*2.2. Synthesis of [Ni(en)2]5[PNb12O40(VO)5](OH)5*·*18H2O* (**1**)

Ni(CH3COO)2·4H2O (0.2240 g, 0.9 mmol) was dissolved in distilled water (0.83 mL), and to this en (0.13 mL) was added, obtaining a purple solution. Then the resulting solution was added dropwise to K7HNb6O19·13H2O (0.1507 g, 0.11 mmol) aqueous solution (8 mL) containing VOPO4·2H2O (0.0534 g, 0.27 mmol) and NaVO3 (0.0329 g, 0.27 mmol) under stirring. The pH value of the mixture was adjusted to 10.50 using 2 M NaOH. The mixture was transferred to a Teflon-lined autoclave (23 mL) and kept at 160 ◦C for 72 h, and then slowly cooled to room temperature after 24 h, and brown block crystals of **1** were isolated in about 17.5% yield (based on Nb). Anal. Calcd (%) for **1**: C, 7.02; H, 3.56; N, 8.18; Ni, 8.57; P, 0.90; V, 7.44; Nb, 32.56. Found: C, 7.57; H, 3.47; N, 7.83; Ni, 8.06; P, 0.48; V, 7.05; Nb, 32.34. FT-IR (cm<sup>−</sup>1): 3451 (s), 2947 (w), 2883 (w), 1589 (s), 1460 (m), 1395 (w), 1330 (w), 1282 (s), 1233 (w), 1112 (w), 1032 (s), 942 (s), 870 (s), 805 (w), 700 (s), 627 (w), 498 (w), 473 (w).
