Spin crossover in mononuclear and binuclear iron(III) complexes with pentadentate Schiff-base ligands

Article abstract of DOI:10.1016/S0009-2614(00)00701-6

A series of mononuclear hexacoordinate iron(III) complexes, [Fe(⁵L)(py)]BPh₄, and binuclear hexacoordinate iron(III) complexes, [(⁵L)Fe(μ²-bpy)Fe(⁵L)](BPh₄) ₂, has been prepared and their magnetic properties were investigated; the pentadentate ligands were derivatives of ⁵L=saldptn=N,N′-bis(2-hydroxybenzyliden)-1,7-diamino-4- azaheptane. Temperature variation of the effective magnetic moment for them shows that a spin transition from the low-spin to the high-spin state occurs. The magnetic data were fitted to an Ising-like model appropriate for the mono- and binuclear systems.

Full text of DOI:10.1016/S0009-2614(00)00701-6

28 July 2000
Ž
.
Chemical Physics Letters 325 2000 411–419
www.elsevier.nlrlocatercplett
ž /
Spin crossover in mononuclear and binuclear iron III complexes
with pentadentate Schiff-base ligands
b
a
a
Roman Boca a,), Yutaka Fukuda , Milan Gembicky , Radovan Herchel ,
ˇ
´
a
c
d
b
Rastislav Jarosciak , Wolfgang Linert , Franz Renz , Jyunko Yuzurihara
ˇˇ
a
Department of Inorganic Chemistry, SloÕak Technical UniÕersity, SK-812 37 BratislaÕa, SloÕakia
b
Department of Chemistry, Faculty of Science, Ochanomizu UniÕersity, Tokyo, Japan
Institute of Inorganic Chemistry, Vienna UniÕersity of Technology, Vienna, Austria
Department of Inorganic and Analytical Chemistry, Johannes Gutenberg UniÕersity, D-55099 Mainz, Germany
c
d
Received 28 March 2000; in final form 9 June 2000
Abstract
Ž
.
w
Ž5 .Ž .x
Ž .
A series of mononuclear hexacoordinate iron III complexes, Fe L py BPh4, and binuclear hexacoordinate iron III
wŽ5
.
Ž
.
Ž5 .xŽ
L Fe m -bpy Fe L BPh4 2, has been prepared and their magnetic properties were investigated; the
.
2
complexes,
X
5
Ž
.
pentadentate ligands were derivatives of LssaldptnsN,N -bis 2-hydroxybenzyliden -1,7-diamino-4-azaheptane. Temper-
ature variation of the effective magnetic moment for them shows that a spin transition from the low-spin to the high-spin
state occurs. The magnetic data were fitted to an Ising-like model appropriate for the mono- and binuclear systems. q 2000
Elsevier Science B.V. All rights reserved.
Ž
.
1. Introduction
ligand bearing a charge q. The Schiff-base Fe III
complexes spanning that class are exemplified by
w
Ž
. x
The family of the spin crossover systems involv-
Fe X-saleten Y
Fe
systems and abbreviated as
2
IIIŽ3
ing iron complexes is very numerous 1–4 . The
LyI
the tridentate monoanion saleten is
.2 xq
Ž
w
x
w
Ž
.
most frequently observed is the case of the Ss0 to
a condensation product of substituted salicylalde-
. w x
Ž .
Ss2 transition for Fe II complexes. However, the
spin transitions involving Fe III centres are well
documented Ss1r2 to Ss5r2 transition by a
few examples among which the tris dithiocarbamato
hyde with N-ethylethylenediamine 6 .
An interesting behaviour is observed for Fe X-
.Ž .x
Ž
.
w Ž
Ž
.
salen NO complexes which belong to the class of
IIIŽ4 .Ž yI.x
Fe
operates between the low-spin and the intermediate-
Ž . w x
spin states Ss1r2 to Ss3r2 transition 7 . An
LyII
A
systems: the spin transition
Ž
.
w
Ž
.
