Electronic structure of low-spin ferric chlorins: Characterization of bis(dimethylphenylphosphine)(meso-tetraphenyl-chlorinato)iron(III) triflate

Article abstract of DOI:10.1021/ic0013328

The title compound has been prepared by the reaction of dimethylphenylphosphine and (mesotetraphenylchlorinato) iron(III) triflate. Its structural characteriztion demonstrates that the low-spin iron (III) atom is centerd in the heme plane with a radial expansion of the chlorinato core.

Full text of DOI:10.1021/ic0013328

4494
Inorg. Chem. 2001, 40, 4494-4499
II,15 all from Escherichia coli, to perform diverse biological
Electronic Structure of Low-Spin Ferric
Chlorins: Characterization of
Bis(dimethylphenylphosphine)(meso-tetraphenyl-
chlorinato)iron(III) Triflate
reactions. A chlorin is a hydroporphyrin with one reduced
pyrrole double bond. There is currently limited data available
of the physical properties of iron chlorins to explain their
biological properties. In contrast, much more information is
available with iron porphyrins. Thus it has been accepted that
most of the low-spin iron(III) have a (dxy)2 (dxz, dyz)3 ground
state. However, we reported that when two molecules of tert-
butylisocyanide are bound to ferric tetraphenylporphyrin, the
1H NMR spectrum is indicative of a low spin complex which
shows the chemical shifts of the pyrrole protons in the
diamagnetic area.16,17 The hyperfine shifts were separated into
their dipolar and contact contributions. The separated compo-
nents reflect the very low magnetic anisotropy of the iron, and
the unusual orientation of the unpaired spin density when the
nitrogen axial ligands are exchanged for isocyanide ligands leads
to complete reverse localization. Subsequently, it was recognized
Marwan Kobeissi,† Loˆıc Toupet,‡ and
Ge´rard Simonneaux*,†
Laboratoire de Chimie Organome´tallique et Biologique,
URA CNRS 415, Universite´ de Rennes 1,
35042 Rennes Cedex, France, and Groupe de Physique
Cristalline, UA CNRS 040804, Universite´ de Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
ReceiVed NoVember 27, 2000
1
Introduction
that the unusual H NMR behavior results from the formation
of an unusual (dxz, dyz)4 (dxy)1 ground state.18,19 It should be
underlined that a similar (dxz, dyz)4 (dxy)1 situation was also
reported with low-basicity cyanopyridine complexation to
ferriporphyrins. The change in ground state of low-spin iron-
(III) from (dxy)2 (dxz, dyz)3 to (dxz, dyz)4 (dxy)1 electron config-
uration occurs progressively through the series of pyridine
complexes of both Fe(III)TPP and Fe(III)TMP complexes, the
low-basicity pyridines stabilizing the unusual (dxz, dyz)4 (dxy)1
state.3,20 The axial EPR spectra, with g > g| are also indicative
of a (dxz, dyz)4 (dxy)1 state. With the TPP compound, the X-ray
structure shows that the two axial ligands have relative
perpendicular orientations along with an extensively S4-ruffled
porphyrin core which is related to electronic factors rather than
steric factors. This electronic contribution may be due to the
partial delocalization of the (dxy)1 unpaired electron into the 3a2u-
(π) orbital of the porphyrin ring, which is made possible by
the twisting of the nitrogen pz orbitals of the nitrogen out of
the plane of the porphyrin ring, as suggested recently.3,20 The
Mo¨ssbauer spectrum shows also an unusually small positive to
large negative quadrupole splitting in the ferric tetraphenylpor-
phyrinato complexes.3
There is currently a large interest in determining what factors
affect the electronic structure of low-spin hemoproteins. Thus
an attractive attempt regarding the structure-function relation-
ship is to substitute oxygen by various exogen ligands.1 Several
small nonphysiological ligands have been examined by various
spectroscopic methods in hope of gaining information about
structural and reactivity perturbations of the heme environment.
