Dimerization of Ethylene by Nickel Phosphino–Borate Complexes

Article pubs.acs.org/Organometallics

Dimerization of Ethylene by Nickel Phosphino−Borate Complexes Dmitry V. Gutsulyak, Andrew L. Gott, Warren E. Piers,* and Masood Parvez Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada S Supporting Information *

ABSTRACT: Trifluoroborate-functionalized phosphine ligands react with a variety of nickel(II) precursors to cleanly yield a number of κ2(P,F)-bound nickel complexes, which were characterized crystallographically. In comparison to related palladium complexes, ancillary ligands in the nickel complexes were observed to be generally more weakly bound, and the trifluoroborate ligands were more easily displaced by coordinating solvents that did not cause a similar displacement in a related palladium system. Such weaker ligand coordination resulted in a much faster dimerization of ethylene. Experiments conducted under constant ethylene pressure saw the suppression of the isomerization of 1-butene observed in related palladium complexes; higher oligomers were also generated under such conditions.

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INTRODUCTION Transition-metal-catalyzed olefin polymerization/oligomerization has been a prime driver in the field of organometallic chemistry for many years. Soon after the discovery of the first highly active [P,O] nickel catalysts for ethylene oligomerization by Keim and co-workers in the 1970s,1 the well-known SHOP (Shell Higher Olefin Process) system was industrialized.2 Since then, a large number of nickel catalysts with bidentate [P,O] ligands have been developed.3 One of the attractive features of the SHOP-type [P,O] catalysts is the ability to operate without addition of any cocatalysts (aluminum alkyls, ligand scavengers), which circumvents the generation of undesirable waste products.3d Some of the latest reports on the [P,O] type catalysts are devoted to nickel complexes of the type I, which were found to be highly active in ethylene polymerization4 when supported by the sulfonated phosphine ligand system first reported by Drent and co-workers.5 On the basis of the extensively documented catalytic properties of their palladium analogues,6−11 the comparatively unexplored class of nickel complexes may also have potential in catalyzing the copolymerization of ethylene with polar monomersan area of great current interest12where the ultimate goal remains a catalytic system that yields polymeric materials with novel physical properties, with predictable levels of comonomer incorporation via suitable tuning of the catalyst.

others have recently reported structurally related phosphine− trifluoroborate [P,F] palladium compounds of the type II,13 which demonstrate only moderate catalytic activity in the dimerization of ethylene. Herein, we report the first examples of the analogous [P,F] nickel complexes, which exhibit significantly enhanced catalytic activity in comparison to their palladium analogues.

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RESULTS AND DISCUSSION P-aryl-substituted potassium trifluoroborates (II) were previously reported by ourselves13a and Jordan,13b but the resulting palladium compounds were generally unreactive toward ethylene in comparison to the more electron-rich P-alkylsubstituted aryltrifluoroborates. We therefore chose to focus on the latter ligand system in this work, using isopropyl substituents. In comparison to their palladium analogues, which were inert to salt metathesis with ligand potassium phosphinoborates in our hands, NMR-scale reactions confirmed that nickel chloride containing precursors react over the course of several hours with such ligand precursors to form the corresponding nickel complexes. Thus, treatment of the previously reported zwitterionic HPiPr2[C6H4(2-BF3)]13a with KN(SiMe3)2 cleanly furnished the potassium salt 1 as a white solid in essentially quantitative yield, and reaction between potassium phosphine−borate 1 (Scheme 1) and NiClPh(PPh3)2 cleanly yielded the borate nickel complex 2 in good isolated yield. NMR-scale reactions suggest that these reactions proceed with complete conversion. Coordination of the phosphine−borate ligand to the nickel center was implied by the observed 31P and 19F NMR spectra. Two doublets of quartets, distorted by second-order effects,

Inspired by the success of the phosphine-sulfonate [P,O] complexes in a very wide variety of catalytic systems, we and © XXXX American Chemical Society

Received: April 5, 2013

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Scheme 1. Synthesis of the Phosphine−Trifluoroborate Nickel Complexes

Figure 1. Molecular structure of 2. Hydrogen atoms have been omitted for clarity. Displacement ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ni(1)− C(13) = 1.891(3), Ni(1)−F(1) = 1.9666(17), Ni(1)−P(2) = 2.2373(8), Ni(1)−P(1) = 2.2452(8), B(1)−F(1) = 1.482(4), B(1)− F(2) = 1.399(5), B(1)−F(3) = 1.369(4); C(13)−Ni(1)−P(2) = 87.18(9), F(1)−Ni(1)−P(2) = 89.31(6), C(13)−Ni(1)−P(1) = 91.08(9), F(1)−Ni(1)−P(1) = 92.27(6).

