Structural characterization of a new samarium–sodium heterometallic coordination polymer

The crystal structure is reported of a new heterometallic samarium compound comprised of alternating SmIII and NaI metal centers bridged by o-vanillin ligands to create a helical chain.

Lanthanide-containing materials are of interest in the field of crystal engineering because of their unique properties and distinct structure types.In this context, a new samarium-sodium heterometallic coordination polymer, poly[tetrakis(� 2 -2-formyl-6-methoxyphenolato)samarium(III)sodium(I)], {[SmNa(C 8 H 7 O 3 ) 4 ]•solvent} n (Sm-1), was synthesized and crystallized via slow evaporation from a mixture of ethanol and acetonitrile.The compound features alternating Sm III and Na I ions, which are linked by ortho-vanillin (o-vanillin) ligands to form a mono-periodic chain-like coordination polymer.The chains propagate along the [001] direction.Residual electron density of disordered solvent molecules in the void space could not be reasonably modeled, thus the SQUEEZE function was applied.The structural, vibrational, and optical properties are reported.

Chemical context
The synthesis of lanthanide compounds with 2-hydroxy-3methoxy benzaldehyde (o-vanillin) ligand derivatives is of great interest in the field of crystal engineering because of their photophysical and magnetic properties (Chaudhari et al., 2012;Song et al., 2017;Novitchi et al., 2012;Albrecht, 2001).In crystal engineering, the ligand of choice has a large effect on the dimensionality of lanthanide-containing compounds owing to their high-coordination environments (Bunzli & Piguet, 2002).For example, ligands with multiple binding sites are ideal because of their ability to bridge metal centers or act as capping ligands (Heuer-Jungemann et al., 2019;Cheng & Yang, 2017).o-Vanillin is a popular ligand for heterometallic synthesis due to its ability to generate a variety of compounds through its multiple binding sites (carboxylate and methoxy groups; Andruh, 2015).While there is an extensive library of lanthanide and o-vanillin-containing compounds, ranging in dimensionality from small molecules to coordination polymers (CPs) and metal organic frameworks (MOFs) (CSD, version 2021.3.0;Groom et al., 2016), we are not aware of any reports containing o-vanillin, Sm III and Na I , and have found only a single report containing both o-vanillin and Sm III (Griffiths et al., 2016).However, heterometallic lanthanide-transitionmetal compounds with o-vanillin have been reported (Costes et al., 2015(Costes et al., , 2018;;Kırpık et al., 2019).These compounds crystallize as discrete molecular dinuclear units.To the best of our knowledge, the only reported lanthanide-Na I -o-vanillincontaining compound crystallized as an aggregate structure with a hydrophobic cavity (Li et al., 2022).The lanthanide-Na I -o-vanillin compound isolated by Li et al. is vastly different from the structure described here, [SmNa(C 8 H 7 O 3 ) 4 ]•solvent (Sm-1).Herein we report the synthesis, crystal structure, and characterization of an interesting new samarium-sodium heterometallic CP synthesized with o-vanillin ligands.

Structural commentary
The compound [SmNa(C 8 H 7 O 3 ) 4 ]•solvent (Sm-1) crystallizes in the P2 1 /c space group.The asymmetric unit features one crystallographically unique Sm III and Na I metal center, and four o-vanillin ligands (Fig. 1).Each metal center is coordinated by eight oxygen atoms, each displaying a distorted square-antiprismatic geometry with a local C 1 symmetry (Fig. 1).The Sm III metal centers are bound to four o-vanillin ligands (� 2 ) with an average Sm-O bond length of 2.395 (2) A ˚.The Na I cations are bound to six o-vanillin ligands, two of which are bidentate (� 2 ) and four are monodentate (� 1 ), with average Na-O bond lengths of 2.530 (4) A ˚.The metal-to-oxygen bond distances are typical of those reported in similar systems (Ma et al., 2021;Peng et al., 2011).The Sm III and Na I atoms alternate and are bridged together by three � 2 -o-vanillin ligands that each display unique bonding environments through the phenoxo, aldehydic, and methoxy groups (see Fig. S1 in the supporting information).The first ovanillin ligand binds the alternating Sm III and Na I atoms through the phenoxo and aldehydic groups, leaving the methoxy group uncoordinated, Fig. S1a.The second o-vanillin ligand bridges the Sm III and Na I atoms using the phenolic group, with the aldehydic and methoxy groups binding solely to the Sm III and Na I atoms, respectively, Fig. S1b.Lastly, the third o-vanillin ligand bridges the alternating Sm III and Na I atoms via the aldehydic and phenoxo groups while the methoxy group binds solely to an adjacent Na I atom, Fig. S1c.This creates a bimetallic helical chain that propagates along the [001] direction (Fig. 2).The potential solvent area volume of Sm-1 is 10.6% per unit cell (calculated using PLATON; Spek, 2020).

Supramolecular features
The structure was analyzed for non-covalent interactions and no evidence for �-� interactions was observed.However, a series of close atom contacts (C-H� � �C) are present between adjacent chains (Table 1).The supramolecular chains are stabilized primarily through C-H� � �C interactions, allowing the stacking of adjacent chains in the structure.

