Sodium iodine(V) oxyfluoride, NaIO₂F₂

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 I^V atom has m2m symmetry with a stereochemically active lone electron pair. The O and F atoms (both with m symmetry) are bonded to the I^V 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.

S1. Comment

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 I^V 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 O1^iv [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 I^V and Te^IV 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 = (a_rutile)^1/2, b = (a_rutile)^1/2 [(b_rutile)^1/2 ?] and c = (c_rutile)^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).

S2. Experimental

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

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

	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.]
	Fig. 2. (a) The anionic polyhedron around the I1^5+ 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.]
	Fig. 3. A projection of the structure of NaIO2F2 along [001], showing the pseudo-cubic subcell.
	Fig. 4. The parallel chains of the I1 and Na1 polyhedra of NaIO2F2.
	Fig. 5. The rutile structure, for comparison with the NaIO2F2.

Sodium iodine(V) oxyfluoride

Crystal data

NaIO2F2	F(000) = 392
Mr = 219.89	Dx = 3.943 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2	Cell parameters from 3200 reflections
a = 6.9287 (10) Å	θ = 4.9–30.0°
b = 7.2735 (13) Å	µ = 8.65 mm^−1
c = 7.3503 (13) Å	T = 293 K
V = 370.42 (11) Å^3	Irregular tablet, colourless
Z = 4	0.10 × 0.10 × 0.02 mm

Data collection

Nonius KappaCCD diffractometer	312 independent reflections
Radiation source: fine-focus sealed tube	303 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
Detector resolution: 9 pixels mm^-1	θmax = 30.0°, θmin = 4.9°
CCD scans	h = −9→9
Absorption correction: multi-scan (SADABS; Bruker 2001)	k = −10→10
Tmin = 0.478, Tmax = 0.846	l = −10→10
3335 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.010	w = 1/[σ^2(Fo^2) + (0.0157P)^2 + 2.0799P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.023	(Δ/σ)max < 0.001
S = 0.75	Δρmax = 0.40 e Å^−3
312 reflections	Δρmin = −0.57 e Å^−3
22 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0188 (6)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
I1	0.5000	0.03831 (3)	0.2500	0.01246 (9)
Na1	0.0000	0.0000	0.0000	0.0199 (3)
O1	0.5000	0.1881 (2)	0.0612 (2)	0.0216 (4)
F1	0.2108 (2)	0.0372 (2)	0.2500	0.0296 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I1	0.01676 (12)	0.01230 (13)	0.00831 (11)	0.000	0.000	0.000
Na1	0.0251 (7)	0.0171 (6)	0.0173 (7)	0.000	0.000	0.0018 (5)
O1	0.0361 (10)	0.0156 (7)	0.0130 (8)	0.000	0.000	0.0040 (7)
F1	0.0184 (8)	0.0444 (11)	0.0261 (8)	0.0052 (7)	0.000	0.000

Geometric parameters (Å, º)

I1—O1^i	1.7647 (17)	Na1—F1^vii	2.3627 (11)
I1—O1	1.7647 (17)	Na1—F1	2.3627 (11)
I1—F1	2.0040 (17)	Na1—F1^viii	2.3627 (11)
I1—F1^ii	2.0040 (17)	Na1—F1^iii	2.3627 (11)
I1—O1^iii	2.8185 (19)	Na1—Na1^ix	3.6752 (6)
I1—O1^iv	2.8185 (19)	Na1—Na1^x	3.6752 (6)
Na1—O1^v	2.3124 (18)	O1—Na1^xi	2.3124 (18)
Na1—O1^vi	2.3124 (18)	F1—Na1^x	2.3627 (11)
O1^i—I1—O1	103.72 (12)	F1—Na1—F1^viii	76.35 (7)
O1^i—I1—F1	90.14 (3)	O1^v—Na1—F1^iii	92.23 (6)
O1—I1—F1	90.14 (3)	O1^vi—Na1—F1^iii	87.77 (6)
O1^i—I1—F1^ii	90.14 (3)	F1^vii—Na1—F1^iii	76.35 (7)
O1—I1—F1^ii	90.14 (3)	F1—Na1—F1^iii	103.65 (7)
F1—I1—F1^ii	179.55 (10)	F1^viii—Na1—F1^iii	180.0
O1^i—I1—O1^iii	177.62 (6)	O1^v—Na1—Na1^ix	101.21 (4)
O1—I1—O1^iii	73.90 (8)	O1^vi—Na1—Na1^ix	78.79 (4)
F1—I1—O1^iii	89.87 (3)	F1^vii—Na1—Na1^ix	38.95 (3)
F1^ii—I1—O1^iii	89.87 (3)	F1—Na1—Na1^ix	141.05 (3)
O1^i—I1—O1^iv	73.90 (8)	F1^viii—Na1—Na1^ix	141.05 (3)
O1—I1—O1^iv	177.62 (6)	F1^iii—Na1—Na1^ix	38.95 (3)
F1—I1—O1^iv	89.87 (3)	O1^v—Na1—Na1^x	78.79 (4)
F1^ii—I1—O1^iv	89.87 (3)	O1^vi—Na1—Na1^x	101.21 (4)
O1^iii—I1—O1^iv	108.48 (7)	F1^vii—Na1—Na1^x	141.05 (3)
O1^v—Na1—O1^vi	180.000 (17)	F1—Na1—Na1^x	38.95 (3)
O1^v—Na1—F1^vii	92.23 (6)	F1^viii—Na1—Na1^x	38.95 (3)
O1^vi—Na1—F1^vii	87.77 (6)	F1^iii—Na1—Na1^x	141.05 (3)
O1^v—Na1—F1	87.77 (6)	Na1^ix—Na1—Na1^x	180.0
O1^vi—Na1—F1	92.23 (6)	I1—O1—Na1^xi	139.35 (10)
F1^vii—Na1—F1	180.00 (9)	I1—F1—Na1	128.21 (4)
O1^v—Na1—F1^viii	87.77 (6)	I1—F1—Na1^x	128.21 (4)
O1^vi—Na1—F1^viii	92.23 (6)	Na1—F1—Na1^x	102.11 (6)
F1^vii—Na1—F1^viii	103.65 (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) x−1/2, y−1/2, z; (vi) −x+1/2, −y+1/2, −z; (vii) −x, −y, −z; (viii) −x, y, z; (ix) −x, −y, z−1/2; (x) −x, −y, z+1/2; (xi) x+1/2, y+1/2, z.