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As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, the previously unknown structure of NaIO2F2 corresponds to a new structure type based on isolated IO2F2- polyhedra forming sheets separated by Na+ layers. The sodium ion is octa­hedrally coordinated with 2/m site symmetry, while the IV atom has m2m symmetry with a stereochemically active lone electron pair. The O and F atoms (both with m symmetry) are bonded to the IV atoms in a fully ordered manner. A comparison with the structure of ferroelastic KIO2F2 and with structures based on hexa­gonal close packing of anions, mainly rutile-type and FeTeO3F-type, reveals differences that are attributed to the smaller ionic radius of Na+ and the ordering of the Na and I cations.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270108007087/sq3127sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270108007087/sq3127Isup2.hkl
Contains datablock I

Comment top

The existence of sodium iodine(V) oxyfluoride, NaIO2F2, has been reported without structural characterization (Nikolaeva et al., 1988). As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, we report here the structure of NaIO2F2. Iodine(V) phases are expected to present some similarities with their tellurium(IV) homologues because of their almost identical ionic radius, in spite of their lower thermal stability.

In NaIO2F2, the Na+ atom is in a sixfold coordination, at the centre of a nearly regular NaO2F4 octahedron (Fig. 1 and Table 1). Each Na+ atom is surrounded by four F atoms forming a square base and two apical O atoms.

The IV cation is surrounded by four anions, namely two equatorial O atoms, and two axial F atoms at a longer distance (Table 1). Full O/F anionic order on the O1 and F1 sites is evidenced by bond-valence calculations (Brown, 1981) ? (Table 2). The corresponding polyhedron (Fig. 2) can be described as a trigonal bipyramid, IO2F2E, the fifth corner of which is occupied by the lone pair, E. The introduction of two weaker I1—O1 bonds turns the IO2F2E trigonal bipyramid into a pentagonal bipyramid, IO4F2E, but if the lone pair E is not included, the I5+ environment can also be considered as an IO4F2 octahedron distorted by the repulsion of this lone pair on O1i and O1iv [symmetry codes: (i) x, −y, −z; (iv): −x + 1, −y, z + 1/2] (Fig. 2). This IO2F2 polyhedron is essentially the same as those in KIO2F2 (Abrahams & Bernstein, 1976) and [N(CH3)4][IO2F2][HF2] (Gerken et al., 2004). Weak I1—O1 bonds are also systematically present in these structures.

A comparison between IV and TeIV is very meaningful because their lone pairs have the same stereochemical activity (Glay et al., 1975). Some isolated polyhedra displaying such activity that can be compared with IO2F2 include TeOF42− in Cs2TeOF4 (Jansen & Kessler, 2001), and IOF3 (Edwards & Taylor, 1974), TeF4 (Kniep et al., 1984) and IF5 (Burbank & Jones, 1974) in the corresponding phases.

Depending on the respective distribution of O and F anions in these structures, the classical umbrella effect resulting from the stereochemical activity of the lone pair E is more or less marked. In TeF4, the axial angle F2—Te1—F4 is 161.31 (19)° and the equatorial angle F1—Te1—F3 is 87.59 (21)°. In IF5, the average of the axial F—I1—F angles is 162.97 (63)°. In oxyfluorides, these angles increase, e.g. in IOF3 the axial F1—I1—F2 angle is 165.92 (11)° and the equatorial O1—I1—F3 angle is 102.02 (13)°; the average value of axial F—Te1—F angles in TeOF42− is 177.55°. These last two polyhedra are the most similar to the IO2F2 configuration, in which the axial angle F1—I1—F1iii is 179.55 (10)° and the equatorial angle O1—I1—O1iii is 103.72 (12)° [symmetry code: (iii) −x + 1, y, −z + 1/2]. Therefore, in these oxyfluorides the umbrella effect, characterized in a first approximation by axial angles lower than 180°, is partly or almost completely counterbalanced by the repulsion between the equatorial O anions and the axial F anions.

