Chromium(II) Pincer Complexes with Dearomatized PNP and PNC

Article pubs.acs.org/Organometallics

Chromium(II) Pincer Complexes with Dearomatized PNP and PNC Ligands: A Comparative Study of Their Catalytic Ethylene Oligomerization Activity Thomas Simler,† Pierre Braunstein,*,† and Andreas A. Danopoulos*,†,‡ †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS) and ‡Institute for Advanced Study (USIAS), Université de Strasbourg, Strasbourg CEDEX 67081, France S Supporting Information *

ABSTRACT: Monodeprotonation of the 2,6-bis(di-tert-butylphosphinomethyl) pyridine (tBuPNtBuP) at the α-lutidinyl-CH2 position with 1 equiv of KCH2C6H5 and concomitant dearomatization of the heterocycle afforded K(tBuP*NatBuP) (tBuP*= di-tert-butyl vinylic P donor, tBuP = PtBu2, Na = anionic amido N donor); its transmetalation with [CrCl2(THF)2] afforded the CrII complex [Cr(tBuP*NatBuP)Cl] (A). The X-ray diffraction analysis of A established a slightly distorted squareplanar coordination geometry at the metal center and confirmed retention of the dearomatized coordinated ligand. The catalytic activity of A in ethylene oligomerization was studied and compared with that of the related CrII complexes [Cr(tBuP*NaCNHC)Cl] (B) and [Cr{Cr(tBuP*NatBuP*)Cl}2] (C) previously reported [Chem. Commun., 2015, 51, 10699 and Dalton Trans., 2016, 45, 2800, respectively].

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Scheme 1. Reported Transmetalation of tBuP*NaCNHC and tBu P*NatBuP* Anionic Pincer Ligands to Access the Corresponding CrII Complexesa

INTRODUCTION Following the seminal work of Milstein and co-workers on pyridine-based pincer complexes1 and the demonstration that ligand dearomatization/rearomatization can occur during catalytic steps involving bases and/or X−H reagents (X = H, B, N, O, and Si) or substrates (e.g., CO2, nitriles, and O2),2 metal complexes containing dearomatized pincer ligands are attracting increasing attention, frequently leading to metal−ligand cooperation in catalysis.3 In particular, complexes of mid and late transition metals with proton-responsive multifunctional ligands4 were found to promote topical catalytic reactions,5 such as (de)hydrogenation reactions6 and CO2 reduction (using Ir,7 Re,8 or Fe9 metal centers). Relevant complexes with earthabundant metals are appealing, and in this context, the recent emphasis on Mn species is particularly noteworthy.10 It is noticeable that very few examples of structurally characterized11 proton-responsive dearomatized pyridine-based pincer complexes with early/mid transition metals have been described, mainly with Cr,12 Zr,13 Mn,10b and Re;14 examples of dearomatized La complexes have also been reported.15 This scarcity is possibly due to synthetic difficulties; the typical route employed to access dearomatized pincer complexes involves treatment of the precoordinated ligand with an external strong base (e.g., KOtBu, KN(SiMe3)2, or Li-alkyl), a methodology that may suffer from reduced regioselectivity, especially with electrophilic metal centers. We recently described an alternative strategy to access dearomatized pincer complexes based on the transmetalation of dearomatized, anionic ligand entities from well-defined, isolated Li or K salts (Scheme 1).12,13,16 Using this methodology, CrII complexes [Cr(tBuP*NaCNHC)Cl] (B) and [Cr{Cr(tBuP*NatBuP*)Cl}2] © XXXX American Chemical Society

a

DiPP = 2,6-diisopropylphenyl.12,13

(C) (P* = vinylic P donor, tBuP = PtBu2, and Na = anionic amido donor), both featuring a dearomatized ligand backbone, were successfully isolated and structurally characterized. In addition to their intrinsic importance related to occasionally unique structural and reactivity features, chromium complexes are especially interesting as precatalysts in ethylene oligomerization and may lead to the selective formation of linear α-olefins Received: August 29, 2016

