Linear α-Olefins Obtained with Palladium(II) Complexes Bearing a

Article pubs.acs.org/Organometallics

Linear α‑Olefins Obtained with Palladium(II) Complexes Bearing a Partially Oxidized Tetraphosphane Werner Oberhauser,*,† Gabriele Manca,† Andrea Ienco,† Christof Strabler,‡ Johannes Prock,‡ Alexander Weninger,‡ Rene Gutmann,‡ and Peter Brüggeller*,‡ †

Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di Ricerca CNR di Firenze, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy ‡ Institut für Allgemeine, Anorganische und Theoretische Chemie, Universität Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria S Supporting Information *

ABSTRACT: The coordination of Pd(II) to the 1,3-trans- and 2,3trans-dioxides and the trioxide of cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane gave two dinuclear Pd(II) complexes, which are structural isomers, and a mononuclear complex, respectively. The latter complexes proved to be suitable precatalysts for the oligomerization of ethylene to linear α-olefins (98% selectivity). The different catalytic activity of the structural isomers was shown to depend on the dynamic behavior of the molecular structure.

P

phosphonium salt, originating from the reaction between a diphosphane and benzyl bromide, and the successive reaction with aqueous NaOH give acceptable yields of phosphane−phosphane oxide.8 Both latter synthesis methods fail when tri- and tetraphosphane ligands are concerned. Hence, some of us developed a new Co halide-mediated oxidation of cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb); the selectivity of the oxidation reaction was steered by the type of halide employed.9

olydentate (P−O) ligands containing pairs of phosphorus and oxygen donor atoms (i.e., neutral and anionic oxygen atoms) have proven to be suitable ligands for a variety of metalcatalyzed organic transformations.1 A neutral oxygen donor may be regarded as an intramolecular solvent molecule forming only weak metal−oxygen bonds that may be cleaved reversibly. As a result, these hemilable or hybride ligands provide empty coordination sites, when needed, in the course of the catalytic cycle without separation of the oxygen donor from the complex fragment. (P−O) ligands containing an anionic oxygen atom (i.e., alkoxide, carboxylate, and sulfonate) in combination with Ni or Pd have been shown to be suitable for (i) olefin oligomerization (i.e., SHOP process)2 and polymerization reactions,3 (ii) non-strictly alternating CO−olefin copolymerization reactions,4 and (iii) copolymerization reactions between ethylene and polar olefins.5 On the other hand, (P−O)−Pd complexes containing a phosphane−phosphane oxide chelating ligand have been used to catalyze the homopolymerization of ethylene and the copolymerization of ethylene with polar olefins, giving linear functionalized polymers with randomly inserted polar functional groups along the polymer chain.6 The main problem concerning the synthesis of phosphane−phosphane oxide ligands consists of the lack of selectivity of the oxidation reaction when conventional oxidants (i.e., O2, H2O2, or Br2/H2O) are used.7 As a result, mixtures containing the unreacted diphosphane and the corresponding mono- and dioxide are obtained, which are generally difficult to separate. In this context, Grushin introduced a synthesis protocol that foresees a Pd(OAc)2 (OAc = acetate)catalyzed monoxidation step that is highly selective for aromatic diphosphanes.7 In addition, the treatment of a monoquaternary © 2014 American Chemical Society

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RESULTS AND DISCUSSION The one-pot aerobic synthesis of partially oxidized dppcb, mediated by CoCl2, gave a mixture of two dioxides (i.e., 1,3- and 2,3-trans-dppcbO2) L1 and L2, respectively, and the trioxide dppcbO3 (L3) in an almost 1:1:1 molar ratio (Scheme 1).10 L2 was removed from the latter ligand mixture upon its coordination to [Pd(CH3CN)4](BF4)2, giving a Pd−bischelate complex of the type [Pd(κ2P,P′-L2)2](BF4)2,10 which is, in contrast to L1 and L3, not soluble in toluene and can thus be separared by a simple filtration process. The formation of a Pd−bischelate complex, built up by two five-membered Pd−(PP) palladacycles, is the driving force for the selectivity of the latter reaction. The remaining ligand mixture (i.e., L1 and L3) was reacted with [PdClMe(η4-COD)] in CH2Cl2, producing a mixture of the corresponding di- and mononuclear Pd(II) complexes [Pd2Cl2Me2L1] (1) and [PdClMeL3] (2) (Scheme 1), respectively, which were then separated from each other by Received: May 29, 2014 Published: July 29, 2014 4067

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Scheme 1. Synthesis of 1−6

of the former complex, conducted in DMF, giving thus free ligand L2, which was then reacted with [PdClMe(η4-COD)] in CH2Cl2, giving a light orange solid in 76% yield. The 31P{1H} nuclear magnetic resonance (NMR) spectra, acquired in CD2Cl2 at room temperature, showed for 1 a doublet

exploiting the toluene solubility of 2 that was much higher than that of 1. Both latter complexes were isolated as beige solids in 69 and 66% yields, respectively. The Pd−bischelate complex [Pd(κ2P,P′-L2)2](BF4)2 was successively converted into 3 (Scheme 1) by a cyanolysis reaction 4068

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Figure 1. Variable-temperature 31P{1H} NMR spectra of 1 at room temperature (a) and 213 K (b), 4 at room temperature (c) and 213 K (d), and 5 at room temperature (e) and 213 K (f). Asterisks denote the 77Se satellites present in the spectrum of 5.

