Tetrathiafulvalene-based azine ligands for anion and metal cation coordination

Summary The synthesis and full characterization of two tetrathiafulvalene-appended azine ligands, namely 2-([2,2’-bi(1,3-dithiolylidene)]-4-yl)-6-((2,4-dinitrophenyl)hydrazono)methyl)pyridine (L1) and 5-([2,2’-bi(1,3-dithiolylidene)]-4-yl)-2-((2,4-dinitrophenyl)hydrazono)methyl)pyridine (L2) are described. The crystal structure of ligand L1 indicates that the ligand is completely planar with the presence of a strong intramolecular N3–H3···O1 hydrogen bonding. Titration experiments with inorganic anions showed that both ligands are suitable candidates for the sensing of fluoride anions. Ligand L2 was reacted with a Re(I) cation to yield the corresponding rhenium tricarbonyl complex 3. In the crystal structure of the newly prepared electroactive rhenium complex the TTF is neutral and the rhenium cation is hexacoordinated. The electrochemical behavior of the three compounds indicates that they are promising for the construction of crystalline radical cation salts.


Introduction
Tetrathiafulvalene (TTF) is known to have excellent electrondonating properties resulting in stable radical cation (TTF •+ ) and dication (TTF 2+ ) species from two sequential and reversible oxidation processes. The huge interest in the synthesis of TTF and its very numerous derivatives [1] has been initiated by the high electrical conductivity discovered in a chloride salt of TTF [2] and metallic behavior in the charge-transfer complex with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) [3]. These systems have played a major role for the preparation of molecular materials designed for various applications. They have been, for example, used as electron donor molecules to prepare electrically (super)conducting crystalline materials [4][5][6][7], as solar energy systems [8,9] or even as donor moieties in nonlinear optical (NLO) materials [10,11]. In the last decades one of the biggest challenges in materials science is devoted to the preparation of multifunctional molecular materials that can potentially exhibit, in solution and/or in solid state, synergy or coexistence between two or more different physical properties. In order to address this issue, many efforts have been devoted to the association of a binding or coordinating unit to the redoxactive TTF moiety. This strategy has led for example, in solid state, to the preparation of electroactive metal complexes that combine magnetic and electrical properties [12][13][14][15][16][17]. In solution, TTF-based redox-responsive receptors for neutral and/or charged guest sensing applications have been prepared [18][19][20][21][22]. On this ground, chemosensors capable of recognizing anionic and/or cationic species constitute an important area of increasing research in supramolecular chemistry, considering the ubiquitous properties of both anions and metal cations [23][24][25][26][27]. In fact, anions are involved in a large number of biological and chemical processes [28][29][30][31]. Note that fluoride is of particular interest among the other inorganic anions because of its both beneficial (e.g., preventing dental caries and treatment of osteoporosis) and detrimental (e.g., fluorisis) effects on human health [32][33][34]. Many TTF derivatives have been used to coordinate or to bind separately metal cations [35] or inorganic anions [36], however only few examples were used for both inorganic anions and metal cations coordination [37].
Herein we report the synthesis and electronic properties of two new multifunctional TTF-based azine ligands that integrate distinctive functional groups as depicted in Scheme 1 and thus capable of coordinating both metal cations as well as inorganic anions. The sensing studies for inorganic anions are discussed. Metal cation complexation studies of the new ligands afforded the formation and the structural characterization of a neutral rhenium complex.

Results and Discussion
Synthesis of the ligands L1 and L2 The protocol followed for the synthesis of the new azine ligands L1 and L2 is summarized in Scheme 2. The reaction of 6-bromo-2-pyridinecarboxaldehyde or 5-bromo-2-pyridinecarbox-

Crystal structure description
Details about data collection and structure refinement are given in Table 1. Crystallographic data for the structural analysis have been deposited within the Cambridge Crystallographic Data Centre, CCDC 1055120 (ligand L1) and CCDC 1055119 (complex 3).
In the solid state, the packing arrangement in L1 is characterized by the self-assembly between the TTF donor unit and the dinitrophenyl acceptor unit forming head-to-tail dimers. Moreover, the plane-to-plane distance between the donor and acceptor moieties is d = 3.39 Å, showing an evident overlap that is comparable to the reported intermolecular charge-transfer complexes [35]. This overlap develops along a-axis forming infinite columns ( Figure 2). These columns are linked together along c-axis through hydrogen bonding occurring between TTF-C-H and NO 2 (TTF-C-H1···O3-NO 2.493(6) Å).
In the b-direction, the columns of stacked head to tail molecules are connected laterally through S···O heteroatom contacts (d = 3.16 Å, Figure S1, Supporting Information File 1) resulting in alternating stacks with a "zig zag" like manner with an angle of rotation of 135.6° ( Figure 3).

