Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo

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Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo-bridged Dinuclear Cerium Complexes with Pentadentate Schiff-base Ligands under Dioxygen Atmosphere Satoru Shirase, Koichi Shinohara, Hayato Tsurugi, and Kazushi Mashima ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01718 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo-bridged Dinuclear Cerium Complexes with Pentadentate Schiff-base Ligands under Dioxygen Atmosphere Satoru Shirase, Koichi Shinohara, Hayato Tsurugi,* and Kazushi Mashima* Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan. ABSTRACT: Ionic mononuclear and neutral dinuclear complexes of cerium(III) 3-L1―3-L9 bearing a series of dianionic pentadentate Schiff-base ligands were synthesized and characterized, and used as catalysts for N-oxyl radical-free aerobic alcohol oxidation. Reactions of Ce(NO3)3·6H2O with ortho-tert-butyl-substituted sterically hindered ligands NH(CH2CH2N=CHC6H2-3-(tBu)-5-R2-2OH)2 (L1H2: R2 = tBu; L2H2: R2 = OMe; and L3H2: R2 = H) in the presence of triethylamine afforded the corresponding anionic cerium complexes [HNEt3][Ce(L1-3)(NO3)2] (3-L1—3-L3), whereas complexation with sterically less-hindered ligands, such as NH(CH2CH2N=CHC6H2-3-R1-5-R2-2-OH)2 (L4H2: R1 = OMe, R2 = H; L5H2: R1 = H, R2 = tBu; L6H2: R1 = H, R2 = OMe; L7H2: R1 = H, R2 = H; L8H2: R1 = H, R2 = NO2; and L9H2: R1 = tBu, R2 = NO2), afforded neutral dinuclear complexes [Ce(L4-9)(NO3)]2 (3-L4— 3-L9). Among these newly prepared complexes, complex 3-L1 was selected as the best catalyst for oxidizing primary and secondary alcohols under dioxygen atmosphere without any N-oxyl radicals such as TEMPO to produce the corresponding carbonyl compounds, where the oxo-bridged dinuclear complex worked as a catalyst while maintaining its dinuclear skeleton during the catalytic cycle. In addition, an intramolecular redox process between the two cerium centers through the bridging oxygen atom played a key role in forming the ligand phenoxide radical mediated TEMPO-free alcohol oxidation reaction. Keywords：cerium catalysts, aerobic alcohol oxidation, oxo-bridged dinuclear complex, Schiff-base ligand, co-catalyst free oxidation

1. INTRODUCTION Oxidation of primary and secondary alcohols to the corresponding carbonyl compounds is a critical transformation in organic synthesis. A variety of oxidants, such as peracids, peroxides, and chlorinated compounds, have been utilized so far, but these reagents require harsh conditions and coproduce more than stoichiometric amounts of waste.1 A recent requirement for organic reactions is the development of environmentally friendly synthetic protocols, and a notable alternative is aerobic catalytic oxidation in which dioxygen is an oxidant and H2O is the sole byproduct. Various transition metal complexes, including Cu,2 Co,3 Fe,4 Ru,5 Pd,6 and V, 7 have been intensively investigated as catalysts for oxidative transformations using dioxygen. 8 Almost all of these transition metal complexes require co-catalysts, however, such as stable N-oxyl radicals: Semmelhack first reported that a copper complex in combination with 2,2,6,6-tetramethylpyperidine-N-oxyl (TEMPO) as a co-catalyst became a catalyst for a practical aerobic oxidation of alcohols, wherein the copper catalyst mediated a one-electron redox and TEMPO concurrently functioned to abstract the α-hydrogen atoms of alcohols (Figure 1a).9 Since this pioneering work, many highly active copper/nitroxyl radical catalyst systems have been extensively developed, and high catalytic performance has been realized as unique cooperative effects of both the metals and organic co-catalysts, although the use of costly additives such as N-oxyl radicals should be reduced and eliminated (Figure 1a).2 In this context, Kitajima10 and Stack11 independently demonstrated that salen-type copper complexes are effective as catalysts for oxidizing alcohols without any co-catalysts; during the catalytic cycle, the formation of phenoxide radicals at the redox-active salen ligands plays an important role for abstracting α-hydrogen atoms from alcohols, in which the metal and the ligand cooperatively support the oxidation reaction (Figure 1b). In most cases, including these examples, phenoxide radicals are

essential for achieving the co-catalyst-free aerobic alcohol oxidation reaction,12 which is resemble to the natural enzymatic system for alcohol oxidation such as Galactose oxidase. 13 Accordingly, the development of such an oxidation process by metal catalysts without any co-catalysts is a long-standing and challenging task.

Figure 1. Transition metal catalyst systems for aerobic alcohol oxidation (a) with a co-catalyst and (b) without any co-catalyst.

