Multielectron redox chemistry of iron porphyrinogens

Article abstract of DOI:10.1021/ja043132r

Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E_1/2 = -0.57 V vs NHE in CH₃CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E_p = +0.77 V vs NHE in CH₃CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C^α-C ^α bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Moessbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker σ-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.

Full text of DOI:10.1021/ja043132r

Published on Web 03/12/2005
Multielectron Redox Chemistry of Iron Porphyrinogens
Julien Bachmann and Daniel G. Nocera*
Contribution from the Department of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge Massachusetts 02139-4207
Received November 15, 2004; E-mail: nocera@mit.edu
Abstract: Iron octamethylporphyrinogens were prepared and structurally characterized in three different
oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions.
Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined.
The iron center is easily oxidized by a single electron (E1/2 ) -0.57 V vs NHE in CH3CN) when confined
within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single
four-electron step (Ep ) +0.77 V vs NHE in CH3CN); one of the ligand-based electrons is intercepted for
the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation
equivalents in the macrocycle are stored in CR-CR bonds of spirocyclopropane rings, formed between
adjacent pyrroles. EPR, magnetic and Mo¨ssbauer measurements, and DFT computations of the redox
states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin
states, whereas the fully oxidized ligand possesses a weaker σ-donor framework, giving rise to high-spin
iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders
a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.
Introduction
the redox properties of iron porphyrins have been investigated
intensely as isolated cofactors,27,28 in the environments of de
Iron porphyrin, or heme, is a ubiquitous and versatile protein
cofactor. By controlling the heme axial coordination and peptidic
environment, proteins may tune cofactor reactivity to selectively
perform redox-derived functions as varied as single-electron
transfer (cytochromes),1-13 dioxygen coordination and transport
(hemoglobin and myoglobin),14-17 dioxygen activation and
substrate oxidation (cytochrome P-450),18-23 and peroxide and
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novo designed29-37 and natural proteins,38-51 and in the context
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Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
Chart 1
exception of Compound I, which possesses an iron-oxo whose
oxidizing power is augmented by a single oxidizing equivalent
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A R T I C L E S
Bachmann and Nocera
step to deliver porphyrin from porphyrinogen.73-75 Lying at the
fully oxidized terminus of the red branch, the annular ring
system of porphyrin displays the aforementioned restricted
metal-based redox activity. Two other lines of chemistry evolve
from the common porphyrinogen origin as a result of tetrapy-
rrole-based redox and acid-base transformations. As elaborated
by Floriani and Gambarotta using group I, II, IV, and f-block
ions,76-83 the green branch results from Lewis acid-base
properties of the fully reduced tetrapyrrole macrocycle, which
increases its Lewis basicity from η1 to η3 and η5 by tilting
pyrroles inward toward the core. The blue branch arises from
two-electron oxidation of porphyrinogen to yield the so-called
“artificial porphyrins”.84-89 As we have revealed for porphy-
rinogen containing the element zinc,90 the tetrapyrrole macro-
cycle undergoes discrete two-electron oxidations with accom-
panying formation of one and two spirocyclopropane function-
alities.
organic redox cooperativity engenders a multielectron chemistry
of the “artificial hemes” that is unavailable to native heme
cofactors.
Experimental Section
Materials. All compounds were handled, reactions were performed,
and analytical samples were prepared in inert atmosphere using standard
Schlenk, drybox, and vacuum-line techniques. Solvents were purchased
from VWR Scientific Products and purified using a Braun solvent
purification system or using standard solvent purification techniques.91
Deuterated solvents were purchased from Cambridge Isotope Labora-
tories, degassed, dried, and distilled by procedures similar to those used
for nonisotopically enriched solvents. Ferrocenium tetrafluoroborate
(FcBF4) was purified as previously described.90 Other reagents were
purchased from Aldrich and used as received. Elemental analyses were
conducted at H. Kolbe Mikroanalytisches Laboratorium (Mu¨hlheim a.
d. Ruhr, Germany). Preparative methods for the d0 and meso-
perdeuteriomethyl (d24) versions of each compound were identical; in
each case, only one of both is described. Similarly, the only spectro-
scopic differences observed between both isotopic versions appeared
in NMR and IR: reported are the NMR and IR data for the meso-
perdeuteriomethyl version.
In contrast to the porphyrins of the red branch of Chart 1,
the spectroscopic and redox properties of tetrapyrrole parent
compounds on orthogonal branches remain largely unexplored,
due principally to the presence of noninnocent ions and ancillary
ligands. This problem has been especially pervasive for the blue
branch of the artificial porphyrins, which have been isolated as
salts of iron halide and polynuclear copper halide counteranions
and with axial halide ligands. We now describe the synthesis
of the sodium and tetrafluoroborate salts of porphyrinogen
macrocycles containing iron. Owing to the absence of redox-
active counterions and axial ligands, the oxidation-reduction
chemistry of the iron porphyrinogens has been unveiled by
electrochemistry, and the various oxidation states have been
electronically characterized by spectroscopy and computation.
It transpires that iron porphyrinogens possess a one-electron
metal-based and four-electron ligand-based chemistry. Metal-
Synthesis of Na(THF)2[LFe]. A milky white suspension of 16.48
g (20.47 mmol) d24-Na4(THF)4L in 600 mL of THF was charged with
3.89 g (30.71 mmol, 1.5 equiv) of anhydrous FeCl2 beads (-10 mesh).
The resulting mixture was stirred gently overnight with a large magnetic
stir bar so that the FeCl2 beads were maintained at the bottom of the
flask. Over the course of the first hour of stirring, the mixture turned
light green, then beige, brown, and eventually deep red. A gray mixture
of NaCl and Fe metal was filtered from the reacted solution; the filtrate
was evaporated and dried to yield a solid, which was resuspended in
400 mL of pentane and subsequently stirred for 2 h. The d24-Na(THF)2-
[LFe] product was isolated by filtration and dried in vacuo. Yield: 13.02
g (95%) of a deep red powder. The crude product was used for further
syntheses. A sample rerystallized from a THF/pentane mixture was
used to obtain the following analytical data. 1H NMR (500 MHz, THF-
d8): δ ) 13.1 (ax CD2H), 3.62 (s, THF-R), 1.78 (s, THF-â), -72.2
2
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(br, pyrrole). H NMR (76.8 MHz, THF): δ ) 13.1 (s, 12H, Meax),
-27.9 (s, 12H, Meeq). UV-vis λmax/nm (ꢀ/M-1 cm-1) in CH3CN: 332
(1200), 427 (4800), 526 (3900), ∼570 (sh). IR νmax/cm-1 ) 3096 m
(pyrrole C-H), 2220 s (C-D), 2128 m (C-D), 2060 w (C-D), 1049
s, 917 m, 889 m, 790 w, 750 s. Anal. Calcd for C36H24D24FeN4NaO2:
C 64.36, N 8.34, Fe 8.31; Found: C 64.14, N 8.32, Fe 8.43.
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Synthesis of Na2(THF)2[LFe]. Na(THF)2[LFe] (3.00 g, 4.63 mmol),
Na metal (0.11 g, 4.63 mmol, 1 equiv) and naphthalene (0.30 g, 2.32
mmol, 0.5 equiv) were mixed in 200 mL of THF. The deep red mixture
was refluxed until the foam was white. The cooled deep brown solution
was passed through a frit to remove remaining solid Na and the filtrate
was brought to dryness by evaporation. The solid Na2(THF)2[LFe]
residue was resuspended in 100 mL of hexane, stirred for 2 h, and
eventually filtered and dried in vacuo. Yield: 2.80 g (90%) of brown
2284.
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1322-1332.
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1617-1618.
1
powder. H NMR (500 MHz, THF-d8): δ ) 12.1 (s, ∼0.1H, CD2H),
(81) Campazzi, E.; Solari, E.; Scopelliti, R.; Floriani, C. Inorg. Chem. 1999,
38, 6240-6245.
3.62 (s, 8H, THF-â), 1.78 (s, 8H, THF-â), -13.3 (br, 8H, pyrrole). 2H
NMR (76.8 MHz, THF): δ ) 12.1 (s, 12H, Meax), -46.9 (s, 12H,
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42, 814-818.
