Novel Cofacial Ruthenium(II) Porphyrin Dimers and Tetramers

Article abstract of DOI:10.1021/ic971361r

A series of cofacially arranged ruthenium(II) porphyrin dimers 1-5 having a variety of axial ligands such as CO, pyridine, and 4-cyanopyridine, were synthesized. Porphyrin tetramers, 6 and 7, which have pyridylporphyrin ligands at the axial positions of the parent cofacial ruthenium(II) dimers. were also prepared. These porphyrin dimers and tetramers were characterized by ¹H NMR spectroscopy. ESI (electrospray ionization)-mass spectroscopy, and elemental analysis. The ruthenium porphyrin dimers and tetramers exhibited characteristic electrochemical and spectroscopic properties caused by interactions between the porphyrin subunits. Stepwise oxidations of the porphyrin rings or the ruthenium ions in the cofacial dimer skeltons were observed in the cyclic voltammograms. The potential differences (ΔE°' mV) of the oxidation steps were larger than 260 mV for all the porphyrin oligomers. The Soret bands of the cofacial dimers were significantly broadened by excitonic interactions between the two porphyrin subunits. Furthermore, the mixed-valence states of 3-7 showed specific intervalence charge-transfer (IT) bands between the Ru(II) and Ru(III) cores in the near-IR region at around 1500 nm.

Full text of DOI:10.1021/ic971361r

4986
Inorg. Chem. 1998, 37, 4986-4995
Novel Cofacial Ruthenium(II) Porphyrin Dimers and Tetramers
Kenji Funatsu,† Taira Imamura,*,† Akio Ichimura,‡ and Yoichi Sasaki†
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 October 28, 1997
A series of cofacially arranged ruthenium(II) porphyrin dimers 1-5 having a variety of axial ligands such as CO,
pyridine, and 4-cyanopyridine, were synthesized. Porphyrin tetramers, 6 and 7, which have pyridylporphyrin
ligands at the axial positions of the parent cofacial ruthenium(II) dimers, were also prepared. These porphyrin
dimers and tetramers were characterized by 1H NMR spectroscopy, ESI (electrospray ionization)-mass
spectroscopy, and elemental analysis. The ruthenium porphyrin dimers and tetramers exhibited characteristic
electrochemical and spectroscopic properties caused by interactions between the porphyrin subunits. Stepwise
oxidations of the porphyrin rings or the ruthenium ions in the cofacial dimer skeltons were observed in the cyclic
voltammograms. The potential differences (∆E°′ mV) of the oxidation steps were larger than 260 mV for all the
porphyrin oligomers. The Soret bands of the cofacial dimers were significantly broadened by excitonic interactions
between the two porphyrin subunits. Furthermore, the mixed-valence states of 3-7 showed specific intervalence
charge-transfer (IT) bands between the Ru(II) and Ru(III) cores in the near-IR region at around 1500 nm.
Introduction
has been paid to the two Bchlb molecules called “special pairs”
which are arranged cofacially with an average ring separation
∼3.3 Å between the pyrrole rings I.4 To mimic the “special
pairs”, many porphyrin dimers and oligomers bound covalently
by organic spacers, such as phenyl substituents, were synthesized
and their potential photochemical properties were sought.4,5
Recently self-assembly of metal-porphyrins with pyridyl,6,7
imidazolyl,8 or oxo-substituents9,10 were used for the construc-
tion of metal-porphyrin oligomers containing cofacially arranged
zinc6 and iron9 porphyrin dimers. Crystal structures of the two
zinc dimers [Zn(2-PyPOR)]2 showed that the pyrrole rings
overlap each other, and the mean plane separations between
the 24 atom porphyrin cores were ca. 3.3 Å.6a Hence the zinc
Assembly of multicomponents is essential for the construction
of the functional systems such as molecular electronic devices,
redox-active materials, and light-harvesting structures.1 In
addition, since the structure of the photosynthetic reaction center
of Rhodopseudomonas ViVidis was determined by X-ray crystal-
lography,2 many researchers have taken a great interest in
experimental and theoretical examinations of the electronic
structure of the photosynthetic reaction center.3 Special attention
† Hokkaido University.
‡ Osaka City University.
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S0020-1669(97)01361-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/29/1998

Novel Cofacial Ru(II) Porphyrin Dimers and Tetramers
Inorganic Chemistry, Vol. 37, No. 19, 1998 4987
Figure 1. Structures of cofacially arranged ruthenium(II) porphyrin dimers and tetramers.
N2 gas after dehydration using 4-Å molecular sieves. Dichloromethane
and toluene were used without further purification. Silica gel (Wakogel
C-300) and alumina (Wolem, neutral, activity III) were used for column
chromatography. TLC silica plates were purchased from Tokyo Kasei.
Measurements. 1H NMR spectra were recorded on a JEOL-EX270
spectrometer. IR spectra (KBr method) were measured with a Hitachi
270-50 infrared spectrophotometer. UV-vis spectra were recorded
on a Hitachi U-3410 or U-3000 spectrophotometer. Near-IR absorption
spectra were also recorded on a Hitachi U-3410 spectrophotometer.
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
were performed with a BAS CV-50W voltammetry analyzer. The data
were digitized and stored in a personal computer. The working and
the counter electrodes for the cyclic voltammetry measurements were
a platinum disk (i.d. ) 1.6 mm) and a platinum wire, respectively.
Cyclic voltammograms and differential pulse voltammograms were
recorded at a scan rate of 100 and 30 mV/s at 20 °C, respectively. The
dimers capture the structural character of “special pairs”.
Unfortunately, zinc(II) ion is relatively labile for substitution,
and the zinc-porphyrin oligomers in solution are in equilibrium
with monomers. Therefore we have paid more attention to the
less labile metal ions of ruthenium(II) and osmium(II).
A
variety of perpendicularly linked ruthenium(II) and osmium-
(II) porphyrin oligomers were synthesized and investigated.7c,11
We report herein the synthesis, characterization, and singular
spectral and electrochemical properties of a new series of
cofacially arranged ruthenium porphyrin dimers with axial
ligands of CO, pyridine, or 4-cyanopyridine (isonicotinonitrile,
PyCN), 1-5 as shown in Figure 1. Porphyrin tetramers, 6 and
7, derived from the cofacial porphyrin dimers, are also reported.
Experimental Section
Materials. Pyrrole and p-tolualdehyde for the preparation of
porphyrins, H2(2-Py)T3P, H2(2-Py)tB3P, H2(3-Py)tB3P, and H2(4-Py)-
tB3P,12 were purchased from Wako and used without further purifica-
tion. 4-tert-Butylbenzaldehyde and 2-, 3-, and 4-pyridinecarbaldehyde
for the preparation of the porphyrins were purchased from Aldrich.
Triruthenium(0) dodecacarbonyl, Ru3(CO)12, and H2OEP were also
purchased from Aldrich. Ethanol was purified by distillation under
(12) Abbreviations: OEP ) 2,3,7,8,12,13,17,18-octaethylporphyrinato di-
anion; TPP ) 5,10,15,20-tetraphenylporphyrinato dianion; TTP)5,-
10,15,20-tetratolylporphyrinato dianion; 4-PyT3P ) 5-(4-pyridyl)-
10,15,20-tritolylporphyrinato dianion; POR ) general porphyrin
dianion; H2tB4P ) 5,10,15,20-tetra(4-tert-butyl)phenylporphyrin; H2-
(4-Py)P3P ) 5-(4-pyridyl)-10,15,20-triphenylporphyrin; H2(2-Py)T3P
) 5-(2-pyridyl)-10,15,20-tritolylporphyrin; H2(2-Py)tB3P ) 5-(2-
pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrin; H2(3-Py)tB3P )
5-(3-pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrin; H2(4-Py)tB3P
) 5-(4-pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrin; Py ) py-
ridine; PyCN ) 4-cyanopyridine (isonicotinonitrile); azpy ) 4,4′-
azopyridine; TBA(PF6) ) n-tetrabutylammonium hexafluorophosphate;
CV ) cyclic voltammogram; DPV ) differential pulse voltammogram.
(11) (a) Kimura, A.; Funatsu, K.; Imamura, T.; Kido, H.; Sasaki, Y. Chem.
Lett. 1995, 207. (b) Funatsu, K.; Kimura, A.; Imamura, T.; Ichimura,
A.; Sasaki, Y. Inorg. Chem. 1997, 36, 1625. (c) Kariya, N.; Imamura,
T.; Sasaki, Y. Inorg. Chem. 1997, 36, 833.

4988 Inorganic Chemistry, Vol. 37, No. 19, 1998
Funatsu et al.
sample solutions in 0.1 M (TBA)PF6‚CH2Cl2 ((TBA)PF6 ) tetrabutyl-
ammonium hexafluorophosphate) were deoxygenated by a stream of
argon. The reference electrode was Ag/0.01 M [Ag(CH3CN)2]PF6, 0.1
M (TBA)PF6 (acetonitrile), or Ag/AgCl. Redox potentials obtained
were corrected by the potential of a ferrocenium/ferrocene couple (0.352
V).
