Tris­(methyl­ammonium) hydrogen­phosphate di­hydrogenphosphate

The title compound, 3CH₆N⁺·HPO₄²⁻·H₂PO₄⁻, aggregates with the moieties interconnected by O—H...O and N—H...O hydrogen bonds, with O...O and N...O distances in the ranges 2.5366 (16)–2.5785 (14) and 2.7437 (16)–2.9967 (18) Å, respectively. Three C—H...O hydrogen bonds are also present, with C...O distances in the range 3.2310 (18)–3.3345 (17) Å. All H atoms are ordered. Structures with ordered hydrogenphosphate and di­hydrogen­phosphate components are rare.

S1. Comment

Interest in the present study was influenced by properties observed in n-alkylammoniumdihydrogenphosphates [C_nH_2n+1NH₃]⁺[H₂PO₄]. In these compounds, ferroelastic phase transitions arise, as discovered by Kroupa & Fuith (1993, 1994). Phase transitions and ferroelastic switching are related to the hydrogen-bond patterns in these derivatives (see, for example, Kasatani et al., 1998; Fábry et al., 2000), and so a goal in this field is to prepare compounds in this class and determine their crystal structures. However, the structure of methylammoniumdihydrogenphosphate, (II), the simplest compound in the series, has not been reported to date. Crystals of the title compound, (I), grew accidentally during our attempts to grow crystals of (II). We present the crystal structure of (I) here.

The most interesting feature of compound (I) is the co-crystallization of the dihydrogenphosphate and hydrogenphosphate anions in an ordered arrangement. Although this is not unprecedented, the simultaneous occurrence of both anions in an ordered way is quite rare. Among inorganic structures present in the Inorganic Crystal Structure Database (ICSD, 2004) there are only two hits, namely oxonium phyllo-hexakis[dihydrogenphosphato(V)] bis[hydrogenphosphato(V)]trialuminate tetrahydrate, (H₃O)[Al₃(H₂PO₄)₆(HPO₄)₂](H₂O)₄ (Brodalla & Kniep, 1980; R = 0.051), and aluminium phyllo-dihydrogenphosphate hydrogenphosphate hydrate, Al(H₂PO₄)(HPO₄)(H₂O) (Kniep et al., 1978; R = 0.033). In both compounds, the O atoms of the anions are simultaneously coordinated to the Al atoms.

Among the compounds listed in the Cambridge Structural Database (CSD, Version 5.26; Allen, 2002) there are only three structures with the title anions and with R factors ≤ 0.05, namely tris(hordenine) monohydrogenphosphate dihydrogenphosphate monohydrate (refcode KAWMOK; Mukhopadhyay et al., 1989), hexakis(melaminium) tetrakis(dihydrogenphosphate) monohydrogenphosphate tetrahydrate (refcode XORYAE; Janczak & Perpétuo, 2002) and tris(2-ammonioethyl)amine dihydrogenphosphate monohydrogenphosphate (refcode WAKBAM; Dakhlaoui et al., 2004). The P—O bond lengths of these structure determinations correlate well with those in (I).

