Isshin
Yoshida
a,
Yuji
Kikukawa
*a,
Ryoji
Mitsuhashi
b and
Yoshihito
Hayashi
a
aDepartment of Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan. E-mail: kikukawa@se.kanazawa-u.ac.jp
bInstitute of Liberal Arts and Science, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
First published on 1st May 2024
Clarification and control of the active sites at the atomic/molecular level are important to develop nanocatalysts. The catalytic performance of two oxidation states of nitrate-incorporating octadecavanadates, [V18O46(NO3)]5− (V18) and [V18O46(NO3)]4− (V18ox), and a copper-substituted one, [Cu2V16O44(NO3)]5− (Cu2V16), in selective oxidation was investigated. Both V18 and V18ox possessed the same double-helical structures and one of two tetravalent vanadium sites of V18 was oxidized in V18ox. The comparison of the mobility of the incorporated nitrate reveals that tetravalent vanadium centres show stronger interaction with the incorporated anions than pentavalent ones. The oxidation reaction with V18ox proceeded more smoothly with tert-BuOOH as an oxidant than that with V18. The reactivity and selectivity of the oxidation of 2-cyclohexen-1-ol were different among the derivatives. V18ox showed the higher reactivity with 72% selectivity to epoxide. With V18, reactivity was lower but higher selectivity to epoxide was achieved. In the presence of Cu2V16, 2-cyclohexen-1-one was selectively obtained with 81% selectivity. The order of the reactivity for cyclooctene was V18ox, V18 and Cu2V16. These results shows that the cap part of the double-helix acts as the active site. Even though the vanadium–oxygen species exhibit the same structures, the catalytic properties can be controlled by changing the valence of vanadium and metal substitution.
Nitrate-incorporating octadecavanadate [V18O46(NO3)]5− (V18) possesses helicity.21 The original procedure for synthesis of V18 involved the oxidation of dark purple decavanadate [V10O26]4− in nitromethane in the presence of nitrate. Recently, the Streb group reported a new precursor by heating yellow decavanadate [H3V10O28]3− and reaction with nitrate to yield V18.22 Generally, the structures of polyoxovanadates vary under slightly different synthetic conditions. The fact that the V18 structure is obtained with the different procedures means that V18 can be considered to be one of the stable polyoxovanadates. The charge of V18 is 5−, showing the presence of two V4+ ions. From the bond valence sum (BVS) values, the V4+ sites are localized on the top and bottom parts of the helix. From the cyclic voltammetry, V18 shows multi-step reversible redox properties. The stability, the localized valence, and the redox properties led us to investigate the effect of the valence of the vanadium centre of the cap parts on the oxidation catalytic properties of the V18 helix. In addition, we recently reported a derivative of V18, dicopper-substituted polyoxovanadate (Cu2V16) and its catalytic activity for alcohol oxidation (Fig. 1).23 In this work, preparation of the oxidized species of V18 (V18ox) and reactivity of the local vanadium site are investigated with a comparison of the catalytic performance among the derivatives.
With reference to the electrochemical investigation, synthesis of the oxidized V18 by a chemical reaction was investigated. In the previous report, V18 was synthesized in nitroethane. During the reaction, the synthetic solution colour changed from purple to brown and finally to green, indicating that the reaction contains oxidation and reduction processes of the polyoxovanadate.21 Without addition of NO3−, V18 was also obtained by the reaction of [V10O26]4− and tert-BuOOH (TBHP) in nitromethane at 85 °C for 2 days. Its IR spectrum showed the typical peaks at 1342 and 1359 cm−1 due to the incorporated NO3− in V18, suggesting that nitromethane is oxidized to form NO3− during the reaction (Fig. S3†). Through the experimental facts, we made the hypothesis that the over oxidized product was reduced by the nitroalkane in the previous report, and that without the nitroalkane, oxidized products are prepared. To obtain the oxidized products, we selected acetonitrile as the solvent. As we expected, the brown colour of the synthetic solution was maintained. Upon the addition of diethyl ether, a brown product was obtained.
