Magnetic resonance spectroscopic investigations of the electronic ground and excited states in strongly nonplanar iron(III) dodecasubstituted porphyrins

Article abstract of DOI:10.1021/ic049089q

A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)Fe^III, octaethyltetraphenylporphyrin, (OETPP)Fe^III, its perfluorinated phenyl analogue, (F₂₀OETPP)Fe^III, and tetra-(β, β′-tetramethylene)-tetraphenylporphyrin, (TC₆TPP)Fe ^III, have been prepared and characterized by ¹H NMR spectroscopy: chloride, perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation, and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes consideration of a thermally accessible excited state. The results show that, although the ground state differs for various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the excited state often has S = 3/2 (or S = 5/2 in the cases where the ground state has S = 3/2). The EPR spectra (4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show large-g_max signals with g_max = 3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of low-spin (LS) (d _xy)²(d_xz,d_yz)³ ground states for both. The bis-cyanide complex also yields a large-g_max EPR spectrum with g = 3.49 and other features that could suggest that some molecules have the (d_xz,d_yz)⁴(d _xy)¹ ground state. The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S = 5/2 ground-state systems with admixture of 1-10% of S = 3/2 character.

Full text of DOI:10.1021/ic049089q

Inorg. Chem. 2005, 44, 2848−2866
Magnetic Resonance Spectroscopic Investigations of the Electronic
Ground and Excited States in Strongly Nonplanar Iron(III)
Dodecasubstituted Porphyrins
Liliya A. Yatsunyk, Nikolai V. Shokhirev, and F. Ann Walker*
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721-0041
Received July 9, 2004
A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)FeIII, octaethyltetraphe-
nylporphyrin, (OETPP)FeIII, its perfluorinated phenyl analogue, (F20OETPP)FeIII, and tetra-(
â′-tetramethylene)-
â
,
tetraphenylporphyrin, (TC6TPP)FeIII, have been prepared and characterized by 1H NMR spectroscopy: chloride,
perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments
have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the
complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation,
and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects
contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded
version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes
consideration of a thermally accessible excited state. The results show that, although the ground state differs for
various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the
3
5
excited state often has S
(4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show “large-gmax” signals with gmax
3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of
) / (or S ) / in the cases where the ground state has S )
3/2). The EPR spectra
2 2
)
low-spin (LS) (dxy)2(dxz,dyz)3 ground states for both. The bis-cyanide complex also yields a large-gmax EPR spectrum
with g
)
3.49 and other features that could suggest that some molecules have the (dxz,dyz)4(dxy)1 ground state.
5
The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S
ground-state systems with admixture of 1
)
/
2
3
−
10% of S
)
/
character.
2
Introduction
contrast, NMR spectroscopy is extremely useful for inves-
tigating the ambient-temperature solution structures and spin
states of a wide range of metalloporphyrins. As part of an
ambient-temperature study, it is often possible not only to
characterize the electronic ground state, but also to determine
the possible existence of a thermally accessible excited state,
as we have shown elsewhere.7-9 This is because of the
Saddle-shaped iron(III) porphyrinate complexes have been
shown to be very promising models of the heme centers in
the cytochrome bc1 complex and other heme proteins.1,2 For
this reason, a detailed investigation of their NMR and frozen
solution EPR spectra is highly desirable. EPR spectroscopy
is an excellent technique for characterizing the electronic
ground state of paramagnetic heme complexes at 4.2 K, as
we have shown in recent studies of the molecular structures
and EPR spectra of a number of octaalkyltetraphenylpor-
phyrinate complexes of Fe(III) in the solid state.1-6 In
(4) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 4341-4352.
(5) Yatsunyk, L. A. Ph.D. Dissertation, University of Arizona, Tucson,
AZ, 2003.
(6) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, K.
M.; Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc.
2001, 123, 6564-6578.
(7) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-
17804.
(8) Banci, L.; Bertini, I.; Luchinat, C.; Pierattelli, R.; Shokhirev, N. V.;
Walker, F. A. J. Am. Chem. Soc. 1998, 120, 8472-8479.
(9) Nesset, M. J.; Cai, S.; Shokhireva, T. Kh.; Shokhirev, N. V.; Jacobson,
S. E.; Jayarai, K.; Gold, A.; Walker, F. A. Inorg. Chem. 2000, 39,
532-540.
* To whom correspondence should be addressed. E-mail: awalker@
u.arizona.edu.
(1) Walker, F. A. Chem. ReV. 2004, 104, 589-615 and references therein.
(2) Yatsunyk, L. A.; Carducci, M. D.; Walker, F. A. J. Am. Chem. Soc.
2003, 125, 15986-16005.
(3) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757-777.
2848 Inorganic Chemistry, Vol. 44, No. 8, 2005
10.1021/ic049089q CCC: $30.25
© 2005 American Chemical Society
Published on Web 03/23/2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
temperature dependence of the paramagnetic contribution to
the chemical shift of a given proton, δpara, also known as
the hyperfine or isotropic shift10-14
be determined by fitting the temperature dependence of the
proton chemical shifts, assuming that the diamagnetic shift
of each proton type is known. A program that carries out
this fitting procedure, with least-squares minimization of the
errors between experimental and calculated shifts, has been
created in our laboratory and is available on the Internet.18
In the present work, the following Fe(III) porphyrin
systems have been investigated as a function of temperature
δpara ) δobs - δdia
(1)
where δobs is the observed chemical shift of the proton of
interest and δdia is the diamagnetic shift of the same proton,
measured for the corresponding metal complex that is
diamagnetic [Co(III) or low-spin Fe(II) in place of Fe(III),
for example]. δpara consists of two contributions: the contact
(through-bond) and the pseudocontact (through-space, also
called the electron-nuclear dipolar) terms10-13
1
by H NMR spectroscopy: octaethyltetraphenylporphyrin
(OETPP); octaethyltetra(perfluoro)phenylporphyrin (F20-
OETPP), in which all H positions of the phenyl rings of
OETPP are replaced by F; octamethyltetraphenylporphyrin
(OMTPP); and tetra-(â,â′-tetramethylene)tetraphenylpor-
phyrin (TC6TPP). Depending on the nature of the axial
ligands, the complexes can adopt different ground-state
electron configurations, either predominantly high-spin (five-
coordinate chloride complexes), low-spin with the (dxy)2-
(dxz,dyz)3 configuration (six-coordinate iron(III) porphyrinates
with basic pyridines and imidazoles, a number of whose
structures have been reported1), low-spin complexes with the
(dxz,dyz)4(dxy)1 ground state (six-coordinate iron(III) por-
δpara ) δcon + δpc
(2)
Each of these terms can usually be estimated with fairly high
accuracy, as described in detail elsewhere.10-13 Both of these
terms usually have an inverse temperature dependence
resulting from the Curie law14-16
δpara ) C/T
(3)
phyrinates with two CN-, 4-CNPy,3 or t-BuNC4 axial
Because the contact term dominates the paramagnetic shifts
3
ligands), or intermediate-spin (IS) complexes, S )
/
2
of all spin states of Fe(III), a nearly linear 1/T dependence
(OETPP with 4-CNPy, THF, or other weakly coordinating
ligands such as perchlorate).3,5,19
5
is generally observed even for S ) /2 Fe(III) complexes,14
which have the largest zero-field splitting constants for this
metal and oxidation state.10
The goals of the present work were to assign all resonances
and to analyze the temperature dependence of the chemical
shifts of the highly saddled iron(III) porphyrinate complexes
to determine unambiguously their electronic ground and
excited states. EPR spectroscopy was utilized in most cases
to determine the electronic ground state of the complexes
of interest at 4.2 K; a number of the EPR spectra, especially
those of crystalline samples, have been published
previously.1-4,6 As will be seen, there are some cases in
which the electronic ground state at 4.2 K differs from that
observed over the temperature range accessible for solution
NMR investigations (180-303 K in CD2Cl2).
However, as we showed previously,7-9 a number of model
heme complexes have a thermally accessible excited state
that causes at least some curvature of the Curie plot,7,8,17
and can sometimes show extremely curved chemical shift
dependence,7,9 when plotted as a function of inverse absolute
temperature. Expansion of the Curie law to include the
contribution from the Boltzmann population of this thermally
accessible excited state yields the following expression, if
the 1/T2 contribution to the high-spin state is neglected7
21/kT
δpara ) (1/T)[W1C1 + W2C2e-E /kT]/[W1 + W2e-E
]
(4)
21
Experimental Section
where E21 is the energy separation between the ground and
excited states; W1 and W2 are the multiplicities of the ground
and excited states, respectively () 2S + 1 in each case);
and C1 and C2 are the coefficients determined for each state.
These coefficients are approximately equal to the Curie
coefficients of each (eq 3), except for the small contribution
from the pseudocontact contribution to δpara. C1 and C2 can
Synthesis and Sample Preparation for NMR Spectroscopy.
The syntheses of (OMTPP)FeCl, (OETPP)FeCl, and (TC6TPP)-
FeCl were carried out as described elsewhere;2 (F20OETPP)FeCl
was a gift from Dr. C. J. Medforth, University of California, Davis,
CA. Conversion from chloride to perchlorate anion was done
according to previously described procedures.3,9
NMR samples of (OMTPP)FeCl, (OETPP)FeCl, (F20OETPP)-
FeCl, and (TC6TPP)FeCl and their perchlorate counterparts were
prepared by dissolving 2-5 mg of each compound in 0.3 mL of
CD2Cl2 in a 5-mm NMR tube (Wilmad WGH-07). Bis-ligated
complexes were prepared by subsequent addition of 3-6 equiv of
the desired ligand. In some cases, higher amounts of the axial ligand
were necessary to ensure full complex formation at ambient
temperatures. The Na[FeOETPP(CN)2] sample was prepared by
dissolving 3-5 mg of (OETPP)FeCl in 0.3 mL of DMF-d7 in an
NMR tube and adding 2 drops of D2O saturated with NaCN.
(10) La Mar, G. N.; Walker, F. A. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: New York, 1979; Vol. IV, pp 61-157.
(11) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of Paramagnetic
Molecules. Applications to Metallobiomolecules and Models; Elsevi-
er: New York, 2001.
(12) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Plenum Press: San Diego, CA, 2000; Vol. 5,
Chapter 36, pp 81-183.
(13) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544.
(14) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286-301.
(15) Walker, F. A.; La Mar, G. N. Ann. N. Y. Acad. Sci. 1973, 206, 328-
348.
1
(16) This is true except in the case where S > /2 and there is a relatively
large zero-field splitting, in which case the pseudocontact term has a
C′/T2 dependence and the contact term has a C/T dependence.10-14
(17) Basu, P.; Shokhirev, N. V.; Enemark, J. H.; Walker, F. A. J. Am.
Chem. Soc. 1995, 117, 9042-9055.
(18) Shokhirev, N. V.; Walker, F. A. http://www.shokhirev.com/nikolai/
programs/prgsciedu.html, 2004.
(19) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2849

Yatsunyk et al.
NMR Spectroscopy. Most of the work presented here was
to be expected, the approximate temperatures at which
dynamic processes were visibly active, and in cases of the
Lewis base complexes, whether enough ligand had been
added to maintain the desired complex throughout the
temperature range. Then, a DQF-COSY (or magnitude-
mode COSY, depending on the relaxation times of the
protons of interest) spectrum was recorded at a temperature
where all peaks were reasonably sharp and well-resolved,
and phenyl resonances were assigned on the basis of the
presence of phenyl-m-o and m-p cross-peaks. In the cases
of the various (OETPP)FeIII, (F20OETPP)FeIII, and (TC6TPP)-
FeIII complexes, methylene and methyl protons could be
assigned unambiguously from the same COSY spectra by
detecting the cross-peaks between geminal diastereotopic
methylene protons, as well as between each of these protons
and CH3 (for OETPP, F20OETPP) or CH2(â) (for TC6TPP)
groups. For bis-ligated complexes, NOESY and/or ROESY
experiments were utilized to assign the bound-ligand peaks.
Because of the presence of ligand exchange (dissociation of
the axial pyridine/imidazole ligands and binding of free
pyridine/imidazole to the iron porphyrin) above certain
temperatures, chemical-exchange (CE) cross-peaks were
observed between free- (F) and bound-(L) ligand resonances.
Because the free-ligand assignments were known, the CE
cross-peaks allowed unambiguous assignment of the bound-
ligand resonances. Utilizing NOE cross-peaks in NOESY
or ROESY experiments, the assignments based on COSY
carried out using a Varian Unity-300 NMR spectrometer operating
1
at 299.957 MHz H frequency and equipped with a broad-band
inverse probe and a Varian variable-temperature unit. The temper-
ature was calibrated using the standard Wilmad methanol sample.
Bruker DRX-500 and -600 NMR spectrometers were used for
ambient- to high-temperature experiments, because the gradient
1
probes cannot be cooled lower than -20 °C. H 1D spectra were
referenced to the residual solvent peak (CD2Cl2, 5.32 ppm; CDCl3,
7.24 ppm; C2D2Cl4, 5.91 ppm; and DMF-d7, 8.02 ppm), and 2D
spectra were referenced to a specific signal from a diamagnetic
species in the 1D spectra.
1
Homonuclear H 2D NMR spectra were acquired at a number
of temperatures between +60 and -90 °C depending on the sample,
using standard pulse sequences, with 512 complex points in the
directly detected dimension, and 128 t1 increments in the indirectly
detected dimension (states mode). ROESY experiments were
performed only in the cases where a relatively narrow spectral
window could be used; for complexes with wide spectral windows,
ROESY spin lock experiments could not be performed. The probe
coil was tuned to the proton frequency, and the pulse width of the
90° proton pulse and the relaxation time, T1, of each proton signal
1
in a 1D H spectrum were determined at each temperature before
the 2D experiments were run. The mixing times in the NOESY/
ROESY experiments were set to the average T1 of the protons that
are close to the paramagnetic center, i.e., the fast-relaxing protons
(usually phenyl-o, some bound-ligand protons, methylene protons
in OETPP and TC6TPP, and methyl protons in OMTPP). The
relaxation delays in 2D experiments were set so that the total recycle
time was larger than or equal to the T1 of the phenyl-p protons,
which were typically the slowest-relaxing protons of the complex,
or the average of the T1 relaxation times of the free-ligand protons.
