Difference in spin crossover pathways among saddle-shaped six-coordinated iron(III) porphyrin complexes

Article abstract of DOI:10.1021/ic0300969

The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L₂]⁺ and [Fe(TBTXP)L₂]⁺ have been examined in solution by ¹H NMR, ¹³C NMR, and EPR spectroscopy and by magnetic measurements. While [Fe(OMTPP)(DMAP) ₂]⁺ and [Fe(TBTXP)(DMAP)₂]⁺ maintain the low-spin (S = 1/2) state, [Fe(OMTPP)(THF)₂]⁺ and [Fe(TBTXP)(THF)₂]⁺ exhibit an essentially pure intermediate-spin (S = 3/2) state over a wide range of temperatures. In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S = 3/2 to S = 1/2 as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is similar to that of [Fe(OETPP)PY₂]⁺ reported in our previous paper (Ikeue, T. ; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620) in the context that all these complexes exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of [Fe(OMTPP)L₂]⁺ and [Fe(TBTXP)L ₂]⁺ (L = Py, 4-CNPy) revealed, however, that these complexes adopt the less common (d_xz, d_yz) ⁴(d_xy⁾¹ electron configuration at low temperature in contrast to [Fe(OETPP)Py₂]⁺ which shows the common (d_xy)²(d_xz, d_yz) ³ electron configuration. These observations have been attributed to the flexible nature of the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores can ruffle the porphyrin ring and adopt the (d_xz, d_yz)⁴(d _xy)¹ electron configuration at low temperature. Therefore, this study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways.

Full text of DOI:10.1021/ic0300969

Inorg. Chem. 2003, 42, 5560−5571
Difference in Spin Crossover Pathways among Saddle-Shaped
Six-Coordinated Iron(III) Porphyrin Complexes
,†,§
Takahisa Ikeue,† Yoshiki Ohgo,† Owendi Ongayi,‡ M. Grac¸a H. Vicente,‡ and Mikio Nakamura*
Department of Chemistry, School of Medicine, Toho UniVersity, Tokyo 143-8540, Japan,
DiVision of Biomolecular Science, Graduate School of Science, Toho UniVersity,
Funabashi 274-8510, Japan, and Department of Chemistry, Louisiana State UniVersity,
Baton Rouge, Louisiana 70803-1804
Received March 13, 2003
The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ have
been examined in solution by 1H NMR, 13C NMR, and EPR spectroscopy and by magnetic measurements. While
[Fe(OMTPP)(DMAP)2]+ and [Fe(TBTXP)(DMAP)2]+ maintain the low-spin (S ) 1/2) state, [Fe(OMTPP)(THF)2]+ and
[Fe(TBTXP)(THF)2]+ exhibit an essentially pure intermediate-spin (S ) 3/2) state over a wide range of temperatures.
3
In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S ) /2 to S )
1/2 as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is
similar to that of [Fe(OETPP)Py2]+ reported in our previous paper (Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi,
M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617−2620) in the context that all these complexes
exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of
[Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ (L ) Py, 4-CNPy) revealed, however, that these complexes adopt the less
common (d , d )4(d )1 electron configuration at low temperature in contrast to [Fe(OETPP)Py2]+ which shows the
xz yz
xy
common (d )2(d , d )3 electron configuration. These observations have been attributed to the flexible nature of
xy
xz yz
the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores
can ruffle the porphyrin ring and adopt the (d , d )4(d )1 electron configuration at low temperature. Therefore, this
xz yz
xy
study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways.
5
Introduction
five-coordinate high-spin (S ) /2) complexes. Maltempo4
discussed a quantum mechanically spin admixed S ) 3/2, 5/2
state and suggested that the S ) /2 state is an important
Several factors control the spin states of iron(III) porphyrin
complexes.1-3 Among these, the number and nature of axial
ligands are the most important factors. Strong axial ligands
such as cyanide, imidazole, and pyridine lead to the formation
of low-spin (S ) 1/2) six-coordinate complexes. In contrast,
anionic ligands such as Cl- and F- lead to the formation of
3
contributor to the spin state of cytochromes c′, which are
recognized as histidine ligated five-coordinate iron(III)
complexes.4-6 In fact, we have recently reported that the
mono(imidazole) complexes of iron(III) porphyrins exhibit
the S ) 3/2, 5/2 admixed intermediate-spin state in a series of
[Fe(TMP)(RIm)]+ complexes, where RIm indicates alkyl
substituted imidazoles and benzimidazoles.7-9 In general, the
S ) /2 character increases in five-coordinate complexes as
the ligand field of anionic axial ligands weakens. Thus, Reed
and co-workers ranked the relative field strengths of weak
* To whom correspondence should be addressed. E-mail:
mnakamu@med.toho-u.ac.jp.
3
† School of Medicine, Toho Universirty.
‡ Louisiana State University.
§ Division of Biomolecular Science, Graduate School of Science, Toho
University.
(1) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.
(2) Scheidt, W. R.; Gouterman, M. In Iron Porphyrin, Part I; Lever, A.
B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; pp
89-139.
(3) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
3, Chapter 16, pp 49-112.
(4) Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540-2547.
(5) Fujii, S.; Yoshimura, T.; Kamada, H.; Yamaguchi, K.; Suzuki, S.;
Shidara, S.; Takakuwa, S. Biochim. Biophys. Acta 1995, 1251, 161-
169.
(6) De´me´ne´, H.; Tsan, P.; Gans, P.; Marion, D. J. Phys. Chem. B 2000,
104, 2559-2569.
5560 Inorganic Chemistry, Vol. 42, No. 18, 2003
10.1021/ic0300969 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/15/2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
Scheme 1 Saddle and Ruffle Conformations for Nonplanar Distortion
Scheme 2
in the Porphyrin Corea
a Filled circles (b) correspond to atoms above the least squares plane
(calculated for the 24 atoms of the porphyrin core), and open circles (O)
represent atoms below the plane. Atoms not represented by circles are in
the plane.
anionic ligands (X) in Fe(TPP)X on the basis of the
spectroscopic and magnetic properties and called the hier-
archy a magnetochemical series.10,11 Recent studies have
shown that the deformation of the porphyrin ring is also an
important factor in determining the spin state;12-14 a quite
pure intermediate-spin state was observed in highly nonplanar
six-coordinate complexes with weak axial ligands such as
the saddled [Fe(OETPP)(THF)2]+ and the ruffled [Fe(TiPrP)-
(THF)2]+ complexes.12 The saddled and ruffled conforma-
tions are shown in Scheme 1. These results were attributed
to the short Fe-Np (Np: nitrogen atoms of porphyrin) bond
lengths caused by the nonplanarity of the porphyrin ring,12-17
and the weak coordination ability of the axial ligands.10,11
The pronounced S4 saddled structure of the OETPP ring
state; we and others have shown that the ruffled deformation
stabilizes the (dxz, dyz)4(dxy)1 state,19-29 while the saddled
deformation stabilizes the (dxy)2(dxz, dyz)3 state.29 In this paper,
we report on the spin states of other saddle-shaped por-
phyrin complexes, [Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+,
in which the axial ligands are substituted pyridines and THF
(Scheme 2). We also report that the magnetic behavior of
[Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ is significantly dif-
ferent from that of [Fe(OETPP)L2]+ despite the structural
similarity of these complexes.
Experimental Section
3
stabilizes the S ) /2 state even in the presence of nitrogen
1
General Procedure. H and 13C NMR spectra were recorded
bases. Therefore, [Fe(OETPP)(4-CNPy2)]+ shows a quite
pure intermediate-spin state over a wide range of tempera-
tures in CD2Cl2 solution.18 In sharp contrast, the ruffled
porphyrin complex [Fe(TiPrP)(4-CNPy)2]+ shows a typical
low-spin character with (dxz, dyz)4(dxy)1 electron configuration
over a wide range of temperatures.19 Obviously, the defor-
mation mode of the porphyrin ring significantly influences
the electron configuration of low-spin iron(III) and the spin
1
on a JEOL LA300 spectrometer operating at 300.4 MHz for H.
Chemical shifts were referenced to the residual peak of dichlo-
1
romethane (δ ) 5.32 ppm for H and 53.8 ppm for 13C). EPR
spectra were measured at 4.2 K with a Bruker E500 spectrometer
operating at X-band and equipped with an Oxford helium cryostat.
