A self-assembled bis(pyrrolo)tetrathiafulvalene-based redox active square

Abstract

The synthesis and full characterization of a redox-active π-donating molecular square is described, through coordination-driven self-assembly of a new key bis(pyrrolo)TTF building block with Pt(II) salts; the electrochemical behaviour of the square is consistent with noninteracting bis(pyrrolo)TTF units.

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Introduction

The coordination-driven self-assembly methodology has proven to be of particular interest for preparing molecular polygons and polyhedrons of predetermined shapes,¹ such systems being otherwise very challenging to synthesize through classical covalent multistep synthesis. These supramolecular assemblies generally call for a combination of rigid di-, tri- or tetra pyridyl substituted bridging ligands and complementary metallic salts of defined geometries (e.g. square planar Pt(II) and Pd(II) salts). Beside the synthetic challenge of preparing large size host architectures, they offer unique opportunities for new applications which are directly correlated to the nature of their two constituting partners (ligand and metal complexes), such as molecular recognition.¹ In this context, some examples of supramolecular polygons involving redox-active units have been described, with electroactivity located on the corner (i.e. the metal itself or an organometallic ligand (e.g.ferrocene) directly bound to the metal).^1,2Ferrocene unit can also be found as a pendant group on the bridging ligand between two metallic corners.³ Noteworthy, only few examples of polygons involving side walls that are themselves redox-active are described,⁴ and in this case, they involve reducible ligands such as 4,4′-bipyridine or perylene diimide. This spatial organization allows a control over the charge state of the cavity. However, to the best of our knowledge, functional metallosupramolecular squares built from highly π-donating side-walls are unknown to date, and would constitute a challenging reciprocal complement to previous π-electron accepting supramolecular or covalent macrocycles among which is found the famous cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺),^5a,b extensively used in donor–acceptor [2]-rotaxane chemistry.^5c The π-donating ability of tetrathiafulvalene (TTF) derivatives is well-established, and has projected this unit as a key redox building block in various molecular and supramolecular switchable systems.⁶ In particular, TTF derivatives are very easily oxidized according to two reversible one e⁻ steps.

We propose here the preparation of a redox-active π-donating supramolecular square by self-assembling of a new TTF-based ligand with Pt(II) phosphine corners. Several TTF derivatives bearing two pyridyl coordinating moieties are described.⁷ These systems have not been selected to our purpose since they exist as two Z/E isomers (Scheme 1), known to be very difficult to separate and which undergo a Z/Eisomerization after oxidation or protonation.⁸ We have instead focused our interest on the non-isomerizable derivative 1 as a key redox-active ligand. This system involves the bis(pyrrolo)TTF skeleton (BPTTF)⁹ as a basic framework, a unit which occupies a growing place among electroactive supramolecular architectures.^6a,10

Scheme 1 (a) The two isomeric Z and E forms in bis-substituted TTF; (b) N,N′-bis-substituted BPTTF.

Results and discussion

We recently described the synthesis of the key thione derivative 3¹¹ (Scheme 2), starting from 4-formyl-5-(diethoxymethyl)-1,3-dithiole-2-thione¹² and involving a three step procedure.^9b The target bis-pyridyl BPTTF derivative 1 is obtained by self-coupling of thione 3 in refluxing trimethylphosphite. DFT calculations (Gaussian® 2003, Becke3LYP (B3LYP)) have been carried out on compound 1 (Fig. 1). The electronic density in the HOMO of 1 is located on the central donating skeleton and on the lateral pyrrole motifs, illustrating the strong π-delocalization in BPTTF derivatives compared to the parent TTF framework; the LUMO is essentially located on both pyridyl and pyrrole heterocycles. The corresponding energies are very similar to that of unsubstituted BPTTF, anticipating a comparably high π-donating ability.¹⁰

Scheme 2 Synthesis of bis(pyridyl)-BPTTF ligand 1.

Fig. 1 Frontier orbitals of compound 1.