All 2D NMR data acquired on the Unity 300, with the exception
of COSY and DQF-COSY data, were processed using the Felix
2000 software package (Accelerys) with zero-filling to twice the
original data size in both dimensions and Gaussian apodization
before each of the two Fourier transformations, followed by baseline
correction (the baseline points were detected using the FLATT
procedure20). COSY and DQF-COSY data were processed using
the VNMR software package, with zero-filling and squared sine-
bell apodization before each of the two Fourier transforms, followed
by baseline correction. Data acquired on the Bruker DRX-500 and
-600 instruments were processed using the Xwinnmr software
package. Fitting of the data for the Curie plots was done using the
two-level temperature-dependent fitting program created in this
laboratory.7,18
1
data could be confirmed. The importance of 1D H spectra
should not be underestimated, and their accurate integration
also contributed to peak assignments.
Another important source of information about the as-
signment of proton resonances is the spin-lattice relaxation
times, T1. The dipolar relaxation rate is inversely proportional
to the sixth power of the distance (r) between the proton
and the paramagnetic center (1/T 1/r6);21 thus the T1 value
decreases dramatically upon even a small decrease of r.
Protons attached to carbons that carry a high spin density
[such as â-pyrrole C’s for the high-spin (HS) or low-spin
(LS) (dxy)2(dxz,dyz)3 ground state] and ligand protons right
above the porphyrin core (2,6-H in 4-Me2NPy, 2-H and 4-H
in 1-MeIm) have very short T1 relaxation times (1-10 ms).
Protons that are farther from the electron density delocalized
onto the porphyrin core have longer T1 values (50-150 ms),
and finally, protons of free ligands and solvents have T1 on
the order of 0.5-1 s, or longer, in the absence of chemical
exchange. In accord with these findings, the spin-spin
relaxation times, T2, for the protons that are close to the
paramagnetic center are very short, and because of the
inverse relationship between T2 and the line width, these
proton signals are broad or at least much broader than the
other peaks in the spectra.
EPR Spectroscopy. EPR spectra were recorded on a Bruker
ESP-300E EPR spectrometer (operating at 9.4 GHz) equipped with
an Oxford Instruments ESR 900 continuous-flow helium cryostat.
Microwave frequencies were measured using a Systron-Donner
frequency counter. Spectra were obtained for samples in frozen
CD2Cl2 and DMF solutions. Typical values for microwave power,
modulation frequency, and modulation amplitude were 0.2 mW,
100 kHz and 1.011 G, respectively.
Results
A. Five-Coordinate Fe(III) Complexes. (OMTPP)FeCl
was studied in the temperature range from +35 to -93 °C,
and example 1D 1H spectra at 21 and -80 °C are shown in
Figure 1. They are consistent with the C2V symmetry of the
1
I. Assignment of H NMR Resonances. Proton reso-
nances for all complexes studied were assigned as follows.
First, the 1D 1H spectrum was surveyed over the entire liquid
range of the solvent to determine the range of chemical shifts
(21) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear
Magnetic Resonance in One and Two Dimensions; Clarendon Press:
Oxford, U.K., 1992; Chapter 9.
(20) Gu¨ntert, P.; Wu¨thrich, K. J. Magn. Reson. 1992, 96, 403-407.
2850 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
Table 1. Chemical Shifts for Selected Resonances in Five-Coordinate Octaalkyltetraphenyl Complexes at +30 °Ca
b
complex
δpyr
δm
δp
δo
δm - δp
δm - δo
(OMTPP)FeCl
(OETPP)FeCl
(F20OETPP)FeCl
(TC6TPP)FeCl
(OEP)FeCld
49.93
35.26
40.01
53.85
37.2
81.0
59.4
44.86
12.92
12.89
(-89.05)c
13.00
-
12.15
7.56
7.23
7.70
9.2
7.56
7.00
(-82.92)c
7.55
-
6.20
9.36
9.78
9.16
11.9
-
8.56
9.26
(-39.09)c
7.96
-
6.40
11.88
12.97
11.40
7.7
+5.36
+5.89
(-6.13)c
+5.45
-
+6.0
-1.80
-2.55
-1.46
-4.2
-
+4.36
+3.64
(-49.46)c
+5.04
-
+5.8
-4.32
-5.74
-3.7
+2.7
-
(TPP)FeCle
(OMTPP)FeClO4
f
(OETPP)FeClO4
f
(TC6TPP)FeClO4
88.77
13.0
g
(TPP)FeClO4
g
(ETIO)FeClO4
∼62 CH3,
-
-
∼42, ∼29 CH2
a The chemical shifts for δpyr, δm and δo are average chemical shifts. δpyrr indicates methyl (for OMTPP) or methylene [CH2(out) for OETPP and
b
F20OETPP and CH2(R) for TC6TPP] groups directly attached to pyrrole â-C’s. 19F-phenyl shifts measured at 23 °C. d Data from ref 15 at 29 °C; a somewhat
c
higher value of 41.4 ppm (30 °C) is reported in ref 48. e Data from ref 6 at 25 °C. f Data for 23 °C. g Data from ref 28; T ) 26 °C.
by the difference in pseudocontact shifts, because of the “up”
(toward the FeIII and Cl-) and “down” (away from the FeIII
and Cl-) position of the methyl groups.
The EPR spectrum (X-band, 4.2 K, frozen CD2Cl2
solution) of (OMTPP)FeCl is indicative of a predominantly
5
high-spin (S ) /2) ground state, consistent with the tem-
perature-dependent NMR shifts just discussed. It is charac-
terized by gx ) 6.65, gy ) 5.30 [g ) (gx + gy)/2 ) 5.98],
and gz )1.98. Similar values are obtained for the (OMTPP)-
FeCl complex in frozen THF solution [gx ) 6.6, gy ) 5.4
(g ) 6.0), and gz )1.99]. According to Maltempo and
Moss,23 pure HS and IS states are characterized by g values
of 6 and 4, respectively. Therefore, the reduction of the g
value due to quantum-mechanical S ) 5/2, 3/2 mixing can be
expressed as a combination of these two limiting g values
according to the equation23
g ) 6(a5/2)2 + 4(b3/2)2
(5)
where (a5/2)2 and (b3/2)2 are the coefficients of the HS and
IS states, respectively, indicating the amount of each in the
ground state of the complex. The value of (a5/2)2 can be
calculated from g by the following expression23
Figure 1. 1H NMR spectra of (OMTPP)FeCl at 21 and -80 °C, together
with the downfield part of the NOESY/EXSY spectrum at -80 °C, with
5-ms mixing time; * represents impurities.
(a5/2)2 ) (g - 4)/2
(6)
The EPR g values obtained are consistent with 0-1.25%
mixing of the IS state into the HS ground state of the
(OMTPP)FeCl complex.
complex in solution. The proton chemical shifts are sum-
marized in Table S1 (Supporting Information), together with
the relaxation times, T1. The chemical shifts of all protons
of (OMTPP)FeCl at +30 °C are included in Table 1, where
the various five-coordinate complexes can be compared.
Above -26 °C, there is one signal representing all eight
methyl groups. It splits into two resonances as the temper-
ature is lowered because of slow porphyrin ring inversion,
for which the rates have been measured and are reported in
the accompanying article.22 The two methyl signals show
significantly different chemical shifts of 80.3 and 70.9 ppm,
a difference of 9.4 ppm, at -70 °C. As the temperature is
lowered, this difference increases and reaches 10.9 ppm at
-90 °C. The two different chemical shifts are mainly caused
(OETPP)FeCl has been investigated in detail by Cheng
et al.,24 Schu¨nemann et al.,25 and our laboratory.6 Here, we
compare the chemical shifts of this complex for two different
solvents, CD2Cl2 and C2D2Cl4, both of which were used for
the kinetic studies described in the accompanying article.22
CD2Cl2 was used in the temperature range from -90 to +35
°C, and C2D2Cl4 from ambient temperature to +70 °C. No
dependence of kinetic parameters on the nature of the solvent
(for these two solvents only) was observed;22 however, the
(23) Maltempo, M. M.; Moss, T. H. ReV. Biophys. 1976, 9, 181-215.
(24) Cheng, R.-J.; Chen, P.-Y.; Gau, P.-P.; Chen, C.-C.; Peng, S.-M. J.
Am. Chem. Soc. 1997, 119, 2563-2569.
(25) Schu¨nemann, V.; Gerdan, M.; Trautwein, A. X.; Haoudi, N.; Mandon,
D.; Fischer, J.; Weiss, R.; Tabard, A.; Guilard, R. Angew. Chem., Int.
Ed. 1999, 38, 3181-3183.
(22) Yatsunyk, L. A.; Ogura, H.; Walker, F. A. Inorg. Chem. 2005, 44,
2867-2881.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2851

Yatsunyk et al.
of chemical-exchange (CE) cross-peaks in the NOESY
spectra at all temperatures above 0 °C. They arise from
chemical exchange between the “inner-up” and “outer-down”
and “outer-up” and “inner-down” methylene protons and
characterize ring inversion in (OETPP)FeCl, which is
described in detail in the accompanying article.22
(F20OETPP)FeCl was characterized in the temperature
range from -10 to +100 °C in CD2Cl2 and C2D2Cl4. Peak
assignments and T1 values for two temperatures are presented
in Table S3 (Supporting Information), and the chemical shift
at +30 °C for the pyrrole-CH2 protons is included in Table
1. The T1 relaxation times decrease with decreasing temper-
ature and, in general, are shorter than those for the (OETPP)-
FeCl complex. The 1D 1H spectrum and the downfield part
of the NOESY spectrum at +5 °C are shown in Figure S2
(Supporting Information). It is interesting to note that the
presence of fluoro substituents on all positions of the phenyl
rings influences the electron distribution around the porphyrin
core, which is manifested in a change of the relative positions
of the methylene protons as compared to those in the
(OETPP)FeCl analogue. Whereas in the NOESY spectra of
the latter, CE cross-peaks were observed between CH2(1)
and CH2(3) as well as between CH2(2) and CH2(4), in the
NOESY spectra of (F20OETPP)FeCl, CH2(1) and CH2(4),
as well as CH2(2) and CH2(3) are in chemical exchange with
each other. These cross-peaks carry information about the
ring inversion in (F20OETPP)FeCl and the slightly larger
cross-peak volumes compared to the peak volume for the
(OETPP)FeCl sample indicate similar but somewhat higher
flexibility of the porphyrin core in the fluorinated analogue
(kex ) 73 vs 16 s-1 at 25 °C).22
Figure 2. 1H NMR spectra of (OETPP)FeCl in CD2Cl2 and C2D2Cl4 at
+30 °C. A wider spread of methylene signals is observed for the complex
in the latter solvent.
chemical shifts are solvent-dependent, especially in the case
of the methylene resonances. The 1H NMR spectra of
(OETPP)FeCl in the two different solvents at +30 °C are
shown in Figure 2. Peak assignments and T1 values are
presented in Table S2 (Supporting Information), and the
chemical shifts of all protons in this complex at +30 °C are
included in Table 1. Proton chemical shifts in CD2Cl2 found
in this study are essentially identical to those reported
previously.24 For (OETPP)FeCl in C2D2Cl4, a wider spread
of the methylene resonances is clearly observed. The rest of
the peaks have only somewhat different chemical shifts as
compared to the same sample in CD2Cl2 (Figure 2). Despite
the difference in chemical shifts of the methylene protons
in the two solvents, the relaxation times are solvent-
independent (Table S2). Short relaxation times are observed
for the methylene as well as for the phenyl-o protons.
Relaxation times for protons at the same position in both
(OETPP)FeCl and (OMTPP)FeCl complexes are very similar
(Tables S2 and S1, respectively).
(TC6TPP)FeCl. Detailed NMR characterization of the
(TC6TPP)FeCl complex is presented elsewhere,27 but the
temperature-dependent fitting for this complex is included
below.
(OETPP)FeClO4, (OMTPP)FeClO4, and (TC6TPP)-
FeClO4. The chemical shifts for these complexes at +30
°C are included in Table 1. Assignments were made on the
basis of 1D peak intensities and COSY spectra. The chemical
shifts of proton resonances of all three of these complexes
at four temperatures were included in Table 4 of ref 3.
Comparison of phenyl-H shifts to those of (TPP)FeClO4 and
the analogous etioporphyrin complex, (ETIO)FeClO4, re-
ported earlier by Goff and Shimomura28 is included in Table
1.
In (OETPP)FeCl in CD2Cl2, CH2(1) and CH2(4) represent
one geminal pair, and CH2(2) and CH2(3) the other, as was
demonstrated by the two-bond cross-peaks in the COSY
spectra.6 In accord with this fact, we were able to detect two
sets of very weak NOE cross-peaks between the protons from
the same geminal pair in the NOESY spectra at 10, 5, and
0 °C (Figure S1, Supporting Information); these NOEs were
not observed previously.6,26 NOE cross-peaks are stronger
at lower temperatures and with longer mixing times. The
pattern of NOE cross-peaks is the same as the COSY pattern.
In addition to the NOEs, there are two relatively strong sets
In the EPR spectra (X-band, 4.2 K, CD2Cl2), 6.14, 4.0,
and 1.99 signals are observed for (OMTPP)FeClO4, and 6.33,
5.3, and 1.99 signals for (OETPP)FeClO4. These values
suggest that the complexes are spin-admixed species, with
5
53.5% and 90.8% S ) /2 character, respectively.
