NMR and EPR studies of low-spin Fe(III) complexes of meso-tetra-(2,6- disubstituted phenyl)porphyrinates complexed to imidazoles and pyridines of widely differing basicities

Inorg. Chem. 2005, 44, 7468−7484

NMR and EPR Studies of Low-Spin Fe(III) Complexes of

meso-Tetra-(2,6-Disubstituted Phenyl)Porphyrinates Complexed to

Imidazoles and Pyridines of Widely Differing Basicities

C. Todd Watson,^†Sheng Cai,^‡Nikolai V. Shokhirev, and F. Ann Walker*

Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721-0041

Received May 10, 2005

A series of bis-axially ligated complexes of iron(III) tetramesitylporphyrin, TMPFe(III), tetra-(2,6-dibromophenyl)-

porphyrin, (2,6-Br₂)₄TPPFe(III), tetra-(2,6-dichlorophenyl)porphyrin, (2,6-Cl₂)₄TPPFe(III), tetra-(2,6-difluorophenyl)-

porphyrin, (2,6-F₂)₄TPPFe(III), and tetra-(2,6-dimethoxyphenyl)porphyrin, (2,6-(OMe)₂)₄TPPFe(III), where the axial

ligands are 1-methylimidazole, 2-methylimidazole, and a series of nine substituted pyridines ranging in basicity

from 4-(dimethylamino)pyridine (pK_a(PyH⁺)

) ) 1.45 and 1.1,

9.70) to 3- and 4-cyanopyridine (pK_a(PyH⁺)

respectively), have been prepared and characterized by EPR and ¹H NMR spectroscopy. The EPR spectra, recorded

at 4.2 K, show “large g_max”, rhombic, or axial signals, depending on the iron porphyrinate and axial ligand, with the

g_maxvalue decreasing as the basicity of the pyridine decreases, thus indicating a change in electron configuration

from (d_xy)²(d_xz,d_yz)³to (d_xz,d_yz)⁴(d_xy)¹through each series at this low temperature. Over the temperature range of the

NMR investigations (183

−

313 K), most of the high-basicity pyridine complexes of all five iron(III) porphyrinates

-pyrrole spin densities,

−0.017, that are indicative of the S )

1/2 (d_xy)²(d_xz,d_yz)³electron configuration, while the temperature

exhibit simple Curie temperature dependence of their pyrrole-H paramagnetic shifts and

â

F_C≈ 0.015

dependences of the pyrrole-H resonances of the lower-basicity pyridine complexes (pK_a(PyH⁺) < 6.00) show significant

deviations from simple Curie behavior which could be fit to an expanded version of the Curie law using a temperature-

dependent fitting program developed in this laboratory that includes consideration of a thermally accessible excited

state. In most cases, the ground state of the lower-basicity pyridine complexes is an S

) 1/2 state with a mixed

(d_xy)²(d_xz,d_yz)³/(d_xz,d_yz)⁴(d_xy)¹electron configuration, indicating that these two are so close in energy that they cannot

be separated by analysis of the NMR shifts; however, for the TMPFe(III) complexes with 3- and 4-CNPy, the

ground states were found to be fairly pure (d_xz,d_yz)⁴(d_xy)¹electron configurations. In all but one case of the intermediate-

to low-basicity pyridine complexes of the five iron(III) porphyrinates, the excited state is found to be S

)

3/2, with

3

1

a (d_xz,d_yz) (d_xy) (d_z) electron configuration, lying some 120

−

680 cm^-higher in energy, depending on the particular

2

porphyrinate and axial ligand. Full analysis of the paramagnetic shifts to allow separation of the contact and

pseudocontact contributions could be achieved only for the [TMPFe(L)₂]⁺series of complexes.

Introduction

heme proteins.^1-11For this reason, a detailed investigation

of their NMR and frozen solution EPR spectra is highly

desirable. While EPR spectroscopy is an excellent technique

Synthetic ferriheme complexes of various types have been

shown to be very promising models of the heme centers in

the mitochondrial cytochrome bc₁complex and other related

(1) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.

Chem. Soc. 1986, 108, 5288-5297.

(2) Walker, F. A.; Benson, M. J. Phys. Chem. Soc. 1982, 88, 3495-3499.

(3) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem.

Soc. 1991, 113, 5497-5510.

(4) 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.

(5) 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.

* To whom correspondence should be addressed. Phone: 520-621-8645.

Fax: 520-626-9300. E-mail: awalker@u.arizona.edu.

^†Current address: Department of Chemistry, MS324, Unversity of

Arkansas, 5210 Grand Ave., Fort Smith, AR 72913-3649. E-mail:

cwatson@uafortsmith.edu.

^‡Current address: Division of Immunology, City of Hope National

Medical Center, 1500 E Duarte Rd., Duarte, CA 91010. E-mail: SCai@

coh.org.

7468 Inorganic Chemistry, Vol. 44, No. 21, 2005

10.1021/ic0507316 CCC: $30.25

Published on Web 09/03/2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

for characterizing the electronic ground state of ferriheme

complexes at 4.2 K, as we have shown in studies of the

molecular structures and EPR spectra of a number of

porphyrinate complexes of Fe(III) in the solid state,^1,3-5,9,11

NMR spectroscopy is extremely useful for investigating the

ambient-temperature solution structures and spin states of a

wide range of metalloporphyrins.^6-8,12-15As 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.^8,12-15This is because of the

temperature dependence of the paramagnetic contribution to

the chemical shift of a given proton, also known as the

hyperfine or isotropic shift, as shown below.

The chemical shifts of protons in the NMR spectra of

paramagnetic molecules are the sum of two contributions,

the first of which is the diamagnetic shift, the chemical shift

of the protons of interest in the absence of unpaired electrons

in the molecule. An accurate approximation of the diamag-

netic shift can be obtained from a diamagnetic compound

that is similar to the paramagnetic one being studied,⁶herein

the Co(III) analogues of the Fe(III) complexes of this study.

The second contribution to the observed chemical shift is

the paramagnetic shift, which is also known as the isotropic

or hyperfine shift^6,16-19

the delocalization of the unpaired electron through

bonds^6,16-19

δ_con) A g âS(S + 1)µ₀/12πγ_Nhk_BT

(4)

In this equation, A is the electron-nuclear hyperfine coupling

constant, g is the average g value, S is the total spin of the

metal (which is 1/2 for the systems of interest in this work),

â is the Bohr magneton (9.2741 × 10^-28J G^-1), h is Planck’s

constant divided by 2π (1.6784 × 10^-35J s), k_Bis the

Boltzmann constant (1.3806 × 10^-23J K^-1), µ₀is the

permittivity of a vacuum, and γ_Nis the magnetogyric ratio

of the NMR nucleus. If the delocalization occurs through σ

bonds the sign of the contact shift is positive.²⁰This results

in a positive shift of the resonance.^6,16If the delocalization

occurs through π bonds, the sign of the contact shift for a

proton attached to a carbon that is part of the π system is

negative^6,16-22and the observed shift is negative.^6,16-19

The other contribution to the paramagnetic shift of eq 3

is the pseudocontact shift. This results from the through-

space interaction between the unpaired electron at the metal

center and the nucleus being studied and may be ap-

proximated as the following if the second-order Zeeman

contribution to the magnetic susceptibilities is small and the

ground state is well-separated from any excited states^6,23

2

δ_pc) [â²S(S + 1)µ₀/72πk_BT]{[2g_zz

-

δ_obs) δ_dia+ δ_para

δ_para) δ_iso) δ_hf

(1)

(2)

(g_xx²+ g_yy²)](3cos²θ - 1)/r³+

3(g_xx²- g_yy²)(sin²θcos2Ω)/r³} (5)

The paramagnetic shift can be subdivided into two parts,

the contact and electron-nuclear dipolar (pseudocontact)

contributions^6,16-19

For systems with axial symmetry or 4-fold-symmetric

macrocyclic complexes with rapidly rotating axial ligands,

the last term of eq 5, the rhombic part, averages to zero,

leaving^6,16-19

δ_para) δ_con+ δ_pc

(3)

The contact term represents the paramagnetic shift of

the resonance from the diamagnetic position caused by

δ_pc) [â²S(S + 1)µ₀/72πk_BT][2g_zz

-

2

(g_xx²+ g_yy²)](3cos²θ - 1)/r³(6)

(6) Walker, F. A. Proton NMR and EPR Spectroscopy of Paramagnetic

Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith,

K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.

5, Chapter 36, pp 81-183.

In eqs 5 and 6, g_zz, g_yy, and g_xxare the principal g values

of the metal complex, where g_zzis the g value along the

molecular z axis, which is aligned with the heme normal, r

is the distance from the metal center to the proton of interest,

and θ is the angle between a line along the magnetic z axis,

usually taken to be the molecular z axis, and a line running

through the metal center and the position of interest. Goff

has shown²⁴that for bis-imidazole complexes of iron(III)

(7) La Mar, G. N.; Bold, T. J.; Satterlee, J. D. Biochim. Biophys. Acta

1977, 498, 189-207.

(8) Basu, P.; Shokhirev, N. V.; Enemark, J. H.; Walker, F. A. J. Am.

Chem. Soc. 1995, 117, 9042-9055.

(9) Yatsunyk, L. A.; Carducci, M. D.; Walker, F. A. J. Am. Chem. Soc.

2003, 125, 15986-16005.

(10) Walker, F. A. Coord. Chem. ReV. 1999, 185-186, 471-534.

(11) Walker, F. A. Chem. ReV. 2004, 104, 589-615.

(12) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-

17804.

2

porphyrinates the quantity [2g_zz- (g_xx+ g_yy²)] is very

similar whether calculated from EPR g values or from NMR

data, suggesting that the second-order Zeeman term²³is

probably quite small for these bis-imidazole complexes.

However, we found, in the work discussed below, that the

(13) 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.

(14) Banci, L.; Bertini, I.; Luchinat, C.; Pierattelli, R.; Shokhirev, N. V.;

Walker, F. A. J. Am. Chem. Soc. 1998, 120, 8472-8479.

(15) Yatsunyk, L. A.; Shokhirev, N. V.; Walker, F. A. Inorg. Chem. 2005,

44, 2848-2866.

(16) La Mar, G. N.; Walker, F. A. NMR Studies of Paramagnetic

Metalloporphyrins. In The Porphyrins, Vol. IV; Dolphin, D., Ed.;

Academic Press: New York, 1979; pp 61-157.

(17) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of Paramagnetic

Molecules. Applications to Metallobiomolecules and Models; Elsevi-

er: New York, 2001.

(20) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance;

Harper and Row: New York, 1967; p 80.

(21) McLachlan, A. D. Mol. Phys. 1958, 1, 233.

(22) Chestnut, D. B. J. Chem. Phys. 1958, 29, 43-47.

(23) Horrocks, W. D.; Greenberg, E. S. Mol. Phys. 1974, 27, 993-999.

(24) Goff, H. M. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron

Porphyrins, Part One; Lever, A. B. P., Gray, H. B., Eds; Addison-

Wesley: Reading, MA, 1983; p 249.

(18) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544.

(19) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286-301.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7469

Watson et al.

Scheme 1

EPR g values appear to overestimate the magnitude of the

ambient temperature pseudocontact shifts of the bis-pyridine

complexes of the iron porphyrinates of this study.

As is evident from eqs 4-6, both the contact and

pseudocontact shifts usually have inverse temperature de-

pendences resulting from the Curie law^6,16-19,25

δ_para) C/T

(7)

where C is the Curie coefficient.