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We recently focused our attention on cerium complexes, because the cerium atom adopts two stable oxidation states, +3 and +4, and its one-electron redox couple can be utilized for a large number of redox processes.14 Actually, Ce(IV)-based compounds, such as cerium ammonium nitrate (CAN), are widely used as versatile oxidants in organic transformations, bioinorganic reactions, and electron-transfer chemistry.15 Although CAN is commonly used as a stoichiometric oxidant, CAN becomes a catalyst for the aerobic oxidation of alcohols in the presence of TEMPO.16 This unique one-electron redox nature of the cerium atom, similar to that of the copper metal, prompted us to search for a cerium complex capable of catalytically oxidizing alcohols to the corresponding aldehydes. We previously reported that a neutral cerium(IV) complex 1 bearing a pentadentate Schiff-base ligand performs as an excellent catalyst in the presence of TEMPO, and isolated a neutral mononuclear Ce(III) complex 2 as an intermediate species that was prepared by reducing the Ce(IV) complex 1 with an organosilicon reducing reagent (Figure 1a).17 Herein, we report a catalyst system of ionic mononuclear and neutral dinuclear complexes of cerium(III) bearing Schiff-base ligands without any co-catalyst for oxidizing primary and secondary alcohols to the corresponding carbonyl compounds. In addition, we identified a unique nature of oxo-bridged dinuclear cerium complexes in which one of two cerium centers functioned as a catalytically active site while the other cerium center supported an intramolecular redox process for forming the ligand phenoxide radical as well as proceeding this TEMPO-free oxidation reaction. 2. RESULTS AND DISCUSSION 2-1. Synthesis of Ionic Mononuclear Cerium Complexes An ionic mononuclear cerium(III) complex, [HNEt3][Ce(L1)(NO3)2] (3-L1), was prepared in 83% yield by treating Ce(NO3)3·6H2O with a pentadentate Schiff-base ligand, NH(CH2CH2N=CHC6H2-3-R1-5-R2-2-OH)2 (L1H2: R1 = R2 = t Bu), in the presence of 2 equiv of NEt3 (Scheme 1). Complex 3-L1 was characterized by NMR measurements, despite its paramagnetic nature, and a single crystal X-ray diffraction study (Figure 2; vide infra) with its combustion analysis. Its 1H NMR spectrum in CD3CN at 30 °C displayed four paramagneticallyshifted broad singlets centered at δ 30.95, 18.15, 11.22, and 42.10, which were assignable to an imine proton, two aromatic protons, and an amine proton of L1; two singlets at δ 4.26 and 4.26 due to two magnetically non-equivalent tBu groups; and four broad resonances at δ -16.25, -6.37, 3.26, and 4.54 due to four dissymmetric –CH2CH2– protons of the ligand. Three ethyl groups of the triethylammonium salt were observed as a rather sharp triplet at δ 1.14 and a quartet at δ 2.06, attributed to the methyl and methylene protons, respectively. The observed chemical shift values were almost the same as those for mononuclear cerium(III) complex 2 with the same Schiff-base ligand, indicating that the coordination environment of L1 in complex 3-L1 was almost the same as that of 2. Advantages of complex 3-L1 were air stability in solid state and easy handling for catalytic reactions compared with 2, which decomposed quickly upon exposure to air. Similarly, mononuclear ionic cerium complexes [HNEt3][Ce(L2)(NO3)2] [3-L2: L2 = NH(CH2CH2N=CHC6H2-3-tBu-5-OMe-2-O)2] and [HNEt3][Ce(L3)(NO3)2] [3-L3: L3 = NH(CH2CH2N=CHC6H33-tBu-2-O)2]) were prepared in 89% and 95% yields, respectively, by treating Ce(NO3)3·6H2O with the corresponding Schiff-base ligands L2H2 and L3H2, whose phenoxy rings have

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tert-butyl substituents at the ortho-positions and different substituents at the para-positions. These complexes were fully characterized by their spectral data, combustion analyses, and a crystallographic study for 3-L3.

Scheme 1. Synthesis of Mononuclear Cerium(III) Complexes 3-L1―3-L3 with Pentadentate Schiff-base Ligands. Figure 2 shows the crystal structure of the anionic part of 3L1, in which the cerium atom adopts a nine-coordinated geometry by three nitrogen atoms and two oxygen atoms in a distorted penta-coordinated equatorial plane of the ligand and two ҝ2-(O,O)-NO3 anions at both apical sites. The dihedral angle (39.3°) between the two phenoxy rings indicates a deformation of the penta-coordinated equatorial plane due to steric repulsion between the two tert-butyl groups of the two phenoxy rings. The distances of Ce―O(phenoxide) (2.29—2.30 Å) are normal

