- Email: [email protected]
Palladium(II) complexes of (pyridyl)imine ligands as catalysts for the methoxycarbonylation of olefins
Inorganica Chimica Acta xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Palladium(II) complexes of (pyridyl)imine ligands as catalysts for the methoxycarbonylation of olefins Zethu Zulua, George S. Nyamatoa,b, Thandeka A. Tshabalalaa, Stephen O. Ojwacha,
⁎
a b
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa Department of Physical Sciences, University of Embu, P.O Box 6-60100, Embu, Kenya
ARTICLE INFO
ABSTRACT
Keywords: Palladium Structures Olefins Methoxycarbonylation Esters
Reactions of 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2) and 3-methoxy-N-((pyridin-2-yl)methylene)propan-1-amine (L3) ligands with either [PdCl2(COD)] or [PdCl (Me)(COD)] produced the corresponding monometallic complexes [PdCl2(L1)] (1), [PdClMe(L1)] (2), [PdCl2(L2)] (3) and [PdCl2(L3)] (4). The solid state structure of complex 1 confirmed the bidentate coordination mode of L1, giving a distorted square planar geometry. All the complexes (1–4) formed active catalysts for the methoxycarbonylation of higher olefins to give linear and branched esters. The catalytic behavior of complexes 1–4 were influenced by both the complex structure and olefin chain length.
1. Introduction Functionalization of olefins constitute a fundamental basis of today’s chemical industry and is responsible for the production of a wide range of fine and bulk chemical products [1–3]. Carbonylation of olefins represents one of the most important olefin transformation reactions that is mostly applied to produce useful products such as esters, which are key reagents, for the industrial production of solvents, detergents, cosmetics and pharmaceuticals [4,5]. In terms of production, besides hydroformylation and related transformations [6–8], alkoxycarbonylation reactions constitute the next largest industrial applications in the area of homogeneous catalysis [9]. In this regard, the palladium-catalyzed methoxycarbonylation of 1-alkenes is an active area of research [10–14] that is currently receiving appreciable attention. To date, palladium complexes are the most widely used catalysts in the methoxycarbonylation reactions due to their high catalytic activity and stability [13,15]. It is generally known that the catalytic behavior of coordination compounds is influenced by the ancillary ligands [16]. Thus, by finetuning the electronic and steric properties of the ligands, the activity and stability of the metal center can be adjusted [17–19]. For example, the combination of hard and soft donor atoms within the same ligand framework makes such a ligand a good candidate for active and stable catalysts [5]. While phosphine-donor ligands have traditionally been used in the preparation of palladium catalysts for methoxycarbonylation reactions, attention is currently shifting to mixed
⁎
nitrogen–phosphine donor palladium complexes due to their perceived stability and tolerance to impurities [20,21]. Thus palladium complexes, for example, of the type [PdCl2(Ph2PNHpy-k2-P,N)] and [PdCl (Ph2PNHpyk2-P,N)(PPh3)]Cl have been reported to form active and stable catalysts in the methoxycarbonylation of styrene [22]. Although a number of ligands have so far been developed, their rational design aimed at affording higher catalytic activities and stability continues to be an important topic in this area [23]. One approach that can be employed to achieve a balance between catalyst activity and stability is via the use of potential hemi-labile ligands. These systems have the ability to stabilize the active metal center, without compromising catalytic activity [5,24]. In a recent report, for example, hemi-labile ligands based on bidentate P^O-ligands were shown to influence palladium(II) catalyzed methoxycarbonylation of 1alkenes to afford mainly branched esters [25]. In other previous work, it has been shown that a combination of O or S donors with P or N donors form good hemi-labile ligands in palladium(II) catalyzed organic transformations [26–28]. For example, Mecking and Keim reported palladium η3-allyl complexes chelated by hemi-labile ligands based on bidentate P,O-ligands [29] as catalysts in ethylene dimerization. Even though this approach has been rarely applied in methoxycarbonylation reactions, we believe it could provide a promising avenue for the design and development of stable palladium catalysts for these reactions. Recently, we reported on the methoxycarbonylation of olefins catalyzed by palladium(II) complexes bearing (benzimidazolylmethyl)
Corresponding author. E-mail address: [email protected] (S.O. Ojwach).
https://doi.org/10.1016/j.ica.2019.119270 Received 14 June 2019; Received in revised form 24 October 2019; Accepted 7 November 2019 Available online 09 November 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zethu Zulu, et al., Inorganica Chimica Acta, https://doi.org/10.1016/j.ica.2019.119270
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
1H, 6-py, 3JHH = 8.0); 13C {1H} NMR (DMSO‑d6): 59.19 (CH2-N), 62.10 (CH2-O), 128.55 (3-py-C), 128.98 (5-py-C), 141.84 (4-py-C), 150.56 (6py-C), 156.33 (2-py-C), 172.12 (C]N), ESI-MS: m/z (%) 293 [(M+ - Cl, 65%)]. FT-IR (cm−1): υ(C]N)imine = 1650 Anal. Calcd for C8H10Cl2N2OPd: C, 29.34; H, 3.08; N, 8.55. Found: C, 29.05; H, 3.14; N, 8.29.
amine [20] and (phenoxy)imine [30] ligands in which the catalytic performances of these complexes were controlled by the ligand architecture. In our continued pursuit for both active and stable catalysts for methoxycarbonylation of 1-alkenes, we herein report the syntheses of palladium(II) complexes of iminopyridine ligands, containing potential hemi-labile ether or hydroxy pendant donor groups. The structural elucidation of the complexes, effect of complex structure and olefin substrate on the activity and regioselectivity of the resultant catalysts are herein discussed.
2.2.3. Synthesis of [PdCl2(L3)] (4) Complex 4 was prepared according to the procedure described for 1 using L3 (0.06 g, 0.35 mmol) and [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (20 mL). Complex 4 was isolated as a yellow solid. Yield = 0.087 g (70%).1H NMR (400 MHz, DMSO‑d6): δH (ppm): 2.06 (q, 2H, CH2, 3JHH = 6.6); 3.25 (s, 3H, CH3-O); 3.42 (t, 2H, CH2-O, 3 JHH = 6.1); 3.80 (t, 2H, CH2-N, 3JHH = 6.9); 7.89 (t, 1H, 5-py, 3 JHH = 7.4); 8.12 (d, 1H, 3-py, 3JHH = 7.6); 8.37 (t, 1H, 4-py, 3 JHH = 7.8); 8.59 (s, 1H, CeH); 8.99 (d, 1H, 6-py, 3JHH = 5.6 Hz). 13C {1H} NMR (DMSO‑d6): 30.20 (CH2), 57.13 (CH2-N), 58.30 (CH3-O), 69.26 (CH2-O), 128.66 (3-py), 129.00 (5-py), 141.78 (4-py), 150.57 (6py), 156.34 (2-py), 172.01 (C]N). ESI-MS: m/z (%) 201 [(M+ + Na+, 100%)]. FT-IR (cm−1): υ(C]N)imine = 1648. Anal. Calcd for C10H14Cl2N2OPd: C, 33.78; H, 3.97; N, 7.88. Found: C, 33.75; H, 4.01; N, 7.84.
2. Experimental section and methods 2.1. General instrumentation and material All synthetic manipulations were performed under nitrogen atmosphere using standard Schlenk line techniques. All solvents were purchased from Merck and were dried and distilled prior to use. Diethyl ether, hexane and toluene were dried over sodium wire and benzophenone, methanol and absolute ethanol over calcium oxide, while dichloromethane was dried and distilled over phosphorus pentoxide. Methylheptanoate, methylnonanoate, sodium hydroxide (NaOH) and DMSO‑d6 were purchased from Merck Chemicals. Ethanolamine, 2methoxyethylamine, 2-pyridinecarboxaldehyde, olefins, hydrochloric acid, p-TsOH (≥98.5%), PPh3 (99%) and CDCl3 were purchased from Sigma Aldrich and used as received. [PdCl2(COD)] [31], [PdCl(Me) (COD)] [32], ligands 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2), 3-methoxyN-((pyridin-2-yl)methylene)propan-1-amine (L3), and complex [PdClMe(L1)] (2) were prepared by following our recently published procedures [28]. 1H NMR and 13C {1H} NMR spectra were recorded on a Bruker Ultrashield 400 (1H NMR 400 MHz, 13C {1H} NMR 100 MHz) spectrometer in CDCl3 solution at room temperature and chemical shifts (δ) were determined relative to internal TMS and are given in ppm. The infrared spectra were recorded on a PerkinElmer Spectrum 100 in the 4000–400 cm−1 range. Elemental analyses were performed on a Thermal Scientific Flash 2000 and mass spectra were recorded on an LC premier micromass spectrometer.
2.3. X-ray crystallography X-ray data collection for complex 1 was recorded on a Bruker Apex Duo equipped with an Oxford Instruments Cryojet operating at 100(2) K and an Incoatec microsource operating at 30 W power. Crystal and structure refinement data are given in Table 1. The data were collected with Mo Kα (λ = 0.71073 Å) radiation at a crystal-to-detector distance of 50 mm. The data were collected using omega and phi scans with exposures taken at 30 W X-ray power and 0.50° frame widths using APEX2 [33]. The data were reduced with the programme SAINT [33] using outlier rejection, scan speed scaling, as well as standard Lorentz and polarisation correction factors. A SADABS semi-empirical multiscan absorption correction [33] was applied to the data. Direct methods, SHELXS-2016 [34] and WinGX [35] were used to solve the data. All non-hydrogen atoms were located in the difference density map and refined anisotropically with SHELXL-2016 [34]. All hydrogen atoms were included as idealized contributors in the least squares process. Their positions were calculated using a standard riding model with C–Haromatic distances of 0.93 Å and Uiso = 1.2 Ueq and C–Hmethylene distances of 0.99 Å and Uiso = 1.2 Ueqand C–Hmethyl distances of 0.98 Å and Uiso = 1.5 Ueq.
