Spin Transition in [Fe(DPEA)(NCS)2], a Compound with the New Tetradentate Ligand (2-Aminoethyl)bis(2-pyridylmethyl)amine (DPEA): Crystal Structure, Magnetic Properties, and M?ssbauer Spectroscopy

Article abstract of DOI:10.1021/ic9615133

The new spin transition compound [Fe^II(DPEA)(NCS)₂], where DPEA [(2-aminoethyl)bis(2-pyridylrnethyl)amine] is a new tetradentate ligand, has been synthesized, and its structure, magnetic properties, and M?ssbauer spectra have been investigated. The crystal structure has been determined by X-ray diffraction at 298 K. The compound crystallizes in the monoclinic system, space group is P2₁/c, with Z = 4, a = 9.358(1) ?, b = 11.812(2) ?, r = 17.135(2) ?, and β-94.5(4)°. The distorted [FeN₆] octahedron is formed from four nitrogen atoms belonging to DPEA and two provided by the cis thiocyanate groups. The two pyridine rings of DPEA are in mer positions. Each molecule is linked to its neighbors by hydrogen-bonding interactions as well as by numerous van der Waals contacts supposed to be responsible for the cooperativity of the system. Variable-temperature magnetic susceptibility measurements (20-290 K) have evidenced a relatively abrupt S = 2 = S = 0 transition centered at T_1/2 = 138 K. The thermal variation of the high spin state fraction observed by M?ssbauer spectroscopy is in agreement with that obtained from magnetic susceptibility measurements. The fitting of M?ssbauer and magnetic data with the Ising-like model allowed us to determine the energy gap between the high-spin and low-spin states (Aeff = 835 K) and to estimate the variation of the thermodynamic parameters upon spin transition. The calculated variations of enthalpy (ΔH = 6.76 kJ mol^-1) and entropy (AS = 49 J mol^-1 K^-1) associated with the spin transition are in agreement with those previously observed for iron(II) spin-crossover compounds. The spin conversion is found to be close to a first-order phenomenon.

Full text of DOI:10.1021/ic9615133

Inorg. Chem. 1997, 36, 2975-2981
2975
Spin Transition in [Fe(DPEA)(NCS)2], a Compound with the New Tetradentate Ligand
(2-Aminoethyl)bis(2-pyridylmethyl)amine (DPEA): Crystal Structure, Magnetic Properties,
and Mo1ssbauer Spectroscopy
Galina S. Matouzenko,*,† Azzedine Bousseksou,‡ Sylvain Lecocq,§
Petra J. van Koningsbruggen,| Monique Perrin,§ Olivier Kahn,| and Andre´ Collet*,†
Laboratoire de ste´re´ochimie et interactions mole´culaires, EÄ cole normale supe´rieure de Lyon, 46, alle´e
d’Italie, 69364 Lyon cedex 07, France, Laboratoire de chimie de coordination du CNRS, 205, route de
Narbonne, 31077 Toulouse cedex, France, Laboratoire de reconnaissance et organisation mole´culaire,
Universite´ Claude Bernard-Lyon 1, 69622 Villeurbanne cedex, France, and Laboratoire des sciences
mole´culaires, Institut de chimie de la matie`re condense´e de Bordeaux, 33608 Pessac, France
ReceiVed December 20, 1996X
The new spin transition compound [FeII(DPEA)(NCS)2], where DPEA [(2-aminoethyl)bis(2-pyridylmethyl)amine]
is a new tetradentate ligand, has been synthesized, and its structure, magnetic properties, and Mo¨ssbauer spectra
have been investigated. The crystal structure has been determined by X-ray diffraction at 298 K. The compound
crystallizes in the monoclinic system, space group is P21/c, with Z ) 4, a ) 9.358(1) Å, b ) 11.812(2) Å, c )
17.135(2) Å, and â ) 94.5(4)°. The distorted [FeN6] octahedron is formed from four nitrogen atoms belonging
to DPEA and two provided by the cis thiocyanate groups. The two pyridine rings of DPEA are in mer positions.
Each molecule is linked to its neighbors by hydrogen-bonding interactions as well as by numerous van der Waals
contacts supposed to be responsible for the cooperativity of the system. Variable-temperature magnetic susceptibility
measurements (20-290 K) have evidenced a relatively abrupt S ) 2 h S ) 0 transition centered at T1/2 ) 138
K. The thermal variation of the high spin state fraction observed by Mo¨ssbauer spectroscopy is in agreement
with that obtained from magnetic susceptibility measurements. The fitting of Mo¨ssbauer and magnetic data with
the Ising-like model allowed us to determine the energy gap between the high-spin and low-spin states (∆eff
)
835 K) and to estimate the variation of the thermodynamic parameters upon spin transition. The calculated
variations of enthalpy (∆H ) 6.76 kJ mol-1) and entropy (∆S ) 49 J mol-1 K-1) associated with the spin transition
are in agreement with those previously observed for iron(II) spin-crossover compounds. The spin conversion is
found to be close to a first-order phenomenon.
Introduction
relate the characteristics of the spin transition to the electronic
and vibrational properties of the considered material and
High-spin (HS, 5T2) h low spin (LS, 1A1) transitions in iron-
(II) compounds have been extensively studied during the last
two decades. Spin transitions can be induced by physical
perturbations, such as changes in temperature or pressure, and
by light irradiation. These effects are well documented and have
been summarized in several reviews.1 The current interest in
this phenomenon is due to its possible application to molecular
electronics2 and to its implication in certain biological pro-
cesses.3 Several theoretical models have been developed to
particularly to its molecular structure.4 The sensitivity of the
spin state to small perturbations suggests that new coordination
complexes exhibiting spin transition phenomena could be
designed through a fine tuning of the ligands surrounding the
metal. According to Toftlund,1d one of the possible strategies
for engineering such spin transition systems rests on the use of
ligands containing both aliphatic and aromatic nitrogen donor
atoms. The combination in the same ligand of nitrogen atoms
of different nature is expected to generate an intermediate ligand
field force which in turn can produce conditions favoring spin-
state interconversion. Numerous complexes containing such a
combination of ligands have been described.1a,d,5 Recently, we
have used this approach successfully in the case of a pseudo-
* To whom correspondence should be addressed.
