Spin crossover of ferric complexes with catecholate derivatives. Single-crystal X-ray structure, Magnetic and Moessbauer investigations

Article abstract of DOI:10.1039/b418294d

Complexes of general formula [(TPA)Fe(R-Cat)]X·nS were synthesised with different catecholate derivatives and anions (TPA = tris(2-pyridylmethyl) amine, R-Cat^2- = 4,5-(NO₂)₂-Cat^2- denoted DNC^2-; 3,4,5,6-Cl₄-Cat^2- denoted TCC^2-; 3-OMe-Cat^2-; 4-Me-Cat^2- and X = BPh ₄^-; NO₃^-; PF₆ ^-; ClO₄^-; S = solvent molecule). Their magnetic behaviours in the solid state show a general feature along the series, viz., the occurrence of a thermally-induced spin crossover process. The transition curves are continuous with transition temperatures ranging from ca. 84 to 257 K. The crystal structures of [(TPA)Fe(DNC)]X (X = PF₆^-; BPh₄^-) and [(TPA)Fe(TCC)]X·nS (X = PF ₆^-; NO₃^- and n = 1, S = H ₂O; ClO₄^- and n= 1, S = H₂O; BPh₄^- and n = 1, S = C₃H₆O) were solved at 100 (or 123 K) and 293 K. For those two systems, the characteristics of the [FeN₄O₂] coordination core and those of the dioxolene ligands appear to be consistent with a prevailing Fe ^III-catecholate formulation. This feature is in contrast with the large quantum mixing between Fe^III-catecholate and Fe ^III-semiqumonate forms recently observed with the more electron donating simple catecholate dianion. The thermal spin crossover process is accompanied by significant changes of the molecular structures as shown by the average variation of the metal-ligand bond distances which can be extrapolated for a complete spin conversion from ca. 0.123 to 0.156 A. The different space groups were retained in the low- and high-temperature phases. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418294d

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F U L L P A P E R
Spin crossover of ferric complexes with catecholate derivatives.
¨
Single-crystal X-ray structure, Magnetic and Mossbauer
investigations†
S. Floquet,a A. J. Simaan,a E. Rivie`re,a M. Nierlich,b P. T h u e´ry,b J. Ensling,c P. G u¨tlich,c
J.-J. Girerda and M.-L. Boillot*a
a Laboratoire de Chimie Inorganique, UMR 8613, ICMMO, Baˆt 420, Universite´ Paris-Sud,
91405 Orsay, France
b DRECAM/SCM, CEA Saclay, Baˆt.125, 91191 Gif-sur-Yvette Cedex, France
c Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita¨t,
Staudingerweg 9, D-55099 Mainz, Germany
Received 6th December 2004, Accepted 18th March 2005
First published as an Advance Article on the web 7th April 2005
Complexes of general formula [(TPA)Fe(R-Cat)]X·nS were synthesised with different catecholate derivatives and
anions (TPA = tris(2-pyridylmethyl)amine, R-Cat2− = 4,5-(NO2)2-Cat2− denoted DNC2−; 3,4,5,6-Cl4-Cat2− denoted
TCC2−; 3-OMe-Cat2−; 4-Me-Cat2− and X = BPh4−; NO3−; PF6−; ClO4−; S = solvent molecule). Their magnetic
behaviours in the solid state show a general feature along the series, viz., the occurrence of a thermally-induced spin
crossover process. The transition curves are continuous with transition temperatures ranging from ca. 84 to
−
257 K. The crystal structures of [(TPA)Fe(DNC)]X (X = PF6−; BPh4−) and [(TPA)Fe(TCC)]X·nS (X = PF6−; NO3
−
−
and n = 1, S = H2O; ClO4 and n = 1, S = H2O; BPh4 and n = 1, S = C3H6O) were solved at 100 (or 123 K) and
293 K. For those two systems, the characteristics of the [FeN4O2] coordination core and those of the dioxolene
ligands appear to be consistent with a prevailing FeIII–catecholate formulation. This feature is in contrast with the
large quantum mixing between FeIII–catecholate and FeII–semiquinonate forms recently observed with the more
electron donating simple catecholate dianion. The thermal spin crossover process is accompanied by significant
changes of the molecular structures as shown by the average variation of the metal–ligand bond distances which can
˚
be extrapolated for a complete spin conversion from ca. 0.123 to 0.156 A. The different space groups were retained in
the low- and high-temperature phases.
In this paper, we report an experimental investigation of the
Introduction
spin crossover properties of ferric catecholate complexes of the
The properties of ferric catecholate systems were extensively
type [(TPA)Fe(R-Cat)]X (see Scheme 1, the different H2-R-
investigated as they provide interesting functional models of
Cat ligands). Substituents of the catechol group and anions
the intradiol catechol dioxygenases.1,2 These enzymes oper-
were varied as they allow the internal electron transfer or
ate in the sequence of chemical reactions necessary for the
the ligand field strength to be modulated. The spin intercon-
degradation of aromatic compounds, one key step consisting
version shown by these compounds was investigated through
in the intradiol cleavage of the catecholate groups in the
variable-temperature single-crystal X-ray diffraction, magnetic
presence of dioxygene. It is well established that the activity
and Mo¨ssbauer measurements.
for intradiol catechol cleavage depends on the iron–catecholate
interaction, i.e. increases with the semiquinonato character of
the dioxolene ligand and the Lewis acidity of the FeIII ion.2
The dioxygenase reactivity is clearly reinforced in the presence
of electron-donating substituents on the catechol group.3 The
most efficient model system consists of [(TPA)Fe(DBC)]BPh4
(DBC2− = 3,5-di-tert-butylcatecholate dianion and TPA =
tris(2-pyridylmethyl)amine).2
In addition to the iron–catecholate interaction, these com-
plexes may exhibit an electronic lability associated to the spin
multiplicity of the electronic state of the metal ion. Indeed,
the study of the magnetic behaviour of [(TPA)Fe(Cat)]BPh4
(Cat2− = catecholate dianion)4 has revealed the occurrence of an
unexpected thermal high-spin (HS) ↔ low-spin (LS) crossover
in the solid state.5,6 Gradual and incomplete spin crossover
processes were also reported for [(TPA)Fe(DBC)]BPh4 and
[(TPA)Fe(R-cat)]BPh4 (R-Cat = 3- and 4-chlorocatecholates).4,7
Moreover, evidence for an extensive quantum mixing of FeIIICat
and FeIISQ forms in both spin states were provided by DFT
calculations for [(TPA)Fe(Cat)]BPh4.6
Scheme 1 Scheme of the different H2-R-Cat molecules.
