Chromium corroles in four oxidation states

Article abstract of DOI:10.1021/ic010723z

We have isolated and characterized chromium complexes of 5,10,15-tris(pentafluorophenyl)corrole [(tpfc)H₃] (1) in four oxidation states: [(tpfc*)CrO][SbCl₆] (6); [(tpfc)CrO] (2); [(tpfc)CrO][Cp₂Co] (4); and [(tpfc)Cr(py)₂] (3). Complex 6 was prepared both by electrochemical and chemical oxidation of 2; its formulation as a Cr^VO ligand-radical species is based on UV-visible absorption as well as EPR measurements. Cobaltocene reduction of 2 gave 4; it was identified as a diamagnetic d² Cr^IVO complex from its sharp ¹H NMR spectrum. Reaction of 2 with triphenylphosphine yielded a chromium(III) corrole, [(tpfc)Cr(OPPh₃)₂] (5). Owing to its air sensitivity, 5 could not be isolated in the absence of excess OPPh₃. The structure of the Cr^III bis-pyridine complex (3) was determined by X-ray crystallography (Cr-N distances: 1.926-1.952 A, pyrrole; 2.109, 2.129 A, pyridine).

Full text of DOI:10.1021/ic010723z

6788
Inorg. Chem. 2001, 40, 6788-6793
Chromium Corroles in Four Oxidation States
Alexandre E. Meier-Callahan,† Angel J. Di Bilio,† Liliya Simkhovich,‡ Atif Mahammed,‡
Israel Goldberg,*,§ Harry B. Gray,*,† and Zeev Gross*,‡
Beckman Institute, California Institute of Technology, Pasadena, California 91125, Department of
Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology,
Haifa 32000, Israel, and School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
ReceiVed July 9, 2001
We have isolated and characterized chromium complexes of 5,10,15-tris(pentafluorophenyl)corrole [(tpfc)H3] (1)
in four oxidation states: [(tpfc•)CrO][SbCl6] (6); [(tpfc)CrO] (2); [(tpfc)CrO][Cp2Co] (4); and [(tpfc)Cr(py)2]
(3). Complex 6 was prepared both by electrochemical and chemical oxidation of 2; its formulation as a CrVO
ligand-radical species is based on UV-visible absorption as well as EPR measurements. Cobaltocene reduction
of 2 gave 4; it was identified as a diamagnetic d2 CrIVO complex from its sharp 1H NMR spectrum. Reaction of
2 with triphenylphosphine yielded a chromium(III) corrole, [(tpfc)Cr(OPPh3)2] (5). Owing to its air sensitivity, 5
could not be isolated in the absence of excess OPPh3. The structure of the CrIII bis-pyridine complex (3) was
determined by X-ray crystallography (Cr-N distances: 1.926-1.952 Å, pyrrole; 2.109, 2.129 Å, pyridine).
Introduction
We have shown that 2 can transfer atomic oxygen to PPh3 in
aerobic solution to form the first authentic chromium(III)
We have previously reported the facile synthesis of H3(tpfc)
(1) and the utilization of its metal complexes as catalysts for
the activation of hydrocarbons.1,2 Air-stable and fully character-
ized coordination complexes of 1 include [(tpfc)CrO] (2),3
[(tpfc)MnBr],4 and [(tpfc)FeCl].5 The stability of CrV, MnIV,
and FeIV corroles is in quite sharp contrast to the situation with
porphyrins and related macrocycles where lower oxidation states
are more common. Furthermore, our finding of spontaneous
oxidation of chromium and iron corroles to CrVO and FeIVCl
states during synthesis suggests that their MIII complexes could
be employed for dioxygen activation. Indeed, it has been shown
that iron(III) complexes of 1 and other corroles oxidize in air,5,6
but formation of inert FeIV µ-oxo complexes limits the potential
of these molecules as oxygenation catalysts.7
corrole, [(tpfc)Cr(OPPh3)2](5). In the absence of potential
ligands, 5 is readily reoxidized to 2 by dioxygen. Reaction of
5 with pyridine gives [(tpfc)Cr(py)2] (3), which has been
structurally characterized. All in all, we have isolated and
characterized complexes of 1 in four oxidation states: 2,
[(tpfc)CrO]- (4), 3, and [(tpfc•)CrO]+ (6) (Scheme 1).
