Indenyl Functionalized N-Heterocyclic Carbene Complexes of

Feb 7, 2012 - Sasol Technology UK Ltd., Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, U.K.. Organometallics , 2012, 31 (5), pp 1643–165...
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Indenyl Functionalized N-Heterocyclic Carbene Complexes of Chromium: Syntheses, Structures, and Reactivity Studies Relevant to Ethylene Oligomerization and Polymerization Susana Conde-Guadano,‡ Andreas A. Danopoulos,*,† Roberto Pattacini,† Martin Hanton,*,‡ and Robert P. Tooze‡ †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite de Strasbourg, 4 Rue Blaise Pascal, F-67081 Strasbourg Cedex, France ‡ Sasol Technology UK Ltd., Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, U.K. S Supporting Information *

ABSTRACT: The chromium(III) complexes of the type L1CrXY (L1 = 1-(2,6-diisopropylphenyl)-3-[β-(4,7-dimethylindenyl)ethyl]-imidazol-2-ylidene, X, Y = Cl, 2a-Cl; X = Cl, Y = Me, 2a-Me; X = Cl, Y = Ph, 2a-Ph; X, Y = benzyl, 2a-Bz) and L2CrCl2 (L2 = 1-(2,6-diisopropylphenyl)-3-[γ-(4,7-dimethylindenyl)propyl]-imidazol-2-ylidene, 2b-Cl) have been prepared and characterized by analytical and diffraction methods. The salts [L1CrCl(THF)]+[BArF4]− and [L2CrCl(THF)]+[BArF4]− (ArF = 3,5-bis(trifluoromethyl)phenyl) were also characterized, having been obtained by abstraction of chloride, or indirectly a methyl, from 2a-Cl or 2a-Me, and 2b-Cl, by NaBArF4, respectively. Reaction of 2a-Bz with 1 equiv of [H(Et2O)2][BArF4] in THF gave the thermally unstable salt [L1Cr(Bz)(THF)]+[BArF4], [2a-Bz]+[BArF4], which was also characterized crystallographically. The chromium(II) analogue L1CrCl, 3a, was prepared from CrCl2 and L1K, but on prolonged standing in THF, it converted to the dimeric complex 4a. The complexes 2a-Cl, 2b-Cl, 2b-Me, and 2a-Bz in the presence of an Al activator were tested as ethylene polymerization and oligomerization catalysts, in general, showing activity toward polymer formation.

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INTRODUCTION The selective oligomerization and the polymerization of ethene are important chromium-catalyzed processes, and research by academic and industrial groups is aimed at the development of new catalytic systems and the understanding of the underlying mechanism.1 A metallacycle mechanism has been proposed to account for the selectivity of trimerization,2 and well-defined metallacycles of Cr and other metals have been isolated and characterized.3,4 Furthermore, recent deuterium labeling studies gave further support of this mechanism for the selective trimerization reaction.5−7 Similarly, in the tetramerization of ethene,8 deuterium labeling studies have suggested that a metallacycle mechanism is at work,9 although the precise mechanistic trajectory is still the subject of much speculation, with recent suggestions that binuclear chromium μ-alkylenes may be responsible for the observed selectivity.10 The development of selective homogeneous oligomerization catalysts has been dominated by the use of chromium, and the ability of this metal to facilitate unusual catalytic results has recently been further demonstrated by the observation of an “alternating Schulz−Flory” distribution of olefin products.11,12 © 2012 American Chemical Society

There are proposals that an extended metallacyclic or a Cossee−Arlman mechanism may even be operative in specific Cr-catalyzed polymerizations and the Phillips system.11,13−17 Clearly, the role of chromium in selective olefin oligomerization continues to develop, and further understanding of the underlying reasons for these interesting behaviors is required to aid future advances. Important mechanistic questions that have to be addressed in both the oligomerization and the polymerization catalytic systems include: (i) the role of the ligand environment,12 (ii) the Cr oxidation and spin states during the catalytic cycles,15,16,18 and (iii) the role of the nature of the activator on catalytic activity and selectivity.12a,19−21 Obviously these three questions are interlinked and the answers may vary in specific systems. For example, the alternating distribution of olefins that results from the use of bis(imino)thiazole catalysts in conjunction with chromium was shown to be dependent upon ligand structure, while the choice of cocatalyst (MAO, Received: July 17, 2011 Published: February 7, 2012 1643

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Scheme 1. Synthesis of Cr(III) Complexes Described in the Papera

a

For reagents and conditions, see text. DiPP = 2,6-Pri2C6H3. ArF = 3,5-bis(trifluoromethyl)phenyl.

[Ph3C][B(C6F5)4], or [Ph3C][Al(OtBuF)4]) had an equally strong effect.12a The synthesis of well-defined chromium organometallics that show catalytic activity will give better insight into mechanistic questions and the characteristics of the catalytic species. Toward this end, the Cr phosphine stabilized organometallics isolated by Bercaw show oligomerization activity.6,7,22 A series of well-defined pentamethylcyclopentadienyl chromium species prepared by Theopold are ethene polymerization catalysts in the absence of an aluminum activator. Using the same ligand system, further studies have shown that the structurally characterized cation [Cp*Cr(CH2SiMe3)(THF)2]+ is a catalyst precursor, the catalytic species being formed from it, by dissociation of THF. The analogue [Cp*Cr(CH2SiMe3)(Et2O)2]+ is a catalyst precursor in a system that can oligomerize α-alkenes.23,24 The amine donor functionalized cyclopentadienyl complexes of chromium introduced by Jolly showed high activity for ethene polymerization.27a In a computational study of the donor functionalized systems by Jensen, it was concluded that higher activities would have been expected if the amine donor was replaced by an N-heterocyclic carbene, presumably due to a more robust catalyst structure based on a stronger Cr−NHC bond that is maintained even in the presence of high cocatalyst concentrations.27b However, Cr−NHC complexes are relatively rare. Cyclopentadienyl

