The crystal structure of (C2H9N2)2[Zn3(HPO3)4], a three-dimensional zincophosphite framework containing 16-membered rings templated by the unsymmetrical dimethyl hydrazinium cation

Katinaitė, J.; Harrison, W.T.A.

doi:10.1107/S2056989017005758

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Volume 73| Part 5| May 2017| Pages 759-762

https://doi.org/10.1107/S2056989017005758

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The crystal structure of (C₂H₉N₂)₂[Zn₃(HPO₃)₄], a three-dimensional zincophosphite framework containing 16-membered rings templated by the unsymmetrical dimethyl hydrazinium cation

Judita Katinaitė ^a and William T. A. Harrison ^a ^*

^aDepartment of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, Scotland
^*Correspondence e-mail: w.harrison@abdn.ac.uk

Edited by J. Simpson, University of Otago, New Zealand (Received 14 April 2017; accepted 16 April 2017; online 28 April 2017)

The solution-mediated synthesis and crystal structure of 1,1-dimethylhydrazinium tetraphoshonoatotrizincate, (C₂H₉N₂)₂[Zn₃(HPO₃)₄], are described. The anionic [Zn₃(HPO₃)₄]²⁻ framework is built up from alternating ZnO₄ tetrahedra and HPO₃ pseudo-pyramids to generate a three-dimensional 4,3-net encapsulating the C₂H₉N₂⁺ cations. The organic cations, which are protonated at their central N atoms, occupy pores delineated by large 16-membered polyhedral rings and interact with the framework by way of N—H⋯O hydrogen bonds and possible C—H⋯O interactions. One of the zinc ions lies on a crystallographic twofold rotation axis and all the other atoms lie on general positions. The crystal studied was found to be rotationally twinned about the [001] axis in reciprocal space in a 0.585 (5):0.415 (5) ratio.

Keywords: crystal structure; zinc phosphite; unsymmetrical dimethyl hydrazine; open framework.

CCDC reference: 1544227

1. Chemical context

Organically templated zinc phosphites are now a well-established family of open frameworks (e.g.: Phillips et al., 2002 ; Luo et al., 2010 ; Wang et al., 2011 ; Dong et al., 2015 ; Huang et al., 2017 ). As part of our occasional studies in this area (Harrison & McNamee, 2010 ), we now describe the synthesis and structure of the title compound, (I), which represents the first example of a protonated unsymmetrical dimethyl hydrazine (C₂H₈N₂ or UDMH is the neutral molecule and C₂H₉N₂⁺ is the cation) acting as a templating agent for an inorganic open framework. So far as we are aware, the only crystal structures containing C₂H₉N₂⁺ that have been reported previously are molecular salts with different simple counter-ions (Katinaitė & Harrison, 2016 , and references therein).

2. Structural commentary

The asymmetric unit of (I) comprises two zinc cations (Zn1 with site symmetry 2 and Zn2 on a general position), two HPO₃²⁻ hydrogen phosphite groups and one C₂H₉N₂⁺ cation (Fig. 1). Both zinc ions adopt their usual tetrahedral coordination geometries (Table 1) to four nearby O atoms with mean Zn—O separations of 1.942 and 1.945 Å for Zn1 and Zn2, respectively. The range of O—Zn—O bond angles for Zn1 of 100.0 (2)–121.0 (2)° indicates considerable distortion from the ideal tetrahedral value of 109.5°; the spread of values for Zn2 of 99.8 (2)–115.1 (2)° is somewhat smaller. Bond-valence-sum values (in valence units; Brown & Altermatt, 1985 ) for Zn1 and Zn2 of 2.11 and 2.09, respectively, are in adequate agreement with the expected values of 2.00.

Table 1
Selected geometric parameters (Å, °)

Zn1—O4	1.938 (5)	P1—O2	1.504 (5)
Zn1—O4ⁱ	1.938 (5)	P1—O1	1.515 (6)
Zn2—O5ⁱⁱ	1.936 (6)	P1—O3	1.533 (5)
Zn2—O2ⁱⁱⁱ	1.943 (5)	P2—O5	1.500 (6)
Zn2—O6^iv	1.946 (5)	P2—O6	1.520 (5)
Zn2—O1	1.954 (6)	P2—O4	1.529 (5)

