Hexa-μ-acetato-chlorido(μ-N,2-dioxodobenzene-1-carboximidato)-μ3-oxido-tetrairon(III)–water (1/1) and hexa-μ-acetato-(μ-N,2-dioxodobenzene-1-carboximidato)fluorido-μ3-oxido-tripyridinetetrairon(III)–pyridine–water (1/1/0.24)

The title compounds Fe4(C7H4O3)O(C2H3O2)6(C5H5N)3 X where X is either Cl or F were synthesized using a self-assembly reaction in methanol and pyridine with stoicometric addition of salicylhydroxamic acid (H3shi), acetic acid (HOAc), and the appropriate ferric halide salt. The compound is remeniscent of hydroximate binding in metallacrown structures.

The title compounds, [Fe 4 (C 2 H 3 O 2 ) 6 (C 7 H 4 O 3 )FO(C 5 H 5 N) 3 ]ÁC 5 H 5 NÁ0.24H 2 O (1-F) and [Fe 4 (C 2 H 3 O 2 ) 6 (C 7 H 4 O 3 )ClO(C 5 H 5 N) 3 ]ÁH 2 O (1-Cl) were synthesized using a self-assembly reaction in methanol and pyridine with stoichiometric addition of salicylhydroxamic acid (H 3 shi), acetic acid (HOAc), and the appropriate ferric halide salt. The compounds crystallize as solvates, where 1-F has one pyridine molecule that is disordered about a twofold axis and one water molecule with an occupancy of 0.24 (2); and 1-Cl has one water molecule that is disordered over two sites with occupancies of 0.71 (1) and 0.29 (1). The space groups for each analog differ as 1-F crystallizes in Fdd2 while 1-Cl crystallizes in P2 1 . The difference in packing is due to changes in the intermolecular interactions involving the different halides. The two molecules are mostly isostructural, differing only by the torsion of the pyrine ligands and slight orientation changes in the acetate ligands. All of the iron(III) ions are in sixcoordinate octahedral ligand field geometries but each one exhibits a unique coordination environment with various numbers of O (four to six) and N (nought to two) atom donors. Bond-valence sums confirm each iron is trivalent. The hydroximate ligand is bound to three iron(III) ions using a fused chelate motif similar to those in metallacrown compounds.

Chemical context
Examples of hydroximate binding as fused chelate rings has been dominated by a class of coordination compounds known as metallacrowns. First introduced by Pecoraro and, these compounds have since been tuned to explore many applications including host-guest binding, molecular magnetism, and luminescence (Mezei et al., 2007;Chow et al., 2015;Lutter et al., 2018). In particular, iron(III) 9-metallacrown-3 compounds have demonstrated interesting magnetocoolent properties (Chow et al., 2016). Here, we describe two tetra-iron(III) compounds that have a fused chelate motif similar to metallacrowns but that are not examples of metallamacrocycles ( Figs. 1 and 2). Instead, this fused chelate motif is complemented by six acetate ligands, a 3 -oxo ligand, and three pyridine ligands to complete the octahedral ligand fields of the four iron ions. These compounds were a serendipitous discovery from metallacrown synthesis that can be formed with their own rational self-assembly reaction. ISSN 2056-9890

Structural commentary
Each of the iron(III) centers in 1-F and 1-Cl are in sixcoordinate octahedral ligand field geometries and bondvalence sums confirm that each iron ion is trivalent (Zheng et al., 2017). More details are available in Tables 1 and 2. Fe1 is bound to the 2 -oxime oxygen and carbonyl oxygen of shi 3to form a pentagonal chelate ring, an oxygen from an acetate ligand, the nitrogen from two pyridine ligands, and the respective halide for each compound. Fe2 is bound to the imino nitrogen and phenolic oxygen of shi 3to form a hexagonal chelate ring, the 3 -oxo, and an oxygen from three acetate ligands. Fe3 is bound to the 3 -oxo, an oxygen from four acetate ligands, and the nitrogen of a pyridine ligand. Fe4 is bound to the 3 -oxo, the 2 -oxime oxygen of shi 3-, and an oxygen from four acetate ligands. Depictions of these coordination environments are shown in Fig. 3. Generally, for both compounds, the Fe-O (oxo)  ORTEP representation from crystallographic data for 1-Cl. Orange = iron, green = chlorine, light blue = nitrogen, red = oxygen, gray = carbon. Displacement ellipsoids are drawn at the 50% probability level.