Because the contact term dominates the paramagnetic

shifts of all spin states of Fe(III), a nearly linear 1/T

dependence is generally observed even for the S ) 5/2

Fe(III) complexes,¹⁹which have the largest zero-field split-

ting constants for this metal and oxidation state.¹⁶However,

as we have shown previously,^12-15,26-28a number of model

heme complexes have a thermally-accessible excited state

that causes at least some curvature of the Curie plot^{8,12,14,15,27,28}

and can sometimes show extremely curved chemical-shift

dependence.^13,15,26Expansion 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/T²contribution to the high-spin state is neglected^12-16,19

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).^15,26,27

Earlier, many of the same bis-ligand complexes of

iron(III) tetramesitylporphyrinate were prepared and char-

acterized by X-ray crystallography, EPR, Mo¨ssbauer,^3,4,31and

to a limited extent, NMR⁴spectroscopy, although no detailed

analysis of the temperature dependence of the NMR spectra

was made at that time. To further examine the effects of the

axial ligand basicity and the 2,6-phenyl substituents on the

orientation of planar axial ligands and NMR properties of

these supposedly “hindered” porphyrin systems, an investi-

gation of the variable-temperature ¹H NMR spectra of

TMPFe(III) and a series of 2,6-disubstituted tetraphenylpor-

phyrinatoiron(III) with axial ligands were undertaken in this

work. The Mo¨ssbauer spectra of several [((2,6-OCH₃)₂)₄-

TPPFe(L)₂]⁺, [TPPFe(L)₂]⁺, and [TPCFe(L)₂]⁺complexes

that show differences in quadrupole splittings and hyperfine

coupling constants as a function of axial imidazole and

pyridine ligands have also been reported.³²All complexes

are low-spin d⁵, S ) 1/2. However, as shown previously

for several [TMPFe(L)₂]⁺complexes,⁴there are two electron

configurations possible for low-spin Fe(III), the (d_xy)²-

(d_xz,d_yz)³and (d_xz,d_yz)⁴(d_xy)¹configurations, as shown in

Scheme 1. The electron configuration of a given complex

was found to depend on the nature of the porphyrin substitu-

ents and, within a given set of porphyrin substituents, upon

the σ-donor and π-donor/acceptor properties of the axial lig-

ands. The σ-donor strengths, within a series of closely related

ligands such as substituted pyridines of varying basicities

toward the proton, are usually measured by the pK_avalues

of the conjugate acid of the particular pyridine, here abbrev-

iated as pK_a(PyH⁺), or by the gas-phase lone-pair vertical

ionization potentials of the substituted pyridines, which are

in general linearly related to the pK_a(PyH⁺) values.³³

As shown previously for several bis-pyridine complexes

of TMPFe(III), as the basicity of the pyridine decreases from

δ_para) (1/T)[W₁C_n1+ W₂C_n2e^-∆E/kT]/[W₁+ W₂e^-∆E/kT

]

(8)

In this equation, C_n1is the coefficient for the position of

interest for level 1, C_n2is the coefficient for the position of

interest for level 2, W₁and W₂are the statistical weights for

each level () 2S + 1 for each), and ∆E is the energy

separation between the two levels. The coefficients C_n1and

C_n2are approximately equal to the Curie coefficients of each

level, except for the contribution from the pseudocontact

contribution to δ_para, and can be determined by fitting the

temperature dependence of the proton chemical shifts,

assuming that the diamagnetic shift of each proton type is

known. Each of these coefficients can be further decomposed

into the McConnell constant Q_Cand the variable F_C, where

Q_C) -496.8 ppm K for aromatic carbons,^16,20-22and F_Cis

the spin density at the carbon of interest; in general F_Cis

not larger than about 0.02 for the systems of this study. A

program that carries out this fitting procedure, with least-

squares minimization of the errors between the experimental

and calculated shifts, has been created in our laboratory and

is available on the Internet.²⁹

The bis-ligand (imidazoles and pyridines of widely dif-

fering basicities) complexes of several iron(III) octaalkyl-

tetraphenylporphyrinates have been prepared and character-

ized by X-ray crystallography,^9,26-28EPR,^9,14,26-28

NMR,^15,26-28and in some cases, Mo¨ssbauer spectroscopy.³⁰

The NMR investigations of these complexes showed that

(30) Teschner, T.; Yatsunyk, L. A.; Schu¨nemann, V.; Hu, C.; Scheidt, W.

R.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. , Submitted

for publication, 2005.

(25) This is true except in the cases where S > 1/2 and there is a relatively

large zero-field splitting, in which case the pseudocontact term has a

C′/T²dependence and the contact term has a C/T dependence.^6,16-19

(26) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757-777.

(27) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 4341-4352.

(28) Yatsunyk, L. A.; Walker, F. A. J. Porphyrins Phthalocyanines 2005,

9, 214-228.

(31) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;

Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.

(32) Benda, R.; Schu¨nemann, V.; Trautwein, A. X.; Cai, S.; Polam, J. R.;

Watson, C. T.; Shokhireva, T. Kh.; Walker, F. A. J. Biol. Inorg. Chem.

2003, 8, 787-801.

(29) Shokhirev, N. V.; Walker, F. A. http://www.shokhirev.com/nikolai/

programs/prgsciedu.html.

(33) Ramsey, B. G.; Walker, F. A. J. Am. Chem. Soc. 1974, 96, 3314-

3316.

7470 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

scribed previously.^13,35The bis-pyridine and bis-imidazole com-

plexes were produced as follows: Roughly 5 mg of the desired

iron(III) porphyrin perchlorate and 0.35 mL of CD₂Cl₂were added

to a 5 mm NMR tube. Several milligrams of the desired axial ligand

were added to this solution. The ¹H NMR spectrum was then

recorded. At this point, more ligand was titrated into the NMR tube

until the 1D NMR spectra showed free ligand resonances at -60

°C. The iron porphyrinate concentrations were 10 to 15 mM. The

dilution of these concentrations by factors of 2 to 4 caused no

changes in the chemical shifts of the protons of the bis-imidazole

or -pyridine complexes. All ligands were used as received from

Aldrich. The 99.9% CD₂Cl₂was used as received in 1 g sealed

ampules from Cambridge Isotopes.

that of 4-Me₂NPy (pK_a(PyH⁺) ) 9.70³⁴) to that of 4-CNPy

(pK_a(BH⁺) ≈ 1.1⁴), the electronic ground state of the

complex appears to shift fairly smoothly from (d_xy)²(d_xz,d_yz)³

to (d_xz,d_yz)⁴(d_xy)¹, as evidenced by trends in the size of the

largest EPR g value,^3-5the Mo¨ssbauer quadrupole splitting,^3,4

or the NMR pyrrole-H chemical shift.⁴At the time of the

earlier report,⁴we believed that there were only those two S

) 1/2 electron configurations involved, with a variable ∆E

between the two. However, the present work has shown

conclusively that the energy separation between these two

is, in most cases, extremely small, so that a mixed-

configuration S ) 1/2 ground state is present for many of

the complexes of this study, and instead, the excited state

has S ) 3/2. Two limiting types of EPR spectra, each of

which usually consists of only one resolved feature, were

observed in this study; if the single observed feature had its

maximum at g > 3.2, it was called a “large g_max” EPR

spectrum⁴because the largest g value of the normal rhombic

EPR spectra of the low-spin Fe(III) porphyrinates, in which

all three g values are readily observed, typically are not

significantly greater than 3.0. If this feature occurred at g e

2.6, it was shown to be the g of an axial EPR spectrum,

1

The H NMR spectra of [TMPFe(L)₂]ClO₄, [(2,6-Br₂)₄TPPFe-

(L)₂]ClO₄, [(2,6-Cl₂)₄TPPFe(L)₂]ClO₄, [2,6-F₂TPPFe(L)₂]ClO₄,

and [(2,6-(OMe)₂)₄TPPFe(L)₂]ClO₄, where L ) axial ligands

4-CNPy, 3-CNPy, 3-ClPy, 4-HPy, 3-MePy, 4-MePy, 3,5-Me₂Py,

3,4-Me₂Py, 4-Me₂NPy, 1-MeIm, and 2-MeHIm, were recorded on

either a General Electric GN-300 spectrometer operating at 300.100

MHz, a Bruker AM-250 spectrometer operating at 250.068 MHz,

a General Electric GN-500 spectrometer operating at 500.136 MHz,

or a Varian Unity-300 spectrometer operating at 299.952 MHz. The

number of data points was always 16 K, and the spectral width

ranged from 7.5 to 40 kHz as necessary for the particular complex

and spectrometer. The acquisition times ranged from 0.266 to 0.682

s, depending upon the relaxation times of the resonances being

studied. The number of transients collected ranged from 128 to

256, and a 5 Hz line broadening factor to the exponential

multiplication of the FID was usually applied prior to the Fourier

transformation. The spectra of the iron(III) porpyrinates were

recorded at temperatures from -90 to +40 °C in 5 or 10° steps;

the temperature was determined by calibration with the Wilmad

standard methanol variable-temperature sample.

The EPR spectra of the iron(III) porphyrinates listed above were

recorded at 4.2 K 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. The spectra

were obtained for samples in frozen CD₂Cl₂and, in some cases,

toluene solutions. Typical values for microwave power, modulation

frequency, and modulation amplitude were 0.2 mW, 100 kHz and

1.011 G, respectively. The g values obtained from these spectra

are listed in Table 1.

and in some cases, the corresponding g feature was also

||

resolved and observed at g < 2.0.⁴However, it was not

known at the time of that study that the energy separation

of the two electron configurations was so small that

intermediate- to low-basicity pyridines behave as though they

have a mixed-configuration low-spin state over the temper-

ature range of the NMR investigations (183-313 K), and

no energy separation between the two can be determined,

even though the EPR spectra measured at 4.2 K clearly show

a smooth trend.⁴In the present study, we have carefully

analyzed the temperature dependence of the ¹H NMR spec-

tra of the [TMPFe(L)₂]⁺complexes, where L ) a series of

nine pyridines of varying basicities, as well as two imida-

1

zoles, and have extended our investigation of the H NMR

and EPR spectra to include the corresponding complexes

of the following phenyl-substituted iron porphyrinates,

[(2,6-Br₂)₄TPPFe-(L)₂]⁺, [(2,6-Cl₂)₄TPPFe(L)₂]⁺, [(2,6-F₂)₄-

TPPFe(L)₂]⁺, and [(2,6-(OMe)₂)₄TPPFe(L)₂]⁺, in an attempt

to understand the composition of the paramagnetic shifts of

the porphyrin and axial ligand protons and the nature of the

excited states of these complexes. For the octaalkyltetra-

phenylporphyrinates,^15,26,27most of the intermediate- and low-

basicity pyridine complexes of the present study have S )

3/2 excited states.

The synthesis of the bis-substituted pyridine and imidazole

complexes of (2,6-Br₂)₄TPPCo(III), (2,6-Cl₂)₄TPPCo(III), (2,6-F₂)₄-

TPPCo(III), and (2,6-(OMe)₂)₄TPPCo(III) were carried out as

1

described elsewhere.³⁶The 1D H NMR spectra of these diamag-

netic compounds were recorded on a Varian Unity-300 spectrometer

operating at 299.952 MHz. The number of data points was again

16 K, and the spectral width ranged from 3.5 to 4.6 kHz. The

acquisition times ranged from 1.737 to 2.318 s. The number of

transients collected ranged from 128 to 256, and no exponential

multiplication of the FID was applied prior to the Fourier

transformation. The corresponding TMPCo(III) complexes were

synthesized, and their ¹H NMR spectra were reported elsewhere.⁴³

Experimental

TMPH₂and (2,6-Cl₂)₄TPPH₂were purchased from Mid Century

or prepared as described previously;^13,35the (2,6-X₂)₄TPPH₂free-

base porphyrins were synthesized as described previously.^13,35

TMPFeCl, (2,6-Br₂)₄TPPFeCl, (2,6-Cl₂)₄TPPFeCl, (2,6-F₂)₄TPPFeCl,

and 2,6-(OMe)₂)₄TPPFeCl and their perchlorate complexes were

synthesized from the corresponding free-base porphyrins as de-

(36) Polam, J. R.; Shokhireva, T. Kh.; Raffii, K.; Simonis, U.; Walker, F.

A. Inorg. Chim. Acta 1997, 263/1-2, 109-117.

(37) Raitsimring, A. M.; Borbat, P.; Shokhireva, T. Kh.; Walker, F. A. J.

Phys. Chem. 1996, 100, 5235-5244.

(38) Raitsimring, A. M.; Walker, F. A. J. Am. Chem. Soc. 1998, 120, 991-

1002.

(39) Schu¨nemann, V.; Raitsimring, A. M.; Benda, R.; Trautwein, A. X.;

Shokhireva, T. Kh.; Walker, F. A. J. Biol. Inorg. Chem. 1999, 4, 708-

716.

(34) Albert, A. In Physical Methods in Heterocyclic Chemistry; Katritzky,

A. R., Ed.; Academic Press: New York, 1971; Vol. I, pp 1-108.

(35) Watson, C. T. The Effect of Axial Ligands and Porphyrin Substituents

on the Electronic Ground State of Low-Spin Fe(III) Porphyrinates.

Ph.D. Dissertation, University of Arizona, Tuscon, AZ, 1996.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7471

Watson et al.

