Spin crossover in a heptanuclear mixed-valence iron complex

Article abstract of DOI:10.1039/b919120h

The complex [Fe^II{(CN)Fe^IIIL⁵} ₆]Cl₂ consists of a mixed-valence heptanuclear cyanide-bridged unit formed of a Schiff-base pentadentate ligand L⁵ and it shows a spin crossover of the peripheral Fe^III centres. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b919120h

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Spin crossover in a heptanuclear mixed-valence iron complex†
a
a
c
a
d
Roman Bocˇa,*a,b Ivan Salitrosˇ, Jozef Kozˇ´ısˇek, Jorge Linares, Ja´n Moncol and Franz Renz
ˇ
ˇ
Received 16th June 2009, Accepted 2nd December 2009
First published as an Advance Article on the web 12th January 2010
DOI: 10.1039/b919120h
The complex [FeII{(CN)FeIIIL5}6]Cl2 consists of a mixed-
valence heptanuclear cyanide-bridged unit formed of a Schiff-
base pentadentate ligand L5 and it shows a spin crossover of
the peripheral FeIII centres.
1 : 1). The colour change◦d from red-violet to blue. The mixture
was stirred for 2 h at 50 C and left to evaporate spontaneously
for several days at room temperature. Blue polycrystalline powder
was separated by filtration. Yield–90%. Found: C, 55.2; H, 5.01;
N, 13.0. Calc. for C120H126N24O12Fe7Cl2 (Mr = 2558.26 g mol-1) C,
-1
4
≡
˜
56.3; H, 4.96; N, 13.1%. IR (KBr, n/cm ): C N, 2078 (unsplit).
Mononuclear complexes showing spin crossover (SC), predomi-
nantly those of Fe(II), have been studied extensively over a long
period and several reviews about this phenomenon have been
published.1 Recently, there has been interest in dinuclear and
polynuclear systems where the SC interferes with the magnetic
exchange interaction.2 In addition to numerous dinuclear systems,
SC has also been reported for trinuclear, tetranuclear, and some
polynuclear systems.3 Herein we report SC in a mixed-valence
heptanuclear cyanide-bridged complex [FeII{(CN)FeIII(L5)}6]Cl2
formed of a Schiff-base pentadentate blocking ligand L5.
The complex exhibits blue colour, typical of the molecular
Prussian-blue analogues; the first absorption occurs at 17000 cm-1.
The hexacoordination of FeII by CN–FeIII is also confirmed by
-1
≡
˜
the IR spectra (no splitting of the C N band, n = 2078 cm )
and Mo¨ssbauer spectra (the area ratio FeIII : FeII ~ 6 : 1, i.e. ca 14%
of FeII at r.t.). The complex shows a molecular peak in the ESI-
mass spectrum: m/z (K2+/2) = 1244 g mol-1, which matches the
composition [K]Cl2. Molar mass of [K2+] is 2488 g mol-1. K is the
heptanuclear molecular dication [FeII{(CN)FeIII(salpet)}6]2+.
Single-crystal X-ray diffraction studies of [FeII({CN)-
FeIII(salpet)}6]Cl2‡ (hereafter 1) at room temperature re-
The Schiff-condensation of o-salicylaldehyde with an aliphatic
amine pet (1,6-diamino-4-azahexane) at a ratio of 2 : 1 provide the
ligand H2salpet (yellow oil). This ligand is asymmetric, having a
propyl- and an ethyl-bridge to the central nitrogen atom: (OH)Ph-
vealed the presence of
a heptanuclear complex cation
[FeII{(CN)FeIII(salpet)}6]2+ and two chloride anions, which are
disordered among three positions. The heptanuclear species has
=
=
CH N-(CH2)3-NH-(CH2)2-N CH-Ph(OH). Found: C, 70.1; H,
7.12; N, 12.8. Calc. for C19H23N3O2: C, 70.1; H, 7.12; N, 12.9.
