Synthesis of mono-coordinate iron(II)-phen complex via a solid state ligand exchange process from iron(II) oxalate dihydrate at room temperature under mechanical stressing

Article abstract of DOI:10.1081/SIM-200059196

Mono-coordinate iron(II)-1,10-phenanthroline (phen) complex was obtained in a solid state ligand exchange reaction under mechanical stressing by starting from a mixture of iron(II) oxalate dihydrate (FeC₂O ₄·2H₂O) and phen, which was not available via a thermal route or from iron(II) chloride tetra hydrate (FeCl₂· 4H₂O). Copyright

Full text of DOI:10.1081/SIM-200059196

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Synthesis of Mono‐coordinate Iron(II)‐phen Complex
via a Solid State Ligand Exchange Process from Iron(II)
Oxalate Dihydrate at Room Temperature under
Mechanical Stressing
Tadashi Ohshita a , Katsumasa Hirose a , Motoo Komai a & Mamoru Senna a
a Department of Applied Chemistry, Faculty of Science and Technology , Keio University ,
Yokohama, Japan
Published online: 15 Feb 2007.
To cite this article: Tadashi Ohshita , Katsumasa Hirose , Motoo Komai & Mamoru Senna (2005) Synthesis of Mono‐coordinate
Iron(II)‐phen Complex via a Solid State Ligand Exchange Process from Iron(II) Oxalate Dihydrate at Room Temperature under
Mechanical Stressing, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 35:5, 355-358, DOI:
10.1081/SIM-200059196
To link to this article: http://dx.doi.org/10.1081/SIM-200059196
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:355–358, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 0094-5714 print/1532-2440 online
DOI: 10.1081/SIM-200059196
Synthesis of Mono-coordinate Iron(II)-phen Complex via a
Solid State Ligand Exchange Process from Iron(II) Oxalate
Dihydrate at Room Temperature under Mechanical Stressing
Tadashi Ohshita, Katsumasa Hirose, Motoo Komai, and Mamoru Senna
Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
Yokohama, Japan
mono-, di- and tri-coordination of phen to a ferrous ion in a
Mono-coordinate iron(II)-1,10-phenanthroline (phen) complex solid state. Two comparisons are made here, i.e., those
was obtained in a solid state ligand exchange reaction under
mechanical stressing by starting from a mixture of iron(II)
.
oxalate dihydrate (FeC2O4 2H2O) and phen, which was not avail-
able via a thermal route or from iron(II) chloride tetra hydrate
between the starting ferrous materials and between the
thermal and mechanochemical processes.
EXPERIMENTAL
.
(FeCl2 4H2O).
.
Powders of iron(II) oxalate dihydrate (FeC2O4 2H2O, Fe-
Ox) (Kanto Chem., .99.0%) and 1,10-phenanthroline (phen)
(Aldrich, .99.0%) were mixed in the stoichiometric ratio
Fe-Ox/phen ¼ 1/1, 1/2 or 1/3. The mixture was subjected
to mechanical stressing by grinding in a planetary ball-mill
(AGO-II, URF, modified by Nara Machinery) at 300 rpm for
3 h in a water-cooled alumina-lined cylindrical vessel
together with 10 mm zirconia balls in Ar atmosphere. We
also examined the effects of grinding Fe-Ox alone. Heating
was carried out at temperatures between 333 K and 353 K for
5 h in an Ar-sealed glass tube.
Keywords Iron(II) complex, ligand field distortion, mechanical
stressing, solid state ligand exchange, spin crossover
INTRODUCTION
In a solid state, a ligand exchange reaction exhibits a unique
´
´
feature (Fernandez-Bertran et al., 1999; Ohshita et al., 2002;
Paneque et al., 2001), particularly when a molecular crystal
is subjected to anisotropic deformation, inducing disproportio-
nation of the ligand field. We reported that the coordination
compound FeII(phen)3 was obtained from iron(II) chloride
and phen in an almost quantitative yield when we exert mech-
RESULTS
anical shear stress on the powder mixture comprising Effects of Grinding
.
FeCl2 4H2O and phen (Ohshita et al., 2002). Emphasis was
As shown in Figure 1, the intensity of the broad MLCT
absorption band at around 500 nm was significantly increased
by grinding a mixture of Fe-Ox and phen (Madeja and
laid on the role of ligand field distortion under mechanical
stress (Ohshita et al., 2002, 2004; Tsuchiya et al., 1996,
2000, 2001).
II
¨
Konig, 1963). This indicates the coordination of phen to Fe
As we extended our study to change the starting materials,
we found another peculiarity of a solid state ligand exchange
reaction. We focus here on the sequence and mechanisms of
by grinding. We have observed similar kinds of mechanochem-
ical ligand exchange reactions from different combinations,
.
e.g., FeCl2 4H2O-phen as well (Ohshita et al., 2002).
The unique feature of the system Fe-Ox-phen becomes
obvious, however, when we change the stoichiometry of the
starting mixture. Far-IR spectra, shown in Figure 2, reveals
the signal specific to the ring vibration of free phen around
244 cm21 and 411 cm21 when we start from the mixture Fe-
Ox/phen ¼ 1/2 and 1/3 in molar basis (Hutchinson et al.,
1970). We do not observe any bands attributable to di-coordi-
nated phen compounds, typically those at around 280 cm21
(Adams et al., 1982; Takemoto and Hutchinson, 1972, 1973).
The bands from free phen disappear when we start from an
Received 5 January 2005; accepted 19 February 2005.
The authors thank to Drs. H. Inoue, N. Yoshioka and T. Isobe for
valuable discussions. This work was supported by Grant-in-Aid for the
21st Century COE program “KEIO Life Conjugate Chemistry” from
the Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
Address correspondence to M. Senna, Dep. of Applied
Chemistry, Faculty of Science and Technology, Keio University,
3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan. E-mail:
senna@applc.keio.ac.jp.
355