Table 1. EPR g Values of the Iron(III) Porphyrinates of This Study

porphyrinate

(2,6-Cl₂)₄TPP

ligand

pK_a(BH⁺)

TMP

3.48^a

(2,6-Br₂)₄TPP

3.59

(2,6-F₂)₄TPP

(2,6-(OMe)₂)₄TPP

4-Me₂NPy

9.70

3.49

(2.70)2.60

2.29

2.83

2.30

1.86

(1.64) 1.62

4-NH₂Py

4-MePy

9.17

6.02

3.40^b

2.70^c

2.49^c

2.89^b

2.75^c

2.56^c

3.07^b,c

2.59^c

2.64^b

2.53^b

1.54^b

3.17

3.19

2.99

3.25

3.48

3.43

1.90

3-EtPy

4-HPy

5.56

5.22

2.90^b

2.20^b

2.63

3.41

2.52

2.60

3-ClPy

2.84

3.01^b

2.57^b

2.62

2.57

3-CNPy

4-CNPy

1.45

1.1

2.62

2.51

2.60

2.58

2.57

2-MeHIm

1-MeIm

7.56

7.33

3.43

3.36

3.42

2.88

2.30

3.52

2.84

2.29

3.35

3.02

2.28

1.48

2.89^a

2.33^a

1.57^a

(1.49)^d

1.60

1.63

(1.55)^d

(1.64)^d

(1.30)^d

^aRef 3. ^bRef 4. ^cThe two numbers given indicate the range of g values encompassed by the very broad signal. ^dThe number in parentheses is the

smallest g value, calculated from ∑g²) 16.0.

Ambient-temperature ¹H NMR chemical shifts of the free-base,

the TMP complexes have been reported and discussed

previously,^3,4and it was shown that the g_maxvalues (as well

as the Mo¨ssbauer quadrupole splittings) decrease as the basic-

ity of the axial pyridine ligand decreases. However, some

chloroiron(III), and perchloratoiron(III) complexes of the five

porphyrinates under study are listed in Supporting Information

Tables S1, S2, and S3, respectively.

of the EPR spectra are extremely broad (like the one shown

in Figure 1e), and in these cases, we report a range of g val-

ues, as listed in Table 1. As is evident from the g values of

Table 1, the other porphyrinates of this study show similar

ranges of g values for the various axial ligands, except the

4-Me₂NPy complexes of (2,6-F₂)₄TPPFe(III) and (2,6-

(OMe₂))₄TPPFe(III) which have rhombic EPR signals,

presumably because the ortho substituents in these two cases

are small enough to allow the ligands to bind in near-parallel

planes; along the same lines, the 1-MeIm complex of (2,6-

Br₂)₄TPPFe(III) is the only one of the 1-MeIm complexes

to exhibit a single-feature large g_maxEPR signal, presumably

because of the large size of the ortho-bromo substituents,

which must encourage the porphyrinate ring to ruffle and

thus cause even the five-membered ring 1-MeIm ligands to

bind in perpendicular planes.

Results

EPR Spectra of the (2,6-X₂)₄TPPFe(III) Bis-pyridine

and Bis-imidazole Complexes. In Figure 1 are shows

examples of the general EPR spectral types of rhombic (also

called Type II¹⁰), axial (also called Type III¹⁰), and large

g_max(also called Type I¹⁰) porphyrinatoiron(III) bis-ligand

complexes. As has been shown elsewhere,^9-11both rhombic

and “large g_max” EPR spectra are observed for low-spin

ferriheme systems with (d_xy)²(d_xz,d_yz)³electron configurations

and planar axial ligands in parallel and perpendicular planes,

respectively, while axial EPR spectra are observed for low-

spin ferriheme systems with (d_xz,d_yz)⁴(d_xy)¹electron configu-

rations. Most of the complexes of this study (except for those

of 1-MeIm for all but one of the porphyrinates) have axial

ligands in perpendicular planes and thus give “large g_max

”

or Type I¹⁰EPR spectra for the high-basicity pyridine

complexes and axial or Type III¹⁰EPR spectra for the lowest-

basicity pyridine complexes; the complexes of intermediate-

basicity pyridines typically gave broad signals (Figure 1e,

for example) that were difficult to characterize in terms of

the electron configuration of the metal, and because of the

short relaxation times, it has not been possible to determine

unambiguously the electron configuration of these by pulsed

EPR spectroscopy as we have done in other cases of ques-

tionable ground-state complexes.^37-41The g values obtained

for the complexes are listed in Table 1. Those of some of

It has been assumed for some time that g_maxvalues of

greater than 3.2 are indicative of the (d_xy)²(d_xz,d_yz)³electron

configuration,¹for which this g_maxis aligned along the normal

to the heme plane, while for g_maxvalues of 2.6 or less the

electron configuration is (d_xz,d_yz)⁴(d_xy)¹and this g_maxis

oriented in the xy plane and is thus g_x,g_yor g , with g_zbeing

the smallest g value.^4,6,10,37-41These assumptions were

strongly supported by low-temperature near-infrared MCD

spectra of [TMPFe(1-MeIm)₂]⁺, [TMPFe(4-Me₂NPy)₂]⁺, and

[TMPFe(4-CNPy)₂]⁺,⁴²for which the first two complexes

showed fairly intense near-IR bands near 1160 and 1400 nm

at 4.2 K, while the third had extremely weak bands in the

same wavelength region. This difference was explained by

the fact that the porphyrin f Fe(III) charge-transfer transi-

tions that give rise to the NIR bands are allowed when the

hole is in the d_xz,d_yzorbital set but are forbidden when it is

in the d_xyorbital.⁴²They are also strongly supported by the

(40) Astashkin, A. V.; Raitsimring, A. M.; Walker, F. A. J. Am. Chem.

Soc. 2001, 123, 1905-1913.

(41) Astashkin, A. V.; Raitsimring, A. M.; Kennedy, A. R.; Shokhireva,

T. Kh.; Walker, F. A. J. Phys. Chem. A 2002, 106, 74-82.

(42) Cheesman, M. R.; Walker, F. A. J. Am. Chem. Soc. 1996, 118, 8,

7373-7380.

(43) Rafii, K. NMR Studies of a Series of Porphyrinato Cobalt(II) and

Cobalt(III) Complexes. M.S. Thesis, San Francisco State University,

San Francisco, CA, 1993.

7472 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

Figure 1. Examples of frozen solution EPR spectra observed for the low-spin Fe(III) porphyrinate complexes of this study: (a) rhombic, [(2,6-F₂)₄TPPFe-

(1-MeIm)₂]ClO₄; (b) axial with g_zresolved, [TMPFe(4-CNPy)₂]ClO₄; (c) axial with g_zunobserved, [TMPFe(4-HPy)₂]ClO₄; (d) large g_max, [(2,6-F₂)₄TPPFe-

(2-MeHIm)₂]ClO₄; (e) large g_maxwith broad range of g values, [(2,6-Cl₂)₄TPPFe(Py)₂]ClO₄. The g values of these and all other complexes are reported in

Table 1.

paramagnetic shifts of the pyrrole protons at low temperatures

value is near to or less than 2.6 probably have the (d_xz,d_yz)⁴-

(d_xy)¹electron configuration with an unresolved g_zvalue of

1.5 or considerably less. Assuming these limits to hold, the

g values listed in Table 1 indicate that the lowest-basicity

pyridine (4- and 3-cyanopyridine) complexes of all of the

porphyrinates of this study have the (d_xz,d_yz)⁴(d_xy)¹ground

state, while the highest-basicity pyridine (4-Me₂NPy) com-

plexes of all have the (d_xy)²(d_xz,d_yz)³ground state; the basicity

at which each porphyrinate changes ground state appears to

vary somewhat, from pK_a(PyH⁺) values between 2.8 and 6

for TMP and (2,6-Br₂)₄TPP to <5.2 for (2.6-F₂)₄TPP, ∼5.2

for (2,6-Cl₂)₄TPP, and between 5.2 and 6 for (2,6-

(OMe)₂)₄TPP, but they can only be determined by the NMR

studies discussed below if the contact and pseudocontact

contributions to the paramagnetic shift can be separated.

Although these 4.2 K g values, or the magnetic susceptibili-

ties along the three principal axes,²³may vary with temper-

4

of both the [TMPFe(L)₂]⁺and [TPPFe(L)₂]^+6,7series of

complexes and by pulsed EPR studies of related com-

plexes, such as [OEPFe(4-Me₂NPy)₂]⁺,³⁷[OEPFe(HIm)₂]⁺,³⁷

and [TPPFe(HIm)₂]⁺⁴⁰that confirm that the largest g

value for the bis-ligand complexes with high-basicity py-

ridines or imidazoles is indeed aligned along normal to the

plane of the heme and is thus g_z, and by single-crystal

EPR spectroscopy of [TPPFe(4-CNPy)₂]⁺which confirms

that the EPR spectrum is axial with a maximum g value of

2.62 or greater and a minimum g value of 0.92 or less.⁵

Unfortunately, because of the short electron spin relaxa-

tion times and the very deep ¹⁴N modulation for samples

with large g_maxEPR signals, we have thus far been un-

able to use the pulsed EPR techniques to confirm the

orientation of the g tensor either for complexes that have a

g_maxvalue greater than 3.2 or less than 2.6. However, on the

basis of the evidence quoted above, we are convinced that

iron porphyrinate systems with g_maxsignals in which the g

Inorganic Chemistry, Vol. 44, No. 21, 2005 7473

Watson et al.

ature, these values may be helpful in guiding the interpre-

tation of the NMR data obtained in this work and discussed

below.

ground and excited states. The spin states of both the ground

(S ) 1/2 in all cases of this study) and excited states (possible

S ) 1/2, 3/2, or 5/2) are input parameters of this fitting

procedure, and thus, all possible spin states for the excited

state must be considered in each case. Variable-temperature

¹H NMR chemical shifts for all complexes are listed in the

tables of Appendix A of the Ph.D. dissertation of C. T.

Watson.³⁵Additional variable-temperature data for three

[TMPFe(L)₂]⁺complexes (L ) 4-Me₂NPy, Py, 4-CNPy)

were also acquired at more closely spaced temperature

intervals and are essentially identical to those obtained

previously;³⁵the best fits were found to be to the same

excited state, S ) 3/2, and the fitted values of ∆E for the

two data sets were found to be identical.

From the NMR data as a function of temperature, the

program uses eq 8 to calculate the difference in energy (∆E)

between the ground and excited states for the chosen

possibilities of the two. The pseudocontact contribution to

the paramagnetic shift is relatively small for the pyrrole-H

and m-H, but quite large for the o-H of the pyridine ligands,

o-H_Py, because of the small distance (r < 3 Å) between the

o-H_Pyand the iron center (eqs 5 and 6). Thus, for the

pyrrole-H and m-H shifts, the contact contribution dominates,

and no correction has been applied for the pseudocontact

contribution, while for the o-H_Pyshifts the pseudocontact

shift dominates, and no fits to eq 8 using the program

TDFw²⁹were found to converge. Because of the dissociation

of axial ligands at higher temperatures, which introduces a

thermodynamic equilibrium between the low-spin bis-ligand

complex ground state, with its chemical shifts, and the high-

spin mono-ligand complex, with its very different chemical

shifts, in most cases reliable fits could only be obtained using

the chemical shifts at 1000/T > 4.4 K^-1or temperatures

below -46 °C.

The program uses the best-fit energy difference between

the ground and excited states to then calculate the Curie

factors and spin densities at the carbon to which the protons

of interest are bound for the ground and excited states. In

cases where the pseudocontact contribution to the para-

magnetic shift is very small or nonexistent, the Curie factors,

C_n, are proportional to the spin densities, F_C, at the sites of

interest and thus to their molecular orbital coefficients, which

have been interpreted for the ground states of the complexes

of interest to be those for the 3e(π) and 3a_2u(π) frontier

porphyrin orbitals for the (d_xy)²(d_xz,d_yz)³and (d_xz,d_yz)⁴(d_xy)¹

ground states, respectively. However, only the 3-CNPy and

4-CNPy complexes of TMPFe(III) showed clear evidence

of pure (d_xz,d_yz)⁴(d_xy)¹ground states over the temperature

range of the NMR measurements, as evidenced by a very

small or even negative spin density at the â-pyrrole carbons,

even though the 4.2 K EPR data for a number of other

complexes indicate a pure (d_xz,d_yz)⁴(d_xy)¹ground state. In most

series of complexes, the spin densities at the â-pyrrole

carbons simply decrease fairly smoothly as the pK_a(PyH⁺)

decreased, indicating that the energies of the (d_xz,d_yz)⁴(d_xy)¹

and (d_xy)²(d_xz,d_yz)³electron configurations become very

similar, perhaps switch, but never separate enough to allow

thermal isolation of the (d_xz,d_yz)⁴(d_xy)¹state, even at the lowest

¹H NMR Spectra of the (2,6-X₂)₄TPPCo(III) Bis-

1

pyridine and Bis-imidazole Complexes. The H NMR

spectra of the following series of cobalt complexes were

recorded at ambient temperatures: [(2,6-Br₂)₄TPPCo(L)₂]BF₄

where L is 4-CNPy, 3,4-Me₂Py, and 4-Me₂NPy; [(2,6-Cl₂)₄-

TPPCo(L)₂]BF₄where L is 4-CNPy, 3-CNPy, Py, 4-MePy,

3,4-Me₂Py, 4-Me₂NPy, 1-MeIm, and 2-MeHIm; [(2,6-F₂)₄-

TPPCo(L)₂]BF₄where L is 4-CNPy, 3-CNPy, Py, 4-MePy,

3,4-Me₂Py, 4-Me₂NPy, 1-MeIm, and 2-MeHIm; and [(2,6-

(OMe)₂)₄TPPCo(L)₂]BF₄where L is 4-CNPy, 3,4-Me₂Py,

and 4-Me₂NPy. The chemical shifts are summarized in

Supporting Information Tables S4-S6. The ¹H NMR spectra

of a series of [TMPCo(L)₂]BF₄complexes listed in Sup-

porting Information Table S7 have been reported elsewhere.⁴³

The chemical shifts of the proton resonances in these

diamagnetic complexes are used as the diamagnetic shifts

for calculating the paramagnetic shifts of the iron(III)

complexes (eq 1). As can be seen, the chemical shifts follow

very similar patterns from complex to complex, and thus in

cases where specific complexes were not prepared, the

chemical shifts can be estimated to high accuracy from the

chemical shifts of related complexes.

