Photocatalytic Oxygenation of Cyclohexene Initiated by Excitation of

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Photocatalytic Oxygenation of Cyclohexene Initiated by Excitation of [UO2(OPCyPh2)4]2+ under Visible Light Takanori Mashita,† Satoru Tsushima,‡,§ and Koichiro Takao*,† †

Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1-N1-32, O-okayama, Meguro-ku, 152-8550 Tokyo, Japan ‡ Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, 152-8550 Tokyo, Japan § Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf e.V., Bautzner Landstraße 400, 01328 Dresden, Germany

ACS Omega 2019.4:7194-7199. Downloaded from pubs.acs.org by 193.56.67.151 on 04/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Oxygenation reaction of cyclohexene was studied under the presence of a fourfold UO 2 2+ complex with cyclohexyldiphenylphosphine oxide, [UO2(OPCyPh2)4]2+, and blue light irradiation at 436 nm in acetonitrile. As a result, 1,6-hexanedial, cyclohexene oxide, 2-cyclohexen-1-one, and 2-cyclohexen-1-ol were photocatalytically generated as oxygenated products with turnover frequency = 6.7 h−1. In contrast, dimerization of cyclohexene was observed under Ar atmosphere. This implies that a hydrogen atom at the allyl position is abstracted by photoexcited *[UO2(OPCyPh2)4]2+ to give a cyclohexene radical and a U(V) intermediate [UVO2(OPCyPh2)4]+, being well supported by the density functional theory calculation. Under an O2 atmosphere, the former reacts with dissolved O2 to give 2-cyclohexen-1-one and 2-cyclohexen-1ol. Dissolved O2 would be activated by the U(V) intermediate to afford O22− in the end, which drives oxygenation of the CC bond of unreacted cyclohexene.

1. INTRODUCTION Uranium is the most important element in nuclear engineering. Nuclear fuels for the current light water reactors always require enrichment of 235U, the naturally occurring fissile isotope of uranium, because its natural abundance is only 0.72%.1 Up to date, the rest of uranium is simply deposited as waste even after refinement because of its nonfissile nature. In contrast, chemistry of uranium is practically independent of its isotope ratio. Therefore, exploration of novel chemical uses of such uranium resources can be expected.2,3 Recently, sophisticated chemical reactivity of uranyl(VI), UO22+, is focused to explore its potentials in chemical applications.3−6 It is also well known that photoexcited UO22+ is strongly oxidizing.7,8 Furthermore, UO22+−peroxo complexes are frequently reported under the irradiation of sunlight.9−12 Such peroxo complexes still have oxidizing, especially oxygenating, power arising from the O22− ligand. Hence, its application to a novel photocatalytic reaction can be anticipated. Taking into account the typical yellow color of UO22+ and its characteristic green emission, we can expect photochemical activity even under visible light irradiation. The visible-light-driven photocatalysis13,14 and activation of small molecules15−17 are of great interests in coordination chemistry of various transition metals. In the past, development of a visible light responsive photocatalysis involving uranyl(VI) has been challenged by Bakac and Mao.7 According to them, oxygenation of benzene was actually confirmed in the presence of uranyl(VI) ion and hydrogen peroxide, although efficiency of that reaction was not high enough to meet its application. © 2019 American Chemical Society

Very recently, we synthesized a homoleptic fourfold uranyl(VI) complex, [UO2(OPCyPh2)4]2+ (Chart 1), bearing Chart 1. Structure of [UO2(OPCyPh2)4]2+

well-ordered ligand arrangement with specific interligand π−π stacking, and studied its ligand exchange kinetics.18 In the present study, we took this complex as a potential photocatalyst to oxidize or oxygenate an olefin substrate. Cyclohexene was selected as a substrate here because it is a typical olefin compound photochemically stable under visible light and frequently employed for benchmarking. In this communication, we demonstrate a photocatalytic activity of [UO2(OPCyPh2)4]2+ in oxygenation reactions of cyclohexene under visible light irradiation.

