Synthesis and Electronic Structure of Iron Borate Betaine Complexes

Article pubs.acs.org/Organometallics

Synthesis and Electronic Structure of Iron Borate Betaine Complexes as a Route to Single-Component Iron Ethylene Oligomerization and Polymerization Catalysts Brian A. Schaefer,† Grant W. Margulieux,† Margaret A. Tiedemann,† Brooke L. Small,‡ and Paul J. Chirik*,† †

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States Chevron Phillips Chemical Company, 1862 Kingwood Drive, Kingwood, Texas 77339, United States

‡

S Supporting Information *

ABSTRACT: A new route to single-component iron ethylene oligomerization and polymerization catalysts is described. Treatment of readily synthesized iron butadiene complexes with B(C6F5)3 generated the corresponding betaine compounds, active catalysts for the oligomerization and polymerization of ethylene. The electronic structures of a family of iron compounds bearing tridentate, α-diimine phosphine ligands have been determined, including cases where the neutral donor has dissociated from the metal. In iron-catalyzed ethylene oligomerization with these compounds, the hemilability of the chelate has been identified as a catalyst deactiviation pathway.

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INTRODUCTION Aryl-substituted bis(imino)pyridine iron dichloride compounds, (ArPDI)FeCl2, on activation with methylaluminoxane (MAO) are highly active catalysts for the polymerization of ethylene and, in some cases, α-olefins.1 Since the initial independent discoveries by Bennett,2 Brookhart3 and Gibson,4 catalyst improvements have largely relied on empirically driven structure−reactivity relationships.5 Because of the multiple oxidation and spin states available to iron, several proposals have been put forth concerning the identity of the propagating species.6 The synthesis and characterization of well-defined, single-component bis(imino)pyridine iron alkyl cations with weakly coordinating anions definitively established the role of high-spin iron(II)7,8 and are consistent with spectroscopic data obtained with (ArPDI)FeCl2/MAO mixtures (Figure 1).9 Notably, these findings demonstrate that carbon−carbon bond formation, usually confined to the realm of strong-field organometallic complexes, can be achieved by catalysts in a weak ligand field. A related class of iron dihalide compounds bearing αdiimines with pendant neutral donors have been shown, upon activation with MAO, to exhibit high activity for the oligomerization of ethylene (Figure 1).10 These catalysts generate product distributions well-suited for full-range αolefin synthesis, a complementary technology to selective αolefin production methods for ethylene dimerization, trimerization, and even tetramerization.11 Little is known about the nature of the active species of these compounds upon treatment with excess MAO. Establishing the oxidation and © XXXX American Chemical Society

Figure 1. Exploration of the electronic structures and role of the supporting ligand of ethylene oligomerization and polymerization catalysts.

spin states of the iron is essential in understanding the origin of the selectivity of these catalysts as well as in providing insights Received: October 5, 2015

A

DOI: 10.1021/acs.organomet.5b00839 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

naphthalene yielded a green crystalline solid identified as the bis(chelate) monohalide (PhenPNNDip)2FeBr. The solid-state structure established dissociation of the pendant phosphine donors from the iron center (Figure 2).

for the next generation of compounds with improved performance. Here we describe the synthesis of iron borate betaine complexes as a new route to well-defined, singlecomponent iron catalysts for ethylene polymerization and oligomerization. In addition to defining the role of Fe(II) during ethylene oligomerization, these studies have demonstrated the role of the hemilabile ligand in catalyst deactivation pathways and the unique features of MAO in reviving oligomerization activity.

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RESULTS AND DISCUSSION

Synthetic and Electronic Structure Studies. Established routes to bis(imino)pyridine iron alkyl cations have relied on the treatment of the corresponding iron dialkyl with a Bronsted or Lewis acid7 or oxidation of an appropriate neutral iron alkyl precursor.8 In order to prepare the appropriate [PNN] congeners, reductive alkylation chemistry of (PhenPNNDip)FeBr2 was explored (Scheme 1). Monohalide iron chelate compounds have proven to be useful precursors for the synthesis of alkyl complexes.12 Reduction of (PhenPNNDip)FeBr2 with either NaBEt3H or sodium in the presence of a catalytic amount of Figure 2. Solid-state structure of (phenPNNdip)2FeBr with 30% probability ellipsoids. Hydrogen atoms and aryl phosphines have been removed for clarity.

Scheme 1. Reductive Alkylation Chemistry of (PhenPNNDip)FeBr2

Table 1. Computed and Experimental Bond Distances (Å) and Angles (deg) for (phenPNNdip)2FeBr Fe(1)−N(1) Fe(1)−N(2) N(1)−C(1) N(2)−C(2) C(1)−C(2) N(2)−Fe(1)−N(2A) N(1)−Fe(1)−Br(1) N(1)−Fe(1)−N(1A) Br(1)−Fe(1)−N(1A)

exptl

calcd

2.0547(12) 2.0852(12) 1.3183(19) 1.3107(19) 1.453(2) 166.77(7) 116.37(3) 127.26(7) 116.37(3)