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Meeq). UV-vis λmax/nm (ꢀ/M-1 cm-1) in CH3CN: 330 (3300). IR νmax
/
cm-1 ) 3086 m (pyrrole C-H), 2218 s (C-D), 2127 m (C-D), 2060
w (C-D), 1268 s, 1049 s, 1036 s, 919 m, 890 w, 791 w, 750 s. Anal.
Calcd for C36H48FeN4Na2O2: C, 64.48; H, 7.21; N, 8.35; Fe 8.33.
Found: C, 64.35; H, 7.26; N, 8.28; Fe 8.32.
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Synthesis of [L∆∆Fe](BF4)2. d24-Na(THF)2[LFe] (1.86 g, 2.77 mmol)
was dissolved in 50 mL of CH3CN. Because [L∆∆Fe]2+ was found to
be sensitive to ethers, the THF molecules solvating the Na+ cations
had to be removed. The solution was stirred at room temperature for
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Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
10 min, solvent was removed by evaporation, and the remaining solid
was dried, redissolved in 50 mL of CH3CN, and added dropwise to a
50-mL blue CH3CN solution of FcBF4. The green solution was stirred
for 10 min and then again evaporated and dried. The resulting solid
was resuspended in 50 mL of CH2Cl2 and stirred for 2 h at room
temperature, and the beige d24-[L∆∆Fe](BF4)2/NaBF4 solid was separated
from the green Fc/FcBF4 supernatant by filtration The desired product
was extracted from NaBF4 by boiling the crude mixture in 500 mL of
CH2Cl2. The resulting suspension was filtered and concentrated to 100
mL. The product was precipitated by dropwise addition of hexanes,
and it was isolated by filtration and dried in vacuo. A second harvest
of the product, analytically identical to the first, was obtained by
performing the extraction procedure an additional time on the original
d24-[L∆∆Fe](BF4)2/NaBF4 crude mixture. Combined d24-[LFe](BF4)2
yield: 1.05 g (56%) of pale beige powder. 1H NMR (500 MHz, CD3-
Physical Measurements. 1H NMR spectra were recorded at the MIT
Department of Chemistry Instrumentation Facility (DCIF) on a Varian
Inova-500 spectrometer, and 2H NMR spectra were recorded on a
Varian Inova-500 equipped with an inverse probe. 1H NMR chemical
shifts are quoted in ppm relative to tetramethylsilane, and spectra have
been internally calibrated to the monoprotio impurity of the deuterated
2
solvent. H NMR spectra were referenced to the peak of the mono-
deuterio impurity of the natural-abundance solvent; the chemical shift
1
was set to be equal to that observed in H NMR for the monoprotio
impurity of the corresponding deuterated solvent. Spectra were recorded
at 20 °C, and a 30-s delay was used between successive pulses to allow
for quantitative integration of the solvent peaks in 1H NMR. The quality
of the porphyrinogen NMR spectra was sufficiently high that even the
-CD2H isotopic impurity in meso-CD3 (99.9% D) was observable in
1H NMR.
X-band EPR measurements were carried out on frozen solutions at
4.5 K on a Bruker EMX spectrometer; the cavity was maintained at
4.5 K by an Oxford liquid helium cryostat.
Magnetic susceptibilities were determined at 50 Oe on powders
contained in gelatin capsules using a Quantum Design MPMSR2
SQUID susceptometer, which was located at MIT Center for Materials
Sciences and Engineering; the data were corrected for diamagnetism
of the sample holder, and the diamagnetic component of the samples
was evaluated from Pascal’s constants. The inverse molar paramagnetic
susceptibility data, 1/ø, were fitted over the entire temperature range
according to the following
2
CN): δ ) 58.7 (br, 4H, pyrrole), -6.9 (br, 4H, pyrrole). H NMR
(76.8 MHz, CH3CN): δ ) 35.2 (6H, Me), 9.1 (6H, Me), 4.6 (6H, Me),
-2.4 (6H, Me). UV-vis in CH3CN: far UV (λmax < 230 nm) band
tailing into the visible. IR νmax/cm-1 ) 3115 m (pyrrole C-H), 2220
w (C-D), 1575 m (CdN), 1564 m (CdN), 1117 s, 1105 s, 1071 s,
1048 s, 1035 s, 1011 s, 855 m. Anal. Calcd for C28H8D24FeN4: C,
49.59; N, 8.26; Fe, 8.23. Found C, 49.33; N, 8.17; Fe, 8.16.
X-ray Crystal Structure Determinations. X-ray quality crystals
were coated with Paratone N oil and mounted on a glass fiber. X-ray
diffraction data were collected on a Siemens diffractometer equipped
with a CCD detector, using the Mo KR radiation, selected by a graphite
monochromator. The data were integrated to hkl-intensity, and the final
unit cell was calculated using the SAINT v.4.050 program from
Siemens. Solution and refinement were performed with the SHELXTL
v.5.03 suite of programs developed by G. M. Sheldrick and Siemens
Industrial Automation, 1995. Least-squares refinements were applied
to F 2, with hydrogen atoms of organic fragments placed at calculated
positions using a standard riding model and refined isotropically.
Red-black crystals of [(diglyme)2Na][LFeIII] were grown from a
supersaturated diglyme solution and data were collected at -80 °C.
The structure was solved using direct methods; refinement proceeded
1
ø
T - θ
C + øTIP ‚(T - θ)
)
(1)
where øTIP, C, and θ are the temperature-independent paramagnetism,
the Curie constant, and the Weiss constant, respectively.
Mo¨ssbauer absorption data were recorded at 4.2 K on an MS1
spectrometer (WEB Research Co.) equipped with a 57Co source and a
Janus cryostat, and referenced to elemental iron.
UV-visible absorption spectra were recorded on a Spectral Instru-
ments 440 spectrophotometer. Molar absorptivity coefficients were
determined on solutions of submillimolar concentrations of iron
porphyrinogens. IR absorption spectra on solids dispersed in Nujol mulls
were recorded on a Perkin-Elmer 2000 FT-IR spectrophotometer.
Electrochemistry was performed with a standard three-electrode
configuration using a CV-50W potentiostat (Bioanalytical Systems).
The working electrode was Pt, the auxiliary electrode was a Pt wire,
and the reference was an Ag/AgNO3 electrode, which was externally
referenced to the Fc+/Fc couple (E°(Fc+/Fc) ) 0.65 V (in CH3CN)
and 0.69 V (in THF) vs NHE).93 All potentials reported are vs NHE.
The most well-behaved electrochemistry was observed for the tetra-
butylammonium (Bu4N+) salts of the [LFeIII]- and [LFeII]2- anions,
which were prepared by adding a stoichiometric amount of Bu4NCl to
a THF solution of the Na+ salts of the respective complexes. The white
NaCl precipitate was separated from the reaction mixture by filtration,
and the desired product was obtained by evaporating THF from the
filtrate under vacuum. The remaining solid was triturated in hexanes
and isolated by filtration. Cyclic voltammetry (CV), differential pulse
polarography (DPP), and chronocoulometry experiments were con-
ducted on a CH3CN solution containing 1 mM (Bu4N)[LFeIII] and 0.04
M Bu4NBPh4. CV scans were initiated from the solution’s resting
potential at 100 mV/s. The scan rate dependence of CV waves were
recorded over the potential range of 50-600 mV/s. DPP was performed
by polarizing the working electrode at an initial, negative potential for
2 s and then scanning the potential axis in the positive direction. In the
chronocoulometry experiment, the working electrode was poised at 311
mV for 2 s, after which it was jumped to either -899 or 911 mV. The
and converged normally. Red-black crystals of the {[(THF)3FeII(µ-Cl)3-
2+
FeII(THF)3][(THF)4FeII(µ-Cl)2FeII(THF)4]1/2
}
salt of [LFeIII]- were
grown from a supersaturated THF/toluene solution, and data were
collected at -80 °C. The structure was solved using Patterson’s method;
one of the two toluene solvent molecules contained in the crystal was
strongly disordered and could not be properly refined.
Orange crystals containing [LFeII]2- could be grown from pure
ethereal solvents but were too oxygen-sensitive to be manipulated,
turning instantaneously deep red upon breaking of the sealed ampoule
in which they were grown. It was observed that crystals grown from
supersaturated THF solution in the presence of pyridine-N-oxide were
slightly more robust, and a satisfactory dataset for [(pyO)(THF)Na]2-
[LFeII] could be collected at -80 °C. The structure was solved using
direct methods; refinement proceeded and converged normally.
