NMR and EPR studies of chloroiron(iii) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities

Article abstract of DOI:10.1021/ic0515352

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)₂]⁺ (L = Im-d₄, 2-MeHIm, 4-Me₂NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L) ₂]⁺ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)₂]⁺ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)₂]⁺ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)₂]⁺ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the π system), while pyrrole-12,13-CH₂ and -7,18-CH₂ protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me₂NPyH⁺ pK_a = 9.7, H₂Im ⁺ pK_a = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me₂NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (d_xy) ²(d_xz,d_yz)³/(d_xzd _yz)⁴(d_xy)¹ electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (d_xzd_yz) ⁴(d_xy)¹ ground state and that it is ruffled, because ruffling mixes the a_2u(π)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40°C) and thus the negative spin density at the pyrroline-α-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)₂]⁺, whose pyrroline-H chemical shift is -36 ppm at 25°C (Simonneaux, G.; Kobeissi, M. J. Chem. Sac., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me₂NPy adduct.

Full text of DOI:10.1021/ic0515352

Inorg. Chem. 2006, 45, 3519−3531
NMR and EPR Studies of Chloroiron(III) Tetraphenyl-chlorin and Its
Complexes with Imidazoles and Pyridines of Widely Differing Basicities
Sheng Cai, Tatjana Kh. Shokhireva, Dennis L. Lichtenberger, and F. Ann Walker*
Department of Chemistry, The UniVersity of Arizona, Tucson, Arizona 85721-0041
Received September 8, 2005
The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III),
[(TPC)Fe(L)2]+ (L
Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance
)
assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY)
and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF)
calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A.
Inorg. Chem. 2005, 44, 1890
−
1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the
for each bisligand complex, whereas higher spin state was present at
1
ground state is
S
)
/
a
2
NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all
[TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons
that are part of the
π system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts,
as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases
with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having
slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower
(4-Me2NPyH+ pKa
) ) 6.65 (adjusted for the statistical factor of 2 protons)). The temperature
9.7, H2Im+ pKa
dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature
range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations
3
that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S
)
/
excited
2
state in each case that in most cases lies at more than kT at room temperature above the ground state. The
observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate
that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a2u
(
π
)-like orbital of
the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift
of the pyrroline-H ( 6.5 ppm at 40 C) and thus the negative spin density at the pyrroline- -carbons, but the
mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is
36
ppm at 25 C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587 1592). Peak assignments
−
−
°
R
−
°
−
for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange
between this complex and its bis-4-Me2NPy adduct.
Introduction
are heme d1 of the bacterial cytochrome cd1 nitrite reductase/
cytochrome oxidase from Pseudomonas aeruginosa, Para-
There are a number of “green” hemes that catalyze
biological reactions, including the heme d of the cytochrome
bd terminal oxidase of E. coli,1-4 the heme d of the catalase
HPII from E. coli,5-7 and the sulfhemes.8,9 Also included
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* To whom correspondence should be addressed. E-mail: awalker@
u.arizona.edu.
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10.1021/ic0515352 CCC: $33.50
Published on Web 03/29/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 9, 2006 3519

Cai et al.
chlorinatoiron(III) (oxo-OEC),31 and substituted tetraphen-
ylporphyrinatoiron(III) (TPP,32 TMP,33-35 and the octaalkyl-
tetraphenylporphyrins36-39) have been reported. In this work
we report detailed NMR studies of high-spin (TPC)FeCl and
its low-spin complexes with axial ligands (imidazole, 2-
methylimidazole, 4-(dimethylamino)pyridine, pyridine, 4-cy-
anopyridine). We have made full assignments, with the help
of Amsterdam density functional (ADF) calculations, of the
resonances for both low-spin and high-spin complexes,
studied the effect of axial ligands on the electronic structure
of the iron center, and compared our findings to those
reported recently for (OEC)Fe(III)30 and oxo-(OEC)Fe(III)31
complexes. The EPR spectra of these complexes are also
reported. By comparing and contrasting the electronic
properties of these molecules with the electronic properties
of the others, we provide in this work additional information
on the factors that control the electronic properties of all of
these systems.
Chart 1. TPCFeCl with the Numbering Convention of the Atoms of
the Chlorin Ligand Used for Labeling the Proton Resonances
coccus denitrificans, and Thiobacillus denitrificans (a dioxo-
isobacteriochlorin10) and siroheme of E. coli sulfite reductase
heme protein (an isobacteriochlorin11,12). All of these pig-
ments are iron chlorin, isobacteriochlorin, or dioxoisobac-
teriochlorin complexes which have uniquely different struc-
tures. There has previously been some confusion regarding
the classification of reduced hemes and the expectations as
to their electronic properties, and a number of careful struc-
tural investigations were required to elucidate the geometric
and stereoisomeric structures of these macrocycles.1-12 There
have been a number of previous studies of synthetic and
natural iron chlorin and/or isobacteriochlorin complexes by
magnetic resonance techniques, including the seminal paper
of Stolzenberg et al. in 198113 and additional papers from
these authors14-18 as well as from other researchers.19
Understanding the electronic properties of the iron(III)
complexes of each of the individual “green” hemes is an
important step in understanding their mechanisms of action.
Synthesis of 5,10,15,20-tetraphenylchlorinatoiron(III) chlo-
ride ((TPC)FeCl, Chart 1) and trans-2,3,7,8,12,13,17,18-
octaethylchlorinatoiron(III) chloride ((OEC)FeCl) was first
reported and the complexes were studied by 1H NMR
spectroscopy some 25 years ago.13 However, because of the
broad lines and correspondingly short proton relaxation times,
until now full peak assignments of neither the high-spin
chloride complex nor low-spin complexes of (TPC)Fe(III)
with axial ligands have been reported, even though the 1D
1H NMR spectra of tetraphenylchlorinatoiron(III) (TPC),
octaethylchlorinatoiron(III) (OEC),30 mono-oxooctaethyl-
Experimental Section
The 5,10,15,20-tetraphenylchlorin free-base (H2TPC) was pur-
chased from Porphyrin Products Inc. The tetraphenylchlorinatoiron-
(III) chloride ((TPC)FeCl) was synthesized using a modification
of a method developed in this laboratory.40 A total of 20 mg of
H2TPC was dissolved in a minimum amount of chloroform (about
5 mL). An approximate 50-fold excess of ferrous chloride was
dissolved in 30 mL of mixed solvent of 3:1 chloroform/methanol
(v/v). The ferrous chloride solution was mixed with the chlorin
free-base solution in a round-bottom flask. The flask with the
reactant was purged with nitrogen for 5 min, and several crystals
of sodium dithionite were added to remove any traces of oxidizing
agents in the solution. The mixture was refluxed under nitrogen
(21) Ozawa, S.; Watanabe, Y.; Morishima, I. Inorg. Chem. 1992, 31, 4042-
4043.
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313.
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I. J. Am. Chem. Soc. 1994, 116, 634-641.
(24) Ozawa, S.; Watanabe, Y.; Morishima, I. J. Am. Chem. Soc. 1994, 116,
5832-5838.
(25) Kobeissi, M.; Toupet, L.; Simonneaux, G. Inorg. Chem. 2001, 40,
4494-4499.
(26) Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001,
1587-1592.
(27) Simonneaux, G.; Kobeissi, M.; Toupet, L. J. Chem. Soc., Dalton Trans.
2002, 4011-4016.
(28) Kobeissi, M.; Simonneaux, G. Inorg. Chim. Acta 2003, 343, 18-26.
(29) Simonneaux, G.; Kobeissi, M.; Toupet, L. Inorg. Chem. 2003, 42,
1644-1651.
(9) Chatfield, M. J.; La Mar, G. N.; Kauten, R. J. Biochemistry 1987, 26,
6939-6950.
(30) Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44,
1890-1903.
(10) Chang, C. K.; Wu, W. J. Biol. Chem. 1986, 261, 8593-8596.
(11) Crane, B. R.; Siegel, L. M.; Getzoff, E. D. Biochemistry 1997, 36,
12101-12119.
(31) Cai, S.; Belikova, E.; Yatsunyk, L.; Stolzenberg, A. M.; Walker, F.
A. Inorg. Chem. 2005, 44, 1882-1889.
(12) Stroupe, M. E.; Leech, H. K.; Daniels, D. S.; Warren, M. J.; Getzoff,
E. D. Nat. Struct. Biol. 2003, 10, 1064-1073.
(32) La Mar, G. N.; Del Gaudio, J.; Frye, J. S. Biochim. Biophys. Acta
1977, 498, 422-435.
(13) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc.
1981, 103, 4763-4778.
(33) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F.
A.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066-7075.
(34) Watson, C. T. Ph.D. Dissertation, University of Arizona, Tucson, AZ,
1996.
(14) Strauss, S. H.; Pawlik, M. J. Inorg. Chem. 1986, 25, 1921-1923.
(15) Dixon, D. W.; Woehler, S.; Hong, X.; Stolzenberg, A. M. Inorg. Chem.
