Shun
Hayashi
a,
Seiji
Yamazoe
ab,
Kiichirou
Koyasu
ab and
Tatsuya
Tsukuda
*ab
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: tsukuda@chem.s.u-tokyo.ac.jp
bElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan
First published on 2nd February 2016
The base catalytic activity of the decaniobate cluster (TMA)6[Nb10O28]·6H2O (TMA+ = tetramethylammonium cation) was studied theoretically and experimentally. Density functional theory calculations showed that the oxygen atoms in the cluster are highly negatively charged and suggested the possibility that the cluster can act as a base catalyst. We demonstrated for the first time that [Nb10O28]6− actually exhibits base catalytic activity for aldol-type condensation reactions including Knoevenagel and Claisen–Schmidt condensation reactions. The catalytic reactions proceeded under ambient conditions, suggesting that [Nb10O28]6− holds promise as a practical strong base catalyst.
Our simple working hypothesis is that POM having more electronegative oxygen will exhibit stronger base catalysis. The −n/y value of [MxOy]n− represents a negative charge averaged over the oxygen atoms without assuming electronic charge transfer from metal to oxygen and thus provides a lower limit of the negativity of the oxygen atom. On the assumption that the −n/y value gives a measure of the basicity of the POMs, group V POMs (M = V, Nb, Ta) are more promising than group VI POM (M = Mo, W) for base catalysts. For example, polyoxoniobates (PONbs)10,11 such as hexaniobate ([Nb6O19]8−) and decaniobate ([Nb10O28]6−) cluster have more negative −n/y values (−0.42 and −0.21, respectively) than polyoxotungstates (POWs) such as [HGeW10O36]7− (−0.19), [W10O32]4− (−0.13) and [W6O19]2− (−0.11); although, [WO4]2− has a much more negative value of −0.50.
In order to test the hypothesis, we studied in this work homogeneous base catalysis of PONb. As an initial target, we focused on [Nb10O28]6− rather than [Nb6O19]8− although the latter has more negative −n/y value. This is simply because the crystal structure of tetramethylammonium (TMA) salt, that can be dispersed in organic solvent, have been reported only for [Nb10O28]6−.12 In this communication, we theoretically compared negative charges of oxygen atoms of [Nb10O28]6− with those of POWs. Then, we applied (TMA)6[Nb10O28]·6H2O as homogeneous catalyst for aldol-type condensation reactions such as Knoevenagel and Claisen–Schmidt condensation reactions and revealed for the first time that it acted as a Brønsted base catalyst whose strength is comparable to superbases. This result will open up a new possibility of application of PONbs since their catalytic application has been limited to electrocatalysts13 and photocatalysts so far.14–17
The electronic charge of the most negative oxygen atom in [Nb10O28]6− (−0.873) was compared with those of other typical polyoxotungstates such as [W10O32]4−, [W6O19]2−, and [WO4]2− (ref. 5) obtained from the same level of calculation (Fig. 2). The charge of the most negative oxygen atom in [Nb10O28]6− (−0.873) was much more negative than those of [W10O32]4− (−0.753) and [W6O19]2− (−0.721), but was slightly less negative than that of [WO4]2− (−0.934). This comparison suggests that [Nb10O28]6− will show base catalysis as in the case of POWs reported so far.5–9
To test the base catalytic activity of [Nb10O28]6− cluster dispersed in organic solvent, we synthesized the TMA salt, (TMA)6[Nb10O28]·6H2O, according to the reported method.12 The synthesized product was characterized by powder XRD, negative-ion electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. The powder XRD pattern of the product agreed well with that of (TMA)6[Nb10O28]·6H2O reported previously (Fig. 3).12 The ESI-MS of the product in water–MeOH (1:1) exhibited a series of mass peaks assigned to [TMAxH(5−x)Nb10O28]− (x = 2–5) (Fig. 4). The elemental analysis of C, H, N, and Nb agreed with the calculated values for [(CH3)4N]6[Nb10O28]·6H2O to an accuracy of <0.5% (calcd: C, 14.94; H, 4.36; N, 4.36; Nb, 48.2. Found: C, 14.75; H, 4.38; N, 4.27; Nb, 48.7). These results show that (TMA)6[Nb10O28]·6H2O was successfully synthesized.