Experimental Section
All chemicals were obtained from either Aldrich or EM Science
(solvents) and used as received, except for OPPh3 and PPh3, which
were recrystallized before use. Tetrabutylammonium hexafluorophos-
phate was recrystallized from ethanol/ether. The ligand (tpfc)H3 (1)
was prepared according to literature procedures.2 The oxidant dioxinium
hexachloroantimonate (7) was prepared by a standard method.12
Cobaltocene was sublimed before use.
The investigations described herein centered on chromium,
as only CrVO corroles have been fully characterized to date3,8,9
(a report of chromium(III) corroles10 has been questioned11).
[(tpfc)CrO] (2). Compound 1 (60 mg, 75.37 µmol) was dissolved
in about 20 mL of pyridine, and the solution was heated to reflux under
argon. A small amount of CrCl2 (very air-sensitive!) was added to the
hot solution, and the reaction was continued until TLC examination
(silica, n-hexane/CH2Cl2 ) 2:1) indicated the disappearance of the
starting material (about 2 h, a new green spot was obtained above the
spot of corrole; if the reaction is not complete, an additional portion of
CrCl2 can be added). Once 1 was completely consumed, the solvent
was evaporated, and the solid material was dissolved in CH2Cl2 and
filtered to remove inorganic salts. A 3 g portion of silica gel was added
to the filtrate, and the solvent was evaporated. Purification of the product
was performed by column chromatography on silica gel, with a 22:3
mixture of n-hexane and CH2Cl2. The resulting red complex was
recrystallized from CH2Cl2 and n-hexane. The yield from several such
reactions was 35-38 mg (54-59%). MS (+DCI) m/z (% intensity):
861.9 (100%) [MH+]. (-DCI) m/z (% intensity): 861.9 (100%) [M-].
UV-vis (CH2Cl2) λmax [nm] (ꢀ × 10-3 [M-1cm-1]): 402 (81.3), 556
(14.0).
† California Institute of Technology.
‡ Technion-Israel Institute of Technology.
§ Tel Aviv University.
(1) Gross, Z.; Simkhovich, L.; Galili, N. J. Chem. Soc., Chem. Commun.
1999, 599.
(2) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. 1999, 38,
1427.
(3) Meier-Callahan, A. E.; Gray, H. B.; Gross, Z. Inorg. Chem. 2000, 39,
3605.
(4) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.;
Di Bilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132.
(5) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z. Inorg.
Chem. 2000, 39, 2704.
(6) Vogel, E.; Will, S.; Tilling, A. S.; Neumann, L.; Lex, J.; Bill, E.;
Trautwein, A. X.; Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1994,
33, 731.
(7) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Chem. Eur.
J. 2001, 7, 1041.
(8) Matsuda, Y.; Yamada, S.; Murakami, Y. Inorg. Chim. Acta 1980, 44,
L309.
(9) Murakami, Y.; Matsuda, Y.; Yamada, S. J. Chem. Soc., Dalton Trans.
1981, 855.
(11) Erlen, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York,
2000; Vol. 2, p 233.
(10) Boschi, T.; Licoccia, S.; Paolesse, R.; Tagliatesta, P.; Tehran, M. A.;
Pelizzi, G.; Vitali, F. J. Chem. Soc., Dalton Trans. 1990, 463.
(12) Barzilay, C. M.; Sibilia, S. A.; Spiro, T. G.; Gross, Z. Chem. Eur. J.
1995, 1, 222.
10.1021/ic010723z CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/20/2001

Chromium Corroles in Four Oxidation States
Inorganic Chemistry, Vol. 40, No. 26, 2001 6789
Scheme 1
[(tpfc)Cr(py)2] (3). Metal insertion into 1 was performed as
described, but at the end, the solid material was dissolved in a 9:1
mixture of benzene and pyridine and filtered. A 3 g portion of silica
gel was added to the solution, and the solvents were evaporated.
Purification of the product was performed by column chromatography
on silica gel, with a 50:5:1 mixture of n-hexane/CH2Cl2/pyridine as
eluent. The isolated green material was recrystallized from a mixture
of benzene and n-heptane that also contained a few drops of pyridine.
The yield from several such reactions was 49-53 mg (65-70%).
Elemental analysis: found (calcd) for C47H18CrF15N6: C, 56.32%
(56.25%); H, 2.07% (1.81%); N, 8.14% (8.37%). MS (+DCI) m/z (%
intensity): 925.0 (100%) [MH+ - (py)], 846.0 (18%) [MH+ - 2(py)].
