Spectroscopic and oxidation studies of meso-tetraphenyltetrabenzoporphyrin carbonyl complexes of ruthenium(II): CO as the probe to elucidate the bonding characteristics of porphyrins

Article abstract of DOI:10.1021/om960653h

The carbonyl complex of (meso-tetraphenyltetrabenzoporphyrinato)ruthenium(II), Ru(TPTBP)(CO), has been synthesized and characterized by FABMS, UV/vis, ¹H NMR, and IR spectroscopy. Six-coordinate complexes Ru(TPTBP)(CO)(L) with different π-bonding-capability ligands (L = NEt₃, pip, 1-MeIm, py, PBu₃) coordinated trans to CO have been studied. The shifts in v_CO for this series of complexes are consistent with the existence of M → CO π-back-bonding. In contrast to what would be expected by nitrogen basicity, v_CO values for Ru(TPTBP)(CO), Ru(TPP)(CO), and Ru(OEP)(CO) are 1959, 1930, and 1917 cm^-1, respectively. This result suggests that TPTBP should be both a better σ-donor and a better π-acceptor than normal porphyrin systems (P). Oxidation studies of Ru(TPTBP)(CO), Ru(TPTBP)(CO)(py), and Ru(TPTBP)(py)₂ have been carried out both electrochemically and chemically. ¹H NMR, ESR, and electronic spectroscopic studies suggest that there are two different types of oxidation products. The sites of oxidation should both be on the porphyrin ring to give two different types of ruthenium(II) porphyrin π-cation radicals [Ru^II(TPTBP)⁺(L)(L′)]X of A_1u and A_2u character, respectively. In marked contrast to other ruthenium porphyrins reported in the literature, extraplanar ligands in the Ru(TPTBP) system do not affect the site of oxidation (metal vs ring) but only mediate the level of oxidation on the ring (a_1u vs a_2U). These results can be ascribed to the extended π-system and the ring deformation of the TPTBP porphyrin macrocycle and are also consistent with the fact that TPTBP is a stronger π-acceptor than other porphyrin systems.

Full text of DOI:10.1021/om960653h

Organometallics 1997, 16, 2121-2126
2121
Sp ectr oscop ic a n d Oxid a tion Stu d ies of
m eso-Tetr a p h en yltetr a ben zop or p h yr in Ca r bon yl
Com p lexes of Ru th en iu m (II): CO a s th e P r obe to
Elu cid a te th e Bon d in g Ch a r a cter istics of P or p h yr in s
Ru-J en Cheng,* Shang-Ho Lin, and Hsiao-Mei Mo
Department of Chemistry, National Chung-Hsing University,
Taichung, Taiwan 402, Republic of China
Received August 5, 1996X
The carbonyl complex of (meso-tetraphenyltetrabenzoporphyrinato)ruthenium(II), Ru(TPT-
1
BP)(CO), has been synthesized and characterized by FABMS, UV/vis, H NMR, and IR
spectroscopy. Six-coordinate complexes Ru(TPTBP)(CO)(L) with different -bonding-capabil-
ity ligands (L ) NEt3, pip, 1-MeIm, py, PBu3) coordinated trans to CO have been studied.
The shifts in
for this series of complexes are consistent with the existence of M f CO
CO
-back-bonding. In contrast to what would be expected by nitrogen basicity,
values for
CO
Ru(TPTBP)(CO), Ru(TPP)(CO), and Ru(OEP)(CO) are 1959, 1930, and 1917 cm-1, respec-
tively. This result suggests that TPTBP should be both a better -donor and a better
-acceptor than normal porphyrin systems (P). Oxidation studies of Ru(TPTBP)(CO),
Ru(TPTBP)(CO)(py), and Ru(TPTBP)(py)2 have been carried out both electrochemically and
chemically. 1H NMR, ESR, and electronic spectroscopic studies suggest that there are two
different types of oxidation products. The sites of oxidation should both be on the porphyrin
ring to give two different types of ruthenium(II) porphyrin
-cation radicals
[RuII(TPTBP)•+(L)(L′)]X of A1u and A2u character, respectively. In marked contrast to other
ruthenium porphyrins reported in the literature, extraplanar ligands in the Ru(TPTBP)
system do not affect the site of oxidation (metal vs ring) but only mediate the level of oxidation
on the ring (a1u vs a2u). These results can be ascribed to the extended -system and the
ring deformation of the TPTBP porphyrin macrocycle and are also consistent with the fact
that TPTBP is a stronger -acceptor than other porphyrin systems.