2.2. Syntheses of palladium(II) complexes 2.2.1. Synthesis of [PdCl2(L1)] (1) To a solution of [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (10 mL) was added a solution of L1 (0.06 g, 0.35 mmol) in CH2Cl2 (10 mL). The mixture was stirred at room temperature, under nitrogen for 24 h. The yellow precipitate formed was filtered and washed with diethyl ether (2 × 20 mL), filtered and dried to afford 1 as a yellow solid. Recrystallization from CH2Cl2-hexane mixture afforded single crystals suitable for X-ray analysis. Yield = 0.09 g (75%). 1H NMR (400 MHz, DMSO‑d6): δH (ppm): 3.29 (s, 3H, CH3); 3.71 (t, 2H, CH2-N, 3 JHH = 8.0 Hz); 3.91 (t, 2H, CH2-O, 3JHH = 8.0 Hz); 7.89 (t, 1H, 5-py, 3 JHH = 8.0 Hz); 8.18 (d, 1H, 3-py, 3JHH = 8.0 Hz); 8.36 (t, 1H, 4-py, 3 JHH = 8.0 Hz); 8.55 (s, 1H, CeH); 8.99 (d, 1H, 6-py, 3JHH = 8.0 Hz). 13 C {1H} NMR (DMSO‑d6): 58.57 (CH3-O), 58.76 (CH2-N), 69.76 (CH2O), 128.89 (3-py-C), 129.27 (5-py-C), 141.93 (4-py-C), 150.75 (6-py-C), 156.03 (2-py-C), 172.75 (C]N). ESI-MS: m/z (%) 269 [(M+ - Cl2, 100%)]. FT-IR (cm−1): υ(C]N)imine = 1598. Anal. Calcd for C9H12Cl2N2OPd·0.1C6H14: C, 32.93; H, 3.80; N, 8.00. Found: C, 32.98; H, 3.67; N, 7.99.
2.4. Typical procedure for the methoxycarbonylation reactions The methoxycarbonylation catalytic reactions were performed in a 400 mL stainless steel autoclave Parr reactor equipped with a Table 1 Methoxycarbonylation of 1-hexene using complexes 1–4.
2.2.2. Synthesis of [PdCl2(L2)] (3) Complex 3 was prepared according to the procedure described for 1 using L2 (0.05 g, 0.35 mmol) and [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (20 mL). Complex 3 was obtained as a yellow solid. Yield = 0.08 g (70%). 1H NMR (400 MHz, DMSO‑d6): δH (ppm): 3.75 (s, 4H, CH2-N + CH2-O); 7.87 (t, 1H, 5-py, 3JHH = 8.0); 8.18 (d, 1H, 3-py, 3 JHH = 8.0); 8.34 (t, 1H, 4-py, 3JHH = 8.0); 8.48 (s, 1H, CeH); 8.98 (d,
Entry
Catalyst
Time(h)
a
1 2 3 4 5 6
1 2 3 4 1 1
24 24 24 24 12 36
91 62 58 40 47 98
Conv. (%)
b
b/l ratio
39/61 40/60 40/60 35/65 20/80 48/52
TOF h−1 7.6 5.2 4.8 3.3 3.9 8.2
Reaction conditions: pre-catalyst (0.081 mmol), Solvent: methanol. 25 mL and toluene 25 mL; Pd:PPh3:HCl = 1:2:10:200 ; Pd (0.0806 mmol), PPh3 (42.26 mg, 0.1612 mmol), HCl (0.0249 mL) and 1-hexene (2 mL, 16.11 mmol),; p(CO) = 60 bar; temperature: 90 °C; Time: 24 h; a% of 1-hexene converted to esters determined from GC assuming 100% mass balance; bMolar ratio between branched and linear ester determined from GC. 2
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
mechanical stirrer, temperature control unit, sampling valve and internal cooling system. In a typical experiment complex 1 (27.38 mg, 0.08 mmol), PPh3 (42.26 mg, 0.16 mmol), 37% HCl (0.02 mL, 0.80 mmol) and 1-hexene (2 mL, 16.11 mmol), corresponding to Pd:PPh3:HCl:1–hexene molar ratio of 1:2:10:200, were placed in a Schlenk tube. A mixture of toluene (25 mL)-methanol (25 mL) was then added to dissolve them. The mixture was then introduced into the reactor and purged three times with CO, set at required temperature and pressure and then the reaction stirred at 500 rpm. At the end of the reaction time, the reactor was allowed to cool to room temperature and excess CO was vented off. Samples were drawn and filtered using microfilter prior to GC analysis to determine the percentage conversion of the substrate to the products, assuming 100% mass balance. GC–MS was used to determine the identity of the ester products, while the linear and branched esters were assigned using standard authentic samples. The GC analyses was carried out under the following conditions of: 25 m (1.2 mm film thickness) CP-Sil 19 capillary column, injector temperature 250 °C, oven program 50 °C for 4 min, rising to 200 °C at 20 °C/min and holding at 200 °C for 30 min, nitrogen carrier column gas 5 psi.
methine linker (CeH) protons at 3.63 and 8.33 ppm, respectively, the 1 H NMR spectrum of the corresponding complex 1 showed a downfield shift in the peaks at 3.71 and 8.55 ppm, respectively (Fig. S1). A downfield shift of the protons in the aromatic region was also observed with the 5-py-H proton, being recorded at 7.45 ppm in the 1H NMR spectrum of L1, while in the corresponding 1H NMR spectrum of complex 1, it was observed at 7.89 ppm. Similar trends were observed in the 1H NMR spectra of the ligands, L2- and L3, and of their corresponding complexes, 3 and 4. The observed shifting patterns for these complexes have also been reported by Cloete et al., for similar palladium compounds [37]. IR spectra of ligands L1-L3 generally exhibited strong absorption bands in the region 1648–1650 cm−1, indicative of the v(C]N)imine functional group in the ligand motif [30,38]. Upon complexation, a significant shift was observed for the v(C]N)imine band from around 1648–1650 cm−1 in the free ligands L1-L3 to around 1594–1600 cm−1 for complexes 1–4. These IR data falls within the absorption bands previously observed for typical bidentate (pyridyl)imine complexes [28,39]. Mass spectrometry also proved useful in establishing the formation and identity of the ligands and the complexes. For example, the mass spectrometry data of the ligands, L1 (M+ = 164), L2 (M+ = 150) and L3 (M+ = 178), showed m/z = 187, m/z = 173 and m/z = 201 (Fig. S4), respectively, which correspond to sodium coordinated molecular ions that are very much in good correlation with the proposed structures. Moreover, the ESI-MS spectrum of complex 1 showed m/z at 269 as the base peak, corresponding to [M+- 2Cl] fragment, consistent with the loss of two chloride ligands (Supplementary Fig. S5). Elemental analyses data of complexes 1, 3 and 4 were consistent with one ligand motif per metal atom as proposed in Scheme 1, and further established the purity of the bulk materials.
2.5. Density functional theory (DFT) studies DFT calculations were performed in a gas phase to identify the energy-minimized structures based on B3LYP/LANL2DZ (Los Almos National Laboratory 2 double ζ) level theory.16 A split bases set, LANL2DZ for palladium and 6-311G for all other atoms was used to optimize the geometries and energies of the complexes. The Gaussian09 suite of programs was used for all the computations [36]. 3. Results and discussion 3.1. Synthesis of palladium(II) complexes
3.2. Molecular structure of complex 1
Reactions of 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2) and 3-methoxy-N((pyridin-2-yl)methylene)propan-1-amine (L3) with [PdCl2(COD)] or [PdCl(Me)(COD)] resulted in the formation of the corresponding neutral palladium complexes [PdCl2(L1)] (1), [PdClMe(L1)] (2), [PdCl2(L2)] (3) and [PdCl2(L3)] (4) (Scheme 1). We had previously [28] reported the syntheses of complex 2, hence its analytical data and discussion is not given in this contribution. Complexes 1, 3 and 4 were isolated as yellow solids in moderate to high yields (52–80%). The complexes were characterized by NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, and single-crystal X-ray crystallography for compound 1. 1H NMR and 13C NMR spectra of all the synthesized compounds showed all the signature peaks expected of these type of imine ligands (Figs. S1–S3). In addition, 1H NMR spectra of the complexes were very useful in providing preliminary evidence of complexation. This was diagnosed by comparing the signature peaks of the free ligands and the respective complexes. For example, while the 1 H NMR spectrum of L1 gave signature peaks of the N-CH2 and the
Single crystals of complex 1 suitable for X-ray diffraction analysis were grown by slow diffusion of hexane into a CH2Cl2 solution of complex 1 at room temperature. Crystal and structure refinement parameters for complex 1 are given in Table S1, while the molecular structure and bond parameters are shown in Fig. 1. From the solid state structure of complex 1 (Fig. 1), the bidentate N,N′ coordination to the Pd(II) center through the pyridyl and imino nitrogen atoms affords a five-membered chelate ring. The third and fourth coordination sites are occupied by the chloride ligands. The average bond lengths for Pd-Cl bonds in complex 1 were obtained as 2.285(5) Å, while the Pd-Npy and Pd-Nimine bond lengths were found to be 2.026(1) and 2.021(1) Å, respectively. Laine et al. reported similar Pd-Npy and Pd-Nimine bond lengths of 2.028(3) and 2.022(3) Å, respectively, in palladium(II) complexes bearing the unsymmetrical 2,6-bis(1-methylethyl)-N-(2pyridinylmethylene)phenyl-amine ligand [40]. A MOGUL structural data search was also used to validate the Pd-Cl, Pd-Npy and Pd-Nimine bond lengths for complex 1 [41]. The average PdCl bond of 2.284 (5) Å was found to be comparable to 464 complexes of similar moieties, averaging to 2.284 Å with minimum and maximum bond distances of 1.945 and 2.422 Å, respectively [41]. Similarly, the average Pd-Npy and Pd-Nimine bond of 2.026 and 2.021 Å tally well with the average bond distances of 2.039 and 2.019 Å, respectively, calculated from 165 related palladium pyridine complexes [40,42]. The N1Pd1-N2 chelate bond angle of 80.51(16)° is significantly smaller, compared to the N1-Pd-Cl1 and Cl1-Pd1-Cl2 bond angles of 94.01(4)° and 90.96(2)°, respectively. This strained chelate ring thus renders complex 1 to adopt a distorted square-planar geometry. The dimer of complex 1 exhibits two intermolecular hydrogen bonds between the imine hydrogen atom and the methoxy oxygen atom, while the chloride ligands do not participate in any hydrogen bonding (Fig. 1b).
Scheme 1. Synthesis of iminopyridine palladium (II) complexes.
