Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers Sanghamitra Das, Vasudevan Subramaniyan, and Ganesan Mani* Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India 721 302

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S Supporting Information *

ABSTRACT: Unsymmetrical pincers have been shown to be better than the corresponding symmetrical pincers in several catalysis reactions. A new unsymmetrical PNN propincer, 2(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)pyrrole (1), was synthesized from pyrrole through Mannich bases in a good yield. In addition, the new byproduct 2-(3,5dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)-N(hydroxymethyl)pyrrole was also isolated. The reaction of 1 with [PdCl2(PhCN)2] and Et3N in toluene yielded [PdCl{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}] (2). The analogous reaction between 1 and [NiCl2(DME)] or NiX2 (X = Br, I) in the presence of NEt3 in acetonitrile afforded [NiX{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl2 were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited far greater activity on the order of 107 g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer palladium and nickel complexes. This trend was also supported by the iPr group substituted PNP nickel and palladium pincer complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of NMR studies and suggested that the activity increases as the Npyrazole donor increases. Polymers were characterized by 1H NMR, IR, TGA, and powder XRD methods.

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INTRODUCTION Pincer ligands often coordinate to metal atoms in a tridentate fashion and are classified on the basis of the ligating atoms such as PNP, PCP, and NCN among others.1 When the donor atoms are different, typically soft and hard donors, they are usually called unsymmetrical pincers such as PCN, PCO, and PCS.2 In addition, pincers containing the same donor atoms but differ by substituents or having different groups on the periphery of ligand are also called unsymmetrical.3 The main attractive feature of a unsymmetrical pincer is the hemilability: that is, its ability to showcase the bidentate binding mode with a spectator arm.4 This coordination flexibility is one of the key factors by which metal complexes can be judged to be potential precatalysts and to exhibit unique reactivities.5 This, therefore, became a driving force, and a variety of unsymmetrical pincers, their metal complexes, and catalytic properties have been studied.6 Milstein and co-workers have demonstrated the better catalytic activity of the unsymmetrical PNN pincer in the dehydrogenation of primary alcohols to esters and H27 and in the hydrogenation of esters to alcohols.8 Gebbink, Szabó, and co-workers have shown higher turnover numbers for the PCS unsymmetrical pincer in comparison to PCP and SCS ligands in aldol reactions.9 Huang and co-workers have © XXXX American Chemical Society

reported that the benzoquinoline-based NCP pincer iridium complex catalyzes more efficiently the transfer hydrogenation of alkene using ethanol in comparison to the [(PCP)Ir(H)(Cl)] complex.10 Shubina and co-workers have described the superior catalytic behavior of a PCN pincer iridium complex containing a pyrazole N-donor relative to the PCP pincer in the amine−borane dehydrogenation reaction.11 We have been working on pyrrole-based symmetrical NNN12 and PNP13 pincers having pyrazole and diphenylphosphine units on either side of the pyrrole ring. Since these reports, its been our goal to synthesize unsymmetrical pincer with the pyrrole ring. Remarkably, the pyrrole-based pincer ligands have become famous owing to their abilities in stabilizing a variety of metal complexes14 and their catalytic properties.15 Herein, we report a follow-up study detailing the synthesis of the first pyrrole-based unsymmetrical pincer containing mixed donor atoms (N and P) derived from the diphenylphosphine and pyrazole units and its Pd(II) and Ni(II) complexes. We also report their efficiencies at different temperatures in catalyzing norbornene (NB) to polynorborReceived: December 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b03562 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Synthesis of the Unsymmetrical PNN Pincer 6 and New Byproducts Formed in the Course of Its Synthesis

Scheme 2. Synthesis of Unsymmetrical PNN Pincer Pd(II) and Ni(II) Complexes

spectrum. The 31P NMR spectrum showed a singlet at δ 49.9 ppm, which is shifted downfield in comparison to δ 33.8 ppm for the symmetrical pincer palladium complex [PdCl{C4H2N2,5-(CH2PPh2)2-κ3PNP}].13 The coordination-induced chemical shift change is 65.3 ppm in comparison to the free ligand (δ −15.4 ppm), which is larger than that of 50.1 ppm of the symmetrical pincer palladium complex. The κ3-PNN bonding mode is confirmed by the X-ray structure of 8 given in Figure 1 along with selected bond lengths and angles. The geometry around the palladium atom is a distorted square plane formed by two different palladacycles, five- and six-membered, containing P and N donors trans to each other. Interestingly, the twist angle between the pyrrole ring and the palladium square planes is 26.2°, which

nene (PNB) to demonstrate that the unsymmetrical PNN pincer is better than the PNP pincer.

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RESULTS AND DISCUSSION Synthesis of the Unsymmetrical PNN Pincer. The synthesis of the unsymmetrical PNN pincer 6 involves first the preparation of the mono-Mannich base 2 from pyrrole followed by the synthesis of 2-(3,5-dimethylpyrazolylmethyl)pyrrole (3) and 2-(dimethylaminomethyl)-5-(3,5dimethylpyrazolylmethyl)pyrrole (4). Following the synthetic procedure of 2,5-bis(diphenylphosphinomethyl)pyrrole (PNP),13 treatment of 4 with 1 equiv of Ph2PH in toluene under reflux conditions afforded 6 in 60% yield after column chromatographic separation (Scheme 1). In addition, the Mannich reaction of 3 also gave the N-hydroxymethyl derivative 5 as a minor product along with 4. The yield of 5 was improved to 76% when 3 was treated with 2 equiv of HCHO. 4−6 are new compounds and were characterized by spectroscopic and HRMS methods. The methylene groups present in 4−6 feature strikingly distinct peaks in their 1H and 13 C NMR spectra. The 31P NMR spectrum of 6 in CDCl3 showed a singlet at δ −15.4 ppm, which is slightly shifted downfield by 0.9 ppm in comparison to the symmetrical diphosphine ligand 2,5-bis(diphenylphosphinomethyl)pyrrole. Further, treatment of 6 with aqueous H2O2 in toluene afforded the oxidized compound 7, which gives a singlet at δ 31.2 ppm in the 31P NMR spectrum recorded in CDCl3. Metalation and Structural Characterizations. The equimolar reaction of 6, [PdCl2(PhCN)2], and Et3N in toluene afforded the new air-stable unsymmetrical pincer palladium complex 8 in good yield (Scheme 2). The same reaction in the absence of Et3N also gave complex 8, as shown by the 31P NMR spectrum, in which the product is formed by the deprotonation of the pyrrolic NH with concomitant formation of HCl. In the 1H NMR spectrum of 8 in CDCl3, one of the methylene groups appeared as a doublet at δ 3.65 ppm owing to the coupling with the phosphorus atom (2J(P,H) = 13.2 Hz), which was absent in the free ligand

Figure 1. ORTEP diagram of 8 with 50% probability ellipsoids. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): P1−Pd1 2.2274(18), N3−Pd1 1.983(5), N1−Pd1 2.133(5), Cl1−Pd1 2.3174(18); N1−Pd1−N3 87.4(2), N1−Pd1−P1 164.87(15), N3−Pd1−P1 79.73(15), N1−Pd1−Cl1 95.74(15), N3−Pd1−Cl1 172.84(16), P1−Pd1−Cl1 97.96(7). B


