Palladium complexes with 3-phenylpropylamine ligands: synthesis, structures, theoretical studies and application in the aerobic oxidation of alcohols as heterogeneous catalysts

Abstract

The reaction of 3-phenylpropylamine with Pd(OAc)₂ by heating in toluene resulted in the nearly square-planar complex trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂] (1). Complex 1 reacted with NaCl in methanol to obtain the corresponding product trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2). Treatment of 2 with triphenylphosphine in dichloromethane afforded trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(PPh₃)₂]2Cl⁻ (3). All the palladium(II) complexes (1–3) were fully characterized by IR and NMR spectroscopy. In addition, the crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. In these structures, the acetate and chloride ligands are in trans geometry. Density functional theory (DFT) calculations gave bond lengths and angles that were noted as experimental values. Palladium nanoparticles that were derived from complexes (1–3) were supported on cucurbit[6]uril (CB[6]) and identified by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma analysis (ICP) and high-resolution X-ray powder spectroscopy (HR-XPS). CB[6]-supported palladium nanoparticles (NPs) were used as heterogeneous catalysts for the aerobic oxidation of alcohols to the corresponding aldehydes or ketones without over-oxidation. CB[6]-Pd NPs (3) (prepared from complex 3) show better catalytic activity than CB[6]-Pd NPs (1), (2), as a higher yield was observed with them in a relatively short time. Factors such as the amount of catalyst, solvent, temperature and reaction time were all systematically investigated to determine their effects on the yield of catalytic alcohol oxidation reactions. This catalytic system displayed high activity and selectivity toward alcohols in mild conditions. The catalyst was reused five times without any significant loss of catalytic activity.

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Introduction

The selective oxidation of alcohols to the corresponding carbonyl compounds is an important transformation in the chemical industry.^1,2 Traditional oxidizing reagents, which have usually required stoichiometric amounts of inorganic oxidants, such as chromate or permanganate, are expensive and have serious toxicity issues associated with them and are not acceptable from the perspective of green chemistry.^3,4 Homogeneous transition metal-based catalysts such as palladium(II) complexes are known that can promote this reaction without the use of toxic oxidants.⁵

Hydrogen peroxide is considered as one of the least toxic oxidants, because it gives water as the only by-product. It also conforms to environmental concerns, wherein the development of catalytic systems that use molecular oxygen as the oxidant is of considerable interest.^6–8 Molecular oxygen is an inexpensive and abundant oxidant for the oxidation of alcohols.

The most important organic reactions have been performed with homogeneous palladium catalysts. These catalysts possess many merits, such as a high turnover number, high activity and selectivity and excellent yields, but from economic and environmental standpoints, heterogeneous catalysts are highly desirable.^9–14 For this purpose, researchers have immobilized palladium complexes and nanoparticles on various supports, such as polymers,¹⁵ silica,¹⁶ SBA-15/16,¹⁷ MCM-41,¹⁸ alumina,¹⁹ zeolite,²⁰ carbon,²¹ clay,²² PEG²³ and TiO₂,²⁴ to create heterogeneous catalysts, because heterogeneous systems are easy to handle and recover.^25–33

CB[6] compounds are a family of macrocyclic molecules that can act as both supports and capping agents (Fig. 1). CB[6] have been synthesized by the condensation of formaldehyde with glycoluril^34–38 and have two identical portals. At each entrance to the cavity, six polar carbonyl groups are located and provide two negative fringes for bonding to the surface of metals or nanomaterials. In spite of the polar fringed portals, CB[6] is non-polar and therefore nearly insoluble in all common organic solvents and water. In addition, CB[6] is a rigid cyclic structure that has high thermal and chemical stability against many oxidizing agents.³⁹

Fig. 1 Schematic of cucurbit[6]uril.

Herein, we report the synthesis and characterization of palladium complexes derived from 3-phenylpropylamine. Theoretical studies were also carried out to investigate why we obtained complex 2 instead of orthopalladate (as we expected). The optimized molecular parameters were close to those of the real structure as obtained by X-ray analysis. In addition, the LUMO–HOMO energy gap and partial atomic charges of complex 2 (the real product) and complex 2′ (the desired product) were obtained.

These complexes (1, 2 and 3) were used for the preparation of heterogeneous Pd catalysts, which were deposited on CB[6] using a simple method. The supported Pd nanoparticles showed high activity and selectivity in the aerobic oxidation of alcohols under mild conditions. Finally, the catalyst was fully reusable at least five times.

Results and discussion

Scheme 1 shows the complexes prepared in this study and the labelling assigned to the methylene protons.

Scheme 1 (i) Pd(OAc)₂ in toluene (60 °C, 24 h); (ii) excess NaCl in methanol (room temperature, 12 h); (iii) 2PPh₃ in dichloromethane (room temperature, 6 h).

Synthesis and structure of palladium complexes

The reaction of 3-phenylpropylamine Ph(CH₂)₃NH₂ with Pd(OAc)₂ in a 1 : 1 molar ratio was carried out in toluene at 60 °C to produce the palladium complex 1 trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂]. Treatment of complex 1 with an excess of NaCl in methanol afforded the corresponding product trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2). Complex 2 reacted with PPh₃ in a 1 : 2 molar ratio to obtain complex 3 trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(PPh₃)₂]2Cl⁻.⁴⁰ The mononuclear palladium complexes 1, 2 and 3 were characterized by IR and NMR spectroscopy. The spectra for complexes 1, 2 and 3 are given in the ESI (Fig. S1–S9†).

