Synthesis and reactivity of the non-bridged metal-metal bonded rhodium octamethoxyporphyrin dimer

Article abstract of DOI:10.1016/S0022-328X(99)00147-3

Electron-rich rhodium porphyrin alkyls (omp)RhR (R=Me, ⁱPr, omp=2,3,7,8,12,13,17,18-octamethoxyporphyrin dianion) were photolyzed to give the non-bridged metal-metal bonded complex Rh₂(omp)₂. Reaction of (omp)RhH with TEMP

Full text of DOI:10.1016/S0022-328X(99)00147-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 584 (1999) 235–239
Synthesis and reactivity of the non-bridged metal–metal bonded
rhodium octamethoxyporphyrin dimer
Maoqi Feng, K.S. Chan *
Department of Chemistry, Chinese Uni6ersity of Hong Kong, Shatin, NT, Hong Kong
Received in revised form 8 March 1999
Abstract
i
Electron-rich rhodium porphyrin alkyls (omp)RhR (R=Me, Pr, omp=2,3,7,8,12,13,17,18-octamethoxyporphyrin dianion)
were photolyzed to give the non-bridged metal–metal bonded complex Rh2(omp)2. Reaction of (omp)RhH with TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl) also yielded Rh2(omp)2 quantitatively. Rh2(omp)2 reacted with CH3I to produce
(omp)RhMe and (omp)RhI, and with Et3SiH to give (omp)RhSiEt3 and (omp)RhH in 42–52% yield, respectively. Addition of
Rh2(omp)2 with styrene yielded phenylethyl-bridged complexes, (omp)RhCH2CH(Ph)Rh(omp). Rh2(omp)2 reacted with
triphenylphosphine to produce the cationic and anionic RhIII species of [(omp)Rh]+ and [(omp)Rh]−. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Rhodium; Hydride; TEMPO; Photolysis; Metal–metal bond; Porphyrin
been synthesized using this method in over 90% yield
[6].
1. Introduction
Synthesis of the electron-rich rhodium octamethoxy-
Non-bridged d7–d7 metal–metal bonded complexes
porphyrin dimer provides further example to study the
have attracted considerable interest owing to their rich
reactivity of electron-rich metal–metal bonded por-
chemistry. Besides this unique structural aspect, the
phyrin dimer. In this paper we report the synthesis and
metal–metal bond undergoes facile dissociation to yield
chemistry of an electron-rich octamethoxyporphyrin
an extremely reactive metal centered radical. These
(Fig. 1) non-bridged Rh–Rh bonded dimer.
monomers undergo halogen abstraction [1], C–H acti-
vation [2] and olefin insertion [3].
There are three literature methods for synthesizing
rhodium metal–metal bonded complexes: (i) photolysis
2. Results and discussion
of metal alkyls [4]; (ii) aerobic oxidation of rhodium
hydride [5]; and (iii) reaction of metal hydride with
2.1. Synthesis of Rh2(omp)2
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) [6]. The
first method usually requires a long reaction time, while
i
2.1.1. Photolysis of (omp)RhR (R=Me, Pr)
H2omp (omp=2,3,7,8,12,13,17,18-octamethoxypor-
the second method is difficult to handle because over-
phyrin dianion) (see Fig. 1) was prepared according to
oxidation of the air-sensitive metal–metal bonded
the literature method [7]. H2omp and rhodium trichlo-
dimer is possible. The third method is the most conve-
ride hydrate were refluxed in benzonitrile under nitro-
nient and efficient since TEMPO and the TEMPO
reduction coproduct (TEMPOH) are easily removed by
vacuum evaporation. Rh2(oep)2 and Ir2(oep)2 (oep=
2,3,7,8,12,13,17,18-octaethylporphyrin dianion) have
gen for 2 h to give 30% yield. The solubility of
(omp)RhCl was very good in most organic solvents.
