Tri-Tert-Butyl(N-Alkyl)Phosphonium Ionic Liquids: Structure, Properties and Application As Hybrid Catalyst Nanomaterials

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

Daria M. Arkhipova 1,2,* , Vadim V. Ermolaev 2 , Vasili A. Miluykov 2 , Farida G. Valeeva 2, Gulnara A. Gaynanova 2, Lucia Ya. Zakharova 2, Mikhail E. Minyaev 1 and Valentine P. Ananikov 1

1 N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect, 47, 119991 Moscow, Russia; [email protected] (M.E.M.); [email protected] (V.P.A.) 2 Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientiﬁc Center, Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia; [email protected] (V.V.E.); [email protected] (V.A.M.); [email protected] (F.G.V.); [email protected] (G.A.G.); [email protected] (L.Y.Z.) * Correspondence: [email protected]

Abstract: A series of sterically hindered tri-tert-butyl(n-alkyl)phosphonium salts (n-CnH2n+1 with n = 1, 3, 5, 7, 9, 11, 13, 15, 17) was synthesized and systematically studied by 1H, 13C, 31P 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 Citation: Arkhipova, D.M.; family of PILs has a broad variety of structural features, including hydrophobic and hydrophilic Ermolaev, V.V.; Miluykov, V.A.; Valeeva, F.G.; Gaynanova, G.A.; structures that are entirely expressed in the diversity of their properties Zakharova, L.Y.; Minyaev, M.E.; Ananikov, V.P. Tri-tert-butyl(n- Keywords: sterically hindered phosphonium ionic liquids; PdNPs; TEM; Suzuki cross-coupling alkyl)phosphonium Ionic Liquids: reaction; DLS Structure, Properties and Application as Hybrid Catalyst Nanomaterials. Sustainability 2021, 13, 9862. https:// doi.org/10.3390/su13179862 1. Introduction Ionic liquids (ILs) are typically composed of an organic cation (ammonium, imida- Academic Editor: Paula Ossowicz zolium, phosphonium, pyridinium, sulfonium) and inorganic anion [1]. These compounds are of much interest due to the combination of attractive properties such as limited volatil- Received: 28 July 2021 ity and flammability, broad temperature range of the liquid state, thermal and chemical Accepted: 31 August 2021 stability, wide electrochemical potential window, tunable solubility and miscibility and Published: 2 September 2021 highly variable biological activity [2–6]. The main feature of ILs is a high variability of the structure (more than 106 possible compounds), which gives unprecedented opportunities Publisher’s Note: MDPI stays neutral for customization for specific tasks. with regard to jurisdictional claims in The first members of IL as a class attributed to the reports the synthesis of ethanolam- published maps and institutional affil- monium nitrate by Gabriel [7] at the end of the 19th century and ethylammonium nitrate by iations. 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 Copyright: © 2021 by the authors. of new ILs. The main trend for now is developing their industrial application [11]. Licensee MDPI, Basel, Switzerland. One of the promising fields of IL application is energy storage devices [12], as elec- This article is an open access article trolytes in Li-ion batteries [13], including IL-modified polymers [14]. Considering a number distributed under the terms and of requirements for the electrolyte, ILs are often the system of choice for these purposes [15]. conditions of the Creative Commons Attribution (CC BY) license (https:// Currently, sodium-ion batteries are under intensive development [16], and ILs have been creativecommons.org/licenses/by/ proposed to be used as electrolytes in such devices [17]. The IL application in electrochem- 4.0/). istry extends to the modification of electrodes [18] and design of supercapacitors [19].

Sustainability 2021, 13, 9862. https://doi.org/10.3390/su13179862 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 9862 2 of 19

Illustrating the use of ILs is their employment in biomedicine [20] and related areas [21]. The range of their activity is not limited only by antimicrobial ability [22]; ILs also exhibit antitumor [23] and antiviral activity [24]. In biotechnology, ILs serve as an effective solvent for pre-extraction treatment [25] and cellulose dissolution [26]. In tribology, ILs became useful as a lubricant [27] and continued to develop in this direction [28,29]. In general, the role of a tunable solvent for various chemical processes is the main area of application for ILs [30,31]. One of the most remarkable contributions of ILs in the chemical industry is observed in the field of transition metal catalysis [2,32]. In the case of nanosized catalysts, ILs serve as both stabilizers and solvents [33–35]. To enhance catalysis efficiency, the rational design of ILs should be emphasized [36]. The microstructure [37] and ecotoxicity of ILs [38,39] are emerging topics of much attention. The conjunction of hydrogen bonding and ionic interactions of the IL forces microstructural directionality. Such a supramolecular network acts as an “entropic driver” for the design of complex multiscale frames [40]. MacFarlane defines the development prospective of IL study [41]. Electrochemistry and biotechnology are highlighted as the most promising fields of IL adaptation in the industry. The IL capabilities for tuning allow the achievement of specific microstructures and distinct macroscopic functions [37]. Based on the above, ILs are in high demand and require comprehensive study of the novel members of this unique family of compounds [42]. This trend is currently realized in systematic studies of IL lines [43,44]. Particular attention has been paid to phosphonium ionic liquids (PILs) due to their thermal stability. Resistance to decomposition under basic conditions is of special note comparing to nitrogen-containing analogues [45,46]. PILs have found their application in solving urgent problems such as the separation of platinum group metals from used automotive catalysts [47], in fuel desulfurization [48], CO2 sorption [49] and energy storage [50]. In our group, the first examples of sterically hindered PILs were synthesized and described [51]. Earlier we have demonstrated that steric hindrance of PILs plays a key role in nanoparticle stabilization and their catalytic activity in Suzuki cross-coupling reaction. This effect is similar to the catalysis by Pd complexes with sterically demanded phosphines, although the mechanism of the processes is definitely different [52,53]. Previously, we have synthesized and investigated sterically hindered PILs with even numbers of carbon atoms in side chain [54]. Here, we systematically extended the series of sterically hindered PILs with compounds containing odd numbers of methylene fragments in alkyl substituents and characterized them. The obtained PILs were applied to design a catalytic system based on PdNPs. The catalytic hybrid material was characterized with TEM and DLS and successfully tested in Suzuki cross-coupling reaction.

