Crystal structure of catena-poly[[tetra­aqua­magnesium]-μ-(di­hydrogen hypodiphosphato)-κ² O:O′]

[Mg(H₂P₂O₆)(H₂O)₄] is the first alkaline earth hypodiphosphate to be structurally determined. It consists of (H₂P₂O₆)²⁻ anions that are bridged by Mg²⁺ cations into a chain structure. Water mol­ecules complete the octa­hedral coordination sphere of the metal and built up a three-dimensional hydrogen-bonding network.

Abstract

The crystal structure of the title compound, [Mg(H₂P₂O₆)(H₂O)₄]_n, is built up from (H₂P₂O₆)²⁻ anions bridging Mg²⁺ cations into chains extending parallel to [011]. The Mg²⁺ ion is located on an inversion centre and is octa­hedrally coordinated by the O atoms of two (H₂P₂O₆)²⁻ anions and four water mol­ecules. The centrosymmetric (H₂P₂O₆)²⁻ anion has a staggered conformation whereby the tetra­valent phospho­rus atom is surrounded tetra­hedrally by three O atoms and by one symmetry-related P atom. A three-dimensional O—H⋯O hydrogen-bonded network of medium strength involving the P—OH group of the anion and the water mol­ecules is present.

Chemical context  

A considerable number of alkaline metal hypodiphosphates have been characterized in the last few years (Szafranowska et al., 2012 ▸; Wu et al., 2012 ▸; Gjikaj et al., 2012 ▸, 2014 ▸). Until today, the described alkaline metal hypodiphosphates have only been of academic inter­est, with the exception of ammonium and sodium di­hydrogenhypodiphosphates (Collin & Willis, 1971 ▸). The acidic solutions of sodium di­hydrogen­hypo­diphosphate are used for the gravimetric immobilization of uranium(IV) as U₂P₂O₆·3H₂O and UP₂O₇ (Bloss et al., 1967 ▸). Furthermore, ammonium di­hydrogenhypodiphosphate finds a use as a flame retardant (Ruflin et al., 2007 ▸), and its ferroelectricity has recently been discovered (Szklarz et al., 2011 ▸).

The alkaline earth metal hypodiphosphates were first described by Salzer (1878 ▸). Ca₂P₂O₆·2H₂O and BaH₂P₂O₆·2H₂O were first synthesized by Palmer (1961 ▸), but structural data of hypodiphosphates of the alkaline earth metals are still missing. Here, we report the synthesis and the crystal structure of [Mg(H₂P₂O₆)(H₂O)₄].

Structural commentary  

The principal building units in the crystal structure of [Mg(H₂P₂O₆)(H₂O)₄] are [MgO₆] octa­hedra and (H₂P₂O₆)²⁻ anions, forming chains extending parallel to [011] (Fig. 1 ▸). In the chains, each Mg²⁺ cation is bridged by two anions (Fig. 2 ▸). The Mg²⁺ ion is located on an inversion centre and is octa­hedrally coordinated by two (H₂P₂O₆)²⁻ anions and by four water mol­ecules with Mg—O bond lengths ranging from 2.0580 (17) to 2.0646 (18) Å. In the (H₂P₂O₆)²⁻ anion, which is located about an inversion centre and has a staggered conformation, the tetra­valent P atom is surrounded by three O atoms and one symmetry-related P atom with a P—P distance of 2.1843 (12) Å and P—O distances ranging from 1.5013 (16) to 1.5855 (16) Å. All bond lengths and angles of the hypodiphosphate anion are well within the expected ranges (Szafranowska et al., 2012 ▸; Gjikaj et al., 2014 ▸) and are comparable to those found for M ₂P₂O₆·12H₂O (M = Co and Ni; Hagen & Jansen, 1995 ▸; Haag et al., 2005 ▸).

Figure 1.

The crystal structure of the title compound, viewed along [100], showing the chain architecture.

Figure 2.

The mol­ecular entities in the title compound with atom labels and displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x, −y + 1, −z; (ii) −x, −y + 2, −z + 1; (iii) x, y + 1, z + 1.]

Supra­molecular features  

The crystal structure of [Mg(H₂P₂O₆)(H₂O)₄] exhibits a three-dimensional hydrogen-bonded network, in which the (H₂P₂O₆)^2– anions are joined into ribbons along [100] by centrosymmetric pairs of PO3—H3⋯O2 hydrogen bonds (Table 1 ▸ and Fig. 3 ▸). The O⋯O distances between the (H₂P₂O₆)^2– anions and water mol­ecules located between the ribbons range from 2.786 (3) to 2.829 (3) Å), indicating hydrogen bonds of medium strength (Table 1 ▸). These values agree very well with those reported for Rb₂H₂P₂O₆·2H₂O (Wu et al., 2012 ▸).