The samples for the EPR measurement were prepared by the
addition of ca. 10 mol equiv of ligands into the CH2Cl2 solutions
of [Fe(OMTPP)(THF)2]ClO4. The concentrations of EPR samples
were 5-8 mM. The observed EPR spectra had enough quality for
the determination of the g values from the spectra. Solution
magnetic moments of a series of [Fe(OMTPP)L2]ClO4 were
determined by the Evans method in 5-7 mM CD2Cl2 solution.30,31
Alumina (Merck, Brockmann Grade III) was used for column
(7) Abbreviations: ORTPP (R ) M or E), dianion of 2,3,7,8,12,13,17,18-
octaalkyl-5,10,15,20-tetraphenylporphyrin where R is methyl (M) or
ethyl (E); TMP, TPP, and TiPrP, dianions of 5,10,15,20-tetra-
mesitylporphyrin, 5,10,15,20-tetraphenylporphyrin, and 5,10,15,20-
tetraisopropylporphyrin; TBTXP and TBTPP, dianions of 2:3,7:8, 12:
13,17:18-tetrabutano-5,10,15,20-tetra(3,5-dimethylphenyl)porphyrin and
2:3,7:8,12:13,17:18-tetrabutano-5,10,15,20-tetraphenylporphyrin; DMAP,
4-(N,N-dimethylamino)pyridine; Py, pyridine; 3-CNPy, 3-cyanopyri-
(20) Walker, F. A. 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.
t
dine; 4-CNPy, 4-cyanopyridine; HIm, imidazole; BuNC, tert-butyl-
isocyanide.
(21) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(8) Ikezaki, A.; Nakamura, M. Chem. Lett. 2000, 994-995.
(9) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 6225-6236.
(10) Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282.
(11) Evans, D. R.; Reed, C. A. J. Am. Chem. Soc. 2000, 122, 4660-4667.
(12) Ikeue, T.; Saitoh, T.; Yamaguchi, T.; Ohgo, Y.; Nakamura, M.;
Takahashi, M.; Takeda, M. Chem. Commun. 2000, 1989-1990.
(13) Simonato, J.-P.; Pecaut, J.; Pape, L. L.; Oddou, J.-L.; Jeandey, C.;
Shang, M.; Scheidt, W. R.; Wojacynski, J.; Wolowiec, S.; Latos-
Grazynski, L.; Marchon, J.-C. Inorg. Chem. 2000, 39, 3978-3987.
(14) Weiss, R.; Gold, A.; Trautwein, A. X.; Terner, J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Burlington, MA, 2000; Vol. 3, pp 65-96.
(15) Ohgo, Y.; Saitoh, T.; Nakamura, M. Acta Crystallogr. 2001, C57, 233-
234.
(16) Ohgo, Y.; Saitoh, T.; Nakamura, M. Acta Crystallogr. 1999, C55,
1284-1286.
(17) Nakamura, M.; Ikeue, T.; Ohgo, Y.; Takahashi, M.; Takeda, M. Chem.
Commun. 2002, 1198-1199.
(22) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T.; Tajima, K. Inorg.
Chem. 1998, 37, 2405-2414.
(23) Ikezaki, A.; Ikeue, T.; Nakamura, M. Inorg. Chim. Acta 2002, 335,
91-99.
(24) Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
(25) Wolowiec, S.; Latos-Grazynski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(26) Wolowiec, S.; Latos-Grazynski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(27) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
(28) 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.
(29) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
(30) Evans, D. F. J. Chem. Soc. 1959, 2003.
(31) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Molecules in
Biological Systems; Lever, A. B. P., Gray, H. B., Eds.; The Benjamin/
Cummings: Menlo Park, CA, 1983; pp 130-133.
(18) Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.;
Nakamura, M. Angew Chem., Int. Ed. 2001, 40, 2617-2620.
(19) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
Inorganic Chemistry, Vol. 42, No. 18, 2003 5561

Ikeue et al.
chromatography. Analytical thin-layer chromatography was per-
formed using basic alumina or 60 F254 silica gel (precoated sheets,
0.2 mm thick). The syntheses were monitored by TLC and
spectrophotometry. The electronic absorption spectra were measured
in dichloromethane solution using a Perkin-Elmer Lambda 35 UV-
vis spectrophotometer. Mass spectra were obtained at the LSU Mass
Spectrometry Facility. 3,5-Dimethylbenzaldehyde, BF3‚OEt2, dichlo-
rodicyanobenzoquinone (DDQ), 1-nitrocyclohexene, 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU), and ethyl isocyanoacetate were pur-
chased (Aldrich) and used without further purification. All solvents
were dried and purified according to literature procedures.32
Synthesis. 2:3,7:8,12:13,17:18-Tetrabutano-5,10,15,20-tetra(3,5-
dimethylphenyl)porphyrin, H2(TBTXP) was prepared by the fol-
lowing method: Freshly distilled dry dichloromethane was added
to a round-bottom flask fitted with a reflux condenser under argon.
3:4-Butanopyrrole (0.166 g, 1.37 mmol) and 3,5-dimethylbenzal-
dehyde (0.18 mL, 1.37 mmol) were added, and the solution was
stirred at room temperature under a slow steady stream of argon
for 15 min. The flask was shielded from light, and BF3‚OEt2 (0.02
mL, 0.137 mmol) was added.33 This mixture was then stirred for 1
h at room temperature. DDQ (1.50 g, 6.60 mmol) was added to
the reaction flask, and the final solution turned dark pink instantly.
This mixture was refluxed under argon for 1 h to give a dark green
solution. The solvent was reduced to dryness under vacuum, and
the resulting residue was purified by column chromatography using
dichloromethane for elution. Recrystallization from methanol gave
purple crystals of the title porphyrin (0.18 g, 56% yield). 1H NMR
(CDCl3, drop of d-TFA) 8.00 (s, 8H, o-ArH), 7.51 (s, 4H, p-ArH),
2.70 (s, 24H, m-CH3), 2.37, 2.02, 1.72, and 1.29 (broad s, 8H each,
CH2), -0.50 (s, 4H, 4NH). UV-Vis of dication (CH2Cl2, λmax nm)
461 (ꢀ 283 800), 671 (24 700). MS (MALDI) m/e 944.28 (M+).
H2(OMTPP), meso-13C enriched H2(OMTPP), Fe(OMTPP)Cl,
and Fe(TBTXP)Cl were prepared according to the literature.29,34-36
[Fe(OMTPP)(THF)2]ClO4 and [Fe(TBTXP)(THF)2]ClO4 were pre-
pared by addition of a THF solution of AgClO4 to THF solutions
of Fe(OMTPP)Cl and Fe(TBTXP)Cl.29,37 A series of bis-ligated
complexes, [Fe(OMTPP)L2]ClO4 and [Fe(TBTXP)L2]ClO4, were
prepared by addition of 10-15 equiv of the ligand such as DMAP,
Figure 1. 1H NMR spectra obtained upon addition of (a) 3.1 equiv, (b)
5.6 equiv, and (c) 9.0 equiv of 4-CNPy to a CD2Cl2 solution of
[Fe(OMTPP)(THF)2]+ at 298 K.
were observed downfield at 41.5 and 26.1 ppm. Figure 2a
shows the change in chemical shift of these signals upon
addition of 4-CNPy. The gradual upfield shift of the ligand
protons suggests that the coordinated ligand is rapidly
1
exchanging with the free ligand on the H NMR time scale
t
Py, 3-CNPy, 4-CNPy, HIm, and BuNC to a CH2Cl2 or a CD2Cl2
solution of [Fe(OMTPP)(THF)2]ClO4.29
at room temperature.
The porphyrin signals, on the other hand, exhibited only
small changes. For example, the methyl signal appeared at
66.1, 65.7, and 64.7 ppm, and the p-phenyl signal appeared
at 9.7, 9.8, and 9.9 ppm when 3.1, 5.6, and 9.0 equiv of
4-CNPy were added, respectively. Thus, the chemical shifts
of the porphyrin protons are almost independent of the
amount of ligand added, which indicates that [Fe(OMTPP)-
(THF)2]+ is mostly converted into [Fe(OMTPP)(4-CNPy)2]+
at least by the addition of 9.0 equiv of 4-CNPy. It should
be noted, however, that the chemical shifts of the methyl
and p-phenyl protons obtained by addition of excess lig-
and were not significantly different from those of starting
[Fe(OMTPP)(THF)2]+; the chemical shifts of the methyl and
p-phenyl protons in [Fe(OMTPP)(THF)2]+ are 62.0 and 7.6
ppm, respectively. Thus, this conclusion should be taken with
reservation.
Results and Discussion
Formation of the Bis-Ligated Complexes. (i) OMTPP
Complexes. Pyridine usually behaves as a weaker ligand than
imidazole and cyanide. Therefore, it is important to confirm
that the complexes examined in this study actually have bis-
coordination of the ligand even at room temperature. Figure
1
1 shows the H NMR spectra of the sample obtained after
addition of various amounts of 4-CNPy to a CD2Cl2 solution
of [Fe(OMTPP)(THF)2]+ at 298 K. When 3.1 equiv of
4-CNPy was added, two broad signals assigned to 4-CNPy
(32) Perrin, D. D.; Armarego, W. L. F. In Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(33) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt, J. A.
Tetrahedron Lett. 1990, 31, 3719-3722.
(34) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M.
W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851-8857.
(35) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581-592.
(36) Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J.
