NQR and X-ray crystal structure studies of cadmium halide complexes: [C(NH₂)₃]CdI₃ and [4-ClC₆H₅NH₃]₃CdBr₅

2.1 Crystal and molecular structure of [C(NH₂)₃]CdI₃ (1)

The crystal structure determination was carried out at 100 K, and the crystal structure data are listed in Table 1. The projection of the unit cell of 1 down the a axis is shown in Fig. 1. There are three non-equivalent iodine atoms in the crystal structure: the terminal I1 and I2 atoms and a bridging I3 atom. The Cd atom is coordinated tetrahedrally with four I atoms. The CdI₄ tetrahedron is severely distorted with the distances of 274.95(5), 274.21(5), 279.36(5), and 285.68(5) pm for Cd–I1, Cd–I2, Cd–I3, and Cd–I3′, respectively (Table 2). The iodocadmate anions form a single chain with the bridging I3 atoms, which runs through the corners and the center of the ac plane in the c direction. As a characteristic of the chain, the [CdI₄] tetrahedra are connected with each other via two corners. The hydrogen bonding scheme between cations and anions is shown in Fig. 2. The short contact distances are listed in Table 3. The structure of guanidinium cation is normal, and its geometrical values are not included in the table. The cations hydrogen-bonded to the anions are arrayed to bundle the anions so that the repeating unit of the chain c/2 is nearly equal to the size of the cation. This structure is different from those of [C(NH₂)₃]CdBr₃ [9] and [C(NH₂)₃]HgI₃ [8], which take a type 4 structure, and also different from type 5 of [i-(CH₃)₂CHNH₃]CdBr₃ [4] with the same composition described above.

Fig. 1:

Projection of the crystal structure of 1 down the crystallographic a axis.

(Top) Displacement ellipsoids (H atoms with arbitrary radii). (Bottom) Polyhedral representation of the Cd coordination spheres.

Fig. 2:

Hydrogen bond scheme around a [CdI₄] chain in 1.

Possible N–H···I hydrogen bonds are shown by thin lines.

2.2 NQR measurement of [C(NH₂)₃]CdI₃ (1)

Compound 1 showed three ¹²⁷I(m = ±1/2 ↔ ±3/2) NQR lines with the same intensity throughout the observed temperature range between 77 and ca. 350 K as depicted in Fig. 3. This is coincident with the X-ray crystal structure, in which three inequivalent I atoms of two terminal and one bridging type exist. Meanwhile, [C(NH₂)₃]CdBr₃ [9], [C(NH₂)₃]HgBr₃ [8], and the room temperature phase of [C(NH₂)₃]HgI₃ [8] show two NQR lines with the intensity ratio of 1:2 due to one terminal and two equivalent bridging atoms, respectively. Although [C(NH₂)₃]HgI₃ has three NQR lines in the low-temperature phases [8], one terminal and two bridging I atoms exist, showing the different anion structures from 1. For the temperature-dependence curves of NQR frequencies no anomalies are recognized, showing that no phase transitions occur.

Fig. 3:

Temperature dependence of ¹²⁷I (m = ±1/2 ↔ m = ±3/2) NQR frequencies for 1.

The NQR line ν₃ is sensibly lowered by ca. 10 MHz from the average frequency between ν₁ and ν₂ with the separation of ca. 4 MHz. This fact shows that ν₃ can be assigned to the bridging I3 atom because with the angle of Cd–I3···Cd′ = 99.460(16)° (at 100 K: near 90°), the observed NQR frequency is lowered significantly from the ideal value for a Cd–I3 bond owing to the formation of the secondary bond I3···Cd′. For further assignment of the NQR resonance lines, we may consider another secondary bonding, N–H···I–Cd, which also has a possible lowering effect on the frequency of the primary bond, although its effect is usually smaller than that of secondary Cd···I bonding. Table 3 shows that both I1 and I2 atoms take part in hydrogen bonding, whereas the I3 atom does not. However, it seems difficult to do the assignment of ν₁ and ν₂ to I1 or I2 atoms because both atoms are affected by the hydrogen bonds to almost the same extent as that considered in the parameters listed in Table 3. The MO calculations of a [Cd₅I₁₆]^6– moiety as described in the next section shows that the ¹²⁷I (m = ±1/2 ↔ ±3/2) NQR frequencies are in the order of ν(I1) > ν(I2) > ν(I3); the calculated NQR frequencies are about half of the observed ones [10].

