Iridium corroles

Article abstract of DOI:10.1021/ja801049t

This work reports the synthesis and full characterization of 5,10,15-tris-pentafluorophenylcorrolato-iridium(III) bis-trimethylamine 1 and its octabromo derivative 2. The corrole is planar in both cases (the mean deviation from the plane is as low as 0.0371 A for 1 and 0.0325 A for 2), the UV-vis spectra display a split Soret band with a shoulder attributable to an MLCT transition, and cyclic voltammetry reveals that the iridium(II) oxidation state cannot be accessed, while the oxidation to formal iridium(IV) complexes is achieved at much lower potentials than in other coordination environments. Copyright

Full text of DOI:10.1021/ja801049t

Published on Web 05/31/2008
Iridium Corroles
Joshua H. Palmer,† Michael W. Day,† Aaron D. Wilson,† Lawrence M. Henling,† Zeev Gross,*,‡ and
Harry B. Gray*,†
Beckman Institute, California Institute of Technology, Pasadena, California 91125, and Schulich Department of
Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000, Israel
Received February 11, 2008; E-mail: chr10zg@tx.technion.ac.il; hbgray@caltech.edu
Scheme 1. Synthesis of Iridium(III) Corroles
Interest in the chemistry of iridium has accelerated greatly in
recent years,1 owing in part to reports of high-valent oxo and nitrido
species as well as other complexes possessing wide ranging catalytic
activities.2 Concurrent with this increased interest, much effort has
been directed toward the goal of developing new metallocorrole
systems for applications including, but not limited to, medical
diagnostics and therapeutics as well as catalysis.3 This recent surge
in corrole research is due in large part to the development of facile
methods for the synthesis of the stable 5,10,15-tris-pentafluorophe-
nylcorrole (H3tpfc) synthon and of other tris-aryl-substituted
corroles.4,5
First-row transition metal corroles exhibit striking reactivity,
including the activation of O2 by trivalent chromium,6 manganese,7
and iron;8 catalytic reduction of CO2 by iron(I) and cobalt(I);9 and
iron(IV)-mediated aziridination of olefins.10 Metals in high oxida-
tion states enjoy the strong σ-donor environment of corroles; this
property is typified by stable nitrido chromium(VI) and manga-
nese(VI) species.11 Several second-row transition metals also form
stable corrole complexes: (oxo)molybdenum(V);12 ruthenium(III),
as triply bonded Ru-Ru dimers and nitric oxide bound mono-
mers;13 rhodium(III), which catalyzes carbene-transfer reactions;14
and silver(III).15 It is noteworthy that hitherto there has been only
one report of a third-row metallocorrole, an (oxo)rhenium(V)
species.16
We suggest that the shoulders at 448 and 458 nm for 1 and 2,
respectively, are attributable to MLCT transitions, and that couplings
to these excited states give rise to large splittings of the corrole-
based π-π* states. Based on HOMO and LUMO energies extracted
from the redox potentials of 2, the MLCT transitions should indeed
fall in the 400-500 nm region of the visible spectrum.
Cyclic voltammetry (CV, Figure 2)21 reveals that Ir(III) corroles
are very electron-rich: Ir(II) is not electrochemically accessible and
Ir(IV) is obtained at relatively low potentials. Only 2 could be
reduced within the electrochemical window of the solvent and the
reversibility of that process (E1/2 ) -1.21 V vs SCE) is consistent
with the formation of a corrole radical anion rather than Ir(II), as
the latter would rapidly release its axial ligand(s) and most likely
also dimerize.17 All other reversible electron transfer processes are
also obtained at quite positive potentials. Guided by the electro-
chemistry of other metallocorroles,8 the first and second redox
processes of 1 (E1/2 ) +0.66 and +1.28 V vs SCE, respectively)
may tentatively be assigned as metal-centered (IrIII/IrIV) and corrole-
centered (tpfc/tpfc+), respectively. As full bromination at the
ꢀ-pyrrole positions is known to upshift the potentials of metallo-
corroles by a few hundred mV,6,20 the feature at +1.19 V can be
assigned to the IrIII/IrIV couple in 2. Our data show clearly that
Ir(III) is more electron-rich in corroles than in other coordination
environments. The E1/2 value for the [(tpp)Ir]+/[(tpp)Ir]2+ (tpp )
Trianionic ligands should stabilize high oxidation states of
iridium, so an Ir(III) corrole might be as reactive as porphyrinic
Ir(II) toward substrates.17 Herein we report the first fully character-
ized corrolato Ir(III) complex, (tpfc)Ir(III)(tma)2 (1), and an
octabromo-ꢀ-pyrrole derivative, (Br8-tpfc)Ir(III)(tma)2 (2), where
tma ) trimethylamine (Scheme 1).
Compound 1 was obtained in 27% yield via reaction of H3tpfc
with excess [Ir(cod)Cl]2 (cod ) cyclooctadiene) and K2CO3 in hot
THF under Ar to form (tpfc)Ir(I)(cod), which was converted to an
axially tma-ligated Ir(III) complex upon addition of tma N-oxide
and exposure to the atmosphere.18 Full bromination of 1 was
achieved via reaction with excess Br2 in methanol, providing green
crystals of 2 in about 65% yield.19 Both complexes were fully
characterized by spectroscopy, electrochemistry, and X-ray dif-
1
fraction as six-coordinate Ir(III) corroles. The 19F and H NMR
spectra demonstrate that they are diamagnetic (sharp resonances,
Figure 1), possess high symmetry (only one type of ortho-F for
any C6F5 ring), and contain two axial tma groups (18 H atoms at
-2.96 ppm for 1 and -2.59 ppm for 2).
The red shifts of the principal features in the electronic spectrum
of 2 relative to 1 (8-16 nm, Figure 1) are similar to those observed
upon bromination of other metallocorroles,6,20 but the intense Soret
band system is uniquely split, as are the Q-bands (roughly 70 nm).
Figure 1. 1H NMR spectrum of 1 in CD2Cl2 and UV-vis spectra of 1
(red) and 2 (blue) in CH2Cl2. (Inset) The ꢀ-pyrrole proton resonances.
† California Institute of Technology.
‡ TechnionsIsrael Institute of Technology.
9
7786 J. AM. CHEM. SOC. 2008, 130, 7786–7787
10.1021/ja801049t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
obtained free of charge from the CCDC at http://www.ccdc.cam.ac.uk/
data_request/cif. All other Supporting Information is available free of
charge via the Internet at http://pubs.acs.org.
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Figure 3. X-ray structures of 1 (left) and 2 (right): 50% probability
displacement ellipsoids. Average distances (Å), for 1, 2: Ir-N(equatorial),
1.965(9), 1.974(3); Ir-N(axial), 2.185(9), 2.189(3).
tetraphenylporphyrinato) redox couple is about +1.4 V vs SCE,22
and IrIII/IrIV processes in cyclometalated bpy complexes also occur
at much more positive potentials than in 1.23
The molecular structures of 1 and 2 (Figure 3) reveal that their
macrocyclic frameworks are isostructural, with the iridium atom
located in the plane of an essentially flat corrole.24 The Ir-N axial
bonds are about 0.2 Å longer than the in-plane Ir-N equatorial
bonds, as might be expected. The one clear difference between 1
and 2 is that the aryl rings are nearly perpendicular with respect to
the corrole in the latter to avoid steric clash with the bromine atoms.
The structure of 2 is distinctly different from those of analogous
tetraarylporphyrins, where ꢀ-pyrrole bromination induces large
distortions of the macrocycle that produce dramatic red shifts in
UV-vis absorptions and higher reduction potentials.25 For iridium
corroles, the 530 mV upshift in the potential of 2 vs 1 implies major
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(18) 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 refluxed under
argon for 90 min (until the corrole fluorescence was negligible to the eye
upon long-wave 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 chromatog-
raphy of the black solution (silica, 4:1 hexanes/CH2Cl2) provided purple
crystals of (tpfc)Ir(III)(tma)2 (30 mg, 27% yield). 1H NMR (CD2Cl2): δ
8.93 (d, 2H), 8.54 (d, 2H), 8.42 (d, 2H), 8.12 (d, 2H),-2.96 (s, 18H). 19F
NMR (CD2Cl2): δ-139.1 (m, 6H),-156.2 (m, 3H),-164.3 (m, 6H). MS
(ESI): 1105.1 ([M+]), 1046.0 ([M+-tma]), 986.5 ([M+-2tma]). UV-vis (nm):
390, 412, 448 (sh), 572, 638.
We have demonstrated that a corrole can readily accommodate
a 5d transition metal in our work on the first nonorganometallic
Ir(III) porphyrinoid. We also report an X-ray diffraction structure
of a fully brominated derivative. The electron transfer processes
demonstrated for 1 and 2 suggest that they may prove useful as
redox catalysts. Studies are in progress to develop methodologies
for opening an axial coordination site on the metal, a requirement
for testing the catalytic potential of these complexes.
(19) Complex 1 (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 solution provided green crystals of (Br8-tpfc)Ir(III)(tma)2 (15
mg, 63% yield). 1H NMR (CD2Cl2): δ-2.59 (s, 18H). 19F NMR (CD2Cl2):
δ-138.4 (q, 2H),-139.0 (q, 4H),-153.9 (t, 3H),-164.4 (m, 4H),-164.7
(m, 2H). UV-vis (nm): 406, 422, 458 (sh), 580, 646.
(20) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.;
DiBilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132–2134.
(21) In degassed CH2Cl2 solutions under Ar containing 0.3 M NEt4BF4, using
a glassy carbon disk working electrode, a Pt wire auxiliary electrode, and
a Ag/AgCl quasi-reference electrode.
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7, 1979–1983.
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(24) The macrocyclic framework of a fully brominated cobalt corrole also is
flat: Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem.
2001, 66, 550–556.
(25) Ou, Z.; Shao, J.; D’Souza, F.; Tagliatesta, P.; Kadish, K. M. J. Porphyrins
Pthalocyanines 2004, 8, 201–214, and references therein.
Acknowledgment. This work was supported by the US-Israel
BSF (Z.G. and H.B.G.), BP, NSF, CCSER (Gordon and Betty
Moore Foundation), and the Arnold and Mabel Beckman Founda-
tion (H.B.G.).
Supporting Information Available: CCDC 671270 (1) and CCDC
657602 (2) contain supplementary crystallographic data, and can be
JA801049T
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