We previously demonstrated the hemilabile behavior of the phosphine−borate ligands when they are ligated to palladium centers; displacement of the coordinated trifluoroborate moiety was observed to occur on treatment with pyridine derivatives.13,17 We found that the aryltrifluoroborate group in nickel complex 2 is even more labile and can be substituted by weaker donors, such as acetonitrile, with the reversible formation of the zwitterionic complex 2·NCCH3, as shown in Scheme 1. Addition of 1 equiv of CH3CN results in broadening of the signals of 2 in various NMR spectra. Quantitative conversion of 2 into the zwitterionic complex 2·NCCH3 can be achieved in neat acetonitrile, and the structure of 2·NCCH3 was confirmed by X-ray crystallography (Figure 2). 2·NCCH3 crystallized from acetonitrile solution by layering with a diethyl ether/toluene mixture. The trifluoroborate group in 2·NCCH3 was displaced with acetonitrile, and the nickel complex adopts a distortedsquare-pyramidal geometry with the BF3 group located in the apical position. Taking into account the similar van der Waals radii of Ni and Pd atoms,18 a much shorter Ni(1)−F(1) distance of 2.507 Å in 2·NCCH3 in comparison with analogous palladium complexes (Pd−F = 2.939−3.06 Å)13a suggests a significantly stronger interaction of the BF3 group with the nickel center. However, all of the B−F bonds have almost the same length, which is consistent with the zwitterionic nature of the complex 2·NCCH3. The phosphorus atoms retain a trans disposition, but the Ni(1)−P(2) distance becomes shorter (2.2093(9) Å), which reflects the formation of the cationic Ni center upon removal of the negatively charged trifluoroborate group from the P(1)−Ni(1)−C(15) plane. Presumably, the weak secondary Ni−F interaction in 2 exists because of the sterically open apical sites in the complex. Thus, the introduction of steric bulk may diminish or fully prevent this interaction. We anticipated that the synthesis of complex 3 would achieve this, in which the apical sites would be partially blocked by the methyl groups of the coordinated lutidine molecule. Complex 3 was prepared by the treatment of NiClPh(tmeda) with potassium salt 1 in neat 2,6-lutidine and isolated in 47% yield by crystallization from CH2Cl2/hexanes solution. However, similarly to 2, a broad singlet was observed in the 19F NMR spectrum of 3 at δ −173.8 ppm corresponding

were observed in the 31P NMR spectrum at δ +13.2 and +11.3 ppm, with phosphorus−fluorine coupling constants of 16 and 11 Hz, respectively. The free ligand exhibits a 31P NMR chemical shift of δ +6.0 ppm. Coupling of the phosphorus nucleus to the fluorine atoms of the trifluoroborate moiety has been observed previously within free ligands but has not previously been observed upon ligation to metal centers. The relatively high phosphorus−phosphorus coupling constant of 256 Hz is consistent with trans positioning of the two phosphorus atoms in the nickel complex.14 In the 19F NMR spectrum, the chemical shift for the BF3 fluorines in 2 was found significantly upfield (δ −177.3 ppm) in comparison with that of the free phosphine−borate ligand 1 (δ −135.2 ppm), which is consonant with coordination of the BF3 group to the metal center.13 The signal observed in the 19F NMR spectrum is broad even at −78 °C, suggesting rapid exchange of the fluorines in the BF3 moiety in solution. Further structural confirmation of the ligation of the trifluoroborate moiety was obtained from an X-ray crystallographic analysis of complex 2, as seen in Figure 1; X-ray data and processing parameters can be found in Table S2 (Supporting Information). The crystallographic study confirmed a distorted-squareplanar geometry about the nickel center, with phosphorus atoms located trans to one another. Coordination of the BF3 group results in a short Ni(1)−F(1) distance of 1.9666(17) Å and a significant lengthening of the B(1)−F(1) bond (1.482(4) Å) in comparison with the terminal B(1)−F(3) bond of 1.369(4) Å. The third fluorine atom is located in an apical position above the Ni atom with an Ni(1)−F(2) distance of 2.994 Å, which is significantly shorter than the sum of the van der Waals radii (3.10 Å). Such an observation indicates that a weak Ni(1)−F(2) interaction is present in the solid state, which results in a highly distorted κ2 coordination of the BF3 group.15 This secondary interaction may also explain the B(1)−F(2) bond distance of 1.399(5) Å lying in the range between terminal and coordinated B−F bond lengths in 2. Notably, the κ2 coordination mode of the BFn group to a metal is quite rare and, to the best of our knowledge, limited to the coordination of the BF4− ion.16 B

dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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comparison with those in previously reported nickel lutidine catalysts (1.87−1.92 Å),19 which could be a result of steric/ electrostatic repulsion of the coordinated lutidine with the ArBF3 group. As expected, the methyl substituents of the lutidine ligand infringe upon space associated with the apical positions above and below the molecular square plane. In comparison with 2, complex 3 exhibits a longer Ni(1)−F(1) distance of 1.9757(16) Å and a shorter B(1)−F(1) bond (1.474(4) Å). These data are in agreement with a slightly weaker interaction of the BF3 group with the metal center. Interestingly, the B(1)−F(2) and B(1)−F(3) bonds have almost the same length (1.392(4) and 1.388(4) Å), and the Ni(1)−F(2) distance of 3.057 Å is approaching the sum of the van der Waals radii (3.10 Å), which corresponds to only residual (or no) interaction of the nickel center with the apical fluorine atom F(2). In order to prepare a “ligand-free” version of these compounds (i.e., free of triphenylphosphine or lutidine), we also synthesized the phosphine−borate allyl complex 4 by the reaction of [(allyl)NiBr]2 with ligand 1 in chlorobenzene (Scheme 1). The principal spectroscopic features of 4 were resonances observed in the 1H NMR spectrum corresponding to the η3-allyl ligand at δ 2.68, 3.52, and 5.74 ppm. The 31P{1H} NMR spectrum 4 exhibited a resonance at δ 29.0 ppm. Complex 4 slowly decomposed in solution, yielding unidentified paramagnetic impurities, such that an acceptable 13C{1H} NMR spectrum was not obtained. In the presence of acetonitrile, the ArBF3 group is cleanly displaced to yield 4·NCCH3, which was characterized by multinuclear NMR spectroscopy. With the catalyst precursors 2−4 in hand, we evaluated their performance in the ethylene dimerization reaction. Oligomerization of Ethylene. Phosphine−trifluoroborate palladium complexes were previously reported to catalyze dimerization of ethylene to a mixture of butenes.13 To compare the performance of nickel congener 2 to that of its palladium analogue, we conducted ethylene dimerization experiments under the same conditions as reported previously. Accordingly, 4.5 μmol of 2 was dissolved in CD2Cl2 and loaded into a 5 mm NMR tube; the solution was degassed, and 1 atm of ethylene was admitted into the tube. The tube was sealed, and the reaction was followed until the ethylene was consumed. Precatalyst 2 catalyzes the dimerization of ethylene at a significantly faster rate than its palladium analogue; complete consumption of the ethylene occurs in less than 10 min vs the ∼24 h required for the corresponding palladium catalyst at room temperature. Similarly to the Pd-catalyzed reactions,13 the kinetic 1-butene product isomerizes into a thermodynamic mixture of 2-butenes once the ethylene has been consumed (eq 1).