Despite repeated attempts, [L∆∆FeII]2+ would not crystallize with
-
BF4 as the counterion, but exchange of tetrafluoroborate for cobalti-
carborane proceeded smoothly in dichloromethane and afforded orange
single crystals of [L∆∆FeII(NCCH3)][(C2B9H11)2Co]2‚2CH3CN‚2o-C6H4-
Cl2 by slow vapor diffusion of toluene into an o-dichlorobenzene/
acetonitrile solution. The data were collected at 100 K, and the structure
2
was solved using direct methods. Throughout the refinement, the H
atoms were treated as natural-abundance H atoms; in the initial stages
of refinement, all atoms of the cobalticarborane anions (except for the
metal) were treated as borons, then hydrogens were located from the
difference map, and finally the carbon atoms were identified by their
abnormal anisotropic thermal parameters and abnormal bond distances
to hydrogens. In the end, all B-H and C-H distances were fixed to
1.10((0.10) Å, a value measured for C2B9H13 by electron diffraction.92
The crystal was found to be a racemic twin (chiral space group Abm2),
with absolute structure parameter 0.66((0.03).
(92) Mackie, I. D.; Robertson, H. E.; Rankin, D. W. H.; Fox, M. A.; Malget, J.
M. Inorg. Chem. 2004, 43, 5387-5392.
(93) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4733

A R T I C L E S
Scheme 1
Bachmann and Nocera
with FeCl2 beads (-10 mesh) serving as the Fe2+ source.
Reaction mixtures containing the free base porphyrinogen
macrocycle, designated [L]4-, and the FeCl2 beads turn the very
deep red color that is characteristic of the presence of the
[LFeIII]- complex after ∼1 h. The direct insertion product,
[LFeII]2-, was not observed during the course of the reaction
because it is oxidized in situ to [LFeIII]- by FeCl2 (consistent
with the observed formation of black Fe metal powder during
the reaction). Under the heterogeneous synthesis conditions,
[LFeIII]- is isolated as the sodium salt. The absence of iron-
containing countercations was confirmed by elemental analysis,
single-crystal X-ray diffraction analysis, and by Mo¨ssbauer
spectroscopy.
total charge that was passed through the electrode was measured for
250 ms. The resulting curves were analyzed and fitted in their linear
regime by the CV-50W software using a least-squares procedure.
Computational Methods. Density functional theory calculations
(DFT) were performed using the Amsterdam Density Functional
(ADF2002.02) program94,95 on a home-built Linux cluster comprising
60 Intel processors organized in groups of 12 running in parallel. The
generalized gradient approximation was used as implemented in ADF
by the Becke-88 functional for exchange,96 and the Perdew-Wang-91
functional for correlation.97 A basis set of triple-ú Slater-type functions
augmented by a polarization set (TZP) was used for Fe and N atoms
and double-ú with polarization (DZP) for other atoms, with frozen core
approximation; spin restriction was lifted for all S * 0 cases. Energies
reported are gas-phase internal energies at 0 K, without zero-point
vibrational energy, of the entity in the experimental solid-state structure.
Orbitals were visualized using the Molekel v.4.2 Linux-mesa soft-
ware.98,99
Complications arise if the iron center is installed into the
macrocycle according to established procedures.86-88 Tetra-
hydrofuran suspensions of Na4L charged with FeCl2 powder
turn deep red within minutes, and [LFeIII]- is isolable as a salt
Results and Discussion
of FeII -based countercations, [(THF)4Fe(µ-Cl)2Fe(THF)4]1/2
-
Synthesis. Scheme 1 outlines the syntheses of the iron
porphyrinogens that are the focus of this study. Control of the
availability of solubilized FeCl2 is essential to obtaining the
parent complex [LFeIII]- in the absence of iron-based anions.
We find that heterogeneous introduction of Fe2+ ion into solution
with gentle stirring is critical to the success of this chemistry,
2
[(THF)3Fe(µ-Cl)3Fe(THF)3][LFe]2 (see Scheme 1). The very low
solubility of Na4L with respect to FeCl2 in THF presumably
yields to a preponderance of Fe2+ in solution during metalation,
thereby favoring formation of the bimetallic cations followed
by their subsequent metathesis with the Na+ ion. This wayward
chemistry is observed despite a stoichiometric Na4L/FeCl2 ratio
and the employment of methods for the slow addition of FeCl2
to solution. Indeed, the only way we found to prevent the
buildup of Fe2+ in solution was the utilization of the hetero-
geneous reaction conditions described in the foregoing para-
graph. Given the abundance of Fen+ centers in [(THF)4Fe-
(µ-Cl)2Fe(THF)4]1/2[(THF)3Fe(µ-Cl)3Fe(THF)3][LFe]2, different
combinations of formal charges for the constituent Fen+ ions
(94) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra,
C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22,
931-967.
(95) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391-403.
(96) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(97) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M.
R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671-6687.
(98) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. Molekel, v.4.2/3; Swiss
Center for Scientific Computing: Manno, Switzerland, 2000-2002.
(99) Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766-769.
9
4734 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
may satisfy the overall charge balance. The formal oxidation
states in this compound are assigned by inspection of its X-ray
crystal structure, which is supplied as Supporting Information
(Figure S1). The metrics of the [(THF)3Fe(µ-Cl)3Fe(THF)3]n+
complex cation (Table S1, Supporting Information) are identical
to those of the cation in the previously reported salt, [(THF)3FeII-
(µ-Cl)3FeII(THF)3][(THF)SnIVCl5].100 Additionally, the metrics
of the porphyrinogen anion are identical to those of the crystal
structure of [Na(solv)2][LFeIII] (Table S1), thus setting a formal
charge of +2 for the iron centers in the [(THF)4Fe(µ-Cl)2Fe-
(THF)4]2+ complex cation.
metrical measurements for the three structures are given in
Tables 1 and 2, respectively. The structure of [LFeII]2- and
[LFeIII]- are similar to each other and to that previously reported
for the dilithium salt of iron(II) octaethylporphyrinogen. As is
most easily seen from the edge-on perspective, the central iron
ion lies in a nearly perfect planar tetracoordinate environment
defined by the four pyrrole nitrogens (4-N), and no significant
intermolecular contacts are observed. Owing to the sp3 hybrid-
ization of the meso carbons, the pyrroles cant about the
N-Fe-N two-fold axes. The two structures are distinguished
by the Fe-N bond lengths of the macrocycle core. As expected,
the Fe-N bond is significantly longer for the iron center
possessing the greater valence electron count (davg(Fe-N) )
1.933 Å in [LFeII]2- vs davg(Fe-N) ) 1.899 Å in [LFeIII]-).
The only other significant differences in the structures result
directly from the countercations (which are not shown in Figure
1 but included in Figures S3-S6). The Na+ countercation of
[LFeIII]- assumes a pseudo-octahedral coordination defined by
the oxygens of two fac-diglyme solvent molecules. The Na+
countercations of [LFeII]2- are coordinated by diametrically
opposite pyrroles in a η4 fashion (davg(Na‚‚‚N) ) 3.07 Å) is
significantly longer than davg(Na-C) ) 2.80 Å) residing on
opposite faces of the macrocycle. The remaining coordination
sites of the Na+ ions are occupied by three monodentate oxygen
ligands, one from THF and two from symmetry-related bridging
pyridine-N-oxides (which create a coordination polymer in the
lattice). The latter coordinate the alkali cations and attenuate
the very high oxygen sensitivity of the crystals (which in the
absence of pyridine-N-oxide quickly turn from golden orange
to deep red upon manipulation).
The X-ray crystal structures of [LFeII]2- and [LFeIII]- indicate
no direct bonding interaction between the R-carbons of adjacent
pyrroles (davg(CR-CR) ) 2.47 Å in [LFeIII]- and davg(CR-CR)
) 2.50 Å in [LFeII]2-). This observation is consistent with no
oxidation of the macrocycle. Conversely, two pairs of adjacent
CR atoms of the [L∆∆FeII]2+ macrocycle are within single bond
distance of each other (d(CR-CR) ) 1.607(11) Å and
1.553(13) Å). The cyclopropanes are nearly equilateral triangles
with CR-Cmeso-CR angles of 63.3(6)° and 62.3(7)°. These
metrics of the cyclopropane rings are similar to those observed
in previously described dicyclopropane octaethylporphyrinogens
of iron, manganese, and cobalt.84,86-88
The [LFeIII]- complex served as the parent complex for the
other series members. The one-electron reduced congener
[LFeII]2- is afforded in quantitative yield by the reduction of
[LFeIII]- with a stoichiometric or excess amount of sodium
naphthalenide. This material is very air sensitive, sometimes
smoldering upon exposure to air. The Na+ ions may be
exchanged with tetrabutylammonium using Bu4NCl in THF.