Figure 2. ORTEP plot of 1·6.85(C2H4Cl2) and 3·CH2Cl2·0.80(H2O) showing only one isomer. The thermal ellipsoids are shown at a 30% probability level. Solvent molecules and hydrogen atoms, except for the cyclobutane carbon ring, have been omitted, and only the ipso-carbon atoms of the phenyl rings are shown. Selected bond distances (angstroms) and angles (degrees) for 1·6.85(C2H4Cl2): Pd(1)−P(1) = 2.2101(16), Pd(1)−O(1) = 2.207(4), Pd(1)−Cl(1) = 2.3440(17), Pd(1)−C(5) = 2.034(6), Pd(2)−P(3) = 2.2310(15), Pd(2)−O(2) = 2.174(4), Pd(2)−Cl(2) = 2.3417(15), Pd(2)−C(6) = 2.042(6), Cl(1)−Pd(1)−C(5) = 88.3(2), P(1)−Pd(1)−O(1) = 92.10(11), Cl(2)−Pd(2)−C(6) = 88.00(17), and P(3)−Pd(2)−O(2) = 99.38(11). Selected bond distances (angstroms) and angles (degrees) for 3·CH2Cl2·0.8H2O: Pd(1)−P(2) = 2.2567(19), Pd(1)−O(1) = 2.212(4), Pd(1)−Cl(1) = 2.366(2), Pd(1)−C(5) = 2.036(7), Pd(2)−P(3) = 2.2391(18), Pd(2)−O(2) = 2.215(4), Pd(2)−Cl(2) = 2.3543(18), Pd(2)−C(6) = 2.038(6), Cl(1)− Pd(1)−C(5) = 86.0(2), P(2)−Pd(1)−O(1) = 99.36(12), Cl(2)−Pd(2)−C(6) = 86.8(2), and P(3)−Pd(2)−O(2) = 100.92(11).

3, which is the structural isomer of 1, exhibited in the 31P{1H} NMR spectrum two doublets at 30.91 ppm (PO) and 33.02 ppm (P) with only a 3J(P,PO)cis coupling constant of 16.9 Hz. This latter spectroscopic result proves the selective κP,κO-L2 instead of a κ2P-L2 coordination to the PdClMe moiety, which is due to the strong trans influence of the coordinating methyl group.

of doublets at 35.40 ppm (P) and 29.75 ppm (PO) [free ligand, δ(PO) = 27.08 ppm], which are characterized by 3J(P,PO)cis and 3 J(P,PO)trans coupling constants of 17.8 and 7.4 Hz, respectively, and for 2 two doublets at 35.99 ppm (P) and 30.35 ppm (coordinating PO) along with two broad singlets for the uncoordinated PO groups at 25.84 and 19.74 ppm. Compound 4069

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Figure 3. ORTEP plot of 4 and 5·CH2Cl2 showing only one isomer. The thermal ellipsoids are shown at a 30% probability level. Solvent molecules and hydrogen atoms, except for the cyclobutane carbon ring, have been omitted, and only the ipso-carbon atoms of the phenyl rings are shown. Selected bond distances (angstroms) and angles (degrees) for 4: Pd(1)−P(1) = 2.234(2), Pd(1)−S(1) = 2.467(2), Pd(1)−Cl(1) = 2.358(2), Pd(1)−C(5) = 2.057(7), Pd(2)−P(3) = 2.212(2), Pd(2)−S(2) = 2.465(2), Pd(2)−Cl(2) = 2.351(2), Pd(2)−C(6) = 2.039(6), Cl(1)−Pd(1)−C(5) = 85.5(2), P(1)− Pd(1)−S(1) = 107.05(7), Cl(2)−Pd(2)−C(6) = 85.7(2), and P(3)−Pd(2)−S(2) = 98.96(7). Selected bond distances (angstroms) and angles (degrees) for 5·CH2Cl2: Pd(1)−P(1) = 2.213(2), Pd(1)−Se(1) = 2.526(1), Pd(1)−Cl(1) = 2.361(2), Pd(1)−C(5) = 2.080(6), Pd(2)−P(3) = 2.229(2), Pd(2)−Se(2) = 2.548(1), Pd(2)−Cl(2) = 2.355(2), Pd(2)−C(6) = 2.062(6), Cl(1)−Pd(1)−C(5) = 87.2(2), P(1)−Pd(1)−Se(1) = 99.87(5), Cl(2)−Pd(2)−C(6) = 85.7(2), and P(3)−Pd(2)−Se(2) = 107.55(5). 31

P{1H} NMR spectra, acquired in CD2Cl2 at 183 K (Supporting Information), showed for 3 two doublets at 34.33 ppm (P) and 33.64 ppm (PO) with a 3J(P,PO)cis coupling constant of 26.0 Hz, while the analogous NMR spectrum exhibited for 1 two extremely broad humps for P and PO, confirming the presence of many conformations in the slow exchange limit; 3 behaves as expected for a rigid molecular structure. The 1H NMR spectra of 1−3, acquired in CD2Cl2 at room temperature, showed for the Pd-CH3 unit a singlet in the chemical shift range between 0.62 and 0.78 ppm, which is characteristic of the cis location of the methyl group with respect to the coordinated phosphorus atom.4b,5b In addition, the cyclobutane hydrogen atoms of 1 showed at room temperature two broad 1H signals corresponding to axial (ax) (4.58 ppm) and equatorial (eq) cyclobutane hydrogen atoms (4.32 ppm). In contrast, the 1H NMR spectrum of 3 showed for the latter hydrogen atoms well-separated multiplets at 3.61 ppm [H(eq)] and 5.58 ppm [H(ax)]. To study the effect of the chalcogenide type present in 1-related complexes on the dynamic behavior of the overall molecular structure, analogous compounds containing sulfur and selenium instead of oxygen atoms were synthesized by reacting L4 and L5 with [PdClMe(η4-COD)] to give 4 and 5, respectively (Scheme 1). A comparison of the 31P{1H} NMR spectra of 1, 4, and 5 (Figure 1), acquired at room temperature and 213 K, proved that 1 has the most dynamic molecular structure, within the chalcogenide series, while 5 exhibited the most rigid molecular structure, showing already at room temperature four rather broad but distinct 31P signals assigned to P(ax) (37.98 ppm), P(eq) (31.43 ppm), PSe(ax) (27.34 ppm), and PSe(eq) (25.54 ppm) (Figure 1, trace e). When 5 was cooled to 213 K (Figure 1, trace f), 1J(P,Se) coupling constants of 634 and 655 Hz appeared in the corresponding 31P NMR spectra, along with 3J(P,PSe)cis coupling constants of 26.6 and 28.9 Hz, while the 3J(P,PSe)trans coupling constant was smaller (13.6 Hz). In accordance with the 31 1 P{ H} NMR spectrum of 5, acquired at room temperature, the