356
T. OHSHITA ET AL.
SCH. 1. Solid state synthesis on (a) mechanochemical and (b) thermal
processes.
Figure 2, is most likely ascribed to the mono-coordinated
phen compound, although we do not find any literature
support, since the mono-coordinated phen compound was
rarely studied. Charron et al. (1983) has synthesized
Fe(phen)Cl2 by Broomhead-Dwyer synthesis. Their study
FIG. 1. UV-VIS diffuse reflectance spectra of (a) as-mixed and (b) 3 h milled
.
mixture of FeC2O4 2H2O and phen with the molar ratio 1/2.
¨
was restricted, however, to the magnetic properties by Moss-
bauer spectroscopy. Conversely, the present mechanochemical
route does not bring about the di- or tris-coordinated phen
complex even when we gave abundant amount of phen into
the starting mixture.
equimolar mixture of Fe-Ox and phen, as shown in Figure 2,
curve (d).
These spectroscopic features consistently suggest the for-
mation of a mono-coordinated phen complex in a solid-state
under mechanical stressing, i.e., via a mechanochemical
route, as shown in Scheme 1(a). A unique band at around
272 cm21 from the milled mixture, curves (b), (c) and (d) in
While the physical mixture of FeOx and phen with its stoi-
chiometry 1 to 2 is diamagnetic solely due to an intrinsic
property of FeOx (Wrobleski and Brown, 1979), paramagnetic
behavior at high spin states appeared after grinding the
mixture, as shown in Figure 3. This indicates that a tri-coordi-
nated phen complex at low spin states was not formed, and
FIG. 2. Far-infrared absorption spectra of the mixture in the molar ratio of
Fe-Ox to phen,1/2 before grinding (a), and those after grinding a mixture of
the molar ratio 1/3 (b), 1/2 (c) and 1/1 (d).
FIG. 3. Temperature dependence of xM under the external field, 500 mT, for
.
(a) as-mixed and (b) 3 h milled mixture of FeC2O4 2H2O and phen.

SYNTHESIS OF MONO-COORDINATE IRON(II)
357
unreacted FeOx was absent. We therefore suggest that the
mono-coordinated phen complex is predominantly formed
via a mechanochemical route.
Products From a Thermal Process
After heating a mixture of Fe-Ox and phen with a stoichi-
ometry 1 to 2 at temperatures at 353 K for 3 h, IR bands from
free phen at 411 cm21 disappeared, as shown in Figure 4(d).
Instead, we observe a new band at 282 cm21 to be assigned
to the ring vibration of di-coordinated phen (Adams et al.,
1982; Takemoto and Hutchinson, 1972, 1973). Thus, the
di-coordinated phen compounds, never obtained mechano-
chemically, were easily obtained by simply heating a stoichio-
metric mixture at 353 K. Note that we did not observe IR bands
at 272 cm21 attributable to the mono-coordinated phen from
any of the samples obtained by heating. This indicates that
the di-coordinated compound was formed by heating without
FIG. 5. Temperature dependence of effective magnetic moment, meff, under
halting at the stage of the mono-coordinated state, as shown the external field, 500 mT, for (a) as-mixed and (b) heated at 353 K mixture of
.
FeC2O4 2H2O and phen.
in Scheme 1(b).
We observe spin crossover behavior from the above-men-
tioned mixture after heating at 353 K, as shown in Figure 5.
This makes a contrast to the diamagnetic behavior of the
intact mixture. According to the literature (Balasubramanian,
1999), the di-coordinated phen compounds behave as spin
crossover complex compounds. Thus, formation of the di-coor-
dinated phen complex upon heating is obvious, not only from
the Far-IR spectra but also from the change in the magnetic
properties.
DISCUSSION
As shown in Figure 6(a), the binding energy of O1s orbital of
.
intact FeC2O4 2H2O peaks at 532.1 eV and 530.3 eV with its
intensity ratio being about 2 to 1. The peak at the higher
binding energy is attributed to the oxygen atom in the
oxalate ion and the lower one to that of H2O.
Upon grinding Fe-Ox for 3 h, the lower energy peak
decreased its intensity to 11% from ca 33% of the intact
state, with a simultaneous chemical shift to the lower energy
side by 0.8 eV. This implies the dehydration with simultaneous
weakening of the FeII- H2O coordination bonds as a
FIG. 4. Far-infrared absorption spectra of the mixture in the molar ratio of
Fe-Ox to phen,1/2 before grinding (a), and after heating at 343 K (b), 348 K
(c), 353 K (d) for 5 h.
FIG. 6. X-ray photoelectron spectra of O1s orbital of (a) intact and (b)
.
3 h-milled FeC2O4 2H2O.

