*Article* **Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Materials and Their Electrocatalytic Activity for Selective Benzyl Alcohol Oxidation**

**Xiaoxia Li, Ni Zhen, Chengpeng Liu, Di Zhang, Jing Dong, Yingnan Chi \* and Changwen Hu \***

Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, China; 3120191243@bit.edu.cn (X.L.); zhenni1220@163.com (N.Z.); laucouple@163.com (C.L.); dizhang\_pub@163.com (D.Z.); 20200808@btbu.edu.cn (J.D.) **\*** Correspondence: chiyingnan7887@bit.edu.cn (Y.C.); cwhu@bit.edu.cn (C.H.)

**Abstract:** During the controllable synthesis of two vanadium-containing Keggin-type polyoxoniobates (PONbs), [Ni(en)2]5[PNb12O40(VO)5](OH)5·18H2O (**1**) and [Ni(en)3]5[PNb12O40(VO)2]·17H2O (**2**, en = ethylenediamine) are realized by changing the vanadium source and hydrothermal temperature. Compounds **1** and **2** have been thoroughly characterized by single-crystal X-ray diffraction analysis, FT-IR spectra, X-ray photoelectron spectrum (XPS), powder X-ray diffraction (PXRD), etc. Compound **1** contains a penta-capped Keggin-type polyoxoniobate {PNb12O40(VO)5}, which is connected by adjacent [Ni(en)2] 2+ units into a three-dimensional (3D) organic-inorganic framework, representing the first nickel complexes connected vanadoniobate-based 3D material. Compound **2** is a discrete di-capped Keggin-type polyoxoniobate {PNb12O40(VO)2} with [Ni(en)3] 2+ units as counter cations. Compounds **1** and **2** have poor solubility in common solvents and can keep stable in the pH range of 4 to 14. Notably, both **1** and **2** as electrode materials are active for the selective oxidation of benzyl alcohol to benzaldehyde. Under ambient conditions without adding an alkaline additive, compound **1** as a noble metal free electrocatalyst can achieve 92% conversion of benzyl alcohol, giving a Faraday efficiency of 93%; comparatively, **2** converted 79% of the substrate with a Faraday efficiency of 84%. The control experiments indicate that both the alkaline polyoxoniobate cluster and the capped vanadium atoms play an important role during the electrocatalytic oxidation process.

**Keywords:** vanadium-containing polyoxoniobates; electrocatalysis; benzyl alcohol oxidation; nickel complexes
