Next Article in Journal
Enhanced Thermal Stability of Polyphosphate-Dependent Glucomannokinase by Directed Evolution
Next Article in Special Issue
Heterobimetallic Complexes of Bi- or Polydentate N-Heterocyclic Carbene Ligands and Their Catalytic Properties
Previous Article in Journal
Fabrication of TaON/CdS Heterostructures for Enhanced Photocatalytic Hydrogen Evolution under Visible Light Irradiation
Previous Article in Special Issue
Syntheses of Silylene-Bridged Thiophene-Fused Cyclopentadienyl ansa-Metallocene Complexes for Preparing High-Performance Supported Catalyst
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 12
Issue 10
10.3390/catal12101111
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition

by
Thaís R. Cruz
1,
Gustavo H. C. Masson
1,
Kelly A. E. Amorim
2,
Antonio E. H. Machado
3,4,
Beatriz E. Goi
1 and
Valdemiro P. Carvalho-Jr.
1,*
1
Faculdade de Ciências e Tecnologia, UNESP—Univ. Estadual Paulista, Presidente Prudente 19060-900, SP, Brazil
2
Instituto de Química de São Carlos, Universidade de São Paulo, São Carlos 13560-970, SP, Brazil
3
Instituto de Química, Universidade Federal de Uberlândia, Uberlândia 38400-089, MG, Brazil
4
Instituto de Física, Programa de Pós-Graduação em Ciências Exatas e Tecnológicas, Universidade Federal de Catalão, Catalão 75704-020, GO, Brazil
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1111; https://doi.org/10.3390/catal12101111
Submission received: 1 July 2022 / Revised: 6 September 2022 / Accepted: 13 September 2022 / Published: 26 September 2022
(This article belongs to the Special Issue Organometallic Homogeneous Catalysis)
Browse Figures
Versions Notes

Abstract

:
The [Ru(PPh3)2Cl-piperidine(4-aminomethyl)] complex (mono-Ru) was synthesized from [Ru(PPh3)3Cl2] and 4-(aminomethyl)piperidine, whereas the [(PPh3)PdCl(Shiff-pip)] complex (mono-Pd) was obtained by reacting [Pd(PPh3)2Cl2] with its respective Schiff base ligand, both at a 1:1 molar ratio. The heterobimetallic [RuCl2(PPh3)2](μ-Schiff)Pd(PPh3)Cl] complex (Ru/Pd) was synthesized via a one-pot, three-component reaction of mono-Ru, [(Pd(PPh3)2Cl2] and salicylaldehyde. All complexes were fully characterized by FTIR, UV-Vis, and NMR spectroscopy, as well as elemental analysis, MALDI-TOF mass spectrometry, cyclic voltammetry, and computational studies. Ru/Pd was able to polymerize norbornene (NBE) by two different mechanisms: ROMP and vinyl polymerization. The Ru fragment was active for ROMP of NBE, reaching yields of 68 and 31% for mono-Ru and Ru/Pd, respectively, when the [NBE]/[Ru] = 3000 molar ratio and 5 μL EDA addition were employed at 50 °C. The poly(norbornene) (polyNBE) obtained presented an order of magnitude of 104 g mol−1 and Ð values between 1.48 and 1.79. For the vinyl polymerization of NBE, the Pd fragment was active using MAO as a cocatalyst, reaching a yield of 47.0% for Ru/Pd and quantitative yields for mono-Pd when [Al]/[Pd] = 2500 and [NBE]/[Pd] = 20,000 molar ratios were employed, both at 60 °C.

Graphical Abstract

1. Introduction

Complexes based on late-transition metals have been widely used as catalysts in a large number of catalytic reactions, such as the reactions of olefins and acetylenes with hydrogen, carbon monoxide, water, or other nucleophiles, as well as the isomerization, oligomerization, and polymerization of olefins [1,2]. Especially, ruthenium (Ru) and palladium (Pd) metallic centers have emerged as multifunctional catalysts [3,4,5,6,7,8,9,10,11]. These transition-metals have demonstrated high catalytic activity in polymerization reactions [11]. Based on the functionality of these metals, and aiming to increase the range of catalytic reactions, a large number of studies using heterobimetallic complexes composed of Ru(II) and Pd(II) fragments have been conducted [11,12,13]. Furthermore, the use of bimetallic complexes as homogeneous catalysts can increase reactivity via synergistic cooperation [14,15].
A well-defined macromolecular structure is certainly a goal in synthetic polymer chemistry. Ring-opening metathesis polymerization (ROMP) is a good alternative process that can produce high molecular weight and unsaturated polymers from a cyclic olefin, while vinyl polymerization can produce long saturated polymers [16,17,18]. Ruthenium-catalyzed ROMP of norbornene is particularly interesting to obtain unsaturated poly(norbornene) (polyNBE), since it presents high selectivity for olefins and has an electron-rich metallic center [19]. The principal application of these polymers is in the Norsorex® process [20]. PolyNBE can also be obtained by vinyl polymerization using Pd complexes as catalysts. Through this method, it is possible to obtain linear polymers with high molecular weight and saturated chains without the opening ring process [21,22]. Thus, the properties of this polymer are different from those obtained by ROMP, with emphasis on its high heat resistivity and mechanical resistance [20].
In studies conducted by our research group, ROMP has been initiated using non-carbene amine-based Ru complexes, where the active species is obtained in situ through reaction between the Ru complex and ethyl diazoacetate (EDA) [11,23,24,25,26,27]. Recently, heterobimetallic systems for multifunctional catalysts, mainly in the polymerization mechanism, have been addressed [11,28]. This study reports on the synthesis and characterization of a new heterobimetallic complex based on Ru(II) and Pd(II) for ROMP and vinyl polymerization of norbornene. This catalyst was designed to contain a phosphine-Ru(II) fragment to initiate the ROMP reactions and phosphine-Pd(II)-(N,O-Schiff) fragment to initiate the vinyl polymerization reactions (Scheme 1). In addition, the Ru(II) and Pd(II) monometallic fragments were synthesized to relate the catalytic activity to each metallic center and their contributions to the reactions.

