A two-step spin crossover in [(TPA)Fe(III)(cat)]BPh4

Article abstract of DOI:10.1002/(SICI)1521-3773(20000103)39:1<196::AID-ANIE196>3.0.CO;2-W

Measurement of magnetic susceptibility with temperature of a powdered iron(III) catecholate complex clearly indicated a two-step spin-crossover process S = 1/2 mutually implies S = 5/2. The picture shows the plot of χ(M)T against temperature.

Full text of DOI:10.1002/(SICI)1521-3773(20000103)39:1<196::AID-ANIE196>3.0.CO;2-W

COMMUNICATIONS
catecholate dianion) prepared first by Que et al.[2] The latter,
in the presence of O2, gives a nice example of intradiol
dioxygenase activity with cleavage of the intradiol C C
bond.[2a] As a routine check of the spin state, we measured the
magnetic susceptibility of 1 as a function of temperature and
discovered that it exhibits a spin crossover,[3] and moreover
one in two steps. We report here the preliminary study of this
unusual phenomenon.
[6] a) R. G. Chapman, J. C. Sherman, J. Am. Chem. Soc. 1995, 117, 9081 ±
9082; b) R. G. Chapman, G. Olovsson, J. Trotter, J. C. Sherman, J. Am.
Chem. Soc. 1998, 120, 6252 ± 6260.
[7] The value 0.9 is d(pyrazine in 3 ´ pyrazine ´ MeOAc) d(pyrazine in
3 ´ 2pyrazine). The value ‡0.19 is d(CH3COCH3 in 3 ´ pyrazine ´
MeOAc) d(CH3COCH3 in 3 ´ 2MeOAc). The value ‡0.16 is
d(CH3COCH3 in 3 ´ pyrazine ´ MeOAc) d(CH3COCH3 in 3 ´
2MeOAc).
[8] For encapsulation of two or more guest molecules in a single large
chamber, see a) F. C. Tucci, D. M. Rudkevich, J. Rebek, Jr., J. Am.
Chem. Soc. 1999, 121, 4928 ± 4929; b) R. Meissner, X. Garcias, S.
Mecozzi, J. Rebek, Jr., J. Am. Chem. Soc. 1997, 119, 77 ± 85; c) T.
Heinz, D. Rudkevich, J. Rebek, Jr., Nature 1998, 394, 764 ± 766; d) J.
Kang, J. Rebek, Jr., Nature 1997, 385, 50 ± 52; e) J. Kang, J. Santama-
ria, G. Hilmersson, J. Rebek, Jr., J. Am. Chem. Soc. 1998, 120, 7389 ±
7390; f) N. Chopra, J. C. Sherman, Angew. Chem. 1999, 111, 2109 ±
2111; Angew. Chem. Int. Ed. 1999, 38, 1955 ± 1957.
Elemental analysis confirmed that complex 1 is consistent
with the formula [(TPA)Fe(cat)]BPh4 and is unsolvated. The
UV/Vis spectrum of 1 in acetonitrile/DMF (9/1) contains two
broad ligand ± metal charge transfer (LCMT) bands with
1
maxima
at
502 nm
(2680m 1 cm )
and
808 nm
(3890m 1 cm ), similar to those observed in other iron(iii)-
catecholato complexes.[2b] It may be inferred from this result
that the cat ligand chelates the iron(iii) ion in an analogous
manner to DBC in 2.[2a] The structure we propose for 1 is
represented in Figure 1.
1
[9] a) R. G. Chapman, N. Chopra, E. D. Cochien, J. C. Sherman, J. Am.
Chem. Soc. 1994, 116, 369 ± 370; b) R. G. Chapman, J. C. Sherman, J.
Org. Chem. 1998, 63, 4103 ± 4110.
