Cerium(IV) Neopentoxide Complexes - Inorganic Chemistry (ACS

Jul 3, 2017 - Synopsis. While CAN- and CAC-based salt metathesis protocols give ready access to heteroleptic donor-stabilized Ce(IV) neopentoxides lik...
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Cerium(IV) Neopentoxide Complexes Jochen Friedrich, David Schneider, Lorenz Bock, Cac̈ ilia Maichle-Mössmer, and Reiner Anwander* Institut für Anorganische Chemie; Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany S Supporting Information *

ABSTRACT: Treatment of ceric ammonium nitrate (CAN) with varying amounts of sodium neopentoxide led to the isolation of crystalline cerium(IV) complexes [Ce(OCH2tBu)2(NO3)2(HOCH2tBu)2], [Ce(OCH2tBu)3(NO3)(NCCH3)]2, [Ce2(OCH2tBu)7(NO3)]2, and [Ce2(OCH2tBu)9Na-(THF)] featuring CeIV/(OR) ratios of 1:2, 1:3, 1:3.5, and 1:4.5, respectively. The complexes are light-sensitive and prone to ligand redistribution as evidenced by multicomponent NMR spectra as well as the formation of [{Ce(OCH2tBu)4}2(THF)] and the mixed-valent complex [Ce3(OCH2tBu)9(NO3)2]. The CAN protocol also gave access to the isopropoxide derivative [Ce(OiPr)3(NO3)(THF)]2. The reaction of [Et4N]2[CeCl6] (CAC, ceric organoammonium chloride) with different equivalents of Na(OCH2tBu) was also impaired by ligand reorganization and ate complexation as detected for the tetravalent cerium complex [Ce2(OCH2tBu)7Cl2][Et4N]. Protonolysis of [Ce{N(SiHMe2)2}4] with 4 equiv of HOCH2tBu afforded donor-free homoleptic [Ce(OCH2tBu)4]3 in quantitative yield. All complexes were characterized by NMR, DRIFT, and UV−vis spectroscopy, as well as paramagnetic susceptibility measurements, X-ray structure analysis, and elemental analysis.

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Scheme 1. CAN Protocol2,4,6b,8 with Crystallographically Authenticated Cerium(IV) Alkoxides A−E Obtained According to This Route

INTRODUCTION As part of their seminal work on cerium(IV) alkoxide chemistry, Bradley et al. pointed out that of the primary alkoxides [Ce(OR)4] only the neopentoxide was sublimable (260 °C/0.05 mm) which was also corroborated by its molecular complexity of 2.45 as determined by ebullioscopic measurements in benzene.1 Like numerous primary, secondary, and tertiary CeIV alkoxide derivatives, the neopentoxide [Ce(OCH2tBu)4] was obtained via an alcohol interchange on [Ce(OiPr)4(HOiPr)]2.1 The latter isopropoxide derivative could be readily accessed from reactions involving [pyH]2[CeCl6] (py = NC5H5), the appropriate alcohol, and ammonia.1a This ceric organoammonium chloride (CAC) route was shown in the early work by Bradley et al. to be impaired by incomplete Cl/alkoxy exchange as well as pyridine coordination in the case of tertiary alkoxides as analyzed for [CeCl(OR)3(py)] (R = tBu and CMe2Et).1b The pyridinium-based CAC route was later on considered as tedious,2 e.g., the synthesis of [pyH]2[CeCl6] requires at least 2 days,3 and was displaced by the ceric ammonium nitrate (CAN = (NH4)2Ce(NO3)6) route. The CAN route was introduced in 1985 by Gradeff et al. describing the synthesis of [Ce(OR)4] from CAN and sodium alkoxides (methoxide, n-octyloxide) in methanol.2 Some years later, Evans et al. explored this salt metathesis protocol for the synthesis of a series of CeIV tert-butoxides in tetrahydrofuran.4 Until now, crystallographically authenticated CeIV alkoxides originating from the CAN protocol comprise tert-butoxide (A-C),4,5 isopropoxide (D),6 and 2,3-dimethyl-2butoxide (E)7 derivatives (Scheme 1). Heteroleptic and homoleptic tetravalent cerium alkoxides synthesized via the CAN protocol have found many applications, such as precursors for salt metathesis9 and protonolysis reactions,10 as precatalysts in polymerization or © 2017 American Chemical Society

oxidation reactions,11 and precursors for the chemical vapor deposition of CeO2.7,10c,12 In particular, transalcoholysis employing [Ce(OiPr) 4 (HOiPr)] 2 (D) and [{Ce(OtBu)4}2(THF)x] proved to be a viable strategy for the synthesis of donor-functionalized alk(aryl)oxides,10b,11a,12,13 Received: March 31, 2017 Published: July 3, 2017 8114

DOI: 10.1021/acs.inorgchem.7b00828 Inorg. Chem. 2017, 56, 8114−8127

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Inorganic Chemistry fluorinated alkoxides,10c,14 and mixed metallic species.15 Reaction conditions, choice of alcoholic proligand and solvent were found as determining factors, with oxy-cluster formation, as evidenced for [Ce3O(OtBu)10] (F, Chart 1) featuring a

the entire series of the rare-earth elements, featuring a tetrametallic ring structure.19

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RESULTS AND DISCUSSION CAN Protocol. Although thermally rather stable, both with or without donor molecules (“up to about 200 °C under vacuum”),20 CeIV alkoxides lack photostability.21 For this reason, all attempted syntheses as well as the crude and crystalline product samples were carefully protected against extensive exposure to light. Aiming at heteroleptic and homoleptic neopentoxides, the original synthesis (Scheme 1) was adapted for distinct ratios of CAN/Na(OCH2tBu). The 4 equiv reaction, meaning that using 4 equiv of sodium neopentoxide (Scheme 2), resulted in a bright red rubber-like substance, which was identified as the targeted heteroleptic complex [Ce(OCH2tBu)2(NO3)2(HOCH2tBu)x] (1). The ambient-temperature 1H NMR spectrum of 1 in C6D6 was consistent with a fluxional structure in solution, displaying broadened signals for the methyl and methylene groups (Figure S1). Compound 1 decomposed in C6D6 within hours as indicated by the formation of a slightly yellow precipitate (not further characterized) and the appearance of additional proton resonances (including those of paramagnetic species) after ca. 1 week. The ambient-temperature 1H NMR spectrum of 1 in THF-d8 also showed a fluxional structure in solution as revealed by a sharp resonance for the methyl protons and two signals (one strongly, one slightly broadened) for the methylene protons, as well as sharp resonances for the coordinated THF (Figure S2). The chemical shift of the less broadened methylene signal varied for different synthesis attempts and did vanish within some hours (Figure S3). The DRIFT spectrum of compound 1 displayed a broad absorption in the range υ̅ = 3500−3100 cm−1, evidencing the presence of coordinated alcohol (Figure S31). It was not possible to remove all coordinated neopentanol and THF in vacuum without decomposition of the product. Crystallization at −40 °C from a concentrated toluene extract of the crude vacuumdried reaction product yielded a few red crystals of the disolvate [Ce(OCH2tBu)2(NO3)2(HOCH2tBu)2] (1, Figure 1). The molecular structure of 8-coordinate 1 exhibits the same structural motif as the tert-butoxy derivative [Ce(OtBu)2(NO3)2(HOtBu)2] (A) reported previously by Evans et al. (Scheme 1).4 The terminal Ce−O bond lengths of 1 (Ce−O1 2.030(2) Å, Ce−O2 2.029(1) Å) are in the same range as those reported for heteroleptic A (Ce−Oterm 2.023(5) and 2.026(5) Å)4 but shorter than in homoleptic dimeric [Ce(OtBu)4]2 (2.058(3)−2.065(3) Å),9d [Ce(OCMe2iPr)4]2 (E, avg Ce−Oterm 2.085 Å),7 and alcohol-donor containing [Ce(OiPr)4(HOiPr)]2 (D, 2.083(6)−2.188(6) Å).6 As expected, the Ce−O distances of avg 2.463 Å involving the neopentanol are clearly elongated, but they appear significantly shorter than the Ce−O(HOtBu) distances in A (avg 2.525 Å). The cerium (κ2-NO3) bonding is less distorted than that in A, as suggested by less deviating Ce−O bond lengths (2.516(1)− 2.548(1) Å versus 2.515(6)−2.593(6) Å). It is noteworthy that the nitrato ligands in compound 1 exhibit some close contacts to adjacent methylene (O5---H11b 2.569, O9---H16b 2.581 Å) and alcoholic protons (O5---H4 2.844, O9---H3 2.764 Å). The 5 equiv reaction gave an orange powder of putative [Ce(OCH2tBu)3(NO3)] (2, Scheme 2). The ambient-temperature 1H NMR spectrum of crude reaction product 2 in C6D6 showed one signal set for the methyl and methylene protons (Figure S4). Additional signals of lower intensity indicative of

Chart 1. Oligomeric Oxo-alkoxide, Alkoxide, and Nitratealkoxide Complexes Isolated from Decomposition of Homoleptic Alkoxides and Reactions Involving Heteroleptic Alkoxides

typical side-reaction.4,10d In 2011, Aspinall et al. performed the successful transalcoholysis of [{Ce(OtBu)4}2(THF)x] with several donor-functionalized alcohols; however, neopentanol did not yield putative complex “[Ce(OCH2tBu)4(thf)x]”.12 Instead, the Ce3 cluster [Ce3O(OtBu)(OCH2tBu)9] (G, Chart 1) could be crystallized, evidencing incomplete alcohol exchange and oxy-cluster formation. Furthermore, [Ce(OiPr)4(HOiPr)]2 and [Ce(OtBu)2(NO3)2(THF)2] were used to access the first well-defined organocerium(IV) derivatives [(C5H5)3Ce(OiPr)]16 and [(C5H5)3Ce(OtBu)],9b respectively. Alternative synthesis methods for well-defined CeIV alkoxides comprise the oxidation of trivalent precursors, e.g., [Ce(OCtBu3)3], with di-tert-butyl peroxide (DPBO) or 1,4-benzoquinone17 and the recently established alcoholysis of [Ce{N(SiHMe2)2}4] (pKa(HN(SiHMe2)2) = 22.6).18 Herein we present a full account on the synthesis and structural chemistry of hetero- and homoleptic CeIV alkoxide complexes derived from the primary alcohol neopentanol. Following a detailed investigation of the CAN protocol, we also applied salt metathesis reactions using [Et4N]2[CeCl6] and Na(OCH2tBu), as well as the protonolysis reaction of [Ce{N(SiHMe2)2}4] with HOCH2tBu. It is noteworthy that trivalent homoleptic neopentoxide complexes are known for 8115

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Inorganic Chemistry Scheme 2. Synthesis of CeIV Neopentoxide Complexes according to the CAN Protocol

shifts similar to those of compound 1, but the presence of an alcohol adduct was not suggested by DRIFT spectroscopy (Figures S31−33). While initial crystallization attempts from THF, THF/n-hexane, and THF/toluene mixtures failed, dissolving the crude product in toluene resulted in the formation of a slightly yellow precipitate covered by an orange-red solution after several days at −40 °C. The supernatant contained, among other compounds, decomposition product 4 (vide inf ra). Fortunately, repeated crystallization of crude 2 from toluene at −40 °C gave orange-red crystals distinct from decomposition product 4. An X-ray diffraction experiment revealed the molecular composition of [{Ce(OCH 2 tBu) 3 (μ-OCH 2 tBu) 2 Ce(OCH2tBu)2(κ2-NO3)}2] (3, Figure 2), featuring a Ce/ neopentoxy ratio of 1:3.5 and not 1:3 as anticipated. Complex 3 crystallized in the monoclinic space group C2/c, with an inversion center located in the trapezoidal plane Ce1−O1− Ce1′−O1′. The Ce4 nitrate-alkoxy oligomer consists of two symmetry-related Ce2 units bridged via the nitrate ions. The cerium centers of each Ce2 unit are asymmetrically bridged by two μ2-neopentoxy ligands. The coordination environment of the peripheral 6-coordinate cerium centers is completed by three terminal neopentoxy ligands and a nitrate contact, while the inner 7-coordinate cerium centers bear two additional terminal neopentoxy ligands, the κ2-NO3− ion, and another nitrate contact. The Ce−O bond lengths to the terminal neopentoxy ligands (2.0365(1) Å−2.0564(1) Å) are in the typical range as reported for donor-free tetravalent cerium alkoxides.4,6,7,9d A highly asymmetrical bonding of the bridging neopentoxy ligands is documented by markedly distinct Ce−μ2O distances (Ce1−O8 2.2693(1), Ce1−O10 2.24106(1), Ce2− O8 2.4088(1), and Ce2−O10 2.4256(1) Å). The Ce−

Figure 1. ORTEP representation of the molecular structure of [Ce(OCH2tBu)2(κ2-NO3)2(HOCH2tBu)2] (1) with atomic displacement ellipsoids at the 30% level; hydrogen atoms are partially omitted for clarity. H3 and H4 positions have been found in the difference Fourier map. Selected bond lengths [Å] and angles [deg]: Ce−O1 2.030(2), Ce−O2 2.029(1), Ce−O3 2.455(2), Ce−O4 2.471(1), Ce− O5 2.557(1), Ce−O6 2.516(1), Ce−O8 2.533(1), Ce−O9 2.548(1), O3−H3 0.68(2), O4−H4 0.74(3), Ce---H3 2.839, Ce---H4 2.900; O1−Ce−O3 151.72(5), O2−Ce−O4 158.12(5), O1−Ce−N2 77.95(4), O3−Ce−N2 75.25(4), O2−Ce−N1 86.01(4), O4−Ce− N1 74.41(4), Ce−O1−C1 159.9(1), Ce−O2−C6 156.5(1), Ce−O3− C11 128.1(1), Ce−O4−C16 125.2(1).

another CeIV neopentoxide and paramagnetic CeIII species were detected as well, the latter of which could subsequently be assigned to the decomposition product [Ce3(OCH2tBu)9(NO3)2] (4, vide inf ra). The corresponding 1 H NMR spectrum of crude product 2 in THF-d8 showed similar features but slightly broadened signals at lower field (Figure S5). The second minor CeIV species showed chemical 8116

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Inorganic Chemistry

Figure 2. ORTEP representation of the molecular structure of [Ce2(OCH2tBu)7(NO3)]2 (3) with atomic displacement ellipsoids at the 30% level; hydrogen atoms are omitted for clarity; neopentyl groups are in ball and stick representation. Selected bond lengths [Å] and angles [deg]: Ce1−O1 2.595(3), Ce1−O2 2.657(3), Ce1−O1′ 2.682(2), Ce2−O3′ 2.621(3), Ce1−O4 2.046(3), Ce1−O5 2.036(2), Ce2−O6 2.051(3), Ce2−O7 2.056(4), Ce2−O9 2.041(4), Ce1−O10 2.241(3), Ce1−O8 2.270(3), Ce2−O10 2.425(3), Ce2−O8 2.409(3), N1−O1 1.274(4), N1−O2 1.238(4), N1−O3 1.228(4); Ce1−O8− Ce2 108.4(1), Ce1−O10−Ce2 108.7(1), O8−Ce1−O10 74.2(1), O8−Ce2−O10 68.5(1), O1−Ce1−O2 47.8(1).

Figure 3. ORTEP representation of the molecular structure of [Ce3(OCH2tBu)7(NO3)2] (4) with atomic displacement ellipsoids at the 30% level; hydrogen atoms and lattice toluene are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ce1−O1 2.515(3), Ce1−O2 2.039(4), Ce1−O3 2.275(3), Ce1−O4 2.044(3), Ce1−O5 2.462(3), Ce1−O9 2.303(3), Ce2−O1 2.445(3), Ce2−O5 2.496(3), Ce2−O6 2.277(3), Ce2−O7 2.048(4), Ce2−O8 2.044(4), Ce2−O9 2.331(3), Ce3−O1 2.534(3), Ce3−O3 2.420(3), Ce3−O5 2.496(3), Ce3−O6 2.421(3), Ce3−O10 2.550(4), Ce3−O12 2.543(4), Ce3−O13 2.540(4), Ce3−O14 2.541(4); Ce1−O1−Ce2 94.4(1), Ce1−O5−Ce2 94.4(1), Ce1−O9−Ce2 103.4(1), Ce1−O1− Ce3 93.8(1), Ce1−O5−Ce3 96.1(1), Ce2−O1−Ce3 95.1(1), Ce2− O5−Ce3 94.8(1).

O(nitrate) chelate bonding (Ce1−O1 2.5951(1) Å, Ce1−O2 2.6578(1) Å) appears enlarged in comparison with those of monomeric 1 and A,4 but it is closer than the Ce−O(nitrate) bond connecting the Ce2 units (Ce1−O1′ 2.6824(1) Å, Ce2− O3′ 2.6215(1) Å). Complex 3 is a rare CeIV alkoxide derivative featuring bridging NO3 moieties. For comparison, trivalent [{Cu(3-MeOsalpn)}{Ce(NO3)3}]2 (salpn = N,N′-bis(3-methoxysalicylidene)-propylene-1,3-diamine) revealed Ce−κ2− NO 3 and Ce−O(NO 3 )−Ce connecting distances of 2.649(1)/2.681(1) Å) and 2.649(1)/2.6874(9) Å, respectively.22 Less matching respective Ce−O distances were found in [Ce3(Cp*MoO3)2(NO3)7] (2.61(2)−2.82(2) Å; 2.65(3)−2.86(2) Å).23 The DRIFT spectrum of crystalline 3, whether evaporated to dryness under vacuum or not, did not reveal absorptions indicative of trapped alcohol (Figure S33). The ambienttemperature 1H NMR spectrum of crystalline 3 in C6D6 (Figure S6) showed one signal for the methyl protons and two for the methylene protons along with signals of decomposition product 4 (Figure 3). When complex 3 was dissolved in THF-d8, two signal sets for the methyl and methylene protons were detected (Figure S7), in accordance with a donor-induced deoligomerization/separation of 3 into [Ce(OCH2tBu)3(NO3)] (2) and [{Ce(OCH2tBu)4}2(THF)] (6(THF), vide inf ra); for stacked NMR spectra of compounds 2, 3, and 6(THF), see Figure S8. Thus, formation of complex 3 could occur either by oligomerization of a mixture of 2 and 3 involving dissociation of THF (as a result of nonstoichiometric mixtures of CAN and sodium neopentoxide) or via extensive ligand redistribution. The crude reaction mixture, obtained separately from a 5.5 equiv reaction, i.e., targeted synthesis of 3, showed 1H NMR spectra in C6D6 and thf-d8 similar to that of crystalline 3 (Figure S7). Recrystallization of crystalline 3 from toluene resulted in crystals of the aforementioned decomposition product 4 being omnipresent in the 1H NMR spectra discussed above (see also Figures S6 and S10). The X-ray structure analysis of 4 revealed

