Ferromagnetic interactions between triphenylmethyl radicals through an organometallic coupler

Article abstract of DOI:10.1039/a900371a

A diradical consisting of two polychlorinated triphenylmethyl radical units connected by a 1,1'-metallocenylendivinylene bridge has been synthesized and characterized; EPR frozen solution experiments down to liquid helium temperature showed that the organ

Full text of DOI:10.1039/a900371a

Ferromagnetic interactions between triphenylmethyl radicals through an
organometallic coupler
O. Elsner, D. Ruiz-Molina, J. Vidal-Gancedo, C. Rovira* and J. Veciana*
Institut de Ciència dels Materials de Barcelona (CSIC), Campus Universitari de Bellaterra, 08193, Cerdanyola,
Spain. E-mail: vecianaj@icmab.es; cun@icmab.es
Received (in Cambridge, UK) 13th January 1999, Accepted 22nd February 1999
A diradical consisting of two polychlorinated triphenylme-
thyl radical units connected by a 1,1A-metallocenylendiviny-
lene bridge has been synthesized and characterized; EPR
frozen solution experiments down to liquid helium tem-
perature showed that the organometallic unit acts as a
ferromagnetic coupler.
resulting yellow ylide suspension was stirred for ca. 1 h and
after this time ferrocene 1,1A-biscarbaldehyde was added and
stirred for a further 72 h. The reaction mixture was quenched
with HCl and extracted with CHCl3 to give 1,1A-bis{4-
[bis(pentachlorophenyl)methyl]-2,3,5,6-tetrachlorostyryl}fer-
rocene 4 after chromatographic purification from n-hexane–
CHCl3 (1:1). Finally, a THF solution of 4 was treated with an
excess of tetra(n-butyl)ammonium hydroxide. The solution,
which immediately turned purple, was stirred at room tem-
perature for 4 h. Subsequent oxidation of the resulting dianion
with p-chloranil yielded the 4,4A-(1,1A-ferrocenylenedivinyl-
ene)di[a,a-bis(pentachlorophenyl)-2,3,5,6-tetrachlorobenzyl]
diradical 1, which was isolated as a clathrate (1·2C6H6) after
chromatographic purification (n-hexane–CCl4, 1:1) and re-
crystallization from C6H6. This diradical is completely stable in
air both in the solid state and in dilute solutions. The related
monoradical derivative, the 4-(ferrocenylvinylene)-a,a-bis-
(pentachlorophenyl)-2,3,5,6-tetrachlorobenzyl radical 2 was
prepared following the same procedure, from the phosphonium
bromide derivative 3 and ferrocene carbaldehyde. Both radicals
were obtained as dark green microcrystals and were charac-
terized by elemental analysis, cyclic voltammetry, IR, UV–VIS
and EPR spectroscopies.
In the last few years, the development and study of new
coupling units that promote ferromagnetic interactions between
pure organic radicals has been a subject of great interest.1
Metallocenes are excellent candidates to be used as magnetic
couplers not only because of their rich chemistry but also
because they are electroactive species whose oxidation state can
be controlled by means of a chemical or electrochemical
stimulus; moreover their oxidized states are of open-shell
character. However, although such complexes have been
successfully used as building blocks of molecular solids
promoting intermolecular magnetic exchange interactions,2
their use as intramolecular magnetic couplers is new. Recently,
we have reported a novel family of compounds consisting of
two purely organic a-nitronyl aminoxyl radicals connected by
different 1,1A-metallocenylene bridges where the metallocene
units were shown to act as effective magnetic couplers that
transmit the magnetic interactions through their skeletons.3
However, the small spin density located on the metallocene
units of these systems and the presence of intramolecular
hydrogen bonds, which determines the existence of a direct
intramolecular through-space magnetic interaction, lead to the
appearance of an effective antiferromagnetic interaction be-
tween the two a-nitronyl aminoxyl radical units that is very
sensitive to the molecular conformation.4 For this reason, such
complexes were not suitable candidates to study and rationalize
the behaviour of 1,1A-metallocenylene bridges as magnetic
couplers.
O
Cl
Cl
+
–
Br
+
Fe
(C6Cl5)2CH
CH2PPh2
O
Cl
Cl
3
KBuOt
Cl
Cl
CH(C6Cl5)2
In order to overcome both these factors, diradical 1,
consisting of two polychlorinated triphenylmethyl radicals
connected by a 1,1A-ferrocenylendivinylene bridge, was de-
signed. The particular structure and topology of such a diradical
leads to expectation of a non-negligible spin density on the
ferrocene moiety making feasible the magnetic coupling
between the two organic radical units.5 In addition, the location
of both radical units far away from each other avoids any
possibility of having intramolecular contacts, and consequently,
a significant direct through-space magnetic interaction. Here we
report the synthesis and characterization of diradical 1. The
related monoradical 2 has also been synthesized and studied for
comparison purposes.
As outlined in Scheme 1, the synthetic route for preparing
these radical derivatives is based on two main steps. First, a
Wittig reaction between the phosphonium bromide precursor 3
and the corresponding ferrocene carbaldehyde derivative; i.e.
ferrocene 1,1A-biscarbaldehyde and ferrocene monocarbalde-
hyde for radicals 1 and 2, respectively. Second, a subsequent
deprotonation and oxidation to yield the desired radicals. In the
first step of the synthesis of 1, the substituted phosphonium
bromide, a,a-bis(pentachlorophenyl)-2,3,5,6-tetrachloro-aA-
(triphenylphosphonium)-p-xylene bromide 3, was suspended in
dry THF and treated with an equimolar amount of KOBut. The
Cl
Cl
Cl
Cl
Cl
Fe
(C6Cl5)2HC
Cl
4
1
2
Bun4NOH
p-chloranil
Cl
Cl
Cl
•
C(C6Cl5)2
Cl
Cl
Cl
Cl
Cl
Fe
•
(C6Cl5)2C
1
Cl
Cl
•
C(C6Cl5)2
Cl
Cl
Fe
2
Scheme 1
Chem. Commun., 1999, 579–580
579