1988, 27, 3682-3685.
(35) Watson, C. T.; Cai, S.; Shokhirev, N. V.; Walker, F. A. Inorg. Chem.
2005, 44, 7468-7484.
(36) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757-777.
(37) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 4341-4352.
(38) Yatsunyk, L. A.; Shokhirev, N. V.; Walker, F. A. Inorg. Chem. 2005,
44, 2848-2866.
(39) Yatsunyk, L. A.; Walker, F. A. J. Porphyrins Phthalocyanines 2005,
9, 214-228.
(40) Nesset, M. J. M. Ph.D. Dissertation, University of Arizona, Tucson,
AZ, 1994; pp 83-88.
(16) Pawlik, M. J.; Miller, P. K.; Sullivan, E. P.; Levstik, M. A.; Almond,
D. A.; Strauss, S. H. J. Am. Chem. Soc. 1988, 110, 3007-3012.
(17) Sullivan, E. P., Jr.; Grantham, J. D.; Thomas, C. S.; Strauss, S. H. J.
Am. Chem. Soc. 1991, 113, 5264-5270.
(18) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96,
2239-2314.
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3520 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
for 2 h, cooled to room temperature, and washed with water three
times. The product was bubbled with HCl gas to convert any
possible dimer to (TPC)FeCl. Finally, the solvent was evaporated
to dryness, and the solid product was pumped under vacuum
overnight and kept in a drybox for future use.
All samples for NMR studies were prepared in 5 mm NMR tubes
by the addition of methylene chloride-d2 (Cambridge Isotopes) in
the glovebox. Complexes with axial ligands were made by directly
adding an excess of desired axial ligand (iron chlorin/axial ligand
1:4 or larger). Axial ligands used in this work include deuterated
imidazole (Im-d4, Cambridge Isotopes), 2-methylimidazole (2-
MeHIm, Aldrich), 4-(dimethylamino)pyridine (4-Me2NPy, Aldrich),
pyridine (Py, Aldrich), pyridine-d5 (Py-d5, Cambridge Isotopes),
and 4-cyanopyridine (4-CNPy, Aldrich). For complexes with
pyridine and 4-cyanopyridine as axial ligands, an excess of silver
trifluoromethanesulfonate (AgOSO2CF3, 99%, Alfa f SAR) was
also added to improve the binding ability of the ligands by removing
chloride ion. All NMR tubes were sealed tightly before removal
from the glovebox.
1H NMR spectra of the free-base H2TPC were recorded on a
Varian Unity-300 spectrometer operating at 299.955 MHz with the
variable temperature unit set at 10 °C and referenced to the
resonance of the residual solvent protons of CD2Cl2 (5.32 ppm
relative to TMS). Nuclear Overhauser enhancement spectroscopy
(NOESY) and correlation spectroscopy (COSY) spectra were
acquired with the normal Varian pulse sequences with a spectral
width of 3904 Hz in each dimension, an acquisition time of 254
ms, a relaxation delay of 550 ms, a mixing time of 900 ms
(NOESY), and 120 t1 increments. HMQC spectra were recorded
on a Bruker DRX-500 NMR spectrometer at ambient temperatures
with the use of the standard Bruker pulse sequence.
1
Figure 1. 1D H 300 MHz NMR and 2D NOESY spectra of H2TPC in
1
For H NMR measurements of the iron chlorin complexes at
CD2Cl2 (5.32 ppm) at 10 °C. In addition to NOE and J-couplings between
7,18-H and 8,17-H and between both 5,20-o-H and 10,15-o-H and the
overlapping m,p-H (all also shown in the DQF-COSY spectrum of
Supporting Information Figure S1), note the NOEs from 5,20-o-H to
pyrroline-H and 7,18-H and the NOEs from 10,15-o-H to 12,13-H and
8,17-H. The letter S denotes solvent residual proton resonance.
low temperature, the spectra were recorded on the Varian Unity-
300 spectrometer over a temperature range from -90 to +20 °C
and again referenced to the resonance from residual solvent protons.
The temperature was calibrated using the standard Wilmad methanol
and ethylene glycol samples. For other experiments, the spectra
were acquired on the Varian Unity-300 or Bruker DRX-500 NMR
spectrometers. The 1D saturation transfer experiments were carried
out on both the Unity-300 and DRX-500 NMR spectrometers using
the normal nuclear Overhauser effect (NOE) difference pulse
sequence. On both instruments, a selective intermediate level pulse
was irradiated on the specific peaks of the fast relaxing (e.g., high-
spin) species with an irradiation time of 50 ms. On the Unity-300,
the decoupler power was set to 35-40 dB (63 dB is the maximum
power level). On the DRX-500, the pulse level was 45-50 dB (-6
dB is the maximum power level).
ppm region, are shown in Figure 1. The only resonance of
the complex not shown in this spectrum is the single broad
N-H resonance, which occurs at -1.31 ppm. As can be seen,
the three pyrrole-H resonances are found between 8.19 and
8.57 ppm; the two doublets at those extreme chemical shifts
show NOEs with each other. They also show cross-peaks in
the DQF-COSY spectrum shown in Supporting Information
Figure S1, which clearly identifies them as arising from the
7,18- and 8,17-pyrrole-H, although which is which cannot
For electron paramagnetic resonance (EPR) spectroscopy, the
low-spin bisligand complexes of (TPC)Fe(III) were again prepared
in dry CD2Cl2 immediately before the experiments with the use of
the same method as for the NMR studies and frozen in liquid
nitrogen. The EPR spectra were obtained on a CW Bruker ESP-
300E EPR spectrometer operating at X-band using 0.2 mW
microwave power and a 100 kHz modulation amplitude of 2 G. A
Systron-Donner microwave counter was used for frequency calibra-
tion. The EPR measurements were performed at 4.2 K using an
Oxford continuous flow helium cryostat, ESR 900.
3
be determined from this minimal information. The JH-H
coupling constant between these protons on adjacent â-pyr-
role carbons is 4.7 Hz. The singlet at 8.57 ppm thus is due
to the 12,13-pyrrole-H. The singlet at 4.16 ppm with relative
intensity 4 as compared to 2 for each of the pyrrole-H
resonances is clearly the pyrroline-H of the chlorin ring.
Between these pyrroline-H and pyrrole-H regions are the
phenyl-H of H2TPC, and the multiplets have chemical shifts
and relative intensities (in parentheses) ∼7.70 (12), 7.89 (4),
and 8.10 (4) ppm. In comparison, H2TPP has overlapping
m- and p-H resonances at 7.79 (8) and 7.77 (4) ppm,
respectively, and the o-H doublet of doublets at 8.23 (8) ppm
(not shown). From the doublet of doublets coupling patterns
of the multiplets at 7.89 and 8.09 ppm of H2TPC shown in
Results
NMR Spectra and Peak Assignments of H2TPC. The
1D 300 MHz 1H NMR spectrum and the NOESY spectrum
of H2TPC in CD2Cl2, recorded at 10 °C over the 3.6-9.2
Inorganic Chemistry, Vol. 45, No. 9, 2006 3521

Cai et al.
Table 1. Summary of Chemical Shifts of H2TPC
1H chemical
13C chemical
shift
proton
shift
3JH-H coupling
position
(ppm)
(Hz)
(ppm)
2,3-pyrroline
5,20-o-phenyl
5,10,15,20-
4.168 (s)
37
7.892 (dd)
7.0, very small
133
7.683-7.719 (m)
128, 127
m,p-phenyl
10,15-o-phenyl
7,18-pyrrole
12,13-pyrrole
8,17-pyrrole
8.100 (dd)
8.197 (d)
8.393 (s)
8.578 (d)
7.0, very small
4.7
134
124
132
128
4.7
Figure 1 and the DQF-COSY cross-peak pattern shown in
Supporting Information Figure S1, it is clear that the cross-
peaks observed are between each of the two types of o-H
with the strongly overlapping m- and p-H of the two types
of phenyl rings of H2TPC. Thus, the two types of phenyl
rings of H2TPC have their o-H slightly less ring-current
shifted to larger chemical shift than do those of H2TPP, and
one type is less ring-current shifted than the other. HMQC
spectra are shown in Supporting Information Figure S2 for
Figure 2. 1H spectrum of low-spin [(TPC)Fe(Im-d4)2]Cl in CD2Cl2 at 0
°C. The labels “PL”, “7,18”, “12,13”, and “8,17” denote pyrroline, pyrrole-
7,18, pyrrole-12,13, and pyrrole-8,17 protons, respectively.
complex (OMC ) octamethylchlorin) have recently shown
that the spin density at the pyrrole-8,17-carbons is much
larger than that at either the pyrrole-7,18- or -12,13-carbons,30
as shown in Figure 3A. (Separate ADF calculations for
[(TPC)Fe(HIm)2]+ have not been carried out because of the
size of the calculation (40 additional atoms).) Thus, for the
spin-coupled pair of resonances, the pyrrole-8,17-H must
have much larger contact shift than the pyrrole-7,18-H, and
the resonance near -38 ppm thus arises from the pyrrole-
8,17-H. The resonances of the phenyl protons are all located
in the region of 6-7.7 ppm. Their chemical shifts have not
been analyzed in detail because of overlap between the
expected two sets of three phenyl-H resonances.
