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.