In tr od u ction
Tetraphenyltetrabenzoporphyrin (TPTBP) is a struc-
tural hybrid of tetraphenylporphyrin (TPP) and tetra-
benzoporphyrin (TBP). Our previous work showed that,
other than the extended -electron system, macrocycle
of TPTBP is severely distorted into a saddle shape both
in the solid state2 and in solution.3 The consequences
of saddle deformation on the properties of the nonplanar
tetrabenzoporphyrin are significant. As reported previ-
ously, the cumulative effects of conformational changes
and substituent addition are reflected in the red-shifted
optical spectrum, diminished fluorescence,4 increased
basicity (pK3 ) 5.75 for TPTBPH2 versus 3.95 for
TPPH2), and ease of oxidation of the neutral compound
(E1/2 ) +0.42 V for Zn(TPTBP) versus +0.75 V for
Zn(TPP)). These features should result in different
bonding characteristics in the corresponding metal-
loporphyrin complexes. Cis, trans, and metal effects
mediated by the bonding properties of porphyrins have
been reviewed extensively by Buchler et al.5
Metalloporphyrins serve many diverse functions in
biological systems. They may change from an oxygen
carrier to an oxygen activation catalyst just by protein
fine-tuning of the strucuture around the coordination
sphere. The chemical variation of biologically active
molecules represents a very successful method for the
elucidation of structure-property-activity interrela-
tionships. A knowledge of these interrelationships may
help to sort out the essential electronic parameters that
govern the specific action of the heme proteins in
biologic oxygen transport and redox processes. Other
than the electronic and steric effects that can be
introduced through different substituents on the por-
phyrin macrocycle, the functional consequences of non-
planar distortions are of current interest. Fajer sug-
gested that conformational variations provide an
attractively simple mechanism for varying a wide range
of chemical and physical properties of porphyrinic
chromophores and prosthetic groups in vitro and in
vivo.1 These deformations result from intramolecular
steric interactions among the peripheral substituents
in vitro, which are believed to be introduced by the
combinations of hydrogen bonding, axial ligation, and
nearby protein residues in vivo.
As a typical -acid and the widely opened IR region
for
stretching frequencies, CO is a good probe to
CO
elucidate the bonding properties of porphyrins. Carbo-
nyl complexes of iron porphyrins are stable only in the
(2) Cheng, R.-J .; Chen, Y.-R.; Wang, S. L.; Cheng, C. Y. Polyhedron
1993, 12, 1353.
(3) Cheng, R.-J .; Chen, Y.-R.; Chen, C.-C. Heterocycles 1994, 38,
X Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) (a) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J .
J . Am. Chem. Soc. 1988, 110, 7566. (b) Barkigia, K. M.; Gottfried, D.
S.; Boxer, S. G.; Fajer, J . J . Am. Chem. Soc. 1989, 111, 6444. (c) Renner,
M. W.; Cheng, R.-J .; Chang, C. K.; Fajer, J . J . Phys. Chem. 1990, 94,
8508. (d) Chem. Eng. News 1991, 69 (J an 14), 23.
1465.
(4) Cheng, R.-J .; Chen, Y.-R.; Chuang, C.-E. Heterocycles 1993, 34,
1.
(5) Buchler, J . W.; Kokisch, W.; Smith, P. D. Struct. Bonding 1978,
34, 79.
S0276-7333(96)00653-X CCC: $14.00 © 1997 American Chemical Society

2122 Organometallics, Vol. 16, No. 10, 1997
Cheng et al.
of the procedure of Rillema et al. for the synthesis of Ru(TP-
P)(CO).9 In a typical experiment, 100 mg of TPTBPH2 and
100 mg of Ru3(CO)12 in 30 mL of decalin was refluxed under
nitrogen for about 4 h. After it was cooled to room tempera-
ture, the reaction mixture was loaded on a column of silica
gel and eluted with varying ratios of hexane/CH2Cl2/acetone.