B. Six-Coordinate LS Fe(III) (dxy)2(dxz,dyz)3 Ground-
State Complexes. [FeOMTPP(4-Me2NPy)2]Cl. Well-
resolved and reasonably sharp 1D 1H spectra of [FeOMTPP-
(4-Me2NPy)2]Cl can be obtained below -25 °C. The number
(27) Yatsunyk, L. A.; Walker, F. A. J. Porphyrins Phthalocyanines 2005,
in press.
(26) Ogura, H. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2000.
(28) Goff, H. M.; Shimomura, E. J. Am. Chem. Soc. 1980, 102, 31-37.
2852 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
Table 2. Chemical Shifts for Selected Resonances Together with EPR gmax Values for Six-Coordinate OMTPP, OETPP, and TC6TPP Fe(III)
Complexes with the (dxy)2(dxz,dyz)3 Ground State at -50 °C
EPR g
value(s)
a
complex
δpyrr
δp
δm
δo
δm - δp
δm - δo
[FeF20OETPP(4-Me2NPy)2]+
[FeOETPP(4-Me2NPy)2]+
[FeOMTPP(4-Me2NPy)2]+
[FeTC6TPP(4-Me2NPy)2]+
[FeOMTPP(1-MeIm)2]+
13.69
12.50
20.13
27.40
22.15
(-87.14)b
5.96
5.92
6.31
5.97
(-97.66)b
4.89
5.58
6.53
5.18
(-80.05)b
2.89
3.19
4.11
3.23
(-10.52)b
-1.07
(-17.61)b
+2.00
-
3.28
-0.34
+2.39
3.29
3.12
+0.22
-0.75
+2.42
+1.95
∼3.12, 2.83,
2.32, 1.59
3.49, 2.46,
2.27, ∼1.8
-
[FeOETPP(CN)2]-
6.95
NOd
5.77
5.03
2.59
-0.74
+2.44
[FeF20OETPP(t-BuNC)2]+ c
(-82.84)b
(-91.67)b
(-54.33)b
(-7.83)b
(-37.34)b
a δpyrr indicates methyl (for OMTPP) or methylene [CH2(out) for OETPP and F20OETPP and CH2(R) for TC6TPP] groups directly attached to pyrrole
â-C’s. b Phenyl-F chemical shifts were referenced to CF3C6H5 (-63.73 ppm relative to CCl3F). c Data for 23 °C. d Not observed (very broad).
of peaks (only one methyl peak, as well as one peak each
for phenyl-o and -m) is consistent with D2d symmetry of the
complex in solution. Because of this higher symmetry (all
eight methyl groups are chemically and magnetically equiva-
lent), the kinetics of ring inversion could not be investigated
for this or any of the other six-coordinate complexes of
1
OMTPPFeIII.22 The 1D H spectrum of the [FeOMTPP(4-
Me2NPy)2]Cl complex in CD2Cl2 at -60 °C is shown in
Figure S3 (Supporting Information). The methyl peak is
strongly downfield-shifted (20.6 ppm, -60 °C), indicating
high spin density on the pyrrole â-C’s, which is consistent
with the (dxy)2(dxz,dyz)3 electronic ground state of LS iron-
(III). Full peak assignment of the proton resonances in
[FeOMTPP(4-Me2NPy)2]Cl is summarized in Table S4
(Supporting Information), together with the relaxation times,
T1. Phenyl and ligand protons were assigned on the basis of
the DQF-COSY spectrum shown in Figure S4 (Supporting
Information). Chemical shifts, at -50 °C, of all protons for
this and the other six-coordinate complexes of this study, as
well as the g values observed in frozen solution, are
summarized in Table 2.
In the NOESY experiment (T ) -50 °C, τm ) 4 ms;
Figure S5, Supporting Information) the following chemical-
exchange (CE) cross-peaks are observed between free (F)
and ligated (L) 4-Me2NPy: L-F 3,5-H, L-F 2,6-H, and
L-F CH3; these allow unambiguous assignment of all ligand
resonances. A short mixing time in the NOESY experiment
was necessary to observe the L-F 2,6-H cross-peaks.
Because of the fast relaxation of the L 2,6-H resonances,
the diagonal peak is not observed, but the cross-peaks (L-F
2,6-H) are clearly seen (Figure S5). At -80 °C ligand
exchange becomes too slow to be detected by NMR
spectroscopy (no exchange cross-peaks between bound and
free 4-Me2NPy protons). For the analogous [FeOETPP(4-
Me2NPy)2]Cl complex, this temperature was -60 °C,6
indicating its larger binding constant for the axial ligands.
NOE cross-peaks in NOESY experiments are seen for phenyl
resonances (p-m and m-o), free 4-Me2NPy (F 3,5-H-F
CH3, F 2,6-H-F 3,5-H), and the axial ligand spin system
(L 3,5-H-L CH3). There are some very interesting NOEs
between porphyrin methyl and phenyl-m, as well as por-
phyrin methyl and phenyl-o, seen at all temperatures. The
former set of cross-peaks is more intense, whereas the latter
is almost at the noise level. This indicates that the distance
between the porphyrin methyl and phenyl-m is not more than
Figure 3. 1H NMR spectra of [FeF20OETPP(4-Me2NPy)2]Cl in CD2Cl2
recorded at -10 and -70 °C.
5 Å.29 The NOE crossover point (where the NOE cross-peak
sign changes from negative to positive) occurs at -60 °C.
At -70 °C, all NOE cross-peaks for the porphyrin resonances
are positive, but the NOE cross-peaks for free 4-Me2NPy
(F 3,5-H-F CH3, F 2,6-H-F 3,5-H) are still negative. This
is attributed to the difference in the rotational correlation
time (τc) for the small (ligand) and intermediate-sized
(porphyrin) molecules. In general, for small molecules, the
NOE (enhancement) is positive (NOESY cross-peaks are
negative), and for large molecules in any solvent or
intermediate-sized molecules such as the porphyrins of this
study in viscous solvents (or at low temperatures), the NOE
is negative (NOESY cross-peaks are positive).
[FeOETPP(4-Me2NPy)2]Cl. 1D and 2D NMR data and
assignments for this complex in CD2Cl2 have been reported
previously.6 The chemical shifts of the CH2 and phenyl
protons of this complex at -50 °C and the EPR g values
are included in Table 2.
[FeF20OETPP(4-Me2NPy)2]Cl. 1D and 2D NMR data for
this complex resemble those for [FeOETPP(4-Me2NPy)2]Cl6
and are, in general, similar to the results for bis-(4-Me2NPy)
complexes of OMTPP and TC6TPP (discussed above and
below, respectively). Example 1D spectra at -10 and -70
°C are shown in Figure 3, peak assignments are presented
in Table S5 (Supporting Information) together with T1 values
(29) Abraham, R. J.; Fisher, J.; Loftus, P. Introduction to NMR Spectros-
copy; Wiley and Sons: Chichester, U.K., 1988.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2853

Yatsunyk et al.
at -40 and -70 °C, and the CH2 and phenyl-F shifts at -50
°C are included in Table 2. NMR experiments were
performed in two different solvents, CD2Cl2 (from +30 to
-80 °C) and C2D2Cl4 (from +40 to +80 °C). The 2,6-H
resonance of the ligated 4-Me2NPy (expected in the upfield
region between -2 and -4 ppm) was not observed even
with a spectral window extending to -12 ppm.
There are two peaks due to the diastereotopic methylene
protons below -10 °C, suggesting relatively slow kinetics
of ring inversion. Above ambient temperature, the methylene
peaks become so broad that they disappear from the spectra,
and close-to-linear dependence of the chemical shifts in the
Curie plot is observed only for the bound-ligand CH3. T1
values decrease linearly for all of the protons in the [FeF20-
OETPP(4-Me2NPy)2]Cl complex and increase for the free-
ligand protons as the temperature is lowered. Chemical shifts
of the protons of [FeF20OETPP(4-Me2NPy)2]Cl in two
different solvents, CD2Cl2 and C2D2Cl4, follow the same
temperature dependence, in contrast to the solvation effect
observed for (OETPP)FeCl. This suggests that the solvation
effect observed for the five-coordinate complex involves
interaction of solvent with the porphyrin ring mainly in the
vicinity of the open coordination site.
previous studies have indicated that the effects of ortho
substituents on the phenyl rings are not straightforward, i.e.,
that ortho substituents of whatever type are electron-
donating;32,33 to our knowledge, no careful study of the
substituent effect of perfluorophenyl groups in metallopor-
phyrin axial ligand complex formation has yet been reported.
Thus, the larger binding constant for 4-Me2NPy to (F20-
OETPP)FeIII than to the other octaalkyltetraphenylporphy-
rinatoiron(III) compounds of this study might result from a
combination of effects, which also must include the relative
flexibility of the porphyrin and the out-of-plane distortion
of the pyrrole rings; no structures of (F20OETPP)FeIII have
yet been reported to allow the latter factor to be evaluated.
The only set of CE cross-peaks [between the CH2(out)
and CH2(in)] observed in the NOESY spectra is due to ring
inversion. These CE cross-peaks are detected even at -80
°C, suggesting relatively fast kinetics of ring inversion, which
becomes undetectable by NMR methods around -90 °C.22
In the analogous OETPP complex, the macrocycle is much
less flexible, and CE cross-peaks between CH2(out) and CH2-
1
(in) are not observed below -50 °C. Judging from the H
NMR results, the [FeF20OETPP(4-Me2NPy)2]Cl complex has
a fairly flexible porphyrin core combined with stable complex
formation with the 4-Me2NPy axial ligands.
NOESY spectra were acquired in the temperature range
from -20 to -80 °C with 10-70 ms mixing times. There
are no chemical-exchange cross-peaks due to ligand exchange
at any of these temperatures. This fact, coupled with the
observation of sharp free pyridine peaks even at room
Only a few NOE cross-peaks are present in the NOESY
spectra. Those observed are between CH2(out) and porphyrin
CH3, F 2,6-H and F 3,5-H, and L CH3 and L 3,5-H. The
NOE crossover point is around -50 °C, which is consistent
with the data for similar complexes and solvents.
1
temperature in the 1D H spectra, suggest slow kinetics of
ligand exchange and thus high stability of the bis-(4-Me2-
NPy) complex of (F20OETPP)FeIII, at least at any temperature
below -20 °C. In fact, among all of the complexes of this
study, the largest equilibrium constants for ligand binding
were observed for this complex. For other bis-(4-Me2NPy)
complexes, ligand exchange was detected down to much
lower temperatures, namely, -80, -60, and -60 °C for
[FeTC6TPP(4-Me2NPy)2]Cl. The 1H 1D spectra of [FeTC6-
TPP(4-Me2NPy)2]Cl at -20 and -93 °C are shown in Figure
4. One sharp peak is observed for the porphyrin CH2(R)
protons that broadens upon temperature decrease but never
becomes resolved into two resonances over the accessible
temperature range, suggesting a high rate of ring inversion
even at low temperatures and a saddled shape of the
porphyrin core with perpendicular arrangement of axial
ligands over the porphyrin nitrogens. Complete peak assign-
ments and T1 values are presented in Table S6 (Supporting
Information). The relative positions of the proton resonances
in bis-(4-Me2NPy) complexes of iron(III) OMTPP and TC6-
TPP are very similar, with the only difference being the order
of phenyl-H: δp > δm for OMTPP and δm > δp for TC6-
TPP. Longitudinal relaxation times in [FeTC6TPP(4-Me2-
NPy)2]Cl are fairly long for all peaks except L 2,6-H (Table
S6). The T1 values for the free pyridine ligand protons (F
2,6-H and F 3,5-H) and porphyrin CH2(â) increase substan-
tially as the temperature is lowered, but the T1 values of the
protons of the bound ligand, porphyrin phenyls, and CH2-
(R) decrease with decreasing temperature. The T1 of free
4-Me2NPy methyl protons increases rapidly upon cooling
from ambient temperature to -60 °C because of the slowing
1
OMTPP, OETPP and TC6TPP, respectively. When 1D H
spectra were recorded at elevated temperature (from +40 to
+80 °C in C2D2Cl4), free-ligand peaks remained relatively
sharp and began to broaden only at +70 °C, confirming that
ligand exchange is very slow for this complex. The larger
binding constant for 4-Me2NPy to (F20OETPP)FeIII than to
the other octaalkyltetraphenylporphyrinatoiron(III) complexes
might be thought to be due to the electron-withdrawing
properties of the perfluorophenyl substituents, which might
cause higher acidity of the central iron atom and, as a result,
strong interaction with basic ligands. However, as was shown
previously for meta- and para-substituted phenyl complexes
of (TPP)FeCl, the binding constants for replacement of the
coordinated chloride ion by two neutraly-charged Lewis
bases in noncoordinating solvents such as CHCl3 and CH2-
Cl2 are increased by electron-donating substituents, to
stabilize the formal positive charge that is created on
Fe(III) upon loss of the chloride ligand.30,31 Furthermore,
(32) Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.;
Walker, F. A. Inorg. Chem. 1996, 35, 5188-5200.
(33) Koerner, R.; Wright, J. L.; Nesset, M. J. M.; Ding, X. D.; Aubrecht,
K.; Watson, R.; Barber, R. A.; Tipton, A. R.; Norvell, C. J.; Mink, L.
M.; Simonis, U.; Walker, F. A. Inorg. Chem. 1998, 37, 733-745.
(30) Walker, F. A.; Lo, M. W.; Ree, M. T. J. Am. Chem. Soc. 1976, 98,
5552-5560.
(31) Balke, V. L.; Walker, F. A.; West, J. T. J. Am. Chem. Soc. 1985, 107,
1226-1233.