UV-visible absorption spectroscopy
The UV-visible absorption spectra of the ligands L1 and L2 were recorded in a mixture of dichloromethane/acetonitrile solution (9/1, v/v, C = 2 × 10 −5 M) at room temperature ( Figure 4). The two ligands exhibit strong electronic absorption bands between λ = 300 nm and 450 nm which are assigned to the π→π* and n→π* absorption bands resulting from the different units of the two ligands (TTF moiety, pyridyl ring and the dinitrophenylhydrazone group). As compared to ligand L1, L2 exhibits an additional absorption band around λ = 516 nm which is attributed to an intramolecular charge transfer (ICT) excitation from the TTF donor moiety to the dinitrophenylhydrazone accepting group. These results from a strong π-electronic delocalization that occurs in ligand L2 leading to a resonance structure that involves the C=N hydrazone bond as it can be seen in ( Figure S2, Supporting Information File 1).

Theoretical calculations
Theoretical calculations based on density functional theory (DFT) methods have been performed with the Gaussian 09 program [40]. Becke's three-parameter gradient-corrected functional (B3LYP) with 6-31G (d) basis in vacuum was used for   full geometry optimization of the two ligands. The resulting frontier molecular orbitals ( Figure 5) for ligands L1 and L2 indicate that the electron density of the highest occupied molec-ular (HOMO) orbitals develop exclusively on the TTF fragment. The LUMO orbital for ligand L1 is essentially distributed on the nitrophenylhydrazino group with a small participation of the pyridyl ring, while for ligand L2 it is distributed on the π-extended system with a small participation of the external ethylenic atom of the TTF moiety which is confirming the good electronic conjugation in this ligand. The same behavior is observed for the TTF pyridine carboxaldehyde precursors 1 and 2 ( Figure S3, Supporting Information File 1).

Cyclic voltammetry
The electrochemical behavior of the electroactive precursors 1 and 2 as well as of ligands L1 and L2 was investigated by cyclic voltammetry (Figure 6 and Table 3). The measurements in the case of precursors 1 and 2 show two reversible oxidations at E 1ox = +0.26 V, E 2ox = +0.75 V and E 1ox = +0.32 V, E 2ox = +0.77 V vs Ag/Ag + , respectively, that are anodically shifted when compared to the ones of the free TTF because of the presence of the electron deficient pyridinecarboxaldehyde moiety. In addition, E 1ox of 2 is anodically shifted when  compared with E 1ox of 1, indicating a strong π-electron conjugation in precursor 2. As for 1 and 2, ligands L1 and L2 show two reversible oxidations at E 1ox = +0.20 V, E 2ox = +0.70 V and E 1ox = +0.25 V, E 2ox = +0.70 V vs Ag/Ag + , respectively) that are cathodically shifted when compared to the ones of 1 and 2 indicating that the pyridine-hydrazone group is less electron deficient than the corresponding pyridinecarboxaldehyde. In addition, E 1ox of L2 is also anodically shifted when compared with E 1ox of L1 because of the strong π-electron conjugation in ligand L2 and this is in agreement with the bathochromic shift observed for L2 in the UV-visible absorption spectra.