the mixed-valent Ce 3 nitrate-alkoxide complex [{Ce(OCH2tBu)2}2(μ2-OCH2tBu)3(μ3-OCH2tBu)2{Ce(κ2-NO3)2}] (4, Figure 3). The three cerium atoms in complex 4 are interconnected by two μ3- and three μ2-bridging neopentoxy ligands. The 6-coordinate cerium atoms Ce1/2 feature two additional terminal neopentoxy ligands each, while the 8coordinate Ce3 atom accommodates two κ2−NO3 groups. Not unexpectedly, the Ce−O(neopentoxy) distance increases in the order as follows: terminal (avg 2.044 Å) < μ2- (avg 2.338 Å) < μ3-bridging (avg 2.489 Å). The ambient-temperature 1H NMR spectrum of crystalline 4 in toluene-d8 displayed the same proton resonances observed in the previous attempts toward the synthesis of [Ce(OCH2tBu)3(NO3)] (2), ranging between δ = 9.98 ppm and δ = −19.15 ppm (Figure S10). The absence of any alcohol trapped inside the crystalline lattice was corroborated by the DRIFT spectrum of crystalline 4 (Figure S34). It should be noted that the way to the isolation of Ce(IV) alkoxides seems paved with decomposition products featuring oxy and mixed-valent cerium species. Chart 1 depicts complexes F−K as the most relevant of such structurally characterized “degradation products”, while Table 1 lists the parent Ce(IV) alkoxide along with the proposed cause of decomposition and unambiguously characterized decomposition products. The oligomeric mixed-valent nitrate/tert-butoxide complex [Ce3(OtBu)10(NO3)] (H) has been reported independently by England and Lappert et al. and was obtained either from salt metathesis reactions employing equimolar mixtures of [Ce(OtBu)3(NO3)] and Li[N(SiMe3)(C6H3-2,6-Me)] or [Ce(OtBu)2(NO3)2(THF)2] and 2 equiv Li[N(SiMe3)2] (isolated after several months) or by treatment of a 2:1 mixture of [{Ce(OtBu)4}2(THF)x] and [Ce(OtBu)3(NO3)] with 3 equiv 8117

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Inorganic Chemistry Table 1. Products Analyzed From the Degradation of Ce(IV) Alkoxides parent compound

act.

[Ce(OtBu)4]25

ΔTd

[Ce(OiPr)4(HOiPr)]26 [Ce(OiPr)4(HOiPr)]225 1 2, 2(MeCN), 3 6, 6(THF)

hνe,f ΔTg,h hνd hνd hνd,i

Scheme 3. Decomposition Pathways for Ce(IV)−OtBu and Ce(IV)−OCH2tBu Moieties

decomposition products F and K (Chart 1),a,b,c OtBu2,b,d isobutyleneb,d I (Chart 1),a,b,c acetoneb,d J (Chart 1),a,b,c OiPr2c “Ce(III)”,b,d isobutyleneb,d 4,d isobutylene,b,d HOMeb,d isobutylene,b,d,i pivaldehydeb,d

a Method of characterization: X-ray. bMethod of characterization: 1H NMR spectroscopy. cMethod of characterization: FTIR spectroscopy. d Solvent: C6D6. eSolvent: DME/HOiPr. fUnsolvated [Ce(OiPr)4]n is sensitive toward photolysis in DME as well. gSolvent: toluene. h Solvent: Neat. iSolvent: THF-d8.

of [Sn(C5H3-tBu2-1,3)Me3] (accessible in good yield).9a,24 In 1991, Caulton et al. reported that photoreduction of [Ce(OiPr)4(HOiPr)]2 in DME/isopropanol led to the oligomeric oxo-alkoxide complex [Ce4O(OiPr)13(HOiPr)] (I).21 On the basis of thermolysis and photolysis studies, it was concluded that the cerium reduction might occur via light-induced homolytic cleavage of the Ce(IV)−O bond. It was also pointed out that the photoreduction requires the presence of primary or secondary alkoxy ligands since solutions of tert-butoxy and triphenylsiloxy derivatives in DME remained unchanged after 36 h of daylight. Morever, the formation of transient cerium(IV) hydride species was assumed affording Ce(III) and dihydrogen, and acetone and isopropanol were verified by 1 H NMR spectroscopy. In 1996 Sirio et al. isolated the tetravalent cerium oxo-isopropoxy complex [Ce4O(OiPr)14] (J) from the mild thermolysis of solid [Ce(OiPr)4(HOiPr)]2, concomitantly forming 2 equiv of isopropanol (via desolvation) and diisopropylether.25 The tetravalent oxo-alkoxide [Ce3O(OtBu)10] (F) and mixed-valent oxo-alkoxide [Ce3(OtBu)11] (K, Chart 1) were isolated from reaction mixtures aiming at the homoleptic CeIV tert-butoxide complex.5,9d,10d The simultaneous formation of F and K was recently confirmed by the thermal decomposition of preisolated donor-free homoleptic [Ce(OtBu)4]2, generating di-tert-butyl ether and isobutylene as byproducts.9d,26 Ether elimination and Ce(IV)−O bond homolysis were proposed as competing and independent degradation pathways. Isobutylene formation was proposed to originate from the tert-butoxy radical, concomitantly producing another unidentified product.9d Alternatively, oxy-cluster and isobutylene formation in complexes [Ce(OtBu) 4 ] was attributed to a “β-hydrogen” migration as shown in Scheme 3a.12 Monitoring the stability of neopentoxide complexes 1−3, 6, and 6(THF) in C6D6 or THF-d8 by 1H NMR spectroscopy at ambient temperature and without light protection over a period of several days, revealed the formation of isobutylene as well (Figure S30). Moreover, resonances presumably stemming from either dineopentyl ether, methylneopentyl ether or dimethyl ether as well as pivaldehyde were detected. For the decomposition of 2, 2(MeCN), and 3, the 1H NMR spectra revealed also signals assignable to methanol or Ce(IV)-bound methoxy moieties. The resonances of the volatile components vanished upon evaporation of the samples to dryness and remeasurement. Noteworthy is that the resonances of decomposition product 4 revealed an increase within the first ∼4 days of decomposition after which the signals decreased

constantly and disappeared, suggesting 4 as an intermediate of the decomposition process. The formation of isobutylene from the neopentoxide complexes under study can be rationalized on the basis of mechanistic pathways as outlined in Scheme 3. In line with “β-hydrogen” migration proposed for tert-butoxide complexes (Scheme 3a)12,27 would be the respective “β-alkyl” migration, generating a Ce(IV)−OCH3 moiety along with isobutylene (Scheme 3b). The Ce(IV)−OCH3 moiety could engage in ether elimination with adjacent Ce(IV)−OCH3 or Ce(IV)−OCH2tBu moieties producing ether molecules.25,26 It has been shown previously that the thermal decomposition of cerium(III) complex [Ce(OtBu)3]3 (150 °C, vacuum) bearing a sterically bulky alkoxy ligand devoid of accessible β-hydrogen atoms also generates isobutylene. Therefore, homolytic cleavage of the C−C bonds between the quaternary carbons and hydrogen abstraction has been proposed as the degradation pathway most consistent with the experimental data.28 This alternative mechanistic scenario is depicted in Scheme 3c for our Ce(IV) neopentoxide system. Further crystallization attempts of crude [Ce(OCH2tBu)3(NO3)] (2) were successful applying solvent mixtures of MeCN/diethyl ether (1:1) at −40 °C, almost quantitatively yielding huge orange-red crystals. An X-ray diffraction experiment revealed the formation of dimeric acetonitrile adduct [Ce(OCH2tBu)2(μ-OCH2tBu)(κ2-NO3)(NCCH3)]2 (2(MeCN)) featuring the targeted Ce/neopentoxy ratio of 1:3 (Figure 4). The asymmetric unit of 2(MeCN) displays a 7-coordinate cerium center with two terminal neopentoxy, a κ2−O−bonded nitrato, and a MeCN donor ligand. A third neopentoxy ligand bridges the cerium centers of the asymmetric units. The Ce−O bond lengths involving the bridging (Ce−O1 2.305(1) Å) and terminal alkoxy ligands (Ce−O2 2.0599(9) Å and Ce−O3 2.043(1) Å) are in the expected range. The distances between cerium and the methylene hydrogen atoms (fixed positions) of the bridging neopentoxy moieties (Ce′---H1a 3.02 Å and Ce---H1b 3.35 Å) appear relatively short compared to those of the terminal ones (Ce---H6a/H6b/H11a/H11b: 3.82−3.96 Å). This is further reflected in distinct angles of Ce′−O1−C1 (115.19(8)°) and Ce−O1−C1 (128.50(8)°). Moreover, the nitrato ligand once 8118

