Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced

Apr 13, 2017 - Diphosphinoamines (PNP) are commonly used to support Cr-catalyzed ethylene trimerization and tetramerization. Although isomerization of...
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Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced Isomerization of PNP to PPN Ligands Alejo M. Lifschitz, Nathanael A. Hirscher, Heui Beom Lee, Joshua A. Buss, and Theodor Agapie* Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, MC 127-72, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: Diphosphinoamines (PNP) are commonly used to support Cr-catalyzed ethylene trimerization and tetramerization. Although isomerization of PNP to a PPN (iminobisphosphine) species has been established, such reactivity has not been studied in detail in the context of Cr-based selective ethylene oligomerization catalysis. Herein, we show that precursors that are stable as PNP frameworks can isomerize to PPN species in the presence of chlorinated aluminum activators relevant to ethylene oligomerization catalysis. Isomerization changes the pattern of reactivity of the ligands, making them more susceptible to nucleophilic attack by alkyl groups, resulting in a variety of degradation products. The isomerization-mediated degradation of PNP ligands leads to the formation of unwanted polymerization catalysts in ethylene tetramerization systems, thus providing insight into the formation of Cr species that affect the overall selectivity and activity values. For example, independently prepared [R2PNR] leads to potent Cr polymerization catalysts. The susceptibility for isomerization is dependent on the nature of the N-substituent of the PNP precursor. Electron donating N-substituent i-Pr, which disfavors the PPN isomer compared to p-tolyl, and minimization of water contamination correlate with higher oligomerization activity and lower polymer byproducts. More broadly, the present study demonstrates the significant impact that Al-activators can have on the structure and behavior of the supporting ligand leading to detrimental reactivity.

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Scheme 1. Typical Selective Ethylene Trimerization and Tetramerization Catalysts To Generate α-Olefins Require Excess Aluminum-Based Activators and Result in Polymeric and Isomerized Byproducts

atalytic oligomerization of ethylene into linear alpha olefins (LAOs) is an important industrial process that yields valuable commodity products whose utility depends on the LAO chain length.1−4 Selective LAO production is desirable in terms of avoiding energy-intensive separations and disentangling the economics of producing and commercializing each LAO present in mixtures. Selective ethylene oligomerization catalysts proposed to operate via metallacycle-based mechanisms have been the focus of much research in the past two decades.5−20 Systems based on diphosphinoamine (PNP) ligands with pendant ether moieties21 and Cr(III) precursors achieve remarkable activities and selectivities of above 90% for the C6 fraction and 99.9% for 1-hexene within this fraction.10 Related Cr(PNP) catalysts have been demonstrated to also perform ethylene tetramerization, although with lower selectivity (Scheme 1).22 Optimization of such systems has been hampered by the limited understanding of the catalyst activation process and the identity of the catalytic species that give rise to different oligomeric and polymeric products.14,23−28 Due to the paramagnetic nature of the Cr center and the requirement for large excesses of alkyl aluminum activators, there are several aspects of ethylene tetramerization systems that remain poorly understood. There have been studies on the effect that the N- and P-substituents of the PNP ligand have on product selectivity and activity, yet the nature of the catalytically relevant species remains elusive.29−35 Furthermore, while catalytic activity is affected by the activator employed,36−38 the activation process itself is not well understood.39−41 Several Cr complexes have been isolated from reactions with Al reagents, yet it remains unclear whether © 2017 American Chemical Society

those species are representative of the active components in the catalytic mixture because of challenges in observing them in situ.39,41−44 Little has been reported about the fate of the supporting PNP ligands in the presence of typical Al-based activators. Given the PNP−PPN isomerism, which can be affected by the nature of the substituents, protonation state, or metal coordination (Figure 1, top),45−47 conversion to a different ligand framework in the catalytic mixture is a possibility. Such an isomerization process may impact product selectivity in Cr−PNP ethylene oligomerization catalysis. Herein, we show that interactions between Al activators and PNP precursors lead to rearrangement to PPN structures, in Received: March 10, 2017 Published: April 13, 2017 1640