The IR spectrum of complex 1 shows characteristic bands that are due to the acetato ligands at 1573 and 1388 cm⁻¹ (ref. 41) and two peaks that correspond to NH₂ at 3218 and 3118 cm⁻¹. ¹H NMR, ¹³C{¹H} NMR and ³¹P{¹H} NMR spectra of complexes 1, 2 and 3 were recorded in CDCl₃. The ¹H NMR spectrum of complex 1 shows five sets of signals for methylene protons, acetato-methyl and amine protons. The ¹³C{¹H} NMR spectrum of complex 1 exhibits aliphatic and aromatic regions and the carbon signals of CO and CH₃ appear at 180.18 and 23.51 ppm, respectively.

The IR spectrum of complex 2 shows two peaks that correspond to NH₂ at 3223 and 3270 cm⁻¹, whereas peaks due to acetato ligands are not present. The ¹H NMR spectrum of complex 2 displays four sets of signals for methylene protons and amine protons. The ¹³C{¹H} NMR spectrum of complex 2 shows aliphatic and aromatic regions and the carbon signals of CO and CH₃ have disappeared.

The IR spectrum of complex 3 shows two peaks that correspond to NH₂ at 3395 and 3451 cm⁻¹. The ¹H NMR spectrum of complex 3 displays four sets of signals for methylene protons and amine protons. The ³¹P{¹H} NMR spectrum of complex 3 exhibits a peak that corresponds to PPh₃ at 23.26 ppm. Moreover, this spectrum shows an additional peak with lower intensity at 17.86 ppm, which refers to PPh₃ in Pd(PPh₃)₄.⁴² The molar conductance value of complex 3 in acetone (1 × 10⁻³ mol cm⁻³) is 110 Ω⁻¹ cm² mol⁻¹, which indicates that it is a 1 : 2 electrolyte.

The single-crystal structures of 1 and 2 were established by X-ray diffraction. However, attempts to crystallize complex 3 were unsuccessful. The single-crystal X-ray diffraction study of complexes 1 and 2 confirmed the details of the proposed new structures.

Fig. 2 and 3 show ORTEP plots of complexes 1 and 2 with selected bond lengths and angles given in the figure caption. Crystal data and parameters concerning data collection and structure solution and refinement are given in Table S1 of ESI.† Tables S2 and S3 in ESI† contain selected bond lengths and angles for complexes 1 and 2, respectively. H atoms in the figures, as well as in the tables, are omitted for clarity. The palladium metal exhibits a distorted square-planar geometry in both complexes and the two acetates and two chlorides are trans to each other.

Fig. 2 ORTEP diagram of trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂] (1) with 50% probability ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Pd(1)–O(1), 2.019(9); Pd(1)–O(3), 2.019(10); Pd(1)–N(1), 2.057(12); Pd(1)–N(2), 2.059(14). Selected bond angles (°): O(1)–Pd(1)–N(1), 85.3(5); O(3)–Pd(1)–N(1), 94.3(5); O(3)–Pd(1)–N(2), 85.0(5); O(1)–Pd(1)–N(2), 95.4(5); O(1)–Pd(1)–O(3), 179.4(5); N(1)–Pd(1)–N(2), 179.3(8).

Fig. 3 ORTEP diagram of trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2) with 50% probability ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Pd(1)–N(1), 2.0361(15); Pd(1)–N(1ⁱ), 2.0361(15); Pd(1)–Cl(1), 2.3031(5); Pd(1)–Cl(1ⁱ), 2.3031(5). Selected bond angles (°): N(1)–Pd(1)–Cl(1), 88.12(4); N(1)–Pd(1)–Cl(1ⁱ), 91.88(4); N(1ⁱ)–Pd(1)–Cl(1ⁱ), 88.12(4); N(1ⁱ)–Pd(1)–Cl(1), 91.88(4); Cl(1)–Pd(1)–Cl(1ⁱ), 180.0(5); N(1)–Pd(1)–N(1ⁱ), 180.0(5).

As indicated by the ligands at the Pd(II) center of complex 1, the angles subtended vary from 85.0(5)° to 95.4(5)° and from 179.3(8)° to 179.4(5)°. The sum of the bond angles around the palladium is 360.0°. The Pd–N bond lengths, 2.057(12) and 2.059(14) Å, in 1 are close to each other and greater than the values, 2.008(7) and 2.021(6) Å, that were reported for palladium complex 2c.⁴³ Moreover, the Pd–O bond distances (2.019(9) and 2.019(10) Å) in 1 were found to be somewhat longer than the values, 1.953(10) and 1.985(6) Å, that were reported for palladium complex 2c.⁴³

As indicated by the ligands at the Pd(II) center of complex 2, the angles are 88.12(4)°, 91.88(4)°, 88.12(4)° and 91.88(4)° for N(1)–Pd(1)–Cl(1), N(1)–Pd(1)–Cl(1ⁱ), N(1ⁱ)–Pd(1)–Cl(1ⁱ) and N(1ⁱ)–Pd(1)–Cl(1), respectively. The sum of the bond angles around the palladium is 360.0°. In complex 2, the Pd1–N1 (2.0361(15) Å) bond length is the same as the Pd1–N1ⁱ (2.0361(15) Å) distance. The Pd–Cl bond lengths in 2 (2.3031(5) Å) are in the normal range and consistent with the values reported for palladium(II) complexes.⁴⁴

N–H⋯O interactions in the crystal of 1 result in the formation of a three-dimensional structure, as shown in Fig. 4, whereas the N–H⋯Cl interactions that are shown in Fig. 5 result in the formation of a three-dimensional network in the case of 2.

Fig. 4 Intermolecular N–H⋯O interactions in complex 1.

Fig. 5 Intermolecular N–H⋯Cl interactions in complex 2.