Figure 2. Molecular structure of 2·NCCH3. Solvent molecules of crystallization and hydrogen atoms are omitted for clarity. Displacement ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ni(1)−N(1) = 1.894(3), Ni(1)−C(15) = 1.898(3), Ni(1)−P(2) = 2.2093(9), Ni(1)−P(1) = 2.2392(8), B(1)−F(1) = 1.404(4), B(1)−F(2) = 1.403(4), B(1)−F(3) = 1.407(4); N(1)−Ni(1)−P(2) = 95.16(8), C(15)−Ni(1)−P(2) = 86.61(9), N(1)−Ni(1)−P(1) = 88.33(8), C(15)−Ni(1)−P(1) = 90.08(9), N(1)−Ni(1)−P(2) = 95.16(8), C(15)−Ni(1)−P(2) = 86.61(9), N(1)−Ni(1)−P(1) = 88.33(8), C(15)−Ni(1)−P(1) = 90.08(9).

to the coordinated trifluoroborate group. A quartet at δ +18.5 ppm was observed in the 31P spectrum with a phosphorus− fluorine coupling constant of 6 Hz, somewhat smaller than that observed in 2. Unlike 2, however, the reaction of 3 with acetonitrile was not clean and in fact indicated that free lutidine was produced. X-ray structural analysis (Figure 3) of 3 revealed the expected distorted-square-planar geometry about the central nickel atom, with the phosphine ligand located trans to the coordinated lutidine. The Ni(1)−N(1) bond of 1.964(2) Å is quite long in

Figure 3. Molecular structure of 3. Hydrogen atoms have been omitted for clarity. Displacement ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ni(1)− C(13) = 1.892(3), Ni(1)−N(1) = 1.964(2), Ni(1)−F(1) = 1.9757(16), Ni(1)−P(1) = 2.2102(8), B(1)−F(1) = 1.474(4), B(1)−F(2) = 1.392(4), B(1)−F(3) = 1.388(4); C(13)−Ni(1)− N(1) = 92.60(11), N(1)−Ni(1)−F(1) = 82.47(8), C(13)−Ni(1)− P(1) = 91.30(9), F(1)−Ni(1)−P(1) = 93.80(5).

Under these conditions, significant amounts of complex 2 remained intact after the complete consumption of ethylene, which might be indicative of slow catalyst initiation. In these systems, initiation may require dissociation of L (PPh3 in this C

dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX

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Figure 4. 1H NMR spectra of the reaction mixture of 2 with ethylene in CD2Cl2 at 280 K.

case) or the ArBF3 interaction; alternatively, the first insertion of ethylene into the Ni−Ph bond may be the initiation-limiting step. To probe these issues, we monitored the reaction of 2 with ethylene at low temperatures by 1H NMR spectroscopy (Figure 4). Although acetonitrile readily displaces the ArBF3 group in 2 (vide supra), no evidence for a similar substitution with ethylene was found. Indeed, at 220 K, no reaction of 2 with ethylene was observed. Upon slow warming of the sample, a reaction ensued at 280 K, and the formation of styrene and 1butene was observed, along with the formation of a new nickel complex containing the Ni−CH2CH3 fragment. After 10 min, this new species and the original precatalyst 2 are present in about a 1:1 ratio, indicating that the ethyl complex reacts with ethylene more rapidly than the phenyl complex. No evidence for a species analogous to 2·NCCH3 was found, suggesting that, if ethylene displaces the ArBF3 group, insertion is rapid and that the resting state of the catalyst is the ethyl analogue of 2. These observations do not allow us to definitively determine whether the dimerization occurs via dissociation of the ArBF3 group13a or the triphenylphosphine ligand but do show that insertion of ethylene into the Ni−Ph bond is relatively slow. Note that no 2-butenes are apparent in these spectra; these isomers do not appear until the ethylene is essentially consumed. Because the nickel catalysts are significantly more active than the palladium analogues, we were able to significantly reduce the loadings of the catalysts and conduct the catalytic reactions under a constant pressure of ethylene gas (1 atm) on a larger scale. Figure 5 shows a plot of TON vs time for 1-butene production for catalysts 2−4 under these conditions. Under the conditions of a constant ethylene supply to the catalyst, the isomerization of 1-butene is significantly suppressed, with less than 1% of 2-butenes detected by GCMS and NMR spectroscopy in the product mixture after 1 h. As can be seen, catalysts 2 and 4 are quite active, reaching a TON for 1butene production of greater than 2000 after 1 h in each case.

Figure 5. Production of 1-butene (TON) vs time.