Am. Chem. Soc. 1997, 119, 2563-2569.
(37) Ogoshi, H.; Sugimoto, H.; Watanabe, E.; Yoshida, Z.; Maeda, Y.;
Sakai, H. Bull. Chem. Soc. Jpn. 1981, 54, 3414-3419.
More conclusive evidence of the formation of [Fe-
(OMTPP)(4-CNPy)2]+ was obtained from the observed
changes in chemical shift for the THF protons. In [Fe-
(OMTPP)(THF)2]+, the THF protons were observed at 11.6
and 7.6 ppm. Upon addition of 4-CNPy, these signals shifted
upfield and approached those of free THF as is shown in
5562 Inorganic Chemistry, Vol. 42, No. 18, 2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
Figure 2. Change in chemical shifts of (a) 4-CNPy and (b) THF protons in [Fe(OMTPP)(THF)2]+ at 298 K and (c) R-methylene protons in [Fe(TBTXP)-
(THF)2]+ at 273 K upon addition of 4-CNPy.
Figure 2b. When 9.0 equiv of 4-CNPy was added, two sharp
THF signals appeared at 1.93 and 3.80 ppm. The integral
intensities of these signals were similar to those of the ortho
and meta phenyl protons. These results strongly indicate that
the coordinated THF ligands are completely replaced by
4-CNPy at least when 9.0 equiv of the ligand is added.
[Fe(OMTPP)L2]( (L ) HIm, CN-, tBuNC) and [Fe(OETPP)-
t
L2]( (L ) HIm, CN-, BuNC, DMAP, Py, 4-CNPy,
1
THF).12,18,29 The H NMR chemical shifts of [Fe(OETPP)-
(1-MeIm)2]+ have also been recently reported.42 Character-
istic features of the 1H NMR chemical shifts based on these
5
studies are summarized as follows: (i) High-spin (S ) /2)
complexes such as Fe(OMTPP)Cl and Fe(OETPP)Cl show
the methyl and methylene signals fairly downfield, δ 20-
50 ppm, at room temperature.35,36,42-45 (ii) Low-spin (S )
1/2) complexes with the common (dxy)2(dxz, dyz)3 electron
configuration such as [Fe(OMTPP)(HIm)2]+ and [Fe(OET-
PP)(HIm)2]+ also exhibit downfield methyl and methylene
signals, 4-20 ppm at room temperature.29 (iii) Low-spin (S
(ii) TBTXP Complexes. The formation of bis-ligated
complexes in the TBTXP system was also confirmed by
titration experiments. As already mentioned, the methyl
signal of [Fe(OMTPP)]ClO4 showed only a small change
by addition of 4-CNPy. In contrast, the R-methylene signal
of [Fe(TBTXP)]ClO4 exhibited a significant shift. Figure 2c
shows the change in chemical shifts of the R-methylene
protons upon addition of 4-CNPy at 0 °C. When 1.0 equiv
of 4-CNPy was added, the signal moved downfield from 98.9
to 108.4 ppm. Further addition of the ligand up to 64 equiv
caused a gradual upfield shift of the signal until it reached
a constant value of 62.1 ppm. Therefore, it is reasonable to
assume that the signals at 108.4 and 62.1 ppm are the
R-methylene protons of the mono- and bis-adducts, respec-
tively. The chemical shifts of [Fe(TBTXP)(4-CNPy)2]+ at
various temperatures were determined by using the sample
containing 64 equiv of the ligand. Addition of a large excess
of 4-CNPy is not necessary for the formation of the bis-
adduct at lower temperature; the chemical shift of the
R-methylene signal of the sample containing 8 equiv of
4-CNPy is nearly the same as that containing 64 equiv of
the ligand below 230 K. The bis-coordination of the other
porphyrin complexes was similarly confirmed.
1
) /2) complexes with the less common (dxz, dyz)4(dxy)1
electron configuration such as [Fe(OMTPP)(tBuNC)2]+ and
[Fe(OETPP)(tBuNC)2]+ show the meta-phenyl protons at
11-13 ppm, which are similar to those of the high-spin
complexes.29 However, the methyl and methylene signals
appear upfield, 0-10 ppm, in these complexes in contrast
to those of the high-spin complexes. (iv) Intermediate-spin
(S ) 3/2) complexes such as [Fe(OETPP)(THF)2]+ and [Fe-
(OETPP)(4-CNPy)2]+ exhibit downfield shifted methylene
signals, 12-50 ppm.12,19 The presence of downfield shifted
ortho and para signals is another characteristic feature of
intermediate-spin complexes since in the other spin states,
1
5
S ) /2 and S ) /2, the complexes do not show downfield
shifted ortho and para protons; the ortho and para signals
of [Fe(OETPP)(4-CNPy)2]+ appear at 13.9 and 12.0 ppm,
respectively, at room temperature.18
Spin States of [Fe(OMTPP)L2]+ in Solution. (i) H
1
The spin states of a series of complexes [Fe(OMTPP)-
L2]+ (L ) DMAP, Py, and 4-CNPy) were examined on the
NMR Spectra. Porphyrin ring protons give characteristic
1H NMR signals which depend on the electronic state of the
1
paramagnetic metal ions. Thus, H NMR spectroscopy has
(41) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Molecules in
Biological Systems; Lever, A. B. P., Gray, H. B., Eds.; The Benjamin/
Cummings: Menlo Park, CA, 1983; pp 165-229.
been frequently used to determine the spin state of iron(III)
porphyrin complexes.38-41 We have recently reported the 1H
NMR spectra of a series of bis-ligated complexes such as
(42) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, K.
M.; Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc.
2001, 123, 6564-6578.
(43) Nakamura, M.; Yamaguchi, T.; Ohgo, Y. Inorg. Chem. 1999, 38,
(38) Walker, F. A.; Simonis, U. In NMR of Paramagnetic Molecules;
Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1993; Vol.
12, pp 133-274.
(39) Goff, H. M. In Iron Porphyrin, Part I; Lever, A. B. P., Gray, H. B.,
Eds.; Addison-Wesley: Reading, MA, 1983; pp 237-281.
(40) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Substances; Lever,
A. B. P., Ed.; Elsevier: Amsterdam, 1996; pp 29-75.
3126-3131.
5
(44) [Fe(OETPP)Cl] was reported to be an S )
/ spin state with
2
approximately 40% S ) 3/2 spin admixture.36 A later report has shown
that the S ) /2 contribution is much smaller, 4-10%.45
3
(45) Schu¨nemann, V.; Gerdan, M.; Trautwein, A. X.; Haoudi, N.; Mandon,
D.; Fischer, J.; Weiss, R.; Tabard, A.; Guilard, R. Angew Chem., Int.
Ed. 1999, 38, 3181-3183.
Inorganic Chemistry, Vol. 42, No. 18, 2003 5563

Ikeue et al.
Table 1. 1H NMR Chemical Shifts of [Fe(ORTPP)L2]+(R ) Me, Et) and [Fe(TBTXP)L2]+ Taken in CD2Cl2 Solution at 273, 193, and 173 K
Py-CH
193
ortho-H
193
meta-Hc
193
para-H
193
L
273
173
273
173
273
173
273
173
ref
OMTPP
1.8
HIm
19.4
18.8
39.6
65.4
67.0
0.5
23.0
21.8
22.6
36.9
91.3
-3.9
25.1
23.0
23.1
23.5
101.2
-5.1
4.4
4.5
9.1
13.8
13.0
2.5
0.8
1.1
3.5
5.3
19.6
-1.3
5.8
6.1
7.2
6.6
7.0
4.4
5.0
8.0
9.6
5.9
3.9
4.6
8.0
11.5
5.8
17.5
6.4
6.4
8.4
10.5
10.2
4.9
5.4
5.5
6.3
7.9
12.1
3.6
5.0
5.1
6.1
6.8
13.8
3.2
29
DMAPa
Py
2.1
4.1
7.5
17.6
-0.2
this work
this work
this work
this work
29
4-CNPy
THFa
tBuNCa
13.4
16.3
OETPP
1.5
HIma,b
DMAPa,b
Pya
4.1
4.4
10.4
17.2
15.2
10.8
12.0
32.0
45.8
45.7
7.8
2.7
2.6
6.5
21.1
25.7
5.6
12.8
13.1
23.7
64.3
59.8
18.1
2.2
1.9
4.4
22.2
22.9
6.2
13.6
13.4
22.0
70.9
64.1
22.5
4.3
4.6
12.2
14.1
12.7
5.5
0.4
-0.4
2.6
14.9
10.0
7.4
5.5
5.6
5.6
5.0
6.1
3.9
4.2
4.8
3.6
4.4
3.5
3.7
5.7
3.0
3.4
6.5
6.6
10.0
12.5
11.1
6.3
5.6
5.5
6.8
14.6
13.6
6.8
5.3
4.9
5.9
15.3
14.8
6.2
29
18
18
18
12
29
1.2
4.8
14.6
11.1
6.7
4-CNPy
THFa
tBuNC
11.2
11.4
11.0
TBTXP
2.7
HIma
28.1
25.2
27.7
62.1
94.1
98.7
-1.5
32.4
31.5
21.7
14.2
48.7
146.8
-4.6
34.9
34.0
22.0
12.4
27.7
165.8
-1.7
4.7
4.4
5.7
c
12.5
c
2.2
1.6
4.0
4.3
4.5
12.8
d
1.8
1.8
2.4
3.3
4.0
3.7
2.0
1.2
1.2
2.2
2.4
2.9
4.0
1.7
1.0
1.1
2.2
2.4
2.6
4.2
1.6
6.2
6.2
6.9
8.2
9.4
8.7
d
5.4
5.4
6.6
6.6
7.5
9.1
d
5.1
5.0
6.7
6.5
6.9
9.3
d
this work
this work
this work
this work
this work
this work
this work
DMAPa
2.4
4.1
3.7
6.8
12.2
d
Pya
4-CNPy
3-CNPy
THFa
tBuNC
-9.6
d
a Data at 173 K were obtained by the extrapolation from the high temperature data. b Data at 298 K were obtained by the extrapolation from the low
temperature data. c Chemical shifts of m-CH3 in the case of [Fe(TBTXP)L2]+. d Signals are too broad.