1. NaBH /NaOH, N , 50°C, 2 h
4
2
(omp)RhClꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ(omp)RhMe
(1)
2. CH I, N , 0°C, 1 h
3
2
48%
Rhodium octamethoxyporphyrin methyl was pre-
pared from the reductive alkylation of (omp)RhCl.
* Corresponding author. Fax: +852-26035057.
E-mail address: ksc@cuhk.hk (K.S. Chan)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00147-3

236
M. Feng, K.S. Chan / Journal of Organometallic Chemistry 584 (1999) 235–239
After the red solution of (omp)RhCl was reduced to the
brown solution of RhI and oxidative addition with
CH3I, (omp)RhMe was obtained in 48% yield (Eq. (1))
[8]. The synthesis of rhodium octamethoxyisopropyl
was similar to that of (omp)RhMe.
Scheme 1.
was reduced to RhI(omp) and was protonated with HCl
(2 M) to yield (omp)RhH as a red precipitate in 52%
yield (Eq. (3)). When 95% EtOH was used as the
solvent, (omp)RhH formed, remained in solution and
only a little precipitate was produced thus lowering the
yield. When 75% EtOH was used as the solvent, more
precipitate formed and the yield was improved. The
compound (omp)RhH exhibited a characteristic hy-
dride resonance at −50.64 ppm as a doublet with
(2)
Photolysis of (omp)RhMe in C6H6 in the presence of
TEMPO yielded a new species with a methine peak of
the meso-protons at l 9.72 ppm shifted from 10.45 ppm
and methoxy group of b-positions at l 4.82 ppm shifted
from 4.45 ppm in its proton NMR spectrum (Eq. (2)).
These features are highly characteristic of metal–metal
bonded dimers of rhodium porphyrin derivatives be-
cause of increasing shielding of the protons after dimer
formation [9,10]. Besides, the singlet at l 3.60 ppm was
assigned to the methyl group of the coproduct of
TEMPOCH3 which was formed after photolysis, the
value was the same as the literature data [11]. In the
UV–vis spectrum the Soret band was red shifted from
385 to 397 nm after photolysis. TEMPOCH3 and excess
TEMPO were removed by vacuum evaporation.
The photolysis time of (omp)RhMe was much
shorter when TEMPO was used, the reaction time was
reduced to 32 h from more than 72 h when six equiva-
lents of TEMPO was added.
J
Rh–H=43.2 Hz which was similar to the −41.4 ppm
and JRh–H=44 Hz of (oep)RhH [13].
1. NaBH , EtOH (75%), N , 50°C, 2 h
4
2
(omp)RhClꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ(omp)RhH
2. HCl, N , 0°C, 1 h
2
52%
(3)
The compound (omp)RhH reacted with PhCHꢂCH2
to afford (omp)RhCH2CH2Ph in 82% yield (Eq. (4)).
The mechanism is a probable concerted [2+2] addi-
tion, or not purely via a carbon centered intermediate
[14] as opposed to a radical mechanism since no other
isomeric product was observed [1].
C D
6
(omp)RhH+PhCHꢂCH2ꢀꢀꢀꢀ6ꢀꢁ(omp)RhCH2CH2Ph
r.t., 2 h
82%
(4)
The photolysis time of (omp)RhiPr in the presence of
TEMPO was shorter than that of (omp)RhMe and
required only 20 h. The bond strength of Rh–isopropyl
is probably lower than that of Rh–Me due to steric
effects as well as the greater stability of the isopropyl
radical compared to the methyl, and therefore the
Rh–C bond was more easily cleaved [12].
The compound (omp)RhH reacted with TEMPO to
yield Rh2(omp)2 quantitatively in about 4 h at room
1
temperature. The H-NMR of the dimer synthesized in
this way was identical with that of the dimer synthe-
sized from photolysis of (omp)RhR.
