2,4-Di­methyl­anilinium di­hydrogenphosphate

2,4–Di­methyl­anilinium di­hydrogenphosphate, C₈H₁₂N⁺.­H₂PO₄⁻, (I), is monoclinic (P2₁/c). The ions are held together by O—H...O and N—H...O hydrogen bonds. In addition there is one C_methyl—H...O hydrogen bond. The di­hydrogenphosphates and the —NH₃ groups are arranged in sheets which are parallel to the (010) plane. In each sheet there are centrosymmetric pairs of di­hydrogenphosphates held by O—H...O bonds. These pairs of di­hydrogen­phosphates are arranged in columns that are parallel to the unit-cell b axis. These columns are held together via —NH₃ groups through N—H...O bonds. Each hydrogen from an —NH₃ group is donated to a different di­hydrogenphosphate ion. The differential scanning calorimetry experiment showed anomaly during heating at ∼442 K.

Key indicators

• Single-crystal X-ray study
• T = 290 K
• Mean [sigma](C-C) = 0.004 Å
• R factor = 0.034
• wR factor = 0.041
• Data-to-parameter ratio = 11.1

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

The main purpose of this structural study was a determination of arrangement of arylammoniums and dihydrogenphosphates which are held together by hydrogen bonds. R—NH₃⁺ cations (R=aryl, alkyl or H) can bind in various ways to [H₂PO₄]^- anions. Some of these structures show interesting phase transitions and physical properties which are influenced by hydrogen bonding. Examples include [NH₄]⁺[H₂PO₄]^- (e.g. Baur, 1973) or n-alkylammoniumdihydrogenphosphates (Kroupa & Fuith, 1993, 1994; Fábry et al. 2000).

A search in the Cambridge Structure Database System (Allen \& Kennard, 1993) yielded the structure SOMHUX (2-Methyl-4-nitroanilinium dihydrogenmonophosphate) determined by Masse & Levy, 1991. The latter structure is orthorhombic (Pbca) and contains layers of dihydrogenphosphates. Symmetry and structural similarity to room-temperature phases of n-alkylammonium dihydrogenphosphates which also contain layers of dihydrogenphosphates and which are closely related to the prototypic orthorhombic phase (Pbna) made us to think about possibility of phase transitions in other related compounds. Therefore similarly substituted anilines were chosen for investigation, among them 2,4-dimethylaniline.

It was found that in the studied compound there are also sheets of dihydrogenphosphates. These sheets are interconnected by ammonium groups (Fig. 1). Each hydrogen from the ammonium group is donated to a different dihydrogenphosphate ion. In all the compounds the H atoms are ordered at room temperature. The N—H···O contacts are in the interval 2.71 (SOMHUX) – 3.04 Å (e.g. WINKUZ01) while the pertinent angles fall into the interval 153 (FUQNAG) – 177° (SOMHUX). The O—H···O bond lengths are shorter, being in the interval 2.49 (WOBZES) – 2.63 Å (SOMHUX) while the angles fall into the interval 157 (SOMHUX) – 177° (e.g. FUQMOT). (Cambridge Structural Database REFCODES are given in parentheses.)

Despite these similarities, the aforementioned compounds differ in how the sheets composed of dihydrogenphosphates and ammonium groups are interbonded by hydrogen bonds. In the title compound, there are columns of aggregated dihydrogenphosphates parallel to (100) and directed along [010] (Fig. 1). These columns are held together by N—H···O hydrogen bonds while in n-alkylammoniumdihydrogenphosphates as well as in SOMHUX the O—H···O hydrogen-bond network itself makes sheets of H₂PO₄^-. (In the latter compounds, these sheets are also different.)

