Tri- tert -butyl( n -alkyl)phosphonium Ionic Liquids: Structure, Properties and Application as Hybrid Catalyst Nanomaterials

: A series of sterically hindered tri- tert -butyl( n -alkyl)phosphonium salts ( n -C n H 2n+1 with n = 1, 3, 5, 7, 9, 11, 13, 15, 17) was synthesized and systematically studied by 1 H, 13 C, 31 P NMR spectroscopy, ESI-MS, single-crystal X-ray diffraction analysis and melting point measurement. Formation and stabilization palladium nanoparticles (PdNPs) were used to characterize the phosphonium ionic liquid (PIL) nanoscale interaction ability. The colloidal Pd in the PIL systems was described with TEM and DLS analyses and applied in the Suzuki cross-coupling reaction. The PILs were proven to be suit-able stabilizers of PdNPs possessing high catalytic activity. The tri- tert -butyl( n -alkyl)phosphonium salts showed a complex nonlinear correlation of the structure–property relationship. The synthesized family of PILs has a broad variety of structural features, including hydrophobic and hydrophilic structures that are entirely expressed in the diversity of their properties


Introduction
Ionic liquids (ILs) are typically composed of an organic cation (ammonium, imidazolium, phosphonium, pyridinium, sulfonium) and inorganic anion [1]. These compounds are of much interest due to the combination of attractive properties such as limited volatility and flammability, broad temperature range of the liquid state, thermal and chemical stability, wide electrochemical potential window, tunable solubility and miscibility and highly variable biological activity [2][3][4][5][6]. The main feature of ILs is a high variability of the structure (more than 10 6 possible compounds), which gives unprecedented opportunities for customization for specific tasks.
The first members of IL as a class attributed to the reports the synthesis of ethanolammonium nitrate by Gabriel [7] at the end of the 19th century and ethylammonium nitrate by Walden [8] at the beginning of the 20th century. The discovery of moisture-and air-stable ionic liquids in the 1990s resulted in an increased number of publications devoted to ILs-Scopus showed >13,000 publications in this field at the end of 2018 [9,10]. At the same time, Morton and Hamer noted a decrease in the number of patent claims related to the synthesis of new ILs. The main trend for now is developing their industrial application [11].
One of the promising fields of IL application is energy storage devices [12], as electrolytes in Li-ion batteries [13], including IL-modified polymers [14]. Considering a number of requirements for the electrolyte, ILs are often the system of choice for these purposes [15]. Currently, sodium-ion batteries are under intensive development [16], and ILs have been proposed to be used as electrolytes in such devices [17]. The IL application in electrochemistry extends to the modification of electrodes [18] and design of supercapacitors [19]. NMR spectroscopy. NMR spectra were recorded on a Fourier 300 HD instrument (Bruker, Fällanden, Switzerland) at 21 • C ( 1 H 300. 13 MHz, 31 C 75.47 MHz) and AVneo 300 instrument (Bruker, Fällanden, Switzerland) at 21 • C ( 31 P 121.54 MHz). SiMe 4 was used as an internal reference for 1 H and 13 C NMR spectra, and 85% H 3 PO 4 was used as an external reference for 31 P NMR spectra.
Single-crystal X-ray diffraction analysis. X-ray diffraction data for 2b and 3b (Table 1) were collected at 100 K on a Quest D8 diffractometer (Bruker, Bremen, Germany) equipped with a Photon-III area-detector (shutterless ϕand ω-scan technique), using monochromatized Mo K α -radiation. The intensity data were integrated by the SAINT program [55] and corrected for absorption and decay using SADABS [56].
The X-ray diffraction data for 4b were recollected at 100 K on a four-circle Synergy S diffractometer (Rigaku, Wrocław, Poland) equipped with a HyPix600HE area-detector (kappa geometry, shutterless ω-scan technique), using graphite monochromatized Cu K α -radiation; the intensity data were integrated and corrected for absorption and decay by the CrysAlisPro program [57]. The structures were solved by direct methods using SHELXT [58] and refined on F 2 using SHELXL-2018 [59] or OLEX2 [60].
Electrospray ionization mass spectrometry (ESI-MS). ESI-MS measurements were performed using Maxis and MicroTOF II time-of-flight high-resolution mass spectrometers (Bruker Daltonic GmbH, Bremen, Germany) in positive mode in the mass range of m/z 50-3000. Direct syringe injection was used for all analyzed samples in MeCN solution at a flow rate of 5 µL/min. The capillary voltage was −4500 V, spray shield offset was −500 V, nitrogen nebulizer gas was −1 bar, nitrogen drying gas was 4 L·min −1 , and desolvation temperature was 200 • C. The instruments were calibrated with a low concentration tuning mixture (Agilent Technologies G2431A, Santa Clara, CA, USA). Data processing was performed by DataAnalysis 4.0 SP4 software (Bruker Daltonik GmbH, Bremen, Germany).
Melting point measurement. The melting points were measured on a Digital Melting Point Apparatus 1101D (Electrothermal, Stafford, UK).
Transmission electron microscopy (TEM). TEM images of the PdNPs were obtained using a transmission electron microscope HT7700 (Hitachi, Tokyo, Japan) at an accelerating voltage of 100 kV in high resolution mode at a magnification of 80-150 K.
Gas chromatography-mass spectrometry (GC-MS). GC-MS experiments were carried out with a 7890A GC system (Agilent, Santa Clara, CA, USA) equipped with a 5975C mass-selective detector (Agilent, Santa Clara, CA, USA) (electron impact, 70 eV) and an HP-5MS column (30 m × 0.25 mm × 0.25 mm) using He as the carrier gas at a flow of 1.0 mL·min −1 .
Dynamic light scattering (DLS). Nanoparticle size measurements were carried out on a ZetaSizer Nano (Malvern, Worcestershire, UK). The temperature of the cell was 25 • C. The data were analyzed with original ZetaSizer software. The radius (R H ) was calculated according to the Stokes-Einstein relation: D S = k B T/6πηR H , in which D S is the diffusion coefficient, k B is the Boltzmann constant, T is the absolute temperature, and η is the viscosity. All measurements were performed using 633 nm wavelength at the scattering angle of 173 • five times. Only multiple reproducible results were taken into account; therefore, they differed by less than 3%.

