Sterically Hindered Phosphonium Salts: Structure, Properties and Palladium Nanoparticle Stabilization

nanomaterials

Article Sterically Hindered Phosphonium Salts: Structure, Properties and Palladium Nanoparticle Stabilization

Daria M. Arkhipova 1,2,*, Vadim V. Ermolaev 2 , Vasily A. Miluykov 2 , Aidar T. Gubaidullin 2, Daut R. Islamov 2, Olga N. Kataeva 2 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] 2 Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientiﬁc Center, Russian Academy of Sciences, Arbusov Street 8, 420088 Kazan, Russia; [email protected] (V.V.E.); [email protected] (V.A.M.); [email protected] (A.T.G.); [email protected] (D.R.I.); [email protected] (O.N.K.) * Correspondence: [email protected]

Received: 10 November 2020; Accepted: 4 December 2020; Published: 9 December 2020

Abstract: A new family of sterically hindered alkyl(tri-tert-butyl) phosphonium salts (n-CnH2n+1 with n = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20) was synthesized and evaluated as stabilizers for the formation of palladium nanoparticles (PdNPs), and the prepared PdNPs, stabilized by a series of phosphonium salts, were applied as catalysts of the Suzuki cross-coupling reaction. All investigated phosphonium salts were found to be excellent stabilizers of metal nanoparticles of small catalytically active size with a narrow size distribution. In addition, palladium nanoparticles exhibited exceptional stability: the presence of phosphonium salts prevented agglomeration and precipitation during the catalytic reaction.

Keywords: PdNPs; TEM; catalysis; Suzuki cross-coupling reaction; sterically hindered phosphonium salts

1. Introduction Since the beginning of the century, an explosive growth of publications devoted to special organic salts has arisen [1]. Ionic liquids (ILs) are organic salts with a melting point below 100 ◦C based on ammonium, imidazolium, phosphonium, and other cations combined with a variety of counter-ions [2]. They have received particular interest due to their attractive features such as high variability of structure, limited volatility and flammability, broad temperature range of the liquid state, thermal and chemical stability, wide electrochemical potential window and tunable solubility and miscibility [3–5] as well as highly variable biological activity [6]. One extraordinary advantage of organic salts is a structural directionality that is induced through ionic interactions and hydrogen bonding. This supramolecular network becomes an “entropic driver” for the formation of well-defined nanosized objects [7] and complex hierarchical systems [8,9]. From the whole variety of cations, we have chosen the phosphonium one, since the phosphorus atom allows the creation of an organic salt with high steric hindrance directly around the cation charge. Tunable bulkiness around the phosphorus atom is a key property in palladium-catalyzed coupling reactions [10]. The systematic study of a range of phosphonium salts with normal alkyl substituents connected with ionic liquids has become widespread in various fields [11]. Medicine is an important research direction for the application of the ionic liquid [12]. Ionic liquids possess antitumor [13,14] and antiviral activity [15] and could serve as organic antimicrobial additives to the polymeric material [16]. Organic salts have also shown their benefits in material science. Applications have been found

Nanomaterials 2020, 10, 2457; doi:10.3390/nano10122457 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 2457 2 of 20 for them as lubricants [17], in ionic liquid-based polymer electrolytes for Li-ion batteries [18] and supercapacitors [19] and in electrode functionalization [20]. Nanoparticle stabilization for the transition metal catalysis is a key field of application of this class of organic salts [21,22]. Multidisciplinary research at the junction of these booming areas has been shown by the extensive growth of publications devoted to the stabilization of nanoparticles in ionic liquids [23]. Concerning the application of solvent-stabilized nanoparticles, cross-coupling reactions have become one of the most powerful and versatile tools in modern organic chemistry. It is not surprising that metal nanoparticles exhibiting activity in these reactions found such a response [24,25]. A large variety of nanoparticle preparation methods, different materials that can serve as supports or grafting cores, and several media have been studied and discussed [26]. Several studies have supported palladium nanocatalyst for cross-coupling reactions [27,28]. For example, in recent years, different types of nanoparticle support materials were investigated; for example, metal oxides [29–31], zeolites [32–34], carbons [35–37], ordered porous materials [38,39] and synthesized polymers [40] (e.g., ionic polymers [41]) and modified natural polymers [42,43]. There are several studies on mixed composition for palladium nanoparticle (PdNP) support [44–47], and the need for a rational design of catalyst support has been emphasized [48,49]. In pursuit of a rational approach to the creation of a catalytic system based on PdNPs, we have chosen phosphonium salts with a sterically hindered cation with a variable fourth alkyl chain. Tri-tert-butylphosphine was first synthesized and described in 1967 by Hofmann and Schellenbeck [50]. The study of its chemical properties led to the synthesis the first member of sterically hindered phosphonium salts, tri-tert-butyl(methyl)phosphonium iodide. A decade later, Schmidbauer et al. used tri-tert-butyl(methyl)phosphonium bromide as a precursor for tri-tert-butyl(methylene)phosphorene [51]. Originally, phosphonium salt was synthesized from tri-tert-butylphosphine and condensed bromomethane and characterized by physical–chemical methods. In their next study, these authors described the synthesis of the extreme sterically hindered tetra-tert-butylphosphonium tatrafluoroborate through a series of consequent phosphonium–phosphorane transformations [52]. Intermediate substances have been characterized with standard methods, and the structure of the desired product was confirmed with X-ray analysis. Two new salts—tri-tert-butyl(ethyl)phosphonium and tri-tert-butyl(iso-propyl) phosphonium bromides—have been described. Srivastava and co-workers reported the synthesis of tri-tert-butyl(methyl)phosphonium iodide in the coordination sphere of Pt from (t-Bu3P)2Pt and iodomethane [53]. The reaction of [CpFe(CO)2CH2Br] with tri-tert-butylphosphine under mild conditions and following bromide–anion exchange reaction with BAr4− were reported in 1989 [54]. Increasing interest in the field of sterically hindered phosphonium salts has continued in the last decade. A new method of sterically hindered salt synthesis by methylation of tri-tert-butylphosphane hydrobromide with dimethyl acetal has been suggested [55]. We have also described the synthesis of the sterically hindered ionic liquids tri-tert-butyl- and tricyclohexyl(decyl)phosphonium tetrafluroborates [56]. Furthermore, hindered phosphonium salts were conjugated with frustrated t + Lewis pairs ([ethylP Bu3] [(C6F5)3B]2)[57]. This trend led to the emergence of systematic studies devoted to the variety of phosphonium salts [58,59]. In our lab, the first members of sterically hindered phosphonium salts have been synthesized and described [60,61]. Here, for the first time, we describe a systematic study of physical–chemical properties of phosphonium salts combined with the investigation of their influence on PdNP stabilization and the catalytic activity in practically demanding cross-coupling reaction. Nanomaterials 2020, 10, 2457 3 of 20