Alternatively, [LFeIII]- may be oxidized by ferrocenium
tetrafluoroborate (FcBF4) in CH3CN. [L∆∆FeII](BF4)2 is obtained
as the only isolable iron product, regardless of the Na[LFeIII]/
FcBF4 ratio. An insoluble brown byproduct was not character-
ized. Care was taken in the choice of the starting material. Ethers
appear to favor radical processes and exacerbate the subsequent
decomposition of the oxidized product. Consequently, [Na-
(solv)2][LFeIII] where solv ) THF was avoided. Three equiva-
lents of FcBF4 are consumed in the reaction as ascertained by
recovery of the Fc reduction product; for preparative conven-
ience, three equivalents of FcBF4 were therefore employed for
the oxidation. The macrocycle undergoes a selective four-
electron oxidation from [L]4- to L∆∆ (with subsequent reduction
of FeIII to FeII), and under no conditions is a product obtained
with the macrocycle oxidized by two electrons (i.e., [L∆]2-).
Consistent with this result, [L∆∆FeII]2+ and [LFeIII]- do not
undergo a comproportionation reaction. [L∆∆FeII](BF4)2 does
not thermally decompose in the solid state, in acetonitrile
solution, or as a dichloromethane suspension at room temper-
ature. However, salt metathesis attempts in dichloromethane with
sodium tetraphenylborate caused decomposition on the time
scale of dissolution of the reactants (as witnessed by a color
change from yellow to brown and the complete loss of the NMR
signals of [L∆∆FeII]2+). For the purpose of growing large single
crystals suitable for X-ray analysis, sodium cobalticarborane,
Na[(C2B9H11)2Co], was employed for salt metathesis, which
proceeded very cleanly in CH2Cl2.
The presence of the spirocyclopropane rings distorts the
macrocycle and the constituent pyrroles. The presence of
alternating formal single and double bonds in the pyrroles is
plainly evident: NdC double bonds (davg(N(1)-C(4), (N(2)-
C(8)) ) 1.31 Å) are significantly shorter than their neighboring
N-C single bonds (davg(N(1)-C(1), N(2)-C(11)) ) 1.45 Å).
The CR-CR straps constrain the macrocycle along one axis,
causing the 4-N core to deform from a square to a rectangle
( (N(1)-Fe-N(2)) ) 87.3° and (N(1)-Fe-N(1A)) ) 79.5°).
The ruffled conformation of the [L]4- macrocycle is supplanted
by a bowl conformation in [L∆∆], as observed in calixarenes102
and calixpyrroles,103-107 in which all four pyrrole planes are
Published work shows that the iron center of [L∆∆FeII]2+ may
be oxidized when chloride is available to coordinate axially,
yielding [L∆∆FeIIICl]2+ 86
The oxidation, however, does not
.
proceed easily in the absence of noncoordinating, “innocent”
counteranions. For instance, treatment of [L∆∆FeII]2+ with the
strongly oxidizing thianthrenium radical cation101 was ac-
companied by the disappearance of the deep purple color of
the radical cation, but most of the [L∆∆FeII]2+ starting material
was recovered.
Structural Chemistry. The structures of the iron porphy-
rinogen frameworks of [LFeII]2-, [LFeIII]-, and [L∆∆FeII]2+ are
shown in Figure 1. Crystallographic data and selected geo-
(102) Gutsche, C. D.; Muthukrishnan, R. J. Org. Chem. 1978, 43, 4905-4906.
(103) Gale, P. A.; Sessler, J. L.; Lynch, V.; Sansom, P. I. Tetrahedron Lett.
1996, 37, 7881-7884.
(104) Anzenbacher, P., Jr.; Jurs´ıkova´, K.; Lynch, V. M.; Gale, P. A.; Sessler, J.
L. J. Am. Chem. Soc. 1999, 121, 11020-11021.
(100) Janas, Z.; Sobota, P.; Lis, T. J. Chem. Soc. Dalton Trans. 1991, 2429-
2434.
(101) Lee, W. K.; Lio, B.; Park, C. W.; Shine, H. J.; Guzman-Jimenez, I. Y.;
Whitmire, K. H. J. Org. Chem. 1999, 64, 9206-9210.
(105) Sessler, J. L.; Anzenbacher, P., Jr.; Miyaji, H.; Jursikova´, K.; Bleasdale,
E. R.; Gale, P. A. Ind. Eng. Chem. Res. 2000, 39, 3471-3478.
(106) Nielsen, K. A.; Jeppesen, J. O.; Levillain, E.; Becher, J. Angew. Chem.,
Int. Ed. 2003, 42, 187-191.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4735

A R T I C L E S
Bachmann and Nocera
Figure 1. Comparative views of the solid-state structures of the iron porphyrinogen frameworks in [(THF)Na(Opy)]2[LFeII], [Na(diglyme)2][LFeIII], and
[d24-L∆∆FeII(NCCH3)][(C2B9H11)2Co]2: (top) top view, perpendicular to the 4-N plane; (bottom) side view, parallel to the 4-N plane. Thermal ellipsoids are
drawn at the 50% probability level, and H atoms and counterions and solvent molecules are omitted for clarity. The color coding of pink for data associated
with [LFeII]2-, blue for data associated with [LFeIII]-, and green for data associated with [L∆∆FeII(NCCH3)]2+ is maintained throughout this manuscript.
tilted upward relative to the N4 plane. The conformational
change from ruffle to bowl is accompanied by the movement
of the FeII center from the 4-N plane by 0.72 Å. The displaced
FeII center coordinates an axial CH3CN solvent molecule to
engender a distorted square pyramidal coordination geometry.
The four-electron oxidized macrocycle is a much weaker donor
than the parent [L]4- porphyrinogen, as evidenced by long
Fe-N(pyrrole) bonds; the davg(Fe-N(pyrrole)) ) 2.12 Å is 0.18
Å longer than the Fe-N(pyrrole) bond lengths of [LFeII]2- and
is longer than the Fe-N bond to the axial CH3CN (d(Fe-N(3))
) 2.065 Å). All five Fe-N distances in this free [L∆∆FeII-
(NCCH3)]2+ entity, however, are significantly shorter than the
corresponding ones in the previously reported polynuclear
compound [L∆∆FeII(Cl)(Cu4Cl5)], indicating that the interacting
Cu4Cl5- influences the structural properties of the iron porphy-
rinogen moiety.89 Our results establish the stable existence of
and intrinsic metrics for [L∆∆FeIIL]2+ in the absence of an
anionic axial ligand and an interacting counterion.
Paramagnetic 1H and 2H NMR. The porphyrinogen frame-
work was synthesized with deuterated meso methyls, allowing
the solution structures of the series members to be probed by
1H and 2H NMR. As displayed in Figure 2, exceptionally high-
quality NMR spectra are obtained despite the presence of the
paramagnetic iron centers. Owing to the meso d24 substitution,
the hydrogens on the methyl groups and pyrroles can be
investigated in separate experiments and therefore are easily
distinguished from each other.
The single resonance for the eight pyrrole hydrogens observed
1
in the H NMR spectra of [LFeII]2- and [LFeIII]- in solution
(Figure 2, top and middle trace, respectively) is consistent with
the four-fold symmetry of the porphyrinogen in the solid state.
