Pt(II)- and Pt(IV)-bridged cofacial diporphyrins via carbon-transition metal σ-bonds

Article abstract of DOI:10.1021/ja8066385

A directly Pt(IV)-bridged cofacial diporphyrin has been synthesized by the cyclometalation reaction of β-pyridylporphyrin with a Pt(IV) salt. Upon treatment with methylhydrazine, the Pt(IV) bridge is reduced to the Pt(II) center, resulting in a Pt(II)-bridged cofacial dimer with a helicity inversion of the complex as well as change in electronic communication through the metal bridge. Copyright

Full text of DOI:10.1021/ja8066385

Published on Web 10/09/2008
Pt(II)- and Pt(IV)-Bridged Cofacial Diporphyrins via Carbon-Transition Metal
σ-Bonds
Shigeru Yamaguchi, Taisuke Katoh, Hiroshi Shinokubo,* and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan
Received August 25, 2008; E-mail: hshino@kuchem.kyoto-u.ac.jp; osuka@kuchem.kyoto-u.ac.jp
Scheme 1. Synthesis of Pt(IV)-Bridged Cofacial Diporphyrin 3 and
Reduction of Pt(IV) Diporphyrin 3 to Pt(II) Diporphyrin 4a, b, c
Cofacial diporphyrin architectures have attracted considerable
attention due to their involvement in photosynthetic processes1 as
well as potential applications in recognition and activation of small
molecules, sensing, and information storage.2 Although various
spacers have been invented for cofacial diporphyrins, there is no
example of the directly metal-bridged cofacial diporphyrin system
via metal-carbon σ-bonds. However, we envisioned the potential
of redox-active transition metal spacers, which achieve the sensitiv-
ity toward external stimuli and the structural rigidity at the same
time. Furthermore, tunable interactions of two porphyrins depending
on the valence states of the metal bridge would be intriguing.
Metallated porphyrins at their peripheral positions have received
much interest because of their rich properties, such as coordination,
photophysical, and electrochemical properties. Among them, ex-
amples of porphyrins having transition metal-carbon σ-bond on
their skeleton are still scarce.3 The pioneering work of such a
porphyrin has been reported by Arnold and co-workers. We have
recently reported a novel class of ring metalated porphyrins,
porphyrin pincer complexes.4 Matano, Imahori, and co-workers
reported ꢀ-M-C σ-bond bridged dimers of meso-phosphanyl
porphyrin with a trans-configuration.5 Here we wish to report the
construction of a metal-bridged cofacial diporphyrin architecture
through a cyclometalation strategy and their dynamic helical
conformational change by reduction of the bridge.
a 2-Iodopyridine, Pd2(dba)3, PPh3, Cs2CO3, CsF, toluene/DMF/H2O,
reflux. b (Bu4N)2PtCl6, toluene/AcOH, 100 °C. c MeNHNH2, CH2Cl2, RT.
Ar ) 3,5-di-tert-butylphenyl, Bpin ) 3,3,4,4-tetramethyl-2,5-dioxaboranyl.
The synthetic strategy of the Pt-bridged cofacial diporphyrin is
shown in Scheme 1. As a ligand, ꢀ-pyridyl porphyrin 2 is
synthesized from ꢀ-boryl porphyrin 16 via a Suzuki-Miyaura cross-
coupling reaction. The reaction of 2 with a Pt(IV) salt in toluene/
AcOH at 100 °C, followed by repeated column chromatography,
afforded 3 as an air and moisture stable brown solid in 7% yield
along with 2 and unidentified products. Its parent ion peak was
observed at m/z ) 2245.9953 (calcd for (C134H148N10Ni2Pt-
Cl)+ ) 2245.9933 [(M - Cl)+]) in its high resolution electrosplay-
ionization time-of-flight (HR ESI-TOF) mass spectrum. The 1H
NMR spectrum of 3 exhibits only one set of seven porphyrinic
ꢀ-protons, suggesting the high symmetry of 3. Notably, one
ꢀ-proton H1 (7.47 ppm) and o-protons of one aromatic substituent
(6.17 and 5.63 ppm) show a substantial upfield shift, implying the
cofacial geometry of 3. Finally, the X-ray crystallographic analysis
of 3 unambiguously elucidates the structure (Figures 1a and S8a).
The porphyrin macrocycles take a ruffled conformation. Two
porphyrin macrocycles are almost parallel (dihedral angle ) 3° for
4N planes) and exhibit a large lateral shift (Ni-Ni distance ) 5.15
Å). Close packing of two porphyrins was observed with an
interporphyrinic distance of 3.4-3.8 Å for the 4N plane. The Pt
coordination takes a distorted octahedral structure, in which the
C-Pt-C angle is 82.5°. The UV/vis absorption spectrum broadens
and splits due to exciton coupling. A substantial red shift of Q-bands
is observed as well as broadening of the Soret band (Figure S7).
We then attempted reduction of the Pt(IV) bridge of 3 to Pt(II).
To our delight, the reaction of 3 with an excess amount of
Figure 1. X-ray crystal structures. (a) Top views of 3 and (b) 4. Hydrogen
atoms and meso-aryl substituents are omitted for clarity. The thermal