It is interesting that there is one C—H···O contact that should be considered as a hydrogen bond according to the criteria given by Desiraju & Steiner (1999). The angle between the planes C2—C7-H1c7 and C2—C7—O1 (-x,-1/2 + y,1/2 - z) is only 15 (1)° thus indicating attraction of the hydrogen H1c7 to the atom O1. (The distance C7···O1(-x,1/2 + y,1/2 - z) is 3.586 (3) Å; the distance H1c7···O1(-x,1/2 + y,1/2 - z) is 3.48 (2) Å pointing to a possible electrostatic interaction.) The interatomic distances and angles are otherwise normal.

The differential scanning calorimetry experiments [Perkin Elmer DSC 7 using PYRIS software (Perkin-Elmer,1997)] showed interesting behaviour. During heating an anomaly ocurred between 441 and 443 K (aluminium pans, m=9.35 mg, scanning rate 10 K/min, ΔH = 21 J/g). Interestingly, cooling caused no corresponding anomaly. An immediate heating of the sample after it had been cooled down to room temperature did not bring about to the anomaly at \sim442 K.

Nevertheless, after several hours since the last cooling the repeated heating caused partial or full restoration of the anomaly at ~442 K. The degree of restoration depended on time it had elapsed since the last cooling. Both phenomena (i.e. no anomaly during cooling as well as relaxation necessary for restoration of the anomaly at ~442 K) indicate a structural change with much faster kinetics on heating than on cooling.

S2. Experimental

Precipitation of 2,4-dimethylaniline and H₃PO₄. The precipitate was filtered off, dried and dissolved in 96% ethanol from which the single crystals were grown by slow evaporation at room temperature.

S3. Refinement

The O—H, N—H, C(sp²)-H and C(sp³) bond lengths were restrained to 0.85 (2), 0.90 (3), 0.97 (1) and 0.95 (1) Å, respectively. The angles between the H atoms pertinent to the respective methyl groups were restrained to the values 109.5 (1)°.

Computing details

Cell refinement: KM4B8; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: JANA2000; program(s) used to refine structure: JANA2000; molecular graphics: ORTEPIII (Burnett & Johnson, 1996).

Figures

Fig. 1. View of the unit cell of 2,4-dimethylanilinium dihydrogenphosphate along the unit-cell axis b with 30% probability displacement ellipsoids (ORTEPIII; Burnett & Johnson, 1996). For clarity the hydrogen bonds from H3n and H2o are not depicted.

(I)

Crystal data

(C8H9NH3)(H2PO4)	F(000) = 464
Mr = 219.2	Dx = 1.394 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.710688 Å
a = 13.009 (3) Å	Cell parameters from 68 reflections
b = 4.688 (1) Å	θ = 6.1–18.1°
c = 17.607 (4) Å	µ = 0.25 mm^−1
β = 103.60 (2)°	T = 290 K
V = 1043.7 (4) Å^3	Prism, colourless
Z = 4	0.24 × 0.19 × 0.14 mm

Data collection

KUMA4 diffractometer	Rint = 0.055
ω–2θ scans	θmax = 26.0°, θmin = 1.6°
Absorption correction: gaussian Gaussian integration (Coppens, 1965)	h = −16→15
Tmin = 0.942, Tmax = 0.967	k = −5→5
3969 measured reflections	l = 0→21
2050 independent reflections	3 standard reflections every 200 reflections
1258 reflections with I > 3σ(I)	intensity decay: 2.1%

Refinement

Refinement on F	0 constraints
Least-squares matrix: full with fixed elements per cycle	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.034	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 0.041	(Δ/σ)max = 0.0001
S = 1.75	Δρmax = 0.57 e Å^−3
2050 reflections	Δρmin = −0.92 e Å^−3
184 parameters	Extinction correction: Becker & Coppens (1974) type 2
20 restraints	Extinction coefficient: 0.00063 (3)

