Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: Structural and Moessbauer studies

Article abstract of DOI:10.1021/ic030043r

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by Moessbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are ～0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. Moessbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has δ_Fe = 0.08 mm/s, and the six-coordinate complexes exhibit δ_Fe = 0.14 to 0.19 mm/s at 4. 2 K. The five-coordinate complex shows a large quadrupole splitting (ΔE_q = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (ΔE_q = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of ν _CS. Correlations in structural, IR, and Moessbauer results suggest that the sixth ligand effect is primarily induced by changes in σ-bonding. The structure of [Fe(OEP)(CS)(CH₃OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2₁/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, β = 100.453(7)°. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, α = 70.586(1)°, β = 77.242(1)°, γ = 77.959(1)°. [Fe(OEP)(CS)(CH₃OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, α = 90.261(1)°, β = 100.362(1)°, γ = 114.664(1)°.

Full text of DOI:10.1021/ic030043r

Inorg. Chem. 2003, 42, 5202−5210
Effect of the Sixth Axial Ligand in CS-Ligated
Iron(II)octaethylporphyrinates: Structural and Mo1ssbauer Studies
Changsheng Cao,† S. Dahal,† Mayou Shang,‡ Alicia M. Beatty,‡ Wendy Hibbs,‡
,§
,‡
Charles E. Schulz,* and W. Robert Scheidt*
The Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, and Department of Physics, Knox College, Galesburg, Illinois 61401
Received January 31, 2003
The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been
investigated. Six-coordinate complexes have been synthesized and characterized by Mo¨ssbauer and infrared
spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate
parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed.
The 1-methylimidazole and pyridine derivatives exhibit Fe−C(CS) bond distances of 1.703(4) and 1.706(2) Å that
are significantly longer than the 1.662(3) Å reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D.
K. Inorg. Chem. 1982, 21, 1208). The trans Fe−N(ligand) distances of 2.112(3) and 2.1550(15) Å observed for the
1-methylimidazole and pyridine complex are ∼0.13 Å longer than those observed for analogous bis-ligated complexes
and are consistent with a significant structural trans effect for the CS ligand. Mo¨ssbauer investigations carried out
for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features.
All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS.
The five-coordinate derivative has δFe ) 0.08 mm/s, and the six-coordinate complexes exhibit δFe ) 0.14 to 0.19
mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (∆E ) 1.93 mm/s at 4.2 K) which
q
is reduced on coordination of the sixth ligand (∆E ) 0.42−0.80 mm/s at 4.2 K). Addition of a sixth ligand also
q
leads to a small decrease in the value of νCS. Correlations in structural, IR, and Mo¨ssbauer results suggest that
the sixth ligand effect is primarily induced by changes in σ-bonding. The structure of [Fe(OEP)(CS)(CH OH)] is
3
briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P21/n, Z
) 4, a ) 9.5906(5) Å, b ) 16.704(4) Å, c ) 23.1417(6) Å, â ) 100.453(7)°. [Fe(OEP)(CS)(Py)] crystallizes in
the triclinic system, space group P1h, Z ) 5, a ) 13.9073(6) Å, b ) 16.2624(7) Å, c ) 22.0709(9) Å, R )
70.586(1)°, â ) 77.242(1)°, γ ) 77.959(1)°. [Fe(OEP)(CS)(CH OH)] crystallizes in the triclinic system, space
3
group P1h, Z ) 1, a ) 9.0599(5) Å, b ) 9.4389(5) Å, c ) 11.0676(6) Å, R ) 90.261(1)°, â ) 100.362(1)°, γ )
114.664(1)°.
Introduction
than dioxygen. This tight binding of CO is the presumed
result of a strong π-bond between CO and Fe(II). In part,
Carbon monoxide has been used as ligand for exploring
properties of iron porphyrins and heme proteins for over 70
years.1-11 As is well-known, CO binds to the heme of the
respiratory proteins hemoglobin and myoglobin more strongly
(3) Buchler, J. W. In Porphyrins and Metalloporphyrins; Smith, K. M.,
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* To whom correspondence should be addressed. E-mail: Scheidt.1@
nd.edu (W.R.S.). Fax: (574) 631-4044 (W.R.S.).
† University of Notre Dame. These authors contributed equally to this
work.
‡ University of Notre Dame.
§ Knox College.
(1) Keilin, D. Proc. R. Soc., Ser. B. 1925, 98, 312.
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210.
5202 Inorganic Chemistry, Vol. 42, No. 17, 2003
10.1021/ic030043r CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/17/2003

Sixth Ligand in Iron(II)octaethylporphyrinates
this strong binding of CO has led to its use as a probe
molecule in the study of iron porphyrinates and heme
proteins.
function of coordination number, was, in our view, unex-
pectedly large. This variation is distinctly different than that
observed for another π-accepting ligand, NO.29-37
These results have prompted us to investigate how the
π-interaction in a series of thiocarbonyl-ligated iron por-
phyrinates varies as a function of an increase in coordination
number from five to six. We have measured Mo¨ssbauer
spectra for five-coordinate [Fe(OEP)(CS)] and a number of
six-coordinate [Fe(OEP)(CS)(L)] derivatives (L ) Py,
4-CNPy, 4-NMe2Py, 1-MeIm, and Pip) in zero and applied
magnetic field. A pattern similar to that observed for nitrite
is observed; a large quadruple splitting is shown by
[Fe(OEP)(CS)] (∆EQ ) 1.93 mm/s at 4.2 K) with a large
decrease in quadrupole splitting observed on coordination
of a sixth ligand. Values of the quadrupole splitting are
sensitive to the basicity of the sixth ligand. Another sig-
nificant feature of the Mo¨ssbauer spectra of the thiocarbonyl
porphyrinates is the unexpectedly low isomer shift values.
Five-coordinate [Fe(OEP)(CS)] exhibits a low isomer shift
value (δFe ) 0.08 mm/s at 4.2 K); the six-coordinate species
have higher values of isomer shift. The values of the isomer
shifts are much lower than that observed for carbonyl
derivatives.
Although the thiocarbonyl ligand (CS, also called carbon
monosulfide) is isoelectronic with carbon monoxide, CS is
considered to be both a better σ-donor and π-acceptor
compared to the carbonyl ligand. This idea is supported by
several early theoretical12-17 and spectroscopic studies.16-20
There are, however, only a relatively limited number of
transition metal complexes reported,21 so that comparisons
to transition metal carbonyl complexes are limited. Thio-
carbonyl derivatives of iron porphyrinates have been reported
by the groups of Buchler22 and Mansuy.23,24 The structure
of one derivative, five-coordinate [Fe(OEP)(CS)],25,26 has
been reported. The very short Fe-C(CS) distance of 1.662(3)
Å is consistent with the idea that CS is a strong π-accepting
ligand as is the fact that the five-coordinate species is a low-
spin derivative. In this regard, CS appears to be very similar
to the nitrite ligand. Five-coordinate [Fe(TpivPP)(NO2)]- is
also a low-spin iron(II) derivative27 with a short axial Fe-
N(NO2) distance. These properties for the nitrite derivative
were ascribed to a very strong π-accepting character of
-
N-bound NO2 in the iron(II) complex.