Ž
.
iron III compounds the ‘Cambi compounds’ are
w x
well known 5 . These complexes are abbreviated
n
w
IIIŽ2
. x
w
q
hereafter as Fe
LyI , L denotes an n-dentate
analogous property possesses the system Fe-
3
III
Ž
.Ž .xŽ
A
.
w
TMC NO BPh4 which spans the class of Fe -
2
Ž4 .Ž yI .xŽ
.
Ž
L0
Y
systems TMC s 1,4,8,11-
yI
2
.
w x
tetramethyl-1,4,8,11-tetraazacyclotetradecane
8 .
Finally, an Ss3r2 to Ss5r2 spin transition has
IIIŽ2
. x
)
Corresponding author. Fax: q42-1-7-5249-3128; e-mail:
boca@cvtstu.cvt.stuba.sk
LyI Ø sol the
Ž
3
w
been reported for the system Fe
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 00 00701-6

(
)
412
R. Boca et al.rChemical Physics Letters 325 2000 411–419
ˇ
2
5
bidentate monoanion is LyI s 4-morpholine-
[
(
) ]
2.3. Mononuclear complexes Fe LyX py BPh4
X . w x
carbodithionate-S,S 9 .
Our interest is focused to the possible spin transi-
These were prepared by mixing the precursor
w
Ž5
. x
tions involving pentadentate Schiff-base complexes
Fe L-X Cl with the pyridine in the presence of the
sodium tetraphenylborate. Elemental analysis – calc.
Ž .
w
IIIŽ5
. x
yI
of the type Fe
LyII py Y and their binuclear
2
III
w
Ä
Ž
Ž
.
bridged analogues of the type m -bpy Fe -
for XsH, 1 , BC49 H48 N4O2 Fe found : C, 74.3
Ž5
.
42 xŽ
.
LyII
Y
2. Some of these compounds have
yI
Ž
.
Ž
.
.
74.1 ; H, 6.25 6.11 ; N, 7.08 6.95 ; calc. for
Ž .
w
x
Ž
Ž
.
been prepared and investigated previously 10–12 .
Our main interest is focused to a theoretical analysis
in order to obtain microscopic andror macroscopic
Xs3-MeO, 2 , BC51H52 N4O2 Fe found : C, 71.9
Ž
.
Ž
.
.
71.8 ; H, 6.15 6.23 ; N, 6.58 6.49 ; calc. for
Ž .
Ž
.
.
Xs5-Cl, 3 , BC49 H46 N4O2 FeCl2 found : C, 68.4
Ž
.
Ž
.
Ž
.
Ž
thermodynamic parameters describing the spin
68.0 ; H, 5.39 5.41 ; N, 6.51 6.62 .
Ž
.
crossover. Such a set for Fe III systems has not
been derived from the experimental data so far.
[(5
LyX Fe bpy Fe Ly
)
(
)
(
5
2.4. Binuclear complexes
)](
)
X
BPh4
2
Ž
.
The binuclear iron III complexes bridged by bpy
2. Experimental
X
Ž
.
bpys4,4 -bipyridine were obtained by adding a
Ž5
. x
methanolic solution of bpy 1 mmol to Fe L-X Cl
2 mmol in 50 cm of methanol. The mixture was
Ž
.
w
2.1. Ligands H2 L-X
3
Ž
.
stirred at 608C for 10 min and then filtered. A slight
The pentadentate ligand H2 LssaldptnsN,NX-
bis 2-hydroxybenzyliden -1,7-diamino-4-azaheptane
has been prepared by a Schiff-base condensation
between 1,7-diamino-4-azaheptane and the corre-
sponding salicyaldehyde at a ratio of 1:2. By mixing
Ž
.
excess of sodium tetraphenylborate 2.5 mmol in 10
cm3 was added to the filtrate. The resulting crystals
were washed with methanol and diethylether, and
Ž
.
Ž
.
the salicylaldehyde 0.2 mol and 1,7-diamino-4-
3
Ž
.
Ž
.
azaheptane 0.1 mol in methanol 100 cm a yel-
low oily material, saldptn, results.
The ligand H2 L has been modified by substitut-
ing either the position 3 or 5 in the phenyl ring of the
salicylaldehyde, yielding H2 L-X:
X
Ž .
Ž
i
for X s 3-MeO: N,N -bis 2-hydroxy-3-
.
methoxybenzyliden -1,7-diamino-4-azaheptane,
X
Ž .