2. Results and Discussion

2.1. Synthesis and Characterization

Mono-Pd was prepared from the reaction of [Pd(PPh3)2Cl2] with 1 equiv. of Schiff base in methanol under reflux for 12 h to obtain a dark yellow solid (Scheme 2). Prior to the complexation step, the Schiff base ligand was suspended in MeOH and deprotonated with sodium hydroxide at room temperature. Mono-Ru was prepared from the rapid reaction of the [Ru(PPh3)3Cl2] precursor complex with 1 equiv. of 4-(aminomethyl)piperidine in acetone to generate the light pink solid (Scheme 2). The heterobimetallic [RuCl2(PPh3)2](-Schiff)Pd(PPh3)Cl] complex (Ru/Pd) was obtained from the reaction between mono-Ru, [Pd(PPh3)2Cl2] and salicylaldehyde (1:1:1 equiv.) to obtain a Schiff base (N,N,O) via one-pot three-component synthesis, which linked the monometallic fragments (Scheme 2). Mono-Ru, mono-Pd, and Ru/Pd were characterized by elemental analysis, spectroscopic techniques, cyclic voltammetry, and density functional theory (DFT) calculations. All attempts to obtain crystals for X-ray diffraction (XRD) studies have not been successful so far.
The elemental analyses of mono-Ru, mono-Pd, and Ru/Pd agreed well with the molecular formula predicted for these compounds. The FTIR spectrum of mono-Ru revealed two large bands around 3255 cm−1 from asymmetric ν(N-H) stretching, indicating that 4-(aminomethyl)piperidine was coordinated to [Ru(PPh3)3Cl2] (Figure S1). The spectra of both mono-Pd and Ru/Pd showed characteristic absorption bands at 1620 and 1622 cm−1, respectively, from ν(C=N) stretching. These values are different from that of the free Schiff base, which appeared at 1619 cm−1, indicating that the ligand is coordinated by iminic nitrogen in mono-Pd and Ru/Pd. The ν(Pd-Cl) and ν(Ru-Cl) stretches in Ru/Pd at 340 and 298 cm−1, respectively, and suggesting the presence of two different metal centers in this heterobimetallic compound (Figure S1).
The 1H NMR spectra of mono-Ru and Ru/Pd exhibit peaks in the 0.48–3.51 ppm range as multiplets assigned to the CH2 hydrogen groups from the amine ring in the Ru center (Figures S2 and S4). Additionally, mono-Ru shows a triplet around 3.75 ppm, assigned to the NH2 protons from the primary amine. In Ru/Pd, the disappearance of the signal of the NH2 protons, followed by the appearance of a multiplet around 6.78 ppm for the H–C=N group proton, suggests the formation of azomethine nitrogen in Ru–Pd. As for mono-Pd and Ru/Pd (Figure S4), the signal of the azomethine nitrogen merges with the phosphine protons (Figures S3 and S4). The multiplet around 3.48 ppm is assigned to the NH group from the secondary amine for both mono-Ru and Ru/Pd. The 1H NMR spectrum of mono-Pd shows multiplets in the 0.89–2.00 ppm range for the amine ring protons. Mono-Pd, Ru/Pd, and mono-Ru show multiplets between 2.00–0.80 ppm, which are assigned to the CH2 from the primary amine. The peaks in the 6.55−7.30 ppm range for mono-Ru and Ru/Pd are assigned to the phenyl segment from the Schiff base ligand. The 1H NMR spectra of mono-Ru, mono-Pd, and Ru/Pd exhibit signals for the phosphine protons as multiplets in the 5.50–7.80 ppm range.
The 31P{1H} NMR spectra show a singlet at 23.22 ppm for mono-Pd and two singlets at 62.19 and 44.38 ppm for mono-Ru assigned to the square-based pyramid and trigonal bipyramidal structures, as previously reported (Figures S5 and S6) [29]. The 31P{1H} NMR spectrum for Ru/Pd exhibits a peak at 23.12 ppm, consistent with the presence of the Pd fragment in the new species formed (Figure S7). This spectrum also shows a number of signals that can be associated with the presence of different Ru species in the solution. The two peaks at 66.23 and 44.18 ppm in the spectrum confirm the presence of the two structural isomers for the Ru fragment in Ru/Pd, as observed for mono-Ru. The singlet at −4.90 ppm is typical of free PPh3. This is a clear indication of the release of PPh3 from Ru/Pd, suggesting the formation of a dimeric species with one singlet at 40.0 ppm. The free PPh3 and Ru center can undergo a reaction with residual O2 to produce OPPh3 and other Ru species, with sharp peaks at 29.10 and 34.00 ppm, respectively (Figure S7).
The MALDI-TOF mass spectrometry analyses of mono-Ru and Ru/Pd were recorded in CH2Cl2. The mass spectrum of mono-Ru exhibited a peak corresponding to the [mono-Ru]+H+, confirming the formation of the expected mononuclear Ru complex (Figure S8). Ru/Pd exhibited a signal at 13,170,373 m/z consistent with [Ru/Pd]+H+, thus providing strong evidence for the formulation proposed for this heterobimetallic complex (Figure S9).
Computational studies were carried out on the monometallic and heterobimetallic complexes. Theoretical details, involving the selected bond distances and bond angles, are shown in Table 1, and the optimized structures proposed for mono-Pd, mono-Ru, and Ru/Pd are shown in Figure 1. The geometrical parameters of mono-Ru revealed a distorted square-based pyramid similar to those previously reported [30]. Analysis of the P–Ru–P angle shows a value of 163°, indicating trans-positioned phosphines in a distorted geometry. Mono-Pd showed a typical distorted square planar molecular geometry, where the N−Pd−O bond angle and Pd−P bond distance have values close to those predicted for similar complexes [11,19,31,32]. The bond angle and distance values of the monometallic complexes and their fragments in the heterobimetallic species were very similar. This similarity shows that the geometry of the Ru and Pd fragments is conserved in the formation of Ru/Pd.
The UV-vis absorption spectra of mono-Ru, mono-Pd, and Ru/Pd showed two intense absorption bands in the 200–300 nm range related to the intraligand (IL) transfer. MLCT bands can be observed for mono-Ru (λmax at 346 nm) and mono-Pd (λmax at 343 and 394 nm), and they are assigned to d→π* charge transfer transitions. Both monometallic complexes showed bands related to the metallic center or the d-d charge transfer with λmax at 640 and 636 nm for mono-Ru and mono-Pd, respectively. All bands observed in the monometallic species were also observed in Ru/Pd, with a bathochromic shift (red shift). The λmax shift on the absorption spectrum of Ru/Pd confirms that the reaction between mono-Ru, [Pd(PPh3)2Cl2], and salicylaldehyde resulted in a new species containing fragments of both monometallic species. The theoretical absorption spectrum for Ru/Pd was obtained from DFT calculations with dichloromethane as a solvent (Figure 2). The theoretical and experimental absorption spectra of the heterobimetallic species shown in Figure 2 are in excellent agreement. The comparative similarity between the theoretical and experimentally calculated oscillator strengths for Ru/Pd provides reliability to the analysis of the theoretically predicted structure.
Formal oxidation and reduction potentials of mono-Ru, mono-Pd, and Ru/Pd vs. Ag/AgCl were obtained from cyclic voltammograms recorded on a platinum electrode in CH2Cl2 containing 0.1 mol L−1 tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) as a supporting electrolyte. Mono-Ru showed an irreversible oxidation peak at 0.63 V attributed to the metal-centered oxidation of RuII/III, while mono-Pd exhibited an irreversible anodic peak at 0.46 V assigned to the metal-centered oxidation of PdII/III (Figures S10 and S11). Ru/Pd showed two irreversible anodic peaks at 0.44 and 0.69 V, as scanned in the 0.0–0.9 V potential range (Figure 3). Scanning the potential to very positive and negative potentials (from −2.0 to +2.0 V) did not show any addition processes. The oxidation peak at 0.44 V is attributed to the metal-centered oxidation of PdII/III. This redox pair is slightly affected by heteronuclear compound formation. The oxidation peak at 0.69 V attributed to the metal-centered RuII/III couple was cathodically shifted by 50 mV, compared with that observed in the mononuclear Ru center. The presence of the two oxidation peaks associated with the Pd and Ru centers confirms that the heterobimetallic complex was obtained.