2
The H NMR spectrum of these compounds is also consistent
with this four-fold symmetry but two signals of equal intensity
are observed for the meso methyls. The two distinct signals
reflect a symmetry lower than D4h, arising from either (i)
coordination of the Na+ ions to the one side of the macrocycle
or (ii) retention of the D2d conformation of the solid-state
structure for the molecules in solution. The invariance of the
2H NMR spectrum to solvent (THF, diglyme, or acetonitrile),
to encryption of Na+ by 18-crown-6 ether, and to replacement
of Na+ by NBu4 rules out possibility (i); thus, the chair
+
conformation of the tetrapyrrole observed in the solid state is
retained in solution. The two peaks are ascribed to the axial
and equatorial meso methyl positions (axial being defined as
close to perpendicular to the 4-N plane, and equatorial being
defined as close to parallel to that plane). This interpretation is
further corroborated by the relative magnitudes of the para-
magnetic shifts for the axial and equatorial methyl resonances.
The paramagnetic shift, ∆νpc/ν, of a nucleus at a position (r, θ,
φ) relative to the paramagnetic iron center is given by
â2S(S + 1)
4π 9kT
3 cos2 θ - 1
∆νpc µ0
)
‚
‚
‚ (gpar2 - gperp2) (2)
r3
ν
as long as the system has axial symmetry, the electron spin
resides exclusively at iron, and the dipole coupling is purely
(107) Yoon, D.-W.; Hwang, H.; Lee, C.-H. Angew. Chem., Int. Ed. 2002, 41,
1757-1759.
9
4736 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
Table 1. Summary of Crystallographic Data for Iron Porphyrinogens
[L∆∆FeII(NCCH3)][(C2B9H11)2Co]2
2CH3CN 2o-C6H4Cl2
‚
[(THF)Na(Opy)]2[LFeII]
[Na(diglyme)2][LFeIII
]
‚
empirical formula
formula weight
T (K)
C50H58FeN6Na2O4
908.85
183(2)
C40H60FeN4NaO6
771.76
183(2)
C54H85B36Cl4Co2FeN7
1536.96
100(2)
λ (Å)
0.71073
monoclinic
P21/c
20.4824(13)
11.6675(7)
19.2151(12)
90
102.4720(10)
90
0.71073
orthorhombic
Pbca
15.5367(12)
21.1012(16)
25.2473(19)
90
0.71073
orthorhombic
Abm2
23.443(2)
25.146(3)
13.2883(13)
90
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
90
90
90
90
γ (deg)
V (Å3)
4483.6(5)
4
8277.2(11)
8
7833.6(14)
4
Z
crystal size (mm3)
F(000)
0.27 × 0.27 × 0.25
0.45 × 0.45 × 0.45
0.12 × 0.17 × 0.09
1920
3304
3144
absorption coeff. (mm-1
)
0.409
0.423
0.780
θ range (deg) for data collection
2.19-23.28
-21 e h e 22
-12 e k e 8
-21 e l e 21
17385
2.29-23.28
-16 e h e 17
-20 e k e 23
-27 e l e 28
31601
0.87-23.31
-26 e h e 25
-27 e k e 27
-9 e l e 14
19041
index ranges
reflections collected
independent reflns (Rint
completeness to θmax
)
6414 (0.0501)
99.4%
6414/0/764
1.064
5951 (0.0582)
99.8%
5951/0/482
1.479
0.0962, 0.1867
0.0962, 0.1867
0.00334(18)
1.003, -0.605 eÅ-3
4642 (0.0705)
99.5%
4642/1/576
1.111
data/restraints/parameters
GOFc on F 2
R1,a wR2b [I > 2σ(I)]
R1,a wR2b (all data)
extinction coefficient
largest diff. peak, hole
0.0345, 0.0840
0.0419, 0.0876
not used
0.0548, 0.1437
0.0571, 0.1463
not used
0.247, -0.315 eÅ-3
0.827, -0.766 eÅ-3
2
2
2
a R1 ) ∑||Fo - |Fc||/∑|Fo|. b wR2 ) (∑(w(Fo2 - Fc )2)/∑(w(Fo )2))1/2
.
c GOF ) (∑w(Fo2 - Fc )2/(n - p))1/2 where n is the number of data and p is the
number of parameters refined.
Table 2. Selected Crystallographic Metric Parameters of Iron Porphyrinogens (atom numbering scheme provided in Figure 1)
[(THF)Na(Opy)]2[LFeII]
[Na(diglyme)2][LFeIII
]
[L∆∆FeII(NCCH3)][(C2B9H11)2Co]2
‚
2CH3CN
‚2o-C6H4Cl2
Distances/Å
1.896(5)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(4)
Fe(1)-N(3)
N(1)-C(1)
N(1)-C(18)
N(2)-C(3)
N(2)-C(6)
C(1)... C(3)
C(6)... C(8)
C(11)... C(13)
C(16)... C(18)
1.9185(17)
1.9353(17)
1.9371(17)
1.9430(16)
1.376(3)
1.381(3)
1.381(3)
1.385(3)
2.53
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-N(1)
Fe(1)-N(2)
N(1)-C(1)
N(1)-C(4)
N(2)-C(8)
N(2)-C(11)
C(4)... C(8)
Fe(1)-N(1)
2.115(5)
2.117(6)
2.065(8)
1.897(5)
1.899(5)
1.904(5)
1.375(7)
1.393(7)
1.379(7)
1.387(7)
2.47
Fe(1)-N(2)
Fe(1)-N(3)
N(1)-C(4)
N(1)-C(1)
N(2)-C(8)
N(2)-C(11)
C(1)-C(1’)
C(11)-C(11’)
C(4)... C(8)
1.278(8)
1.446(8)
1.334(8)
1.447(9)
1.607(11)
1.553(13)
2.47
2.49
2.49
2.51
C(1)... C(25)
C(11)... C(15)
C(18)... C(22)
2.48
2.48
2.46
Angles/deg
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
N(3)-Fe(1)-N(4)
N(4)-Fe(1)-N(1)
89.55(7)
90.49(7)
90.15(7)
89.82(7)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
N(3)-Fe(1)-N(4)
N(4)-Fe(1)-N(1)
89.6(2)
90.1(2)
90.2(2)
90.2(2)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-N(1A)
N(2A)-Fe(1)-N(2)
N(1A)-Fe(1)-N(2)
N(3)-Fe(1)-N(1)
C(1)-C(15)-C(1’)
C(11)-C(12)-C(11’)
C(4)-C(5)-C(8)
87.27(18)
79.5(3)
78.7(3)
139.77(19)
110.3(2)
63.3(6)
62.3(7)
110.3(4)
through space (“pseudocontact”).108 In the above expression,
µ0 is the vacuum permeability, k Boltzmann’s constant, T the
absolute temperature, S the electron spin quantum number, and
the coordinates of the methyl hydrogens in solution to the
position of the methyl carbon in the solid state, we find that
(∆νpc)ax/(∆νpc)eq ) -3.0 for [LFeIII]- and -4.2 for [LFeII]2-
;
g
par and gper the components of the diagonalized g tensor of the
the observed shift ratio between the axial and methyl resonances
in Figure 2 is -2.5 for [LFeIII]- and -4.6 for [LFeII]2-. Given
the crudeness of the geometric approximation, the agreement
is reasonable.
paramagnetic center that lie parallel and perpendicular to the
molecular axis, respectively. By taking the ratio (∆νpc)ax/
(∆νpc)eq, one only retains the geometric parameters. Equating
The 1H and 2H NMR spectra of [L∆∆FeII]2+ show a doubling
in the number of resonances (Figure 2, bottom trace). This result
(108) Bertini, I.; Luchinat, C. Physical Methods for Chemists; Drago, R. S.,
Ed.; Saunders College: Ft. Worth, 1977; Vol. 3, pp 500-556.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4737

A R T I C L E S
Bachmann and Nocera
1
2
Figure 2. 2H (left) and H (right) NMR spectra of the three paramagnetic iron-d24-porphyrinogen species. The H NMR (77 MHz, CH3CN) probes the
1
meso methyl groups, and the H NMR (500 MHz, CD3CN) probes the â pyrrole sites of the porphyrinogen macrocycles.