1
[10] The H NMR spectra of 1 and DBU in CHCl3/CDCl3 (1/1) at 298 K
gave signals for encapsulated CHCl3 (d ˆ 5.04); thus the free species is
3 ´ 2CHCl3. Upon addition of MeOAc, a new signal for encapsulated
CHCl3 (d ˆ 5.36) appears, in addition to the new signals of encapsu-
lated MeOAc and host; integration yields 3 ´ MeOAc ´ CHCl3. Coop-
erativity appears at first to be lost in CDCl3, but this is because the
second capsule is actually filled with CDCl3, which is ºinvisibleº in the
1H NMR spectra. See the Supporting Information for plots of [3 ´
2CDCl3], [3 ´ MeOAc ´ CDCl3], and [3 ´ 2MeOAc] as a function of
[MeOAc]initial
.
[11] The formation of all complexes reported here is reversible; equili-
brium is reached in minutes in [D5]nitrobenzene at 373 K and in 24 h
in CDCl3 at 298 K. For calculations of Krel, [1]initial ˆ 1.61mm; [Me-
OAc]initial ˆ 32.2mm; [CDCl3]ˆ 12.5m; [3´CDCl3 ´MeOAc]ˆ 0.525mm;
[3 ´ 2MeOAc] ˆ 0.972mm; [3 ´ 2CDCl3] ˆ 0.113mm; [MeOAc]free
ˆ
29.7mm. Krel ˆ [cap ´ MeOAc][CDCl3]/[cap ´ CDCl3][MeOAc], where
cap ˆ any capsule; for example, [cap ´ MeOAc] ˆ [3 ´ CDCl3 ´
MeOAc] ‡ 2[3 ´ 2MeOAc). See the Supporting Information for the
derivation of this equation.
[12] R. G. Chapman, J. C. Sherman, J. Am. Chem. Soc. 1998, 120, 9818 ±
9826.
[13] R. G. Chapman, J. C. Sherman, J. Am. Chem. Soc. 1999, 121, 1962 ±
1963.
‡
Figure 1. Proposed structure for the [(TPA)Fe(cat)] ion.
A Two-Step Spin Crossover in
[(TPA)FeIII(cat)]BPh4**
The temperature dependence of cMT (cM ˆ molar magnetic
susceptibility) of a microcrystalline sample of 1 (Figure 2)
provides evidence for a spin crossover between S ˆ 1ꢁ2 (low
spin) and S ˆ 5ꢁ2 (high spin) states of the iron(iii) ion.[4, 5] The
expected value of cMT for the high-spin iron(iii) ion is
4.375 cm3 mol 1 K. As the room temperature value of cMT is
4.14 cm3 mol 1 K, it is expected that most of the iron(iii) ions
are in the S ˆ 5ꢁ2 state. From the data it can be estimated that
about 3% of the molecules are in the low-spin state at room
temperature. The value of cMT is 0.49 cm3 mol 1 K at 45 K and
stays nearly independent of temperature down to 5 K. This
value is characteristic of low-spin iron(iii) ions and hence of a
complete spin conversion at low temperature. These data
recorded at decreasing and increasing temperatures did not
show any hysteresis effect. Between 45 and 175 K, the cMT
versus T curve reveals that the spin-crossover phenomenon
takes place in two distinct steps centered at 79 and 106 K
(Figure 2, inset). A rather abrupt transformation is observed
between 45 and 94 K (80% of the conversion occur within
Á
A. Jalila Simaan, Marie-Laure Boillot,* Eric Riviere,
Alain Boussac, and Jean-Jacques Girerd*
In our studies of mimics for intradiol catechol dioxyge-
nase[1] we synthesized [(TPA)Fe(cat)]BPh4 (1, TPA ˆ tris(2-
pyridylmethyl)amine, cat ˆ catecholate dianion), a close ana-
logue of [(TPA)Fe(DBC)]BPh4 (2, DBC ˆ 3,5-di-tert-butyl-
Á
[*] Dr. M.-L. Boillot, Prof. J.-J. Girerd, A. J. Simaan, Dr. E. Riviere
Laboratoire de Chimie Inorganique, UMR CNRS 8613
Â
Universite Paris-Sud, 91405 Orsay (France)
Fax : (‡ 33)1-69-15-47-54
E-mail: mboillot@icmo.u-psud.fr, jjgirerd@icmo.u-psud.fr
Dr. A. Boussac
Â
Â
Section de Bioenergetique, URA CNRS 2096
CEA Saclay, 91191 Gif-sur-Yvette (France)
[**] Financial supports from the research fund TMR of the European
Union (Contracts ERB-FMRX-CT 98-0199) and from the CNRS are
gratefully acknowledged.