1

Temperature Dependence of the H Chemical Shifts

of Low-Spin Iron(III) Porphyrinates. It should be noted

that both the contact and pseudocontact terms of the

paramagnetic shift have inverse temperature dependences (eq

4-7). For the pseudocontact shift, however, the temperature

dependence may not be linear with inverse temperature if

the second-order Zeeman term is appreciable²³or if there

are excited states of different spin multiplicities within kT

of the ground state. Since the latter is found to be the case

for some of the complexes of this study, some of the devi-

ations from good linearity are probably a result of this factor.

The temperature dependence of the ¹H NMR shifts is best

shown by a Curie plot (δ_paravs 1000/T), which is linear with

extrapolated intercepts of zero if the Curie law is obeyed.

However, curvature of the Curie plot is often ob-

served.^{8,13-15,44-46}The temperature-dependent fitting pro-

gram, TDFw,²⁹has been created in this laboratory and is

used to fit the expanded Curie-law dependence of the

chemical shift on inverse temperature in the fairly common

cases of curved dependence that result from the presence of

a thermally accessible excited electronic state.^12,15The

program itself can be downloaded from the Internet and used;

a detailed Help section is provided.²⁹In this work, we used

this program to determine the energy difference, ∆E, between

the ground and excited states, the spin state of the latter,

and the spin densities, F_C, at the carbons of interest in the

(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.

(45) Bertini, I.; Luchinat, C.; Parigi, G.; Walker, F. A. J. Biol. Inorg. Chem.

1999, 4, 515-519; 846.

(46) 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.

7474 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

previously.^47-49For [(2,6-Br₂)₄TPPFe(2-MeHIm)₂]⁺, four

pyrrole-H resonances are observed from -90 to +20 °C,

and above the latter temperature, the resonances are too broad

because of the chemical exchange to accurately determine

the chemical shifts. For the [(2,6-Cl₂)₄TPPFe(2-MeHIm)₂]⁺

complex, this extreme broadening occurs above -40 °C.

From -30 to +10 °C, the pyrrole-H resonances are too broad

to be assigned chemical shift values. From +20 to +40 °C,

however, one averaged resonance is observed. For [TMPFe-

(2-MeHIm)₂]⁺, four pyrrole-H resonances are seen from -90

to -10 °C, above which the resonances are too broad for

the chemical shifts to be accurately measured.⁴⁹For [(2,6-

F₂)₄TPPFe(2-MeHIm)₂]⁺and [(2,6-(OMe)₂)₄TPPFe(2-Me-

HIm)₂]⁺, the pyrrole resonance remains a singlet throughout

the -90 to +40 °C temperature range, presumably because

of the smaller effective size of the fluorine and methoxy

groups and thus the apparently much more rapid inversion

of the porphyrin ring and ligand rotation kinetics that go

along with this smaller effective size.⁴⁸

Figure 2. Plot of paramagnetic shift vs 1000/T for the pyrrole-H of the

bis-1-methylimidazole complexes of the five iron(III) porphyrinates. Note

the similar paramagnetic shifts and Curie behavior for all five complexes.

temperature used for the NMR investigations (-90 °C, 183

K). Thus, we can conclude that for all medium- to low-

basicity pyridines the difference in energy between the two

S ) 1/2 ground states is considerably less than kT at 183 K

or ∆E e 40 cm^-1. In all cases, it was found that the

paramagnetic shifts of the bis-imidazole and high-basicity

pyridine complexes were best fit with the simple Curie law

(S ) 1/2, (d_xy)²(d_xz,d_yz)³ground state and no thermally

accessible excited state), while most middle- to low-basicity

pyridines were best fit with a two-level treatment having an

S ) 1/2 mixed (d_xz,d_yz)⁴(d_xy)¹/(d_xy)²(d_xz,d_yz)³electron config-

Supporting Information Figure S1 shows the paramagnetic

shift versus 1000/T for the pyrrole protons of the 2-MeHIm

complexes of the five ferric porphyrinates. In the case where

the pyrrole resonance is split into four peaks, the average

value is used. The paramagnetic shifts for the ortho halo-

genated complexes are very similar, while [(2,6-(OMe)₂)₄-

TPPFe(2-MeHIm)₂]⁺and [TMPFe(2-MeHIm)₂]⁺have more

positive paramagnetic shifts throughout the -90 to +40 °C

temperature range. From +10 to +40 °C the pyrrole-H

paramagnetic shift of [(2,6-(OMe)₂)₄TPPFe(2-MeHIm)₂]⁺

deviates severely from the near-Curie behavior seen for the

other complexes, as rapid on-off ligand exchange with

increasing population of the high-spin five-coordinate inter-

mediate shifts the resonance toward more positive chemical

shift values. As found for the 1-MeIm complexes discussed

above and summarized in Table 2, the temperature-dependent

fitting of the pyrrole-H resonances (of all four lines for the

TMP, (2,6-Br₂)₄TPP, and (2,6-Cl₂)₄TPP complexes and the

single line for the other two) led to the conclusion that there

was no thermally accessible excited state, and thus spin

densities were obtained from one-level (simple Curie) plots.

These yielded somewhat smaller spin densities than were

observed for the 1-MeIm complexes (0.0114 (TMP), 0.0145

((2,6-Br₂)₄TPP), 0.0144 ((2,6-Cl₂)₄TPP), 0.0151 ((2,6-F₂)₄-

TPP), and 0.0132 ((2,6(OMe)₂)₄TPP)).

3

1

2

uration ground state and an S ) 3/2 (d_xz,d_yz) (d_xy) (d_z)

electron configuration excited state with ∆E > 115 cm^-1in

all cases.

NMR Spectra of the Five Fe(III) Porphyrinates Bound

to 1-Methylimidazole and 2-Methylimidazole and Their

Temperature Dependences. The pyrrole-H shifts for the

bis-1-MeIm complexes of the five iron(III) porphyrinates

studied are all very similar, as evidenced by the Curie plots

shown in Figure 2, which are all linear with intercepts near

0 ppm for all five complexes. At low temperatures, the range

of pyrrole-H paramagnetic shifts is less than 5 ppm, with

those of [(2,6-Br₂)₄TPPFe(1-MeIm)₂]⁺being the most nega-

tive, followed by [(2,6-Cl₂)₄TPPFe(1-MeIm)₂]⁺, [(2,6-

(OMe)₂)₄TPPFe(1-MeIm)₂]⁺, then [TMPFe(1-MeIm)₂]⁺, and

finally [(2,6-F₂)₄TPPFe(1-MeIm)₂]⁺. At +40 °C the range

of paramagnetic shifts is less than 2 ppm, with the order of

the paramagnetic shift values nearly the same.

1

Temperature Dependence of the H NMR Spectra of

TMPFe(III) Bound to Pyridines of Different Basicities.

In comparison to the similarity in behavior of all five iron-

(III) porphyrinates with 1-MeIm (and to a lesser extent

2-MeHIm), the NMR spectra and temperature dependences

For all 1-MeIm complexes, it was found that no thermally-

accessible excited state existed, on the basis of the analysis

of the Curie plots using the TDFw program.²⁹One-level fits

(simple Curie behavior) of the pyrrole-H shifts yielded very

similar spin densities for all five complexes (F_C) 0.0156

(TMP), 0.0167 ((2,6-Br₂)₄TPP), 0.0164 ((2,6-Cl₂)₄TPP),

0.0156 ((2,6-F₂)₄TPP), and 0.0159 ((2,6-(OMe)₂)₄-TPP)), as

summarized in Table 2.

1

of the H NMR resonances of the bis-pyridine complexes

ranging in basicity from 4-Me₂NPy (pK_a(PyH⁺) ) 9.70³⁴)

to 4-CNPy (pK_a(PyH⁺) ≈ 1.1⁴) of the five iron(III) porphy-

(47) Walker, F. A.; Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652-8657;

1992, 114, 1929.

Unlike the 1-MeIm complexes of the five iron(III) por-

phyrinates, the bis-2-MeHIm complexes of three of these

porphyrinates have four pyrrole-H resonances at -90 °C

because of the hindered axial ligand rotation, as reported

(48) 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.

(49) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 2787-

2795.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7475

Watson et al.

Table 2. Summary of Results Obtained from the TDFw²⁹Fits of the Temperature Dependence of δ(pyrrole-H) or δ(pyrrole-H and m-H_Ph) for the

[(2,6-X₂)₄TPPFe(L)₂]⁺Complexes of This Study

∆E^c

â-pyrrole carbon spin densities^e

comments on quality or nature of fit

system

GS^a

ES^b

(cm^-1)

MSD^d

[TMPFe(L)₂]⁺

resonances used

L ) 1-MeIm

1/2

-

pyrrole-H, o, p-CH₃, m-H

1: 0.0156; one-level fit

1: 0.0114; one-level fit

L ) 2-MeHIm

L ) 4-Me₂NPy

1: 0.0157; one-level; fair fit;

only data 1000/T > 4.4 used

1: 0.0119; one-level; fit not good

1: 0.0124; one-level; fit good

1: 0.0066; OK for S ) 1/2 d_xy/d_π

2: 0.0165; OK for S ) 3/2

1: 0.0055; OK for S ) 1/2 d_xy/d_π

2: 0.0163; OK for S ) 3/2

1: 0.0053; OK for S ) 1/2 d_xy/d_π

2: 0.0159; OK for S ) 3/2

1: 0.0024; OK for S ) 1/2 d_xy

2: 0.0181; OK for S ) 3/2

1: -0.0008; OK for S ) 1/2 d_xy

2: 0.0193; OK for S ) 3/2

L ) 3,4-Me₂Py

L ) 3,5-Me₂Py

L ) 4-MePy

1/2

-

3/2

-

274

-

0.016

L ) Py

1/2

3/2

338

404

422

439

0.004

0.011

0.008

0.019

L ) 3-ClPy

L ) 3-CNPy

L ) 4-CNPy

[(2,6-Br₂)₄TPPFe(L)₂]⁺

L ) 1-MeIm

L ) 2-MeHIm

L ) 4-Me₂NPy

L ) 3,4-Me₂Py

L ) 3,5-Me₂Py

L ) 4-MePy

L ) 3-MePy

L ) Py

1/2

-

pyrrole-H, m-H

1: 0.0167; one-level fit

1: 0.0145; one-level fit

1: 0.0172; one-level fit

1: 0.0160; one-level fit

1: 0.0155; one-level fit

1: 0.0162; one-level fit

1: 0.0152; one-level fit

1: 0.0082; OK for S ) 1/2 d_π/d_xy

2: 0.0182; OK for S ) 3/2

1: 0.0111; OK for S ) 1/2 d_π/d_xy

2: 0.0170; OK for S ) 3/2

1: 0.0110; OK for S ) 1/2 d_π/d_xy

2: 0.0172; OK for S ) 3/2

1: 0.0077; OK for S ) 1/2 d_xy/d_π

2: 0.0185; OK for S ) 3/2

-

3/2

130

0.032

L ) 3-ClPy

L ) 3-CNPy

L ) 4-CNPy

1/2

3/2

275

328

274

0.015

0.014

0.011

[(2,6-Cl₂)₄TPPFe(L)₂]⁺

L ) 1-MeIm

L ) 2-MeHIm

L ) 4-Me₂NPy

L ) 3,4-Me₂Py

L ) 3,5-Me₂Py

L ) 4-MePy

L) 3-MePy

L ) Py

1/2

-

pyrrole-H, m-H

1: 0.0164; one-level fit

1: 0.0144; one-level fit

1: 0.0171; one-level fit

1: 0.0163; one-level fit

1: 0.0158; one-level fit

1: 0.0153; one-level fit

1: 0.0102; OK for S ) 1/2 d_π/d_xy

2: 0.0173; OK for S ) 3/2

1: 0.0132; OK for S ) 1/2 d_π/d_xy

2: 0.0169; OK for S ) 3/2

1: 0.0127; OK for S ) 1/2 d_π/d_xy

2: 0.0153; OK for S ) 3/2

1: 0.0089; OK for S ) 1/2 d_π/d_xy

2: 0.0189; OK for S ) 3/2

-

3/2

178

0.047

L ) 3-ClPy

L ) 3-CNPy

L ) 4-CNPy

1/2

3/2

449

387

406

0.011

0.029

0.011

[(2,6-F₂)₄TPPFe(L)₂]⁺

L ) 1-MeIm

L ) 2-MeHIm

L ) 4-Me₂NPy

L ) 3,4-Me₂Py

L ) 3,5-Me₂Py

L ) 4-MePy

L ) 3-MePy

L ) Py

1/2

-

pyrrole-H, m-H

1: 0.0156; one-level fit

1: 0.0151; one-level fit

1: 0.0166; one-level fit

-

1: 0.0184; one-level; not a good fit

1: 0.0191; one-level; not a good fit

1: 0.0172; one-level; good fit

1: 0.0179; one-level; not a good fit

1: 0.0161; one-level; good fit

1: 0.0165; one-level; not a good fit

1: 0.0171; OK for S ) 1/2 d_π

2: 0.0019; OK for S ) 1/2 d_xy

1: 0.0112; OK for S ) 1/2 d_xy/d_π

2: 0.0155; OK for S ) 3/2

-

L ) 3-ClPy

-

L ) 3-CNPy

1/2

287

0.022

L ) 4-CNPy

1/2

3/2

368

0.013

[(2,6-(OMe)₂)₄TPPFe(L)₂]⁺

L ) 1-MeIm

1/2

-

pyrrole-H, m-H

1: 0.0159; one-level fit

1: 0.0132; one-level fit

1: 0.0159; one-level; not the best fit

1: 0.0164; one-level;

