Synthesis and properties of rhodium(III) porphyrin cyclic tetramer and cofacial dimer

Article abstract of DOI:10.1021/ic020652j

Rhodium(III) porphyrin complexes, [Rh(4-PyT₃P)Cl]₄ (1) and [Rh(2-PyTB₃P)Cl]₂ (2) (4-PyT₃P = 5-(4-pyridyl)-10,15,20-tritolylporphyrinato dianion, 2-PytB₃P = 5-(2-pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrinato dianion), were self-assembled and characterized by ¹H nuclear magnetic resonance spectroscopy, infrared spectroscopy, and electron spray ionization-mass spectroscopy methods. The spectroscopic results certified that the rhodium porphyrin complexes 1 and 2 have a cyclic tetrameric structure and a cofacial dimeric structure, respectively. The X-ray structure analysis of 1 confirmed the cyclic structure of the complex. The Soret bands of both oligomers were significantly broadened by excitonic interactions between the porphyrin units, compared to those observed for a corresponding analogue of Rh(TTP)(Py)Cl (TTP = 5,10,15,20-tetratolylporphyrinato dianion, Py = pyridine). Stepwise oxidation of the porphyrin rings in the oligomers was observed by cyclic voltammetry. The oligomers 1 and 2 are very stable in solution, and they slowly undergo reactions with pyridine to give corresponding monomer complexes only at high temperatures (～80 °C).

Full text of DOI:10.1021/ic020652j

Inorg. Chem. 2003, 42, 3187−3193
Synthesis and Properties of Rhodium(III) Porphyrin Cyclic Tetramer and
Cofacial Dimer
Keiko Fukushima,† Kenji Funatsu,† Akio Ichimura,‡ Yoichi Sasaki,† Masamitsu Suzuki,†
,†
Tetsuaki Fujihara,† Kiyoshi Tsuge,† and Taira Imamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan, and Department of Chemistry, Faculty of Science,
Osaka City UniVersity, Osaka 558-0022, Japan
Received November 4, 2002
Rhodium(III) porphyrin complexes, [Rh(4-PyT P)Cl]4 (1) and [Rh(2-PytB P)Cl]2 (2) (4-PyT P ) 5-(4-pyridyl)-10,-
3
3
3
15,20-tritolylporphyrinato dianion, 2-PytB P ) 5-(2-pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrinato dianion), were
3
1
self-assembled and characterized by H nuclear magnetic resonance spectroscopy, infrared spectroscopy, and
electron spray ionization−mass spectroscopy methods. The spectroscopic results certified that the rhodium porphyrin
complexes 1 and 2 have a cyclic tetrameric structure and a cofacial dimeric structure, respectively. The X-ray
structure analysis of 1 confirmed the cyclic structure of the complex. The Soret bands of both oligomers were
significantly broadened by excitonic interactions between the porphyrin units, compared to those observed for a
corresponding analogue of Rh(TTP)(Py)Cl (TTP ) 5,10,15,20-tetratolylporphyrinato dianion, Py ) pyridine). Stepwise
oxidation of the porphyrin rings in the oligomers was observed by cyclic voltammetry. The oligomers 1 and 2 are
very stable in solution, and they slowly undergo reactions with pyridine to give corresponding monomer complexes
only at high temperatures (∼80 °C).
Introduction
porphyrins with pyridyl groups as building blocks.6 In a
variety of ruthenium(II) pyridylporphyrin oligomers thus
prepared, cyclic ruthenium porphyrin tetramers7 and cofacial
ruthenium porphyrin dimers,8,9 both without X-ray crystal
structures,10 showed characteristic electrochemical and spec-
troscopic properties caused by interactions between constitu-
ent porphyrin units. Rhodium(III) porphyrins are promising
candidates as another powerful building block that gives
more-stable oligomers with stronger intramolecular interac-
Porphyrin assemblies using axial ligand coordination are
useful in the construction of functional systems such as
molecular electronic devices, redox-active materials, and
light-harvesting structures.1-3 To build up sterically con-
trolled unique metalloporphyrin oligomer systems for these
purposes, versatile central metal ions and substituents of
porphyrin rings have been investigated.4,5 We have adopted
less-substitutable and redox-active metal ions, such as
ruthenium(II), as the central metal ion and coordinative
(5) Sanders, J. K. M. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard R., Eds.; Academic Press: New York, 2000; Vol. 3,
Chapter 22, pp 347-368.
(6) Imamura, T.; Fukushima, K. Coord. Chem. ReV. 2000, 198, 133-
156.
* Author to whom correspondence should be addressed. E-mail:
timamura@sci.hokudai.ac.jp.
† Hokkaido University.
‡ Osaka City University.
(7) Funatsu, K.; Imamura, T.; Ichimura, A.; Sasaki, Y. Inorg. Chem. 1998,
37, 1798-1804.
(1) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 6, Chapter 40, pp 1-42.