3
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
Fig. 1. (a) Molecular structure of complex 1 drawn at 50% probability surfaces. Hydrogen atoms have been rendered as spheres of arbitrary radius. (b) Hydrogen bonding in the dimers of complex 1 viewed down the c-axes. Selected bond length [Å] and angles [°]: Pd(1)-N(1), 2.026(1); Pd(1)N(2), 2.021(1); Pd(1)-Cl(1), 2.292(5); Pd (1)-Cl(2), 2.278(5); N(1)-Pd(1)-N(2), 80.51(6); N(2)-Pd(1)-Cl(2), 94.53(5); Cl(2)Pd(1)-Cl(1), 90.96(2); Cl(1)-Pd(1)-N(1), 94.01(4).
3.3. Methoxycarbonylation of olefins using complexes 1–4 as catalysts 3.3.1. Effect of catalyst structure in the methoxycarbonylation of 1-hexene Recently we reported the methoxycarbonylation of olefins using palladium(II) complexes chelated by (phenoxy)imine ligands bearing alkoxy silane groups in which linear esters were formed as the major products [30]. In this work, we aimed to improve the catalytic activity and selectivity via incorporation of a potentially hemi-labile donor arm in the ligand motif. Preliminary screening of complexes 1–4 in the methoxycarbonylation of 1-hexene at CO pressure of 60 bar, temperature of 90 °C and Pd:PPh3:HCl:1–hexene molar ratio of 1:2:10:200 was carried out (Scheme 2). Although two mechanisms for methoxycarbonylation, hydride ((Pd–H) and methoxycarbonyl (Pd–OMe) mechanism have been proposed [25,43], majority of reports indicate that the hydride mechanism is favored in which protic acid promoter serves as a hydride source. While organic acid promoters such as para-tolyl sulphonic acid are usually preferred, sometimes they do not give active catalyst systems, hence the use of HCl acid. In addition, PPh3 is usually added to stabilize the catalytic active species [44]. In order to establish the role of the PPh3 additive, we acquired an in situ 31P NMR spectrum of the reaction of complex 1 and PPh3/HCl mixture over a 5 h period (Fig. S11). From the 31P NMR spectrum, the formation of a Pd-PPh3 stabilized intermediate was evident from the signals at 24 ppm and 34 ppm (possible cis and trans-isomers) as given in eq. 1. The stability of the PPh3-adduct was affirmed by the zerochange in the spectra over the 5 h period. The signal at −5 ppm corresponds to the free PPh3, and is not surprising since excess amount of the PPh3 was added (Pd:PPh3 = 1:2).
methyl-2-methylhexanoate (branched product) and methyl heptanoate (linear product), Scheme 2. Supplementary Figs. S6 and S7 show typical GC chromatogram and GC–MS spectra of the products obtained. Cole-Hamiltion et al., have developed palladium complexes of bis(di-tert-butylphosphinomethyl)benzene for methoxycarbonylation of terminal or internal alkenes, which under optimal conditions achieve, 100% and 98% conversions using 1-hexene and 1-octene, respectively [45]. Aguirre and co-workers applied palladium(II) complex with P,Ndonor ligand, 2-(diphenylphosphinoamino)pyridine (Ph2PNHpy), as a catalyst precursor for the methoxycarbonylation of 1-hexene at 75 °C and after 24 h, 43% conversion and a branched to linear ratio of 55:45 were detected [22]. In this work, however, reactions run for 12 h gave percentage conversion of 47%, while increasing longer reaction times from 24 h to 36 h marginally improved the conversions from 91% to 98%, respectively (Table 1, entries 1, 5–6). Thus reaction time of 24 h was chosen as the optimum condition. The role of the complex/ligand structure on the performance of the resultant catalysts was evaluated by comparing the catalytic activities of complexes 1–4 (Table 1). From the data, it is evident that the complex structure significantly influenced the catalytic abilities of these complexes in the methoxycarbonylation of 1-hexene. First, we observed that changing from a Pd-Cl system in 1 to a Pd-Me analogue in complex 2, resulted in a drastic drop in percentage conversion from 91% to 62%, respectively (Table 1, entries 1 and 2). The higher catalytic activity of complex 1 relative to complex 2 could be assigned to the electron withdrawing nature of the coordinated chloride ligand in comparison to the electron donating methyl group in complex 2. This has the overall effect of increasing the rate of coordination of 1-hexene substrate co-
Under these reaction conditions, complexes 1–4 formed active catalysts, affording percentage conversions between 40% and 91% (Table 1, entries 1–4), which are slightly higher when compared to conversions between 65% and 85% reported in our previous study [30]. The major products formed, as identified by GC and GC–MS were
ordination to the active metal atom. Another plausible explanation for the observed higher catalytic activity of complex 1 in comparison to 2, could be the ease of formation of the Pd-H species, believed to be the active species [25] from the dichloride complex 1. In order to understand the reason for the higher catalytic activity of 4
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
results mirror those we recently reported for palladium(II) complexes of (phenoxy)imine ligands bearing alkoxy silane groups in which, under similar conditions, regioselectivities of the ester products towards linear products of 58%-65% were realized [30]. 3.3.2. Effect of reaction substrate on methoxycarbonylation of olefins Having established the viability of complexes 1–4 in the methoxycarbonylation of 1-hexene, we turned our attention to the use of higher olefins; 1-heptene, 1-octene, 1-decene and 1-dodecene using complex 1 (Fig. 2). We observed that increasing the chain length of the olefin substrate significantly decreased the resultant catalytic activity of complex 1 (Fig. 2). For example, with 1-hexene as a substrate, conversions of 90% were obtained, while conversions of 47% and 30% were recorded for 1-octene and 1-decene, respectively. This trend can be attributed to the increase in steric hindrance and higher electron density in higher olefins. These two factors combined have the net effect of limiting substrate coordination to the metal center as discussed hitherto. We recently reported similar trends in the methoxycarbonylation of higher olefins using palladium complexes supported by N^O chelating ligands [30]. Increasing the chain length of olefins was also observed to affect the regioselectivity of the ester products, whereby higher alkenes favoured the formation of branched esters. For example, 39% of the branched esters were obtained for 1-hexene, while 60% was obtained when 1dodecene was used as a substrate (Fig. 2). According to Rodriguez et al., the enhanced formation of branched esters is due to the possible isomerisation of higher alkenes to internal alkenes during the reaction [45].
Scheme 2. Methoxycarbonylation of 1-hexene using complexes 1–4 as catalysts.
the PdCl2 complex 1 in relation to the PdClMe complex, 2, we monitored 1H NMR spectra of the reaction between complex 2 and HCl. It is believed that, the reaction of the PdClMe with HCl is likely to result in the formation of the PdCl2 species, followed by generation of the methane gas. From the 1H NMR spectra obtained over the 5 h period (Fig. S12), the disappearance of the Pd-Me (signal at 0.91 ppm) is evident, with only about 5% present after 5 h. This shows that complex 2 is unstable, and may account for its lower catalytic activity in comparison to complex 1. A more important information could also be derived from examination of the catalytic activities of complexes 1 and 3, bearing the OMe and OH pendant donor groups respectively. The better performance of complex 1, bearing the OMe group may be assigned to its hemi-labile nature [46], that allows for substrate coordination, in addition to stabilizing the active intermediate. It has been reported [46,47] that the OH group strongly coordinates to the palladium atom. This has the negative effect of competing with the 1-hexene substrate, accounting for the observed diminished percentage conversions of 58%. This hypothesis is supported by the much lower conversions of 40% observed for complex 4, bearing the propyl-OMe group [28]. The reduction in the catalytic activity of complex 4 relative to complex 1, could also be attributed to the formation of a more stable six-membered chelate ring compared to the five-membered ring in 1. This has the same effect as the OH group in complex 3 of hindering 1-hexene substrate coordination [30]. Reduced electrophilicity of the palladium metal atom in 4, due to the more electron-donating propyl groups, may also be implicated in its diminished catalytic activities [28]. Generally, the structural moiety of the catalysts did not show any direct influence on the regioselectivity of the ester products, since in all cases regioselectivities of 60%-65% towards linear esters were observed. This is not surprising as the steric environment around the metal atoms appears to be largely comparable owing to the remote proximity of the pendant groups from the metal atom (Fig. 1). These
3.3.3. Density functional theoretical calculations of reactivity parameters for complexes 1–4 In order to gain insight into the observed catalytic behavior of structure of complexes 1–4 and the potential hemilability of the ligands, density functional theory (DFT) calculations were performed on the ground-state electronic structures of the potential cationic intermediates 1a-4a, in which ligands L1-L3 adopt tridentate coordination modes (Table 2). The computations were performed using a split basis set LANL2DZ for Pd(II) and 6311G(dp) for the remaining atoms. One of the key features of hemi-labile ligands is the strength of the bond of the metal-pendant donor atom, relative to the incoming substrate [28]. We thus investigated the relationship between the Pd-O bond length and the resultant catalytic activities of the cationic complexes 1a-4a. From the DFT results, we observed that an increase in PdO bond length generally resulted in increased catalytic activities
Fig. 2. Effect of olefin substrate on % conversion and regioselectivity towards linear esters using complex 1 (0.08 mmol) at [Pd]:[PPh3]:[HCl]:[olefin] ratio of 1:2:10:200, PCO, 60 bar; temp, 90 °C, solvent, methanol/toluene (50 mL); time, 24 h. 5
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
Table 2 Theoretical and experimental data for complexes 1a–4a. Complex 1a 2a 3a 4a
a
NBO charge (Pd) a
0.5710(0.368) 0.5650(0.353)a 0.5540(0.364)a 0.5640(0.361)a
Pd-O (Å)
Δε [kcal mol−1]
%Conv
2.1115 2.1754 2. 0945 2.1048
224.42 227.08 226.79 225.06
91 62 58 40
The values in bracket are the NBO charges of neutral complexes 1–4.
