Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb 2 O}, {Nb 4 O 4 } and {Nb 8 O 12 } Cores.

The reactivity of aryl monocarboxylic acids (benzoic, 1- or 2-naphtoic, 4'-methylbiphenyl-4-carboxylic, and anthracene-9-carboxylic acids) as complexing agents for the ethoxide niobium(V) (Nb(OEt) ₅ precursor has been investigated. A total of eight coordination complexes were isolated with distinct niobium(V) nuclearities as well as carboxylate complexation states. The use of benzoic acid gives a tetranuclear core Nb ₄ (μ ₂ -O) ₄ (L) ₄ (OEt) ₈ ] (L=benzoate (1)) with four Nb-(μ ₂ -O)-Nb linkages in a square plane configuration. A similar tetramer, 7, was obtained with 2-naphtoic acid by using a 55 % humid atmosphere synthetic route. Two types of dinuclear brick were identified with one central Nb-(μ ₂ -O)-Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb ₂ (μ ₂ -O)(μ ₂ -OEt)(L)(OEt) ₆ ], with L=1-naphtoate (3) or anthracene-9-carboxylate (5)) or two bridging carboxylate groups ([Nb ₂ (μ ₂ -O)(L) ₂ (OEt) ₆ ], with L=4'-methylbiphenyl-4-carboxylic (4) or anthracene-9-carboxylate (6)). An octanuclear moiety [Nb ₈ (μ ₂ -O) ₁₂ (L) ₈ (η ₁ -L) _4-x (OEt) _4+x ] (with L=2-naphtoate, x=0 or 2; 8) was obtained by using a solvothermal route in acetonitrile; it has a cubic configuration with niobium centers at each node, linked by 12 μ ₂ -O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid ¹ H NMR spectroscopy in order to analyze the esterification reaction, which induces the release of water molecules that further react through oxolation with niobium atoms, in different {Nb ₂ O}, {Nb ₄ O ₄ } and {Nb ₈ O ₁₂ } nuclearities.
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