196
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0196 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2000, 39, No. 1

COMMUNICATIONS
Figure 3. X-band EPR spectra of a powder sample of 1 as a function of
temperature (2.2 ± 100 K). A) Low-field region: gain ˆ 104, frequency
modulation ˆ 100 kHz, amplitude modulation ˆ 9.466 G, microwave pow-
erˆ 2  101 mW; B) high-field region: gain ˆ 103, frequency modulation ˆ
100 kHz, amplitude modulation 4.744 G, microwave powerˆ 5.02 Â
10 3 mW.
Figure 2. Variation of cMT with temperature for 1. Inset: The first
derivative of the cMT versus T curve.
amount only and hence could not be detected with magnetic
measurements.
The magnetic susceptibility of 2, recorded as a function of
the temperature, shows a very smooth spin-crossover proc-
ess.[9] This result could be due to the bulkiness of the tert-butyl
groups which would reduce the cooperativity by keeping
molecules separated.
We are now studying this two-step spin-crossover process of
1 by other spectroscopic methods. In particular, we are
interested in the analysis of the intense LMCT bands
throughout the temperature range of the spin crossover.
Chemical variations of this iron(iii) catecholate complex will
be explored in order to modulate features of the associated
spin-crossover process.
an approximate 27 K range). Around 94 K (cMTˆ
2.32 cm3 mol 1 K), the transition curve presents an intermedi-
ate inflexion point corresponding to a high-spin fraction of
about 48%. The spin crossover at higher temperatures is
smoother. The fact that this magnetic behavior was found with
different solid samples originating from different preparations
allows us to discard the hypothesis of a 1:1 mixture of two
crystalline phases exhibiting spin-crossover processes at
different temperatures.
Two-step spin-crossover processes have been reported only
in a few scarce iron(ii) and iron(iii) complexes.[6, 7] One
possible origin of this peculiar behavior is the existence of
two nonequivalent sites in the unit cell. For some of these
compounds[6b, c, 7] the crystallographic structures at different
temperatures substantiated this explanation. The complexes
in the two local environments give rise to different spin-
crossover processes. To address the origin of the magnetic
behavior reported here, efforts are now directed to obtain
crystals of 1 suitable for structural determination by X-ray
diffraction.
The EPR spectra of 1 in the solid state have been recorded
as a function of temperature (Figure 3). At 2.2 K the spectrum
(gk ˆ 1.91, g? ˆ 2.18) is characteristic of a low-spin axial
iron(iii) species.[4b, 8] This result demonstrates that the low-
spin phase corresponds to a low-spin iron(iii) catecholate
complex. A possible low-spin iron(ii) semiquinonate complex
is ruled out because no radical-type signal was observed. At
higher temperatures, a signal appears at low field while the
low-spin signal disappears. It can be inferred from the
magnetic data that this low-field signal, broadened by spin ±
spin interactions, is due to the high-spin fraction. The
breadth of the signal forbids the evaluation of the zero-
field splitting parameters. At low field and low temper-
atures, weak resonances are detected that we assigned
to high-spin ferric impurities. Under the EPR experi-
mental conditions the signals from these impurities are
amplified, which means that they are present in a small
Experimental Section
TPA was prepared following the described procedure[10] but extracted with
toluene and recrystallized from n-hexane rather than precipitated with
HClO4.
The synthesis of 1 was performed under an argon atmosphere. Catechol
(56 mg, 0.50 mmol) was dissolved in CH3OH (5 mL) and deprotonated by
Et3N (139 mL, 1 mmol); the solution was added to a mixture of FeCl3
(80 mg, 0.49 mmol) and TPA (150 mg, 0.51 mmol) in methanol (15 mL). A
dark blue, microcrystalline powder was precipitated by adding NaBPh4
(170 mg, 0.49 mmol) in methanol and collected by filtration (yield 55%).
Elemental analysis: C42H37BFeN4O2 (%): calcd: C 74.49, H 5.47, B 1.42, Fe
7.23, N 7.24; found: C 74.19, H 5.20, B 1.40, Fe 7.15, N 7.39.
