Axial thiophenolate coordination on diiron(III)bisporphyrin: Influence of heme-heme interactions on structure, function and electrochemical properties of the individual heme center

Article abstract of DOI:10.1021/ic5011677

The binding of a series of substituted thiophenols as axial ligands on a highly flexible ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the pentacoordinate thiophenolato complexes. In the UV-visible spectra of the complexes, the positions of the Soret and band I have been found to be dependent on the pK_a of thiophenols. The alternating shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para- protons in the ¹H NMR spectra, is attributed to negative and positive spin densities, respectively, on thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pK_a of thiophenol and is found to be positively shifted with decreasing pK_a. The effect of the electronic nature of the substituent on the thiophenolate ring has also been demonstrated in which a large potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the Fe(III)/Fe(II) redox couple.

Full text of DOI:10.1021/ic5011677

Article
pubs.acs.org/IC
Axial Thiophenolate Coordination on Diiron(III)bisporphyrin:
Influence of Heme−Heme Interactions on Structure, Function and
Electrochemical Properties of the Individual Heme Center
Debangsu Sil, Firoz Shah Tuglak Khan, and Sankar Prasad Rath*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India
S
* Supporting Information
ABSTRACT: The binding of a series of substituted thiophenols as axial ligands on a highly flexible
ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme
proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the
pentacoordinate thiophenolato complexes. In the UV−visible spectra of the complexes, the positions
of the Soret and band I have been found to be dependent on the pKa of thiophenols. The alternating
shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para- protons in
the 1H NMR spectra, is attributed to negative and positive spin densities, respectively, on
thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate
ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pKa of
thiophenol and is found to be positively shifted with decreasing pKa. The effect of the electronic
nature of the substituent on the thiophenolate ring has also been demonstrated in which a large
potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of
monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and
is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the
thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further
supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato
complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the
Fe(III)/Fe(II) redox couple.
reducing δ-proteobacterium Desulfovibrio vulgaris;5 the diheme
INTRODUCTION
■
Cytochrome rC557 from Escherichia coli;6a bacterial diheme
Multiheme cytochrome c represents an extensive class of
cytochrome c peroxidase;6b C7-type three heme cytochrome
hemoproteins with a consequential role in electron transfer and
(PpcA) from G. sulfurreducens;6c etc. are all multiheme
enzymatic catalysis.1 Understanding the importance of these
cytochromes in which a sulfur atom coordinates to at least
motifs is critical for clarification of the highly optimized
properties of multiheme cytochromes c; however, their
one heme center. The iron center of cytochrome P-450s (P-
450) is also known to have a thiolate coordination.7,8 Substrate
spectroscopic investigation is often complicated by the
binding at the active site of the P-450cam shifts the Fe(III)/
presence of large numbers and efficient coupling of the
individual heme centers.1,2 The simplest member of such a
Fe(II) redox potential to the positive side to be readily reduced
by a physiological reductant. This regulation of the Fe(III)/
family is the diheme cytochrome c (DHC2) from G.
Fe(II) redox potential is very crucial for P-450 to function in
innumerable biological redox processes. The thiolate ligation is,
therefore, expected to significantly influence the property of the
heme iron. Until now, all the thiolato Fe(III) porphyrins
studied as models of cytochromes have been with monohemes.
The effect of such coordination on the diheme/multiheme
cytochromes remains unexplored.
A covalently linked porphyrin dimer can be a useful model of
sulfurreducens, which has two different heme groups connected
via a single polypeptide chain.2 The observed differences in the
porphyrin ring deformations between two heme centers and
the axial ligand orientations in DHC2 have been proposed to
be due to the heme−heme interactions, although the functional
significance of these heme structural arrangements is yet to be
understood. These attractive features have encouraged us to
focus our investigation on the relationship between such
the diheme centers. A sensible choice of the spacer will dictate
interactions and the properties of the metal center as a part of
our ongoing research.3
Sulfur coordination to heme-iron of multiheme is known to
the spatial arrangement, thereby allowing precise control over
the inter-ring interactions and possible electronic communica-
be an integral part of several biological proteins/enzymes.4−7
tions. In the present work, two octaethylporphyrin rings have
been covalently connected via a highly flexible ethane linker
For example, the photosynthetic reaction center (RC) from
Rhodpseudomonas viridis;4a the triheme cytochrome c, DsrJ,
from the purple sulfur bacterium Allochromatium vinosum;4a
Received: May 19, 2014
cytochrome c quinol dehydrogenase, NrfH, from the sulfate-
Published: November 6, 2014
© 2014 American Chemical Society
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that supplements both the horizontal and vertical flexibility of
the bisporphyrin system. Recently, we have reported very
different structures and properties of Fe(III) complexes while
using bisporphyrin architecture as compared to the correspond-
ing monoporphyrin.3 The complete reversal of the ligand field
strength of ClO4− and CF3SO3− in the magnetochemical series
has been observed in a diiron(III)bisporphyrin framework.3a
Investigation of a series of phenolato diiron(III)bisporphyrins
revealed the stabilization of pure intermediate-spin (S = 3/2) of
iron in the 2,4,6-trinitrophenolato complex, while its monop-
orphyrin analog was found to stabilize the high spin state (S =
5/2) only.3b Our group also has reported recently a family of μ-
hydroxo diiron(III)bisporphyrin in which two different spin
states of Fe(III) are stabilized in a single molecular framework,
even though both the cores have exactly the same chemical
entity.3g−i All of these observations are believed to be the
consequences of inter-ring interaction in dihemes.
The present work investigates the binding of a series of
substituted thiophenols as axial ligands on a diiron(III)-
bisporphyrin framework and compares with the corresponding
monoporphyrin and phenolato analogues. Focus will be on how
the inter-ring interactions in a porphyrin dimer influence the
structure, electronic, and redox properties of the individual
heme centers in contrast to its monoheme counterpart. The
change in redox properties of the heme centers on going from a
pentacoordinate to hexacoordinate complex will also be
investigated. Density functional theory (DFT) calculations
have been employed to rationalize the experimental observa-
tions in the present study.
list of the thiophenolato Fe(III)bisporphyrins reported here
along with their abbreviations used.
