Neutral Mono(5-aryl-2-iminopyrrolyl)nickel(II) Complexes as

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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Neutral Mono(5-aryl-2-iminopyrrolyl)nickel(II) Complexes as Precatalysts for the Synthesis of Highly Branched Ethylene Oligomers: Preparation, Molecular Characterization, and Catalytic Studies Claú dia A. Figueira,† Patrícia S. Lopes,† Clara S. B. Gomes,† Joselaine C. S. Gomes,† Luis F. Veiros,† Francisco Lemos,‡ and Pedro T. Gomes*,† Downloaded via 5.188.219.62 on November 16, 2018 at 17:05:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Centro de Química Estrutural, Departamento de Engenharia Química and ‡CERENA, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: New substituted 5-aryl-2-(N-2,6-diisopropylphenylformimino)-1H-pyrrole ligand precursors reacted with the complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] to give rise to new mono(2-iminopyrrolyl) nickel(II) complexes with general formula [Ni{κ2N,N′-5-aryl-NC4H2-2-C(H)N-2,6iPr2C6H3}(o-C6H4Cl)(PPh3)]. The pyrrole 5-aryl substituent is a phenyl or a bulky anthracen-9-yl ring, the first being also modified by para-substituents with electron-releasing (methoxy) or electron-withdrawing (fluorine) characteristics. The new compounds were fully characterized by NMR spectroscopy, elemental analysis, and two of them by single crystal Xray diffraction. The complexes were tested as aluminum-free catalysts for the oligo-/polymerization of ethylene, at different reaction conditions, revealing catalytic activity even when tested as single-component catalysts. The addition of a phosphine scavenger, such as [Ni(COD)2], enhances significantly the catalytic activity. The polyethylene products obtained are hyperbranched low-molecular-weight oligomers with Mn’s in the range 570−3200 g/mol (GPC-based values) and very high branching degrees (80−130 branches/1000 C atoms).

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INTRODUCTION An important breakthrough in the area of polymerization of olefins occurred nearly two decades ago when Grubbs et al. reported neutral aryl or alkyl (R) Ni(II) complexes bearing a monoanionic phenoxyimine chelating ligand and a phosphine or nitrile labile donor (L) (Chart 1, A). These complexes acted

as aluminum-free catalysts for the homo- or copolymerization of α-olefins to high molecular weight polymers, under mild reaction conditions, with activities similar to those of early transition metals.1 The bulkiness of the chelating ligand was a key factor owing to the steric protection provided to the metal center, thus controlling deactivation and chain transfer reactions and facilitating the dissociation of the labile ligand, giving rise to a free coordination site.2 The catalytic action of these neutral complexes contributed to the broadening of the range of olefin-based materials, including, for example, copolymers containing polar monomers for applications as adhesives, paints, or lubricants.3 The ease of synthesis and modifications in both steric and/ or electronic effects of the phenoxyimine ligands led to the development of a large variety of related nickel complexes.4 Many modifications on the phenoxyimine scaffold were performed, as follows: (a) the use of several types of Narylimine groups with different degrees of bulkiness;5 (b) the use of different substituents at the o-phenoxy ring position in order to vary the steric hindrance,6 synthesize heterobimetallic compounds,7 or anchor the complexes in supports such as

Chart 1. General Molecular Structures of Aluminum-Free Nickel(II) α-Olefin Polymerization Catalysts

Received: September 11, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.8b00669 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics silica;8 and (c) the use of bridged bis(o-phenoxyimine) frameworks for the synthesis of binuclear compounds.9,10 Some of these new complexes revealed catalytic inactivity toward ethylene polymerization, even in the presence of phosphine scavengers.11 Others were only active in ethylene oligomerization12 or norbornene polymerization13 when methylaluminoxane (MAO) was used as activator, but several achieved good activities as catalysts for the polymerization of ethylene in the absence of aluminum activators. Owing to these results, a limited number of neutral aryl or alkyl nickel complexes emerged bearing alternative N,O monoanionic chelating ligands, such as 2-anilinotropones (Chart 1, B),14 anilinoperinaphthenones (Chart 1, C),15 βketiminates16 and enolatoimines (Chart 1, D),17 or iminocarboxamidates (Chart 1, E),18 which were capable of promoting the oligo-/polymerization of ethylene in the absence of alkyl aluminum cocatalysts. Apart from these diverse chelating ligands and their internal variations, different L and R ancillary ligands have also been used with nickel. Besides phosphines, ligands such as pyridine or acetonitrile were used as alternative L donors. The methyl and phenyl groups were extensively used as Ni−R ligands for the initiation of the aluminum-free oligo-/polymerization process, although groups such as benzyl6d,18 or naphtyl19 were also reported. In recent years, the interest in highly branched ethylene oligomers20 increased owing to their unique physical properties and potential applications, namely as components in the formulation of high performance synthetic lubricants. High values of branching (N) were usually found in polyethylenes produced with some Al-containing catalyst systems, with values ranging from 40 to 220 branches/1000 C atoms, and high values of Mn’s (2.1 × 104−1.8 × 106 g/mol), for catalyst systems of the type [NiX2(α-diimine)]/MAO,21 or with 90− 200 branches/1000 C atoms (Mn = 430−1200 g/mol) for PEs produced with catalyst systems [NiX2(2-iminopyridine)n]/ AlEt2Cl or MAO (n = 1 or 2).22 For the aluminum-free systems mentioned above (A−D), the oligo-/polyethylenes obtained presented variable microstructures with branching degrees varying from virtually linear to highly branched products, depending on the reaction parameters (temperature and pressure) and catalyst structures, which regulate the competition between chain-walking (chain isomerization) and migratory insertion (chain propagation) steps.23 For example, authors such as Mecking et al. and Marks et al., both using catalysts of the type A, have reported hyperbranched ethylene oligomers with branching numbers in the ranges of 70−80 branches/1000 C (Mn = 2000−6000 g/mol) and 145−150 branches/1000 C (Mn = 2000−3000 g/mol), respectively.24 Values of 8−90 branches/1000 C (Mn = 5 × 104−2.9 × 105 g/ mol) and 17−72 branches/1000 C (Mn = 2 × 104−1.4 × 105 g/mol) were reported for precatalysts of the type B and C, respectively,14,15 whereas ca. 20−80 branches/1000 C (Mn = 1.5 × 103−19 × 104 g/mol) were observed for precatalysts of type D.16b−d,17 Nevertheless, in some of the older works, especially those reporting low molecular weight oligomers, the branching degrees have been overestimated owing to the absence of an end-group correction24a in the corresponding calculations. Neutral Ni(II) complexes bearing anionic N,N chelating 2iminopyrrolyl ligands (Chart 1, F) were also synthesized and tested in the oligo-/polymerization of ethylene, most of them being reported as inactive, although active toward the polymerization of norbornene when activated by MAO.25 In

the specific case of complex [Ni{κ 2 N,N′-2-(N-2,4,6trimethylphenylacetimino)pyrrolyl}(C6H5)(PPh3)], we have reported preliminary tests revealing very low activity in the oligomerization of ethylene when employing [Ni(COD)2] as phosphine scavenger.25a This result was attributed to the lack of the steric protection of the nickel center and to the observed tendency for disproportionation reactions leading to the formation of the corresponding homoleptic complexes [Ni(2iminopyrrolyl)2]. To circumvent this drawback, we decided to synthesize new 2-(N-arylimino)pyrrole ligand precursors substituted with aryl groups at the position 5 of the pyrrole ring26 (see below in Scheme 1). Therefore, in the present work, we describe the synthesis and molecular characterization of a series of complexes of the type [Ni{κ2N,N′-5-aryl-2-(N-2,6-iPr2C6H3formimino)pyrrolyl}(o-C6H4Cl)(PPh3)], which were obtained by the reaction of the corresponding 5-aryl-2-(Narylformimino)pyrrolyl sodium salts with the starting material [Ni(o-C6H4Cl)(PPh3)2Cl]. The resulting new Ni(II) complexes, encompassing 5-aryl substituents of different electronic and steric natures in the supporting chelating ligand and a Nio-chlorophenyl bond for the initiation of the multiple C−C bond formation process, were successfully tested as aluminumfree catalysts for the oligo-/polymerization of ethylene, giving rise to hyperbranched products.

