Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

Article abstract of DOI:10.1021/ic7008495

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO) ₂]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K₁ is much larger than K₂. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K₁ = 267 and K₂ ≈ 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)₂] (C₅₂H ₄₄Fe-N₄O₂S₂): monoclinic, P2 ₁/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930-(4) A, β = 116.246(1)°, V = 1992.95(10) A³, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-N_p distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.

Full text of DOI:10.1021/ic7008495

Inorg. Chem. 2007, 46, 8258−8263
Sulfoxide as a Ligand in Iron(II) Porphyrinates: S- or O-Bound?
Chuanjiang Hu, Bruce C. Noll, and W. Robert Scheidt*
Contribution from The Department of Chemistry and Biochemistry,
UniVersity of Notre Dame, Notre Dame, Indiana 46556
Received May 2, 2007
In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with
tetramethylenesulfoxide (TMSO) has been performed. UV vis spectra at different concentrations of TMSO have
−
shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)],
and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the
equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program
SQUAD gives K1
synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction
study. [Fe(TPP)(TMSO)2] (C52H44Fe N4O2S2): monoclinic, P21/c, a 11.2580(3) Å, b 15.9262(5) Å, c 12.3930-
(4) Å, â ) 116.246(1) , V 2. X-ray crystallography demonstrates the complex is a low-spin
1992.95(10) Å3, Z
bis-TMSO ligated species. The average Fe Np distance is 1.999(4) Å. The most important feature is that TMSO
is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe S distance is 2.2220(3) Å.
)
267 and K2
≈
1. Even though the five-coordinate species is the dominant species under the
−
)
)
)
°
)
)
−
−
Introduction
could be changed when methionine-80 is modified to
methionine-80 sulfoxide in cytochrome c. However, the
nature of the coordination sphere in methionine-80 sulfoxide
cytochrome c was not well established in the previously
reported studies.
As an ambidentate ligand, sulfoxides can coordinate to
metal ions via sulfur or oxygen.5 For methionine sulfoxide
cytochrome c, an NMR6 study by Ivanetich et al.7 suggested
the ligand binding is via the sulfur atom in both the ferrous
and ferric forms. However, Myer concluded that modification
of the methionine to methionine sulfoxide changed the sixth
coordination ligand donor atom to heme iron from sulfur to
the oxygen of methionine-80 sulfoxide for both iron(II) and
The amino acid methionine is a well-known ligand to iron
in the ubiquitous electron-transfer proteins known as the
cytochromes c. The other ligand is a histidine. The relative
ease of losing methionine coordination has been long
recognized and demonstrates the relatively small affinity of
a thioether for iron.1 Nonetheless, the coordination of the
thioether sulfur atom to the physiological oxidation states
of cytochrome c, that is, iron(II) and iron(III), is clear.
In the presence of reactive oxygen species, methionine can
be oxidized to the corresponding sulfoxide.2 In mitochondria
(a major source of reactive oxygen species3), oxidation of
methionine-80 to methionine-80 sulfoxide in the protein
cytochrome c may occur under normal physiological condi-
tions. Though such a transformation in nature has not been
reported, the chemical modification of methionine cyto-
chrome c has been studied and methionine sulfoxide cyto-
chrome c has been shown fully active with cytochrome
oxidase.4 The heme-binding characteristics of the ligand
(5) Calligaris M.; Carugo, O. Coord. Chem. ReV. 1996, 153, 83.
(6) The following abbreviations are used in this paper: NMR, nuclear
magnetic resonance; AcMP8, N-acetylmicroperoxidase-8; TPP, dianion
of meso-tetraphenylporphyrin; TpivPP, dianion of R,R,R,R-tetrakis-
(o-pivalamidophenyl)porphyrin; TDCP(NO2)8P, dianion of meso-
tetrakis(o-dichlorophenyl)-â-octanitroporphyrin; PPIX, dianion of
protoporphyrin IX; Np, porphyrinato nitrogen; TMSO; tetramethyl-
enesulfoxide; THT, tetrahydrothiophen; PMS, pentamethylene sulfide;
THF, tetrahydrogenfuran; EtOH, ethanol; PrOH, propanol; DMSO,
dimethylsulfoxide; DecMS, thioethers n-decylmethyl sulfide; SPh2,
diphenyl sulfide; 1-MeIm, 1-methylimidazole; 1-VinIm, 1-vinylimi-
dazole; 1-BzylIm, 1-benzylimidazole; 4-CNPy, 4-cyanopyridine;
3-CNPy, 3-cyanopyridine; 4-MePy, 4-methylpyridine; Pc, phthalo-
cyanine; HSAB, hard-soft-acid-base; ESR, electronic spin resonance;
18-C-6,1,4,7,10,13,16-hexaoxacyclooctadecane; 222,1,10-diaza-4,7,-
13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane.