Cl2): λmax/nm 392 (Soret), 517, 548. IR (KBr): νCO 1950, 1923 cm-1
.
1H NMR (CD2Cl2-2% CD3OD, 270 MHz): Hmeso δ 9.95 (s); CH3CH2
4.11 (q, 7.56 Hz); CH3CH2 1.94 (t, 7.56 Hz).
Ru(tB4P)(CO)(Py). Anal. Calcd for C66H65N5ORu: C, 75.83; H,
6.27; N, 6.70. Found: C, 75.99; H, 6.18; N, 6.89. UV-vis (CH2Cl2):
λ
max/nm 414 (Soret), 534, 568. IR (KBr): νCO 1948 cm-1 1H NMR
.
(CDCl3, 270 MHz): HPy δ 1.49 (d), 5.15 (t), 6.05 (t); Hâ 8.64 (s); Ho
7.95, 8.15 (d, 7.92 Hz); Hm 7.66, 7.72 (d, 7.92 Hz); Htert-Bu 1.59 (s).
Ru(tB4P)(Py)2. Anal. Calcd for C70H70N6Ru: C, 76.68; H, 6.44;
Synthesis of Porphyrins, H2(2-Py)T3P, H2(2-Py)tB3P, H2(3-Py)-
tB3P, and H2(4-Py)tB3P. Porphyrins containing meso-pyridyl groups
were synthesized with reference to the literature method of meso-pyridyl
derivatives of tetraphenylporphyrin.7b,c,11
N, 7.67. Found: C, 76.41; H, 6.77; N, 7.89. UV-vis (CH2Cl2): λmax
/
nm 413, 422 (Soret), 506. 1H NMR (C6D6, 270 MHz): HPy δ 2.94 (d),
4.51 (t), 5.08 (t); Hâ 8.69 (s); Ho 8.28 (d, 8.25 Hz); Hm 7.61 (d, 8.25
Hz); Htert-Bu 1.44 (s).
H2(2-Py)T3P. p-Tolualdehyde (15 mL), 2-pyridinecarbaldehyde (4.2
mL), and pyrrole (10.3 mL) were condensed in 200 mL of refluxing
propionic acid. The crystalline product of a porphyrin mixture (2.95
g) was obtained after cooling the solution. The monopyridylporphyrin
of H2(2-Py)T3P was isolated by silica gel chromatography. The amount
of H2(2-Py)T3P was 0.83 g (yield: 28%). The desired porphyrin was
Ru(tB4P)(PyCN)2. Anal. Calcd for C72H68N8Ru: C, 75.43; H, 5.98;
N, 9.78. Found: C, 75.61; H, 6.18; N, 9.89. UV-vis (CH2Cl2): λmax
/
nm 407, 418 (Soret), 509, 550-650. IR (KBr): νCN 2236 cm-1 1H
.
1
NMR (C6D6, 270 MHz): HPy δ 2.30 (d), 3.86 (d); Hâ 8.80 (s); Ho 8.83
(d, 8.25 Hz); Hm 7.72 (d, 8.25 Hz); Htert-Bu 1.47 (s).
identified by thin-layer chromatography, elemental analysis, and H
NMR measurements. Anal. Calcd for C46H35N5: C, 83.99; H, 5.36;
N, 10.65. Found: C, 84.07; H, 5.28; N, 10.69. 1H NMR (CDCl3, 270
MHz): HNH δ -2.77 (s, 2H), HCH3 2.65 (s, 9H), Hâ 8.88 (m, 8H), Ho
8.09 (d, J ) 7.9 Hz, 6H), Hm 7.55 (d, J ) 7.9 Hz, 6H), HPy 9.13 (m,
1H), 8.24 (m, 1H), 8.07 (m, 1H), 7.70 (m, 1H) ppm.
Ru(OEP)(PyCN)2. Anal. Calcd for C48H52N6Ru: C, 70.82; H, 6.44;
N, 10.33. Found: C, 71.13; H, 6.71; N, 10.50. UV-vis (CH2Cl2):
λ
max/nm 399 (Soret), 498, 525, 618. IR (KBr): νCN 2230 cm-1 1H
.
NMR (CDCl3, 270 MHz): Hpy δ 1.46 (d), 4.93 (d); Hmeso 9.40 (s);
CH3CH2 3.85 (q, 7.59 Hz); CH3CH2 1.80 (t, 7.59 Hz).
H2(2-Py)tB3P was synthesized by replacing p-tolualdehyde with
4-tert-butylbenzaldehyde used in the synthesis of H2(2-Py)T3P. 4-Tert-
butylbenzaldehyde (12.5 mL), 2-pyridinecarbaldehyde (2.3 mL), and
pyrrole (6.7 mL) were used. A porphyrin mixture (2.20 g) was
obtained. The mixture was separated by using a silica gel column. At
first H2tB4P was eluted by CH2Cl2, and was used for the preparation
of the ruthenium porphyrin monomers of Ru(tB4P)(CO)(MeOH), Ru-
(tB4P)(CO)(Py), Ru(tB4P)(Py)2, and Ru(tB4P)(PyCN)2. The amount of
H2tB4P was 1.0 g. Then H2(2-Py)tB3P was eluted by CH2Cl2 containing
2% EtOH. The amount of H2(2-Py)tB3P was 0.45 g (yield: 20%). Anal.
Calcd for C55H53N5: C, 84.25; H, 6.81; N, 8.93. Found: C, 84.15, H,
6.87, N, 8.66. 1H NMR (CDCl3, 270 MHz): HNH δ -2.75 (s, 2H),
Htert-Bu 1.61 (s, 27H), Hâ 8.88 (m, 8H), Ho 8.15 (d, J ) 8.2 Hz, 6H),
Hm 7.76 (d, J ) 8.2 Hz, 6H), HPy 9.14 (m, 1H), 8.26 (m, 1H), 8.10 (m,
1H), 7.72 (m, 1H).
H2(3-Py)tB3P. 4-tert-Butylbenzaldehyde (12.5 mL), 3-pyridinecar-
baldehyde (2.3 mL), and pyrrole (6.7 mL) were used. A purple product
of a porphyrin mixture (1.55 g) was obtained. The amount of purified
H2(3-Py)tB3P was 0.3 g (yield: 20%). Anal. Calcd for C55H53N5: C,
84.25; H, 6.81; N, 8.93. Found: C, 83.81, H, 7.01, N, 8.52. 1H NMR
(CDCl3, 270 MHz): HNH δ -2.76 (s, 2H), Htert-Bu 1.61 (s, 27H), Hâ
8.89 (m, 8H), Ho 8.15 (d, J ) 7.9 Hz, 6H), Hm 7.77 (d, J ) 7.9 Hz,
6H), HPy 9.47 (m, 1H), 9.04 (m, 1H), 8.53 (m, 1H), 7.75 (m, 1H).
H2(4-Py)tB3P. 4-tert-Butylbenzaldehyde (5.0 mL), 4-pyridinecar-
baldehyde (0.95 mL), and pyrrole (2.76 mL) were used. The purified
H2(4-Py)tB3P (yield: 0.15 g, 16%) was obtained from a purple porphyrin
mixture (0.96 g). Anal. Calcd for C55H53N5: C, 84.25; H, 6.81; N,
8.93. Found: C, 83.96, H, 6.70, N, 8.89. 1H NMR (CDCl3, 270
MHz): HNH δ -2.78 (s, 2H), Htert-Bu 1.61 (s, 27H), Hâ 8.88 (m, 8H),
Ho 8.14 (d, J ) 8.3 Hz, 6H), Hm 7.76 (d, J ) 8.3 Hz, 6H), HPy 9.03
(dd, 2H), 8.17 (dd, 2H).
Synthesis of Cofacial Dimers and Tetramers. [Ru(2-PyT3P)-
(CO)]2 (1). Diethylene glycol monomethyl ether suspension (100 mL)
containing H2(2-Py)T3P (35 mg, 5.32 × 10-5 mol) and Ru3(CO)12 (100
mg, 1.56 × 10-4 mol) was refluxed for 2 h under N2 atmosphere. The
reaction was stopped when the characteristic visible spectral band of
H2(2-Py)T3P around 650 nm was no longer evident. The solution was
cooled and filtered. To the solution, 100 mL of a saturated NaCl
aqueous solution was added. The resulting precipitate was filtered
through a sintered glass funnel, washed with water, and dried at 100
°C in vacuo for 1 h. Because the crude product in solution exhibited
an extra band around 610 nm, because of a chlorin impurity, DDQ
(2,3-dichloro-5,6-dicyano-1,4-benzoquinone) was added to the dichlo-
romethane solution of the crude product, and the suspension was stirred
at room temperature until the band was disappeared.13 The suspension
was filtered to remove insoluble materials, and the filtrate was
chromatographed on a silica gel column with toluene as an eluent. The
first eluted band was collected and evaporated to dryness. The product
was dried at 80 °C in vacuo for 3 h (yield: 9 mg, 22%) Anal. Calcd
for C94H66N10O2Ru2: C, 71.92; H, 4.24; N, 8.92. Found: C, 71.79;
H, 4.36; N, 8.76. 1H NMR (CDCl3, 270 MHz): HPy δ 1.82 (m, 2H),
5.75 (m, 4H), 6.33 (m, 2H), Hâ 5.36 (d, 4.95 Hz, 4H), 8.15 (d, 4.95
Hz, 4H), 8.76 (d, 4.95 Hz, 4H), 8.82 (d, 4.95 Hz, 4H), Ho 7.91, 8.06,
8.44-8.51 (m, 12H), Hm 7.51-7.60, 7.65-7.74 (m, 12H), Htolyl 2.78
(s, 6H), 2.80 (s, 12H).