2. Materials and Methods

2.1. Instrumental NMR spectroscopy. NMR spectra were recorded on a Bruker MSL-400 instrument (BRUKER BioSpin 1 31 GMBH am Silberstreifen, D-76287, Rheinstetten, Germany) at 20 ◦C( H 400 MHz, C 100.6 MHz, 31 1 13 P 161.7 MHz). SiMe4 was used as internal reference for H and C NMR spectra, and 85% H3PO4 as external reference for 31P NMR spectra. X-ray analysis. Data sets for single crystals were collected on a Bruker AXS Kappa APEX Duo diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). Programs used: data collection APEX2, data reduction SAINT, absorption correction SADABS version 2.10, structure solution SHELXT [62], structure refinement by full-matrix least-squares against F2 using SHELXL [63]. The hydrogen atoms were inserted at the calculated positions and refined as riding atoms. The positions of the hydrogen atoms of methyl groups were found using rotating group refinement with idealized tetrahedral angles. The figures were generated using Mercury 4.1 [64] program. Electrospray Ionization Mass Spectrometry (ESI-MS). The ESI-MS measurements were performed using an AmazonX ion trap mass spectrometer (Bruker Daltonic GmbH, Bremen, Germany) in positive (and/or negative) mode in the mass range of 70–3000. The capillary voltage was 3500 V, − nitrogen drying gas—10 L min 1, desolvation temperature 250 C. A methanol/water solution (70:30) · − − ◦ was used as a mobile phase at a flow rate of 0.2 mL/min by binary pump (Agilent 1260 chromatograph, Santa Clara, CA, USA). The sample was dissolved in methanol to a concentration of 10 6 g L 1. − · − The instrument was calibrated with a tuning mixture (Agilent G2431A, Santa Clara, CA, USA). For instrument control and data acquiring the TrapControl 7.0 software (Bruker Daltonik GmbH, Bremen, Germany) was used. Data processing was performed by DataAnalysis 4.0 SP4 software (Bruker Daltonik GmbH, Bremen, Germany). Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). STA449 F3 (NETZSCH, Selb, Germany) were used for the thermal analysis (thermogravimetry/differential scanning calorimetry) in which the variation of the sample mass as a function of temperature and the corresponding heats are recorded. An approximately 7mg sample was placed in an Al (or Al2O3 at temperatures above 600 ◦C) crucible with a pre-hole on the lid and heated from 25 to 600 ◦C. The same empty crucible was used as the reference sample. High-purity argon was used with a gas flow rate of 50 mL/min. TG/DSC measurement were performed at the heating rates of 1–50 K/min. TransmissionElectron Microscopy (TEM). TEM images of the PdNPs before and after the reaction were obtained using a HT7700 (Hitachi, Hitachinaka-shi, Japan) transmission electron microscope. Accelerating voltage 40–120 kV; maximum magnification 800,000; resolution 0.144 nm. X-ray powder diffraction (XRPD). XRPD measurements were performed on an automatic D8 Advance diffractometer (Bruker, Karlsruhe, Germany) equipped with a Vario attachment and Vantec linear PSD using Cu radiation (40 kV, 40 mA) monochromated by a curved Johansson monochromator (λ CuKα1 1.5406 Å). Room temperature data were collected in the reflection mode with a flat-plate sample. Small angle X-ray scattering (SAXS). SAXS data for samples were collected with the AXS Nanostar SAXS system (Bruker, Karlsruhe, Germany) using CuKα (λ 1.5418 Å) radiation from a 2.2 kW X-ray tube (40 kV, 35 mA) coupled with Gobbel mirrors optics and a HiStar 2D area detector. The beam was collimated using three pinholes with apertures of 1000, 400 and 750 µm. The instrument was operated with a sample-to-detector distance of 63.5 cm to provide data at angles 0.1◦ < 2θ < 4.8◦, 1 1 which correspond to 0.007 Å− < s < 0.34 Å− . The value of s is proportional to the inverse of the length 1 scale (s = (4π/λ) sin (θ) in units of Å− ). Gas Chromatography–Mass Spectrometry (GC-MS). The conversion of 1,3,5-triphenylbenzene was estimated by gas chromatography coupled with mass spectrometry on a gas chromatograph 5977A (Agilent, Santa Clara, CA, USA) using electron ionization with an energy of ionizing electrons of 70 eV, the temperature of the ion source 250 ◦C, and an ID-BP5X capillary column (analog of DB-5MS, Nanomaterials 2020, 10, 2457 4 of 20 length 50 m, diameter 0.32 mm, thickness of the phase layer 0.25 µm), and helium as a carrier gas. The mass spectral data were processed using the Xcalibur program. Prior to injection, a sample of the studied substance was dissolved in chromatographically pure ethanol in a concentration of ~10 3 g µL 1 − · − (volume of the injected sample 0.1 µL). The chromatographic regime was as follows: temperature of the injector 250 ◦C, flow split 1:10; temperature-programmed column: initial temperature 120 ◦C (1 min), then heating with a rate of 20 C min 1 to 280 C, and final temperature 280 C (15 min); the flow rate ◦ · − ◦ ◦ of the carrier gas through the column was 2 mL min 1. The temperature of the communication device · − with the mass spectrometer was 280 ◦C.

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), ethyl iodide and n-alkyl bromide (Sigma-Aldrich, St. Louis, MO, USA), phenylboronic acid (Alfa Aesar, Heysham, Lancashire, UK), 1,3,5-tribromobenzene (Alfa Aesar), Pd(OAc)2 (Alfa Aesar), KOH (Himreaktiv, Nizhny Novgorod, Russia), NaBF4 (Himreaktiv, Nizhny Novgorod, Russia) analytical grade, without additional puriﬁcation were used in the work. Detailed synthesis and characterization of the phosphonium salts described in Supplementary Materials.

2.2.1. Typical Procedure for PdNPs Preparation 0.0004 g (0.00178 mmol) of palladium acetate and 0.178 mmol of phosphonium salt was dissolved in 9 mL ethanol and stirred during 20 min at room temperature. The color of solution changes from transparent to light brownish gray.

2.2.2. Typical Procedure for Suzuki Cross-Coupling To the fresh solution of colloidal Pd 0.157 g (0.50 mmol) of 1,3,5-tribrombenzene, 0.276 g (2.25 mmol) of phenyl boronic acid, 0.128 g (2.25 mmol) of potassium hydroxide were added. Reaction mixture was stirred over 7 h at 30 ◦C. Organic compounds were extracted with 9 mL toluene and analyzed by GC-MS.

3. Results and Discussion

3.1. Synthetic Procedures Phosphonium salts with halogen anions were obtained by nucleophilic addition of tri-tert-butylphosphine to respective linear 1-haloalkane (Hal = I or Br) according to the described procedure (Scheme1)[ 65]. The quaternization reaction was conducted under solvent-free conditions except for the cases with solid haloalkanes (C16H33Br, C18H37Br, C20H41Br). For the solid reactants, we applied acetonitrile as a solvent due to its polar aprotic nature. The temperature of the reaction varied, depending on the applied haloalkane, from 0 ◦C for EtI to 90–110 ◦C for liquid 1-bromoalkanes and 80 ◦C for the solid ones. The reaction was ended after the disappearance of the phosphine signal in the 31P NMR spectrum. Compared to tri-n-butylphosphonium salts, the temperature for the synthesis of sterically hindered n t products was higher [58,59] even though the pKa of Bu 3P (7.6) [66] is lower than that of Bu 3P (11.4) [67]. This fact could be explained by steric reasons. Then phosphonium halides were put into a metathesis reaction with weakly coordinating anions, namely tetraﬂuoroborate (Scheme2). The anion exchange reaction was carried out in water at room temperature. The resulting phosphonium salt was precipitated as soon as sodium tetraﬂuoroborate was added to the water solution of the phosphonium halide. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 21

Nanomaterials 2020, 10, 2457 5 of 20 Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 21

