Hybrid azine–oxamic acid molecular design: synthesis, structure, and supramolecular implications

Abstract

Since the 1960s, oxamic acid derivatives and their coordination compounds have been extensively studied, with applications ranging from protective agents for marble to biologically relevant systems. However, no reports exist on hybrid molecules combining an azine group with an oxamic acid-derived group. The azine group is notable for its photochromic properties and π-conjugation, which make it valuable in fields such as biomedicine, optoelectronics, and chemical sensing. This work reports the synthesis and structural characterization of two novel molecules incorporating both azine and oxamic acid-derived groups: the ester form, named NN-Et₂H₂oba, and the acid form, NN-H₄oba. The synthesis was initiated by forming the azine group from 2-nitrobenzaldehyde and hydrazine dichlorohydrate, followed by a reduction of the nitro groups and subsequent addition of the oxamic acid-derived group. Based on the structures obtained by single-crystal X-ray diffraction (SXRD), correlations were established with data from FTIR, ¹H NMR, and elemental analysis. Additionally, given the critical role of intermolecular interactions in determining material properties, a comprehensive study of nature and energies of the non-covalent interactions was conducted using computational methods, including Hirshfeld surface analysis, fingerprint plots, energy framework diagrams, and crystal lattice energy calculations. Intramolecular N–H⋯N hydrogen bonds, present in both molecules, as well as intermolecular C–H⋯O interactions observed in NN-Et₂H₂oba, were found to significantly contribute to the stabilization of molecular planarity. This structural conformation endows the molecules with potential for photophysical applications. Moreover, the supramolecular organization of NN-Et₂H₂oba is primarily stabilized by London dispersive forces, while that of NN-H₄oba is stabilized by both dispersive forces and hydrogen bonds.

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1 Introduction

Oxamic acid derivatives and coordination compounds containing such molecules have been reported in the Cambridge Crystallographic Data Center (CCDC) database since the 1960s.¹ Oxamic acid consists of both an amide and a carboxylic acid functional group, and its derivatives (oxm) are formed by the substitution of hydrogen atoms with organic moieties (Fig. 1a).² A detailed database search revealed a wide variety of structural motifs, particularly when substitutions at the amide nitrogen were considered (Table 1). These compounds have attracted interest due to their structural versatility and their ability to participate in multiple intermolecular interactions.

Fig. 1 Structural drawings of the ester NN-Et₂H₂oba and acid form NN-H₄oba.

Table 1 Structures found in the Cambridge Crystallographic Data Centre (CCDC) database with the oxamate group

Structure	Ref.	Structure	Ref.	Structure	Ref.
R1 = H, Me, Et, Ph, NH2, CH2Ph, COOtBu, (CH2)2NH2, (CH2)2oxm, (CH2)3oxm, CH2CH(Ph)2, CH2COOH, CONH2, COOCH2Ph, CH2COHCH2oxm, pTol, CH2NH3, CH2C≡C, CH2C(CH3)2CH2oxm, CH(CH3)COOH, CH2CF3, CH2C(F2CH3), NHC(NH2)2, (CH2)2NHCH3, cy-hexyloxm, CH2CH(NH3)COOH, CH(PhCH3); R2 = OH, COOH, oxm, Cl, Br, F, CONH(CH2)3COOMe, SO2Phoxm, oxmPhoxm, C=CPhoxm, C≡CPhoxm, N=NPhoxm, SCH3, OCMe, OMe, NH(CO)2NHCH3, N(Phoxm)2, CH2R1, SO2NH3, COOEt, CHNNC(SNH2); R3 = H, Br, OMe, Me, NO2, C≡CPhF; R4 = Cl, Me, NO2, CH(Ph)2, ^iPr; R5 = H, Me, CH(Ph)2, ^iPr; R6 = NH; R7 = Br; R8 = R9 = R10 = Me; R11 = H, Me, Cl, NO2, N(CH3)2, NCHPhOH; R12 = Me, Cl, NO2, N(CH3)2, COOH, NCHPhOH, CF3, COOH; R13 = Cl, COOH; R14 = OH, CF3; R15 = R16 = R17 = R18 = Me; R19 = R20 = COOH; R21 = NH, O; R22 = H, ^tBu; R23 = Me, OMe; R24 = C, N; R25 = S; R26 = Me; R27 = oxmPh(CH3)oxm; oxmPh(CH3)oxmPh(CH3)oxm.
	15–38		39		40
	41		42–63		64–70
	71		12		72,73
	74		75		76
	77		78		79
	80		81		82
	83		84		85
	86		87		88
	89		90		91
	92,93		94		95
	96		97		98
	99,100		101		102
	103		104		105
	106		107		108
	109,110		111		112
	113		114		115
	116		117		110
	118		119		120
	121		122		123

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As a direct consequence of this structural versatility, these compounds have been widely explored in different contexts. Maiore et al.³ demonstrated that molecules such as ammonium N-phenyloxamate, belonging to this class, act as effective protective agents for Carrara marble and biomicritic limestone against acidic agents by reducing the water solubility of calcium salts deposited on the surface. In addition, molecular docking studies revealed strong interactions between the oxamic acid derivative, 2-guanylhydrazido-oxomethanesulfonic acid, and the human butyrylcholinesterase (BChE) receptor – a critical step towards understanding the biological relevance of aminoguanidine derivatives.⁴ Chloroxaloterpin B, another oxamic acid derivative, exhibited potent inhibitory activity against Botrytis cinerea spore germination.⁵ Furthermore, investigations into the linear and nonlinear optical properties of oxamic acid derivatives – such as sulfanilamide [(4-sulfamoylphenyl)carbamoyl]formic acid salt – have demonstrated significant potential for use in advanced optical devices.⁶

