Synthesis, spectroscopic and magnetostructural evidence for the formation of Cu(II) complexes of pyridyl-2-carboxylate (2-pca) and quinolyl-2-carboxylate (2-qca) as a result of a novel oxidative P-dealkylation reaction of diethyl 2-pyridylmethylphosphonat

The interaction of diethyl 2-pyridylmethylphosphonate (2-pmpe) ligand with CuX2 salts unexpectedly leads to the formation of compounds of the formula Cu(2-pca)2 [X=Cl− (1), CH3COO− (3)], and Cu(2-pca)Cl [X=Cl− (2)] (2-pca=pyridine-2-carboxylate ion). The diethyl 2-quinolylmethylphosphonate ligand (2-qmpe) reacts with CuX2 salts to similarly yield compounds of stoichiometry Cu(2-qca)2 · H2O (X=ClO4 − (4)], and for X=Cl Cu(2-qca)2 · H2O (5) and Cu(2-qca)Cl (6), (2-qca=quinoline-2-carboxylate ion). These compounds are products of a novel oxidative P-dealkylation reaction, which takes place on 2-pmpe and 2-qmpe ligands under the used conditions. The compounds were characterized by infrared, ligand field, EPR spectroscopy and magnetic studies. Cu(2-pca)2 exists in two crystalline forms, a blue form (1) and a violet form (3). For 3 the single-crystal structure was determined. The copper atom is four-coordinated in a square-planar geometry. The stack between related (and hence parallel) pca moieties involves interatomic distances of 3.27 Å. Cu(2-qca)2 · H2O also exists in two forms, a green (4) and a blue–green (5). Both these complexes are five coordinated, involve the same CuN2O3 chromophore and are examples of the distortion isomers. Variable-temperature magnetic susceptibility measurements (1.9–300 K) have shown that the antiferromagnetic coupling observed is much stronger in 6 than in 1, 3, 4 and 5. For 2 a ferromagnetic exchange occurs.