Simple dimer containing dissociatively stable mono-imidazole ligated ferrohemes

Article abstract of DOI:10.1039/b717858a

In weakly coordinating solvents Fe^II meso-(N-methylimidazol-2- yl)porphine 1Fe exists as a stable dimer (K_d = 50 ± 30 nM) that binds ligands without undergoing dissociation and is presently the simplest complex in which the mono-imidazole ligation of a ferroheme is enforced without excess imidazole in solution. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717858a

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Simple dimer containing dissociatively stable mono-imidazole ligated
ferrohemesw
Qing-Zheng Yang, Daria Khvostichenko, John D. Atkinson and Roman Boulatov*
Received (in Austin, TX, USA) 19th November 2007, Accepted 12th December 2007
First published as an Advance Article on the web 4th January 2008
DOI: 10.1039/b717858a
In weakly coordinating solvents FeII meso-(N-methylimidazol-
2-yl)porphine 1Fe exists as a stable dimer (Kd = 50 ꢀ 30 nM)
that binds ligands without undergoing dissociation and is pre-
sently the simplest complex in which the mono-imidazole ligation
of a ferroheme is enforced without excess imidazole in solution.
Our objective was to identify simple structural motifs to
enforce dissociatively stable axial coordination of a ferroheme
to a single heterocyclic base without relying on excess of a
sterically hindered ligand (e.g., 2-methylimidazole, 2-MeIm) in
solution. The affinity of a 4-coordinate FeII porphyrin moiety,
Fe(por), for imidazoles (Im) is low (dissociation constant, Kd,
B0.1 mM1) and with the exception of C2-substituted imida-
(1Fe)2 in C6H5Me, CHCl3 or thf, whereas 3Fe and 2Fe were
present predominantly as monomers.8
zoles, FeII(por)(Im) is unstable with respect to a mixture of
FeII(por) and FeII(por)(Im)2.2 Our long-term objective is to
exploit the extensive structure–activity relationship identified
in biomimetic studies of O2 reduction by cytochrome oxidase
to develop simple Fe and Co porphyrin complexes as potential
Pt-free alternatives for O2 reduction catalysts for low-tem-
perature fuel cells. Available literature data3 suggest that, with
few exceptions, enforcing the axial coordination of an Fe or
Co porphyrin by a heterocyclic base (imidazole or pyridine)
throughout the electrocatalytic O2 reduction cycle is the single
most effective strategy to maximize the selectivity, turnover
frequency and turnover numbers and to minimize the over-
potential. The best reported metalloporphyrin-based electro-
catalysts contain an axial imidazole attached covalently to the
macrocycle to achieve intramolecular chelating coordination to
FeII and require multistep synthesis, which precludes their
practical uses. On the other hand, catalytic properties of
surface-adsorbed simple Fe or Co porphyrins are not im-
proved by adding imidazole or pyridine to the aqueous
electrolyte.3
Solution UV–Vis spectra of 1Fe manifested a split Soret of a
pattern typical for (1M)2 dimers (Fig. 1a).4,9,10 The peak at
373 nm is indicative of an FeII porphyrin with a single
imidazole ligand;11 peaks at 548 nm and 575 nm that are
typically observed in such complexes are significantly broa-
dened. From the dilution experiments8 we determined Kd of
(1Fe)2 to be 50 ꢀ 30 nM. Evans measurements revealed 7 ꢀ 1
unpaired electrons per dimer, in accord with the expected2
(and calculated) quintet electronic state of imidazole-ligated
ferroheme, indicating the absence of electronic communica-
tion between the two ferroheme moieties in (1Fe)2.
Affinities of (1Fe)2 to N-methylimidazole (N-MeIm), nitro-
sophenyl (PhNO), and isopropyl isocyanide (iPrNC) were
measured by spectrophotometric titrations of toluene solu-
tions. Titrations with N-MeIm and PhNO proceeded with
well-defined isosbestic points8 suggestive of the interconver-
sion between two chromophores: a 5-coordinate and a
6-coordinate ferroheme. Unit slopes of the Hill plots12
(Fig. 2) indicate that the two binding sites in (1Fe)2 are
independent. The spectral changes in titration of (1Fe)2 with
iPrNC8 could only be modeled with a 3-component system:
Porphyrins 1H2–2H2 are obtained by a previously reported
one-step mixed condensation.4,5 Complexes of these porphy-
rins with many transition metals (but not FeII) have been
reported to form dimers with varying degrees of stability.6,7
Metallation of free bases with FeBr2 in the presence of 2,6-
lutidine in thf proceeded quantitatively. Spectroscopic and
ligand-affinity data suggested that 1Fe existed as a dimer
Department of Chemistry, University of Illinois, 600 S. Mathews
Ave., Urbana, IL 61801, USA. E-mail: boulatov@uiuc.edu;
Fax: +1 217 244 3186; Tel: +1 217 333 4968
w Electronic supplementary information (ESI) available: Details of
syntheses, characterization, spectrophotometric titrations and compu-
tational studies. See DOI: 10.1039/b717858a
Fig. 1 Absorption spectra of (a) (1Fe)2 (black) and (1Zn)2 (blue) and
(b) 3Fe (black) and Fe(tpp) (blue, tpp = tetraphenylporphyrin) in
toluene. All spectra are scaled to the same maximum absorption. All
solutions were 20 mM in the Fe(por) chromophore.
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In contrast to (1Fe)2, the spectroscopic and ligand binding
properties of 3Fe and 2Fe were similar, closely resembling
those of 4-coordinate Fe(tpp). Solution UV–Vis spectra of
either species in toluene up to 100 mM manifested a split Soret
typical of a 4-coordinate ferroheme (Fig. 1). In toluene affinity
of 3Fe and 2Fe for PhNO was low (Kd = 0.10 ꢀ 0.09 mM and
40 ꢀ 30 mM, respectively); and their affinity for iPrNC (Kd =
4 ꢀ 3 mM and 0.9 ꢀ 0.7 mM) and 2-MeIm (Kd = 40 ꢀ 30
mM and 16 ꢀ 7 mM) was comparable to that of Fe(tpp).1 We
did not detect bisadducts, Fe(por)L2 (L = PhNO, 2-MeIm or
iPrNC), which typically do not form with these ligands. Like
Fe(tpp), solutions of 3Fe and 2Fe in C6D5N were diamagnetic.
The undetectably low dimerization constant of 3Fe and 2Fe is
a manifestation of the low affinity of pyridine to a 4-coordi-
nate FeII(por) moiety (Kd 4 1 mM).1
i
Fig. 2 Thermodynamics of ligand binding to (1Fe)2 presented as Hill
plots;12 ln([L]/1 M) is the natural log of the total concentration of the
Four-coordinate monomeric 1Fe and 3Fe bind PrNC with
an identical affinity, suggesting that the affinities of 3Fe and
monomeric 1Fe to PhNO and 2-MeIm or N-MeIm are also
similar.z With this assumption the stability of the dimeric
motif in the presence of RNC, RNO or imidazole can be
evaluated. In solution, (1M)2 dimers (M = Zn, Mg) dissociate
upon exposure to even moderate Lewis bases (e.g., MeOH).16
Whereas (1Fe)2 is 104-fold more stable with respect to the
monomers than (1Fe)2(iPrNC)2, we estimate that (1Fe)2-
(PhNO)2 is more stable than (1Fe)2, and stabilities of
(1Fe)2(N-MeIm)2 and (1Fe)2 are comparable. The dissociation
constants of a bisadduct, KLd, and of (1Fe)2, Kd, are related:
KLd = Kd(KL/K)2, where K and KL are defined by eqns (1) and
(4), respectively. KPhNO of 3FeII is 500-fold lower than that of
individual FeII in (1Fe)2, giving the dissociation constant of
(1Fe)2(PhNO)2 to 2 (1Fe)(PhNO) of 0.2 pM. In contrast, K
(eqns (1) and (2), L = N-MeIm) and K2-MeIm (eqn 4, N = 3)
are identical (within experimental error), suggesting that for-
mation of (1Fe)(N-MeIm)x (x = 1, 2) from (1Fe)2 in the
presence of N-MeIm is probably unfavorable. Using the
literature equilibrium constant1 for the formation of Fe(por)-
(H2O)2 from Fe(por) and the affinity of H2O to imidazole-
ligated 5-coordinate ImFeII(por) we estimate that equilibrium
(5) will remain unfavorable even in pure water. Based on this
analysis, we expect that (1Fe)2 deposited on a graphite elec-
trode in contact with an aqueous electrolyte will remain intact,
thereby enforcing the mono-imidazole ligation of FeII.
i
indicated ligand (PhNO, N-MeIm or PrNC) normalized to 1 M; a is
the fraction of the 6-coordinate, ImFe(por)L, sites. The data were
obtained by spectrophotometric titration of 30 mM solutions of (1Fe)2
in toluene at 27 ꢀ 1 1C under rigorously anhydrous and anaerobic
conditions.
two distinct 5-coordinate and one 6-coordinate ferroheme
chromophores. We assigned them to (1Fe)2, (1Fe)(iPrNC)
and the 6-coordinate part of (1Fe)2(iPrNC) based on the
results of the NMR studies and the UV–Vis spectra of
Fe(tpp)(iPrNC)x (x = 1, 2). The relationship between these
species was adequately described by equilibria (1)–(3) (L =
iPrNC). Because neither (1Fe)2(iPrNC)2 nor (1Fe)(iPrNC)
absorbs below 400 nm, the disappearance of the peak at 373
nm in (1Fe)2 (Fig. 1) allowed us to establish that binding of
iPrNC to the two sites of (1Fe)2 was also independent (Fig. 2).
The affinity of FeII in (1Fe)2 to iPrNC is 104-fold lower than is
typical for a 5-coordinate imidazole-ligated ferroheme13,14 and
102-fold lower than the affinity of 4-coordinate FeII(por), such
as Fe(tpp), 3Fe or 2Fe. The dissociation constant of the
bisadduct, (1Fe)2(iPrNC)2, KPdrNC (eqn 3), was 0.5 ꢀ 0.4 mM,
i.e. 104-fold higher than that of (1Fe)2. From these data and
i
the dissociation constant of (1Fe)2 the affinity of PrNC to 4-
coordinate, monomeric, 1Fe is estimated to be 0.2 mMꢂ1
.
(1Fe)2 + L $ (1Fe)2L, K
(1Fe)2L + L $ (1Fe)2L2, K
(1Fe)2L2 $ 2 (1Fe)L, KdL
(1)
(2)
(3)
NFe + L $ (NFe)L (N = 1–3) KL
(4)
(1Fe)2(H2O)2 + 2H2O $ (1Fe)(H2O)2, Kwd o 1 mMꢂ1 (5)
Despite numerous attempts we were unable to obtain an X-ray
diffraction structure of (1Fe)2 or one of its adducts. To better
understand the structural and electronic properties of (1Fe)2
and its adducts, we optimized dimers 4, 4(MeNC) and
4(MeNC)2 (Fig. 3), as models of (1Fe)2, (1Fe)2(iPrNC) and
(1Fe)2(iPrNC)2, respectively, at the B3LYP/6-31g level.8 Geo-
metries of relevant 5- and 6-coordinate FeII porphyrins calcu-
lated with B3LYP/6-31g agreed well with experimental data.8
Replacement of peripheral aliphatic groups with H atoms is
known to have an insignificant impact on the computed
structural parameters and electronic properties of ferro-
hemes.17 The computations revealed the C2h symmetry of 4
and 4(MeNC)2, in accord with the NMR spectra of (1Fe)2L2
1H-NMR spectra of (1Fe)2(PhNO)2 were consistent with
the dimeric formulation of the adduct:9,15 for example, the
chemical shifts of the imidazole protons in (1Fe)2(PhNO)2
were 7.5 and 2.7 ppm upfield of those protons in 1H2; the b-
pyrrolic protons closest to the imidazole experienced 3.3 ppm
upfield shift, whereas those farthest from the imidazole shifted
8
i
downfield. NMR spectra of (1Fe)2 in the presence of PrNC
(1.1–5 equiv.) revealed the presence of two compounds:
(1Fe)2(iPrNC)2 and (1Fe)(iPrNC), consistent with the results
of spectrophotometric titrations. We observed no binding of
pyridine or 2-MeIm to (1Fe)2 consistent with the ‘tense’ state
of FeII in (1Fe)2 as suggested by DFT calculations (see below).
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30 nM) containing mono-imidazole-ligated ferroheme. The
existence of higher oligomers at the working concentrations
(o100 mM) was inconsistent with the available data. The
molecule is easily accessible synthetically: the free base is
available in one step and metallation is quantitative. Binding
of PhNO to the dimer increases its dissociative stability and
the dimer also binds two molecules of N-methylimidazole or
iPrNC. Spectroscopic studies and DFT calculations showed
that the two centers bind ligands independently. (1Fe)2 pro-
vides a simple route to dissociatively stable mono-imidazole
ligated ferroheme centers that may be of use for Pt-free
catalysis of O2 reduction in low-temperature fuel cells. On
the other hand, FeII meso-(pyrid-2-yl)porphine derivatives do
not dimerize to any appreciable extent.
This work was supported by the University of Illinois,
American Chemical Society Petroleum Research Fund (grant
43354-G3) and by the National Science Foundation through
TeraGrid resources under grants TG-CHE050064 and
TG-CHE060020.
Fig. 3 Chemical structures of dimers 4, 4(MeCN) and 4(MeCN)2 and
the minimum energy structure of 4 at the B3LYP/6-31G level. Colors:
Fe, red; N, blue; C, gray; hydrogen atoms are omitted for clarity.
i
(L = PhNO or PrNC) and (1M)2 (M = Zn, Mg), and an
Notes and references
approximate Cs symmetry of 4(MeNC) and confirmed that the
two binding sites in the dimers are structurally and electro-
nically independent.8 The spin states of the Fe centers in 4
were uncoupled. Upon ligand binding, electronic and struc-
tural changes at the binding site were pronounced and con-
sistent with known properties of FeII porphyrins:2,18,19 the iron
ion became low-spin singlet, with a concomitant decrease in its
displacement from the porphyrin plane (Fe–Ct distance). In
either binding event, the structural and spin state of the
spectator site remained unaffected.8
The ‘tense’ state of the 5-coordinate FeII sites in 4 and
4(MeNC) (Fe–Ct: 0.355–0.359 A vs. 0.335 A in (2-MeIm)Fe-
(porphine)) and unusually small contraction of the Fe–NIm
distance upon MeNC binding (o0.010 A vs. 0.039 A for
(2-MeIm)Fe(porphine)) likely result from steric repulsion
between the two porphyrins of the dimer. The shortest separa-
tion between a pair of carbon atoms in 4(MeNC) and
4(MeNC)2 is 3.331 and 3.206 A, respectively, less than the
sum of the van der Waals radii of two sp2 carbons20 (3.4 A).
These separations are similar to those observed in the crystal
structures of an analog of (2Zn)2 (3.28–3.34 A).4
z 2-MeIm and N-MeIm bind sterically unhindered ferrohemes with
comparable affinities.2
1 M. Tabata and J. Nishimoto, in The Porphyrin Handbook, ed. K.
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The B3LYP/6-31g method underestimates Fe–Ct distances
of 5-coordinate FeII(por),8 and the true Fe–Ct value in (1Fe)2
may be B0.37–0.38 A. Such large displacements of FeII from
the porphyrin core are rare among synthetic imidazole-ligated
porphyrins (the two known examples are in refs. 21 and 22)
and are comparable to those seen in human deoxyhemoglobin
(0.34–0.40 A).23 There is evidence that relative energies of
electronic states of iron(II) porphyrins (which determine ki-
netics of ligand binding) are very sensitive to the Fe–Ct
distance,17 and our dimers may be particularly suited for
biomimetic studies of ligand binding in T-state hemoglobin.
In summary, absorption spectra, dilution experiments, spec-
trophotometric titration data, Evans measurements, and MS
suggest that in solution simple FeII meso-(N-methylimidazol-
2-yl)porphine exists predominantly as a dimer (Kd = 50 ꢀ
23 G. Fermi, M. F. Perutz, B. Shaanan and R. Fourme, J. Mol. Biol.,
1984, 175, 159.
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