Article pubs.acs.org/Organometallics
β‑Diketiminato Nickel Imides in Catalytic Nitrene Transfer to Isocyanides Stefan Wiese, Mae Joanne B. Aguila, Elzbieta Kogut, and Timothy H. Warren* Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057 United States S Supporting Information *
ABSTRACT: The β-diketiminato nickel(I) species [Me3NN]Ni(2-picoline) (1) serves as an efficient catalyst for carbodiimide (RNCNR′) formation in the reactions of a range of organoazides N3R with isocyanides R′NC. [Me3NN]Ni(CNR)2 (R = tBu, Ar (Ar = 2,6-Me2C6H3)) species provide carbodiimides RNCNAr′ upon reaction with Ar′N3 (Ar′ = 3,5-Me2C6H3). Nitrene transfer takes place via the intermediacy of nickel imides. Reaction of [MexNN]Ni(2-picoline) (x = 2 or 3) with Ar′N3 gives the new dinickel imides {[MexNN]Ni}2(μ-NAr′) (4 (x = 3) and 5 (x = 2)) as deep purple, diamagnetic substances. The X-ray structure of {[Me2NN]Ni}2(μ-NAr′) (5) features short Ni−Nimide distances of 1.747(2) and 1.755(2) Å along with a short Ni−Ni distance of 2.7210(3) Å. These dinickel imides 4 and 5 react stoichiometrically with tBuNC to provide the corresponding carbodiimides t BuNCNAr′ in good yield. Azide transfer takes place upon reaction of 1 with TMS-N3 to give the square planar nickel(II) azide [Me3NN]Ni(N3)(2-picoline) (7). Stoichiometric reaction of dinickel dicarbonyl {[Me3NN]Ni}2(μ-CO)2 with organoazides such as Ar′N3 is sluggish, indicating that 1 is not an efficient catalyst for nitrene transfer from organoazides to CO to form isocyanates RNCO.
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INTRODUCTION Late metal imido complexes [M]NR have been implicated in a range of nitrene [NR] transfer reactions to unsaturated substrates and C−H bonds.1 Especially in conjunction with nitrene transfer reagents bearing electron-poor N-substituents such as N-tosyl imidoiodinanes PhINSO2R and sulfonyl azides NNNSO2R, catalytic alkene aziridination (addition to CC double bonds)2,3 and C−H amination (formal insertion into C−H bonds)3,4 represent particularly valuable C−N bond-forming reactions. Synthetic routes to many isolable late transition metal imido complexes [M]NR, however, often employ N-aryl and N-alkyl organoazides N3R in conjunction with a reduced metal precursor [M].1 Perhaps somewhat surprisingly, a limited number of discrete metal imides [M]NR participate in stoichiometric C−H amination5,6 or alkene aziridination.7 Reactions with isocyanides CNR′ or carbon monoxide (CO) with metal imides [M]NR to give carbodiimides RNC NR′ and isocyanates RNCO represent additional stoichiometric nitrene transfer pathways. Especially given the wide synthetic availability of organoazide precursors RNN N,8 the synthesis of carbodiimides and isocyanides from organoazides represents a promising class of C−N bondforming reactions to provide synthetically versatile unsaturated products9 that proceed with loss of environmentally benign N2 (Scheme 1). Early transition metal imido complexes [M]NR′ can mediate the synthesis of carbodiimides from the coupling of two isocyanates R′NCO with loss of CO2,10,11 a method best employed for symmetric carbodiimides. Unsymmetric © XXXX American Chemical Society
Scheme 1. Carbodiimide and Isocyanate Formation from Organoazides via Metal Imides
carbodiimides are typically formed via aza-Wittig reactions of phosphinimines with isocyanates,11 but may also be prepared by addition of silylamines to isocyanates via tin(II) intermediates.12 Importantly, carbodiimides may also be prepared directly from amines by oxidative coupling with isocyanides CNR′ in the presence of heterogeneous and homogeneous precious metal catalysts such as Au surfaces13 with O2 or PdCl2 with Ag2O.14 Renewed interest in later transition metal imido complexes initiated by the isolation and synthetic studies of the mononuclear Ni(II) imide (dtbpe)NiN(2,6-iPr2C6H3)15 (dtbpe =1,2-bis(di-tert-butylphosphino)ethane) revealed that such [M]NR species can undergo stoichiometric nitrene transfer to isocyanides and carbon monoxide (Figure 1). For instance, (dtbpe)NiN(2,6-iPr2C6H3) reacts with benzyl isocyanide (PhCH2NC) and CO to give nickel-bound carbodiimide or isocyanate adducts.16 Peters found that pseudotetrahedral Co(III)17 and Fe(III)18 [M]NAr species react with CO to give RNCO and the corresponding metal dicarbonyl [M](CO)2. Employing a β-diketiminato Received: September 25, 2012
A
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CNR′ and CO suffered from the competition reaction to form diazenes RNNR23 as well as strong binding of the product to the (dtpbe)Ni(0) center.16 In 2009 Holland reported the use of organoazides AdN3 and p-tolylN3 with CO, CNtBu, and CNCy to give the corresponding isocyanates and carbodiimides via Fe(III) imido complexes generated in situ from a dinuclear, bulky β-diketiminato iron precatalyst {LtBuFe}2(μ-N2) (Figure 1).24 Hillhouse described the use of the dinuclear Nheterocyclic carbene {(IPr)Ni(μ-Cl)}2 (IPr = 1,3-di(2,6diisopropylphenyl)imidazolin-2-ylidene) in catalytic nitrene transfer from MesNNN to CNCH2Ph and CNtBu (Figure 1).25 A very recent example involves the use of a zirconium(IV) imide supported by a chelating, redox-active ligand that allows catalytic nitrene transfer from AdN3 and tBuN3 to CNtBu.26 Since we observed efficient transfer of the [NAd] group to CNtBu and CO from the discrete nickel(III) imide [Me3NN]NiNAd that may be prepared from the nickel(I) complexes [Me3NN]Ni(L) (L = 2-picoline27 (1) or 2,4-lutidine;19 Scheme 2), we became interested in the possibility of general catalytic nitrene transfer from organoazides RN3 to isocyanides and carbon monoxide. We describe our efforts herein that illustrate relatively broad organoazide substrate scope in catalytic nitrene transfer to CNtBu and CNAr (Ar = 2,6-Me2C6H3) to form the corresponding carbodiimides. Synthetic studies reveal new dinickel imide intermediates {[MexNN]Ni}2(μ-NAr′) (x = 3 (4) or 2 (5), Ar′ = 3,5-Me2C6H3) as well as complete azide transfer from TMS-N3 to 1 to give [Me3NN]Ni(N3)(2-pic) (7).