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

	x	y	z	Uiso*/Ueq
P	0.08037 (4)	0.05703 (11)	0.11655 (3)	0.02769 (18)
O1	−0.03598 (12)	0.1746 (3)	0.08557 (9)	0.0351 (5)
O2	0.15871 (13)	0.3039 (3)	0.11068 (11)	0.0468 (6)
O3	0.10080 (12)	−0.1884 (3)	0.06608 (8)	0.0366 (5)
O4	0.09002 (12)	−0.0155 (3)	0.20150 (8)	0.0390 (5)
N	0.14708 (15)	0.4949 (4)	0.28896 (12)	0.0331 (7)
C1	0.26217 (16)	0.4775 (4)	0.31971 (12)	0.0295 (7)
C2	0.30343 (18)	0.3005 (4)	0.38333 (12)	0.0332 (7)
C3	0.4134 (2)	0.2859 (5)	0.40772 (14)	0.0435 (9)
C4	0.48118 (18)	0.4390 (5)	0.37308 (14)	0.0429 (8)
C5	0.4361 (2)	0.6158 (5)	0.31124 (16)	0.0484 (10)
C6	0.3272 (2)	0.6360 (5)	0.28401 (15)	0.0421 (9)
C7	0.2329 (2)	0.1319 (6)	0.42350 (18)	0.0523 (10)
C8	0.6008 (2)	0.4085 (9)	0.4009 (2)	0.0716 (14)
H1n	0.1115 (15)	0.523 (4)	0.3265 (11)	0.026 (5)*
H2n	0.1225 (19)	0.330 (4)	0.2625 (14)	0.053 (7)*
H3n	0.131 (2)	0.660 (5)	0.2576 (15)	0.066 (8)*
H1c3	0.4446 (18)	0.161 (4)	0.4508 (10)	0.055 (7)*
H1c5	0.4814 (15)	0.718 (4)	0.2844 (12)	0.055 (7)*
H1c6	0.2939 (16)	0.757 (4)	0.2407 (9)	0.046 (7)*
H1c7	0.1708 (13)	0.053 (4)	0.3903 (13)	0.091 (11)*
H2c7	0.273 (2)	−0.019 (4)	0.4512 (14)	0.112 (13)*
H3c7	0.212 (2)	0.256 (5)	0.4597 (13)	0.126 (15)*
H1c8	0.621 (2)	0.232 (3)	0.3819 (16)	0.16 (2)*
H2c8	0.632 (2)	0.562 (4)	0.3789 (15)	0.125 (16)*
H3c8	0.623 (2)	0.415 (5)	0.4564 (6)	0.102 (12)*
H1o	−0.056 (2)	0.189 (6)	0.0360 (10)	0.064 (9)*
H2o	0.132 (2)	0.466 (4)	0.0963 (19)	0.092 (12)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P	0.0308 (3)	0.0229 (2)	0.0296 (3)	0.0037 (2)	0.0074 (2)	0.0038 (2)
O1	0.0317 (9)	0.0438 (9)	0.0300 (10)	0.0080 (7)	0.0074 (7)	0.0027 (7)
O2	0.0372 (9)	0.0283 (9)	0.0749 (13)	0.0035 (8)	0.0135 (9)	0.0126 (9)
O3	0.0481 (10)	0.0273 (8)	0.0347 (9)	0.0099 (7)	0.0101 (7)	0.0032 (7)
O4	0.0509 (10)	0.0347 (9)	0.0303 (9)	0.0048 (7)	0.0075 (7)	0.0043 (6)
N	0.0335 (11)	0.0350 (13)	0.0322 (11)	0.0007 (8)	0.0106 (9)	0.0011 (9)
C1	0.0308 (11)	0.0310 (12)	0.0279 (11)	−0.0010 (9)	0.0092 (9)	−0.0052 (9)
C2	0.0392 (13)	0.0329 (12)	0.0278 (12)	−0.0002 (10)	0.0086 (10)	0.0006 (9)
C3	0.0417 (15)	0.0514 (16)	0.0353 (15)	0.0092 (12)	0.0050 (12)	0.0023 (11)
C4	0.0335 (13)	0.0575 (14)	0.0373 (13)	0.0026 (12)	0.0074 (10)	−0.0133 (13)
C5	0.0439 (16)	0.0549 (17)	0.0534 (17)	−0.0101 (13)	0.0253 (13)	−0.0038 (13)
C6	0.0429 (15)	0.0440 (14)	0.0418 (15)	0.0022 (12)	0.0147 (12)	0.0085 (11)
C7	0.0492 (17)	0.0524 (17)	0.0562 (18)	0.0014 (14)	0.0144 (14)	0.0200 (14)
C8	0.0338 (16)	0.113 (3)	0.066 (2)	−0.0019 (19)	0.0069 (15)	−0.016 (2)

