Experimental and computational insights of the new hybrid organic–inorganic compound (C₆H₉N₂)₂CdI₄ for advanced optoelectronic applications and biological activities

Abstract

This study reports on the synthesis, structural elucidation, and property investigation of the hybrid organic–inorganic compound (C₆H₉N₂)₂CdI₄, which is composed of 2-amino-5-picolinium cations and discrete cadmium tetraiodide anions. The compound was synthesized using a solution-based method, and its crystal structure was determined through single-crystal X-ray diffraction, revealing a triclinic system with the space group P1̄. Structural analysis shows that the isolated [CdI₄]²⁻ tetrahedra are embedded within the crystal matrix and stabilized by an extensive hydrogen-bonding network formed with the protonated organic cations. Scanning electron microscopy (SEM) revealed a densely agglomerated morphology characterized by irregularly shaped grains. Raman spectroscopy provided insights into the vibrational characteristics. Optical absorption measurements exhibited a pronounced π → π* transition at 307 nm, attributed to the aromatic nature of the organic cation, while the absence of low-energy d–d transitions aligns with the closed-shell electronic configuration of Cd²⁺. Noncovalent interaction analyses based on NCIPLOT and Multiwfn, supported by density functional theory (DFT) calculations, highlighted the key role of hydrogen bonding and van der Waals interactions in stabilizing the crystal lattice and governing molecular packing. Thermal behavior investigated by thermogravimetric and differential scanning calorimetry (TGA/DSC) demonstrated high thermal stability of the compound up to approximately 350 °C. In addition, in vitro antioxidant assays revealed significant activity, with DPPH radical scavenging efficiency reaching about 60% at 1 mg ml⁻¹ and a strong Fe³⁺ reduction capability, indicating effective electron-donor behavior. These combined experimental and theoretical results establish clear structure–property relationships and suggest that (C₆H₉N₂)₂CdI₄ is a promising multifunctional material, offering potential for optoelectronic applications as well as emerging biomedical relevance.

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I. Introduction

Recent advances in experimental and theoretical methods have deepened the study of noncovalent interactions, which govern self-assembly in supramolecular systems. These interactions, including hydrogen bonding,¹ π–π stacking,² van der Waals forces,³ and electrostatic effects,³ dictate the formation, stability, and functionality of molecular assemblies across chemistry, biology, and materials science.

Among these, hydrogen bonding remains one of the most widely studied interactions due to its directional nature, relative strength, and selectivity. It is essential in processes such as protein folding, enzyme–substrate binding, DNA base pairing, and crystal engineering.^4–6 The deliberate use of hydrogen bonding has proven invaluable in crystal engineering, host–guest chemistry, and drug design, where precise control over molecular orientation is paramount.^7–9

In this context, class I organic–inorganic hybrid halides, which rely on weak noncovalent forces to connect organic and inorganic units, have emerged as important platforms for developing functional materials. They combine flexible organic moieties with rigid inorganic frameworks, yielding synergistic optical, electronic, and mechanical properties. Consequently, these materials are actively researched for applications in semiconductors, dielectrics, sensors, and photovoltaics.^10,11

Hybrid compounds containing metal halide frameworks are particularly notable for their tunable electrical conductivity, magnetic ordering, luminescence, and nonlinear optical behavior.¹² Their potential spans diverse fields, including biomedical applications,^13,14 automotive and aerospace industries,^15,16 energy storage,¹⁷ solar energy conversion and LEDs,¹⁸ microelectronics,^19,20 and advanced packaging with barrier or antimicrobial properties.²¹

A key subclass of these materials follows the general formula R₂MX₄, where R is a protonated organic cation, M is a divalent metal cation (e.g., Zn²⁺, Co²⁺, Fe²⁺, Cd²⁺, …), and X is a halide ion. These structures typically feature discrete MX₄ tetrahedra stabilized by hydrogen bonding and van der Waals interactions between organic and inorganic components. This structural motif imparts thermal stability and chemical robustness, enabling property modulation through cation or anion substitution.

Among metal halides, halogenocadmates(II) are especially interesting due to the flexible coordination chemistry of Cd²⁺, which allows diverse structural topologies, from discrete molecular species to extended chains, 2D layers, and 3D networks. This adaptability makes them promising candidates for designing stimuli-responsive materials.²²

Pyridine-based derivatives hold a prominent position in heterocyclic and medicinal chemistry.^23,24 Their electron-rich aromatic systems and nitrogen lone pairs facilitate coordination and hydrogen bonding. Pyridine and its analogs are widely used in bioactive molecules, metal complex ligands, and supramolecular scaffolds. Incorporating them into hybrid structures often enhances stability, binding selectivity, and supramolecular organization. Notably, 2-amino-5-methylpyridine (2A5MP) is recognized for its versatile binding capabilities due to both amino and nitrogen donor groups. Recent molecular docking studies highlight its potential in target-specific binding, particularly in oncology and enzyme inhibition.²⁵

Table 1 summarizes selected hybrid compounds based on 2-amino-5-picoline, illustrating their structural diversity. In all cases, 2-amino-5-picoline serves as the organic component, combined with various divalent metals and halides. The resulting architectures range from discrete units to extended networks, with tunable physical properties that reflect the influence of both metal and halide constituents. Importantly, all reported 2A5MP-based compounds crystallize in centrosymmetric space groups, underscoring their versatility as building blocks for advanced hybrid materials.