The moieties in (I) are interconnected by hydrogen bonds and there are several important points regarding the hydrogen-bond pattern. Firstly, the hydrogen-bond donor atoms O14, O23 and O24 do not act as hydrogen-bond acceptors, except for atom O14 which is involved in a weak C—H···O hydrogen bond (Table 2). Secondly, there is difference between the [HPO₄]²⁻ and [H₂PO₄]⁻ anions in the number of H atoms accepted from the ammonium groups. The O atoms belonging to the [HPO₄]²⁻ anion accept seven H atoms from the ammonium groups, in contrast with the [H₂PO₄]⁻ anion, which accepts only two H atoms of this kind. In addition, the O atoms of the [HPO₄]²⁻ anion accept two other hydroxyl H atoms, so each of these acceptor O atoms of the latter anion is fully saturated by three H atoms. The fact that the [HPO₄]²⁻ anion accepts more H atoms than [H₂PO₄]⁻ can be related to the higher formal negative charge of the former, to the values of the first and second degree dissociation constants pK_a^I and pK_a^II (1.3 and 6.70, respectively; Lide, 1997), and to the obvious fact that a hydroxyl group can accept fewer H atoms than an oxo group. Thirdly, O—H···O hydrogen bonds interconnect the [HPO₄]²⁻ and [H₂PO₄]⁻ anions into columns passing through the structure along the b axis, (010). The methylammonium groups that interconnect all the molecules into a three-dimensional network are attached to these molecules. The higher displacement parameters of atom N3 in one of the methyl ammonium groups can be attributed to a weaker hydrogen bond in comparison with atoms N1 and N2 (cf. Table 1). Consequently, the same holds for atom C3 in comparison with atoms C1 and C2. The P—O···A angles, where A is an acceptor N or O atom, are in the range 108.00 (6)–128.00 (6)°. Finally, three C—H···O hydrogen bonds (Table 3) complete the hydrogen bonding and conform to the criteria given by Desiraju & Steiner (1999).

S2. Experimental

Crystals of (I) were grown from a mixture of methylamine and phosphoric acid in a stoichiometric ratio of 1:1 in an attempt to prepare methylammonium dihydrogenphosphate, (II). The ethanol–water solution was placed in a refrigerator at ~280 K and left there for several months. During this period, part of the sample became dry and possibly partially decomposed, as could be judged from a yellowish taint. From this light-yellowish bulk grew several centimetre-long colourless needle-like crystals of (I), with cross-sections measuring several millimetres. The beaker also contained the desired crystals of (II), of a size of several tenths of a millimetre. The quality of the crystals of (II) was poor, even though (II) is less hygroscopic than (I). Calorimetric experiments were performed on Perkin–Elmer DSC 7 and Pyris Diamond differential scanning calorimeters using PYRIS software (Perkin–Elmer, 2001), with m = 5 mg, a temperature interval of 93–398 K and a scanning rate of 10 K min⁻¹. No structural phase transitions were detected, either on heating or on cooling. The symptoms of decomposition commenced at 378 K.

S3. Refinement

Although all H atoms could be found in the difference Fourier map and their positions and isotropic displacement parameters refined in the final model, some parameters regarding the H atoms were constrained or restrained, as follows. The methylammonium H atoms were constrained and ideal geometry was assumed, with C—H and N—H bond lengths of 0.96 and 0.89 Å, respectively. The O—H bond lengths were restrained to 0.82 (1) Å and the P—O—H angles were restrained to 109.47(s.u.?)°. For all H atoms, U_iso(H) = 1.5U_eq(parent). The structure was examined for rotational disorder in the methyl and ammonium groups and there was no evidence for disorder on analysis of the difference maps and refinement results. A search of the Cambridge Structural Database (Allen, 2002) revealed that disorder of methylammonium molecules or their groups usually corresponds to structures where no or only weak hydrogen bonds are present, such as between halogen groups. As examples, (CH₃NH₃)⁺(HgCl₃)⁻ in the determinations with refcodes QQQBVJ04 and QQQBVJ31 have disordered methyl groups (Korfer & Fuess, 1988).

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2005); cell refinement: CrysAlis RED (Oxford Diffraction, 2005); data reduction: CrysAlis RED; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: JANA2000 (Petříček & Dušek, 2000); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: JANA2000.

Figures

	Fig. 1. A view of the constituent molecules in (I), with anisotropic displacement parameters drawn at the 50% probability level.
	Fig. 2. A view of the unit cell of (I), along the b axis. Methyl H atoms have been omitted.

