Synthesis, crystal structure, thermal analysis, spectroscopic, optical polarizability, and DFT studies, and molecular docking approaches of novel 2-methyl-benzylammonium derivatives for potential anti-inflammatory control

Abstract

This work aims to investigate novel structures based on 2-methylbenzylamine cations [(C₈H₁₂N)₂Co(SCN)₄] (1) and [(C₈H₁₂N)SCN] (2). The novel complexes were characterized and investigated by various techniques such as differential thermogravimetry analysis, FT-IR, UV-visible spectroscopy, impedance complex analysis, molecular modeling based on DFT calculations, and molecular docking as potent anti-inflammatory agents. Based on the reported results of these characterization tools, the desired complex phases were confirmed. These novel compounds were characterized by FTIR analysis, which supported the presence of surface ligand groups of thiocyanates, and UV-visible spectroscopy showed the optical transparencies of the titled compounds in addition to the confirmation of the electronic transition. Complex packing occurred through the N–H⋯S and N–H⋯N H-bond interactions, forming a ring in addition to CH-interactions, resulting in a 3-D network. Finally, molecular docking occurred for both complexes, which suggests that the complexes have anti-inflammatory potential.

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1. Introduction

Cobalt complexes are of great interest due to their diverse architectures, which include a wide variety of assemblies due to their extended configurations such as turbinated networks,¹ molecular drawstrings, chains,² square grids,³ and stairway frameworks.^4–8 Research has been conducted on the design and synthesis of novel compounds based on tricyanic cobaltate, which has a general chemical formula of C₂Co(SCN)₄. The aim of this contribution is twofold: firstly, to investigate how the organic cation (C) can add more interesting properties to the structure, such as protonic conductivity,^9–12 ferroelectricity,¹³ nonlinear optics, and luminescence,^14,15 and secondly, how it can control the inorganic chain [Co(SCN)₄]ⁿ⁻ packing.^16–18 Many antibiotic compounds are either derivatives of known drugs or their metal complexes, underscoring the importance of coordinating ligands. The biological efficacy of these compounds depends not only on the choice of metal but also on the bioactive moiety. Thus, a variety of bioactive ligands have been employed to assess the biological potential of their metal complexes.^19,20 Transition metal complexes from the 3d and 4d series have shown significant interactions with large biological molecules like DNA, RNA, and proteins, displaying strong binding abilities. These complexes have demonstrated promising antimicrobial and anticancer properties.^21,22 Medicinal chemists are increasingly interested in the creation of novel metal-based chemotherapy compounds. Numerous research studies have explored the biological effects of these metal complexes, such as their abilities to combat cancer, bacteria, fungi, and oxidative stress. Some investigations have focused on the synergy between antibiotics and transition metals to improve antibacterial efficacy.

In this present work, single crystals of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)CNS] have been synthesized by room-temperature evaporation. The crystal structures have been determined by X-ray diffraction. Both complexes have been investigated by spectroscopic and optical techniques and characterized using DFT setups. The thermal behaviors were studied and showed phase transition and decomposition in both complexes; additionally, molecular docking suggests that the complexes may act as anti-inflammatory agents.

2. Experimental details

2.1. Synthesis

2.1.1. Preparation of [(C₈H₁₂N)₂Co(SCN)₄]. All of the experimental reagents were used without any further purification. The synthesis of [(C₈H₁₂N)₂Co(SCN)₄] requires a prior preparation of HSCN. To (2 g, 10 mmol) of cobalt chloride, 20 mL of HSCN acid (2 M) was added. Then, 5 mL of a dilute solution of 2-methyl benzylamine (C₈H₁₁N) (2 M) was added dropwise with constant stirring. The final blue solution was left at room temperature. After a few days, blue block-shaped crystals of [(C₈H₁₂N)₂Co(SCN)₄] appeared in the solution.

2.1.2. Preparation of [(C₈H₁₂N)CNS]. The title compound [(C₈H₁₂N)CNS] has been synthesized by reacting an aqueous solution of 2 methylbenzylamine with thiocyanate (HSCN) at 75–85 °C, followed by extraction of [(C₈H₁₂N)CNS] with ethanol, and then recrystallization using ethanol as a recrystallization solvent.

2.2. Structure analysis using X-rays

Single-crystal DRX data were collected on a Bruker-Nonius Kappa CCD diffractometer using Mo-Kα radiation (170 K, λ = 0.71073 A). The crystal structure was solved by direct methods and refined by the full-matrix methods based on F² using the SHELXL program²³ included in the WINGX software package.²⁴ The molecular structures were drawn using Diamond 3.1 supplied by Crystal Impact.²⁵ Crystal data and details of the structure refinement are given in Table 1.

Table 1 Synthesis conditions and crystallographic data for [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN]

Formula	[(C8H12N)2Co(SCN)4]	[(C8H12N)SCN]
Where w = 1/[σ^2(Fo^2) + (0.0215P)^2 + 1.7725P] and P = (Fo^2 + 2Fc^2)/3.
System	Monoclinic	Triclinic
Space group	C2/c	P1̄
a, b, c (Å)	18.6374(5), 5.0271(1), 28.1614(8)	6.5707(3), 10.0248(5), 14.7305(8)
α, β, γ (°)	108.374(2)	100.787(2), 91.137(3), 90.534(3)
V (Å^3)	2503.99(11)	952.88(8)
D x (Mg m^−3)	1.519	1.257
Z	4	4
M r (g mol^−1)	535.62	180.27
T (K)	170	170
θ max, θmin (°)	28.4, 2.3	28.3,0.4
μ (mm^−1)	1.04	0.29
Color	Needle, blue	Block, colourless
Crystal size (mm^3)	0.48 × 0.06 × 0.06	0.40 × 0.36 × 0.22
T max, Tmin	0.746, 0.643	0.746, 0.651
Diffractometer	Bruker-Nonius Kappa CCD	Bruker-Nonius Kappa CCD
Measured reflections	5794	8562
Independent reflections	3102	8562
F(000)	1108	384
h	−24 to 23	−8 to 8
k	−6 to 6	−12 to 13
l	−37 to 37	0 to 19
Parameters refined
R[F^2 > 2σ(F^2)]	0.055	0.058
wR(F^2)	0.112	0.131
S	1.06	1.03
δρmax, δρmin (e Å^−3)	0.48, −0.43	0.50, −0.37
CCDC number	2061208	2087028

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2.3. Physical measurements

2.3.1. Infrared spectroscopy. IR measurements were performed using a NICOLET IR 200 FT-IR spectrometer, with scans run over the range of 400–4000 cm⁻¹.

2.3.2. UV solid state spectroscopy. UV measurement was performed using a PerkinElmer Lambda spectrophotometer. Scans were run over the range of 200–800 cm⁻¹.

2.3.3. The thermal analyses. The thermal analysis spectra were obtained via simultaneous thermogravimetry-differential thermal analysis (TG-DTA) using the PYRIS 1 TGA instrument using 10.1 mg of [(C₈H₁₂N)₂Co(SCN)₄] and 9.80 mg of [(C₈H₁₂N)CNS], at a heating rate of 5 °C min⁻¹ for the titled compound in the temperature range of 300–880 K, with the samples placed in aluminum pans under an inert atmosphere (nitrogen gas).

2.4. Computational details

The geometry optimization of the cobalt complex and thiocyanic salt was carried out using first-principles density functional theory (DFT) computations with the wB97XD functional and the def2SVP basis set. The choice of the ωB97XD/def2SVP method for the theoretical investigation of the electronic and structural studies of 1 and 2 is grounded on its proven accuracy in studying complexes analogous to 1 and 2 as shown by other independent studies.^26–281 and 2 were optimized to the lowest minima to enhance the accuracy of the results obtained and avoid the impact of imaginary frequency.²⁹ Structural optimization was carried out at the ωB97XD/def2SVP level of theory using Gaussian 16³⁰ and visualized using GaussView 6.0.³¹ The visualization of the Frontier molecular orbital (FMO) was carried out using ChemCraft,³² while the visualization of the molecular electrostatic potential (MESP) was carried out using visual molecular dynamics (VMD) software.³³ The analysis of the natural bond orbital (NBO) was carried out using the NBO 7.0 module³⁴ embedded in Gaussian 16.

2.5. Molecular docking protocol

The molecular docking was carried out to investigate the anti-inflammatory potency of 1 and 2 using an in silico molecular docking approach with 1 and 2 as ligands using two anti-inflammatory proteins downloaded from the protein data bank (PDB) viahttps://www.rcsb.org with RCSB codes 1D8A³⁵ and 2QIO,³⁶ respectively. The pre-docking protein preparation of ID8A and 2QIO involved the deletion of water, defining the binding site, deletion of native ligands and extraneous polypeptide chains distal from the SBD sphere (ligand binding site), addition of polar hydrogen, and extraction of the attributes of the SBD sphere along the X, Y, and Z coordinates. All these were accomplished using Biovia Discovery Studio,³⁷ while the in silico receptor–ligand docking was accomplished using AutoDock Vina,³⁸ while the receptor–ligand interaction was visualized using PyMol³⁹ for the 3D interaction and Biovia for the 2D interaction and the H-bond as well.

3. Results and discussion

3.1. Crystallographic study

Crystallographic data and structure refinements of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)CNS] are listed in Table 1.

3.1.1. Crystal structure of [(C₈H₁₂N)₂Co(SCN)₄] (1). The asymmetric unit of (1) comprises one tetra(isothiocyanate) cobalt [Co(NCS)₄]²⁻ anion and two 2-methylbenzylammonium cations (Fig. 1(a)). Selected bond distances and angles are given in Table 2.

Fig. 1 ORTEP views of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b).

Table 2 Selected bond lengths and bond angles of the two cobalt complexes in the title compound

[(C8H12N)2Co(SCN)4]		[(C8H12N)SCN]
Co1–N2^i	1.956(3)	N1–C1	1.491(6)
Co1–N2	1.956(3)	N1–H1C	0.89(3)
Co1–N3^i	1.959(3)	N1–H1D	0.90(3)
Co1–N3	1.959(3)	N1–H1E	0.90(3)
S1–C10	1.627(4)	N2–H2A	0.90(3)
S2–C11	1.634(4)	N2–H2B	0.90(3)
N2–C10	1.154(4)	N2–H2C	0.91(3)
N3–C11	1.157(4)	N4–C25	1.495(7)
N1–C2	1.499(4)
N2i–Co1–N2	115.87(18)	C1–N1–H1C	110(4)
N2i–Co1–N3i	106.41(12)	C1–N1–H1D	108(4)
N2–Co1–N3i	111.76(12)	H1C–N1–H1D	108(5)
N2i–Co1–N3	111.76(12)	C1–N1–H1E	103(3)
N2–Co1–N3	106.41(12)	H1C–N1–H1E	121(5)
N3i–Co1–N3	104.05(18)	H1D–N1–H1E	106(5)
C10–N2–Co1	171.0(3)	N1–C1–C2	111.9(4)
C11–N3–Co1	166.3(3)	N1–C1–H1A	109.2
N2–C10–S1	178.2(4)	C2–C1–H1A	109.2
N3–C11–S2	177.7(3)	N1–C1–H1B	109.2
		C2–C1–H1B	109.2
		H1A–C1–H1B	107.9
Symmetry code: (i) −x + 1, y, −z + 1/2.

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The coordination geometry of the central Co(II) ions can be described as a slightly distorted tetrahedron surrounded by four nitrogen atoms (Fig. S1(a), ESI†) and arranged along the b-axis direction. These anions lie to form anionic layers parallel to the (a,c) plane at y = ¼ and y = ¾. The main geometrical characteristics of the [Co(SCN)₄]²⁻ anion (Fig. S1(a), ESI†) are given in Table 2, where the average Co–N bond distance is 1.956 (3) A° and the N–Co–N bond angles vary in the range of 180.0–106°. These values are in agreement with those found in complexes containing the [M(NCS)_n]^m− anion such as [Ni(SCN)₄]2^− 40 and [In(SCN)₆]³⁻.⁴¹ The SCN⁻ ligands connect to Co(II) atoms using the end-to-end bridging mode, producing a three-dimensional linear chain (3D) along the [100] direction via μ1,4-SCN–thiocyanate bridges (Fig. 2(a)). These cations are located at layers parallel to [001] edges (Fig S2(a), ESI†). The values of the bond distances C(2)–N(1): 1.499 (4) Å and C(2)–C(3): 1.515 (3) Å are in good agreement with those found in similar complexes [C₆H₅CH₂P(Cl)(C₆H₅)₃]₂[Co(NCS)₄],⁴² [2ClBzTPP]₂[Co(NCS)₄],⁴³ (4NO₂BzTPP)₂[Zn(NCS)₄]⁴⁴ and [4NO₂BzPy]₂[Co(NCS)₄].⁴⁵

Fig. 2 (a) Projection along the b-axis of the structure of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN], (b) dotted lines indicate hydrogen bonds.

Each terminal nitrogen atom of the [C₈H₁₂N]⁺ group shares its protons in hydrogen bonding interactions of the type N–H⋯S(SCN), forming a three-dimensional network (Table 3). The N–H(C₈H₁₂N) ⋯S(SCN) distance values vary between 3.360(3) and 3.505(3) Å. The strength of the hydrogen bond can be interpreted according to the criteria concerning the distances N⋯S.⁴⁶ These hydrogen bridging bonds (types: donors and acceptors) and the π–π and CH–π interactions ensure the cohesion and stability of the crystalline edifice (Fig. 3(a)). In addition, the stability of the compound is improved by the CH⋯π interactions between the CH frame and the aromatic rings with a distance of 3.458 Å to 3.993 Å.

Table 3 Hydrogen-bond geometry (Å, °)

D–H⋯A	D–H	H⋯A	D⋯A	D–H⋯A
[(C8H12N)2Co(SCN)4]
N1–H1A⋯S2^ii	0.90(2)	2.59(3)	3.360(3)	144(3)
N1–H1B⋯S1^iii	0.92(2)	2.51(2)	3.403(3)	164(4)
N1–H1C⋯S2	0.91(2)	2.61(2)	3.455(3)	154(3)
N1–H1C⋯S2^iv	0.91(2)	2.90(3)	3.505(3)	125(3)
[(C8H12N)SCN]
N1–H1C⋯N5	0.89(3)	2.02(3)	2.886(7)	163(5)
N1–H1D⋯S1^i	0.90(3)	2.52(3)	3.409(5)	170(5)
N1–H1E⋯S2	0.90(3)	2.53(3)	3.367(5)	155(5)
N2–H2A⋯S2	0.90(3)	2.68(4)	3.409(5)	139(5)
N2–H2B⋯S3	0.90(3)	2.38(3)	3.280(6)	173(5)
N2–H2C⋯S4^ii	0.91(3)	2.50(4)	3.327(5)	151(5)
N3–H3A⋯N6	0.90(3)	1.99(3)	2.876(6)	169(5)
N3–H3B⋯S2^iii	0.92(3)	2.51(3)	3.388(5)	159(5)
N3–H3C⋯S1^ii	0.91(3)	2.64(4)	3.433(4)	147(4)
N4–H4A⋯S1	0.91(3)	2.59(4)	3.386(5)	146(5)
N4–H4B⋯N8	0.91(3)	1.99(3)	2.890(8)	171(6)
N4–H4C⋯N7	0.89(3)	2.21(4)	3.010(6)	149(5)

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Fig. 3 CH–π and π–π stacking in [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b).

3.1.2. Crystal structure of [(C₈H₁₂N)SCN] (2). The asymmetric unit of (2) (Fig. 1(b)) contains one 2-methylbenzyl ammonium cation (C1, C2, C3, C5, C6, C7, or C8/N1) and one thiocyanate anion (S1, C33, or N5). The 2-methylbenzyl ammonium cation (Fig. S2(b), ESI†) ring adopts a slightly distorted chair conformation, with puckering parameters: θ = 120.8 (5)°, while for an ideal chair configuration, θ has a value of 109 to 180°. The bond lengths and bond angles (Table 2) are in normal ranges and are comparable with those reported earlier for similar compounds.^47,48 The different types of H-bonds for [(C₈H₁₂N)CNS] are shown in Fig. 2(b) as dashed light red lines and their relative geometrical parameters are listed in Table 3.

The compound exhibits N–H⋯N and N–H⋯S H-bonds which give rise to 3D chains. Regarding the donor–acceptor bond lengths, all the hydrogen bonds in the studied system are found to be weak (D–A > 3 Å) and primarily involve electrostatic interactions.⁴⁹ The intermolecular hydrogen bonding interactions link neighboring thiocyanate anions through N–H⋯N hydrogen bonds with lengths from 1.99(3) Å to 2.21(4) Å and through N–H⋯S hydrogen bonds with lengths from 2.38(3) Å to 2.52(3) Å (Table 3), contributing to the R²₆(16) rings as shown in Fig. S3 (ESI†), in addition to the CH–π stacking interactions (Fig. 3(b)) between aromatic rings which provides more stability to the three-dimensional framework with a stacking distance of 3.872 Å.

[(C₈H₁₂N)₂Co(SCN)₄]: symmetry codes: (ii) x, y + 1, z; (iii) x, y − 1, z; (iv) −x + 1/2, y + 1/2, −z + 1/2.

[(C₈H₁₂N)SCN]: symmetry code: (i) x − 1, y, z; (ii) x, y + 1, z; (iii) x + 1, y, z.

3.2. Structural analysis

The structural properties (bond length and bind angle) of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] were investigated theoretically using GaussView and compared with the experimental data to compare the degree of consonance or dissonance complemented with the calculation of the root mean square deviation (RMSD). Table 4 shows the structural properties of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] with special emphasis on the experimental and theoretical bond length and bond angle of the selected bond labels, while Fig. 4 shows the optimized structures of [(C₈H₁₂N)₂Co(SCN)₄] (Fig. 4(a)) and [(C₈H₁₂N)SCN] (Fig. 4(b)), respectively.

Table 4 The bond length and bond angles of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] investigated at the ωB97XD/def2SVP level of theory

Bond length (Å)
[(C8H12N)2Co(SCN)4]			[(C8H12N)SCN]
Atom label	Experimental	Theoretical	Atom label	Experimental	Theoretical
Co1–N2	1.956	1.874	S2–C34	1.656	1.640
Co1–N3	1.959	1.820	N1–C1	1.491	1.176
S1–C10	1.627	1.611	C3–C4	1.396	1.393
N2–C10	1.154	1.182	C6–C7	1.386	1.340
N3–C11	1.157	1.180	N2–C9	1.467	1.490
RMSD		0.068			0.130

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Bond angle (°)
[(C8H12N)2Co(SCN)4]			[(C8H12N)SCN]
Atom label	Experimental	Theoretical	Atom label	Experimental	Theoretical
C10–N2–Co1	171.0	159.021	C1–N1–H1	110.000	103.511
C11–N3–Co1	166.3	153.064	C17–N3–H3	108.000	111.375
N2–C10–S1	178.2	178.111	C7–C6–C5	118.800	119.992
N3–C11–S2	177.7	177.261	C4–C5–C6	120.300	119.207
N1–C2–C3	110.4	111.384	C6–C7–H7	119.500	119.396
RMSD		7.301			3.058

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Fig. 4 The optimized structures of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b) optimized at the ωB97XD/def2SVP level and visualized using ChemCraft.

As presented in Table 4, the five intramolecular bonds considered in 1 were Co₁–N₂, Co₁–N₃, S₁–C₁₀, N₂–C₁₀, and N₃–C₁₁. The first three were longer in the experimental data, while in the theoretical data, they were slightly shorter with differences in magnitude of 0.082 Å, 0.139 Å, and 0.016 Å, respectively. This implies that the theoretical values were 4.2%, 7.1%, and 1.0% different from the corresponding experimental values. The last two bonds were slightly longer theoretically than experimentally with differences in magnitude being 0.028 Å and 0.023 Å, respectively, which further implies that the experimental values were 2.4% and 1.9% lower than the theoretical magnitudes, respectively. Conversely, the first four of the five bond labels considered in [(C₈H₁₂N)SCN] were higher experimentally than theoretically. Experimentally, the magnitudes of the S₂–C₃₄, N₁–C₁, C3–C4, and C6–C₇ bond labels were 1.565 Å, 1.491 Å, 1.396 Å and 1.386 Å, respectively, while the theoretical values of 1.640 Å, 1.176 Å, 1.393 Å and 1.340 Å were 0.016 Å, 0,315 Å, 0.003 Å and 0.046 Å, respectively, which correspond to the magnitude differences of 1.0%, 21. 1%, 0.2%, and 3.3%, respectively. Furthermore, the N₂–C₉ bond had a theoretical value of 1.490 Å and a slightly lower experimental magnitude of 1.467 Å. Overall, the RMSD of [(C₈H₁₂N)₂Co(SCN)₄] was found to be lower with a value of 0.068 compared to that of 2 with a higher value of 0.130.

The bond angles of the selected intramolecular bond labels were also studied at the same level of theory and a great degree of harmony between the experimental and theoretical values was observed as shown by the not-so-high RMSD of 7.301 and 3.058 for [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN], respectively. In [(C₈H₁₂N)₂Co(SCN)₄], the C₁₀–N₂–Co₁, C₁₁–N₃–Co₁, N₂–C₁₀–S₁, and N₃–C₁₁–S₂ bond angles were experimentally higher than the theoretical values with the differences in the values being 11.979, 13.236, 0.089 and 0.439, respectively, thereby indicating that the last two showed a higher degree of consonance compared to the first two. On the other hand, the N₁–C₂–C₃ bond angle was theoretically higher with a theoretical value of 11.384, while the experimental value was 0.88343 degrees lower with a magnitude of 110.4. Conversely, the [(C₈H₁₂N)SCN] bond angles showed a higher degree of closeness between the experimental and theoretical values. The C₁–N₁–H1, C₄–C₅–C₆, and C₆–C₇–H₇ bond angles were higher experimentally than theoretically with the differences of 6.5, 1.1, and 0.1 degrees, respectively, while the C₁₇–N₃–H₃ and C₇–C₆–C₅ bond angles were higher theoretically than experimentally with differences in value being 3.4 and 1.2 degrees, respectively.

3.3. Infrared spectroscopy investigation

Fig. 5 displays the IR spectrum of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN]. The characteristic vibrations can prove the presence of the thiocyanate ligand and its binding mode to the Co(II) ion center for the formation of the anionic complex [Co (SCN)₄]. The strong band at 2079 cm⁻¹ can be assigned to the stretching vibration of the C≡N bond of the thiocyanic ligand. The band at 840 cm⁻¹ is attributed to the C–S bond stretching vibration. The band at 490 cm⁻¹ is ascribed to the bending vibration of N–C–S. This vibrational assignment of the thiocyanate indicates the binding of the thiocyanate ligand to the metal (II) center via its N-terminal atom. The assignment of these bands to thiocyanate vibrations and the determination of its coordination mode are based on previously reported results such as those for (C₂N₆H₁₂) [Co (NCS)₄]. H₂O⁵⁰ and [2NAPMeTPP]₂[Co(NCS)₄].⁵¹

Fig. 5 Infrared absorption spectra of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b).

The spectrum also shows characteristic vibrations for 2-methylbenzylamine. The broad bands in the range of 3600–2300 cm⁻¹ correspond to the stretching vibrations of the organic and hydroxyl groups ν(N–H) and ν(C–H). The band at 1504 cm⁻¹ corresponds to ν(C=C) stretching vibrations. The band at 1450 cm⁻¹ can be assigned to the CH₂ deformation.⁵²

The bands at 1244 and 1180 cm⁻¹ can be attributed to the ring deformation. The weak bands at 1166 and 1021 cm⁻¹ can be assigned to the CH₂ twisting. The weak band at 870 cm⁻¹ can be attributed to the ring deformation.⁵³

3.4. Solid-state spectroscopy

The luminescence properties have been tested for the solid states of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)CNS] at room temperature, in the region [200–800 nm]. As depicted in Fig. 6, the compounds show different luminescence behaviors; the two characteristic bonds for [(C₈H₁₂N)₂Co(SCN)₄] at 598, 456, and 402 nm are assigned to d → d*, charge transfer and n → π* transitions, respectively, and for [(C₈H₁₂N)SCN], one characteristic bond at 290 nm is assigned to charge transfer transition.

Fig. 6 Solid state UV-vis spectra of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b).

The calculation of E_g revealed that [(C₈H₁₂N)₂Co(SCN)₄] exhibits semiconductor behavior with a gap energy value of 2.09 eV and [(C₈H₁₂N)SCN] exhibits isolant behavior with a gap energy of 3.8 eV (Fig. S4, ESI†). These behaviors are probably due to the interactions in the molecular solid, the charge transfer between the central metal and its coordinated ligands, and especially due to the presence of the thiocyanate anions that may affect the emission.⁵⁴

3.5. Electronic properties

3.5.1. Quantum reactivity descriptors. Following the advances in the modern-day understanding of the Frontier molecular orbital (FMO) theory⁵⁵ of chemical reactivity, there is a global consensus on the indispensable pertinence of the two major frontier molecular orbitals, namely, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to the reactivity and invariably the stability of a compound.^56,57Table 5 presents the electronic properties of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] as well as the magnitudes of their global quantum reactivity descriptors, while Fig. 7 is a pictorial illustration of the HOMOs and LUMOs of [(C₈H₁₂N)₂Co(SCN)₄] (Fig. 7(a)) and [(C₈H₁₂N)SCN] (Fig. 7(b)), respectively.

Table 5 The electronic properties and the quantum descriptors of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] investigated at the ωB97XD/def2SVP level of theory

Complex	HOMO (eV)	LUMO (eV)	E gp (eV)	IP (eV)	EA (eV)	S (eV)	η (eV)	μ (eV)	χ (eV)	ω (eV)
A1	−7.645	−4.640	3.005	7.645	4.640	0.333	1.502	6.142	−6.142	12.557
A2	−5.665	−5.319	0.346	5.665	5.319	2.894	0.173	5.492	−5.492	87.281

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Fig. 7 the HOMO–LUMO graphics of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b) visualized using ChemCraft.

Fundamentally, the difference between the HOMO and the LUMO is generally known as the energy gap, E_gp, and can be used to theoretically compare the chemical reactivity of compounds because the higher the energy gap the less reactive the compound and the greater the stability, while a lower energy gap indicates higher reactivity and lower stability. [(C₈H₁₂N)₂Co(SCN)₄] has an E_gp of 3.005 eV and that of [(C₈H₁₂N)SCN] has a calculated value of 0.346 eV. This deductively implies that [(C₈H₁₂N)₂Co(SCN)₄] is predicted to have a lower reactivity compared to [(C₈H₁₂N)SCN] and may be accounted for by the presence of Co as a complexation metal in [(C₈H₁₂N)₂Co(SCN)₄], while in [(C₈H₁₂N)SCN] without a complexing metal, the reactivity is higher. This also implies that [(C₈H₁₂N)₂Co(SCN)₄] is more stable than [(C₈H₁₂N)SCN]. This variation in the comparative reactivity and stability strength of the Co complex and [(C₈H₁₂N)SCN] is further elucidated by the chemical softness and chemical hardness which give more information about the reactivity and stability of a compound, respectively. This implies that the higher the chemical softness, the higher the reactivity and vice versa, while the higher chemical hardness corresponds to the greater stability for both complexes; [(C₈H₁₂N)₂Co(SCN)₄] with a chemical softness of 0.333 eV is equally predicted to be less reactive than [(C₈H₁₂N)SCN] with a chemical softness of 2.894 eV, while the lower chemical hardness of [(C₈H₁₂N)SCN] corroborates this. Furthermore, application of Koopman's approximations can be used to obtain certain other quantum descriptors namely: chemical potential (μ), chemical hardness (η), chemical softness (S), electrophilicity index (ω), and electronegativity (χ) using eqn (1)–(7).⁵⁸

IP = −E_HOMO (1)

EA = −E_LUMO (2)

χ = −μ (7)

3.5.2. Natural bond orbitals (NBOs). The study of natural bond orbitals is aimed at investigating the inter-molecular charge transfer from donor orbitals to acceptor orbitals and in this specific case, this is done using the Second Order Perturbation Theory of the Fock Matrix in the NBO basis.³⁴Table 6 presents the results of the NBO analysis of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN], including the transitions, the donor orbitals, acceptor orbitals, occupancies as well as stabilization energies which were computed based on eqn (8).where f_ij is the off-diagonal element on the Fock matrix, E_(j) − E_(i) are the diagonal elements and E⁽²⁾ represent the second order stabilization energy of the system.⁵⁹ From Table 6, the transitions with the greatest contribution to the stability of 1 are LP₍₃₎ → LP₍₁₎*, σ → LP₍₁₎*, σ → LP₍₄₎* and σ* → σ*. The LP₍₃₎ → LP₍₁₎* transition from S₁₃ to C₆ had the highest stabilization energy of 177.17 kcal mol⁻¹, thereby implying that this transition contributed the most to the orbital stabilization of [(C₈H₁₂N)₂Co(SCN)₄]. Furthermore, the σ → LP₍₁₎* transition with an E⁽²⁾ of 138.90 kcal mol⁻¹ was due to the electron transfer from the complexing metal, Co₁ to C₆. The σ → LP₍₄₎* transition occurred twice with notable stabilization energies of 135.80 and 135.53 kcal mol⁻¹, respectively, while the σ* → σ* transition from Co₁–N₃ to Co₁–N₅ had an E⁽²⁾ of 113.42 kcal mol⁻¹. Conversely, in [(C₈H₁₂N)SCN], the E⁽²⁾ values were comparatively lower than those of [(C₈H₁₂N)₂Co(SCN)₄] which equally corroborates the FMO observation that this complex is predicted to be comparatively more stable but less reactive than [(C₈H₁₂N)SCN]. The values of the stabilization energies of [(C₈H₁₂N)SCN] ranged from 10.45 kcal mol⁻¹ to 69.59 kcal mol⁻¹ and were due to five different transitions namely: LP₍₃₎ → π*, π* → σ*, LP₍₂₎ → π*, LP₍₁₎ → σ* and π* → σ*. The orbital transition with the highest contribution to the stability of [(C₈H₁₂N)SCN] is the LP₍₃₎ → π* with an E⁽²⁾ of 69.69 kcal mol⁻¹, while the π* → σ* transition had the least contribution with an E⁽²⁾ of 10.45 kcal mol⁻¹.

Table 6 The natural bond orbital (NBO) of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] investigated at the ωB97XD/def2SVP level of theory

Transition	Donor	Occupancy	Acceptor	Occupancy	E ^(2) (kcal mol^−1)	E (j)−(i) (a.u.)	F(i,j) (a.u.)
[(C8H12N)2Co(SCN)4]
LP(3) → LP(1)*	S13	1.63739	C6	0.88420	177.17	0.15	0.167
σ → LP(1)*	Co1	1.61138	C6	0.88420	138.90	0.28	0.201
σ → LP(4)*	Co1–N5	0.21653	Co1	0.39697	135.80	0.11	0.196
σ → LP(4)*	Co1–N3	0.30559	Co1	0.39697	135.53	0.12	0.195
σ* → σ*	Co1–N3	0.30559	Co1–N5	0.21653	113.42	0.01	0.069
[(C8H12N)SCN]
LP(3) → π*	S3	1.76244	N1–C2	0.22839	69.59	0.41	0.150
π* → σ*	N1–C2	0.18127	N1–C3	0.06469	37.65	0.46	0.335
LP(2) → π*	S3	1.76463	N1–C2	0.18127	37.08	0.58	0.133
LP(1) → σ*	S3	1.98291	N1–C2	0.06469	15.56	1.54	0.140
π* → σ*	N1–C2	0.18127	C2–S3	0.01068	10.45	0.10	0.091

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3.5.3. Molecular electrostatic potential (MESP). The intra-molecular differential charge distribution is a phenomenon that can be accounted for by the fact that the nucleophilicity or electrophilicity of the atoms that constitute a molecule cannot be the same, hence in a molecule, the regions with a greater electron-pulling tendency are usually characterized by their possession of lone pairs, high electronegativity as well as high electron affinity.^60,61 Structurally, [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] have some structural resemblance, but due to the inherent structural differences, they have different positions of the nucleophilic and electrophilic regions as illustrated by the red and blue regions, respectively, in Fig. 8.

Fig. 8 The graphical illustration of the molecular electrostatic potentials of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b) visualized using VMD.

3.6. Molecular docking analysis

The bioactivity of a compound depends on its structural complementarity with the target receptor^62–64 and can be investigated using the in silico molecular docking simulation as this approach can provide useful insights into the binding strength and affinity of bonding between a receptor and a ligand.⁶⁵ The results as provided in Table 7 summarize the binding affinity (kcal mol⁻¹) and amino acid residues for the receptor–ligand interaction between [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] with 1D8A and 2QIO, respectively, while Fig. 9 shows the 2D, 3D and the H-bond of the molecular docking as carried out in this study. As presented in Table 7, the anti-inflammatory potentials of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] were specific for 1D8A and 2QIO, respectively, as the docking scores of 1D8A-(a) for[(C₈H₁₂N)₂Co(SCN)₄] and the 2QIO-(b) for [(C₈H₁₂N)SCN] interactions had 0.0 binding affinity in all the 9 poses. Table 7 shows that in the 1D8A-(a) interaction, the mean binding affinity was −5.0 kcal mol⁻¹, while the first and ninth pose binding affinities were −5.4 and −4.6 kcal mol⁻¹ with the amino acid residues arising from the interaction of 1 with the 1D8A receptor being a two-fold interaction of 1 with the alanine 189 (ALA:189) of the polypeptide chain B of 1D8A at distances of 2.47 Å and 2.91 Å, respectively. On the other hand, the 2QIO-(b) interaction had a slightly greater mean binding affinity of −5.1 kcal mol⁻¹ with the first and ninth poses having binding affinities of −5.5 kcal mol⁻¹ and −5.3 kcal mol⁻¹, while the amino acid residues were three and were at the alanine 190 (ALA:190) of the polypeptide chain C with interaction bond lengths of 2.58 Å, 2.63 Å and 2.97 Å, respectively. Based on the foregoing, the anti-inflammatory potential of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] has been established using in silico bioactivity prediction.

Table 7 The binding affinity and amino acid residues of the receptor–ligand interactions of [(C₈H₁₂N)₂Co(SCN)₄] and [(C₈H₁₂N)SCN] with 1D8A and 2QIO

MODE	Anti-inflammatory potential
	Binding affinity (kcal mol^−1)	Amino acid residue	Binding affinity (kcal mol^−1)	Binding affinity (kcal mol^−1)	Binding affinity (kcal mol^−1)	Amino acid residue
	1D8A-A1	1D8A-A1	2QIO-A1	1D8A-A2	2QIO-A2	2QIO-A2
1	−5.5	B:ALA:189:O (2.91 Å and 2.47 Å)	0.0	0.0	−5.5	C:ALA:190:O (2.97 Å, 2.58 Å and 2.63 Å)
2	−5.4		0.0	0.0	−5.5
3	−5.1		0.0	0.0	−5.3
4	−4.9		0.0	0.0	−5.2
5	−4.9		0.0	0.0	−5.0
6	−4.8		0.0	0.0	−4.9
7	−4.7		0.0	0.0	−4.9
8	−4.7		0.0	0.0	−4.9
9	−4.6		0.0	0.0	−4.8
Mean	−5.0		0.0	0.0	−5.1

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Fig. 9 The graphical illustration of the molecular docking interaction between 1D8A and 2QIO with [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b); molecular docking analysis was performed using Biovia Discovery Studio, while the in silico receptor–ligand docking was accomplished using AutoDock Vina, while the receptor–ligand interaction was visualized using PyMol for the 3D interaction and Biovia for the 2D interaction and the H-bond as well.

3.7. Thermal properties

DTA and TGA were carried out using the PYRIS 1 TGA instrument. The thermal curves of the two coordination compounds are given in Fig. 10(a) and (b).

Fig. 10 DTA/TG curves of [(C₈H₁₂N)₂Co(SCN)₄] (a) and [(C₈H₁₂N)SCN] (b).

[(C₈H₁₂N)₂Co(SCN)₄] shows two endothermic peaks at 390 K and 470 K. The first one corresponds to the phase transition without the loss of weight detected by differential thermogravimetry and the second one corresponds to the degradation of the organic part and some of the thiocyanate ligands from the metal compound, as confirmed by TGA analysis. The decomposition in the [420–480 K] range has the same variation for the compound (1), and the decomposition of the resulting Co(NCS)₄ is carried out at a higher temperature.⁶⁶ During the process, the SO group, free from hydrogen bonding, is oxidized to SO₂ at [520–560 K].^66–68

For [(C₈H₁₂N)SCN], the TGA studies shows a phase transition at 323 K without any weight loss. The decomposition process begins at 453 K for [(C₈H₁₂N)SCN], during this endothermic process, the mass loss in this step can be attributed to the successive decomposition of the titled complex. This endothermic step takes place when the intermolecular N–H⋯S hydrogen bonding breaks apart during the deamination process and one of the three N–H groups per complex molecule is no longer engaged in hydrogen bonding. During the latest process, the SO group is oxidized to SO₂ at 520 K.

4. Conclusion

Novel crystalline compounds [(C₈H₁₂N)₂Co(SCN)₄] (1) and [(C₈H₁₂N)SCN] (2) were synthesized and analyzed using XRD single crystallography, FTIR spectroscopy, differential thermal gravimetric analysis, complex impedance measurements, and magnetic studies. The crystal structure (1) shows that Co(II) atoms are bridged by four SCN⁻ anions generating 3-D chains assured by hydrogen bonds, van der Waals, and CH–π interactions. Crystal (2) shows that intermolecular cohesion is ensured by N–H⋯S, N–H⋯N hydrogen bonds and CH–π stacking interactions. Furthermore, frontier molecular orbital analysis and NBO analyses have been calculated using the DFT method.

TGA/DTA coupled analysis shows that these compounds undergo a phase transition at 390 K for (1) and at about 323 K for (2). Optical properties were investigated through absorption measurements. It is found that the gap energy value of the formed compounds is 2.09 eV for (1) and 3.80 eV for (2); according to these results, we can predict that our finding is a semiconductor. The molecular docking occurred for both complexes, suggesting that the complexes have an anti-inflammatory potential.

Data availability

CCDC 2061208 and 2087028 contain the supplementary crystallographic data for the obtained compounds. The DSC diagram and B3LYP-converged geometry, and the antibacterial activities of the title complex are also provided in the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors extend their appreciation to the Researchers Supporting Project, King Saud University, Riyadh, Saudi Arabia, for funding this work through grant No. RSP2024R353.

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Footnote

† Electronic supplementary information (ESI) available: CCDC 2061208 and 2087028 contain the supplementary crystallographic data for the obtained compounds. The DSC diagram and B3LYP-converged geometry and the antibacterial activities of the title complex. CCDC 2061208 and 2087028. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj03197k

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