Notably, however, the 1-butenes constitute only 60−70% of the product mixture and 20−30% hexenes is also produced by these two catalysts.20 In contrast, the lutidine-ligated precatalyst 3, while much less active, does not produce any hexenes, as determined by GCMS. In all cases, the curves show that the increase in TON vs time is not linear, indicating catalyst degradation over the course of the 1 h time frame of these experiments. The behavior of these phosphine−trifluoroborate nickel complexes as highly active ethylene dimerization catalysts contrasts markedly with that of analogous phosphine−sulfonate complexes, which are known to be very effective ethylene polymerization catalysts.4 The trifluoroborate and sulfonate D

dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX

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Figure 6. DFT optimized structures of 2 (left) and 2-SO3 (right) with selected NBO charges (B3LYP/lanl2dz). dichloromethane-d2 were purchased from Cambridge Isotopes and dried over the appropriate drying agent before vacuum transfer into a sealed bomb in a glovebox. HPiPr2(C6H4(2-BF3K)),13a 1b,13a NiClPh(PPh3)2,23 and NiClPh(tmeda)24 were prepared according to published procedures. [NiBr(allyl)]2 and potassium bis(trimethylsilyl)amide were purchased from Aldrich and used as received. NMR spectra were obtained on Bruker Avance II and III spectrometers. 1H and 13C{1H} NMR spectra were referenced to tetramethylsilane, 11B{1H} NMR spectra to BF3·OEt2, 19F NMR spectra to CFCl3, and 31P NMR spectra to 85% aqueous phosphoric acid. Full structural assignment was confirmed by 2-D NMR spectra where necessary. Elemental analyses and mass spectra were obtained by the Instrumentation Facility of the Department of Chemistry, University of Calgary. Quoted elemental analysis results are the average of two separate determinations. PiPr2(C6H4-(2-BF3K)) (1). A 50 mL round-bottomed flask was charged with a solid mixture of HPiPr2(C6H4(2-BF3)) (1.00 g, 4.2 mmol) and KN(SiMe3)2 (0.83 g, 4.2 mmol). Tetrahydrofuran (10 mL) was added via vacuum transfer, and the reaction mixture was warmed to ambient temperature and stirred overnight. The reaction mixture was filtered, washed repeatedly with pentane, and dried under vacuum to yield the title compound as a white solid. Yield: 1.09 g (3.9 mmol, 93%). 1H NMR (400 MHz, CDCl3, 298 K): δ 0.79 ppm (d of d, 6H, 2 × CH3 of iPr, 3JPH = 14 Hz, 3JHH = 7 Hz), 0.96 (d of d, 6H, 2 × CH3 of iPr, 3JPH = 14 Hz, 3JHH = 7 Hz), 1.99 (m, 2H, CH of iPr), 7.12−7.20 (m, 2H, aryl C-H), 7.40 (d, 1H, aryl C-H, 3JHH = 7 Hz), 7.59 (d, 1H, aryl C-H, 3JHH = 7 Hz). 13C{1H} NMR (100.6 MHz, CDCl3, 298 K): δ 20.0 ppm (d, CH3 of iPr, 3JCP = 11 Hz), 20.5 (CH3 of iPr, 3JCP = 11 Hz), 24.7 (d, CH of iPr, 1JCP = 9 Hz), 126.4, 127.7, 131.5 (all aryl C-H), 132.0 (d, aryl C−H, 3JCP = 6 Hz); carbon adjacent to boron not observed. 11B{1H} NMR (128.4 MHz, CDCl3, 298 K): δ +7.0 ppm (s, br, PiPr2(C6H4(2-BF3K))). 19F NMR (376.6 MHz, CDCl3, 298 K): δ −135.0 ppm (s, br, PiPr2(C6H4(2-BF3K))). 31 1 P{ H} NMR (162.0 MHz, CDCl3, 298 K): δ +6.0 ppm (s, PiPr2(C6H4(2-BF3K))). ESI-MS (negative): calcd for [M−] m/z 261.1199, found 261.1193. [κ2(P,F)-i‑PrNiPh(PPh3)] (2). A 50 mL round-bottomed flask was charged with a solid mixture of NiClPh(PPh3)2 (0.115 g, 0.165 mmol) and 1 (0.050 g, 0.17 mmol). Dichloromethane (15 mL) was added via vacuum transfer, and the reaction mixture was allowed to warmed to ambient temperature and stirred overnight. The resulting orange solution was filtered, and the filtrate was concentrated under vacuum. Hexane was layered over the concentrated solution. Orange crystals of the product were obtained overnight at −30 °C. The crystals were separated from the solution by decantation, washed with hexane, and dried under vacuum. Yield: 0.090 g (81%). 1H NMR (400 MHz, 298

groups have different electronic properties, which may influence the electron density on the metal center and overall reactivity of the metal complex. Indeed, according to DFT calculations (B3LYP/lanl2dz), the nickel center in complex 2 is more electron deficient (NBO charge +0.338) than in the analogous sulfonate complex 2-SO3 (NBO charge +0.294) (, Figure 6). These results suggest that the ArBF3 group is a weaker donor to the nickel center than the ArSO3 group, perhaps facilitating β-hydride elimination in the butyl (and hexyl) chains formed after one or two insertions of ethylene.21 This hypothesis requires more sophisticated inquiry for verification but points to the potential utility of such chelating phosphino−trifluoroborate ligands.