monocarbene complexes were prepared by Jolly and Tilset and studied as ethene polymerization catalysts.27a,28 There are three additional examples of chromium complexes bearing indenyl ligands functionalized with N-donor pyridyl, imidazolimine, and quinolinyl groups that were studied as alkene polymerization catalysts.29 Furthermore, chromium alkyls stabilized by chelating methylene dicarbene have been recently reported.30,31 “Pincer” pyridine dicarbene complexes of chromium when activated with MAO exhibit interesting oligomerization activity, possibly via an extended metallacyclic mechanism.15 We have recently reported the synthesis of potassium βethyl- and γ-propyl-dimethylindenyl-functionalized N-heterocyclic carbenes and the synthesis of new complexes with these ligands and early transition metals, such as Zr, Ti, and V.32−34 The well-defined geometry of the metals in various oxidation states supported by the tethered ligands, the known acceleration of catalytic reactions by indenyl complexes, and the indirect tuning of metal−NHC bonding by the adjustment of the length of the tether seem to be attractive features for catalyst design. In this paper, we describe the synthesis and characterization of a new class of Cr(III) and Cr(II) complexes with dimethylindenyl-functionalized N-heterocyclic carbenes and the isolation of Cr organometallic derivatives relevant to 1644

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catalysis. We also present studies of selected new complexes as precatalysts in polymerization and oligomerization reactions.

The Cr−C(NHC) bond lengths are 2.154(2) and 2.089(4) Å for 2a-Cl and 2b-Cl, respectively.29−31,33 The difference in Cr− C(NHC) bond lengths may be ascribed to (i) increased strain and inferior directionality in the Cr−NHC σ-bonding in the two carbon atom tethered NHC of 2a-Cl, (ii) variation in the π-contribution of the NHC−Cr bonding, and (iii) different interelectronic repulsions involving nonbonding metal and ligand orbitals. Structural features associated with these rationalizations include the small difference between the exocyclic and endocyclic angles (Scheme 4) [Δ(exoendo)] formed by the coordination of the NHC to the metal in 2a-Cl and 2b-Cl (6.15° and 5.99°, respectively), and the dramatic difference between the interplanar angles of the NHC and indenyl planes (42.5° and 25.5°, in 2a-Cl and 2b-Cl, respectively). Substitution of the chlorides in 2a-Cl and 2b-Cl with aryls or alkyls leading to new organochromium species was undertaken in order to more closely approximate the species that are involved in chromium-catalyzed oligomerization and polymerization reactions. Chromium NHC complexes with σ-aryls or σalkyls are very rare and their stability and reactivity virtually unknown.30,31a The occurrence of the alkylimidazolium elimination reaction, which has been established in certain late metal N-heterocyclic carbene alkyls and aryls, has not been systematically studied for other metals.31b,c On the other hand, pentamethylcyclopentadienyl alkyls of chromium have been studied as models for polymerization catalysts by Theopold.23 Two synthetic strategies based on the reaction of the potassium salts 1a or 1b with Cr(CH3)Cl2(THF)3 or the displacement of one or two chlorides in 2a-Cl or 2b-Cl by nucleophilic organomagnesium organometallics in salt metathetical reactions were explored. The reaction of 2a-Cl with diphenylmagnesium in cold tetrahydrofuran gave good yields of the dark red-purple, paramagnetic, air-sensitive mixed chloro phenyl complex 2a-Ph. Prolonged exposure (>1 day) of the complex to chlorinated solvents led to decomposition. Characterization of 2a-Ph was carried out analytically and crystallographically (see Figure 3).

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RESULTS AND DISCUSSION The new complexes and the chemical transformations leading to them are given in Scheme 1 (using Cr(III) as starting materials) and in Scheme 3 (using Cr(II) as starting materials). Synthesis of Cr(III) Complexes. The reaction of the potassium salts 1a and 1b, available in multigram quantities as previously described,33 with CrCl3(THF)3 in THF gave good yields of the green, paramagnetic Cr(III) species 2a-Cl and 2bCl. Because their low solubility in ethereal and hydrocarbon solvents, the products were obtained as analytically pure powders after extraction in dichloromethane and filtration to remove inorganic salts. They were characterized by analytical and X-ray diffraction methods. The structures of 2a-Cl and 2b-Cl are shown in Figures 1 and 2, respectively.

Figure 1. Molecular structure of 2a-Cl; thermal ellipsoids are at the 40% level. Selected bond lengths (Å) and angles (deg): C(1)−Cr(1) = 2.154(2), Cl(1)−Cr(1) = 2.2436(6), Cl(2)−Cr(1) = 2.3074(6); N(2)−C(1)−N(1) = 99.77(17), N(2)−C(1)−Cr(1) = 133.17(16), N(1)−C(1)−Cr(1) = 127.00(14), Cl(2)−Cr(1)−Cl(1) = 94.10(2).

Figure 3. Molecular structure of 2a-Ph; thermal ellipsoids are at the 30% level. Only the ipso-carbon atom of the 2,6-Pri2C6H3 is shown for clarity. Selected bond lengths (Å) and angles (deg): C(29)−Cr(1) = 2.049(6), Cr(1)−C(1) = 2.099(5), Cr(1)−Cl(2) = 2.2840(19); N(2)−C(1)−N(1) = 104.6(4), C(29)−Cr(1)−Cl(2) = 95.62(18), C(1)−Cr(1)−Cl(2) = 103.48(16), C(29)−Cr(1)−C(1) = 94.9(2).