UV−visible spectroscopy comes in handy, particularly in
predicting the geometry of the thiophenolato complex in
solution. Splitting of the Soret band is frequently discerned for
the sundry porphyrin dimers and trimers in linear spatial
orientations.10 For example, splitting of the Soret band was
obtained for the anti conformation of bis(zinc porphyrin) as a
result of coordination of the axial ligands to the zinc ions.10
Although the Soret band is not split into two distinct peaks in
the present complexes, probably due to overlapping of B⊥ and
B∥ transitions, a shoulder is seen to be accompanying the Soret
band. The shoulder, although weak, indicates anti conformation
in solution, which has been further confirmed by the single
crystal X-ray structure (vide infra) in the solid. Dark purple
crystalline solids of the molecules were deposited by slow
diffusion of n-hexane into a benzene solution of the complexes
in good yields and are structurally characterized. The solid state
1
structures are also preserved in solution as reflected in the H
NMR spectra in CDCl3 (vide infra). Detailed synthetic
procedures of all the complexes along with their character-
izations are given in the Experimental Section.
We also have synthesized a series of thiophenolato
complexes of the mono-porphyrin analogues in order to
compare with the related thiophenolato Fe(III)bisporphyrins
reported here. Scheme 2 lists the thiophenolato Fe(III)
porphyrin along with their abbreviations used in the present
work. Although 4a and 4b were known previously, 4c and 4d
are new. The difference in properties of the bisporphyrin
complexes compared to the monoporphyrin counterpart would
provide unequivocal evidence of the role played by inter-ring
interactions in a porphyrin dimer.
The UV−vis spectral data for the Soret and Q-bands of the
thiophenolato complexes 2a−2d are summarized in Table 1.
The wavelength maxima of the Soret and band I8b (∼600−670
nm) for the complexes 2a−2d have been plotted as a function
of pKa of thiophenols in Figure 2, which is found to be linearly
correlated. Such linear correlation for the Soret band and band
I have also been obtained for the thiophenolato complexes
(4a−4d) of the monomeric counterpart. Band I has been
assigned to porphyrin to iron, a1u(π) and a2u(π) to eg(dπ),
charge transfer transitions.8b,c With increasing electron
donating ability of the ligand, the dπ orbitals of iron rise in
energy and thereby shift the charge transfer transition to higher
energy, i.e., shorter wavelength. A thiophenol with better
electron donating ability, i.e., greater pKa value, will increase the
dπ energy level and, therefore, raise the charge transfer
transition energy which eventually shift the absorbance to a
shorter wavelength. A similar relationship was also observed
earlier.8b,12
RESULTS AND DISCUSSIONS
■
μ - O x o - s y n - 1 , 2 - b i s [ 5 - ( 2 , 3 , 7 , 8 , 1 2 , 1 3 , 1 7 , 1 8 -
octaethylporphyrinato)iron(III)]ethane, 1, was synthesized
using a procedure reported earlier.3c UV−vis spectroscopic
data of 1 show a Soret band at 399 nm and Q-band at 579 nm
in dichloromethane which suggest the face-to-face orientation
of the porphyrin macrocycles. Upon the addition of thiophenol
into the dichloromethane solution of 1, a large change in the
UV−visible spectra has been observed which results in an
intense blue-shifted Soret band at 391 nm along with a
shoulder at 413 nm, and three Q-bands arise at 511, 541, and
633 nm due to the formation of five-coordinate9 complex 2a.
Similar spectral changes have also been observed for other
thiophenolato complexes 2b−2d reported here, and Figure 1
compares the UV−visible spectra between 1 and 2d in
dichloromethane. Scheme 1 shows the synthetic outline and
We also have plotted the wavelength maxima of the Soret
and band I for the phenolato analog of the dihemes 5a−5e
(Chart 1) in Figure 2 as a function of pKa of the substituted
phenol. Although a similar trend to that described above has
been obtained here also, a good fitting, however, was not
obtained. It is interesting to note that a notably large deviation
from linearity is observed in the case of 5d, which is probably
due to a change of iron spin state from a high (5/2) to
intermediate spin (3/2) state.
Crystallographic Characterizations. Dark purple crystal-
line solids of the molecules were deposited by slow diffusion of
n-hexane in a benzene solution of the complexes in the air at
room temperature. Figures 3, 4, S2, and S3 demonstrate the X-
Figure 1. UV−vis spectra (at 298 K) of 1 (green line) and 2d (red
line) in dichloromethane.
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Scheme 1
Scheme 2
ray structures of 2a, 2d, 2b, and 2c, respectively. In all of the
complexes, two iron centers, each in a five-coordinate square-
pyramidal geometry, are present in anti conformations. Except
2c, which crystallizes in a triclinic crystal system with the P1
space group, other molecules crystallize in the monoclinic
crystal system with the P21/c space group. Selected bond
distances and angles are reported in Table 2. The packing
diagrams of the complexes are shown in Figure 5 (for 2c) and
Figures S4−S6 (for complexes 2a, 2b, and 2d, respectively).
Figure 5 also reveals the intermolecular interaction between
two thiophenolate rings (with an average distance of 3.41 Å) of
the neighboring molecules. The thiophenolate rings are nearly
cofacial to each other with a small offset favoring strong π−π
interaction.
The average Fe−Np bond distances for 2a, 2b, 2c, and 2d
are 2.059(2), 2.052(4), 2.058(2), and 2.053(6)Å, respectively,
which fall within the range observed for a high-spin (S = 5/2)
Fe(III) porphyrin.3,13 These values are comparable to the Fe−
Np bond distance reported for axial thiophenolato coordination
̅
Figure 2. Correlation between pKa values of conjugate acid of the
thiophenols and phenols3b with the wavelength maxima of
■
bisthiophenolatodiiron(III)bisporphyrins ( ), bisphenolatodiiron-
▲
(III)bisporphyrins ( ), and thiophenolatoiron(III)monoporphyrins
●
( ) for (A) Soret band and (B) band I.
to iron(III) monoporphyrins also.8 For example, the reported
8d
Fe−Np bond distances for 4a8a and FeIII(TPP)(SPh)·C6H6
are 2.057 and 2.063 Å, respectively.
The Fe−S bond distances observed for 2a and 2b are
2.2829(11) and 2.2842(19) Å, respectively, while for 2c and 2d,
Table 1. UV−Visible Spectral Data of Complexes 2a−2d
thiophenols (complex)
thiophenol (2a)
pKa
Soret band (nm)
band IV (nm)
band III (nm)
band I (nm)
a
6.61
7.22
5.17
3.64
390
391
385
383
511
514
515
519
541
546
550
552
633
628
645
657
b
b
b
2,4,6-trimethylthiophenol (2b)
2,6-dichlorothiophenol (2c)
2,3,4,5,6-pentafluorothiophenol (2d)
a
b
pKa was obtained from ref 11. pKa values were obtained from “calculated properties” for each compound in SciFinder.