† Electronic supplementary information (ESI) available: Fig. S1 shows
the powder X-ray diffraction patterns recorded at room temperature for
Experimental
Ferric salts, NaPF6, NaBPh4, H2TCC, H2Cat, H2-3-OMe-Cat,
H2-4-CH3-Cat and solvents were purchased from Aldrich, Acros
the 1a, 2a, 3a, 4a and 5a compounds. See http://www.rsc.org/suppdata/
dt/b4/b418294d/
1 7 3 4
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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or Fluka chemicals and used without further purification.
TPA and H2DNC were prepared according to the procedures
described in the literature.8,9 Complexes [(TPA)Fe(R-Cat)]BPh4
(R-Cat = Cat, 3-OMe-Cat, 4-CH3-Cat) (noted 3a–5a) were
synthesised under an argon atmosphere.
thesised under an argon atmosphere according to the procedure
described in ref. 4. The microcrystalline powders were isolated
by filtration under an inert atmosphere, washed with methanol
and dried under vacuum. Anal. Calcd. for FeC48H42N4O2B (3a,
773.52 g mol−1): C 74.19, H 5.20, N 7.39, Fe 7.15, B 1.40. Found
C 74.49, H 5.47, N 7.24, Fe 7.23, B 1.40. Anal. Calcd. for
FeC49H44N4O3B (4a, 803.55 g mol−1): C 73.24, H 5.52, N 6.97, Fe
6.95, B 1.35. Found C 72.61, H 5.49, N 7.13, Fe 6.95, B 1.40. For
FeC49H44N4O2B (5a, 787.56 g mol−1): C 74.73, H 5.63, N 7.11,
Fe 7.10, B 1.37. Found C 74.90, H 5.65, N 7.18, Fe 7.10, B 1.00.
Preparation of [(TPA)Fe(R-Cat)]X·S (R-Cat2− = DNC2−,
TCC2−, X− = BPh4−, PF6−, NO3 and S = solvent)
−
H2-R-Cat (0.255 mmol) was dissolved in methanol (10 mL) and
deprotonated by 0.510 mmol of Et3N (71.5 lL). This solution
was slowly added to a mixture of Fe(NO3)3·9H2O (103 mg,
0.255 mmol) and TPA (74 mg, 0.255 mmol) in methanol (10 mL).
Cyclic voltametry measurements
These data were recorded with an EGG PAR potentiostat
(M273 model). The counter electrode was an Au wire, the
working electrode, a glassy carbon disk, was carefully polished
before each voltamogram with a 1 lm diamond paste, sonicated
in an ethanol bath and then washed carefully with ethanol.
The reference electrode was a saturated calomel electrode. The
solvent used was distilled acetonitrile where NBu4PF6 was added
to obtain a 0.1 M supporting electrolyte. The experiments were
carried out at RT.
[(TPA)Fe(DNC)]BPh4 (1a) and [(TPA)Fe(TCC)]BPh4 (2a).
The solution of [(TPA)Fe(R-Cat)]+ was stirred at 50 ◦C for
15 min before the addition of one equivalent of NaBPh4 in
methanol (10 mL). The microcrystalline powder (1a, dark green
and 2a·acetone, purple) was isolated by filtration, washed with
methanol and dried under vacuum. Single crystals suitable
for X-ray diffraction were obtained by the evaporation of a
(1/1) methanol/acetone solution of complex. Anal. Calcd. for
FeC48H40N6O6B (1a, 863.52 g.mol−1): C 66.76, H 4.67, N 9.73.
Found C 66.63, H 4.62, N 9.73. For FeC48H38N4O2Cl4B (2a,
911.31 g.mol−1): 63.26, H 4.20, N 6.15. Found C 63.23, H 4.57,
N 6.21.
UV-vis measurements
These were performed using a Varian Cary 5E double-beam
spectrophotometer at RT. The solutions were prepared under
an argon atmosphere.
[(TPA)Fe(DNC)]PF6 (1b) and [(TPA)Fe(TCC)]PF6 (2b). The
complex salts were prepared at room temperature with an
addition of an excess of NaPF6 (0.3 mmol for 1b, 3 mmol
for 2b) in 15 mL of (1 : 2) methanol/water. Large single
crystals of 1b were obtained from the mixture upon standing
overnight. For 2b, the crude solid was recrystallised from a
filtered methanol solution. Deep purple platelets of 2b were
isolated by filtration, washed with water and dried in air.
Anal. Calcd for FeC24H20N6O6PF6 (1b, 689.28 g.mol−1): C
41.80, H 2.93, N 12.19. Found C 41.73, H 2.96, N 12.09. For
FeC24H18N4O2Cl4PF6 (2b, 737.04 g mol−1): C 39.11, H 2.46, N
7.60. Found C 38.77, H 2.45, N 7.42.
X-Ray measurements
Diffraction data collection was performed on a Nonius diffrac-
tometer equipped with a CCD detector. The lattice parameters
were determined from ten images recorded with 2◦ U scans and
later refined on all data. A 180◦ U range was scanned, with
2◦ steps, with a crystal-to-detector distance fixed at 30 mm.
Data were corrected for Lorentz polarisation. The structure was
solved by direct methods and refined by full-matrix least-squares
on F2 with anisotropic thermal parameters for all non H-atoms
(except in the case of acetone solvent atoms of 2a·acetone for
which isotropic thermal parameters were chosen), H atoms were
introduced (except in the case of H atoms of the water molecule
of 2c·H2O and 2d·H2O) at calculated positions and constrained
to ride on their parent C atoms. All calculations were performed
on an O2 Silicon graphics station with the SHELXTL package.10
The labelling of the donor atoms is shown in Scheme 2. The
temperature values chosen for the measurements (T1 = 293 K,
T2 = 173, 123 or 100 K) correspond to different fractions of HS
species. The fraction of HS iron(III) ions can be estimated from
[(TPA)Fe(DNC)]NO3·H2O (1d·H2O) and [(TPA)Fe(TCC)]-
NO3·H2O (2d·H2O). The solution of complex diluted with
water (10 mL) was slowly evaporated. Dark green needles
of 1d·H2O or deep purple crystals of 2d·H2O were obtained
after several days, filtered, washed with water and dried in air.
Anal. Calcd. for FeC24H22N7O10 (1d·H2O, 624.32 g mol−1): C
46.17, H 3.55, N 15.70. Found C 46.04, H 3.47, N 15.76. For
FeC24H20N5O6Cl4 (2d·H2O, 672.10 g mol−1): C 42.99, H 2.78, N
10.45. Found C 42.80, H 2.80, N 10.44.
the magnetic data (assuming that vMTHS = 4.375 cm3 mol−1
K
and vMTLS ≈ 0.375 cm3 mol−1 K for pure HS and LS species,
respectively). At T1(T2), it is equal to 0.95(0.09) for 1a; 0.83
(0.02) for 1b; (1) for 2a·acetone, 0.80 (0.03) for 2b; (0.02) for
2c·H2O and 0.71 (0.01) for 2d·H2O.