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indicative of binding to the paramagnetic Cr(III) center. Resulting complexes 3 and 4 were characterized by singlecrystal X-ray diffraction (XRD). Both 1 and 2 lead to dinuclear structures in the solid state in which the Cr(III) centers are bridged by two μ2-chloride anions. Complex 3 displays moderately contracted Cr−P and Cr−Cr distances, as compared to complex 4, while the bite angles of the ligands in 4 are slightly larger than in 3 by 1.6° (Figure 2). These structures are similar to PNP−Cr complexes previously reported.22,50

Figure 1. Factors affecting the PPN vs PNP isomerization (top)45−48 and the effect of Al reagents reported here (bottom).

contrast to the reverse conversion typically observed upon metal coordination.45,49 The isomerized species are prone to subsequent reactivity, ultimately resulting in undesired but highly active ethylene polymerization catalysts. We investigate the effect of N-substitution and the nature of the Al reagents on the isomerization process. Moisture and halides increase the ability of Al alkyl precursors to promote the isomerization of PNP to PPN. The electronic basis for PNP to PPN isomerization and implications for catalysis are discussed.

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RESULTS AND DISCUSSION Synthesis of Chromium Complexes. Both p-tolyl and ipropyl PNP ligands, 1 and 2 respectively, were utilized as representative examples of the large number of N-aryl and Nalkyl ligands that have been scrutinized in the literature for ethylene oligomerization (Scheme 2).29−35 Chromium complexes of these phosphines were prepared as previously reported for related PNP ligands.22 Treatment of CrCl3(THF)3 with PNP in toluene leads to the disappearance of the resonance in the 31P{1H} NMR spectra corresponding to the free phosphines (δ = 69 ppm for 1 and 48 ppm for 2),

Figure 2. Solid state structures of 3 (A) and 4 (B) with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å) and angles (deg): for 3 Cr1−P1 2.495(4), Cr1−P2 2.407(4), Cr1−Cl1 2.278(5), Cr1−Cl3 2.285(4), Cr1−Cl2 2.351(5), Cr1−Cl2′ 2.383(4), P1−Cr1−P2 66.62(1), P1−N1−P2 103.61(5); for 4 Cr1−P1 2.450(5), Cr1−P2 2.503(6), Cr1−Cl1 2.295(6), Cr1−Cl2 2.350(4), Cr1−Cl2′ 2.409(5), Cr1−Cl3 2.271(6), P1−Cr1−P2 66.44(2), P1− N1−P2 105.52(7). H atoms have been omitted for clarity.

Scheme 2. PNP Ligands (1 and 2) and PNP−Cr(III) (3 and 4) Precatalysts Employed in This Study

Reactions of 1 with Molecular Al Alkyl Reagents. Certain electron withdrawing N-substituents have been shown to stabilize PPN isomers rather than the more common PNP frameworks (Figure 1).47,51 Furthermore, while no PNP to PPN isomerization has been reported for PNP ligands employed in catalysis, interconversion between the two kinds of frameworks has been shown to be driven in one example by protonation of pyridine in a PNP−pyridine derivative.46 This 1641

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nitrogen (Figure 3). The Al-methyl group is disordered, displaying partial (12%) chloride occupancy, corresponding to

rearrangement is likely facilitated by the delocalization of the negative charge developed on the PPN nitrogen onto the positively charged pyridinium moiety. Phosphacyclic moieties have been reported to support the PPN isomer in the Nprotonated and deprotonated forms.48 The protonated version maintains the PPN linkage upon coordination to Pt, while the deprotonated version isomerizes to PNP upon binding to Pd.48 In the context of Cr-catalyzed ethylene oligomerization, this isomerization process may occur upon interaction with Lewis acidic Al activators relevant to ethylene oligomerization catalysis. The interaction of phosphine 1 with molecular Al species was studied. The 31P{1H} NMR spectrum of a mixture of 1 and 30 equiv of AlMe3 exhibits a modest shift upfield indicative of an interaction between the phosphine and the Lewis acidic Al compound, without changes in the PNP linkage (Scheme 3).

