Crystal structure and characterization of a new lanthanide coordination polymer, [Pr2(pydc)(phth)2(H2O)3]·H2O

The crystal structure and thermogravimetric stability of [Pr2(pydc)(phth)2(H2O)3]·H2O, a two-dimensional coordination polymer with a novel coordination mode of pyridine-2,5-dicarboxylate (pydc2−), are reported.


Chemical context
Lanthanide coordination polymers (LnCPs) have attracted widespread interest because of their unique properties and wide range of potential applications, such as in luminescent temperature sensing (Rocha et al., 2016), catalysis (Sinchow et al., 2022), gas detection (Thammakan et al., 2023) and drug delivery (Wei et al., 2020).However, the high coordination numbers of the trivalent lanthanides (Ln III ) and the versatility in their coordination geometries complicates the control of intermolecular interactions and the prediction of coordination polymer frameworks.In addition, the synthesis of these frameworks is also influenced considerably by differences in synthetic procedures and conditions such as solvents, pH, reaction temperature and time, among other factors (Sinchow et al., 2019).Organic ligands are utilized as a template for the structural design, to direct the framework architecture.Among the organic ligands available, polycarboxylic acids are notably the most used because they are hard base ligands and can facilitate diverse coordination modes.In this work, pyridine-2,5-dicarboxylic acid (H 2 pydc) and phthalic acid (H 2 phth) were chosen to be the structure-directing ligands.Relevant structures include, for example, [Pr 3 (phen) 2 (phth)

Structural commentary
The asymmetric unit of [Pr 2 (pydc)(phth) 2 (H 2 O) 3 ]•H 2 O is composed of two Pr III metal centers, one molecule of pydc 2À , two molecules of phth 2À , three coordinated water molecules and a non-ligated water molecule (Fig. 1).The Pr1 ion is ninefold coordinated to one N atom from pydc 2À and eight O atoms from four phth 2À , two pydc 2À and one water molecule to form a {Pr (1)NO 8 } motif that can be described as a distorted tricapped trigonal prism.The Pr2 ion is also ninefold coordinated, being surrounded by nine O atoms from three phth 2À , one pydc 2À and two water molecules in a distorted tricapped trigonal-prismatic {Pr(2)O 9 } motif.The Pr-O bond lengths are in the range 2.413 (3)-2.691(3) A ˚and the Pr-N bond is 2.696 (3) A ˚(Table 1), in accordance with a previous report for Pr III  These dimers are then connected by the carboxyl groups of phth 2À in a � 3 -� 2 :� 1 :� 1 :� 1 fashion to form a mono-periodic chain also extending in the b-axis direction.These chains are connected through a novel coordination mode for pydc 2- involving a � 1 -� 1 :� 1 carboxyl group at one side and a � 2 -� 1 :� 1 carboxyl group together with the pyridyl N atom coordinated on the other side to form a {[Pr 2 (pydc)(phth) 2 (H 2 O) 3 ]} n layer extending in the ( 101) plane (Fig. 2a).

Thermogravimetric analysis
The thermogravimetric curve of the title compound shows four steps of weight loss in the temperature range 30 � C to 1000 � C (Fig. 4).The first step occurs at 100-185 � C with a 6.0% weight loss attributed to the removal of one hydrogen-bonded water and two coordinated water molecules (calc.6.4%).The second step observed at 300-350 � C is due to the loss of the other coordinated water molecule (exp.2.5%, calc.2.1%).This step is possibly due to the removal of O14W, which is held by both strong and weak hydrogen-bonding interactions.The next step of weight loss occurs in the temperature range 400-580 � C and represents a higher weight loss of 37.3%.This step can be attributed to the pyrolysis of the organic ligands (two phth 2À ligands, calc.38.7%).The last step of weight loss, from 580 to 1000 � C, could be due to the elimination of the bridging pydc 2À ligand to form praseodymium oxide residues (exp.14.7%, calc.19.5%).

Database survey
A search for the title compound in the Cambridge Structural Database (CSD version 5.44, April 2023;Groom et al., 2016) using CONQUEST software (version 2023.2.0;Bruno et al., 2002) did not match with any reported structures.Regarding organic ligands, there were 123 structures of lanthanide coordination polymers that included pydc 2À .Among these structures, interestingly, there were none in which pydc 2À adopts the same coordination mode as in the title compound (Sinchow et al., 2019).This new mode of coordination acts as a � 3 -bridge to link three Pr III ions and facilitates the formation of a di-periodic coordination framework.Regarding phth 2À , there were 118 structures deposited in the CSD, none of which
Thermogravimetric analyses (TGA) were carried out using a Mettler Toledo TGA/DSC 3+, with a heating rate of 20 � C min À 1 , ramping from 30 to 1100 � C under a nitrogen gas flow.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.All hydrogen atoms of aromatic rings and water molecules were positioned geometrically and refined using a riding model with U iso (H) = 1.2-1.5Ueq (C,O).

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 crystal structure was solved using the dual-space algorithm with the SHELXT program (Sheldrick, 2015a) and refined on F 2 by the full-matrix least-squares technique using the SHELXL program (Sheldrick, 2015b) via the Olex2 interface (Dolomanov et al., 2009).

Table 3
Experimental details.