2.183 2.186 1.306 1.299 1.483 161.27 111.16 124.44 124.39

The metrical parameters (Table 1) of the α-diimines coupled with the Mössbauer parameters (δ = 0.70 mm/s, ΔEQ = 2.42 mm/s) and X-band EPR spectrum (Figure 3) establish that the overall S = 3/2 iron compound is best described as a high-spin iron(II) compound with one neutral and one monoreduced αdiimine.13 DFT calculations utilizing the B3LYP functional were performed to gain additional insight into the electronic structure of (phenPNNdip)2FeBr. A broken-symmetry solution, BS(4,1), where the integers designate the number of unpaired spins on two distinct spin centers, was found to be the lowest in energy. This corresponds to an electronic structure depiction with four α spins located at the metal center and one β spin distributed equally over the two ligands. The bonding situation can be visualized by the Mulliken spin density plot shown in Figure 4. Positive spin density is located mainly at the iron center (ρFe = +3.70), while negative spin density is distributed over both of the ligands (ρL = −0.70), suggestive of a delocalized mixed-valence system. The signs of the spins are arbitrary. Mössbauer parameters calculated from this optimized structure of δ = 0.84 mm/s and ΔEQ = 3.22 mm/s are slightly B


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Organometallics

The iron alkyl complex (PhenPNNDip)FeCH2SiMe3 was obtained in approximately 95% purity, albeit in low isolated yield, from washing the mixture of products with cold hexane. This procedure generated sufficient material for additional characterization. A solution magnetic moment of 3.7 μB (benzene-d6, 23 °C) established an S = 3/2 ground state, analogous to the case for related aryl-substituted bis(imino)pyridine iron alkyl complexes.12 Twelve paramagnetically shifted resonances were observed in the benzene-d6 1H NMR spectrum. Both the 57Fe Mössbauer isomer shift of 0.53 mm/s and rhombic signal observed by X-band EPR spectroscopy (see the Supporting Information) also support a high-spin iron alkyl complex. In the absence of crystallographic data, determination of the redox state of the [PNN] chelate and assignment of the oxidation state of the metal are tenuous. Oxidation of (PhenPNNDip)FeCH2SiMe3 was explored to generate the targeted iron alkyl cation. Addition of [Cp2Fe][B(C6H3-3,5-(CF3)2)4] produced a mixture of two products, as judged by Mössbauer spectroscopy. One, accounting for approximately 30% of the mixture, exhibited parameters (δ = 0.49 mm/s, ΔEQ = 0.83 mm/s) consistent with formation of the desired iron cation.8 Unfortunately, identification of the second complex has not been accomplished. Importantly, addition of excess ethylene gas to this mixture produced a Schulz−Flory value (C10/C8) of 0.6, similar to the value produced from (PhenPNNDip)FeBr2/MAO mixtures, suggesting that, similar to the case for bis(imino)pyridine complexes, highspin iron(II) alkyl complexes are catalytically competent (Scheme 1). Because the synthesis of iron alkyl and the corresponding alkyl cation were complicated by formation of bis(chelate) compounds, alternative routes to single-component oligomerization catalysts were explored. Emphasis was placed on a method that would avoid reductive conditions which result in formation of these impurities. We have previously reported that reduction of various [(PNN)FeBr2] derivatives in the presence of carbon monoxide yields the desired carbonyl compounds without competing formation of bis(chelate) iron byproducts.14 Butadiene, much like carbon monoxide, has also proven to be a sufficiently stabilizing, strong-field ligand that favors mono(chelate) products and provides an entry point for catalytic evaluation.15 Erker and co-workers reported a strategy for activating group 4 metallocene and α-diimine nickel16 butadiene compounds by treatment with B(C6F5)3, yielding the corresponding betaine compounds, which are active singlecomponent catalysts for alkene polymerization. With the intent of extending this approach to iron, syntheses of (PNN)Fe(η4C4H6) compounds were conducted. Stirring a THF solution of (PhenPNNDip)FeBr2 with excess butadiene in the presence of 0.5% Na(Hg) followed by extraction into diethyl ether, filtration, and recrystallization produced (PhenPNNDip)Fe(η4C4H6) as a purple, diamagnetic solid in 92% yield. The benzene-d6 1H NMR spectrum exhibits the number of resonances expected for a C1-symmetric iron compound with five distinct peaks for the butadiene ligand, suggesting a static structure in solution at 23 °C. A single 31P resonance was observed at 70.0 ppm, and zero-field 57Fe Mö ssbauer parameters (δ = 0.29 mm/s, ΔEQ = 1.39 mm/s) are consistent with a low-spin iron(II) compound. A NOESY NMR experiment was conducted in benzene-d6 at 23 °C to assign the geometry of the butadiene ligand. The data are most consistent with a trans configuration, similar to that previously

Figure 3. X-band EPR spectrum of (PhenPNNDip)2FeBr recorded in toluene glass at 10 K. Conditions: microwave frequency 9.381 GHz, power 2.0 mW, modulation amplitude 4 G. Spectroscopic parameters: E/D = 0.33, gx = 2.01, gy = 2.02, gz = 2.05, gstrain = 0.12, 0.04, 0.05.

Figure 4. Spin density plot obtained from a Mulliken population analysis (red, positive spin density; yellow, negative spin density) for (phenPNNdip)2FeBr.

higher than the experimentally determined values and are likely a consequence of the elongated metal−ligand bond distances. Given the complications observed with the isolation of bis(chelate) iron complexes using reduction chemistry, direct alkylation reactions were studied. Addition of 2 equiv of LiCH2SiMe3 to a toluene slurry of (PhenPNNDip)FeBr2 followed by extraction into diethyl ether produced a mixture of products identified as (PhenPNNDip)2Fe and (PhenPNNDip)FeCH2SiMe3. In a typical procedure, the iron alkyl was slightly favored (60:40) over (PhenPNNDip)2Fe, but other ratios were obtained if the reaction conditions were modified. The bis(chelate) complex (PhenPNNDip)2Fe was more reliably synthesized from addition of 2 equiv of sodium in the presence of a catalytic amount of naphthalene to (PhenPNNDip)FeBr2. A combination of Mössbauer spectroscopy, magnetic measurements, DFT calculations, and X-ray diffraction on a related derivate (see the Supporting Information) support an S = 1 compound best described as a low-spin Fe(II) compound with two noninteracting one-electron-reduced [PNN] chelates. C