Figure 2. Molecular structure of 2b-Cl; thermal ellipsoids are at the 40% level. Selected bond lengths (Å) and angles (deg): C(1)−Cr(1) = 2.089(4), Cl(1)−Cr(1) = 2.2765(10), Cl(2)−Cr(1) = 2.3022(11); N(1)−C(1)−Cr(1) = 124.8(3), N(2)−C(1)−Cr(1) = 130.7(3), Cl(1)−Cr(1)−Cl(2) = 98.30(4).

Complex 2a-Ph also adopts a distorted tetrahedral geometry. The bonding pattern of the Cr with the indenyl ring is analogous to 2a-Cl [Cr−C(indenyl) in the range of 2.214(6)− 2.386(6) Å]. The Cr−C(NHC) [2.100(5)Å] and the Cr− C(Ph) [2.050(6) Å] bond lengths are not unusual.29,30 The increased (Δexoendo = 9.6°) and interplanar NHC−indenyl angle (56.8°) may be ascribed to the increased steric demands of the phenyl group. Attempts to substitute the second chloride in 2a-

Both complexes are mononuclear with the metal centers adopting distorted tetrahedral geometries, assuming that the indenyl centroid is occupying one coordination site. The Cr− C(indenyl) bond lengths, (in the ranges of 2.227(2)−2.326(2) Å in 2a-Cl and 2.242(4)−2.355(4) Å in 2b-Cl) are comparable to known cyclopentadienyl chromium(III) dichloride complexes and in support of a pentahapto−indenyl coordination.36 1645

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Å] bond lengths are comparable with those of the previously discussed complexes. The Cr−C(carbene) distance at 2.119(5) Å is within the observed range in the literature.24 The Cr− benzyl groups [2.164(4) and 2.113(5) Å] are characteristic of carbon−chromium σ-bonds and within the observed range for several complexes in the literature. The angles subtended at the benzylic carbons C29 and C36 discount any interaction of the aromatic rings with the metal (ca. 114−120°). Interestingly, the close similarity of the Cr−C(carbene) and Cr−C(benzyl) distances support the involvement of similar interactions between the metal and C atoms in both cases, presumably of predominantly σ-nature with minimal π-components. The formation of alkyl chromium(III) cationic complexes is a necessary step toward the active species for oligomerization and polymerization catalysis via a Cossee−Arlman mechanism. However, few examples of alkyl chromium cations have been reported, primarily by Theopold.23 To establish whether the indenyl functionalized NHC can stabilize alkyl cations, we tried to form the cations either by chloride abstraction from 2a-Me and 2b-Me or by alkanolysis from 2a-Bz. In addition, the formation of cationic alkyls may lead to the development of activator-free organochromium polymerization catalysts. Attempts to abstract the chloride from 2a-Me and 2b-Me with B(C6F5)3 were not successful. However, the reaction of 2a-Me and 2b-Me with NaBAr F 4 (Ar F = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane/tetrahydrofuran was completed after 3 days at room temperature, when the purple solution turned dark green in color. X-ray diffraction crystallography provided evidence that, surprisingly, the methyl group was abstracted from the metal instead of the chloride (Scheme 2). The analytically pure green powders of chloride

Ph by phenyl via reaction with excess MgPh2 were unsuccessful, resulting in recovery of the staring materials. The mixed methyl chlorides 2a-Me and 2b-Me, analogues of 2a-Cl and 2b-Cl, respectively, were easily prepared by the reaction of Cr(CH3)Cl2(THF)3 with 1a and 1b. The structure of 2a-Me was determined crystallographically and is shown in Figure 4.

Figure 4. Molecular structure of 2a-Me; thermal ellipsoids are at the 40% level. Selected bond lengths (Å) and angles (deg): Cr(1)−C(29) = 2.151(6), Cr(1)−C(1) = 2.070(6), Cr(1)−Cl(2) = 2.287(2); C(1)−Cr(1)−Cl(2) = 103.91(17), C(1)−Cr(1)−C(29) = 94.24(2), Cr(1)−C(1)−N(1) = 134.3(4), Cr(1)−C(1)−N(2) = 122.5(4).

The structural features of 2a-Me are analogous to the parent 2a-Cl with respect to the metal coordination geometry, Cr− indenyl and Cr−NHC bonding [Cr−C(indenyl) bond lengths in the range of 2.273(6)−2.341(7) Å, Cr−C(carbene) 2.070(6) Å]. The Cr−C(CH3) bond length [2.150(6) Å] is similar to reported values.27,35 The violet air-sensitive dibenzyl chromium complex 2a-Bz was prepared by the reaction of the dichloride 2a-Cl at low temperature in THF with freshly prepared dibenzyl magnesium solution in diethyl ether. Complex 2a-Bz is insoluble in petrol and diethyl ether and soluble in toluene and THF; under an inert atmosphere or vacuum as a solid, it is stable at room temperature for days. It was characterized by X-ray crystallography (shown in Figure 5).

Scheme 2. Proposed Steps Leading to the formation of (2aCl+)(BArF4)

cations (2a-Cl+)(BArF4) and (2b-Cl+)(BArF4) were isolated in good yields after standard workup, as described in the Experimental Section. The structure of the three carbon bridged cation (2b-Cl+)(BArF4) is shown in Figure 6. It adopts a distorted tetrahedral geometry with the indenyl five-membered ring centroid occupying one position. The bonding asymmetry of the indenyl on the Cr center is more pronounced than in the previously discussed complexes, as can be inferred from the wider range of Cr−C(indenyl) bond distances [2.199(8)−2.386(10) Å]. The Cr−O(THF) [2.060(6) Å] bond length is similar to the corresponding distances for Cr(III) cations reported.24 The Cr−C(carbene) distance [2.062(10) Å] is within the range observed previously and above. The Cr−Cl bond length [2.260(3) Å] is slightly shorter than that in the neutral dichloride complexes 2a-Cl and 2b-Cl, in agreement with the cationic nature of the metal. The tetraarylborate anion does not exhibit any close contacts to the chromium cation. The reason behind the favored formation of chloride rather than methyl cations is obscure. Methyl cations are known to be very reactive species (see also below); they may be formed in the first place and react quickly with dichloromethane to form the more stable observed cations (2a-Cl+)(BArF4) and (2b-