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

Then phosphonium halides were put into a metathesis reaction with weakly coordinating anions, namely tetrafluoroborate (Scheme 2). The anion exchange reaction was carried out in water at room temperature. The resulting phosphonium salt was precipitated as soon as sodium tetrafluoroborate was added to the water solution of the phosphonium halide. Scheme 2. The anion exchange reaction. Scheme 2. The anion exchange reaction. New compounds were obtained in high yields (up to 98%; see ESI for details). The phosphonium saltsNew are whitecompounds or yellowish were obtained crystalline in orhigh amorphous yields (up solids to 98%; that aresee solubleESI for in details). chloroform, The dichloromethane,phosphonium salts acetone, are white ethanol, or methanolyellowish and crystalline almost insoluble or amorphous in diethyl solids ether andthat petroleumare soluble ether. in chloroform, dichloromethane, acetone, ethanol, methanol and almost insoluble in diethyl ether and

3.2.petroleum NMR Spectroscopy ether. Scheme 2. The anion exchange reaction. 3.2.1.3.2. NMR31P NMRSpectroscopy Spectra NewThe 31 P{compounds1H} NMR spectrawere ofobtained the alkyl(tri- in hightert-butyl) yields phosphonium (up to 98%; halides see (TableESI for1) eachdetails). exhibited The 3.2.1. 31P NMR Spectra phosphoniuma singlet at 49.71 salts0.41 are ppm, white with or oneyellowish exception, crystalline1a, where or theamorphous signal was solids at 50.53 that ppm. are Thesoluble31P{1 H}in ± chloroform,NMRThe spectra 31P{ dichloromethane,1H} of theNMR alkyl(tri- spectra tertacetone, of-butyl) the ethanol, alkyl(tri- phosphonium metertthanol-butyl) tetraﬂuoroborates and phosphonium almost insoluble allhalides exhibited in diethyl (Table a ether singlet 1) eachand at petroleum49.10exhibited0.07 a ether. singlet ppm, withat 49.71 one ± exception, 0.41 ppm, 1bwith, where one exception, the signal 1a was, where at 49.96 the ppm. signal The was chemical at 50.53 shiftsppm. ± ofThe1a 31andP{1H}1b NMRappear spectra in a lowerof the magneticalkyl(tri-tert ﬁeld,-butyl) possibly phosphonium due to the tetrafluoroborates lack of electron density all exhibited of ethyl a 3.2.substituentsinglet NMR at 49.10Spectroscopy to compensate ± 0.07 ppm, for with the one cationic exception, charge 1b on, where the phosphorus the signal was atom. at 49.96 ppm. The chemical shifts of 1a and 1b appear in a lower magnetic field, possibly due to the lack of electron density of 31 1 31 1 3.2.1.ethylTable substituentP NMR 1. H Spectra NMR to compensate chemical shifts for (δ the(ppm), cationic CDCl charge3, 400 MHz, on the 20 ◦phosphorusC) and P{ H} atom. NMR shifts (δ (ppm), CDCl , 161.7 MHz, 20 C). The 313 P{1H} NMR spectra◦ of the alkyl(tri-tert-butyl) phosphonium halides (Table 1) each 1 31 1 exhibitedTable a 1. singlet HPhosphonium NMR at chemical 49.71 Salt ± 0.41shifts ppm, (δ (ppm), with CDCl one3 ,exception, 400 MHz, 20 1a °C), where and P{theH} signal NMR shiftswas at (δ 50.53(ppm), ppm. Chemical Shift δ of 31 CDCl3, 161.7 MHz, 20 °C. Chemical Shift δ P NMR, ppm The 31P{1H}N NMR spectra Cation of the Anion alkyl(tri-α-Protonstert-butyl)1H NMR, phosphonium ppm tetrafluoroborates all exhibited a singlet at 49.10 ± 0.07 ppm, with one exception, 1b, where the signal was at 49.96 ppm. The chemical Phosphonium1a Salt+ I− 2.93 50.53 Chemical Shift δ t-Bu3P C2H5 Chemical Shift δ of α-protons 1H NMR, ppm shiftsN of 1a1bCation and 1b appearAnion in BFa lower4− magnetic field,2.58 possibly due to the lack 49.96 of electron31P NMR, density ppm of 2a + Br− 2.65 49.54 ethyl substituentt-Bu to 3compensateP C4H−9 for the cationic charge on the phosphorus atom. 1a 2b I BF4− 2.312.93 49.17 50.53 t-Bu3P+C2H5 − 1b 3a + BF4 Br− 2.612.58 49.51 49.96 t-Bu3P C6H13 Table 1. 1H NMR chemical shifts (δ (ppm), CDCl3, 400 MHz, 20 °C) and 31P{1H} NMR shifts (δ (ppm), 2a 3b Br− BF4− 2.322.65 49.16 49.54 + CDClt-Bu34a, 161.73P C 4MHz,H9 + 20 °C. Br− 2.55 49.30 t-Bu3P C8H174− 2b 4b BF BF4− 2.292.31 49.08 49.17 3a Phosphonium5a Salt+ Br − Br− 2.562.61 49.78 Chemical 49.51 Shift δ 3 + 6t-Bu133P C10H21 1 t-Bu5bP C H BFChemical4− Shift δ2.29 of α-protons H NMR, ppm 49.08 3bN Cation AnionBF4− 2.32 31P NMR, 49.16 ppm 6a + Br− 2.59 50.06 4a t-Bu3P C12BrH− −25 2.55 49.30 1a 6b + I BF4− 2.272.93 49.03 50.53 t-But-Bu33P +C82H175 4b 7a + BF4−− Br− 2.682.29 50.01 49.08 1b t-Bu3P CBF14H429 2.58 49.96 7b BF4− 2.27 49.08 5a t-Bu3P+C10H21 Br−− 2.56 49.78 2a 8a Br Br 2.652.65 49.53 49.54 t-Bu3P+C4H9 + − t-Bu3P C16H33− 2b 8b BF4 BF4− 2.292.31 49.07 49.17 3a 9a + Br− Br− 2.622.61 50.12 49.51 + t-Bu3P C18H37 t-Bu9b3P C6H13 BF4− 2.29 49.08 3b BF4− 2.32 49.16 10a + Br− 2.54 49.51 t-Bu3P C20H−41 4a 10b Br BF4− 2.332.55 49.11 49.30 t-Bu3P+C8H17 4b BF4− 2.29 49.08 5a t-Bu3P+C10H21 Br− 2.56 49.78

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5b BF4− 2.29 49.08 6a Br− 2.59 50.06 t-Bu3P+C12H25 6b BF4− 2.27 49.03 7a Br− 2.68 50.01 t-Bu3P+C14H29 7b BF4− 2.27 49.08 8a Br− 2.65 49.53 t-Bu3P+C16H33 8b BF4− 2.29 49.07 9a Br− 2.62 50.12 t-Bu3P+C18H37 9b BF4− 2.29 49.08 10a Br− 2.54 49.51 Nanomaterialst-Bu32020P+C,2010H, 245741 6 of 20 10b BF4− 2.33 49.11

Although tri- tri-terttert-butylphosphine-butylphosphine is is extremely extremely sensitive sensitive to to oxidation oxidation in inair air and and flammability, ﬂammability, it requiresit requires exceptional exceptional care care in in handling handling and and an an inert inert atmosphere atmosphere to to achieve high puritypurity 3131P{1H}H} NMR spectra of the phosphonium salts containing a single peak.