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RESULTS AND DISCUSSION Catalytic Nitrene Transfer. As probed by the commercially available isocyanides CNtBu and CNAr, the monovalent [Me3NN]Ni(2-picoline) (1) precatalyst exhibits relatively broad azide substrate scope (Table 1). In our initial screen, we employed a 5 mol % loading of 1 relative to the isocyanide (ca. 0.2 M) and organoazide (ca. 0.2 M) reactants for 30 min in Et2O at room temperature (RT). A range of aryl azides examined in combination with tert-butylisocyanide (CNtBu) gave excellent yields (79−99%; Table S1). In contrast, the more electron-poor aryl isocyanide CNAr in combination with aryl azides delivered moderate yields (46−84%), with particularly poor performance by the sterically hindered azide MesN3 (3%). Despite the ability of [Me3NN]NiNAd to participate in efficient stoichiometric transfer (Scheme 2),19 attempted catalytic carbodiimide formation with the bulky organoazide AdN3 met with little success. Reactions with the aryl isocyanide CNAr typically proceed more sluggishly and require longer reaction times (17 h) to obtain high yields with 5 mol % 1. Thus, to allow for comparisons between CNtBu and CNAr substrates, a standard reaction time of 17 h at RT was employed (Table 1). While most reactions took place readily at room temperature, mesityl azide required heating at 60 °C in benzene for 24 h to achieve a 73% yield with CNAr. On the other hand, the more basic isocyanide CNtBu proved uniformly more reactive, allowing a low catalyst loading of 1 mol % 1 in most cases. We also examined the primary alkyl azide PhCH2CH2N3 as well as the electron-deficient sulfonylazide TsN 3 and benzoylazide PhC(O)N3, which each represent classes of azides that have not been reported for catalytic nitrene transfer to isocyanides. Hillhouse et al. reported the use of TsN3 for their NHC Ni system but only in the stoichiometric reaction of {(IPr)Ni(μ-Cl)}2 with TsN3 to form {(IPr)Ni}2(Cl)(μ-Cl)(μ-
Figure 1. Selected first-row, late metal imides in stoichiometric and catalytic nitrene transfer to CNR′ and CO.
supporting ligand, we demonstrated nitrene transfer from the nickel imide [Me3NN]NiNAd to CNtBu and CO to give AdNCNtBu and AdNCO (Scheme 2)19 as well as Scheme 2. Synthesis and Reactivity of [Me3NN]NiNAd with CNtBu and CO
from the dicopper nitrene {[Me3NN]Cu}2(μ-NAr′) (Ar′ = 3,5Me2C6H3) to CNtBu to give Ar′NCNtBu.20 Hillhouse deployed an especially bulky N-heterocyclic carbene to stabilize a two-coordinate imide that transfers a sterically encumbered imido ligand to CO from (IPr*)NiN(dmp) (Figure 1).6 Other related examples include the reaction of CO with the bridging dinuclear Ru(II) imide {Cp*Ru}2(μ-NPh)(μ-CO) to give PhNCO21 as well as a novel example of CO and CNtBu addition to a terminal Fe(IV) nitride to give bound isocyanate and carbodiimate anions [NCO]− and [N CNtBu]−.22 Early attempts to employ (dtbpe)NiNR intermediates in catalytic nitrene transfer from organoazides RNNN to B
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74% and 34% isolated yield, respectively (Scheme 3). The two νCN bands observed in the IR spectrum of each isocyanide
Table 1. Carbodiimide Formation from Organoazides and Isocyanides Catalyzed by 1c
Scheme 3. Synthesis of NiI Isocyanide Adducts 2 and 3
adduct are consistent with the presence of two coordinated isocyanide ligands owing to symmetric and antisymmetric combinations of the individual CN oscillators. These bands occur at νCN = 2080 and 2112 cm−1 for [Me3NN]Ni(CNtBu)2 (2) and at νCN = 1997 and 2025 cm−1 for [Me3NN]Ni(CNAr)2 (3). The νCN stretches are found at lower energy than in free CNtBu and CNAr (2131 and 2123 cm−1, respectively), indicating that a significant degree of back-bonding occurs in the adducts 2 and 3. EPR glass spectra of 2 at 89 K in toluene glass (with a drop of CNtBu) indicate a pseudoaxial environment with g1 = 2.32 and g2 ≈ g3 = 2.17 (Figure S7). Similarly, 3 also shows an axial environment at 89 K in toluene glass (with excess CNAr) with g1 = 2.27 and g2 = g3 = 2.17 (Figure S9). Isotropic solution or frozen glass EPR measurements of pure 2 and 3 in the absence of added isocyanide give more complex spectra. This is likely a result of facile partial dissociation of these isocyanides from 2 and 3 indicated by the presence of free isocyanide in NMR spectra of 2 and 3 at RT in benzene-d6. While this may reflect the dissociation of one isocyanide from the bis(isocyanide) complexes 2 and 3 in solution to give a mixture of the corresponding mono- and bis(isocyanide) complexes, β-diketiminato Ni(I) arene complexes are also known.30 The X-ray structure of [Me3NN]Ni(CNAr)2 (3) (Figure 2) shows a distorted tetrahedral coordination at the Ni center with
60 °C in benzene, 17 h. b5 mol % 1, 17 h. cGeneral conditions: 1 or 5 mol % 1 in ether at RT for 17 h.
