Electroactive Bisiminopyridine Ligands: Synthesis and Complexation Studies

The condensation reaction of


Introduction
Tetrathiafulvalene (TTF) and its derivatives have attracted much interest because of their electron-donating ability and their attractive reversible redox properties.They have therefore been widely used as donor components in the preparation of molecular conductors and superconductors [1][2][3].
In order to obtain electroactive transition metal complexes with original structural and electronic properties, we report herein the synthesis of the TTF 2 -BisIm-Py (3) that can act as an (N,N,N) pincer.The TTF based donor with monoaniline moiety, (4-(6,7-dimethyldithio-tetrathiafulvalene)-aniline) (2), has been chosen for the Schiff base condensation reaction with 2,6-diformylpyridine.The reaction of this TTF based tridentate pincer with zinc metal cation afforded a new electroactive metal complex.
The TTF 2 -BisIm-Py pincer was reacted with one equivalent of zinc chloride (ZnCl 2 ) in a dicloromethane/acetonitrile solvents mixture to afford good quality single crystals of [Zn(TTF 2 -BisIm-Py) Cl 2 ] (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.

UV-Vis Spectroscopy
The electronic absorption spectra of the TTF 2 -BisIm-Py pincer (3) and of the [Zn(TTF 2 -BisIm-Py) Cl 2 ] (4) complex were recorded in dichloromethane solution (~2•10 −5 M at room temperature; Figure 6).The TTF 2 -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 (ZnCl 2 ), 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.Crystals 2012, 2 343

Electrochemistry
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 (H 2 O < 1 ppm, O 2 < 1 ppm) CH 3 CN (5.10 −4 M) with Bu 4 NPF 6 (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 TTF 2 -BisIm-Py (3) show two reversible oxidations waves (at E 1/2 1 = 0.400 V and E 1/2 2 = 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].

X-Ray Structure Determinations
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 F 2 .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.

General
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 13 C) and Bruker Avance DRX 300 automatic spectrometer (operating at 300 MHz for 1H, and 75 MHz for 13 C).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.

Conclusions
As a first step towards the realization of multifunctional materials, a new TTF based pincer has been synthesized and its chelating ability has been investigated in respect to transition metal cations such as Zn(II).The crystal structure of the newly prepared electroactive zinc complex reveals that the TTF is neutral and the zinc cation is pentacoordinated.The two chlorines are involved in a set of hydrogen bonds giving rise to a 2D supramolecular grid arrangement.The electrochemical behavior of both the ligand and the neutral metal complex suggests that these compounds are useful candidates for the preparation of chemically and/or electrochemically oxidized radical cation crystalline salts.Further studies will be performed in order to investigate the complexation ability of this novel electroactive pincer on other metal cations such as Cu(II), Fe(II), Co(II), etc.

Figure 3 .
Figure 3.The crystal structure of complex 4 with the labeling scheme of the atoms.Hydrogen atoms have been omitted for clarity.

Figure 5 .
Figure 5.The crystal packing in 4 showing the non-conventional hydrogen bonds (red dashed lines) and S•••S (yellow dashed lines) interactions.

Figure 6 .
Figure 6.Electronic absorption spectra of the pincer 3 and the zinc complex 4.

Table 2 .
Crystal Data and Structure Refinement for compound 4.