Silver(II) fluorosulfate: A thermally fragile ferromagnetic derivative of divalent silver in an oxa-ligand environment

Article abstract of DOI:10.1002/ejic.201100077

Several synthetic pathways and characterization of silver(II) fluorosulfate are reported. The title compound crystallizes in the monoclinic space group, P2₁/c, with a = 10.5130(4) A, b = 7.7524(3) A, c = 8.9366(4) A, β = 117.867(2)° [V = 643.88(5) A³, Z = 4, d_calcd. = 3.15 gcm^-3] in a structure type related to that of AgF₂. Puckered [Ag(SO₃F)₂] sheets are present in the crystal structure with two oxygen atoms of the fluorosulfate anions utilized for bonding within the sheet; the third oxygen atom serves as a linker to the adjacent sheet. Terminal fluorine atoms form small cavities in the structure. The S-O stretching region of the vibrational (IR and Raman) spectra is rich in bands, thus confirming the structural complexity of Ag(SO ₃F)₂. Ag(SO₃F)₂ is a soft ferromagnet with a Curie temperature of 24.8 K and it shows a single broad electron spin resonance (ESR) with g = 2.183 at T = 293 K. The intrasheet magnetic superexchange constant, J, derived from magnetization measurements, equals +1.0 meV per formula unit. Density functional theory calculations suggest that the superexchange occurs through the OO moiety of the Ag-O-S-O-Ag bridge while omitting the S atom, and the yield is J = +1.1 meV. The Coulomb-corrected local spin density approximation (LSDA+U) calculations predict a direct electronic band gap at the Fermi level of 1.05 eV. Large magnetic moments reside on O atoms attached to Ag^II; in consequence, Ag(SO ₃F)₂ is thermally unstable; at room temperature or in the presence of strong acids its dark brown crystals slowly decompose at the surface to a black mixed-valence Ag₃(SO₃F)₄. Very fast exothermic decomposition of Ag(SO₃F)₂ with emission of a fluorosulfuryl radical (SO₃F^′·) occurs above 120 °C as confirmed by simultaneous thermogravimetric, calorimetric and evolved gas analyses. Ag(SO₃F)₂ exhibits a puckered sheet structure and it shows 2D ferromagnetism below 25 K; DFT methods predict it to be a magnetic semiconductor with the band gap at the Fermi level of approximately 1.05 eV.Magnetic superexchange take place through the OO moiety of the structural O-S-O bridge while omitting the sulfur atom, similarly as for related AgSO₄.

Full text of DOI:10.1002/ejic.201100077

FULL PAPER
DOI: 10.1002/ejic.201100077
Silver(II) Fluorosulfate: A Thermally Fragile Ferromagnetic Derivative of
Divalent Silver in an Oxa-Ligand Environment
[a]
[b]
[c]
[d]
ˇ ´
Przemysław J. Malinowski, Mariana Derzsi, Zoran Mazej, Zvonko Jaglicic,
Piotr J. Leszczyn´ski,[b] Tomasz Michałowski,[a] and Wojciech Grochala*[a,b]
Keywords: Silver / Oxidation / Density functional calculations / Fluorosulfate
Several synthetic pathways and characterization of silver(II)
fluorosulfate are reported. The title compound crystallizes in
the monoclinic space group, P21/c, with a = 10.5130(4) Å, b
magnetization measurements, equals +1.0 meV per formula
unit. Density functional theory calculations suggest that the
superexchange occurs through the OO moiety of the Ag–O–
S–O–Ag bridge while omitting the S atom, and the yield is J
=
7.7524(3) Å, c = 8.9366(4) Å, β = 117.867(2)° [V =
643.88(5) Å3, Z = 4, dcalcd. = 3.15 gcm–3] in a structure type
related to that of AgF2. Puckered [Ag(SO3F)2] sheets are
present in the crystal structure with two oxygen atoms of the
fluorosulfate anions utilized for bonding within the sheet; the
third oxygen atom serves as a linker to the adjacent sheet.
Terminal fluorine atoms form small cavities in the structure.
The S–O stretching region of the vibrational (IR and Raman)
= +1.1 meV. The Coulomb-corrected local spin density
approximation (LSDA+U) calculations predict a direct elec-
tronic band gap at the Fermi level of 1.05 eV. Large magnetic
moments reside on O atoms attached to AgII; in consequence,
Ag(SO3F)2 is thermally unstable; at room temperature or in
the presence of strong acids its dark brown crystals slowly
decompose at the surface to a black mixed-valence Ag3-
spectra is rich in bands, thus confirming the structural com- (SO3F)4. Very fast exothermic decomposition of Ag(SO3F)2
plexity of Ag(SO3F)2. Ag(SO3F)2 is a soft ferromagnet with a
Curie temperature of 24.8 K and it shows a single broad elec-
tron spin resonance (ESR) with g = 2.183 at T = 293 K. The
intrasheet magnetic superexchange constant, J, derived from
with emission of a fluorosulfuryl radical (SO3F·) occurs above
120 °C as confirmed by simultaneous thermogravimetric,
calorimetric and evolved gas analyses.
