The crystal structure of a mononuclear PrIII complex with cucurbit[6]uril

A reaction between cucurbit[6]uril (CB6) with an excess of Pr(NO3)3·6H2O lead to the isolation of new mononuclear complex [Pr(CB6)(NO3)(H2O)5](NO3)2·9.56H2O (1), which crystallizes in the P21/n space group. The asymmetric unit of 1 contains two crystallographically independent [Pr(CB6)(NO3)(H2O)5]2+ complex cations, four nitrate counter-anions for charge balance and 19.12 interstitial water molecules. The coordination environments of the PrIII ions in 1 are formed by two carbonyl O atoms from bidentate cucurbit[6]uril units, two oxygen atoms from the bidentate nitrate anion and five water molecules.


Chemical context
Cucurbit[n]urils (CB[n]s) are 3D cyclic organic molecules possessing a rigid hydrophobic macrocyclic cavity, which is available for the uptake of various guest molecules via noncovalent interactions (Lin et al., 2020).Recently, the main interest in cucurbit [n]uril chemistry was due to their possible applications in selective catalysis (Nandi et al., 2017), molecular recognition (Barrow et al., 2015) and drug delivery (Das et al., 2019).The presence of several carbonyl oxygen atoms on both sides of the macrocyclic ring makes cucurbit[n]urils attractive ligand platforms for the design of discrete and polymeric coordination compounds, which can provide accessible channels due to the peculiarities of the arrangement of the cucurbit[n]urils in the crystal structure (Ni et al., 2013).The design of lanthanide(III) complexes with cucurbit[n]urils is particularly interesting because of the possible applications in molecular magnetism (Ren et al., 2013) and luminescence (Matsumoto et al., 2022).Depending on the size of the macrocyclic cavity and the lanthanide(III) ionic radii, cucurbit [n]urils usually provide two to six oxygen atoms in the coordination sphere of the lanthanide ions (Zhang et al., 2019(Zhang et al., , 2020;;Liang et al., 2013b;Zheng & Liu, 2017).In the majority of cases, the interaction between cucurbit[n]urils and lanthanide(III) salts leads to the formation of discrete mononuclear assemblies with one coordinated cucurbit[n]uril (Ren et al., 2013;Ni et al., 2015); however, examples of poly-nuclear complexes and coordination polymers have also been reported (Zhang et al., 2019;Zhang et al., 2020;Liang et al., 2013a,b).Several lanthanide-containing complexes with cucurbit[6]urils have been reported previously (Ren et al., 2013;Zheng & Liu, 2017;Shan et al., 2016;Yang et al., 2016;Xiao et al., 2016).In the absence of additional bridging organic ligands or anionic complexes, the interaction between Ln III salts and cucurbit [6]uril leads to the formation of mononuclear complexes (Ren et al., 2013;Yang et al., 2016;Kovalenko et al., 2021;Samsonenko et al., 2002).
Since cucurbit [6]uril is poorly soluble in water, lanthanide complex formation is usually observed in the presence of strong mineral acids (da Silva et al., 2014) or a substantial excess of the lanthanide salt reaching 25-fold excess (Ren et al., 2013).The observed structure of lanthanide(III)-cucurbit [6]uril complexes depends upon a number of factors, which include an excess of the lanthanide salt in the reaction mixture, the counter-anion, reaction temperature and crystallization conditions.
In this work we report synthesis and crystal structure of a new mononuclear Pr III complex with cucurbit[6]uril, [Pr(CB6)(NO 3 )(H 2 O) 5 ](NO 3 ) 2 •9.56 H 2 O (1), which was synthesized in the presence of a lowered tenfold Pr III excess and is not isomorphous to previously reported Ln III complexes with cucurbit[6]uril.