2854 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
Figure 5. X-band EPR spectrum at 4.2 K of (a) [FeTC6TPP(4-Me2NPy)2]-
Cl in frozen CD2Cl2 and (b) Na[FeOETPP(CN)2] in frozen DMF-d7. Typical
large-gmax signals with g ) 3.12 and 3.49, respectively, are observed,
indicating LS (dxy)2(dxz,dyz)3 ground states. The signal at g ) 4.3 is due to
non-heme Fe3+. Additional signals with g ) 2.46, 2.27, and ∼1.8 are due
to some species with (dxz,dyz)4(dxy)1 ground states.
[FeOMTPP(1-MeIm)2]Cl is D2d, despite the unsymmetrical
nature of the 1-methylimidazole ligand, indicating that the
metal and porphyrin do not sense the ligand asymmetry. As
a result, all eight methyl groups are represented by one
resonance in the NMR spectra; [FeOETPP(1-MeIm)2]Cl has
D2d symmetry as well.6 Large positive shifts are observed
for the methyl resonance (22.45 ppm at -60 °C), that move
further downfield as the temperature is lowered (Figure 6).
The methyl protons in [FeOMTPP(4-Me2NPy)2]Cl exhibit
very similar behavior (Table S4). In addition, the relative
positions of the phenyl (δp > δm > δo) and bound-ligand
resonances for the two complexes are very similar as well
(Tables S4 and S7). These data, along with the EPR results
(both complexes have a large-gmax EPR signal at 4.2 K in
CD2Cl2, with gmax ) 3.20 and 3.12 for [FeOMTPP(4-Me2-
NPy)2]Cl and [FeOMTPP(1-MeIm)2]Cl, respectively1), are
consistent with the (dxy)2(dxz, dyz)3 ground state, for which
the spin density is concentrated at the pyrrole â-carbon
positions. In addition, the latter complex also has a quite
strong rhombic EPR signal in the frozen solution sample,1
with g values of 2.83, 2.32, and 1.59. This is in agreement
with the fact that molecular structures having both “parallel”
(19.5° dihedral angle) and perpendicular (90°) axial ligand
plane orientations have been obtained for crystals of [FeOMT-
PP(1-MeIm)2]Cl grown from different solvent systems.1
Figure 4. 1H NMR spectra of [FeTC6TPP(4-Me2NPy)2]Cl in CD2Cl2
recorded at -20 and -93 °C, together with peak assignments.
of chemical exchange and then decreases linearly with 1/T
as the temperature is lowered further because of the increase
in solvent viscosity.
The NOESY/EXSY spectrum (T ) -20 °C, τm ) 60 ms;
Figure S6, Supporting Information) shows three pairs of
significant chemical-exchange cross-peaks, namely, F-L
CH3, F-L 3,5-H, and F-L 2,6-H, making the axial ligand
peak assignment straightforward. The NOESY spectrum also
shows NOE cross-peaks between CH2(R) and CH2(â) and
between CH2(R) and phenyl-o protons. The phenyl protons
were assigned by the cross-peaks in the DQF-COSY
spectrum (Figure S7, Supporting Information) and the
J-coupling pattern observed in the 1D spectra.
According to NOESY and ROESY data that were acquired
in the temperature range from -20 to -90 °C at 10 °C
intervals and with mixing times of 40-80 ms,34 axial ligand
exchange becomes undetectable on the chemical shift time
scale below -60 °C. Macrocycle ring inversion is still very
fast even at -90 °C. The NOE crossover point occurs at
-50 °C, as for all other bis-ligated octaalkyltetraphenyl
porphyrins, because of the similarity in molecule size and
solvent. Other NOE cross-peaks include phenyl-o-phenyl-
m; L CH3 - 3,5-H; F CH3 - 3,5-H; F 2,6-H - 3,5-H (NOE
cross-peaks were observed only below -60 °C).
NOESY and ROESY experiments were performed in the
temperature range from -60 to -90 °C with 15-50 ms
mixing times depending on the temperature. At lower
temperatures, shorter mixing times were used, as the T1
values of all porphyrin and bound-ligand protons decrease
sharply with decreasing temperature (Table S7). Only for
the free-ligand protons do the T1 values increase exponen-
tially with decreasing temperature, as a result of the decrease
in the rate of chemical exchange with the bound ligand.
Chemical exchange between free and bound 1-MeIm is
reflected in the NOESY and ROESY spectra by the presence
of following CE cross-peaks: L-F CH3, L 5H-F 4,5-H,
and L-F 2-H. Unfortunately, no CE cross-peaks due to the
ring inversion could be observed in NOESY or ROESY
spectra because of the chemical and magnetic equivalence
In the EPR spectrum of [FeTC6TPP(4-Me2NPy)2]Cl (Fig-
ure 5 top), a “large-gmax” signal with g ) 3.12 indicates a
low-spin Fe(III) complex with a (dxy)2(dxz,dyz)3 ground state
and perpendicular arrangement of axial pyridine ligands.
1
[FeOMTPP(1-MeIm)2]Cl. The H 1D NMR spectra for
[FeOMTPP(1-MeIm)2]Cl in CD2Cl2 at -60 and -90 °C are
shown in Figure 6. Complete peak assignments as well as
T1 values at two different temperatures are presented in Table
S7 (Supporting Information). The apparent symmetry of
(34) Mixing times given are for NOESY experiments; τm in ROESY
experiments was usually set to one-half of the mixing time used for
the NOESY spectrum for the same temperature.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2855

Yatsunyk et al.
Figure 6. 1H NMR spectra for [FeOMTPP(1-MeIm)2]Cl in CD2Cl2 at -60 and -90 °C, together with peak assignments.
of all eight methyl groups in [FeOMTPP(1-MeIm)2]Cl. The
following NOE cross-peaks were observed in the NOESY
and ROESY experiments: o-m, m-p, and o-porphyrin
CH3. At very low temperature (-90 °C), there is a very weak
NOE between phenyl-m and porphyrin CH3, indicating that
these two protons spend some time within 5 Å or less of
each other. The NOE crossover point is at -60 °C, which
correlates nicely with the temperatures found for other
systems in this study.
The L 5-H and 4-H resonances were the most difficult to
assign. One is relatively sharp and has a CE cross-peak to
free-ligand F 4,5-H; the second is very broad with short T1
(on the order of 1 ms) and shows no cross-peaks in the 2D
spectra. By relaxation properties only, the first peak was
assigned to L 5-H (farther from the paramagnetic center),
and the second to L 4-H (close proximity to the paramagnetic
center). L 2-H, which is approximately as far from the
paramagnetic center as L 4-H, has a similar peak shape and
T1 value, but a very different chemical shift because of the
difference in π-orbital coefficients for the two corresponding
carbons. A similar situation is observed for other 1-MeIm
complexes.6,12,35 In fact, the behavior and characteristics
(position, shape, and T1) of the 1-MeIm 2-H proton is very
similar to those of L 2,6-H of 4-Me2NPy coordinated to
OMTPPFeIII, TC6TPPFeIII (this work), or OETPPFeIII.6
Ligand exchange in [FeOMTPP(1-MeIm)2]Cl is relatively
fast and can still be detected by the presence of CE cross-
peaks in 2D NOESY and ROESY spectra at -90 °C. On
the other hand, ligand exchange in the analogous bis-(4-Me2-
NPy) complex is already too slow to be detected by NOESY
at -80 °C (in the presence of approximately the same
concentration of axial ligand). This difference is largely due
to the larger binding constant and higher basicity of 4-Me2-
NPy [pKa(PyH+) ) 9.7] as compared to 1-MeIm [pKa-
(1-MeImH+) ) 7.3336].
1
Na[FeOETPP(CN)2]. H NMR spectra at +50 and -30
°C are shown in Figure S8 (Supporting Information), together
with peak assignments obtained from integration, J-coupling
patterns, and COSY spectra, an example of which is shown
in the lower part of Figure S8. Because of the much longer
T1 values, J couplings of phenyl and methylene resonances
are resolved at nearly all temperatures used (from +80 to
-57 °C). The presence of one methylene peak in the
spectrum of Na[FeOETPP(CN)2] that broadens upon tem-
perature decrease suggests a fast rate of ring inversion in
this complex. This is the only example of an (OETPP)FeIII
complex studied that displays fast ring inversion (single
methylene peak) at all accessible temperatures.
Nakamura et al.19 have investigated 1H and 13C NMR and
EPR properties of the same complex but with a different
(35) Isaac, M. F.; Lin, Q.; Simonis, U.; Suffian, D. J.; Wilson, D. L.;
(36) Albert, A. In Physical Methods in Heterocyclic Chemistry; Katritzky,
A. R., Ed.; Academic Press: New York, 1971; Vol. I, pp 1-108.
Walker, F. A. Inorg. Chem. 1993, 32, 4030-4041.
2856 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
counterion, Bu4N[FeOETPP(CN)2] in CD2Cl2; for compari-
son, H NMR chemical shifts for Na[FeOETPP(CN)2] in
saddled octaalkyltetraphenylporphyrin (OATPP) macrocy-
cles, which have methyl or methylene substituents at the eight
â-pyrrole positions and phenyl groups at the four meso
positions, are not as straightforward to determine as are those
of the corresponding simple tetraphenylporphyrins. Unlike
the TPPFeIII complexes,10-13 no matter what the electron
configuration of the OATPPFeIII complex is, the â-CH3 or
-CH2 resonances invariably have positive chemical shifts,
and the descriptors “more positive” or “less positive” are
not hard and fast numbers that can unambiguously differenti-
ate an S ) 5/2 from an S ) 3/2 or an S ) 3/2 from an S ) 1/2
ground-state system, or even differentiate clearly between
the two possible S ) 1/2 ground states. Furthermore, because
of the possible existence of a thermally accessible excited
state that might be significantly populated at the temperatures
of the NMR investigations, a careful study of the temperature
dependence of the 1H chemical shifts of these complexes is
required to confirm the ground-state electron configuration
and to point to possible excited states that might contribute
to the observed shifts. In addition, because the rotation of
ethyl groups attached to the â-pyrrole positions of the
porphyrin ring is known to be hindered at low tempera-
tures,15,35 at the beginning of this study, we assumed that
only the phenyl-H resonances of the OETPP complexes could
be used to accurately assign the ground- and excited-state
spin and to determine the energy separation between them
using eq 4. However, the temperature dependence of phenyl
resonances is usually so small that the energy separation
determined from the fit is not well-defined. Thus, we
experimented with using the methyl signal(s) of OMTPPFeIII
complexes, whose temperature dependence is much stronger
than that of the phenyl-H, to see how well the energy
separations were reproduced with and without inclusion of
the methyl shift(s), and we eventually found that even
OETPPFeIII, F20OETPPFeIII, and TC6TPPFeIII complexes also
yielded what appear to be reliable results when the R-CH2
resonances were included in the fit, as described below.
An important criterion for determining the nature of the
excited state (and in some cases, the ground state as well)
was the sign and magnitude of the Curie factors C1 and C2
of eq 4 obtained for the â-pyrrole substituent(s), either CH3
as in the case of OMTPPFeIII or R-CH2 as in the case of the
other three porphyrins of this study. These Curie factors can
be converted to approximate spin densities (approximate
because no attempt has been made to separate the relatively
small pseudocontact contribution from the relatively large
contact contribution to the paramagnetic shifts) using the
form of the McConnell equation41 appropriate for the current
studies
1
DMF-d7 and Bu4N[FeOETPP(CN)2] in CD2Cl2 are presented
in Table S8 (Supporting Information). In general, chemical
shifts are similar in the two cases; however, the phenyl-m
chemical shift decreases with decreasing temperature for Na-
[FeOETPP(CN)2] (this work) but stays almost at the same
value for Bu4N[FeOETPP(CN)2].19
Among 2D experiments, only the COSY spectrum was
recorded. It showed cross-peaks for the phenyl spin system,
o-m and m-p, and a set of cross-peaks for CH2-CH3
(Figure S8). Neither NOESY nor ROESY experiments were
performed because assignments could be made using 1D 1H
and 2D COSY data only. The bound ligands have no protons,
and therefore, it is impossible to study ligand exchange by
1H NMR spectroscopy.
The EPR spectrum of Na[FeOETPP(CN)2] in DMF-d7 at
4.2 K is shown in Figure 5, lower trace. It contains a large-
g
max signal with g ) 3.49, indicating the (dxy)2(dxz,dyz)3 ground
state of the LS Fe(III) and near degeneracy of the porphyrin
π orbitals (dxz and dyz) due to the axial symmetry of the bound
cyanide ligands. Similar signals with g ) 3.31, 3.48, and
3.70 were observed for Bu4N[FeOETPP(CN)2]19 and Bu4N-
[FeOMTPP(CN)2]19 in CD2Cl2 and for K[FeTPP(CN)2]37 in
DMF-d7, respectively. In our EPR spectrum of Na[FeOETPP-
(CN)2], there are some additional signals with g ) 2.46, 2.27,
and ∼1.8, as well as a small free-radical signal at g ) 2.00.
The g ≈ 1.8 signal might be the second component of the g
) 3.49 large-gmax signal, whereas the g ) 2.46 and 2.27
signals are typical of the (dxz,dyz)4(dxy)1 ground state some-
times shown by bis-cyanide complexes of iron por-
phyrinates.19,38-40 Integration of these and the large-gmax peaks
cannot be used for estimation of the amount of each species
in solution of Na[FeOETPP(CN)2] because of the short
relaxation times of large-gmax signals, which makes them
appear much smaller than the other signals in the spectrum,
as well as the fact that we do not know the third g value and
thus do not know where the spectrum ends. Nevertheless, it
is clear that the species giving rise to the free-radical signal
is due to only a very small amount of radical (<0.1%).
[FeOMTPP(t-BuNC)2]ClO4, [FeOETPP(t-BuNC)2]ClO4,
[FeTC6TPP(t-BuNC)2]ClO4, [FeOMTPP(4-CNPy)2]ClO4,
[FeOETPP(4-CNPy)2]ClO4, and [FeTC6TPP(4-CNPy)2]-
ClO4. The NMR spectra of these complexes have been
reported and assigned, and temperature-dependent fitting of
their chemical shifts has been reported previously,3,4 but
further discussion of the analysis of the temperature depen-
dence is included at the end of the following section.