Figure 5. Molecular structure of 2a-Bz; one of the two similar independent molecules in the asymmetric unit is shown. Thermal ellipsoids are at the 40% level; only the ipso-C of the Pri2C6H3 group is depicted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−C(1) = 2.117(5), Cr(1)−C(36) = 2.164(4), Cr(1)−C(29) = 2.103(5), C(1)−N(1) = 1.376(6), C(1)−N(2) = 1.359(6); C(1)− Cr(1)−C(29) = 95.66(2), C(1)−Cr(1)−C(36) = 104.69(18), Cr(1)− C(1)−N(2) = 121.85(3), Cr(1)−C(1)−N(1) = 135.14(4).

Complex 2a-Bz adopts a distorted tetrahedral geometry assuming one coordination site is occupied by the indenyl centroid. The Cr−C(indenyl) [between 2.274(5) and 2.415(5) 1646

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Figure 6. Molecular structure of the cation in (2b-Cl+)(BArF4); the isopropyl groups of the 2,6-diisopropylphenyl have been omitted for clarity. Thermal ellipsoids are at the 40% level. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) = 2.266(3), Cr(1)−C(1) = 2.077(10), Cr(1)−O(1) = 2.052(6), C(1)−N(1) = 1.348(11), C(1)− N(2) = 1.371(11); C(1)−Cr(1)−O(1) = 101.39(3), C(1)−Cr(1)− Cl(1) = 92.44(3), Cr(1)−C(1)−N(1) = 132.67(6), Cr(1)−C(1)− N(2) = 123.54(6).

Figure 7. Molecular structure of the cation in (2a-Bz+)(BArF4); thermal ellipsoids are at the 30% level. Only the ipso-C of the Pri2C6H3 group is shown for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−C(1) = 2.134, Cr(1)−O1 = 2.077, Cr(1)−C(29) = 2.104; O(1)−Cr(1)−C(1) = 103.76, C29−Cr(1)−C(1) = 95.54, N(1)−C(1)−N(2) = 106.36, N(1)−C(1)−Cr(1) = 135.23, N(2)− C(1)−Cr(1) =118.24, Cr(1)−C(29)−C(30) = 119.83.

information neither on the selectivity of the protonation nor the site of initial attack. (i.e., C(benzyl), C(NHC), or metal). The accuracy of the metrical data determined by X-ray diffraction is low, but it appears that the Cr−C bond lengths in (2a-Bz+)(BArF4) are similar to the analogous bonds n the 2aBz. For example, the Cr−C(NHC) distance [2.130(5) Å in (2a-Bz+)(BArF4) and 2.119(5) Å in 2a-Bz], the Cr− C(indenyl) [between 2.214(3) and 2.3755(18) Å in (2aBz+)(BArF4) and 2.274(5) and 2.415(5) Å in 2a-Bz] and the Cr−C(benzyl) [2.111(4) Å in (2a-Bz+)(BArF4) and 2.164(4) and 2.113(5) Å in 2a-Bz] do not provide monotonic metrical trends that could be ascribed to the cationic nature of the metal; the Cr−O(THF) [2.070(2) Å] bond length is similar to the corresponding distances for the Cr(III) cations reported24 and the THF coordinated in the cation (2a-Cl+)(BArF4) discussed above. Therefore, the coordination of this solvent might play an important role, stabilizing chromium(III) cation complexes with this ligand system. It would have been very interesting to test the activity of cation (2a-Bz+)(BArF4) in the oligo/polymerization of ethylene, especially under activator-free conditions. However, this proved impossible due to its high thermal instability, which makes it impossible to handle without substantial decomposition. Syntheses Starting with Cr(II) Complexes. The oxidation state of Cr in active polymerization and, especially, oligomerization catalysts is ambiguous and the subject of ongoing debate and experimentation; indeed, it has been shown that the oxidation state is directly involved in the type of catalysis that results.25 To gain a better insight into the coordination of indenyl functionalized NHC on chromium, we also studied its coordination chemistry with Cr(II) metal centers. CrCl2 was reacted with the potassium salt 1a in cold tetrahydrofuran. The lime-colored suspension turned into an olive green solution, which was concentrated and cooled at −30 °C for 1 week to afford a good crop of light brown crystals of complex 3a. This compound was extremely air- and moisturesensitive; insoluble in petrol, diethyl ether, and toluene; slightly soluble in tetrahydrofuran; and unstable in dichloromethane. However, 3a reacted at room temperature in tetrahydrofuran over long periods (>2 days) to give the dinuclear complex 4a in which an oxygen atom acts as a bridge (Scheme 3). This