of thermodynamic stability at ambient (p, T) conditions. In-
deed, theoretical DFT calculations predict that AgII is cap-
able of transferring the hole from its 4d9 set to the 2p band
of oxygen in many pseudobinary inorganic connections in
the gas phase.[3] Nevertheless, several thermodynamically
metastable derivatives of AgII in the solid state with oxygen
in the first coordination sphere of a metal have been re-
ported, notably AgII[AgIIIO2]2,[4] AgII(SO3F)2,[5] AgII-
Introduction
Fluorides constitute by far and away the majority of in-
organic compounds of divalent silver.[1] Although AgII is a
strong oxidant (E0[AgII/AgI] = +1.98 V vs. NHE[2]), it can-
not compete in oxidizing power with fluorine (E0[F2/2F–] =
+3.05 V vs. NHE[2]), hence the majority of the fluorides of
AgII are thermally stable to several hundred degrees centi-
grade. However, the corresponding redox potential of the
oxide/peroxide redox pair (E0[H2O2, 2H+/2H2O] = +1.76 V
vs. NHE) is less positive than that of the AgII/AgI pair,
which places oxygen-based connections of AgII at the verge
[7]
(SO3CF3)2,[6] AgSO4 and AgII(pz)2(S2O8), in which pz =
pyrazine.[8]
The electronic and magnetic properties of oxa derivatives
of AgII are often surprising due to substantial 4d(Ag)–
2p(O) orbital mixing, which vastly exceeds the one seen for
analogous CuII oxide derivatives. In consequence, AgII-
[AgIIIO2]2 is supposedly metallic[4] whereas AgSO4 shows
an unusually strong antiferromagnetic (AFM) coupling,
[a] Faculty of Chemistry, University of Warsaw,
Pasteur 1, 02093 Warsaw, Poland
Fax: +48-22-5540801
E-mail: wg22@cornell.edu
[b] Interdisciplinary Center for Mathematical and Computational which persists up to the temperature of thermal decomposi-
tion of this compound.[7] The remaining AgII(SO3F)2[5] and
Modeling, University of Warsaw,
Pawinskiego 5a, 02106 Warsaw, Poland
[6]
AgII(SO3CF3)2 were only partially characterized in the
[c] Department of Inorganic Chemistry and Technology, Jozˇef
Stefan Institute,
previous literature reports: (i) limited data is available on
the positions of IR bands; (ii) the value of the Curie tem-
perature for ferromagnetic AgII(SO3F)2 was initially given
as 20 K[5] but was then inconsistently reported as 40 K else-
where.[9] Crystal structures and other important properties
of this compound were not studied.
Jamova 39, 1000 Ljubljana, Slovenia
[d] University of Ljubljana, Faculty of Civil and Geodetic
Engineering, and Institute of Mathematics, Physics and
Mechanics,
Jadranska 19, 1000 Ljubljana, Slovenia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100077.
Eur. J. Inorg. Chem. 2011, 2499–2507
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2499

W. Grochala et al.
FULL PAPER
Due to the paucity of AgII complexes as well as to extend
our previous work on compounds of silver(II) with oxa li-
gands,[7,10] we explored various synthetic pathways and
thoroughly characterized the chemical reactivity and physi-
cal properties of pseudobinary fluorosulfate and triflate de-
rivatives of AgII. In the current work we report the crystal
structure of AgII(SO3F)2, full vibrational (IR and Raman)
and electron spin resonance (ESR) spectra, characterize its
magnetic and thermal properties and interpret our findings
with the help of periodic DFT calculations. In a parallel
4Ag(SO3F)2 + HSO3F Ǟ Ag3(SO3F)4 + [AgSO3F·HSO3F] +
3/2(SO3F)2Ȇ
(4)
If the reaction was stopped after four days, AgF2 and
Ag(SO3F)2 were the main crystalline phases with just a
small amount of Ag3(SO3F)4 impurity.
Very pure samples of Ag(SO3F)2, free of any paramag-
netic impurities and used later for magnetic measurements,
were obtained using the published reaction[5] [Equation (5)].
2AgSO3F + (SO3F)2 Ǟ 2Ag(SO3F)2
(5)
report we describe related mixed-valent Ag3(SO3F)4.[11]
A
AgSO3F precursor was obtained in a reaction between
AgF (Sigma–Aldrich) and a large excess amount of HSO3F
(Sigma–Aldrich). Remnants of the acid were washed away
with a mixture of CF3COOH and (CF3CO)2O in a volu-
metric ratio 1:3 followed by drying for 24 h under vacuum.
The second substrate of the reaction [Equation (5)],
(SO3F)2, was obtained from thermal decomposition of
Xe(SO3F)2.[14]
contribution on related AgII(SO3CF3)2 will follow
shortly.[12]
Results and Discussion
Synthesis
The first preparation of AgII(SO3F)2 relied on the oxi-
dation of various Ag-containing precursors with a very re-
active sulfuryl peroxide for 168 h at 70 °C,[5] see for example
Equation (1).
Crystal Structure
Ag(SO3F)2 (Figure 1) grown from dark brown saturated
HSO3F as a product of the reaction [Equation (3)] formed
long, beautiful needle-shaped single crystals, intergrown in
the form of hedgehogs (Figure 2). The needles were ex-
tremely mechanically fragile and chemically reactive and
they instantly decompose to colourless AgISO3F if taken
away from supernatant and placed inside a quartz capillary.