358
T. OHSHITA ET AL.
One-dimensional Ferromagnetic Exchange in Fe(phen)Cl2. Inorg.
Chim. Acta. 1983, 68, 233–236.
consequence of grinding, while keeping the Fe-Ox bond
strength eventually unchanged (Avvakumov et al., 1994).
This serves as evidence of preferential substitution of H2O
by phen upon grinding the mixture of Fe-Ox and phen.
This kind of anisotropic change in the bond strength is less
likely by heating. Based on the scale of the spectrochemical
series, the bond strengths of Fe-O(H2O) and Fe-O(Ox22) are
quite similar and weaker than that of phen (Huheey et al.,
1993). This results in the formation of di-coordination
complex, being thermodynamically more stable, by heating
the 1:2 molar ratio mixture.
´
´
Fernandez-Bertran, J.; Castellanos-Serra, L.; Yee-Madeira, H.;
Reguera, E. Proton Transfer in Solid State: Mechanochemical
Reactions of Imidazole with Metallic Oxides. J. Solid State
Chem. 1999, 147 (2), 561–564.
Hutchinson, B.; Takemoto, J.; Nakamoto, K. Metal Isotope Effect on
Metal-ligand Vibrations. II. Tris Complexes of 2,20-Bipyridine
and 1,10-Phenanthroline. J. Am. Chem. Soc. 1970, 92 (11),
3335–3339.
Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Prin-
ciples of Structure and Reactivity, 4th Ed.; Harper Collins College
Publishers: New York, 1993; p. 405.
We previously reported the formation of the tri-coordination
phen complex upon grinding
¨
¨
Madeja, K.; Konig, E. Zur Frage der Bindungsverhaltnisse in Kom-
plexverbindungen des Eisen(II) mit 1,10-Phnanthrolin. J. Nucl.
Chem. 1963, 25, 377–385.
a
mixture comprising
.
.
FeCl2 4H2O and phen. In FeCl2 4H2O, there is no such an
anisotropy. Note that the anisotropy in Fe-Ox is attributed to
the existence of the 2-D coplanar networking of Fe-Ox bonds
(Wrobleski and Brown, 1979).
Ohshita, T.; Nakajima, D.; Tsukamoto, A.; Tsuchiya, N.; Isobe, T.;
Senna, M.; Yoshioka, N.; Inoue, H. Role of Molecular Strain on
The Solid-State Synthesis of Coordination Compounds from
Iron(II) Chloride Tetrahydrate and 1,10-Phenanthroline under
Mechanical Stress. Ann. Chim. Sci. Mat. 2002, 27 (6), 91–101.
Ohshita, T.; Tsukamoto, A.; Senna, M. Change in The Magnetic Prop-
erties of [FeII(phen)3](PF6)2 in the Solid State by Combining
Grinding and Annealing. Phys. Stat. Sol. 2004, 201 (4), 762–768.
CONCLUSION
The mono-coordinated phen complex was formed from the
.
powdered mixture of FeC2O4 2H2O and 1,10-phenanthroline
only by grinding, even when we started from the mixture in
excess stoichiometry of phen. This makes a contrast with our
previous observation to form the tri-coordinated phen
´
´
Paneque, A.; Fernandez-Bertran, J.; Reguera, E.; Yee-Madeira, H.
Mechanochemical Synthesis of Hemin-Imidazole Complexes.
Trans. Met. Chem. 2001, 26 (1–2), 76–80.
.
complex from FeCl2 4H2O and phen. The 2-D network struc-
Takemoto, J.; Hutchinson, B. Effect of Magnetic Crossover on The
Low-frequency IR Spectrum of [Fe(1,10-Phenanthroline)2
(NCS)2]. Inorg. Nucl. Chem. Lett. 1972, 8 (9), 769–772.
Takemoto, J.; Hutchinson, B. Low-frequency Infrared Spectra of
Complexes which Exhibit Magnetic Crossover. I. Iron(II) Com-
plexes of 1,l0-Phenanthroline and 2,20-Bipyridine. Inorg. Chem.
1973, 12 (3), 705–708.
ture, due to chelate bridging bonds, causes anisotropy in crys-
tallographical and mechanical properties of the oxalate. The
anisotropy, in turn, results in the unusual preference to the
ligand exchange of phen with H2O under mechanical stressing.
This explains the unique feature of the mono-coordinated phen
complex formation by grinding. This kind of anisotropy-
induced phenomenon does not occur in a conventional
thermal process.
Tsuchiya, N.; Isobe, T.; Senna, M.; Yoshioka, N.; Inoue, H. Mechan-
ochemical Effects on The Structures and Chemical States of
.
[Fe(phen)3](NCS)2 H2O. Solid State Commun. 1996, 99 (8),
525–529.
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