Supramolecular features
Both compounds crystallize as solvates where 1-F has one pyridine (N5 C35-39) and a 0.24 (2) occupancy water molecule (O17), and 1-Cl has one disordered water molecule (O17 and O17A). The pyridine in 1-F is disordered on a special position (twofold axis). This pyridine interacts with the main moiety via a hydrogen bond from an acetate C11-H11A bond to N5 on the pyridine. The pyridine also forms a hydrogen bond using using C39-H39 to donate to O6 from an acetate. The solvent water in 1-F has two hydrogen bonds, where the O17-H17E bond donates to O13 on an acetate, and the C17-H17A bond on an acetate donates to O17. The solvent water in 1-Cl is disordered over two sites with occupancies of 0.71 (1) and 0.29 (1) for the major and minor contributors. The major water site has three hydrogen bonds including: (i) the O17-H17D bond donating to Cl1, (ii) the O17-H17E bond donating to O3 in an acetate, and (iii) the C26-H26 bond of a pyridine donating to O17. The minor contributor has two hydrogen bonds, one where the C26-H26 bond in a pyridine donates to O17A and where the C6-H6 bond on shi 3donates to O17A. Details of all hydrogen bonds, including distances and angles, are summarized in Tables 5 and 6.
The main moieties also have intermolecular hydrogenbonding interactions. In 1-F, the C13-H13A bond on an acetate donates to F1, the C15-H15B bond on an acetate donates to F1, the C19-H19B bond on an acetate donates to F1, the C19-H19C bond of an acetate donates to O11 of an acetate, the C23-H23 bond on a pyridine donates to O5 of an acetate, the C26-H26 bond of a pyridine donates to O6 of an acetate, and the C31-H31 bond of a pyridine donates to O2 on shi 3-. In 1-Cl, the C15-H15C bond on an acetate donates to O15 from an acetate, the C19-H19A bond on an acetate donates to O11 from an acetate, the C21-H21 bond on a pyridine donates to O4 from an acetate, and the C23-H23 bond from a pyridine donates to Cl1. In addition to hydrogen bonding, 1-F hasstacking between the pyridine containing N2 and C20-C24 and the pyridine containing N4 and C30-C34. There is nostacking observed in 1-Cl.

Table 6
Hydrogen-bond geometry (Å , ) for 1-Cl. Symmetry codes: (i) x; y; z þ 1; (ii) Àx; y þ 1 2 ; Àz þ 1; (iii) Àx; y À 1 2 ; Àz þ 1; (iv) Àx; y À 1 2 ; Àz þ 2; (v) Àx À 1; y À 1 chemistry of fluorine compared to chlorine. Fluorine has a smaller radius and is more electronegative than chlorine, and these properties have an effect on the overall packing of the compounds in their lattice. Essentially, the molecules of 1-F pack tighter than those of 1-Cl. The main moiety intermolecular hydrogen bonds discussed above demonstrate this difference. For 1-F, there are seven intermolecular hydrogen bonds where three of the hydrogen bonds involve fluorine (Fig. 5). However, in 1-Cl there are four intermolecular hydrogen bonds and only one of these hydrogen bonds involves the chlorine (Fig. 6). In addition, the difference in radius and electronegativity results in different lengths for hydrogen-bonding interactions, where 1-F has proton-tofluorine distances of 2.49, 2.54, and 2.60 Å while 1-Cl has a proton-to-chlorine distance of 2.97 Å for their respective intermolecular hydrogen bonds. Since the molecules of 1-F pack more tightly than those of 1-Cl, their orientation is fixed such that all of the fluorine atoms of adjacent molecules point towards the same direction of the unit cell and is enforced by stacking of pyridine ligands (Fig. 5). In 1-Cl, adjacent layers of molecules point their chlorine atoms in opposite directions as there is less interaction between the molecules, likely due to pair-opposing molecular dipoles (Fig. 6). This observation also suggests that 1-F may have a crystallographic net dipole since all of the fluorines point in the same general direction.