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CONCLUSIONS A range of nickel phosphino−trifluoroborate complexes were prepared and structurally characterized. It was demonstrated that phosphine−trifluoroborate nickel complexes effectively catalyze the oligomerization of ethylene under very mild conditions without the addition of ligand scavengers and cocatalysts. The catalytic activity of the nickel complexes was observed to be significantly higher than that of the previously reported analogous palladium catalysts and comparable to that of other neutral SHOP type ethylene oligomerization catalysts.3d,e However, the best of the cationic catalysts exhibit higher activity.22 The tendency toward oligomerization vs polymerization may be related to the electron deficiency engendered at the nickel center by these poorly donating ArBF3 groups.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise indicated, all manipulations were carried out in either an MBraun inert-atmosphere glovebox or on a greaseless dual-manifold vacuum line using Teflon (Kontes) needle valves and swivel-frit type glassware. Tetrahydrofuran, toluene, and hexanes were dried using a Grubbs-type solvent purification system and stored in evacuated bombs over sodium/ benzophenone prior to use. Acetonitrile was predried over 4 Å molecular sieves before vacuum transfer into a separate evacuated bomb over 4 Å molecular sieves. Chlorobenzene and dichloromethane were predried over calcium hydride, before vacuum transfer into a separate evacuated bomb over calcium hydride. 2,6-Lutidine was dried over 4 Å molecular sieves. Acetonitrile-d3, chloroform-d3, and E

dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

K, CD2Cl2): δ 1.02−1.12 ppm (m, 6H, 2 × CH3 of iPr), 1.20−1.30 (m, 6H, 2 × CH3 of iPr), 2.14 (m, 2H, 2 × CH of iPr), 6.50−6.60 (m, 3H, aryl C-H of Ni(C6H5)), 7.10 (d, 2H, aryl C-H of Ni(C6H5), 3JHH = 7 Hz), 7.26 (t, 1H, aryl C-H, 3JHH = 8 Hz), 7.30 (m, 7H, aryl C-H), 7.41−7.51 (m, 10H, aryl C-H), 7.56 (d, 1H, aryl C-H, 3JHH = 8 Hz). 13 C{1H} NMR (100.6 MHz, 298 K, CD2Cl2): δ 18.4 ppm (s, CH3 of i Pr), 19.0 (d, CH3 of iPr, 2JCP = 3 Hz), 24.9 (d, CH of iPr 1JCP = 18 Hz), 25.0 (d, CH of iPr, 1JCP = 18 Hz), 122.8 (t, aryl C-H, 3JCP = 3 Hz), 126.8 (s, aryl C-H), 127.2 (d, aryl C-H, 3JCP = 4 Hz), 128.5 (d, aryl C-H, 2JCP = 9 Hz), 128.7 (d, aryl C-H, 2JCP = 9 Hz), 128.9 (d of d, aryl C-H, 3JCP = 8 Hz, 4JCP = 2.0 Hz), 129.5 (d, aryl Cq, 1JCP = 10 Hz), 129.8 (d, aryl Cq, 1JCP = 10 Hz), 129.9 (s, aryl C-H), 130.8 (d, aryl CH), 4JCP = 2 Hz), 132.5 (m, aryl C-H), 131.1 (d, aryl C-H, 4JCP = 2 Hz), 134.7 (d of d, aryl C-H, 3JCP = 8 Hz, 4JCP = 3 Hz), 138.2 (t, aryl C-H, 4JCP = 3 Hz), carbon adjacent to boron not observed. 11B{1H} NMR (128.4 MHz, 298 K, CD2Cl2): δ +3.9 ppm (s, br, C6H4BF3Ni). 19 F NMR (376.6 MHz, 298 K, CD2Cl2): δ −178.9 ppm (s, br, C6H4BF3Ni). 