A facile synthesis of rhodium(III) porphyrin-silyls

Article abstract of DOI:10.1016/S0022-328X(98)00812-2

Rhodium(III) porphyrin-silyls [Me₃SiRhT(p-X)PP (X=H, Me)] were synthesized from the reactions of the rhodium(I) porphyrin anions, generated from the reduction of the rhodium(III) porphyrin chlorides with the sodium amalgam in toluene, with degassed Me₃SiCl at room temperature. A single crystal structure of (5,10,15,20-tetraphenylporphyrinato)(trimethylsilyl)rhodium(III) (1) showed that the Rh-Si bond length is equal to 2.305 ?.

Full text of DOI:10.1016/S0022-328X(98)00812-2

Journal of Organometallic Chemistry 568 (1998) 257–261
A facile synthesis of rhodium(III) porphyrin–silyls
Andy K.-S. Tse, Bo-Mu Wu, Thomas C.W. Mak, Kin Shing Chan *
Department of Chemistry, The Chinese Uni6ersity of Hong Kong, Hong Kong, China
Received 26 May 1998
Abstract
Rhodium(III) porphyrin–silyls [Me3SiRhT(p-X)PP (X=H, Me)] were synthesized from the reactions of the rhodium(I)
porphyrin anions, generated from the reduction of the rhodium(III) porphyrin chlorides with the sodium amalgam in toluene,
with degassed Me3SiCl at room temperature.
nato)(trimethylsilyl)rhodium(III) (1) showed that the Rh–Si bond length is equal to 2.305 A. © 1998 Elsevier Science S.A. All
A
single crystal structure of (5,10,15,20-tetraphenylporphyri-
˚
rights reserved.
Keywords: Rhodium; Porpyhrin–silyls; Structures
1. Introduction
sometimes heterogenous nature of the systems [7]. Fur-
thermore, the isolation of stable compounds in the
reaction mixtures may not always be the active interme-
diates. Since the fundamental requirement for the de-
sign of an appropriate catalyst is not well understood
and may be due to the somewhat primitive state of our
understanding of the chemistry of the transition metal–
silyl bond [8], the synthesis of rhodium silyls has mainly
been achieved through the oxidation of silanes into low
valent rhodium metal centers [3]. We are motivated to
study the properties of the rhodium–silyl porphyrin
complexes and now report the facile synthesis of
rhodium porphyrin silyls from the reductive silylation
of rhodium porphyrin chlorides.
The studies of intermediates as well as the measure-
ments of the bond dissociation energies are important
in understanding chemical reactions [1]. Transition
metal silyl complexes have been proposed to be vital
in catalytic processes such as hydrosilylation [2], sil-
ane polymerization [3] and silylformylation [4]. Tran-
sition metal catalyzed hydrosilylation of olefins is of
paramount importance for the formation of silicon
carbon bonds. Rhodium(I) complexes such as
RhCl(PPh3)3, HRh(CO)(PPh3), [Rh(CO)2Cl]2, [Rh(C2
H4)2Cl]2, Rh(acac)(CO)2 etc., have been demonstrated
to be powerful catalysts in the hydrosilylation of olefins
[5]. Rhodium–cobalt clusters like Co2Rh2(CO)12,
Rh4(CO)12 and (tBuNC)RhCo(CO)4 catalyze the silyl-
formylation of hydrosilanes and 1-hexyne at 10 atm of
CO to give (Z)-1-silyl-2-formyl-1-hexenes [4].
2. Result and discussion
The involvement of metal–silyl intermediates are
proposed. However, the detection of intermediates is
hampered by the fact high catalytic rates [6] and the
Rhodium(III) porphyrin–silyls were prepared from
the reaction of nucleophilic rhodium porphryin anions
with trimethylsilyl chloride in toluene. Since the reac-
tion of Me3SiLi–HMPA with cobalt porphyrin chlo-
rides did not yield the cobalt porphyrin silyls but the
cobalt porphyrin phosphoryl complex [9], the use of
silyl lithium would not be expected to be fruitful. The
* Corresponding author. Fax: +852 26035057; e-mail:
ksc@cuhk.hk
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00812-2

