Not So Similar: Different Ways of Nb(V) and Ta(V) Catecholate Complexation

The reactions between catechol (H2cat) and niobium(V) or tantalum(V) precursors in basic aqueous solutions lead to the formation of catecholate complexes of different natures. The following complexes were isolated and characterized by single-crystal X-ray diffraction (SCXRD): (1) (NH4)3[NbO(cat)3]∙4H2O; (2) K2[Nb(cat)3(Hcat)]·2H2cat·2H2O; (3) Cs3[NbO(cat)3]·H2O; (4) (NH4)4[Ta2O(cat)6]·3H2O; (5) Cs2[Ta(cat)3(Hcat)]·H2cat; (6) Cs4[Ta2O(cat)6]·7H2O. The isolated crystalline products were characterized by elemental analysis, X-ray powder diffraction (XRPD), FTIR, and TGA. The structural features of these complexes, such as {Ta2O} unit geometry, Cs-π interactions, and crystal packing effects, are discussed.

Catecholates belong to redox-active ligands, which can be reversibly oxidized in two consecutive steps and can act as electron reservoirs for activation for both substrates and metal centers. They can also be used to design novel magnetic and optic materials [30][31][32]. Regarding the catecholates, it was apparently Rosenheim who in 1932 reported the dissolution of niobic acid in alkaline catechol (C 6 H 4 (OH) 2 , H 2 cat) solutions [33]. Later, in 1959, Brown and Land studied this reaction with UV-VIS techniques [34,35]. This approach was used in the analytical chemistry of Nb and Ta [36]. Several salts of [NbO(cat) 3 ] 3− and [Ta 2 O(cat) 6 ] 4− were isolated but not structurally characterized [37].
The redox behavior of Nb(V) and Ta(V) complexes with catecholate derivatives under air-free conditions is quite interesting but rare [38,39]. The most straightforward results in the field belong to Ta(V) complexes with N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide, which is active in the four-electron oxidative formation of aryl diazenes [40].
Interestingly, Nb and Ta, which are commonly regarded as "chemical twins", give products with different stoichiometry. In this work, we took up these studies in order to clarify the nature of the catecholate complexes formed by Nb(V) and Ta(V) in aqueous solutions. Nb 2 O 5 ·xH 2 O slightly dissolves in the solution of catechol in aqueous ammonia upon refluxing in an argon atmosphere. As the reaction proceeds, the solution turns from colorless to red. Slow cooling of the reaction mixture gives yellowish crystals of (NH 4 ) 3 [NbO(cat) 3 ]·4H 2 O (1). The product was isolated by suction filtration and characterized with single-crystal X-ray diffraction (SCXRD). The crystal structure of [NbO(cat) 3 ] 3− is shown in Figure 1.

Results and Discussion
The redox behavior of Nb(V) and Ta(V) complexes with catecholate derivatives air-free conditions is quite interesting but rare [38,39]. The most straightforward res the field belong to Ta(V) complexes with N,N-bis (3,5-di-tert-butyl-2-phenoxide) which is active in the four-electron oxidative formation of aryl diazenes [40].
Interestingly, Nb and Ta, which are commonly regarded as "chemical twins products with different stoichiometry. In this work, we took up these studies in o clarify the nature of the catecholate complexes formed by Nb(V) and Ta(V) in aq solutions.

