Xanthene-modified and hangman iron corroles

Article abstract of DOI:10.1021/ic101943h

Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining the oxidation state of

Full text of DOI:10.1021/ic101943h

1368 Inorg. Chem. 2011, 50, 1368–1377
DOI: 10.1021/ic101943h
Xanthene-Modified and Hangman Iron Corroles
Matthias Schwalbe,†,§ Dilek K. Dogutan,† Sebastian A. Stoian,†,‡ Thomas S. Teets,† and Daniel G. Nocera*,†
†Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139, United States, and ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth
Avenue, Pittsburgh, Pennsylvania 15213, United States. §Current address: Department of Chemistry, Humboldt
€
Universitat zu Berlin, Brook-Taylor-Strasse 2, 12489, Berlin; E-mail: matthias.schwalbe@chemie.hu-berlin.de
Received September 25, 2010
Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated
reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining
the oxidation state of the metal center. Investigation of their electronic structure using 57Fe Mossbauer spectroscopy in
€
conjunction with density functional theory (DFT) calculations reveals the non-innocence of the corrole ligand. Although
these iron corroles contain a formal Fe(IV) center, the deprotonated corrole macrocycle ligand is one electron oxidized.
The electronic ground state of these complexes is best described as an intermediate spin S = 3/2 Fe(III) site strongly
antiferromagnetically coupled to the S = 1/2 of the monoradical dianion corrole [Fe(III)Cl-corroleþ•]. We show here that
iron corroles as well as xanthene-modified and hangman xanthene iron corroles are redox active and catalyze the
disproportionation of hydrogen peroxide via the catalase reaction, and that this activity scales with the oxidation
potential. The meso position of corrole macrocycle is susceptible toward nucleophilic attack during catalase turnover.
The reactivity of peroxide within the hangman cleft reported here adds to the emerging theme that corroles are good at
catalyzing two-electron activation of the oxygen-oxygen bond in a variety of substrates.
Introduction
has emphasized porphyrin10-14 and salen15,16 macrocycles.
We have expanded the applicability of the hangman con-
struct by developing methods to deliver hangman corrole
xanthene (HCX).17 Corroles, when compared to porphyrins,
lack one meso bridge carbon atom and hence have a smaller
macrocycle cavity. Moreover, the four macrocycle nitrogens
bear three protons, as compared to only two in porphyrins
and thus, when deprotonated, corroles have a higher charge,
and they are able to stabilize metals in higher oxidation
states.18,19 Although the formal oxidation state of iron in
metallocorrole is high, it does not necessarily describe the
actual metal electron count. For instance, iron chloride
corrole complexes may be described as either [Fe(IV)Cl-
corrole] or [Fe(III)Cl-corroleþ•]. Since both formulations
lead to triplet ground states with rather similar spectroscopic
The proton-coupled electron transfer (PCET) chemistry
that is essential to small molecule activation is achieved by
hangman macrocycles by positioning a proton-donor/accep-
tor group from the scaffold of a xanthene backbone over the
macrocyclic redox-active site.1-4 The presence of a proton
transfer group over the active site of a redox platform permits
proton and electron transfer to be coupled in the process of
activating molecules bound within the hangman cleft. The
hangman construct is especially effective for oxygen-oxygen
bond activation because the delivery of a proton from the
hanging group promotes the two-electron heterolytic clea-
vage of the O-O bond.1,2,4-9 Heretofore, the hangman motif
*To whom correspondence should be addressed. E-mail: nocera@
mit.edu.
(11) McGuire, R., Jr.; Dogutan, D. K.; Teets, T. S.; Suntivich, J.; Shao-
Horn, Y.; Nocera, D. G. Chem. Sci. 2010, 1, 411–414.
(12) Chang, C. J.; Yeh, C.-Y.; Nocera, D. G. J. Org. Chem. 2002, 67,
1403–1406.
(13) Yeh, C.-Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2001, 123,
1513–1514.
(1) Rosenthal, J.; Nocera, D. G. Acc. Chem. Res. 2007, 40, 543–553.
(2) Rosenthal, J.; Nocera, D. G. Prog. Inorg. Chem. 2007, 55, 483–544.
(3) Chang, C. J.; Chang, M. C. Y.; Damrauer, N. H.; Nocera, D. G.
Biochim. Biophys. Acta 2004, 1655, 13–28.
(4) Reece, S. Y.; Hodgkiss, J. M.; Stubbe, J.; Nocera, D. G. Philos. Trans.
R. Soc. B 2006, 361, 1351–1364.
(5) Chang, C. J.; Chng, L. L.; Nocera, D. G. J. Am. Chem. Soc. 2003, 125,
1866–1876.
(6) Liu, S.-Y.; Soper, J. D.; Yang, J. Y.; Rybak-Akimova, E. V.; Nocera,
D. G. Inorg. Chem. 2006, 45, 7572–7574.
(7) Yang, J. Y.; Nocera, D. G. J. Am. Chem. Soc. 2007, 129, 8192–8198.
(8) Yang, J. Y.; Nocera, D. G. Tetrahedron Lett. 2008, 49, 4796–4798.
(9) Yang, J. Y.; Nocera, D. G. ChemSusChem. 2008, 1, 941–949.
(10) Chng, L. L.; Chang, C. J.; Nocera, D. G. Org. Lett. 2003, 5, 2421–
2424.
(14) Dogutan, D. K.; Bediako, D. K.; Teets, T. S.; Schwalbe, M.; Nocera,
D. G. Org. Lett. 2010, 12, 1036–1039.
(15) Liu, S.-Y.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 5278–5279.
(16) Yang, J. Y.; Bachmann, J.; Nocera, D. G. J. Org. Chem. 2006, 71,
8706–8714.
(17) Dogutan, D. K.; Stoian, S.; McGuire, R. Jr.; Schwalbe, M. S.; Teets,
T. S.; Nocera, D. G. J. Am. Chem. Soc. 2010, 132, 131-140.
(18) Gross, Z.; Gray, H. B. Comments Inorg. Chem. 2006, 27, 61–72.
(19) Wasbotten, I.; Ghosh, A. Inorg. Chem. 2006, 45, 4910–4913.
r
pubs.acs.org/IC
Published on Web 01/18/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1369
properties, a combined experimental and theoretical analysis
is often required to differentiate between the two possible
configurations.20 Short Fe-N distances and an unperturbed
macrocyclic scaffold in pentafluorophenyl and octaethyl
corroles support a Fe(IV)-corrole formulation.21-24 Subse-
quent studies have revised this view.20,25-29 Chemical shifts
Absorption spectral measurements were made on DCM
solutions of each compound using a Cary 5000 UV-vis-NIR
spectrometer from Varian employing the software Cary Wi-
nUV. Quartz cells with a 10 mm path length were used. Steady
state emission spectra were recorded on an automated Photon
Technology International (PTI) QM 4 fluorimeter equipped
with a 150-W Xe arc lamp and a Hamamatsu R928 photomul-
tiplier tube. Excitation light was wavelength selected with glass
filters. Solution samples for absorption and emission experi-
ments were prepared under ambient conditions in DCM and
contained in screw-cap quartz fluorescence cells. Lifetime mea-
surements were performed on THF solutions prepared under
nitrogen atmosphere and then subject to freeze-pump cycles.
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,
15-bis(tert-butyl-phenyl) Corrole (4a). Method 1. Bromoxanthene
aldehyde 1 (129 mg, 0.30 mmol) and 4-tert-butylphenyldipyrro-
methane 3a (209 mg, 0.75 mmol) were dissolved in 10 mL of DCM.
The condensation reaction commenced with the addition of 10 mL
of 1.3 mM solution of TFA in DCM followed by stirring at room
temperature for 7 h. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ, 170 mg, 0.75 mmol) was then added, and the reaction
mixture was stirred for an additional 15 min. The solution was
loaded directly onto a silica gel column and eluted with DCM,
and the first luminescent band was collected. Further purification
by column chromatography on silica was accomplished using
DCM/hexane (4:5). The resulting dark violet solid was suspended
in methanol, filtered and dried in vacuo (58 mg, 20% yield based
on 1). 1H NMR (500 MHz, CDCl3, 25ꢀC): δ=8.97 (d, J= 4.0 Hz,
2H), 8.89 (d, J = 5.0 Hz, 2H), 8.65 (s(br), 2H), 8.51 (d, J = 4.5 Hz,
2H), 8.35 (s(br), 4H), 8.04 (d, J = 2.5 Hz, 1H), 7.83 (d, J = 8.5 Hz,
4H), 7.80 (d, J = 2.5 Hz, 1H), 7.42 (d, J = 2.5 Hz, 1H), 7.10 (d, J =
2.5 Hz, 1H), 1.90 (s, 6H), 1.59 (s, 18H), 1.53 (s, 9H), 1.27 (s, 9H).
HR(ESI)-MS (MHþ) (M = C62H65N4OBr): Calcd m/z =
963.4429, found 963.4404. MS(MALDI-TOF) (Mþ): 962.45.