[Ru(2-PytB3P)(CO)]2 (2). [Ru(2-PytB3P)(CO)]2 was synthesized
by a method similar to that of [Ru(2-PyT3P)(CO)]2 using H2(2-Py)-
tB3P in place of H2(2-Py)T3P. Ru3(CO)12 (100 mg, 1.56 × 10-4 mol)
and H2(2-Py)tB3P (60 mg, 7.61 × 10-5 mol) were used (yield: 20 mg,
29%). Anal. Calcd for C112H102N10O2Ru2: C, 73.82; H, 5.64; N, 7.69.
Found: C, 73.79; H, 5.84; N, 7.79. 1H NMR (CDCl3, 270 MHz): HPy
δ 1.79 (m, 2H), 5.74 (m, 4H), 6.33 (m, 2H), Hâ 5.38 (d, J ) 4.95 Hz,
4H), 8.18 (d, J ) 4.95 Hz, 4H), 8.79 (d, J ) 4.95 Hz, 4H), 8.85 (d, J
) 4.95 Hz, 4H), Ho,m 8.56 (m, 4H), 7.71-8.14 (m, 20H), Htert-Bu 1.68
(s, 18H), 1.69 (s, 36H).
Synthesis of Monomers. Ruthenium porphyrin monomers were
synthesized with reference to the literature methods of Ru(TPP)(CO)-
(EtOH),13a Ru(OEP)(CO)(MeOH),13a Ru(OEP)(CO)(Py),14 and Ru-
(OEP)(Py)2.14
[Ru(2-PyT3P)(Py)]2 (3). In the synthesis of [Ru(2-PyPOR)(L)]2 (L
) Py, PyCN, H23-PytB3P, H24-PytB3P), the toluene solution containing
the parent complex of [Ru(2-PyPOR)(CO)]2 and a corresponding ligand
L was photoirradiated using a medium-pressure mercury lamp. In 3,
the solution (700 mL) containing [Ru(2-PyT3P)(CO)]2 (32 mg, 2.04 ×
10-5 mol) and pyridine (3.5 µL, 4.08 × 10-5 mol) was irradiated for
2 h with stirring and vigorous Ar bubbling14 at the temperatures between
0 and 5 °C. The solution was changed in color from red to brown
upon irradiation. The brown solution was filtered and evaporated to
dryness. The resulting solid was dissolved in a small amount of toluene
and separated by an alumina column (activity III) with toluene as an
eluent. The first eluted brown band was collected and evaporated to
dryness. The resulting deep-purple solid was recystallized from toluene-
methanol, and dried at 110 °C in vacuo for 2 h (yield: 28 mg, 82%).
Ru(tB4P)(CO)(MeOH). Anal. Calcd for C62H64N4O2Ru: C, 74.59;
H, 6.46; N, 5.61. Found: C, 74.88; H, 6.80; N, 5.79. UV-vis (CH2-
Cl2): λmax/nm 413 (Soret), 532. IR (KBr): νCO 1946 cm-1 1H NMR
.
(CDCl3-2% CD3OD, 270 MHz): Hâ δ 8.71 (s); Ho 8.04, 8.14 (d, 7.92
Hz); Hm 7.72 (d, 7.92 Hz); Htert-Bu 1.60 (s).
Ru(OEP)(CO)(MeOH). Anal. Calcd for C38H48N4O2Ru: C, 65.77;
H, 6.97; N, 8.08. Found: C, 66.00; H, 7.01; N, 7.93. UV-vis (CH2-
(13) (a) Collman, J. P.; Barnes, C. E.; Brothers, P. J.; Collins, T. J.; Ozawa,
T.; Gallucci, J. C.; Ibers, J. A. J. Am. Chem. Soc. 1984, 106, 5151.
(b) Rousseau, K.; Dolphin, D. Tetrahedron Lett. 1974, 4251.
(14) (a) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. J. Am.
Chem. Soc. 1978, 100, 3015. (b) Sovocol, W.; Hopf, E. R.; Whitten,
D. G. J. Am. Chem. Soc. 1972, 94, 4350.

Novel Cofacial Ru(II) Porphyrin Dimers and Tetramers
Inorganic Chemistry, Vol. 37, No. 19, 1998 4989
analogue Ru(TPP)(CO)(Py)15 and perpendicularly linked ru-
thenium porphyrin oligomers.11 ESI-MS exhibited fragment
peaks at molecular weights of the cofacial carbonyl dimers, 1
and 2, at m/z+ 1570.8 (relative abundance: 92%) and 1824.0
(25%), respectively. 1H NMR spectra were especially diagnostic
for the structure of the cofacial ruthenium porphyrin dimers.
The signal patterns of 1 and 2 are almost the same as each other,
except the signals of the meso-phenyl substituents. In both
systems, four 2-pyridyl protons and four â-pyrrole protons were
observed in a significantly higher region due to the shielding
effect of the facing porphyrin π-conjugated system, as observed
in the similar zinc cofacial dimer systems.6a Although some
signals of the 2-pyridyl protons overlapped, integral intensities
of the signals and the signal correlation on H-H COSY
measurements cleaned up the problem. The chemical shift
values of 1 and 2 are listed in Table 1.
Anal. Calcd for C102H76N12Ru2: C, 73.23; H, 4.64; N, 10.05. Found:
C, 73.40; H, 4.83; N, 9.83.
[Ru(2-PytB3P)(Py)]2 (4). [Ru(2-PytB3P)(Py)]2 was synthesized
using [Ru(2-PytB3P)(CO)]2 in place of [Ru(2-PyT3P)(CO)]2. [Ru(2-
PytB3P)(CO)]2 (25 mg, 1.37 × 10-5 mol) and pyridine (2.2 µL, 2.75
× 10-5 mol) were used (yield: 23.5 mg, 89%). Anal. Calcd for
C120H112N12Ru2: C, 74.89; H, 5.87; N, 8.74. Found: C, 74.81; H, 6.35;
N, 8.23. 1H NMR (C6D6, 270 MHz): HPy δ 3.47 (d, 2H), 5.14 (t, 2H),
5.46 (m, 4H), HR-Py 1.86 (d, 4H), Hâ-Py 4.05 (t, 4H), Hγ-Py 4.72 (t,
2H), Hâ 5.97 (d, J ) 4.95 Hz, 4H), 8.50 (d, J ) 4.95 Hz, 4H), 8.93
(m, 8H), Ho,m 8.93-8.81 (dd, J ) 1.65, 7.92 Hz, 4H), 8.34-7.62 (dd,
J ) 1.65, 7.92 Hz, 20H), Htert-Bu 1.62 (s, 36H), 1.54 (s, 18H).
[Ru(2-PytB3P)(PyCN)]2 (5). [Ru(2-PytB3P)(CO)]2 (26 mg, 1.43 ×
10-5 mol) and 4-cyanopyridine (3.0 mg, 2.88 × 10-5mol) were used
(yield: 23.0 mg, 81%). Anal. Calcd for C122H110N14Ru2: C, 74.21;
H, 5.62; N, 9.93. Found: C, 74.16; H, 5.88; N, 9.67. 1H NMR (C6D5-
CD3, 270 MHz): HPy δ 3.11 (d, 2H), 5.16 (t, 2H), 5.54 (d, 4H), 5.62
(t, 2H), HR-Py 1.32 (dd, 4H), Hâ-Py 3.63 (dd, 4H), Hâ 5.76 (d, J ) 4.95
Hz, 4H), 8.36 (d, J ) 4.95 Hz, 4H), 8.79 (m, 8H), Ho,m 7.76-8.76 (dd,
J ) 1.65, 7.92 Hz, 24H), Htert-Bu 1.65 (s, 36H), 1.58 (s, 18H). IR
Cofacial Bispyridine Dimers, 3 and 4. Infrared spectra of
3 and 4 showed no carbonyl stretches, indicating the complete
replacement of carbonyl ligands by pyridine in the synthetic
procedures. Solubility of 4 in nonpolar solvents such as toluene
was much higher than that of 3 due to the solubility effect of
tert-butyl substituents, which was in contrast to the case of the
cofacial carbonyl dimers, 1 and 2. ESI-MS was a very useful
tool for the characterization of these cofacial porphyrin dimers
as has been often used to characterize multinuclear metal
complexes.16 ESI-MS exhibited fragment peaks corresponding
to the molecular weights of both the dimers with 100% relative
(KBr): νCN 2232 cm-1
.
