Structures and reactivity patterns of group 9 metallocorroles

Article abstract of DOI:10.1021/ic901164r

Group 9 metallocorroles 1-M(PPh₃) and 1-M(py)₂ [M = Co(III), Rh(III), Ir(III); 1 denotes the trianion of 5,10,15- trlspentafluorophenylcorrole] have been fully characterized by structural, spectroscopic, and electrochemical methods.

Full text of DOI:10.1021/ic901164r

9308 Inorg. Chem. 2009, 48, 9308–9315
DOI: 10.1021/ic901164r
Structures and Reactivity Patterns of Group 9 Metallocorroles
Joshua H. Palmer,† Atif Mahammed,‡ Kyle M. Lancaster,† Zeev Gross,*,‡ and Harry B. Gray*,†
†Beckman Institute, California Institute of Technology, Pasadena, California 91125, and ‡Schulich Faculty of
Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received June 17, 2009
Group 9 metallocorroles 1-M(PPh3) and 1-M(py)2 [M = Co(III), Rh(III), Ir(III); 1 denotes the trianion of 5,10,15-tris-
pentafluorophenylcorrole] have been fully characterized by structural, spectroscopic, and electrochemical methods.
Crystal structure analyses reveal that average metal-N(pyrrole) bond lengths of the bis-pyridine metal(III) complexes
˚
˚
˚
increase from Co (1.886 A) to Rh (1.957 A)/Ir (1.963 A); and the average metal-N(pyridine) bond lengths also
˚
˚
˚
increase from Co (1.995 A) to Rh (2.065 A)/Ir (2.059 A). Ligand affinities for 1-M(PPh3) axial coordination sites
increase dramatically in the order 1-Co(PPh3) < 1-Rh(PPh3) < 1-Ir(PPh3). There is a surprising invariance in the M(+/
0) reduction potentials within the five- and six-coordinate corrole series, and even between them; the average M(+/0)
potential of 1-M(PPh3) is 0.78 V vs Ag/AgCl in CH2Cl2 solution, whereas that of 1-M(py)2 is 0.70 V under the same
conditions. Electronic structures of one-electron-oxidized 1-M(py)2 complexes have been assigned by analysis of
electron paramagnetic resonance spectroscopic measurements: oxidation is corrole-centered for 1-Co(py)2 (g =
2.008) and 1-Rh(py)2 (g = 2.003), and metal-centered for 1-Ir(tma)2 (gzz = 2.489, gyy = 2.010, gxx = 1.884, gav
2.128) and 1-Ir(py)2 (gzz = 2.401, gyy = 2.000, gxx = 1.937, gav = 2.113).
=
Introduction
catalyze the reduction of carbon dioxide.7 These reactivity
patterns highlight the role of strong corrole σ-donation in the
activation of low-valent metal centers.8 This same electronic
property, which accounts for the unusual stability of nitrido
chromium(VI) and manganese(VI) complexes,9 clearly is an
important factor in metallocorrole-catalyzed processes.10
Complexes of triarylcorroles with group 13-15 elements
have been characterized as well, often with a focus on
photophysical properties;11 importantly, a gallium(III) cor-
role has been shown to be both an in vivo imaging agent and
an anticancer drug candidate.12 Second-row metallocorroles
have not been investigated extensively, although there are
reports of silver(III),13 ruthenium(III),14 and rhodium(III)
Systematic investigations of transition metal corrole com-
plexes have accelerated recently because of major advances in
synthetic methods.1-3 Many of these complexes, especially of
first-row transition metals, exhibit striking reactivity: iron-
(IV) derivatives remain the only non-copper catalysts for the
aziridination of olefins by chloramine-T;4 manganese(III)
catalyzes oxygenations via observable (oxo)manganese(V);5
chromium(III) mediates the aerobic oxidation of thiophenol
to diphenyl disulfide;6 and iron(I) and cobalt(I) corroles
*To whom correspondence should be addressed. E-mail: chr10zg@tx.
technion.ac.il (Z.G.), hbgray@caltech.edu (H.B.G.).
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Boschi, T.; Smith, K. M. Chem. Commun. 1999, 1307–1308. (c) Gross, Z.; Galili,
N.; Simkhovich, L.; Saltsman, I.; Botoshansky, M.; Blaser, D.; Boese, R.;
Goldberg, I. Org. Lett. 1999, 1, 599–602. (d) Koszarna, B.; Gryko, D. T. J.
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r
pubs.acs.org/IC
Published on Web 09/08/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9309
derivatives, with the latter functioning as a catalyst
in carbene-transfer reactions.15 An (oxo)molybdenum(V)
corrole also has been characterized.16
Third-row transition metal corroles are extremely rare,
with fully characterized (oxo)Re(V) and Ir(III) compounds
representing the only known oxidation states.17,18 We pre-
dicted that trianionic corroles, on the basis of their ability to
stabilize numerous metals in high oxidation states,19 would
greatly stabilize iridium(III) and that these derivatives would
exhibit strikingly different redox properties than iridium(III)
porphyrins or organoiridium(III) complexes such as
[Ir(ppy)3] (ppy = 2-phenylpyridine, Figure 1). Indeed, Ir(III)
corroles can be stabilized even by weakly donating axial
ligands, whereas Ir(III) porphyrins are only stable when the
metal is further coordinated by organometallic ligands [such
as in (por)Ir(CO)Cl and (por)Ir-R, where por stands for the
porphyrin dianion and R = alkyl or hydride)].20
Figure 1. Complexesof2-phenylpyridine, porphyrins, and corroleswith
iridium(III).
