Triangular {Mn(OH)}36 + fragment encapsulated in trivacant [A-R-SiW9O34]10 − ligand

[{MnIII(OH)(H2O)}3SiW9O34]4 − 1a, has been isolated from the reaction of Na9[β-SiW9O34H]·23H2O and [Mn3O(O2CMe)6(HIm)3](O2CMe) (Im = imidazole C3H4N2) in aqueous solution. The trimetal-substituted polyoxometalate (TMSP) 1 has been fully characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, and infrared spectroscopy. This monomeric anion is composed of one trilacunary Keggin fragment A-α-SiW9O34 and three manganese ions encapsulated resulting in a triangular {Mn(OH)}36 + ring core. Experimental structural and magnetism aspects of the material are reported and discussed.