The crystal structures of three derivatives, [Fe(OEP)-
(CS)(1-MeIm)], [Fe(OEP)(CS)(Py)], and [Fe(OEP)(CS)-
(CH3OH)], were solved during the course of the investigation.
All show Fe-C(CS) distances that are longer than the
1.662(2) Å value of the five-coordinate species and thus
suggest a weaker interaction of CS with the iron center in
the six-coordinate complexes. The Fe-L bond distances trans
to CS are all longer than expected for iron(II) species and
show that the CS ligand exerts a strong trans effect.
The nature of the strong π-accepting character of the nitrite
ligand was found to be diminished substantially when an
additional ligand was coordinated to form six-coordinate
derivatives.28 This was seen in the Mo¨ssbauer spectra where
a large quadrupole splitting was observed for five-coordinate
[Fe(TpivPP)(NO2)]- and much smaller quadrupole splitting
values for the six-coordinate derivatives [Fe(TpivPP)(NO2)-
(Py)] and [Fe(TpivPP)(NO2)(PMS)]. This was interpreted in
terms of variable π-bonding exhibited by the nitrite ligand.
The apparent differences in π-bonding were also supported
by the structural data.28 The variation in π-bonding, as a
Experimental Section
General Information. Tetrahydrofuran (THF) was dried and
distilled over sodium/benzophenone. Chlorobenzene (C6H5Cl) was
washed with concentrated sulfuric acid, aqueous NaHCO3, and
water till neutral, dried overnight over MgSO4, and distilled over
P2O5. THF, C6H5Cl, 2,4,6-trimethylpyridine, and CSCl2 were
degassed before use by three freeze/pump/thaw cycles. Pyridine
was dried over KOH, distilled under vacuum, and used immediately.
Piperidine was dried and distilled over calcium hydride. 1-Methyl-
imidazole was dried and distilled over sodium. 4-Cyanopyridine
was recrystallized from a dichloromethane/ethyl ether mixture.
4-(Dimethylamino)pyridine was recrystallized from toluene. 57Fe2O3
(95% enriched) was purchased from Cambridge Isotope Labora-
tories and used as received. Octaethylporphyrin was purchased from
Midcentury Chemicals and was used as received. All other
chemicals were used as received from Aldrich or Fisher. Most
(11) Salzmann, R.; McMahon, M. T.; Godbout, N.; Sanders, L. K.;
Wojdelski, M.; Oldfield, E. J. Am. Chem. Soc. 1999, 121, 3818.
(12) Richards, W. G. Trans. Faraday Soc. 1967, 63, 257.
(13) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755.
(14) Hubbard, J. L.; Lichtenberger, D. L. Inorg. Chem. 1980, 19, 3865.
(15) Ziegler, T. Inorg. Chem. 1986, 25, 2721.
(16) Lichtenberger, D. L.; Fenske, R. F. Inorg. Chem. 1976, 15, 2015.
(17) Saillard, J. Y.; Grandjean, D.; Caillet, P.; le Beuze, A. J. J. Organomet.
Chem. 1980, 190, 371.
(18) Butler, I. S. Acc. Chem. Res. 1977, 10, 359.
(19) Andrews, M. A. Inorg. Chem. 1977, 16, 496.
(20) Woodward, S. S.; Angelici, R. J.; Dombek, B. D. Inorg. Chem. 1978,
17, 1634.
(21) Moltzen, E. K.; Klabunde, K. J. Chem. ReV. 1988, 88, 391.
(22) Buchler, J. W.; Kokisch, W.; Smith, P. D.; Tonn, B. Z. Naturforsch.
1978, 33b, 1371.
(23) Mansuy, D.; Battioni, J.-P.; Chottard, J.-C. J. Am. Chem. Soc. 1978,
100, 4311.
(24) Battioni, J.-P.; Chottard, J.-C.; Mansuy, D. Inorg. Chem. 1982, 21,
2056.
(29) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17.
(30) Bohle, D. S.; Hung, C.-H. J. Am. Chem. Soc. 1995, 117, 9584.
(31) Bohle, D. S.; Debrunner, P. G.; Fitzgerald, J.; Hansert, B.; Hung, C.-
H.; Thompson, A. J. J. Chem. Soc., Chem. Commun. 1997, 91.
(32) Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404.
(33) Scheidt, W. R.; Duval, H. F.; Neal, T. J.; Ellison, M. K. J. Am. Chem.
Soc. 2000, 122, 4651.
(25) Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208.
(26) Abbreviations: OEP, dianion of octaethylporphyrin; TPP, dianion of
tetraphenylporphyrin; TpivPP, dianion of R,R,R,R-tetrakis(o-pival-
amidophenyl)porphyrin; TTP, dianion of tetratolylporphyrin; Py,
pyridine; 4-CNPy, 4-cyanopyridine; 4-NMe2Py, 4-(dimethylamino)-
pyridine; 1-MeIm, 1-methylimidazole; Pip, piperidine; Np, porphyrinato
nitrogen; PMS, pentamethylene sulfide; Iz, indazole; t-BuCN, tert-
butyl isocyanide.
(34) Scheidt, W. R.; Lee, Y. J.; Hatano, K. J. Am. Chem. Soc. 1984, 106,
3191.
(27) Nasri, H.; Wang, Y.; Huynh, B. H.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 717.
(28) Nasri, H.; Ellison, M. K.; Krebs, C.; Huynh, B. H.; Scheidt, W. R. J.
Am. Chem. Soc. 2000, 122, 10795.
(35) Scheidt, W. R.; Piciulo, P. L. J. Am. Chem. Soc. 1976, 98, 1913.
(36) Scheidt, W. R.; Brinegar, A. C.; Ferro, E. B.; Kirner, J. F. J. Am.
Chem. Soc. 1977, 99, 7315.
(37) Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1999, 121, 5210.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5203

Cao et al.
manipulations were carried out under argon using a double-manifold
vacuum line, Schlenk-ware, and cannula techniques. IR spectra were
recorded on a Nicolet-Nexus 670 FT-IR spectrophotometer as KBr
pellets. UV-vis spectra were recorded on a Perkin-Elmer Lambda
6 spectrophotometer. The solid-state Mo¨ssbauer samples were
immobilized in Apiezon M grease. Spectra were obtained for
[Fe(OEP)(CS)] using iron at normal and 95% enrichment in 57Fe
while spectra for [Fe(OEP)(CS)(Py)], [Fe(OEP)(CS)(1-MeIm)],
[Fe(OEP)(CS)(4-CNPy)], [Fe(OEP)(CS)(4-NMe2Py)], and [Fe(OEP)-
(CS)(Pip)] were obtained from enriched samples that had been
prepared for another experiment. For the enriched iron derivatives,
due to the small quantity of sample required, thin, dilute grease
mulls were used to make the sample as uniform as possible.
Mo¨ssbauer measurements were performed on a constant acceleration
spectrometer at 4.2 K with zero field and up to 9 T in a magnetic
field parallel to the γ-ray beam (Knox College).
