Synthesis and dynamic structure of multinuclear Rh complexes of porphyrinoids

Article abstract of DOI:10.1039/b717022j

Large porphyrinoids with three Rh(CO)₂ groups at their dipyrrin units shows two-step metal transposition through the macrocycle in solution and the equilibrium between the C_3v-isomer and the C_s-isomer depends on the solven

Full text of DOI:10.1039/b717022j

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Synthesis and dynamic structure of multinuclear Rh complexes of
porphyrinoidsw
Jun-ichiro Setsune,*a Masayuki Todaa and Takafumi Yoshidab
Received (in Cambridge, UK) 5th November 2007, Accepted 23rd January 2008
First published as an Advance Article on the web 19th February 2008
DOI: 10.1039/b717022j
Large porphyrinoids with three Rh(CO)2 groups at their di-
pyrrin units shows two-step metal transposition through the
macrocycle in solution and the equilibrium between the
C3v-isomer and the Cs-isomer depends on the solvent polarity.
purification on silica gel. The X-ray crystallography of 4
showed that three Rh(CO)2 groups are ligated by three
dipyrrin units of the macrocycle.z The Rh(CO)2 groups are
pushed out of the dipyrrin mean planes by the steric constraint
of the 1,4-phenylene linkers and sitting on the same face of the
macrocycle (Fig. 1). The three dipyrrin units are almost
coplanar and the 1,4-phenylene linkers are tilted by
42.61–52.31 with respect to the adjacent dipyrrin mean planes
to make a bowl-shaped macrocycle structure.
Increasing the ring size is a challenge in the chemistry of
expanded porphyrins from the viewpoint of accommodating
polyatomic entities within the ring.1 We have previously
synthesized porphyrinoids with 16, 20, and 24 pyrrole units
using a 2,20-bipyrrole building block.2 However, it is generally
more difficult to synthesize larger porphyrinoids by increasing
the number of pyrrole units within a ring. Thus, we are also
interested in synthesizing large porphyrinoids by introducing a
spacer in the 2,20-bipyrrole building block.3 Metal complexes
of large porphyrinoids are of great interest because a definite
spatial arrangement of multiple metals can be constructed.4
We describe here such multinuclear complexes of the ex-
panded rosarin and octaphyrin having the 1,4-phenylene
spacers where the Rh(CO)2 group passes through the macro-
cycle and this oscillating movement of the Rh(CO)2 group
depends on the ring size, the number of metals, and the solvent
polarity.
The dipyrrole 15 was reacted with benzaldehyde in the
presence of trifluoroacetic acid (TFA) at room temperature
for 24 h and then oxidized with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ). The expanded rosarin 2 with three
dipyrrole units was obtained in 61% yield along with the
expanded octaphyrin 3 in 8% yield (Scheme 1). It is remark-
able that the yield of 2 by this Rothemund-type synthesis is as
high as that of the parent rosarin(1.0.1.0.1.0) reported origin-
ally by Sessler.6 The UV-vis bands of 2 and 3 at 504 and
550 nm, respectively, are similar to those at 473 and 569 nm
of the corresponding rosarin(1.0.1.0.1.0) and octaphyr-
in(1.0.1.0.1.0.1.0), respectively.2 Reaction of 2 and 3 with
[Rh(CO)2Cl]2 (5-fold molar equiv.) and K2CO3 (excess) in
CH2Cl2 for 24 h at room temperature gave the tris(dicarbo-
nylrhodium) complex 4 and tetrakis(dicarbonylrhodium) com-
plex 5, respectively, in good yields after chromatographic
1
The H NMR spectrum of 4 in CD2Cl2 at 20 1C showed a
pair of multiplets at 2.50 and 2.25 ppm due to the diaster-
eotopic methylene protons of the ethyl groups next to the 1,4-
phenylene linkers and a pair of singlets at 8.14 and 7.36 ppm
due to the 1,4-phenylene protons, which are consistent with
the C3v symmetric structure in accord with the X-ray crystal-
lography. Since the 1,4-phenylene protons appear as a single
singlet at 7.80 ppm for 2 but as two singlets for 4, rotation of
the 1,4-phenylene linker is inhibited at room temperature on
the NMR time scale by the steric constraint with three
Rh(CO)2 groups. The observed chemical shifts for the inner
and outer phenylene protons of 4 are not evidently indicative
of antiaromaticity expected for 36p macroring system. This is
in contrast with the chloroCd(II) p-benziporphyrin where the
1,4-phenylene ring incorporated in the 18p macroring system
a Department of Chemistry, Graduate School of Science, Kobe
University, 1-1 Rokkodai-cho, Nada-ku, 657-8501 Kobe, Japan.