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A.K.-S. Tse et al. / Journal of Organometallic Chemistry 568 (1998) 257–261
reaction of RhI porphyrin anion in THF, a polar
solvent, with Me3SiCl was not suitable since THF is
known to be polymerized by Me3SiCl [10]. The nucle-
ophilic rhodium(I) porphyrin anion [RhI(por)], was
generated successfully from the reduction of the rhodiu-
m(III) porphyrin chlorides 3 and 4 by sodium amalgam
in toluene for 2 days [11] and reacted with Me3SiCl at
room temperature (r.t.) to produce the corresponding
rhodium(III) porphyrin–silyls [Me3SiRh(tpp) 5 and
Me3SiRh(tpp) 6] in 29 and 37% yield, respectively (Eq.
1).
Table 1
Chemical shifts and 13C–1H coupling constants of complex 6
Carbon
Chemical shift, l/ppm
JC–H (Hz)
1
2
3
4
5
6
7
123.38
128.67
134.55
137.84
134.20
127.96
22.22
141.4
159.3
164.9
141.4
127.1
Meso
h
144.20
139.94
131.83
−1.09
i
141.4
120.5
Me3Si carbon
the line widths at half maximum were 1590 and 2226
Hz, respectively. The upfield shifted signal was due to
the porphyrin ring current effect and the magnitude
was comparable to the literature value of
Et3SiRh(OEP) [12].
The structure of 5 was confirmed by a single crystal
X-ray diffraction study (Fig. 2). Crystals were grown
from a chloroform-dichloromethane-hexane solution.
Details of data collection and processing parameters
was given in Table 2. The coordination sphere of the
rhodium atom shows a square pyramidal geometry with
four porphyrinato nitrogen atoms occupying the basal
sites and the silicon atom of trimethylsilyl group resid-
ing at the axial site. The mean bond length of the
1
Complexes 5 and 6 exhibit characteristic H and 13C
resonances. In the 1H-NMR, these two complexes 5
and 6 showed high field singlets at −3.82 and −3.81
ppm, respectively with each corresponding to nine pro-
tons which are consistent with the presence of
timethylsilyl groups. The upfield shift to −3.81 ppm is
due to the porphyrin ring current effect on the Me3Si
group. The 13C-NMR spectra of complexes 5 and 6
showed high field signals at −1.72 and −1.74 ppm,
respectively corresponding to the trimethylsilyl carbons.
An interesting aspect in the 13C-NMR spectra for 6 was
that all the tolyl carbons are non-equivalent with four
singlets for the C2–C6 and C3–C5 positions (Fig. 1).
The chemical shift difference of C2–C6 and C3–C5
carbons was due to the orthogonal nature of the 18 y
porphyrin macrocycle and the tolyl group in this five
coordinated rhodium complex.
˚
Rh–N bonds is 2.016 A which agrees with the five-co-
ordinated organorhodium(III) porphyrin [13]. The Rh–
˚
Si length is 2.035 A which is similar to the triethylsilyl
rhodium porphyrin–silyl bond lengths [13]. The por-
phyrin ring is close to planar with a mean deviation of
˚
0.0756 A from the basal plane for the four nitrogens,
whereas the nitrogen atoms deviate alternatively above
˚
and below the plane by 0.0232 A (Table 3).
Recently, a non-concerted radical example of the
reaction of a rhodium porphyrin dimer with a silane to
yield rhodium(III) porphyrin-silyls has been docu-
mented [12]. However, the relatively inaccessible
rhodium porphryin dimer starting material is required
[13]. In summary, rhodium(III) porphyrin–silyls have
been synthesized from the reaction of rhodium por-
phryin anion and Me3SiCl.
The gate-decouped 13C-NMR spectrum of complex 6
aids us in assigning all the carbons of the porphyrin
ring and the Me3Si group. The chemical shifts and
13C–1H coupling constants are summarized in Table 1.
The 29Si-NMR spectra of complexes 5 and 6 showed
an upfield singlet at −116.75 and −116.81 ppm and
3. Experimental
UV–vis spectra were recorded on a Hitachi U-3300
spectrophotometer using CH2Cl2 as the solvent. 1H-
NMR spectra were recorded on a Bruker WM 250 (250
MHz) spectrometer. Chemical shifts (l) were reported
with reference to the residual CHCl3 (l 7.24 ppm) in
CDCl3 and the coupling constant (J) was reported in
Fig. 1. General structural formula of porphyrin–silyl comlpexes.