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Chart 1
Figure 5. Diagram illustrating the packing of 2c molecules in the unit
cell and the π−π interaction between the thiophenolate rings of two 2c
molecules (H atoms have been omitted for clarity).
the values are, respectively, 2.3136(10) and 2.316(2) Å. These
values are comparable to the Fe−S bond distance reported so
far for Fe(III)porphyrin with thiophenolato coordination.8 For
example, the reported Fe−S bond distances in 4a and
FeIII(OEP)(S-2-CF3CONHC6H4) are 2.299(3) and 2.327(4)
Å, respectively.8a,e,f However, the Fe−S distance has been found
to increase with increasing electron withdrawing substituents
on the thiophenols, this is due to the fact that an electron
withdrawing substituent with a strong −I effect reduces the
electron donating ability of the iron bound sulfur atom and
thereby increases the Fe−S bond length. The Fe−S−C bond
angle for 2a is 102.85(13)°, which is, however, identical to
102.5(3)°, observed in 4a.8a The Fe−S−C angle for 2b is
105.3(2)°, while for 2c and 2d, the angles are 101.27(10) and
102.5(3)°, respectively. As can be seen, the change in Fe−S−C
angle is rather small in the series.
Figure 3. A perspective view of 2a showing 50% thermal contours for
all non-hydrogen atoms at 100 K (H atoms have been omitted for
clarity).
Five-coordinate iron(III) porphyrins are known to exist as
high-spin (S = 5/2), intermediate-spin (S = 3/2), and also as a
quantum mechanical spin admixed state with varying
proportions of S = 3/2 and S = 5/2 states.3,13−15 The structural
parameters important for identifying the spin state of five-
coordinate iron(III) porphyrins are the displacement of the
iron from the mean plane of the C20N4 pophyrinato core (Fe···
Ctp) and the average Fe−Np distance.3,13 For the high-spin
case, the typical Fe−Np and Fe···Ctp distances are ≥2.045 and
≥0.42 Å, respectively. On the other hand, values reported for
five-coordinate spin-admixed iron(III) porphyrinates are in the
range of 1.961−2.038 Å for Fe−Np and 0.10−0.36 Å for the
displacements of the iron (Fe···Ctp), varying according to the
amount of S = 3/2 and 5/2 character present in the system.
The average Fe−Np and Fe···Ctp distances are 2.059(2) Å and
0.61 Å for 2a, 2.052(4) Å and 0.58 Å for 2b, 2.058(2) Å and
0.41 Å for 2c, and 2.053(5) Å and 0.55 Å for 2d, respectively,
and the spin state can thus be assigned as a pure high-spin state
of iron.
Table 3 compares the structural parameters of thiophenolato
diiron(III)bisporphyrin complexes reported here along with
their phenolato and monoporphyrin analogues. As can be seen,
the average Fe−Np bond distances of the phenolato complexes
5a−5c are in general greater than those of corresponding
thiophenolato complexes 2a−2d. The only exception being the
case of 5d,3b where the iron(III) center is in an intermediate-
spin (S = 3/2) state resulting in a smaller Fe−Np distance. The
Fe−S bond distances in 2a−2d are larger compared to the Fe−
O distances observed in their phenolato counterparts, 5a−5d.
This indicates that thiophenols are weaker donors than
Figure 4. A perspective view of 2d showing 50% thermal contours for
all non-hydrogen atoms at 100 K (H atoms have been omitted for
clarity).
Table 2. Selected Bond Distance (Å) and Angles (deg)
bond distances
2a
2b
2c
2d
Fe1−N1
Fe1−N2
Fe1−N3
Fe1−N4
Fe1−S1
2.064(2)
2.054(2)
2.062(2)
2.057(2)
2.2829(11)
2a
2.049(4)
2.049(4)
2.056(4)
2.055(4)
2.2842(19)
2b
2.052(2)
2.055(2)
2.062(2)
2.062(2)
2.3136(10)
2c
2.041(6)
2.067(6)
2.054(6)
2.050(6)
2.316(2)
2d
bond angles
N1−Fe1−N2
N1−Fe1−N3
N1−Fe1−N4
N2−Fe1−N3
N2−Fe1−N4
N3−Fe1−N4
Fe1−S1−C38
87.70(8)
150.02(9)
85.47(8)
86.09(8)
154.46(9)
87.63(8)
102.8513)
88.05(16)
154.84(19)
85.31(17)
85.51(17)
151.2(2)
89.44(9)
155.87(9)
85.87(9)
88.1(2)
156.5(2)
85.8(2)
86.5(2)
152.2(3)
88.4(2)
102.5(3)
86.09(9)
155.27(9)
88.35(9)
88.70(17)
105.3(2)
101.27(10)
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Table 3. Selected Structural Parameters for Five-Coordinate FeIII(porphyrin)thiophenolate/phenolate Complexes
a
b
c
d
e
f
f
g
complex
Fe−Np
Fe−S/O (L)
Fe−S/O−C
ΔFe
Δ24
Cm
Cβ
Fe···Fe
ref
24
2a
2b
2c
2d
5a
5b
5c
2.059(2)
2.052(4)
2.058(2)
2.053(6)
2.057(3)
2.077(3)
2.066(5)
2.075(5)
1.972(3)
2.057
2.2829(11)
2.2842(19)
2.3136(10)
2.316(2)
1.916(2)
1.864(3)
1.807(4)
1.813(4)
2.000(2)
2.299
102.85(13)
105.3(2)
101.27(10)
102.5(3)
134.4(2)
152.0(3)
165.7(4)
162.7(4)
127.2(2)
102.51
0.61
0.58
0.41
0.55
0.51
0.52
0.55
0.56
0.23
0.51
0.48
0.20
0.19
0.16
0.19
0.12
0.08
0.20
0.13
0.26
0.07
0.05
0.37
0.37
0.30
0.35
0.25
0.07
0.40
0.24
0.53
0.12
0.06
0.16
0.15
0.11
0.15
0.11
0.13
0.15
0.11
0.21
0.07
0.06
9.76
9.63
9.46
9.75
9.67
10.09
9.94
this work
this work
this work
this work
3b
3b
core I
core II
3b
5d
9.39
3b
8a
8e
FeIII(OEP)(SPh)
FeIII(OEP){S-2,6-
(CF3CONH)2C6-
H3}
2.048
2.356
104.55
FeIII(OEP)(S-2-
2.047
2.063
2.042
2.327
2.315
2.333
103.88
103.59
105.86
0.40
0.47
0.47
0.02
0.04
0.27
0.04
0.04
0.02
0.03
0.05
0.53
8e
8d
8g
CF3CONHC6H4)
FeIII(TPP)(SPh)·
C6H6
FeIII(TPP){S-2,6-
(CF3CONH)2C6-
H3}·1/2CH2Cl2
FeIII(TPP)(S-
2,3,5,6-F4C6H1)
2.058
2.299
103.67
0.45
0.06
0.14
0.05
8g
a
b
c
d
Average value in Å. Distance (in Å) of axial ligand L. Angle (in deg). Displacement of iron from the least-squares plane of C20N4 porphyrinato
e
f
core. Average displacement of atoms from the least-squares plane of the C20N4 porphyrinato core. Average displacement of the respective carbons
from the mean plane of the C20N4 porphyrinato core. Nonbonding distance (in Å) between two Fe(III) centers in a molecule.