Ag + (SO3F)2 Ǟ Ag(SO3F)2
(1)
Here we have synthesized the title compound by using
two novel reaction pathways. The first one, methathetic,
was based on a ligand exchange between KSO3F and
[13]
Ag(SbF6)2
tion (2).
in anhydrous HF (aHF) according to Equa-
2KSO3F(aHF) + Ag(SbF6)2(aHF) Ǟ Ag(SO3F)2ȇ + 2KSbF6(aHF)
(2)
Solid reagents [colourless KSO3F, ocean blue Ag-
(SbF6)2] were mixed in a fluorinated ethylene propylene
(FEP) reactor and aHF was condensed as a solid over the
mixture at 77 K. The temperature was raised slowly above
the melting point of aHF (–84 °C). As aHF melted, the
droplets progressively dissolved reagents and a brown col-
our immediately appeared. The mixture was heated slowly
to room temperature, and the KSbF6 byproduct was
washed away with new portions of aHF, thereby leaving in-
soluble brown residue of Ag(SO3F)2. The yield was close to
100% (based on the mass of washed-off KSbF6). The prod-
uct contained very few impurities as suggested by its Ra-
man spectrum.
Figure 1. Powder XRD pattern of Ag(SO3F)2 obtained from Equa-
tion (3): experimental and differential curves are shown.
The second synthetic method was based on the reaction
between AgF2 and HSO3F at room temperature or slightly
above it [but less than 50 °C; Equation (3)].
AgF2 + 2HSO3F Ǟ Ag(SO3F)2 + 2HFȆ
(3)
Figure 2. Long, needle-shaped dark brown single crystals of
The reaction was, however, very slow and the conversion Ag(SO3F)2 grown from HSO3F as a product of the reaction [Equa-
tion (3)] over several days, intergrown in the form of hedgehogs and
of larger conglomerates.
was not completed even after four months. Simultaneously,
partial decomposition of brown Ag(SO3F)2 to black Ag3-
(SO3F)4 could be noticed on the surface of the as-forming
product, according to Equation (4).
The crystal structure of silver(II) fluorosulfate (Table 1,
Figure 3) was therefore solved from the powder data. Re-
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Silver(II) Fluorosulfate
straints on geometry of fluorosulfuric anions were ap-
The short S=O bonds (ca. 1.418 Å) are localized at oxy-
plied.[15] The sample used for refinement was obtained in a gen atoms to form long, more ionic bonds to AgII cations.
reaction [Equation (3)] that lasted for four days and it con- The longer S–O bonds (ca. 1.48 Å) involve oxygen atoms
tained a considerable amount of AgF2 [24.4(2) wt.-%] and that form short, more covalent bonds to AgII.
some AgIIAgI2(SO3F)4 [9.1(2) wt.-%] (weight fractions were
Ag(SO3F)2 forms puckered layers with the [AgS2] sublat-
obtained from refinement). Only lattice vectors were refined tice, which is topologically very similar to the [AgF2] sublat-
[8]
for both minority phases [AgIIAgI2(SO3F)4 and AgF2[16]]. tice of AgF2.[16] The [Ag(SO3F)2] sheets are interconnected
through the longest Ag–O bonds just like the [AgF2] sheets
Table 1. Crystallographic data for Ag(SO3F)2.
are linked through the longest Ag–F bonds, thus enabling
an intersheet magnetic exchange.[16] The AgII cations are
Space group
V [Å3]
a [Å]
b [Å]
c [Å]
β [°]
P21/c (no.14)
643.88(5)
10.5130(4)
7.7524(3)
8.9366(4)
117.867(2)
4
linked within each sheet through OSO linkers from the
SO3F groups; direct Ag–O–Ag bridges are absent. As for
several other fluorosulfates with modestly covalent metal–
oxygen bonding, such as SnII(SO3F)2[18] or AuIII(SO3F)3,[19]
fluorine is not bound to metal cation. Instead, terminal flu-
orine atoms stick towards one another while forming very
narrow channels along the a crystallographic axis (Fig-
ure 3). The shortest F···F separation is 3.12(2) Å, which,
compared to the doubled van der Waals radius of the F
atom of 2.94 Å, leaves only 0.16 Å of free space. Thus, even
the smallest hydrophobic atom, that of helium, cannot
travel through the F channels of Ag(SO3F)2.
Z
Rp
Rwp
0.0244
0.0321
R(Bragg)
GOF
0.0276
1.14
R(Ag1–O) [Å]
R(Ag2–O) [Å]
R(S1–O) [Å]
R(S2–O) [Å]
R(S1–F1) [Å]
R(S2–F2) [Å]
2.053(18)
2.07(3)
1.48(3)
1.48(3)
1.55(2)
1.55(2)
2.14(3)
2.154(19)
1.48(2)
1.48(3)
2.61(2)
2.49(2)
1.417(18)
1.418(19)
Vibrational Spectra
Structural complexity, low local C1 symmetry of SO3F
moieties and the presence of two independent fluorosulfuric
anions in the crystal structure are revealed in the rich-in-
bands vibrational spectra of Ag(SO3F)2 (Figure 4).[20] The
positions of the major IR bands are in fair agreement with
those reported previously,[5] but we were able to identify
more bands than in the previous study and provide Raman
wavenumbers as well (Table 2).[5] It is clear that mutual ex-
clusion principle applies, since the strongest IR bands are
Raman-inactive and vice versa (recall, an inversion centre
is present).
Figure 3. Crystal structure of Ag(SO3F)2. (left) The crystallo-
graphic unit cell; (right) view along the a axis emphasizing the pres-
ence of F channels. Grey balls: Ag, yellow balls: S, blue balls: O,
green balls: F. Selected long (apical) Ag–O bonds are marked with
red dotted lines.
The unit cell of Ag(SO3F)2 (Figure 3) contains two crys-
tallographically independent Ag cations and two distinct
types of fluorosulfuric anions. The Ag2+ cations are each
coordinated by six oxygen atoms in the form of a distorted
elongated octahedron; the four short Ag–O distances range
between 2.053(18) and 2.154(19) Å, whereas the longer api-
cal ones are between 2.49(2) and 2.61(2) Å. The short Ag–
O separations are close to those found for other oxo com-
pounds of divalent silver, such as AgII(AgO2)2 (2.052–
2.086 Å)[4] or AgIISO4 (2.094–2.198 Å).[7] Regretfully, the
crystal structure of related Cu(SO3F)2 is not available for
comparison.
Figure 4. The IR and Raman spectra of Ag(SO3F)2. The asterisk
(*) marks the strongest bands of the Ag3(SO3F)4 impurity.
The dimensionless Jahn–Teller distortion parameter, D,
The wavenumbers of characteristic IR-active S–O
ranges from 1.245 for Ag1 to 1.179 for Ag2, and it is thus stretching and O–S–O deformation modes reconfirms pres-
similar to the those observed for AgII in a fluoride environ- ence of covalently bound tridentate fluorosulfuric anions.[21]
ment.[17]
Specific are:[22]
Eur. J. Inorg. Chem. 2011, 2499–2507
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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W. Grochala et al.
FULL PAPER
Table 2. IR and Raman bands of Ag(SO3F)2 with intensities and Our own results confirmed the existence of these salts;[11]
assignment together with the theoretical DFT values (LDA; for
however, we note that they cannot be formed in a direct
reaction between the corresponding pseudobinary fluoro-
sulfates, see for example Equation (6).
GGA see the Supporting Information).[a]
IR (ref.[5])
IR
Raman
DFT
(LDA)
Assignment
SO3 str.
1320 (s, br.)
1185 (vs, br.)
1320 (w, sh)
1313 (s)
1188 (s, sh)
1176 (vs, br.)
1340 (s)
1309 (w)
1239 (vw)
1194 (vw) 1282–1026
1134 (s)
Ag(SO3F)2 + 2Ag(SO3F) Ǟ Ag3(SO3F)4
(6)
This is due to a lack of thermal stability of the Ag-
(SO3F)2 precursor (see next section). Even if the solid–solid
reaction is conducted for two weeks at a temperature as low
as 80 °C, complete decomposition takes place of dark
brown Ag(SO3F)2 to colourless AgI-containing products.
It is conceivable that, as with AgF2, Ag(SO3F)2 might
behave like a weak Lewis base towards stronger Lewis acids,
for example, HSO3CF3 or HSbF6. Indeed, displacement re-
actions using these acids [see Equations (7) and (8)] are very
fast and efficient and they result in, respectively, insoluble
brown Ag(SO3CF3)2[6] and an ocean-blue-coloured solution
of Ag(SbF6)2 in an excess amount of aHF (together with
colourless AgI-containing residues).
1070 (s, br.)
820 (ms)
1070 (s)
1046 (m, sh)
838 (w)
826 (w, sh)
820 (s)
615 (w, sh)
608 (m)
1086 (w)
1069 (s)
838 (s)
819 (vw)
813–773
587–522
SF str.
SO3F def.
604 (w)
598 (s)
587 (s)
558 (s)
434 (s)
428 (s)
415 (vw)
410 (w)
595 (m)
560 (m)
434 (w)
421 (w)
419–392
279–34
SO3F rock.
255 (s)
236 (s)
AgO str.
and lattice
Ag(SO3F)2 + 2HSO3CF3 Ǟ Ag(SO3CF3)2 + 2HSO3F
(7)
238 (vs)
219 (vs)
141 (w)
118 (w)
91 (m)
138 (w)
114 (w)
Ag(SO3F)2 + 2HSbF6 Ǟ Ag(SbF6)2 + 2HSO3F
(8)
78 (w)
Reactions between Ag(SO3F)2 and HSO3CF3 or HSbF6
could not be stopped at the intermediate stage of
[Ag(SO3F)+]L– salts (in which L = SO3CF3, SbF6). In view
of lack of thermal and thermodynamic stability of Ag-
(SO3F)2 (see next section), it is conceivable that compounds
that contain an even more acidic and coordinatively less
saturated Ag(SO3F)+ moiety would be even more thermally
unstable than Ag(SO3F)2, and they should be sought at low
temperatures only.
[a] v: very, s: strong, m: medium, w: weak, br.: broad, sh: shoulder,
str = stretching, def = deformation, rock = rocking.
(i) a strong band above 1100 cm–1 that corresponds to S–O
stretching is split into three centred at 1313, 1180 and
around 1170 cm–1, thus making this part of spectrum sim-
ilar to that of Cu(SO3F)2,[22,23]
(ii) the band above 1300 cm–1 that corresponds to S=O
stretching points to the presence of oxygen that is bound
more weakly to AgII but more strongly to S,
Ag(SO3F)2 is not soluble in any common inorganic or
[25]
organic solvent. Similarly to AgF2
it vigorously reacts
(iii) the O–S–O deformation modes are stiffened with ν5
with water and even with anhydrous sulfuric or nitric acid.