1

H NMR spectrum of the latter compound showed two distinct H singlets for the Pd-CH3 unit centered at 0.62 and 0.71 ppm, confirming the asymmetric molecular structure of 5. The single-crystal X-ray structure analyses of 1·6.85(C2H4Cl2)11 and 3·CH2Cl2·0.80(H2O)11 exhibited in the asymmetric unit two conformers, while in the ORTEP plot, only one conformer of each is shown (Figure 2). Both molecular structures share (i) a folded cyclobutane carbon ring with folding angles of 145.2°/149.7° [1·6.85(C2H4Cl2)] and 151.7°/154.9° [3·CH2Cl2· 0.80(H2O)] and (ii) the cis coordination of the Pd-CH3 group with respect to the phosphane phosphorus atom, which is in accordance with the 1H NMR singlet found for Pd-CH3 in the corresponding 1 H NMR spectra. Both molecular structures differ significantly in the range of observed P−Pd−O bite angles [i.e., between 92.10(11)° and 100.48(11)° in 1·6.85(C2H4Cl2) and between 99.36(12)° and 101.17(12)° in 3·CH2Cl2·0.80(H2O)]. The smaller bite angle range in the latter case is indicative of a more rigid molecular structure in 3. In addition, 1·6.85(C2H4Cl2) showed shorter intramolecular contacts between the Pd-CH3 hydrogen atoms and the ortho hydrogen atoms of the nearest phenyl rings ranging between 2.299 and 2.443 Å, compared to those of 3·CH2Cl2·H2O (i.e., ranging between 2.446 and 2.779 Å). The molecular structures of 411 and 5·CH2Cl211are shown in Figure 3. Similar to 1·6.85(C2H4Cl2), 4 showed two conformers in the asymmetric unit, both with a folded cyclobutane carbon ring (i.e., folding angle of 146.4°/153.1°), while 5·CH2Cl2 consists of only one isomer (i.e., folding angle of 149.9°). The increase in the size of the six-membered ring, caused by the elongation of the Pd−X bond distances (X = O, S, and Se) from 2.212−2.215 (O) to 2.526−2.548 Å (Se), leads to a rigid asymmetric molecular conformation of 5 (i.e., the longer the Pd−X bond length, the more the six-membered ring is twisted), which is clearly shown by the significantly different P−Pd−Se bite angles ranging between 99.87(5)° and 107.55(5)°. This latter crystallographic result is in complete accordance with the spectroscopic results found for 5. 1

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with dppeO-based catalysts.13 On the other hand, Nozaki’s Pd(II)− [1,2-bis(diphenylphosphino)benzene monoxide]-based catalysts lead exclusively to polymers.6 The structurally flexible precatalysts 1 and 2 showed under identical experimental conditions (i.e., the same Pd content) an almost identical catalytic performance in terms of activity and product selectivity (Table 1, entries 3 and 4 vs entries 7 and 8). This latter catalytic result is strongly indicative of two independently operating Pd(II) centers in 1. To rationalize the significantly lower catalytic activity of 3 compared to those of 1 and 2, operando high-pressure (HP)NMR experiments (Figure 4) were conducted in CD2Cl2 with 1 and 3 in

1−3 were successfully applied to catalyze the oligomerization reaction of ethylene in toluene. The catalytically active cationic Pd-Me species5f,12 were obtained “in situ” by reacting the precatalysts with NaBAr′4 {Ar′ = B[3,5-(CF3)2(C6H3)]4} in the presence of ethylene. Unlike 1−3, 4 and 5 proved to be unstable under the applied catalytic conditions because they released elemental sulfur and selenium in solution, respectively. The catalytic performance of 1−3 was compared under identical catalytic conditions to that of [PdClMe(κ1P,κ1O-dppeO)] (6) [dppeO = 1,2-bis(diphenylphosphanyl)ethane monoxide], which is characterized by a flexible six-membered palladacycle.13 The catalytic results obtained with 1−3 and 6 are listed in Table 1. Table 1. Ethylene Oligomerization Catalyzed by 1−3 and 6 entry

precatalyst

t (h)

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

1 1 1 1 1 1 2 2 3 3 3 6 6 6

3 3 1 3 6 3 1 3 1 3 6 3 1 3

a

p(C2H4) (psi)

TOF

150 450 750 750 750 750 750 750 750 750 750 450 750 750

1253 2335 4080 4010 3970 156 3950 3890 2180 1920 1760 1993 7260 4557

b

linear olefins (%)/α-olefins (%)c

α(β)d

92/97 94/98 nd 96/98 95/98 nd nd 96/97 nd 96/98 94/97 55/nd nd 60/nd

0.76(0.32) 0.78(0.28) nd 0.80(0.25) 0.80(0.25) nd nd 0.79(0.27) nd 0.79(0.27) 0.79(0.27) 0.18(4.6) nd 0.19(4.3)

