Single crystal synthesis and structure of various transitionmetal ¯uorides with divalent and tetravalent cations

M. Kraus, M. MuÈller, R. Fischer, R. Schmidt, D. Koller, B.G. MuÈller*

Institut fuÈr Anorganische und Analytische Chemie I der Justus-Liebig-UniversitaÈt Giessen,

Heinrich-Buff-Ring 58, D-35392 Giessen, Germany

Received 9 May 1999; received in revised form 23 June 1999; accepted 21 July 1999

Abstract

The following abstract summarizes various methods for the preparation of single crystals of mainly ionic, high melting transition metal

¯uorides, with di- and tetravalent cations and their structure. # 2000 Elsevier Science S.A. All rights reserved.

Keywords: Single crystal synthesis; Structure investigations

1. Introduction

The conditions for synthesis for binary as well as complex

transition metal ¯uorides depends mainly upon their oxida-

tion state or, more precisely, their individual redox-poten-

tials of cations involved. For example, titanium(II)¯uoride,

i.e., in the form of NaTiZr2F11 can be obtained only under

strictly reducing conditions (Ti2�/Ti � ÿ1.6 V, pH � 0) that

is, under strict exclusion of O2, HF, H2O and N2. On the

other hand, NaAgZr2F11 can only be synthesized by reaction

with elemental ¯uorine (Ag2�/Ag � �1.4 V, pH � 0), i.e.,

under strictly oxidizing conditions. For the preparation of

single crystals, each crystal requires individual, speci®c,

however, at any rate, extensive measures concerning the

equipment.

2. Complex fluorides with Ti2�, Sm2�

Numerous binary as well as complex halogenides

(Hal. � Cl, Br, I) with transition metals in low oxidation

states have been known for some time, and are mostly

structurally well characterized. However, information con-

cerning corresponding ¯uorides are rare or lacking entirely.

It seems to be based upon the fact that, obviously for

energetic reasons (lattice energy), the low-valent ¯uorides

are unstable in view of disproportionation, for example 2

`̀ MF''!M �MF2 (Cu), 3 `̀ MF''! 2 M �MF3 (Au, In,

Ga), 3 `̀ MF''!M � 2 MF3 (Sc, Ti, Tm etc.).

Therefore, all mentioned compounds Ð at least as solid

states Ð are still unknown today.

Similar to numerous ¯uorides in high oxidation states (for

example, Cs2CoF6 [1], Cs2CuF6 [2]), sometimes the low

oxidation states can be `̀ caught'' that is stabilized, by

complexation. The prerequisites to achieve that are:

1. Suitable reduction compounds, often the individual

metal itself.

2. A suitable complexing component, which will not be

reduced under reaction conditions.

3. A structure type formed thereby, which is sufficiently

stable, in this case, which offers the optimum coordina-

tion conditions in view of their radii for all ions.

4. Suitable, i.e., inert material, which, under reaction

conditions reacts neither with fluorides used nor with

the material of the wall (metal, ceramics).

For the preparation of the ®rst complex ¯uorides with

Ti2�, numerous systems were tested under various condi-

tions, but so far, only those of the formula TiMF6 (M � Zr,

Hf) as well as NaTiM2F11 (M � Zr, Hf) were obtained. The

preparation was achieved as explained below:

2 TiF3 � Ti-powder� 3MF4

!700ÿ800�C

3ÿ4 weeks3 TiMF6 �orange-yellow single crystals� (1)

3 NaF� 2TiF3 � Ti-powder� 6 MF4 !700ÿ800�C

3ÿ4 weeks

3 NaTiM2F11 �orange-yellow single crystals� (2)

Journal of Fluorine Chemistry 101 (2000) 165±171

*Corresponding author.

0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 5 4 - 2

in sealed Ti-tubes welded under an inert gas atmosphere

(He), M � Zr, Hf.

Some results of the single crystal X-ray data are sum-

marized in Tables 1 and 2, details of the structure in

illustration 1±3.

The structure (see Figs. 1±3) of NaM2�M4�2 F11

(M2� � Ti, V, Mn, Fe, Co, Ni, Cu, Pd, Ag, M4� � Zr, Hf

[3]) is built-up by layers of equatorial edge sharing poly-

hedra (M2�: octahedra, M4�: pentagonal bi-pyramid, Na�:

hexagonal bi-pyramid), linked together by trans-standing

Fÿ to three±dimensional lattice.

