Crystal structure of hexakis(N,N-dimethylformamide-κO)iron(III) μ-chlorido-bis(trichloridocadmium)

Octahedral [Fe(DMF)6]3+ and tetrahedral-based [Cd2Cl7]3− ions stack alternately along the c-axis direction in this rare example of a [Fe(DMF)6]3+-containing ionic salt.

The title compound, [Fe(C 3 H 7 NO) 6 ][Cd 2 Cl 7 ], crystallizes in the trigonal space group R3 and is assembled from discrete [Fe(DMF) 6 ] 3+ cations (DMF = N,Ndimethylformamide) and [Cd 2 Cl 7 ] 3À anions. In the cation, the iron(III) atom, located on a special position of 3 site symmetry, is coordinated by six oxygen atoms from DMF ligands with all Fe-O distances being equal [2.0072 (16) Å ]. A slight distortion of the octahedral environment of the metal comes from the cis O-Fe-O angles deviating from the ideal value of 90 [86.85 (7) and 93.16 (7) ] whilst all the trans angles are strictly 180 . The central Cl atom of the [Cd 2 Cl 7 ] 3À anion is also located on a special position of 3 site symmetry and bridges two corner sharing, tetrahedrally coordinated Cd II atoms. The two Cd atoms and the central Cl atom are colinear. The two sets of terminal chloride ligands on either side of the dumbbell-like anion are rotated relative to each other by 30 . In the crystal, the cations and anions, stacked one above the other along the c-axis direction, are held in place principally by electrostatic interactions. There are also C-HÁ Á ÁCl and C-HÁ Á ÁO interactions, but these are rather weak. Of the six crystal structures reported to date for ionic salts of [Fe(DMF) 6 ] n+ cations (n = 2, 3), five contain Fe II ions. The title compound is the second example of a stable compound containing the [Fe(DMF) 6 ] 3+ cation. The existence of both [Fe(DMF) 6 ] 2+ and [Fe(DMF) 6 ] 3+ cations shows that the DMF ligand coordination sphere can accommodate changes in the charge and spin states of the metal centre.

Chemical context
In our ongoing research into the new functions and applications of coordination compounds with Schiff-base ligands, we have utilized a synthetic scheme involving a zerovalent metal as the source of metal ions, together with another metal salt, in order to prepare new heterometallic complexes Vassilyeva et al., 2018Vassilyeva et al., , 2021. In a typical procedure, the metal powder undergoes oxidative dissolution in air to generate metal ions that then interact with the second metal salt and pre-formed ligand. The condensation reaction between the Schiff-base precursors occurs in situ without isolation of the imine. Dioxygen from the air is reduced to form a water molecule with participation of protons donated by the imine, which is capable of deprotonation. By using the above scheme, new homo-and heterometallic Co III , Co III /Zn II and Co III /Cd II complexes with a Schiff-base ligand derived from 2-hydroxy-3-methoxybenzaldehyde (ovanillin) and the simple amine methylamine have been prepared (Nesterova et al., 2018(Nesterova et al., , 2019. Comparative studies ISSN 2056-9890 of their catalytic behaviours in oxidation reactions of alkanes with H 2 O 2 and m-chloroperoxybenzoic acid were undertaken to elucidate the role of the second (inactive) metal centre (Cd) in the catalytic performance of the heterometallic compounds. Given the remarkable catalytic activity of the Schiff base Fe III metal complexes mimicking the Fe-containing enzymes that oxidize alkanes in nature (Nesterov et al., 2015), we decided to extend our work and replace the cobalt centre with iron in a heterometallic core supported by the above Schiff-base ligand.
To facilitate formation of the desired compound, an additional basic agent, N-phenyldiethanolamine, was introduced following the previous successful participation of diethanolamine in the formation of a mixed-ligand Schiff base Ni II /Zn II dimer (Vassilyeva et al., 2021). In the latter compound, the deprotonated aminoalcohol molecules provide additional alkoxo-bridges between the metal centres. The use of aminoalcohol deprotonation in reactions employing zerovalent metals in the synthesis of heterometallics was established by a number of us several years ago (Vassilyeva et al., 1997;Buvaylo et al., 2005Buvaylo et al., , 2012. In the present work, the treatment of cadmium powder and FeCl 3 Á6H 2 O with a solution of the in situ-formed Schiff base in open air worked a different way than expected and led to the isolation of the title compound, the mixed-metal ionic salt [Fe III (DMF) 6 ][Cd 2 Cl 7 ], (1), the identity of which was established by X-ray crystallography and confirmed by chemical analysis.