Article

Organometallics established by X-ray crystallography in bis(imino)pyridine and bis(aldimino)pyridine iron compounds.15,17 To generate an active, single-component catalyst for ethylene oligomerization, a toluene solution of (PhenPNNDip)Fe(η4C4H6) was treated with B(C6F5)3. A red, paramagnetic solid identified as (PhenPNNDip)Fe(η3-C4H6-B(C6F5)3) was isolated in 85% yield. An identical procedure was used to synthesize the bis(imino)pyridine analogue (iPrPDI)Fe(η3-C4H6-B(C6F5)3), and a dark blue-green compound was obtained in 84% yield (Figure 5). Benzene-d6 solution magnetic moments (23 °C) of

Figure 6. Zero-field 57Fe Mossbauer spectra for (phenPNNDip)Fe(η3C4H6-B(C6F5)3) (top) and (iPrPDI)Fe(η3-C4H6-B(C6F5)3) (bottom).

The solid-state structure of (iPrPDI)Fe(η3-C4H6-B(C6F5)3) (Figure 5) demonstrates formation of an η3-allyl ligand lifted from the idealized iron chelate plane, resulting in a molecular geometry best described as distorted square pyramidal with the pyridine(diimine) nitrogen atoms and C(37) of the allyl defining the basal plane. Distortions to the bond distances of the bis(imino)pyridine (Nimine−Cimine = 1.312(2), 1.306(2) Å; Cimine−Cipso = 1.451(3), 1.450(3) Å) are diagnostic of redox activity and are consistent with one-electron chelate reduction. Metrical parameters are reported in Table 2. This observation is in contrast with the neutral chelates previously observed with cationic bis(imino)pyridine iron alkyl complexes used as singlecomponent ethylene polymerization precatalysts.8 Table 2. Computed and Experimental Bond Distances (Å) and Angles (deg) for (iPrPDI)Fe(η3-C4H6-B(C6F5)3) Figure 5. Synthesis of iron betaine compounds and solid-state structure of (iPrPDI)Fe((η3-C4H6-B(C6F5)3) with 30% probability ellipsoids. Hydrogen atoms and [iPr] aryl substituents are omitted for clarity.

Fe(1)−N(1) Fe(1)−N(2) Fe(1)−N(3) Fe(1)−C(35) Fe(1)−C(36) Fe(1)−C(37) N(1)−C(2) N(3)-C-(8) C(2)−C(3) C(7)−C(8) C(35)−C(36) C(36)−C(37) N(2)−Fe(1)−C(35) N(2)−Fe(1)−C(37)

3.2 (PNN) and 3.0 (PDI) μB establish S = 1 compounds, demonstrating a weakening of the ligand field upon activation of the butadiene ligand. Both compounds exhibit observable and slightly paramagnetically shifted 1H NMR spectra in benzene-d6 solution. The PNN derivative exhibits 14 paramagnetically shifted resonances over a range of about 60 ppm in the 1H NMR spectrum. The zero-field 57Fe Mössbauer parameters recorded in the solid state at 80 K (PNN, δ = 0.46 mm/s, ΔEQ = 2.41 mm/s; PDI, δ = 0.56 mm/s, ΔEQ = 2.66 mm/s) establish formation of single iron products with both chelates and are consistent with high-spin iron compounds (Figure 6).

a

D

exptl

calcd

calcda

2.1255(15) 1.8997(15) 2.1033(15) 2.0906(19) 2.0658(18) 2.2028(18) 1.312(2) 1.306(2) 1.451(3) 1.450(3) 1.413(3) 1.393(3) 168.14(7) 107.37(7)

2.257 1.972 2.250 2.118 2.214 2.674 1.306 1.305 1.460 1.464 1.420 1.404 169.56 120.73

2.282 1.963 2.260 2.091 2.066 2.203 1.301 1.303 1.462 1.465 1.415 1.407 168.47 109.77

The calculation was performed with a geometry constraint. DOI: 10.1021/acs.organomet.5b00839 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics The electronic structure of (iPrPDI)Fe(η3-C4H6-B(C6F5)3) was further studied with DFT calculations using the B3LYP functional.18 The Fe(1)−C(37) and −C(38) distances were constrained to experimentally determined values in the calculations. In the absence of such constraints, geometry optimization lengthened these values to 2.214 and 2.674 Å, well outside the distances observed in the solid-state structure. The energy difference between the constrained and unconstrained solutions was essentially indistinguishable, with the constrained geometry solution being less stable by only 1.3 kcal mol−1. A broken-symmetry (4,2) solution19 was lowest in energy, corresponding to a high-spin Fe(II) (SFe = 2) center engaged in antiferromagnetic coupling to both chelate (SPDI = 1/2) and allyl (S allyl = 1/2) radical anions accounting for the experimentally observed S = 1 ground state. A spin density plot for (iPrPDI)Fe(η3-C4H6-B(C6F5)3) is reported in Figure 7.

Figure 8. Qualitative molecular orbital diagram for (iPrPDI)Fe(η3C4H6-B(C6F5)3) from a B3LYP DFT calculation.

iron product. X-ray diffraction established the formation of [(iPrPDI)Fe(THF)2]2[(F5C6)3BC4H6B(C6F5)3] (Figure 9). To

Figure 7. Spin density plot for (iPrPDI)Fe(η3-C4H6-B(C6F5)3) obtained from a Mulliken population analysis (red, positive spin density; yellow, negative spin density).