(2) Balzani, V.; Scandola, F. In ComprehensiVe Supramolecular Chem-
istry; Atwood, J., MacNicol, D., Davies, E., Vo¨gtle, F., Lehn, J.-M.,
Eds.; Pergamon Press: Oxford, U.K., 1996; Vol. 10, Chapter 23, pp
687-746.
(3) Wojaczynski, J.; Latos-Grazynski, L. Coord. Chem. ReV. 2000, 204,
113-171.
(4) Chou, J.-H.; Kosal, M. E.; Nalwa, H. S.; Rakow, N. A.; Suslick, K.
S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: New York, 2000; Vol. 6, Chapter 41, pp
43-131.
(8) Funatsu, K.; Imamura, T.; Ichimura, A.; Sasaki, Y. Inorg. Chem. 1998,
37, 4986-4995.
(9) Imamura, T.; Funatsu, K.; Ye, S.; Morioka, Y.; Uosaki, K.; Sasaki,
Y. J. Am. Chem. Soc. 2000, 122, 9032-9033.
(10) The formation of a cyclic zinc porphyrin tetramer from 5-(4-pyridyl)-
10,15,20-triphenylporphyrinato zinc(II) in solution was also clarified
by variable temperature 1H NMR measurements.7 Very recently, in
the preparation of the present paper, the structure of the cyclic tetramer
was confirmed for 5-(4-pyridyl)-15-(3,5-di-tert-butylphenyl)porphy-
rinato zinc(II) by X-ray crystal analysis (Tsuda, A.; Nakamura, T.;
Sakamoto, S.; Yamaguchi, K.; Osuka, A. Angew. Chem., Int. Ed. 2002,
41, 2817-2821).
10.1021/ic020652j CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/15/2003
Inorganic Chemistry, Vol. 42, No. 10, 2003 3187

Fukushima et al.
Scheme 1
X-ray Crystallography. The crystals of [Rh(4-PyT3P)Cl]4 (1)
were obtained as CHCl3 and C6H14 solvates, via the diffusion of
pentane into the chloroform solution. Structure data were collected
on a Rigaku model AFC-8S diffractometer with a Mercury CCD
area detector using graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) at 120 K and processed using the Crystal Clear
software program.14 Final cell parameters were obtained from a
least-squares analysis of reflections with I > 10σ(I). The crystal
structure was solved by direct methods and expanded using Fourier
and difference Fourier techniques. Atomic coordinates and aniso-
tropic thermal parameters of the non-hydrogen atoms were refined
by full-matrix least-squares calculations. The hydrogen atoms were
placed at calculated positions. All calculations were performed using
the teXsan crystallographic software package.15
tions, because of their less-substitutable character and the
small ionic radius of the Rh(III) ion. Rhodium(III) porphyrins
have been practically utilized as versatile building blocks to
assemble structurally unique oligomers such as heterometallic
porphyrin arrays11 and a cofacial dimer12 with two pyridyl
groups at trans-meso positions of the constituent porphyrin
unit.
Herein, a new cyclic rhodium(III) pyridylporphyrin tet-
ramer and a cofacial dimer are reported (Scheme 1). The
structure of the cyclic tetramer was successfully determined
by X-ray crystallography. The rhodium(III) porphyrin oli-
gomers showed strong intramolecular interactions in UV-
vis spectra and electrochemical data.
Materials. Free-base porphyrins containing meso-pyridyl groupss
H2(2-Py)T3P, H2(2-Py)tB3P, and H2(4-Py)T3Pswere synthesized
and characterized by 1H NMR measurements.8 Rhodium porphyrin
monomer, Rh(TTP)(Py)Cl, was synthesized by the reaction of
[RhI(CO)2Cl]2 and H2TTP, with reference to the literature method,16
and characterized by spectral methods.
Experimental Section
Instrumentation. UV-vis spectra were recorded on Hitachi
model U-3410 or model U-3000 spectrophotometers. 1H NMR
spectra were recorded on a JEOL model EX270 spectrometer.
Cyclic voltammograms (CVs) were recorded with a BAS model
CV-50W voltammetry analyzer at a scan rate of 100 mV s-1 at 20
°C. The working and counter electrodes for the cyclic voltammetry
measurements were a platinum disk (inside diameter of 1.6 mm)
and a platinum wire, respectively. The sample solutions in 0.1 M
TBA(PF6)-CH2Cl2 were deoxygenated by a stream of argon.13 The
reference electrode was Ag/AgCl. Redox potentials obtained were
corrected by the potential of a ferrocenium/ferrocene couple (0.352
V).
