Crystal structures of three anionic lanthanide–aluminium [3.3.1] metallacryptate complexes

The three [3.3.1] metallacryptate complexes [Hpy]2[GdAl6(H2shi)2(shi)7(py)1.855(H2O)2]·7.396py·H2O, 1, [Hpy]2[DyAl6(H2shi)2(shi)7(py)1.891(H2O)2]·7.429py·H2O, 2, and [Hpy]2[YbAl6(H2shi)2(shi)7(py)1.818(H2O)2]·7.386py·H2O, 3, where Hpy+ is pyridinium, shi3− is salicylhydroximate, and py is pyridine, consist of an aluminium-based metallacryptand that captures an Ln III ion in the central cavity. The metallacryptand portions are comprised of an Al—N—O repeat unit; thus, they can be considered three-dimensional metallacrowns. The encapsulated Ln III ions are nine-coordinate with a spherical capped-square-antiprism geometry, while the six AlIII ions are all octahedral. Four of the AlIII ions are chiral centers with 2 Δ and 2 Λ stereoconfigurations. The remaining two AlIII ions have trans chelate rings from two different shi3− ligands.


Chemical context
Since the first report of metallacrowns (MC) in 1989, the chemistry of these molecules has grown to include a number of different structural types and subsequent applications (Pecoraro, 1989). The first and archetypal metallacrowns are based on a macrocyclic metal-nitrogen-oxygen repeat unit and these molecules typically bind a metal ion in the central cavity similar to crown ethers (Pecoraro et al., 1997). However, the structural variation has expanded to include molecules such as azametallacrowns with a metal-nitrogen-nitrogen repeat, collapsed MCs -structures without a central cavity and ISSN 2056-9890 metal ion, inverse MCs that bind a non-metal atom in the central cavity, and MC-like metallacryptands -complexes with an M-N-O repeat unit in a three-dimensional pattern (Mezei et al., 2007). As structures have become more varied, so have the properties and applications of the molecules. Potential applications include single-molecule magnetism, magnetorefrigeration, MRI contrast agents, host-guest complexes, gasand solvent-sorption materials, and optical imaging agents (Nguyen & Pecoraro, 2017;Lutter et al., 2018b;Pavlishchuk et al., 2017;Atzeri et al., 2016). The properties of the MCs are derived from the interplay of the central metal ion and the ring metal ions. Often a transition metal is used for the ring metal ions, while either a lanthanide or transition-metal ion is captured in the central cavity. The use of lanthanide ions in particular can yield interesting single-molecule magnet or luminescent properties with the correct choice of the ring metal. For instance, the combination of paramagnetic Dy III and Mn III ions leads to MCs with single-molecule magnet properties (Zaleski et al., 2004(Zaleski et al., , 2007Boron et al., 2016;Cao et al., 2016), while the use of Zn II or Ga III in combination with Ln III ions can lead to luminescent MCs as the closed-shell electron configuration of the ring ions does not quench the radiation emitted by the Ln III ions (Jankolovits et al., 2011;Chow et al., 2016;Martinić et al., 2017). Indeed, several Ln III -Ga III MCs have garnered attention as optical imaging agents (Nguyen et al., 2018;Lutter et al., 2018aLutter et al., , 2019Lutter et al., , 2020. To better understand the properties of the central Ln III ion in an MC framework (M-N-O repeat unit), we sought to isolate the Ln III ion from paramagnetic ring metal ions as these ions complicate the magnetism of the complexes and quench any luminescence. Thus, any magnetic or spectroscopic properties would be that of the Ln III ion inside an MC ligand environment. One suitable metal is aluminum(III) as the charge of this ion should allow substitution for Mn III and Ga III ions while maintaining overall molecular charge balance. In addition, aluminum has not been explored in MC chemistry in detail as only two other aluminum-based metallacryptates have been reported to date (Travis et al., 2020). Herein we present three [3.3.1] metallacryptate complexes [Hpy] 2 [GdAl 6 (H 2 shi) 2 (shi) 7 (py) 1.855 (H 2 O) 2 ]Á7.396pyÁH 2 O, 1, [Hpy] 2 [DyAl 6 (H 2 shi) 2 (shi) 7 (py) 1.891 (H 2 O) 2 ]Á7.429pyÁH 2 O, 2, and [Hpy] 2 [YbAl 6 (H 2 shi) 2 (shi) 7 (py) 1.818 (H 2 O) 2 ]Á7.386pyÁH 2 O, 3, where Hpy + is pyridinium, shi 3À is salicylhydroximate, and py is pyridine. Complexes 1-3, which are isomorphous, differ from the previous aluminum-based metallacryptates in that 1-3 are discrete molecules, while the latter structures are twodimensional networks of metallacryptates. Future studies will investigate the magnetic properties of 1-3 to understand the behavior of the Ln III ions in a metallacrown-like framework.