Table 1 Selected hybrid compounds based on 2-amino-5-picoline: chemical formula, crystal system, and space group

Compound	System	Space group	Ref.
(C6H9N2)2MnI4	Triclinic	P1̄	26
(C6H9N2)2CoCl4	Monoclinic	P21/c	27
(C6H9N2)2MnBr4	Triclinic	P1̄	28
(C6H9N2)2CdCl4	Triclinic	P1̄	29
(C6H9N2)FeCl4	Monoclinic	P21/c	30
(C6H9N2)2[SbBr4]Br	Monoclinic	C2/m	31
(C6H9N2)2Sb2I8	Triclinic	P1̄	32
(C6H9N2)2SnCl6	Monoclinic	P21/c	33
(C6H9N2)2ZnCl4	Monoclinic	P21/c	34

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This study focuses on (C₆H₉N₂)₂CdI₄, a new iodide-based member of the 2A5MP family. The use of iodide extends the chemistry toward heavier, more polarizable halides, enabling a systematic evaluation of halide effects on structure and properties within the same organic framework.

Beyond optoelectronics, organic–inorganic hybrid halometalates have attracted recent attention for their biological activity, particularly their ability to scavenge reactive oxygen species (ROS). This behavior is often linked to electron or hydrogen donation from organic ligands coordinated to metal–halide frameworks. In the present compound, the electron-rich 2-amino-5-picolinium cation combined with discrete [CdI₄]²⁻ units suggests potential antioxidant functionality. Accordingly, we evaluated the antioxidant properties of (C₆H₉N₂)₂CdI₄ using DPPH radical scavenging and ferric reducing power assays, extending the multifunctional relevance of R₂MX₄ hybrids into biomedical applications.

This work adopts an integrated experimental and theoretical approach to elucidate structure–property relationships in (C₆H₉N₂)₂CdI₄. Single-crystal X-ray diffraction is combined with Raman and UV-vis spectroscopy, density functional theory (DFT/TD-DFT) calculations, and noncovalent interaction analyses using RDG/NCIPLOT and Multiwfn. This comprehensive strategy reveals how hydrogen bonding and van der Waals interactions govern lattice stability and electronic structure, while highlighting the tunability of optical, thermal, and antioxidant properties through judicious metal and halide selection.

II. Experimental details

1. Synthesis and single-crystal X-ray characterization of (C₆H₉N₂)₂CdI₄

The hybrid compound (C₆H₉N₂)₂CdI₄ was synthesized using a slow evaporation method, designed to promote the growth of well-defined single crystals. The detailed procedure is illustrated in Fig. 1. First, 0.1 g of cadmium iodide (CdI₂) was dissolved in 5 ml of hydroiodic acid (HI)-acidified distilled water under gentle stirring. Separately, 0.059 g of 2-amino-5-picoline (C₆H₈N₂) was dissolved in 5 ml of the same solvent. Both solutions were stirred until completely clear, indicating full dissolution of all components.

Fig. 1 Schematic diagram for the synthesis process of (C₆H₉N₂)₂CdI₄.

The two solutions were then combined under continuous stirring for 5 minutes, followed by the addition of 2–3 drops of concentrated HI to maintain an acidic environment. The resulting mixture was stirred for an additional 15 minutes at room temperature to ensure complete homogenization. The solution was then allowed to stand undisturbed at ambient temperature for several days, during which yellow plate-like single crystals formed due to slow solvent evaporation and controlled supersaturation.

The overall chemical reaction can be represented as:

2(C₆H₉N₂) + CdI₂ + 2HI → (C₆H₉N₂)₂CdI₄ (1)

A well-formed single crystal of (C₆H₉N₂)₂CdI₄, measuring 0.29 × 0.145 × 0.100 mm³, was selected using a polarizing microscope and mounted on a Bruker APEX II four-circle area detector diffractometer. X-ray diffraction data were collected at 296(2) K using Mo Kα radiation (λ = 0.71073 Å), monochromated with a graphite crystal. Data acquisition was performed using the APEX2 software suite.³⁵ The reflections were measured within a 2θ range of 2.580° to 27.493°, and empirical absorption corrections were applied using the SADABS program,³⁶ based on a multi-scan analysis. The crystal structure was solved by direct methods using SHELXT-2018, which enabled the localization of heavy atoms. Subsequent refinement of the structure and determination of lighter atom positions were carried out with SHELXL-2014, both included in the WINGX software package.³⁷ A summary of the experimental conditions and refinement parameters is provided in Table 2.

Table 2 Crystallographic data and structure refinements of (C₆H₉N₂)₂CdI₄

Formula	(C6H9N2)2CdI4
Color/shape	Yellow/platelets
Formula weight (gmol^−1)	838.30
Crystal system	Triclinic
Space group	P1̄
Density	2.481
Crystal size (mm)	0.29 × 0.145 × 0.100
Temperature (K)	296(2)
Diffractometer	Bruker APEXII
a (Å)	8.541(2)
b (Å)	9.446(2)
c (Å)	15.217(4)
α (°)	83.619(12)
β (°)	89.011(13)
γ (°)	66.923(11)
V (Å^3)	1122.0(5)
Z	2
Radiation type	Mo Kα (0.71073 Å)
Absorption correction	Multi-scan
θ range for data collection (°)	2.580 ≤ θ ≤ 27.493
Measured reflections	31 347
Independent reflections	5127
Observed data [I > 2σ(I)]	4252
Index ranges	h = −11 → 11
	k = −12 → 12
	l = −19 → 19
F(000)	756
Number of parameters	192
R1	0.0271
wR2	0.0675
Goof	1.093

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CCDC 2284327 contains the supplementary crystallographic data for (C₆H₉N₂)₂CdI₄.