P1—O1—Zn2	128.0 (3)	P2—O4—Zn1	123.6 (3)
P1—O2—Zn2ⁱⁱⁱ	140.3 (3)	P2—O5—Zn2^v	138.4 (4)
P1—O3—Zn1	137.2 (3)	P2—O6—Zn2^vi	120.8 (3)
Symmetry codes: (i) $[-x, y, -z+{\script{1\over 2}}]$ ; (ii) $[-x+{\script{1\over 2}}, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (iii) $[-x+{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]$ ; (iv) $[x+{\script{1\over 2}}, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (v) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (vi) $[x-{\script{1\over 2}}, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Figure 1
The asymmetric unit of (I)

expanded to show the zinc coordination polyhedra (50% displacement ellipsoids). For symmetry codes, see Table 1

Both phosphorus atoms in (I) display their expected HPO₃ pseudo-tetrahedral geometries with mean P—O distances (1.517 Å for P1 and 1.516 Å for P2) and O—P—O angles (112.7° for P1 and 112.6° for P2) that are consistent with previous results (Dong et al., 2015). P1 deviates from its pyramid of attached O atoms by 0.418 (4) and the equivalent deviation for P2 is 0.420 (3) Å.

The structure of (I) is completed by the charge-balancing C₂H₉N₂⁺ cation, which is protonated at the central (methylated) N2 atom, as is most commonly seen for this species (Katinaitė & Harrison, 2016). The C—N and N—N bond lengths are indistinguishable and N2 deviates from the plane of N1, C1 and C2 by 0.434 (8) Å.

In the extended framework structure of (I), the zinc- and phosphorus-centred building units strictly alternate: every O atom forms a Zn—O—P bridge (mean angle = 131.4°), thus there are no `dangling' Zn—OH₂, P=O or P—OH bonds as found in some zincophosphite frameworks (Shi et al., 2004 ; Liu et al., 2008 ), which is fully consistent with the 3:4 Zn:P stoichiometry of the anionic [Zn₃(HPO₃)₄]²⁻ component of the structure (Harrison & McNamee, 2010). In addition, there are no Zn—N bonds (direct metal-to-template links) in (I); compare Kirkpatrick & Harrison (2004 ), Lin et al. (2004 ) and Harrison (2006 ).

The polyhedral connectivity in (I) can be broken down as follows: the Zn2, P1 and P2 polyhedra form four-ring (i.e.: a loop of two Zn atoms and two P atoms) chains, with the zinc atoms as the linking nodes, which propagate alternately in the [ $[\overline{1}]$ 10] and [110] directions with respect to the c-axis direction. Atom Zn1 serves to link these criss-cross chains into a three-dimensional open framework. If the template is omitted, a PLATON (Spek, 2009 ) analysis indicates that 878 Å³ (43.3%) of the unit cell is `empty space' and the `framework density' (FD) (number of Zn and P atoms per 1000 Å³; Brunner & Meier, 1989 ) of (I) is 13.8. This low FD is comparable to that of the unusual open-framework MAPSO-46, which contains Mg, Al, P and Si as its tetrahedral framework atoms (Bennett & Marcus, 1988 ). When the template is included in the calculation, PLATON indicates no free space, suggesting that the template is a `snug fit' within the inorganic framework of (I).

In the extended structure, large 16-ring pores (Figs. 2 and 3) are apparent in the framework, which alternately propagate in [ $[\overline{1}]$ 10] and [110] with respect to the c-axis direction. Measured atom-to-atom, the 16-ring has a dimension of ∼5.7 × 14.6 Å. Pairs of template cations lie roughly in the plane of the 16-rings and interact with framework oxygen atoms by way of N—H⋯O hydrogen bonds (Table 2). It is notable that the H⋯O separation for the charge-assisted N2⁺—H3N⋯O3 bond is much shorter than the H⋯O separations for the terminal N1H₂ grouping. Within the asymmetric unit, an R₂²(7) loop is apparent (Fig. 1). Possible weak C—H⋯O interactions (Table 2) consolidate the structure.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N⋯O1	0.91	2.34	3.130 (9)	146
N1—H2N⋯O6^vii	0.91	2.35	3.133 (9)	144
N2—H3N⋯O3	1.00	1.79	2.762 (8)	163
C1—H1A⋯O1^v	0.98	2.50	3.474 (11)	173
C1—H1C⋯O5^viii	0.98	2.50	3.295 (11)	138
C2—H2C⋯O2^ix	0.98	2.43	3.355 (9)	157
Symmetry codes: (v) $[-x+{\script{1\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (vii) $[x+{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ ; (viii) -x, -y, -z; (ix) $[x, -y+1, z-{\script{1\over 2}}]$ .