Geometric parameters (Å, º)

P—O1	1.583 (2)	C3—C4	1.385 (3)
P—O2	1.562 (2)	C3—H1c3	0.97 (2)
P—O3	1.515 (2)	C4—C5	1.384 (3)
P—O4	1.510 (2)	C4—C8	1.525 (3)
O1—H1o	0.85 (2)	C5—C6	1.388 (3)
O2—H2o	0.85 (2)	C5—H1c5	0.97 (2)
N—H1n	0.90 (2)	C6—H1c6	0.97 (2)
N—H2n	0.92 (2)	C7—H1c7	0.95 (2)
N—H3n	0.95 (2)	C7—H2c7	0.95 (2)
C1—N	1.470 (2)	C7—H3c7	0.95 (2)
C1—C2	1.396 (2)	C8—H1c8	0.95 (2)
C1—C6	1.383 (3)	C8—H2c8	0.95 (2)
C2—C3	1.395 (3)	C8—H3c8	0.95 (1)
C2—C7	1.508 (4)
O1—P—O2	107.77 (9)	H1n—N—H3n	103 (2)
O1—P—O3	110.36 (9)	H2n—N—H3n	113 (2)
O1—P—O4	105.7 (1)	C2—C3—H1c3	119 (1)
O2—P—O3	108.5 (1)	C4—C3—H1c3	118 (1)
O2—P—O4	109.2 (1)	C4—C5—H1c5	119 (1)
O3—P—O4	115.14 (9)	C6—C5—H1c5	119 (1)
N—C1—C2	119.7 (2)	C1—C6—H1c6	118 (1)
N—C1—C6	118.8 (2)	C5—C6—H1c6	123 (1)
C1—C2—C3	116.7 (2)	C2—C7—H1c7	116 (1)
C1—C2—C7	121.8 (2)	C2—C7—H2c7	108 (2)
C2—C3—C4	123.5 (2)	C2—C7—H3c7	107 (2)
C3—C4—C5	117.5 (2)	H1c7—C7—H2c7	109 (2)
C1—C6—C5	119.4 (2)	H1c7—C7—H3c7	109 (2)
C3—C4—C8	121.2 (2)	H2c7—C7—H3c7	109 (2)
C4—C5—C6	121.4 (2)	C4—C8—H1c8	108 (2)
P—O1—H1o	114 (2)	C4—C8—H2c8	108 (2)
P—O2—H2o	117 (2)	C4—C8—H3c8	112 (2)
C1—N—H1n	113 (1)	H1c8—C8—H2c8	110 (2)
C1—N—H2n	110 (2)	H1c8—C8—H3c8	110 (2)
C1—N—H3n	109 (2)	H2c8—C8—H3c8	110 (2)
H1n—N—H2n	109 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1o···O3^i	0.85 (2)	1.76 (2)	2.605 (2)	175 (2)
O2—H2o···O3^ii	0.85 (3)	1.72 (2)	2.563 (2)	170 (3)
N—H1n···O1^iii	0.90 (2)	2.14 (2)	3.030 (2)	169 (2)
N—H2n···O4	0.92 (2)	1.94 (2)	2.848 (2)	172 (2)
N—H3n···O4^ii	0.95 (2)	1.82 (2)	2.766 (2)	175 (2)
C7—H1c7···O1^iv	0.95 (2)	2.60 (2)	3.315 (3)	132 (2)

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