Ž
ii for X s 5-MeO: N,N -bis 2-hydroxy-5-
.
methoxybenzyliden -1,7-diamino-4-azaheptane,
X
Ž .
Ž
iii for Xs5-Cl: N,N -bis 2-hydroxy-5-chloro-
.
benzyliden -1,7-diamino-4-azaheptane.
5
[
(
) ]
2.2. Precursors Fe LyX Cl
Ž
.
A solution of anhydrous iron III chloride in
3
Ž
.
.
methanol 50 cm was added to a solution of H2 L-X
10 mmol in methanol 50 cm . The mixture was
stirred at 508C for 10 min and then triethylamine 20
mmol was added. The resulting solution was stirred
3
Ž
Ž
.
Ž
.
at 508C for 1 h until black crystals precipitated.
These were collected, washed with methanol and
diethyl ether, and dried in vacuum.
2
III
2q
w
Ž
. x
L
2
Fig. 1. A molecular structure of m -bpy Fe
complex.

(
)
R. Boca et al.rChemical Physics Letters 325 2000 411–419
413
ˇ
Ž
.
Ž
.
Fig. 2. A relationship between the energy levels of the Heisenberg exchange left and the Ising-like model of the spin crossover right for a
Ž
.
binuclear Fe III complex. Degeneracies of the respective energy levels are given in parenthesis.
dried in vacuum. Elemental analysis – calc. for
Ž .
Here the symbol s denotes a fictitious spin with
ˆA
Ž
.
XsH, 4 , B2C98 H94 N8O4 Fe2 found : C, 74.4
eigenvalues "1 for the high-spin and low-spin states,
respectively; its thermal average entering the mean-
field approximation, which is a continuous variable,
Ž
.
Ž
.
Ž
.
74.5 ; H, 5.99 5.99 ; N, 7.08 7.08 ; calc. for
Ž .
Ž
.
Xs5-Cl, 5 , B2C98 H90 N8O4 Fe2Cl4 found : C,
Ž
.
Ž
.
Ž
.
² :
68.5 68.3 ; H, 5.28 5.32 ; N, 6.52 6.54 ; calc. for
Ž .
is sA . The intracomplex A–B coupling parameter
Ž
.
Ž .
JAB adopts negative positive values for an ‘antifer-
Xs3-MeO, 6 , B2C102 H102 N8O8 Fe2 found : C,
Ž
.
Ž
.
Ž
.
Ž
.
72.0 71.6 ; H, 6.04 6.09 ; N, 6.59 6.66 ; calc. for
Ž .
romagnetic’ ‘ferromagnetic’ coupling of the ficti-
tious spins. The intermolecular interaction parame-
ters are J for A . . . A and B . . . B pairs, and JX for
Ž
.
Xs5-MeO, 7 , B2C102 H102 N8O8 Fe2 found : C,
Ž
.
Ž
.
Ž
.
72.0 72.3 ; H, 6.04 6.25 ; N, 6.59 6.49 .
X
Ž
.
Ž
The distance d N–N in 4,4 -bipyridine is 715 pm
and the estimate of the Fe–Fe separation is d Fe–
Fe sd N–N q2=200 pms1115 pm Fig. 1 .
The X-ray structure analysis will be published else-
A . . . B and B . . . A, respectively these are eventu-
Ž
.
ally neglected . The formation energy of the high-spin
.
Ž
.
Ž
.
Ž .
Fig. 2 .
LS
state is D0 sEHS
–
²
:
q
With the help of the substitutions ms sA
X
X
w
x
²
: ² : ² :
sB , ns sA – sB , JqsJqJ , and JysJ–J
where 13 .
the energy levels result:
2.5. Magnetic measurements
E1 sELL syD0 qmJqyJAB
;
2a
2b
2c
2d
Ž
Ž
Ž
Ž
.
.
.
.
The magnetic susceptibility for powder samples
has been recorded using the Faraday balances in the
temperature range from 4.2 K until 280 K.
E2 sELH snJyqJAB
E3 sEHL synJyqJAB
E4 sEHH sqD0 ymJqyJAB
;
;
.