Ag(SO3F)2 is decomposed by dimethylformamide, N-meth-
ylformamide, tetrahydrofuran, nitromethane (with evol-
ution of unidentified colourless gas[26]) and perfluoropyr-
idine. In contrast to AgF2,[27] Ag(SO3F)2 resists prolonged
action of hexane, cyclohexane and sulfolane (tetra-
hydrothiophene 1,1-dioxide) and unlike AgF2 it is decom-
posed by CCl4 to AgCl within half an hour. Surprisingly,
and similarly to AgF2, Ag(SO3F)2 is inert toward tert-butyl
alcohol and perfluoro-tert-butyl alcohol (despite the fact
that peroxides of these alcohols are known).
Ag(SO3F)2 easily oxidizes I2 and Br2 to yield the corre-
sponding silver(I) halides. Reaction with CeIII sulfate in
HSO3F is much slower but after one week partial conver-
sion to characteristic orange-coloured CeIV complexes can
be noticed. Ag(SO3F)2 is reduced within 3 h by solution of
above 600 cm–1 and ν3 above 560 cm–1,
(iv) the S–F stretching frequencies are well above 800 cm–1,
which is consistent with the fact that all F atoms are ter-
minal and thus indirectly suggests a more covalent Ag–O
bonding. The absorption band is narrow and intense, in
contrast to those observed for ionic fluorosulfates (broad
and weak).[24]
The Raman spectrum (Figure 4) is also rich in bands due
to the presence of four formula units (FUs) inside the crys-
tallographic unit cell and concomitant Davidoff splitting
and/or Fermi resonances (coupling of different tones, over-
tones or combination modes).
Chemical Reactivity
In most reactions we have attempted, Ag(SO3F)2 behaves nitrosobenzene in hexane. None of the redox reactions
like a moderately strong Lewis acid and a strong oxidizer. studied may be stopped at intermediate stage that corre-
For example, it has been known that Ag(SO3F)2 may form sponds to a mixed-valence AgI/AgII fluorosulfate; full re-
pseudoternary salts with fluorosulfates of AgI and KI.[5] duction of AgII to AgI is always observed.
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Silver(II) Fluorosulfate
Thermal Analysis
stable with respect to the products of decomposition. De-
composition is thus a spontaneous kinetically controlled
process that occurs slowly even at room temperature. As a
result, the surfaces of Ag(SO3F)2 crystals are often covered
with black Ag3(SO3F)4 salt. However, all attempts in the
synthesis of Ag3(SO3F)4 by means of controlled thermal de-
composition of Ag(SO3F)2 at temperatures not exceeding
80 °C have failed.[11] AgI(SO3F)2 has been obtained in all
cases.
Leung and Aubke[5] reported the temperature of the ther-
mal decomposition of AgII fluorosulfate as 210 °C but they
did not specify chemical nature of the products of decom-
position. Our own thermogravimetric analysis (TGA)/
differential scanning calorimetry (DSC) measurements
(Figure 5) suggest, however, that Ag(SO3F)2 slowly decom-
poses thermally even at 30 °C (faster at T Ͼ 120 °C), thus
it is thermally less stable than previously reported. Decom-
position leads to the evolution of yellowish fumes of
(SO3F·) radical and to AgI fluorosulfate according to Equa-
tions (9) and (10) (theoretical values of mass loss related to
initial substrate are given in brackets).
Magnetic Properties and ESR Spectra
The presence of paramagnetic Ag2+ species gives rise to
magnetic ordering phenomena in solids. The plot of the
temperature dependence of magnetic susceptibility for sil-
ver(II) fluorosulfate in the 2–150 K temperature range is
presented in Figure 6.
Ag(SO3F)2 Ǟ AgSO3F + 1/2(SO3F)2Ȇ (–32.4 wt.-%)
(9)
(SO3F)2 Ǟ 2SO3FȆ
(10)
Figure 5. The TGA/DSC profiles of Ag(SO3F)2 measured in Ar
flow at 5 Kmin–1 heating rate.
Decomposition does not stop at the Ag3(SO3F)4 stage
Figure 6. Magnetic susceptibility of Ag(SO3F)2 versus temperature
measured in magnetic field of H = 1000 Oe.
[11]
since this compound is equally unstable as Ag(SO3F)2
and it decomposes to AgISO3F. Further heating results in
decomposition of AgISO3F to Ag2SO4 with evolution of
SO2F2 [Equation (11)], similarly to the process observed for
M(SO3F)2 (M = Sr, Ba).[28]
Ag(SO3F)2 orders ferromagnetically with a Curie tem-
perature, Θ, of 24.8 K estimated from a Curie–Weiss curve
fit above 150 K. At first approximation the magnetic super-
exchange constant, J, for a 2D system such as Ag(SO3F)2
2AgSO3F Ǟ Ag2SO4 + SO2F2Ȇ (–16.7 wt.-%)
(11)
Ag2SO4 is the ultimate product of thermal decomposi- can be estimated from the mean-field approximation (i.e.,
tion at 400 °C as confirmed by XRD and IR spectroscopy Curie–Weiss law) and high-temperature series expansion by
as well as the presence of the characteristic sharp endother- Baker et al.[30] as Θ/2, thus yielding a J value of 1.0 meV.
mic peak of its phase transition at 425 °C in the DSC pro-
The magnetic behaviour of Ag(SO3F)2 proves to be dif-
ferent from that of its antiferromagnetic (AFM) AgSO4 sib-
file[29] (Figure 5).