1
the pyrrole-, pyrroline-, and phenyl-H, all of whose H and
13C chemical shifts are listed in Table 1.
The NOESY spectrum of Figure 1 shows NOE cross-peaks
between the pyrroline-H and the o-H multiplet at 7.89 ppm
and also very weak NOE cross-peaks with the pyrrole-H
doublet at 8.20 ppm, thus identifying that pyrrole-H doublet
arises from the 7,18-H. In a like manner, the singlet pyrrole-H
resonance at 8.39 ppm shows NOE cross-peaks with the o-H
resonance at 8.10 ppm, as does the pyrrole-H doublet at 8.58
ppm, thus identifying the latter as the 8,17-pyrrole-H. The
chemical shifts and assignments are summarized in Table
1. The assignment of the pyrrole-H resonances of H2TPC,
as a diamagnetic precursor of the iron(III) complexes of TPC,
allows fitting of the temperature dependence of the pyrro-
line-H and each of the pyrrole-H resonances of these
complexes.
NMR Spectra and Peak Assignments of the Low-Spin
(TPC)Fe(III) Complexes. [(TPC)Fe(Im-d4)2]Cl. Upon the
addition of axial ligands, the high-spin complex (TPC)FeCl
changes color from brown to green and becomes a low-spin,
S ) 1/2 (TPC)Fe(III) bisimidazole complex. Figure 2 shows
the 1D 1H NMR spectrum of low-spin [(TPC)Fe(Im-d4)2]Cl
in CD2Cl2 at 0 °C. It shows four paramagnetically shifted
resonances, one at positive and three at negative chemical
shifts. By integration, the resonance at +39 ppm must be
that of the pyrroline protons; the same assignment of the
bis-1-MeIm complex has been reported previously.27 In the
COSY spectrum of [(TPC)Fe(Im-d4)2]Cl, obtained at +30
°C and shown in Supporting Information Figure S3, very
weak cross-peaks were observed between the two pyrrole-H
resonances at -2 and -32 ppm, which corresponds to -2
and -38 ppm at 0 °C in Figure 2. As for the free-base H2-
TPC, these two resonances can be assigned to the adjacent
pyrrole-7,18 and pyrrole-8,17 protons. Thus the third reso-
nance with negative chemical shift is assigned to the pyrrole-
12,13-H. Although from the COSY spectrum alone we
cannot distinguish which of the first two resonances are from
protons attached to pyrrole-7,18 and which to pyrrole-8,17,
ADF calculations on the corresponding [(OMC)Fe(HIm)2]+
[(TPC)Fe(2-MeHIm)2]Cl.
[(TPC)Fe(2-MeHIm)2]Cl
shows a similar 1D spectrum (Figure 4) to that of [(TPC)-
Fe(Im-d4)2]Cl, except that the magnitude of the chemical
shifts is smaller at the same temperature, indicating smaller
spin delocalization to the R-pyrroline and â-pyrrole carbons
in this complex. In the COSY spectrum shown in Supporting
Information Figure S4, as seen for [(TPC)Fe(Im-d4)2]Cl,
Figure S3, there are also weak cross-peaks between the
pyrrole-7,18-H and pyrrole-8,17-H. In a similar fashion,
resonance assignments of the pyrroline and all the pyrrole
protons can be made. Although [(TPC)Fe(2-MeHIm)2]Cl and
[(TPC)Fe(Im-d4)2]Cl have different chemical shifts, the order
of the chemical shifts of the three pyrrole-H resonances is
same for the two (from the least to the most negative
chemical shifts: in magnitude, 7,18 > 12,13 > 8,17). This
suggests that these two complexes have similar electronic
structures but with less spin density delocalized to the chlorin
ring in the case of the 2-MeHIm ligands.
The assignments of the bound axial 2-MeHIm ligands can
be made from chemical exchange cross-peaks observed
between the bound and free axial ligands in the NOESY/
EXSY spectrum shown in Supporting Information Figure S5.
As for [(TPC)Fe(Im-d4)2]Cl, the phenyl proton peaks are
tightly grouped and strongly overlapping between +6 and
+7.7 ppm. Thus, their chemical shifts have not been analyzed
in detail.
[(TPC)Fe(4-Me2NPy)2]Cl. This complex has a similar 1H
NMR spectrum to those of the former two complexes, with
chemical shifts intermediate between the two, as shown in
Supporting Information Figure S6. As for the former two
complexes, the assignments of [(TPC)Fe(4-Me2NPy)2]Cl
3522 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
Figure 3. Spin densities obtained from the ADF calculations for (A) [(chlorin)Fe(ImH)2]+ in the planar geometry, with electron configuration (dxy)2-
(dxz,dyz)3 and the antisymmetric chlorin 3e(π)-like orbital contributing significantly, (B) [(chlorin)Fe(MeNC)2]+ in the planar geometry, with metal electron
configuration (dxz,dyz)4(dxy)1 and some contribution from a mixture of chlorin orbitals, and (C) [(chlorin)Fe(MeNC)2]+ in the ruffled geometry, with metal
electron configuration (dxz,dyz)4(dxy)1 and the chlorin 3a2u(π)-like orbital contributing significantly, all shown for a generic chlorin core. The red and blue
colors represent positive and negative spin density, respectively. Adapted from Figure 8 of ref 30.
Figure 4. 1D 1H NMR spectrum of low-spin [(TPC)Fe(2-MeHIm)2]Cl in
CD2Cl2 at 0 °C. “FL” denotes free axial ligand in the solution; “BL” denotes
bound axial ligand. The N-H of the axial ligands are not observed, probably
due to their fast proton exchange rate with traces of water in the solution,
which leads to very broad peaks. Asterisks denote resonances arising from
the minor amount of [(TPP)Fe(2-MeHIm)2]Cl present in the sample due to
reoxidation of TPC to TPP upon iron insertion.
were also made from the COSY (Supporting Information
Figure S7) and NOESY (Figure S8) spectra. The intermediate
chemical shifts of the pyrroline and pyrrole-8,17 protons of
[(TPC)Fe(4-Me2NPy)2]Cl between those of [(TPC)Fe(2-
MeHIm)2]Cl and [(TPC)Fe(Im-d4)2]Cl are a point that will
1
Figure 5. 1D H NMR spectrum of low-spin (a) [(TPC)Fe(Py)2]Cl and
be considered further in the discussion section.
(b) [(TPC)Fe(Py-d5)2]Cl in CD2Cl2 at 0 °C. “BL-m”, “BL-p”, and “BL-o”
denote meta, para, and ortho protons of the bound axial ligands, respectively;
“m”, “p” and “o” denote meta, para, and ortho protons of the meso phenyl
groups, respectively. Asterisks denote resonances arising from the minor
amount of [(TPP)Fe(Py)2]Cl and [(TPP)Fe(Py-d5)2]Cl present in the samples
due to reoxidation of TPC to TPP upon iron insertion. The letter X represents
residual H2O.
[(TPC)Fe(Py)2]+. Pyridine is a relatively weak base that
binds to (TPC)FeCl only when the chloride ion has been
-
replaced with CF3SO3 by the addition of AgOSO2CF3.
1
Figure 5a shows the 1D H spectrum of [(TPC)Fe(Py)2]+.
To simplify the spectrum and help make assignments, the
complex with deuterated pyridine as the axial ligands,
[(TPC)Fe(Py-d5)2]+, was also studied by 1H NMR spectros-
copy (Figure 5b). The peak assignments were made from
COSY (Supporting Information Figure S9) and NOESY
(Figure S10) spectra. The order of the chemical shifts of the
pyrroline and pyrrole protons is still the same as those of
the former three complexes, although the magnitude of the
chemical shifts of the pyrroline and pyrrole-8,17 protons is
much smaller. The temperature dependence of the chemical
shifts of the three types of pyrrole-H and the pyrroline-H
does not follow the Curie law, as will be discussed in detail
below.
from the COSY spectrum, as shown in Supporting Informa-
tion Figure S9. Since there are two kinds of meso-carbons,
in total six resonances were observed from the phenyl
protons. The COSY spectrum shows four pairs of cross-
peaks, two of which are between m- and o-H, the other two
from m- and p-H. The two resonances with half intensity as
compared to the other four can be assigned to p-H; the two
resonances which have cross-peaks with both o- and p-H
are the m-H, and the other two are the o-H. The resonances
of the phenyl p- and m-H in Figure 5a appear to be smaller
than they should be. This is because the phenyl p- and m-H
are farther away from the iron center as compared to the
o-H, leading to longer relaxation times (for p-H, T1 ≈ 250
ms). Under the conditions of the 1D NMR experiments
Unlike the former three complexes, for [(TPC)Fe(Py)2]+
the phenyl protons are well-resolved and can be assigned
Inorganic Chemistry, Vol. 45, No. 9, 2006 3523

Cai et al.