A

DOI: 10.1021/acs.organomet.6b00685 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (LAOs).17 The latter are valuable building blocks with a range of industrial applications and serve as comonomers for the production of linear low-density polyethylene (C4−C8), plasticizers (C6−C10), surfactants (C12−C20), detergents, or synthetic lubricants.18 Selective and nonselective ethylene oligomerization processes are known with chromium catalysts in the presence of alkyl aluminum cocatalysts.19 The postulated reaction mechanisms in these cases depend on several ligand-modulated factors, including the stabilization of the oxidation states accessible during the catalytic cycle.20 Furthermore, the role of the diverse species generated by different cocatalysts is not yet fully understood.21 The electronic analogy between N-heterocyclic carbene (NHC) and phosphine ligands22 and the specific thermodynamic and kinetic properties of CNHC−M versus P−M bonds have triggered considerable interest in catalytic studies involving NHC complexes and recently in complexes bearing both NHC- and P-donors. In the latter case, synergism of the two functionalities has been occasionally evidenced.23 Attempts to suppress catalyst deactivation of Cr−NHC oligomerization catalysts led also to the introduction of chelate ligand structures involving at least one NHC functionality,24 i.e., bidentate (CNHCCNHC,25 CNHCC,26 CNHCN,20d,27 CNHCP,28 and CNHCS)20d and tridentate (C NHC NC NHC , 20a,d,29 PNC NHC , 12 C NHC SC NHC , 20d and NCNHCN)30 ligand scaffolds. To date, CrIII pincer complex [Cr(CNHCNCNHC)Cl3] (Figure 1) is one of the most active

distributions of oligomers without the formation of any insoluble polymer. Herein, we describe the synthesis of new complex [Cr(tBuP*NatBuP)Cl] (A) with a dearomatized pincer ligand and compare its activity in ethylene oligomerization with that of previously reported complexes B12 and C13 which also feature dearomatized ligands.

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RESULTS AND DISCUSSION Synthesis of A. The anionic and dearomatized tBuP*NatBuP ligand was obtained as a potassium salt ({1-E/1-Z} and 2, Scheme 2) by treatment of neutral ligand 2,6-bis(di-tertbutylphosphinomethyl)pyridine tBuPNtBuP with benzylpotassium, using the methodology recently developed.16 The potential of the two potassium complexes as transmetalation reagents was demonstrated by the synthesis of CrII complex A. The reaction of either {1-Z/1-E} or 2 with [CrCl2(THF)2] (1:1 reactant ratio) in THF at low temperatures afforded [Cr(tBuP*NatBuP)Cl] (A) in very good yield. This complex is paramagnetic (μeff = 4.8 ± 0.2 μB in C6D6 solution, Evans’ method32) and dark purple X-ray quality single crystals were grown from pentane (Scheme 2). The structure of A (Figure 2) features a CrII center in a distorted square planar coordination geometry (P−Cr−P* angle of 165.16(7)°; sum of the coordination angles at Cr of 360.0°) and the anionic tridentate (tBuP*NatBuP) ligand in a pincer-type (κP*,κNa,κP) coordination mode. Only the Z configuration of the exocyclic double bond in A is observed, which shows that facile E → Z isomerization occurs during the transmetalation process. The Cr−Na bond distance in A (2.082(5) Å) is intermediate between the Cr−Npyridine distance in [Cr(PhPN PhP)Cl2 ] (2.164(2) Å)31 and the Cr−Na distances in C (2.052(6) and 2.058(6) Å).13 Such differences may be due to the overall negative charge of the ligand, [zero in [Cr(PhPNPhP)Cl2], one in A and two in C]. Although the coordinated tBuP*NatBuP ligand is nonsymmetric with respect to the plane perpendicular to the metal coordination plane and passing through the Cl−Cr−N vector, the Cr−P1 and Cr−P2 distances at the pincer arms are very similar. There is no interaction between the Cr and the α-CH2P or α′-CHP carbon atom, in contrast to the situation observed in the closely related ZrIV complex bearing the doubly deprotonated tBuP*NatBuP* ligand.13 The C−C bond distances within the heterocycle range from 1.361(9) to 1.433(9) Å and are alternating around values associated with single and double bond, pointing to dearomatization of the ring in A. Consistently, the C5−C15 bond distance of 1.398(8) Å is significantly shorter than the C1−C6 single bond of 1.506(9) Å. The P2−C15 distance (1.755(7) Å) is shorter than P1−C6 (1.821(8) Å) or P−CH2 distances in [Cr(PhPNPhP)Cl2] (1.829(2) and 1.834(2) Å).31 It is comparable to P−Csp2 bond distances in related pincer-type complexes12,13,16 and in coordinated α-phosphino carbanions33 and is consistent with charge stabilization due to negative hyperconjugation onto the phosphine moiety and