The partition function of the system is constructed as
follows:
3. Theoretical models
4
The fitting procedure for the binuclear complexes,
Zs
g exp yE rkT
Ž .
Ý
w
x
i
i
outlined elsewhere 14 , is based on the Ising-like
spin Hamiltonian of the form
i
s1
sgL2S exp yE rkT qr exp yE rkT
Ž
.
Ž
.
1
eff
2
D0
ˆ
²
:
s
ˆA
HsyJAB s Ps
q
s qs yJ sA
Ž
ˆA ˆB
.
Ž
ˆA ˆB
.
Ž
qreff exp yE rkT qr2 exp yE rkT
2
Ž
.
Ž
.
3
eff
4
yJX sA s q s
s
1
Ž .
²
:
s
ˆB
²
:
²
:
.
ˆA
3
Ž .
q sB
ˆB
.
Ž
.
B

(
)
414
R. Boca et al.rChemical Physics Letters 325 2000 411–419
ˇ
where the effective degeneracy ratio, reff , occurs
reff sgHSrgLS
and consequently only a single implicit equation
results
.
4
Ž .
²
:
A
y1qreff exp y D y2 J s rkT
Ž
.
0
²
:
s
sA
.
The latter quantity, in fact, includes a degeneracy of
the electronic states as well as that of the vibrational
²
:
1qreff exp y D y2 J s rkT
Ž
.
0
A
8
Ž .
Ž
.
states; consequently reff )3 holds true for Fe III .
Ž
.
The thermal average of the individual formal
Finally,
xHS s 1q s r2
spin values are calculated as follows:
²
:
9
Ž .
Ž
.
4
A
²
:
s
sA
s
g exp yE rkT
Ž .
Ý
A ,i
i
i
holds true for the high-spin mole fraction.
The microscopic parameters interrelate to the en-
thalpy and entropy of the spin transition as follows
i
s1
sgL2S yexp yE rkT yr exp yE rkT
.
Ž
.
Ž
1
eff
2
D HsNA D0
10
11
Ž
Ž
.
.
qreff exp yE rkT
Ž
.
3
DSsR ln reff
qre2ff exp yE rkT rZ
5a
Ž
.
Ž
.
4
and the equilibrium temperature becomes expressed
as
and
2
²
:
sB sgLS yexp yE rkT qr exp yE rkT
.
Ž
.
Ž
Tc sD HrDS .
12
Ž
.
1
eff
2
Ž
Thus, the greater the D H, the greater the Tc at the
yreff exp yE rkT
Ž
.
3
.
constant DS . For a vanishing cooperativity parame-
qre2ff exp yE rkT rZ .
5b
Ž
.
Ž
.
ter J the Arrhenius plot
4
xHS
Such a pair of the coupled equations can be solved
ln Ksln
sf 1rT
13
Ž
.
Ž
.
w
x
ž 1yx
/
by an iterative procedure 14 . The starting values in
HS
Ž0.
the heating direction are sA s sB Ž0. sy1.
²
:
²
:
is a straight line that determines D H and DS. For
J)0, however, a non-linear dependence is obtained.
The greater J, the greater the steepness of the spin
transition expressed, for instance, by xHS versus T
function.
For the given temperature point and the trial set of
free parameters JAB , J, JX, D0, and reff the first
Ž
.
Ž1.
²
:
Ž .
estimate sA
is evaluated according to Eq. 5a ;
is calculated with the help of Eq. 5b
Ž1.
²
:
Ž
.
then sB
Ž0.
Ž1.
²
:
²
:
by using sB
and sA
on the right side.
Ž1.
Ž1.
The high-spin mole fraction can be derived from
the experimental susceptibility data in the following
way. The expression for the molar magnetic suscep-
tibility is
²
:
²
:
Having the first estimates sA
and sB
deter-
mined, the procedure is repeated until a selfconsis-
tency is reached. Typically 20–50 iterations yield a
convergence under the threshold of 10y4. When
X
²
:
²
:
JsJ are assumed, the values of sA and sB are
xmol sxHS xHS q 1yx
x
14
Ž
.
Ž
.