(-DCI) m/z (% intensity): 844.9 (100%) [M- - 2(py)]. UV-vis (9:1
benzene/pyridine) λmax [nm] (ꢀ × 10-3 [M-1 cm-1]): 418 (33.7), 433
(38.9), 482 (23.5), 610 (12.9), 642 (14.0). 19F NMR (188 MHz) (C6D6,
δ in ppm): -133.1, -138.8 (br s, br s, 6F, ortho-F), -154.8 (s, 1F,
para-F), -155.3 (s, 2F, para-F), -161.8 (s, 6F, meta-F).
Compound 3 also was prepared from 2 by dissolving the latter (8.1
mg, 11.6 µmol) in 25 mL of CH2Cl2/pyridine (9:1) in the presence of
excess PPh3 (50 mg, 191 µmol). Stirring of the solution for several
hours (the color changes from red to dark green), followed by solvent
evaporation, yielded a residue that was dissolved in hexane/CH2Cl2/
pyridine (220:30:2) and eluted through silica gel with the same solution
mixture. The dark green band was collected and recrystallized from
CH2Cl2/pyridine to yield 10.1 mg of 3 (84.7% yield).
Millimolar solutions of [(tpfc)Cr(III)(py)2] for EPR measurements
were freshly prepared in CH2Cl2-pyridine or CD2Cl2-pyridine-d5 10:1
mixtures (pyridine proportions higher than 10% were avoided because
they produced larger spectral line widths). The solutions were frozen
as quickly as possible in order to minimize [(tpfc)Cr(III)(py)2] oxidation.
A magnetically dilute sample of 3 was obtained by cocrystallization
with the isomorphous complex [(tpfc)Co(III)(py)2]13 in a 0.5:10 Cr(III)/
Co(III) ratio. The cocrystals were transferred into a standard EPR tube
and crushed with a long quartz rod to form a microcrystalline powder.
Compound 3 gave very fragile single crystals via recrystallization
from benzene/n-heptane/pyridine. Diffraction data were obtained on a
Nonius Kappa CCD diffractometer at low temperature (∼110 K) to
minimize atomic thermal displacements and increase signal-to-noise.
The crystal structure was solved and refined by standard crystallographic
techniques.
and wR2 ) 0.152 for 3845 reflections with F > 4σ(F), wR2 ) 0.216
for all data. The incorporated benzene solvent (one molecule in a general
position and another located on inversion) is severely disordered in
the lattice. The pentafluorophenyl substituent, located between the N22-
and N23-pyrrole rings, also reveals excessively large thermal displace-
ment parameters that reflect the partial disorder of this fragment.
[Cp2Co][(tpfc)CrO] (4). A 25 mL two-neck flask was loaded under
argon with 2 (16.1 mg, 18.7 µmol) and an excess of freshly sublimed
cobaltocene; it was then evacuated. Vacuum transfer of acetone, fol-
lowed by stirring for a few minutes (once the solvent had thawed) and
solvent removal in vacuo yielded a residue that was redissolved in a
minimum of CH2Cl2. Hexanes were added to precipitate a pink solid.
The pink pellet obtained after centrifugation and supernatant removal
was dried under vacuum to afford 15.7 mg of 4 (79.9% yield). UV-vis
(CH2Cl2) λmax [nm] (ꢀ × 10-3 [M-1cm-1]): 432 (176.3), 542 (19.7), 578
(38.9). 1H NMR (300 MHz) (CD2Cl2 δ in ppm): 9.13 (d, J ) 4.0 Hz,
pyrrole-H), 8.77 (d, J ) 4.0 Hz, pyrrole-H), 8.72 (d, J ) 4.7 Hz, pyrrole-
H), 8.60 (d, J ) 4.7 Hz, pyrrole-H).19F NMR (282 MHz) (CD2Cl2 δ in
ppm): -134.3 (m, 3F, ortho-F), -135.7 (m, 3F, ortho-F), -151.9 (m,
3F, para-F), -159.8 (m, 3F, meta-F), -160.2 (m, 3F, meta-F).
[(tpfc)Cr(OPPh3)2] (5). Solutions containing this air-sensitive com-
pound were prepared as follows: a 1 mM solution of 2 in degassed
toluene was allowed to react with 1 mM OPPh3/1 mM PPh3 for 30
min in the same solvent under argon. Alternatively, 5 was prepared by
dissolving a small amount of 3 (to form a ∼10 µM solution) in a
degassed 1 mM OPPh3 solution in toluene under argon. Ligand
exchange was complete in 30 min.