Cl+)(BArF4) (Scheme 2). Isolation of intermediates to corroborate this hypothesis was attempted. The reaction was stopped before the solution turned into green, filtered through Celite concentrated, and layered with petrol, but no crystalline species were obtained. Another interesting aspect in the synthesis of (2a-Cl+)(BArF4) and (2b-Cl+)(BArF4) is that a small amount of the hard THF is necessary for the cation formation, demonstrating the enthalpic contribution of the dative bond in the overall formation of the cationic species. The reaction of 2a-Me and NaBArF4 under the same conditions, but in the absence of THF, gave the dichloride complex 2a-Cl cocrystallized with NaBArF4, as was shown by crystallographic and elemental analysis of the isolated product. The dibenzyl chromium complex 2a-Bz was reacted with [H(Et2O)2][BArF4] at −78 °C in tetrahydrofuran. The reaction mixture turned immediately from violet to orange. After workup, the benzyl complex (2a-Bz+)(BArF4) was isolated as fine orange needles by crystallization of a concentrated diethyl ether solution at −30 °C. The complex is thermally unstable, and its characterization was only possible by X-ray crystallography. All the steps, including crystal selection and data collection, were carried out at low temperature. Even after taking these handling precautions, decomposition was not completely eliminated, having an adverse effect on the quality of the X-ray diffraction data set. However, the technique unequivocally established the identity of the complex and gave a reasonable set of bond lengths and angles for the cation (2aBz+)(BArF4). The diffraction pattern showed a lot of background diffuse scattering, probably from disordered solvent, and the Bragg reflections were broad. As a result, the structure, especially the thermal parameters, is of low quality. A diagram of the molecule is shown in Figure 7. The metal in complex (2a-Bz+)(BArF4) adopts a distorted tetrahedral geometry analogous to all the chromium complexes discussed previously. The coordination sphere comprises one cyclopentadienyl ring of the indenyl system, one NHC ligand, one anionic benzyl group, and one coordinated THF. Therefore, the formation of the cation resulted by the abstraction of one toluene molecule after the reaction with the electrophilic acid. We could not obtain additional 1647

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Scheme 3. Synthesis of Cr Complexes Starting from CrCl2a

a

For reagents and conditions, see text. DiPP = 2,6-Pri2C6H3.

reactivity was reproducible under the experimental conditions employed. The mechanism of this oxygenation is unknown but thought to involve either oxygenation from adventitous oxygen or oxygen atom transfer from the THF molecule. Complex 4a was crystallized by slow vapor diffusion of petrol into a concentrated solution of 3a in THF at room temperature. Both complexes 3a and 4a were characterized by X-ray diffraction techniques, and diagrams of the molecules are shown in Figures 8 and 9, respectively.

Complex 3a adopts a distorted three coordinate geometry assuming that the indenyl five-membered ring centroid occupies one coordination site. The Cr−C(indenyl) bond lengths [between 2.259(4) and 2.395(4) Å] are similar to the corresponding distances for the Cr(III) complexes discussed above and in the range observed in other Cr(II) complexes.27 The Cr−C(NHC) and Cr−Cl distances [2.085(4) Å] and [2.2879(11) Å], respectively, are also within the observed ranges in the literature and in complexes discussed above.29 Complex 4a is a dinuclear centrosymmetric dimer in which each Cr center adopts a distorted tetrahedral geometry. The Cr−C(indenyl) [2.239(7), 2.278(7), and 2.274(7) Å] and Cr− C(NHC) [2.108(8) Å] bond lengths are within the expected and previously reported values;26,29 the distances between the Cr and the nonbonding C atoms of the five-membered indenyl ring are 2.458 and 2.444 Å. Therefore, the hapticity of the fivemembered ring is better described as 3 for 4a. The formal oxidation state of the Cr centers in 4a is 3. The observed structural motive of [(Cr-Cl)2(μ-O)] in 4a is very rare. To our knowledge, there is only one example in this category.31 Binuclear species with bridging alkoxides are more common. An alternative formulation of 4a as bearing a (μ-OH) group (rather than μ-O) appears less likely in view of the significantly shorter Cr−O bond lengths observed in 4a compared to the Cr(μ-OH)Cr entries found in the Cambridge Crystallographic Database. The collection of structural data for the new complexes with varied features (length of tether, chloride vs. alkyde substitution, cationic charge and metal oxidation state) justifies an attempt toward rationalizing the observed trends. Meaningful comparisons can be made between the complexes (i) with the same type of substitution and different lengths of the tether (2a-Cl vs 2b-Cl), (ii) the dichloride (2b-Cl) and the analogous cation in (2b-Cl+)(BArF4) and the dibenzyl 2a-Bz and the cation in (2a-Bz+)(BArF4), (iii) the Cr(III) complexes with a variable degree of chloride and alkyde substitution (2aCl, 2a-Me, 2a-Ph, 2a-Bz), and (iv) the analogous Cr(III) and Cr(II) complexes (2b-Cl and 3a). The presence of the longer tether, [2a-Cl vs 2b-Cl], gives rise to a smaller interplanar angle ϕ1 (Scheme 4) between the

Figure 8. Molecular structure of 3a; thermal ellipsoids are at the 40% level (one of the two molecules in the asymmetric unit are shown). Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) = 2.2896(11), Cr(1)−C(1) = 2.075(4), C(1)−N(1) = 1.380(6), C(1)−N(2) = 1.365(5); Cl(1)−Cr(1)−C(1) = 104.00(11), Cr(1)− C(1)−N(1) = 133.95(3), Cr(1)−C(1)−N(2) = 123.2(3).

Scheme 4 Figure 9. Molecular structure of 4a; thermal ellipsoids are at the 40% level. Only the ipso-C of the Pri2C6H3 group is shown for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) = 2.311(2), Cr(1)−C(1) = 2.108(8), C(1)−N(1) = 1.374(9), C(1)−N(2) = 1.348(9), Cr(1)−O(1) = 1.810(2); C(1)−Cr(1)−Cl(1) = 102.19(2), Cr(1)−C(1)−N(1) = 135.87(6), Cr(1)−C(1)−N(2) = 118.81(5), O(1)−Cr(1)−Cl(1) = 98.69(7), Cr(1)−O(1)−Cr(1) = 148.19(7). 1648

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Table 1. Results of Ethene Polymerization Using Precatalysts 2a-Cl, 2b-Cl, and 2b-Mea entry

precat

P (bar)

1 2 3 4 5 6

2a-Cl 2a-Cl 2b-Cl 2b-Cl 2b-Me 2b-Me

8 40 8 40 8 40

a

T (°C)

time (min)

TON (g) (g Cr)−1

activity (g) (g Cr)−1 hr−1

activity (g) (g Cr)−1 hr−1 bar−1

TON (mol) (mol Cr)−1

activity (mol) (mol Cr)−1 hr−1

activity (g)(g Cr)−1 hr−1 bar−1

liq (%)

PE (%)

PE (g)

20 60 20 60 20 60

51 24 19 14 41 19

30 200 162 900 26 600 146 300 41 300 103 800

35 500 404 600 85 200 636 000 60 600 323 200

4438 10115 10650 15900 7575 8080

56 000 302 000 49 400 271 100 76 600 192 300

65 800 749 900 157 900 1 178 700 112 300 599 100

8225 18 748 19 738 29 468 14 038 14 978

0 0 0 0 0 0

100 100 100 100 100 100

7.850 42.361 9.925 38.028 10.738 26.978

General conditions: PhCl solvent (74 mL), 5 μmol of precatalyst, MMAO-3A activator (500 equiv).

