Two multi-Rh-incorporated polyoxometalates [NH 2 (CH 3 ) 2 ] 10 [Na 4 (H 2 O) 8 ]H 3 [As 4 W 42 O 142 (OH) 4 (CH 3 COO) 2 -Rh 3 (H 2 O) 4 ]•13H 2 O•4[NH(CH 3 ) 2 ] (1) and [K 4 Na(H 2 O) 6 ]KH 10 [As 4 W 40 O 140 Rh 4 (H 2 O) 4 ]•34H 2 O (2) have been synthesized in acetate buffer solution. Polyanion 1a is built up atop of an acetate-modified rectangular framework [As 4 W 42 O 142 (OH) 4 (CH 3 COO) 2 ] 26− , while polyanion 2a contains a pure inorganic cryptand [As 4 W 40 O 140 ] 28− . All Rh atoms of these two compounds share the same hexa-coordinate distorted-octahedral geometry and are embedded into their cavities through As−Rh bonding with a bond length around 2.304(4)−2.436(5) Å. Besides, they not only represent structural novelty but also demonstrate controllable proton conduction properties. Catalysts 1 and 2 can catalyze cycloaddition of epoxides with CO 2 in a solvent-free system in conjunction with 1-ethyl-1-methylpyrrolidinium bromide.