† EÄ cole normale supe´rieure de Lyon (UMR CNRS and ENS-Lyon No.
117; chaire de l’Institut universitaire de France).
‡ CNRS (UP CNRS 8241).
§ Universite´ Claude Bernard-Lyon 1 (ESA Q5078).
| Institut de chimie de la matie`re condense´e de Bordeaux.
X Abstract published in AdVance ACS Abstracts, May 15, 1997.
(1) (a) Goodwin, H. A. Coord. Chem. ReV. 1976, 18, 293. (b) Gu¨tlich, P.
Struct. Bonding 1981, 44, 83. (c) Ko¨nig, E.; Ritter, G.; Kulshreshtha,
S. K. Chem. ReV. 1985, 85, 219. (d) Toftlund, H. Coord. Chem. ReV.
1989, 94, 67. (e) Zarembowitch, J.; Kahn, O. New J. Chem. 1991, 15,
181. (f) Ko¨nig, E. Struct. Bonding. 1991, 76, 51. (g) Gu¨tlich, P.;
Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024.
(h) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(2) Kahn, O.; Codjovi, E. Phil. Trans. R. Soc. Lond. A 1996, 354, 359.
(3) (a) Tang, S. C; Koch, S.; Papaefthymiou, G. C., Foner, S.; Frankel,
R. B.; Ibers, J. A.; Holm, R. H. J. Am. Chem. Soc. 1976, 98, 2412.
(b) Morishima, I.; Iizuka, T. J. Am. Chem. Soc. 1974, 96, 5270. (c)
Dose, E. V.; Tweedle, M. F.; Wilson, L. J.; Sutin, N. J. J. Am. Chem.
Soc. 1977, 99, 3886. (d) Backes, W. L.; Sligar, S. G.; Schenkman, J.
B. Biochemistry 1982, 21, 1324. (e) Ficher, M. T.; Sligar, S. G.
Biochemistry 1987, 26, 4797.
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(b) Bari, R. A.; Sivardie`re, J. Phys. ReV. B 1972, 5, 4466. (c) Sasaki,
N.; Kambara, T. J. Phys. Soc. Jpn. 1987, 49, 1806. (d) Sorai, M.;
Seki, S. J. Phys. Soc. Jpn. 1972, 33, 575. (e) Bolvin, H.; Kahn, O.
Chem. Phys. 1995, 192, 295. (f) Bousseksou, A.; Constant-Machado,
H.; Varret, F. J. Phys. 1 (Fr.) 1995, 5, 747.
(5) (a) Toftlund, H.; Yde-Andersen, S. Acta Chem. Scand. A 1981, 35,
575. (b) Chang, H. R.; McCusker, J. K.; Toftlund, H.; Wilson, S. R.;
Trautwein, A. X.; Winkler, H.; Hendrickson, D. N. J. Am. Chem. Soc.
1990, 112, 6814. (b) McCusker, J. K.; Toftlund, H.; Rheingold, A.
L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1797. (d) Al-
Obaidi, A. H. R.; McGarvey, J. J.; Taylor, K. P.; Beel, S. E. J.; Jensen,
K. B.; Toftlund, H. J. Chem. Soc. Chem. Comm. 1993, 536. (e)
McCusker, J. K.; Rheingold, A. L.; Hendrickson, D. N. Inorg. Chem.
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1996, 35, 5055.
S0020-1669(96)01513-3 CCC: $14.00 © 1997 American Chemical Society

2976 Inorganic Chemistry, Vol. 36, No. 14, 1997
Matouzenko et al.
Table 1. Crystallographic Data for [Fe(DPEA)(NCS)2
Scheme 1a
chem formula
a, Å
b, Å
C16H18N6S2Fe
9.358(1)
11.818(2)
17.135(2)
90.0(5)
94.5(4)
90.0(5)
1889(1)
4
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
fw
414.33
P21/c
298
1.540 56
1.457
85.63
space group
temp, K
λ(Cu KR), Å
F
calc, g‚cm-3
µ, cm-1
a Key: (i) Zn and AcOH in EtOH, 60-70 °C, 40%; (ii) N-(2-
bromoethyl)phthalimide in toluene, 15 h reflux; (iii) hydrazine hydrate
in EtOH, reflux, 41%.
Ra
0.066
0.073
b
Rw
2
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
Chart 1. Stereoisomers of [Fe(DPEA)(NCS)2]a
none of these complexes have been characterized by X-ray
crystallography.
Experimental Section
Chemistry. All reagents and solvents used in this study are
commercially available and were used without further purification. [Fe-
(py)4(NCS)2] (py ) pyridine) was prepared according to the described
procedure.10 All syntheses involving Fe(II) species were carried out
in deoxygenated solvents under an inert atmosphere of N2 using Schlenk
type vessel or glovebox techniques. Elemental analyses (C, H, N, S,
and Fe determinations) were performed at the Service Central de
Microanalyse du CNRS in Vernaison, France. 1H NMR spectra were
recorded in CDCl3 on a Bruker AC200 spectrometer operating at 200
MHz.
a Only 1, where the pyridine ligands are meridionally arranged, was
isolated in this work.
Ligand Synthesis. (a) Bis(2-pyridylmethyl)amine (DPA).
A
octahedral iron(II) complex of a C3-cyclotriveratrylene ligand,
for which different spin states have been found to coexist in
the crystal lattice.6
mixture of 2-(aminomethyl)pyridine (10.3 mL, 0.1 mol), zinc powder
(20 g), and glacial acetic acid (20 mL) in 100 mL of 95% ethanol was
vigorously stirred at 60-70 °C under argon. To this mixture, pyridine-
2-carboxaldehyde (9.5 mL, 0.1 mol) was gradually added over a period
of 2 h. Then further amounts of zinc powder (40 g) and glacial acetic
acid (40 mL) were added over a period of 2 h, and stirring at 60-70
°C was continued for 5 h. The resulting mixture was allowed to stand
at room temperature overnight, and the precipitate was filtered off. The
filtrate was evaporated under reduced pressure to a syrup which was
made alkaline with an excess of aqueous NaOH. The organic layer
was extracted with ether, dried over MgSO4, and evaporated to give a
light brown oil which was distilled to give 8 g (40%) of a pale yellow
syrup of DPA, bp 139-141 °C (1 Torr). 1H NMR (CDCl3): δ 8.52-
7.04 (m, 8H, aromatic H’s), 3.93 (s, 4H, CH2), 2.55 (s, 1H, NH).