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Inorganic Chemistry lies between the values found for the symmetrical NNN (38.9°)12 and PNP (13.9°)13 pincer palladium complexes. The P1−Pd−N1 angle of 164.87(15)° also remains between the values found for the symmetrical complexes (172.7(3)° (NNN)12 and 162.3(1)° (PNP)).13 The same trend is seen in the reported analogous Pd(II) complexes. The P−Pd−N angle in the unsymmetrical [(PCN)PdCl] complex containing one phosphinite and one pyrazole donor is 166.6°,16 which is greater than the value (∼160.3°) found in the symmetrical PCP-bis(phosphinite) palladium complexes17 but is lower than the values found in NCN-bis(pyrazolylmethyl)benzene (177.3°) 18 and bis(dimethylpyrazolylmethyl)pyridine (175.8°)19 Pd(II) complexes. In addition, the Pd−Npyrazole bond distance of 2.133(5) Å is slightly longer than that (2.041(5) Å) found in the NNN pincer palladium complex. Conversely, the Pd−P distance of 2.2274(18) Å is slightly shorter than that (2.3074(16) Å) of the PNP pincer complex, indicating a stronger bond in 8. This bond length difference could be due to the strong trans effect of P in comparison to N.4a,20 The pyrrolide N−Pd distance of 1.983(5) Å is in the range reported for the symmetric complexes [(PNP)PdCl] (1.981(4) Å),13 [(NNN)PdCl] (1.958(7) Å),12 and [2,5bis(N-cyclohexyl-1-(p-tolyl)methanimine)pyrrolide)PdCl] (1.879(3) Å).21 It is close to that of an isoindolide nitrogen coordinated palladium complex (1.972(3) Å).22 However, it is shorter than the amide N−Pd distance found in the analogous pincer palladium(II) complex [PdCl{N(SiMe2CH2PPh2)2}] (2.063(2) Å),23 indicating the strong bond formed by the pyrrolide nitrogen atom. The analogous 1:1 reaction between 6 and [NiCl2(DME)] or NiX2 (X = Br, I) in acetonitrile in the presence of NEt3 under reflux conditions afforded the pincer nickel complexes 9−11 in good yields (Scheme 2). These complexes are diamagnetic, air stable, and soluble in toluene, CH2Cl2, CHCl3, ether, THF, and acetone and have been characterized by both spectroscopic and X-ray methods. Their 1H spectra featured a doublet around δ 3.4 ppm owing to the coupling with the phosphorus atom for the phosphorus-CH2 protons, whereas the pyrazole-CH2 protons appeared as a singlet. The 31P NMR spectra of 9−11 displayed singlets at δ 30.6, 35.0, and 44.6 ppm, respectively, which are, unlike the palladium complex 8, close to the values (δ 30.6, 35.0, and 42.6 ppm) shown by their corresponding symmetrical PNP nickel complexes.13 The X-ray structure of 10 is given in Figure 2 together with selected bond distances and angles, and that of 11 is given in Figure S36 in the Supporting Information. The X-ray structure revealed the typical pincer structures containing two fused rings created by the pyrazole and phosphorus arms. The geometry around the nickel atom in both complexes 10 and 11 is not a perfect square plane; it is distorted toward a tetrahedron, as shown by their trans angles ranging from 160.6(2) to 166.2(2)°. Thus, the nickel atom is located 0.041 and 0.064 Å above the nickel square plane defined by the coordinated atoms in 10 and 11, respectively, owing to the steric conflict between the pyrazole methyl and halide atoms. In both structures, the pyrazole arm lies well above the pyrrole ring mean plane with an N1−N2−C6−C7 torsion angle of 51.3° (10), while the Ph2P arm remains slightly below the pyrrole ring plane with a P1−C11−C10−N3 torsion angle of −24.0° (10), owing to the steric hindrance among the phosphorus phenyl, pyrazole methyl, and the halide groups. The twist angle between the mean pyrrole ring and the nickel square planes in 10 is 26.1°, higher than that (27.2°) found in

Figure 2. ORTEP diagram of 10 with 50% probability ellipsoids. H atoms and crystal lattice diethyl ether are omitted for clarity. Selected bond lengths (Å) and angles (deg): P1−Ni1 2.1613(11), N3−Ni1 1.861(3), N1−Ni1 1.969(3), Br1−Ni1 2.3215(7); N1−Ni1−N3 91.12(13), N1−Ni1−P1 166.06(11), N3−Ni1−P1 82.77(10), N1− Ni1−Br1 97.50(10), N3−Ni1−Br1 162.47(10), P1−Ni1−Br1 91.85(4).

11. In addition, both twist angles are larger than those (13.8°) found in [(PNP)NiX] (X = Br, I) complexes containing the symmetric PNP pincer ligand.13 Interestingly, Ni−P distances (2.1613(11) Å (10) and 2.1647(19) Å (11)) are shorter than the distances (2.253(1)−2.1955(9) Å) found in the nickel(II) pincer complexes of type [(PNP)NiX] (X = halides) containing symmetrical PNP ligands,13,14c,f,15a,24 owing to the strong trans effect of the P atom in comparison to the N atom. Further, the pyrrolide N−Ni distances (1.861(3) Å (10) and 1.847(6) Å (11)) are shorter than the amide N−Ni distances found in the analogous pincer nickel(II) complexes (1.924(2) Å in [NiCl{N(SiMe2CH2PPh2)2}]23 and 1.895(3) and 1.912(5) Å in [NiX{N(o-C6H4PPh2)2}] (X = Cl, Br)),25 indicating the strong bond formed by the pyrrolide N atom. Vinyl Polymerization of Norbornene. The vinyl norbornene polymer has several applications and has a cost advantage over related materials.26 Hence, a variety of precatalyst metal complexes have been developed, among which palladium27 and nickel28 complexes have been shown to possess the highest activities29 and even catalyze polymerization of polar group substituted norbornene and copolymerization reactions.30 Remarkably, the polymerization of polar norbornenes by the cationic allyl palladium complex in the absence of any cocatalyst has also been reported.31 Recently, we reported the highly active palladium complexes of o-bis(3,5-dimethylpyrazolylmethyl)phenolate for the vinyl norbornene polymerization reactions.32 The norbornene polymerization behaviors of the unsymmetrical PNN pincer complexes 8−11 were studied and compared with those of symmetrical NNN and PNP pincer palladium and nickel complexes to demonstrate the difference caused by a change in the donor atoms. The polymerization reaction with 1 μmol of the precatalyst 8 in the presence of a 1000 molar excess of MMAO in toluene yielded 28% of PNB (Table 1, entry 1). A dramatic improvement in the yield of PNB (97%) was obtained when the amount of catalyst 8 was increased to 2 μmol (entry 2). In addition, further improved activity was found when the concentration of NB was doubled under these conditions (entry 3). In contrast, when the C


Article

Inorganic Chemistry Table 1. Polymerization of Norbornene Using the Palladium Complex 8 in the Presence of Cocatalyst MMAOa

Table 2. Polymerization of Norbornene Using the Nickel Complexes 9−11 in the Presence of MMAOa

entry

8 (μmol)

NB (g)

[Al]/[8]

PNB (g)

yield (%)

activity (×107)d

1b 2b 3b 4b 5b 6c 7c 8c 9c 10c

1.0 2.0 2.0 2.0 2.0 1.0 2.0 2.0 2.0 2.0

1 1 2 2 2 2 2 3 2 2

1000 1000 1000 500 250 1000 1000 1000 500 250

0.284 0.968 1.679 0.254 0.036 0.370 1.97 2.256 0.727 0.264

28 97 84 13 2 19 99 75 36 13

0.1 0.6 1.7 0.8 0.1 2.2 5.9 7.0 2.2 0.8

entry

cat. (μmol)

[Al]/[cat.]

PNB (g)

yield (%)

activity (×107)b

1 2 3c 4d 5 6 7 8 9 10 11e 12 13 14 15

10 (1.0) 10 (1.0) 10 (1.0) 10 (1.0) 10 (1.0) 10 (1.0) 10 (2.0) 10 (3.0) 10 (1.0) 10 (0.5) 10 (1.0) 9 (1.0) 9 (0.5) 11 (1.0) 11 (0.5)

500 1000 1000 1000 1500 2000 500 333 200 1000 1000 1000 1000 1000 1000

1.543 1.614 1.085 0.881 1.537 1.21 1.497 1.486 0.749 1.452 0.495 1.270 1.266 1.491 0.863

77 81 54 44 77 60 75 74 37 73 25 63 63 74 43

9.3 9.7 6.5 5.3 9.3 7.3 4.7 3.0 4.5 17.5 3.0 7.6 15.3 9.0 10.4

a

All reactions were carried out with 2 g of NB in 5 mL of CH2Cl2 at room temperature for 1 min unless specified otherwise. bIn units of g of PNB mol−1 h−1. c10 mL of CH2Cl2. d20 mL of CH2Cl2. eIn toluene.

a

All reactions were carried out at room temperature for 1 min, except entries 1−3, for which the reaction times were 15, 5, and 3 min, respectively. bReaction carried out in 5 mL of toluene. cReaction carried out in 5 mL of CH2Cl2. dIn units of g of PNB mol−1 h−1.