Examples using iron porphyrin models include mainly carbon
monoxide, cyanide, isocyanides, and to a less extent phos-
phines.2 So far, experimental investigations have mostly focused
on the electronic structure of low-spin iron(III) porphyrins.3
Studies of the electronic structures with simple iron chlorins
which might serve as models for naturally occurring iron chlorin
proteins have been more limited.4-13 This is related to the
synthetic difficulty of preparing pure compounds due to the
oxygen sensitivity of reduced macrocycle.
Nature utilizes iron chlorins such as heme d, found in a
terminal oxidase complex,14 or a catalase, hydroxyperoxidase
† Laboratoire de Chimie Organome´tallique et Biologique.
‡ Groupe de Physique Cristalline.
(1) Simonneaux, G. Coord. Chem. ReV. 1997, 165, 447-474.
(2) Simonneaux, G.; Bondon, A. In The Porphyrin Handbook; Khadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2000; Vol.5, p 299-322.
To find out the general conditions for the formation of low-
spin ferric chlorin complexes with the (dxy)2 (dxz, dyz)3 or
(dxz,dyz)4 (dxy)1 ground state, we have decided to examine a large
series of low-spin iron(III) chlorin complexes bearing various
different axial ligands by means of 1H NMR and EPR
spectrocopies and X-ray crystallography. We previously ob-
served an unusual electronic structure for [(TPC)Fe(t-BuNC)2]-
(3) Walker, F. A. The Porphyrin Handbook; Khadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
5, p 81-183.
(4) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc.
1981, 103, 4763-4778.
CF3SO3 by H NMR.21 We have now further extended our
(5) Strauss, S. H.; Silver, M. E.; Long, K. M.; Thompson, R. G.; Hudgens,
R. A.; Spartalian, K.; Ibers, J. A. J. Am. Chem. Soc. 1985, 107, 4207-
4215.
1
studies to the preparation and complete spectral characterization
(NMR and EPR) of [(TPC)Fe(PPh(Me)2)2]CF3SO3. Its X-ray
(6) Strauss, S. H.; Pawlik, M. J. Inorg. Chem. 1986, 25, 1921-1923.
(7) Strauss, S. H.; Pawlik, M. J.; Skowyra, J.; Kennedy, J. R.; Anderson,
O. P.; Spartalian, K.; Dye, J. L. Inorg. Chem. 1987, 26, 724-730
(8) Pawlik, M. J.; Miller, P. K.; Sullivan, E. P.; Levstik, M. A.; Almond,
D. A.; Strauss, S. H. J. Am. Chem. Soc. 1988, 110, 3007-3012.
(9) Dixon, D. W.; Woehler, S.; Hong, X.; Stolzenberg, A. M. Inorg. Chem.
1988, 27, 3682-3685.
(15) Chiu, J. T.; Loewen, P. C.; Switala, J. G., Gennis, R. B.; Timkovich,
R. J. Am. Chem. Soc. 1989, 111, 7046-7050.
(16) Simonneaux, G.; Hindre´, F.; Le Plouzennec, M. Inorg. Chem. 1989,
28, 823-825.
(17) Geze, C.; Legrand, N.; Bondon, A.; Simonneaux, G. Inorg. Chim.
Acta 1992, 195, 73-76.
(10) Licoccia, S.; Chatfield, M. J.; La Mar, G.; Smith, K. M.; Mansfield,
K. E.; Anderson, R. R. J. Am. Chem. Soc. 1989, 111, 6087-6093.
(11) Ozawa, S.; Watanabe, Y.; Nakashima, S.; Kitagawa, T.; Morishima,
I. J. Am. Chem. Soc. 1994, 116, 634-641.
(18) 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.
(12) Ozawa, S.; Watanabe, Y.; Morishima, I. J. Am. Chem. Soc. 1994, 116,
5832-5838.
(19) 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.
(13) Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
(20) 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.
(14) Timkovich, R.; Cork, M. S.; Gennis, R. B.; Johnson, P. Y. J. Am.
Chem. Soc. 1985, 107, 6069-6075.