Table 2. 1H Chemical Shifts (ppm) of the [(TPC)Fe(L)2]+ Complexes of This Study in CD2Cl2 at 233 K
ligand L
imidazole-d4
pyrroline-H
8,17-H
12,13-H
7,18-H
o-H
m-H
p-H
44.9
34.0
26.2
15.1
-46.5
-39.8
-29.2
-19.8
-4.5
-9.8
-8.7
-4.8
-1.0
+1.6
∼+7.7
∼+6.7
∼+6.0
4-(dimethylamino)pyridine
2-methylimidazole
pyridine
∼+7.7
∼+6.7
∼+6.0
-8.7
∼+7.7
∼+6.7
∼+6.0
-1.0
+12.4
+16.7
∼+12a,b
+5.2, +5.7
+3.3, +2.2
∼-3a
+9.8, +9.6
+13.3, +12.3
∼+17a
+6.5, +6.7
+4.9, +4.8
∼+1a
4-cyanopyridine
-6.5
+4.5
tert-butylisocyanidea
∼-42a
∼+7a,b
∼+5a,b
a Chemical shifts at 233 K estimated from ref 26. b Order of pyrrole-H resonances not determined;26 assumed to be the same as for [(TPC)Fe(4-CNPy)2]+.
1
Figure 6. 1D H NMR spectrum of low-spin [(TPC)Fe(4-CNPy)2]Cl in
CD2Cl2 at -40 °C. The asterisk marks an impurity, probably traces of water.
shown in Figure 5 (total time period for one transient is 100
ms), the relaxation delay was not long enough for the
magnetization of m- and p-H to relax to equilibrium. The
relative resonance intensity of phenyl o-, m-, and p-H is 2:2:1
when a long delay time (1 s) is used.
It should be noted that the phenyl-H resonances are shifted
from their diamagnetic positions in an alternating fashion,
with the m-H appearing at more positive chemical shift than
its diamagnetic counterpart, while the o- and p-H appear at
more negative chemical shifts than their diamagnetic coun-
terparts. Thus, the phenyl-H shift differences δm - δo and
δm - δp are positive, which indicates positive spin density
at the meso-carbons,41 as discussed in greater detail in the
Discussion.
Figure 7. EPR spectra of low-spin (TPC)Fe(III) complexes with the axial
ligands of this study.
peak assignments. The temperature dependence of the three
pyrrole-H and the pyrroline-H chemical shifts is anti-Curie,
as will be discussed further below. Again, the pattern of shifts
is that of the phenyl-H shift differences δm - δo and δm -
δp being positive, which indicates positive spin density at
the meso-carbons. This finding supports the assignment of
the ground-state electron configuration in this case as
(dxz,dyz)4(dxy)1 (see discussion section).
[(TPC)Fe(4-CNPy)2]+. 4-CNPy is the weakest-binding of
the axial ligands studied in this work. Even in the presence
of AgOSO2CF3, it binds to (TPC)Fe(III) well only below
1
-20 °C. The 1D H NMR spectrum recorded at -40 °C
shown in Figure 6 for [(TPC)Fe(4-CNPy)2]+ is quite different
from those of other complexes discussed above. Although
the order of the chemical shifts of the three pyrrole-H peaks
from positive to negative is still 7,18 >12,13 > 8,17, it is
found that the 7,18- and 12,13-H of this complex have
positive chemical shifts and the 8,17-H chemical shift is
much less negative than for all other complexes. Furthermore,
the pyrroline-H of this complex has a negative chemical shift;
thus the order of pyrrole-H resonances remains the same as
that of the other complexes of this study, while the
pyrroline-H resonance has moved from most positive to most
negative chemical shift at -40 °C (see Table 2). The peak
assignments were also accomplished by COSY (Supporting
Information Figure S11) and NOESY experiments (Figure
S12). As for [(TPC)Fe(Py)2]+, the resonances from the
phenyl protons are also well resolved, which enables full
EPR Spectra of the Low-Spin (TPC)Fe(III) Complexes.
The rhombic signal in the EPR spectrum of [(TPC)Fe(HIm)2]-
Cl shown in Figure 7 is typical of low-spin Fe(III) porphy-
rinates and chlorinates with (dxy)2(dxzdyz)3 ground states.30,42
And although broader, the [(TPC)Fe(2-MeHIm)2]+ and
[(TPC)Fe(4-Me2NPy)2]+ complexes have very similar ob-
served g values to those of [(TPC)Fe(HIm)2]+, suggesting
that they also have the same ground state. However, for
[(TPC)Fe(Py)2]+ and [(TPC)Fe(4-CNPy)2]+, the EPR signals
are broad and poorly resolved. As reported recently for the
corresponding complexes of (TMP)Fe(III),33-35 these two
complexes may not have a pure (dxy)2(dxzdyz)3 or (dxy)1(dxzdyz)4
ground state, even though their ambient-temperature 1H NMR
shifts, discussed above, are consistent with an S ) 1/2 ground
state. The same broad EPR signals were found not only for
the (TMP)Fe(III) complexes of the same ligands33,35 but also
3524 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
Figure 8. Temperature-dependent fitting plots of the chemical shifts of the pyrroline- and three pyrrole-H to eq 1 for (A) [(TPC)Fe(4-Me2NPy)2]+, (B)
[(TPC)Fe(Im-d4)2]+, (C) [(TPC)Fe(Py)2]+, and (D) [(TPC)Fe(4-CNPy)2]+. In all plots, 9 represents pyrroline-H, 1 represents 7,18-pyrrole-H, 2 represents
12,13-pyrrole-H, and b represents 8,17-pyrrole-H. The best-fit spin densities and ∆E values are summarized in Table 3.
3
for (TPP)Fe(III) complexes having 2,6-substituents on the
four phenyl rings.34,35
dition, a thermally accessible excited state having S ) /2
5
or S ) /2 is also possible. As we have shown previous-
ly,35-38,43-45 a number of ferriheme complexes have a ther-
mally accessible excited state that causes at least some cur-
vature of the Curie plot and can sometimes show extremely
curved chemical shift dependence.35,36,44 Expansion of the
Curie law to include the contribution from the Boltzmann
population of this thermally accessible excited state yields
the following expression,43 if the 1/T2 contribution to the
intermediate- or high-spin excited state is neglected:46-49
Temperature Dependence of the Chemical Shifts of
Low-Spin (TPC)Fe(III) Complexes. The non- and anti-
Curie behavior observed for [(TPC)Fe(Py)2]+ and [(TPC)-
Fe(4-CNPy)2]+, as well as the deviations of the intercepts
of the Curie plots of even the bisimidazole (this work) and
bis-1-methylimidazole27 complexes, indicates the existence
of a thermally accessible excited state for all of these (TPC)-
1
Fe(III) complexes. Two S ) /2 electron configurations are
possible for iron(III) porphyrinates and chlorinates: the more
commonly observed (dxy)2(dxzdyz)3 and the now not-so-
uncommon (dxy)1(dxzdyz)4 electron configuration.41,48 In ad-
δcon ) (1/T)[W1Cn1 + W2Cn2 e-∆E/(kT)]/[W1 + W2 e-∆E/(kT)
]
(1)
In this equation Cn1 is the coefficient for the position of
interest for level 1, Cn2 is the corresponding coefficient for
(41) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544.
(42) Astashkin, A. V.; Raitsimring, A. M.; Walker, F. A. J. Am. Chem.
Soc. 2001, 123, 1905.
(43) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-
(48) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: Burlington, MA, 1999; Vol.
V, pp 81-183.
17804.
(44) Nesset, M. J. M.; Cai, S.; Shokhireva, T. Kh.; Shokhirev, N. V.;
Jacobson, S. E.; Jayaraj, K.; Gold, A.; Walker, F. A. Inorg. Chem.
2000, 39, 532-540.
(45) Banci, L.; Bertini, I.; Luchinat, C.; Pierattelli, R.; Shokhirev, N. V.;
Walker, F. A. J. Am. Chem. Soc. 1998, 120, 8472-8479.
(46) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286-301.
(47) La Mar, G. N.; Walker, F. A. NMR Studies of Paramagnetic
Metalloporphyrins. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. IV, pp 61-157.
1
(49) When S > /2 and there is a relatively large zero-field splitting, the
pseudocontact term has a C′/T2 dependence and the contact term has
a C/T dependence.46-48
(50) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance
with Applications to Chemistry and Chemical Physics; Harper and
Row: New York, 1967; p 80.
(51) McLachlan, A. D. Mol. Phys. 1958, 1, 233.
(52) Chestnut, D. B. J. Chem. Phys. 1958, 29, 43-47.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3525

Cai et al.