Materials
All procedures associated with the preparation of the starting reagents, synthesis, and isolation of products were carried out under inert atmosphere using the standard Schlenk technique.

Typical Procedure for PdNP Preparation
First, 0.0004 g (0.00178 mmol) of palladium acetate and 0.178 mmol of PIL (Supplementary Materials, Table S10) were dissolved in 9 mL ethanol and stirred for 20 min at room temperature. The color of the solution changed from transparent colorless to light brownish gray.

General Procedure for Suzuki Cross-Coupling
To a fresh solution of colloidal Pd, 0.16 g (0.5 mmol) of 1,3,5-tribrombenzene, 0.28 g (2.2 mmol) of phenylboronic acid, and 0.13 g (2.2 mmol) of potassium hydroxide were added. The reaction mixture was stirred over 7 h at 30 • C. Organic compounds were extracted with 9 mL toluene and analyzed by GC-MS.

Synthetic Procedures
The quaternization reaction of tri-tert-butylphosphine and respective linear 1-haloalkane (Hal = I or Br) was conducted under solvent-free conditions except for the cases with solid haloalkanes. Acetonitrile (for C 15 H 31 Br) and DMF (for C 17 H 35 Br) were applied as a solvent (Scheme 1). The temperature of the reaction was varied depending on the applied haloalkane, from −70 • C for MeI [61] to 90-100 • C for liquid 1-bromoalkanes and 80 • C for the solid ones. The reaction was completed after disappearance of the phosphine signal in the 31 P NMR spectrum.
To a fresh solution of colloidal Pd, 0.16 g (0.5 mmol) of 1,3,5-tribrombenzene, 0.28 g (2.2 mmol) of phenylboronic acid, and 0.13 g (2.2 mmol) of potassium hydroxide were added. The reaction mixture was stirred over 7 h at 30 °С. Organic compounds were extracted with 9 mL toluene and analyzed by GC-MS.

Synthetic Procedures
The quaternization reaction of tri-tert-butylphosphine and respective linear 1-haloalkane (Hal = I or Br) was conducted under solvent-free conditions except for the cases with solid haloalkanes. Acetonitrile (for C15H31Br) and DMF (for C17H35Br) were applied as a solvent (Scheme 1). The temperature of the reaction was varied depending on the applied haloalkane, from −70 °C for MeI [61] to 90-100 °C for liquid 1-bromoalkanes and 80 °C for the solid ones. The reaction was completed after disappearance of the phosphine signal in the 31 P NMR spectrum. Scheme 1. The quaternization reaction of tri-tert-butylphosphine and 1-haloalkane.
The metathesis reaction of phosphonium halide to weakly coordinating tetrafluoroborate-anion was carried out in water at room temperature (Scheme 2). Being poorly soluble in water, phosphonium tetrafluoroborates precipitated as soon as sodium tetrafluoroborate was added to the solution of phosphonium halide, which is readily soluble in water. New compounds were obtained in high yields (up to 98%; see Supplementary Materials for details). The phosphonium salts are white or yellowish crystalline or amorphous solids that are soluble in chloroform, dichloromethane, acetone, ethanol, methanol, DMF, DMSO and almost insoluble in diethyl ether and petroleum ether.

31 P NMR Spectral Data
The 31 P{ 1 H} NMR spectra of the tri-tert-butyl(n-alkyl)phosphonium salts (Table 1) exhibited a singlet in the range of 47.7-51.3 ppm. The chemical shifts of phosphonium salts weakly depend on the anion. Phosphonium halides have chemical shifts mostly in lower magnetic fields than phosphonium tetrafluoroborates, on average 1-2 ppm. At the same time, the length of the alkyl substituent in the phosphonium cation also has a weak influence on the chemical shift in 31 P{ 1 H} NMR spectra. The difference is not more than 3.6 ppm. The chemical shifts of 1a and 1b appear in a lower magnetic field, possibly due to Scheme 1. The quaternization reaction of tri-tert-butylphosphine and 1-haloalkane.
The metathesis reaction of phosphonium halide to weakly coordinating tetrafluoroborate-anion was carried out in water at room temperature (Scheme 2). Being poorly soluble in water, phosphonium tetrafluoroborates precipitated as soon as sodium tetrafluoroborate was added to the solution of phosphonium halide, which is readily soluble in water.
To a fresh solution of colloidal Pd, 0.16 g (0.5 mmol) of 1,3,5-tribrombenzene, 0.28 g (2.2 mmol) of phenylboronic acid, and 0.13 g (2.2 mmol) of potassium hydroxide were added. The reaction mixture was stirred over 7 h at 30 °С. Organic compounds were extracted with 9 mL toluene and analyzed by GC-MS.

Synthetic Procedures
The quaternization reaction of tri-tert-butylphosphine and respective linear 1-haloalkane (Hal = I or Br) was conducted under solvent-free conditions except for the cases with solid haloalkanes. Acetonitrile (for C15H31Br) and DMF (for C17H35Br) were applied as a solvent (Scheme 1). The temperature of the reaction was varied depending on the applied haloalkane, from −70 °C for MeI [61] to 90-100 °C for liquid 1-bromoalkanes and 80 °C for the solid ones. The reaction was completed after disappearance of the phosphine signal in the 31 P NMR spectrum. Scheme 1. The quaternization reaction of tri-tert-butylphosphine and 1-haloalkane.
The metathesis reaction of phosphonium halide to weakly coordinating tetrafluoroborate-anion was carried out in water at room temperature (Scheme 2). Being poorly soluble in water, phosphonium tetrafluoroborates precipitated as soon as sodium tetrafluoroborate was added to the solution of phosphonium halide, which is readily soluble in water. New compounds were obtained in high yields (up to 98%; see Supplementary Materials for details). The phosphonium salts are white or yellowish crystalline or amorphous solids that are soluble in chloroform, dichloromethane, acetone, ethanol, methanol, DMF, DMSO and almost insoluble in diethyl ether and petroleum ether.