[(tpfc•)CrO] (6). This complex was prepared both by chemical and
electrochemical oxidation of 2. Chemical oxidation: excess dioxinium
hexachloroantimonate was added to a CH2Cl2 solution of 2 cooled to
-78 °C (dry ice/acetone); 6 was precipitated by adding cold hexanes.
Electrochemical oxidation (for EPR): 2 (1-2 mg) was dissolved in
0.1 M tetrabutylammonium hexafluorophosphate/CH2Cl2 and oxidized
at -78 °C (acetone/dry ice) in an electrochemical cell with platinum
grid electrodes; the potential was scanned slowly (10 mV/s) from 0 to
1.5 V, and then a sample was transferred into an EPR tube, which was
placed in liquid nitrogen.
Spectroscopy. UV-vis spectra were measured on a HP 8452
spectrophotometer. X-band EPR spectra were obtained on a Bruker
EMX spectrometer equipped with a rectangular cavity working in the
TE102 mode. Variable temperature measurements were conducted with
an Oxford continuous-flow helium cryostat (temperature range 3.8-
300 K). Accurate frequency values were provided by a frequency
counter built in the microwave bridge.
Crystal data for 3: (C47H18CrF15N6)‚1.5(C6H6), Mr ) 1120.8,
monoclinic, space group C2/c (No. 15), a ) 40.705(1), b ) 8.740(1),
c ) 26.999(2) Å, â ) 98.19(1)°, V ) 9507.2(12) Å3, Z ) 8, Fcalcd
)
1.566 g cm-3, 2θmax ) 50.8°, 7986 unique reflections. Final R1 ) 0.093,
The absolute values of the [(tpfc)Cr(III)(py)2] spin-Hamiltonian
parameters were extracted from simulations of EPR spectra. Calcula-
tions were carried out on a PC using FORTRAN code14 based on
(13) Mahammed, A.; Giladi, I.; Goldberg, I.; Gross, Z. Chem. Eur. J., in
press.

6790 Inorganic Chemistry, Vol. 40, No. 26, 2001
Meier-Callahan et al.
Gladney’s general EPR fitting program15 and adapted for simulations
of EPR spectra of randomly oriented spin quartets. Transition fields
and the corresponding average transition moments16 were computed
in the magnetic field domain17,18 by matrix diagonalization of the
3
S ) /2 spin Hamiltonian:
5
4
H ) â(gzHzSz + gxHxSx + gyHySy) + D S2z - + E(Sx2 - Sy2 )
(
)
D and E are defined in terms of the components of the diagonal fine-
structure tensor as follows: Dzz ) (2/3)D; Dxx ) (1/3)D + E; Dyy
)
(1/3)D - E. In the absence of an applied magnetic field, the energy
separation between the two Kramers doublets (zero-field splitting) is
given by 2(D2 + 3E2)1/2 53Cr(III) hyperfine splitting (abundance 9.54%)
.
and higher order terms were omitted in the spin Hamiltonian.
Electrochemistry. Voltammetric measurements were made on a
CHI660 workstation with a normal three-electrode configuration,
consisting of a glassy carbon electrode, an Ag/AgCl reference electrode,
and a Pt-wire auxiliary electrode. Samples were in the millimolar range
in 0.1 M [Bu4N]PF6/CH2Cl2 solution at room temperature. Spectro-
electrochemical experiments were performed with the same solutions,
but in a 0.1 mm path length cell using two gold minigrids as electrodes
and a Ag/AgCl reference electrode.
Figure 1. Cyclic voltammogram of 2 in 0.1 M TBAPF6/CH2Cl2
solution (22 °C). Scan rate: 25 mV/s.
Results and Discussion
We have shown previously that the reaction between 1 and
chromium hexacarbonyl, followed by aerobic workup, affords
the very stable CrVO corrole 2 in good chemical yields.3 This
complex has been fully characterized by spectroscopic methods
and X-ray crystallography. Our findings for 2 are similar to the
observations of Matsuda et al. on a chromium complex of a
different corrole,8 but they are in conflict with the report of a
stable chromium(III) corrole10 (the latter work also was
questioned in a review on metallocorroles11). Matsuda also
reported that a CrVO corrole can be oxidized to a state that was
formulated as CrVI on the basis of UV-vis changes and the
disappearance of the d1 EPR signal. As this highly oxidized
chromium corrole might also be a CrVO corrole radical, we
decided to examine the properties of the product of one-electron
oxidation of 2 as well as complexes in oxidation states lower
than CrVO. First, we describe the electrochemistry of 2, as its
oxidation and reduction potentials are useful starting points for
most of the syntheses.