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RESULTS AND DISCUSSION Ligand Precursors, Coordination Reaction, and Molecular Characterization. Besides the variations at the N-imino function substituent, former modifications on the 2iminopyrrolyl ligand scaffold toward the steric crowding at the metal center involved the substitution of the pyrrolyl carbon 5, adjacent to the ring nitrogen, which included only alkyl groups.27 Very recently, we started introducing aryl substituents at position 5 of the 2-iminopyrrolyl ring, being successful in the stabilization of highly unsaturated bis- and mono(5-aryl-N-arylformimino)pyrrolyl complexes of Fe(II) and Co(II).26a,b The synthesis of the 5-aryl substituted 2-(Narylformimino)pyrrole ligand precursors is based on a sequential synthetic strategy in which the last step encompasses a condensation reaction between 5-aryl-2formylpyrrole derivatives28 and substituted anilines (2,6diisopropylaniline in the case of the present work) in refluxing toluene. The new 2-iminopyrrole ligand precursors, II and III (Scheme 1), prepared in the present work were obtained in moderate to high yields (38 and 74%), and were fully characterized by NMR spectroscopy (1H, 13C, and 19F for III), elemental analysis, and single-crystal X-ray diffraction (see text, Figures S1 and S2 and Tables S1−S3). Ligand precursors I−IV were then deprotonated with an excess of sodium hydride, in refluxing THF for 2 h, to give the corresponding in situ prepared 2-iminopyrrolyl sodium salts INa−IVNa that were further reacted with the square planar complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl], yielding aryl-nickel(II) complexes 1−4 (Scheme 1). The starting material trans-[Ni(o-C6H4Cl)(PPh3)2Cl] was obtained as a yellow solid from the stoichiometric oxidative addition of o-dichlorobenzene to [Ni(COD)2] in the presence of PPh3 (2 equiv) in good yields (80%), and characterized by elemental analysis and 1H, 13C, and 31P NMR spectroscopies. The 1H spectrum reveals six different broad resonances at approximately the same chemical shifts of its analogue transB


Article

Organometallics Scheme 1. Synthesis of 5-Aryl-2-(N-2,6-diisopropylphenylformimino)pyrrole Ligand Precursors I−IV and Their Corresponding (o-Chlorophenyl) Nickel(II) Complexes 1−4

[Ni(C6H5)(PPh3)2Cl],29 the only difference being the resonance related to the Hortho of the Ni−C6H4Cl, which now integrates for one proton. The 31P{1H} spectrum is a sharp singlet at 21.63 ppm, similar to the value reported for the referred analogue. Toluene solutions of ligand sodium salts INa−IVNa were reacted with trans-[Ni(o-C6H4Cl)(PPh3)2Cl] at −20 °C, which after workup resulted in the isolation of corresponding (ochlorophenyl) nickel(II) complexes 1−4 as orange solids in low to moderate yields (23−66%). These complexes are partially soluble in n-hexane and soluble in Et2O and toluene. They were fully characterized by NMR spectroscopy (1H, 13C, 31 P, and 19F for 3) and elemental analysis. Red crystals suitable for X-ray diffraction were obtained for compounds 2 and 4 from diethyl ether and toluene solutions, respectively, at −20 °C. Both complexes crystallized in the monoclinic crystal system, in P21 and P21/n space groups, the asymmetric units containing two independent molecules (A and B). Perspective views of the molecular structures of molecules A are given in Figure 1. The details of the crystal structure determinations are listed in Table S4, and selected bond distances and angles for 2 and 4 are given in Tables S5 and S6. As commonly observed, the molecular structure of 2 shows the triphenylphosphine group located trans to the N-imino substituent of the iminopyrrolyl chelating ligand, whereas in the molecular structure of 4, the triphenylphosphine group adopted the less common cis position to the N-imino substituent, very likely to minimize the strong stereochemical constraints imposed by the 5-(anthracen-9-yl) substituent of the coordinated pyrrolyl ring. This is in accordance with the 1 H NMR spectra of complexes 1−3, where the resonance of the iminic proton is a doublet due to the 4JPH coupling with the trans positioned phosphorus atom of the phosphine ligand, whereas in complex 4 the resonance is a singlet. The presence of these new 5-aryl substituents at the bidentate iminopyrrolyl ligands induces a large overall hindrance around the Ni(II) center. For the chelate, a 38− 42° dihedral angle between the 5-(4-methoxyphenyl) plane and the pyrrolyl rings is observed in complex 2, values that doubled when compared to the same dihedral angle for the ligand precursor II (15−18°). In complex 4, due to the high hindrance of the anthracen-9-yl group, this value is 70° for molecule B and almost perpendicular (86.6(2)°) for molecule A of the asymmetric unit. On the imine side, the dihedral angle between the pyrrolyl ring and the 2-N-arylimine group is

Figure 1. Molecular structures of complexes (a) 2 and (b) 4, molecules A, using 50% probability level ellipsoids. Hydrogen atoms have been omitted for clarity.

almost perpendicular (80−86°) in both complexes as expected, owing to the very bulky ortho-isopropyl groups. In complex 2, the phosphine ligand is on the same side of the 5-(4-methoxyphenyl)pyrrolyl ring (i.e., trans to the imine moiety), inducing a higher steric hindrance when compared to the related 5-unsubstituted iminopyrrolyl nickel complexes.25 This fact increases the cis angle N1−Ni1−P1 (103° instead of ca. 90°) and shortens the trans angle N2−Ni1−P1 (152−157° instead ca. 180°), the Ni1−P1 bond being also slightly longer (ca. 2.17 Å) than the average for 5-unsubstituted 2iminopyrrolyl Ni complexes (ca. 2.14 Å). In fact, an important C


Article

Organometallics

the 31P minor resonance of complex 4 at 11.90 ppm corresponds to the trans isomer, analogous to those of 1−3, and the major resonance at 18.87 ppm to the cis isomer. Conversely, the tiny 31P resonances exhibited by compounds 1−3 in the range δ 16−17 are attributed to the corresponding cis isomers. Variable-temperature (VT) NMR experiments performed on 4, from −80 to +50 °C, show essentially invariant 1H and 31P NMR spectra with no special alteration in the resonances shapes. However, a slight decrease in the minor isomer 31P resonance relative area from 2:1 at room temperature to 2:0.8 at −60 °C points to a possible very slow chemical exchange process operating between the two cis and trans isomers. This is corroborated by the 1H−1H NOESY NMR spectrum of complex 4 (Figure S11), which reveals off-diagonal peaks correlating each resonance of the 5-(anthracen-9-yl)-2iminopyrrolyl and o-chlorophenyl ligands of an isomer with the corresponding resonances of the other one, clearly indicating chemical exchange. This very slow exchange process is depicted in Scheme 2, its origin probably involving the

feature to note in the molecular structure of 2 is that the geometry around the Ni(II) center is essentially square planar, but owing to the volume constraints imposed, there is a tetrahedral distortion on the Ni−PPh3 bond. Distortions from coplanarity of ca. 26−32° are observed between the planes defined by P1−Ni1−Cipso2 and N1−Ni1−N2, while reported 5-unsubstituted 2-iminopyrrolyl nickel complexes show square planar geometries with dihedral angles of 17.55 and 13.48°.25 Similar distortion values (ca. 16−19°) are observed for complex 4 due to the formation of the less common isomer upon coordination, with the PPh3 coordinating trans to the pyrrolyl ring, although longer Ni1−P1 bond lengths of 2.20 and 2.19 Å (for molecules A and B, respectively) are observed. In complex 4, the existence of stereochemical reasons may determine the formation of the cis isomer observed in the X-ray structure. However, the clear existence of intermolecular π−π stacking interactions between aryl rings (with distances between 3.3 and 3.8 Å)30 in both molecules of the asymmetric unit may also be important driving forces for the preference of this isomer. Interactions such as those observed between the 5(anthracen-9-yl) and the o-chlorophenyl rings (3.513(3) Å for A and 3.468(3) Å for B, respectively), between the ochlorophenyl and one phenyl of the PPh3 ligand (3.683(3) and 3.769(3) Å), and between another phenyl of the PPh3 and the phenyl ring of the 2-N-arylimine group (3.676(3) and 3.518(4) Å) are likely confer more stability to that isomer. In solution, complexes 1−3 have in general similar 1H (Figures S3−S5) and 13C NMR spectra, exhibiting C 1 symmetry, owing to the presence of the o-chlorine atom in the phenyl ligand. Besides the inequivalence of all the ochlorophenyl ligand resonances in both the 1H and 13C spectra, it is also possible to observe an asymmetric environment from the resonances of the 2-N-(2,6-diisopropylphenylformimino) substituent of the bidentate ligand, as evidenced in the 1H and 1H−1H COSY (Figure S6) NMR spectra of complex 3. The 2-N-(2,6-diisopropylphenyl) ring has different signals for the meta-protons and also for the methine and methyl protons of the isopropyl groups (two meta-H, two iPr−CH, and four iPr−CH3 signals), each corresponding to a different resonance in the 13C spectra. Complex 4, containing the 5-(anthracen-9-yl) substituent at the pyrrolyl ring, gave an inseparable mixture of two isomers displaying fairly complex 1H (Figure S8) and 13C spectra. The 1H−1H NOESY NMR spectra of complexes 1−3 (Figure S8) point unequivocally to PPh3 ligands coordinating trans to the imine group, each showing spatial correlations between the o-proton of the o-chlorophenyl ring and the protons of one isopropyl group of the N-2,6-iPr2C6H3 arm, as well as with the o-phenyl protons of the PPh3 ligand. However, in the 1H−1H NOESY NMR spectrum of complex 4 (Figure S9) all the evidence point to a cis coordination in the major isomer, with the PPh3 o-phenyl protons showing spatial correlations with the isopropyl protons of the N-2,6iPr2C6H3 arm and also with the o-proton of the o-chlorophenyl ring. Moreover, the 31P NMR spectrum of 4 (Figure S10) exhibits two resonances at δ 18.87 (major isomer) and 11.90 (minor isomer), in a molar ratio 2:1 at room temperature, whereas compounds 1−3 exhibit 31P resonances at δ 12.05, 11.99, and 12.43, respectively, although containing very minor peaks at δ 16.35 (0.05:1), 16.46 (0.08:1), and 16.87 (0.03:1). Taking into account that the 31P spectra of these compounds show resonances in the range 12−12.5 ppm, it can be assumed that

