The protocol followed for the synthesis of the new (2,6-bis(4-(4',5'-bis(methylthio)-[2,2'-bi(1,3-dithiolylidene)]-4-yl)aniline)-iminopyridine) pincer (3, TTF₂-BisIm-Py) is presented in Figure 1.

The starting stannic derivative (6,7-dimethyldithio-2-trimethyltin-tetrathiafulvalene) was synthesized as previously described [25] and was reacted with 1-iodo-4-nitrobenzene under Stille coupling conditions to afford (1-(6,7-dimethyldithio-tétrathiafulvalene)-4-nitro-benzene) (1) in 67% yield [26]. Reduction of the nitro group to the amine was achieved using a mixture of HCl ethanolic solution in the presence of metallic tin [26]. The obtained amine (2) was further used in a condensation reaction with 2,6-diformylpyridine to afford the TTF₂-BisIm-Py pincer (3).

The TTF₂-BisIm-Py pincer was reacted with one equivalent of zinc chloride (ZnCl₂) in a dicloromethane/acetonitrile solvents mixture to afford good quality single crystals of [Zn(TTF₂-BisIm-Py) Cl₂] (4) (Figure 2) that were suitable for the X-ray crystal structure determination.

Single-crystal diffraction analysis revealed that the asymmetric unit (Figure 3) contains half of the molecule of complex (4). Within this complex the zinc ion is pentacoordinated, the coordination sphere being formed by the three nitrogen atoms of the pincer and two chlorines ions. For a more accurate description of the stereochemistry around the zinc ion, the τ parameter (τ = [(α− α')/60] [27], where α and α' are the transoid angles formed by the metal ion and the donor atoms within the basal plane), also known as Adisson’s parameter, was calculated to be τ = 0.47. This indicates an intermediate between the two geometries corresponding number of five, square pyramidal and trigonal bipyramidal.

In the coordination polyhedron, Zn–(N/Cl)ligand distances are in the 2.052(5)–2.351(3) Å range. A selection of relevant bond lengths and angles are given in Table 1. In the complex, the planarity of the ligand is disrupted by the TTF-(SMe)₂ units, as proven by the value of the dihedral angle least-squares planes of the pyridyl group and the dithiole ring (23.55°). In the solid state, each of the two chlorine atoms coordinated to the zinc ion are involved in non-conventional hydrogen bonds with hydrogen atoms from other two neighboring molecules to generate a grid-like sheet represented in Figure 4.

Within this supramolecular layer developed in the ab crystallographic plane, each of the three six-membered aromatic rings of the ligand are stacked in a face to face manner (centroid···centroid 3.56 Å) with two other rings forming the triads highlighted in Figure 5. The crystal packing (Figure 5) reveals a segregation between the TTF units and the rest of the molecule that can be associated with the presence of numerous sulfur-sulfur interactions (intralayer S···S: S2···S3 3.59 Å, S2···S6 3.66 Å, S4···S6 3.58 Å) reinforcing the above-mentioned layers but also connecting them into a 3D network (interlayer S···S: S3···S5 3.73 Å, S5···S5 3.30 Å).

The electronic absorption spectra of the TTF₂-BisIm-Py pincer (3) and of the [Zn(TTF₂-BisIm-Py) Cl₂] (4) complex were recorded in dichloromethane solution (~2·10⁻⁵ M at room temperature; Figure 6). The TTF₂-BisIm-Py pincer exhibits two strong electronic absorption bands at λ = 269 nm and 336 nm which are assigned to the π→π* absorption bands of the TTF and the phenyl ring. The broad band observed in the visible region at λ_max = 433 nm is characteristic of the intramolecular charge transfer transition (ICT) from the highest occupied molecular orbital in the two TTFs to the lowest unoccupied molecular orbital in the electron-accepting pyridyl unit [20,28] and is responsible for the dark red color of this compound. Upon complexation of the pincer with zinc chloride (ZnCl₂), the electronic absorption spectrum presents the same features as the free ligand in the high energy region (<400 nm). However, the ICT transition is red shifted by about 80 nm indicating some effect on the electron acceptor behavior of this ligand upon complexation of zinc chloride. In addition, a shoulder (around 385 nm) is appearing, which is probably due to conformational change (all trans to all cis) of the pincer after complexation with zinc chloride.

The electrochemical behaviors of the new pincer (3) and of the new complex (4) were examined by cyclic voltammetry (CV). The CV was performed in a three-electrode cell equipped with a platinum millielectrode, a platinum wire counter-electrode and a silver wire used as quasi-reference electrode. The electrochemical experiments were carried out under dry and oxygen-free atmosphere (H₂O < 1 ppm, O₂ < 1 ppm) CH₃CN (5.10⁻⁴ M) with Bu₄NPF₆ (TBAP) (0.1 M) as supporting electrolyte. The voltammograms were recorded on an EGG PAR 273A potentiostat with positive feedback compensation. Based on repetitive measurements, absolute errors on potentials were estimated around ±5 mV.

Cyclic voltammetry measurements (Figure 7) in the case of the pincer TTF₂-BisIm-Py (3) show two reversible oxidations waves (at E_1/2¹ = 0.400 V and E_1/2² = 0.758 V vs. SCE). These potentials are anodically shifted from those of the free TTF (0.37 V vs. SCE) [1c)] because of the presence of the electrodeficient pyridine ring. The two oxidation bands, being broad, suggest very small differences between the oxidation potentials of the two TTF units present in the molecule. This behavior is similar to the one observed for bis(TTF) donors, described earlier by Avarvari et al. [29], separated by a triazine ring.