Figure 2. Molecular structures of 3-L1 and 3-L3 with 50% probability ellipsoids. All hydrogen atoms and HNEt3+ are omitted for clarity. Selected bond lengths (Å) and angles (degree) for 3L1: Ce—N1 2.6697(18), Ce—N2 2.7411(19), Ce—N3 2.6438(17), Ce—O1 2.3022(14), Ce—O2 2.2852(13), Ce—O3 2.6620(15), Ce—O4 2.6504(15), Ce—O6 2.566(2), Ce—O7 2.6615(17); N1—Ce—N2 64.71(6), N2—Ce—N3 63.10(6), N1—Ce—N3 123.99(5), O1—Ce—O2 106.24(5), O3—Ce— O4 48.03(4), O6—Ce—O7 48.23(6), O1—Ce—N2 69.04(5),

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O2—Ce—N3 68.83(5); those for 3-L3: Ce—N1 2.6618(17), Ce—N2 2.6860(15), Ce—N3 2.6544(13), Ce—O1 2.3006(10), Ce—O2 2.3127(14), Ce—O3 2.6287(15), Ce—O4 2.6046(11), Ce—O6 2.6939(11), Ce—O7 2.6469(12); N1—Ce—N2 63.30(4), N2—Ce—N3 64.91(5), N1—Ce—N3 127.83(5), O1—Ce—O2 103.70(4), O3—Ce—O4 48.87(5), O6—Ce— O7 47.91(3), O1—Ce—N1 69.35(4), O2—Ce—N3 69.44(4). for a single bond compared with that reported for other cerium(III) complexes (2.16—2.28 Å).17,18 The distances of Ce―N(amine, imine) (2.64―2.74 Å) in 3-L1 are typical for dative interaction of Ce―N (2.67―2.70 Å).17,18 The distances between the cerium atom and oxygen atoms of one of the two nitrate Ce―O(nitrate) (2.57―2.66 Å) are also normal for cerium(III) nitrate complexes (2.53—2.67 Å), whereas those of the other Ce―O(nitrate) (2.65―2.66 Å) were somewhat elongated compared with those of 2 (2.59—2.61 Å) due to hydrogen bonding between a hydrogen atom of [HNEt3]+ and nitrate oxygens.17,18 The molecular structure of 3-L3 is essentially the same as that of 3-L1, and some of the structural data are listed in Figure 2. 2-2. Synthesis of Neutral Dinuclear Cerium Complexes We found that sterically less-hindered pentadentate ligands afforded the corresponding neutral dinuclear cerium(III) complexes. Reaction of Ce(NO3)3·6H2O with L4H2 (R1 = OMe; R2 = H) in the presence of NEt3 resulted in the formation of [Ce(L4)(NO3)]2 (3-L4) in 83% yield (Scheme 2). Complex 3L4 was characterized by NMR measurements, and its dinuclear structure was revealed by mass spectroscopy of 3-L4, displaying a peak for monocation, [M - NO3]+ (m/z = 1080.1), together with a single crystal diffraction analysis (Figure 3; vide infra). No NMR signals assignable to the ammonium salt were detected, distinguishing the structure of 3-L4 from the anionic mononuclear structure of 3-L1―3-L3. Due to the paramagnetic nature of 3-L4, all signals in its 1H NMR spectrum in DMSO-d6 were upfielded, downfielded, and broadened; four broad singlets observed at δ 22.52, 12.80, 9.16 and 9.01 were assigned to an imine proton and three aromatic protons of L4; and four broad signals centered at δ 6.40, 5.26, -0.44 and -8.22 corresponded to ethylene protons; and a broad singlet at δ 2.02 was attributed to the four OMe groups, suggesting that the four iminophenolato moieties were magnetically equivalent. Such a symmetric NMR pattern might be due to a rapid exchange of the cerium atom between the oxygen atoms of µ-phenoxide and phenoxide. Similarly, other less-hindered ligand such as L5 (R1 = H; R2 = tBu), L6 (R1 = H; R2 = OMe), L7 (R1 = R2 = H), and L8 (R1 = H; R2 = NO2) gave the corresponding neutral dinuclear cerium complexes [Ce(L5―8)(NO3)]2 (3-L5―3-L8) (Scheme 2), while a rather bulky but electron-deficient ligand L9 (R1 = tBu; R2 = NO2) bearing a tert-butyl group at the ortho position also produced a neutral dinuclear complex [Ce(L9)(NO3)]2 (3-L9). Complex 3-L4 was crystallized from N,N’-dimethylformamide (DMF) layered by Et2O at room temperature to give yellow crystals of 3-L4·(dmf)2. Figure 3 shows the dinuclear structure of 3-L4, in which one of two phenoxy oxygen atoms of the ligand coordinates to the other cerium atom to hold a dinuclear unit. The cerium atom adopts a nine-coordinated geometry of one ҝ2-(O,O)-NO3 anion, one ҝ1-O-DMF, three nitrogen atoms, and three oxygen atoms of the Schiff-base ligand. The distances of Ce―O(µ-phenoxide) (2.45―2.51 Å) are in good accordance