The process at Ep ) 0.77 V (vs NHE) corresponds to a
multielectron reaction; its peak current is ∼3 times greater than
that of the reversible process. A more accurate measurement of
the redox equivalency for both electrode processes is afforded
by chronocoulometry. Using the Cottrell equation, plots of Q
vs t1/2 for potential steps encompassing the Ep ) -0.55 and Ep
) 0.77 redox processes yield a linear response with slopes in
1:2.7 ratio, consistent with the DPP result. The identities of these
two electrode processes are revealed by performing electro-
chemical experiments on solutions of [L∆∆FeII]2+: differential
pulse polarograms exhibit identical peaks with those displayed
in Figure 3. Taken together, the DPP, CV, and chronocoulo-
metric results lead us to the following electrochemical assign-
ments:
[LFeIII]- + e- f [LFeII]2- E1/2 ) -0.57 V
[L∆∆FeII]2+ + 3e- f [LFeIII]- Ep ) +0.77 V
(3)
(4)
Figure 3. Cyclic voltammetric (red and gray) and differential pulse
polarographic (black) traces of a 1 mM CH3CN solution of Bu4N[LFeIII]
and 0.04 M Bu4NBPh4. The cyclic voltammogram was recorded at a sweep
rate of 100 mV/s and the differential pulse voltammogram at a sweep rate
of 20 mV/s, pulse amplitude of 20 mV, pulse period of 200 ms, pulse width
of 50 ms, and sample width of 17 ms. The color-coded stripe identifies the
predominant iron porphyrinogen species in each potential range.
We note that peak potential measurements for quasi-reversible
and irreversible processes in DPP will be shifted to more
negative potentials by an activation overpotential. Notwithstand-
ing, fast cyclic voltammetric scans of the [L∆∆FeII]2+/[LFeIII]-
is in line with the observed lowering in symmetry of the solid-
state structure upon oxidation of the macrocycle. Thus, the
symmetry properties of the porphyrinogen system are revealed
in their quintessential simplicity by appropriate deuteration of
the tetrapyrrole.
couple indicate that Ep is near E1/2
.
The porphyrinogen macrocycle is very electron-donating as
reflected by the reduction potential of the iron residing at its
core. The reduction potential of reaction 3 is more negative than
that of low-potential ferrodoxins (Fd), whose ferric forms are
among the most difficult iron cofactors to reduce in biology
(E1/2(FdFeIII/II) ) 0 to -0.5 V vs NHE).109 With regard to its
porphyrin relatives, iron porphyrinogen is ∼0.5 to 1.3 V more
difficult to reduce than heme cofactors (depending on the nature
of the axial ligands).
Reaction 4 provides a measure of ligand-centered redox
processes. The porphyrinogen can react in two-electron steps
with the concomitant formation of a C-C bond between the R
carbons of adjacent pyrroles; each cyclopropane thus formed
is a two-equivalent hole reservoir for the porphyrinogen
framework. The reduction potentials of half-reactions 3 and 4
Electrochemistry. Figure 3 shows the cyclic voltammogram
of [LFeIII]- in CH3CN. Two oxidation-reduction events are
observed. Reversible electrochemical behavior for the cyclic
voltammetric process at E1/2 ) -0.57 V (vs NHE) is indicated
by linear plots of the cathodic and anodic currents vs υ1/2 (scan
rate, υ: 50-600 mV s-1) (Figure S2) and values of 1.2 ( 0.2
for the ratios of the anodic and cathodic peak currents. The redox
process at more positive potential is characterized by a
significant increase in the current response and an irreversible
wave form. As is often the case, a potential for the redox process
is afforded by differential pulse polarography. An anodically
scanning electrode immersed in CH3CN solutions of [LFeIII]-
yields two peaks in the differential pulse polarogram. The
process at Ep ) -0.55 V (vs NHE) corresponds to the reversible
E1/2 ) -0.57 V wave observed in the cyclic voltammogram.
(109) Battistuzzi, G.; D’Onofrio, M.; Borsari, M.; Sola, M.; Macedo, A. L.;
Moura, J. J. G.; Rodrigues, P. J. Biol. Inorg. Chem. 2000, 5, 748-760.
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4738 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
reproduces the absorption spectra of [LFeII]2-, [LFeIII]-, and
[L∆∆FeII]2+ in CH3CN. The intense color of [LFeIII]- arises from
two bands at 427 and 526 nm that dominate the absorption
profile; a ∼570 nm shoulder is observed on the trailing edge of
the lowest-energy absorption band. The molar absorption
coefficients of both bands suggest a charge-transfer parentage
for the transitions. A ligand-to-metal character for this charge-
transfer concurs with the very reducing nature of the [L]4- ligand
and the presence of an oxidizing FeIII center. The featureless
profiles of both [LFeII]2- and [L∆∆FeII]2+ support this LMCT
assignment: reduction of the metal and oxidation of the ligand
would cause a LMCT to blue shift into the ultraviolet spectral
region. Accordingly, a charge-transfer absorption is not observed
for either compound.
Figure 4. UV-visible absorption spectra of Na2(THF)2[LFeII] (pink
spectrum), Na(THF)2[LFeIII] (blue spectrum), and [L∆∆FeII](BF4)2 (green
spectrum) in CH3CN.
More quantitative insight into the frontier molecular orbitals
of the iron porphyrinogens is provided by DFT calculations.
Full geometry optimizations on [Na(THF)2]2[LFeII] and [Na-
(THF)2][LFeIII] were performed, and the Na+ ions and their
attendant coordination spheres were removed to afford the
isolated porphyrinogen frameworks. The optimized geometry
and the details of the MO diagram for these calculations were
found to reproduce those obtained for single-point calculations
of [LFeII]2- and [LFeIII]- at the geometries imposed by the
crystal structures of [(THF)Na(Opy)]2[LFeII] and [Na(digly)2]-
[LFeIII]. On the basis of these results, the electronic structure
of [L∆∆FeII(NCCH3)]2+ was determined from a single-point
calculation at the geometry defined by the crystal structure of
[L∆∆FeII(NCCH3)][(C2B9H11)2Co]2. This approach allowed us
to circumvent the significant computational effort and complex-
ity associated with treating the [(C2B9H11)2Co]- anions. Orbital
energies and Mulliken populations for the iron porphyrinogens
are listed in the Supporting Information (Tables S26-S31).
Figure 5 displays the energy level diagrams and the Kohn-
Sham representations of the frontier molecular orbitals of
[LFeII]2- and [LFeIII]-. Because spin restriction is lifted, there
is no common spatial wave function for each R/â spin pair,
and accordingly, two manifolds of “orbitals” are represented
for the R and â spinors. Pairs of spinors with antiparallel spin
and similar spatial extension for the metal-based functions are
correlated by dashed lines. We note that the Kohn-Sham
orbitals differ from those derived from a Hartree-Fock (HF)
formalism by the inclusion of the exchange correlation energy;110
however, comparative analysis establishes that the shapes and
symmetries of the Kohn-Sham orbitals accord well with those
calculated by more traditional HF and extended Hu¨ckel ap-
proximations,111,112 and indeed these orbitals have found
widespread use in electronic structure descriptions.113-120 The
offer the potential for the overall four-electron ligand-based
reduction
[L∆∆FeII]2+ + 4e- f [LFeII]2- E1/2 ) 0.43 V
(5)
The four-electron reduction of the porphyrinogen macrocycle
was previously ascertained by inserting the redox-inactive Zn2+
ion into the porphyrinogen macrocycle.90 For this system the
consecutive two-electron processes were observed at E1/2
-
([L∆∆ZnII]2+/[L∆ZnII]) ) 0.63 V and E1/2([L∆ZnII]/[LZnII]2-) )
0.21 V vs NHE. Combining these two redox couples yields a
potential for the overall four-electron ligand-centered reduction
[L∆∆ZnII]2+ + 4e- f [LZnII]2- E1/2 ) 0.42 V (6)
The consistency between the ligand redox potentials for half-
reactions 5 and 6 for macrocycles encrypting redox-active and
redox-inactive metals suggests that the frontier orbitals of the
ligand are not significantly perturbed by the metal orbitals
residing at its core.
The inability to observe the partially oxidized porphyrinogen
species for iron, when it is available to zinc, suggests that the
disproportionation of [L∆FeII] to the observed iron porphyrino-
gens is favorable
3[L∆FeII] f 2[LFeIII]- + [L∆∆FeII]2+
(7)
We can assess the viability of this contention if we assume that
the potentials for the two-electron ligand transformations are
the same for iron and zinc, i.e., E1/2([L∆∆MII]2+/[L∆MII]) ) 0.63
V and E1/2([L∆MII]/[LMII]2-) ) 0.21 V vs NHE for M ) Zn
and Fe. This assumption is reasonable, given that we have
already shown the reduction potentials for half-reactions 5 and
6 to be nearly equivalent. On this basis, we calculate the
following reduction potential:
(110) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys. Chem. 1996, 100, 12974-
12980.