10.1021/ic0013328 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/11/2001

Notes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4495
Table 1. Crystallographic Data for Fe(TPC)[P(Me)2Ph]2CF3SO3 2
2 (C5H12)
structure has also been determined which is one of few stucture
determinations of a low-spin ferric chlorin complex.(13)
A
empirical formula 2(C61H52FeF3N4P2O3S) V, Å3
2796.2(2)
1
comparison with [(TPP)Fe (PPh(Me)2)2]ClO4 shows a small
distortion of the chlorin ring in the former complex.22
fw
2263.83
triclinic
P1
14.1570 (4)
14.1860 (6)
16.1270 (8)
75.202(3)
77.913(2)
63.979(2)
Z
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
F calcd g.cm-3 1.345
µ cm-1
T (K)
Rw
4.36
294
0.187
0.071
Experimental Section
General Information. As a precaution against the formation of the
µ-oxo dimer [Fe(TPC)]2O,7,23 all reactions were carried out in dried
solvents in Schlenk tubes under an Ar atmosphere. Solvents were
final R
1
distilled from appropriate drying agents and stored under argon. H
NMR spectra were recorded on a Bruker AC 300P spectrometer in
CD2Cl2 or CDCl3 at 300 MHz. Tetramethylsilane was used as the
internal reference. The temperatures are given within 1 K. EPR spectra
were recorded in CH2Cl2 on a Bruker EMX 8/2,7 spectrometer operating
at X-band frequencies. Samples were cooled to 4.2 K in a stream of
helium gas in frozen CH2Cl2, the temperature of which was controlled
by an Oxford Instruments ESR 900 cryostat. Visible spectra were
measured on a Uvikon 941 spectrometer in CH2Cl2.
conformationally different cations and the two disordered anions.26 The
two resulting complexes are designated 2A and 2B. After anisotropic
refinement, some peaks were found and assigned to the nonstoichio-
metric pentane (30%). The whole structure was refined by the full-
matrix least-squares techniques27 (use of F magnitude; x, y, z, âi,j for
Fe, N, P, and C atoms; x, y, z for triflate anions and riding mode for
hydrogen atoms; 1320 variables and 8606 observations with I > 2.0σ-
Reagents. The following iron chlorins were prepared by literature
methods: (TPC)FeCl24 and (TPC)FeCF3SO3.21 P(Me)2Ph is com-
mercially available.
(I); calc w ) 1/[σ2(Fo)2 + (0.125P)2 + 2.06P] where P ) (Fo + 2
2
Fc2)/3 with the resulting R ) 0.071, Rw ) 0.187 and Sw ) 1.029
(residual ∆F < 0.76 eA-3). Atomic scattering factors were from the
International Tables for X-ray crystallography.28 ORTEP views were
realized with PLATON98.29
Synthesis. (TPC)Fe[P(Me)2Ph]2 1. A solution of (TPC)FeCl (0.1
g, 0.14 mmol) in dichloromethane was reduced under argon by Zn-
Hg amalgam. The solution was then filtered and 8 equiv of dimeth-
ylphenylphosphine added by a syringe to in situ (TPC)Fe species.
Hexane addition (30 cm3) was added gradually and the solution set
aside for crystallization. Fine crystals were collected by filtration.
Yield: 0.094 g (70%). UV-vis (toluene): λmax/nm 370 (ꢀ 13.7 dm3
Results and Discussion
Preparation of Compounds 1 and 2. The synthesis of the
symmetric bis-phosphine species (TPC)Fe(PMe2Ph)2 1 is a
variation of our method, previously reported for the synthesis
of (TPP)Fe(PMe2Ph)2,30 and the reduction of Fe(III) to Fe(II)
was carried out with Zn-Hg amalgam. The compound is
obtained as a crystalline solid with 70% yield and characterized
1
mmol-1 cm-1), 444 (ꢀ 62.4) 569 (ꢀ 7.9), 609 (ꢀ 17.1), 654 (ꢀ 4.3). H
NMR (δ, CDCl3, ppm, 273 K): 8.2 (2H, Hpyrr), 7.8 (4H, Ho), 7.6 (m,
20 H, Hpyrr, Ho,m,p), 4.35 (4H, pyrrolidine). Ligand: 6.9 (Hp), 6.7 (Hm),
4.9 (Ho) and -2.35 (Me). FAB MS (m/z): [M]+ 946.3, [M -
P(Me)2Ph]+ 808.3, [M - 2P(Me)2Ph] 670.3.
