Synthesis and moessbauer characterization of octahedral iron(II) carbonyl complexes Fel2(CO)3L and Fel2(CO) 2L2: Developing models of the [Fe]-H2ase active site

Article abstract of DOI:10.1021/ic9017882

A series of mono- and disubstituted complexes, Fel₂(CO) _xL_4-x, x = 2 or 3, is conveniently accessed from simple mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with Fel ₂(CO)₄, firs

Full text of DOI:10.1021/ic9017882

Inorg. Chem. 2009, 48, 11283–11289 11283
DOI: 10.1021/ic9017882
€
Synthesis and Mossbauer Characterization of Octahedral Iron(II) Carbonyl
Complexes FeI2(CO)3L and FeI2(CO)2L2: Developing Models of the [Fe]-H2ase
Active Site
Bin Li,†,‡ Tianbiao Liu,† Codrina V. Popescu,*,§ Andrey Bilko,§ and Marcetta Y. Darensbourg*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77843, ‡School of Chemical
§
Engineering and Technology, Tianjin University, Tianjin 300072, P.R. China, and Department of Chemistry,
Ursinus College, Collegeville, Pennsylvania 19426
Received September 7, 2009
A series of mono- and disubstituted complexes, FeI2(CO)xL4-x, x = 2 or 3, is conveniently accessed from simple
mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with FeI2(CO)4, first reported by Hieber in 1928.
The highly light sensitive complexes yield to crystallization and X-ray diffraction studies for six complexes showing
them to be rudimentary structural models of the monoiron hydrogenase, [Fe]-H2ase or Hmd, active site in native
II
II
€
(Fe (CO)2) or CO-inhibited (Fe (CO)3) states. Diatomic ligand (ν(CO)) vibrational and Mossbauer spectroscopies
are related to those reported for the Hmd active site. The importance of a serial approach for relating such parameters
in model compounds to low spin FeII in the diverse ligation of enzyme active sites is stressed.
Introduction
are of interest as it performsa stereoselective, hydride transfer
reaction to a unique substrate.9 Furthermore, as a monoiron
complex, it should offer insight into the role of iron in the
binuclear [FeFe]- and [NiFe]-H2ases.11 Synthetic challenges
exist in the variety of ligand donors at the single active iron of
the Hmd site as described above, and in the strategic
positioning of substrate that exists within the confinement
of the protein matrix.
While X-ray crystallography gives a snapshot of the [Fe]-
H2ase enzyme active site,10 the specific reason(s) for such a
diverse ligation environment as described above, as it affects
function through management of the electron density at iron,
is more apt to be found in spectroscopic analyses, specifically
For decades now, Hieber0s FeIIX2(CO)4 (X = Cl, Br, and
I) complexes have been utilized as synthons for development