* Towhomcorrespondenceshouldbeaddressed.E-mail: scheidt.1@nd.edu.
(1) Taler, G.; Schejter, A.; Navon, G.; Vig, I.; Margoliash, E. Biochemistry
1995, 34, 14209.
(2) Gilbert, D. L.; Colton, C. A.; Eds. ReactiVe Oxygen Species in
Biological Systems; Kluwer Academic/Plenum: New York, 1999.
(3) Funkel, T.; Holbrook, N. J. Nature 2000, 408, 239.
(4) Ivanetich, K. M.; Bradshaw, J. J.; Kaminsky, L. S. Biochemistry 1976,
15, 1144.
(7) Ivanetich, K. M.; Bradshaw, J. J.; Fazakerley, G. V. Biochem. Biophys.
Res. Com. 1976, 72, 433.
8258 Inorganic Chemistry, Vol. 46, No. 20, 2007
10.1021/ic7008495 CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/08/2007

Sulfoxide as a Ligand in Iron(II) Porphyrinates
Table 1. Brief Crystallographic Data and Data Collection Parameters
for [Fe(TPP)(TMSO)2]
formula
C52H44FeN4O2S2
fw, amu
876.88
a, Å
11.2580(3)
b, Å
15.9262(5)
c, Å
12.3930(4)
â, deg
116.246(1)
1992.95(10)
V, Å3
space group
Z
P21/c
2
1.461
Dc, g/cm3
F(000)
916
0.534
µ, mm-1
crystal dimens, mm3
radiation
temp, K
0.38 × 0.33 × 0.20
Mo KR, λ ) 0.71073 Å
100(2)
total data collected
abs corr
unique data
unique observed data [I > 2σ(I)]
refinement method
final R indices [I > 2σ(I)]
final R indices (all data)
24 711
semiempirical from equivalents
4085 (Rint ) 0.0200)
3846
Figure 1. UV-vis spectra taken under argon in 5% EtSH/toluene solution.
The concentration of [Fe(TPP)] is 0.86 × 10-4 mol/L, the TMSO
concentrations in 10-3 mol/L (a) 1.13. (b) 2.26. (c) 3.39. (d) 5.08. (e) 7.23.
(f) 9.95. (g) 14.2. The enlarged spectra from 480 to 730 nm are measured
in the 10-mm UV cell.
full-matrix least-squares on F2
R1 ) 0.0275, wR2 ) 0.0783
R1 ) 0.0293, wR2 ) 0.0799
iron(III).8 They concluded that in the neutral-to-alkaline pH
range, the low-spin spectrum is a consequence of coordina-
tion of heme iron by the sulfoxide oxygen. In recent work
on AcMP8, Lushington et al.9 suggested that methionine-80
sulfoxide does not coordinate to iron(III) in the methionine-
80 sulfoxide cytochrome c on the basis of the comparison
of spectroscopic data for the DMSO complex of Fe(III)-
AcMP8 with published data for the methionine-80 sulfoxide
form. Their vacuum quantum chemical density functional
theory calculations suggested that coordination of DMSO
to Fe(II)-AcMP8 via oxygen is enthalpically favored over
sulfur coordination; resonance Raman spectroscopic data in
aqueous solution concurred with this conclusion. They
claimed that the unfavorable steric environment near the
heme iron in cytochrome c discriminates against coordination
of methionine-80 sulfoxide.
Therefore, it is of fundamental importance to understand
the coordination mode of sulfoxide to heme iron. Surpris-
ingly, the interaction of sulfoxide ligand with iron(II)
porphyrin has not been directly investigated. This is in
distinct contrast to the situation with iron(III) where coor-
dination of sulfoxide through oxygen has been long estab-
lished.10,11 Reed and Scheidt have studied the sulfoxide-
ligated iron(III) complex, [Fe(TPP)(TMSO)2]ClO4 (TMSO
) tetramethylenesulfoxide), and the magnetic, Mo¨ssbauer,
ESR, and structure data demonstrate that it is a high-spin
O-ligated species. To our knowledge, no structural data on
sulfoxide-ligated iron(II) porphyrinates have been reported
so far. As part of a more general program of characterization
of high-spin iron(II) porphyrinates,12 our primary goal was
to prepare five-coordinate TMSO-ligated species, but to our
surprise, a six-coordinate low-spin S-ligated iron porphyri-
nate, [Fe(TPP)(TMSO)2], was obtained. We present herein
a study of the equilibria of Fe(TPP) with TMSO and the
molecular structure of [Fe(TPP)(TMSO)2].