Despite the wide variety of crystal structures reported for oxamic acid derivatives, no studies to date have described the presence of azine groups in combination with oxamic acid moieties. The azine group has a general structure represented by R₁R₂C=N–N=CR₃R₄ and is particularly notable for its photochromic properties, undergoing thermal and photochemical isomerization at the C=N bond.⁷ Azines – especially those with nitrogen-containing aromatic systems – constitute a class of compounds that promote π-conjugation, enabling efficient light absorption and energy transfer. Additionally, they participate in π–π stacking interactions, which have given rise to a significant degree of research interest in azine-containing molecules as building blocks in supramolecular chemistry and functional molecular architectures.⁸ Azine derivatives have also been employed in the design of multifunctional aggregation-induced emission luminogens (AIEgens), owing to their high selectivity and sensitivity for picric acid detection via hydrogen bonding interactions – highlighting their potential in biomedicine, optoelectronics, and chemical sensing.⁹ Furthermore, a study by Diyali et al. reported antidiabetic properties in azine derivatives.¹⁰

Given the growing interest in multifunctional molecules, the integration of azine groups with oxamic acid-derived compounds represents a promising strategy for the development of hybrid compounds with enhanced structural and functional versatility.^11–14 In this context, the present work reports the synthesis and characterization of two novel molecules incorporating both functional groups: the ester NN-Et₂H₂oba (Fig. 1b) and the acid form NN-H₄oba (Fig. 1c). These species exhibit conformational versatility and can adopt antiperiplanar (NN-Et_xH_4−xoba^anti) or gauche (NN-Et_xH_4−xob^gauche) arrangements due to free rotation around the central single nitrogen–nitrogen bond of the azine group (Fig. 1d and e). In addition, this study investigates the nature and energies of intermolecular interactions within their crystal structure – an aspect of relevance, as numerous studies have demonstrated the critical role such interactions play in determining material properties and potential applications. These interactions were analyzed using Hirshfeld surface mapping, fingerprint plots, and lattice energy–structure diagrams. As these are newly synthesized compounds, the study also includes characterization of their thermal properties (melting points) and spectroscopic features using ¹H NMR (nuclear magnetic resonance) and IR (infrared) spectroscopy.

2 Experimental section

2.1 Materials

Hydrazine dihydrochloride (≥98%, CAS: 5341-61-7), sodium hydroxide (97%, CAS: 1310-73-2), 2-nitrobenzaldehyde (98%, CAS: 552-89-6), ethanol (99.5%, CAS: 64-17-5), metallic zinc (98%, CAS: 7440-66-6), ammonium chloride (99%, CAS: 12125-02-9), methanol (99.8%, CAS: 67-56-1), dichloromethane (99.5%, CAS: 75-09-2), ethyl chlorooxoacetate (98%, CAS: 4755-77-5), tetrahydrofuran (99%, CAS: 109-99-9), potassium hydroxide (>90%, CAS: 1310-58-3), hydrochloric acid (37%, CAS: 7647-01-0), and dimethyl sulfoxide (99.9%, CAS: 67-68-5) were used. All chemicals and solvents used were of analytical grade and used without further purification in the syntheses.

2.2 Synthesis

2.2.1 Diethyl 2,2′-(((1E,1′E)-hydrazine-1,2-diylidenebis(methaneylylidene))bis(2,1-phenylene))bis(oxamate) – NN-Et₂H₂oba. A mixture of 1.679 g (16.0 mmol) of hydrazine dichlorohydrate, 1.280 g (32.0 mmol) of NaOH and 2 mL of water was added to a solution of 4.836 g (32.0 mmol) of 2-nitrobenzaldehyde (compound 1 – Scheme 1) in 150 mL of ethanol. The reaction yielded a yellow solid, which was filtered, washed with hot water, and dried at room temperature. This yellow solid corresponds to compound 2 (Scheme 1). Yield: 4.225 g – 88.6%. The reduction of nitro groups was carried out using zinc as a catalyst. 3.903 g (13.1 mmol) of 2, 10.278 g (157 mmol) of metallic zinc, and 8.409 g (157 mmol) of NH₄Cl were added to 200 mL of methanol. The reaction mixture was stirred at 50 °C for 3 hours. The resulting mixture was filtered, and the solid residue was washed with dichloromethane. The resultant washing liquid was then combined with the filtrate, and the solid residue was discarded. Subsequently, 200 mL of water were added to the filtrate, and the product was extracted with dichloromethane until the aqueous phase became colorless. The solvent was removed by rotary evaporation, yielding a dark-yellow solid (compound 3) – Scheme 1. Yield: 2.358 g – 75.6%. For the next step, 415.9 µL (4.20 mmol) of ethyl chlorooxoacetate was slowly added to a solution of compound 3 (0.502 g; 2.10 mmol) in 50 mL of dichloromethane, maintained in an ice bath. After the addition was complete, the mixture was refluxed at 40 °C for 2 hours. The solvent was then removed by rotary evaporation, and the resulting solid was washed with methanol at 50 °C. The light-yellow solid (NN-Et₂H₂oba – Scheme 1) was filtered and dried at room temperature. Yield: 0.550 g – 59.5%. Single crystals suitable for X-ray diffraction were obtained by recrystallization with tetrahydrofuran. m. p. 229 °C. IR (KBr, cm⁻¹): 3235 [ν(N–H)], 3114 and 3073 [ν(C–H_arom)], 2976 and 2935 [ν(C–H_alif)], 1735 [ν(C=O_ester)], 1710 [ν(C=O_amide)], 1602, 1583 and 1535 [ν(C–C_arom)], 1283 [ν(C–N)], 1175 and 1110 [ν(C–O)], 757 [δ(C–H_ortho)]. ¹H NMR (200 MHz, CDCl₃): 13.49 (s, 2H), 9.04 (s, 2H), 8.82 (d, J = 8.3 Hz, 2H), 7.57 (m, 4H), 7.28 (ddd, J = 0.9, 7.3 Hz, 2H). ¹³C NMR (400 MHz, dmso-d₆): 164.2, 160.0, 154.9, 137.4, 133.8, 132.7, 120.7, 120.2, 62.8, 13.8 ppm. Anal. calcd for C₂₂H₂₂N₄O₆ (NN-Et₂H₂oba) (%): C, 60.3; H, 5.1; N, 12.8. Found (%): C, 59.2; H, 5.0; N, 12.7.

