New Polymer Syntheses Part 57 : Thermally Stable New Ferrocene-Polyazomethines , Synthetic Methodology , and Characterization

A new interesting category of higher thermally stable polyazomethines containing ferrocene in the polymers main chain 6a–e was synthesized by solution-polycondensation reaction of 1-(m-formylphenyl)-1󸀠󸀠-(5-formyl-2-methoxyphenyl)-ferrocene monomer 4 with different aliphatic and aromatic diamines. A model compound 5 was synthesized from dialdehyde monomer 4 with aniline and was characterized by elemental and spectral analyses. The desirable resulting polymers were characterized by elemental and spectral analyses, in addition to solubility measurement using different solvents. The thermal properties of these polymers were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)measurements.The redox behaviours were studied for the ferrocene polymers in comparison with both the parent ferrocene monomer and the model compound by using cyclic voltammetry (CV).


Introduction
Metal-containing polymers have emerged as an important category of polymeric materials.The impetus for the development of these materials is based on the premise that polymers-containing metals are expected to possess properties significantly different from those of conventional organic polymers.Ferrocene remains, after about 55 years since its discovery, a molecule of intrigue and continued interest.The sustained interest in ferrocene is in part due to the rich chemistry of iron (II) center and the variety of synthetic methods available for functionalizing the cyclopentadienyl ligands [1][2][3][4][5][6][7][8][9].Ferrocene derivatives with good donor abilities (i.e., nitrogen, sulfur, oxygen, phosphorus, etc.) have attracted much interest, as the coordination of a metal to these heteroatoms generates multicenter molecules which have potential in many different areas.Amongst these N-substituted ligands ferrocenyl Schiff bases play pivotal role in the field of catalysis [10][11][12][13][14][15][16][17].
A class of the particularly attractive polymers is the aromatic polyazomethines or Schiff base polymers because they show good mechanical strength, attractive thermal stability and fiber-forming properties [18].They are synthesized by condensation polymerization of dialdehydes or diketones with diamines to give high-molecular weight polymers.However, poor solubility in organic solvents limited their practical applications in various fields [19].A few groups of researchers have synthesized soluble polyazomethines by introducing alkyl or alkoxy groups into the polymer chains and investigated their structure and physical properties such as electric conductivity and mechanical properties [20].In last years, there has been increasing interest in the synthesis of new polymers with unusual thermal, optical, and mechanical properties.Among these ones, polyazomethines were investigated with respect to their properties including thermal stability [21], optical and electrical response [22][23][24][25], conductivity [26,27], syn-anti isomerization [28], opto(electronic) and liquid crystal properties [29][30][31], and film or fiber forming ability [32,33].The inclusion of ferrocene units into polymeric arrays has attracted much attention due to the electronic donating ability, reversible redox chemistry, steric properties, and ready functionalization of this stable fragment.
The approach chosen in this study deals with the synthesis and characterization of new polyazomethines containing ferrocene moiety in the polymers main chain.The electrochemical behaviours of these polymers were investigated by cyclic voltammetry.Moreover solubility, thermal stability, and morphology of the synthesized polymers had been examined and the data were discussed.

Instrumentation.
Fourier transform Infrared spectrophotometer (FT-IR) spectra were recorded on Nicolet 6700-Thermo Fisher Scientific, by using the KBr pellet technique. 1H-NMR spectra were recorded on a Bruker AM 200 ( 1 H: 200 MHz) spectrometer, and chemical shifts are reported as  values (ppm) relative to internal Me 4 Si.Highresolution MALDI-TOF mass spectrometry (MS) analysis was performed by the MS-service of the Laboratorium für Organische Chemie at ETH Zürich.Elemental analyses were performed by the Mikrolabor of the Laboratorium für Organische Chemie, ETH Zürich.The samples were dried rigorously under vacuum prior to analysis to remove stronglyadhering solvent molecules.High-resolution thermogravimetric analysis (TGA) was performed on a Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) and differential thermal gravimetric (DTG) were carried out in air with Shimadzu DTG-60 at heating rate of 10 ∘ C/min in air.All measurements were carried out in an air-stream under the same conditions.The mass loss was plotted against increasing temperature as well as its first derivative (DTG) that represents the change in decomposition rate.Differential scanning calorimetry (DSC) measurements were performed using the DSC Q1000 differential scanning calorimeter from TA Instruments in a temperature range of −80-+200 ∘ C with a heating rate of 10 ∘ C min −1 .Samples of a total weight ranging between 3 and 10 mg were closed into aluminum pans of 40 L, covered by a holed cap, and analyzed under a nitrogen atmosphere.The glass transition temperature (  ) was taken in the second heating run.CV was measured on a cyclic voltammetric analyzer using glassy carbon electrode (GCE), Bioanalytical System, USA, starting potential 0.0 V and final potential +1.5 V, at Analytical Chemistry Unit ACAL, Assiut University, Egypt.