2. Crystal morphology, thermal stability, and spectroscopy (Raman, UV-visible) analyses: instrumentation and procedures

The crystal's surface morphology was investigated using a TESCAN Vega III scanning electron microscope (SEM). Elemental composition was analyzed through energy-dispersive X-ray spectroscopy (EDX), utilizing the Bruker Quantax 100 Easy EDX system. This system is equipped with a Bruker XFlash 410 M Silicon Drift Detector, offering a high energy resolution of 133 eV for Mn Kα at 100 kcps. The EDX setup is fully integrated into the SEM platform, enabling precise and efficient compositional analysis.

The thermal stability and mass variation of the samples were investigated using a PerkinElmer STA6000 differential scanning calorimeter under a controlled nitrogen environment. The analyses were performed with a uniform heating rate of 10 °C min⁻¹, during which the responses of the specimen and the reference were recorded concurrently.

Raman spectra were recorded using a Horiba LabRam HR Evolution Raman microscope, equipped with a 633 nm excitation laser operating at an approximate power of 50 mW at the sample surface. A 100× objective lens was utilized to focus the laser to a spot size of about 1 µm, ensuring high spatial resolution. Instrument calibration was performed using a silicon reference crystal, with the characteristic Raman band set at 520.5 cm⁻¹. Each final spectrum represents the average of 10 successive acquisitions, each with an integration time of 5 seconds, to enhance signal-to-noise ratio and ensure spectral reliability.

UV-visible absorption measurements were carried out using a HITACHI U-3300 spectrophotometer (both in absorbance and reflectance modes, slit width of 20 nm, and LISR-3100 integrated sphere; BaSO₄ was used as a reference), covering the spectral range from 250 to 400 nm.

3. Determination of in vitro antioxidant activities

3.1. DPPH radical scavenging activity. The free radical-scavenging potential of the (C₆H₉N₂)₂CdI₄ compound was evaluated following the method of Bersuder et al.³⁸ In brief, 500 µl of the sample at various concentrations (0.05–1 mg ml⁻¹) was combined with 375 µl of ethanol (99.5%) and 125 µl of DPPH solution (0.02% in ethanol, 99.5%). The resulting mixtures were incubated at room temperature in the dark for 1 hour. After incubation, the absorbance was measured at 517 nm using a UV-visible spectrophotometer. The DPPH radical scavenging percentage was calculated as:
where A_c represents the absorbance of the control solution and A_s is the absorbance of the sample mixed with DPPH. Vitamin C was used as the positive control.

3.2. Reducing power assay. The iron(III) reducing capacity of (C₆H₉N₂)₂CdI₄ was evaluated following the method of Yildirim et al.³⁹ In brief, 1 ml of the sample at various concentrations (0.05–1 mg ml⁻¹) was combined with 1.25 ml of phosphate buffer (0.2 M, pH 6.6) and 2.5 ml of 1% (w/v) potassium ferricyanide solution. The mixture was incubated at 50 °C for 20 minutes to allow reduction of Fe³⁺. Subsequently, 2.5 ml of 10% (w/v) trichloroacetic acid was added to stop the reaction, and the samples were centrifuged at 10 000 × g for 10 minutes. From the resulting supernatant, 2.5 ml was mixed with 2.5 ml of distilled water and 0.5 ml of 0.1% (w/v) ferric chloride solution. The absorbance of the resulting solution was measured at 700 nm using a UV-visible spectrophotometer. Vitamin C was employed as the reference standard.

III. Results and discussion

1. Structure description

In a previous study,²⁹ the hybrid compound (C₆H₉N₂)₂CdCl₄ was successfully synthesized and structurally characterized. It crystallizes at room temperature in the triclinic crystal system, adopting the centrosymmetric space group P1̄. The refined unit cell parameters are: a = 7.7127(3) Å, b = 8.6907(3) Å, c = 15.0834(5) Å, α = 94.472(2)°, β = 91.016(2)°, γ = 111.725(1)°, with a unit cell volume of V = 935.16(6) ų. The crystal structure consists of isolated [CdCl₄]²⁻ tetrahedra, each charge-compensated by two protonated 2-amino-5-picolinium (C₆H₉N₂)⁺ cations. These units are interconnected via hydrogen bonding and weak intermolecular interactions, contributing to the cohesion of the lattice.

In the present investigation, a structural change was observed upon halide substitution, where chloride ions were replaced by iodide ions to yield the new compound (C₆H₉N₂)₂CdI₄ under investigation. Remarkably, this iodide-based compound retains the same triclinic crystal system and P1̄ space group. However, due to the significantly larger ionic radius and higher polarizability of I⁻ compared to Cl⁻, notable changes in the unit cell dimensions were observed. As listed in Table 2, the refined parameters are: a = 8.541(2) Å, b = 9.446(2) Å, c = 15.217(4) Å, α = 83.619(12)°, β = 89.011(13)°, γ = 66.923(11)°, with a unit cell volume of V = 1122.0(5) ų and Z = 2. This expansion reflects the influence of iodide on the molecular packing and overall structural framework.

The asymmetric unit (Fig. 2) contains discrete [CdI₄]²⁻ tetrahedra, where Cd²⁺ is coordinated to four iodide ligands. Each anion is surrounded by two (C₆H₉N₂)⁺ cations, forming a dense hydrogen-bonding network via N–H⋯I contacts. π⋯π stacking between adjacent pyridine rings further stabilizes the packing, leading to a three-dimensional supramolecular framework.

Fig. 2 The asymmetric unit of (C₆H₉N₂)₂CdI₄. Dashed lines represent hydrogen bonds.

To better understand the crystalline architecture, a structural projection along the crystallographic a-axis is shown in Fig. 3. This view reveals a regular alternation of cationic and anionic layers parallel to the (001) plane.

Fig. 3 Projection of the structure of (C₆H₉N₂)₂CdI₄ along the crystallographic a-axis.

The anionic moiety [CdI₄]²⁻ exhibits a distorted tetrahedral geometry. The Cd–I bond lengths range between 2.7461(8) Å and 2.8130(8) Å, while the I–Cd–I bond angles vary from 105.00(2)° to 116.69(2)° (Table 3). These values are consistent with the reported [CdI₄]²⁻ structure.^8,40

Table 3 Selected bond distances (Å) and angles (°)

Distances (Å)		Angles (°)
Cd1–I1	2.7461(8)	I1–Cd1–I2	112.91(2)
Cd1–I2	2.7980(7)	I1–Cd1–I3	116.69(2)
Cd1–I3	2.7600(8)	I1–Cd1–I4	105.00(2)
Cd1–I4	2.8130(8)	I2–Cd1–I3	105.31(1)
N1–C2	1.367(8)	I2–Cd1–I4	109.61(2)
N1–C6	1.326(6)	I3–Cd1–I4	107.12(2)
C1–C3	1.508(9)	C2–N1–C6	123.4(5)
C2–C3	1.350(9)	C1–C3–C2	121.1(6)
C3–C4	1.402(7)	C1–C3–C4	122.5(6)
C4–C5	1.340(9)	C2–C3–C4	116.4(5)
C5–C6	1.388(8)	C3–C4–C5	121.9(6)
C6–N2	1.343(7)	C4–C5–C6	120.5(5)
C7–C9	1.492(9)	N1–C6–C5	117.1(5)
N3–C8	1.342(8)	N1–C6–N2	119.7(5)
N3–C12	1.332(6)	C5–C6–N2	123.2(5)
C8–C9	1.359(8)	C8–N3–C12	124.3(5)
C9–C10	1.392(6)	N3–C8–C9	121.3(5)
C10–C11	1.341(9)	C7–C9–C8	121.6(5)
C11–C12	1.419(8)	C7–C9–C10	122.5(5)
C12–N4	1.330(8)	C8–C9–C10	115.9(5)
		C9–C10–C11	122.8(5)
		C10–C11–C12	119.7(5)
		N3–C12–C11	115.9(5)
		N3–C12–N4	120.2(5)
		C11–C12–N4	123.9(5)

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To quantify the tetrahedral distortion, the Baur distortion index was calculated:⁴¹

where, d = Cd–I distance, a = I–Cd–I angle, m = average value, n₁ = 4 and n₂ = 6 for tetrahedral environment.

The calculated distortion indices, DI(Cd–I) = 0.0094 and DI(I–Cd–I) = 0.0332, suggest that the [CdI₄]²⁻ unit deviates slightly from an ideal tetrahedral configuration, where all bond lengths and angles would be identical (i.e., ∼109.5° and equivalent Cd–I distances). The small yet noticeable deviation in both bond lengths and angles indicates that the tetrahedron is subtly distorted. This distortion arises mainly due to intermolecular interactions, particularly N–H⋯I hydrogen bonds formed between the [CdI₄]²⁻ anion and the surrounding (C₆H₉N₂)⁺ cations.

The [CdI₄]²⁻ anionic unit can theoretically adopt several geometries, such as tetrahedral, square planar, or seesaw configurations. To determine the actual geometry of the cadmium-centered polyhedron in the present compound, we employed the distortion index τ₄, which quantifies the deviation from ideal geometries. This parameter ranges from 0 for a perfect square planar shape to 1 for a perfect tetrahedron. It is calculated using the following formula:⁴²

where α and β are the two largest bond angles within the coordination polyhedron, and θ is the ideal tetrahedral angle (109.5°). For the [CdI₄]²⁻ unit, using α = 89.011° and β = 83.619°, the τ₄ value is calculated as 0.957. This τ₄ value, very close to 1, clearly indicates that the [CdI₄]²⁻ unit adopts an almost ideal tetrahedral geometry.

The geometric parameters of the protonated amine cations, summarized in Table 3, are consistent with those observed in similar cadmium-based hybrid structures.²⁹ The overall structural cohesion is primarily maintained through N–H⋯I hydrogen bonding interactions between the organic cations and the [CdI₄]²⁻ tetraiodocadmate(II) anions, as well as π⋯π stacking interactions between the aromatic systems of neighboring organic moieties.

Each (C₆H₉N₂)⁺ cation participates in multiple hydrogen bonding interactions with three distinct [CdI₄]²⁻ anions, as depicted in Fig. 4(a). Conversely, each [CdI₄]²⁻ tetrahedron is stabilized by six hydrogen bonds formed with adjacent organic cations (Fig. 4(b)). The distances between nitrogen and iodine atoms involved in hydrogen bonding (N⋯I) span from 3.545 to 3.807 Å, while the corresponding N–H⋯I bond angles range from 150.12° to 171.54°, reflecting the directional nature and varying strengths of these interactions (Table 4).

Fig. 4 Hydrogen bonds established by (a) the protonated amine and (b) the metallic tetrahedron in (C₆H₉N₂)₂CdI₄.

Table 4 Hydrogen bonding geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
a 1 + x, y, z.b x, 1 + y, z.
N2–H2B⋯I2^a	0.86	2.919	3.765	168.22
N2–H2A⋯I1	0.86	2.932	3.703	150.12
N1–H1⋯I4	0.859	2.73	3.545	159.01
N3–H3A⋯I3	0.86	2.754	3.602	169.32
N4–H9⋯I4^b	0.86	2.954	3.807	171.54

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In addition to hydrogen bonding, π⋯π stacking interactions are present, adopting a parallel-displaced configuration between their aromatic rings, commonly observed in such supramolecular assemblies.^43,44 The shortest centroid-to-plane distance between two adjacent aromatic rings is measured at 3.882 Å, which falls within the typical range for effective π⋯π interactions, contributing further to the stability and packing of the crystal lattice.

2. SEM/EDX results

Scanning electron microscopy (SEM) images at increasing magnifications (100×, 1000×, 5000×) reveal a densely agglomerated microstructure composed of irregular polyhedral grains with rough, compact surfaces (Fig. 5), suggesting strong intergranular adhesion.

Fig. 5 Scanning electron microscopy images of (C₆H₉N₂)₂CdI₄ at different magnifications: (a) 100×, (b) 1000×, and (c) 5000×.

Complementary EDX analysis, presented in Fig. 6, was conducted to verify the elemental composition and spatial distribution of constituent atoms. The EDX spectrum in Fig. 6(a) confirms the presence of carbon (C), nitrogen (N), cadmium (Cd), and iodine (I), as expected based on the molecular formula. Notably, no additional peaks were detected, indicating the absence of impurities. As is commonly known, hydrogen atoms are not detectable via EDX due to their low atomic number and lack of significant X-ray emission.⁴⁵

Fig. 6 (a) EDX spectrum of the synthesized (C₆H₉N₂)₂CdI₄ material, and (b)–(f) EDX element distribution charts.

Fig. 6(b)–(f) show the elemental mapping results, which demonstrate a homogeneous distribution of all detected elements across the sample surface on the micrometer scale. This uniform elemental distribution corroborates the compositional purity of the synthesized hybrid material and supports the consistency of its crystal structure.

3. Thermal stability evaluation

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of (C₆H₉N₂)₂CdI₄ were carried out in the 50–450 °C temperature range at a heating rate of 10 °C min⁻¹ under nitrogen. The corresponding curves are presented in Fig. 7. The sample shows a nearly constant mass and an almost featureless DSC baseline up to about 350 °C, indicating the absence of phase transitions or significant degradation in this interval and confirming a wide stability window for the title compound. Above this temperature, a sharp endothermic event is observed in the DSC trace, accompanied by a steep weight loss in the TGA curve, which can be ascribed to the onset of irreversible thermal decomposition of (C₆H₉N₂)₂CdI₄.

Fig. 7 Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of (C₆H₉N₂)₂CdI₄.

4. Vibrational analysis

The room-temperature Raman spectrum of the synthesized compound is presented in Fig. 8. The vibrational mode assignments, listed in Table 5, were made by comparison with reported spectra of structurally related compounds.^{8,27,29,46–50}

Fig. 8 Raman spectrum of (C₆H₉N₂)₂CdI₄ at room temperature.

Table 5 Observed Raman frequencies (cm⁻¹) and band assignments for (C₆H₉N₂)₂CdI₄

Raman shift (cm^−1)	Assignment
ν, stretching; δ, bending; γ, rocking; w, wagging; s., symmetric; as., antisymmetric.
3312	νNH2 as.
3265	νNH2 s.
3196	νN–H
3145–3024	νCH ring
2955	νCH3 as.
2863/2919	νCH3 s.
1662	δNH2
1620	δN–H
1548–1460	νCC ring as.; δCH ring
1410	δCH3 as.
1386	δCH3 s.
1344	νCNC as. ring
1320	δCH ring
1234	νC–N(H2)
1155	γNH2
987	γCH ring
854	νC–C(H3)
792	γN–H
757	δCCC ring
723	γCH3
645	wNH2
305–480	Ring deformations and torsions
147	νCdI as.
118	νCdI s.
110	δI–Cd–I as.
54	δI–Cd–I s.

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In the low-frequency region (below 400 cm⁻¹), the observed Raman bands are primarily associated with vibrational motions of the inorganic [CdI₄]²⁻ tetrahedral units. Specifically, the peaks appearing at 118 cm⁻¹ and 147 cm⁻¹ correspond to the symmetric and asymmetric Cd–I stretching modes, respectively. These values are consistent with those reported for other tetrahedral Cd²⁺ complexes containing iodide ligands.^8,47,48 Additionally, the bands located at 54 cm⁻¹ and 110 cm⁻¹ can be attributed to I–Cd–I bending modes, reflecting the internal flexibility of the [CdI₄]²⁻ framework.

In the mid and high frequency regions (above 400 cm⁻¹), the Raman spectrum is dominated by vibrational modes of the organic (C₆H₉N₂)⁺ cation. The rocking vibration of the methyl group (γCH₃) is observed at 725 cm⁻¹. The amino group rocking (γNH₂) and wagging (wNH₂) modes are identified at 1155 cm⁻¹ and 645 cm⁻¹, respectively. Vibrational modes involving the C–N–C moiety within the aromatic ring are also observed experimentally: the antisymmetric and symmetric CNC stretching modes (νCNC) appear at 1344 cm⁻¹, while the exocyclic C–N stretching mode (νC–N) is located at 1234 cm⁻¹. The methyl group deformation vibrations are seen at 1386 cm⁻¹ (symmetric bending mode) and 1410 cm⁻¹ (antisymmetric modes). The δNH₂ scissoring and in-plane N–H bending (δN–H) vibrations are detected at 1662 cm⁻¹ and 1620 cm⁻¹, respectively. These assignments are consistent with those typically reported for protonated aminopyridine derivatives.^27,29,50

In the high wavenumber range, the spectrum exhibits bands related to C–H stretching modes of the methyl group at 2863 cm⁻¹ and 2919 cm⁻¹ (symmetric stretching), and at 2960 cm⁻¹ (antisymmetric stretching). The aromatic C–H stretching vibrations appear in their expected spectral region, spanning 3024 to 3145 cm⁻¹. Notably, broad and low intensity features centered at 3652, 3312, and 3196 cm⁻¹ are attributed to N–H stretching vibrations, including both symmetric and antisymmetric νNH₂ modes and the νN–H ring stretches. The broadening of these bands indicates that the amino and ring N–H groups are involved in strong hydrogen bonding, in agreement with the crystallographic data.

As a whole, the observed vibrational spectrum aligns well with those expected for the [CdI₄]²⁻ anion and (C₆H₉N₂)⁺ cation.

5. Optoelectronic properties of (C₆H₉N₂)₂CdI₄ compound

The UV-visible absorption spectrum of the hybrid compound (C₆H₉N₂)₂CdI₄, shown in Fig. 9, displays a distinct and intense absorption band centered at 307 nm. This band is assigned to a π → π* transition, attributed to the aromatic moiety of the 2-amino-5-methylpyridinium cation.²⁹ Such transitions are characteristic of organic chromophores with conjugated π-electron systems and significantly contribute to light-harvesting efficiency in hybrid architectures.⁵¹ In general, organic–inorganic halometalates of formula (C₆H₉N₂)₂MX₄, where [MX₄]²⁻ represents a tetrahedral metal halide anion, exhibit composite absorption features. The organic π-system dominates absorption in the UV region, while the metal-halide unit contributes through charge-transfer (CT) or d–d transitions depending on the nature of M. For example, (C₆H₉N₂)₂CoCl₄ exhibits strong π → π* bands in the range of 200–400 nm, in addition to charge-transfer and d–d transitions extending up to 800 nm.²⁷ In contrast, (C₆H₉N₂)₂CdCl₄, incorporating the d¹⁰ closed-shell Cd²⁺ ion, lacks low-energy d–d transitions and exhibits only a strong π → π* absorption,²⁹ leading to a sharper and more distinct band edge.

Fig. 9 TD-DFT validation of the experimental UV-visible spectra of the (C₆H₉N₂)₂CdI₄ compound: monomer and dimer absorption.

To evaluate the optical bandgap (E_g), a key parameter in determining a material's suitability for optoelectronic applications such as UV photodetectors, light-emitting diodes, or dielectric insulators,^52–54 the absorption coefficient (α) was extracted using the Beer–Lambert relation

where A is the absorbance and l is the sample thickness. The nature of the transition was then deduced using the Tauc formalism:

(αhν)^1/n = β(hν − E_g) (5)

where n is the exponent indicating the type of transition (n = 1/2 for direct and n = 2 for indirect transition), and β is a constant related to disorder.

Among the tested models, the most accurate and well-fitted plot corresponds to n = 1/2, obtained by plotting (αhν)² versus photon energy (hν). This result aligns with the Tauc model, confirming that the compound exhibits a direct allowed electronic transition. Plotting (αhν)² versus photon energy (hν) yields a linear region, whose extrapolation to ((αhν)² = 0) (Fig. 10) gives the direct bandgap E_g = 3.76 eV for (C₆H₉N₂)₂CdI₄. This value is slightly higher than the 3.43 eV gap observed for its chlorinated analogue (C₆H₉N₂)₂CdCl₄,²⁹ suggesting that despite iodide's greater polarizability and ionic radius,^26,55,56 factors that generally narrow the bandgap, the extended Cd–I bonds and reduced orbital overlap in [CdI₄]²⁻ may elevate the conduction band minimum or depress the valence band maximum, thereby widening E_g. Additional structural factors such as hydrogen bonding, anion polarizability, and crystalline packing also influence the electronic landscape of these materials.^57,58

Fig. 10 Tauc plot for (C₆H₉N₂)₂CdI₄.

A broader comparison across various (C₆H₉N₂)₂MX₄ compounds further emphasizes the tunability of optical properties via metal and halide substitution. For example, transition metals with partially filled d-orbitals, such as Mn²⁺ (d⁵), Fe²⁺ (d⁶), and Co²⁺ (d⁷), introduce low-energy d–d transitions and mid-gap states that significantly reduce E_g, as evidenced by values ranging from 2.21²⁶ to 2.47 eV³⁰ for Mn and Fe-based chlorides. Conversely, closed-shell d¹⁰ cations like Cd²⁺ exhibit wider gaps (3.43–3.76 eV) due to the absence of such transitions. Furthermore, halide substitution affects E_g through changes in ionic size and orbital character:^59,60 replacing Cl⁻ with Br⁻ or I⁻ generally increases the valence-band energy due to the higher energy and polarizability of the heavier halides; however, this does not always translate to a smaller bandgap, as structural and orbital interactions modulate the outcome.^61,62 As summarized in Table 6, these variations enable precise optical tuning across the UV-visible spectrum. Overall, the (C₆H₉N₂)₂CdI₄ compound, with its high bandgap, direct transition, and sharp π → π* absorption edge, represents a compelling candidate for wide-bandgap optoelectronic applications, while also highlighting the structural versatility and design flexibility of organic–inorganic halometalates.

Table 6 Band gap energies for (C₆H₉N₂)₂CdI₄ compared to those reported in previous works

Compound	Band gap Eg (eV)	Ref.
(C6H9N2)2CdCl4	3.43	29
(C6H9N2)2CoCl4	3.47	27
(C6H9N2)FeCl4	2.47	30
(C6H9N2)2MnI4	2.21	26
(C6H9N2)2MnBr4	2.38	28
(C6H9N2)2CdI4	3.76	This work

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To better understand the optoelectronic properties of the (C₆H₉N₂)₂CdI₄ compound and to explain the experimental results, time-dependent density functional theory (TD-DFT) simulations were carried out. GaussView 5⁶³ was utilized to prepare input files and to visualize different computed results. The Gaussian 09⁶⁴ program was employed in these calculations within the IEFPCM solvation model⁶⁵ using water as solvent. Both the monomer and dimer absorption spectra of the investigated compound (C₆H₉N₂)₂CdI₄ were computed using the TD-B3LYP-D3/LanL2DZ level of theory.^66,67 The calculated absorption spectra are depicted in Fig. 9, together with the experimental spectrum, and a significant correlation between the calculated and experimental data can be noticed. In addition to the monomer, the dimeric form was considered to account for possible intermolecular interactions (such as hydrogen bonding and halogen bridging) that can persist in solution and strongly influence the electronic structure. Compared to the monomer, the dimer showed a red-shifted absorption peak. This red shift reflects the impact of intermolecular interactions on the electronic structure and provides a better match with the experimental data. Indeed, the dimer's maximum absorption wavelength (300 nm) is close to the experimental value of 307 nm, supporting the idea that aggregated species may coexist with monomers in solution.

Table 7 lists the major electronic transitions for the first three excited states that contributed to the absorption spectra for both the dimer and the monomer. The dimer transition analysis demonstrated that the HOMO → LUMO transition occurs at 340.21 nm with a low oscillator strength (0.0003).

Table 7 Simulated absorption wavelengths, excitation energies, oscillator strengths, and major contributions for monomer and dimer compounds, using TD-B3LYP-D3/LanL2DZ level of theory in water as solvent

Compound	Excited state	Wavelength (nm)	Energy (cm^−1)	f	MO contributions
Monomer	S1	328.40	30450.87	0.0008	H → L+1 (59%)
	S2	328.02	30486.35	0.0001	H−1 → L (45%)
	S3	324.75	30792.85	0.0013	H−1 → L+1 (45%)
Dimer	S1	340.21	29393.47	0.0003	H → L (89%)
	S2	340.12	29401.53	0.0014	H → L+1 (91%)
	S3	338.76	29519.29	0.0007	H → L+2 (88%)

---

HOMO and LUMO orbitals for the monomer and dimer of the studied compound were analyzed to gain insight into its electronic characteristics. The isosurfaces of the frontier molecular orbitals (FMOs) are shown in Fig. 11, highlighting the features of a π-conjugated system. The LUMO is delocalized over the acceptor group, corresponding to the organic moiety, for both the monomer and the dimer, whereas the HOMO is mainly concentrated on the donor core, represented by the inorganic CdI₄ group. For the monomer, the calculated HOMO and LUMO energy levels are −6.26 eV and −2.02 eV, respectively, and for the dimer, they are −5.98 eV and −2.01 eV. The HOMO–LUMO gap decreases slightly from 4.23 eV for the monomer to 3.97 eV for the dimer, suggesting enhanced conjugation and a tendency toward red-shifted optical properties. To provide more insight into the reactivity of the compound under investigation, a series of global chemical reactivity descriptors, including global hardness (η), chemical potential (μ), and global electrophilicity index values (ω), have been derived through the use of the HOMO and the LUMO energy values. These parameters are a nice way to show how chemical reactivity and structural strength are related. The equations listed below specify these parameters:^30,32,68

andThe simulated values of global hardness, chemical potential, and electrophilicity index are 1.98 eV, −3.99 eV, and 4.01 eV, respectively (Table 8). These indicate moderate electronic kinetic stability (high η, low polarizability) and a propensity for intramolecular charge transfer from the electron-rich organic cation (higher-lying HOMO) to the [CdI₄]²⁻ acceptor unit (moderate ω), consistent with the TD-DFT transition analysis. The negative chemical potential confirms the overall thermodynamic stability of the dimer.

Fig. 11 The molecular frontier orbitals HOMO and LUMO computed with TD/B3LYP-D3/LANL2DZ method in water as solvent of the monomer (a) and the dimer (b) units for the studied compound (C₆H₉N₂)₂CdI₄.

Table 8 Global chemical reactivity descriptors for the dimer structure of the (C₆H₉N₂)₂CdI₄ compound, calculated at the TD-B3LYP-D3/LanL2DZ level of theory

Parameters	Values
EHOMO (eV)	−5.98
ELUMO (eV)	−2.01
HOMO–LUMO gap (eV)	3.97
Chemical potential (eV)	−3.99
Global hardness (eV)	1.98
Electrophilicity index (eV)	4.01

---

To investigate the nature of the intermolecular interactions and the non-covalent interactions in the (C₆H₉N₂)₂CdI₄ compound, we investigated the reduced density gradient (RDG) for the dimer system.^69–71 Multiwfn⁷² software was employed to do the RDG computations, enabling the extraction of the electron density and the gradient data from the calculated wavefunction. The VMD⁷³ program was then used to present the RDG isosurfaces. Distinct peaks in the RDG profile were found by the RDG analysis of the (C₆H₉N₂)₂CdI₄ dimer, indicating the presence of several intermolecular interactions that control the system's stability (see Fig. 12). However, significant peaks linked to hydrogen bonding were seen between the iodine atoms of the CdI₄ unit and the nitrogen atoms of the (C₆H₉N₂)₂ groups. This interaction was shown as a blue area on the RDG isosurface, indicating strong attractive forces that play a major role in stabilizing the structure. Furthermore, weaker van der Waals interactions have been observed, as shown by green-colored RDG areas. These dispersion forces between the organic and the inorganic components further stabilize the studied compound. Regions of strong repulsion were also detected; these were identified by red RDG areas that suggested steric hindrance or unfavorable spatial arrangements. The RDG analysis reveals a delicate equilibrium between these interactions, where steric repulsion affects the system's spatial arrangement while hydrogen bonding and van der Waals forces maintain it. The importance of non-covalent interactions in supporting the dimer's stability and structural integrity is shown by this detailed investigation.

Fig. 12 RDG graph (a) and isosurface plot (b) of the (C₆H₉N₂)₂CdI₄ compound.

6. Evaluation of in vitro biological activity

Oxidative stress, arising from the excessive generation of reactive oxygen species (ROS) during normal metabolic processes, is known to cause severe damage to biological systems through lipid peroxidation, protein modification, and activation of stress-related signaling pathways. Antioxidants play a vital protective role by neutralizing free radicals through the donation of electrons or hydrogen atoms, thereby preventing the propagation of oxidative chain reactions.^74,75 In view of the growing interest in developing effective antioxidant materials, the antioxidant potential of the (C₆H₉N₂)₂CdI₄ compound was systematically assessed using complementary in vitro assays.

The free radical scavenging ability of (C₆H₉N₂)₂CdI₄ was first evaluated using the DPPH assay, a widely employed method based on the reduction of the stable nitrogen-centered DPPH radical (Fig. 13). The scavenging activity increased steadily with concentration, demonstrating a clear dose-dependent response. At a final concentration of 1 mg ml⁻¹, the compound exhibited approximately 60% radical scavenging efficiency, indicating its strong capacity to quench free radicals and interrupt radical chain reactions. This behavior suggests that (C₆H₉N₂)₂CdI₄ can effectively delay the onset of lipid oxidation, highlighting its potential as a promising antioxidant agent.

Fig. 13 DPPH radical scavenging activity of (C₆H₉N₂)₂CdI₄ at different concentrations.

To further elucidate the antioxidant mechanism, the reducing power of (C₆H₉N₂)₂CdI₄ was investigated through the Fe³⁺/ferricyanide reduction assay. This method evaluates the electron-donating ability of antioxidant compounds by monitoring the conversion of Fe³⁺ to Fe²⁺. As shown in Fig. 14, the reducing power of the compound increased markedly with increasing concentration, reaching a maximum absorbance of approximately 0.7 at 700 nm for a concentration of 1 mg ml⁻¹.

Fig. 14 Reducing power effect of (C₆H₉N₂)₂CdI₄ at different concentrations.

The results obtained from both assays reveal a consistent antioxidant behavior, in which (C₆H₉N₂)₂CdI₄ acts as a reductone by donating electrons or hydrogen atoms to stabilize reactive species. The close agreement between the DPPH scavenging activity and the reducing power measurements further supports the existence of a direct correlation between antioxidant efficiency and electron-donating capability. Moreover, the enhanced activity observed may be attributed to the synergistic interaction between the organic ligand and the bismuth-iodide inorganic framework, which likely facilitates charge transfer processes. Overall, these findings demonstrate that (C₆H₉N₂)₂CdI₄ exhibits significant antioxidant potential, making it a suitable candidate for further investigation in antioxidant-related applications.

IV. Conclusion

The hybrid organic–inorganic compound (C₆H₉N₂)₂CdI₄ was successfully synthesized and thoroughly characterized through complementary experimental and computational approaches. Single-crystal X-ray diffraction revealed a triclinic structure comprising discrete [CdI₄]²⁻ tetrahedra stabilized by extensive N–H⋯I hydrogen bonds, π-stacking interactions, and van der Waals contacts between protonated 2-amino-5-picolinium cations and inorganic anions, as confirmed by RDG/NCIPLOT analysis. Raman spectroscopy verified the characteristic Cd–I stretching modes, while SEM/EDX demonstrated compositional purity and homogeneous elemental distribution.

Optical measurements established a direct-allowed bandgap of 3.76 eV with a sharp π → π* absorption edge at 307 nm attributable to the aromatic cation, validated by TD-DFT calculations on the dimer model. The wide bandgap, absence of low-energy d–d transitions, and minimal orbital overlap between isolated tetrahedra suggest low exciton binding energy suitable for UV optoelectronics. TGA/DSC analysis confirmed exceptional thermal stability with no mass loss or endothermic events up to 350 °C, substantially exceeding typical hybrid perovskites and enabling thin-film processing.

The compound also exhibited potent in vitro antioxidant activity, achieving 60% DPPH radical scavenging efficiency at 1 mg ml⁻¹ and strong ferric reducing power, reflecting synergistic electron/hydrogen donation from the electron-rich organic ligand and redox-accessible CdI₄²⁻ framework. These combined properties, wide-bandgap semiconductivity, high thermal robustness, validated electronic structure, and biological activity, position (C₆H₉N₂)₂CdI₄ as a promising multifunctional material for UV photodetection, transparent electronics, and antioxidant applications.

Author contributions

Arafet Ghoudi: writing – original draft, validation, software, methodology, investigation. Walid Taouali: writing – original draft, validation, software, methodology, investigation. José A. Paixão: writing – original draft, visualization, formal analysis. Noweir Ahmad Alghamdi. formal analysis, validation, writing review. Rui Fausto: writing – review & editing, visualization, validation. Abderrazek Oueslati: writing – review & editing, visualization, validation, investigation, formal analysis, data curation.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors confirm that the data used to support the findings of this study are included within the article and are available from the corresponding author upon reasonable request.

CCDC 2284327 contains the supplementary crystallographic data for this paper.⁷⁶

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