Figure 2
Fragment of (I)

showing a 16-ring channel occupied side-by-side by two C₂H₉N₂⁺ template cations.

Figure 3
The unit-cell packing in (I)

viewed approximately along [110] with the framework represented topologically (i.e. as Zn—P links).

3. Database survey

A survey of of the Cambridge Structural Database (Groom et al., 2016 : updated to April 2017) for organically templated zinc phosphite frameworks (those containing a Zn—O—P—H fragment) revealed 172 matches.

4. Synthesis and crystallization

Caution! UDMH is toxic, potentially carcinogenic and may form explosive mixtures with oxidizing agents: all appropriate safety precautions should be taken when handling it. Zinc oxide (1.63 g), phosphorus acid (1.64 g) and 20 ml of a 1.0 M aqueous UDMH solution were mixed in a 1:1:1 molar ratio in a sealed PTFE bottle and heated to 353 K for 24 h and then cooled to room temperature over a few hours. Product recovery by vacuum filtration yielded some colourless blocks of (I) accompanied by an unidentified white powder.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. The N-bound H atoms were located in difference maps, relocated to idealized locations (N—H = 0.91–1.00 Å) and refined as riding atoms. The other hydrogen atoms were placed geometrically (P—H = 1.32, C—H = 0.98 Å) and refined as riding atoms. The constraint U_iso(H) = 1.2U_eq(carrier) or 1.5U_eq(methyl carrier) was applied in all cases. The methyl groups were allowed to rotate, but not to tip, to best fit the electron density. The crystal chosen for data collection was found to be rotationally twinned about the [001] axis in reciprocal space in a 0.585 (5):0.415 (5) ratio.

Table 3
Experimental details

Crystal data
Chemical formula	(C₂H₉N₂)₂[Zn₃(HPO₃)₄]
M_r	638.24
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	15.1154 (5), 8.7269 (3), 16.1675 (6)
β (°)	108.156 (1)
V (Å³)	2026.48 (12)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	3.90
Crystal size (mm)	0.19 × 0.11 × 0.05

Data collection
Diffractometer	Rigaku Mercury CCD
Absorption correction	Multi-scan (SADABS; Sheldrick, 2004 )
T_min, T_max	0.527, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	2273, 2273, 2169
(sin θ/λ)_max (Å⁻¹)	0.650

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.068, 0.239, 1.22
No. of reflections	2273
No. of parameters	127
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	1.51, −1.24
Computer programs: CrysAlis PRO (Rigaku, 2015 ), SHELXS97 (Sheldrick, 2008 ), SHELXL2014 (Sheldrick, 2015 ), ORTEP-3 for Windows (Farrugia, 2012 ), ATOMS (Shape Software, 2005 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1544227

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2056989017005758/sj5526sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017005758/sj5526Isup2.hkl

checkCIF report

Computing details top

Data collection: CrysAlis PRO (Rigaku, 2015); cell refinement: CrysAlis PRO (Rigaku, 2015); data reduction: CrysAlis PRO (Rigaku, 2015); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and ATOMS (Shape Software, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

1,1-Dimethylhydrazinium tetraphoshonoatotrizincate(II) top

Crystal data top

(C₂H₉N₂)₂[Zn₃(HPO₃)₄]	F(000) = 1280
M_r = 638.24	D_x = 2.092 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 15.1154 (5) Å	Cell parameters from 6272 reflections
b = 8.7269 (3) Å	θ = 2.6–27.6°
c = 16.1675 (6) Å	µ = 3.90 mm⁻¹
β = 108.156 (1)°	T = 100 K
V = 2026.48 (12) Å³	Block, colourless
Z = 4	0.19 × 0.11 × 0.05 mm

Data collection top

Rigaku Mercury CCD diffractometer	2169 reflections with I > 2σ(I)
ω scans	θ_max = 27.5°, θ_min = 2.7°
Absorption correction: multi-scan (SADABS; Sheldrick, 2004)	h = −19→18
T_min = 0.527, T_max = 1.000	k = −11→11
2273 measured reflections	l = −11→20
2273 independent reflections

Refinement top

Refinement on F²	Hydrogen site location: mixed
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.068	w = 1/[σ²(F_o²) + (0.1587P)² + 13.6003P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.239	(Δ/σ)_max < 0.001
S = 1.22	Δρ_max = 1.51 e Å⁻³
2273 reflections	Δρ_min = −1.24 e Å⁻³
127 parameters	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.011 (2)
Primary atom site location: structure-invariant direct methods

Special details top

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.