C D
6
6
(omp)RhH+TEMPOꢀꢀꢀꢀꢀꢀꢀꢀꢁRh2(omp)2
(5)
A
plausible mechanism for the synthesis of
quantitatively
Rh2(omp)2 was shown in Scheme 1. The Rh–alkyl
bond underwent photolytically initiated homolysis to
yield RhII(omp) and the methyl radical. The methyl
radical was trapped by TEMPO to yield 2,2,6,6-te-
tramethylpiperidine-1-methoxy. The RhII(omp) under-
went dimerization to yield Rh2(omp)2.
2.2. Properties of Rh2(omp)2
Rh2(omp)2 underwent typical oxidative addition with
organic substrates. Reaction of Rh2(omp)2 with CH3I
afforded (oep)RhMe and (omp)RhI in 44 and 32%
yield, respectively (Eq. (6)). When Rh2(omp)2 reacted
with triethylsilane, (omp)Rh(SiEt3) and (omp)RhH
were produced in 52 and 33% yield, respectively (Eq.
(7)). Styrene underwent facile insertion reaction into the
Rh–Rh bond of Rh2(omp)2 to give (omp)RhCH2-
CH(Ph)Rh(omp) in 85% yield (Eq. (8)). The chemical
shifts of the meso-methine protons in the two porphyrin
rings were 9.78 and 10.56 ppm and the diastereotopic
methylene protons binding to Rh were split into two
peaks.
2.1.2. Reaction of (omp)RhH with TEMPO
The compound (omp)RhH was synthesized by the
method similar to the synthesis of (oep)RhH [5]. A
suspension of (omp)RhCl in EtOH (aq.) (75%) solution
When excess triphenylphosphine was added into
Rh2(omp)2 in C6D6, the chemical shift of meso-methine
protons had shifted from 9.76 to 9.78 and 10.56 ppm,
Fig. 1. Molecular structure of H2omp.

M. Feng, K.S. Chan / Journal of Organometallic Chemistry 584 (1999) 235–239
237
and that of methoxy protons had shifted from 4.85 to
4.1. Synthesis of octamethoxyporphyrin H2omp
4.38 and 4.67 ppm. The shifts were due to the het-
erolytic cleavage of Rh2(omp)2 into Rh(omp)+and
Rh(omp)− as observed similarly in the reaction of
Rh2(oep)2 with pyridine [15]. This heterolytic cleavage
of RhII dimer into RhI and RhIII monomers by PPh3
may involve ligand-induced polarization of the Rh–Rh
bond [16].
H2omp was prepared and characterized as Ref. [7].
4.2. Synthesis of rhodiumIII octamethoxyporphyrin
(omp)RhCl
C D
6
6
H2omp (60 mg, 0.11 mmol) and RhCl3·3H2O (43 mg,
0.16 mmol) were added into PhCN (10 ml) in a 50 ml
flask and the suspension was heated to reflux for 2 h
under N2. Then PhCN was vacuum distilled off. The
dark brown residue was separated by column chro-
matography using CH2Cl2 as the eluent to give
(omp)RhCl as a purple red solid (22.5 mg, 30% yield).
Rf=0.62 (CH2Cl2); FAB MS (NBA) 686 [M+]; 1H-
NMR (CDCl3, 250 MHz) l 4.82 (s, 24H), 10.26 (s, 4H);
UV–vis (CH2Cl2, nm, log m) 392 (5.46), 518 (4.63), 553
(4.66); HRMS Calc. for RhC28H28N4O8Cl: 686.0665.
Found: 686.0665.