31P{1H} NMR (162 MHz, 298 K, CD2Cl2): δ 13.2 (d of q, PPh3, 2JPP = 257 Hz, 4JPF = 16 Hz), 11.3 (d of q, PiPr2(C6H4BF3), 2 JPP = 257 Hz, 4JPF = 11 Hz). Anal. Calcd for C36H38BF3NiP2·0.1CH2Cl2: C, 64.94; H, 5.77. Found: C, 64.98; H, 6.00. [κ2(P,F)-i‑PrNiPh(NCCH3)(PPh3)] (2·NCCH3). A solution of complex 2 (0.030 g, 0.045 mmol) in acetonitrile (1 mL) was layered with a mixture of Et2O and toluene (1/1). Yellow crystals were obtained after 3 days at −30 °C. Yield: 0.010 g (∼30%). Quantitative conversion of 2 into [κ2(P,F)-i‑PrNiPh(NCCD3)(PPh3)] was observed by NMR spectroscopy in CD3CN. 1H NMR (400 MHz, 298 K, CD3CN): δ 1.02 ppm (m, br, 6H, 2 × CH3 of iPr), 1.18 (m, br, 6H, 2 × CH3 of i Pr), 2.15 (s, br, 2H, 2 × CH of iPr), 6.53 (s, br, 3H, aryl C-H of Ni(C6H5)), 7.12 (t, 1H, aryl C−H, 3JHH = 8 Hz), 7.20 (s, br, 2H, aryl C-H of Ni(C6H5)), 7.24−7.38 (m, 8H, aryl C-H), 7.38−7.52 (m, 9H, aryl C-H), 7.78 (d, 1H, aryl C−H, 3JHH = 8 Hz). 13C{1H} NMR (100.6 MHz, 298 K, CD3CN): δ 19.1, 20.5 ppm (both s, CH3 of iPr), 24.4 (d, CH of iPr, 1JCP = 21 Hz), 122.6 (s), 125.9 (d, 3JCP = 5 Hz), 127.3 (s), 129.2 (s), 129.4 (bs), 131.0 (s), 131.2 (s), 135.12 (bs), 136.0 (m), 139.2 (s), all aryl C-H and Cq), carbon adjacent to boron not observed. 11 1 B{ H} NMR (128.4 MHz, 298 K, CD3CN): δ 4.1 ppm (s, br, C6H4BF3Ni). 19F NMR (376.6 MHz, 298 K, CD3CN): δ −137.6 ppm (s, br, C6H4BF3Ni). 31P{1H} NMR (162.0 MHz, 298 K, CD3CN): δ 26.3 ppm (d, PPh3, 2JPP = 251 Hz), 21.3 (d, PiPr2(C6H4BF3), 2JPP = 251 Hz). Anal. Calcd for C38H41BF3NNiP2: C, 65.18; H, 5.90; N, 2.00. Found: C, 63.00; H, 5.80; N, 2.21 (complex 2·NCCH3 contains cocrystallized solvent molecules in the solid state, confirmed by X-ray analysis). [κ2(P,F)-i‑PrNiPh(2,6-lutidine)] (3). A 5 mL round-bottomed flask was charged with a solid mixture of NiClPh(tmeda) (0.067 g, 0.23 mmol) and 1 (0.070 g, 0.23 mmol). Dry 2,6-lutidine (1 mL) was added via syringe, and the reaction mixture was stirred for 1 h at room temperature. The resulting dark orange solution was evaporated under vacuum, and the residue was extracted with CH2Cl2 (2 × 5 mL). The extracted fractions were combined and concentrated under vacuum. A few drops of 2,6-lutidine were added to the solution, followed by layering with hexane. Yellow-red crystals were obtained at −30 °C after a few days. The crystals were separated from the solution by decantation, washed with hexane, and dried under vacuum. Yield: 0.050 g (43%). 1H NMR (400 MHz, 298 K, CD2Cl2): δ 1.03 ppm (d of d, 6H, 2 × CH3 of iPr, 3JPH = 15 Hz, 3J\HH = 7 Hz), 1.37 (d of d, 6H, 2 × CH3 of iPr, 3JPH = 15 Hz, 3JHH = 7 Hz), 2.49 (m, 2H, 2 × CH of iPr), 4.00 (s, 6H, 2 × CH3 of lutidine), 6.72 (m, 1H, aryl C-H), 6.78 (m, 2H, aryl C-H), 7.08 (d, 2H, aryl C-H, 3JHH = 8 Hz), 7.31 (t, br, aryl C-H, 1H, 3JHH = 8 Hz), 7.42 (t, br, aryl C-H, 1H, 3JHH = 8 Hz), 7.51−7.45 (m, 3H, aryl C-H), 7.54 (t, 1H, aryl C-H, 3JHH = 8 Hz), 7.68 (d, br, 1H, aryl C-H, 3JHH = 8 Hz). 13C{1H} NMR (100.6 MHz, 298 K, CD2Cl2): δ 18.1 ppm (m, CH3 of iPr), 23.3 (d, CH of iPr, 1JCP = 27 Hz), 25.9 (s, CH3 of lutidine), 123.5 (d, aryl C-H, 4JCP = 2 Hz), 123.7 (d, aryl C-H, 4JCP = 1.0 Hz), 125.1 (d, aryl Cq, 1JCP = 30 Hz), 126.2 (s, aryl C-H), 126.7 (d, aryl C-H, 3JCP = 6 Hz), 130.1 (d, aryl C-H, 4JCP = 2 Hz), 130.5 (s, aryl C-H), 133.3 (d, aryl C-H, 2JCP = 13 Hz), 138.6,

