Trans-Catena-Poly[[(Bis-(μ-N,N’-Bis[(Pyridin-3-yl)Methyl]Ethanediamide))-Diaqua-Cadmium(II)] Bis(Nitrate) Tetrahydrate)]

1. Introduction

During the past decades, oxamic acid derivatives, i.e., oxalic acid monoamides, have encountered a flourishing interest due to the variety of their applications, ranging from medicine [1,2] to the conservation and restoration of cultural heritage [3,4,5,6]. Oxalyl diamides (oxalamides), i.e., oxalic acid diamides, have been widely employed in synthetic organic chemistry [7,8] and coordination chemistry [9,10]. In particular, N,N’-bis(pyridin-3-ylmethyl)oxalamide (L) was isolated in two polymorphs [11], as a hydrate [12], and it was structurally characterized in about fifteen different cocrystals and about thirty coordination compounds, where the pyridine N-atoms were directly involved in the coordination of transition metal ions as varied as Cu^II [13], Zn^II [14], Ni^II [15], Ag^I [16], Au^I [17], Co^II [18,19], and Pd^II [20]. In addition, four Cd^II complexes bearing the L oxalamide ligand in combination with carboxylate and dithiophosphato ancillary ligands have been reported to date [18,21].

We report here on the synthesis and the spectroscopic and structural solid-state characterization of the the coordination polymer (CP) [Cd(L)₂(H₂O)₂](NO₃)₂·4H₂O]_∞.

2. Results

N,N’-bis(pyridin-3-ylmethyl)oxalamide (L) was prepared in quantitative yield by refluxing pyridin-3-ylmethylamine and diethyloxalate in 2:1 molar ratio in water solution [6,16]. Crystals of compound 1 were grown by slow evaporation of an acetonitrile/water mixture of L and Cd(NO₃)₂·4H₂O in a 1:3 molar ratio (Scheme 1; Tables S1–S4). Compound 1 was characterized by elemental analysis, melting point determination, and FT-IR spectroscopy. Single crystal X-ray diffraction analysis established 1 as [Cd(L)₂(OH₂)₂](NO₃)₂·4H₂O, crystallized in the triclinic space group P$\overline{1}$ (Figure 1).

The asymmetric unit of compound 1 features a half-occupied Cd^II ion located on an inversion centre that is coordinated by a donor molecule L interacting through a pyridine N1 atom [Cd1–N1 = 2.3365(10) Å], a water molecule bound to the metal ion through the O8 atom [Cd1–O8 = 2.2991(9) Å], a nitrate and two co-crystallized water molecules. The L unit displays an antiperiplanar conformation of the oxalamide core with an O=C–C=O torsion angle of 174.7(1)° (Table S4), as previously found in the crystal structure determinations of different N,N′-dialkyloxalamides [22,23]. The C–C, C=O, and C–N bond lengths [1.533(1), 1.229(1)/1.233(1), and 1.325(1)/1.330(1) Å, respectively] are very close to the average values calculated for the 253 differently substituted free oxalamides deposited at the Cambridge Structural Database [CSD; average distances: C–C, 1.53(2); C–O, 1.23(1); C–N, 1.33(1) Å] [24]. Notably, all the 29 compounds containing L that were structurally characterized show the oxalamide in the same antiperiplanar conformation.

In the crystal structure of compound 1 each ligand unit bridges two symmetry-related Cd atoms (Cd and Cd^iv in Figure 1; Cd^iv–N4 = 2.3366(10) Å; ^iv = +x, +y, −1+z), so that each Cd atom shows a pseudo-octahedral coordination achieved by four N atoms lying on the meridian coordination plane and two trans-disposed water molecules. This coordination results in the formation of a CP featuring a ribbon-like motif propagating along the c-axis with Cd nodes shared between Cd₂L₂ links and self-complementary hydrogen bonds between oxalamides of adjacent L units forming a ${\mathrm{R}}_2^2\left(10\right)$ motif (interaction a in Figure 2 and Figure 3 and Table 1). In the crystal packing, adjacent ribbons are connected via face-to-face slipped π–π stacking interactions between pyridyl rings with distances between 3.70 and 3.82 Å, as shown in Figure 2b. The charge of the cationic ribbons is balanced by NO₃^– anions that, in combination with both coordinated and co-crystallized water molecules, define an intricate hydrogen bonded motif (interactions b-h in Figure 3 and Table 1).

A comparison between the solid-state FT-IR spectrum of compound 1 and that of the ligand L clearly shows the vibrational bands typical of C–H (2940–3050 cm^–1), N–H (3302 cm^–1), and C=O (1653–1655 cm^–1) stretching vibrations. A small but detectable shift in the C–N vibration of the pyridine ring was observed on passing from L to compound 1 (1525 and 1517 cm^–1, respectively) as a consequence of N-coordination (Figure S1). As expected, the broad band due to the O–H stretching mode of water molecules (3500 cm^–1) and those typical of the nitrate anion (1385 cm^–1) could only be envisaged in the FT-IR spectrum of compound 1.