Preparation of [Fe(OEP)(CS)]. [Fe(OEP)Cl] was prepared by
a modified literature method.3,38 The five-coordinate thiocarbonyl
complex was prepared according to the method reported by
Buchler.22 Typically, 50 mg of [Fe(OEP)Cl] was dissolved in 25
mL of THF and degassed three times. The solution was transferred
via cannula to 0.2 mL of sodium amalgam (1%). Immediate
reduction occurred with a color change of the solution from brown
to red. Thiophosgene (0.1 mL, degassed three times) was added
over a period of 10 min, and the solution was stirred for 24 h under
argon at room temperature. The THF solution was decanted from
the amalgam residue. The amalgam residue was washed twice with
THF, and the combined solution was evaporated to dryness under
vacuum. The product was extracted with toluene and purified by
column chromatography (neutral alumina, 50-200 µ, activity level
4). After evaporating the solvent in a vacuum at 60 °C, the
compound was washed with hexane (yield 33-40%). λmax(nm) in
Preparation of [57Fe(OEP)(CS)(L)] (L ) Py, 1-MeIm, Pip,
4-CNPy, and 4-NMe2Py). The iron-enriched six-coordinate deriva-
tives were prepared by the same procedure as already described.
The νCS values for the derivatives are the following: [57Fe(OEP)-
(CS)(Py)], 1280 cm-1; [57Fe(OEP)(CS)(1-MeIm)], 1271 cm-1
;
[(57Fe(OEP)(CS)(4-CNPy)], 1284 cm-1
;
[57Fe(OEP)(CS)(4-
NMe2Py)], 1279 cm-1; [57Fe(OEP)(CS)(Pip)], 1279 cm-1
.
X-ray Structure Determinations. [Fe(OEP)(CS)(1-MeIm)]. A
red block-shaped crystal of [Fe(OEP)(CS)(1-MeIm)] was used for
data collection on an Enraf-Nonius FAST area detector diffrac-
tometer at 130 K using methods and procedures for small molecules
standard in this lab.40 The structure was solved in the space group
P21/n by direct methods with SHELXS-86.41 After all non-hydrogen
atoms were refined anisotropically, most of the hydrogen atoms
were located by the difference Fourier synthesis. However, in the
least-squares refinement, all hydrogen atoms were put in idealized
positions and refined with riding models (C-H ) 0.95 Å for R2CH,
0.99 Å for R2CH2, 0.98 Å for RCH3). The structure was refined
against F2 by the SHELXL-93 program.41 The structure was
completely ordered.
[Fe(OEP)(CS)(Py)] and [Fe(OEP)(CS)(CH3OH)]. Crystalline
samples of [Fe(OEP)(CS)(Py)] and [Fe(OEP)(CS)(CH3OH)] were
placed in inert oil, mounted on glass pins, and transferred to the
cold gas stream of the diffractometer. Crystal data were collected
and integrated using a Bruker Apex system, with graphite mono-
chromated Mo KR (λ ) 0.71073 Å) radiation at 100 K. The
structures were solved by direct methods using SHELXS-97 and
refined using SHELXL-97 (Sheldrick, G. M., University of Go¨t-
tingen). Non-hydrogen atoms were found by successive full-matrix
least-squares refinement on F2 and refined with anisotropic thermal
parameters. Hydrogen atom positions were located from difference
Fourier maps, and a riding model with fixed thermal parameters
[uij ) 1.2Uij(eq) for the atom to which they are bonded] was used
for subsequent refinements. The asymmetric unit of [Fe(OEP)(CS)-
(Py)] contains two and one-half iron-porphyrin units, with the
Fe(II) ion of the half unit located at a crystallographic inversion
center and the corresponding axial ligands also disordered over two
positions (50% occupancies). This porphyrin ring, containing
Fe(1)-3, is also disordered due to a 2-fold disorder of two of the
unique ethyl substituents, so that the two ethyl groups, one unique
methylene bridge and four unique carbon atoms of the porphyrin
backbone, are present in 65%/35% relative occupancies. The
porphyrin ring containing Fe(1)-1 includes two disordered sections,
arising from two disordered ethyl groups a piece and including the
corresponding â carbons, with site occupancies of 51%/49% and
65%/35%, respectively. The porphyrin ring containing Fe(1)-2 has
one 73%/27% disorder of two ethyl groups and their â carbons,
and one 50%/50% disorder of the methyl group of one ethyl
substituent. The axial substituents of both of these porphyrin rings
(attached to Fe(1)-1 and Fe(1)-2) are completely ordered. The
asymmetric unit of [Fe(OEP)(CS)(CH3OH)] contains one-half of
an iron-porphyrin unit,42 with the position of the Fe(II) ion located
at the crystallographic inversion center. Therefore, the methanol
and CS ligands are disordered and were refined with 50% site
occupancies.
CH2Cl2: 387, 516, 551 (reported, 388, 518, 551). IR (KBr), νCS
:
1292 cm-1 (reported: 1292 cm-1).22
Preparation of [Fe(OEP)(CS)(L)] (L ) Py, 1-MeIm, Pip,
4-CNPy, and 4-NMe2Py). The six-coordinate thiocarbonyl com-
plexes were prepared by recrystallization (slow evaporation) from
the CHCl3/CH3OH (3:1) solution of [Fe(OEP)(CS)] (40 mg)
containing 0.1 mL of pyridine, 1-methylimidazole, or piperidine.
The 4-cyanopyridine and 4-(dimethylamino)pyridine derivatives
were prepared by slow evaporation of the CHCl3/CH3OH solution
containing five-coordinate [Fe(OEP)(CS)] and the respective ligand
in 1:4 molar ratio. λmax(nm, log ꢀ) in CH2Cl2: [Fe(OEP)(CS)(1-
MeIm)], 408, 531, 557 (reported, 410, 530, 560); [Fe(OEP)(CS)-
(Py)], 407, 528, 559 (reported, 408, 529, 560); [Fe(OEP)(CS)(Pip)],
409, 529, 559 (reported, 410, 529, 560);22 [Fe(OEP)(CS)(4-CNPy)],
405, 5.190; 529, 4.148; 556, 4.285; [Fe(OEP)(CS)(4-NMe2Py)],
410, 5.195; 530, 4.122; 560, 4.170. νCS for [Fe(OEP)(CS)(Py)],
1280 cm-1 (reported, 1282 cm-1); νCS for [Fe(OEP)(CS)(1-MeIm)],
1272 cm-1 (reported, 1275 cm-1); νCS for [Fe(OEP)(CS)(Pip)], 1280
cm-1 (reported, 1278);22 νCS for [Fe(OEP)(CS)(4-CNPy)], 1284
cm-1; νCS for [Fe(OEP)(CS)(4-NMe2Py)], 1279 cm-1
.
Preparation of [57Fe(OEP)Cl] and [57Fe(OEP)(CS)]. The
enriched iron porphyrin [57Fe(OEP)Cl] derivative was prepared
following the reported method.39 Enriched (95%) 57Fe2O3 and
H2OEP were used in 1.8:1 molar ratio, and the completion of metal
insertion was monitored by UV-vis spectra. The reaction mix-
ture was stirred at 80°, for 12 h to ensure complete metalation.