Experimental Section
General Information. Chlorobenzene was washed with con-
centrated sulfuric acid, then with water until the aqueous layer was
neutral, dried with MgSO4, and distilled twice over P2O5 under
argon. Hexanes were distilled over sodium benzophenone. Ethaneth-
iol (Aldrich) was used as received. All other chemicals were used
as received from Aldrich or Fisher. meso-Tetraphenylporphyrin
(H2TPP) was prepared according to Adler et al.13 [Fe(TPP)Cl] was
prepared according to a modified Adler preparation;14,15[Fe(TPP)]2O
was prepared from [Fe(TPP)Cl].15 All manipulations were per-
formed under argon.
Synthesis of [Fe(TPP)(TMSO)2]. [Fe(II)(TPP)] was prepared
by reduction of [Fe(TPP)]2O (32 mg, 0.024 mmol) with ethanethiol
(∼1 mL) in ∼ 6 mL of chlorobenzene. The chlorobenzene solution
was stirred for 3 days, then transferred into a Schlenk flask
containing tetramethylenesulfoxide (0.5 mL, 5.6 mmol) by cannula,
and stirred for another hour. X-ray quality crystals were obtained
in sealed 8 mm diameter glass tubes by liquid diffusion using
hexanes as the nonsolvent.
Equilibrium Constant Determination. UV-vis spectra were
recorded on a Perkin-Elmer Lambda 19 UV-vis-near-IR spec-
trometer in a specially designed combined 1- and 10-mm inert
atmosphere cell. A solution of the four-coordinate iron(II) porphyrin
was prepared as above. UV-vis spectra of Fe(TPP) in different
concentrations of TMSO were measured. For the low-concentration
region, TMSO was titrated into [Fe(II)(TPP)] in a 5% EtSH/toluene
(8) Myer, Y. P.; Kumar, S.; Kinnally, K.; Pande, J. J. Protein Chem. 1987,
6 (4), 321.
(9) Lushington. G. H.; Cowley, A. B.; Silchenko, S.; Lukat-Rodgers, G.
S.; Rodgers, K. R.; Benson, D. R. Inorg. Chem. 2003, 42, 7550.
(10) Mashiko, T.; Kastner, M. E.; Spartalian, K.; Scheidt, W. R.; Reed, C.
A. J. Am. Chem. Soc. 1978, 100, 6354.
(11) Muthusamy, M.; Andersson, L. A.; Sun, J.; Loehr, T. M.; Thomas,
C. S.; Sullivan, E. P.; Thomson, M. A.; Long, K. M.; Anderson, O.
P.; Strauss, S. H. Jr. Inorg. Chem. 1995, 34, 3953.
(13) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(14) (a) Adler, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Inorg. Nucl.
Chem. 1970, 32, 2443. (b) Buchler, J. W. In Porphyrins and
Metalloporphyrins; Smith, K. M., Ed.; Elsevier Scientifc Publishing:
Amsterdam, The Netherlands, 1975; Chapter 5.
(15) (a) Fleischer, E. B.; Srivastava, T. S. J. Am. Chem. Soc. 1969, 91,
2403. (b) Hoffman, A. B.; Collins, D. M.; Day, V. W.; Fleischer, E.
B.; Srivastava, T. S.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 3620.
(12) Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schulz, C. E.;
Scheidt, W. R. J. Am. Chem. Soc. 2005, 1 27, 5675.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8259

Hu et al.
Figure 2. ORTEP diagram of [Fe(TPP)(TMSO)2]. The hydrogen atoms of the porphyrin ligand have been omitted for clarity; 50% probability ellipsoids
are depicted.
solution; the EtSH was added to prevent oxidation of the iron.16
For the high-concentration region, the [Fe(II)(TPP)] solution was
added into the concentrated TMSO solution. In order to obtain
spectra with appropriate intensity for both the Soret and Q bands,
several different concentrations of iron porphyrin were used. The
concentrations of TMSO for UV-vis measurements range from
1.13 × 10-3 to 10.7 mol/L. The equilibrium constants for these
equilibria were calculated with the nonlinear least-squares program
SQUAD.17 SQUAD calculates best values for the equilibrium
constants by employing multiple-wavelength absorption data for
varying concentrations of the reactants. Absorbance data, at 10-
nm increments spanning the visible region, were input into SQUAD.
including negative intensities. The structure was refined in space
group P21/c. The iron atom is located at a crystallographic inversion
center, and the asymmetric unit contains half an iron porphyrin
and one TMSO ligand. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were added with the standard
SHELXL-97 idealization methods. The program SADABS21 was
applied for the absorption correction. Brief crystal data are listed
in Table 1. Complete crystallographic details, atomic coordinates,
anisotropic thermal parameters, and hydrogen atom coordinates are
given in the Supporting Information.