Electrochemistry. The cyclic voltammogram of 2 displays
two well-defined waves, separated by about 1.1 V (Figure 1,
E1/2(ox) ) 1.24 V, E1/2(red) ) 0.11 V). Both processes are
reversible, as demonstrated by the near unity ratio ipc/ipa as well
as the linear dependence of the peak current on the square root
of the scan rate. This indicates that relatively stable species have
been formed.
The spectroelectrochemical results for 2 are shown in Figure
2. Upon oxidation at an applied potential of 1.64 V, the 402
nm absorption band (analogous to the Soret band in porphyrins)
exhibits a large intensity reduction and shifts to 392 nm (Figure
2a). In addition, the band at 556 nm (analogous to the Q-band
in porphyrins) disappears, and weak new bands appear at 650
nm. All these changes are normally associated with ligand
oxidation; that is, they indicate that 2 is oxidized to [(tpfc•)-
CrVO]+ (6). Importantly, the spectrum of 2 is fully recovered
upon reversal of the applied potential to about 1.0 V.
Figure 2. a. Spectroelectrochemical oxidation of a solution of 2 in
0.1 M TBAPF6/CH2Cl2 at an applied potential of 1.64 V vs Ag/AgCl
(22 °C). Inset: Spectral changes upon treatment of a CH2Cl2 solution
of 2 with 7. The marked band (/) is due to the excess organic radical
present. b. Spectroelectrochemical reduction of a solution of 2 in 0.1
M TBAPF6/CH2Cl2 at an applied potential of 0.0 V vs Ag/AgCl (22
°C). Inset: spectrum obtained after treatment of 2 with Cp2Co.
The response to reduction is completely different (Figure
2b): the Soret band undergoes a very large increase in intensity
and a 32 nm red shift from 402 to 434 nm; in addition, a sharp
Q-band appears at 580 nm. The final spectrum is very similar
to that of an (oxo)chromium(IV) porphyrin,19 thereby indicating
that 2 is reduced at the metal to form [(tpfc)CrIVO]- (4).
Our formulation of 4 as a CrIVO species accords with a
literature report,9 but we differ in the assignment of the
electronic structure of 6. Thus, we decided to isolate the
reduction and the oxidation products of 2 for further spectro-
scopic characterization, especially by EPR and NMR. For
example, metal-centered oxidation of 2 would give a diamag-
netic chromium(VI) corrole, that is, one that would be NMR-
active and EPR-silent, whereas a radical formed by corrole
oxidation would have very different properties.
(14) Bonomo, R. P.; Di Bilio, A. J.; Riggi, F. Chem. Phys. 1991, 151,
323.
(15) Swalen, J. D.; Gladney, H. M. IBM J. Res. DeV. 1964, 8, 515.
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21, 842.

Chromium Corroles in Four Oxidation States
Inorganic Chemistry, Vol. 40, No. 26, 2001 6791
1
Figure 4. 19F and H NMR (inset) spectra of [(tpfc)CrIVO]- (4) in
acetone-d6 solution (22 °C).
Figure 3. EPR spectrum of electrochemically generated 6 in 0.1 M
TBAPF6/CH2Cl2 frozen solution (30 K). Inset: total signal intensity
vs temperature (the solid line was calculated for ∆E ) 9 cm-1).
CrVO (d1) and corrole radical electrons are coupled in 6 to give
a triplet ground state, as the temperature-dependent EPR spectra
confirm that ∆E[E(S)0) - E(S)1)] is positive (Figure 3,
inset).30 Analysis of the decrease in signal intensity with
Oxidation and Reduction Products of 2. The half-wave
oxidation potential of 2 is 1.24 V: thus, for the preparation of
6, we used the hexachloroantimonate salt of the cation radical
of p-dibenzodioxin (7) (Scheme 2), a powerful oxidant (E1/2
temperature from 4 to 40 K gives ∆E ) 9 cm-1
.