basis of the spectroscopic characteristics already described.
The chemical shifts in Table 1 and the Curie plots for the
methyl and meta-proton signals in Figure 3A clearly indicate
that [Fe(OMTPP)(DMAP)2]+ is in the low-spin state at least
below 273 K because of the similarity of the chemical shifts
and their temperature dependence to those of [Fe(OMTPP)-
(HIm)2]Cl; the latter has been fully characterized as the low-
spin complex with (dxy)2(dxz, dyz)3 electron configuration.29
In contrast, the methyl signal of [Fe(OMTPP)Py2]+ exhibited
a curious temperature dependence as shown in Figure 3Aa.
While it appeared fairly downfield, 53.1 ppm at room
temperature, it moved upfield and approached the methyl
signal of [Fe(OMTPP)(HIm)2]+ at 173 K. The methyl signal
of [Fe(OMTPP)(4-CNPy)2]+ also appeared fairly downfield,
61.9 ppm at 313 K. This signal moved downfield as the
temperature was lowered from 313 to 273 K and then shifted
upfield as the temperature was further lowered from 273 to
173 K. At 173 K, the lowest temperature examined in this
NMR study, the methyl signal of [Fe(OMTPP)(4-CNPy)2]+
appeared very close to that of the low-spin [Fe(OMTPP)-
(HIm)2]+ complex. These results suggest that the spin
transition from the S ) 3/2 to the S ) 1/2 took place both in
[Fe(OMTPP)Py2]+ and [Fe(OMTPP)(4-CNPy)2]+ in the
temperature range 313-173 K as observed in the case of
[Fe(OETPP)(Py)2]+ reported in our previous paper.18 The
presence of the downfield shifted ortho and para protons at
room temperature and their upfield shift at lower temperature
also support the spin transition. The unexpected result was
obtained, however, from the Curie plots for the meta signals
shown in Figure 3Ab. As the temperature was lowered, the
Curie line of the meta signal of [Fe(OMTPP)(4-CNPy)2]+
moved away from that of [Fe(OMTPP)(HIm)2]+, and ap-
proached that of [Fe(OMTPP)(tBuNC)2]+. Similar temper-
ature dependence was observed in [Fe(OMTPP)(Py)2]+
although the deviation from the Curie line for [Fe(OMTPP)-
(HIm)2]+ was less pronounced than that for [Fe(OMTPP)-
(4-CNPy)2]+. Since [Fe(OMTPP)(tBuNC)2]+ has been well
characterized as the low-spin complex with a quite pure (dxz,
1
dyz)4(dxy)1 electron configuration on the basis of H NMR,
13C NMR, and EPR spectroscopy,29 the most reasonable
explanation is that the spin transition takes place from the S
3
1
) /2 to the S ) /2 in both [Fe(OMTPP)(4-CNPy)2]+ and
[Fe(OMTPP)Py2]+,18,45-47 and that the S ) /2 complexes
1
formed at low temperature adopt the (dxz, dyz)4(dxy)1 electron
configuration. This is in sharp contrast to the case of the
corresponding OETPP complex, [Fe(OETPP)Py2]+, where
3
1
the spin transition occurs from the S ) /2 to the S ) /2
with the (dxy)2(dxz, dyz)3 electron configuration.18 The reasons
for the difference in spin crossover pathways between the
OMTPP and OETPP complexes will be discussed later. It is
worthwhile to compare the electronic ground state of [Fe-
(OMTPP)(4-CNPy)2]+ with that of [Fe(TMP)(4-CNPy)2]+
reported by Walker et al.48,49 Comparison of the H NMR
1
chemical shifts of these complexes clearly demonstrates that
the (dxz, dyz)4(dxy)1 contribution in [Fe(TMP)(4-CNPy)2]+ is
much larger than that in [Fe(OMTPP)(4-CNPy)2]+; the
chemical shifts of the meta signals of [Fe(TMP)(4-CNPy)2]+
and [Fe(OMTPP)(4-CNPy)2]+ are 14.6 and 9.6 ppm, respec-
tively, at 193 K. In other words, the energy difference
between the dxy and dπ(dxz, dyz) orbitals is much larger in
(46) Hodges, K. D.; Wollmann, R. G.; Kessel, S. L.; Hendrickson, D. N.;
VanDerveer, D. G.; Barefield, E. K. J. Am. Chem. Soc. 1979, 101,
906-917.
(47) Koch, W. O.; Schu¨nemann, V.; Gerdan, M.; Trautwein, A. X.; Kru¨er,
H.-J. Chem. Eur. J. 1998, 4, 686-691.
(48) 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.
(49) Safo, M. K.; Walker, F. A.; Raitsimring, A. M.; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760-7770.
5564 Inorganic Chemistry, Vol. 42, No. 18, 2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
Figure 3. Curie plots for some signals of (A) [Fe(OMTPP)L2]+ and (B) [Fe(TBTXP)L2]+ in CD2Cl2 solution. (A) (a) Py-CH3 and (b) m-H; (B) (a) R-CH2
t
and (b) m-CH3 signals: 0, DMAP; 4, Py; O, 4-CNPy; +, 3-CNPy; 2, THF; b, HIm; 9, BuNC.
[Fe(TMP)(4-CNPy)2]+ than in [Fe(OMTPP)(4-CNPy)2]+.
Similarly, the meta signal of [Fe(TMP)Py2]+ appears at 9.5
ppm, which is 1.5 ppm more downfield than that in [Fe-
(OMTPP)Py2]+; note that the downfield shift of the meta
signal is caused by the increase in the (dxz, dyz)4(dxy)1
contribution.23,29,48 These results are consistent with our recent
finding that saddle-shaped complexes resist changing their
electron configuration from the common (dxy)2(dxz, dyz)3 to
the less common (dxz, dyz)4(dxy)1 even if the axial ligands have
weak σ-donating and strong π-accepting characters.29
In the case of [Fe(OMTPP)(THF)2]+, the methyl signal
appeared fairly downfield, 62.1 ppm at room temperature,
and moved further downfield in proportion to 1/T. Thus, the
complex is expected to be in the intermediate-spin state as
in the case of [Fe(OETPP)(THF)2]+.12 The presence of the
downfield shifted ortho and para signals, 17.6 and 12.1 ppm
at 193 K, respectively, is another piece of evidence showing
that the complex is in the intermediate-spin state.12,18
Figure 4. Temperature dependence of the effective magnetic moments
of a series of [Fe(OMTPP)L2]+ species determined in CD2Cl2 solution by
the Evans method.
(ii) Solution Magnetic Moments. Temperature depen-
dence of the effective magnetic moments of [Fe(OMTPP)-
L2]+ (L ) DMAP, Py, 4-CNPy, THF) has been determined
by the Evans method in CD2Cl2 solution and is given in
Figure 4. [Fe(OMTPP)(DMAP)2]+ exists almost exclusively
as the low-spin complex in the temperature range 213-280
K. [Fe(OMTPP)Py2]+ also exists as the low-spin complex
Inorganic Chemistry, Vol. 42, No. 18, 2003 5565

Ikeue et al.