(13) Abbreviations: H2TTP ) 5,10,15,20-tetratolylporphyrin; H2TPP )
5,10,15,20-tetraphenylporphyrin; H2(4-Py)T3P ) 5-(4-pyridyl)-10,15,-
20-tritolylporphyrin; H2(2-Py)T3P ) 5-(2-pyridyl)-10,15,20-tritolylpor-
phyrin; H2(4-Py)P3P ) 5-(4-pyridyl)-10,15,20-triphenylporphyrin;
H2(2-Py)T3P ) 5-(2-pyridyl)-10,15,20-tritolylporphyrin; TTP ) 5,-
10,15,20-tetratolylporphyrinato dianion; TPP ) 5,10,15,20-tetraphe-
nylporphyrinato dianion; OEP ) 2,3,7,8,12,13,17,18-octaethylpor-
phyrinato dianion; 4-PyT3P ) 5-(4-pyridyl)-10,15,20-tritolylporphyrinato
dianion; 2-PytB3P ) 5-(2-pyridyl)-10,15,20-tri(4-tert-butyl)phenylpor-
phyrinato dianion; Py ) pyridine; TBA(PF6) ) n-tetrabutylammonium
hexafluorophosphate.
(14) Data Processing System Software; Rigaku Co.: Tokyo, 1999.
(15) Single-Crystal Structure Analysis Software, Version 1.6; Molecular
Structure Co.: The Woodlands, TX, 1993.
(16) Grass, V.; Lexa, D.; Momenteau, M.; Saveant, J.-M. J. Am. Chem.
Soc. 1997, 119, 3536-3542.
(11) Redman, J. E.; Feeder, N.; Teat, S. J.; Sanders, J. K. M. Inorg. Chem.
2001, 40, 2486-2499.
(12) (a) Aoyama, Y.; Kamohara, T.; Yamagishi, A.; Toi, H.; Ogoshi, H.
Tetrahedron Lett. 1987, 28, 2143-2146. (b) Aoyama, Y.; Yamagishi,
A.; Tanaka, Y.; Toi, H.; Ogoshi, H. J. Am. Chem. Soc. 1987, 109,
4735-4737.
3188 Inorganic Chemistry, Vol. 42, No. 10, 2003

Synthesis and Properties of Rh(III) Porphyrin Species
Anal. Calcd for C53H41N5ClRh (Rh(TTP)(Py)Cl): C, 71.83; H,
4.66; N, 7.90; Cl, 4.00. Found: C, 71.59; H, 5.04; N, 8.03; Cl,
1
3.87. H NMR (CDCl3, 270 MHz): HCH3 2.69 (s, 12H), Ho 8.17,
8.20 (dd, 8H), Hm 7.49-7.56 (m, 8H), Hâ 8.88 (s, 8H), H2,6-Py
0.96 (d, 2H), H3,5-Py 5.03 (t, 2H), H4-Py 6.02 (t, 1H) ppm. UV-
vis (CH2Cl2), λmax/nm (ꢀ/103 M-1 cm-1): 427 (276), 536.5 (24.1),
571 (7.84).
[Rh(4-PyT3P)Cl]4 (1). The tetramer 1 was prepared with
reference to the methods for Rh(TTP)(Py)Cl 16 and [Ru(4-PyT3P)-
(CO)]4.7 [RhI(CO)2Cl]2 (100 mg, 0.26 mmol) and H2(4-Py)T3P (100
mg, 0.15 mmol) were dissolved in 100 mL of toluene under an
argon atmosphere in a glovebox. The solution was removed from
the glovebox and refluxed for 5 h under an argon atmosphere,
filtered through a sintered glass to remove the remaining rhodium
complex, and evaporated to dryness. The resulting solid material
was again dissolved in dichloromethane. The solution was warmed,
mixed with a large excess of 2,3-dichloro-5,6-dicyano-p-benzo-
quinone (DDQ), and chromatographed on a silica gel column using
0.4% methanol-dichloromethane as an eluent. The first eluted red
band was collected and evaporated to dryness. The solid product
was recrystallized from dichloromethane-pentane and dried at 100
°C in vacuo for 4 h (yield: 41%, 48 mg).
Figure 1. ESI-MAS spectrum of [Rh(4-PyT3P)Cl]4 (1) with 0.5 mass
unit intervals. The bar graph shows a calculated spectrum of the dication
of the tetramer, ([Rh(4-PyT3P)Cl]42+).
Table 1. Crystallographic Data for 1‚10CHCl3‚C5H12‚2C6H14
empirical formula
temp/K
cryst syst
space group
a/Å
C211H182Cl34N20Rh4
123
triclinic
P1h (No. 2)
15.972(1)
17.631(2)
19.856(2)
86.924(3)
86.302(1)
81.108(1)
5507.2(8)
1
Anal. Calcd for C184H132N20Cl4Rh4: C, 69.57; H, 4.19; N, 8.82;
Cl, 4.46. Found: C, 69.36; H, 4.69; N, 8.20; Cl, 4.19. UV-vis
(CH2Cl2), λmax/nm (ꢀ/103 M-1 cm-1): 429 (686), 537 (76.7), 576
b/Å
c/Å
R/deg
1
(23.4). H NMR (CDCl3, 270 MHz): HCH3 2.64 (s, 12H), 2.74 (s,
â/deg
γ/deg
24H), Ho 8.01-8.16 (m, 24H), Hm 7.49-7.77 (m, 24H), Hâ 7.00
(d, 8H), 8.52 (d, 8H), 8.73 (d, 8H), 8.95 (d, 8H), H3,5-Py 0.32 (d,
4H), 0.89 (d, 4H), H2,6-Py 5.56 (dd, 4H), 5.96 (dd, 4H) ppm.