Fig. 2 Temperature dependence of IppT of diradical 1. The closed circles
represent the experimental data and the continuous line the fit of
experimental data to the Bleaney–Bowers equation.
triplet species and appeared to be symmetrical, indicating that
this complex has a low (if any) anisotropy. The forbidden Dms
= ±2 transition characteristic of a triplet species, was also
observed at the half-field region of the spectrum and the
intensity of the corresponding signal (Ipp), obtained by double
integration, was measured in the range 4–100 K. The reproduci-
bility of the results was confirmed by two independent
experiments. Since the quantity IppT is proportional to the
population in the triplet state, the fact that IppT (Fig. 2) increases
with decreasing temperature indicates that the ground state of 1
is the triplet state and the singlet state should be regarded as a
thermally accessible excited state.7 A separation of +10 ±2 K (7
cm21) between both states was obtained from the fitting of the
data in Fig. 2 to a Bleaney–Bowers equation.8
In conclusion, we have shown that 1,1A-ferrocenylene bridges
can act as ferromagnetic couplers when radical units with a
suitable topology are connected to them. This concept can be
extended to the synthesis of novel metallocene complexes
bearing other organic and inorganic units providing a valuable
access to this interesting class of materials.
Fig. 1 Cyclic voltammograms recorded in a CH2Cl2 solution containing
NBu4PF6 (0.l M) of (a) monoradical 2 and (b) diradical 1.
The cyclic voltammetric response of 2 shows one oxidation
and one reduction process. The reversible process at +587 mV
arises from the oxidation of the ferrocene unit while the
reversible reduction process occurring at 2177 mV is asso-
ciated with the reduction of the triphenylmethyl radical unit.
Cyclic voltammetry of diradical 1 (Fig. 1) shows one reversible
oxidation process at +666 mV and one reversible reduction
process at 2181 mV that involves the simultaneous transfer of
two electrons. The oxidation process was assigned, as in the
monoradical species, to the oxidation of the ferrocene unit,
while the reduction process was assigned to the reduction of
both triphenylmethyl radical units. The fact that the oxidation
process of the ferrocene unit of diradical 1 appears at a higher
potential value than those observed for monoradical 2 and the
unsubstituted ferrocene is the first direct evidence for the
presence of an electronic interaction between the radical and the
ferrocene units. Nevertheless, EPR spectroscopy provides more
detailed and definitive information about the electronic struc-
ture as well as of the intramolecular electron–electron inter-
actions in these compounds. X-Band EPR isotropic spectra of
radicals 1 and 2 were obtained in toluene–CH2Cl2 (1:1). The
spectra of both complexes at room temperature showed lines
corresponding to the coupling of the unpaired electrons with the
different nuclei with non-zero magnetic moments; i.e. with 1H
and naturally abundant 13C isotope at the a and aromatic
positions. Computer simulation gave the isotropic g-values
(giso) and the isotropic hyperfine coupling constants (ai) of the