A projection along the [001] direction (Fig. 3) shows that the Na+ cations and IO2F2 complex anions form a CsCl-like lattice (Hyde & Andersson, 1989). The pseudo-cubic Na1 subcell is in fact tetragonal, with a = 5.02 Å and c = 3.68 Å, and its centre is occupied by an IO2F2 complex anion which is responsible for the distortion.

The IO4F2E pentagonal bipyramids (or IO4F2 distorted octahedra), incorporating two weak I1—O1 bonds, form linear [IO2F2]n chains extending along the c axis through O—O edge sharing. Along this same axis, NaO2F4 octahedra share F—F edges, also forming parallel linear chains alternating with [IO2F2]n ones and shifted by c/4 (Fig. 4). Successive I1 and Na1 chains are connected along the [100] direction through F1 corners forming sheets. Along [010], these sheets are interconnected only via O1 corners of NaO2F4 octahedra (Fig. 3). In this way, the structure can be described as a smooth three-dimensional array of nearly perfect edge-sharing NaO2F4 octahedra and distorted IO4F2 octahedra. From this point of view, the structure of NaIO2F2 is derived from a rutile superstructure (Fig. 5) by a cationic I1/Na1 ordering in such a way that a = (arutile)1/2, b = (arutile)1/2 [(brutile)1/2 ?] and c = (crutile)1/2. This kind of structure is well adapted to cations presenting a stereochemically active electronic lone pair. For example, the structure of BiOF and PbFCl is described by Hyde & Andersson (1989) as being derived from the rutile type. The puckering of the anionic hexagonal close-packed planes, common in rutile phases and in TiO2 itself, accommodates in NaIO2F2 the distortion resulting from the lone-pair activity of I5+.

In the MIO2F2 series, before the present work, only the KIO2F2 structure was known. Described in the polar Pca21 space group, it is also derived from CsCl, but the much greater size of K+ compared with Na+ leads to a complete separation of corrugated planes of IO2F2 polyhedra, alternating with slightly twisted K+ square layers.

There is no evidence for noncentrosymmetry in NaIO2F2, as attempts to refine in a subgroup of Cmcm did not lead to a significantly better refinement. Moreover, no unusually large atomic displacement parameters for F and O are detected as in KIO2F2.

In conclusion, in NaIO2F2, as in KIO2F2, the IO2F2 complex ion is present and seems to be a very stable unit. The crystal structure of NaIO2F2, in spite of some analogies with KIO2F2, is clearly different as a consequence of the difference in size between Na+ and K+. If weak I1—O1 bonds are considered, the CsCl-derived stacking of isolated M+ and IO2F2 ions is better described as a three-dimensional array of octahedra derived from the TiO2 rutile type with cationic Na1/I1 long-range ordering. A parallel can also be established with the structure of FeTeO3F (Laval et al., 2008), which is derived from the α-PbO2 type (to which belongs TiO2 under high pressure) but with Fe/Te long-range ordering. It would be interesting to see if NaIO2F2 under pressure presents the classical rutile α-PbO2 transition between these two main types of hexagonal close-packed anionic array (Hyde & Andersson, 1989).