[Ru(2-PyT3P)(H23-PytB3P)]2 (6). The solution containing [Ru(2-
PyT3P)(CO)]2 (27 mg, 1.72 × 10-5 mol) and H2(3-Py)tB3P (27 mg,
3.44 × 10-5 mol) was irradiated with UV-visible light for 3 h. The
solution obtained was filtered. The resulting solid was dried, dissolved
in a small amount of toluene, and purified by silica gel column
chromatography using toluene as eluent. The first eluted brownish-
purple band was collected and evaporated to dryness. The resulting
deep-purple solid was dried at 100 °C in vacuo for 2 h (yield: 27 mg,
54%). Anal. Calcd for C202H172N20Ru2: C, 78.72; H, 5.63; N, 9.09.
Found: C, 78.82; H, 6.00; N, 8.66. 1H NMR (C6D6, 270 MHz): HPy
δ 3.22 (d, 2H), 3.64 (t, 2H), 4.40 (m, 2H), 4.50 (t, 2H), 2.30 (d, 2H),
6.02 (d, 2H), Hâ 5.73 (d, J ) 4.95 Hz, 4H), 8.25 (d, J ) 4.95 Hz, 4H),
8.78 (d, 4H), 8.86 (d, 4H), 6.74 (d, 4H), 8.57 (d, 4H), 8.93 (d, 4H),
8.99 (d, 4H), Ho,m 6.65-8.63 (dd, J ) 1.65, 7.92 Hz, 48H), Htert-Bu
1.45 (s), 1.47 (s), Htolyl 2.35 (s), 2.45 (s).
[Ru(2-PyT3P)(H24-PytB3P)]2 (7). [Ru(2-PyT3P)(CO)]2 (27 mg, 1.72
× 10-5 mol) and H2(4-Py)tB3P (27 mg, 3.44 × 10-5 mol) were used
(yield: 23 mg, 46%). Anal. Calcd for C202H172N20Ru2: C, 78.72; H,
5.63; N, 9.09. Found: C, 78.56; H, 6.02; N, 8.84. 1H NMR (C6D6,
270 MHz): HPy δ 3.70 (d, 2H), 5.23 (t, 2H), 5.67 (t, 2H), 5.94 (d, 2H),
2.29 (d, 4H), 5.08 (d, 4H), Hâ 6.99 (d, J ) 4.95 Hz, 4H), 8.43 (d, J )
4.95 Hz, 4H), 8.85 (m, 8H), 6.21 (d, 4H), 8.54 (d, 4H), 9.07 (m, 8H),
Ho,m 7.04-8.85 (dd, J ) 1.65, 7.92 Hz, 48H), Htert-Bu 1.41 (s), 1.45
(s), Htolyl 2.55 (s), 2.58 (s).
1
abundances, that is, m/z+ ) 1672.3 for 3 and 1923.3 for 4. H
NMR spectra certified the cofacial structure of 4 depicted in
Figure 1. The solubility in C6D6 was high enough for the
measurements. The integral intensity ratios and the signal
correlations obtained by H-H COSY measurements distin-
guished the 2-pyridyl and axial pyridine signals (Figure S1).
Similarly to the carbonyl dimers of 1 and 2, the 2-pyridyl and
â-pyrrole proton signals of 4 were observed in the high magnetic
field region at 1-6 ppm as listed in Table 1. Besides the four
signals of 2-pyridyl protons, the three proton signals of R-(4H),
â-(4H), and γ-positions (2H) of the axial pyridine ligands were
also observed in the high magnetic region of 1 to 5 ppm, which
are significantly higher than those of free pyridine signals (7-9
ppm).
Cofacial Bis(4-cyano)pyridine Dimer, 5. Cofacial bis(4-
cyano)pyridine dimer, 5, was fairly soluble in dichloromethane,
chloroform, toluene, and benzene. Elemental analysis of 5
satisfied the composition. ESI-MS exhibited a fragment cor-
responding to the molecular weight of 5 (m/z+ 1974.6) with
100% relative abundance. The 1H NMR spectrum of 5 showed
a â-pyrrole signal and four 2-pyridyl proton signals in the high
magnetic field (1-6 ppm), as listed in Table 1, which was
similar to the other cofacia dimers. Two double-doublet signals
of the 4-cyanopyridine protons also appeared in the magnetic
field (1.32 and 3.63 ppm) significantly higher than those of free
4-cyanopyridine (7.55 and 8.83 ppm in CDCl3). The fact that
only four 2-pyridyl and two 4-cyanopyridine proton signals were
Results and Discussion
Characterization of Ruthenium Porphyrin Oligomers,
1-7. With the exception of the cofacial bispyridine dimer 3,
the other cofacial ruthenium porphyrin dimers were character-
1
ized by H NMR spectroscopy, IR spectroscopy, electrospray
ionization mass spectrometry (ESI-MS), and microanalysis. In
the case of 3, much lower solubility than the other cofacial
dimers made accurate analyses of the 1H NMR spectra difficult.
However, all the data from ESI-MS and microanalysis, and the
similarities in the UV-vis spectral and cyclic voltammetric
features of 3 and 4 supported the formation of a cofacial dimeric
structure for 3.
Cofacial Carbonyl Dimers, 1 and 2. Solubility of both the
cofacial carbonyl dimers in nonpolar solvents such as dichlo-
romethane, chloroform, toluene, and benzene was unexpectedly
similar, though the solubility of the dimer 2 with tert-butyl
substituents, was much lower than that of a monomeric analogue
of Ru(TPP)(CO)(Py). Elemental analyses of the cofacial
carbonyl dimers agreed well with their respective compositions
as detailed in the Experimental Section. Infrared spectra of 1
and 2 showed characteristic carbonyl stretches around 1955
cm-1. This value is almost the same as those of a monomer
(15) (a) Bonnet, J. J.; Eaton, S. S.; Eaton, G. R.; Holm, R. H.; Ibers, J. A.
J. Am. Chem. Soc. 1973, 95, 2141. (b) Little, R. G.; Ibers, J. A. J.
Am. Chem. Soc. 1973, 95, 8583.
(16) (a) Manna, J.; Whiteford, J. A.; Stang, P. J.; Muddiman, D. C.; Smith,
R. D. J. Am. Chem. Soc. 1996, 118, 8731. (b) Stang, P. J.; Cao, D.
H.; Chen, K.; Gray, G. M.; Muddiman, D. C.; Smith, R. D. J. Am.
Chem. Soc. 1997, 119, 5163. (c) Fujita, M.; Nagao, S.; Ogura, K. J.
Am. Chem. Soc. 1995, 117, 1649. (d) Whiteford, J. A.; Rachlin, E.
M.; Stang, P. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2524. (e)
Stang, P. J.; Persky, N. E. J. Chem. Soc., Chem. Commun. 1997, 77.
(f) Romero, M. F.; Ziessel, R.; D-Gervais, A.; Dorsseelaer, A. V. J.
Chem. Soc., Chem. Commun. 1996, 551.

4990 Inorganic Chemistry, Vol. 37, No. 19, 1998
Funatsu et al.