3.2.2. 11HH NMR NMR Signals Signals for for C( C(αα)-H)-H Protons Protons We found thatthat thetheα α-protons-protons are are very very sensitive sensitive to to anion anion exchange exchange (Figure (Figure1). 1). The The chemical chemical shift shift of ofα-protons α-protons of phosphoniumof phosphonium tetrafluoroborates tetrafluoroborates appears appears in higher in higher magnetic magnetic fields fields (2.30 (2.300.03 ± ppm)0.03 ppm) than ± thanphosphonium phosphonium bromides bromides (2.61 (2.610.07 ± ppm) 0.07 ppm) (Table (Table1). This 1) is. This caused is caused by the by presence the presence of hydrogen of hydrogen bonds ± bondsbetween betweenα-protons α-protons and bromide and bromide anion bearing anion high bearing electron high a ffielectronnity, while affinity, the weakly while coordinatedthe weakly coordinatedtetrafluoroborate tetrafluoroborate anion interacts anio withn interacts the surrounding with the ionssurrounding via non-covalent ions via Coulombnon-covalent forces Coulomb [56]. forces [56].

Figure 1. α-protons in the sterically hindered phosphonium salts. Figure 1. α-protons in the sterically hindered phosphonium salts. The exception is α-protons of 1a and 1b, where the peak of α-protons are at 2.93 ppm and 2.58 ppm, The exception is α-protons of 1a and 1b, where the peak of α-protons are at 2.93 ppm and 2.58 respectively, as mentioned above, due to the lack of electron density of ethyl substituent to compensate ppm, respectively, as mentioned above, due to the lack of electron density of ethyl substituent to for the cationic charge on the phosphorus atom. compensate for the cationic charge on the phosphorus atom. 3.3. X-ray Analysis 3.3. X-ray Analysis Crystals suitable for X-ray structural analysis were obtained for two phosphonium salts 1a and 10b.Crystals Crystal packing suitable of for5b X-ray[56] wasstructural published analysis earlier, were and obtained here it is for compared two phosphonium with the new salts ones 1a of and1a 10band. 10bCrystal(Figures packing2–4). of 5b [56] was published earlier, and here it is compared with the new ones of 1a and1a 10bpossesses (Figures an 2–4). ionic crystal lattice with cations and anions at the points indicated in Figure2. The crystal1a possesses packing an is ionic compact crystal and lattice ordered. with cations and anions at the points indicated in Figure 2. The crystalCrystal packing packing is ofcompact5b is more and disorderedordered. than the previous one (Figure3)[ 56]. Due to a linear alkyl substituent in the structure of phosphonium salt, the location of the molecules in the crystal is more complex. The molecules of 10b are arranged “head to tail” in the crystal (Figure4). The long alkyl substituents areNanomaterials strictly 2020 parallel, 10, x toFOR each PEER other. REVIEW Hydrophilic (the head group) and hydrophobic (alkyl side-chain)7 of 21 regions are divided in the crystal packing.

Figure 2. Crystal packing of 1a. Figure 2. Crystal packing of 1a.

Figure 3. Crystal packing of 5b.

Figure 4. Crystal packing of 10b.

Crystal packing of 5b is more disordered than the previous one (Figure 3) [56]. Due to a linear alkyl substituent in the structure of phosphonium salt, the location of the molecules in the crystal is more complex. The molecules of 10b are arranged “head to tail” in the crystal (Figure 4). The long alkyl substituents are strictly parallel to each other. Hydrophilic (the head group) and hydrophobic (alkyl side-chain) regions are divided in the crystal packing.

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Figure 2. Crystal packing of 1a.

Nanomaterials 2020, 10, 2457 Figure 2. Crystal packing of 1a. 7 of 20

Figure 3. Crystal packing of 5b. Figure 3. Crystal packing of 5b. Figure 3. Crystal packing of 5b.

Figure 4. CrystalCrystal packing of 10b.. Undoubtedly, the difference in the crystal packing of phosphonium salt affects their melting Crystal packing of 5b is more Figuredisordered 4. Crystal than packing the previous of 10b. one (Figure 3) [56]. Due to a linear temperature. It should be noted that in the studied structures, anions are located at the hydrogen atom alkyl substituent in the structure of phosphonium salt, the location of the molecules in the crystal is of theCrystalα-carbon packing atom of of 5b the isn -alkylmore disordered substituent. than the previous one (Figure 3) [56]. Due to a linear more complex. alkyl substituent in the structure of phosphonium salt, the location of the molecules in the crystal is The molecules of 10b are arranged “head to tail” in the crystal (Figure 4). The long alkyl more3.4. Melting complex. Point substituents are strictly parallel to each other. Hydrophilic (the head group) and hydrophobic (alkyl The molecules of 10b are arranged “head to tail” in the crystal (Figure 4). The long alkyl side-chain)The melting regions temperatures are divided (Tinm the) of crystal the synthesized packing. phosphonium salts were measured. The trend substituents are strictly parallel to each other. Hydrophilic (the head group) and hydrophobic (alkyl of Tm as a function of the number of carbon atoms in the n-alkyl substitution on the phosphonium side-chain)cation is presented regions inare Figure divided5. in the crystal packing. The melting points of the phosphonium salts reflected the type of crystal packaging, as expected. The first members of the series had high melting points above 200 ◦C. With an increase in the length of the alkyl chain, the symmetry of the molecules decreased, as well as the ordering of the crystal packing, which led to a drop in Tm. The long alkyl substituents up to 20 carbon atoms were oriented strictly parallel to each other in the crystal packing, leading to the slight growth of the melting point. A similar discussion has been previously reported on the number of 1-alkyl-3-methylimidazolium tetrafluoroborates [68]. The general trend was a decrease in melting points from methyl to n-octyl chains, followed by an increase in melting points and formation of a smectic liquid crystal phase with an increasing number of carbon atoms in n-alkyl substituent. The melting points of phosphonium bromides in most cases are lower than those of phosphonium tetrafluoroborates. The presence of multiple hydrogen bonds C–H F leads to a gain in intermolecular ··· interaction and as a consequence an increase in the melting temperature. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 21

Undoubtedly, the difference in the crystal packing of phosphonium salt affects their melting temperature. It should be noted that in the studied structures, anions are located at the hydrogen atom of the α-carbon atom of the n-alkyl substituent.

3.4. Melting Point

The melting temperatures (Tm) of the synthesized phosphonium salts were measured. The trend Nanomaterials 2020, 10, 2457 8 of 20 of Tm as a function of the number of carbon atoms in the n-alkyl substitution on the phosphonium cation is presented in Figure 5.