2. Materials and Methods 2.1. Instruments NMR spectroscopy. NMR spectra were recorded on a Fourier 300 HD instrument (Bruker, Fällanden, Switzerland) at 21 ◦C(1H 300.13 MHz, 31C 75.47 MHz) and AVneo ◦ 31 300 instrument (Bruker, Fällanden, Switzerland) at 21 C( P 121.54 MHz). SiMe4 was 1 13 used as an internal reference for H and C NMR spectra, and 85% H3PO4 was used as an external reference for 31P NMR spectra. Single-crystal X-ray diffraction analysis. X-ray diffraction data for 2b and 3b (Table1) were collected at 100 K on a Quest D8 diffractometer (Bruker, Bremen, Germany) equipped with a Photon-III area-detector (shutterless ϕ- and ω-scan technique), using monochromatized Mo Kα-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 Kα-radiation; the intensity data were integrated and corrected for absorption and decay Sustainability 2021, 13, 9862 3 of 19

by the CrysAlisPro program [57]. The structures were solved by direct methods using SHELXT [58] and refined on F2 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 (RH) was calculated according to the Stokes–Einstein relation: DS = kBT/6πηRH, in which DS is the diffusion coefficient, kB 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%.

2.2. 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. Tri-tert-butylphosphine (Dal-Chem, Nizhny Novgorod, Russia), methyl and propyl iodides and n-alkyl bromides (Sigma-Aldrich, Burlington, MA, USA), phenylboronic acid (Alfa Aesar, Heysham, UK), 1,3,5-tribromobenzene (Alfa Aesar, Heysham, UK), Pd(OAc)2 (Alfa Aesar, Heysham, UK), KOH (Himreaktiv, Stary Oskol, Russia), NaBF4 (Himreaktiv, Stary Oskol, Russia) analytical grade, without additional puriﬁcation were used in the work. Detailed synthesis and characterization of the phosphonium salts are described in Supplementary Materials.

2.2.1. 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.

2.2.2. 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. Sustainability 2021, 13, x FOR PEER REVIEW 4 of 19 Sustainability 2021, 13, x FOR PEER REVIEW 4 of 19

To a fresh solution of colloidal Pd, 0.16 g (0.5 mmol) of 1,3,5-tribrombenzene, 0.28 g (2.2 mmol)To a fresh of phenylboronic solution of colloidal acid, andPd, 0.160.13 gg (0.5(2.2 mmol)mmol) ofof 1,3,5-tribrombenzene, potassium hydroxide 0.28 were g added.(2.2 mmol) The ofreaction phenylboronic mixture waacid,s stirred and 0.13 over g 7(2.2 h at mmol) 30 °С. ofOrganic potassium compounds hydroxide were were ex- Sustainability 2021, 13, 9862 added.tracted withThe reaction 9 mL toluene mixture and wa analyzeds stirred by over GC–MS. 7 h at 30 °С. Organic compounds4 ofwere 19 extracted with 9 mL toluene and analyzed by GC–MS. 3. Results and Discussion 3. Results and Discussion 3.3.1. Results Synthetic and Procedures Discussion 3.1. Synthetic Procedures 3.1. SyntheticThe quaternization Procedures reaction of tri-tert-butylphosphine and respective linear 1-haloal- kaneThe The(Hal quaternization quaternization = I or Br) was reaction conductedreaction of tri- of tertunder tri--butylphosphinetert solven-butylphosphinet-free and conditions respective and respective except linear for 1-haloalkane linear the cases 1-haloal- with kane (Hal = I or Br) was conducted under solvent-free conditions except for the cases with (Halsolid = haloalkanes. I or Br) was conductedAcetonitrile under (for C solvent-free15H31Br) and conditions DMF (for except C17H35 forBr) thewere cases applied with as a solidsolidsolvent haloalkanes. haloalkanes. (Scheme 1). Acetonitrile Acetonitrile The temperature (for (for C15 CH of1531H theBr)31Br) re andaction and DMF DMF was (for varied(for C17 HC3517 dependingBr)H35Br) were were applied on applied the as applied a as a solventhaloalkane,solvent (Scheme (Scheme from1). 1). − The70 The °C temperature temperaturefor MeI [61] of thetoof 90–100the reaction reaction °C was for was variedliquid varied depending1-bromoalkanes depending on the on and applied the 80 applied °C for haloalkane, from −70 ◦C for MeI [61] to 90–100 ◦C for liquid 1-bromoalkanes and 80 ◦C for thehaloalkane, solid ones. from The − 70reaction °C for wasMeI completed[61] to 90–100 afte °Cr disappearance for liquid 1-bromoalkanes of the phosphine and 80signal °C for in the solid ones. The reaction was completed after disappearance of the phosphine signal in the solid31 ones. The reaction was completed after disappearance of the phosphine signal in the31 P NMR spectrum. thethe 31PP NMRNMR spectrum. spectrum.

Scheme 1. The quaternization reaction of tri-tert-butylphosphine and 1-haloalkane. SchemeScheme 1. 1.The The quaternization quaternization reaction reaction of tri-of tri-terttert-butylphosphine-butylphosphine and and 1-haloalkane. 1-haloalkane.

TheThe metathesis metathesis reaction reaction of phosphonium of phosphonium halide halide to weakly to weaklycoordinating coordinating tetrafluoroborate-anionThe metathesis was carried reaction out in of water phosphonium at room temperaturehalide to weakly (Scheme coordinating 2). Being tetrafluorob-poorly solu- tetrafluoroborate-anionorate-anion was carried was out carried in water out at in room water temperature at room temperature (Scheme (Scheme 2). Being2). poorly Being solu- poorlyble in solublewater, inphosphonium water, phosphonium tetrafluoroborates tetrafluoroborates precipitated precipitated as soon as soon as assodium sodium tetra- ble in water, phosphonium tetrafluoroborates precipitated as soon as sodium tetra- tetrafluoroboratefluoroborate was was added added to the to thesolution solution of phosphonium of phosphonium halide, halide, which which is readily is readily soluble fluoroborate was added to the solution of phosphonium halide, which is readily soluble solublein water. in water. in water.

