Hybrid Chlorides with Methylhydrazinium Cation: [CH₃NH₂NH₂]CdCl₃ and Jahn-Teller Distorted [CH₃NH₂NH₂]CuCl₃

Abstract

The synthesis, structural, phonon, optical, and magnetic properties of two hybrid organic-inorganic chlorides with monoprotonated methylhydrazinium cations (CH₃NH₂NH₂⁺, MHy⁺), [CH₃NH₂NH₂]CdCl₃ (MHyCdCl₃), and [CH₃NH₂NH₂]CuCl₃ (MHyCuCl₃), are reported. In contrast to previously reported MHyM^IICl₃ (M^II = Mn²⁺, Ni²⁺, and Co²⁺) analogues, neither compound undergoes phase transitions. The MHyCuCl₃ has a crystal structure familiar to previous crystals composed of edge-shared 1D chains of the [CuCl₅N] octahedra. MHyCuCl₃ crystallizes in monoclinic P2₁/c symmetry with MHy⁺ cations directly linked to the Cu²⁺ ions. The MHyCdCl₃ analogue crystallizes in lower triclinic symmetry with zig-zag chains of the edge-shared [CdCl₆] octahedra. The absence of phase transitions is investigated and discussed. It is connected with slightly stronger hydrogen bonding between cations and the copper–chloride chains in MHyCuCl₃ due to the strong Jahn–Teller effect causing the octahedra to elongate, resulting in a better fit of cations in the accessible space between chains. The absence of structural transformation in MHyCdCl₃ is due to intermolecular hydrogen bonding between two neighboring MHy⁺ cations, which has never been reported for MHy⁺-based hybrid halides. Optical investigations revealed that the bandgaps in Cu²⁺ and Cd²⁺ analogues are 2.62 and 5.57 eV, respectively. Magnetic tests indicated that MHyCuCl₃ has smeared antiferromagnetic ordering at 4.8 K.

1. Introduction

Hybrid organic–inorganic materials (mostly halides) have received a great deal of attention in recent years due to their enormous application potential in the field of high tech industry, particularly as materials for electronic and optoelectronic devices [1,2]. Many of them exhibit controllable optical [3], electric [4], ferroelectric [5,6,7], switchable dielectric [6], and magnetic [8] properties.

Hydrazine is an interesting small inorganic molecule capable of forming an onium or double onium cation that can be incorporated into crystal structures [9,10]. Compounds containing onium hydrazine cations (Hy⁺) are well known and have been the focus of many investigations [9,10]. The degree of methylation of a hydrazine molecule affects its chemical properties and ability to bond in the crystal lattice. Simple organic methylhydrazinium cation (MHy⁺) has just been recognized as an object of interest due to its small enough size to form a three-dimensional (3D) organic–inorganic perovskites [11,12,13,14,15]. Only four organic cations have so far matched the size and shape parameters required to form a 3D perovskite architecture with divalent metal ions. Next to MHy⁺, these include the methylammonium (MA⁺) [16,17,18,19,20], formamidinium (FA⁺) [16,18,20,21,22], and aziridinium (AZ⁺) [23] cations.

There is also a class of halogenide perovskites of larger monovalent metals (M^I = Na, K, Rb, Cs) that may form a 3D network with the general formula AM^IX₃·0.5H₂O, where A stands for bivalent organic cation. In such networks, the bigger dodecahedral gap accommodates larger organic cations, such as dabconium, methyldabconium, 3-aminopyrrolidinium, piperazinium, or methylpiperazinium cations [24,25,26,27].

Recent research has shown that MHy⁺-containing hybrid coordination polymers may also form low-dimensional counterparts, such as layered (2D) [28] or chain (1D) [29,30] architectures. The most intriguing aspect is that the MHy⁺ ligand may interact differently with inorganic metal–ligand subnetworks in the accessible space: (i) organic cation can simply fill the available space and connect with an inorganic network of metal–ligand octahedra [M^IIX₆] through N–H···X (X = oxygen, halide) hydrogen bonds (HBs), as reported in [MHy]M^II(HCOO)₃ (M^II = Mn²⁺, Mg²⁺, Fe²⁺, Zn²⁺) [13], [MHy]Mn(H₂PO₂)₃ [31], or in MHyPbI₃ [29]; (ii) organic cation can be strongly bound with the subnetwork of inorganic octahedra, resulting in additional short N···M^II contacts, as reported in MHyPbX₃ (X = Cl⁻, Br⁻) [14,15]; (iii) one of the N atoms can directly be in the first coordination sphere of the metal, forming non-uniform octahedra of the [M^IIX₅N] type, which has so far only been found for the MHyM^IICl₃ (M^II = Mn²⁺, Co²⁺, Ni²⁺) crystals [30]. The third type of rare coordination has also been found for hybrid coordination polymers with Hy⁺, (Hy)₃MnX₅ (X = Cl⁻, Br⁻) [32], and 1,1,1-trimethylhydrazinium (Me₃Hy⁺) [33] cations.

In this study, we synthesized new phases of the hybrid chlorides MHyM^IICl₃ that include Cu²⁺ and Cd²⁺ ions. We undertook a comprehensive physicochemical examination to determine why the structural features, including coordination type and interactions of MHy⁺ cations with the metal–chloride framework, of those two analogues vary from other known counterparts. The goal of this work is also to understand why Cu²⁺ and Cd²⁺ analogues do not exhibit phase transitions (PTs) when compared to other members of this family of chlorides.

2. Results and Discussion

2.1. Structural Properties

MHyCuCl₃ adopted monoclinic P2₁/c symmetry. It is yet another example of hybrid MHyM^IICl₃ compounds with M^II = Co²⁺, Ni²⁺, Mn²⁺ reported to date [30], in which the terminal N atom of MHy⁺ is a co-creator of M^II first coordination sphere. In other words, [CuCl₅N] octahedra were formed. The octahedra were arranged by edge-sharing, parallel chains propagating along the [010] direction (Figure 1a). The P2₁/c phase was isostructural to the low temperature (measured at 100–120 K) phases of Co²⁺, Ni²⁺, and Mn²⁺ analogues [30]. All atoms occupied general positions of C₁ site symmetry. The Cu–Cl distances were 2.2691(10)–2.815(1) Å, while the Cu–N bond length was equal to 2.061(3) Å. The Cu–Cl distances had a much wider range (~0.55 Å) than their Co²⁺ (0.06 Å), Ni²⁺ (0.02 Å), and Mn²⁺ (0.07 Å) counterparts [30]. Indeed, an axial elongation of the octahedra was observed (Figure S1), pointing out the presence of the Jahn–Teller effect, characteristic of Cu²⁺ compounds with octahedral geometries [34]. The MHy⁺ cations were positionally ordered and anchored in the structure by several N–H···Cl HBs (green dashed lines in Figure 1a,b). Both terminal and middle NH₂ groups interacted with chlorine ion acceptors from neighboring chains, stabilizing the crystal structure in [100] and [001] directions (with donor-acceptor (D···A) distances of 3.426(3) Å and 3.269(3) Å, respectively). The HBs within the chains were also present with D···A distances of 3.157(3)–3.631(3) Å. The intermolecular interactions lead to angular and (combined with Jahn–Teller effect) bond length distortion of the octahedra, as indicated by octahedral angle variance and bond length distortion values of σ² = 20.7 deg² and Δ = 0.1225, respectively. Both values were calculated using the VESTA program [35].

The second newly obtained compound reported herein, i.e., MHyCdCl₃, crystallized in triclinic, centrosymmetric P$\overline{1}$ symmetry. The motif of MHyCdCl₃ consisted of inorganic [CdCl₃⁻]_∞ double chains propagating along the [100] direction, separated by the MHy⁺ cations. All atoms adopted C₁ site symmetry. The chains were composed of edge-sharing octahedra with Cd–Cl distances of 2.523(1)–2.712(1) Å. The MHy⁺ cations were anchored via N–H···Cl (Figure 1c) and N–H···N (Figure 1d) HBs with D···A distances of 3.168(4)–3.384(4) and 2.975(5) Å, respectively. It is worth noting that both the N1 and N2 atoms of MHy⁺ were engaged in creating a 3D network of HBs. An interplay between inorganic and organic constituents affected the octahedra symmetry, expressed in σ² and Δ values of 17.8 deg² and 0.021, respectively. Analogous atomic alignment, i.e., with 1D edge-sharing double chains, was reported for MHyPbI₃ [29]. Higher symmetry of MHyPbI₃ (monoclinic, P2₁/c) is associated with the presence of larger metal cations and halide anions, which leads to enlarged interatomic distances and, therefore, weaker HBs and less distorted octahedra (σ² = 6.32 deg²). Analogous alignment was also reported for [C₃H₇N₂S]CdCl₃, where C₃H₇N₂S⁺ is the 2-amino-4,5-dihydro-3H⁺-1,3-thiazolium cation [36].

The phase purity of the MHyCdCl₃ bulk sample was confirmed by a good match of its PXRD pattern (R_exp = 1.70, R_prof = 6.26, wR_prof = 11.22, GOF = 6.6) with the simulated one based on the single crystal structure (Figure 2). The measured PXRD pattern of the MHyCuCl₃ bulk sample was also in good agreement (R_exp = 1.52, R_prof = 5.44, wR_prof = 9.85, GOF = 6.5) with the calculated one based on the single-crystal data. The Pawley refinement method was used to obtain the fitted profiles. The PXRD analysis results also revealed the negligible presence of another phase (CuCl₂·H₂O) in an amount of about 2%; the peak from the additional phase is marked with an asterisk in Figure 2.

2.2. Phonon Properties

Table S1 defines and lists the 24 internal (13A′ + 11A″) and 6 external (3A′ + 3A″) vibrational modes of the free MHy⁺ ion with C_s symmetry. The presence of two MHy⁺ ions in the primitive cell doubles the number of modes corresponding to MHy⁺ with the factor group symmetry C_i in the triclinic MHyCdCl₃ crystal with Z = 2. As a result, the internal and external modes are increased to 48 (24A_g + 24A_u) and 12 (6A_g + 6A_u), respectively. Since the number of modes corresponding to MHy⁺ cations with the C_2h symmetry is increased by 4 times in the monoclinic MHyCuCl₃ crystal with Z = 4, the number of modes corresponding to MHy⁺ cations with the C_2h symmetry is 96 (24A_g + 24A_u + 24B_g + 24B_u) and 24 (6A_g + 6A_u + 6B_g + 6B_u), respectively. Similar considerations apply to metal cations M^II and chloride ligands Cl⁻, which have 6 (3A_g + 3A_u) and 18 (9A_g + 9A_u) modes in the triclinic MHyCdCl₃ crystal, respectively, and 12 (3A_g + 3A_u + 3B_g + 3B_u) and 36 (9A_g + 9A_u + 9B_g + 9B_u) modes in the monoclinic MHyCuCl₃ crystal.

To summarize, the total number of expected vibrational modes for MHyCdCl₃ is 84 (42A_g + 42A_u), which includes 81 optical (42A_g + 39A_u) and 3 acoustic (3A_u), and 168 (42A_g + 42A_u + 42B_g + 42B_u) for MHyCuCl₃, which includes 165 optical (42A_g + 41A_u + 42B_g + 40B_u) and 3 acoustic. Because g-type modes are only Raman-active and u-type modes are only IR (infrared)-active, the number of expected bands in the Raman (IR) spectrum of MHyCdCl₃ is 42 (39). These values are 84 and 81 for the MHyCuCl₃ analogue, respectively. The number of observed bands in the room-temperature (RT) spectra is lower than expected (Figure 3, Table S2). This effect is caused by the overlapping of closely spaced bands caused by the low factor group (Davydov) splitting.

The proposed assignment, presented in Table S2, is based on a comparison with literature sources, including MHyM^IICl₃ (M^II = Co²⁺, Ni²⁺, Mn²⁺) [30], 3D and 2D lead halides comprising the MHy⁺ cation [12,29,37], MHyMn(H₂PO₂)₃ [31], and MHyM^II(HCOO)₃ (M^II = Mg²⁺, Fe²⁺, Mn²⁺, and Zn²⁺) [13], as well as density functional calculations (DFT) calculations performed for the MHy⁺ ion [31]. The majority of IR and Raman bands corresponding to the internal vibrations of MHy⁺ are observed in typical ranges as reported in the literature.

The IR and Raman spectra of MHyCuCl₃ are qualitatively very similar to those of MHyM^IICl₃ (M^II = Co²⁺, Ni²⁺, Mn²⁺) [30]. The differences involving wavenumber up- or downshifts reaching a few cm⁻¹ for MHyCuCl₃ are mainly due to the higher mass, different ionic radius, the strength of HBs, and Jahn–Teller effect activity of Cu²⁺ ions in an octahedral configuration. The most pronounced variations were observed for the so-called MHy⁺-cage mode, which can be correlated with the parameter defined as the space available for cation per formula unit V_Z [30]. This mode was described as a torsional mode with a strong sensitivity to ligand type due to coupling with the inorganic cage via HBs bonds [37], which is similar to the behavior of the MA⁺-cage mode, which has been initially reported for the MAPbX₃ (X = Br⁻, I⁻) hybrids [38]. In our previous paper concerning MHyM^IICl₃ (M^II = Co²⁺, Ni²⁺, Mn²⁺), the MHy⁺-cage mode clearly correlates to the ionic radius of M^II and metal electronegativity, i.e., it was observed as an IR band at 515 cm⁻¹ for MHyMnCl₃, 563 cm⁻¹ for MHyCoCl₃, and at 595 cm⁻¹ for MHyNiCl₃ [30]. Taking into account the IR and Raman spectra of MHyCdCl₃ and MHyCuCl₃, we assigned the MHy⁺-cage mode to IR bands at 386 and 606 cm⁻¹, as well as Raman bands at 389 and 614 cm⁻¹, respectively (see Table S2). The correlation of these values with the ionic radius is higher for MHyCuCl₃ and much lower for MHyCdCl₃. This is not surprising considering that MHy⁺ is not a direct ligand for the metal cation in the MHyM^IICl₃ series. As a result, the torsional vibrational energy is predicted to differ.

Another interesting feature that can be analyzed using vibrational spectroscopy is the strength of HBs. The positions of IR and Raman bands corresponding to stretching vibrations of both NH₂ and NH₂⁺ groups for the MHyCuCl₃ analogue have very similar energies in comparison to the previously studied MHyM^IICl₃ (M^II = Co²⁺, Ni²⁺, Mn²⁺) series. However, for MHyCuCl₃, most of the bands observed above 3000 cm⁻¹ have the lowest positions among these four isostructural analogues. It proves our SCXRD analysis, showing that the MHyCuCl₃ analogue creates the shortest D···A contacts and thus forms the strongest hydrogen bonds.

As mentioned before, the vibrational spectra of MHyCdCl₃ differ most strongly in this region, suggesting that the HB network has different properties. Crystallographic data showed that the MHy⁺ cations do not enter the first coordination sphere of Cd²⁺ ions, therefore, they have more freedom in the metal–chloride framework. This leads to the formation of direct, short, and strong N–H···N HBs between MHy⁺ cations, which has never been reported for an MHy⁺ cation so far. We think that these stronger contacts are responsible for the formation of the broad bands observed in spectra above 2750 cm⁻¹, which are absent for other representatives of the MHyM^IICl₃ group.

2.3. Optical Properties

The diffuse reflectance spectra of MHyM^IICl₃ (M^II = Cd²⁺, Cu²⁺) are shown in Figure 4. In the low-wavelength range, MHyCdCl₃ exhibits an intense absorption band with a maximum at 212 nm and a weaker band at 275 nm, similar to FACd(H₂PO₂)₃ (FA⁺ = formamidinium) [39] and [TPrA]Cd(dca)₃ (TPrA⁺ = tetrapropylammonium, dca⁻ = dicyanamide) [40]. MHyCuCl₃ has a much wider low-wavenumber band, with maxima at 249, 330, 379, and 410 nm previously assigned to the ligand-to-metal charge transfer from Cl⁻ ligand to Cu²⁺ centers for (C₅H₁₄N₂)[CuCl₄] (C₅H₁₄N₂²⁺ = diprotonated 1,4-diazacycloheptane), (N(CH₃)₄)CdX₃:Cu²⁺ (N(CH₃)₄⁺ =tetramethylammonium, X = Cl⁻, Br⁻), and MA₂CuCl_xBr_4-x (MA⁺ = methylammonium) [41,42,43]. The diffuse reflectance spectrum of MHyCuCl₃ also has a very broad absorption band with a maximum of about 850 nm, which is caused by Cu²⁺ d-d transitions in the D_4h coordination centers [41,43,44]. The presence of this band for MHyCuCl₃, which is composed of CuCl₆ octahedra, indicates a strong Jahn–Teller effect causing octahedra elongation, as confirmed by SCXRD studies (see Figure S1).

Both spectra show the presence of several bands above 1500 nm that corresponds to MHy⁺ overtones and combinations of vibrational modes.

The diffuse reflectance spectra of MHyM^IICl₃ (M^II = Cd²⁺, Cu²⁺) were further recalculated to determine the bandgap energy using the Kubelka–Munk function F(R) = (1 − R)²/2R, where R is the reflectance. The results are presented in Figure S2, and the obtained bandgap energies are 2.62 and 5.57 eV for MHyCuCl₃ and MHyCdCl₃, respectively. The obtained values are within the ranges reported previously for (C₅H₁₄N₂)[CuCl₄] (2.56 eV) [41], FACd(H₂PO₂)₃ (5.42 eV) [39], and [TPrA]Cd(dca)₃ (5.02 eV) [40].

2.4. Magnetic Properties

Figure 5 summarizes the results of the magnetic property measurements carried out for MHyCuCl₃, which are very similar to those obtained for MHyMnCl₃ [30]. As can be inferred from panel (a), the compound with copper exhibited paramagnetic behavior in almost the entire temperature range studied, and its magnetic susceptibility χ(T) could be described by the Curie–Weiss law χ(T) = C/(T − θ_p) down to a few kelvins with the least-squares fitting parameters C = 0.42(2) emu mol⁻¹ K (Curie constant) and θ_p = −1.7(4) K (Curie–Weiss temperature); see the thick solid line in Figure 5a. The effective magnetic moment μ_eff derived from the Curie constant C was about 1.83(1) μ_B, which was lower than μ_eff = 3.55 μ_B expected for a free Cu²⁺ ion with the 3d⁹ electron configuration (i.e., for S = 1/2, L = 2, J = 5/2, g_J = 1.2) and only slightly higher than the spin-only magnetic moment of 1.73 μ_B (i.e., calculated for S = 1/2, L = 0, J = 1/2, g_J = 2), suggesting a non-negligible, yet still very small, orbital contribution to μ_eff. The estimated value of the magnetic moment was, in turn, very close to the averaged experimental value of μ_eff reported for paramagnetic salts containing non-interacting Cu²⁺ ions, i.e., 1.83 μ_B. Moreover, the fitted value of C was nearly the same as the RT value of the product χT, i.e., 0.41 emu mol⁻¹ K, and the negative sign of θ_p, which indicated the presence of antiferromagnetic correlations in MHyCuCl₃, was in full agreement with the concave shape of the χT(T) curve observed down to the lowest temperature studied (see Figure 5a, right axis).

At low temperatures, MHyCuCl₃ exhibited a broad anomaly in the temperature variation of the magnetic susceptibility with a maximum at T* = 4.8 K (Figure 5b), being very similar in shape to that found in its counterpart with Mn²⁺, yet much smaller [30]. In addition, here, χ(T) was independent of the magnetic field H (at least in low fields) and showed no difference between the curves taken in the ZFC and FC regimes, which suggests antiferromagnetic ordering. The antiferromagnetic-like character of the observed anomaly was confirmed by the linear field dependence of the magnetization (Figure 5c,d), exhibiting only a small change in its slope of about 3 kOe. In the highest field applied (70 kOe), magnetization M is far from any saturation and achieves a value corresponding to only 0.45 μ_B, which is much smaller than μ_ord = 3 μ_B and 1 μ_B expected for full and spin-only magnetic moment of Cu²⁺, respectively.

2.5. Effect of Cd²⁺ and Cu²⁺ on the Properties

Cd²⁺ and Cu²⁺ coordination chemistry significantly differs from Ni²⁺, Co²⁺, and Mn²⁺, which has a significant influence on the structures of MHyCdCl₃ and MHyCuCl₃ in comparison to previously described compounds. To begin, unlike the other metals discussed, Cd²⁺ has a d¹⁰ close-shell electronic configuration. Additionally, according to Shannon [45], the ionic radius of Cd²⁺ is greater (95 pm) than that of Ni²⁺, Co²⁺, Mn²⁺, and Cu²⁺ (69–83 pm). Because of these features, the metal cation in MHyCdCl₃ is coordinated by six chloride anions, and the MHy⁺ cation is excluded from the first coordination sphere. MHyCdCl₃ has a triclinic structure, with the MHy⁺ cations occupying the voids in the metal–chloride framework. Furthermore, MHy⁺ cations are stacked in chains that are linked by stronger intermolecular HB.

The distinct behavior of Cd²⁺ and Cu²⁺ analogues has previously been observed for other hybrids. Among the AmM^II(HCOO)₃ (Am⁺ = NH₄⁺; M^II = Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, and Zn²⁺) formates [46], only AmCd(HCOO)₃ and AmCu(HCOO)₃ members adopt the orthorhombic symmetry, which is lower compared to other hexagonal family members. Furthermore, as the sole representative, the AmCd(HCOO)₃ crystal exhibits no PT and adopts a perovskite-like topology [47] in contrast to other members, which have chiral-like crystal architecture.

A similar outcome effect has been observed for [DMA]M^II(HCOO)₃ (DMA⁺ = dimethylammonium; M^II = Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, and Zn²⁺) formates [46], where only [DMA]Cu(HCOO)₃ and [DMA]Cd(HCOO)₃ do not undergo PTs. However, the structural symmetry of [DMA]Cd(HCOO)₃ is hexagonal in this example, as it is for all members except for [DMA]Cu(HCOO)₃, which crystallizes in the lower orthorhombic phase. In this case, the lack of PT has been attributed to the large size of the cavity occupied by DMA⁺ and weak HB contacts between cations and the cadmium-formate framework [48].

To determine the reason for the absence of PTs in the MHyCdCl₃ and MHyCuCl₃, we compared the estimated structural parameters presented in

Table 1. Comparison of tolerance factors (TFs), octahedral angle variance σ², and bond length distortion Δ for MHyM^IICl₃.

MII	TF	Δ	σ2 (deg2)
Mn2+ [30]	1.16	0.0240	32.093
Co2+ [30]	1.23	0.0311	18.368
Ni2+ [30]	1.26	0.0350	9.569
Cu2+	1.24	0.1225	20.661
Cd2+	1.14	0.0210	17.839

. As one can see, the tolerance factor (TF) is insufficient to explain this phenomenon, as the values for MHyCdCl₃ and MHyCuCl₃ are comparable to those reported for MHyM^IICl₃ (M^II = Mn²⁺, Co²⁺, Ni²⁺) [30]. A detailed examination of σ² and Δ, which characterize the framework flexibility, reveals that the Jahn–Teller effect prevents the emergence of PT. This elongation also causes the shortest Cu–Cu distances between octahedral chains propagating along the [100] and [001] directions to be 6.988(1) and 6.505(1) Å, respectively. The M^II–M^II distances or MHyM^IICl₃ (M^II = Mn²⁺, Co²⁺, Ni²⁺) are comparable along the [001] direction (6.489(1)–6.580(1) Å) but much longer along the [100] direction (7.156(1)–7.338(1) Å) [30]. The closest Cu–Cu distances (3.601(1)–3.661(1) Å) along the chain are comparable or slightly higher than the M^II–M^II distances reported for M^II = Mn²⁺, Co²⁺, Ni²⁺, which ranged from 3.455(1) to 3.630(1) Å [30]. Furthermore, among the other members, the Cu–N bond is the shortest (2.061(3) Å) and contributes the most covalent bonding (2.116(3), 2.194(2), and 2.365(2) Å for Ni²⁺, Co²⁺, and Mn²⁺, respectively). The substantially elongated [CuCl₅N] octahedra change the available space for MHy⁺ cations and allow them to fit better in the accessible void, resulting in a slightly stronger network of HBs. This alignment in the metal-halide network might possibly be due to the ordered state of the MHy⁺ cations, which has been reported for MHyM^IICl₃ (M^II = Mn²⁺, Co²⁺, Ni²⁺) in the low-temperature (LT) phase [30].

The inability of Cd²⁺ ions to bind MHy⁺ also makes these cations better fit into the existing spaces in the network and can get close enough to form a HB between two adjacent cations. The existence of this unique contact, which is stronger than the HBs normally formed between organic cations and ligands, efficiently prevents cation disorder and allows the crystal to adopt lower triclinic symmetry with double edge-connected zig-zag octahedral chains, which is impossible for Ni²⁺, Mn²⁺, and Co²⁺ transition metal cations.

3. Materials and Methods

3.1. Synthesis

Methylhydrazine (98%, Sigma-Aldrich, Saint Louis, MS, USA), hydrochloric acid (35–38%, Avantor Performance Materials, Gliwice, Poland), cadmium(II) (98%, Sigma-Aldrich, Saint Louis, MS, USA), and copper(II) (98%, Sigma-Aldrich, Saint Louis, MS, USA) chlorides were obtained commercially and used without additional purification.

In order to grow MHyM^IICl₃ (M^II = Cu²⁺, Cd²⁺) crystals, 1 mmol of M^IICl₂ was digested in hydrochloric acid. The solution was then dropwise treated with methylhydrazine (1.5 mmol, 0.2 mL). The resulting mixture was left undisturbed at RT in order to slowly evaporate the solvent. After 7–30 days, the colorless and green crystals of MHyCdCl₃ and MHyCuCl₃, respectively, were harvested from the solution and air-dried.

3.2. Single-Crystal and Powder X-ray Diffraction

Single-crystal X-ray diffraction (SCXRD) experiments were carried out with MoKα radiation using an Xcalibur four-circle diffractometer (Oxford Diffraction, Abingdon, UK), an Atlas CCD detector, and graphite-monochromated MoKα radiation. Absorption was corrected by multi-scan methods using CrysAlis PRO 1.171.39.46 (Rigaku Oxford Diffraction, 2018, Tokyo, Japan). Empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was applied. Hydrogen atoms were initially placed based on the local geometry and refined using a riding model. The crystal structure was solved in Olex2 1.5 [49] using SHELXT-2014/4 [50] and refined with SHELXL-2018/3 [51]. The unit cell of MHyCuCl₃ (monoclinic, P2₁/c with a = 6.9879(5) Å, b = 7.2032(5) Å, c = 13.0105(9) Å, β = 96.022(7)°, V = 651.3(1) ų, Z = 4) was chosen with respect to the analysis of diffraction pattern and systematic extinction rules (Figure S3). MHyCdCl₃ (triclinic, P$\overline{1}$ with a = 3.8660(1) Å, b = 9.3519(9) Å, c = 10.0790(3) Å, α = 106.146(6)°, β = 90.080(3)°, γ = 93.734(5)°, V = 349.21(4) ų, Z = 2) was treated as a two-domain non-merohedral twin with twin fraction (BASF) equal to 0.5358(11). Experimental details and selected geometric parameters are presented in Tables S3–S5. The main components of the crystal structures of both compounds are presented in Figure S4. The CIF files of reported structures can be found in the CCDC Database with deposition numbers 2047529 for MHyCdCl₃ and 2047531 for MHyCuCl₃.

Powder X-ray diffraction (PXRD) experiments were performed in the reflection mode on a PANalytical X’Pert diffractometer (Almelo, The Netherlands) equipped with a a PIXcel solid-state linear detector using Ni filtered CuKα radiation (λ = 1.54184 Å). The X-ray diffraction patterns were generated at 30 mA and 40 kV. For the processing of the PXRD data, the program X’Pert High Score Plus (PANalytical, Almelo, The Netherlands) was involved [52].

3.3. Spectroscopic Measurements

IR spectra in the range of 4000–400 cm⁻¹ (mid-IR) were measured using a Nicolet iS50 infrared spectrometer (Waltham, MA, USA) using a KBr pellet for MHyCdCl₃ and a nujol suspension for MHyCuCl₃ due to the reactivity of the compound with KBr. The far-IR spectra in the 400–50 cm⁻¹ were measured as a nujol suspension on the polyethylene plate for both compounds. The spectral resolution was set to 2 cm⁻¹.

Raman spectra in the 3500–50 cm⁻¹ range with 2 cm⁻¹ resolution were measured using a Bruker FT MultiRAM spectrometer (Billerica, MA, USA) equipped with the YAG:Nd laser operating at 1064 nm.

The absorption spectra in the back-scattering mode in the UV-VIS range were measured using an Agilent Cary 5000 spectrophotometer (Santa Clara, CA, USA) equipped with a PryingMantis™ diffuse reflectance accessory.

3.4. Magnetic Measurements

Magnetization of randomly oriented single crystals of MHyCuCl₃ was measured using a commercial Quantum Design MPMS XL magnetometer (San Diego, CA, USA) from RT down to 2 K and in applied magnetic fields up to 70 kOe. The diamagnetic background coming from a sample holder was found to be weak and negligible in comparison to the signal coming from the samples; hence its subtraction was omitted. Moreover, no diamagnetization corrections were made to the data reported here.

4. Conclusions

The MHyM^IICl₃ (M^II = Cd²⁺, Cu²⁺) hybrid organic–inorganic compounds were synthesized using a conventional technique of crystallization from solution by gradual evaporation. The MHyCuCl₃ analogue crystallizes in the monoclinic P2₁/c symmetry, according to the single-crystal X-ray diffraction measurement. The structure is similar to previously reported M^II = Mn²⁺, Ni²⁺, and Co²⁺ structures in that it is made up of edge-sharing 1D chains of the [CuCl₅N] octahedra running in the [010] direction. MHy⁺ cations in MHyCuCl₃ are effectively coordinated by metal ions, as in the three analogues mentioned. MHyCuCl_3, as a single representative, on the other hand, experiences no PT, and the MHy⁺ cations are ordered at RT. The absence of PTs was attributed to the significant Jahn–Teller effect, which was supported by single-crystal X-ray diffraction and diffuse reflectance measurements. The elongation of the [CuCl₅N] octahedra results in a better fit of organic cations in the accessible space as well as a considerably stronger network of HBs that prevent the disorder. Magnetic measurements revealed that MHyCuCl₃ exhibits only smeared antiferromagnetic ordering at roughly 4.8 K and no ferromagnetic correlations up to the highest field investigated. Optical studies confirmed a strong Jahn–Teller distortion and revealed a bandgap of 2.62 eV for this material.

The cadmium analogue differs significantly from the other representatives in the MHyM^IICl₃ class of hybrids. The MHyCdCl₃ compound crystallizes in triclinic $P\overline{1}$ symmetry, and its crystals are comprised of double zig-zag chains that propagate along the [100] direction. The MHy⁺ cations are ordered at RT, and the MHyCdCl₃ crystal, like MHyCuCl₃, does not experience PTs. In this case, the absence of structural transition was attributed to the presence of a unique HB contact between two neighboring MHy⁺ cations, which inhibits cation disorder. This type of contact is possible because, uniquely in this case, the organic cations are not included in the first coordination sphere of the Cd²⁺ ion. According to optical investigations, MHyCdCl₃ has a bandgap energy of 5.57 eV.