Recrystallization from CH2Cl2/hexane gave pure Ru(TPTB-
P)(CO) with an average yield of 57%. MS: (M)+ m/e 942, (M
- CO)+ m/e 914. HRMS calcd for C61H36ON4Ru: (M)+ m/e
942.1933; found m/e 942.1937 (∆ ) 0.4).
presence of excess CO or in the solid state. The group
homologue of iron, ruthenium, is expected to show a
much stronger metal-to-ligand -back-bonding in its +II
oxidation state and form more stable complexes with
CO than with iron. Both cis and trans effects should
be exaggerated with ruthenium porphyrins. On the
other hand, resonance Raman studies showed that
-back-bonding to the porphyrin appears to be quanti-
tatively similar for Ru(II) and Fe(II).6 Therefore,
Ru(TPTBP)(CO) will be the first candidate for these
series of systematic studies.
Bis(p yr id in e)(t et r a p h en ylt et r a b en zop or p h yr in a t o)-
r u th en iu m (II), Ru (TP TBP )(p y)2. Following earlier work-
ers10 a solution of 100 mg of Ru(TPTBP)(CO) in 70 mL of
pyridine was irradiated with a 450 W mercury lamp (Pyrex
filter) for 12 h, while the solution was continually flushed with
nitrogen. The solution was then reduced to dryness, followed
by column chromatography and recrystallization from CH2Cl2/
hexane. An average yield of 68% was obtained. MS: (M)+
m/e 1072, (M - 2py)+ m/e 914. HRMS calcd for C70H46N6Ru:
(M)+ m/e 1072.2827; found m/e 1072.2833 (∆ ) +0.5 ppm).
C a r b o n y l ( p y r i d i n e ) ( t e t r a p h e n y l t e t r a b e n z o -
p or p h yr in a to)r u th en iu m (II), Ru (TP TBP )(CO)(p y). In a
typical experiment, 100 mg of Ru(TPTBP)(CO) in 30 mL of
CH2Cl2 was treated with 9 L of pyridine at room temperature.
The solution was then reduced to dryness, followed by recrys-
tallization from CH2Cl2/hexane to give Ru(TPTBP)(CO)(py) in
quantitative yield. The complete conversion of Ru(TPTB-
P)(CO) into Ru(TPTBP)(CO)(py) was checked by TLC and
Exp er im en ta l Section
P h ysica l Mea su r em en ts. 1H NMR spectra were recorded
on a Varian Gemini-200 or VXR-300 spectrometer. Electronic
spectra were measured with a Hitachi U-3210 spectrophotom-
eter. Infrared spectra were recorded on a Hitachi IR-270
spectrophotometer as a KBr disk or in CH2Cl2 solution. ESR
spectra were recorded at the X band with a Bruker spectrom-
eter. Mass spectra were obtained using a J EOL J MS-HX110
high-resolution double-focusing mass spectrometer equipped
for fast atom bombardment analysis.
Cyclic voltammetry was performed in CH2Cl2 solution that
was 0.10 M in supporting electrolyte and 1.5-3.5 mM in
metalloporphyrin. The supporting electrolyte was tetra-n-
butylammonium perchlorate. A conventional three-electrode
system was used. Two Pt buttons served as the working
electrode and the counter electrode. All potentials in this
paper are reported vs the SCE. Measurements were made
with a Bioanalytical Systems Model BAS CV-27 potentiostat.
Chemical oxidations of ruthenium porphyrin complexes in
dilute CH2Cl2 solutions were carried out by a spectrophoto-
metric titration procedure using I2 or Br2 solutions. A sto-
ichiometric amount of oxidant is enough for complete oxida-
tion.
1
confirmed by H NMR, UV/vis, and IR spectra (Figure 1b and
Table 1). MS: (M)+ m/e 1021, (M - py)+ m/e 942, (M - py -
CO)+ m/e 914. HRMS: calcd for C66H41ON5Ru (M)+ m/e
1021.2355; found m/e 1021.2364 (∆ ) +0.9 ppm).