The magnetic moment of TiZrF6, with �eff(298 K) �2.80 B.M., is only slightly below the spin only value of

2.89 B.M.

Orange±red single crystals of SmHfF6 were obtained

using a similar method by heating of

Sm-powder� 2SmF3 � 3HfF4 !800�C

3ÿ4 weeks3SmHfF6

(3)

in sealed tantalum-tubes, welded under He-atmosphere. The

compound is iso-typic with EuZrF6 and some crystallo-

graphic data are listed in Table 3.

3. Pd(NbF6)2 and Au(AuF4)2 (Au3F8)

Although Pd2�, Au2� and Au3� are in rather low (med-

ium) oxidation states against ¯uorine, the starting materials

Table 1

Lattice constants, space group and number of formula units

Compound a (pm) b (pm) c (pm) û spc.gr. z

TiZrF6 821.0 ± ± ± Fm �3 ma 4

TiHfF6 392.2 ± ± ± Pm �3 ma 1

NaTiZr2F11 918.0 682.6 780.8 116.2 C2/m 2

NaTiHf2F11 917.8 681.1 779.3 116.1 C2/m 2

aReO3-type ordered (TiZrF6) or statistically disordered (TiHfF6 high temperature phase).

Table 2

Inter-atomic distances [d(M±F)] and coordination number (CN)

Compound d (Ti2�±F) (pm) d (M4�±F) (pm) CN (Ti2�) CN (M4�) CN (Na�)

TiZrF6 210.9 199.7 6 6 ±

TiHfF6 196.1 196.1 6 6 ±

NaTiZr2F11 210.2±212.8 193.9±214.4 6 7 8

NaTiHf2F11 210.5±212.2 194.3±213.6 6 7 8

Fig. 1. Structure of TiZrF6.

Fig. 2. Polyhedra layers inNaM2�M4�2 F11.

166 M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171

PdF3 and AuF3 are only obtainable by reaction of

these metals or halogenides (e.g., PdCl2, AuCl3) with

elemental ¯uorine. Further conversion is then carried out

using palladium or gold tubes, welded under an Ar-atmo-

sphere.

2 PdF3 � Pd-powder � 6NbF5

!350�C

ca: 21 d3 Pd�NbF6�2 �blue single crystals� (4)

8AuF3 � Au-powder

!400�C

ca:30d3 Au�AuF4�2 �brown single crystals� (5)

Au(AuF4)2 was always obtained as a by-product during

synthesis of complex ¯ouroaurates [4]. Pd(NbF6)2 is neither

iso-typic with Pd(SbF6)2 nor Ag(NbF6)2 [5] but forms an

individual structural type. Au(AuF4)2, on the other hand,

crystalizes in the same structure type as Pd(AuF4)2 or

Ag(AuF4)2. The most important crystallographic data are

listed in Table 4.

Figs. 4±6 show the environment of M2� (M � Pd, Ag,

Au) or details of the individual structures, respectively.Fig. 3. Structure of NaM2�M2F11.

Fig. 4. Environment of M2�[AuF4]2 (M � Pd2�, Ag2�, Au2�).

M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171 167

Fig. 5. Structure of M2�[AuF4]2 (M � Pd2�, Ag2�, Au2�), view [100].

Table 3

Crystallographic data of SmHfF6

a (pm) b (pm) c (pm) z Specific gravity d (Sm2�±F) (pm) d (Hf4�±F) (pm)

753.9 1101.5 534.9 4 Cmma 6 � 251.3 4 � 201.2

4 � 289.2 4 � 220.9

Table 4

Crystallographic data and inter-atomic distances for Pd(NbF6)2 and Au(AuF4)2

Compound a (pm) b (pm) c (pm) û Specific gravity z CN M2� CN M3�/M5�

Pd(NbF6)2 516.3 516.3 1891.7 ± P31c 2 6 6

Au(AuF4)2 527.2 1070.7 537.4 90.6 P21/n 2 4 � 2 4

d (Au2�±F) 2 � 210.0 2 � 214.1 2 � 265.1

d (Pd2�±F) 3 � 213.9 3 � 214.9

d (Au3�±F) 2 � 189.6 2 � 196.2

d (Nb5�±F) 3 � 181.0 3 � 197.5

Fig. 6. Structure of M2�[AuF4]2 (M � Pd2�, Ag2�, Au2�), view [001].