SchemeScheme 2. 2.The The anion anion exchange exchange reaction. reaction. Scheme 2. The anion exchange reaction. NewNew compounds compounds were were obtained obtained in highin high yields yields (up to(up 98%; to 98%; see Supplementary see Supplementary Materi- Mate- alsrials for Newfor details). details). compounds The The phosphonium phosphonium were obtained salts salts arein high whiteare whiteyields or yellowish or (up yellowish to 98%; crystalline seecrys Supplementarytalline or amorphous or amorphous Mate- solidsrialssolids for that that details). are are soluble soluble The inphosphonium in chloroform, chloroform, dichloromethane, salts dichloromethane, are white or acetone, yellowish acetone, ethanol, crysethanol,talline methanol, methanol, or amorphous DMF, DMF, DMSODMSOsolids andthat and almostare almost soluble insoluble insoluble in chloroform, in in diethyl diethyl ether dichloromethane, ether and and petroleum petroleum acetone, ether. ether. ethanol, methanol, DMF, DMSO and almost insoluble in diethyl ether and petroleum ether. 3.2. NMR Spectroscopy 3.2. NMR Spectroscopy 3.2.1.3.2. NMR31P NMR Spectroscopy Spectral Data 3.2.1. 31P NMR Spectral Data 3.2.1.The 31P31 NMRP{1H} Spectral NMR spectra Data of the tri-tert-butyl(n-alkyl)phosphonium salts (Table1) exhibitedThe a31P{ singlet1H} NMR in the spectra range of the 47.7–51.3 tri-tert ppm.-butyl( Then-alkyl)phosphonium chemical shifts of phosphonium salts (Table 1) ex- The 31P{1H} NMR spectra of the tri-tert-butyl(n-alkyl)phosphonium salts (Table 1) ex- saltshibited weakly a singlet depend in the on therange anion. of 47.7–51.3 Phosphonium ppm. halidesThe chemical have chemical shifts of shifts phosphonium mostly in salts hibited a singlet in the range of 47.7–51.3 ppm. The chemical shifts of phosphonium salts lowerweakly magnetic depend fields on the than anion. phosphonium Phosphonium tetrafluoroborates, halides have onchemical average shifts 1–2 ppm. mostly At in the lower weakly depend on the anion. Phosphonium halides have chemical shifts mostly in lower samemagnetic time, fields the length than of phosphonium the alkyl substituent tetrafluor in theoborates, phosphonium on average cation 1–2 also ppm. has At a weak the same influencetime,magnetic the on lengthfields the chemical thanof the phosphonium alkyl shift substituent in 31P{ 1tetrafluorH} NMRin the oborates,spectra.phosphonium The on differenceaverage cation 1–2also is notppm. has more a At weak thanthe influ-same time, the length of the alkyl substituent in the phosphonium cation also has a weak influ- 3.6ence ppm. on Thethe chemicalchemical shifts shift of in1a 31andP{1H}1b NMRappear spectra. in a lower The magnetic difference field, is possibly not more due than to 3.6 31 1 theenceppm. lack onThe of the electron chemical chemical density shifts shift of of thein 1a methylandP{ H} 1b NMR substituent appear spectra. in toa compensatelower The differencemagnetic for the field,is cationicnot possibly more charge than due 3.6 to onppm. the phosphorusThe chemical atom. shifts of 1a and 1b appear in a lower magnetic field, possibly due to Sustainability 2021, 13, x FOR PEER REVIEW 5 of 19

Sustainability 2021, 13, 9862 the lack of electron density of the methyl substituent to compensate for the cationic5 of 19 charge on the phosphorus atom.

3.2.2.3.2.2.1 H1H NMR NMR Spectral Spectral Data Data UnlikeUnlike31 P{31P{1H}1H} NMR NMR spectra, spectra, anion anion exchange exchange has a has great a inﬂuencegreat influence on the chemical on the chemical shiftsshifts in in1H 1H NMR NMR spectra spectra (Table (Table1), especially 1), especially for the forα-protons. the α-protons. The α-protons The α-protons appeared appeared 1 inin the theH 1H NMR NMR spectra spectra as aas multiplet a multiplet (Figure (Figure1). 1).

FigureFigure 1. 1.α -Protonsα-Protons in stericallyin sterical hinderedly hindered PILs. PILs.

Table 1. 1H NMR chemical shifts (δ (ppm), CDCl , 300.13 MHz, 21 ◦C) and 31P{1H} NMR chemical Table 1. 1H NMR chemical shifts (δ (ppm), CDCl3 3, 300.13 MHz, 21 °C) and 31P{1H} NMR chemical shifts (δ (ppm), CDCl , 121.54 MHz, 21 ◦C). shifts (δ (ppm), CDCl3 3, 121.54 MHz, 21 °C).

Phosphonium Salt Chemical Shift of Chemical Shift Phosphonium Salt Chemical1 Shift of α- 31 Chemical Shift N Cation Anion α-Protons H NMR, ppm P NMR, ppm Protons 1H NMR, ppm 31P NMR, ppm N Cation −Anion 1a + I 2.11 51.3 t-Bu3P CH3 − − 1b1a BF4 I 1.902.11 51.0 51.3 3 + 3 − 2a t-Bu+ P CH I 2.60 50.7 1b t-Bu3P C3H7 −BF4− 1.90 51.0 2b BF4 2.33 49.2 − 3a + Br − 2.54 48.8 2a t-Bu3P C5H11 − I 2.60 50.7 3b t-Bu3P+C3H7 BF4 2.32 49.2 − − 4a2b + Br BF4 2.562.33 49.4 49.2 t-Bu3P C7H15 − 4b BF4 2.29 47.7 3a − Br− 2.54 48.8 5a + + Br 2.49 49.3 t-But-Bu3P 3CP9HC195H11 − 5b BF4 4− 2.27 47.7 3b −BF 2.32 49.2 6a + Br 2.49 50.6 t-Bu3P C11H23 − − 6b4a BF4 Br 2.282.56 48.9 49.4 t-Bu3P+C7H15 − 7a + Br − 2.46 50.7 4b t-Bu3P C13H27 −BF4 2.29 47.7 7b BF4 2.26 48.9 − − 8a5a + Br Br 2.502.49 50.6 49.3 t-Bu3P C15+H31 − 8b t-Bu3P C9H19 BF4 2.27 48.8 5b −BF4− 2.27 47.7 9a + Br 2.48 50.6 t-Bu3P C17H35 − 9b6a BF4 Br− 2.262.49 48.8 50.6 t-Bu3P+C11H23 6b BF4− 2.28 48.9 The signal of α-protons in phosphonium tetrafluoroborates appears in a higher mag- 7a ± Br− 2.46 ± 50.7 netic field (2.30 0.03 ppm)+ than that in phosphonium bromides (2.53 0.07 ppm). This is t-Bu3P C13H27 α related7b to the presence of hydrogen bondsBF4− between -protons2.26 and halogen anions bearing 48.9 high electron affinity. The weakly coordinated tetrafluoroborate anion interacts with the − surrounding8a cations via noncooperativeBr Coulomb forces [622.50]. 50.6 3 + 15 31 t-Bu P C H 1 The8b chemical shift of α-protons inBFH4− NMR spectra moves2.27 towards a higher magnetic 48.8 field with the elongation of the alkyl substituent (Figure2) due to the compensation of 9a Br− 2.48 50.6 electronic densityt-Bu withdrawn3P+C17H35 by the cationic charge on the phosphorus atom. The exception is the9bα-protons of 1a and 1b, where theBF peaks4− were at 2.11 ppm2.26 and 1.90 ppm, respectively, 48.8 due to belonging to the α-protons of the methyl group. 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 1H NMR spectra moves towards a higher magnetic field with the elongation of the alkyl substituent (Figure 2) due to the compensation of Sustainability 2021, 13, x FOR PEER REVIEW 6 of 19

