Sodium peroxodi­sulfate

The asymmetric unit of disodium peroxodisulfate 2Na⁺·S₂O₈²⁻ consists of a single Na⁺ cation and half of a peroxodisulfate dianion, the latter lying across a crystallographic inversion centre. The crystal structure is isostructural with that of potassium peroxodisulfate and it is composed of layers of mol­ecules, partitioned by the Na⁺ cations, parallel to the (01) plane of the triclinic cell. Neighbouring mol­ecules within each layer are bridged end-to-end by pairs of short S⋯O inter­molecular contacts [S⋯O = 3.074 (2) Å].

Comment

Sodium peroxodisulfate, (I), readily forms SO₄ radicals in hot aqueous solution. It is a powerful oxidizing and bleaching agent and it can also be used as a polymerization promoter, as well as providing a cleaner alternative to ferric chloride for copper etchant solutions (Serguchev et al., 1980).

Compund (I) crystallizes from aqueous solution in the triclinic space group P, with one Na⁺ cation and half of the peroxodisulfate dianion, the latter located across an inversion centre, in the asymmetric unit (Fig. 1). Its crystal structure, which is isostructural with that of potassium peroxodisulfate, K₂S₂O₈, [Naumov et al., 1997; ICSD (Belsky et al., 2002) refcode 54024] is composed of layers of peroxodisulfate anions, which are aligned parallel to the (01) plane and partitioned by corrugated layers of Na⁺ cations (Fig. 2).

The intra­molecular S—O distances and O—S—O bond angles for the peroxodisulfate dianion are very simlar to those reported for the potassium analogue (see Table 1). However, the cation environments for the two analogues are quite different. In potassium peroxodisulfate, the K⁺ cations are coordinated by nine O atoms with inter­atomic distances ranging from 2.751 (3) to 3.347 (3) Å. For sodium peroxo­disulfate, the Na⁺ cations are coordinated by six O atoms, with Na—O inter­atomic distances between 2.340 (2) and 2.596 (2) Å. It is inter­esting to note that in the sodium analogue, the O atom involved in the intra­molecular peroxo bond, O1, is not involved in the cation coordinate environment, although it does exhibit the shortest Na—O distance outside this range [Na1—O1 = 3.167 (2) Å]. The K1—O1 distance in the potassium analogue [3.089 (3) Å] is of an inter­mediate length compared with the other O atoms defining the coordination environment. The overall effect of the Na—O coordination environment in sodium peroxodisulfate is the formation of a three-dimensional network. This is indicated by the polyhedral plot shown in Fig. 3. The Na⁺ cations form layers of distorted edge-sharing octa­hedra (shown as the blue polyhedra in Fig. 3), while the tetra­hedra formed by each end of the dianions (shown as the yellow polyhedra in Fig. 3) form corner-sharing bridges between the layers.

Perhaps the most striking difference between the two structures concerns the S⋯O intra­molecular contact distances. Within the layers, neighbouring anions are aligned end-to-end so that pairs of relatively short S⋯O contacts are formed. In the sodium analogue these contacts are extremely short [S1⋯O3 = 3.074 (2) Å] (Fig. 4), while in the crystal structure of the potassium analogue these contacts are significantly longer [S1⋯O3 = 3.417 (3) Å].

Figure 1 The structure of sodium peroxodisulfate, showing 30% probability ellipsoids [symmetry code: (i) −x + 1, −y + 2, −z + 1].

Figure 2 The packing of sodium peroxodisulfate, viewed along the a axis.

Figure 3 A polyhedral representation of the crystal structure of sodium peroxodisulfate. The structure is viewed along the c axis, with the a axis directed to the right and the b axis directed upwards. The blue polyhedra indicate the layers of distorted edge-sharing NaO6 octa­hedra while the yellow tetra­hedra indicate the SO4 groups of the peroxodisulfate dianion.

Figure 4 The dashed lines indicate short S⋯O contacts in sodium peroxodisulfate. This view is approximately perpendicular to (01).

Experimental

The sample of sodium peroxodisulfate was prepared from anhydrous starting material (of 99% purity, as received from Aldrich) and recrystallized from an aqueous solution by slow evaporation. A suitable crystal was selected from the resulting batch. The sample was cooled to 150 K using an Oxford Cryosystems low-temperature device (Cosier & Glazer, 1986) during data collection.