2. RESULTS AND DISCUSSION Figure 1 shows progress of the photochemical reaction. The concentration of cyclohexene gradually decreased as a function Received: March 7, 2019 Accepted: April 9, 2019 Published: April 19, 2019 7194

DOI: 10.1021/acsomega.9b00635 ACS Omega 2019, 4, 7194−7199

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[UO2(OPCyPh2)4]2+ and photoirradiation, meaning that a source of oxygen in this reaction is atmospheric O2. Interestingly, bi-2-cyclohexen-1-yl (Chart 2), which can be Chart 2. Structure of Bi-2-cyclohexen-1-yl

regarded as a cyclohexene dimer, was obtained as another product under this anaerobic condition, providing an important insight to clarify the reaction mechanism as discussed later. Furthermore, despite only 3 mM loading of [UO2(OPCyPh2)4]2+, 60.2 mM cyclohexene was converted within 3 h of photoirradiation, indicating that this reaction proceeds photocatalytically. In summary, all of the experimental results clearly demonstrate that the cyclohexene oxygenation observed in Figure 1 is catalytically driven by the photoexcited [UO2(OPCyPh2)4]2+ under the visible-light irradiation at 436 nm. The turnover frequency (TOF) up to 3 h was 6.7 h−1. In order to clarify the mechanism of this photocatalytic reaction, light absorption and emission behavior of [UO2(OPCyPh2)4]2+ in CH3CN were examined. A UV−vis absorption spectrum of [UO2(OPCyPh2)4]2+ in CH3CN is shown in Figure 2a. The characteristic vibronic absorption

Figure 1. Reaction progress of photochemical oxygenation of 0.1 M cyclohexene with 3 mM [UO2(OPCyPh2)4]2+ in acetonitrile under 0.1 MPa O2 atmosphere and photoirradiation at 436 nm.

of photoirradiation time. Products detected by gas chromatography−mass spectrometry (GC−MS) were 1,6-hexanedial, cyclohexene oxide, 2-cyclohexen-1-one, and 2-cyclohexen-1-ol, as shown in Figure 1. These results indicate that the oxygenation reactions of cyclohexene proceed under the current condition. In addition, the slight difference between cyclohexene conversion and sum of yields of these products has been observed (Scheme S1, Supporting Information), suggesting that undetected products should also be present in the reaction mixture. However, such unknown species should be minor components because sum of selectivity of the detected products is up to 98.9% at 3 h (Scheme S1, Supporting Information). Among these main products, 1,6hexanedial is most predominantly generated with 58.2% selectivity and 35.2% yield. This dialdehyde is a useful precursor for the synthesis of other more valuable C6 compounds such as adipic acid.19 The yields of 2-cyclohexen-1-one and 2-cyclohexen-1-ol were almost the same at the beginning of the photochemical reaction up to 30 min, whereas the formation of 2-cyclohexen-1-one became more predominant compared with that of 2-cyclohexen-1-ol with the elapse of time. Cyclohexene oxide was not detected during initial 30 min, whereas it became the second major product with 20.6% selectivity and 12.5% yield at 3 h. To confirm the role of [UO2(OPCyPh2)4]2+ in the photochemical reaction of Figure 1, several blank tests were performed. In the absence of [UO2(OPCyPh2)4]2+, no reactions have been observed and cyclohexene remained unchanged even after 3 h under photoirradiation. This indicates that photodegradation of cyclohexene is negligible and that [UO2(OPCyPh2)4]2+ is mandatory to be present to promote the oxygenation of the substrate. We also confirmed that OPCyPh2 does not exhibit any remarkable absorption at >300 nm as shown in Figure S2 (Supporting Information), implying that it is hard to expect any photochemical reactivity of OPCyPh2 under photoirradiation at 436 nm. Even under the presence of [UO2(OPCyPh2)4]2+, cyclohexene was not converted in the dark. These results imply that this cyclohexene oxygenation is initiated by photoexcitation of [UO2(OPCyPh2)4]2+. Finally, under 0.1 MPa Ar atmosphere, the above-mentioned oxygenated products shown in Figure 1 were not observed despite the presence of

Figure 2. UV−vis absorption spectrum of [UO2(OPCyPh2)4]2+ in CH 3 CN (a) and emission spectra (λ ex at 300 nm) of [UO2(OPCyPh2)4]2+ (1.0 × 10−4 M) in CH3CN under absence (blue) and presence of 0.1 M cyclohexene (red) (b).