Preparation of [(TPA)Fe(DNC)]ClO4·H2O (1c·H2O) and
[(TPA)Fe(TCC)]ClO4·H2O (2c·H2O). CAUTION: Perchlo-
rate salts combined with organic ligands are potentially ex-
plosive and should be handled with care. The mixture of
Fe(ClO4)3.10H2O (136.3 mg, 0.255 mmol) and TPA (74 mg,
0.255 mmol) in methanol (30 mL) was heated at 60 ◦C to
prevent the precipitation of [(TPA)Fe(H2O)2](ClO4)3. To this
solution was added a methanol solution (10 mL) of H2-R-Cat
(0.255 mmol) previously deprotonated by 0.510 mmol of Et3N
(71.5 lL). After stirring for 30 min at 60 ◦C, the reaction mixture
was allowed to cool to room temperature. This solution was di-
luted with water (10 mL) and allowed to stand at 4 ◦C overnight.
A dark green powder of 1c·H2O resulted and was isolated by
filtration, washed with water and dried in air. Single crystals of
2c·H2O suitable for X-ray diffraction were obtained from the
dark purple powder dissolved in a methanol/water solution.
Anal. Calcd. for FeC24H22N6O11Cl (1c·H2O, 661.76 g.mol−1): C
43.57, H 3.35, N 12.71. Found C 43.83, H 3.15, N 12.92. For
FeC24H20N4O7Cl5 (2c·H2O, 709.54 g.mol−1): C 40.63, H 2.84, N
7.90. Found C 40.37, H 2.58, N 7.82.
Scheme 2 Scheme of the coordination sphere and labels.
Magnetic measurements
The temperature dependence of the magnetic susceptibility have
been carried out on samples of typical weight 15–20 mg using
a Quantum Design SQUID magnetometer (MPMS5S model)
calibrated against a palladium sample.
Mo¨ssbauer measurements
[(TPA)Fe(R-Cat)]BPh4 (R-Cat = Cat: 3a, R-Cat = 3-OMe-
Cat: 4a, R-Cat = 4-CH3-Cat: 5a). These complexes were syn-
A constant-acceleration type Mo¨ssbauer spectrometer equipped
with a 1024-channel analyser operating in the time scale mode,
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2
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and a 25 mCi 57Co/Rh source was employed. The isomer shifts
reported here are relative to a-Fe at room temperature. Spectra
of the samples (thickness of about 5 mg Fe cm−2) were collected
between 293 and 30 K by means of a combined He continuous
flow/bath cryostat. The Mo¨ssbauer spectra were analysed by
means of a computer program (RECOIL).11
Results and discussion
X-Ray diffraction
All complexes under study exhibit a thermal spin crossover
process except 2a·acetone, which remains in the HS state at any
temperature (vide infra). Therefore the structures of 1a, 1b, 2b
and 2d·H2O were determined at two temperatures (T1 = 293 K,
T2 = 123 or 100 K) whereas the one of 2a·acetone was solved at
173 K. For 2c·H2O, the measurement was carried out in the LS
phase (HS fraction = 0.02) at 123 K. Crystal data collection and
refinement parameters for all complexes are given in Table 1.
Selected bond lengths and angles are listed in Table 2.
2b crystallises in the orthorhombic space group Pna2(1)
(Z = 4). Both 2c·H2O and 2d·H2O crystallise as a monoclinic
space group P2(1)/c (Z = 4) with similar unit cell volume and
parameters. These latter are also very close to those found for
2b. Finally 1a and 2a·acetone are described by P-1 triclinic space
groups (Z = 2) with different unit cell parameters.
By decreasing the temperature, the space groups of all these
complexes are retained and significant variations of the unit
cell parameters and volumes are observed. For example, the
variation between 293 and 123 (or 100) K of the unit cell volume
per formula unit (DV) is found to be 30.6 (1b); 37.4 (2d·H2O);
3
˚
38.6 (2b) and 40.0 A (1a). Such values are the sum of two
contributions: the volume change corresponding to the thermal
spin crossover (vide infra) and the lattice thermal expansion.
Molecular structures of 1a, 1b, 2a·acetone, 2b and 2d·H2O in the
HS form
For this set of complexes, prevailingly in the HS form at 293 K,
the FeN4O2 core possesses distorted octahedral geometries
(1a, Fig. 1) similar in nature to those previously described for
the related HS ferric catecholate compounds.2 The common
structural features are (i) large values of the metal–ligand
bond lengths, (ii) bond angles showing marked deviations
from the octahedral geometry, (iii) an asymmetry of the metal
environment due to the coordination of the tripodal ligand (see
the bond lengths and angles in Table 2). The mean values of
selected bond lengths are reported in Table 3. From the data
collected in Tables 2 and 3, the structural parameters of 1a
and 2a·acetone are fully consistent with those characterised
for HS FeIII catecholate systems (ꢀFe–Namineꢁ = 2.151–2.246 A,
˚
2,12
˚
˚
ꢀFe–Npyridineꢁ = 2.109–2.227 A, ꢀFe–Oꢁ = 1.907–1.979 A).
The slight discrepancies observed for 1b and 2b, 2d·H2O
indicate the presence of some amount of LS species at 293 K
that agrees with the results of the magnetic measurements.
The characteristics of the dioxolene ligands can be analysed
through the values of the C–O bond length and those of
the aromatic ring.12,13 The mean values of the C–O and C–C
Fig. 1 The molecular structure of 1a at 100 K.
