**1. Introduction**

Polyoxoniobates (PONbs), as a unique branch of polyoxometalates (POMs), have drawn widespread attention in the past few decades due to their diverse structures and multiple applications in catalysis, nuclear-waste treatment, and virology [1,2]. Nevertheless, compared with other POM members, such as polyoxotungstates, polyoxomolybdates, and polyoxovanadates, the development of PONbs is relatively slow due to the lack of soluble Nb precursors, and their low reactivity and narrow working pH range [3]. Recently, great progress has been made in isopolyoxoniobates and some large clusters, such as {Nb27O76}, {Nb32O96}, {Nb52O150}, {Nb81O225}, {Nb114O316}, and the highest nuclearity {Nb288O768(OH)48(CO3)12} have been reported [4–7]. In 2002, the first Keggin-type PONb {(Ti2O2)SiNb12O40} was successfully synthesized by Nyman et al., marking the beginning of heteropolyoxoniobate chemistry [8]. After that, a series of heteropolyoxoniobates were reported and the Keggin-type {XNb12O40} <sup>n</sup><sup>−</sup> (X = Si, Ge, P) are the most extensively studied [9,10].

In the periodic table, Nb and V are neighbors with similar ionic radius and electronegativity, and their hydrolysis and condensation can be performed under alkaline

**Citation:** Li, X.; Zhen, N.; Liu, C.; Zhang, D.; Dong, J.; Chi, Y.; Hu, C. Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Materials and Their Electrocatalytic Activity for Selective Benzyl Alcohol Oxidation. *Molecules* **2022**, *27*, 2862. https:// doi.org/10.3390/molecules27092862

Academic Editor: Xiaobing Cui

Received: 14 April 2022 Accepted: 29 April 2022 Published: 30 April 2022

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conditions. Inspired by these similarities, in 2011, we synthesized the first Keggin-type vanadium-containing PONb {VNb12O40(VO)2} stabilized by Cu complex units [11]. Since then, a series of vanadium-containing Keggin-type PONbs and their derivatives have been reported, including {XNb12O40(VO)2} (X = Si, Ge, P, V), {XNb12O40(VO)4} (X = As, V), {PNb10V2O40(VO)4}, {XNb12O40(VO)6} (X = P, V), {VNb14O42L2} (L = CO3 <sup>2</sup>−, NO3 <sup>−</sup>), {XNb8V4O40(VO)4} (X = P, V, As), {AsNb9V3O40(VO)4}, {TeNb9V2O37}, and {TeNb9V3O39}, where the introduced V acts as central, capping, or/and substituted atoms [12–23]. Notably, most of the clusters were modified by metal-complex units. The use of the metal-complex unit not only contributes to the isolation of novel Keggin-type PONbs, but can also link the discrete PONb clusters into extended structures. In general, most transition metals tend to hydrolyze rapidly into precipitation under alkaline conditions, and thereby the coexistence of transition metal ions with basic PONbs is a challenge [3]. As the Cu ion can tolerate the alkaline synthesis conditions combining with its Jahn–Teller effect, Cu-complexes are the dominated metal organic units in the synthesis of PONb-based hybrids [24–28]. In contrast, Ni-complexes were seldom used and the extended structure based on V-containing PONb hybrids and a Ni-complex is rare.

Compared with other POM members, the catalytic properties of PONbs are not extensively explored. Due to their Brønsted basicity, PONbs have been used to promote the hydrolysis of chemical warfare agents [29,30]. The introduction of V endows basic PONb clusters with interesting redox properties. For example, a double-anion cluster {PNb12O40(VO)2(V4O12)2} was successfully prepared in our group, which can effectively promote the basic hydrolysis of the nerve agent simulant and the oxidative decontamination of the sulfur mustard simulant [31]. Then, we found that the organic-inorganic hybrids based on {PNb12O40(VO)2} were active for the selective oxidation of benzyl-alkanes to ketone [14]. Our investigation indicates that the V atoms of {V5Nb23O80} and {V6Nb23O81} play a key role in the selective oxidation of the sulfur mustard simulant [32]. Owing to their fast and reversible electron transfer behavior, POMs are also a kind of promising electrocatalyst [33–35]. Recently, the covalent triazine framework immobilized {PMo10V2O40} shows excellent activities in the electrocatalytic oxidation of benzyl alcohols and ethylbenzene [36,37]. However, the electrocatalytic activity of vanadium-containing PONbs is nearly unexplored.

Herein, we report the controllable synthesis and structural characterization of two vanadium-containing Keggin-type PONbs: [Ni(en)2]5[PNb12O40(VO)5](OH)5·18H2O (**1**) and [Ni(en)3]5[PNb12O40(VO)2]·17H2O (**2**, en = ethylenediamine) (Scheme 1). Compound **1** contains a Keggin-type PONb capped by five vanadyl groups, which was further connected by [Ni(en)2] 2+ units into a three-dimensional (3D) organic-inorganic framework. Compound **2** is a di-capped discrete vanadoniobate cluster with a [Ni(en)3] 2+ unit as count cations. Interestingly, **1** and **2** as electrode materials can catalyze the selective oxidation of benzyl alcohol to benzaldehyde under alkaline additive free conditions and compound **1** is more active than **2**. The control experiments show that both the capped V atom and the basic PONb cluster contribute to the enhancement of electrocatalytic activity.

**Scheme 1.** Controllable synthesis of **1** and **2**.