Table 3. Polymerization of Norbornene Using the Palladium Complex 8 and Nickel Complexes 9−11 with EtAlCl2 as a Cocatalysta

aluminum ratio was decreased, the yield of polymer was also decreased, which could be due to a decrease in chain transfer rate (vide infra). When the polymerization was carried out in CH2Cl2, higher yield and activity were obtained (Table 1, entries 6−10). The reactions in CH2Cl2 showed the same trend of decrease in activity with a decrease in the aluminum content, consistent with the case for the toluene reactions. Notably, the reaction in CH2Cl2 gave a near-quantitative yield (99%) of polynorbronene with an activity of 5.9 × 107 g of PNB mol−1 h−1 (entry 7). This is greater than the 84% yield with activity 1.7 × 107 g of PNB mol−1 h−1 from the toluene reaction. The polymerization behavior of the nickel complexes 9−11 is summarized in Table 2. The reaction with the aluminum to complex 10 mole ratio of 500 in CH2Cl2 gave a 77% yield of PNB with an activity of 9.3 × 107 g of PNB mol−1 h−1 (entry 1). When the aluminum ratio is increased to 1000, both the yield and activity are increased slightly (entry 2). However, a further increase in the aluminum content (ratios of 1500 and 2000) resulted in decreased yields and activities. In addition, the yields and activities decreased with an increased amount of complex 10 (entries 7 and 8) or a lower amount of aluminum ([Al]/[M] = 200) (entry 9) or lower concentration of norbornene (entries 3 and 4). Gratifyingly, an excellent activity of 1.7 × 108 g of PNB mol−1 h−1 (entry 10) was obtained with 0.5 μmol of complex 10, indicating its highly active nature. Conversely, the polymerization reaction in toluene gave poor yield and activity, as observed with palladium pincer complex 8. In addition, as shown in Table 2, although similar polymerization behaviors were obtained with the chloride 9 and iodide complex 11, the bromide complex 10 remains a better catalyst in terms of greater yields and activities. Using the optimized conditions obtained with MMAO reactions, the polymerization reactions with EtAlCl2 as a cocatalyst were carried out, and the results are given in Table 3. When EtAlCl2 was added to the mixture of palladium complex

entry

cat. (μmol)

PNB (g)

yield (%)

activity (×107)d

b

8 (2.0) 8 (2.0) 9 (1.0) 9 (1.0) 10 (1.0) 10 (1.0) 11 (1.0) 11 (1.0) [(NNN)PdCl] (2.0) [(PNP)PdCl] (2.0) [(PNP)NiBr] (1.0) [(iPrPNP)NiBr] (1.0)

0.893 1.446 1.418 0.897 1.872 0.565 1.876 0.864 1.976 0.471e 1.174e 0.972e

45 72 71 45 94 28 94 43 99 24 59 49

2.7 4.4 8.5 5.4 11.3 3.4 11.3 5.2 5.9 1.4 7.1 5.8

1 2c 3b 4c 5b 6c 7b 8c 9c 10c 11b 12b a

All reactions were carried out with 2 g of NB and [MMAO]/[cat.] = 1000 at room temperature for 1 min. bIn 5 mL of CH2Cl2. cIn 5 mL of toluene. dIn units of g of PNB mol−1 h−1. egel-like solid dried in open air only.

and norbornene in CH2Cl2, the solution turned black, probably due to palladium metal particles, and hence gave lower activity in comparison to the MMAO reactions discussed above. Conversely, in the cases of nickel complexes, the addition of EtAlCl2 resulted in a color change to yellow and yielded polymer with >90% monomer conversion in CH2Cl2 (entries 5 and 7). Both the palladium- and nickel-catalyzed reactions in toluene afforded relatively lower yields. In addition, the activity shown by [(NNN)PdCl] is greater than those of PNN and PNP palladium complexes (vide infra). Having observed excellent activities of the unsymmetrical pincer Pd(II) and Ni(II) complexes, we became interested in studying the effect of the pincer framework on the norbornene polymerization reactions. The catalytic performances of the symmetrical pincer complexes chloro[2,5-bis(3,5dimethylpyrazolylmethyl)pyrrolido]palladium(II) ([(NNN)PdCl]), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II) ([(PNP)PdCl]), chloro[2,5-bisD


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Inorganic Chemistry

Table 4. Comparison of the Catalytic Norbornene Polymerization Performances of the Symmetrical (NNN and PNP) as Well as Unsymmetrical (PNN) Pyrrole-Based Pincer Palladium(II) and Nickel(II) Complexesa entry

cat.

solvent

time (min)

quantity of cat. (μmol)

PNB (g)

yield (%)

activity (×107)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

[(NNN)PdCl] [(NNN)PdCl] [(NNN)PdCl] [(PNN)PdCl] [(PNN)PdCl] [(PNN)PdCl] [(PNP)PdCl] [(iPrPNP)PdCl] [(PNN)NiCl] [(PNP)NiCl] [(PNN)NiBr] [(PNP)NiBr] [(PNP)NiBr] [(iPrPNP)NiBr] [(iPrPNP)NiBr] [(PNN)NiI] [(PNP)NiI] [NiCl2(DME)] [NiCl2(DME)] NiBr2 NiBr2 PdCl2 [PdCl2(PhCN)2] [PdCl2(COD)]

CH2Cl2 CH2Cl2 toluene CH2Cl2 CH2Cl2 toluene CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

1 1 1 1 1 3 1 1 1 1 1 1 10 1 10 1 1 1 10 1 10 1 1 1

1.0 2.0 2.0 1.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.808 1.998 1.572 0.370 1.97 1.679 0.281 0.221 1.270 0.314 1.614 0.473 0.849 0.189 0.768 1.491 0.689 0.542 0.680 0.087 0.451 0.156 0.408 0.354

90 100 79 19 99 84 14 11 64 16 81 24 42 9 38 75 34 27 34 2 22 8 20 18

10.9 6.0 4.7 2.2 5.9 1.7 0.8 0.7 7.6 1.9 9.7 2.8 0.5 1.1 0.5 0.9 4.1 3.3 0.4 0.5 0.3 0.9 2.5 2.1

All reactions were carried out with 2 g of NB in 5 mL of solvent with [MMAO]/[cat.] = 1000 at room temperature. bIn units of g of PNB mol−1 h−1. a

more space for the monomer binding and facilitates chain growths. Therefore, greater activities and yields were obtained with the pyrazole-substituted pincer metal complexes. In order to confirm the role of steric factors in causing the activity difference, we synthesized the reported complexes chloro[2,5bis(diisopropylphosphinomethyl)pyrrolido]palladium(II) ([(iPrPNP)PdCl]) and bromo[2,5-bis(diisopropylphosphinometh yl ) py r ro lido ]n ic k el ( II) ([(iPrPNP)NiCl]) and carried out the polymerization reactions. It is found that the yield (11%) and activity (0.7 × 107 g of PNB mol−1 h−1) of the isopropyl-substituted pincer palladium complex (entry 8) are relatively lower than the phenyl-substituted PNP pincer palladium complex activity (0.8 × 107 g of PNB mol−1 h−1) and yield (14%) (entry 7). This is further supported by the nickel complex reactions. The phenylsubstituted pincer nickel complex gave a 23% yield of PNB, whereas the iPr group substituted nickel complex yielded a negligible amount of polymer. This activity difference is conceivable, given the Tolman cone angle (θ) of iPr (114°) being greater than the phenyl group’s cone angle (105°).33 In addition, a similar trend was also observed for EtAlCl2 cocatalyst reactions (see Table 3). Furthermore, polymerizations using [NiCl 2 (DME)], NiBr 2 , PdCl 2 , and [PdCl2(PhCN)2] in CH2Cl2 gave poor yields of polynorbornene under the same conditions, indicating the important role of the pincer ligands in stabilizing active metal centers involved in the course of polymerization cycles. Effect of Temperature on NB Polymerization. The effect of temperature on NB polymerization is summarized in Table 5. In the cases of [(PNN)PdCl]- and [(NNN)PdCl]catalyzed reactions, the yield and activity of PNB decreased as the temperature was increased from room temperature to 60

(diphenylphosphinomethyl)pyrrolido]nickel(II) ([(PNP)NiCl]), bromo[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) ([(PNP)NiBr]), and iodo[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) ([(PNP)NiI]) together with those of the pincer complexes [(iPrPNP)PdCl]24a and [(iPrPNP)NiBr],14f containing the diisopropylphosphinomethyl group , are given in Table 4. The quantitative conversion of norbornene to polynorbornene was observed with 2 μmol of the symmetrical NNN pincer palladium complex in CH2Cl2 (entry 2, Table 4). Although a similar yield (99%) was obtained with the unsymmetrical PNN palladium complex 8, the yield of polynorbornene was very low (14%) when the symmetrical PNP palladium complex was used under the same conditions. The activity difference is then attributed to the number of pyrazole N donors present in the pincer complexes. The activity decreases as the pyrazole ring is replaced with a PPh2 group in the NNN pincer ligand. This decreasing trend was also found with the corresponding nickel pincer complexes. The yields of polymer (64, 81, and 75%) given by the unsymmetrical nickel complexes 9−11, respectively, are greater than those (16, 24, and 34%) given by the symmetrical PNP nickel pincer complexes. Hence, the activity decrease with the PNP- or PPh2-substituted pincer metal complexes can be explained by steric crowding around the active metal atom. The substituents attached to the phosphorus atom have a three-dimensional orientation, which effectively shields the active metal center; as a result, the sterically encumbered norbornene monomer finds it difficult to bind for subsequent transformations, resulting in the decreased activity. On the other hand, the pyrazole rings in either the NNN or PNN ligand are oriented in one direction only; hence, this creates E


Article

Inorganic Chemistry Table 5. NB Polymerization at Varied Temperaturesa entry

[Al]/[cat.]

