Polyoxoniobates and Polyoxotantalates as Ligands—revisited

This short review summarizes our contribution to the coordination chemistry of noble metals (organometallic fragments of Rh, Ir, Ru and hydroxo Pt(IV)) and polyoxocomplexes of niobium and tantalum.

The high charge density means that [M6O19] 8− anions are expected to act as efficient ligands towards various metals.This is indeed so, and a few examples of coordination of [Nb6O19] 8− to Mn(IV) and Ni(IV) were reported, as early as in 1967-1969 [18][19][20][21][22][23][24], and Co(III), reported only in 2011 [30], giving complexes of the type [M(Nb6O19)2] n− , where two hexaniobate units (each acting as tridentate ligand) sandwich an octahedrally coordinated central metal ion.There is also a strong association of hexaniobate with alkali metal cations, even in water, where the existence of neutral [A8Nb6O19] (A = Rb, Cs) species was established by Small Angle X-ray Scattering (SAXS) [31].Curiously enough, no tantalate analogues of [M(Nb6O19)2] n− could be obtained [32].It is doubtless highly desirable to expand this still rather modest list of existing complexes.However, strongly basic conditions, necessary to prevent hydrolysis of hexaniobate (and precipitation of niobium oxide), severely limit the choice of suitable metal precursors (they would simply give insoluble hydroxides).This drawback can be obviated in two different ways.First, in order to limit the hydrolysis and ensuing condensation, a part of the coordinating positions of a transition metal can be occupied by a polydentate (or polyhaptic) ligand (or simply by a strongly coordinated unidentate ligand, such as CO).This can be achieved for Co(III), Cr(III), Cu(II) and Ni(II) in the presence of polyamines [33][34][35][36][37]. Particularly attractive for this purpose are the isolobal fragments d 6 -fac-{ML3}, where ML3 stands for {M(CO)3} + (M = Mn, Re), {(arene)Ru} 2+ , {Cp*Rh} 2+ , {Cp*Ir} 2+ .In fact, a decade ago Pope et al. used [M6O19] 8− (M = Nb, Ta) as robust inorganic tridentate ligands in reactions with [Mn(CO)3(CH3CN)3]ClO4 or [Re(CO)5Br], which gave rise to trans-[(M'(CO)3)2M6O19] 6− (M' = Mn, Re; M = Nb, Ta) [38].Proust et al. reported the syntheses of a series of complexes combining {(arene)Ru} 2+ and [Nb6O19] 8− with Ru/POM stoichiometries ranging from 1:1-4:1 [39].Coordination of organometallic fragments, such as {(p-cymene)Ru} 2+ or {Cp*Rh} 2+ , to POM adds extra possibilities for the synthesis of hybrid (i.e., combining chemically very different moieties within the same structure) complexes.This development reflects not only academic interest in such compounds, being a logical development of POM chemistry in general, but also is driven by the well-known catalytic properties of many POM complexes of transition metals, in particular, with noble metals [40].
The second way is to use a metal which forms stable well-defined and water soluble hydroxocomplexes, such as Pt(IV), for which both [Pt(OH)4(H2O)2] and [Pt(OH)6] 2− exist as individual species [41].In this case, a particular attraction for coordinating Pt(IV) to [Nb6O19] 8− comes from the reports about use of PONb in photocatalysis.In this case, a photoactivator is necessary, which role is typically fulfilled by [PtCl6] 2− [42].From this point of view, direct coordination of Pt(IV) to PONb is very desirable.Pt(IV), being octahedral and sufficiently oxophilic, could either substitute a Nb atom in the hexaniobate structure, similar to [H2Pt IV V9O28] 5− which is simply a Pt(IV)-monosubstituted classical decavanadate [43,44], or be "grafted" as a capping atom.As we will see, the latter is the case.
In this short review, we focus on the recent advances in the field of group V (in particular, Nb and Ta) POM complexes, concentrating on the synthesis of Lindqvist-based POM with incorporated noble metal centers and their potential applications as electrocatalysts for water oxidation.The second part addresses related studies with heteropolyniobiate homologues coordinating noble metal fragments.The third part deals with the current use of analytical tools to unravel the integrity and reaction dynamics in the solution and selected examples are discussed.

