Synthesis and crystal structure of bis[μ-N,N-bis(2-aminoethyl)ethane-1,2-diamine]bis[N,N-bis(2-aminoethyl)ethane-1,2-diamine]-μ4-oxido-hexa-μ3-oxido-octa-μ2-oxido-tetraoxidotetranickel(II)hexatantalum(V) nonadecahydrate

The crystal structure of the title compound consists of discrete {[Ni2(κ4-tren)(μ- κ3-tren)]2Ta6O19} cluster molecules that are linked by intermolecular O—H⋯O and N—H⋯O hydrogen bonding into layers extending parallel to the bc plane.

Synthesis and crystal structure of bis[l-N,N-bis(2aminoethyl)ethane-1,2-diamine]bis[N,N-bis(2aminoethyl)ethane-1,2-diamine]-l 4 -oxido-hexa-l 3oxido-octa-l 2 -oxido-tetraoxidotetranickel(II)hexatantalum(V) nonadecahydrate Dana ( 4 -tren)(-3 -tren)] 2 Ta 6 O 19 }Á19H 2 O (tren is N,N-bis(2-aminoethyl)-1,2-ethanediamine, C 6 H 18 N 4 ). In its crystal structure, one Lindqvist-type anion {Ta 6 O 19 } 8-(point group symmetry 1) is connected to two Ni II cations, with both of them coordinated by one tren ligand into discrete units. Both Ni II cations are sixfold coordinated by O atoms of the anion and N atoms of the organic ligand, resulting in slightly distorted [NiON 5 ] octahedra for one and [NiO 3 N 3 ] octahedra for the other cation. These clusters are linked by intermolecular O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonding involving water molecules into layers parallel to the bc plane. Some of these water molecules are positionally disordered and were refined using a split model. Powder X-ray diffraction revealed that a pure crystalline phase was obtained but that on storage at room-temperature this compound decomposed because of the loss of crystal water molecules.

Chemical context
The investigation of synthesis conditions and crystal structures of new inorganic-organic hybrid polyoxidometalates (POMs) of V, Nb, Ta, Mo or W is still an emerging research field in inorganic chemistry. The enormous variety of their structural, physical and chemical properties and the resulting potential applications are reflected in the large number of reported compounds (Tagliavini et al., 2021;Streb, 2012;Bijelic et al., 2019;Yamase, 2013;Kö nig, 2020;Č olović et al., 2020;Monakhov et al., 2015). Within the POM family, polyoxidoniobates and -tantalates have a special position because of their challenging synthesis conditions, i.e. high pH values are required as a result of the high stability of their respective oxides. This is the reason why we have been engaged in the research field of POM chemistry for several years, with the aim in developing new synthesis routes, also with an increasing focus on the PONb and POTa chemistry (Mü scher-Polzin et al., 2020a,b;Dopta et al., 2018aDopta et al., ,b, 2020. Most of the POMs are usually synthesized by solvothermal reactions using slightly soluble metal oxides. It turned out that the use of water-soluble compounds as precursor materials is more effective for generating new compounds, which opens the possibility of developing more efficient syntheses at room temperature (Dopta et al., 2020;Mahnke et al., 2018a,b). Some transition metal (TM) decorated POTas have also been synthesized by slow crystallization at room temperature (Guo et al., 2011;Li et al., 2019), which is characterized by long reaction times and high sensibility for parameter changes during reaction. To overcome these drawbacks, we were interested in the possibility of faster crystallization times. To achieve this goal, we used preformed TM complexes and a special combination of different solvent gradients in the reaction vessel. Appropriate TM complexes are based on the tetradentate ligand N,N-bis(2aminoethyl)-1,2-ethanediamine (tren), which offers coordination flexibility, providing two free coordination sites in an octahedral environment, with the possibilities for further ligation to O atoms of POMs or acting as charge-balancing cations. Based on that reasoning, an aqueous solution of K 8 [Ta 6 O 19 ]Á16H 2 O was reacted with the preformed complex [Ni(tren)(H 2 O)Cl]ClÁH 2 O at room temperature, leading to crystallization of violet needle-like crystals of the title compound, which was characterized by single-crystal X-ray diffraction. Comparison of the experimental powder X-ray diffraction pattern with that calculated from single crystal data revealed that a pure crystalline phase had formed. However, the relatively high background indicated the presence of some amount of an amorphous phase (see Fig. S1 in the supporting information). This is in line with the observation that the title compound is very unstable in air, which might be traced back to the loss of crystal water molecules, and was the reason why further investigations were not performed.

Structural commentary
The crystal structure of {[Ni 2 ( 4 -tren)(-3 -tren)] 2 Ta 6 O 19 }Á-19H 2 O consists of one Lindqvist-type anion {Ta 6 O 19 } 8-, located on a center of inversion, as well as two Ni II cations, two N,N-bis(2-aminoethyl)-1,2-ethanediamino ligands and nineteen water molecules that are located in general positions (Figs. 1 and 2). Some of the water O atoms are positionally disordered and were refined using a split model without locating their attached hydrogen atoms.

Figure 4
View of the hydrogen-bonded chains running parallel to [010]. Intermolecular hydrogen bonding is indicated by dashed lines. In the left part, hydrogen atoms were omitted for clarity.

Synthesis and crystallization
0.03 mmol of Ni[(tren)(H 2 O)Cl]ClÁH 2 O were dissolved in 1 ml of a 4:1 DMSO:water solution (v/v) and subsequently transferred into a 5 ml snap-cap glass tube. Then 1 ml of a 3:1 mixture (v/v) of DMSO and water and a solution of 0.0125 mmol of K 8 {Ta 6 O 19 }Á16H 2 O in 1 ml of water (pH = 12.3) were added slowly, one after the other, into the tube, which then was closed and left at room temperature. After a few days, pink-violet needle-shaped crystals were filtered off and washed with mother liquor.

Experimental details
The PXRD measurement was performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-and N-bound hydrogen atoms were refined with idealized positions with U iso (H) = 1.2U eq (C,N) using a riding model. Some of the hydrogen atoms belonging to water molecules were located in a difference-Fourier map. Their bond lengths were set to ideal values and they were refined with U iso (H) = 1.5U eq (O). Some of the water atoms (O16-O19) are positionally disordered and were refined using a split model with 50% occupation for each of the corresponding sites; O20 was refined with one position and an occupation of 50%. The hydrogen atoms of water molecules that could not be located were considered in the calculation of the molecular formula.