Three　α-Keggin　heteropolymolybdates　with　the　formula　[(C　5H4NH)COOH]3[PMo12O40]　1,　{[Sm(H2O)4(pdc)]3}{[Sm(H2O)　3(pdc)]}[SiMo12O40]·3H2O　2　and　{[La(H2O)4(pdc)]4}[PMo12O　40]F　3　(H2pdc　=　pyridine-2,6-dicarboxylate),　have　been　synthesized　under　hydrothermal　condition　and　characterized　by　single　crystal　X-ray　diffraction　analyses,　elemental　analyses,　inductively　coupled　plasma　atomic　emission　spectroscopy　(ICP-AES),　IR,　thermal　gravimetric　analyses,　thermal　infrared　spectrum　analyses　and　powder　X-ray　diffraction　(PXRD)　analyses.　Single　crystal　X-ray　diffraction　indicates　all　three　compounds　comprise　ball-shaped　Keggin　type　[XMo12O40]n-　polyoxometalates　(POMs)　(n　=　3,　X　=　P;　n　=　4,　X　=　Si,　respectively)　with　different　types　of　carboxylic　ligands　derived　from　H2pdc,　and　these　cluster　anions　are　isostructural.　In　order　to　explore　structural　characteristics,　Rhodamine　B　photocatalytic　(RhB)　degradation　and　two-dimensional　infrared　correlation　spectroscopy　(2D-IR　COS)　tests,　are　investigated　for　1,　2　and　3.　In　RhB　degradation,　all　compounds　show　good　photocatalytic　activity.　For　1,　the　activity　mainly　comes　from　POMs.　While　in　2　and　3,　POMs＇　photocatalytic　activity　is　enhanced　by　the　Ln(iii)-pdc　metal-organic　frameworks.　Structural　properties　like　POM＇s　stability　and　magnetic　sensitivity　are　discussed　by　2D-IR　COS　under　thermal/magnetic　perturbations.　©　2012　The　Royal　Society　of　Chemistry.