1 2 3 4 5 6

1000 1000 1000 500 400 400

7 8 9 10

1000 1000 1000 1000

11 12 13 14

1000 1000 1000 1000

15 16 17 18 19 20 21 22 23 24

1000 1000 1000 1000 100 100 200 300 400 500

25 26 27 28 29 30

1000 1000 1000 1000 400 400

PNB (g)

yield (%)

activity (×106)b

[cat.] = [(PNN)PdCl] (2.0 μmol) 1.218 61 2.4 1.755 88 3.5 0.578 29 1.2 0.397 20 0.8 0.390 19 0.8 0.147 7 0.3 [cat.] = [(NNN)PdCl] (2.0 μmol) 1.608 80 3.2 1.998 >99 4.0 0.384 19 0.8 0.114 6 0.2 [cat.] = [(PNP)PdCl] (2.0 μmol) 0.214 11 0.4 0.382 19 0.8 0.820 41 1.6 0.566 28 1.1 [cat.] = [(PNN)NiBr] (1.0 μmol) 0.515 26 2.1 0.695 35 2.8 0.974 49 3.9 1.335 67 5.3 0.405 20 0.4 0.559 28 0.6 0.688 34 2.7 0.950 47 3.8 1.231 61 4.9 1.310 65 5.2 [cat.] = [(PNP)NiBr] (1.0 μmol) 0.318 16 1.3 0.451 22 1.8 0.895 45 3.6 1.286 64 5.1 0.389 19 1.5 0.881 44 3.5

lower than those given by PNN and NNN pincer complexes at room temperature. In these palladium-catalyzed high-temperature reactions, the solution turned black, probably indicating decomposition of the catalyst and formation of palladium metal particles, which accounts for the lower yield and activity for the complexes containing the more N donating PNN and NNN pincers. In contrast to this, the more P donating PNP pincer palladium complex was able to stabilize adequately the active species formed at 60 °C to give improved activity. In the case of [(PNN)NiBr], the yield and activity of PNB increased as the temperature was increased from 0 to 100 °C for the given aluminum mole ratio (Figure 3B). The same trend was observed when the aluminum content was increased from a 100 to 1000 molar ratio for a given temperature (100 °C). In addition, the [(PNP)NiBr] pincer also showed an increased activity with temperature, and its activity at 100 °C is quite close to that of the unsymmetrical PNN complex, as given in Figure 3B. NMR Study and Rationalization of Polymerization. These observations can be explained if the PNN and NNN pincers are regarded as sterically less encumbered in comparison to the PNP pincer and are readily able to form an equilibrium between the precatalyst and MMAO to give an activated complex formed via the alkyl−halide exchange reaction, as indicated in Chart 1. A similar equilibrium was suggested to explain the increased activity in the presence of a large excess of MAO or with an increase in temperature for NB or olefin polymerization reactions.34 The 31P NMR spectrum of a mixture of 10 and MMAO (10 or 100 equiv) in toluene-d8 showed only one peak around δ 38.0 ppm, and when the amount of MMAO is 4 equiv, it appeared at δ 33.5 ppm. This is in the range found for 10, [(PNP)NiBr], and its alkyl derivatives.14c Further, its 1H NMR spectrum featured a doublet around δ 3 ppm for the P-bound methylene protons with the coupling constant J(PH) = 11.6 Hz similar to the 1H NMR spectrum of 10, in addition to shifts of other protons in the PNN ligand. Furthermore, the 31P NMR spectrum of a mixture of PNN and MMAO (10 equiv) in toluene-d8 showed a peak at δ −11.2 ppm, which did not appear for the above mixtures. This suggests that the nickel pincer complex did not decompose in the presence of MMAO to give the free PNN. Similar 1H and 31P NMR spectra were obtained for a mixture of 8 and MMAO (10 equiv). These strongly suggest that the chelation bonding mode of PNN

T (°C) 0 25 60 60 60 100 0 25 60 100 0 25 60 100 0 25 60 100 60 100 100 100 100 100 0 25 60 100 60 100

a

All reactions were carried out with 2 g of NB in 5 mL of toluene for 15 min. bIn units of g of PNB mol−1 h−1.

and 100 °C for the given aluminum ratio (400 or 1000 molar excess), as shown in Figure 3A. Conversely, [(PNP)PdCl] exhibited increased yield and activity at 60 °C in comparison to its room-temperature activity. However, the yield is decreased considerably at 100 °C and its best activity is still

Figure 3. Effect of temperature on polymerization behaviors of NNN, PNN, and PNP pincer Pd and Ni complexes with an [Al]/[M] ratio of 1000. F


Article

Inorganic Chemistry Chart 1. Proposed Activation Equilibrium of Precatalyst in the Presence of MMAO To Give an Active Catalysta

The symbol □ denotes a vacant site for NB binding and subsequent transformations.

a

using the phosphine arm (κ2P,Npyrrole) is retained in the presence of an excess amount of MMAO. The 31P NMR spectrum of a mixture of 10, MMAO (100 equiv), and NB (2 equiv) in toluene-d8 showed two peaks, δ 37.7 (s) and −4.9 (br s) ppm. A similar spectrum was obtained even after the same mixture was heated at 60 °C. Hence, the new broad peak around −5 ppm could be due to a new nickel complex containing both NB and PNN pincer formed in the course of catalysis. Its 1H NMR spectrum showed mainly peaks due to MMAO protons, as it is in a very large excess amount and the bound NB protons could not be identified. Conversely, the 31P NMR spectrum of the palladium complex 8 in the presence of 100 equiv of MMAO and 2 equiv of NB showed more peaks, indicating decomposition. Although it is difficult to suggest the coordination of the pyrazole N to the aluminum atom in MMAO from the change in the chemical shift value of PNN, the M−Npyrazole bond distances in both Ni and Pd complexes are longer than those found in the symmetrical complexes and the pyrazole arm forms a six-membered ring, indicating their labilities in 8−11. Therefore, the M−Npyrazole bond can also break in the presence of a strong Lewis acid (Al3+ in MMAO) and the activated complex decomposes to give lower yield and activity at high temperatures for palladium complexes. In addition, the polymerization of NB in CH2Cl2 in the absence of MMAO using the in situ generated [(PNN)Ni(Me)] or [(PNP)Ni(Me)] from the reaction between [(PNN)NiBr] or [(PNP)NiBr] and MeMgBr in THF, respectively, gave no precipitate of polymer. Hence, MMAO is required to activate the nickel or palladium complex to form an active metal complex having the structure [(PNNκP,N)Ni(R)] (Chart 1) containing one vacant site for NB binding and subsequent transformations. Polymer Characterization. All MMAO-cocatalyzed reactions gave powdery materials which are insoluble in several solvents such as THF, CH2Cl2, CH3CN, MeOH, and CH3COOEt. However, polymers formed by nickel catalysts are slightly soluble in CDCl3 and their 1H NMR spectra showed no peak around 5.5 ppm. Therefore, polymers were characterized by ATR, TGA, and powder XRD methods. ATR spectra showed no band in the region 1620−1680 cm−1 and a band at 941 cm−1 for polymers formed at room temperature as well as at high temperatures, suggesting the vinyl type polynorbornene. In addition, TGA showed one major degradation temperature at around 421 °C with a mass loss of 89% (Figure 4). The same type of TGA was observed for the vinyl polynorbornene reported previously by us32 and others.35 Conversely, polymers obtained with EtAlCl2 reactions were of a gel type material that became very hard upon drying under vacuum and then in open air. This polymer is also insoluble in the aforementioned solvents. Its TGA showed two degradation points around 122 and 420 °C with mass losses of

Figure 4. TGA profiles of the polynorbornene obtained with [EtAlCl2]/[(PNN)PdCl] (red line), [EtAlCl2]/[(PNN)NiCl] (light blue line), [EtAlCl2]/[(PNN)NiBr] (black line), [MMAO]/[(PNN)PdCl] (magenta line), and [MMAO]/[(PNN)NiCl] (dark blue line) in the presence of a 1000 molar excess of cocatalyst in CH2Cl2.