Figure 3. Solid state structure of 7b/7c with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å) and angles (deg): P1−P2 2.2193(9), P1−N1 1.628(2), N1−Al1 1.911(2), P2−P1−N1 107.55(8), P1−N1−Al1 125.6(1). Partial occupancy of Cl and CH3 was modeled for Cl1/C1 ligand of Al. H atoms have been omitted for clarity.

Scheme 3. (A) PNP Compound 1 Binds to AlMe3 without Isomerization to PPN. (B) Isomerization of PNP Compound 1 to PPN (7) Induced by Coordination to a Lewis Acidic Al Center

the formation of 7c and cocrystallization with 7b. The P−P distance in 7b/7c (2.2193(9) Å) is similar to that observed for a pyridinium modified PPN species and slightly shorter than in other PPN isomers modified with electron withdrawing Nsubstituents.51 On the other hand, the P−N distance in 7b/7c is longer by an average of 0.05 Å when compared to other examples of metal-free PPN motifs, indicating weakening of the P−N interaction by binding of Al. The short distance between the N-tolyl and one of the P-phenyl groups (3.26 Å) is in the range of π−π interactions.52 Given that excess Al activator is typically used in catalytic systems, the effect of adding AlMe3, which does not promote isomerization of PNP to PPN, was investigated. A mixture of 1 and AlMe3 (50 equiv) was treated with 2 equiv of AlClMe2 (Scheme 4A). No isomerization was observed over 2 days, indicating that AlMe3 acts as an inhibitor of the isomerization promoted by AlClMe2. This suggests that the excess aluminum activator employed in catalysis could serve the role of shielding the ligand from more acidic Al species that may result from the activation process. Excess AlMe3 may compete for association

The spectrum remains the same upon monitoring for several days (Figure S14). Given that a single peak is observed in the 31 P NMR spectrum, κ2-(P,P) coordination is proposed (compound 5), although a dynamic process, that is fast on the NMR time scale, involving switching between binding κ1 to the different P donors cannot be ruled out. As the aluminum activator may include halides, for example by ligand substitution with CrCl3(THF)3, 1 was treated with AlClMe2 in toluene (Scheme 3). Initially, the 31P{1H} NMR spectrum exhibited a broadened resonance slightly shifted upfield indicative of coordination of 1 to Al (6). In the course of a day, two sets of doublets appear in the 31P{1H} NMR spectrum of the mixture (δ = 38.0 and −10.8 ppm, 1JP−P = 293 Hz; 38.9 and −11.3 ppm, 1JP−P = 293 Hz). The presence of doublets with large one bond P−P coupling is consistent with isomerization. The observation of two sets of doublets by 31 1 P{ H} NMR and two peaks for the Al−CH3 by 1H NMR spectroscopy is consistent with ligand exchange, corresponding to a mixture of 7a and 7b. The conversion to species with higher halide content indicates a propensity to bind chloride to Al upon coordination of the PPN isomer. XRD analysis of single crystals obtained from this mixture revealed the formation of a PPN isomer coordinated to an Al center via

Scheme 4. PNP to PPN Isomerization Dependence on the Addition of AlMe3: (A) Isomerization Can Be Prevented by Addition of AlMe3 to the PNP Isomer. (B) Once the PPN Adduct Is Formed, the Excess AlMe3 Does Not Drive the Back-Isomerization to the Native PNP Isomer