of FeII carbonyl complexes.1 Recent applications of FeIII2-
(CO)4 are in the preparation of biomimetics of [NiFe]- and
the monoiron hydrogenase or [Fe]-H2ase.2-4 The latter, also
called Hmd,5-10 contains a low-spin FeII active site sur-
rounded by two cis-oriented CO ligands, a cysteinyl-S, an
organic pyridone bidentate ligand (N and acyl carbon), and a
H2O (or an open site) trans to the acyl group as sketched in
Figure 1A. Synthetic analogues of the [Fe]-H2ase active site
*To whom correspondence should be addressed. E-mail: cpopescu@
ursinus.edu (C.V.P.); marcetta@mail.chem.tamu.edu (M.Y.D.).
(1) Hieber, W.; Bader, G. Ber. Deutsch. Chem. Ges. 1928, 61, 1717–1722.
(2) Verhagen, J. A. W.; Lutz, M.; Spek, A. L.; Bouwman, E. Eur. J. Inorg.
Chem. 2003, 3968–3974.
(3) Li, Z. L.; Ohki, Y.; Tatsumi, K. J. Am. Chem. Soc. 2005, 127, 8950–
8951.
€
ν(CO) IR and Mossbauer spectroscopies. For the [NiFe]-
and [FeFe]-H2ase active sites in various redox levels, ν(CO)
IR spectroscopy has been helpful to this understanding
through the correlation of signals from the enzymes with
those of smallmoleculesynthetic analogues or biomimetics.11
(4) Obrist, B. V.; Chen, D. F.; Ahrens, A.; Schunemann, V.; Scopelliti, R.;
Hu, X. L. Inorg. Chem. 2009, 48, 3514–3516.
€
As Mossbauer spectroscopy is variable in its response to
oxidation state, to spin state, and to changes in electron
density at iron, it is particularly necessary that synthetic
analogues be developed in which changes are systematic
and interpretable for their fundamental properties. At this
time such correlations between methodical structural
changes and parameters for well-characterized low-spin iron
complexes are limited, at best.
(5) Lyon, E. J.; Shima, S.; Boecher, R.; Thauer, R. K.; Grevels, F. W.; Bill,
E.; Roseboom, W.; Albracht, S. P. J. J. Am. Chem. Soc. 2004, 126, 14239–
14248.
(6) Shima, S.; Lyon, E. J.; Thauer, R. K.; Mienert, B.; Bill, E. J. Am.
Chem. Soc. 2005, 127, 10430–10435.
(7) Shima, S.; Thauer, R. K. Chem. Rec. 2007, 7, 37–46.
(8) Shima, S.; Pilak, O.; Vogt, S.; Schick, M.; Stagni, M. S.;
Meyer-Klaucke, W.; Warkentin, E.; Thauer, R. K.; Ermler, U. Science
2008, 321, 572–575.
(9) Vogt, S.; Lyon, E. J.; Shima, S.; Thauer, R. K. J. Biol. Inorg. Chem.
2008, 13, 97–106.
The FeIIX2(CO)4 complexes have the following advan-
tagesas precursors for model complexes of [Fe]-H2ase: (a) the
(10) Hiromoto, T.; Ataka, K.; Pilak, O.; Vogt, S.; Stagni, M. S.;
Meyer-Klaucke, W.; Warkentin, E.; Thauer, R. K.; Shima, S.; Ermler, U.
FEBS Lett. 2009, 583, 585–590.
(11) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245–2274.
r
2009 American Chemical Society
Published on Web 10/27/2009
pubs.acs.org/IC