UV-vis, nm (ε ꢀ 10-3 M-1 cm-1): 421 (132), 568 (17), 617 (15),
1
of the H NMR signals, EPR spectra of reduced species,
magnetic susceptibility measurements, 57Fe Mossbauer spec-
€
troscopy, and theoretical calculations of corrolates support
the radical dianion state of the corrole macrocycle and an
intermediate-spin ferric center.
We report here the preparation of xanthene-modified
corroles (XC) and HCX macrocycles of iron and show that
these species possess a redox non-innocent XC and HCX
57
€
ligand. Fe Mossbauer spectroscopy in conjunction with
Density Functional Theory (DFT) calculations indicate that
these iron chloride xanthene corroles are best described as
[Fe(III)Cl-corroleþ•].20,30 The redox chemistry of iron chlo-
ride XC and HCX platforms has been exploited to drive the
catalytic dismutation of hydrogen peroxide. The turnover
frequency of the initial dismutation reaction indicates the
ease of the reduction of the high valent iron oxo corrole.
Experimental Section
General Methods. Silica gel for column chromatography was
obtained from Whatman Inc. (Silica Gel 60, 230-400 μm mesh).
All solvents (tetrahydrofuran (THF), toluene, dichloromethane
(DCM), dimethylformamide (DMF), hexane, and methanol)
and most starting materials were obtained from Sigma-Aldrich
and used without further purification. Compounds 1,12 2,12
3a-3c,31 5a,17 and 6a17 were prepared according to literature
procedures.
1H NMR spectra were recorded at ambient temperature on a
Varian Mercury 300 or 500 MHz spectrometer. All spectra were
referenced to tetramethylsilane (TMS) or deuterated chloro-
form (CDCl3) as an internal standard (measured values for δ are
given in parts per million (ppm) and for J in Hertz (Hz)).
Elemental analysis was performed by Midwest Microlab La-
boratories, Indiana. Electrospray ionization (ESI) mass spectra
were obtained using a Bruker Daltonics APEXIV 4.7 T FT-
ICR-MS instrument at the DCIF facility of MIT. Matrix-
assisted laser desorption/ionization-time of flight (MALDI-
TOF) spectra were recorded on a Bruker Omniflex instrument
with a reflectron accessory.
650 (14) in DCM. Anal. Calcd for C62H65N4OBr 0.5H2O: C, 76.68;
3
H, 6.85; N, 5.77. Found: C, 76.50; H, 6.90; N, 5.65.
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,
15-bismesityl corrole (4b). Method 1 was followed by using 3b
(198 mg, 0.75 mmol) in place of 3a. Two column chromatographs
(silica, ethylacetate/hexane = 1:25) afforded a violet solid, which
was washed with methanol and dried in vacuo to furnish a purple
solid (64 mg, 23% yield based on 1). 1H NMR (300 MHz, CDCl3,
25 ꢀC): δ = 8.87 (d, J = 4.2 Hz, 2H), 8.42 (m, 4H), 8.32 (d, J = 4.2
Hz, 2H), 8.00 (d, J = 2.4 Hz, 1H), 7.76 (d, J = 2.4 Hz, 1H), 7.37 (d,
J = 2.1 Hz, 1H), 7.25 (m, 4H), 7.03 (d, J = 2.1 Hz, 1H), 2.59 (s,
6H),1.99(s, 6H), 1.91(s, 6H), 1.86(s, 6H), 1.49(s, 9H), 1.23(s, 9H).
HR(ESI)-MS (MHþ) (M = C60H61N4OBr): Calcd m/z =
933.4102, found 933.3979. MS(MALDI/TOF) (Mþ): 932.57.
UV-vis, nm (ε ꢀ 10-3 M-1 cm-1): 409 (125), 424 (109), 567
(14), 603 (11), 634 (7) in DCM. Anal. Calcd for C60H61N4OBr: C,
77.15; H, 6.58; N, 6.00. Found: C, 76.91; H, 6.52; N, 5.86.
(20) Ye, S.; Tuttle, T.; Bill, E.; Simkhovich, L.; Gross, Z.; Thiel, W.;
Neese, F. Chem.;Eur. J. 2008, 14, 10839–10851.
(21) Caemelbecke, E. V.; Will, S.; Autret, M.; Adamian, V. A.; Lex, J.;
Gisselbrecht, J.-P.; Gross, M.; Vogel, E.; Kadish, K. M. Inorg. Chem. 1996,
35, 184–192.
(22) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z.
Inorg. Chem. 2000, 39, 2704–2705.
(23) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41, 5433–
5439.
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,15-
bis(pentafluorophenyl) corrole (4c). Method 1 was followed by using
3c (234 mg, 0.75 mmol) in place of 3a. Column chromatography
(silica, DCM/hexane = 1:2) afforded a dark purple solid, which
was washed with methanol and dried in vacuo (72 mg, 24% yield
based on 1). 1H NMR (500 MHz, CDCl3, 25 ꢀC): δ = 9.12 (d, J =
4.0Hz, 2H), 8.68(m, 4H), 8.57(s(br),2H), 8.02(d,J= 2.5 Hz, 1H),
7.85 (d, J=2.5 Hz, 1H), 7.40 (d, J=2.5 Hz, 1H), 7.06 (d, J=2.5 Hz,
1H), 1.89 (s, 6H), 1.54 (s, 9H), 1.24 (s, 9H). HR(ESI)-MS
(MHþ) (M=C54H39F10N4OBr): Calcd for m/z=1029.2220, found
1029.2242. MS(MALDI/TOF) (MHþ): 1030.37. UV-vis, nm (ε ꢀ
10-3 M-1 cm-1): 414 (134), 560 (21), 612 (12) in DCM. Anal. Calcd
(24) Simkhovich, L.; Gross, Z. Inorg. Chem. 2004, 43, 6136–6138.
(25) Cai, S.; Licoccia, S.; D’Ottavi, C.; Paolesse, R.; Nardis, S.; Bulach,
V.; Zimmer, B.; Kh. Shokhireva, T.; Walker, F. A. Inorg. Chim. Acta 2002,
339, 171–178.
€
(26) Zakharieva, O.; Schunemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.;
Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636–6648.
(27) Cai, S.; Walker, F. A.; Licoccia, S. Inorg. Chem. 2000, 39, 3466–3478.
(28) Nardis, S.; Paolesse, R.; Licoccia, S.; Fronczek, F. R.; Vicente,
M. G. H.; Shokhireva, T. K.; Cai, S.; Walker, F. A. Inorg. Chem. 2005,
44, 7030–7046.
(29) Walker, F. A.; Licoccia, S.; Paolesse, R. J. Inorg. Biochem. 2006, 100,
810–837.
(30) Steene, E.; Wondimagegn, T.; Ghosh, A. J. Phys. Chem. B 2001, 105,
11406–11413.
(31) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J. S. Org. Process Res. Dev. 2003, 7, 799–812.
for C54H39F10N4OBr H2O: C, 61.90; H, 3.94; N, 5.35. Found: C,
3
62.07; H, 3.91; N, 5.24.
General Synthesis of FeCl Corroles 8. Method 2. A solution
of 4 (0.15 mmol) and FeCl2 (380 mg, 3.00 mmol) in 20 mL of
dimethylformamide (DMF) was degassed with argon for 20 min.

1370 Inorganic Chemistry, Vol. 50, No. 4, 2011
Schwalbe et al.