Table 1. Hydrogen-bond geometry (, ).

DHA	DH	HA	D A	DHA
O3H3O2i	0.79(4)	1.94(4)	2.687(2)	157(3)
O4H4AO2ii	0.82(3)	2.00(3)	2.817(2)	169(3)
O4H4BO1iii	0.85(4)	1.94(4)	2.786(2)	173(3)
O5H5AO2iv	0.75(4)	2.03(4)	2.768(3)	165(3)
O5H5BO3v	0.77(4)	2.08(4)	2.829(3)	165(4)

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

Figure 3.

The hydrogen bonds between (H₂P₂O₆)^2– anions and water mol­ecules in the title compound. The symmetry codes are as in Table 1 ▸.

Synthesis and crystallization  

Disodium di­hydrogenhypodiphosphate was prepared according to Leininger & Chulski (1953 ▸). An aqueous solution of hypodi­phospho­ric acid was obtained by passing a saturated solution of disodium di­hydrogenhypodiphosphate through a cation-exchange resin (Dowex 50WX2 50–100). About 40 ml of an aqueous solution of hypodi­phospho­ric acid (H₄P₂O₆) were collected in the pH range 1.5–3.5 and subsequently added to magnesium carbonate (117 mg) at room temperature. Colourless block-shaped crystals of the title compound were obtained after several days at 278 K.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. All hydrogen atoms were located in a difference Fourier map and were refined isotropically without restraints.

Table 2. Experimental details.

Crystal data
Chemical formula	[Mg(H2P2O6)(H2O)4]
M r	256.33
Crystal system, space group	Triclinic, P
Temperature (K)	223
a, b, c ()	5.1486(15), 6.595(2), 7.096(2)
, , ()	112.31(2), 98.55(2), 98.28(2)
V (3)	215.09(11)
Z	1
Radiation type	Mo K
(mm1)	0.61
Crystal size (mm)	0.28 0.25 0.23
Data collection
Diffractometer	Stoe IPDS-II
Absorption correction	Numerical (X-SHAPE and X-RED; Stoe Cie, 1999 ▸, 2001 ▸)
T min, T max	0.843, 0.869
No. of measured, independent and observed [I > 2(I)] reflections	2193, 799, 739
R int	0.057
(sin /)max (1)	0.609
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.036, 0.094, 1.15
No. of reflections	799
No. of parameters	81
H-atom treatment	All H-atom parameters refined
max, min (e 3)	0.60, 0.53

Computer programs: X-AREA (Stoe Cie, 2002 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), DIAMOND (Brandenburg, 2012 ▸), PLATON (Spek, 2009 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 1408335

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

supplementary crystallographic information

Crystal data

[Mg(H2P2O6)(H2O)4]	Z = 1
Mr = 256.33	F(000) = 132
Triclinic, P1	Dx = 1.979 Mg m−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 5.1486 (15) Å	Cell parameters from 3841 reflections
b = 6.595 (2) Å	θ = 3.2–25.7°
c = 7.096 (2) Å	µ = 0.61 mm−1
α = 112.31 (2)°	T = 223 K
β = 98.55 (2)°	Block-shaped, colourless
γ = 98.28 (2)°	0.28 × 0.25 × 0.23 mm
V = 215.09 (11) Å3

Data collection

Stoe IPDS-II diffractometer	799 independent reflections
Radiation source: fine-focus sealed tube	739 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.057
ω–scans	θmax = 25.7°, θmin = 3.2°
Absorption correction: numerical (X-SHAPE and X-RED; Stoe & Cie, 1999, 2001)	h = −6→6
Tmin = 0.843, Tmax = 0.869	k = −8→8
2193 measured reflections	l = −8→8

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.094	All H-atom parameters refined
S = 1.15	w = 1/[σ2(Fo2) + (0.0594P)2 + 0.0577P] where P = (Fo2 + 2Fc2)/3
799 reflections	(Δ/σ)max < 0.001
81 parameters	Δρmax = 0.60 e Å−3
0 restraints	Δρmin = −0.53 e Å−3