Synthesis and characterization of CB[6]-Pd NPs

Our goal for the synthesis of CB[6]-Pd NPs was to use simple methods. To achieve this goal, CB[6] was prepared in water and Pd nanoparticles were dispersed on CB[6] in green solvents such as water and ethanol (Scheme 2). In these reactions, palladium complexes 1–3 were used as precursors for preparation of the catalysts.

Scheme 2 Preparation of CB[6]-Pd NPs 1–3.⁴⁷

Previous studies show that CB[6] has two identical portals lined by carbonyl groups, which provide two negative fringes that are capable of binding to the surface of metals or other nanostructures.^45,46 Palladium ions can bind to carbonyl groups at the portals of CB[6] before a reducing agent (NaBH₄) is added to the reaction mixture.^45,46 When a reducing agent is added to the reaction mixture, a high percentage of Pd(II) is transformed into Pd(0). This synthesis procedure is based on the physical adsorption of Pd(0) on the surface of the CB[6] support via electronic interaction between electrons from Pd(0) and vacant orbitals of C=O. However, electrostatic interaction between surface atoms of Pd and CB[6] can effectively prevent the Pd NPs from agglomeration.

The XRD pattern of fresh CB[6]-Pd NPs (3) exhibits a broad reflection peak that corresponds to CB[6] at about 23° (ref. 48) and also shows the peaks at 40°, 46°, 68° and 82° that corresponds to (111), (200), (220) and (311) crystallographic planes of Pd(0) nanoparticles with a face-centred cubic Pd lattice (Fig. 6a). This is in good agreement with results obtained from the literature (Fig. 6b).⁴⁹

Fig. 6 X-ray powder diffraction patterns of (a) CB[6]-Pd NPs (3) and (b) Pd(0).⁴⁶

Fig. 7 shows HR-XPS spectra of CB[6]-Pd NPs (3). The Pd 3d spectra show the characteristic Pd 3d_5/2 and 3d_3/2 doublet peaks. The expected positions of Pd(0) and Pd(II) species, at around 335.3 (ref. 50 and 52) and 337.6 eV,^50,51 respectively, are shown.

Fig. 7 XPS spectra of (a) high resolution Pd 3d of fresh CB[6]-Pd NPs (3) and (b) CB[6]-Pd NPs (3) after the fifth run.

The Pd(II) peaks were fitted with a Gaussian–Lorentzian peak shape, whereas the Pd(0) peaks were fitted with an asymmetric shape. The fitting indicates that Pd in fresh CB[6]-Pd NPs (3) is a mixture of Pd(II) and Pd(0) states (Fig. 7a). After the fifth run, Pd(0) is observed in higher proportions, which indicates that after catalytic cycles Pd(II) cations were transformed into Pd(0).

HR-XPS N 1s spectra of fresh CB[6]-Pd NPs (3) and CB[6]-Pd NPs (3) after the fifth run show a single peak at around 400.0 eV, which is attributed to C–N bonds⁴⁹ from CB[6] and the ligand (Fig. S10†). After the fifth run, C–N bonds are observed in lower proportions, which indicate that after catalytic cycles Pd(II) was transformed into Pd(0) and the ligand was removed from complex 3.

Fig. 8 shows FTIR spectra of palladium complex 3 (a), CB[6] (b) and CB[6]-Pd NPs (c). From Fig. 8a, it is clear that the two bands at around 2924 and 2849 cm⁻¹ could be attributed to asymmetric and symmetric stretching of CH₂ groups of 3-phenylpropylamine, respectively. The peak at 3048 cm⁻¹ corresponds to stretching vibrations of CH in the aromatic rings in palladium complex 3. The three strong bands that were observed near 744, 693 and 518 cm⁻¹ in the complex were attributed to the coordinated PPh₃ ligand.⁵³ The FTIR spectrum of CB[6] (Fig. 8b) displays a peak at 1726 cm⁻¹. This peak was assigned to carbonyl stretching vibrations, regarding which no changes between CB[6]-Pd NPs (3) (Fig. 8c) and CB[6] (Fig. 8b) were observed, which proves that there is no direct bonding between CB[6] and metal atoms.⁵⁴ In addition, the absorption bands at 693 and 518 cm⁻¹ demonstrated the presence of PPh₃ in CB[6]-Pd NPs (3) in lower proportions (Fig. 8c) compared with palladium complex 3 (Fig. 8a).

Fig. 8 FTIR spectra of: (a) palladium complex 3; (b) CB[6]; (c) CB[6]-Pd NPs.

A FE-SEM image of the corresponding CB[6]-Pd NPs (3) confirms the uniform distribution of Pd NPs on CB[6] (Fig. 9).

Fig. 9 FE-SEM image corresponding to CB[6]-Pd NPs (3).

The corresponding TEM images reveal that the Pd NPs were formed and well dispersed on the surface of the support (Fig. 10). The average particle size of Pd(0) in fresh CB[6]-Pd NPs, based on the sizes of more than 243 particles, was 3.11 nm (Fig. 10a). TEM images after the fifth run are shown in Fig. 10b. The average particle size of Pd(0) in CB[6]-Pd NPs after use was 2.62 nm. Moreover, the TEM images show that aggregation of Pd NPs was not observed even after fifth run.

Fig. 10 TEM images of (a) fresh CB[6]-Pd NPs (3) and (b) CB[6]-Pd NPs (3) after the fifth run: the histograms illustrate the size distribution of Pd nanoparticles.