Fig. 3 Powder XRD pattern of the product and simulated pattern from reported crystal structure.12 |
We applied (TMA)6[Nb10O28]·6H2O as a homogeneous catalyst for the Knoevenagel condensation reaction. This is a fundamental coupling reaction to form carbon–carbon double bonds between active methylene compounds such as nitriles (donors) and carbonyl compounds (acceptors). The key step in the reaction is proton abstraction from donors by base catalysts. We studied whether (TMA)6[Nb10O28]·6H2O catalyzed the coupling reaction between benzaldehyde (1) and various nitriles having different pKa values. The results of the catalytic reactions are summarized in Table 1. [Nb10O28]6− cluster afforded in good yields coupling products with nitriles 2a and 2b (entries 1 and 2) whose pKa values are 12.3 and 16.0 even at mild temperature T = 313 K (entries 1-1 and 2-1). Negative-ion ESI-MS of the catalyst after the reaction (entry 2-1) exhibited a series of mass peaks of [TMAxH(5−x)Nb10O28]− (x = 1–3) (Fig. S1†),18 suggesting that the catalysts did not decompose. Turnover frequencies for these reactions (entries 1-2 and 2-2) were calculated to be 66 and 28 h−1 at 343 K, respectively, from the kinetic data (Fig. S2†).18 [Nb10O28]6− cluster showed base catalytic activity for nitriles 2c (pKa = 19.4, entry 3), 2d (pKa = 19.5, entry 4), 2e (pKa = 21.9, entry 5) and 2f (pKa = 23.8, entry 6), although the yields gradually decreased with an increase in the pKa values of the nitriles. However, [Nb10O28]6− cluster did not show any activity for the reaction of 2g (pKa = 32.5, entry 7).
Entry | Donor | pKa | Temp. (K) | Yield (%) |
---|---|---|---|---|
a Reaction conditions: catalyst (1 mol% with respect to 1), 1 (1.0 mmol), 2 (1.0 mmol), MeOH 1 mL, 343 K, 24 h. | ||||
1-1 | 12.3 | 313 | 61 | |
1-2 | 343 | 88 | ||
2-1 | 16.0 | 313 | 77 | |
2-2 | 343 | 88 | ||
3 | 19.4 | 343 | 61 | |
4 | 19.5 | 343 | 39 | |
5 | 21.9 | 343 | 35 | |
6 | 23.8 | 343 | 8 | |
7 | 32.5 | 343 | 0 |
Catalytic activity of (TMA)6[Nb10O28]·6H2O for Claisen–Schmidt condensation reaction between 1 and acetophenone (2h, pKa = 24.7) was also studied. As shown in Scheme 1, [Nb10O28]6− also acted as a homogeneous base catalyst for Claisen–Schmidt condensation.
Scheme 1 Claisen–Schmidt condensation catalyzed by (TMA)6[Nb10O28]·6H2O. aReaction conditions: catalyst (1 mol% with respect to 1), 1 (1.0 mmol), 2 (1.0 mmol), MeOH 1 mL, 343 K, 24 h. |
Although the yields of the reactions with 2f (pKa = 23.8) and 2h (pKa = 24.7) were low compared to typical base catalysts such as hydroxide and alkoxide,19,20 the fact that [Nb10O28]6− cluster could activate less acidic nitriles indicated its strong basicity. To the best of our knowledge, [Nb10O28]6− cluster is the first example of a strong base catalyst that can activate 2f and 2h among various POMs. The base strength of [Nb10O28]6− is as high as 24.7 if we assume that the base strength of a catalyst is comparable to the pKa values of methylene groups from which protons can be abstracted. This value is comparable to those of superbases, whose base strengths are defined to be larger than 26. However, those superbases are generally very sensitive to water and CO2 and exhibit strong basicities only under rigorously controlled conditions.21 In contrast, [Nb10O28]6− cluster showed base catalysis even when it was used in non-hydrated solvent under ambient conditions. This novel feature suggests that the PONbs hold promise as a practical strong base catalyst.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00338a |
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