Figure 1. Structures of the symmetrical [Cr(CNHCNCNHC)Cl3] and [Cr(PhPNPhP)Cln] (n = 2 and 3) chromium pincer complexes (top)20a,31 and the dearomatized CrII pincer complexes studied in this work (bottom).12,13

precatalysts for the oligomerization of ethylene, with activities reaching 40 000 g mmol−1 bar−1 h−1.20a Using P-donor ligands, Gambarotta and co-workers examined the activity of symmetrical [Cr(PhPNPhP)Cln] (n = 2 and 3) pincer complexes in catalytic ethylene oligomerization (Figure 1);31 it was found that both CrII and CrIII complexes were moderately active, leading to

Scheme 2. Synthesis of the Potassium Salts {1-E/1-Z} or 2 and Transmetalation to the CrII Pincer Complex [Cr(tBuP*NatBuP)Cl] (A)

B


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Organometallics

typical conditions (starting temperature: 30 °C, reaction time 35 min, 10 bar of ethylene, 400 equiv of methylaluminoxane (MAO) as cocatalyst), A was poorly active (less than 500 g(C2H4) g(Cr)−1 h−1) and produced mostly polymers (entry 1). In contrast, under similar conditions B was ca. 6 times more active (entry 2) and produced smaller amounts of polyethylenes (44.5 wt %). Interestingly, precatalyst B gave rise to a more selective system, affording mainly hexenes with a high selectivity for α-olefins (>92%). When ethyl aluminum dichloride (EADC) was used as cocatalyst instead of MAO, almost only polyethylenes were produced (entry 3). Similar observations have already been reported for other chromium-based systems.21,28 Compared to the system B/MAO, the activity of the trinuclear complex C/MAO was doubled, and almost no polyethylene was formed (entry 4), using the same total amount of Cr for comparison (4.0 × 10−5 mol of Cr, i.e., 1.3 × 10−5 mol of C). In this case, a high exotherm was evident after a long induction period (typically 20−25 min), which led to a maximum temperature of 65−70 °C. A longer reaction time (60 min) resulted in a slightly lower activity (entry 5). Interestingly, when the reaction was started at 50 °C instead of 30 °C, a much shorter induction period was observed (ca. 5 min) with an activity of up to 6700 g(C2H4) g(Cr)−1 h−1 (entry 6) and overall high selectivity in α-olefins (86−100%) (Figure 3). The molecular mass distribution of the oligomers produced by the system C/MAO, 50 °C is of the Schultz−Flory type with a K value of 0.9, where K = kprop/(kprop + kch transfer) = moles of Cn+2/moles of Cn. In a reference experiment, the activity of CrCl2 was also examined under the same conditions (Table 1, entry 7). Interestingly, it was moderately active and provided good selectivity in α-olefins but with a different distribution (K = 0.6) (Figure 3) and higher amounts of polyethylenes than those of the system C/MAO, 50 °C. The lower K value for the system

Figure 2. Structure of [Cr(tBuP*NatBuP)Cl] (A) with thermal ellipsoids at 40% probability. H atoms have been omitted for clarity, except for α−CH (C15) and α−CH2 (C6). Selected bond distances (Å) and angles (deg): Cr1−Cl1 2.302(2), Cr1−N1 2.082(5), Cr1−P1 2.450(2), Cr1−P2 2.440(2), P1−C6 1.821(8), P2−C15 1.755(7), C1−C6 1.506(9), C5−C15 1.398(8), C1−C2 1.371(9), C2−C3 1.383(9), C3−C4 1.361(9), C4−C5 1.433(9), C1−N1 1.368(8), N1−C5 1.392(7); P1−Cr1−P2 165.16(7), N1−Cr1−P1 82.6(1), N1−Cr1− P2 82.6(1), P1−Cr1−Cl1 96.48(7), P2−Cr1−Cl1 98.32(7), Cr1−N1− C5 118.8(4), Cr1−N1−C1 122.6(4), C1−C6−P1 115.1(5), C5−C15− P2 119.5(5). Sum of the angles around Cr1: 360.0°.

polarization effects.34 A dominant single-bond character of the P−Csp2 bond in A is also fully consistent with the corresponding Wiberg bond indexes.12,13,16,35 Catalytic Ethylene Oligomerization Using A−C. The catalytic activities of A−C (Figure 1) were evaluated in ethylene oligomerization, and the results are compiled in Table 1. Under