Aerobic oxidation of alcohols by CB[6]-Pd NPs

The selective oxidation of alcohols to the corresponding carbonyl compounds is one of the most important transformations in the synthesis of fine chemicals. Several excellent catalysts for the oxidation of alcohols to the corresponding aldehydes or ketones have been disclosed, but most of these require heating and toxic oxidants, which violate the requirements of “green chemistry”.

We first examined the ability of CB[6]-Pd NPs (Cat 1–3) to catalyze the aerobic oxidation of benzyl alcohol. Initially, we focused our attention on the aerobic oxidation of benzyl alcohol under identical reaction conditions (90 °C, toluene as solvent, K₂CO₃ as base, in air) to compare catalyst 3 with catalysts 1 and 2 (Table 1).

Table 1 Optimizing the catalyst for the oxidation reaction of benzyl alcohol^a

Entry	Catalyst	Conversion^b (%)
a Reaction conditions: alcohol (1 mmol), K2CO3 (1 mmol), toluene (6 mL), catalyst (0.05 mol% Pd), 90 °C, 23 h, in air.b Determined by GC using biphenyl as an internal standard.c Catalyst (0.025 mol% Pd).
1	1	36
2	2	39
3	3	60
4	3	52^c
5	PPh3	5

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Although all catalysts were able to produce the corresponding benzaldehyde in good yield within the indicated time, catalyst 3 exhibited higher catalytic activity than that of catalysts 1 and 2 (Table 1, entries 1, 2 and 3). This is due to the fact that by changing the ligands on the transition metal atom, catalytic properties can be altered. Moreover, bulky electron-rich phosphines tend to improve the reaction performance and they also act as catalysts themselves.^55–57 For further investigation, a blank experiment with only PPh₃ was performed and it was found that PPh₃ could not act as a catalyst on its own (Table 1, entry 5). In addition, two different amounts of CB[6]-Pd NPs (3) (0.025 and 0.05 mol% Pd) were used, keeping all other reaction parameters fixed, namely, temperature (90 °C), toluene (6 mmol), K₂CO₃, and reaction time (23 h) in air. In fact, the amount of catalyst had a significant effect on the oxidation of benzyl alcohol (Table 1, entries 3 and 4). The results are shown in Table 1 and indicate 52% and 60% conversion corresponding to 0.025 and 0.05 mol% Pd catalyst, respectively. The lower conversion of benzyl alcohol to benzaldehyde with 0.025 mol% Pd catalyst may be due to there being fewer catalytic sites. Therefore, 0.05 mol% Pd catalyst was taken to be optimal.

Our further investigation concerned the oxidation of benzyl alcohol using CB[6]-Pd NPs (3), toluene and atmospheric air as a source of molecular oxygen in the presence of various bases (Table 2).

Table 2 Optimizing the reaction conditions for the oxidation reaction of benzyl alcohol using CB[6]-Pd NPs (3)^a

Entry	Solvent	Pd (mol%)	Base	Temp. (°C)	Time (h)	Conv.^b (%)
a Reaction conditions: benzyl alcohol (1 mmol), base (1 mmol), solvent (6 mL), catalyst (0.025 or 0.05 mol% Pd), in air.b Determined by GC using biphenyl as an internal standard.c Under air bubbling.d Conversion to benzoic acid.e The reaction was carried out in the presence of CB[6].f The reaction was carried out in the presence of Pd(OAc)2 supported on CB[6].g The reaction was carried out in the presence of PdCl2 supported on CB[6].
1	Toluene	0.05	K2CO3	40	4	85^c
2	Toluene	—	K2CO3	90	23	10
3	Toluene	0.05	—	rt	10	75^c
4	Toluene	0.05	K2CO3	rt	6	68^c
5	Toluene	0.025	K2CO3	90	6	70^c
6	Toluene	0.05	K2CO3	90	6	77^c (18)^d
7	Toluene	0.05	K2CO3	90	23	60
8	Toluene	0.05	K3PO4·3H2O	90	23	42
9	Toluene	0.05	NaOH	90	23	41
10	Toluene	0.05	KOH	90	23	13
11	Toluene	0.05	Na2CO3	90	23	28
12	EtOH : H2O (4 : 1)	0.05	K2CO3	90	23	31
13	EtOH : H2O (2 : 1)	0.05	K2CO3	90	23	43
14	EtOH : H2O (1 : 1)	0.05	K2CO3	90	23	55
15	Acetone	0.05	K2CO3	90	23	4
16	DMF	0.05	K2CO3	90	23	8
17	MeOH	0.05	K2CO3	90	23	8
18	EtOH	0.05	K2CO3	90	23	4
19	CH3CN	0.05	K2CO3	90	23	3
20^d	Toluene	0.05	K2CO3	40	4	—^c^,^e
21^e	Toluene	0.05	K2CO3	40	4	68^c^,^f
22^f	Toluene	0.05	K2CO3	40	4	54^c^,^g

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It is reasonable that a base deprotonates the alcohol and starts the catalytic reaction; therefore, the oxidation of alcohols over palladium-based catalysts generally needs a base as a promoter.⁵⁸ Therefore, different bases were used to promote the oxidation of benzyl alcohol and K₂CO₃ was found to be the most effective (Table 2, entry 7).