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agent, AlMe3 (vide supra). Addition of excess AlMe3 leads to complete conversion to species that do not display the PNP motif. The 31P spectrum displays major peaks at 24 and −24 ppm. The peak at 24 ppm is reproduced upon treatment of [Ph2PN(p-tolyl)]Li with 2 equiv of AlMe2Cl followed by 50 equiv of AlMe3, suggesting that it is a [Ph2PN(p-tolyl)]− complex or a degradation product derived from it. The peak at −24 ppm is assigned to Al-coordinated PPh2Me. These products indicate again that the PPN isomer is susceptible to cleavage by alkylation. In contrast, compound 1 does not degrade upon treatment with AlMe3 (vide supra). Reactions of 1 with M-MAO. To test whether ligand isomerization and degradation take place with the activator used in catalysis, reactivity with modified methylaluminoxane type 3A (M-MAO, formula: [(CH3)0.7(i-C4H9)0.3AlO]n) was investigated. 1 was treated with 30 equiv of M-MAO in toluene, resulting in only a minor shift in the 31P{1H} NMR spectrum, indicative of association with the Lewis acidic M-MAO, but no isomerization. Given the difference in reactivity of the halide coordinated Al reagent compared to AlMe3 (vide supra), generation of halogenated M-MAO was attempted. CrCl3(THF)3 was employed as a source of chloride to exchange with alkyl groups on M-MAO, as might occur in an actual catalytic system. Toward this end, a mixture of M-MAO and 1 was treated with CrCl3(THF)3. Cyclam was added to sequestrate the paramagnetic Cr(III) centers which hinder the detection of the P-containing products by 31P NMR spectroscopy. Upon filtration through glass wool, the filtrate displays a major peak at 69 and minor peaks at 29 and −27 ppm in the 31 P NMR spectrum. The major peak corresponds to 1, while the minor ones correspond to products of PNP cleavage (Ph2PNH(p-tolyl) or Al coordinated [Ph2PNH(p-tolyl)]−, and PPh2Me). The formation of PPh2Me and the PN moiety indicates cleavage of PNP. A similar outcome is observed when treating 1 with M-MAO in the presence of equimolar amounts of water, also yielding PPh2Me (and its oxidation product, OPPh2Me). With certain batches of M-MAO, peaks displaying features reminiscent of the PPN isomer are also observed, suggesting that isomerization precedes cleavage. This demonstrates that PNP binding, isomerization to PPN, and degradation, which were characterized above in a stepwise fashion with molecular alkyl aluminum complexes, can all be driven by M-MAO upon increasing its Lewis acidity via either halogenation or further hydrolysis. Reactions of 2 with Alkyl Al Reagents. Similar experiments were performed with compound 2, for comparison to 1. Mixing 2 with 3 equiv of AlClMe2 in toluene does not lead to isomerization of the ligand to PPN (Scheme 6), but rather results in simple coordination to the Al center, as evidenced by a modest shift in the 31P{1H} NMR spectrum and the solid state structure of 8 (Figure 4). For comparison to 1, 2 was treated with M-MAO and water, as described above for 1. Peaks corresponding to the degradation of the PNP motif were not

to the ligand scaffold or directly react with Al byproducts to generate species less reactive toward isomerization of PNP to PPN. The reversibility of the isomerization was tested by adding excess AlMe3 (50 equiv) to the preformed Al(PPN) adduct 7a/ 7b. The Al(PPN) adduct is the major species observed at short reaction times (3 h). This result indicates that, once isomerized, the PPN ligand is kinetically or thermodynamically trapped and the process is not reversed by the excess isomerization inhibitor AlMe3. Therefore, if the ligand necessary for catalysis is the PNP isomer, the activation procedure must prevent isomerization. Furthermore, at longer reaction times, further conversion is observed (vide infra). Overall, this is the first observation that phosphine precursors that are stable as PNP species can be converted to PPN isomers via interaction with a metal-based Lewis acid. The difference in reactivity of Al reagents highlights the importance of assessing the nature of the species generated in solution upon precatalyst activation, for example, as a result of halide abstraction from the chromium precursor. Most importantly, isomerization to PPN species results in reactivity distinct from that of parent PNP ligands. For instance, 1 displays no reactivity with methylmagnesium bromide after 16 h, as probed via 31P{1H} NMR spectroscopy (Scheme 5a). Scheme 5. Conversion to PPN Isomer Leads to New Reactivity toward Nucleophiles: (A) The Native PNP Framework Is Unreactive toward an Alkyl Grignard Nucleophile. (B) The PPN Al Adduct 7a/7b Is Cleaved into Monophosphine and PN Species