a
N,κ1-O:NSO2Tol).25 Despite the possibility of primary aliphatic azides to form metal imines via α-H migration,28 PhCH2CH2N3 cleanly forms the corresponding carbodiimides. Tosylazide requires a higher catalyst loading with CNtBu and gives the highest yield with the aryl isocyanide (95%; entry e). Benzoyl azides are a substrate class that has not been heavily investigated in late metal nitrene chemistry, perhaps due to the thermal Curtius arrangement that gives PhNCO and N2.29 Nonetheless we demonstrate high and moderate carbodiimide yields employing this nitrene source with CNtBu and CNAr, respectively (Table 1, entry g). Insights into Catalytic Mechanism: Isocyanide Complexes. We performed a series of synthetic investigations involving the catalyst [Me3NN]Ni(2-pic) (1) and both isocyanide and azide reagents to probe new nickel species that could reasonably participate in the catalytic reactions. The addition of 2.2 equiv of CNtBu or 3.7 equiv of CNAr to [Me3NN]Ni(2-pic) (1) gives [Me3NN]Ni(CNtBu)2 (2) or [Me3NN]Ni(CNAr)2 (3) as brown crystals from pentane in
Figure 2. X-ray structure of [Me3NN]Ni(CNAr2,6Me2)2 (3).
symmetric Ni−CNAr bonds shown by the nearly identical Ni− C bond distances of 1.863(3) and 1.866(3) Å. The Ni−Nβ‑dik bond distances are 1.956(2) and 1.963(2) Å. The twist angle of 67.6° between Nβ‑dik−Ni−Nβ‑dik and Cisocy−Ni−C planes illustrates the distorted tetrahedral geometry at nickel. We find that both [Me3NN]Ni(CNtBu)2 (2) and [Me3NN]Ni(CNAr)2 (3) react at RT in ether with the representative azide N3Ar′ (Ar′ = 3,5-Me2C6H3) to give the corresponding carbodiimides (Scheme 4). For instance, [Me3NN]Ni(CNtBu)2 C
dx.doi.org/10.1021/om300909n | Organometallics XXXX, XXX, XXX−XXX
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Scheme 4. Reaction of NiI Isocyanide Adducts with N3Ar′
(2) reacts with 2 equiv of N3Ar′ to give Ar′NCNtBu in 97% yield as quantified by 1H NMR spectroscopy (Scheme 4). Monitored by UV−vis spectroscopy under catalytically relevant conditions in ether, the dinickel imide 4 forms upon addition of N3Ar′ to 2 or 3. Addition of 20 equiv of CNR′ to [Me3NN]Ni(pic) (1) results in the formation of the isocyanide adducts [Me 3 NN]Ni(CNR′) 2 (2 and 3), which react immediately to produce the dinickel imide {[Me3NN]Ni}2(μNAr′) (4) upon subsequent addition of N3Ar′ to these solutions (Figures S17, S18). The dinickel imide 4 is also observed as the major Ni-containing species after standing 1 h at RT. Synthesis of New Dinickel Imides. On the basis of stoichiometric studies between CNtBu and [Me3NN]Ni NAd,19 we anticipated that related nickel imides might be generally active for nitrene group transfer. To probe the nature of the nickel nitrene species formed upon reaction of 1 with aryl azides, we added N3Ar′ to 2 equiv of 1, which gives {[Me3NN]Ni}2(μ-NAr′) (4), as substantiated by NMR and elemental analysis (Scheme 5). Despite the diamagnetic nature
Figure 3. X-ray structure of {[Me2NN]Ni}2(μ-NAr′) (5).
possesses modestly longer Cu−Nnitrene distances of 1.794(5) and 1.808(5) Å. We find that {[Me2NN]Ni}2(μ-NAr′) (5) exhibits related temperature-dependent 1H NMR chemical shifts in toluene-d8. For instance, at 20 °C the imido o-H, backbone C-Me, and backbone C−H resonances appear at δ 1.87, 0.30, and 4.74 ppm, which shift downfield with cooling to −60 °C to δ 3.88, 1.16, and 5.00 ppm. In contrast, the nitrene p-Ar-H signal shifts upfield from δ 7.34 ppm at 20 °C to δ 6.78 pm at −60 °C (Figures S12, S13). Both {[Me3NN]Ni}2(μ-NAr) (4) and {[Me2NN]Ni}2(μNAr′) (5) are rather reactive toward isocyanides. Each reacts quickly with excess CNtBu to give the carbodiimide tBuN CNAr′ in 88% and 69% yield, respectively (Scheme 6).
Scheme 5. Synthesis of Dinickel Imides 4 and 5
Scheme 6. Isocyanide Transfer to Nickel Imides
of 4 in solution (μeff = 0.0(2) μB at RT), variable-temperature H NMR spectra of 4 in toluene-d8 show somewhat unusual chemical shifts. For instance, at 20 °C the imido o-H, backbone C-Me, and backbone C−H resonances appear at δ 0.16, −0.40, and 4.71 ppm, which shift downfield with cooling to −60 °C to δ 3.26, 0.94, and 5.08 ppm. In contrast, the nitrene p-Ar-H signal shifts upfield from δ 8.01 ppm at 20 °C to 7.18 ppm at −60 °C (Figures S10, S11). To better establish the nature of this putative dinickel nitrene species, we sought the crystal structure of 4. Unfortunately, numerous attempts were unsuccessful, which motivated the synthesis of the analogue {[Me2NN]Ni}2(μ-NAr′) (5), which lacks p-Me groups on the β-diketiminato N-aryl rings. Reaction of 2 equiv of [Me2NN]Ni(2-pic) with N3Ar′ in Et2O results in an immediate color change from red to dark purple. The product {[Me2NN]Ni}2(μ-NAr′) (5) may be crystallized from Et2O as dark purple crystals suitable for X-ray diffraction in 80% yield (Scheme 5). The X-ray structure of 5 (Figure 3) reveals the dinuclear nature of this imido species with a Ni−Ni′ separation of 2.7120(3) Å and Ni−Nimide distances of 1.747(2) and 1.755(2) Å. While the Ni−Nimide distances are quite similar to the related dinickel alkylimide {[Me3NN]Ni}2(μ-NAd) (1.738(3) and 1.758(3) Å), 27 the Ni−Ni distance is significantly longer (2.4872(7) Å). Nonetheless, the Ni−Ni distance is shorter than in the related dicopper arylnitrene {[Me3NN]Cu}2(μ-NAr′)20 (Cu···Cu = 2.911(1) Å), which 1