350

300

250

200 BF4

Tm, °C 150 Br

100

0 0 2 4 6 8 10 12 14 16 18 20 n

Figure 5. Melting points as a function of the number of carbons, n, in the fourth alkyl substituent on the phosphorus atom. 3.5. PdNP Stabilization and TEM Sample Preparation The melting points of the phosphonium salts reflected the type of crystal packaging, as expected.PdNPs The proved first members to be superior of the catalysts series had in organichigh melting chemistry points especially above 200 due °C. to theirWith “cocktail-type”an increase in thebehavior length forof the cross-coupling alkyl chain, the reactions. symmetry However, of the mo thermodynamicallylecules decreased, as unstable well as the nanoparticles ordering of the are crystalsusceptible packing, to aggregation which led to and a drop require in theTm. presenceThe long ofalkyl a stabilizing substituents agent. up to Some 20 carbon organic atoms salts were orientedshown to strictly be excellent parallel stabilizers to each for other metal in nanoparticles.the crystal packing, The fine-tuning leading to of the the organicslight growth salt structure of the meltingallows one point. to vary the NP size and consequently its catalytic activity [69]. There are several articles devotedA tosimilar the relationship discussion between has the been nature ofpreviously the stabilizer reported and the NPon size. the These number studies haveof 1-alkyl-3-methylimidazoliumreported the influence of the tetrafluoroborates cation structure [70 [68]], the. The anion general volume trend [71 was] as a well decrease as both in ofmelting them pointstogether from [72]. methyl Earlier, to we n-octyl have shownchains, that followed steric hindranceby an increase of the in phosphonium melting points salt and influences formation PdNPs’ of a smecticcatalytic liquid activity crystal [73]. phase with an increasing number of carbon atoms in n-alkyl substituent. TheFor themelting present points study, weof chosephosphonium palladium bromides acetate as thein sourcemost ofcases nanoparticles. are lower The than procedure those of phosphoniumPdNP formation tetrafluoroborates. represents the dissolution The presence of palladium of multiple salt hydrogen and corresponding bonds C–H···F phosphonium leads to a salt gain in inthe intermolecular organic solvent. interaction We tested and two as techniques a conseque ofnce nanoparticles an increase deposition in the melting on the temperature. grid: a “nanofishing” technique and the droplet technique. Pd(OAc)2 and 9b were dissolved in ethyl alcohol. The sample for TEM3.5. PdNP measurement Stabilization was and prepared TEM Sample after 20Preparation min of stirring of the solution. PdNPs“Nanofishing” proved is ato special be superior technique catalysts developed in fororganic studying chemistry liquid-phase especially colloidal due systems to their and “cocktail-type”dispersed nanoparticles behavior [74 ].for The cross-coupling procedure includes reactions. immersing However, a copper thermodynamically grid into the studied unstable solution nanoparticlesfor 30–40 s and are then susceptible rinsing the to gridaggregation in acetone and to removerequire excessthe presence organic of components a stabilizing and agent. drying Some the organicsample insalts the were air. shown to be excellent stabilizers for metal nanoparticles. The fine-tuning of the organicThe salt droplet structure technique allows is aone standard to vary approach the NP size including and consequently the application its ofcatalytic 2–3 small activity drops [69]. of a Therestudied are solution several containingarticles devoted nanoparticles to the relationship onto a copper between grid. the After nature rinsing of the in acetone,stabilizer the and grid thewas NP size.dried These in air studies and investigated have reported by TEM. the influence of the cation structure [70], the anion volume [71] as The solution of colloidal palladium prepared using 9b was applied to compare these two methods. The sample prepared using the “nanofishing” technique showed that the agglomerates of PdNPs are small and well dispersed over the copper grid (Figure6a,b). The average size of individual PdNPs was 2.81 0.80 nm. ± Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 21 well as both of them together [72]. Earlier, we have shown that steric hindrance of the phosphonium salt influences PdNPs’ catalytic activity [73]. For the present study, we chose palladium acetate as the source of nanoparticles. The procedure of PdNP formation represents the dissolution of palladium salt and corresponding phosphonium salt in the organic solvent. We tested two techniques of nanoparticles deposition on the grid: a “nanofishing” technique and the droplet technique. Pd(OAc)2 and 9b were dissolved in ethyl alcohol. The sample for TEM measurement was prepared after 20 min of stirring of the solution. “Nanofishing” is a special technique developed for studying liquid-phase colloidal systems and dispersed nanoparticles [74]. The procedure includes immersing a copper grid into the studied solution for 30–40 s and then rinsing the grid in acetone to remove excess organic components and drying the sample in the air. The droplet technique is a standard approach including the application of 2–3 small drops of a studied solution containing nanoparticles onto a copper grid. After rinsing in acetone, the grid was dried in air and investigated by TEM. The solution of colloidal palladium prepared using 9b was applied to compare these two methods. The sample prepared using the “nanofishing” technique showed that the agglomerates of Nanomaterials 2020, 10, 2457 9 of 20 PdNPs are small and well dispersed over the copper grid (Figure 6a,b). The average size of individual PdNPs was 2.81 ± 0.80 nm.

Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 21

60 c 50

Distribution, % Distribution, 20

0 123456 PdNPs size, nm

Figure 6. TEMTEM of of PdNPs using the “n “nanoﬁshing”anofishing” technique ( a,b) and size distribution graph (c).

When the droplet technique was used, the sample contained the agglomerates of PdNPs that were located in a thick massive layer of the phosphonium salt (Figure 7a). These kinds of samples were opaque and hardly suitable for the TEM analysis. The average size of an individual nanoparticle was 3.37 ± 0.85 nm, which is close to the results of the “nanofishing” technique. It should be noted that individual nanoparticles were much more difficult to locate, and the fraction available for analysis was smaller than for the “nanofishing” procedure.

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60 c 50

Distribution, % Distribution, 20

0 123456 PdNPs size, nm Nanomaterials 2020, 10, 2457 10 of 20

Figure 6. TEM of PdNPs using the “nanofishing” technique (a,b) and size distribution graph (c). When the droplet technique was used, the sample contained the agglomerates of PdNPs that were located inWhen a thick the massivedroplet technique layer of thewas phosphoniumused, the sample salt contained (Figure 7thea). agglomerates These kinds of of PdNPs samples that were were located in a thick massive layer of the phosphonium salt (Figure 7a). These kinds of samples opaque and hardly suitable for the TEM analysis. The average size of an individual nanoparticle was were opaque and hardly suitable for the TEM analysis. The average size of an individual 3.37 0.85 nm, which is close to the results of the “nanofishing” technique. It should be noted that nanoparticle± was 3.37 ± 0.85 nm, which is close to the results of the “nanofishing” technique. It individualshould nanoparticlesbe noted that individual were much nanoparticles more difficult were to locate,much more and thedifficult fraction to locate, available and fortheanalysis fraction was smalleravailable than forfor analysis the “nanofishing” was smaller procedure. than for the “nanofishing” procedure.

Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 21

50 45 c 40 35 30 25 20

Distribution, % Distribution, 15 10 5 0 123456 PdNPs size, nm

FigureFigure 7. TEM 7. TEM of of PdNPs PdNPs using using the the dropletdroplet technique (a (a,b,b) and) and size size distribution distribution graph graph (c). (c).

Thus, the “nanofishing” technique was chosen for the TEM samples preparation as the most optimal when phosphonium salt acts as a stabilizer. Acetonitrile, chloroform, and ethanol were tested as media for the formation of PdNPs. Palladium acetate and 6b were dissolved in 9 mL of each solvent. The sample for TEM measurement was prepared using the “nanofishing” technique after 20 min of stirring the solution. We did not observe the formation of single PdNPs in acetonitrile and chloroform (Figures 8 and 9). According to the literature, both chloroform [75] and acetonitrile [76] can solvate Pd (OAc)2 well in its tri-nuclear form without decomposition. While no particles were observed in acetonitrile (Figure 8), the agglomerates of particles were detected in chloroform (Figure 9).

Nanomaterials 2020, 10, 2457 11 of 20

Thus, the “nanoﬁshing” technique was chosen for the TEM samples preparation as the most optimal when phosphonium salt acts as a stabilizer. Acetonitrile, chloroform, and ethanol were tested as media for the formation of PdNPs. Palladium acetate and 6b were dissolved in 9 mL of each solvent. The sample for TEM measurement was prepared using the “nanoﬁshing” technique after 20 min of stirring the solution. We did not observe the formation of single PdNPs in acetonitrile and chloroform (Figures8 and9). According to the literature, both chloroform [75] and acetonitrile [76] can solvate Pd (OAc)2 well in its tri-nuclear form without decomposition. While no particles were observed in acetonitrile (Figure8), Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 21 theNanomaterials agglomerates 2020, 10 of, x FOR particles PEER REVIEW were detected in chloroform (Figure9). 12 of 21

Figure 8. TEM sample prepared using the “nanofishing” procedure: Pd (OAc)2 and 6b in acetonitrile. FigureFigure 8. 8.TEM TEM samplesample preparedprepared usingusing thethe “nanoﬁshing”“nanofishing” procedure:procedure: Pd (OAc) 2 andand 6b6b inin acetonitrile. acetonitrile.