1 7 3 6
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2

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D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2
1 7 3 7

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˚
Table 3 The average metal–ligand bond lengths (A) for [(TPA)Fe(R-Cat)]X·S complexes, S1 = H2O, S2 = acetone with the corresponding values of
the vMT product (cm3 mol−1 K)
T/K
c HS
vMT
ꢀFe–Oꢁ
Fe–Nam
ꢀFe–Nimꢁ
ꢀFe–Lꢁ
ꢀC–Oꢁ
ꢀC–Cꢁ
C19–C24
C21–C22
1a
293
100
293
100
173
293
123
123
293
100
0.86
0.09
0.83
0.02
1
0.80
0.03
0.02
0.71
0.01
3.823
0.794
3.699
0.530
4.292
3.632
0.806
0.470
3.264
0.480
1.93
1.90
1.92
1.89
1.935
1.91
1.89
1.89
1.91
1.89
2.182(4)
2.018(3)
2.153(4)
1.995(3)
2.191(5)
2.138(13)
1.993(6)
1.990(3)
2.129(5)
1.992(9)
2.12
1.98
2.08
1.96
2.123
2.113
1.93
1.96
2.08
1.96
2.07
1.96
2.04
1.94
2.07
2.05
1.93
1.94
2.03
1.94
1.333
1.335
1.32
1.33
1.33
1.356
1.339
1.330
1.330
1.32
1.39
1.39
1.39
1.39
1.39
1.40
1.39
1.40
1.395
1.39
1.422(6)
1.422(5)
1.426(7)
1.422(5)
1.430(9)
1.47(2)
1.427(12)
1.422(5)
1.405(8)
1.39 (2)
1.398(6)
1.391(5)
1.375(8)
1.404(5)
1.394(10)
1.36(2)
1.415(13)
1.403(5)
1.395(9)
1.431(19)
1a
1b
1b
2a·acetone
2b
2b
2c·H2O
2d·H2O
2d·H2O
Molecular packing of 1a and 1b at 100 K
˚
bond lengths (see Table 3) span over the ranges 1.32–1.36 A and
˚
The molecular packing of 1b at 100 K consists of pairs of cationic
complexes with anions. As shown in Fig. 2, the NO2 groups
are twisted by 40.3 and 39.3◦ with respect to the catecholate
aromatic plane that can be accounted for by the steric effect
and the different molecular contacts with aromatic carbon
atoms (O4–C8; 3.199, O5–C10; 3.220, O5–C19; 3.301 and O5–
1.39–1.40 A, respectively. These characteristics are comparable
to those reported for a number of FeIII–catecholate systems.14
However this analysis shows the existence of some borderline
values of ꢀC–Cꢁ (for 2b and 2c·H2O samples) and rather high
˚
C19–C24 and C21–C22 bond length values (1.403–1.427 A)
in the low temperature data (see for example, 2c·H2O, 1b and
2b). These discrepancies are mainly observed with complexes of
group 2. They might be indicative of a very small contribution of
the Fe(II)semiquinonate configuration within the ground state.
˚
C24; 3.218 A). The non-coplanarity precludes any significant
p-stacking interactions between the catecholate rings although
they are located in quasi-parallel planes. For 1a, the packing is
similar but the cationic complexes are more efficiently separated
−
by the bulky BPh4 anions. The presence of molecular contacts
between the NO2 group and the pyridine rings may contribute to
stabilise this NO2 group close to the aromatic plane. The second
one is found to be significantly twisted (twist angles 21.7 and
57.6◦). As a general feature in the molecular packing of 1b and
1a, some aromatic rings are found in almost parallel planes but
Molecular structures of 1a, 1b, 2b, 2c·H2O and 2d·H2O in the
LS form
As shown in Tables 2 and 3, the metal–donor atom bond
lengths of the FeN4O2 cores are significantly shortened and
the deviation of bond angles to 90 and 180◦ are reduced
when the temperature decreases from room temperature to 123
or 100 K. These observations that reflect the net increase of
the ligand field strength associated to the (t2g)3(eg)2 ↔ (t2g)5
conversion of the metal ion,15 are consistent with the magnetic
data (vide infra). Indeed, the variation of bond lengths and
angles roughly compare with those characterised for ferric spin
crossover complexes with similar FeN4O2 coordination core
in the LS state.16–18 Note that the variation of the N4FeN3
bond angle and the Fe–N1 bond length (N1 = Namine) are
˚
the distances between the ring centroids (dg > 4.11 and 4.27 A
for 1a and 1b, respectively) and the dihedral angles between
the planes correspond to very small p-stacking interactions. In
contrast, a number of intermolecular contacts are observed in
both structures.
equal or larger than 10◦ (see for example 2b) and 0.14 A (2b,
˚
2d·H2O) respectively. Consequently, they can be considered as
good probes for analysing the spin crossover process. Merkel
and coworkers12 have reported the crystalline structure of
[(TPA)Fe(TCC)]ClO4·nH2O (with n = 0.25) at 170 K. These
data which are comparable to those here reported for 2c·H2O
characterise a low-spin metal ion (from the magnetic data, the
HS fraction at 170 K is equal to 0.06). The LS FeIII catecholate
systems are characterised by the following mean values: ꢀFe–
˚
˚
Namineꢁ = 1.990–2.018 A, ꢀFe–Nimineꢁ = 1.935–1.977 A, ꢀFe–
˚
Oꢁ = 1.885–1.896 A. If the difference of HS fractions in the
temperature range used for the X-ray measurements is taken
into account, the variation of the mean metal–donor atom bond
length ꢀFe–Lꢁ can be extrapolated for a complete spin crossover
Fig. 2 The intermolecular contacts involving the NO2 groups of 1b at
100 K.
˚
process to 0.123 (1b) < 0.128 (1a) ∼ 0.129 (2d·H2O) < 0.156 A
(2b) (the value determined from optimised geometries of 3a is
6
˚
0.157 A). These variations span over the range reported for
Molecular packing of 2a·acetone, 2b and 2d·H2O
the Schiff base ferric complexes undergoing a spin crossover,
17,18
˚
(0.12–0.15 A).
As it can be noted for 3a, the variations of
The molecular packing of complexes 2b (shown in Fig. 3 at
123 K) and 2d·H2O consists of layers of anions (including the
solvent molecules for the solvated complexes) alternating with
layers of cations organised in pairs. This latter is made up of
complexes with opposite orientations along their N1–Fe–O1
axis which results in some assembling of the hydrophobic TCC
groups of the two complexes and the polar coordination cores
with the sheets of anions. Although the different pairs of cations
the mean metal–donor atom bond lengths observed for 1a and
1b are associated to a large change of the Fe–N1 (N1 = Namine
)
and Fe–N2 bonds that are located trans to the Fe–O bonds (i.e.:
DFe–N1 > DFe–N2 > DFe–N3 and DFe–N4). For 2b, the change
of the bond lengths takes place through very large Fe–N2 and
Fe–N3 variations, whereas for 2d·H2O, it involves rather small
Fe–N1 and Fe–N4 variations.