about 58% and 36%, respectively (Figure 4). The powder XRD pattern of polymers obtained with MMAO reactions featured two broad peaks at 2θ = 9−11° and 18−19°, whereas that formed from EtAlCl2 gave one merged broad peak around 17.5°, indicating the noncrystalline nature as reported in the literature (see Figures S45−48 in the Supporting Information).27b,c

■

CONCLUSIONS In conclusion, 6 years after the report of the symmetrical NNN and PNP pincers, the first unsymmetrical pyrrole-based PNN pincer was synthesized in an analogous manner but in a stepwise fashion. Their palladium(II) and nickel(II) complexes were synthesized and structurally characterized. A systematic study of norbornene vinyl polymerization was carried out using both symmetrical and unsymmetrical pyrrole-based pincer metal complexes to check their catalytic efficiencies at different temperatures. The PNN pincer palladium and nickel complexes gave very high yields with activities in the range of 107 g of PNB mol−1 h−1, which remains close to those of the NNN pincer complex. Interestingly, the symmetrical N-donor pincer complex showed a far greater activity and quantitative yield in comparison to the symmetrical P-donor pincer complexes. This suggests that, as the number of N-donors increases, the yield and activity increase, following the order NNN > PNN > PNP at room temperature. At higher temperatures, these Pd and Ni complexes behaved differently. At 60 °C, the palladium complex showed the reverse order: NNN < PNN < PNP. The observed variation in the yield and G


Article

Inorganic Chemistry

and the residue was loaded onto a silica gel column chromatograph. Elution using methanol/chloroform (10/90 v/v) afforded 4 as a colorless oily compound after the solvents were removed under vacuum (7.100 g, 30.56 mmol, 99%). 1H NMR (CDCl3, 400 MHz): δ 2.18 (s, 6H, CH3), 2.20 (s, 3H, CH3), 2.21 (s, 3H, CH3), 3.34(s, 2H, CH2), 5.07 (s, 2H, CH2), 5.76 (s, 1H, pyrazole CH), 5.88 (d, J(H,H) = 5.6, 1H, pyrrole β-CH), 5.98 (d, J(H,H) = 4.8, 1H, pyrrole β-CH), 8.87 (br s, 1H, NH). 13C NMR (CDCl3, 100.6 MHz): δ 11.5, 13.1, 45.0, 46.4, 56.8, 105.8, 106.7, 107.7, 126.7, 130.1, 139.0, 148.0. ATRIR (cm−1): ν 3197 (m), 3123 (m), 3100 (m), 3000 (m), 2984 (m), 2964 (m), 2942 (m), 2922 (m), 2851 (m), 2809 (s), 2780 (m), 2761 (s), 2718 (m), 1657 (w), 1592 (w), 1547 (s), 1460 (vs), 1437 (vs), 1418 (vs), 1388 (s), 1356 (vs), 1304 (m), 1272 (s), 1256 (s), 1214 (w), 1197 (s), 1172 (m), 1142 (m), 1123 (w), 1097 (w), 1039 (s), 1020 (vs), 994 (s), 955 (w), 935 (w), 848 (vs), 825 (s), 777 (vs), 744 (s), 722 (w), 683(s), 667 (w), 634 (m), 621 (w), 469 (w). HRMS (+ESI): calcd m/z for [M + H+] C13H21N4 233.1761, found 233.1775. Synthesis of 2-(Dimethylaminomethyl)-5-(3,5-dimethylpyrazolylmethyl)-N-hydroxymethylpyrrole (5). Formaldehyde (40 wt % aqueous solution, 0.86 mL, 11.4 mmol) and dimethylamine hydrochloride (0.465 g, 5.70 mmol) were placed in a round-bottom flask, cooled to 0 °C in an ice bath, and stirred for 30 min. To this solution was added slowly a solution of 2-(3,5dimethylpyrazolylmethyl)pyrrole (1.000 g, 5.707 mmol) in methanol (60 mL), and the mixture was stirred at room temperature for 48 h. The reaction was quenched by adding a 20% aqueous solution of NaOH (0.250 g, 6.25 mmol). After 1 h the volatiles were removed under vacuum and the resulting residue was extracted into dichloromethane (3 × 30 mL). The solvent was removed again, and the residue was loaded onto a silica gel column chromatograph. Elution using methanol/chloroform (10/90 v/v) afforded 5 as a colorless oily compound after the solvents were removed under vacuum (1.140 g, 4.345 mmol, 76%). 1H NMR (CDCl3, 400 MHz): δ 2.18 (s, 6H, CH3), 2.23 (s, 6H, CH3), 3.44 (s, 2H, CH2), 5.17 (s, 2H, CH2), 5.48 (s, 2H, CH2), 5.76 (s, 1H, pyrazole CH), 5.94 (d, J(H,H) = 3.2, 1H, pyrrole β-CH), 6.01 (d, J(H,H) = 3.2, 1H, pyrrole β-CH). 13 C NMR (CDCl3, 50.3 MHz, ppm): δ 11.5, 14.0, 44.5, 45.0, 55.7, 67.1, 105.5, 108.4, 109.9, 128.3, 130.6, 139.3, 147.5. FT-IR (KBr, cm−1): ν 3201 (s), 3133 (s), 3107(s), 3036 (s), 2984 (s), 2922 (s), 2871 (s), 2786 (s), 2731 (m), 2606 (w), 1709 (w), 1657 (m), 1592 (m), 1553 (s), 1479 (s), 1462 (s), 1424 (s), 1382 (s), 1301(vs), 1262 (m), 1220 (w), 1149 (m), 1061 (s), 1029 (vs), 1007 (vs), 981 (m), 806 (s), 776 (vs), 735 (s), 702 (m), 660 (m). HRMS (+ESI): calcd m/z for [M + H+] C14H23N4O 263.1866, found 263.1857. Synthesis of 2-(3,5-Dimethylpyrazolylmethyl)-5(diphenylphosphinomethyl)pyrrole (6). To a solution of 2(dimethylaminomethyl)-5-(3,5-dimethylpyrazolylmethyl)pyrrole (1.000 g, 4.304 mmol) in toluene (30 mL) was added Ph2PH (0.75 mL, 0.802 g, 4.31 mmol), and the mixture was heated at 115 °C for 24 h. The reaction mixture was cooled to room temperature, and the solvent was removed under vacuum. The resulting residue was dissolved in a minimum amount of diethyl ether and subjected to flash silica gel column chromatography. Elution was first with petroleum ether and then with diethyl ether. The solvent was removed to give 6 as a colorless solid, which was washed with pentane (2 × 20 mL) and dried under vacuum (0.970 g, 2.60 mmol, 60%). Mp: 109 °C. 1H NMR (CDCl3, 400 MHz): δ 2.16 (s, 3H, CH3), 2.21 (s, 3H, CH3), 3.32 (s, 2H, CH2), 4.98 (s, 2H, CH2), 5.70 (br s, 1H, pyrrole β-CH), 5.76 (s, 1H, pyrazole CH), 5.90 (br s, 1H, pyrrole βCH), 7.30 (m, 10H, C6H5), 8.30 (br s, 1H, NH). 13C NMR (CDCl3, 125.7 MHz): δ 11.0, 13.6, 28.0 (d, J(C,P) = 15 Hz, CH2), 46.0, 105.6, 107.1 (d, J(C,P) = 5.0 Hz, pyrrole β-CH), 107.2, 126.0, 128.2 (d, J(C,P) = 8.8 Hz, pyrrole α-CH), 128.5 (d, J(C,P) = 6.2 Hz, phenyl), 128.8, 132.8 (d, J(C,P) = 17.6 Hz, phenyl), 138.5 (d, J(C,P) = 15.1 Hz, phenyl), 138.8, 147.6. 31P{1H}NMR (CDCl3, 161.9 MHz): δ −15.4 (s). FT-IR (KBr, cm−1): ν 3435 (s), 3197 (s), 3141 (s), 3003 (s), 2934 (s), 1590 (w), 1545 (s), 1478 (s), 1460 (s), 1434 (vs), 1378 (m), 1351 (m), 1300 (m), 1271 (s), 1206 (m), 1182 (m), 1115 (m), 1089 (m), 1030 (m), 994 (m), 921 (m), 801 (s), 773 (s), 739 (s),

activity of different pincer frameworks is explained using steric factors with the support of Tolman cone angles and NMR studies. Studies of other olefin and copolymerization reactions using these pincer metal complexes are in progress.