Table 2. Results of Ethene Polymerization Catalysis Using 2a-Bza entry

solvent

equiv act salt

equiv AlEt3

T (°C)

1 2 3 4 5

toluene toluene toluene cumene toluene

1 0 1 2.5 0

0 0 100 100c 100

11/30 30/60 8 12 15

TEXOb (°C)

time (min)

TON (mol)(mol Cr)−1

activity (mol)(mol Cr)−1 hr−1

activity (mol)(mol Cr)−1 hr−1 bar−1

liq (%)

PE (%)

75 35 15

67 67 13 50 4

0 0 14 000 7800 6800

0 0 63 200 9400 30 600

0 0 7900 1175 3825

0 0 0

100 100 100

General conditions: solvent (60 mL), 20 μmol of 2a-Bz, [(Et2O)2H][BArF4] as activator salt, 8 bar ethene. bMaximum temperature reached during initial exotherm. cAdded after 20 min.

a

Given the isolation of the relatively stable chromium(III) dialkyl species 2a-Bz and the characterization of the cationic chromium(III) alkyl compound (2a-Bz)(BArF4), an opportunity to probe mechanistic aspects of catalysis was presented. By using 2a-Bz under catalytically relevant conditions, reactions were undertaken to explore if catalysis could be initiated without the use of an aluminum reagent. The reactions were performed in an autoclave maintained under 1 bar of ethene during initial addition of reagents and 8 bar during catalysis. Table 2 contains the conditions examined and the results obtained. In the first attempt, a solution of [(Et2O)2H][BArF4] was added to 2a-Bz in toluene as the solvent at 11 °C. The reaction was then warmed to 30 °C after pressurization to 8 bar of ethene, and the reaction mixture was stirred for 1 h (Table 2, entry 1). No solid products were formed, and GC analysis of the gas headspace and liquid fraction did not reveal the formation of any liquid products derived from ethene. GC-MS analysis of the liquid fraction revealed the presence of diethyl ether as expected, but also 1,3-(CF3)2C6H4, which suggests at least some decomposition of the borate anion under these conditions. Table 2, entry 2, describes an attempt to use no activator salt, but elevated temperature in a bid to achieve a reductive elimination of the two benzyl groups furnishing a chromium(I) complex, but again, no catalysis was observed. Table 2, entry 3, was a repeat of entry 1, but with the addition of 100 equiv of AlEt3. Under these conditions, catalysis was immediately apparent from the significant exotherm that occurred, and exclusive polymerization was observed, similar to that seen with 2a-Cl, albeit with lower activity and productivity. The use of the [(Et2O)2H][BArF4] activator salt in conjunction with 2a-Bz should evolve toluene as a byproduct. Thus, in order to probe if this was occurring under the conditions used, a reaction was performed in cumene as the solvent, using 2.5 equiv of the activator salt to maximize toluene evolution (Table 2, entry 4). After 20 min, the reaction was sampled, and GC-MS revealed the presence of toluene. (The cumene solvent and stock solutions of all reagents were screened by GC-MS to ensure that they were entirely free of

indenyl and NHC planes. In addition, the Cr−C(NHC) bond is slightly shorter in the longer tether ligand, which may be ascribable to a different degree of π-bonding or metal−ligand interelectronic repulsions. Surprisingly, there is no correlation of the Δ(exoendo) and ϕ1. In the dichlorides and the dibenzyl complexes, there are two unequal Cr−Cl and Cr−benzyl bonds, respectively; the shortest of the two is also found in the cations (2b-Cl+)(BArF4) and (2a-Bz+)(BArF4), respectively, implying that the weaker bond may be possibly breaking on cation formation. The stepwise replacement of Cl by alkyls (2a-Cl, 2a-Me, 2a-Bz) does not have a monotonic effect on the Cr−C(NHC) distance, but only results in the increase of the range of and the “average” Cr−indenyl bond lengths. Finally, the Cr−C(NHC) distance is shorter in the monomeric Cr(II) complex 3a than in the analogous 2a-Cl, possibly due to the softer character of the Cr(II) species. Catalytic Studies. Complexes 2a-Cl, 2b-Cl, and 2b-Me were screened for ethene oligo/polymerization under two sets of conditions (8 bar and 20 °C versus 40 bar and 60 °C), and the results are summarized in Table 1 below. The catalysis was performed in chlorobenzene as a solvent to ensure solubility of the precatalysts and utilized MMAO-3A as the aluminum activator. The data in Table 1 reveals that all three precatalysts were active for the exclusive polymerization of ethene, no liquid products being detectable in any of the reactions (despite extentive extraction of the polymer product). The higher temperature and pressure conditions gave significantly increased activity and productivity compared with the milder process parameters. All reactions were allowed to continue until cessation of ethylene uptake, which is believed to have occurred due to encapsulation of the active species in the growing polymer rather than catalyst deactivation. This is based upon the observation that continued vigorous stirring after initial cessation of ethylene uptake gave brief periods of consumption, presumably as active catalyst centers were re-exposed to ethylene. Hence, the higher productivities observed at the elevated temperatures are partly accounted for by the increased solubility of the polymer under these conditions. 1649