Table 3. Summary of Results Obtained from the TDFw53 Fits of the Temperature Dependence of the Three δ(Pyrrole-H) and δ(Pyrroline-H) of the
[(TPC)Fe(L)2]+ Complexes of This Study
pyrroline-R and â-pyrrole carbon spin densities, FC,e
a
b
system
SG
SE
∆E,c cm-1
msdd
comments on quality or nature of fit
1
3
L ) Im-d4
/
/
238
0.054
1: 0.0210, 0.0143 av
(0.0064, 0.0308, 0.0057). OK for S ) 1/2 dπ/dxy.
(0.0102, 0.0215, 0.0066). OK for S ) 3/2.
2
2
2: 0.0117, 0.0127 av
1: 0.0152, 0.0121 av
2: 0.0103, 0.0115 av
1: 0.0091, 0.0098 av
2: 0.0032, 0.0080 av
1: 0.0032, 0.0042 av
2: 0.0205, 0.0206 av
1
1
1
1
3
3
3
3
L ) 4-Me2NPy
L ) 2-MeHIm
L ) Py
/
/
/
/
/
/
/
/
132
639
700
563
0.050
0.083
0.149
0.174
(0.0071, 0.0275, 0.0017). OK for S ) 1/2 dπ/dxy.
(0.0083, 0.0206, 0.0057). av FC too small for S ) 3/2.
(0.0081, 0.0182, 0.0030). OK for S ) 1/2 dπ/dxy.
(0.0061, 0.0130, 0.0049). av FC too small for S ) 3/2.
(0.0043, 0.0115, -0.0032). OK for S ) 1/2 dxy/dπ.
(0.0048, 0.0402, 0.0169). OK for S ) 3/2.
2
2
2
2
2
2
2
2
L ) 4-CNPy
1: -0.0101, -0.0010 av (0.0011, 0.0011, -0.0050). OK for S ) 1/2 dxy.
2: 0.0335, 0.0133 av
(0.0060, 0.0386, 0.0029). OK for S ) 3/2.
a Ground-state spin (SG); electron configuration may be (dxy)2(dxz,dyz)3 (abbreviated dπ), (dxz,dyz)4(dxy)1 (abbreviated dxy), or a mixture of the two, as
b
3
1
1
2
marked in the comments at the far right. Excited-state spin (SE); all systems appear to have the electron configuration (dxz,dyz) (dxy) (dz ) . c Best fit energy
difference between ground and excited state ((30%). d msd ) mean-square deviation of the data points from the best fit. e Spin densities, FC, obtained from
fits, using the program TDFw, to eq 1; 1 ) ground state, 2 ) excited state. Spin densities are given in the order pyrroline-H, average of three pyrrole-H,
with individual pyrrole-H spin densities in parentheses in the order 7,18; 8,17; 12,13. Comments indicate how the spin density distribution is interpreted in
terms of the sizes of the coefficients for each pyrrole position.
level 2, W1 and W2 are the statistical weights for each level
() 2S + 1 for each), and ∆E is the energy separation
between the two levels. The coefficients Cn1 and Cn2 are
approximately equal to the Curie coefficients of each level
(except for the small contribution of the pseudocontact
contribution to the paramagnetic shifts47,48), and they can be
determined by fitting the temperature dependence of the
proton chemical shifts, assuming that the diamagnetic shift
of each proton type is known. The coefficients Cn1 and Cn2
can be further subdivided into the McConnell constant QC
and the variable FC, where QC ) -496.8 and 591.4 ppm K
for protons attached to aromatic and aliphatic carbons,
respectively,47,50-52 and FC is the spin density at the carbon
of interest; in general FC is not larger than about 0.03 for
the systems of this study. 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.53
By entering the diamagnetic shifts of the free-base ligand
for each proton type (Table 1) and the contact shift versus
inverse temperature data for each complex into the two-level
fitting program, specifying the ground-state spin as S ) 1/2,
and testing each possible spin S for the excited state, we
calculate FC1, FC2, and ∆E for each spin choice. Choosing S
However, upon viewing the spin densities calculated from
the fits, we found those FC2 values for the excited state S )
5/2 to be unreasonably small. Thus, the reasonable spin state
3
was found to be S ) /2 in each case. The same result was
found as in previous studies of most octaalkyltetraphenyl-
porphyrinatoiron(III),36-39 (OEC)Fe(III) and OEPFe(III),30
(TMP)Fe(III),35 and four ((2,6-X2)4TPP)Fe(III)35 series of
complexes with axial ligands of varying basicities. The
3
results of the S ) /2 excited state fits for FC1, FC2, and ∆E
are summarized in Table 3. Checks for contributions to the
temperature dependence from ligand on-off equilibria at
near ambient temperatures were made by not including
higher-temperature chemical shift data in the fitting. It was
found that including data points from the entire temperature
range (T ) 303-183 K) yielded the same ∆E and spin
densities FC1 and FC2 as if only the T < 253 K data were
used for Im-d4 and 4-Me2NPy but that only T < 253 K
chemical shifts could be used for the other three complexes.
And because the pseudocontact shifts are small and have
not been accurately determined, yet are expected to have
the same temperature dependence as the contact shift, the
paramagnetic shift instead of contact shift was used for fitting
the temperature dependence.
In Figure 8 are shown the Curie plots of the pyrroline and
three pyrrole-H of the (TPC)Fe(III) complexes with 4-Me2-
NPy, Im-d4, Py, and 4-CNPy ligands. In parts A and B of
Figure 8, respectively, the proton resonances of [(TPC)Fe-
(4-Me2NPy)2]+ and [(TPC)Fe(Im-d4)2]+ exhibit approximate
Curie behavior, which before the advent of the 2-level fitting
procedure43 would have been passed off as simple Curie
behavior, even though the intercepts of such linear plots were
not quite the same as the diamagnetic shifts. In contrast, in
Figure 8D, the proton resonances of [(TPC)Fe(4-CNPy)2]+
show anti-Curie behavior. [(TPC)Fe(Py-d5)2]+ shows behav-
ior between these extremes. Table 3 summarizes the fitting
results for all complexes of this study. Both [(TPC)Fe(4-
CNPy)2]+ and [(TPC)Fe(Py)2]+ are seen to have a ground
state with much smaller spin densities on all pyrrole and
pyrroline carbons, but most especially on the 12,13- and 1,4-
carbons than for [(TPC)Fe(4-Me2NPy)2]+ and [(TPC)Fe(Im-
1
) /2 for the excited state as well as the ground state (to
cover the possibility that the ground-state electron config-
uration was (dxy)2(dxz,dyz)3 while the excited-state electron
configuration was (dxy)1(dxz,dyz)4 or vice versa) did not lead
to convergence of the fits, whereas choosing S ) 3/2 or S )
5/2 for the excited state led to convergence in all cases.
(53) Shokhirev, N. V.; Walker, F. A. http://www.shokhirev.com/nikolai/
programs/prgsciedu.html.
(54) Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T.;
Raffii, K.; Simonis, U.; Walker, F. A. J. Phys. Chem. A 1997, 101,
2778-2886.
(55) Polam, J. R.; Shokhireva, T. Kh.; Raffii, K.; Simonis, U.; Walker, F.
A. Inorg. Chim. Acta 1997, 263 (1,2) 109-117.
(56) Yatsunyk, L. A.; Ogura, H.; Walker, F. A. Inorg. Chem. 2005, 44,
2867-2881.
(57) La Mar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A. J. Am. Chem.
Soc. 1973, 95, 63-75.
(58) Shokhireva, T. Kh.; Shokhirev, N. V.; Walker, F. A. Biochemistry
2003, 42, 679-693.
(59) Cai, S. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2001.
3526 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
d4)2]+, and an excited state with large spin densities on these
two sets of carbons. This is consistent with the ground state
of these two complexes having a largely (dxzdyz)4(dxy)1
electron configuration, probably a mixture of (dxy)2(dxz,dyz)3
and (dxz,dyz)4(dxy)1 for the former and quite pure (dxz,dyz)4-
3
(dxy)1 for the latter. The excited state has S ) /2 with a
3
1
1
2
probable (dxzdyz) (dxy) (dz ) electron configuration. As men-
tioned above, the spin densities obtained for the excited states
of all complexes clearly are not consistent with S ) 5/2; the
FC values are much too small for the high-spin configuration.
An average spin density of 0.014-0.018 has been found for
3
the S ) /2 excited states of a series of (TMP)Fe(III) and
four 2,6-disubstituted (TPP)Fe(III) complexes.35 Thus, es-
pecially for the bis-2-methylimidazole complex of (TPC)-
Fe(III), the spin density obtained for the â-pyrrole carbons
is much too small (Table 3), and the fit may be considered
unreliable. This is not surprising, considering the dynamics
of ring inversion that are probably occurring over the same
temperature range54,55 and may affect the temperature
dependence of the resonances.56 The bis-Im-d4 and -4-Me2-
NPy complexes have (dxy)2(dxzdyz)3 ground-state electron
3
configurations and the same S ) /2 excited state with the
same electron configuration, while the pyridine complex has
much smaller spin density coefficients than the two just
mentioned, yet larger than the 4-CNPy complex. It thus
appears to have a mixed ground-state configuration of
(dxz,dyz)4(dxy)1/(dxy)2(dxz,dyz)3. Therefore, the fits of the NMR
spectra to the expanded version of the Curie law, eq 1,
provide more definitive information about the electronic
ground state of the low-basicity pyridine complexes than do
the EPR spectra of the same complexes discussed above.