HS
LS
necessarily equal.
and then
The formal spin value are interrelated to the
high-spin mole fraction as follows:
xTyxLS
xHSTyxLST
T
xHS
s
.
15
Ž
.
²
:
²
:
xHS s 2q s q sB r4 .
6
Ž .
Ž
.
A
In the case of the mononuclear complexes the Ising-
Such a formula is usually applied when constant
low-temperature limit and high-temperature limit are
experimentally available. Subsequently, the fitting of
w
x
like spin Hamiltonian simplifies to 15
D0
the function xHS s f D0, JAB, J, JX,reff is done.
Ž
.
ˆ
²
:
s
ˆA
Hs
s yJ s
7
Ž .
ˆA
A
2
However, our low-temperature data show a non-con-

(
)
R. Boca et al.rChemical Physics Letters 325 2000 411–419
415
ˇ
Table 1
Characteristic functions for the magnetic susceptibility of binuclear systems
a
Ž .
a
SA,SB
FS
,S
A
B
exp a
Ž .
Ž .
1r2,1r2
1q3 exp a
exp a q5 exp 3a q14 exp 6a
Ž .
Ž
.
Ž
.
3r2,3r2
1q3 exp a q5 exp 3a q7 exp 6a
.
Ž .
Ž
.
Ž
exp a q5 exp 3a q14 exp 6a q30 exp 10a q55 exp 15a
.
Ž .
Ž
.
Ž
.
Ž
.
Ž
5r2,5r2
1q3 exp a q5 exp 3a q7 exp 6a q9 exp 10a q11 exp 15a
.
Ž .
Ž
.
Ž
.
Ž
.
Ž
a
The argument asJexrkT accounts for the true magnetic exchange interaction.
. Ž .
mediate-spin , and 5r2 high-spin with the spin-only
stant value of xLST owing to an antiferromagnetic
exchange inside the binuclear complex. Therefore
the individual-spin magnetic susceptibility for a bin-
Ž
.
s.o. effective magnetic moment being meffrmB s
1.7, 3.9, and 5.9, respectively. The measured temper-
w
x
uclear system was expressed via the formula 16
ature variation of the effective magnetic moment per
X
complex, meffrmB s798 xmol
T
1r2, is displayed in
.
Ž
NA m0 m2B 2 gS2
Fig. 3.
xS s
FS
a
Ž .
16
Ž
.
A ,SB
k
T
Ž
For two mononuclear complexes XsH and Xs
.
Ž
.
3-MeO the low-temperature data below 100 K
Ž .
a are given
where the characteristic functions FS
A ,SB
show almost constant value of the effective magnetic
moment; this, however, exceeds the spin-only value
for Ss1r2 so that a magnetogyric factor g)2 is
Ž
in Table 1 for the argument asJexrkT the funda-
mental physical constants adopt their usual meaning .
The characteristic functions for the magnetic sus-
ceptibility of a mononuclear species in low-spin and
the high-spin states account for the Curie–Weiss law
.
Ž
.
needed. For Fe III in a strong ligand field of octahe-
2
dral symmetry the ground state is T, i.e., magnetic
active. The six magnetic levels might be thermally
accessible also on a symmetry lowering and when
dealing only with the lowest Kramers doublet, the
presence of the remaining levels will reflect itself in
NA m0 m2B g2S Sq1
Ž
.
S
xS s
17
Ž
.
k
3 TyQ
Ž .
These assumptions allow a direct fitting of the func-
the increasing value of geff. Therefore, geff,ŽSs1r2.
)
Ž
.
tion xmol s f D0 , J,reff ; gLS ,Q LS , gHS for the
2 should be considered as a free parameter. On the
mononuclear species and xmol s f D0, JA B , J, JX,
6
Ž
contrary, for Ss5r2 the ground state is A1 and
.
reff ;gLS, Jex , gHS for the binuclear species, respec-
thus geff,ŽSs5r2. f2 can be safely assumed. A small
decrease of the effective magnetic moment below 10
K can be attributed to the intermolecular interactions
tively. In order to avoid an overparametrization of
the model, some additional simplifications will be
done as explained in discussion. An important note
follows: while the parameters gLS , Jex , gHS describe
the magnetic interactions inside the binuclear com-
plex, the set of parameters D0, JA B , J, JX,reff is char-
acteristic for the Ising-like spin crossover model and
Ž
and covered by the Weiss constant Q paramagnetic
.