On the other hand, treatment of 7a/7b with 1 equiv of methylmagnesium chloride leads to a mixture of species (Scheme 5b). Specifically, the 31P{1H} NMR spectrum of the reaction mixture shows three new resonances at 69, 29, and −26 ppm, in addition to starting material, 7a. These products are assigned to free PNP (1), a PN product resulting from PNP cleavage, and PPh2Me, respectively. PNP (1) is proposed to form upon alkylation of the Al center resulting in a metal site that does not support the PPN isomer and promotes isomerization back to PNP. Alkylation at the terminal P site results in PPh2Me and a metal coordinated [R2PNR′]− ligand.53 Treatment of independently prepared [Ph2PN(ptolyl)]Li with 2 equiv of AlClMe2 in toluene yields a species resonating at 30 ppm in the 31P{1H} NMR spectrum, assigned to the Al coordinated [R2PNR′]− ligand. Treatment with methylmagnesium bromide does not cause further transformation of this species after 20 h. The observed reactivity suggests that isomerization to PPN renders the ligand susceptible to decomposition in alkylating environments. The propensity for cleavage was further tested with an Al alkylating

Scheme 6. Reaction of 2 with AlClMe2

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doublets at 52.3 and 21.4 ppm (JP−P = 11.7 Hz). The small coupling constant observed here is also consistent with maintaining the PNP motif. This synthetic protocol allows the isolation of X-ray quality crystals (Figure 5). XRD studies

Figure 4. Solid state structure of 8 with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å) and angles (deg): P1−Al1 2.7198(8), P2−Al1 2.4570(6), P1−Al1−P2 64.41(2), P1−N2−P2 108.47(8). H atoms have been omitted for clarity.

observed in this case. The N-alkyl substituted PNP framework is clearly more robust than the N-aryl substituted version in the presence of Lewis acids and alkylating agents. Reactions with Brønsted Acids. Compounds 1 and 2 clearly show distinct reactivities with Al-based Lewis acids. To gain more insight into the ligand behavior, reactions with Brønsted acids were investigated (Scheme 7). Diphosphines 1

Figure 5. Solid state structure of 10+[AlCl4]− with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å) and angles (deg): P1−N1 1.645(1), P2−N1 1.748(2), P1−N1−P2 125.88(9). H atoms have been omitted for clarity.

reveal a PNP group protonated at one of the P centers. The anion is [AlCl4]−. As with Al Lewis acids, these experiments with Brønsted acids demonstrate that alkyl substitution makes the PNP motif less prone to PPN isomerization. It is worth noting that PPN isomerization has been proposed previously for diphosphine 2, upon treatment with trityl cation.50 The final product in that case, assigned as a PPN species, is reported to have peaks at 53 and 22 ppm in the 31P NMR spectrum. These peaks are notably close to species 10+, which displays the PNP linkage. In situ generated species 10+, in the presence of water, leads to further conversion to a new complex. Crystallographic characterization shows a PPO species with an AlCl3 moiety coordinated to O (11, Figure 6). A potential mechanism of

Scheme 7. Reactions with Brønsted Acids: (A) PNP to PPN Isomerization with 1. (B) Protonation without Isomerization with 2, Followed by Conversion to PPO, in the Presence of H2O and AlCl3

Figure 6. Solid state structure of 11 with thermal ellipsoids at the 50% probability level, obtained from the degradation of 2 and resulting in P−P bond formation. Selected bond lengths(Å): P1−P2 2.1861(9). H atoms have been omitted for clarity.