[57Fe(OEP)(CS)] was prepared by the same procedure as already
(40) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314.
(41) Programs used in this study included SHELXS-86 (Sheldrick, G. M.;
Acta Crystallogr., Sect. A 1990, A46, 467), SHELXL-93 (Sheldrick,
G. M.; J. Appl. Crystallogr., in preparation), and local modifications
of Johnson’s ORTEP2. Scattering factors were taken from: Inter-
national Tables for Crystallography; Wilson, A. J. C., Ed.; Kluwer
Academic Publishers: Dordrecht, 1992; Vol. C.
described, from 57Fe(OEP)Cl. νCS: 1291 cm-1
.
(38) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
(39) Landergren, M.; Baltzer, L. Inorg. Chem. 1990, 29, 556.
(42) For these triclinic systems, there are two asymmetric units per unit
cell.
5204 Inorganic Chemistry, Vol. 42, No. 17, 2003

Sixth Ligand in Iron(II)octaethylporphyrinates
Table 1. Crystallographic Details for [Fe(OEP)(CS)(L)] Derivatives
[Fe(OEP)(CS)-
(1-MeIm)]
Fe(OEP)(CS)- [Fe(OEP)(CS)-
(Py)
(CH3OH)]
empirical formula C41H50FeN6S
C42H49FeN5S
711.77
C38H52FeN4OS
668.75
fw
714.78
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21/n
9.5906(5)
16.704(4)
23.1417(6)
90
100.453(7)
90
3645.9(8)
4
triclinic
P1h
triclinic
P1h
13.9073(6)
16.2624(7)
22.0709(9)
70.5860(10)
77.2420(10)
77.9590(10)
4541.8(3)
5
9.0599(5)
9.4389(5)
11.0676(6)
90.2610(10)
100.3620(10)
114.6640(10)
842.72(8)
1
R (deg)
â (deg)
γ (deg)
V, (Å3)
Z
D
calcd, g/cm3
1.302
1.301
1.318
wavelength, Å
0.71073
0.71073
0.71073
Figure 1. ORTEP diagram of [Fe(OEP)(CS)(1-MeIm)] displaying the atom
labeling scheme. Thermal ellipsoids of all atoms are contoured at the 50%
probability level. Hydrogen atoms have been omitted for clarity.
cryst size (mm3) 0.22 × 0.14 × 0.03 0.3 × 0.2 × 0.2 0.5 × 0.3 × 0.2
temp, K
130(2)
22290
7882
R1 ) 0.0735
wR2 ) 0.1742
R1 ) 0.0987
wR2 ) 0.1902
100(2)
49146
22494
R1 ) 0.0458
100(2)
9096
4178
R1 ) 0.0587
reflns collected
indep reflns
final R indices
[I > 2σ(I)]
R indices
wR2 ) 0.1106 wR2 ) 0.1395
R1 ) 0.0587
R1 ) 0.0608
(all data)
wR2 ) 0.1185 wR2 ) 0.1403
A brief description of crystallographic details for all three
derivatives is given in Table 1 while complete information is given
in the Supporting Information.
Results
The synthesis, molecular structures, IR, UV-vis, and
Mo¨ssbauer spectra of several six-coordinate thiocarbonyl
iron(II) porphyrinates are reported. The six-coordinate thio-
carbonyl complexes exhibit UV-vis spectra with red-shifted
Soret and visible absorption bands compared to those of the
five-coordinate derivative. The CS stretching frequency is
found to be lower in all the six-coordinate thiocarbonyl
complexes (1272-1284 cm-1) compared to 1292 cm-1
observed for [Fe(OEP)(CS)]. Solid-state Mo¨ssbauer meas-
urements were made on six samples, [Fe(OEP)(CS)] and
[Fe(OEP)(CS)(L)] (L ) 1-MeIm, Py, 4-CNPy, 4-NMe2Py,
and Pip). The Mo¨ssbauer data will be discussed subsequently.
The molecular structures of three six-coordinate deriva-
tives, [Fe(OEP)(CS)(1-MeIm)], [Fe(OEP)(CS)(Py)], and
[Fe(OEP)(CS)(CH3OH)], have been determined. A labeled
ORTEP diagram of [Fe(OEP)(CS)(1-MeIm)] is given in
Figure 1. Crystals of [Fe(OEP)(CS)(Py)] contain two and
one-half independent molecules per asymmetric unit;42 the
axial ligands in the two full molecule in the unit are ordered
while the half molecule in the unit has the axial ligands
disordered over two positions. The three molecules are
distinguished with atom labels followed by an underscore
one(_1), two(_2), or three(_3) (half molecule); identical
labeling schemes are used in all three. A labeled ORTEP
diagram of molecule 2 of [Fe(OEP)(CS)(Py)] is given in
Figure 2; ORTEP diagrams of the other two molecules are
given in the Supporting Information, as is the diagram for
[Fe(OEP)(CS)(CH3OH)]. A summary of selected bond
distances and angles for the thiocarbonyl derivatives is given
in Table 2.
Figure 2. ORTEP diagram of molecule 2 of [Fe(OEP)(CS)(Py)] displaying
the atom labeling scheme; this labeling scheme is used for all three
molecules. Thermal ellipsoids of all atoms are contoured at the 50%
probability level. Hydrogen atoms have been omitted for clarity.
Table 2. Selected Bond Lengths, Bond Angles, and Iron Displacements
for [Fe(OEP)(CS)(L)] Derivatives
[Fe(OEP)(CS)(L)]
L:
a
1-MeIm
Py(1)
Py(2)
Py(3)
CH3OH
Fe-N(1)b 1.981(3) 2.002(3) 2.0034(15) 2.0048(8) 2.000(2) 1.992(2)
Fe-N(2)b 1.989(3) 1.996(3) 2.0050(15) 2.0043(14) 1.998(2) 1.994(2)
Fe-N(3)b 1.979(3) 2.000(3) 2.0072(16) 2.0039(14)
Fe-N(4)b 1.980(2) 2.006(3) 2.0048(14) 2.0108(14)
b
(Fe-Np)av 1.982(5) 2.001(4) 2.005(2)
2.006(3)
1.7042(18) 1.889(8) 1.800(12)
1.999(1) 1.993(1)
Fe-C(CS)b 1.662(3) 1.703(4) 1.707(2)
Fe-Lb
2.112(3) 2.1469(18) 2.1550(15) 1.959(3) 2.089(7)
1.559(3) 1.563(4) 1.565(2) 1.5626(18) 1.603(8) 1.576(12)
0.23 0.10
0.03-0.10c 0.0-0.03c 0.0d 0.0d
C-Sb
b
∆
Fe
Fe-C-Se 176.3(2) 172.2(2) 176.24(13) 174.57(12) 175.4(5) 175.4(6)
C-Fe-Le
175.32(3) 177.66(8) 176.94(7) 177.8(3) 168.8(8)
tw tw tw tw tw
ref
25f
a Five-coordinate species. b Values in Å. c Range defined by choice of
disordered atoms. d Value required by inversion symmetry. e Value in deg.
f tw ) this work.
displays the average values for each chemically unique type
of bond length and bond angle. Similar diagrams for the
pyridine and methanol derivatives are given in the Supporting
Information. In contrast to the strongly ruffled core of
[Fe(OEP)(CS)(1-MeIm)], the porphyrin cores in the remain-
ing derivatives are more nearly planar.