Monitored by UV−vis spectroscopy in ether, sequential addition of 0.5 equiv of N3Ar′ to 1 to give in situ generated 4 followed by addition of excess CNAr cleanly gives the isocyanide adduct [Me3NN]Ni(CNAr)2 (3), while addition of excess CNtBu does not lead to [Me3NN]Ni(CNtBu)2 (2) (Figures S19, S20). It is possible that the more electron-rich carbodiimide product interacts with the [Me3NN]Ni fragment, hindering association with CNtBu. Nonetheless, the high catalytic yield in nitrene transfer from N3Ar′ to CNtBu (99%, Table 1, entry b) would not suggest appreciable product inhibition during catalysis. DFT Studies: Insight into Dinickel Imides. DFT studies (ADF2007.1−BP/ZORA/TZ2P(+)) conducted on a model of {[Me3NN]Ni}2(μ-NAr′) (4) obtained through the optimization of the crystal structure of {[Me2NN]Ni}2(μ-NAr′) (5) predict the S = 0 and S = 1 forms to be close in energy, favoring the S = 0 form by only 0.7 kcal/mol in electronic energy. Experimentally, this energy difference is higher (>1.5 kcal/mol) based on the magnetic moment of 0.0(2) μB measured for 4 and 5 in benzene-d6 at RT. These two forms are predicted to be very similar in structure (Figure S17), with the only substantial D
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difference being slightly longer Ni−Nimide distances in 4-S=1 (Ni−N: 1.782 and 1.785 Å) compared to 4-S=0 (Ni−N: 1.738 and 1.739 Å). The distribution of unpaired electron density in the S = 1 spin state of 4 (Figure 4) is correlated with the directions of the
Scheme 7. Synthesis and X-ray Structure of [Me3NN]Ni(N3)(2-picoline) (6)
somewhat out of the normal diamagnetic range for related square planar Ni(II) β-diketiminato compounds. Significantly, the peaks for bound 2-picoline are essentially broadened into the baseline at RT. These observations suggest that 2-picoline reversibly dissociates from 6, giving the presumably paramagnetic [Me3NN]Ni(N3). A solution magnetic moment measurement in benzene-d6 in the presence of a large excess of 2-picoline (ca. 100 equiv) gives μeff = 0.81 μB. Supporting this suggestion that the three-coordinate [Me3NN]Ni(N3) would be high spin (S = 1), a related three-coordinate nickel(II) chloride complex [β-dik]Ni−Cl bearing an exceptionally bulky β-diketiminate ligand is high spin with a solution magnetic moment μeff = 3.1 μB.35 Strong π-donor ligands such as amides −NR1R2 are typically required to give low-spin, three-coordinate β-diketiminato Ni(II) complexes.19,27,31 Cooling to −80 °C results in sharpening of broad peaks to allow for assignment of the 1H and 13C NMR spectra, though reversible dissociation/association of 2-pic may not be completely frozen out (Figure S14). For instance, the diastereotopic sets of backbone Me and N-Ar p-Me peaks are not resolved and appear at δ 1.37 and 2.12 ppm, and the N-Ar o-Me groups appear as a broad resonance centered at δ 2.66 ppm. The 2-Me group of the bound 2-pic ligand appears at δ 4.166 ppm (Figure S14). Scouting Reactions with CO. Scouting experiments seeking to prepare isocyanates RNCO from organoazides N3R and CO catalyzed by 1 unfortunately met with little success. We believe that the poor performance of 1 in catalytic nitrene transfer to CO is due to the formation of a rather stable dinuclear nickel carbonyl complex. Addition of excess CO to [Me3NN]Ni(2-picoline) (1) quickly gives the d9−d9 dimer {[Me2NN]Ni}2(μ-CO)2 (7), which may be isolated in 62% yield as black crystals from ether (Scheme 8). This bridging carbonyl had been previously prepared in the addition of CO to [Me3NN]NiNAd to give OCNAd and 7 in 76% yield.19 The X-ray structure of {[Me2NN]Ni}2(μ-CO)2 (7) reveals a rather short Ni−Ni distance of 2.401(2) Å in which metal center may be viewed as distorted trigonal pyramidal to accommodate bulky aryl groups on the N-donor atoms (Scheme 8). The two carbonyl groups are not symmetrically bound to the metal center, with Ni−C bond distances of 1.759(4) and 2.078(4) Å. Both CO ligands are coordinated in a bent fashion with M−C−O angles of 160.8(3)o and 121.8(3)o, one being significantly more obtuse that the other. The relatively high CO stretching frequencies νCO = 1931 and 1893 cm−1, especially for a bridging species, do not suggest an extremely strong back-bonding interaction.
Figure 4. Spin density plot of 4-S=1. Excess spin α in blue; spin β in red (0.001 isovalue).