Subsequent heating to reflux (110-120 ꢀC) for 4 h resulted in a
color change from dark green to dark red. This reaction can also be
conducted in a microwave on a solution of 3 mL of DMF for 45 min
at 120 ꢀC. DMF was removed under vacuum, and the resulting
product was dissolved in 30 mL of DCM. This solution was
washed with water and brine to remove residual DMF, dried over
Na2SO4, and concentrated to dryness. The resulting crude product
was chromatographed (silica, diethylether) to afford the corre-
sponding hangman μ-oxo dimer as a major brown band. The
collected fraction was concentrated to dryness, and DCM (30 mL)
and 7% HCl(aq) (10 mL) were added. The mixture was stirred
vigorously for several hours. The organic phase was washed with
water and brine, and dried over Na2SO4. The resulting solution
was concentrated to dryness, and the resulting product was
suspended in methanol, filtered, and dried in vacuum.
reaction mixture during the synthesis of 7a. Crude 4a (0.15
mmol) and FeCl2 (380 mg, 3.0 mmol) in 3 mL of DMF were
reacted for 1 h at 120 ꢀC in a microwave. DCM (30 mL) was
added, and the solution was washed four times with water to
remove residual DMF. The resulting organic fraction was
stirred vigorously with 30 mL of 1 M NaOH (aq) overnight.
The organic fraction was washed with water and brine and
concentrated to dryness. Subsequent column chromatography
(silica, DCM) afforded μ-oxo dimer 7a as a first major brown
band. A smaller green band appeared behind the first band, and
it was collected separately. Further purification on a second
column (silica, DCM:hexane (2:1)) afforded pure 10. HR(ESI)-
MS (Mþ) (M = C62H65N4O2Br): Calcd for m/z = 977.4364,
found 977.4326. MS(MALDI/TOF) (M): 978.56. UV-vis, nm
(ε ꢀ 10-3 M-1 cm-1): 407 (42), 683 (6.4) in DCM.
X-ray Crystallographic Details. The crystals were mounted on
a Bruker three circle goniometer platform equipped with an APEX
2 detector. A graphite monochromator was employed for wave-
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,
5-bis(tert-butylphenyl)corrolato Iron Chloride (8a). Yield: 99 mg
(63%). HR(ESI)-MS (Mþ) (M = C62H62N4OBrFe): Calcd for
m/z = 1015.3467, found 1015.3442 (major peak; a minor peak was
observed at 1048.37). MS(MALDI/TOF) (Mþ): 1015.37. UV-
vis, nm (ε ꢀ 10-3 M-1 cm-1): 360 (52), 418 (57), 637 (5.1) in DCM.
Anal. Calcd for C62H62N4OBrClFe: C, 70.89; H, 5.95; N, 5.33.
Found: C, 70.64; H, 6.03; N, 5.37.
˚
length selection of the Cu KR radiation (λ = 1.54178 A, isocorrole
˚
10) or Mo KR radiation (λ = 0.71073 A, Corrole(Mes)3FeCl and
8a). The data were processed and refined using the program SAINT
supplied by Siemens Industrial Automation. Structures were solved
by direct methods in SHELXS and refined by standard difference
Fourier techniques in the SHELXTL program suite (6.10 v.,
Sheldrick G. M., and Siemens Industrial Automation, 2000).
Hydrogen atoms bound to carbon were placed in calculated
positions using the standard riding model and refined isotropically.
Hydrogen atoms bound to nitrogen or oxygen were located in the
difference map and refined semifreely; all non-hydrogen atoms were
refined anisotropically. The structure of Corrole(Mes)3FeCl con-
tained solvent molecules which were modeled as two-part disorders.
The 1-2 and 1-3 distances of all disordered parts were restrained
to be similar using the SADI command; the rigid-bond restraints
SIMU and DELU were also used on disordered parts. The crystal
of 10 was found to be partially disordered, and a heavily disordered
solvent molecule was removed using the SQUEEZE operation in
PLATON.32
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,15-
bismesityl-corrolato Iron Chloride (8b). Yield: 106 g (69%).
HR(ESI)-MS (Mþ) (M = C60H58N4OBrFe): Calcd for m/z =
987.3154, found 987.3164 (major peak; a minor peak was
observed at 1022.2866). MS(MALDI/TOF) (MþCl)þ: 1022.19.
UV-vis, nm (ε ꢀ 10-3 M-1 cm-1): 362 (38), 389 (42), 625 (3.6) in
DCM. Anal. Calcd for C60H58N4OBrClFe 1.5H2O: C, 68.67;
3
H, 5.86; N, 5.34. Found: C, 68.87; H, 6.01; N, 5.10.
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,15-
bis(pentafluorophenyl)corrolato Iron Chloride (8c). Yield: 121
mg (72%). HR(ESI)-MS (Mþ) (M = C54H36N4OBrF10Fe):
Calcd for m/z = 1083.1242, found 1083.1272 (major peak; a minor
peak was observed at 1120). MS(MALDI/TOF) (MþCl)þ:
1118.12. UV-vis, nm (ε ꢀ 10-3 M-1 cm-1): 367 (48), 395 (53),
Crystallographic Data for 8a. C62H62N4OBrClFe 2C6H6,
3
623 (4.8) in DCM. Anal. Calcd for C54H36N4OBrClF10Fe H2O:
3
M = 1206.58, green crystals, monoclinic, space group P21/n,
˚
C, 57.09; H, 3.37; N, 4.93. Found: C, 56.93; H, 3.41; N, 4.80.
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,
15-bis(tert-butylphenyl)corrolato Iron-μ-oxo Dimer (7a). Dimer
7a was obtained by stirring a solution of 8a in dichloromethane
vigorously overnight with a 0.1 M aqueous solution of NaOH.
Subsequent extraction with water and brine, followed by drying
the organic phase over NaSO4 and removal of solvent to dryness
yields 7a in almost quantitative yield. HR(ESI)-MS (MHþ)
(M=C124H124N8O3Br2Fe2): Calcd for m/z = 2046.6964, found
2046.6960. MS(MALDI/TOF) (MH)þ: 2046.35. UV-vis, nm (ε ꢀ
10-3 M-1 cm-1): 388 (105), 518 (19) in DCM.
˚
˚
a=15.746(2) A, b=13.8863(19) A, c=29.932(4) A, R=90.00ꢀ,
3
˚
β=99.118(2)ꢀ, γ=90.00ꢀ, V=6462.1(15) A , Z=4, T=100(2) K,
R1=0.0785, wR2=0.1884, GOF=1.052.
Crystallographic Data for 10. C62H65N4O2Br, M = 978.09,
˚
green crystals, triclinic, space group P1, a = 6.10300(10) A, b =
˚
˚
14.2694(3) A, c = 33.1609(6) A, R = 85.023(2)ꢀ, β = 89.002(2)ꢀ,
γ = 85.403(2)ꢀ, V = 2867.55(9) A , Z = 2, T = 100(2) K, R1 =
0.0792, wR2 = 0.1856, GOF = 1.148.
3
˚
Crystallographic Data for Corrole(Mes)3FeCl. C46H41N4ClFe
C6H12 C6H6, M=902.38, green crystals, monoclinic, space group
3
3
˚
˚
˚
P21/n, a = 11.9567(12) A, b = 20.250(2) A, c = 20.050(2) A, R =
90.00ꢀ, β= 99.108(2)ꢀ, γ = 90.00ꢀ, V = 4793.3(8) A , Z = 4, T =
100(2) K, R1=0.0693, wR2=0.1856, GOF=1.034.
10-(4-(5-Carboxy-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,
15-bis(tert-butylphenyl)corrolato Iron Chloride (9a). Method 2 was
followed with 6a as the starting material. Purification with column
chromatography (silica, DCM:EtOAc (2:1)) gave 98 mg (65%) of
9a. HR(ESI)-MS (Mþ) (M = C63H63N4O3Fe): Calcd for m/z =
979.4259, found 979.4240. MS(MALDI/TOF) (Mþ): 979.49
UV-vis, nm (ε ꢀ 10-3 M-1 cm-1): 360 (39.5), 419 (41.5), 638
(3.5) in DCM. Anal. Calcd for C63H63N4O3ClFe: C, 74.51; H, 6.25;
N, 5.52. Found: C, 74.42; H, 6.38; N, 5.48.
3
˚
Cyclic Voltammetry. All cyclic voltammograms (CV) were
performed on DCM solutions containing 0.1 M NBu4PF6
(tetrabutylammonium hexafluorophosphate) and the corrole
compound under argon atmosphere at ambient temperature.