C a r b o n y l (1 -m e t h y l i m i d a z o l e )(t e t r a p h e n y l t e t -
r a ben zop or p h yr in a to)r u th en iu m (II), Ru (TP TBP )(CO)-
(MeIm ). This complex and all the following series of com-
plexes were generated in situ for spectroscopic studies. Typi-
cally, a 0.5% CH2Cl2 solution of 1-methylimidazole (68 L,
0.0041 mmol) was added in small portions to a solution of
Ru(TPTBP)(CO) (3.5 mg, 0.0037 mmol) in CH2Cl2 (17.5 mL).
The conversion of Ru(TPTBP)(CO) into Ru(TPTBP)(CO)(MeIm)
was monitored by UV/vis and IR spectra. Similar titration
was carried out in CDCl3 for 1H NMR studies. Spectroscopic
data are collected in Table 1.
C a r b o n y l (p i p e r i d i n e )(t e t r a p h e n y l t e t r a b e n z o -
p or p h yr in a to)r u th en iu m (II), Ru (TP TBP )(CO)(p ip ). This
complex was prepared with piperidine as above. Spectroscopic
data are collected in Table 1.
C a r b o n y l(t r ie t h y la m in e )(t e t r a p h e n y lt e t r a b e n zo -
por ph yr in ato)r u th en iu m (II), Ru (TP TBP )(CO)(NEt3). This
complex was prepared in a similar manner, except that
addition of excess triethylamine (∼20 equiv) is necessary for
the formation of Ru(TPTBP)(CO)(NEt3). The conversion of
Ru(TPTBP)(CO) into Ru(TPTBP)(CO)(NEt3) was monitored by
UV/vis and IR spectra. Similar titration was carried out in
CDCl3 for 1H NMR studies. Spectroscopic data are collected
in Table 1.
C a r b o n y l(t r i-n -b u t y lp h o s p h in e )(t e t r a p h e n y lt e t -
r a ben zop or p h yr in a to)r u th en iu m (II), Ru (TP TBP )(CO)-
(P Bu 3), a n d Bis(tr i-n -bu tylp h osp h in e)(tetr a p h en yltet-
r aben zopor ph yr in ato)r u th en iu m (II), Ru (TP TBP )(P Bu 3)2.
A 2% CH2Cl2 solution of tri-n-butylphosphine (43 L, 0.0042
mmol) was added in ∼10 L portions to a solution of Ru(TPT-
BP)(CO) (4 mg, 0.0042 mmol) in CH2Cl2 (20 mL). The
conversion of Ru(TPTBP)(CO) into Ru(TPTBP)(CO)(PBu3) was
monitored by UV/vis and IR spectra. Similar titration was
carried out in CDCl3 for 1H NMR studies. Addition of more
than 1 equiv of tri-n-butylphosphine induces decarbonylation
and formation of Ru(TPTBP)(PBu3)2. Ru(TPTBP)(PBu3)2 was
The preparation of Zn(TPTBP) was a modification of the
procedures published previously by Kopranekov et al.7 and
Ichimura et al.8
(Tetr a p h en yltetr a ben zop or p h yr in a to)zin c, Zn (TP T-
BP ). A homogeneous mixture of phthalimide (0.735 g, 5
mmol), phenylacetic acid (1.5 g, 10 mmol), and zinc benzoate
(1.51 g, 5 mmol) was put into a 25 mL round-bottom flask and
heated at 360 °C for 40 min under an N2 atmosphere. After
the mixture was cooled, the dark green solid was reduced to a
powder, washed several times with hot water and hexane, and
finally dissolved in CH2Cl2 and the solution filtered. This
procedure also yields a substantial amount of the triphenyl
derivative. Purification was accomplished by repeated chro-
matography of the filtrate with different ratios of hexane/
CH2Cl2, first on a column of aluminum oxide (Merck 1097) and
then on a column of silica gel. The product was finally
recrystallized from CH2Cl2/hexane, and the average yield was
around 10%. MS: (M)+ m/e 876.
Tetr a p h en yltetr a ben zop or p h yr in , TP TBP H2. Zn(TPT-
BP) (100 mg) was dissolved in about 50 mL of CH2Cl2, treated
with 100 mL of 15% HCl aqueous solution, and then neutral-
ized with sodium acetate. Further purification was carried
out by chromatography on a column of silica gel with hexane/
CH2Cl2 and recrystallization from CH2Cl2/hexane. MS: (M +
H)+ m/e 815.