Table 1. Oligomerization of Ethylene Using the Cr(II) Complexes A−C as Precatalyst and Comparison with CrCl2a overall selectivity (wt %) entry

precat.

activityb

TOFc

C4 (α-C4)

C6 (α-C6)

C8 (α-C8)

C10 (α-C10)

C10+

PE

1 2 3 4 5 6 7

A B Bd C Ce Cf CrCl2

480 3050 950 6050 4650 6700 1600

900 5650 1750 11250 8650 12450 2950

4.3 (99) 14.4 (100) 1.2 (99) 19.5 (99) 30.2 (100) 25.8 (100) 32.8 (100)

3.9 (87) 19.4 (96) 1.0 (60) 26.2 (93) 36.4 (94) 34.2 (95) 29.7 (96)

5.6 (31) 13.1 (94) 1.2 (13) 23.4 (93) 20.3 (89) 21.7 (89) 20.3 (92)

1.2 (43) 7.2 (92) 1.7 (1) 21.5 (80) 10.7 (83) 11.8 (86) 9.3 (94)

0.4 1.4 0.1 8.0 0.7 4.9 2.4

84.6 44.5 94.8 1.4 1.7 1.6 5.4

Conditions unless otherwise stated: starting temperature 30 °C, 35 min run, 10 bar of C2H4, 4 × 10−5 mol of Cr, toluene solvent (9.4 mL), and MAO toluene solution (total volume 20.0 mL). bIn g(C2H4) g(Cr)−1 h−1. cIn mol(C2H4) mol(Cr)−1 h−1. dUsing EADC as cocatalyst. eA 60 min run. fStarting temperature: 50 °C a

Figure 3. Comparison of the selectivity in α-olefins for the different systems (left) and oligomer distribution for the system C/MAO, 50 °C (right). C