A formal diagram showing the displacement of atoms (in
units of 0.01 Å) from the 24-atom mean plane of [Fe(OEP)-
(CS)(1-MeIm)] is given in Figure 3. The diagram also
The axial ligand plane in [Fe(OEP)(CS)(1-MeIm)] makes
a dihedral angle of 21.9°, with the closest Fe-Np vector.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5205

Cao et al.
atom mean plane in five-coordinate [Fe(OEP)(CS)] is
relatively small at 0.23 Å, owing to the low-spin state of
iron(II), the displacement is decreased to 0.10 Å in [Fe(OEP)-
(CS)(1-MeIm)]. Similar or smaller values of the out-of-plane
displacement are also found for the pyridine derivative; the
precision of values is limited by slight disorder in the
porphyrin cores. All displacements are on the thiocarbonyl
side of the porphyrin plane. Concomitant with the decrease
in the out-of-plane iron displacement, there is an increase
of >0.02 Å in the average Fe-Np distance of the six-
coordinate derivatives (see Table 2). The radius of the central
hole, Ct‚‚‚N, increases from 1.969 Å in five-coordinate
[Fe(OEP)(CS)] to 1.998 Å in the six-coordinate species.
The Fe-N bond distances trans to Fe-C(CS) are relatively
long. The Fe-N(1-MeIm) distance is found to be 2.112(3)
Å for [Fe(OEP)(CS)(1-MeIm)] and Fe-N(Py) distances of
2.1469(18) and 2.1550(15) Å are found for [Fe(OEP)(CS)-
(Py)]. Estimates of the magnitude of the increase in Fe-
N(L) can be determined by comparison with a series of bis-
ligated six-coordinate iron(II) porphyrinate complexes bonded
to either imidazole43 or pyridine ligands.44 The bis-imidazole
complexes43 display axial Fe-N(L) bonds ranging from
2.004(2) to 2.017(4) Å; a lengthening of ∼0.10 Å is thus
inferred. The Fe-N(L) bond in [Fe(Por)(L)2], where L is
either pyridine or a substituted pyridine,44 varies from
1.996(2) to 2.039(1) Å, with a median value of 2.019 Å.
This gives a lengthening of ∼0.12 Å for the Fe-N(Py) bond.
The bond lengthening is similar to that observed for the
iron(II) nitrosyl porphyrinates where the Fe-N(L) distances
of the ligand trans to nitrosyl are lengthened by >0.2 Å.
The thiocarbonyl ligand thus appears to have a significant
structural trans effect similar to the nitrosyl ligand in iron(II)
porphyrinates.35,36,45 The six-coordinate derivatives show a
most significant structural feature: a general lengthening of
the Fe-C(CS) bond on coordination of the sixth ligand. The
1-methylimidazole derivative exhibits an Fe-C(CS) distance
of 1.703(4) Å, a distance significantly longer than the Fe-
C(CS) distance of 1.662(3) Å observed for five-coordinate
[Fe(OEP)(CS)].25 Similar values are seen in the [Fe(OEP)-
(CS)(Py)] structure where Fe-C(CS) distances of 1.707(2)
and 1.704(2) Å are observed for the two molecules exhibiting
ordered axial ligands. These values (average ) 1.705(2) Å)
appear to be reliable as can be attested by the constant value
of the C-S bond distance of 1.56 Å. Thus, there is a ∼0.04
Figure 3. Formal diagram of the porphinato core displaying the
perpendicular displacements, in units of 0.01 Å, of the core atoms from
the 24-atom mean plane of [Fe(OEP)(CS)(1-MeIm)]. Positive values of
displacments are toward the CS ligand. Also given on the diagram are the
averaged values for the chemically unique bond distances and angles for
the core.
The corresponding dihedral angles in [Fe(OEP)(CS)(Py)] are
42.9° (molecule 1), 39.1° (molecule 2), and 35.0° (molecule
3). The FeCS angles are close to linear with values of of
172.2(2)°, in the 1-methylimidazole complex, and values of
176.2(1)°, 174.6(1)°, and 174.6(12)°, in the pyridine deriva-
tive. Complete listings of bond lengths and bond angles are
included in the Supporting Information.
Discussion
A major objective of this investigation was to examine
the effect of adding a sixth ligand on the Fe-C(CS) bonding
in thiocarbonyl iron(II) porphyrinate derivatives. Accord-
ingly, a number of [Fe(OEP)(CS)(L)] derivatives with neutral
nitrogen donors (pyridine, substituted pyridines, piperidine,
and 1-methylimidazole) have been prepared. The vibrational
spectra show that the addition of a sixth ligand has a small
effect on the C-S stretching frequency. All six-coordinate
derivatives have νCS at lower frequency than the 1292 cm-1
of five-coordinate [Fe(OEP)(CS)], with a maximum decrease
of 20 cm-1.
The molecular structures of two nitrogen-ligated deriva-
tives, [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)], have
been determined along with that of a methanol derivative,
[Fe(OEP)(CS)(CH3OH)]. This latter derivative, with the
weakly binding methanol ligand, unfortunately has sym-
metry-required disorder of the axial ligands and provides no
detailed metrical information about the axial ligand distances.
The pyridine derivative has an unusual crystallographic
feature with two and one-half molecules in the asymmetric
unit.42 We first compare the new six-coordinate structures
with the parent five-coordinate species.
(43) Several bis-ligated iron(II) complexes of general formula [Fe(Por)-
(L)2] where L is an imidazole derivative have been structurally
characterized. The values of Fe-N(L) are as follows: (a) [Fe(TPP)-
(1-MeIm)2], 2.014(5) Å, Hoard, J. L., as a personal communication
to W.R.S.; (b) [Fe(TPP)(1-VinIm)2], 2.004(2) Å, [Fe(TPP)(1-BzIm)2],
2.017(4) Å, Safo, M. K.; Scheidt, W. R.; Gupta, G. P. Inorg. Chem.
1990, 29, 626.
(44) Several bis-ligated iron(II) complexes of general formula [Fe(Por)-
(L)2] where L is either pyridine or a substituted pyridine have been
structurally characterized. The values of Fe-N(L) are as follows: (a)
[Fe(TPP)(Py)2], 2.037(1) Å, Li, N.; Coppens, P.; Landrum, J. Inorg.
Chem. 1988, 27, 482. (b) [Fe(TPP)(Py)2]‚2Py, 2.039(1) Å, Ni, L.;
Petricek, V.; Coppens, P.; Landrum, J. Acta Crystallogr., Sect. C 1985,
C41, 902. (c) [Fe(TMP)(4-MePy)2], 2.010(2) Å, [Fe(TMP)(4-CNPy)2],
1.996(2) Å, [Fe(TMP)(3-CNPy)2], 2.026(2) Å, Safo, M. K.; Nesset,
M. J. M.; Walker, F. A.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1997, 119, 9438.
As expected, the addition of a sixth ligand leads to a
decrease in the out-of-plane displacement of the iron atom.