attributed to ligand-to-metal charge transfer (LMCT) in UO22+ was observed in the range from 360 to 510 nm.20 Emission spectra of [UO2(OPCyPh2)4]2+ are shown in Figure 2b. Although [UO2(OPCyPh2)4]2+ should be excited at 436 nm to simulate the reaction system of Figure 1, strong Raman scattering from the solvent remarkably disturbs the emission spectroscopy (Figure S3, Supporting Information). For this reason, we selected 300 nm excitation, where there is no absorption of cyclohexene and the solvent. [UO2(OPCyPh2)4]2+ (1.0 × 10−4 M) shows its characteristic emission spectrum with the vibronic structures at 480−600 nm. These signals attributed to the light emission from the 3 LMCT excited state of UO 2 2+ . Even at the higher concentration of [UO2(OPCyPh2)4]2+ (3 mM), which is the same with the actual photoreaction system of Figure 1, the similar light emission has also been observed, as shown in Figure S4a (Supporting Information), whereas the vibronic emission spectrum is significantly distorted by the concen7195


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reaction kinetics.18 However, the identical products shown in Figure 1 have also been obtained with similar selectivity in the photochemical reaction catalyzed by [UO2(OPPh3)4]2+, where no specific interactions are present in its molecular structure (Table S1, Supporting Information). Thus, the current photocatalytic reaction seems to be little affected by the intramolecular π−π stacking in [UO2(OPCyPh2)4]2+. Consequently, replacement of OPCyPh2 with OPMe3 in our DFT calculations is a proper treatment to simulate the actual experimental system under reduced computational cost. We optimized structures of the singlet ground state (I) and the lowest triplet states (II, III) of [UO2(OPMe3)4]2+− cyclohexene adducts (Figure S6, Table S2, Supporting Information). For the lowest triplet states, we considered two models before and after the hydrogen abstraction. Through the excitation from singlet to triplet, the UOax distances increase from 1.80 to 1.86 Å, which is in line with the LMCT excitation step shown in Scheme 1. In contrast, the U− O distances in the equatorial plane at the triplet state (II, 2.28−2.30 Å) are not very different from those at the singlet one (I, 2.29−2.30 Å). Any interactions between cyclohexene and Oax of UO22+ are unlikely to be significant at the singlet state I (H···O distance: 3.16 Å). In contrast, one of the allyl H atoms of cyclohexene tends to come closer to Oax at the lowest triplet state II with H···Oax distance: 2.57 Å and C−H···Oax angle: 174.1°. According to Steiner,24,25 this interaction can be assigned to the C−H···Oax hydrogen bond. Indeed, the hydrogen abstraction from the allylic carbon (state II) to Oax (state III) is energetically favorable by 28.5 kcal·mol−1. In the UO22+−alcohol adduct, the H-abstraction by excited uranyl occurred spontaneously,22 whereas it is not the case for the current [UO2(OPMe3)4]2+−cyclohexene adduct. This implies that there is certain reaction barrier even if it is exothermic. From the elongated U−OaxH distance (2.10 Å) in state III and the spin density α−β which shows localization of the unpaired electron on UOOH and on the cyclohexene residue in a roughly half-to-half fashion, it can be concluded that resulting species are [UVO(OH)(OPMe3)4]2+ and radical 3 (Chart 1). This result is similar to what we have found previously in the case of photoreduction of UO22+ by alcohols and BH3X− (X = H and CN).22,23 After the hydrogen abstraction by the photoexcited [UO2(OPMe3)4]2+, the deprotonation occurs to give [UVO2(OPMe3)4]+ (state IV) as a U(V) intermediate. This oxidation state of U is so unstable that it immediately reacts with dissolved O2 to afford O2•− and UO22+. This should be the photoinitiated O2 activation scheme in the current system. The resulting radical 3 may also readily react with dissolved O2 under the aerobic condition. The DFT calculation suggested that the reaction between 3 and O2 gives 2cyclohexen-1-one and OH• radical through the intersystem crossing from quartet to doublet in terms of total spin degeneracy of the system (Scheme S2, Supporting Information). The OH• radical is still possible to give 2-cyclohexen-1ol through the reaction with another radical 3. This reaction scheme is in line with the similar product yields of these ketone and alcohol products within the initial 30 min of Figure 1. We also have to pay attention that 2-cyclohexen-1-ol may also be reactive with the photoexcited [UO2(OPCyPh2)4]2+ in a manner similar to alcohols as we demonstrated previously.22 Therefore, this alcohol product would be consumed in the reaction system, where the expected product is 2-cyclohexen-1-

