A decatungstate-type polyoxoniobate with centered manganese: [H2Mn(IV)Nb10O32]8- as a soluble tetramethylammonium salt.

A highly symmetric Mn(IV)-centered polyoxoniobate [H2Mn(IV)Nb10O32](8-) was synthesized via hydrothermal methods as a soluble tetramethylammonium salt. The structure is similar to decatungstate structure [W10O32](4-), except for the central heteroatom. The cluster is stable between 4 < pH < 10, as was characterized with ESI-MS and UV-Vis spectroscopy.

Scheme 1B) where two W 5 O 18 units are linked by a central lanthanide ion. 10 Pope et al. synthesized a [Mn(CO) 3 ] + -capped hexaniobate more recently. 11 However, complete encapsulation of manganese within the PONb framework has not yet been achieved. Echoing the goals of the Pope et al., 11 manganese substitution in PONb might benefit attempts to sequester dangerous nuclides, such as 99 Tc, in nuclear waste as niobates are generally stable at basic-pH conditions.
Herein we report a new type of Mn IV -substituted PONb cluster, [H 2 Mn IV Nb 10 O 32 ] 8− [MnNb 10 , Scheme 1C], as a tetramethylammonium (TMA) salt, TMA 8 [H 2 Mn IV Nb 10 O 32 ]·22H 2 O (1). This cluster ion can be viewed as two MnNb 5 Lindqvist motifs condensed by bridging oxygen atoms and sharing a Mn IV site. We note that the structure of this cluster is similar to decatungstate ion, 12  The compound 1 was synthesized by hydrothermal reaction of the mixture of hydrous niobium oxide, KMnO 4 , TMAOH at 110°C for 3 days. We note that higher reaction temperatures lead to decomposition of the cluster over increased reaction times. The resulting brown solution after reaction was washed with isopropanol and extracted with ethanol. Purification to get analytically pure compound of 1 was challenging due to the similar solubility of 1 and dark colored impurity. The crystals of 1 were obtained as olive-gold colored crystals in the dark-brown viscous oily product. The dark, oily product was carefully washed with minimal amounts of ethanol to isolate crystals, since the dark colored impurity was only slightly more soluble in ethanol than 1. We note that the previously reported reaction condition to obtain MnNb 12 sandwich complex 8 involved lower temperature (∼90°C) and shorter reaction times (less than an hour) compared to the conditions used to synthesize 1. We suspect that the Mn IV encapsulated by the niobates results from a solution driven to a thermodynamically more stable end point by longer reaction times and higher temperatures.
The structure of 1 was determined by single crystal X-ray crystallography. ‡ The cluster in 1 has manganese at the center surrounded by ten oxo-niobate frameworks, possessing idealized D 4h symmetry [Scheme 1C, Fig We note that the cluster MnNb 12 has similar, but more regular, Mn-O lengths of 1.863 (26) Electrospray-ionization mass spectrometry (ESI-MS) confirmed the identity of the cluster in the solution [Fig. 2]. The mass spectrum of 1 shows series of peaks for −4, −3 and −2 charged ions. The series of −4 ion peaks can be explained as due to fragmentation of the relatively highly charged However, 1 exhibits the same ESI-MS spectra without contact with methanol. Thus actual existence of methylated cluster in our system is unlikely and we believe that the peaks corresponding to CH 3 + adduct originated from the fragmentation of TMA during ionization. The color of the aqueous solution of 1 is yellow and distinctly different from the reported orange color of MnNb 12 ,  The stability of the cluster was examined by using both UV-Vis and ESI-MS titration experiments. When dissolved into solution at 2 mM concentrations, the solution reaches a natural pH of 8.7. Adding acid or base to this solution leads to decomposition of the cluster, as is evident in the decreased abundance of peaks assignable to clusters in ESI-MS of 2 mM and 0.2 mM solutions [ Fig. 3 and Fig. S5, † respectively]. When titrating with base, peaks assignable to the clusters significantly decrease in abundance at pH > 11 for a 2 mM solution of 1 and pH > 9 for 0.2 mM solution of 1. ESI-MS spectra taken during an acid titration of 1 indicates that the cluster is stable until pH ∼ 5, however, the initial golden yellow color of the solution started to fade at pH < 7 and changed to orange-pink color near pH ∼ 4 during the titration of a 2 mM solution of 1 with acid, while the yellow solution color did not change with base addition until pH ∼ 13. In addition to decomposition, we suspect that protonation of structural oxygens is affecting the color.
ESI-MS of the acidified solution indicated an increase in peaks assignable to H + adducts of 1, with a decrease of the CH 3 + -adduct peak. The increase in CH 3 + adduct peaks during base addition is expected and attributable to the increased TMA concentration, added to the solution as a TMAOH base and then fragmented in the ionization step of ESI-MS [ Fig. 3 and We suspect that these indicate decomposition of the cluster, in agreement with the ESI-MS results. The absorption spectra did not change significantly until acidified to pH 4.4. Below this pH, overall absorption gradually increased until the compound finally decomposed to form a colored precipitate that we assume is hydrous, manganese-niobium oxide. The behavior is similar in the higher concentration solutions, but the 2 mM solution begins to flocculate as early as pH 6.5, reflecting the higher sample concentration and existence of background salt (0.1 M TMACl), which was not used in ESI-MS titration experiment.
In summary, we report a new Mn IV -substituted PONb, [H 2 Mn IV Nb 10 O 32 ] 8− as a soluble TMA salt. It has a relatively wide stability between 4 < pH < 10 in aqueous solutions, suggesting a robust use in applications, possibly complementing the decatungstate ion. This work was supported by an NSF CCI grant through the Center for Sustainable Materials Chemistry, number CHE-1102637.