C O M M U N I C A T I O N S
carbon and two nitrogen atoms around the meso carbon of each
core, are 3.32 Å for 3a and 3.51 Å for 3b.
geometry optimizations were performed for three different isomers,
cis-cis, trans-cis, and trans-trans dimers, at the B3LYP/6-31G(d)
level. Among these isomers, the cis-cis dimer has the smallest
energy, indicating that this is the predominant structure for 3a. The
HOMO-LUMO gaps decrease as the order of cis-cis (2.53 eV),
trans-cis (2.23 eV), and trans-trans (2.14 eV) (Figure S17).
Compared to the HOMO-LUMO gap of 3b (2.07 eV), the larger
HOMO-LUMO gap of 3a is accounted for by the cis-cis isomer
of isobacteriochlorin freebase dimer. The cis-cis conformation is
also supported by the diffraction analysis: the X-ray structure is
better suited with the calculated cis-cis structure. Variable-
temperature 1H NMR analysis of 3a from -90 to 25 °C in CD2Cl2
confirmed the presence of two species at lower temperatures,
probably indicating NH tautomerization of 3a (Figure S14).
In summary, we investigated solvent dependent keto-enol
tautomerism of 2,18-dihydroxyporphyrins. Furthermore, we found
unique reactivity of them to produce isobacteriochlorin dimers upon
oxidation. This is the first example of directly linked isobacterio-
chlorin dimers, which will open up new chemistry of isobacterio-
chlorin as well as face-to-face porphyrin dimers. The plausible
mechanism for the formation of 3 probably involves dimerization
of quinone-like diketoporphyrin, which is generated by the oxidative
dehydrogenation of 2 (Scheme S1). Further investigation on the
quinone-like diketoporphyrin intermediate is currently under way
in our laboratory.
Figure 2 shows UV-vis absorption spectra of 3a, 3b, and 3c in
dichloromethane. Free base 3a shows splitting Soret bands at 425
and 485 nm and Q bands at 584, 631, and 674 nm. The splitting of
the Soret band may be caused by exciton coupling between two
closely located isobacteriochlorin units. In comparison to 3a, the
metal complexes 3b and 3c exhibit slight red-shifted and splitting
Soret-like bands at 418 and 453 nm, and 450 and 469 nm,
respectively. More interestingly, substantially red-shifted lowest Q
bands are observed at 759 and 751 nm, respectively, indicating a
significant change in the HOMO-LUMO gap induced by meta-
lation.
Figure 1. X-ray crystal structure of 3b: (a) top view and (b) side view.
Thermal ellipsoids are at 50% probability level. Meso substituents are
omitted for clarity.
Acknowledgment. This work was supported by Grant-in-Aids
for Scientific Research (No. 18685013) from MEXT, Japan, and
S.H. acknowledges the Research Fellowships of JSPS for Young
Scientists.
Supporting Information Available: General procedures, spectral
data for compounds, and CIF files. This material is available free of
References
(1) Crossley, M. J.; Hardwig, M. M.; Sternhell, S. J. Org. Chem. 1988, 53,
1132 For a meso-hydroxyporphyrin, see: Esdaile, L. J.; Senge, M. O.; Arnold,
D. P. Chem. Commun. 2006, 4192.
(2) (a) Murphy, M. J.; Siegel, L. M.; Kamin, H.; Rosenthal, D. J. Biol. Chem.
1973, 248, 2801. (b) Chang, C. K. J. Biol. Chem. 1985, 260, 9520.
(3) (a) Scott, A. I.; Irwin, A. J.; Siegel, L. M.; Shoolerly, J. N. J. Am. Chem.
Soc. 1978, 100, 7987. (a) Stolzenberg, A. M.; Spreer, L. O.; Holm, R. H.
J. Am. Chem. Soc. 1980, 102, 364. (b) Wu, W.; Chang, C. K. J. Am. Chem.
Soc. 1987, 109, 3148.
(4) Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8264.
(5) (a) Yamaguchi, S.; Kato, T.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc.
2007, 129, 6392. (b) Hisaki, I.; Hiroto, H.; Shinokubo, H.; Osuka, A. Angew.
Chem., Int. Ed. 2007, 46, 5125. (c) Song, J.; Jang, S. Y.; Yamaguchi, S.;
Sanker, J.; Hiroto, S.; Aratani, N.; Shin, J.-Y.; Easwaramoorthi, S.; Kim,
K. S.; Kim, D.; Shinokubo, H.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47,
6007.
Figure 2. UV-vis absorption spectra of 3a, 3b, and 3c in dichloromethane.
To investigate the intramolecular electronic interaction between
two isobacteriochlorin moieties, cyclic voltammetry was performed
in dichloromethane solution with Bu4NPF6 as electrolyte (Figure
S16). For 3b, two reversible oxidation potentials at 0.42 and 0.65
V and an irreversible reduction potential at -1.34 V were observed
(vs ferrocene/ferrocenium). The splitting oxidation potentials
indicate the existence of strong π-electronic intramolecular interac-
tions of 3b. Such splitting oxidation potentials were also observed
for 3a at 0.45 and 0.69 V, but the irreversible reduction potential
was substantially lowered to -1.52 V. Thus, the HOMO-LUMO
gaps were estimated for 3a, 3b, and 3c to be 1.96, 1.77, and 1.65
V, respectively. This tendency is in good agreement with that
observed in absorption spectra. To investigate the origin of the much
larger HOMO-LUMO gap of 3a than 3b and 3c, MO calculations
were performed. Dioxoisobacteriochlorin has two cis and trans
isomers along with NH tautomerism (Chart 1). Consequently,
(6) For related dimerization of oxophlorins, see: (a) Fuhrhop, J.-H.; Baumgartner,
E.; Bauer, H. J. Am. Chem. Soc. 1981, 103, 5854. (b) Khoury, R. G.;
Jaquinod, L.; Nurco, D. J.; Pandey, R. K.; Senge, M. O.; Smith, K. M. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2496.
(7) The crystallographic data of 3a: C127H147N8O4Cl9, M ) 2168.58, triclinic,
j
space group P1 (No. 2), a ) 14.333(2), b ) 16.550(3), c ) 27.977(5) Å, R
) 93.212(3), ꢀ ) 103.856(3), γ ) 110.975(3)°, V ) 5942.3(17) Å3, Z ) 2,
T ) 90(2) K, Fcalcd ) 1.212 g/cm, R1 ) 0.0937 (I > 2σ(I)), wR2 ) 0.3056
(all data), GOF ) 1.047. 3b: C127H143N9O4Ni2Cl4, M ) 2110.96, triclinic,
j
space group P1 (No. 2), a ) 13.188(5), b ) 14.388(5), c ) 16.656(7) Å, R
) 66.993(12), ꢀ ) 86.554(14), γ ) 78.472(12)°, V ) 2849.9(19) Å3, Z )
1, T ) 123 K, Fcalcd ) 1.230 g/cm, R1 ) 0.0860 (I > 2σ(I)), wR2 ) 0.2661
(all data), GOF ) 1.029.
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