[Li 6 (C 2 N 2 H 8 ) 11.5 ][Fe 4 Se 8 ]: the first lithium-containing chalcogenidotetraferrate synthesized in solution

The octaselenidotetraferrate(II/III) with six Li atoms as counter-ions chelated by ethylenediamine (en, C 2 H 8 N 2 ), {[Li 6 (en) 11.5 ][Fe 4 Se 8 ]} n , was synthesized from Li, FeSe and Se in an ethylenediamine solution at room temperature. Its crystal structure was determined at 100 K and has triclinic ( P 1) symmetry. The [Fe 4 Se 8 ] 6 (cid:2) anions show a connectivity comparable to a distorted tetrahedral cluster structure. Contact distances are given, and central structure features compared to literature known selenidoferrates with the same anion motif.


Chemical context
Alkali metal chalcogenido ferrates A x [Fe y Ch z ] (A = LiÁ Á ÁCs; Ch = OÁ Á ÁTe) show [FeCh 4 ] tetrahedra to be the most prominent coordination motif of the anionic moiety. These can be isolated, connected into chains, layers, or three-dimensional networks. A special motive is the connection of four such [FeCh 4 ] (Ch = S, Se, Te) units via three shared edges each to form a tetramer. This secondary structure can be described as a distorted heterocubane [Fe 4 Ch 4 ] xÀ core, with one additional chalcogenido ligand on every iron corner of the heterocubane (Schwarz & Rö hr, 2015). Another approach to describe those tetramers can be as a distorted tetrahedral star (stella quadrangula; Stü ble et al., 2016).
For the compound presented herein, an initial attempt was to use the solubility of Li and Se in ethylenediamine to form in situ Li 2 Se, which would then react with iron(II) selenide to a lithium selenidoferrate(II). Instead, the solution approach yielded the mixed-valent tetraferrate [Li 6 (en 11.5 )][Fe 4 Se 8 ].
The Se-Fe-Se angles for the bridging Se atoms are in the range of 101.23 (2)-108.66 (3) and therefore smaller than the ideal tetrahedron angle. As a result of the contraction of the heterocubane along the crystallographic b-axis, the Se-Fe-Se angles are the smallest in this direction, 101.23 (2)-101.90 (2) . The Se-Fe-Se angles of the terminal Se atoms are in the range of 108.31 (2)-119.59 (3) .
Five of the six crystallographically independent Li cations are tetrahedrally coordinated by one amine group of four ethylenediamine molecules. The ethylenediamine molecules bridge adjacent lithium cations to form an infinite threedimensionally connected network of 3 1 [Li 6 (en) 11.5 ] 6+ . The last Li cation is coordinated by three amine groups of three independent ethylenediamine molecules with the fourth coordination site occupied by one of the terminal selenido ligands (Se3) as depicted in Fig. 4. The coordination of Se to the Li cation results in the non-integer number of ethylenediamine ligands in the sum formula. As a result of the individual coordination environments of lithium ions, a complicated 6-nodal net with point symbol {3.6.7 3 .8} 2 {3.7 3 .8 2 }{4 2 .6.8 2 .9}{4 2 .6 3 .8}{6.8.9} is obtained (ToposPro V 5.4.1.0;Blatov et al., 2014).

Supramolecular features
The anion packing can be described as layers stacked in an AB type. The Li coordinated to the terminal Se ligand pointing in one direction defines the A layer, while the ligand pointing in the opposite direction defines the B layer. The layers are shifted with respect to each other such that the anions of one layer are placed between the anions of the other layer. The formation of those layers correlates with the inversion centre in the unit cell containing two asymmetric units. The Liethylenediamine network is located both in between and within those layers surrounding the anions (see Fig. 5). It is stabilized by classical N-HÁ Á ÁSe and non-classical C-HÁ Á ÁSe hydrogen bonds (Table 1).
The hydrogen bonds between the ethylenediamine molecules of the cationic network and the selenium atoms of the anion can be described by graph-set theory (e.g. Etter et al., 1990). In graph-set theory, a hydrogen-bond network can be described by a pattern designator (G), the pattern's degree (r) and the number of donors (d) and acceptors (a): G a d (r). G can be S (intramolecular), C (chains), R (rings) or D (non-cyclic) and r is the number of atoms before repetition. To be able to determine the graph sets, the Li-N coordination was also counted as a 'bond' in the network, so rings or chains can contain Li atoms. The anionic moiety was not counted as taking part in the network; every acceptor Se atom was looked at individually. Only the smallest/simplest pattern for each set is given. For Se1 the graph set is three times R 1 2 (8), for the donor pairs N7, N14; N14, N16; N7, N16. Se2 can be described by two sets of R 1 2 (4) for N10, N18; N22, N23 and two R 1 2 (5) for N7, N13 and N21, N23. Se3 is bound in two sets of R 1 2 (4) to N2, N8; N15, N17 and in R 1 2 (5) to N2, N15 and R 1 2 (9) to N2, N24. Se4 has two sets, R 1 2 (8) N2, N8 and R 1 2 (12) N2, N6/N6A (the split positions of N6 are counted in a single set). Se5 has five sets of classical hydrogen bonds: three sets of R 1 2 (4) for N1, N19; N3, N19 and N12, N13; two sets of R 1 2 (5) for N2, N22 and N16, N19 and one set of non-classical hydrogen bonds D for C6. Se6 has one set of classical hydrogen bonds, R 1 2 (5) to N1, N18 and one set of non-classical D to C20. Se7 has one set of R 1 2 (5) for N4, N17 and one R 1 2 (7) for N12, N24A and one R 1 2 (9) N12, N20. Se8 has three classical sets of hydrogen bonds: one   Atom labelling of the anionic moiety in [Li 6 (en 11.5 )][Fe 4 Se 8 ] with selected distances. Green: Fe, red: Se, ellipsoids are drawn with 70% probability. set of R 1 2 (4) to N4, N21, C 2 2 (4) for N5, N9 and R 1 2 (5) to N10, N14 and two non-classical sets, C 2 2 (8) to C1, C10/C10A (the split positions of C10 are counted in a single set) and C 2 2 (10) C1, C13.