Cobaltocene (E1/2 ) -0.83 V vs Ag/AgCl)31 was chosen for
the reduction of 2, mainly because its oxidized form, cobalto-
cenium, is a nonnucleophilic counterion. The reaction proceeded
smoothly on a semisynthetic scale, and 4 was isolated as a
cobaltocenium salt (Scheme 2). Importantly, the absorption
spectrum is virtually identical to the one obtained spectroelec-
Scheme 2
1
trochemically (Figure 2b, inset). The H NMR spectrum of 4
(Figure 4) clearly supports CrVO reduction. The â-pyrrole
protons are not shifted from their diamagnetic position and are
sharp; indeed, they display 4.0-4.7 Hz J-coupling constants, a
very strong indication that 4 is diamagnetic. Owing to strong
π(Ã) donation,3 a CrIV corrole is expected to have a (dxy)2
ground state.32,33
We conclude that reduction and oxidation of 2 are metal-
and corrole-centered reactions, respectively. Complex 4 is a low-
spin d2 oxo corrole, analogous to CrIVO,32 RuVIO2,34 MnVN,35-38
and MnVO39 porphyrins and salens, and it is isoelectronic with
the MnVO complex of 1.40
)1.41 V)12 that does not interfere with NMR/EPR measure-
ments. Compound 7 is a crystalline blue solid, stable at -20
°C for weeks and of limited solubility in CH2Cl2. The last
property is crucial for isolating the product without any para-
magnetic impurity. Treating a micromolar solution of [(tpfc)-
CrVO]/CH2Cl2 with aliquots of the oxidant in the same solvent
results in the formation of a species with an electronic spectrum
identical to the one seen in the electrochemical experiments
(Figure 2a, inset). In sharp contrast to the characteristic room-
temperature EPR spectrum of the d1 CrVO complex 2, that of 6
is only observable in frozen solutions and is very different
(shown in Figure 3 for the electrochemically generated com-
plex). Even though the region around g ∼ 2 is obscured by the
strong signal of 2, the spectrum has a feature at half-field (g ∼
4) attributable to a spin-triplet species. This finding rules out
metal oxidation (d0 chromium(VI) would be diamagnetic and
EPR-silent); indeed, this evidence strongly points to ligand
oxidation.
Chromium(III) Corroles 3 and 5. CrVO complexes of
porphyrins and salens oxygenate olefins.41,42 The much less
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compound I, where a paramagnetic FeIVO (d4) center is coupled
to a porphyrin radical.20-22 Even more closely related is a VIVO
(d1) porphyrin radical species.23 In addition, there have been
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(41) Groves, J. T.; Kruper, J. W. J. J. Am. Chem. Soc. 1979, 101, 7613.
(42) Srinivasan, K.; Kochi, J. K. Inorg. Chem. 1985, 24, 4671.

6792 Inorganic Chemistry, Vol. 40, No. 26, 2001
Meier-Callahan et al.
Figure 5. Molecular structure of 3: the Cr-corrole core is flat, and
the axial pyridines are nearly coplanar; CrIII-N distances are 1.926-
(4)-1.952(4) Å (pyrrole) and 2.109(4)-2.129(4) Å (pyridine).
Figure 6. UV-vis spectra of [(tpfc)Cr(py)2] (solid line) and [(tpfc)-
Cr(OPPh3)2] (dashed line) in toluene solution (22 °C).
Table 1. Selected Structural Parameters for Related [(tpfc)MIII(py)2]
Complexes (Diffraction Measurements at 110 K)
[(tpfc)Cr(py)2] [(tpfc)Fe(py)2] [(tpfc)Co(py)2]
M-N(pyrrole) bond
length range (Å)
1.926-1.952 1.865-1.923 1.873-1.900
M-N(pyridine) bond 2.109, 2.129 2.028, 2.032
lengths (Å)
1.994, 1.994
N21‚‚‚N23 (Å)
N22‚‚‚N24 (Å)
axial-py twist
angle (deg)
3.871
3.860
10.8
3.783
3.770
8.8
3.766
3.775
2.3
positive CrVO/CrIVO potential of 2 (0.11 V) relative to porphyrin
and salen complexes (0.47-0.76 V)42,43 suggests that its
reactivity will be much lower. This is indeed the case: 2 does
not react with styrene, norbornene, or cyclooctene. Accordingly,
we looked for a much stronger oxophile to reduce 2 to a
chromium(III) corrole: treatment of 2 with triphenylphosphine
produces very large absorption spectral changes with the final
spectrum reminiscent of chromium(III) porphyrins.44 Product
5 is [(tpfc)Cr(OPPh3)2], which is extremely air-sensitive and
could not be isolated as a pure material; it was manipulated in
solutions containing excess OPPh3.