Electron spray ionization-mass spectroscopy (ESI-MS): 1588.1
V/Å3
Z
d
calcd/g cm-3
1.391
no. of unique reflns
no. of observed reflns
R1, wR2a
21 837
14 524
0.102, 0.234
(m/Z+) for [Rh(4-PyT3P)Cl]42+, 3177.8 (m/Z+) for [Rh(4-PyT3P)-
+
Cl]4
.
[Rh(2-PytB3P)Cl]2 (2). [RhI(CO)2Cl]2 (100 mg, 0.26 mmol) and
H2(2-Py)tB3P (80 mg, 0.1 mmol) were dissolved in 80 mL of
toluene in a glovebox under an argon atmosphere. The solution
was removed from the glovebox, refluxed for 3 h under an argon
atmosphere, passed through a sintered glass, and chromatographed
on a silica gel column. Using dichloromethane as an eluent, a pale
pink colored band was eluted first and followed by the elution of
a red band. The red band was collected and evaporated to dryness.
The solid material obtained was dried for 4 h in vacuo (yield: 48%,
45 mg).
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
,
2
2
2
w ) {σ2(Fo ) + [0.05 (max(Fo , 0) + 2Fc )/3]2}-1
.
Section. The tetramer 1 showed an ESI-MS peak at 1588.1
(m/Z+) with 0.5 mass unit, which corresponds to the dication
of the tetramer ([Rh(4-PyT3P)Cl]42+), in addition to the peak
of [Rh(4-PyT3P)Cl]4+ at 3177.8 (m/Z+) (Figure 1). Although
the ESI-MS spectrum with 0.5 mass unit somewhat
overlapped the spectrum with 1.0 mass unit of dimer
+
fragments ([Rh(4-PyT3P)Cl]2 ), the spectral feature with 0.5
Anal. Calcd for C110H102N10Cl2Rh2: C, 71.77; H, 5.59; N, 7.61;
Cl, 3.85. Found: C, 71.03; H, 5.77; N, 7.71; Cl, 3.79. UV-vis
(CH2Cl2), λmax/nm (ꢀ/103 M-1 cm-1): 421 (232), 547 (45.4), 586
(8.98). 1H NMR (CD2Cl2, 270 MHz): Ht-Bu 1.71 (s, 18H), 1.73 (s,
36H), Ho,m 7.81-8.13 (m, 24H), Hâ 5.48 (d, 4H), 8.49 (d, 4H),
9.07 (d, 4H), 9.13 (d, 4H), H3-Py 1.43 (d, 2H), H4,5-Py 5.66 (m,
4H), H6-Py 6.33 (d, 2H) ppm. ESI-MS: 1841.01 (m/Z+) for [Rh-
(2-PytB3P)Cl]2+, 1804.06 (m/Z+) for [Rh2(2-PytB3P)2Cl]+.
The treatment of [RhI(CO)2Cl]2 with H2(2-Py)T3P gave another
dimer complex, [Rh(2-PyT3P)Cl]2, which has a broad Soret band
at 420.5 nm and Q-bands at 547.5 and 586.0 nm in dichloromethane.
By the addition of pyridine to the solution, the UV-vis spectrum
slowly changed to give a spectrum of the corresponding monomer.
However, this complex could not be characterized in detail, because
of its low solubility in organic solvents, which prevented measure-
mass unit indicated the formation of the tetramer. The
cofacial dimer 2 showed a parent peak of [Rh(2-PytB3P)-
+
Cl]2 at 1841.01 (m/Z+). 1H NMR spectra of 1 and 2
confirmed the cyclic tetramer structure and the cofacial dimer
structure, respectively, and showed the high symmetry of
these frameworks in solution. The sharpness of the NMR
signals does not contradict diamagnetism of the oligomers;
that is, the valence of the Rh ion is 3+.
Cyclic Tetramer 1. The structure of tetramer 1 was
determined by X-ray structure analysis. The crystallographic
data of [Rh(4-PyT3P)Cl]4 and selected bond lengths and
angles are listed in Tables 1 and 2. The molecular structure
of the tetramer 1 has a crystallographically imposed inversion
center, and two porphyrin units, labeled Rh1por and Rh2por,
are independent. One unit cell contains ten chloroform, two
hexane, and one pentane molecules as crystal solvents, other
than one tetramer molecule. The structure analysis revealed
that each pyridyl group of a rhodium porphyrin unit is
coordinated to the Rh atom of an adjacent porphyrin unit to
1
ment of the H NMR spectrum.