g
phenols. The Fe−S−C angle in the complexes 2a−2d, though
much smaller than the Fe−O−C angles in the phenolato
counterparts 5a−5d,3b is similar to the Fe−S−C angles in the
monoporphyrin complexes reported in the literature. The
difference in the Fe−S/O−C bond angles as we shift from
thiophenols to phenols are probably due to the shorter Fe−O
distances compared to Fe−S distances. As the aromatic ring is
drawn closer to the iron center, the steric interactions with the
porphyrin ring increase, which subsequently increase the Fe−
O−C angle in order to minimize the steric effect.
The porphyrin macrocycles are all distorted in the diiron-
(III)bisporphyrins reported in the present work in which the
bridging meso carbons are displaced most. The core
deformation of the thiophenolato complexes is similar to that
observed in the case of their phenolato counterparts3b and is
mostly ruffled. This can be best appreciated by turning to
Figure 6, which compares the out-of-plane displacements in
units of 0.01 Å of the porphyrin core atoms of 2a−2d, 5a
Figure 6. Atom deviations (in units of 0.01 Å) from the least-squares
plane of the C20N4 porphyrinato core in (A) 2a, 5a,3b and 4a8a and
(B) 2b, 2c, and 2d. The horizontal axis represents the atom number in
the macrocycle (the numbering scheme is shown in Figure S7)
showing the bond connectivity between atoms.
(phenolato analog of 2a), and 4a (which is the monomeric
analog of 2a). While the porphyrin macrocycle in thiopheno-
lato and phenolato complexes are highly distorted, the ring is
planar in the related monomeric thiophenolato complex 4a. As
evident, the interaction between two rings in diheme results in
larger ring deformation of the individual porphyrin centers.
This has also been reflected in the average atom displacements
from the mean porphyrin plane (Δ24) and iron displacements
Fe
24
therein (Δ ), which are found to be 0.20 and 0.61 Å,
respectively, for 2a while for 4a the values are 0.07 and 0.51 Å.
Fe
24
It is also to be noted here that both the Δ24 and Δ are more in
the thiophenolato complexes compared to the corresponding
phenolato complexes of diiron(III)bisporphyrin.
̈
Mossbauer. Mossbauer parameters are one of the most
powerful probes to determine the spin states of the iron(III)
̈
porphyrins.3,13 Figure 7 demonstrates the Mossbauer spectrum
of the microcrystalline samples of 2a at 298 K, which shows a
small quadrupole splitting [δ (ΔEq): 0.28 (0.46) mm/s]
Figure 7. Zero-field Mossbauer spectra of microcrystalline samples of
̈
̈
2a at 298 K.
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characteristic of the high-spin nature of Fe(III). Thus,
state of iron which were then carefully simulated (a
representative simulated spectrum is shown in Figure S10) to
obtain the following g values: g⊥ = 2.38 and gII = 1.90 for 6a, g⊥
= 2.42 and gII = 1.85 for 6b, g⊥ = 2.36 and gII = 1.92 for 6c, and
g⊥ = 2.37 and gII = 1.91 for 6d. Although no other structural
data are available for the six-coordinate low-spin complex, it is
expected that one axial ligand will bind as a thiophenolate,
while the other one will bind as a thiophenol as also observed in
the X-ray structure of FeIII(TPP)(C6H5S)(C6H5SH).8h Figure 9
compares the EPR spectra of 2a, 6a, and 6b under identical
conditions.
Mossbauer parameters are consistent with the results obtained
̈
from the single-crystal X-ray structure of the complex (vide
supra).
EPR. The EPR measurements were carried out at 120 K in
both the solid and solution phases in dichloromethane, which
show a similar spectral pattern for 2a to 2d; Figure 8 shows
Figure 8. X-band EPR spectra in CH2Cl2 (at 120 K): (A) 2a and (B)
2b.
representative spectra of 2a and 2b, respectively. All the spectra
are axially symmetric, and a careful simulation of these spectra
(a representative simulated spectrum is shown in Figure S8)
provided the following g values: g⊥ = 5.95 and gII = 2.00 for 2a,
g⊥ = 5.90 and gII = 1.99 for 2b, g⊥ = 5.90 and gII = 2.00 for 2c,
and g⊥ = 5.85 and gII = 2.00 for 2d. These results provide
unequivocal evidence of the high-spin (S = 5/2) nature of the
iron in the complexes in both the solid and solution
phases.3,13−15 The EPR spectra of the thiophenolato complexes
4a to 4d also bear the signature of a high-spin state in both a
solid and solution under identical conditions.
However, the addition of excess thiophenol to the dichloro-
methane solution of 2 results in the formation of a six-
coordinate low-spin complex 6 as depicted in Scheme 3. The
UV−visible spectra of the pentacoordinate complex 2a and the
hexacoordinate complex 6a are compared in Figure S9. A red
shifting of the Soret band was observed upon formation of the
hexacoordinate low-spin complex. The complexes 6a−6d
produce axial EPR spectra at 120 K typical for a low-spin
Figure 9. X-band EPR spectra in CH2Cl2 (at 120 K) (A) 2a, (B) 6a,
and (C) 6b.
Such conversion from the pentacoordinate high-spin Fe(III)
complex to a hexacoordinate low-spin one is also observed in
the case of cytochrome P-450. The addition of a water
molecule at the vacant sixth coordination site of the cystine
bound pentacoordinate high-spin iron(III) porphyrin of P-450
converts to hexacoordinated low-spin complex.8h,16 It is to be
noted that complete conversion to the low-spin complexes 6c
and 6d was not achieved even after the addition of a large
excess of corresponding thiophenols to 2c and 2d, respectively.
A possible explanation could be the weakening of binding
ability of the thiophenols with increasing acidity. However, in
the case of the phenolato analogues 5a−5d, the addition of a
large excess of phenols did not produce any change in the EPR
Scheme 3
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signal. Thus, the formation of the six-coordinate complex does
not take place in solution in the case of the five-coordinate
phenolato complexes.
assigned as high-spin. For 2a, eight diastereotopic methylene
signals are observed at 33.5, 35.2, 37.1, 39.5, 41.9, 43.9, and
48.1 ppm; two meso- resonances are observed at −45.6 and
−54.9 ppm in a 2:1 intensity ratio, while the bridging
methylene signal is obtained at 72.5 ppm. Similarity in the
nature and positioning of the 1H NMR peaks with the chloride
and phenolato complexes confirms the high-spin (S = 5/2)
nature of iron in 2a in solution as also observed in the solid
1H NMR. The structure and properties of the complexes in
solution can be obtained from their 1H NMR spectra in CDCl3.
