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Iron Stereochemistry in Oxygen-Containing Compounds. V. N. Serezhkin, L. B. Serezhkina, and D. V. Pushkin. Samara State University, Samara, Russia.

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2006, Vol. 32, No. 3, pp. 180–191. © Pleiades Publishing, Inc., 2006. Original Russian Text © V.N. Serezhkin, L.B. Serezhkina, D.V. Pushkin, 2006, published in Koordinatsionnaya Khimiya, 2006, Vol. 32, No. 3, pp. 188–200.

Iron Stereochemistry in Oxygen-Containing Compounds V. N. Serezhkin, L. B. Serezhkina, and D. V. Pushkin Samara State University, Samara, Russia Received April 24, 2005

Abstract—The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to analyze the coordination of 1598 sorts of Fe atoms in the structure of 985 oxygen-containing compounds. The iron atoms whose oxidation number varies from 1 to 6 can coordinate 2 to 8 oxygen atoms in the crystal structure, giving rise to the FeOn coordination polyhedra shaped like square antiprisms (n = 8), octahedra or trigonal prisms (n = 6), square pyramids or trigonal bipyramids (n = 5), tetrahedra or squares (n = 4), triangles (n = 3), or dumbbells (n = 2). The effect of the valence state and the coordination number of iron on the VDP parameters was studied. A general linear correlation between the solid angles of the VDP faces corresponding to Fe–O bonds and the corresponding interatomic distances, which vary over a broad range (1.63–2.77 Å), was established. It is shown in relation to the A2Fe2O5 oxides (A = Ca, Sr) that VDP characteristics make it possible to reveal essential differences in the crystal structures even for isostructural compounds. The FeIVO6 octahedra, unlike isoelectronic MnIIIO6 or CrIIO6 octahedra, display no Jahn–Teller effect. In was shown in relation to CaFeO3 that VDP parameters can be used for the quantitative description of the charge disproportionation in the crystal structure. DOI: 10.1134/S1070328406030043

Within the framework of our research into the stereochemistry of 3d metals [1, 2], this study is devoted to analysis of compounds containing iron atoms surrounded by oxygen in the crystal structures. The procedures for selection and analysis of the structural information carried out using Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres are the same as used previously [1, 2]. By now, the structures of more than 17 thousand compounds that contain both iron and oxygen atoms are known [3, 4]. Of these, the imposed requirements: (1) a crystal structure containing FeOn polyhedra determined to an R-factor of ≤0.1; (2) known coordinates of all atoms; and (3) no random distribution of iron or oxygen, were met for 983 compounds, which served as the objects of crystal chemical analysis. The structures of these compounds, whose list is available from the authors ([email protected]), contained 1595 crystallographically different sorts of iron atoms, in particular, 3 Fe(VI), 1 Fe(V), 45 Fe(IV), 1008 Fe(III), 479 Fe(II), 1 Fe(I), and 58 Fe(II,III) atoms. The designation Fe(II, III) corresponding to the formally non-integer oxidation number 2.5 is used in those cases where crystallographically indiscernible Fe(II) and Fe(III) atoms occupy one position in the structure. Examples of such compounds are ErFe2O4 {67700}, β-Fe2(PO)4O {80556}, and K3Fe2O4 {94466} whose crystals contain only one sort of iron atoms. Here and below, the numbers in braces are the digital or letter codes under which a compound can be identified unambiguously in the databases [3, 4].

Using the unit cell parameters, space groups, and the coordinates of the basis atoms and also the TOPOS software [5], we calculated the VDP characteristics for all the basis atoms in 983 compounds and found the coordination numbers (C.N.s) of these atoms by the intersecting spheres method [6]. Generally, a VDP of some atom A has the composition AXnZm, where X are chemically bonded atoms, n is the coordination number (C.N.) of the A atoms, Z are atoms whose VDP share faces with the VDP of the A atom but the corresponding contacts are not chemical bonds, the sum n + m being equal to the total number of VDP faces [5–7]. All pair interatomic contacts of any A atom can be classified unambiguously and objectively into valence A–X and nonvalence A/Z contacts (the slash implies the presence of a common face of the VDP of chemically nonbonded atoms indicated on the right and on the left of the slash) using the method of intersecting spheres [6], which operates with the VDP characteristics of all atoms present in the crystal structure. According to the results (Table 1), iron atoms exhibit C.N.s of 2, 3, 4, 5, 6, or 8 with respect to oxygen. The combinatorially topological type of the coordination polyhedra (CP) of iron atoms with C.N. ≥ 4 was determined by searching for isomorphism of the edge network graph of the corresponding VDP by the method described previously [8]. When assigning a CP to a particular geometric type, as described previously [8], “simplified” VDP with the number of faces equal to the iron C.N. calculated by the method of intersecting

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181

Table 1. Characteristics of the VDP of the iron atoms in the oxygen environment* Atom

C.N. Coordination polyhedron

The number of Fe atoms

Nf

Nnb VVDP , Å3 SVDP , Å2 RSD, Å

DA, Å

G3

Fe(VI)

4

Tetrahedron

3

10(0) 1.5

7.5(1)

26.3(6) 1.216(5) 0.03(1) 0.100(1)

Fe(V)

4

Tetrahedron

1

11

8.5

27.7

Fe(IV)

4

Tetrahedron

3

13(1) 2.3

9.7(3)

29.2(8) 1.32(1)

0.05(1) 0.096(2)

5

Square pyramid

2

14(0) 1.8

8.9(1)

26.3(1) 1.29(1)

0.30(1) 0.0999(2)

6

Octahedron

43

10(2) 0.7

7.1(2)

22.2(3) 1.19(1)

0.001(3) 0.08332(2)

4

Tetrahedron

177

16(4) 3.0 10.7(6)

31(1)

1.36(2)

0.07(9) 0.096(4)

5

Square pyramid

13

13(2) 1.6

9.7(5)

28(1)

1.32(2)

0.15(6) 0.091(2)

5

Trigonal bipyramid

55

11(3) 1.2

9.7(5)

28(1)

1.32(2)

0.09(5) 0.092(3)

6

Octahedron

759

9(2) 0.5

8.3(4)

24.6(7) 1.26(2)

8

Square antiprism

4

9(1) 0.1

8.2(1)

23.1(2) 1.251(5) 0.02(1) 0.0810(1)

4

Tetrahedron

2

12(0) 2.0 11.8(1)

32.4(1) 1.41(1)

0.05(1) 0.0919(1)

5

Trigonal bipyramid

11

13(2) 1.6 10.9(3)

31.5(9) 1.38(1)

0.04(4) 0.0934(5)

6

Octahedron

45

25.4(3) 1.28(1)

0.01(1) 0.0836(4)

3

Triangle

4

Tetrahedron

4

Square

5

Trigonal bipyramid

6

Octahedron

6

Trigonal prism

8

Square antiprism

2

Dumbbell

Fe(III)

Fe(II,III)

Fe(II)

Fe(I)

1.8

8(2) 0.3

8.7(1)

1.27

0.12

0.0982

0.02(2) 0.0841(8)

5

14(2) 3.7 15(2)

37(5)

1.53(7)

0.06(3) 0.098(7)

12

15(4) 2.8 12(1)

32(3)

1.43(5)

0.10(7) 0.092(5)

5

14(2) 2.5 14(2)

33(5)

1.48(9)

0.01(2) 0.096(11)

14(2) 1.8 11.0(7)

30(1)

1.38(3)

0.12(4) 0.090(2)

12(3) 1

9.9(6)

27(1)

1.33(3)

0.03(4) 0.084(1)

3

15(4) 1.5 10.8(6)

29(2)

1.37(3)

0.01(1) 0.093(6)

14

12(1) 0.5 10.4(2)

27.1(3) 1.35(1)

0.01(4) 0.082(1)

