The importance of a β-β bond for long-range antiferromagnetic coupling in directly linked copper(II) and silver(II) diporphyrins

Article abstract of DOI:10.1002/anie.200501943

(Figure Presented) Interaction over a distance: Long-range antiferromagnetic interactions between distant copper(II) and silver(II) ions are observed only for triply and singly linked diporphyrins with at least one β-β bond, which underscores the critical importance of this direct bond for antiferromagnetic coupling. These results can be qualitatively accounted for in terms of the spin distribution of copper(II) and silver(II) porphyrins (see picture).

Full text of DOI:10.1002/anie.200501943

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great number of covalently linked oligomacrocycles (e.g.
porphyrins or phthalocyanines)have so far been prepared,
only a few such molecules have been tested for the
exploitation of long-range magnetic coupling.[2,3] One diffi-
culty in the use of these metalated macrocycles for magnetic
coupling may be exemplified by the work of Eaton et al., in
which a ꢀJ value of only about 0.5 cmꢀ1 was detected for a
face-to-face coplanar bis-CuII diporphyrin with a short center-
to-center distance of around 4.1 Š.[2a] However, suitably
arranged intervening bridges may help magnetic communi-
cation between distant metal centers.[3]
In recent years, we have explored a series of directly
linked diporphyrins, including the meso–meso singly linked
diporphyrin 1,[4a] the meso–b singly linked diporphyrin 2,[4b,e]
the meso–b, b–meso doubly linked diporphyrin 3,[4c,e] and the
meso–meso, b–b, b–b triply linked diporphyrin 4.[4d,e,f] Because
of their direct covalent linkages, these diporphyrins exhibit a
large electronic interaction, which increases in the order 2 <
1 ! 3 ! 4, as judged from the absorption spectra.[4] In these
diporphyrins, two metal centers that are connected by various
s-bond networks are kept strictly apart due to the center-to-
center distances of 8.34, 8.91, 8.60, and 8.42 Š for 1, 2, 3, and 4,
respectively.[5] Herein we report antiferromagnetic coupling
in bis-CuII and bis-AgII complexes of 1–4. It is known that the
unpaired spin of both CuII and AgII porphyrins is localized in
the dx2ꢀy2 orbital, which leads to a situation where the unpaired
electrons can be delocalized into the porphyrinic p-electronic
network only through a s-bond pathway.[6] In this context, the
diporphyrins 1–4 constitute a nice set for systematic studies on
the dependence of antiferromagnetic interactions on a direct
linkage.
Metal Porphyrins
DOI: 10.1002/anie.200501943
The Importance of a b–b Bond for Long-Range
Antiferromagnetic Coupling in Directly Linked
Copper(ii) and Silver(ii) Diporphyrins**
Takahisa Ikeue, Ko Furukawa, Hiroshi Hata,
Naoki Aratani, Hiroshi Shinokubo, Tatsuhisa Kato,*
and Atsuhiro Osuka*
Magnetic exchange coupling between distant metal centers is
a major topic in the field of magnetochemistry.[1] Although a
The free-base diporphyrins 1–4 were prepared by the
reported methods[4] and metalated with Cu(OAc)2 and
AgOAc to afford 1Cu–4Cu and 1Ag–4Ag, respectively
(Scheme 1). The effective magnetic moments (cT)at 300 K
[*] Prof. Dr. T. Kato
Department of Chemistry
Graduate School of Science
Josai University
Sakado 350–0295 (Japan)
Fax: (+81)49-271-7985
E-mail: rik@mail.josai.ac.jp
Dr. T. Ikeue, H. Hata, Dr. N. Aratani, Prof. Dr. H. Shinokubo,
Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University
and
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. K. Furukawa
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
[**] This work was partly supported by Grants-in-Aid from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan (no.
15350022 and 21st Century COE on Kyoto University Alliance for
Chemistry) and by the International Innovation Center of Kyoto
University.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Directly linked porphyrins 1M–5M. M: CuII, AgII.
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were determined to be 0.82–0.87 emuKmolꢀ1 for 1Cu–4Cu,
porphyrins, similarly to the case of 4Cu (see SI). The least-
squares fit with the Bleaney–Bowers equation gave a ꢀJ value
of 3.64 cmꢀ1 for 4Ag (Figure 1b).
thus indicating the presence of two magnetically uncoupled
spin doublets (Figure 1a; see also Supporting Information
(SI)). Variable-temperature magnetic susceptibility measure-
ments revealed that the cT values of 1Cu, 2Cu, and 3Cu are
nearly temperature-independent in the range 2–300 K, with
essentially constant values of about 0.8 emuKmolꢀ1.
The marked differences observed between the magnetic
coupling behaviors of 4M and 1M–3M led us to consider the
importance of a b–b bond for effective long-range magnetic
coupling, since such a linkage only exists in 4M. We thus
prepared a new b–b singly linked diporphyrin 5M from a b-
borylated porphyrin precursor.[7] Both bis-metalated com-
plexes 5Cu and 5Ag exhibit long center-to-center distances of
around 9.63 Š and the lowest excitonic coupling in the
absorption spectra in the series. Nevertheless, as shown in
Figure 2, both the cT values of 5Cu and 5Ag exhibit
Figure 1. Variable-temperature magnetic susceptibility measurements
in the range 2–300 K: a) 3Cu and 3Ag, b) 4Cu and 4Ag. The solid
lines represent the fitting curves based on the Bleaney–Bowers
equation.
Figure 2. Variable-temperature magnetic susceptibility measurements
in the range 2–300 K: a) 5Cu and b) 5Ag. The solid lines represent the
fitting curves based on the Bleaney–Bowers equation.
The mean g values in the EPR spectra of 1Cu, 2Cu, and
3Cu in frozen toluene at 4.0 K were 2.08, 2.13, and 2.13 (see
