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

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This journal iscThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 165–168 165

Cite this: New J. Chem ., 2011, 35, 165–168

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

Jean-Yves Balandier, Marcos Chas, Se´bastien Goeb, Paul I. Dron, David Rondeau, Ahmed Belyasmine, Nuria Gallego and Marc Salle´*

Received (in Montpellier, France) 11th July 2010, Accepted 7th September 2010 DOI: 10.1039/c0nj00545b

The synthesis and full characterization of a redox-activep-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.

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).^1,2 Ferrocene 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,⁴and in this case, they involve reducible ligands such as 4,4⁰-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 highlyp-donating side-walls are unknown to date, and would constitute a challenging reciprocal complement to previous p-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 p-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 esteps.

We propose here the preparation of a redox-active p-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.⁷These systems have not been selected to our purpose since they exist as twoZ/Eisomers (Scheme 1), known to be very diﬃcult to separate and which undergo aZ/Eisomerization after oxidation or protonation.⁸We have instead focused our interest on the non-isomerizable derivative1as a key redox-active ligand.

This system involves the bis(pyrrolo)TTF skeleton (BPTTF)⁹as a basic framework, a unit which occupies a growing place among electroactive supramolecular architectures.^6a,10

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-thione¹²and involving a three step procedure.^9b Scheme 1 (a) The two isomeric ZandE forms in bis-substituted TTF; (b) N,N⁰-bis-substituted BPTTF.

Scheme 2 Synthesis of bis(pyridyl)-BPTTF ligand1.

Universite´ d’Angers, CNRS UMR 6200, MOLTECH-Anjou, 2 Bd Lavoisier, 49045, Angers Cedex, France.

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Tel: +33 241735439

wElectronic supplementary information (ESI) available: Additional NMR, FT-ICR data as well as molecular modeling data. See DOI:

10.1039/c0nj00545b

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166 New J. Chem., 2011, 35, 165–168 This journal iscThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

The target bis-pyridyl BPTTF derivative1is obtained by self- coupling of thione 3 in reﬂuxing trimethylphosphite. DFT calculations (Gaussian^s 2003, Becke3LYP (B3LYP)) have been carried out on compound 1 (Fig. 1). The electronic density in the HOMO of1is located on the central donating skeleton and on the lateral pyrrole motifs, illustrating the strong p-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 highp-donating ability.¹⁰

Reaction of 1 with a cis-blocked ethylene diamine Pd(II) complex (enPd(NO₃)₂) in DMSO-d₆could 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, ESIw), 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 1C 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 complex2, isolated in a 31% yield after ﬁltration (Scheme 3). Molecular square2is 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-d6are illustrative of the formation of a discrete supramolecule (Fig. 2). Only two AA⁰XX⁰ signals are observed for pyridyl protons (H_a= 8.75 ppm and H_b= 7.28 ppm) in accordance with pyridine units coordinated to dpppPt(OTf)₂.¹³ Metal binding is furthermore conﬁrmed by a downﬁeld shift of the CH2P signal in 2 relative to dpppPt(OTf)2 complex

(Table S1, ESIw). The¹⁹F spectrum as well as the³¹P 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,¹⁵in 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 them/z1640 peak is observed in the ESI-FTICR mass spectrum of2(Fig. 3, right and Table S2, ESIw). The comparison with a theoretical isotopic distribution calculated for a tricharged species (Dm = 0.33 u) confirms the transfer into the gas-phase of the molecular square structure [(1–Ptdppp)45OTf]³⁺. Other charged species are also observed in the ESI mass spectra of2 such as [1–PtdpppOTf]⁺atm/z1192 and [(1–Ptdppp)₂OTf]³⁺

at m/z 745. They correspond to a quarter and a half of a square, respectively (Table S2, ESIw), 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.

Cyclic voltammetry of 2¹⁶ was carried out in CH₂Cl₂/ CH3CN (1/1 v/v) (Bu4NPF60.1 M), and allows observation of two redox processes corresponding to the reversible oxidation of the four BPTTF moieties (E¹ox= 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.^4bSquare2presents 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 Vvs.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)¹⁷experiments performed on this model system and using ferrocene as an internal reference, have Fig. 1 Frontier orbitals of compound1.

Scheme 3 Synthesis of square2.

Fig. 2 ¹H,³¹P and¹⁹F NMR signals of2(acetone-d6).

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This journal iscThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 165–168 167 shown that each BPTTF unit behaves independently, displaying

two successive 1ereversible processes for each BPTTF unit upon oxidation.¹¹ Unfortunately, because of adsorption phenomena, TLCV studies failed in the case of compound2, and did not allow to probe coulometric data. Considering the similar electrochemical CV responses between2and the model half-molecule, it is nevertheless reasonable to anticipate that square2integrating 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 p-donating macrocyclic system complementary to the extensively studied reducible ones generally based on the bipyridinium unit.

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 square2for C60 was studied by UV-visible titration. Increasing amounts of C60 were added to a solution of2in dichloromethane (Fig. S3, ESIw). The titration curve shows the expected decrease of the absorption band at 395 nm

corresponding to disappearing of receptor2, but did not allow observation of the complex, expected atca.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, ESIw). 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 ofca.22 A˚ were found between facing BPTTF units, and a value of 32.5 A˚ for the diagonal of the square. This corresponds to a cavity size which is roughly double compared to the parent square built from a 4,4⁰-bipyridine bridging ligand,¹³which 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 p-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,¹⁹this 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 reduciblep-electron deﬁcient supramolecular or covalent macrocyclic analogues.

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

Experimental

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

Dithiole thione8(267 mg, 1.07 mmol) is dissolved in freshly distilled trimethylphosphite (20 mL). The reaction mixture is Fig. 3 Left: DOSY experiment of2(acetone-d6), 500 MHz, 298 K; right: ESI-FT-ICR mass spectrum of2in acetone. Comparison of the experimental isotopic pattern of the triply charged ion [(1–Ptdppp)45OTf]³⁺(up) with the calculated isotope pattern (bottom).

Fig. 4 Deconvoluted cyclic voltammogram of 2 (5 10⁴ M), NBu4PF6(10¹M),v= 100 mV s¹,vs.AgCl/Ag, CH3CN/CH2Cl2

(1/1).

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168 New J. Chem., 2011, 35, 165–168 This journal iscThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

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 produce1as an orange powder (61 mg, 0.14 mmol). Yield 26%; mp 4 260 1C; C20H12N4S4: MALDI-TOF: M⁺: 435.99; IR (KBr): 1645, 1593, 1511, 1316 cm¹. 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, ESIw), 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_a), 8.09 (d, 4 H, J = 5.0 Hz, H_b), 7.91 (s, 4 H, H_pyrrole).

Square[(1–Ptdppp)₄]⁸⁺8OTf2

To a solution of dpppPt(OTf)₂(21.9 mg, 24mmol) in CH₂Cl₂ (12 mL) is added ligand 1 (9.6 mg, 22 mmol). 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 mmol). Yield 31%; mp 4 260 1C; ¹H NMR (acetone-d₆): 8.75 (d, 16 H, J = 5.16 Hz, H_a), 7.89 (m, 36 H, Haro), 7.44 (m, 60 H, Haro + Hpyrrole), 7.28 (d, 16 H,J= 5.16 Hz, Hb), 3.49 (m, 16 H, CH2P), 2.35 (m, 8 H, CH2);³¹P NMR (acetone-d6):13.4 (JPt–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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