A stable six-coordinate (tpfc)Ir(tma)2 complex (1-Ir(tma)2,
Figure 1) was obtained via the reaction of [Ir(cod)Cl]2 with
5,10,15-tris-pentafluorophenylcorrole (H3tpfc), followed by
treatment with tma-N-oxide.21 Treatment of 1-Ir(tma)2 with
elemental bromine in methanol yielded an octabromo com-
plex (Br8-tpfc)Ir(tma)2 (1b-Ir(tma)2, Figure 1); both iridium-
(III) derivatives were characterized by nuclear magnetic
resonance (NMR) spectroscopy, mass spectrometry (MS),
X-ray crystallography, UV-vis spectroscopy, and cyclic
voltammetry (CV). The planar macrocyclic framework in
both 1-Ir(tma)2 and 1b-Ir(tma)2 is quite unlike that of
porphyrins, which tend to saddle or ruffle when bromi-
nated.22 In addition, the electrochemical data suggested that
the metal, rather than the macrocycle, is oxidized to Ir(IV) in
Figure 2. Axially ligated metal(III) corroles (py = pyridine).
both complexes, in contrast with recent findings for analo-
gous cobalt(III) corroles.23
Here we report the synthesis and characterization of
corroles 1-Ir(py)2 and 1-Ir(PPh3), whose properties are
compared with those of isostructural cobalt(III) and
rhodium(III) analogues 1-Co(py)2, 1-Rh(py)2, 1-Co(PPh3),
and 1-Rh(PPh3) (Figure 2). Each set, 1-M(py)2 or 1-M-
(PPh3), represents a rare example of an entire transition
metal group with the same oxidation state and coordination
number, another being the Group 8 (por)M-CO series.24 Our
research has focused on the substitutional lability of Group 9
metallocorroles and the sites of oxidation reactions, as both
change markedly within the group. We have found the
following: Co(III) corroles are far more substitutionally
labile than analogous Rh(III) or Ir(III) derivatives; the
affinity of five-coordinate 1-M(PPh3) for a sixth ligand
increases dramatically down the row; and oxidation of
1-M(py)2 occurs primarily on the metal rather than on the
macrocyclic ligand only in the case of iridium (IrIII/IV). We
report extensive electronic absorption, electrochemical, and
electron paramagnetic resonance (EPR) data that support
these conclusions.
(15) Saltsman, I.; Simkhovich, L.; Balazs, Y.; Goldberg, I.; Gross, Z.
Inorg. Chim. Acta 2004, 357, 3038–3046.
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2006, 45, 386–394.
(17) Tse, M. K.; Zhang, Z.; Mak, T. C. W.; Chan, K. S. Chem. Commun.
1998, 1199–1200.
(18) Palmer, J. H.; Day, M. W.; Wilson, A. D.; Henling, L. M.; Gross, Z.;
Gray, H. B. J. Am. Chem. Soc. 2008, 130, 7786–7787.
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Phthalocyanines 2004, 8, 1091–1105. (b) Goldberg, D. P. Acc. Chem. Res.
2007, 40, 626–634. (c) Kerber, W. D.; Goldberg, D. P. J. Inorg. Biochem. 2006,
100, 838–857. (d) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987–1999. (e)
Ghosh, A.; Steene, E. J. Inorg. Biochem. 2002, 91, 423–436. (f) Kadish, K. M.;
Shen, J.; Fremond, L.; Chen, P.; El Ojaimi, M.; Chkounda, M.; Gros, C. P.; Barbe,
J. M.; Ohkubo, K.; Fukuzumi, S.; Guilard, R. Inorg. Chem. 2008, 47, 6726–6737.
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(b) Li, B.; Chan, K. S. Organometallics 2008, 27, 4034–4042. (c) Cui, W.; Li, S.;
Wayland, B. B. J. Organomet. Chem. 2007, 692, 3198–3206. (d) Yanagisawa,
M.; Tashiro, K.; Yamasaki, M.; Aida, T. J. Am. Chem. Soc. 2007, 129, 11912–
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6671. (i) Zhai, H.; Bunn, A.; Wayland, B. Chem. Commun. 2001, 1294–1295. (j)
Shi, C.; Mak, K. W.; Chan, K. S.; Anson, F. C. J. Electroanal. Chem. 1995, 397,
321–324. (k) Collman, J. P.; Chng, L. L.; Tyvoll, D. A. Inorg. Chem. 1995, 34,
1311–1324. (l) Kadish, K. M.; Deng, Y. J.; Yao, C.-L.; Anderson, J. E.
Organometallics 1988, 7, 1979–1983.
(21) Abbreviations: [Ir(cod)Cl]2, H3(tpfc), tpfc, Br8-tpfc, t-4bpa, tma-N-
oxide, tma, and py stand for iridium(I) cyclooctadiene chloride dimer, 5,10,15-
tris-pentafluorophenylcorrole, 5,10,15-tris-pentafluorophenylcorrolato trianion,
2,3,7,8,12,13,17,18-octabromo-5,10,15-tris-pentafluorophenylcorrolato trianion,
tris(4-bromophenyl)aminium hexachloroantimonate, trimethylamine N-oxide,
trimethylamine, and pyridine, respectively.