E-mail: setsunej@kobe-u.ac.jp; Fax: +81-78-803-5770;
Tel: +81-78-803-5683
b Department of Frontier Research and Technology, Headquarters for
Innovative Cooperation and Development, Kobe University
w Electronic supplementary information (ESI) available: Synthesis
and spectroscopic data for 1, 2, 3, 4, 5, and 6, including ROESY
NMR for 4 and VT 1H NMR spectra for 6. See DOI: 10.1039/
b717022j
Scheme
complexes.
1
Synthesis of expanded rosarin and octaphyrin Rh
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Fig. 1 X-Ray crystal structure of 4 with thermal ellipsoids at the 50%
level (a top view (left) and a side view (right)). The peripheral ethyl and
phenyl groups and solvent molecules are omitted for clarity. Rh1–Rh2
7.227(1); Rh2–Rh3 6.751; Rh3–Rh1 7.206; O2–O3 3.079; O4–O5
2.909; O6–O1 3.015 A.
Fig. 3 UV-Vis spectra of 4 in various compositions of CH2Cl2/
toluene.
The NMR signals due to the methylene protons coalesced to
a single resonance at 2.47 ppm at 80 1C in toluene-d8, and at
the same time the signals due to the 1,4-phenylene protons
coalesced to a single resonance at 8.05 ppm at 80 1C (Fig. 2).
These changes indicate that the Rh(CO)2 group passes
through the macrocycle rapidly on the NMR time scale at
80 1C to make both faces of the macrocycle magnetically
equivalent (Scheme 2, equilibrium between 4(C3v) and 4(Cs)).
On the other hand, a shoulder signal at 8.25 ppm due to the
second component at 20 1C was split into six resonances at 9.4,
9.1, 8.8, 7.8, 7.4, and 7.3 ppm at ꢁ75 1C. This splitting pattern
is consistent with the Cs-symmetric structure where three
Rh(CO)2 groups are distributed to both faces of the macro-
cycle and the rotation of the 1,4-phenylene linker is restricted.
The coalescence of these six signals can be rationalized in
terms of the transposition of one of the two Rh(CO)2 groups
on the same face into the other face of the macrocycle, which
not only equalizes the magnetic circumstances of both faces of
the macrocycle but also causes three 1,4-phenylene groups to
be magnetically equivalent (Scheme 2, equilibrium between
4(Cs)-a, 4(Cs)-b, and 4(Cs)-c). Therefore, the D3h-symmetric
spectral pattern is seen at 20 1C for the Cs-symmetric isomer.
Since three Rh(CO)2 units are on the same face of the
macrocycle in the C3v-isomer, the dipole moment of the C3v-
isomer is larger than that of the Cs-isomer. Therefore, the C3v-
is tilted by 451 but shows a large chemical shift difference
(6.6 ppm at 298 K) between the inner and outer phenylene
protons.7 A pair of multiplets at 2.56 and 2.33 ppm due to the
C3v symmetric isomer in toluene-d8 at 20 1C are overlapped
with a multiplet at 2.56 ppm due to the second component
with a molar ratio of 1 : 1 (Fig. 2). Two singlets at 8.22 and
7.53 ppm due to the 1,4-phenylene protons of the C3v
symmetric isomer are also overlapped with a broad signal at
8.25 ppm in toluene-d8 at 20 1C. These solvent-dependent
spectral changes were observed in the UV-vis spectra of 4 as
well as in the NMR spectra. A single absorption band in the
visible region was observed at 556 nm in CH2Cl2, whereas a
major band at 578 nm with a shoulder band at 556 nm were
observed in toluene (Fig. 3). Equilibrium between two species
is indicated by the presence of an isobestic point at 567 nm
when the volume ratio of CH2Cl2 and toluene was changed
continuously without changing concentration.