Database survey
The o-vanillin ligand is widely used in coordination chemistry with over 70 structures containing o-vanillin and lanthanides reported in the Cambridge Structural Database (CSD, version 2021.3.0;Groom et al., 2016).A survey of structures containing samarium and o-vanillin resulted in only one compound, a heterometallic and heteroleptic cluster containing Sm III and Na I metal centers bound by 2-(E)-{[(2-hydroxyphenyl)imino]-methyl}-6-methoxyphenol ligands (Griffiths et al., 2016).In this compound, the o-vanillin ligands act as capping ligands and are bidentate (� 2 ) in fashion, whereas in Sm-1, the o-vanillin ligands act as bridging ligands that connect the Sm III and Na I atoms to form a mono-periodic CP.

Synthesis and crystallization
The compound Sm-1 was synthesized by dissolving 10 mg of Sm III chloride hexahydrate (SmCl 3 •6H 2 O, Strem Chemicals, 99.9%) in 208.5 mL of hydrochloric acid (HCl, Sigma Aldrich, 37% w/w).The mixture was slowly heated to dryness, and the residue was dissolved in 500 mL of hydrobromic acid (HBr, Aldrich, 48% w/w ACS reagent).The solution was gently heated to dryness and once cooled, the residue was dissolved in 655 mL ethanol (Fisher,200 proof) to form a 0.042 M Sm III solution with a pH near 1.4 (Solution A).A 0.105 M o-vanillin solution (Solution B) was prepared by dissolving o-vanillin (TCI, >99.0%) in an ethanol/acetonitrile (1:1, acetonitrile: Fisher, 99.5% certified ACS) mixture.The following were added to a 4 mL glass reaction vial: 100 mL Solution A, 400 mL Solution B, and 33.4 mL 0.5 M NaOH (aqueous, Sigma Aldrich, >98.0%), yielding a yellow solution with a pH of 7.7.The vial was covered with parafilm that had a small slash in it to allow slow evaporation of the solvent.After 4 days, yellow acicular crystals grew from the reaction solution in radial bursts (Fig. 3).The synthesis of Sm-1 has an 80% yield.Several synthetic variations were explored to improve the singlecrystal diffraction quality.Adding an additional equivalent of NaOH brought the initial pH to �8.5 and yielded the same phase, but the crystals were too small for single-crystal studies.Decreasing the NaOH equivalents (in the pH range of 2-4) did not yield any quality crystalline product upon evaporation.In addition, simply starting with SmCl 3 •6H 2 O salt, instead of the HCl/HBr Sm stock protocol, indeed crystallized Sm-1; however, these were also too small for individual manipulation.Although not reported here, the synthesis was developed as an analogue for transuranic chemistry, in which strong acid stock solutions are a practicality and serve as redox control.

Experimental details
Sm-1 crystals were harvested, washed with ethanol, and mounted to MiTeGen MicroMounts from immersion oil.Data were collected on a Bruker D8 Venture diffractometer equipped with a Photon III detector using a Mo anode microfocus source (diamond I�S 3.0) and ' and !scans, at 100 K.The collection strategy was calculated factoring in the known symmetry and collected with at least triplicate multiplicity.The data were reduced using SAINT (Bruker, 2014) and multi-scan absorption correction was applied using SADABS (Krause et al., 2015), both within the APEX4 software (Bruker, 2014).Using Olex2 (Dolomanov et al., 2009), the structure was solved with the SHELXT (Sheldrick, 2015a) structure solution program and refined with the SHELXL (Sheldrick, 2015b) refinement package using least-squares minimization.Additional experimental and instrumentation details on powder X-ray diffraction, infrared spectroscopy, and diffuse reflectance spectroscopy can be found in the supporting information.

Refinement
Crystal data, data collection, and structure refinement details of Sm-1 are summarized in Table 2.The H atoms associated with the carbon atoms were affixed to the respective parent atoms using a riding model.Residual electron density of disordered solvent molecules in the void space could not be reasonably modeled, thus the SQUEEZE function was applied via PLATON (Spek, 2015(Spek, , 2020)).A total of 47 electrons were accounted for by SQUEEZE and removed.This amounts to about 2 solvent molecules (acetonitrile and/or ethanol) per unit cell.While most of the reaction medium was acetonitrile and ethanol, water molecules are also possible from the aqueous NaOH spike.The Sm-1 single crystals diffracted weakly, perhaps owing to the small crystal size.Attempts to crystallize and select higher quality single crystals were unsuccessful.Bond-valence analysis on the metal centers yields summations of 3.30 and 0.98 for Sm III and Na I , respectively (Brown & Altermatt, 1985;Yee et al., 2019).

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.

Figure 1
Figure 1Top: The asymmetric unit of Sm-1.The Sm, Na, C, and O atoms are depicted as orange, teal, black, and red ellipsoids, respectively.The displacement ellipsoids are drawn at 50% probability.The hydrogen atoms are removed for clarity.Bottom: The coordination environment of the Sm III and Na I metal centers, represented as orange and teal polyhedra, respectively.

Figure 2
Figure 2 Polyhedral representation of Sm-1 showing the propagation of the chains along the [001] direction.The Sm III and Na I atoms are represented as orange and teal polyhedra, respectively.The oxygen atoms are represented by red spheres and the carbon atoms are represented in stick form.Hydrogen atoms have been omitted for clarity.

Figure 3
Figure 3Microscope image of Sm-1 crystals with scale for reference.

Table 1
Atom pairs and distances (A ˚).

Table 2
Experimental details.