Tris(methylammonium) hydrogenphosphate dihydrogenphosphate

Crystal data

3CH6N^+·HO4P^2^−·H2O4P^−	F(000) = 616
Mr = 289.2	Dx = 1.577 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2yn	Cell parameters from 20259 reflections
a = 12.3571 (6) Å	θ = 3.3–26.4°
b = 6.5465 (2) Å	µ = 0.39 mm^−1
c = 15.1231 (6) Å	T = 120 K
β = 95.556 (4)°	Prism, colourless
V = 1217.65 (9) Å^3	0.41 × 0.27 × 0.11 mm
Z = 4

Data collection

Oxford Model CCD area-detector diffractometer	2498 independent reflections
Radiation source: fine-focus sealed tube	2187 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.026
Detector resolution: 8 pixels mm^-1	θmax = 26.4°, θmin = 3.3°
Rotation method data acquisition using ω scans	h = −15→15
Absorption correction: analytical [CrysAlis RED (Oxford Diffraction, 2005); method by Clark & Reid (1995)]	k = −8→8
Tmin = 0.877, Tmax = 0.937	l = −18→18
20259 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.026	Hydrogen site location: difference Fourier map
wR(F^2) = 0.080	H atoms treated by a mixture of independent and constrained refinement
S = 1.89	Weighting scheme based on measured s.u.'s w = 1/[σ^2(I) + 0.0009I^2]
2498 reflections	(Δ/σ)max = 0.001
154 parameters	Δρmax = 0.29 e Å^−3
6 restraints	Δρmin = −0.39 e Å^−3
69 constraints

Special details

Experimental. The crystal was put into "Special-Glass" capillaries produced by Wolfgang Mueller, Schöwalde bei Berlin because of hygroscopicity of the sample.

Refinement. All the H atoms were discernible in difference Fourier maps and could be refined to reasonable geometry. Despite of it the methyl and ammonium H atoms were constrained to ideal tetrahedral positions. The N—H and C—H distances were constrained to 0.89 and 0.96 Å, respectively, while their isotropic displacement parameters were kept as 1.5 multiple of equivalent displacement parameter of the atom to which the hydrogen is bonded. As to the hydroxyl H atoms their restrained distance equals to 0.82 Å, while their isotropic displacement parameters were also kept as 1.5 multiple of equivalent displacement parameter of the pertinent O atoms.