Structural commentary
The structures of 1-3 are isomorphous with varying degrees of disorder in the metallacryptate and amount of lattice pyridine molecules . The overall structure of the complexes is akin to that of a [3.3.1] cryptand, where the numbers indicate the number of ether oxygen atoms in each carbon-oxygen chain between the nitrogen atoms (Lehn, 1978;Krakowiak et al., 1993). [1][2][3]the [3.3.1] nomenclature is derived from the number of oxygen atoms in each Al-N-O chain between the anchoring Al III ions (Fig. 4). Al1 and Al2 are analogous to the nitrogen atoms of a cryptand, while the remaining Al III ions form the metallacryptand. There are two longer O-N-Al-O-N-Al-O-N linkages and one shorter N-O linkage between Al1 and Al2. The metal-nitrogen-oxygen repeat unit of each chain between Al1 and Al2 is that of an archetypal metallacrown; thus, the Al-N-O chains of 1-3 can be considered three-dimensional metallacrowns, i.e. metallacryptands. As these complexes bind a metal ion in the central cavity, they are more accurately described as metallacryptates just as the term cryptates is used to describe cryptands that bind a central guest. Each metallacryptate unit consists of one Ln III ion and six Al III ions (total charge of 21+) and of seven triply deprotonated shi 3À ligands and two H 2 shi À (total charge of 23-). The metallacryptate dianion is charge-balanced by the presence of two pyridinium ions (total 2+ charge) in the lattice. The hydroximate groups of the seven shi 3À  The single-crystal X-ray structure of [Hpy] 2 [YbAl 6 (H 2 shi) 2 (shi) 7 -(py) 1.855 (H 2 O) 2 ]Á7.386pyÁH 2 O, 3, with displacement ellipsoids at the 50% probability level. See Fig. 1 for additional display details.

Figure 4
The (a) metallacryptand and (b) metallacryptate views of 1 highlighting the [3.3.1] connectivity between the metal ions. See Fig. 1 for additional display details.
central Ln III ion. The two remaining H 2 shi À ligands also bind with their oxime oxygen to the central Ln III ion but do not participate in the metallacryptand shell. For the H 2 shi À ligands, the oxime oxygen atoms are deprotonated, while the oxime nitrogen and phenolate oxygen atoms remain protonated. As the oxime nitrogen atoms are protonated, they do not bind to Al III ions and are not involved in the N-O topology, although the H 2 shi À ligands do serve to bridge the central Ln III to the Al III ions. Beyond the overall molecular charge considerations, the oxidation state assignments of the Ln III and Al III ions are also confirmed by bond-valence-sum (BVS) values (Table 1; Brese & O'Keeffe, 1991;Trzesowska et al., 2004). For 1-3, a section of the main molecule is disordered induced by presence or absence of a pyridine ligand coordinated to Al6. In the absence of the pyridine moiety, an H 2 shi À ligand (associated with N4) moves into the space otherwise occupied by the pyridine and the ligand's protonated phenol oxygen atom coordinates to Al6. When the pyridine is bound to Al6, the phenol oxygen atom remains uncoordinated and hydrogen bonded to to a lattice pyridine molecule (associated with N15). The movement of the H 2 shi À ligand induces movement for the Ln III ion, for Al4, which also binds the same H 2 shi À ligand, and for one of the shi 3À ligands (associated with N9) coordinated to Al4. For 1-3 the occupancy ratio of the metallacryptand portions refined to 0.8550 (13) :0.1450 (13), to 0.8909 (13) :0.1091 (13), and to 0.8181 (14): 0.1819 (14), respectively. The structural description below will focus on the major moiety of each complex. A full description of disorder treatment can be found in the Refinement section.