Experimental top

NaIO2F2 was prepared by progressive evaporation at 373 K of a 2:1 molar mixture of NaF and I2O5 dissolved in hydrofluoric acid (40%) in a Teflon beaker. After full evaporation, transparent air-stable single crystals suitable for X-ray studies were obtained, growing on the surface of a second pink-coloured amorphous phase.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: DIRAX/LSQ (Duisenberg, 1992); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The coordination polyhedron of the Na1+ cation in NaIO2F2. [Symmetry codes: (i) x, −y, −z; (ii) x − 1/2, y − 1/2, z; (v) x − 1/2, −y + 1/2, − z; (vi) −x, y, −z + 1/2; (vii) −x, −y, z − 1/2.]
[Figure 2] Fig. 2. (a) The anionic polyhedron around the I15+ cation in the structure of NaIO2F2. The arrow indicates the direction to which the lone pair E points. Dotted lines represent the weak I1—O1 bonds. [Symmetry codes: (i) x, −y, −z; (iii) −x + 1, y, −z + 1/2; (iv) −x + 1, −y, z + 1/2.]
[Figure 3] Fig. 3. A projection of the structure of NaIO2F2 along [001], showing the pseudo-cubic subcell.
[Figure 4] Fig. 4. The parallel chains of the I1 and Na1 polyhedra of NaIO2F2.
[Figure 5] Fig. 5. The rutile structure, for comparison with the NaIO2F2.
Sodium iodine(V) oxyfluoride top
Crystal data top
NaIO2F2F(000) = 392
Mr = 219.89Dx = 3.943 Mg m3
Orthorhombic, CmcmMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2Cell parameters from 3200 reflections
a = 6.9287 (10) Åθ = 4.9–30.0°
b = 7.2735 (13) ŵ = 8.65 mm1
c = 7.3503 (13) ÅT = 293 K
V = 370.42 (11) Å3Irregular tablet, colourless
Z = 40.10 × 0.10 × 0.02 mm
Data collection top
Nonius KappaCCD
diffractometer
312 independent reflections
Radiation source: fine-focus sealed tube303 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.021
Detector resolution: 9 pixels mm-1θmax = 30.0°, θmin = 4.9°
CCD scansh = 99
Absorption correction: multi-scan
(SADABS; Bruker 2001)
k = 1010
Tmin = 0.478, Tmax = 0.846l = 1010
3335 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.010 w = 1/[σ2(Fo2) + (0.0157P)2 + 2.0799P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.023(Δ/σ)max < 0.001
S = 0.75Δρmax = 0.40 e Å3
312 reflectionsΔρmin = 0.57 e Å3
22 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.0188 (6)
Crystal data top
NaIO2F2V = 370.42 (11) Å3
Mr = 219.89Z = 4
Orthorhombic, CmcmMo Kα radiation
a = 6.9287 (10) ŵ = 8.65 mm1
b = 7.2735 (13) ÅT = 293 K
c = 7.3503 (13) Å0.10 × 0.10 × 0.02 mm
Data collection top
Nonius KappaCCD
diffractometer
312 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker 2001)
303 reflections with I > 2σ(I)
Tmin = 0.478, Tmax = 0.846Rint = 0.021
3335 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01022 parameters
wR(F2) = 0.0230 restraints
S = 0.75Δρmax = 0.40 e Å3
312 reflectionsΔρmin = 0.57 e Å3
Special details top

Experimental. Intensity data were collected with a Nonius Kappa-CCD diffractometer using a monochromated Mo—Kα radiation. These intensities were corrected for absorption effects by using a multi-scan method (SADABS, Bruker 2001). Structure solution by direct methods in the Cmcm space group, followed by refinement of atomic coordinates and anisotropic thermal parameters, were performed using respectively the SHELXS97 and SHELXL97 programs (Sheldrick, 2008).