The mononuclear precursor [Fe(salpet)Cl] was synthesized as
follows. A methanol solution of H2salpet (5 mmol in 15 cm3) was
combined with a methanol solution of FeCl3·6H2O (5 mmol in
30 cm3) accompanied with a colour change to dark-violet. Then
triethylamine (10 mmol) was added. After 30 min of stirring at
50 ◦C the reacting mixture was cooled to room temperature and a
dark-brown crystalline powder precipitated. This was separated by
filtration and washed with cold methanol and diethylether. Yield–
85%. Found: C, 54.3; H, 5.10; N, 10.1. Calc. for C19H21N3O2FeCl
(Mr = 414.69 g mol-1) C, 55.0; H, 5.10; N, 10.1%. IR (KBr,
an inversion centre (Fig. 1). The Fe1–C bond lengths are in the
II
˚
range 1.879(9)-1.932(10) A, Fe –C–N bond angles are in the range
177.3(8)-179.3(8)◦, which deviate only slightly from linearity;
these are similar to only reported {FeIIFeIII6} type complex
[FeII{(CN)FeIII(saldptm)}6]Cl2·17.25CH3OH (saldptm = bis(3-
(salicylideneamino)propyl)methylamine, refcode: EKIJUD).6 The
˚
cyanide C–N bond distances are in the range 1.128(9)–1.144(9) A,
and they match other complexes with bridging cyanide bonds.6,7
-1
=
˜
˜
n/cm ): N–H, 3230; C N, 1631, 1620, 1597. UV-Vis (Nujol,
-1
n/cm ) 18000, 24000, 27500, 40000. According to the magnetic
susceptibility, magnetization, Mo¨ssbauer spectra, and metal–
ligand distances this complex is high-spin (S = 5/2).
The mononuclear precursor dissolved in methanol (1 mmol
in 50 cm3) was combined with a methanol–water solution of
K4[Fe(CN)6]·3H2O (0.167 mmol in 10 cm3 of CH3OH–H2O =
aInstitute of Inorganic Chemistry, Institute of Physical Chemistry, Slovak
Technical University (FCHPT), SK-812 37, Bratislava, Slovakia. E-mail:
roman.boca@stuba.sk; Tel: 00421 2 59325 610
bDepartment of Chemistry, University of SS Cyril and Methodius, Trnava,
Slovakia
cLaboratoire GEMAC, UMR CNRS 86, Universite´ de Versailles, 78035
Versailles, France
dInstitute of Inorganic Chemistry, Leibniz University, D-30167, Hannover,
Germany
† Electronic supplementary information (ESI) available: electron, IR,
ESI, and Mo¨ssbauer spectra. CCDC reference number 633798. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b919120h
Fig.
1
X-ray
structure
of
the
heptanuclear
complex
[FeII{(CN)FeIII(salpet)}6]2+ (hydrogen atoms omitted for clarity).
2198 | Dalton Trans., 2010, 39, 2198–2200
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Table 1 Mo¨ssbauer-spectra parameters for the {FeIIFeIII6} complex 1a
T/K Centre
DEQ/mm s-1
d/mm s-1
Area/%
xHS
20
LS-Fe(III)
2.514
1.006
[0.15]
2.475
0.951
[0.15]
2.466
0.979
[0.15]
2.433
0.986
[0.15]
2.452
0.901
[0.15]
2.412
0.937
[0.15]
—
0.103
0.297
-0.057
0.103
0.312
-0.100
0.104
0.285
-0.063
0.100
0.275
-0.019
0.050
0.270
-0.175
0.043
0.246
-0.099
—
21.5
58.9
19.6
29.2
57.8
13.0
25.3
60.8
13.9
23.8
61.5
14.7
12.5
73.1
14.4
7.7
0.73
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
LS-Fe(III)
HS–Fe(III)
LS-Fe(II)
78
0.66
0.70
0.72
0.85
0.91
1.00
120
150
180
200
300
78.6
13.7
0
72.2
27.8
0.876
0.154
0.201
-0.162
a Estimate for the high-spin mole fraction of Fe(III): xHS = AHS/(AHS + ALS
)
[under the hypothesis of uniform Debye–Waller factors].