Niobium Complexes
Hydrated Nb2O5·xH2O or hexametalate alkali-metals salts A8[M6O19]·nH2O wer for the reaction with catechol (C6H4O2, H2cat) under air-free conditions. We used di bases (ammonia or different alkalis) to adjust the reaction medium. All synthetic are summarized in the Experimental section.
Switching to KOH as a reaction medium results in the crystallization of K 2 [Nb(cat) 3 (Hcat)]·2H 2 cat·2H 2 O (2) by slow cooling of the reaction solution. The structure of [Nb(cat) 3 (Hcat)] 2− is shown in Figure 2, left.
In this study, complexes with Nb V coordination environments have not been reported. In this complex anion, Nb has CN 7 with a pentagonal-bipyramidal coordination polyhedral, but without any Nb=O bond. Instead, this position is occupied by a phenolic oxygen from a monodentately coordinated Hcat − anion. The Nb-O axial bond distances are 1.9623(9) Å (Hcat) and 2.0033(9) Å (cat). The equatorial Nb-O bond distances fall into a 2.0033(9)-2.1211(10) Å interval. Moreover, the anion structure is stabilized by the intramolecular H-bond, d(H18-O6) = 1.922(2) Å. This structure can be regarded as a kind of intermediate between [NbO(cat) 3 ] 3− and [Nb(cat) 4 ] 3− . It should be noted that only a single complex of Nb with 4 O-donor bidentate ligands, and this is of Nb IV , has been structurally characterized [46]. In the crystal structure, K + , H 2 O, [Nb(cat) 3 (Hcat)] 2− , and neutral H 2 cat molecules combine into linear neutral associates (Figure 2, right).  In this study, complexes with Nb V coordination environments have not been reported. In this complex anion, Nb has CN 7 with a pentagonal-bipyramidal coordination polyhedral, but without any Nb=O bond. Instead, this position is occupied by a phenolic oxygen from a monodentately coordinated Hcat − anion. The Nb-O axial bond distances are 1.9623(9) Å (Hcat) and 2.0033(9) Å (cat). The equatorial Nb-O bond distances fall into a 2.0033(9)-2.1211(10) Å interval. Moreover, the anion structure is stabilized by the intramolecular Hbond, d(H18-O6) = 1.922(2) Å. This structure can be regarded as a kind of intermediate between [NbO(cat)3] 3− and [Nb(cat)4] 3− . It should be noted that only a single complex of Nb with 4 O-donor bidentate ligands, and this is of Nb IV , has been structurally characterized [46]. In the crystal structure, K + , H2O, [Nb(cat)3(Hcat)] 2− , and neutral H2cat molecules combine into linear neutral associates (Figure 2, right).
The use of Cs8[Nb6O19]·14H2O and 2M CsOH under the same reaction conditions leads to the crystallization of Cs3[NbO(cat)3]·H2O (3). The geometry of this Nb complex is practically the same as 1. The most interesting feature of this structure is the location of Cs + cations. Curiously, crystallization from the aqueous solution does not supply water molecules to the cesium coordination sphere. Instead, Cs + prefers Cs + -π interactions with the aromatic catechol rings of adjacent [NbO(cat)3] 3− complexes ( Figure 3). A search of the Cambridge Structural Database [47] yielded few other examples of such coordination, but in all cases, Cs + was involved in both Cs-π and Cs-OH2 bonding [48,49].

Tantalum Complexes
In the case of Ta, we used only hexatantalates in the reactions with catechol. Using

Tantalum Complexes
In the case of Ta, we used only hexatantalates in the reactions with catechol. K8[Ta6O19]·16H2O as a tantalum source and ammonia as a base resulted in the form of (NH4)4[Ta2O(cat)6]·3H2O (4), from which single crystals were isolated and charact by SCXRD. In the crystal structure, a slightly bent {Ta-O-Ta} 8+ binuclear unit is coordi with six cat 2− ligands ( Figure 4). The orientational disorder of the {Ta(cat)3} moiety two close positions (0.5/0.5 occupancies) gives two closely similar geometries [Ta2O(cat)6] 4− complexes in the structure: (i) more linear but less symmetrical, wi following parameters: Ta1