In addition, various solvents were investigated in the presence of K₂CO₃ and it was found that solvents, such as acetonitrile (CH₃CN), N,N-dimethylformamide (DMF), acetone, MeOH and EtOH, were not effective (Table 2, entries 15–19). The conversion of benzyl alcohol was low when H₂O : EtOH (1 : 1, 1 : 2, and 1 : 4) was used as the solvent (Table 2, entries 12–14) and toluene was the best choice for this catalytic reaction (Table 2, entry 7). In the presence of air, the reaction time needed to be prolonged to 23 h at 90 °C. To reduce the reaction time, the catalytic oxidation of benzyl alcohol was studied by performing the reaction with benzyl alcohol under air bubbling. To determine the effect of air bubbling on the oxidation reaction of benzyl alcohol, three different temperatures (rt, 40 °C and 90 °C) were investigated. The results show that 68%, 85%, and 77% conversion was achieved, corresponding to rt, 40 °C and 90 °C, (Table 2, entries 1, 4 and 6). At an elevated temperature (90 °C), >95% conversion of benzyl alcohol was found with 77% selectivity for benzaldehyde and 18% selectivity for benzoic acid, which is due to over-oxidation of benzaldehyde to benzoic acid at an elevated temperature (Table 2, entry 6). Moreover, in the presence of CB[6] as the catalyst, the reaction did not occur (Table 2, entry 20).

For further investigation, Pd(OAc)₂ and PdCl₂ were also supported on CB[6] and in comparison with the present catalyst (CB[6]-Pd NPs (3)), provided lower conversion (Table 2, entries 21 and 22).

The scope of the catalytic system was subsequently extended to the aerobic oxidation of a variety of alcohols under optimized conditions (Table 3). Several aromatic alcohols were employed to study the general application of CB[6]-Pd NPs (3) for the oxidation of alcohols and the results are listed in Table 3, entries 1–8. The selectivity for the corresponding aldehyde or ketone was high for all alcohol oxidation reactions. High efficiency was obtained for the oxidation of 2-phenylethanol, wherein 100% conversion was observed at 2 h (Table 3, entry 2). CB[6]-Pd NPs (3) exhibit higher catalytic activity for substituted aromatic alcohols containing electron-donating groups (e.g. –OCH₃) than those containing electron-withdrawing groups (such as –NO₂) (Table 3, entries 7 and 8). Moreover, this catalytic system was inactive with linear alcohols (Table 3, entries 9 and 10). The results show that CB[6]-Pd NPs (3) were highly active and selective for the oxidation of alcohols that have at least one benzene ring.

Table 3 Aerobic oxidation of alcohols using CB[6]-Pd NPs (3)^a

Entry	Substrate	Time (h)	Conv.^b (Yield)^c (%)
a Reaction conditions: alcohol (1 mmol), K2CO3 (1 mmol), toluene (6 mL), catalyst (0.05 mol% Pd), 40 °C, under air bubbling.b Determined by GC using biphenyl as an internal standard.c Isolated yields.
1	Benzyl alcohol	4	85 (81)
2	2-Phenylethanol	2	100 (94)
3	1-Phenylethanol	4	98 (91)
4	4-Hydroxybenzyl alcohol	4	96 (90)
5	2-Chlorobenzyl alcohol	4	77 (75)
6	2-Hydroxybenzyl alcohol	4	80 (78)
7	4-Nitrobenzyl alcohol	4	84 (81)
8	4-Methoxybenzyl alcohol	4	90 (83)
9	3-Hexanol	10	Trace
10	2-Methyl-1-propanol	10	Trace

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The catalytic activity of complex 3 was also studied in the aerobic oxidation of a variety of alcohols using the optimized heterogeneous reaction conditions. As presented in Table 4, in these heterogeneous systems, different alcohols react to obtain the corresponding products. In comparison with homogeneous systems, the oxidation of alcohols in the presence of CB[6]-Pd NPs (3) (heterogeneous system) could be considered an efficient and versatile catalytic reaction for the aerobic oxidation of a variety of alcohols.

Table 4 Aerobic oxidation of alcohols using complex 3^a

Entry	Substrate	Time (h)	Conv.^b
a Reaction conditions: alcohol (1 mmol), K2CO3 (1 mmol), toluene (6 mL), complex 3 (0.05 mol%), 40 °C, under air bubbling.b Determined by GC using biphenyl as an internal standard.
1	Benzyl alcohol	4	88
3	1-Phenylethanol	4	100
4	4-Hydroxybenzyl alcohol	4	93
5	2-Chlorobenzyl alcohol	4	76
7	4-Nitrobenzyl alcohol	4	88
8	4-Methoxybenzyl alcohol	4	93
9	3-Hexanol	10	Trace

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Heterogeneity and recyclability

Further experiments were performed to determine the recyclability of the solid catalyst. Consecutive oxidations of benzyl alcohol under air bubbling were also carried out at 40 °C using toluene as the solvent. The results are summarized in Fig. 11. At a palladium loading of 0.05 mol%, CB[6]-Pd NPs (3) afforded a conversion of 85% under air bubbling after 4 h. After the first reaction cycle, the CB[6]-Pd NPs (3) catalyst was recovered by centrifugation and washed with ethanol and water three times. The catalyst was dried overnight at 100 °C and for the second reaction cycle, fresh benzyl alcohol was added and the other conditions were the same as in the first reaction cycle. A conversion of 87% was achieved after 4 h. From the third to the fifth reaction, no significant change was observed in selectivity and only a slight decrease in conversion was observed, which showed that the catalyst was stable and could be regenerated for repeated use.

Fig. 11 Recyclability test for catalyst in the aerobic oxidation of alcohols.

ICP-OES analysis showed that the palladium content of fresh CB[6]-Pd NPs (3) was 11.9 wt%. The palladium content after five consecutive reaction cycles was almost the same as that of the fresh catalyst and palladium could not be detected in the filtrate after the reaction. Moreover, a hot filtration test confirmed that the reaction did not continue after the catalyst was removed. These results confirm the heterogeneous character of the catalytically active species, which limits pollution of the environment by the catalyst.