with those reported for Ce(III)―O(µ-phenoxide) (2.45 Å),20 and the distance of Ce―O(phenoxide) (2.31 Å) is almost the same as that of the ionic mononuclear cerium complexes 3-L1 and 3-L3. The distances of Ce―N(amine, imine) (2.65―2.67 Å) are typical noncovalent distances of Ce―N (2.67―2.70 Å). The distances of Ce―O(nitrate) (2.65―2.70 Å) are also normal for cerium(III) nitrate complexes (2.53—2.67 Å), and somewhat elongated compared with 2 (2.59—2.61 Å) due to the trans effect of the oxygen atoms of the bridging phenoxy ligands.

Scheme 2. Synthesis of Dinuclear Cerium(III) Complexes 3-L4 ―3-L9 with Pentadentate Schiff-base Ligands.

Figure 3. Molecular structure of 3-L4·(dmf)2 with 50% probability ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (degree): Ce1—N1 2.6747(19), Ce1—N2 2.645(2), Ce1—N3 2.6701(19), Ce1— O1 2.3083(17), Ce1—O2 2.453(2), Ce1—O2* 2.5148(18), Ce1—O3 2.5013(17), Ce1—O4 2.646(2), Ce1—O5 2.699(2); N1—Ce1—N2 64.62(6), N1—Ce1—N3 63.75(7), O1—Ce1— O2 92.84(7), O1—Ce1—N2 69.33(7), O2—Ce1—N3 66.87(7), O4—Ce1—O5 47.71(6). 2-3. Cerium-catalyzed Oxidation of Alcohols to Aldehydes under Dioxygen Atmosphere With a series of ionic mononuclear cerium(III) complexes 3L1—3-L3 and neutral dinuclear cerium(III) complexes 3-L4— 3-L9 in hand, we tested the catalytic performance of these complexes for selective oxidation of 4-methylbenzyl alcohol (4a) to 4-methylbenzaldehyde (5a), and the results are summarized in Table 1. Upon starting from the bulky substituted ligand system, under the conditions including complex 3-L1 (5 mol% catalyst loading), MS 4Å, DMF, 10 h, 100 °C, and O2 (atmospheric pressure), 4a was oxidized to give 5a in quantitative yield (entry 1). The substituents at the para-position of the phenoxy group

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of complexes 3-L2, 3-L3, and 3-L9 sensitively affected the catalytic activity: complex 3-L2 having a methoxy group at the para-position gave 5a in 84% yield and 3-L3 with a phenyl group was less effective (44% yield), whereas 3-L9 bearing an electron-withdrawing group (NO2) exhibited almost no catalytic activity (entries 2―4). Similar substituent effects at the para-position on catalytic activity were observed for complexes 3-L5―3-L8 that had less bulky ligands, though their catalytic activities were relatively lower than those of 3-L1―3-L3 (entries 5―8). Acceleration of the catalytic activity by the bulky and electron-donating substituents at the 3- and 5-positions of the phenoxy ring strongly suggested that phenoxide radicals might be involved in the alcohol oxidation process; this is a similar tendency to the reported TEMPO-free copper(II) catalyst system that the phenoxide radical is stabilized by the bulky and electron-donating substituents at the 3- and 5-positions, where ligand phenoxide radials played a crucial role for the oxidation process.12 Thus, we selected 3-L1 as the best catalyst for cocatalyst free cerium-catalyzed alcohol oxidation under atmospheric pressure of O2. Finally, we checked the catalytic oxidation of 4a using 3-L1 in the presence of TEMPOH as an additive, giving 5a in 95% yield with a slightly lower catalytic activity compared to 3-L1 without TEMPOH. Table 1. Oxidation of 4-Methylbenzyl alcohol to 4-Methylbenzylaldehyde by Cerium Complexes under Dioxygen Atmosphere.a,b

entry

Ce complex

condition

conv. (%)