and 2 were treated with [H(OEt2)2][B(C6H3-3,5-(CF3)2)3] ([H(OEt2)2][BAr′4]) in toluene solution, and analyzed by 31 1 P{ H} NMR spectroscopy. For 1, the NMR spectrum shows two broad doublets at 34.8 and −12.6 ppm (JP−P = 294 Hz) (Figure S6). The coupling constant is indicative of a P−P bond, and therefore isomerization to the PPN isomer.46 Mixing solutions of [H(OEt2)2][BAr′4] and 2, in contrast, generates a 31 1 P{ H} NMR spectrum with two singlets, at 51.3 and 21.0 ppm, suggesting that the PNP moiety is retained (Figure S7). Protonated 2 can also be generated upon treatment with AlCl3 in the presence of water (see Supporting Information for procedure to generate 10+[AlCl4]−). The 31P{1H} NMR spectrum of the resulting species, 10+[AlCl4]−, has two

formation could involve a PPN intermediate that is hydrolyzed to substitute O for NiPr. Although this reactivity pathway is likely not relevant to the catalytic conditions, the formation of 11 indicates that even 2 can undergo degradation, albeit more slowly than 1. Overall, these observations highlight that water can result in PNP ligand isomerization and degradation to different products from both M-MAO and Brønsted acid additives. 1644

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Figure 8). Each Cr center in 12 is coordinated by a κ2-(P,P) PNP ligand, a Me group, and three bridging chlorides. The

Electronic Comparison of 1 and 2. To address the difference in reactivity between 1 and 2, the two isomers were studied by computation. Electrostatic potential maps with simplified models (PH2NHPH2/PH2PH2NH and PMe2NMePMe2/PMe2PMe2NMe Figures 7 and S22, respec-

Figure 8. Solid state structure of 12 with thermal ellipsoids at the 50% probability level. AlCl4 counteranion and H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr1− P1 2.4411(8), Cr1−P2 2.4776(8), Cr1−Cl1 2.3831(7), Cr1−Cl2 2.4458(7), Cr1−Cl3 2.3813(7), P1−Cr1−P2 67.66(2), P1−N1−P2 106.91(12).

Figure 7. Electrostatic potentials mapped onto electron density isosurfaces (isovalue = 0.004 e−/Å3) of model PH2NHPH2 (left) and isomerized PH2PH2NH (right).

PPN motif is not observed. The high isolated yield of 12 in these experiments suggests that the primary mode of reactivity of AlClMe2 corresponds to partial methylation of Cr and halide abstraction. While in the absence of the Cr(III) center the ligand isomerizes to a PPN species, in this case 1 remains as a PNP adduct. Even though these results do not rule out the possibility of the PNP ligand isomerizing upon coordination to the Cr center to generate an undetected minor Cr−PPN species, they do indicate that the major Cr species generated displays κ2-(P,P) PNP binding. Potentially, PNP to PPN isomerization starting from 3 could also occur via a mechanism involving dissociation of 1 from Cr followed by reaction with Al species. Implications to Catalysis. When CrCl3(THF)3 and either 1 or 2 are mixed in toluene under ethylene pressure (60 psi), in the presence of 300 equiv of M-MAO activator, both ligands give rise to catalysts selective for 1-hexene and 1-octene production (Table 1). The catalytic setup employed consists of a Fischer−Porter pressure vessel connected to an ethylene feed through a line equipped with a Matheson PUR-Gas In-Line Purifier Combi Oxygen/Moisture trap. Under these conditions, both ligands lead to significant amounts of polyethylene, with this behavior being more pronounced for tolyl N-substituted 1. The activity and selectivity values are similar to those observed in previously published studies using a similar catalytic setup and low pressures of ethylene.31 Overall, these observations follow the same trend previously observed in the literature, in which N-alkyl substitution improves the activity of the resulting tetramerization catalysts and concomitantly decreases the amount of polymeric byproducts.54 The generation of polymer has significant impact on large-scale industrial setups in terms of reactor fouling.55 The experiments above provide a number of insights about the factors affecting the formation of active catalytic species based on the chemistry of the Al activators and the supporting PNP ligands. They suggest that the Lewis acidity of the activator can vary across the activation process, enabling a stronger interaction with the ligand upon reaction with trace moisture and the halogenated Cr precursor.56 This stronger interaction may help the Al center better compete with Cr for coordination to the PNP framework, dissociating the