Reaction of 1 with a cis-blocked ethylene diamine Pd(II) complex (enPd(NO₃)₂) in DMSO-d₆ could be monitored by ¹H NMR. The initial spectrum, which corresponds to a complex mixture, progressively converges to a simple set of five signals after four hours at rt (Fig. S2, ESI†), as expected from the formation of a polygon symmetric structure. Attempts to isolate the complex from the NMR solution failed. On the other hand, the use of the more kinetically inert Pt(II) metal complex (enPt(NO₃)₂) only leads to a mixture of complexes even after heating at 100 °C for two weeks in DMSO-d₆. On the contrary, the reaction of 1 with an equimolar amount of cis-blocked complex dpppPt(OTf)₂ in methylene chloride leads to the rapid dissolution of the ligand, and subsequent precipitation of a yellow powder corresponding to the target complex 2, isolated in a 31% yield after filtration (Scheme 3). Molecular square 2 is soluble in different organic solvents (acetone, acetonitrile, nitromethane, DMF, DMSO), and could be characterized through various techniques. Multinuclear NMR (¹H, ³¹P, ¹⁹F) analyses in acetone-d₆ are illustrative of the formation of a discrete supramolecule (Fig. 2). Only two AA′XX′ signals are observed for pyridyl protons (H_α = 8.75 ppm and H_β = 7.28 ppm) in accordance with pyridine units coordinated to dpppPt(OTf)₂.¹³Metal binding is furthermore confirmed by a downfield shift of the CH₂P signal in 2 relative to dpppPt(OTf)₂ complex (Table S1, ESI†). The ¹⁹F spectrum as well as the ³¹P NMR (−13.4 ppm, accompanied by ¹⁹⁵Pt–³¹P satellites) exhibits a single signal (−79.2 ppm), illustrating the occurrence of a single discrete structure. Diffusion-ordered spectroscopy (DOSY) NMR has become a valuable tool for investigating large molecules.¹⁴ A DOSY experiment performed on 2 supports the occurrence of a single polygon species, as illustrated by the alignment of a single set of diffusion coefficient value in the spectrum (Fig. 3, left). NMR data do provide an insight into the symmetry of the polygon, but are not enough to extract polygon size information. The use of mass spectrometry to characterize metal-assembled polygons has been demonstrated,¹⁵ in particular through the use of the high resolving power of a Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer. In our case, an experimental isotopic pattern centered on the m/z 1640 peak is observed in the ESI-FTICR mass spectrum of 2 (Fig. 3, right and Table S2, ESI†). The comparison with a theoretical isotopic distribution calculated for a tricharged species (Δm = 0.33 u) confirms the transfer into the gas-phase of the molecular square structure [(1–Ptdppp)₄·5OTf]³⁺. Other charged species are also observed in the ESI mass spectra of 2 such as [1–Ptdppp·OTf]⁺ at m/z 1192 and [(1–Ptdppp)₂·OTf]³⁺at m/z 745. They correspond to a quarter and a half of a square, respectively (Table S2, ESI†), and their presence can be attributed either to a formation in solution prior to the electrospray process or to fragmentations due to in-source collision induced dissociations.

Scheme 3 Synthesis of square 2.

Fig. 2 ¹H, ³¹P and ¹⁹F NMR signals of 2 (acetone-d₆).

Fig. 3 Left: DOSY experiment of 2 (acetone-d₆), 500 MHz, 298 K; right: ESI-FT-ICR mass spectrum of 2 in acetone. Comparison of the experimental isotopic pattern of the triply charged ion [(1–Ptdppp)₄·5OTf]³⁺ (up) with the calculated isotope pattern (bottom).

Cyclic voltammetry of 2¹⁶ was carried out in CH₂Cl₂/CH₃CN (1/1 v/v) (Bu₄NPF₆ 0.1 M), and allows observation of two redox processes corresponding to the reversible oxidation of the four BPTTF moieties (E¹_ox = 630 mV et E²_ox = 920 mV) which behave independently (Fig. 4). In addition, the Pt(II) corner appears electrochemically inert in this window of potentials.^4b Square 2 presents a good stability under its different oxidation states, as shown by the shape of the CV which remains unchanged upon recurrent scanning of the potentials between 0.3 and 1.1 V vs.AgCl/Ag, a behavior which is identical to previous observations made on the half-molecule, built from the association of two redox BPTTF entities and one Ptdppp complex.¹¹ Thin Layer Cyclic Voltammetry (TLCV)¹⁷ experiments performed on this model system and using ferrocene as an internal reference, have shown that each BPTTF unit behaves independently, displaying two successive 1e⁻ reversible processes for each BPTTF unit upon oxidation.¹¹ Unfortunately, because of adsorption phenomena, TLCV studies failed in the case of compound 2, and did not allow to probe coulometric data. Considering the similar electrochemical CV responses between 2 and the model half-molecule, it is nevertheless reasonable to anticipate that square 2 integrating four BPTTF units, behaves similarly, with each of the four BPTTF units capable to reversibly generate a dicationic species, thus providing a reversible control over the charge state of the cavity. This renders this self-assembled square structure particularly attractive as a very easily oxidisable π-donating macrocyclic system complementary to the extensively studied reducible ones generally based on the bipyridinium unit.

Fig. 4 Deconvoluted cyclic voltammogram of 2 (5 × 10⁻⁴ M), NBu₄PF₆ (10⁻¹ M), v = 100 mV s⁻¹, vs.AgCl/Ag, CH₃CN/CH₂Cl₂ (1/1).