unpaired electrons with the different nuclei with non-zero
magnetic moments. The giso values for diradical 1 and
monoradical 2 were 2.0028 and 2.0033, respectively which are
very close to that observed for other polychlorotriphenylmethyl
radicals.6 More interesting is the comparison of the isotropic
hyperfine coupling constant values with the hydrogen atoms of
the ethylene moieties and some of the carbon nuclei of the
triphenylmethyl unit. The values of the coupling constant of
diradical 1, a1(1H) ≈ 0.80 G (2H), a2(1H) @ 0.30 G (2H) and
a(13C) ≈ 13.0 G (1 Ca) are approximately half those found for
monoradical 2, a1(11H) ≈ 1.77 G (1H), a2(11H) ≈ 0.57 G (1H)
and a(13C) ≈ 29.0 G (1 Ca) . It is then possible to conclude that
the two electrons in diradical 1 are interacting with a magnetic
exchange coupling constant, J, that fulfills J > > ai. It is also
worth noting that the coupling constants of diradical 1 are not
exactly half of those observed for the monoradical 2 when
measured under identical experimental conditions, suggesting
that both molecules have small differences in the conformations
of their bridges. The spectrum of diradical 1 in frozen toluene–
CH2Cl2 (1:1) showed the characteristic fine structure of a
This work was supported by grant from DGES (project
PB96-0802-C02-01), CIRIT (project SGR 96-00106) and the
3MD Network of the TMR program of the E.U. (contract
ERBFMRX CT980181).
Notes and references
1 D. Gatteschi, O. Kahn, J. S. Miller and F. Palacio, Molecular Magnetic
Materials, Kluwer Academic, Dordrecht, 1991; O. Kahn, Molecular
Magnetism, VCH Publishers, Weinheim, 1993; A. Rajca, Chem. Rev.,
1994, 94, 871; H. Iwamura and N. Koga, Acc. Chem. Res., 1993, 26,
346.
2 J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl., 1994, 33, 385
and references therein.
3 O. Jürgens, J. Vidal-Gancedo, C. Rovira, K. Wurst, C. Sporer, B.
Bildstein, H. Schottenberger, P. Jaitner and J. Veciana, Inorg. Chem.,
1998, 37, 4547.
4 The spin density on the metallocene units linked to the a-carbon atom of
a-nitronyl aminoxyl radicals is very small because the SOMO orbital has
a node on this carbon atom and therefore the spin density is transmitted
only by a spin polarization mechanism.
5 By contrast, for diradical 1 both unpaired electrons can be delocalized by
conjugation onto the ferrocene unit according to the particular topology
of the diradical promoting a larger magnetic coupling.
6 O. Armet, J. Veciana, C. Rovira, J. Riera, J. Castañer, E. Molins, J. Rius,
C. Miravitlles, S. Olivella and J. Brichfeus, J. Phys. Chem., 1987, 91,
5608.
7 To obtain accurate temperature measurements that ensure the validity of
the experimental results, the spectrometer was equipped both with a
flowing-helium Oxford ESR-900 cryostat (4.2–100 K), controlled by an
Oxford ITC4 temperature control unit, and with a calibrated custom-
made double temperature control system for determining accurately the
sample temperature. Additional precautions to avoid undesirable satura-
tion effects and spectral line broadening were also taken.
8 R. L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1986, p. 71.
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Chem. Commun., 1999, 579–580