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Organometallics

AVANCE III − 400 MHz or AVANCE I − 500 MHz equipped with a cryogenic probe). Downfield shifts are reported as positive and are referenced using signals of the residual protio solvent (1H). Effective magnetic moments were measured using the Evans’ method corrected for the diamagnetic contributions.32 Elemental analyses were performed by the “Service de microanalyses”, Université de Strasbourg. Synthesis of [Cr(tBuP*NatBuP)Cl] (A). To a suspension of [CrCl2(THF)2] (0.053 g, 0.20 mmol) in THF (5 mL) precooled at −78 °C was added a solution of 2 (0.123 g, 0.20 mmol) in THF (10 mL). The resulting dark red-purple solution was allowed to reach r.t. and was stirred for 1 h. The volatiles were evaporated in vacuo and the solid residue was extracted with a mixture of pentane and Et2O and filtered. After evaporation of the volatiles the solid residue was collected. Yield: 0.083 g (0.17 mmol), 86%. Slow evaporation of a concentrated pentane solution gave dark red-purple crystals of A. 1H NMR (400.13 MHz, C6D6, δ) 26.2 (br s, 2H), 22.5−3.5 (br m, 38H), −4.7 (s, 1H), −18.1 (s, 1H). Magnetic susceptibility (Evans’ method, 400.13 MHz, C6D6, 298 K) μeff = 4.8 ± 0.2 μB (consistent with 4 unpaired electrons). Anal. Calcd for C23H42ClCrNP2 (481.99): C, 57.32; H, 8.78; N, 2.91. Found: C, 57.09; H, 8.74; N, 3.03. General Procedure for the Catalytic Ethylene Oligomerization. All catalytic reactions were performed in a magnetically stirred (1200 rpm) 145 mL stainless steel autoclave. A 125 mL glass container was used to avoid corrosion of the autoclave walls. The preparation of the precatalyst solution was adapted to the nature and the amount of the cocatalyst. With MAO, a total amount of 4 × 10−5 mol of Cr (i.e., 4.0 × 10−5 mol of A (0.019 g), B (0.022 g), CrCl2 (0.005 g), or 1.3 × 10−5 mol of C (0.014 g)) was dissolved in 9.4 mL of toluene, and the solution was injected into the reactor under an ethylene flux. In the case of CrCl2, a suspension of anhydrous CrCl2 in toluene was injected. Then, 10.7 mL of a 1.5 × 10−3 M cocatalyst toluene solution, corresponding to 400 equiv of MAO, was added to the reactor to reach a total volume of 20.1 mL with the precatalyst solution. With AlEtCl2 (EADC), 4 × 10−5 mol of Cr complex (B) was dissolved in 10 mL of toluene and the solution was injected into the reactor under an ethylene flux, followed by 5 mL of an 8 × 10−5 M cocatalyst solution corresponding to 10 equiv of EADC. The total volume of the solution inside the reactor was 15 mL. The catalytic reactions were started at 30 °C unless otherwise stated. No cooling of the reactor was applied during the reaction. After injection of the catalyst and cocatalyst solutions under a constant low flow of ethylene, which is considered as time t0, the reactor was immediately pressurized to 10 bar of ethylene. The temperature increased, owing solely to the exothermicity of the reaction. The 10 bar working pressure was maintained through a continuous feed of ethylene from a bottle placed on a balance to allow continuous monitoring of the ethylene uptake. At the end of each test (35 or 60 min), a dry ice bath was used to rapidly cool the reactor. When the inner temperature reached 0 °C, the ice bath was removed, allowing the temperature to slowly rise to 18 °C. The gaseous phase was then transferred into a 10 L polyethylene tank filled with water. An aliquot of this gaseous phase was transferred into a Schlenk flask, previously evacuated, for GC analysis. The amount of ethylene consumed was thus determined by differential weighing of the ethylene bottle (accuracy of the scale, 0.1 g). From this amount of ethylene was subtracted the ethylene remaining in the gaseous phase (calculated using the GC analysis). Although this method is of limited accuracy, it was used throughout and gave satisfactory reproducibility. The reaction mixture in the reactor was quenched in situ by the addition of ethanol (10 mL), transferred into a Schlenk flask, and separated from the metal complexes by trap-to-trap evaporation (20 °C, 0.8 mbar) into a second Schlenk flask previously immersed in liquid nitrogen in order to avoid loss of product for GC analysis. When GC analysis of the liquid phase was carried out by collecting a small amount of the catalytic mixture and quenching it with 5% aqueous hydrochloric acid solution, slightly different activity and distribution of oligomers (C4−C20) were observed. For GC analyses, 1-heptene was used as an internal reference. X-ray Crystallography. A summary of the crystal data, data collection, and refinement for A is given in Table 2. The crystal was mounted on a glass fiber with grease from Fomblin vacuum oil. Data sets

CrCl2/MAO may be rationalized by the absence of ligand steric protection around the metal center.36 The increased activity of the B/MAO system compared to that of A/MAO may be due to the hemilabile behavior of the nonsymmetric tBuP*NaCNHC with plausible selective decoordination of the phosphine arm under catalytic conditions.37 The long induction period that was seen in the system C/MAO implies composition/structural changes of the complex in the presence of MAO and/or ethylene leading to the generation of the catalytically active species. However, no attempt was made to characterize reactive intermediates. The activities observed in C/MAO compare well to those reported for divalent [Cr(PhPNPhP)Cl2] (4125−5500 g g(Cr)−1 h−1)31 but are inferior to those of the state of the art CrIII-bis(carbene)pyridine pincer complex.20a,36 In the chromium complexes with the latter family of ligands, the activity was found to be highly dependent on the wingtip functionalization of the NHC, with different steric bulks leading to dramatic decreases in the catalyst activity. Interestingly, CrIII complexes usually display superior catalytic performances in ethylene oligomerization to their corresponding CrII analogues.28,31

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CONCLUSIONS The focus of this study was to extend the recently developed transmetalation route involving K(tBuP*NatBuP) to CrII in order to access catalytically relevant dearomatized pyridine-based pincer complexes. Interestingly, during the successful transmetalation which afforded A, an easy E → Z isomerization of the exocyclic double bond was observed, since A with the Z stereoisomeric ligand was exclusively obtained. The comparative catalytic study has shown that the nature of the side arm in the pincer complexes influences the activity and selectivity of the catalyst in ethylene oligomerization: Complex A with two P-donors was poorly active and mostly produced polymers, while the activity and selectivity for α-olefins was moderately increased with complex B, which contains wingtips with one P and one CNHC donors. Interestingly, trinuclear CrII complex C was the most active catalyst in the series and produced only a small amount of polyethylenes. The origin of this may be traced to the involvement of multinuclear complexes, a fact well-documented in other areas of catalysis,38 polymerization catalysis39 that has also been alluded to in the catalytic chromium-catalyzed selective ethylene tetramerization.17b In view of the scope and versatility of the new transmetalation route with K(tBuP*NatBuP), it can be anticipated that further catalytically relevant dearomatized pyridine-based pincer complexes will become accessible and represent potential candidates for metal−ligand cooperation.3c