Results and Discussion
Elemental analyses of 1 and 2 were consistent with their
respective compositions, as detailed in the Experimental
Inorganic Chemistry, Vol. 42, No. 10, 2003 3189

Fukushima et al.
Table 2. Selected Atomic Distances (Å) and Angles (deg) for Complex
the Rh ions is 0.01 Å for both Rh1por and Rh2por. The bond
distances around the Rh atoms in Rh1por and Rh2por are
comparable to those found in other rhodium porphyrins.11
The dihedral angles of adjacent rhodium porphyrin units were
95.9° and 84.1°, and the distances between cofacially located
porphyrins were 9.61 (Rh1por-Rh1′por) and 8.08 Å
(Rh2por-Rh2′por). Two pentane and four chloroform mol-
ecules are incorporated in the cavity formed by four rhodium
porphyrin units (see Figure S1 in the Supporting Informa-
tion).
The tetramer 1 has Ci symmetry in the crystal. If the
tetramer has an idealized symmetric structuresthat is, C4h
symmetrysthe pyridyl groups would be coordinated verti-
cally to the neighboring porphyrin units, as observed in the
zinc porphyrin tetramer system,10 in which the dihedral
angles of coordinating pyridyl groups and zinc porphyrin
units are 87°-93°. However, in the tetramer 1, the dihedral
angles of coordinating pyridyl groups and rhodium porphy-
rins were 87.8° and 108.6° for Rh1por and Rh2por,
respectively. The latter angle largely deviated by 18.6° from
the idealized C4h structure (Figure 2b). By this distortion,
the peripheral tolyl groups are located asymmetrically against
the virtual C4 axis and mirror plane.
1a
Rh1-N1
Rh1-N2
Rh1-N3
Rh1-N4
Rh1-N10′
Rh1-Cl1
2.034(7)
2.029(7)
2.030(7)
2.024(7)
2.058(7)
2.315(2)
Rh2-N5
Rh2-N6
Rh2-N7
Rh2-N8
Rh2-N9
Rh2-C12
2.081(7)
2.017(6)
2.032(7)
2.021(6)
2.022(7)
2.324(2)
Rh1‚‚‚Rh2
Rh1‚‚‚Rh2′
9.684(1)
9.764(1)
N1-Rh1-N2
N2-Rh1-N3
N3-Rh1-N4
N4-Rh1-N1
N1-Rh1-N3
N2-Rh1-N4
Cl1-Rh1-N10′
N1-Rh1-N10′
N2-Rh1-N10′
N3-Rh1-N10′
N4-Rh1-N10′
90.3(3)
89.5(3)
90.4(3)
89.8(3)
179.7(3)
179.8(3)
179.8(2)
89.3(3)
89.5(3)
90.9(3)
90.4(3)
N6-Rh2-N7
N7-Rh2-N8
N8-Rh2-N9
N9-Rh2-N6
N6-Rh2-N8
N7-Rh2-N9
Cl2-Rh2-N5
N5-Rh2-N6
N5-Rh2-N7
N5-Rh2-N8
N5-Rh2-N9
90.1(3)
89.9(3)
89.8(3)
90.2(3)
178.9(3)
179.2(3)
177.7(2)
92.2(3)
86.3(3)
88.9(3)
92.9(3)
a Symmetry transformation used to generate equivalent atoms: ′ -x, -y,
-z.
As shown in Figure 3, the cyclic tetramer molecules stack
with eight peripheral tolyl groups to form the channel
structure along the a-axis. The tetramer chains along this
a-axis also contact neighboring chains with four tolyl groups
sticking out vertically toward the a-axis. By this molecular
arrangement, not only the channels composed of the in-
tramolecular cavity but also the channels between neighbor-
ing tetramer units run along the a-axis. The crystal solvents
are accommodated in these channels (see Figure S2 in the
Supporting Information). In this crystal packing, the distor-
tion of tetramer molecules eliminates the confliction of
mutual tolyl groups.
In CDCl3, the 1H NMR signals of the pyridyl and â-pyrrole
groups showed very large upfield shifts, which indicates the
coordination of rhodium porphyrin units to the axial positions
of adjacent rhodium porphyrin units through pyridyl groups
to form the cyclic tetramer (see Figure S3 in the Supporting
Information). Each rhodium porphyrin unit in the tetramer
framework is equivalent in solution. Pyridyl protons in the
3,5- and 2,6-positions gave two sets of upfield-shifted double-
doublet peaks distinctly at ∼0.5 and 6 ppm, respectively.