Results
X-ray Structure Determination. Single-crystal experiments
were carried out on a Bruker Apex system with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). A red crystal
with the dimensions 0.38 × 0.33 × 0.20 mm3 was used for the
structure determination. The crystalline sample was placed in inert
oil, mounted on a glass pin, and transferred to the cold gas stream
of the diffractometer. Crystal data were collected at 100 K. The
structure was solved by direct methods using SHELXS-9718 and
refined against F2 using SHELXL-97;19,20 subsequent difference
Fourier syntheses led to the location of all the remaining non-
hydrogen atoms. For the structure refinement, all data were used
Figure 1 shows the UV-vis spectra of Fe(TPP) with
TMSO concentrations ranging from 0 to 1.49 × 10-1 mol/
L; the highest concentrations of TMSO are not illustrated.
Before the addition of TMSO, the spectrum of Fe(II)(TPP)
shows two Soret bands at 419 and 442 nm and a relative
weaker band at 537 nm in the Q-band region (500-700 nm).
During the titration, the electronic spectra show that the Soret
band becomes a single sharp band at 432 nm with increased
intensity. The band at 537 nm decreases, and new bands
show up at 560 and 603 nm. Clear isosbestic points are
observed in both the Soret and Q-band regions. When the
concentration of TMSO is over about 0.45 mol/L, the Soret
band begins to decrease and finally shifts to 429 nm with a
relatively weaker intensity in 10.7 mol/L of TMSO (95%).
The bands at 560 and 603 nm decrease, and the band at 530
nm increases by the increasing of TMSO concentration.
Crystals were obtained from solutions where the TMSO-
to-Fe(TPP) concentration ratio was ∼220. X-ray crystal-
lography shows that the isolated complex is a six-coordinate
(16) Even under these dilute solution conditions, we see no spectral changes
resulting from the presence of EtSH and we conclude that EtSH does
not coordinate to iron. The relatively long times of the titrations do
require EtSH.
(17) Leggett, D. J. In Computational Methods for the Determination of
Formation Constants; Leggett, D. J., Ed.; Plenum: New York, 1985;
Chapter 6.
(18) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(19) Sheldrick, G. M. Program for the Refinement of Crystal Structures;
Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(20) R1 ) ∑||Fo| - |Fc||/∑|Fo| and wR2 ) {[w(Fo - Fc )2]/[wFo ]}1/2
.
2
2
4
The conventional R factors R1 are based on F, with F set to 0 for
negative F2. The criterion of F2 > 2σ(F2) was used only for calculating
R1. R factors based on F2(wR2) are statistically about twice as large
as those based on F, and R factors based on all data will be even
larger.
(21) Sheldrick, G. M. Program for Empirical Absorption Correction of
Area Detector Data; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
1996.
8260 Inorganic Chemistry, Vol. 46, No. 20, 2007

Sulfoxide as a Ligand in Iron(II) Porphyrinates
Figure 3. Diagram illustrating the core conformation and iron displacement
of the title complex. The displacements of the iron and the atoms of the
porphyrin core from the mean plane defined by the 24 porphyrin atoms are
given. The position of the S-O bond with respect to directions defined by
the Fe-Np directions is shown: solid lines are above the plane, dashed
lines are below the plane.
Figure 4. UV-vis spectra taken under argon. (a) Fe(II)(TPP) (8.6 × 10-5
mol/L). (b) Fe(II)(TPP) (3.8 × 10-5 mol/L) in 4.3 × 10-2 mol/L TMSO
solution. (c) Fe(II)(TPP) (3.0 × 10-5 mol/L) in 10.7 mol/L TMSO solution.
