*2.3. Synthesis of [Ni(en)3]5[PNb12O40(VO)2]*·*17H2O* (**2**)

Compound **2** was synthesized by a similar procedure to that of **1**, but without adding NaVO3. The pH value of the mixture was adjusted to 10.50 using 2 M NaOH and transferred to a Teflon-lined autoclave (23 mL), kept at 140 ◦C for 72 h, and then slowly cooled to room temperature. Brown block crystals of **2** were isolated in about 21.8% yield (based on Nb). Anal. Calcd (%) for **2**: C, 10.53; H, 4.54; N, 12.28; Ni, 8.58; P, 0.91; V, 2.98; Nb, 32.59. Found: C, 10.46; H, 4.42; N, 11.84; Ni, 8.31; P, 0.64; V, 2.77; Nb, 32.86. FT-IR (cm<sup>−</sup>1): 3425 (s), 2945 (w), 2875 (w), 1593 (s), 1469 (m), 1363 (w), 1318 (w), 1275 (s), 1233 (w), 1106 (w), 1027 (s), 947 (s), 877 (s), 815 (w), 709 (s), 638 (w), 505 (w), 469 (w).