Sensing properties of the azine ligands for anions
It is known that phenylhydrazone groups are able to act as optical sensors particularly for fluoride anions [41][42][43][44][45]. Thus, the colorimetric sensing abilities of the two ligands L1 and L2 were investigated by adding various anions such as hydrogensulfate, acetate, iodine and fluoride (used as tetrabutylammonium salts) in a mixture of dichloromethane/acetonitrile (9:1, v/v). Addition of increasing amounts of F − causes a dramatic change in color from yellow to violet that can be observed by the naked eye ( Figure S4 in Supporting Information File 1), which is accompanied by the formation of a new broad absorption band centered at about 510 nm in the case of ligand L1. In the case of ligand L2, addition of F − (Figure 7) causes also a dramatic change in color from light orange to violet that can be observed by the naked eye ( Figure S5 in Supporting Information File 1), that is accompanied by a decrease of the intense absorption band centered at about 380 nm and the increase of the ICT absorption band centered around 540 nm. This change is likely due to the deprotonation of the hydrazone nitrogen which causes an enhancement of charge transfer from the TTF unit and the deprotonated nitrogen to the electron poor 2,4-dinitrophenyl moiety [36]. A remarkable feature is the occurrence of a quite well defined isosbestic point at 420 nm and 447 nm for L1 and L2, respectively, indicating that L1 or L2 coexist with only one species upon addition of TBAF. Note that upon addition of other inorganic anions such as bromide, chloride or hydrogensulfate we have observed a negligible absorption changes while in the case of acetate anion a moderate absorption changes are obtained ( Figures S6 and S7 in Supporting Information File 1) [46]. Treatment of an electrolytic solution of ligand L1 or L2 with an increasing amount of fluoride anion (tetrabutylammonium fluoride trihydrate in a CH 2 Cl 2 /CH 3 CN mixture) involve the presence, as previously seen for fluoride anion sensing [47], and mainly on the first cycle, of the pre-wave superimposed on the wave of oxidation of the ligands. We clearly see on the second cycle a negligible change of the oxidation potential of the ligand which is very likely because of the large distance between the TTF and the fluoride coordinating unit ( Figure S8 in Supporting Information File 1).
In order to get further supports to the observed optical sensing and to get deeper insights into the interactions between L1 or L2 and fluoride, 1 H NMR titration experiments were performed in DMSO-d 6 ( Figure 8 and Figure S9 in Supporting Information File 1). The measurements indicate that the N-H peak disappears after addition of one equivalent of TBAF while the other aromatic proton resonances of L1 or L2 exhibit an upfield shift. These results tend to be consistent with the deprotonation of the N-H group and the delocalization of the negative charge over the π-conjugated system as previously observed TTF dinitrophenylhydrazone [36]. Note that there is no change in the 1 H NMR spectrum observed for other anions.

Synthesis and crystal structure of a neutral rhenium complex
Few metal complexes based on ruthenium cations have been previously prepared with dinitrophenylazine type ligands [48,49]. These reports indicate that the pyridinedinitrophenylazine type ligands are good candidates for the formation of metal complexes. We have therefore investigated the complexation of L1 and L2 with various metal cations and we succeeded in the crystallization of a neutral rhenium metal complex with ligand L2. Thus, the equimolar reaction between L2 and the [Re(CO) 5 Cl] precursor performed in refluxing toluene, under no light and inert atmosphere, afforded a mononuclear neutral complex 3 described as [ReL2(CO) 3 Cl]·0.5H 2 O as a dark precipitate [50]. Single crystals of 3 were obtained by recrystallization from acetone/hexane solution. Details about data collection and structure refinement are given in Table 1. As expected, the resulting metal complex 3 is composed of one ligand L2 coordinated to Re(CO) 3 Cl fragment through two nitrogen atoms of the pyridine and the C=N hydrazone group (Figure 9). Upon complexation, the ligand acquires a cis-conformation of the hydrazinopyridine moiety in contrast to the trans-conformation observed for free ligand L1 (see Figure 1 and Figure 9). Within the complex, the rhenium center is surrounded by the bidentate chelating L2 ligand, three carbonyl ligands arranged in a facial fashion, and a chlorine atom and its coordination sphere presents the expected, although slightly distorted, octahedral geometry. The angle formed by the rhenium center and N atoms equals to 73.6(5)° which is smaller than the angle of 90°a dopted in an ideal octahedron. In addition, in the complex the C-Re-C angles identified as C19-Re1-C20, C19-Re-C21, C20-Re-C21 are 87.7°, 91.7° and 90.9°, respectively, which are close to 90° indicating that CO ligands are almost linearly coordinated to the rhenium(I) cation. The length of the two Re-N bonds are (N1-Re1 2.20(1) Å) and N2-Re1 2.18(1) Å, and the formal double bond character C=N is maintained (C12-N2 1.32(2) Å). All three Re-CO bond lengths are very close, and the Re-C-O angles present minor deviations from linear structure, values ranging from 165(2)° to 177(2)° (  Table 4).
The UV-visible absorption spectrum of the rhenium complex 3 recorded in a mixture of dichloromethane/acetonitrile (9:1, v/v) at room temperature (c 1.1 × 10 −4 M) presents the same features as the free ligand L2 with a red shift of the different absorption bands ( Figure S10 in Supporting Information File 1). The ICT trasnsition suffers a bathochromic shift by about 100 nm as compared with the free ligand which indicates an increase of the electron acceptor effet of the ligand upon complexation with rhenium which acts as strong Lewis acid.
After complexation, the redox behavior of the TTF moiety is maintained. We note a positive shift of the two oxidation potentials in the case of complex 3 by about 90 mV and 100 mV ( Figure S11 in Supporting Information File 1). This increase of the oxidation potential suggests that the rhenium fragment is acting as an electron acceptor by decreasing the electron density on the TTF unit. This behavior is in agreement with the electronic absorption experiments and confirms the strong electronic conjugation in ligand L2. The electrochemical behavior observed for L2 and its corresponding rhenium complex 3 indicate that this compounds are valuable candidates for the electrochemical formation of air-stable radical cation crystalline salts [16].