1 7 3 8
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2

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The relative variation of the unit cell volume is found between
3.9 (1a) and 5.7% (2d·H2O). These values are similar to those
reported for ferrous and ferric spin crossover complexes.15
Magnetic properties
In Fig. 4a,b is shown the temperature dependence of the vMT
product (vM being the molar magnetic susceptibility corrected
for the diamagnetic contributions) for solid samples of 1a–d, 2b–
d. S, 4a and 5a (S = H2O for 2c and 2d). In Fig. 4b, the magnetic
properties of [(TPA)Fe(Cat)]BPh4 3a previously reported4 are
plotted for comparison. The magnetic measurements are super-
imposable in the cooling and the heating modes and no hysteresis
effect is detected. In Table 4 are gathered the vMT values
observed in the high and low temperature limits together with
an estimation of the transition temperature T1/2 (corresponding
to the half conversion of the metal ions c HS = c LS = 0.5)
and the residual HS fraction at low temperature.19 These
magnetic data reveal that the compounds of general formula
[(TPA)Fe(R-Cat)]X undergo a thermal spin crossover process
as a general feature. Indeed, [(TPA)Fe(TCC)]BPh4·acetone is
the only example of a complex existing in the HS state at any
temperature. In contrast to the transition curve reported for 3a,
those of 1a–d,2b–d. S, 4a and 5a are of continuous type as they
spread over a temperature range equal to or larger than 200 K.
The weak cooperative character of the transition curves
can be accounted for by the absence of any significant p-
stacking interactions observed in the molecular packing of the
studied compounds. This feature contrasts to the solid state
characteristics of 3a for which strong p-stacking interactions
and numerous intermolecular contacts lead to a cooperative
transition.6
Fig. 3 The molecular packing of 2b at 123 K in the ab plane. The
displacement ellipsoids are shown at the 50% probability level.
The thermodynamical parameters of the transition process
can be estimated from the Slichter and Drickamer equation
based on the regular solution model.20,21 The enthalpy and
entropy variations (see Table 4) are found in the range of
5.3–11.2 kJ mol−1 and 26–51 J mol−1 K−1, respectively. The
DS values exceed the pure electronic contribution Rln(6/2) =
9.13 J mol−1 K−1 resulting from the difference in the spin
degeneracies of the HS and LS states of a ferric complex. In
addition, they are in agreement with those previously reported
for ferric complexes (DS = 16–18 and 34–41 J mol−1 K−1).22 As
expected for gradual transformations, the interaction parameter
C is relatively small as it varies between 0 and RT1/2. We
note that this treatment, based on the regular solution model
proposed by Slichter and Drickamer, fails to give reasonable
thermodynamical parameters (with a non negative value of C)
for 1d·H2O, 4a and 5a. The absence of structural data for these
compounds precludes any analysis of these results.
are regularly spaced with almost parallel aromatic rings in the
interlayer space, no significant p-stacking interactions can be
detected. For example, a weak interaction between the pyridine
◦
˚
rings is observed for 2d·H2O with dg = 3.892 A and a = 4.97 at
100 K. A moderate hydrogen bonding interaction is identified
−
for 2d·H2O between the oxygen atoms of the H2O and NO3
molecules. This interaction, characterised by O6w◦–O4 distances
˚
of 2.849 A and N5–O4–O6w angles of 122.05 , contributes
to stabilise the anionic layer. In the molecular packing of
2a·acetone, the cations [(TPA)Fe(TCC)]+ are well separated from
−
each other by the bulky BPh4 anions. The most significant p-
stacking interaction is found between the two pyridine rings
◦
˚
bearing the N4 atom (at 293 K, dg = 3.627 A and a = 0.00 ).
The molecular packing of 2c·H2O at 123 K compares with
those determined at 293 K for 2b and 2d·H2O. A moderate
−
hydrogen bonding interaction is observed between ClO4 and
˚
˚
the water molecule (Ow–O6 distance of 2.709(7) A and Ow–O6–
The transition temperatures increase as 5a < 3a < 1a < 4a
∼ 1c·H2O < 2b < 1b < 2d·H2O < 2c·H2O ∼ 1d·H2O. The
complexes [(TPA)Fe(R-Cat)]BPh4 (noted a, as X− = BPh4−) with
catecholate bearing electron-donating and electron-withdrawing
◦
Cl5 134.84 , Ow–O4(2−x,1−y,1−z) 2.850(6) A, O4–Ow–O6
◦
115.1(2) ). A number of intermolecular contacts are observed
for 2b, 2d·H2O, 2a·acetone and 2c·H2O.
Table 4 Experimental vMT values measured at low and high temperatures. Analysis of these data according to ref. 19. Estimation of the fraction of
residual HS species and determination of the thermodynamical parameters (T1/2, DH/kJ mol−1, C/J mol−1 and DS/J mol−1 K−1) according to the
Slichter and Drickamer method.20,21
Compound T/K vMT/cm3mol−1
K
c HS at low T
T/K vMT/cm3 mol−1
K
T
1/2/K DH/kJ mol−1 c /kJ mol−1
DS/J mol−1 K−1
1a
10
10
10
10
55
10
5
5
10
5
0.449
0.465
0.580
0.579
—
3.603
0.698
0.400
0.453
0.634
0.682
0.02
0.02
0.05
0.05
—
293
350
300
350
300
300
300
300
350
300
300
4.188
4.021(3.757)
3.933
3.670(3.164)
4.296
4.013
3.661
3.394
3.734(3.351)
3.874
3.818
143
224
172
257
—
7.2
11.2
6.2
—
0.200
0.519
0.812
—
51
50
36
—
1b
1c·H2O
1d·H2O
2a·acetone
2a
—
—
2b
0.08
0.01
0.02
0.06
0.07
204
253
245
168
84
5.3
9.1
9.8
—
0.907
1.434
0.907
—
26
36
40
—
—
2c·H2O
2d·H2O
4a
5a
5
—
—
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2
1 7 3 9

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Fig. 4 (a, b): Temperature dependence of the vMT product for solid samples of 1a (᭹), 1b (᭜), 1c·H2O (ꢀ), 1d·H2O (ꢁ), 2b (ꢂ), 2c·H2O (ꢂ), 2d·H2O
(ꢃ), 4a(x) and 5a (+). The magnetic properties of [(TPA)Fe(Cat)]BPh4 (3a) previously reported4 were plotted for comparison (full line). For clarity,
the labels of the hydrated compounds were noted w instead of H2O.
substituents are characterised either by a spin crossover process
or by a HS ground state (2a). The sequence of transition
temperatures 4a > 1a > 3a > 5a (for 2a·acetone, T1/2 = 0)
clearly shows that the spin crossover processes are dominated
by solid state effects rather than the molecular characteristics.
