Synthesis and crystal structure of 4-fluoro­benzyl­ammonium di­hydrogen phosphate, [FC₆H₄CH₂NH₃]H₂PO₄

The crystal structure of 4-fluoro­benzyl­ammonium di­hydrogen phosphate, [FC₆H₄CH₂NH₃]H₂PO₄, consists of layers of 4-fluoro­benzyl­ammonium cations and di­hydrogen phosphate anions that alternate along the c axis, connected by hydrogen bonds into a three-dimensional network.

Abstract

The asymmetric unit of the title salt, [p-FC₆H₄CH₂NH₃]⁺·H₂PO₄ ⁻, contains one 4-fluoro­benzyl­ammonium cation and one di­hydrogen phosphate anion. In the crystal, the H₂PO₄ ⁻ anions are linked by O—H⋯O hydrogen bonds to build corrugated layers extending parallel to the ab plane. The FC₆H₄CH₂NH₃ ⁺ cations lie between these anionic layers to maximize the electrostatic inter­actions and are linked to the H₂PO₄ ⁻ anions through N—H⋯O hydrogen bonds, forming a three-dimensional supra­molecular network. Two hydrogen atoms belonging to the di­hydrogen phosphate anion are statistically occupied due to disorder along the OH⋯HO direction.

Chemical context  

A hybrid compound is a material that involves both organic and inorganic components blended in the solid state on the mol­ecular scale. Such materials allow the combination of the intended properties of both the organic and inorganic components when they self-assemble in the crystal. The resulting properties do not simply consist of the sum of the individual contributions, since they also strongly depend on the nature of the inter­actions established by the different components within the structure. The nature of the inter­actions has been used to divide organic–inorganic hybrid materials into two different classes, both of them being of technological inter­est. In class I, organic and inorganic components are connected together through strong chemical covalent or iono-covalent bonds; in class II, the two components are assembled by weaker inter­actions, such as hydrogen bonds and/or van der Waals and Coulombic inter­actions.

In particular, in considering hybrid systems belonging to class II, derivatives from ortho­phospho­ric acid (H₃PO₄) are often associated with functionalized organic mol­ecules (amines or amides) to produce organic–inorganic materials with potentially forceful hydrogen-bonding inter­actions between donor (D) and acceptor (A) components. Among these hybrid phosphates, the di­hydrogen phosphates have received great inter­est over recent years. Indeed, these compounds can be considered the most stable organic phosphates and also the first to be studied in more detail. They have a technological inter­est in many realms, such as magnetism, electricity, optics and in biomaterials research (Adams, 1977 ▸; Hearn & Bugg, 1972 ▸).

In these compounds, the acidic di­hydrogen phosphate anion H₂PO₄ ⁻, through the formation of O—H⋯O hydrogen bonds, gives rise to various topologies of anionic substructures. In the crystal structure of 2-ammonium­benzamide di­hydrogen phosphate (Belghith et al., 2015 ▸), the H₂PO₄ ⁻ tetra­hedra are associated in pairs, forming centrosymmetric finite units, while in 2,3-di­methyl­anilinium di­hydrogen phosphate (Rayes et al., 2004 ▸), they form a network composed of hydrogen-bonded chains. Two-dimensional anionic layers are observed in 4-chloro­anilinium di­hydrogen phosphate (Dhaouadi et al., 2008 ▸) and in 2-methyl­piperazinediium di­hydrogen phosphate (Choudhury et al., 2000 ▸), while in the crystal structure of imidazolium di­hydrogen phosphate (Blessing et al., 1986 ▸), the H₂PO₄ ⁻ anions are linked by hydrogen bonds to form a three-dimensional cage-type network, inside which the cations are trapped. The varieties of the observed arrangements suggest that selected packing architectures can be designed by choosing an appropriate amine.

In order to enrich the knowledge of such kinds of hybrid materials and to investigate the effect of hydrogen bonds on chemical and structural features, we report here synthesis and crystal structure analysis of the novel organic di­hydrogen phosphate, (p-FC₆H₄CH₂NH₃)⁺·H₂PO₄ ⁻.

