Redox-coupled complexation of 23-phospha-21-thiaporphyrin with group 10 metals: a convenient access to stable core-modified isophlorin-metal complexes.

Redox-Coupled Complexation of 23-Phospha-21-thiaporphyrin with Group 10
Metals: A Convenient Access to Stable Core-Modified Isophlorin-Metal Complexes
Yoshihiro Matano,*,† Takashi Nakabuchi,† Shinya Fujishige,‡ Haruyuki Nakano,‡ and Hiroshi Imahori†,§,|
Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan,
Department of Chemistry, Graduate School of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan, Institute for Integrated
Cell-Material Sciences, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and, Fukui Institute for Fundamental Chemistry,
Kyoto UniVersity, Sakyo-ku, Kyoto 606-8103, Japan
Received September 30, 2008; E-mail: matano@scl.kyoto-u.ac.jp
Isophlorins (N,N′-dihydroporphyrins) are a class of [20]annulenes,
whose structure, optical/electrochemical properties, and aromaticity
have received continuing interest in relation to the 18π porphyrin
chemistry.1-4 This class of compounds is also expected to show
characteristic coordination behavior derived from tetradentate macro-
cyclic ligands. Vaid et al. and Brothers et al. recently succeeded in
preparing silicon(IV) isophlorin complexes [Si(TPP)(L)2; TPP: tet-
raanion of tetraphenylporphyrin; L ) THF or pyridine]5 and a
diboranyl isophlorin complex [B2(TTP); TTP: tetraanion of tetra-p-
tolylporphyrin],6 respectively, by 2e-reduction of the corresponding
metalloporphyrins and disclosed their antiaromatic character on the
basis of experimental and theoretical results.7,8 Except for these
examples, however, the coordination chemistry of isophlorins still
remains unveiled because of their intrinsic nature to undergo 2e-
oxidation (aromatization to porphyrins) under ambient conditions.9 We
anticipated that replacement of pyrrole nitrogens with other heteroa-
toms, namely core-modification,10 would bring chemical stability to
isophlorin complexes by alternating the coordinating abilities, charges,
and redox properties of their 20π macrocyclic platforms. Here, we
report the first synthesis of core-modified isophlorin-metal complexes,
which relies on the redox-coupled complexation of a P,S,N2-hybrid
porphyrin with zerovalent group 10 metals, and the structure and
aromaticity of their 20π systems.
In this study, we used P,S,N2-hybrid 111 and S2,N2-hybrid 2 as
core-modified porphyrin free bases. Compound 2 was newly
prepared via BF3-promoted [3 + 1] condensation of a 2,5-
bis(hydroxymethyl)thiophene with a thiatripyrrane12 (Scheme S1
in Supporting Information). When treated with 1 equiv of Pd(dba)2
(dba ) dibenzylideneacetone) in CH2Cl2 at room temperature, 1
was consumed completely within a minute (checked by TLC). After
removal of the solvent, the residue was subjected to column
chromatography on alumina to give Pd-P,S,N2 complex 3Pd as a
dark green solid in 95% yield (Scheme 1). In sharp contrast, no
complexation took place between 2 and Pd(dba)2 even after
refluxing in 1,2-dichlorobenzene for 5 h. This indicates that the
core-phosphorus atom in 1 contributes significantly to the facile
complexation with palladium. Treatment of 1 with Ni(cod)2 (cod
) 1,5-cyclooctadiene) and excess Pt(dba)2 under suitable reaction
conditions produced 3Ni and 3Pt, respectively (Scheme 1).
Complexes 3Pd and 3Pt were found to be stable in air, whereas
3Ni gradually decomposed in solution.