Polymer Syntheses
2.5.1.General Procedure.In a three-necked flask equipped with a condenser, dry nitrogen inlet, outlet, and dropping funnel, a mixture of 1-(m-formylphenyl)-1  -(5-formyl-2-methox-phenyl)ferrocene (4) (1.0 mmol) and diamine (1.1 mmol) was dissolved in 30 mL absolute EtOH in presence of few drops piperidine.The reaction mixture was stirred in an oil bath at 80 ∘ C under nitrogen for 6-10 h.A solid product separated out during reflux, which was filtered off, washed well with hot ethanol, was dried under high vacuum.

Synthesis of Model
Compound.Before attempting polymerization, model compound 5 was synthesized by the interaction of 1 mole of monomer 4 with 2 moles of aniline in EtOH, piperidine as described in Scheme 2. The structure of this model compound was confirmed by elemental and spectral analysis.The FT-IR spectrum showed a new characteristic absorption band at 1622 cm −1 due to CH=N stretching in addition to other characteristic absorption bands of the Fe-Cp stretching vibrations, at 1074, 975 and 823 cm −1 .The 1 H-NMR spectrum of model compound 5 (in CDCl 3 ) was in accordance with the proposed structure (Figure 1).It showed the molecular ion peak in the matrixassisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum.[35] by the interaction of one mole of monomer 4 with one mole of diamine compounds in presence of few drops piperidine give new Fcpolyazometines 6a-e as described in Scheme 3.

Polymerization and Polymer Characterization. Using solution polycondensation technique
The structures of the resulting polymers were also established from elemental analyses and spectral data.The elemental analyses of all the polymers coincided with the characteristic repeating units of each polymer; the data are included in the experimental part.The FT-IR spectral data of all polyazomethine derivatives showed characteristic absorption band at 3078-3085 cm −1 for C-H aromatic, at 2833-2926 cm −1 for C-H aliphatic 1619-1646 cm −1 due to C=N, bands of the Fe-Cp stretching vibrations at 1079, 970 and 815 cm −1 .In addition, other characteristic absorption bands due to specific groups present in the various polymers.Also the 1 H-NMR spectra of Fc-polyazomethine 6a, 6b,and 6e (in CDCl 3 ) were in accordance with the proposed structures (Figure 2, 1 H-NMR of Fc-polyazomethine 6b).

Solubility.
The solubility of the Fc-polyazomethines 6ad was tested using solvents including dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), chloroform, benzene, hexane, and formic acid.50 mg of the polymer were added to 1 mL of the chosen solvent and the solution analyzed by visual inspection.The results are shown in Table 1.All Fcpolyazometines 6a-e are clearly less soluble in polar, aprotic solvents DMSO and DMF except Fc-polyazomethines which contain aliphatic moieties 6a-c are virtually insoluble in DMSO.All Fc-polyazomethines 6a-e are soluble in hexane and formic acid.All Fc-polyazomethines which contain aliphatic moieties 6a-c are freely soluble in chloroform and benzene except 6c which is partially soluble in chloroform.All Fcpolyazomethines which contain aromatic moieties 6d-e are partially soluble in chloroform and benzene except 6e which is freely soluble in chloroform.The study succeeded in inserting the ferrocene moieties in the polymer main chain.As expected, the presence of such moieties in the polymer backbone will lead to increasing chain packing distances and decreasing interchain interactions, such as hydrogen bonding, thereby increasing the solubility compared with other polyazomethines [36].It can be clarified that the solubility of Fc-polyazomethines 6a-e is based on the aliphatic or aromatic spacers in the polymer backbone.It was found that the presence of aliphatic spacers in the polymer backbone increase the flexibility of the polymer chains and allow the solvent molecules to go through them which increase the probability of solubility.