The relatively low amount of spin density observed at the iron center is likely due to the large overlap observed between the allylic fragment and the metal center. A qualitative molecular orbital diagram for (iPrPDI)Fe(η3-C4H6-B(C6F5)3) is presented in Figure 8. DFT-computed 57Fe Mössbauer parameters of δ = 0.63 mm/s and ΔEQ = 3.16 mm/s were obtained from this electronic structure description and are in good agreement with the experimentally determined values. While X-ray-quality crystals of (PhenPNNDip)Fe(η3-C4H6-B(C6F5)3) have remained elusive, the similarity in magnetic moments and 57Fe Mö ssbauer parameters support an identical ground-state electronic structure. Both classes of iron betaine complexes proved sensitive to coordinating solvents. Addition of THF to (PhenPNNDip)Fe(η3C4H6-B(C6F5)3) resulted in 70% re-formation of (PhenPNNDip)Fe(η4-C4H6) along with 30% of an unidentified compound. With (iPrPDI)Fe(η3-C4H6-B(C6F5)3), dissolution in THF or addition of approximately 20 equiv of THF to a benzene-d6 solution of the compound resulted in loss of 1 equiv of butadiene with concomitant formation of a green, paramagnetic

Figure 9. Synthesis, solid-state structure (30% probability ellipsoids), and EPR spectroscopic data for [(iPrPDI)Fe(THF)2]2[(F5C6)3BC4H6B(C6F5)3]. The second [PDI]Fe counterion, hydrogen atoms, and selected aryl substituents have been omitted for clarity.

our knowledge, this is the first example of this unusual dianion. The geometry about each iron cation is best described as distorted square pyramidal with no close contacts with the dianion. The metrical parameters of the chelate are consistent with a neutral bis(imino)pyridine. The zero-field 57Fe Mössbauer spectrum exhibits a single quadrupole doublet centered at 1.05 mm/s, consistent with a high-spin Fe(I) center and the value previously reported for [(iPrPDI)Fe(OEt2)][(B(C6F5)4].20 The X-band EPR spectrum E


Article

Organometallics Table 3. Ethylene Oligomerization and Polymerization Performance of Iron Betaine Compoundsa

compoundb 3

(1)-Fe(η -C4H6-B(C6F5)3) (1)-FeBr2/1000 MAO (1)2-FeBr/1000 MAO (1)2-Fe/1000 MAO (1)2-Fe (2)- Fe(η3-C4H6-B(C6F5)3)c (2)-Fe(CH3)+c,d

Tav/Tmax (°C)

yield (g)

PI or K(C10/C8)e

productivity (g mmol−1 h−1)

26/34 40/72 48/67 39/69 23/28 85/97 67/74

0.023 5.18 2.66 3.64 0 8.31 10.55

0.68 0.68 0.58 0.70 N/A 2.55 3.09

13.4 6214 3190 4368 0 9972 6330

Polymerizations were carried out for 10 min using 0.005 mmol of catalyst in 10 mL of toluene with 430 psi of ethylene at 23 °C in a 50 mL Parr reactor, unless otherwise noted. bThe notation (1) designates use of PhenPNNDip, and (2) designates iPrPDI. cPolymerization was carried out in a 300 mL Parr reactor with 175 mL of toluene. dPolymerization was carried out using 0.01 mmol of catalyst. eThe polydispersity index was determined by high-temperature GPC in o-dichlorobenzene; Schulz−Flory distributions were determined by GC. a

In summary, a new route to single-component iron ethylene oligomerization and polymerization catalysts has been discovered by treatment of readily available butadiene compounds with B(C6F5)3. For the bis(imino)pyridine example, the electronic structure is different from those of previously reported iron alkyl cations and involves a chelate radical anion, demonstrating the increased field strength of the betaine. Isolation of single-component [PNN]-based catalysts demonstrated the impact of the hemilabile ligand, which through dissociation opens pathways to bis(chelate) iron compounds and ultimately catalyst deactivation.

was recorded in toluene glass (10 K) and exhibits a rhombic signal consistent with an S = 3/2 iron compound. The spectrum was simulated using an S = 3/2 spin Hamiltonian formalism with large zero-field splitting parameters (D ≫ hν ≈ 0.3χμ−1), full rhombicity (E/D = 0.33), and g values of gx = 2.03, gy = 2.23, and gz = 1.93 (gav = 2.06) (Figure 9). Evaluation of the iron betaine complexes for catalytic oligomerization and polymerization of ethylene is reported in Table 3. Also included are the results of ethylene oligomerization with ( Phen PNN Dip )FeBr 2 , ( Phen PNN Dip ) 2 FeBr, and (PhenPNNDip)2Fe each activated with 1000 equiv of MAO. The catalytic activity of the previously reported singlecomponent bis(imino)pyridine iron methyl complex [(iPrPDI)FeCH3][BPh4]8 is also included for comparison. The bis(imino)pyridine betaine complex ( iPrPDI)Fe(η3-C 4H6-B(C6F5)3) proved to be an exceptionally active single-component ethylene polymerization catalyst, and significant exotherms were observed upon addition of ethylene. The polymer obtained from this reaction had a weight-average molecular weight of 89000 and a polydispersity of 2.55. The iron betaine complex (PhenPNNDip)Fe(η3-C4H6-B(C6F5)3) proved to be an effective single-component catalyst for ethylene oligomerization, yielding a Schulz−Flory distribution of α-olefin products that are indistinguishable from that of the iron dibromide on activation with excess MAO. One stark difference between the single-component and MAO-activated catalysts is the reduced activity of the former. The poor activity observed with (PhenPNNDip)Fe(η3-C4H6-B(C6F5)3) suggests catalyst deactivation. In light of the preference for formation of bis(chelate) iron compounds upon alkylation, it is likely that similar compounds result in deactivation in the singlecomp on ent examples. Control experiment s with (PhenPNNDip)2Fe produced no ethylene oligomerization or polymerization activity, identifying such compounds as potential deactivation pathways from single-component catalyst precursors. With excess MAO, however, the bis(chelate) iron compounds (PhenPNNDip)2FeBr and (PhenPNNDip)2Fe both proved to be active for catalytic ethylene oligomerization. These results demonstrate that if bis(chelate) iron compounds form during more commonly used (PhenPNNDip)FeBr2-MAO ethylene oligomerizations, the activator can convert them back to an active propagating species. This effect is absent in the singlecomponent cases, and therefore lower productivities are observed.