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were performed under dry argon atmosphere using standard Schlenk techniques or a MBraun glovebox. THF and diethyl ether were dried by refluxing over sodium/benzophenone ketyl and distilled under an argon atmosphere prior use. Pentane was dried by passing through columns of activated alumina and subsequently purged with argon. The solvents were stored, after drying, over potassium mirror in the glovebox until use. C6D6 was distilled over KH and degassed by freeze−pump− thaw cycles. The synthesis and characterization of {1-E/1-Z}, 2,16 B,12 and C13 were performed as described previously. [CrCl2(THF)2] was prepared by continuous extraction of commercial anhydrous CrCl2 (Aldrich) into dry THF for 3 days, followed by evaporation of the volatiles from the suspension of the extracted complex and drying the light green-gray residue under vacuum for ca. 20 min. NMR spectra were recorded on Bruker spectrometers (AVANCE I − 300 MHz, D


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Privileged Pincer-Metal Platform: Coordination Chemistry & Applications; Topics in Organometallic Chemistry Series, Vol. 54; Springer: Cham, Switzerland, 2016. (2) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840−10841. (3) (a) van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8832−8846. (b) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588−602. (c) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236−12273. (4) Crabtree, R. H. New J. Chem. 2011, 35, 18−23. (5) (a) van der Vlugt, J. I. Eur. J. Inorg. Chem. 2012, 2012, 363−375. (b) Gunanathan, C.; Milstein, D. Chem. Rev. 2014, 114, 12024−12087. (c) Werkmeister, S.; Neumann, J.; Junge, K.; Beller, M. Chem. - Eur. J. 2015, 21, 12226−12250. (6) Zell, T.; Milstein, D. Acc. Chem. Res. 2015, 48, 1979−1994. (7) Tanaka, R.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14168−14169. (8) Vogt, M.; Nerush, A.; Diskin-Posner, Y.; Ben-David, Y.; Milstein, D. Chem. Sci. 2014, 5, 2043−2051. (9) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; BenDavid, Y.; Milstein, D. Angew. Chem., Int. Ed. 2011, 50, 9948−9952. (10) (a) Mukherjee, A.; Nerush, A.; Leitus, G.; Shimon, L. J. W.; Ben David, Y.; Espinosa Jalapa, N. A.; Milstein, D. J. Am. Chem. Soc. 2016, 138, 4298−4301. (b) Nerush, A.; Vogt, M.; Gellrich, U.; Leitus, G.; BenDavid, Y.; Milstein, D. J. Am. Chem. Soc. 2016, 138, 6985−6997. (c) Mastalir, M.; Glatz, M.; Gorgas, N.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Chem. - Eur. J. 2016, 22, 12316− 12320. (11) The Cambridge Structural Database, accessed Aug. 2016: Allen, F. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380−388. (12) Simler, T.; Danopoulos, A. A.; Braunstein, P. Chem. Commun. 2015, 51, 10699−10702. (13) Simler, T.; Frison, G.; Braunstein, P.; Danopoulos, A. A. Dalton Trans. 2016, 45, 2800−2804. (14) Vogt, M.; Nerush, A.; Iron, M. A.; Leitus, G.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2013, 135, 17004−17018. (15) Sugiyama, H.; Korobkov, I.; Gambarotta, S.; Mö ller, A.; Budzelaar, P. H. M. Inorg. Chem. 2004, 43, 5771−5779. (16) Simler, T.; Karmazin, L.; Bailly, C.; Braunstein, P.; Danopoulos, A. A. Organometallics 2016, 35, 903−912. (17) (a) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641−3668. (b) Peitz, S.; Aluri, B. R.; Peulecke, N.; Müller, B. H.; Wöhl, A.; Müller, W.; Al-Hazmi, M. H.; Mosa, F. M.; Rosenthal, U. Chem. - Eur. J. 2010, 16, 7670−7676. (c) McGuinness, D. S. Chem. Rev. 2011, 111, 2321−2341. (d) van Leeuwen, P. W. N. M.; Clément, N. D.; Tschan, M. J. L. Coord. Chem. Rev. 2011, 255, 1499−1517. (e) Agapie, T. Coord. Chem. Rev. 2011, 255, 861−880. (f) Breuil, P.-A. R.; Magna, L.; Olivier-Bourbigou, H. Catal. Lett. 2015, 145, 173−192. (18) Forestière, A.; Olivier-Bourbigou, H.; Saussine, L. Oil Gas Sci. Technol. 2009, 64, 649−667. (19) (a) Britovsek, G. J. P.; Malinowski, R.; McGuinness, D. S.; Nobbs, J. D.; Tomov, A. K.; Wadsley, A. W.; Young, C. T. ACS Catal. 2015, 5, 6922−6925. (b) Gong, M.; Liu, Z.; Li, Y.; Ma, Y.; Sun, Q.; Zhang, J.; Liu, B. Organometallics 2016, 35, 972−981. (c) Zheng, M.; Wu, H.; Zhang, X.; Zhang, J. J. Organomet. Chem. 2016, 817, 21−25. (d) Coxon, A. G. N.; Köhn, R. D. ACS Catal. 2016, 6, 3008−3016. (20) (a) McGuinness, D. S.; Gibson, V. C.; Wass, D. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716−12717. (b) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem. Soc. 2004, 126, 14712− 14713. (c) Wass, D. F. Dalton Trans. 2007, 816−819. (d) McGuinness, D. S.; Suttil, J. A.; Gardiner, M. G.; Davies, N. W. Organometallics 2008, 27, 4238−4247. (e) Radcliffe, J. E.; Batsanov, A. S.; Smith, D. M.; Scott, J. A.; Dyer, P. W.; Hanton, M. J. ACS Catal. 2015, 5, 7095−7098. (f) Härzschel, S.; Kühn, F. E.; Wöhl, A.; Müller, W.; Al-Hazmi, M. H.; Alqahtani, A. M.; Müller, B. H.; Peulecke, N.; Rosenthal, U. Catal. Sci.