Figure 7. (a) Frozen solution EPR spectrum of [(tpfc)Cr(III)(py)2]
(solvent 10:1 CD2Cl2/pyridine-d5, 77 K). Instrumental parameters: ν
) 9.4878 GHz, modulation frequency ) 100 kHz, modulation ampli-
tude ) 12 G, microwave power ) 8 mW, 10 scans. Sharp absorption
at ∼341.5 mT is assigned to CrVO (2) formed by oxidation of [(tpfc)-
Cr(III)(py)2] in air. Resonance at high resolution (inset): nine-line multi-
plet is centered at g ∼ 1.985 with a nitrogen superhyperfine coupling
constant of 2.90(2) × 10-4 cm-1 (note that multiplet is not resolved in
nondeuterated solvent).3 (b) Experimental vs computer simulated EPR
spectra of a polycrystalline sample of [(tpfc)Cr(III)(py)2] cocrystallized
with [(tpfc)Co(III))(py)2] (exptl, 3.9 K). Instrumental parameters: ν )
9.4795 GHz, modulation frequency ) 100 kHz, modulation amplitude
) 10 G, microwave power ) 6.4 mW, 15 scans. The features at ∼180-
190 mT are likely due to large crystallites (it proved difficult to produce
a good powder with the limited amount of sample available); the weak
feature at 342.5 mT and those below 100 mT are due to unknown
impurities. Simulated spectrum computed with a Lorentzian function.
(c) Relationship between the simulated spectrum and the angular
dependence of the transition fields in the D-tensor reference frame:
solid lines, angular dependence in zx plane (θ ) 0 f π/2, φ ) 0);
dotted lines, angular dependence in zy plane (θ ) 0 f π/2, φ ) π/2).
Substitution of the axial OPPh3 in 5 was followed by changes
in Soret absorption: upon addition of pyridine, 5 is converted
to 3 (Figure 6).45 The absence of isosbestic points indicates the
(43) Fujii, H.; Yoshimura, T.; Kamada, H. Inorg. Chem. 1997, 36, 1122.
(44) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am.
Chem. Soc. 1977, 99, 8195.
(45) Aliquots from a stock solution of 2 in toluene were injected into 2
mL toluene solutions that contained 10.6 mM OPPh3, 0.9 mM PPh3,
and variable concentrations of pyridine (in the range 9.9-693 µM).
In each case, the solution absorption spectrum was measured 15 min
later to confirm that 2 was fully reduced and equilibrated with all
potential ligands.

Chromium Corroles in Four Oxidation States
Inorganic Chemistry, Vol. 40, No. 26, 2001 6793
presence of [(tpfc)Cr(OPPh3)(py)] in appreciable amounts.
Extracting equilibrium constants was not attempted, as such
analysis is by no means trivial.46 Qualitatively, the affinity of
chromium(III) for pyridine is higher than for triphenylphosphine
oxide.
Co(III)(py)2]13 (Figure 7b,c). We found a nearly isotropic g and
a slightly rhombic fine-structure tensor: gz ) 1.967(9), gx ) gy
) 1.989(9), |D| ) 0.2802(6) cm-1, |E| ) 0.01835(15) cm-1, 4
K (these values change little in the range 4-150 K). The
estimated zero-field splitting parameters for the compound in
frozen solution are |D| ∼ 0.230(5) cm-1 and |E| ∼ 0.02 cm-1 49
;
It is apparent that 2 is obtained by air oxidation of CrIII and
that pyridine coordination inhibits this reaction. Accordingly,
metal insertion into 1 was performed in pyridine, and the same
solvent was utilized in all workup steps: 3 was obtained in good
yield, and X-ray quality crystals were grown from a benzene/
n-heptane/pyridine solution. Although the resolution was not
very high, the basic structural aspects of this first authentic
chromium(III) corrole were revealed; 3 features a flat corrole
macrocycle and nearly coplanar mutual alignment of the two
coordinated pyridines (Figure 5). The largest deviation from
the mean plane defined by all the corrole atoms is only 0.14 Å.
These features are very similar to those found in crystal
structures of related bis-pyridine complexes of iron(III) and
cobalt(III).7,13 Chromium(III) has a somewhat larger radius than
the other two ions, yet it is still located perfectly in the plane
defined by the four inner nitrogen atoms. This is accompanied,
however, by a systematic increase in Cr-N compared to M-N
distances in related complexes (Table 1). As observed for 2,3
the bond distances between Cr and the corrole nitrogens are
relatively short (1.92-1.94 Å) when compared to CrII porphy-
rins, which have M-Npyrrole bonds in the range 2.012-2.141
Å.47 Interestingly, the M-Npyridine bonds are of similar lengths
in both cases (2.026-2.121 Å).