Scheme 1 Synthesis steps of NN-Et₂H₂oba.

2.2.2 2,2′-(((1E,1′E)-hydrazine-1,2-diylidenebis(methaneylylidene))bis(2,1-phenylene))bis(oxamic) – NN-H₄oba. The acid form NN-H₄oba was obtained via hydrolysis of the corresponding ester – NN-Et₂H₂oba (Scheme 2). A suspension of 0.550 g (1.30 mmol) of NN-Et₂H₂oba in 120 mL of a mixture containing 80 mL of water, 40 mL of ethanol, and 0.296 g (5.20 mmol) of KOH was stirred at 60 °C for 2 hours. After this period, 231.4 µL of HCl 37% (2.85 mmol) was added to the solution mixture, which was then stirred for 1 h at room temperature. The resulting light-yellow solid was filtered, washed with water, and dried at room temperature. Yield: 0.442 g – 91.8%. Single crystals suitable for X-ray diffraction were obtained by recrystallization with dimethyl sulfoxide. m. p. 214 °C. IR (KBr, cm⁻¹): 3255 [ν(N–H)+ ν(O–H)], 3108 [ν(C–H_arom)], 1766 [ν(C=O_acid)], 1679 [ν(C=O_amide)], 1605, 1579 and 1543 [ν(C–C)_arom], 1354 [ν(C–N)], 1310 [ν(C–O)], 767 [δ(C–H_ortho)]. ¹H NMR (200 MHz, dmso-d₆, TMS), δ (ppm): 13.11 (s, 2H), 8.96 (s, 2H), 8.60 (d, J = 8.4 Hz, 2H), 7.67 (m, 4H) 7.36 (ddd, J = 1.0, 7.5 Hz, 2H). Anal. calcd for C₁₈H₁₄N₄O₆·dmso (NN-H₄oba) (%): C, 52,2; H, 4,4; N, 12.2. Found (%): C, 51.8; H, 4.1; N, 12.9.

Scheme 2 Hydrolysis of the NN-Et₂H₂oba to obtain compound NN-H₄oba.

2.3 Instrumentation

Melting points (uncorrected) were determined using an MP90 Melting Point System (Mettler Toledo) in open capillary tubes. The IR spectrum of NN-Et₂H₂oba was recorded on a Bomen Michelson MB100 spectrometer using KBr pellets, with a resolution of 4 cm⁻¹ and 16 scans. For NN-H₄oba, the IR spectrum was acquired in attenuated total reflectance (ATR) mode at the IMBUIA beamline of the Brazilian Synchrotron Light Laboratory (LNLS) that is part of the Brazilian Center for Research in Energy and Materials (CNPEM), with a resolution of 2 cm⁻¹ and 256 scans. ¹H NMR spectra were recorded at 200 MHz on a Bruker DPX-200 spectrometer at ambient temperature using CDCl₃ for NN-Et₂H₂oba and dmso-d₆ for NN-H₄oba. The ¹³C NMR spectrum of the NN-Et₂H₂oba was recorded at 400 MHz on a Bruker Avance III spectrometer at ambient temperature using dmso-d₆. Chemical shifts were expressed in ppm and referenced to tetramethylsilane (δTMS = 0.00). Elemental analyses (C, H, and N) were performed using a Thermo Fisher Scientific Flash EA 1112 Series elemental analyzer.

2.4 X-ray data collection and refinements

Single crystals of NN-Et₂H₂oba and NN-H₄oba were mounted on Micro-mesh supports and cooled under a nitrogen stream on a Bruker D8 Venture diffractometer equipped with a Photon II C7 detector, a Cu-Kα µS-microsource radiation (λ = 1.54178 Å), and a Göbbel mirror monochromator. Intensity data were measured by thin-slice ω- and ϕ-scans and processed with the APEX4 software package.¹²⁴ The structure was refined by full-matrix least-squares methods on F²'s using SHELXL¹²⁵ and solved by the intrinsic phasing methods implemented in SHELXT.¹²⁶ Absorption corrections were applied using a multi-scan method.¹²⁷ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located in different Fourier maps and refined isotropically without constraints. Neutral atom scattering factors were taken from standard literature sources.¹²⁸ All crystallographic calculations were carried out using the WinGX¹²⁹ suite, and structural figures were generated using Mercury.¹³⁰ Crystallographic data and structure refinement parameters for NN-Et₂H₂oba and NN-H₄oba are summarized in Table 2. The crystallographic data have been deposited in the Cambridge Structural Database under deposition numbers CCDC 2478688 and 2478689.

