Functionalization of Krebs-Type Polyoxometalates with N,O-Chelating Ligands: A Systematic Study

The first organic derivatives of 3d-metal-disubstituted Krebs-type polyoxometalates have been synthesized under mild bench conditions via straightforward replacement of labile aqua ligands with N,O-chelating planar anions on either preformed or in situ-generated precursors. Nine hybrid clusters containing carboxylate derivatives of five- or six-membered aromatic N-heterocycles as antenna ligands have been obtained as pure crystalline phases and characterized by elemental and thermal analyses, infrared spectroscopy, and single-crystal X-ray diffraction. They all show the general formula [{M^IIL­(H₂O)}₂(WO₂)₂(<i>B</i>-β-XW₉O₃₃)₂]^<i>n–</i> and can be classified as follows: <b>1-SbM</b>, where L = 1<i>H</i>-imidazole-4-carboxylate (imc), X = Sb^III, <i>n</i> = 12, and M^II = Mn, Co, Ni, Zn; <b>1-TeM</b>, where L = imc, X = Te^IV, <i>n</i> = 10, and M^II = Mn, Co; <b>2-SbNi</b>, where L = 1<i>H</i>-pyrazole-3-carboxylate (pzc), X = Sb^III, <i>n</i> = 12, and M^II = Ni; and <b>3-SbM</b>, where L = pyrazine-2-carboxylate (pyzc), X =Sb^III, <i>n</i> = 12, and M^II = Co, Zn. The 3d-metal-disubstituted tungstotellurate­(IV) skeleton of compounds <b>1-TeM</b> is unprecedented in polyoxometalate chemistry. The stability of these hybrid Krebs-type species in aqueous solution has been confirmed by ¹H NMR spectroscopy performed on the diamagnetic <b>1-SbZn</b> and <b>3-SbZn</b> derivatives. Our systematic study of the reactivity of disubtituted Krebs-type polyoxotungstates toward diazole-, pyridine-, and diazinecarboxylates demonstrates that organic derivatization is strongly dependent on the nature of the ligand, as follows: imc displays a “universal ligand” character, as functionalization takes place regardless of the external 3d metal and heteroatom; pzc and pyzc show selectivity toward specific 3d metals; pyridazine-3-carboxylate and pyrimidine-4-carboxylate promote partial decomposition of specific precursors, leading to [M^IIL₂(H₂O)₂] complexes; and picolinate is inert under all conditions tested

Functionalization of Krebs-Type Polyoxometalates with N,O-Chelating Ligands: A Systematic Study

The first organic derivatives of 3d-metal-disubstituted Krebs-type polyoxometalates have been synthesized under mild bench conditions via straightforward replacement of labile aqua ligands with N,O-chelating planar anions on either preformed or in situ-generated precursors. Nine hybrid clusters containing carboxylate derivatives of five- or six-membered aromatic N-heterocycles as antenna ligands have been obtained as pure crystalline phases and characterized by elemental and thermal analyses, infrared spectroscopy, and single-crystal X-ray diffraction. They all show the general formula [{M^IIL­(H₂O)}₂(WO₂)₂(<i>B</i>-β-XW₉O₃₃)₂]^<i>n–</i> and can be classified as follows: <b>1-SbM</b>, where L = 1<i>H</i>-imidazole-4-carboxylate (imc), X = Sb^III, <i>n</i> = 12, and M^II = Mn, Co, Ni, Zn; <b>1-TeM</b>, where L = imc, X = Te^IV, <i>n</i> = 10, and M^II = Mn, Co; <b>2-SbNi</b>, where L = 1<i>H</i>-pyrazole-3-carboxylate (pzc), X = Sb^III, <i>n</i> = 12, and M^II = Ni; and <b>3-SbM</b>, where L = pyrazine-2-carboxylate (pyzc), X =Sb^III, <i>n</i> = 12, and M^II = Co, Zn. The 3d-metal-disubstituted tungstotellurate­(IV) skeleton of compounds <b>1-TeM</b> is unprecedented in polyoxometalate chemistry. The stability of these hybrid Krebs-type species in aqueous solution has been confirmed by ¹H NMR spectroscopy performed on the diamagnetic <b>1-SbZn</b> and <b>3-SbZn</b> derivatives. Our systematic study of the reactivity of disubtituted Krebs-type polyoxotungstates toward diazole-, pyridine-, and diazinecarboxylates demonstrates that organic derivatization is strongly dependent on the nature of the ligand, as follows: imc displays a “universal ligand” character, as functionalization takes place regardless of the external 3d metal and heteroatom; pzc and pyzc show selectivity toward specific 3d metals; pyridazine-3-carboxylate and pyrimidine-4-carboxylate promote partial decomposition of specific precursors, leading to [M^IIL₂(H₂O)₂] complexes; and picolinate is inert under all conditions tested