Structural commentary  

The title hybrid salt crystallizes in the Pbcn space group with one para-fluoro­benzyl­ammonium cation and one di­hydrogen phosphate anion in the asymmetric unit (Fig. 1 ▸). Analysis of the P—O bond lengths clearly reveals the double-bond character of the P—O2 inter­action [1.492 (4) Å], suggesting at the same time the possible protonation of the remaining O atoms showing longer bonds [P1—O1 = 1.561 (4), P1—O3 = 1.543 (4) and P1—O4 = 1.535 (4) Å]. This is confirmed by the presence of electron density peaks close to these oxygen atoms, compatible in terms of height and distance from hydrogen atoms. However, the refinement showed half occupancy for two of the three hydrogen atoms, in agreement with charge neutrality and geometric considerations (both are disordered over two positions along the O–H⋯H–O direction involving the same oxygen atom in two adjacent anions). This explains the shorter P—O3 and P—O4 bond lengths, when compared with P1—O1, revealing at the same time the composition of the resulting anion as H₂PO₄ ⁻. The organic cation exhibits a regular configuration, with distances and angles in accordance to literature data (Wang et al., 2015 ▸; Klapötke et al., 2003 ▸).

Figure 1.

The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 50% probability level. The two half-filled H atoms have a site-occupation factor of 0.5.

Supra­molecular features  

The presence in the title compound of a number of donor and acceptor sites leads to the formation of a complex O—H⋯O and N—H⋯O hydrogen-bonding system (Table 1 ▸) which, supported by electrostatic and van der Waals inter­actions, gives rise to the formation of a stable three-dimensional supra­molecular network. O1—H1O⋯O2, O3—H3O⋯O3 and O4—H4O⋯O4 hydrogen bonds connect each di­hydrogen phosphate unit to an adjacent one, which results in the formation of an infinite two-dimensional corrugated layer of anions extending parallel to the ab plane (Fig. 2 ▸). In the inorganic supra­molecular layers, rings with a graph-set ring motif (Etter, 1990 ▸) of (16) are found, lying at z ∼1/4 and 3/4. The 4-fluoro­benzyl­ammonium cations are trapped between the anionic layers to maximize the electrostatic inter­actions and are linked to the H₂PO₄ ⁻ anions through N1—H1A⋯O2, N1—H1B⋯O3 and N1—H1C⋯O4 hydrogen bonds, forming (12) graph-set motifs with the O—H⋯O bonds. The cations are anchored on both sides of the H₂PO₄ ⁻ anionic layer, resulting in the stacking of an alternating organic–inorganic supra­molecular network (Fig. 3 ▸) along the c axis. Within the organic network, the dipolar character of the 4-fluoro­benzyl­ammonium mol­ecule leads to an alternating anti­parallel mol­ecular stacking along the a axis that prevents significant π–π inter­actions between the aromatic rings but promotes van der Waals inter­actions as the unique inter­molecular inter­actions between the organic mol­ecules.

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1O⋯O2i	0.82	1.75	2.569 (5)	172
O3—H3O⋯O3ii	0.82	1.67	2.483 (5)	168
O4—H4O⋯O4iii	0.82	1.71	2.523 (5)	174
N1—H1A⋯O2iv	0.89	1.91	2.785 (6)	169
N1—H1B⋯O3v	0.89	1.96	2.831 (6)	167
N1—H1C⋯O4	0.89	2.03	2.900 (6)	164

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

Figure 2.

A layer of H₂PO₄ ⁻ anions, parallel to the ab plane, formed by hydrogen bonds displaying (16) graph-set ring motifs.

Figure 3.

Projections of the [FC₆H₄CH₂NH₃]H₂PO₄ structure along the a axis (left) and the b axis (right), showing the alternate stacking of inorganic and organic layers along the c axis.