A potentiostat/galvanostat from CH Instruments was used
to record CVs. A three compartment cell was outfitted with a
0.07 cm2 glassy carbon button electrode as the working elec-
trode, a platinum wire as the auxiliary electrode, and Ag/AgCl
as the reference electrode. Reported potentials were referenced
to a ferrocenium/ferrocene (Fcþ/Fc) internal standard. CVs
were collected at scan rates of 10-100 mV/s.
5,10,15-Trismesitylcorrolato Iron Chloride. Method 2 was
followed with 5,10,15-trimesitylcorrole as starting material.
Purification by column chromatography (silica, DCM, DCM:
methanol (24:1)) gave 79 mg (71%) of product. HR(ESI)-MS
(Mþ) (M = C46H41N4Fe): Calcd for m/z = 705.6894, found
705.2561. MS(MALDI/TOF) (MCl): Calcd for C46H41N4FeCl
m/z = 740.2369, found, 740.38, found (Mþ) 705.31. UV-vis,
nm (ε ꢀ 10-3 M-1 cm-1): 361 (35.5), 387 (38), 625 (2.8).
10-(4-(5-Bromo-2,7-di-tert-butyl-9,9-dimethylxanthenyl))-5,15-
bis(tert-butylphenyl)-5-hydroxo Isocorrole (10). 10 was isolated
as a side product in less than 5% yield after work up of a crude
Spectroscopy. Samples for transient emission were contained
within high-vacuum cells consisting of a 2-mm path length clear
fused-quartz cell, which was connected to a 10-cm3 solvent
reservoir via a graded seal. The two chambers were isolated
(32) Sluis, P. v. d.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–201.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1371
Scheme 1
from the environment and from each other by high-vacuum
Teflon valves. An aliquot of 100 μL of sample dissolved in dry
THF was added to the cell. The THF in the reservoir was subject
to three freeze-pump-thaw cycles (10-6 Torr) and transferred
back to the cell by vacuum transfer.
Excitation of the samples was accomplished with the output
of a 45 fs Coherent Libra HE operating at 1 kHz to produce 4 mJ of
800 nm light. Visible wavelengths were generated using a Coherent
OPerA Solo tunable over the entire visible range of the spectrum,
with pulse energies of approximately 20 μJ. Average excitation
power at the sample was attenuated to 5 μJ to avoid photodamage.
Transient emission lifetime kinetics were measured on a Hamamat-
su C4334 Streak Scope streak camera (2D, y-axis is time and the
x-axis is nm). The emission was collected at the magic angle
(θ = 54.7ꢀ) over a 20 to 50 ns time window where the spectral axis
(100-nm range) was centered on the emission peak.
tails of the calculation are provided in the Supporting Information.
The stability of the ground state was tested by time-dependent (TD)
DFT. Electronic charge and spin distributions were monitored with
Mulliken population analysis. The 57Fe isomer shift was calculated
using the calibration given by Vrajmasu et al.;36 electric field
gradient (EFG) parameters ΔEQ and η, as well as the AFC and
ASD of the hyperfine coupling tensor were evaluated with the
properties modules of the Gaussian 09 program.
Catalytic Assay. Hydrogen peroxide dismutation reactions
were performed at room temperature in a sealed (PTFE septum)
20 mL reaction vial equipped with a magnetic stirbar and a capillary
gas delivery tube linked to a graduated buret filled with water. The
reaction vial was charged with 2 mmol of the iron corrole catalyst,
50 mmol of 1,5-dicyclohexylimidazole, 3.0 mL of CH2Cl2, and
1.0 mL of CH3OH. The dicyclohexylimidazole is used as a ligand to
axially coordinate iron opposite the hangman cleft. The solution
was stirred for 5-10 min to ensure gas pressure equilibration. An
aliquot of 10.4 M (30%) aqueous H2O2 (0.2 mL) was added to the
reaction mixture via syringe, and the reaction mixture was stirred
vigorously. The time was set to zero immediately after addition of
H2O2. The O2 evolution reaction was monitored volumetrically,
and the TON for produced O2 (n) was calculated using the ideal gas
equation. The identity of the oxygen gas was confirmed indepen-
dently by gas chromatography.
€
Variable-field, variable-temperature Mossbauer spectra were
recorded in the temperature range from 4.2-180.0 K and
magnetic fields up to 8.0 T using a constant acceleration spectro-
meter fitted with a liquid helium cooled Janis Research Super-
Varitemp cryostat and a superconducting magnet. Spectral
simulations were performed using the WMOSS software
(Edina, MN). The isomer shifts are quoted relative to iron metal
at room temperature.
Density Functional Theory (DFT). Computations were carried
out using the quantum-chemical software package Gaussian 09.33
To investigate the electronic structure of 8a, a simplified structural
model was constructed in which the methyl and 4-tert-butyl side
groups of the xanthene backbone were replaced with hydrogen
atoms and the tert-butyl groups of the 4-tert-butylphenyl-meso
substituents were replaced with methyl groups. Geometry optimi-
zation and single point calculations were performed with the
B3LYP/6-311 g hybrid functional/basis set combination.34,35 De-
Results and Discussion
Synthesis and Characterization. Free base and iron XC
and HCX complexes were prepared by the synthetic strategy
outlined in Scheme 1. These corroles are delivered in a
streamlined condensation reaction that we have previously
reported.17 Briefly, xanthene dibromide (or monobromo-
monomethoxycarbonyl xanthene) was converted into bro-
moxanthene aldehyde 1(respectively 2) by lithiation followed
by reaction with DMF.12 Substituted dipyrromethanes
(33) Gaussian 09, Revision A2; Gaussian, Inc.: Wallingford, CT, 2009; full
citation in the Supporting Information.
(34) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377.
(35) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1998, 37, 785–789.
ꢀ
€
(36) Vrajmas-u, V. V.; Munck, E.; Bominaar, E. L. Inorg. Chem. 2003, 42,
5974–5988.

1372 Inorganic Chemistry, Vol. 50, No. 4, 2011
Schwalbe et al.
(R = 4-tert-butylphenyl (3a), mesityl (3b), and penta-
fluorophenyl (3c) were synthesized by the Lindsey meth-
od of condensing commercially available pyrrole and aryl
aldehydes.31,37 The condensation of 1 or 2 with the cor-
responding aryl dipyrromethane in dichloromethane/tri-
fluoroacetic acid mixture furnishes the xanthene corrole
compounds in about 20% yield (Supporting Information,
Table S1) under optimized conditions, which were ascer-
tained from the 4-tert-butylphenyl system to be 37.5 mM
DPM, 15 mM 1, 0.6 mM TFA, 7 h stirring at room
temperature and quenching with 2,3-dichloro-5,6-dicya-
nobenzoquinone (DDQ entry 7, Supporting Information,
Table S1). We note that concentrations of reactants and
Lewis acid, which are lower than those usually employed
in corrole synthesis, led to improved yields of the
xanthene modified corroles (entries 1-7, Supporting
Information, Table S1).38 In addition, variation of the
reaction conditions by using longer reaction times (entries
8 and 10, Supporting Information, Table S1), hydrochlo-
ric acid in a water-methanol mixture as a solvent (entries
9 and 10, Supporting Information, Table S1), or p-
chloranil as an oxidizing agent (entry 4) did not improve
the yield as has been found for other corrole compounds.39
1H NMR spectra of the free base XC complexes (aryl =
4-tert-butylphenyl (4a), mesityl (4b), and pentafluorophenyl
(4c)) show two sets of signals in the aromatic region between
7 and 9 ppm corresponding to four proton signals assignable
to the xanthene backbone and four signals assignable to the
corrole macrocycle (Supporting Information, Figures S7-
S9). Additional proton signals in the aromatic region appear
for the mesityl substituent (two singlets) and for the 4-tert-
butylphenyl substituent (two multiplets). In the aliphatic
region, signals are clearly distinguished for the two methyl
and two 4-tert-butyl substituents of the xanthene backbone
as well as one additional signal for the 4-tert-butyl groups on
the macrocycle of 4a and three signals for the methyl groups
of 4b. For the latter complex, the mesityl peaks are split
owing to constricted rotation about the Cipso-Cmeso bond of
the aryl groups and lowered symmetry above and below the
corrole plane that is imposed by the xanthene scaffold.
Pyrrolic (3H) and carboxylic acid (1H) protons were not
observed in 1H NMR spectra recorded at room temperature.
