTTF complex 2 has been converted through a cycloaddition−retroelectrocyclization (CA-RE) reaction in the presence of TCNQ into a novel D−A−D′−A−D pentad complex 3 where electronic interplay has been evidenced between the extended TCNQs and the organometallic fragment, leaving the two TTF redox centers in a nonconjugated, perpendicular orientation. The pentad complex 3 exhibits an original and unprecedented intramolecular coupling between the two extended TCNQ across a noninnocent organometallic bridge. Moreover, an unexpected intense intramolecular charge transfer occurs between the less electron-rich donor fragment, the Ru center, and the extended TCNQ moieties.
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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.orglett.7b02818. Experimental procedures and characterization data for complexes 2 and 3, computational details, and crystallographic data (PDF) Accession Codes
CCDC 1563024−1563025 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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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Dominique Lorcy: 0000-0002-7698-8452 D
DOI: 10.1021/acs.orglett.7b02818 Org. Lett. XXXX, XXX, XXX−XXX