Crystal data

• 2Na⁺·O₈S₂²⁻

• M_r = 238.11

• Triclinic,

• a = 4.780 (2) Å

• b = 5.575 (2) Å

• c = 6.091 (3) Å

• α = 101.871 (7)°

• β = 103.337 (7)°

• γ = 97.418 (7)°

• V = 151.89 (11) ų

• Z = 1

• D_x = 2.603 Mg m⁻³

• Mo Kα radiation

• Cell parameters from 680 reflections

• θ = 7–57°

• μ = 1.02 mm⁻¹

• T = 150 K

• Needle, colourless

• 0.20 × 0.05 × 0.05 mm

Data collection

• Bruker SMART diffractometer

• φ and ω scans

• Absorption correction: multi-scanSADABS (Sheldrick, 2004)T_min = 0.67, T_max = 0.95

• 1326 measured reflections

• 696 independent reflections

• 590 reflections with I > 2σ(I)

• R_int = 0.021

• θ_max = 28.9°

• h = −6 → 6

• k = −7 → 7

• l = −8 → 7

Refinement

• Refinement on F

• R[F² > 2σ(F²)] = 0.038

• wR(F²) = 0.034

• S = 1.06

• 590 reflections

• 56 parameters

• Modified Chebychev polynomial (Watkin, 1994; Prince, 1982) with the coefficients 1.89, −1.11, 1.17

• (Δ/σ)_max < 0.001

• Δρ_max = 0.48 e Å⁻³

• Δρ_min = −0.42 e Å⁻³

Table 1
Selected geometric parameters (Å, °)

S1—O1	1.6392 (19)
S1—O2	1.4389 (18)
S1—O3	1.4408 (19)
S1—O4	1.4396 (19)
O1—O1^i	1.479 (3)
Na1—O3	2.376 (2)
Na1—O3^ii	2.380 (2)
Na1—O2^iii	2.389 (2)
Na1—O2^iv	2.476 (2)
Na1—O4^v	2.340 (2)
Na1—O4^vi	2.596 (2)

O1—S1—O2	97.30 (11)
O1—S1—O3	105.92 (11)
O1—S1—O4	106.87 (11)
O2—S1—O3	115.78 (12)
O2—S1—O4	115.65 (12)
O3—S1—O4	113.08 (11)
O1^i—O1—S1	106.26 (17)
Symmetry codes: (i) -x+1, -y+2, -z+1; (ii) -x, -y+1, -z; (iii) x-1, y-1, z; (iv) -x+1, -y+1, -z; (v) -x+1, -y+1, -z+1; (vi) x-1, y, z.

Indexing with GEMINI (Sparks, 1999) revealed that the sample was twinned non-merohedrally with two domains. The data set was integrated using the orientation matrix of the stronger subset of reflections, corresponding to the larger domain. During refinement, the ROTAX procedure, as implemented in the CRYSTALS refinement package (Cooper et al., 2002), was used to identify the relationship between the two domains. This could be expressed by the matrix (00, 00, 0.667 0.523 1), which corresponds to a twofold rotation about the c* axis. Subsequent refinement indicated that the twin fraction of the second domain was 0.379 (8).

Data collection: SMART (Bruker Nonius, 2001); cell refinement: SAINT; data reduction: SAINT (Bruker Nonius, 2003); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molec­ular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS and PLATON (Spek, 2003).

Supporting information

Computing details

Data collection: SMART (Bruker–Nonius, 2001); cell refinement: SAINT; data reduction: SAINT (Bruker–Nonius, 2003); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS and PLATON (Spek, 2003).

disodium peroxodisulfate

Crystal data

2Na^+·O8S2^2^−	Z = 1
Mr = 238.11	F(000) = 118
Triclinic, P1	Dx = 2.603 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 4.780 (2) Å	Cell parameters from 680 reflections
b = 5.575 (2) Å	θ = 7–57°
c = 6.091 (3) Å	µ = 1.02 mm^−1
α = 101.871 (7)°	T = 150 K
β = 103.337 (7)°	Needle, colourless
γ = 97.418 (7)°	0.20 × 0.05 × 0.05 mm
V = 151.89 (11) Å^3

Data collection

Bruker SMART diffractometer	590 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
φ and ω scans	θmax = 28.9°, θmin = 3.6°
Absorption correction: multi-scan SADABS (Sheldrick, 2004)	h = −6→6
Tmin = 0.67, Tmax = 0.95	k = −7→7
1326 measured reflections	l = −8→7
696 independent reflections

Refinement

Refinement on F	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.038	Modified Chebychev polynomial (Watkin, 1994; Prince, 1982) with the coefficients 1.89, -1.11, 1.17
wR(F^2) = 0.034	(Δ/σ)max = 0.000217
S = 1.06	Δρmax = 0.48 e Å^−3
590 reflections	Δρmin = −0.42 e Å^−3
56 parameters

Special details

Refinement. ABSTM02_ALERT_3_C The ratio of expected to reported Tmax/Tmin(RR') is < 0.90 T min and Tmax reported: 0.670 0.950 T min(prime) and Tmax expected: 0.814 0.950 RR(prime) = 0.824 Please check that your absorption correction is appropriate. PLAT061_ALERT_3_C Tmax/Tmin Range Test RR' too Large ············. 0.82

The crystal was a fine needle and the absorption correction correspondingly anisotropic.