31 P NMR Spectral Data
The 31 P{ 1 H} NMR spectra of the tri-tert-butyl(n-alkyl)phosphonium salts (Table 1) exhibited a singlet in the range of 47.7-51.3 ppm. The chemical shifts of phosphonium salts weakly depend on the anion. Phosphonium halides have chemical shifts mostly in lower magnetic fields than phosphonium tetrafluoroborates, on average 1-2 ppm. At the same time, the length of the alkyl substituent in the phosphonium cation also has a weak influence on the chemical shift in 31 P{ 1 H} NMR spectra. The difference is not more than 3.6 ppm. The chemical shifts of 1a and 1b appear in a lower magnetic field, possibly due to New compounds were obtained in high yields (up to 98%; see Supplementary Materials for details). The phosphonium salts are white or yellowish crystalline or amorphous solids that are soluble in chloroform, dichloromethane, acetone, ethanol, methanol, DMF, DMSO and almost insoluble in diethyl ether and petroleum ether.

31 P NMR Spectral Data
The 31 P{ 1 H} NMR spectra of the tri-tert-butyl(n-alkyl)phosphonium salts (Table 1) exhibited a singlet in the range of 47.7-51.3 ppm. The chemical shifts of phosphonium salts weakly depend on the anion. Phosphonium halides have chemical shifts mostly in lower magnetic fields than phosphonium tetrafluoroborates, on average 1-2 ppm. At the same time, the length of the alkyl substituent in the phosphonium cation also has a weak influence on the chemical shift in 31 P{ 1 H} NMR spectra. The difference is not more than 3.6 ppm. The chemical shifts of 1a and 1b appear in a lower magnetic field, possibly due to the lack of electron density of the methyl substituent to compensate for the cationic charge on the phosphorus atom.

1 H NMR Spectral Data
Unlike 31 P{ 1 H} NMR spectra, anion exchange has a great influence on the chemical shifts in 1 H NMR spectra (Table 1), especially for the α-protons. The α-protons appeared in the 1 H NMR spectra as a multiplet ( Figure 1). Sustainability 2021, 13, x FOR PEER REVIEW 5 of 1 the lack of electron density of the methyl substituent to compensate for the cationic charg on the phosphorus atom.

1 H NMR Spectral Data
Unlike 31 P{ 1 H} NMR spectra, anion exchange has a great influence on the chemica shifts in 1 H NMR spectra (Table 1), especially for the α-protons. The α-protons appeare in the 1 H NMR spectra as a multiplet (Figure 1).  The signal of α-protons in phosphonium tetrafluoroborates appears in a higher mag netic field (2.30 ± 0.03 ppm) than that in phosphonium bromides (2.53 ± 0.07 ppm). This related to the presence of hydrogen bonds between α-protons and halogen anions bearin high electron affinity. The weakly coordinated tetrafluoroborate anion interacts with th surrounding cations via noncooperative Coulomb forces [62].
The chemical shift of α-protons in 1 H NMR spectra moves towards a higher magnet field with the elongation of the alkyl substituent ( Figure 2) due to the compensation o  The signal of α-protons in phosphonium tetrafluoroborates appears in a higher magnetic field (2.30 ± 0.03 ppm) than that in phosphonium bromides (2.53 ± 0.07 ppm). This is related to the presence of hydrogen bonds between α-protons and halogen anions bearing high electron affinity. The weakly coordinated tetrafluoroborate anion interacts with the surrounding cations via noncooperative Coulomb forces [62].
The chemical shift of α-protons in 1 H NMR spectra moves towards a higher magnetic field with the elongation of the alkyl substituent ( Figure 2) due to the compensation of electronic density withdrawn by the cationic charge on the phosphorus atom. The exception is the α-protons of 1a and 1b, where the peaks were at 2.11 ppm and 1.90 ppm, respectively, due to belonging to the α-protons of the methyl group. electronic density withdrawn by the cationic charge on the phosphorus atom. The exception is the α-protons of 1a and 1b, where the peaks were at 2.11 ppm and 1.90 ppm, respectively, due to belonging to the α-protons of the methyl group.
As the number of methylene groups increases, the shift of the methyl group at the end of the fourth alkyl substituent moves towards a higher magnetic field until it reaches 0.88 ppm ( Figure 2). The methylene protons belonging to carbon atoms starting from the fourth one from the phosphorus atom have multiple peaks in the region 1.35-1.20 ppm.

13 C{ 1 H} NMR Spectral Data
A particular pattern in 13 C NMR spectra for the investigated PILs is observed. For convenient discussion, we propose labeling the carbon atoms according to Figure 3. The spectra of 1b, 2b and 3b are distinct; for the rest of the PILs, the peak positions are close to each other. The chemical shift of quaternary carbon atom Cq appears as a doublet at 39.3 ppm with a first-order P-C spin-spin coupling constant of 29.4 ± 0.3 Hz ( Figure 3, Table 2). There is one exception for 1b, whose peak appears at 37.9 ppm, and the first-order P-C coupling constant is 32.1 Hz. As the number of methylene groups increases, the shift of the methyl group at the end of the fourth alkyl substituent moves towards a higher magnetic field until it reaches 0.88 ppm ( Figure 2). The methylene protons belonging to carbon atoms starting from the fourth one from the phosphorus atom have multiple peaks in the region 1.35-1.20 ppm.