Single crystals suitable for X-ray crystallographic analysis were obtained from the mixed solvent of acetonitrile and ethyl acetate in the presence of 4 equiv. of {Et4N}BF4 (Fig. 1 and Table S1†). Two tetra-n-butylammonium (TBA) and two tetraethylammonium (TEA) cations per one polyoxovanadate were observed. The total number of cations was one less than for the original V18. The anion was isostructural with the original V18. These results showed the formation of one-electron-oxidized products (V18ox). The double helical structure was capped by two vertex-sharing VO5 units. The distances between the capping vanadium atoms of V18ox and V18 were 8.49 and 8.55 Å, respectively. The angles of the inserted nitrate triangles from the line between the capping vanadium atoms of V18ox and V18 are 5° and 12°, respectively (Fig. S4†). These results show the inside volume of the sphere of the V18ox is smaller than that of V18. The bond lengths between the capping vanadium atoms, V1 and V2, and the surrounding oxygen atoms in V18ox are 1.851(3), 1.873(3), and 1.584(4) Å, and 1.904(3), 1.925(3), and 1.597(4) Å, respectively. The bond valence sum values of V1 and V2 are 5.22 and 4.23, respectively, showing that one of the capped vanadium atoms is oxidized.29,30
Thermogravimetry analysis of TBA salts of V18ox and V18 showed no weight loss under 200 °C except for that of hydrated water (Fig. S5†), suggesting their stability below 200 °C. The IR spectrum of V18ox synthesized by the chemical reaction was the same as that obtained by the electrochemical method. It shows the typical split peaks at 1342 and 1360 cm−1 due to the incorporated nitrate (Fig. S6†). The bond length differences of the cap parts led to the different intensities of each peak. The temperature-dependent IR spectra of TBA salts of V18 and V18ox were measured under vacuum conditions (Fig. 3 and Fig. S7†). Although no shifts of the peaks due to nitrate in V18 were observed even at 200 °C, the peak at 1342 cm−1 due to nitrate in V18ox at room temperature was shifted to 1346 cm−1 at 200 °C. After cooling the original peak position was retrieved. With increasing the temperature, the spectra in the region between 700 and 900 cm−1 due to the polyoxovanadate frameworks were slightly changed, and the original spectra were recovered after cooling. These results indicated the higher mobility of the incorporated nitrate in V18ox with the distortion of the polyoxovanadate framework than that in V18. Recently we reported the mobility of the incorporated nitrate in Cu2V16 below room temperature.18 Although the inside cavity of Cu2V16 was smaller than that of V18, the incorporated nitrate of Cu2V16 showed smoother mobility. Among the derivatives, the helical body was identical. Therefore, the mobility of the nitrate strongly depended on the cap parts. Tetravalent vanadium centres showed stronger interactions with the incorporated anion than pentavalent ones.
The UV/vis spectrum of V18ox showed a halved intensity of the band around 600–900 nm due to IVCT of V18ox in comparison with that of V18, matching the V18ox obtained by the electrochemical method (Fig. 2). Upon addition of {n-Bu4N}I into the solution of V18ox, the intensity in the IVCT region increased and in the presence of 2.25 equivalents of {n-Bu4N}I with respect to V18ox, the intensity at 900 nm was identical to that of V18, showing the reduction of V18ox to give V18 (Fig. S8†).
Entry | Substrate | Catalyst | Time/min | Sulfoxide yielda/% | Sulfone yielda/% |
---|---|---|---|---|---|
The typical catalytic reaction conditions: substrate (1 mmol), TBA salts of catalysts (5 μmol), 5.5 M decane solution of TBHP (1 mmol), acetonitrile (2 mL), internal standard (0.2 mmol), 32 °C, 800 rpm with a Teflon-coated magnetic stirrer bar, under Ar. Yields were determined by GC with the internal standard method and/or 1H NMR (see the ESI†). The reaction was carried out in a screw-top test tube.a Yields were determined by GC with naphthalene as an internal standard.b Yields were determined by GC with naphthalene as an internal standard and 1H NMR.c Yields were determined by 1H NMR. | |||||
1 | V18ox | 10 | 91 | 1 | |
2 | V18 | 10 | 25 | 1 | |
3 | 60 | 89 | 2 | ||
4 | [V10O26]4− | 60 | 47 | 9 | |
5 | [NO3]− | 60 | 19 | 3 | |
6 | Cu2V16 | 10 | 34 | 2 | |
7 | 60 | 88 | 2 | ||
8 | — | 60 | 8 | — | |
9b | V18ox | 2 | 89 | 1 | |