2.2. Polymerization Reactions

Norbornene (NBE) can be polymerized in different ways. Each route leads to its own polymer type, which is different in structure and properties. In this study, the main interest in heterobimetallic complexes lies in their possible application as bifunctional catalysts for the vinyl polymerization and ROMP of NBE. In this sense, mono-Ru, mono-Pd, and Ru/Pd were evaluated as catalytic precursors to produce different poly(NBE)s (Scheme 3). The catalytic behaviors of Ru/Pd and its Pd monometallic fragment (mono-Pd) were investigated in vinyl polymerization under different [MAO]/[Pd] and [NBE]/[Pd] ratios and temperatures (30, 60, or 90 °C). Ru/Pd and its Ru monometallic fragment mono-Ru were evaluated in ROMP reactions under different [EDA]/[Ru] and [NBE]/[Ru] ratios and temperatures (25 or 50 °C).

2.2.1. Vinyl Polymerization Reactions

Catalyst activities and polymer yields significantly depended on the applied reaction conditions for the polymerization of NBE, using mono-Pd and Ru/Pd, as shown in Figure 4 and Table 2. Initially, experiments on NBE polymerization were carried out in the absence of MAO (Al), and no catalytic activity for mono-Pd and Ru/Pd was observed (Table 2, Entries 1 and 11). Variation of the Al/mono-Pd and Al/Ru/Pd molar ratio showed considerable effect on the polymerization reaction. The yields of poly(NBE) increased with the increasing the Al/catalyst ratio for both complexes (Figure 4). However, the catalytic activity slightly decreased when the Al/Pd ratio was increased to 3000, since higher amounts of MAO deactivate the catalytic species. The concentration of NBE should meet some conditions to achieve satisfactory polymerization. Greater yields of poly(NBE) were obtained when the NBE/Pd molar ratio was increased from 10,000 to 20,000. Catalytic activity significantly decreased for molar ratios >20,000. Performance of the catalysts was also influenced by reaction temperature, and the highest activity was achieved at 60 °C for Ru/Pd (Table 2, Entry 9). For mono-Pd, quantitative yields were achieved at 30 and 60 °C (Table 2, Entries 9 and 5). A decrease in catalytic activity was observed at higher temperatures (90 °C), probably due to the decomposition of the active species (Table 2, Entry 10 and 20). Under all investigated conditions, mono-Pd showed higher catalytic activity than Ru/Pd. Vinyl polymerization was also performed using mono-Ru as pre-catalyst to evaluate the contribution of the Ru center to this heterobimetallic complex. The reaction was carried out using [Al]/[mono-Ru] = 2500 and [NBE]/[mono-Ru] = 20,000 molar ratio at 60 °C for 15 min. The experiment evidenced that the Ru fragment is not able to catalyze NBE vinyl polymerization, since no polymer yield was observed. FTIR analyses of the poly(NBE)s were performed. The characteristic bands of these polymers were similar, at ~942 cm−1. These absorptions were identified to the ring system of bicyclic heptane [33]. No absorption was observed in the 735–960 cm−1 range, which is characteristic of the ROMP structure of polyNBEs [34,35,36]. Therefore, the catalytic polymerization of NBE occurs via vinyl addition.

2.2.2. ROMP Reactions

Initially, the influence of the Pd-metal center on ROMP reactions was evaluated using mono-Pd as a pre-catalyst, and no polymer yield was observed when [NBE]/[mono-Pd] = 3000 molar ratio and VEDA = 10 μL were employed at 50 °C for 30 min. The absence of activity indicates that the Pd fragment has no effect on the ROMP reaction. The use of EDA was indispensable to obtain any ROMP activity. Mono-Ru and Ru/Pd were inactive in the absence of a carbene source. When EDA was added to generate the active species (Ru=C), polymer formation was observed. The catalytic activity of mono-Ru and Ru/Pd improved with increasing VEDA (up to VEDA = 5 μL) and declined VEDA > 5 μL (Figure 5). An excessive amount of EDA (VEDA >5 μL) causes an uncontrolled coordination of EDA to the Ru center, as verified by the decrease in the yields and Mn values, followed by the increase in the Ð values [29,37]. The [NBE]/[Ru] molar ratio was investigated to achieve the most efficient experimental condition for narrower poly(NBE)s. The yields increase when increasing the [NBE]/[Ru] molar ratio, starting from 1000, with yields <20%, reaching yields of 59 and 24% at 3000 for mono-Ru and Ru/Pd, respectively (Table 3). A decrease in the yields was observed for [NBE]/[Ru] = 5000. This behavior has been described in ROMP reactions conducted with similar complexes [18,37,38,39]. Aiming to reach a more efficient induction period, catalytic tests were also performed at 50 °C using the [NBE]/[Ru] = 3000 molar ratio and VEDA = 5 μL for 30 min. The increase in temperature generated higher catalytic activity for mono-Ru and Ru/Pd, which indicates that the induction period depends on the temperature to initiate polymerization (Table 3). For a better understanding of the differences in catalysis using mono-Ru and Ru/Pd, the kinetics of the induction period was investigated by UV-vis from the monitoring of {mono-Ru + EDA → mono-Ru=C} and {Ru/Pd + EDA → C=Ru/Pd} reactions (Figures S12 and S13). The spectra were recorded every 30 s and kobs values were determined for mono-Ru and Ru/Pd, 9.24 × 10−4, and 7.60 × 10−3 s−1, respectively. The similarity in kobs values indicate that differences in catalytic activity should not be related to the rate of formation of the active species.
Results for the ROMP and vinyl polymerization of NBE showed higher activity using monometallic species as pre-catalysts, compared with that using the heterobimetallic complex. Aiming to find correlations between the structural features and catalytic behaviors of the monometallic and heterobimetallic species, the steric properties of the active species were assessed using computational studies. Steric contour maps illustrate the steric bulk distribution for active species around the Pd- and Ru-metal centers (Figure 6).
As described earlier, the vinyl polymerization is not related to the Ru fragment. In this sense, the Pd center accessibility in the olefin coordination step was investigated, considering the active species for mono-Pd and Ru/Pd. Thus, the chloride and phosphine ligands were removed, and the methyl group was inserted in the Pd center (Figure 5). The difference in the monometallic and heterobimetallic systems is quantified by the percentage of buried volume, % VBur, which is 62.6 for mono-Pd and 63.0 for Ru/Pd. Steric influence on ROMP reactions was evaluated considering the Ru center with the loss of two PPh3 and addition of EDA to mono-Ru and Ru/Pd (Figure 6). In contrast to the behavior of heterobimetallic complex in vinyl polymerization, a more pronounced difference was found. The % VBur is only 81.4 for mono-Ru and 84.5 for Ru/Pd. As expected, the steric contour maps revealed that the heterobimetallic system is less accessible to the olefin coordination than the corresponding monometallic system.