PLAT029_ALERT_3_C _diffrn_measured_fraction_theta_full Low ······. 0.97

This value is only marginally smaller than the ideal (0.99).

PLAT432_ALERT_2_C Short Inter X···Y Contact S1.. O3.. 3.07 A ng.

This short intermolecular contact bridges persulfate molecules end-on to form infinite chains. This interaction is similar to that observed by Naumov et al. (1997) for the isostructural potassium persulfate where the molecules are also bridged by short S···O intermolecular bonds. In this instance, however, the distance is 3.417 A ng.

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

	x	y	z	Uiso*/Ueq
Na1	0.0359 (2)	0.31397 (19)	0.20685 (18)	0.0131
S1	0.61154 (13)	0.78124 (11)	0.24744 (11)	0.0080
O1	0.5785 (4)	1.0388 (3)	0.4180 (3)	0.0143
O2	0.7722 (4)	0.8959 (3)	0.1113 (3)	0.0113
O3	0.3176 (4)	0.6526 (3)	0.1263 (3)	0.0111
O4	0.7701 (4)	0.6451 (3)	0.3949 (3)	0.0130

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0141 (5)	0.0136 (5)	0.0102 (5)	0.0005 (4)	0.0012 (4)	0.0032 (4)
S1	0.0088 (3)	0.0082 (3)	0.0068 (3)	0.00165 (19)	0.0025 (2)	0.00122 (19)
O1	0.0223 (10)	0.0094 (8)	0.0132 (9)	−0.0009 (7)	0.0125 (8)	0.0006 (7)
O2	0.0135 (9)	0.0106 (8)	0.0112 (8)	0.0013 (7)	0.0063 (7)	0.0031 (6)
O3	0.0099 (9)	0.0106 (8)	0.0114 (8)	0.0004 (7)	0.0016 (7)	0.0017 (7)
O4	0.0119 (9)	0.0165 (9)	0.0116 (9)	0.0046 (7)	0.0013 (7)	0.0065 (7)

Geometric parameters (Å, º)

S1—O1	1.6392 (19)	Na1—O3^ii	2.380 (2)
S1—O2	1.4389 (18)	Na1—O2^iii	2.389 (2)
S1—O3	1.4408 (19)	Na1—O2^iv	2.476 (2)
S1—O4	1.4396 (19)	Na1—O4^v	2.340 (2)
O1—O1^i	1.479 (3)	Na1—O4^vi	2.596 (2)
Na1—O3	2.376 (2)	Na1—Na1^ii	3.565 (2)
O1—S1—O2	97.30 (11)	O2^iii—Na1—Na1^ii	121.43 (7)
O1—S1—O3	105.92 (11)	O2^iv—Na1—Na1^ii	73.92 (6)
O1—S1—O4	106.87 (11)	O4^vi—Na1—Na1^ii	79.90 (6)
O2—S1—O3	115.78 (12)	O4^v—Na1—O3	103.99 (8)
O2—S1—O4	115.65 (12)	O3^ii—Na1—O3	82.92 (8)
O3—S1—O4	113.08 (11)	O2^iii—Na1—O3	153.31 (8)
O1^i—O1—S1	106.26 (17)	O2^iv—Na1—O3	77.27 (7)
O4^v—Na1—O3^ii	154.76 (8)	O4^vi—Na1—O3	85.88 (8)
O4^v—Na1—O2^iii	97.98 (8)	Na1^ii—Na1—O3	41.50 (5)
O3^ii—Na1—O2^iii	83.62 (7)	Na1^vii—O2—Na1^iv	102.44 (7)
O4^v—Na1—O2^iv	126.29 (8)	Na1^vii—O2—S1	127.66 (11)
O3^ii—Na1—O2^iv	78.77 (7)	Na1^iv—O2—S1	127.67 (11)
O2^iii—Na1—O2^iv	77.56 (7)	Na1—O3—Na1^ii	97.08 (8)
O4^v—Na1—O4^vi	77.36 (7)	Na1—O3—S1	128.52 (11)
O3^ii—Na1—O4^vi	79.00 (7)	Na1^ii—O3—S1	134.38 (11)
O2^iii—Na1—O4^vi	113.94 (8)	Na1^v—O4—Na1^viii	102.64 (7)
O2^iv—Na1—O4^vi	153.52 (7)	Na1^v—O4—S1	137.82 (12)
O4^v—Na1—Na1^ii	139.91 (7)	Na1^viii—O4—S1	117.76 (11)
O3^ii—Na1—Na1^ii	41.42 (5)

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