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 (Å2) top
xyzUiso*/Ueq
I10.50000.03831 (3)0.25000.01246 (9)
Na10.00000.00000.00000.0199 (3)
O10.50000.1881 (2)0.0612 (2)0.0216 (4)
F10.2108 (2)0.0372 (2)0.25000.0296 (4)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
I10.01676 (12)0.01230 (13)0.00831 (11)0.0000.0000.000
Na10.0251 (7)0.0171 (6)0.0173 (7)0.0000.0000.0018 (5)
O10.0361 (10)0.0156 (7)0.0130 (8)0.0000.0000.0040 (7)
F10.0184 (8)0.0444 (11)0.0261 (8)0.0052 (7)0.0000.000
Geometric parameters (Å, º) top
I1—O1i1.7647 (17)Na1—F1vii2.3627 (11)
I1—O11.7647 (17)Na1—F12.3627 (11)
I1—F12.0040 (17)Na1—F1viii2.3627 (11)
I1—F1ii2.0040 (17)Na1—F1iii2.3627 (11)
I1—O1iii2.8185 (19)Na1—Na1ix3.6752 (6)
I1—O1iv2.8185 (19)Na1—Na1x3.6752 (6)
Na1—O1v2.3124 (18)O1—Na1xi2.3124 (18)
Na1—O1vi2.3124 (18)F1—Na1x2.3627 (11)
O1i—I1—O1103.72 (12)F1—Na1—F1viii76.35 (7)
O1i—I1—F190.14 (3)O1v—Na1—F1iii92.23 (6)
O1—I1—F190.14 (3)O1vi—Na1—F1iii87.77 (6)
O1i—I1—F1ii90.14 (3)F1vii—Na1—F1iii76.35 (7)
O1—I1—F1ii90.14 (3)F1—Na1—F1iii103.65 (7)
F1—I1—F1ii179.55 (10)F1viii—Na1—F1iii180.0
O1i—I1—O1iii177.62 (6)O1v—Na1—Na1ix101.21 (4)
O1—I1—O1iii73.90 (8)O1vi—Na1—Na1ix78.79 (4)
F1—I1—O1iii89.87 (3)F1vii—Na1—Na1ix38.95 (3)
F1ii—I1—O1iii89.87 (3)F1—Na1—Na1ix141.05 (3)
O1i—I1—O1iv73.90 (8)F1viii—Na1—Na1ix141.05 (3)
O1—I1—O1iv177.62 (6)F1iii—Na1—Na1ix38.95 (3)
F1—I1—O1iv89.87 (3)O1v—Na1—Na1x78.79 (4)
F1ii—I1—O1iv89.87 (3)O1vi—Na1—Na1x101.21 (4)
O1iii—I1—O1iv108.48 (7)F1vii—Na1—Na1x141.05 (3)
O1v—Na1—O1vi180.000 (17)F1—Na1—Na1x38.95 (3)
O1v—Na1—F1vii92.23 (6)F1viii—Na1—Na1x38.95 (3)
O1vi—Na1—F1vii87.77 (6)F1iii—Na1—Na1x141.05 (3)
O1v—Na1—F187.77 (6)Na1ix—Na1—Na1x180.0
O1vi—Na1—F192.23 (6)I1—O1—Na1xi139.35 (10)
F1vii—Na1—F1180.00 (9)I1—F1—Na1128.21 (4)
O1v—Na1—F1viii87.77 (6)I1—F1—Na1x128.21 (4)
O1vi—Na1—F1viii92.23 (6)Na1—F1—Na1x102.11 (6)
F1vii—Na1—F1viii103.65 (7)
Symmetry codes: (i) x, y, z+1/2; (ii) x+1, y, z; (iii) x, y, z; (iv) x+1, y, z+1/2; (v) x1/2, y1/2, z; (vi) x+1/2, y+1/2, z; (vii) x, y, z; (viii) x, y, z; (ix) x, y, z1/2; (x) x, y, z+1/2; (xi) x+1/2, y+1/2, z.

Experimental details

Crystal data
Chemical formulaNaIO2F2
Mr219.89
Crystal system, space groupOrthorhombic, Cmcm
Temperature (K)293
a, b, c (Å)6.9287 (10), 7.2735 (13), 7.3503 (13)
V3)370.42 (11)
Z4
Radiation typeMo Kα
µ (mm1)8.65
Crystal size (mm)0.10 × 0.10 × 0.02
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker 2001)
Tmin, Tmax0.478, 0.846
No. of measured, independent and
observed [I > 2σ(I)] reflections
3335, 312, 303
Rint0.021
(sin θ/λ)max1)0.703
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.010, 0.023, 0.75
No. of reflections312
No. of parameters22
Δρmax, Δρmin (e Å3)0.40, 0.57

Computer programs: COLLECT (Nonius, 1998), DIRAX/LSQ (Duisenberg, 1992), EVALCCD (Duisenberg et al., 2003), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999).

Selected bond lengths (Å) top
I1—O11.7647 (17)Na1—O1ii2.3124 (18)
I1—F12.0040 (17)Na1—F12.3627 (11)
I1—O1i2.8185 (19)
Symmetry codes: (i) x, y, z; (ii) x1/2, y1/2, z.
Bond valences top
AtomNa1I1Vij
O10.2531.903 + 0.1102.27
O10.2531.903 + 0.1102.27
F12 × 0.1570.7181.07
F12 × 0.1570.7181.07
Vij1.145.46
 

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