tration effect. In a 3D emission spectrum (Figure S4b), the emission is still observed under 436 nm excitation. This indicates that [UO2(OPCyPh2)]42+ can be excited even in the current reaction system of Figure 1. When 0.1 M cyclohexene was added to the sample solution of Figure 2b, the emission spectrum was significantly quenched (red line). Such a quenching phenomenon was also observed visually, as shown in Figure S5 (Supporting Information). This result indicates that energy transfer from excited *[UO2(OPCyPh2)4]2+ to cyclohexene actually proceeds to initiate the photochemical reaction observed in Figure 1. As mentioned above, the oxygenation of cyclohexene shown in Figure 1 does not proceed under Ar atmosphere. On the other hand, bi-2-cyclohexen-1-yl (Chart 2) was detected as another product, which has never occurred in the aerobic conditions. Bi-2-cyclohexen-1-yl seems to be a dimerized product of cyclohexene in which the carbon atoms at allyl positions are connected with each other. The occurrence of bi2-cyclohexen-1-yl under Ar atmosphere and the emission quenching observed in Figure 2 suggest that one of the hydrogen atoms at the allyl position of cyclohexene is abstracted through the energy transfer from excited *[UO2(OPCyPh2)4]2+ to cyclohexene. There are three hydrogen positions available to be abstracted, as shown in Chart 3. Chart 3. Structures of possible cyclohexene radicals.

Density functional theory (DFT) calculations for these cyclohexene radical isomers suggest that radical 3 is most stable compared to other isomers (radical 1: +15.03 kcal/mol, radical 2: +27.62 kcal/mol). This is in line with the stabilization of radical 3 expectable from its resonance structure, as shown in Chart 3. Previously, we studied the excited states of UO22+ in the framework of Kohn−Sham DFT as well as time-dependent DFT.21−23 We have shown that these methods are valid to study the photoreduction of UO22+ and to identify the chargetransfer states as well as reaction pathways. Here, we followed the same practice and studied the photoexcited states of [UO2(OPMe3)4]2+, which was employed as an analogue of [UO2(OPCyPh2)4]2+ to reduce computation cost, and its reaction with cyclohexene (Scheme 1). The π−π stacking interaction found in [UO2(OPCyPh2)4]2+ may also affect its photochemical reactivity as observed in the ligand exchange Scheme 1. Hydrogen Abstraction from Cyclohexene to Excited UO22+ Unit (L = OPCyPh2, OPMe3)a

a

Charge of each uranyl complex is omitted for simplicity. 7196


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the UO22+−peroxo complex seems to be actually formed in the reaction system. Considering the catalytic nature of the current system, the UO22+−peroxo complex should play an important role to convert cyclohexene to 1,6-hexanedial and cyclohexene oxide through oxygenation of the CC bond, although further details of the mechanism to give these oxygenated products have not been fully clarified yet. In the last decades, many efforts have been dedicated to the activation of small molecules including O2.16 One of the typical cases of photodriven O2 activation should be the “Pacman”type Fe(II)−porphyrin catalyst for aerobic olefin oxidation reported by Nocera and co-workers,15 where cyclohexene was photocatalytically converted to cyclohexene oxide, 2-cyclohexene-1-ol, and 2-cyclohexene-1-one with TOF = 150 h−1 under visible light irradiation. Compared with this, our system is actually less efficient in terms of TOF (=6.7 h−1). However, uniqueness of our uranyl photocatalysis is the formation of 1,6hexanedial as a main product, which has never been obtained in the systems reported so far. Most recently, Arnold and coworkers also reported photocatalytic oxidation of hydrocarbons by a uranyl−phenanthroline complex under visible light at 420 nm.31 Although mechanistic aspects have not been clarified yet, we can still expect various potentials of UO22+ as a photocatalyst driven even under visible light.