The FeIII–N–C bond angles of 165.3(8)–173.8(6)◦ deviate more
from linearity that the analogous bond angles in EKIJUD,6 but
they span the range 150–180◦ for other complex with bridging
cyanide ligands.7 The distances between FeII and FeIII atoms
are 5.026(3) (Fe1 ◊ ◊ ◊ Fe4), 5.045(3) (Fe1 ◊ ◊ ◊ Fe2), and 5.055(2)
III
˚
(Fe1 ◊ ◊ ◊ Fe3) A. The geometry around the peripheral Fe ions is
distorted due the nature of salpet pentadentate ligand. The bond
distances betweenperipheralFeIII and donor atoms are in the range
III
Fig. 2 Mo¨ssbauer spectra at different temperatures for the heptanuclear
{FeIIFeIII6} complex. From top to bottom: T = 20, 78, 120, 150, 180, 200,
and 300 K.
˚
˚
2.013(10)-2.058(9) A for Fe –N(cyanide), 1.823(10)-1.960(6) A
III
III
˚
for Fe –O(phenolate), 2.020(13)-2.161(9) A for Fe –N(amine),
III
˚
and 1.832(15)-2.110(8) A for Fe –N(imine), respectively. How-
ever, one pair of FeIII–N(cyanide) bonds in trans position is shorter
than the remaining pairs which indicates that the crystallographic
centre Fe4 is more close to the LS state (s = 1/2).
The Mo¨ssbauer spectra for the heptanuclear complex (Fig. 2,
Table 1) taken at T = 78 K show a coexistence of the low-spin
Fe(III) [29%, DEQ = 2.47, d = 0.10] and of the high-spin Fe(III)
[58%, DEQ = 0.95, d = 0.31], along with the presence of the
low-spin Fe(II) [13%, DEQ = 0.15, d = -0.10]. The Mo¨ssbauer
parameters are in mm s-1 (calibrated to a-Fe at r.t). The central
Fe(II) centre remains low-spin over all temperatures studied (20–
300 K). On heating the area-fractions associated with individual
quadrupole doublets of six peripheral Fe(III) centres alter in favour
of the high-spin Fe(III); the {DEQ, d} parameters stay almost
constant. This is an unambiguous proof that thermally-induced
spin crossover occurs. At T = 200 K the low-spin fraction of
Fe(III) almost disappears (less than 8%) and at room temperature
it is below a threshold of detection.
Fig. 3 Magnetic functions for the heptanuclear complex: left—tem-
perature dependence of the effective magnetic moment; right—field
dependence of the magnetization (dashed lines are guides for eyes).
Magnetic susceptibility measurements were done using a
SQUID magnetometer (Quantum Design) at B = 0.1 T between
T = 2 K and 300 K. Raw susceptibility data were corrected for
the effective magnetic moments stays approximately constant but
for T < 30 K it drops down. Six uncoupled Fe(III) centres with the
spins s = 5/2 provide a high-temperature limit of meff/mB = g[6s(s +
1)]1/2 = 14.5. Assuming 4 HS and 2 LS centres (xHS = 0.67), the
theoretical value of meff = 12.1 mB is close to that observed around
100 K. The central Fe(II) coordinated by six cyano ligands stays
underlying diamagnetism using the set of Pascal constants: cdia
=
-18.0 ¥ 10-9 m3 mol-1 (SI units). The effective magnetic moment
(Fig. 3) shows a gradual decrease from the value of meff = 14.3 mB at
T = 300 K to meff = 11.7 mB at T = 100 K. Below this temperature
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low-spin (s = 0) and it is magnetically silent. Magnetization taken
at 2 K saturates to N > 16 unpaired electrons that probably refer
to 3HS+3LS (N = 18).
1 H. A. Goodwin, Coord. Chem. Rev., 1976, 18, 293; P. Gu¨tlich, Struct.
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As the Mo¨ssbauer spectra show a coexistence of the low-
spin and high-spin Fe(III) centres even at T = 20 K, a more
elaborated spin crossover model needs to be developed. In the
present case of six magnetoactive Fe(III) centres we are left with
seven referential (electronic) states: LLLLLL, LLLLLH (6¥),
LLLLHH (15¥), LLLHHH (20¥), LLHHHH (15¥), LHHHHH
(6¥), and HHHHHH separated by six Di differences. While the first
reference state (six centres of S = 1/2) involves 26 = 64 magnetic
energy levels, the last one (six centres of S = 5/2) involves 66 =
46656 magnetic energy levels. Therefore the partition function not
only contains a tremendous number (69952) of different terms,
but also a large number of parameters (Di, gi, and Ji)
2 J. A. Real, J. Zarembowitch, O. Kahn and X. Solans, Inorg. Chem., 1987,
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6−N
(2
N
)
⋅6
6







6
Z(Bk ) =
⋅
exp[−(D0 +...+ DN +e
)/ kT ]
i,k
6-N
[L
N
]
∑ ∑
H


ˇ
ˇ
N
N =0 

I. Salitrosˇ, R. Bocˇa, L. Dlha´nˇ, M. Gembicky´, J. Kozˇ´ısˇek, J. Linares, J.
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ˇ
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815.