1
by H NMR (see above).
[(TPC)Fe(PPh(Me)2)2]CF3SO3 2. To a solution of 0.1 g (0.12 mmol)
of (TPC)FeCF3SO3 in 2.5 mL of dichloroethane was added 2.5 equiv
(43.5 µL) of P(Me)2Ph by a syringe under stirring at room temperature.
After being stirred for 15 mn, the solution became green. Then 5 mL
of pentane was added and the solution was set aside overnight for
crystallization at 0 °C. Purple crystals of Fe(TPC)[P(Me)2Ph]2CF3SO3
were collected by filtration and washed with hexane. Yield: 0.097 g
(73%). UV-vis (CH2Cl2): λmax/nm 371 (ꢀ 33.7 dm3 mmol-1 cm-1),
437 (ꢀ 65.6), 605 (ꢀ 13.5).
Two major difficulties may be encountered in preparing
phosphine ferric complexes of chlorins. First, oxidation of the
chlorin ring to the porphyrin ring may occur, as was previously
reported with imidazole ligands.12 Second, a possible autore-
duction of the ferric state may occur. This situation was
previously encountered with ferric porphyrins.22,31Using triflate,
a weak axial ligand, as an intermediate, allowed us to solve
this problem. Thus, a similar strategy was used to prepare low-
spin ferric tetraphenylchlorins. Starting from (TPC)Fe(CF3SO3)
in dichloroethane solution, [(TPC)Fe(PPh(Me)2)2]CF3SO3 2 was
readily obtained from P(Me)2Ph addition in 70% yield. 2
exhibited two Soret bands, one at 437 nm and the second in
the near-ultraviolet region (371 nm). It is interesting to note
that hyperporphyrin spectra were reported for bis(phosphine)
iron(III) porphyrin complexes, the highest wavelength being red-
shifted (442 nm).22 Suitable crystals were obtained by diffusion
of hexane in the dichloroethane solution of 2 in a thin tube.
Structure Determination. The crystal structure of 2 is
comprised of two independent ion pairs in the asymmetric unit
of structure. The iron chlorines differ in the orientation of the
phenyl rings of the axial ligands with respect to the porphyrin
plane. Compound 2A has its phosphine ligands with the aromatic
Single-Crystal Structure Determination on Fe(TPC)-
[P(Me)2Ph]2CF3SO3 2. The X-ray study was carried out on a NONIUS
Kappa CCD diffractometer using graphite monochromatized Mo KR
radiation. The cell parameters were obtained with Denzo and Scalepack25
with 10 frames (Φ rotation: 1° per frame). Crystallographic data are
presented in Table 1. Crystals of the compound were obtained as
reported above. The data collection (2θmax ) 60°, 91 frames via 2.0°
φ rotation and 60 s per frame, 46 frames via 2.0° ω rotation and 60 s
per frame, range hkl: h, 0.18; k, -16.18; l, -20.21, gave 14 211
reflections. The data reduction leads to 13 322 independent reflections
from which 8606 reflections satisfied I > 2.0σ(I). The structure was
solved with SIR-97, which reveals the non-hydrogen atoms of the two
(21) Starting from Fe(TPC)Cl24 in tetrahydrofuran solution, Fe(TPC)CF3-
SO3 3 was readily obtained from AgCF3SO3 addition in 70% yield.
The [Fe(TPC)]CF3SO3 complex exhibited UV-vis spectra with a Soret
band at 402 nm (ꢀ ) 66 dm3 mmol-1 cm-1) and a characteristic chlorin
second band at 649 nm (ꢀ ) 9 dm3 mmol-1 cm-1). 1H NMR δ, CDCl3,
ppm, 273 K: 114 (br, 2H, Hpyrr), 88 (br, 2H, Hpyrr), 58 (br, 2H, Hpyrr),
25.5 (br, 4H, pyrrolidine), 12.35 (br, 8H, Ho); 11 (br, 4H, Hm) and
10.3 (br, 4H, Hm); 9.6 (br, 2H, Hp); 9.2 (br, 2H, Hp). EPR: g ) 5.87
and 1.99. Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans.,
in press.