Figure 9.9.TEM TEM samples samples prepared prepared using using the “nano the “nan ﬁshing”o fishing” procedure: procedure: Pd (OAc) Pd2 and (OAc)6b in2 chloroform.and 6b in Figure 9. TEM samples prepared using the “nano fishing” procedure: Pd (OAc)2 and 6b in chloroform. Ethanolchloroform. proved to promote the formation and stabilization of the PdNPs with 6b (Figure 10), and theEthanol average proved size wasto promote 2.01 0.72the formation nm. The PdNPs and stabilization were located of inthe large PdNPs groups with and 6b possessed(Figure 10), a Ethanol proved to promote± the formation and stabilization of the PdNPs with 6b (Figure 10), anduniform the average size. Individual size was nanoparticles, 2.01 ± 0.72 nm. as The well PdNP as thes aggregateswere located and in molecules large groups of phosphonium and possessed salt, a and the average size was 2.01 ± 0.72 nm. The PdNPs were located in large groups and possessed a wereuniform available size. Individual in the ethanol nanoparticles, solution, and as itwell was as of interestthe aggregates to study and these molecules systems inof morephosphonium detail. uniform size. Individual nanoparticles, as well as the aggregates and molecules of phosphonium salt, were available in the ethanol solution, and it was of interest to study these systems in more salt, were available in the ethanol solution, and it was of interest to study these systems in more detail. detail.

Nanomaterials 2020, 10, 2457 12 of 20 NanomaterialsNanomaterials 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 13 13 of of 21 21

Figure 10. TEM samples prepared using the “nanofishing” procedure: Pd (OAc)2 and 6b in ethanol. Figure 10. TEMTEM samples samples prepared using the “n “nanoﬁshing”anofishing” procedure: Pd (OAc) 2 andand 6b6b inin ethanol. ethanol.

TheThe samplesample sample ofof of the the the PdNPs PdNPs PdNPs and and and5b 5b in5b ethanolin in ethanol ethanol was was analyzedwas analyzed analyzed with X-raywi withth X-Ray powderX-Ray powder powder diffraction diffraction diffraction (XRPD) (XRPD)method(XRPD) (Figuremethod method 11(Figure (Figure). According 11). 11). According According to XRPD data,to to XRPD XRPD the sample data, data, the isthe a sample multicomponentsample is is a amulticomponent multicomponent system consisting system system of consistingamorphousconsisting of of and amorphous amorphous several crystallineand and several several phases, crystalline crystalline the presence phases, phases, the ofthe whichpresence presence is evidenced of of which which is byis evidenced evidenced the pronounced by by the the pronounceddipronouncedfference in thedifference difference half-width in in the ofthe severalhalf-width half-width reflections. of of several several The reflections. searchreflections. for suitableThe The search search experimental for for suitable suitable X-ray experimental experimental diffraction X-raypatternsX-ray diffraction diffraction for identification patterns patterns infor for the identification identification PDF-2 database in in the the led PDF-2 PDF-2 to a suitabledatabase database substance—the led led to to a asuitable suitable crystalline substance—the substance—the form of crystallinepalladiumcrystalline Pd,form form syn., of of palladium Codepalladium No. 00-001-1201—whosePd, Pd, syn., syn., Code Code No. No. 00-001-1201—whose di00-001-1201—whoseffraction peak positions diffraction diffraction are shown peak peak in positions thepositions figure are are as shownverticalshown in bars.in the the Thefigure figure diff asuse as vertical vertical nature bars. ofbars. most The The of diffuse thediffuse palladium nature nature of di of ffmost ractionmost of of peaksthe the palladium palladium indicates diffraction itsdiffraction nanostructure. peaks peaks indicatesInindicates addition, its its ananostructure. comparisonnanostructure. was In In made addition, addition, between a acomp comp the experimentalarisonarison was was made andmade calculated between between XRD-spectrathe the experimental experimental (Figure and and S1 calculatedincalculated Supplementary XRD-spectra XRD-spectra Materials). (Figure (Figure S1 S1 in in Supplementa Supplementaryry Materials). Materials).

(a()a ) (b(b) )

FigureFigure 11.11. 11. ExperimentalExperimental Experimental powder powder powder di ﬀdiffraction ractiondiffraction pattern pattern pattern of a of sampleof a asample sample PdNPs PdNPs PdNPs and 5b and inand ethanol 5b 5b in in (ethanol aethanol); enlarged ( a();a); enlargedpartenlarged of the part dipartﬀ ractogramof of the the diffractogram diffractogram (b). Red vertical ( b(b).). Red barsRed vertical showvertical the bars bars positions show show the of the the positions positions interference of of the the peaks interference interference corresponding peaks peaks correspondingtocorresponding the crystalline to to formthe the crystalline crystalline of palladium form form Pd, of of syn.,palladium palladium Code No.Pd, Pd, syn., 00-001-1201. syn., Code Code No. No. 00-001-1201. 00-001-1201.

OtherOther possible possible crystalline crystalline components componentscomponents present presentpresent in inin the thethe testtest test samplesample sample includeinclude include thethe the usedused used salt salt and and the the productsproducts ofof of itsits its interactioninteraction interaction or or co-crystallizationor co-crystallization co-crystallization with wi with theth the solvent.the solvent. solvent. These These These were were were not analyzednot not analyzed analyzed at this at at stage this this stageofstage the of research.of the the research. research. TheThe quantitative quantitative assessment assessmentassessment of ofof the thethe sizes sizessizes of ofof pa palladiumpalladiumlladium Pd PdPd (0) (0) crystallites crystallites from fromfrom powder powder X-ray X-ray diffractiondidiffractionffraction data datadata was waswas carried carriedcarried out based outout based onbased analysis onon analys ofanalys theis profilesis ofof thethe of diprofilesprofilesffraction ofof reflections diffractiondiffraction corresponding reflectionsreflections correspondingtocorresponding the crystalline toto form thethe crystalline ofcrystalline palladium formform for of theof palladiumpalladium given sample. forfor thethe The givengiven crystallite sample.sample. sizes TheThe obtained crystallitecrystallite from sizessizes the obtainedprofilesobtained of from difromffraction the the profiles profiles lines corresponding of of diffraction diffraction to lines dilinesfferent co correspondingrresponding families of planes to to different different are somewhat families families di of ffoferent planes planes but are areare somewhatsomewhat different different but but are are close close to to each each other, other, in indicatingdicating the the absence absence of of noticeable noticeable anisometry anisometry of of palladiumpalladium crystallites.crystallites. InIn general,general, theirtheir sizessizes areare inin thethe rangerange ofof 8.8–17.88.8–17.8 nmnm (Table(Table S1S1 inin SupplementarySupplementary Materials). Materials).