1 2 3 4 5 6 7 8 9 10 11

3-L1 3-L2 3-L3 3-L9 3-L5 3-L6 3-L7 3-L8 3-L1 3-L1 3-L1

with 10 mol% TEMPOH under air under Ar

>99 84 44 10 64 81 43 17 95 91 no reaction

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substituted benzyl alcohols 4h,i were oxidized to 5h,i in good yields after 20 h. Ortho-substituted benzyl alcohols 4j―m were oxidized to 5j―m in moderate to excellent yields and showed similar reactivity to the para-substituted benzyl alcohols 4a―e despite their large steric hindrances. Table 2. Catalytic Oxidation of Benzyl Alcohols.a,b

a

Alcohol (0.200 mmol), 3-L1 (0.010 mmol), MS 4Å (40 mg), 0.5 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an internal standard. bNMR yields. c20 h. Oxidation of secondary benzyl alcohols 4n—q was slow, and the corresponding ketones were obtained after a longer reaction time with 8 mol% catalyst (Table 3). An activated secondary benzyl alcohols 4n and 4o were effectively oxidized to give 5n and 5o in 81% and >99% yield, respectively, whereas secondary benzyl alcohols 4p and 4q were less reactive to afford 5p and 5q in 37% and 52% yield. Our catalytic system can be applied to oxidation of aliphatic alcohols (4r—4t). Cinnamyl alcohol (4r) was smoothly converted to 5r in good yield of 89%. Secondary aliphatic alcohols 4s and 4t were respectively oxidized to the corresponding ketones 5s and 5t in moderate to excellent yields of >99% and 42% yield. Table 3. Catalytic Oxidation of Secondary Benzyl Alcohols and Aliphatic Alcohols. a,b

a

4a (0.200 mmol), [Ce] (0.010 mmol on metal), MS 4Å (40 mg), 0.50 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an internal standard. bNMR yields. We next screened various substituted benzyl alcohols as substrates and the results are shown in Table 2. We increased the temperature for benzyl alcohol derivatives using the best catalyst 3-L1 to improve the product yields for a series of substrates. Functionalized benzyl alcohols were oxidized to the corresponding aldehydes under our oxidation system. Benzyl alcohol and its derivatives bearing an electron-donating substituent at the para-position afforded the corresponding aldehydes in excellent yields (Table 2, 5b,c). Para-fluoro and trifluoromethyl-substituted benzyl alcohol derivatives 4d,e were slowly oxidized to form the corresponding aldehydes 5d,e in moderate yields of 63% and 80% after 20 h. Meta-methyl and methoxysubstituted benzyl alcohol 4f,g were converted to the corresponding aldehydes 5f,g in 90% and 86% yield, whereas meta-

a

Alcohol (0.200 mmol), 3-L1 (0.016 mmol), MS 4Å (40 mg), 0.50 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an internal standard. bNMR yields. c5 mol%. 2-4. Reaction of Cerium(III) Complexes with Dioxygen Giving Dinuclear Peroxo and Oxo Complexes We conducted stoichiometric reactions to elucidate how O2 played a role on activating catalyst precursors 3-L1—3-L3. In fact, we exposed three ionic mononuclear Ce(III) complexes 3L1—3-L3 to O2, and we isolated the corresponding peroxobridged dinuclear Ce(IV) complexes [Ce(L1―3)(NO3)]2(µη2:η2-O2) (6-L1—6-L3) (Scheme 3). Bubbling dioxygen into the

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solution of 3-L2 in CH3CN resulted in a rapid color change from yellow to purple, and the corresponding peroxo complex 6-L2 was isolated as purple microcrystalline solids in 40% yield. The UV-Vis spectrum of 6-L2 showed characteristic absorption at 540 nm, corresponding to those found for the dinuclear cerium(IV) µ-peroxo complex 6-L1.17 The same experimental procedure was applied to the complexes 3-L1 and 3-L3 to prepare the corresponding peroxo complexes 6-L1 and 6-L3 in 86% and 44% yields, respectively, though 6-L1 was already prepared by the reaction of a neutral cerium(III) complex 2 with O2 in CH3CN and crystallographically characterized.17 The UV-Vis spectrum of 6-L3 was almost the same as that of 6-L1 and 6-L2, displaying characteristic absorptions at 331 nm and 515 nm.

(1.33―1.51 Å).17,21 By changing the oxidation state of the cerium center from +3 to +4, the distances of the Ce—O(phenoxide) bonds are shortened (2.17 Å, ca. 0.11－0.14 Å) compared with the anionic Ce(III) complexes 3-L1 and 3-L3, which is consistent with the two-electron reduction of O2 to form a -2:2peroxo structure. We measured the velocity for the conversion of 3-L1 under atmospheric pressure of dioxygen to 6-L1 at -40 °C in CH3CN, and Figure 5 shows the time-dependent UV-Vis spectra for 3L1. The absorption around 365 nm for 3-L1 rapidly decreased with an increase in new absorptions at 340 and 530 nm due to 6-L1, where two isosbestic points at 354 and 399 nm were detected during the transition. The reaction rate constant (kobs) for the formation of 6-L1 was 7.58 x 10-4 [s-1].

3500 0.35

12000 1.2

Scheme 3. O2 Activation by Cerium(III) Complexes 3-L1―3L3 to form Peroxo-bridged Dinuclear Ce(IV) Complexes 6L1―6-L3.