Scheme 2. Very Slow Chemical Exchange Process Observed by NMR between the Major (4cis) and the Minor (4trans) Isomers of Complex 4 Corresponding to an Isomerization cis−trans in Relation to the Imine Ligand Moiety

dissociation of the PPh3 of one of the coordination sites and recoordination in the other site, with possible slow rotation of the o-chlorophenyl ligand about its bond to Ni and/or the 2,6diisopropylphenyl group around its bond to the iminic nitrogen. A possible explanation for the higher content of isomer 4cis in the mixture is the lower hindrance exerted by the anthracenyl group on the o-chlorophenyl ligand than on the bulkier PPh3. The stability difference between the cis and the trans isomers of complexes 2 and 4 was studied by means of DFT calculations.31 For each isomer, the two possible conformers arising from the two orientations of the chorine atom (“up” or “down”) were addressed (Figure S12). In the case of the complex with the bulky anthracenyl group (complex 4) the energy values obtained for all four structures are within 2.7 kcal/mol, and more interestingly, the more stable conformers for each isomer have similar stability, 4cis being only 0.1 kcal/ mol more stable than 4trans. This small energy difference is in good accordance with the presence of the two isomers in solution, clearly observed by NMR. Also, the 31P NMR shifts calculated for the two isomers show the same trend observed in the experimental ones, the chemical shift of the trans isomer being 4 ppm lower than the one obtained for 4 cis , corroborating the conclusions based on NMR data referred above. In contrast, equivalent calculations performed for complex 2 indicate a much wider energy range for the four molecules (6.5 kcal/mol), and more importantly, the trans D


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Organometallics

Table 1. Oligo-/Polymerization of Ethylene Catalyzed by Complexes 1−4 and Characterization of the Corresponding Synthesized Oligo-/Polymersa

entry

cat. or cat.*b

T (°C)

PE (g)

activity (kgPE/(molNi h bar))

Mn (NMR) (g/mol)c

Mn (GPC) (g/mol)d

Đd

Ne

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 1* 1* 2 2 2* 2* 3 3 3* 3* 4 4 4* 4*

25 50 25 50 25 50 25 50 25 50 25 50 25 50 25 50

0.50 0.67 1.16 0.83 0.50 0.70 1.09 0.80 0.12 0.46 0.80 0.29 0.15 1.36 0.89 1.90

2.79 3.70 6.43 4.59 2.79 3.88 6.07 4.43 0.67 2.58 4.64 1.67 0.82 7.55 4.95 10.58

380 380 700 500 350 480 700 490 340 440 750 370 1500 920 1900 960

610 610 1200 830 620 600 1300 890 570 640 1300 630 2800 2200 3200 2100

1.1 1.1 1.4 1.3 1.2 1.1 1.4 1.3 1.1 1.2 1.5 1.2 1.5 1.5 1.5 1.5

97 94 111 104 85 109 110 97 81 107 109 88 128 116 127 117

Experimental conditions: P(abs) = 9 bar; [Ni] = 10−2 mmol; reaction time = 2 h; solvent: toluene, 50 mL. bCatalyst X* = Catalyst X + 2 equiv of [Ni(COD)2] cCalculated from 1H NMR intensity ratios of unsaturated end groups vs. overall integral. dDetermined by GPC/SEC (calibration with polystyrene standards); chromatograms are shown in Figures S35−S50; Đ = Mw/Mn. eNumber of branches/1000 C atoms, determined by 1H NMR (spectra in Figures S13−S28) and corrected for methyl end-groups (equation S3). a

selected samples being also analyzed by 13C{1H} NMR.The organic substances produced by catalysts 1−3 are low density oils, with low molecular weights (Mn between 570 and 1300 g/ mol; GPC/SEC chromatograms in Figures S35−S46) and very high branching degrees (80−130 branches/1000 C). The PEs obtained with 4 are more viscous, their molecular weights being 2−4 times higher (Mn ranging from 2100 to 3200 g/mol; GPC/SEC chromatograms in Figures S47−S50) than those obtained with precatalysts 1−3, using the same reaction conditions. The branching degrees of the products originated from 4 are also slightly higher than those obtained in the remaining cases. Nevertheless, all the PE products obtained in this work can be considered as hyperbranched polyethylenes (see below).20 The increase in the reaction temperature decreases the molecular weights, in agreement with the fact that temperature promotes chain transfer reactions (via β-hydrogen transfer). The dispersity (Đ = Mw/Mn) values are not significantly affected, the oligomers showing in general narrow molecular weight distributions, typical of well-behaved single-site catalysts, although the products generated by catalyst 3, particularly those in the absence of [Ni(COD)2], show some bimodal nature (see GPC/SEC chromatograms in the Supporting Information). However, for the oligoethylenes with Mn < 1000 g/mol, their Mn values may have been overestimated and the corresponding Mw/Mn underestimated, because the lower molecular weight side of the chromatograms (longer retention times) overlap the permeation limit of the columns (ca. 250 g/mol). Moreover, as the GPC/SEC analyses were calibrated with polystyrene standards, the determined molecular weights are nonabsolute values. Therefore, an endgroup analysis of the PEs by 1H NMR spectroscopy was also carried out, according to the state-of-the-art-literature22c,24a (see Table 1 and the Supporting Information). For example,

isomer (2trans) is now 3.3 kcal/mol more stable than its counterpart, 2cis, considering the most stable conformer in each case. This is accordance with a vestigial presence of 2cis in solution, detected by NMR. Moreover, the calculated 31P NMR shifts show the trend experimentally observed with the δ value obtained for 2trans 4 ppm below the one calculated for 2cis. Catalytic Oligo-/Polymerization of Ethylene. Complexes 1−4 were studied as catalysts for the oligo-/polymerization of ethylene. The tests were performed at 9 bar of ethylene (absolute pressure) for 2 h in the presence or absence of the phosphine scavenger [Ni(COD)2] (2 equiv) and at two different temperatures, 25 or 50 °C (see Table 1). Blank tests were also performed in the same experimental conditions, using only [Ni(COD)2] and a mixture of [Ni(COD)2]/PPh3 (2:1), revealing inactivity in all the tested conditions. In general, using the same reaction conditions, the results obtained for precatalysts 1−3 are relatively similar, in either the absence or the presence of phosphine abstractor [Ni(COD)2] (catalyst systems 1*−3*). The introduction of electron-releasing or -withdrawing substituents in the para position of the 5-phenyl ring, such as the methoxy (in 2) or the fluorine (in 3), does not influence significantly the catalytic activities (although 3 or 3* always present slightly poorer catalytic performances) or the type of polyethylenes (PEs) produced, in comparison with the 5-phenyl unsubstituted 2iminopyrrolyl N,N′-bidentate ligand (in 1). Conversely, the increase in the steric bulkiness, when changing from a 5-phenyl substituent (in 1−3) to a 5-(anthracen-9-yl) substituent (in 4), increases very significantly not only the catalytic activity (at 50 °C) but also the molecular weight of the PEs obtained. The oligo-/polymers obtained were characterized by gel permeation chromatography/size-exclusion chromatography (GPC/SEC) and by 1H NMR spectroscopy, with some E


Article

Organometallics Figure S13 shows the 1H NMR spectrum of the PE obtained with single-component catalyst 1, at 25 °C and 9 bar, the magnification of which, in the range 6.0−4.6 ppm (inset in Figure S13), allows the identification of the vinyl end-group unsaturations in the PE backbone. The terminal vinyl group(CH2CH−) proton resonances are assigned at 5.70 and 4.90 ppm, whereas those of the internal vinyl group (−CHCH−) protons appear at 5.35 ppm. These types of resonances are also identified in the 1H NMR spectra of all the PE samples produced in this work, as shown in Figures S14− S28. A general trend observed is that the GPC/SEC Mn values are 1.5−2.0 times higher, in accordance with what is reported in the literature.22c,24 The branching degrees (N) of the oligomers (Table 1) were determined by 1H NMR spectroscopy and calculated using the methyl end-groups correction (equation S3),24a owing to their low molecular weights. The high values of N obtained are not particularly affected by the nature of the catalyst, whereas an increase in the reaction temperature seems to lead just to a slight decrease of the branch content. Selected samples of PEs were also characterized by 13C{1H} NMR spectroscopy and their microstructure analyzed in terms of type and distribution of the branches along the main chain. One set of reaction conditions (25 °C and 9 bar of ethylene) was chosen in order to compare the products obtained with the four catalyst systems, and the PEs obtained with catalyst 4 were compared for different experimental conditions, such as 25 and 50 °C, and the presence or absence of phosphine scavenger [Ni(COD)2]. These results are summarized in Table 2.

exactly the same microstructure. However, the stereochemical bulkiness of the 5-aryl substituent influences the oligomers microstructure, since varying the 5-substituent from 5-phenyl groups (in 1*−3*) to the bulky 5-anthracen-9-yl (in 4*) causes a significant increase in the percentage of methyl branches, along with a general decrease of longer branches. The presence or absence of [Ni(COD)2] does not substantially affect the microstructure, but the temperature also exerts some influence. An increase in the reaction temperature from 25 to 50 °C, using catalyst 4*, causes a significant decrease of the methyl branches and an increase in the contents of longer branches (from ethyl to long). The bulkiness of the 5-(anthracen-9-yl) substituent can limit the rate of the chain-walking mechanism (chain isomerization process responsible for the branching), giving rise mainly to the formation of methyl branches, but the use of higher reaction temperatures allows a higher content of longer branches. In order to have a more realistic insight into the behavior of the catalyst systems along the reaction time, namely their lifetime and robustness, we decided to measure the ethylene consumption for some selected catalyst systems and reaction conditions. The kinetic profiles for the uptake of ethylene gas in the oligo-/polymerization of ethylene catalyzed by systems 1*−3* (1−3/[Ni(COD)2]), at 25 °C and 9 bar of ethylene, are shown in Figure 2. The three systems have similar catalytic

Table 2. Microstructural Analysis (Branches Distribution) of the PEs Obtained with Catalyst Systems 1*−4* (1−4/ [Ni(COD)2] (2 equiv))a type of branches (%)c entry

cat.

Nb

methyl

ethyl

propyl

butyl

sec-butyl

long

3 7 11 13 15 16

1* 2* 3* 4d 4* 4*e

111 110 109 128 127 117

43 41 43 57 62 46

6 7 6 7 6 7

3 3 3 3 2 4

21 22 20 15 12 18

10 10 10 7 7 10

17 17 18 11 11 15

Figure 2. Catalytic profiles for the oligo-/polymerization of ethylene catalyzed by systems 1*−3* (1−3/[Ni(COD)2] (2 equiv)), at 25 °C and constant ethylene pressure of 9 bar (absolute).

At 25 °C and constant ethylene pressure of 9 bar. bNumber of branches/1000 C atoms, determined by 1H NMR (Table 1). c Determined by 13C{1H} NMR (spectra in Figures S29−S34 and using equation S11). dPE obtained in a catalytic reaction performed with single-component catalyst 4. ePE obtained in a catalytic reaction performed at 50 °C. a

profiles, exhibiting an increase in the reaction rate up to their activity maxima, in the range 70−120 min, followed by a plateau, though exhibiting a smooth decay, and then ending up in a fast and full deactivation occurring in the last 30 min of reaction. Although, the activities of these three catalyst systems are relatively similar, the 5-(p-methoxyphenyl) donor substituent in the pyrrolyl ring seems to confer more robustness and longer lifetime to precatalyst 2. Complex 1 was also monitored at different experimental conditions, such as the presence or absence of [Ni(COD)2] and at temperatures of 25 or 50 °C, as depicted in Figure 3. An increase in the reaction temperature to 50 °C considerably enhances the ethylene consumption for catalyst system 1*, drastically changing the catalyst kinetic profile (Figure 3, green points) in comparison to that exhibited at 25 °C (red points). After a sharp increase in the reaction rate,

The 13C{1H} NMR spectra of the selected PE samples are shown in Figures S29−S34, being complex spectra owing to their high degrees of branching. The identification of the majority of the peaks is possible due to the previously reported assignments.32 Branches with lengths varying from methyl (isolated and paired) to butyl, and longer branches can also be found. It is also evident the presence of a quite significant amount of sec-butyl groups in all the samples analyzed (see Table 2), which is the basic motif of a branch-on-branch structure, clearly indicative of PEs with hyperbranched microstructures.24a,22c The PEs obtained with catalyst systems 1*−3* (1−3/ [Ni(COD)2]) using the same reaction conditions gave almost F


Organometallics

Article

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CONCLUSIONS

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EXPERIMENTAL SECTION

In contrast to their previously reported 5-unsubstituted derivatives, the new 5-aryl-2-(N-2,6-diisopropylphenylimino)pyrrolyl Ni(II) complexes herein synthesized and characterized are effective aluminum-free catalysts for the oligo-/polymerization of ethylene. Their catalytic behavior is observed either when acting as single-component catalysts or, more efficiently, in the presence of the phosphine scavenger [Ni(COD)2], exhibiting catalytic activities similar to other reported welldefined aluminum-free nickel(II) catalysts. The ethylene oligomers obtained combine very high degrees of branching (in the range 80−130 branches/1000 C atoms) and molecular weights typically below 1300 g/mol (GPC-based) for the catalyst systems based on precatalysts 1−3. The PEs obtained with precatalyst 4, which contains a bulky 5-(anthracen-9-yl) substituent on the pyrrolyl moiety, exhibit higher molecular weights, in the range of ca. 2000−3000 g/mol, this precatalyst also giving rise to the highest catalytic activities observed for these catalyst systems. The microstructure analysis of the PEs by 13C{1H} NMR spectroscopy confirms very high branch contents (80−130 branches/1000 C atoms), corresponding approximately to an equal distribution of methyl and longer branches (≥C4, including sec-butyl branches). Nevertheless, the bulkiest precatalyst used (complex 4), which gives rise to the highest branching degrees produced in this work, induces an increase in the methyl branch content at the expense of longer branches. The presence of a quite significant amount of secbutyl groups in all the samples analyzed, which is the basic motif of a branch-on-branch structure, is clearly indicative of PEs with hyperbranched microstructures. Nickel(II) compounds 1−4 synthesized in this work are among the scarce number of well-defined aluminum-free precatalysts capable of producing hyperbranched ethylene oligomers.

Figure 3. Plots of the ethylene uptake vs time for the oligo-/ polymerization of ethylene catalyzed by system 1* (1/[Ni(COD)2] (2 equiv)), at 25 and 50 °C, and by 1 acting as single-component catalyst, at 50 °C, at a constant ethylene pressure of 9 bar (absolute).

with a maximum at ca. 25 min and about an order of magnitude higher than at 25 °C, a fast deactivation occurred in the next 45 min of reaction, with the catalyst lifetime being reduced by a third. Using the same experimental conditions (50 °C, 9 bar) but without the presence of the phosphine scavenger [Ni(COD)2] (golden points), catalyst 1 exhibited a reaction profile similar to that of 1*, being though much less active, with a consumption maximum observed at ca. 15 min and about half of the activity of that of 1*. In fact, [Ni(COD)2] has a good ability to scavenge the PPh3 decoordinated from precatalyst 1, minimizing its competition with ethylene monomer at the catalytic Ni(II) centers. The catalyst activity and lifetime increase significantly with the use of [Ni(COD)2], since the presence of triphenylphosphine in the reaction medium can also contribute to the deactivation of the catalyst via the occurrence of disproportionation reactions. The oligomerization of ethylene catalyzed by 2-iminopyrrolyl nickel complexes 1−4 likely follows the mechanism widely described in the literature for the cationic-type αdiimine23,33 and 2-iminopyridine22c Ni(II) catalysts or the neutral aryl or alkyl Ni(II) catalysts containing phenoxyimine and other mononegative chelating ligands.1,23d Accordingly, the dissociation or abstraction of the PPh3 labile group leaves a vacant coordination position at the metal center, enabling the coordination of ethylene to the nickel center and subsequent migratory insertion into the Ni-o-chlorobenzene bond, which corresponds to the polymerization initiation. Multiple repetitions of these two sequential steps allow the chain growth of the oligomer. However, β-hydrogen elimination is a favored process, generating an hydride metal species containing a coordinated unsaturated oligomer chain, which evolve via two paths: (a) The hydride returns to the oligomeric chain through an 1,2-hydride shift reaction, giving rise to a branched alkyl group (chain isomerization) and if a sequence of continuous β-hydrogen elimination/1,2-hydride shift reactions (chain-walking) is favored, then longer branches are produced, giving rise to hyperbranched products. (b) At some point, the unsaturated oligomer chain is released from the metal center, leaving a new free coordination site, where a new chain can start (chain transfer step).

General Considerations. Experiments involving air- and/or moisture-sensitive materials were carried out under dinitrogen atmosphere using a glovebox, dual vacuum/nitrogen lines, and standard Schlenk techniques. The dinitrogen gas was supplied in cylinders (Air Liquide) and purified by passage through 4 Å molecular sieves. Solvents were predried with 4 Å molecular sieves and purified by refluxing over a drying agent (sodium/benzophenone for diethyl ether, THF, and toluene; calcium hydride for n-hexane and npentane), and further distilled and stored under dinitrogen in J. Young ampules. Hexamethydisiloxane (HMDSO) was dried with 4 Å molecular sieves, distilled trap-to-trap and stored under dinitrogen in a J. Young ampule. Solvents and solutions were transferred by a positive pressure of nitrogen with stainless-steel cannulae and mixtures were filtered through modified cannulae fitted with glass fiber filter disks. Unless otherwise stated, the reagents were acquired from commercial suppliers (e.g., Aldrich, Fluka, Merck) and used without further purification. The 2,6-diisopropylaniline reagent was dried over calcium hydride, distilled trap-to-trap, and stored under dinitrogen. Sodium hydride was purchased in 60% mineral oil dispersion, washed several times with n-hexane, dried under vacuum, and stored under nitrogen. Starting materials such as [Ni(COD)2] and 5-aryl-2formylpyrrole precursors of I−IV were prepared as described in the literature.26,28,34 Ligand precursors I and IV were also synthesized as described previously.26 The sodium salts of I−IV, namely, INa−IVNa, were synthesized in situ, but IINa and IIINa were also isolated and characterized by NMR (see the Supporting Information). The NMR spectra were recorded on a Bruker Avance III 300 (1H, 300.130 MHz; 13C, 75.468 MHz; 19F, 282.404 MHz; 31P, 121.495 G


Article

Organometallics

Hz, Ph−CmetaCl), 123.7 (t, 4JCP = 2.0 Hz, Ph−Cmeta). 31P{1H} NMR (121 MHz, CD2Cl2): δ 21.65. Anal. Calcd for C42H34Cl2NiP2: C 69.08, H 4.69. Found: C 69.13, H 4.67. General Method for the Synthesis of [Ni(o-C6H4Cl){κ2N,N′-5aryl-NC4H2-2-C(H)N-2,6-iPr2(C6H3)}(PPh3)] Complexes (1−4). NaH (1.12 equiv) was weighed to a Schlenk tube and a suspension of the ligand precursor in THF (10 mL) was added dropwise. The mixture was stirred at 85 °C for 2 h and subsequently allowed to cool to room temperature. The THF solution was filtered and evaporated to dryness and the solvent changed to toluene. The toluene solution (20 mL) of the sodium salt (ca. 3.5% excess) was added dropwise to a suspension of the starting material trans-[Ni(o-C6H4Cl)(PPh3)2Cl] in toluene (30 mL) at −30 °C, and the resulting mixture stirred overnight. The resulting dark red solution was filtered and evaporated to dryness, the residue washed, and the remaining product subsequently extracted, both operations at −10 °C and with appropriate solvents. The resulting orange-red solution was concentrated and stored at −20 °C, yielding the desired pure product. Complex 1. Deprotonation of I (0.390 g, 1.18 mmol) with NaH (0.032 g, 1.32 mmol), gave in situ sodium salt INa as a yellow-brown powder that reacted with complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] (0.831 g, 1.14 mmol). The residue was first washed with n-hexane, but the crude was all soluble. The solution was stored at −20 °C, and a red orange oily solid precipitated. The solution was filtered and npentane added to wash the solid, giving 1 as an orange powder. Yield: 0.569 g (66%). 1H NMR (300 MHz, CD2Cl2): δ 7.65 (d, 4JHP = 5.4 Hz, 1H, NCH), 7.43 (d, 3JHH = 6.6 Hz, 2H, 5-Ph−Hortho), 7.27− 7.22 (m, 9H, PPh3−Hpara + PPh3−Hortho), 7.11 (t, 3JHH = 6.6 Hz, 6H, PPh3−Hmeta), 7.00 (d, 3JHH = 7.6 Hz, 1H, N−Ph−HmetaA), 6.96−6.87 (m, 4H, 5-Ph−Hpara + 5-Ph−Hmeta + N−Ph−Hpara), 6.85 (d, 3JHH = 3.4 Hz, 1H, H3 pyrr), 6.67 (d, 3JHH = 7.4 Hz, 1H, N−Ph−HmetaB), 6.37 (d, 3JHH = 7.6 Hz, 1H, NiPh−HmetaA), 6.28 (t, 3JHH = 7.1 Hz, 1H, NiPh−Hpara), 6.17 (d, 3JHH = 3.3 Hz, 1H, H4 pyrr), 5.89 (t, 3JHH = 7.1 Hz, 1H, NiPh−HmetaB), 5.65 (d, 3JHH = 7.1 Hz, 1H, NiPh−Hortho), 4.37 (dt, 3JHH = 13.2, 6.4 Hz, 1H, CHA(CH3)2), 3.43 (dt, 3JHH = 13.2, 6.5 Hz, 1H, CHB(CH3)2), 1.44 (dd, 3JHH = 12.9, 6.7 Hz, 6H, CHA(CH3)2), 0.91 (dd, 3JHH = 11.6, 6.7 Hz, 6H, CHB(CH3)2). 13 C{1H} NMR (75 MHz, CD2Cl2): δ 161.2 (NCH), 153.7 (C5 pyrr), 146.4 (N−Ph−Cipso), 144.3 (N−Ph−CorthoB), 141.8 (d, 3JCP = 3.2 Hz, NiPh−CorthoCl), 141.8 (N−Ph−CorthoA), 141.2 (d, 3JCP = 3.2 Hz, C2 pyrr), 140.4 (d, 2JCP = 49.7 Hz, NiPh−Cipso), 140.4 (d, 3JCP = 2.1 Hz, NiPh−Cortho), 137.5 (5-Ph−Cipso), 133.9 (d, 2JCP = 9.8 Hz, PPh3−Cortho), 131.8 (d, 1JCP = 43.8 Hz, PPh3−Cipso), 129.6 (d, 4JCP = 2.4 Hz, PPh3−Cpara), 128.1 (5-Ph−Cortho + 5-Ph−Cmeta), 127.9 (d, 3 JCP = 9.8 Hz, PPh3−Cmeta), 126.8 (d, 4JCP = 2.7 Hz, NiPh−CmetaA), 125.8 (5-Ph−Cpara), 125.6 (N−Ph−Cpara), 124.0 (d, 5JCP = 1.0 Hz, NiPh−Cpara), 123.2 (d, 4JCP = 2.9 Hz NiPh−CmetaB), 122.7 (N−Ph− CmetaB), 122.5 (N−Ph−CmetaA), 119.3 (C3 pyrr), 114.7 (C4 pyrr), 29.2 (CAH(CH3)2), 27.8 (CBH(CH3)2), 27.0 (CAH(CH3)(CH3)), 26.0 (CBH(CH3)(CH3)), 23.8 (CAH(CH3)(CH3)), 22.0 (CBH(CH3)(CH3)). 31P{1H} NMR (121 MHz, CD2Cl2): δ 12.05. Anal. Calcd for C47H44N2NiPCl·0.08C6H14: C 74.15, H 5.89, N 3.65. Found: C 74.50, H 6.22, N 3.51. Complex 2. Deprotonation of II (0.429 g, 1.19 mmol) with NaH (0.032 g, 1.33 mmol), gave the in situ sodium salt IINa as a greenyellowish powder that reacted with complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] (0.833 g, 1.14 mmol). The residue was washed with nhexane and the remaining orange solid extracted with diethyl ether, yielding 2 as a brown-orange solid. Yield: 0.404 g (45%). A second crop was obtained through further concentration of the filtrate and storage at −20 °C, leading to the formation of crystals suitable for Xray diffraction. 1 H NMR (300 MHz, CD2Cl2): δ 7.61 (d, 4JHP = 5.6 Hz, 1H, N CH), 7.34−7.22 (m, 11H, 5-Ph−Hortho + PPh3−Hpara + PPh3−Hortho), 7.15−7.10 (m, 6H, PPh3−Hmeta), 6.99 (d, 3JHH = 7.3 Hz, 1H, N−Ph− HmetaA), 6.89 (t, 3JHH = 7.7 Hz, 1H, N−Ph−Hpara), 6.83 (d, 3JHH = 3.7 Hz, 1H, H3 pyrr), 6.67 (d, 3JHH = 7.2 Hz, 1H, N−Ph−HmetaB), 6.45− 6.38 (m, 3H, 5-Ph−Hmeta + NiPh−HmetaA), 6.28 (t, 3JHH = 7.4 Hz, 1H, NiPh−Hpara), 6.08 (d, 3JHH = 3.6 Hz, 1H, H4 pyrr), 5.88 (t, 3JHH = 7.0 Hz, 1H, NiPh−HmetaB), 5.58 (d, 3JHH = 7.7 Hz, 1H, NiPh−Hortho),