electronic density withdrawn by the cationic charge on the phosphorus atom. The excep- Sustainability 2021, 13, x FOR PEER REVIEWtion is the α-protons of 1a and 1b, where the peaks were at 2.11 ppm and 1.90 ppm,6 of re-19 spectively, 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 Sustainability 2021, 13, 9862 end of the fourth alkyl substituent moves towards a higher magnetic field until it reaches6 of 19 electronic density withdrawn by the cationic charge on the phosphorus atom. The excep- 0.88 ppm (Figure 2). The methylene protons belonging to carbon atoms starting from the tion is the α-protons of 1a and 1b, where the peaks were at 2.11 ppm and 1.90 ppm, re- fourth one from the phosphorus atom have multiple peaks in the region 1.35–1.20 ppm. spectively, 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.

1 1 3 ◦ FigureFigure 2. 2.High High ﬁeld field area area of of the the HH NMR NMR spectra spectra (CDCl (CDCl3,, 300.13 300.13 MHz, MHz, 21 21 °C)C)of of1b 1b,,2b 2b,,3b 3band and9b 9b..

3.2.3.As 13C{ the1H} number NMR Spectral of methylene Data groups increases, the shift of the methyl group at the end of the fourth alkyl substituent13 moves towards a higher magnetic ﬁeld until it reaches A particular pattern in C NMR spectra for the investigated PILs is observed. For 0.88convenient ppm (Figure discussion,2). The we methylene propose protons labeling belonging the carbon to atoms carbon according atoms starting to Figure from 3. the The Figure 2. High field area of the 1H NMR spectra (CDCl3, 300.13 MHz, 21 °C) of 1b, 2b, 3b and 9b. fourthspectra one of from1b, 2b the and phosphorus 3b are distinct; atom havefor the multiple rest of peaksthe PILs, in the the region peak positions 1.35–1.20 are ppm. close

to each13 other.1 3.2.3.3.2.3.13 C{C{1H} NMR Spectral Data 13 AA particular particular pattern pattern in in13 CC NMRNMR spectraspectra forfor the the investigated investigated PILs PILs is is observed. observed. For For convenientconvenient discussion, discussion, wewe propose propose labeling labeling the the carbon carbon atoms atoms according according to to Figure Figure3 .3. The The spectraspectra of of1b 1b, 2b, 2band and3b 3bare are distinct; distinct; for for the the rest rest of of the the PILs, PILs, the the peak peak positions positions are are close close to eachto each other. other.

Figure 3. Atom numbering used for the phosphonium cation in discussion of the 13C NMR spectra.

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 1313 FigureFigurecoupling 3. 3.Atom Atom constant numbering numbering is 32.1 used used Hz. for for the the phosphonium phosphonium cation cation in in discussion discussion of of the the CC NMR NMR spectra. spectra.

TheThe chemicalchemical shiftshift ofof quaternary quaternary carbon carbon atom atom C Cq qappears appears asasa a doublet doublet at at 39.3 39.3 ppm ppm withwith aa ﬁrst-order first-order P-C P-C spin–spin spin–spin coupling coupling constantconstant ofof 29.429.4± ± 0.30.3 HzHz (Figure(Figure3 ,3, Table Table2). 2). ThereThere is is one one exception exception for for1b 1b,, whose whose peak peak appears appears at at 37.9 37.9 ppm, ppm, and and the the ﬁrst-order first-order P-C P-C couplingcoupling constant constant is is 32.1 32.1 Hz. Hz. Sustainability 2021, 13, 9862 7 of 19

13 1 ◦ Table 2. C{ H} NMR chemical shifts (δ (ppm), CDCl3, 75.47 MHz, 21 C), with coupling constants JPC (Hz).

PIL Cq Ct Ca Cb Cc Cm Cx Cy Cz 1 1 1b 37.9 (d, JPC = 32.1) 29.1 0.4 (d, JPC = 45.8) ------1 1 2 3 2b 39.2 (d, JPC = 29.4) 29.8 20.4 (d, JPC = 35.2) 18.9 (d, JPC = 6.4) 16.3 (d, JPC = 14.2) ---- 1 1 2 3 3b 39.3 (d, JPC = 29.1) 29.8 18.5 (d, JPC = 35.3) 24.7 (d, JPC = 6.7) 33.7 (d, JPC = 12.6) - - 22.2 13.8 1 1 2 3 4b 39.3 (d, JPC = 29.2) 29.8 18.5 (d, JPC = 35.2) 25.1 (d, JPC = 6.6) 31.7 (d, JPC = 12.7) 28.8 31.5 22.6 14.0 1 1 2 3 5b 39.3 (d, JPC = 29.3) 29.8 18.5 (d, JPC = 35.2) 25.0 (d, JPC = 6.6) 31.7 (d, JPC = 12.4) 29.3; 29.2 31.8 22.6 14.1 1 1 2 3 6b 39.3 (d, JPC = 29.1) 29.8 18.5 (d, JPC = 34.9) 25.1 (d, JPC = 6.6) 31.7 (d, JPC = 12.6) 29.6; 29.4; 29.3; 29.2 31.9 22.7 14.1 1 1 2 3 7b 39.3 (d, JPC = 29.4) 29.8 18.5 (d, JPC = 35.0) 25.0 (d, JPC = 6.7) 31.7 (d, JPC = 12.6) 29.6; 29.3; 29.2 31.9 22.7 14.1 1 1 2 3 8b 39.3 (d, JPC = 29.4) 29.8 18.5 (d, JPC = 35.2) 25.0 (d, JPC = 6.7) 31.7 (d, JPC = 12.6) 29.7–29.5; 29.4; 29.2 31.9 22.7 14.1 1 1 2 3 9b 39.3 (d, JPC = 29.6) 29.8 18.5 (d, JPC = 35.1) 25.0 (d, JPC = 6.5) 31.7 (d, JPC = 12.5) 29.7–29.5; 29.4–29.3; 29.2 31.9 22.7 14.1 Sustainability 2021, 13, x FOR PEER REVIEW 7 of 19

