Synthesis and structure of trans-2,5-di­methyl­piperazine-1,4-diium di­hydrogen diphosphate

In the title salt, C₆H₁₆N₂²⁺ ·H₂P₂O₇²⁻, the complete dication is generated by a crystallographic centre of symmetry with the methyl groups in equatorial orientations. The complete dianion is generated by a crystallographic twofold axis with the central O atom lying on the axis: the P—O—P bond angle is 135.50 (12)°. In the crystal, the di­hydrogen diphosphate anions are linked by O—H⋯O hydrogen bonds, generating (001) layers. The organic cations bond to the inorganic layers by way of N—H⋯O and C—H⋯O hydrogen bonds. A Hirshfeld surface analysis shows that the most important contributions for the crystal packing are from O⋯H/H⋯O (60.5%) and H⋯H (39.4%) contacts.

1. Chemical context

Hybrid organic–diphosphate-based materials have received attention due to their role in catalytic, adsorption, ion-exchange, optical and biological processes (Chen & Munson, 2002; Ballarini et al., 2006). Protonated diphosphate forms such as HP₂O₇^3– or H₂P₂O₇^2– have the ability to associate with organic cations by means of ionic and non-covalent inter­actions (Desiraju, 1989; Steiner, 2002) to generate different supra­molecular architectures. Among the many diphosphate structures templated with organic cations are (C₂H₁₀N₂)·H₂P₂O₇ (Averbuch-Pouchot & Durif, 1993), (C₅H₆N₂O₂)₂·H₂P₂O₇ (Toumi Akriche et al., 2010), (C₈H₁₂N)₂·H₂P₂O₇, (Marouani et al., 2010 (C₈H₁₂NO)₂·H₂P₂O₇ (Elboulali et al., 2013) and (C₆H₅CH₂NH₃)₂·H₂P₂O₇ (Saad et al., 2014). As part of our ongoing studies of these compounds, we now report the synthesis and structure of the title compound, C₆H₁₆N₂²⁺·H₂P₂O₇^2–, (I).

2. Structural commentary

The asymmetric unit of (I) comprises a half diphosphate anion completed by a crystallographic twofold rotation axis (atom O4 lies on the axis) and half a trans-2,5-di­methyl­piperazine-1,4-dium cation completed by inversion symmetry (Fig. 1). The phospho­rous atom adopts a distorted tetra­hedral geometry with bond lengths and angles in the range 1.4916 (11)–1.5884 (8) Å and 102.44 (9)–116.60 (8)°, respectively. The longest P—O distance corresponds to the bridging oxygen atom [P1—O4 = 1.5884 (8) Å], the inter­mediate one is the P—OH bond [P1—O1 = 1.5389 (15) Å], whereas the shortest bonds correspond to terminal oxygen atoms [P1—O2 = 1.4916 (11) and P1—O3 = 1.4924 (14) Å] in agreement with previous di­hydrogen diphosphate structures elaborated by our group (Toumi Akriche et al., 2010; Saad et al., 2014). The PO₄ tetra­hedra are fused by atom O4 forming a bent P₂O₇ unit [P1—O4—P1ⁱ = 135.50 (12)°; symmetry code: (i) −x + 1, y, −z + )] and staggered conformation [O1—P1—P1ⁱ—O2ⁱ = 42.8 (1)°], close to those previously observed for diphos­phates with twofold symmetry (Marouani et al., 2010; Charfi & Jouini, 1997). In the cation, the piperazine ring adopts a chair conformation with puckering parameters Q = 0.2770 Å, θ = 90° and φ = 142° in which the methyl substituents occupy equatorial sites. The bonds and angles in the cation [N/C—C in the range 1.484 (2)–1.514 (2) Å and N/C—C/N—C, in the range 108.68 (11)–112.80 (12)°] show no significant difference from those reported in other trans-2,5-di­methyl­piperazine based salts (Landolsi & Abid, 2021; Gatfaoui et al., 2014).

Figure 1 The mol­ecular structure of (I) with displacement ellipsoids drawn at the 50% probability level. Symmetry codes: (i) −x + 1, y, −z + ; (ii) −x + 1, −y + 1, −z + 1. The H atoms are presented as small spheres of arbitrary radius. Hydrogen bonds are shown as green dotted lines.