tively) show that isomerization results in a significant redistribution of electronic density within the PNP framework. The PPN isomer displays a relative increase in electronic density around the N atom. This is consistent with the binding of Al or protons through N rather than P. Furthermore, the more electron withdrawing N-substituents are expected to stabilize the PPN isomer relative to more electron rich substituents. The robustness of the N-alkyl PNP motif compared to N-aryl PNP may result from the unfavorable charge distribution that accompanies isomerization to PPN for the more electron rich alkyl substituent. On the other hand, electronic density in the native PNP structure is less localized on N and more delocalized onto the P atoms. Thus, the success of 2 in supporting ethylene oligomerization catalysts may be in part due to its lower propensity to convert to the PPN isomer, which serves to eliminate the possibility of degradation by nucleophilic attack of the isomerized species. Reactions of Cr−PNP Complex with AlClMe2. As the solid state structures of 3 and 4 show, the PNP moieties of 1 and 2 do not isomerize upon binding to the CrCl3 motif. Compound 3, supported by more isomerization-prone ligand 1, was further investigated. To test whether the isomerization might occur while bound to Cr, reactions of 3 with a small excess of AlClMe2, the Al reagent demonstrated to generate the PPN motif with 1 (vide supra), were performed. When using 2 or 3 equiv of the activator, cationic dinuclear Cr(III) complex 12 was obtained in 87% and 65% yield, respectively (Scheme 8, Scheme 8. Reaction of PNP−Cr(III) Complex 3 with AlClMe2

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Organometallics Table 1. Ethylene Oligomerization and Polymerization Trialsa ligand

activityb

PEc

C6 c

C8c

C10+c

1-hexene in C6c

1-octene in C8c

1 2 2d 2e o-CF3−PPN (13) Ph2PP(O)Ph2 Ph2PMe Ph2P−N(tolyl)Lif Ph2P−N(tolyl)Lig Ph2P−N(tolyl)Lih none

85 1700 340 350 118 68 45 110 1432 5044 160

31 3 15 74 100 100 100 100 100 100 100

24 38 32 10

43 54 53 15

2 5 99 >99 >99 >99

a Notes: CrCl3(THF)3 (0.8 mM), ligand (1 equiv., 0.8 mM), 7% Al M-MAO solution in heptane (1.35 mL, 300 equiv), 60 psi C2H4, toluene (10 mL total volume), 25 °C, 1 h reaction time. Overall productivity is reported in grams of overall products per grams of Cr used. bActivity reported in g/g Cr. cProduct distributions are shown as wt %. d10 equiv of H2O. e50 equiv of H2O. f1 equiv of [Ph2PN(p-tolyl)]Li. g2 equiv of [Ph2PN(p-tolyl)]Li. h 3 equiv of [Ph2PN(p-tolyl)]Li.

supporting ligand from the catalyst.57 Furthermore, the present observations suggest that the PNP framework can react under activation conditions via isomerization, and that the degree to which this happens will depend on the nature of the Nsubstituent. Isomerization will be mitigated in systems with a large excess of unreacted activator, and it might be prevented altogether for 2 via judicious choice of activator. Nevertheless, the ligand may further degrade upon isomerization by nucleophilic attack with alkylating species. A number of catalytic ethylene oligomerization experiments were carried out to test the effect of potential ligand degradation products on the catalytic ethylene oligomerization. [Ph2PN(p-tolyl)]−, Ph 2PP(O)Ph 2, and PPh2 Me are all degradation products observed in the experiments described above, and were tested for catalytic performance. Additionally, a PPN species stable as this isomer in metal-free form, Ph2PPPh2−N−(o-CF3C6H5) (13),51 was investigated. In all cases, 300 equiv of M-MAO was employed as activator. Under these conditions, it was found that the use of (Ph2PP(O)Ph2, [Ph2PN(p-tolyl)]Li, or 13 as supporting ligands led to polymerization activity only (Table 1). [Ph2PN(p-tolyl)]− species are particularly active, with their productivity values being several times higher than the overall oligomerization and polymerization activity of the parent system using 1. Cr catalysts displaying ligands with PN motifs that support polymerization and oligomerization have been previously reported.58 Iminobisphosphine 13, which is stable as a PPN isomer without a coordinated Lewis acid, only gives rise to polymerization activity. Addition of water to the catalytic mixture also results in a significant change in selectivity toward polymerization. In the absence of a supporting phosphine ligand only modest polymerization activity is observed. When combined, these results suggest that ligand precursor degradation or dissociation from the Cr center may be responsible for the undesired production of polymeric byproducts. Notably, the ability of [Ph2PN(p-tolyl)]− to support a very active polymerization catalyst, albeit when excess ligand is present, indicates that if even a small portion of the PNP precursor degrades, a substantial amount of polymer could form. The ligand framework (1) that is most prone to PNP to PPN isomerization and degradation is also the ligand that leads to more polymer product. This correlation suggests that partial ligand degradation could be responsible for poor catalyst