The carbon atom of methyl groups in tert-butyl Ct 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]. Three carbon atoms in methylene groups close to the phosphorus atom (Ca, Cb, Cc) 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 at- tribute the chemical shift in the 13C NMR spectra with the highest value of the coupling constant to the carbon atom Ca. The peculiar feature of the 13C 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 Sustainability 2021, 13, 9862 chemical shift of Ca of 2b is located at 20.4 ppm. Along with elongation of the alkyl chain,8 of 19 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 pointedThe out carbon that atomthe P-C of spin–spin methyl groups coupling in constanttert-butyl for C tCdoesb is less not than have that the for P-C Cc (6.4–6.7 spin–spin couplingHz and 12.4–14.2 constant despiteHz respectively). it being located This could two bondsbe caused from by the the phosphorus steric effect atomof tert (Figure-butyl 4). Thisgroups effect (shielding could be or related the possibility to the quaternary of realizing carbon only one atom certain in between conformation) them [62 or]. by the influence of the anion.

13 ◦ FigureFigure 4. 4. SelectedSelected region of of the the 13C CNMR NMR spectra spectra (CDCl (CDCl3, 75.473, 75.47 MHz, MHz, 21 °C) 21 of C)1b, of 2b1b, 3b, 2b and, 3b 9band; 9b; peakspeaks are are assigned assigned accordingaccording Figure 33..

Three carbon atoms in methylene groups close to the phosphorus atom (C ,C ,C ) ap- The chemical shift of the terminal methyl group of the linear alkyl chain Cz isa sensitiveb c pearto the as affinity doublets to with the phosphorus the following atom, P-C spin–spinand it moves coupling towards constant the lower values: magnetic35.1 ± field0.2 Hz, 12.6until± it0.1 reaches Hz, 6.6 14.1± 0.1 ppm, Hz which (Figure is4 typi). Oncal the for assumption the methyl group of 1-D of and alkanes. 2-D NMR spectroscopy (COSY,The HSQC, signals HMBC), of the middle we specified carbon atoms the signals (Cm) in of the these alkyl carbon chain appeared atoms. We in attributethe range the 13 chemicalof 28.8–29.7 shift ppm in the(FigureC NMR4). spectra with the highest value of the coupling constant to 13 the carbon atom Ca. The peculiar feature of the C NMR spectra of 1b is the position of the Ca peak in the high field (0.4 ppm, Table2) 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. The chemical shift of the terminal methyl group of the linear alkyl chain Cz 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 (Cm) in the alkyl chain appeared in the range of 28.8–29.7 ppm (Figure4).

3.3. 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 S1–S13 in Supplementary Materials). The asymmet- + − ric unit of 2b contains one [t-Bu3PPr] cation and one disordered [BF4] anion (Z’ = 1) (Figure5 ). It might be noted that only the use of AgBF4 allowed us to obtain pure crystal 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 substitutional disorder of the anion were formed, in which the iodine atom is located nearly at the boron atom Sustainability 2021, 13, x FOR PEER REVIEW 9 of 19

3.3. 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 S1–S13 in Supplementary Materials). The asym- + − 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 crystal + + − − 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 substitutional Sustainability 2021, 13, 9862 9 of 19 disorder of the anion were formed, in which the iodine atom is located nearly at the boron − − atom position. However, the [BF4]−/I− substitutional disorder was not detected for compounds 3b and 4b containing a longer n-alkyl chain. The asymmetric unit of 3b (Figure 6) − − position.and 4b (Figure However, 7) the consists [BF4] /Iof threesubstitutional (Z’ = 3) disorderand of four was not(Z’detected = 4) crystallographically for compounds non- 3b and 4b containing a longer n-alkyl+ chain.− The asymmetric unit of 3b (Figure6) and 4b equivalent [t-Bu3P(n-CnH2n+1)]+[BF4]− ion pairs, correspondingly. In case of 4b, six out of (Figureeight ions7) consists were ofentirely three (Z’ disordered = 3) and of (Figures four (Z’ = S8–S13, 4) crystallographically Tables S8 and non-equivalent S9 in Supplementary [t-Bu P(n-C H )]+[BF ]− ion pairs, correspondingly. In case of 4b, six out of eight ions Materials).3 n 2n+1 4 were entirely disordered (Figures S8–S13, Tables S8 and S9 in Supplementary Materials). EnlargementEnlargement of of the the alkyl alkyl chain chain likely likely leads leads to increase to increase in a number in a number of crystallograph- of crystallograph- icallyically independent independent ion ion pairs pairs (Z’) (Z’) and and disorder disorder augmentation. augmentation. The The crystal crystal packing packing of of steri- stericallycally hindered hindered PILs PILs with with signiﬁcantlysignificantly different different length length of alkyl of alkyl substituent substituent was reported was reported earlierearlier [ 54[54].].

− − FigureFigure 5. 5.Crystal Crystal structure structure of of2b 2b. The. The disorder disorder of theof the BF 4BF4anion anion is notis not shown. shown. Hereinafter Hereinafter the the prob- probabilityability of thermal of thermal ellipsoids ellipsoids is is set set to to the the 50% level.level.

