Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb₂O}, {Nb₄O₄} and {Nb₈O₁₂} Cores

Abstract

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 Nb4(μ2‐O)4(L)4(OEt)8] (L=benzoate (1)) with four Nb−(μ2‐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−(μ2‐O)−Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb2(μ2‐O)(μ2‐OEt)(L)(OEt)6], with L=1‐naphtoate (3) or anthracene‐9‐carboxylate (5)) or two bridging carboxylate groups ([Nb2(μ2‐O)(L)2(OEt)6], with L=4’‐methylbiphenyl‐4‐carboxylic (4) or anthracene‐9‐carboxylate (6)). An octanuclear moiety [Nb8(μ2‐O)12(L)8(η1‐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 μ2‐O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid 1H 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 {Nb2O}, {Nb4O4} and {Nb8O12} nuclearities.

Molecular niobium(v) complexes with mononuclear {Nb1} and dinuclear oxo species {Nb2O} connected through aryl di- or tetra-carboxylate linker

“…It is assigned to the ν (CO) coming from the formation of an ethyl-pyromellitate ester. Such formation was previously reported on the synthesis of a tetranuclear niobium acetate 51 or aryl-carboxylates 42 compounds and is expected to come from the reaction of the acid with the ethoxide groups (forming ethyl-ester and water) coming from the niobium precursor. As a result, it indicates the complexation of the carboxylic ligand to the niobium centers releasing some of the ethoxy molecules allowing the esterification reaction.…”

“…The reaction of the aromatic polycarboxylic ligands with the niobium( v ) ethoxide precursor lead to the formation of three crystalline molecular compounds containing a number of niobium centers going from four to eight which is comparable from this aspect to most of the complex obtained with various monotopic ligands in literature or very recently, in our group. 42 In addition to the carboxylate complexation, the common feature is the occurrence of one μ-oxo groups bridging two adjacent niobium centers in all dimeric {Nb 2 O} units. The generation of this bridging oxygen atoms is due to the reaction between the carboxylic acid ligands and leaving ethoxide groups from the niobium {Nb(OEt) 5 } precursor generating a small quantity of water (by-product of the esterification reaction) thus allowing the following condensation reaction forming the niobium dimer: 2 × Nb(OEt) + H 2 O → Nb–O–Nb + 2 × EtOH.…”

“…39 This arrangement generates through the BHMP molecule, a tetranuclear species [(BHMP){Nb 2 O(OEt) 5 } 2 ], without any additional μ-oxo groups linking the niobium atoms. Another example of such an association of niobium carboxylate from the alkoxide route is also illustrated in a mixed bismuthniobium moiety, 40 41 Following our recent work on this topic, using aromatic monocarboxylic acid as ligands, 42 we decided to work with polytopic acids as ligands. Herein, we report on the investigation of the reactivity of polycarboxylic aromatic acids with niobium penta-ethoxide Nb(OEt) 5 precursor as the niobium(V) metal source.…”

“…Following our recent work on this topic, using aromatic monocarboxylic acid as ligands, 42 we decided to work with polytopic acids as ligands. Herein, we report on the investigation of the reactivity of polycarboxylic aromatic acids with niobium penta-ethoxide Nb(OEt) 5 precursor as the niobium( v ) metal source.…”

Isolation of a Dodecanuclear Polyoxo Cluster {Nb12O21} Showing a Rare Case of Five‐fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

“…It led to the formation of an hexadecaniobate cluster [Nb 16 O 28 (OEt) 12 (piv) 12 ] [49] . In our recent work on this topic, we have used several aromatic monocarboxylic and polycarboxylic acid ligands to complex niobium(IV) tetrachloride and niobium(V) ethoxide precursors, successfully increasing the nuclearity of the niobium centers in the final compounds up to the formation of an octameric {Nb 8 O 12 } moieties [50–52] …”