Table 1. 1H NMR Chemical Shift Values of Ruthenium(II) Porphyrin Dimers and Tetramers at 23 °C
complex
(solvent)
tolyl(CH3)
or tert-Bu
pyridine or pyridyl
(axial ligand)
2-pyridyl
â-pyrrole
phenyl (o, m)
7.51-8.51
NH
[Ru(2-Py)T3P)(CO)]2 (1)
1.82 (d, 2H)
5.75 (m, 4H)
6.33 (t, 2H)
5.36 (d. 4H)
8.15 (d, 4H)
8.76 (d, 4H)
8.82 (d, 4H)
5.38 (d, 4H)
8.18 (d, 4H)
8.79 (d, 4H)
8.85 (d, 4H)
5.97 (d, 4H)
8.50 (d, 4H)
8.93 (m, 8H)a
5.76 (d, 4H)
8.36 (d, 4H)
8.79 (m, 8H)
2.78 (s)
2.80 (s)
(CDCl3)
δ/ppm vs TMS 0 ppm
[Ru(2-PytB3P)(CO)]2 (2)
1.79 (d, 2H)
5.74 (m, 4H)
6.33 (t, 2H)
7.70-8.56
1.68 (s)
1.69 (s)
(CDCl3)
δ/ppm vs TMS 0 ppm
[Ru(2-PytB3P)(Py)]2 (4)
(C6D6)
δ/ppm vs C6H6 7.2 ppm
[Ru(2-PytB3P)(PyCN)]2 (5)
3.47 (d, 2H)
5.14 (t, 2H)
5.45 (m, 4H)
3.11 (d, 2H)
5.16 (t, 2H)
5.54 (d, 2H)
5.62 (t, 2H)
3.22 (d, 2H)
3.64 (t, 2H)
4.40 (m, 2H)
4.50 (t, 2H)
7.62-8.93
7.76-8.75
1.54 (s)
1.62 (s)
1.86 (d, 4H)
4.05 (t, 4H)
4.72 (t, 2H)
1.32 (d, 4H)
3.63 (d, 4H)
1.58 (s)
1.65 (s)
(C6D5CD3)
δ/ppm vs C6H5CH3 2.1 ppm
[Ru(2-PyT3P)(H23-PytB3P)]2 (6)
5.73 (d, 4H)
8.25 (d, 4H)
8.78 (d, 4H)
8.86 (d, 4H)
6.74 (d, 4H)
8.57 (d, 4H)
8.93 (d, 4H)
8.99 (d, 4H)
6.99 (d, 4H)
8.43 (d, 4H)
8.85 (m, 8H)
6.21 (d, 4H)
8.54 (d, 4H)
9.07 (m, 8H)
6.65-8.63
1.45 (s)
1.47 (s)
(tert - Bu)
2.35 (s)
2.45 (s)
(tolyl)
2.30 (d, 2H)
4.40 (m, 2H)
6.02 (d, 2H)
-2.56
-2.60
(C6D6)
δ/ppm vs C6H6 7.2 ppm
[Ru(2-PyT3P)(H24-PytB3P)]2 (7)
(C6D6)
3.70 (d, 2H)
5.23 (t, 2H)
5.67 (t, 2H)
5.94 (d, 2H)
8.85-7.04
1.41 (s)
1.45 (s)
(tert - Bu)
2.55 (s)
2.58 (s)
(tolyl)
2.29 (d, 4H)
5.08 (d, 4H)
δ/ppm vs C6H6 7.2 ppm
a The multiplet signal is due to overlap between the signals of the â-pyrrol and phenyl groups.
observed, clearly excluded the coordination of 4-cyanopyridine
by the cyano nitrogen. The 1H NMR data of the perpendicularly
linked oligomers11 and the bispyridine dimer 4 supported the
formation of the cofacial dimer 5, [Ru(2-PytB3P)(NC5H4-
CN)]2. The infrared spectrum of 5 showed a characteristic sharp
stretch band of it’s cyano substituents (νCN) at 2232 cm-1, while
intensities and signal correlations obtained by H-H COSY.
Although the proton signals of the 2-positions of the axial
3-pyridyl groups of 6 were not assigned by the H-H COSY
because of the absence of vicinal couplings, four pyridyl proton
signals of the cofacial dimer subunits and three pyridyl proton
signals of the axial 3-pyridylporphyrin subunits were assigned.
The integral intensities and the other signals assigned to seven
pyridyl protons and eight â-pyrrole protons indicated that 6 has
the porphyrin tetrameric structure and that the axial porphyrin
ligands are rotating around the ruthenium ions in the NMR time
scale.
ν
CN of free 4-cyanopyridine appeared at 2236 cm-1, that is, there
was little difference between the two bands. It was noted that
when benzonitrile was coordinated to the Ru(II) ion in the
pentaammine complexes, the νCN of free benzonitrile (2231
cm-1) shifted to a significantly lower frequency (2188 cm-1),
due to strong π-back-bonding from Ru(II) ion to the cyano
substituent.17 These results also verified that 4-cyanopyridine
was coordinated to Ru(II) ion through the nitrogen atom of the
pyridyl group instead of the cyano group.
These spectroscopic and analytical results apparently revealed
that the ruthenium porphyrin oligomers, 1-7, have cofacial
dimeric frameworks and a variety of corresponding axial ligands
(CO, Py, PyCN, Py-POR).
Bispyridylporphyrin Tetramers, 6 and 7. Elemental
analyses of 6 and 7 were satisfactory. ESI-MS of 6 and 7 gave
fragment peaks of the same molecular weight at m/z+ 3081.8.
Relative abundances of the fragments were 70% for 6 and 100%
for 7. 1H NMR spectra of 6 and 7 revealed that the oligomers
took on porphyrin tetrameric structures. In both the tetramers,
the ratios of integral intensities between CH3 (tolyl) and tert-
butyl proton signals were 1:3, indicating the composition of 1:1
(2:2) for the ruthenium porphyrin and axial porphyrin subunits.
Integral intensities of the other proton signals also agreed well
with the 1:1 composition. The signals for the pyridyl and
â-pyrrole protons of the ruthenium porphyrin and axial por-
phyrin subunits were observed at high magnetic fields as listed
in Table 1. In the case of 7, four pyridyl proton signals of the
cofacial dimer subunits and two pyridyl proton signals of the
axial 4-pyridylporphyrin subunits were distinguished by integral
Electrochemical Studies. Redox potentials of the cofacial
carbonyl ruthenium porphyrin dimers, 1 and 2, are listed in Table
2. The redox behavior of 1 and 2 were almost the same and
very characteristic in the stepwise oxidation of the porphyrin
rings owing to the strong interactions between the two porphyrin
rings. Both the carbonyl dimers exhibited four one-electron
oxidation waves from 650 to 1500 mV (Figure S2). The former
two waves and the latter two waves were assigned to the first
and second porphyrin ring oxidation processes, respectively. The
average redox potentials of the former two waves (E1(av)) and
those of the latter two waves (E2(av)) in both 1 and 2 were not
so different from those of the first and second porphyrin
oxidation waves of Ru(tB4P)(CO)(Py). The differences, (E2(av)
- E1(av) values), in the average redox potentials of 1 and 2 were
585 and 601 mV, respectively. These values were similar to
those of Ru(tB4P)(CO)(Py) and Ru(TPP)(CO)(Py)18 as listed
in Table 2.
(17) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 3rd ed.; A Wiley-Interscience: New York,
1978. (b) Clark, R. E.; Ford, P. C. Inorg. Chem. 1970, 9, 227.
Redox potentials of the cofacial bispyridine porphyrin dimers,
3 and 4, are listed in Table 3. Figure 2a and b showed that the

Novel Cofacial Ru(II) Porphyrin Dimers and Tetramers
Inorganic Chemistry, Vol. 37, No. 19, 1998 4991
Table 2. Redox Potentials E°′ and ∆E°′ (in mV) Values of
different from that of the free porphyrins H2(3-Py)tB3P and H2-
(4-Py)tB3P with respect to reversibility. The free ligands show
irreversible porphyrin ring oxidation, because the one-electron
ring oxidation caused nucleophilic attack of the pyridyl group
on the â-positions of the other porphyrin rings to give
polymerized species.21 On the other hand, in the tetramers of
6 and 7, the polymerization was inhibited by the coordination
of the pyridyl groups to ruthenium ions, so that the reversible
porphyrin ring oxidation progressed.11b Both the complexes, 6
and 7, exhibited two one-electron waves of the Ru(III/II)
processes. Potential differences, ∆E°′, of the processes were
more than 320 mV. These ∆E°′ values of 6 and 7 were larger
than those of 3-5 by more than 30 mV. Besides the two Ru-
(III/II) processes, two additional reversible processes appeared
at around 950-1300 mV. Each of these processes apparently
mediated 2-electron transfers judging from the current heights
in cyclic voltammograms and differential pulse voltammograms
as shown in Figures 3 and S3. The processes could be assigned
to the overall 2-electron oxidations of each of the axial porphyrin
rings, because the cofacial ruthenium porphyrin rings must be
oxidized at higher potential regions and the processes must be
irreversible as experienced in the systems of 3-5. Namely the
results indicated that the two axial porphyrin ligands in each of
6 and 7 were oxidized at the same potentials. Irreversible
oxidation waves were actually observed at more positive
potentials of 1612 mV for 6 and 1480 mV for 7. The
irreversible waves were assigned to the oxidations of the
ruthenium porphyrin rings of 6 and 7. The shifts to the positive
direction in the oxidation processes of the ruthenium porphyrin
rings might result from the plus charges on the axial porphyrin
ligands. The positive shift of 6 is larger than that of 7, and
could come from the differences in proximity of the ruthenium
porphyrins from the π-conjugated systems of the axial porphy-
rins, that is, 6 is apparently closer than 7. The difference in
the interactions between the core ruthenium porphyrins and the
axial porphyrins in these two systems was reflected by the redox
potentials in the oxidation steps of the axial porphyrins of 6
(989 mV) and 7 (963 mV), that is, the potentials were the reverse
of the free porphyrins H23-PytB3P (834 mV) and H24-PytB3P
(902 mV).