Reactivity of [M 6 O 19 ] 8− towards Noble Metal Complexes
In this section, studies of reactivity of the Lindqvist [M6O19] 8− (M = Ta and Nb) POM towards a noble metal complex are discussed, with an emphasis on finding suitable experimental conditions for noble metal grafting, the main structural features of the resulting solids, as well as their behavior in aqueous solution.2) can be readily isolated in high yields (80% for Nb and 70% for Ta) [45].Crystal structures of both complexes contain hybrid bi-capped organometallic-POM anions trans-[{Cp*Rh}2M6O19] 4− (Figure 1) with the typical Lindqvist-type oxide metal core (metal octahedron with one μ6-, twelve μ2-and six terminal oxygen ligands) decorated at an opposite pair of the triangular {M3O3} faces with two capping organometallic fragments {Cp*Rh} 2+ .In both cases, exclusively trans-isomers crystallize.No cis-isomers were observed in solution or in solid.Both 1 and 2 produce basic solutions in water; 3 mM solutions have the pH value of 8.8, which is much lower in comparison to that produced by alkali metal salts of parent [M6O19] 8− of comparable concentration (10.5-11.0).The ESI(-) mass spectra of aqueous solutions of 1 and 2 display prominent peaks assigned to the [{Cp*Rh}2M6O19 + 2H] 2− dianions, accompanied by minor signals from the singly-charged [{Cp*Rh}2M6O19 + 3H] − anions.However, the protonation degree observed by ESI-MS does not necessarily mirrors that in the solution because of the influence of the ESI process itself on the protonation [46].For example, the most efficient ionization mechanism observed by ESI-MS for the decaniobate [Nb10O28] 6− ion was the uptake of up to three protons, whereas solution studies indicated no protonation [26,47].
It is worth stressing that such condensation is unknown for free, non-coordinated hexatantalate.Perhaps, the reduced charge of the 1:1 complex [{(C6H6)Ru}Ta6O19] 6− facilitates interaction between two negatively charged species; coordination of electron-accepting fragment {(C6H6)Ru} 2+ may affect electronic density on the oxygen atoms of the POM part as well.
Decreasing the POM/Ru ratio to 1:2 leads to the bis-capped complex trans-[{(C6H6)Ru}2Ta6O19]   Oxidation process generates a significant rise in the current for a catalytic process mediated by the complex.The current in the presence of the platinum niobate is several times higher than for the blank solution.This electrochemical behavior is similar to the reported for the nonatungstate complex with a tetrarhodium-oxo core [51].

Reactivity of Heteropolyniobates towards Noble Metal Complexes
In 1979, S.K. Ray reported that reaction of K7H[Nb6O19]•13H2O with H6TeO6 gave a heteropolyniobate formulated as K8H2[TeNb12O38]•13H2O [52].This formula was deduced uniquely from elemental analysis.Such stoichiometry could imply that this compound would be analogous with the known heteropolyniobates [(Nb6O19)2M] n− (M = Co, Ni, Mn, Pt) with octahedral Te VI sandwiched between two hexaniobates.We were curious if it were really so and repeated these experiments, only to find out that the purported "dodecaniobate" was in fact [(OH)TeNb5O18] 6− , which can be regarded simply as a substituted hexaniobate with one {NbO} 3+ vertex replaced with a {TeOH} 5+ vertex.In fact, this complex had been already identified by W. Casey and co-workers, who prepared it from Nb2O5•xH2O, Me4NOH and H6TeO6 in methanolic solution [53].Perhaps, the authors of [52] dealt with an approximately stoichiometric mixture of [(OH)TeNb5O18] 6− and [Nb6O19] 8− , which led them to the wrong conclusion about the composition of the product.Surface charge density of telluroniobate is lower than in the hexaniobate, which means that their reactivity might be different.In order to check the significance of this difference, reactivity studies towards noble metal complexes have been extended in our group to the telluroniobate [(OH)TeNb5O19] 6− .