The structures of 3M were characterized by NMR and mass
spectrometry. The salient feature of their 1H spectra is negligible
diatropic/paratropic ring current effects derived from the π electron
circuit. For instance, meso, �, and P-phenyl protons of 3Pd are
observed at δ 6.01 (JPH ) 37.6 Hz), 5.58-6.13, and 7.65-8.37,
respectively, and they are upfield-shifted (∆δ ) -4.4 to -2.5 for
meso, �) or downfield-shifted (∆δ ) +6.1 to +2.4 for P-phenyl)
as compared to the corresponding protons of 1 (Figure S1b). The
slight differences in chemical shifts among 3Ni, 3Pd, and 3Pt imply
that the central metal does not perturb the ring current effect in
3M dramatically (Figure S1).13 In the 31P{1H} NMR spectra, a
singlet (3Ni and 3Pd) or doublet (3Pt, 1JP-Pt ) 3190 Hz) peak
appears at δ 35.1-61.9, and it is deshielded relative to the 31P peak
of 1 (δ 18.6). These data show that 3M essentially possesses
nonaromatic character.
The crystal structure of 3Pdwas unambiguously elucidated by X-ray
diffraction analysis.14 As shown in Figure 1a, the palladium center
adopts a square planar geometry with the sum of N-Pd-X (X ) P,
S) bond angles of 360°. Because of this coordination, the phosphorus
and sulfur atoms are displaced by 0.95 and 0.93 Å, respectively, from
the mean π plane composed of the peripheral 20 carbon atoms (Figure
1b). The Pd-X (X ) N, S) bond lengths are very close to those
observed for a square planar Pd(II)-P,S,N2-hybrid calixphyrin com-
plex,15 and the peripheral C-C bond alternation well explains the 20π
valence-bond structure for the P,S,N2-hybrid ligand (Figure 1c). These
data suggest that oxidation states of the metal center and the hybrid π
ligand in 3Pd are +2 and -2, respectively. The whole structure of
3Pd differs considerably from the structures of ruffled Si(TPP)(THF)25a
and Ge(TPP)(py)27 and planar meso-pentafluorophenyl core-modified
O4- and O2,S2-isophlorins,4 all of which were reported to be antiaro-
matic. Variable-temperature 1H measurement of 3Pd in CDCl3 did
not show noticeable spectral change between -50 and 50 °C,
suggesting that the distorted structure of 3Pd is basically maintained
in solution. Indeed, the UV-vis absorption spectrum of 3Pd shows
broad Soret-like bands at λmax 375-414 nm and no detectable Q bands
(Figure S2), which is characteristic of highly ruffled, nonaromatic 4nπ
porphyrinoids.1,3,4,16 The electrochemical oxidation processes of 3Pd
occurred reversibly at -0.26 and +0.06 V vs Fc/Fc+ (Figure S3).17
The Pd coordination shifts the first oxidation potentials of 1 to the
negative side by 0.71 V, reflecting the 20π isophlorin structure of 3Pd.
† Graduate School of Engineering, Kyoto University.
‡ Kyushu University.
§ Institute for Integrated Cell-Material Sciences, Kyoto University.
| Fukui Institute for Fundamental Chemistry, Kyoto University.
Scheme 1. Synthesis of P,S,N2-Isophlorin-metal Complexes 3M
Published on Web 11/14/2008
10.1021/ja807742g CCC: $40.75  2008 American Chemical Society16446 9 J. AM. CHEM. SOC. 2008, 130, 16446–16447

The Pt complex 3Pt exhibited similar optical and electrochemical
properties (λmax 312-407 nm; Eox ) -0.24 and +0.07 V).