II. Temperature-Dependent Fitting of the Proton
Chemical Shifts. The ground-state spin and electron con-
figuration of each of the Fe(III) complexes of the highly
C ) KFC
(7)
where FC is the spin density at the carbon attached to the
â-pyrrole carbon in the ground (1) or excited (2) state, as
sensed by the chemical shift(s) of the protons also bound to
that carbon, and the constant K is taken as +591.4 MHz for
methyl and methylene carbons and -496.8 MHz for the
(37) Innis D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.
(38) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(39) Wolowiec, S.; Latos-Graz˘yn´ski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(40) Wolowiec, S.; Latos-Graz˘yn´ski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(41) McConnell, H. M. J. Chem. Phys. 1956, 24, 764-766.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2857

Yatsunyk et al.
phenyl carbons.8,10-13 For methylene carbons, this is defi-
nitely an approximation, for the value of K depends on the
dihedral angle θ between the C-H vector and the pπ orbital
of the â-pyrrole carbon.8,10-13 Calculated spin densities are
overestimated by use of K ) 591.4 MHz for methylene
carbons, but because it is difficult to estimate accurately the
average angle θ of the CH2 protons over the temperatures
of the NMR studies an exact value or even a range of values
cannot be given. In any case, for purposes of the fits to eq
4, this is not a serious problem, as it affects the spin densities
determined by this fitting procedure to an extent of only 10-
20%.
greater than that of the excited state, and thus comments
made about the spin densities of the excited state in the right-
most column of summary Table 3 should not be taken as
seriously as comments about the spin densities of the ground
state.
As is shown clearly in this work, the temperature depend-
ences of the proton chemical shifts of the complexes under
study are usually not linear, as expected by the simple Curie
law (eq 3), but are often non-Curie or even anti-Curie. The
expanded version of the Curie law (eq 4),7 however, applied
using a program developed in our laboratory for this purpose,
TDFw,18 usually allows least-squares fitting of the chemical
shifts, with output that includes the best estimate of the
energy separation E21, as well as the mean-square deviation
of the data points from the best fitting line, and the spin
densities at relevant carbon positions on the macrocycle or
the axial ligands for both the ground and excited state. Part
of the input to the program is the spin states of the ground
and the excited state; the spin state of the ground state (E1)
can usually be defined by the nature of the EPR g values,
and then, if the spin state is not known for the excited state,
various possible spin states for E2 can be probed to see if
some can be ruled out. As mentioned above, the sign and
magnitude of the spin densities obtained from the fitting
procedure offer important guidance in this process. The best
fitting results obtained are summarized in Table 3, and
complete tabulation of all fits attempted is presented in Table
S9 (Supporting Information), where entries highlighted in
red are not acceptable and entries highlighted in blue are
those that are presented in Table 3.
Spin densities should be positive for methyl and methylene
5
3
groups13 and should be larger for S ) /2 than for S ) /2
2
2
states (because of the presence of a σ-symmetry dx -y
unpaired electron, in addition to the two dπ unpaired
electrons);13 larger for S ) 3/2 than for (dxy)2(dxz,dyz)3 S ) 1/2
states (because of the presence of two as compared to one
3
dπ unpaired electrons, at least for most S )
/ iron
2
porphyrinates);13 and considerably larger for the latter than
for S ) /2 states having a (dxz,dyz)4(dxy)1 electron configu-
1
ration (because of the absence of dπ unpaired electrons).13
In fact, the spin densities for the (dxz,dyz)4(dxy)1 electron
configuration states might even be found to be negative,
because there is practically no predicted contact shift at the
â-pyrrole positions for these complexes and the sign of the
magnetic anisotropy of the pseudocontact term is opposite
1
that for the (dxy)2(dxz,dyz)3 S )
/
state systems.12,13 In
2
addition, there seems to be some other small, as yet
unexplained contribution to the â-pyrrole substituent protons
shift that is opposite in sign to that expected for the contact
shift; this has been seen especially in the larger-than-
diamagnetic pyrrole-H shifts of [FeTPP(RNC)2]+ com-
plexes,42,43 but also, in retrospect, for the larger-than-
diamagnetic pyrrole-CH2 shift of [FeOEP(t-BuNC)2]+.44
Perhaps it is caused by spin polarization from the large spin
density at the meso carbons of (dxz,dyz)4(dxy)1 electron
configuration systems, but if so, then it is transmitted from
the meso C to the pyrrole R-C and on to the â-C, with
reversal of sign of the spin density observed or expected at
the meso C. Although the reason(s) for this negative spin
density are not understood at this time, its existence should
not be considered evidence of an unacceptable fit in cases
of the (dxz,dyz)4(dxy)1 ground state, as long as its magnitude
is much smaller than the spin densities observed for the (dxy)2-
(dxz,dyz)3 ground state. However, finding negative spin
densities for methyl or methylene carbons for spin states
A. Five-Coordinate Fe(III) Complexes. (OMTPP)FeCl.
A two-level fit7 to eq 4 of the chemical shift data was used
for the averaged methyl plus five phenyl-H resonances above
coalescence and the two methyl plus five phenyl-H reso-
nances below coalescence of the methyl peaks to analyze
the temperature dependence more fully. From the EPR data
discussed above, we know that this complex has a largely S
) 5/2 ground state, admixed with a small amount of S ) 3/2
character, and we can thus assume with a high degree of
5
confidence that the spin state of E1 can be taken as /2 and
3
that of E2 can be taken as /2. The best fit to all variable-
temperature data is shown in Figure 7, where the fit is
5
consistent with the ground state being largely S ) /2 and
3
the excited state being largely S ) /2, with an energy
separation between them of 215 cm-1 when all data points
are used. If the lowest-temperature data point above methyl
peak coalescence (1000/T ) 3.925) and the highest-temper-
ature data points below coalescence (1000/T ) 4.115) are
deleted for the methyl groups, because chemical exchange
due to porphyrin ring inversion22 clearly affects their
chemical shifts, the energy separation obtained from the fit
is 205 cm-1. The same energy separation is obtained if only
the data below coalescence are used, but without the two
data points at 1000/T ) 4.115. If only the phenyl-H data,
which show a much smaller temperature dependence than
the methyl resonances, are used, the best fit for the energy
separation is 204 cm-1. All of these values are nearly
indistinguishable and certainly within the experimental error
1
other than the S ) /2 (dxz,dyz)4(dxy)1 configuration is an
indication that the ground (or, more usually, excited) state
has been incorrectly chosen. It should also be mentioned that
the reliability of the spin densities of the ground state is much
(42) Simonneaux, G.; Hindre, F.; Le Plouzennec, M. Inorg. Chem. 1989,
28, 823-825.
(43) Simonneaux, G.; Schu¨nemann, V.; Morice, C.; Carel, L.; Toupet, L.;
Winkler, H.; Trautwein, A. X.; Walker, F. A. J. Am. Chem. Soc. 2000,
122, 4366-4377
(44) Walker, F. A.; Nasri, H.; Turowska-Tyrk, I.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109-12118.
2858 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
1
Table 3. Summary of Two-Level Fitting Results for H NMR Chemical Shifts of Octaalkyltetraphenylporphyrinatoiron(III) Complexesa
E1
E2
E21,b
peaks used,
system
(GS) (ES) cm-1
MSDc
0.030
1000/T range usedd
â-substituent spin densities,e comments
5
5
3
3
(OMTPP)FeCl, CD2Cl2
/
/
199
133
all Ph-H, 3 CH3 w/o 2 pts
1: 0.0285, 0.0239, avg 0.0262, a.c. 0.0268;
2
2
2
2
(lowest-T a.c., highest-T b.c.) F reasonable for S ) 5/2
2: 0.0119, 0.0215, avg 0.0167, a.c. 0.0108;
F reasonable for S ) 3/2
(OETPP)FeCl, CD2Cl2
/
/
0.090
4 Ph-H, 4 CH2
1: 0.0249, 0.0148, 0.0147, 0.0022, avg 0.0142;
F small for S ) 5/2
2: 0.0073, 0.0331, 0.0256, 0.0500, avg 0.0290;
F large for S ) 3/2
5
3
5
5
(F20OETPP)FeCl
(TC6TPP)FeCl
/
/
/
/
NA
260
0.358
0.072
4CH2, all Ph-F
all Ph-H, R-CH2,
2 â-CH2
one-level fit
2
2
2
2
1: 0.0359, 0.0323, avg 0.0341; F large for S ) 3/2
2: 0.0131, 0.0164, avg 0.0148 ; F small for S ) 5/2
1: 0.0145, 0.0281, avg 0.0213 ; F OK for S ) 3/2
2: 0.0419, 0.0019, avg 0.0219 ; F ∼small for S ) 5/2
1: 0.0313 ; F large for S ) 3/2
3
3
1
1
5
5
3
3
(OETPP)FeClO4
/
/
/
/
/
/
/
/
785
741
445
638
0.028
0.110
0.010
0.039
R-CH2 + Ph-H
2
2
2
2
2
2
2
2
(OMTPP)FeClO4
CH3 a.c., Ph-H
CH3, all Ph-H
2: 0.0044 ; F very small for S ) 5/2
1: 0.0054 ; F reasonable for S ) 1/2 dπ
2: 0.0120 ; F reasonable for S ) 3/2
1: 0.0031, 0.0009, avg 0.0020 ; F reasonable
for S ) 1/2 dπ
[FeOMTPP(4NMe2Py)2]Cl
[FeOETPP(4NMe2Py)2]Cl
all Ph-H, R-CH2 only,
1000/T > 4
2: 0.0129, -0.0010, avg 0.0060 ; F small for S ) 3/2
1: 0.0035, -0.0009, avg 0.0013 ; F small for
S ) 1/2 dπ
1
3
[FeF20OETPP(4NMe2Py)2]Cl
/
/
423
0.016
CH2,CH3, Ph-F,
1000/T > 3.5
2
2
2: 0.0069, 0.0034 ; F small for S ) 3/2 ES
1: 0.0079 ; F reasonable for S ) 1/2 dπ
2: 0.0154 ; F reasonable for S ) 3/2
1: 0.0070 ; F reasonable for S ) 1/2 dπ
2: 0.0235 ; F reasonable for S ) 3/2
1: 0.0001 ; F reasonable for S ) 1/2 dxy
2: 0.0065 ; F reasonable for S ) 1/2 dπ
1: 0.0194, 0.0044, avg 0.0119 ; F reasonable for
S ) 3/2
1
1
1
3
3
3
1
5
[FeTC6TPP(4NMe2Py)2]Cl
[FeOMTPP(1-MeIm)2]Cl
Na[FeOETPP(CN)2]
/
/
/
/
/
/
/
/
462
741
169
273
0.012
0.003
0.012
R-,â-CH2, all Ph-H
2
2
2
2
2
2
2
2
CH3 + all Ph-H,
1000/T > 4.2
CH2 + all Ph-H,
1000/T > 3.0
[FeOETPP(4-CNPy)2]ClO4
0.0022 all Ph-H, CH2
2: 0.0214, 0.0113, avg 0.0164 ; F reasonable for
S ) 5/2
1
1
1
1
5
5
3
3
[FeOMTPP(4-CNPy)2]ClO4
[FeTC6TPP(4-CNPy)2]ClO4
[FeOMTPP(t-BuNC)2]ClO4
[FeOETPP(t-BuNC)2]ClO4
/
/
/
/
/
/
/
/
681
935
707
123
0.033
0.005
0.025
0.026
CH3, all Ph-H,
1000/T > 5.0
R-CH2, all Ph-H,
1000/T > 4.9
CH3, all Ph-H,
1000/T > 4.4
all Ph-H, 2 CH2,
1000/T > 4.0
1: -0.0009 ; F reasonable for S ) 1/2 dxy
2: 0.1899 ; F large for S ) 5/2
2
2
2
2
2
2
2
2
1: 0.0006 ; F reasonable for S ) 1/2 dxy
2: 0.1071 ; F large for S ) 5/2
1: -0.0022 ; F reasonable for S ) 1/2 dxy
2: 0.0127 ; F reasonable for S ) 3/2
1: 0.0243, 0.0228, avg 0.0236; F much too large
for S ) 1/2 dxy
2: -0.0047, 0.0005, avg -0.0021; F much too small
or S ) 3/2
1
3
[FeF20OETPP(t-BuNC)2]ClO4
[FeTC6TPP(t-BuNC)2]ClO4
/
/
/
/
271
430
0.176
0.027
CH2, all Ph-F,
1000/T > 4.0
1: 0.0037, 0.0072, avg 0.0055 ; F too large for
S ) 1/2 dxy
2
2
2
2
2:-0.0034, 0.0093, avg 0.0030 ; F much too small for
S ) 3/2
1
1
R,â-CH2, all Ph-H,
1000/T > 4, w/o
lowest-T R-CH2 pt
1: -0.0025 ; F reasonable for S ) 1/2 dxy
2: 0.0060 ; F reasonable for S ) 1/2 dπ
a Calculated using the program TDFw.18 b Energy difference between the ground and excited states. c Mean square deviation, a rough indication of the
quality of the fit. d Peaks used in the fit; a.c. ) above coalescence, b.c. ) below coalescence, referring to CH3 resonances in these two temperature regimes;
1000/T range was often >4.0 because of the possibility of ligand exchange or other processes that yielded an obviously unusual temperature dependence.
e avg ) average of CH2 or CH3 spin densities.
expected ((5-10%) for the fitting, and it is gratifying to
see that the phenyl-only data yield a fit that is similar to
that obtained from all data (excluding the points affected by
chemical exchange due to porphyrin inversion). Thus, this
data set is an example of one that has good accuracy and
for which the spins S of the ground and excited states are
known. With regard to the spin densities obtained for the
ground state, the methyl-carbon spin density determined
above coalescence of the methyl resonances, 0.0279, is very
similar to the average spin density of the two separate methyl
carbons below coalescence, 0.0261. However, the same does
not hold true for the excited state, where the spin density of
the methyl carbon above coalescence is 0.0020, whereas the
average below coalescence is 0.0170. For the best fit, where
one data point each just above and below coalescence was
deleted, for which the results are presented in Table 3, both
ground-state (0.0268, 0.0262) and excited-state (0.0108,
0.0167) spin densities above and the average below coales-
cence are much more similar, and, not surprisingly, the MSD
is much smaller (0.030 as compared to 0.116) than that for
the complete data set.