A.K.-S. Tse et al. / Journal of Organometallic Chemistry 568 (1998) 257–261
259
Hertz (Hz). 13C-NMR spectra were obtained on either a
Bruker WM 250 (62.9 MHz) or Bruker AMX 500 (125
MHz) spectrometer and referenced to the residual CHCl3
(l 77.00 ppm) in CDCl3. 29Si-NMR spectra was taken on
a Bruker AMX 500 (99 MHz) spectrometer and the
chemical shift (l) was referenced to the external standard
TMS (0.00 ppm). 31P-NMR spectra was recorded on a
Bruker AMX 500 (202 MHz) spectrometer and the
chemical shift (l) was referenced to the external standard
H3PO4 (0.00 ppm). FAB MS spectra were recorded on
a Joel JMS-HX 110 Mass Spectrometer using m-ni-
trobenzyl alcohol (NBA) as the matrix at National
Tsing-Hua University, Taiwan. Elemental Analysis were
performed by Medac, Department of Chemistry, Brunel
University, United Kingdom.
Table 2
Crystal data for Rh(tpp)SiMe3
5
Empirical formula
Crystal system
Space group
(C47H37N4RhSi) (1/2CHCl3)
Triclinic
P1 (no. 2)
848.5
12.723(3)
13.005(3)
15.091(3)
100.46(1)
102.52(1)
116.47(1)
2
Formula weight
˚
a (A)
˚
b (A)
˚
c (A)
h (°)
i (°)
k (°)
Z
F(000)
870
2068.6(10)
1.362
3
˚
V (A )
Dcalc. (g/cm3)
Crystal size (mm)
Radiation
0.30×0.30×0.50
Mo–Kh (u=0.71073 A)
˚
Unless otherwise noted, all materials were obtained
from commercial suppliers and used without further
purification. THF and toluene were distilled from the
sodium benzophenone ketyl and sodium, respectively.
Monochromator
Highly oriented graphite
crystal
0.577
7692
v (mm−1
Number of unique reflections
)
Number of reflections with I\10| 5425
Number of variables
R, Rw (obs data)
Goodness-of-fit
531
0.056, 0.068
1.18
Largest and mean shift HD/|
0.101, 0.001
1.05 to −0.91
Res extrema in final diff map
3
˚
(e/A )
1
2
2
2
R=ꢀꢁFo–ꢁFoꢂ/ꢀꢁFoꢁ, wR=[ꢀw2(ꢁFoꢁ–ꢁFcꢁ /ꢀw2ꢁFoꢁ ] .
HMPA was distilled from CaH2. The complex Me3SiCl
was distilled from CaH2, stored in a Telfon stoppered
round-bottomed flask and was degassed by the freeze-
pump-thaw method (three cycles) immediately prior to
use. Silica gel (70–230 mesh) was used for column
chromatography. Thin layer chromatography (TLC) was
performed on Merck percoated silica 60F254 plates.
3.1. Rhodium(III) tetraphenylporphyrin chloride
ClRh(tpp) (3) [14]
To a hot solution of H2(tpp) 1 (0.30 g, 0.48 mmol) in
PhCN (30 ml), RhCl3 ·3H2O (0.25 g, 0.95 mmol) was
Table 3
o
˚
Bond lengths (A) and angles ( ) of 5
˚
Bond length (A)
Rh(1)–N(1)
Rh(1)–N(3)
Rh(1)–Si(1)
Si(1)–C(46)
2.025(5)
2.014(5)
2.305(2)
1.879(7)
Rh(1)–N(2)
Rh(1)–N(4)
Si(1)–C(45)
Si(1)–C(47)
2.009(5)
2.017(5)
1.846(10)
1.857(8)
Bond angle (°)
Si(1)–Rh(1)–N(1)
N(1)–Rh(1)–N(2)
N(1)–Rh(1)–N(3) 174.2(2)
Si(1)–Rh(1)–N(4) 177.2(2)
Rh(1)–Si(1)–C(45) 111.4(3)
C(45)–Si(1)–C(47) 107.8(4)
94.7(1)
89.9(2)
Si(1)–Rh(1)–N(2)
Si(1)–Rh(1)–N(3)
N(2)–Rh(1)–N(3)
N(3)–Rh(1)–N(4)
Rh(1)–Si(1)–C(46) 112.8(2)
Rh(1)–Si(1)–C(47) 110.1(3)
91.3(1)
91.1(1)
90.0(2)
90.1(2)
Fig. 2. ORTEP drawing of (5,10,15,20-tetraphenylporphyri-
nato)(trimethylsilyl) rhodium(III) (5), (a) and edge view, (b) hydro-
gens have been omitted for clarity.