SI), respectively, which were reproduced as a simple sum of an
isolated CuII porphyrin. In contrast, the cT value of 4Cu was
found to drop sharply at temperatures below 20 K and to
reach a value of 0.34 emuKmolꢀ1 at 2.0 K (Figure 1b). This
temperature dependence is indicative of Curie law behavior
with a weak antiferromagnetic coupling between the two
copper(ii)ions. A least-squares fit with the Bleaney–Bowers
equation gave a ꢀJ value of 1.43 cmꢀ1 for 4Cu. In line with
the cT measurement, the EPR spectrum of 4Cu at the mean g
value of 2.11 could not be reproduced as a sum of isolated CuII
porphyrins but as two magnetically interacting CuII porphyr-
ins. This means that the spectrum of 4Cu is due to the
thermally populated S = 1 triplet state above the S = 0 ground
state.
The magnetic properties of 1Ag–4Ag were also exam-
ined. In the range 2–300 K, the cT values of 1Ag–3Ag are
temperature independent (ca. 0.8 emuKmolꢀ1; Figure 1a),
thus indicating the presence of two magnetically uncoupled
spin doublets, while the cT values of 4Ag exhibit a sharp drop
below 20 K, reaching a value of 0.09 emuKmolꢀ1 at 2.0 K (see
SI and Figure 1b). The mean g values in the EPR spectra of
1Ag–4Ag in frozen toluene are 2.06, 2.07, 2.06, and 2.06,
respectively. The spectra of 1Ag–3Ag were reproduced as a
simple sum of isolated AgII porphyrins, whereas that of 4Ag
could be reproduced as two magnetically interacting AgII
temperature-dependent behavior at low temperature, reach-
ing values of 0.65 and 0.24 emuKmolꢀ1 at 2.0 K, respectively.
The least-squares fit of the cT values with the Bleaney–
Bowers equation provided ꢀJ values of 0.55 and 1.73 cmꢀ1,
respectively. These results clearly indicate the critical role of a
direct b–b bond in the long-range antiferromagnetic coupling.
Although excited triplet states are present in 4Cu, 4Ag,
5Cu, and 5Cu, no EPR signal was observed at the half field.
This can be accounted for in terms of a small fine-structure
interaction j D j in the triplet states (see SI)of these
diporphyrins.[8] The j D j value should become smaller with
increasing spin–spin distance. Actually, a simple estimation of
j D j for 4M and 5M led to a prediction that the “forbidden
transition” at the half field is hardly observed in these cases
(see SI); this seems to be a general feature of diporphyrins
with a large center-to-center distance.
The critical importance of a direct b–b bond can be
explained in terms of the spin densities of CuII and AgII
porphyrin monomers, as calculated by the DFT method at
the B3LYP level. The 6-31G* basis set was employed except
for Cu and Ag, for which a basis set consisting of the Stuttgart
effective core potential was used. In both cases, the calcu-
lations confirmed that the unpaired electron in the dx2ꢀy2
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orbital develops only at the b position and not at the
meso position (see SI), thus highlighting the importance of a
direct b–b bond. In fact, the calculated singly occupied MOs
(SOMOs; Figure 3)of a model compound for 4Cu, in which
CuII and AgII porphyrin arrays is an attractive subject that is
actively being pursued in our laboratory.
Received: June 6, 2005
Published online: October 7, 2005
Keywords: copper · magnetochemistry · metal–metal
.
interactions · porphyrinoids · silver
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[3] Some directly linked oligoporphyrins have been used for spin
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Figure 3. Two calculated SOMOs of a model for 4Cu in the S=1 state.
[5] The center-to-center distances were estimated from the X-ray
crystal structures of the related diporphyrins given in referen-
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[6] G. N. La Mar, F. A. Walker in The Porphyrins, Vol. IVB (Ed.: D.
Dolphin), Academic Press, New York, 1979, p. 57.
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all meso substituents were replaced with hydrogen, indicate
that the spin orbital of the copper porphyrin interacts only
through the b–b bond in the S = 1 state (Figure 3a). In
addition, the DFT calculations support our interpretation of
the EPR measurements, namely that the S = 0 ground state
lies below the S = 1 state.
The antiferromagnetic couplings are larger in the AgII
complexes than in their CuII counterparts. Since the spin
distribution patterns are similar due to the spin location in the
dx2ꢀy2 orbital, the observed difference may be attributed to
different spin densities. The spin density distribution via the s-
contact contribution can be estimated by a 2H NMR method.
Typically, the 2H NMR spectrum of CuII(tpp)exhibits the
pyrrole b-2H signal at around d = 41 ppm as a broad signal,[9]
while the signals of AgII(tpp)are too broad to be detected,
thus indicating that the s-contact contribution is larger for
AgII porphyrin than CuII porphyrin. The fact that 5Ag lies
roughly on the long-range limit predicted by the Coffman–
Buettner equation[10] suggests that the direct b–b bond allows
an effective s-bond pathway for long-range antiferromagnetic
coupling between distal paramagnetic metal ions in porphyr-
ins (see SI).
In conclusion, we have shown that antiferromagnetic
coupling is only effective for 4Cu, 4Ag, 5Cu, and 5Ag, thus
underlining the crucial importance of a direct b–b bond.
However, even in the extensively p-conjugated diporphyrins
4Cu and 4Ag, the long-range antiferromagnetic interaction is
considered to propagate via a b–b s-bond pathway. These
results will be quite useful for further molecular design of
magnetically coupled molecules. The exploration of higher
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