(26) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 1998, 31, 74-77.
(27) Sheldrick, G. M. SHELX97. Program for the refinement of crystal
structures; Go¨ttingen University: Germany, 1997.
(28) International Tables for X-ray crystallography; Kluwer Academic
Publishers: Norwell, MA, 1992; Vol. C.
(22) Simonneaux, G.; Sodano, P. Inorg. Chem. 1988, 27, 3956-3959.
(23) Fleischer, E. B.; Palmer, J. M.; Srivastava, T. S.; Chatterjee, A. J.
Am. Chem. Soc. 1971, 93, 3162-3167.
(24) Feng, D.; Ting, Y. S.; Ryan, M. D. Inorg. Chem. 1985, 24, 612-617.
(25) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(29) Spek, A. L. PLATON. A multipurpose crystallographic tool; Utrecht
University: Utrecht, The Netherlands, 1998.
(30) Sodano, P.; Simonneaux, G.; Toupet, L. J. Chem. Soc., Dalton Trans.
1988, 2615-2620.
(31) La Mar, G.; Del Gaudio, J. J. Bioinorg. Chem. 1977, 2, 207-226.

4496 Inorganic Chemistry, Vol. 40, No. 17, 2001
Notes
Figure 1. Perspective view for the cations (a) 2A and (b) 2B in crystal
2.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Fe(TPC)[P(Me)2Ph]2CF3SO3 2
Molecule A
Fe(1) P(1)
Molecule B
2.350(3) Fe(61) P(61)
Figure 2. Atom labels and formal diagrams of the chlorinato core
showing the distances for the cations (a) 2A and (b) 2B.
2.356(4)
2.358(4)
2.046(9)
1.974(9)
2.012(10)
1.997(9)
1.479(16)
1.327(13)
1.385(13)
1.353(14)
Fe(1) P(2)
Fe(1) N(1)
Fe(1) N(2)
Fe(1) N(3)
Fe(1) N(4)
C(2) C(3)
C(7) C(8)
C(12) C(13)
C(17) C(18)
P(1) Fe(1) P(2) 179.4(2)
N(1) Fe(1) P(1) 89.9(3)
N(2) Fe(1) P(1) 91.5(3)
N(3) Fe(1) (P1) 89.6(3)
N(4) Fe(1) P(1) 89.1(3)
2.348(3) Fe(61) P(62)
2.0009(8) Fe(61) N(61)
1.971(9) Fe(61) N(62)
2.022(10) Fe(61) N(63)
1.992(9) Fe(61) N(64)
1.446(16) C(62) C(63)
1.368(19) C(67) C(68)
1.373(18) C(72) C(73)
1.362(16) C(77) C(78)
N(3), and N(4) (average Fe-N distance: 1.995(9) Å) in A.
Similar situation was found in B: Fe-N(61) ) 2.046(9) Å and
average Fe-N distance: 1.994(9) Å. In [(TPP)Fe(P(Me)2Ph)2]-
ClO4,32 the two different Fe-N distances, 1.992(2) Å and 1.988-
(2) Å, average 1.990(2) Å in agreement with other low-spin
iron(III) porphyrin structures (range: 1.970(14)-2.000(6) Å.33
Thus there is a core-hole expansion of the macrocycle due to
the reduced pyrrole. Such a situation has been previously
reported for high spin iron(III) quinoxalinotetraphenylporphy-
rin13 and iron(III)dioxoisobacterio chlorin,34 and discussed with
molecular mechanics calculations.35,36
P(61) Fe(61) P(62) 179.5(2)
N(61) Fe(61) P(61) 90.7(3)
N(62) Fe(61) P(61) 89.2(3)
N(63) Fe(61) P(61) 87.5(3)
N(64) Fe(61) P(61) 91.2(3)
groups parrallel to two trans pyrrole rings; 2B has its phosphine
ligands with the aromatic groups parallel to two phenyls of TPC.