3. Supra­molecular features

The extended structure of (I) is built up from hydrogen-bonded sheets of diphosphate anions extended along the crystallographic c-axis direction at z = 1/4 and 3/4 with the cations lying between these anionic sheets featuring extensive hydrogen bonds, so as to build a three-dimensional supra­molecular network (Fig. 2). A projection along the c axis at z = 1/4 of the inorganic sheet (Fig. 3) shows that O—H⋯O hydrogen bonds (Table 1) link adjacent di­hydrogen diphos­phate anions to develop anionic layers extending parallel to (001). Electrostatic, medium-strength N—H⋯O and weaker C—H⋯O inter­actions (Table 1) between the organic cations and these anionic layers are responsible for the three-dimensional crystal structure.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯O3^i	0.88 (3)	1.61 (3)	2.460 (2)	161 (3)
N1—H1A⋯O2^ii	0.90 (2)	1.86 (2)	2.7454 (17)	167 (2)
N1—H1B⋯O2	0.90 (2)	1.87 (2)	2.7543 (17)	168 (2)
C1—H1C⋯O3^iii	0.982 (19)	2.353 (19)	3.193 (2)	143.0 (15)
C1—H1D⋯O1^iv	0.970 (19)	2.41 (2)	3.205 (2)	139.1 (15)
C2—H2⋯O3^v	1.00 (2)	2.53 (2)	3.344 (2)	138.3 (15)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

Figure 2 The projection along the b-axis direction of the crystal packing of (I). Hydrogen bonds are shown as dotted lines.

Figure 3 The layered di­hydrogen diphosphate self-assembly, viewed along the c axis at z = 1/4. The O—H⋯O hydrogen bonds are shown as green dotted lines.

4. Hirshfeld surface analysis

Fig. 4a shows the Hirshfeld surface generated with Crystal Explorer (Wolff et al., 2012) mapped over d_norm with red spots corresponding to short inter-atomic contacts. The fingerprint plots illustrated in Fig. 4b and 4c with characteristic spikes indicate that the major inter-atomic contributions to the structure are from O⋯H/H⋯O (60.5%) and H⋯H (39.4%) contacts in accordance with the structure topology.

Figure 4 Hirshfeld surface (a) mapped with dnorm and fingerprint plots showing the major inter-contacts contributions of (b) O⋯H/H⋯O and (c) H⋯H.

5. Synthesis and experimental

The monocrystals of (I) were synthesized into two steps. Firstly, di­phospho­ric acid, H₄P₂O₇, was obtained from Na₄P₂O₇ (26 mg, 50 ml H₂O) by using an ion-exchange resin (Amberlite IR 120). Then, the fresh di­phospho­ric acid solution was neutralized with trans-2,5-di­methyl­piperazine base in a 1:1 molar ratio at low temperature. The resulting solution was slowly evaporated at room temperature for several days until colourless needle-shaped crystals of (I) were grown.

6. IR spectrum

The IR spectrum of (I) was collected at room temperature using a Perkin–Elmer Spectrum BXII spectrometer (KBr method) between 400 and 4000 cm⁻¹ (Fig. 5). The spectrum exhibits broad bands between 2376 and 3027 cm⁻¹, which can be assigned to the stretching modes of the –CH₃, –CH₂–, –CH– and (–NH₂)⁺ groups of the organic cation (Silverstein et al., 1974). The broadness of these bands in (I) is indicative of the presence of a hydrogen-bonding network. The bending vibrations of these groups are observed in the region 1321–1631 cm⁻¹. The inter­nal modes of the (H₂P₂O₇)^2– anion appear in the range 410–1265 cm⁻¹ (Harcharras et al., 1997; Kamoun et al., 1992). The elongation modes of the PO₂ and PO₃ terminal groups occur between 1265 and 975 cm⁻¹, whereas the terminal P—O stretching vibration of the PO₂ group is observed at 1155 cm⁻¹ and those at 1093 and 1051 cm⁻¹ are attributed to the symmetric and asymmetric terminal P—O stretching vibration of the PO₃ group (Sarr & Diop, 1987). The rocking of the PO₂ and PO₃ deformation modes occur between 491 and 627 cm⁻¹. The symmetric and asymmetric elongation modes of the P—O—P bridge for the diphosphate group with a bent conformation are observed as ν_as(P—O—P) = 911 and 975 cm⁻¹ and ν_s(P—O—P)= 721 cm⁻¹. The simultaneous activity of these vibration modes confirms the results obtained by X-ray concerning the bent geometry and the C₂ symmetry of the diphosphate anion. With regard to the Laza­rev (1972) correlation between the P—O—P bridge stretching frequencies and the bridge angle value as Δ = (ν_as– ν_s)/(ν_as+ ν_s) = 0.12, and by simple extrapolation of the graph of Δ = f(α) given by Rulmont et al. (1991), we can estimate the calculated P—O—P angle of 136° in excellent agreement with the value of 135.50 (12)° found for (I).