3. Experimental Section

3.1. General Remarks

Unless otherwise stated, all syntheses and manipulations were performed under nitrogen (5.0-nitrogen, Air Liquide) atmosphere, following standard Schlenk techniques. Toluene and dichloromethane (CH2Cl2) were distilled in a calcium hydride system and stored under nitrogen. Norbornene (NBE), ethyldiazoacetate (EDA), 4-(aminomethyl)piperidine (pipNH2), salicylaldehyde, methylaluminoxane (MAO), and triphenylphosphine were obtained from Aldrich and used as acquired. The [RuCl2(PPh3)3] and [PdCl2(PPh3)2] complexes were prepared following the literature, and their purity was verified by satisfactory elemental analysis and spectroscopic examination [40,41].

3.2. Analyses

Elemental analyses were performed on a Perkin-Elmer CHN 2400 instrument. Infrared spectra were obtained on a Perkin Elmer Frontier instrument equipped with a diamond ATR module. The absorption spectra were recorded on a Shimadzu (model UV-1800) spectrophotometer using 1 cm-path length quartz cells. The 1H and 31P{1H} NMR spectra were obtained on a 500 MHz Bruker Avance III instrument (Bruker BioSpin, Bremen, Germany). Resonance frequency was 500.16 MHz for 1H NMR and 202.46 MHz for 31P{1H}. Chemical shifts were referenced using TMS and reported in ppm (δ). The analyses were performed in CDCl3. The chemical shifts are listed in ppm downfield of TMS and referenced from the solvent peaks or TMS. The signals were labeled as s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, and m = multiplet. MALDI-TOF analyses were carried out on a Bruker Daltonics Autoflex III Smartbeam. The molecular weights and molecular weight distribution of the polymers were determined by gel permeation chromatography using a Shimadzu Prominence LC system equipped with a LC-20AD pump, DGU-20A5 degasser, CBM-20A communication module, CTO-20A oven at 40 °C, and RID-10A detector equipped with two Shimadzu columns (GPC-805: 30 cm, Ø = 8.0 mm). The retention time was calibrated according to poly(methyl methacrylate) standards using HPLC-grade THF as eluent at 40 °C, with a flow rate of 1.0 mL min−1. Electrochemical measurements were performed using an Autolab PGSTAT204 potentiostat with a stationary platinum disk and wire as working and auxiliary electrodes, respectively. The reference electrode was Ag/AgCl. The measurements were performed in CH2Cl2 with 0.1 mol L−1 of n-Bu4NPF6 at 25 °C ± 0.1.

3.3. Computation Details

The structures of the compounds under study were optimized and had their vibrational frequencies calculated at the density functional theory (DFT) level using the hybrid functional M06 implemented in the Gaussian 09 software and DGDZVP basis set [42,43,44]. The influence of the solvent on the optimizations was evaluated using the IEFPCM model [45]. The electronic spectra were calculated for the first 30 singlet electronic states, also considering the molecules solvated in dichloromethane (IEFPCM), using the CAM-B3LYP density functional combined with valence triple-zeta quality with double polarized def2-TZVPP basis set [46]. The topographic steric maps of active species in vinyl polymerization and ROMP reactions were obtained using the SambVca 2.1 software [47] from the optimized structures of mono-Pd, mono-Ru and Ru/Pd.

3.4. ROMP Procedure

In a typical ROMP experiment, 2.4 or 1.1 µmol of mono-Ru and Ru/Pd, respectively, were dissolved in CHCl3 (2 mL) with an appropriate amount of NBE (1.12 g for mono-Ru and 0.53 g for Ru/Pd), followed by addition of a carbene source (EDA). The polymerization was performed for different times (5–60 min). The reaction mixture was stirred at 25 or 50 °C in a silicon oil bath. At room temperature, 10 mL methanol were added, and the precipitated polymer was filtered, washed with methanol, and dried in a vacuum oven at 40 °C, until constant weight was reached. The reported yields are average values from catalytic runs were performed at least three times, and the listed values are the arithmetic averages. The isolated polyNBEs were dissolved in THF for GPC data interpretation.

3.5. Vinyl Polymerization Procedure

In a typical procedure, 2.5 µmol of mono-Pd or Ru/Pd in 1.0 mL toluene, 1.80 g of NBE in 3 mL toluene, and an appropriated amount of toluene were added to a Schlenk flask (50 mL) under nitrogen atmosphere. After stirring at 30, 60, or 90 °C for 10 min, a certain amount of MAO was loaded into the polymerization system using a syringe, and the reaction was started. After 15 min, acidic ethanol (Vethanol:Vconc.HCl = 20:1) was added to terminate the reaction. The PNB was isolated by filtration, washed with ethanol, and dried at 100 °C for 24 h under vacuum. For all polymerization procedures, the total reaction volume was 10.0 mL.

3.6. Synthesis of [RuCl2(PPh3)2(piperidine-4(aminomethyl)] (mono-Ru)

A 100 mL Shlenck flask equipped with a stirring bar and [RuCl2(PPh3)3] (0.41 mmol) was purged three times with vacuum/argon cycles. Then, 20 mL acetone, previously purged with Ar(g), was added to the system. The solution was stirred before addition of 4-(aminomethyl)piperidine (0.41 mmol). The mixture was kept at RT for 12 h. Then, the solvent was removed by filtration and the green powder was washed with ethyl ether (3 × 5 mL) and dried under vacuum for 2 h to give mono-Ru. Elemental analysis for C42H44Cl2N2Ru: calculated C 62.22; H 5.47; N 3.46; found: C 62.48; H 5.83; N 3.21. UV–Vis λmax nm (εmax [102 L mol−1 cm−1]): 230 (1.26), 260 (0.97), 346 (0.15), 640 (0.09). FTIR (cm−1): 3255 υ(N−H), 3058 υ(C−H), 1585–1433 υ(C=C), 1089 υ(P-C), 287 υ(Ru-Cl). 1H NMR (CDCl3, 500.16 MHz, δ/ppm): 1H NMR (500.16 MHz, CDCl3): δ = 7.12−6.70 (m, 30H, CH Ar), 3.75 (t, 2H NH2), 3.48 (s, 1H, NH), 3.14−0.48 (m, 11H, CH2 amine) ppm; 31P{1H} NMR (CDCl3, 202.46 MHz, δ): 62.19, 44.38. MALDI-TOF (CHCl3) calcul./found (m/z): [mono-Ru]+H+ 811,1479/811,1387. EPR: No signal was observed. Yield: 63%.