NMR Spectra and Peak Assignments of High-Spin
(TPC)FeCl. Figure 9A shows the 1D 1H NMR spectrum of
high-spin (TPC)FeCl in CD2Cl2 at 303 K. Partial peak
assignments of pyrroline-H and phenyl-H have been pub-
lished previously.16 However, the assignments of the three
pyrrole-H resonances have not been reported before. The
average pyrrole-H chemical shift is close to that of the
pyrrole-H in (TPP)FeCl57 (seen as a minor impurity in Figure
9A and Figure 9Ba because of the ease of reoxidation of
(TPC)FeCl to (TPP)FeCl), indicating that these two S ) 5/2
complexes have the same electronic structure.
1
Figure 9. (A) 1D H NMR spectrum of high-spin (TPC)FeCl in CD2Cl2
at 303 K. The small peak marked with an asterisk arises from the impurity
(TPP)FeCl produced during iron insertion. The “m” stands for the m-H of
the meso phenyl groups. (B) Saturation transfer experiments on the mixture
of high-spin (TPC)FeCl and low-spin of [(TPC)Fe(4-Me2NPy)2]Cl at 303
K: (a) the control spectrum; (b-d) difference spectra.
tunately, as for other Fe(III) porphyrinates and chlorinates,30
no evidence of monoaxial ligand complexes in the solution
is detected in the NMR spectra. By irradiating, one by one,
the pyrrole-H resonances of (TPC)FeCl and observing the
corresponding resonances of the low-spin species in the
difference spectrum, which arise from chemical exchange,
we can assign the peaks of (TPC)FeCl.
It is difficult to assign the three pyrrole proton peaks of
high-spin (TPC)FeCl using normal 2D NMR techniques
because of their very short relaxation times (T1 ≈ 1-2 ms).
Since full peak assignments (with the help of ADF calcula-
tions) have been made on the low-spin species, we can make
full assignments for the high-spin species from the informa-
tion provided by chemical exchange between the low- and
high-spin species30,58 by saturation transfer experiments. As
reported previously,30,58 the method is to make a mixture of
low-spin and high-spin complexes by adding less axial ligand
than necessary for full conversion of all of the high-spin
complex to low-spin, 2 equiv if the equilibrium constant for
ligand binding is very large. As a result, only part of the
(TPC)FeCl will be converted to the low-spin species (e.g.,
if (TPC)FeCl/axial ligand ≈ 1:1, at best the ratio of
concentrations of (TPC)FeCl/[(TPC)Fe(L)2]+ ≈ 1:1). For-
The axial ligand used for saturation transfer studies cannot
bind too strongly (as does imidazole) or too weakly (as does
4-CNPy), because the chemical exchange between high- and
low-spin complexes either will not be observed or will lead
to severe broadening of the low-spin complex resonances,
respectively, in these cases. We found that 4-Me2NPy is an
appropriate ligand for the saturation transfer experiments;
pyridine would also work for this purpose. Figure 9B shows
the control and all difference spectra from saturation transfer
experiments on the mixture of (TPC)FeCl and [(TPC)Fe(4-
Me2NPy)2]Cl. The assignments of the three pyrrole-H peaks
are given on the control spectrum.
In Figure 9Bb, besides the peak irradiated (pyrrole-7,18)
and the corresponding pyrrole-7,18 proton peak of the low-
spin complex, there is another negative peak observed, which
is at the position of the o-H of the bound axial ligand 4-Me2-
Inorganic Chemistry, Vol. 45, No. 9, 2006 3527

Cai et al.
NPy. Actually, if the pyridine o-H resonance of the complex
is irradiated, the difference spectrum also contains a negative
peak appearing at the position of the pyrrole-7,18 protons
of (TPC)FeCl. These signals cannot arise from chemical
exchange nor from an NOE (the NOE difference signal
usually has the same sign as the irradiated peak at temper-
atures above -50 °C for intermediate-sized molecules such
as (TPC)FeCl, and its magnitude is much smaller). In fact,
even when we used pure (TPC)FeCl and irradiated at the
blank position of the chemical shift of the o-H peak of the
complex (-16 ppm), where there was then no peak, we still
obtained a negative peak at 84 ppm (the chemical shift of
the pyrrole-7,18-H). This peak disappeared when the same
experiment was carried out on the DRX-500, and we have
shown conclusively that it is an instrumental artifact of the
Unity-300.59 Thus the extra negative-phase peak in saturation
transfer difference spectrum b of Figure 9B located at -16
ppm is an artifact.
supported by the electron nuclear double resonance (EN-
DOR) and electron spin echo envelope modulation (ESEEM)
study42 on the bisimidazole complexes of (TPC)FeCl and
(TPP)FeCl. In that work it was found that both [(TPP)Fe-
(HIm)2]Cl and [(TPC)Fe(HIm)2]Cl have a (dxy)2(dxzdyz)3
ground state, even though this disagrees with the definitions
of Taylor based upon g values.42,62 The spin delocalization
pattern obtained in those systems is also consistent with the
1H chemical shifts found in this work.
The decrease in the magnitude of the chemical shifts from
[(TPC)Fe(Im-d4)2]Cl to [(TPC)Fe(2-MeHIm)2]Cl might be
explained on the basis of the expected trend of the stability
of the metal orbitals according to the donor abilities of the
axial ligands. The observation that the magnitude of the
chemical shifts is smaller in this case is consistent with
2-MeHIm being a stronger donor than HIm or Im-d4. A
stronger σ donor destabilizes the metal and places more metal
d character and less chlorin character in the SOMO, as
illustrated by comparison of the spin densities of the stronger
ligand donor case, Figure 3A, with the weaker σ donor case,
Figure 3B. However, although a stronger σ donor toward
the proton, 2-MeHIm is “hindered” in binding to iron
porphyrinates, as is evidenced by the slightly longer Fe-
Nax bonds found for [TPPFe(2-MeHIm)2]+ (2.015(4), 2.010-
(4) Å),63 [TMPFe(1,2-Me2Im)2]+ (2.004(5), 2.004(5) Å),64
[OMTPPFe(2-MeHIm)2]+ (2.007(7), 2.010(7) Å in one
structure, 2.006(5), 2.032(5) Å in another),65 and [OETPPFe-
(2-MeHIm)2]+ (2.09(2), 2.09(2) Å)66 as compared to those
for “nonhindered” imidazole complexes of iron porphyrinates
such as [TMPFe(1-MeIm)2]+ (1.975(3) Å for both ligands
in one structure and 1.965(3) Å for both in the other),67
parallel-[TMPFe(5-MeHIm)2]+ (1.978(6), 1.961(5) Å in one
molecule, 1.980(5), 1.985(5) Å in the other),68 and perpen-
dicular-[TMPFe(5-MeHIm)2]+ (1.957(6), 1.973(6) Å),68 an
average of 0.037 Å longer bond for the 2-MeHIm ligand.
While only a small difference, it may be sufficient to weaken
the σ-donor strength of 2-MeHIm toward Fe(III) somewhat
when the metal is in a highly aromatic macrocyclic ring such
as a chlorin or a porphyrin where the Fe-Nax bond lengths
must be increased to accommodate the steric requirements
of the 2-MeHIm ligands, such that 2-MeHIm is only a
slightly stronger σ donor than Im-d4.
Discussion
Electronic Structure of the Low-Spin (TPC)Fe(III)
Complexes. The large negative chemical shifts of the
pyrrole-8,17-H and the large positive chemical shifts of the
pyrroline-H of [(TPC)Fe(Im-d4)2]Cl indicate that there is an
unpaired electron in the predominantly dyz orbital with partial
delocalization into the appropriate π orbitals of the chlorin
ring. In this discussion we define a coordinate system
centered at the metal atom, with the z axis perpendicular to
the mean plane of the chlorin ring and the x axis coinciding
with the approximate C2 axis of the molecule that runs from
the metal to bisect the two pyrroline carbon atoms (Chart
1). As found for the pyrrole protons of low-spin [(TPP)Fe-
(HIm)2]C1,60,61 the chemical shifts of the pyrrole proton
resonances are dominated by the contact interaction. This is
supported by the ADF calculations reported previously30 and
shown in Figure 3, where it is seen that the overlap between
the 3e(π)-like and 1a1u(π)-like orbitals of the chlorin ring
with the dyz orbital of the iron center30 leads to large spin
density on the 8,17-pyrrole and the pyrroline carbons and
smaller spin densities at the pyrrole-7,18 and pyrrole-12,13
positions. The spin densities at the meso-carbons are also
very small, consistent with the NMR results (the closely
spaced phenyl-H resonances between 6 and 7.7 ppm, Figure
2). Figure 3B shows the spin density distribution calculated
for metal orbitals that are relatively more stable than those
in Figure 3A. Note that the 7,18-carbon positions have
negatiVe spin density, while the 8,17- and 12,13-carbon
positions have more positive spin density in comparison to
Figure 3A. A relative stability of the metal orbitals inter-
mediate between that obtained in Figure 3A and that obtained
in Figure 3B reproduces the order of the chemical shifts of
the 8,17-pyrrole, 12,13-pyrrole, and 7,18-pyrrole positions
of (TPC)Fe(III) with imidazole ligands.