(111) Stowasser, R.; Hoffmann, R. J. Am. Chem. Soc. 1999, 121, 3414-3420.
(112) Hamel, S.; Duffy, P.; Casida, M. E.; Salahub, D. R. J. Electron Spectrosc.
Relat. Phenom. 2002, 123, 345-363.
[L∆FeII] + e- f [LFeIII]- E1/2 ) 0.99 V
(8)
(113) Lehnert, N.; Neese, F.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am.
Chem. Soc. 2002, 124, 10810-10822.
(114) Chen, P.; Root, D. E.; Campochiaro, C.; Fujisawa, K.; Solomon, E. I. J.
Am. Chem. Soc. 2003, 125, 466-474.
With this in hand, the driving force for reaction 7 is determined
to be ∆E(7) ) +0.36 V. In contrast to the zinc porphyrinogen
case, the availability of the +3 metal oxidation state and the
stability of the [LMIII]- species drives disproportionation
reaction 7, thus preventing the isolation of [L∆FeII].
Electronic Structure of Iron Porphyrinogens. Solutions of
[LFeIII]- are blood red, whereas those of [LFeII]2- are pale
yellow and those of [L∆∆FeII]2+ even paler in color. Figure 4
(115) Chisholm, M. H.; Gallucci, J.; Hadad, C. M.; Huffman, J. C.; Wilson, P.
J. J. Am. Chem. Soc. 2003, 125, 16040-16049.
(116) Gray, T. G.; Rudzinski, C. M.; Meyer, E.; Holm, R. H.; Nocera, D. G. J.
Am. Chem. Soc. 2003, 125, 4755-4770.
(117) Gray, T. G.; Rudzinski, C. M.; Meyer, E. E.; Nocera, D. G. J. Phys. Chem.
A 2004, 108, 3238-3243.
(118) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am.
Chem. Soc. 2004, 126, 10013-10020.
(119) Manke, D. R.; Loh, Z.-H.; Nocera, D. G. Inorg. Chem. 2004, 43, 3618-
3624.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4739

A R T I C L E S
Bachmann and Nocera
Figure 5. Molecular orbital diagram for [LFeII]2- (left) and [LFeIII]- (right), with metal-centered functions highlighted in color. Because spin restriction is
lifted, there is not a common spatial wave function for each R/â spin pair, and two manifolds of “orbitals” are drawn separately. The metal-centered spatial
wave functions are similar for [LFeII]2- and [LFeIII]- and for R and â spinors. The orbitals shown in the middle of the diagram are depicted at the 95%
probability level for the â spinors of [LFeIII]-.
relative energies and characters of the molecular orbitals for
the iron porphyrinogen in both FeIII and FeII oxidations states
are similar. As expected, the d-orbitals of the more electropos-
itive FeIII center are slightly stabilized with respect to those of
its FeII congener. Both [LFeII]2- and [LFeIII]- are predicted to
assume intermediate spin arising from the electronic configura-
the ligand-based oxidation is not affected by the nature of the
metal ion as evidenced by the similarity of the redox potentials
for half-reactions 5 and 6.
The ligand field of the oxidized porphyrinogen, L∆∆, is
significantly weakened relative to that of [L]4-, to the extent
that FeII in [L∆∆FeII(NCCH3)]2+ assumes a high-spin electron
configuration. Figure 6 displays the Kohn-Sham orbitals of
[L∆∆FeII(NCCH3)]2+. The ∼4 eV span for the d-orbital splitting
in [LFe]n- complexes is reduced to ∼1.5-2 eV in [L∆∆FeII-
(NCCH3)]2+. The diminished ligand field arises from the
confluence of a smaller metal-ligand σ interaction and a
decreased basicity of the oxidized ligand. The former effect
results from the distortion of the L∆∆ porphyrinogen macrocycle
from the perfect square coordination geometry that characterizes
the [L]4- ligand environment. The most significant perturbation
2
2
2
0
2
1
2
0
2
2
2
2
2
2
tions (dxy) (dz ) (dxz,dyz) (dx -y ) and (dxy) (dz ) (dxz,dyz) (dx -y ) ,
respectively (Table S32). The dxy, dxz, and dyz orbitals are
2
effectively nonbonding as is the dz orbital in the absence of an
2
2
axial ligand. Conversely, the dx -y is strongly destabilized by
its σ-antibonding interaction with the pyrrole lone pairs, thus
accounting for the inaccessibility of the high spin state. We find
that >99% of the overall unpaired spin is carried within the d
orbital manifold, with little spin observed about the porphy-
rinogen periphery. This result is highlighted by the Kohn-Sham
representations for the frontier molecular orbitals, which are
similar for the R and â spinors of both [LFeII]2- and [LFeIII]-.
For brevity, the spatial representations for the spinors of [LFeIII]-
are depicted in the center of Figure 5. These frontier orbitals
exhibit nearly exclusive metal d orbital character with no
contribution from the porphyrinogen framework. This result is
consistent with electrochemical measurements, which reveal that
2
2
of this distortion is the stabilization of the dx -y orbital, thus
allowing for its occupation by an unpaired electron, and this
yields a high-spin state (S ) 2). The decreased basicity is a
direct consequence of the oxidation of the macrocycle by four
electrons. As for the complexes of the fully reduced ligand, more
than 99% of the total unpaired spin is localized on the iron
center.
The nature of the symmetry lowering of the macrocycle upon
four-electron oxidation is abundantly clear from the comparison
(120) Gray, T. G.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc. 2004, 12,
9760-9768.
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4740 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
Figure 6. Molecular orbital diagram for [L∆∆FeII(NCCH3)]2+ with metal-
centered functions highlighted in color. Because spin restriction is lifted,
there is not a common spatial wave function for each R/â spin pair, and
two manifolds of “orbitals” are drawn separately. The metal-centered spatial
wave functions are depicted at the 95% probability level for the R spinors
of [L∆∆FeII(NCCH3)]2+
.
of the frontier molecular orbitals of [L]4- and L∆∆ shown in
Figure 7. The [L]4- macrocycle in [LFeII]2- and [LFeIII]-
features four, essentially degenerate, HOMOs that arise from
the four possible combinations of the a2 π orbitals of the
constituent pyrroles to yield one completely bonding combina-
tion (with respect to the interpyrrole CR‚‚‚CR interactions), a
completely antibonding one, and each of the last two featuring
two bonding and two antibonding nearest-neighbor CR‚‚‚CR
interactions. In the ruffled conformation of [L]4-, the four
pyrroles are electronically isolated from each other, and the four
linear combinations are isoenergetic and completely occupied
(eight electrons) to give a net bond order of zero. As schemati-
cally represented in Chart 2, four-electron oxidation of this set
of frontier molecular orbitals lifts the four-fold degeneracy (left
panel). The four remaining electrons pair up in the two orbitals
that are bonding with respect to the newly established CR-CR
bonds, yielding a net bond order of two. If only the nodal
structure between pyrroles is represented (bottom left panel),
then the overall stereoelectronic transformation between [L]4-
and L∆∆ becomes reminiscent of the classic problem in physical
organic chemistrysthe distortion of the square cyclobutadiene
dianion (with four electrons in two, filled, degenerate π orbitals)
to the rectangular cyclobutadiene upon removal of half of its
highest-lying π electrons (bottom right panel). The tetrapyrrole
Figure 7. Comparison of relevant ligand-based Kohn-Sham orbitals of
π-type symmetry for [L]4- and L∆∆
.
oxidation is a four-electron equivalent of that of the cyclo-
butadiene dianion; in both cases, the oxidized species has to
distort from the four-fold symmetric geometry to lift the frontier
orbital degeneracy, yielding two diametrically situated bonds.