Figure 5 The infrared spectrum of (I).

7. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in a difference Fourier map and their positions were freely refined.

Table 2
Experimental details

Crystal data
Chemical formula	C6H16N2^2+·H2O7P2^2−
Mr	292.16
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	293
a, b, c (Å)	10.2557 (5), 8.3978 (5), 13.7681 (7)
β (°)	91.422 (4)
V (Å^3)	1185.42 (11)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	0.39
Crystal size (mm)	0.30 × 0.20 × 0.10
Data collection
Diffractometer	Xcalibur diffractometer with Sapphire3 CCD detector
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2019)
Tmin, Tmax	0.979, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	9376, 2255, 1738
Rint	0.040
(sin θ/λ)max (Å^−1)	0.769
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.041, 0.103, 1.05
No. of reflections	2255
No. of parameters	114
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.59, −0.39
Computer programs: CrysAlis PRO (Rigaku OD, 2019), SHELXS97 (Sheldrick, 2008), SHELXL2018/3 (Sheldrick, 2015), DIAMOND (Brandenburg & Putz, 2005) and WinGX publication routines (Farrugia, 2012).

Supporting information

Computing details

trans-2,5-Dimethylpiperazine-1,4-diium dihydrogen diphosphate

Crystal data

C6H16N2^2+·H2O7P2^2^−	F(000) = 616
Mr = 292.16	Dx = 1.637 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 10.2557 (5) Å	Cell parameters from 2302 reflections
b = 8.3978 (5) Å	θ = 4.0–31.8°
c = 13.7681 (7) Å	µ = 0.39 mm^−1
β = 91.422 (4)°	T = 293 K
V = 1185.42 (11) Å^3	Needle, colorless
Z = 4	0.30 × 0.20 × 0.10 mm