(Table 2). For example, Pd–O bond length of 2.1115 Å in 1a and 2.0945 Å in 3a corresponds to conversions of 91% and 58%, respectively. This is consistent with stronger coordination of OH group relative to OCH3 to the palladium(II) atom, thus limiting substrate accessibility to the palladium atom in 3a. A similar trend was observed by Kress et al., in ethylene oligomerization reaction, where the complex containing the OH group is inactive, while the analogous complex, bearing the OCH3 pendant arm, is active [48]. Examination of the Pd-O bond lengths of 2.1115 Å in 1a and 2.1048 Å in 4a and their catalytic activities (91% and 40%, respectively, also revealed a similar trend. Complex 1a contains a five-membered ring, while 4a contains a sixmembered ring, hence the higher catalytic activities observed for 1a may be attributed to its longer bond distance. However, the small difference in bond length of 0.0067 Å may not sufficiently explain the large differences in their percentage conversions of 91% and 40%, suggesting other factors such as electrophilicity of the metal atom may play a role. Thus to further rationalize the experimental results, we examined the charge on the Pd metal atom in the cationic derivatives - (1a-a) and its inherent influence on catalytic activities. Comparison of complexes 1a, 3a and 4a reveals that higher catalytic activities were obtained with an increase in the positive charge of the metal atom. This is expected and has been attributed to enhanced coordination of the substrate to the palladium(II) center [28]. For example, complex 1a, carrying the greatest NBO charge of 0.571 was the most active. The lower NBO charge of complex 2a can be attributed to the methyl group, coordinated to the palladium metal center as opposed to the electron withdrawing chloro ligand in 1a. In addition, the higher conversion of 1a relative to 4a may also be rationalized from the higher NBO charge in 1a of 0.5710 compared to the value of 0.5610 in 4a. The HOMO–LUMO energy gaps were also investigated to study their influence on the catalytic activities of the respective complexes (Table 2). A smaller energy difference between 1-hexene’s HOMO and pre-catalysts 1a–4a’s LUMO should promote coordination of the 1-hexene substrate to the metal center [18]. The results obtained in Table 2 are not in agreement with the expected behaviour [49] since the catalytic activities did not vary linearly with the energy gaps (Δε). Thus the HOMO–LUMO energy gaps did not have a profound effect on the resultant catalytic activities of the complexes. This data implies that the methoxycarbonylation reactions catalyzed by complexes 1–4 were charged controlled, rather than frontier-orbital directed. The DFT data in Table 2, therefore, supports the hemi-labile nature of the ligands, and the role of the pendant group in controlling the respective catalytic activities of the complexes.
4. Conclusions In conclusion, we have prepared three new neutral palladium complexes of (pyridyl)imine ligands. The ligands adopt bidentate coordination mode to give square planar palladium complexes. All the complexes form active catalysts in the methoxycarbonylation of higher olefins to give linear and branched esters. The nature of the ligand motif played a major role in controlling the catalytic performance of the complexes, largely controlled by the coordination abilities of the pendant donor atom. DFT studies support the hemi-lability of the (imino) pyridine ligands in the methoxycarbonylation reactions. In addition, the olefin chain length affected both the catalytic activity and regioselectivity of the methoxycarbonylation reactions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors thank the DST-NRF (South Africa), Center of Excellence in Catalysis (c*change) and the University of KwaZulu-Natal for financial support. Appendix A. Supplementary data Supplementary information contains NMR and GC-MS spectral, spectra for the ligands and complexes, GC chromatograms, GC-MS spectral data of the methoxycarbonylation products, crystal collection and refinement data and CCDC number 1960949 for complex 1. Supplementary data to this article can be found online at https://doi. org/10.1016/j.ica.2019.119270. References [1] S.C. Stouten, T. Noël, Q. Wang, M. Beller, V. Hessel, Catal. Sci. Technol. 6 (2016) 4712–4717. [2] L. Wu, Q. Liu, I. Fleischer, R. Jackstell, M. Beller, Nature Commun. 5 (2014) 3091. [3] K. Dong, X. Fang, S. Gülak, R. Franke, A. Spannenberg, H. Neumann, R. Jackstell, M. Beller, Nature Commun. 8 (2017) 14117. [4] S. Boyde, Green Chem. 4 (2002) 293–307. [5] K. Kumar, J. Darkwa, Polyhedron 138 (2017) 249–257. [6] R. Franke, D. Selent, A. Börner, Chem. Rev. 112 (2012) 5675–5732. [7] J.R. Zbieg, E. Yamaguchi, E.L. McInturff, M.J. Krische, Science 336 (2012) 324–327. [8] P.W. Van Leeuwen, C. Claver, Rhodium Catalyzed Hydroformylation, Springer
6
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al. Science & Business Media, 2002. [9] J. Liu, Z. Wei, H. Jiao, R. Jackstell, M. Beller, ACS Centr. Sci. 4 (2017) 30–38. [10] G. Kiss, Chem. Rev. 101 (2001) 3435–3456. [11] J.D. Nobbs, C.H. Low, L.P. Stubbs, C. Wang, E. Drent, M. van Meurs, Organometallics 36 (2016) 391–398. [12] M. Schmidt, T. Pogrzeba, L. Hohl, A. Weber, A. Kielholz, M. Kraume, R. Schomäcker, Mol. Catal. 439 (2017) 1–8. [13] X.-F. Wu, X. Fang, L. Wu, R. Jackstell, H. Neumann, M. Beller, Acc. Chem. Res. 47 (2014) 1041–1053. [14] K. Dong, R. Sang, X. Fang, R. Franke, A. Spannenberg, H. Neumann, R. Jackstell, M. Beller, Ang. Chem. Int. Ed. 129 (2017) 5351–5355. [15] A. Brennführer, H. Neumann, M. Beller, ChemCatChem 1 (2009) 28–41. [16] J. Durand, B. Milani, Coord. Chem. Rev. 250 (2006) 542–560. [17] K. Dong, R. Sang, Z. Wei, J. Liu, R. Dühren, A. Spannenberg, H. Jiao, H. Neumann, R. Jackstell, R. Franke, Chem. Sci. 9 (2018) 2510–2516. [18] G.S. Nyamato, S.O. Ojwach, M.P. Akerman, Dalton Trans. 45 (2016) 3407–3416. [19] C. Bianchini, A. Meli, Coord. Chem. Rev. 225 (2002) 35–66. [20] T.A. Tshabalala, S.O. Ojwach, Trans. Met. Chem. 43 (2018) 339–346. [21] T.A. Tshabalala, S.O. Ojwach, M.A. Akerman, J. Mol. Catal. A: Chem. 406 (2015) 178–184. [22] P.A. Aguirre, C.A. Lagos, S.A. Moya, C. Zúñiga, C. Vera-Oyarce, E. Sola, G. Peris, J.C. Bayón, Dalton Trans. (2007) 5419–5426. [23] J. Liu, H. Li, R. Dühren, J. Liu, A. Spannenberg, R. Franke, R. Jackstell, M. Beller, Angew. Chem. Int. Ed. 129 (2017) 12138–12142. [24] P.J. Deckers, B. Hessen, J.H. Teuben, ngew. Chem. Int. Ed. 40 (2001) 2516–2519. [25] C. Arderne, l.A. Guzei, C.W. Holzapfel, T. Bredenkamp, ChemCatChem 8 (2016) 1084–1093. [26] C.M. McGuirk, J. Mendez-Arroyo, A.M. Lifschitz, C.A. Mirkin, J. Ame. Chem. Soc. 136 (2014) 16594–16601. [27] A.E. Díaz-Álvarez, C. Vidal, F.J. Suárez, J. Díez, V. Cadierno, P. Crochet, Organometallics 34 (2015) 3670–3677.
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
7
G.S. Nyamato, S.O. Ojwach, M.P. Akerman, Organometallics 34 (2015) 5647–5657. S. Mecking, W. Keim, Organometallics 15 (1996) 2650–2656. S.O. Akiri, S.O. Ojwach, Inorg. Chim. Acta 489 (2019) 236–243. D. Drew, J. Doyle, A.G. Shaver, Inorganic Syntheses: Reagents for Transition Metal Complex and Organometallic Syntheses, 28 (1990) 346–349. R.E. Rulke, J.M. Ernsting, A.L. Spek, C.J. Elsevier, P.W. van Leeuwen, K. Vrieze, Inorg. Chem. 32 (1993) 5769–5778. Bruker, in, Madison, Wisconsin, USA, 2012. G.M. Sheldrick, in, University of Göttingen, Göttingen, Germany, 2016. L.J. Farrugia, Appl. Cryst. 45 (2012) 849–854. M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. Petersson, Inc.: Wallingford, CT, USA, (2009). J. Cloete, S.F. Mapolie, J. Mol. Catal. A Chem. 243 (2006) 221–225. W. Zhang, W.-H. Sun, B. Wu, S. Zhang, H. Ma, Y. Li, J. Chen, P. Hao, J. Organomet. Chem. 691 (2006) 4759–4767. J.M. Benito, E. de Jesús, F.J. de la Mata, J.C. Flores, R. Gómez, P. Gómez-Sal, Organometallics 25 (2006) 3876–3887. T.V. Laine, M. Klinga, M. Leskelä, Eur. J. Inorg. 1999 (1999) 959–964. I.J. Bruno, J.C. Cole, M. Kessler, J. Luo, W.S. Motherwell, L.H. Purkis, B.R. Smith, R. Taylor, R.I. Cooper, S.E. Harris, J. Chem. Info. Comp. Sci. 44 (2004) 2133–2144. D. Kim, Y. Song, S. Kim, H.-J. Lee, H. Lee, J. Coord. Chem. 67 (2014) 2312–2329. I.A. Shuklov, N.V. Dubrovina, J. Schulze, W. Tietz, K. Kühlein, A. Börnera, Arkivoc (2012) 66–75. M. Carvajal, G. Miscione, A. Accardi, J. Novoa, A. Bottoni, Mol. Phys. 104 (2006) 805–831. C.J. Rodriguez, D.F. Foster, G.R. Eastham, D.J. Cole-Hamilton, Chem. Commun. (2004) 1720–1721. P.-Y. Shi, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Organometallics 21 (2002) 3203–3207. M.K. Ainooson, I.A. Guzei, L.C. Spencer, J. Darkwa, Polyhedron 53 (2013) 295–303. S.P. Meneghetti, P.J. Lutz, J. Kress, Organometallics 18 (1999) 2734–2737. P. Huo, W. Liu, X. He, H. Wang, Y. Chen, Organometallics 32 (2013) 2291–2299.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Palladium(II) complexes of (pyridyl)imine ligands as catalysts for the methoxycarbonylation of olefins Zethu Zulua, George S. Nyamatoa,b, Thandeka A. Tshabalalaa, Stephen O. Ojwacha,
⁎
a b
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa Department of Physical Sciences, University of Embu, P.O Box 6-60100, Embu, Kenya
ARTICLE INFO
ABSTRACT
Keywords: Palladium Structures Olefins Methoxycarbonylation Esters
Reactions of 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2) and 3-methoxy-N-((pyridin-2-yl)methylene)propan-1-amine (L3) ligands with either [PdCl2(COD)] or [PdCl (Me)(COD)] produced the corresponding monometallic complexes [PdCl2(L1)] (1), [PdClMe(L1)] (2), [PdCl2(L2)] (3) and [PdCl2(L3)] (4). The solid state structure of complex 1 confirmed the bidentate coordination mode of L1, giving a distorted square planar geometry. All the complexes (1–4) formed active catalysts for the methoxycarbonylation of higher olefins to give linear and branched esters. The catalytic behavior of complexes 1–4 were influenced by both the complex structure and olefin chain length.