Database survey
Two other compounds in the Cambridge Structural Database (Groom et al., 2016) feature the same hydroximate coordination motif to three iron(III) ions shown in 1-F and 1-Cl, where both are iron(III) 9-metallacrown-3 compounds (Chow et al., 2016): HADWOB and HADWUH. HADWOB is a 9metallacrown-3 with three benzoate ligands that bridge the ring and central iron(III) ions and three methanol molecules that are bound to ring iron(III) ions. HADWUH is a set of two 9-metallacrown-3 compounds with three isophthalate ligands that bridge the ring and central iron(III) ions as well as spanning the two rings into a dimeric structure. These structures are adaptations of another iron(III) 9-metallacrown-3 reported in 1989 . The other major motif of a 3 -oxo combined with -acetato ligands on iron is not found in the Cambridge Structural Database.

Synthesis and crystallization
Fe 4 (shi)O(OAc) 6 (pyridine) 3 F (1-F): To a flask was added salicylhydroxamic acid (0.0766 g, 0.500 mmol, 1 equiv) and iron(III) fluoride trihydrate (0.3338 g, 2.000 mmol, 4 equiv). These solids were dissolved in a mixture of methanol (10 mL) and pyridine (2 mL), resulting in a dark-purple solution. Glacial acetic acid (0.200 mL, 3.50 mmol, 7 equiv) was added immediately, and the resulting solution was stirred for 1 h. The reaction mixture was gravity filtered using Whatman #2 filter paper, and the filtrate was allowed to evaporate slowly. After about one week, purple plates were obtained and diffracted. These plates were collected using vacuum filtration with #2 Whatman filter paper and a water aspirator and allowed to dry for 1 h before stopping the vacuum. Synthetic yield = 27% research communications Representation of the crystal packing in 1-F from crystallographic data. Hydrogen-bond pairs involving the halide are shown as spheres and the Fe-F bond is bolded for emphasis. Pyridines that havestacking are bolded. Orange = iron, yellow = fluorine, light blue = nitrogen, red = oxygen, gray = carbon.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. The absolute structure for both compounds were determined by refinement of the Flack parameter. For 1-F, the pyridine containing N5 and C35-C39 is disordered around a special position (twofold axis) that was refined using a PART À1 command. The displacement parameters of these atoms were restrained with an esd of 0.01 using the ISOR command in SHELXL to limit excessive prolate character in displacement ellipsoids due to the disorder on a special position. A partial-occupancy water molecule containing O17 was refined to have an occupancy of 0.24 (2). Hydrogen atoms on O17 were located on the difference map and distances were restrained to 0.84 (2) Å for O-H bonds in water using a DFIX command in SHELXL. In addition, the distance between H-H atoms in the water molecule was restrained to 1.35 (2) Å using a DANG command in SHELXL. These restraints maintain reasonable geometry for a water molecule. Final refinement required an additional geometric constraint using the AFIX 3 command in SHELXL research communications Table 7 Experimental details.  (16) Computer programs: APEX2 (Bruker, 2004), SAINT (Bruker, 2001), SHELXT2018/3 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), and SHELXTL (Sheldrick, 2008).
to stabilize the positions of these 0.24 (2) occupancy hydrogen atoms. For 1-Cl, one disordered water molecule containing O17 was refined using a PART command and refined occupancies of 0.71 (1) and 0.29 (1). Hydrogen atoms for the water were found on the difference map and O-H bonds were restrained to 0.84 (2) Å using a DFIX command in SHELXL.
The distance between H-H atoms in the water molecule were restrained to 1.35 (2) Å using a DANG command in SHELXL. These restraints maintain reasonable geometry of the water molecules.

Computing details
For both structures, data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXT2018/3 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).  (2) 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. Refinement. Refined as a two-component inversion twin. A lattice pyridine containing N5 and C35 through C39 lies on a twofold axis and was refined using a PART -1 command. The partial occupancy of lattice water O17 was refined using a free variable, and the H atoms were found on the difference map and refined with DFIX and DANG commands.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.   (2)

Hexa-µ-acetato-chlorido(µ-N,2-dioxodobenzene-1-carboximidato)-µ 3 -oxido-tetrairon(III)-water (1/1) (1-Cl)
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. Refinement. Refined as a two-component inversion twin. A lattice water containing O17 was refined over two sites with a PART command and the occupancies were refined using a free variable. H atoms were found on the difference map and refined with DFIX and DANG commands. Minor disorder of the pyridine containing N4 was not refined and is likely due to the disorder of the lattice water.