performance, although other explanations such as different electronic effects on the active catalyst cannot be ruled out. Even for the more robust ligand, 2, addition of water in substoichiometric amounts relative to M-MAO results in substantially increased polymer formation. This effect may also be traced to potential decomposition of 2 as demonstrated under certain conditions with Al reagents. Overall, these results suggest that an increased propensity of certain PNP ligands to isomerize to PPN due to N-substituent electronics or strong Lewis acids in the reaction mixture, followed by further degradation, is one of the reasons behind polymer formation.

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CONCLUSIONS We have demonstrated that PNP ligands that support Cr catalysts for selective ethylene oligomerization do not represent an innocent ancillary framework, but rather they are prone to isomerization and degradation via PPN intermediates. This isomerization behavior has been characterized by triggering it with different Al Lewis acidic species for the first time and without the need for synthetic modifications of the PNP substituents. The isomerization and degradation pathway was studied in a stepwise fashion, demonstrating that PNP frameworks can act as precursors for active species that differ in the nature of the supporting ligand, giving rise to undesired polymerization activity. When comparing ligands with different N-substituents, the changes in polymerization selectivity and activity depend on the ability of the PNP framework to access a PPN intermediate. The propensity for isomerization is related to the ability of the N-substituents to stabilize the partial negative charge in the PPN isomers, with aryl substituents more strongly favoring isomerization as compared to alkyl substituents. The nature of the Al activator has a large effect on the PNP isomerization. While the trialkyl aluminum reagents tested herein do not lead to isomerization, hydrolyzed and chlorinated versions promote it. With catalytically relevant activator, MMAO, halides from Cr precursors or excess water result in Al species that drive PNP isomerization. Upon isolation of a welldefined Al complex supported by a PPN framework, we have shown that the isomerized ligand displays higher reactivity and propensity toward degradation in alkylating environments. Most importantly, the degradation products support ethylene polymerization and not oligomerization catalysis.59,60 Therefore, the PNP isomerization and degradation directly impacts catalytic performance. Ligand precursors less prone to isomer1646

DOI: 10.1021/acs.organomet.7b00189 Organometallics 2017, 36, 1640−1648

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ization (and, therefore, degradation) and rigorously minimizing water contamination are important factors in limiting the degree of polymerization. Although we have not yet seen evidence for the PPN motif coordinated to Cr,48 it cannot be ruled out for the active Cr species that has been elusive thus far despite years of study by several groups. While the work presented herein focuses on ethylene trimerization/tetramerization systems, the broader use of PNP and other potentially noninnocent ligands in catalysis highlights the importance of understanding the chemistry of the ligand framework and its isomerization-assisted transformations initiated by additional reagents in multicomponent reaction mixtures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00189. Experimental details, synthetic procedures, spectra, and crystallographic information (PDF) Structure information (XYZ) Crystal data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Theodor Agapie: 0000-0002-9692-7614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to The Dow Chemical Company and Caltech for funding. T.A. .is grateful for Sloan, Dreyfus, and Cottrell fellowships. J.A.B. is grateful to the NSF for a Graduate Research Fellowship. We thank Michael K. Takase and Lawrence M. Henling for assistance with crystallography, and Sean Ewart, David Laitar, and Mari Rosen for insightful discussions.

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