Selective Si-C bond cleavage on a diorganosilicon porphyrin complex bearing different axial ligands

Article abstract of DOI:10.1246/cl.2009.362

Tetraphenylporphyrinato methylphenylsilicon complex (abbreviated to Si(TPP)MePh) was synthesized and structurally characterized. Although the Si-Ph bond of Si(TPP)MePh is much longer than the Si-Me bond, photolysis and chemical oxidation of Si(TPP)MePh cl

Full text of DOI:10.1246/cl.2009.362

362
Chemistry Letters Vol.38, No.4 (2009)
Selective Si–C Bond Cleavage on a Diorganosilicon Porphyrin Complex
Bearing Different Axial Ligands
Shintaro Ishida, Kimio Yoshimura, Hideyuki Matsumoto, and Soichiro Kyushinꢀ
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu 376-8515
(Received January 5, 2009; CL-090013; E-mail: kyushin@chem-bio.gunma-u.ac.jp)
Tetraphenylporphyrinato methylphenylsilicon complex (ab-
39% isolated yield. The structure of Si(TPP)MePh was unequiv-
ocally determined by spectroscopic evidence8 and X-ray struc-
tural analysis (Figure 1).9
breviated to Si(TPP)MePh) was synthesized and structurally
characterized. Although the Si–Ph bond of Si(TPP)MePh is
much longer than the Si–Me bond, photolysis and chemical ox-
idation of Si(TPP)MePh cleaved the Si–Me bond selectively and
gave new phenyl-substituted silicon porphyrins. Theoretical
studies of a model compound showed that significant ꢀ–ꢁ
orbital interaction raised the HOMO and is responsible for the
silicon–carbon bond cleavage. The instability of phenyl radical
to methyl radical caused selective bond cleavage.
PhMgCl
Si(TPP)MePh
39%
Si(TPP)ClMe
ð1Þ
THF, rt
In the solid state, the geometry around hexacoordinated
silicon is distorted, and the porphyrin ring is slightly ruffled.
The average displacement for the meso carbon atoms from the
least-squares plane of four nitrogen atoms (N4 plane) is
6a
˚
˚
0.47 A, similar to that of Si(TPP)Ph2 (0.48 A). Short contacts
are found between ortho carbons of axial ligand and nitrogen
Diorganosilicon porphyrin complexes are a unique class of
neutral hexacoordinated silicon species having two axial organic
ligands at the trans positions.1 Although silicon porphyrins and
their derivatives would be attractive materials for surface mod-
ification2 and photodynamic therapy,3,4 the number of organo-
silicon porphyrins is quite limited and only homoleptic axial
ligands have been known. Kadish and co-workers have reported
octaethylporphyrinato dimethylsilicon and diphenylsilicon com-
plexes.5 Aida and co-workers have reported several symmetrical
diorganosilicon tetraphenylporphyrin complexes Si(TPP)R2
(R ¼ Et, n-Pr, CH2SiMe3, Ph, CH=CH2, and CꢁCPh)
(Chart 1).6 They found that axial alkyl ligands on silicon porphy-
rin complexes are highly photolabile and form the corresponding
tetracoordinated silyl diradicals.6c
˚
atoms at 3.107–2.984 A, within the sum of van der Waals radii
(3.25 A),10 indicating severe steric repulsion exists in these posi-
˚
tions and is responsible for the ruffled structure. Si–Me and
˚
Si–Ph bond lengths are 1.905(3) and 1.940(3) A, which are much
longer than standard tetravalent silicon–carbon bond lengths
10
˚
(1.87 A), showing typical 3c-4e hypervalent bonding. The
remarkably long Si–Ph bond can be attributed to a weak 3c-4e
bond, which is readily deformed by steric repulsion.
In sharp contrast to simple prediction of the bond lengths,
when a C6D6 solution of Si(TPP)MePh was irradiated by a
100 W halogen lamp for 7 min at room temperature in the pres-
ence of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), selec-
tive Si–Me bond cleavage occurred to give Si(TPP)Ph(TEMPO)
and MeTEMPO in quantitative yields (Scheme 1).11,12 Si(TPP)-
Me(TEMPO) was not observed in the resulting mixture. Since an
intense broad ESR signal was observed after irradiation at room
temperature without TEMPO, persistent pentacoordinated silyl
radical might be formed by homolysis of a Si–C bond in
Si(TPP)MePh.13 Irradiation of Si(TPP)MePh in carbon tetra-
chloride formed only Si(TPP)ClPh12 by chlorine abstraction
from carbon tetrachloride, showing typical silyl radical charac-
Since a key electronic characteristic of hexacoordinated sil-
icon is three-center four-electron (3c-4e) bonding using a 3p or-
bital of silicon, axial ligands can interact with each other. There-
fore, a diorganosilicon porphyrin complex bearing different
axial ligands would be a good probe for their reactivity and
selectivity. We report herein synthesis, structure, and electronic
characteristics of a diorganosilicon porphyrin bearing different
axial ligands, Si(TPP)MePh, and its selective Si–Me bond cleav-
age by photolysis and oxidation. Theoretical studies of porphy-
rinato methylphenylsilicon complex (abbreviated to Si(Por)-
MePh) as a model compound indicate that ꢀ–ꢁ orbital interac-
tion plays an important role in the properties of the silicon por-
phyrin.
