RETRACTED ARTICLE: Synthesis and characterization of trimeric phosphazene based ionic liquids with tetrafluoroborate anions and their thermal investigations

The quaternized compounds (PzIL1–9) reacted with sodium tetrafluoroborate (NaBF4) to generate phosphazene based ionic liquids (PzILs), PzIL1a–9a. The newly synthesized ionic compounds (PzIL1a–9a) were verified using elemental CHN analyses and functional and spectroscopic (FTIR and 1H, 13C, 31P-NMR) analyses techniques. The thermal properties of PzIL1a–9a were investigated using thermogravimetric analysis (TGA). According to the initial decomposition temperature values calculated based on the TGA thermograms, PzIL7a (213 °C) was recognized to be more thermally stable than the other PzILs studied. PzIL1a–9a exhibited good solubility in the water and demonstrate a typical dielectric relaxation behavior, conductivity levels for both low and high-frequency regions. AC conductivity mechanisms and dielectric relaxation behavior of each sample are investigated by fabricating parallel plate capacitors.


Measurements.
Elemental analyses (C, H, N) were performed using a LECO CHNS-932 elemental analyzer. The Fourier transform infrared (FTIR) spectra of all the PzILs were monitored with a Jasco 430 FT-IR Spectrometer in KBr pellets in the 4,000-400 cm −1 region. 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra were recorded on Agilent 600 MHz Premium COMPACT NMR spectrometer (tetramethylsilane (TMS) as an internal standard for 1 H and 85% H 3 PO 4 as an external standard for 31  . Free cyclotriphosphazene bases and cyclic trimers fully substituted by aliphatic and aromatic substituents with terminal tertiary amino functions have been synthesized and subsequently quaternized by treatment with methyl iodide. The quaternized derivatives (PzIL1-9) were obtained according to published papers 20,21 .       13    Dielectric measurements. The dielectric properties were measured by using a parallel plate capacitor with an impedance analyzer (Hewlett Packard 4194A) in a frequency range between 10 2 and 10 7 Hz. All measurements were performed at room temperature. To investigate their dielectric properties, all ILs solved in ethyl acetate and ultrasonicated for 30 min to provide a homogeneous solution. ILs were sandwiched by using precleaned Indium tin oxide (ITO) coated glasses. Teflon spacer (t = 0.075 mm) was used to fix the thickness of ILs. The electrode area of all samples has remained the same.

Results and discussion
Syntheses and characterization. PzILs were prepared by reaction of individual polyiodo ionic compounds (PzIL1-9) and NaBF 4 aqueous solutions at ambient temperature. However, as PzILs (PzIL1a-9a) obtained were dissolved in water, the solution mixture was evaporated to dryness and extracted with acetone. PzILs that appeared highly hygroscopic were dried under dynamic vacuum for several days 17,20,21 . However, it was impossible to keep them in a dehydrated state after exposure to the laboratory and during their transport for chemical analysis. PzILs containing the BF 4 − anions are soluble in water and very polar organic solvents. We have followed this behavior by FTIR spectroscopy, elemental analysis, and thermal analysis techniques. www.nature.com/scientificreports/ The physical state of ILs with a common phosphazenium cation at 25 °C is usually dependent on anion; for example, compounds containing iodide (I − ), hexafluorophosphate (PF 6 − ), and BF 4 − as anions are generally isolated as solids or waxy solids [16][17][18]20,21,30 . However, in the presence of the NTf 2 − and trifluoromethylsulfonate (OTf − ) anions, PzIL s are insoluble in water and are obtained as liquids 17,20,21,30 .