only data below 1000/T ) 4.4 used

L ) 2-MeHIm

L ) 4-Me₂NPy

L ) 3,4-Me₂Py

7476 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

Table 2 (Continued)

∆E^c

â-pyrrole carbon spin densities^e

comments on quality or nature of fit

system

GS^a

ES^b

(cm^-1)

MSD^d

resonances used

[(2,6-(OMe)₂)₄TPPFe(L)₂]⁺

L ) 3,5-Me₂Py

L ) 4-MePy

1/2

-

1: 0.0161; one-level;

only data below 1000/T ) 4.4 used

1: 0.0156; one-level;

only data below 1000/T ) 4.5, but fit not good

1: 0.0180; one-level fit

-

L ) 3-MePy

L ) Py

1/2

-

3/2

-

388

-

0.097

1: 0.0118; OK for S ) 1/2 d_π/d_xy

2: 0.0157; OK for S ) 3/2

L ) 3-ClPy

L ) 3-CNPy

L ) 4-CNPy

1/2

3/2

119

409

682

0.019

0.011

0.019

1: 0.0079; OK for S ) 1/2 d_xy/d_π

2: 0.0127; OK for S ) 3/2

1: 0.0043; OK for 1/2 d_xy/d_π

2: 0.0224; large for S ) 3/2

1: 0.0085; OK for S ) 1/2 d_xy/d_π

2: 0.0210; fit only for 1000/T > 4.5

^aGS ) spin (S) of the ground state. ^bES ) spin (S) of the excited state. ^cBest fit energy ((30%). ^dMSD ) mean-square deviation of the data points

from the best fit. ^eSpin densities obtained from fits, using the program TDFw, to eq 8; 1 ) ground state, 2 ) excited state.

decreases. The curvature of the Curie plots becomes more

pronounced for the middle-basicity pyridines, and for the

lowest-basicity pyridine complexes, the pyrrole-H Curie plots

exhibit anti-Curie behavior (Figure 3).

Table 2 summarizes the ∆E values calculated from the

fits of the plots of paramagnetic shifts vs the inverse

temperature for all complexes, including the series of

[TMPFe(L)₂]⁺. It was found that plots exhibiting close to

true Curie behavior give a shallow minimum in the two-

level fits and are best fit as one-level systems having no

thermally accessible excited state. However, despite one-

level fits, except for L ) 1-MeIm and 4-Me₂NPy, the spin

densities were smaller than the typical F_Cat ∼0.0156. For

example, for L ) 3,4-Me₂Py and 3,5-Me₂Py, the F_Cvalues

are 0.0119 and 0.0124, respectively. For L ) 4-MePy, Py,

Figure 3. Plot of paramagnetic shift vs 1000/T for the pyrrole-H of the

[TMPFe(L)₂]⁺complexes where L ) XPy, showing the anti-Curie behavior

of the low-basicity pyridine complexes and approximately Curie behavior

for the high-basicity pyridine complexes.

3-ClPy, and the cyanopyridines, the temperature dependence

was consistent with a two-level fit with the ground state being

S ) 1/2 and the excited state being S ) 3/2. Throughout the

series of lower-basicity pyridine complexes, the values of

F_Cof the ground state decrease steadily with decreasing

ligand basicity (0.0066, 0.0055, 0.0053, 0.0024, and -0.0008,

respectively), indicating a ground state that has a decreasing

contribution from the (d_xy)²(d_xz,d_yz)³electron configuration

and an increasing contribution from the (d_xz,d_yz)⁴(d_xy)¹electron

configuration as the basicity of the ligand decreases; the 3-

and 4-CNPy complexes appear to have quite pure (d_xz,d_yz)⁴-

(d_xy)¹configurations. The negative value of F_Cfor the

4-CNPy complex appears to result from a small contact

contribution (+) and a pseudocontact contribution of similar

magnitude but opposite sign (-), as reported earlier.¹⁵At

the same time, the values of F_Cfor the excited state generally

increase somewhat (0.0165, 0.0163, 0.0159, 0.0181, 0.0193,

respectively) but are still close enough to F_C≈ 0.0156 to

indicate a single d_πunpaired electron for the S ) 3/2 excited

rinates show somewhat different behaviors. The temperature

dependence of the pyrrole-H, m-H_Ph, and o-H_Pyare discussed

in detail for the complexes of TMPFe(III), and then,

comparisons in the behavior of the other four series of

complexes are presented briefly. Before doing so, however,

we wish to point out that the ortho substituents on the phenyl

rings of all five of the iron porphyrinates of this study are

large enough that phenyl rotation is not expected to occur

in any of the complexes; this was shown conclusively by

Eaton and Eaton for a series of (TMP)MX and five-

coordinate tetra-(pentafluorrophenyl)porphyrinatometal com-

plexes in 1975.⁵⁰Thus, phenyl rotation is not expected to

contribute in any way to the temperature dependence of the

paramagnetic shifts of these complexes.

â-Pyrrole Protons. Figure 3 shows the pyrrole-H para-

magnetic shift vs 1000/T for the nine bis-pyridine complexes

of TMPFe(III). For the higher-basicity pyridines, where the

ligands are 4-Me₂NPy, 3,4-Me₂Py, and 3,5-Me₂Py, it can

be seen that at low temperatures the pyrrole-H chemical shift

is very negative and the resonances move toward the

diamagnetic region linearly as the inverse temperature

3

1

2

state and thus (d_xz,d_yz) (d_xy) (d_z) electron configurations in

all cases. From the best fits, it is found that these complexes

give converged fitted ∆E values that generally increase with

decreasing basicity of the ligand: 274 cm^-1for the 4-MePy

complex and 439 cm^-1for the 4-CNPy complex.

The linear relationship between the pyrrole-H δ_paraand

the pK_aof the protonated pyridine ligands (pK_a(PyH⁺)) at

(50) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660-3666.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7477

Watson et al.

Figure 5. Plot of paramagnetic shift vs 1000/T for the o-H_Pyof the

[TMPFe(L)₂]⁺complexes. Note that the shifts of the low-basicity pyridine

resonances are more positive than those of the higher-basicity pyridine

resonances.

Figure 4. Plot of the paramagnetic shift of the pyrrole-H resonances of

the [TMPFe(L)₂]⁺complexes vs pK_a(PyH⁺) of the axial ligands at -90,

-30, and +40 °C. Note the decreasing slope of the best-fit line as the

temperature increases.

-80 °C that was reported previously⁴exists throughout the

temperature range studied, as shown in Figure 4. Although

the slopes of these plots are negative throughout the

temperature range of the measurements, they become less

negative as the temperature increases.

m-Phenyl Protons (m-H_Ph). The pattern of increasing

paramagnetic shift with decreasing axial pyridine ligand

basicity at a given temperature seen for the pyrrole-H Curie

plots is also seen in the m-H_PhCurie plots shown in

Supporting Information Figure S2. The plots for each

complex are linear, with only slight curvature at higher

temperatures. The slope of the m-H Curie plot of Figure S2

for [TMPFe(4-Me₂NPy)₂]⁺is negative, while that of [TMPFe-

(3,4-Me₂Py)₂]⁺is close to zero, and thereafter the slopes

become more positive as the axial pyridine basicity decreases

further. The TDFw program²⁹was not used to analyze these

m-H_Phshifts alone because of the very small temperature

dependence. Rather, the m-H_Phshifts were analyzed together

with the pyrrole-H paramagnetic shifts discussed above.

Ortho Protons of the Axial Pyridine Ligands (o-H_Py).

Figure 5 shows the Curie plots for the o-H_Pyresonances of

the TMPFe(III) complexes of the nine pyridine ligands. For

the complexes with unsymmetrical pyridine axial ligands,

the value plotted is the average for the two different o-H_Py

resonances. For the complexes where the axial ligands are

4-CNPy through 3,4-Me₂Py, the Curie plots show smooth

shifts of the o-H_Pyresonance to less negative values as the

temperature is increased. A slight curvature toward more

positive paramagnetic shifts can also be seen at the higher

temperatures. In contrast, the o-H_Pyresonance of the [TMPFe-

(4-Me₂NPy)₂]⁺complex shifts in a negative direction with

increasing temperature until about 0 °C and then begins to

curve in a positive direction with further-increasing temper-

ature, likely the result of ligand dissociation. Hence, the

temperature-dependent behavior of the o-H_Pyresonance is

entirely different from those of the pyrrole-H and m-H_Ph

resonances. As will be shown in detail below, this is because

Figure 6. Plot of the paramagnetic shift of the o-H_Pyat -60 °C vs pyridine

basicity for the [TMPFe(L)₂]⁺complexes showing the slight change in slope

of the line at pK_a(PyH⁺) ≈ 5-6.

the o-H_Pyparamagnetic shifts have the largest pseudocontact

contributions of all of the protons of these complexes, and

the pseudocontact shift is sensitive to the structure and g

anisotropy (eqs 5 and 6) rather than through-bond spin

delocalization (eq 4).

A plot of the o-H_Pyparamagnetic shifts at -60 °C vs pK_a-

(PyH⁺), Figure 6, as well as at all other temperatures, shows

an abrupt change in slope of the best-fit line near the pK_aof

the conjugate acid of pyridine itself (Py, 5.22³⁴). This change

in slope suggests a change in the electron configuration of

the metal and thus a change in the relative contributions of

the contact and pseudocontact interactions (eq 3). As the

temperature is further raised, the o-H_Pyresonances move

somewhat further positive, but there is very little pK_a(PyH⁺)

dependence on the o-H_Pyparamagnetic shifts for the lowest-

basicity pyridine complexes. Figure 5 shows that, in opposi-

tion to what is seen for the pyrrole-H shifts, the more basic

pyridine o-H resonances deviate from Curie behavior the

most, while the low-basicity pyridine complexes show fairly

linear Curie behavior for the o-H_Pyshifts. The o-H_Pyfits to

7478 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

eq 8 alone did not converge for most of the complexes for

any possible excited state.

of the same 2,6-F₂series, the best fit was to a mixed S )

1/2 ground state (F_C) 0.0112) and an S ) 3/2 excited state

(F_C) 0.0155) with ∆E ) 368 cm^-1. For Py itself and the

lower-basicity pyridine complexes of the 2,6-Br₂, 2,6-Cl₂and

2,6-(OMe)₂series, the pyrrole-H shifts are best fit with a

two-level plot, with reduced F_Cfor the S ) 1/2 ground state

that decreases as the basicity of the pyridine decreases and

suggests a mixed d_π/d_xyground state for each and a larger

F_Cfor the S ) 3/2 excited state (0.0157 to 0.0224) which

suggests, in all cases, one unpaired d_πelectron and a (d_xz,d_yz)³-

(d_xy) (d_z) electron configuration for all. The ∆E values range

from 119 to 682 cm^-1among all the low-basicity pyridine

complexes of the 2,6-Br₂, 2,6-Cl₂, and 2,6-(OMe)₂series.