The solid state structures are also preserved in solution as
1
reflected in their H NMR spectra. The signals are broad in
general and shifted both upfield and downfield regions. The
basic resonance pattern of porphyrin core for the thiophenolato
complexes grossly resemble the pattern followed by meso
substituted five-coordinate Fe(III) porphyrins of type
XFeIII(meso-R-OEP).3b It is, therefore, expected that there
should be two meso- resonances in 2:1 intensity ratio, eight
methylene resonances, and four equally intense methyl
resonances for a five-coordinate complex. The eight methylene
resonances arise from the diastereotopic nature of these
protons which occurs whenever the two sides of the porphyrin
are inequivalent.
1
(vide supra). Similar H NMR spectra are also observed for
other thiophenolato complexes (2b−2d) reported here and are
compared in Figure 11. As can be seen, the nature and
positions of methylene and meso protons bear the signature of a
high-spin (S = 5/2) nature for all of the complexes.
The 1H NMR spectrum of 2a is compared in Figure 10 with
the previously reported complex of ethane-bridged diiron(III)-
Figure 11. 1H NMR spectra of (A) 2a, (B) 2b, (C) 2c, and (D) 2d in
CDCl3 at 295 K.
Chemical shifts of the CH3 protons of the ethyl substituent
are also known to be sensitive of spins. CH3 signals of 2a
appear at 7.9 and 8.3 ppm. Methyl proton signals of 2b−2d
have also appeared in a similar spectral region. The downfield
shifting of such resonances are due to delocalization of spin up
to the CH3 protons via σ bonds.3b,17 Therefore, the positions of
methyl resonances further support the spin state assignment for
the complexes as shown above.
Figure 10. H NMR spectra of (A) 7, (B) 5a,3b and (C) 2a (inset
3d
1
shows the proton numbering scheme) in CDCl3 at 295 K.
The thiophenolate resonances of 2 are, however, shifted to
both upfield and downfield regions in the 1H NMR as reflected
in Figures 10 and 11, which indicate π-spin delocalization from
the Fe(III) center to the thiophenolate ligand.3a,b,e,9a The ortho
protons of the axial phenolate ligand are closest to the
paramagnetic Fe center and have extremely small T1 (spin−
lattice relaxation) values resulting in a very broad signal at
−85.9 ppm. The meta and para proton resonances are relatively
sharp and are found at 56.4 and −82.7 ppm, respectively. In
order to assign the peaks of the thiophenolate moiety
successfully, various substituted thiophenols have been used
here, and Figure 11 demonstrates the 1H NMR spectra of 2a−
2d. As can be seen, the ortho and para protons show an upfield
shift, while meta protons show a downfield one. Similar
observations are also reported when substituted phenol and
bisporphyrin with phenolate (5a)3b and chloride (7 in Chart
2)3d axial ligands in which the spin state of iron was already
Chart 2
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catechols are used as axial ligands.3b,e,9,18 Figure S11
demonstrates the Mulliken spin densities, using DFT, of the
thiophenolato carbons of 2a in which spin densities are
observed as positive at ortho and para positions and negative at
the meta position. Therefore, the ortho and para protons should
be shifted upfield, while the meta proton is in the downfield
shifted redox potential of 2c (−0.584 V vs SCE) is close to that
of substrate-free cytochrome P-450cam
.
Coulometric reduction of the thiophenolato complexes in
dichloromethane at a constant potential has also been
performed, and the progress of the reaction was monitored
continuously by UV−visible spectroscopy as shown in Figure
S13 for 2c, as a representative case. A gradual decrease of Soret
band characteristics of Fe(III)porphyrin and the appearance of
a new Soret band corresponding to Fe(II) confirms the
corresponding potential as that of the Fe(III)/Fe(II) redox
couple.19 The positively shifted Fe(III)/Fe(II) redox couple of
complex 2a (E1/2 = −0.76 V) compared to that of phenolato
complex 5a (E1/2 = −0.92 V) suggests that thiophenol is a
weaker electron donor than phenol, which is due to back-
donation of electron density from the iron center to an empty d
orbital of sulfur. Since oxygen does not have a low-lying 3d
orbital, similar back-donation of the electron from iron to
oxygen is not possible with phenol. Therefore, the electron
density of iron in 2a should be lower than that in 5a, which
explains why the iron center of the former complex is more
reducible.
1
region, which is, however, observed in the H NMR of the
molecules. The alternating shift pattern, which is of the
opposite sign of the chemical shifts for meta versus ortho and
para protons, is also indicative of π-spin delocalization on the
thiophenolato ligand.
1
The H NMR spectra of the monoporphyrin counterpart
4a−4d under identical conditions are presented in Figure S12.
1
The H NMR spectra of 4a and 4b in C6D6 were reported
earlier.8e As displayed in Figure S12, two methylene and one
meso proton signal are observed in the complexes, which is,
however, expected for the five-coordinate complex. The two
methylene resonances arise from the diastereotopic nature of
these protons, which occurs whenever the two sides of the
porphyrin are inequivalent due to axial coordination. The ortho
and para protons of the thiophenolate are upfield shifted, while
the meta proton is downfield shifted, as seen in the case of
diheme analogues 2a−2d.
For complex 73d and complexes 5a−5e,3b it has been
observed that on moving from monoheme to diheme, the
Fe(III)/Fe(II) redox potential is shifted toward the more
negative value. For complexes 2a−2d, however, such a trend
has not been observed. With thiophenol having a lower pKa, the
Fe(III)/Fe(II) potential of the diheme was found to follow the
reverse trend, i.e., more positively shifted compared to its
monoheme analogues. Thus, the pKa of the coordinated
thiophenols plays a crucial role in deciding the nature of shift of
the Fe(III)/Fe(II) redox couple.