After complexation, the oxidation potentials of the zinc metal complex (4) are slightly positively shifted (about 20 mV and 25 mV), suggesting that the zinc metal cation is affecting the oxidation potential of the TTF moieties by decreasing the electron density on the TTF units, a fact also suggested by the electronic absorption spectra. As a consequence of these electrochemical behaviors, we expect radical cation salts of both the TTF pincer and the corresponding zinc metal complex to form readily upon electrochemical oxidation into air-stable crystals [15].

Details about data collection and solution refinement are given in Table 2. X-ray diffraction measurements were performed on a Bruker Kappa CCD diffractometer, operating with a Mo-Kα (λ = 0.71073 Å) X-ray tube with a graphite monochromator. The structures were solved (SHELXS-97) by direct methods and refined (SHELXL-97) by full matrix least-square procedures on F². All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations but not refined. CCDC reference number: 871185 for 4. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

All the reactions were carried out under Argon and using HPLC solvents. Nuclear magnetic resonance spectra were recorded on a Bruker Avance DRX 500 spectrometer (operating at 500.04 MHz for 1H and 125 MHz for ¹³C) and Bruker Avance DRX 300 automatic spectrometer (operating at 300 MHz for 1H, and 75 MHz for ¹³C). Chemical shifts are expressed in parts per million (ppm) downfield from external TMS. The following abbreviations are used: s, singlet; d, doublet; t, triplet. MALDI- TOF MS spectra were recorded on Bruker Biflex-IIITM apparatus, equipped with a 337 nm N2 laser. Elemental analyses were recorded using a Flash 2000 Fisher Scientific Thermo Electron analyzer.

1-Iodo-4-nitro-benzene (0.44 g, 1.78 mmol), 4-(trimethylstannyl)-4',5'-bis(methylthio)-tetrathiafulvalene (0.98 g, 2.14 mmol), and Pd(PPh₃)₄ as catalyst (10% mmol) were mixed in 40 mL of dry toluene and refluxed overnight under argon. The mixture was filtered over Celite and silica, washed with toluene and dichloromethane and dried under vacuum. The residue was purified by column chromatography (pentane/dichloromethane 1/1) to give 0.50 g of dark blue solid, yield: 67%. ¹H-NMR (300 MHz, CDCl₃) δ/ppm: 8.22 (d, 2H, J³ = 9 Hz), 7.53 (d, 2H, J³ = 9 Hz), 6.78 (s, 1H), 2.45 (s, 3H), 2.44 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃) δ/ppm: 147.0, 138.0, 133.7, 127.7, 126.6, 124.3, 118.3, 112.2, 109.6, 19.2; MS (MALDI-TOF): m/z: 416.9 (M_th = 416.91)

To a solution of compound 1 (0.25 g, 0.59 mmol in 20 mL ethanol), Sn (0.14 g, 1.2 mmol) and HCl 35% (0.31 mL) were added and the mixture was refluxed for 4 h under argon. After the removal of the solvent, the crude was solubilised in ethyl acetate and washed several times with a NaOH solution (1 M) and water. The residue was purified by column chromatography with dichloromethane as eluent to give 0.23 g of an orange solid, yield: 99%. ¹H-NMR (300 MHz, CDCl₃) δ/ppm: 7.18 (d, 2H, J³ = 8.7 Hz), 6.62 (d, 2H, J³ = 8.4 Hz), 6.26 (s, 1H), 3.79 (broad s, 2H) 2.44 (s, 3H), 2.43 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃) δ/ppm:146.9, 136.2, 127.6, 127.5, 122.8, 115.4, 114.9, 109.4, 106.3, 29.7, 19.2; MS (MALDI-TOF): m/z: 386.9 (M_th = 386.94).

Compound 2 (0.115 g, 0.3 mmol) was solubilised in 50 mL ethanol, 2,6-Pyridinedicarboxaldehyde (0.019 g, 0.14 mmol) was added, followed by 3 drops of acetic acid and the mixture was refluxed overnight. After cooling the reaction, a red precipitate was formed which was filtered, washed with additional ethanol and dried to afford 116 mg of a red solid, yield: 94%. ¹H-NMR (300 MHz, CDCl₃) δ/ppm: 8.67 (s, 2H), 8.29 (d, 2H, J³ = 7.7 Hz), 7.95 (t, 1H, J³ = 7.7 Hz), 7.46 (d, 4H, J³ = 8.4 Hz), 7.31 (d, 4H, J³ = 8.4 Hz), 6.55 (s, 2H), 2.45 (s, 6H), 2.44 (s, 6H); ¹³C-NMR (75 MHz, CDCl₃) δ/ppm:160.2, 154.4, 150.6, 135.4, 130.9, 127.2, 123.5, 121.7, 113.2, 19.2; λ_max(CH₂Cl₂)/nm 269 (ε/dm³ mol⁻¹ cm⁻¹ 49260), 336 (60870), 433 (12670). Selected IR bands (cm⁻¹): 2916, 1621, 1567, 1454, 1411, 1212, 958, 835, 769. MS (MALDI-TOF): m/z: 873.6 (M_th = 872.89).

In a test tube, a solution of the pincer 3 (20 mg, 0.023 mmol) in CH₂Cl₂ (5 mL) was mixed with a solution of ZnCl₂ (3.1 mg, 0.023 mmol) in CH₃CN (3 mL) and ultrasonicated for 2 min. On top of the resulting solution a layer of 1:1 diethyl-ether mixture was added which led to the formation of good quality single crystals of complex 3 after 1 week. Yield: 18 mg (77%). λ_max(CH₂Cl₂)/nm 267 (ε/dm³ mol⁻¹ cm⁻¹ 40750), 333 (42040), 385 sh, 550 (7290). Selected IR bands (cm⁻¹): 2916, 1622, 1568, 1457, 1418, 1215, 966, 888, 773.