Each Ln III ion is nine-coordinate with seven of the oxime oxygen atoms provided by the seven shi 3À ligands, which participate in the formation of the metallacryptand and form bridges to all six Al III ions, and the two remaining oxime oxygen atoms are provided by the two H 2 shi À ligands. Each oxime oxygen atom of the H 2 shi À ligands also serves as a -bridge between the central Ln III ion and an Al III ion. Based on a SHAPE (SHAPE 2. 1;Llunell et al., 2013;Pinsky & Avnir, 1998;Casanova et al., 2004) analysis of the Ln III coordination sphere, the geometry can best be described as a spherical capped square antiprism (Table 2; Fig. 5). The continuous shape measure (CShM) values for this geometry (1.083 for 1, 0.991 for 2, and 0.931 for 3) are below or near 1.0, where a value less than 1.0 typically indicates only minor distortions from the ideal shape (Cirera et al., 2005).
The six Al III ions of each metallacryptate are six-coordinate, all with an octahedral geometry as indicated by the CShM values (Table 3). Two of the Al III ions have trans shi 3À ligands, while the remaining four Al III are chiral centers (Fig. 6). For Al3 and Al6, the coordination sphere consists of two transchelate rings of two different shi 3À ligands. A six-membered Table 1 Average bond length (Å ) and bond-valence-sum (BVS) values (v.u.) used to support assigned oxidation states of the lanthanide and aluminium ions of 1-3.  Figure 5 Polyhedral view of the spherical capped square antiprism coordination geometry for Gd1 of 1. See Fig. 1 for additional display details. ring comprised of an oxime nitrogen atom and phenol oxygen atom of a shi 3À ligand is opposite of a five-membered chelate ring composed of oxime and carboxylate oxygen atoms of a different shi 3À ligand. The coordination is completed by an oxygen atom of a water molecule binding opposite to that of a nitrogen atom of a pyridine molecule. For Al1, Al2, A4, and Al5, the coordination sphere consists of three cis chelate rings in a propeller configuration with two Ã and two Á stereoconfigurations per metallacryptand. Al1 and Al5 both have a Ã stereoconfiguration but different types of chelate rings. For Al1 the coordination is completed by one five-membered and two six-membered chelate rings from three shi 3À ligands, while for Al5 the coordination is completed by two five-membered rings from shi 3À and H 2 shi À ligands and one six-membered ring from a shi 3À ligand. Both Al2 and Al4 have a Á stereoconfiguration, but different types of chelate rings. For Al2 the coordination is completed by two five-membered and one sixmembered chelate rings from three shi 3À ligands, while for Al4 the coordination is completed by two five-membered rings from shi 3À and H 2 shi À ligands and one six-membered ring from a shi 3À ligand. For the Ã and Á Al III ions, the types of oxygen and nitrogen atoms comprising the five-and sixmembered chelate rings are the same as in Al3 and Al6. For 1-3, several pyridine molecules are located in the lattice. Some of the pyridine molecules are fully occupied and ordered (associated with N13, N18, N19, and N20, while others are disordered and/or partially occupied. The lattice pyridine molecules associated with N15, N23, and N25 are correlated with the disorder of the metallacryptate (the presence or absence of the pyridine coordinated to Al6). In addition, two other pyridine molecules are independently disordered with a shared occupancy ratio (N14, N17 vs N22, N24). Lastly, the pyridine molecule associated with N21 is partially occupied with 1:1 disorder around an inversion center. Complete details pertaining to the treatment of the pyridine disorder including occupancy ratios are described in the Refinement section.  Table 3 Continuous Shapes Measures (CShM) values for the geometry about the six-coordinate ring Al III ions in 1-3.