Iron insertion was achieved by refluxing the free base 4
or 6a with an excess of iron dichloride in DMF as previously
described.22 The iron chloride complexes (R = 4-tert-butyl-
phenyl (8a, 9a), mesityl (8b), and pentafluorophenyl (8c))
were isolated in 60% yield after purification by column
chromatography. The presence of the paramagnetic iron
Figure 1. X-ray crystal structure of 8a, with thermal ellipsoid plots
shown at 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond distances (A) and angles (deg): Fe-N1 1.921, Fe-N2
1.901, Fe-N3 1.903, Fe-N4 1.928, Fe-Cl 2.245, N1-Fe-N3 153.6,
N4-Fe-N2 154.2, N2-Fe-Cl 103.7, N4-Fe-Cl 101.5.
˚
Figure 2. X-ray crystal structure of Corrole(Mes)3FeCl, with thermal
ellipsoid plots shown at 50% probability level. Hydrogen atoms are
omitted for clarity.
and Corrole(Mes)3FeCl are shown in Figures 1 and 2,
respectively. Both compounds show similar structural fea-
tures to known iron chloride corrole complexes.28,41 The
five coordinate iron center is considerably displaced above
the N4-plane of the macrocycle, which has a slight saddle
form (Figures 1 and 2). The distance of the Fe atom from
the best-fit N4-plane is virtually identical in both structures:
1
center led to interpretable but very complex H NMR
spectra with signals in the -30 to þ100 ppm region. The
structure of the iron corrole complexes was, however,
revealed by X-ray crystal analysis.
˚
˚
0.4213(17) A for Corrole(Mes)3FeCl and 0.4181(21) A for
8a. The Fe-N distances for the two structures are nearly
identical, ranging from 1.892(3) to 1.929(4) A. These
Compounds Corrole(Mes)3FeCl40 and Corrole(C6F5)3-
FeCl22 were prepared as reference systems to elucidate the
influence of the xanthene backbone. Crystals of Corrole-
(Mes)3FeCl were grown from a methanol/dichloromethane
solution. Crystals of 8a were obtained by slow evaporation
of a benzene solution of the complex. The structures of 8a
˚
metrics are in accord with those observed for [Fe(tpfc)Cl]
(tpfc = tris(pentafluorophenyl)corrole, Corrole(C6F5)3FeCl)
and [Fe(oec)Cl] with oec = octaethylcorrole,21 which
have nearly identical Fe-N distances and similar puck-
˚
ering of the Fe atom out of the N4-plane (0.367 A). In the
solid-state structure of 8a, the xanthene backbone stands
almost perpendicular to the macrocyclic plane, while the
(37) Dogutan, D. K.; Zaidi, H. S. H.; Thamyongkit, P.; Lindsey, J. S.
J. Org. Chem. 2007, 72, 7701–7714.
(38) Gryko, D. T.; Jadach, K. J. Org. Chem. 2001, 66, 4267–4375.
(39) Koszarna, B.; Gryko, D. T. J. Org. Chem. 2006, 71, 3707–3717.
(40) Gryko, D. T.; Koszarna, B. Org. Biomol. Chem. 2003, 1, 350–357.
(41) 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, 731–735.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1373
Figure 3. X-ray crystal structure of 10, with thermal ellipsoid plots
shown at 50% probability level. Hydrogen atoms bonded to carbon are
omitted for clarity.
aryl substituents are twisted about 60ꢀ out of the macro-
cyclic plane. Excepting the axial chloride found opposite to
the bromide of the xanthene scaffold, the overall hangman
scaffold of 8a and 9a is similar to that for the hangman
corrole framework, which has recently been published.17
No cations or anions are present in either crystal structure;
this observation is consistent with a formal Fe(IV) center.
MALDI-TOF-MS indicated the presence of a μ-oxo
intermediate 7, which was not typically isolated. This
compound was transformed directly into 8 by treating a
DCM solution of 7 with aqueous HCl. Only for the 4-tert-
butylphenyl substituted XC dimer 7a was the μ-oxo dimer
isolated and characterized by standard analytical techni-
ques. The NMR spectrum of 7a shows signals in the ex-
pected range of 0 to 10 ppm in contrast to Fe(IV)Cl corrole
compounds such as 8a and 9a, which show markedly
shifted NMR signals (-45 to 30 ppm). A second corrole
10 could also be isolated from treating crude 8a with
sodium hydroxide. The crystal structure of 10 shown in
Figure 3 reveals that OH attacks the carbon at the meso
position of the corrole. Isocorrole formation from hydrox-
ide42 and alkoxide attack43,44 is known to occur from the
oxidized corrole macrocycle. This reaction sequence may
also be envisioned for the formation of 10. The resulting
isocorrole framework exhibits similar metrics to that pro-
ducedfrom anattempted demetalationof a corrolatosilver
complex.42 The addition of the hydroxy group breaks the
symmetry of the corrole macrocycle; the symmetry low-
Figure 4. Electronic absorptionspectra of the (a) iron XC(8a(solid line,
black), 8b (dashed line, blue), 8c (dotted line, red) and (b) the 3H XC (4a
(solid line, black), 4b (dashed line, blue), 4c (dotted line, red)) series in
DCM. (c) Emission spectra of the 3H XC (4a (solid line, black), 4b
(dashed line, blue), 4c (dotted line, red)) series in DCM.
longer maintained. Moreover, two single resonances are
observed for the pyrrole N-H protons at 16.0 and 16.2
ppm and a distinct singlet for the O-H proton at 2.6 ppm.
This observation contrasts that of the pyrrolic N-H
protons of the parent complex 4, which are not detected
by 1H NMR owing to their delocalization about the
corrole core under conditions of fast exchange. As shown
in Supporting Information, Figure S3 the localized pyrrole
protons in 10 establish an extended array in the solid state
structure via a hydrogen bond network.
Spectroscopy. The UV-vis absorption spectra of the
corroles offer an interesting comparison to their porphyrin
relatives. Figure 4a shows the absorption spectra of the iron
XC series of compounds. The Soret band, centered about
420 nm, is split and has a considerably attenuated extinction
coefficient when compared to those of analogous porphyr-
ins (ca. 200,000 M-1 cm-1). This splitting of the Soret band
is attributed to the lower symmetry of the corrole macro-
cycle ring.20,45 The λabs,max of the series (see Table 1) follows
the trend 8b ∼ 8c >8a. This trend stems from the twisting of
the aryl substituents out of the macrocyclic plane as a result
of steric demands exerted by the mesityl and pentafluo-
rophenyl substituents in meso positions. As shown in
1
ering is clearly observed in the H NMR spectrum. All
β-pyrrolic protons are inequivalent, and each gives rise to
1
two sets of distinguishable H resonances: four doublets
and four doublet of doublets (4J(N,H) = 2 Hz, see Sup-
porting Information, Figure S10) appear in the region
between 6 and 7 ppm, indicating that aromaticity is not
(42) Stefanelli, M.; Shen, J.; Zhu, W. H.; Mastroianni, M.; Mandoj, F.;
Nardis, S.; Ou, Z. P.; Kadish, K. M.; Fronczek, F. R.; Smith, K. M.;
Paolesse, R. Inorg. Chem. 2009, 48, 6879–6887.
(43) Nardis, S.; Pomarico, G.; Fronczek, F. R.; Vicente, M. G. H.;
Paolesse, R. Tetrahedron Lett. 2007, 48, 8643–8646.
(44) Pomarico, G.; Xiao, X. A.; Nardis, S.; Paolesse, R.; Fronczek, F. R.;
Smith, K. M.; Fang, Y. Y.; Ou, Z. P.; Kadish, K. M. Inorg. Chem. 2010, 49,
5766–5774.
(45) Shen, J.; El Ojaimi, M.; Chkounda, M.; Gros, C. P.; Barbe, J. M.;
Shao, J.; Guilard, R.; Kadish, K. M. Inorg. Chem. 2008, 47, 7717–7727.

1374 Inorganic Chemistry, Vol. 50, No. 4, 2011
Schwalbe et al.