1
Fig. 2 Variable temperature H NMR spectra of 4 in toluene-d8.
Scheme 2 Two-step metal transposition equilibrium.
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the Cs-isomer. On the other hand, four metals are fixed in
the Rh4 complex of the expanded octaphyrin. This work
shows that the highly flexible molecular structure character-
istic of big macrocycle can be configurated by metallation,
which is the basis for binding large guest molecules and then
bringing to cooperative reactions by multiple metals using
large porphyrinoids.
We acknowledge the financial support by Grant-in-Aid for
Scientific Research (No. 16033240 and No. 18550058) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan. J.S. is also grateful to the CREST pro-
gram (the Japan Science and Technology Agency) and the
VBL project (Kobe University).
Notes and references
1
Fig. 4 Variable temperature H NMR spectra of 5 in toluene-d8.
z Crystal data for 4 C93H87N6O6Rh3ꢂCHCl3ꢂC3H6O, M = 1870.86, T
= 300(2) K, monoclinic, space group P21, a = 10.3920(6), b =
31.0783(17), c = 14.7868(8) A, b = 109.9440(10)1, V = 4489.2(4) A3,
Z = 2, rcalc = 1.384 g cmꢁ3, m(Mo-Ka) = 0.692 mmꢁ1, reflections
isomer is favored in CH2Cl2 that is a more polar solvent than
toluene. It is reasonable that the crystal of 4 obtained from
polar solvents (CHCl3 and acetone) shows the C3v-structure in
the X-ray analysis.
collected: 16261, final R indices [I 4 2s(I)]: R1 = 0.0444, wR2
=
0.1077, R indices (all data) R1 = 0.0497, wR2 = 0.1144. CCDC
666201. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b717022j
The monorhodium complex 6 was obtained in 31% yield
when an equimolar mixture of 2 and [Rh(CO)2Cl]2 was stirred
for 3 h in CH2Cl2 at room temperature. The Rh(CO)2 group of
6 is rapidly oscillating between two faces of the macrocycle
even at ꢁ40 1C in toluene-d8 as evidenced by the C2v sym-
metric 1H NMR spectral pattern where six quartets and six
triplets due to the peripheral ethyl groups were observed. Since
the protons due to the 1,4-phenylene linker appeared as a AB
quartet at 8.02 and 7.84 ppm and a singlet at 7.66 ppm at
20 1C, the 1,4-phenylene groups are freely rotating at
20 1C (ESIw).
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The conformational change of the expanded octaphyrin 3 is
very fast in solution to give a simple 1H NMR pattern similar
to that of 2; only one singlet for the 1,4-phenylene protons and
a pair of quartets for methylene protons. Coordination of four
Rh(CO)2 groups to 3 fixes the macrocycle conformation
probably in the S4 symmetric saddle shape. This is evidenced
by the 1H NMR spectrum of 5; two singlets at 7.63 and 8.08 ppm
for the 1,4-phenylene protons, two pairs of multiplets due to the
diastereotopic methylene protons. Since this spectral pattern in
toluene-d8 was not changed between 80 1C and ꢁ60 1C,
metal is not moving through the macrocycle of 5 (Fig. 4).
In summary, the expanded rosarin and octaphyrin showed
unique coordination chemistry. The Rh3 complex of the
expanded rosarin exists as the C3v-isomer in CH2Cl2 as well
as in the crystal state, but relatively slow metal transposition
passing through macrocycle was observed in the toluene
solution to cause interchange between the C3v-isomer and
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