temperature-dependent 1H NMR chemical shifts observed for 4 and 5. There is excess spin α in the vicinity of the imide N-aryl o-C-H units as well as the β-diketiminato backbone C-H, which each shift downfield with decreasing temperature. On the other hand, there is excess spin β at the nitrene N-aryl p-C atom correlated with an upfield shift of the corresponding p-C-H resonance with decreasing temperature. The chemical shifts of the imido o-H resonances for 4 and 5, which appear at δ 0.16 and 1.87 ppm at 20 °C, respectively, clearly move toward their “diamagnetic” positions (ca. δ 7 ppm) with decreasing temperature, suggesting that this temperature-dependent behavior is likely due to contact shifts from a minute amount of the S = 1 form that increasingly contributes at elevated temperature. Related behavior has been seen in the monomeric, low-spin nickel(II) amide [Me3 NN]Ni-NPh 2, which is calculated to possess a relatively low lying triplet state.31 Transfer of the Azido Group to Nickel. Reaction of [Me3NN]Ni(2-picoline) (1) with trimethylsilyl azide demonstrates an extreme case in which the whole azide functionality is transferred to the metal center.32−34 This reaction generates the square planar nickel(II) azide species [Me3NN]Ni(N3)(2-pic) (6), which may be isolated from ether in 44% yield as red crystals. The X-ray crystal structure of 6 (Scheme 7) shows a four-coordinate, pseudo square planar coordination environment at nickel with Ni−Nβ‑dik distances of 1.912(2) and 1.900(2) Å. The Ni−N distances to the azide and 2-picoline Ndonors are just slightly longer at 1.939(2) and 1.923(2) Å, respectively. The square planar geometry is only slightly distorted, shown by the slight twist angle between the N1−Ni− N2 and N3−Ni−N4 planes of 17.3°. The linear azide moiety is mildly unsymmetrical, with N4−N5 and N5−N6 distances of 1.213(3) and 1.168(3) Å, though the shorter N5−N6 distance may also be due to greater librational motion at the free terminus of the azide group. [Me3NN]Ni(N3)(2-pic) (6) is not stable in solution when redissolved after isolation: excess 2-picoline must be added for spectroscopic characterization. Broad 1H NMR signals are observed at RT in toluene-d8 spread over δ −2 to 9 ppm, E
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employed with low (1 mol %) catalyst loadings of 1 to give good to excellent yields of the corresponding carbodiimides. The scope of catalytic carbodiimide formation established for 1 is considerably broader than initial reports employing Fe(I) βdiketiminato and bridging NHC Ni(I) catalysts: the former examined the azides AdN3 and TolN3 with the alkyl isocyanides CNtBu and CNCy,24 and the later examined MesN3 with CNCH2Ph and CNtBu.25 Synthetic studies focusing on understanding the interaction of the isocyanide and organoazide reagents with the nickel(I) precatalyst [Me3NN]Ni(2-pic) (1) reveal the formation of bis(isocyanide) complexes [Me3NN]Ni(CNR′)2 (R′ = tBu (2), R′ = Ar2.6‑Me2 (3)) as well as bridging dinickel arylimides {[MexNN]Ni}2(μ-NAr′) (x = 3 (4), x = 2 (5)). Importantly, the bis(isocyanide) species 2 and 3 react with the organoazide N3Ar, and the dinickel arylimides 4 and 5 react with the isocyanide tBuNC employed in this study to deliver the corresponding carbodiimides. Thus, these studies indicate that bis(isocyanide) and nickel imido complexes may participate in catalytic carbodiimide formation upon reaction with organoazides and isocyanides, respectively. Some electron-poor organoazides did not perform as well as their electron-rich counterparts, and the use of N3TMS with isocyanides led to no carbodiimide formation. It is possible that the lower nucleophilicity of electron-poor organoazides makes it difficult for them to compete against isocyanide for access to the catalyst’s coordination sphere. In the case of N3TMS, synthetic studies reveal that the azide group is oxidatively transferred to the nickel center to form the nickel(II) azide complex [Me3NN]Ni(N3)(2-picoline) (6), which is inactive in catalytic nitrene transfer to isocyanides with organoazides. Such reactivity appears to be not entirely uncommon, with related examples resulting in metal-azide formation starting from βdiketiminato Fe(II)32 and Ge(II)34 hydrides as well as with an Al(I) species.33 The picoline ligand of 6 appears to be rather labile, requiring added 2-picoline to enhance its stability in solution. The relative difficulty of employing CO in catalytic nitrene transfer reactions employing precatalyst 1 is likely due to the bridging nature of the nickel(I) carbonyl species {[Me3NN]Ni}2(μ-CO)2 (7) that results from exposure of 1 to CO. Coupled with the somewhat poor nucleophilicity of organoazides N3R, the dinuclear nature of 7 leads to extremely sluggish reactivity with organoazides. To enhance reactivity, the bridging structure of the {[Me3NN]Ni}2(μ-CO)2 resting state should be prevented. For instance, Holland’s successful catalytic nitrene transfer from AdN3 to CO by a very bulky βdiketiminato Fe(I) complex results in mononuclear [Fe](CO)2 and [Fe](CO)3 species following catalysis.24 Holland et al. have shown that a more sterically demanding β-diketiminato ligand with N-Ar o-iPr groups can form a T-shaped Ni(I) carbonyl complex [iPr2NN]Ni(CO).36 The use of such bulky ligands could also enhance isocyanide dissociation from [Ni](CNR′)2 complexes during catalytic carbodiimide formation as well as favor terminal nickel(III) imido intermediates. While this may enhance the rate of nitrene transfer, it could also favor side reactions involving C−C and C−N coupling reactions at the paryl position of radical [NiIII]NAr intermediates observed by Bai and Stephan.30
Scheme 8. Synthesis and X-ray Structure of {[Me3NN]Ni}2(μ-CO)2 (7)
The 1H NMR spectrum of 7 in toluene-d8 at −90 °C (Figure S15) is consistent with its solid-state structure (Scheme 8), giving rise to six methyl signals (eight expected) since the two Ni fragments can be interconverted by a C2-axis in solution; separation of two backbone-Me signals was not achieved. Warming to −75 °C results in the coalescence of the p-Me signals at δ 2.37 and 2.13 ppm in the low-temperature limit, consistent with a concerted twisting motion of each [Me3NN] Ni fragment about its respective Ni−Ni vector, corresponding to an activation barrier ΔG⧧ = 9.4(4) kcal/mol at this temperature (Figure S16). All o-Me resonances coalesce at −23 °C, likely a result of reversible dissociation/reassociation of a [Me3NN]Ni moiety from the {[Me3NN]Ni(μ-CO)}2 dimer 7 (Figure S16) with an activation barrier ΔG⧧ = 11.3(3) kcal/mol at this temperature (ca. 30 mM). Stoichiometric reaction of {[Me3NN]Ni}2(μ-CO)2 (7) with 3,5-dimethylphenylazide gave a 13% yield of the corresponding carbodiimide after 3 h at RT (Scheme 9). A catalytic attempt Scheme 9. Sluggish Reactivity of 7 with N3Ar
employing 1 equiv of 3,5-dimethylphenylazide N3Ar′ and either 1 or 6 atm CO in benzene at 60 °C for 24 h with 5 mol % [Me3NN]Ni(2-pic) (1) resulted essentially in no yield of the anticipated isocyanate OCNAr′. Motivated by the previously reported stoichiometric reaction of [Me3NN]Ni NAd with excess CO resulting in a 76% yield,19 a catalytic reaction was performed with AdN3 under similar conditions with no observable product.