C a r b o n y l(t e t r a p h e n y lt e t r a b e n z o p o r p h y r in a t o )-
r u t h en iu m (II), R u (TP TBP )(CO). The preparation and
characterization of ruthenium(II) carbonyl porphyrin com-
plexes have been described in the literature. The method we
used for insertion of ruthenium into TPTBP is a modification
(6) Kim, D.; Su, Y. O.; Spiro, G. Inorg. Chem. 1986, 25, 3993-3997.
(7) Kopranekov, V. N.; Dashkevich, S. N.; Lukyanets, E. A. Zh.
Obshch. Khim. 1981, 51, 2513.
(8) Ichimura, K.; Sakuragi, M.; Morii, H.; Yasuike, M.; Fukui, M.;
Ohno, O. Inorg. Chim. Acta 1991, 186, 95.
(9) Rillema, D. P.; Nagle, J . K.; Barringer, L. F., J r.; Meyer, T. J . J .
Am. Chem. Soc. 1981, 103, 56.
(10) Antipas, A.; Buchler, J . W.; Gouterman, M.; Smith, P. D. J .
Am. Chem. Soc. 1978, 100, 3015.

Tetraphenyltetrabenzoporphyrin Complexes of Ru
Organometallics, Vol. 16, No. 10, 1997 2123
1
Ta ble 1. Electr on ic, H NMR, a n d IR Sp ectr a l Da ta for Ru (TP TBP ) Com p lexes
1H NMR
UV/vis
complex
R
o
m, p
CO
Zn(TPTBP)
Ru(TPTBP)(CO)
460
607, 651
576, 622
578, 620
578, 620
574, 618
578, 622
584, 624
544, 616
556, 624
7.16
7.05
7.0
7.26
7.2
7.2
7.2
7.25
7.2
7.1
7.0
7.0
8.28
8.3
7.89
7.9
7.89
7.89
7.9
418, 446
422, 448
424, 446
426,446
428, 448
438, 458
372, 410, 436
332, 458
1959
1968
1965
1956
1930
1978
Ru(TPTBP)(CO)(py)
Ru(TPTBP)(CO)(MeIm)
Ru(TPTBP)(CO)(pip)
Ru(TPTBP)(CO)(NEt3)
Ru(TPTBP)(CO)(PBu3)
Ru(TPTBP)(py)2
8.22, 8.34
8.22, 8.34
8.25, 8.35
8.25, 8.35
8.2, 8.28
8.24
7.0
7.05
7.04
6.95
6.8
7.9
7.85
7.82
7.82
Ru(TPTBP)(PBu3)2
6.8
8.2
generated quantitatively with 2 equiv of PBu3 (86 L, 0.0085
mmol). Spectroscopic data are summarized in Table 1.
Resu lts a n d Discu ssion
Sp ect r oscop ic Ch a r a ct er iza t ion s of R u t h en -
iu m (II) Tetr a p h en yltetr a ben zop or p h yr in s. The
preparation and characterization of RuP(CO)L, RuP-
(py)2, and RuP(PBu3)2 complexes for tetraphenylpor-
phyrin (TPP) and octaethylporphyrin (OEP) systems
have been described.11 These procedures work similarly
for tetraphenyltetrabenzoporphyrin (TPTBP).
FAB/MS is suitable for the confirmation of ruthenium
insertion. Other than CO, most axial ligands trans to
CO are too weak to remain coordinated during ioniza-
tion. However, most axial ligands are easily identified
in 1H NMR spectra. All resonances corresponding to
axial ligands are upfield-shifted through the anisotropic
ring current effect of the porphyrin macrocycle. Typical
1H NMR spectra are shown in Figure 1 for Ru(TPTB-
P)(CO), Ru(TPTBP)(CO)(py), and Ru(TPTBP)(py)2.