R E T R
The spin systems and the 31 P{ 1 H} NMR data of the PzILs (PzIL1a-9a) are presented in Table 1. The 31 P spectra of PzIL1a, PzIL4a, and PzIL7a are illustrated in Fig. 1, as an example. The 31 P spectra of the other PzILs were analyzed, taking into account of Figs. S1 and S2. The 2 J PP /Δν values of these compounds are calculated and listed in Table 1. The average coupling constant, 2 J PP , value (57.5 Hz) of the PzIL1a-6a (containing the five-membered spirocyclic ring) is slightly larger than those of the six-membered ones (PzIL7a-9a) ( 2 J PP = 53.2 Hz). As expected, www.nature.com/scientificreports/ PzILs (PzIL1a-9a) have AX 2 spin system which give rise to one triplet {PN(spiro)/PA} and one doublet {P(OR) 2 / PX}. δP(spiro) (ca. 22.44) of PzIL7a-9a is smaller than those of the PzILs (PzIL1a-6a); δP(spiro) (ca. 27.26). The assignments of the chemical shifts, multiplicities, and coupling constants were elucidated from the 13 C and 1 H-NMR spectra (Figs. S3-S14) of the PzILs and presented in "Experiment" section. The J coupling constants and δ shifts of C 1 , C 2 /C 6 , C 3 /C 5, and C 4 carbons of the PzILs (PzIL1a-9a) were observed in good agreement with literature values 1,20,21,30 for the compounds and did not change very much. The average J FC and/or J PC values of C 1 , C 2 /C 6 , C 3 /C 5 and C 4 carbons were estimated as 1 J FC = 243.2 Hz, 2 J FC = 21.3 Hz, 3 , 2a, 4a, 5a, 7a, and 8a were observed at ~ 52.72 ppm, while the -PhNCH 3 + chemical shifts of the PzIL3a, 6a and 9a have appeared at ~ 50.57 ppm.
On the other hand, the 1 H NMR data of the PzILs (PzIL1a-9a) were reported in the "Experiment" section, and the expected J coupling constants and δ shift values of hydrogen atoms were elucidated. The 3 J HH , 3 J FH, and 4 J FH and δ shifts of H 2 /H 6 and H 3 /H 5 protons of the FPh groups of the PzILs (PzIL1a-9a) were interpreted, and these values were found to be following the literature findings 1,20,21,27 . The average 3 J HH , 3 2a, 4a, 5a, 7a, and 8a were observed at ~ 3.08 ppm, while the -PhNCH 3 + signals of the PzIL3a, 6a, and 9a have appeared at ~ 4.32 ppm.
The νPN bands observed in the ranges of 1,228-1,219 cm −1 and 1,189-1,124 cm −1 , respectively, refer to the νasymm. and νsymm. stretching vibrations of the P=N bonds of the phosphazene skeletons (Figs. S15-S17) 31  Thermal studies. Table S1 gives the details of thermal behavior, according to the primary thermograms (TG) (Fig. 2) and derivative thermograms (DTG) (Fig. S18) for the PzILs (PzIL1a-9a). It can be seen that all the PzILs are decomposed in three steps (Table S1). It is understood that water molecules are separated from the structure in the first step. Most of the mass loss for all compounds occurs in the second step (about 52-93%). In Fig. 2, although there is no visible difference among the decomposition temperatures of ILs, it is seen that the compounds PzIL7a-9a begin to degrade at higher temperatures when Table S1 is examined (213, 189 and 193 °C, respectively). Additionally, we can say that decomposition temperature changes depending on the alkoxy or aryloxy group. The decomposition temperature increases with increased the alkoxy chain length increase for PzIL1a, 2a, and PzIL4a, 5a. For example, PzIL5a begins to decompose at 158 °C, while PzIL4a begins to decompose at 109 °C. But this is not true for PzIL7a and PzIL8a (213, 189 °C, respectively).
When the thermal stability of the PzILs with the same cationic part is compared, it is seen that the ILs containing NTf 2 − anions have higher thermal stability. This is followed by PzILs containing the I − anions and the BF 4 − anions, respectively. For example, the decomposition temperature of the PzIL4a (109 °C) is lower than that of PzILs containing NTf 2 − anions (292 °C) and I − anions (236 °C) as compared to ILs having the same cationic part in the literature 20,21 . Dielectric properties. Frequency-dependent dielectric permittivity and AC electrical conduction evaluation of all samples were investigated. Dielectric response against the time-dependent external electric field is typically given by complex permittivity, where, ω is angular frequency, ε ′ and ε ′′ are real and imaginary part of complex permittivity respectively. The real part of dielectric constant ε ′ is attributed to the in-phase polarization and imaginary part, ε ′′ represents outof-phase polarization called a dielectric loss component. Frequency dependence of the real part of the complex dielectric constant on log-scale is given in Fig. 3a. Real dielectric constant values differ according to the variation of side and the main chain of the samples. The Low-frequency dielectric constant of each sample is very high. Moreover, there is a sharp decrease in dielectric constant with increasing frequency. This phenomenon cannot be associated with a direct molecular motion as for the bulk but attribute the electrode polarization which is due to the diffusion of ions at the interface. Polarization of ILs can be attributed to ion transport and reorientation of dipolar ions. The universal relaxation behavior of ILs is observed for each sample by the spectral measurement. ε 0 and ε ∞ are the static and high-frequency components of the real part of dielectric constant. Relaxation time (τ(s)) and shape parameter (α) are calculated and given in Table 2. Relaxation time is estimated by using ε ′ vs. frequency plots. Also, shape parameter is found by fitting the ε ′ vs. frequency plots according to Debye's relaxation formula, given in Eq. (2). www.nature.com/scientificreports/ The frequency-dependent electrical conductivity variation of each sample is given in Fig. 3b. All newly synthesized ILs have almost the same conductivity behavior; typical electrolyte response. The random barrier model is used to define the charge transport mechanism of ILs. Conductivity mechanism of ILs can be analyzed by using universal law of conductivity; σ (ω) = σ 0 + Aω n . Where σ 0 and Aω n are DC and AC conductivity related components respectively, A is a constant and n is the exponent factor. Exponent factor could be varied between 0 to 1 and defines ion-environment interaction for this study. A σ 0 and n were estimated by fitting the bulk properties reflected regions (the region outside of the electrode polarization) according to the universal law.
All estimated values are given in Table 2. In high-frequency region hopping conduction exponent factor values vary between 0.72 and 0.92. Hopping conduction region of PzIL2a and PzIL3a is shifted to higher frequencies. This could be attributed to the higher conductivity levels of these samples.  www.nature.com/scientificreports/ maintained, have been synthesized and characterized. The different PzILs exhibit different thermal behavior due to structural and functional group differences, but all the PzILs have almost the same thermal decomposition pattern. PzILs with BF 4 − anions are less stable than PzILs with NTf 2 − and I − anions. Electrical conductivities and dielectric behaviors of PzILs were also determined. The static dielectric constant, shape parameter, relaxation time and exponent factor of all newly synthesized ILs were determined using a parallel plate impedance spectroscopy technique. All samples demonstrate typical conduction behavior of ILs. DC and hopping conduction mechanisms are predominant for each sample. With suitable design, these PzILs can offer unique solutions to a variety of technologies, from the organic field-effect transistor to lithium-ion battery production.