Reaction [(OH)TeNb5O19] 6− with {(C6H6)Ru} 2+
Reactivity studies on [(OH)TeNb5O18] 6− were also extended to [(C6H6)RuCl2]2 as source of the {(C6H6)Ru} 2+ .ESI-MS can be conveniently used to monitor the advance of the reaction.Grafting of one and two {(C6H6)Ru} 2+ fragments can be immediately inferred from the characteristic m/z shifts to higher values, as illustrated in Figure 6.Moreover, the uptake of one or two Ru-containing fragments confers a new characteristic isotopic pattern due to multiple isotopes of the Ru at the natural abundance.Formation of single and double capped complexes [(OH)TeNb5O18{(C6H6)Ru}]

Analytical Tools to Study Solution Speciation in Nb and Ta POM Chemistry
One of the general features of the reactivity of POMs is their lability in aqueous solutions up to the point that such solutions can be regarded as virtual libraries of different building blocks, which equilibrate together and can form different structures depending on counter cations, pH and etc.The simplest case is protonation and [Nb6O19] 8− in water have been reported and important differences in the solution behavior have been observed due to the distinctive propensity to aggregation with counteractions for M = Ta and Nb [61].Uv-vis spectroscopy complemented with DFT calculations have been also reported [62].More complicated processes connected with rearrangement of POM backbone can take place [63][64][65].It is obvious that proving the identity between species existing in solid state and solution is a very important part of this chemistry.However, the study of solution behavior of polyniobates and polytantalates is a challenging task.These species are colorless, diamagnetic, show no redox behavior and are, under most conditions, in a dynamic equilibrium.In this section, useful analytical tools that can provide complementary signatures for the Nb and Ta POM characterization in solution are discussed.In particular, the usefulness of 1 H NMR, 1 H NMR Diffusion Ordered Spectroscopy (DOSY), capillary electrophoresis and ESI-MS in concert is underlined.An illustrative example is given to demonstrate the complementarily of all techniques to determine stoichiometry and solution dynamics.
The possibilities of NMR are restricted to the use of 17 O, which unfortunately has very low natural abundance and sensitivity.It was used, for example, to follow the conversion of several niobates in water [66,67].However, coordination of organometallic fragments to niobates and tantalates provides new opportunities for use of NMR for solution studies.In particular, structural information can be gained from 1 H NMR signals of the organic ligands attached to the grafted heterometal, which would tell us about the overall symmetry of the compound.Moreover, it is possible to estimate diffusion coefficients from 1 H NMR DOSY of the organic part that can be subsequently used to calculate molecular weight of a particular species.This 1 H DOSY NMR technique is becoming a powerful tool for the determination of the size (hydrodynamic radius) and the molecular weight of inorganic clusters [68][69][70].This technique has not been virtually used in the chemistry of Nb and Ta POM prior to our work.
Another useful tool for solution studies is capillary electrophoresis (CE).It is rarely used in the chemistry of POMs [71,72].The possibilities opened up by the use of CE for POMs mixtures were studied in a few cases [73].Although in these works CE revealed itself as a promising analytical tool, its potential for POM chemistry is far from being fully exploited.Combination of CE, HPLC and other methods (in particular ESI-MS and NMR) can provide sufficient information about quantitative and qualitative composition of solutions, numbers of POM species, and their stability and evolution with time.
Accurate determination of the formulae and structure of new compounds remains an important issue in POM chemistry.In this respect, use of soft ionization mass spectrometric techniques, and in particular, ESI techniques, is becoming widespread.Excellent reviews on the use of soft ionization MS techniques can be found elsewhere; for example, Cronin et al. illustrated the versatility of ESI and cryospray ionization techniques not only for characterization of final POM assemblies, but also to prove mechanisms of self-assembly of supramolecular POM-based architectures in solution [47,50].Ohlin et al. reported an important work on this topic highlighting different approaches to obtain reliable information on the dynamic solution of POM on the basis of ESI-MS [47].However, there are several potential limitations to using ESI-MS alone for the investigation of POM.This is the case when dynamic speciation involving hydroxy-bridged POM species, the pH value, ionic strength or the counteraction plays an important role.In addition, during the ESI process itself, the samples undergo dramatic changes in pH and concentration, which may perturb the speciation equilibrium.Moreover, due to the high charge state and molecular weight, the use of ESI-MS spectrometers equipped with high resolution analysers is important to facilitate unambiguous molecular compositions' determination.In this part, we will discuss a case study, where we used ESI-MS, 1 H DOSY NMR and CE techniques, combined together, in order to elucidate the solution behavior of the {(C6H6)Ru} 2+ /[Ta6O19] 8− system to afford compounds 9 and 10 [48].5.85 ppm).These results necessarily imply two equilibria.The first corresponds to cis-trans isomerization process, strongly displaced towards the trans isomer.The latter corresponds to the de-coordination of a {(C6H6)Ru} 2+ moiety.This hypothesis is supported by the observation of a small broad peak at 5.4 ppm, attributed to [(C6H6)2Ru2(OH)3] + (see Equation 2).