(22) Ou, Z.; Shao, J.; D’Souza, F.; Tagliatesta, P.; Kadish, K. M. J.
Porphyrins Pthalocyanines 2004, 8, 201–214, and references therein.
(23) Kadish, K. M.; Shen, J.; Fremond, L.; Chen, P.; El Ojaimi, M.;
Chkounda, M.; Gros, C. P.; Barbe, J. M.; Ohkubo, K.; Fukuzumi, S.;
Guilard, R. Inorg. Chem. 2008, 47, 6726–6737.
Experimental Section
Materials. Silica gel for column chromatography (Silica Gel
60, 63-200 μm mesh) was obtained from EMD Chemicals, as
were all solvents (tetrahydrofuran (THF), toluene, CH2Cl2,
hexanes, and methanol). Most starting materials for syntheses
were from Sigma-Aldrich and used without further purification.
Exceptions are pyrrole and pentafluorobenzaldehyde, which
were both purified by vacuum distillation before use. Tetrabu-
tylammonium hexafluorophosphate, which was used as a
supporting electrolyte in the CV experiments, was also from
(24) Brown, G. M.; Hopf, F. R.; Meyer, T. J.; Whitten, D. G. J. Am.
Chem. Soc. 1975, 97, 5385–5390.

9310 Inorganic Chemistry, Vol. 48, No. 19, 2009
Palmer et al.
Sigma-Aldrich and used without further purification. Electro-
des for CV were obtained from CH Instruments.
5,10,15-Tris-pentafluorophenylcorrolato-iridium(III) Triphe-
nylphosphine, 1-Ir(PPh3). H3tpfc (40 mg), [Ir(cod)Cl]2
(170 mg), and K2CO3 (70 mg) were dissolved/suspended in 75
mL of degassed THF, and the mixture was heated at reflux
under argon for 90 min. Triphenylphosphine (260 mg dissolved
in 5 mL THF) was added, and the solution was heated at reflux
for another half hour under laboratory atmosphere before being
allowed to cool to room temperature. Column chromatography
of the deep green mixture (silica, 3:1 hexanes/CH2Cl2) afforded
a bright red-orange solution, which could be evaporated to give
(tpfc)Ir(III)(PPh3) (30 mg, 64% yield) as a ruby-colored solid.
1H NMR (CDCl3): δ 8.67 (d, 2H, J = 4.5), 8.36 (d, 2H, J = 5.1),
8.18 (d, 2H, J = 5.1), 8.00 (d, 2H, J = 4.5), 6.98 (t, 3H, J = 7.2),
Syntheses. The synthesis of 5,10,15-tris-pentafluorophenyl-
corrole (the H3tpfc ligand) was accomplished by a simplified
version of the standard procedure outlined in reference 1c. 25
The cobalt(III) and rhodium(III) corroles were available from
previous studies.15,26 Compounds 1-tma and 2-tma have only been
reported in a previous communication,18 hence their syntheses are
summarized below along with those of the new corroles.
5,10,15-Tris-pentafluorophenylcorrolato-iridium(III) Bis-tri-
methylamine, 1-Ir(tma)2. H3tpfc (80 mg), [Ir(cod)Cl]2 (335 mg),
and K2CO3 (140 mg) were dissolved/suspended in 150 mL of
degassed THF, and the mixture was heated at reflux under
argon for 90 min (until corrole fluorescence was negligible to the
eye upon long-wavelength irradiation with a hand-held lamp).
Tma N-oxide (110 mg) was added, and the solution was allowed
to slowly cool to room temperature while open to the laboratory
atmosphere. Column chromatography of the black mixture
(silica, 4:1 hexanes/CH2Cl2) provided an auburn solution, from
which purple crystals of (tpfc)Ir(III)(tma)2 (30 mg, 27% yield)
could be grown by slow evaporation. 1H NMR (CDCl3): δ 8.90
(d, 2H, J= 4.2), 8.50 (d, 2H, J= 5.1), 8.38 (d, 2H, J=4.5),8.09(d,
2H, J = 4.2), -2.95 (s, 18H). 19F NMR (CDCl3): δ -138.38 (m,
6F), -154.89 (m, 3F), -163.27 (m, 6F). MS (ESI): 1105.1 ([M+]),
1046.0 ([M+-tma]), 986.5 ([M+-2tma]). UV-vis (CH2Cl2, nm, ε Â
10-3 M-1 cm-1): 388 (47), 412 (56), 572 (14), 640 (5.3).
3
6.69 (t, 6H, J = 6.9), 4.52 (d/d, 6H, J = 19.5, 4J = 3.6). 19F
NMR (CDCl3): δ -137.44 (m, 6F), -154.05 (m, 3F), -162.54
(m, 3F). MS (ESI): 1248.1 ([M+]). UV-vis (CH2Cl2, nm, ε Â
10-3 M-1 cm-1): 398 (66), 554 (8.8), 588 (6.7).
