Regioselective β-metalation of meso-phosphanylporphyrins. Structure and optical properties of porphyrin dimers linked by peripherally fused phosphametallacycles

Article abstract of DOI:10.1021/ja710542e

meso-Diphenylphosphanylporphyrins were successfully prepared via Pd-catalyzed C-P cross-coupling reaction of the corresponding meso-iodoporphyrins with diphenylphosphane. The meso-phosphanylporphyrins underwent regioselective metalation at the β carbon to produce novel classes of porphyrin dimers linked by peripherally fused phosphametallacycles. A Pd-mononucler complex was structurally characterized by X-ray crystallography, which revealed a flat structure of the Pd-linked porphyrin π systems. Both experimental and theoretical results demonstrate that the orbital interaction between the pyrrolic p_π orbitals and the metal d_π orbital affects optical and electrochemical properties of the metal-linked coplanar porphyrin dimers. Copyright

Full text of DOI:10.1021/ja710542e

Published on Web 03/18/2008
Regioselective â-Metalation of meso-Phosphanylporphyrins. Structure and
Optical Properties of Porphyrin Dimers Linked by Peripherally Fused
Phosphametallacycles
Yoshihiro Matano,*,† Kazuaki Matsumoto,† Yoshihide Nakao,† Hidemitsu Uno,‡
Shigeyoshi Sakaki,† and Hiroshi Imahori†,§
Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto
615-8510, Japan, Integrated Center for Sciences, Ehime UniVersity, Matsuyama 790-8577, Japan, and Institute for
Integrated Cell-Material Sciences, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan
Received November 22, 2007; E-mail: matano@scl.kyoto-u.ac.jp
Scheme 1. Synthesis and Reactions of
meso-Phosphanylporphyrins
Peripherally metalated porphyrins bearing a carbon-metal
σ-bond play a crucial role in transition-metal-catalyzed function-
alization of porphyrin rings.1 This class of compounds should also
provide valuable information on electronic communication between
the porphyrin π-system and the metal d orbitals attached at the
periphery. Arnold and co-workers investigated the structures and
fundamental properties of meso-η1-palladio- and platinioporphyrins,
which were prepared by oxidative addition of the corresponding
zerovalent metals to C-Br bonds of meso-bromoporphyrins.2
Recently, Osuka, Shinokubo, and co-workers synthesized the pincer-
type meso-η1-palladioporphyrins via meso-C-H activation directed
by two neighboring 2-pyridyl groups.3 However, the number of
structurally characterized examples, especially those containing a
â-C-M bond,4,5 is still limited. Here we report the first examples
of â-η1-palladio- and platinioporphyrins, which are formed by
regioselective metalation at the â carbon of meso-phosphanylpor-
phyrins.6 Most importantly, the coplanar porphyrin dimers linked
by peripherally fused phosphametallacycles have been found to
exhibit characteristic optical and electrochemical properties derived
from the pπ-dπ orbital interaction.
Table 1. Complexation of 2a,b with Pd(II) and Pt(II) Salts
entry
2
MX2
‚L/solvent
2/MX2
product (yielda)
1
2
2a
2a
2b
2a
Pd(OAc)2/toluene
Pd(OAc)2/toluene
Pd(OAc)2/toluene
PtCl2(cod)/CH2Cl2
1/0.5
1/1
1/0.5
1/0.5
5a (55%), 6a (23%)
5a (trace), 6a (73%)
5b (70%)
3
4b
7a (62%)
a Isolated yield based on 2a,b. b Et3N was added.
The Pd-catalyzed C-P cross-coupling reaction of meso-iodopor-
phyrins 1a,b1c,e with diphenylphosphane in MeCN-THF produced
the corresponding meso-phosphanylporphyrins 2a,b as air-sensitive
substances (Scheme 1). For instance, 2a was oxidized rapidly in
air to meso-phosphorylporphyrin 3a.7 Due to difficulty of isolating
2a,b at this stage, the crude reaction mixtures were subsequently
treated with elemental sulfur, affording meso-thiophosphorylpor-
phyrins 4a,b as air-stable solids in 87-92% yields based on 1a,b.
Desulfurization of 4a,b with excess P(NMe2)3 in refluxing toluene
reproduced 2a,b quantitatively. Remarkably, this two-step protocol
enabled us to isolate pure 2a,b in 90-95% isolated yields (based
on 4a,b) by simple reprecipitation under inert atmosphere.8
Treatment of 2a,b with palladium(II) and platinum(II) salts
yielded novel classes of porphyrin dimers 5-7, which contain two
phosphametallacycle linkages as depicted in Figure 1 and Scheme
S1 (Supporting Information). The complexation of 2a with 0.5 equiv
of Pd(OAc)2 in toluene afforded Pd-mononuclear complex 5a and
bis-µ-acetato-bridged Pd-dinuclear complex 6a in 55% and 23%
yield, respectively (Table 1, entry 1). When 2a was slowly added
to a toluene solution of 1 equiv of Pd(OAc)2, 6a was formed
predominantly in 73% yield (entry 2). By contrast, 2b reacted with
0.5 equiv of Pd(OAc)2 to produce Pd-mononuclear complex 5b
exclusively (entry 3). The complexation of 2a with 0.5 equiv of
PtCl2(cod) (cod ) 1,5-cyclooctadiene) in the presence of Et3N gave
Figure 1. Structures of â-η1-palladio- and platinioporphyrins 5-9.