Figure 4. 31P{1H}(HP)NMR spectra of 1 and 3 acquired in CD2Cl2 at room temperature (a) and after activation of the precatalysts in the presence of ethylene (450 psi) (b).

the presence of NaBAr′4 and ethylene pressure (450 psi). The 31P NMR spectra of the neutral precatalysts in the absence of NaBAr′4 and ethylene pressure are shown as traces a in Figure 4. Upon addition of NaBAr′4 to a ethylene-saturated CD2Cl2 solution of 1 and 3, followed by pressurization with ethylene (450 psi), the latter precatalysts were converted at room temperature into cationic species (1′)2+ and (3′)2+, respectively, as shown by the corresponding 31P{1H} NMR spectra (Figure 4, traces b). The 31P NMR signals assigned to P and PO of the latter cationic species shifted in opposite directions with respect to the corresponding chemical shifts found for the precatalysts (traces b vs traces a), which is consistent with an elongated Pd−P bond distance and a shortened Pd−O bond distance in the cationic species compared to those in 1 and 3 as shown with model compounds A and B+, respectively (see below). The corresponding 1H NMR spectra showed for (3′)2+ a singlet at 0.62 ppm that was assigned to the methyl group located in the cis position with respect to the coordinating phosphorus atom. No formation of oligomers was found with (3′)2+ at room temperature. In contrast, in the case of (1′)2+, the formation of linear α-olefins with a selectivity analogous to that of the bulk catalytic reactions was observed. When the catalytic solutions were heated to 323 K, no phosphorus-containing Pd species could be intercepted, confirming a fast catalytic reaction at the latter reaction temperature. From the operando (HP)NMR study, we can infer that (i) (1′) 2+ and (3′)2+ are related cationic Pd-alkyl species {only to (3′)2+ can we assign a compound of the formula [Pd2Me2(C2H4)2(κP, κO-L2)][B(Ar′4)]2}, (ii) unlike (1′)2+, (3′)2+ does not insert ethylene at room temperature, and (iii) the 31P{1H} NMR spectra of (1′)2+ and (3′)2+ acquired in the presence of ethylene confirm the κP,κO-coordination mode

a

Catalytic conditions: Pd (0.0055 mmol), toluene (30.0 mL), 353 K, and NaBAr′4 (0.015 mmol). bTOF expressed as millimoles of C2H4 consumed per millimole of Pd per hour. cPercentage of linear and α-olefins determined by 13C{1H} NMR spectroscopy (CDCl3) and gas chromatography analysis. dα is the millimoles of Cn+2 per millimole of Cn, and β = (1 − α) × α−1. eAt 323 K.

On the basis of an analysis of the catalytic data reported in Table 1, we found that 1−3 gave linear oligomers characterized by an α value of 0.78 and a selectivity ranging between 92 and 96% depending on the ethylene pressure applied as shown for 1 (Table 1, entries 1, 2, and 4). Gas chromatography−mass spectrometry (GC−MS) analysis of the toluene solution of the obtained oligomers proved that mostly methyl-branched ethylene oligomers were formed as minor byproducts. Moreover, 13C{1H} NMR and GC spectra of the oligomer material, obtained with 1 and 3 (Supporting Information), revealed an α-olefin selectivity of 98% (Table 1), proving the low isomerization activity of the latter catalysts. The high selectivity of the latter catalysts for linear olefins leads to them outperforming five-membered-based Pd−(P-PO) catalysts studied by Keim et al. which are known to be much more selective than the six-membered counterparts.13 These catalysts are also more productive than typical Keim-type Ni catalysts (i.e., TOF < 1000).2b,e In addition, 1−3 showed a much higher α value (i.e., probability of chain propagation)14 of the Schulz−Flory oligomer chain length distribution compared to that of 6 (i.e., 0.80 vs 0.19). As a consequence, the β value (i.e., ratio of the chain transfer rate to the propagation rate)14 is 17 times higher in 1−3 than in 6. The latter precatalyst gives maximal C12 olefins (α = 0.19), which is in accordance with Keim’s catalytic results obtained 4071

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Scheme 2. Proposed Catalytic Cycle for the 1−3-Catalyzed Olefin Oligomerization Reaction

of L1 and L2, which contrasts the dynamic behavior (i.e., κP,κOdppeO vs κP-dppeO coordination mode) observed for Pd(II)dppeO-based ethylene oligomerization catalysts, even in the presence of only 15 psi of ethylene.15 In addition, the latter ligand dynamic fosters the formation of the catalytically inactive chelate species of the formula [Pd(κ1P,κ1O-dppeO)2(BAr′4)2].16 In fact,

6 showed a significant activity decay with time, which contrasts with the high catalyst stability found mainly for 1 and 2 (Table 1, entries 13 and 14 vs entries 3−5 and entries 7 and 8). The catalytic cycle proposed for 1−3-catalyzed ethylene oligomerization reactions has been constructed by means of a theoretical study using as a starting model compound a mononuclear 4072

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CONCLUSIONS Dinuclear Pd(II) complexes 1 and 3 bear two structural isomers of dppcb dioxide (i.e., 1,3-trans-dppcbO2 and 2,3-transdppcbO2), while the coordination of the dppcb trioxide to Pd(II) gave the mononuclear counterpart (2). The molecular structures of 1 and 3 showed in solution a different dynamic behavior, 3 being more rigid than 1. In fact, via elongation of the Pd−X bond (X = O, S, or Se) upon substitution of O with S and Se in 1 analogue structures, the Se derivative showed an asymmetric molecular structure in solution and in the solid state. 1−3 were successfully applied to catalyze the oligomerization of ethylene, yielding linear α-olefins with a selectivity of >96% (i.e., Ni-based ethylene oligomerization catalysts give mainly highly branched products)20 and an α value of 0.78, outperforming even the most selective five-membered palladaring with a coordinated (P-PO) ligand. Although no significant difference in selectivity among 1−3 was observed, 1 and 2 showed high catalyst stability with time (i.e., much higher than that of Ni-based20 and Fe-based21 ethylene polymerization catalysts) and a catalytic activity significantly higher than that of 3, because of a faster insertion of ethylene into the Pd− alkyl bond in 1 and 2 than in 3.