2.3 MO calculations for the [Cd₅I₁₆]^6– moiety as the model for the crystal structure of [C(NH₂)₃]CdI₃ (1)

Details of the MO calculations are described in the Experimental Section. In the calculations, the crystal field effect was not considered, and the bridging I3 and I3′ atoms are not identical. The ¹²⁷I (m = ±1/2 ↔ m = ±3/2) NQR frequencies and asymmetric parameters η are calculated as follows: 58.70 MHz, 0.05 for I1; 49.73 MHz, 0.01 for I2; 36.17 MHz, 0.22 for I3; 38.57 MHz, 0.35 for I3′. The calculated frequencies are about half of the observed values. Meanwhile, the angles between the z principal axes of the EFGs are mostly in good agreement with the bond angles of the crystal structure as listed in Table 4. It is interesting that the direction of the z axis of the I3 atom is normal to the Cd–I3···Cd′ plane, as observed in the case of the z axis of the bridging Br atom in Al₂Br₆ [11]. Further, the angle between the z axes of the I3 and I3′ atoms is in agreement with the torsion angle of Cd–I3···Cd′–I3′, 114.20°, as listed in Table 4.

2.4 Crystal and molecular structure of [4-ClC₆H₅NH₃]₃CdBr₅ (2)

The crystal structure determination was carried out at 100 K, and the crystal structure data are included in Table 1. A view of the crystal structure down the crystallographic a axis is shown in Fig. 4. The anion structure of 2 is the same as that in [H₂NNH₃]₃CdBr₅ (type 2), that is, a single zigzag chain of cis corner-sharing [CdBr₆] octahedra, running along the b axis at x = 1/2, z = 1/4, and x = 1/2, z = 3/4, as shown in Fig. 4, with five inequivalent Br atoms Br1 to Br5. The bridging bond lengths of 285.98(3) and 291.07(3) pm for Cd–Br3 and Cd–Br3′, respectively, are longer by ca. 20 pm compared to the terminal bond lengths of 273.76(3), 273.11(3), 269.26(3), and 272.22(3) pm for Cd–Br1, Br2, Br3, and Br5, respectively. The structures of the three inequivalent 4-ClC₆H₄NH₃⁺ cations are all normal, and their geometrical values are not listed in the table. The molecular planes of all cations are situated nearly normal to the bc plane at x = 0. The cations form organic double layers in which the chlorine end of the cation is directed toward the middle of the double layer and the NH₃ end toward the anion chains, as shown in Fig. 5. The chain structure of the anions is sustained by the N–H···Br hydrogen bonds between cations and anions, as shown in Fig. 6, with N–H···Br contacts listed in Table 3.

Fig. 4:

Projection of the crystal structure of 2 down the crystallographic a axis.

(Top) Displacement ellipsoids (H atoms with arbitrary radii). (Bottom) Polyhedral representation of the Cd coordination spheres.

Fig. 5:

Projection of the unit cell of 2.

Fig. 6:

Hydrogen bond scheme around a [CdBr₆] chain in 2.

Possible N–H···Br hydrogen bonds are shown by thin lines.