Table 2. 13C NMR Chemical Shifts of [Fe(ORTPP)L2]+ (R ) Me, Et)
Taken in CD2Cl2 Solution at 298, 193, and 173 K
meso
193
Py-R
193
Py-â
L
298
173
298
298 193
ref
OMTPP
128.4
HIm
DMAP
Py
29.9 -36.6 -57.5
74.8 173.0 161.3 29
49.9 163.7 147.2 this work
42.1 214.4 157.8 this work
48.0
3.4 -13.9
109.8
-8.8 163.3
180.1. 204.6
422.5
4-CNPy -67.9 224.3
242.5 109.0 242.0 188.1 this work
THF
-0.5 -104.7 -138.8a
b
b
b
b
this work
tBuNC
701.4 1153a
-206.5 -484.0a 112.8 77.2a 29
OETPP
HIm
DMAP
Py
7.3 -65.1 -87.1
2.5 -52.7 -81.1
163.0 130.5 167.0 148.5 29
167.9 122.1 175.9 159.4 18
384.1 224.0 243.8 211.2 18
469.6 664.0 265.6 328.1 18
-186.2 -88.0 -73.6
4-CNPy -235.6 -499.5 -510.6
THF
-87.0 -291.4a
b
b
b
b
this work
tBuNC
419.3 340.8
232a
-3.7
74.3 144.2 137.0 29
a Data obtained by the extrapolation from high temperature. b Signals
are too broad to detect.
Figure 5. 13C NMR spectra of [Fe(OMTPP)(4-CNPy)2]+ taken in CD2-
Cl2 solution at (a) 298 K and (b) 173 K.
at 213 K, although the intermediate-spin complex increases
steeply as the temperature is raised. In the case of [Fe-
(OMTPP)(4-CNPy)2]+, both the low-spin and intermediate-
spin complexes exist comparably even at 213 K. The
intermediate-spin complex increased with the temperature
and became the sole entity at 280 K. The effective magnetic
moments of [Fe(OMTPP)(THF)2]+ are almost constant, 4.1-
4.2 µB, in the temperature range 213-280 K, indicating that
the complex is in the intermediate-spin state. Thus, the results
on the effective magnetic moments are consistent with those
Figure 6. Curie plots for the meso-13C signals of [Fe(OMTPP)L2]+ and
[Fe(OETPP)(4-CNPy)2]+ taken in CD2Cl2 solution. The following symbols
are used for the [Fe(OMTPP)L2]+ complexes: 0, DMAP; 4, Py; O,
4-CNPy; b, HIm; 9, tBuNC. The symbol + indicates the meso-carbon signal
of [Fe(OETPP)(4-CNPy)2]+.
1
obtained by H NMR spectroscopy.
(iii) 13C NMR Spectra. Figure 5 shows the 13C NMR
spectra of [Fe(OMTPP)(4-CNPy)2]+ taken at 298 and 173
K. The signals were assigned on the basis of the acquisition
of the proton-coupled 13C NMR spectra. The meso-carbon
signals were assigned using meso-13C enriched complexes.
The R-pyrrole and ipso-phenyl carbons of [Fe(OMTPP)-
(DMAP)2]+ and [Fe(OMTPP)Py2]+ were conveniently as-
signed on the basis of their coupling with the adjacent meso-
carbons in the 13C NMR spectra of the meso-13C enriched
complexes.29 In the inset of Figure 5 are given the 13C NMR
spectra of the meso-13C enriched complexes taken at 298
and 173 K. Table 2 lists the 13C NMR chemical shifts of the
meso, Py-R, and Py-â carbons in [Fe(OMTPP)L2]+ taken at
298, 193, and 173 K along with those of the corresponding
carbons in [Fe(OETPP)L2]+.18,29 Figure 6 shows the Curie
plots for the meso-carbon signals of a series of [Fe(OMTPP)-
the meso signal of [Fe(OMTPP)(4-CNPy)2]+ showed a
completely different temperature dependence. It appeared
fairly upfield, -67.9 ppm at 298 K, and moved further
upfield until the temperature was lowered to 253 K where it
reached -138 ppm. The presence of a fairly upfield shifted
meso signal is one of the most characteristic features of the
intermediate-spin complexes which discriminates them from
the complexes in different spin states such as S ) 1/2 and S
5
) /2; the meso-carbon signals are usually observed at δ )
0-100 ppm in the (dxy)2(dxz, dyz)3-type low-spin com-
plexes,29,38-41δ > 200 ppm in the (dxz, dyz)4(dxy)1-type low-
spin complexes,19,22,23,29 and ca. 500 ppm in the high-spin
complexes.38-41 Some low-spin complexes with quite pure
(dxz, dyz)4(dxy)1 electron configuration such as [Fe(TPP)-
(tBuNC)2]+ exhibit the meso signals fairly downfield, at
around 800 ppm.29 Thus, the 13C NMR data indicate that
the major part of [Fe(OMTPP)(4-CNPy)2]+ is in the S ) 3/2
state at 298-253 K. As the temperature was further lowered
below 253 K, the meso signal of [Fe(OMTPP)(4-CNPy)2]+
1
L2]+ complexes. As suggested from the H NMR chemical
shifts, [Fe(OMTPP)(DMAP)2]+ is in the low-spin state since
the chemical shift of the meso signal and its temperature
dependence are quite similar to those of [Fe(OMTPP)-
(HIm)2]+ over the temperature range examined.29 In contrast,
5566 Inorganic Chemistry, Vol. 42, No. 18, 2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
Figure 7. EPR spectra of (A) [Fe(OMTPP)L2]+ and (B) [Fe(TBTXP)L2]+ taken in frozen CH2Cl2 solutions at 4.2 K.
1
steeply moved downfield and reached 423 ppm at 173 K.
The large downfield shift of the meso signal strongly
indicates that the low-spin complex with (dxz, dyz)4(dxy)1
electron configuration exists as a major component.19,22,23,29,50
Thus, the spin transition takes place from the S ) 3/2 to the
We have already mentioned on the basis of the H NMR
and solution magnetic moments that both [Fe(OMTPP)-
(THF)2]+ and [Fe(OMTPP)(4-CNPy)2]+ exhibit an essentially
pure intermediate-spin state at room temperature. The
chemical shifts of the meso-carbons are, however, quite
different; while [Fe(OMTPP)(THF)2]+ showed the meso
signal at -0.5 ppm at room temperature, [Fe(OMTPP)(4-
CNPy)2]+ exhibited it at -67.9 ppm. Even the meso signal
1
S ) /2 with (dxz, dyz)4(dxy)1 electron configuration as the
temperature is lowered.
Similar spin transition has been observed in [Fe(OMTPP)-
of [Fe(OMTPP)Py2]+, which has much smaller S ) /2
3
3
Py2]+ although the population of the S )
/ state is
2
character than [Fe(OMTPP)(THF)2]+, appeared more upfield,
-8.8 ppm. A similar tendency has been observed in the
OETPP series; although both [Fe(OETPP)(THF)2]+ and
[Fe(OETPP)(4-CNPy)2]+ are pure intermediate-spin com-
plexes, the chemical shifts of their meso signals are very
different, -87 and -236 ppm, respectively, at 298 K.18 The
reason for this difference in meso-carbon shifts will be
discussed in detail later in this paper. Nevertheless, the 13C
NMR results are totally consistent with those obtained by
1H NMR and magnetic measurements.
1
comparable to that of the S ) /2 even at 298 K as it is
revealed from the chemical shift of the meso signal, -8.8
ppm. As already mentioned, the meso signals of [Fe-
(OMTPP)(HIm)2]+ and [Fe(OMTPP)(4-CNPy)2]ClO4 ap-
peared at 29.9 and -67.9 ppm, respectively; the former
complex is considered to be in the pure low-spin state with
(dxy)2(dxz, dyz)3 electron configuration,29 while the latter is in
an essentially pure intermediate-spin state at 298 K based
on the effective magnetic moment shown in Figure 4. As
the temperature was lowered to 173 K, the meso signal
moved downfield from -8.8 to 180.1 ppm, suggesting that
the complex is in the low-spin state with (dxz, dyz)4(dxy)1
electron configuration. Thus, the temperature dependence of
the magnetic behavior of [Fe(OMTPP)Py2]+ and [Fe-
(OMTPP)(4-CNPy)2]+ resembles that of [Fe(OETPP)Py2]+
in the sense that all these complexes exhibit a novel spin
(iv) EPR Spectra. Figure 7A shows the EPR spectra of
[Fe(OMTPP)L2]+ taken at 4.2 K in frozen CH2Cl2 solution.
Table 3 lists the EPR g values for [Fe(OMTPP)-L2]+. The g
values for saddled [Fe(OETPP)L2]+ and ruffled [Fe(TiPrP)-
L2]+ are also listed for comparison.12,18,19,29,51 EPR spectros-
copy has been frequently used to determine the electronic
state of iron(III) porphyrins. While high-spin complexes give
signals at g ) 6 and 2, intermediate-spin complexes exhibit
signals at g ) 4 and 2.52 In the case of six-coordinate low-
spin complexes, there are three types of EPR spectra.20,53,54
3
1
transition from the S ) /2 to the S ) /2; [Fe(OETPP)(4-
CNPy)2]+ exhibits the spin transition only in the solid state
below 180 K.18 It should be emphasized again that the
electron configurations of the low-spin complexes formed
at low temperature are different; [Fe(OMTPP)Py2]+ and
[Fe(OMTPP)(4-CNPy)2]+ adopt the (dxz, dyz)4(dxy)1 electron
configuration while [Fe(OETPP)Py2]+ forms the (dxy)2(dxz,
dyz)3 electron configuration.