dx.doi.org/10.1021/om200641u | Organometallics 2012, 31, 1643−1652

Organometallics

Article

Technologies 5973N MSD Mass Spectrometric instrument with an EI source. Complex 2a-Cl. To a stirred solution of CrCl3(THF)3 (0.57 g, 1.5 mmol) in THF (30 mL) was added at −78 °C via a cannula a precooled solution of 1a (0.66 g, 1.5 mmol) in the same solvent (30 mL). On mixing, the color of the reaction mixture changed from purple to dark green. The mixture was allowed to warm to room temperature and stirred overnight. Evaporation of the volatiles under reduced pressure, extraction of the green residue in dichloromethane (3 × 30 mL), filtration of the organic extracts through Celite, and removal of the solvent under reduced pressure gave the analytically pure product as a dark green powder. The product was recrystallized by diffusion of petrol into a dichloromethane solution of 2a-Cl. Yield: 0.37 g, 47%. Anal. Calcd (%): C, 64.62; H, 6.34; N, 5.38. Found (%): C, 64.70; H, 6.25; N, 5.38. Magnetic susceptibility (Evans’ method in dichloromethane): 3.6 BM. 1H NMR, (CD2Cl2): δ 72.9 (1H), 43.2 (1H), 9.8 (1H), 8.9 (1H), 6.9 (3H), 1.4(18H, m), −4.0 (3H), −41.0 (2H). Complex 2b-Cl. To a stirred solution of CrCl3(THF)3 (0.46 g, 1.2 mmol) in THF (30 mL) was added via a cannula at −78 °C a precooled solution of 1b (0.55 g, 1.2 mmol) in the same solvent (30 mL). On mixing, the color of the reaction mixture changed from purple to dark green. It was allowed to warm to room temperature and stirred overnight. Evaporation of the volatiles under reduced pressure, extraction of the green residue in dichloromethane (3 × 30 mL), filtration of the organic extracts through Celite, and evaporation of the solvent under reduced pressure gave the analytically pure product as a dark green powder. The product was recrystallized by diffusion of petrol into a dichloromethane solution of 2b-Cl. Yield: 0.36 g, 51%. Calcd (%): C, 65.17; H, 6.55; N, 5.24. Found (%): C, 65.10; H, 6.54; N, 5.30. The 1H NMR spectrum in CD2Cl2 at room temperature is featureless. Magnetic susceptibility (Evans’ method in dichloromethane): 3.7 BM. Complex 2a-Me. To a stirred solution of CrMeCl2(THF)3 (0.31 g, 0.87 mmol) in THF (20 mL) at −78 °C was added via a cannula a precooled solution of 1a in the same solvent (0.38 g, 0.87 mmol in 20 mL). On completion of the addition, the green reaction mixture was stirred at this temperature for 1 h, was allowed to warm up to −20 °C, and stirred for 4 h. The intense red colored solution was stored overnight at −25 °C. Evaporation of the volatiles under reduced pressure, extraction of the red residue in cold toluene (3 × 20 mL, −78 °C), filtration of the organic extracts through Celite, removal of the solvent under reduced pressure, followed by washing the residue with cold petrol (2 × 10 mL, −78 °C), and drying under vacuum gave the analytically pure product as a brown-red powder. The product was recrystallized by diffusion of diethylether into a concentrated THF solution of 2a-Me at −25 °C. Yield: 0.20 g, ca. 60%. Calcd (%): C, 69.67; H, 7.21; N, 5.60. Found (%): C, 69.59; H, 7.17; N, 5.55. The 1 H NMR spectrum in C6D6 at room temperature is featureless. Magnetic susceptibility (Evans’ method in toluene): 3.4 BM. Complex 2b-Me. To a stirred solution of CrMeCl2(THF)3 (0.18 g, 0.5 mmol) in THF (10 mL) at −78 °C was added via a cannula a precooled solution of 1b (0.22 g, 0.5 mmol) in the same solvent (10 mL). After completion of the addition, the green mixture was stirred at this temperature for 1 h, then allowed to warm to −20 °C and stirred for 4 h. The intense red colored solution was stored overnight at −25 °C. Evaporation of the volatiles under reduced pressure, extraction of the red residue in toluene (40 mL), filtration of the organic extracts through Celite, removal of the solvent under reduced pressure, followed by washing the residue with petrol (2 × 10 mL), and drying under vacuum gave the analytically pure product as a brown-red powder. Yield: 0.18 g, ca. 70%. Calcd (%): C, 70.10; H, 7.40; N, 5.45. Found (%): C, 69.95; H, 7.34; N, 5.38. The 1H NMR spectrum in C6D6 at room temperature is featureless. Magnetic susceptibility (Evans’ method in toluene): 3.5 BM. Complex 2a-Ph. To a stirred solution of 2a-Cl (0.25 g, 0.5 mmol) in THF (10 mL) at −78 °C was added a solution of diphenylmagnesium in THF (0.62 mL, 0.76M). On completion of the addition, the green mixture was allowed to warm up to −20 °C and stirred at this temperature for 2 h, when the color turned to dark

trace toluene before use.) After the sample was taken, AlEt3 was added to the reaction and the autoclave repressurized to 8 bar with ethene, where upon gas uptake was observed over a further 30 min period. This reaction again produced exclusively polyethylene. To verify that the observation of toluene was reproducible, in a Schlenk tube a mixture of [(Et2O)2H][BArF4] and 2a-Bz (5:2 ratio) was stirred in cumene, the initially purple solution turning black with concomitant formation of an oil. GC-MS analysis again revealed the presence of toluene, which was quantified relative to the diethyl ether detected with a ratio of 5:1.4 (Et2O/toluene), indicating the extent to which the benzyl groups are abstracted under these conditions. Finally, Table 2, entry 5, describes catalysis using AlEt3, but no activator salt, and as can be seen, catalysis does occur, but with decreased activity and productivity compared with entry 3. Taken together, entries 1 and 4 suggest that, although the use [(Et2O)2H][BArF4] with 2a-Bz does abstract at least one benzyl group from chromium, this alone is insufficient to induce catalytic activity. In contrast, as demonstrated by entry 5, trialkylaluminium in isolation does facilitate polymerization, but less effectively than when used in conjunction with a weakly coordinating anion (entry 3), suggesting that the active species is cationic. Presumably, in the case where AlEt3 is used in isolation, an aluminate anion is formed in situ as a byproduct of the benzyl group abstraction, (i.e., [AlEt3Bz]−).