2.5 NQR measurement of [4-ClC₆H₅NH₃]₃CdBr₅ (2)

Compound 2 shows five ⁸¹Br (ν₁ to ν₅) and three ³⁵Cl (ν₆ to ν₈) NQR lines, in accordance with the crystal structure, in the temperature range between 77 and ca. 295 K, as depicted in Fig. 7. The continuous changes of the ν-vs.-T curves indicate that no phase transitions occur in the observed temperature range. The ν₄(⁸¹Br) and ν₅(⁸¹Br) lines are closely situated and almost overlapping throughout the observed temperature range. In the figure, the ν₄(⁷⁹Br) and ν₅(⁷⁹Br) lines are also plotted, which show slight but recognizable frequency differences between them owing to ν (⁷⁹Br)/ν (⁸¹Br) = 1.19707. The three ³⁵Cl NQR lines, ν₆ to ν₈, are also very closely located, indicating that the corresponding Cl atoms have quite similar chemical environments.

Fig. 7:

Temperature dependence of ^79,81Br and ³⁵Cl NQR frequencies for 2.

The figure inserted shows the ν_4,5 doublet(⁷⁹Br).

All ν-vs.-T curves are different from that of a normal case where the frequencies are decreasing monotonously with increasing temperature due to the increased torsional vibrations. ν₂ shows an eye-catching anomalous positive temperature dependence, and all the ν-vs.-T curves of the other lines (including the ³⁵Cl lines) also show anomalous, slight positive curvatures. These anomalies can be produced by the cation motions that are excited with increasing temperatures and followed by changes in the hydrogen bonding in which all Br atoms are involved (Table 3). The NQR frequencies of terminal bonds generally correlate in reciprocal proportion with the bond lengths, but the differences between the terminal bond lengths of Cd–Br1, Cd–Br2, Cd–Br4, and Cd–Br5 are quite small, within ±3 pm, which makes the assignment of a frequency to each atom only from the bond distances difficult. However, the unique appearance of the ⁸¹Br NQR resonance of the Br atoms of the anion can allow some assignments. It is remarked that ν₂ and ν₃ increase their separation symmetrically upward and downward with the horizontal center line of about 42 MHz with increasing temperature. This suggests that each Br2 and Br4 atom in the trans position is to be attributed to one of these lines. The trans effect describes that the average NQR frequency between two atoms located in the trans positions in the octahedral coordination complex is maintained nearly constant during the temperature change because the bonding electrons of two bonds with a central atom are delocalized in the 3c–4e MO. Meanwhile, the positive temperature dependence of ν₂ can be understood by breaking the hydrogen bonds with increasing temperature, which results in an increase of the NQR frequency of the related Br atom because the hydrogen bonds exert a shift to lower the frequency. Both Br2 and Br4 are involved in the hydrogen bonding with short distances of N···Br 336.4 and 321.0 pm for the former and 326.9 pm for the latter. Br1 and Br5 are situated in the trans positions relative to the bridging atoms Br3′ and Br3, respectively. It is notable that the frequency separation from the center line of 42 MHz is approximately twice as large for ν₁ than for ν_4,5. Therefore, the closely situated lowest doublet lines ν_4,5 must be assigned to Br1 and Br5 and the highest line ν₁ to Br3. That the frequency of the bridging atom Br3 is the highest is attributable to the bond angle Cd–Br3···Cd′ = 164.604(9)° (at 100 K: near 180°), which suggests a significant increase of its NQR frequency in contrast to the bridging I atom in [C(NH₂)₃]CdI₃ (1). The MO calculations of a [Cd₃Br₁₆]^10– moiety, as described in the next section, show that the ⁸¹Br NQR frequencies are in the order of ν(Br3) > ν(Br2) ≈ ν(Br4) > ν(Br1) ≈ ν(Br5), but the calculated NQR frequencies are about half of the observed ones [10].

According to an early work [12], an isolated [CdBr₄]^2– tetrahedron is stable even in the gas phase, and therefore a [CdBr₆]^4– octahedron in the crystal appears to be maintained by Coulomb interactions. Hydrogen bond assists also in the stabilization of complex halogenocadmate anions.