(51) Ikeue, T.; Yamaguchi, T.; Ohgo, Y.; Nakamura, M. Chem. Lett. 2000,
342-343.
(52) Palmer, G. In Iron Porphyrin, Part II; Lever, A. B. P., Gray, H. B.,
Eds.; Addison-Wesley: Reading, MA, 1983; pp 43-88.
(53) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888-6898.
(50) Nakamura, M.; Ikeue, T.; Ikezaki, A.; Ohgo, Y.; Fujii, H. Inorg. Chem.
1999, 38, 3857-3862.
(54) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.
Chem. Soc. 1986, 108, 5288-5297.
Inorganic Chemistry, Vol. 42, No. 18, 2003 5567

Ikeue et al.
complex with (dxy)2(dxz, dyz)3 electron configuration also exists
as a minor component. The EPR spectrum of [Fe(OMTPP)-
(THF)2]+ shown in Figure 7Ad exhibits two signals at g )
4.09 and 1.97, indicating that the complex is in a quite pure
intermediate-spin state as in the case of the corresponding
OETPP complex [Fe(OETPP)(THF)2]+.12 Therefore, the EPR
results are consistent with those obtained by other methods
Table 3. EPR g Values of Nonplanar Six-Coordinate Iron(III)
Porphyrin Complexes Taken at 4.2 K in Frozen CH2Cl2 Solution
porphyrin
OMTPP
L
g1
g2
g3
ref
HIm
DMAP
Py
2.84
3.21
2.53
2.52
4.09
2.20
2.72
3.24
3.39
4.28
4.01
2.29
2.99
3.12
2.54
2.50
2.49
4.05
2.21
2.55
2.54
2.52
2.41
3.99
2.16
2.31
2.06
2.53
2.52
4.09
2.17
2.37
1.58
29
this work
this work
this work
this work
29
29
51
18
18
1.85
1.82
1.97
1.95
1.64
4-CNPy
THF
tBuNC
HIm
1
such as H and 13C NMR spectroscopy and the magnetic
OETPP
TBTXP
data.
DMAP
Pya
2.08
3.80
4.01
2.25
2.29
Spin States of [Fe(TBTXP)L2]+ in Solution. (i) 1H NMR
Spectra. The chemical shifts of a series of [Fe(TBTXP)-
L2]+ (L ) DMAP, Py, 4-CNPy, 3-CNPy, THF, HIm, and
tBuNC) species are listed in Table 1. The Curie plots of the
R-methylene and meta-methyl signals are given in Figure
3B. These results suggest that the spin state of [Fe(TBTXP)-
L2]+ resembles that of [Fe(OMTPP)L2]+ when the axial
ligand is the same. For example, [Fe(TBTXP)(THF)2]+
adopts the S ) 3/2 state as is revealed by the downfield shifted
R-methylene signal, 166 ppm at 173 K, along with its
linearity in the Curie plots. A fairly large downfield shift of
the R-methylene signal as compared with that of [Fe-
(OMTPP)(THF)2]+, 101 ppm at 173 K, should be attributed
to the difference in dihedral angle between the H-CR-C(â-
pyrrole) plane and the pyrrole ring; one of the R-methylene
protons in the cyclohexene ring of the TBTXP complex is
supposed to be parallel to the pz orbital at the â-pyrrole
carbon,59,60 and therefore suffers a large contact shift.38-40
Similarly, [Fe(TBTXP)(DMAP)2]+ and [Fe(TBTXP)(HIm)2]+
4-CNPya
THF
2.08
2.00
1.92
12
29
tBuNC
HIm
this work
this work
this work
this work
this work
this work
this work
29
19
19
19
12
DMAP
Py
2.54
2.50
2.49
4.05
2.21
2.55
2.54
2.52
2.41
3.99
2.16
3-CNPy
4-CNPy
THF
1.99
1.94
tBuNC
HIm
TiPrP
DMAP
Py
1.60
1.79
1.97
1.96
4-CNPy
THF
tBuNC
29
a Taken for the solid samples.
The first is the rhombic spectrum consisting of three signals,
and it is observed in complexes carrying two planar axial
ligands aligned in a parallel fashion above and below the
porphyrin ring. The second is the so-called large gmax type
spectrum in which two planar ligands are oriented perpen-
dicularly. Complexes with linear ligands such as cyanide also
show large gmax type.20,21,50,55 While these two types of spectra
are observed in complexes with (dxy)2(dxz, dyz)3 electron
configuration, the third known as the axial type is observed
in complexes with (dxz, dyz)4(dxy)1 electron configuration; the
latter complexes generally have axial ligands with low lying
π* orbitals and/or a strongly ruffled porphyrin ring.20,22,29
The EPR spectrum of [Fe(OMTPP)(DMAP)2]+ shown in
Figure 7Aa is classified as the large gmax type as in the case
of [Fe(OETPP)(DMAP)2]+.18,42,51 Thus, [Fe(OMTPP)(D-
MAP)2]+ is a low-spin complex with (dxy)2(dxz, dyz)3 electron
configuration at 4.2 K. In contrast, the EPR spectrum of [Fe-
(OMTPP)(4-CNPy)2]+ shown in Figure 7Ac is classified as
the axial type, which indicates that the complex adopts the
(dxz, dyz)4(dxy)1 electron configuration. The lack of signals
corresponding to the intermediate-spin complex and/or the
low-spin complex with (dxy)2(dxz, dyz)3 electron configuration
should be attributed to the temperature effect. This means
that the populations of these species decrease at lower
temperature and become negligibly small at 4.2 K, the
temperature at which the EPR spectra are measured. Al-
though the EPR spectrum of [Fe(OMTPP)Py2]+ shown in
Figure 7Ab consists of at least two components, the large
gmax and axial types, the major part is the axial type as in
the case of [Fe(OMTPP)(4-CNPy)2]+.55-57 Thus, [Fe(OMT-
PP)Py2]+ exists mainly as the low-spin complex with (dxz,
dyz)4(dxy)1 electron configuration at 4.2 K although the
1
adopt the S ) /2 spin state over a wide temperature range
as observed in [Fe(OMTPP)(DMAP)2]+ and [Fe(OMTPP)-
(HIm)2]+. In contrast to the complexes already mentioned,
the Curie plots for the R-CH2 and m-CH3 protons of [Fe-
(TBTXP)(3-CNPy)2]+, [Fe(TBTXP)(4-CNPy)2]+, and [Fe-
(TBTXP)Py2]+ exhibited curvatures, which suggests a spin
crossover phenomenon between the S ) 3/2 and the S ) 1/2.
Since the Curie lines of the R-methylene signals in these
complexes moved away from those of [Fe(TBTXP)(HIm)2]+
and [Fe(TBTXP)(DMAP)2]+ at lower temperature, and
approached the Curie line of [Fe(TBTXP)(tBuNC)2]+ as
shown in Figure 3B, the electron configuration of the low-
spin complexes formed at 173 K should be represented as
(dxz, dyz)4(dxy)1. Therefore, the temperature dependence of the
magnetic behavior in [Fe(TBTXP)(4-CNPy)2]+ and [Fe-
(TBTXP)Py2]+ is quite similar to that of the corresponding
OMTPP complexes.
(ii) EPR Spectra. The EPR spectra of a series of [Fe-
(TBTXP)L2]+ complexes were taken in frozen CH2Cl2
solutions at 4.2 K and are shown in Figure 7B. The g values
(56) The low-spin complexes with the (dxy)2(dxz, dyz)3 and (dxz, dyz)4(dxy)1
electron configurations should be considered as isomers and exist as
the different entities at 4.2 K;57 the former maintains the saddled
structure while the latter has the saddled structure with ruffled
contribution.58
(57) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 2761-2768.
(58) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.;
Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31-41.
(59) Barkigia, K. M.; Renner, M. K.; Furenlid, L. R.; Medforth, C. J.; Smith,
K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627-3635.
(60) Finikova, O. S.; Cheprakov, A. V.; Carroll, P. J.; Dalosto, S.;
Vinogradov, S. A. Inorg. Chem. 2002, 41, 6944-6946.
(55) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.