C1
N1
N2
The reaction of Si(TPP)ClMe7 with phenylmagnesium chlo-
ride afforded Si(TPP)MePh as a green photosensitive solid in
N4
Si1
N3
C2
R1
R1
Ph
Ph
N
N
N
Si
N
N
Si N
Figure 1. ORTEP drawing of Si(TPP)MePh (30% thermal ellip-
soids; benzene molecules and hydrogen atoms are omitted for
N
N
Ph
Ph
˚
clarity). Selected bond lengths (A) and angles (deg): Si1{N1 ¼
R2
Si(TPP)R1R2
R2
Si(Por)R1R2
1:974ð2Þ, Si1{N2 ¼ 1:996ð2Þ, Si1{N3 ¼ 1:975ð2Þ, Si1{N4 ¼
1:982ð2Þ, Si1{C1 ¼ 1:905ð3Þ, Si1{C2 ¼ 1:940ð3Þ; C1{Si1{C2 ¼
178:8ð1Þ, C1{Si1{N1 ¼ 89:6ð1Þ, N3{Si1{N4 ¼ 90:6ð1Þ.
Chart 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.4 (2009)
363
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100.
(i)
Si(TPP)Ph(TEMPO) + MeTEMPO
(iv)
(ii)
Si(TPP)ClPh
Si(TPP)Ph(n-Pr)
Si(TPP)MePh
2
3
Germanium porphyrin complexes were applied for photodynamic
therapy: K. M. Kadish, Q. Y. Xu, J.-M. Barbe, J. E. Anderson, E.
Wang, R. Guilard, J. Am. Chem. Soc. 1987, 109, 7705.
H. Horiuchi, T. Tanaka, K. Yoshimura, K. Sato, S. Kyushin, H.
Matsumoto, H. Hiratsuka, Chem. Lett. 2006, 35, 662.
(iii)
Si(TPP)(OH)Ph
(i) h
(ii) h
ν
ν
(7 min), TEMPO, C6D6
(10 min), CCl4
(iii) TCNQ, CH2Cl2
(iv) n-PrMgBr, THF
4
5
6
K. M. Kadish, Q. Y. Xu, J.-M. Barbe, R. Guilard, Inorg. Chem.
1988, 27, 1191.
Scheme 1.
a) J.-Y. Zheng, K. Konishi, T. Aida, Inorg. Chem. 1998, 37, 2591.
b) J.-Y. Zheng, K. Konishi, T. Aida, Chem. Lett. 1998, 453. c) J.-Y.
Zheng, K. Konishi, T. Aida, J. Am. Chem. Soc. 1998, 120, 9838.
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4085. b) K. M. Kane, F. R. Lemke, J. L. Petersen, Inorg. Chem.
1997, 36, 1354.
Spectroscopic data for Si(TPP)MePh: green crystalline solid,
1H NMR (300 MHz, C6D6): ꢂ 8.97 (s, 8H, pyrrole), 7.90 (d, 8H,
J ¼ 6:7 Hz, o-H in meso-Ph), 7.41–7.28 (m, 12H, m-H and p-H
in meso-Ph), 5.46 (t, 1H, J ¼ 7:8 Hz, axial p-Ph), 5.07 (t, 2H,
J ¼ 7:8 Hz, axial m-Ph), 1.11 (d, 2H, J ¼ 7:8 Hz, axial o-Ph),
ꢂ6:38 (s, 3H, axial SiMe); 13C NMR (126 MHz, CD2Cl2): ꢂ
150.4, 143.6, 140.5, 133.8, 131.0, 127.4, 126.3, 126.2, 122.0,
121.7, 117.9, 10.8; 29Si NMR (99 MHz, CD2Cl2): ꢂ ꢂ197:4; IR
(KBr, cmꢂ1): 1600, 1490, 1440, 1350, 1260; UV–vis (CH2Cl2):
7
8
Figure 2. KS-HOMO of Si(Por)MePh.
ter.14 No significant decomposition of Si(TPP)Ph(TEMPO) and
Si(TPP)ClPh was observed even for thirty-minute irradiation.