The enlarged spectra from 480 to 730 nm are measured in the 10-mm UV
cell.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for for
[Fe(TPP)(TMSO)2]
Because the iron(II) porphyrinates with different coordination
numbers have substantially different absorption spectra,22
UV-vis spectroscopy can be used to monitor the reaction
and determine the equilibrium constants. As shown in Figures
1 and 4, both the Soret band and Q bands show remarkable
changes during the titration. The spectrum with TMSO
concentration at 4.3 × 10-2 mol/L shows a sharp Soret band
at 432 nm and three bands in the Q-band region: two broad
bands with similar intensities at around 540 and 555 nm and
one at 603 nm with about half the intensity of the other two
bands. This spectral pattern, including intensity, is similar
to that observed for imidazole-ligated five-coordinate high-
spin iron(II) species.22 We can thus conclude that the
dominant species in this solution is a five-coordinate high-
spin species. Analysis by the SQUAD program suggests [Fe-
(TPP)(TMSO)] is 89% of the total iron species at this
concentration. The spectrum in 10.7 mol/L TMSO is
substantially different. The single Soret band is shown at a
similar position with lower intensity. The Q bands show
distinct differences in extinction coefficients: the major peak
at 530 nm is about twice as intense as the shoulder peak at
560 nm. These features are the same as features observed in
the six-coordinate nitrogen-based species,22 which suggests
the dominant species in this solution is six-coordinate low-
spin [Fe(TPP)(TMSO)2]. Analysis by the SQUAD program
confirms [Fe(TPP)(TMSO)2] is 91% of the total iron species
at this concentration. Values of the extinction coefficients
are calculated by the SQUAD program: λmax (log ꢀ): 432-
(5.38), 540(4.03), 560 (4.03), 603(3.61) for [Fe(TPP)-
(TMSO)]; 429(5.16), 530 (4.13), 560(3.96) for [Fe(TPP)-
(TMSO)2].
Fe(1)-N(1)
Fe(1)-S(1)
S(1)-C(4)
2.0020(11) Fe(1)-N(2)
2.2220(3) S(1)-O(1)
1.7981(14) S(1)-C(1)
1.9956(11)
1.4794(11)
1.8076(15)
N(2)-Fe(1)-N(1)
N(2)-Fe(1)-S(1)
O(1)-S(1)-C(4)
C(4)-S(1)-C(1)
90.55(5)
90.63(3)
106.54(7)
91.76(7)
N(1)-Fe(1)-S(1)
87.62(3)
O(1)-S(1)-Fe(1) 118.91(5)
O(1)-S(1)-C(1)
C(2)-C(1)-S(1)
C(1)-S(1)-Fe(1) 114.27(5)
106.57(7)
107.22(10)
C(4)-S(1)-Fe(1) 115.18(5)
species. An ORTEP diagram is provided in Figure 2, and
selected bond distances and angles are listed in Table 2. The
porphyrin molecule has inversion symmetry, the iron(II) atom
is at an inversion center and coordinated to four pyrrole
nitrogens and two sulfur atoms of TMSO. The average Fe-
Np distance is 1.999(4) Å, and the Fe-S distance is 2.2220-
(3) Å. Coordination at the sulfur atom is pyramidal, with an
O-S-C bond angle of 106.54(7)°, a C-S-C angle of
91.76(7)°, a Fe-S-O angle of 118.91(5)°, and an average
Fe-S-C angle of 114.7(6)°. The S-O distance is 1.4794-
(11) Å. A complete listing of bond distances and angles is
given in the Supporting Information.
The displacement of each atom of the porphyrin core from
the 24-atom mean plane is shown in Figure 3. Also included
on the diagrams of Figure 3 are the individual Fe-Np bond
lengths. An analogous diagram showing atomic displace-
ments from the mean plane of the four pyrrole nitrogen atoms
is given in Figure S1 of the Supporting Information.
Discussion
Spectral Studies. There are three species involved in the
reaction of TMSO with Fe(II)(TPP): four-coordinate Fe-
(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-
coordinate [Fe(II)(TPP)(TMSO)2].
The spectral changes in Figure 1 suggests that they relate
to reaction 1. The appearance of isosbestic points indicates
only one major species formed at these concentrations, a five-
coordinate species. It also suggests that the equilibrium
K1
Fe(II)(TPP) + TMSO y
\z Fe(II)(TPP)(TMSO)
(1)
Fe(II)(TPP)(TMSO) +
(22) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.;
Bumenberg, E.; Linder, R. E.; LaMar, G. N.; Del, Gaudio, J.; Lang,
G.; Spartalian, K. J. Am. Chem. Soc. 1980, 102, 4182.
K2
TMSO y\z Fe(II)(TPP)(TMSO)2 (2)
Inorganic Chemistry, Vol. 46, No. 20, 2007 8261

Hu et al.
Table 3. Selected Bond Distances (Å) and Angles (deg) for [Fe(TPP)(TMSO)2] and Related Species
spin stateb
ref
a
complex
Fe-Np
Fe-Sax
Fe-Oax
S-O
[Fe(TPP)(TMSO)2]
1.999(4)
2.046(6)
2.035(9)
1.996(6)
1.985(3)
1.982(6)
2.057(4)
2.068(9)
2.067(7)
2.074(6)
2.092(5)
2.2220(3)
1.4794(11)
1.522(9)
1.539(6)
LS
HS
HS
LS
LS
LS
HS
HS
HS
HS
HS
tw
10
11
25
25
25
36
37
38
39
39
[Fe(TPP)(TMSO)2] ClO4
Fe(OEP)(DMSO)2]PF6
[Fe(TPP)(THT)2]
[Fe(TPP)(THT)2]ClO4
[Fe(TPP)(PMS)2]ClO4
[Fe(II)(TPP)(THF)2]
[Fe(II)(F8TPP)(THF)2]
[Fe(TTP)(THF)2]
2.078(13)
2.082(5)
2.336(3)
2.357(27)
2.341(1)
2.351(3)
2.314(3)
2.3208(8)
2.153(11)
2.152(2)
[Fe(TDCP(NO2)8P)(EtOH)2]
[Fe(TDCP(NO2)8P)(PrOH)2]
a Average Fe-Np distance. b LS, low spin; HS, high spin.
constant K1 is much larger than K2. The analysis of the
spectral data by the SQUAD program supports this. The
result gives formation constants log K1 ) 2.43 ( 0.06 and
log â2 ) log(K1K2) ) 2.41 ( 0.16. This gives K1 ) 267
and K2 ) 0.95. A value of K2 ≈ 2300 has been reported
from a photolysis experiment.23 It is likely that the equilib-
rium constant (in pure TMSO) is actually that of K1.
All the above suggests three species in solution are
dominant at different concentrations of TMSO. Six-
coordinate [Fe(TPP)(TMSO)2] is the dominant species in
solution at very high concentrations of TMSO, while the five-
coordinate species is the dominant species at relatively low
concentrations. In order to prepare a five-coordinate species,
the synthesis was performed at 0.75 mol/L of TMSO. At
that concentration, the dominant species in solution is five-
coordinate [Fe(TPP)(TMSO)], but the six-coordinate species
was crystallized and verified by X-ray crystallography. Its
crystallization is probably due to solubility issues even
though it is not the major species in solution. Finally, it
should be noted that our spectroscopic data do not define
the coordination modes (O vs S) nor that in a mixed ligand
complex such as a nitrogen donor/sulfoxide that iron(II)
would definitely be coordinated by the sulfur.
Molecular Structure. In the crystal structure, the average
Fe-Np distance in [Fe(TPP)(TMSO)2] is 1.999(4) Å. This
distance is that expected for a low-spin iron(II) species.24
Low-spin iron(II) examples for comparison include a thio-
ether-ligated species, [Fe(TPP)(THT)2] (1.996(6) Å);25 sev-
eral imidazole-ligated species, [Fe(TPP)(1-VinIm)2] (2.001(2)
Å), [Fe(TPP)(1-BzylIm)2] (1.993(9) Å), and [Fe(TPP)(1-
MeIm)2] (1.997(6) Å);26 and several pyridine-ligated species,
[Fe(TMP)(4-CNPy)2] (1.992(1) Å), [Fe(TMP)(3-CNPy)2]
(1.996(0) Å), and [Fe(TMP)(4-MePy)2] 1.988(0) Å).27
The low-spin state of [Fe(TPP)(TMSO)2] is in distinct
contrast to the bis(sulfoxide) complexes of iron(III) porphy-
rinates that are all known to be high spin.10,11 The significant
difference between the iron(II) and iron(III) porphyrinate
systems is the ligation mode of the sulfoxide ligands:
O-bound in iron(III) and S-bound in iron(II). Qualitatively,
such a difference in axial coordination mode could be
predicted by Pearson’s HSAB principles.28 The harder iron-
(III) is expected to bind the hard oxygen donor atom of the
sulfoxide, consistent with observation. Iron(II), on the other
hand, is considered a borderline acid and either O- or
S-ligated sulfoxide species might be anticipated. Indeed, both
O-29,30 and S-bound31,32 sulfoxide complexes of iron(II) are
known. However, the “hardness’’ and “softness’’ of a metal
ion can be dramatically modified by the nature of the other
ligands, depending on their σ-donating or π-accepting
properties.33,34 For the case of porphyrin derivatives, the
highly delocalized π system will tend to make iron(II) softer
and favor sulfur ligation as observed.