(b) (2-Aminoethyl)bis(2-pyridylmethyl)amine (DPEA). To a
solution of the above DPA (8 g, 0.04 mol) in toluene (50 mL) were
added 5.6 mL (0.04 mol) of triethylamine and 10.2 g (0.04 mol) of
N-(2-bromoethyl)phthalimide. The mixture was refluxed under argon
overnight and was then concentrated under reduced pressure. The solid
residue was washed with 50 mL of cold water and then 50 mL of ether,
to give 7.5 g (50%) of N-(2-(bis(2-pyridylmethyl)amino)ethyl)-
phthalimide as a beige solid. 1H NMR (CDCl3): δ 8.43-7.00 (m,
8H, aromatic H’s), 7.83-7.69 (m, 4H, aromatic H’s), 3.84 (s, 4H, CH2),
3.84-3.79 (t, 2H, CH2), 2.89-2.81 (t, 2H, CH2). To this product
dissolved in 50 mL of absolute ethanol was added 0.98 mL (0.02 mol)
of hydrazine monohydrate. The mixture was refluxed under argon until
formation of a white gelatinous precipitate. When the formation of
the latter appeared complete, it was decomposed by heating with 12
M HCl (8 mL). The precipitate (phthalyl hydrazide hydrochloride)
was filtered off, and the solution was concentrated under vacuum. The
residue was made basic with aqueous NaOH, and the organic material
was extracted with ether. The ethereal solution was dried over KOH
pellets and evaporated under vacuum, to give 4.1 g (41%) of practically
pure DPEA (yellow oil). 1H NMR (CDCl3): δ 8.49-7.04 (m, 8H,
aromatic H’s), 3.76 (s, 4H, CH2), 2.75-2.55 (m, 4H, CH2), 1.45 (s,
2H, NH2).
In this paper we report the synthesis of (2-aminoethyl)bis-
(2-pyridylmethyl)amine (DPEA), a new tetradentate ligand in
which the conditions mentioned above are fulfilled (Scheme
1). This ligand was converted to the iron(II) complex [Fe-
(DPEA)(NCS)2] (1) (Chart 1), which showed a sharp spin
transition centered at 138 K. The single-crystal X-ray structure,
magnetic susceptibility, and Mo¨ssbauer spectroscopy of this
complex were investigated, and the spin-state conversion was
analyzed in the light of the Ising-like electron-vibrational
model.7 So far, cooperative spin transitions in cis-diisothiocy-
anatoiron(II) complexes have only been observed in [FeL2-
(NCS)2], where L is a bidentate ligand (bipy, phen, etc.).8 The
[FeL(NCS)2] complexes of tetradentate ligands known before
this work mainly give rise to spin equilibrium phenomena,9 and
(6) Matouzenko, G.; Ve´riot, G.; Dutasta, J. P.; Collet, A.; Jordanov, J.;
Varret, F., Perrin, M., Lecocq, S. New J. Chem. 1995, 19, 881.
(7) Bousseksou, A.; Nasser, J.; Linares, J.; Boukheddaden, K.; Varret, F.
J. Phys. 1 (Paris) 1992, 2, 1381.
(8) (a) Ko¨nig, E.; Madeja, K.; Watson, K. J. J. Am. Chem. Soc. 1968, 90,
1146. (b) Ko¨nig, E.; Madeja, K. Inorg. Chem. 1967, 6, 48. (c) Ko¨nig,
E.; Watson, K. J. Chem. Phys. Lett. 1970, 6, 457. (d) Sorai, M.; Seki,
S. J. Phys. Chem. Solids 1974, 35, 555. (e) Ko¨nig, E.; Ritter, G.; Irler,
W. Chem. Phys. Lett. 1979, 66, 336. (f) Ganguli, P.; Gu¨tlich, P.;
Mu¨ller, E. W. J. Chem. Soc., Dalton Trans. 1981, 441. (g) Gallois,
B.; Real, J. A. Hauw, C.; Zarembowitch, J. Inorg. Chem. 1990, 29,
1152. (h) Claude, R.; Real, J. A., Zarembowitch, J.; Kahn, O.; Ouahab,
L.; Grandjean, D.; Boukheddaden, K.; Varret, F.; Dworkin, A. Inorg.
Chem. 1990, 29, 4442. (i) Granier, T.; Gallois, B.; Gaultier, J.; Real,
J. A.; Zarembowitch, J. Inorg Chem. 1993, 32, 5305. (j) Real, J. A.;
Mun˜oz, M. K.; Andre´s, E.; Granier, T.; Gallois, B. Inorg. Chem. 1994,
33, 3587.
(9) (a) Højland, F.; Toftlund, H.; Yde-Andersen, S. Acta Chem. Scand. A
1983, 37, 251. (b) Toftlund, H.; Pedersen, E.; Yde-Andersen, S. Acta
Chem. Scand. A 1984, 38, 693. (c) Yu, Z.; Schmitt, G.; Bo¨res, N.;
Gu¨tlich, P. J. Phys.: Condens. Matter. 1995, 7, 777. (d) Buchen, T.;
Toftlund, H.; Gu¨tlich, P. Chem. Eur. J. 1996, 2, 1129.
(10) Erickson, N. E.; Sutin, N. Inorg. Chem. 1966, 5, 1834.

