Synthesis and crystal structure of a heterobimetallic nickel–manganese 12-metallacrown-4 methanol disolvate monohydrate compound

The metallacrown compound, NiII(OAc)2[12-MCMn(III)N(shi)-4](CH3OH)6·2CH3OH·H2O, where MC is metallacrown, shi3− is salicylhydroximate, and −OAc is acetate, has an overall square shape that captures an NiII ion in a central cavity while the ring of the metallamacrocycle contains four MnIII ions. Two acetate anions, which are located on opposite faces of the MC, form bridges between the central NiII ion and two of the ring MnIII ions.


Chemical context
Since their recognition in 1989 by Pecoraro, metallacrowns (MC) have proven to be a versatile class of metallamacrocycles with applications such as single-molecule magnets, magnetorefrigerants and optical imaging agents (Mezei et al., 2007;Nguyen & Pecoraro, 2017;Lutter et al., 2018). The archetypal metallacrown consists of a cyclic metal-nitrogenoxygen repeat unit that generates a central cavity that is capable of binding a metal ion. Initially, homometallic compounds were produced; however, heterobimetallic systems were soon generated that typically contained transition-metal ions in the ring metal position and either alkali or lanthanide ions captured in the central cavity of the MC (Pecoraro et al., 1997;Mezei et al., 2007). In addition, heterotrimetallic systems that bind both alkali and lanthanide ions have been reported since 2014 (Azar et al., 2014). One area lacking is the use of two different transition-metal ions in an archetypal MC. While several examples of heterobimetallic 3d 'collapsed' metallacrowns, species without a central MC cavity and thus no central metal ion (Psomas et al., 2001;Gole et al., 2010), and inverse metallacrowns, species that bind a non-metal atom in the central MC cavity to the ring metal ions (Szyrwiel et al., 2013;Shiga et al., 2014;Zhang et al., 2014;Nesterova et al., 2015), have been reported, only two heterobimetallic 3d archetypal 12-MC-4 compounds have been described to date. In 2014, Happ and Rentschler reported a Cu II (DMF) 2 Cl 2 [12-MC Fe(III)N(shi) -4](DMF) 4 Á2DMF compound ISSN 2056-9890 that contains Fe III ions in the ring positions and a Cu II ion captured in the central MC cavity (Happ & Rentschler, 2014). Recently we described the structure of (TMA) 2 {Mn(OAc) 2 -[12-MC Mn(III)Cu(II)N(shi) -4](CH 3 OH)}Á2.90CH 3 OH that consists of alternating Cu II and Mn III ions about the MC ring and an Mn II ion bound to the central MC cavity (Lewis et al., 2020). Herein we report a third heterobimetallic 3d archetypal 12-MC-4 compound: Ni II (OAc) 2 [12-MC Mn(III)N(shi) -4](CH 3 OH) 6 Á-2CH 3 OHÁH 2 O, 1, that contains ring Mn III ions and a Ni II ion captured in the central MC cavity. Future work will focus on the magnetic properties of the compound as the similar Mn(OAc) 2 [12-MC Mn(III)N(shi) -4] (Zaleski et al., 2011) and {Mn(OAc) 2 [12-MC Mn(III)Cu(II)N(shi) -4]} 2À (Lewis et al., 2020) systems behave as single-molecule magnets.