Database survey  

A search of the Cambridge Structural Database (Version 5.37; last update February 2016; Groom et al., 2016 ▸) for related compounds showed that [FC₆H₄CH₂NH₃]·H₂PO₄, is isotypic with 4-chloro­benzyl­ammonium di­hydrogen phosphate (Dhaouadi et al., 2005 ▸). The main difference concerns the hydrogen atoms of the di­hydrogen phosphate anion. These, ordered on two sites in the latter structure, are located over three positions for the title structure, two of which show half occupancy. In spite of this difference, the resulting anionic framework and the linking of the cations are analogous in both cases. A similarly organized anionic layer is formed by self-assembly of H₂PO₄ ⁻ units in the structure of octane-1,8-di­ammonium bis­(di­hydrogen phosphate) (Mrad et al., 2011 ▸). Although the amine used is of different nature, the compound crystallizes in the same space group Pbcn and, approximately similar to the present case, two hydrogen atoms were found to be shared along the O—H—O bonding direction involving two H₂PO₄ ⁻ groups. The difference in the organic moiety is reflected in a different anchoring of the cations on the anionic layers, building in this case a three-dimensional hydrogen-bonded network.

Synthesis and crystallization  

Crystals of the title compound were grown by dissolving in water p-fluoro­benzyl­amine (purity 99%, Sigma–Aldrich) and ortho­phospho­ric acid (85%_wt, d = 1.7 kg cm⁻³) in a 1:1 molar ratio. The resulting mixture was heated slightly (330 K) under constant stirring for 3 h to obtain a clear solution. Schematic­ally the reaction can be written as follows:

F(C₆H₄)CH₂NH₂ + H₃PO₄ → [FC₆H₄CH₂NH₃]·H₂PO₄

The solution thus obtained was placed in a Petri dish and kept for crystallization at room temperature without disturbance. Single crystals of the title compound, suitable for X-ray diffraction analysis, were obtained after one week (yield 82%).

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The H atoms were located in a difference Fourier map and refined as riding, with O—H = 0.82 Å, N—H = 0.89 Å, C—H = 0.93 and 0.97 Å. A rotating model was used for the OH and ammonium groups. The di­hydrogen phosphate H atoms were refined with U _iso(H) = 1.5U _eq(O), those of the ammonium H atoms with U _iso(H) = 1.5U _eq(N), and the remaining ones with U _iso(H) = 1.2U _eq(C). Two H atoms were found to be disordered over two positions along the O—H⋯H—O direction involving the same oxygen atom in two adjacent anions and refined with half occupancy. An outlier (524) was omitted in the last cycles of the refinement.

Table 2. Experimental details.

Crystal data
Chemical formula	C7H9FN+·H2PO4 −
M r	223.14
Crystal system, space group	Orthorhombic, P b c n
Temperature (K)	294
a, b, c (Å)	7.1630 (8), 9.1309 (10), 29.694 (3)
V (Å3)	1942.1 (4)
Z	8
Radiation type	Mo Kα
μ (mm−1)	0.29
Crystal size (mm)	0.36 × 0.31 × 0.27
Data collection
Diffractometer	Bruker SMART CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2008 ▸)
T min, T max	0.813, 0.846
No. of measured, independent and observed [I > 2σ(I)] reflections	19365, 1803, 1780
R int	0.031
(sin θ/λ)max (Å−1)	0.606
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.077, 0.170, 1.33
No. of reflections	1803
No. of parameters	131
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.52, −0.65

Computer programs: APEX2 and SAINT (Bruker, 2008 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2014 (Sheldrick, 2015b ▸), ORTEP-3 for Windows (Farrugia, 2012 ▸) and VESTA (Momma & Izumi, 2011 ▸).

