By slow evaporation single crystals of the studied compound were obtained from a solution of 4,4’-diaminodiphenylmethan [Sigma-Aldrich] and sulfuric acid (98 wt% from Fluka) as obtained from commercial sources without further purification. A solution of 4,4’-diaminodiphenylmethan (1.98 g, 0.01 mol) in 200 mL of water was added to a solution of sulfuric acid. The resulting solution was filtered through paper and left at room temperature. After several days, transparent and good single crystals were produced, which were filtered and air dried.

The crystal of the title compound is built up from 4,4’-diamoniumdiphenyl- methan cations, two types of sulfate anions with different ionization state, i.e. [HSO₄]⁻ monoanions and [SO₄]²⁻ dianions, and from water molecules. Figure 1 showed that the asymmetric unit of (C₁₃H₁₆N₂)_1.5HSO₄SO₄·H_1.5O was formed by two crystallographically different cations, two independent anions and one water molecule. The main geometrical features of different entities are indicated in Table 2.

A view of the structure projected along the b axis is reported in Figure 2. The atomic arrangement of the structure of the DDPS was described by a three-

dimensional network of structural units formed by clusters [HS₂O₈]³⁻ sulfate, water molecules and organic cations. The mineral skeleton of this compound is formed by basic [SO₄]²⁻ and acidic [HSO₄]⁻ groups which are interconnected via O(4)−H・・・O(8) hydrogen bond leading to the formation of isolated clusters in the (a, b) plane. The same type of hydrogen bonds O−H・・・O links these mineral entities together, in pairs, via two water molecules which share a hydrogen atom located on a C₂ axis, to form a finite chains parallel to c axis (Figure 3).

The short distance O(4)・・・O(8) = 2.5151(6) Å, shows that this hydrogen bond was considered strong. Distances and bond angles describing the anions [SO₄]²⁻ and [HSO₄]⁻ are illustrated in Table 2. The lengths of S−O bonds in are close to literature average [13] value of 1.4082(4) Å. S−O(H) bond in is longer (1.5520(4)

Å), which is typical value, however, one of the remaining S−O bonds (S1−O4; 1,5520(4) Å), is slightly shorter than the others S1−O2 and S1−O3; this fact can be explained by the location of a proton on oxygen O(4) of the anion , this characteristic is in line with those observed in the protonated oxoanions [14] [15] [16] . The mean value of S−O distances and angles O−S−O are: 1.459(4) Å, 109.37(20) and 1.458(4) Å, 109.46(19) respectively for the tetrahedra S(1)O4 and S(2)O4. These values are, also, in good agreement with those observed for similar anionic groups [17] [18] . The calculated average values of the distortion indices [19] , corresponding to the different angles and distances in both S(1)O4 and S(2)O4 tetrahedron [DI(OSO) = 0.0100 - 0.0316, DI(SO) = 0.0076 - 0.0291 and DI(OO) = 0.0602 - 0.0087] exhibit a pronounced distortion of the O−S−O angles compared to S−O and O・・・O distances. So, each sulfate group can be considered as a rigid regular arrangement of oxygen atoms, with a displacement of the S atom from its centroid. The interaction of the sulfuric acid with the organic molecule (C₁₃H₁₄N₂) leads to the protonation of the two nitrogen atoms of the 4,4’ diaminodiphenylmethan and the formation of two cations (C₁₃H₁₆N₂)²⁺ cristallographically independent, respectively denoted: A{C(1) C(7)} and B{C(8) C(20)}.

Main geometrical characteristics of these cations are summarized in Table 2. With regards to the organic cations, the A cation has a local symmetry, in which the C(1) atom is located on a C₂ axis. Each sulfate anion is linked to its organic groups neighbors via N−H・・・O hydrogen bonds. Furthermore, the tetrahedron HS(1)O₄ is involving in an only one O−H・・・O hydrogen bond with the water molecule and in four N−H・・・O hydrogen bond with three organic cations neighbors. The tetrahedron S(2)O₄ is engaged in six hydrogen bonds of the same type from six organic cations. Thus, hydrogen bonding plays a significant role in linking the organic molecules with the anionic sheets made by HSO₄ and SO₄ moieties. This interaction contributes to the cohesion and the stability of the structure. All the D (donor)−H・・・A (acceptor) hydrogen bonds are listed in Table 3, with an upper limit of 2.458(7) Å for H・・・A distance and a lower limit of 87(12) ˚ for the D−H・・・A bond angles. Consequently, this atomic arrangement exhibits two types of intermolecular interactions: O−H・・・O and N−H・・・O, considered respectively as strong and weak hydrogen bonds. In this case, the titled compound contains two hydrogen bonds of the first type and ten of the second one. The N(1)−H(1)N(1)・・・O(7) interaction is considered as a strong hydrogen bond [N(1)−H(1)N(1)・・・O(7) = 1.757(6) Å] [20] . This latter provides the connection of the organic cations to the cluster [H₂S₂O₈]²⁻. All these interactions play a fundamental role in the process of the creation and the stability of a three-dimen- sional framework of (C₁₃H₁₆N₂)_1.5HSO₄SO₄·H_1.5O.