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were carried out using standard high-vacuum-line, Schlenk, or cannula techniques or in an M. Braun inert-atmosphere drybox containing an atmosphere of purified nitrogen. The M. Braun drybox was equipped with a cold well designed for freezing samples in liquid nitrogen. Solvents for air- and moisture-sensitive manipulations were dried and deoxygenated using literature procedures.21 Sodium triethylborohydride (1.0 M in toluene, Aldrich) was used without further purification. Tris(pentafluorophenyl)borane (Aldrich) was dried for 24 h on the high-vacuum line prior to use. Deuterated solvents for NMR spectroscopy were distilled from sodium metal under an atmosphere of argon and stored over 4 Å molecular sieves. The following compounds were prepared as described previously: (PhenPNNdip)FeBr2 and (3,5‑Me‑PhPNNMes)FeCl210 and (iPrPDI)Fe(C4H6).17 1 H NMR spectra were recorded on a Varian Inova 400 spectrometer operating at 399.860 MHz. All chemical shifts are reported relative to SiMe4 using 1H (residual) chemical shifts of the solvent as a secondary standard. All 1H NMR coupling constants are reported in Hz. 13C NMR spectra were recorded on a Bruker 500 spectrometer operating at 125.71 MHz. 13C chemical shifts are reported relative to SiMe4 using chemical shifts of the solvent as a secondary standard where applicable. 31P NMR spectra were collected on a Bruker 300 AVANCE spectrometer operating at 299.763 MHz and were referenced to 85% H3PO4 as an external standard. For all paramagnetic compounds, the peak width at half-height is reported in hertz. Infrared spectroscopy was conducted on a Thermo-Nicolet iS10 FT-IR spectrometer calibrated with a polystyrene standard. Elemental analyses were performed at Robertson Microlit Laboratories, Inc., in Ledgewood, NJ. Continuous wave (CW) EPR spectra were recorded at 10 K on an X-band Bruker EMXPlus spectrometer equipped with an EMX standard resonator and a Bruker PremiumX microwave bridge. The spectra were simulated using EasySpin for MATLAB. Gel permeation chromatography was performed at 150 °C in 1,2dichlorobenzene at 1.0 mL/min on a Viscotek HT-GPC Modular 350A instrument equipped with three SEC columns (Viscotek CLM6210 columns), refractive index, and two-angle light-scattering F