The detected observables (magnetization, magnetic susceptibil-
ity, high-spin mole fraction, heat capacity) will involve all these
parameters. It is not a realistic target to fix these parameters
reliably using the data which are presently available. The estimate
that at T = 78 K the complex under study contains ca 58% of
the high-spin Fe(III) fraction could result from a manifold occu-
pation of the reference states (LLLLLL, LLLLLH, LLLLHH,
LLLHHH, LLHHHH, LHHHHH, and HHHHHH).8 This is the
raison d’etre why the refinement of the structure cannot reach a
better R-factor.
3 R. Herchel, R. Bocˇa, M. Gembicky´, J. Kozˇ´ısˇek and F. Renz, Inorg.
Chem., 2004, 43, 4103; E. Breuning, M. Ruben, J.-M. Lehn, F. Renz, Y.
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Hyperfine Interact., 2008, 184, 259.
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stable carbides and nitrides when cyanides burn in a commercial C–H–
N analyzer (FlashEA 1112, ThermoFinnigan). The resulting material
exhibits identical electron/IR spectra independent of whether Fe(II)- or
Fe(III)-hexacyanide has been used in synthesis.
5 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112.
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2001, 40, 3836; G. Rogez, A. Marvilliers, E. Riviere, J.-P. Audiera, F.
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Angew. Chem., Int. Ed., 2000, 39, 2885.
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K. S. Murray, Chem. Commun., 2001, 333; R. J. Parker, L. Spicia, B.
Moubaraki, K. S. Murray, D. C. R. Hockless, D. A. Rae and A. C.
Willis, Inorg. Chem., 2002, 41, 2489; R. J. Parker, L. Spiccia, S. R. Batten,
J. D. Cashion and G. D. Fallon, Inorg. Chem., 2001, 40, 4696; J. P.
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Grant agencies (Slovakia: VEGA 1/0213/08, APVV 0006-07,
COST-0006-06, VVCE 0004-07; Germany: Leibniz University
ZFM; EU: Structural Funds, Interreg IIIA) are acknowledged
for the financial support.
Notes and references
‡ Crystallographic data: X-ray single-crystal data for 1 were collected
at room temperature using Oxford Diffraction Gemini R CCD diffrac-
˚
tometer with graphite-monochromated Mo-Ka radiation (l = 0.7107 A).
The structure of [FeII{(CN)FeIII(salpet)}6]Cl2 was solved and refined by
SHELX-97 package.5 Chloride anions are disordered in three positions
(one is special position). Crystal data for [FeII{(CN)FeIII(salpet)}6]Cl2:
¯
C
120H126Cl2Fe7N24O12, monoclinic P1, a = 14.443(3), b = 14◦.480(3),
˚
c = 14.842(3) A, a = 105.15(3), b = 105.21(3), g = 92.04(3) , V =
2873.0(10) A , Z = 1, Dc = 1.479 g cm-3, m = 0.978 mm-1, F(000) =
3
˚
1326, T = 293(2) K, 2qmax = 26.4◦ (-18 ≤ h ≤ 17,-18 ≤ k ≤ 18,-16 ≤ l ≤
18). Final results (for 692 parameters and 277 restraints) were R1 = 0.1153
and wR2 = 0.3098 for 4122 reflections with I > 2s(I), and R1 = 0.2325,
wR2 = 0.3482 and S = 1.033 for all 11646 reflections. Reference number is
CCDC 633798.
8 Averaged Fe–N (Fe–O) distances in the heptanuclear complex at room
temperature are shorter than those in the mononuclear high-spin
˚
complex: 2.119 (1.927) versus 2.147 (1.943) A.
2200 | Dalton Trans., 2010, 39, 2198–2200
This journal is
The Royal Society of Chemistry 2010
©