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

	x	y	z	Uiso*/Ueq
P1	0.19501 (3)	0.73150 (6)	0.41049 (2)	0.00787 (12)
P2	0.51339 (3)	0.73305 (6)	0.27457 (2)	0.00897 (12)
O11	0.17508 (9)	0.59157 (16)	0.32974 (7)	0.0133 (3)
O12	0.15801 (8)	0.95190 (15)	0.38813 (7)	0.0124 (3)
O13	0.13977 (8)	0.65071 (16)	0.49030 (7)	0.0118 (3)
O14	0.31993 (9)	0.73327 (16)	0.44353 (7)	0.0131 (3)
O21	0.46611 (9)	0.84779 (16)	0.34766 (7)	0.0150 (3)
O22	0.63476 (9)	0.69617 (17)	0.28747 (7)	0.0135 (3)
O23	0.45432 (9)	0.51903 (16)	0.26175 (7)	0.0164 (3)
O24	0.49032 (9)	0.85079 (16)	0.18322 (7)	0.0132 (3)
HO14	0.35458 (16)	0.773 (3)	0.4034 (4)	0.0196*
HO24	0.4336 (8)	0.916 (2)	0.1833 (5)	0.0198*
HO23	0.4187 (12)	0.5159 (11)	0.2131 (5)	0.0246*
N1	0.07623 (10)	0.24473 (19)	0.50257 (8)	0.0115 (4)
C1	0.11490 (13)	0.1895 (3)	0.59560 (10)	0.0175 (5)
N2	0.76111 (10)	0.84154 (18)	0.15763 (8)	0.0122 (4)
C2	0.84935 (13)	0.6880 (2)	0.15541 (10)	0.0155 (5)
N3	0.32008 (12)	0.2363 (2)	0.35783 (10)	0.0191 (4)
C3	0.38950 (15)	0.2592 (3)	0.44346 (13)	0.0236 (5)
H1N1	0.099195	0.369796	0.490831	0.0172*
H2N1	0.003905	0.241632	0.495626	0.0172*
H3N1	0.102447	0.155862	0.465544	0.0172*
H1N2	0.715738	0.802835	0.196709	0.0183*
H2N2	0.724958	0.851416	0.103965	0.0183*
H3N2	0.789774	0.962333	0.173527	0.0183*
H1N3	0.269947	0.140856	0.363594	0.0286*
H2N3	0.287589	0.354635	0.343402	0.0286*
H3N3	0.361235	0.199654	0.315353	0.0286*
H1C1	0.088247	0.056292	0.608844	0.0263*
H2C1	0.088473	0.287757	0.635435	0.0263*
H3C1	0.192942	0.1887	0.602608	0.0263*
H1C2	0.889099	0.71354	0.105054	0.0233*
H2C2	0.81834	0.55356	0.15082	0.0233*
H3C2	0.89762	0.697845	0.208934	0.0233*
H1C3	0.441976	0.365448	0.437859	0.0283*
H2C3	0.344766	0.294165	0.489695	0.0283*
H3C3	0.426491	0.132932	0.457992	0.0283*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P1	0.0076 (2)	0.0076 (2)	0.0085 (2)	0.00006 (13)	0.00095 (14)	0.00000 (14)
P2	0.0078 (2)	0.0099 (2)	0.0093 (2)	0.00062 (13)	0.00097 (14)	−0.00064 (14)
O11	0.0152 (5)	0.0121 (5)	0.0129 (5)	0.0021 (4)	0.0024 (4)	0.0038 (4)
O12	0.0147 (5)	0.0091 (5)	0.0130 (5)	−0.0018 (4)	−0.0001 (4)	−0.0010 (4)
O13	0.0103 (5)	0.0134 (5)	0.0118 (5)	0.0011 (4)	0.0026 (4)	−0.0027 (4)
O14	0.0082 (5)	0.0191 (6)	0.0121 (5)	0.0009 (4)	0.0021 (4)	−0.0025 (4)
O21	0.0136 (6)	0.0178 (6)	0.0143 (5)	0.0024 (4)	0.0051 (4)	0.0028 (4)
O22	0.0098 (5)	0.0132 (5)	0.0176 (6)	−0.0010 (4)	0.0015 (4)	−0.0038 (4)
O23	0.0202 (6)	0.0144 (6)	0.0134 (5)	0.0061 (4)	−0.0040 (4)	−0.0021 (4)
O24	0.0132 (5)	0.0140 (5)	0.0127 (5)	−0.0035 (4)	0.0022 (4)	−0.0033 (4)
N1	0.0106 (6)	0.0116 (6)	0.0126 (6)	0.0001 (5)	0.0022 (5)	−0.0009 (5)
C1	0.0164 (8)	0.0219 (8)	0.0135 (8)	0.0001 (6)	−0.0026 (6)	−0.0020 (6)
N2	0.0119 (6)	0.0119 (6)	0.0126 (6)	−0.0010 (5)	0.0008 (5)	0.0000 (5)
C2	0.0141 (8)	0.0151 (8)	0.0170 (8)	0.0024 (6)	−0.0004 (6)	0.0011 (6)
N3	0.0247 (8)	0.0131 (7)	0.0210 (7)	0.0042 (6)	0.0105 (6)	0.0016 (6)
C3	0.0222 (9)	0.0166 (9)	0.0312 (10)	−0.0012 (7)	−0.0021 (7)	0.0029 (7)

Geometric parameters (Å, º)