Pd complex characterized by a six-membered palladacycle that is fused to a folded cyclobutane carbon ring with a fixed folding angle of 150.7° (Scheme 2). This simplification of the model compound is justified by the experimental fact that at least in the case of 1 both Pd centers were found to perform equally. All calculated structural parameters and activation energies reported herein take into account solvent effects deriving from toluene. The model compound of the precatalyst (A) that is characterized by Pd−P and Pd−O bond distances of 2.23 and 2.32 Å, respectively, is transformed into monocationic palladium species B+ (step a). This latter reaction occurs in the presence of NaBAr′4 and ethylene through an associative activation process that comprises the coordination of ethylene to A (0.8 kcal mol−1), and successive chloride abstraction, which is associated with an energy cost of 29.2 kcal mol−1. B+ is characterized by an elongation of the Pd−P bond distance and shortening of the Pd−O bond distance to 2.30 and 2.25 Å, respectively, compared to those of A. In fact, the NMR spectroscopically intercepted compound (3′)2+ is a B+ analogue. The latter model compound undergoes an isomerization reaction (step b) yielding C+, which is destabilized by 7.6 kcal mol−1 with respect to B+ but which inserts ethylene (step c) more easily into the Pd−CH3 bond than B+ [i.e., +12.9 kcal mol−1 (C+) vs +32.3 kcal mol−1 (B+)] due to a longer Pd−CH3 bond distance and a more activated CC bond of the coordinated ethylene in C+ compared to B+, which fosters methyl migration. In this context, strong repulsive H···H interactions, as found in the molecular structure of 1·6.85(C2H4Cl2), may positively influence this latter isomerization reaction. Analogous isomerization reactions have been found to be crucial for chain propagation in Pd phosphine sulfonate-mediated ethylene polymerization,17 ethylene−methyl acrylate copolymerization,18 and the nonalternating CO−ethylene copolymerization reactions.19 The resulting tricoordinated Pd species (Bprop+) leads upon ethylene coordination to cationic [Pd(propyl)(ethylene)]+ isomers D+ and E+, with D+ being more stable than E+. Multipleethylene insertion (step f) leads to (D+)′ and (E+)′. In fact, the NMR spectroscopically identified compound (1′)2+ is a (D+)′ analogue. The Pd−β-hydride elimination reaction, which is the termination reaction, involves the three-coordinated Pd species BR+, which is characterized by an alkyl group coordinating trans with respect to the phosphane oxide oxygen atom.18 We studied the Pd−β-hydride elimination by using the successfully optimized Bpentyl+ species, which is a BR+ analogue (Supporting Information), while the isomeric species (i.e., with the pentyl group trans to the coordinated phosphorus atom) was not obtained. Although no transition state for the Pd−β-hydride elimination reaction was found, we explored the potential energy surface (PES) by a relaxed scan of the Pd−Hβ distance and found that the latter process is associated with an energy cost of 8.5 kcal mol−1, giving the three coordinated Pd−H species [BH(trans)+] and free olefin (step h). The latter Pd−H converts into the more stable isomer [BH(cis)+] (step i), which then coordinates ethylene giving the cationic [PdH(ethylene)]+ species F+. The isomer of F+ (G+) inserts ethylene into the Pd−H bond leading to the threecoordinated species Bethyl+ (step l). The coordination of ethylene to the latter species gives cationic [Pd(ethyl)(ethylene)]+ isomers H+ and I+, which are D+ and E+ analogues, respectively. Multipleethylene insertion in the more reactive isomer I+ (step o) closes the catalytic cycle.

