η2-porphyrin Ru(II) π complexes

Article abstract of DOI:10.1021/ja104842h

Highly stable η^2-porphyrin Ru(II) π complexes have been synthesized. The formation of the η² π bond has been facilitated through π-π stacking interactions between the porphyrin and the planar ligand on Ru. π coordination of ruthenium

Full text of DOI:10.1021/ja104842h

Published on Web 07/01/2010
η2-Porphyrin Ru(II) π Complexes
Shigeru Yamaguchi,† Hiroshi Shinokubo,*,‡ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan, and Department
of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Nagoya 464-8603, Japan
Received June 3, 2010; E-mail: hshino@apchem.nagoya-u.ac.jp; osuka@kuchem.kyoto-u.ac.jp
Abstract: Highly stable η2-porphyrin Ru(II) π complexes have
been synthesized. The formation of the η2 π bond has been
facilitated through π-π stacking interactions between the por-
phyrin and the planar ligand on Ru. π coordination of ruthenium
to porphyrin results in dramatic changes in the aromaticity and
electronic properties. These properties are controllable via fine-
tuning of the ligand on ruthenium.
Porphyrins are an important and extensively investigated class
of ligands in which the central cavity is employed as the coordina-
tion site. In contrast, research on metal coordination at the porphyrin
periphery is an emerging area.1 One of the characteristic features
of a porphyrin is strong π-π stacking interactions with another
π-conjugated molecule due to their extended π-conjugated framework.
Metal-π-arene complexes have received much attention in
organometallic chemistry. However, there are few examples of
porphyrins bearing a π-coordinated metal on the core.2 Among the
Figure 1. X-ray crystal structures of 2H [(a) top view and (b) side view]
and 7 [(c) top view and (d) side view]. The counteranion of 2H, the meso-
metal-π-arene complexes, creation of η2 coordination is particularly
aryl substituents, and hydrogen atoms have been omitted for clarity. The
thermal ellipsoids are scaled to the 50% probability level.
difficult. Precise design of the metal center in terms of electron
density and coordination environment is required for synthesis of
η2 π-arene complexes.3 Not surprisingly, no example of an η2-
porphyrin metal π complex has been reported to date. One of the
serious problems is instability of η2 π coordination. Here we report
the synthesis of η2-porphyrin metal π complexes through stabiliza-
tion by π-π stacking interactions between the porphyrin and the
planar ligand on the metal.
S15 in the Supporting Information). Importantly, none of the expected
σ complexes of 1H were detected in the reaction mixture. This is
probably because such cyclometalated σ complexes would lose
stabilization by π-stacking interactions.
1
In the H NMR spectrum of 2H, a set of three peaks at δ )
6.60, 5.47, and 4.62 ppm was assigned to the central pyridine moiety
of the terpyridine. These signals were substantially upfield-shifted
by the shielding effect of the aromatic ring current of the porphyrin.
Generally, π complexation induces some degree of dearomatization
of an arene. In fact, the nucleus-independent chemical shift (NICS)
value of 2H at the center of the porphyrin was calculated to be
δ ) -11.2 ppm, which is slightly larger than that of porphine (δ
) -15.5 ppm).8 The meso proton at δ ) 6.60 ppm exhibited an
upfield shift relative to that of 1H (10.67 ppm) due to the electron
donation from the Ru center, the decrease in aromaticity of the
porphyrin ring, and the shielding effect of the terpyridyl moiety.
The π complexation on the macrocycle largely influences the
UV-vis absorption spectrum (Figure 2). The Soret band of 2H is
ill-defined, and the Q band exhibits a substantial red shift relative
to that of 1H.
We serendipitously achieved the synthesis of highly stable η2-
porphyrin metal π complexes in the course of our investigation of
peripherally cyclometalated porphyrins.4 According to the known
procedure for cyclometalation of 2-phenylpyridine derivatives,5 ꢀ-(2-
pyridyl)porphyrin 1H4 was reacted with Ru(terpyridyl) cation in
toluene/n-BuOH solution at 100 °C. To our surprise, the product was