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DISCUSSION AND CONCLUSIONS The nickel(I) β-diketiminate [Me3NN]Ni(2-pic) (1) catalyzes nitrene transfer from a diverse array of organoazides RN3 to isocyanides CNR′ (R′ = tBu or Ar) to give carbodiimides RN CNR′. Especially in conjunction with the electron-rich isocyanide CNtBu, aryl, alkyl, tosyl, and carbonyl azides may be
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EXPERIMENTAL SECTION
General Procedures. All experiments were carried out in a dry nitrogen atmosphere using an MBraun glovebox and/or standard F
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{[Me3NN]Ni}2(μ-NAr′) (4). To a solution of [Me3NN]Ni(2-pic) (1) (0.235 g, 0.484 mmol) in 3 mL of Et2O at −35 °C was added a solution of 3,5-dimethylphenylazide (0.036 g, 0.242 mmol) in 2 mL of Et2O at −35 °C. An immediate color change from red to dark purple took place, and gas bubbles evolved. All volatiles were immediately removed in vacuo. The remaining residue was taken up in 10 mL of pentane, and all volatiles were again removed in vacuo. This step is necessary to remove as much free 2-picoline as possible to maximize the isolated yield of this substance. The remaining dark purple residue was taken up in cold Et2O and filtered through Celite. The resulting solution was concentrated, layered with cold pentane, and allowed to crystallize overnight at −35 °C to afford a dark purple solid in 76% yield (0.167 g, 0.185 mmol). 1H NMR (toluene-d8, 20 °C): δ 8.010 (s, 1H, p-NAr-H), 7.546 (s, 8H, m-Ar-H), 4.707 (s, 2H, C-H backbone), 3.411 (s, 12H, p-Ar-CH3), 3.243 (s, 24H, o-Ar-CH3), 2.597 (s, 6H, mNAr-CH3), 0.159 (s, 2H, o-NAr-H), −0.403 (s, 12H, backbone CH3);. 13 C{1H} NMR (C6D6; RT, 400 MHz): δ 156.61, 146.81, 139.18, 126.67, 110.81, 25.05, 19.21, 15.67. μeff (C6D6) = 0.0(2) μB. UV−vis (Et2O, 25 °C): λmax = 375 nm (ε = 18 000 M−1 cm−1) and 529 nm (ε = 4500 M−1 cm−1). Anal. Calcd for C54H67N5Ni2: C, 71.78; H, 7.47; N, 7.75. Found: C, 71.58; H, 7.59; N, 8.04. {[Me2NN]Ni}2(μ-NAr′) (5). To a solution of [Me2NN]Ni(2-picoline) (0.490 g, 1.07 mmol) in 8 mL of Et2O at −35 °C was added a cold solution of 3,5-dimethylphenylazide (0.079 g, 0.536 mmol) in 3 mL of Et2O at −35 °C. An immediate color change from red to dark purple took place, and gas bubbles evolved. All volatiles were immediately removed in vacuo. The remaining residue was taken up in 10 mL of pentane, and all volatiles were again removed in vacuo. This step is necessary to remove as much free 2-picoline as possible to provide high crystalline yields. The remaining dark purple residue was taken up in cold Et2O and filtered through Celite. The resulting solution was concentrated, layered with cold pentane, and allowed to crystallize overnight at −35 °C to afford a dark purple solid in 80% yield (0.362 g, 0.428 mmol). 1H NMR (toluene-d8, 20 °C): δ 7.555 (d, 8H, m-ArH), 7.286 (s, 1H, p-Ar-H), 6.578 (t, 4H, p-Ar-H), 4.768 (s, 2H, backbone C-H), 2.791 (s, 24H, o-Ar-CH3), 2.294 (s, 6H, m-Ar-CH3), 1.940 (s, 2H, o-Ar-H), 0.341 (s, 12H, backbone CH3). 13C{1H} NMR (C6D6; RT): δ 157.05, 147.38, 141.30, 127.65, 122.82, 35.46, 23.50, 17.87. μeff (C6D6) = 0.0(2). UV−vis (Et2O, 25 °C): λmax = 408 nm (ε = 11 000 M−1 cm−1) and 527 nm (ε = 6900 M−1 cm−1). Anal. Calcd for C50H59N5Ni2: C, 70.87; H, 7.02; N, 8.26. Found C, 71.19; H, 7.30; N, 8.18. [Me3NN]Ni(N3)(2-picoline) (6). A chilled (−35 °C) solution of [Me3NN]Ni(2-pic) (1) (0.196 g (0.404 mmol) was prepared in 8 mL of Et2O, to which a chilled (−35 °C) solution of TMSN3 (1.00 mL, 0.868 g, 7.53 mmol) was added. The solution was allowed to warm to RT. During this time the red color of the starting material changed hues to a magenta red. The reaction mixture was allowed to stir for 1 h at RT, after which time all volatiles were removed in vacuo. The red solid was taken up in Et2O and passed through Celite. Then the intense red solution was concentrated to afford red crystals at −35 °C suitable for single-crystal X-ray analysis in 44% yield (0.092 g, 0.176 mmol). All NMR and UV−vis spectra were taken with excess 2picoline (∼5−10 equiv); otherwise the compound decomposes to a black suspension. 1H NMR (toluene-d8, −80 °C): δ 7.591 (br s, 1H, opic-H), 7.171 (s, 1H, pic-H), 7.089 (s, 1H, pic-H), 6.54 (br s, m-Ar-H), 6.311 (br, 1H, pic-H), 6.007 (br, 1H, pic-H), 5.934 (br, 1H, pic-H), 4.717 (s, 1H, backbone C-H), 4.166 (s, 3H, pic-CH3), 2.661 (br, 12H, o-Ar-CH3), 2.120 (s, 6H, p-Ar-CH3), 1.367 (s, 6H, backbone-CH3). 13 C{1H} NMR (toluene-d8, −80 °C): δ 158.47, 150.46, 149.50, 148.45, 135.96, 135.50, 133.29, 132.61, 123.00, 120.60, 25.00, 24.67, 24.34, 24.00 (includes free pic). μeff (C6D6 and 100 equiv of 2picoline) = 0.81 μB. IR: νN3 = 2040 cm−1 (in the presence of excess 2pic). UV−vis (Et2O with 100 equiv of 2-picoline; 25 °C): λmax = 540 nm (ε = 3800 M−1 cm−1). Anal. Calcd for C29H36N6Ni: C, 66.05; H, 6.88; N, 15.94. Found: C, 66.34; H, 7.19; N, 15.59. {[Me3NN]Ni}2(μ-CO)2 (7). A solution of [Me3NN]Ni(2,4-lutidine) (1) (0.250 g (0.501 mmol) was prepared in 10 mL of Et2O, to which CO (48.9 mL @ 1 atm, 298 K; 2.00 mmol, 8 equiv) was added. The solution immediately took on an extremely dark green hue, and the