Nuclear Magnetic Resonance Spectroscopy. 1H and 19F NMR
data were obtained on CDCl3 solutions of each compound at
room temperature using a Varian Mercury 300 MHz NMR
spectrometer. 1H chemical shifts are reported relative to solvent
peaks and 19F chemical shifts are reported relative to a saved,
external CFCl3 standard.
Mass Spectrometry. Measurements were made on CH3OH
solutions of each compound by electrospray ionization into a
Thermofinnigan LCQ ion trap mass spectrometer.
2,3,7,8,12,13,17,18-Octabromo-5,10,15-tris-pentafluorophe-
nylcorrolato-iridium(III) Bis-trimethylamine, 1b-Ir(tma)2. Com-
pound 1-Ir(tma)2 (15 mg) and Br2 (70 μL) were dissolved in 20
mL of MeOH and stirred overnight. Column chromatography
(silica, 4:1 hexanes/CH2Cl2) of the red mixture provided a ruddy
solution from which purple crystals of (Br8-tpfc)Ir(III)(tma)2
(15 mg, 63% yield) could be grown by addition of methanol
X-ray Crystallography. Concentrated CH2Cl2/CH3OH solu-
tions of corroles 1-Ir(tma)2, 1b-Ir(tma)2, and 1- Ir(py)2 were
allowed to undergo slow evaporation from scintillation vials.
The resultant crystals were mounted on a glass fiber using
Paratone oil and then placed on a Bruker Kappa Apex II
diffractometer under a nitrogen stream at 100 K. The
SHELXS-97 program was used to solve the structures.
1
followed by slow evaporation. H NMR (CDCl3): δ -2.60 (s,
Cyclic Voltammetry. CV measurements were made with a
WaveNow USB Potentiostat/Galvanostat (Pine Research Ins-
trumentation) using Pine AfterMath Data Organizer software.
A three electrode system consisting of a platinum wire working
electrode, a platinum wire counter electrode, and an Ag/AgCl
reference electrode, was employed. The CV measurements were
made using dichloromethane solutions, 0.1 M in tetrabutylammo-
nium perchlorate (TBAP, Fluka, recrystallized twice from abso-
lute ethanol), and 10-3 M substrate under an argon atmosphere at
ambient temperature. The scan rate was 0.1 V/s and the E1/2 value
for oxidation of ferrocene under these conditions was 0.55 V.
UV-visible spectroelectrochemical measurements were made
on dichloromethane solutions, 0.5 M in TBAP, and 0.1-0.3 mM
in substrate with an optically transparent platinum thin-layer
electrode working electrode, a platinum wire counter electrode,
and an Ag/AgCl reference electrode under an argon atmosphere
at ambient temperature. Potentials were applied with a Wave-
Now USB Potentiostat/Galvanostat. Time-resolved UV-visible
spectra were recorded with a Hewlett-Packard Model 8453 diode
array rapid-scanning spectrophotometer.
18H). 19F NMR (CDCl3): δ -137.78 (d/d, 2F, 3J = 35.1, 4J =
18.3), -138.54 (d/d, 4F, 3J = 33.9, 4J = 17.1), -152.89 (m, 3F),
-163.38 (m, 4F), -163.70 (m, 2F). MS (ESI): 1616.4 ([M+
-
2tma]). UV-vis (CH2Cl2, nm, ε Â 10-3 M-1 cm-1): 404 (61),
424 (70), 580 (16), 654 (7.3).
5,10,15-Tris-pentafluorophenylcorrolato-iridium(III) Bis-pyr-
idine, 1-Ir(py)2. H3tpfc (40 mg), [Ir(cod)Cl]2 (170 mg), and
K2CO3 (70 mg) were dissolved/suspended in 75 mL of degassed
THF, and the mixture was heated at reflux under argon for 90
min. Pyridine (1 mL) was added, and the solution was allowed to
slowly cool to room temperature while open to the laboratory
atmosphere. Column chromatography of the forest green mix-
ture (silica, 4:1 hexanes/CH2Cl2 followed by 3:2 hexanes/
CH2Cl2) afforded a bright green solution, from which thin,
green crystals of (tpfc)Ir(III)(py)2 (26 mg, 50% yield) could be
grown by addition of methanol followed by slow evaporation.
1H NMR (CDCl3): δ 8.84 (d, 2H, J = 4.5), 8.53 (d, 2H, J = 4.8),
8.32 (d, 2H, J = 4.8), 8.17 (d, 2H, J = 4.5), 6.21 (t, 2H, J = 7.8),
5.19 (t, 4H, J = 7.0), 1.72 (d, 4H, J = 5.1). 19F NMR (CDCl3): δ
-138.68 (m, 6F), -154.84 (t, 2F, J = 22.2), -155.20 (t, 1F, J =
22.2), -163.28 (m, 4F), -163.65 (m, 2F). MS (ESI): 1144.1
([M+]). UV-vis (CH2Cl2, nm, ε Â 10-3 M-1 cm-1): 390 (28),
412 (43), 582 (12), 619 (6.5).