ellipsoids are scaled to the 50% probability level.
MeNHNH2 in CH2Cl2 proceeded smoothly to furnish 4 in 65%
yield, which was formed via a formal “slipping motion” of the two
porphyrin macrocycles (Scheme 1).7 HR ESI-TOF mass exhibited
its parent ion peak at m/z ) 2233.0161 (calcd for (C134H148N10-
1
Ni2PtNa)+ ) 2233.0151 [(M + Na)+]). In the H NMR, 4 also
displays one set of proton peaks of porphyrin macrocycles. The
doublet signal of ꢀ-protons H2 and H3 of 4 appeared in the upfield
region (6.97 and 6.52 ppm), while the singlet peak of H1 of 3 was
upfield shifted. These changes clearly indicate the lateral slipping
motion from Pt(IV)-dimer 3 to Pt(II)-dimer 4. Finally, the cofacial
porphyrin geometry of 4 was confirmed by X-ray crystallographic
analysis (Figures 1b and S8b). The coordination of the Pt(II) center
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10.1021/ja8066385 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
S14). Further investigation on the electrochemical behavior is
currently underway.12
In conclusion, we have achieved the synthesis of Pt(IV)- and Pt(II)-
bridged cofacial diporphyrins. Both platinum centers force two
porphyrin macrocycles to be in close proximity by the stable two Pt-C
σ-bonds supported by the pyridyl groups. At the same time, the
platinum bridge offers a conformationally flexible nature to the
complexes due to the susceptibility of platinum toward redox reaction.
These complexes also exhibit helical chirality. We are now focusing
on the applications of these Pt-bridged cofacial diporphyrins in
asymmetric catalysis and material science.
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research (Nos. 18685013 and 20037034) from
MEXT, Japan. S.Y. appreciates the JSPS Research Fellowships for
Young Scientists. We also thank Prof. Hitoshi Tamiaki (Ritsumei-
kan Univ.) for CD measurement.
Figure 2. CD spectra of 3M (red) and 4P (green).
is square planar. The length of C-Pt bonds of 4 is 1.993 Å, which
is shorter than that of 3 (2.044 Å). The two porphyrin macrocycles
stand somewhat apart from each other (dihedral angle ) 35.4° for
4N planes and Ni-Ni distance ) 7.208 Å) in comparison to the
overlapped structure of 3. The macrocycles also take a ruffled
conformation, and the averages of the mean plane deviations of 3
and 4 are almost the same (3, 0.314 Å and 4, 0.323 Å for 4N plane).
The UV/vis absorption spectrum of 4 shows broadening, splitting,
and a red shift (∆λ ) 15 nm for the Soret band as compared to 2,
Figure S7).
These Pt-bridged cofacial diporphyrins 3 and 4 have inherent
helical chirality. We succeeded to separate all the enantiomers by
the chiral HPLC. The CD spectra of enantiomers exhibit distinct
Cotton effects which are unpredictable by the empirical method
(Figures 2 and S11).8 No racemization of enantiomers was observed
in both the solid states and solution at least for 1 month. This
stability is probably due to the bulkiness of the porphyrin as well
as robust Pt-C and Pt-N bonds. Furthermore, we found helicity
inversion by reduction of enantiomers: reduction of M spiral of 3
(3M) predominantly yielded one enantiomer of 4 (4P) in 77% ee.9
The CD pattern of 4P matches well with the calculated spectrum
by the TD-DFT method (Figure S15).
The electronic coupling of the porphyrin moieties of 3 and 4
was examined by cyclic voltammetry along with 2 (Figure S11, V
vs ferrocene/ferrocenium ion pair).10 Pt(II)-bridged dimer 4 shows
distinct splitting of the first oxidation potential: the reversible waves
were observed at 0.13 and 0.36 V as one-electron processes and at
0.72 V as a two-electron process. In sharp contrast, two reversible
waves were observed at 0.33 and 0.67 V for Pt(IV)-bridged dimer
3. This different behavior highlights the importance of the oxidation
state of the Pt-bridge and thus involvement of d-orbitals in the
interporphyrinic interaction. The lower potential of 4 than that of
2 (Eox1 ) 0.48 V) is mainly due to the electron-donating character
of the Pt(II) atom.11 In addition, the first oxidation potential of 3 is
higher than that of 4 because of less electron-donating ability of
Pt(IV) as compared to Pt(II). The square planar Pt(II) center has a
Supporting Information Available: General procedures, spectral
data for compounds and absorption spectra. CIF files for the X-ray
analysis of 3, 3M, and 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
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2
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2
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