(OETPP)FeCl. Again, the EPR spectra discussed above
5
are consistent with a largely S ) /2 ground state admixed
3
with a small amount of S ) /2 character (4-10%).6,25 The
excited state is assumed to be S ) 3/2. A two-level fit of one
o-phenyl-H (the other is buried under other resonances for
more than half the temperature range), two m-H, and one
p-H resonances in CD2Cl2 was consistent with a mainly
Inorganic Chemistry, Vol. 44, No. 8, 2005 2859

Yatsunyk et al.
in (OETPP)FeCl, but in this case, the measurements were
made on 200 or 300 MHz spectrometers.24 This means that
the difference in frequency, ∆ν, of the resonances being
averaged is larger for the latter, and thus, for coalescence to
occur at about the same temperature as for (OEP)FeCl, a
lower barrier to rotation must exist for the OETPP than for
the OEP complex. Although at first glance it might seem
strange that the more congested-looking OETPP complex
would have a lower barrier to ethyl rotation, it is true that
the large deviation of the â-carbons from the plane of the
porphyrin actually moves them away from the phenyl groups
and thus removes much of the steric hindrance to ethyl group
rotation that one might have thought would be present. This
is a particular case of a more general study45 that showed
that out-of-plane deformability of the macrocycle is important
in lowering the activation energy for rotation of peripheral
substituents on a porphyrin. Overall, a straightforward
relationship between the position of the substituent being
rotated (meso or â) and the symmetry of the deformation
mode (ruffling or saddling) required to lower the rotational
barrier was shown: ruffling lowers the barrier for meso-
substituent rotation by moving the meso positions out-of-
plane, whereas saddling (which moves the pyrrole â-positions
out-of-plane) appears to lower the barrier for rotation of
â-substituents.45
Two-level fitting of the four CH2 and two CH3 resonances
to eq 4 yielded an energy separation of E21 ) 232 cm-1,
whereas fitting of the four CH2 resonances alone yielded an
energy separation of 173 cm-1. Fitting of the five fluorine
resonances was consistent only with a one-level fit, i.e.,
simple Curie behavior (eq 3) and no thermally accessible
excited state. Thus, we can say that, at best, the separation
between ground and excited states is small, if there is an
excited state. Plots of all 1H and 19F shifts, fit to the simple
Curie law, are shown in Figure S10A and B (Supporting
Information).
Figure 7. Plot of the fit of the chemical shifts of (OMTPP)FeCl to the
two-level fitting program, eq 4. The value of E21 obtained from this fit is
215 cm-1. The methyl-C spin densities determined for the ground state are
0.0270 above coalescence of the methyl signals and 0.0288 and 0.0234
(average 0.0261) below coalescence; for the excited state, they are 0.0020
above coalescence and 0.0070 and 0.0270 (average 0.0170) below
coalescence. Ideally, the average spin densities above and below coalescence
for the ground state should be the same, as is also true for the excited state;
in this case, although the ground-state spin densities are quite similar, those
for the excited state are not. See text for further discussion.
S ) 5/2 ground state and a mainly S ) 3/2 excited state, with
the latter lying 151 cm-1 higher in energy. When the four
R-CH2 resonances were added to the fitting procedure, the
energy separation between ground and excited states dropped
to 133 cm-1, a 12% smaller value. Although it is likely that
there is some contribution from hindered rotation of the ethyl
groups of (OETPP)FeCl at very low temperatures to the
calculated energy of the excited state, this might be offset
by the small temperature dependence of the phenyl protons.
Thus we can assume that the true energy separation is
bracketed by the values 133 and 151 cm-1, both of which
are suitably small values that are consistent with the spin-
admixed description reported previously for this complex.24,25
The plot of the fit of all data is shown in Figure S9
(Supporting Information).
For the same complex in C2D2Cl4, for which the NMR
spectra were recorded over the temperature range from +20
to +80 °C, there was considerable scatter in the temperature-
dependence data points and poor convergence was observed
for the fits, and thus no conclusions can be made about the
effect of solvent on the spin state and excited-state energy
separation.
(TC6TPP)FeCl. As for the above-described chloro-
iron(III) complexes, the EPR spectrum of this complex
5
3
indicates a mainly S ) /2 ground state with some S ) /2
admixture.27 The proton chemical shifts show a strong
temperature dependence, that is somewhat nonlinear for all
protons. Two-level temperature-dependent fitting of the five
phenyl-H’s according to eq 4 yielded a value of E21 ) -74
cm-1, indicating that the spin states of the ground and excited
states should be reversed. Fitting of the two R-CH2, two
â-CH2, and five phenyl-H resonances assuming a ground
5
3
state with S ) /2 and an excited state with S ) /2 yielded
an energy separation of 138 cm-1, but the spin density
coefficients for the R-CH2 protons in the excited state were
negative and of magnitude similar to those of the ground
state (ground state 0.0390, 0.0345; excited state -0.0315,
-0.0145), also indicating that the assumed ground spin state
is not correct. If the ground state is instead assumed to have
(F20OETPP)FeCl. A simple Curie plot (eq 3) for all
resonances in (F20OETPP)FeCl in CD2Cl2 shows a linear
dependence of all chemical shifts with inverse temperature;
however, the proton shifts do not extrapolate to the dia-
magnetic positions at infinite temperature (T-1 ) 0), although
the fluorine shifts do. The observed temperature dependence
may be due to the hindered rotation of the ethyl groups. Such
hindered rotation was first used to explain the non-Curie
behavior of the methylene signals in (OEP)FeCl, for which
the methyl and methylene protons coalesce at 65 °C and
around 100 °C, respectively, on a 100 MHz NMR spec-
trometer.15 These temperatures are similar to those observed
3
5
S ) /2 and the excited state S ) /2, the energy separation
between the two is found to be 262 cm-1, and the spin density
(45) Medforth, C. J.; Haddad, R. E.; Muzzi, C. M.; Dooley, N. R.; Jaquinod,
L.; Shyr, D. C.; Nurco, D. J.; Olmstead, M. M.; Smith, K. M.; Ma,
J.-G.; Shelnutt, J. A. Inorg. Chem. 2003, 42, 2227-2241.
2860 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
coefficients calculated for the R-CH2 protons in both the
ground and excited states are found to be positive (ground
state 0.0356, 0.0321; excited state 0.0131, 0.0164), with those
of the ground state being larger than those of the excited
state, as had been expected for the initial supposition, S )
yielded E21 ) 638 cm-1 (MSD ) 0.039). If only the three
phenyl-H resonances were used for the two-level fitting, the
energy separation was found to be 778 cm-1 (MSD ) 0.012),
whereas if only the m- and p-phenyl-H resonances were
utilized for the two-level fit, the energy separation was found
to be 478 cm-1 (MSD ) 0.010). Despite the smaller MSD,
the small temperature dependence of the phenyl-H and the
fact that only two peaks were used for the fit means that
these data are inherently not as reliable as the more complete
set, for which the fit is shown in Figure S15 (Supporting
Information).
[FeF20OETPP(4-Me2NPy)2]Cl. A simple Curie plot for
the R-CH2 resonances shows a linear dependence below -10
°C, and the two-level fits using all 1H data, including ligand-
H, below 1000/T ) 3.5 yielded an E21 value of 354 cm-1
(MSD ) 0.026), shown in Figure S16 (Supporting Informa-
tion). Without the ligand protons, over the same temperature
range, E21 ) 420 cm-1 (MSD ) 0.020). Inclusion of the
temperature dependence of the phenyl-F resonances yielded
E21 ) 423 cm-1 (MSD ) 0.016); the 19F shifts show an
extremely small temperature dependence, which is smaller
than and opposite to that shown by (F20OETPP)FeCl and
[FeF20OETPP(t-BuNC)2]ClO4 (see below, Discussion sec-
tion).
3
5/2 and /2, respectively. A plot of the data points and the
best fit is provided in Figure S11 (Supporting Information).
The order of spin states is not in agreement with the EPR
data, and the spin densities of ground and excited states are
reversed in magnitude.27 The only reasonable explanation
for the NMR measurements’ showing the ground state to
3
have S ) /2 while the EPR measurements indicate that it
5
has S ) /2 is that there is a thermal equilibrium between
3
the two spin states that makes the S ) /2 state lowest in
energy over the temperature range of the NMR experiments,
but we know of no precedence for such a possibility.
(OETPP)FeClO4 and (OMTPP)FeClO4. Using the two-
level fitting program, both complexes adopt the intermediate-
spin ground state, S ) 3/2, with an S ) 5/2 excited state lying
781 and 742 cm-1 higher in energy (Figures S12 and S13,
Supporting Information). The presence of a thermally ac-
cessible excited state causes non-Curie behavior for most
protons.
B. Six-Coordinate LS Fe(III) (dxy)2(dxz,dyz)3 Ground-
State Complexes. [FeOMTPP(4-Me2NPy)2]Cl. At temper-
atures below -25 °C, a simple Curie plot shows a linear
dependence for all resonances except L 2,6-H, with nondia-
magnetic shift intercepts at 1000/T ) 0. The axial ligand
2,6-H protons (L 2,6-H) have very complicated temperature
dependence, with different slopes in the low- and high-
temperature regimes due to the effect of ring inversion and
ligand rotation and possibly also ligand exchange. The
observed temperature dependence of proton resonances in
[FeOMTPP(4-Me2NPy)2]Cl, except for the L 2,6-H, is best
fit with eq 4 assuming a ground state having S ) 1/2 with a
large spin density at the pyrrole-CH3 and a very small spin
density at the meso-phenyl-H, indicative of the (dxy)2(dxz,dyz)3
[FeTC6TPP(4-Me2NPy)2]Cl. All resonances except L
2,6-H show linear behavior in the simple Curie plot, but only
the phenyl protons (meta, ortho, and para) extrapolate to
nearly diamagnetic positions. Large deviations from the
diamagnetic shifts are observed for CH2(R) and L 2,6-H
intercepts. The L 2,6-H resonance, assigned by the presence
of chemical-exchange cross-peaks with F 2,6-H in the
NOESY spectrum, is very broad, which is consistent with
the 1/r6 dependence of dipolar relaxation by the paramagnetic
center.12 Two-level fitting of all phenyl-H and R- and â-CH2
resonances, Figure S17 (Supporting Information), was con-
1
sistent with the ground state being S ) /2 and the excited
3
state being S ) /2 and lying 462 cm-1 higher in energy
3
electron configuration, and an excited state having S ) /2
(MSD ) 0.012). If only the phenyl-H were used, the energy
separation between ground and excited states was found to
be 482 cm-1 (MSD ) 0.006). Including all ligand-H
resonances as well yielded E21 ) 364 cm-1 (MSD ) 0.022).
As in other cases described both above and below, the
ligand-H temperature dependences seem to be somewhat
different from those of the porphyrin macrocycle, suggesting
that additional processes (ligand exchange, ligand rotation,
macrocycle inversion) might influence the temperature
dependence of these resonances, and therefore, in general,
the ligand resonances were not included in the best fits to
eq 4.
[FeOMTPP(1-MeIm)2]Cl. Above -40 °C, ligand ex-
change, and most likely fast ring inversion as well, broadens
the proton signals. The resonances belonging to the porphyrin
CH3, phenyl-o, and axial-ligand methyl (L CH3) and 2H (L
2-H) shift strongly with temperature. All other resonances
show much smaller temperature dependences. Fitting the CH3
and phenyl-H resonances at 1000/T > 4.2 to the expanded
Curie law treatment (eq 4) indicates an S ) 1/2 ground state
with spin density mainly at the pyrrole-CH3 (0.0070) and
with a larger spin density at the pyrrole-CH3 and, again, a
very small spin density at the meso-phenyl-H. Using all
resonances, the two-level fit showed that the excited state
lies 251 cm-1 above the ground state, but a larger-than-
desired MSD of 0.043 was observed. The best fit, shown in
Figure S14 (Supporting Information), was found to be for
all porphyrin protons, excluding the axial ligand protons,
1
which yielded an energy separation between the S ) /2
ground and S ) 3/2 excited states of E21 ) 445 cm-1 (MSD
) 0.010) and spin density coefficients for the methyl carbons
of 0.0054 and 0.0120 for the ground and excited states,
respectively, that are consistent with the expected spin
1
3
densities of the S ) /2 and /2 spin states involved.
[FeOETPP(4-Me2NPy)2]Cl. The temperature dependence
of the proton chemical shifts reported elsewhere6 has not
previously been fit to the two-level expression. Such fits,
using all resonances, for temperatures below -23 °C (1000/T
> 4.0), were consistent with the ground state having S ) 1/2
and the excited state S ) 3/2, with a separation between them
of 741 cm-1 (MSD ) 0.046). Excluding the ligand protons
Inorganic Chemistry, Vol. 44, No. 8, 2005 2861

Yatsunyk et al.
program, but with the insights gained by investigating the
series of complexes of this study, we make a few additional
comments about these systems here. For [FeOETPP(4-
CNPy)2]ClO4, we found that the phenyl-H temperature
dependence obeys the simple Curie law (eq 3), with S )
3/2.3 However, if the CH2 resonances are included in a two-
level fit, with an S ) 3/2 ground state, but including an excited
5
state having S ) /2, we find that this state lies 273 cm-1
higher in energy (MSD ) 0.022). The spin densities for the
ground (0.0194, 0.0044; average 0.0119) and excited (0.0214,
0.0113; average 0.0164) states are consistent with the ground
3
5
and excited states having S ) /2 and /2, respectively. For
the possibility of the ground-state electron configuration
being S ) 2 Fe(II) antiferromagnetically coupled to an S )
1/2 porphyrin radical, as was suggested might be the case in
the original study,3 the spin densities obtained from that fit
are not consistent with this possibility (i.e., they are positive
for both ground and excited states).
Figure 8. Plot of the fit of the chemical shifts of [FeOMTPP(1-MeIm)2]-
Cl to the two-level fitting program, eq 4. The value of E21 obtained from
this fit is 741 cm-1 (MSD ) 0.003). The methyl-C spin density determined
1
for the S )
/ ground state is 0.0070, and that for the excited state
2
(S ) 3/2) is 0.0235. Including all ligand resonances in the fit yielded a value
of E21 ) 577 cm-1 (MSD ) 0.023) and spin densities of 0.0068 and 0.0146
for the ground and excited states, respectively.
For [FeOMTPP(4-CNPy)2]ClO4, we found that the ground
state had S ) 1/2 with a (dxz,dyz)4(dxy)1 electron configuration,
as shown by the EPR spectra, and the excited state, which
was some 650 cm-1 higher in energy (MSD ) 0.033), had
S ) 3/2.3 We now find that there is also an acceptable solution
an S ) 3/2 excited state with more spin density at the pyrrole-
CH3 (0.0235) lying 741 cm-1 to higher energy, Figure 8
(MSD ) 0.003). Using the phenyl-H resonances only, the
energy gap between ground and excited states was calculated
to be 704 cm-1 (MSD ) 0.003), a value fairly similar to
that obtained by including the pyrrole-CH3 resonance.
Na[FeOETPP(CN)2]. The temperature dependence of the
chemical shifts of Na[FeOETPP(CN)2] shows that all peaks,
except those for the porphyrin methylenes, have close-to-
linear inverse temperature dependences; two-level fitting
according to eq 4 of the phenyl-H-only data below 1000/T
5
that has the excited state lying S ) /2, that lies some 680
cm-1 higher in energy than the ground state (MSD ) 0.033).
In both cases, the spin density of the ground state is
calculated to be negative (-0.0012 and -0.0009, respec-
tively), whereas that for the excited state is large and positive
(0.3352 and 0.1899, respectively), larger than expected for
3
5
the S ) /2 and /2 cases, respectively, but most especially
too large for the S ) 3/2 excited state. However, the magnetic
susceptibility data reported earlier3 are more consistent with
1
1
3
) 3.0, assuming an S ) /2 ground state and an S ) /2
excited state, yielded E21 ) 184 cm-1 (MSD ) 0.009).
Including the CH2 resonances in this plot yielded E21 ) 169
cm-1 (MSD ) 0.012) and spin density coefficients for the
ground and excited states (FC1 ) 0.0001, FC2 ) 0.0065) that
are consistent with the complex having a (dxz,dyz)4(dxy)1 S )
the S ) /2 excited state.
For [FeTC6TPP(4-CNPy)2]ClO4, we also found previously
that the ground state had S ) 1/2 with a (dxz,dyz)4(dxy)1 electron
configuration, as shown by the EPR spectra, and the excited
state, which was some 900 cm-1 higher in energy (MSD )
3
0.005), had S ) /2.3 We now find that there is also an
1
1/2 ground state and a (dxz,dyz)3(dxy)2 S ) /2 excited state
5
acceptable solution that has the excited state S ) /2, that
lies some 940 cm-1 higher in energy (MSD ) 0.005), and
as in the case above, the spin density of the ground state is
small, although positive (0.0006), whereas that of the excited
state is large and positive (0.1071), consistent with the S )
5/2 state. Again, the magnetic susceptibility data reported
(Figure S18, Supporting Information). However, this does
not entirely agree with the EPR spectrum, which shows a
large-gmax signal that is indicative of the (dxy)2(dxz,dyz)3 ground
state (Figure 5), although additional features (g ) 2.46, 2.27,
and 1.8) are also observed that are consistent with the
(dxz,dyz)4(dxy)1 electron configuration. Thus, it is possible that
this is another case, like that of [TPPFe(OCH3)(OO-t-Bu)]-,46
where there might be a thermodynamic equilibrium between
the two different S ) 1/2 electron configurations that would
cause the ambient-temperature NMR data to be fit to the
spin density distribution expected for the species that are
thermodynamically stable in this temperature range.
3
earlier3 are more consistent with the S ) /2 excited state.
For [FeOMTPP(t-BuNC)2]ClO4, we find no change from
what we reported previously,4 i.e., that the ground state has
S ) 1/2 with a (dxz,dyz)4(dxy)1 electron configuration, as shown
3
by the EPR spectra, and the excited state has S ) /2 and
lies 707 cm-1 higher in energy. The spin density of the
methyl protons in the ground state is calculated to be negative
(-0.0022), whereas that for the excited state is much larger
and positive (0.0127). In this case, we know that the ground
state has a (dxz,dyz)4(dxy)1 electron configuration and, hence,
the negative spin density is expected because of the small
spin density on the â-carbon and the additional contribution
due possibly to polarization, mentioned above; however, the
magnitude of the negative spin density is larger than might
C. Six-Coordinate LS Fe(III) (dxz,dyz)4(dxy)1 Ground-
State Complexes. The bis-(4-CNPy)3 and bis-(t-BuNC)4
complexes of this series of iron(III) octaalkyltetraphenyl-
porphyrins have previously been fit to eq 4 using the TDF
(46) Rivera, M.; Caignan, G. A.; Astashkin, A. V.; Raitsimring, A. M.;
Shokhireva, T. Kh.; Walker, F. A. J. Am. Chem. Soc. 2002, 124, 6077-
6089.
2862 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
have been expected, in comparison to the 4-CNPy complexes
just discussed.
Discussion
All five-coordinate chloride complexes (OMTPP)FeCl,
(OETPP)FeCl, (F20OETPP)FeCl, and (TC6TPP)FeCl exhibit
very similar NMR and EPR behaviors. The low effective
symmetry of each, C2V, results in magnetically nonequivalent
methylene and methyl groups, as well as phenyl-o and -m
protons [two different peaks are also observed for o- and
m-fluorine resonances in the 19F NMR spectrum of (F20-
OETPP)FeCl].47 All complexes have relatively large positive
shifts for the methylene and methyl groups directly attached
to the pyrrole â-carbons (Table 1). This is indicative of large
spin delocalization to the pyrrole â-positions. In the case of
a pure HS state, all five d orbitals of Fe(III) are half-filled,
creating the possibility for spin delocalization from both dσ
For [FeOETPP(t-BuNC)2]ClO4, we find the same spin
states and ground-state electron configuration, with E21
)
130 cm-1 higher in energy (MSD ) 0.048),4 but rather
strange spin densities on the CH2: for the ground state,
0.0226 and 0.0022 below coalescence and 0.0035 above, and
for the excited state, -0.0043 and +0.0005 below coales-
cence and 0.0022 above. These spin densities are not
consistent with the electron configurations involved. [FeF20-
OETPP(t-BuNC)2]ClO4 shows a similar pattern of inconsis-
tent spin densities. No other combination of spin state choices
for ground and excited levels produces an acceptable result
in terms of spin densities, and we are thus left with no
explanation for the spin densities determined by the fitting
process. In any case, as reported previously, the EPR
spectrum of [FeOETPP(t-BuNC)2]+, measured at 4.2 K, is
indicative of a (dxz,dyz)4(dxy)1 ground-state system.4 It is not
clear why no fits for either of these complexes, with any
combination of ground and excited spin states, yields an
acceptable solution.
(dx -y ) and dπ (dxz, dyz) and dz 48 (see below) Fe(III) orbitals.
Therefore, the observed shifts for methyl and methylene
groups directly attached to the pyrrole â-carbons are the
balance of the contribution of the dσ and dπ unpaired
electrons, both of which cause downfield shifts for the
protons one carbon away from the pyrrole â-positions. The
average chemical shifts for the methylene protons in (OET-
PP)FeCl, (F20OETPP)FeCl, and (OEP)FeCl15 are similar
because of similar patterns of spin delocalization for all three
complexes. Such similarity seems reasonable in the case of
the two former complexes because we can expect similar
values of the average angle of the CH2 protons with respect
to the pπ orbital of the â-carbon for them, but in the case of
(OEP)FeCl, the core geometry (planar) and rate of ethyl
group rotation (probably slower for the planar porphyrin ring)
are so different from those of both OETPPs that the similarity
in methylene shifts might be coincidental. For chloro-
iron(III) complexes of (OMTPP)FeCl and (OETPP)FeCl,
two-level fitting of the temperature dependence of the
â-pyrrole substituent resonances and the phenyl-H yield
ground states having largely S ) 5/2 and excited states having
2
2
2
For [FeTC6TPP(t-BuNC)2]ClO4, the best fit was found to
be (neglecting the lowest-temperature data point for the
R-CH2 resonance) with both ground and excited spin states
having S ) 1/2, but with the two different electron configura-
tions, with E21 ) 430 cm-1 (MSD ) 0.027) and spin densities
for the ground and excited states of -0.0025 and 0.0060,
respectively. This seems to be reasonable for ground- and
excited-state electron configurations of (dxz,dyz)4(dxy)1 and
(dxz,dyz)3(dxy)2, respectively. These are the same ground- and
excited-state electron configurations as found in a detailed
NMR and Mo¨ssbauer spectroscopic study of [FeTTP(2,6-
XylylNC)2]+, a complex that, although shown by its crystal
structure to be very nonplanar, is highly ruffled rather than
saddled.43 Hence, it appears that [FeTC6TPP(t-BuNC)2]+,
predicted to be the least saddled of the octaalkyltetraphe-
nylporphyrinates of this study, has an energy level structure
similar to that of the meso-only-substituted porphyrinate with
similar axial ligands. However, we want to illustrate the
sensitivity of the fitting procedure in this case to a single
data point. When the lowest-temperature data point for the
R-CH2 resonance were included, there was no convergence
3
S ) /2, with energy separations of 200 and 130 cm-1,
respectively. On the other hand, the (TC6TPP)FeCl complex
has ground and excited spin states reversed, with an energy
separation of ∼270 cm-1, and there is no accessible excited
state for the (F20OETPP)FeCl complex in the temperature
range studied.
The meso-phenyl-H shift differences for (OMTPP)FeCl,
(OETPP)FeCl, and (TC6TPP)FeCl, δm - δp and δm - δo,
are relatively large and positive (see Table 1), suggesting
some π spin density at the meso C’s, as expected for dπ spin
delocalization to the porphyrin 4e(π*) orbital (Fe f P π*
back-bonding). However, because the a2u(π) orbital also has
large spin density at the meso positions, these NMR data
1
with the S ) /2,1/2 spin state combination, whereas for the
ground state having S ) 1/2 and excited state having S ) 3/2,
E21 was calculated to be 108 cm-1 (MSD ) 0.036). The spin
densities calculated for this latter case are strange, -0.0084
for the ground state (too large negative) and +0.0006 for
the excited state (much too small positive), which encourages
one to closely scrutinize the fit obtained, which then leads
to the recognition that the lowest-temperature data point for
the R-CH2 resonance is markedly off the line of the best fit
2
cannot exclude spin transfer through a dz -a2u bonding
interaction for these five-coordinate complexes where the
metal is out of the plane of the porphyrin nitrogens, as was
pointed out recently by Cheng and co-workers.48
The perchlorate complexes, (OMTPP)FeClO4 and (OET-
PP)FeClO4, have intermediate-spin ground states and high-
1
3
to this S ) /2, /2 combination and that the best fit should
be sought without including this single data point. Hence,
careful scrutiny of the quality of the temperature-dependence
data is required in all cases.
(47) Yatsunyk, L. A.; Walker, F. A. Inorg. Chim. Acta 2002, 337, 266-
274.
(48) Cheng, R.-J.; Chen, P.-Y.; Lowell, T.; Liu, T.; Noodlemann, L.; Case,
D. A. J. Am. Chem. Soc. 2003, 125, 6774-6783.
Comparison of the results obtained for each of the
compounds investigated is included in the Discussion section.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2863

Yatsunyk et al.
spin excited states, with large separations between ground-
and excited-state energies (∼740 and ∼790 cm-1, respec-
tively).
the phenyl-H shift differences, with δm - δp of similar
magnitude for H and F nuclei (Table 1), but with δm - δo
(though also of opposite sign to the phenyl-H shift differ-
ence), being more than 10 times larger in magnitude (-49.5
ppm average for the two phenyl-F’s). Comparing the
phenyl-F shift differences of the (F20OETPP)FeCl complex
to those of the corresponding bis-4-Me2NPy and bis-t-BuNC
complexes, those of the chloroiron complex are the largest,
those of the bis-t-BuNC complex are somewhat smaller, and
those of the bis-4-Me2NPy complex are the smallest.
