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.