To gain a deeper insight into the aromaticity of the P,S,N2-
isophlorin ring in 3M, we performed density functional theory
(DFT) calculations of Pd and Pt model complexes (3M-m; M )
Pd, Pt) at the B3LYP/6-31G(d,p)+LANL2DZ level (For details,
see Supporting Information). As indicated in Figures S4 and S5,
the optimized geometry and the bond lengths of 3Pd-m are close
to those of 3Pt-m. The natural atomic orbital occupancies of d
orbitals calculated for 3M-m indicate that the formal oxidation
state of their central metals is +2 (Table S1). The absolute values
of nucleus-independent chemical shifts (NICS) at the center of two
adjacent heterole rings of 3M-m are much smaller than those of
free-base model 1-m (Figure S8c) and close to zero, suggesting
that the 20π P,S,N2-isophlorin ring in 3M-m possesses nonaro-
matic character in terms of the magnetic criterion. Although the
loss of paratropicity is ascribable to the distortion from planarity
of the ring system,18 other factors should also be taken into
consideration. To reveal the effects of core elements and central
metals on the paratropicity of isophlorin π circuit, we calculated
NICS values of imaginary Pd-S2,N2 and Mg-P,S,N2 complexes
(Pd-S2,N2-m and Mg-P,S,N2-m) as references (Figures S6 and
S7). At the optimized structures, Pd-S2,N2-m shows a NICS value
of -0.12, whereas Mg-P,S,N2-m displays positive NICS values
of +2.66 and +3.03.19 These results imply that the lack of
paratropic ring currents in the Ni-, Pd-, and Pt-P,S,N2 complexes
is due in a part to the central group 10 metals.
To evaluate the coordinating ability of 1, we next calculated heat
of formation of 3Pd-m by the DFT method. As shown in Scheme
S2, formation of 3Pd-m and dba from 1-m and Pd(dba)2 yields a
negative energy (∆E0 ) -7.1 kcal mol-1), suggesting that the Pd-
complexation of 1-m is thermodynamically favorable. On the other
hand, the Pd-complexation of S2,N2-free-base model 2-m yields a
positive energy (∆E0 ) +9.1 kcal mol-1). Evidently, the core-
phosphorus atom plays crucial roles in enhancing the electron-accepting
ability of the 18π porphyrin ring and stabilizing the 20π isophlorin
ring due to high P-Maffinity.20It must be emphasized that incorporation
of P and S atoms changes the charge of fully anionic 20π system from
-4 to -2. Thus, 3M are regarded as neutral M(II) complexes.
In summary, we have revealed that isophlorin-metal complexes
are readily accessible by redox-coupled complexation of P,S,N2-
hybrid porphyrin. Most importantly, group 10 metals, which are
weakly reducing as compared to traditionally used Na/Hg, Mg, and
Zn, have proven to reduce the 18π porphyrin ring efficiently. The
experimental and theoretical results represent that the P,S,N2-
isophlorin complexes possess nonaromaticity in terms of both
geometrical and magnetic criteria. The present study demonstrates
that core-modification with phosphorus could be a promising
methodology to develop coordination chemistry of 4nπ porphyrinoids.
Acknowledgment. This work was supported by Grant-in-Aid
for Science Research on Priority Areas (No. 20038039) from
MEXT, Japan. T.N. thanks a JSPS fellowship for young scientists.
Supporting Information Available: Experimental details, CIF file
for 3Pd, and DFT computational results. This material is available free
of charge via the Internet at http://pubs.acs.org.
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(14) C41H29N2PPdS, MW ) 719.09, monoclinic, P21/n, a ) 9.352(3) Å, b )
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(I > 2.00σ(I)), GOF ) 1.010.
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JA807742G
Figure 1. (a) Top and (b) side views of 3Pd. Hydrogen atoms and meso-
phenyl groups (b) are omitted for clarity; bond lengths (Å) and bond angles
(deg): Pd-P, 2.1830(11); Pd-N1, 2.060(3); Pd-N2, 2.070(3); Pd-S,
2.2664(11); P-Pd-N1, 88.14(9); P-Pd-N2, 90.65(9); S-Pd-N1, 90.02(9);
S-Pd-N2, 91.15(9). (c) Bond lengths determined by X-ray (3Pd; black)
and DFT calculation (3Pd-m; blue) and NICS values (red).
J. AM. CHEM. SOC. 9 VOL. 130, NO. 49, 2008 16447
C O M M U N I C A T I O N S