Article

Organometallics

min at room temperature before the volatiles were removed in vacuo. The product was extracted from the solid residue with diethyl ether and filtered through a plug of Celite. Removal of the solvent in vacuo yielded 0.073 g (87%) of the title product. Anal. Calcd for C76H74FeN4P2: C, 78.61; H, 6.42; N, 4.82. Found: C, 78.46; H, 6.41; N, 4.47. Magnetic susceptibility (benzene-d6, 293 K): μeff = 2.45 μB. No 1H NMR resonances were observed. Preparation of (PhenPNNDip)FeCH2SiMe3. A 20 mL scintillation vial was charged with a stirbar, 0.400 g (0.521 mmol) of (PhenPNNDip)FeBr2, and 10 mL of toluene. The mixture was chilled in the freezer to −35 °C. A separate vial was charged with 0.098 mg of ((trimethylsilyl)methyl)lithium (1.041 mmol) and 5 mL of toluene. The lithium reagent was then added dropwise to the chilled solution of (PhenPNNDip)FeBr2. The resulting purple solution was stirred for 15 min at room temperature before being filtered through Celite, and volatiles were removed. The crude product was washed with cold hexane. Removal of residual solvent in vacuo yielded 0.174 g (48%) of the title product. Anal. Calcd for C42H48FeN2PSi: C, 72.52; H, 6.95; N, 4.03. Found: C, 72.21; H, 6.72; N, 3.72. Magnetic susceptibility (benzene-d6, 293 K): μeff = 3.7 μB. 1H NMR (300 MHz, C6D6, 23 °C): δ −16.02 (2 peaks, Δν1/2 = 51.8 Hz, Δν1/2 = 433 Hz), −13.26 (Δν1/2 = 56 Hz), −8.03 (Δν1/2 = 19 Hz), −4.14 (Δν1/2 = 19 Hz), −1.87 (Δν1/2 = 40 Hz), −0.59 (Δν1/2 = 253 Hz), 11.72 (Δν1/2 = 303 Hz), 14.70 (Δν1/2 = 55 Hz), 69.68 (Δν1/2 = 179 Hz), 79.88 (Δν1/2 = 52 Hz), 81.05 (Δν1/2 = 52 Hz), 85.42 (Δν1/2 = 222 Hz) ppm. Preparation of (PhenPNNDip)Fe(η4-C4H6). A thick-walled vessel was charged with 4.0 g of mercury, approximately 10 mL of THF, and a stir bar. Sodium (0.046 g, 2.004 mmol) was cut into small pieces and added slowly to the rapidly stirred slurry. The resulting amalgam was stirred for an additional 30 min to ensure complete dissolution. A solution of (PhenPNNDip)FeBr2 (0.770 g, 1.002 mmol) in 10 mL of THF was added to the reaction vessel, which was then sealed. The resulting mixture was brought to −196 °C and the vessel was evacuated. A 10 equiv amount of of 1,3-butadiene (10.02 mmol) was then introduced via gas bulb addition, and the reaction mixture was stirred for 6 h. The resulting purple mixture was then decanted away from the amalgam, and the volatiles were removed in vacuo. The resulting solid was dissolved in diethyl ether and passed through a pad of Celite. The solvent was removed in vacuo to give 0.611 g (92%) of a purple solid identified as (PhenPNNDip)Fe(C4H6). Anal. Calcd for C42H43FeN2P: C, 76.13; H, 6.54; N, 4.23. Found: C, 75.82; H, 6.81; N, 4.21. 1H NMR (300 MHz, benzene-d6, 23 °C): δ −3.03 (m, 1H, butadiene CH2), −2.82 (m, 1H, butadiene CH2), 0.83 (d, 3JHH = 6.7 Hz, 3H, iPr CH3), 0.93 (d, 3JHH = 6.7 Hz, 3H, iPr CH3), 1.23 (d, 3JHH = 6.8 Hz, 3H, iPr CH3), 1.38 (m, 1H, butadiene CH2), 1.54 (d, 3JHH = 6.8 Hz, 3H, iPr CH3), 2.42 (m, 1H, butadiene CH2), 3.04 (sept, 3JHH = 6.7 Hz, 1H, iPr CH), 3.28 (m, 1H, N−CH2) 3.38 (m, 1H, P-CH2), 3.86 (m, 1H, P-CH2), 4.10 (m, 1H, N-CH2), 4.12 (m, 1H, iPr CH), 6.22 (d, 3JHH = 7.0 Hz, 1H, Ar), 6.42 (m, 2H, butadiene CH), 6.63 (t, 3 JHH = 7.5 Hz, 1H, Ar), 6.72 (m, 1H, Ar), 6.80 (t, 3JHH = 7.8 Hz, 1H, Ar), 7.01 (m, 5H, Ar), 7.09 (m, 2H, Ar), 7.23−7.44 (m, 5H, Ar), 7.49 (t, 3JHH = 8.1 Hz, 2H, Ar) ppm. One Ar-H was not located. 13C{1H} NMR (125.71 MHz, benzene-d6, 23 °C): δ 23.1 (Ar-CHMe2), 25.2 (Ar-CHMe2), 25.3 (Ar-CHMe2), 26.6 (Ar-CHMe2), 27.9 (Ar-CHMe2), 28.0 (Ar-CHMe2), 32.7 (d, 2JPC = 11.9 Hz, butadiene CH2), 37.3 (d, 2 JPC = 8.4 Hz, butadiene CH2), 40.5 (d, 1JPC = 34.2 Hz, P-CH2), 50.5 (d, 2JPC = 3.7 Hz, N−CH2), 82.9 (butadiene CH), 97.5 (butadiene CH), 118.9 (Ar-CH), 119.2 (Ar-CH), 123.7 (Ar-CH), 123.9 (Ar-CH), 124.6 (Ar-CH), 125.0 (Ar-CH), 126.4 (Ar-CH), 127.5 (Ar-CH), 128.4 (Ar-CH), 128.5 (Ar-CH), 128.6 (Ar-CH), 128.7 (Ar-CH), 129.3 (ArCH), 131.2 (Ar-C), 132.1 (Ar-C), 132.6 (Ar-C), 132.7 (Ar-CH), 132.8 (Ar-CH), 132.9 (Ar-CH), 133.0 (Ar-CH), 133.8 (Ar-C), 136.0 (ArC), 141.2 (Ar-C), 141.9 (Ar-C), 142.1 (Ar-C), 151.2 (Ar-C), 155.7 (Ar-C), 156.6 (Ar-C) ppm. One Ar-CH was not located. 31P{1H} (300 MHz, benzene-d6, 23 °C): δ 70.0 (br s) ppm. Preparation of (PhenPNNDip)Fe(η3-C4H6-B(C6F5)3). A 20 mL scintillation vial was charged with a stirbar, 0.155 g (0.234 mmol) of (PhenPNNDip)Fe(η4-C4H6), and 10 mL of toluene. The mixture was chilled in the freezer to −35 °C. A separate vial was charged with 0.120 g (0.234 mmol) of tris(pentafluorophenyl)boron and 5 mL of toluene.