Although the displacement of the iron atom from the 24-
(45) Wyllie, G. R. A.; Schulz, C. E.; Scheidt, W. R. Submitted for
publication.
5206 Inorganic Chemistry, Vol. 42, No. 17, 2003

Sixth Ligand in Iron(II)octaethylporphyrinates
Table 3. Solid-State Mo¨ssbauer Parameters for Thiocarbonyl and
Related Derivatives
ture of a five-coordinate carbonyl derivative is unavailable.
However, the Fe-C(CO) distance is 1.706(5) Å in [Fe(Deut)-
(CO)(THF)]54 where the trans ligand is the weakly binding
tetrahydrofuran ligand.
The differences in the Fe-C(CS) and Fe-C(CO) bond
distances immediately point toward a better bonding capabil-
ity of the CS ligand compared to the CO ligand. The
relatively short Fe-C and relatively long C-S bond distance
point to significantly stronger Fe f C π-donation in the CS
complexes relative to the CO complexes.55 The differences
in Fe f C π-donation are also clearly reflected in the
Mo¨ssbauer spectra of the thiocarbonyl and carbonyl iron
porphyrinates.
Mo¨ssbauer spectra were obtained for five-coordinate
[Fe(OEP)(CS)] and several six-coordinate complexes with
neutral nitrogen donor ligands. Measurements were carried
out in zero applied field at room temperature and 4.2 K and
in a 9.0 T applied magnetic field at 4.2 K. The Mo¨ssbauer
data for the thiocarbonyl complexes are summarized in Table
3 along with a number of related iron porphyrinate deriva-
tives for comparison. Fits to the experimental data in the
applied field show that five-coordinate [Fe(OEP)(CS)] and
all its six-coordinate derivatives have the expected diamag-
netic (low-spin) ground state.
[Fe(OEP)(CS)] is found to exhibit a very small value for
the isomer shift (δFe ) 0.08 mm/s at 4.2 K and -0.03 mm/s
at 293 K). The isomer shift is the smallest yet reported for
a formally iron(II) porphyrinate complex. The isomer shift
is comparable to values found for some iron porphyrin
complexes with iron in formal oxidation states of +3 and
+4, especially those with good π-accepting ligands.68 It is
Iron(II) Derivatives
complexa
[Fe(OEP)(CS)]
∆Eq
δFe
T (K)
VCS
ref
tw
b
b
c
1.95
1.93
0.90
0.80
0.67
0.57
0.65
0.65
-0.03
0.08
0.03
0.19
0.04
0.15
0.05
0.19
0.06
0.19
0.03
0.14
0.28
0.20
0.18
0.35
0.35
0.41
0.42
0.41
293
4.2
293
4.2
293
4.2
293
4.2
293
4.2
293
4.2
293
293
295
4.2
100
4.2
1292
1284
1280
1279
1278
1271
]Fe(OEP)(CS)(4-CNPy)]
[Fe(OEP)(CS)(Py)]
tw
tw
tw
tw
tw
[Fe(OEP)(CS)(Pip)]
[Fe(OEP)(CS)(4-NMe2Py)]
[Fe(OEP)(CS)(1-MeIm)]
0.497
0.44
0.47
0.42
0.57
0.35
0.53
1.24
1.26
2.28
1.18
0.93
[Fe(TPP)(CO)(Py)]
[Fe(TPP)(CO)(1-MeIM)]
[Fe(TPP)(CO)(Pip)]
[Fe(TPP)(NO)]
56
56
57
58
31
28
28
28
[Fe(OEP)(NO)]
[Fe(TpivPP)(NO2)]-
[Fe(TpivPP)(NO2)(PMS)]-
[Fe(TpivPP)(NO2)(Py)]-
4.2
4.2
Iron(III) Derivatives
b
b
complexa
∆Eq
δFe
T (K)
ref
[Fe(TPP)(NO2)(NO)]
1.37
1.36
1.36
1.48
1.43
0.56
0.57
1.55
1.64
1.63
1.65
1.99
1.92
1.67
1.98
2.06
2.12
0.02
0.13
0.13
0.01
0.09
0.05
0.14
0.13
0.20
0.12
0.20
-0.07
0.02
0.08
0.16
0.18
0.13
293
77
59
60
59
59
59
61
61
62
62
62
62
62
62
63
63
63
63
4.2
293
4.2
293
4.2
293
4.2
293
4.2
293
4.2
300
120
4.2
120
[Fe(TpivPP)(NO2)(NO)]
[Fe(OEP)(NO(p-C6H4F)]
[Fe(OEP)(NO)]ClO4
[Fe(OEP)(NO)]ClO4‚CHCl3
[Fe(OEP)(NO)(Iz)]ClO4
[Fe(OEP)(t-BuNC)2]+
(49) Kim, K.; Fettinger, J.; Sessler, J. L.; Cyr, M.; Hugdahl, J.; Collman,
J. P.; Ibers, J. A. J. Am. Chem. Soc. 1989, 111, 403.
(50) Kim, K.; Ibers, J. A. J. Am. Chem. Soc. 1991, 113, 6077.
(51) Slebodnick, C.; Duval, M. L.; Ibers, J. A. Inorg. Chem. 1996, 35,
3607.
(52) Slebodnick, C.; Fettinger, J. C.; Peterson, H. B.; Ibers, J. A. J. Am.
Chem. Soc. 1996, 118, 3216.
(53) Salzmann, R.; Ziegler, C. J.; Godbout, N.; McMahon, M. T.; Suslick,
K. S.; Oldfield, E. J. Am. Chem. Soc. 1998, 120, 11323.
(54) Scheidt, W. R.; Haller, K. J.; Fons, M.; Mashiko, T.; Reed, C. A.
Biochemistry 1981, 20, 3653.
(55) Richardson, J. W., Jr.; Angelici, R. J.; Jacobson, R. A. Inorg. Chem.
1987, 26, 452.
[Fe(TPP)(t-BuNC)2]+
Iron(III 1/2, IV) Derivatives
{[Fe(TPP)]2N}
1.08
1.08
1.15
2.04
1.88
0.10
0.17
0.04
-0.13
0.10
300
6
293
293
131
64
{[Fe(TTP)]2N}
{[Fe(TTP)]2N}SbCl6
{[Fe(TPP)]2C}
65
65
66
a Abbreviations given in ref 26. b Value in mm/s. c Value in cm-1
.
(56) Havlin, R. H.; Godbout, N.; Salzmann, R.; Wojdelski, M.; Arnold,
W.; Schulz, C. E.; Oldfield, E. J. Am. Chem. Soc. 1998, 120, 3144.
(57) James, B. R.; Sams, J. R.; Tsin, T. B.; Reimer, K. J. J. Chem. Soc.,
Chem. Commun. 1978, 746.
(58) Nasri, H.; Ellison, M. K.; Chen, S.; Hyunh, B. H.; Scheidt, W. R. J.
Am. Chem. Soc. 1997, 119, 6274.
(59) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 1999, 38,
100.
(60) Settin, M. F.; Fanning, J. C. Inorg. Chem. 1988, 27, 1431.