13 C{ 1 H} NMR Spectral Data
A particular pattern in 13 C NMR spectra for the investigated PILs is observed. For convenient discussion, we propose labeling the carbon atoms according to Figure 3. The spectra of 1b, 2b and 3b are distinct; for the rest of the PILs, the peak positions are close to each other. electronic density withdrawn by the cationic charge on the phosphorus atom. The exception is the α-protons of 1a and 1b, where the peaks were at 2.11 ppm and 1.90 ppm, respectively, due to belonging to the α-protons of the methyl group. As the number of methylene groups increases, the shift of the methyl group at the end of the fourth alkyl substituent moves towards a higher magnetic field until it reaches 0.88 ppm ( Figure 2). The methylene protons belonging to carbon atoms starting from the fourth one from the phosphorus atom have multiple peaks in the region 1.35-1.20 ppm.

13 C{ 1 H} NMR Spectral Data
A particular pattern in 13 C NMR spectra for the investigated PILs is observed. For convenient discussion, we propose labeling the carbon atoms according to Figure 3. The spectra of 1b, 2b and 3b are distinct; for the rest of the PILs, the peak positions are close to each other. The chemical shift of quaternary carbon atom Cq appears as a doublet at 39.3 ppm with a first-order P-C spin-spin coupling constant of 29.4 ± 0.3 Hz (Figure 3, Table 2). There is one exception for 1b, whose peak appears at 37.9 ppm, and the first-order P-C coupling constant is 32.1 Hz. The chemical shift of quaternary carbon atom C q appears as a doublet at 39.3 ppm with a first-order P-C spin-spin coupling constant of 29.4 ± 0.3 Hz (Figure 3, Table 2). There is one exception for 1b, whose peak appears at 37.9 ppm, and the first-order P-C coupling constant is 32.1 Hz.  The carbon atom of methyl groups in tert-butyl C t does not have the P-C spin-spin coupling constant despite it being located two bonds from the phosphorus atom ( Figure 4). This effect could be related to the quaternary carbon atom in between them [62].
troscopy (COSY, HSQC, HMBC), we specified the signals of these carbon atoms. We attribute the chemical shift in the 13 C NMR spectra with the highest value of the coupling constant to the carbon atom Ca. The peculiar feature of the 13 C NMR spectra of 1b is the position of the Ca peak in the high field (0.4 ppm, Table 2) due to its high shielding. The chemical shift of Ca of 2b is located at 20.4 ppm. Along with elongation of the alkyl chain, it moves towards a higher field (18.5 ppm). In contrast, the signals of Cb and Cc shift to the lower field (from 18.9 to 25.1 ppm for Cb and from 16.3 to 31.7 ppm for Cc). It should be pointed out that the P-C spin-spin coupling constant for Cb is less than that for Cc (6.4-6.7 Hz and 12.4-14.2 Hz respectively). This could be caused by the steric effect of tert-butyl groups (shielding or the possibility of realizing only one certain conformation) or by the influence of the anion.  Three carbon atoms in methylene groups close to the phosphorus atom (C a , C b , C c ) appear as doublets with the following P-C spin-spin coupling constant values: 35.1 ± 0.2 Hz, 12.6 ± 0.1 Hz, 6.6 ± 0.1 Hz (Figure 4). On the assumption of 1-D and 2-D NMR spectroscopy (COSY, HSQC, HMBC), we specified the signals of these carbon atoms. We attribute the chemical shift in the 13 C NMR spectra with the highest value of the coupling constant to the carbon atom C a . The peculiar feature of the 13 C NMR spectra of 1b is the position of the C a peak in the high field (0.4 ppm, Table 2) due to its high shielding. The chemical shift of C a of 2b is located at 20.4 ppm. Along with elongation of the alkyl chain, it moves towards a higher field (18.5 ppm). In contrast, the signals of C b and C c shift to the lower field (from 18.9 to 25.1 ppm for C b and from 16.3 to 31.7 ppm for C c ). It should be pointed out that the P-C spin-spin coupling constant for C b is less than that for C c (6.4-6.7 Hz and 12.4-14.2 Hz respectively). This could be caused by the steric effect of tert-butyl groups (shielding or the possibility of realizing only one certain conformation) or by the influence of the anion.
The chemical shift of the terminal methyl group of the linear alkyl chain C z is sensitive to the affinity to the phosphorus atom, and it moves towards the lower magnetic field until it reaches 14.1 ppm, which is typical for the methyl group of alkanes.
The signals of the middle carbon atoms (C m ) in the alkyl chain appeared in the range of 28.8-29.7 ppm (Figure 4).

Single-Crystal X-ray Diffraction Analysis
The structures of 2b, 3b and 4b were determined by the single-crystal X-ray diffraction analysis (Tables S1-S9 and Figures  position. However, the [BF 4 ] − /I − substitutional disorder was not detected for compounds 3b and 4b containing a longer n-alkyl chain. The asymmetric unit of 3b ( Figure 6) and 4b (Figure 7) consists of three (Z' = 3) and of four (Z' = 4) crystallographically non-equivalent [t-Bu 3 P(n-C n H 2n+1 )] + [BF 4 ] − ion pairs, correspondingly. In case of 4b, six out of eight ions were entirely disordered ( Figures S8-S13, Tables S8 and S9 in Supplementary Materials).
Enlargement of the alkyl chain likely leads to increase in a number of crystallographically independent ion pairs (Z') and disorder augmentation. The crystal packing of sterically hindered PILs with significantly different length of alkyl substituent was reported earlier [54]. metric unit of 2b contains one [t-Bu3PPr] + cation and one disordered [BF4] − anion (Z' = 1 ( Figure 5). It might be noted that only the use of AgBF4 allowed us to obtain pure crysta of 2b, whereas in the absence of Ag + , crystals of [t-Bu3PPr] + [BF4] − xI − (1−x) (x = 0.94-0.96; e.g see 2b', Figure S2, Tables S4 and S5 in Supplementary Materials) with a substitutiona disorder of the anion were formed, in which the iodine atom is located nearly at the boro atom position. However, the [BF4] − /I − substitutional disorder was not detected for com pounds 3b and 4b containing a longer n-alkyl chain. The asymmetric unit of 3b ( Figure 6 and 4b (Figure 7) consists of three (Z' = 3) and of four (Z' = 4) crystallographically non equivalent [t-Bu3P(n-CnH2n+1)] + [BF4] − ion pairs, correspondingly. In case of 4b, six out o eight ions were entirely disordered ( Figures S8-S13, Tables S8 and S9 in Supplementar Materials).
Enlargement of the alkyl chain likely leads to increase in a number of crystallograph ically independent ion pairs (Z') and disorder augmentation. The crystal packing of steri cally hindered PILs with significantly different length of alkyl substituent was reporte earlier [54].    (Figures S8-S13, Tables S8 and S9 in Supplementar  Materials). Enlargement of the alkyl chain likely leads to increase in a number of crystallograph ically independent ion pairs (Z') and disorder augmentation. The crystal packing of steri cally hindered PILs with significantly different length of alkyl substituent was reported earlier [54].