Supplementary Material

CCDC reference: 1516161

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

C7H9FN+·H2PO4−	Dx = 1.526 Mg m−3
Mr = 223.14	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pbcn	Cell parameters from 4573 reflections
a = 7.1630 (8) Å	θ = 5.5–37.8°
b = 9.1309 (10) Å	µ = 0.29 mm−1
c = 29.694 (3) Å	T = 294 K
V = 1942.1 (4) Å3	Prism, colourless
Z = 8	0.36 × 0.31 × 0.27 mm
F(000) = 928

Data collection

Bruker SMART CCD diffractometer	1780 reflections with I > 2σ(I)
ω scan	Rint = 0.031
Absorption correction: multi-scan (SADABS; Bruker, 2008)	θmax = 25.5°, θmin = 1.4°
Tmin = 0.813, Tmax = 0.846	h = −8→8
19365 measured reflections	k = −11→11
1803 independent reflections	l = −35→35

Refinement

Refinement on F2	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.077	H-atom parameters constrained
wR(F2) = 0.170	w = 1/[σ2(Fo2) + 11.9717P] where P = (Fo2 + 2Fc2)/3
S = 1.33	(Δ/σ)max < 0.001
1803 reflections	Δρmax = 0.52 e Å−3
131 parameters	Δρmin = −0.65 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	Occ. (<1)
P1	0.7518 (2)	0.61709 (14)	0.29940 (4)	0.0247 (3)
O1	0.7675 (6)	0.7342 (4)	0.33752 (11)	0.0346 (9)
H1O	0.7332	0.8139	0.3279	0.052*
O2	0.8598 (5)	0.4862 (4)	0.31440 (13)	0.0328 (9)
O3	0.8281 (5)	0.6833 (4)	0.25529 (12)	0.0325 (9)
H3O	0.9425	0.6786	0.2553	0.049*	0.5
O4	0.5439 (5)	0.5863 (4)	0.29115 (12)	0.0336 (9)
H4O	0.5230	0.5869	0.2640	0.050*	0.5
N1	0.2305 (6)	0.3951 (5)	0.31025 (13)	0.0304 (10)
H1A	0.1169	0.4344	0.3092	0.046*
H1B	0.2441	0.3322	0.2876	0.046*
H1C	0.3158	0.4655	0.3079	0.046*
C1	0.2552 (9)	0.3166 (6)	0.35386 (16)	0.0334 (12)
H1D	0.3705	0.2609	0.3531	0.040*
H1E	0.1530	0.2482	0.3580	0.040*
C2	0.2606 (8)	0.4220 (6)	0.39291 (16)	0.0297 (11)
C3	0.0985 (9)	0.4714 (7)	0.4122 (2)	0.0427 (15)
H3	−0.0159	0.4388	0.4013	0.051*
C4	0.1037 (11)	0.5698 (8)	0.4480 (2)	0.0541 (19)
H4	−0.0059	0.6043	0.4610	0.065*
C5	0.2724 (12)	0.6139 (8)	0.46325 (19)	0.0543 (18)
C6	0.4362 (11)	0.5685 (8)	0.4453 (2)	0.0558 (19)
H6	0.5498	0.6013	0.4566	0.067*
C7	0.4285 (9)	0.4709 (7)	0.4093 (2)	0.0442 (15)
H7	0.5388	0.4383	0.3961	0.053*
F1	0.2783 (8)	0.7094 (5)	0.49869 (14)	0.0887 (17)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
P1	0.0248 (6)	0.0201 (6)	0.0293 (6)	0.0016 (5)	−0.0057 (6)	0.0010 (5)
O1	0.043 (2)	0.0255 (18)	0.0348 (19)	0.0029 (18)	−0.0126 (19)	−0.0025 (16)
O2	0.0263 (19)	0.0222 (17)	0.050 (2)	0.0002 (16)	−0.0069 (18)	0.0051 (17)
O3	0.0229 (18)	0.042 (2)	0.0326 (19)	0.0039 (18)	0.0007 (16)	0.0095 (17)
O4	0.0226 (18)	0.048 (2)	0.0299 (19)	−0.0048 (18)	−0.0031 (16)	0.0030 (18)
N1	0.029 (2)	0.032 (2)	0.030 (2)	−0.001 (2)	0.001 (2)	−0.0028 (19)
C1	0.037 (3)	0.027 (3)	0.036 (3)	0.004 (3)	−0.002 (3)	0.002 (2)
C2	0.032 (3)	0.029 (3)	0.028 (2)	−0.001 (2)	−0.003 (2)	0.003 (2)
C3	0.039 (3)	0.050 (4)	0.040 (3)	−0.001 (3)	0.003 (3)	−0.003 (3)
C4	0.063 (5)	0.061 (4)	0.039 (4)	0.013 (4)	0.009 (3)	−0.006 (3)
C5	0.082 (5)	0.048 (4)	0.033 (3)	0.003 (4)	−0.006 (4)	−0.010 (3)
C6	0.060 (5)	0.060 (4)	0.047 (4)	−0.011 (4)	−0.013 (4)	−0.009 (3)
C7	0.042 (3)	0.050 (4)	0.041 (3)	0.007 (3)	−0.004 (3)	−0.003 (3)
F1	0.127 (4)	0.084 (3)	0.055 (2)	0.002 (3)	−0.010 (3)	−0.039 (2)