Cyclic Voltammetry. Cyclic voltammetry was performed at
298 K under N2 in CH2Cl2 using 0.1 M tetra(n-butyl)-
ammonium hexafluorophosphate (TBAH) as the supporting
electrolyte. The Fe(III)/Fe(II) redox potential of 2a, 2b, 2c,
and 2d are observed at −0.76, −0.90, −0.54, and −0.36 V vs
Ag/AgCl, respectively, and three representative spectra have
been shown in Figure 12. The Fe(III)/Fe(II) redox potentials
A plot of Fe(III)/Fe(II) potential of 2a−2d against pKa of
the thiophenols is shown in Figure 13, in which a linear
Figure 13. Plots of pKa of thiophenols/phenols vs E1/2 [Fe(III)/
■
Fe(II)] in V of bisthiophenolatodiiron(III)bisporphyrins ( ),
▲
bisphenolatodiiron(III)biporphyrins ( ), and thiophenolatoiron(III)-
●
monoporphyrins ( ).
Figure 12. A portion of the cyclic voltammograms of (A) 2e, (B) 2a,
and (C) 2b at 298 K in CH2Cl2 (scan rate 100 mV/s) with 0.1 M
tetra(n-butyl)ammonium hexafluorophosphate as a supporting electro-
lyte. The reference electrode was Ag/AgCl.
relationship has been observed. Such linear relationships have
also been obtained for the monomeric counterpart 4a−4d. The
electrochemical data suggest that the thiophenolato complex
with a more acidic thiophenol has a more positively shifted
Fe(III)/Fe(II) couple. Increasing the number of electron
withdrawing substituents on the thiophenol ring increases its
acidity and thus shifts the Fe(III)/Fe(II) redox potential more
toward positive, while the electron donating substituent leads to
an opposite effect. A similar pKa dependence of the Fe(III)/
Fe(II) redox couple has been obtained (Figure 13) for the
of 4b, 4c, and 4d have also been measured and are listed in the
Experimental Section, while that of 4a (−0.68 V vs SCE in
dichloromethane) has been obtained from the literature.8f The
redox potentials of native cytochrome P-450cam were reported
to be −0.415 V vs SCE for substrate binding mode and −0.572
V vs SCE for nonsubstrate binding mode.8e,i The positively
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phenolato analogues 5a−5e also;3b a deviation from linearity
has been observed for 5d due to a change of metal spin.
Shifting of the Fe(III)/Fe(II) redox couple is, however,
known in the literature;8 for example, in the case of
FeIII(TPP){S-2,6-(CF3CONH)2C6H3}, FeIII(OEP)(S-2-
CF3CONHC6H4), FeIII(OEP){S-2,6-(CF3CONH)2C6H3},
FeIII(OEP)(S-2-CF3CONHC6H4), and 4a, the Fe(III)/Fe(II)
redox potentials are −0.19, −0.41, −0.25, −0.52, and −0.68 V
respectively vs SCE in dichloromethane.8b,f,g The variation in
the Fe(III)/Fe(II) redox potential has been suggested as a
result of a hydrogen bond (NH····S) while the electronic effect
by ortho and para substituent on the thiophenol was found to
be insignificant.8b,f Among complexes of FeIII(OEP){S-2,6-
(CF3CONH)2C6H3}, FeIII(OEP)(S-2-CF3CONHC6H4), and
4a, a variation of 430 mV in the Fe(III)/Fe(II) couple was
obtained by the influence of intramolecular NH····S hydrogen
bonding.8b,f In the present investigation, however, the effect of
substituent on the thiophenol ring toward the Fe(III)/Fe(II)
redox potential has been explored varying the substituent from
electron donating (2b) to electron withdrawing (2c, 2d).
Modulating electronic property of the substituent leads to a
wide potential window of 540 mV for Fe(III)/Fe(II) redox
couple in the series 2a−2d. However, in the case of monomeric
analogues 4a−4d, the variation in Fe(III)/Fe(II) redox
potential on varying such electronic properties of thiophenols
is only 270 mV, which is half of the potential range observed in
diheme. Such an increase in the potential range on going from
monoheme to diheme can be attributed as a result of heme−
heme interaction present in dihemes.
center is found to significantly influence the Fe(III)/Fe(II)
redox potential.3b The control of heme redox potential by
controlling the spin state is crucial for the functioning of
cytochrome P450.8h,k
Computational Studies. Density functional calculations
have been carried out using the B3LYP hybrid functional20−22
using the Gaussian 03, revision B.04, package.23 The single
point energy calculations were performed for 2a, 5a, and 7
using the LANL2DZ basis set for iron atoms and the 6-31G**
basis set for all other atoms. Atom coordinates of 2a and 5a
have been obtained from the respective crystal structures while
geometry optimization of 7 has been performed with the help
of DFT. Figure 15 represents a qualitative plot of the relative
Cyclic voltammogram of the six-coordinate complex 6a has
also been measured under identical conditions, in which the
Fe(III)/Fe(II) redox couple was observed at −0.27 V (versus
Ag/AgCl). The large positive shift of 490 mV in the Fe(III)/
Fe(II) redox potential of 6a, as compared to 2a, suggests that
the reduction of the iron center becomes much easier upon
coordination of an extra thiophenol at the sixth position which
eventually leads to changing the metal spin from high to low.
Figure 14 compares the cyclic voltammograms of both five-
(2a) and six-coordinate complexes (6a). Such a positive shift in
the Fe(III)/Fe(II) redox potential can be attributed to the
contributions from the sixth axial coordination as well as a
change of metal spin (from high to low-spin). Earlier, it was
demonstrated that the change in the spin state of the Fe(III)
Figure 15. A plot showing the relative energies of the LUMOs for 2a,
5a, and 7.
energy of LUMOs for 2a, 5a, and 7. Reduction of metal-
loporphyrin systems can be seen as an addition of electrons to
the LUMO; therefore, a compound with a low energy LUMO
will be easier to reduce. The LUMO of 2a was found to be
lowest in energy and that of 7 being highest with 5a in between.
This explains the positive shifting of the Fe(III)/Fe(II) couple
as we move from 7 to 5a to 2a, which also supports our
experimental observation. As can be seen in Figure 16, the
Figure 16. A plot showing the HOMOs for (A) 2a and (B) 5a.
HOMO is more localized on the thiophenolate ligand with
significant amplitude on the sulfur atom in 2a, while it is more
localized on the porphyrin ring in 5a, which suggests the back-
donation of an electron in the thiophenolato complex.
CONCLUSIONS
■
Syntheses, structure, and properties of a series of diiron(III)-
bisporphyrins with axial thiophenolate coordination have been
reported. The porphyrin macrocycle in the diheme complexes
is found to be more distorted compared to the monoheme
analogues. Electrochemical data reveal a good linear relation-
Figure 14. A portion of the cyclic voltammograms of (A) 2a and (B)
6a at 298 K in CH2Cl2 (scan rate 100 mV/s) with 0.1 M tetra(n-
butyl)ammonium hexafluorophosphate as the supporting electrolyte.
The reference electrode was Ag/AgCl.