FigureFigure 6. 6.Crystal Crystal structure structure of 3b of. 3b One. One of three of three non-equivalent non-equivalent ion pairs ion is pairs shown. is shown.

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Figure 7. Crystal structure of 4b. One of four crystallographically non-equivalent ion pairs is shown.

3.4. 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 anFigure increase 7.7.Crystal Crystal in the structure structuremethylene of of4b .4bgroups One. One of four ofin fourthe crystallographically alkylcrystallographically chain, Tm non-equivalent diminishes non-equivalent step- ion pairs ion is shown. pairs is shown. wise and reaches a minimum. This fact is connected with the gain of disordering in the 3.4. Melting Point crystal lattice. The3.4. next Melting slight Point increase in the melting point of long-chained PILs is caused The melting point (Tm) of the synthesized PILs was measured. The trend of Tm as by hydrophobic intermolecular interactions. The corresponding dependence of Tm on the a functionThe melting of the number point (T ofm carbon) of the atoms synthesized in the alkyl PILs substituent was measured. of PIL is The presented trend inof Tm as a elongation of alkyl substituents in the series of homologous compounds was reported ear- functionFigure8. of the number of carbon atoms in the alkyl substituent of PIL is presented in lier [54,63]. Figure 8. The first two members of the PIL series have high melting points above 200 °C and 350 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 300 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 250 crystal lattice. The next slight increase in the melting point of long-chained PILs is caused

С by hydrophobic intermolecular interactions. The corresponding dependence of Tm on the 200 elongation of alkyl substituents in the series of homologous compounds was reported earlier [54,63]. BF4 150 Br 350 melting point, ° 100 300 50 250

0 С 0123456789101112131415161718200 n BF4 150 Figure 8. Melting points as a function of the number of carbons, n, in the fourth alkyl substituent on the phosphorus atom. Br melting point, ° ◦ 100The 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.50 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 0 0123456789101112131415161718 n

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lattice. The next slight increase in the melting point of long-chained PILs is caused by Figure 8. Melting points as a function of the number of carbons, n, in the fourth alkyl substituent hydrophobic intermolecular interactions. The corresponding dependence of Tm on the on the phosphorus atom. 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 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 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 (approxi- the row. It should be noted that 4a and 4b have almost the same melting point (approx- mately 125 ◦°C). The Tm values of phosphonium bromides in most cases are lower than imately 125 C). The Tm values of phosphonium bromides in most cases are lower than thosethose for for phosphonium phosphonium tetrafluoroborates. tetrafluoroborates. The The fluorine fluorine atoms atoms have have short short contacts contacts with with hy- drogenhydrogen atoms atoms belonging belonging to the to linearthe linear alkyl alkyl chain, chain, resulting resulting in an increasein an increase in intermolecular in intermo- interactions.lecular interactions. It is remarkable It is remarkable that 6a, 7athat, 8a 6aand, 7a,9a 8amelt and below9a melt 100 below◦C. 100 °C.

3.5.3.5. PdNPPdNP StabilizationStabilization andand TEMTEM SampleSample PreparationPreparation PILsPILs have have been been proven proven to to be be an an effective effective stabilizer stabilizer of of metal metal nanoparticles nanoparticles [ 5[5,64].,64]. The The applicationapplication of of stabilized stabilized NPs NPs is is widespread widespread in thein the area area of catalysisof catalysis [65] [65] and and other other areas. areas. ILs couldILs could be also be involvedalso involved in the in creation the creation of novel of novel nanocomposites nanocomposites with biopolymers with biopolymers for drug for anddrug gene and delivery gene delivery [66]. In [66]. addition, In addition, ILs could ILs becould applied be applied for magnetic for magnetic NP stabilization NP stabiliza- to extracttion to heavyextract metal heavy ions metal [67 ].ions [67]. ForFor the the currentcurrent study, study, PdNPs PdNPs were were obtained obtained by by the the reduction reduction of of palladium palladium acetate acetate in in ethanolethanol inin thethe presencepresence ofof thethe correspondingcorresponding PILPIL (Supplementary(Supplementary Materials,Materials, TableTable S10).S10). AccordingAccording to to previous previous investigation, investigation, ethanol ethanol is is the the solvent solvent of of choice choice for for efﬁcient efficient synthesis synthesis ofof hybrid hybrid NP NP systems systems [ 54[54].]. TheThe PdNPPdNP synthesissynthesis procedureprocedure does does not not require require any any reducing reducing agent,agent, elevatedelevated temperaturetemperature oror inertinert atmosphere.atmosphere. TheThe PdNPPdNP sizesize waswas estimatedestimated usingusing transmissiontransmission electronelectron microscopymicroscopy (TEM)(TEM) (Figure(Figure9 ).9). The The “nanoﬁshing” “nanofishing” technique technique was was usedused toto capture capture nanoparticles nanoparticles from from the the liquid liquid phase. phase. The The copper copper grid grid was was immersed immersed in in a a colloidalcolloidal solution solution of of the the PdNPs PdNPs for for 30 s,30 rinsed s, rinsed with with acetone acetone to remove to remove organic organic components compo- andnents dried and in dried air. in air.

Figure 9. TEM image of PdNPs stabilized with 9b. Figure 9. TEM image of PdNPs stabilized with 9b.

Here,Here, the the key key point point of of the the present present study stud shouldy should be emphasized.be emphasized. A common A common procedure proce- involvesdure involves drying drying the solution the solution and and then then using using the the obtained obtained solid solid phase phase for for microscopic microscopic analysis.analysis. However, in such a a case, case, nanopartic nanoparticlesles may may form form during during the the drying drying process, process, or ortheir their morphology morphology may may substantially substantially change change upon upon drying. drying. In the In thepresent present study, study, we cap- we capturedtured nanoparticles nanoparticles directly directly from from solution solution without without carrying carrying out out a destructivedestructive isola-isolation/dryingtion/drying process.process.