Similarly to the chloroiron complex, which has spin delo-
1
Selected H NMR chemical shifts, together with large-
gmax or normal rhombic EPR g values, for four bis-4-
(dimethylamino)pyridine, one bis-1-methylimidazole, and
one bis-cyanide complexes of iron(III) OMTPP, OETPP, F20-
OETPP, and TC6TPP are presented in Table 2 and indicate
the (dxy)2(dxz,dyz)3 ground state for all complexes with the
unpaired electron in one of the dπ orbitals. The large
downfield shifts observed for the methyl or methylene
resonances in these complexes correspond to the large upfield
shift of the pyrrole protons in various [Fe(TPP)(L)2]+
species,13 suggesting that the major spin densities are at the
â-pyrrole carbon atoms. The differences observed in phenyl
proton chemical shifts (Table 2) are small and negative for
δm - δp (except in the case of [FeTC6TPP(4-Me2NPy)2]Cl)
and small and positive for δm - δo in all complexes,
indicating negligible spin density at the porphyrin meso
positions, in agreement with the nodal properties of the
3e(π) orbitals of the porphyrin. Similar patterns of phenyl
shifts are observed for TPPFeIII complexes with axial ligands
that give rise to the (dxy)2(dxz,dyz)3 ground state.13 Two-level
fitting of the temperature dependence of the chemical shifts
for the bis-4-Me2NPy and the bis-1-MeIm complexes shows
that the ground state has S ) 1/2 with a (dxy)2(dxz,dyz)3 electron
2
calization to the meso carbons via interaction of the dz
unpaired electron with the 3a2u(π) orbital,48 the bis-t-BuNC
complex, with the dxy unpaired electron that can also interact
with the 3a2u(π) orbital if the complex is able to ruffle,49
has a large value of δm - δo of -37.34 ppm, whereas δm -
δp is relatively small (-7.83 ppm). In contrast, both phenyl-F
shift differences for [FeF20OETPP(4-Me2NPy)2]+ complex,
with its unpaired electron in a dπ orbital, which interacts with
the 3e(π) porphyrin orbital that has nodes at the meso
positions, are of similar size, intermediate between those of
the δm - δp and δm - δo shift differences of the other two
complexes (-10.52 and -17.61 ppm, respectively). It is also
interesting to note that the Curie plots for the phenyl-F’s of
(F20OETPP)FeCl and [FeF20OETPP(t-BuNC)2]+ have posi-
tive slopes, whereas those of [FeF20OETPP(4-Me2NPy)2]+
have negative slopes; the signs of the slopes follow those
expected for the sign of the pseudocontact shift contribu-
tion,47 whereas the magnitudes of the slopes decrease in the
order (F20OETPP)FeCl > [FeF20OETPP(t-BuNC)2]+ > [FeF20-
OETPP(4-Me2NPy)2]+. We previously estimated the relative
sizes of the pseudocontact contributions to the isotropic shifts
for the fluorines of the three compounds of this study and a
number of others47 and concluded that all of them that are
known to have large spin densities on the meso carbon to
which the phenyl group is attached exhibit larger-than-
expected o-phenyl-F shifts. Although we could not determine
whether these large o-phenyl-F shifts were caused by a
ligand-centered pseudocontact (dipolar) shift arising from
large spin density at the meso carbon or a “through-space
contact shift” arising from direct electron-cloud overlap
between the unpaired electron density at the meso carbon
and the o-phenyl-F, in all cases, the sign of the large
o-phenyl-F isotropic shift was the same as the sign of the
spin density at the meso carbon;47 this includes the tris-
(pentafluorophenyl)corrolatoiron(III) chloride complex, which
has large negative spin densities at the three meso carbons
3
configuration and the excited state has S ) /2 in all cases.
The energy separations between ground and excited states
for the bis-4-Me2NPy complexes roughly parallel the rigidity
of the porphyrin ring, as determined by the kinetics of
porphyrin ring inversion,22 with OETPP > OMTPP, TC6-
TPP, F20OETPP (640 > 450, 460, 420 cm-1, respectively),
suggesting that rapid porphyrin deformation changes might
3
lower the barrier to spin state change to S ) /2. In these
comparisons, we must keep in mind that the error in
measurement of E21 is probably on the order of 10% of the
value obtained, at least in most cases. Comparing the
[FeOMTPP(4-Me2NPy)2]+ and [FeOMTPP(1-MeIm)2]+ com-
3
plexes, the S ) /2 excited state lies at much higher energy
in the latter (740 cm-1) than in the former (450 cm-1),
1
indicating a “purer” S ) /2 spin state for the bis-1-MeIm
complex over the temperature range of the NMR studies.
For the [FeOETPP(CN)2]- complex, where both ground
1
and excited states are found to have S ) /2, but different
electron configurations, the small value of E21 determined
from two-level fitting to eq 4, 170 cm-1, means that, at
ambient temperatures where NMR spectra are measured, a
significant fraction of the complex ions have the (dxz,dyz)3-
(dxy)2 excited-state electron configuration and thus the
phenyl-H shift differences do not appear markedly different
from those observed for the (dxy)2(dxz,dyz)3 ground-state
complexes having 4-Me2NPy or 1-MeIm ligands.
The phenyl-F shifts of (F20OETPP)FeCl, [FeF20OETPP-
(4-Me2NPy)2]+, and [FeF20OETPP(t-BuNC)2]+ have been
reported previously at other temperatures,47 but are included
in Tables 1 (23 °C) and 2 (30 and 23 °C) of this work. The
phenyl-F shift differences, δm - δp and δm - δo, of (F20-
OETPP)FeCl are negative in sign as compared to those of
3
due to antiferromagnetic coupling between S ) /2 Fe(III)
and a corrolate(2-•) radical.50
Detailed discussion of the EPR spectral type for most of
the complexes in this study is presented elsewhere,1,2 together
with structural information and polycrystalline EPR spectra.
Briefly, the large-gmax type of EPR spectra with g ) 3.12-
(49) Safo, M. K.; Walker, F. A.; Raitsimring, A. M.; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760-7770.
(50) Zakharieva, O.; Schu¨nemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.;
Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636-
6648.
2864 Inorganic Chemistry, Vol. 44, No. 8, 2005

Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins
3.49 observed for frozen CD2Cl2 solutions of all six
complexes (Table 2) and the rhombic signal also observed
for the bis-1-methylimidazole complex are direct evidence
for the (dxy)2(dxz,dyz)3 ground state and support the conclusions
obtained from NMR spectra; the second signal observed for
the bis-cyano complex (g ) 2.46, 2.27, and unknown) is
consistent with some molecules having the (dxz,dyz)4(dxy)1
electron configuration. It should be noted that the EPR
spectra determine the electronic ground state of the low-
spin iron(III) complexes at 4.2 K. Thus, at higher temper-
atures, where the NMR spectra are obtained, the electron
configuration might be different, as for the complexes with
spin crossover3 or a thermodynamic equilibrium,46 as appears
to be the case for Na[FeOETPP(CN)2]. However, in general,
most low-spin iron(III) complexes preserve their ground state
over wide temperature ranges, and NMR and EPR data are
in good agreement with each other, as in most cases of the
present study. An acceptable exception for Na[FeOETPP-
(CN)2] is the presence of an equilibrium between the two S
them diverge or converge to high energies.51 As a result,
the functions are highly curved at small 1/T, and the Curie
factors determined are unreasonably large. This is the case
for the 4-CNPy complexes of OMTPPFeIII and TC6TPPFeIII,
and it is not possible to clearly determine what the spin state
of the higher-energy state is or whether additional factors
affect the temperature dependence of the chemical shifts. The
latter is why only very low-temperature data were used in
those fits,3 because of the possibility that ligand dissociation
5
as a means of reaching the S ) /2 state was affecting the
observed shifts.
Conclusions
1
Combined analysis of the H NMR and EPR data has
revealed that most highly nonplanar five-coordinate chlor-
5
oiron(III) porphyrins adopt the high-spin (S ) /2) state
admixed with 0-10% of the IS (S ) 3/2) state. Spin
delocalization to the pyrrole â-positions results in large
downfield shifts of the protons of methyl and methylene
groups directly attached to the pyrrole rings. The pattern of
phenyl shifts indicates some amount of spin delocalization
1
) /2 electron configuration. However, we can find no
acceptable explanation for the bis-tert-butylisocyanide com-
plexes of OETPPFeIII and its perfluorophenyl analogue.
With regard to the fits to the expanded version of the Curie
law, eq 4,7 we have found that two-level fitting can be used
in two ways:
1. Strong. The experimental data are accurate, the
measurement interval is wide, the spin states of ground and
excited levels are known, and convergence is good. In this
case, we can formulate a positive hypothesis about the
electronic structure and consider the numeric values obtained
as reliable data. This is the situation for many of the
complexes of this study.
2. Weak. The experimental data are not good enough and
convergence is poor (or there is no convergence). In this
case, we can still use the TDF procedure if the data derived
from the fit do not contradict the existing hypothesis. We
can state this if the values from the hypothesis allow fitting
within the experimental errors.51 This is the situation for (F20-
OETPP)FeCl and (OMTPP)FeClO4, whereas reasonable fits
are not found for the (OETPP)FeCl data in C2D2Cl4, for
which no convergence of the fit was obtained, and the
[FeOETPP(t-BuNC)2]ClO4 and [FeF20OETPP(t-BuNC)2]-
ClO4 complexes (Table 3), which were found to have
unreasonable spin densities for all combinations of spin states
probed. For [FeTC6TPP(t-BuNC)2]ClO4, we found that the
fits obtained were extremely sensitive to the use of one data
point that deviated from all fitting lines and that, if this data
point were not included, a fit was obtained that is similar to
that found for the bis-isocyanide complexes of a nonsaddled
iron porphyrinate, [FeTTP(2,6-XylylNC)2]+,43 i.e., that the
ground- and excited-state electron configurations are S )
1/2 (dxz,dyz)4(dxy)1 and (dxz,dyz)3(dxy)2, respectively.
2
to the porphyrin meso positions due to Fe(dz ) f porphyrin
3a2u(π) interactions. Thus, except for some admixture of the
IS state, the magnetic resonance characteristics of these
chloroiron complexes do not differ from those of less highly
substituted porphyrinates. The exception to this rule is (TC6-
TPP)FeCl, which has an EPR spectrum similar to those of
the other complexes, but for which fitting the 1H NMR shifts
to a two-level expansion of the Curie law yields a ground
3
5
state of S ) /2 and excited state of S ) /2, although the
spin densities obtained from the fit are too large for the
ground state and too small for the excited state. Both
3
perchlorate complexes have S ) /2 ground states and S )
5/2 excited states.
Six-coordinate octaalkyltetraphenylporphyrinatoiron(III)
complexes with two 4-Me2NPy or 1-MeIm ligands are low-
spin, with the common (dxy)2(dxz,dyx)3 ground state, and
exhibit spin delocalization due to Fe(dπ) f porphyrin 3e(π)
interactions, as seen with the same bis-ligand complexes of
both “parents”, octaethylporphyrinato- and tetraphenylpor-
phyrinatoiron(III).10,12,13 The chemical shift pattern in the 1H
NMR spectra and the spin density coefficients obtained from
the two-level temperature-dependent fitting indicate the
largest spin delocalization in the ground state to the pyrrole
â-positions and little or no spin delocalization to the
porphyrin meso positions; however, whereas the two parent
complex ions, [FeOEP(L)2]+ and [FeTPP(L)2]+, have chemi-
cal shifts that strictly obey the Curie law,10,12 the bis-4-Me2-
NPy and -1-MeIm complexes of octaalkyltetraphenylpor-
phyrinatoiron(III) require two-level fitting of the temperature
dependence of their proton resonances to eq 4, and the
3
excited state in each case is found to have S ) /2.
EPR spectroscopy shows that Na[FeOETPP(CN)2] in
DMF-d7 has a mixture of the two S ) /2 ground states,
We should usually be somewhat suspicious of fits with
excited-state energies significantly larger than 500 cm-1.
Often in such cases, the dependence of the MSD on E21 is
very shallow, and the fits are unstable, which might make
1
whereas NMR spectroscopy shows that, over the temperature
range studied, it has the S ) 1/2 ground state with a (dxz,dyz)4-
1
(dxy)1 electron configuration and an S ) /2 (dxz,dyz)3(dxy)2
(51) See detailed discussion in the Help section of the program TDFw.18
electron configuration for the excited state; the results from
Inorganic Chemistry, Vol. 44, No. 8, 2005 2865

Yatsunyk et al.
the two spectroscopic methods are best rationalized as being
due to a thermodynamic equilibrium that somewhat favors
the (dxy)2(dxz,dyz)3 ground state at 4.2 K and the (dxz,dyz)4-
(dxy)1 ground state at NMR temperatures, with the (dxz,dyz)3-
(dxy)2 excited-state configuration not very high in energy (170
cm-1). The bis-4-CNPy complexes of all iron(III) porphyrins
[FeTTP(2,6-XylylNC)2]+.43 For [FeOETPP(t-BuNC)2]+ and
[FeF20OETPP(t-BuNC)2]+, no solution was found from the
fits that was consistent with any reasonable excited state.
Acknowledgment. We thank the National Institutes of
Health, Grant DK-31038 (F.A.W.), for support of this
research. The authors also thank Dr. C. J. Medforth for
providing the sample of (F20OETPP)FeCl. This article was
written while F.A.W. was a Visiting Professor and Alexander
von Humboldt Senior Awardee in Science in the Physics
Institute of the University of Lu¨beck, and she thanks
Professor Alfred X. Trautwein for his hospitality and
friendship.
1
studied except that of OETPPFeIII have S ) /2 (dxz,dyz)4-
3
5
(dxy)1 ground states and either S ) /2 or /2 for the excited
states; the OETPPFeIII complex has an S ) 3/2 ground state
and either no or an S ) 5/2 excited state at quite high energy.
The bis-t-BuNC adducts of all complexes studied have S )
1/2 (dxz,dyz)4(dxy)1 ground states, but the nature of the excited
state differs considerably among the series: for [FeOMTPP-
3
(t-BuNC)2]+, the excited state has S ) /2 and lies at quite
Supporting Information Available: Figures S1-S18 and
Tables S1-S9 are available free of charge via the Internet at
http://pubs.acs.org.
high energy (∼710 cm-1), whereas for [FeTC6TPP-
(t-BuNC)2]+, the excited state lies at lower energy (430 cm-1)
1
and has the other S ) /2 electron configuration, (dxz,dyz)3-
(dxy)2, as does the related nonsaddled bis-isocyanide complex
IC049089Q
2866 Inorganic Chemistry, Vol. 44, No. 8, 2005