Supramolecular features
For 1-3, similar numerous hydrogen bonds and weak C-HÁ Á ÁO interactions exist within each metallacryptate, between the Hpy + ions and the metallacryptate, and between the lattice pyridine molecules and the metallacryptate (Tables 4-6). The protonated oxime nitrogen atom of each of the two H 2 shi À ligands forms a hydrogen bond with itself by interacting with the phenolate oxygen atom of the same H 2 shi À ligand (N4-H4NÁ Á ÁO12 and N5-H5NÁ Á ÁO15). The protonated phenolate oxygen atoms of the H 2 shi À ligands form hydrogen bonds to the nitrogen atom of lattice pyridine molecules (O12-H12OÁ Á ÁN15, O15-H15Á Á ÁN14, O15-H15Á Á ÁN22). The water molecules coordinated to the Al III ions (O28 and O29) form hydrogen bonds to the nitrogen atom of lattice pyridine molecules (N13, N18, and N19) and to the oxime oxygen atom of a shi 3À ligand (O16). The lattice water molecule (O30) also forms a hydrogen bond to a lattice pyridine molecule (N20).
The pyridinium ions form hydrogen bonds to the carboxylate oxygen atom of shi 3À ligands (N12-H12AÁ Á ÁO9 and N16-H16Á Á ÁO27). The shi 3À ligands and pyridine molecules form several different types of C-HÁ Á ÁO interactions. The carbonhydrogen atom of a benzene ring of a shi 3À ligand forms an interaction with a carboxylate oxygen atom of a shi 3À ligand on a neighboring metallacryptate [C62-H62Á Á ÁO17 i ; symmetry code: (i) x + 1, y, z]. The carbon-hydrogen atom of a coordinated pyridine ligand forms an interaction with the oxime oxygen atom of a shi 3À ligand (C68-H68Á Á ÁO4). The carbon-hydrogen atoms of lattice pyridine molecules form interactions with the coordinated water molecule (C74-H74Á Á ÁO28) and with phenolate oxygen atoms of shi 3À ligands (associated C94 and C130).

Database survey
A survey of the Cambridge Structural Database (CSD version 5.41, update March 2020, Groom et al., 2016 reveals that there are three comparable metallacryptates. All are based on the [3.3.1] metallacryptand structure with six metal ions and seven shi 3À ligands forming the metallacryptand and the structures encapsulate an Ln III ion in the central cavity to form a metallacryptate. One structure is an individual molecule as in 1-3 and is based on gallium(III) as the ring metal ions (DIBLOS;Lutter et al., 2018a). However, the molecule contains one H 2 shi À and one Hshi 2À ligand to help encapsulate a central Tb III ion, and charge balance is maintained by three triethylammonium cations. Furthermore, the synthetic scheme for compounds 1-3 is based on the TbGa 6 molecule as the solvent choice and the ratios between the reactants are the same for both set of compounds (Lutter et al., 2018a). The other two metallacryptate structures are closely related and are also based on aluminum(III) as in [1][2][3] Table 4 Hydrogen-bond geometry (Å , ) for 1. Symmetry codes: (i) x þ 1; y; z; (ii) Àx; Ày þ 1; Àz þ 1.