Cyclic voltammetry of Si(TPP)MePh in dichloromethane
showed an irreversible first oxidation wave at lower potential
(Epa ¼ þ0:73 V vs. Ag/AgCl) than a reversible first oxidation
wave (E1=2 ¼ þ1:07 V) of the corresponding free base TPPH2.
Si(TPP)MePh was treated with one equivalent tetracyano-
quinodimethane (TCNQ) as a one-electron oxidant in the dark
to form Si(TPP)(OH)Ph in 91% yield after usual work-up.15
One-electron oxidation of Si(TPP)MePh and a trace amount of
water in the reaction vessel also caused selective Si–Me bond
cleavage.
In order to obtain insight into the electronic structure of di-
organosilicon porphyrin complexes, DFT calculations were car-
ried out using Si(Por)MePh as a model compound.16 The highest
occupied Kohn–Sham orbital (KS-HOMO) of Si(Por)MePh is
shown in Figure 2, which is formed by significant interaction
between the porphyrin ꢁ orbital and 3c-4e ꢀSi{C orbital. The
3c-4e bonding orbital and porphyrin ꢁ system can interact effec-
tively with each other because of their favorable geometry and,
therefore, raise their KS-HOMO. Consequently, one-electron
removal from the KS-HOMO by irradiation or oxidation weak-
ened the axial bond. Evaluation of the bond dissociation energies
by DFT calculations supports the selective Si–Me bond cleav-
age. Cleavage of a Si–Me bond is by 38.1 kJ/mol more favorable
than that of a Si–Ph bond probably owing to the instability of
phenyl radical.
ꢃ
max/nm (log ") 342 (4.53), 452 (5.49), 559 (3.44), 602 (4.00),
649 (4.40); Anal. Found: C, 82.55; H, 5.34; N, 7.32%. Calcd for
51H36N4Si: C, 83.57; H, 4.95; N, 7.64%.
C
.
Crystal data for Si(TPP)MePh C6H6: Mr ¼ 811:04, monoclinic,
˚ ˚
9
P21=n, T ¼ 173 K, a ¼ 13:1694ð9Þ A, b ¼ 23:401ð2Þ A, c ¼
ꢃ
3
˚
˚
14:416ð1Þ A, ꢄ ¼ 102:327ð1Þ , V ¼ 4340:1ð5Þ A , Z ¼ 4, R1 ¼
0:0826 (all data), wR2 ¼ 0:1623 (all data), GOF ¼ 1:229. Crystal-
lographic data have been deposited with Cambridge Crystal-
lographic Data Centre as supplementary publication no. CCDC-
717651.
10 a) A. Bondi, J. Phys. Chem. 1964, 68, 441. b) L. E. Sutton, Tables of
Interatomic Distances and Configurations in Molecules and Ions,
The Chemical Society, London, 1958.
11 These experimental details and physical data are in Supporting
Information, which is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
12 Isolation of Si(TPP)Ph(TEMPO) and Si(TPP)ClPh was failed
owing to their instability toward chromatography. Chemical
trap of Si(TPP)ClPh using 1-propylmagnesium bromide gave
Si(TPP)Ph(n-Pr) in 59% yield.
13 According to the reaction products, participation of tetracoordi-
nated silyl biradical6c in the reaction of Si(TPP)MePh may be ruled
out.
14 C. Chatgilialoglu, C. H. Schiesser, in The Chemistry of Organic
Silicon Compounds, ed. by Z. Rappoport, Y. Apeloig, John Wiley
& Sons, Chichester, 2001, Vol. 3, Chap. 4.
15 Further oxidation of Si(TPP)MePh using an excess amount of
TCNQ proceeded, and Si(TPP)(OH)26b was obtained in 75% yield.
Structures of Si(TPP)(OH)Ph and Si(TPP)(OH)2 were determined
by NMR spectroscopy and X-ray structural analysis. The experi-
mental details are described in Supporting Information.
16 All calculations were carried out at the B3LYP/6-31G(d) level
with Gaussian 03 package programs. Details and full reference of
Gaussian 03 are described in Supporting Information.
This work was supported by Grants-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
References and Notes
Reviews on hypervalent silicon compounds: a) C. Chuit, R. J. P.
1
Published on the web (Advance View) March 21, 2009; doi:10.1246/cl.2009.362