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CH2(b): 69.6. p-H: −90.5; mt-H: 66.7 ppm. E1/2 (Fe3+/Fe2+), V:
−0.54.
ship between the Fe(III)/Fe(II) redox couple and pKa of
thiophenol. The Fe(III)/Fe(II) redox couple has been shifted
more positively in the thiophenolato complex compared to
their phenolato analogue due to the presence of low-lying
vacant 3d orbitals in sulfur which facilitates the back-donation
of an electron from the iron center. Changing the electronic
nature of the substituents on the thiophenolate ring in diheme
has been found to change the Fe(III)/Fe(II) redox potential up
to 540 mV (in contrast to the value of only 270 mV in case of
monoheme analogues). The pKa of the coordinated thiophenol
has been found to play a crucial role in deciding the nature of
the shift of the Fe(III)/Fe(II) redox couple, while the inter-ring
interaction decides the extent of shifting. The large difference in
structural, chemical, and electrochemical properties of the
diheme as compared to the monoheme analog provide
unequivocal evidence of the role played by heme−heme
interaction in diheme.
Additions of excess thiophenol to the dichloromethane
solution of five-coordinate thiophenolate complex results in the
formation of a six-coordinate low-spin complex which shows a
large positive shift of 0.49 V in the Fe(III)/Fe(II) redox couple.
Such a large positive shift is attributed to the contributions
from the sixth axial coordination as well as change of iron spin
(from high to low).
2d. Yield: 98 mg (75%). Anal. Calcd (found): C, 64.50 (64.33); H,
5.66 (5.75); N, 7.00 (7.15). UV−vis (dichloromethane) [λmax, nm (ε,
M−1 cm−1)]: 383 (1.28 × 105), 418 (1.06 × 105), 519 (1.85 × 104),
552 (1.70 × 104) 657 (1.58 × 104).1H NMR (CDCl3, 295 K) meso-H:
−46.7, −60.2; CH3: 7.5, 7.9, 8.2; CH2: 36.5, 36.9, 41.1, 44.3, 45.6,
46.1; CH2(b): 63.6 ppm. E1/2 (Fe3+/Fe2+), V: −0.36.
4b. UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 382 (9.1
× 104), 504 (1.2 × 103), 533 (1.0 × 103), 632 (7.8 × 102). E1/2 (Fe3+/
Fe2+), V: −0.72.
4c. Yield: 95 mg (74%). Anal. Calcd (found): C, 65.80 (65.63); H,
6.18 (6.27); N, 7.31(7.25). UV−vis (dichloromethane) [λmax, nm (ε,
M−1 cm−1)]: 377 (1.3 × 105), 514 (6.1 × 103), 534 (5.8 × 103), 643
(9.8 × 102). 1H NMR (CDCl3, 295 K) meso-H: −48.7; CH3: 8.8; CH2:
40.1, 46.9; p-H: −90.2; mt-H: 67.3 ppm. E1/2 (Fe3+/Fe2+), V: −0.56.
4d. Yield: 99 mg (75%). Anal. Calcd (found): C, 64.04 (64.15); H,
5.63 (5.75); N, 7.11 (7.03). UV−vis (dichloromethane) [λmax, nm (ε,
M
−1 cm−1)]: 374 (1.54 × 105), 510 (7.8 × 103), 534 (6.5 × 103), 647
1
(1.2 × 103). H NMR (CDCl3, 295 K) meso-H: −47.9; CH3: 7.6, 7.3;
CH2: 42.0, 47.9 ppm. E1/2 (Fe3+/Fe2+), V: −0.45.
Theoretical Calculation. DFT calculations have been carried out
by employing a B3LYP20−22 hybrid functional using the Gaussian 03,
revision B.04, package.23 The method used was Becke’s three
parameter hybrid exchange functional; the nonlocal correlation
provided by the Lee, Yang, and Parr expression; and Vosko, Wilk,
and Nuair 1980 correlation functional (III) for local correction.20−22
The basis set was LanL2DZ for the iron atom and 6-31G** for carbon,
nitrogen, sulfur, oxygen, and hydrogen atoms. All the calculations were
performed with a multiplicity of 11, and no structural relaxations were
carried out. Atom coordinates of 2a and 5a3b have been obtained from
the respective crystal structures. In the absence of X-ray structure,
geometry optimization of 7 has been performed with the help of DFT
using the initial atom coordinates from the X-ray structure of trans-1,2-
bis[chloroiron(III)5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]-
ethene.3e Visualization of the molecular orbitals and the corresponding
diagrams were made using the Avogadro software.25
EXPERIMENTAL SECTION
■
Materials. μ-Oxo-syn-1,2-bis[5-(2,3,7,8,12,13,17,18-
octaethylporphyrinato)iron(III)]ethane, 1; μ-oxo-bis-
[(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III)], 3;
(thiophenolato)(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III),
4a; (2,4,6-trimethylthiophenolato)(2,3,7,8,12,13,17,18-
octaethylporphyrinato)iron(III), 4b; and 1,2-bis[(chloro){5-
(2,3,7,8,12,13,17,18-octaethylporphyrinato)}iron(III)]ethane, 7, were
prepared using the methods reported earlier.8b,3c,16,24 Reagents and
solvents were purchased from commercial sources and purified by
standard procedures before use.
Instrumentation. UV−vis spectra were recorded on a PerkinElm-
er UV/vis spectrometer. Electron paramagnetic resonance (EPR)
spectra were obtained on a Bruker EMX EPR spectrometer. Elemental
(C, H, and N) analyses were performed on a PerkinElmer 2400II
Synthesis. Complexes 2a−2d were prepared using the general
procedure; details for one representative case have been described
below.