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EXPERIMENTAL SECTION

General Procedures. All reactions were conducted under a nitrogen atmosphere using the standard Schlenk technique. [PdClMe(η4-COD)],22 1,3-trans-dppcbS2,23 and 1,3-trans-dppcbSe224 were prepared according to literature methods, while [Pd(CH3CN)4](BF4)2 was purchased from Aldrich. 1H and 31P{1H} NMR spectra were recorded on a Bruker Avance DRX-300 spectrometer operating at 300.13 and 121.98 MHz, respectively. 13C{1H} NMR spectra of the isolated oligomers (i.e., after the evaporation of toluene) were acquired in CDCl3 on a Bruker Avance DRX-400 spectrometer at 100.62 MHz. Operando (HP)NMR spectroscopic experiments were conducted on the same spectrometer using a 10 mm BB probe and a 10 mm sapphire tube (Saphikon, Milford, NH), equipped with a homemade titanium highpressure charging head.25 Chemical shifts (δ) are reported in parts per million relative to TMS (1H NMR) or 85% H3PO4 (31P{1H} NMR). FAB-MS spectra were recorded on a Finnigan MAT-95 spectrometer, using 3-nitrobenzyl alcohol (NOBA) as a matrix. Elemental analyses were conducted with a NA 1500 Carlo Erba elemental analyzer. GC analyses were conducted on a Shimadzu GC 2010 Plus apparatus equipped with an SPB-1 Supelco fused silica capillary column (30 m × 0.25 mm × 0.25 μm) and a flame ionization detector. GC−MS analyses were performed on a Shimadzu QP2010S apparatus equipped with the same capillary column as the gas chromatograph. Synthesis of 1−3. The ligand mixture of L1−L3 (1:1:1 L1:L2:L3 molar ratio) was obtained following a reported synthesis procedure.9 The latter ligand mixture (0.300 g) was dissolved in CH2Cl2 (20.0 mL), and solid [Pd(CH3CN)4](BF4)2 (0.0280 g, 0.0630 mmol) was added while the mixture was vigorously stirred, which was continued for 30 min at ambient temperature, followed by the complete evaporation of the solvent. The obtained yellow residue was successively suspended in toluene (30.0 mL), stirred for 30 min at room temperature, and filtered. This procedure was repeated three times. The toluene-insoluble compound formed was the mononuclear Pd−bischelate complex with the formula [Pd(κ2-P,P′-L2)2](BF4)2,9 which was converted into 3 as described below. The combined filtrates were dried via evaporation, and the isolated solid consisted of an approximately equimolar amount of L1 and L3.9 To this ligand mixture (0.117 g, 0.141 mmol) was added [PdClMe(η4-COD)] (0.0559 g, 0.211 mmol) dissolved in CH2Cl2 (15.0 mL), and the reaction mixture was stirred for 2 h at room temperature. The solvent was thereafter completely removed by evaporation and the residue stirred in toluene (30.0 mL) for 1 h and then filtered. The obtained residue was again suspended and stirred in toluene (30.0 mL) for 1 h, filtered, and dried under vacuum. The isolated beige powder contained 1, while the combined filtrates consisted of 2. The powder of 1 was recrystallized from 1,2-dichloroethane, giving beige crystals that were 4073

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Organometallics

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collected and then dried under vacuum. [Pd(κ2-P,P′-L2)2](BF4)2 (113.0 mg, 0.059 mmol), which was obtained as described above, was dissolved in DMF (5.0 mL), and after the addition of NaCN (28.7 mg, 0.59 mmol), the obtained solution was stirred for 2 h at room temperature. Afterward, the solvent was removed completely, water (20.0 mL) added, and the obtained suspension stirred for 2 h. Then the solid was filtered off, washed with water, and dried under vacuum at room temperature, yielding L2 as an off-white solid. The yield of L2 was 75.3 mg (77%). [PdClMe(η4-COD)] (48.2 mg, 0.181 mmol) and L2 (75.3 mg, 0.090 mmol) were dissolved in deaerated CH2Cl2 (10.0 mL), and the mixture was stirred for 2 h at room temperature. The obtained solution was then filtered and the solvent completely removed, giving a solid compound that was suspended in toluene and stirred for 1 h followed by its filtration and drying, yielding 3 as a light orange solid. 1. Yield: 69.1 mg (66%). Anal. Calcd for C54H50Cl2O2P4Pd2· 3.4C2H4Cl2: C, 49.46; H, 4.35. Found: C, 49.37; H, 4.41. Positive ion FAB-MS: m/z 1103.2 [M − Cl]+, 961.3 [M − Pd − 2Cl]+. 2. Yield: 55.1 mg (69%). Anal. Calcd for C53H47ClO3P4Pd: C, 63.80; H, 4.75. Found: C, 63.75; H, 4.79. Positive ion FAB-MS: m/z 962.2 [M − Cl]+. 3. Yield: 77.8 mg (76%). Anal. Calcd for C54H50Cl2O2P4Pd2: C, 56.98; H, 4.39. Found: C, 57.10; H, 4.42. Positive ion FAB-MS: m/z 1052.29 [M − 2Cl − Me]+. Single crystals of 1·6.85(C2H4Cl2) and 3·CH2Cl2·0.80(H2O), suitable for a single-crystal X-ray structural analysis, were obtained by gas-phase diffusion of n-hexane into a 1,2-dichloroethane or dichloromethane solution of 1 and 3, respectively, at room temperature. NMR Data for 1. 1H NMR (300.13 MHz, CD2Cl2, 298 K): δ 0.70 (s, 6H, PdCH3), 4.32 [br s, 2H, CH(eq)], 4.58 [br s, 2H, CH(ax)], 7.11−7.46 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 298 K): δ 35.40 [dd, 3J(P,PO)cis = 17.8 Hz, 3J(P,PO)trans = 7.4 Hz, P], 29.75 [dd, 3J(P,PO)cis = 17.8 Hz, 3J(P,PO)trans = 7.4 Hz, PO]. 1H NMR (300.13 MHz, CD2Cl2, 213 K): δ 0.42 (s, 6H, PdCH3), 4.20 [br s, 2H, CH(eq)], 4.50 [br s, 2H, CH(ax)], 6.50−7.70 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 213 K): δ 37.6 (br s, P), 31.20 (br s, PO). NMR Data for 2. 1H NMR (300.13 MHz, CD2Cl2, 298 K): δ 0.62 (s, 3H, PdCH3), 3.82 [br s, 2H, CH(eq)], 4.05 [br s, 2H, CH(ax)], 7.02−7.64 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 298 K): δ 35.99 [d, 3J(P,PO)cis = 18.5 Hz, P], 30.35 [d, 3J(P,PO)cis = 18.5 Hz, PO], 25.84 (s, PO), 19.74 (s, PO). NMR Data for 3. 1H NMR (300.13 MHz, CD2Cl2, 298 K): δ 0.78 (s, 6H, PdCH3), 3.61 [m, 2H, CH(eq)], 5.58 [m, 2H, CH(ax)], 7.08− 7.60 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 298 K): δ 33.02 [d, 3J(P,PO)cis = 16.9 Hz, P], 30.91 [d, 3J(P,PO)cis = 16.9 Hz, PO]. Synthesis of 4. In a Schlenk flask under a nitrogen atmosphere, [PdClMe(η4-COD)] (55.6 mg, 0.210 mmol) was added to a deaerated CH2Cl2 solution (10.0 mL) of 1,3-trans-dppcbS2 (90.0 mg, 0.105 mmol). The resulting solution was stirred at room temperature for 1 h and then concentrated to a small volume (2.0 mL), and upon addition of diethyl ether (15.0 mL), the product precipitated from the solution, which was separated from the solid by filtration. The obtained off-white solid was washed with diethyl ether (15.0 mL) and dried in a stream of nitrogen. Yield: 98.3 mg (80%). Anal. Calcd for C54H50Cl2P4S2Pd2: C, 60.96; H, 4.70. Found: C, 61.04; H, 4.73. 1H NMR (300.13 MHz, CD2Cl2, 298 K): δ 0.68 (s, 6H, PdCH3), 3.51 [m, 2H, CH(eq)], 3.88−4.05 [br s, 2H, CH(ax)], 6.70−7.80 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 298 K): δ 35.80 (br s), 29.62 (br s). Both latter 31P{1H} NMR signals were present in a 3:1 integral ratio. 1H NMR (300.13 MHz, CD2Cl2, 213 K): δ 0.40 (s, 3H, PdCH3), 0.67 (s, 3H, PdCH3), 3.51 [m, 2H, CH(eq)], 3.88 [m, 2H, CH(ax)], 6.70−7.80 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 213 K): δ 38.65 [dd, 3J(P,PS)cis = 26.1 Hz, 3 J(P,PS)trans = 14.3 Hz, P], 37.02 [d, 3J(P,PS)cis = 30.4 Hz, P], 35.45 [dd, 3 J(P,PS)cis = 30.4 Hz, 3J(P,PS)trans = 14.3 Hz, PS], 30.02 [d, 3J(P,PS)cis = 26.1 Hz, PS]. Synthesis of 5. In a Schlenk flask under a nitrogen atmosphere, [PdClMe(η4-COD)] (44.5 mg, 0.168 mmol) was added to a deaerated CH2Cl2 (10.0 mL) solution of 1,3-trans-dppcbSe2 (80.0 mg, 0.084 mmol). The resulting solution was stirred at room temperature for 1.5 h. Afterward, the solution was concentrated to a small volume (1.0 mL), and upon