Curie temperature .
The remaining mononuclear complex for Xs5-Cl
shows features of an intermediate-spin ground state
4
Ž
.
A2 C4v group of symmetry . A valuable discussion
Ž
.
its nature is non-magnetic most probably elastic .
w x
of such a case can be found elsewhere 3 . Again
g
eff,ŽSs3r2. )2 can be expected. A possible zero-field
splitting can be simply accounted for considering the
Weiss constant Q.
4. Results and discussion
The results of the fitting procedure are listed in
The possible spin states for the mononuclear
Table 2 and displayed in Fig. 4. In all cases gHS s
2.0023 has been fixed.
Ž
.
Ž
.
Ž
Fe III species are Ss1r2 low-spin , 3r2 inter-

(
)
416
R. Boca et al.rChemical Physics Letters 325 2000 411–419
ˇ
Ž
.
Ž
.
Fig. 3. Measured temperature dependence of the effective magnetic moment per unit formula for a series of mononuclear left and
Ž
.
binuclear right complexes with substituted ligand L–X.
In accordance with expectations, gLS overcomes
the free-electron value for 1 through 3. A small value
and D0, both are very sensitive for the consideration
Ž
of the cooperativity parameter J compare fitted
Ž
.
.
of the Weiss constant QLS: y1 K describes a drop
of the effective magnetic moment at the lowest
temperatures. The value of the effective degeneracy
ratio, reff , highly overreaches the its minimum value
given by the electronic degeneracy only, i.e. 3.0 for
parameters, for instance, for 1a and 1b . However, a
reliable determination of J requires experimental
data along the whole transition curve, which is not
the present case, except 3.
For the complex 3 the model of the spin-crossover
has been improved by considering the molecular
Ž
.
Fe III . This indicates an important contribution of
w
x
the vibrational partition function. The values of reff
vibrations as described elsewhere 17,18 ; the param-
Table 2
Calculated magnetic parameters for mononuclear complexes
Parameters
Complex, substituent
XsH
Xs3-MeO
Xs5-Cl
Model a
Fitted
gLS
QLSrK
reff
1a
1b
2a
2b
3a
3b
3c
b
2.63
y0.56
112
1540
–
2.63
y0.56
7.1
627
197
2.98
y1.12
79
1204
–
2.98
y1.12
4.4
406
168
2.35
y0.67
110
386
–
2.35
y0.67
28
265
42
2.35
y0.67
b
Ž
.
Ž
.
hvLS rks797 hvHS rks514
Ž
Ž
.
D0rk rK
2246
.
Jrk rK
64
c
Rr%
Derived
0.76
0.71
0.79
0.72
0.81
0.67
0.65
DSrJ Ky1 moly1
D HrkJ moly1
TcrK
39.3
12.8
326
16.3
5.2
319
36.3
10.0
276
12.3
3.4
275
39.1
3.2
82
27.7
2.2
80
78
a
Model a, no cooperativity; model b, cooperativity parameter; model c, cooperativityqmolecular vibrations. Error estimates: 1.0 for
D0, reff , and J; 0.01 for QLS and gLS
.
b
Fixed parameters using the low-temperature data.
Discrepancy factor of the regression.
c

(
)
R. Boca et al.rChemical Physics Letters 325 2000 411–419
417
ˇ
Ž
.
Fig. 4. Fit of the magnetic data for mononuclear complexes left : XsH, 1, Xs3-MeO, 2, and Xs5-Cl, 3; empty symbols –
Ž
.
experimental data, full circles – calculated points, dashed line – theoretical curve; Arrhenius plot for 3 right .
eter set 3c consists of gLS , QLS, D0, J, hÕLS, and
hÕHS the last pair accounts for a mean vibrational
energy for the low-spin and the high-spin state,
moment meffrmB ;3 in the temperature interval of
Ž
Ž
.