Melting Point
The melting point (Tm) of the synthesized PILs was measured. The trend of Tm as a function of the number of carbon atoms in the alkyl substituent of PIL is presented in Figure 8.
The first two members of the PIL series have high melting points above 200 °C and melt with decomposition that is caused by the pronounced ionic nature of the compounds The moderate size of the phosphonium cation and its relative symmetry cause high lattice energy. With an increase in the methylene groups in the alkyl chain, Tm diminishes step wise and reaches a minimum. This fact is connected with the gain of disordering in the crystal lattice. The next slight increase in the melting point of long-chained PILs is caused by hydrophobic intermolecular interactions. The corresponding dependence of Tm on the elongation of alkyl substituents in the series of homologous compounds was reported ear lier [54,63].

Melting Point
The melting point (T m ) of the synthesized PILs was measured. The trend of T m as a function of the number of carbon atoms in the alkyl substituent of PIL is presented in Figure 8.

Melting Point
The melting point (Tm) of the synthesized PILs was measured. The trend of Tm as a function of the number of carbon atoms in the alkyl substituent of PIL is presented in Figure 8.
The first two members of the PIL series have high melting points above 200 °C and melt with decomposition that is caused by the pronounced ionic nature of the compounds. The moderate size of the phosphonium cation and its relative symmetry cause high lattice energy. With an increase in the methylene groups in the alkyl chain, Tm diminishes stepwise and reaches a minimum. This fact is connected with the gain of disordering in the crystal lattice. The next slight increase in the melting point of long-chained PILs is caused by hydrophobic intermolecular interactions. The corresponding dependence of Tm on the elongation of alkyl substituents in the series of homologous compounds was reported earlier [54,63]. The first two members of the PIL series have high melting points above 200 • C and melt with decomposition that is caused by the pronounced ionic nature of the compounds. The moderate size of the phosphonium cation and its relative symmetry cause high lattice energy. With an increase in the methylene groups in the alkyl chain, T m diminishes stepwise and reaches a minimum. This fact is connected with the gain of disordering in the crystal lattice. The next slight increase in the melting point of long-chained PILs is caused by hydrophobic intermolecular interactions. The corresponding dependence of T m on the elongation of alkyl substituents in the series of homologous compounds was reported earlier [54,63].
The anion has a moderate influence on the melting point of PIL. The difference is minor for medium-length chained ionic liquids and slightly greater for outer members in the row. It should be noted that 4a and 4b have almost the same melting point (approximately 125 • C). The T m values of phosphonium bromides in most cases are lower than those for phosphonium tetrafluoroborates. The fluorine atoms have short contacts with hydrogen atoms belonging to the linear alkyl chain, resulting in an increase in intermolecular interactions. It is remarkable that 6a, 7a, 8a and 9a melt below 100 • C.

PdNP Stabilization and TEM Sample Preparation
PILs have been proven to be an effective stabilizer of metal nanoparticles [5,64]. The application of stabilized NPs is widespread in the area of catalysis [65] and other areas. ILs could be also involved in the creation of novel nanocomposites with biopolymers for drug and gene delivery [66]. In addition, ILs could be applied for magnetic NP stabilization to extract heavy metal ions [67].
For the current study, PdNPs were obtained by the reduction of palladium acetate in ethanol in the presence of the corresponding PIL (Supplementary Materials, Table S10). According to previous investigation, ethanol is the solvent of choice for efficient synthesis of hybrid NP systems [54]. The PdNP synthesis procedure does not require any reducing agent, elevated temperature or inert atmosphere. The PdNP size was estimated using transmission electron microscopy (TEM) (Figure 9). The "nanofishing" technique was used to capture nanoparticles from the liquid phase. The copper grid was immersed in a colloidal solution of the PdNPs for 30 s, rinsed with acetone to remove organic components and dried in air. The anion has a moderate influence on the melting point of PIL. The difference is minor for medium-length chained ionic liquids and slightly greater for outer members in the row. It should be noted that 4a and 4b have almost the same melting point (approximately 125 °C). The Tm values of phosphonium bromides in most cases are lower than those for phosphonium tetrafluoroborates. The fluorine atoms have short contacts with hydrogen atoms belonging to the linear alkyl chain, resulting in an increase in intermolecular interactions. It is remarkable that 6a, 7a, 8a and 9a melt below 100 °C.