MHz) or on a Bruker Avance III 400 (1H, 400.130 MHz; 13C, 100.613 MHz; 19F, 376.498 MHz; 23Na, 105.842 MHz) spectrometers. Deuterated solvents were dried over molecular sieves (4 Å to CDCl3 and CD2Cl2; 3 Å to CD3CN), degassed by the freeze−pump− thaw method and stored under inert atmosphere in J. Young ampules. For air- and/or moisture-sensitive materials, samples were prepared inside the glovebox in J. Young tubes. Spectra were referenced internally using the residual-protio resonance (1H) and the solvent resonance (13C) relative to tetramethylsilane (δ = 0). 19F, 23Na, and 31 P were referenced using CFCl3 (δ = 0), NaCl 1 M (δ = 0), and H3PO4 85% (δ = 0), respectively. For IINa and IIINa, the sodium spectra were obtained after digital subtraction of the characteristic 23 Na resonance of the 5 mm tube glass material, measured in a blank experiment performed with an empty tube. The chemical shifts are quoted in δ (ppm), and coupling constants J in Hz. Multiplicities were abbreviated as follows: broad (br), singlet (s), doublet (d), doublet of doublets (dd), triplet (t), heptet (h), and multiplet (m). In order to perform nuclei resonance assignments, 2D NMR experiments were also performed (1H−1H COSY, 1H−1H NOESY, 1H−13C HSQC, and/or 1H−13C HMBC). All spectra were recorded at an average room temperature of 22 °C, with exception of the VT experiments performed for complex 4, recorded at a range of decreasing temperatures from 22 to −80 °C and of increasing temperatures from 22 to 50 °C. The polyethylene samples were characterized by solution NMR using a mixture of solvents C6D6/TCB (1:3) (TCB = 1,2,4trichlorobenzene), and recorded at 90 °C in a 5 mm sample tube. All the 1H spectra were recorded with an acquisition time of 1.5 s and a delay of 4.0 s, whereas in the 13C{1H} spectra a 2.0 s acquisition time and a 5.0 s delay were used. For the 13C{1H} spectra, the chemical shifts were referenced internally to the methylene group resonance at 30.00 ppm. The elemental analyses were performed in a Fisons Instrument Mod EA-1108, at the Laboratório de Análises (IST). For each compound two independent determinations were executed. General Synthesis of 5-Aryl-2-(N-2,6-diisopropylphenylformimino)-1H-pyrroles (II and III). Compounds 5-aryl-2-formyl-1Hpyrrole reagent, 2,6-diisopropylaniline (1.2 equiv), and a catalytic amount of p-toluenesulfonic acid (PTSA) (5 mol %) were suspended in toluene in a round-bottomed flask. The apparatus was closed by fitting a Soxhlet extractor half-filled with preactivated molecular sieves 4 Å, a condenser, and a CaCl2 guard tube, and the mixture refluxed at ca. 130 °C for the time required to achieve maximum conversion. After cooling down to room temperature, the solvent was removed under vacuum. The crude was purified by conventional solvent recrystallization or column chromatography. The detailed synthesis and characterization of these ligand precursors are given in the Supporting Information. Synthesis of trans-[Ni(o-C6H4Cl)(PPh3)2Cl]. A solution of [Ni(COD)2] (2.75 g, 10 mmol) in THF (100 mL) was prepared in a degassed round-bottomed flask immersed in a bath at −20 °C. Triphenylphosphine (10.5 g, 40 mmol) was grounded until becoming a fine powder and then added under a counterflow of dinitrogen to the flask. During the addition the reaction medium immediately turned to red. The reaction mixture was then stirred for 1 h at room temperature. The latter mixture was again cooled down to −20 °C, and a solution of o-dichlorobenzene (1.13 mL, 10 mmol) in THF (5 mL) was added dropwise. The mixture was allowed to warm up to room temperature and left stirring overnight. The solvent was removed under vacuum, and the crude red solid washed with nhexane (3 × 50 mL) to give a yellow solid that was dried under vacuum. Yield: 5.84 g (80%). 1H NMR (300 MHz, CD2Cl2): δ 7.64− 7.62 (m, 12H, PPh3−Hortho), 7.48−7.33 (m, 6H, PPh3−Hpara), 7.30− 7.25 (m, 12H, PPh3−Hmeta), 6.96 (d, 3JHH = 6.7 Hz, 1H, Ph−Hortho), 6.31−6.24 (m, 2H, Ph−Hmeta), 6.05 (d, 3JHH = 7.3 Hz, 1H, Ph− Hpara). 13C{1H} NMR (75 MHz, CD2Cl2): δ 150.7 (t, 2JCP = 34.5 Hz, Ph−Cipso), 141.6 (t, 3JCP = 4.6 Hz, Ph−CorthoCl), 138.6 (t, 3JCP = 4.0 Hz, Ph−Cortho), 135.0 (t, 2JCP = 5.5 Hz, PPh3−Cortho), 131.8 (t, 1JCP = 21.7 Hz, PPh3−Cipso), 130.0 (PPh3−Cpara), 128.1 (t, 3JCP = 4.7 Hz, PPh3−Cmeta), 127.9 (t, 5JCP = 2.5 Hz, Ph−Cpara), 124.5 (t, 4JCP = 2.0 H


Article

Organometallics 4.45−4.31 (m, 1H, CHA(CH3)2), 3.69 (s, 3H, O−CH3), 3.48−3.34 (m, 1H, CHB(CH3)2), 1.44−1.40 (m, 6H, CHA(CH3)2), 0.93−0.88 (m, 6H, CHB(CH3)2). 13C{1H} NMR (75 MHz, CD2Cl2): δ 160.9 (NCH), 158.4 (5-Ph−Cpara), 153.6 (C5 pyrr), 146.5 (N−Ph− Cipso), 144.3 (N−Ph−CorthoB), 141.9 (N−Ph−CorthoA), 141.7 (d, 3JCP = 2.7 Hz, NiPh−CorthoCl), 140.7 (d, 3JCP = 3.5 Hz, C2 pyrr), 140.4 (d, 2 JCP = 53.2 Hz, NiPh−Cipso), 140.4 (d, 3JCP = 2.1 Hz, NiPh−Cortho), 133.9 (d, 2JCP = 9.9 Hz, PPh3−Cortho), 131.9 (d, 1JCP = 43.6 Hz, PPh3−Cipso), 130.6 (5-Ph−Cipso), 129.6 (d, 4JCP = 2.4 Hz, PPh3− Cpara), 129.2 (5-Ph−Cortho), 127.9 (d, 3JCP = 9.8 Hz, PPh3−Cmeta), 126.6 (d, 4JCP = 2.7 Hz, NiPh−CmetaA), 125.5 (N-Ph−Cpara), 123.9 (d, 5 JCP = 1.0 Hz, NiPh−Cpara), 123.2 (d, 4JCP = 2.9 Hz NiPh−CmetaB), 122.7 (N−Ph−CmetaB), 122.4 (N−Ph−CmetaA), 119.4 (C3 pyrr), 114.3 (C4 pyrr), 113.6 (5-Ph−Cmeta), 55.5 (O-CH3), 29.2 (CAH(CH3)2), 27.8 (CBH(CH3)2), 27.0 (CAH(CH3)(CH3)), 26.0 (CBH(CH3)(CH3)), 23.7 (CAH(CH3)(CH3)), 22.1 (CBH(CH3)(CH3)). 31P NMR (121 MHz, CD 2 Cl 2 ): δ 11.99. Anal. Calcd for C48H46N2NiOPCl: C 72.79, H 5.85, N 3.54. Found: C 72.92, H 5.91, N 3.52. Complex 3. Deprotonation of III (0.436 g, 1.25 mmol) with NaH (0.034 g, 1.40 mmol), gave in situ sodium salt IIINa as a light yellow powder that reacted with complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] (0.873 g, 1.20 mmol). The residue was washed with n-hexane, and the remaining orange solid extracted with diethyl ether, yielding 3 as a dark orange crystalline solid. Yield: 0.455 g (49%). 1H NMR (300 MHz, CD2Cl2): δ 7.65 (d, 3JHH = 5.5 Hz, 1H, NCH), 7.39−7.34 (m, 2H, 5-Ph−Hortho), 7.31−7.23 (m, 9H, PPh3−Hpara and PPh3− Hortho), 7.13 (t, 3JHH = 6.8 Hz, 6H, PPh3−Hmeta), 6.99 (d, 3JHH = 7.5 Hz, 1H, N−Ph−HmetaA), 6.89 (t, 3JHH = 7.6 Hz, 1H, N−Ph−Hpara), 6.83 (d, 3JHH = 3.4 Hz, 1H, H3 pyrr), 6.67 (d, 3JHH = 7.5 Hz, 1H, N− Ph−HmetaB), 6.57 (t, 3JHH = 8.7 Hz, 2H, 5-Ph−Hmeta), 6.37 (d, 3JHH = 7.6 Hz, 1H, NiPh−HmetaA), 6.27 (t, 3JHH = 7.3 Hz, 1H, NiPh−Hpara), 6.11 (d, 3JHH = 3.4 Hz, 1H, H4 pyrr), 5.89 (t, 3JHH = 7.1 Hz, 1H, NiPh−HmetaB), 5.62 (d, 3JHH = 7.4 Hz, 1H, NiPh−Hortho), 4.45−4.32 (m, 1H, CH A (CH 3 ) 2 ), 3.47−3.34 (m, 4.4H, CH 2 Et 2 O + CHB(CH3)2), 1.45 (d, 3JHH = 6.7 Hz, 3H, CHA(CH3)(CH3)), 1.39 (d, 3JHH = 6.6 Hz, 3H, CHA(CH3)(CH3)), 1.16 (t, 3JHH = 7.0 Hz, 5H, CH3 Et2O), 0.92 (d, 3JHH = 7.2 Hz, 3H, CHB(CH3)(CH3)), 0.90 (d, 3 JHH = 7.1 Hz, 3H, CHB(CH3)(CH3)). 13C{1H} NMR (75 MHz, CD2Cl2): δ 161.7 (d, 1JCF = 243.7 Hz, 5-Ph−Cpara), 161.3 (NCH), 160.5 (5-Ph−Cipso), 152.5 (C5 pyrr), 146.3 (N−Ph−Cipso), 144.1 (N−Ph−CorthoB), 141.8 (N−Ph−CorthoA), 141.6 (d, 3JCP = 2.8 Hz, NiPh−CorthoCl), 141.2 (d, 2JCP = 3.3 Hz, C2 pyrr), 140.2 (d, 2JCP = 50.0 Hz, NiPh−Cipso), 140.2 (d, 3JCP = 2.2 Hz, NiPh−Cortho), 134.0 (d, 2JCP = 9.9 Hz, PPh3−Cortho), 131.8 (d, 1JCP = 43.7 Hz, PPh3−Cipso), 129.8 (d, 4JCP = 2.4 Hz, PPh3−Cpara), 129.5 (d, 3JCF = 7.9 Hz, 5-Ph− Cortho), 128.0 (d, 3JCP = 9.8 Hz, PPh3−Cmeta), 126.8 (d, 4JCP = 2.6 Hz, NiPh−CmetaA), 125.7 (N−Ph−Cpara), 124.0 (NiPh−Cpara), 123.4 (d, 4 JCP = 2.9 Hz, NiPh−CmetaB), 122.8 (N−Ph−CmetaB), 122.5 (N−Ph− CmetaA), 119.3 (C3 pyrr), 114.7 (d, 2JCF = 18.0 Hz, 5-Ph−Cmeta), 114.6 (C4 pyrr), 66.1 (CH2 Et2O), 29.3 (CAH(CH3)2), 27.8 (CBH(CH3)2), 27.0 (CAH(CH3)(CH3)), 26.0 (CBH(CH3)(CH3)), 23.7 (CAH(CH3)(CH3)), 22.1 (CBH(CH3)(CH3)), 15.5 (CH3 Et2O). 19F NMR (282 MHz, CD2Cl2): δ −118.31 (s). 31P NMR (121 MHz, CD2Cl2): δ 12.43. Anal. Calcd for C47H43FN2NiPCl·0.82C4H10O· 0.16[Si(CH3)2O]: C 71.28, H 6.17, N 3.29. Found: C 71.30, H 6.19, N 3.28. Complex 4. Deprotonation of IV (0.646 g, 1.50 mmol) with NaH (0,040 g, 1.68 mmol), gave in situ sodium salt IVNa as a yellow powder that reacted with complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] (1.059 g, 1.45 mmol). The residue was washed with n-pentane, and the remaining brown-orange solid extracted with diethyl ether. As an unreacted amount of starting material precipitated together with the product, several extractions with small amounts of cold HMDSO allowed the removal of the impurity. A last recrystallization in diethyl ether at −20 °C afforded 4 (0.288 g, 23%) as a dark orange crystalline solid. An intermediate crop was evaporated and recrystallized in toluene, affording crystals suitable for X-ray diffraction at −20 °C. The bulk material of 4 is composed by a mixture of two isomers A and