The magnitude of the shifts is larger than that of the [Ru-
(4-PyT3P)(CO)]4 system, as discussed in the latter section
of “Intramolecular Interactions”. One set of the 3,5- and 2,6-
protons in a pyridyl group is directed to the outside of the
framework, and the other set of protons is directed to the
inside of the framework. The integral intensity of each
pyridyl proton signal is equivalent to four protons. This
assignment was verified by H-H COSY measurements. The
â-pyrrole protons gave four signals resulting from their
positions. The integral intensity of each â-pyrrole signal
corresponds to eight protons. Thus, the 1H NMR results show
that the tetramer 1 is stably present in solution.
Figure 2. (a) Top view and (b) side view of the molecular structure of
[Rh(4-PyT3P)Cl]4 (1). Thermal ellipsoids are drawn to illustrate a 50%
probability surface.
give the cyclic structure (Figure 2a). Each rhodium porphyrin
ring is almost planar. The largest deviation from the 24-
atom mean-square plane is 0.22 Å, and the displacement of
Cofacial Dimer 2. The dimer complex of [Rh(2PyT3P)-
Cl]2 synthesized preliminary was hardly soluble in organic
3190 Inorganic Chemistry, Vol. 42, No. 10, 2003

Synthesis and Properties of Rh(III) Porphyrin Species
Figure 4. UV-vis spectra of [Rh(4-PyT3P)Cl]4 (1) and Rh(4-PyT3P)-
(Py)Cl in toluene at 20 °C. Rh(4-PyT3P)(Py)Cl was formed by the addition
of a large amount of pyridine (250 equiv per porphyrin unit) to the tetramer
solution at 80 °C.
groups shifted from 8-9 ppm for corresponding monomers
to 0-6 ppm of high magnetic fields, as observed in cofacial
porphyrin dimers such as analogous rhodium,12a ruthenium,8
and zinc complexes.17,18 The magnitude of the shifts is
comparable with the corresponding ruthenium porphyrin
dimer system (vide infra). â-Proton signals also shifted to
higher magnetic fields in the order of â1 > â2 > â3 > â4,
where â1-protons are the closest to the center of another
porphyrin unit.
Stability of Rhodium Porphyrin Oligomers in Solution.
1
The H NMR measurements of 1 at various temperatures
verified the rotation of the pyridyl groups at ∼100 °C without
cleavage of the tetrameric framework. However, the addition
of a large amount of pyridine to the solution of the tetramer
1 at high temperatures caused decomposition of the frame-
work to give the monomer. Different from the ruthenium
system, in which decomposition of the framework occurred
even at room temperature in the presence of 100 equiv of
pyridine,7 the decomposition of the rhodium porphyrin
tetramer 1 needs higher concentrations of pyridine and higher
temperatures. By the addition of 250 equiv of pyridine per
rhodium porphyrin unit to the toluene solution of 1 at 80
°C, the UV-vis spectrum changed to a sharper spectrum in
12 h, as shown in Figure 4. In the progress of the reaction,
the peaks (λmax ) 429.5, 538.0, 575.5 nm) slightly shifted
to shorter wavelengths (λmax ) 427.0, 537.5, 573.5 nm) to
give the corresponding monomer of Rh(4-PyT3P)(Py)Cl. The
half-widths of the peak of 1 at 429.5 nm and of Rh(4-PyT3P)-
(Py)Cl at 427.0 nm are 1800 and 1130 cm-1, respectively.
The final spectrum of the product, Rh(4-PyT3P)(Py)Cl, is
1.4 times as intense as the Soret band of the initial complex
1.
Figure 3. (a) View from a-axis and (b) side view of the molecular structure
of [Rh(4-PyT3P)Cl]4 (1).
The dimer 2 is also robust in solution. When the toluene
solution of 2 containing 1000 equiv of pyridine per porphyrin
solvents; therefore, the dimer complex 2 of [Rh(2-PytB3P)-
Cl]2 with bulky substituents was prepared and characterized.
1
The assignment of the H NMR spectrum was achieved by
(17) Kobuke, Y.; Miyaji, H. J. Am. Chem. Soc. 1994, 116, 4111-4112.
(18) Stibrany, R. T.; Vasudevan, J.; Knapp, S.; Potenza, J. A.; Emge, T.;
Schugar, H. J. J. Am. Chem. Soc. 1996, 118, 3980-3981.
H-H COSY measurements (see Figures S4 and S5 in the
Supporting Information). The proton signals of the 2-pyridyl
Inorganic Chemistry, Vol. 42, No. 10, 2003 3191

Fukushima et al.
Figure 5. UV-vis spectra of [Rh(2-PytB3P)Cl]2 (2) and Rh(4-PytB3P)-
(Py)Cl in toluene at 20 °C. Rh(4-PytB3P)(Py)Cl was formed by the addition
of a large amount of pyridine (1000 equiv per porphyrin unit) to the dimer
solution at 80 °C.