Table 2 Summary of the crystal data and refinement details for NN-Et₂H₂oba and NN-H₄oba

Compound	NN-Et2H2oba	NN-H4oba
Formula	C22H22N4O6	C18H14N4O6, 4(C2H6OS)
Formula weight	438.43	694.86
Crystal system	Triclinic	Monoclinic
Space group	P1̄	P2(1)/c
a (Å)	7.4167(2)	13.269(4)
b (Å)	11.8025(3)	5.3039(6)
c (Å)	13.2319(4)	24.374(4)
α (°)	114.5540(10)	90.00
β (°)	92.4900(10)	105.583(15)
γ (°)	96.2670(10)	90.00
Z	2	2
Wavelength (Å)	1.54178	1.54178
Volume (Å^3)	1042.16(5)	1652.3(6)
T (K)	100(2)	100(2)
D calc (g cm^−3)	1.397	1.397
Crystal size (mm^3)	0.336 × 0.111 × 0.061	0.798 × 0.163 × 0.152
F (000)	460	732
µ (mm^−1)	0.865	3.144
θ range (°)	4.2–74.0	3.5–70.9
Refl. collected	28 270	73 279
Refl. indep. (Rint)	4192 (0.038)	3183 (0.044)
Refl. obs. [I > 2σ(I)]	3631	3106
Restraints/parameters	0/377	0/251
Goodness-of-fit	1.121	1.061
R, wR [I > 2σ(I)]	0.038, 0.086	0.033, 0.088
R, wR (all data)	0.047, 0.091	0.034, 0.089
Largest diff. peak and hole (e Å^−3)	0.24, −0.20	0.43, −0.38
Reflections weighted	w = [s^2(Fo^2) + (0.0211P)^2 + 0.6738P]^−1	w = [σ^2(Fo^2) + (0.0516P)^2 + 0.9103P]^−1
	where P = (Fo^2 + 2Fc^2)/3	where P = (Fo^2 + 2Fc^2)/3

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2.5 Hirshfeld surface (HS) analysis, two-dimensional fingerprint plots, and energy-framework diagrams

The Hirshfeld surface analyses^131,132 and the non-covalent interaction energy¹³³ calculations for NN-H₄oba and NN-Et₂H₂oba were performed using the crystallographic information files (CIF) obtained from single-crystal X-ray diffraction. All computations were carried out with CrystalExplorer 21.5.¹³⁴ The energy framework analyses were based on density functional theory (DFT) calculations using the B3LYP/6-31G(d,p) level of theory.

3 Results and discussion

3.1 General characterization

The ¹H NMR and ¹³C NMR spectra of NN-Et₂H₂oba are shown in Fig. S1, while the ¹H NMR spectrum of NN-H₄oba is presented in Fig. S2. For NN-Et₂H₂oba, the ¹H NMR spectrum displays eight distinct signals, consistent with the proposed molecular structure. The singlets at 9.04 and 13.49 ppm are attributed to hydrogens 1 and 6, respectively. Hydrogen 2 appears as a doublet centered at 8.82 ppm (J = 8.2 Hz, 2H). The aromatic region (hydrogens 3–5) shows hydrogen 3 as a triplet of doublet (J = 0.9, 7.4 Hz, 2H), while hydrogens 4 and 5 appear as a triplet and a doublet, respectively, overlapping into a single broadened signal. The quartet at 4.48 ppm and the triplet at 1.48 ppm correspond to the ethyl group, further supporting ester formation. In the ¹³C NMR spectrum, eleven signals corresponding to the characteristic carbon atoms of the NN-Et₂H₂oba ester are observed. Resonances in the 150–165 ppm region are attributed to the carbon atoms of the oxamic acid-derived group and to the C=N carbon of the azine unit. The signals associated with the aromatic ring carbons appear in the 120–140 ppm range. The formation of the ester is confirmed by the presence of characteristic CH₃ and CH₂ carbon signals of the ethyl group at 13.8 and 62.8 ppm, respectively. In the spectrum of the acid NN-H₄oba, hydrogens 3 and 5 appear as a triplet of doublets at 7.36 ppm (J = 1.0, 7.5 Hz, 2H) and a doublet at 8.60 ppm (J = 8.43 Hz, 2H), respectively. The multiplet centered at 7.67 ppm corresponds to hydrogens 2 and 4. The azine –C=N– hydrogen appears at 8.96 ppm and the amide N–H at 13.11 ppm.¹³⁵