Rh2(omp)2+CH3Iꢀꢀꢀꢀꢁ(omp)RhMe+(omp)RhI
(6)
r.t., 2 h
C D
44%
32%
6
6
Rh2(omp)2+Et3SiHꢀꢀꢀꢀꢁ(omp)RhSiEt3+(omp)RhH
r.t., 2 h
52%
33%
(7)
Rh2(omp)2+PhCHꢂCH2
C D
6
6
ꢀꢀꢀꢀꢁ(omp)RhCH2CH(Ph)Rh(omp)
(8)
r.t., 2 h
85%
C D
Rh2(omp)2+2PPh3ꢀꢀꢀꢀꢁ(omp)Rh+(PPh3)
6
6
r.t., 2 h
+(omp)Rh−(PPh3)
(9)
3. Conclusions
4.3. Synthesis of octamethoxyporphyrinato methyl
rhodiumIII (omp)RhMe
An electron-rich metal–metal bonded porphyrin
dimer Rh2(omp)2 has been synthesized by the reaction
of (omp)RhH with TEMPO and photolysis of
(omp)RhR (R=Me, Pr). The oxidative addition of
Rh2(omp)2 with MeI, Et3SiH, and styrene is typical of
the chemistry of a metal–metal bonded dimer.
i
To a degassed solution of (omp)RhCl (24 mg, 0.035
mmol) in EtOH (10 ml) was added nitrogen-purged
NaBH4 (15 mg, 0.39 mmol) solution in NaOH (0.2 M,
1 ml) through a cannular, slowly. The red solution
changed into brown quickly. The brown solution was
stirred under nitrogen at 0°C for 1 h, then methyl
iodide (0.1 ml, 0.16 mmol) was added and red precipi-
tate was produced immediately. The reaction mixture
was extracted with CH2Cl2 (3×20 ml) and purified by
column chromatography using a solvent mixture of
CH2Cl2:hexane (1:2) as the eluent to give (omp)RhMe
(11 mg, 48%). Rf=0.86 (CH2Cl2). FAB MS (relative
intensity, %) 666 ([M+], 51), 651 (63), 620 (31), 591
4. Experimental
All manipulations were performed under nitrogen by
vacuum line techniques or in a Braun MB 150 M
1
glovebox. H-NMR spectra were recorded on either a
Bruker WM-250 (250 MHz), Bruker DPX-300 (300
MHz) or Bruker AMX-500 (500 MHz) spectrometer.
Chemical shifts (l) were reported with reference to the
residual CHCl3 (l 7.24 ppm) in CDCl3 or C6H6 (l 7.15
ppm) in C6D6, the coupling constant (J) was reported
in Hz. Low-resolution mass spectra were recorded on a
VG7070F mass spectrometer. High-resolution mass
spectra were recorded on a Bruker APEX 47e FT-ICR
mass spectrometer. IR spectra were obtained on a
Perkin–Elmer 1600 FT-IR spectrophotometer. UV–vis
spectra were recorded on a Hitachi U-3300 spectropho-
tometer. Yields were reported in either isolated yield or
NMR yield using the residue solvent proton as the
internal standard.
Unless otherwise noted, all materials were obtained
from commerical suppliers and used without further
purification unless otherwise specified. Rhodium
trichloride (RhCl3·3H2O) was obtained from Johnson
Matthey. Benzene-d6 was vacuum distilled from
sodium.
1
(19), 561 (8), 413 (17), 391 (26); H-NMR (C6D6, 300
MHz) l 6.04 (d, 3H, J=2.7 Hz), 4.45 (s, 24H), 10.45
(s, 4H); UV–vis (CH2Cl2, nm, log m) 385 (4.94), 549
(4.50); HRMS Calc. for RhC29H31N4O8: 666.1191.
Found: 666.1208.
4.4. Synthesis of octamethoxyporphyrinato isopropyl
rhodiumIII (omp)Rh(iPr)
The procedure was similar to the synthesis of
Rhomp(Me). Yield 32%. Rf=0.82 (CH2Cl2). FAB MS
(NBA) 694 [M+]; 1H-NMR (C6D6, 300 MHz) l −4.35
(d, 6H, J=4.4 Hz), −2.35 (d, 1H, J=2.4 Hz), 4.42 (s,
24H), 10.49 (s, 4H); UV–vis (CH2Cl2, nm) 384, 514,
548; HRMS Calc. for RhC31H35N4O8: 694.1504.
Found: 694.1469.