B, in a ratio 2:1 (A/B), which is invariant with temperature (from −80 to 45 °C) in the NMR spectra. The major isomer, containing the imine group located cis to PPh3, is represented by A, whereas B represents the trans (minor) isomer. 1 H NMR (300 MHz, CD2Cl2): δ 8.42 (d, 3JHH = 7.9 Hz, 1H, H1 anthrB), 8.28 (d, 3JHH = 8.7 Hz, 2H, H1 anthrA), 8.03 (s, 2H, H10 anthrA), 7.95 (s, 1H, H10 anthrB), 7.87−7.78 (m, 7H, H4 anthrA + H8 anthrA + H3 anthrB + NCHB), 7.72−7.59 (m, 6H, H5 anthrA + H7 anthrB + H2 anthrA + H5 anthrB), 7.50−7.37 (m, 17H, NCHA + H3 anthrA + H2 anthrB + PPh3A−Hortho), 7.30−7.25 (m, 3H, H6 anthrA + H6 anthrB), 7.21−7.08 (m, 13H, H7 anthrA + H4 anthrB + PPh3A− Hpara + PPh3B−Hpara + H8 anthrB), 7.01−6.93 (m, 17H, H3 pyrrA + H3 pyrrB + N−PhA−Hpara + PPh3A−Hmeta), 6.86−6.80 (m, 13H, PPh3B−Hortho + PPh3B−Hmeta + N−PhB−Hpara), 6.72 (d, 3JHH = 7.3 Hz, 4H, N−PhA−Hmeta), 6.64 (d, 3JHH = 7.1 Hz, 2H, N−PhB−Hmeta), 6.34 (d, 3JHH = 7.1 Hz, 1H, NiPhB−HmetaCl), 6.18−6.17 (m, 2H, H4 pyrrA), 6.09−6.00 (m, 2H, NiPhB−Hpara + H4 pyrrB), 5.49 (t, 3JHH = 7.5 Hz, 1H, NiPhB−Hmeta), 5.20−5.03 (m, 7H, NiPhA−HmetaCl + NiPhA−Hortho + NiPhB−Hortho + NiPhA−Hpara), 4.88−4.76 (m, 1H, CHB1(CH3)2), 4.66 (t, 3JHH = 7.3 Hz, 2H, NiPhA−Hmeta), 4.61−4.52 (m, 2H, CHA 1(CH3)2), 3.48−3.32 (m, 3.2H, CH2 Et2O + CHB2(CH3)2), 3.19−3.10 (m, 2H, CHA2(CH3)2), 1.50−1.43 (m, 6H, CHB1(CH3)(CH3) + CHB1(CH3)(CH3)), 1.33 (d, 3JHH = 6.7 Hz, 6H, CHA1(CH3)(CH3)), 1.24 (d, 3JHH = 6.4 Hz, 6H, CHA1(CH3)(CH3)), 1.19−1.12 (m, 13.3H, CHB2(CH3)(CH3) + CH3 Et2O + CHA2(CH3)(CH3)), 0.89 (d, 3JHH = 6.4 Hz, 3H, CHB2(CH3)(CH3)), 0.79 (d, 3JHH = 6.7 Hz, 6H, CHA2(CH3)(CH3)). 13C{1H} NMR (75 MHz, CD2Cl2): δ 163.9 (NCHA), 161.7 (NCHB), 149.6 (N− Ph−CorthoA), 149.5 (N−Ph−CorthoB), 148.3 (q), 146.7 (q), 143.7 (q), 142.3 (d, 3JCP = 4.1 Hz, NiPhA−CorthoCl), 141.5 (q), 140.7 (d, 3JCP = 3.7 Hz, NiPhB−CorthoCl), 139.6 (d, 3JCP = 2.9 Hz, NiPhB−Cortho), 139.0 (d, 3JCP = 3.6 Hz, NiPhA−Cortho), 138.7 (q), 138.0 (q), 134.5 (q), 134.2 (d, 2JCP = 10.1 Hz, PPh3A−Cortho), 133.7 (d, 2JCP = 10.4 Hz, PPh3B−Cortho), 132.6 (q), 132.0 (q), 131.5 (d, 1JCP = 23.4 Hz, PPh3B− Cipso), 130.7 (q), 130.0 (d, 1JCP = 41.9 Hz, PPh3A−Cipso), 130.0 (q), 129.5 (PPh3A−Cpara + C1 anthrB), 128.8 (C1 anthrA), 128.3 (PPh3B− Cpara + C10 anthrB), 127.7 (PPh3A−Cmeta + PPh3B−Cmeta), 126.3 (NiPhA−CmetaCl), 125.9 (C10 anthrA), 125.6 (NiPhB−CmetaCl), 125.3 (anthr), 125.1 (anthr), 125.0 (anthr), 124.7 (anthr), 124.4 (N−PhA− Cmeta), 124.2 (N−PhB−Cpara), 123.7 (N−PhB−Cmeta), 123.0 (NiPhB− Cmeta), 122.8 (NiPhB−Cpara), 122.4 (NiPhA−Cpara), 122.2 (N−PhA− Cpara), 122.0 (NiPhA−Cmeta), 119.7 (C4 pyrrB), 119.1 (C3 pyrrB), 118.2 (d, 4JCP = 3.3 Hz, C4 pyrrA), 117.9 (C3 pyrrA), 66.1 (CH2 Et 2 O), 29.5 (CH B 1 (CH 3 ) 2 ), 29.2 (CH A 1 (CH 3 ) 2 ), 28.3 (CHA2(CH3)2), 27.8 (CHB2(CH3)2), 26.3 (CHB1(CH3)(CH3)), 26.0 (CH A 2 (CH 3 )(CH 3 )), 25.8 (CH A 1 (CH 3 )(CH 3 )), 23.7 (CHB2(CH3)(CH3)), 23.1 (CHB1(CH3)(CH3)), 22.4 (CHB2(CH3)(CH3)), 22.0 (CHA2(CH3)(CH3)), 21.9 (CHA1(CH3)(CH3)), 15.52 (CH3 Et2O). 31P NMR (121 MHz, CD2Cl2): δ 18.87 PPh3A, 11.90 PPh3B. Anal. Calcd for C55H48N2NiPCl·0.6[Si(CH3)2O]: C 74.46, H 5.74, N 3.10. Found: C 74.86, H 5.70, N 2.85. X-ray Data Collection. Crystallographic and experimental details of the crystal structure determinations are listed in Tables S1 and S4. The crystals were selected from the growing environment (under a dinitrogen flow, when sensitive to air and/or moisture) and covered with polyfluoroether oil. Crystallographic data were collected using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) on a Bruker AXS-KAPPA APEX II diffractometer equipped with an Oxford Cryosystem open-flow nitrogen cryostat, at 150 K. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT on all observed reflections. Absorption corrections were applied using SADABS.35 Structure solution and refinement were performed using direct methods with program SIR200436 and SHELXL,37 both included in the software package WinGX v2014.1.38 Despite crystals of ligand precursor III showed poor diffracting power, size, and crystal quality (Rint of 0.152), the structure refined to a perfect convergence. Non-hydrogen atoms were refined anisotropically and the hydrogen atoms, except the NH protons, were inserted in idealized positions and allowed to refine riding on the parent carbon atom. Figures of the molecular structures were generated using I


Article

Organometallics Mercury 3.9, software available at the Cambridge Crystallographic Data Centre. Data have been deposited with the CCDC under deposition numbers 1833543 for II, 1833544 for III, 1833545 for 2 and 1833546 for 4. The supplementary crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Catalytic Oligo-/Polymerization Tests. Polymerizations were carried out in 300 mL Miniclave Drive Büchi pressure reactors, equipped with magnetically stirred glass reaction vessels and with external protective meshes. The reaction vessels were previously dried in the oven at 140 °C and degassed, and freshly distilled toluene (50 mL) was added under a counterflow of nitrogen. The predefined reaction temperature was set and the solvent allowed to equilibrate. A quick degassing was applied and the reactor was filled with an ethylene atmosphere of 5 bar (absolute pressure). For the experiments with the cocatalyst [Ni(COD)2], a 10−2 M toluene solution (2 mL) of the latter was added, immediately followed by a 10−2 M toluene solution (1 mL) of 1−4 with a glass syringe. For the catalytic reactions in the absence of cocatalyst, only the toluene solution of catalyst 1−4 was added. The pressure was raised up to 9 bar (absolute pressure) and maintained during the 2 h of reaction. The ethylene supply was closed at the end of the catalytic test and the gas pressure slowly vented to the fume hood. The reaction medium was quenched with methanol while stirring and the products were isolated by removal of the volatiles, yielding pale yellow to colorless oils that were dried until constant weight in a vacuum oven, at room temperature. Several blank tests were performed, only using 2 equiv of [Ni(COD)2] and with a mixture of [Ni(COD)2]:PPh3 (2:1), all revealing inactivity in the tested conditions (25 or 50 °C and 9 bar of ethylene). The ethylene gas uptake was monitored by means of a Bronkhorst EL-Flow mass flowmeter (ref: F-111CM-500-AGD-88-K), which was connected to an acquisition computer through a RS232 interface. The flowmeter software (FlowDDE, FlowView and FlowPlot) allowed the monitoring and acquisition of the data in the computer. The reaction procedure was the same as described above, with the reactor pressurized and allowed to equilibrate for 10 min before the flow meter valve was opened and the data acquisition began, followed by the addition of the solution catalyst. GPC/SEC Analyses. Gel permeation chromatography/size exclusion chromatography (GPC/SEC) analyses were performed in a HPLC Waters chromatograph, containing an isocratic pump Waters 1515 and a refractive index detector Waters 2414. The oven was stabilized at 30 °C and the elution of samples was carried out through two PolyPore columns, protected by a PolyPore Guard column (Polymer laboratories). The software Empower performed the acquisition and data processing. THF was used as eluent, at a flow rate of 1.0 mL/min. Before use, the solvent was filtrated through 0.45 μm PTFE membranes Fluoropore (Millipore) and degassed in an ultrasound bath for 15−20 min. The PE samples were filtered through 0.20 μm PTFE filters Durapore (Millipore). Molecular weights were calibrated relative to polystyrene standards (TSK Tosoh Co.); therefore, it should be taken into account that deviations can occur when polyethylene samples were analyzed, owing to differences in the hydrodynamic volumes of the two polymers. Computational Details. All details are given in the Supporting Information.

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tion (NMR spectra, equations, and GPC/SEC chromatograms) (PDF) Cartesian coordinates (XYZ) Accession Codes

CCDC 1833543−1833546 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Luis F. Veiros: 0000-0001-5841-3519 Pedro T. Gomes: 0000-0001-8406-8763 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Fundaçaõ para a Ciência e Tecnologia for financial support (Projects PTDC/EQU-EQU/110313/2009, UID/QUI/00100/2013, and UID/ECI/04028/2013) and for fellowships to C.A.F., P.S.L., C.S.B.G., and P.T.G. (SFRH/ BPD/112340/2015, SFRH/BD/88639/2012, SFRH/BPD/ 107834/2015, and SFRH/BSAB/140115/2018, respectively).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00669. Detailed synthetic procedures (II and III), NMR data (II, III, IINa, IIINa), and spectra (1−4), X-ray diffraction discussion (II and III) and data (II, III, 2, and 4), computational details, and polyethylenes characterizaJ


Article

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Organometallics

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Neutral Mono(5-aryl-2-iminopyrrolyl)nickel(II) Complexes as

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