3000 0.3

ε

10000 1

ε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8000 0.8

2500 0.25

6000 0.6

2000 0.2

y = 7.58E-04x + 1.60E-01 R² = 0.984 70

4000 0.4

120

170

220

time [s]

2000 0.2 0 300

400

500 600 wavelength (nm)

700

Figure 5. Time-dependent UV-Vis spectra upon bubbling O2 to 3-L1 solution (0.1 mM in CH3CN) at -40 °C (kobs = 7.58 x 10-4 [s-1]). The UV transition was recorded in every 20 sec. The formation rate of 6-L1 from 3-L1 was calculated based on the absorption at λ = 530 nm.

Figure 4. Molecular structure of complex 6-L2 with 50% probability ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (degree): Ce—N1 2.637(7), Ce—N2 2.620(7), Ce—N3 2.541(7), Ce—O1 2.172(6), Ce—O2 2.175(5), Ce—O3 2.655(6), Ce—O4 2.541(6), Ce—O6 2.339(3), Ce—O7 2.288(2), O6—O7 1.484(10); N1—Ce—N2 62.5(2), N2—Ce—N3 65.4(3), O1— Ce—O2 95.5(2), O1—Ce—N1 69.5(2), O2—Ce—N3 71.4(2), O3—Ce—O4 48.71(19), Ce1―O6―Ce1* 138.9(4), Ce1―O7―Ce1* 146.3(4). The crystal structure of complex 6-L2 is shown in Figure 4. Two cerium centers are bridged by side-on bounded peroxo ligand to form a dinuclear structure. Bond distances around the cerium center are typical for the cerium(IV) complexes. The distances of the Ce(IV)―O(peroxo) bonds (2.29―2.34 Å) around the µ-peroxo moiety are in good accordance with those reported for 6-L1 and other dinuclear Ce(IV) µ-peroxo complexes (2.24―2.38 Å).17,21 The distance of the O―O(peroxo) bond (1.48 Å) is almost the same to 6-L1, and is long enough to be characterized as an O—O single bond that is typically found in other dinuclear µ-η2:η2-peroxo complexes of cerium

Peroxo-bridged dicerium complex 6-L1 was further converted to the oxo-bridged dicerium complex 7-L1 upon heating the CH3CN solution of 6-L1 at 70 °C. In fact, heating the reaction mixture of the peroxo complex 6-L1 with or without 4methylbenzyl alcohol produced the corresponding oxo-bridged dinuclear cerium(IV) complex, [Ce(L1)(NO3)]2(µ-O) (7-L1) (Scheme 4), which were characterized by spectroscopic methods and crystallographic analysis (vide infra), though the detailed reaction mechanism for the conversion from 6-L1 to 7-L1 was not clear. No transformation from 6-L1 to 7-L1 was observed at room temperature. In the 1H NMR spectrum of 7-L1, a singlet resonance for the imine protons of the Schiff-base ligand was observed at  7.98, while two singlet signals due to the phenoxy moieties were detected at  7.60 and 7.10, suggesting a symmetric structure in solution. In addition, we found two singlet signals assignable to the tert-butyl groups at the orthoand para-positions of the phenoxy rings. All the signals for 7L1 were detected in the region typical for diamagnetic complexes, indicating the formation of dinuclear Ce(IV)—Ce(IV) species. A cyclic voltammogram of oxo-bridged dinuclear Ce(IV) complex 7-L1 showed one reversible oxidation wave at -0.273 V(vs Fc) assignable to the redox behavior of Ce(IV) and Ce(III).19 Figure 6 shows the molecular structure of 7-L1, in which each cerium center adopts a distorted 8 coordination geometry and each cerium unit correlates with a C2 operation. The distance of the Ce–O(µ-oxo) bond (2.08 Å) is typical for Ce(IV)–O(µ-

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17000

5000

15000

y = -5.03x + 4660 R² = 0.998

4500

13000

ε

oxo) (2.07―2.18 Å) bonds.20 The Ce―O―Ce angle (169°) is more acute than that in the Ce(III) μ-oxo dinuclear complex [CeCp*2(thf)]2(μ-O) (Cp* = η5-C5Me5) (176°),22 probably due to the steric repulsion of the pentadentate ligand. Dinuclear Ce(IV) μ-oxo complexes are uncommon, although bis(µoxo)dinuclear complexes as well as polynuclear oxo bridged cerium complexes were previously reported.21-23 The distances of the Ce–O(phenoxide) bond (2.20―2.21 Å) are slightly elongated compared with those of peroxo-bridged dinuclear Ce(IV) complex 6-L1, probably due to the steric repulsion between the supporting ligands. The distance of the Ce–O(nitrate) bond (2.53 Å) is reasonable compared with that of reported Ce(IV) complexes, such as complex 1 (2.51―2.52 Å).17

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Figure 7. Time-dependent UV-Vis spectra of 6-L1 to 7-L1 (0.2 mM in CH3CN) at 70 °C (kobs = 6.41 x 10-3 [s-1]). The UV transition was recorded in every 30 seconds. The formation rate of 7-L1 from 6-L1 were calculated based on the absorption at λ = 580 nm. Scheme 4. Synthesis of Oxo-bridged Dinuclear Ce(IV) Complex 7-L1.