Table 1. Summary of the Photophysical Properties of Free Base and Iron XC Compounds
λ
em,max/nmb
a
cmpd
λ
abs,max/nm (ε/103 M-1 cm-1
)
CH2Cl2
THF
τ/nmc
4a
4b
4c
7a
8a
8b
8c
9a
421 (132), 568 (17.0), 617 (15.0), 650 (14.0)
409 (125), 424 (109), 567 (14.0), 603 (11.0), 634 (7.0)
414 (134), 560 (21.0), 612 (12.0), 630 (8.6)
388 (105), 518 (19.0)
360 (52.0), 418 (57.0), 637 (5.1)
362 (38.0), 389 (42.0), 625 (3.6)
669
646
651
683
657
665
4.7
5.2
4.1
367 (48.0), 395 (53.0), 623 (4.8)
360 (39.0), 419 (42.0), 636 (3.5)
a Absorption maxima in dichloromethane at room temperature. b Emission maxima in indicated solvent at room temperature. c Emission lifetime of XC
compound in THF that had been subject to freeze-pump thaw cycles to remove oxygen; measurements were performed at room temperature (λexc = 400 nm).
€
porphyrins, the twisting of the rings attenuates electronic
coupling between the aromatic meso substituents and the
macrocycle ring.13,22,46,47 With the absence of ortho substit-
uents on the 4-tert-butylphenyl ring of 8a, this steric hin-
drance is removed, and electronic delocalization onto the
aromatic rings from the corrole is possible, thus resulting in
red-shifts of the Soret band. Spectral differences among the
series in the Q-band region are less pronounced. We note
that the Soret and the Q bands of the iron corroles (see
Figure 4a for 8 and Supporting Information, Figure S1 for
7a, respectively) exhibit a strong hypsochromic shift as well
as a broadening and a lower absorption cross-section of the
Soret band when compared to that of the free base corrole
(Figure 4b), though hydroxylation also leads to attenuation
of the Soret band (Supporting Information, Figure S2).
Moreover, the electronic properties of the metalated and
free base systems differ in one other important aspect. The
absence of low-lying dd states in free base XC compounds
removes efficient non-radiative decay pathways from the
system; consequently, the free base XC corroles are lumi-
nescent (Figure 4c). The λem,max of the series (Table 1)
follows the same trend as that of the absorption spectra,
4b ∼ 4c > 4a. The emission spectra exhibit a noticeable red-
shift of about 15 nm for the compounds dissolved in THF
(see Table 1). The emission lifetime of the free base XC is
about 4 ns in THF at room temperature under oxygen free
conditions (see Table 1).
Table 2. Zero Field Mossbauer Parameters of Complexes 7a, 8a-c, 9a and
Related FeCl Corrole Compoundsa
compound
T/K
δ/mm s-1
ΔEQ/mm s-1
8a
8b
8c
9a
4.2
4.2
4.2
4.2
80
80
4.2
77
0.19
0.18
0.19
0.20
0.18
0.19
0.21
0.19
0.04
0.02
2.91
2.84
2.95
2.97
2.94
2.88
3.02
2.99
2.24
2.35
[FeCl(tpfc)] b
[FeCl(tdcc)] b
[FeCl(7,13-Me2Et6corr)] c
[FeCl(OECorr)] d
7a
4.2
77
[(OECorr)Fe]2(μ-O) d
a Tabulated values are limited to iron-corrole complexes that have
either chloride or bridging oxo axial ligands. b Data taken from ref 20.
c Data taken from ref 26. d Data taken from ref 41.
formulations give triplet ground states that are well isolated
from excited states, and no definitive conclusion can be
drawn about the nature of the ground state on the basis of
spin quantum number alone.
€
Mossbauer spectra recorded at 4.2 K in zero applied
magnetic field for 7a, 8a-c and 9a consist of similar
quadrupole doublets, whose isomer shifts, δ, and quadru-
pole splittings, ΔEQ, are listed in Table 2. Weak magnetic
fields (<0.1 T) have little effect on the spectra, which are
consistent with zero or integer spin of systems with an even
number of electrons. Variable-field, variable-temperature
€
Mossbauer spectroscopy analysis was carried out for 8a. A
An authentic assignment of the oxidation state of the
metal center is confounded by the redox non-innocence of the
ligand. The formal Fe(IV)Cl formulation engenders a
S = 1 triplet ground state. But the same spin multiplicity
may be obtained for a Fe(III) corrole-radical formulation
[FeIIICl(corroleþ•)], where the ferric ion has an intermediate
series of spectra were recorded at temperatures between 4.2
and 150 K in variable fields ranging from 0.0 to 8.0 T. These
spectra were analyzed in the framework of an S = 1 spin
Hamiltonian,
ꢀ
ꢁ
3
2
2
2
2
E
^
D
spin configuration SFe = /2 that is antiferromagnetically
coupled to the Scorroleþ• = /2 spin of the oxidized corrole
^
^
^
^
^
^
1
H ¼ D Sz - þ ðSx - SyÞ þ βS g B þ S A I
3 3
3
3
3
ligand. Direct exchange interactions between metal ions and
ligand spins may be quite strong, typically in the range of
hundreds if not thousands of wavenumbers.48 DFT estimates
for the magnitude of the putative SFe =3/2, Scorroleþ• =1/2 ex-
change interaction J = þ1328 cm-1 for [FeCl(tpfc)]. Mag-
netic susceptibility analysis yields the value J = þ544 cm-1
for [FeCl(tpfc)] and J ∼ þ350 cm-1 for [FeCl(7,13-Me2Et6-
^
^
þ βngnI B þ HQ
3
ꢀ
ꢁ
0
eQVz z
0
2
2
2
15
^
HQ
^
^
^
¼
3Iz0 - þ ηðIx0 þ Iy0 Þ
12
4
corr)], using the JS1 S2 convention.20,26 Therefore, both
3
rffiffiffiffiffiffiffiffiffiffiffiffi
η2
0
eQVz z
0
ΔEQ
¼
1 þ
(46) Palmer, J. H.; Mahammed, A.; Lancaster, K. M.; Gross, Z.; Gray,
H. B. Inorg. Chem. 2009, 48, 9308–9315.
2
3
(47) Eikey, R. A.; Khan, S. I.; Abu-Omar, M. M. Angew. Chem., Int. Ed.
2002, 41, 3592–3595.
The choice of a spin state S = 1 is justified both by the
room temperature magnetic moment of μeff = 2.83 μB, as
(48) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.;
€
Munck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181–10192.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1375
Figure 6. Spin-density plot calculated for the ground state of 8a.
is a rather soft parameter. Fitting of the set of variable-field
spectra yields the ZFS parameters D = þ16.5 ( 0.5 cm-1
and E/D=0.12 ( 0.02, and the hyperfine coupling constants
Ax=-24.0 ( 0.2 T, Ay=-26.7 ( 0.2 T, and Az=þ1.0 (
2.0 T. While rather satisfactory simulations are obtained
with EFG, A, and ZFS tensors of which the principal axes
are collinear, further improvement in the fit is achieved
by rotating the EFG tensor about the x-axis of ZFS with
βEFG = 18ꢀ (see Supporting Information, Table S5), in
particular in the representation of the low energy feature in
the 7.0-8.0 T, 4.2 K spectra. We note that at 4.2 K and fields
up to 8.0 T, the combined Boltzmann population of the
excited doublet is less than 0.05, consequently the spectra
calculated in fast or slow relaxation regime are identical.
€
Figure 5. Variable-field, variable-temperature Mossbauer spectra of 8a
recorded for a 57Fe enriched, solid state sample. The applied field is aligned
parallel to the incident γ beam. The solid lines are best fits obtained using an
S = 1 spin Hamiltonian with D=16.5 cm-1, E/D=0.12, gx = gy = gz =
2.00 δ=0.193 mm/s, ΔEQ=2.911 mm/s, η = 0.0, Ax=-24.0 T, Ay=-26.7
T, Az=þ1.0 T and a rotated EFG tensor such that REFG=γEFG=0.0ꢀ,
€
Mossbauer spectra recorded for 7a (Supporting Infor-
βEFG=18.0ꢀ.
mation, Figure S5) indicate that the bridging oxo ligand
mediates a strong antiferromagnetic coupling between the
two Slocal=1 local iron sites to yield a Stotal=0 ground state.