3.7. Synthesis of [PdCl(PPh3)(N,O-Schiff)] (mono-Pd)

A solution of NaOH (0.05 mmol) in methanol was added to a solution of Schiff base in methanol, and the reaction mixture was stirred at RT for 2 h. The deprotonated ligand mixture was transferred using a cannula to a 50-mL two-necked flask fitted with a reflux condenser containing the [PdCl2(PPh3)2] (0.05 mmol) precursor. The mixture was stirred and refluxed for 4 h. A dark yellow precipitate was then filtered and washed with methanol and ethyl ether and dried under vacuum. Elemental analysis for C31H31ClNOPPd: calculated C 61.40; H 5.15; N 2.31; found: C 61.39; H 5.18; N 2.31. UV–Vis λmax nm (εmax [102 L mol−1 cm−1]): 235 (3.16), 277 (1.29), 343 (0.30), 394 (0.25). FTIR (cm−1): 3052 υ(C−H)ar, 2919–2844 υ(C−H), 1620 υ(C=N), 1488–1437 υ(C=C), 1084 υ(P−C), 352 υ(Pd−Cl). 1H NMR (CDCl3, 500.16 MHz, δ/ppm): δ = 7.75−7.45 (m, 16H, PPh3, and H-C=N), 7.27−7.26 (t, 1H, Phsalyc), 7.24−7.18 (dd, 1H Phsalyc) 6.90−6.85 (d, 1H Phsalyc), 6.62−6.56 (t, 1H Phsalyc), 3.60−3.54 (d, 2H CH2 amine), 2.00−1.90 (m, 1H, C-H amine), 1.90–0.89 (m, 10 H, CH2 amine ring) ppm; 31P{1H} NMR (CDCl3, 202.46 MHz, δ): 23.22. EPR: No signal was observed. Yield: 86%.

3.8. Synthesis of [RuCl2(PPh3)2](μ-Schiff)Pd(PPh3)Cl] (Ru/Pd)

Mono-Ru (0.57 mmol) and [Pd(PPh3)2Cl2] (0.57 mmol) were added to a previously oven-dried 100 mL Shlenck flask equipped with a stirrer, and three vacuum/argon cycles were applied to this system. Then, a previously de-aired solution of salicylaldehyde (0.57 mmol) and dichloromethane was added. The mixture was stirred at RT for 12 h and filtered. The dark red powder was washed with diethyl ether and dried under vacuum for 2 h. Elemental analysis for C67H62Cl3N2P3PdOPRu: calculated C 61.06; H 4.74; N 2.13; found: C 61.05; H 4.73; N 2.41. UV–Vis λmax nm (εmax [102 L mol−1 cm−1]): 233 (3.80), 355 (0.37), 395 (0.29), 636 (0.07). FTIR (cm−1): 3264 υ(N−H); 3052 υ(C−H)ar, 2964–2869 υ(C−H), 1622 υ(C=N), 1481–1437 υ(C=C), 1083 υ(P−C), 340 υ(Pd−Cl), 298 υ(Ru−Cl). 1H NMR (CDCl3, 500.16 MHz, δ/ppm): 7.80−6.75 (m, 46 H, PPh3 and H−C=N), 6.78–6.27 (dd, 2H, Phsalyc), 6.30−6.10 (2t, 2H, Phsalyc), 3.53–3.45 (m, 1H, secondary amine), 2.00−0.80 (m, 11 H, 5CH2, 1CH amine). 31P{1H} NMR (CDCl3, 202.46 MHz, δ): 66.23, 44.18, 38.96, 32.59. MALDI-TOF (CHCl3) cal/found (m/z): -[Ru/Pd+H+]+: 1317,1292/1317,0373. EPR: No signal was observed. Yield: 64%.

4. Conclusions

The new heterobimetallic complex (Ru/Pd) and its monometallic fragments (mono-Ru and mono-Pd) were successfully synthesized and characterized by elemental analysis, spectroscopic techniques, cyclic voltammetry, DFT calculations, and MALDI-TOF. An excellent agreement between theoretically and experimentally calculated UV-vis spectra was obtained, which suggests that the Ru moiety presents a square-based pyramidal structure, while the Pd moiety presents a distorted planar square structure. Both fragment structures were preserved in the bimetallic species. The cyclic voltammetry of Ru/Pd exhibits two irreversible processes at 0.44 and 0.69 V assigned to the RuII/III and PdII/III redox pair, with a slight shift, compared with those from the monometallic species. The Ru/Pd system displays catalytic activity for NBE polymerization via ROMP and addition, reaching yields of 31 and 47%, respectively, in the best observed conditions. The monometallic Ru and Pd fragments proved to be active for NBE polymerization via ROMP (yield of 68%) and addition (quantitative yield). Monometallic systems were more active for both polymerizations, due to their lower hindrance effect, which facilitates olefin coordination.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12101111/s1, Figure S1. FTIR spectra of complexes mono-Pd, mono-Ru and Ru/Pd; Figure S2. 1H NMR spectrum of mono-Ru in CDCl3; Figure S3. 1H NMR spectrum of mono-Pd in CDCl3; Figure S4. 1H NMR spectrum of Ru/Pd in CDCl3; Figure S5. 31P{1H} NMR spectrum of mono-Ru in CDCl3; Figure S6. 31P{1H} NMR spectrum of mono-Pd in CDCl3; Figure S7. 31P{1H} NMR spectrum of mono-Ru in CDCl3; Figure S8. MALDI-TOF spectroscopy of mono-Ru from CH2Cl2 solution. Matrix: α-cyano-4-hydroxycinnamic acid; Figure S9. MALDI-TOF spectroscopy of Ru/Pd from CH2Cl2 solution. Matrix: α-cyano-4-hydroxycinnamic acid; Figure S10. Cyclic voltammograms of mono-Ru from CH2Cl2 solutions at 25 °C; Figure S11. Cyclic voltammograms of mono-Pd from CH2Cl2 solutions at 25 °C; Figure S12. Kinetic study of mono-Ru in presence of EDA monitored by electronic spectroscopy every 30 seconds and (insert) dependence of ln(Abst -Absinf) as a function of time at 316 nm; Figure S13. Kinetic study of Ru/Pd in presence of EDA monitored by electronic spectroscopy every 30 seconds and (insert) dependence of ln(Abst -Absinf) as a function of time at 297 nm.

Author Contributions

T.R.C. worked on the synthesis and characterization of the complexes, carried out the polymerizations and kinetic experiments, interpreted the results and took the lead in writing the manuscript; G.H.C.M. worked on the synthesis and characterization of the complexes, interpreted the results and took the lead in writing the manuscript; K.A.E.A. performed the 1H and 31P{1H} NMR experiments and contributed to the final version of the manuscript; A.E.H.M. performed the theoretical studies and contributed to the final version of the manuscript; B.E.G. provided critical feedback and helped shape the research, analysis and manuscript; V.P.C.-J. supervised the project, conceived the study and were in charge of overall direction and planning. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by São Paulo Research Foundation (FAPESP) 2018/06340-1, 2021/13128-1 (V.P.C.-J.) and 2021/11873-1 (B.E.G.). This work was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior–Brasil (CAPES)–Finance Code 001.