The experimental values of mass loss for steps I and II ling.[7] The magnetic ordering temperature is much lower
of thermal decomposition (28.8 and 15.0%) agree fairly than for AgSO4 (here ordering is limited by thermal decom-
well with the theoretical values, assuming the occurrence of position at ca. 400 K). These differences can be rationalized
the reaction in Equation (9) at the first stage (32.4%) and while taking into account different geometries of the mag-
of the reaction in Equation (11) at the second stage netic superexchange pathways for both compounds (Fig-
(16.7%). The fact that the observed mass losses are slightly ure 7). For AgSO4 the superexchange goes through the
smaller that the theoretical ones may be explained by spon- double O–O bridge with both Ag–O–O and O–O–Ag
taneous partial decomposition of unstable Ag(SO3F)2 spec- angles close to 180°; the corresponding angles are much
imen held for 1 h in a stream of Ar gas inside the TGA smaller for Ag(SO3F)2 and one of them is in fact closer to
analyzer during conditioning of the sample.
90° (113–118°), hence its favour for the FM-type intrasheet
The first step of thermal decomposition of Ag(SO3F)2 exchange. In addition, the Ag–O distances between the lay-
is more exothermic than the second one. In any case, the ers are not very large (ca. 2.5 Å), whereas the angles of the
exothermicity of decomposition and evolution of gases intersheet Ag–O–O–Ag bridge deviate from 180° (154 and
(SO3F, SO2F2) renders Ag(SO3F)2 thermodynamically un- 98°), thus favouring an intersheet FM superexchange.
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W. Grochala et al.
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culations using local spin density approximation (LSDA)
and Coulomb-corrected local spin density approximation
(LSDA+U) methods. Two different projections of spin den-
sity within the crystallographic unit cell are shown in Fig-
ure 10. The electronic band structure and electronic density
of states are shown in Figure 11. Magnetic moments on
atoms are listed in Table 3.
Figure 7. Geometries of the magnetic superexchange pathways
(Ag···OO···Ag) for Ag(SO3F)2 (left) and related AgSO4 (right).
The magnetic hysteresis measurement at 5 K (Figure 8)
shows negligible hysteresis, hence Ag(SO3F)2 may be lab-
elled as a soft ferromagnet. The saturated magnetic moment
of 1.08 μB per FU agrees with the value of around 1 μB per
FU expected for a 4d9 transition-metal cation.
Figure 10. Two projections of spin density, ρsp, within the crystallo-
graphic unit cell as calculated for Ag(SO3F)2 at the LSDA+U level:
–0.03 eÅ–3 Ͻ ρsp Ͻ +0.66 eÅ–3. Projections emphasize the ferro-
magnetic character of both the intra- and intersheet superexchange.
Figure 8. Magnetic hysteresis (M vs. H) of Ag(SO3F)2 at 5 K. The
expanded region around (0,0) is shown in the inset.
The ESR spectra measured between 2.4 and 293 K are
shown in Figure 9. The spectrum taken at room tempera-
ture shows only one broad line with g = 2.183; the g factor
characteristic of Ag(SO3F)2 is different than the one of
2.119 measured for related AgI2AgII(SO3F)4.[11] As the tem-
perature is lowered the ESR band gets sharper, more in-
tense and eventually splits in three. At 17.2 K, just below
the temperature of ferromagnetic ordering (24.8 K), g fac-
tors have values of g1 = 2.269, g2 = 2.212 and g3 = 2.136.
The rhombic g tensor is of course not unexpected given the
local 2+2+2 environment of AgII. However, at 2.4 K the
ESR signal is split again with g factors of g1 = 5.065, g2 =
2.422 and g3 = 1.248.
Figure 11. Electronic band structure at the LSDA level (left), at the
LSDA+U level (middle) and electronic density of states (DOS) at
the LSDA+U level (right) for Ag(SO3F)2. Spin majority channel:
red, spin minority channel: dark blue. Colour code for partial DOS:
summed contribution from all O atoms: light blue, both Ag atoms:
violet.
Table 3. The magnetic moments on atoms as calculated for the FM
ground state of Ag(SO3F)2 at the LSDA+U level.
Atom
μ [μB]
+0.51
+0.50
0.00
Atom
μ [μB]
+0.08
+0.09
+0.02
Ag1
O1, O1Ј
O2, O2Ј
O3, O3Ј
Ag2
S1, S2, F1, F2
The ferromagnetism of Ag(SO3F)2 arises from the mag-
netic superexchange of unpaired Ag(4d) electrons that re-
side at x2–y2 orbitals (localized within mutually inclined
[AgO4] squares) through the O–O bridges of the fluorosul-
furic anions (Figure 10). The values of magnetic moments
on Ag1/Ag2 atoms as obtained from the LSDA+U calcula-
tions are 0.50–0.51 μB, and thus larger than those calculated
for ferrimagnetic AgSO4 (Ϯ0.43–0.45 μB).[7] Simulta-
neously, the calculated values of magnetic moments on
Figure 9. ESR spectra of Ag(SO3F)2 at five different temperatures
[K].