Most oxygen-ligated iron(II) porphyrinates are species with
THF or alcohol as the axial ligands.35-39 Contrasting with
the aforementioned low-spin S-ligated iron(II) porphyrinates,
their central irons are in the high-spin state with an expanded
porphyrin core, as shown in Table 3. As can be seen in the
Figure 3, the porphyrin core is very planar. The S-O bond
is close to eclipsing the N(1)N(1′) direction, (N(1′) is a
symmetry related pyrrole nitrogen of N(1)). The angle
between the projection of the S-O bond, and the Fe-Np
vector is 5.8°. The orientation is also illustrated in Figure 3.
The six-coordinate, S-ligated state of [Fe(TPP)(TMSO)2]
is clearly seen in Figure 2. Scheidt and Reed had noted earlier
that the Fe-S distances in a wide variety of iron porphyrinate
derivatives was insensitive to a number of factors that could
(23) Larsen, R. W.; Wheeler, P. E.; Findsen, E. W. Inorg. Chim. Acta 2001,
319, 1.
(24) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543.
(25) Mashiko, T.; Reed, C. A.; Haller, K. J.; Kastner, M. E.; Scheidt, W.
R. J. Am. Chem. Soc. 1981, 103, 5758.
(26) Safo, M. K.; Scheidt, W. R.; Gupta, G. P. Inorg. Chem. 1990, 29,
626.
(27) Safo, M. K.; Nesset, M. J. M.; Walker, F. A.; Debrunner, P. G.;
Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 9438.
(28) Pearson, R. G. Coord. Chem. ReV. 1990, 100, 403.
(29) Lippard, S. J.; Carson, E. C. Inorg. Chem. 2006, 45, 837.
(30) Muller, A.; Bogge, H.; Schimanski, U.; Penk, M.; Nieradzik, K.;
Dartmann, M.; Krickemeyer, E.; Schimanski, J.; Romer, C.; Romer,
M.; Dornfeld, H.; Wienboker, U.; Hellmann, W.; Zimmermann, M.
Monatsh. Chem. 1989, 120, 367.
(31) Calderazzo, F.; Pampaloni, G.; Vitali, D.; Collamati, I.; Dessy, G.;
Fares, V. J. Chem. Soc., Dalton Trans. 1980, 1965-1969.
(32) Carni, C.; Pelizzi C.; Pelizzi, G.; Predieli, G.; Tarasconi, P.; Vitali, F.
J. Chem. Soc. Chem. Commun. 1990, 613.
(33) Alessio, E.; Milani, B.; Bolle, M.; Mestroni, G.; Faleschini, P.; Todone,
F.; Geremia, S.; Calligaris, M. Inorg. Chem. 1995, 34, 4722.
(34) Calligaris, M.; Faleschini, P.; Todone, F.; Alessio, E.; Geremia, S. J.
Chem. Soc. Dalton Trans. 1995, 653.
(35) Ohgo, Y.; Saitoh, T.; Nakamura, M. Acta Cryst. C. 2001, 57, 233.
(36) Reed, C. A.; Mashiko, T.; Scheidt, W. R.; Spartalian, K.; Lang, G. J.
Am. Chem. Soc. 1980, 102, 2302.
(37) Thompson, D. W.; Kretzer, R. M.; Lebeau, E. L.; Scaltrito, D. V.;
Ghiladi, R. A.; Lam, K.-C.; Rheingold, A. L.; Karlin, K. D.; Meyer,
G. J. Inorg. Chem. 2003, 42, 5211.
(38) Hu, C.; Noll, B. C.; Scheidt, W. R. Acta Crystallogr. 2005, E61, m830.
(39) Barkigia, K. M.; Palacio, M.; Sun, Y.; Nogues, M.; Renner, M. W.;
Varret, F.; Battioni, P.; Mansuy, D.; Fajer, J. Inorg. Chem. 2002, 41,
5647.
8262 Inorganic Chemistry, Vol. 46, No. 20, 2007

Sulfoxide as a Ligand in Iron(II) Porphyrinates
be expected to lead to differing Fe-S distances: all observed
distances are ∼2.35 Å. The relatively short Fe-S distance
of 2.2220(3) Å in [Fe(TPP)(TMSO)2] was therefore rather
unexpected and can be compared to 2.338(1), 2.334(1) Å in
[Fe(TPP)(THT)2],25 2.367(3) Å in K(18-C-6)[Fe(TpivPP)-
(SC6HF4)(O2)], 2.365(2) Å in Na(18-C-6)[Fe(TpivPP)(SC6-
HF4)(O2)],40 2.324(2) Å in Na(222)[Fe(TpivPP)(SC2H5)],41
and 2.380(2) Å in K(222)[Fe(TpivPP)(PMS)(NO2)].42 It is
thus seen that the Fe-S distance in [Fe(TPP)(TMSO)2] is
more than 0.1 Å shorter than all previously observed FeII-S
distances. A similar situation has been seen in ruthenium-
(II) porphyrinates, although the difference between Ru-S
distances in the S-ligated sulfoxide and thioether complexes
is about 0.05 Å. The observed distances are Ru-S ) 2.319-
(1) Å in [Ru(II)(OEP)(DMSO)2],43 and 2.376(1) and 2.371-
(1) Å in [Ru(OEP)(DecMS)2] and [Ru(OEP)(SPh2)2],44
respectively.
The shortening of the Fe-S bond in the S-bound sulfox-
ides relative to analogous thioether derivatives can be
associated with either a strengthening of Fe-S dπ-pπ back-
bonding, enhanced σ-bonding, or possible steric effects.
Steric effects appear to have no influence as nonbonded
distances between the TMSO ligand or the tetrahydrothiophene
ligand and the porphyrin in the respective six-coordinate
complexes are similar. Increased π-back-bonding has been
proposed for Ru(II)-DMSO nonporphyrin complexes;5,45,46
such π bonding is associated with a lengthening of the S-O
bond relative to the value in uncomplexed sulfoxides.
However, in [Fe(TPP)(TMSO)2], the S-O distance (1.4794-
(11) Å) is close to that in free TMSO (1.484(3) Å).47 Thus,
π-back-bonding does not appear to be making a significant
contribution to the shortness of the Fe-S bond in [Fe(TPP)-
(TMSO)2]. Enhanced σ-bonding of the TMSO is the most
possible reason for the shortening of the Fe-S bond. If there
is enhanced σ-bonding in TMSO complexes, the TMSO will
be expected to be more basic than tetrahydrothiophen which
has been observed.48 This enhanced σ-bonding has also been
suggested by Lushington et al.9 in a DFT calculation.
Summary
A study of the equilibria of Fe(TPP) with TMSO has been
performed. Equilibrium constants were obtained as K1 ) 267
and K2 ) 0.95. At low concentration, the spectra show
isosbestic points during the titration. Three species in solution
are dominant at different TMSO concentrations. Under the
synthetic conditions, the dominant species in solution is five-
coordinate species. But probably due to solubility issues, the
six-coordinate species, [Fe(II)(TPP)(TMSO)2], was crystal-
lized. X-ray crystallography shows that is a sulfur-ligated
low-spin species, not like the oxygen-ligated high-spin [Fe-
(III)(TPP)(TMSO)2]ClO4. The average Fe-Np distance is
1.999(4) Å, and the Fe-S distance is 2.2220(3) Å.
Acknowledgment. We thank the National Institutes of
Health for support of this research under Grant No. GM-
38401. W.R.S. thanks Professor Kenton R. Rodgers for a
useful discussion.
Supporting Information Available: Figure S1 displays dis-
placements form the N4 plane; Tables S1-S6 give complete
crystallographic details, atomic coordinates, bond distances and
angles, anisotropic temperature factors, and fixed hydrogen
atom positions; and X-ray crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(40) Schappacher, M.; Ricard, L.; Fischer, J.; Weiss, R.; Bill, E.; Montiel-
Montoya, R.; Trautwein, A. X. Eur. J. Biochem. 1987, 168, 419.
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R.; Bill, E.; Trautwein, A. X. Inorg. Chem. 1989, 28, 4639.
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1988, 27, 2414.
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(46) March, F. C.; Ferguson, G. Can. J. Chem. 1971, 49, 3590.
(47) Forga´cs, G.; Schultz, G.; I. Hargittai, I.; Jalsovszky, I.; Kucsman, A.;
J. Chem. Soc., Faraday Trans. 1989, 2, 85, 303.
(48) Curci, R.; Di Furia, F.; Levi, A.; Lucchini, V.; Scorrano, G.; J. C. S.
Perkin II 1975, 341.
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