found to be the η2-porphyrin Ru(II) π complex 2H, which was obtained
in 57% yield as a brown solid after separation by silica gel column
chromatography under ambient conditions. This is quite unusual
because η2 π-arene complexes are generally unstable except with
electron-rich metals.3 The structure was unambiguously elucidated by
single-crystal X-ray diffraction analysis (Figure 1). The Ru(II) atom
is bound to both a meso carbon atom and an adjacent R-carbon of the
porphyrin in a distinct η2 manner.6 Remarkably, the terpyridyl moiety
and the porphyrin macrocycle are almost parallel, and the interplanar
distance between them is quite close (∼3.3 Å), suggesting that the
structure is stabilized by π-π stacking interactions. The bond lengths
of the Ru(II)-C bonds are 2.294 and 2.342 Å, respectively, which
are slightly longer than those of an η2 π-alkene Ru(II) complex (2.196
and 2.260 Å).7 Because of the π complexation, the deviations of the
meso and R-carbons from the mean plane are relatively large (Figure
The unusual stability of the η2 π bonds is also highlighted by the
central metalation of 2H. Metalation of the central cavity of 2H with
Zn(OAc)2 and Cu(OAc)2 afforded 2Zn and 2Cu in high yields without
decomposition of the η2 π coordination (Scheme 1).
This procedure allows the introduction of functionalized terpyridyl
moieties into the η2 π complex. As shown in Scheme 1, π complexes
bearing electron-donating and electron-withdrawing groups on terpy-
ridyl were successfully synthesized (3, 62% yield; 4, 73% yield; 5,
29% yield). In the cyclic voltammograms, two reversible oxidation
waves were observed, and the first oxidation potentials of 1H, 2H, 3,
† Kyoto University.
‡ Nagoya University.
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9992 J. AM. CHEM. SOC. 2010, 132, 9992–9993
10.1021/ja104842h  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of an η2-Porphyrin Ru π Complex Bearing a
Carbon-Metal σ Bonda
a (a) LiCl, 1,2-dichloroethane/methanol, 80 °C.
at 3.22 and 2.70 ppm (vs 0.59 ppm for 2H). The NICS value for 7
was calculated to be -8.6 ppm, supporting the weaker aromaticity of
7 than 2H (NICS ) -11.2 ppm).
Figure 2. UV-vis absorption spectra of 1H (red) and 2H (blue) in CH2Cl2.
Scheme 1. Synthesis of η2-Porphyrin Ru π Complexesa
In summary, we have established a novel strategy for the
synthesis of highly stable η2-porphyrin metal π complexes. The
formation of these π complexes is facilitated through π-π
interactions between the porphyrinic macrocycle and the planar Ru
complexes. π coordination of ruthenium to porphyrin results in
dramatic changes in the aromaticity and electronic properties.
Importantly, these properties are controllable via modification of
the ligand on the Ru center. Further extension of the chemistry of
η2-porphyrin metal π complexes is currently underway.
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research (18685013 and 20037034) from MEXT, Japan.
H.S. acknowledges Asahi Glass Foundation for financial support. S.Y.
appreciates the JSPS Research Fellowships for Young Scientists.
Supporting Information Available: General procedures, spectral
data for compounds, absorption spectra, and CIF files from the X-ray
analyses of 2H and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
a (a) (i) n-BuOH/toluene, 100 °C; (ii) KPF6, CH2Cl2/MeOH, rt. (b)
M(OAc)2 (M ) Cu, Zn), MeOH, rt.
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at -0.08 V, which is a much lower potential than that of 2H (Eox1
)
0.38 V). In the 1H NMR spectrum of 7, the meso proton at 5.27 ppm
exhibited a substantial upfield shift relative to that of 2H (δ ) 6.60
ppm) due to the higher donation ability of the Ru center of 7. The
higher electron density also results in stronger back-donation of the
Ru center to the macrocycle, and consequently, the π-bond lengths in
7 (2.223 and 2.198 Å) are shorter than those in 2H (2.294 and 2.342
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(10) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc. 1976, 98, 3326.
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