Structural commentary
Compound (1), [Fe(C 3 H 7 NO) 6 ][Cd 2 Cl 7 ], crystallizes in the trigonal space group R3 and is assembled from discrete [Fe(DMF) 6 ] 3+ cations (DMF = N,N-dimethylformamide) and [Cd 2 Cl 7 ] 3À anions. In the cation, the iron(III) atom sits on a special position of 3 site symmetry and is coordinated by six oxygen atoms from the DMF ligands with all the Fe-O bond lengths being equal at 2.0072 (16) Å ( Fig. 1, Table 1). The octahedral environment of the metal is slightly distorted as a result of the cis O1-Fe1-O1 angles deviating from the ideal value of 90 [86.85 (7) and 93.16 (7) ] while all the trans angles are strictly 180 . The central Cl atom of the [Cd 2 Cl 7 ] 3À anion, Cl1, is also located on a special position of 3 site symmetry and bridges two corner-sharing, tetrahedrally coordinated Cd II atoms. The two Cd atoms and the central Cl atom are colinear (Cd1-Cl1-Cd1 vi angle = 180 ) and the bridging Cd1Á Á ÁCd1 vi distance is 5.0752 (3) Å (Fig. 1). The two sets of terminal chloride ligands, Cl2, on either side of the dumbbell-like anion are rotated relative to each other by 30 . Around each Cd atom, the bridging Cd-Cl1 distance at 2.5377 (3) Å is 0.1 Å longer than that of the terminal Cd-Cl2 distance (2.4358 (5) Å ) and the Cl2-Cd1-Cl1 and Cl2-Cd1-Cl2 i angles are 107.547 (14) and 111.325 (13) , respectively, which are very close to the ideal value of 109 . The bond lengths and angles of the DMF ligands are similar to those found in [Fe(DMF) 6 ](ClO 4 ) 3 (Houlton et al., 2015).

Supramolecular features
In the crystal, the cations and anions are stacked one above the other along the c-axis direction (Fig. 2) (7) Symmetry codes:

Synthesis and crystallization
2-Hydroxy-3-methoxy-benzaldehyde (0.3 g, 2 mmol) was stirred magnetically with CH 3 NH 2 ÁHCl (0.14 g, 2 mmol) and N-phenyldiethanolamine (0.36 g, 2 mmol) in methanol (20 mL) in a 50 mL conical flask at 303 K for 20 min. A fine Cd powder (0.11 g, 1 mmol) and dry FeCl 3 Á6H 2 O (0.27 g, 1 mmol) were introduced to the flask, and the mixture was kept stirring at 333 K to achieve dissolution of the zerovalent metal (1 h). The resulting dark blue-green solution was then filtered and allowed to evaporate at room temperature. After a week, the solution was diluted with DMF (7 mL) since it was thickening and filtered again. Dark-green octahedral crystals of (1) formed over two months after successive addition of Pr i OH (4 mL) and diethyl ether (4 mL) in several portions. The crystals were filtered off, washed with diethyl ether and finally dried in air. Yield (based on Fe): 0.13 g (64%). Analysis calculated for C 18 H 42 FeN 6 O 6 Cd 2 Cl 7 (967.37) : C 22.35,H 4.38,N 8.69%. Found: C 22.86,H 4.30, C 8.36%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Anisotropic displacement parameters were refined for all non-hydrogen atoms. All the carbon-bound hydrogen atoms were placed in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom [C-H = 0.95 Å , U iso (H) = 1.2U eq (C) for CH and C-H = 0.98 Å , U iso (H) = 1.5U eq (C) for CH 3 ].

Hexakis(N,N-dimethylformamide-κO)iron(III) µ-chlorido-bis(trichloridocadmium)
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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )