!H NMR and EPR Studies of the Electronic Structure of Low-Spin Iron(III) Isocyanide Mesotetraphenylchlorin Complexes: a (dxz,dyz)4(dxy)1 configuration from 293 to 4 K

Article abstract of DOI:10.1039/b100925g

Reaction of (tetraphenylchlorinato)iron(III) chloride with eight equivalents of tert-butyl isocyanide and 2,6-xylyl isocyanide in the presence of zinc amalgam afforded Fe(TPC)(t-BuNC)2 1 and Fe(TPC)(2,6-xylylNC)2 2, respectively. The synthesis and characterization of the trifluoromethanesulfonato derivatives of (tetraphenylchlorinato)iron(III) 3, bis(tert-butyl isocyanide)(tetraphenylchlorinato)iron(III) 4, and bis(2,6-xylyl isocyanide)(tetraphenylchlorinato)iron(III) 5 are reported: [Fe(TPC)]CF3SO3 3, [Fe(TPC)(t-BuNC)2]CF3SO3 4 and [Fe(TPC)(2,6-xylylNC)2]CF3SO3 5. The 1H NMR isotropic shifts at 20 deg C of the complexes 4 and 5, varied from 5 ppm for 4 to 8 ppm for 5 rather than the expected -10 to -30 ppm, based on previously studied bis-ligated complexes of low-spin iron(III) chlorins. EPR spectra of [Fe(TPC)(t-BuNC)2]CF3SO3 4 in solution are axial, with g_<*> = 2.15 and g_<*> = 1.97 at 4 K, Σg² = 14.7. All physical properties are consistent with a low-spin iron(III) with an unusual ground-state configuration (d_xz,d_yz⁴/d_xy)¹.

Full text of DOI:10.1039/b100925g

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1H NMR and EPR studies of the electronic structure of low-spin
iron(III) isocyanide mesotetraphenylchlorin complexes:
a (dxz,dyz)4(dxy)1 configuration from 293 to 4 K†
Gérard Simonneaux* and Marwan Kobeissi
Laboratoire de Chimie Organométallique et Biologique, UMR CNRS 6509,
Université de Rennes 1, 35042 Rennes cedex, France. E-mail: simonnea@univ-rennes1.fr
Received 26th January 2001, Accepted 4th April 2001
First published as an Advance Article on the web 23rd April 2001
Reaction of (tetraphenylchlorinato)iron() chloride with eight equivalents of tert-butyl isocyanide and 2,6-xylyl
isocyanide in the presence of zinc amalgam afforded Fe(TPC)(t-BuNC)2 1 and Fe(TPC)(2,6-xylylNC)2 2, respectively.
The synthesis and characterization of the trifluoromethanesulfonato derivatives of (tetraphenylchlorinato)iron() 3,
bis(tert-butyl isocyanide)(tetraphenylchlorinato)iron() 4, and bis(2,6-xylyl isocyanide)(tetraphenylchlorinato)-
iron() 5 are reported: [Fe(TPC)]CF3SO3 3, [Fe(TPC)(t-BuNC)2] CF3SO3 4 and [Fe(TPC)(2,6-xylylNC)2]CF3SO3 5.
The 1H NMR isotropic shifts at 20 ЊC of the pyrrole protons of the two complexes 4 and 5, varied from 5 ppm for 4
to 8 ppm for 5 rather than the expected Ϫ10 to Ϫ30 ppm, based on previously studied bis-ligated complexes of low-
spin iron() chlorins. EPR spectra of [Fe(TPC)(t-BuNC)2]CF3SO3 4 in solution are axial, with g⊥ = 2.15 and g|| = 1.97
at 4 K, Σg2 = 14.7. All physical properties are consistent with a low-spin iron() with an unusual ground-state
configuration (dxz,dyz)4(dxy)1.
porphyrins have a (dxy)2(dxz,dyz)3 ground state. However it
Introduction
should be underlined that an unusual (dxz,dyz)4(dxy)1 situation
The study of the binding of small ligands to heme proteins has
was first reported with low-basicity 4-cyanopyridine complex-
played an important role in our ability to understand protein–
ation to ferriporphyrins.34 The axial EPR spectra, with g⊥ > g||
substrate interactions in general.1,2 Probably the most familiar
are also indicative of a (dxz,dyz)4(dxy)1 state. When two mol-
examples of bioinorganic chemistry are the oxygen carriers
ecules of tert-butyl isocyanide are bound to iron() tetra-
such as hemoglobins. The simplicity of the ligands (O2, CO,
phenylporphyrin, the 1H NMR spectrum is indicative of
NO, CNϪ and CNR) and the wealth of structural data available
a low spin complex which shows the chemical shifts of the
for heme proteins have led to a better understanding of these
pyrrole protons in the diamagnetic region.35 Subsequently, it
systems. Thus an attractive objective in connection with
was recognized that the unusual 1H NMR behavior results from
structure–function relationships is to substitute oxygen by vari-
the formation of a pure (dxz,dyz)4(dxy)1 ground state.36 With the
ous exogenous ligands. Surprisingly though there are numerous
TPP compound, the X-ray structure shows an extensively
examples of iron porphyrin complexes bearing isocyanide
S4-ruffled porphyrin core which is related to electronic factors
ligands,3 the use of isocyanides as structural and functional
rather than steric factors. This electronic contribution may be
probes of reduced-ring hemes such as iron chlorins and iron
due to the partial delocalization of the (dxy)1 unpaired electron
isobacteriochlorins is still rare.4 In this article, the purpose is
into the 3a2u(π) orbital of the porphyrin ring, which is made
to compare the isocyanide complexation on iron chlorin and
possible by the twisting of the nitrogen pz orbitals of the nitro-
iron porphyrin, both metallomacrocycles being considered as
gen out of the plane of the porphyrin ring, as suggested
models of active site of heme proteins.
recently.36,37 More recently, several groups reported also low-
Investigations of the structural and spectroscopic properties
spin iron() porphyrins with an unusual (dxz,dyz)4(dxy)1 ground
of model compounds such as iron() chlorins4–20 and
state due in part to meso-substitution.38–40
isobacteriochlorins7,10,12,15,21–23 have been useful in understand-
Despite their g-values, which according to the Proper Axis
ing the active site of numerous heme enzymes such as heme d
system of Taylor,6 have been classified as having (dxz,dyz)4(dxy)1
found in a terminal oxidase complex24–27 or a catalase, hydroxy-
ground states for many years,5,6 the chlorins studied thus far by
peroxidase II,28 all from Escherichia coli. In sulfmyoglobin, a
nonfunctional form of myoglobin, the porphyrin macrocycle
1H NMR spectroscopy clearly have a (dxz,dyz)3(dxy)2 ground
state.33 We have previously reported unusual low-spin iron()
has been reduced to a chlorin by addition of a sulfur atom to a
porphyrin bis-isocyanide complexes showing the (dxz,dyz)4(dxy)1
pyrrole ring.29 Various hemoproteins such as myoglobins,30,31
ground state,35 and now describe the preparation of new low-
31
horseradish peroxidase and cytochrome b5 have also been
reconstituted with iron chlorin prosthetic groups. A chlorin
is a hydroporphyrin with one reduced pyrrole double bond.
There are currently not enough results available of the physical
spin iron() chlorin bis-isocyanide complexes also showing the
unusual (dxz,dyz)4(dxy)1 electronic structure. This result contrasts
sharply with the electronic structure of low-spin iron()
tetramesitylchlorin bis-imidazole analogues18 which show,
properties of iron chlorins to explain their biological properties
1
according to H NMR spectroscopy, a probable (dxz,dyz)3(dxy)2
and their electronic ground state is still problematic.32 In
ground state.
contrast, much more information is available with iron por-
phyrins.33 Thus it has been accepted that most low-spin iron()
Results and discussion
† Abbreviations
used:
TPC = 7,8-dihydro-5,10,15,20-tetraphenyl-
Syntheses
porphyrin dianion (tetraphenylchlorin), TMC = 7,8-dihydro-5,10,15,
20-tetra(2,4,6-trimethylphenyl)porphyrin dianion (tetramesitylchlorin),
TPP = 5,10,15,20-tetraphenylporphyrin dianion.
The synthesis of symmetric bis-isocyanide complexes in the
iron() state, Fe(TPC)(CNR)2, was via a variation of our
DOI: 10.1039/b100925g
J. Chem. Soc., Dalton Trans., 2001, 1587–1592
This journal is © The Royal Society of Chemistry 2001
1587

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Scheme 1
method, previously reported for the synthesis of Fe(TPP)-
(CNR)2.35 Starting from Fe(TPC)Cl,41 the reduction of Fe()
to Fe() was carried out with zinc amalgam under argon. The
compounds were obtained as crystalline solids in 70–80% yield
and characterized by 1H NMR (see below). The stability of the
reduced complexes was satisfactory at ambient temperature so
that it was not necessary to add excess ligand in the solution to
record NMR spectra. This result was expected since it was
previously reported that the stability constants for ligand
binding to metal complexes generally increase with increasing
saturation of the macrocycle.42,43
Two major difficulties may be encountered in preparing
iron() isocyanide complexes of chlorins. First oxidation of the
chlorin ring to a porphyrin ring may occur, as previously
reported with imidazole ligands.17 Second, isocyanide is prob-
ably a weakly coordinating ligand for the iron() state due to
the lower basicity of the ligand44,45 in comparison to imidazole
and pyridine, thus making iron() complexation difficult. This
situation was previously encountered, with iron() porphy-
rins.35 Using perchlorate or triflate, a weak axial ligand, as
an intermediate, allowed us to solve this problem. Thus, a
similar stategy was employed to prepare low-spin iron()
tetraphenylchlorins.
Starting from Fe(TPC)Cl41 in tetrahydrofuran solution,
[Fe(TPC)]CF3SO3 3 was readily obtained, after addition of
AgCF3SO3, in 78% yield. The [Fe(TPC)]CF3SO3 complex
exhibited a UV–VIS spectra with a Soret band at 402 nm (ε = 66
dm3 mmolϪ1 cmϪ1) and a characteristic chlorin second band at
649 nm (ε = 9 dm3 mmolϪ1 cmϪ1). Addition under argon of
4 equiv. of tert-butyl isocyanide to [Fe(TPC)]CF3SO3 in
dichloromethane afforded the hexacoordinated complex [Fe-
(TPC)(t-BuNC)2]CF3SO3 4 (Scheme 1). Precipitation of purple
crystals occurred and the product was collected by filtration
(81% yield). The [Fe(TPC)(t-BuNC)2]CF3SO3 complex exhib-
ited a UV–VIS spectrum with a Soret band at 414 nm (ε = 66 dm3
mmolϪ1 cmϪ1) and a characteristic chlorin band at 648 nm
Fig. 1 Proton NMR spectrum of [Fe(TPC)]CF3SO3 3 in CD2Cl2 at
273 K. Assignment of the various resonances are indicated; X indicates
the residual solvent and impurity peaks.
also suggest that the observed isocyanide stretching frequencies
are influenced by the reduction of the porphyrin ring
(cis-macrocycle influence). Thus, isocyanide ligands bonded to
iron() chlorins (ν(C᎐N) 2117 cmϪ1) have lower CN stretching
᎐
᎐
᎐
frequencies than those in Fe(TPP)(t-BuNC)2 (ν(C᎐N 2129
᎐
cmϪ1).47 As expected, the more electron donating the macro-
cycle the lower the (CN) frequency due to an increase in the
π* population of the CN bond of the iron() complexes.
1H NMR spectroscopy
(ε = 9 dm3 mmolϪ1 cmϪ1). [Fe(TPC)(2,6-xylylNC)2]CF3SO3
5
1
was prepared by a similar method, using 2,6-xylylNC instead of
t-BuNC.
The H NMR spectrum of Fe(TPC)(t-BuNC)2 1 displays two
groups of signals corresponding to the chlorin ring protons:
(8.19, 8.18 and 7.85 (Hpyr); 7.80 (Ho); 7.56–7.6 (Hmϩp); 4.02
(Hpyrrolidine) and to the ligand (Ϫ0.07 ppm). These chemical
IR spectroscopy
20
shifts are very similar to those found for Fe(TPC)(PMe2Ph)2
The IR spectra of the new complexes [Fe(TPC)(CNR)2]CF3SO3
are different from that of [Fe(TPC)]CF3SO3, exhibiting a major
and are as expected for diamagnetic iron() chlorin derivatives.
The protons of the ligand are shifted to high field (vs. free
ligand) due to the ring current shift of the macrocycle. Similar
results were obtained with complex 2.
additional band at ca. 2180 cmϪ1 due to ν (C᎐N) in Nujol. The
᎐
᎐
᎐
ν(C᎐N) stretching frequency of CNR is increased upon
᎐
1
coordination of the isocyanide to the metal, increasing from
2130 cmϪ1 for the free ligand46 to 2192 cmϪ1 in [Fe(TPC)-
(t-BuNC)2]CF3SO3. This increase of the frequency indi-
cates a higher bond order in the complex than in the free ligand
which is attributed to the donor properties of isocyanides and
to the concomitant decrease in the σ* population of the CN
bond. These results are consistent with the presence of a posi-
tive formal charge on the compound in the former case. By
contrast, the isocyanide frequency decreases in the reduced
A representative H NMR spectrum of [Fe(TPC)]CF3SO3 3
is shown in Fig. 1. The pyrrole (pyr) proton resonances for 3 at
114, 88 and 58 ppm (273 K) are within the range of 50–120 ppm
found for the pyrrole resonances in other high-spin chlorin
complexes.14 These values are much further downfield than
the values of 40–10 ppm found for spin-admixed (S = 3/2,5/2)
complexes such as [Fe(TPP)]CF3SO3 and [Fe(TPP)]ClO4.48
A
magnetically simple molecule is expected to follow Curie-law
behavior in that a plot of the chemical shifts vs. 1/T is linear
with an intercept equal to the resonance in the diamagnetic
complex Fe()(TPC)(t-BuNC) (ν(C᎐N) 2117 cmϪ1). The data
᎐
᎐
2
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Table 1 Observed proton shifts for [Fe(TPC)(t-BuNC)2]CF3SO3 4 and Fe(TPC)(t-BuNC)2 1 (δ, CD2Cl2, ppm) and relative differences
Chlorin
Ho
HoЈ
Hm
HmЈ
Hp
HpЈ
Hpyrro
Hpyr
Hpyr
Hpyr
t-Bu
(∆H/H)a
(∆H/H)b
(∆H/H)iso
1.3
7.79
Ϫ6.6
Ϫ0.39
7.76
Ϫ8.15
12.9
7.6
5.39
14.7
7.6
7.1
3.07
7.6
Ϫ4.5
3.42
7.6
Ϫ4.18
Ϫ36.0
4.02
Ϫ40.0
5.11
7.85
Ϫ2.89
7.15
8.18
Ϫ0.85
7.92
8.19
Ϫ0.73
Ϫ0.81
Ϫ0.07
Ϫ0.74
c
a Chemical shifts for [Fe(TPC)(t-BuNC)2]CF3SO3 4 at 298 K with TMS as internal references. b Chemical shifts for Fe(TPC)(t-BuNC)2 1 at 298 K
with TMS as internal reference. c Isotropic shift of 4 relative to the diamagnetic complex Fe(TPC)(t-BuNC)2 1 as reference. A medium value of 8 ppm
for the chemical shifts of the diamagnetic pyrroles was used since the relative assignment was not possible at this stage.
Fig. 3 Proton NMR spectrum of [Fe(TPC)(t-BuNC)2]CF3SO3 4 in
CD2Cl2 at 298 K. Assignment of the various resonances are indicated;
Fig. 2 Plot of chemical shift vs. reciprocal temperature for [Fe(TPC)]-
X indicates the residual solvent and impurity peaks.
CF3SO3 3 in CD2Cl2.
complex. The absolute values of the isotropic shift should
increase in proportion to 1/T. This is illustrated in the form of the
Curie law plot in Fig. 2 showing negative intercepts at Ϫ43, Ϫ67
and Ϫ77.8 ppm for the pyrrole protons and Ϫ23 ppm for the
pyrrolidine protons. In contrast, the pyrrole proton signals of
[Fe(TPP)]CF3SO3 show reverse Curie law behavior in moving
upfield as the temperature is lowered.48 However the remarkable
deviations of the Curie plot intercepts from the diamagnetic
standard positions are more consistent with the contribution of
both S = 5/2 and S = 3/2 spin states in the description of the
electronic state. Also the curvature of the plot for the most
downfield pyrrole resonance (see Fig. 2) and a small solution
magnetic moment (µ = 5.1 µB) for a pure high spin state prob-
ably require the contribution of both S = 5/2 and 3/2 spin states.
Furthermore it was recently reported that other thermally
accessible excited states can be populated with perchlorato-
iron() complexes of a series of 2,6-disubstituted tetraphenyl-
porphyrin ligands.48 The same two level approach could also be
applied to our system with a ground state largely S = 5/2 and a
probable large separation between this ground state and a pos-
sible S = 3/2 excited state. The theoretical treatment49 is beyond
the scope of this current work and would require data from a
complete series of substituted chlorin complexes. We, however,
can state that the change from a porphyrin ligand to a chlorin
ligand induces only partly a change from the intermediate spin
state to a high spin state.
Fig. 4 Plot of chemical shift vs. reciprocal temperature for [Fe(TPC)-
(t-BuNC)2]CF3SO3 4 in CD2Cl2.
are found in a downfield position at 5.11, 7.15 and 7.92 ppm
(293 K). This is in contrast with the pyrrole protons of [Fe-
(TMC)(MeIm)2]ClO4 (MeIm = N-methylimidazole) (Ϫ47.4,
Ϫ12.3 and Ϫ2.4 ppm)17 and provides strong proof for a differ-
ent electronic structure in these derivatives. Magnetic measure-
ments via the Evans’ method were made for CD2Cl2 solutions
of [Fe(TPC)(t-BuNC)2]CF3SO3 4 employing TMS as the refer-
ence (293 K).50 The solution magnetic moment (µ = 1.92 µB) is
compatible with the low-spin state S = 1/2.
A representative 1H NMR spectrum of [Fe(TPC)(t-BuNC)2]-
1
CF3SO3 4 is shown in Fig. 3 and H NMR isotropic shifts are
listed in Table 1. The peaks for the phenyl protons of the por-
phyrin ring are fully assigned by proton COSY experiments.
For isocyanide axial ligands, measurements of the relative
intensities and relative line-widths fully determine the assign-
ment. The shift of the isocyanide ligand is unchanged in the
presence of excess ligand. Hence axial ligand dissociation is not
expected to be significant at ambient temperature.
In addition analysis of the curve in the Curie plot was made
for [Fe(TPC)(t-BuNC)2]CF3SO3 4. The temperature depend-
ences of the chemical shifts of the protons in CD2Cl2 are shown
in Fig. 4. The chemical shifts vary linearly with 1/T, but the
extrapolated lines do not pass through the diamagnetic value at
1/T = 0 and the apparent intercepts of the best fit lines are
negative (Ϫ4 ppm) for one pyrrole proton while the two other
pyrrole protons show very small temperature dependence
(intercepts at 6.2 and 4.7 ppm). The temperature dependency of
However, the spectrum of [Fe(TPC)(t-BuNC)2]CF3SO3
4
shows unexpected behavior in that the pyrrole proton signals
J. Chem. Soc., Dalton Trans., 2001, 1587–1592
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Table 2 Observed proton shifts for [Fe(TPC)(2,6-xylyNC)2]CF3SO3 5 and Fe(TPC)(2,6-XylylNC)2 2 (δ, CD2Cl2, ppm) and relative differences
Chlorin
Ho
2,6-Xylyl
HoЈ
Hm
HmЈ
Hp
HpЈ
Hpyrro
Hpyr
Hpyr
Hpyr
Me
Hp
Hm
(∆H/H)a
(∆H/H)b
(∆H/H)iso
Ϫ0.42
7.99
Ϫ8.41
Ϫ2.51
7.81
Ϫ10.32
13.95
7.66
6.29
16.12
7.66
8.46
1.63
7.66
Ϫ6.03
2.02
7.66
Ϫ5.64
—
4.12
—
Ϫ3.29
7.85
Ϫ11.3
Ϫ2.0
8.27
Ϫ10.0
3.3
8.29
Ϫ4.7
1.20
0.79
0.41
7.11
6.67
0.44
6.50
6.49
0.01
c
a Chemical shifts for [Fe(TPC)(2,6-xylylNC)2]CF3SO3 5 at 298 K with TMS as internal references. Hpyrrolidine not detected. b Chemical shifts for
Fe(TPC)(2,6-xylylNC)2 2 at 298 K with TMS as internal reference. c Isotropic shift of 2 relative to the diamagnetic complex Fe(TPC)(2,6-xylylNC)2 as
reference. We used a medium value of 8 ppm for the chemical shifts of the diamagnetic pyrroles since the relative assignment was not possible at this
stage.
the pyrrole resonances based upon the Curie plot shown in
Fig. 4 leads to isotropic shift sign reversal.This sign reversal of
the pyrrole shift was previously observed as recently reported
for low-spin iron() quinoxalinoporphyrin complexes.19
In order to better characterize the bis-isocyanide iron chlorin
structure, a comparison of the present 1H NMR results to data
for the corresponding porphyrin complexes35 has been made.
The averaged chemical shifts of [Fe(TPC)(t-BuNC)2]CF3SO3
are similar to those reported for [Fe(TPP)(t-BuNC)2]ClO4 (9.73
ppm).35 Thus the small isotropic shift for the pyrrole protons
(∆δ = Ϫ2 ppm for 4, ∆δ = 1.3 ppm for [Fe(TPP)(t-BuNC)2]-
ClO4 35) favors the interpretation that negligible spin density
resides on the pyrrole carbons and so accounts for the observed
chemical shifts in the diamagnetic region. As previously
reported by Walker,33 this pattern of isotropic shifts observed
and the weak temperature dependence of the pyrrole protons
Fig. 5 EPR spectrum of [Fe(TPC)(t-BuNC)2]CF3SO3 4 as a CH2Cl2
glass, recorded at 4 K.
are indicative of a (dxz,dyz)4(dxy)1 ground state. It is interesting
that the mechanism of spin transfer appears here to give rise
to a result largely different from that observed for low-spin
NMR data discussed above, we have found that the ground
iron() bisimidazole complexes of synthetic chlorins.18,33 In
states of the two complexes [Fe(TPC)(t-BuNC)2]CF3SO3
4
this latter case for example, one of the pyrrole proton signals of
[Fe(TMC)(Im)2]ClO4 (Im = imidazole) are found in a high field
position (see above) due probably to a large contact contribu-
tion in this position. The chemical shift observed for the tert-
(g⊥ = 2.15 and g|| = 1.97) and [Fe(TPP)(t-BuNC)2]ClO4 (g⊥ =
2.21 and g|| = 1.93)36 are of the (dxy)1 type. This is supported by
the fact that the EPR spectra of their CH2Cl2 solutions are
quite similar.
butyl isocyanide axial ligands (δCH = Ϫ0.81 ppm) is close to
3
those expected for groups located in the shielding area of the
porphyrin ring (δCH = Ϫ0.07 ppm). A similar situation is
observed with xylyl3 isocyanide ligands. In this case, the
isotropic shifts of the ligated isocyanide are very close to zero
(see Table 2). Thus, there appears to be essentially no spin
density on the axial ligands, as expected because of the
orthogonality of the isocyanide pπ orbitals and the metal dxy
orbital. In contrast, the description of the electronic structure
should account for a large spin density at the pyrrolidine
position and a large spin density at the meso position. As
previously suggested, this situation may indicate that there is a
significant spin delocalization to the a2u(π) orbital, which is
possible only if the porphyrin ring is ruffled causing twisting of
the nitrogen pz orbitals from the normal of the porphyrin ring.37
Conclusion
In conclusion, these spectroscopic observations are indicative
of a metal-based electron in the dxy orbital for the [Fe(TPC)-
(t-BuNC)2]CF3SO3 4 at all temperatures. Thus the change in
ground state of low-spin Fe() from the usual (dxy)2(dxz,dyz)3 to
the unusual (dxz,dyz)4(dxy)1 electron configuration occurs both
with porphyrin and chlorin macrocycles and seems largely
related to the π-acceptor properties of the isocyanide ligand.
The bis-cyano iron() quinoxalinoporphyrin complex which
formally has a chlorin-like structure also show this unusual
ground state.19 In contrast, it should be underlined that the
coordination of t-BuNC to iron() isobacteriochlorin chloride
induces a reversible electronic rearrangement resulting in the
reduction of iron() to iron() with the formation of a
π-radical due to the presence of both the tetrahydroporphyrin
macrocycle and the chloride ligand.4
EPR spectroscopy
It has been recognized that the EPR g values of low-spin
ferriporphyrins provide valuable information about the orbital
of the unpaired electron.51 EPR properties of [Fe(TPC)(Im)2]ϩ
and other low-spin complexes formed from iron() tetra-
phenylchlorin derivatives have been reported.5,6,52 Although the
apparent extent of rhombicity is smaller for hydroporphyrin
than for porphyrin complexes with the same set of axial ligands,
all of the hydroporphyrin complexes evidence rhombic spectra.
In contrast, the EPR spectrum of [Fe(TPC)(t-BuNC)2]CF3SO3
4 is axial in frozen solution with g⊥ = 2.15 and g|| = 1.97 at 4 K
(Fig. 5) . The relative energies of the three t2g d orbitals can be
calculated from the g values in solution, using a general theory
elaborated by Taylor.6 Thus ∆/λ is negative (Ϫ12.4) also indi-
cating that the ground state is largely (dxy)1. On the basis of the
Experimental
General information
As a precaution against the formation of the µ-oxo dimer
[Fe(TPC)]2O,13,53 all reactions were carried out in dried solvents
in Schlenk tubes under an Ar atmosphere. Solvents were
distilled from appropriate drying agents and stored under
1
argon. H NMR spectra were recorded on a Bruker AC 300P
spectrometer in CD2Cl2 or CDCl3 at 300 MHz. Tetramethyl-
silane was used as internal reference. The temperatures are
given within 1 K. EPR spectra were recorded in CH2Cl2 on
a Bruker EMX 8/2,7 spectrometer operating at X-band
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frequencies. Samples were cooled to 4.2 K in a stream of helium
gas in frozen CH2Cl2, the temperature of which was controlled
by an Oxford Instruments ESR 900 cryostat. Visible spectra
were measured on a Uvikon 941 spectrometer in CH2Cl2.
was added 2.5 equiv. of 2,6-xylyl isocyanide under stirring at
room temperature. After 15 min, the solution became dark
green. Then 8 ml of hexane was added and the solution was
set aside two days for crystallization at 0 ЊC. Purple crystals
of 5 were collected by filtration and washed with hexane. The
yield was 89 mg (67%). UV–VIS (toluene): λmax/nm 414 (ε 76
dm3 mmolϪ1 cmϪ1), 606 (ε 11.2), 654 (ε 11.6). FAB MS (m/z):
Reagents
The free-base hydroporphyrins were prepared as previously
reported.54,55 The iron chlorin Fe(TPC)Cl was prepared by
a literature method.41 tert-Butyl isocyanide and 2,6-xylyl
isocyanide are commercially available from Aldrich.
ϩ
Ϫ1
᎐
670, [M Ϫ 2 (2,6-xylylNC) Ϫ CF SO ] . IR ν(C᎐N) 2162 cm
᎐
3
3
(Nujol).
References
Syntheses
1 E. Antonini and M. Brunori, Hemoglobin and Myoglobin in their
Reactions with Ligands, North-Holland, Amsterdam, 1971.
2 T. L. Poulos, in The Porphyrin Handbook, eds. K. M. Kadish,
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3 G. Simonneaux and A. Bondon, in The Porphyrin Handbook, eds.
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107, 4207.
Fe(TPC)(t-BuNC)2 1. A solution of Fe(TPC)Cl (0.1 g, 0.14
mmol) in 15 ml of dichloromethane was reduced under argon
by Zn–Hg amalgam at room temperature. After a reaction time
of 60 min, the solution was then filtered and 8 equiv. of tert-
butyl isocyanide added by a syringe to the Fe(TPC) species. The
solution was then stirred for 12 hours. Hexane (30 cm3) was
then added gradually and the solution set aside for crystalliz-
ation. Fine crystals were collected by filtration after 3 days.
Yield 0.1 g ( 85%). UV–VIS (CHCl3): λmax/nm 427 (ε 110 dm3
mmolϪ1 cmϪ1), 608 (ε 17).
1H NMR (δ, CD2Cl2, ppm) 8.19 (2H, s, Hpyr); 8.18 (2H, d,
Hpyr); 7.85 (2H, d, Hpyr); 7.79, 7.76 (8H, m, Ho); 7.6 (12H, d,
Hmϩp); 4.02 (4H, s, Hpyrrolidine); Ϫ0.07 (18H, s, H ligand). FAB
MS (m/z): [M Ϫ 2 t-BuNC]ϩ 670. IR ν(CN) 2117 cmϪ1 (Nujol).
Fe(TPC)(2,6-xylylNC)2 2. A solution of Fe(TPC)Cl (0.1 g,
0.14 mmol) in 15 cm3 of dichloromethane was reduced under
argon by Zn–Hg amalgam at room temperature. After a reac-
tion time of 60 min, the solution was then filtered and 8 equiv.
of xylyl isocyanide added by syringe to the Fe(TPC) species.
The solution was then stirred for 12 hours. Hexane (30 cm3) was
then added gradually and the solution set aside for crystalliz-
ation. Fine crystals were collected by filtration after 3 days.
Yield 0.09 g (69%). UV-VIS (CHCl3): λmax/nm 427 (ε 80 dm3
mmolϪ1 cmϪ1), 610 (ε 11).
11 L. A. Andersson, T. M. Loehr, C. K. Chang and A. G. Mauk, J. Am.
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Almond and S. H. Strauss, J. Am. Chem. Soc., 1988, 110, 3007.
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17 S. Ozawa, Y. Watanabe and I. Morishima, J. Am. Chem. Soc., 1994,
116, 5832.
1H NMR (δ, CD2Cl2, ppm) 8.29 (2H, s, Hpyr); 8.27 (2H, d,
Hpyr); 7.85 (2H, d, Hpyr); 7.99; 7.81 (8H, m, Ho); 7.66 (12H, d,
Hmϩp); 6.67 (2H, t, Hp ligand); 6.49 (4H, d, Hm ligand); 4.12
(4H, s, Hpyrrolidine); 0.79 (12H, s, CH3 ligand). FAB MS (m/z):
[M Ϫ 2(2,6-xylylNC)]ϩ 670. IR ν(CN) 2111 cmϪ1 (Nujol).
18 S. Ozawa, Y. Watanabe, S. Nakashima, T. Kitagawa and
I. Morishima, J. Am. Chem. Soc., 1994, 116, 634.
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Biological Inorganic Chemistry Conference, Toulouse, July 17–20,
2000.
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8782.
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[Fe(TPC)]CF3SO3 3. A solution of Fe(TPC)Cl (0.3 g, 0.42
mmol) and AgCF3SO3 in 20 ml of tetrahydrofuran was stirred
under argon at 60 ЊC. After a reaction time of 15 min, the solu-
tion was then filtered. Hexane (40 cm3) was added gradually
and the solution set aside for crystallization. Fine crystals were
collected by filtration after one night. Yield 0.27 g (78%). UV–
VIS (toluene): λmax/nm 402 (ε 66 dm3 mmolϪ1 cmϪ1), 606 (ε 13),
649 (ε 9); (δ, CDCl3, ppm, 273 K): 114 (2H, br, Hpyr), 88 (2H, br,
Hpyr), 58 (2H, br, Hpyr), 25.5 (4H, br, Hpyrrolidine), 12.35 (8H, br,
Ho); 11 (4H, br, Hm) and 10.3 (4H, br, Hm); 9.6 (2H, br, Hp); 9.2
(2H, br, Hp). EPR: g = 5.87 and 1.99.
[Fe(TPC)(t-BuNC)2]CF3SO3 4. To a solution of 100 mg (122
µmol) of [Fe(TPC)]CF3SO3 3 in 4 ml of dichloromethane was
added 2.5 equiv. of tert-butyl isocyanide by syringe under stir-
ring at room temperature. After 15 min, the solution became
dark green. Then 8 ml of hexane was added and the solution
was set aside for two days at 0 ЊC for crystallization. Purple
crystals of 4 were collected by filtration and washed with hex-
ane. The yield was 98 mg (81%). UV–VIS (toluene): λmax/nm
414 (ε 66 dm3 mmolϪ1 cmϪ1), 599 (ε 10.7), 648 (ε 9). FAB MS
(m/z): 819 [M Ϫ 2 t-BuNC]ϩ, 670 [M Ϫ 2 t-BuNC Ϫ CF3SO3]ϩ.
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IR ν(C᎐N) 2192 cmϪ1 (Nujol).
᎐
᎐
34 M. K. Safo, F. A. Walker, A. M. Raitsimring, W. P. Walters, D. P.
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[Fe(TPC)(2,6-xylylNC)2]CF3SO3 5. To a solution of 100 mg
(122 µmol) of [Fe(TPC)]CF3SO3 3 in 4 ml of dichloromethane
J. Chem. Soc., Dalton Trans., 2001, 1587–1592
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