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EXPERIMENTAL SECTION

All reactions and manipulations were carried out using standard Schlenk-line techniques under a nitrogen atmosphere or nitrogenfilled glovebox. Petroleum ether (bp 40−60 °C) and other solvents were distilled under an N2 atmosphere according to the standard procedures. Norbornene, modified methylaluminoxane (MMAO-12, 7 wt % in toluene), and EtAlCl2 (25 wt % in toluene) were purchased from Aldrich and used as received, except for norbornene, which was distilled over sodium under vacuum. Other chemicals were obtained from commercial sources and used as received. All solvents were dried and distilled under an N2 atmosphere using standard procedures. 2(Dimethylaminomethyl)pyrrole,36 [PdCl2(COD)],37 [PdCl2(PhCN)2],38 [NiCl2(DME)],39 Ph2PH,40 chloro[2,5-bis(diisopropylphosphinomethyl)pyrrolido]palladium(II), 24a and bromo[2,5-bis(diisopropylphosphinomethyl)pyrrolido]nickel(II)14f were prepared by following the reported procedures. Other chemicals were obtained from commercial sources and were used without further purification. 1H NMR (200, 400, and 500 MHz) and 13C NMR (50.3, 100.6, 125.7, and 150.9 MHz) spectra were recorded on Bruker ACF200, AV400, AV500, and AV600 spectrometers. Chemical shifts were referenced with respect to the chemical shifts of the residual protons present in deuterated solvents. H3PO4 (85%) was used as an external standard for 31P NMR measurements. FTIR and ATR spectra were recorded using a PerkinElmer Spectrum Rx instrument. High-resolution mass spectra (ESI) were recorded using a Xevo G2 Tof mass spectrometer (Waters). Thermogravimetric analysis (TGA) was recorded on a TG 209 F3 Tarsus instrument (Netzsch) in which polymer samples were heated from room temperature to 800 °C at a rate of 10 or 20 K min−1 under a nitrogen atmosphere. Powder XRD data were collected by using a Bruker AXS D8 diffractometer. Synthesis of 2-(3,5-Dimethylpyrazolylmethyl)pyrrole (3). To a xylene solution (30 mL) of 2-(dimethylaminomethyl)pyrrole (5.000 g, 40.26 mmol) was added 3,5-dimethylpyrazole (3.800 g, 39.53 mmol), and the solution was refluxed at 145 °C for 15 h. The solution was then cooled to room temperature and the solvent was removed under vacuum. The resulting residue was loaded onto a silica gel column and eluted with an ethyl acetate/petroleum ether mixture (1/ 2 v/v). The solvent was removed from the first fraction to give 3 as a white fluffy solid (4.200 g, 23.97 mmol, 59.5% or 60%). Mp: 132−134 °C. 1H NMR (CDCl3, 400 MHz): δ 2.20 (s, 3H, CH3), 2.22 (s, 3H, CH3), 5.11 (s, 2H, CH2), 5.77 (s, 1H, pyrazole CH), 6.10 (d, J(H,H) = 2.4, 2H, pyrrole β-CH), 6.71 (t, J(H,H) = 1, 1H, pyrrole α-CH), 9.07 (br s, 1H, NH). 13C NMR (CDCl3, 100.6 MHz): δ 11.2, 13.6, 45.7, 105.5, 106.9, 108.1, 119.0, 126.9, 139.3, 148.0. FT-IR (KBr, cm−1): ν 3169 (vs), 3130 (vs), 3101 (vs), 2971 (s), 2938 (s), 1570 (m), 1560 (w), 1548 (s), 1508 (w), 1458 (s), 1420 (s), 1380 (m), 1348 (m), 1301 (m), 1274 (s), 1209 (m), 1138 (s), 1097 (m), 1031 (s), 986 (m), 957 (w), 927 (w), 882 (m), 866 (w), 836 (m), 813 (m), 796 (s), 738 (vs), 726 (vs), 682 (m), 632 (w), 609 (m), 470 (w). HRMS (+ESI): calcd m/z for [M + H+] C10H14N3 176.1182, found 176.1185. Synthesis of 2-(Dimethylaminomethyl)-5-(3,5dimethylpyrazolylmethyl)pyrrole (4). Formaldehyde (40 wt % aqueous solution, 2.30 mL, 30.6 mmol) and dimethylamine hydrochloride (2.514 g, 30.83 mmol) were placed in a round-bottom flask, cooled to 0 °C in an ice bath, and stirred for 30 min. To this solution was added slowly a solution of 2-(3,5dimethylpyrazolylmethyl)pyrrole (5.400 g, 30.82 mmol) in methanol (60 mL), and the mixture was stirred at room temperature for 48 h. The reaction was quenched by adding a 20% aqueous solution of NaOH (1.250 g, 31.25 mmol). After 1 h, the volatiles were removed under vacuum and the resulting residue was extracted into dichloromethane (3 × 30 mL). The solvent was removed again, H


Article

Inorganic Chemistry

OCH2), 5.09 (s, 2H, CH2), 5.80 (br s, 1H, pyrrole β-CH), 5.83 (s, 1H, pyrazole CH), 5.93 (br s, 1H, pyrrole β-CH), 7.43−7.96 (m, 10H, C6H5). 13C NMR (CDCl3, 125.7 MHz): δ 11.7, 15.5, 16.2, 32.6 (d, J(C,P) = 25.1 Hz, CH2), 45.9, 66.0, 103.1 (d, J(C,P) = 11.3 Hz, pyrrole β-CH), 107.7, 128.0, 129.0 (d, J(C,P) = 8.8 Hz, phenyl), 129.3, 129.7, 131.3, 132.9 (d, J(C,P) = 7.5 Hz, phenyl), 136.7, 140.0, 152.2. 31P{1H} NMR (CDCl3,161.9 MHz): δ 30.6 (s). ATR-IR (cm−1): ν 3074 (w), 3052 (w), 2964 (w), 2929 (w), 2858 (w), 1964 (w), 1550 (m), 1486 (m), 1463 (m), 1434 (m), 1421 (m), 1395 (m), 1382 (m), 1379 (m), 1346 (m), 1308 (w), 1272 (m), 1253 (m), 1181 (w), 1159 (w), 1104 (s), 1075 (m), 1052 (m), 1036 (m), 1023 (m), 997 (m), 939 (w), 916 (w), 842 (m), 812 (m), 777 (w), 738 (vs), 706 (s), 689 (vs), 676 (s), 634 (m), 631 (m), 605 (m), 576 (w), 553 (w), 524 (vs), 479 (s), 473 (m). Anal. Calcd for C23H23ClN3NiP: C, 59.21; H, 4.97; N, 9.01. Found: C, 59.18; H, 4.903; N, 9.01. Synthesis of [NiBr{C 4 H 2 N-2-(CH 2 Me 2 pz)-5-(CH 2 PPh 2 )κ3P,N,N}] (10). The above procedure was followed with NiBr2 (0.058 g, 0.26 mmol) to give complex 10 as crystals upon crystallization from ether (0.068 g, 0.12 mmol, 46%). Mp: >150 °C. 1H NMR (CDCl3, 400 MHz): δ 1.21 (t, J(H,H) = 7 Hz, 3H, OCH2CH3), 2.26 (s, 3H, CH3), 2.72 (s, 3H, CH3), 3.43 (d, J(H,H) = 11.6, 2H, CH2), 3.49 (q, J(H,H) = 7 Hz, 4H, OCH2), 5.11 (s, 2H, CH2), 5.82 (d, J(H,H) = 3.2 Hz, 1H, pyrrole β-CH), 5.84 (s, 1H, pyrazole CH), 5.96 (d, J(H,H) = 3.2 Hz, 1H, pyrrole β-CH), 7.43− 7.97 (m, 10H,C6H5). 13C NMR (CDCl3, 100.6 MHz): δ 11.7, 15.5, 17.4, 33.7 (d, J(C,P) = 31.2 Hz, CH2), 45.9, 66.0, 102.9 (d, J(C,P) = 10.0 Hz, pyrrole β-CH), 107.7, 127.9, 128.9 (d, J(C,P) = 10 Hz, phenyl), 129.8, 130.3, 131.2 (d, J(C,P) = 3 Hz, phenyl), 133.1 (d, J(C,P) = 9 Hz, phenyl), 136.6 (d, J(C,P) = 7 Hz, phenyl), 140.2, 152.2. 31P{1H} NMR (CDCl3,161.9 MHz): δ 35.0 (s). ATR-IR (cm−1): ν 3071 (w), 3052 (w), 2964 (w), 2919 (w), 2858 (w), 1984 (w), 1715 (w), 1592 (w), 1573(w), 1550 (m), 1486 (m), 1466 (m), 1434 (m), 1392 (m), 1375 (m), 1340 (m), 1272 (m), 1259 (m), 1230 (w), 1185 (w), 1155 (w), 1104 (s), 1071 (m), 1049 (m), 1029 (m), 1020 (m), 997 (m), 942 (m), 832 (m), 780 (m), 731 (s), 706 (m), 689 (vs), 657 (m), 631 (m), 612 (m), 518 (vs), 482 (s), 466 (m). Anal. Calcd for C23H23BrN3NiP: C, 54.06; H, 4.54; N, 8.22. Found: C, 54.37; H, 4.75; N, 7.88. Synthesis of [NiI{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}] (11). The above procedure was followed with NiI2 (0.081 g, 0.26 mmol) to give complex 11 as crystals upon crystallization from ether (0.076 g, 0.12 mmol, 46%). Mp: >150 °C. 1H NMR (CDCl3, 500 MHz): δ 1.22 (t, J(H,H) = 7 Hz, 3H, OCH2CH3), 2.28 (s, 3H, CH3), 2.77 (s, 3H, CH3), 3.46−3.51 (m, 6H, OCH2 and 2H, CH2), 5.12 (s, 2H, CH2), 5.85 (d, J(H,H) = 3 Hz, 1H, pyrrole β-CH), 5.86 (s, 1H, pyrazole CH), 6.01 (d, J(H,H) = 2.5 Hz, 1H, pyrrole β-CH), 7.43− 7.94 (m, 10H, C6H5). 13C NMR (CDCl3, 125.7 MHz): δ 11.7, 15.5, 19.7, 35.2 (d, J(C,P) = 28.9 Hz, CH2), 46.0, 66.1, 102.6 (d, J(C,P) = 10.0 Hz, pyrrole β-CH), 107.8 (d, J(C,P) = 10.0 Hz), 127.8, 128.7 (d, J(C,P) = 11.3 Hz, phenyl), 131.3, 133.5 (d, J(C,P) = 8.8 Hz), 136.6 (d, J(C,P) = 7.5 Hz, phenyl), 140.5, 152.1. 31P{1H} NMR (CDCl3, 161.9 MHz): δ 44.6 (s). ATR-IR (cm−1): ν 3045 (w), 2961 (w), 2925 (w), 2903 (w), 2861 (w), 2165 (w), 1715 (m), 1553 (m), 1466 (m), 1434 (m), 1401 (m), 1392 (m), 1379 (m), 1343 (m), 1308 (w), 1269 (m), 1259 (m), 1178 (w), 1159 (w), 1117 (w), 1097 (m), 1071 (m), 1055 (w), 1016 (w), 1000 (m), 945 (m), 916 (w), 877 (w), 842 (m), 783 (m), 738 (vs), 709 (m), 689 (vs), 673 (m), 628 (m), 608 (m), 521 (vs), 492 (s), 486 (s), 473 (m), 463 (m). Anal. Calcd for C23H23IN3NiP: C, 49.51; H, 4.15; N, 7.53. Found: C, 49.92; H, 4.50; N, 7.09. General Procedure for Norbornene Polymerization. The nickel or palladium complex was placed in a 100 mL Schlenk flask and was dried under vacuum for 20 min. To this was added CH2Cl2 or toluene followed by norbornene. The polymerization reaction began after addition of MMAO-12 using a syringe at room temperature. Precipitation occurred usually within 1 min. The reaction was stopped by adding an MeOH/HCl (10 mL/1 mL) mixture. The precipitate was filtered, washed with MeOH several times, and then dried under vacuum at 80 °C for 4 h to give a powdery polynorbornene polymer.