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Inorganic Chemistry

the methyl group signal occurred at −85 °C, affording two signals of 1:2 ratio for bridging and terminal neopentoxy ligands. Alkoxy-nitrato exchange reactions of CAN-derived species [Ce(OR)x(NO3)4−x] (R = iPr and tBu; x = 1−4) or [Ce3−x(OtBu)12−3xNax] (x = 1 and 2) with NH4NO3 were independently described by Gradeff et al.2 and Evans et al.4 as a proper means for controlling the Ce/alkoxy ratio. Therefore, we examined whether 3 (Ce/neopentoxy ratio 1:3.5) can be converted to 2(MeCN) (Ce/neopentoxy ratio 1:3) through addition of NH4NO3. Accordingly, complex 3, which was generated from a 5.5 equiv reaction, was treated with a stoichiometric amount of NH4NO3 in a solvent mixture of MeCN/diethyl ether (1:1) over 48 h under careful exclusion of light. A sticky orange substance was obtained after filtration and evaporation of the supernatant to dryness, affording an orange powder similar to that obtained from the 5 equiv reaction, namely, putative 2. The apparent formation of complex 2 was also revealed by the 1H NMR spectra recorded in THF-d8 and C6D6 at ambient temperature; however, the DRIFT spectrum displayed distinct absorptions in the range υ̅ = 3410−3197 cm−1 assignable to OH and NH stretching vibrations. Although the intensity of the latter NH bands decreased upon subsequent extractions by toluene, a crystalline product could not be obtained. Hence, the envisaged alkoxy-nitrato exchange reactions seem less straightforward for the cerium neopentoxy complexes under study. Given the apparent problems with isolating satisfying amounts of crystalline product with a Ce/neopentoxy ratio of 1:3, namely, [Ce(OCH2tBu)3(NO3)] (2), according to the original CAN route in THF, we wondered whether this might originate from the inherent properties of the neopentoxy moiety. Hence, we examined the 5 equiv reaction with Na(OiPr), aiming at the isolation of heteroleptic isopropoxy derivative [Ce(OiPr)3(NO3)] (5, Scheme 4). Compound 5 was

Figure 4. ORTEP representation of the molecular structure of [Ce(OCH2tBu)3(NO3)(NCCH3)]2 (2(MeCN)) with atomic displacement ellipsoids at the 30% level; hydrogen atoms are partially omitted for clarity. Selected bond lengths and atomic distances [Å] and angles [deg]: Ce−O1 2.305(1), Ce−O1′ 2.3762(9), Ce−O2 2.0599(9), Ce−O3 2.043(1), Ce−O4 2.551(1), Ce−O5 2.496(1), Ce−N2 2.583(2), Ce---C1 3.392; Ce−O1−Ce1′ 109.28(4), Ce−O1− C1 128.50(8), Ce′−O1−C1 115.19(8), Ce−O2−C6 174.12(9), Ce− O3−C11 168.9(1), Ce−O1′−C1′ 115.19(8), O1−Ce−O1′ 70.72(4), O4−Ce−O5 50.60(4).

more exhibits close interatomic contacts to neighbored methylene hydrogen atoms (O4---H1a′ 2.61 Å; distance N2---H1b 3.38 Å) of the bridging neopentoxy ligands. The DRIFT spectrum of crystalline 2(MeCN) (Figure S35) showed characteristic bands of coordinated acetonitrile at υ̅ = 2304, 2276 cm−1, which were still present after exposure of the sample to vacuum. The ambient-temperature 1H NMR spectrum of crystalline 2(MeCN) in C6D6 displayed one sharp signal each for the methyl and methylene protons at 1.06 and 4.70 ppm, respectively (Figure S11), similarly shifted as in crude [Ce(OCH2tBu)3(NO3)] (2). The coordinated MeCN was detected at 0.65 ppm. After several hours, the formation of a precipitate was observed in the NMR tube. Remeasurement of this sample showed slightly shifted proton resonances (similar to those of crystalline 3) and several new paramagnetically shifted signals, which were partly assignable to mixedvalent complex 4 (Figure S12). After 5 days, no further changes were observed (Figure S12). Stirring another sample of 2(MeCN) either for 1 week in toluene or in the presence of an equimolar amount of neopentanol for 2 days in toluene did not increase the amount of 4. However, exposing the sample to sunlight for a period of 2 h resulted in a markedly changed 1H NMR spectrum, wherein crude [Ce(OCH2tBu)3(NO3)] (2) and [Ce3(OCH2tBu)9(NO3)2] (4) could no longer be detected. An ambient-temperature 1H NMR spectrum of 2(MeCN) in THF-d8 confirmed the occurrence of decomposition and rearrangement processes (Figures S13 and S14). After 5 days, the signal set of [{Ce(OCH2tBu)4}2(THF)] (6(THF)) could be detected as well as broadened resonances due to an ongoing decomposition and generation of a further unknown product (Figure S14). A VT (variable temperature) 1H NMR spectroscopic study of 2(MeCN) in toluene-d8 clearly indicated broadening of the methylene protons at −70 °C and splitting into two signals for the bridging and terminal neopentoxy ligands at −80 °C, with the signal of the terminal moieties showing a further splitting at −90 °C (Figure S15). Splitting of

Scheme 4. Synthesis of CeIV Isopropoxide Complexes 5 and 5(THF) according to the CAN Protocol

previously synthesized (but not characterized) and used in a metathesis reaction with Li[N(SiMe3)2] affording the tetravalent cerium alkoxy-amide complex [Ce(OiPr)3{N(SiMe3)2}]2.9c In the present study, compound 5 was obtained as a bright red rubber-like substance, and in contrast to the neopentoxy derivative 2, crystallization from a concentrated mixture of toluene and THF at −40 °C was successful over a period of several weeks. The solid-state structure of [Ce(OiPr)2(μOiPr)(κ2-NO3)(THF)]2 (5(THF)) as revealed by X-ray crystallography shows a dimeric molecular arrangement of 7coordinate cerium centers similar to those of 2(MeCN) with the nitrato and the donor molecules positioned trans to each other (Figure 5), as well as Ce−O distances in the expected 8119

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Inorganic Chemistry

impaired by its low solubility, but nevertheless, one set of sharp resonances for the methyl and methylene protons could be detected (Figure S20). Dissolving 6(THF) in THF-d8 gave a similar 1H NMR spectrum with sharp resonances for the methyl and methylene protons (Figure S21). A VT 1H NMR spectroscopic study of 6(THF) in toluene-d8 was affected by its low solubility as well, but clearly showed a higher agglomeration of 6(THF) and formation of a second minor component over time. Again, the coalescence and splitting phenomena were difficult to interpret (Figures S22 and S23). The DRIFT spectrum of the crude reaction product 6(THF) did not reveal absorptions indicative of the presence of any trapped alcohol (Figure S38). The elemental analysis matched well a compound of composition [{Ce(OCH2tBu)4}2(THF)]. Crystallization occurred from THF and THF/n-hexane mixtures as well as toluene, but single crystals suitable for an X-ray diffraction study were not obtained. Crystallization from the concentrated pale yellow n-hexane extract at −40 °C yielded yellow crystals of ate complex [{Ce(OCH2tBu)2}2(μ2OCH2tBu)3(μ3-OCH2tBu)2Na(THF)] (7(THF), Figure 6, 13% yield). However, 7(THF) could be isolated in 80% yield according to the CAN protocol using 6.5 equiv of Na(OCH2tBu) instead of the aforementioned 6 equiv amount.4

Figure 5. ORTEP representation of the molecular structure of [Ce(OiPr)3(NO3)(THF)]2 (5(THF)) with thermal ellipsoids at 30% probability level; hydrogen atoms are partially omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ce−O1 2.502(1), Ce− O2 2.544(1), Ce−O4 2.053(1), Ce−O5 2.068(1), Ce−O6 2.523(1), Ce−O7 2.315(1), Ce−O7′ 2.337(1), N1−O1 1.278(2), N1−O2 1.274(2), N1−O3 1.218(2), Ce---H1 3.99, Ce---H4 3.83, Ce---H7 3.09, Ce---H7′ 4.27; Ce−O7−Ce′ 109.45(4), O7−Ce−O7′ 70.55(4), O1−Ce−O2 50.78(4), Ce−O4−C1 167.48(1), Ce−O5−C4 165.98(1), Ce−O7−C7 118.81(9), Ce−O7′−C7′ 131.40(9).