Up to temperatures of 120 K, no evidence for the popula-
tion of an excited state is observed. The isomer shift δ =
0.04 mm/s of 7a is lower than for the chloro iron corrolato
complexes, indicating a higher Fe(IV) character in the former
species. Complex 7a has a positive ΔEQ =þ2.24 mm/s and a
small η ≈ 0.2, values similar to those for 8a.
determined by NMR using Evans method,49 and by the
€
similarity of the zero field Mossbauer parameters with
those reported for other FeCl corrole complexes, which
have been shown to have a S = 1 ground state.20,26,41
The hyperfine splitting observed in the 8.0 T, 150.0 K
spectrum is dominated by the nuclear Zeeman and quad-
rupole interactions. The magnetic hyperfine interactions
have a minor effect on the spectra because the thermal
spin expectation value, ÆSæth ∼ 1/T, and the associated
The range of isomer shifts obtained for FeCl corrole
complexes overlaps both with those reported for authen-
tic SFe = 1 Fe(IV) species and of low-spin ferric com-
pounds.50-54 Therefore, to provide further aid in inter-
internal field, Bint = A ÆSæth/gnβn, are small at this tempera-
3
ture. Under these conditions, the sign of ΔEQ is unambigu-
ously determined and found to be positive. Moreover, simu-
lations of the 8.0 T, 100.0-150.0 K spectra yield an asym-
metry parameter η ≈ 0, implying that the electric field
gradient (EFG) tensor is axial. The internal field of the 4.2
K spectra does not saturate even in strong applied fields up to
8.0 T. This behavior is generally found when the zero field
splitting (ZFS) is large and positive and, in the present case,
implies that the lowest sublevel of the spin multiplet is |S=1,
MS = 0æ. The internal field results from mixing of the excited
|S=1, MS =(1æmagnetic doublet into the ground sublevel
by the applied field. These qualitative observations are
supported by the spectral simulations shown in Figure 5.
We find a hyperfine coupling tensor A that is quasi-axial with
|Ax| ∼ |Ay| . |Az|. At 4.2 K, the simulated splitting and
spectral pattern are strongly correlated with Ax,y, whereas Az
€
preting Mossbauer data, the electronic ground state of 8a
was examined with spin-unrestricted DFT calculations.
The spin density plot calculated for the S = 1 ground
state of the geometry-optimized model (Figure 6) reveals
that the DFT solution is best described as a broken
symmetry (BS) state with the majority spin (R, spin-up)
localized on the [FeCl] moiety and the minority spin
(50) Collins, T. J.; Fox, B. G.; Hu, Z. G.; Kostka, K. L.; Munck, E.;
Rickard, C. E. F.; Wright, L. J. J. Am. Chem. Soc. 1992, 114, 8724–8725.
(51) Rohde, J. U.; In, J. H.; Lim, M. H.; Brennessel, W. W.; Bukowski,
€
M. R.; Stubna, A.; Munck, E.; Nam, W.; Que, L. Science 2003, 299, 1037–
1039.
(52) Justel, T.; Muller, M.; Weyhermuller, T.; Kressl, C.; Bill, E.;
Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber, B.;
Wieghardt, K. Chem.;Eur. J. 1999, 5, 793–810.
(53) Kostka, K. L.; Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard,
€
C. E. F.; Wright, L. J.; Munck, E. J. Am. Chem. Soc. 1993, 115, 6746–6757.
(49) Garland, C. W.; Nibler, J. W. Shoemaker, D. P. Experiments in
Physical Chemistry, 8th ed.; McGraw-Hill: New York: 2003, pp 371-379.
(54) Keutel, H.; Kapplinger, I.; Jager, E. G.; Grodzicki, M.; Schunemann,
V.; Trautwein, A. X. Inorg. Chem. 1999, 38, 2320–2327.

1376 Inorganic Chemistry, Vol. 50, No. 4, 2011
Schwalbe et al.
Table 3. Electrochemical Summary of Fe Corrole Complexesa
compound
7a b
8a
8b
8c
FeCl Cor-Mes3
FeCl Cor-(C6F5)3
9a
E1/Vb
E2/Vb
0.46 Vc
0.51 V
0.54 V
0.73 V
0.54 V
0.88 V
0.53 V
-0.91 V
-0.51 V
-0.56 V
-0.26 V
-0.59 V
-0.07 V
-0.44 V
a Potential vs Fcþ/Fc in DCM and 0.1 M TBAPF6. b E2 is assigned to
the reduction of the corrole cation, and E1 is ascribed to metal reduction.
c An additional redox event E20 = 0.02 V falls between E1 and E2 that is
consistent with a second metal based redox process in the dimer, see
Supporting Information, Figure S4.
Figure 7. Cyclic voltammogram of 8a in CH2Cl2 with 0.1 M TBAPF6 as
the supporting electrolyte. The reference potential is ferrocenium/ferro-
cene. The scan was initiated at the crosshair, and the potential was
Our results agree well with published DFT,20,26 and ab
initio59 studies of FeCl corrole complexes, which also are
best described as intermediate-spin Fe(III) corrole radical
cation systems.
scanned anodically at a rate of 0.1 V s-1
.
(β, spin-down) delocalized on the corrole ring. Analysis of
the gross orbital population and Mulliken spin density
of þ2.63 indicate an intermediate-spin d5 configuration
for the iron site, namely, |(x2-y2)2(xz)R(yz)R(z2)R|. The
corrole ligand is one-electron oxidized and has an un-
paired β-electron with a cumulated spin-density of -0.79;
this β-electron occupies an extended molecular orbital
with large contributions from the pz atomic orbitals of the
pyrrole N and of the meso C atoms (see Supporting
Information, Table S4). This electronic structure is con-
firmed by TD-DFT calculations which yield an energy
gap of ≈8000 cm-1 between the highest occupied dβ
orbital (x2-y2) and the closely spaced {yz/xz} β-unoccu-
pied orbital doublet; the highest occupied πβ corrole
orbital is ∼7000 cm-1 below (x2-y2).
Oxidation-Reduction Chemistry. Electrochemical inves-
tigations clearly show the influence of aryl meso-substitution
on the reduction potentials of XC compounds. The cyclic
voltammogram (CV) of 8a that is shown in Figure 7 is typical
of the iron corrole XC series and the A3 corrole complexes;
Table 3 lists the measured redox potentials for all complexes
examined in this study. The initial scan begins anodically
from a potential where [Fe(III)-corroleþ•] is the electroactive
species. On this basis, it is logical thatthefirstwave(scanning
anodically) corresponds to the corrole-based reduction
potential and the second wave corresponds to the metal based
reduction process. Two reversible redox events are observed.
These results are in accordance with the electrochemistry of
unmodified corroles for which the low-potential CV wave
has been assigned to a Fe(III)/“Fe(IV)” couple and the high
potential CV wave has been assigned to the corroleþ•/corrole
couple.60 Whereas 4-tert-butylphenyl and mesityl meso-
substituted corroles exhibit similar reduction potentials,
the introduction of the strongly electron withdrawing penta-
fluorophenyl substituent shifts the potential range by 0.2-
0.3 V to more positive potentials. The μ-oxo dimer 7a shows
three reversible CV waves (Supporting Information, Figure
S4), which have been assigned to one corrole based oxidation
at higher potentials and two Fe(III)/“Fe(IV)” processes at
lower potential.60 Alternatively, owing to the strong coupling
of Fe centers across the oxo bridge, one metal based and two
ligand based redox events is also plausible with the second
metal based reduction shifted considerably anodic owing to
strong coupling.
The predicted isomer shift δcalc = 0.18 mm/s is in good
agreement with the experimentally observed value δexp
0.19 mm/s. The estimated EFG tensor is quasi-axial, ηcalc
=
=
0.05, with the large, positive component eQVzz/2 =
þ3.28 mm/s oriented along the Fe-Cl vector and the
smaller, negative components being roughly aligned with
the NFeN medians (see Supporting Information, Table S3).
Predicted isomer shifts have an uncertainty of 0.1 mm/s and
quadrupole splitting of 0.5 mm/s.55-58 Qualitative agree-
ment is obtained for the calculated hyperfine coupling
tensor A59 (Supporting Information, Table S6). The A-ten-
sor is the sum of the isotropic Fermi-contact term AFC, the
traceless spin-dipolar tensor ASD and the orbital tensor AL.
The large energy gap between the occupied and unoccupied
dβ orbitals suggests that the orbital component AL to the
hyperfine tensor is small and that the magnitude of the ZFS
is mainly determined by the interaction of the ground orbital
state with excited orbital states of different spin multipli-
cities. The calculated ASD is nearly axial and collinear to the
EFG tensor such that their largest, positive components are
aligned. Along this direction, ASDz overcomes the dominant
(negative) contact contribution, AFC, to yield an overall
hyperfine coupling tensor A that has a small and positive
component along the Fe-Cl vector and two large negative
components parallel to the plane of the corrole ligand.