Refinement. Refined as a 2-component twin with components rotated about (001) in reciprocal space

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Zn1	0.0000	0.16565 (13)	0.2500	0.0076 (4)
Zn2	0.36570 (6)	0.37073 (9)	0.48484 (5)	0.0096 (4)
P1	0.15444 (13)	0.3885 (2)	0.37817 (11)	0.0091 (5)
H1	0.1147	0.5248	0.3662	0.011*
P2	−0.01597 (13)	−0.0917 (2)	0.11747 (11)	0.0088 (5)
H2	−0.0253	−0.1970	0.1728	0.011*
O1	0.2557 (4)	0.4152 (7)	0.3858 (3)	0.0193 (12)
O2	0.1393 (5)	0.3246 (6)	0.4592 (3)	0.0180 (12)
O3	0.1081 (4)	0.2975 (7)	0.2948 (3)	0.0160 (11)
O4	0.0428 (4)	0.0362 (6)	0.1728 (3)	0.0146 (11)
O5	0.0321 (4)	−0.1690 (6)	0.0604 (4)	0.0169 (12)
O6	−0.1139 (4)	−0.0379 (6)	0.0684 (3)	0.0124 (10)
C1	0.1591 (7)	0.2877 (11)	0.0880 (5)	0.028 (2)
H1A	0.1821	0.1825	0.1002	0.043*
H1B	0.1847	0.3339	0.0452	0.043*
H1C	0.0909	0.2867	0.0649	0.043*
C2	0.1529 (8)	0.5365 (10)	0.1562 (6)	0.029 (2)
H2A	0.1818	0.5963	0.2091	0.043*
H2B	0.0852	0.5358	0.1439	0.043*
H2C	0.1683	0.5827	0.1072	0.043*
N1	0.2898 (6)	0.3663 (9)	0.2063 (5)	0.0249 (17)
H1N	0.3067	0.3880	0.2642	0.030*
H2N	0.3170	0.4346	0.1793	0.030*
N2	0.1884 (5)	0.3781 (7)	0.1691 (4)	0.0132 (13)
H3N	0.1608	0.3280	0.2110	0.016*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Zn1	0.0064 (7)	0.0080 (6)	0.0076 (6)	0.000	0.0008 (4)	0.000
Zn2	0.0093 (6)	0.0109 (5)	0.0078 (5)	−0.0032 (3)	0.0015 (4)	0.0003 (3)
P1	0.0106 (10)	0.0085 (8)	0.0076 (8)	−0.0011 (6)	0.0021 (7)	0.0010 (6)
P2	0.0085 (9)	0.0093 (8)	0.0077 (8)	0.0008 (6)	0.0014 (7)	0.0001 (6)
O1	0.020 (3)	0.024 (3)	0.012 (2)	−0.006 (2)	0.001 (2)	0.002 (2)
O2	0.033 (3)	0.009 (2)	0.013 (2)	−0.002 (2)	0.009 (2)	0.0000 (18)
O3	0.013 (3)	0.023 (3)	0.012 (2)	−0.010 (2)	0.004 (2)	−0.006 (2)
O4	0.015 (3)	0.014 (2)	0.014 (2)	0.000 (2)	0.002 (2)	−0.006 (2)
O5	0.025 (3)	0.012 (2)	0.016 (2)	0.005 (2)	0.009 (2)	−0.0021 (19)
O6	0.008 (3)	0.016 (2)	0.013 (2)	0.000 (2)	0.0020 (19)	0.004 (2)
C1	0.039 (5)	0.024 (4)	0.015 (3)	−0.003 (4)	−0.002 (4)	−0.008 (3)
C2	0.048 (6)	0.020 (4)	0.024 (4)	0.005 (4)	0.020 (4)	0.006 (3)
N1	0.019 (4)	0.034 (4)	0.020 (3)	−0.004 (3)	0.004 (3)	0.002 (3)
N2	0.017 (4)	0.013 (3)	0.011 (3)	−0.004 (2)	0.008 (3)	−0.001 (2)

