POMs, as a kind of nano-sized metal-oxide clusters, were recently spotlighted in the field of catalysis and green chemistry, owing to their unique physicochemical properties and tunable structure [
24–
34]. Over the past two decades, there were some reports demonstrating POMs as ideal candidates in facilitating the Knoevenagel condensation reaction [
19–
23,
35]. The study of POMs catalysts of Knoevenagel condensation reaction could date back to the 2007 [
21], when Mizuno's group first reported that the basic POMs, (TBA)
4[
γ-SiW
10O
34(H
2O)
2] and (TBA)
7[H(
γ-SiW
10O
32)
2(
μ-O)
4], could efficiently catalyze the condensation of benzaldehyde and malononitrile. Subsequently, they presented the highly negatively charged POMs (TBA)
6[
γ-H
2GeW
10O
36] could act as an excellent catalyst for Knoevenagel condensation reaction [
22]. Inspired by mentioned work, Song
et al. developed two tri-lacunary POMs showing satisfactory performance of the condensation reaction of ethyl cyanoacetate and benzaldehyde [
36]. Aside from polytungstates catalysts, polyoxoniobates were also investigated as potential catalysts for Knoevenagel condensation reaction for their outstanding basicity [
37–
40]. In 2016, Tsukuda and Wang groups presented polyoxoniobates (TMA)
6[Nb
10O
28] and Na
16[SiNb
12O
40]·
xH
2O showing high catalytic activities for Knoevenagel condensation reaction, respectively [
37,
38]. After that, our group and Tsukuda group successively applied Lindqvist-type POMs, K
7HNb
6O
19·13H
2O and (TBA)
8[Ta
6O
19], in catalyzing the condensation reaction of benzaldehyde and ethyl cyanoacetate, and obtained satisfactory catalytic performances [
39,
40]. Compared to basic catalysts, catalysts containing Lewis acid and base sites are expected to give premium catalytic activities for their concurrent substrate activation and nucleophilic attack [
41–
44]. With that, transition-metal (TM) substituted POMs could be ideal catalysts towards Knoevenagel condensation reaction, where TM cores act as Lewis acid sites and nucleophilic surface-oxygen atoms work as the Lewis base sites, meanwhile their synergistic effect could be beneficial in accelerating the reaction process [
45]. Based on this idea, very recently, our group reported polyoxomolybdates-based catalysts demonstrating impressivecatalytic activities in accelerating Knoevenagel condensation reaction due to the synergy of TM and POMs [
19,
20]. In addition, lanthanide (Ln) also have good Lewis acid ability, implying great potentials to further improve the performance of POMs-based catalysts though there are few related reports yet [
45]. Therefore, we were strongly motived to explore novel Ln-substituted POMs from the viewpoints of both catalytic effect (design the satisfactory catalysts for Knoevenagel condensation reaction) and synthesis strategies (expand the POMs family).