1. Introduction Functionalization of olefins constitute a fundamental basis of today’s chemical industry and is responsible for the production of a wide range of fine and bulk chemical products [1–3]. Carbonylation of olefins represents one of the most important olefin transformation reactions that is mostly applied to produce useful products such as esters, which are key reagents, for the industrial production of solvents, detergents, cosmetics and pharmaceuticals [4,5]. In terms of production, besides hydroformylation and related transformations [6–8], alkoxycarbonylation reactions constitute the next largest industrial applications in the area of homogeneous catalysis [9]. In this regard, the palladium-catalyzed methoxycarbonylation of 1-alkenes is an active area of research [10–14] that is currently receiving appreciable attention. To date, palladium complexes are the most widely used catalysts in the methoxycarbonylation reactions due to their high catalytic activity and stability [13,15]. It is generally known that the catalytic behavior of coordination compounds is influenced by the ancillary ligands [16]. Thus, by finetuning the electronic and steric properties of the ligands, the activity and stability of the metal center can be adjusted [17–19]. For example, the combination of hard and soft donor atoms within the same ligand framework makes such a ligand a good candidate for active and stable catalysts [5]. While phosphine-donor ligands have traditionally been used in the preparation of palladium catalysts for methoxycarbonylation reactions, attention is currently shifting to mixed
⁎
nitrogen–phosphine donor palladium complexes due to their perceived stability and tolerance to impurities [20,21]. Thus palladium complexes, for example, of the type [PdCl2(Ph2PNHpy-k2-P,N)] and [PdCl (Ph2PNHpyk2-P,N)(PPh3)]Cl have been reported to form active and stable catalysts in the methoxycarbonylation of styrene [22]. Although a number of ligands have so far been developed, their rational design aimed at affording higher catalytic activities and stability continues to be an important topic in this area [23]. One approach that can be employed to achieve a balance between catalyst activity and stability is via the use of potential hemi-labile ligands. These systems have the ability to stabilize the active metal center, without compromising catalytic activity [5,24]. In a recent report, for example, hemi-labile ligands based on bidentate P^O-ligands were shown to influence palladium(II) catalyzed methoxycarbonylation of 1alkenes to afford mainly branched esters [25]. In other previous work, it has been shown that a combination of O or S donors with P or N donors form good hemi-labile ligands in palladium(II) catalyzed organic transformations [26–28]. For example, Mecking and Keim reported palladium η3-allyl complexes chelated by hemi-labile ligands based on bidentate P,O-ligands [29] as catalysts in ethylene dimerization. Even though this approach has been rarely applied in methoxycarbonylation reactions, we believe it could provide a promising avenue for the design and development of stable palladium catalysts for these reactions. Recently, we reported on the methoxycarbonylation of olefins catalyzed by palladium(II) complexes bearing (benzimidazolylmethyl)
Corresponding author. E-mail address: [email protected] (S.O. Ojwach).
https://doi.org/10.1016/j.ica.2019.119270 Received 14 June 2019; Received in revised form 24 October 2019; Accepted 7 November 2019 Available online 09 November 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zethu Zulu, et al., Inorganica Chimica Acta, https://doi.org/10.1016/j.ica.2019.119270
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
1H, 6-py, 3JHH = 8.0); 13C {1H} NMR (DMSO‑d6): 59.19 (CH2-N), 62.10 (CH2-O), 128.55 (3-py-C), 128.98 (5-py-C), 141.84 (4-py-C), 150.56 (6py-C), 156.33 (2-py-C), 172.12 (C]N), ESI-MS: m/z (%) 293 [(M+ - Cl, 65%)]. FT-IR (cm−1): υ(C]N)imine = 1650 Anal. Calcd for C8H10Cl2N2OPd: C, 29.34; H, 3.08; N, 8.55. Found: C, 29.05; H, 3.14; N, 8.29.
amine [20] and (phenoxy)imine [30] ligands in which the catalytic performances of these complexes were controlled by the ligand architecture. In our continued pursuit for both active and stable catalysts for methoxycarbonylation of 1-alkenes, we herein report the syntheses of palladium(II) complexes of iminopyridine ligands, containing potential hemi-labile ether or hydroxy pendant donor groups. The structural elucidation of the complexes, effect of complex structure and olefin substrate on the activity and regioselectivity of the resultant catalysts are herein discussed.
2.2.3. Synthesis of [PdCl2(L3)] (4) Complex 4 was prepared according to the procedure described for 1 using L3 (0.06 g, 0.35 mmol) and [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (20 mL). Complex 4 was isolated as a yellow solid. Yield = 0.087 g (70%).1H NMR (400 MHz, DMSO‑d6): δH (ppm): 2.06 (q, 2H, CH2, 3JHH = 6.6); 3.25 (s, 3H, CH3-O); 3.42 (t, 2H, CH2-O, 3 JHH = 6.1); 3.80 (t, 2H, CH2-N, 3JHH = 6.9); 7.89 (t, 1H, 5-py, 3 JHH = 7.4); 8.12 (d, 1H, 3-py, 3JHH = 7.6); 8.37 (t, 1H, 4-py, 3 JHH = 7.8); 8.59 (s, 1H, CeH); 8.99 (d, 1H, 6-py, 3JHH = 5.6 Hz). 13C {1H} NMR (DMSO‑d6): 30.20 (CH2), 57.13 (CH2-N), 58.30 (CH3-O), 69.26 (CH2-O), 128.66 (3-py), 129.00 (5-py), 141.78 (4-py), 150.57 (6py), 156.34 (2-py), 172.01 (C]N). ESI-MS: m/z (%) 201 [(M+ + Na+, 100%)]. FT-IR (cm−1): υ(C]N)imine = 1648. Anal. Calcd for C10H14Cl2N2OPd: C, 33.78; H, 3.97; N, 7.88. Found: C, 33.75; H, 4.01; N, 7.84.
2. Experimental section and methods 2.1. General instrumentation and material All synthetic manipulations were performed under nitrogen atmosphere using standard Schlenk line techniques. All solvents were purchased from Merck and were dried and distilled prior to use. Diethyl ether, hexane and toluene were dried over sodium wire and benzophenone, methanol and absolute ethanol over calcium oxide, while dichloromethane was dried and distilled over phosphorus pentoxide. Methylheptanoate, methylnonanoate, sodium hydroxide (NaOH) and DMSO‑d6 were purchased from Merck Chemicals. Ethanolamine, 2methoxyethylamine, 2-pyridinecarboxaldehyde, olefins, hydrochloric acid, p-TsOH (≥98.5%), PPh3 (99%) and CDCl3 were purchased from Sigma Aldrich and used as received. [PdCl2(COD)] [31], [PdCl(Me) (COD)] [32], ligands 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2), 3-methoxyN-((pyridin-2-yl)methylene)propan-1-amine (L3), and complex [PdClMe(L1)] (2) were prepared by following our recently published procedures [28]. 1H NMR and 13C {1H} NMR spectra were recorded on a Bruker Ultrashield 400 (1H NMR 400 MHz, 13C {1H} NMR 100 MHz) spectrometer in CDCl3 solution at room temperature and chemical shifts (δ) were determined relative to internal TMS and are given in ppm. The infrared spectra were recorded on a PerkinElmer Spectrum 100 in the 4000–400 cm−1 range. Elemental analyses were performed on a Thermal Scientific Flash 2000 and mass spectra were recorded on an LC premier micromass spectrometer.
2.3. X-ray crystallography X-ray data collection for complex 1 was recorded on a Bruker Apex Duo equipped with an Oxford Instruments Cryojet operating at 100(2) K and an Incoatec microsource operating at 30 W power. Crystal and structure refinement data are given in Table 1. The data were collected with Mo Kα (λ = 0.71073 Å) radiation at a crystal-to-detector distance of 50 mm. The data were collected using omega and phi scans with exposures taken at 30 W X-ray power and 0.50° frame widths using APEX2 [33]. The data were reduced with the programme SAINT [33] using outlier rejection, scan speed scaling, as well as standard Lorentz and polarisation correction factors. A SADABS semi-empirical multiscan absorption correction [33] was applied to the data. Direct methods, SHELXS-2016 [34] and WinGX [35] were used to solve the data. All non-hydrogen atoms were located in the difference density map and refined anisotropically with SHELXL-2016 [34]. All hydrogen atoms were included as idealized contributors in the least squares process. Their positions were calculated using a standard riding model with C–Haromatic distances of 0.93 Å and Uiso = 1.2 Ueq and C–Hmethylene distances of 0.99 Å and Uiso = 1.2 Ueqand C–Hmethyl distances of 0.98 Å and Uiso = 1.5 Ueq.