¹Symmetrycode: i: x − 1/2, −y + 3/2, z − 1/2; ii: −x + 3/2, y + 1/2, −z + 3/2; iii: x − 1/2, y + 3/2; ziv: −x + 3/2, −y + 3/2, −z + 2; v: x − 1/2, y + 1/2, z vi: x, −y + 1, z − 1/2; vii: x − 1, y, z.

Recently, several researches on the vibrational properties of organic sulfates containing HSO₄ and SO₄ groups had been widely investigated [21] [22] .

These studies have been focused, essentially, on the relationship between symmetry considerations and normalmodes vibrations of these groups. The common result is that these groups loose theirs free state symmetry (T_d for SO₄ and C_3v for HSO₄) to be C₁ under the effect of its incorporation in the complex and its interaction with its crystalline environment. As a result, the vibrational properties are affected by moving the degeneracy of some normal modes, splitting of some IR active modes and appearance of some IR inactive modes. Here, we recall that the free ion SO₄, in its ideal Td symmetry are ν₁ = 981 cm⁻¹, ν₂ = 451 cm⁻¹, ν₃ = 1104 cm⁻¹ and ν₄ = 614 cm⁻¹ [23] . In this work, an attempt to assign frequencies to different stretching vibrations and deformation of the organic cation is performed based on previous work [24] [25] . A broad band extending from 3700 to 2500 cm⁻¹ is observed in the IR spectrum (Figure 4). This broad band must be due to the symmetric and asymmetric stretching modes of CH, NH₃, NH₂, OH, H₂O and NH₃ bending and rocking may occur in the ranges 1640 - 1520 and 933 - 722 cm⁻¹. The out-of-plane OH bending mode ʋ (OH) appears in infrared spectrum at the average 3744 to 3780 cm⁻¹. The shifting of the stretching and bending vibrations of the NH₃ group from the free state value confirms the formation of hydrogen bonds of varying strengths in the crystal. Frequencies in the range 933 - 722 cm⁻¹ are attributed to ρ(CH), ρ(NH₃), ρ(NH₂), γ(CH) and γ(C=C). The frequency bands in the region 410 - 484 cm⁻¹ are attributed to the symmetric deformation vibration of δs(SO₄). The asymmetric deformation symmetry δas(SO₄) was observed in the area 602 - 671 cm⁻¹.

While that connected to the symmetry of valence SO₄ group is presented by the band 992 cm⁻¹. Bands observed in the 992 - 1186 cm⁻¹, on asymmetrical valence vibration νs(SO₄) δas(SO₄) region.

The simultaneous TG?DTA analysis curves of (C₁₃H₁₆N₂)_1.5HSO₄SO₄·H_1.5O were carried out in air at a heating rate of 5˚C/min on a sample of 21.5 mg placed in a platinum crucible and heated from ambient to 325˚C. The result obtained during the decomposition of the title compound is illustrated in Figure 5. This study shows that this complex is thermally stable up to 185˚C. The TGA curve revealed the presence of two steps weight losses. The first decomposition, from 185˚C to 205˚C which is accompanied by one shoulder on the DTA curve at 197˚C, corresponds to the loss of the water molecules (experimental weight loss: 3.72% and theoretical weight loss: 3.42%) and leading to anhydrous framework of (C₁₃H₁₆N₂)_1.5HSO₄SO₄. The second stage, starts at about 220˚C and ends above 300˚C, is assigned to the degradation of the organic entities (experimental loss: 65.46% and theoretical loss: 60.85%). This phenomenon is accompanied by one endothermic peak observed on the DTA curve at about 238˚C.