Magnetic susceptibility measurements were carried out with a Quantum
Design SQUID magnetometer (Model MPMS5S) calibrated against a
palladium standard. The independence of the susceptibility value with
regard to the applied magnetic field was checked at room temperature.
EPR spectra were recorded at several temperatures with a Bruker ESP300
X-band spectrometer equipped with a Bruker ESR910 cryostat.
Received: July 21, 1999 [Z13762]
Â
Á
[1] a) P. Mialane, E. Anxolabehere-Mallart, G. Blondin, A. Nivorojkine,
J. Guilhem, L. Tchertanova, M. Cesario, N. Ravi, E. Bominaar, J. J.
Girerd, E. Münck, Inorg. Chim. Acta 1997, 263, 367; b) P. Mialane, L.
Tchertanova, F. Banse, J. Sainton, J. J. Girerd, Inorg. Chem., in press;
c) P. Mialane, PhD thesis, University of Paris-XI-Orsay (France), 1997.
[2] a) H. G. Jang, D. D. Cox, L. Que, Jr, J. Am. Chem. Soc. 1991, 113, 9200;
b) D. D. Cox, S. J. Benkovic, L. M. Bloom, F. C. Bradley, M. J. Nelson,
L. Que, Jr, D. E. Wallick, J. Am. Chem. Soc. 1988, 110, 2026.
Angew. Chem. Int. Ed. 2000, 39, No. 1
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0197 $ 17.50+.50/0
197

COMMUNICATIONS
[3] P. Gütlich, A. Hauser, H. Spiering, Angew. Chem. 1994, 106, 2109;
Angew. Chem. Int. Ed. Engl. 1994, 33, 2024.
[4] a) M. F. Tweedle, L. J. Wilson, J. Am. Chem. Soc. 1976, 98, 16, 4824;
b) M. D. Timken, D. N. Hendrickson, E. Sinn, Inorg. Chem. 1985, 24,
3947.
only in anionic but also in electrically neutral platinum(ii)
complexes can the platinum atom act as a hydrogen-bond
acceptor.
Two approaches of a water molecule to 1 or 2 along the
zaxis (defined as the normal to the platinum coordination
plane through the Pt atom) were considered: approach I with
the O atom oriented toward Pt and approach II with the O H
vector directed toward Pt, as shown for complex 2 in Figure 1.
[5] W. O. Koch, V. Schünemann, M. Gerdan, A. X. Trautwein, H. J.
Krüger, Chem. Eur. J. 1998, 4, 686.
[6] a) C. P. Köhler, R. Jakobi, E. Meissner, L. Wiehl, H. Spiering, P.
Gütlich, J. Phys. Chem. Solids 1990, 51, 239; b) D. Boinnard, A.
Bousseksou, A. Dworking, J. M. Savariault, F. Varret, J. P. Tuchagues,
Inorg. Chem. 1994, 33, 271; c) Y. Garcia, PhD thesis, University of
Bordeaux I (France), 1999; d) J. A. Real, H. Bolvin, A. Bousseksou,
A. Dworkin, O. Kahn, F. Varret, J. Zarembowitch, J. Am. Chem. Soc.
1992, 114, 4650.
[7] V. V. Zelentsov, Sov. Sci. Rev. Sect. B. 1987, 10, 485.
[8] C. Kim, K. Chen, J. Kim, L. Que, Jr, J. Am. Chem. Soc. 1997, 119, 5964.
[9] A. J. Simaan, unpublished results.
[10] B. G. Gafford, R. A. Holwerda, Inorg. Chem. 1989, 28, 60.
Figure 1. Approaches of H2O, using complex 2 as an example. Approach I:
O atom directed toward Pt; approach II with O H vector directed toward Pt.