range. Once again, the nitrato ligand seems to exhibit close contacts to neighbored methine hydrogen atoms (O1---H7 2.872 Å, fixed hydrogen positions) of the bridging isopropoxy ligands. The DRIFT spectrum of crude 5 (Figure S36) did not reveal any OH-alcoholic stretching vibrations but rather a pronounced C−H bond absorption at lower energy (υ̅ = 2613 cm−1), indicative of secondary interactions.29 The ambient-temperature 1H NMR spectrum of crude [Ce(OiPr)3(NO3)] (5) in C6D6 displayed one septet for the methine protons and one doublet for the methyl groups (Figure S16). Similar to the case of [Ce(OCH2tBu)3(NO3)] (2), a precipitate formed after approximately 1 day. The formation of this precipitate is accompanied by paramagnetically shifted signals for both the methine and methyl resonances in the 1H NMR spectrum. However, the 1H NMR spectrum of crude 5 in THF-d8 was more complicated, displaying two broadened signals and one sharp multiplet for the methine protons and three broadened methyl group resonances (Figure S17). Apparently, ligand redistribution and formation of at least one new species occurred. Crystalline 5(THF) (dissolved in toluene-d8) was further examined by VT 1H NMR spectroscopy (Figures S18 and S19). The ambient temperature spectrum showed a septet at 5.17 ppm and a broadened resonance at 5.06 ppm for the terminal and bridging methine protons, respectively. Also, the methyl protons revealed separate signals for the terminal (broadened doublet at 1.38 ppm) and bridging moieties (broadened singlet at 1.25 ppm). Although coalescence phenomena clearly occurred at lower temperature and were particularly visible for the methyl protons, the signal splitting proved difficult to interpret (Figures S18 and S19). The two multiplets for coordinated THF were found at 3.68 and 1.41 ppm. Our attempts to synthesize putative homoleptic neopentoxide [Ce(OCH2tBu)4] (6) according to the CAN protocol using 6 equiv of Na(OCH2tBu) resulted in the formation of a product mixture containing mainly the THF adduct [{Ce(OCH 2 tBu) 4 } 2 (THF)] (6(THF)) and ate complex [Ce2(OCH2tBu)9Na(THF)] (7(THF)). Recording the ambient-temperature 1H NMR spectrum of 6(THF) in C6D6 was

Figure 6. ORTEP representation of the molecular structure of [Ce2(OCH2tBu)9Na(THF)] (7(THF)) with atomic displacement ellipsoids at the 30% level; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ce1−O5 2.370(4), Ce1− O6 2.093(4), Ce1−O7 2.095(7), Ce1−O8 2.168(4), Ce1−O9 2.409(3), Ce2−O1 2.185(4), Ce2−O2 2.423(3), Ce2−O3 2.106(9), Ce2−O4 2.085(4), Ce2−O5 2.351(4), Ce2−O9 2.378(3), Na1−O1 2.470(6), Na1−O2 2.368(4), Na1−O8 2.659(5), Na1−O9 2.367(4), Na1−O10 2.271(7); Ce1−O2−Ce2 96.2(1), Ce1−O5−Ce2 99.1(1), Ce1−O9−Ce2 97.2(1), Ce1−O2−Na1 90.4(1), Ce1−O8−Na1 88.4(1), Ce1−O9−Na1 90.2(1), Ce2−O1−Na1 91.1(2), Ce2−O2− Na1 88.1(1), Ce2−O9−Na1 89.1(1), O2−Ce1−O5 70.6(1), O2− Ce2−O5 70.6(1), O2−Ce1−O9 67.3(1), O2−Ce2−O9 67.4(1), O2− Na1−O9 68.5(1).

The molecular structure of 7(THF) comprises three trigonally arranged metal centers with 6-coordinate cerium and 5-coordinate sodium centers, comparable to those of previously reported tert-butoxide complexes [Ce2(OtBu)9Na], [Ce2(OtBu)9K(THF)2] (C, Scheme 1), [Th2(OtBu)9Na], and [U2(OtBu)9Na].4,5,30 The Ce−O distances to the terminal and bridging neopentoxy ligands (Ce−Oterm 2.085−2.106 Å; Ce− Oμ 2.351 and 2.370 Å; Ce−Oμ3 2.378−2.423 Å; Na−Oμ 2.470 and 2.659 Å; Na−Oμ3 2.367 and 2.368 Å) are shorter than 8120

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Inorganic Chemistry those found for [Ce2(OtBu)9K(THF)2] (Ce−Oterm 2.103− 2.118 Å; Ce−Oμ 2.376 and 2.413 Å; Ce−Oμ3 2.421−2.432 Å; K−Oμ 2.897 and 2.946 Å; K−Oμ3 2.748 and 2.775 Å).5 The ambient-temperature 1H NMR spectrum of crystalline 7(THF) in C6D6 showed four signals for the methylene (4:2:2:1 ratio) and three for the methyl protons (6:2:1 ratio, Figure S24). While the former methylene peaks are attributable to four terminal, two μ3(CeCeNa)-, two μ2(CeNa)-, and one μ2(CeCe) bridging neopentoxy ligand, the methyl peaks of the two μ2(CeNa) moieties seem to merge with the terminal ones. CAC Protocol. Given our general interest in the feasibility of salt metathesis strategies toward cerium alkoxides, we were curious whether the CAC protocol based on easily obtainable [Et4N]2[CeCl6]3 would provide access to heteroleptic CeIV neopentoxy derivatives of type [Ce(OCH2tBu)x(Cl)y] (x + y = 4). The CAC route, as depicted in Scheme 5, would have the major advantage of not generating any alcohol or ammonia, which can potentially form adduct complexes (cf., CAN route).4

Figure 7. ORTEP representation of the molecular structure of [Ce2(OCH2tBu)7Cl2][Et4N] (8) with atomic displacement ellipsoids at the 30% level; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ce1−Cl1 2.6897(9), Ce2−Cl2 2.7022(8), Ce1−O1 1.918(11), Ce1−O2 2.000(4), Ce1−O5 2.358(2), Ce1−O6 2.324(2), Ce1−O7 2.345(2), Ce2−O3 2.069(3), Ce2−O4 2.061(3), Ce2−O5 2.345(2), Ce2−O6 2.394(2), Ce2−O7 2.329(2), Cl1---N1 4.834; Ce1−O5−Ce2 95.28(8), Ce1−O6−Ce2 94.87(8), Ce1−O7−Ce2 96.08(8), O5−Ce1−O6 70.74(9), O5− Ce1−O7 70.12(7), O6−Ce1−O7 73.66(8).

Scheme 5. CAC Route Employing [Et4N]2[CeCl6] and Na(OCH2tBu)

(2.690(1) and 2.702(1) Å) are in the range of those detected in other oxygen-bonded cerium(IV) complexes like [Ce{N(iPr2PO)2}3Cl]3b (2.6852(9) Å), [{N[CH2CH2NCH(2-O-3,5tBu 2 C 6 H 2 )] 3 }CeCl] 3 1 (2.793(1) Å), [Ce(OCMe 2 CH 2 (CNCH 2 CH 2 N(2,6-iPr 2 C 6 H 3 )(N(SiMe 3 ) 2 ) 2 Cl) 3 2 (2.643(7) Å), [{Li3(THF)5}{(BINOLate)3CeCl}(THF)]33 (2.667(2) Å), [Ce{Co(η 5 -C 5 H 5 ){P(O)(OEt) 2 } 3 } 2 Cl 2 ] 3b (2.709(8) and 2.712(8) Å), and [Ce{((tBuNO)C6H4CH2)3N}Cl]34 (2.7436(8) Å). The closest distance of trapped [Et4N]+ to the anionic fragment is found between the nitrogen and a chlorido ligand (∼4.7 Å). The reaction of [Et4N]2[CeCl6] with 2 equiv of Na(OCH2tBu) in acetonitrile was less successful. The ambienttemperature 1H NMR spectrum of the crude product in THFd8 showed several broadened signals for the methylene and methyl protons, among other signals similar to those of 6(THF). In addition, the chemical shift of the [Et4N] resonances was reminiscent of those of complex 8. An X-ray diffraction analysis of crystals obtained from an acetonitrile solution at −40 °C revealed the starting material [Et4N]2[CeCl6] (Table S3), cocrystallized with four molecules of MeCN per unit cell.35 The reformation of [Et4N]2[CeCl6] is in accordance with extensive ligand reorganization, which is also corroborated by the detection of 6(THF) by 1H NMR spectroscopy. Silylamide Elimination Protocol: Use of [Ce{N(SiHMe2)2}4] as a Precursor. Given the successful protonolysis reactions of [Ce{N(SiHMe2)2}4] with alcohols, reported previously by us and the Schelter group, we eventually aimed at the donor-free non-ate homoleptic cerium neopentoxide complex by applying the reaction shown in Scheme 6.18d Upon addition of neopentanol to [Ce{N(SiHMe2)2}4], a visible reaction occurred instantly, as indicated by a color change from deep purple to bright yellow. Evaporation of the volatiles left compound 6 quantitatively as a yellow powder. The ambient-temperature 1H NMR spectrum of 6 in C6D6 revealed a fluxional structure in solution (Figures S26 and S27), although the bridging and terminal neopentoxy ligands showed superimposed signal sets of broadened and sharp resonances at δ = 4.62/4.60 ppm and δ = 1.17/1.13 ppm for the methylene