The IR spectra of NN-Et₂H₂oba and NN-H₄oba are presented in Fig. S3. In both spectra, the band corresponding to the out-of-plane angular deformation of the ortho-substituted aromatic ring hydrogen is observed at 757 cm⁻¹ for NN-Et₂H₂oba and at 767 cm⁻¹ for NN-H₄oba. It is also possible to identify the bands that indicate the presence of the oxamate group corresponding to the C–O and C–N stretching vibrations (1110 cm⁻¹, 1175 and 1283 cm⁻¹ for NN-Et₂H₂oba and 1310 cm⁻¹ and 1354 cm⁻¹ for NN-H₄oba). Furthermore, the most intense bands in the spectrum are associated with the C=O stretching of the oxamate groups (1710 and 1735 cm⁻¹ for NN-Et₂H₂oba; 1676 and 1766 cm⁻¹ for NN-H₄oba). The bands corresponding to C–C_arom stretching vibrations were identified at 1535, 1583, and 1602 cm⁻¹ for the ester, and at 1543, 1579, and 1605 for the acid. In both spectra, the presence of bands associated with C–H_arom stretching vibrations was observed (3073 and 3114 cm⁻¹ for NN-Et₂H₂oba and 3108 cm⁻¹ for NN-H₄oba) while the bands originating from C–H_aliph stretching vibrations were observed only in the NN-Et₂H₂oba spectrum (2976 and 2935 cm⁻¹), confirming the presence of the ethyl ester group. Spectra exhibit N–H stretching vibrations at 3235 cm⁻¹ for the ester and at 3255 cm⁻¹ for the acid.¹³⁶ The observed bands are consistent with the structures determined by SXRD.

3.2 The molecular and supramolecular structures of NN-Et₂H₂oba and NN-H₄oba

The molecular structures of NN-Et₂H₂oba and NN-H₄oba determined from SXRD are shown in Fig. 2. A summary of the crystallographic data and refinement parameters is provided in Table 2. Both molecules adopt an antiperiplanar conformation (NN-Et_xH_4−xoba^anti), probably due to the steric hindrance imposed by the oxamic acid-derived groups. Furthermore, a center of symmetry is located at the midpoint of each molecule, between the N–N bonds of the azine group.

Fig. 2 (a) NN-Et₂H₂oba; (b) NN-H₄oba. Thermal ellipsoids are drawn at the 80% probability level.

NN-Et₂H₂oba crystallizes in the triclinic space group P1̄. The crystal structure reveals two crystallographically independent molecules in the asymmetric unit, each represented by half a molecule, with no solvent molecules present. Bond lengths within the azine group agree with the values reported for similar molecules (see Table S1).¹³⁷ Similarly, the bond lengths in the oxamic acid-derived group are consistent with literature data, particularly with the values reported by Pim et al.¹³⁸ for aromatic oxamic acid derived in ester form (H₂Et₂L) (see Table S1). Among the two crystallographically distinct units, one is essentially planar (lower unit – Fig. 3a and b), whereas the other exhibits a slight deviation from planarity, with an interplanar angle of α = 8.38° between the plane of the azine group (C20–N4–N4–C20) and that of the oxamic acid-derived group (N3–C13–C12–O4) (upper unit – Fig. 3a and b).

Fig. 3 Representation of the plane formed by the azine group (gray) in (a) and (c), and the angle formed between this plane and the plane defined by the oxamic acid-derived group (red) in (b) and (d), for NN-Et₂H₂oba and NN-H₄oba, respectively.

The acid form, NN-H₄oba, crystallizes in the monoclinic space group P2(1)/c, with an inversion center imposed by symmetry within the azine group. As a result, the asymmetric unit comprises half of the NN-H₄oba molecule together with two dmso molecules. The C9–N2 and N2–N2 bond lengths are very similar to those in NN-Et₂H₂oba (see Table S1), resulting in similar structural features. The same trend is observed for the oxamic acid-derived groups, whose bond lengths show minimal deviations from those in the ester and from the literature data for H₃opba¹³⁹ (Table S1). The molecule adopts a quasiplanar conformation (Fig. 3c), with a dihedral angle of α = 3.22° (Fig. 3d).

The three-dimensional packing of NN-Et₂H₂oba (Fig. 4 and Fig. S4) is governed by intra- and intermolecular hydrogen bonds together with dispersive interactions. Intramolecular N–H⋯N interactions involving amide and azine groups are observed in both crystallographically independent molecules in the bc plane (N1–H⋯N2 and N3–H⋯N4; Fig. 4a), with distances of 1.88(2) and 1.931(19) Å and angles of 140.0(2)° and 136.9(2)° (Table 3), consistent with moderate interactions.¹⁴⁰ This interaction restricts intramolecular rotation in azine bonds, which may be useful in applications such as aggregation-induced emission.⁹ Peng et al.¹⁴¹ demonstrated the application of a salicylaldehyde azine as a fluorophore, in which these properties originated in part from hydrogen bonding between the hydroxyls of the aromatic ring and the nitrogen of the azine, forming interactions similar to N–H⋯N. Intermolecular C–H⋯O hydrogen bonds (C4–H⋯O3 and C15–H⋯O6; Fig. 4a) connect adjacent molecules between the oxygen from oxamic acid-derived groups and the aromatic rings, forming a supramolecular chain. This results in a hetero-R₂²(12) ring motif, with donor–acceptor distances of 2.413(18) Å and 2.390(18) Å, and angles of 133.9(13)° and 136.4(14)°, respectively. Together with the intramolecular N–H⋯N contacts, these interactions stabilize the molecular planarity. The supramolecular chains pack along the a-axis through London dispersion forces (Fig. 4b), involving hydrogen, carbon, and oxygen contacts. Among these, a weak carbonyl–carbonyl n → π* interaction^142,143 is observed, which is attributed to the partial donation of electron density from the non-bonding electron pair (n) located on the oxygen atom of the carbonyl group to the antibonding π* orbital of the neighboring carbon atom from carbonyl. These contacts adopt the geometry of a sheared-parallel motif.¹⁴² Specifically, interactions between C12–C13⋯O3 and C12ⁱⁱ–C13ⁱⁱ⋯O3ⁱ (Fig. 4b) are identified, with contact distances of 3.157(2) and 3.653(2) Å [symmetry code: (i) 1 − x, 1 − y, 1 − z; (ii) −x, 1 − y, 1 − z]. In a previous work by Morales–Santana et al.,¹⁴⁴ a molecule containing the oxamate group showed a similar interaction between carbon and sulfur, classified as a π–hole interaction.