No correlation can be found for example with the Hammett
constants (r (NO2)> r (Cl)> r (H)> r (CH3)> r (OCH3)
corresponding to NO2 (1), Cl (2), H (3), CH3 (5) and OCH3(4)
substituents) which indicate in this order the decrease of the
electron-withdrawing character of the R groups and hence
some increase of the electronic density of the catecholate ring.23
Accordingly, this feature contrasts with the analysis of some
electronic spectral and electrochemical data collected in solution
which confirm the relationship between the low-energy LMCT
bands and the Lewis basicity of the catecholate groups.24,25 One
of the unexpected solid state effects can be the twist of the two
NO2 groups characterised in the crystal structure of 1a, that
accounts for a very small electron-withdrawing effect. It has
been verified from the 293 K powder X-ray diffraction patterns
that no common structural feature can be found for the series of
compounds [(TPA)Fe(R-Cat)]BPh4 (see ESI†).
In Fig. 5 is plotted the unit cell volume per spin crossover
molecule (V/Z) as a function of the transition temperature
3
˚
Fig. 5 Plot of the unit cell volume per metal ion (V(T)/Z in A ) as a
function of the transition temperature T1/2. When the crystal structure
was solved at two temperatures corresponding to prevailing HS and LS
species, the two values of the volume were reported. The dotted line was
drawn in order to show the main tendency.
T
1/2. It shows that the larger the unit cell volume, the lower
the T1/2 value. The unit cell volume obviously decreases with
the size of the monovalent anion (a (BPh4−)> b (PF6−)> c
(ClO4−)> d (NO3−)).26 It turns out that the spin crossover cation
undergoes a larger chemical pressure in solids formed with
smaller anions.27 The general tendency shown by these data
is an increase of the transition temperatures under the effect
of the lattice pressure. Consistently, the compound 2a·acetone
characterised by a HS ground state contains both the larger
spherical anion BPh4− and an acetone molecule which also plays
a role of spacer. However, these effects of chemical pressure do
not rule out weaker contributions due to the anion or solvent
molecules whose subtle effects were first identified in the study
of [Co(trpy)2]2+, [Fe(paptH)2]2+ and [Fe(pic)3]2+.28
superposition of two distinct quadrupole doublets indicates that
the interconversion between the spin states is slow relative to
the hyperfine frequencies (≈108 Hz) associated with the nuclear
energy schemes.
The compounds are characterised in the low- and
high-temperature phases by specific Mo¨ssbauer parameters
(DEQ(LS) = 1.5–2.0 mm s−1; dIS(LS) = 0.20–0.27 mm s−1 and
DEQ(HS) = 0.4–1.1 mm s−1; dIS(HS) = 0.24–0.37 mm s−1). These
data roughly compare to those observed for 3a,6 except that the
DEQ(HS) and dIS(HS) values are found to be smaller for the
complexes with R-substituted catecholate. In addition, the set
of DEQ(LS) and dIS(HS) parameters are slightly smaller for this
family of catecholate complexes than those reported for ferric
complexes of biological interest29 with [FeIIIN4O2] coordination
cores (for the LS state, DEQ = 2.0–3.0 mm s−1; dIS = 0.10–
To sum up, the molecular electronic effects due to different
catecholate substituents cannot account for the magnetic be-
haviours observed for these complexes in the solid state. The
latter are dominated by lattice effects (packing and chemical
pressure, arrangement and intermolecular contacts).
0.25 mm s−1 and the HS state, DEQ = 0.5–1.5 mm s−1; dIS
=
0.40–0.60 mm s−1). The observation, that the area fraction in
the 130 K spectrum of 4a (Fig. 6) is not fully covered by the
fitting, may well be due to a valence tautomerism process30 or
to an extensive quantum mixing between the Fe(III)–catecholate
and the Fe(II)–semiquinonate configurations as recently shown
for 3a.6 Indeed, the UV-vis data collected in solution24 show
Mo¨ssbauer measurements
Variable temperature 57Fe Mo¨ssbauer spectra were recorded for
2b, 4a, 5a. In Table 5 are provided the least-squares fitting
parameters. In Fig. 6 are shown the Mo¨ssbauer spectra of 4a
taken at 130 and 78 K. In the studied temperature range, the
1 7 4 0
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2

View Article Online
Table 5 Selected 57Fe Mo¨ssbauer fitted parameters of complexes [(TPA)Fe(R-Cat)]X. Isomer shifts are relative to a−iron. The HS fraction is based
on the evaluation of the areas AHS and ALS of the HS and LS resonance lines, assuming equal Lamb–Mo¨ssbauer factors for the two spin states
Compound
T/K
dIS(LS)/mm s−1
DEQ(LS)/mm s−1
dIS(HS)/mm s−1
DEQ(HS)/mm s−1
c HS
[(TPA)Fe(TCC)]PF6 2b
293
200
100
—
0.20(1)
0.212(2)
—
1.51(23)
2.019(4)
0.249(34)
0.25(35)
—
0.826
0.89(30)
—
1
0.77
0
[(TPA)Fe(MeOCat)]BPh4 4a
130
78
0.267(15)
0.268(15)
1.634(25)
1.618(25)
0.367(22)
—
0.932(22)
—
0.43
0
[(TPA)Fe(MeCat)]BPh4 5a
210
30
—
0.255(2)
—
1.764(2)
0.342(16)
—
1.052(14)
—
1
0
groups (FeIII–TCC and FeIII–DNC complexes) and the FeN4O2
core appear to be closer to those of a ferric catecholate
configuration than a quantum mixing between the catecholate
and semiquinonate forms observed with the more donating
catecholate Cat2−.6 The data recorded at low temperatures show
that the changes of metal–ligand bond lengths and bond angles
are consistent with the spin crossover process characterised by
the magnetic measurements.
It is of interest to note that a spin crossover process due to
species with a prevailing FeIIICat character is here identified
with the less donating catecholate groups. The results obtained
with the more donating catecholate groups should be further
analysed and discussed with regard to these ones. Indeed
the spin crossover process is also exhibited by a compound
described by a strong quantum mixing of the FeIIICat and
FeIISQ configurations and a coupling between spin-crossover
and charge transfer, i.e. the valence tautomerism cannot be
discarded for the more donating catecholate groups.
Acknowledgements
Fig. 6 Mo¨ssbauer spectra of 4a recorded at 130 and 78 K.
We thank Xavier Ottenwaelder for providing the H2DNC ligand,
Re´gis Guillot for his assistance and Kamel Boukheddaden
for interesting comments. The European Science Foundation,
the European TMR programme (ERB-FMRX-CT98-0199) are
gratefully acknowledged.
that the charge transfer between the catecholate group and the
metal ion of compound 4a, is reinforced by the presence of
the electron-donating CH3O group in comparison to 3a. These
hypotheses should be investigated by using detailed Mo¨ssbauer
measurements in the presence of a magnetic field and an enriched
57Fe compound.