Finally, hybrid-DFT calculations shown that the lone pairs (LPs) of electrons on the pyridine nitrogen atoms of the ligand L, which feature remarkably negative natural charges (average charge Q_N = –0.481 |e|, Table S7), are available for coordination (Figure S2). Notably, the model complex cation [Cd(L)₄(OH₂)₂]²⁺ was successfully optimized (Figure S3) and shown the same structural features as the complex unit in compound 1. The terminal pyridine N-atoms in [Cd(L)₄(OH₂)₂]²⁺ display natural charges ranging between –0.471 and –0.491 |e| (Table S8), demonstrating that the N-atoms on the terminal pyridine rings are available for further coordination and thus accounting for the formation of the CP in compound 1.

3. Materials and Methods

3.1. General

All the reagents and solvents were used without further purification. N,N’-bis(pyridin-3-ylmethyl)oxalamide was synthesized as previously reported [16]. Fourier-Transform infrared (FT-IR) spectroscopic measurements were recorded at room temperature on a Thermo-Nicolet 5700 spectrometer on KBr pellets, with a KBr beam-splitter and KBr windows (4000–400 cm^–1; resolution 2 cm⁻¹). Melting point determinations were carried out on a FALC mod. C apparatus. DFT calculations were carried out both on L and the model compound [Cd(L)₄(OH₂)₂]²⁺ at DFT level with the commercial suite of programs Gaussian 16 [25] by adopting the hybrid mPW1PW hybrid functional [26]. The def2SVP basis sets [27,28] were adopted for all the atomic species. Vibrational frequencies were calculated at the optimized geometries. BS data were extracted from the EMSL BS Library [29]. The memory required for each calculation was evaluated by the GaussMem cross-platform (Linux, macOS, Windows) program as a function of the number of shared processors, the total number of basis set functions, and a memory threshold depending on the highest angular momentum basis function [30,31]. Molecular geometry optimization for compound 1 was performed starting from structural data. Charge distributions were evaluated at the NBO level [32,33,34] at the optimized geometries. GaussView [35] was used to investigate the Kohn-Sham molecular orbital composition and charge distribution. X-ray diffraction data for compound 1 were collected at 100(2) K by means of ω scans with a Bruker D8 Venture diffractometer equipped with a PHOTON II area detector. Data reduction was carried out with SAINT v8.37[36] and SADABS-2016/2 [37] and the structure was solved with the ShelXT [38] solution program using dual methods. The model was refined by iterative cycles of least-squares refinement on F² ShelXL [39] 2018/3 and by using Olex2 1.5 [40] as the graphical interface.

Crystal data for compound 1: C₂₈H₄₀CdN₁₀O₁₆, (Mr = 885.10 g mol⁻¹) triclinic, P -1 (No. 2), a = 9.1489(9) Å, b = 9.2546(9) Å, c = 11.6226(12) Å, α = 91.023(4)°, β = 112.441(4)°, γ = 93.497(4) °, V = 906.96(16) ų, T = 100(2) K, Z = 1, µ(Mo Kα) = 0.688 mm⁻¹, 44270 reflections measured, 4674 unique (R_int = 0.0390) which were used in all calculations. The final wR₂ was 0.0425 (all data) and R₁ was 0.0167 [F² ≥ 2 σ(F²)].

3.2. Synthesis of Compound 1

To 3 mL of a CH₃CN solution of N,N’-bis(pyridin-3-ylmethyl)oxalamide (5.0×10^–3 mol/L), a 1.0×10^–1 mol/L solution of Cd(NO₃)₂⋅4H₂O in water was added (donor/Cd molar/ratio 1:3). A colourless crystalline precipitate formed in 24 h and was isolated from the mother liquor, gently washed with CH₃CN and air dried. A portion of the crystals was placed on a glass slide and coated with a perfluoroether oil. A crystal suitable for X-ray diffraction analysis was selected and mounted on a MiTeGen loop. M.p.: 232 °C. FT-MIR (KBr pellet, 4000–400 cm^–1): 3508w, 3288m, 3057vw, 2926vw, 2399vw, 2395vw, 1763vw, 1662s, 1606w, 1517m, 1452w, 1385vs, 1263w, 1236w, 1188vw, 1128vw, 1049vw, 985vw, 825m, 750vw, 696m, 648w, 511vw, 438vw, 407vw cm^–1.

4. Conclusions

Compound 1 was synthesized by the self-assembly of N,N’-bis(pyridin-3-ylmethyl)oxalamide and cadmium nitrate and its crystal structure elucidated by single crystal X-ray diffraction analysis. The crystal structure of compound 1 consists of a CP featuring cationic 1D-ribbons whose charge is counterbalanced by nitrate anions. Each Cd node shows a pseudo-octahedral coordination achieved by four pyridine N-atoms and two trans-disposed water molecules. The crystal packing results from the cooperation of π-π stacking interactions and hydrogen bonds involving oxalamides, water molecules and nitrates. Compound 1 confirms the potential of dipicolyloxalmides as flexible ligands for a variety of coordination compounds and opens new perspectives in the field of the crystal engineering of CPs and metal-organic frameworks.