Nanomaterials 2020, 10, 2457 13 of 20

Nanomaterialsclose to each 2020 other,, 10, x indicating FOR PEER REVIEW the absence of noticeable anisometry of palladium crystallites. In general, 14 of 21 their sizes are in the range of 8.8–17.8 nm (Table S1 in Supplementary Materials). Additionally,Additionally, the the analysis analysis of of the the size size and and shape shape of ofthe the obtained obtained PdNPs PdNPs in EtOH in EtOH solutions solutions of 10b of and10b and5b salts5b saltswas wasperformed performed by small by small angle angle X-ray X-ray scattering scattering (SAXS). (SAXS). According According to SAXS to SAXSdata, both data, samplesboth samples are characterized are characterized as heterogeneous, as heterogeneous, with with the presence the presence of the of therandomly randomly oriented oriented particles particles in solutionsin solutions with with dimensional dimensional characteristics characteristics in inthe the range range of of 20–28 20–28 nm nm (Figures (Figures S2 S2 and and S3, S3, Table Table S2 S2 in in SupplementarySupplementary Materials).Materials). TheThe increased increased particle particle sizes sizes compared compared with thewith data the of powderdata of di ﬀpowderraction diffractionmay indicate may aggregation indicate aggregation or a more complex or a more morphology complex morphology of the resulting of the nanoparticles, resulting nanoparticles, in particular, inthe particular, formation the of doubleformation layers of do onuble their layers surface. on their surface.

3.6.3.6. Catalytic Catalytic Suzuki Suzuki Cross-Coupling Cross-Coupling Reaction Reaction Earlier,Earlier, we reported that PdNPs stabilized with the the sterically sterically hindered hindered phosphonium phosphonium salts salts were were thethe effective eﬀective catalyst catalyst in in Suzuki Suzuki [77] [77 ]and and Sonogashir Sonogashiraa [78] [78 cross-coupling] cross-coupling reactions. reactions. The The wide wide scope scope of substratesof substrates involved involved in inthe the reaction reaction resulted resulted in in high high conversion, conversion, e.g., e.g., chloroarenes, chloroarenes, and and successful recyclingrecycling of of the catalytic system was achieved [60]. [60]. TheThe cross-coupling reaction reaction of of 1,3,5-tribromobenzene and and phenylboronic phenylboronic acid acid (Scheme (Scheme 33)) waswas carriedcarried out out using using in insitu situ prepared prepared PdNPs PdNPs under under mild mild conditions conditions (30 °C, (30 0.36◦C, mol% 0.36 [Pd], mol% 7 h). [Pd], Mainly 7 h). tri-substitutedMainly tri-substituted benzene benzene was formed was formedas a product; as a product; the products the products of mono- of mono- and di-substitution and di-substitution were detectedwere detected in the in amounts the amounts 0–1.55% 0–1.55% and and 1.9–8.24% 1.9–8.24% consequently. consequently. The The PdNPs PdNPs showed showed high high catalytic catalytic activityactivity and and considerable considerable stability. stability. No No “Pd “Pd black” black” precipitation precipitation was was observed observed in inthe the system system before before or afteror after the thereaction. reaction. It should It should be po beinted pointed out that out a that challenging a challenging model model reaction reaction was used was in used this study, in this wherestudy, wherethree C–C three bonds C–C bondsshould should be made be madein a one-pot in a one-pot manner manner under under mild mildconditions. conditions. Below, Below, we describewe describe the theresults results of cross-coupling of cross-coupling reaction reaction for forparticular particular studied studied metal metal nanoparticles. nanoparticles.

SchemeScheme 3. 3. SuzukiSuzuki cross-coupling cross-coupling reaction reaction used used as as a a model model reaction. reaction.

3.7.3.7. PdNP PdNP Size Size Dynamics Dynamics duri duringng the the Cross-Coupling Cross-Coupling Reaction Reaction TheThe TEM sample preparation of the PdNPs stabilized by the sterically hindered phosphonium saltsalt was was carried carried out out immediately immediately after after the the preparatio preparationn of of the the catalytic catalytic system system (20 (20 min min of of stirring the the phosphonium saltsalt and and palladium palladium acetate) acetate) and 7and h after 7 h the after onset the of Suzukionset reactionof Suzuki using reaction “nanofishing” using “nanofishing”techniques. For techniques. each phosphonium For each salt, phosphonium 100–500 nanoparticles salt, 100–500 were nanoparticles processed to calculate were processed the average to calculatesize and standardthe average deviation size and (Table standard2). deviation (Table 2). The average size of the PdNPs before the Suzuki reaction is in a narrow range of 2.0–3.7 nm (except 1b). DespiteTable the di 2.ff Theerence average in the size structures of PdNPs of before phosphonium and after Suzuki salt and reaction. a strong difference in their properties, they all show the ability to stabilize very small nanoparticles, and consequently, the catalyst Phosphonium The Average Size of The Average Size of PdNPs after Suzuki Entryhas a large active surface area. The standard deviation is only 0.7–1.8 nm, which indicates a high uniformity of nanoparticlesSalt in the solution.PdNPs, nm Reaction, nm 1 1b stabilizes1b PdNPs 2–3 times larger6.43 ± and2.65 with a wider distribution interval. 5.96 ± 2.08 Some large PdNPs (up2 to 15 nm) were2b observed when 2b3.69was ± 1.35 used as a stabilizer. This is perhaps 4.25 ± 2.21 associated with the high3 ionicity of3b these compounds thatnot doobserved not provide strong stabilization 3.57 of nanoparticles, ± 2.35 so the Pd atoms4 aggregate4b in bigger frameworks3.72 than ± 1.13 in the presence of other phosphonium not observed salts. 5 Nanoparticles5b were not found in2.78 the ± solution 0.87 containing 3b. The experiment 4.23 ± 1.36 was repeated twice with6 the same result.6b The highly dynamic2.01 ± nature0.72 of stabilized metal clusters 3.57 in ± the1.62 liquid phase may prevent7 trapping7b from the solution and2.56 detection ± 0.73 by microscopy. 2.67 ± 0.83 8 8b 3.11 ± 1.08 3.59 ± 1.27 9 9b 2.81 ± 0.80 2.15 ± 0.66 10 10b 3.62 ± 1.57 4.59 ± 2.46

Nanomaterials 2020, 10, 2457 14 of 20

Nanomaterials 2020, 10,Table x FOR 2.PEERThe REVIEW average size of PdNPs before and after Suzuki reaction. 15 of 21

The Average Size of The Average Size of PdNPs after TheEntry average Phosphonium size of the PdNPs Salt before the Suzuki reaction is in a narrow range of 2.0–3.7 nm PdNPs, nm Suzuki Reaction, nm (except 1b). Despite the difference in the structures of phosphonium salt and a strong difference in 1 1b 6.43 2.65 5.96 2.08 their properties, they all show the ability to stabil±ize very small nanoparticles, and± consequently, the 2 2b 3.69 1.35 4.25 2.21 catalyst has a large active surface area. The standard± deviation is only 0.7–1.8 nm,± which indicates a 3 3b not observed 3.57 2.35 ± high uniformity4 of nanoparticles 4b in the solution.3.72 1.13 not observed ± 1b stabilizes5 PdNPs 5b 2–3 times larger and2.78 with0.87 a wider distribution interval. 4.23 1.36Some large PdNPs ± ± (up to 156 nm) were observed 6b when 2b was used2.01 as0.72 a stabilizer. This is perhaps 3.57 associated1.62 with the ± ± high ionicity7 of these compounds 7b that do not2.56 provide0.73 strong stabilization of 2.67 nanoparticles,0.83 so the Pd ± ± 8 8b 3.11 1.08 3.59 1.27 atoms aggregate in bigger frameworks than in the± presence of other phosphonium± salts. 9 9b 2.81 0.80 2.15 0.66 Nanoparticles were not found in the solution± containing 3b. The experiment± was repeated twice 10 10b 3.62 1.57 4.59 2.46 with the same result. The highly dynamic nature± of stabilized metal clusters in the± liquid phase may prevent trapping from the solution and detection by microscopy. AfterAfter Suzuki Suzuki reaction, reaction, the the size size ofof PdNPsPdNPs andand thethe standard deviation deviation increased increased insignificantly insignificantly (Table(Table2). 2). This This indicates indicates the the high high stability stability of of the the formedformed nanoparticlesnanoparticles during during the the reaction reaction and and the the effiefficiencyciency of of the the phosphonium phosphonium salts salts as as stabilizing stabilizing agents.agents. InIn the the solution solution containingcontaining 3b,3b, thethe nanoparticles nanoparticles were were detected detected after afterthe catalytic the catalytic reaction. reaction. The ThePdNPs PdNPs were were probably probably formed formed in inthe the process process of ofSuzu Suzukiki reaction reaction or orwhen when dynamic dynamic properties properties of ofthe the systemsystem were were changed, changed, which which allowed allowed their their detection.detection. Interestingly,Interestingly, the the PdNPs PdNPs were were not not found found in in the the presence presence of 4bof 4bafter after the the Suzuki Suzuki reaction, reaction, which which may may indicate their agglomeration/precipitation or a change toward molecular stabilization with indicate their agglomeration/precipitation or a change toward molecular stabilization with inefficient inefficient trapping from solution. Since the “palladium black” was not detected during the trapping from solution. Since the “palladium black” was not detected during the experiment the experiment the former case is unlikely. former case is unlikely. XRPD and SAXS analysis of the system of the PdNPs and 5b in ethanol were performed in the XRPD and SAXS analysis of the system of the PdNPs and 5b in ethanol were performed in the solution with doubled concentration. The observed particles were larger than those measured by solutionTEM technique with doubled due to concentration. the agglomeration The observed process. particles were larger than those measured by TEM techniqueAll duethe catalytic to the agglomeration systems showed process. high catalytic activity in the studied Suzuki reaction (Figure 12).All The the conversion catalytic systems was in showed the range high of catalytic 61–93% activity(excluding in the 3b studied). The control Suzuki experiment reaction (Figure in the 12 ). Theabsence conversion of the was phosphonium in the range salt of was 61–93% also (excludingcarried out,3b for). Thewhich control the conversion experiment was in only the absence around of the6%. phosphonium salt was also carried out, for which the conversion was only around 6%.

100 7 Conversion, % PdNPs size, nm 90 6 80 70 5 60 4 50 3 40 Conversion, % 30 2 20 1 10 0 0 2 4 6 8 10 12 14 16 18 20 n

FigureFigure 12. 12.The The PdNP PdNP size size and and the the conversion conversion ofof the Su Suzukizuki reaction reaction dependin dependingg on on the the phosphonium phosphonium saltsalt structure. structure.

Nanomaterials 2020, 10, 2457 15 of 20

The solution containing 3b before the Suzuki reaction where PdNPs were not observed by “nanofishing” technique led to the lowest conversion (23%) (Table2, Entry 3). Overall, the sizes of the PdNPs varied within the range of 2.0–6.4 nm, and the quantity of the surface Pd atoms in the corresponding nanoparticles may change significantly [79]. However, the conversion does not depend on this change. The results indicate that in the presence of phosphonium salt, catalytically active species can be generated from a large variety of PdNPs. The mechanism of PdNP-catalyzed cross-coupling reactions typically involves dynamic interconversion of catalytic species and leaching [80]. High catalytic efficiency observed for metal NPs of different sizes may be evidence for a dynamic catalytic system, where catalytically active centers can be formed in situ due to oxidative addition of the reagent and leaching [81–83]. This was previously discussed in the literature, including the discovery of “cocktail”-type catalytic systems for Pd-catalyzed cross-coupling reactions [80,84]. PdNPs were shown to play the role of convenient reservoirs of catalytically active species. In such cases, variations in the size of PdNPs did not significantly affect the overall performance of the reaction as soon as in situ generation of active centers became efficient. Dynamic “cocktail”-type catalytic systems have several practical advantages, such as good scope and easy practical usage. The present study shows that phosphonium salts possess excellent compatibility with PdNPs and facilitate the catalytic reaction. The reasons for a difference in conversions should be sought in the environment of nanoparticles in the solution and the availability of their surface for the reactants. Since the stabilization of nanoparticles is carried out with the molecules of phosphonium salt and they are in the nearest surrounding, it is worth paying close attention to their properties in the solution. According to the literature, phosphonium salts have microheterogeneity [85], which is a special characteristic of the phosphonium salts containing the long alkyl substituents. Each molecule of such phosphonium salts is divided into hydrophobic and hydrophilic parts, which allows them to form aggregates in the solution. 9b and 10b form microreactors in which the substrates are concentrated around the nanoparticles. This leads to a higher conversion compared to phosphonium salts with medium-length alkyl chains (Figure 12). At the same time, TEM shows that the micelles/microreactors agglomerate into large aggregates consisting of hundreds to thousands of PdNPs surrounded by the molecules of phosphonium salt.

4. Conclusions A series of novel phosphonium salts containing sterically hindered cations based on tri-tert-butylphosphine with varied n-alkyl substituent were synthesized, along with a detailed study of their 1H, 13C and 31P NMR, mass spectra, melting points and X-ray analysis. The sterically hindered phosphonium salts support the formation of the PdNPs’ small average size with a narrow size distribution of 2.01 0.72 to 6.43 2.65 nm. The PdNPs demonstrate high ± ± catalytic activity in Suzuki cross-coupling. This is an important practical advantage, since variation in the side-chain length of phosphonium salts allows one to control their physical properties (for example, solubility in polar or non-polar solvents), but the NPs’ stabilization effect will remain. In such a case, this class of compounds may serve as universal NP stabilizers with tunable sidechain. The average size of PdNPs after the catalytic reaction slightly enhances the point about their excellent stability. The high catalytic efficiency observed for metallic NPs, regardless of their size within the range of ~2–6 nm, indicates a dynamic catalytic system. Catalytically active sites can be formed in different processes of catalyst evolution, where PdNPs play the role of a convenient reservoir of catalytically active species. In this case, variation in the size of the PdNPs does not significantly affect the overall catalytic performance. The wide scope of the applicable reactions and convenient practical usage make the application of dynamic “cocktail”-type catalytic systems advantageous.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/12/2457/s1, the synthetic procedure and characterization of each phosphonium salt, Crystal data, X-ray Powder diffraction and Small angle X-ray scattering, Figure S1: Experimental diffraction pattern of Pd@5b sample and theoretical Nanomaterials 2020, 10, 2457 16 of 20 calculated curve, Figure S2: Screenshots of the 2D small-angle scattering pattern of the Pd@10b sample and SAXS diffraction intensity profiles (Intensity vs. s) at 23 ◦C, Table S1: The sizes of palladium crystallites, calculated from the parameters of diffraction peaks, for Pd@5b sample, Table S2: Calculated parameters for the samples Pd@10b and Pd@5b, Figure S3: The fitting of experimental SAXS curve for samples Pd@5b and Pd@10b in the case of Guinier-Porod model. Author Contributions: D.M.A., V.V.E., A.T.G., D.R.I. carried out the experimental work and measured and analyzed the data. V.P.A., V.A.M., O.N.K. 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. Conflicts of Interest: The authors declare no conflict of interest.

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