Tantalum Complexes
In the case of Ta, we used only hexatantalates in the reactions with catecho K8[Ta6O19]·16H2O as a tantalum source and ammonia as a base resulted in the fo of (NH4)4[Ta2O(cat)6]·3H2O (4), from which single crystals were isolated and chara by SCXRD. In the crystal structure, a slightly bent {Ta-O-Ta} 8+ binuclear unit is coor with six cat 2− ligands (   O bond distance is 1.993(2) Å, indicating a slight axial compression of the {TaO7} bipyramid. Cs + -π interactions in the crystal structure of 5 is shown in Figure 5, right.  On the other hand, the reluctance of Ta to form {TaO} 3+ upon complexation of catecholate is in line with well-known differences in the chemistry of Nb and Ta: in HCl, they form [NbOCl 5 ] 2− and [TaCl 6 ] − , respectively; TaOCl 3 is much less stable than NbOCl 3 ; in 2-3% HF solutions, Nb is present as [NbOF 5 ] 2− , while Ta forms [TaF 7 ] 2− . Quantum chemical calculations show that in [MOCl 5 ] 2− (M = Nb, Ta, Pa, Db), the Ta=O bond is the weakest. This may be due to relativistic effects, which strongly stabilize the d-level in Ta and thus make it less accessible for dative π-bonding [55].
Reported synthetic and structural data for Nb(V) and Ta(V) catecholate complexes can thus be presented in the following scheme (Figure 7). On the other hand, the reluctance of Ta to form {TaO} 3+ upon complexation of catecholate is in line with well-known differences in the chemistry of Nb and Ta: in HCl, they form [NbOCl5] 2− and [TaCl6] − , respectively; TaOCl3 is much less stable than NbOCl3; in 2-3% HF solutions, Nb is present as [NbOF5] 2− , while Ta forms [TaF7] 2− . Quantum chemical calculations show that in [MOCl5] 2− (M = Nb, Ta, Pa, Db), the Ta=O bond is the weakest. This may be due to relativistic effects, which strongly stabilize the d-level in Ta and thus make it less accessible for dative π-bonding [55].
Reported synthetic and structural data for Nb(V) and Ta(V) catecholate complexes can thus be presented in the following scheme (Figure 7).
None of the reported complexes can be recrystallized from aqueous solutions either in air or in air-free conditions. After dissolution in water in air, the obtained solution immediately changes color from yellow to dark brown (practically black). The 1 H and 13 C NMR spectra of such solutions cannot be adequately assigned due to the presence of a huge number of species. The same type of speciation was found after dissolution of the corresponding solid samples in water under air-free conditions. The solid samples of isolated compounds change color from yellow to dark brown in an air atmosphere. In argon, the color change does not proceed. The phase purity check by XRPD using initial and aged samples does not reflect any significant difference, indicating changes only to the surface of the crystals.
None of the reported complexes can be recrystallized from aqueous solutions either in air or in air-free conditions. After dissolution in water in air, the obtained solution immediately changes color from yellow to dark brown (practically black). The 1 H and 13 C NMR spectra of such solutions cannot be adequately assigned due to the presence of a huge number of species. The same type of speciation was found after dissolution of the corresponding solid samples in water under air-free conditions.
The solid samples of isolated compounds change color from yellow to dark brown in an air atmosphere. In argon, the color change does not proceed. The phase purity check by XRPD using initial and aged samples does not reflect any significant difference, indicating changes only to the surface of the crystals.

General Information
The catechol and ammonia solutions were of commercial quality (Sigma Aldrich, St. Louis, MO, USA) and were used as purchased. The K7[HNb6O16]·13H2O and Cs8[Nb6O19]·14H2O were synthesized using the same data as previously reported by Abramov et al. [57]. All syntheses described below were carried out in an Ar atmosphere using the Schlenk technique. The Nb2O5·xH2O was prepared by acidifying K7[HNb6O16]·13H2O aqueous solution and was used as freshly prepared.
An elemental analysis was carried out on a Eurovector EA 3000 CHN analyzer. The FT-IR spectra were recorded on an FT-801 spectrometer (Simex, Novosibirsk, Russia). The

General Information
The catechol and ammonia solutions were of commercial quality (Sigma Aldrich, St. Louis, MO, USA) and were used as purchased. The K 7 [HNb 6 O 16 ]·13H 2 O and Cs 8 [Nb 6 O 19 ]·14H 2 O were synthesized using the same data as previously reported by Abramov et al. [57]. All syntheses described below were carried out in an Ar atmosphere using the Schlenk technique. The Nb 2 O 5 ·xH 2 O was prepared by acidifying K 7 [HNb 6 O 16 ]·13H 2 O aqueous solution and was used as freshly prepared.
An elemental analysis was carried out on a Eurovector EA 3000 CHN analyzer (Pavia, Italy). The FT-IR spectra were recorded on an FT-801 spectrometer (Simex, Novosibirsk, Russia). The TGA experiments were conducted using a NETZSCH TG 209 F1 device in an Al crucible by heating a sample from 20 to 300 • C with a 10 • C gradient. The TGA data are summarized in Supporting Information (Figures S5-S10).