139.0 (both s, aryl C-H), 159.6 (s, aryl Cq), carbon adjacent to boron not observed. 11B{1H} NMR (128.4 MHz, 298 K, CD2Cl2): δ +2.6 ppm (s, br, C6H4BF3Ni). 19F NMR (376.6 MHz, 298 K, CD2Cl2): δ −173.8 ppm (s, br, C6H4BF3Ni). 31P{1H} NMR (162.0 MHz, 298 K, CD2Cl2): δ +18.5 ppm (q, PiPr2(C6H4BF3), 4JPF = 6 Hz). Anal. Calcd for C25H32BF3NNiP: C, 59.58; H, 6.40; N, 2.78. Found: C, 59.74; H, 6.58; N, 2.50. Attempted Generation of [κ2(P,F)-i‑PrNiPh(NCCD3)(2,6-lutidine)] (3·NCCD3). A J. Young NMR tube was charged with 3 (0.010 g, 0.020 mmol), and CD3CN (0.6 mL) was added in a glovebox. According to NMR data, 50% of the starting complex was consumed with the release of free lutidine. No signals of the noncoordinated BF3 group were observed in the 19F NMR spectrum of the reaction mixture. [κ2(P,F)-i‑PrNi(allyl)] (4). A 50 mL round-bottomed flask was charged with a solid mixture of [(allyl)NiBr]2 (0.070 g, 0.39 mmol) and 1 (0.117 g, 0.39 mmol). Dry chlorobenzene (10 mL) was added via syringe, and the reaction mixture was stirred for 1 h at room temperature. The resulting brown reaction mixture was filtered, and the filtrate was evaporated under vacuum. The residue was extracted with toluene (3 × 5 mL) and concentrated under vacuum. Hexane was layered over the solution; a yellow precipitate was obtained overnight at −30 °C. The precipitate was separated from the solution by decantation, washed with hexane, and dried under vacuum. Yield: 0.105 g (75%). (Slow decomposition of the compound in solution results in the formation of unidentified paramagnetic impurities, which results in the overall broadness of all signals in NMR spectra.) 1H NMR (400 MHz, 298 K, CD2Cl2): δ 1.17−1.32 ppm (m, 12H, 4 × CH3 of iPr), 2.30 (m, 2H, 2 × CH of iPr), 2.68 (s, br, 2H, CH2CHCH2), 3.52 (s, br 2H, CH2 CHCH 2 ), 5.74 (m, 1H, CH2CHCH2), 7.34 (m, 1H, aryl C-H of C6H4BF3), 7.43 (m, 1H, aryl C-H of C6H4BF3), 7.71 (m, 1H, aryl C-H of C6H4BF3). 13C{1H} NMR (100.6 MHz, 298 K, CD2Cl2): δ 18.8, 19.4 ppm (both s, br, CH3 of iPr), 25.7 (d, br, CH of iPr, 1JCP = 26 Hz), 113.8, 127.2, 127.4, 130.2, 131.5, 133.2 (all s, br, aryl C-H and aryl Cq). Several resonances could not be located due to the broadness of signals. 11B{1H} NMR (128.4 MHz, 298 K, CD2Cl2): δ +3.7 ppm (s, br, C6H4BF3Ni). 19F NMR (376.6 MHz, 298 K, CD2Cl2): δ −186.1 ppm (s, br, C6H4BF3Ni). 31P{1H} NMR (162 MHz, 298 K, CD2Cl2): δ 29.0 (s, br, PiPr2(C6H4BF3)). Anal. Calcd for C15H23BF3NiP·0.2C6H5Cl: C, 50.76; H, 6.31. Found: C, 50.62; H, 6.41. Generation of [κ2(P,F)-i‑PrNi(allyl)(NCCD3] (4·NCCD3). A J. Young NMR tube was charged with 4 (0.010 g, 0.028 mmol), and CD3CN (0.6 mL) was added in a glovebox. Full conversion of 4 into 4·NCCD3 was observed by NMR spectroscopy. 1H NMR (400 MHz, 298 K, CD3CN): δ 0.95−1.08 ppm (dd, 6H, 2 × CH3 of iPr, 3JPH = 13.3 Hz, 3JHH = 6.8 Hz), 1.12−1.22 (dd, 6H, 2 × CH3 of iPr, 3JPH = 15.9 Hz, 3JHH = 6.8 Hz), 2.43 (m, 2H, 2 × CH of iPr), 2.58 (s, br, 2H, CH2CHCH2), 3.60 (s, br 2H, CH2 CHCH 2 ), 5.49 (m, 1H, CH2CHCH2), 7.20 (m, 1H, aryl C-H of C6H4BF3), 7.32 (m, 2H, aryl C-H of C6H4BF3), 7.72 (m, 1H, aryl C-H of C6H4BF3). 13C{1H} NMR (100.6 MHz, 298 K, CD3CN): δ 18.7 ppm (s, CH3 of iPr), 19.2 (d, CH3 of iPr, 2JPC = 5.5 Hz), 24.3 (d, br, CH of iPr, 1JCP = 22.5 Hz), 113.2 (s), 125.7 (d, J = 6.0 Hz), 127.4 (d, J = 35.4 Hz), 129.5 (d, J = 2.2 Hz), 131.4 (s), 136.3 (d, J = 12.4 Hz). 11B{1H} NMR (128.4 MHz, 298 K, CD3CN): δ +3.9 ppm (q, br, C6H4BF3Ni, 1JBF = 55 Hz). 19F NMR (376.6 MHz, 298 K, CD3CN): δ −137.3 ppm (s, br, C6H4BF3). 31 1 P{ H} NMR (162 MHz, 298 K, CD3CN): δ 33.6 (q, br, PiPr2(C6H4BF3), 4J = 14.1 Hz). Oligomerization of Ethylene. Method 1 (NMR Scale Reactions). A J. Young NMR tube was charged with precatalyst (4.5 μmol), and CD2Cl2 (0.6 mL) was added in a glovebox. The NMR tube was then attached to a high-vacuum line and degassed via repeated freeze− pump−thaw cycles, before the admission of ethylene at 1 atm pressure. The reaction was monitored by NMR spectroscopy. Method 2. In a 100 mL round-bottom flask was added toluene (30 mL) followed by addition of 300 μL of a precatalyst solution in CH2Cl2 (1.5 × 10−2 M). The flask was degassed via repeated freeze− pump−thaw cycles. In the flask was added ethylene at 1 atm pressure. The pressure of ethylene was maintained during the reaction. The F

dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX

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Schnetmann, I. Proc. Natl. Acad. Sci. 2011, 108, 8955. (e) Runzi, T.; Guironnet, D.; Gottker-Schnetmann, I.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 16623. (f) Runzi, T.; Frohlich, D.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 17690. (g) Guironnet, D.; Caporaso, L.; Neuwald, B.; Gottker-Schnetmann, I.; Cavallo, L.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 4418. (h) Bouilhac, C.; Runzi, T.; Mecking, S. Macromolecules 2010, 43, 3589. (i) Zhang, D.; Guironnet, D.; Gottker-Schnetmann, I.; Mecking, S. Organometallics 2009, 28, 4072. (j) Guironnet, D.; Roesle, P.; Runzi, T.; Gottker-Schnetmann, I.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 422. (7) (a) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi, T.; Chung, L. W.; Morokuma, K. J. Am. Chem. Soc. 2012, 134, 13912. (b) Nakamura, A.; Kageyama, T.; Goto, H.; Carrow, B. P.; Ito, S.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 12366. (c) Nakamura, A.; Munakata, K.; Ito, S.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2011, 133, 6761. (d) Kanazawa, M.; Ito, S.; Nozaki, K. Organometallics 2011, 30, 6049. (e) Kageyama, T.; Ito, S.; Nozaki, K. Chem. Asian J. 2011, 6, 690. (f) Nozaki, K.; Kusumoto, S.; Node, S.; Kochi, T.; Chung, L. W.; Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030. (g) Ito, S.; Nozaki, K. Chem. Rec. 2010, 10, 315. (h) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088. (i) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215. (j) Ito, S.; Munakata, K.; Nakamura, A.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14606. (k) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 8948. (l) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 7770. (m) Kochi, T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25. (8) (a) Cai, Z. G.; Shen, Z. L.; Zhou, X. Y.; Jordan, R. F. ACS Catal. 2012, 2, 1187. (b) Conley, M. P.; Jordan, R. F. Angew. Chem., Int. Ed. 2011, 50, 3744. (c) Shen, Z. L.; Jordan, R. F. Macromolecules 2010, 43, 8706. (d) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 15450. (e) Vela, J.; Lief, G. R.; Shen, Z. L.; Jordan, R. F. Organometallics 2007, 26, 6624. (f) Luo, S.; Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 8946. (9) (a) Piche, L.; Daigle, J. C.; Rehse, G.; Claverie, J. P. Chem. Eur. J. 2012, 18, 3277. (b) Daigle, J. C.; Piche, L.; Arnold, A.; Claverie, J. P. ACS Macro Lett. 2012, 1, 343. (c) Daigle, J. C.; Piche, L.; Claverie, J. P. Macromolecules 2011, 44, 1760. (d) Piche, L.; Daigle, J. C.; Poli, R.; Claverie, J. P. Eur. J. Inorg. Chem. 2010, 4595. (e) Kryuchkov, V. A.; Daigle, J. C.; Skupov, K. M.; Claverie, J. P.; Winnik, F. M. J. Am. Chem. Soc. 2010, 132, 15573. (f) Skupov, K. M.; Hobbs, J.; Marella, P.; Conner, D.; Golisz, S.; Goodall, B. L.; Claverie, J. P. Macromolecules 2009, 42, 6953. (g) Skupov, K. M.; Piche, L.; Claverie, J. P. Macromolecules 2008, 41, 2309. (h) Skupov, K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.; Conner, D.; Goodall, B. L.; Claverie, J. P. Macromol. Rapid Commun. 2007, 28, 2033. (10) (a) Chen, C.; Anselment, T. M. J.; Frohlich, R.; Rieger, B.; Kehr, G.; Erker, G. Organometallics 2011, 30, 5248. (b) Anselment, T. M. J.; Wichmann, C.; Anderson, C. E.; Herdtweck, E.; Rieger, B. Organometallics 2011, 30, 6602. (c) Anselment, T. M. J.; Anderson, C. E.; Rieger, B.; Boeddinghaus, M. B.; Fassler, T. F. Dalton Trans. 2011, 40, 8304. (d) Haras, A.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics 2006, 25, 946. (e) Hearley, A. K.; Nowack, R. A. J.; Rieger, B. Organometallics 2005, 24, 2755. (11) (a) Luo, R.; Newsham, D. K.; Sen, A. Organometallics 2009, 28, 6994. (b) Borkar, S.; Newsham, D. K.; Sen, A. Organometallics 2008, 27, 3331. (c) Newsham, D. K.; Borkar, S.; Sen, A.; Conner, D. M.; Goodall, B. L. Organometallics 2007, 26, 3636. (d) Liu, S.; Borkar, S.; Newsham, D.; Yennawar, H.; Sen, A. Organometallics 2007, 26, 210. (12) Nakamura, A.; Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K. Acc. Chem. Res. 2013, DOI: 10.1021/ar300256h. (13) (a) Gott, A. L.; Piers, W. E.; Dutton, J. L.; McDonald, R.; Parvez, M. Organometallics 2011, 30, 4236. (b) Kim, Y.; Jordan, R. F. Organometallics 2011, 30, 4250. (14) Pregosin, P. S. NMR Spectroscopy in Organometallic Chemistry; Wiley-VCH: Weinheim, Germany, 2012. (15) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088.

reaction was monitored by taking 1 mL aliquots of the solution at regular time intervals and analyzing them by NMR spectroscopy/GCMS using CH2Cl2 as internal standard. X-ray Crystallography. Single crystals of 2, 3, and 2·NCCH3 were grown as described in the relevant part of the Experimental Section. Diffraction data for 2, 3, and 2·NCCH3 were collected on a Bruker P4/RA/SMART 1000 CCD diffractometer using Mo Kα radiation at −100 °C. All data were corrected for absorption using SADABS,25 and the structures were solved in SHELXS9726 and refined using fullmatrix least squares on F2 using SHELXL97.26 Crystal structure diagrams were plotted using ORTEP 3.27

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Tables and CIF files giving details on the quantum chemical calculations and crystallographic data for 2, 2·NCCH3, and 3. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel: (+1) 403-220-5746. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank LANXESS GmbH for their generous support of this work. W.E.P. also thanks the Canada Council of the Arts for a Killam Research Fellowship (2012−2014). We acknowledge Dr. Benedikt Neue for assistance with the DFT calculations.

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REFERENCES

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dx.doi.org/10.1021/om400288u | Organometallics XXXX, XXX, XXX−XXX