260
A.K.-S. Tse et al. / Journal of Organometallic Chemistry 568 (1998) 257–261
added and the mixture refluxed for 1 h to give a red
solution. After removal of the solvent by high vac-
uum, the crude product was chromatographed over
silica gel using CHCl3 as eluent to give 3 as a red
solid (0.35 g, 0.47 mmol, 95%). Rf=0.60 (1:1 hexane/
CH2Cl2); 1H-NMR (CDCl3, 250 MHz) l 7.95 (m,
12H), 8.12 (d, 8H, J=4.3 Hz), 8.96 (s, 8H).
(M+). Anal. Calc. for C51H45N4RhSi: C, 72.50; H,
5.37; N, 6.33. Found: C, 72.50; H, 5.37, N, 6.63.
3.4. 5,10,15,20-Tetraphenylporphyrinato trimethylsilyl
rhodium(III) Me3SiRh(tpp) (5)
The preparation of Me3SiRh(tpp) 5 followed a pro-
cedure similar to that of 6 described above. A de-
gassed solution of Rh(tpp)Cl 3 (0.10 g, 0.13 mmol)
and Na/Hg (4%, 3 g) in freshly distilled toluene (50
ml) was degassed and stirred for 2 days under N2
and was added to a solution of Me3SiCl (0.84 g, 7.8
mmol) to give orange crystals (0.030 g, 29%). Rf=
0.53 (CH2Cl2:hexane=1:3); 1H-NMR (CDCl3, 250
MHz) l −3.82 (s, 9H), 7.69 (m, 12H), 8.08 (d, 4H,
J=7.7 Hz), 8.18 (d, 4H, J=7.7 Hz), 8.63 (s, 8H);
13C-NMR (CDCl3, 62.9 MHz) l −1.72, 122.79,
126.62, 126.72, 127.60, 131.28, 133.64, 133.95, 142.20,
143.41; 29Si-NMR (CDCl3, 99.4 MHz) l −116.81;
UV–vis (CH2Cl2), umax, nm (log m) 406 (4.66), 513
(3.61); FAB MS 788 (M+). Anal. Calc. for
C47H37N4RhSi: C, 71.56; H, 4.73; N, 7.10. Found: C,
71.24; H, 5.52; N, 6.34.
3.2. Rhodium(III) tetratolylporphyrin chloride ClRh(ttp)
(4) [15]
To a hot solution of H2(ttp) 2 (0.30 g, 0.48 mmol)
in PhCN (30 ml), RhCl3 ·3H2O (0.25 g, 0.95 mmol)
was added and the mixture refluxed for 1 h to give a
red solution. After removal of solvent by high vac-
uum, the crude product was chromatographed over
silica gel using CHCl3 as eluent to give 4 as a red
solid (0.36 g, 0.45 mmol, 96%). Rf=0.78 (1:1 hexane/
CH2Cl2); 1H-NMR (CDCl3, 250 MHz) l 2.70 (s,
12H), 8.07 (d, 4H, J=7.5 Hz), 8.20 (d, 4H, J=7.5
Hz), 8.94 (s, 8H).
3.3. 5,10,15,20-Tetratolylporphyrinato trimethylsilyl
rhodium (III) Me3SiRh(ttp) (6)
4. Supplementary material
The preparation of Me3SiRh(ttp) 6 was described
as a typical procedure. A solution of Rh(ttp)Cl 4
(0.100 g, 0.133 mmol) and Na/Hg (4%, 3 g) in freshly
distilled toluene (50 ml) was degassed by the freeze-
pump-thaw method (three cycles). The red solution
was stirred at r.t. under N2 for 3 days to give a
greenish–brown solution. This solution was added
into a degassed solution of Me3SiCl (0.86 g, 7.98
mmol) via a cannular under N2 at r.t. in the absence
of light. The solution was stirred for 20 min and
CH2Cl2 was added. The mixture was washed with wa-
ter (2×50 ml) and dried (MgSO4). The solvent was
removed by high vacuum and the resulting residue
was chromatographed over silca gel using CH2Cl2/
hexane (1:4) as the eluent. The first fraction was col-
lected, evaporated and dried to give an orange solid.
It was recrystallized by CH2Cl2/hexane to give orange
crystals (0.042 g, 37%). Rf=0.54 (CH2Cl2:hexane=
X-ray diffraction data of complex 5 has been de-
posited in the Cambridege Crystallographic Data
Centre as supplementary material.
Acknowledgements
We thank the Research Grants Council of Hong
Kong (CUHK 300/94 P) for financial support.
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