716 (m), 699 (vs), 625 (w), 533 (w), 503 (s), 464 (m), 418 (m). HRMS (+ESI): calcd m/z for [M + H+] C23H25N3P 374.1781, found 374.1768. Synthesis of 2-(Diphenylphosphorylmethyl)-5-(3,5dimethylpyrazolylmethyl)pyrrole (7). To a solution of 2(diphenylphosphinomethyl)-5-(3,5-dimethylpyrazolylmethyl)pyrrole (0.100 g, 0.268 mmol) in toluene (20 mL) was added aqueous H2O2 (0.20 mL, 1.8 mmol, 30% (w/w) in water). The solution was stirred for 16 h, and the solvent was removed under vacuum to give 7 as a colorless sticky solid (0.075 g, 0.19 mmol, 71%). 1H NMR (CDCl3, 400 MHz): δ 2.04 (s, 3H, CH3), 2.09 (s, 3H, CH3), 3.46 (d, J(H,P) = 12, 2H, CH2), 4.89 (s, 2H, CH2), 5.53 (s, 1H, pyrrole β-CH), 5.64 (s, 1H, pyrazole CH), 5.74 (s, 1H, pyrrole β-CH), 7.26−7.47 (m, 10H, C6H5), 9.60 (br s, 1H, NH). 13C NMR (CDCl3, 150.9 MHz): δ 11.2, 13.7, 30.2 (d, J(C,P) = 69.4, CH2), 46.1, 105.6, 107.3, 108.8 (d, J(C,P) = 7.5, pyrrole β-CH), 121.8(d, J(C,P) = 9, pyrrole α-CH), 127.5, 128.7 (d, J(C,P) = 12.1, phenyl), 131.1 (d, J(C,P) = 9, phenyl), 131.8 (d, J(C,P) = 99.6, phenyl), 132.1 (d, J(C,P) = 3, phenyl), 138.9, 147.7. 31P{1H} NMR (CDCl3, 161.9 MHz): δ 31.2 (s). FT-IR (KBr, cm−1): ν 3385 (br m), 3028 (m), 2963 (vs), 2871 (m), 1687 (m), 1610 (m), 1511 (s), 1466 (s), 1404 (m), 1386 (m), 1361 (m), 1229 (br s), 1115 (m), 1063 (m), 1016 (s), 902 (m), 829 (vs), 760 (w), 713 (w), 572 (s). Synthesis of [PdCl{C 4 H 2 N-2-(CH 2 Me 2 pz)-5-(CH 2 PPh 2 )κ3P,N,N}] (8). To a solution of [PdCl2(PhCN)2] (0.100 g, 0.261 mmol) and 2-(diphenylphosphinomethyl)-5-(3,5dimethylpyrazolylmethyl)pyrrole (6; 0.100 g, 0.268 mmol) in toluene (20 mL) was added dropwise triethylamine (0.08 mL, 0.574 mmol) with stirring at room temperature. The solution was stirred for 24 h, and the solvent was removed under vacuum. The resulting residue was dissolved in dichloromethane (10 mL), washed with water (3 × 30 mL), and dried over anhydrous Na2SO4. The solution was then filtered, and the solvent was evaporated again under vacuum to give a deep yellow residue (0.094 g, 0.18 mmol, 69%). Suitable single crystals for an X-ray diffraction study were obtained by slow evaporation of a solution of 8 in dichloromethane/petroleum ether (20/80 v/v). Mp: >150 °C. 1H NMR (CDCl3, 400 MHz): δ 2.31 (s, 3H, CH3), 2.64 (s, 3H, CH3), 3.67 (d, J(H,H) = 13.2, 2H, CH2), 5.00 (s, 2H, CH2), 5.86 (s, 1H, pyrazole CH), 5.93 (d, J(H,H) = 1.6, 1H, pyrrole β-CH), 6.03 (d, J(H,H) = 1.6 Hz, 1H, pyrrole β-CH), 7.45− 7.89 (m, 10H, C6H5). 13C NMR (CDCl3, 100.6 MHz): δ 11.9, 15.5, 34 (d, J(C,P) = 34.2 Hz, CH2), 46.8, 102.7 (d, J(C,P) = 14.1 Hz, pyrrole β-CH), 106.4, 107.2, 127.9, 128.8, 129.1 (d, J(C,P) = 11 Hz, phenyl), 131.8 (d, J(C,P) = 2 Hz, phenyl), 133.2 (d, J(C,P) = 11 Hz, phenyl), 136.0 (d, J(C,P) = 5 Hz, phenyl), 140.2, 152.3. 31P{1H} NMR (CDCl3, 161.9 MHz): δ 49.9 (s). ATR-IR (cm−1): ν 1720 (w), 1548 (m), 1463 (m), 1434 (m), 1408 (m), 1393 (m), 1373 (m), 1350 (m), 1278 (m), 1109 (m), 1067 (m), 1047 (m), 998 (m), 940 (w), 834 (s), 798 (m), 747 (s), 732 (s), 716 (s), 684 (vs), 636 (m), 517 (vs), 487 (s), 477 (s). HRMS (+ESI): calcd m/z for [M − Cl]+ C 23 H 23 N 3 PPd + 478.0659, found 478.0674. Anal. Calcd for C23H23ClN3PPd: C, 53.71; H, 4.51; N, 8.17. Found: C, 53.91; H, 4.50; N, 7.79. Synthesis of [NiCl{C 4 H 2 N-2-(CH 2 Me 2 pz)-5-(CH 2 PPh 2 )κ3P,N,N}] (9). To a solution of 2-(diphenylphosphinomethyl)-5(3,5-dimethylpyrazolylmethyl)pyrrole (0.100 g, 0.268 mmol) in acetonitrile (20 mL) was added triethylamine (0.08 mL, 0.574 mmol) with stirring at room temperature. To this solution was added [NiCl2(DME)] (0.058 g, 0.26 mmol), and the mixture was refluxed for 18 h. The solution was cooled to room temperature, and then the solvent was removed under vacuum. The resulting residue was dissolved in dichloromethane (20 mL). The solution was washed with water (3 × 10 mL), dried over anhydrous Na2SO4, and then filtered over Celite. The solvent was removed again under vacuum to give an orange-red residue which was extracted into diethyl ether. Upon cooling to −4 °C for 1−2 days, complex 9 was obtained as deep orange crystals containing one ether molecule (0.078 g, 0.14 mmol, 54%). Mp: >150 °C. 1H NMR (CDCl3, 400 MHz): δ 1.21 (t, J(H,H) = 7 Hz, 3H, OCH2CH3), 2.25 (s, 3H, CH3), 2.66 (s, 3H, CH3), 3.41 (d, J(H,H) = 11.6 Hz, 2H, CH2), 3.48 (q, J(H,H) = 7 Hz, 4H, I