The 3 equiv reaction in acetonitrile aiming at putative complex [Ce(OCH2tBu)3Cl] resulted in yellow-brown solid 8, insoluble in noncoordinating solvents. The ambient-temperature 1H NMR spectrum of the crude reaction product in acetonitrile-d3 showed several broadened and overlapping resonances for the methylene protons and two slightly broadened signals for the methyl groups (Figure S25). In addition, sharp signals for NEt4 moieties were detected revealing an approximate neopentoxy/NEt4 ratio of 7:1. The respective 1H NMR spectrum of crude product 8 in THF-d8 showed similar chemical shifts for the alkoxide moieties as observed for 6(THF). Combined 1H NMR spectroscopic and elemental analysis suggested that 8 might have the composition [Ce2(OCH2tBu)7Cl1+x][Et4N]x (x = 1−2). Fortunately, crystallization of 8 from THF at −40 °C gave yellow crystals suitable for X-ray structure analysis, proving a molecular composition of [{CeCl(OCH2tBu)2}2(μ-OCH2tBu)3][Et4N] (8, Figure 7). The Ce−O(neopentoxy) bond lengths in the anionic [Ce2(OCH2tBu)7Cl2] fragment with 6-coordinate cerium centers are slightly longer than those in dimeric 2(MeCN) with 7-coordinate cerium centers. The Ce−Cl distances 8121

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Inorganic Chemistry Scheme 6. Protonolysis of [Ce{N(SiHMe2)2}4] with 4 equiv of Neopentanol in n-Hexane Yielding Trimeric [Ce(OCH2tBu)4]3 (6)

and methyl protons, respectively. A VT 1H NMR spectroscopic study in toluene-d8 clearly proved a higher agglomeration of 6 at lower temperatures (Figure S27), but the observed signal pattern (difficult to interpret >6 signals for the methylene protons and >3 signals for the methyl protons) was distinct from that detected for 6(THF). Interestingly, the ambienttemperature 1H NMR spectrum of crystalline 6 (synthesized in n-hexane) in THF-d8 was consistent with that of 6(THF) (Figure S28). Also, when the protonolysis of [Ce{N(SiHMe2)2}4] with neopentantol was performed in THF, the ambient-temperature 1H NMR spectrum of the obtained solid product in C6D6 was identical to that of 6(THF) from the CAN route (Figure S29). Attempts to collect single-crystalline 6(THF) derived from the silylamine elimination protocol were unsuccessful, corroborating the idea that the presence of THF is disadvantageous for crystallization. As for all other complexes under study, the DRIFT spectrum showed a further weak absorption at υ̅ = 2689 cm−1 in addition to the undisturbed C−H bond vibrations, which is in the region of CH vibrations engaged in β-H---Ce interactions (Figure S37). Single crystals of [Ce3(OCH2tBu)8(μ-OCH2tBu)4] (6) could be obtained from a concentrated solution in n-hexane at −40 °C (Figure 8). Complex 6 crystallizes as a trimer in the triclinic space group P1̅, with the three cerium atoms featuring a bent arrangement (Ce1−Ce2−Ce3A 113.57(3) Å). For comparison, low-valent rare-earth metal silylamides were shown to form similar trimeric bent structures.36 The peripheral cerium centers Ce1 and Ce3 in 6 are 5-coordinate (three terminal and two bridging neopentoxy ligands), adopting a distorted square pyramidal coordination geometry. The central Ce2 is coordinated in a distorted octahedral fashion by two terminal and four bridging neopentoxy ligands. The terminal Ce−O bond lengths (1.979(3)−2.087(3) Å, avg 2.063 Å) are shorter than those in [Ce2(OCMe2iPr)6(μ-OCMe2iPr)2] (E, avg Ce−Oterm 2.085 Å) and [(OCtBu3)3Ce(μ-OC6H4O)Ce(OCtBu3)3] (avg Ce− Oterm 2.091 Å).7,9d,17 The bridging Ce−Oμ bond lengths range from 2.25(2) to 2.359(2) Å (avg 2.329 Å). In contrast to 2(MeCN), 3, 5(THF), [Ce(OCMe2iPr)3(μ-OCMe2iPr)]2 (E),7 and [Ce(OtBu)4]2,9d the bridging ligands in 6 are symmetrically arranged (avg Oμ−Ce−Oμ 67.1°, Ce−Oμ−Ce 112.4°). It appears that the increased charge density (higher positive charge and therefore smaller radii) of CeIV versus CeIII has a greater effect on the terminal Ce−O distances than on the bridging. The terminal Ce−Oterm distances in the trivalent complex [Ce2(OCH2tBu)2(μ-OCH2tBu)4]2 are comparatively longer (CeIII−Oterm 2.146 Å) than the bridging ones (CeIII−Oμ

Figure 8. ORTEP representation of the molecular structure of [Ce(OCH2tBu)4]3 (6) with atomic displacement ellipsoids at the 30% level; hydrogen atoms and disordered atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ce1−O1 2.072(3), Ce1− O2 2.057(3), Ce1−O3 2.078(3), Ce1−O4 2.358(2), Ce1−O5 2.337(3), Ce2−O4 2.322(3), Ce2−O5 2.359(2), Ce2−O6 2.081(2), Ce2−O7 2.087(3), Ce2−O8 2.25(2), Ce2−O9 2.340(11), Ce3−O8 2.30(3), Ce3−O9 2.371(11), Ce3−O10 1.979(3), Ce3−O11 2.076(6), Ce3−O12 2.072(6), Ce1---Ce2 3.8468(3), Ce2---Ce3 3.896(2); Ce1−O4−Ce2 110.59(10), Ce1−O5−Ce2 109.98(9), Ce2−O9−Ce3, 111.6(4), O4−Ce1−O5 69.58(8), Ce2−O8−Ce3 117.8(11), Ce1−Ce2−Ce3 113.57(3).

avg 2.382 Å).19c The O−Ce−O angles observed in complex 6 are almost identical to those reported for the trivalent complexes [Ln(OCH2tBu)3]4 (Ln = La−Lu).19a,b,d Volatility of Homoleptic CeIV Neopentoxide. For [Ce(OCH2tBu)4]3 (6) and [{Ce(OCH2tBu)4}2(THF)] (6(THF)) sublimation tests were performed at 150 °C using high vacuum (5 × 10−5 mbar). While donor-free 6 sublimed virtually quantitatively and without any significant decomposition, thermal displacement of THF and the formation of significant amounts of neopentanol were observed for the respective thermal treatment of 6(THF). All relevant 1H NMR spectra are compiled in Figure S26. Overall, these findings are in line with the early investigations by Bradley et al.1a UV−Vis Spectroscopic Characterization. In general, all complexes under study showed sensitivity to light accompanied by typical color changes, a behavior found previously for other β-proton containing CeIV alkoxides.21 The UV−visible spectra of complexes 1−8 were recorded in toluene (Figure 9a) or acetonitrile (Figure 9b), taking into consideration solubility issues, the high energy absorption between 200 and 300 nm, and the comparability with [Et4N]2[CeCl6].35b In toluene, all complexes displayed ligand to metal charge transfer (LMCT) bands with one broad absorption maximum at about 300 nm. Of note is that nitrate-containing neopentoxides [Ce(OCH 2 tBu) 2 (NO 3 ) 2 (HOCH 2 tBu) x ] (1) and [Ce(OCH2tBu)3(NO3)] (2) displayed a red-shift of the onset to ∼500 nm compared to those of [Ce(OiPr)3(NO3)] (5) at ∼450 nm and nitrate-free complexes [Ce(OCH2tBu)4]3 (6), [Ce(OCH2tBu)4(THF)] (6(THF)), and [Ce2(OCH2tBu)9Na(THF)] (7(THF)) at 400−450 nm. The molar absorptivities of the nitrate-free complexes were much higher than those of 8122

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(OR) ratio to 1:6 resulted in THF adduct [{Ce(OCH2tBu)4}2(THF)], which eluded crystallographic characterization. Crystallization of the products from the CAN protocol is affected by (light-induced) decomposition, ligand rearrangement, and ate complex formation as evidenced by the isolation of mixed-valent complex [Ce3(OCH2tBu)9(NO3)2] or sodium neopentoxide-implemented [Ce2 (OCH2 tBu) 9 Na(THF)]. Applying the CAC protocol, treatment of [Et4N]2[CeCl6] with Na(OCH2tBu) gave a rare heteroleptic cerium(IV) chloride/alkoxide complex, namely, [Ce2(OCH2tBu)7Cl2][Et4N]. Finally, homoleptic tetravalent cerium neopentoxide could be accessed by performing a protonolysis of [Ce{N(SiHMe2)2}4] with neopentanol. Donor-free cerium(IV) neopentoxide crystallizes as a Ce3 oligomer featuring the connectivity [(RO)3Ce(μ-OR)2Ce(OR)2(μ-OR)2Ce(OR)3], and it can be sublimed without decomposition. Hence, the degree of oligomerization and the thermal behavior are in line with what has been found by Bradley et al. in 1956.1a