Figure 6. Cyclic voltammograms of rhodium porphyrin complexes in 0.1
M TBA(PF6)-CH2Cl2 solution. Plot a shows data for Rh(TTP)(Py)Cl, plot
b shows data for [Rh(4-PyT3P)Cl]4 (1) ((s) experimental data and (‚‚‚)
simulation), and plot c shows data for [Rh(2-PytB3P)Cl]2 (2).
unit was left to stand at 80 °C, the broad band at 420.0 nm
(half-width of 2480 cm-1) changed to a sharp spectrum of
Rh(2-PytB3P)(Py)Cl with a peak at 428 nm (half-width of
1075 cm-1) (Figure 5). The final spectrum of the monomer,
Rh(2-PytB3P)(Py)Cl, shows a 2.27-fold increase in intensity,
compared to the dimer 2. It became obvious that the rhodium
porphyrin oligomers are very stable to substitution at room
temperature. This is a remarkable result because, in ruthe-
nium porphyrin systems, the same reaction proceeds in a
few minutes at room temperature.
Intramolecular Interactions. The intramolecular interac-
tions in 1 and 2 were studied by cyclic voltammetry and
UV-vis spectral measurements and discussed in comparison
to the structural data and the 1H NMR spectra. The tetramer
1 showed a broad oxidation peak of the porphyrin rings at
∼1100 mV (Figure 6b). The spectral simulation clarified that
the oxidation mediates four one-electron steps and proceeds
step by step. The first ring oxidation proceeds at 1013 mV
(the half-wave potential is denoted as E°′). From the redox
data shown in Table 3, it is obvious that there are strong
intramolecular interactions between the constituent porphyrin
units; that is, the difference in the half-wave potentials
between the first ring oxidation and the fourth ring oxidation
is ∼173 mV. The value of 173 mV in 1 is significantly larger
than that of the ruthenium system (128 mV).7 The differences
of the potentials between the first and second oxidation steps,
the second and third oxidation steps, and the third and fourth
oxidation steps are 64, 64, and 45 mV, respectively. In the
[Ru(4-PyT3P)(CO)]4 complex,7 the first porphyrin ring
oxidation occurs at 998 mV ,19 followed by stepwise oxida-
tion of the remaining porphyrins. The differences of the
potentials in the ruthenium system increase as the increment
of the oxidation step increases (that is, 18, 49, and 61 mV,
respectively).7 The oxidation number of the central metals
Table 3. Redox Potentials and Potential Differencesa
first ring oxidation
complex
E°′/mV
(∆E°′/mV)
(∆E°t′otal/mV)
Rh(TTP)(Py)Cl
[Rh(4-PyT3P)Cl]4 (1)
955
1013
1077
1141
1186
0
0
64
64
45
173
[Rh(2-PytB3P)Cl]2 (2)
998
265
1263
a E°′ denotes the half-wave potential vs Ag/AgCl in 0.1 M TBA(PF6)-
CH2Cl2. Redox potentials were corrected by the potential of a ferrocenium/
ferrocene couple (0.352 V).
must be responsible for the difference in oxidation behavior
between the two systems, even for porphyrin ring oxidation,
although the electrostatic effects on the porphyrin rings are
not formally different between Ru(+2)CO(0)-Por(-2) and
Rh(+3)Cl(-1)-Por(-2). Although the value of the half-wave
potential (1013 mV at the first oxidation of 1) is 58 mV
larger than that of the corresponding monomer of Rh(TTP)-
(Py)Cl without pyridyl groups (955 mV), there is no essential
difference in the ease of the first oxidation between the
tetramer and the corresponding monomer with a pyridyl
group. The replacement of a tolyl group of H2TTP by a
pyridyl group should increase the potential by 45 mV, as
suggested by the difference in the reduction potential between
H2TPP and H2(4-Py)P3P.20
The UV-vis spectral intramolecular interactions of the
tetramer 1 are stronger than those of the ruthenium tetramer
complex, [Ru(4-PyT3)(CO)]4; that is, the formation of the
tetrameric structure in the rhodium system 1 has a small
molar absorptivity (1.7 × 105 M-1 cm-1 per porphyrin unit),
which corresponds to 62% of Rh(TTP)(Py)Cl (2.8 × 105
M-1 cm-1), whereas the ruthenium tetramer has 71% (2.0
(19) This value was corrected by adding 263 mV to the original value of
735 mV vs Ag/Ag+.
(20) Williams, G. N.; Williams, R. F. X.; Lewis, A.; Hambright, P. J. Inorg.
Nucl. Chem. 1979, 41, 41-44.