addition of diethyl ether (10.0 mL), the product precipitated from the solution. The solid product was separated from the solution by filtration, washed with diethyl ether (10.0 mL), and dried in a stream of nitrogen, yielding 5 as yellowish powder. Yield: 81.9 mg (77%). Anal. Calcd for C54H50Cl2P4Se2Pd2: C, 51.31; H, 3.95. Found: C, 51.55; H, 4.02. 1H NMR (300.13 MHz, CD2Cl2, 298 K): δ 0.62 (s, 3H, PdCH3), 0.71 (s, 3H, PdCH3), 3.45 [m, 2H, CH(eq)], 4.08 [m, 2H, CH(ax)], 6.41− 8.22 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 298 K): δ 37.98 (br s, P), 31.43 [d, 3J(P,PSe)cis = 24.3 Hz, P], 27.34 (br s, PSe), 25.54 [d, 3J(P,PSe)cis = 24.3 Hz, PSe]. 1H NMR (300.13 MHz, CD2Cl2, 213 K): δ 0.43 (s, 3H, PdCH3), 0.62 (s, 3H, PdCH3), 3.38 [d, 3J(H,H) = 8.1 Hz, 2H, CH(eq)], 3.97 [d, 3J(H,H) = 8.1 Hz, 2H, CH(ax)], 6.27− 8.00 (m, 40H, ArH). 31P{1H} NMR (121.98 MHz, CD2Cl2, 213 K): δ 37.37 [dd, 3J(P,PSe)cis = 28.9 Hz, 3J(P,PSe)trans = 13.6 Hz, P], 31.66 [d, 3J(P,PSe)cis = 26.6 Hz, P], 28.32 [dd, 3J(P,PSe)cis = 26.6 Hz, 3 J(P,PSe)trans = 13.6 Hz, 1J(Se,P) = 634 Hz, PSe], 27.42 [d, 3J(P,PSe)cis = 28.9 Hz, 1J(Se,P) = 655 Hz, PSe]. Single crystals of 4 and 5·CH2Cl2, suitable for a single-crystal X-ray structural analysis, were obtained by gasphase diffusion of n-hexane into a dichloromethane solution of 4 and 5. Operando (HP)NMR Experiments with 1 and 3 in CD2Cl2. In separate NMR experiments, 1 (0.01 mmol) and 3 (0.01 mmol) were dissolved in deaerated CD2Cl2 (2.0 mL) in a Schlenk tube under a nitrogen atmosphere and successively transferred to a 10 mm sapphire tube that afterward was inserted into the NMR probe at room temperature. After the acquisition of 1H and 31P{1H} NMR spectra, the sapphire tube was removed from the NMR probe. The CD2Cl2 solution was then saturated with ethylene at room temperature followed by the addition of NaBAr′4 (0.025 mmol). The sapphire tube was then pressurized with ethylene to 450 psi and inserted into the NMR probe for the acquisition of 1H and 31P{1H} NMR spectra at room temperature. Afterward, the sapphire tube was heated to 323 K, followed by the acquisition of NMR spectra. Afterward, the NMR probe was cooled to room temperature and a couple of final NMR spectra were acquired. Then the sapphire tube was removed from the NMR probe, excess ethylene vented off, and the viscous CD2Cl2 solution analyzed by GC. Catalytic Ethylene Polymerization. A stainless steel autoclave (320.0 mL) equipped with a mechanical stirrer, a pressure controller, and a temperature controller was charged with the precatalyst (0.0055 mmol of Pd) and NaBAr′4 (0.015 mmol). The autoclave was then sealed, evacuated, and charged by suction with toluene (30.0 mL), which had been previously been saturated with ethylene at room temperature. Afterward, the autoclave was pressurized to 75 psi and heated to the desired reaction temperature by means of an oil bath. Once the reaction temperature had been reached, the ethylene pressure was adjusted to the desired pressure and held by continuous feeding of the autoclave with ethylene from a reservoir. The pressure drop in the reservoir was registered during the catalytic experiments. After the desired reaction time, the autoclave was successively cooled to 273 K by means of an ice/acetone mixture, the excess ethylene vented off, and the toluene solution subjected to GC and GC−MS analyses, using n-heptane as an external standard. Afterward, toluene was evaporated at room temperature from the solutions by means of a vacuum pump; the waxy residue was analyzed after being dissolved in CDCl3, and 1H and 13 C{1H} NMR spectra were acquired.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and NMR characterization of 6; variable-temperature 31 1 P{ H} NMR spectra of 1 and 3; 13C{1H} NMR spectra and gas chromatograms of the ethylene oligomerization products obtained with 1 and 3; 1H (HP)NMR spectra of 1, (1′)2+, 3, and (3′)2+; single-crystal X-ray structural analyses of 1·6.85(C2H4Cl2), 3·CH2Cl2·0.80(H2O), 4, and 5·CH2Cl2; and a theoretical study. This material is available free of charge via the Internet at http://pubs.acs.org. 4074