18

42.8

0.02

35 405

1

8.0 20.2

1.69

0.0951

* The following parameters are indicated for each sort of Fe atoms: C.N. is the coordination number with respect to O atoms; Nf is the average number of VDP faces; Nnb is the average number of non-valence contacts per Fe–O bond; VVDP is the VDP volume; SVDP is the total surface area of the VDP; RSD is the radius of a sphere whose volume is equal to VVDP; DA is the displacement of the Fe nucleus from the geometric center of gravity of its VDP; G3 is the dimensionless second moment of inertia of a VDP. The root-mean-square deviations are given in parentheses.

spheres were considered. Therefore, small faces corresponding to non-valence Fe/Z contacts were removed. In the compounds under study, 1595 iron atoms form 9 CP having different shape (Table 1). The only known example of a compound with C.N. 2 giving rise to linear symmetric O–Fe–O dumbbells is K3FeO2 {73215} containing Fe(I) atoms. The triangular coordination (C.N. 3), which is also unusual, was found only for Fe(II). Depending on the Fe : O ratio, the crystal structure contains isolated FeO3 groups (the Na9(FeIIO3)(FeIIIO4) {412212} and Na4FeO3 {1410}, {23635} structures) or nearly planar dimers of two triangles sharing a vertex (Cs6Fe2O5) {73134} and Cs2K4(Fe2O5) {65942}). Tetrahedra (C.N. 4) are found for Fe atoms with any oxidation number from +2 to +6. Note that for C.N. of 4, only Fe(II) atoms may form not only tetrahedral but also square coordination, detected RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

both in the natural mineral gillespite, BaFeSi4O10 {6256}, {31202}, {31203}, and in synthetic perovskite-like mixed oxides, CaFe3(TiO3)4 {79277} and CaFeTi2O6 {79353}. For C.N. 5, two types of iron CP exist, a square pyramid and a trigonal bipyramid. The first one is formed by Fe(IV) and Fe(III), while the second one, by Fe(III), Fe(II,III), and Fe(II). The most abundant iron C.N. is 6, which corresponds most often to an octahedral coordination (Fe(IV), Fe(III), Fe(II,III), and Fe(II)). Only for Fe(II) atoms with C.N. 6, is the CP shaped like a trigonal prism possible (for III

example, Fe 2 FeII(P2O7)2) {71129}). The highest C.N. of 8 with a square antiprism geometry is encountered only in Fe(II) or Fe(III) compounds, in particular, in the structure of natural almandite Fe3(Al2Si3O12) {96735}– {96738}, Fe(II) tetra-tert-butyl-tetrakis(carbamoylVol. 32

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182

SEREZHKIN et al.

Table 2. Characteristics of Fe–O and Fe/O contacts in the structure of oxygen-containing iron compounds

The number of Fe atoms

the number of bonds

Fe–O bonds

Fe(VI)

4

3

12

Fe(V)

4

1

Fe(IV)

4

3

12

1.76–1.86

1.81(3)

5

2

10

1.85–1.92

6

43

258

4

177

5 5

Atom C.N.

Fe(III)

6

r(Fe–O), Å range 1.63–1.66

average

Non-valence Fe/O contacts* Ω(Fe–O), %

the number of average contacts

range

1.647(7) 24.0–24.9

24.4(3)

6

4 1.714–1.724 1.720(5) 23.1–24.3

23.7(6)

1

21.0–24.0

22.9(8)

6

1.87(3)

18.1–19.3

19.1(4)

0

1.85–2.00

1.92(2)

15.6–17.4

16.6(1)

0

708

1.72–2.04

1.87(4)

16.9–25.2

23(1)

1188

13

65

1.69–2.08

1.97(7)

17.7–23.6

19(1)

0

55

275

1.77–2.46

1.97(9)

9.0–23.3

19(2)

759 4554

1.65–2.67

2.02(7)

7.3–22.5

17(1)

r(Fe/O), Å range

averaverrange* age age

3.88–4.35 4.0(2) 4.07

Ω(Fe/O), %

4.07

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