As can be seen in Figure 1b,c, the intensities of the
upfield-shifted resonances clearly indicate the number
of coordinated pyridine groups in the complexes to be
one and two, respectively. The split ortho resonances
in Figure 1a,b further confirm different coordination
environments on two sides of the porphyrin plane. With
the coordination of the second pyridine in the bis(pyri-
dine) complex, R-py shifts downfield while -py and -py
shift upfield. This pattern suggests a stronger and
shorter Ru-py bond in the bis(pyridine) complex than
in the carbonyl pyridine complex. This is consistent
with the fact that strong trans influence from CO will
make the trans pyridine weak. On the other hand,
without the coordination of CO, all peaks from R, -
protons of the porphyrin macrocycle shift upfield for
Ru(TPTBP)(py)2. The increased charge density on the
porphyrin ring has been suggested as a result of
enhanced metal to porphyrin -back-bonding.5
F igu r e 1. 200 MHz 1H NMR spectra of (a) Ru(TPTB-
P)(CO), (b) Ru(TPTBP)(CO)(py), and (c) Ru(TPTBP)(py)2
taken in CDCl3 at room temperature.
resonances and the corresponding UV/vis and IR spec-
tral changes (vide infra).
NMR data for the series of ruthenium(II) complexes
we studied are summarized in Table 1. No peaks
corresponding to the coordinated ligand were detected
with the addition of 1 equiv of NEt3 to the Ru(TPTB-
P)(CO) solution. Therefore, Ru(TPTBP)(CO)(NEt3) was
generated in solution in the presence of excess ligand
to avoid ligand dissociation in dilute solution. Coordi-
nation of NEt3 is visible from minor changes of the ring
Typical electronic absorption spectra are shown in
Figure 2 for Zn(TPTBP), Ru(TPTBP)(py)2, Ru(TPTB-
P)(CO), and Ru(TPTBP)(CO)(py). Table 1 also includes
peak optical absorption data for all the ruthenium(II)
complexes we studied. In comparison with the normal
type spectrum of Zn(TPTBP), the Q bands of carbonyl
complexes of ruthenium(II) are very prominent as well
as substantially blue-shifted (ca. 30 nm). The ratio
ꢀmax(B)/ꢀmax(Q) is anomalously low compared to that for
normal metalloporphyrins. There is a pronounced
shoulder to the blue side of the Soret band; otherwise,
the spectrum would seem to be typically hypso.10 The
electronic absorption data of these Ru(II) carbonyl
(11) (a) Tsutsui, M.; Ostfeld, D.; Hoffman, L. M. J . Am. Chem. Soc.
1971, 93, 1820. (b) Tsutsui, M.; Ostfeld, D.; Francis, J . N.; Hoffman,
L. M. J . Coord. Chem. 1971, 1, 115. (c) Chow, B. C.; Cohen, I. A.
Bioinorg. Chem. 1971, 1, 57. (d) Brown, G. M.; Hopf, F. R.; Ferguson,
J . A.; Meyer, T. J .; Whitten, D. G. J . Am. Chem. Soc. 1973, 95, 5939.
(e) J ames, B. R.; Dolphin, D.; Leung, T. W.; Einstein, F. W. B.; Willis,
A. C. Can. J . Chem. 1984, 62, 1238.

2124 Organometallics, Vol. 16, No. 10, 1997
Cheng et al.
F igu r e 2. UV/vis spectra of (a) Zn(TPTBP), (b) Ru(TPTBP)(py)2, (c) Ru(TPTBP)(CO), and (d) Ru(TPTBP)(CO)(py) taken
in CH2Cl2.
Ta ble 2. Com p a r ison of νCO Va lu es for Ru P (CO)
porphyrin complexes can be interpreted in terms of a
Com p lexes w ith Ba sicities of P or p h yr in F r ee Ba se
Gouterman four-orbital model modified by the inclusion
of the metal d orbitals. Increased metal-to-porphyrin
-back-bonding promotes mixing of eg( *) and eg(d )
complex
Ru(TPTBP)(CO)
Ru(TPP)(CO)
Ru(OEP)(CO)
CO(cm-1
pK3
)
1959
5.75
1930a
3.95
1917a
4.36
orbitals, with the result that the
f * transitions fall
a
at higher energies than those of a metal complex
without this -bond interaction. The magnitude of the
hypsochromic shifts has been correlated with the -do-
nor ability of the metal ion or the -acceptor ability of
the porphyrin ring.10 The amount of the shifts for the
TPTBP system is about twice the similar shifts for the
TPP system (i.e. 30 nm vs 16 nm). This may suggest
that TPTBP is a stronger -acceptor than TPP.