Acknowledgments

We would like to thank Eduardo René Pérez Gonzales (UNESP) and Luana Ribeiro dos Anjos (UNESP) and also to Benedito dos Santos Lima-Neto for their NMR analysis.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Bender, M.L. Mechanism of Homogeneous Catalysis from Proton to Proteins; Wiley: New York, NY, USA, 1971; pp. 19–320. [Google Scholar]
  2. Collman, J.P. Patterns of organometallic reactions related to homogeneous catalysis. Acc. Chem. Res. 1968, 1, 136–143. [Google Scholar] [CrossRef]
  3. Singh, K.S. Recent advances in C–H bond functionalization with ruthenium-based catalysts. Catalysts 2019, 9, 173. [Google Scholar] [CrossRef]
  4. Montgomery, T.P.; Johns, A.M.; Grubbs, R.H. Recent advancements in stereoselective olefin metathesis using ruthenium catalysts. Catalysts 2017, 7, 87. [Google Scholar] [CrossRef]
  5. Clapham, S.E.; Hadzovic, A.; Morris, R.H. Mechanisms of the H2-hydrogenation and transfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes. Coord. Chem. Rev. 2004, 248, 2201–2237. [Google Scholar] [CrossRef]
  6. Bellina, F.; Carpita, A.; Rossi, R. Palladium catalysts for the Suzuki cross-coupling reaction: An overview of recent advances. Synthesis 2004, 2004, 2419–2440. [Google Scholar] [CrossRef]
  7. Seki, M. Recent advances in Pd/C-catalyzed coupling reactions. Synthesis 2006, 2006, 2975–2992. [Google Scholar] [CrossRef]
  8. Kim, A.N.; Stoltz, B.M. Recent advances in homogeneous catalysts for the asymmetric hydrogenation of heteroarenes. ACS Catal. 2020, 10, 13834–13851. [Google Scholar] [CrossRef] [PubMed]
  9. Walter, M.D.; Moorhouse, R.A.; Urbin, S.A.; White, P.S.; Brookhart, M. γ-agostic species as key intermediates in the vinyl addition polymerization of norbornene with cationic (allyl) Pd catalysts: Synthesis and mechanistic insights. J. Am. Chem. Soc. 2009, 131, 9055–9069. [Google Scholar] [CrossRef] [PubMed]
  10. Zhang, Y.; Wang, J. Recent developments in Pd-catalyzed reactions of diazo compounds. Eur. J. Organ. Chem. 2011, 2011, 1015–1026. [Google Scholar] [CrossRef]
  11. Gois, P.D.D.S.; Maia, J.I.P.; Masson, G.H.C.; Martins, D.M.; Machado, A.E.D.H.; Goi, B.E.; Carvalho, V.P.D., Jr. Monometallic and heterobimetallic ruthenium (II) and palladium (II) complexes based on a pyridine-hydrazone ligand as bifunctional catalysts for ROMP of norbornene and ethylene polymerization. Appl. Organomet. Chem. 2022, 36, 6491. [Google Scholar] [CrossRef]
  12. Lin, S.-C.A.; Liu, Y.-H.; Peng, S.-M.; Liu, S.-T. Hetero-bimetallic complexes based on an anthyridine ligand preparation and catalytic activity. Organometallics 2019, 39, 123–131. [Google Scholar] [CrossRef]
  13. Bitzer, M.J.; Kühn, F.E.; Baratta, W. Tandem Suzuki–Miyaura/transfer hydrogenation reaction catalyzed by a Pd–Ru complex bearing an anionic dicarbene. J. Catal. 2016, 338, 222–226. [Google Scholar] [CrossRef]
  14. Sabater, S.; Mata, J.A.; Peris, E. Hydrodefluorination of carbon–fluorine bonds by the synergistic action of a ruthenium–palladium catalyst. Nat. Commun. 2013, 4, 1–7. [Google Scholar] [CrossRef]
  15. Mata, J.A.; Hahn, F.E.; Peris, E. Heterometallic complexes, tandem catalysis and catalytic cooperativity. Chem. Sci. 2014, 5, 1723. [Google Scholar] [CrossRef]
  16. Sun, W.-H.; Yang, H.; Li, Z.; Li, Y. Vinyl polymerization of norbornene with neutral salicylaldiminato nickel (II) complexes. Organometallics 2003, 22, 3678–3683. [Google Scholar] [CrossRef]
  17. Calderon, N. Ring-opening polymerization of cycloolefins. J. Macromol. Sci. Rev. Macromol. Chem. 1972, 7, 105–159. [Google Scholar] [CrossRef]
  18. Grubbs, R.H. Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003; Volume 3. [Google Scholar]
  19. Sutthasupa, S.; Shiotsuki, M.; Sanda, F. Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials. Polym. J. 2010, 42, 905–915. [Google Scholar] [CrossRef]
  20. Janiak, C.; Paul, G.L. The vinyl homopolymerization of norbornene. Macromol. Rapid Commun. 2001, 22, 479–493. [Google Scholar] [CrossRef]
  21. Mathew, J.P.; Reinmuth, A.; Melia, J.; Swords, A.N.; Risse, W. (η3-allyl) palladium (II) and palladium (II) nitrile catalysts for the addition polymerization of norbornene derivatives with functional groups. Macromolecules 1996, 29, 2755–2763. [Google Scholar] [CrossRef]
  22. Liu, H.; Haibin, Y.; Xiaochao, S. Synthesis of nickel and palladium complexes with diarylamido-based unsymmetrical pincer ligands and application for norbornene polymerization. Dalton Transact. 2019, 48, 609–617. [Google Scholar] [CrossRef]
  23. Fonseca, L.R.; Sá, J.L.S.; Carvalho, V.P.; Lima-Neto, B.S. Cross-link in norbornadiene-based polymers from ring-opening metathesis polymerization with pyrrolidine-based Ru complex. Polym. Bull. 2018, 75, 3705–3721. [Google Scholar] [CrossRef]
  24. Silva, R.A.N.; Borim, P.; Fonseca, L.R.; Lima-Neto, B.S.; Sá, J.L.S.; Carvalho, V., Jr. Non-carbene complex [RuCl2(PPh3)2(azocane)] as active catalyst precursor for ROMP and ATRP. Catal. Lett. 2017, 147, 1144–1152. [Google Scholar] [CrossRef]
  25. Afonso, M.B.A.; Cruz, T.R.; Silva, Y.F.; Pereira, J.C.A.; Machado, A.E.; Goi, B.E.; Carvalho, V.P., Jr. Ruthenium (II) complexes of Schiff base derived from cycloalkylamines as pre-catalysts for ROMP of norbornene and ATRP of methyl methacrylate. J. Organomet. Chem. 2017, 851, 225–234. [Google Scholar] [CrossRef]