Electronic Structure Measurements. The electronic struc-
3
ture calculations predict S ) 1, /2, and 2 ground states for
[LFeII]2-, [LFeIII]-, and [L∆∆FeII]2+, respectively (Figures 5 and
6, Table S32). Experimentally, the half-integer spin of [LFeIII]-
could be assessed easily by EPR: the spectrum of (Bu4N)[LFeIII]
is shown in the top of Figure 8. It is similar those of FeIII (S )
3/2) ions residing in the square ligand fields of corroles and
porphyrins in the absence of axial ligands.121,122 The observed
g values of 4.2 (at ∂A/∂H ) 0) and 1.95 (at ∂2A/∂H2 ) 0) are
(121) Sakai, T.; Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura,
M. J. Am. Chem. Soc. 2003, 125, 13028-13029.
(122) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41, 5433-
5439.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4741

A R T I C L E S
Chart 2
Bachmann and Nocera
typical of an intermediate-spin FeIII (gx ) gy ) 4, gz ) 2) with
large axial zero-field splitting (zfs) (D . hν) but no rhombic
asymmetry (E/D ) 0).123 These zfs features are in agreement
with the D2d symmetry of the tetrapyrrole complexes. Per the
results of Sakai et al.,121 the small high-field deviation from
gz ) 2 indicates a single-electron occupancy for dxz, dyz and
dz ; this result is also obtained theoretically, as shown in Figure
2
5. Spectra of more dilute samples exhibit a very complicated
fine structure which we did not try to interpret. Consistent with
these EPR results, the magnetism of [LFeIII]- complies with
the Curie-Weiss law, including temperature-independent para-
magnetism (TIP), yielding an effective moment of µeff ) 3.7
â, which is near the spin-only value for S ) 3/2 (µeff ) 3.87, g
) 2).
As integer-spin systems, [LFeII]2- and [L∆∆FeII]2+ are less
amenable to EPR investigation. Indeed, (Bu4N)2[LFeII] was
found to be EPR silent, consistent with the presence of a large
zero-field splitting (zfs) (D . hν, E/D ) 0). The observation
of an EPR spectrum (Figure 8, bottom) for [L∆∆FeII](BF4)2
indicates rhombic distortion from a square coordination, E/D
* 0. The formation of both cyclopropane rings, observed by
crystallography and by NMR, presumably results in |ms| state
mixing, giving rise to otherwise forbidden transitions. A Curie-
Weiss fit of the magnetism of [L∆∆FeII](BF4)2 yields an overall
µeff ) 5.0 â (µeff(calcd) ) 4.9 â for spin-only S ) 2).
Definitive experimental oxidation state and spin state assign-
ments for the inorganic centers of [LFeII]2- and [L∆∆FeII]2+ were
afforded by their Mo¨ssbauer spectra, displayed in Figure 9. The
single quadrupole doublet exhibited by [LFeII]2- (δ ) 0.35 mm/
s, ∆Eq ) 2.34 mm/s) is characteristic of an intermediate spin,
S ) 1. Although this spin state is relatively rare for FeII, it is
by far the most prevalent in the few reported square planar
complexes of that ion, and the Mo¨ssbauer characteristics lie
within the range spanned by these complexes.124-126 The
(123) Palmer, G. In Physical Methods in Bioinorganic Chemistry; Que, L., Jr.,
Ed.; University Science Books: Sausalito, CA, 2000; p 153.
(124) Oh, Y.; Shin, B.-C.; Swenson, D.; Goff, H. M.; Kang, S. K. Acta
Crystallogr. 2004, C60, m57-m59.
(125) Strauss, S. H.; Silver, M. E.; Long, K. M.; Thompson, R. G.; Hudgens,
R. A.; Spartalian, K.; Ibers, J. A. J. Am. Chem. Soc. 1985, 107, 4207-
4215.
(126) Riley, D. P.; Stone, J. A.; Busch, D. H. J. Am. Chem. Soc. 1977, 99,
767-767.
Figure 8. Frozen solution X-band (9.385 GHz) EPR spectra of (Bu4N)-
[LFeIII] (blue spectrum) in frozen THF (4.5 K) and [L∆∆FeII](BF4)2 (green
spectrum) in frozen CH3CN (4.5 K). [LFeII]2- is EPR silent.
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4742 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Multielectron Redox Chemistry of Iron Porphyrinogens
A R T I C L E S
ligand-based redox activity therefore proceeds in a single four-
electron step. The oxidation of the tetrapyrrole is accompanied
by a distortion of the 4-N core from a square to a rectangle,
and this structural deformation is manifest to the electronic
structure of the different iron porphyrinogens.
The ability of the tetrapyrrole macrocycle to support redox
transformations in addition to the metal establishes “artificial
hemes” as more versatile redox cofactors than hemes. This redox
activity between the metal and the ligand is decoupled from
the acid-base activity of the iron center. Large variations in
the overall redox content can be accommodated without any
changes in the metal’s primary coordination sphere. This is
drastically different from the usual observation of a metal center
that will have its coordination geometry and/or number change
upon multielectron oxidation. For the case of iron hemes,
oxidation is typically accompanied by an increase in metal-
substrate bond order (e.g., oxo). It can be anticipated that iron
porphyrinogens bearing halogen- or chalcogen-based ligand
substrates will maintain more labile metal-substrate bonds in
high oxidation states. We are currently pursuing such ideas in
the context of the activation of small molecules.
Figure 9. 57Fe Mo¨ssbauer spectra of powdered Na2(THF)2[LFeII] (top) and
[L∆∆FeII](BF4)2 (bottom) at 4.2 K.
Mo¨ssbauer spectrum of the [L∆∆FeII]2+ cation (δ ) 1.11 mm/
s, ∆Eq ) 4.66 mm/s) is most intriguing. The large quadrupole
splitting clearly indicates a high spin state, S ) 2;127 but also,
this ∆Eq is to the best of our knowledge the largest reported to
date, surpassing the value of 4.55 mm/s that is observed for the
Acknowledgment. We dedicate this work to the memory of
Prof. Carlo Floriani, a pioneer in the chemistry of porphyrino-
gens. We thank B. M. Bartlett for acquiring the SQUID data,
Prof. S. J. Lippard for use of his Mo¨ssbauer instrument, and
Dr. J. Kuzelka and Dr. S. Yoon for their help with its use. This
work was supported by a grant from the National Science
Foundation (CHE-0132680). J.B. gratefully acknowledges MIT
for a pre-doctoral Presidential Fellowship and the Shell Oil
Company for a research fellowship.
i
distorted tetrahedral FeII (S ) 2) center in [Fe3(SC6H2 Pr3-2,4,6)4-
{N(SiMe3)2}2].128
Conclusions
Three oxidation states of iron porphyrinogen have been
unveiled in the absence of any redox or spectroscopically active
counterions. A metal-centered FeII/III redox activity, reminiscent
of heme porphyrins, is observed. Owing to the highly reducing
nature of the tetrapyrrole framework, the FeIII/II reduction
potential, however, is nearly one volt more negative than that
of a typical heme cofactor. This metal-based redox activity is
augmented by a multielectron chemistry of the porphyrinogen
macrocycle. Two cyclopropane rings, formed from the C-C
coupling between the R carbons of adjacent pyrroles, function
as two separate two-hole reservoirs within the porphyrinogen
framework. The two-electron oxidized, singly CR-CR bonded,
intermediate is unstable with respect to disproportionation; the
Supporting Information Available: Crystal structure of
[(THF)3FeII(µ-Cl)3FeII(THF)3][(THF)4FeII(µ-Cl)2FeII(THF)4]1/2
-
[LFeIII]2‚2PhCH3; cyclic voltammetry of the [LFeIII]-/[LFeII]2-
redox couple; atom numbering schemes, tables of crystal data,
atomic coordinates, bond lengths and angles, anisotropic thermal
parameters and hydrogen coordinates for [Na(digly)2][LFeIII],
[(THF)3FeII(µ-Cl)3FeII(THF)3][(THF)4FeII(µ-Cl)2FeII(THF)4]1/2
-
[LFeIII]2‚2PhCH3, [(THF)Na(Opy)]2[LFeII], and [L∆∆FeII(NCMe)]-
[(C2B9H11)2Co]2‚2MeCN‚2o-C6H4Cl2; energies of the frontier
MOs and Mulliken population analysis for [LFeII]2-, [LFeIII]-,
and [L∆∆FeII(NCMe)]2+. X-ray crystallographic files, in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(127) Mu¨nck, E. In Physical Methods in Bioinorganic Chemistry; Que, L., Jr.,
Ed.; University Science Books: Sausalito, CA, 2000; p 291.
(128) Evans, D. J. Chem. Phys. Lett. 1996, 255, 134-136.
JA043132R
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