The pyridines present a somewhat different situation from
the imidazoles. The intermediate magnitude of the chemical
shifts of [(TPC)Fe(4-Me2NPy)2]Cl compared to [(TPC)Fe-
(Im-d4)2]Cl and [(TPC)Fe(2-MeHIm)2]Cl does not follow
(62) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.
(63) Scheidt, W. R.; Kirner, J. F.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963-1968.
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Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
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2001, 123, 6564-6578.
The ground state and peak assignments of [(TPC)Fe(Im-
d4)2]Cl and [(TPC)Fe(2-MeHIm)2]Cl made here are also
(67) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 5497-5510.
(60) Satterlee, J. D.; La Mar, G. N. J. Am. Chem. Soc. 1976, 98, 2804-
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(61) La Mar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1782-
1790.
3528 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
from the expected greater donor strength of 4-Me2NPy
compared to the imidazoles, and the trend of decreasing
magnitude of the chemical shifts with decreasing donor
strength of the substituted pyridines is opposite to the trend
for the substituted imidazoles, if only σ basicity is considered.
Considering first the donor strength of 4-Me2NPy as
compared to the two imidazoles of this study, four different
sets of physical data indicate that 4-Me2NPy is the stronger
donor: (1) the pKa of the conjugate acid of the ligand, pKa-
(BH+), for which 4-Me2NPy is the most basic; pKa(BH+) )
9.7, 6.65, and 7.56,69 respectively, for 4-Me2NPy, HIm, and
2-MeHIm;70,71 (2) the equilibrium constant for binding of
two ligands to a given Fe(III) porphyrinate in a common
ring that alters the electronic interactions and energies. These
three factors may not be independent. With regard to
π-acceptor abilities, photoelectron spectroscopy studies in
our laboratories of CpMn(CO)2L complexes, where L is a
variety of substituted pyridines, show that the π-acceptor
abilities of a series of pyridines have a significant effect in
stabilizing the metal orbitals.76 The π-acceptor abilities of
pyridines and imidazoles have been mentioned in other
contexts also,77-91 but there is no clear evidence of the
relative π-acceptor abilities of these ligands to stabilize metal
orbitals. The π-acceptor abilities of 4-Me2NPy and the
imidazoles with the Fe(III) center of these molecules are
expected to be much smaller than observed in the CpMn-
(CO)2L molecules and probably insufficient to explain the
trends, although further investigation is needed.
As for geometry changes of the chlorin ring, the energy
required to ruffle the chlorin ring is very small (22 cm-1
calculated for (OMC)Fe(MeNC)2, where OMC ) octa-
methylchlorin).30 The low frequency of the ruffle vibrational
mode and the similar energies of the (dxy)2(dxzdyz)3 and
(dxzdyz)4(dxy)1 configurations assist vibronic coupling of these
configurations in the ground-state wave function.30 The steric
tendency of axial ligands to cause ruffling of the TPC ring
is expected to increase in the order Im-d4 < 4-Me2NPy <
2-MeHIm, since five-membered ring imidazoles have their
R-hydrogens moved considerably away from interference
with the porphyrin π system than do six-membered ring
pyridine ligands. This is the same order as the decreasing
magnitude of the chemical shifts among these three strong-
field ligands. Furthermore, the contribution of the (dxzdyz)4-
(dxy)1 electron configuration to the ground state is promoted
not only by the ruffling of the chlorin ring but also by π
stabilization of the dxzdyz orbitals relative to the dxy orbital
by the axial ligands. In previous studies of low-spin Fe(III)
tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP)
complexes33-35 with 4-CNPy ligands, the ground state of the
solvent, usually expressed as log â2III, for which 4-Me2NPy
III
has the largest values; for (TPP)Fe(III) in DMF, log â2
)
6.8,72 5.5,73 and 5.2,72 for 4-Me2NPy, HIm, and 2-MeHIm,
respectively, although it should be noted that a different anion
was used for the HIm measurement (Cl- vs ClO4 for the
-
other two, even though this should not be an important factor
in DMF solution, where the anion is already dissociated and
two DMF molecules are bound before N-donor axial ligands
are added); (3) the energies of the axial ligand σ orbitals,
for which that of 4-Me2NPy is less stable (9.1 eV ionization
energy from gas-phase photoelectron spectroscopy)74 than
those of HIm or 2-MeHIm (∼10.30 and ∼10.08 eV,
respectively);75 (4) the energies of the axial ligand π-donor
orbitals, for which the symmetric π-donor orbital of 4-Me2-
NPy is less stable in energy (7.82 eV)74 than the comparable
orbitals of HIm and 2-MeHIm (the π orbitals with large
electron density on the nitrogen that bears the lone pair ionize
in the same region as the lone pair at ∼10.30 and ∼10.08
eV, respectively).75 These four sets of data are consistent in
indicating that 4-Me2NPy is both a stronger σ and a stronger
π donor than either of the two imidazoles, in contrast to the
1
intermediate magnitude of the H NMR chemical shifts of
the 4-Me2NPy complex relative to the two imidazole
complexes.
(76) Weisser, J. T.; Gruhn, N. E.; Walker, F. A.; Lichtenberger, D. L.
Abstracts of Papers, 221st National Meeting of the American Chemical
Society, San Diego, CA, April 1-5, 2001; American Chemical
Society: Washington, DC, 2001; INOR-508.
The lack of correlation between the σ or π donor strengths
of these ligands and the magnitudes of the chemical shifts
may suggest that (a) 4-Me2NPy has greater π-acceptor ability
than the imidazoles to stabilize the metal orbitals, which
counters the effect of the σ-donor ability, that (b) the π-donor
ability of 4-Me2NPy is greater than that of either of the
imidazoles, thus increasing the spin delocalization to the
pyridine ligand through ligand to metal π donation at the
expense of spin delocalization to the chlorin ring, or that (c)
the pyridines induce a slight geometry change in the chlorin
(77) Nelson, S. M.; Shepherd, T. M. Inorg. Chem. 1965, 4, 813-817.
(78) Herberhold, M.; Alt, H. Justus Liebigs Ann. Chem. 1976, 292-299.
(79) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 4021-4023.
(80) Ries, W.; Bernal, I.; Quast, M.; Albright, T. A. Inorg. Chim. Acta
1984, 83, 5-15.
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1984, 14, 339-353.
(82) Shepherd, R. E.; Proctor, A.; Henderson, W. W.; Myser, T. K. Inorg.
Chem. 1987, 26, 2440-2444.
(83) Aroney, M. J.; Clarkson, R. M.; Hambley, T. W.; Pierens, R. K. J.
Organomet. Chem. 1992, 426, 331-342.
(84) Schoenherr, T.; Degen, J. Z. Naturforsch., A: Phys. Sci. 1990, 45,
161-168.
(69) The two imidazole pKa(BH+) values have been corrected for the
statistical factor that results from the two protons present in the
protonated form.
(85) Clarke, M. J.; Bailey, V.; Doan, P.; Hiller, C.; LaChance-Galang, K.
J.; Daghlian, H.; Mandal, S.; Bastos, C. M.; Lang, D. Inorg. Chem.
1996, 35, 4896-4903.
(70) Albert, A. In Physical Methods in Heterocyclic Chemistry; Katritzky,
A. R., Ed.; Academic Press: New York, 1971; Vol. I, pp 1-108.
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A. R., Ed.; Academic Press: New York, 1971; Vol. III, pp 1-26.
(72) Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.;
Walker, F. A. Inorg. Chem. 1996, 35, 5188-5200.
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5552-5560.
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Daff, S.; Chapman, S. K. J. Biol. Chem. 2004, 279, 48876-48882.