■

CONCLUSIONS This paper described the synthesis of indenyl functionalized NHC complexes of Cr and various organometallic derivatives thereof, including rare and occasionally thermally unstable dialkyls and cationic alkyl organometallics. The complexes 2aCl, 2b-Cl, 2a-Me, and 2a-Bz in the presence of aluminum activators act as moderately active37 ethylene polymerization catalysts rather than the initially targeted oligomerization catalysts, and in this respect, they resemble the pentamethylcyclopentadienyl chromium species prepared by Theopold, which serve as models for the homogeneous chromium ethylene polymerization catalysts.

■

EXPERIMENTAL SECTION

General Methods. Elemental analyses were carried out by the London Metropolitan University microanalytical laboratory. All manipulations were performed under nitrogen in a Braun glovebox or using standard Schlenk techniques, unless stated otherwise. Solvents were dried using standard methods and distilled under nitrogen prior to use. The light petroleum used throughout had a bp of 40−60 °C. The starting materials were prepared according to literature procedures: 1a and 1b,32,33 CrMeCl2(THF)3,38 dibenzylmagnesium,39 H(Et2O)2BArF4, and NaBArF4.40 An ether solution of Ph2Mg was prepared by the addition of a stoichiometric amount of dioxane into an ether solution of PhMgCl, filtration of MgCl2(dioxane)2, and retitration of the filtrate. Catalytic experiments were performed in 250 mL Buchi Miniclaves equipped with stainless steel vessels with integral thermal-fluid jackets, internal cooling coils, and mechanical mixing via gas-entraining stirrers. Ethylene (Grade 4.5) was supplied by Linde and passed through oxygen and moisture-scrubbing columns prior to use; ethylene flow was measured using a Siemens Sitrans F C Massflo system (Mass 6000-Mass 2100) and the data logged. Liquid phase sample, GC-FID analysis was performed using an Agilent Technologies 6850N GC System equipped with a PONA column (50 m × 0.20 mm × 0.50 μm) using hydrogen as a carrier gas. GC-MS analysis was performed on an Agilent Technologies 6890N GC system equipped with a PONA column (50 m × 0.20 mm × 0.50 μm) column, coupled to an Agilent 1650

dx.doi.org/10.1021/om200641u | Organometallics 2012, 31, 1643−1652

Organometallics

Article

red. After storing at −25 °C for 48 h, the volatiles were removed under reduced pressure. The red residue was extracted in dichloromethane, filtered through Celite, and dried under vacuum to afford the red analytically pure product. Yield: 0.12 g, 41%. Anal. Calcd (%): C, 72.73; H, 6.77; N, 4.99. Found (%): C, 72.69; H, 6.83; N, 4.90. X-ray quality crystals were obtained by crystallization from tolune at −30 °C. The 1H NMR spectrum in C6D6 at room temperature is featureless. Magnetic susceptibility (Evans’ method in toluene): 3.5 BM. Complex 2a-Bz. To a stirred solution of 2a-Cl (0.20 g, 0.38 mmol) in THF (10 mL) at −78 °C was added dropwise a solution of freshly made dibenzyl magnesium in ether (6.4 mL, 0.06 M). After completion of the addition, the green mixture was allowed to warm up to −30 °C and stirred at this temperature for 3 h, when the color turned into dark violet. After keeping the reaction mixture at −25 °C for 2 days, the volatiles were evaporated under reduced pressure and the violet residue was extracted in toluene (20 mL) and filtered through Celite. Toluene was evaporated in vacuo and the residue washed with petrol and dried to give an analytically pure compound. Yield: 0.09 g, 37%. Anal. Calcd (%): C, 79.87; H, 7.45; N, 4.44. Found (%): C, 79.80; H, 7.52; N, 4.36. The 1H NMR spectrum of the analytically pure sample in toluene-d8 is not assignable based on the static structural model established by the crystal structure; this may be due to fluxionality in solution and the possible occurrence of broad signals that are hidden in the baseline. Peaks were observed at δ 38.0, 7.0, 1.4, −5.0, −10.1, −23.2, and −33.5. Magnetic susceptibility (Evans’ method in toluene): 3.1 BM. X-ray quality crystals were obtained by slow (1 week) diffusion of ether into a solution of the 2aBz in THF at −25 °C. Complex (2b-Bz+)(BArF4). A solution of [H(Et2O)2][BArF4] (0.33 g, 0.3 mmol) in cold THF (20 mL, −78 °C) was added via a cannula to the purple solution of 2a-Bz (0.20 g, 0.3 mmol) in the same solvent. The brown reaction mixture was stirred at −78 °C for 2 h and kept at −30 °C overnight. After removal of the volatiles under reduced pressure, the brown residue was washed with cold petrol (2 × 15 mL, −78 °C) and dried under vacuum. Crystallization was accomplished by maintaining a concentrated diethylether solution of the solid at −30 °C for 24 h. The above solutions and the crystals are thermally unstable and were always handled at low temperatures (