Ground- and excited-state reactivity of iron porphyrinogens

Article abstract of DOI:10.1021/ic0616636

The iron(II) porphyrinogen dication, [L^ΔΔFe ^II]²⁺, is a multielectron oxidant featuring the metal center in its reduced state and the ligand as the redox reservoir. Oxidations break the ligand's redox-active C-C bonds. Extremely short-lived excited states are consistent with extensive structural reorganization that accompanies charge-transfer excitation of the porphyrinogen.

Full text of DOI:10.1021/ic0616636

Inorg. Chem. 2007, 46, 607−609
Ground- and Excited-State Reactivity of Iron Porphyrinogens
Julien Bachmann, Justin M. Hodgkiss, Elizabeth R. Young, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307
Received September 2, 2006
The iron(II) porphyrinogen dication, [L∆∆FeII]2+, is a multielectron
oxidant featuring the metal center in its reduced state and the
ligand as the redox reservoir. Oxidations break the ligand’s redox-
active
C−C bonds. Extremely short-lived excited states are
consistent with extensive structural reorganization that accompanies
charge-transfer excitation of the porphyrinogen.
Figure 1. Porphyrinogen ligand used in this study, in its lowest (left) and
highest (right) oxidation states.
of [L∆∆FeII]2+ from +59 and -7 ppm to +55 and -9 ppm,
respectively (Figure S1). Alternatively, the same NMR
signals are obtained upon three-electron oxidation of Na-
[LFeIII] with AuCl3, KAuCl4, or PhICl2. The simple shift of
the 1H â-pyrrole resonances shows that the symmetry of the
macrocycle is maintained in the reaction and its oxidation
state is not affected,9 consistent with the axial coordination
of Cl- to Fe. Crystallographic analysis of the isolated material
proves its identity to be [L∆∆FeII-Cl](FeCl4) (Figures 2 and
S3). The UV-visible spectrum of the complex is independent
The redox chemistry of coordination compounds is
expanded when derived from a metal and a ligand working
in concert. For such cases, redox activity is usually derived
from (a) one-electron ligand-based redox couples and (b)
the involvement of frontier orbitals of mixed metal-ligand
character.1-5 The porphyrinogen (Figure 1) actively partici-
pates in redox chemistry and does so in discrete two-electron
steps by storing multielectron equivalents with the formation
or breaking of one or two cyclopropane C-C bonds that
are localized on the porphyrinogen ring.6,7 Although transi-
tion-metal complexes of oxidized porphyrinogen have been
structurally established for several metal-halo inorganic
centers,6-8 the general coordination and redox chemistry of
such compounds have remained largely unexplored. We
recently described an electron-transfer series for iron
porphyrinogen,7b
-
of the synthetic path; the conspicuous bands of the FeCl4
counterion are observed at 313 and 363 nm.10 The anion is
produced by the presence of excess chloride, which causes
extrusion of the Fe ion from a fraction of the metallopor-
phyrinogen units and limits the isolated yield to 56%. The
powder Mo¨ssbauer spectrum of [L∆∆FeII-Cl](FeCl4) displays
(in addition to the FeCl4- signal)11 large quadrupole splitting
and isomer shift, δ ) 1.0 mm/s, ∆Eq ) 3.8 mm/s, similar to
those observed for [L∆∆FeII](BF4)2, corresponding to high-
spin Fe(II).7b
[LFeII]2- -8 [LFeIII]-
-8 [L∆∆FeII]2+
(1)
-e
-3e
This Communication reports on the reactivity originating
from the four-electron-oxidized iron porphyrinogen dication,
[L∆∆FeII]2+, and provides the first excited-state studies of
porphyrinogen coordination compounds.
The reactivity of [L∆∆FeII]2+ switches from ligand sub-
stitution chemistry to redox chemistry upon reaction with
Unlike its reduced congeners [LFeII]2- and [LFeIII]-,
[L∆∆FeII]2+ reacts with anionic ligands. Exposure of
[L∆∆FeII](BF4)2 to a stoichiometric or excess amount of Bu4-
NCl in acetonitrile causes a shift in the 1H â-pyrrole signals
(6) (a) Jubb, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1992, 114, 6571-6573. (b) De Angelis, S.; Solari, E.; Floriani,
C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1994, 116, 5691-
5701. (c) De Angelis, S.; Solari, E.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. J. Am. Chem. Soc. 1994, 116, 5712-5713. (d) Crescenzi,
R.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1999, 121, 1695-1706.
* To whom correspondence should be addressed. E-mail: nocera@mit.edu.
(1) Holm, R. H.; Balch, A. L.; Davison, A.; Maki, A. H.; Berry, T. E. J.
Am. Chem. Soc. 1967, 89, 2866-2874 and references cited therein.
(2) Ray, K.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc.
2005, 127, 5641-5654 and references cited therein.
(3) Pierpont, C. G. Coord. Chem. ReV. 2001, 216, 99-125.
(4) Gutlich, P.; Dei, A. Angew. Chem., Int. Ed. 1997, 36, 2734-2736.
(5) (a) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005,
44, 5559-5561. (b) Haneline, M. R.; Heyduk, A. F. J. Am. Chem.
Soc. 2006, 128, 8410-8411.
(7) (a) Bachmann, J.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 2829-
2837. (b) Bachmann, J.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127,
4730-4743. (c) Bachmann, J.; Nocera, D. G. Inorg. Chem. 2005, 44,
6930-6932.
(8) Bhattacharya, D.; Dey, S.; Maji, S.; Pal, K.; Sarkar, S. Inorg. Chem.
2005, 44, 7699-7701.
(9) L4- displays fourfold rotational symmetry in solution and in the solid
state, L∆∆ twofold, and L∆2- none.
(10) Shapley, P. A.; Bigham, W. T.; Hay, M. T. Inorg. Chim. Acta 2003,
345, 255-260.
10.1021/ic0616636 CCC: $37.00
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Inorganic Chemistry, Vol. 46, No. 3, 2007 607

COMMUNICATION
and P(n-Bu)3. On the other hand, its reactions with P(C6F5)3,
tetracyanoethane, water, pyridinium chloride, and Ph4PBr
yield [LFeIII]- as the single paramagnetic product (see the
crystal structure of (Ph4P)[LFeIII], Figures 2 and S4). This
product likely results from single-electron redox reactions
of the iron porphyrinogen species with substrate and solvent.
A well-defined chemistry of [L∆∆FeII]2+ is obtained with
iodide as the reductant. On the basis of the previously
determined redox potential of [L∆∆FeII]2+ (vs NHE), the
thermodynamics of reaction (4) are favorable:12
I3- + 2e- f 3I-
[L∆∆FeII]2+ + 3e- f [LFeIII]-
2[L∆∆FeII]2+ + 9I- f 2[LFeIII]- + 3I3 ∆E ≈ +0.24 V (4)
E° ) +0.53 V
(2)
E° ) +0.77 V (3)
-
Upon mixing [L∆∆FeII](BF4)2 with Bu4NI, the characteristic
UV-visible signature of triiodide immediately appears at
291 and 362 nm. The fate of the iron porphyrinogen
byproduct was found to depend strongly on the experimental
conditions. Under slow addition of a cold solution (-40 °C)
of [L∆∆FeII](BF4)2 to excess Bu4NI (g6 equiv), more than
80% of [L∆∆FeII]2+ is converted to [LFeIII]- according to eq
4. The remaining product is accounted for by LH4, which
presumably forms by H• atom abstraction from the solvent.
However, when a stoichiometric amount of I- (4.5 equiv) is
added to [L∆∆FeII]2+ at room temperature, all of the porphy-
rinogen is found as LH4. The stoichiometry of the reaction
was determined spectroscopically (Figure 3) to yield a
I-/[L∆∆FeII]2+ ratio of 9:2, as expected on the basis of eq 4.
The reduction of [L∆∆FeII]2+ to [LFeIII]- by I- is contrasted
to the recently reported8 oxidation of iron(III) tetrakis(meso-
pentane-1,5-diylporphyrinogen), [*LFeIII]-, by excess I2 to
yield [*L∆∆FeII-I](I3)‚I2. We note that the iodide complex
was originally formulated8 as [*L∆∆FeII-I]+(I3)-‚(I2)+(I3)-.
A review of the CIF file shows a miscount in the ions placed
at special positions and indicates the correct formulation to
be [*L∆∆FeII-I](I3)‚I2.
Figure 2. Crystal structures (thermal ellipsoids at 50% probability, H atoms
and solvents omitted) of [L∆∆FeIICl](FeCl4) (top: orthorhombic, Pbca, R1
[I > 2σ(I)] ) 0.046) and (Ph4P)[LFeIII] (bottom: monoclinic, P21/c, R1 [I
> 2σ(I)] ) 0.059). The asymmetric unit of the [L∆∆FeIICl](FeCl4) crystal
contains two chemically identical ion pairs, only one of which is displayed.
Full crystallographic data and color figures are available as Supporting
Information.
Figure 3. UV-visible spectra of [L∆∆FeII](BF4)2 and Bu4NI reaction
mixtures of varying stoichiometries n (I-:[L∆∆FeII]2+). The bands at 291
and 362 nm are due to I3- and those at 245 nm to I-. Inset: 362-nm signal
as a function of the stoichiometry, with linear fits intersecting at n ) 4.5.
1
alcoholates such as KOCMe3 or NaOMe. H NMR reveals
The excited states of the iron porphyrinogens were
examined by transient absorption spectroscopy. Density
functional theory (DFT) calculations identify that the lowest-
energy allowed transition of [LFeIII]- is derived from a one-
electron ligand-to-metal charge transfer (LMCT) from the
pyrroles to empty d(Fe) orbitals.7b Figure 4 displays the
transient absorption spectrum of [LFeIII]-. A prominent
bleach of the ground-state absorption is superimposed on a
broad positive transient signal. Beyond the initial spike
attributed to pump-probe cross-correlation, the transient state
relaxes back to the initial state monoexponentially with a τ
) 13((3) ps lifetime at all wavelengths probed (λprb ) 500-
806 nm; Figure S3 ). Because the transient spectrum is more
or less a bleach of the ground state, identifying the chemical
nature of the excited state (e.g., a charge transfer [L‚FeII]- 13
vs a vibrational excited state14 of [LFeIII]-) is difficult.
reduced [LFeIII]- (â-pyrrole peak at -72 ppm) as the only
observable product. Reduction is prevented with the silano-
late NaOSiMe3; elemental analysis of the isolated material
is consistent with the axially coordinated complex [L∆∆FeII-
OSiMe3](OSiMe3). The δ ) 0.4 mm/s and ∆Eq ) 1.0 mm/s
values from the Mo¨ssbauer spectrum are also consistent with
the presence of a Fe(II) oxidation state. In solution, rotation
around the Fe-O bond appears to be sterically hindered, as
1
evidenced by four distinct H NMR resonances for the
â-pyrroles (δ ) +107, +89, +84, and +27 ppm; Figure
S1). The silanolate, [L∆∆FeII-OSiMe3]+, reacts with F- in
dichloromethane to yield a solid precipitate, which redis-
solves in acetonitrile. Its NMR spectrum shows the presence
of a single paramagnetic product, with 2-fold symmetry (1H
â-pyrrole, +20 and +15 ppm; 2H methyl, +7, +4, -4, and
-21 ppm), and a large quadrupole splitting but a small
isomer shift (δ ) 0.3 mm/s; ∆Eq ) 3.2 mm/s) is observed
in the Mo¨ssbauer spectrum. The compound does not react
with electron-rich substrates such as cis-cyclooctene, PPh3,
(11) Crystal packing forces seem to distort the anion slightly from its ideal
tetrahedral symmetry: δ ) 0.3 mm/s; ∆Eq ) 0.3 mm/s.
(12) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press:
Oxford, U.K., 1988; p 937.
608 Inorganic Chemistry, Vol. 46, No. 3, 2007

COMMUNICATION
Figure 5. Reactivity originating from oxidized iron porphyrinogen.
Figure 4. Transient absorption decay signal of Na[LFeIII] (black dots)
probed at 530 nm upon excitation at 403 nm, with an exponential fit (gray
line). Inset: transient spectrum at t ) 5 ps (black) compared to the ground-
state absorption (dotted gray trace); the horizontal line defines the zero.
the two-electron CR-CR bonds of the ligand function as the
multielectron reservoir, allowing the oxidation state of the
metal to be minimally affected. The porphyrinogen is unusual
compared to typical coordination compounds in that the
highly oxidizing center is located on the ligand and not the
metal. Thus, the metal remains a poor Lewis acid even when
the compound is highly oxidized. Other consequences of
ligand-based redox equivalency are evident in the photo-
physics of these compounds. The excited states of oxidized
chloroiron complexes of porphyrins and porphyrinogen
exhibit widely different reactivities despite similar lifetimes.