Database survey
A search of the Cambridge Structural Database (CSD version 5.43; Groom et al., 2016) for compounds showing the motif of a heterocubane core containing iron and chalcogens yielded 548 results. Most of those are structures containing sulfidoferrate heterocubane cores, which are investigated with regard to iron-sulfur proteins. For a more direct comparison, only compounds comprising one terminal ligand on each iron atom and solely Se as the chalcogen were chosen, as the bond lengths inside the cubane core differ significantly depending on whether S or Se is part of the heterocubane. First, selenido ferrates, which only differ in the anionic moiety, are compared. In the second step, we analyse compounds that contain organic ligands on the terminal Se atoms.
Regarding the elemental ratios of alkali metal to Fe and Se, [Li 6 (en 11.5 )][Fe 4 Se 8 ] is identical to K 6 [Fe 4 Se 8 ] (ICSD: 430631; Stü ble et al., 2016). In the following, the bond lengths of the anion are compared to the ones in K 6 [Fe 4 Se 8 ], despite the differences in the cationic moiety. The Fe-Se bonds to the terminal Se atoms in the potassium selenido tetraferrate are in the range of 2.320 (2)-2.307 (2) Å and therefore shorter than the respective bonds in the title compound, lithium selenido tetraferrate [2.3251 (8)-2.3502 (7) Bronger et al., 1983). In K 6 [Fe 4 Se 8 ], 24 cations form a distorted cube around the anions. In the title compound, no such regular shape can be observed for the surrounding cations, probably due to the more complex cation motif with the ethylenediamine ligands interlinking the alkali metal ions.
Comparing the bond angles and distances of the anionic moiety to homologues with aliphatic or arylic ligands at the terminal Se atoms, the bond lengths inside the heterocubane core are in a similar range: Fe-Se distances are found to be 2.3573 (12)

Synthesis and crystallization
500 mg (3.71 mmol, 1 eq.) of iron(II) selenide, 293 mg (3.71 mmol, 1 eq.) of Se and 52 mg (7.49 mmol, 2 eq.) of Li were stirred in 50 mL of dry ethylenediamine under an argon atmosphere. The reaction mixture was stirred for 24 h, during which time several colour changes from brown to green to brown occur; these are most likely associated with the various intermediate polyselenide anions. After 24 h, a final colour change to brown was observed and the solution was filtered and allowed to stand for 16 weeks to afford crystallization. A crystal suitable for X-ray diffraction analysis was chosen in Paratone oil under a light microscope. The obtained crystals were comparably large and visible without a microscope. Breaking the crystal typically yielded very small crystallites that immediately decomposed because of their air and research communications Acta Cryst. (2022). E78, 1204-1208 Fuss and Thiele [Li 6 (C 2 H 8 N 2 ) 11.5 ][Fe 4 Se 8 ] 1207 Table 1 Hydrogen-bond geometry (Å , ).  (6)  145 Symmetry codes: (i) x; y þ 1; z; (ii) Àx þ 1; Ày þ 1; Àz þ 1; (iii) Àx; Ày; Àz þ 1; (iv)

D-HÁ
moisture sensitivity. Therefore, a crystal larger than the X-ray beam was used for analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal was found to comprise two pieces, mis-aligned by $1.6 . For the purpose of integration, and processing, facilities for handling twinning by non-merohedry (namely the HKLF 5 data format in SHELXL) were attempted. In combination with the experimentation with different integration box sizes with common-volume overlap of 4% did not result in an improved dataset.
One of the en ligands is disordered, the disorder was treated with RIGU and DELU restraints. H atoms were treated by a mixture of independent and constrained refinement.

Poly[[tricosa(µ-ethylenediamine)dodecalithium] bis[octaselenidotetraferrate(II/III)]]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.