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 [{MIIL­(H2O)}2(WO2)2(B-β-XW9O33)2]n– and can be classified as follows: 1-SbM, where L =
1H-imidazole-4-carboxylate (imc), X = SbIII, n = 12, and MII = Mn, Co, Ni, Zn; 1-TeM, where L = imc, X = TeIV, n = 10, and MII = Mn, Co; 2-SbNi, where L
= 1H-pyrazole-3-carboxylate (pzc), X = SbIII, n = 12, and MII = Ni; and 3-SbM, where L = pyrazine-2-carboxylate (pyzc), X =SbIII, n = 12, and MII = Co, Zn. The 3d-metal-disubstituted
tungstotellurate­(IV) skeleton of compounds 1-TeM 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 1-SbZn and 3-SbZn 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 [MIIL2(H2O)2] complexes; and picolinate is inert under all conditions
tested.