Structural commentary
The title metallacrown compound is positioned about an inversion center located on the Ni II ion that resides in the central MC cavity (Fig. 1). The metallacrown macrocycle possesses an Mn III -N-O repeat unit that generates an approximately square molecule due to the fused five-and sixmembered chelate rings of the shi 3À ligand that place the Mn III ions 90 o relative to each other. The oxime oxygen atoms of the shi 3À ligands generate the MC cavity and also bind the central Ni II ion. Two acetate anions, which bind on opposite faces of the MC, tether the Ni II ion to the MC by forming three atom bridges to two of the ring Mn III ions. In addition to average bond lengths and bond-valence-sum (BVS) values (Table 1; Liu & Thorp, 1993), the oxidation state assignments of the Ni II and Mn III ions are supported by overall molecular charges, where one Ni II and four Mn III ions are counterbalanced by four shi 3À and two acetate anions.
The central Ni II ion is six-coordinate with an octahedral geometry as verified by a SHAPE analysis (SHAPE 2.1; Llunell et al., 2013;Pinsky & Avnir, 1998). Continuous shape measure (CShM) values of less than 1.0 indicate only minor distortions from the ideal geometry (Cirera et al., 2005); thus, the CShM value of 0.164 for the octahedral geometry clearly defines the shape about the Ni II ion ( Table 2). The coordination environment is comprised of four oxime oxygen from four shi 3À ligands in the equatorial plane and two axial carboxylate oxygen atoms of two acetate anions. As mentioned above, the acetate anions bind on opposite faces of the MC and connect the Ni II ion to two Mn III ions (Mn2). The acetate binding motif is different than the analogous homometallic Mn II (OAc) 2 [12-MC Mn(III)N(shi) -4](DMF) 6 Á2DMF, where the acetate anions bind on the same face of the MC and the central Mn II ion exhibits a geometry that is best described as a trigonal prism (Lah & Pecoraro, 1989). Table 1 Average bond length (Å ) and bond-valence-sum (BVS) values (v.u.) used to support assigned oxidation states of the nickel and manganese ions of 1.

Figure 1
The molecular structure of the title compound, (a) top view with only the metal atoms and shi 3À ligands labeled for clarity and (b) side view with only the metal atoms and axial ligands labeled for clarity. The displacement ellipsoids are at the 50% probability level. For clarity, hydrogen atoms and solvent molecules have been omitted. [Color scheme: yellow-Ni II , green-Mn III , red-O, blue-N and gray-C; symmetry code: (i) Àx + 1, Ày + 1, Àz + 1.] The ring Mn III ions (Mn1 and Mn2) are six-coordinate with a tetragonally distorted octahedral geometry ( Table 2). The Jahn-Teller elongation, typical of a high-spin 3d 4 ion, is located along the z-axis of each Mn III ion. For both Mn1 and Mn2, the equatorial coordination environment is composed of trans chelate rings from two shi 3À ligands. A five-membered chelate ring is generated from an oxime oxygen atom and a carbonyl oxygen atom of one shi 3À ligand, and a sixmembered chelate ring is produced by an oxime nitrogen atom and a phenolate oxygen atom of the second shi 3À ligand. For Mn1 the axial atoms are oxygen atoms from two methanol molecules, while for Mn2 the axial atoms are an oxygen atom from a methanol molecule and a carboxylate oxygen atom from an acetate anion.
In addition, solvent methanol and water molecules are located in the structure, and the methanol molecules form hydrogen bonds to the metallacrown. The water molecule associated with O13 is slightly offset from and disordered about an inversion center.

Supramolecular features
The coordinated and interstitial methanol molecules of 1 participate in several hydrogen bonds (Figs. 2 and 3, Table 3). The hydroxyl group of the methanol molecule associated with O9 and coordinated to Mn1 forms a hydrogen bond to an oxygen atom (O12) of an interstitial methanol molecule. In addition, the hydroxy group of another methanol molecule associated with O10 and coordinated to Mn1 forms an intramolecular hydrogen bond to a carboxylate oxygen atom (O7) of an acetate anion. Also the hydroxyl group of the interstitial methanol molecule associated with O12 forms a hydrogen bond to the other carboxylate oxygen atom (O8) of the acetate anion. Lastly, a one-dimensional chain of metallacrowns is mediated by the hydroxyl group of a methanol molecule associated with O11 and coordinated to Mn2 that forms a hydrogen bond to a carboxylate oxygen atom (O5) of a shi 3À ligand of a neighboring metallacrown. As a symmetryequivalent hydrogen bond also occurs on the opposite side of the MC, a one-dimensional chain is established (Fig. 3). The connection between the neighboring MCs, the hydrogen bonds between the MC and the interstitial methanol molecules, and pure van der Waals forces contribute to the overall packing of 1.