Comparison with other studies

The catalytic performance of the catalyst CB[6]-Pd NPs (3) for the oxidation of alcohols was compared with some previously reported results using different types of support (Table 5).^15,59–63 As shown in Table 5, in comparison with other supported palladium catalysts, CB[6]-Pd NPs (3) exhibited comparable yields with a low catalyst loading in a short reaction time. In comparison with previous study, this catalytic system provides a higher turnover frequency (TOF). In addition, the preparation process of the catalyst CB[6]-Pd NPs (3) is very simple and environmentally friendly.

Table 5 Comparison with reported results for the Suzuki reaction on supported Pd catalysts

Entry	Catalyst^a	Reaction conditions solvent/base/temp./time/oxidant	Yields^b/%	TOF^b (h^−1)	References
a Data in parentheses indicate mol% Pd used.b Turnover frequency: mmol aldehyde product/(mmol catalyst × h reaction time).
1	Pd@SBA-15 (0.4)	Toluene/K2CO3/80 °C/3.5 h/O2	83	59	59
2	PdNPs/PS (0.5)	Toluene/K2CO3/85 °C/15 h/—	98	13	15
3	PdHAP-0 (0.6)	Trifluorotoluene/—/90 °C/1 h/O2	99	165	60
4	Pd/MIL-101 (1.5)	Toluene/—/80 °C/1.5 h/O2	99	44	61
5	MS-PdG-G (1)	Toluene/K2CO3/110 °C/10 h/O2	99	10	62
6	Pd(TOP)/MB-H2O2	Solvent-free/—/80 °C/24 h/—	29	36	63
7	CB[6]-Pd NPs (0.05)	Toluene/K2CO3/40 °C/4 h/air bubbling	81	405	This work

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Theoretical approaches

To determine the structure of the prepared molecule 2 in the gas phase, theoretical calculations were performed. In the previous sections, we described the synthetic methods for the preparation of palladium complexes. According to our previous studies,^44b with these reaction conditions, we expected to obtain an orthopalladated complex, but X-ray crystal structure analysis confirmed that the geometry was trans instead. Therefore, optimizations of two structures (the real (2) and expected (2′) products) were performed to find a reason why the trans product is formed.

The optimized parameters of 2 are in good agreement with the experimental values determined by X-ray analysis. The most important parameters that were calculated for the optimized structure of 2 are listed in Table 6. A comparison between calculated parameters (2*) and X-ray structure parameters (2) confirms that this structure is close to the real structure.

Table 6 Selected bond lengths (Å) and bond angles (°) for compound 2 (calculated (2*) and X-ray structure (2))

	2	2*
Bond lengths
Pd1–N1	2.0361(15)	2.058
Pd1–N1^i	2.0361(15)	2.057
Pd1–Cl1	2.3031(5)	2.466
Pd1–Cl1^i	2.3031(5)	2.466
Bond angles
N1–Pd1–Cl1	88.12(4)	89.9
N1^i–Pd1–Cl1	91.88(4)	90.1
N1–Pd1–Cl1^i	91.88(4)	90.1
N1^i–Pd1–Cl1^i	88.12(4)	89.9
Cl1–Pd1–Cl1^i	180.0(5)	180.0
N1–Pd1–N1^i	180.0(5)	180.0

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According to the X-ray structure, complex 2 has a trans geometry and calculation of the Gibbs free energies of both complexes showed that the trans structure (2) could be obtained more easily than the orthopalladate. The calculated values of ΔG for the reactions that produce 2 and 2′ are −263.72 and −132.35 kcal mol⁻¹, respectively, and the ΔH values are −305.40 and 151.25 kcal mol⁻¹ in the gas phase. Therefore, the Gibbs free energy confirms that the trans compound must be the major product.

We also employed population analyses for 2 and 2′ to determine the energies of the frontier molecular orbitals (FMOs). Graphic representations of the LUMO and HOMO and their related energies for both structures are shown in Fig. 12. The energy gaps of 2 and 2′ are 0.0610 and 0.1647 eV, respectively. By comparing the energies of the frontier orbitals, the LUMO–HOMO energy gap in 2 is less than that in 2′, which shows that this structure is more reactive. For a more exact determination of the partial atomic charges, NBO calculations were used. The results of these calculations for complex 2 show that the partial charges of Cl atoms in complex 2 are about −0.5. Moreover, the charge of Pd in this complex is 0, which demonstrates good interaction.

Fig. 12 Frontier molecular orbital diagrams of complexes 2 and 2′.

The second-order perturbation energies (E₂) that were obtained from the NBO calculations show that the Pd-ligand bonds are stronger in 2 compared with 2′. The E₂ interaction energy between Cl and Pd is 172 kcal mol⁻¹ and that between N and Pd is 154 kcal mol⁻¹ for complex 2, whereas the same values for complex 2′ are 142 and 2 kcal mol⁻¹, respectively (Pd–C is a real covalent bond and its energy should be added to the E₂ energy). All these calculations confirm the production of complex 2.

Experimental

General

All reactants were purchased from Merck Chemical Company and Aldrich and used as received. Solvents were used without further purification or drying.

Fourier transform infrared (FTIR) spectra were obtained in KBr pellets with a Jasco FT/IR 680 Plus instrument. NMR spectra were obtained with a Bruker spectrometer at 400.13 MHz (¹H). Elemental analysis was performed on a LECO CHNS-932 apparatus. The molar conductance of complex 3 was measured in acetone at 1 × 10⁻³ M using an Elmetron CC-505 conductivity meter.