In the latter case, charge-transfer photoexcitation is followed
by fast back-transfer. The extensive structural reorganization
of the ligand and bond-breaking/-making within it that
accompany redox events such as that potentially derived from
charge-transfer excitation appear to be sufficiently demanding
that they cannot compete with vibrational relaxation of the
electronic excited state.16 Consequently, the compound is
photostable.
Fast excited-state dynamics are also observed for
[L∆∆FeIICl]+. The lowest unoccupied molecular orbitals of
[L∆∆FeIICl]+ are ligand-based, and the highest occupied
molecular orbitals correspond to the nonbonding electron of
the chloride ligand, with metal-centered singly occupied
molecular orbitals at intermediate energies (Figure S2),
suggesting the presence of a chloride-to-porphyrinogen
charge-transfer contribution. A weak positive signal at 806
nm appears promptly after 550-nm excitation and subse-
quently decays monoexponentially with a lifetime of τ )
30((10) ps. This transient state does not support excited-
state bimolecular reactivity. Indeed, no photoreaction is
observed upon steady irradiation into the absorption manifold
of [L∆∆FeIICl]+, even in the presence of H• atom donors such
as terpinene. We note that a photoreaction is observed when
the absorption envelope of the FeCl4- counterion is excited
(350 nm < λ < 450 nm). H• atom abstraction from terpinene
-
by [FeCl4 ]* leads to its quantitative aromatization to
Acknowledgment. This work was supported under a
grant from the National Science Foundation (Grant CHE-
0132680) and made use of the Shared Experimental Facilities
at MIT’s CMSE, a MRSEC Program of the National Science
Foundation (Grant DMR 02-13282).
cymene.
Nonradiative decay of iron porphyrinogens occurs before
the geometric reorganization and bonding changes attendant
to the two-electron transformations of the ligand can occur.
This observation contrasts the photochemistry of iron por-
phyrins. Chloroiron(III) porphyrins, PFeIII-Cl, undergo facile
metal-ligand bond homolysis to PFeII + Cl• upon chloride-
to-iron(III) LMCT excitation,15 despite the short excited-state
lifetime (shorter than that reported here for [L∆∆FeIICl]+).
In the case of the porphyrins, excitation is localized in the
Cl-Fe bond that is being activated. Therefore, the structural
rearrangement caused by the excited state directly activates
the chemically relevant vibrational mode. Consistent with
this interpretation, π f π* excitation of the porphyrin ring
of PFeIII-Cl does not result in an efficient photochemistry.
In the case of the porphyrinogens, the excitation occurs
between bonds that are weakly coupled, i.e., the Fe-Cl bond
and the CR-CR bonds of the porphyrinogen ring. We expect
that this phototransformation is highly nonadiabatic, thus
accounting for the lack of porphyrinogen photochemistry.
Supporting Information Available: Experimental procedures,
NMR spectra, DFT results, ultrafast spectroscopy data, and tables
of crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0616636
(13) The spectra of [LFeII]2- and aromatic cation radical of L show no
structured absorption in the visible region (see ref 7b).
(14) Bilsel, O.; Milam, S. N.; Girolami, G. S.; Suslick, K. S.; Holten, D.
J. Phys. Chem. 1993, 97, 7216-7221.
(15) (a) Hendrickson, D. N.; Kinnaird, M. G.; Suslick, K. S. J. Am. Chem.
Soc. 1987, 109, 1243-1244. (b) Suslick, K. S.; Watson, R. A. Inorg.
Chem. 1991, 30, 912-919. (c) Suslick, K. S.; Watson, R. A. New J.
Chem. 1992, 16, 633-642.
(16) Because the porphyrinogen ligand is redox-active, charge-transfer
excited states can, in general, be described in a first approximation as
transient states in which electron transfer has occurred between two
parts of the species, one of which is the macrocycle in the case of
iron porphyrinogens. Electron transfer to and from the porphyrinogen
ligand is accompanied by bond-breaking/-making events, and therefore
charge-transfer excitation in iron porphyrinogens is coupled to
structural reorganization. Such coupling between nuclear and electronic
coordinates would facilitate efficient nonradiative relaxation, thus
circumventing possible further chemically relevant transformations.
The reaction chemistry of [L∆∆FeII]2+ is summarized in
Figure 5. The neutral charge of the oxidized L∆∆ macrocycle
makes the cationic iron center prone to axial coordination.
[L∆∆FeII]2+ undergoes facile multielectron reduction to
[LFeIII]- by iodide and alcoholates. In such transformations,
Inorganic Chemistry, Vol. 46, No. 3, 2007 609