Thus the orientation of the ligands with respect to the N(1)-
Fe-N(3) axis is not equivalent for the two independent
molecules. The angle, defined as the angle between the
N-Fe-N axis and the projection of the ligand normal onto the
porphyrin plane, is 53.4(2)° for molecule A and 172.8(1)° for
molecule B. The structures of 2A and 2B are shown in Figure
1. Individual values of bond distances and angles for 2A and
2B are given in Table 2.
The C(2)-C(3) (2A: 1.446(16) Å, 2B: 1.479(16) Å) distance
in the pyrroline ring is longer than the usual values of the three
remaining pyrroles (average value of 1.367(18) Å) and reflects
the sp3 hybridization of the corresponding pyrroline atoms. Such
(32) Toupet, L.; Sodano, P.; Simonneaux, G. Acta Crystallogr. 1990, C46,
1631-1633.
(33) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.
(34) Barkigia, K. M.; Chang, C. K.; Fajer, J.; Renner, M. W. J. Am. Chem.
Soc. 1992, 114, 1701-1707.
(35) Kaplan, W. A.; Suslick, K. S.; Scott, R. A. J. Am. Chem. Soc. 1991,
113, 9824-9827.
The metal-nitrogen distance to the reduced pyrrole, N(1),
2.009 (8) Å, is longer than the remaining three nitrogens: N(2),
(36) Eschenmoser, A. Ann. N.Y. Acad. Sci. 1986, 471, 108

Notes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4497
Figure 3. 1H NMR spectrum of [(TPC)Fe (PPh(Me)2)2]CF3SO3 2 recorded at 283 K in CD2Cl2.
Table 3. Observed and Isotropic Shifts of [Fe(TPC)(PMe2Ph)2]CF3SO3 2 (δ, CD2Cl2, ppm)
proton chlorin
Ho
Ho′
Hm
Hm′
Hp
Hp′
Hpyrro
Hpyr
Hpyr
Hpyr
(∆H/H)a
6.85
7.6
-0.75
6.98
7.8
-0.82
7.62
7.6
0.02
7.71
7.6
0.11
7.10
7.6
-0.5
Hm
5.28
7.44
7.6
-0.16
64.6
4.35
60.25
0.66
7.6
-7.14
0.66
7.6
-7.14
-57.86
8.2
-65.66
(∆H/H)b
c
(∆H/H)iso
proton ligand
Ho
Hp
Me
(∆H/H)a
(∆H/H)b
(∆H/H)iso
8.3
4.9
3.4
9.32
6.9
2.42
-13.2
-2.35
-10.85
6.7
-1.42
c
a Chemical shift of [(TPC)Fe(PMe2Ph)2]CF3SO3 at 298 K with TMS as internal reference. Abbreviations used: Ho and Ho′ ) ortho-protons; Hm
and Hm′ ) meta-protons; Hp and Hp′ ) para-protons. b Chemical shift of (TPC)Fe(PMe2Ph)2 at 298 K with TMS as internal reference. c Isotropic
shift of 2 with diamagnetic complex (TPC)Fe(PMe2Ph)2 as reference. We used a medium value of 7.8 ppm for the chemical shifts of the diamagnetic
pyrroles since the relative assignment was not possible at this stage.
a situation was previously observed with two iron chlorins: the
ferrous octaethylchlorin (OEC)Fe (C-C distance: 1.508(7) Å)5
and with the µ-oxo complex [(TPC)Fe]2O (C-C distance:
1.419(9) Å).7
The pyrrole and pyrroline rings are only slighly displaced
above and below the mean plane of the chlorin (maximum
displacement: 0.12(2) Å for 2A and 0.15(2) Å for 2B). Thus
the conformation of the chlorin macrocycle can be described
as weakly distorted. Figure 2 gives out of plane distances for
the atoms in the chlorin core from the mean chlorin plane.
The axial Fe-P distances are very similar to those in the
iron(III) complex of tetraphenylporphyrin containing the same
axial ligand (2.350(1) Å).32 but are longer than in the iron(II)
complex of TPP containing the same ligand (2.284(1) Å).30
1H NMR Spectroscopy. The 1H NMR spectrum of [(TPC)-
Fe(PPh(Me)2)2]CF3SO3 2 at 297 K is shown in Figure 3. The
peaks for the phenyl protons of the porphyrin ring are assigned
completely by 2D COSY spectra (Supporting Information). For
Figure 4. Curie plot of the chemical shifts vs reciprocal temperature
the phosphine ligands and the pyrroles, the relative intensities
and 2D COSY determine the assignment. The shifts of the
phosphine ligands are independent of the presence of excess
ligand, and hence axial ligand dissociation does not appear to
be significant at ambient temperature. Magnetic measurements
using the method of Evans37,38 were made for 0.03 M CD2Cl2
solutions of 2 at 297 K, employing Me4Si as the reference. The
solution magnetic moment m ) 1.87µB) is compatible with a
of [(TPC)Fe (PPh(Me)2)2]CF3SO3 2 in CD2Cl2.
-12.3, and -47.1 ppm, 203 K).12 The hyperfine shifts, obtained
by referencing the observed shift to that of the corresponding
diamagnetic complex (TPC)Fe(PPh(Me)2)2 are summarized in
Table 3. The hyperfine shifts for symmetrical low-spin ferric
porphyrins are known to consist of large contact shifts and
smaller upfield dipolar shifts due to the magnetic anisotropy.
In chlorin, the symmetry is lost but La Mar and co-workers
have suggested that such a situation is also highly probable with
low-spin ferric chlorins.10 Thus they have noted that the HOMO
in ferric chlorins (a1u) is symmetry and energitically situated
for interactions with the iron dπ orbitals.39
1
low-spin state, S ) /2.
The spectrum of [(TPC)Fe (PPh(Me)2)2]CF3SO3 2 shows the
pyrrole proton signals at 0.66 (4H) and -57.8 (2H) ppm (293
K). This is quite different from the pyrrole proton signals of
another low-spin Fe(III) chlorinates at ambient temperature [Fe-
(QTPP)(Im)]+ (-12.2, -15.6 and -21.7 ppm, 293 K)13 but
close to the pyrrole proton signals of [(TMC)Fe(Im)2]Cl (-2.4,
For the saturated pyrroline ring, the protons are expected to
exhibit low-field π contact shifts.10 The observed large low-
(37) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
(38) Sur, S. K. J. Magn. Reson. 1989, 82, 169-173.
(39) Chatfield, M. J.; La Mar, G. N.; Parker, W. O.; Smith, K. M.; Leung,
H. K.; Morris, I. M. J. Am. Chem. Soc. 1988, 110, 6352-6358.

4498 Inorganic Chemistry, Vol. 40, No. 17, 2001
Notes
Figure 5. EPR spectrum of [(TPC)Fe(PPh(Me)2)2]CF3SO3 2 in a CH2Cl2 glass, recorded at 4 K.
field shift (δ ) 69 ppm) agrees with an important π metal
bonding involving a molecular orbital of the chlorin derived
from the a1u of a porphyrin. Nevertheless, this contact shift is
much higher than those of the corresponding [(TMC)Fe(Im)2]-
Cl (δ ) 38.3 ppm)12 and pyropheophorbide a methylester iron-
(III) (δ ) 24 and 28 ppm).10 Since the contact shift has been
shown to arise predominantly from spin delocalization into the
bonding dxz and dyz orbitals,10 this result agrees with a (dxy)2-
(dxz,dyz)3 electronic structure in this aryl phosphine derivatives
(see below).
The isotropic shift obtained for the phosphine methyl groups
of PMe2Ph in [(TPP)Fe[P(Me)2Ph]2]ClO4 is slightly upfield
(-2.5 ppm).22 A dramatic change is observed for [TPC)Fe-
(P(Me)2Ph)2]CF3SO3. In comparison with the porphyrin, the
signal is at a stronger field (δ ) - 14 ppm), implying that
contact shift dominates. This large upfield methyl proton contact
shifts agree with unpaired spin density on the methyl group
occurring from occupied π-symmetry dxz and dyz orbitals of the
metal. This results also confirms the (dxy)2 (dxz, dyz)3 ground
state, both in porphyrin and chlorin complexes
Analysis of the curve in the Curie plot was made for the
[TPC)Fe(P(Me)2Ph)2]CF3SO3 complex. The temperature de-
pendences of the chemical shifts of the protons in CD2Cl2 are
shown in Figure 4. A magnetically simple molecule is expected
to follow Curie-law behavior in that a plot of the chemical shifts
vs 1/T is linear with an intercept equal to the resonance in the
diamagnetic complex. Plots of ortho, meta, and para signals of
meso-aryl protons of the chlorin ring are reasonably linear which
intercept respectively at 8.03, 7.4, and 7.34 ppm. However these
protons show only small temperature dependence, consistent
with a weak spin density on the meso position. For the pyrrole
resonances, the chemical shifts vary linearly with 1/T, but the
extrapoled lines do not pass through the diamagnetic value at
1/T ) 0.
[(TPP)Fe(Imidazole)2]+ and other low-spin complexes formed
from tetraphenylchlorin derivatives have been reported.40-42 The
apparent extent of rhombicity is smaller for hydroporphyrin than
for porphyrin complexes with the same set of axial ligands but
all of the hydroporphyrin complexes evidence rhombic spectra.
In contrast, the EPR spectrum of [(TPP)Fe(P(Me)3)2]ClO4 is
axial in frozen solution with g ) 3.5 at 4 K.43 This gmax signal
is consistent with near degeneracy of the dxz and dyz as expected
for low-spin ferric complexes with axial ligands that have
effective cylindrical symmetry. It should also be underlined that
previous NMR results showed that the ferrous hydroporphyrin
complex FeTPC is magnetically different from FeTPP despite
their common S ) 1 ground state: it is rhombic instead of axial.6
Compound 2 shows the principal g values g1 ) 2.506, g2 )
2.374, and g3 ) 1.743. As expected, the reduction of the
porphyrin to a chlorin-type ring removes the degeneracy of the
dπ orbitals. These values are also quite similar to those of the
bis-imidazole complex of iron(III) tetraphenylchlorin which
shows the principal g values at g1 ) 2.49, g2 ) 2.39, and g3 )
1.75.40,41,44 Since Walker and co-workers in a recent ENDOR
and ESEEM study demonstrate that the bis(imidazole) com-
plexes of iron(III) tetraphenylchlorin and tetraphenylporphyrin
have the same order of g-values and the same electronic ground
state,44 it seems reasonable to propose also a (dxy)2 (dxzdyz)3
ground state for [TPC)Fe(P(Me)2Ph)2]CF3SO3, in agreement
1
with the H NMR study.
In conclusion, these spectroscopic observations are indicative
of a metal-based electron in the dπ orbitals for [TPC)Fe-
(P(Me)2Ph)2]CF3SO3 complex at any temperature. Thus the
change in ground state of low-spin Fe(III) from the usual (dxy)2
(dxz,dyz)3 to the unusual (dxz,dyz)4 (dxy)1 electron configuration
(41) Peisach, J.; Blumberg, W. E.; Adler, A. D. Ann. N.Y. Acad. Sci. 1973,
206, 310-327.
EPR Spectroscopy. It has been recognized that the EPR g
values of low-spin ferriporphyrins provide valuable information
about the orbital of the unpaired electron.3,40 EPR properties of
(42) Muhoberac, B. B. Arch. Biochem. Biophys. 1984, 233, 682-697.
(43) Walker, F. A.; Simonis, U. In Biological Magnetic Resonance; Berliner,
L. E., Reuben, J., Ed.; Plenum: New York, 1993; Vol. 12, p 13-
274.
(44) Astashkin, A. V.; Raitsimring, A. M.; Walker, F. A. J. Am. Chem.
Soc. 2001, 123, 1905-1913.
(40) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.

Notes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4499
angles; plots of chemical shifts versus 1/T and 2D COSY spectrum.
This material is available free of charge via the Internet at http:
//pubs.acs.org.
which was previously suggested to occur from porphyrin to
chlorin macrocycles41 is not observed with phosphine ligands.
Supporting Information Available: Tables of parameters, posi-
tional and thermal parameters for all atoms, anisotropic displacement
parameters for non-hydrogen atoms, and bond distances and bond
IC0013328