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3.6.3.6. PdNPPdNP BehaviorBehavior duringduring SuzukiSuzuki ReactionReaction FreshlyFreshly prepared prepared PdNPs PdNPs were were involved involved in the Suzukiin the cross-couplingSuzuki cross-coupling reaction (Scheme reaction3) as(Scheme a catalyst. 3) as Pda catalyst. (0.36 mol%) Pd (0.36 and mol%) the molar and ratiothe molar of stabilizer/Pd ratio of stabilizer/Pd 100/1 were 100/1 used were as optimalused as (Tableoptimal S10). (Table The S10). GC–MS The method GC–MS was meth usedod was to estimate used to the estimate conversion the conversion degree of 1,3,5-tribromobenzenedegree of 1,3,5-tribromobenzene into 1,3,5-triphenylbenzene. into 1,3,5-triphenylbenzene. It should be It notedshould that be triphenylben-noted that tri- zenephenylbenzene was formed was as aformed main product. as a main product.

SchemeScheme 3.3. SuzukiSuzuki cross-couplingcross-coupling reactionreaction ofof 1,3,5-tribromobenzene1,3,5-tribromobenzeneand andphenylboronic phenylboronic acid. acid.

TheThe challengingchallenging tri-substitutedtri-substituted substratesubstrate andand mildmild conditionsconditions ofof thethe catalyticcatalytic reactionreaction werewere chosenchosen toto emphasizeemphasize thethe differencedifference betweenbetween thethe catalyticcatalytic systemssystems basedbased onon PILsPILs ofof differentdifferent structures. structures. The The conversion conversion of of 1,3,5-tribromobenzene 1,3,5-tribromobenzene into into 1,3,5-triphenylbenzene 1,3,5-triphenylbenzene is ais good a good benchmarking benchmarking system, system, which which we usedwe used here here to analyze to analyze the behavior the behavior of the of particular the par- studiedticular studied catalytic catalytic system system in the cross-coupling in the cross-coupling reaction. reaction. TheThe preparationpreparation of of the the PdNPs PdNPs for for TEM TEM analysis analysis was was carried carried out out after after 20 min20 min of stirring of stir- thering phosphonium the phosphonium salt and salt palladium and palladium acetate acetate in ethanol in ethanol and 7 and h after 7 h the after start the of start the cross-of the couplingcross-coupling reaction reaction using theusing “nanofishing” the “nanofishing technique.” technique. For each For entry, each100–500 entry, 100–500 nanoparticles nano- wereparticles processed were toprocessed calculate to the calculate average sizethe andaverage standard size deviationand standard (Table deviation3, Figures (Table S14–S31 3, inFigures Supplementary S14–S31 in Materials). Supplementary There wereMaterials) aggregates. There and were individual aggregates PdNPs and in individual the TEM images.PdNPs in Most the ofTEM them images. were uniformMost of them and had were clear uniform edges. and had clear edges.

TableTable 3. 3.The The averageaverage sizesize ofof PdNPsPdNPs beforebefore and and after after the the Suzuki Suzuki reaction. reaction.

The Average Size of TheThe Average Average Size Size of of PdNPs PdNPs after EntryEntry PIL The Average PIL Size of PdNPs, nm PdNPs, nm SuzukiSuzuki Reaction, Reaction, nm nm 1 1 1b 1b 7.3 ± 1.5 7.3 ± 1.5 3.0 3.0 ±± 1.21.2 2 2b 5.1 ± 3.0 5.0 ± 1.6 2 2b 5.1 ± 3.0 5.0 ± 1.6 3 3b 3.7 ± 0.9 4.6 ± 1.3 3 4 3b 4b 3.7 ± 0.9 2.8 ± 0.9 4.6 2.8 ±± 1.31.0 4 5 4b 5b 2.8 ± 0.9 4.5 ± 3.1 2.8 5.8 ±± 1.03.0 5 6 5b 6b 4.5 ± 3.1 3.2 ± 0.8 5.8 2.9 ±± 3.01.0 ± ± 6 7 6b 7b 3.2 ± 0.8 4.1 1.2 2.9 3.7 ± 1.01.1 8 8b 4.2 ± 1.2 3.5 ± 1.0 7 9 7b 9b 4.1 ± 1.2 3.6 ± 1.4 3.7 2.6 ±± 1.10.9 8 8b 4.2 ± 1.2 3.5 ± 1.0 9 9b 3.6 ± 1.4 2.6 ± 0.9 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 These systems were studied in terms of nanoparticle stabilization by coating agents electrosteric (a transitional one) [68,69]. The electrostatic type of NP stabilization is typical of various natures. There are three types of NP stabilization: electrostatic, steric and elec- for ionic compounds. The hydrophobic long alkyl chain in the structure provides steric stabilization.trosteric (a transitional Extreme one) electrostatic [68,69]. The and electr stericostatic types type lead of to NP the stabilization strong stabilization is typical for of NPsionic and compounds. affect their The surface hydrophobic inacceptability long alkyl [70 ,ch71ain]. The in the combined structure type provides of stabilization steric stabi- is electrostericlization. Extreme when electrostatic the compound and has steric an types ionic chargelead to asthe well strong as hydrophobic stabilization alkylof NPs chain. and Mostlyaffect their the ILssurface provide inacceptability electrosteric [70,71]. stabilization. The combined The series type of bulkyof stabilization phosphonium is electro- salts understeric when investigation the compound has structural has an ionic diversity, charge and as all well types as hydrophobic of stabilization alkyl are chain. represented Mostly inthe a row.ILs provide electrosteric stabilization. The series of bulky phosphonium salts under investigationPILs with has short structural alkyl chains diversity, and hydrophilic and all types properties of stabilization usually provideare represented electrostatic in a stabilization.row. The compounds 1b and 2b have a stronger ionic nature in the range and, as a consequence,PILs with short could alkyl not chains effectively and hydrophili prevent PdNPc properties agglomeration. usually provide The NP electrostatic size in 1b afterstabilization. the reaction The decreasedcompounds more 1b and than 2b twice have (Table a stronger3, entry ionic 1), whichnature couldin the berange explained and, as