  26. Chaves, H.K.; Ferraz, C.P.; Carvalho, V., Jr.; Lima-Neto, B.S. Tuning the activity of alternative Ru-based initiators for ring-opening metathesis polymerization of norbornene and norbornadiene by the substituent in 4-CH2R-piperidine. J. Mol. Catal. A Chem. 2014, 385, 46–53. [Google Scholar] [CrossRef]
  27. Cruz, T.R.; Silva, R.A.; Machado, A.E.; Lima-Neto, B.S.; Goi, B.E.; Carvalho, V.P. New dmso–ruthenium catalysts bearing N-heterocyclic carbene ligands for the ring-opening metathesis of norbornene. New J. Chem. 2019, 43, 6220–6227. [Google Scholar] [CrossRef]
  28. Masson, G.H.C.; Cruz, T.R.; Gois, P.D.S.; Martins, D.M.; Lima-Neto, B.S.; Oliveira, G.S.; Machado, A.E.H.; Bernardo-Gusmão, K.; Goi, B.E.; Carvalho, V.P., Jr. Ruthenium–nickel heterobimetallic complex as a bifunctional catalyst for ROMP of norbornene and ethylene polymerization. New J. Chem. 2021, 45, 11466–11473. [Google Scholar] [CrossRef]
  29. Matos, J.M.E.; Lima-Neto, B.S. Piperidine as ancillary ligand in the novel [RuCl2(PPh3)2(piperidine)] complex for metathesis polymerization of norbornene and norbornadiene. J. Mol. Catal. A Chem. 2004, 222, 81–85. [Google Scholar] [CrossRef]
  30. Fernandes, R.J.; Silva, T.B.; Lima-Neto, B.S.; Haiduke, R.L. Structural and kinetic insights into the mechanism for ring opening metathesis polymerization of norbornene with [RuCl2(PPh3)2(piperidine)] as initiator complex. J. Mol. Catal. A Chem. 2015, 410, 58–65. [Google Scholar] [CrossRef]
  31. Cui, J.; Zhang, M.; Zhang, Y. Amino-salicylaldimine–palladium(II) complexes: New and efficient catalysts for Suzuki and Heck reactions. Inorg. Chem. Commun. 2010, 13, 81–85. [Google Scholar] [CrossRef]
  32. Agrahari, B.; Layek, S.; Ganguly, R.; Pathak, D.D. Synthesis, crystal structures, and application of two new pincer type palladium (II)-Schiff base complexes in CC cross-coupling reactions. Inorg. Chim. Acta 2018, 471, 345–354. [Google Scholar] [CrossRef]
  33. Kennedy, J.P.; Makowski, H.S.J. Carbonium ion polymerization of norbornene and its derivatives. J. Macromol. Sci. Chem. 1967, 1, 345. [Google Scholar] [CrossRef]
  34. Barnes, D.A.; Benedikt, G.M.; Goodall, B.L.; Hung, S.S.; Kalamarides, H.A.; Lenhard, S.; McIntosh, L.H.; Selvy, K.T.; Shick, R.A.; Rhodes, L.F. Addition polymerization of norbornene-type monomers using neutral nickel complexes containing fluorinated aryl ligands. Macromolecules 2003, 36, 2623–2632. [Google Scholar] [CrossRef]
  35. Patil, A.O.; Zushma, S.; Stibrany, R.T.; Rucker, S.P.; Wheeler, L.M. Vinyl-type polymerization of norbornene by nickel(II) bisbenzimidazole catalysts. J. Polym. Sci. Part A Polym. Chem. 2003, 41, 2095–2106. [Google Scholar] [CrossRef]
  36. Zhao, C.; Ribeiro, M.R.; de Pinho, M.N.; Subrahmanyam, V.S.; Gil, C.L.; de Lima, A.P. Structural characteristics and gas permeation properties of polynorbornenes with retained bicyclic structure. Polymer 2001, 42, 2455–2462. [Google Scholar] [CrossRef]
  37. Carvalho-Jr, V.; Ferraz, C.P.; Lima-Neto, B.S. Electronic synergism in [RuCl2(PPh3)2(amine)] complexes differing the reactivity for ROMP of norbornene and norbornadiene. J. Mol. Catal. A Chem. 2010, 333, 46–53. [Google Scholar] [CrossRef]
  38. Grela, K. (Ed.) Olefin Metathesis: Theory and Practice, 1st ed.; Wiley: Hoboken, NJ, USA, 2014. [Google Scholar]
  39. Slugovc, C. The ring opening metathesis polymerisation toolbox. Macromol. Rapid Commun. 2004, 25, 1283–1297. [Google Scholar] [CrossRef]
  40. Hallman, P.S.; Stephenson, T.A.; Wilkinson, G. Tetrakis(triphenylphosphine)dichlororuthenium(II) and Tris(triphenylphosphine)dichlororuthenium(II). Inorg. Synth. 1970, 12, 237. [Google Scholar]
  41. Zahrtmann, N.; Claver, C.; Godard, C.; Riisager, A.; Garcia-Suarez, E.J. Selective oxidative carbonylation of aniline to diphenylurea with ionic liquids. Chem. Cat. Chem. 2018, 10, 2450–2457. [Google Scholar] [CrossRef]
  42. Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215–241. [Google Scholar]
  43. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision E.01; Gaussian, Inc.: Wallingford, CT, USA, 2013. [Google Scholar]
  44. Godbout, N.; Salahub, D.R.; Andzelm, J.; Wimmer, E. Optimization of Gaussian-type basis sets for local spin density functional calculations. Part I. Boron through neon, optimization technique and validation. Can. J. Chem. 1992, 70, 560. [Google Scholar] [CrossRef]
  45. Tomasi, J.; Mennucci, B.; Cancès, E. The IEF version of the PCM solvation method: An overview of a new method addressed to study molecular solutes at the QM ab initio level. J. Mol. Struct. Theochem. 1999, 464, 211–226. [Google Scholar] [CrossRef]
  46. O’Boyle, N.M.; Tenderholt, A.L.; Langner, K.M. cclib: A library for package-independent computational chemistry algorithms. Comp. Chem. 2007, 29, 839–845. [Google Scholar] [CrossRef] [PubMed]
  47. Falivene, L.; Cao, Z.; Petta, A.; Serra, L.; Poater, A.; Oliva, R.; Cavallo, L. Towards the online computer-aided design of catalytic pockets. Nat. Chem. 2019, 1, 872–879. [Google Scholar] [CrossRef] [PubMed] [Green Version]
Catalysts 12 01111 sch001 550
Scheme 1. The Ru/Pd heterobimetallic complex and its monometallic fragments: mono-Pd and mono-Ru.
Scheme 1. The Ru/Pd heterobimetallic complex and its monometallic fragments: mono-Pd and mono-Ru.
Catalysts 12 01111 sch001
Catalysts 12 01111 sch002 550
Scheme 2. Overview of reactions to obtain mono-Ru, mono-Pd, and Ru/Pd.
Scheme 2. Overview of reactions to obtain mono-Ru, mono-Pd, and Ru/Pd.
Catalysts 12 01111 sch002
Catalysts 12 01111 g001a 550Catalysts 12 01111 g001b 550
Figure 1. Views of the optimized structures of mono-Pd, mono-Ru, and Ru/Pd.