Inorganic Chemistry

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In the case of polymerization at higher temperature, the precatalyst and NB in toluene were placed in a preheated oil bath and then MMAO was added. In the nickel-catalyzed reactions, the reaction mixture turned to a viscous liquid until 15 min, whereas a colorless precipitate began to form after 2 min in the case of palladiumcatalyzed reactions. An MeOH/HCl (10 mL/1 mL) mixture was added, and solid polymers were dried as above. In the case of EtAlCl2 as cocatalyst, the reaction mixture is a suspension and gel-like precipitate is formed after quenching with MeOH/HCl mixture. The precipitate was initially dried under vacuum at 50 °C. The resulting polymer still appears to be a gellike material which became a very hard one piece of chunk slowly upon drying in open atmosphere at room temperature. X-ray Crystallography. Single crystals suitable for X-ray diffraction for 8, 10 and 11 were obtained from the solvents mentioned in their respective synthetic procedures. Data collections were performed using a Bruker APEX-II or D8 Venture APEX3 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The space group for each structure was obtained by the XPREP program. The structures were then solved by SIR-9241 or SHELXT42 available in WinGX, which successfully located most of the non-hydrogen atoms. Subsequently, least-squares refinements were carried out on F2 using SHELXL-2018/1 (Sheldrick, 2018)43 to locate the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were fixed in calculated positions. In the structure of 8, one of the phenyl rings is disordered over two positions with occupation factors of 56 and 44%. Diethyl ether in the crystal lattices of structures 10 and 11 is disordered. This disorder was successfully handled with EADP and SADI restraints. The structure refinement data are given in Table S1.

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REFERENCES

(1) (a) Albrecht, M.; van Koten, G. Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts. Angew. Chem., Int. Ed. 2001, 40, 3750−3781. (b) van der Boom, M. E.; Milstein, D. Cyclometalated Phosphine-Based Pincer Complexes: Mechanistic Insight in Catalysis, Coordination, and Bond Activation. Chem. Rev. 2003, 103, 1759−1792. (c) Selander, N.; Szabó, K. J. Catalysis by Palladium Pincer Complexes. Chem. Rev. 2011, 111, 2048−2076. (2) (a) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.; Milstein, D. A PCN Ligand System. Exclusive C−C Activation with Rhodium(I) and C−H Activation with Platinum(II). Organometallics 1997, 16, 3981− 3986. (b) Ebeling, G.; Meneghetti, M. R.; Rominger, F.; Dupont, J. The trans−Chlorometalation of Hetero−Substituted Alkynes: A Facile Entry to Unsymmetrical Palladium YCY′ (Y, Y′ = NR2, PPh2, OPPh2, and SR) “Pincer” Complexes. Organometallics 2002, 21, 3221−3227. (c) Niu, J.-L.; Hao, X.-Q.; Gong, J.-F.; Song, M.-P. Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C−H activation and some catalytic applications. Dalton Trans. 2011, 40, 5135−5150. (3) (a) Asay, M.; Morales-Morales, D. Non-symmetric pincer ligands: complexes and applications in catalysis. Dalton Trans. 2015, 44, 17432−17447. (b) Khake, S. M.; Jagtap, R. A.; Dangat, Y. B.; Gonnade, R. G.; Vanka, K.; Punji, B. Mechanistic Insights into Pincer−Ligated Palladium−Catalyzed Arylation of Azoles with Aryl Iodides: Evidence of a PdII−PdIV−PdII Pathway. Organometallics 2016, 35, 875−886. (c) Li, Y.; Yu, X.; Wang, Y.; Fu, H.; Zheng, X.; Chen, H.; Li, R. Unsymmetrical Pincer N−Heterocyclic Carbene− Nitrogen−Phosphine Chelated Palladium(II) Complexes: Synthesis, Structure, and Reactivity in Direct Csp2−H Arylation of Benzoxazoles. Organometallics 2018, 37, 979−988. (4) (a) Fleckhaus, A.; Mousa, A. H.; Lawal, N. S.; Kazemifar, N. K.; Wendt, O. F. Aromatic PCN Palladium Pincer Complexes. Probing the Hemilability through Reactions with Nucleophiles. Organometallics 2015, 34, 1627−1634. (b) Hemilability of PCN-type pincer complexes: Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Nucleophilic De− coordination and Electrophilic Regeneration of “Hemilabile” Pincer−Type Complexes: Formation of Anionic Dialkyl, Diaryl, and Dihydride PtII Complexes Bearing No Stabilizing π−Acceptors. Chem. - Eur. J. 2004, 10, 4673−4684. Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Pincer “Hemilabile” Effect. PCN Platinum(II) Complexes with Different Amine “Arm Length. Organometallics 2005, 24, 1082−1090. (5) Braunstein, P.; Naud, F. Hemilability of Hybrid Ligands and the Coordination Chemistry of Oxazoline-Based Systems. Angew. Chem., Int. Ed. 2001, 40, 680−699. (6) (a) de Boer, S. Y.; Korstanje, T. J.; La Rooij, S. R.; Kox, R.; Reek, J. N. H.; van der Vlugt, J. I. Ruthenium PNN(O) Complexes: Cooperative Reactivity and Application as Catalysts for Acceptorless Dehydrogenative Coupling Reactions. Organometallics 2017, 36, 1541−1549. (b) Gafurov, Z. N.; Kagilev, A. A.; Kantyukov, A. O.; Balabaev, A. A.; Sinyashin, O. G.; Yakhvarov, D. G. Classification and synthesis of nickel pincer complexes. Russ. Chem. Bull. 2018, 67, 385− 394. (c) Lawrence, M. A. W.; Green, K.-A.; Nelson, P. N.; Lorraine, S. C. Review: Pincer ligands−Tunable, versatile and applicable. Polyhedron 2018, 143, 11−27. (d) Stadler, B. M.; Puylaert, P.; Diekamp, J.; van Heck, R.; Fan, Y.; Spannenberg, A.; Hinze, S.; de Vries, J. G. Inexpensive Ruthenium NNS-Complexes as Efficient Ester Hydrogenation Catalysts with High CO vs.CC Selectivities. Adv. Synth. Catal. 2018, 360, 1151−1158. (7) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127, 10840− 10841. (8) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Efficient Homogeneous Catalytic Hydrogenation of Esters to Alcohols. Angew. Chem., Int. Ed. 2006, 45, 1113−1115. (b) Zhang, J.; Balaraman, E.; Leitus, G.; Milstein, D. Electron−Rich PNP− and PNN−Type

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03562. NMR, IR, crystal structure and refinement, TGA, powder XRD, and crystallographic data (PDF) Accession Codes

CCDC 1875470−1875472 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected] (G.M.). Tel: +91 3222 282320. Fax: +91 3222 282252. ORCID

Sanghamitra Das: 0000-0003-4890-9849 Vasudevan Subramaniyan: 0000-0001-8130-6789 Ganesan Mani: 0000-0002-0782-6484 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the DST, New Delhi, India, for financial support, and DST FIST Level-II grant (No. SR/FST/CSII026/2013) for the 500 MHz NMR instrument. We are also thankful to Prof. M. C. Das, Department of Chemistry, IITKharagpur, for a TGA discussion. J


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Inorganic Chemistry

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