238
M. Feng, K.S. Chan / Journal of Organometallic Chemistry 584 (1999) 235–239
4.5. Photolysis experiment
4.8. Reaction of (omp)RhH with TEMPO
In a glovebox, (omp)RhMe (50 mg, 0.075 mmol) and
TEMPO (70 mg, 0.45 mmol, six equivalents) in de-
gassed benzene (10 ml) was loaded into a 50 ml flask
fitted with a vacuum-line-adapted Teflon valve. The
tube was closed, removed from the glovebox. Then it
was irradiated using a mercury lamp. Progress of the
The compound (omp)RhH (5 mg, 8 mmol) was dis-
solved in C6D6 (0.7 ml) in an NMR tube, and then
TEMPO (6 mg, 40 mmol) was added. The red solution
changed into dark red. The reaction was monitored by
1H-NMR. After 4 h, Rh2(omp)2 formed quantitatively
by NMR integration. 1H-NMR of Rh2(omp)2 was iden-
tical with that obtained from the photolysis experiment.
1
photolysis was monitored by H-NMR by aliquots of
about 0.4 ml of reaction mixture. These samples were
transferred through a rubber septum of a NMR tube,
the solvent was evaporated by vacuum and then ben-
zene-d6 (ca. 0.4 ml) was added. After the rhodium
methyl was completely consumed, the reaction mixture
was evaporated to dryness under vacuum. Rh2(omp)2
was obtained (39 mg, 81% yield). H-NMR (C6D6, 300
MHz) l 4.81 (s, 48H), 9.74 (s, 8H); UV–vis (C6H6, nm)
397, 519, 555.
4.9. Reactions of Rh2(omp)2 with Et3SiH
General procedure: in a glovebox, Rh2(omp)2 (4 mg,
6 mmol), Et3SiH (0.1 ml) and benzene-d6 (0.5 ml) were
loaded into a 5 mm diameter NMR tube fitted with a
vacuum-line-adapted Teflon valve. The tube was closed
and removed from the glovebox. It was degassed by the
freeze-pump-thaw method (three cycles), and then the
tube was sealed under vacuum. The reaction was moni-
1
1
tored by H-NMR. After about 2 h, (omp)RhH (33%
4.6. Synthesis of rhodiumIII octamethoxyporphyrin
hydride (omp)RhH
NMR yield) and (omp)RhSiEt3 (52% NMR yield) were
produced. Separation of the residue by column chro-
matography gave pure (omp)RhSiEt3 using CH2Cl2 as
eluent, while (omp)RhH was oxidized and remained in
the column. (omp)RhSiEt3: Rf=0.64 (CH2Cl2); 1H-
NMR (C6D6, 300 MHz) l −3.23 (q, 6H, J=7.8 Hz),
−1.14 (t, 9H, J=7.9 Hz), 4.41 (s, 24H), 10.44 (s, 4H);
FAB MS 765 [M+ −1]; UV–vis (hexane, nm) 385, 512,
545, 617.
To a degassed solution of (omp)RhCl (24 mg, 0.035
mmol) in EtOH (75%, 10 ml) was added nitrogen-
purged NaBH4 (15 mg, 0.39 mmol) solution in NaOH
(1 M, 1 ml) slowly through a cannular. The red solution
changed brown quickly. The brown solution was stirred
under nitrogen at 50°C for 2 h, cooled down to 0°C,
and then HCl (2 M, 5.0 ml) was added. Red precipitate
was produced immediately. After filtration under nitro-
gen with a cannular, the red solid was washed with
degassed H2O (4.0 ml) and MeOH (1.0 ml), respec-
tively. The red solid was then dried over vacuum (11.9
4.10. Reaction of Rh2(omp)2 with CH3I
The procedure was the same as that in Section 4.9.
