Effects of Polyoxometalates on the Hydroxylation of N-Heterocyclic Ligands.

Four polyoxometalate (POM)-based Cu complexes with the hydroxylated pyridine analogue 3-(2-hydroxylpyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl (btpo), H3{[Cu2(btpo)2](PW12O40)}·2H2O (1), H3{[Cu2(btpo)2](PMo12O40)}·2H2O (2), H2{[Cu2(btpo)2](SiW12O40)·SO4 (3), and H4{[Cu4(btpo)4](SiW12O40)}·8H2O (4), were synthesized hydrothermally under acid conditions. Single-crystal X-ray structural analysis reveals that 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl was hydroxylated into btpo in compounds 1-4, again providing structural evidence for the Gillard mechanism, in which H2O as a weak nucleophile can attack the α-C atom of N-heterocyclic molecules at a lower pH value (ca. 1.0) with the help of POMs.