Electronic Structure: A DFT Picture
To better understand the nature of magnetism and elec- bridging O1/O2 atoms of 0.08–0.09 μB are slightly smaller
tronic structure of Ag(SO3F)2 we have carried out DFT cal- than those calculated for respective atoms in AgSO4
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Silver(II) Fluorosulfate
(Ϯ0.09–0.10 μB). These observations are in agreement with valent silver, we have synthesized silver(II) fluorosulfate
what might be anticipated from chemical character of (first reported and briefly characterized in 1978[5]) and suc-
2–
SO3F– and SO4 anions: the former one contains one cessfully solved its crystal structure from the powder X-ray
highly electronegative fluorine atom and bears a single data. Monoclinic Ag(SO3F)2 adopts a puckered-sheet struc-
negative charge, in contrast to the latter anion, which con- ture related to that of AgF2. The vibrational (IR and
tains only less electronegative oxygen atoms and bears a Raman) spectra confirm the presence of tridentate fluoro-
double negative charge. In consequence, sulfate dianion is sulfuric anions with a reduced local C1 symmetry, covalent-
more susceptible to oxidation by potent AgII oxidizer than ly bound to a transition-metal cation. Two oxygen atoms
is fluorosulfate anion, and thus chemical bonding between are used for intrasheet bridging of the adjacent AgII cation,
AgII and these oxa-anions somewhat differs; the ionic char- whereas the third one links the AgII cations from adjacent
acter is more pronounced for the AgII···OSO2F– bonding.
sheets. F atoms are terminal.
The more ionic character of the AgII···O bonding has
The presence of paramagnetic AgII cations is revealed by
some impact on the strength of magnetic superexchange. strong ESR absorption with g = 2.183 at room temperature.
The intrasheet J estimated at the LSDA level of +1.1 meV Ag(SO3F)2 is a soft ferromagnet with a Curie temperature
per FU (thus +2.2 meV per each pair of interacting AgII of 24.8 K and magnetic superexchange constant of approxi-
centres[31]) is in very good agreement with the experimental mately 1.0 meV per FU. Density functional theory calcula-
value of +1.0 meV per FU. The value of J is thus much tions show that the magnetic superexchange occurs through
smaller (as far as absolute value is concerned) than the ex- the O–O moiety of the Ag–O–S–O–Ag bridge while omit-
perimental value of –9.5 meV per pair of AgII cations for ting the S atom, with J = +1.1 meV. Large magnetic mo-
1D ferrimagnetic AgSO4.[7] This is only partially related to ments reside on O atoms attached to AgII (close to 0.1 μB),
a slightly worse capability of SO3F– anions to transfer mag- thus facilitating spontaneous (exothermic) thermal decom-
netic superexchange compared to SO42– anions. The geome- position of the title compound with emission of fluoro-
try of Ag–O–O–Ag bridges (with one O–O–Ag angle of sulfuryl radical (SO3F·), which takes place (slowly) even at
160° substantially departing from 90° and thus favouring room temperature or in an acidic environment. Ag(SO3F)2
AFM rather than FM superexchange) is to be blamed for is a powerful oxidant that readily decomposes upon action
that. Indeed, as we show in a separate contribution,[11] the of many common organic solvents, and it oxidizes elemen-
potential of SO3F– anions to transfer magnetic superex- tal Br2 (E0 for Br2/BrI = +1.60 V vs. NHE) and even CeIII
change is better revealed for antiferromagnetic Ag3(SO3F)4 (E0 for CeIII/CeIV = +1.76 V vs. NHE).
with its intrachain J value of –7.5 meV per pair of AgII
cations due to the appropriate geometry of the bridging
moieties.
Experimental Section
LSDA calculations predict an unrealistically small value
of the band gap at the Fermi level of 0.25 eV (Figure 11).
The fact that the LSDA calculation does not converge to
General: All reactions were carried out under an Ar atmosphere
(Ͻ0.1 ppm O2 and Ͻ1 ppm H2O) as provided by using a glovebox
from MBraun or in a fluoropolymer vacuum line.
metallic solution indicates that supplementation of the
IR spectra were taken with a Vertex 80V vacuum spectrometer
LSDA method with the on-site electron repulsion U term
from Bruker. For the mid-IR region (500–7500 cm–1) the powdered
is not necessary to stabilize the semiconducting magnetic
samples were placed on the surface of AgCl windows, whereas for
state. However, the LSDA+U calculation leads to a larger
far-IR (50–650 cm–1) the samples were placed on polyethylene win-
direct band gap at Γ of 1.05 eV, which is consistent with
dows.
the dark-brown colour of the compound (unfortunately, the
Raman spectra were recorded with a Horiba Jobin Yvon LabRam-
experimental estimate of the band gap is currently unavail-
HR Raman microspectrometer with a 632.8 nm He:Ne laser excit-
ing beam. The power of the beam was reduced to a value of
[7]
able). The value of 1.05 eV compares with 0.82 eV obtained
previously at the same level of theory for black AgSO4
0.17 mW to avoid photo- and thermal decomposition of the sam-
which again indicates the more ionic character of Ag-
ples.