P1—O11	1.5274 (11)	N1—H3N1	0.89
P1—O12	1.5412 (11)	C1—H1C1	0.96
P1—O13	1.5369 (11)	C1—H2C1	0.96
P1—O14	1.5758 (11)	C1—H3C1	0.96
P2—O21	1.5008 (12)	N2—H1N2	0.89
P2—O22	1.5130 (11)	N2—H2N2	0.89
P2—O23	1.5831 (11)	N2—H3N2	0.89
P2—O24	1.5836 (11)	C2—H1C2	0.96
N1—C1	1.4859 (19)	C2—H2C2	0.96
N2—C2	1.485 (2)	C2—H3C2	0.96
N3—C3	1.490 (2)	N3—H1N3	0.89
O14—HO14	0.820 (8)	N3—H2N3	0.89
O23—HO23	0.820 (10)	N3—H3N3	0.89
O24—HO24	0.820 (11)	C3—H1C3	0.96
N1—H1N1	0.89	C3—H2C3	0.96
N1—H2N1	0.89	C3—H3C3	0.96
O11—P1—O12	111.38 (6)	H1C2—C2—H2C2	109.47
O11—P1—O13	111.84 (6)	H1C2—C2—H3C2	109.47
O11—P1—O14	109.58 (6)	H2C2—C2—H3C2	109.47
O12—P1—O13	110.46 (6)	H1N3—N3—H2N3	109.47
O12—P1—O14	108.87 (6)	H1N3—N3—H3N3	109.47
O13—P1—O14	104.45 (6)	H2N3—N3—H3N3	109.47
O21—P2—O22	115.89 (6)	H1C3—C3—H2C3	109.47
O21—P2—O23	108.99 (6)	H1C3—C3—H3C3	109.47
O21—P2—O24	110.42 (6)	H2C3—C3—H3C3	109.47
O22—P2—O23	108.31 (6)	P1—O14—O21	123.43 (6)
O22—P2—O24	106.42 (6)	P2—O23—O12^i	117.46 (6)
O23—P2—O24	106.38 (6)	P2—O24—O11^ii	116.05 (6)
H1N1—N1—H2N1	109.47	N1^iii—O13—P1	116.67 (6)
H1N1—N1—H3N1	109.47	N1—O13—P1	122.05 (6)
H2N1—N1—H3N1	109.47	N1^iv—O12—P1	127.96 (6)
H1C1—C1—H2C1	109.47	N2—O22—P2	118.02 (6)
H1C1—C1—H3C1	109.47	N2^v—O22—P2	127.16 (6)
H2C1—C1—H3C1	109.47	N2^vi—O13—P1	117.48 (6)
H1N2—N2—H2N2	109.47	N3—O11—P1	108.05 (6)
H1N2—N2—H3N2	109.47	N3^ii—O11—P1	123.42 (6)
H2N2—N2—H3N2	109.47	N3^iv—O12—P1	117.12 (6)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O14—HO14···O21	0.820 (8)	1.752 (6)	2.5366 (16)	159.5 (5)
O23—HO23···O12^i	0.820 (10)	1.771 (9)	2.5785 (14)	167.7 (7)
O24—HO24···O11^ii	0.820 (11)	1.765 (12)	2.5735 (15)	168.3 (12)
N1—H2N1···O13^iii	0.89	1.94	2.7677 (16)	154
N1—H1N1···O13	0.89	1.91	2.7825 (16)	168
N1—H3N1···O12^vii	0.89	1.94	2.8337 (16)	177
N2—H1N2···O22	0.89	1.91	2.7916 (17)	172
N2—H3N2···O22^viii	0.89	1.86	2.7437 (16)	173
N2—H2N2···O13^ix	0.89	1.93	2.8141 (15)	175
N3—H2N3···O11	0.89	2.08	2.9416 (17)	163
N3—H1N3···O12^vii	0.89	1.92	2.8044 (18)	173
N3—H3N3···O11^i	0.89	2.31	2.9967 (18)	134
C2—H1C2···O14^ix	0.96	2.53	3.2310 (19)	130
C2—H2C2···O22^v	0.96	2.56	3.3347 (19)	137
C3—H3C3···O21^vii	0.96	2.58	3.243 (2)	126

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