Conclusion
Two multifunctional ligands which associate an electrondonating TTF unit with an electron-accepting dinitrophenyl group as well as a coordinating pyridine azine moiety were successfully synthesized. Ligand L2 exhibit a strong electronic conjugation between the donor and the acceptor resulting in the occurrence of an intramolecular charge transfer (ICT) band between the two fragments. Single crystals of ligand L1 have been obtained and its crystal structure indicates the ligand is completely planar with the occurrence of a strong intramolecular as well as intermolecular hydrogen bonding. Inorganic anions titration experiments showed that the two ligands are suitable candidates for the sensing of fluoride anions. Metal cation-coordination experiments afforded the obtaining of a neutral electroactive rhenium(I) complex. The crystal structure of this complex indicates the formation of dimers that are connected through strong hydrogen bonding. The electrochemical behavior of both the ligands and the neutral rhenium(I) complex suggests that crystalline radical cation salts can be readily obtained upon chemical and/or electrochemical oxidation. The complexation abililty of the two novel electroactive ligands toward transition metal cations such as Cu(II), Fe(II), Co(II), etc is in progress.

Experimental
General information NMR spectra were recorded on a Bruker Avance DRX 300 spectrometer operating at 300 MHz for 1 H NMR and 75 MHz for 13  X-ray single-crystal diffraction data for complex 3 were collected at 180 K on an Agilent SuperNova diffractometer equipped with Atlas CCD detector and mirror monochromated micro-focus Cu K α radiation (λ = 1.54184 Å). For ligand L1, crystal data were collected at 293 K on a Bruker KappaCCD diffractometer, equipped with a graphite monochromator utilizing MoKα radiation (λ = 0.71073Å). The two structures were solved by direct methods, expanded and refined on F 2 by full matrix least-squares techniques using SHELX97 programs (G.M. Sheldrick, 1998). All non-H atoms were refined anisotropically and the H atoms were included in the calculation without refinement. Multiscan empirical absorption was corrected using the SADABS program (Bruker AXS area detector scaling and absorption correction, v2008/1, Sheldrick, G.M., (2008)) for ligand L1 and using the CrysAlisPro program (CrysAlisPro, Agilent Technologies, V1.171.37.35g, 2014) for complex 3. For ligand L1, the structure refinement showed disordered electron density which could not be reliably modeled and the program PLATON/SQUEEZE were used to remove the scattering contribution corresponding to dimethyl sulfoxide solvent from the intensity data. The assumed solvent composition (3 DMSO per asymmetric unit) was used in the calculation of the empirical formula, formula weight, density, linear absorption coefficient and F(000). For complex 3, the largest difference peak and hole of 2.29 eÅ −3 observed is relatively high and it can be attributed to bad absorption correction. As the Gaussian absorption method does not improve the refinement, we have chosen the empirical absorption correction. This residual electronic density is located around the Re metal ion.

Supporting Information
Supporting Information File 1 Additional analytical data.