Cofacial Carbonyl Dimersa
por oxidn. 1 por oxidn. 2 E2(av)
complex
E1 (∆E°′)
E2 (∆E°′) - E1(av)
[Ru(2-PyT3P)(CO)]2 (1)
660 [1]
(259)
1286 [1]
(176)
585
919 [1]
662 [1]
(274)
1462 [1]
1316 [1]
(174)
1482 [1]
1360
[Ru(2-PytB3P)(CO)]2 (2)
601
936 [1]
Ru(TPP)(CO)(Py) (vs SSCE)b 810
550
571
Ru(tB4P)(CO)(Py)
675
1246
a Redox potentials were obtained from (Epa + Epc)/2(E°′). The values
were corrected by the potential of a ferrocenium/ferrocene couple (352
mV). The numerals in brackets are the numbers of electrons transferred
which were evaluated from the wave heights of CV or DPV. b Reference
18a.
redox behavior of 3 and 4 were almost the same, that is, stepwise
four one-electron oxidations proceeded in both 3 and 4. The
former two one-electron waves were assigned to Ru(III /II)
similar to those of Ru(tB4P)(Py)2 and Ru(TPP)(Py)2.18 Although
the latter two one-electron waves were irreversible processes,
the waves could be assigned to the first porphyrin ring oxidation
steps.
The potential differences (∆E°′, over 270 mV) in each redox
process of the cofacial carbonyl and bispyridine dimers were
significantly (20 to 60 mV) larger than those of the cyclic
tetramers such as [Ru(4-PyT3P)(CO)]4 and [Ru(4-PyT3P)-
(Py)]4.19 The strong interactions between the two ruthenium
porphyrin subunits in these cofacial dimers apparently result
from the proximity between the two ruthenium porphyrin
subunits. We believe that the cofacial arrangement and overlap-
ping of the two porphyrin planes, as observed in the X-ray
structures of cofacial zinc porphyrin dimers,6a are essential for
the strong interactions. The redox profile of 5 was similar to
those of 3 and 4, that is, 5 exhibited two reversible one-electron
waves of Ru(III/II) and two irreversible one-electron waves of
the ruthenium porphyrin ring oxidation processes. The poten-
tials of Ru(III/II) in 5 were shifted to the positive direction
relative to that of 4 due to the stronger electron-withdrawing
abilities (more strong π-acidity) of the coordinated 4-cyanopy-
ridine. It is interesting that the potentials of the ruthenium
porphyrin ring oxidations were negatively shifted, relative to
that of 4. Similar correlation in the potential shifts of Ru(III/
II) and the porphyrin ring oxidations was observed between Ru-
(TBP)(azpy)2 and Ru(TBP)(pyz)2.20 The redox potential of
Ru(III/II) of Ru(TBP)(azpy)2 is more positive than that of Ru-
(TBP)(pyz)2 and the porphyrin ring oxidation of Ru(TBP)(azpy)2
is more negative than that of Ru(TBP)(pyz)2. These results
suggested that the π-acidity of the axial pyridyl ligands
controlled the strength of the interaction between the π or π*
orbitals of the porphyrin rings and the d orbitals of the ruthenium
ion.
In the region of negative potentials, redox processes were
observed as shown in Figure 3. In the case of 7, two reduction
processes of the axial porphyrin ligands were observed, similar
to those of the free 4-pyridylporphyrin, though the redox
potentials were shifted slightly in positive direction. The
reduction processes of 7 mediated two-electron transfers,
indicating that two axial porphyrin ligands are reduced at the
same potential with no interactions between the axial porphyrins,
which is similar to the case of the perpendicularly arranged
porphyrin dimers and trimers.11b On the other hand, the redox
profile of the reduction processes of the axial porphyrins in 6
was significantly different from that of free 3-pyridylporphyrin
as shown in Figure 3a, that is, a reduction wave was observed
at -1363 mV in 6. In addition, the number of electrons
transferred in this process was determined to be one by the
current ratio between the reduction process and Ru(III/II)
processes, suggesting that the two axial porphyrins interact via
the cofacial ruthenium porphyrin dimer subunits. This result
might be brought about by proximal interactions through the
π* orbitals of the porphyrin π-conjugated systems between the
axial 3-pyridylporphyrins and the core ruthenium porphyrins.
Redox potentials of the tetramers, 6 and 7, and free axial
porphyrin ligands are also tabulated in Table 3. Cyclic
voltammograms of 6 and 7 are given in Figure 3a and b. The
redox behavior of the axial porphyrin ligands of 6 and 7 was
(18) (a) Brown, G. M.; Hopf, F. R.; Ferguson, J. A.; Meyer, T. J.; Whitten,
D. G. J. Am. Chem. Soc. 1973, 95, 5939. (b) Felton, R. H. In The
Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol.
5, Chapter 3. (c) Brown, G. M.; Hopf, F. R.; Meyer, T. J.; Whitten,
D. G. J. Am. Chem. Soc. 1975, 97, 5385. (d) Pacheco, G. M.; James,
B. R.; Rettig, S. J. Inorg. Chem. 1995, 34, 3477.
(19) Funatsu, K.; Imamura, T.; Ichimura, A.; Sasaki, Y. Inorg. Chem. 1998,
37, 1798.
(20) Marvaud, V.; Launay, J. P. Inorg. Chem. 1993, 32, 1376.
(21) Giraudeau, A.; Ruhlmann, L.; El Kahef, L.; Gross, M. J. Am. Chem.
Soc. 1996, 118, 2969.

4992 Inorganic Chemistry, Vol. 37, No. 19, 1998
Funatsu et al.
redn.
Table 3. Redox Potentials E°′ and ∆E°′ (in mV) Values of Cofacial Bispyridine Dimers and Tetramersa
axial por
Ru(III/II)
por oxidn
(∆E°′)
complex
(∆E°′)
oxidn.
[Ru(2-PyT3P)(Py)]2 (3)
23 [1]
(296)
319 [1]
15 [1]
(290)
305 [1]
116 [1]
(269)
384 [1]
10 [1]
(328)
338 [1]
52 [1]
(321)
373 [1]
210
1284 (Epa) [1]
(209)
1493 (Epa) [1]
1255 (Epa) [1]
(207)
1462 (Epa) [1]
1230 (Epa) [1]
(123)
1353 (Epa) [1]
1612 (Epa)
[Ru(2-PytB3P)(Py)]2 (4)
[Ru(2Py)tB3P)(PyCN)]2 (5)
[Ru(2-PyT3P)(H23-PytB3P)]2 (6)
[Ru(2-PyT3P)(H24-PytB3P)]2 (7)
989 [1 × 2]
-1363 [1]
1279 [1 × 2]
1480 (Epa)
963 [1 × 2]
-1580 [1 × 2]
-1233 [1 × 2]
1208 [1 × 2]
Ru(TPP)(Py)2 (vs SSCE)b
Ru(tB4P)(Py)2
Ru(tB4P)(PyCN)2
Ru(OEP)(PyCN)2
H23-PytB3P
1260
1121
1165
1080
82
326
107
834
1138
902
1301 (Epa)
-1692
-1351
-1648
-1290
H24-PytB3P
a Redox potentials were obtained from (Epa + Epc)/2(E°′). The values were corrected by the potential of a ferrocenium/ferrocene couple (352
mV). The numerals in brackets are the numbers of electrons transferred, which were evaluated from the wave heights of CV or DPV. b Reference
18a.
Figure 3. (a) Cyclic voltammogram of [Ru(2-PyT3P)(H23-PytB3P)]2
(6) in 0.1 M TBA(PF6) CH2Cl2 solution. (b) Cyclic voltammogram of
Figure 2. (a) Cyclic voltammogram of [Ru(2-PytB3P)(Py)]2 (4) in 0.1
[Ru(2-PyT3P)(H24-PytB3P)]2 (7) in 0.1 M TBA(PF6) CH2Cl2 solution.
M TBA(PF6) CH2Cl2 solution. (b) Differential pulse voltammogram
of [Ru(2-PytB3P)(Py)]2 (4) in 0.1 M TBA(PF6) CH2Cl2 solution.
with an absorption maximum at 412 nm as shown in Figure 4.
Similar porphyrin dimers [Zn(2-PyPOR)]2 and an imidazole
tethered cofacial zinc porphyrin dimer [Zn(OEP-im)]2 showed
apparent exciton splittings of the Soret bands with a splitting
UV-Vis Spectroscopy. UV-vis spectroscopic data are
listed in Table 4. UV-vis spectra of the cofacial ruthenium
porphyrin dimers were very characteristic. Each of the cofacial
carbonyl dimers 1 and 2 exhibited a significant broad Soret band
range of 890 to 1040 cm-1 6a
.