2.6 MO calculations for the [Cd₃Br₁₆]^10– moiety as the model for the crystal structure of [4-ClC₆H₅NH₃]₃CdBr₅ (2)

The details of MO calculations are described in the Experimental Section. In the calculations, the crystal field effect was not considered, and the bridging Br3 and Br3′ atoms are not identical. The ⁸¹Br NQR frequency and asymmetric parameters η are calculated as follows: 19.23 MHz, 0.00 for Br1; 28.43 MHz, 0.00 for Br2; 64.96 MHz, 0.05 for Br3; 64.85 MHz, 0.04 for Br3′; 29.66 MHz, 0.00 for Br4; 19.60 MHz, 0.00 for Br5. The calculated frequencies are about half of the observed values. Meanwhile, the angles between the z principal axes of the EFGs are mostly in good agreement with the angles found in the crystal structure as listed in Table 4. The direction of the z axis of the Br3 atom in 2 corresponds nearly to that of the Cd–Br3···Cd′ bond.

4.1 Sample preparations

1 was prepared by mixing 0.06 mol of guanidine carbonate (first grade; Wako) and 0.12 mol of CdCO₃ (practical grade; Wako) with 0.30 mol of HI as 40 % aqueous HI solution (special grade; Wako), and 2 was prepared by mixing of 0.18 mol of 4-chloroaniline (special grade; Wako) and 0.06 mol of CdCO₃ with 0.40 mol of HBr as 30 % aqueous HBr solution (special grade; Wako). In both cases, large colorless crystals appeared upon removing water from the solutions by P₂O₅ in desiccators at room temperature. Elemental analyses: calcd. C 2.17, H 1.09, N 7.59; found C 2.26, H 1.08, N 7.68 for 1; calcd. C 24.08, H 2.35, N 4.68; found C 24.08, H 2.36, N 4.69 for 2.

4.2 NQR measurements

The ^81,79Br NQR spectra were recorded by a homemade super-regenerative-type NQR spectrometer at temperatures above 77 K. The resonance frequencies were determined by the counting method. The accuracy of the frequency measurements is estimated to be within ±0.05 MHz.

4.3 Crystal structure determination

All measurements were made on a Bruker D8 Venture Photon area detector (MoKα radiation, λ = 71.073 pm). Data were collected at 100 K and processed using Olex2 1.2 [14]. The structure was solved by Direct Methods [15] and expanded using Fourier techniques. All calculations were performed using the Olex2 1.2 crystallographic software package except for the refinement, which was performed using Shelxl [16]. Positional and displacement parameters and other experimental details can be obtained from the Cambridge Crystallographic Data Centre [17].

4.4 MO calculations

MO calculation on PM3 basis was carried out using WinGamess, a quantum chemistry package [18]. The population analysis was done for the 4p orbitals of Br atoms bonded to the central Cd atom in a [Cd₃Br₁₆]^10– moiety and 5p orbitals of I atoms bonded to the central Cd atom in a [Cd₅I₁₆]^6– moiety. These two moieties are cut off from the crystal structures having no symmetry, and therefore, the bridging I3 and I3′ atoms in 1 are not identical. For the bridging Br3 and Br3′ atoms in 2, the situation is the same. The number of unbalanced p-electrons, U_pi = |n_i – (n_j + n_k)/2| (i, j, k = x, y, z), was calculated from the population of p_i orbitals, n_i, which were obtained after diagonalization of the population matrix with the off-diagonal values making estimation of the contribution from overlap integrals like S = ∫pipj dτ to be 0.5. The quadrupole coupling constant e²Qq h^–1 was calculated from multiplication of the maximum value of U_pi by e²Qq_atomh^–1 (¹²⁷I, 2292.712 MHz; ⁸¹Br, 643.02 MHz) [10]. The crystal field effect was not considered. Meanwhile, the directions of the principal axes of the EFGs could be calculated from eigenvectors of the population matrixes. Facio 19.1.4 [19, 20] and Mercury 3.6 were used for the support of MO calculation [21].