Special details

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

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
P	0.05636 (10)	0.66957 (9)	0.00780 (8)	0.0119 (2)
Mg	0.0000	1.0000	0.5000	0.0118 (3)
O1	0.1302 (3)	0.8332 (3)	0.2329 (2)	0.0158 (4)
O2	0.2710 (3)	0.6732 (3)	−0.1158 (2)	0.0167 (4)
O3	−0.2044 (3)	0.6995 (3)	−0.1200 (2)	0.0180 (4)
O4	0.3263 (3)	0.9689 (3)	0.6850 (3)	0.0233 (4)
O5	0.2204 (3)	1.2973 (3)	0.5191 (3)	0.0207 (4)
H3	−0.345 (8)	0.697 (6)	−0.086 (6)	0.041 (9)*
H4A	0.326 (6)	0.895 (5)	0.756 (5)	0.019 (7)*
H4B	0.491 (7)	1.027 (5)	0.699 (5)	0.024 (7)*
H5A	0.229 (6)	1.410 (6)	0.605 (6)	0.026 (8)*
H5B	0.220 (7)	1.323 (6)	0.422 (6)	0.041 (10)*

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
P	0.0114 (3)	0.0134 (4)	0.0117 (4)	0.0031 (2)	0.0061 (2)	0.0046 (2)
Mg	0.0107 (5)	0.0141 (6)	0.0101 (5)	0.0033 (4)	0.0042 (4)	0.0035 (4)
O1	0.0155 (8)	0.0170 (8)	0.0141 (8)	0.0036 (6)	0.0072 (6)	0.0041 (6)
O2	0.0152 (8)	0.0201 (8)	0.0171 (8)	0.0049 (6)	0.0096 (6)	0.0075 (6)
O3	0.0137 (8)	0.0277 (9)	0.0188 (8)	0.0081 (7)	0.0082 (6)	0.0131 (7)
O4	0.0128 (9)	0.0347 (10)	0.0299 (10)	0.0040 (7)	0.0037 (7)	0.0219 (9)
O5	0.0295 (9)	0.0169 (9)	0.0150 (8)	0.0016 (7)	0.0097 (7)	0.0052 (8)

Geometric parameters (Å, º)

P—O1	1.5013 (16)	Mg—O5ii	2.0646 (18)
P—O2	1.5122 (15)	Mg—O5	2.0646 (18)
P—O3	1.5855 (16)	O3—H3	0.79 (4)
P—Pi	2.1843 (12)	O4—H4A	0.82 (3)
Mg—O4ii	2.0580 (17)	O4—H4B	0.85 (4)
Mg—O4	2.0580 (17)	O5—H5A	0.75 (4)
Mg—O1	2.0637 (15)	O5—H5B	0.77 (4)
Mg—O1ii	2.0637 (15)
O1—P—O2	116.02 (9)	O1ii—Mg—O5ii	88.52 (7)
O1—P—O3	112.90 (9)	O4ii—Mg—O5	90.25 (8)
O2—P—O3	106.05 (9)	O4—Mg—O5	89.75 (8)
O1—P—Pi	108.73 (7)	O1—Mg—O5	88.52 (7)
O2—P—Pi	108.36 (7)	O1ii—Mg—O5	91.48 (7)
O3—P—Pi	104.04 (7)	O5ii—Mg—O5	180.0
O4ii—Mg—O4	180.0	P—O1—Mg	147.48 (9)
O4ii—Mg—O1	88.56 (7)	P—O3—H3	123 (2)
O4—Mg—O1	91.44 (7)	Mg—O4—H4A	128 (2)
O4ii—Mg—O1ii	91.44 (7)	Mg—O4—H4B	125.9 (19)
O4—Mg—O1ii	88.56 (7)	H4A—O4—H4B	106 (3)
O1—Mg—O1ii	180.00 (7)	Mg—O5—H5A	124 (3)
O4ii—Mg—O5ii	89.75 (8)	Mg—O5—H5B	121 (3)
O4—Mg—O5ii	90.25 (8)	H5A—O5—H5B	104 (4)
O1—Mg—O5ii	91.48 (7)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O2iii	0.79 (4)	1.94 (4)	2.687 (2)	157 (3)
O4—H4A···O2iv	0.82 (3)	2.00 (3)	2.817 (2)	169 (3)
O4—H4B···O1v	0.85 (4)	1.94 (4)	2.786 (2)	173 (3)
O5—H5A···O2vi	0.75 (4)	2.03 (4)	2.768 (3)	165 (3)
O5—H5B···O3vii	0.77 (4)	2.08 (4)	2.829 (3)	165 (4)

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