Refinement
The structures of 1 and 3 were solved by isomorphous replacement from the analogue 2. For 1-3, a section of the main molecule is disordered induced by presence or absence of a pyridine ligand coordinated to Al6. In the absence of the pyridine moiety, an H 2 shi À ligand (associated with N4) moves into the space otherwise occupied by the pyridine, and the phenol oxygen atom coordinates to the aluminum (Al6). The movement of the H 2 shi À ligand induces movement for the Ln III ion, for Al4, which also binds the same H 2 shi À ligand, and for one of the shi 3À ligands (associated with N9) coordinated to Al4. For 3, atoms C24B and C27B of the H 2 shi À were constrained to have identical ADPs. The substantial movement of the H 2 shi À ligand induces a shift of the solvate pyridine (associated with N15) that is hydrogen bonded to the phenol oxygen of the H 2 shi À . The two Ln III ions were constrained to have identical ADPs. Equivalent sections of the two disordered moieties were restrained to have similar geometries. Another solvate pyridine molecule was included in the disorder and refined as threefold disordered (associated with N15, N23, and N25). The major disorder component solvate pyridine ring was refined as additionally disordered (associated with N15 and N23). The nitrogen atoms of these two moieties were constrained to share positions and ADPs. The minor disorder component solvate pyridine ring (associated with N25) was constrained to resemble an ideal hexagon with C-C distances of 1.39 Å . The disordered pyridine rings were restrained to have similar geometries as another, not disordered pyridine ring. U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. For 1, subject to these conditions, the occupancy ratio refined to 0.8550 (13):0.1450 (13). The occupancy rates for the additionally split pyridine ring (associated with N15 and N23) are 0.531 (3) and 0.324 (3). For 2, the occupancy ratio refined to 0.8909 (13):0.1091 (13). The occupancy rates for the additionally split pyridine ring (associated with N15 and N23) are 0.539 (3) and 0.352 (3). For 3, the occupancy ratio refined to 0.8181 (14):0.1819 (14). The occupancy rates for the additionally split pyridine ring (associated with N15 and N23) are 0.391 (3) and 0.324 (3).
Two other solvate pyridine rings are independently disordered with a shared occupancy ratio (N14, N17 vs N22, N24). The disordered moieties were restrained to have similar geometries as another, not disordered pyridine ring. U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. For 1, subject to these conditions, the occupancy ratio for the pyridine molecules refined to 0.613 (9)  Another solvate pyridine (associated with N21) is 1:1 disordered around an inversion center. The disordered moieties were restrained to have similar geometries as another, not disordered pyridine ring. U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. For 1, subject to these conditions, the occupancy rate refined to 2 Â 0.396 (4). For 2, the occupancy rate refined to 2 Â 0.429 (4). For 3, the occupancy rate refined to 2 Â 0.386 (5).
Water hydrogen-atom positions and some amine hydrogenatom positions were refined and O-H and selected N-H distances were restrained to 0.84 (2) and 0.88 (2) Å , respectively. Some water, amine and phenol hydrogen-atom positions were further restrained based on hydrogen-bonding considerations (phenol hydrogen atoms were placed in calculated positions, but were allowed to rotate around the C-O axis). 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). Additional crystal data, data collection, and structure refinement details are summarized in Table 7.