Figure 6. Molecular structure of complex 7-L1 with 50% probability ellipsoids. All hydrogen atoms and methyl groups of tBu substituents are omitted for clarity. Selected bond distances (Å) and angles (degree); Ce—N1 2.621(3), Ce—N2 2.606(3), Ce— N3 2.600(3), Ce—O1 2.211(2), Ce—O2 2.196(2), Ce—O3 2.525(2), Ce—O4 2.531(2), Ce—O6 2.0804(4); N1—Ce—N2 62.56(9), N2—Ce—N3 65.55(9), O1—Ce—O2 93.87(8), O1—Ce—N1 71.20(8), O2—Ce—N3 69.96(7). O3—Ce—O4 50.38(9), Ce1―O6―Ce1* 168.99(16). Thermolysis of 6-L1 was monitored by UV-Vis spectra for determining the formation rate of the corresponding oxobridged complexes 7-L1 (Figure 7).17 Absorption for peroxo complex 6-L1 gradually decreased with the appearance of new absorptions at λmax = 331 and 486 nm. During this transition, we found an isosbestic point at 491 nm. Formation of 7-L1 from 6-L1 was much slower than the peroxo formation from 3-L1 and required a higher temperature. We further checked the effects of O2 and benzyl alcohol for the thermolysis of 6-L1 to 7-L1:17 no rate accelerations were observed for the transition, suggesting that interactions of O2 and benzyl alcohol with cerium were not involved in the oxo-bridged formation.

We tested both of peroxo- and oxo-bridged dinuclear Ce(IV) complexes 6-L1 and 7-L1 for the catalytic alcohol oxidation reaction under the optimal conditions (Scheme 5). We found that the complexes 6-L1 and 7-L1 showed catalytic activity for oxidation of 4a, giving 5a in 88% and 85% yield, respectively, suggesting that 3-L1 was readily converted to 7-L1 via 6-L1 upon exposure to dioxygen. Thus, 7-L1 was an actual catalyst precursor of the catalytic cycle (vide infra).

Scheme 5. Catalytic Activity of Complexes 6-L1 and 7-L1: 4a (0.200 mmol), [Ce] (0.0050 mmol), MS 4Å (40 mg), 0.5 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an internal standard. 2-5. Kinetic Study of the Alcohol Oxidation by Oxo-bridged Dinuclear Ce(IV) Complex 7-L1 We investigated the catalytic reaction profile for the oxidation of 4a by oxo-bridged dinuclear Ce(IV) complex 7-L1 to clarify the dependence of the catalyst concentration. Under O 2 at atmospheric pressure, the initial reaction rate constants (kobs) were determined by monitoring the amount of 4-methylbenzaldehyde under five different catalyst concentrations. A plot of the kobs showed a linear dependence on the initial concentration of 7-L1 in the range of 1.9—7.4 mM (Figure 8a), clearly indicating that the reaction was first-order for catalyst 7-L1. Thus, during the reaction, the oxo-bridged dinuclear structure was kept intact without any dissociation into the monomeric form. In addition, the kobs was linearly fitted to the initial concentration of 4methylbenzyl alcohol in the range of 2.0―6.0 M (Figure 8b). Thus, the reaction rate was first-order in both of [7-L1] and [alcohol]; a rate law obeying kobs[7-L1][alcohol].

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kobs [min-1]

3.00E-04

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2.50E-04

2.00E-04 1.50E-04 y = 2.80E-05x + 6.58E-05 R² = 0.997

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(b)

1.60E-02 1.20E-02 8.00E-03 y = 2.71E-03x - 3.21E-04 R² = 0.995

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Figure 8. Reaction rate dependence on (a) the catalyst concentration and (b) the alcohol concentration. 2-6. Reaction Mechanism Scheme 6 shows a plausible mechanism for the co-catalyst-free cerium-catalyzed alcohol oxidation reaction starting from the oxo-bridged dinuclear Ce(IV) complex 7-L1, generated from 3L1 and O2 (vide supra). It is assumed that complex 7-L1 reacts with 4a to form a dinuclear Ce(IV) benzylalkoxy complex A with a release of HNO3 because of the 1st order dependence on the concentration of 7-L1 for the catalytic reaction as well as the stability of the oxo-bridged dinuclear skeleton in 7-L1 when heated in the presence of 4a under Ar atmosphere. In the presence of external radical sources such as TEMPO, abstraction of H-atom at the -position to the oxygen atom proceeds by the radical, as our observation for Ce(IV)/TEMPO/O2-catalyzed alcohol oxidation.17 In the absence of TEMPO, formation of the phenoxide radical in the ligand framework is indispensable for the catalytic transformation, similar to the Galactose oxidase.13 Thus, in this benzylalkoxy-bound dinuclear Ce(IV) species A, one of the two cerium centers accepts one-electron to be reduced to Ce(III) with a phenoxide radical at the Schiff-base lig-