Table 2. Crystal Data, Data Collection, and Refinement for A compound chemical formula CCDC Number

[Cr(tBuP*NatBuP) Cl] (A) C23H42ClCrNP2

temperature (K)

173(2)

space group

Pna21

1494912

no. of formula units per unit cell, Z absorption coefficient (μ/mm) no. of reflections measured no. of independent reflections Rint final R1 values (I > 2 σ(I)) final wR(F2) values (I > 2 σ(I)) final R1 values (all data) final wR(F2) values (all data) goodness of fit on F2

4

formula Mass

481.96

crystal system

orthorhombic

a (Å)

22.961(3)

b (Å) c (Å) α (deg)

8.1876(10) 14.2393(17) 90

β (deg) γ (deg)

90 90

unit cell volume (Å3)

2676.9(6)

0.656 13535 5368 0.0687 0.0679 0.1011 0.1077 0.1135 1.087

were collected at 173(2) K on a Bruker APEX-II CCD Duo diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.71073 Å). The cell parameters were determined (APEX2 software)40 from reflections taken from three sets of 12 frames, each at 10 s exposure. The structures were solved by direct methods using the program SHELXS-2013.41 The refinement and all further calculations were carried out using SHELXL-2013.41b The H atoms were introduced into the geometrically calculated positions (SHELXL-2013 procedures) unless stated otherwise and refined riding on the corresponding parent atoms. The non-H atoms were refined anisotropically, using weighted full-matrix leastsquares on F2.

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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.6b00685. Crystallographic information file for A (CIF)

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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 The USIAS, CNRS, Université de Strasbourg, Région Alsace, and Communauté Urbaine de Strasbourg are acknowledged for fellowships and a Gutenberg Excellence Chair (2010−11) to AAD. We thank the CNRS and the MESR (Paris) for funding and for a Ph.D. grant to T.S. We are grateful to the Service de Radiocristallographie (Ms. Corinne Bailly) for determination of the crystal structure and to Mr. Marc Mermillon-Fournier for technical assistance in catalysis. We thank a reviewer for interesting suggestions. Dedicated to Prof. Gerard van Koten for his outstanding career and leading role in the field of pincer ligands.

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REFERENCES

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Chromium(II) Pincer Complexes with Dearomatized PNP and PNC

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