detectors. Polymer melting point transition temperatures (Tm) were measured on a PerkinElmer DSC 7 instrument on the second heating cycle at a heating rate of 10 °C/min. Solid-state magnetic moments were determined using a Johnson Matthey Magnetic Susceptibility Balance, collected at 295 K, unless otherwise noted. Solution magnetic moments were determined by the method of Evans at 295 K using a ferrocene standard unless otherwise noted.22 Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil in a drybox, transferred to a nylon loop, and then quickly transferred to the goniometer head of a Bruker SMART APEX DUO or D8 VENTURE DUO diffractometer system equipped with molybdenum and copper X-ray tubes (λ = 0.71073 and 1.54184 Å, respectively). Preliminary data revealed the crystal system. The data collection strategy was optimized for completeness and redundancy using the Bruker COSMO software suite. The space group was identified, and the data were processed using the Bruker SAINT+ program and corrected for absorption using SADABS. The structures were solved using direct methods (SHELXS) completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures. All DFT calculations were performed with the ORCA program package.23 The geometry optimizations of the complexes and singlepoint calculations on the optimized geometries were carried out at the B3LYP level24−26 of DFT. The all-electron Gaussian basis sets were those developed by the Ahlrichs group.27−29 Triple-ζ quality basis sets def2-TZVP with one set of polarization functions on cobalt and on the atoms directly coordinated to the metal center were used. For the carbon and hydrogen atoms, polarized split-valence def2-SV(P) basis sets were used, which were of double-ζ quality in the valence region and contained a polarizing set of d functions on the non-hydrogen atoms. Auxiliary basis sets to expand the electron density in the resolution-of-the-identity (RIJCOSX) approach30−32 were chosen to match the orbital basis.33−35 Numerical frequencies were calculated at the same level of theory to confirm the optimized geometries (no imaginary frequencies) and to derive thermochemical data. Throughout this paper we describe our computational results by using the broken-symmetry (BS) approach by Ginsberg and Noodleman.19 Because several broken-symmetry solutions to the spin-unrestricted Kohn−Sham equations may be obtained, the general notation BS(m,n)36 has been adopted, where m (n) denotes the number of spin-up (spin-down) electrons at the two interacting fragments. Canonical and corresponding orbitals, as well as spin density plots, were generated with the program Molekel.37 Preparation of (PhenPNNDip)2FeBr. A 20 mL scintillation vial was charged with a stirbar, 0.400 g (0.521 mmol) of (PhenPNNDip)FeBr2, and 10 mL of tetrahydrofuran. The mixture was chilled in the freezer to −35 °C. In a separate vial, 0.066 mg of naphthalene (0.521 mmol) and an excess of sodium metal were stirred in THF for 3 h. The resulting green sodium naphthalenide solution was then added dropwise to the chilled solution of (PhenPNNDip)FeBr2. The resulting green solution was stirred for 15 min at room temperature before the volatiles were removed in vacuo. The product was extracted from the solid residue with diethyl ether and filtered through a plug of Celite. Removal of the solvent in vacuo yielded 0.170 g (52%) of the title product. Recrystallization from a diethyl ether solution at −35 °C furnished crystals suitable for X-ray diffraction. Anal. Calcd for C76H74BrFeN4P2: C, 73.55; H, 6.01; N, 4.51 Found: C, 73.06; H, 6.25; N, 4.74. Magnetic susceptibility (benzene-d6, 293 K): μeff = 3.62 μB. 1H NMR (300 MHz, C6D6, 23 °C): δ −10.18 (Δν1/2 = 30 Hz), −6.73 (Δν1/2 = 44 Hz), −3.75 (Δν1/2 = 63 Hz), 1.75 (Δν1/2 = 20 Hz), 4.02 (Δν1/2 = 42 Hz), 5.05 (Δν1/2 = 31 Hz), 5.95 (Δν1/2 = 23 Hz), 8.68 (Δν1/2 = 23 Hz), 56.97 (Δν1/2 = 83 Hz), 61.94 (Δν1/2 = 22 Hz), 64.85 (Δν1/2 = 73 Hz), 73.00 (Δν1/2 = 27 Hz) ppm. Preparation of (PhenPNNDip)2Fe. A 20 mL scintillation vial was charged with a stirbar, 0.090 g (0.073 mmol) of (PhenPNNDip)2FeBr, and 10 mL of diethyl ether. The mixture was chilled in the freezer to −35 °C. A separate vial was charged with 0.007 g (0.073 mmol) of ((trimethylsilyl)methyl)lithium and 5 mL of diethyl ether. The lithium reagent was then added dropwise to the chilled solution of (PhenPNNDip)2FeBr. The resulting violet solution was stirred for 15 G