10b | V18 | 2 | 3 | — | |
11b | V18 | 120 | 91 | 2 | |
12 | V18ox | 5 | 82 | 9 | |
13 | V18 | 5 | 9 | — | |
14 | 120 | 86 | 5 | ||
15b | V18ox | 10 | 76 | 1 | |
16b | V18 | 10 | 10 | — | |
17b | 120 | 76 | 5 | ||
18c | V18ox | 15 | 93 | 3 | |
19c | V18 | 120 | 93 | 3 | |
20c | V18ox | 15 | 97 | 1 | |
21c | V18 | 120 | 90 | 2 | |
22 | V18ox | 3 | 73 | 7 | |
23 | V18 | 3 | 31 | 4 | |
24 | 10 | 83 | 4 | ||
25 | V18ox | 10 | 93 | 2 | |
26 | V18 | 10 | 64 | 1 | |
27 | 15 | 90 | 3 | ||
28 | V18ox | 5 | 94 | 3 | |
29 | V18 | 5 | 18 | 1 | |
30 | 20 | 81 | 3 | ||
31 | V18ox | 5 | 98 | 1 | |
32 | V18 | 5 | 33 | 8 | |
33 | 20 | 94 | 2 |
In the presence of V18 and V18ox, the oxidation of various kinds of sulfides with TBHP proceeded to give the corresponding sulfoxides and sulfones. In all cases, the reaction with V18ox proceeded more efficiently than that with V18. Aromatic sulfides with electron-donating groups required short reaction times. Phenyl vinyl sulfide gave the corresponding sulfoxide and sulfone with retention of the CC double bond. The small difference in the reactivity of diphenyl sulfide between V18 and V18ox indicates that the steric hinderance of the two phenyl groups prevents the substrate from approaching the active site of the cap part of V18ox. The oxidation of cyclic and linear aliphatic sulfides also proceeded to afford the corresponding sulfoxides.
Next, oxidation of alcohols was investigated. For the oxidation of 1-phenyl ethanol, the yield of acetophenone after 24 h with V18ox was 50% (Scheme 1). Cu2V16 showed a higher catalytic performance than V18ox and the reaction hardly proceeded in the presence of V18. The order of the catalytic activity for the alcohol oxidation was different from that of sulfide oxidation.
Scheme 1 Oxidation of 1-phenyl ethanol. Reaction time was 24 h. Naphthalene was used as an internal standard. The typical catalytic reaction conditions: substrate (1 mmol), TBA salts of catalysts (5 μmol), 5.5 M decane solution of TBHP (1 mmol), acetonitrile (2 mL), internal standard (0.2 mmol), 32 °C, 800 rpm with a Teflon-coated magnetic stirrer bar, under Ar. Yields were determined by GC with the internal standard method and/or 1H NMR (see the ESI†). The reaction was carried out in a screw-top test tube. |
Oxidation of 2-cyclohexen-1-ol, a kind of allylic alcohol possessing two functional groups (CC, and OH) which can be oxidized, was investigated (Scheme 2). With V18ox, the total yield of the products reached 76% in 24 h. The ketone to epoxide selectivity was 28:72. Although the total yield with V18 was lower under the same conditions, the selectivity toward epoxidation was higher than that with V18ox. In the presence of Cu2V16, selective alcohol oxidation proceeded. With the immobilization of metal cations into the polyoxometalate frameworks, the preference of the selectivity changed dramatically.31,32
Scheme 2 Oxidation of 2-cyclohexen-1-ol. Reaction time was 24 h. Chlorobenzene was used as an internal standard. The typical catalytic reaction conditions: substrate (1 mmol), TBA salts of catalysts (5 μmol), 5.5 M decane solution of TBHP (1 mmol), acetonitrile (2 mL), internal standard (0.2 mmol), 32 °C, 800 rpm with a Teflon-coated magnetic stirrer bar, under Ar. Yields were determined by GC with the internal standard method and/or 1H NMR (see the ESI†). The reaction was carried out in a screw-top test tube. |
In the case of the epoxidation of cyclooctene, the order of the reactivity was V18ox, V18 and Cu2V16 (Scheme 3).
Scheme 3 Epoxidation of cyclooctene. Naphthalene was used as an internal standard. The typical catalytic reaction conditions: substrate (1 mmol), TBA salts of catalysts (5 μmol), 5.5 M decane solution of TBHP (1 mmol), acetonitrile (2 mL), internal standard (0.2 mmol), 32 °C, 800 rpm with a Teflon-coated magnetic stirrer bar, under Ar. Yields were determined by GC with the internal standard method and/or 1H NMR (see the ESI†). The reaction was carried out in a screw-top test tube. |
The comparison of oxidation catalytic activity among the derivatives revealed that the changing of the oxidation state or the constituent metal cation provided control of the reactivity.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2341040. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nr01243g |
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