The pyrrole-H shifts for the pyridine complexes vs pK_a-

(PyH⁺) at three temperatures are shown in Supporting

Information Figure S7 for the 2,6-Br₂series, Figure S8 for

the 2,6-F₂series, and Figure S9 for the 2,6-(OMe)₂series of

complexes: the 2,6-Cl₂plot was so similar to that of the

2,6-Br₂plot that it is not shown.

m-Phenyl Protons of the Four [2,6-(X₂)₄TPPFe(L)₂]⁺

Complexes. The m-H_PhCurie plots of the [(2,6-Br₂)₄TPPFe-

(L)₂]⁺complexes shown in Supporting Information Figure

S10 are similar to those of [TMPFe(L)₂]⁺except that, in this

case, only the [(2,6-Br₂)₄TPPFe(4-CNPy)₂]⁺complex has a

positive slope. Those of the other complexes are very similar,

and for all series, the Curie plots parallel those of the

respective pyrrole-H plots, as shown in Supporting Informa-

tion Figures S10-S13.

Ortho Protons of the Pyridine Axial Ligands. The o-H_Py

Curie plots for the [(2,6-Br₂)₄TPPFe(L)₂]⁺complexes in

Supporting Information Figure S14, those for the 2,6-Cl₂

series in Figure S15, those for the 2,6-F₂series in Figure

S16, and those for the 2,6-(OMe)₂series in Figure S17 show

smooth shifts toward a less negative paramagnetic shift with

increasing temperature for the lower basicity pyridines, as

seen for the [TMPFe(L)₂]⁺complexes, although there are

some differences in the order of the temperature-dependent

shifts of the individual ligand complexes and an increasing

overlap of the Curie plots for lower-basicity ligand complexes

in the order Br ) Cl < F < OMe.

That the temperature dependences of the ligand resonances,

in general, behave differently than those of the porphyrin

resonances (because of ligand on-off, ligand rotation, and

porphyrin inversion kinetics) has been convincingly shown

for a series of bis-ligand complexes of several octaalkyl-

tetraphenylporphyrinatoiron(III).¹⁵Furthermore, in the case

of the iron(III) porphyrinates of this study, the strongly

changing pseudocontact shift contribution, which is some-

what apparent from the g values in Table 1 and will be

discussed in more detail below, begins with relatively large

negative values for the most basic pyridines, where the value

1

2

of g_|²- g is positive, passes through zero near the pK_a-

(PyH⁺) of 4-MePy or Py, and becomes increasingly positive

as the pK_a(PyH⁺) decreases further, where the value of g_|²

- g ²is negative, a behavior that is quite different from that

expected for the contact shift, which appears to simply

decrease smoothly with decreasing pK_a(PyH⁺). This pseudo-

contact shift sign dependence is magnified in the case of

the o-H_Pybecause they are very close to the paramagnetic

metal center (r < 3 Å), and thus the use of ligand resonances

to attempt to determine ∆E will not be considered further.

1

Temperature Dependence of the H NMR Spectra of

the [(2,6-Br₂)₄TPPFe(L)₂]ClO₄, [(2,6-Cl₂)₄TPPFe(L)₂]-

ClO₄, [(2,6-F₂)₄TPPFe(L)₂]ClO₄, and [(2,6-(OMe)₂)₄TPPFe-

(L)₂]ClO₄Complexes where L ) Pyridines of Different

Basicities. Because all of these porphyrinate complexes

showed similar behavior to those of the TMP complexes and

to each other, their spectra are discussed together.

â-Pyrrole Protons. All of the complexes behave similarly,

as shown in Supporting Information Figure S3 for [(2,6-Br₂)₄-

TPPFe(L)₂]ClO₄, Figure S4 for [(2,6-Cl₂)₄TPPFe(L)₂]ClO₄,

Figure S5 for [(2,6-F₂)₄TPPFe(L)₂]ClO₄, and Figure S6 for

[(2,6-(OMe)₂)₄TPPFe(L)₂]ClO₄. For all four sets of com-

plexes, the Curie plots are linear for L ) 4-Me₂NPy and a

number of other higher-basicity pyridines, while the tempera-

ture dependences of the lower-basicity pyridine complexes

show a slight curvature toward more positive paramagnetic

shifts at higher temperatures, and this curvature becomes

more pronounced as the basicity of the pyridine decreases.

Unlike the [TMPFe(L)₂]⁺complexes, the pyrrole-H slopes

are all negative except the one for the 3-CNPy complex of

the dimethoxy porphyrinate, Figure S6. As Table 2 shows,

the temperature dependence of the higher-basicity pyridine

complexes are best fit with a simple one-level Curie plot,

with the spin densities at the â-pyrrole positions, F_C, ranging

from 0.0180 to 0.0152 in random order for L ) 4-Me₂NPy

to 3-MePy for the 2,6-Br₂, 2,6-Cl₂and 2,6-(OMe)₂series,

while the fits of all of the [(2,6-F₂)₄TPPFe(L)₂]⁺complexes,

except those having L ) 3- and 4-CNPy, are best fit with a

linear Curie dependence with F_Cat the â-pyrrole carbons

ranging from 0.0191 (3,5-Me₂Py) to 0.0161 (Py). The

3-CNPy complex of the 2,6-F₂series was the only case in

this entire study of five porphyrinates for which the two-

level fit was most consistent: the ground state was found to

be S ) 1/2 (d_xy)²(d_xz,d_yz)³and the excited-state was S ) 1/2

(d_xy)¹(d_xz,d_yz)⁴with ∆E ) 287 cm^-1. For the 4-CNPy complex

In the plot of paramagnetic shift vs pK_a(PyH⁺) at -60 °C

(Supporting Information Figure S18), it is apparent that the

change in slope of the best-fit line near the pK_a(PyH⁺) found

in the [TMPFe(L)₂]⁺complexes (Figure 6) is present in the

[(2,6-Br₂)₄TPPFe(L)₂]⁺complexes as well but is not as

pronounced as observed for the TMP complexes; the change

in slope is smaller for the Cl₂complexes (Supporting

Information Figure S19) and is imperceptible for the F₂

complexes (Supporting Information Figure S20). However,

for the [(2,6-(OMe)₂)₄TPPFe(L)₂]⁺series the plot of δ_para

-

(o-H_Py) vs pK_a(PyH⁺), Figure 7, shows much less scatter in

the low-basicity pyridine data than in the data for the [(2,6-

F₂)₄TPPFe(L)₂]⁺complexes (Supporting Information Figure

S20). Thus the change in slope of the best-fit line at pK_a-

(PyH⁺) ≈ 5 is much more evident.

Conclusions from Temperature-Dependent Fitting of

the Pyrrole-H Paramagnetic Shifts. Because of the rela-

tively small temperature range over which ligand exchange

Inorganic Chemistry, Vol. 44, No. 21, 2005 7479

Watson et al.

state reverse in energy. Only in this TMP series of complexes

do the two electron configurations of the ground state

separate in energy from each other significantly enough that

we see the spin densities at the â-pyrrole carbons that are

expected for a pure (d_xz,d_yz)⁴(d_xy)¹ground state for the lowest-

basicity pyridines.

Discussion

In all of the bis-imidazole and high-basicity pyridine

complexes of the five iron(III) porphyrinates under study,

the electronic ground state of the metal center is low-spin

d⁵S ) 1/2 with a (d_xy)²(d_xz,d_yz)³electron configuration. The

magnetic symmetry about the metal center is approximately

tetragonal for porphyrin complexes that have the axial ligands

lying in perpendicular planes. This is the case for all of the

hindered imidazole (2-MeHIm) and substituted pyridine

complexes of TMPFe(III), for which a number of structures

are available,^3,4,31and it is also expected for (2,6-Br₂)₄TPPFe-

(III) and (2,6-Cl₂)₄TPPFe(III) on the basis of the large g_max

EPR signals observed for their bis-2-MeHIm and 4-Me₂NPy

complexes (Figure 1, Table 1); by analogy, the lower-basicity

pyridine complexes of these same porphyrinates are also

expected to have axial ligands in perpendicular planes.

However, for (2,6-F₂)₄TPPFe(III) and (2,6-(OMe)₂)₄TPPFe-

(III), while the bis-2-MeHIm complexes clearly also have

axial ligands in perpendicular planes, as evidenced by the

large g_maxEPR signals, the bis-4-Me₂NPy complexes appear

to have ligands in parallel planes on the basis of the rhombic

EPR signals observed. Nevertheless, for the bis-4-MePy

complexes of each, the EPR signal is of the large g_maxtype

(Table 1), even though the steric requirements of 4-MePy

binding to the iron(III) porphyrinates are not different from

those of 4-Me₂NPy, and thus we cannot say with certainty

whether the ligands are in parallel or perpendicular planes;

in fact, these could be cases in which the dihedral angle

between ligand planes is close to the 57° angle which we

Figure 7. Plot of the paramagnetic shift of the o-H_Pyat -60 °C vs pyridine

basicity for the [(2,6-(OMe)₂)₄TPPFe(L)₂]⁺complexes, showing a definite

change in slope at pK_a(PyH⁺) ≈ 5.

did not affect the chemical shifts and the relatively small

number of data points obtained over that temperature range,

the accuracy of the ∆E values is in general only about (30%.

Despite this, several general conclusions can be made. It is

found that for the low-basicity pyridine complexes of all (2,6-

X₂)₄TPPFe(III) complexes the ground state is either (d_xz,d_yz)⁴-

(d_xy)¹or mixed (d_xz,d_yz)⁴(d_xy)¹/(d_xy)²(d_xz,d_yz)³with an energy

separation too small to allow us to differentiate the two states,

and the thermally accessible excited state is S ) 3/2, (d_xz,d_yz)³-

1

2

(d_xy) (d_z) , with one single exception. For higher-basicity

pyridine complexes, the ground state is (d_xy)²(d_xz,d_yz)³and

the excited state is the same S ) 3/2 electron configuration.

For the highest-basicity pyridine and 1-MeIm, as well as

2-MeHIm complexes, the only reasonable fit was to simple

Curie dependence, indicating no thermally accessible excited

state. From the best fits of the Curie plots for the low-basicity

pyridine complexes, it is found that these complexes give

converged fitted ∆E values that generally increase as the

basicity of the pyridine decreases. That the ∆E should

become larger as the basicity of the pyridine decreases

suggests that although the σ basicity of the pyridine toward

have recently shown to be the point at which the EPR spectral

51

type changes from normal rhombic to large g_max

.

For

imidazole and the higher-basicity pyridine ligands, the

unpaired electron is in an e-symmetry d_π(d_xzor d_yz) orbital.³

Previous studies of low-spin iron(III) porphyrinates have

shown that large negative chemical shifts at the pyrrole

position arise from spin density in the 3e(π) molecular orbital

of the porphyrin ring.^6,16These orbitals have the proper

symmetry to interact with the d_xz, d_yz(d_π) metal orbitals, and

the spin density can be delocalized into the 3e(π) porphyrin

orbitals through porphyrin to iron π donation. The 3e(π)

orbitals have large coefficients at four of the â-pyrrole

carbons and small coefficients at the other four, with the

two orbitals having large and small coefficients reversed.

Because of rapid axial pyridine ligand rotation and simul-

taneous ruffle-inversion of the porphyrin ring,^47-49on a time

scale of millions of times per second for these metal(III)

porphyrinates at ambient temperatures,⁵²the spin density

2

the proton decreases, the energy of the d_zorbital of the metal

appears to remain fairly constant (Scheme 1), but instead,

the energy of the d_πorbitals drops so that the ∆E between

lowest and highest energy of these orbitals actually increases

in many (but not all) cases.

For the [TMPFe(L)₂]⁺complexes, the g values measured

at 4.2 K suggest that the change in the electronic ground

state occurs at or somewhat above the pK_a(PyH⁺) of 4-MePy

(6.02) (Table 1). At the point of the change of ground state,

the g anisotropy is zero, the d_xz, d_yz, and d_xyorbitals are equal

in energy, and there is an equally mixed (d_xy)²(d_xz,d_yz)³/

(d_xz,d_yz)⁴(d_xy)¹ground state. From the change in slope of the

plot of the paramagnetic shift of the o-H_Pyvs pK_a(PyH⁺)

shown in Figure 5, which appears to occur between pK_a5

and 6, we can conclude that this is the point at which the g

anisotropy, and thus the pseudocontact shift, changes sign,

and at which the two electron configurations of the ground

(51) Yatsunyk, L. A.; Dawson, A.; Carducci, M. D.; Walker, F. A. J. Am.

Chem. Soc. Submitted for publication, 2005.

(52) Polam, J. R.; Shokhireva, T. Kh.; Raffii, K.; Simonis, U.; Walker, F.

A. Inorg. Chim. Acta 1997, 263/1-2, 109-117.

7480 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

present at all eight â-pyrrole carbons is equal and is the

average of the large and small spin densities of the individual

3e(π) orbitals, on the time scale of the NMR experiments.

Thus, the spin density can delocalize to the pyrrole positions

and cause the large negative paramagnetic shifts seen for

the pyrrole-H resonances of [PorFe(L)₂]⁺complexes having

bis-imidazole or higher-basicity pyridine axial ligands.^6,16

Bis-1-MeIm and -2-MeHIm Complexes of All Five

Porphyrinates. For all five iron(III) porphyrinates, the

temperature dependences of δ_parafor the bis-1-MeIm com-

plexes exhibit simple Curie behavior with no thermally

accessible excited state. The â-pyrrole spin densities range

from 0.0156 to 0.0167 (Table 2), a very small range of less

than (3% difference. Thus, these are the “purest” low-spin

iron(III) bis-ligand complexes, which have (d_xy)²(d_xz,d_yz)³

electron configurations with no contributions from other

possible electron configurations. In contrast, while none of

the bis-2-MeHIm complexes of the five iron porphyrinates

appear to have thermally-accessible excited states, their spin

densities at the â-pyrrole carbons are in general smaller

(0.0151-0.0114, Table 2), suggesting an increasing contri-

bution from the (d_xz,d_yz)⁴(d_xy)¹electron configuration in the

order of the decreasing value of F_C. This contribution appears

to be a thermal mixing of the (d_xz,d_yz)⁴(d_xy)¹configuration

with the (d_xy)²(d_xz,d_yz)³electron configuration with a very

small energy difference, ∆E. The Curie plots of the four

[TMPFe(2-MeHIm)₂]⁺pyrrole-H resonances exhibit no

evidence of the curvature that would suggest a well-separated

but still thermally-accessible excited state.

The magnitude of the g value of the large g_maxfeature in

the EPR spectra of the bis-2-MeHIm complexes of the five

porphyrinates (Table 1) decreases in the same order as the

value of F_Clisted above (g ) 3.52, F_C) 0.0151 ((2,6-F₂)₄-

TPP), 3.43, 0.0145 ((2,6-Br₂)₄TPP), 3.42, 0.0144 ((2,6-Cl₂)₄-

TPP), 3.35, 0.0132 ((2,6-(OMe)₂)₄TPP), 3.17, 0.0114 (TMP)),

and previous studies of more bulky phenyl-substituted

tetraphenylporphyrinates have suggested that extreme ruffling

tends to push even bis-imidazole-coordinated iron porphy-

rinates toward the (d_xz,d_yz)⁴(d_xy)¹electron configuration.^53,54

It is also possible that the value of g_maxis also an indicator

of the dihedral angle between the axial ligand planes, as we

have recently found in X-ray crystallographic and ground

crystalline EPR studies of other iron(III) porphyrinates.⁵¹

However, there are clearly electronic as well as steric

contributions to the observed electron configurations of these

bis-2-MeHIm complexes. The (2,6-F₂)₄TPPFe(III) complex

is clearly not as ruffled as the (2,6-Br₂)₄TPPFe(III) complex

on the basis of the fact that, as mentioned above in the EPR

section, the latter complex with 1-MeIm is the only one that

exhibits a large g_maxEPR spectrum at 4.2 K, indicating that

the 1-MeIm ligands are in perpendicular planes.

are dominated by the contact interaction.⁶The range of -90

°C δ_paravalues for the complexes whose Curie plots are

shown in Figure 2 indicates that there are major differences

in the spin density at the pyrrole position as a function of

ligand basicity. The pyrrole-H δ_parafor the 4-Me₂NPy

complex at -88 °C is -42.5 ppm. For the 4-CNPy complex

(the least basic pyridine studied), the pyrrole-H δ_paraat the

same temperature is -3.6 ppm. Figure 4, a plot of δ_paraat

-90, -30 and +40 °C vs pK_a(PyH⁺), shows that there is a

smooth transition from large negative to small negative

paramagnetic shift values as the basicity of the axial ligand

decreases at each of these temperatures.

For low-basicity pyridine complexes of TMPFe(III), such

as [TMPFe(4-CNPy)₂]⁺, it is found, from the EPR spectra,^4,5

that the d_xyorbital is of higher energy than the degenerate

(d_xz,d_yz) set and contains the unpaired electron.^5,44The d_xy

orbital, however, does not have the proper symmetry to

overlap with any porphyrin orbital unless the TMPFe(III)

bis-pyridine complexes are S₄ruffled.⁵In this case, the lobes

of the d_xyorbital, which are in the mean plane of the

porphyrin ring, have the correct symmetry for partial overlap

with the nitrogens of the the 3a_2u(π) porphyrin orbital

because a component of each of the nitrogen p_zorbitals lies

in the xy plane of the porphyrin ring. This allows porphyrin-

to-metal π electron donation, and spin density can thus be

delocalized to the 3a_2u(π) molecular orbital. This porphyrin

molecular orbital has small electron density at the â-pyrrole

positions and large orbital coefficients at the meso carbons.⁶

This results in small shifts for pyrrole-H resonances from

their diamagnetic positions for the [TMPFe(L)₂]⁺complexes

with lower-basicity pyridine axial ligands and large negative

shifts of the m-H resonances of [OEPFe(t-BuNC)₂]⁺or large

negative shift differences of porphyrin phenyl-H and (δ_m-

δ_p) or (δ_m- δ_o) of [TPPFe(t-BuNC)₂]⁺.¹⁸The orbital overlap

is made possible by the S₄ruffling of the porphyrin core

found in these complexes.^5,18,44

The Curie plots of the pyrrole-H resonances of the series

of pyridine complexes of TMPFe(III) (Figure 3) show clearly

that there is a range of behaviors, from Curie to anti-Curie,

as the basicity of the pyridine decreases. The inverse

temperature dependence seen in eq 4 for the contact and eqs

5 and 6 for the pseudocontact components of the paramag-

netic shift suggests that there should be a smooth linear shift

of the pyrrole-H resonance toward zero paramagnetic shift

as the temperature is raised. This can clearly be seen for

both the 4-Me₂NPy and 3,5-Me₂Py complexes of TMPFe-

(III), where the Curie plots for these two ligand complexes,

shown in Figure 8, are linear and extrapolate to near 0 ppm

at 1/T ) 0. The lowest-basicity pyridine complex pyrrole-H

resonances show anti-Curie behavior, with curved lines and

very negative apparent intercepts for the lowest-temperature

part of the plots. This difference from the expected Curie

behavior can be explained by two possible effects: (1) the

presence of a thermally-accessible excited state and (2) the

rapid ligand on-off exchange with a five-coordinate inter-

mediate of increasingly significant concentration as the

temperature is raised. Each of these processes results in a

[TMPFe(L)₂]⁺Complexes Where L ) Substituted

Pyridine. The pyrrole-H paramagnetic shifts for high-basicity

pyridines (i.e., those having the (d_xy)²(d_xz,d_yz)³ground state)

(53) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura, N. Inorg.

Chim. Acta 1994, 224, 113-124.

(54) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T.; Tajima, K. Inorg.

Chem. 1998, 37, 2405-2414.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7481

Watson et al.

Scheidt et al.⁵⁵found that a related six-coordinate complex,

[OEPFe(3-ClPy)₂]⁺, has an intermediate-spin iron center, and

the bis-4-Me₂NPy and 4-CNPy complexes of OETPPFe(III),

OMTPPFe(III) and TC₆TPPFe(III)¹⁵show S ) 3/2 excited

states. It is thus not surprising that S ) 3/2 excited states

are observed for the complexes in the present study.

The other process that causes nonlinear behavior in the

Curie plots is ligand dissociation because of the decreasing

size of the equilibrium constants for bis-ligand complex

formation as the temperature is raised, combined with rapid

on-off ligand exchange between the bis- and mono-ligand

complexes, which averages the chemical shifts for the five-

and six-coordinate complexes. The ¹H NMR spectra for many

of these complexes, especially the lower-basicity pyridine

complexes, show broadening of the pyrrole-H, o-H_Py, and

free ligand resonances as the temperature is raised above

-30 °C, a clear sign of a kinetic process involving an

intermediate with a different spin state. For strong donors,

such as the more basic pyridine ligands of this study, the

five-coordinate mono-ligand complex is expected to be high-

spin, S ) 5/2, as seen in the five-coordinate TMPFeCl

complex. In the chloroiron(III) high-spin species, the pyr-

role-H resonance lies at +81 ppm at room temperature (Table

S2), and a mono-pyridine complex would be expected to

have a similar pyrrole-H chemical shift. As the temperature

is raised to the point where chemical exchange of the six-

coordinate low-spin complex with the five-coordinate high-

spin complex becomes apparent by broadening of the free

and bound ligand resonances, the position of the pyrrole-H

resonance of the (ostensibly) low-spin complex shifts in a

positive direction more rapidly than expected and the

temperature dependence plot thus curves toward more

positive chemical shifts than expected for Curie behavior.

For the lower-basicity pyridines, however, the five-

coordinate complex could have the intermediate-spin S )

3/2 state (and of course, a spin-admixed state would also be

possible). This S ) 3/2 state is seen in the five-coordinate

Figure 8. Curie plot showing the similar temperature dependence of the

pyrrole-H and m-H_Phof [TMPFe(1-MeIm)₂]⁺and [TMPFe(4-Me₂NPy)₂]⁺.

different electronic state for the complex as the temperature

is raised and thus different paramagnetic shifts for the pyrrole

protons.

The presence of a thermally accessible electronic excited

state has been discussed in the Results section, where fitting

of the paramagnetic shifts to eq 8 was used to estimate the

energy separation between ground and excited states and the

nature of the latter, as well as the spin density coefficients

for the â-pyrrole carbons for both the ground and excited

states. It has been shown previously by EPR and Mo¨ssbauer

spectroscopies,^4,10as well as MCD spectroscopy,^10,42all

recorded at 4.2 K, that the ground-state electronic configu-

ration of [TMPFe(4-CNPy)₂]⁺has the unpaired electron in

the d_xyorbital, while the excited state, found by temperature-

dependent fitting of the paramagnetic shifts in this work,

has S ) 3/2, with â-pyrrole spin densities, F_C, that are similar

to or slightly larger than those of the S ) 1/2 (d_xy)²(d_xz,d_yz)³

ground-state complexes (F_C) 0.0159-0.0193). As men-

tioned in the Results section, this similarity suggests that

the electron configuration of the S ) 3/2 excited state is

3

1

2

1

2

(d_xz,d_yz) (d_xy) (d_z) rather than (d_xy) (d_xz,d_yz) (d_z) , where the

spin densities at the â-pyrrole carbons should be ap-

proximately double these quantities because of the presence

of two d_πunpaired electrons. Thus, the unpaired electron in

the d_πorbital has the proper symmetry to overlap with one

of the 3e(π) molecular orbitals of the porphyrin, resulting

in spin delocalization to the pyrrole positions of a magnitude

similar to that observed for the S ) 1/2 (d_xy)²(d_xz,d_yz)³ground-

state complexes. This causes the resonance to shift toward

less positive paramagnetic shifts as the temperature is

increased. This explains the anti-Curie behavior seen in the

4-CNPy and 3-CNPy complexes of TMPFe(III), as well as

the trend seen as the slopes of the lines move smoothly from

negative to positive as the basicity of the ligand decreases

through the series 4-MePy to 4-CNPy. By the time the

3-CNPy and 4-CNPy complexes are reached, the ground state

has switched to a quite pure S ) 1/2 (d_xz, d_yz)⁴(d_xy)¹electron

configuration with the excited state continuing to be the S

-

TMPFeOClO₃complex, with OClO₃being a fairly weak-

1

field axial ligand. (The H NMR spectra of the porphina-

toiron(III) perchlorate complexes used in this study are

summarized in Supporting Information Table S3.) With the

low-basicity pyridines also being fairly weak-field ligands,

it would not be surprising if their five-coordinate TMP-

Fe(III) complexes were also intermediate-spin or spin-

2

admixed. Because the d_{x -y}orbital is not populated for the

S ) 3/2 (or is only partially populated for spin-admixed)

iron porphyrinates, the five-coordinate mono-pyridine com-

plexes have similar or more negative pyrrole-H shifts than

those of the S ) 1/2 (d_xy)²(d_xz,d_yz)³ground-state complexes,

depending upon the electron configuration of the S ) 3/2 or

spin-admixed S ) 3/2, 5/2 five-coordinate complex, although

this does depend on the 2,6-phenyl substituents,¹³as shown

in Table S3. Thus, the ligand on-off exchange with an

intermediate with a different spin state is a major factor which

must be considered in interpreting the temperature-depen-

dence data for the lower-basicity pyridines; at temperatures

3

1

2

) 3/2 (d_xz,d_yz) (d_xy) (d_z) electron configuration. An increase

in the population of this excited state thus shifts the pyrrole-H

resonance in a negative direction as the temperature increases.

(55) Scheidt, W. R.; Geiger, D. D.; Hayes, R. G.; Lang, G. J. Am. Chem.

Soc. 1983, 105, 2625-32.

7482 Inorganic Chemistry, Vol. 44, No. 21, 2005

Magnetic Spectroscopy of S ) 1/2 Iron(III) Porphyrinates

above -46 °C (1000/T ) 4.4), in this study, it is evident

that ligand dissociation plays a role in determining the

observed chemical shifts of all porphyrin resonances, while

for the higher-basicity pyridines, this does not appear to be

the case. The latter ligands bind to TMPFe(III) with larger

equilibrium constants,⁵⁶and evidence of chemical exchange,

such as line broadening in the NMR spectra, is not observed

until the temperature is raised to +30 to +40 °C. Neverthe-

less, the temperature-dependent fitting of the Curie plots used

data recorded at below -45 °C in all cases.

To investigate the pseudocontact contribution to the

paramagnetic shift, we examined the o-H resonances of the

axial pyridine ligands in detail. The pseudocontact shift

exhibits r^-3dependence (eq 6), and thus the protons closest

to the iron center have the largest pseudocontact shifts; o-H_Py

are only on average 2.98 Å from the Fe center.^3,4To

determine if there is a trend in the shifts of the o-H_Py

resonances of the different complexes, we constructed plots

of the o-H_Pyparamagnetic shift vs pK_a(PyH⁺) at -60 °C for

each of the iron(III) porphyrinate series (Figures 6, 7,

S18-S20). From these plots it is evident that in each case

there is a change in the slope of the line at roughly the pK_a

of the conjugate acid of pyridine itself, with steeper slope

for the higher-basicity pyridines and a less steep slope for

the lower-basicity pyridines.

The 4.2 K EPR data for the [TMPFe(L)₂]⁺complexes with

pyridine axial ligands (Table 1), together with NMR,^4,7

MCD,⁴²and pulsed EPR data,^37,40show that the sign of the

magnetic anisotropy changes throughout the series of ligands.

Thus, for basic pyridine complexes, such as [TMPFe(4-Me₂-

NPy)₂]⁺, g_z> g_x, g_y, indicating that the unpaired electron is

in the d_xz,d_yzorbital set, while for the weakly basic pyridine

complexes, such as [TMPFe(4-CNPy)₂]⁺, g_x, g_y> g_z,

indicating that the unpaired electron is in the d_xyorbital. A

change in sign of the magnetic anisotropy will change the

sign of the pseudocontact shift (eq 6). The o-H_Pyresonances

are the most sensitive to changes in the pseudocontact shift,

and because there is a pronounced change in slope of the

o-H_Pyparamagnetic shift vs pK_a(PyH⁺), it can be reasoned

that this change in the sign of the magnetic anisotropy occurs

between a pK_aof 5 and 6 for the TMP complexes. It should

be noted, however, that the ligand basicity at which the

thermally accessible S ) 3/2 excited state begins to be

detected is similar, and thus, the changes in the electron

configuration and thermal excitation appear to occur hand-

in-hand. This has less influence on our understanding of the

TMPFe(III) complexes than of the other four iron porphy-

rinates, as will become evident below.

though the gross magnetic behavior of all five sets of

porphyrinate complexes appears to be fairly similar. The

separation method and results are discussed in detail in the

Supporting Information. As a summary, we can say that the

plot of paramagnetic shift of the o-H_Pyresonance vs pK_a-

(PyH⁺) (Figure 6) and the paramagnetic shift of the m-H_Ph

resonance of the bis-(4-Me₂NPy) complex of TMPFe(III)

(which was assumed to be totally pseudocontact in na-

ture)^6,16,24play central roles in separating the contact and

pseudocontact shifts.

With the assumption that the m-H_Phparamagnetic shift is

totally pseudocontact in nature, the pseudocontact shift of

the o-H_Pyof [TMPFe(4-Me₂NPy)₂]⁺could be calculated from

the ratio of the geometric factors for the m-H_Phand o-H_Py

protons for the TMPFe(III) complex (Supporting Information

Table S8). From that value of δ_pcof the o-H_Py, δ_confor these

protons could immediately be calculated from δ_parafor the

same protons (eq 3). Now, with the assumption that δ_pc

becomes zero at a ligand basicity close to that of Py or

4-MePy, the δ_paraof the o-H_Pyfor that complex is identically

equal to δ_con. Using these two contact shift data points, a

straight line representing the dependence of δ_conon pyridine

basicity can be drawn in Figure 6 to produce a new version

of it (Supporting Information Figure S21) which allows δ_con

and δ_pcof the o-H_Pyof each complex to be determined. Then,

using the geometric factors for the o-H_Py, m-H_Ph, and

pyrrole-H from Supporting Information Table S8, the values

of δ_pcfor the o-H_Pycan be used to calculate those for the

m-H_Phand pyrrole-H of each complex.

Thus, the change in the slope of the plot of δ_para(o-H_Py) at

a pK_a(PyH⁺) close to that of Py or 4-MePy (Figure 6) is

indicatiVe of a change in sign of the g anisotropy, eq 6, and

hence a change in the relatiVe energy ordering of the d_xy

and d_πorbitals of the metal. The results, presented in

Supporting Information Table S9 for the nine TMPFe(III)

complexes using 4-MePy as the point at which the g

anisotropy changes sign, show that the pyrrole-H contact shift

at -60 °C decreases in magnitude from -27.7 ppm for L )

4-Me₂NPy to -14.6 ppm for L ) 4-CNPy, while the

pseudocontact shift of the same protons varies from -8.6 to

+6.4 ppm over the same series.

[(2,6-Br₂)₄TPPFe(L)₂]ClO₄, [(2,6-Cl₂)₄TPPFe(L)₂]ClO₄,

[(2,6-F₂)₄TPPFe(L)₂]ClO₄, and [(2,6-(OMe)₂)₄TPPFe(L)₂]-

ClO₄Complexes. For all of these complexes, it can be seen

from the spin densities at the â-pyrrole carbons (Table 2)

that all low-basicity pyridines have mixed S ) 1/2 (d_xy)²-

(d_xz,d_yz)³/(d_xz,d_yz)⁴(d_xy)¹ground states over the temperature

range of the NMR measurements plus a thermally-accessible

S ) 3/2 excited state that contributes signficantly to the NMR

paramagnetic shifts, even at -60 °C (213 K). Thus, none of

the low-basicity pyridine complexes of these four iron(III)

porphyrinates has a pure (d_xz,d_yz)⁴(d_xy)¹ground state over the

temperature of the NMR measurements. The EPR spectra,

however, show that at 4.2 K the complexes from L ) 4-HPy

to 4-CNPy all have the (d_xz,d_yz)⁴(d_xy)¹ground state.

Separation of the Contact and Pseudocontact Shifts for

[TMPFe(L)₂]ClO₄. The ability to separate the contact and

pseudocontact shifts of a series of related metalloporphyrinate

complexes is an obvious sign that the magnetic properties

of the systems of interest are clearly understood. In the

present study, the TMP complexes are the only ones that

permit this separation with any degree of confidence, even

For all of the [(2,6-X₂)₄TPPFe(L)₂]⁺complexes, the o-H_Py

δ_paravs pK_a(PyH⁺) plot was examined to obtain evidence

for the basicity at which the g anisotropy changes sign. And

(56) Nesset, M. J. M. The Effect of Axial Ligands and Porphyrin

Substituents on the Dynamic and Electronic Properties of Model

Hemes. Ph.D. Dissertation, University of Arizona, Tuscon, AZ 1994.

Inorganic Chemistry, Vol. 44, No. 21, 2005 7483

Watson et al.

in all cases, there is a change in slope of the pK_a(PyH⁺)

near the parent Py data point (5.22). If the pseudocontact

shift is zero at this point and the [(2,6-X₂)₄TPPFe(4-Me₂-

NPy)₂]⁺complex of each has the purest (d_xy)²(d_xz,d_yz)³ground

state, the o-H_Pycontact shift for [(2,6-X₂)₄TPPFe(4-Me₂-

NPy)₂]⁺at -60 °C can be calculated and a similar deter-

mination of the pseudocontact and contact shifts of the lower-

basicity pyridines to that discussed above for the TMPFe(III)

complexes could be carried out. However, such treatments

show increasingly (rather than decreasingly) negative contact

shifts for the pyrrole-H resonance as the basicity of the

pyridine is decreased, which are inconsistent with the

expectation that the contact shift should become less negative

as the transition to the (d_xz,d_yz)⁴(d_xy)¹ground-state becomes

more complete, as found for the [TMPFe(L)₂]⁺complexes

discussed above. Other assumptions, including using the spin

density determined for the pyrrole â-carbons of the bis-4-

CNPy complexes of each iron porphyrinate (Table 2) to

estimate the degree of transition from the (d_xy)²(d_xz,d_yz)³to

the (d_xz,d_yz)⁴(d_xy)¹ground state throughout the series of

pyridine complexes for each at best gave a fairly constant

contact shift throughout the series. Thus, it is clear that the

mixed-orbital ground state of most lower-basicity pyridine

complexes and the presence of a thermally accessible S )

3/2 excited state in each case preclude the separation of the

pseudocontact and contact contributions to the paramagnetic

shifts of the ground states of the four [(2,6-X₂)₄TPPFe(L)₂]⁺

series of iron(III) porphyrinates, and the changes in slope of

the o-H_Pyparamagnetic shift vs pK_a(PyH⁺) observed for these

four series of iron(III) porphyrinates are likely to be the result

of the increasing contribution from the S ) 3/2 excited state

as the ligand basicity is decreased.

of the [TMPFe(4-NMe₂Py)₂]⁺complex, which demonstrates

Curie behavior, to the [TMPFe(4-CNPy)₂]⁺complex which

shows definite anti-Curie behavior resulting from a quite pure

(d_xz,d_yz)⁴(d_xy)¹ground state with thermal population of the S

) 3/2 (d_xz,d_yz) (d_xy) (d_z) excited state. For [(2,6-Br₂)₄TPPFe-

(L)₂]⁺, [(2,6-Cl₂)₄TPPFe(L)₂]⁺, [(2,6-F₂)₄TPPFe(L)₂]⁺, and

[(2,6-(OMe)₂)₄TPPFe(L)₂]⁺, the complexes with higher-

basicity pyridine axial ligands also show Curie behavior,

while the low-basicity complexes show non-Curie behavior.

None show the anti-Curie behavior seen for [TMPFe(4-

CNPy)₂]⁺and [TMPFe(3-CNPy)₂]⁺(Figure 3).

3

1

2

When one attempts to separate the contact from the

pseudocontact shifts using the δ_para(o-H_Py) vs pK_a(PyH⁺)

plots, it becomes apparent that the contact shift line calculated

by assuming the m-H_Phshift of [TMPFe(4Me₂NPy)₂]⁺is

totally pseudocontact in nature and then using the geometric

factors of the m-H_Phand the 4-NePy δ_para) δ_conpoint of

the o-H_Pyvs pK_a(PyH⁺) plot of Figure S21 leads to the best

estimate of the contact shifts. However, the same procedure

does not work for the other four series of Fe(III) porphyri-

nates.

Acknowledgment. The financial support of the National

Institutes of Health (Grant DK-31038) is gratefully acknowl-

edged. The authors wish to thank Professor Michael F.

Brown of the University of Arizona and Professor Ursula

Simonis of San Francisco State University for use of NMR

spectrometers for part of this work. This paper was written

while F.A.W. was an Alexander von Humboldt Awardee in

Science in the Physics Institute at the University of Lu¨beck.

She wishes to thank her host, Professor Alfred X. Trautwein,

for his hospitality and friendship during this stay.

Conclusions. The ¹H NMR spectra of the [TMPFe(L)₂]⁺

and [(2,6-X₂)₄TPPFe(L)₂]⁺complexes as a function of

temperature exhibit a range of behaviors. The [TMPFe(L)₂]⁺

paramagnetic shifts encompass every situation seen in the

other complexes, from the purest (d_xy)²(d_xz,d_yz)³ground state

Supporting Information Available: Figures S1-S20 and

Tables S1-S7. This material is available free of charge via the

Internet at http://pubs.acs.org.

IC0507316

7484 Inorganic Chemistry, Vol. 44, No. 21, 2005

Article Doi

NMR and EPR studies of low-spin Fe(III) complexes of meso-tetra-(2,6- disubstituted phenyl)porphyrinates complexed to imidazoles and pyridines of widely differing basicities

DOI: 10.1021/ic0507316

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Article abstract of DOI:10.1021/ic0507316

Full text of DOI:10.1021/ic0507316

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