Pt-mononuclear complex 7a in 62% yield (entry 4). All the products
were fully characterized by MS and NMR spectroscopies. In the
MS spectra, intense molecular ion peaks were detected. The 31P
NMR spectra displayed single peaks at δ 46.7-51.1,8 indicating
that two phosphorus atoms coordinate to the palladium or platinum
center equivalently. The 31P-195Pt coupling constant of 2834 Hz
observed for 7a suggests that the two phosphine ligands are
coordinated in a trans geometry. The appearance of seven kinds of
1
peripheral â protons (each 2H) in the H NMR spectra of 5-7
reveals that one of the â-H atoms of the porphyrin ring is replaced
by the Pd(II) or Pt(II) salt through the complexation (Figures S24-
S27, Supporting Information). It is likely that the meso-phosphanyl
group directs the P-ligated metal center to activate the neighboring
â-C-H bond regioselectively.
† Graduate School of Engineering, Kyoto University.
‡ Ehime University.
The structure of 5b was unambiguously elucidated by X-ray
crystallography.9 As shown in Figures 2 and S1 (Supporting
§ Institute for Integrated Cell-Material Sciences, Kyoto University.
9
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J. AM. CHEM. SOC. 2008, 130, 4588-4589
10.1021/ja710542e CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
cyclic voltammograms of 5a and 7a displayed appreciably broad-
ened or split cathodic and anodic waves for their electrochemical
oxidation processes (Figure S19, Supporting Information),12 sug-
gesting that the π-radical cations delocalize over the metal-linked
two porphyrin rings. The relatively large splitting potential (∆Eox
) 0.06 V) observed for 7a indicates that the delocalization between
the Pt-linked porphyrin π systems occurs more efficiently than that
between the Pd-linked π systems.
In summary, we have successfully applied the phosphane-directed
regioselective â-C-H activation by Pd(II) and Pt(II) salts to the
synthesis of new classes of porphyrin dimers linked by the
peripherally fused phosphametallacycles. The present results dem-
onstrate that the pπ-dπ orbital interaction at the peripheral
â-carbon-metal bond potentially affects the optical and electro-
chemical properties of the metal-linked coplanar porphyrin π
systems.
Figure 2. Top view (upper) and side view (lower) of 5b. Hydrogen atoms,
10,20-meso-aryl groups, solvents, and MeOH (top view) are omitted for
clarity: Pd-Câ, 2.046(4) Å; Pd-P, 2.2904(14) Å; Câ-Pd-P, 81.84(11)°
and 98.16(11)°; Pd-P-Cmeso, 104.85(13)°.
Acknowledgment. This work was partially supported by Grants-
in-Aid (No. 17350018 and No. 461) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. We thank Dr.
Motoo Shiro for X-ray crystallography.
Supporting Information Available: Experimental details, CIF file
for 5b, and DFT computational results. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) For example, see: (a) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J.
Am. Chem. Soc. 1993, 115, 2513-2515. (b) Lin, V. S.-Y.; DiMagno, S.
G.; Therien, M. J. Science 1994, 264, 1105-1111. (c) Shultz, D. A.;
Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63, 769-774. (d)
Shanmugathasan, S.; Johnson, C. K.; Edwards, C.; Matthews, E. K.;
Dolphin, D.; Boyle, R. W. J. Porphyrins Phthalocyanines 2000, 4, 228-
232. (e) Odobel, F.; Suzenet, F.; Blart, E.; Quintard, J.-P. Org. Lett. 2000,
2, 131-133. (f) Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc.
2005, 127, 8264-8265.
(2) (a) Arnold, D. P.; Sakata, Y.; Sugiura, K.; Worthington, E. I. Chem.
Commun. 1998, 2331-2332. (b) Arnold, D. P.; Healy, P. C.; Hodgson,
M. J.; Williams, M. L. J. Organomet. Chem. 2000, 607, 41-50. (c)
Hodgson, M. J.; Healy, P. C.; Williams, M. L.; Arnold, D. P. J. Chem.
Soc., Dalton Trans. 2002, 4497-4504. (d) Hartnell, R. D.; Edwards, A.
J.; Arnold, D. P. J. Porphyrins Phthalocyanines 2002, 6, 695-707. (e)
Hartnell, R. D.; Arnold, D. P. Organometallics 2004, 23, 391-399. (f)
Hartnell, R. D.; Arnold, D. P. Eur. J. Inorg. Chem. 2004, 1262-1269.
(3) Yamaguchi, S.; Katoh, T.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc.
2007, 129, 6392-6393.