Spin Transition in [Fe(DPEA)(NCS)2]
Inorganic Chemistry, Vol. 36, No. 14, 1997 2977
Table 3. Selected Bond Distances (Å) for [Fe(DPEA)(NCS)2]a
Fe1-N1
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N5
Fe1-N6
S1-C1
2.115(4)
2.155(5)
2.186(4)
2.177(4)
2.231(4)
2.215(5)
1.690(7)
1.029(7)
1.616(5)
1.151(7)
1.507(8)
1.453(9)
1.480(8)
1.471(7)
1.486(8)
N3-C30
N3-C34
C30-C31
C31-C32
C32-C33
C33-C34
C34-C35
N4-C40
N4-C44
C40-C41
C41-C42
C42-C43
C43-C44
C44-C45
1.353(7)
1.344(6)
1.359(8)
1.349(9)
1.39(1)
1.378(7)
1.524(8)
1.341(6)
1.345(7)
1.366(9)
1.38(1)
N1-C1
S2-C2
N2-C2
N6-C53
C53-C52
C52-N5
N5-C35
N5-C45
1.35(1)
1.394(9)
1.495(8)
a Estimated standard deviations in the least significant digits are given
in parentheses.
Figure 1. Perspective view of the [Fe(DPEA)(NCS)2] molecule,
showing the atom-numbering scheme. The ellipsoids enclose 50%
probability.
Table 4. Selected Angles (deg) for [Fe(DPEA) (NCS)2]a
N1-Fe1-N2
N1-Fe1-N6
N1-Fe1-N5
N1-Fe1-N3
N1-Fe1-N4
Fe1-N6
95.6(2)
95.1(2)
174.1(2)
103.5(2)
104.2(2)
167.4(2)
90.2(2)
93.7(2)
85.6(2)
79.2(2)
90.3(2)
C52-N5-C35
C52-N5-C45
C35-N5-C45
Fe1-N3-C30
Fe1-N3-C34
C30-N3-C34
N3-C30-C31
C30-C31-C32
C31-C32-C33
C32-C33-C34
N3-C34-C33
N3-C34-C35
C33-C34-C35
N5-C35-C34
Fe1-N4-C40
Fe1-N4-C44
C40-N4-C44
N4-C40-C41
C40-C41-C42
C41-C42-C43
C42-C43-C44
N4-C44-C43
N4-C44-C45
C43-C44-C45
N5-C45-C44
110.0(4)
114.0(4)
111.0(4)
127.0(3)
116.2(3)
116.6(4)
123.1(5)
120.3(6)
118.4(6)
118.9(5)
122.7(5)
114.6(4)
122.7(5)
109.6(4)
126.1(4)
115.0(3)
119.0(4)
123.1(6)
117.6(6)
120.4(7)
119.6(7)
120.3(5)
117.1(5)
122.5(5)
111.6(5)
Table 2. Fractional Atomic Coordinates and Isotropic Equivalent
Displacement Parametersa,b for Non-Hydrogen Atoms of
[Fe(DPEA)(NCS)2]
atoms
x/a
y/b
z/c
Beq
Fe1
S1
N1
C1
S2
N2
C2
N6
0.05443(7)
0.3293(2)
0.1546(4)
0.2242(6)
0.2782(2)
0.1119(5)
0.1812(5)
-0.0003(6)
-0.0362(7)
-0.1233(7)
-0.0606(4)
-0.1681(4)
-0.2321(6)
-0.3747(7)
-0.4595(7)
-0.3975(5)
-0.2527(5)
-0.1753(7)
0.2242(4)
0.17248(6)
-0.1638(2)
0.0179(3)
-0.0486(6)
0.1934(1)
0.1668(4)
0.1786(4)
0.2190(4)
0.3433(6)
0.3717(5)
0.3356(3)
0.1239(3)
0.0255(5)
0.0061(6)
0.0860(6)
0.1882(6)
0.2036(4)
0.3124(5)
0.2979(3)
0.2805(6)
0.3631(7)
0.4694(8)
0.4899(6)
0.4021(5)
0.4163(5)
0.36660(4)
0.4284(1)
0.4008(3)
0.4119(3)
0.12195(8)
0.2473(3)
0.1953(3)
0.4859(3)
0.4864(4)
0.4152(4)
0.3425(2)
0.3355(2)
0.3531(3)
0.3375(4)
0.3017(4)
0.2823(3)
0.3000(3)
0.2809(4)
0.3848(2)
0.4213(3)
0.4307(4)
0.4014(4)
0.3669(4)
0.3580(3)
0.3145(4)
3.39(1)
7.31(5)
4.5(1)
5.3(1)
5.44(3)
5.6(1)
4.1(1)
4.4(1)
5.8(2)
5.7(1)
4.09(9)
3.92(9)
4.7(1)
5.6(1)
6.0(2)
5.2(1)
4.1(1)
5.3(1)
3.88(9)
4.8(1)
6.4(2)
7.1(2)
5.9(2)
4.5(1)
5.8(2)
N2-Fe1-N5
N2-Fe1-N3
N2-Fe1-N4
N6-Fe1-N5
N6-Fe1-N3
N6-Fe1-N4
N5-Fe1-N3
N5-Fe1-N4
N3-Fe1-N4
Fe1-N1-C1
S1-C1-N1
Fe1-N2-C2
S2-C2-N2
Fe1-N6-C53
N6-C53-C52
C53-C52-N5
Fe1-N5-C52
Fe1-N5-C35
Fe1-N5-C45
85.3(2)
75.2(1)
77.0(1)
C53
C52
N5
152.2(1)
166.2(5)
176.1(6)
158.2(4)
179.2(5)
108.4(4)
109.3(5)
114.0(5)
108.1(3)
106.6(3)
106.7(3)
N3
C30
C31
C32
C33
C34
C35
N4
C40
C41
C42
C43
C44
C45
0.3544(5)
0.4563(7)
0.4212(8)
0.2891(8)
0.1898(6)
0.0461(7)
a Estimated standard deviations in the least significant digits are given
in parentheses.
a sample of 100 mg of microcrystalline powder enclosed in a 2 cm
diameter cylindrical plastic sample holder, the size of which had been
determined to optimize the absorption. Variable-temperature spectra
were obtained in the 80-305 K range, by using a MD306 Oxford
cryostat, the thermal scanning being monitored by an Oxford ITC4
servocontrol device ((0.1 K accuracy). A least-squares computer
program was used to fit the Mo¨ssbauer parameters and determine their
standard deviations of statistical origin (given in parentheses).11
Solution and Refinement of the X-ray Structure. The structure
of the above described complex was determined at room temperature
(ca. 298 K). The crystal was sealed in a Lindemann capillary and
mounted on a four-circle diffractometer (CAD4 Enraf-Nonius) with
Cu anticathode and graphite monochromator. Crystal data and refine-
ment results are summarized in Table 1. Cell parameters were
determined from 25 reflections with 2θ angles between 6.4 and 51.5°.