Structural commentary
The title complex 1 was prepared and isolated as colorless crystals according to a modified procedure for the analoguous Dy III complex with cucurbit[6]uril, using Pr(NO 3 ) 3 •6H 2 O for 1 (Ren et al., 2013).The previously reported synthetic strategy employed a 25-fold excess of the Ln III salt in order to promote the solubility of cucurbit [6]uril.In this work we introduced ultrasonication before placing the reaction mixture in hydrothermal conditions, which allowed the excess of the Ln III salt needed for the cucurbit [6]uril to be dissolved to be decreased.
The obtained complex [Pr(CB6)(NO 3 )(H 2 O) 5 ](NO 3 ) 2 •9.56 H 2 O (1) crystallizes in the P2 1 /n space group, while previously reported complexes obtained as outcomes of interactions between Ln(NO 3 ) 3 and cucurbit [6]uril crystallized in the orthorhombic Pna2 1 space group (Ln = Gd, Dy, Ho and Yb) or in the monoclinic P2 1 /n space group in the case of Ln III (Samsonenko et al., 2002;Ren et al., 2013).The different space group in the case of 1 may be caused by a variation of the synthetic conditions or the different lanthanide(III) radii.
The unit cell of complex 1 contains eight Pr III -cucurbituril cationic [Pr(CB6)(NO 3 )(H 2 O) 5 ] 2+ complex molecules (Fig. 1) per unit cell, non-coordinated water molecules and two nitrate anions per complex cation for charge balance.There are two crystallographically independent complex cations in the asymmetric unit of 1, however, the differences in the coordination environments of the Pr III ions are minor.The Pr III ions in the complex 1 are nonacoordinated.Two coordination positions of the Pr III ions are occupied by two carbonyl oxygen atoms from the coordinated CB6 ligands, two positions contain oxygen atoms from the bidentate nitrate anions and the remaining five positions are occupied by oxygen atoms from the coordinated water molecules.The macrocyclic cucurbit[6]uril coordinates in bidentate mode, which is typical for Ln III -cucurbit[n]uril complexes without additional ligands.The carbonyl oxygen atoms on the opposite side of the macrocycle remain uncoordinated.
The Pr-O carbonyl bond distances in complex 1 are typical for cucurbit[6]uril complexes with Ln III ions (Samsonenko et al., 2002;Ren et al., 2013;da Silva et al., 2014;Lin et al., 2019).The observed bond distances between the Pr III ions and the nitrate oxygen atoms are typical for bidentately coordinated nitrate anions to Pr III ions (Pavlishchuk et al., 2019).However, minor differences in the geometrical parameters of the coordination spheres of Pr1A and Pr1B are observed (Table 1, (Fig. 2).According to the calculations performed with Shape 2.1 software (Casanova et al., 2005, Table 2), the nona-

Figure 2
Coordination environments of the (a) Pr1A and (b) Pr1B ions in complex 1.

Figure 3
Fragment of the crystal structure viewed along the a axis.

Table 2
Continuous shape calculations for nonacoordinated Pr III ions in complex 1 performed with Shape 2.1 software (Casanova et al., 2005).
In summary, we have synthesized a new Pr III complex with the macrocyclic cucurbit[6]uril ligand, which crystallizes in space group P2 1 /n, while previously reported complexes with other lanthanide ions Ln III = La, Gd, Dy, Ho and Yb crystallized in P2 1 /n or Pna2 1 .The crystal structure of 1 contains [Pr(CB6)(NO 3 )(H 2 O) 5 ] 2+ complex cations, two non-coordin- ated nitrates per cation and non-coordinated water molecules.Subtle differences in the bond distances and angles in the Pr III coordination spheres leads to the observation of two crystallographically different types of Pr III ions.The composition of the coordination sphere of two types of nonacoordinated Pr III ions in 1 is the same, however the symmetry of the coordination environment of Pr1A and Pr1B ions is different.

Synthesis and crystallization
Cucurbit[6]uril was obtained by a modified procedure (Zbruyev et al., 2023).Cucurbit[6]uril (C 36 H 36 N 24 O 12 •10H 2 O, CB6, 11.8 mg, 0.01 mmol) and Pr(NO 3 ) 3 •6H 2 O (42 mg, 0.1 mmol) were placed in a closed vial containing 3 mL of water and ultrasonicated for the enhancement of macrocycle solubility.The obtained suspension was heated for 1 h at 358 K in a sand bath, which was accompanied by dissolution of macrocyclic ligand.Afterwards the observed clear solution had been heated at 363 K for 2 h.Slow cooling in the sand bath led to the formation of colorless crystals of 1 in one day.IR spectra of CB6 and complex 1 are shown in Fig. 5.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were placed in calculated positions and refined as riding.T min = 0.892, T max = 1.000 75330 measured reflections 20820 independent reflections 17014 reflections with I > 2σ(I)

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.Geometric parameters (Å, º)

Figure 5 IR
Figure 5IR spectra of CB6 and complex 1 recorded in a KBr pellet.

Table 4
Experimental details.