Compound 1 is a one-dimensional coordination polymer: The asymmetric unit contains two Pr³⁺ cations, three doubly-deprotonated (C₇H₃NO₄)²⁻ (pdc²⁻) dianions, one singly-deprotonated (C₇H₄NO₄)⁻ (Hpdc⁻) anion, one 3-hydroxylpyridinium (H3hp⁺) cation and twelve water molecules (Figure 1).

The coordination geometries of the praseodymium ions in 1 are very different: Pr1 is coordinated by two O,N,O-tridentate pdc²⁻ dianions and the O,N,O-tridentate Hpdc⁻ anion to generate a fairly regular tri-capped trigonal prismatic PrO₆N₃ coordination geometry with the N atoms serving as the caps protruding through the prismatic side-faces (Figure 2). Each ligand forms one Pr–O bond to the “top” triangular face and one to the bottom as well as the metal–nitrogen bond. The dihedral angle between the O1/O5/O11 and O3/O7/O9 triangular faces is 5.00(18)° and the metal ion is displaced by 1.6622(14) Å from the first triplet of O atoms and by −1.7184(14) Å from the second. The Pr–N bond lengths (Table 1) (mean = 2.606 Å) are longer than all but one of the Pr–O bonds (mean = 2.510 Å).

The geometrical parameters for the three chelating ligands (containing atoms N1, N2 and N3) are similar to those in related structures [6] and all the carboxylate groups are close to coplanar with their attached rings. The presence of the (un-removed) carboxylate proton in the N3-containing mono-ion is clearly indicated in terms of the C20–O10 and C20–O9 bond lengths of 1.291(4) Å and 1.231(4) Å, respectively.

The Pr2 species in 1 is coordinated by an O,N,O-tridentate pdc²⁻ dianion (containing N4), four water molecules and two bridging monodentate-O pdc⁻ species to generate a PrO₈N coordination polyhedron that approximates to a mono-capped square anti-prism (Figure 3). The mean Pr–O separation is 2.498 Å and the four longer bonds are to the water molecules, although the differences are very small. The capping atom, O21, is part of a water molecule and protrudes through the O2/O15/O18/O20 square face of the anti-prism. The dihedral angle between this face and the O8/O13/O19/N4 face is 6.79(13)° and Pr2 is displaced from the two faces by −0.9083(13) Å and 1.5041(13) Å, respectively.

The bridging N1- and N2-containing pdc²⁻ dianions lead to infinite chains of alternating Pr1- and Pr2-centred polyhedra, which propagate in [100] (Figure 4). The Pr1⋯Pr2 separations are 6.4122(3) Å (via the N1 ligand) and 6.9775(3) Å (via the N2 ligand).

To complete the structure of 1, an N–H⋯O and numerous O–H⋯O and hydrogen bonds occur (see supplementary materials for a full list). The 3-hydroxypyridinium (H3hp⁺) cation appears to play a significant role in establishing the structure of 1. As well as forming N–H⋯O and O–H⋯O hydrogen bonds, which serve to crosslink the [100] chains, it participates in two aromatic π–π stacking interactions (Figure 5). This might be described as an “intercalation” of the H3hp⁺ species between the two pendant ligands (N3 attached to Pr1 and N4 attached to Pr2) of the alternating metal atoms in the chain.

The TGA results (Figure 6) are consistent with the stoichiometry of the crystal: The first weight loss of about 17.8% (calculated value 17.2%) corresponds to the loss of the four coordinated and eight non-coordinated water molecules between 75 and 190 °C. The second weight loss of 7.6% up to 280 °C results from the removal of the 3-hydroxypyridinum ion (calcd. 7.2%). The third weight loss of about 49.4% is due to successive release of decomposition products from the pyridine dicarboxylate anions (calcd. 49.1%), yielding Pr₂O₃ as the final product (25.3% residual weight; calc. 26.5%).

The C, H, N elemental analysis was carried out with a Vario Micro Cube (Elementar, Germany). IR data (KBr pellet) were recorded on a Perkin–Elmer FTIR 180 spectrophotometer over the range 4000–400 cm⁻¹. TGA data (25–1000 °C) were recorded under continuous nitrogen flow, with a ramp-rate of 10 °C·min⁻¹ using a SDT Q600 instrument (TA Instruments, USA).

A mixture of pyridine 2,6-dicarboxylic acid (84 mg, 0.5 mmol), 3-hydroxypyridine (45 mg, 0.5 mmol), PrCl₃·6H₂O (45 mg, 0.125 mmol) and 0.2 mL formic acid and 7 mL distilled water was sealed in a 23 mL Teflon-lined autoclave and heated to 170 °C for 12 h and cooled to room temperature at 30 °C·h⁻¹ to yield a green solution. Green needles of 1 slowly grew as the water evaporated over two weeks and were recovered by vacuum filtration, rinsing with water and dried at room temperature. Analysis (%) calc. for C₃₃H₄₃N₅O₂₉Pr₂: N 5.58; C 31.54; H 3.43; Found: N 5.69; C 31.73; H 3.61. Yield ca. 65%. IR (cm⁻¹): 3572–3288 (broad, strong) water O–H stretch; 1630 ν_asym(C=O); 1415, 1359 ν_sym(C=O).

The single-crystal data for 1 (green needle 0.40 × 0.05 × 0.05 mm) were collected using a Bruker Kappa APEX II [10] CCD diffractometer (graphite monochromated Mo Kα radiation, λ = 0.71073 Å) at room temperature. Data reduction with SAINT [10] then proceeded and the structure was solved by direct methods with SHELXS [11]. The resulting atomic model was developed and refined against |F|² with SHELXL [11] and the “observed data” threshold for calculating the R(F) residuals was set as I > 2σ(I). The C- and N-bound bound H atoms were placed in idealised locations (C–H = 0.96–0.97 Å, N–H = 0.86 Å) and refined as riding atoms. The O-bound H atoms were located in difference maps and refined as riding atoms in their as-found relative locations. The constraint U_iso(H) = 1.2U_eq(carrier) was applied in all cases. The structural model was analysed and validated with PLATON [12] and full refinement details are given in the deposited cif.

Crystal data for 1: C₃₃H₄₃N₅O₂₉Pr₂, M_r = 1255.54, triclinic, (No. 2), Z = 2, a = 13.2567(1) Å, b = 13.6304(2) Å, c = 13.6409(2) Å, α = 89.695(1)°, β = 63.049(1)°, γ = 86.105(1)°, V = 2191.16(5) ų, F(000) = 1252, T = 293(2) K, ρ_calc = 1.903 g·cm⁻³, μ = 2.304 mm⁻¹, 18565 reflections recorded (6.0° ≤ 2θ ≤ 55.0°; −17 ≤ h ≤ 16, −17 ≤ k ≤ 17, −17 ≤ l ≤ 17), R_Int = 0.021, 9996 merged reflections, 8617 with I > 2σ(I), 622 variable parameters, R(F) = 0.033, wR(F²) = 0.084, w = 1/[σ²(F_o²) + (0.0392P)² + 3.8222P], where P = (F_o² + 2F_c²)/3, min./max. ∆ρ = −1.06, +2.32 e Å⁻³. Cambridge Structural Database deposition number: CCDC-885694.