The redox chemistry of Hangman platforms may be
manifested in the activation of small molecules by proton-
coupled electron transfer (PCET).1,2,61 The disproportio-
nation of H2O2 to O2 and H2O is one important PCET
process that is catalyzed by a wide variety of metallopro-
teins and enzymes.62 As shown in Table 4 and shown in
Supporting Information, Figure S21, the iron XC and
HCX complexes catalyze this dismutation reaction. The
time course of oxygen production (Supporting Information,
Figure S21) is similar for each of the corroles: the reaction
(55) Neese, F. Inorg. Chim. Acta 2002, 337, 181–192.
(56) Godbout, N.; Havlin, R.; Salzmann, R.; Debrunner, P. G.; Oldfield,
E. J. Phys. Chem. A 1998, 102, 2342–2350.
(57) Neese, F. J. Phys. Chem. 2003, 118, 3939–3948.
(58) Sinnecker, S.; Neese, F.; Noodleman, L.; Lubitz, W. J. Am. Chem.
Soc. 2004, 126, 2613–2622.
(60) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Chem.;
Eur. J. 2001, 7, 1041–1055.
(61) Rosenthal, J.; Chng, L. L.; Fried, S. D.; Nocera, D. G. Chem.
Commun 2007, 2642–2644.
(59) Roos, B. O.; Veryazov, V.; Conradie, J.; Taylor, P. R.; Ghosh, A.
J. Phys. Chem. B. 2008, 112, 14099–14102.
(62) Nicholls, P.; Fita, I.; Loewen, P. C. Adv. Inorg. Chem. 2001, 51, 51–
106.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1377
Table 4. Catalytic Data for H2O2 Disproportionation of XC and HCX Iron
reduction of a high valent state, and hence in principle a more
oxidizing center should be more adept at dismutation. How-
ever, Gross and co-workers have recently shown that Fe
corroles are potent dismutation catalysts beginning from the
þ3 oxidation state of the metal. The key intermediate in
dismutation in Gross’s system will therefore likely access either
[Fe(IV)-corroleþ•] or [Fe(V)-corrole], with the former more
likely. Conversely, in the study here, 8 and 9 are one-electron
oxidized above Gross’ system. Hence, dismutation will access
[Fe(V)-corroleþ•]. While such an intermediate is expected to be
more potent for dismutation, comparison of the far greater
activity of Gross’ system to that reported here suggests that
formation of the higher oxidation state is more difficult to
achieve, and a thermodynamic barrier is imposed on the
dismutation. Moreover, the TON number in Gross’ system is
limited by Fe-O-Fe formation, but the corrole remains
intact; thus, higher TON can be achieved by averting dimer-
ization. In contrast, the systems reported here show degrada-
tion of the corrole framework. This result suggests that a more
potent oxidizing intermediate such as [Fe(V)-corroleþ•] is more
susceptible to oxidative degradation of the macrocycle. This is
commonly observed in porphyrin chemistry where the meso
position is susceptible of oxidation.66-68 These results taken
together suggest that [M(III)-corrole], and not [M(III)-
corroleþ•] (or [M(IV) corrole]), is the proper precatalyst state
needed for effective dismutation by corrole complexes.
Corroles and Related Porphyrin Analoguesa
compound
TOF/min-1 b
TON (% conversion) c
FeCl(TMP)d
FeCl-Cor(C6F5)3
FeCl-Cor(Mes)3
FeCl-HPX-Mesd
0.2
79
15
102
15
21
30
16
27
162
173
95
533
91
96
99
70
147
8a
8b
8c
9a
d
FeCl-HPX-C6F5
a In solutions of dichloromethane/methanol (3:1), for further details
see the Supporting Information. b TOF recorded over initial 1 min
reaction time. c After 60 min reaction time; significant decomposition
of corrole over 60 min. d Data taken from ref 61, TMP = tetramesi-
tylporphyrin, Mes = mesityl.
proceeds quickly during the initial stages of the reaction
and over time the total amount of oxygen produced
approaches an asymptotic limit. Oxygen cessation is
likely a result of catalyst decomposition. Analysis of the
reaction mixture by UV-vis and MS shows no sign of the
formation of dimeric species 7 such as has been recently
observed for water-soluble Fe corroles63 or starting com-
pound. Indeed, we cannot find any higher molar mass
fragment and the color of the solution is completely
bleached. We suspect the decomposition might go via
an intermediate like 10. As indicated by 10, the meso
positions of the corrole macrocycle are susceptible to
attack by oxygen species; this type of decomposition
pathway is prevalent for N4 macrocycles that access high
oxidation states.42-44
The similar activities of 8a to 9a indicate that the
hanging group has little effect of PCET reactivity. This
result contrasts Hangman porphyrins and salens in which
the carboxylic hanging group proved to be a prerequisite
for high dismutation activity. We believe that the in-
stability of the corrole may overwhelm the benefits of
the Hangman effect on promoting dismutation activity,
though this contention demands greater scrutiny.
Finally, thehanging group has been shown to be important
for preorganizing the peroxide within the metal-oxo hanging
cleft.6 The same may be true here for the corroles, but the
thermodynamic barriers and macrocycle instability for a
[Fe(III)-corroleþ•] precursor appear to subvert the benefits
afforded by the hangman effect. These results suggest that the
hangman effect will be more fully realized for [M(III)-
corrole] systems.
Acknowledgment. M.S. thanks the German Academic
Exchange Service (DAAD) for fellowship support within
the Postdoc-Programme. The developments of new syn-
thetic methods and synthesis of the new compounds were
performed under the sole sponsorship of Eni S.p.A under
the Eni-MIT Alliance Solar Frontiers Program, and charac-
terization work was executed under the Division of Chemi-
cal Sciences, Geosciences, and Biosciences, Office of Basic
Concluding Remarks
Hangman iron corroles are synthesized in good yields from
a one-pot condensation of dipyrromethane with the aldehyde
of a xanthene spacer followed by metal insertion using micro-
Energy Sciences of the U.S. Department of Energy through
€
recorded at Carnegie Mellon University, in Prof. Eckard
57
Grant DE-FG02-05ER15745. Fe Mossbauer spectra were
€
wave irradiation. Mossbauer spectroscopy as well as DFT
€
Munck’s laboratory; this research has been supported by the
calculations of the iron corroles axially ligated by chloride are
consistent with the description of a [Fe(III)Cl-corroleþ•] in
which the corrole ligand is redox non-innocent. The iron
corrole complexes promote the catalytic dismutation of hydro-
gen peroxide. The peroxide dismutation reactivity reported
here adds to the emerging theme that corroles are good at
catalyzing two-electron activation of the O-O bond by dis-
mutation in a variety of substrates including superoxide64 and
peroxynitrite.65 The rate determining step in dismutation
reactions is often the oxidation of peroxide by a metal-oxo
center, two electrons above the initial state of the macrocycle
complex.6 In this case, the reaction is facilitated by the ease of
U.S. NIH (EB001475 to E.M.). S.A.S. wishes to thank Prof.
Emile Bominaar for useful discussions. We thank Cassandra
Cox and Andrew Horning for lifetime measurements. T.S.
T. acknowledges the Fannie and John Hertz Foundation for
a graduate research fellowship.
Supporting Information Available: Full experimental details
for synthesis, NMR, MALDI-MS and high resolution ESI-MS
of corrole compounds, crystallographic data, electronic spectra,
CVs, computational details, and additional Mossbauer spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
€
(63) Mahammed, A.; Gross, Z. Chem. Commun. 2010, 46, 7040–7042.
(64) Eckshtain, M.; Zilbermann, I.; Mahammed, A.; Saltsman, I.; Okun,
Z.; Maimon, E.; Cohen, H.; Meyerstein, D.; Gross, Z. Dalton Trans. 2009,
7879–7882.
(66) Bhuyan, J.; Sarkar, S. Chem.;Eur. J. 2010, 16, 10649–10652.
(67) Balch, A. L.; Noll, B. C.; Zovinka, E. P. J. Am. Chem. Soc. 1992, 114,
3380–3385.
(68) Smith, K. M.; Brown, S. B.; Troxler, R. F.; Lai, J.-J. Tetrahedron
Lett. 1980, 21, 2763–2766.
(65) Mahammed, A.; Gross, Z. Angew. Chem., Int. Ed. 2006, 45, 6544–6547.