Funding information
Funding for this research was provided by: Faculty Professional Development Council, Pennsylvania State System of Higher Education (grant to C. M. Zaleski); National Science Foundation (grant No. CHE 1625543 to M. Zeller). 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. The structure was solved by isomorphous replacement from its Dy analogue, RR1_65. A section of the main molecule is disordered induced by presence or absence of an aluminum coordinated pyridine ligand. In the absence of the pyridine moiety a salicylate ligand swings into the space otherwise occupied by the pyridine, and the phenol oxygen atom coordinates to the aluminum (Al6). The movement of the salicylate ligand induces shifts to the Gd ion, to the aluminum ion the other two oxygen atoms are coordinated to (Al4) and one of the other salicylate ligands coordinated to Al4. The substantial movement of the first salicylate ligand induces a shift of the solvate pyridine its phenol oxygen is hydrogen bonded to. Another solvate pyridine molecule (adjacent to the second disordered salicylate ligand) was included in the disorder. The two Gd ions were constrained to have identical ADPs. Equivalent sections of the two disordered moieties were restrained to have similar geometries. The minor solvate pyridine ring was constrained to resemble an ideal hexagon with C-C distances of 1.39 Angstrom. The major moiety solvate pyridine ring was refined as additionally disordered. The nitrogen atoms of these two moieties were constrained to share position and ADP. Its two moieties and the other disordered pyridyl ring were restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy ratio refined to 0.8550 (13) to 0.1450 (13). The occupancy rates for the additionally split ring are 0.531 (3) and 0.324 (3). Two other solvate pyridyl rings are independently disordered with a shared occupancy ratio (N14, N17 vs N22, N24). The disordered moieties were restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy ratio for the one disordered in a general position refined to 0.613 (9) to 0.387 (9). Another solvate pyridyl ring is 1:1 disordered around an inversion center and partially occupied. The disordered moieties were restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy rate refined to two times 0.396 (4). Water H atom positions and some amine H atom positions were refined and O-H and selected N-H distances were restrained to 0.84 (2) and 0.88 (2) Angstrom, respectively. Some water, amine and phenol H atom positions were further restrained based on hydrogen bonding considerations (phenol H atoms were placed in calculated positions, but were allowed to rotate around the C-O axis).

Bis(pyridinium) diaquadipyridinehexakis[µ 3 -salicylhydroximato(3-)]bis[µ 2salicylhydroximato(1-)]hexaaluminiumytterbium-pyridine-water (1/7.386/1) (3)
Crystal data (C 5  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. The structure was solved by isomorphous replacement from its Dy and Gd analogues, RR1_61 and RR1_65. A section of the main molecule is disordered induced by presence or absence of an aluminum coordinated pyridine ligand. In the absence of the pyridine moiety a salicylate ligand swings into the space otherwise occupied by the pyridine, and the phenol oxygen atom coordinates to the aluminum (Al6). The movement of the salicylate ligand induces shifts to the Yb ion, to the aluminum ion the other two oxygen atoms are coordinated to (Al4) and one of the other salicylate ligands coordinated to Al4. The substantial movement of the first salicylate ligand induces a shift of the solvate pyridine its phenol oxygen is hydrogen bonded to. Another solvate pyridine molecule (adjacent to the second disordered salicylate ligand) was included in the disorder. Atoms C24B and C27B were constrained to have identical ADPs. The two Yb ions were constrained to have identical ADPs. Equivalent sections of the two disordered moieties were restrained to have similar geometries. The minor solvate pyridine ring was constrained to resemble an ideal hexagon with C-C distances of 1.39 Angstrom. The major moiety solvate pyridine ring was refined as additionally disordered. The nitrogen atoms of these two moieties were constrained to share position and ADP. Its two moieties and the other disordered pyridyl ring were restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy ratio refined to 0.8181 (14) to 0.1819 (14). The occupancy rates for the additionally split ring are 0.391 (3) and 0.324 (3). Two other solvate pyridyl rings are independently disordered with a shared occupancy ratio (N14, N17 vs N22, N24). The disordered moieties were 14 restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy ratio for the one disordered in a general position refined to 0.473 (7) to 0.527 (7). Another solvate pyridyl ring is 1:1 disordered around an inversion center and partially occupied. The disordered moieties were restrained to have similar geometries as another not disordered pyridine ring. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy rate refined to two times 0.386 (5). Water H atom positions and some amine H atom positions were refined and O-H and selected N-H distances were restrained to 0.84 (2) and 0.88 (2) Angstrom, respectively. Some water, amine and phenol H atom positions were further restrained based on hydrogen bonding considerations (phenol H atoms were placed in calculated positions, but were allowed to rotate around the C-O axis).