(SO3F)2. Inspection of the partial density of states (pDOS)
XRD powder patterns were obtained with a D8 discover dif-
for Ag(SO3F)2 (Figure 11) reveals that the states below the
fractometer from Bruker equipped with a Cu cathode and a parallel
Fermi level are predominated by the contribution from oxy-
beam setting provided by Göbel mirrors. The sample was loaded
into a thin (0.3 mm diameter, 0.01 mm thick walls) quartz capillary
gen atoms, whereas those above the Fermi level are pre-
dominated by the contribution from Ag, just as was calcu-
from Hilgenberg. The structure solution was performed with
lated previously for AgSO4.[7] Ag(SO3F)2 thus has a some-
Jana2006[32] coupled with Expo2004.[33] Topas 4.2 (Bruker AXS,
what more pronounced charge-transfer insulator character
than AgSO4 (a Mott–Hubbard insulator).
2009) was used to generate starting structural model for subsequent
refinement. We employed Vesta to generate the structure visualiza-
tions.[34]
Further details on the crystal structure investigations may be ob-
tained from the Fachinformationszentrum Karlsruhe, 76344 Egg-
enstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail:
crysdata@fiz-karlsruhe.de), on quoting the depository number
Conclusions
In seeking to fill the gap in the data regarding synthesis,
reactivity and characterization of rare oxa-derivatives of di- CSD-422414.
Eur. J. Inorg. Chem. 2011, 2499–2507
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W. Grochala et al.
FULL PAPER
Thermal analysis was performed with an STA 409PG TGA/DSC
analyzer from Netzsch coupled with an Aëolos QMS 403C mass
spectrometer (Netzsch) for evolved gas analysis. Samples (ca.
10 mg) were loaded into alumina crucibles. Heating rates of 1, 5
and 10 Kmin–1 were applied.
sulting in a quasi-3D magnetic character of this system at low tem-
peratures.
Phonon frequencies at the centre of the Brillouin zone were calcu-
lated with VASP at the PAW LDA and PAW GGA-PBE level using
atomic displacement of 0.002 Å with forces preoptimized down to
10–4 eVÅ–1 (for comparison of LDA phonon DOS and of experi-
mental IR and Raman spectra, see the Supporting Information).
Magnetic measurements were conducted with a superconducting
quantum interference device magnetometer MPMS-XL-5 from
Quantum Design equipped with a 50 kOe magnet, and operating
in the temperature range 2–300 K.
Supporting Information (see footnote on the first page of this arti-
cle): More computational DFT results including the phonon den-
sity of states and assignment of vibrational bands are given.
ESR spectra were taken with an ESP 300E spectrometer (frequency
9.5 GHz; X-band) from Bruker for samples sealed in Ar atmo-
sphere inside a 4 mm thick quartz capillary.
Acknowledgments
Solid-state density functional theory (DFT) calculations were per-
formed using the VASP code[35] with the projector-augmented wave
method (PAW)[36] as implemented in the MedeA package. For the
exchange-correlation part of the Hamiltonian, the local density
approximation (LDA) in its local spin density approximation
(LSDA) variant was applied, since we have noticed previously that
LDA allows for much better reproduction than generalized gradi-
ent approximation (GGA) of the unit-cell parameters of related
AgSO4[37] (for the GGA-PBE and GGA-PW91 results, see the Sup-
porting Information). During the full geometry optimization (cell
and atomic parameters), the ionic relaxation was continued until
the forces on individual atoms were less than 0.002 eVÅ–1. The
electronic iterations convergence was set to 10–7 eV per atom by
using the standard blocked Davidson algorithm and reciprocal
space projection operators. The spacing between the points for the
k-points mesh generation was around 0.5 Å–1 (Monhorst pack of
5ϫ5ϫ5). The valence electrons were described by plane waves
with a kinetic energy cutoff of 600 eV, thereby yielding satisfactory
convergence of total energy. Four different schemes of magnetic
ordering were tested: one ferromagnetic: (i) further denoted as FM,
with FM intrasheet and FM intersheet coupling, and three antifer-
romagnetic ones: (ii) AFM1 with AFM intrasheet and AFM in-
tersheet coupling, (iii) AFM2 with AFM intrasheet and FM in-
tersheet coupling, (iv) AFM3, with FM intrasheet and AFM in-
tersheet coupling, as well as (v) a nonmagnetic cell. Total optimiza-
tions at the LSDA level for the above-mentioned unit cells have
showed that the FM solution corresponds to a global energy mini-
mum with all AFM solutions higher in total energy, and the non-
magnetic solution pushed above the FM one at as much as
35.4 meV per FU. For this reason, the unit cell of the FM solution
optimized at the LSDA level was used for subsequent single-point
calculations of energy of all magnetic ordering schemes in question.
This work is dedicated to Professor Felix Aubke in recognition of
his significant contribution to chemistry of fluorosulfates. The pro-
ject “Quest for Superconductivity in Crystal-Engineered Higher
Fluorides of Silver” is operated within the Foundation for Polish
Science “TEAM” Program cofinanced by the EU European Re-
gional Development Fund. This work has been partly supported
by the Slovenian Research Agency (ARRS) within the research
program P1-0045 Inorganic Chemistry and Technology. Calcula-
tions have been performed on ICM supercomputers.
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Silver(II) Fluorosulfate
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Received: January 22, 2011
Published Online: April 26, 2011
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