Three reflections were measured every 1 h as a control of the crystal
quality and every 100 reflections as an intensity control. The 7903
reflections were measured and merged (R ) 0.046). The 3566 unique
reflections were used in the refinement. The “SDP”12 package supplied
by Nonius was used to correct the intensities from absorption and usual
a Estimated standard deviations in the least significant digits are given
b
4
in parentheses. Beq ) /3∑i∑jâijaiaj.
Complex Synthesis. To 10 mL of a solution of [Fe(py)4(NCS)2]
(48.8 mg, 0.1 mmol) in methanol was added 5 mL of an ethanol solution
containing 24.2 mg (0.1 mmol) of DPEA. The resulting green brown
solution was allowed to stand overnight at room temperature, to give
yellow brown prismatic crystals of 1 which were collected by filtration,
washed with ethanol, and dried under vacuum. The crystal used in
the X-ray structure determination was selected from this sample. Anal.
Calcd for C16H18N6S2Fe: C, 46.38; H, 4.38; N, 20.28; S, 15.48; Fe,
13.48. Found: C, 46.20; H, 4.52; N, 20.01; S, 15.59; Fe, 13.13.
Physical Measurements. Magnetic Measurements. Magnetic
susceptibilities were measured in the temperature range 20-300 K with
a fully automated Manics DSM-8 susceptometer equipped with a TBT
continuous-flow cryostat and a Drusch EAF 16 UE electromagnet
operating at ca. 1.7 T. Data were corrected for magnetization of the
sample holder and for diamagnetic contributions, which were estimated
from the Pascal constants.
(11) Varret, F. Proceedings of the International Conference on Mo¨ssbauer
Effect Applications. Jaipar, India, 1981; Indian National Science
Academy: New Delhi, 1982.
(12) Frenz, B. A. Stucture determination package; Enraf-Nonius: Delft,
The Netherlands, 1982.
Mo1ssbauer Spectra. The variable-temperature Mo¨ssbauer measure-
ments were obtained on a constant-acceleration spectrometer with a
25 mCi source of 57Co (Rh matrix). Isomer shift values (IS) are given
with respect to metallic iron at room temperature. The absorber was

2978 Inorganic Chemistry, Vol. 36, No. 14, 1997
Matouzenko et al.
Figure 2. Stereoview of the packing diagram for [Fe(DPEA)(NCS)2].
Lorentz polarization; MULTAN13 was used to solve the structure by
use of direct methods, and the “SHELX-76” program14 was used for
the refinement by full-matrix least-squares methods. Non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms were refined
isotropically. Hydrogen coordinates, anisotropic thermal parameters
of non-hydrogen atoms, remaining bond distances and angles, and least-
square planes are deposited as Supporting Information.
between cis nitrogen atoms are found in the range 75.2(1)-
104.2(2)°, and those between trans nitrogen atoms are in the
range 152.2(1)-174.1(2)°; these values strongly deviate from
the values of 90 and 180 that would be observed in an ideal
octahedron. This severe distortion of the [FeN6] core from Oh
symmetry is certainly due to the steric constraints caused by
the coordination of the tetradentate ligand. A similar feature
has been observed for high-spin complexes [FeL2(NCS)2]
containing bidentate ligands.8g-j,15 The NCS groups are almost
linear (N1-C1-S1 ) 176.1(6)°, N2-C2-S2 ) 179.2(5)°),
whereas the Fe-N-C linkages are appreciably bent (Fe-N1-
C1 ) 166.2(5)°, Fe-N2-C2 ) 158.2(4)°).
Stereochemistry. The tetradentate ligand DPEA being
coordinated to the iron(II) atom forms three five-membered
chelate rings. One of these rings includes the aliphatic
ethanediamine fragment; the other two contain the (pyridyl-
methyl)amino groups of the ligand. Models show that in this
octahedral complex the cis arrangement of two NCS groups is
the only one possible. The pyridine rings of the ligand can
have either meridional or facial arrangement (1 and 2, respec-
tively, in Chart 1). The facial arrangement leads to a chiral
complex (resolvable into enantiomers), whereas the meridional
one is achiral. Only complex 1 where the pyridine ligands are
meridionally arranged was isolated in this work. This achiral
complex shows a residual chirality in the crystal state due to
the twist conformation of the five-membered ethanediamine
metallocycle, and in the P21/c unit cell two pairs of symmetry
related left-handed and right-handed conformations exist (Figure
2).
Results
Ligand and Complex Syntheses. The new ligand DPEA
was obtained in ca. 16% overall yield by the straightforward
synthesis shown in Scheme 1. The Schiff base formed in situ
from pyridine-2-carboxaldehyde and 2-(aminomethyl)pyridine
was reduced to the corresponding amine (DPA) in the presence
of zinc powder and acetic acid (40%). Then reaction of DPA
with N-(2-bromoethyl)phthalimide followed by hydrazinolysis
provided the tetradentate ligand DPEA (41%) as a yellow syrup,
essentially pure for the next step. The desired complex [Fe-
(DPEA)(NCS)2] was obtained by mixing equimolecular amounts
of [Fe(py)4(NCS)2] and DPEA in a 2:1 mixture of methanol
and ethanol from which crystals suitable for the physical studies
were obtained. These crystals were not solvated, and their
elemental analysis (C, H, N, S, Fe) was consistent with the
expected composition.