Figure 3. UV-vis absorption spectra of 2a (red), 5a (purple), 6a (green),
and 7a (blue) in toluene. The inset shows HOMO-4 of 9.
Information), the Pd center in the phosphapalladacycles adopts a
distorted square planar geometry (ΣC-Pd-P ) 360°) with Ci sym-
metry. As a consequence, two porphyrin rings are almost on
the same plane with a Zn-Zn distance of 12.1 Å.10 The Pd-C
bond length [2.046(4) Å] of 5b is comparable to the reported
value [2.05(2) Å] of Arnold’s meso-η1-palladioporphyrin2a and
longer than those [1.969(6)-1.977(7) Å] of Osuka’s pincer-type
meso-η1-palladioporphyrins.3
(4) Smith, K. M.; Langry, K. C.; Minnetian, O. M. J. Org. Chem. 1984, 49,
4602-4609.
(5) Quite recently, Sugiura, Arnold, and co-workers reported the regioselective
synthesis and crystal structure of â-HgCl-substituted porphyrins. See:
Sugiura, K.-i.; Kato, A.; Iwasaski, K.; Miyasaka, H.; Yamashita, M.; Hino,
S.; Arnold, D. P. Chem. Commun. 2007, 2046-2047.
(6) Matano, Y.; Matsumoto, K.; Terasaka, Y.; Hotta, H.; Araki, Y.; Ito, O.;
Shiro, M.; Sasasmori, T.; Tokitoh, N.; Imahori, H. Chem. Eur. J. 2007,
13, 891-901.
The UV-vis absorption spectra of phosphanylporphyrin 2a and
the Pd-dinuclear complex 6a displayed relatively narrow Soret
bands at λmax 426 and 438 nm, respectively (Figures 3 and S2,
Supporting Information). In sharp contrast, the Pd- and Pt-
mononuclear complexes 5a and 7a showed rather broad absorptions
at the Soret-band regions (λmax ) 426 and 422 nm). To gain a deep
insight into the character of these transitions, we performed time-
dependent density functional theory (TD-DFT) calculations of their
model complexes 8 and 9 and 5,10,15,20-tetraphenylporphyrina-
tozinc(II) (TPPZn) (Figures S3-S14 and Tables S1-S4, Supporting
Information). Notably, the excitations from HOMO-4 largely
contribute to Soret bands of 8 and 9, which are split or broadened
as compared to that calculated for TPPZn, although the calculated
excitation energies are somewhat larger than the observed values.11
As visualized in Figures S5-S8 (Supporting Information) and 3
(inset), the HOMO-4 in 8 and 9 involves antibonding character
between the pyrrolic pπ orbitals and the metal dπ orbital, which
implies possible electronic communication between the coplanar
porphyrin π systems through the peripheral â-C-M bonds. Indeed,
(7) Arnold suggested the formation of a meso-phosphanylporphyrin as the
intermediate in their synthesis of meso-phosphorylporphyrins. See: (a)
Atefi, F.; Locos, O. B.; Senge, M. O.; Arnold, D. P. J. Porphyrins
Phthalocyanines 2006, 10, 176-185. (b) Atefi, F.; McMurtrie, J. C.;
Turner, P.; Duriska, M.; Arnold, D. P. Inorg. Chem. 2006, 45, 6479-
6489.
(8) δP (162 MHz; CDCl3 or CD2Cl2) of 2a, 2b, 5a, 5b, 6a, and 7a are -5.4,
-6.0, 51.1, 48.6, 50.5, and 46.7 (1JP-Pt ) 2834 Hz), respectively.
(9) C140H134N8O4P2PdZn2, P1h, a ) 11.030(5) Å, b ) 16.207(8) Å, c ) 16.613-
(8) Å, R ) 97.695(8)°, b ) 91.885(8)°, γ ) 98.247(9)°, V ) 2909(2) Å3,
Z ) 1, Dc ) 1.308 g cm-3, 13199 obsd, 764 variables, Rw ) 0.1690, R
) 0.0691 (I>2.00σ(I)), GOF ) 1.064. Although the quality of crystal-
lographic data is not at the publishable level, a similar planar structure of
7a was confirmed by X-ray diffraction analysis.
(10) The zinc atom is deviated from the 24-atom mean plane (0.32 Å) due to
the coordination by the methanol-oxygen.
(11) The calculated wavelengths of Soret bands are red-shifted when solvation
effects are incorporated. In a Zn analog of 8, the splitting of Soret band
becomes much smaller, indicating that the pπ-dπ orbital interaction
between porphyrin and bridging Pd atom is important to lead to a broad
Soret band of 8. For details, see the Supporting Information.
(12) Eox and Ered (vs Fc/Fc+; in CH2Cl2 with 0.1 M nBu4N+PF6-; Ag/Ag+
[0.01 M AgNO3 (MeCN)]): +0.35/+0.60 V and -1.82 V for 5a; +0.42/
+0.67 V and -1.71 V for 6a; +0.31/+0.37/+0.57 V and -1.83 V for
7a.
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