A　3D　organic-inorganic　hybrid　compound,　(2-MepyH)(3)　[{Fe(1,10-phen)(3)}(3)][{Pr4Sb12O18(OH)　Cl-11.5}(TDC)(4.5)({Pr4Sb12O18(OH)Cl-9.5}　Cl)]3(2-Mepy)28H(2)O　(1;　2-Mepy=2-methylpyridine,　1,10-phen=1,10-phenanthroline,　H2TDC=thiophene-2,5-dicarboxylic　acid),　was　hydrothermally　synthesized　and　structurally　characterized.　Unusually,　two　kinds　of　high-nuclearity　clusters,　namely　[(Pr4Sb12O18　(OH)Cl-11)(COO)(5)](5-)　and　[(Pr4Sb12O18　(OH)Cl-9)Cl(COO)(5)](4-),　coexist　in　the　structure　of　compound　1;　two　of　the　latter　clusters　are　doubly　bridged　by　two　(2)-Cl-　moieties　to　form　a　new　centrosymmetric　dimeric　cluster.　An　unprecedented　spontaneous　and　reversible　single-crystal-to-single-crystal　transformation　was　observed,　which　simultaneously　involved　a　notable　organic-ligand　movement　between　the　metal　ions　and　an　alteration　of　the　bridging　ion　in　the　dimeric　cluster,　induced　by　guest-release/re-adsorption,　thereby　giving　rise　to　the　interconversion　between　compound　1　and　the　compound　(2-MepyH)(3)[{Fe(1,10-phen)(3)}(3)][{Pr4Sb12O18(OH)Cl-11.5}(TDC)(4)({Pr4Sb12O18Cl10.5(TDC)(0.5)(H2O)(1.5)}O-0.5)]　25H(2)O　(1).　The　mechanism　of　this　transformation　has　also　been　discussed　in　great　detail.　Photocatalytic　H-2-evolution　activity　was　observed　for　compound　1　under　UV　light　with　Pt　as　a　co-catalyst　and　MeOH　as　a　sacrificial　electron　donor.