Article

Organometallics The boron reagent was then added dropwise to the chilled solution of (PhenPNNDip)Fe(η4-C4H6). The resulting red-purple solution was stirred for 15 min at room temperature before the volatiles were removed in vacuo. The product was extracted from the solid residue with diethyl ether and filtered through a plug of Celite. Removal of the solvent in vacuo yielded 0.233 g (85%) of the title product. Anal. Calcd for C60H43BF15FeN2P: C, 61.34; H, 3.69; N, 2.38. Found: C, 61.40; H, 3.93; N, 2.29. Magnetic susceptibility (benzene-d6, 293 K): μeff = 3.2 μB. 1H NMR (400 MHz, benzene-d6, 23 °C): δ −7.63 (Δν1/2 = 23 Hz), −4.30 (Δν1/2 = 55 Hz), −0.55 (Δν1/2 = 46 Hz), 2.75 (Δν1/2 = 21 Hz), 3.66 (Δν1/2 = 16 Hz), 4.98 (Δν1/2 = 18 Hz), 5.89 (Δν1/2 = 19 Hz), 8.55 (Δν1/2 = 17 Hz), 9.69 (Δν1/2 = 24 Hz), 14.61 (Δν1/2 = 24 Hz), 26.68 (Δν1/2 = 32 Hz) 30.59 (Δν1/2 = 23 Hz), 43.58 (Δν1/2 = 32 Hz), 46.38 (Δν1/2 = 23 Hz) ppm. 19F (300 MHz, benzene-d6, 23 °C): δ −128.6 (ortho), −159.5 (para), −165.5 (meta) ppm. 11B (300 MHz, benzene-d6, 23 °C): δ −12.1 ppm. Preparation of (iPrPDI)Fe(η3-C4H6-B(C6F5)3). A 20 mL scintillation vial was charged with a stirbar, 0.200 g (0.338 mmol) of (iPrPDI)Fe(η4-C4H6), and 10 mL of toluene. The mixture was chilled in the freezer to −35 °C. A separate vial was charged with 0.173 g (0.338 mmol) of tris(pentafluorophenyl)boron and 5 mL of toluene. The boron reagent was then added dropwise to the chilled solution of (iPrPDI)Fe(η4-C4H6). The resulting dark blue-green solution was stirred for 15 min at room temperature before the volatiles were removed in vacuo. The product was extracted from the solid residue with diethyl ether and filtered through a plug of Celite. Removal of the solvent in vacuo yielded 0.315 g (84%) of the title product. Anal. Calcd for C55H49BF15FeN3: C, 59.85; H, 4.47; N, 3.81. Found: C, 59.68; H, 4.80; N, 3.58. Magnetic susceptibility (benzene-d6, 293 K): μeff = 3.0 μB. 1H NMR (400 MHz, benzene-d6, 23 °C): δ −28.03 (Δν1/2 = 81 Hz), −24.85 (Δν1/2 = 141 Hz), −13.93 (Δν1/2 = 60 Hz), −2.89 (Δν1/2 = 141 Hz), −0.77 (Δν1/2 = 47 Hz), 2.87 (Δν1/2 = 82 Hz), 3.46 (Δν1/2 = 78 Hz), 4.25 (Δν1/2 = 54 Hz), 4.63 (Δν1/2 = 25 Hz), 4.99 (Δν1/2 = 49 Hz), 8.50 (Δν1/2 = 49 Hz), 8.87 (Δν1/2 = 54 Hz), 12.74 (Δν1/2 = 56 Hz), 15.29 (Δν1/2 = 67 Hz), 34.20 (Δν1/2 = 86 Hz), 42.98 (Δν1/2 = 90 Hz) ppm. 19F (300 MHz, benzene-d6, 23 °C): δ −148.4 (ortho), −162.8 (para), −166.3 (meta) ppm. Preparation of [(iPrPDI)Fe(THF)2]2[(F5C6)3BC4H6B(C6F5)3] (2)Fe(THF)2. A 20 mL scintillation vial was charged with a stirbar, 0.315 g (0.285 mmol) of (iPrPDI)Fe(η3-C4H6-B(C6F5)3), and 10 mL of THF. The mixture immediately turned bright green and was stirred for 15 min at room temperature before the volatiles were removed in vacuo, yielding 0.344 g (99%) of the title product. Anal. Calcd for C122H124B2F30Fe2N6O4: C, 60.01; H, 5.12; N, 3.44. Found: C, 60.27; H, 5.37; N, 3.22. Magnetic susceptibility (benzene-d6, 293 K): μeff = 3.82 μB. 1H NMR (300 MHz, benzene-d6, 23 °C): δ −243.8 (Δν1/2 = 397 Hz, 6H, C(CH3)), −11.98 (Δν1/2 = 185 Hz), −10.68 (Δν1/2 = 631 Hz), −0.49 (Δν1/2 = 113 Hz, 24H, iPr CH3), 57.17 (Δν1/2 = 194 Hz, 2H, m-pyr), 325 (Δν1/2 = 1628 Hz, 1H, p-pyr) ppm. 19F (300 MHz, benzene-d6, 23 °C): δ −131.5 (ortho), −165.9 (para), −169.6 (meta) ppm (formation of THF-BArF also observed at −133.1, −155.0, and −163.0 ppm). Representative Procedure for Single-Component Ethylene Polymerization for Table 1. A 50 mL stainless steel autoclave and glass insert were dried in an oven at 200 °C and then cooled inside the drybox. After cooling, toluene (10 mL) and catalyst (5 μmol) were added. The autoclave was then sealed and charged with ethylene (30 bar), and the mixture was stirred for 10 min. The reaction was terminated by slowly venting the autoclave over a few minutes. The mixture was filtered and washed with MeOH. The solids were dried under vacuum at 100 °C overnight. The molecular weight and polydispersity were determined by size exclusion chromatography. Representative Procedure for MAO-Activated Ethylene Polymerization for Table 1. A 50 mL stainless steel autoclave and glass insert were dried in an oven at 200 °C and then cooled inside the drybox. After cooling, the autoclave was charged with 1000 equiv of a 10 wt % solution of MAO. Extra toluene was added to bring the reaction volume up to 10 mL. The catalyst (5 μmol) was dissolved in a small amount of dichloromethane or toluene and charged to an NMR tube, which was then bound to the stirring shaft of the autoclave. The

autoclave was then sealed and charged with ethylene (30 bar), and the reactor stirrer was started, resulting in breakage of the NMR tube and catalyst activation. The reaction mixture was stirred for 10 min. The reaction was terminated by slowly venting the autoclave over a few minutes. The mixture was filtered and washed with MeOH. The solids were dried under vacuum at 100 °C overnight. The molecular weight and polydispersity were determined by size exclusion chromatography.

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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.5b00839. Additional NMR spectroscopic, X-ray crystallographic and DFT outputs (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.J.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Chevron Phillips Chemical for financial support. We thank Dr. Carsten Milsmann for assistance with quantum chemical analysis.

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REFERENCES

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