EPR Spectroscopy. Solutions for EPR were prepared
by adding 50 μL of a 10 mM CH2Cl2 solution of iodine to 100
μL of a 1 mM CH2Cl2 solution of the corrole being examined,
ensuring complete one-electron oxidation of the substrate. To
rule out side reactions with iodine, sub-stoichiometric amounts
of the radical cation tris(4-bromophenyl)aminium hexachlor-
oantimonate (t-4bpa) also were employed for oxidations. Spec-
tra taken of the products obtained from reactions with both
oxidants were virtually identical. EPR spectroscopy was per-
formed using a Bruker EMX Biospin instrument, with a Gunn
diode microwave source. Solutions were pre-cooled by rapid
freezing in liquid nitrogen; spectra of samples at 20 K were
obtained using liquid helium as a coolant. The SPINCOUNT
package was used to simulate EPR parameters.27
(25) A 140 μL portion of a solution of 0.5 mL of trifluoroacetic acid in 5
mL of CH2Cl2 was added to 1.73 mL of warm (liquid) pentafluorobenzal-
dehyde, with rapid stirring. Addition of 1.46 mL of freshly distilled pyrrole
resulted in the rapid formation of a viscous red solution. After 10 min, 20 mL
of CH2Cl2 was added and the mixture was allowed to stir briefly, followed by
slow addition of 3.84 g of DDQ to oxidize the newly formed macrocycle.
Purification was accomplished by successive chromatographic treatments
with 6.5:3.5 CH2Cl2/hexanes and 8.5:1.5 CH2Cl2/hexanes on silica, followed
by recrystallization from hot pentane.
(26) (a) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z.
Inorg. Chem. 2000, 39, 2704–2705. (b) Mahammed, A.; Giladi, I.; Goldberg, I.;
Gross, Z. Chem.;Eur. J. 2001, 7, 4259–4265.
(27) Golombek, A. P.; Hendrich, M. P. J. Magn. Reson. 2003, 165, 33–48.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9311
Figure 3. 1H (red) and 19F (blue inserts) NMR spectra of 1-Ir(py)2.
UV-vis Absorption Spectroscopy. Absorption spectral mea-
surements were made on solutions of each compound in
CH2Cl2, using a Hewlett-Packard 8452A Diode Array Spectro-
photometer. Extinction coefficients were calculated from mea-
surements on corroles at variable concentrations in CH2Cl2
solutions. UV-vis spectroscopic titrations were performed by
stepwise addition of small aliquots of CH2Cl2 solutions of t-4bpa
to CH2Cl2 solutions of each corrole. The t-4bpa oxidant was
employed rather than iodine owing to overlapping absorptions
because of excess reagent in the latter case.
the diamagnetic ring current of the aromatic corrole. The
1-Ir(py)2 pyridine proton resonances are more shifted
than those of PPh3 in Ir(PPh3) (both relative to their
positions in the absence of a metal center) because of the
proximity of py protons to the corrole ring. The coordi-
nation number (six for the bis-pyridine complex 1-Ir(py)2
and five for the triphenylphosphine complex 1-Ir(PPh3))
1
can be deduced from H NMR spectra via integration
of the relevant proton resonances. The same information
can also be extracted from 19F NMR spectra (Figure 3,
inset), since the C2v symmetry of 1-Ir(py)2 requires
equivalence of above- and below-plane ortho- and meta-
fluorine atoms on each C6F5 ring.
Results and Discussion
Iridium(III) Corroles. The insertion of the metal ion
into the free base corrole H3(tpfc) was achieved
via heating with [Ir(cod)Cl]2 and K2CO3 in THF under
an inert atmosphere. This procedure was followed by
addition of pyridine or triphenylphosphine and opening
the reaction mixture to the laboratory atmosphere to
effect aerobic oxidation (see Supporting Information).
Owing to their low-spin d6 electronic configurations,
iridium(III) corroles display highly resolved NMR spec-
tra that are characteristic of diamagnetic complexes,28
The electronic absorption spectra of six-coordinate
metal(III) corroles are shown in Figure 4. The spectra
of 1-Co(py)2 and 1-Rh(py)2 are similar, each with one
major Soret band and Q bands near 580 and 600 nm,
whereas the spectrum of 1-Ir(py)2 exhibits highly struc-
tured Soret absorption with Q bands that are red-shifted
by about 20 nm compared to those of the 3d and 4d
analogues. We emphasize that 1-Co(py)2 is in equilibrium
with the monopyridine complex 1-Co(py) in CH2Cl2
solution [which is not the case for 1-Rh(py)2 or 1-Ir-
(py)2].26b The spectrum of 1-Co(py)2 in 5% pyridine
(where only the bis-ligated form exists in solution) is in
the Supporting Information. As the main components of
the electronic spectra are attributable to corrole-based
π-π* transitions in all cases, the differences likely reflect
1
as shown in Figure 3 for 1-Ir(py)2. The H NMR spec-
tra reveal four β-pyrrole CH proton resonances as doub-
lets with J coupling constants of about 4.5 Hz at 8-9 ppm
and axial ligand resonances at high field attributable to
(28) Balazs, Y. S.; Saltsman, I.; Mahammed, A.; Tkachenko, E.; Golubkov,
G.; Levine, J.; Gross, Z. Magn. Reson. Chem. 2004, 42, 624–635.

9312 Inorganic Chemistry, Vol. 48, No. 19, 2009
Palmer et al.
Figure 4. Electronic absorption spectra of six-coordinate bis-pyridine
metal(III) corroles (top) and five-coordinate PPh3-ligated metal(III)
corroles (bottom); 2.5 μM in CH2Cl2 solution.