2.2. Syntheses of palladium(II) complexes 2.2.1. Synthesis of [PdCl2(L1)] (1) To a solution of [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (10 mL) was added a solution of L1 (0.06 g, 0.35 mmol) in CH2Cl2 (10 mL). The mixture was stirred at room temperature, under nitrogen for 24 h. The yellow precipitate formed was filtered and washed with diethyl ether (2 × 20 mL), filtered and dried to afford 1 as a yellow solid. Recrystallization from CH2Cl2-hexane mixture afforded single crystals suitable for X-ray analysis. Yield = 0.09 g (75%). 1H NMR (400 MHz, DMSO‑d6): δH (ppm): 3.29 (s, 3H, CH3); 3.71 (t, 2H, CH2-N, 3 JHH = 8.0 Hz); 3.91 (t, 2H, CH2-O, 3JHH = 8.0 Hz); 7.89 (t, 1H, 5-py, 3 JHH = 8.0 Hz); 8.18 (d, 1H, 3-py, 3JHH = 8.0 Hz); 8.36 (t, 1H, 4-py, 3 JHH = 8.0 Hz); 8.55 (s, 1H, CeH); 8.99 (d, 1H, 6-py, 3JHH = 8.0 Hz). 13 C {1H} NMR (DMSO‑d6): 58.57 (CH3-O), 58.76 (CH2-N), 69.76 (CH2O), 128.89 (3-py-C), 129.27 (5-py-C), 141.93 (4-py-C), 150.75 (6-py-C), 156.03 (2-py-C), 172.75 (C]N). ESI-MS: m/z (%) 269 [(M+ - Cl2, 100%)]. FT-IR (cm−1): υ(C]N)imine = 1598. Anal. Calcd for C9H12Cl2N2OPd·0.1C6H14: C, 32.93; H, 3.80; N, 8.00. Found: C, 32.98; H, 3.67; N, 7.99.
2.4. Typical procedure for the methoxycarbonylation reactions The methoxycarbonylation catalytic reactions were performed in a 400 mL stainless steel autoclave Parr reactor equipped with a Table 1 Methoxycarbonylation of 1-hexene using complexes 1–4.
2.2.2. Synthesis of [PdCl2(L2)] (3) Complex 3 was prepared according to the procedure described for 1 using L2 (0.05 g, 0.35 mmol) and [PdCl2(COD)] (0.10 g, 0.35 mmol) in CH2Cl2 (20 mL). Complex 3 was obtained as a yellow solid. Yield = 0.08 g (70%). 1H NMR (400 MHz, DMSO‑d6): δH (ppm): 3.75 (s, 4H, CH2-N + CH2-O); 7.87 (t, 1H, 5-py, 3JHH = 8.0); 8.18 (d, 1H, 3-py, 3 JHH = 8.0); 8.34 (t, 1H, 4-py, 3JHH = 8.0); 8.48 (s, 1H, CeH); 8.98 (d,
Entry
Catalyst
Time(h)
a
1 2 3 4 5 6
1 2 3 4 1 1
24 24 24 24 12 36
91 62 58 40 47 98
Conv. (%)
b
b/l ratio
39/61 40/60 40/60 35/65 20/80 48/52
TOF h−1 7.6 5.2 4.8 3.3 3.9 8.2
Reaction conditions: pre-catalyst (0.081 mmol), Solvent: methanol. 25 mL and toluene 25 mL; Pd:PPh3:HCl = 1:2:10:200 ; Pd (0.0806 mmol), PPh3 (42.26 mg, 0.1612 mmol), HCl (0.0249 mL) and 1-hexene (2 mL, 16.11 mmol),; p(CO) = 60 bar; temperature: 90 °C; Time: 24 h; a% of 1-hexene converted to esters determined from GC assuming 100% mass balance; bMolar ratio between branched and linear ester determined from GC. 2
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
mechanical stirrer, temperature control unit, sampling valve and internal cooling system. In a typical experiment complex 1 (27.38 mg, 0.08 mmol), PPh3 (42.26 mg, 0.16 mmol), 37% HCl (0.02 mL, 0.80 mmol) and 1-hexene (2 mL, 16.11 mmol), corresponding to Pd:PPh3:HCl:1–hexene molar ratio of 1:2:10:200, were placed in a Schlenk tube. A mixture of toluene (25 mL)-methanol (25 mL) was then added to dissolve them. The mixture was then introduced into the reactor and purged three times with CO, set at required temperature and pressure and then the reaction stirred at 500 rpm. At the end of the reaction time, the reactor was allowed to cool to room temperature and excess CO was vented off. Samples were drawn and filtered using microfilter prior to GC analysis to determine the percentage conversion of the substrate to the products, assuming 100% mass balance. GC–MS was used to determine the identity of the ester products, while the linear and branched esters were assigned using standard authentic samples. The GC analyses was carried out under the following conditions of: 25 m (1.2 mm film thickness) CP-Sil 19 capillary column, injector temperature 250 °C, oven program 50 °C for 4 min, rising to 200 °C at 20 °C/min and holding at 200 °C for 30 min, nitrogen carrier column gas 5 psi.
methine linker (CeH) protons at 3.63 and 8.33 ppm, respectively, the 1 H NMR spectrum of the corresponding complex 1 showed a downfield shift in the peaks at 3.71 and 8.55 ppm, respectively (Fig. S1). A downfield shift of the protons in the aromatic region was also observed with the 5-py-H proton, being recorded at 7.45 ppm in the 1H NMR spectrum of L1, while in the corresponding 1H NMR spectrum of complex 1, it was observed at 7.89 ppm. Similar trends were observed in the 1H NMR spectra of the ligands, L2- and L3, and of their corresponding complexes, 3 and 4. The observed shifting patterns for these complexes have also been reported by Cloete et al., for similar palladium compounds [37]. IR spectra of ligands L1-L3 generally exhibited strong absorption bands in the region 1648–1650 cm−1, indicative of the v(C]N)imine functional group in the ligand motif [30,38]. Upon complexation, a significant shift was observed for the v(C]N)imine band from around 1648–1650 cm−1 in the free ligands L1-L3 to around 1594–1600 cm−1 for complexes 1–4. These IR data falls within the absorption bands previously observed for typical bidentate (pyridyl)imine complexes [28,39]. Mass spectrometry also proved useful in establishing the formation and identity of the ligands and the complexes. For example, the mass spectrometry data of the ligands, L1 (M+ = 164), L2 (M+ = 150) and L3 (M+ = 178), showed m/z = 187, m/z = 173 and m/z = 201 (Fig. S4), respectively, which correspond to sodium coordinated molecular ions that are very much in good correlation with the proposed structures. Moreover, the ESI-MS spectrum of complex 1 showed m/z at 269 as the base peak, corresponding to [M+- 2Cl] fragment, consistent with the loss of two chloride ligands (Supplementary Fig. S5). Elemental analyses data of complexes 1, 3 and 4 were consistent with one ligand motif per metal atom as proposed in Scheme 1, and further established the purity of the bulk materials.
2.5. Density functional theory (DFT) studies DFT calculations were performed in a gas phase to identify the energy-minimized structures based on B3LYP/LANL2DZ (Los Almos National Laboratory 2 double ζ) level theory.16 A split bases set, LANL2DZ for palladium and 6-311G for all other atoms was used to optimize the geometries and energies of the complexes. The Gaussian09 suite of programs was used for all the computations [36]. 3. Results and discussion 3.1. Synthesis of palladium(II) complexes
3.2. Molecular structure of complex 1
Reactions of 2-methoxy-N-((pyridin-2-yl)methylene)ethanamine (L1), 2-((pyridin-2-yl)methyleneamino)ethanol (L2) and 3-methoxy-N((pyridin-2-yl)methylene)propan-1-amine (L3) with [PdCl2(COD)] or [PdCl(Me)(COD)] resulted in the formation of the corresponding neutral palladium complexes [PdCl2(L1)] (1), [PdClMe(L1)] (2), [PdCl2(L2)] (3) and [PdCl2(L3)] (4) (Scheme 1). We had previously [28] reported the syntheses of complex 2, hence its analytical data and discussion is not given in this contribution. Complexes 1, 3 and 4 were isolated as yellow solids in moderate to high yields (52–80%). The complexes were characterized by NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, and single-crystal X-ray crystallography for compound 1. 1H NMR and 13C NMR spectra of all the synthesized compounds showed all the signature peaks expected of these type of imine ligands (Figs. S1–S3). In addition, 1H NMR spectra of the complexes were very useful in providing preliminary evidence of complexation. This was diagnosed by comparing the signature peaks of the free ligands and the respective complexes. For example, while the 1 H NMR spectrum of L1 gave signature peaks of the N-CH2 and the
Single crystals of complex 1 suitable for X-ray diffraction analysis were grown by slow diffusion of hexane into a CH2Cl2 solution of complex 1 at room temperature. Crystal and structure refinement parameters for complex 1 are given in Table S1, while the molecular structure and bond parameters are shown in Fig. 1. From the solid state structure of complex 1 (Fig. 1), the bidentate N,N′ coordination to the Pd(II) center through the pyridyl and imino nitrogen atoms affords a five-membered chelate ring. The third and fourth coordination sites are occupied by the chloride ligands. The average bond lengths for Pd-Cl bonds in complex 1 were obtained as 2.285(5) Å, while the Pd-Npy and Pd-Nimine bond lengths were found to be 2.026(1) and 2.021(1) Å, respectively. Laine et al. reported similar Pd-Npy and Pd-Nimine bond lengths of 2.028(3) and 2.022(3) Å, respectively, in palladium(II) complexes bearing the unsymmetrical 2,6-bis(1-methylethyl)-N-(2pyridinylmethylene)phenyl-amine ligand [40]. A MOGUL structural data search was also used to validate the Pd-Cl, Pd-Npy and Pd-Nimine bond lengths for complex 1 [41]. The average PdCl bond of 2.284 (5) Å was found to be comparable to 464 complexes of similar moieties, averaging to 2.284 Å with minimum and maximum bond distances of 1.945 and 2.422 Å, respectively [41]. Similarly, the average Pd-Npy and Pd-Nimine bond of 2.026 and 2.021 Å tally well with the average bond distances of 2.039 and 2.019 Å, respectively, calculated from 165 related palladium pyridine complexes [40,42]. The N1Pd1-N2 chelate bond angle of 80.51(16)° is significantly smaller, compared to the N1-Pd-Cl1 and Cl1-Pd1-Cl2 bond angles of 94.01(4)° and 90.96(2)°, respectively. This strained chelate ring thus renders complex 1 to adopt a distorted square-planar geometry. The dimer of complex 1 exhibits two intermolecular hydrogen bonds between the imine hydrogen atom and the methoxy oxygen atom, while the chloride ligands do not participate in any hydrogen bonding (Fig. 1b).
Scheme 1. Synthesis of iminopyridine palladium (II) complexes.