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Sustainability 2021, 13, 9862 13 of 19 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 amountby aggregation of small ofNPs the of large approximately PdNPs, their 3 precipitationnm remained and in the excluding reaction from mix- the system. At ture. Despite thethe same same mean time, PdNP an amount size of of 2b small, the NPssize ofdistribution approximately decreased 3 nm remained during the in the reaction catalytic reaction,mixture. which has Despite a similar the same explanation mean PdNP as occurred size of 2b with, the 1b size. distribution decreased during Chain elongationthe catalytic results reaction, in a higher which contribution has a similar of explanationsteric stabilization as occurred and, with as a1b con-. sequence, a decrease Chainin the mean elongation PdNP results size and in asize higher distribution. 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 pre- Compound 5b has a middle alkyl substituent length and steric reasons begin to prevail vail in PdNP stabilization. Due to its molecular nature, 5b could stabilize larger PdNPs as in PdNP stabilization. Due to its molecular nature, 5b could stabilize larger PdNPs as well well as smaller ones,as smaller and the ones, size and distribution the size distribution significantly signiﬁcantly increased. increased. In Figures In Figures S22 and S22 and S23 (in S23 (in SupplementarySupplementary Materials), Materials), we observe we observea bimodal a bimodal system systemof the particles: of the particles: the first the ﬁrst group group has mean hassizes mean of 3–5 sizes nm, of and 3–5 nm,the second and the has second sizes has of sizes11–13 of nm, 11–13 respectively. nm, respectively. PILs 6b–9b, duePILs to steric6b–9b, reasons,due to could steric stabilize reasons, PdNPs couldstabilize in the initial PdNPs stage in theof nucle- initial stage of nu- ation, providing cleation,the presence providing of a large the presencenumber of asmall large particles number (2.6–4.2 of small nm) particles with(2.6–4.2 a nar- nm) with a row size distribution.narrow size distribution. The change in PdNPThe change size depending in PdNP size on dependingthe length onof the lengthfourthof substituent the fourth substituentobserved observed in in the present articlethe present is similar article to the is similar trends to described the trends previously described previouslyfor PILs with for PILseven withnum- even numbers bers of carbon atomsof carbon in the atoms side inalkyl the chain side alkyl [54]. chain [54]. The implication Theof the implication bulky PIL of for the PdNP bulky stabilization PIL for PdNP allows stabilization good and allows moderate good and moderate conversion of 34–62%conversion (Figure of 34–62%10). Catalytic (Figure systems 10). Catalytic including systems PILs including with medium-sized PILs with medium-sized alkyl substituentsalkyl possess substituents slightly possesshigher slightlyactivity higherthan other activity PILs. than The other dynamic PILs. Thenature dynamic of nature of the nanocatalyst requires a balance between its activity and stability that could be provided the nanocatalyst requires a balance between its activity and stability that could be pro- by a stabilizer combining electrostatic and steric properties. vided by a stabilizer combining electrostatic and steric properties.

70 8

60 7

6 50

5 40 4 30

Conversion, % 3 PdNP size, nm

20 2

10 1

0 0 1357911131517 n conversion PdNP size

Figure 10. The PdNP size and the conversion of the Suzuki reaction depend on the phosphonium Figure 10. The PdNP size and the conversion of the Suzuki reaction depend on the phosphonium salt structure. salt structure. 3.7. 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

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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 (Table4, 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).

Table 4. The size of nanocomposites in ethanol solution.

Entry PIL Size of Nanocomposites (nm) 1 4b 309 ± 2 2 5b 371 ± 2 3 6b 219 ± 3 4 7b 842 ± 5 5 8b 304 ± 2 6 9b 238 ± 2

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.

4. Conclusions A family of new phosphonium ionic liquids containing bulky cations based on tri- tert-butylphosphine with varied n-alkyl substituents was synthesized and characterized. A detailed study of their 1H, 13C and 31P 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, Sustainability 2021, 13, 9862 15 of 19

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, 1H/13C/31P 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 S2. Bond lengths for 2b/Å. Table S3. Bond angles for 2b/◦. Table S4. Bond lengths for 2b’/Å. Table S5. Bond angles for 2b’/◦. Table S6. Bond lengths for 3b/Å. Table S7. Bond angles for 3b/◦. Table S8. Bond lengths for 4b/Å. Table S9. Bond angles for 4b/◦. Table S10. The mass of PIL − used in the procedure of catalyst preparation. Figure S1. The structure of 2b. The [BF4] 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 [BF4] /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 t + cation [ Bu3P(n-C5H11)] in 3b. The displacement ellipsoids are set to the 50% probability level. t + Figure S5. The second crystallographically non-equivalent cation [ Bu3P(n-C5H11)] in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S6. The third crystallographically t + non-equivalent cation [ Bu3P(n-C5H11)] in 3b. The displacement ellipsoids are set to the 50% − probability level. Figure S7. Three crystallographically non-equivalent anions [BF4] in 3b. The displacement ellipsoids are set to the 50% probability level. Figure S8. Crystallographically non- t + − equivalent cations [ Bu3P(n-C7H15)] and anions [BF4] in 4b. The disorder is not shown. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. Figure S9. t + The first crystallographically non-equivalent cation [ Bu3P(n-C7H15)] in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. Figure S10. The t + second crystallographically non-equivalent cation [ Bu3P(n-C7H15)] in 4b. Hydrogen atoms are omitted. The displacement ellipsoids are set to the 50% probability level. The A/B disorder ratio t is 0.904(2): 0.096(2) for three Bu fragments and 0.772(4):0.228(4) for the n-C7H15 aliphatic chain. t + Figure S11. The third crystallographically non-equivalent cation [ Bu3P(n-C7H15)] 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 t + cation [ Bu3P(n-C7H15)] 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. Four − crystallographically non-equivalent anion [BF4] in 4b. The displacement ellipsoids are set to the 50% probability level. The A/B disorder ratio for atoms B2, F5..F8 is 0.884(8):0.116(8). The A/B/C disorder ratio for atoms F10..F12 is 0.515(3):0.198(3):0.087(2). The A/B/C disorder ratio for atoms B4, F13..F16 is 0.753(3):0.133(3):0.114(2). 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 Sustainability 2021, 13, 9862 16 of 19

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. Author Contributions: D.M.A., V.V.E., F.G.V., G.A.G., M.E.M. carried out the experimental work and measured and analyzed the data. V.P.A., V.A.M., L.Y.Z. created the idea, designed and planned the study, and supervised the whole project. All authors discussed the results, contributed to writing the manuscript and approved the final version of the manuscript for submission. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by RFBR (project number 19-33-60074). Acknowledgments: The authors gratefully acknowledge the CSF-SAC FRC KSC RAS for providing necessary facilities to carry out this work. Crystal structure determination was performed in the Department of Structural Studies of Zelinsky Institute of Organic Chemistry, Moscow. Conflicts of Interest: The authors declare no conflict of interest.

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