Figure 1. Views of the optimized structures of mono-Pd, mono-Ru, and Ru/Pd.
Catalysts 12 01111 g001aCatalysts 12 01111 g001b
Catalysts 12 01111 g002 550
Figure 2. Electronic absorption spectra of mono-Ru (blue dashed line), mono-Pd (orange dotted line), and Ru–Pd (marron solid line) and experimentally calculated oscillator strength (f) (dark red vertical lines) for Ru–Pd. Conditions: [Ru/Pd] = [mono-Ru] = [mono-Pd] = 1.0 × 10−4 mol L−1; CH2Cl2 as solvent, at 25 °C.
Figure 2. Electronic absorption spectra of mono-Ru (blue dashed line), mono-Pd (orange dotted line), and Ru–Pd (marron solid line) and experimentally calculated oscillator strength (f) (dark red vertical lines) for Ru–Pd. Conditions: [Ru/Pd] = [mono-Ru] = [mono-Pd] = 1.0 × 10−4 mol L−1; CH2Cl2 as solvent, at 25 °C.
Catalysts 12 01111 g002
Catalysts 12 01111 g003 550
Figure 3. Cyclic voltammogram of Ru–Pd obtained in n-Bu4NPF6/CH2Cl2 0.1 mol L−1 vs. Ag/AgCl at a scan rate of 100 mV s−1; [Ru/Pd] = 1.0 × 10−3 mol L−1.
Figure 3. Cyclic voltammogram of Ru–Pd obtained in n-Bu4NPF6/CH2Cl2 0.1 mol L−1 vs. Ag/AgCl at a scan rate of 100 mV s−1; [Ru/Pd] = 1.0 × 10−3 mol L−1.
Catalysts 12 01111 g003
Catalysts 12 01111 sch003 550
Scheme 3. Schematic representation of different types of polymerizations of NBE.
Scheme 3. Schematic representation of different types of polymerizations of NBE.
Catalysts 12 01111 sch003
Catalysts 12 01111 g004 550
Figure 4. Catalytic activity as function of Al/Pd molar ratio. Vtotal = 10 mL, [NBE]/[Pd] = 20,000, mono-Pd and Ru/Pd = 2.5 μmol, and polymerization reaction at 30 °C for 15 min.
Figure 4. Catalytic activity as function of Al/Pd molar ratio. Vtotal = 10 mL, [NBE]/[Pd] = 20,000, mono-Pd and Ru/Pd = 2.5 μmol, and polymerization reaction at 30 °C for 15 min.
Catalysts 12 01111 g004
Catalysts 12 01111 g005 550
Figure 5. Yield dependence as a function VEDA, at 25 °C, for ROMP of NBE with and Ru/Pd in CHCl3 for 30 min; [NBE]/[Ru] = 3000.
Figure 5. Yield dependence as a function VEDA, at 25 °C, for ROMP of NBE with and Ru/Pd in CHCl3 for 30 min; [NBE]/[Ru] = 3000.
Catalysts 12 01111 g005
Catalysts 12 01111 g006 550
Figure 6. Topographic steric maps of active species in vinyl polymerization and ROMP reactions and their respective optimized structures. Active species of mono-Pd and mono-Ru (left) and active species of Ru/Pd (right). The iso-contour curves are in Å.
Figure 6. Topographic steric maps of active species in vinyl polymerization and ROMP reactions and their respective optimized structures. Active species of mono-Pd and mono-Ru (left) and active species of Ru/Pd (right). The iso-contour curves are in Å.
Catalysts 12 01111 g006
Table
Table 1. Theoretical bond distances (Å) and bond angles (°) for mono-Ru, mono-Pd, and Ru/Pd.
Table 1. Theoretical bond distances (Å) and bond angles (°) for mono-Ru, mono-Pd, and Ru/Pd.
Geometric Parametersmono-RuRu/Pdmono-Pd
Ru(1)−Cl(1)2.44922.4528-
Ru(1)−Cl(2)2.45102.4623-
Ru(1)-P(1)2.44972.4539-
Ru(1)-P(2)2.45832.4308-
Ru(1)-N(1)2.14252.1361-
Cl(1)−Ru(1)−Cl(2)168.420170.521-
Cl(1)−Ru(1)−N(1)85.65585.857-
Cl(2)−Ru(1)−N(1)98.720103.557-
P(1)−Ru(1)−P(2)163.994162.371-
Pd(1)−Cl(3)-2.35832.3580
Pd(1)−N(2)-2.07552.0773
Pd(1)−O(1)-2.02242.0172
Pd(1)−P(3)-2.29532.2971
N(2)−Pd(1)−O(1)-89.25189.664
P(3)−Pd(1)−Cl(3)-86.59086.369
Table
Table 2. Polymerization of NBE with mono-Pd and Ru/Pd activated by MAO a.
Table 2. Polymerization of NBE with mono-Pd and Ru/Pd activated by MAO a.
ComplexEntryAl/Pd[NBE]/[Pd]T (°C)Activity bYield (%)
mono-Pd1020,00030--
2100020,000302.431.9
3150020,000303.444.7
4200020,000305.472.34
5250020,000307.5100
6300020,000306.687.2
7250010,000303.183.0
8250030,000306.759.7
9250020,000607.5100
10250020,000904.052.8
Ru/Pd11020,00030--
12100020,000300.45.1
13150020,000300.810.4
14200020,000301.114.2
15250020,000301.723.2
16300020,000301.621.7
17250010,000300.923.0
18250030,000301.08.7
19250020,000603.547.0
20250020,000900.33.6
a Polymerization conditions: Vtotal = 10 mL; reaction time: 15 min; complexes: 2.5 μmol. b (106 g mol−1 Pd h−1).
Table
Table 3. ROMP of NBE with mono-Ru and Ru/Pd a.
Table 3. ROMP of NBE with mono-Ru and Ru/Pd a.
ComplexEntryVEDA (μL)[NBE]/[Ru]T (°C)Yield (%)Mn b
(104 g mol−1)		Рb
mono-Ru10300025---
2230002554.22.11.65
3530002559.05.91.53
4730002539.86.81.62
51030002521.63.51.76
6510002519.52.11.48
7550002528.72.71.54
8570005022.63.11.48
9530005068.05.91.61
Ru/Pd100300025---
11230002511.22.11.63
12530002523.92.91.58
13730002510.42.21.71
14103000255.11.91.79
1551000258.61.81.48
1655000259.92.11.54
17570005022.63.01.48
18530005031.24.51.61
a Reactions performed in CHCl3 and 30 min. b Determinate by GPC.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Cruz, T.R.; Masson, G.H.C.; Amorim, K.A.E.; Machado, A.E.H.; Goi, B.E.; Carvalho-Jr., V.P. Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition. Catalysts 2022, 12, 1111. https://doi.org/10.3390/catal12101111

AMA Style

Cruz TR, Masson GHC, Amorim KAE, Machado AEH, Goi BE, Carvalho-Jr. VP. Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition. Catalysts. 2022; 12(10):1111. https://doi.org/10.3390/catal12101111

Chicago/Turabian Style

Cruz, Thaís R., Gustavo H. C. Masson, Kelly A. E. Amorim, Antonio E. H. Machado, Beatriz E. Goi, and Valdemiro P. Carvalho-Jr. 2022. "Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition" Catalysts 12, no. 10: 1111. https://doi.org/10.3390/catal12101111

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop