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

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 MHyMIICl3 (MII = 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.


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.
In this study, we synthesized new phases of the hybrid chlorides MHyM II Cl 3 that include Cu 2+ and Cd 2+ 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 Cu 2+ and Cd 2+ analogues do not exhibit phase transitions (PTs) when compared to other members of this family of chlorides.

Structural Properties
MHyCuCl 3 adopted monoclinic P2 1 /c symmetry. It is yet another example of hybrid MHyM II Cl 3 compounds with M II = Co 2+ , Ni 2+ , Mn 2+ 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, [CuCl 5 N] octahedra were formed. The octahedra were arranged by edge-sharing, parallel chains propagating along the [010] direction ( Figure 1a). The P2 1 /c phase was isostructural to the low temperature (measured at 100-120 K) phases of Co 2+ , Ni 2+ , and Mn 2+ analogues [30]. All atoms occupied general positions of C 1 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 Co 2+ (0.06 Å), Ni 2+ (0.02 Å), and Mn 2+ (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 Cu 2+ 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 NH 2 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 σ 2 = 20.7 deg 2 and ∆ = 0.1225, respectively. Both values were calculated using the VESTA program [35].
The phase purity of the MHyCdCl3 bulk sample was confirmed by a good match of its PXRD pattern (Rexp = 1.70, Rprof = 6.26, wRprof = 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 (Rexp = 1.52, Rprof = 5.44, wRprof = 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. The phase purity of the MHyCdCl 3 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 MHyCuCl 3 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 (CuCl 2 ·H 2 O) in an amount of about 2%; the peak from the additional phase is marked with an asterisk in Figure 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 MHyCdCl 3 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 MHyCuCl 3 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 MHyCdCl 3 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 MHyCuCl 3 crystal.
To summarize, the total number of expected vibrational modes for MHyCdCl 3 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 MHyCuCl 3 , which includes 165 optical (42A g + 41A u + 42B g + 40B u ) and 3 acoustic. Because g-type modes are only Ramanactive and u-type modes are only IR (infrared)-active, the number of expected bands in the Raman (IR) spectrum of MHyCdCl 3 is 42 (39). These values are 84 and 81 for the MHyCuCl 3 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 IR and Raman spectra of MHyCuCl 3 are qualitatively very similar to those of MHyM II Cl 3 (M II = Co 2+ , Ni 2+ , Mn 2+ ) [30]. The differences involving wavenumber upor downshifts reaching a few cm −1 for MHyCuCl 3 are mainly due to the higher mass, different ionic radius, the strength of HBs, and Jahn-Teller effect activity of Cu 2+ 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 MAPbX 3 (X = Br − , I − ) hybrids [38]. In our previous paper concerning MHyM II Cl 3 (M II = Co 2+ , Ni 2+ , Mn 2+ ), 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 MHyMnCl 3 , 563 cm −1 for MHyCoCl 3 , and at 595 cm −1 for MHyNiCl 3 [30]. Taking into account the IR and Raman spectra of MHyCdCl 3 and MHyCuCl 3 , 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 MHyCuCl 3 and much lower for MHyCdCl 3 . This is not surprising considering  The proposed assignment, presented in Table S2, is based on a comparison with literature sources, including MHyM II Cl3 (M II = Co 2+ , Ni 2+ , Mn 2+ ) [30], 3D and 2D lead halides comprising the MHy + cation [12,29,37], MHyMn(H2PO2)3 [31], and MHyM II (HCOO)3 (M II = Mg 2+ , Fe 2+ , Mn 2+ , and Zn 2+ ) [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 II Cl3 (M II = Co 2+ , Ni 2+ , Mn 2+ ) [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 Cu 2+ ions in an octahedral configuration. The most pronounced variations were observed for the socalled MHy + -cage mode, which can be correlated with the parameter defined as the space available for cation per formula unit VZ [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 II Cl3 (M II = Co 2+ , Ni 2+ , Mn 2+ ), 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 II Cl3 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 NH 2 and NH 2 + groups for the MHyCuCl 3 analogue have very similar energies in comparison to the previously studied MHyM II Cl 3 (M II = Co 2+ , Ni 2+ , Mn 2+ ) series. However, for MHyCuCl 3 , 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 MHyCuCl 3 analogue creates the shortest D···A contacts and thus forms the strongest hydrogen bonds.
As mentioned before, the vibrational spectra of MHyCdCl 3 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 Cd 2+ 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 II Cl 3 group.
Both spectra show the presence of several bands above 1500 nm that corresponds to MHy + overtones and combinations of vibrational modes.  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 II Cl 3 (M II = Cd 2+ , Cu 2+ ) 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 Figure 5 summarizes the results of the magnetic property measurements carried out for MHyCuCl 3 , which are very similar to those obtained for MHyMnCl 3 [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 Cu 2+ ion with the 3d 9 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 Cu 2+ 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 MHyCuCl 3 , 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).

Effect of Cd 2+ and Cu 2+ on the Properties
Cd 2+ and Cu 2+ coordination chemistry significantly differs from Ni 2+ , Co 2+ , and Mn 2+ , 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, Cd 2+ has a d 10 close-shell electronic configuration. Additionally, according to Shannon [45], the ionic radius of Cd 2+ is greater (95 pm) than that of Ni 2+ , Co 2+ , Mn 2+ , and Cu 2+ (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 Cd 2+ and Cu 2+ analogues has previously been observed for other hybrids. Among the AmM II (HCOO)3 (Am + = NH4 + ; M II = Cd 2+ , Co 2+ , Cu 2+ , Fe 2+ , Mg 2+ , Mn 2+ , Ni 2+ , and Zn 2+ ) formates [46], only AmCd(HCOO)3 and AmCu(HCOO)3 members At low temperatures, MHyCuCl 3 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 Mn 2+ , 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 Cu 2+ , respectively.

Effect of Cd 2+ and Cu 2+ on the Properties
Cd 2+ and Cu 2+ coordination chemistry significantly differs from Ni 2+ , Co 2+ , and Mn 2+ , which has a significant influence on the structures of MHyCdCl 3 and MHyCuCl 3 in comparison to previously described compounds. To begin, unlike the other metals discussed, Cd 2+ has a d 10 close-shell electronic configuration. Additionally, according to Shannon [45], the ionic radius of Cd 2+ is greater (95 pm) than that of Ni 2+ , Co 2+ , Mn 2+ , and Cu 2+ (69-83 pm). Because of these features, the metal cation in MHyCdCl 3 is coordinated by six chloride anions, and the MHy + cation is excluded from the first coordination sphere. MHyCdCl 3 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.
A similar outcome effect has been observed for [DMA]M II (HCOO) 3 (DMA + = dimethylammonium; M II = Cd 2+ , Co 2+ , Cu 2+ , Fe 2+ , Mg 2+ , Mn 2+ , Ni 2+ , and Zn 2+ ) 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 MHyCdCl 3 and MHyCuCl 3 , we compared the estimated structural parameters presented in Table 1. As one can see, the tolerance factor (TF) is insufficient to explain this phenomenon, as the values for MHyCdCl 3 and MHyCuCl 3 are comparable to those reported for MHyM II Cl 3 (M II = Mn 2+ , Co 2+ , Ni 2+ ) [30]. A detailed examination of σ 2 and ∆, 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 II Cl 3 (M II = Mn 2+ , Co 2+ , Ni 2+ ) 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 = Mn 2+ , Co 2+ , Ni 2+ , 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 Ni 2+ , Co 2+ , and Mn 2+ , respectively). The substantially elongated [CuCl 5 N] 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 II Cl 3 (M II = Mn 2+ , Co 2+ , Ni 2+ ) in the low-temperature (LT) phase [30]. The inability of Cd 2+ 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 Ni 2+ , Mn 2+ , and Co 2+ transition metal cations.
In order to grow MHyM II Cl 3 (M II = Cu 2+ , Cd 2+ ) crystals, 1 mmol of M II Cl 2 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 MHyCdCl 3 and MHyCuCl 3 , respectively, were harvested from the solution and air-dried.

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 MHyCuCl 3 (monoclinic, P2 1 /c with a = 6.9879 (5) Figure S4. The CIF files of reported structures can be found in the CCDC Database with deposition numbers 2047529 for MHyCdCl 3 and 2047531 for MHyCuCl 3 .
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].

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 MHyCdCl 3 and a nujol suspension for MHyCuCl 3 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.

Magnetic Measurements
Magnetization of randomly oriented single crystals of MHyCuCl 3 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.

Conclusions
The MHyM II Cl 3 (M II = Cd 2+ , Cu 2+ ) hybrid organic-inorganic compounds were synthesized using a conventional technique of crystallization from solution by gradual evaporation. The MHyCuCl 3 analogue crystallizes in the monoclinic P2 1 /c symmetry, according to the single-crystal X-ray diffraction measurement. The structure is similar to previously reported M II = Mn 2+ , Ni 2+ , and Co 2+ structures in that it is made up of edge-sharing 1D chains of the [CuCl 5 N] octahedra running in the [010] direction. MHy + cations in MHyCuCl 3 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 [CuCl 5 N] 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 MHyCuCl 3 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 II Cl 3 class of hybrids. The MHyCdCl 3 compound crystallizes in triclinic P1 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 MHyCdCl 3 crystal, like MHyCuCl 3 , 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 Cd 2+ ion. According to optical investigations, MHyCdCl 3 has a bandgap energy of 5.57 eV.