PdNP Stabilization and TEM Sample Preparation
PILs have been proven to be an effective stabilizer of metal nanoparticles [5,64]. The application of stabilized NPs is widespread in the area of catalysis [65] and other areas. ILs could be also involved in the creation of novel nanocomposites with biopolymers for drug and gene delivery [66]. In addition, ILs could be applied for magnetic NP stabilization to extract heavy metal ions [67].
For the current study, PdNPs were obtained by the reduction of palladium acetate in ethanol in the presence of the corresponding PIL (Supplementary Materials, Table S10). According to previous investigation, ethanol is the solvent of choice for efficient synthesis of hybrid NP systems [54]. The PdNP synthesis procedure does not require any reducing agent, elevated temperature or inert atmosphere. The PdNP size was estimated using transmission electron microscopy (TEM) (Figure 9). The "nanofishing" technique was used to capture nanoparticles from the liquid phase. The copper grid was immersed in a colloidal solution of the PdNPs for 30 s, rinsed with acetone to remove organic components and dried in air. Here, the key point of the present study should be emphasized. A common procedure involves drying the solution and then using the obtained solid phase for microscopic analysis. However, in such a case, nanoparticles may form during the drying process, or their morphology may substantially change upon drying. In the present study, we captured nanoparticles directly from solution without carrying out a destructive isolation/drying process. Here, the key point of the present study should be emphasized. A common procedure involves drying the solution and then using the obtained solid phase for microscopic analysis. However, in such a case, nanoparticles may form during the drying process, or their morphology may substantially change upon drying. In the present study, we captured nanoparticles directly from solution without carrying out a destructive isolation/drying process.

PdNP Behavior during Suzuki Reaction
Freshly prepared PdNPs were involved in the Suzuki cross-coupling reaction (Scheme 3) as a catalyst. Pd (0.36 mol%) and the molar ratio of stabilizer/Pd 100/1 were used as optimal (Table S10). The GC-MS method was used to estimate the conversion degree of 1,3,5-tribromobenzene into 1,3,5-triphenylbenzene. It should be noted that triphenylbenzene was formed as a main product.

PdNP Behavior during Suzuki Reaction
Freshly prepared PdNPs were involved in the Suzuki cross-coupling reaction (Scheme 3) as a catalyst. Pd (0.36 mol%) and the molar ratio of stabilizer/Pd 100/1 were used as optimal (Table S10). The GC-MS method was used to estimate the conversion degree of 1,3,5-tribromobenzene into 1,3,5-triphenylbenzene. It should be noted that triphenylbenzene was formed as a main product. The challenging tri-substituted substrate and mild conditions of the catalytic reaction were chosen to emphasize the difference between the catalytic systems based on PILs of different structures. The conversion of 1,3,5-tribromobenzene into 1,3,5-triphenylbenzene is a good benchmarking system, which we used here to analyze the behavior of the particular studied catalytic system in the cross-coupling reaction.
The preparation of the PdNPs for TEM analysis was carried out after 20 min of stirring the phosphonium salt and palladium acetate in ethanol and 7 h after the start of the cross-coupling reaction using the "nanofishing" technique. For each entry, 100-500 nanoparticles were processed to calculate the average size and standard deviation (Table 3, Figures S14-S31 in Supplementary Materials). There were aggregates and individual PdNPs in the TEM images. Most of them were uniform and had clear edges. These systems were studied in terms of nanoparticle stabilization by coating agents of various natures. There are three types of NP stabilization: electrostatic, steric and electrosteric (a transitional one) [68,69]. The electrostatic type of NP stabilization is typical for ionic compounds. The hydrophobic long alkyl chain in the structure provides steric stabilization. Extreme electrostatic and steric types lead to the strong stabilization of NPs and affect their surface inacceptability [70,71]. The combined type of stabilization is electrosteric when the compound has an ionic charge as well as hydrophobic alkyl chain. Mostly the ILs provide electrosteric stabilization. The series of bulky phosphonium salts under investigation has structural diversity, and all types of stabilization are represented in a row.
PILs with short alkyl chains and hydrophilic properties usually provide electrostatic stabilization. The compounds 1b and 2b have a stronger ionic nature in the range and, as The challenging tri-substituted substrate and mild conditions of the catalytic reaction were chosen to emphasize the difference between the catalytic systems based on PILs of different structures. The conversion of 1,3,5-tribromobenzene into 1,3,5-triphenylbenzene is a good benchmarking system, which we used here to analyze the behavior of the particular studied catalytic system in the cross-coupling reaction.
The preparation of the PdNPs for TEM analysis was carried out after 20 min of stirring the phosphonium salt and palladium acetate in ethanol and 7 h after the start of the crosscoupling reaction using the "nanofishing" technique. For each entry, 100-500 nanoparticles were processed to calculate the average size and standard deviation ( Table 3, Figures S14-S31 in Supplementary Materials). There were aggregates and individual PdNPs in the TEM images. Most of them were uniform and had clear edges. These systems were studied in terms of nanoparticle stabilization by coating agents of various natures. There are three types of NP stabilization: electrostatic, steric and electrosteric (a transitional one) [68,69]. The electrostatic type of NP stabilization is typical for ionic compounds. The hydrophobic long alkyl chain in the structure provides steric stabilization. Extreme electrostatic and steric types lead to the strong stabilization of NPs and affect their surface inacceptability [70,71]. The combined type of stabilization is electrosteric when the compound has an ionic charge as well as hydrophobic alkyl chain. Mostly the ILs provide electrosteric stabilization. The series of bulky phosphonium salts under investigation has structural diversity, and all types of stabilization are represented in a row.
PILs with short alkyl chains and hydrophilic properties usually provide electrostatic stabilization. The compounds 1b and 2b have a stronger ionic nature in the range and, as a consequence, could not effectively prevent PdNP agglomeration. The NP size in 1b after the reaction decreased more than twice (Table 3, entry 1), which could be explained by aggregation of the large PdNPs, their precipitation and excluding from the system. At the same time, an amount of small NPs of approximately 3 nm remained in the reaction mixture. Despite the same mean PdNP size of 2b, the size distribution decreased during the catalytic reaction, which has a similar explanation as occurred with 1b.
Chain elongation results in a higher contribution of steric stabilization and, as a consequence, a decrease in the mean PdNP size and size distribution.
Compound 5b has a middle alkyl substituent length and steric reasons begin to prevail in PdNP stabilization. Due to its molecular nature, 5b could stabilize larger PdNPs as well as smaller ones, and the size distribution significantly increased. In Figures S22 and S23 (in Supplementary Materials), we observe a bimodal system of the particles: the first group has mean sizes of 3-5 nm, and the second has sizes of 11-13 nm, respectively.
PILs 6b-9b, due to steric reasons, could stabilize PdNPs in the initial stage of nucleation, providing the presence of a large number of small particles (2.6-4.2 nm) with a narrow size distribution.
The change in PdNP size depending on the length of the fourth substituent observed in the present article is similar to the trends described previously for PILs with even numbers of carbon atoms in the side alkyl chain [54].
The implication of the bulky PIL for PdNP stabilization allows good and moderate conversion of 34-62% ( Figure 10). Catalytic systems including PILs with medium-sized alkyl substituents possess slightly higher activity than other PILs. The dynamic nature of the nanocatalyst requires a balance between its activity and stability that could be provided by a stabilizer combining electrostatic and steric properties. a consequence, could not effectively prevent PdNP agglomeration. The NP size in 1b after the reaction decreased more than twice (Table 3, entry 1), which could be explained by aggregation of the large PdNPs, their precipitation and excluding from the system. At the same time, an amount of small NPs of approximately 3 nm remained in the reaction mixture. Despite the same mean PdNP size of 2b, the size distribution decreased during the catalytic reaction, which has a similar explanation as occurred with 1b.
Chain elongation results in a higher contribution of steric stabilization and, as a consequence, a decrease in the mean PdNP size and size distribution.
Compound 5b has a middle alkyl substituent length and steric reasons begin to prevail in PdNP stabilization. Due to its molecular nature, 5b could stabilize larger PdNPs as well as smaller ones, and the size distribution significantly increased. In Figures S22 and  S23 (in Supplementary Materials), we observe a bimodal system of the particles: the first group has mean sizes of 3-5 nm, and the second has sizes of 11-13 nm, respectively.
PILs 6b-9b, due to steric reasons, could stabilize PdNPs in the initial stage of nucleation, providing the presence of a large number of small particles (2.6-4.2 nm) with a narrow size distribution.
The change in PdNP size depending on the length of the fourth substituent observed in the present article is similar to the trends described previously for PILs with even numbers of carbon atoms in the side alkyl chain [54].
The implication of the bulky PIL for PdNP stabilization allows good and moderate conversion of 34-62% ( Figure 10). Catalytic systems including PILs with medium-sized alkyl substituents possess slightly higher activity than other PILs. The dynamic nature of the nanocatalyst requires a balance between its activity and stability that could be provided by a stabilizer combining electrostatic and steric properties.