References
1 M. Costas, M. P. Mehn, M. P. Jensen and L. Que, Jr., Chem. Rev.,
2004, 104, 939; H. J. Kru¨ger, in Biomimetic Oxidations Catalyzed by
Transition Metal Complexes, ed. B. Meunier, Imperial College Press,
London, 2000, 363; T. Funabiki, Oxygenases and Model systems in
Catalysis by Metal complexes, ed. R. Ugo and B. R. James, Kluwer
Academic Publishers, Dordrecht, 1997, vol 19, pp. 1–393.
2 D. D. Cox and L. Que, Jr., J. Am. Chem. Soc., 1988, 110, 8085; D. D.
Cox, S. J. Benkovic, L. M. Bloom, F. C. Bradley, M. J. Nelson, L.
Que, Jr. and D. E. Wallick, J. Am. Chem. Soc., 1988, 110, 2026; H. G.
Jang, D. D. Cox and L Que, Jr., J. Am. Chem. Soc., 1991, 113, 9200;
L. Que, Jr. and R. Y. N. Ho, Chem. Rev., 1996, 96, 2607.
Conclusion
The complexes [(TPA)Fe(R-Cat)]X were synthesised with dif-
ferent anions and catecholate ligands. They form a family of
systems exhibiting spin crossover processes in the solid state.
The magnetic behaviours, determined from variable temperature
magnetic susceptibility measurements, consist of very smooth
S = 1/2 ↔ S = 5/2 transition curves (R-Cat2− = DNC2− and
TCC2−) which indicate weak cooperative interactions. Although
the electron-donating or electron-withdrawing character of
the catechol substituents leads to different FeIII←Cat charge
transfer interactions and reactivities, it cannot account for the
observed shift of the transition temperatures. The analysis of
the structural data obtained for some FeIII–TCC and FeIII–
DNC complexes allowed a correlation between the T1/2 values
and the unit cell volume to be shown. This feature means that
the relative stabilisation of the LS state is mainly due to the
chemical pressure experienced by the complex under the effect
of the lattice. From these crystal structures, the characteristics
of the dioxolene groups substituted with electron withdrawing
3 M. Duda, M. Pascaly and B. Krebs, Chem. Commun., 1997, 835.
4 A. J. Simaan, M.-L. Boillot, E. Rivie`re, A. Boussac and J.-J. Girerd,
Angew. Chem., Int. Ed., 2000, 39, 196.
5 P. Gu¨tlich, H. Spiering and A. Hauser, in Inorganic Electronic Struc-
ture and Spectroscopy, vol II, ed. E. I. Solomon and A. B. P. Lever,
VCH, Weinheim, 1999, p. 575; Molecular magnetism, ed. O. Kahn,
VCH, New York, 1993, p. 53; P. van Koningsbruggen, Y. Maeda and
H. Oshio in Spin-Crossover in Transition Metal Compounds, Topics in
Current Chemistry, eds: P Gu¨tlich, H. A. Goodwin, Springer, Berlin,
2004, 233, p. 259; M. Bacci, Coord. Chem. Rev., 1988, 86, 245; J. K.
Beattie, Adv. Inorg. Chem., 1988, 32, 1; H. Toftlund, Coord. Chem.
Rev., 1989, 94, 67; J. Zarembowitch, New J. Chem., 1992, 16, 255.
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2
1 7 4 1

View Article Online
6 A. J. Simaan, M.-L. Boillot, R. Carrasco, J. Cano, J.-J. Girerd, T. A.
Mattioli, J. Ensling, H. Spiering and P. Gu¨tlich, Chem., Eur. J., 2005,
11, 1779.
7 T. Funabiki, A. Mizoguchi, T. Sugimoto, S. Tada, M. Tsuji, H.
Sakamoto and S. Yoshida, J. Am. Chem. Soc., 1986, 108, 2921; T.
Funabiki and T. Yamazaki, J. Mol. Catal. A: Chem., 1999, 150, 37;
T. Funabiki, A. Fukui, Y. Hitomi, M. Higuchi, T. Yamamoto, T.
Tanaka, F. Tani and Y. Naruta, J. Inorg. Biochem., 2002, 91, 151;
T Funabiki, D Sugio, N. Inui, M. Maeda and Y. Hitomi, Chem.
Commun., 2002, 412.
18 P. G. Sim, E. Sinn, R. H. Petty, C. L. Merill and L. J. Wilson, Inorg.
Chem., 1981, 20, 1213; H. Oshio, K. Toriumi, Y. Maeda and Y.
Takashima, Inorg. Chem., 1991, 30, 4252; Y. Maeda, H. Oshio, K.
Toriumi and Y. Takashima, J. Chem. Soc., Dalton Trans, 1991, 1227;
Y. Maeda, Y. Noda, H. Oshio and Y. Takashima, Bull. Chem. Soc.
Jpn., 1992, 65, 1825.
19 The magnetic data were analysed from vMT = c 1(vMT)HS + (1 − c1)
(vMT)SCO where c 1 is the fraction of species being in the HS state at
any temperature; vMTSCO corresponds to the spin crossover process
and vMTHS (vMTLS) is equal to 4.375 (0.375) cm3 mol−1 K.
8 B. G. Gafford and R. A. Holwerda, Inorg. Chem., 1989, 28, 60.
9 J. Ehrlich and M. T. Bogert, J. Org. Chem., 1947, 522; J. Marquet, M.
Moreno-Manas, A. Vallribera, A. Virgili, J. Bertran, A. Gonzalez-
Lafont and J. M. Lluch, Tetrahedron, 1987, 43, 351; T. J. Collins,
R. D. Powell, C. Slebodnick and E. S. Uffelman, J. Am. Chem. Soc.,
1991, 113, 8419.
10 G. M. Sheldrick, SHELXTL-97, program for the refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
11 K. Lagarec and D. G. Rancourt, Recoil- Mo¨ssbauer Spectral Analysis
Software for Windows (version 1.0), Department of Physics, Univer-
sity of Ottawa, Canada, 1998.
12 L. Que, Jr., R. C. Kolanczyk and L. S. White, J. Am. Chem. Soc.,
1987, 109, 5373; W. O. Koch and H. J. Kru¨ger, Angew. Chem., Int.
Ed. Engl., 1996, 34, 2671; P. Mialane, L. Tchertanov, F. Banse, J.