10–100 K Fig. 2 . A drop below 10 K indicates an
Ž
.
antiferromagnetic interaction Jex -0 leading to the
.
respectively . In this case the quality of the fit is
Ss0 ground state.
perfect as seen also from the Arrhenius plot ln K
On temperature increase above 100 K a spin
Ž
.
Ž
.
Ž
.
versus 1rT Fig. 4 . Evidently, deviation from the
transition to SA,SB s 5r2,5r2 becomes evident;
linearity, which represents cooperativeness as well as
its saturation value
w
x
the effect of molecular vibrations 17,19 , is well
covered by the theoretical model. The reason why
the vibrations can be relevantly introduced for com-
NA m0 m2B gH2 S 2P105
xHS
s
19
Ž
.
k
T
36
Ž
w x.
pound 3 is the following according to 17 : the low
value of the equilibrium temperature, which implies
a small energy difference between the LS and HS
'
yielding meffrmB s gHS 3P2P105r36 ; 8.38,
however, is not reached until 300 K.
The results of the fitting procedure are presented
in Table 3 and are exemplified in Fig. 5. In all cases
gHS s2.0023 has been fixed and the intermolecular
interaction parameters, J and JX, deleted from the
consideration. Eventually, an approximation JqsJ
qJX f2 Jav and JysJ–JX f0 has been applied.
It can be seen that the quality of the fit is very
good for XsH and 5-Cl. The remaining binuclear
complexes suffer for the lack of the high-temperature
data that prevents a reliable determination of the spin
crossover parameters.
lowest vibronic levels. The site formation enthalpy
X
Ž
.Ž
.
then relaxes to D0 sD0 q 15r2 hÕHS yhÕLS so
that the electronic level difference D0rks2246 K is
reduced to DX0rks124 K.
The characteristic susceptibility function for
Ž
.
Ž
.
SA,SB s 1r2,1r2 pair in the high-temperature
Ž
.
0 s1r4 and then the
limit becomes F1r2,1r2
a
Curie-like behaviour is predicted
NA m0 m2B gL2S
xLS
s
18
Ž
.
k
2T
3krN m x T s g
Ž
.
In the last binuclear complex 7, Xs5-MeO the
spin crossover starts from the intermediate-spin case
SA,SB s 3r2,3r2 . Analogously to the mononu-
clear case for 3, Xs5-Cl, the site formation energy
'
yielding meff
s
3r2 mB.
(
Ž
.
A
0
LS
Ž
.
Ž
.
Such behaviour is observed for three binuclear com-
Ž
.
plexes 4, 5, and 6 showing the effective magnetic

(
)
418
R. Boca et al.rChemical Physics Letters 325 2000 411–419
ˇ
Table 3
Calculated magnetic parameters for binuclear complexes
Parameter
Complex, substituent
XsH
Xs5-Cl
Xs3-MeO
Xs5-MeO
Model a
Fitted
4a
4b
5a
5b
6a
7a
b
gLS
2.42
y2.80
–
167
1301
1.32
2.42
y2.80
46
26
823
1.15
2.38
y2.32
–
26
894
1.31
2.38
y2.32
78
2.8
287
2.43
y2.45
1.91
y0.73
b
Ž
Ž
reff
.
X
Jexrk rK
.
JrksJ rk rk
–
–
d
d
w
x
w
x
3.0
931
1.70
3.0
493
1.70
Ž
.
D0rk rK
c
Rr%
Derived
1.19
DSrJ Ky1 moly1
D HrkJ moly1
TcrK
42.6
10.8
254
27.0
6.8
253
27.1
7.4
275
8.6
2.4
276
9.1
7.7
9.1
4.1
w
x
w
x
w
x
w
x
847
449
a
Model a, no cooperativity; model b, cooperativity parameter. Error estimates: 1.0 for D0, reff and J; 0.01 for Jex and gLS
Fixed parameters using the low-temperature data.
Discrepancy factor of the regression.
.
b
c
d
Values in brackets were fixed.
Ž
.
D0 or the enthalpy of the transition D H is only a
transition is rather gradual and centred at quite high
temperature. The cooperativity parameter is much
lower than the enthalpy of the transition that reflects
in the absence of the steepness and the eventual
hysteresis. Therefore the binuclear character of the
compounds under study does not result in any spe-
cific effect like a two-step process.