(61) Richter-Addo, G. B.; Wheeler, R. A.; Hixon, C. A.; Chen, L.; Khan,
M. A.; Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. J. Am. Chem.
Soc. 2001, 123, 6314.
(62) Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2000, 39,
5102.
(63) Walker, F. A.; Nasri, H.; Turowska-Tyrk, I.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109.
(64) Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 1747.
(65) Li, M.; Shang, M.; Ehlinger, N.; Schulz, C. E.; Scheidt, W. R. Inorg.
Chem. 2000, 39, 580.
(66) English, D. R.; Hendrickson, D. N.; Suslick, K. S. Inorg. Chem. 1983,
23, 367.
Å lengthening of Fe-C(CS) upon coordination of a trans
neutral nitrogen ligand.
The Fe-C(CS) bond length of 1.703(4) Å observed for
[Fe(OEP)(CS)(1-MeIm)] is much shorter than the Fe-C(CO)
bond length of 1.744(5) Å found for [Fe(OEP)(CO)(1-
MeIm)].11 Similarly, the Fe-C distances of 1.707(2) and
1.704(2) Å observed for the thiocarbonyl complex, [Fe(OEP)-
(CS)(Py)], are again shorter than the Fe-C distance of
1.77(2) Å observed for [Fe(TPP)(CO)(Py)].46 This 1.77 Å
value is in the range of Fe-C(CO) distances for a number
of other six-coordinate carbonyl complexes.47-53 The struc-
(46) Peng, S.-M.; Ibers, J. A. J. Am. Chem. Soc. 1976, 98, 8032.
(47) Caron, C.; Mitschler, A.; Rive`re, G.; Ricard, L.; Schappacher, M.;
Weiss, R. J. Am. Chem. Soc. 1979, 101, 7401.
(48) Ricard, L.; Weiss, R.; Momenteau, M. J. Chem. Soc., Chem. Commun.
1986, 818.
(67) Rhynard, D.; Lang, G.; Spartalian, K.; Yonetani, T. J. J. Chem. Phys.
1979, 71, 3715.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5207

Cao et al.
tempting to suggest that the extremely low value of the
isomer shift is due solely to the π-accepting character of the
axial thiocarbonyl ligand. However, decreases in the isomer
shift in a similar series of complexes can be related to both
increased σ-donating and π-accepting character of the
ligands.69 Mo¨ssbauer spectra for the six-coordinate thio-
carbonyl complexes provide evidence for the importance of
σ-bonding as well as π-bonding for contributions to the
extremely low isomer shift.
ting. The large temperature effect on the quadrupole splitting
suggests the importance of a low-lying excited state for these
complexes.
The large decrease in quadrupole splitting upon increase
in coordination number is similar to that seen for iron(II)
nitrite complexes. Five-coordinate [Fe(TpivPP)(NO2)]- ex-
hibits a quadrupole splitting of 2.28 mm/s at 4.2 K while
the six-coordinate complexes [Fe(TpivPP)(NO2)(PMS)]- and
[Fe(TpivPP)(NO2)(Py)]- show smaller quadrupole splittings
of 1.18 and 0.93 mm/s, respectively, at 4.2 K. The differences
were attributed to a decrease in the importance of Fe donation
to the π-acceptor nitrite upon addition of the sixth ligand.28
A comparable explanation for the changes in quadrupole
splitting would appear applicable to the thiocarbonyls: a
decrease in the importance of Fe donation to the π-acceptor
CS upon addition of the sixth ligand. In the nitrite system,
the rhombicity in the π-interaction between nitrite and
iron(II) was also considered to be an important factor in the
large value of the quadrupole splitting. Clearly, rhombicity
in π-bonding for (linear) thiocarbonyl complexes is not
applicable since both Fe dπ-orbitals are involved in π-
bonding to CS. Thus, while the rhombic dπ-donation in the
Fe-N(NO2) π-interaction may have a significant effect on
the value of the quadrupole splitting in five-coordinate
[Fe(TpivPP)(NO2)]-, the thiocarbonyl results suggest that the
rhombicity is neither the only nor even the major contributor
to the large quadrupole splitting in these iron(II) porphyr-
inates. As shown in the next paragraphs, the variation in
quadrupole splitting for the six-coordinate thiocarbonyl
derivatives is not consistent with large changes in the
character of the axial π-bonding and hence must primarily
be due to cis effects.70
The six-coordinate thiocarbonyl complexes show substan-
tial variation in quadrupole splitting with ∆Eq values ranging
from 0.80 to 0.42 mm/s at 4.2 K (Table 3). These variations
in quadrupole splitting might be expected to reflect parallel
differences in d-electron density on iron, differences caused
by π-bonding. However, the clear relationship between ligand
basicity and quadrupole splitting is not consistent with axial
ligand π-effects. A linear relationship between ligand pKa
and room temperature quadrupole splitting for the three
pyridine derivatives studied shows the most basic pyridine
derivative has the lowest value of quadrupole splitting, not
the highest, the opposite of that expected for π-effects. The
three derivatives encompass the entire pKa range of pyri-
dines.71 This relationship is by no means an exact one; the
ligands piperidine and 1-methylimidazole only fit the cor-
relation in a general way. Piperidine, the most basic of the
ligands studied, does not have the smallest value of quad-
rupole splitting; the 1-methylimidazole derivative does.
Figure 4 gives a plot of quadrupole splitting versus ligand
pKa; the line shown is the best fit to the three pyridine values.
The quadrupole splitting values are also correlated with
the CS stretching frequency. Values of νCS are given in Table
The axial ligands used include three pyridine derivatives,
4-cyanopyridine, pyridine, and 4-(dimethylamino)pyridine
(these span the basicity range of pyridine derivatives),
piperidine, a strong, pure σ-donor ligand, and 1-methyl-
imidazole, a strongly basic ligand that is also a modest
π-donor. Mo¨ssbauer parameters of the six-coordinate thio-
carbonyl complexes show increases in the isomer shift
relative to the parent [Fe(OEP)(CS)]. Values for the five
derivatives studied are given in Table 3. The increase in
isomer shift is similar for all five complexes with δFe ranging
from 0.14 to 0.19 mm/s at 4.2 K, values that are nonetheless
still quite small for formally iron(II) species. Since the most
basic and least basic sixth ligands give the same value of
isomer shift (0.19 mm/s), we conclude that both σ- and
π-effects must contribute to the changed isomer shift values
in the six-coordinate complexes. As discussed in a following
paragraph, a decreased σ-component of the Fe-C(CS) bond
and not a changed π-component must be regarded as the
primary cause for the decreased isomer shift in the six-
coordinate species compared to the five-coordinate complex.