DLS Data of the Catalytic System
The catalytic composition includes palladium, PIL and solvent molecules. TEM images show assembled clusters of PdNPs that could contain PILs as well. One may suggest the presence of such metal-organic aggregates in solution before and during the catalytic reaction. We used an affordable DLS technique to determine the size of the nanocomposite in solution.
Catalytic systems including 4b-9b were studied, and bimodality was found to be a characteristic feature. Two groups of particles of various sizes were observed. The hydrodynamic diameter of metal-organic aggregates changes regularly in the PIL row. The catalytic composition consisting of 4b, 5b, 8b and 9b was characterized by a large amount (94-98%) of smaller particles measuring 238-371 nm ( Table 4, Figures S32-S37 in Supplementary Materials). The proportion between small and large aggregates changes for 6b. This system contains 68% small particles (219 nm), and 32% of the remaining particles have a mean size of 678 nm. In the case of 7b, the distribution in the system became completely monomodal (large aggregates with an average size of 840 nm). Notably, the catalyst based on 7b demonstrates the lowest conversion in the row. At the same time, it is the only monomodal system with larger particles. Consequently, metal-organic aggregates of approximately 800 nm weakly promote the Suzuki reaction compared to the above smaller particles. The prevalence of small particles in the solution contributes to the catalytic activity of PdNPs (6b and 9b). However, the system with 8b includes small particles and shows only slightly increased conversion compared to the 7b system. Thus, the correlation between the aggregate size and the conversion is not clear. Most likely, the inner structure of observable metal-organic aggregates is essential.
Concerning the study of the catalytic system, one may note the following discussion. The sizes of the PdNPs varied in the range of 2.6-7.3 nm, which can affect the catalytic activity through the quantity of the surface active atoms of Pd [72], but this was not the case. A similar scenario appears when DLS measurement of the catalytic system was performed. The complexity and multifactorial nature of the catalytic process indicate that different palladium particles could be reservoirs for active species [73]. The stage of oxidative addition of the reagent to Pd or the leaching process results in small active clusters or even atomic Pd [74,75]. The high catalytic activity of such dissimilar catalysts indicates the presence of a dynamic catalytic system [74]. In this situation, the initial size of the PdNPs does not significantly influence the catalytic activity.
In case of the metal-organic particles, their inner structure is more essential rather than their size. The nearest environment of the Pd(0) consisting of the PIL ions influences the catalytic activity. It is caused by migration of the reactant to the surface of PdNP, reverse movement of the reaction products, Pd clusters and atoms leaching. The supramolecular network of the metal-organic aggregate derives from the structure directionality of the particular PIL that is known in the literature as microheterogeneity [37,76] and possessed by all types of ILs. For this reason an extensive study of the PIL microstructure is required.