3192 Inorganic Chemistry, Vol. 42, No. 10, 2003

Synthesis and Properties of Rh(III) Porphyrin Species
× 105 M-1 cm-1) of the corresponding monomer (2.8 × 105
M-1 cm-1). Thus, both electrochemical and UV-vis spectral
measurements showed that, in the tetramer systems, the
interactions of the rhodium tetramer 1 are stronger than those
of the ruthenium tetramer. This result must be mainly
ascribed to the shorter interplanar separation, originated from
the smaller ionic radius of the Rh(III) ion,21 of the constituent
porphyrin planes in the rhodium(III) tetramer system. Actu-
ally, the distance of the (Cl)Rh-N(Pyridyl) bond (2.07 Å
on average) in the tetramer 1 is significantly shorter than
that of the (CO)Ru-N(Py or Pyridyl) bond in Ru(TPP)(CO)-
(Py) (2.19 Å)22 and Ru(OEP)(CO)(H24-PyP3P) (2.23 Å).23
This assumption explains well the relatively strong intramo-
lecular interactions in 1 and is consistent with the larger 1H
NMR upfield shifts (0.32, 0.89 ppm) of 3,5-pyridyl protons
in [Rh(4-PyT3P)Cl]4, compared with those (0.95, 1.68 ppm)24
of [Ru(4-PyT3P)(CO)]4.
On the other hand, in the dimer systems, there is no
difference between the rhodium system and the ruthenium
system in the magnitude of intramolecular interactions
evaluated electrochemically and UV-vis spectrally. The CV
of 2 is shown in Figure 6c. The solubility in CH2Cl2 is very
low; therefore, the concentration of the sample solution was
adjusted to 0.25 mM. As expected from the electrochemical
results of ruthenium systems,8 the constituent porphyrin rings
of 2 were also oxidized stepwise. One of the two porphyrin
units in 2 was first oxidized at 998 mV, followed by the
second one-electron oxidation of the other porphyrin unit at
1263 mV. The difference in potential between the first
oxidation step and the second oxidation step is 265 mV,
which is comparable to 274 mV of [Ru(2-PytB3P)(CO)]2.8
The Soret band of 2 is extremely broadened by excitonic
interactions between constituent porphyrins; that is, the molar
absorptivity per each constituent porphyrin unit is 1.16 ×
105 M-1 cm-1 (half-width of 2490 cm-1), which is 42% of
2.76 × 105 (half-width of 1130 cm-1) for the corresponding
monomer. The strong intramolecular interaction spectrally
observed (UV-vis) for 2 is again almost the same as for
the ruthenium dimer system. The strong geometrical require-
ments for porphyrin-porphyrin interactions are the result
of π-σ attractions that overcome π-π repulsions;25 thus, a
favorable interplanar separation could be attained. For zinc
porphyrin dimers, it was predicted to be 3.4 Å for the 24-
atom porphyrin unit.25 Actually, the interplanar separation
of a cofacial zinc porphyrin dimer was 3.3 Å.18 This result
suggests that the interplanar separations in both the rhodium
porphyrin dimer and the ruthenium porphyrin dimer are not
so different from each other, which may give rise to almost
the same intramolecular interactions in the dimer systems.
As expected, the chemical shifts of the pyridyl protons in
both the dimer systems are almost the same, that is, H3-Py
1.43 (1.79), H4,5-Py 5.66 (5.74), and H6-Py 6.33 (6.33) ppm
for [Rh(2-PytB3P)Cl]2 (2) ([Ru(2-PytB3P)(CO)]2).8
Conclusion
Rhodium(III) porphyrin tetramer and dimer compounds
were synthesized and characterized. The structure of the
tetramer was determined by X-ray crystallography. UV-
vis spectra of these oligomers showed broad Soret bands, as
observed in the systems of the corresponding ruthenium
porphyrin oligomers. These rhodium porphyrin oligomers
underwent reactions at high temperatures with large amounts
of pyridine to give corresponding monomer complexes.
Sharpening and increasing intensity in the Soret bands,
accompanied by the progress of the monomerizations in these
oligomers, indicates the presence of excitonic interactions
between the porphyrin units. Electrochemical analyses
revealed that the ring oxidation processes in these systems
proceed stepwise. The potential difference (∆E°′total) of the
oxidation step in the dimer system was 265 mV. This value
is very large, compared to that obtained by the simulation
for the system of the cyclic tetramer.
(21) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. The ionic
radius (0.665 Å) of the Rh(III) ion is smaller than that (0.68 Å) of the
Ru(III) ion; therefore, the ionic radius of the Rh(III) ion must be
significantly smaller than that of the Ru(II) ion.
(22) Little, R. G.; Ibers, J. A. J. Am. Chem. Soc. 1973, 95, 8583-8590.
(23) Funatsu, K.; Kimura, A.; Imamura, T.; Ichimura, A.; Sasaki, Y. Inorg.
Chem. 1997, 36, 1625-1635.
(24) The values are the upfield shifts of 2,6-pyridyl protons of [Ru(2-
PytB3P)(CO)]2 in ref 7, where the term “2,6-pyridyl protons” was used
instead of “3,5-pyridyl protons”.
Supporting Information Available: X-ray crystallographic files
in CIF format and five figures (Figures S1-S5). This material is
available free of charge via the Internet at http://pubs.acs.org.
IC020652J
(25) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
Inorganic Chemistry, Vol. 42, No. 10, 2003 3193