1
elemental analyzer. H NMR spectra were recorded on a JEOL 500
MHz instrument. The spectra for paramagnetic molecules were
recorded over a 100-kHz bandwidth with 64 K data points and a 5 ms
90° pulse. For a typical spectrum, between 2000 and 3000 transients
were accumulated with a 50-μs delay time. The residual 1H resonances
of the solvents were used as a secondary reference. Cyclic
voltammetric studies were performed on a BAS Epsilon electro-
chemical workstation in dichloromethane with 0.1 M tetrabutylammo-
nium hexafluorophoshate (TBAH) as a supporting electrolyte, and the
reference electrode was Ag/AgCl and the auxiliary electrode was a Pt
wire. The concentration of the compounds was on the order of 10−3
M. The ferrocene/ferrocenium couple occurs at E1/2 = +0.45 (65) V
versus Ag/AgCl under the same experimental conditions. 57Fe
Syntheses of 2a. A total of 100 mg of 1 (0.082 mmol) was
dissolved in 100 mL of dichloromethane, and thiophenol (18.07 mg,
0.164 mmol) was added to it. The mixture was then stirred for 30 min
under nitrogen. During the progress of the reaction, the green solution
changed to bright red, and the resulting solution was then evaporated
to complete dryness. The solid thus obtained was then dissolved in a
minimum volume of benzene and carefully layered with n-hexane. On
standing for 6−8 days in air at room temperature, a dark purple
crystalline solid was formed, which was collected by filtration, washed
well with the mother liquor, and dried in a vacuum. Yield: 88 mg
(76%). Anal. Calcd (found): C, 72.60 (72.79); H, 7.09 (7.35); N, 7.88
(7.91). UV−vis (dichloromethane) [λmax, nm (ε, M−1 cm−1)]: 390
(1.2 × 105), 413 (1.05 × 105), 511 (2.52 × 104), 541 (2.45 × 104) 633
Mossbauer spectra were recorded using a Wissel 1200 spectrometer
̈
and a proportional counter. 57Co(Rh) in a constant acceleration mode
was used as the radioactive source. Isomer shifts (δ) are given related
to α-iron foil at room temperature.
1
(1.95 × 104). H NMR (CDCl3, 295 K) meso-H: −45.6, −54.9; CH3:
7.9, 8.3; CH2: 33.5, 35.2, 37.1, 39.5, 41.9, 43.9, 48.1; CH2(b): 72.5. o-
H: −85.9; p-H: −82.7; mt-H: 56.4 ppm. E1/2 (Fe3+/Fe2+), V: −0.76.
2b. Yield: 90 mg (73%). Anal. Calcd (found): C, 73.38 (73.55); H,
7.50 (7.84); N, 7.44 (7.48). UV−vis (dichloromethane) [λmax, nm (ε,
X-ray Structure Solution and Refinement. Single-crystal X-ray
data were collected at 100 K on a Bruker SMART APEX CCD
diffractometer equipped with CRYO Industries low-temperature
apparatus, and intensity data were collected using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The data
integration and reduction were processed with SAINT software.26 An
absorption correction was applied.27 The structure was solved with the
direct method using SHELXS-97 and was refined on F2 by full-matrix
least-squares technique using the SHELXL-97 program package.28
Non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were included in calculated positions. In the refinement,
hydrogens were treated as riding atoms using SHELXL default
M
−1 cm−1)]: 391 (1.46 × 105), 415 (1.3 × 105), 514 (2.6 × 104), 546
1
(2.51 × 104) 628 (1.91 × 104). H NMR (CDCl3, 295 K) meso-H:
−44.9, −62.6; CH3: 7.3, 7.4; CH2: 30.9, 33.7, 35.1, 37.6, 38.4, 42.7;
CH2(b): 85.4; mt-H: 77.7 ppm. E1/2 (Fe3+/Fe2+), V: −0.90.
2c. Yield: 110 mg (86%). Anal. Calcd (found): C, 66.24 (66.49); H,
6.21 (6.60); N, 7.19 (7.22). UV−vis (dichloromethane) [λmax, nm (ε,
M−1 cm−1)]: 385 (1.37 × 105), 410 (1.12 × 105), 515 (1.98 × 104),
550 (1.75 × 104), 645 (1.62 × 104). H NMR (CDCl3, 295 K) meso-
1
H: −47.9, −64.2; CH3: 7.7, 7.9; CH2: 35.5, 37.4, 40.1, 42.9, 47.6, 51.8;
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Table 4. Crystal Data and Data Collection Parameters
2a
2b
2c
2d
formula
T, K
C86H100Fe2N8S2
100(2)
C92H112Fe2N8S2
100(2)
C86H96Cl4Fe2N8S2
100(2)
C86H90F10Fe2N8S2
100(2)
fw
1421.56
1505.72
1559.33
1601.48
monoclinic
P21/c
cryst syst
space group
a, Å
monoclinic
P21/c
monoclinic
P21/c
triclinic
P1
̅
14.517(5)
17.068(5)
14.872(5)
90.000(5)
90.352(5)
90.000(5)
3685(2)
Mo Kα (0.71073)
2
14.850(5)
17.254(5)
15.114(5)
90.000(5)
93.611(5)
90.000(5)
3865(2)
Mo Kα (0.71073)
2
11.627(5)
12.813(5)
14.097(5)
67.275(5)
73.455(5)
84.451(5)
1856.7(13)
Mo Kα (0.71073)
1
14.647(5)
17.029(5)
14.927(5)
90.000(5)
91.512(5)
90.000(5)
3722(2)
Mo Kα (0.71073)
2
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
radiation (λ, Å)
Z
d
calcd, g cm−3
1.281
1.294
1.395
1.429
F(000)
μ, mm−1
1512
1608
820
1672
0.502
0.483
0.644
0.525
no. of unique data
no. of params refined
GOF on F2
6834
7162
6864
6848
450
480
468
495
1.031
1.051
1.027
1.067
a
R1 [I > 2σ(I)]
0.0479
0.0840
0.0465
0.1096
a
R1 (all data)
0.0646
0.1343
0.0673
0.1434
b
wR2 (all data)
0.1270
1.019 and −0.383 e.Å−3
0.2376
0.854 and −0.463 e.Å−3
0.1182
0.509 and −0.296 e.Å−3
0.3302
2.742 and −0.648 e.Å−3
largest diff. peak and hole
∑[w(Fo2 − Fc2)2]
∑[w(Fo2)2]
∑ || F |−| F ||
a
b
o
c
R1 =
wR2 =
∑ |F |
o
parameters. Crystallographic data and data collection parameters are
given in Table 4.
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ASSOCIATED CONTENT
* Supporting Information
■
S
UV−visible spectral change upon gradual addition of
thiophenol to a dichloromethane solution of 1 (Figure S1);
X-ray structures of 2b and 2c (Figures S2 and S3); packing
diagrams of 2a, 2b, and 2d (Figures S4−S6); atom numbering
scheme used for out-of-plane displacement plots (Figure S7);
experimental and simulated EPR spectra of 2b (Figure S8);
UV−visible spectra of 2a and 6a in dichloromethane (Figure
S9); experimental and simulated EPR spectra of 6b (Figure
1
S10); Mulliken spin density plots for 2a (Figure S11); H
NMR spectra of complexes 4a−4d (Figure S12); UV−visible
spectral change upon one-electron reduction of 2e (Figure
S13). X-ray crystallographic details in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sprath@iitk.ac.in.
■
Notes
The authors declare no competing financial interest.
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DEDICATION
■
Dedicated to Professor Marilyn M. Olmstead on the occassion
of her 70th birthday.
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