one. This should be the reason why the deviation in yields of these products has been observed at >1 h in Figure 1. Our next concern is whether a peroxo complex of UO22+ is involved in the current photocatalytic reaction. In the former time, it has been reported repeatedly that a peroxo complex is colored in orange.10,11,26,27 In the current photocatalytic reaction system, the color of the sample solution also turned from pale yellow to orange, suggesting that the UO22+−peroxo species occurs in the reaction mixture. Although we have made various attempts to crystallize this orange-colored species, crystallization of such a compound is unsuccessful up to now. Instead, UV−vis absorption spectra of the reaction mixture were recorded to clarify what happens on the parent [UO2(OPCyPh2)4]2+ under the photoirradiation. As shown in Figure S7 (Supporting Information), the absorption intensity in the UV region increased with elapse of reaction time, whereas the vibronic bands of the UO22+ unit are still detectable at 410, 425, and 440 nm even at 3 h. Most importantly, a new signal appears at 350 nm through the photoirradiation. Such an absorption band was commonly found in the sample solutions in which the UO22+−peroxo species occurs.26,27 Therefore, a UO22+−peroxo complex should also be formed in the current photocatalytic system through the photoinduced activation of the dissolved O2 discussed above. Although the reported UO22+−peroxo complexes exhibit range of nuclearities either monomeric or polymeric,11,12,26−30 it is hard to know further details in the current system.

4. EXPERIMENTAL SECTION [UO2(OPCyPh2)4](ClO4)2·EtOH was prepared as we reported previously.18 At 298 K, an acetonitrile solution (2 mL, Kanto) dissolving [UO2(OPCyPh2)4]2+ (3 mM) and cyclohexene (100 mM, TCI) in a Pyrex glass test tube with 0.1 MPa O2 atmosphere was subjected to photoirradiation at 436 nm from the 250 W ultrahigh pressure mercury lamp (REX-250, Asahi Spectra). Reaction products and residual cyclohexene were identified and determined by GC−MS (GCMS-QP2010, SHIMADZU). Further experimental details are described in the Supporting Information together with the photograph of experimental setup (Figure S1, Supporting Information). Quantum chemical calculations were performed using Gaussian 16 program32 employing the DFT by using a conductor like polarizable continuum model.33 Structure optimizations were performed at the B3LYP level.34,35 In case stacking interaction becomes important, calculations were performed at the B97D3 level which include empirical dispersion corrections (D3 version of Grimme’s dispersion with the original D3 damping function).36 The energyconsistent small-core effective core potential (ECP) and the corresponding basis set suggested by Küchle et al. were used for U,37 whereas large-core ECP was used on C, N, and O.38 The most diffuse basis functions on uranium with the exponents 0.005 (all s, p, d, and f type functions) were omitted. These basis functions had a very small effect on the reaction energies (less than 1 kJ mol−1). For O, we added one d function to the basis set. For H, the 6-31G basis set was used. The spin−orbit effects and basis set superposition error corrections were neglected. All of the calculations were performed on a TSUBAME 3.0 supercomputing system at the Tokyo Institute of Technology.

3. CONCLUSIONS In summary, the most plausible reaction mechanism for the photocatalytic oxygenation of cyclohexene by [UO2(OPCyPh2)4]2+ is summarized in Scheme 2. At the first Scheme 2. Photocatalytic Reaction Mechanism of the Oxidation of Cyclohexene over [UO2(OPCyPh2)4]2+

step, [UO2(OPCyPh2)4]2+ is photoexcited by the visible light irradiation at 436 nm. The triplet excited state of *[UO2(OPCyPh2)4]2+ is abstracting the hydrogen atom at the allyl position of cyclohexene to give [U V O(OH)(OPCyPh2)4]2+ as well as the cyclohexene radical 3. After the deprotonation, the [UVO2(OPCyPh2)4]+ intermediate readily activates dissolved O2. Although the details of the further reduction of O2•− to O22− has not been clarified yet,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00635. 7197


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Experimental details with photograph, absorption spectrum of OPCyPh 2 , emission spectra of [UO2(OPCyPh2)4]2+ under different conditions; selectivity of main products in Figure 1 at 3 h; photographs of luminescence quenching with addition of cyclohexene, optimized structures, and selected bond lengths of [UO2(OPMe3)4]2+−cyclohexene adducts; results of cyclohexene oxygenation catalyzed by photoexcited [UO2(L)4]2+ (L = OPCyPh2 and OPPh3); and UV− vis absorption spectra of the sample solution of Figure 1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81 3 5734 2968. ORCID

Satoru Tsushima: 0000-0002-4520-6147 Koichiro Takao: 0000-0002-0952-1334 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Tokyo Tech World Research Hub Initiative (WRHI) Program of Institute of Innovative Research, Tokyo Institute of Technology.

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REFERENCES

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