Ž
.
Ž
Ž
.
half of the value for the SA,SB s 1r2,1r2 to
Ž
.
Ž
.
SA,SB s 5r2,5r2 transition when the same
.
model is considered, e.g. data set 7a vs. 6a .
On conclusion, the Ising-like model is successful
in fitting the magnetic data for binuclear Fe III
complexes exhibiting the spin crossover. The spin
Ž
.
Fig. 5. A fit of the magnetic data for XsH, 4, Xs3-MeO, 6, Xs5-Cl, 5, and Xs5-MeO, 7; empty symbols – experimental data, full
circles – calculated points, dashed line – theoretical curve.

(
)
R. Boca et al.rChemical Physics Letters 325 2000 411–419
419
ˇ
w x
7
F.V. Wells, S.W. McCann, H.H. Wickman, S.L. Kessel,
Acknowledgements
Ž
.
D.N. Hendrickson, R.D. Feltham, Inorg. Chem. 21 1982
2306.
Thanks are do the European Community Training
w x
Ž .
8
w x
R.J. Butcher, E. Sinn, J. Am. Chem. Soc. 98 1976 2440.
Ž
.
9 D.P. Rininger, N.V. Duffy, R.C. Weir, E. Gelerinter, J.
and Mobility of Researchers Network TMR project
Ž
.
Stanford, D.L. Uhrich, Chem. Phys. Lett. 52 1977 102.
Ž
.
‘Thermal and Optical Spin State Switching’ TOSS
w
x
10 N. Matsumoto, S. Ohta, C. Yoshimura, A. Ohyoshi, S.
supported by the European Community under the
Contract number ERB-FMRX-CT98-0199 and to the
European Science Foundation under the project
‘Molecular Magnets’.
Kohata, H. Okawa, Y. Maeda, J. Chem. Soc. Dalton Trans.
Ž
.
1985 2575.
w
w
x
11 A. Ohyoshi, J. Honbo, N. Matsumoto, S. Ohta, S. Sakamoto,
Ž
.
Bull. Chem. Soc. Jpn. 59 1986 1611.
x
12 S. Ohta, C. Yoshimura, N. Matsumoto, H. Okawa, A.
Ž
.
Ohyoshi, Bull. Chem. Soc. Jpn. 59 1986 155.
w
w
x
13 Y. Fukuda, J. Yuzurihara, to be published.
x
Ž
.
14 A. Bousseksou, F. Varret, J. Nasser, J. Phys. I. 3 1993
References
1463.
w
w
w
w
w
x
15 A. Bousseksou, J. Nasser, J. Linares, K. Boukheddaden, F.
w x
Ž .
P. Gutlich, Struct. Bonding 44 1981 83.
¨
Ž .
1
w x
2
Varret, J. Phys. I. 2 1992 1381.
Ž
.
x
E. Konig, Struct. Bonding 76 1991 51.
¨
16 R. Boca, Theoretical Foundations of Molecular Magnetism,
ˇ
w x
3
O. Kahn, Molecular Magnetism, VCH, New York, 1993.
Elsevier, Amsterdam, 1999.
w x
x
4
P. Gutlich, A. Hauser, H. Spiering, Angew. Chem. 106
¨
17 A. Bousseksou, H. Constant-Machado, F. Varret, J. Phys. I. 5
Ž
.
Ž
.
1994 2109.
1995 747.
w x
5
Ž
.
x
L. Cambi, L. Szego, Ber. Dtsch. Chem. Ges. B 64 1931
2591.
M.S. Haddad, W.D. Federer, M.W. Lynch, D.N. Hendrick-
18 R. Boca, M. Boca, R. Jarosciak, M. Vrbova, Mol. Cryst. Liq.
ˇ
ˇ
ˇˇ
´
Ž
.
Cryst. 335 1999 551.
w x
6
x
19 G. Lemercier, A. Bousseksou, S. Seigeuric, F. Varret, J.-P.
Ž
.
Ž
.
son, Inorg. Chem. 20 1981 131.
Tuchagues, Chem. Phys. Lett. 226 1994 289.