3
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
Fig. 1. (a) Molecular structure of complex 1 drawn at 50% probability surfaces. Hydrogen atoms have been rendered as spheres of arbitrary radius. (b) Hydrogen bonding in the dimers of complex 1 viewed down the c-axes. Selected bond length [Å] and angles [°]: Pd(1)-N(1), 2.026(1); Pd(1)N(2), 2.021(1); Pd(1)-Cl(1), 2.292(5); Pd (1)-Cl(2), 2.278(5); N(1)-Pd(1)-N(2), 80.51(6); N(2)-Pd(1)-Cl(2), 94.53(5); Cl(2)Pd(1)-Cl(1), 90.96(2); Cl(1)-Pd(1)-N(1), 94.01(4).
3.3. Methoxycarbonylation of olefins using complexes 1–4 as catalysts 3.3.1. Effect of catalyst structure in the methoxycarbonylation of 1-hexene Recently we reported the methoxycarbonylation of olefins using palladium(II) complexes chelated by (phenoxy)imine ligands bearing alkoxy silane groups in which linear esters were formed as the major products [30]. In this work, we aimed to improve the catalytic activity and selectivity via incorporation of a potentially hemi-labile donor arm in the ligand motif. Preliminary screening of complexes 1–4 in the methoxycarbonylation of 1-hexene at CO pressure of 60 bar, temperature of 90 °C and Pd:PPh3:HCl:1–hexene molar ratio of 1:2:10:200 was carried out (Scheme 2). Although two mechanisms for methoxycarbonylation, hydride ((Pd–H) and methoxycarbonyl (Pd–OMe) mechanism have been proposed [25,43], majority of reports indicate that the hydride mechanism is favored in which protic acid promoter serves as a hydride source. While organic acid promoters such as para-tolyl sulphonic acid are usually preferred, sometimes they do not give active catalyst systems, hence the use of HCl acid. In addition, PPh3 is usually added to stabilize the catalytic active species [44]. In order to establish the role of the PPh3 additive, we acquired an in situ 31P NMR spectrum of the reaction of complex 1 and PPh3/HCl mixture over a 5 h period (Fig. S11). From the 31P NMR spectrum, the formation of a Pd-PPh3 stabilized intermediate was evident from the signals at 24 ppm and 34 ppm (possible cis and trans-isomers) as given in eq. 1. The stability of the PPh3-adduct was affirmed by the zerochange in the spectra over the 5 h period. The signal at −5 ppm corresponds to the free PPh3, and is not surprising since excess amount of the PPh3 was added (Pd:PPh3 = 1:2).
methyl-2-methylhexanoate (branched product) and methyl heptanoate (linear product), Scheme 2. Supplementary Figs. S6 and S7 show typical GC chromatogram and GC–MS spectra of the products obtained. Cole-Hamiltion et al., have developed palladium complexes of bis(di-tert-butylphosphinomethyl)benzene for methoxycarbonylation of terminal or internal alkenes, which under optimal conditions achieve, 100% and 98% conversions using 1-hexene and 1-octene, respectively [45]. Aguirre and co-workers applied palladium(II) complex with P,Ndonor ligand, 2-(diphenylphosphinoamino)pyridine (Ph2PNHpy), as a catalyst precursor for the methoxycarbonylation of 1-hexene at 75 °C and after 24 h, 43% conversion and a branched to linear ratio of 55:45 were detected [22]. In this work, however, reactions run for 12 h gave percentage conversion of 47%, while increasing longer reaction times from 24 h to 36 h marginally improved the conversions from 91% to 98%, respectively (Table 1, entries 1, 5–6). Thus reaction time of 24 h was chosen as the optimum condition. The role of the complex/ligand structure on the performance of the resultant catalysts was evaluated by comparing the catalytic activities of complexes 1–4 (Table 1). From the data, it is evident that the complex structure significantly influenced the catalytic abilities of these complexes in the methoxycarbonylation of 1-hexene. First, we observed that changing from a Pd-Cl system in 1 to a Pd-Me analogue in complex 2, resulted in a drastic drop in percentage conversion from 91% to 62%, respectively (Table 1, entries 1 and 2). The higher catalytic activity of complex 1 relative to complex 2 could be assigned to the electron withdrawing nature of the coordinated chloride ligand in comparison to the electron donating methyl group in complex 2. This has the overall effect of increasing the rate of coordination of 1-hexene substrate co-
Under these reaction conditions, complexes 1–4 formed active catalysts, affording percentage conversions between 40% and 91% (Table 1, entries 1–4), which are slightly higher when compared to conversions between 65% and 85% reported in our previous study [30]. The major products formed, as identified by GC and GC–MS were
ordination to the active metal atom. Another plausible explanation for the observed higher catalytic activity of complex 1 in comparison to 2, could be the ease of formation of the Pd-H species, believed to be the active species [25] from the dichloride complex 1. In order to understand the reason for the higher catalytic activity of 4
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
results mirror those we recently reported for palladium(II) complexes of (phenoxy)imine ligands bearing alkoxy silane groups in which, under similar conditions, regioselectivities of the ester products towards linear products of 58%-65% were realized [30]. 3.3.2. Effect of reaction substrate on methoxycarbonylation of olefins Having established the viability of complexes 1–4 in the methoxycarbonylation of 1-hexene, we turned our attention to the use of higher olefins; 1-heptene, 1-octene, 1-decene and 1-dodecene using complex 1 (Fig. 2). We observed that increasing the chain length of the olefin substrate significantly decreased the resultant catalytic activity of complex 1 (Fig. 2). For example, with 1-hexene as a substrate, conversions of 90% were obtained, while conversions of 47% and 30% were recorded for 1-octene and 1-decene, respectively. This trend can be attributed to the increase in steric hindrance and higher electron density in higher olefins. These two factors combined have the net effect of limiting substrate coordination to the metal center as discussed hitherto. We recently reported similar trends in the methoxycarbonylation of higher olefins using palladium complexes supported by N^O chelating ligands [30]. Increasing the chain length of olefins was also observed to affect the regioselectivity of the ester products, whereby higher alkenes favoured the formation of branched esters. For example, 39% of the branched esters were obtained for 1-hexene, while 60% was obtained when 1dodecene was used as a substrate (Fig. 2). According to Rodriguez et al., the enhanced formation of branched esters is due to the possible isomerisation of higher alkenes to internal alkenes during the reaction [45].
Scheme 2. Methoxycarbonylation of 1-hexene using complexes 1–4 as catalysts.
the PdCl2 complex 1 in relation to the PdClMe complex, 2, we monitored 1H NMR spectra of the reaction between complex 2 and HCl. It is believed that, the reaction of the PdClMe with HCl is likely to result in the formation of the PdCl2 species, followed by generation of the methane gas. From the 1H NMR spectra obtained over the 5 h period (Fig. S12), the disappearance of the Pd-Me (signal at 0.91 ppm) is evident, with only about 5% present after 5 h. This shows that complex 2 is unstable, and may account for its lower catalytic activity in comparison to complex 1. A more important information could also be derived from examination of the catalytic activities of complexes 1 and 3, bearing the OMe and OH pendant donor groups respectively. The better performance of complex 1, bearing the OMe group may be assigned to its hemi-labile nature [46], that allows for substrate coordination, in addition to stabilizing the active intermediate. It has been reported [46,47] that the OH group strongly coordinates to the palladium atom. This has the negative effect of competing with the 1-hexene substrate, accounting for the observed diminished percentage conversions of 58%. This hypothesis is supported by the much lower conversions of 40% observed for complex 4, bearing the propyl-OMe group [28]. The reduction in the catalytic activity of complex 4 relative to complex 1, could also be attributed to the formation of a more stable six-membered chelate ring compared to the five-membered ring in 1. This has the same effect as the OH group in complex 3 of hindering 1-hexene substrate coordination [30]. Reduced electrophilicity of the palladium metal atom in 4, due to the more electron-donating propyl groups, may also be implicated in its diminished catalytic activities [28]. Generally, the structural moiety of the catalysts did not show any direct influence on the regioselectivity of the ester products, since in all cases regioselectivities of 60%-65% towards linear esters were observed. This is not surprising as the steric environment around the metal atoms appears to be largely comparable owing to the remote proximity of the pendant groups from the metal atom (Fig. 1). These
3.3.3. Density functional theoretical calculations of reactivity parameters for complexes 1–4 In order to gain insight into the observed catalytic behavior of structure of complexes 1–4 and the potential hemilability of the ligands, density functional theory (DFT) calculations were performed on the ground-state electronic structures of the potential cationic intermediates 1a-4a, in which ligands L1-L3 adopt tridentate coordination modes (Table 2). The computations were performed using a split basis set LANL2DZ for Pd(II) and 6311G(dp) for the remaining atoms. One of the key features of hemi-labile ligands is the strength of the bond of the metal-pendant donor atom, relative to the incoming substrate [28]. We thus investigated the relationship between the Pd-O bond length and the resultant catalytic activities of the cationic complexes 1a-4a. From the DFT results, we observed that an increase in PdO bond length generally resulted in increased catalytic activities
Fig. 2. Effect of olefin substrate on % conversion and regioselectivity towards linear esters using complex 1 (0.08 mmol) at [Pd]:[PPh3]:[HCl]:[olefin] ratio of 1:2:10:200, PCO, 60 bar; temp, 90 °C, solvent, methanol/toluene (50 mL); time, 24 h. 5
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al.
Table 2 Theoretical and experimental data for complexes 1a–4a. Complex 1a 2a 3a 4a
a
NBO charge (Pd) a
0.5710(0.368) 0.5650(0.353)a 0.5540(0.364)a 0.5640(0.361)a
Pd-O (Å)
Δε [kcal mol−1]
%Conv
2.1115 2.1754 2. 0945 2.1048
224.42 227.08 226.79 225.06
91 62 58 40
The values in bracket are the NBO charges of neutral complexes 1–4.