Description of the Structure. The complex [Fe(DPEA)-
(NCS)2] crystallizes in the monoclinic space group P21/c (Z )
4). The perspective view of the molecule is shown in Figure 1
and corresponds to structure 1 in Chart 1. Fractional atomic
coordinates and selected bond lengths and angles are given in
Tables 2-4, respectively.
Crystal Packing. Figure 2 displays a stereoview of the
crystal structure of 1. The packing consists of layers of
molecules parallel to the bc plane. In each layer the cohesion
is due to weak hydrogen bonds involving hydrogens H60 and
H61 of the amino group N6, which are linked to the thiocyanate
sulfur atoms belonging to two different adjacent molecules.
Between the layers, in the a direction, cohesion is achieved by
van der Waals interactions. The hydrogen bonds data and the
intermolecular distances shorter than 3.70 Å are listed in Tables
5 and 6, respectively.
[FeN6] Octahedral Geometry. Two of the six nitrogen
atoms coordinated to the Fe(II) center belong to the cis
thiocyanate groups and four belong to the tetradentate ligand.
The latter is coordinated by its two pyridine nitrogen atoms in
apical position and by two aliphatic amino groups in adjacent
position. The Fe-N distances involving the thiocyanate groups
(Fe-N1 ) 2.115(4) Å, Fe-N2 ) 2.155(5) Å) are shorter than
those involving the pyridine rings (Fe-N3 ) 2.186(4) Å, Fe-
N4 ) 2.177(4) Å) and the aliphatic amino groups (Fe-N5 )
2.231(4) Å, Fe-N6 ) 2.215(5) Å). The N-Fe-N angles
Magnetic Susceptibility Data. Variable-temperature mag-
netic susceptibility data were collected in both cooling and
warming modes in the range 20-290 K. The magnetic
(13) Main, P.; Fiske, S. J.; Hull, S. E.; Lessinger, L.; Germain, G.; Declercq,
J. P.; Woolfson, M. M. MULTAN 80, A System of Computer Programs
for the Automatic solution of Crystal Structure from X-ray Diffraction
Data; University of York, England, and University of Louvain,
Belgium, 1980.
(14) Sheldrick, G. M. SHELX-76, Computer Program for Crystal Structure
Determination; University of Cambridge: Cambridge, England, 1976.
(15) (a) Roux, C.; Zarembowitch, J.; Gallois, B.; Granier, T.; Claude, R.
Inorg. Chem. 1994, 33, 2273. (b) Real, A.; Zarembowitch, J.; Kahn,
O.; Solans, X. Inorg. Chem. 1987, 26, 2939.

Spin Transition in [Fe(DPEA)(NCS)2]
Inorganic Chemistry, Vol. 36, No. 14, 1997 2979
Table 5. Interatomic Distances (Å) and Angles (deg) for the
Hydrogen-Bonding Interactions in [Fe(DPEA)(NCS)2]a
D-H‚‚‚A
D-H
H‚‚‚A
D‚‚‚A
D-H‚‚‚A
N6-H61‚‚‚S1b
0.76(6)
0.95(5)
2.84(6)
2.63(6)
3.58(1)
3.51(2)
165(5)
156(5)
N6-H60‚‚‚S2b
a Estimated Standard deviations in the least significant digits are
given in parentheses. b Symmetry operations: N6‚‚‚S1, -x, -y, -z +
1
1
1; N6‚‚‚S2, +x, -y + /2, + z + /2.
Table 6. Intermolecular Contacts (Å) shorter than 3.70 Åa
atoms
sym operators
-x, -y, -z
translations
dist
C1‚‚‚N6
0, 0, 1
0, -1, 0
0, -1, 0
0, -1, 0
0, -1, 0
0, -1, 0
0, 0, -1
0, 0, -1
-1, 0, 0
-1, 0, 0
-1, 0, 0
1, 1, 1
3.47(1)
3.68(1)
3.43(1)
3.65(1)
3.66(1)
3.59(1)
3.62(1)
3.57(1)
3.61(1)
3.62(1)
3.62(1)
3.60(1)
3.65(1)
C1‚‚‚C35
N2‚‚‚C45
C31‚‚‚C43
C33‚‚‚C43
C32‚‚‚C43
S2‚‚‚C53
S2‚‚‚C40
C32‚‚‚C40
C33‚‚‚C40
C33‚‚‚C41
C41‚‚‚C42
C42‚‚‚C42
-x, + y + 1/2, -z + 1/2
-x, + y + 1/2, -z + 1/2
-x, + y + 1/2, -z + 1/2
-x, + y + 1/2, -z + 1/2
-x, + y + 1/2, -z + 1/2
+ x, -y + 1/2, + z + 1/2
+ x, -y + 1/2, + z + 1/2
+ x, + y, + z
+ x, + y, + z
+ x, + y, + z
-x, -y, -z
-x, -y, -z
1, 1, 1
a Estimated standard deviations in the least significant digits are given
in parentheses.
Figure 4. Selected Mo¨ssbauer spectra of [Fe(DPEA)(NCS)2] obtained
in the cooling mode. The solid lines represent fitted curves.
complete. In the warming mode the same magnetic behavior
as in the cooling one was observed and no thermal hysteresis
was detected.
Mo1ssbauer Spectroscopy. The Mo¨ssbauer studies were
performed between 80 and 305 K. Representative Mo¨ssbauer
spectra of [Fe(DPEA)(NCS)2] obtained in the heating mode are
shown in Figure 4. The low- and the high-temperature main
doublets are typical for high-spin and low-spin iron(II), respec-
tively. The isomer shift (IS) values are 0.507(7) mm s-1 at 80
K and 1.169(1) mm s-1 at 305 K. The quadrupole splitting
(∆EQ) values are in agreement with those previously observed
for iron(II) spin-crossover compounds.1b At intermediate tem-
peratures the contributions of the two spin states are well
resolved and the spectra consist of two doublets in thermal
equilibrium with each other. No spectrum deformation or line
broadening was observed in this temperature range (80-305
K), indicating that the LS h HS conversion rates are slow
compared to the hyperfine frequencies (∼108 s-1).16 The 80 K
spectrum evidences the absence of residual high-spin fraction,
while the 305 K spectrum indicates the presence of a small
residual low-spin fraction (6(1)%). Detailed values of the
Mo¨ssbauer parameters resulting from the least-squares fitting
procedures are listed in Table 7 for a representative set of
temperatures.