Schlenk techniques. Molecular sieves (4A) were activated in vacuo at 180 °C for 24 h. Diethyl ether and tetrahydrofuran (THF) were first sparged with nitrogen and then dried by passage through activated alumina columns. Pentane was first washed with concentrated HNO3/ H2SO4 to remove olefins, stored over CaCl2, sparged, and then passed through activated alumina columns. Benzene, toluene, and ethylbenzene were purchased anhydrous and stored over 4A molecular sieves. All deuterated solvents were sparged with nitrogen, dried over activated 4A molecular sieves, and stored under nitrogen. Celite was dried overnight at 200 °C under vacuum. 1H and 13C NMR spectra were recorded on a Varian MR 400 or Mercury 300 MHz spectrometer (400 or 300 MHz; 100.47 or 75.4 MHz, respectively). All NMR spectra were recorded at room temperature unless otherwise noted and were indirectly referenced to residual solvent signals or TMS as internal standards. UV−vis spectra were measured on a Varian Cary 50 or 100 spectrophotometer, using airtight quartz cuvettes with screw-cap tops or Teflon stoppers. Solution EPR spectra were recorded on a JEOL JES-FA200 continuous wave spectrometer equipped with an X-band Gunn oscillator bridge and a cylindrical mode cavity employing a modulation frequency of 100 kHz. GC-MS spectra were recorded on a Varian Saturn 3900, and elemental analyses were performed on a Perkin-Elmer PE2400 microanalyzer at Georgetown. IR measurements were performed on a Perkin-Elmer Spectrum One FT-IR spectrometer using thin films of the sample on a NaCl plate evaporated from pentane or ether. All reagents were obtained commercially unless otherwise noted. [Me3NN]Ni(2-picoline)27 and [Me2NN]Ni(2-picoline)37 were prepared by literature procedures. General Procedure for Catalytic Formation of Carbodiimides and Characterization of Carbodiimide Products. A solution of organoazide (0.843 mmol) along with tert-butylisocyanide (1.2 equiv, 1.01 mmol) or 2,6-dimethylphenylisocyanide (1.1 equiv, 0.927 mmol) was prepared in 3 mL of Et2O and chilled to −35 °C. A second solution of [Me3NN]Ni(2-picoline) (1) (0.0422 mmol = 5 mol % or 0.00843 mmol = 1 mol %) was prepared in 3 mL of Et2O and similarly chilled to −35 °C. The two cooled solutions were added together and allowed to react under the described conditions in Table 1 or Table S1, typically at RT for 30 min or 17 h. The reactions were then quenched with 5 mL of CH2Cl2 to oxidize the catalyst to insoluble {[Me3NN]Ni}2(μ-Cl)2.38 All volatiles were removed in vacuo. The remaining oil was taken up in CH2Cl2 and filtered through Celite. All volatiles were removed in vacuo. Quantification was performed by addition of 1 equiv of internal standard (anthracene, naphthalene, or anisole) followed by 1H NMR analysis in CDCl3. Preparation of Compounds. [Me3NN]Ni(CNtBu)2 (2). A solution of tert-butylisocyanide (0.038 g, 0.458 mmol) in 2 mL of Et2O at −35 °C was added to a solution of [Me3NN]Ni(2-pic) (1) (0.100 g, 0.206 mmol) in 5 mL of Et2O at −35 °C. An immediate color change from red to brownish-orange took place. The reaction mixture was allowed to stir at RT for 30 min. All volatiles were removed in vacuo, and the resulting crude product was crystallized from pentane at −35 °C. Brown crystals were recovered in 74% yield (0.085 g, 0.152 mmol). IR: νCN = 2080, 2112 cm−1. μeff (C6D6) = 1.63 μB. UV−vis (Et2O, 25 °C): λmax = 475 nm (ε = 910 M−1 cm−1). Anal. Calcd for C33H47N4Ni: C, 70.97; H, 8.48; N, 10.03. Found: C, 70.67; H, 8.35; N, 9.75. EPR of 2 with excess tBuNC in toluene frozen glass at 89 K: g1 = 2.32 and g2 = g3 = 2.17. [Me3NN]Ni(CNAr)2 (3). A solution of 2,6-dimethylphenylisocyanide (0.099 g, 0.756 mmol) in 4 mL of Et2O at −35 °C was added to a solution of [Me3NN]Ni(2-pic) (1) (0.100 g, 0.206 mmol) in 5 mL of Et2O at −35 °C. The reaction mixture was allowed to stir at RT overnight, during which time a color change from red to dark brown took place. All volatiles were removed in vacuo, and the resulting crude product was crystallized from pentane at −35 °C. Brown crystals were recovered in 34% yield (0.045 g, 0.0692 mmol). νCN = 1997, 2025 cm−1. μeff (C6D6) = 1.51 μB. UV−vis (Et2O, 25 °C): λmax = 543 nm (ε = 1900 M−1 cm−1). Anal. Calcd for C41H47N4Ni: C, 75.24; H, 7.24; N, 8.56. Found: C, 75.71; H, 7.41; N, 8.44. EPR of 3 with excess CNAr in toluene frozen glass at 89 K: g1 = 2.27 and g2 = g3 = 2.17. G