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EXPERIMENTAL SECTION

General Procedures. All operations were performed under rigorous exclusion of oxygen and moisture in an argon atmosphere, using standard Schlenk, high-vacuum, and glovebox techniques (MB Braun MB150B-G-I; 99%, 63.5 mg) was crystallized twice from a concentrated n-hexane solution at −35 °C to give yellow crystals (23.5 mg, 0.04 mmol, 37%). Anal. Calcd for C60H132Ce3O12: C 49.16, H 9.08. Found: C 49.68, H 9.08. 1H NMR (C7D8, 500 MHz, 26 °C) δ: 4.52 (start of deconvolution of CH2 group), 4.50 (s, 24H, CH2), 1.08 (start of deconvolution of tBu group), 1.05 (s, 108H, CH3) ppm. Evans (C6D6): χmol = 8.94 × 10−4 emu·mol−1, χmol·T = 0.27 emu·K·mol−1, μeff = 1.46 μB, Δ = 2.83 Hz, c = 0.0321 mol·L−1. DRIFT (KBr, cm−1): 3052 (w), 3022 (w), 2950 (s), 2863 (s), 2822 (m), 2689 (w), 1596 (w), 1491 (m), 1478 (m), 1458 (m), 1445 (m), 1393 (m), 1361 (m), 1092 (s), 1065 (s), 1020 (s), 700 (m), 595 (m), 437 (w). Sy nt hesi s o f [ {Ce(OCH 2 t B u) 4 } 2 ( T H F) ] ( 6 ( T H F )) a nd [Ce2(OCH2tBu)8Na(THF)] (7(THF)). Applying the CAN protocol in THF (8 mL),4 CAN (250 mg, 0.45 mmol) was reacted with 6 equiv of Na(OCH2tBu) (301 mg, 2.74 mmol). After stirring the reaction mixture for 2 h at ambient temperature, centrifugation and evaporation of the supernatant yielded a yellow powder (236 mg). Extraction of the yellow powder with n-hexane gave a yellow-brown solution containing 7(THF) and a bright yellow residue, putative [{Ce(OCH2tBu)4}2(THF)] (6(THF)) (115 mg, 0.11 mmol, 48%). Crude product 6(THF): Anal. Calcd for C44H97Ce2O9: C 50.36, H 9.22. Found: C 50.29, H 9.21. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.53 (s, 8H, CH2), 4.04 (s (br), 2H, OC4H8), 1.56 (s (br), 2H, OC4H8), 1.16 (s, 36H, CH3) ppm. 1H NMR (THF-d8, 400.6 MHz, 26 °C) δ: 4.33 (s, 8H, CH2), 3.62 (m, 2H, OC4H8), 1.77 (m, 2H, OC4H8), 0.96 (s, 36H, CH3) ppm. Evans (C6D6, only a fractional amount of 10 mg was dissolved): Δ = 0.72 Hz; (THF-d8): χmol = 6.39 × 10−4 emu· mol−1, χmol·T = 0.19 emu·K·mol−1, μeff = 1.24 μB, Δ = 1.58 Hz, c = 0.0155 mol·L−1. DRIFT (KBr, cm−1): 2948 (s), 2861 (s), 2817 (m), 2686 (w), 1477 (m), 1391 (m), 1359 (m), 1109 (m), 1069 (s), 933 (w), 900 (w), 751 (w), 593 (s), 437 (s).

differs due to a nonstoichiometric neopentanol and THF inclusion. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.49 (s (br), 4H, CH2), 0.92 (s, 18H, CH3) ppm. 1H NMR (THF-d8, 400.6 MHz, 26 °C) δ: 3.98 (s (br), 4H, CH2), 3.76 (s (br), 4H, CH2, neopentanol), 3.62 (m, 4H, THF), 1.77 (m, 4H, THF), 0.84 (s, 36H, CH3) ppm. Evans (C6D6): χmol = 4.20 × 10−4 emu·mol−1, χmol·T = 0.12 emu·K·mol−1, μeff = 1.00 μB, Δ = 9.68 Hz, c = 0.0396 mol·L−1; (THF-d8): χmol = 4.11 × 10−4 emu·mol−1, χmol·T = 0.12 emu·K·mol−1, μeff = 0.99 μB, Δ = 9.00 Hz, c = 0.0386 mol·L−1. DRIFT (KBr, cm−1): 3327 (w), 2955 (s), 2900 (m), 2863 (m), 2689 (w), 1652 (w), 1522 (s), 1472 (s), 1393 (s), 1366 (s), 1261 (s), 1085 (s), 1048 (s), 1020 (s), 935 (w), 866 (m), 809 (w), 740 (m), 669 (w), 615 (m), 570 (w), 438 (m). Synthesis of [Ce(OCH2tBu)3(NO3)] (2). Applying the CAN protocol in THF (8 mL),4 CAN (548 mg, 1.00 mmol) was reacted with 5 equiv of Na(OCH2tBu) (551 mg, 5.00 mmol). After stirring for 2 h at ambient temperature, centrifugation, and evaporation of the supernatant, a yellow-orange powder was obtained. After extraction with toluene and evaporation of the solvent, crude product 2 (427 mg, 0.92 mmol, 92%) was obtained. Assuming a CAN/Na(OtBu)-analogous reaction,4 this 1:5 reaction should have resulted in the formation of putative tris(neopentoxy) complex [Ce(OCH2tBu)3(NO3)] (2) with additional THF or alcohol donor coordination. Analysis of crude product 2 obtained from the toluene extract: Anal. Calcd for C15H33O6Ce1N1: C 38.87, H 7.18, N 3.02. Found: C 40.78, H 8.04, N 2.67. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.58 (s, br, 6H, CH2), 4.01 (m, THF), 1.44 (m, THF), 1.04 (s, br, 27H, CH3). 1H NMR (THF-d8, 26 °C) δ: 4.33 (s, br, 1H, CH2), 4.03 (s, br, 5H, CH2), 0.84 (s, br, 27H, CH3) ppm. Evans (C6D6): χmol = 3.35 × 10−4 emu·mol−1, χmol·T = 0.10 emu·K·mol−1, μeff = 0.89 μB, Δ = 3.39 Hz, c = 0.0418 mol·L−1; (THF-d8): χmol = 3.23 × 10−4 emu·mol−1, χmol·T = 0.10 emu· K·mol−1, μeff = 0.88 μB, Δ = 2.41 Hz, c = 0.0374 mol·L−1. DRIFT (KBr, cm−1): 2952 (s), 2898 (m), 2866 (m), 2838 (m), 1522 (s), 1478 (m), 1395 (m), 1362 (m), 1256 (m), 1083(s), 1060 (s), 1019 (s), 934 (w), 602 (s), 565 (w), 440 (w). Crystallization of crude product 2 (360 mg, 0.78 mmol) from a toluene extract at −40 °C resulted on one occasion in orange-red crystals of [Ce2(OCH2tBu)7(NO3)]2 (3) suitable for X-ray diffraction analysis and sufficient for spectroscopic and elemental analyses. Anal. Calcd for C70H154Ce4N2O20: C 44.15, H 8.15, N 1.47. Found: C 44.25, H 8.08, N 1.78. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.85−4.72 (2 s (br), 14H, CH2), 1.14 (s (br), 63H, CH3). 1H NMR (THF-d8, 26 °C) δ: 4.33 (s, br, 6H, CH2), 4.15 (s, br, 8H, CH2), 0.96 (s, br, 36H, CH3), 0.83 (s, br, 36H, CH3) ppm. DRIFT (KBr, cm−1): 2950 (s), 2903(m), 2864 (m), 2687 (w), 1520 (m), 1477 (s), 1362 (m), 1305 (w), 1257 (w), 1215 (w), 1071 (s), 1053 (s), 1020 (s), 998 (m), 619 (m), 597 (m), 439 (w). Crystallization of crude product 2 (132 mg, 0.28 mmol) from a 1:1 mixture of acetonitrile and diethyl ether at −40 °C gave large orange crystals of [Ce(OCH2tBu)3(NO3)(NCCH3)]2 (2(MeCN)) (103 mg, 0.20 mmol, 72%) suitable for X-ray diffraction analysis. Anal. Calcd for C17H36Ce1N2O6: C 40.47, H 7.19, N 5.55. Found: C 40.86, H 7.37, N 5.66. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.70 (s, 6H, CH2), 1.06 (s, 27H, CH3), 0.65 (s, 3H, CH3CN) ppm. 1H NMR (THF-d8, 400.6 MHz, 26 °C) δ: 4.53 (s, 1H), 4.09 (br (s), 4H, CH2), 3.49 (s, 1H), 1.94 (s, CH3CN), 0.82 (s, 27H, CH3) ppm. Evans (THF-d8): χmol = 3.43 × 10−4 emu·mol−1, χmol·T = 0.10 emu·K·mol−1, μeff = 0.90 μB, Δ = 1.74 Hz, c = 0.0278 mol·L−1. DRIFT (KBr, cm−1): 2953 (s), 2900 (s), 2866 (m), 2681 (w), 2304 (w), 2276 (w), 1528 (m), 1502 (m), 1476 (m9, 1466 (m), 1368 (m), 1279 (m), 1255 (m), 1088 (s), 1060 (s), 1021 (s), 989 (s), 935 (s), 601 (m), 552 (w), 445 (m). Synthesis of Ce3(OCH2tBu)9(NO3)2(C7H8) (4). Recrystallization of samples of 2 or 3 (formerly measured by 1H NMR spectroscopy in C6D6) from toluene at −40 °C, as well as recrystallization of 3 from toluene at −40 °C, resulted in the formation of a slightly yellow precipitate within some days. Filtration left a luminous red solution from which some red single crystals of 4 suitable for X-ray diffraction analysis were grown. Anal. Calcd for C52H107Ce3N3O15: C 43.96, H 7.59, N 1.97. Found: C 44.13, H 7.20, N 2.37. 1H NMR (C7D8), 400.6 MHz, 26 °C) δ: 9.98 (s, 2H, CH2), 5.86 (dd, 11.23 Hz, 10H, CH2), 4.73 (s, 2H, CH2), 3.69 (s, 9H, CH3), 1.60 (s, 18H, CH3), 1.55 (s, 8124