Samples of compound 3 display rich EPR spectra, charac-
teristic of chromium(III) complexes (S ) 3/2) with a zero-field
splitting similar in magnitude to the microwave quantum (at X
band hν ∼ 0.3 cm-1).48 The EPR spectrum in frozen solution
(Figure 7a) shows broad resonances at 3.8 K and above (spectral
resolution in deuterated methylene chloride/pyridine is slightly
improved relative to a nondeuterated solvent mixture). Thus,
to extract the spin-Hamiltonian parameters of 3 reliably, we
measured and simulated the EPR spectrum of a polycrystalline
sample of [(tpfc)Cr(III)(py)2] diluted in isomorphous [(tpfc)-
these values accord with those extracted from the solid-state
experiment, indicating similar structures in both states. Impor-
tantly, the spin-Hamiltonian parameters of [(tpfc)Cr(III)(py)2]
are close to those reported for chromium(III) porphyrin species
[(TPP)Cr(III)(Cl)(X)] (X ) pyridine and other neutral N donors,
|D| ∼ 0.156 ( 0.012 cm-1; X ) neutral O- or S-donors, |D| ∼
0.232 ( 0.004 cm-1; |E| e 0.013 < 0 cm-1) in frozen solution.44
Concluding Remarks
We have successfully isolated and characterized chromium
complexes of 1 in four different oxidation states. The one-
electron oxidation and reduction reactions of 2 are corrole- and
metal-centered (giving 6 and 4, respectively), and oxygen-atom
transfer from 2 to triphenylphosphine affords CrIII, leading to
the isolation of 5 and 3. Aerobic oxidation of CrIII corroles to
CrVO species is reminiscent of the chemistry of chromium
porphyrins (note the lower oxidation states relative to corroles),50
wherein CrII complexes oxidize readily in aerobic environments
to relatively inert CrIVO species. In experiments now underway,
we are investigating whether it is possible to tune the reactivities
of CrVO and CrIII corroles to the point where both oxygen-
atom transfer (CrVO + PPh3 f CrIII + OPPh3) and aerobic
oxo regeneration (CrIII + 1/2O2 f CrVO) are facile. Our goal is
to elucidate the factors that control the rates of CrVO and CrIII
redox reactions in the hope that ultimately we will be able to
design active catalysts for aerobic oxygenation of olefins and
other hydrocarbons.
Acknowledgment. This work was supported by the NSF
(H.B.G.), the Bayer Corporation, and the Israel Science Founda-
tion (Z.G.).
Supporting Information Available: Tables of crystal data, struc-
ture refinement details, atomic coordinates, anisotropic thermal param-
eters, bond lengths, and bond angles for 3. This material is available
free of charge via the Internet at http://pubs.acs.org.
(46) Sideris, E. E.; Valsami, G. N.; Koupparis, M. A.; Macheras, P. E.
Pharm. Res. 1992, 9, 1568.
(47) Scheidt, W. R.; Brinegar, A. C.; Kirner, J. F.; Reed, C. A. Inorg. Chem.
1979, 18, 3610.
IC010723Z
(48) Note that six-coordinate chromium(III) complexes are characterized
by quasiisotropic g factors (<2) and zero-field splittings in the 0 to
∼2.5 cm-1 range (McGarvey, B. R. In Transition Metal Chemistry;
Carlin, R. L., Ed.; Dekker: New York, 1966; Vol. 3. Pedersen, E.;
Toftlund, H. Inorg. Chem. 1974, 13, 1603. Bencini, A.; Gatteschi, D.
In Transition Metal Chemistry; Nelson G. A., Figgis B. N., Eds.;
Dekker: New York, 1982; Vol. 8, ref 43.).
(49) The relative intensities of some resonances in the frozen solution EPR
spectrum were not reproduced well in the simulations. This is
suggestive of D-tensor strain, which means that a distribution of spin-
Hamiltonian parameters is needed to obtain a fit of the EPR line shape.
Rotational freedom of the pyridine ligands is a probable cause of strain.
(50) Reed, C. A.; Kouba, J. K.; Grimes, C. J.; Cheung, S. K. Inorg. Chem.
1978, 17, 2666.