Conclusions
A family of new phosphonium ionic liquids containing bulky cations based on tritert-butylphosphine with varied n-alkyl substituents was synthesized and characterized. A detailed study of their 1 H, 13 C and 31 P NMR spectra, melting points, mass spectra and single-crystal X-ray diffraction analysis was performed. The diversity of represented PIL members allows us to demonstrate a nonlinear relationship between structure and properties. Despite their similar structure, notable differences occurred in NMR spectra, melting points, size and distribution of stabilized PdNPs as well as in their catalytic performance. PILs were declared tunable compounds with desired properties and an effective PdNP stabilizer, which are advantages for their usage in industrial and academic practice. PILs are capable of forming and stabilizing aggregates with Pd(0), which are clearly identified by TEM and DLS analyses. The size and catalytic activity of metal-organic aggregates depend on the PIL structure at the molecular level and on the nature of the intermolecular interaction.
The appropriate prospect for the study of sterically hindered PILs is their microstructure investigation. Obviously, the possible practical use of PILs requires new insight into their impact on the environment. Therefore, their toxicity against microorganisms and human cells should be estimated.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/su13179862/s1, Materials, Synthetic procedures, 1 H/ 13 C/ 31 P NMR spectra for all compounds. Crystal data, data collection and structure refinement details for 2b, 2b', 3b and 4b are summarized in Table S1. The structures have been deposited at the Cambridge Crystallographic Data Center with the reference CCDC numbers 2099141-2099145; they also contain supplementary crystallographic data. These data can be obtained free of charge from the CCDC via https://www.ccdc.cam.ac.uk/ structures/ (accessed on 01 september 2021). Table S1. Crystal data and structure refinement for 2b.  Table S10. The mass of PIL used in the procedure of catalyst preparation. Figure S1. The structure of 2b. The [BF 4 ] − anion is disordered over two positions (A and B) with the disorder ratio of 0.855(9):0.145 (9). The displacement ellipsoids are set to the 50% probability level. Figure S2. The structure of 2b'. The disorder ratio for the [BF 4 ] − /I − positional disorder is 0.9614(4):0.0386 (4). The displacement ellipsoids are set to the 50% probability level. Figure S3. Crystallographically non-equivalent ions of 3b. The displacement ellipsoids are set to the 50% probability level. Figure S4. The first crystallographically non-equivalent cation [ t Bu 3 P(n-C 5 H 11 )] + in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S5. The second crystallographically non-equivalent cation [ t Bu 3 P(n-C 5 H 11 )] + in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S6. The third crystallographically non-equivalent cation [ t Bu 3 P(n-C 5 H 11 )] + in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S7. Three crystallographically non-equivalent anions [BF 4 ] − in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S8. Crystallographically nonequivalent cations [ t Bu 3 P(n-C 7 H 15 )] + and anions [BF 4 ] − in 4b. The disorder is not shown. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. Figure S9. The first crystallographically non-equivalent cation [ t Bu 3 P(n-C 7 H 15 )] + in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. Figure S10. The second crystallographically non-equivalent cation [ t Bu 3 P(n-C 7 H 15 )] + in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. The A/B disorder ratio is 0.904(2): 0.096(2) for three t Bu fragments and 0.772(4):0.228(4) for the n-C 7 H 15 aliphatic chain. Figure S11. The third crystallographically non-equivalent cation [ t Bu 3 P(n-C 7 H 15 )] + in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. The A/B disorder ratio is 0.6176(16):0.3824 (16). Figure S12. The fourth crystallographically non-equivalent cation [ t Bu 3 P(n-C 7 H 15 )] + in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. The A/B disorder ratio is 0.8810(16):0.1190 (16). Figure S13 Figure S14. TEM image (a) and size distribution (b) of PdNPs in 1b before the Suzuki reaction. Figure S15. TEM image (a) and size distribution (b) of PdNPs in 1b after the Suzuki reaction. Figure S16. TEM image (a) and size distribution (b) of PdNPs in 2b before the Suzuki reaction. Figure S17. TEM image (a) and size distribution (b) of PdNPs in 2b after the Suzuki reaction. Figure S18. TEM image (a) and size distribution (b) of PdNPs in 3b before the Suzuki reaction. Figure S19. TEM image (a) and size distribution (b) of PdNPs in 3b after the Suzuki reaction. Figure S20. TEM image (a) and size distribution (b) of PdNPs in 4b before the Suzuki reaction. Figure S21. TEM image (a) and size distribution (b) of PdNPs in 4b after the Suzuki reaction. Figure S22. TEM image (a) and size distribution (b) of PdNPs in 5b before the Suzuki reaction. Figure S23. TEM image (a) and size distribution (b) of PdNPs in 5b after the Suzuki reaction. Figure S24. TEM image (a) and size distribution (b) of PdNPs in 6b before the Suzuki reaction. Figure S25. TEM image (a) and size distribution (b) of PdNPs in 6b after the Suzuki reaction. Figure S26. TEM image (a) and size distribution (b) of PdNPs in 7b before the Suzuki reaction. Figure S27. TEM image (a) and size distribution (b) of PdNPs in 7b after the Suzuki reaction. Figure S28. TEM image (a) and size distribution (b) of PdNPs in 8b before the Suzuki reaction. Figure S29. TEM image (a) and size distribution (b) of PdNPs in 8b after the Suzuki reaction. Figure S30. TEM image (a) and size distribution (b) of PdNPs in 9b before the Suzuki reaction. Figure S31. TEM image (a) and size distribution (b) of PdNPs in 9b after the Suzuki reaction. Figure S32. The size distribution of metal-organic aggregates in solution of catalytic system based on 4b. Figure S33. The size distribution of metal-organic aggregates in solution of catalytic system based on 5b. Figure S34. The size distribution of metal-organic aggregates in solution of catalytic system based on 6b. Figure S35. The size distribution of metal-organic aggregates in solution of catalytic system based on 7b. Figure S36. The size distribution of metal-organic aggregates in solution of catalytic system based on 8b. Figure S37. The size distribution of metal-organic aggregates in solution of catalytic system based on 9b.