Several electron-rich receptors incorporating TTF units or derivatives (extended-TTF, BPTTF)^6e,11,18 have been shown to present good binding ability for electrodeficient C60. The binding affinity of square 2 for C60 was studied by UV-visible titration. Increasing amounts of C60 were added to a solution of 2 in dichloromethane (Fig. S3, ESI†). The titration curve shows the expected decrease of the absorption band at 395 nm corresponding to disappearing of receptor 2, but did not allow observation of the complex, expected at ca. 600 nm; instead the solution becomes turbid, presumably due to precipitation.

In order to evaluate the cavity size of such polygon, DFT calculations have been performed on the Pd square (1/enPd(NO₃)₂) (Fig. S4, ESI†). From this study, the metal centers show a square planar geometry, in which bond distances and angles are in the usual range. Plane to plane distances of ca. 22 Å were found between facing BPTTF units, and a value of 32.5 Å for the diagonal of the square. This corresponds to a cavity size which is roughly double compared to the parent square built from a 4,4′-bipyridine bridging ligand,¹³ which makes this system one of the largest squares assembled to date, and which justifies the lack of visible interaction between redox units during CV analyses.

Conclusions

A redox-active molecular square which incorporates the highly π-donating bis(pyrrolo)-TTF unit has been synthesized. Besides to constitute the first synthetic response to a TTF-square target structure suggested by Fabre a decade ago in conclusion of a review,¹⁹ this system allows a control over the charge state of the corresponding cavity upon reversible oxidation. As an easily oxidisable macrocycle, it constitutes therefore a promising reciprocal complement to the extensively used reducible π-electron deficient supramolecular or covalent macrocyclic analogues.

Association studies of this metalla-ring with various π-accepting species are underway.

Experimental

Bis-(N-(4-pyridyl)pyrrolo[3,4-d])tetrathiafulvalene 1

Dithiole thione 8 (267 mg, 1.07 mmol) is dissolved in freshly distilled trimethylphosphite (20 mL). The reaction mixture is refluxed for 4 h. After cooling, methanol (30 mL) is added and the resulting solid is separated by filtration (frit No 4), and rinsed successively with methanol, dichloromethane, acetone, and diethyl ether, to produce 1 as an orange powder (61 mg, 0.14 mmol). Yield 26%; mp > 260 °C; C₂₀H₁₂N₄S₄: MALDI-TOF: M^+˙: 435.99; IR (KBr): 1645, 1593, 1511, 1316 cm⁻¹. Though insoluble in common organic solvents, a ¹H NMR analysis could be carried out in DMSO-d₆, by adding BF₃/Et₂O to a suspension of 1 in DMSO-d₆ (Fig. S1, ESI†), producing therefore in situ a more soluble bis(N(pyridyl)-BF₃) derivative. ¹H NMR (DMSO-d₆): 8.83 (d, 4 H, J = 5.0 Hz, H_α), 8.09 (d, 4 H, J = 5.0 Hz, H_β), 7.91 (s, 4 H, H_pyrrole).

Square [(1–Ptdppp)₄]⁸⁺·8OTf⁻2

To a solution of dpppPt(OTf)₂ (21.9 mg, 24 μmol) in CH₂Cl₂ (12 mL) is added ligand 1 (9.6 mg, 22 μmol). The reaction mixture is stirred for 5 days at rt in the darkness. The resulting solid formed along the reaction is filtered off (frit No 4) and rinsed with CH₂Cl₂ (10 mL), to afford a yellow-orange powder (9.3 mg, 1.7 μmol). Yield 31%; mp > 260 °C; ¹H NMR (acetone-d₆): 8.75 (d, 16 H, J = 5.16 Hz, H_α), 7.89 (m, 36 H, H_aro), 7.44 (m, 60 H, H_aro + H_pyrrole), 7.28 (d, 16 H, J = 5.16 Hz, H_β), 3.49 (m, 16 H, CH₂P), 2.35 (m, 8 H, CH₂); ³¹P NMR (acetone-d₆): −13.4 (J_Pt–P = 3049 Hz); ¹⁹F NMR (acetone-d₆): −79.2; C₁₉₃H₁₅₂F₁₅N₁₆O₁₅P₈Pt₄S₂₁: FT-ICR: [1–3OTf]³⁺ (calc.): 1640.4075; [1–3OTf]³⁺ (tr.): 1640.4002.

Acknowledgements

The authors gratefully acknowledge Dr C. Afonso for his help during ESI-FTICR analyses (SM3P platform), Pr P. Richomme and Dr E. Levillain for their assistance in DOSY and TLCV analyses, respectively, and Dr D. Amabilino for fruitful discussions. This work has been financially supported by ANR-PNANO (ANR-07-NANO-030-01).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional NMR, FT-ICR data as well as molecular modeling data. See DOI: 10.1039/c0nj00545b

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