Scanning electron microscopy (SEM) studies were carried out at 15 kV using a HITACHI S-4160 instrument (Japan). Transmission electron micrographs (TEM) were obtained with a JEOL JEM 1010 microscope operating at an accelerating voltage of 100 kV. The palladium content of the catalyst was measured by a inductively coupled plasma (ICP-OES) analyzer (Perkin Elmer 7300DV spectrometer). The X-ray powder diffraction (XRD) pattern was recorded using a Scintag X-ray diffractometer with a 1.54 Å (Cu Kα) X-ray radiation source. HR-XPS experiments were performed using a SPECS Sage HR 100 spectrometer with a non-monochromatic X-ray source (magnesium Kα line with an energy of 1253.6 eV and an applied power of 250 W), which was placed perpendicular to the analyzer axis and calibrated using the 3d_5/2 line of Ag with a full width at half maximum (FWHM) of 1.1 eV. An electron flood gun was used during measurements to neutralize charging effects. The resolution that was selected for the spectra of the different elements used pass energy of 15 eV and 0.15 eV per step. All measurements were made in an ultrahigh vacuum (UHV) chamber at a pressure below 8 × 10⁻⁸ mbar. In the fittings, Gaussian–Lorentzian and asymmetric functions were used (after a Shirley background correction).

Conversions were monitored using an Agilent Technologies 6890N gas chromatograph equipped with a flame ionization detector (FID) and an HB-50⁺ column (length = 30 m, inner diameter = 320 μm, and film thickness = 0.25 μm). Products were identified by comparison with authentic samples.

Crystallography

Crystals that were suitable for the X-ray molecular structure determination of 1 and 2 were obtained by the diffusion method for solutions of 1 and 2 in dichloromethane/n-hexane (1 : 3 v/v) and chloroform/n-hexane (1 : 3 v/v), respectively. The X-ray diffraction experiment was performed at 120 K with the use of an Agilent Gemini single-crystal diffractometer (Cu Kα radiation). The structure was solved using SuperFlip software⁶⁴ and further refined with Jana2006.⁶⁵ MCE software⁶⁶ was used for visualization of Fourier maps. The structure was refined by the full-matrix least-squares method on the F-squared value. The atoms of the palladium complex were refined anisotropically. The positions of hydrogen atoms were kept in the expected geometry with U_iso set to 1.2 times the U_eq value of the parent atom. The atoms O₁, O₂ and O₃ were refined isotropically and represented disordered solvent. The centers of these atoms do not represent actual atomic positions.

Computational methods

The DFT method was applied to optimize the structures and calculate molecular and spectral parameters of the compounds in the gas phase. The Gaussian 09 program package⁶⁷ was employed for optimizing the structures and calculation of molecular properties. To perform DFT calculations, Becke's three-parameter exchange functional⁶⁸ was used in combination with the Lee–Yang–Parr correlation functional (B3LYP) with the LANL2DZ basis set.⁶⁹ Molecules were used without any symmetry restriction and C₁ symmetry was assumed for all molecules. NBO analyses⁷⁰ were carried out as implemented in the Gaussian program package using the B3LYP/LANL2DZ level of theory.

Synthesis of trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂] (1)

Pd(OAc)₂ (0.0448 g, 0.2 mmol) was added to a solution of 3-phenylpropylamine (28 μL, 0.2 mmol) in toluene (10 mL) and the resulting mixture was heated under reflux for 24 h. The solvent was then evaporated and the resulting yellow solid residue was dissolved in n-hexane (10 mL) and CH₂Cl₂ (2 mL). A pale yellow solid immediately precipitated. The mixture was stirred for 2 h at room temperature and the resulting suspension was filtered. The pale yellow solid thus obtained was air-dried to obtain (1).

Complex 1: yellow solid, yield: 0.06533 g, 66%; mp 135 °C. IR: ν_max/cm⁻¹ 1573 and 1388 (CO), 3218 and 3118 (NH₂). ¹H NMR (400 MHz, CDCl₃, 25 °C, TMS): δ (ppm): 1.85 (s, 3H, MeCO₂), 2.04 (m, 2H, H₂), 2.53 (m, 2H, H₁), 2.60 (t, ³J_HH = 7.6 Hz, 2H, H₃), 3.77 (m, 2H, NH₂), 7.17–7.30 (m, 5H, C₆H₅). ¹³C{¹H} NMR (CDCl₃, 25 °C, TMS): δ (ppm): 23.51 (Me), 32.50 (C₂), 32.90 (C₃), 43.23 (C₁), 126.15–140.85 (C_aromatic), 180.18 (CO). Anal. calcd for C₂₂H₃₂N₂O₄Pd: C, 53.39; H, 6.52; N, 5.66. Found: C, 53.26; H, 6.65; N, 5.73.

Synthesis of trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2)

To a suspension of 1, methanol was added excess NaCl and the resulting mixture was stirred for 12 h at room temperature. The solvent was then evaporated and the resulting yellow solid residue was dissolved in n-hexane (10 mL) and CH₂Cl₂ (2 mL). The mixture was stirred for 2 h at room temperature and the resulting suspension was filtered. The pale yellow solid thus obtained was air-dried to obtain (2).

Complex 2: yellow solid, yield: 0.04656 g, 52%; mp 170 °C. IR: ν_max/cm⁻¹ 3223 and 3270 (NH₂). ¹H NMR (400 MHz, CDCl₃, 25 °C, TMS): δ (ppm): 2.03 (m, 2H, H₂), 2.67 (m, 2H, H₁), 2.69 (m, 2H, H₃), 2.78 (m, 2H, NH₂), 7.17–7.31 (m, 5H, C₆H₅). ¹³C{¹H} NMR (CDCl₃, 25 °C, TMS): δ (ppm): 32.85 (C₂), 33.10 (C₃), 44.87 (C₁), 126.22–140.65 (C_aromatic). Anal. calcd for C₁₈H₂₆Cl₂N₂Pd: C, 48.29; H, 5.85; N, 6.26. Found: C, 47.79; H, 5.92; N, 6.16.