Mo¨ssbauer spectra have been measured at room temper-
ature for several comparable carbonyl complexes: [Fe(TPP)-
(CO)(Py)],56 [Fe(TPP)(CO)(1-MeIm)],56 and [Fe(TPP)(CO)-
(Pip)].57 Significantly higher isomer shifts are found, δFe
)
0.28, 0.20, and 0.18 mm/s at 293 K, respectively, for an
increase of 0.15-0.20 mm/s compared to the range (0.03-
0.06 mm/s) of the thiocarbonyl derivatives. The isomer shifts
of the carbonyls themselves are also lower (by roughly 0.15-
0.20 mm/s) compared to the value of δFe ) 0.41-0.51 mm/s
generally observed for typical low-spin iron(II) porphyrin-
ates.68 The ∼0.15 mm/s increases in isomer shift between
members of the series CS,L > CO,L > bis neutral nitrogen
donors are dominated by decreasing Fe f L(axial) π-bonding
in the order CS > CO > nitrogen donor.
The five-coordinate thiocarbonyl complex exhibits a large
value of the Mo¨ssbauer quadrupole splitting constant, ∆Eq
) 1.93 mm/s at 4.2 K and 1.95 mm/s at 293 K, a relatively
large value for a low-spin iron(II) species. A large decrease
in quadrupole splitting is observed on coordination of a sixth
ligand to [Fe(OEP)(CS)]; the quadrupole splitting values for
the six-coordinate complexes range from 0.80 to 0.42 mm/s
(Table 3). Moreover, all derivatives, except the piperidine
derivative and the parent five-coordinate complex, show a
substantial temperature dependence for the quadrupole split-
(68) Debrunner, P. G. In Iron Porphyrins; Lever, A. B. P., Gray, H. B.,
Eds.; VCH Publishers Inc.: New York, 1983; Part 3, Chapter 2.
(69) Shenoy, G. K. Mo¨ssbauer Effect Isomer Shifts. In Mo¨ssbauer
Spectroscopy Applied to Inorganic Chemistry; Long, G. J., Ed.; Plenum
Press: New York, 1984; pp 57-76.
(70) Buchler, J. W.; Kokisch, W.; Smith, P. D. Struct. Bonding (Berlin)
1978, 34, 79.
(71) Schoefield, K. S. Hetero-Aromatic Nitrogen Compounds; Plenum
Press: New York, 1967; p 146.
5208 Inorganic Chemistry, Vol. 42, No. 17, 2003

Sixth Ligand in Iron(II)octaethylporphyrinates
the measured 57Fe quadrupole splitting constants by the
expression
2(∆Eq)
Vzz )
(2)
η2
eQ 1+
x
3
where e is the charge on the electron, η is the asymmetry
parameter (Vxx - Vyy/Vzz), and Q is the quadrupole moment
of the 57Fe nucleus in its excited state. Measurements in
applied magnetic field show that the asymmetry parameter
η is effectively zero for all CS complexes studied and that
the sign of Vzz is always positive. Expression 1 shows that
2
increased populations of the dz , dxy, and dxz orbitals and a
2
2
Figure 4. Plot displaying the relationship between ligand pKa and
Mo¨ssbauer quadrupole splitting constant for the six-coordinate [Fe(OEP)-
(CS)(L)]. The line is the best fit to the three pyridine derivatives.
decreased population of the dx -y orbital lead to decreased
values of Vzz and hence decreased quadrupole splitting
(expression 2). Since an increase in the population of dxy
and dxz from decreased axial π-bonding has already been
shown to be unlikely, as demonstrated by the νCS values,
any increase in these populations must come from porphyrin
2
2
f Fe π-bonding. A decrease in the dx -y population must
come from decreased σ-bonding between iron and porphyrin,
probably in response to the increased σ-bonding from the
sixth ligand. The porphyrin-iron σ- and π-effects are counter
to each other and must be be approximately equivalent. The
2
involvement of dz is consistent with the σ-donation and the
general pKa dependence of the sixth ligand, while the changes
in iron-porphyrin bonding are cis effects induced by addition
of the sixth ligand.
These observations allow two important conclusions. First,
the small changes in axial Fe f C π-bonding on addition of
a sixth ligand means that the ∼0.04 Å increase in Fe-C
bond length in the six-coordinate species can only be the
result of decreased σ-donation by the CS ligand. Clearly,
the strong structural trans effect of CS must result from
σ-donation competition, and CS remains a strong σ-donor.
Second, the large d-electron distribution changes consistent
with the large change in the quadrupole splitting must arise,
in large part, from cis effects induced by binding the ligand
trans to CS. The cis effects are both an increase in the
porphyrin f Fe π-donation and a decrease in the σ-bonding.
Figure 5. Plot displaying the relationship between νCS and Mo¨ssbauer
quadrupole splitting constant for the six-coordinate [Fe(OEP)(CS)(L)]. The
line is the best fit to all derivatives.
3; stretching frequencies decrease with increasing pKa of the
added ligand.72 Figure 5 displays the correlation: a decreased
value of the quadrupole splitting is related to a decreased
value of νCS. Any decrease in νCS is expected to arise from
an increased population of the CS π*-orbitals, which in turn
derives from increased Fe f C π-donation. But the small
changes in values of νCS suggest only a very modest decrease
in the C-S bond strength for the set of compounds studied,
even when the five-coordinate complex is included. Thus,
we can conclude that the change in Fe f C π-donation upon
addition of a sixth ligand must also be very small.
Conclusion
The structural and Mo¨ssbauer studies of these iron(II)
octaethylporphyrinate thiocarbonyl complexes show the
effects of adding the sixth ligand are primarily the result of
changes induced by the σ-bonding of the axial ligand. The
large quadrupole splitting and exceptionally small isomer
shifts observed for the [Fe(OEP)(CS)] derivatives indicate
that CS must be an exceptionally good π-bonding ligand and
that the character of the axial Fe f C(CS) π-bonding is little
affected by the sixth ligand.
The variation in quadrupole splitting can be analyzed in
terms of the Vzz component of the electric field gradient. Vzz
is approximately given by the expression
1
2
Vzz ) k nx2-y2 - nz2 + nxy - (nxz + nyz)
(1)
[
]
where k is the scaling factor and the ni values are the effective
Acknowledgment. We thank the National Institutes of
Health for the support of this research under Grant GM-
38401 to W.R.S. Funds for the purchase of the FAST area
populations of the 3d orbitals.74 Values of Vzz are related to
(72) The variation of νCS with the basicity of the trans ligand has been
noted in earlier studies.23,24 A similar pattern has been noted for
[Fe(por)(CO)(L)] systems.73
(74) Bancroft, J. M.; Platt, R. H. AdV. Inorg. Chem. Radiochem. 1972, 15,
(73) Alben, J. O.; Caughey, W. S. Biochemistry 1968, 7, 175.
59.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5209

Cao et al.
angles, anisotropic temperature factors, and fixed hydrogen atom
positions for [Fe(OEP)(CS)(1-MeIm)], [Fe(OEP)(CS)(Py)], and
[Fe(OEP)(CS)(CH3OH)] (PDF). X-ray crystallographic information
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
detector diffractometer was provided through NIH Grant RR-
06709 to the University of Notre Dame.
Supporting Information Available: Figures S1-S7 giving
ORTEP diagrams and mean plane drawings for [Fe(OEP)(CS)(Py)]
and [Fe(OEP)(CS)(CH3OH)] and Tables S1-S18, giving complete
crystallographic details, atomic coordinates, bond distances and
IC030043R
5210 Inorganic Chemistry, Vol. 42, No. 17, 2003