In contrast, 1 and 2 showed no
exciton splittings, though the frameworks of the dimers were

Novel Cofacial Ru(II) Porphyrin Dimers and Tetramers
Inorganic Chemistry, Vol. 37, No. 19, 1998 4993
λ
max/nm (ꢀ; 104 M-1 cm-1/ꢀ per subunit)
Table 4. UV-Vis Data of Ruthenium Porphyrin Oligomer
complex
solvent
[Ru(2-PyT3P)(CO)]2 (1)
[Ru(2-PytB3P)(CO)]2 (2)
[Ru(2-PyT3P)(Py)]2 (3)
[Ru(2-PytB3P)(Py)]2 (4)
[Ru(2-PytB3P)(PyCN)]2 (5)
[Ru(2-PyT3P)(H23-PytB3P)]2 (6)
[Ru(2-PyT3P)(H24-PytB3P)]2 (7)
H23-PytB3P
H24-PytB3P
Ru(tB4P)(CO)(Py)
Ru(tB4P)(Py)2
Ru(tB4P)(PyCN)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
C6H5CH3
C6H5CH3
C6H5CH3
C6H5CH3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
413 (22.4/11.2), 539 (3.79/1.90), 579 (0.71/0.36)
413 (23.3/11.7), 539 (4.02/2.01), 575 (0.74/0.37)
403 (19.6/9.8), 421 (sh.), 508 (3.03/1.52)
405 (18.7/9.4), 422 (sh.), 508 (2.83/1.42)
403 (17.4/8.7), 418 (17.3/8.7), 508 (3.86/1.93), 621 (1.41/0.71), 665 (1.15/0.58)
407 (sh. 37.2), 421 (60.9), 516 (6.00), 552 (3.52), 591 (2.03), 650 (1.62)
407 (sh. 38.9), 421 (73.5), 513 (7.13), 550 (3.41), 589 (2.21), 652 (1.56)
420 (46.8), 516 (1.98), 551 (1.03), 593 (0.58), 650 (0.52)
420 (45.9), 516 (1.98), 551 (0.95), 591 (0.59), 649 (0.45)
414 (27.0), 534 (2.05), 568 (0.59)
413 (16.0), 422 (16.5), 506 (2.29)
407 (sh. 10.1), 418 (20.8), 509 (2.83), 550-650 (broad band)
399 (12.1), 498 (1.59), 525 (3.29), 618 (1.69)
Ru(OEP)(PyCN)2
Figure 4. UV-vis spectral change of carbonyl dimer 1 in CH2Cl2 by
the addition of excess pyridine (100 equiv) at 23 °C.
almost the same as those of the cofacial zinc porphyrin dimers.
On addition of a large amount of pyridine to the solutions of 1
and 2, the Soret bands sharpened and increased in intensity
without change in the absorption maxima. The change is due
to the pyridine substitution reactions to form the corresponding
monomers, Ru(2-PyT3P)(CO)(Py) and Ru(2-PytB3P)(CO)(Py),
as shown in Figure 4. The final spectra were almost the same
as those of Ru(tB4P)(CO)(Py) and Ru(TPP)(CO)(Py).15a Half-
widths of the Soret bands of these monomers were ca. 18 nm,
which was less than half of the half-widths (45 nm) of 1 and 2.
Furthermore, the absorption intensities of the monomer spectra
were more than twice of those of 1 and 2. These characteristics
of the Soret bands in 1 and 2 are likely to result mainly from
excitonic interactions between the two porphyrin π-conjugated
systems.22 The Q-bands of 1 and 2 showed red shifts by ca.
250 cm-1 relative to those of the corresponding monomers.
Similar red shifts (100-150 cm-1) were reported in the systems
of the cofacial zinc dimers.
Figure 5. (a) UV-vis spectra of bis(4-cyano)pyridine dimer and
monomers in CH2Cl2 at 23 °C. Solid line: [Ru(2-PytB3P)(PyCN)]2 (5).
Dashed line: Ru(tB4P)(PyCN)2. Dotted line: Ru(OEP)(PyCN)2. (b)
UV-vis spectral changes of [Ru(2-PytB3P)(PyCN)]2 5 by one electron
oxidative titration with Ce(IV) in CH2Cl2 at 23 °C. Solid line: [RuII,II
-
(2-PytB3P)(PyCN)]2. Dashed line: [RuII,III(2-PytB3P)(PyCN)]2+. Dotted
line: [RuIII,III(2-PytB3P)(PyCN)]2
.
2+
The Soret bands and Q-bands of the bispyridine dimers 3
and 4 were also significantly broad relative to those of the
corresponding monomers, Ru(2-PyT3P)(Py)2 and Ru(2-PytB3P)-
(Py)2. In addition, the Soret bands of 3 and 4 were blue shifted
by 7-9 nm relative to the corresponding monomers. Figure 5
shows the characteristic UV-vis spectrum of bis(4-cyanopy-
ridine) dimer 5. The Soret band and Q-band were also
significantly broadened relative to the corresponding monomer
of Ru(tB4P)(PyCN)2. Although the feature of the porphyrin π
- π* transition (the Soret band and Q-band) of 5 was similar
to those of 3 and 4, extra broad bands were observed for 5 at
621 and 665 nm. These bands were also observed for monomer
analogues of Ru(tB4P)(PyCN)2 (550-650 nm) and Ru(OEP)-
(PyCN)2 (618 nm) as shown in Figure 5a and Table 4. From
the facts described later, the bands were assigned to a metal to
ligand charge-transfer band (MLCT: Ru(dπ) - cyanopyridine-
(π*)). The bands exhibited solvent dependence, e.g., the band
of 5 shifted from 614 nm in toluene (dielectric constant: 2.4)
to 621 nm in dichloromethane (8.9) and 625 nm in acetone
(20.7).23 The oxidation of Ru(II) to Ru(III) in these three
biscyanopyridine complexes using (NH4)2Ce(NO3)6 as an
oxidizing agent decreased the intensities of the bands as shown
in Figure 5b. Ru(OEP)(Py)2 exhibits a MLCT band (Ru(dπ)
- Py(π*)) at around 450 nm, because the eight ethyl substituents
of the OEP rings with electron donating abilities raise the eg*
orbitals of the porphyrins above the lowest pyridine π* orbital.24
On the other hand, the MLCT bands were not observed for Ru-
(22) (a) Kasha, M.; Rawls, H. L.; El-Bayoumi, M. A. Pure Appl. Chem.
1965, 11, 371. (b) Kasha, M.; Radiat. Res. 1963, 20, 55.
(23) Wollmann, H. Pharmazie 1974, 29, 708.

4994 Inorganic Chemistry, Vol. 37, No. 19, 1998
Funatsu et al.
(TPP)(Py)2 because the four meso-phenyl substituents are
electron-withdrawing and stabilize the eg* orbitals of the
porphyrin below the pyridine π* orbital.24 The lowest π* orbital
of 4-cyanopyridine with an electron-withdrawing cyano group
must be lower than the lowest π* orbital of pyridine. Thus the
shift of the MLCT band (Ru(dπ) - cyanopyridine(π*)) of Ru-
(OEP)(PyCN)2 (618 nm) to lower energy relative to the Ru-
(OEP)(Py)2 systems (450 nm) is rationalized. In addition, since
the lowest π* orbital of 4-cyanopyridine may be below the eg*
orbitals of the meso-arylporphyrin rings of tB4P and 2-PytB3P,
MLCT bands (Ru(dπ) - cyanopyridine(π*)) were observed for
5 (621 nm) and Ru(tB4P)(PyCN)2 (550-650 nm). In fact, a
ruthenium(II) meso-tetraarylporphyrin complex having two axial
azopyridine ligands, Ru(TBP)(azpy)2, exhibited the MLCT band
(Ru(dπ) - azpy(π*)) in the lower energy region at around 800
nm, since the lowest π* orbital of azopyridine is more stable
than those of 4-cyanopyridine and pyridine.20
UV-vis spectra of the porphyrin tetramers 6 and 7 were
measured in toluene, because these tetramers were unstable in
dichloromethane. In both the tetramers, four peaks of the
Q-bands of the axial porphyrin ligands were observed in the
region of 500-650 nm as shown in Table 4 and Figure S4.
Q-bands due to the core cofacial ruthenium porphyrin subunits
should also be in this region, though no peaks were observed.
Besides the Q-bands, another broad band was observed for 7 at
around 650-750 nm. Similar broad bands appeared in the same
region in previously reported perpendicularly linked porphyrin
trimers, Ru(OEP)(H24-PyP3P)2 and Ru(TTP)(H24-PyP3P)2.11b
Figure 6. (a) Near-infrared absorption spectral changes in the systems
of [Ru(2-PytB3P)(Py)]2 (4) accompanied by one-electron oxidative
titration with Ce(IV) in CH2Cl2 at 23 °C. (b) Near-infrared absorption
spectral change of Ru(TPP)(Py)2 by stepwise 0.25-electron oxidative
titrations with Ce(IV) in CH2Cl2 at 23 °C. Two spiky peaks at around
1650 nm were doubly generate vibrations of the stretches of C-H bonds
in CH2Cl2.