Fig. 4 (a) Hydrogen bonds and (b) dispersion interactions stabilizing the supramolecular structure in NN-Et₂H₂oba.

Table 3 Hydrogen bonds involving oxamic acid-derived groups in NN-Et₂H₂oba and NN-H₄oba

Interactions	Distance H⋯A (Å)	Angle (°)
Symmetry code: (i) −1 + x, −1 + y, z; (ii) x, y, 1 + z; (iii) x, 1 + y, z; (iv) x, 1.5 − y, 0.5 + z; (v) 2 − x, 0.5 + y, 1.5 − z.
NN-Et2H2oba
C4–H5⋯O3^i	2.390(18)	136.4(14)
C15–H16⋯O6^ii	2.413(18)	140.0(17)
N1–H6⋯N2	1.88(2)	133.9(13)
N3–H17⋯N4	1.931(19)	136.9(17)
NN-H4oba
C11–H⋯O1	2.52(2)	136.4(15)
C11–H⋯O1^iii	2.44(2)	154.7(17)
C12–H⋯O2^iv	2.39	155.5
C13–H⋯O3^iv	2.48	149.5
C10^v–H⋯O3^iv	2.59(2)	152.2(17)
O2–H⋯O4	1.54(3)	179(2)
C10–H⋯O5	2.43(2)	155.0(16)
C11^v–H⋯O5^iii	2.44(2)	148.8(16)
C11–H⋯O5	2.44(2)	148.8(16)
C11^v–H⋯O5^v	2.44(2)	148.8(16)
C12–H⋯O5^v	2.63	178.5
N1–H2⋯N2	1.97(2)	139.7(17)

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The supramolecular structure of NN-H₄oba (Fig. 5a) exhibits the same intramolecular N1–H⋯N2 hydrogen bond between amide and azine groups, as in NN-Et₂H₂oba, with a bond length of 1.97(2) Å and an angle of 139.7(17)°, which contributes to the molecular planarity and similar functional implications. The packing in the bc plane is further stabilized by an extended hydrogen-bond network involving oxamic acid oxygen atoms and crystallization dmso molecules (Fig. 5a and Fig. S5a), comprising both strong and moderate interactions (Table 3). In the b direction (Fig. S5a), these hydrogen bonds are also involved; however, the molecular units within the layers are connected through interactions similar to those observed for the ester form [C2–C1⋯O4^iv 3.006(2) and C1–C2⋯O4^iv 3.104(3) Å; symmetry code: (iv) x, −1 + y, z], forming a hetero-R²₁(3) ring.¹⁴⁵ In this arrangement, the carbonyl group interacts with the sulfoxide group, with the oxygen atom from the sulfoxide group acting as an electron-density donor toward the π* orbital carbon from the carbonyl group. This interaction gives rise to a zigzag-type supramolecular arrangement (Fig. 5b). In this case, the n → π* interaction occurs between the NN-H₄oba unit and the dmso molecule, in contrast to NN-Et₂H₂oba, where the interaction takes place between the units themselves through carbonyl groups. Furthermore, the adopted geometry of the interaction is perpendicular.¹⁴² Consequently, the interaction in the acid form is stronger than that in the ester form, due to reduced steric hindrance.

Fig. 5 (a) Hydrogen bonds that form the bc plane. (b) n → π* interactions stabilizing the supramolecular structure in the b direction in NN-H₄oba.

The Hirshfeld surfaces (HSs) and fingerprint plots of NN-Et₂H₂oba and NN-H₄oba were mapped using the normalized contact distance (d_norm), defined from d_e (external distance) and d_i (internal distance). The color coding on the HSs indicates the proximity of interatomic contacts relative to the sum of the van der Waals radii: red = smaller, blue = larger, and white = similar distance. The 2D fingerprint plots and percentage contributions are shown in Fig. S6 and S7. In NN-Et₂H₂oba, the absence of crystallization solvents results in interactions exclusively between the NN-Et₂H₂oba units. HSs (Fig. 6a) display bright red spots associated with medium-strength hydrogen bonds between amide oxygen and the aromatic hydrogen – previously identified in Fig. 4. These O⋯H/O⋯H interactions contribute 19.5% to the overall supramolecular characteristics (Fig. S6). The remaining interactions are mainly dispersive (Fig. 6b and c), involving H⋯H, C⋯C, H⋯C, and C⋯O atoms, represented by small light-red spots. Among these, an n → π* interaction (Fig. 6c) – previously described in Fig. 4b – is also identified. Ethyl groups enable additional hydrogen-based interactions. Major contributions are H⋯H/H⋯H (43.2%) and C⋯C/C⋯C (19.7%), while C⋯O/C⋯O (5.3%) and C⋯C/C⋯C (3.0%) are less significant. Similar results have been reported in the literature for molecules containing the azine moiety, in which H⋯H/H⋯H contacts are likewise identified as the dominant contributors to the crystal packing, accounting for 44.8%¹⁴⁶ and 48.0%¹⁴⁷ of the total intermolecular interactions.

Fig. 6 Views of the Hirshfeld surfaces mapped over d_norm for NN-Et₂H₂oba. Emphasis on (a) hydrogen-bonding interactions and (b) and (c) dispersive-type interactions.

In the NN-H₄oba (Fig. 7), bright red spots indicate strong hydrogen bonds between the NN-H₄oba and dmso1a. The dmso molecules are identified according to the sulfur symmetry code (1 refers to Sⁱ and 2 to Sⁱⁱ): 1a (1 − x, 1 − y, 1 − z), 1b (1 − x, 2 − y, 1 − z), 1c (−1 + x, y, z) and 2a (1 − x, −1/2 + y, 1.5 − z). Additional interactions with the dmso1b, dmso1c, and dmso2a molecules appear as light red spots (Fig. 7). These visible O⋯H/O⋯H and C⋯O/C⋯O interactions contribute 25.7% and 5.0%, respectively, to the supramolecular features (Fig. S7). Other major contributions are H⋯H/H⋯H (33.4%) and C⋯H/C⋯H (20.2%), followed by N⋯H (5.6%) and C⋯C (4.5%), which, although smaller, still influence the supramolecular organization.

Fig. 7 Views of the Hirshfeld surfaces mapped over d_norm for NN-H₄oba. The dmso molecules are identified according to the symmetry code of the S atom (1 corresponds to Sⁱ and 2 to Sⁱⁱ): 1a (1 − x, 1 − y, 1 − z), 1b (1 − x, 2 − y, 1 − z), 1c (−1 + x,y,z) and 2a (1 − x, −1/2 + y, 1.5 − z).

The interaction energy results for NN-Et₂H₂oba and NN-H₄oba are summarized in Table 4 (lines 1–10). Only contacts with total interaction energy (E_tot) values greater than −15.0 kJ mol⁻¹ are discussed. The molecular environments are shown in Fig. S8 and S9. For NN-Et₂H₂oba, the highest stabilization energy (E_tot = 104.6 kJ mol⁻¹ – line 5) is mainly due to London dispersion involving hydrogen and carbon atoms, as well as n → π* interactions between the carbonyl oxygen of the oxamic acid-derived moiety and the carbon atoms of the same group. The second most stabilizing interaction is the R₂²(12) heterocyclic ring formed by hydrogen bonds between aromatic hydrogen atoms and the carbonyl oxygen of the amide, with E_tot = 37.4 kJ mol⁻¹ (line 3). In all these interactions, as well as in the remaining three interactions, dispersion energy (E_dis) is greater than electrostatic energy (E_ele). Dispersion contributions (%E_dis) range from 50.1% to 85.6%, confirming that London dispersion dominates the stabilization of the NN-Et₂H₂oba structure.

Table 4 Calculated interaction energies (kJ mol⁻¹), %E_comp contributions to stabilization energy for selected hydrogen bonds and close contacts in NN-Et₂H₂oba and NN-H₄oba. The four components: −E_ele, −E_pol, −E_dis and E_rep are reported as unscaled energy values, whereas −E_tot is scaled and is calculated according to the footnote

Line	Units	Interaction	−Eele	−Epol	−Edis	E rep	−Etot	%Eele^c	%Edis^c	R
a Asymmetric unit 1. b Asymmetric unit 2. c The % contribution of the component (electrostatic or dispersion) (%Ecomp) is calculated through (Ecomp/Estab) × 100, in which Estab = Eele + Epol + Edis. d R = distance between centroids in Å; Etot = keleEele + kpolEpol + kdisEdis + krepErep (scale factors: kele: 1.057; kpol: 0.740; kdis: 0.871; krep: 0.618).
NN-Et2H2oba
1	NN-Et2H4oba^a⋯NN-Et2H4oba^b		5.4	1.1	25.2	15.2	19.1	17.0	79.5	11.98
2	NN-Et2H4oba^b⋯NN-Et2H4oba^a		5.3	1.8	15.9	0.0	20.8	23.0	69.1	13.90
3	NN-Et2H4oba^a⋯NN-Et2H4oba^a		13.5	7.0	20.6	0.0	37.4	32.8	50.1	13.23
4	NN-Et2H4oba^a⋯NN-Et2H4oba^a		3.4	1.4	28.6	0.0	29.6	10.2	85.6	13.24
5	NN-Et2H4oba^a⋯NN-Et2H4oba^b		27.4	6.4	134.9	75.5	104.6	16.2	80.0	3.71
NN-H4oba
6	NN-H4oba⋯dmso1a		110.2	24.8	19.1	143.3	62.8	71.5	12.4	6.57
7	NN-H4oba⋯dmso1b		12.0	5.7	27.7	26.2	24.8	26.4	61.0	4.96
8	NN-H4oba⋯dmso2a		7.6	2.9	79.1	34.2	58.0	8.5	88.3	9.02
9	NN-H4oba⋯dmso1c		13.5	2.4	10.4	11.0	18.3	51.3	39.5	8.66
10	NN-H4oba⋯NN-H4oba		7.6	2.9	79.1	34.2	57.9	8.5	88.3	5.30

---

In the case of NN-H₄oba, the shortest hydrogen bond corresponds to the most stabilizing interaction, the heterocyclic ring R₂²(9) formed between NN-H₄oba and dmso1a (E_tot = 62.8 kJ mol⁻¹; line 6), consistent with literature on similar oxamic acid-derived groups and dmso molecules rings.¹⁴⁴ Two further interactions contribute significantly to supramolecular stabilization: (i) hydrogen bonding between the oxygen atoms of the oxamic acid-derived and the methyl hydrogen atoms of dmso2a, forming R²₂(10) (E_tot = 58.0 kJ mol⁻¹; line 8), with E_dis = 88.3% and e E_ele = 8.5%; and (ii) a purely dispersive contact between NN-H₄oba units themselves (E_tot = 57.9 kJ mol⁻¹; line 10), also dominated by 88.3% dispersion forces. Additional contributions arise from the R²₁(3) and R²₂(6), and R²₁(6) motifs, with E_tot values of 24.8 kJ mol⁻¹ (line 7) and 18.3 kJ mol⁻¹ (line 9). Therefore, both systems benefit from dispersive contributions. NN-Et₂H₂oba is mainly stabilized by dispersive interactions and steric effects from ethyl groups. In contrast, NN-H₄oba presents the largest contribution from directional hydrogen bonds with active participation of the crystalline solvent, complemented by London dispersion energies comparable to hydrogen bonds. Small changes in the crystalline environment can have a substantial impact on the supramolecular energy balance.

The data in Table 3 are represented in the energy framework diagrams in Fig. 8, constructed for clusters of nearest-neighbor molecules within 3.8 Å of a central NN-Et₂H₂oba or NN-H₄oba fragment. In both crystals, dispersion energy (green) predominates over electrostatic energy (red), as also reflected in the total energy frameworks (blue). This dominance agrees with the fingerprint plots, which show high frequency of H⋯H and C⋯H contacts. Moreover, most hydrogen bonds in both structures involve carbon atoms as donors, highlighting the key role of weak, cumulative noncovalent interactions in stabilizing the supramolecular structures. These results contrast with the findings reported by Morales et al.,¹⁴⁴ in which systems containing only the oxamate group and solvent molecules were predominantly electrostatic interactions. One hypothesis to explain our results is that the planarity of the molecules—favored by the intramolecular hydrogen bonds between the azine nitrogen and the amide hydrogen—also contributes to spatially enhancing dispersive interactions.

Fig. 8 Energy-framework diagrams for E_dis, E_ele, and E_tot for a cluster of nearest-neighbor molecules in (a) NN-Et₂H₂oba and (b) NN-H₄oba. All diagrams use the same cylinder scale of 50 for energies and no interaction was cut off.

4 Conclusion

The synthetic routes for hybrid compounds containing azine and oxamic acid derivatives, NN-Et₂H₂oba and NN-H₄oba, have been successfully established. Their crystal structures, elucidated by single-crystal X-ray diffraction, reveal that both molecules adopt an antiperiplanar conformation. Notably, the acidic form of the molecule incorporates solvent molecules within the crystal lattice, while the ester form crystallizes as solvent-free molecular units, which significantly altered the forces involved in stabilizing the supramolecular structure. Hirshfeld surface analysis, two-dimensional fingerprint plots, and energy framework diagrams collectively indicate that, for NN-Et₂H₂oba, the structure stability is partly governed by hydrogen bonding but is predominantly dictated by dispersive interactions. In contrast, the supramolecular stabilization of NN-H₄oba is mainly attributed to hydrogen bonding with the crystallization dmso molecules. Despite these differences, energy framework analyses confirm that dispersion forces are the dominant contributors to the overall stabilization in both structures, significantly favored by high molecular planarity and intramolecular N-H⋯N interactions, which restrict rotational freedom in the azine portion. This conformational restriction, previously associated with aggregation-induced emission in azine-based systems, underscores the relevance of these structural characteristics for potential photophysical applications. A detailed understanding of these intermolecular interactions is crucial for the rational design of new molecular architectures with tailored physicochemical properties, particularly in fields such as biomedicine, sensing technologies, and optoelectronic materials.

Author contributions

Ketlyn W. Borth: methodology, formal analysis, data curation, investigation, writing – original draft, and software. Letícia dos S. Leal: methodology. Grazielli da Rocha: formal analysis and data curation. Verônica de C. Teixeira: writing – review & editing, supervision, project administration, funding acquisition, and conceptualization. Tatiana R.G. Simões: writing – review & editing, supervision, project administration, funding acquisition, and conceptualization.

Conflicts of interest

No conflicts of interest exist.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: ¹H NMR spectra, IR spectra, SXRD structural data, fingerprints, and data from calculations of intermolecular interactions. Crystallographic data for compounds NN-Et₂H₂oba and NN-H₄oba were deposited at the Cambridge Crystallographic Data Center (CCDC), with numbers CCDC 2478688 (NN-Et₂H₂oba) and 2478689 (NN-H₄oba). See DOI: https://doi.org/10.1039/d5nj04656d.

CCDC 2478688 and 2478689 contain the supplementary crystallographic data for this paper.^148a,b

Acknowledgements

The authors acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação Araucária, Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, #2021/08111-2), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Projects 1 – Finance Code 001, for financial support. They also thank the IMBUIA beamline from the LNLS/CNPEM for the support during the IR measurements under proposal #20232001.

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