Reference 11d.
BP)(CO) reduces the metal-to-CO back-bonding, as
evidenced by higher CO stretching frequencies. Actu-
ally, part of the effect due to increased -acidity is
compensated by the increased -basicity. Larger por-
phyrin-to-metal -donation should increase the metal-
to-CO back-bonding. The difference in
between
CO
Ru(TPP)(CO) and Ru(OEP)(CO) is mainly induced by
different -basicities, as evidenced by the values of pK3.
Therefore, TPTBP is apparently a stronger -donor and
-acceptor than other porphyrin-related macrocycles.
Oxid a tion Stu d ies of Ru (TP TBP )(CO), Ru (TP T-
BP )(CO)(p y), a n d Ru (TP TBP )(p y)2. Cyclic voltam-
mograms were measured for Ru(TPTBP)(CO), Ru(TPT-
BP)(CO)(py), and Ru(TPTBP)(py)2 complexes in CH2Cl2
solution with TBAP as the electrolyte. The bis(pyridine)
species show three reversible one-electron oxidations,
while the other two species reveal only two reversible
one-electron oxidations within the solvent limits (Table
3). The first one-electron-oxidation process could be
accomplished chemically by the addition of Br2 or I2 in
CH2Cl2 solution. Visible spectra of the one-electron-
oxidation products of Ru(TPTBP)(CO) and Ru(TPTB-
P)(py)2 are shown in Figure 3. Oxidation products of
both Ru(TPTBP)(CO) and Ru(TPTBP)(CO)(py) gave
very similar spectral changes. All the visible spectral
changes reveal several isosbestic points with less in-
tense absorptions, similar to those of other well-
characterized porphyrin -cation radicals. These por-
phyrin -cations exhibited a typical ESR signal for a
In Figure 2b, the bis(pyridine) complex, which shows
other intense absorption besides normal bands, gives a
very clear example of the hypso/hyper type spectrum.
Dramatic differences were found in comparing the
visible spectrum of Ru(TPTBP)(py)2 with that of
Ru(TPP)(py)2,11d consistent with different -bonding
capabilities between TPTBP and TPP.
data for Ru(TPTBP)(CO)(L) complexes are sensi-
CO
tive to the bonding ability of trans ligands L. L ligands
with stronger -acidity will compete for the -electron
density from the central metal, thus reducing MfCO
-back-bonding. On the basis of the available
data
CO
for these series of Ru(TPTBP)(CO)(L) complexes, as
shown in Table 1, the following sequence of -acidity of
trans ligands can be obtained: PBu3 > py > 1-MeIm >
pip > NEt3.
Comparison of
data between different porphyrin
CO
systems can be informative (Table 2). The higher
-acidity of TPTBP compared to other porphyrin-related
macrocycles is apparent in the CO stretching frequen-
cies of metal carbonyl complexes of these macrocycles.
Greater metal-to-macrocycle back-bonding in Ru(TPT-

Tetraphenyltetrabenzoporphyrin Complexes of Ru
Organometallics, Vol. 16, No. 10, 1997 2125
Ta ble 3. Electr och em ica l a n d EP R Va lu es for
Ru th en iu m P or p h yr in Com p lexes
E1/2
O1
PRuII/ PRuII/
g(77 K) after
first oxidn
complex
PRuIII P•+RuII O2
O3
Ru(TPTBP)(CO)
Ru(TPTBP)(CO)(py)
Ru(TPTBP)(py)2
Ru(TPP)(CO)c
0.49
0.56
0.36
0.82
0.81
0.93
1.10
0.94 1.38
1.21
1.36
1.26
2.000a
1.999a
2.000b
2.004
2.007
Ru(TPP)(CO)(py)c
c
Ru(TPP)(py)2
0.21
0.08
Ru(OEP)(CO)c
0.64
1.21
2.004
c
Ru(OEP)(py)2
a
b
ESR active at room temperature. ESR silent at room tem-
perature. c Reference 11d.