Figure 2
Intramolecular hydrogen bonding (dashed lines) between the coordinated methanol molecule (O10) and a carboxylate oxygen atom (O7) of the bridging acetate anion, and intermolecular hydrogen bonding (dashed lines) of the interstitial methanol molecule (O12) with a coordinated methanol molecule (O9) and a carboxylate oxygen atom (O8) of the bridging acetate anion. [Symmetry codes: (i) Àx + 2, Ày + 1, Àz compounds. The first reported manganese-nickel MC is a 'collapsed' metallacrown as it does not contain a central cavity. The structure has an M-N-O repeat unit but two of the oxime oxygen atoms bind to ring metal ions across the potential central cavity, thus collapsing the cavity and preventing the binding of a central metal ion. The compound [12-MC Ni(II)Mn(III)N(shi)2(pko)2 -4](OAc) 2 (QOCXAH; Psomas et al., 2001), where pko À is 2,2 0 -dipyridylketonoximate, contains both Mn III and Ni II ions in the MC ring positions with the metals arranged in an alternating pattern. The two other compounds can both be considered dimers of inverse 9-MC-3 systems, where each MC binds a 3 -O in the central cavity instead of a metal ion. In both compounds, two inverse 9-MC-3 units, each based on an Mn III 2 Ni II core, are linked together to form a dimer. The main difference between the structures is the MC framework ligand: salicylaldoxime (XIFGUQ; Szyrwiel et al., 2013) or 5-chlorosalicylaldehyde oxime (LOKHIE; Zhang et al., 2014). Thus, 1 represents the only manganesenickel archetypal MC structure type as 1 contains a central metal ion.
Tetraethylammonium acetate tetrahydrate (4 mmol, 1.0462 g) and salicylhydroxamic acid (2 mmol, 0.3063 g) were dissolved in 4 mL of DMF and 4 mL of methanol, resulting in a clear orange solution. In two separate vessels, nickel(II) acetate tetrahydrate (0.125 mmol, 0.0312 g) was dissolved in 4 mL of DMF and 4 mL of methanol resulting in a green-blue solution and manganese(II) acetate tetrahydrate (2 mmol, 0.4909 g) was dissolved in 4 mL of DMF and 4 mL of methanol resulting in an clear orange solution. The manganese(II) acetate solution was then added to the tetraethylammonium acetate/salicylhydroxamic acid solution resulting in a brown solution. The nickel(II) acetate was then immediately added and no color change was detected; however, the formation of a precipitate was observed. The mixture was then left to stir overnight and subsequently gravity filtered the next day. The filtrate was a dark orange-brown solution and no precipitate was recovered. Slow evaporation of the filtrate at room temperature afforded X-ray quality dark-brown block-shaped crystals after 16 weeks. A small fraction of crystals and mother liquor were separated for analysis by single crystal X-ray diffraction. The remaining crystals were washed with cold DMF and dried. The percent yield was 22% based on nickel(II) acetate tetrahydrate.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. An electron density region disordered around an inversion center was refined as a water molecule located slightly offset of the inversion center. The water hydrogen-atom positions were initially refined and O-H and HÁ Á ÁH distances were restrained to 0.84 (2) and 1.36 (2) Å , respectively, and further restrained based on hydrogen-bonding considerations while a damping factor was applied. In the final refinement cycles the hydrogen atoms were constrained to ride on the oxygen carrier atom. The displacement parameters of the water O atom were restrained to be close to isotropic. For the methanol molecules, the O-H bond distance was also restrained to 0.84 (2) Å . The U iso values for the O-H hydrogen atoms (water and methanol) were set to a multiple of the value of the carrying oxygen atom (1.5 times). All other hydrogen atoms were placed in calculated positions and refined as riding on their carrier atoms with C-H distances of 0.95 Å for sp 2 carbon atoms and 0.98 Å for methyl carbon atoms. The U iso values for hydrogen atoms were set to a multiple of the value of the carrying carbon atom (1.2 times for sp 2 -hybridized carbon atoms or 1.5 times for methyl carbon atoms).    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. A single electron density disordered around an inversion center was refined as a water molecule located slightly offset of the inversion center. The water H atom positions were initially refined and O-H and H···H distances were restrained to 0.84 (2) and 1.36 (2) Angstrom, respectively and further restrained based on hydrogen bonding considerations while a damping factor was applied. In the final refinement cycles the H atoms were constrained to ride on the oxygen carrier atom.