5568 Inorganic Chemistry, Vol. 42, No. 18, 2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
Scheme 3. Schematic Representation of the Change in Energy Levels of the Iron d Orbitals at Ambient Temperature in Solution Expected When
Planar TPP Is Replaced by Saddled ORTPP (R ) Me, Et), or When the Axial Ligand in [Fe(ORTPP)L2]ClO4 Changes from DMAP to Py, to 4-CNPy,
and Then to THFa
a Labels a-g indicate the stabilization or destabilization of the iron d orbitals caused by the ligand change: (a) destabilization due to short Fe-Np bond
lengths caused by the saddled deformation; (b) stabilization due to the weak ligand field of axial ligand; (c) stabilization due to the iron dπ and ligand pπ*
interactions; (d) destabilization due to the iron dxy and porphyrin a2u interaction; (e) destabilization due to the increase in the interaction between iron dπ and
porphyrin 3eg orbitals; (f) destabilization due to the lack of the iron dπ and ligand pπ* interactions; (g) stabilization due to the lack of the iron dxy and
porphyrin a2u interaction.
are listed in Table 3. The results are similar to those in the
corresponding [Fe(OMTPP)L2]+ complexes with the same
axial ligand. Both the DMAP and HIm complexes show the
low-spin state with (dxy)2(dxz, dyz)3 electron configuration
while the Py, 3-CNPy, and 4-CNPy complexes exhibit the
low-spin state with (dxz, dyz)4(dxy)1 electron configuration at
due to the stronger dπ-3eg interactions as compared with
those in the planar complexes having longer Fe-Np bond
distances.29 The effective overlaps of the dπ and 3eg orbitals
expected for the saddled conformation could further strengthen
these interactions.29 In this situation, if the axial DMAP
ligand is replaced by Py and then by 4-CNPy ligands, the
t
2
4.2 K. As expected, the BuNC showed a typical axial
energy level of the dz orbital drops to a great extent due to
spectrum with g ) 2.21 and g ) 1.94, suggesting that the
the weak ligand field strength of 4-CNPy. Concomitantly,
the energy levels of the dπ orbitals also drop due to the iron-
(dπ)-ligand(pπ*) interactions although the degree of decrease
in the dπ orbitals is expected to be much smaller than that in
||
complex is in a quite pure low-spin state with (dxz, dyz)4-
(dxy)1 electron configuration. In the case of [Fe(TBTXP)-
(THF)2]+, two signals were observed at g ) 4.05 and 1.99,
indicating that the complex is in the intermediate-spin state
as in the case of [Fe(OMTPP)(THF)2]+ and [Fe(OETPP)
(THF)2]+ complexes.
2
2
the dz orbital. As a result, the dz and dπ orbitals in [Fe-
(ORTPP)(4-CNPy)2]+ are located quite close to each other
in the energy diagram, which results in the formation of the
intermediate-spin complex at room temperature. When the
temperature is lowered, an iron-ligand bond contraction can
occur.61,64,65 This contraction could cause the increase in
Reasons for the Novel Magnetic Behavior in Saddled
+
+
2
[Fe(ORTPP)L ] (R ) Me, Et) and [Fe(TBTXP)L2] . In
the present paper as well as in our previous papers,18,61 we
have reported that saddle-shaped complexes [Fe(OMTPP)-
L2]+ (L ) Py, 4-CNPy) and [Fe(OETPP)L2]+ (L ) Py)
commonly exhibit a spin crossover process between the S
) 3/2 and S ) 1/2 in solution. The magnetic behavior of these
complexes can be interpreted in terms of the difference in
energy levels of the five d orbitals. In the following
discussion, we explain why these complexes exhibit a novel
spin transition as the temperature is lowered. Scheme 3 is a
qualitative illustration of how the energy levels of the d
orbitals vary when going from planar [Fe(TPP)(DMAP)2]+
to saddled [Fe(ORTPP)(DMAP)2]+, [Fe(ORTPP)Py2]+, [Fe-
(ORTPP)(4-CNPy)2]+, and then [Fe(ORTPP)(THF)2]+. In the
2
energy level of the dz orbital and induce the spin transition
from the S ) 3/2 to the S ) 1/2. It should be noted here that,
while [Fe(OETPP)Py2]+ adopts the (dxy)2(dxz, dyz)3 electron
configuration, [Fe(OMTPP)Py2]+ and [Fe(OMTPP)(4-CN-
Py)2]+ have the (dxz, dyz)4(dxy)1 electron configuration at low
temperature.
The difference in the electron configurations between the
OMTPP and OETPP systems at low temperature can be
explained as follows. When DMAP is replaced by Py, the
energy levels of the dπ orbitals are lowered due to the iron-
(dπ)-ligand(pπ*) interactions in both systems. These com-
plexes are further stabilized by the dxy-a2u interaction caused
by the ruffling of the porphyrin core. In the previous paper,
we mentioned that the OETPP core is more rigid than the
OMTPP core on the basis of the difference in the rotation
barriers of the coordinated 2-MeIm ligands between [Fe-
2
2
saddled complexes, the dx -y orbital is destabilized due to
the short Fe-Np bond distances.36,42,45,61-63 The short Fe-
Np bond distances also destabilize the dπ(dxz, dyz) orbitals
(61) Ohgo, Y.; Ikeue, T.; Nakamura, M. Inorg. Chem. 2002, 41, 1698-
1700.
(62) Cheng, R.-J.; Chen, P.-Y. Chem. Eur. J. 1999, 5, 1708-1715.
(63) Barkigia, K. M.; Renner, M. W.; Fajer, J. J. Porphyrins Phthalocya-
nines 2001, 5, 415-418.
(64) Scheidt, W. R.; Geiger, D. K.; Haller, K. J. J. Am. Chem. Soc. 1982,
104, 495-499.
(65) Ellison, M. K.; Nasri, H.; Xia, Y.-M.; Marchon, J.-C.; Schulz, C. E.;
Debrunner, P. G.; Scheidt, W. R. Inorg. Chem. 1997, 36, 4801-4811.
Inorganic Chemistry, Vol. 42, No. 18, 2003 5569

Ikeue et al.
Scheme 4. Schematic Representation of the Change in Energy Levels of the Iron d Orbitals at Ambient Temperature Expected When Planar TPP Is
Replaced by Ruffled TiPrP, or When the Axial Ligand in [Fe(TiPrP)L2]ClO4 Changes from DMAP to Py, to 4-CNPy, and Then to THFa
a Labels a-g indicate the stabilization or destabilization of the iron d orbitals expected for the ligand change: (a) destabilization due to short Fe-Np bond
lengths caused by the ruffled deformation; (b) stabilization due to the weak ligand field of axial ligand; (c) stabilization due to the iron dπ and ligand pπ*
interactions; (d) destabilization due to the iron dxy and porphyrin a2u interaction; (e) stabilization due to the decrease in interactions between the iron dπ and
porphyrin 3eg orbitals; (f) destabilization due to the lack of the iron dπ and ligand pπ* interactions; (g) stabilization due to the lack of the iron dxy and
porphyrin a2u interaction.
(OMTPP)(2-MeIm)2]+ and [Fe(OETPP)(2-MeIm)2]+; the
meso-carbon signal of the latter complex splits into three
signals below -60 °C due to the hindered rotation of the
ligand while the corresponding signal in the former complex
maintains its singlet even at -100 °C.29 Another set of
evidence supporting the rigidity of the OETPP core as
compared to the OMTPP core comes from the difference in
chemical shifts of the meso-13C signals between [Fe-
(OMTPP)(tBuNC)2]+ and [Fe(OETPP)(tBuNC)2]+; the meso-
13C signals were observed at 701 and 419 ppm at 25 °C,
respectively. A larger downfield shift of the meso-13C signal
in the OMTPP complex corresponds to the greater (dxz, dyz)4-
(dxy)1 contribution, which in turn corresponds to the larger
ruffling of the OMTPP core.29 We can then explain why only
the OMTPP complexes, [Fe(OMTPP)Py2]+ and [Fe(OMTPP)-
(4-CNPy)2]+, adopt the (dxz, dyz)4(dxy)1 electron configuration;
the OMTPP complexes can ruffle the porphyrin core with
relatively small energy and change the electron configuration
from (dxy)2(dxz, dyz)3 to (dxz, dyz)4(dxy)1.66
two signals below 0 °C. In contrast, the R-methylene signal
of [Fe(TBTXP)(DMAP)2]+ maintained a broad singlet even
at -80 °C. A similar difference in temperature dependence
of the R-methylene signals was observed for the HIm, Py,
and 4-CNPy complexes. Thus, the ring inversion of the
TBTXP complexes takes place more rapidly than that of the
OETPP complexes, which in turn indicates the flexible nature
of the TBTXP core as compared to the OETPP core. The
present study therefore reveals that the rigidity of the
porphyrin ring plays a crucial role in determining the spin
crossover pathways.
Reasons for the Formation of [Fe(ORTPP)(THF)2]+-
(R ) Me, Et) with an Essentially Pure Intermediate-Spin
State. As shown in Scheme 3, the replacement of 4-CNPy
with a much weaker THF ligand could induce several
changes in the energy levels of the d orbitals: (i) stabilization
2
of the dz orbital, (ii) stabilization of the dxy orbital, and (iii)
2
destabilization of the dπ orbitals. The stabilization of the dz
orbital is attributed to the weak ligand field of THF as
compared with 4-CNPy. The latter two effects, ii and iii,
are caused by the lack of π-accepting capability of THF.
Because the iron (dπ) and ligand (pπ*) interactions are fairly
weak in [Fe(ORTPP)(THF)2]+, the energy levels of the dπ
orbitals would be raised when going from [Fe(ORTPP)(4-
CNPy)2]+ to [Fe(ORTPP)(THF)2]+. As a result, the dπ
orbitals are located far above the dxy orbital, which in turn
weakens the iron (dxy) and porphyrin (a2u) interactions and
hence stabilizes the iron (dxy) orbital; note that the destabi-
lization of the iron dxy orbital is caused by the dxy-a2u
interaction in the low-spin complexes with (dxz, dyz)4(dxy)1
electron configuration.48,49 Therefore, the three d orbitals, dxz,
The flexibility of the TBTXP core as compared with that
of the OETPP core can be understood from the results
reported by Smith, Shelnutt, and co-workers; the inversion
barrier of [Ni(TBTPP)], which is structurally similar to
TBTXP, is much smaller than that of [Ni(OETPP)] as
1
determined by the line shape analysis of the H NMR
signals.33 In the present case, the temperature dependence
of the diastereotopic R-methylene protons also showed a
large difference in 1H NMR line shape between [Fe(TBTXP)-
L2]+ and [Fe(OETPP)L2]+. For example, a broad R-meth-
ylene signal of [Fe(OETPP)(DMAP)2]+ at 25 °C splits into
(66) Our preliminary result on the X-ray crystallographic analysis of [Fe-
(OMTPP)Py2]ClO4 has revealed that the saddled deformation of this
complex is much smaller than that of [Fe(OETPP)Py2]ClO4 previously
reported;61 the average deviation of the â-pyrrole carbon atoms from
the least-squares porphyrin plane is 1.04 Å in the former while it is
1.22 Å in the latter. The result is another piece of evidence supporting
the rigid nature of the OETPP core as compared to the OMTPP core.
Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura, M. To
be published.
2
dyz, and dz , are located quite close to each other, resulting
in the formation of an essentially pure intermediate-spin
complex over a wide range of temperatures.
In the previous section, we mentioned that the upfield shift
of the meso-carbon signal is the most characteristic feature
of intermediate-spin complexes from the viewpoint of 13C
5570 Inorganic Chemistry, Vol. 42, No. 18, 2003

Saddle-Shaped Six-Coordinated Fe(III) Porphyrins
NMR spectroscopy. Although both [Fe(ORTPP)(THF)2]+
and [Fe(ORTPP)(4-CNPy)2]+ are in the S ) 3/2 spin state at
least at room temperature, the chemical shifts of the meso-
carbon signals are quite different; they are -0.5 (R ) Me)
and -87.0 (R ) Et) ppm for [Fe(ORTPP)(THF)2]+ and
-67.9 (R ) Me) and -235.6 (R ) Et) ppm for [Fe(ORTPP)-
(4-CNPy)2]+. As previously mentioned, the dπ orbitals of
[Fe(ORTPP)(THF)2]+ are located far above those of the
corresponding [Fe(ORTPP)(4-CNPy)2]+ as shown in Scheme
3. Therefore, the iron(dπ)-porphyrin(3eg) interactions are
more effective in [Fe(ORTPP)(4-CNPy)2]+ than in [Fe-
(ORTPP)(THF)2]+ due to the smaller energy gap in the
former complexes. Since the 3eg orbitals have large coef-
ficients at the pyrrole carbon and nitrogen and zero spin
density at the meso-carbon atoms,38 the strong iron(dπ)-
porphyrin(3eg) interactions in [Fe(ORTPP)(4-CNPy)2]+ in-
duce larger spin densities on the pyrrole carbon atoms, which
in turn causes a large downfield shift of the pyrrole carbon
signal and an upfield shift of the adjacent meso-carbon
signal.19,23,29,67 As a result, the meso signals of the 4-CNPy
complexes appear at greater magnetic field at room temper-
ature than those of the corresponding THF complexes.
Difference in Magnetic Properties between Saddled
a wide range of temperatures, while the saddled [Fe(ORTPP)-
(4-CNPy)2]+ exhibits the spin transition from S ) 3/2 to S )
1/2 at lower temperature. As already mentioned, the replace-
ment of 4-CNPy by a much weaker THF ligand could
2
stabilize the dz and dxy orbitals and destabilize the dπ orbital.
As a result, four d orbitals of [Fe(TiPrP)(THF)2]+ are located
very close in the energy diagram as shown in Scheme 4,
resulting in the formation of a pure intermediate-spin
complex over a wide range of temperatures: 1H NMR,
δ(Py-H) ) -52.1 ppm (233 K); EPR, g ) 3.99 and g )
1.97 (4.2 K); SQUID, µeff ) 3.90 ( 0.10 µB (50-300 K);
Mo¨ssbauer, IS (76 K) ) 0.34 mm s-1, and QS (76 K) )
3.71 mm s-1; X-ray, av Fe-Np ) 1.967(12) Å.12,15
Conclusions
1
Combined analyses using H NMR, 13C NMR, and EPR
spectroscopy and magnetic measurements revealed that the
saddle-shaped [Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ (L )
DMAP, Py, 4-CNPy, THF) complexes exhibit different spin
states in solution which depend on the axial ligands. While
1
the DMAP and THF complexes maintain the S ) /2 and S
) 3/2 spin states, respectively, over a wide temperature range,
the Py and 4-CNPy complexes exhibited a spin transition
3
1
+
2
from S ) /2 to S ) /2 as the temperature decreased.
Therefore, the magnetic behavior of these complexes is very
similar to that of the corresponding [Fe(OETPP)L2]+ recently
reported. The low-spin [Fe(OMTPP)L2]+ and [Fe(TBTXP)-
L2]+ (L ) Py and 4-CNPy) complexes formed at low
temperature exhibited, however, a different electron config-
uration from that of the corresponding [Fe(OETPP)L2]+; the
former adopt the (dxz, dyz)4(dxy)1 while the latter exhibit the
(dxy)2(dxz, dyz)3 electron configuration. These results have been
explained in terms of the difference in rigidity of the
porphyrin cores: while the OMTPP and TBTXP complexes
can ruffle the porphyrin core with relatively small energy
and change the electronic ground state from (dxy)2(dxz, dyz)3
to (dxz, dyz)4(dxy)1, the OETPP complexes maintain the saddled
structure due to the core rigidity.
[Fe(ORTPP)L ] (R ) Me, Et) and Ruffled [Fe(TRP)-
L2]+. The energy levels of the d orbitals in ruffled complexes
such as [Fe(TiPrP)L2]+ are influenced differently by the
porphyrin ring.19 The ruffling of the planar porphyrin ring
causes three major effects on the d orbitals of bis-ligated
2
2
iron(III) porphyrins: (i) destabilization of the dx -y orbital
due to the short Fe-Np bond distances,15,16,68-70 (ii) stabiliza-
tion of the dπ orbitals due to the less effective overlap
between the iron (dπ) and porphyrin (3eg) orbitals,29 and (iii)
destabilization of the dxy orbital due to the iron(dxy)-
porphyrin(a2u) interaction.48,49 Consequently, the energy levels
of the dπ and dxy orbitals are reversed when going from planar
[Fe(TPP)(DMAP)2]+ to ruffled [Fe(TiPrP)(DMAP)2]+ com-
plexes as shown in Scheme 4. In this situation, even if the
axial DMAP ligand is replaced by a much weaker Py or
2
4-CNPy, the energy gap between the dπ and the dz orbital
Acknowledgment. This work was supported by the Grant
in Aid (14540521) for Scientific Research to M.N. from
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. M.N. also thanks the Futaba Memorial Founda-
tion for financial support. T. I. is grateful to the JSPS
Research Fellowship for young scientists. Thanks are due
to the Research Center for Molecular Materials, the Institute
for Molecular Science (IMS), and Louisiana State University.
is still large enough to maintain the low-spin state. Since
the 4-CNPy ligand has low-lying pπ* orbitals, it stabilizes
2
not only the dz orbital by its weak σ-donating ability but
also the dπ orbitals by its strong π-accepting ability.
Therefore, the ruffled [Fe(TiPrP)(4-CNPy)2]+ maintains the
S ) 1/2 state with (dxz, dyz)4(dxy)1 electron configuration over
(67) Goff, H. M. J. Am. Chem. Soc. 1981, 103, 3714-3722.
(68) Scheidt, W. R.; Kirner, J. L.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963-1968.
(69) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohnrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
(70) Ohgo, Y.; Ikeue, T.; Saitoh, T.; Nakamura, M. Chem. Lett. 2002, 432-
433.
Supporting Information Available: Curie plots of the ortho
and para protons in [Fe(OMTPP)L2]+. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC0300969
Inorganic Chemistry, Vol. 42, No. 18, 2003 5571