Sainton and J.-J. Girerd, Inorg. Chem, 2000, 39, 2440; D. H. Jo, Y.-
M. Chiou and L. Que, Jr., Inorg. Chem., 2001, 40, 3181; M. Pascaly,
M. Duda, F. Schweppe, K. Zurlinden, F. K. Mu¨ller and B. Krebs,
J. Chem. Soc., Dalton Trans, 2001, 828; M. Merkel, F. K. Mu¨ller and
B. Krebs, Inorg. Chim. Acta, 2002, 337, 308; M. Merkel, M. Pascaly,
M. Wieting, M. Duda and A. Rompel, Z. Anorg. Allg. Chem., 2003,
629, 2216.
20 C. P. Slichter and H. G. Drickamer, J. Chem. Phys., 1972, 56, 2142.
21 The variation of the Gibbs free energy (see ref. 20) associated to the
LS ↔ HS transformation of an assembly of N molecules is DGHL
=
GHS − GLS = DHHL + C(1 − 2c HS) − TDSHL, where DHHL, DSHL
are the enthalpy and entropy variations (T1/2 = DHHL/DSHL), C
is an interaction term and c HS, the HS fraction. The equilibrium
condition is defined by the implicit equation ln[(1 − c HS)/c HS] =
[DHHL + C(1 − 2c HS)]/RT − DSHL/R. Depending on the value of
C/2RT1/2, the transition is gradual without hysteresis (C/2RT1/2 <
1), the transition is abrupt without hysteresis (C/2RT1/2 = 1) and
the transition is abrupt with hysteresis (C/2RT1/2 > 1). The c HS vs.
T plot derived from the experimental data was analysed with a mean
squared fitting procedure.
22 A. J. Conti, K. Kaji, Y. Nagano, K. M. Sena, Y. Yumoto, R. K.
Chadha, A. L. Rheingold, M. Sorai and D. N. Hendrickson, Inorg.
Chem., 1993, 32, 2681; M. Sorai, J. Phys. Chem. Solids, 1990, 51, 941;
V. I. Shipilov, V. V. Zelentsov, V. M. Zhdanov and V. A. Turdakin,
JETP Lett. (Engl. Transl.), 1974, 19, 294.
23 J. March, in Advanced Organic Chemistry, Reactions, Mechanisms and
Structure, International Student Edition, McGraw-Hill Inc., 1977.
24 The energies of the first charge transfer electronic transitions were
determined at room temperature for acetonitrile solutions of 3a,
4a and 5a. The values of energies (cm−1) and molar extinction
coefficients (M−1 cm−1) were equal to 19 841(2050) and 12 531(2980)
for 3a; 15 625(2300) and 11 848(2460) for 4a; 19 841(2460) and 11
601(3180) for 5a. The electrochemical potentials of the [(TPA)Fe(R-
Cat)]+ complexes in acetonitrile with 0.1 M NBu4PF6 were measured
at room temperature. The E1/2 values (V/ECS) for the metal-centered
redox process FeIII(R-Cat)+/FeII(R-Cat) were equal to −0.53 (5a);
−0.52 (4a); −0.51 (3a); −0.17 (2a) and −0.05 (1a).
13 M. Buchanan, S. L. Kessel, H. H. Downs, C. G. Pierpont and D. N.
Hendrickson, J. Am. Chem. Soc., 1978, 100, 7894; S. R. Boone, G. H.
Purser, H. R. Chang, M. D. Lowery, D. N. Hendrickson and C. G.
Pierpont, J. Am. Chem. Soc., 1989, 111, 2292; A. S. Attia, B. J.
Conklin, C. W. Lange and C. G. Pierpont, Inorg. Chem., 1996, 35,
1033; W. O. Koch, V. Schu¨nemann, M. Gerdan, A. X. Trautwein and
H. J. Kru¨ger, Chem. Eur. J., 1998, 4, 1255.
14 From the literature (references 12 and 13), the HS FeIIICat (or
FeIISQ) systems are characterised by the following mean values of
˚
their bond lengths ꢀC–Oꢁ = 1.295–1.355 A (1.275–1.290), ꢀC–Cꢁ =
25 Y. Hitomi, M. Yoshida, M. Higuchi, H. Minami, T. Tanaka and T.
Funabiki, J. Inorg. Biochem., 2005, 99, 755.
26 D. M. P. Mingos and A. L. Rohl, Inorg. Chem., 1991, 30, 3769.
27 H. Spiering, E. Meissner, H. Ko¨ppen and E. W. Mu¨ller, Chem.
Phys., 1982, 68, 65; J. J. Kolnaar, PhD Thesis, University of Leiden,
1998.
28 M. Sorai, J. Ensling, K. M. Hasselbach and P. Gu¨tlich, Chem. Phys.,
1977, 20, 197; G. A. Renovitch and W. A. Baker, J, J. Am. Chem.
Soc., 1967, 89, 6377; H. A. Goodwin, Coord. Chem. Rev., 1976, 18,
293; S. Kremer, W. Henke and D. Reinen, Inorg. Chem., 1982, 21,
3013.
˚
1.389–1.399 A (1.402–1.440) and more specifically C21–C22 = 1.369–
˚
1.400 A (1.383–1.446). It should be noted that the labels C21 and C22
correspond to the C4 and C5 carbon atoms of the catecholate groups
in the structural data reported in the literature.
15 E. Ko¨nig, Prog. Inorg. Chem., 1987, 35, p. 527; E. Ko¨nig, Structure
and Bonding, Springer-Verlag, Berlin, 1991, 76, p. 51.
16 From the structural data obtained with ferric Schiff base complexes
(in ref. 17 and 18), the average of the metal ligand bond lengths
of LS (HS) species is expected to vary as ꢀFe–Oꢁ = 1.879–1.895
˚
˚
(1.908–1.939) A; ꢀFe–Nimineꢁ = 1.930–1.944 (2.081–2.125) A; and ꢀFe–
˚
Namineꢁ = 1.999–2.046 (2.173–2.215) A.
29 E. Mu¨nck in Physical Methods in Bioinorganic Chemistry, ed. L. Que,
17 M. D. Timken, D. N. Hendrickson and E. Sinn, Inorg. Chem., 1985,
24, 3949; Y. Nishida, K. Kino and S. Kida, J. Chem. Soc., Dalton
Trans, 1987, 1957; A. J. Conti, R. K. Chadha, K. M. Sena, A. L.
Rheinhold and D. N. Hendrickson, Inorg. Chem., 1993, 32, 2670.
Jr., University Science Books, Sausalito, 2000, p. 287.
30 D. N. Hendrickson and C. G. Pierpont in Spin-Crossover in Transition
Metal Compounds, Topics in Current Chemistry, ed. P. Gu¨tlich and
H. A. Goodwin, Springer, Berlin, 2004, vol. 234, p. 63.
1 7 4 2
D a l t o n T r a n s . , 2 0 0 5 , 1 7 3 4 – 1 7 4 2