and while the other cerium forms (bridging-oxo)(nitro)cerium(IV) to generate A’. Dissociation of the oxo-bridged dinuclear structure to the monomeric form during the redox process was excluded due to the reversible wave of the complex 7-L1 in the cyclic voltammogram.19 The next step is that the phenoxide radical abstracts an -hydrogen atom from the benzylalkoxy moiety bound to the cerium(III) atom to form benzylalkoxycerium(III) species B with a carbon radical  to the oxygen atom. Subsequent homolysis of the Ce—O(alkoxy) bond in species B proceeds to afford benzaldehyde with one-electron reduction of the oxo-bridged dicerium center, producing [Ce(III)]——[Ce(III)]+ dinuclear intermediate C. Thus, the oxobridged dinuclear motif plays an important role for the electronic communication between the two cerium centers in cocatalyst free alcohol oxidation. Further reaction of zwitterionic complex C with dioxygen generates a hydroperoxo species of Ce(IV)―Ce(III) D by a concerted one-electron reduction of O2 and hydrogen abstraction from the phenol moiety of the ligand. Such the phenoxide radical-assisted H-atom transfer from the alkoxymetal species to O2 was reported for co-catalyst-free alcohol oxidation by copper(II) and zinc(II) complexes with Schiff-base ligands.12 Intermediate A’ is regenerated by an exchange reaction of the hydroperoxy ligand in D and 4a, giving hydrogenperoxide. In this catalytic cycle, the rate-determining step is expected to be an H-atom abstraction by the ligand phenoxide radical, based on the KIE value (2.08) for the oxidation of PhCH2OH/PhCD2OH.19 During the catalytic reaction, it is assumed that MS 4Å functions for not only decomposing the in situ-generated hydrogen peroxide as a heterogeneous catalyst24 but also trapping the in situ-generated HNO3 as a weak base to prevent a reverse reaction from A to 7-L1. 3. CONCLUSION We herein report a TEMPO-free cerium complex-catalyzed oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, as well as the synthesis of a series of cerium catalyst precursors 3-L1 ― 3-L9 bearing a pentadentate Schiff-base ligand with electron-donating and –withdrawing substituents on the phenoxy ring. In relation to the mechanistic study, we successfully isolated oxo-bridged dicerium complex

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7-L1 upon exposure of 3-L1 to dioxygen and further heating, as a pre-catalyst for the alcohol oxidation, and proposed oxobridged dicerium alkoxy complex A as the key intermediate. To the best of our knowledge, this is the first example of the oxobridged dinuclear cerium complex that worked as a catalyst for the aerobic oxidation of alcohols while maintaining its dinuclear skeleton, in which an electronic communication through the bridging oxygen atom is a key step. Further exploitation of the oxo-bridged dinuclear complexes to catalyze other reactions is ongoing in our laboratory.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org. All synthetic procedures, experimental details, spectral characterization, kinetic procedures, data and additional discussion (PDF) Structure data for 3-L1 (CIF) Structure data for 3-L3 (CIF) Structure data for 3-L4 (CIF) Structure data for 6-L2 (CIF) Structure data for 7-L1 (CIF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected] ORCID Satoru Shirase: 0000-0002-6932-085X Koichi Shinohara: 0000-0003-1622-0756 Hayato Tsurugi: 0000-0003-2492-7705 Kazushi Mashima: 0000-0002-1316-2995

Funding Sources JSPS KAKENHI Grant Nos. JP15KT0064 (Grant-in-Aid for Scientific Research on Innovative Areas (B)) and JP15H05808 (Precisely Designed Catalysis with Customized Scaffolding).

ACKNOWLEDGMENT S. S. thanks the financial support by the JSPS Research Fellowships for Young Scientists. This work was supported by JSPS KAKENHI Grant Nos. JP15KT0064 (Grant-in-Aid for Scientific Research on Innovative Areas (B)) to H. T. and JP15H05808 (Precisely Designed Catalysis with Customized Scaffolding) to K. M. We thank Prof. Dr. Sonja Herres-Pawlis in RWTH Aachen University for a fruitful discussion.

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