Synthesis of (NH 4 ) 3 [NbO(cat) 3 ]·4H 2 O (1):
A total of 2 g of catechol (18.6 mmol) was added to a suspension of 1 g Nb 2 O 5 ·xH 2 O (3.1 mmol) in 20 mL of H 2 O. After the addition of 4 mL of NH 3 ·H 2 O, the resulting reaction mixture was refluxed for 1.5 h until the clear-red solution formed. Small yellowish crystals formed after the solution was kept at 5 • C for 3 days. The crude product was collected by filtration on a glass filter, washed with diethyl ether, and dried in vacuo. The typical yield was 0.740 g (23% based on Nb 2 O 5 ). Anal. Calc. for C 18

Synthesis of (NH 4 ) 4 [Ta 2 O(cat) 6 ]·3H 2 O (4):
A total of 1 g of catechol (9.1 mmol) was added to a solution of 1 g K 8 [Ta 6 O 19 ]·16H 2 O (0.5 mmol) in 20 mL of H 2 O. After adding 5 mL of NH 3 ·H 2 O, the resulting mixture was refluxed for 2 h. After that, the hot yellow solution was filtered from the white precipitate, and 50 mL of EtOH was added to the solution. Crystals formed after the solution was kept below 5 • C for one week. The crude product was collected by filtration on a glass filter, washed with diethyl ether, and dried in vacuo. The typical yield was 0.485 g (26% based on hexatantalate). Anal. Calc. for C 36

Synthesis of Cs 2 [Ta(cat) 3 (Hcat)]·H 2 cat (5):
A total of 1.46 g of catechol (13.3 mmol) was added to a solution of 1 g Cs 8 [Ta 6 O 19 ]·14H 2 O (0.37 mmol) in 20 mL of H 2 O. After adding 5 mL of NH 3 ·H 2 O, the resulting mixture was refluxed for 2 h. After that, the hot yellow solution was filtered from the white precipitate. Crystals formed after the solution was cooled down to room temperature. The crude product was collected by filtration on a glass filter, washed with diethyl ether, and dried in vacuo. The typical yield was 0.643 g (26% based on hexatantalate). Anal. Calc. for C 30

Synthesis of Cs 4 [Ta 2 O(C 6 H 4 O 2 ) 6 ]·7H 2 O (6):
A total of 0.73 g of catechol (6.63 mmol) was added to a solution of 1.0 g Cs 8 [Ta 6 O 19 ]·14H 2 O (0.37 mmol) in 20 mL of H 2 O. After adding 5 mL of NH 3 ·H 2 O, the resulting mixture was refluxed for 2 h. After that, the hot yellow solution was filtered from the white precipitate, and 50 mL of EtOH was added to the solution. Crystals formed after the solution was kept below 5 • C for one week. The crude product was collected by filtration on a glass filter, washed with diethyl ether, and dried in vacuo. The typical yield was 0.485 g (26% based on hexatantalate). Anal. Calc. for C 36

X-ray Diffraction on Single Crystals
The crystallographic data and refinement details are given in Table S1. Structures were solved by SHELXT [58] and refined by a full-matrix least-squares treatment against |F| 2 in anisotropic approximation with SHELXL 2019/3 [59] in the ShelXle program [60]. The main bond distances are given in Table S2. The crystal packing projections are given in Supporting Information (Figures S11-S15). The ellipsoid plots for all complexes are collected in SI (Figures S16-S21).

Conclusions
This research reports preparation protocols for Nb V and Ta V catecholate complexes in aqueous solutions. The reaction conditions presented provide opportunities to use hydrated metal oxides or hexametalates as starting materials. The direct reaction with catechol is a new point in the chemistry of polyoxoniobates and tantalates, generating many possibilities for future research. The choice of a reagent for basic media generation is very important and affects the formation of the final product. The load of catechol is also very important and allows one to control the formation of {M(cat) 3  In order to obtain more extensive information about intermediate or more complex species, additional studies are needed. Another challenging point is catching the species generated during the solvation of the reported complexes. At the current stage, NMR data show the formation of plenty of complexes after the dissolution of the titled compounds in water or DMSO.