168 M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171

4. Ag(II)-compounds: AgMF6 (M � Ti, Pd, Pt, Sn) [6]and Ag2F5 (AgIIFAgIIIF4)

All Ag(II) ¯uorides decompose when heated in the

absence of elemental ¯uorine at already comparatively

low temperatures. In addition to that, some compounds of

this type decompose into volatile tetra¯uorides MF4

(M � Ti, Mn, Ge, Sn, Pb) or penta¯uorides MF5 (M � Cr,

Rh, Pt) at higher temperatures.

AgIMF5 Ar;T�300�CAgMF6

F2Ar; T�400�C

!F2=Ar;T>400�CAgF2 �MF4 " AgF2 �MF5 (6)

Table 5

AgMF6, AgFAgF4, lattice constants (pm) and color (P�1)

Compound a (pm) b (pm) c (pm) � (pm) � (pm) (pm) z Color

AgSnF6 563.2 532.2 519.9 120.1 90.7 114.1 1 Blue

AgTiF6 567.5 516.1 516.0 117.0 91.2 118.5 1 Blue

AgPdF6 501.5 508.7 996.5 89.6 103.1 120.8 2 Dark-green

AgPtF6 503.5 509.8 100.4 90.1 103.0 120.5 2 Dark-violet

AgFAgF4a 499.9 110.87 735.7 90.1 106.5 90.2 4 Brown

aCompounds AgMF6 (M � Ge, Pb, Cr, Mn, Rh) have been synthesized too but with still unknown lattice constants.

Fig. 7. Unit cell of AgMF6 (M � Sn, Ti, Pd). The stucture of AgFAgF4 is very complicated (low space group symmetry, great number of formular types per

unit cell). Therefore, no further details are given here.

Table 6

AgMF6, AgFAgF4, inter-atomic distances and coordination numbers (single crystal data)

Compound d (Ag2�±F) (pm) d (Ag3�±F) (pm) d (M4�±F) (pm) CN (Ag2�) CN (Ag3�) CN (M4�)

AgTiF6 212.2(2�) ± 184.2(2�) 4 � 2 ± 6

218.1(2�) ± 189.1(2�)

229.8(2�) ± 190.6(2�)

AgSnF6 210.4(2�) ± 192.4(2�) 4 � 2 ± 6

210.6(2�) ± 197.9(2�)

241.0(2�) ± 198.1(2�)

AgPdF6 208.3±242.2 186.3±193.0 4 � 2 ± 6

AgFAgF4 ±

Ag(1) 203.6±257.0 ± 2 � 4 ±

Ag(2) 201.2±261.9 ± 2 � 4 ±

Ag(3) 200.8±237.0 ± 2 � 4 ±

Ag(4) ± 189.5±91.6 ± 4

Ag(5) ± 189.7±192.4 ± 4

Ag(6) ± 189.2±192.0 ± 4

M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171 169

Therefore, in any case, working in closed systems (auto-

claves) is essential because every known material reacts

with F2 under higher pressures or temperatures (T > 4508C,

p � 10±100 bar, t � 3±4 weeks). Due to the insolubility

of all compounds AgMF6 in any solvent including HF at

room temperature, one is forced to achieve suitable crystals

Fig. 8. Layers of corner-linked Ag2�F6-octahedra.

Fig. 9. Coordination of Ag2�(1) by Ag3�F4-groups (similar: Ag2�(2), Ag2�(3)).

170 M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171

under solvothermal conditions (T � 4008C, p(F2) > 10 bar,

p(HF) � 200 bar, t � 3±4 days). So far, only single crystals

of AgTiF6 (blue), AgSnF6 (blue) and AgPdF6 (dark green)

were able to be prepared. They are isotypic with CuSnF6 or

CuPtF6 [7]. As by-product during synthesis of AgRhF6-

crystals, additional dark brown crystals of the formula

Ag2F5 (AgIIFAgIIIF4) were obtained. The following Tables

5 and 6 give the most important crystallographic data as well

as the interatomic distances.

Figs. 7±9 re¯ect details of the structures.

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[4] R. Schmidt, B.G. MuÈller, Z. Anorg. Allg. Chem. 625 (1999) 605±608.

[5] B.G. MuÈller, Angew.Chem. Int. Ed. Engl. 26 (1987) 689.

[6] B.G. MuÈller, R. Hoppe, Z. Anorg. Allg. Chem. 392 (1972) 37±41.

[7] F. SchroÈter, B.G. MuÈller, Z. Kristallogr. 196 (1991) 261±268.

M. Kraus et al. / Journal of Fluorine Chemistry 101 (2000) 165±171 171