(omp)RhMe (44% NMR yield) and (omp)RhI (32%
NMR yield) were produced. Pure products were sepa-
rated by column chromatography, at first CH2Cl2 was
used as solvent to elute (omp)RhMe and then diethyl
ether was used to elute (omp)RhI. (omp)RhI: Rf=0.18
1
mg, 52% yield). H-NMR (C6D6, 300 MHz) l −50.64
(d, 1H, JRh–H=43.2 Hz), 4.41 (s, 24H), 10.54 (s, 4H);
UV–vis (C6H6, nm) 385, 542, 620.
1
(CH2Cl2); H-NMR (C6D6, 300 MHz) l 4.86 (s, 24H),
4.7. Reaction of (omp)RhH with PhCHꢂCH2
10.24 (s, 4H); FAB MS (NBA) 778 [M+].
To a solution of (omp)RhH (4 mg, 6 mmol) in
degassed C6H6 (1.0 ml) was added styrene (0.1 ml) with
a microsyringe. The red solution changed into dark red.
After 2 h, the mixture was evaporated to dryness under
vacuum and the residue was separated by column chro-
matography using CH2Cl2 as the eluent to give
(omp)Rh(CH2CH2Ph) (3.8 mg, 82% yield). Rf=0.54
(CH2Cl2); 1H-NMR (300 MHz, C6D6) l −5.62 (dt,
2H, JRh–H=3.0, JH–H=7.5 Hz, RhCH2), −3.95 (t,
2H, J=8.1 Hz, RhCH2CH2), 4.28 (d, 2H, J=6.9 Hz,
ortho-H), 4.73 (s, 24H, OMe), 6.15 (m, 2H, meta-H in
C6H5), 6.29 (m, 1H, para-H), 9.89 (s, 4H, meso-H in
omp); FAB MS (relative intensity, %) 756 ([M+], 2),
651 ([M+ −CH2CH2Ph], 13); UV–vis (benzene, nm)
384, 512, 548, 712.
4.11. Reaction of Rh2(omp)2 with PhCHꢂCH2
Rh2(omp)2 (4 mg, 6 mmol) was dissolved in benzene-
d6 (0.5 ml) in an NMR tube, PhCHꢂCH2 (0.1 ml) was
added with a microsyringe. The reaction was monitored
1
by H-NMR. After 2 h, (omp)RhCH2CH(Ph)Rh(omp)
was produced in 85% yield. 1H-NMR (300 MHz, C6D6)
l −11.19 (m, 1H), −9.54 (m, 1H), −8.71 (m, 1H,
CHH%CHPh), −0.30 (d, 1H, JH–H=7.5 Hz, ortho-H),
0.93 (d, 1H, JH–H=7.5 Hz, ortho%-H), 4.49 (s, 24H,
OMe), 4.80 (s, 24H, OMe), 5.05 (dd, 1H, J=2.4, 9.0
Hz, meta-H), 5.60 (dd, 1H, J=2.4, 9.0 Hz, meta-H%),
6.62 (dd, 1H, J=2.4, 9.0 Hz, para-H), 9.78 (s, 4H,
meso-H in OMP), 10.56 (s, 4H, meso-H in omp);
UV–vis (benzene, nm) 386, 516, 549, 713.

M. Feng, K.S. Chan / Journal of Organometallic Chemistry 584 (1999) 235–239
239
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5305.
4.12. Reaction of Rh2(omp)2 with PPh3
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Rh2(omp)2 (4 mg, 6 mmol) was dissolved in ben-
zene-d6 (0.5 ml) in an NMR tube, PPh3 (5.4 mg, 20
1
mmol) was added. The reaction was monitored by H-
NMR.
After
3
h,
(omp)Rh+PPh3
and
(omp)Rh−PPh3 was produced. 1H-NMR (300 MHz,
C6D6) l 4.38 (s, 24H, OMe in (omp)Rh−), 4.67 (s,
24H, OMe in (omp)Rh+), 7.08–7.35 (m, PPh3), 9.61
(s, 4H, meso-H in (omp)Rh−), 10.76 (s, 4H, meso-H
in (omp)Rh+).
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Acknowledgements
We thank the Croucher Foundation of Hong Kong
for financial support.
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