DOI: 10.1021/acs.inorgchem.7b00828 Inorg. Chem. 2017, 56, 8114−8127

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Inorganic Chemistry

equipped with a fine focus sealed tube and curved graphite monochromator using Mo Kα radiation (λ = 0.71073 Å). The data collection strategy was determined using COSMO339 employing ωand ϕ-scans. Raw data were processed using APEX40 and SAINT;41 corrections for absorption effects were applied using Sadabs.42 The structure was solved by direct methods (SHELXS-1997/2013)43 and refined against all data by full-matrix least-squares methods on F2 (SHELXLE-2014).44 All graphics were produced employing ORTEP345 and POV-Ray.46 Further details of the refinement and crystallographic data are listed in Tables S2 and S3 and in CCDC 1540690− 1540698. The neopentyl moieties of the neopentoxide complexes 3, 4, 6, 7(thf), and 8 were partly or completely disordered. Thus, strong restraints had to be used in some cases.

Evaporation of the extract and subsequent recrystallization from tolue ne at −40 °C gave clear yellowish crystals of [Ce2(OCH2tBu)9Na(THF)] (7(THF)) (68 mg, 0.05 mmol, 13%). Anal. Calcd for C49H107Ce2Na1O10: C 50.75, H 9.30. Found: C 49.55, H 9.57. 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.85 (s (br), 2H, CH2), 4.73 (s, 4H, CH2), 4.51 (s, 8H, CH2), 4.37 (s (br), 4H, CH2), 3.55 (m, 4H, OC4H8), 1.36 (m, 4H, OC4H8), 1.35 (s, 9H, CH3), 1.19 (s, 18H, CH3), 1.13 (s (br), 54H, CH3) ppm. Evans (C6D6): χmol = 6.32 × 10−4 emu·mol−1, χmol·T = 0.19 emu·K·mol−1, μeff = 1.23 μB, Δ = 1.40 Hz, c = 0.0160 mol·L−1. DRIFT (KBr, cm−1): 2949 (s), 2861 (m), 2815 (m), 2688 (w), 1477 (m), 1392 (m), 1360 (m), 1100 (s), 1068 (s), 1020 (s), 593 (m), 432 (w). Synthesis of [Ce2(OCH2tBu)7Cl2][Et4N] (8). To a solution of [Et4N]2[CeCl6] (200 mg, 0.33 mmol) in MeCN (5 mL) were added 3 equiv of Na(OCH2tBu) (108 mg, 0.98 mmol) dissolved in MeCN (3 mL). The resulting yellow-brown suspension was stirred for 2 h at ambient temperature. Centrifugation gave a colorless precipitate and a yellow-brown supernatant. Evaporation of the supernatant and subsequent extraction with THF left a greyish residue. Evaporation of the THF extract gave the crude product in form of a dark yellow powder (151 mg, 0.14 mmol, 85%). Crystallization at −40 °C from THF or MeCN gave single-crystalline 8 suitable for X-ray structure analysis. Crude product (8): Anal. Calcd for [Ce2(OCH2tBu)7Cl2][Et 4 N] C 4 3 H 9 7 Ce 2 Cl 2 NO 7 : C 47.32, N 1.28, H 8.96; [Ce2(OCH2tBu)7Cl3][Et4N]2 C51H117Ce2Cl3N2O7: C 48.73, N 2.23, H 9.38. Found: C 47.26, N 2.11, H 8.89. 1H NMR (MeCN-d3, 400.6 MHz, 26 °C) δ: 4.47−4.35 (m, 14H, CH2), 3.17 (q, 8H, CH2CH3), 1.21 (d v t, 12H, CH2CH3), 0.94 (d, 63H, CH3) ppm. Evans: (THFd8): χmol = 6.39 × 10−4 emu·mol−1, χmol·T = 0.19 emu·K·mol−1, μeff = 1.24 μB, Δ = 1.06 Hz, c = 0.0156 mol·L−1. 1H NMR spectroscopy and elemental analysis suggested that the crude product contained approximately 1.25 equiv of [Et4NCl]. DRIFT (KBr, cm−1): 2964 (s), 2897 (m), 2857 (m), 2682 (w), 1400 (m), 1393 (m), 1358 (m), 1279 (m), 1112 (m), 1091 (s), 1068 (s), 1021 (m), 1002 (m), 902 (w), 726 (w), 594 (s), 561 (m), 440 (m). Attempted Synthesis of 2(MeCN) from Crude 3 via Alkoxynitrato Exchange with NH4NO3. Applying the CAN protocol in THF (5 mL),4 CAN (181.1 mg, 0.33 mmol) was reacted with 5.5 equiv of Na(OCH2tBu) (200 mg, 1.82 mmol). After stirring the reaction mixture for 2 h at ambient temperature, centrifugation and evaporation of the supernatant afforded a yellow-orange substance. Subsequent extraction with toluene under stirring overnight left putative crude product 3 (103.5 mg, 0.05 mmol, 72%). 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.86−4.63 (3 s (br), 14H, CH2), 1.11 (s (br), 63H, CH3). 1H NMR (THF-d8, 26 °C) δ: 4.33 (s, br, 6H, CH2), 4.15 (s, br, 8H, CH2), 0.96 (s, br, 36H, CH3), 0.83 (s, br, 36H, CH3) ppm. Crude 3 (99.2 mg, 0.05 mmol) was dissolved in 2.5 mL of Et2O, NH4NO3 (8.3 mg, 0.10 mmol) dissolved in 2.5 mL of MeCN added, and the mixture stirred for 48 h at ambient temperature. The obtained slightly yellow-orange reaction mixture was filtered, and the filtrate was evaporated to dryness resulting in an orange solid (94.5 mg, 0.21 mmol, 98% of putative crude 2). 1H NMR (C6D6, 400.6 MHz, 26 °C) δ: 4.59 (s, br, 6H, CH2), 1.05 (s, br, 27H, CH3). 1H NMR (THF-d8, 26 °C) δ: 4.33 (s, br, 1H, CH2), 4.10 (s, br, 5H, CH2), 0.85 (s, br, 27H, CH3) ppm. DRIFT (KBr, cm−1): 3410−3197 (w) 2950 (s), 2864 (m), 2686 (w), 1694 (m), 1518−1462 (m), 1393 (m), 1360 (m), 1263 (m), 1215 (w), 1062 (s), 1020 (s), 934 (w), 899 (w), 812 (w), 739 (w), 600 (m), 565 (m), 437 (m). Extraction of the orange solid (86.4 mg) by toluene and subsequent evaporation of the supernatant to dryness produced again an orange solid (76.5 mg), displaying less intense NH bands in the DRIFT spectrum. The extraction residue (4.4 mg) was identified as NH4NO3 by DRIFT spectroscopy. Crystal Data Collection, Structure Solution, and Refinement. Suitable single crystals for X-ray structure analyses were selected in a glovebox, coated with Paratone-N HR2−643, and fixed on a nylon loop fiber. Data for compounds 1, 2(MeCN), 4, 5(THF), 6, 7(THF), 8, and [Et4N]2[CeCl6](MeCN)2 were collected on a Bruker APEX DUO instrument equipped with an IμS microfocus sealed tube and QUAZAR optics for MoKα radiation (λ = 0.71073 Å). Data for compound 3 were collected on a Bruker SMART APEX II instrument

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00828. 1 H NMR and DRIFT spectra, magnetic susceptibility measurements, X-ray crystallographic data for 1−8 and [Et4N]2[CeCl6](MeCN)2 (PDF) Accession Codes

CCDC 1540690−1540698 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Reiner Anwander: 0000-0002-1543-3787 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS In memory of Donald C. Bradley, protagonist of metal alkoxide chemistry. We thank the German Science Foundation (Grant AN 238/16-1) for funding, Dr. A. Berkefeld for helpful discussions, and Dr. K. Eichele and K. Strohmaier for measuring VT 1H NMR spectra.

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REFERENCES

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