[Pr₂(pdc)₃(Hpdc)(H₂O)₄]_n·n(H3hp)·8n(H₂O), a One-Dimensional Coordination Polymer Containing PrO₆N₃ Tri-Capped Trigonal Prisms and PrO₈N Mono-Capped Square Anti-Prisms (H₂pdc = Pyridine 2,6-Dicarboxylic Acid, C₇H₅NO₄; 3hp = 3-Hydroxy Pyridine, C₅H₅NO)

1. Introduction

Coordination polymers with potentially useful physical properties such as sorption and catalytic activity can be produced by careful choice of ligands, metal ions, the solvent and the pH of the solution [1,2,3]. Pyridine-2,6-dicaboxylic acid (H₂pdc, C₇H₅NO₄) is a versatile ligand that can chelate to, or bridge between, metal ions and can furnish different types of infinite-chain polymers [4,5,6,7]. As an extension of such studies, we now describe the solution-phase synthesis, crystal structure and characterization of the title compound, [Pr₂(pdc)₃(Hpdc)]_n·n(C₅H₆NO)·8n(H₂O), 1, in which the two Pr³⁺ ions have very different coordination geometries. Previously reported complexes containing Pr³⁺ and de-protonated ligands derived from H₂pdc include [Pr(pdc)(Hpdc)(H₂O)₂]_n·nH₂O [8], [Pr₂(pdc)₃(H₂O)₃]_n·nH₂O [9], [Pr₃(pdc)₄(Hpdc)(H₂O)₈]_n·8nH₂O [9]. These show various ligand and water bonding modes to the metals, but all of them lead to nine-coordinate polyhedra for the metal atoms and the bridging ligands lead to infinite chains in the crystal.

2. Results and Discussion

2.1. Crystal Structure of 1

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).

Figure 1. The asymmetric unit of 1 (50% displacement ellipsoids). The uncoordinated water molecules and the C-bound H atoms are omitted for clarity.

Figure 1. The asymmetric unit of 1 (50% displacement ellipsoids). The uncoordinated water molecules and the C-bound H atoms are omitted for clarity.

Figure 2. Detail of 1 showing the tri-capped trigonal prismatic coordination of Pr1 (50% displacement ellipsoids). The orchid-colored lines indicate the trigonal prism of O atoms.

Figure 2. Detail of 1 showing the tri-capped trigonal prismatic coordination of Pr1 (50% displacement ellipsoids). The orchid-colored lines indicate the trigonal prism of O atoms.

Table 1. Selected bond-distances (Å) in 1.

Pr1–O7	2.469(2)	Pr1–O11	2.472(2)
Pr1–O5	2.499(2)	Pr1–O3	2.501(2)
Pr1–O1	2.518(2)	Pr1–O9	2.601(2)
Pr1–N2	2.594(3)	Pr1–N1	2.604(3)
Pr1–N3	2.620(3)
Pr2–O8 #1	2.448(2)	Pr2–O2	2.469(2)
Pr2–O15	2.477(2)	Pr2–O13	2.489(2)
Pr2–O19	2.490(3)	Pr2–O20	2.501(3)
Pr2–O18	2.525(2)	Pr2–O21	2.582(3)
Pr2–N4	2.604(3)

Symmetry code: ^#1x – 1, y, z.

Table 1. Selected bond-distances (Å) in 1.

Pr1–O7	2.469(2)	Pr1–O11	2.472(2)
Pr1–O5	2.499(2)	Pr1–O3	2.501(2)
Pr1–O1	2.518(2)	Pr1–O9	2.601(2)
Pr1–N2	2.594(3)	Pr1–N1	2.604(3)
Pr1–N3	2.620(3)
Pr2–O8 #1	2.448(2)	Pr2–O2	2.469(2)
Pr2–O15	2.477(2)	Pr2–O13	2.489(2)
Pr2–O19	2.490(3)	Pr2–O20	2.501(3)
Pr2–O18	2.525(2)	Pr2–O21	2.582(3)
Pr2–N4	2.604(3)

Symmetry code: ^#1x – 1, y, z.

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.

Figure 3. Detail of 1 (50% displacement ellipsoids) showing the mono-capped square anti-prismatic coordination of Pr2. O18, O19, O20 and O21 are parts of water molecules. The orchid lines indicate the square anti-prism. O8* is generated by the symmetry operation (x − 1, y, z).

Figure 3. Detail of 1 (50% displacement ellipsoids) showing the mono-capped square anti-prismatic coordination of Pr2. O18, O19, O20 and O21 are parts of water molecules. The orchid lines indicate the square anti-prism. O8* is generated by the symmetry operation (x − 1, y, z).

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).

Figure 4. Fragment of a [100] chain of alternating Pr1- and Pr2-centred polyhedra in 1.

Figure 4. Fragment of a [100] chain of alternating Pr1- and Pr2-centred polyhedra in 1.

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.

Figure 5. Fragment of the structure of 1 showing the π–π stacking interactions (open pink lines) of the “intercalated” H3hp⁺ cation: The separations of its ring centroid with those of the N3- and N4-containing rings (indicated by pink spheres) are 3.596(2) Å and 3.647(3) Å, respectively.

Figure 5. Fragment of the structure of 1 showing the π–π stacking interactions (open pink lines) of the “intercalated” H3hp⁺ cation: The separations of its ring centroid with those of the N3- and N4-containing rings (indicated by pink spheres) are 3.596(2) Å and 3.647(3) Å, respectively.

2.2. Thermal Analysis

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%).

Figure 6. TGA trace for 1.

Figure 6. TGA trace for 1.

3. Experimental Section

3.1. Instrumental

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).

3.2. Synthesis

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).

3.3. Single-Crystal Data Collection and Analysis

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.

4. Conclusions

The synthesis and crystal structure of the title one-dimensional coordination polymer have been described, in which the coordination polyhedra of the two nine-coordinate praseodymium ions are very different. The “guest” 3-hydroxypyridinium cation appears to play an important role in establishing the structure in terms of inter-chain hydrogen bonds and intra-chain intercalated π–π stacking interactions.