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volatiles were removed in vacuo. Recrystallization from ether at −35 °C provided black crystals in 62% yield (0.130 g, 0.155 mmol). This species is identical to that prepared from [Me3NN]NiNAd and excess CO.19 1H NMR (toluene-d8, −90 °C): δ 6.873 (br, 1, Ar-H), 6.767 (br, 2, Ar-H), 6.527 (br, 1, Ar-H), 4.992 (s, 1, backbone C-H), 3.365 (s, 6, o-Ar-CH3), 2.374 (s, 12, o-Ar-CH3 and p-Ar-CH3), 2.138 (s, 6, p-Ar-CH3), 2.024 (s, 6, o-Ar-CH3), 1.695 (s, 6, o-Ar-CH3), 1.601 (s, 12, backbone-CH3). 13C{1H} NMR (toluene-d8, −60 °C): δ 160.55, 149.19, 137.44, 133.45, 131.68, 129.94, 98.22 (backbone-CH), 23.70, 21.27 (backbone-CH3), 20.27, 18.79. IR: νCO = 1931, 1893 cm−1. Stoichiometric Carbodiimide Formation. Ar′NCNtBu from [Me3NN]Ni(CNtBu)2 and N3Ar′. [Me3NN]Ni(CNtBu)2 (2) (0.245 g, 0.439 mmol) was dissolved in 10 mL of Et2O and chilled to −35 °C. To this chilled solution was added 3,5-dimethylphenylazide (129 mg, 0.877 mmol). This reaction mixture was allowed to stir for 30 min at RT, after which all volatiles were removed in vacuo. Then the remaining solid was taken up in Et2O and passed through Celite. All volatiles were removed in vacuo, and anisole (95.3 μL, 0.095 g, 0.877 mmol) was added as a 1H NMR standard. The product was quantified via 1H NMR (97% yield, essentially complete consumption of all CNtBu initially bound to [Me3NN]Ni) as well as observed via GC/ MS: m/z (CI mode) = 203 (M + 1). Ar′NCNAr from [Me3NN]Ni(CNAr)2 and N3Ar′. [Me3NN]Ni(CNAr)2 (3) (0.040 g, 0.0611 mmol) was dissolved in 5 mL of Et2O and chilled to −35 °C. To this chilled solution was added a solution of 3,5-dimethylphenylazide (0.018 g, 0.122 mmol) at −35 °C. This reaction mixture was allowed to stir for 30 min at RT. All volatiles were removed in vacuo. Then the remaining solid was taken up in Et2O and passed through Celite. All volatiles were removed in vacuo, and anisole (13.2 μL, 0.013 g, 0.122 mmol) was added as a 1H NMR standard. Unfortunately, overlapping peaks did not allow for ready 1H NMR quantification. The product was clearly observed via GC/MS m/ z (CI mode) = 251 (M + 1). Ar′NCNtBu from {[Me3NN]Ni}2(μ-NAr′) and CNtBu. To a chilled (−35 °C) solution of {[Me3NN]Ni}2(μ-NAr′) (4) (0.048 g, 0.0531 mmol) in 5 mL of Et2O was added tert-butylisocyanide (24 μL, 0.018 g, 0.212 mmol). The reaction was allowed to stir at RT for 30 min, after which time the reaction was quenched with 2 mL of CH2Cl2. All volatiles were removed in vacuo, and the remaining film was taken up in Et2O and passed through Celite. All volatiles were removed in vacuo. To this oil was added 10 equiv of anisole for 1H NMR quantification (57.7 μL, 0.057 g, 0.531 mmol) to indicate an 88% yield of Ar′NCNtBu. Ar′NCNtBu from {[Me2NN]Ni}2(μ-NAr′) and CNtBu. To a chilled (−35 °C) solution of {[Me2NN]Ni}2(μ-NAr′) (5) (0.040 g, 0.0472 mmol) in 5 mL of Et2O was added tert-butylisocyanide (10 μL, 0.007 g, 0.0884 mmol). The reaction was allowed to stir at RT for 30 min, after which time the reaction was quenched with 2 mL of CH2Cl2. All volatiles were removed in vacuo, and the remaining film was taken up in Et2O and passed through Celite. All volatiles were removed in vacuo. To this oil was added anisole (51.2 μL; 0.472 mmol) for 1H NMR quantification to indicate the formation of Ar′NCNtBu in 69% yield. DFT Calculations. The DFT calculations employed the Becke− Perdew exchange−correlation functional39 using the Amsterdam Density Functional suite of programs (ADF 2007.01).40,41 Slatertype orbital (STO) basis sets employed for H, C, and N atoms were of triple-ζ quality augmented with two polarization functions (ZORA/ TZ2P), while an improved triple-ζ basis set with two polarization functions (ZORA/TZ2P+) was employed for the Ni atom. Scalar relativistic effects were included by virtue of the zero-order regular approximation (ZORA).42 The 1s electrons of C and N as well as the 1s−2p electrons of Ni were treated as frozen core. The VWN (Vosko, Wilk, and Nusair) functional was used for LDA (local density approximation).43 Somewhat tighter than default convergence (ΔE = 1 × 10−3 hartree, max. gradient = 1 × 10−3 hartree/Å, max. Cartesian step = 1 × 10−2 Å) and integration (4 significant digits) parameters were employed for geometry optimizations.
Experimental X-ray coordinates for {[Me2NN]Ni}2(μ-NAr′) (5) were used as the starting point for the geometry optimization of lowspin {[Me3NN]Ni}2(μ-NAr) (4-S=0) in a restricted (S = 0) calculation after p-Me groups were installed on the β-diketiminato N-aryl rings. The geometry optimization for the S = 1 form of {[Me2NN]Ni}2(μ-NAr) (4-S=1) employed the DFT-optimized coordinates for 4-S=0 as a starting point in an unrestricted (S = 1) calculation specifying two unpaired electrons (spin α − spin β). ADFview40 was used to prepare the three-dimensional representations of the structures shown in Figure S21 as well as the spin density plot shown in Figure 4.
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ASSOCIATED CONTENT
S Supporting Information *
Complete synthetic and kinetics details, additional characterization data for compounds including EPR spectra, computational methods, and X-ray details with fully labeled thermal ellipsoid plots for 3, 5, 6, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to NSF (CHE-1012523) for support of this work as well as for an award of an EPR spectrometer (CHE0840453).
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REFERENCES
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