11284 Inorganic Chemistry, Vol. 48, No. 23, 2009
Li et al.
desired FeII oxidation state; (b) intrinsic carbonyls (as FeII
complexes are not always amenable to extrinsic CO
binding);12 and (c) facile CO replacement and halide abstrac-
tion reactivity that permits structural modifications.
Herein, we report the syntheses and structures of a series of
FeI2(CO)3L complexes (L = N-heterocyclic carbenes (NHC),
phosphines, pyridine) and FeI2(CO)2(NkN) (NkN =
R-diamines). These complexes are used for spectroscopic
reference points as described above and they can, as well, be
regarded as useful precursors for advanced models.
Results and Discussion
Synthesis of FeI2(CO)3L (L = IMes (1), SIMes (2), IMe
(3), PMe3 (6), PPh3 (7), PCy3 (8), P(OEt)3 (9), and Py
(10)) and FeI2(CO)2(NkN) ((NkN) = bipyridine (4) and
phenanthroline (5)). Scheme 1 outlines the scope of our
synthetic endeavor. The monosubstituted FeI2(CO)3P
(P = PPh3 and PMe3) complexes have been reported, how-
ever, possibly due to their light and heat sensitivity,
without structural characterization.13-15
N-Heterocyclic carbenes are regarded as alternatives to
phosphines in organometallic chemistry with certain
advantages deriving from their distinctive electronic,
chemical, and steric properties.16 Thus, FeI2(CO)3NHC
complexes with NHC = IMes (1,3-bis(2,4,6-trimethyl-
phenyl)imidazol-2-ylidene), complex 1; the saturated ana-
logue of IMes or SIMes (1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylide), complex 2; and IMe = 1,3-bismethyli-
midazol-2-ylidene, complex 3, were prepared and are re-
ported in this study. They are easily synthesized as follows:
to a solution of FeI2(CO)4 in hexane, the freshly prepared
NHC ligand in hexane is slowly added. After stirring for 1 h
at 22 °C, products precipitate out of solution (Scheme 1),
reflecting a solubility property of the FeI2(CO)3L com-
plexes consistent with higher polarity as compared to
FeI2(CO)4.
The reactions of FeI2(CO)4 with 2,20-bipyridine af-
forded the disubstituted complex, FeI2(CO)2(bipy), com-
plex 4, under the same conditions as above. Due to the
poor solubility of 1, 10-phenanthroline, phen, in hexane,
FeI2(CO)2(phen), 5, was prepared in CH2Cl2 using equal
amounts of phen and FeI2(CO)4. The simple mixing
procedure was used to prepare monosubstituted phos-
phine and pyridine derivatives of FeI2(CO)4 (FeI2-
(CO)3L, L = PMe3 (6),14,15 PPh3 (7),13,15 PCy3 (8),
P(OEt)3 (9), Py (10)).14 It should be mentioned that in
the case of FeI2(CO)3PMe3 (6) fast addition of PMe3 leads
to the formation of FeI2(CO)2(PMe3)2. All of these com-
plexes are light and heat sensitive; therefore, they are
stored in the dark at -40 °C in a glovebox.
Figure 1. (A) Active site of Hmd or [Fe]-H2ase with its CO stretching
frequencies.5,6 (B) Infrared spectra of mono-FeII complexes 1, 6, 8, 10,
and 12.
each complex. Figure 1B displays the ν(CO) infrared
spectra of selected complexes 1, 6, 8, 10, and 12 (see the
Supporting Information for the ν(CO) infrared spectra of
complexes 2-4, 7, 9, and 11). All ν(CO) IR data for 1-12
are listed in Table 1. According to the ν(CO) IR patterns,
monosubstituted complexes can be classified into two
subtypes of geometries: fac-FeI2(CO)3L (L = PMe3 (6);
py (10)) and mer-FeI2(CO)3L (L = IMes (1); SIMes (2);
IMe (3); PPh3 (7); PCy3 (8); P(OEt)3 (9)). The patterns of
the two types are very distinctive; the first band of the fac-
FeI2(CO)3L complexes is strong while for mer-FeI2-
(CO)3L it is weak. The assignments are well-supported
by crystal structures (vide infra) of complexes 1-3 and 10.
The pattern of the fac-FeI2(CO)3L complexes matches
well to that of the CO-inhibited active site of [Fe]-H2ase or
CO-Hmd.5 Therefore, as structural models these com-
plexes provide evidence for the geometry of the CO-
inhibited state of the enzyme. The CO frequencies of the
fac-FeI2(CO)3L complexes (e.g., (10), 2095, 2049, 2023
cm-1) are ca. 20 cm-1 higher than those of CO-Hmd
(2074, 2020, 1981 cm-1).
Infrared Spectroscopy. In comparison to the FeI2(CO)4
starting material, the CO-substituted complexes exhibit
v(CO) frequencies at lower wavenumbers with positions
dependent on the donor abilities of the various substitu-
ent ligands and with patterns reflecting the symmetry of
(12) Hardman, N. J.; Fang, X.; Scott, B. L.; Wright, R. J.; Martin, R. L.;
Kubas, G. J. Inorg. Chem. 2005, 44, 8306–8316.
(13) Cohen, I. A.; Basolo, F. J. Inorg. Nucl. Chem. 1966, 28, 511–520.
(14) Pankowski, M.; Bigorgne, M. J. Organomet. Chem. 1977, 125, 231–
252.
(15) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Venturi, C.; