Cyclohexylammonium Hexaisothiocyanatonickelate(II) Dihydrate as a Single-Source Precursor for High Surface Area Nickel Oxide and Sulfide Nanocrystals

Abstract

Cyclohexylammonium hexaisothiocyanatonickelate(II) dihydrate, (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O, was synthesized, for the first time, by a four-step method in a yield of 95%. The compound was fully characterized by elemental microanalysis, Fourier transform infrared (FTIR), ultraviolet-visible-near infrared (UV-Vis-NIR), and nuclear magnetic resonance (NMR) spectroscopy and thermogravimetry. A single crystal X-ray diffraction (SXRD) gave the monoclinic space group P2₁/c with a = 15.8179 (5) Å, b = 10.6222 (3) Å, c = 13.8751 (4) Å, β = 109.362 (1)°, V = 2199.45 (11) ų, Z = 2 (T = 293 K) for this novel hybrid organic–inorganic compound. The title compound was employed as a single-source precursor for the synthesis of mesoporous, high surface area nickel oxide (53 Å; 452 m²/g) and nickel sulfide (46 Å; 220 m²/g) via pyrolysis under air at 550 °C or helium atmosphere at 500 °C, respectively. X-ray powder diffraction (XRPD) demonstrated the nanocrystalline nature of both NiO and NiS with an average crystallite size of 16 and 54 nm, respectively. Scanning electron microscope (SEM) indicated the formation of agglomerated, quasi-spherical particles of nickel oxide and agglomerated flake-like structures of nickel sulfide. The high surface area, porosity, and nanocrystallinity of both NiO and NiS, obtained via this approach, are promising for a wide spectrum of applications.

1. Introduction

For more than 100 years, the triatomic linear thiocyanate [SCN]⁻ or isothiocyanate [NCS]⁻ ligand has captured the attention of many researchers due to its low toxicity and varied coordination modes to metal ions. It can bind to the metal center as a terminal monodentate linker via N- or S-atom, according to Pearson’s hard-soft acid-base (HSAB) classification concept. Furthermore, it binds to metal cations in bridging mode through nitrogen and sulfur atoms, M—NCS—M [1,2]. These different modes of coordination lead to the formation of compounds with major diversity in chemical, physical, and structural properties. Therefore, in the context of correlating thiocyanate complex structures with their properties, Christian et al. prepared manganese, cobalt, and nickel thiocyanate complexes, which exhibited cooperative magnetic phenomena. Such complexes dramatically changed their magnetic properties upon thermal decomposition [3]. In addition, manganese, iron, and nickel thiocyanate complexes with 4-ethylpyridine, as a co-ligand, were synthesized and investigated to show cooperative magnetic phenomena [4]. In another application, the [Ni(SCN)₄(2-methylpiperazine)₂] complex was effective against the activities of bacteria such as E. coli, S. typhimurium, E. feacium, and C. albicans [5]. The complex of [Ni(NCS)₂(para-phenylpyridine)₄] showed remarkable isomeric selectivity for ortho-xylene over meta- and para-xylene from a ternary mixture, and a similar selectivity for meta- over para-xylene from a binary mixture [6].

Hybrid organic–inorganic compounds, based on the bulky anionic complex of hexaisothiocyanatonickelate(II), [Ni(NCS)₆]⁴⁻, have also drawn considerable attention due to their promising applications [7,8,9,10,11,12,13], depending on the identity of the organic cation. For instance, when the organic cation was 1-butyl-3-methylimidazolium, BMIM+, the resultant compound exhibited ionic liquid nature and reversible thermochromic behavior [7]. However, when the organic cation was tetrakis(triethylammonium), the eventually produced compound was a useful precursor for the synthesis of anhydrous nickel thiocyanate [8]. The piperidinium cation resulted in a new compound, which could be used for the preparation of nickel sulfide via thermal decomposition under nitrogen [9]. Incorporation of triphenylmethylphosphonium cation in a compound with [Ni(NCS)₆]⁴⁻ produced ferroelectric, piezoelectric material, which was suitable for harvesting energy upon its integration, 15 wt.%, in thermoplastic polyurethane [10]. On the other hand, introducing trimethylammonium gave a suitable material for a dielectric switch, tuned by frequency, as a result of phase transition [11].

On the basis of the organic cation, cyclohexylammonium is a unique building block due to its capability to form various dimensional structures, depending on the identity of its counter anion and the presence of solvent of crystallization molecules, via its hydrogen donor nature [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. Another attractive feature of cyclohexylammonium is its usefulness as a soft organic template for the creation of mesopores, after its thermal decomposition, within the produced ceramic [26,38,39].

Metal sulfides have received high attention as semiconductors and for other applications. Dithiocarbamate of general formula (R₂CNS₂⁻) was extensively reported as a single-source precursor (SSP) for metal sulfides [40,41,42,43,44] in the form of thin films or nanoparticles [45,46]. Although there have been thorough investigations of several thiocyanate or isothiocyanate metal compounds, none of them has been utilized as an SSP for metal sulfides or oxides [3,8,12,13,47,48,49,50,51,52,53]. Recently, we have reported cyclohexylammonium tetraisothiocyanatocobaltate(II) as an SSP for cobalt sulfide (CoS) and tricobalt tetraoxide (Co₃O₄) nanoparticles [26], while cyclohexylammonium hexaisothiocyanatochromate(III) sesquihydrate as an SSP for nanocrystalline chromium sulfide (Cr₂S₃) and chromium oxide (Cr₂O₃) [38].

In this work, cyclohexylammonioum hexaisothiocyanatonickelate(II) dihydrate, (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O was synthesized, characterized, and utilized as an SSP, via its thermal decomposition under two different atmospheres, for high surface area, mesoporous, nanocrystalline nickel sulfide (NiS) and nickel oxide (NiO).

2. Material and Methods

Nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O; purum p.a.; crystallized; ≥98.0%, Sigma-Aldrich, St. Louis, MI, USA], sodium thiocyanate (NaNCS; pure; 99%; Riedel-de Haën AG, Seelze, Germany), cyclohexylamine [C6H11NH2; >99.0%(GC); TCI], hydrochloric acid (HCl; 37%; certified AR for analysis; Fisher Chemical; Fisher Scientific, Waltham, MA, USA), and ethanol absolute (EtOH; GRG, Shibuya-ku, Japan; 99.9%; PETROCHEM) were commercially available and were used without further purification.

Synthesis of (C₆H₁₁NH₃Cl): cyclohexylammonium chloride (C₆H₁₁NH₃Cl) was prepared by the direct reaction of 110 mmol of cyclohexylamine (12.6 mL), dissolved in 100 mL of water of 110 mmol of hydrochloric acid (~9.2 mL of 37% HCl), diluted to 100 mL, in an ice bath, was added gradually to the cyclohexylamine solution over a period of 30 min. Colorless crystals of cyclohexylammonium chloride (14.9 g; quantitative yield) were obtained by evaporation of water solvent at room temperature.

Synthesis of (C₆H₁₁NH₃NCS): cyclohexylammonium thiocyanate (C₆H₁₁NH₃NCS) was obtained from the metathesis reaction between cyclohexylammonium chloride and sodium thiocyanate in ethanol medium, as it was described in the literature [26,34,38]. The synthesis of C₆H₁₁NH₃NCS was the second step in our approach. In a typical reaction, a solution of sodium thiocyanate NaSCN (8.107 g; 100 mmol) in EtOH (350 mL) was added to a solution of cyclohexylammonium chloride (C₆H₁₁NH₃Cl;13.563 g; 100 mmol) in EtOH (350 mL) with stirring at room temperature. Afterward, a precipitate of NaCl byproduct was filtered off, while the filtrate was left for slow evaporation at room temperature to obtain the desired product of C₆H₁₁NH₃NCS, which was purified by recrystallization with a yield of 99%.

Synthesis of Ni(NCS)₂: A solution of sodium thiocyanate (1.6214 g; 20 mmol) in EtOH (50 mL) was poured, with stirring, into a solution of Ni(NO₃)₂·6H₂O (2.908 g; 10 mmol) in EtOH (50 mL) at room temperature. Then, a clear, light green ethanolic solution was obtained after the filtration of the precipitated sodium nitrate (NaNO₃) with a yield of 1.68 g (99%).

Synthesis of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O: cyclohexylammonium hexaisothiocyanatonickelate(II) dihydrate was prepared by a ligand addition reaction, where 40 mmol (6.3304 g) of C₆H₁₁NH₃NCS crystals was added to [Ni(NCS)₂]_(EtOH) under continuous magnetic stirring until the completion of the reaction, as it was indicated by the complete dissolution of C₆H₁₁NH₃NCS. Green crystals of the target compound were obtained by the room-temperature evaporation of EtOH over a period of a week. The crystals were purified by dissolving them in the minimum amount of EtOH, followed by filtration of the undissolved white crystals of NaNO₃ (proven by XRD analysis), and then by the room-temperature evaporation of EtOH to give 8.0174 g of the target compound crystals (95% yield). The overall yield of the multistep reactions of the new target compound was 93%. The synthesis procedure is illustrated in Scheme 1.

Our preparation method of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O is straightforward and cost-effective because it is performed at room temperature. On the other hand, thermal methods are required for the preparation of the reported hybrid organic–inorganic compounds of [Ni(NCS)₆]⁴⁻ [7,9]. Moreover, our method gave excellent yield, while the synthesis procedures of [(C₂H₅)₄N]₄[Ni(NCS)₆] [18], [(C₆H₅)₃PMe]₄[Ni(NCS)₆] [10], [(H₃C)₃NH]₄[Ni(NCS)₆] [11], and (BMIM)₄[Ni(NCS)₆] [7] resulted in yields of 36%, 55%, 80%, and 93%, respectively.

2.1. Elemental Microanalysis

A PerkinElmer Series II CHNS/O analyzer was used to quantify carbon, hydrogen, nitrogen, sulfur, and oxygen in the novel organic–inorganic hybrid compound. On the other hand, an Agilent 700 series inductively-coupled plasma-optical emission (ICP-OE) spectroscopy was used to quantify the nickel content.

2.2. Fourier Transform Infrared (FTIR) Spectrophotometry Analysis

A PerkinElmer Spectrum GX was used for FTIR analysis. All samples were ground to a fine powder before they were mixed with 10–15 weight times of FTIR-grade potassium bromide (KBr). The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was recorded in the range of 400–4000 cm⁻¹.

2.3. UV-Visible-Near Infrared Absorbance Spectrophotometry (UV-Vis-NIR)

A Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer was used to record the liquid-state UV-Vis-NIR absorbance spectrum for the new compound in the range of 350–1100 nm.

2.4. Nuclear Magnetic Resonance (NMR) Spectroscopic Analyses

Proton and carbon NMR analyses were carried out on a Joel 600 MHz by using deuterated methanol.

2.5. Single Crystal X-ray Diffraction Measurements

The data collections were carried out at 293K and 120K, on a Bruker D8 Venture diffractometer by using MoK_α radiation. Single crystals of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O, suitable for X-ray diffraction, were selected on the basis of the size and the sharpness of the diffraction spots. Data processing and all refinements were performed with the Jana 2006 program package [54]. A multi-scan-type absorption correction was applied by using SADABS in Apex3 software package Bruker 2012 and the crystal shape was determined with the video microscope.

2.6. Thermal Gravimetric Analysis (TGA)

The Q50 thermal gravimetric analyzer (TGA) was used to study the thermal decomposition of the new compound under two different atmospheres: air and helium. Both TGA analyses were carried out at a 5.0 °C/min ramping rate from room temperature to 800 °C.

3. Results and Discussion

3.1. Elemental Analysis

Satisfactory elemental analyses were obtained for all the elements present. Calculated and found values are compiled in

Table 1. Elemental microanalysis for (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O.

Element	Theoretical (wt/wt%)	Experimental (wt/wt%)
C	42.70	42.57
H	7.17	7.12
N	16.60	16.58
S	22.80	22.65
O	3.79	3.68
Ni	6.95	6.86

. Furthermore, crystal structure determination by single-crystal X-ray diffraction (SCXRD) resulted in the same molecular formula.

3.2. FTIR Analysis

The FTIR spectrum in Figure 1 shows different vibrational frequencies for ν_O—H, ν_N—H, ν_C—H, ν_C=N, ν_C—N, and ν_C=S. These frequencies represent the cation, anion, and crystallization water molecules within the crystal of the synthesized novel, hybrid organic–inorganic compound. The spectrum displays two broad, medium intensity bands at 3555 and 3438 cm⁻¹, which might correspond to the stretching of ν_O—H for the crystallization water molecules. The broadness of these peaks indicates their participation in forming intermolecular hydrogen bonds. This assumption regarding hydrogen bonds was confirmed by a careful analysis of the crystal structure, which was determined by SCXRD. The strong sharp band at 1585 cm⁻¹ could be ascribed to the water bending mode. The strong, broad peaks in the range of 3150–3000 cm⁻¹ might be attributed to the asymmetric stretching vibrations of ν_N—H of the cyclohexylammonium cations. This bathochromic shift in the asymmetric stretch of ν_N—H could be owing to the positively charged nitrogen atom and to the hydrogen bonds with the other moieties in the compound, as it was verified by SCXRD. We could observe the N—H scissoring at 1585 cm⁻¹ (strong), wagging at 1321 and 1282 cm⁻¹ (medium), and twisting at 919 cm⁻¹ (medium). The peak at 2936 cm⁻¹ would be assigned to the asymmetric stretching of ν_C—H, while the peak at 2858 cm⁻¹ might be assigned to the symmetric stretching of ν_C—H of the cyclohexylammonium [55]. The C—H deformation could be ascribed to the bands at 1495 and 1448 cm⁻¹, while its wagging at 1227 cm⁻¹, its twisting at 1120 and 1065 cm⁻¹, its bending at 869 cm⁻¹, and its rocking at 841, 792, and 546 cm⁻¹. The stretching vibrations of ν_C—N might be assigned to the bands at 1034 and 1000 cm⁻¹, while its bending at 495 cm⁻¹ [56,57,58]. The ring deformation could be observed at 1174, 890, and 790 cm⁻¹. On the other hand, the ring breathing could be detected at 956 cm⁻¹. The band at 2235 cm⁻¹ might be an indication for some absorbed atmospheric CO₂ [59,60]. The ligand of N=C=S exhibited strong asymmetric stretching vibrations for ν_N=C at 2091 and 2063 cm⁻¹ [61], which were evidence for the binding of the N-terminal of NCS ligands to Ni(II) ion center, while the splitting of the band at 2091 and 2063 cm⁻¹, circled in Figure 1, might indicate the distortion of the octahedral structure of the anionic complex of [Ni(NCS)6]⁴⁻. On the other hand, the symmetric stretching of the NCS ligand could be observed at 921 cm⁻¹, with weak stretching vibrations for ν_C=S at 774 and 786 cm⁻¹, and bending at 475 cm⁻¹ [3,62]. These observed bands of NCS ligand, shifted towards lower wavenumbers, implied some loss of CN triple bond character (2091 cm⁻¹) and coordination to nickel ion center via N-terminal, while the shift towards higher wavenumbers for the CS bond implied some gain of C=S double bond character [63]. Thus, FTIR spectrophotometry verified the formation of the target hybrid organic–inorganic compound, which was also confirmed by UV-Vis-NIR absorbance and SCXRD.

3.3. UV-Vis-NIR Analysis

Figure 2 shows the UV-Vis-NIR absorbance spectrum of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O, in methanol, at room temperature, as a function of energy and wavenumber. The spectrum exhibited three distinguished peaks at 1.22 eV (9842.25 cm⁻¹; ν₁), 1.97eV (15,898.25 cm⁻¹; ν₂), and 3.17 eV (25,575.45 cm⁻¹; ν₃), where all these peaks were assigned to spin-allowed transitions. The ν₁ peak corresponds to the ³A_2g(F) → ³T_2g(F) transition, the ν₂ to the ³A_2g(F) → ³T_1g(F) transition, and the ν₃ the ³A_2g(F) → ³T_1g(P) transition, as per the Tanabe–Sugano diagram for octahedral d⁸ metal ion center [12,13]. With considering the significance of the interactions between metal and ligand and the separating energy of ~1.0 eV between each two successive absorption bands, we could assign the ν₃ transition to ligand–metal charge transfer (LMCT) because it was the strongest in terms of energy and intensity, ascribe the ν₂ transition to ligand field (d-d) band, and attribute the ν₁ transition to both of d-d and LMCT, where part of LMCT is symmetrically banned for the perfectly octahedral hexaisothiocyanatonickelate(II) and feebly permitted in the real distorted structure of this complex, and hence, ν₁ transition was observed stronger and more intense than ν₂ transition. Furthermore, the ratio between the positions of two successive peaks was ~1.61, indicating the formation of octahedral Ni(II) complex without tetragonal distortion [13]. All these observations, inferred from the UV-Vis-NIR spectrum, confirmed the formation of the anionic complex of [Ni(NCS)₆]⁴⁻ and were in agreement with the above FTIR spectrum and with the crystal structure determination by SCXRD.

3.4. NMR Spectra Analyses

The 1H- and 13C-NMR spectra of the new hybrid organic–inorganic compound of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O are displayed and discussed in the supplementary information (Figures S1 and S2; Tables S1 and S2).

3.5. SCXRD Analyses

At room temperature, the compound (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O crystallizes in the monoclinic crystal system. The extinction conditions observed agree with the P2₁/c space group. Most of the atomic positions were found by using the superflip program. With isotropic atomic displacement parameters (ADPs), the residual factors converged to the value R(F) = 0.27, wR(F²) = 0.5432, and S = 4.95 for 98 refined parameters and 6892 observed reflections. At this stage of the refinement, the chemical formula (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O was not equilibrated. By refining the anisotropic ADPs of all the atoms, the residual factors converged to the value R(F) = 0.1044, wR(F²) = 0.252, and S = 2.31 for 223 refined parameters. The difference-Fourier maps showed very weak residues at distances close to 0.9Å from several carbon, nitrogen, and oxygen atoms and were attributed to the H atoms. Restrictions were applied on their positions and (ADPs) and the extinction parameter was refined. The chemical formula became (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O and the residual factors decreased to the final values, given in the supplementary information. The atomic positions and the anisotropic ADPs are given in the supplementary information, respectively.

Since at room temperature very large ADPs were observed for most of the carbon atoms, a second set of data was collected at 120 K. A large decrease of the cell volume from 2199.45(11) to 2134.77(11) ų was observed without any change in the symmetry. Therefore, the room temperature structure was used as a starting model for the refinement of the low temperature structure. A significant decrease in the ADPs was observed. However, one of the two C₆H₁₄N still showed large carbon ADPs. Consequently, these carbon atoms were split without any constraints on their ADPs and led to the final residual factors, given in

Table 2. Experimental details.

Crystal Data
Chemical formula	(C6H11NH3)4[Ni(NCS)6]·2H2O	(C6H11NH3)4[Ni(NCS)6]·2H2O
Mr	843.9	843.9
Crystal system, space group	Monoclinic, P21/c	Monoclinic, P21/c
Temperature (K)	293	120
a, b, c (Å)	15.8179 (5), 10.6222 (3), 13.8751 (4)	15.6486 (5), 10.4641 (3), 13.7387 (4)
β (°)	109.362 (1)	108.393 (1)
V (Å3)	2199.45 (11)	2134.77 (11)
Z	2	2
Radiation type	Mo Kα	Mo Kα
µ (mm−1)	0.76	0.79
Crystal size (mm)	0.23 × 0.19 × 0.07	0.23 × 0.19 × 0.07
Data collection
Diffractometer	D8 venture diffractometer	D8 venture diffractometer
Absorption correction	Multi-scan SADABS	Multi-scan SADABS
Tmin, Tmax	0.694, 0.747	0.695, 0.746
No. of measured, independent and observed [I > −4σ(I)] reflections	39757, 6892, 6892	31049, 5537, 5537
Rint	0.028	0.026
(sin θ/λ)max (Å−1)	0.722	0.677
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.072, 0.168, 1.54	0.036, 0.096, 1.18
No. of reflections	6892	5537
No. of parameters	230	277
No. of restraints	3	2
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
(Δ/σ)max	0.107
Δρmax, Δρmin (e Å−3)	0.50, −0.39	0.18, −0.23

. The atomic positions and the ADPs are given in the supplementary information (Tables S1–S4). Further details on the structure refinement may be obtained from the Cambridge Crystallographic Data Centre (CCDC), by quoting the Registry No. CCDC 2,131,541 [64].

The unit cell of the title compound (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O consists of two formula units (Z = 2), each of which comprises two water molecules, the anionic complex of hexaisothiocyanatonickelate(II) (the inorganic moiety) and four cyclohexylammonium cations (the organic moiety). Half of the cations show disorder on the carbon atomic positions (see Figure 3). Selected bond lengths and angles are given in

Table 3. Selected angles and bond length of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O.

Bond Length (Å)			Angles Degree
Ni1	N1	2.092	N1	Ni1	N2	86.77
Ni1	N2	2.099	N1	Ni1	N3	89.30
Ni1	N3	2.082	N1	Ni1	N1	180.00
Ni1	N1	2.092	Ni1	N1	C1	146.4
Ni1	N2	2.099	Ni1	N2	C2	153.4
Ni1	N3	2.082	Ni1	N3	C3	156.4
N1	C1	1.155(2)	N1	C1	S1	179.0(1)
N2	C2	1.154(2)	N2	C2	S2	178.6(1)
N3	C3	1.153(2)	N3	C3	S3	178.4(1)
C1	S1	1.630(1)
C2	S2	1.637(1)
C3	S3	1.620(1)

.

The hexaisothiocyanatonickelate(II) anion displayed a regular octahedron geometry. The angle value of 180° was shown perfectly linear for N—Ni—N octahedron axis, while marginal deviations from linearity were shown as 179.0(1), 178.6(1), and 178.4(1)° for the N1—C1—S1, N2—C2—S2, and N3—C3—S3 ligand angle values, respectively. These values were similar to those previously reported for this anionic complex with tetrakis(triethylammonium) cation [8]. Hydrogen bonds among cation, anion, and water molecules were observed. They interconnect these molecules to form a 2D-layered structure, as shown in Figure 4, where the crystal structure view, along the b-axis, illustrates clearly that each layer of the octahedral [Ni(NCS)₆]⁴⁻ is interacting with two layers of (C₆H₁₁NH₃)⁺. All possible hydrogen bonds are listed in

Table 4. List of possible hydrogen bonds at 120K for (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O.

Donor	Hydrogen	Acceptor	D-H Distance, Å	H…A Distance, Å	D-A Distance, Å	A-H…D Angle, °
O1	H1O1	S1	0.866(15)	2.459(19)	3.3043(14)	166(3)
N5	H2N5	N1	0.87	2.35	3.0924(19)	143.14
N5	H2N5	N2	0.87	2.39	3.0791(17)	135.97
N5	H3N5	O1	0.87	1.97	2.8375(15)	172.00
N4	H2N4	N3	0.87	2.31	3.0838(18)	148.52

.

Three types of hydrogen bonds were observed in this crystal structure, as shown in Figure 5. The first type of interaction was the N—H···N and corresponded to the hydrogen bond between ammonium cation and the thiocyanate nitrogen. The second type of interaction was the N—H···O and corresponded to the bond between ammonium cation and a water molecule. The third type of interaction was the O—H···S and corresponded to the bond between a water molecule and thiocyanate sulfur. The second and third hydrogen bonding types were only observed with the equatorial thiocyanate ligands. All these bonds, detected by SCXRD, were in agreement with the observed vibrational bands in the FTIR spectrum.

At room temperature, the single crystal of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O compound showed a strong disorder in the doubly attached ring. For this reason, data were also collected at 120 K. The atomic positions of the disordered structures collected at 293 K are given in Tables S3 and S5, while those collected at 120 K are displayed in Tables S4 and S6. As the low temperature measurement did not suppress the disorder, a theoretical ordered structure of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O was built. All the disordered atoms with partial occupancy were ordered manually in their average positions and considered full occupancy. This enabled us to simulate the X-ray powder diffraction patterns for the disordered and ordered structures of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O by using the “Mercury” program, as shown in Figure 6A,B, respectively.

Figure 7a shows the distribution of all atoms inside the primitive cell except the hydrogen atoms, while Figure 6B gives an example of the disorder in the carbon atoms of cyclohexyl ring. The first pattern, shown in Figure 6A, was generated in the presence of complete disorder and the second one, shown in Figure 6B, was generated by removing only the disorder in the carbon of cyclohexyl ring. Comparing the two simulated patterns in Figure 6A,B revealed that removing the disorder on the doubly attached carbons of the ring either reduced the intensities of most peaks or disappeared them. However, there were some exceptions of the observation, where the peak intensities at 2-θ ~10.8 and 11.7 degrees were enhanced and a new peak was arisen at ~11.25 degrees, as the result of reducing the disorder. As the disorder was enhanced at 293 K, where the XRPD experiment was conducted, we observed that the experimental XRPD pattern (Figure 6C) had more peaks than the simulated one and the two peaks at ~10.8 and 11.7 degrees disappeared. The unit cell of the monoclinic lattice consists of two formula units. The structure of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O at 120 K deviated slightly from the standard of two formula units per primitive cell due to the disorder not only in the doubly attached rings, as shown in Figure 7b, but also in the number of other atoms in the primitive cell, as shown in

Table 5. The number of atoms in the primitive cell for the standard two lattice points per primitive cell case and the disordered one.

Atom	Standard	Disorder
O	4	3
C (radial)	12	12
C (ring)	44	48
Ni	2	2
N	20	20
S	12	12

.

The simulated X-ray diffraction pattern can be used as a reference standard when there is a good match between the simulated pattern of the structure and the experimental one. By comparing the experimental XRPD pattern (Figure 6C) with the simulated ones of Figure 6A or Figure 6B, one can observe that the number of peaks of the experimental pattern exceeded those of the simulated ones due to the disorder in the number of carbon and oxygen atoms, as illustrated in Table 5, which obviously broke the standard of two formula units per primitive cell [26,38]. The more atoms, existing inside the primitive cell, might create different families of Bragg reflection planes, and consequently, generate more X-ray diffraction peaks. In addition, increasing the number of atoms inside the primitive cell produced Bragg reflection planes with smaller interplanar d-spacing or larger diffracted angles 2-theta. This explanation tells us why there were some peaks at the positions larger than 35° in the experimental pattern, while they were absent in the simulated patterns. However, there were some peaks in the low 2-theta region that were absent in the simulated patterns such as the peaks at ~5.8 and 7.7 degrees. These peaks were not because of the presence of impurities due to the reactants of cyclohexylammonium thiocyanate, sodium thiocyanate, and nickel nitrate hexahydrate or the byproduct of sodium nitrate, as shown in Figure 8, which compares the XRD patterns of the reactants, byproduct, and experimental XRPD of our new hybrid organic–inorganic compound. Thus, we excluded the existence of impurities and concluded that the disorder might create new families of Bragg’s reflection planes, which either led to forming new peaks or disappearing some others by constructive and destructive interferences.

3.6. TG Analysis

The TGA showed two different multi-stage decomposition thermograms under air and helium, as shown in Figure 9. About 4.5% weight loss under both atmospheres was observed in the temperature range of 50–112 °C, which could be due to the loss of two crystallization water molecules and the formation of the anhydrous form of the compound, (C₆H₁₁NH₃)₄[Ni(NCS)₆]. The second step, under helium, was observed as a sharp decay in the range of 112–200 °C, accounting for ~70.5% weight loss under helium due to the loss of all the four cations of cyclohexylammonium and three of the isothiocyanate ligands. The third step was observed in the range of 200–322 °C, accounting for ~12% weight loss due to the loss of two isothiocyanate ligands. The fourth step was observed in the range of 322–437 °C accounting for ~2% weight loss under helium due to the loss of CN moiety and the formation of nickel sulfide (NiS) with a remaining weight percentage of 11.0 (calculated 10.75%). The formation of NiS was also confirmed by XRPD.

On the other hand, the second step, under air, represented a loss of one cyclohexylammonium of ~12% in the temperature range of 112–180 °C. The third step could be attributed to the loss of the remaining three cyclohexylammonium (~35.6%) in the temperature range of 180–242 °C. The fourth step corresponded to the loss of two isothiocyanate ligands between 242 °C and 375 °C. The last step could be owing to the loss of the remaining four isothiocyanate ligands and the formation of nickel oxide (NiO) at 550 °C with a remaining weight percentage of ~9.0 (calculated 8.85%). The formation of NiO was confirmed by XRPD, too.

3.7. XRPD Microstructure of NiO and NiS

The XRD pattern, obtained after the pyrolysis of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O at 550 °C under air (Figure 10a), coincided with the XRPD pattern of cubic NiO. The peaks at 2θ = 37.2, 43.3, 62.9, 75.5, and 79.6°, corresponded, respectively, to the crystallographic planes of (111), (200), (220), (511) and (222), as assigned by the (JCPDS card no. 47-1049) [65] (Figure 10a). No impurity peaks were observed in the pattern. Moreover, the relatively sharp XRD peaks indicated the formation of crystalline structure and the peak broadness connoted nanostructures. By applying the Scherrer equation, the crystallite size was calculated:

$$D=\frac{0.9\lambda }{\beta cos\theta }$$

(1)

The symbols λ, θ, and β stand for the wavelength of the Cu K_α (1.5406 Å), the diffraction angle, and the full width at half maximum (FWHM) in radians, respectively. By using the highest intensity peaks of (111), (200), and (220), an average D value of 16.34 nm was obtained. The d-spacing (d), the lattice parameter (a), and the microstrain (ε) were obtained by using Equations (2)–(4), respectively [66].

$$d=\frac{\lambda }{2 sin\theta }$$

(2)

$$d=\frac{a}{\sqrt{h^2+k^2+l^2}}$$

(3)

$$\epsilon =\frac{\beta }{4 tan\theta }$$

(4)

The tabulated values (

Table 6. Crystallite size and lattice parameters of NiO and NiS.

Sample	β	D (nm)	D* (nm)	d (Å)	L. Parameters		ε	ε*
					a	c
NiO	0.5027	16.34	30.80	2.2530	4.180		0.0055	0.0026
NiS	0.2077	53.79	72.98	2.3050	3.421	5.322	0.0021	0.0015

D* (crystallite size) and ε* (microstrain) calculated by the Williamson–Hall method.

) are in agreement with previous data.

The XRD pattern in Figure 10b, obtained after the pyrolysis of (C₆H₁₁NH₃)₄[Ni(NCS)₆]·2H₂O at 500 °C under helium, matched well with the JCPDS reference 01-075-0613 of the rhombohedral α-NiS. The peaks at 2θ = 30.2, 34.7, 45.8, 53.5, 60.7, 62.6, 63.2, 66.6, and 70.5° corresponded to the crystallographic plane of (100), (101), (102), (110), (103), (200), (201), (004), and (202), respectively [67]. The relatively high crystallinity of the synthesized α-NiS nanocrystalline was evidenced by the relatively sharp peaks of the pattern, while their less broadening reflected large size NPs. The average crystallite size (D) was estimated to be 53.79 nm, considering the high intensity peaks: (100), (101), (102) and (110). The lattice parameters a and c were calculated according to the expression [68]:

$$\frac{1}{d_{hkl}^2}=\frac{4}{3}\left(\frac{h^2+hk+k^2}{a^2}\right)+\frac{l^2}{c^2}$$

(5)

To calculate the microstrain of the nanocrystalline NiO and NiS, the uniform deformation model UDM of the Williamson–Hall method, presuming crystals isotropicity, was used, as given in Equation (6) [69]. This method implies that the properties are independent of the crystallographic direction of measurement.

$${\beta }_{hkl}cos{\theta }_{hkl}=\frac{k\lambda }{D}+4\epsilon sin{\theta }_{hkl}$$

(6)

Plotting ${\beta }_{hkl}cos{\theta }_{hkl}$ against $4sin{\theta }_{hkl}$ (Figure 11a,b) provides linear relationship, where the slope and intercept can provide the microstrain and the crystallite size, respectively. For NiO and NiS, the crystallite sizes (D*) were 30.80 and 72.98 nm, respectively, while the microstrain (ε) values were 0.0026 for NiO and 0.0015 for NiS. The difference in the average crystallite sizes, estimated via Scherrer’s and W–H methods can be attributed to the deviation in particle size distribution averaging, as more peaks are considered in the latter [70].

3.8. Morphological Study

Figure 12A portrays the SEM image of slightly aggregated quasi-spherical NiO particles of an almost uniform size of 70 nm [71]. The EDX spectrum (Figure 12B) confirmed the formation of the NiO, as manifested by the sharp peaks at the binding energy of Ni and O. The presence of carbon in the EDX spectrum might be attributed to the carbon tape, used to mount the sample on the SEM holder. Figure 12C shows that the NiS forms flake-like structures. Vijaya et al. [72] attributed the formation of nano-flakes to the electrostatic interactions and van der Waals forces, which caused the nanoparticles to aggregate as nanosheets and then to nanoflakes-like owing to the alteration of nucleation rate. The elemental composition of the NiS (Figure 12D) is portrayed as Ni and S peaks, supporting the formation of NiS with the 1:1 stoichiometry of Ni and S in the nanostructures.

3.9. Porosity Analysis

The N₂ adsorption–desorption measurements, at liquid N₂ temperature of 77 K, were performed to determine the specific surface area and to probe the mesoporosity and the textural characteristics of the fabricated NiO and NiS. The as-recorded adsorption–desorption isotherms are displayed in Figure 13A (NiO) and Figure 13B (NiS), where the isotherms suggested archetypal type-II sorption patterns with type-H₃ hysteresis loops, indicating mesoporous nanostructures as per the IUPAC definition [73], which was additionally substantiated by the Barrett–Joyner–Halenda (BJH) pore size distribution of 7.0 and 10 nm for NiO and NiS, respectively. The prominent step at the adsorption branch combined with the sharp decay of the desorption branch was concrete evidence of mesoporous material [74]. A sudden increase in the adsorbed N₂, perceived at P/P₀ greater than 0.8, is commonly linked to the capillary condensation, designating good sample homogeneity and relatively smaller pore size, as the P/P₀ inflection point is influenced by the pore size. The textural properties of the nanocrystalline mesoporous NiO and NiS samples are provided in

Table 7. Specific surface area and porosity parameters of NiO and NiS.

Sample	SBET (m2/g)	Pore Volume (cm3/g)	Pre Size, (Å)
NiO	451.45	0.472	53.33
NiS	219.92	0.170	45.96

. The results demonstrated that NiO had enhanced porosity, as reflected by larger BET surface area, pore volume, and pore size. A similar dramatically increased porosity of NiO NPs was attributed to the removal of surfactant residues and hydroxyl groups out of the pore system during calcination [75]. For both isotherms, the adsorption and desorption isotherms completely overlapped in the low and intermediate relative pressure ranges for NiO, and the hysteresis loop subsists in the high relative pressure region (P/P₀ > 0.8), which was mainly due to the presence of ink-bottle type and/or slit-shaped pores. Such ink-bottle type of pores possess a larger pore size in the bottle body, leading to the occurrence of hysteresis in the high relative pressure region [76]. On the other hand, no hysteresis loop was observed for NiS, indicating cylindrical closed-end pores, as per the Kelvin equation, which conjectures the similarity of the condensed phase and the uniform bulk liquid at the interface, separating the dense adsorbate region and the gas-like region, when the pore fills as when it empties. This phenomenon results in the coincidence of the desorption branch of open pores with the adsorption branch of the closed-end pores, and thus, no hysteresis loop exists [77].

4. Conclusions

The novel hybrid organic–inorganic compound cyclohexylammonioum hexaisothiocyanatonickelate(II) dihydrate was synthesized at room temperature in four reaction steps with an overall yield of 93%. The title compound has been characterized fully by spectroscopic techniques, the crystal and molecular structures have been determined at 295 and 120 K. High surface area, mesoporous, nanocrystalline nickel oxide, and sulfide were obtained by pyrolysis in air and helium, respectively. These results show that cyclohexylammonioum hexaisothiocynatonickelate(II) is a promising single-source precursor for the facile production of mesoporous, nanocrystallined NiO and NiS for application-oriented research.