trans-[{(C6H6)Ru}2Ta6O19
ESI(−) mass analysis of Na10-10 in water reveals only the presence of the monomeric 9 6-species, thus indicating that the dimerization depicted in Eq. 1 is a reversible process ongoing from the solution to the solid state and vice versa.The electrophorograms recorded for aqueous solutions of Na 10 -10 and Na 4 -9 are presented in Figure 8.The solutions of Na 10 -10, to which some amount of Na 4 -9 has been added as reference, give two well-resolved peaks (Figure 7).The second peak corresponds to 9, which was confirmed by UV-spectroscopy and by the standard addition method.The first peak must therefore correspond to the monomeric [{(C6H6)Ru}Ta6O19] 6− species (or its dominant protonated form), because the signal from the dimer with its larger size and charge, should appear after the signal of 10, and with a longer retention time.

Conclusions
The chemistry of niobium and tantalum polyoxoanions still belongs to a little researched area of inorganic chemistry.Many unusual complexes still remain hidden, and isolation of principally new [(Ta6O18)2(µ-O)] 14− backbone is a tantalyzing hint at this potential richness.In this short review, we summarized our recent research focused on the coordination of noble metals to hexametalates.Grafting of {Cp*M} 2+ (M = Rh, Ir) on the surface of Lindqvist-type hexametalates (including [(OH)TeNb5O19] 6− ) was studied with full characterization of the hybrid complexes in solid state and solution.Coordination of {(C6H6)Ru} 2+ to [Ta6O19] 8− gives unprecedented [({(C6H6)Ru}Ta6O18)2 (µ-O)] 10− .Complexation of Pt(IV) with [Nb6O19] 8− was studied with different techniques.New dimeric complex [(Nb6O19{Pt(OH)2})2] 12− is obtained when hexaniobate reacts with Pt(IV) in 1:1 molar ratio, while increasing the Pt/Nb6 ratio to 2:1 gives sandwich-type complex [Pt(Nb6O19)2] 12− .We used different, both traditional and non-traditional, analytical tools for studying the solution behavior of individual reaction products and reaction mixtures, a challenging task for the systems where few analytical methods are readily available.The 1 H, 13 C and DOSY NMR techniques combined with ESI-MS and CE highlight are adequate tools for characterization of complex solutions.An illustrative example dealing with the {(C6H6)Ru} 2+ /[Ta6O19] 8− system demonstrates complementarity of all these techniques for determination of stoichiometry and solution dynamics, an approach both promising and highly recommendable to trace unambiguously the identity of large inorganic systems [74].
] 6− with {Cp*Rh} 2+ and {Cp*Ir} 2+ We investigated reaction of [(OH)TeNb5O18] 6− with [Cp*MCl2]2 as sources of {Cp*M} 2+ (M = Rh, Ir).Despite the difference in the surface charge density between [(OH)TeNb5O18] 6− and [Nb6O19] 8− , formation of double-capped complexes takes place as easily as with hexaniobate.Crystal structure of Na2[{Cp*Rh}2Te(OH)Nb5O18]•24H2O (14) shows coordination of two {Cp*Rh} 2+ fragments, with the Te atom localized in the central M4 plane of Lindqvist-type anion as illustrated in Figure 5. Two other metal positions are fully occupied by niobium.However, this disorder corresponds to four different orientations of a single isomer which is the only possible structure for the trans complex in the solid state.