Figure 6. Electronic absorption spectra of metal(III) corroles in CH2Cl2
solution demonstrate: (a, top), reversible transformations between
1-Co(py)2 and 1-Co(PPh3) upon addition of PPh3 and pyridine; (b, middle),
formation of 1-Rh(PPh3)2; (c, bottom), formation of 1-Ir(PPh3)2. Only a
representative selection of the many individual traces (25 to 350 eq. ligand)
that are shown in the bottom panel are specifically identified.
Figure 5. Structure of 1-Ir(py)2 (H atoms omitted).
˚
Table 1. Metal-Nitrogen Bond Lengths (A) in 1-Co(py)2, 1-Rh(py)2, 1-Ir(py)2,
1-Ir(tma)2, and 1b-Ir(tma)2
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. and
copies can be obtained on request, free of charge, by
quoting the publication citation and the deposition num-
ber 657603.] is very similar to those of fully characterized
1-Co(py)2 and 1-Rh(py)2 analogues.26b 1-Ir(py)2 has a
planar macrocyclic framework (root-mean-square atom-
M-N(corrole)
M-N(axial)
reference
1-Co(py)2
1-Rh(py)2
1-Ir(py)2
1-Ir(tma)2
1b-Ir(tma)2
1.873(4)-1.900(4)
1.938(5)-1.976(5)
1.947(2)-1.979(2)
1.940(3)-1.981(3)
1.959(2)-1.989(2)
1.994(4)-1.995(4)
2.060(5)-2.071(5)
2.052(2)-2.066(2)
2.184(3)-2.186(3)
2.186(2)-2.192(2)
26b
15
this work
18
18
˚
ic deviation of 0.04 A out of the plane defined by the N4
increased mixing with MLCT in the order Co < Rh < Ir.
Red-shifted Q bands also are exhibited by five-coordinate
1-Ir(PPh3), while the Soret bands differ markedly, as
follows: 1-Co(PPh3) has a highly split system, 1-Rh(PPh3)
displays less splitting but is red-shifted, and 1-Ir(PPh3)
exhibits a single, relatively sharp Soret band.
The structures of 1-Ir(tma)2 and 1b-Ir(tma)2 have been
reported previously.18 The newly obtained structure of 1-
Ir(py)2 [Crystallographic data have been deposited at the
coordination core) with an in-plane Ir(III) and two
virtually parallel axial pyridines (Figure 5). The axial
Ir-N bond lengths of 1-Ir(py)2 are much shorter than
those of 1-Ir(tma)2, likely reflecting the π-acceptor cap-
ability of the pyridine ligands. 1-Ir(py)2 and 1-Rh(py)2
have similar M-N bond lengths; those of 1-Co(py)2 are
slightly shorter (Table 1).
Substitution and Addition Reactions. Previous investi-
gations have revealed that 1-Co(PPh3) and 1-Rh(PPh3)

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9313
Figure 7. Cyclic voltammograms: (a, top) 1-M(PPh3); (b, bottom)
1-M(py)2, in CH2Cl2 solutions.
Figure 8. Absorption spectral changes accompanying oxidation of
1-Co(PPh)3 (top) and 1-Rh(PPh)3 (bottom)byt-4bpa in CH2Cl2 solutions.
Table 2. Reduction Potentials (CH2Cl2, TBAP, V vs Ag/AgCl) for Group 9
1-Rh(PPh3) and 1-Ir(PPh3) to those of 1-Rh(py)2 and 1-
Ir(py)2 suggests that the products are six-coordinate bis-
triphenylphosphine species, 1-Rh(PPh3)2 and 1-Ir(PPh3)2.
While many corrole-chelated metal(III) ions possess
surprisingly low affinities for a sixth ligand,26b,30,31 our work
demonstrates that this property is not as pronounced for 4d
and especially for 5d metals.
Metal(III) Corrolesa
1-M(PPh3)2
E1/2
1-M(py)2
E1/2
M = Co
M = Rh
M = Ir
0.83
0.79
0.72
M = Co
M = Rh
M = Ir
0.67
0.72
0.71
a Under virtually identical conditions, E1/2 for ferrocene is 0.55 V; and
iodine has Epa of 0.89 V and Epc of 0.39 V.
Electrochemistry. Our earlier work focused on the
differences between the β-pyrrole-unsubstituted complex
1-Ir(tma)2 and its fully brominated analogue, 1b-Ir(tma)2.
The latter displays one oxidation (at 1.19 V vs SCE) and
one reduction (at -1.21 V vs SCE) within the solvent
potential window of CH2Cl2, while two reversible oxida-
tions (at 0.66 and 1.28 V vs SCE) were observed for the
former.18 The current emphasis is on the M(+/0) reduc-
tion potential of each of the complexes, because it should
correspond to either a metal-centered (MIII/MIV, where
react differently with pyridine: both ligand substitution
and addition to Co(III) yield 1-Co(py)2, but Rh(III)
undergoes only addition to give 1-Rh(PPh3)(py).26b,29
The reactivity of 1-Ir(PPh3) is similar to that of Rh(III),
that is, addition of pyridine produces a mixed-ligand
complex (see Supporting Information). Additional con-
firmation of the substitutional lability of the Co(III)
corrole is the observation (Figure 6a) that addition of
PPh3 to 1-Co(py)2 rapidly yields 1-Co(PPh3) [addition of
excess pyridine regenerates the bis-pyridine complex;
the spectrum changes slightly because of the absence of
5-coordinate 1-Co(py) under these conditions]. Complemen-
tary information was obtained by examination of spectral
changes upon the addition of PPh3 to five-coordinate me-
tallocorroles. While addition of a 100,000-fold excess of
triphenylphosphine produced only minor changes in the
spectrum of 1-Co(PPh3) (see Supporting Information), the
spectra of 1-Rh(PPh3) (Figure 6b) and 1-Ir(PPh3)
(Figure 6c) changed completely after the addition of 6300
and 350 equiv, respectively. The similarity of the visible
spectra obtained upon addition of triphenylphosphine to
M = Co, Rh, or Ir) or a ligand-centered (tpfc/tpfc+•
)
process. The CVs of all complexes are shown in Figure 7;
and Table 2 lists the corresponding reduction potentials.
The results reveal that six-coordinate 1-M(py)2 complexes
are oxidized at lower potentials than five-coordinate 1-
M(PPh3) and that the differences become larger in the
order Ir < Rh < Co. It is surprising that there are only
very small differences in reduction potentials for the two
series: 0.70 ( 0.03 for 1-M(py)2 and 0.78 ( 0.06 for
1-M(PPh3). Very large potential shifts would be expected
for metal-based processes (MIII/IV), as documented by
the finding that RhIII/IV reduction potentials are 0.2 to
(30) Bendix, J.; Dmochowski, I. J.; Gray, H. B.; Mahammed, A.;
Simkhovich, L.; Gross, Z. Angew. Chem., Int. Ed. 2000, 39, 4048–4051.
(31) Kowalska, D.; Liu, X.; Tripathy, U.; Mahammed, A.; Gross, Z.;
Hirayama, S.; Steer, R. P. Inorg. Chem. 2009, 48, 2670–2676.
(29) Simkhovich, L.; Goldberg, I.; Gross, Z. J. Porphyrins Phthalocya-
nines 2002, 6, 439–444.

9314 Inorganic Chemistry, Vol. 48, No. 19, 2009
Palmer et al.
Figure 9. Absorption spectral changes accompanying oxidation of
1-Co(py)2 (top) and 1-Rh(py)2 (bottom) by t-4bpa in CH2Cl2 solutions.
Reaction with the Co(III) complex was terminated after addition of 0.6 equiv,
owing to product instability.26b
0.3 V more positive than IrIII/IV potentials in cyclometa-
lated complexes.32 A caveat is that the central metal ion
should have some effect on the potential even in the case
of macrocycle-centered oxidation, as observed previously
for both corroles and porphyrins.33,34
Electronic Structures of One-Electron-Oxidized
Metallo(III) Corroles. Changes in electronic spectra upon
M(III) corrole oxidation can used as a tool for analyzing
whether an electron is removed primarily from a metal or
ligand orbital. One-electron oxidation of 1-Co(PPh3) or
1-Rh(PPh3) by t-4bpa (Figure 8) was marked by substan-
tial intensity reductions of the M(III) Soret and Q bands
and formation of a broad absorption system at 690 (ε =
4000 M-1 cm-1) for cobalt and 710 nm (ε = 2300 M-1
cm-1) for rhodium. These long-wavelength absorption
features, which appear at 673 nm (ε = 3000 M-1 cm-1 for
Co and 4100 M-1 cm-1 for Rh) in the spectra of chemi-
cally oxidized 1-Co(py)2 and 1-Rh(py)2 complexes
(Figure 9), represent signature spectroscopic features
for corrole-centered oxidations.
Absorption spectral changes upon oxidation of
iridium(III) corroles are substantially different from
those of 3d and 4d analogues. Oxidations of 1-Ir(PPh3),
1-Ir(py)2, and 1-Ir(tma)2 by t-4bpa in CH2Cl2 were per-
formed, and their progress was tracked by UV-visible
spectroscopy (Figure 10). In the case of the five-coordi-
nate complex 1-Ir(PPh3), oxidation causes a significant
Figure 10. Absorption spectral changes accompanying oxidation of
1-Ir(PPh3) (top), 1-Ir(py)2 (middle), and 1-Ir(tma)2 by t-4bpa.
reduction in the intensity of the Soret system, accompa-
nied by broadening, and the Q-bands are slightly red-
shifted. For each of the six-coordinate complexes 1-Ir-
(py)2 and 1-Ir(tma)2, the Soret band blue shifts over 30 nm
(peaks at 366 nm) and the Q-bands all but vanish. A weak
absorption appears at 695 nm (ε = 3000 M-1 cm-1) in the
spectrum of 1-Ir(PPh3); and a similar system is centered at
685 nm (ε = 2600 M-1 cm-1) in the spectra of the six-
coordinate complexes; these features are red-shifted from
the 673 nm band observed upon oxidation of either 1-
Co(py)2 or 1-Rh(py)2, suggesting that the site of oxidation
is different for Ir(III). As it is well established that
substantial blue shifts of Soret bands accompany metal-
centered oxidations [Note that a 32 nm blue shift in
the Soret peak occurs upon oxidation of [1-CrIV(O)]- to
[1-CrV(O)] (see ref 35).] of metallocorroles,5b,35 an Ir(IV)
oxidation state is indicated for each of the one-electron-
oxidized Ir(III) complexes.