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(75) Ramsey, B. G. J. Org. Chem. 1979, 44, 2093-2097.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3529

Cai et al.
complexes was found to be (dxzdyz)4(dxy)1, as shown for the
chlorin complex in Figure 3C for the ruffled geometry. In
this case, the spin density at the pyrrole positions is much
smaller than expected for the (dxy)2(dxzdyz)3 configuration. The
trends for the pyrrole-H chemical shifts of the substituted
pyridine complexes in this study are consistent with increas-
ing contribution of the spin density represented by the
(dxzdyz)4(dxy)1 configuration, as shown in Figure 3C, in the
ground state as the σ-donor ability of the pyridine decreases
and the π-acceptor ability correspondingly increases.
sponding [(OEC)Fe(L)2]+ complexes, where the pyrrole-CH2
chemical shifts increase in the order Im-d4 < 4-Me2NPy <
Py < 4-CNPy and the pyrroline-H chemical shifts increase
in the same order.30 In the case of the octaethylchlorin
complexes of pyridine and 4-cyanopyridine, it was found in
that study that the spin state over the NMR temperature range
was not S ) /2, but rather either S ) /2 or S ) /2. 30 That
alkyl-substituted porphyrin complexes of lower-basicity
pyridine ligands are often not low-spin at either ambient or
low temperatures has been shown previously.36,92-95 How-
ever, all of the TPC complexes of this study and those
1
3
5
For [(TPC)Fe(4-CNPy)2]+, which is the weakest σ donor
and the strongest π acceptor of the ligands in this study, the
chemical shifts indicate that the ground-state wave function
is now dominated by the spin density distribution in Figure
3C, with the (dxzdyz)4(dxy)1 configuration and the ruffled
geometry. The most negative pyrrole-H resonance (also the
8,17-H) at -40 °C is at -4.5 ppm, whereas it is at -46.5
ppm for [(TPC)Fe(Im-d4)2]Cl at the same temperature. There
are two additional differences between the 1H NMR spectra
of these two complexes: (i) while the six expected phenyl-H
resonances of [(TPC)Fe(Im-d4)2]+ are clustered between 6
and 7.7 ppm, those of [(TPC)Fe(4-CNPy)2]+ are well-
resolved, with the two types of m-H (13.2, 12.2 ppm) having
more positive chemical shifts than those of the o-H (3.0,
2.1 ppm) and p-H (4.9, 4.8 ppm), which thus yields δm -
δo and δm - δp chemical shift differences that are positive;
and (ii) while the pyrroline-H of [(TPC)Fe(Im-d4)2]+ have a
very positive chemical shift (+44.9 ppm at -40 °C), those
of [(TPC)Fe(4-CNPy)2]+ have a negative chemical shift
(-6.5 ppm at -40 °C). A negative chemical shift for the
pyrroline-H is opposite the expected sign of the contact shift
for the protons attached to an aliphatic carbon that is attached
to a carbon that is part of the π system of the macrocycle.47,48
With regard to (i), the positive chemical shift differences
δm - δo and δm - δp indicate that there is significant positive
spin density at the meso-carbons,41 as is consistent with spin
delocalization from the metal dxy orbital to the 3a2u(π)-like
orbital of a ruffled TPC ring, Figure 3C. And with regard to
(ii), the negative chemical shift of the pyrroline-H of [(TPC)-
Fe(4-CNPy)2]+ indicates negatiVe spin density at the R-car-
bons of the pyrroline ring.30,48 As shown in Figure 3C, ADF
calculations have shown that for a ruffled macrocycle
conformation, there is negative spin density at the R-carbons
of the pyrroline ring.30 The magnitude of this negative spin
density is smaller than that reported for [(TPC)Fe(t-
BuNC)2]+, which has a chemical shift of -36 ppm for the
pyrroline-H at +25 °C or about -42 ppm at -40 °C.26 Thus,
the “purity” of the (dxz,dyz)4(dxy)1 ground state of [(TPC)Fe-
(4-CNPy)2]+ is considerably less than that of [(TPC)Fe(t-
BuNC)2]+. This is also consistent with the EPR spectra of
the two, very broad for [(TPC)Fe(4-CNPy)2]+, with g ≈ 2.68,
2.36, 1.7-1.6 or smaller (Figure 7), but very sharp for
[(TPC)Fe(t-BuNC)2]+, with g ) 2.15 and g| ) 1.97. 26
It should be noted that the trends observed in this work
for the magnitudes of the chemical shifts of the pyrroline-
and pyrrole-H of the [(TPC)Fe(L)2]+ complexes as a function
of ligand L (Im-d4 > 4-Me2NPy > 2-MeHIm > Py >
4-CNPy) are opposite those observed earlier for the corre-
1
reported previously25-28 have been found to have S ) /2
over the temperature range of the NMR measurements, albeit
with a thermally accessible excited state of S ) /2 for the
complexes of this study and probably the others as well.
3
Electronic Structure of High-Spin (TPC)FeCl. (TPC)-
FeCl and (TPP)FeCl are similar in electronic structure. In
both, the chemical shifts of the pyrrole protons are dominated
by the (positive) contact shifts that result from σ-spin
delocalization, with a smaller contact shift contribution
(negative) arising from π-spin delocalization. The average
chemical shift of the three pyrrole-H resonances of high-
spin (TPC)FeCl (+73 ppm) is fairly similar to the chemical
shift of the (TPP)FeCl pyrrole-H (+79 ppm). This suggests
that the amount of spin density delocalized from the iron
center to the chlorin ring is nearly the same as that
delocalized to the porphyrin ring in (TPP)FeCl but that it is
distributed quite asymmetrically among the three pyrrole
positions of the chlorin ring.16 The order of the three
pyrrole-H chemical shifts of (TPC)FeCl is 7,18 > 12,13 >
8,17, with the pyrrole-7,18 the most positive. This is the same
as the order of the three pyrrole-H chemical shifts of [(TPC)-
Fe(Im-d4)2]Cl, with pyrrole-8,17 the most negative. This
indicates that the π spin delocalization mechanism is the
same for [(TPC)Fe(Im-d4)2]Cl and (TPC)FeCl, since the
2
2
σ-delocalization contribution that results from the dx -y
electron should be equal for all pyrrole-carbons. From the
order of the pyrrole-H chemical shifts we can conclude that
high-spin (TPC)FeCl also has larger π spin density on the
pyrrole-8,17-carbons, leading to larger (in magnitude) nega-
tive π contact shift and thus smaller overall chemical shift.
For the pyrrole-7,18-H, the observed chemical shift is the
largest of the three, and thus the π spin density and the
contact shift (in magnitude) from this unpaired π spin are
the smallest of the three pyrrole-H resonances. As in [(TPC)-
Fe(Im-d4)2]Cl, it is also the 3e(π)-like orbital that is involved
in the interaction with the appropriate dπ orbital of the metal
center and thus contributes to the π spin distribution.
The small negative chemical shift of the pyrroline-H is
probably completely a result of the pseudocontact contribu-
tion to the paramagnetic shift, which is expected to be
negative,46-49 indicating essentially zero contact shift for
(92) Scheidt, W. R.; Geiger, D. K.; Hayes, R. G.; Lang, G. J. Am. Chem.
Soc. 1983, 105, 2625-2632.
(93) Kintner, E. T.; Dawson, J. H. Inorg. Chem. 1991, 30, 4892-4897.
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(95) Geiger, D. K.; Scheidt, W. R. Inorg. Chem. 1984, 23, 1970.
3530 Inorganic Chemistry, Vol. 45, No. 9, 2006

Studies of Chloroiron(III) Tetraphenylchlorin Complexes
these protons. This suggests essentially zero π spin density
at the pyrroline R-carbons of (TPC)FeCl.
(dxy)2(dxzdyz)3 to (dxzdyz)4(dxy)1 with weakening of the σ- and
π-donor ability and strengthening of the π-acceptor ability
of the ligands. The 4-CNPy complex, which represents the
weakest σ donor and strongest π acceptor, is believed to be
ruffled and thus has negative spin density at the R-pyrroline
carbons, because of mixing of the a2u-like π orbital of the
chlorin in the (dxzdyz)4(dxy)1 configuration.
Conclusions
The electronic structure of the low-spin [(TPC)Fe(L)2]+
complexes with L ) imidazoles is predominantly the (dxy)2-
(dxzdyz)3 configuration, and the extent of spin delocalization
to the TPC ring depends mostly on the σ- and π-donor ability
of the axial ligand. This is a result of σ- and π-donor effects
that modulate the d-orbital energies of the metal, such that
4-Me2NPy and the nonhindered imidazole that are strong
donors increase the metal character and decrease the ligand
character of the spin density. Steric effects (most important
for 2-MeHIm, somewhat important for the pyridines, and
relatively unimportant for HIm or Im-d4) and distortions of
the TPC ring may also play a role. The chemical shifts of
the pyrroline and pyrrole-8,17-H resonances of these com-
plexes are dominated by the contact shifts from π spin
delocalization which arises from the interaction between the
3e(π)-like orbital of the chlorin ring and the appropriate dπ
orbital of the metal center. For L ) lower-basicity pyridines
the ground state of the metal center tends to change from
(TPC)FeCl is a high-spin Fe(III) complex. The chemical
shifts of the pyrrole-H are dominated by the contact
contribution from σ spin delocalization. The 3e(π)-like
orbitals of the chlorin ring are involved in π interaction with
the iron center.
Acknowledgment. The National Institutes of Health,
Grant DK-31038 (F.A.W.), and National Science Foundation,
Grant CHE0416004 (D.L.L.) are gratefully acknowledged.
Supporting Information Available: Figures S1-S12 showing
COSY, HMQC, NOESY, and saturation transfer difference NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0515352
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