(Table 2). For example, Pd–O bond length of 2.1115 Å in 1a and 2.0945 Å in 3a corresponds to conversions of 91% and 58%, respectively. This is consistent with stronger coordination of OH group relative to OCH3 to the palladium(II) atom, thus limiting substrate accessibility to the palladium atom in 3a. A similar trend was observed by Kress et al., in ethylene oligomerization reaction, where the complex containing the OH group is inactive, while the analogous complex, bearing the OCH3 pendant arm, is active [48]. Examination of the Pd-O bond lengths of 2.1115 Å in 1a and 2.1048 Å in 4a and their catalytic activities (91% and 40%, respectively, also revealed a similar trend. Complex 1a contains a five-membered ring, while 4a contains a sixmembered ring, hence the higher catalytic activities observed for 1a may be attributed to its longer bond distance. However, the small difference in bond length of 0.0067 Å may not sufficiently explain the large differences in their percentage conversions of 91% and 40%, suggesting other factors such as electrophilicity of the metal atom may play a role. Thus to further rationalize the experimental results, we examined the charge on the Pd metal atom in the cationic derivatives - (1a-a) and its inherent influence on catalytic activities. Comparison of complexes 1a, 3a and 4a reveals that higher catalytic activities were obtained with an increase in the positive charge of the metal atom. This is expected and has been attributed to enhanced coordination of the substrate to the palladium(II) center [28]. For example, complex 1a, carrying the greatest NBO charge of 0.571 was the most active. The lower NBO charge of complex 2a can be attributed to the methyl group, coordinated to the palladium metal center as opposed to the electron withdrawing chloro ligand in 1a. In addition, the higher conversion of 1a relative to 4a may also be rationalized from the higher NBO charge in 1a of 0.5710 compared to the value of 0.5610 in 4a. The HOMO–LUMO energy gaps were also investigated to study their influence on the catalytic activities of the respective complexes (Table 2). A smaller energy difference between 1-hexene’s HOMO and pre-catalysts 1a–4a’s LUMO should promote coordination of the 1-hexene substrate to the metal center [18]. The results obtained in Table 2 are not in agreement with the expected behaviour [49] since the catalytic activities did not vary linearly with the energy gaps (Δε). Thus the HOMO–LUMO energy gaps did not have a profound effect on the resultant catalytic activities of the complexes. This data implies that the methoxycarbonylation reactions catalyzed by complexes 1–4 were charged controlled, rather than frontier-orbital directed. The DFT data in Table 2, therefore, supports the hemi-labile nature of the ligands, and the role of the pendant group in controlling the respective catalytic activities of the complexes.
4. Conclusions In conclusion, we have prepared three new neutral palladium complexes of (pyridyl)imine ligands. The ligands adopt bidentate coordination mode to give square planar palladium complexes. All the complexes form active catalysts in the methoxycarbonylation of higher olefins to give linear and branched esters. The nature of the ligand motif played a major role in controlling the catalytic performance of the complexes, largely controlled by the coordination abilities of the pendant donor atom. DFT studies support the hemi-lability of the (imino) pyridine ligands in the methoxycarbonylation reactions. In addition, the olefin chain length affected both the catalytic activity and regioselectivity of the methoxycarbonylation reactions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors thank the DST-NRF (South Africa), Center of Excellence in Catalysis (c*change) and the University of KwaZulu-Natal for financial support. Appendix A. Supplementary data Supplementary information contains NMR and GC-MS spectral, spectra for the ligands and complexes, GC chromatograms, GC-MS spectral data of the methoxycarbonylation products, crystal collection and refinement data and CCDC number 1960949 for complex 1. Supplementary data to this article can be found online at https://doi. org/10.1016/j.ica.2019.119270. References [1] S.C. Stouten, T. Noël, Q. Wang, M. Beller, V. Hessel, Catal. Sci. Technol. 6 (2016) 4712–4717. [2] L. Wu, Q. Liu, I. Fleischer, R. Jackstell, M. Beller, Nature Commun. 5 (2014) 3091. [3] K. Dong, X. Fang, S. Gülak, R. Franke, A. Spannenberg, H. Neumann, R. Jackstell, M. Beller, Nature Commun. 8 (2017) 14117. [4] S. Boyde, Green Chem. 4 (2002) 293–307. [5] K. Kumar, J. Darkwa, Polyhedron 138 (2017) 249–257. [6] R. Franke, D. Selent, A. Börner, Chem. Rev. 112 (2012) 5675–5732. [7] J.R. Zbieg, E. Yamaguchi, E.L. McInturff, M.J. Krische, Science 336 (2012) 324–327. [8] P.W. Van Leeuwen, C. Claver, Rhodium Catalyzed Hydroformylation, Springer
6
Inorganica Chimica Acta xxx (xxxx) xxxx
Z. Zulu, et al. Science & Business Media, 2002. [9] J. Liu, Z. Wei, H. Jiao, R. Jackstell, M. Beller, ACS Centr. Sci. 4 (2017) 30–38. [10] G. Kiss, Chem. Rev. 101 (2001) 3435–3456. [11] J.D. Nobbs, C.H. Low, L.P. Stubbs, C. Wang, E. Drent, M. van Meurs, Organometallics 36 (2016) 391–398. [12] M. Schmidt, T. Pogrzeba, L. Hohl, A. Weber, A. Kielholz, M. Kraume, R. Schomäcker, Mol. Catal. 439 (2017) 1–8. [13] X.-F. Wu, X. Fang, L. Wu, R. Jackstell, H. Neumann, M. Beller, Acc. Chem. Res. 47 (2014) 1041–1053. [14] K. Dong, R. Sang, X. Fang, R. Franke, A. Spannenberg, H. Neumann, R. Jackstell, M. Beller, Ang. Chem. Int. Ed. 129 (2017) 5351–5355. [15] A. Brennführer, H. Neumann, M. Beller, ChemCatChem 1 (2009) 28–41. [16] J. Durand, B. Milani, Coord. Chem. Rev. 250 (2006) 542–560. [17] K. Dong, R. Sang, Z. Wei, J. Liu, R. Dühren, A. Spannenberg, H. Jiao, H. Neumann, R. Jackstell, R. Franke, Chem. Sci. 9 (2018) 2510–2516. [18] G.S. Nyamato, S.O. Ojwach, M.P. Akerman, Dalton Trans. 45 (2016) 3407–3416. [19] C. Bianchini, A. Meli, Coord. Chem. Rev. 225 (2002) 35–66. [20] T.A. Tshabalala, S.O. Ojwach, Trans. Met. Chem. 43 (2018) 339–346. [21] T.A. Tshabalala, S.O. Ojwach, M.A. Akerman, J. Mol. Catal. A: Chem. 406 (2015) 178–184. [22] P.A. Aguirre, C.A. Lagos, S.A. Moya, C. Zúñiga, C. Vera-Oyarce, E. Sola, G. Peris, J.C. Bayón, Dalton Trans. (2007) 5419–5426. [23] J. Liu, H. Li, R. Dühren, J. Liu, A. Spannenberg, R. Franke, R. Jackstell, M. Beller, Angew. Chem. Int. Ed. 129 (2017) 12138–12142. [24] P.J. Deckers, B. Hessen, J.H. Teuben, ngew. Chem. Int. Ed. 40 (2001) 2516–2519. [25] C. Arderne, l.A. Guzei, C.W. Holzapfel, T. Bredenkamp, ChemCatChem 8 (2016) 1084–1093. [26] C.M. McGuirk, J. Mendez-Arroyo, A.M. Lifschitz, C.A. Mirkin, J. Ame. Chem. Soc. 136 (2014) 16594–16601. [27] A.E. Díaz-Álvarez, C. Vidal, F.J. Suárez, J. Díez, V. Cadierno, P. Crochet, Organometallics 34 (2015) 3670–3677.
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
7
G.S. Nyamato, S.O. Ojwach, M.P. Akerman, Organometallics 34 (2015) 5647–5657. S. Mecking, W. Keim, Organometallics 15 (1996) 2650–2656. S.O. Akiri, S.O. Ojwach, Inorg. Chim. Acta 489 (2019) 236–243. D. Drew, J. Doyle, A.G. Shaver, Inorganic Syntheses: Reagents for Transition Metal Complex and Organometallic Syntheses, 28 (1990) 346–349. R.E. Rulke, J.M. Ernsting, A.L. Spek, C.J. Elsevier, P.W. van Leeuwen, K. Vrieze, Inorg. Chem. 32 (1993) 5769–5778. Bruker, in, Madison, Wisconsin, USA, 2012. G.M. Sheldrick, in, University of Göttingen, Göttingen, Germany, 2016. L.J. Farrugia, Appl. Cryst. 45 (2012) 849–854. M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. Petersson, Inc.: Wallingford, CT, USA, (2009). J. Cloete, S.F. Mapolie, J. Mol. Catal. A Chem. 243 (2006) 221–225. W. Zhang, W.-H. Sun, B. Wu, S. Zhang, H. Ma, Y. Li, J. Chen, P. Hao, J. Organomet. Chem. 691 (2006) 4759–4767. J.M. Benito, E. de Jesús, F.J. de la Mata, J.C. Flores, R. Gómez, P. Gómez-Sal, Organometallics 25 (2006) 3876–3887. T.V. Laine, M. Klinga, M. Leskelä, Eur. J. Inorg. 1999 (1999) 959–964. I.J. Bruno, J.C. Cole, M. Kessler, J. Luo, W.S. Motherwell, L.H. Purkis, B.R. Smith, R. Taylor, R.I. Cooper, S.E. Harris, J. Chem. Info. Comp. Sci. 44 (2004) 2133–2144. D. Kim, Y. Song, S. Kim, H.-J. Lee, H. Lee, J. Coord. Chem. 67 (2014) 2312–2329. I.A. Shuklov, N.V. Dubrovina, J. Schulze, W. Tietz, K. Kühlein, A. Börnera, Arkivoc (2012) 66–75. M. Carvajal, G. Miscione, A. Accardi, J. Novoa, A. Bottoni, Mol. Phys. 104 (2006) 805–831. C.J. Rodriguez, D.F. Foster, G.R. Eastham, D.J. Cole-Hamilton, Chem. Commun. (2004) 1720–1721. P.-Y. Shi, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Organometallics 21 (2002) 3203–3207. M.K. Ainooson, I.A. Guzei, L.C. Spencer, J. Darkwa, Polyhedron 53 (2013) 295–303. S.P. Meneghetti, P.J. Lutz, J. Kress, Organometallics 18 (1999) 2734–2737. P. Huo, W. Liu, X. He, H. Wang, Y. Chen, Organometallics 32 (2013) 2291–2299.