(32) Calogero, G.; Giufridda, G.; Serroni, S.; Ricevuto, V.; Campagna, S.
Inorg. Chem. 1995, 34(3), 541–545.
(33) Gross, Z. J. Biol. Inorg. Chem. 2001, 6, 733–738.
(34) Fuhrhop, J. H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc. 1973,
95, 5140–5147.
(35) Meier-Callahan, A. E.; di Bilio, A. J.; Simkhovich, L.; Mahammed,
A.; Goldberg, I.; Gray, H. B.; Gross, Z. Inorg. Chem. 2001, 40, 6788–6793.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9315
bona fide iridium(IV) corrole. The spectrum of the pro-
duct of oxidation of 1-Ir(py)2 under the same conditions
also is highly anisotropic, albeit with less rhombicity, with
gzz = 2.401, gyy = 2.000, gxx = 1.937 (Figure 11d).
Interpretation of the EPR spectra of oxidized five-
coordinate complexes 1-Co(PPh3), 1-Rh(PPh3), and 1-
Ir(PPh3) is problematic. Oxidized 1-Co(PPh3) and 1-Rh-
(PPh3) appear to be corrole radicals (with isotropic g
tensors centered near the free-electron value), but the
electronic structure of oxidized 1-Ir(PPh3) cannot be
assigned, as its EPR spectrum is obscured by signals likely
generated from side reactions with iodine.
Concluding Remarks
Our investigation is the first to focus on the reactivity
patterns of an isostructural series of corrole-chelated transi-
tion metals. The results shed new light on the chemical
reactivity and stability of high oxidation states of metallo-
corroles. There is a clear trend within each series in terms of
ligand substitution (only cobalt(III) corroles appear labile at
room temperature) and ligand addition to the five-coordinate
1-M(PPh3) (Co < Rh < Ir), whereas all complexes display
similar reduction potentials. The unexpected insensitivity of
reduction potentials to metal identity has been interpreted in
terms of different centers of oxidation for each complex.
Absorption and EPR spectral data demonstrate that the first
oxidation is corrole-centered for cobalt and rhodium, but
metal-centered for iridium. Our demonstration that the
reactivity patterns of Ir(III) corroles are so different from
those of 3d and 4d analogues suggests that increased efforts
should be made to obtain complexes of other 5d metals, as it
is likely that such materials will exhibit rich and in some cases
unexpected physical and chemical properties that will further
expand the range of useful applications for these electroni-
cally unique macrocycles.
Figure 11. EPR spectra taken at 20 K in frozen toluene solutions
(CH2Cl2 was added to Co and Rh toluene solutions with t-4bpa) of the
chemically oxidized forms of (clockwise from top left): (a) 1-Co(py)2; (b)
1-Rh(py)2; (c) 1-Ir(tma)2; (d) 1-Ir(py)2. The blue traces are experimental
spectra; the black traces are simulations performed using the SPINCOUNT
package.
EPR Spectra. Carbon-based organic radicals, such as
those obtained from oxidation of corrole complexes with
non-redox-active metals like gallium(III), exhibit EPR
spectra with isotropic g values near 2.0023.30 In stark
contrast, low-spin d5 metal complexes with organic li-
gands display highly anisotropic EPR signals, often with
hyperfine splittings attributable to electron-nucleus inter-
actions.36
Complexes 1-Co(py)2, 1-Rh(py)2, 1-Ir(py)2, and 1-Ir-
(tma)2 were oxidized chemically with 5 equiv of elemental
iodine in CH2Cl2, and EPR spectra of the resultant
solutions were recorded at 20 K. Each of the spectra of
the oxidized cobalt and rhodium complexes displays a
single, narrow band [centered at g = 2.008 (Co,
Figure 11a) or g = 2.003 (Rh, Figure 11b), respectively]
clearly attributable to a corrole-based radical. Such a
radical signature is not found in the EPR spectrum of
either of the oxidized 1-Ir(py)2 and 1-Ir(tma)2 comp-
lexes. The oxidation product of 1-Ir(tma)2 exhibits
a highly rhombic spectrum (Figure 11c), with g tensor
components (gzz = 2.489, gyy = 2.010 gxx = 1.884) that
are very similar to those of low-spin d5 metalloporphyr-
inoids such as bis-amine-iron(III) porphyrins37 and
(tpfc)Fe(py)2,38 thereby demonstrating the presence of a
Acknowledgment. This work was supported by the
Center for Chemical Innovation Grant NSF CHE-
0802907, U.S.-Israel BSF (Z.G. and H.B.G.), BP,
CCSER (Gordon and Betty Moore Foundation), and
the Arnold and Mabel Beckman Foundation. We thank
Dr. Angelo Di Bilio for help with measurements of low-
temperature EPR spectra; and Drs. Lawrence M. Henling
and Michael W. Day for assistance with the acquisition
and analysis of crystallographic data.
Supporting Information Available: Additional information as
noted in the text. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Pinzino, C.; Ucello-Barretta, G.; Zanello, P. Organometallics 1995, 14, 3275–
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