Geometric parameters (Å, º)

P1—O2	1.492 (4)	C1—H1E	0.9700
P1—O4	1.535 (4)	C2—C3	1.371 (8)
P1—O3	1.543 (4)	C2—C7	1.372 (8)
P1—O1	1.561 (4)	C3—C4	1.391 (9)
O1—H1O	0.8200	C3—H3	0.9300
O3—H3O	0.8200	C4—C5	1.352 (11)
O4—H4O	0.8200	C4—H4	0.9300
N1—C1	1.491 (6)	C5—C6	1.354 (11)
N1—H1A	0.8900	C5—F1	1.367 (7)
N1—H1B	0.8900	C6—C7	1.393 (9)
N1—H1C	0.8900	C6—H6	0.9300
C1—C2	1.508 (7)	C7—H7	0.9300
C1—H1D	0.9700
O2—P1—O4	113.9 (2)	H1D—C1—H1E	108.0
O2—P1—O3	112.5 (2)	C3—C2—C7	119.2 (5)
O4—P1—O3	106.3 (2)	C3—C2—C1	120.7 (5)
O2—P1—O1	107.1 (2)	C7—C2—C1	120.2 (5)
O4—P1—O1	108.1 (2)	C2—C3—C4	120.6 (6)
O3—P1—O1	108.8 (2)	C2—C3—H3	119.7
P1—O1—H1O	109.5	C4—C3—H3	119.7
P1—O3—H3O	109.5	C5—C4—C3	118.2 (7)
P1—O4—H4O	109.5	C5—C4—H4	120.9
C1—N1—H1A	109.5	C3—C4—H4	120.9
C1—N1—H1B	109.5	C4—C5—C6	123.4 (6)
H1A—N1—H1B	109.5	C4—C5—F1	118.4 (7)
C1—N1—H1C	109.5	C6—C5—F1	118.2 (7)
H1A—N1—H1C	109.5	C5—C6—C7	117.7 (7)
H1B—N1—H1C	109.5	C5—C6—H6	121.2
N1—C1—C2	111.4 (4)	C7—C6—H6	121.2
N1—C1—H1D	109.4	C2—C7—C6	121.0 (6)
C2—C1—H1D	109.4	C2—C7—H7	119.5
N1—C1—H1E	109.4	C6—C7—H7	119.5
C2—C1—H1E	109.4

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O···O2i	0.82	1.75	2.569 (5)	172
O3—H3O···O3ii	0.82	1.67	2.483 (5)	168
O4—H4O···O4iii	0.82	1.71	2.523 (5)	174
N1—H1A···O2iv	0.89	1.91	2.785 (6)	169
N1—H1B···O3v	0.89	1.96	2.831 (6)	167
N1—H1C···O4	0.89	2.03	2.900 (6)	164

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