In both 6 and 7, the Soret bands of the ruthenium porphyrin
dimer core and the axial porphyrin subunits overlapped with
each other. Although the Soret bands of the ruthenium
porphyrin dimer core and the axial porphyrin ligands appeared
at 407 nm as a shoulder and 421 nm, respectively, the molar
absorptivities of 6 and 7 at 421 nm (60.9 × 104 for 6 and 73.5
× 104 M-1 cm-1 for 7) were much smaller than the values
expected for two axial porphyrin subunits (larger than 90 ×
104 M-1 cm-1), even despite overlapping of the Soret bands of
the ruthenium porphyrin dimer subunits to some extent. Similar
decreases in molar absorptivity have also been commonly
observed in perpendicularly linked porphyrin trimers, Ru(OEP)-
(H24-PyP3P)2 and Ru(TTP)(H24-PyP3P)2.11b In addition, the
decrease in the absorptivity of 6 was much larger than that of
7. It may be more feasible for π-conjugated systems of the
axial 3-pyridylporphyrin ligands to interact with those of the
ruthenium porphyrin dimer subunits than between perpendicu-
larly coordinated 4-pyridylporphyrins and the ruthenium por-
phyrin dimer subunits.
values indicated that their mixed-valence states (one-electron
oxidized complexes) were stable with respect to disproportion-
ation.25
Since bulk electrolysis in some cofacial dimers caused
adsorption on the electrodes, these dimers were chemically
oxidized by Ce(IV) or I2 to yield the corresponding dimers with
mixed valence states. In the cases of the cofacial carbonyl
dimers of 1 and 2, low solubility of the neutral dimer complexes
and instability of the mixed-valence states interfered with the
measurements of clear visible and near-IR spectra for the mixed-
valence states. In 3, 4, and 5, visible and near-IR spectral
changes accompanied by oxidative titrations with Ce(IV) or I2
were followed successfully. The spectral changes in 3 and 4
matched to the theoretical stoichiometries. On the other hand,
in 5, the oxidizing agents needed more than a stoichiometric
amount to obtain the mixed valence state species, because of
the high redox potentials of Ru(III/II) in these systems. The
visible spectral change in the system of 4 caused by the
oxidations of ruthenium ions was similar to that of 5 except
that the spectral change occurred at around 600 nm. The visible
spectra of the one- and two-electron oxidized complexes derived
from 4 or 5 almost returned to the initial spectra of the parent
complexes on reduction by cobaltcene.
Properties of Mixed-Valence State. As described in the
section “Electrochemical Studies”, the cofacial dimers and
tetramers exhibited stepwise oxidations at the ruthenium por-
phyrin rings or metal centers due to strong interactions between
the cofacially arranged ruthenium porphyrin subunits. The
comproportionation constants (Kc) of the mixed-valence dimers
estimated from ∆E values (mV) were 2.4 × 104 and 3.3 × 104
for 1 and 2 in the first porphyrin ring oxidation processes,
respectively. The constants were also evaluated to be 3.5 ×
104, 8.0 × 104, 10.0 × 104, 35.1 × 104, and 26.7 × 104, in the
Ru(III/II) processes for 3, 4, 5, 6, and 7, respectively. These
Spectral change of 4 and Ru(TPP)(Py)2 in the near-IR regions
by the titrimetric oxidation with Ce(IV) are shown in Figure
6a and b, respectively. Although both the dimer and the neutral
monomer had no bands in the region of 1000-2500 nm, one-
electron oxidation of 4 gave a new broad band in the near-IR
region with a peak maximum at 1500 nm. The broad band
increased in intensity on addition of Ce(IV) and attained
maximum intensity with the stoichiometric addition for complete
(24) (a) Schick, G. A.; Bocian, D. F. J. Am. Chem. Soc. 1984, 106, 1682.
(b) Vitols, S. E.; Roman, J. S.; Ryan, D. E.; Blackwood, M. E., Jr.;
Spiro, T. G. Inorg. Chem. 1997, 36, 764.
(25) Richrdson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278.

Novel Cofacial Ru(II) Porphyrin Dimers and Tetramers
Inorganic Chemistry, Vol. 37, No. 19, 1998 4995
one-electron oxidation. Then the broad band decreased upon
further addition and finally disappeared to give new two bands
between 1500 and 2500 nm. The end point of the completion
of the spectral changes fitted the stoichiometry for the two-
electron transfer. The spectrum of the two-electron oxidized
complex of 4 with two weaker peaks at 1500-2500 nm was
similar to that of [RuIII(TPP)(Py)2]+. These results revealed
that the characteristic broad band at 1500 nm of the one-electron
oxidized complex was an intervalence charge-transfer (IT) band.
Similar broad bands were also observed for the other cofacial
dimers, whose peak maxima were 1491 nm for 3 and 1570 nm
for 5. Porphyrin tetramers of 6 and 7 gave also broad bands at
around 1500 nm in the course of oxidation. However addition
of oxidizing agents to solutions containing 6 or 7 caused
simultaneous oxidation of the axial porphyrin ligands besides
the oxidation of the ruthenium ions. In fact, the solution of
fully oxidized 6 or 7 gave a new visible spectrum with a Soret
band at around 445 nm, indicating the formation of π cation
radicals derived from the axial porphyrin ligands.26
Although the origin of the bands is not clear, we tentatively
assigned the bands to LMCT from the porphyrin rings arranged
cofacially to Ru(III).
Conclusion
A variety of cofacial ruthenium porphyrin dimers, 1-5, with
axial CO, pyridine, or 4-cyanopyridine ligands, and tetramers,
6 and 7, with axial pyridylporphyrin ligands were synthesized.
These new oligomers were characterized mainly by spectral
1
methods such as H NMR and ESI-MS measurements. Elec-
trochemical properties of the oligomers were characteristic.
Oxidations of the porphyrin rings or the ruthenium ions at the
first stages proceeded stepwise by the strong interactions
between the cofacial dimer subunits. Splittings in the redox
potentials (∆E, mV) were over 260 mV for all oligomers. An
interaction between the axial 3-pyridylporphyrin ligands in 6
was observed in the reduction process of the axial 3-pyridylpor-
phyrins, which was in contrast to the noninteracting axial
4-pyridylporphyrin ligands in 7. Important factors of the strong
interaction of the oligomers are the cofacial arrangement
between constituent porphyrin planes and the overlap of the
π-conjugated systems of the porphyrin rings. Interactions
between the porphyrin subunits were represented in UV-vis
spectra by significant broadenings in the Soret bands, though
apparent exciton splittings were not observed in contrast to the
zinc dimers, [Zn(2-PyPOR)]2 and [Zn(im-POR)]2. In 5 and the
monomer analogues, Ru(OEP)(PyCN)2 and Ru(tB4P)(PyCN)2,
MLCT bands from Ru(II) to the axial cyanopyridine ligands
were observed. Mixed-valence states (Ru(III,II)) in the systems
of 3-7 exhibited intervalence charge-transfer bands in the near-
IR region at around 1500 nm. The intervalence charge transfer
most likely proceeds across the overlapping pyrrole rings of
the two ruthenium porphyrin rings.
Many mixed valence multinuclear metal complexes such as
Creutz-Taube ions show IT bands in the near-IR region.27
Interactions between the metal centers occur across the conju-
gated bridging ligands. The dπ(metal) - pπ*(bridging ligand)
interactions are important factors to strengthen the interactions
between metal centers.27,28 There are dπ(Ru) - pπ*(porphyrin)
interactions in the Ru(POR)(Py)2 systems.24 Furthermore, in
the systems of 3-7, electronic communication of the π-conju-
gate system between the ruthenium porphyrin rings and 2-py-
ridyl substituents must be cut off, because of the perpendicular
geometries between the porphyrin planes and the 2-pyridyl rings.
Hence intervalence charge transfer (IT) between Ru(II) and Ru-
(III) in 3-7 most likely occurs across the overlapping pyrrole
rings of the two ruthenium porphyrin rings through the π*
orbitals of the ruthenium porphyrin rings.
Figure 6a also shows that the two-electron oxidized com-
plexes in the systems of 3-7 have prominent bands at around
1000 nm which were not observed in [RuIII(TPP)(Py)2]+.
Acknowledgment. We acknowledge funds from a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Science, Sports and Culture, Japan (No. 08454206) and the 1994
Kawasaki Steel 21st Century Foundation.
(26) (a) Peychal-Heiling, G.; Wilson, G. S. Anal. Chem. 1971, 43, 545.
(b) Peychal-Heiling, G.; Wilson, G. S. Anal. Chem. 1971, 43, 550.
(27) (a) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086. (b) Creutz,
C. Prog. Inorg. Chem. 1983, 30, 1. (c) Ward, M. D. Chem. Soc. ReV.
1995, 121.
(28) (a) Ernst, S.; Kasack, V.; Kaim, W. Inorg. Chem. 1988, 27, 1146. (b)
Kaim, W.; Kasack, V. Inorg. Chem. 1990, 29, 4696.
Supporting Information Available: Figures of H-H COSY of 4,
cyclic voltammogram of 1, differential pulse voltammogram of 6, and
UV-vis spectra of 6 and 7 (Figures S1, S2, S3, and S4) (4 pages).
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