Data collection

Xcalibur diffractometer with Sapphire3 CCD detector	2255 independent reflections
Radiation source: fine-focus sealed X-ray tube, Enhance (Mo) X-ray Source	1738 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.040
Detector resolution: 16.0318 pixels mm^-1	θmax = 33.1°, θmin = 3.5°
ω scans	h = −15→15
Absorption correction: multi-scan (CrysAlisPro; Rigaku OD, 2019)	k = −12→12
Tmin = 0.979, Tmax = 1.000	l = −21→21
9376 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.103	All H-atom parameters refined
S = 1.05	w = 1/[σ^2(Fo^2) + (0.038P)^2 + 1.0885P] where P = (Fo^2 + 2Fc^2)/3
2255 reflections	(Δ/σ)max = 0.001
114 parameters	Δρmax = 0.59 e Å^−3
0 restraints	Δρmin = −0.38 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
P1	0.64197 (4)	0.21184 (5)	0.26767 (3)	0.02554 (12)
O1	0.72741 (18)	0.0974 (2)	0.20950 (10)	0.0587 (5)
O2	0.67285 (11)	0.20328 (14)	0.37398 (7)	0.0287 (2)
O3	0.64629 (15)	0.37300 (17)	0.22218 (10)	0.0449 (3)
O4	0.5000	0.1402 (2)	0.2500	0.0500 (6)
H1	0.757 (3)	0.012 (4)	0.240 (2)	0.084 (10)*
N1	0.61378 (12)	0.40835 (16)	0.52268 (9)	0.0238 (3)
C1	0.62451 (15)	0.57743 (19)	0.49245 (11)	0.0248 (3)
C2	0.51107 (15)	0.62671 (18)	0.42671 (10)	0.0239 (3)
C3	0.5174 (2)	0.8018 (2)	0.40212 (19)	0.0443 (5)
H1A	0.683 (2)	0.385 (3)	0.5625 (16)	0.042 (6)*
H1B	0.622 (2)	0.345 (3)	0.4707 (16)	0.039 (6)*
H1C	0.6259 (18)	0.642 (2)	0.5518 (14)	0.028 (5)*
H1D	0.7042 (19)	0.588 (2)	0.4565 (14)	0.032 (5)*
H2	0.510 (2)	0.559 (3)	0.3668 (15)	0.039 (5)*
H3A	0.516 (3)	0.862 (4)	0.460 (2)	0.084 (10)*
H3B	0.442 (3)	0.833 (3)	0.359 (2)	0.064 (8)*
H3C	0.603 (3)	0.817 (3)	0.3694 (19)	0.061 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P1	0.0296 (2)	0.0287 (2)	0.01809 (17)	0.00277 (15)	−0.00450 (13)	0.00137 (14)
O1	0.0792 (12)	0.0686 (11)	0.0291 (7)	0.0334 (9)	0.0148 (7)	−0.0004 (7)
O2	0.0323 (6)	0.0344 (6)	0.0189 (5)	0.0073 (5)	−0.0055 (4)	−0.0012 (4)
O3	0.0556 (9)	0.0387 (7)	0.0400 (7)	−0.0063 (6)	−0.0100 (6)	0.0169 (6)
O4	0.0415 (11)	0.0244 (9)	0.0823 (15)	0.000	−0.0347 (10)	0.000
N1	0.0225 (6)	0.0279 (6)	0.0210 (5)	0.0053 (5)	−0.0025 (4)	0.0010 (5)
C1	0.0233 (6)	0.0276 (7)	0.0233 (6)	−0.0022 (5)	−0.0009 (5)	0.0008 (5)
C2	0.0252 (7)	0.0260 (7)	0.0206 (6)	0.0019 (5)	−0.0006 (5)	0.0031 (5)
C3	0.0479 (11)	0.0303 (9)	0.0548 (12)	0.0016 (8)	0.0029 (10)	0.0144 (9)

Geometric parameters (Å, º)

P1—O2	1.4916 (11)	N1—C1	1.484 (2)
P1—O3	1.4924 (14)	N1—C2^ii	1.5020 (19)
P1—O1	1.5389 (15)	C1—C2	1.514 (2)
P1—O4	1.5884 (8)	C2—N1^ii	1.5020 (19)
O4—P1^i	1.5884 (8)	C2—C3	1.510 (2)
O2—P1—O3	116.60 (8)	P1—O4—P1^i	135.50 (12)
O2—P1—O1	111.76 (8)	C1—N1—C2^ii	112.80 (12)
O3—P1—O1	108.96 (9)	N1—C1—C2	111.58 (12)
O2—P1—O4	107.67 (5)	N1^ii—C2—C3	109.69 (14)
O3—P1—O4	108.40 (8)	N1^ii—C2—C1	108.68 (11)
O1—P1—O4	102.44 (9)	C3—C2—C1	111.29 (15)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3^iii	0.88 (3)	1.61 (3)	2.460 (2)	161 (3)
N1—H1A···O2^iv	0.90 (2)	1.86 (2)	2.7454 (17)	167 (2)
N1—H1B···O2	0.90 (2)	1.87 (2)	2.7543 (17)	168 (2)
C1—H1C···O3^v	0.982 (19)	2.353 (19)	3.193 (2)	143.0 (15)
C1—H1D···O1^vi	0.970 (19)	2.41 (2)	3.205 (2)	139.1 (15)
C2—H2···O3^i	1.00 (2)	2.53 (2)	3.344 (2)	138.3 (15)

Symmetry codes: (i) −x+1, y, −z+1/2; (iii) −x+3/2, y−1/2, −z+1/2; (iv) −x+3/2, −y+1/2, −z+1; (v) x, −y+1, z+1/2; (vi) −x+3/2, y+1/2, −z+1/2.