Synthesis of trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(PPh₃)₂]2Cl⁻ (3)

To a suspension of palladium complex 2 (0.275 g, 0.5 mmol), dichloromethane (15 mL) was added PPh₃ (0.262 g, 1 mmol). The resulting solution was stirred for 6 h and then filtered through a plug of MgSO₄. The filtrate was concentrated to ca. 2 mL and n-hexane (10 mL) was added to precipitate 3 as a pale yellow solid, which was collected and air-dried.

Complex 3: yellow solid, yield: 0.05047 g, 56%; mp 171 °C (dec.). ΔM (Ω⁻¹ cm² mol⁻¹)/110. IR: ν_max/cm⁻¹ 3395 and 3451 (NH₂). ¹H NMR (400 MHz, CDCl₃, 25 °C, TMS): δ (ppm): 1.28 (m, 2H, H₂), 2.09 (m, 2H, H₁), 2.75 (t, ³J_HH = 7.2 Hz, 2H, H₃), 3.06 (m, 2H, NH₂), 7.28–7.75 (m, C₆H₅). ³¹P{¹H} NMR (CDCl₃, 25 °C): δ (ppm): 23.26 (s, 1P, PPh₃ (complex 3)), 17.85 (s, 1P, PPh₃ (Pd(PPh₃)₄)).

Preparation of glycoluril and CB[6]

Glycoluril and CB[6] were prepared according to previous reports.^71,72 For the preparation of glycoluril, to a solution of urea (60 g, 1 mol) in water (100 mL) was added a 40% aqueous solution of glyoxal (50 g, 0.345 mol) and concentrated HCl (8.6 mL). The resulting solution was heated at 90 °C until a heavy precipitate was formed. Then, the reaction mixture was allowed to cool to room temperature and filtered. The precipitate was washed with copious amounts of water (200 mL) followed by acetone to remove residual water and dried under vacuum.

For the synthesis of CB[6], a stirred mixture of glycoluril (1.5 g, 10.6 mmol), 37% aqueous formaldehyde solution (2.4 mL), concentrated sulfuric acid (1.43 mL) and water (10 mL) was heated for several hours at 160 °C. The resulting reaction mixture was cooled to room temperature and poured into water (25 mL). A pale yellow precipitate was formed, which was filtered off and dissolved in concentrated hydrochloric acid and then the solution was diluted with water. A precipitate was formed, which was washed several times with water and dried at 130 °C.

Preparation of CB[6]-Pd NPs

CB[6]-Pd NPs catalysts were prepared according to the literature method.⁷³ For this purpose, 0.1 mmol palladium complex (1, 2 or 3) and 0.1 mmol (0.099 g) CB[6] were mixed in 10 mL water at room temperature. The mixture was stirred for 30 min until a uniform brown suspension was formed. After this, freshly prepared NaBH₄ in ethanol (1 mmol in 10 mL) was rapidly added to the reaction mixture and reduction occurred instantaneously. The reduction was indicated by a color change from brown to black. The mixture was stirred at room temperature for another 3 h. The final reaction product was separated by centrifugation and washed several times with aqueous ethanol (v/v, 1 : 1) to remove excess salt. CB[6]-Pd NPs were dried at 70 °C for 10 h.

General procedure for the aerobic oxidation of alcohols

Benzyl alcohol (1 mmol), base (1 mmol), solvent (6 mL) and catalyst (0.05 mol% Pd) were combined in a dry flask and the mixture was stirred at a given temperature in air or under air bubbling for a given time. After the reaction, the liquid was filtered and analyzed by GC to determine the conversion and selectivity.

Recycling procedure for CB[6]-Pd NPs

Benzyl alcohol was used to test the recyclability of CB[6]-Pd NPs (3) in the aerobic oxidation of alcohols. After the first run was complete, the CB[6]-Pd NPs (3) catalyst was separated by centrifugation and washed several times with ethanol and water to remove adsorbed organic substrates and salt. The catalyst was dried overnight at 70 °C prior to being reused. Then, the catalyst was used for the second run without additional activation and the same process was repeated for the next run.

Conclusions

In conclusion, we prepared three palladium complexes that were derived from primary 3-phenylpropylamine, we developed a simple method for their dispersion in the form of palladium nanoparticles on the surface of CB[6], and we tested the application of these nanoparticles for the aerobic oxidation of alcohols in toluene using air as a source of molecular oxygen. The most promising catalytic effects were exhibited by nanoparticles of complex 3, CB[6]-Pd NPs (3), which were evaluated as an efficient catalyst for the aerobic oxidation of alcohols. Importantly, all reactions were performed at 40 °C and the corresponding products were obtained in good or excellent yields. The small size of the Pd particles explains the remarkably high activity of this particular catalyst. TEM and ICP of the regenerated catalyst showed that it is stable under the present reaction conditions.

Acknowledgements

This study was supported by the Department of Chemistry, Isfahan University of Technology and the University of Vigo. We gratefully acknowledge the funding support received for this project from the Iranian Nanotechnology Initiative Council. Crystallography was supported by the Czech Science Foundation project 15-12653S.

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Footnote

† Electronic supplementary information (ESI) available: NMR and FT-IR spectra of all complexes (1–3); single-crystal data of 1 and 2 (CCDC 1060354 and 1060510), HR-XPS N 1s spectra of fresh CB[6]-Pd NPs (3) and CB[6]-Pd NPs (3) after the fifth run. CCDC 1060354 and 1060510. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17249g

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