Geometric parameters (Å, º) top

Zn1—O4	1.938 (5)	O2—Zn2ⁱⁱⁱ	1.943 (5)
Zn1—O4ⁱ	1.938 (5)	O5—Zn2^v	1.936 (6)
Zn1—O3ⁱ	1.945 (5)	O6—Zn2^vi	1.946 (5)
Zn1—O3	1.945 (5)	C1—N2	1.475 (9)
Zn2—O5ⁱⁱ	1.936 (6)	C1—H1A	0.9800
Zn2—O2ⁱⁱⁱ	1.943 (5)	C1—H1B	0.9800
Zn2—O6^iv	1.946 (5)	C1—H1C	0.9800
Zn2—O1	1.954 (6)	C2—N2	1.474 (10)
P1—O2	1.504 (5)	C2—H2A	0.9800
P1—O1	1.515 (6)	C2—H2B	0.9800
P1—O3	1.533 (5)	C2—H2C	0.9800
P1—H1	1.3200	N1—N2	1.465 (10)
P2—O5	1.500 (6)	N1—H1N	0.9100
P2—O6	1.520 (5)	N1—H2N	0.9100
P2—O4	1.529 (5)	N2—H3N	1.0000
P2—H2	1.3200

O4—Zn1—O4ⁱ	108.7 (3)	P1—O3—Zn1	137.2 (3)
O4—Zn1—O3ⁱ	121.0 (2)	P2—O4—Zn1	123.6 (3)
O4ⁱ—Zn1—O3ⁱ	100.0 (2)	P2—O5—Zn2^v	138.4 (4)
O4—Zn1—O3	100.0 (2)	P2—O6—Zn2^vi	120.8 (3)
O4ⁱ—Zn1—O3	121.0 (2)	N2—C1—H1A	109.5
O3ⁱ—Zn1—O3	107.4 (4)	N2—C1—H1B	109.5
O5ⁱⁱ—Zn2—O2ⁱⁱⁱ	99.8 (2)	H1A—C1—H1B	109.5
O5ⁱⁱ—Zn2—O6^iv	115.1 (2)	N2—C1—H1C	109.5
O2ⁱⁱⁱ—Zn2—O6^iv	110.8 (2)	H1A—C1—H1C	109.5
O5ⁱⁱ—Zn2—O1	107.5 (3)	H1B—C1—H1C	109.5
O2ⁱⁱⁱ—Zn2—O1	114.1 (3)	N2—C2—H2A	109.5
O6^iv—Zn2—O1	109.3 (2)	N2—C2—H2B	109.5
O2—P1—O1	114.2 (3)	H2A—C2—H2B	109.5
O2—P1—O3	115.0 (3)	N2—C2—H2C	109.5
O1—P1—O3	108.9 (3)	H2A—C2—H2C	109.5
O2—P1—H1	106.0	H2B—C2—H2C	109.5
O1—P1—H1	106.0	N2—N1—H1N	109.3
O3—P1—H1	106.0	N2—N1—H2N	109.2
O5—P2—O6	113.4 (3)	H1N—N1—H2N	109.5
O5—P2—O4	112.6 (3)	N1—N2—C2	114.3 (7)
O6—P2—O4	111.9 (3)	N1—N2—C1	108.3 (6)
O5—P2—H2	106.1	C2—N2—C1	112.4 (7)
O6—P2—H2	106.1	N1—N2—H3N	107.2
O4—P2—H2	106.1	C2—N2—H3N	107.2
P1—O1—Zn2	128.0 (3)	C1—N2—H3N	107.2
P1—O2—Zn2ⁱⁱⁱ	140.3 (3)

O2—P1—O1—Zn2	2.2 (6)	O5—P2—O4—Zn1	177.2 (3)
O3—P1—O1—Zn2	−127.8 (4)	O6—P2—O4—Zn1	48.1 (4)
O1—P1—O2—Zn2ⁱⁱⁱ	−86.8 (7)	O6—P2—O5—Zn2^v	110.4 (6)
O3—P1—O2—Zn2ⁱⁱⁱ	40.2 (8)	O4—P2—O5—Zn2^v	−18.0 (7)
O2—P1—O3—Zn1	28.0 (7)	O5—P2—O6—Zn2^vi	−68.4 (4)
O1—P1—O3—Zn1	157.6 (5)	O4—P2—O6—Zn2^vi	60.3 (4)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···O1	0.91	2.34	3.130 (9)	146
N1—H2N···O6^vii	0.91	2.35	3.133 (9)	144
N2—H3N···O3	1.00	1.79	2.762 (8)	163
C1—H1A···O1^v	0.98	2.50	3.474 (11)	173
C1—H1C···O5^viii	0.98	2.50	3.295 (11)	138
C2—H2C···O2^ix	0.98	2.43	3.355 (9)	157

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

Acknowledgements

We thank the EPSRC National Crystallography Service (University of Southampton) for the X-ray data collection.

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