O H ´´´ PtII: Hydrogen Bond with a Strong
Dispersion Component**
The interaction energies at the Hartree ± Fock (EHF) and at
the MP2 (EMP2) levels were evaluated as the difference
between the total energy of the two interacting species and
the sum of the total energies of the individual molecules,
corrected for the basis set superposition error. In addition, an
estimate of the electrostatic component (EES) was obtained as
the sum of Coulomb terms qiqj/rij between atomic charges
determined by optimizing the fit to the MP2 electrostatic
potential of the isolated molecules. To an approximation, the
interaction energy EMP2 can be considered to be the sum of the
electrostatic (EES), exchange-repulsion (EEX), polarization
Ï
Á
Jirí Kozelka,* Jacqueline Berges, Roger Attias, and
Jann Fraitag
In the past decade, crystallographic and spectroscopic
evidence has accumulated showing that d8 metal ions such
as platinum(ii) are capable of acting as hydrogen-bond
acceptors.[1, 2] One may ask whether water could undergo
similar hydrogen bond like interactions with solvated plati-
num complexes. We report here an ab initio study, at the
Mùller-Plesset second-order perturbation (MP2) level, of the
interaction between a water molecule and two platinum(ii)
complexes [Pt(NH3)4]2‡ (1) and trans-[Pt(OH)2(NH3)2] (2).
Our calculations indicate that a linear HO H ´´´ Pt interaction
is stabilized by a strong dispersion component. In the case of
(EPOL), and charge-transfer (ECT) components, plus the
[4±6]
contribution of electron correlation effects ECORR
(Eq. 1).
EMP2 ˆ EES‡EEX‡EPOL‡ECT‡ECORR
(1)
2‡
the HO H ´´´ Pt(NH3)4 interaction, the dispersion compo-
It is common practice to associate the interaction energy
nent considerably reduces the electrostatic repulsion, while
for the HO H ´´´ Pt(OH)2(NH3)2 approach, it adds to the
electrostatic attraction, giving rise to a hydrogen bond like
determined at the Hartree ± Fock level EHF with the sum
[5]
EES‡EEX‡EPOL‡ECT
.
The difference EMP2 EHF can be
therefore used as an estimate for ECORR. In Figure 2, the
interaction energies EMP2, EHF, and EES are plotted against the
Pt ´´´ O separation, together with the differences EMP2 EHF
1
interaction with a dissociation energy of about 4 kcalmol . It
is suggested for the previously reported complex cis-
[PtBr2(gly-N)2] ´ H2O (gly ˆ NH2CH2COOH) that a similar
hydrogen bond between a water molecule and an uncharged
platinum central atom occurs in the solid state. Therefore, not
and EHF EES
.
Approach I (Figure 2a, b): In approach I, both EHF and
EMP2 curves quite closely follow the EES curve at long and
intermediate distances, indicating that the interaction is
mainly determined by electrostatic (long-range) and ex-
change-repulsion (short-range) energy. For the approach I/1
[*] Dr. J. Kozelka, Dr. R. Attias, J. Fraitag
Laboratoire de Chimie et Biochimie Pharmacologiques
et Toxicologiques, UMR CNRS 8601
the electrostatic energy is negative, giving rise to a profound
Á
45, rue des Saints-Peres, 75270 Paris (France)
1
(
11 kcalmol ) energy minimum at a Pt ´´´ O distance of
Fax: (‡33)1-42-86-83-87
about 3.3 Š, whereas approach I/2 is purely repulsive. Agree-
ment with the deep minimum for approach I/1 is found in the
Cambridge Crystallographic Data Centre (CCDC), which
contains several entries of dicationic platinum(ii) tetraamine
complexes with axial water ligands at distances between 3 and
3.5 Š. The structure of [Pt(py)4]Cl2 ´ 3H2O (py ˆ pyridine),
determined by neutron diffraction,[7] (Figure 3) for example,
features a square-planar PtN4 coordination, which is extended
E-mail: kozelka@biomedicale.univ-paris5.fr
Á
Dr. J. Berges
Â
Laboratoire de Chimie Theorique
Â
Universite Pierre et Marie Curie
4, place Jussieu, 75005 Paris (France)
[**] We are indebted to Drs. J. Langlet, P. Pyykkö, A. Pullman, J. Caillet,
and C. Giessner-Prettre for stimulating discussions and helpful
comments. Computer time from the IDRIS center of CNRS is
gratefully acknowledged.
198
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0198 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2000, 39, No. 1