Figure 3. Thermal variation of øMT for [Fe(DPEA)(NCS)2].
properties of complex 1 in the form of øΜT vs T are presented
in Figure 3. The magnetic measurements revealed a relatively
abrupt spin transition between the high spin (S ) 2) and low
spin (S ) 0) states of the complex. At room temperature (290
K) the magnitude of øΜT (3.56 cm3 mol-1 K; µeff ) 5.36 µB)
corresponds to a quintet spin state. This value slowly decreases
upon cooling to 150 K, the sharp spin conversion occurring
between 150 and 130 K. The spin transition temperature T1/2
(temperature for which the high-spin fraction is equal to 0.5) is
found at 138 K, with 55% of the spin transition taking place
within 10 K (140-130 K). This magnetic behavior implies that
a relatively high cooperativity between molecules exists in the
crystal. At low temperature (20 K) the value øMT reaches 0.1
cm3 mol-1 K (µeff ) 0.89 µB), which is close to the temperature-
independent paramagnetism value expected for low-spin iron-
(II) complexes. Although this value could also be explained
by the presence of about 2.5% of high-spin iron(II) fraction,
the absence of residual paramagnetism at low temperature
revealed by Mo¨ssbauer spectroscopy (Vide infra) as well as the
closeness of experimental øMT to the expected value for a pure
singlet spin state suggests that the spin transition is indeed quasi
The nHS values obtained through magnetic susceptibility
measurements were calibrated with the 293 K nHS(∼AHS/Atot)
value obtained from the Mo¨ssbauer measurements. The thermal
variation of nHS resulting from the magnetic susceptibility
measurements (see Figure 5) agrees perfectly with that obtained
from the Mo¨ssbauer study. Figure 5 clearly shows that the spin-
state conversion is relatively steep and achieved essentially
(16) (a) Adler, P.; Hauser, A.; Vefn, A.; Spiering, H.; Gu¨tlich, P. Hyperfine
Interact. 1989, 47, 343. (b) Adler, P.; Spiering, H.; Gu¨tlich, P. J. Chem.
Phys. Lett. 1994, 226, 289.

2980 Inorganic Chemistry, Vol. 36, No. 14, 1997
Matouzenko et al.
Table 7. Least-Squares-Fitted Mo¨ssbauer Dataa,b for [Fe(DPEA)(NCS)2]
low-spin state
high-spin state
T (K)
IS (mm s-1
)
∆EQLS (mm s-1
)
Γ/2 (mm s-1
)
IS (mm s-1
)
∆EQHS (mm s-1
)
Γ/2 (mm s-1
)
AHS/Atot (%)
80
135
140
150
230
0.505(1)
0.488(1)
0.487(1)
0.462(3)
0.375
0.382(1)
0.378(2)
0.390(2)
0.413(4)
0.4
0.115(1)
0.117(1)
0.119(1)
0.126(3)
0.23(5)
0
23
51
80
95
1.073(4)
1.079(1)
1.073(1)
1.032(1)
2.04(1)
0.135(6)
0.137(2)
0.136(1)
0.126(1)
2.000(2)
1.951(2)
1.687(2)
a IS, isomer shift; ∆EQ, quadrupole splitting; Γ, half-height width of the line; AHS/Atot, area ratio. b With their statistical standard deviations given
in parentheses; italicized values were fixed to the fit; isomer shift values refer to metallic iron at 300 K. A: base-line-corrected data area.
Figure 5. Thermal variation of the high-spin fraction (nHS) of [Fe-
Figure 6. Arrhenius plot of the eqilibrium constant obtained by
(DPEA)(NCS)2]. Open and black circles correspond to magnetic
Mo¨ssbauer spectroscopy for [Fe(DPEA)(NCS)2].
susceptibility and to Mo¨ssbauer spectroscopy data, respectively. The
solid line represents a least-squares fit with the two-level Ising-like
model.
the opening of the Fe(NCS)2 angle (95.6(2)°) with respect to a
value close to 90° that would be expected in an unstrained
complex.
between 130 and 150 K. The spin transition temperature, T1/2
,
A series of cis-diisothiocyanatoiron(II) complexes with tet-
radentate ligands of the bis(2-pyridylmethyl)diamine and tris-
(2-pyridylmethyl)amine (TPA) types has been described by
Toftlund et al.9 These complexes display either temperature-
is close to 138 K. The transition temperature T1/2 found from
the Mo¨ssbauer spectroscopy is identical with that given by
magnetic measurements.