dx.doi.org/10.1021/om500573q | Organometallics 2014, 33, 4067−4075

Organometallics

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Article

wR(F2) = 0.1361. X-ray crystallographic data for 4: C54H50Cl2P4Pd2S2, M = 1170.66, triclinic, P1̅, T = 243(2) K, a = 12.9989(4) Å, b = 16.7551(5) Å, c = 26.836(1) Å, α = 73.752(1)°, β = 81.623(2)°, γ = 74.561(2)°, V = 5392.9(3) Å3, Z = 4, R[F2 > 2σ(F2)] = 0.0426, wR(F2) = 0.0949. X-ray crystallographic data for 5·CH2Cl2: C55H52Cl4P4Pd2Se2, M = 1349.37, triclinic, P1̅, T = 243(2) K, a = 12.6896(4) Å, b = 13.2844(4) Å, c = 18.6285(6) Å, α = 102.671(2)°, β = 93.340(2)°, γ = 114.708(2)°, V = 2743.19(16) Å3, Z = 2, R[F2 > 2σ(F2)] = 0.0405, wR(F2) = 0.0986. (12) Malinoski, J. M.; Brookhart, M. Organometallics 2003, 22, 5324− 5335. (13) Brassat, I.; Keim, W.; Killat, S.; Möthrath, M.; Mastrorilli, P.; Nobile, C. F.; Suranna, G. P. J. Mol. Catal. A: Chem. 2000, 157, 41−58. (14) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997, 16, 2005−2007. (15) Mecking, S.; Keim, W. Organometallics 1996, 15, 2650−2656. (16) Coyle, R. J.; Slowvokhotov, Y. L.; Antipin, M. Y.; Grushin, V. V. Polyhedron 1998, 17, 3059−3070. (17) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088−14100. (18) Haras, A.; Anderson, G. D. W.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics 2006, 25, 4491−4497. (19) (a) Haras, A.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics 2006, 25, 946−953. (b) Haras, A.; Michalak, A.; Rieger, B.; Ziegler, T. J. Am. Chem. Soc. 2005, 127, 8765−8774. (20) Gao, R.; Sun, W.-H.; Redshaw, C. Catal. Sci. Technol. 2013, 3, 1172−1179. (21) Zhang, W.; Sun, W.-H.; Redshaw, C. Dalton Trans. 2013, 42, 8988−8997. (22) Rülke, E. R.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769−5778. (23) Stampfl, T.; Haid, R.; Langes, C.; Oberhauser, W.; Bachmann, C.; Kopacka, H.; Ongania, K.-H.; Brüggeller, P. Inorg. Chem. Commun. 2000, 3, 387−392. (24) Stampfl, T.; Czermak, G.; Gutmann, R.; Langes, C.; Kopacka, H.; Ongania, K.-H.; Brüggeller, P. Inorg. Chem. Commun. 2002, 5, 490−495. (25) Bianchini, C.; Meli, A.; Traversi, A. Ital. Pat. FI A,000,025, 1997.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.M. and A.I. acknowledge ISCRA-CINECA HP Grants “HP10BEG2NO” and “HP10CHEVJ8” and the Centro Ricerca Energia e Ambiente, Colle Val d’Elsa (SI), for computational resources. This research was financially supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF) and the Forschungsförderungsgesellschaft (FFG) (Vienna, Austria), the Tiroler Wissenschaftsfonds (TWF) (Innsbruck, Austria), and the companies VERBUND AG and D. Swarovski KG.

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dx.doi.org/10.1021/om500573q | Organometallics 2014, 33, 4067−4075