TGA and DSC Studies.
The thermal behaviour of the Fc-polyazomethines 6a-d was evaluated by TGA in air with heating rate of 10 ∘ C/min.TGA curves show a small weightloss in the range 0.5-4.5% starting at 50 ∘ C until 235 ∘ C except Fc-polyazomethine 6a.Table 2 shows the temperatures for the various % weight loss.The initial decomposition of these polymers (10% loss) is considered to be polymerdecomposition temperature (PDT) [35], which occurred in range 140 ∘ C to 435 ∘ C. The maximum polymer degradation temperature (PDT max ) corresponds to the temperature at which the maximum rate of weight loss occurred.PDT max for Fc-polyazomethines 6a-d were 439 ∘ C, 432 ∘ C, 516 ∘ C, 373 ∘ C, and 402 ∘ C, respectively.The degradation steps involve the scission of azomethine groups; scission of many bonds with the liberation of free shorter chains depending upon the       4).From the TGA data it is noticed that the fast degradation step in all Fc-polyazomethines 6a-e did not start before 277.61 ∘ C. Thus, it is quite acceptable to claim that these polymers are thermally stable till high temperature.
The thermal behaviour of two selected polymers, Fcpolyazomethine 6b and 6e as containing different aliphatic and aromatic spacers in the main chain was studied by differential scanning calorimetry (DSC) which revealed that these compounds have   values at 55 ∘ C and 150 ∘ C, respectively, indicating that Fc-polyazomethine containing aromatic spacers has higher   values than that which contains aliphatic spacers and this may be due to the more flexibility effect of the aliphatic spacers on the polymer chains.

Electrochemical Study.
The electrochemical behaviours of some ferrocene derivatives were investigated by cyclic voltammetry showing interesting behaviour [34,37,38], this encouraged us to study the electrochemical behaviour of some new polyazomethines with ferrocene moiety in the main chain.The electrochemical redox properties of the newly synthesized polyazomethines 6a-e based ferrocene moiety were studied by cyclic voltammetry at room temperature in (dry CH 2 Cl 2 : absolute ethanol, 1 : 1) solutions, using glassy carbon electrode as working electrode, Pt gauze as a counter electrode and Ag/AgCl as a reference electrode, and tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte.The electrochemical data of the investigated compounds were compared with those of Fcmonomer 4 and Fc-model compound 5, the results are summarized in Table 3.The Cyclic voltammetric behaviour of these compounds showed one cathodic peak and the corresponding oxidation peak in the potential range of 250-650 mV at the glassy carbon electrode.The separation of the anodic and the cathodic peak potentials, Δ p , were 188, 134, 170, 37, 164, 164, and 164 mV at 20 mVs −1 for Fcmonomer 4, Fc-model 5, and Fc-polyazomethines 6a-e, respectively.These values are larger than that expected for a reversible two-electron transfer reaction, which is given by 57/z mV, where z is the number of electrons transferred in the process [39], indicating that the irreversibility of the electron-transfer process was maintained under this condition.The formal potential,  0 , taken as the average of  pc and  pa were 343, 413, 443, 401, 385, 385, and 428 mV for Fc-monomer 4, Fc-model 5, and Fc-polyazomethines 6a-e, respectively. 0 shifted to more positive potentials by ca. 100 mV for Fc-polyazomethine 6d in comparing to Fc-monomer 4. This revealed that the reduction of Fcpolyazomethine 6d becomes more easily at the glassy carbon electrode.One couple of redox waves is observed clearly in the cyclic voltammograms for all compounds in the potential range ca.0.00-1.50V.The couple redox waves in the potential range ca.240-530 mV is due to the redox process of ferrocene/ferrocenium + system.Also, there is another anodic wave for all compounds in potential range 540-650 mV, this is due to the conjugation through the conjugated aromatic system.The second anodic wave did not appear in the case of Fc-polyazomethines 6b and c, due to the increasing of methyl spacer which cut the conjugation through the conjugated aromatic system, on the other hand in case of Fc-polyazomethine 6e this anodic wave increased because of the presence of ether aromatic linkage and the lone pairs of electron relay the conjugation through the conjugated aromatic system.

Conclusion
In this research a new interesting series of thermally stable Fcpolyazomethines 6a-e was synthesized via polycondensation reactions of ferrocene-dialdehyde derivative with different diamines.The ferrocene-dialdehyde was prepared through three-step reactions.Ferrocene was converted to ferrocenediester derivative through Gomberg's arylation reaction.Reduction of ferrocene-diester derivative using LAH converted it to ferrocene-dialcohol derivative.The latter was oxidized with activated MnO 2 in dry CHCl 3 to get the ferrocenedialdehyde derivative.The thermal behaviour of these new Fe-polyazomethines showed that the polymer decomposition temperature (PDT) of them did not start before 340 ∘ C for Fe-polyazomethine 6a at 140 ∘ C, which indicated that these new Fe-polyazomethines are thermally-stable till high temperature.Also, the electrochemical properties of these new Fe-polyazomethines were studied in comparison with both Fc-monomer 4 and Fc-model 5 by CV.SEM study of Fe-polyazomethines 6b indicated that its surface possesses a globular structure and for Fe-polyazomethines 6d the surface possesses a globular structure with some coalescence present.The research team is interested in modifying the chemical structure of these polymers aiming to improve their chemical and physical properties.This work is presently in progress.