dependent spin equilibrium (5T2
h
1A1) or a temperature-
Discussion
independent spin state (high spin or intermediate). In the last
case the high-spin state is stabilized by the presence in the
ligands of methyl substituents in proximity of the nitrogen donor
atoms. The TPA ligand differs from the DPEA ligand by the
replacement of a 2-aminoethyl group by a 2-pyridylmethyl
fragment. The magnetic behavior of the [Fe(TPA)(NCS)2]
complex in the temperature range 77-300 K is characterized
by a gradual conversion from high to low spin starting
immediately below room temperature, whereas for the present
[Fe(DPEA)(NCS)2] complex there is no noticeable increase of
the low-spin state fraction down to about 170 K. This is
certainly in line with the decrease of the ligand field strength
in DPEA due to the substitution of one aliphatic nitrogen atom
for an aromatic one in the TPA ligand, resulting in a 2:2
aliphatic:aromatic ratio in the former vs 1:3 in the latter. The
much more abrupt character of the spin transition in [Fe(DPEA)-
(NCS)2] than in [Fe(TPA)(NCS)2] indicates that a higher degree
of cooperativity exists in the former. The Arrhenius plot of
the equilibrium constant (Keq ) nHS/(1 - nHS)) obtained from
Mo¨ssbauer spectroscopy for [Fe(DPEA)(NCS)2] is shown in
Figure 6. It can be clearly seen that the spin conversion is
cooperative. The reversed S shape observed is a signature of
the departure from an ideal solution behavior (linear plot).20
The data assembled in Tables 5 and 6 show that, among the six
The new spin transition complex [Fe(DPEA)(NCS)2] (1)
illustrates the interest of using ligands that combine aliphatic
and aromatic nitrogen donor atoms in their structure to design
compounds presenting spin-state transformations. Numerous
complexes with bidentate,17 tridentate,18 tetradentate,9 and
hexadentate1d,5 ligands that meet this requirement have been
described. The [Fe(DPEA)(NCS)2] complex discussed here is
the first structurally characterized representative of the family
of Fe(II) cis-diisothiocyanato complexes with tetradentate
ligands.9 The cis configuration of the NCS groups is required
to avoid the important strain that would result from three
meridionally fused chelates. Among diisothiocyanatoiron(II)
spin transition complexes only two examples of trans NCS
groups are known, one with monodentate ligands15a and another
with bidentate bridging ligands.19 Nevertheless, there is still
some strain in the [Fe(DPEA)(NCS)2] complex, as judged from
(17) (a) Renovitch, G. A.; Baker, W. A. J. Am. Chem. Soc. 1967, 89, 6377.
(b) Baker, A. T.; Goodwin, H. A. Austr. J. Chem. 1984, 37, 1157.
(18) (a) Boylan, M. J.; Nelson, S. M.; Deeney, F. A. J. Chem. Soc. A 1971,
976. (b) Goodwin, H. A.; Mather, D. W. Austr. J. Chem. 1974, 27,
965. (c) Ko¨nig, E.; Ritter, G.; Irler, W.; Goodwin, H. A. J. Am. Chem.
Soc. 1980, 102, 4681. (d) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K.;
Waigel, J.; Goodwin, H. A. Inorg. Chem. 1984, 23, 1896.
(19) (a) Vreugdenhil, W.; Haasnoot, J. G.; Kahn, O.; Thuery, P.; Reedijk,
J. J. Am. Chem. Soc. 1987, 109, 5272. (b) Vreugdenhil, W.; Diemen,
J. H.; Graaff, R. A. G. de; Haasnoot, J. G.; Reedijk, J.; Kraan, A. M.
van der; Kahn, O.; Zarembowitch, J. Polyhedron 1990, 9, 2971.
(20) Lemercier, G.; Bousseksou, A.; Seigneuric, S.; Varret, F.; Tuchagues,
J. P. Chem. Phys. Lett. 1994, 226, 289.

Spin Transition in [Fe(DPEA)(NCS)2]
Inorganic Chemistry, Vol. 36, No. 14, 1997 2981
nitrogen atoms connected to the metal center, only the aliphatic
ones N2 and N6 participate in the intermolecular interactions,
the pyridinic N3 and N4 being protected by the aromatic rings.
The hydrogen bonds involving the N6 atom, which cannot occur
in the [Fe(TPA)(NCS)2] complex, possibly represent the most
important channel responsible for the cooperativity in complex
1, others being provided by the intermolecular C‚‚‚C contacts.
We also suspect that the flexibility of the ligand amino aliphatic
chain can contribute to the cooperativity effects.
The calculated variations of enthalpy and entropy fall within
the limits ∆H ≈ 6-15 kJ mol-1 and ∆S ≈ 37-65 J K-1 mol-1
found for mononuclear iron(II) spin transition compounds from
calorimetric measurements.1e,g In spite of the absence of thermal
hysteresis, evidenced by the magnetic measurements in cooling
and heating modes, the value of the ratio Jcoop/T1/2 ) 0.9
indicates that the spin-state conversion in 1 is close to a first-
order spin-state transition for which Jcoop/T1/2 ) 1.22 It suggests
that small structural or chemical modifications of the [Fe-
(DPEA)(NCS)2] complex could lead to a first-order transition.
Studies of new systems that involve either modifications of the
polydentate ligand or replacement of the two NCS groups by
bidentate ligands are in progress.
The nHS fraction obtained from Mo¨ssbauer data could be fitted
by the mean-field equation of the two level Ising-like model7
formally equivalent to the macroscopic model4f,21 (Figure 5).
The parameter values for the best least-squares fit are ∆eff
)
835 K, Jcoop ) 124 K, and reff ) 391; Jcoop is the ferromagnetic-
like intermolecular coupling parameter (cooperativity param-
eter),7 ∆eff ) EHS - ELS is the energy splitting of two levels of
a fictious spin (1, and reff ) gHS/gLS is the ratio of effective
degeneracies of electron-vibrational levels in high-spin and
low-spin states.4f,21 With the values of the parameters reff and
Acknowledgment. We are grateful to the CNRS, De´parte-
ment des sciences chimiques, for a grant to G.M.
Supporting Information Available: A full presentation of crystal-
lographic data and experimental parameters for [Fe(DPEA)(NCS)2]
(Table S1), general displacement parameter expressions, B’s (Table
S2), positional parameters and isotropic displacement parameters for
the hydrogen atoms (Table S3), bond distances and angles (Tables S4
and S5, respectively), and least-squares planes (Table S5) (6 pages).
Ordering information is given on any current masthead page.
∆
eff and using the formal relationship between the macroscopic
(entropy and enthalpy) and microscopic parameters (∆eff and
reff), we can thus estimate the molar entropy and enthalpy
changes upon spin-state conversion as ∆S ) R ln reff ) 49 J
K-1 mol-1 and ∆H ) N∆eff ) 6.76 kJ mol-1
.
IC9615133
(21) (a) Bousseksou, A.; Nasser, J.; Linares, J.; Boukheddaden, K.; Varret,
F. Mol. Cryst. Liq. Cryst. 1993, 243, 269. (b) Bousseksou, A. Ph.D.
Thesis, Universite´ Paris 6, 1992.
(22) Wajnflasz, J.; Pick, R. J. Phys. C 1971, 32, 1.