1
F igu r e 4. 200 MHz H NMR spectral changes during the
one-electron oxidation of (a) Ru(TPTBP)(CO)(py) and (b)
Ru(TPTBP)(py)2 with Br2 in CDCl3.
complexes were ring-oxidized to form two different types
of porphyrin -cation radicals. The carbonyl complexes
are predominantly of A1u character, and the bis(pyri-
dine) complex is mainly of A2u character.
It is well-documented that the CO complex of ruthe-
nium(II) porphyrin electrochemically or chemically un-
dergoes ring oxidation to form the -cation radical
[RuIIP•+(CO)L]+. However, in normal porphyrin sys-
tems, one-electron oxidation of carbonyl-free RuIIP(py)2
species occurs at the metal to give [RuIIIP(py)2]+.11d The
presence of CO in ruthenium porphyrins causes large
anodic shifts for the first oxidation (∼0.6 V from Table
3), making the CO complexes much more difficult to
oxidize and resulting in a change in oxidation site. For
the TPTBP system, the corresponding anodic shift is no
larger than 0.2 V and is more consistent with oxidation
at the same site, namely, the TPTBP ligand.
Due to the extended -system and the ring deforma-
tion of the TPTBP porphyrin macrocycle, it is reasonable
that energy levels of the highest occupied molecular
orbitals (a1u and a2u) are higher than that of the metal
F igu r e 3. Visible spectral changes during the one-electron
oxidation of (a) Ru(TPTBP)(CO) with Br2 and (b) Ru(TPT-
BP)(py)2 with I2 in CH2Cl2.
free radical at 77 K. Although radicals of the carbonyl
complexes gave single-line ESR signals with g ≈ 2.0 at
room temperature, the radical of the bis(pyridine)
complex showed a less intense signal with g ≈ 2.0 only
at 77 K.
d
orbitals in both CO and bis(pyridine) complexes.
Differences in -back-bonding ability between CO and
pyridine are not sufficient to invert the relative energy
of metal d and TPTBP MO levels; they only exchange
the order of a1u and a2u
MO’s. Previous ENDOR
1
Consistent with the ESR results, while the H NMR
studies showed that Zn(TPTBP) was oxidized to give an
A2u -cation radical and suggested that in the TPTBP
system a2u should be higher than a1u in energy.1c
However, with the strong -acid CO coordinated at the
axial position, porphyrin will engage more as a -donor;
the a2u MO which is mainly responsible for the PfM
donation might be depressed in energy and result in
an a1u -cation radical upon oxidation.
In contrast to other ruthenium porphyrins reported
in the literature, extraplanar ligands in the Ru(TPTBP)
system do not affect the site of oxidation (metal vs ring)
but only mediate the level of oxidation on the ring (a1u
spectrum of the oxidation product of the bis(pyridine)
complex was well-resolved (Figure 4b), parts of the
spectra of the corresponding carbonyl complexes were
beyond detection due to too much broadening (Figure
1
4a). Similar H NMR and ESR spectral features have
been reported for [RuII(OEP)•+(CO)]Br and [RuII(O-
EP)•+(CO)]ClO4 -cation radicals which possess pre-
dominant A2u and A1u character, respectively.12 All
these results suggest that both CO and bis(pyridine)
(12) Morishima, I.; Takamuki, Y.; Shiro, Y. J . Am. Chem. Soc. 1984,
106, 7666.

2126 Organometallics, Vol. 16, No. 10, 1997
Cheng et al.
vs a2u). This rationalization is also consistent with the
above data that TPTBP, in comparison to normal
porphyrins, is an exceptionally good -acceptor in the
absence of CO. At this stage, it is hard to rationalize
whether the saddle-shaped deformation or extended
-system of TPTBP is mainly responsible for its -acid-
ity. The consequences of these special bonding charac-
teristics of TPTBP on the coordination chemistry of
other transition-metal complexes are currently under-
way in our laboratory.
Ack n ow led gm en t. This work was supported by the
National Science Council of the Republic of China
(Grant No. NSC85-2113-M005-018).
OM960653H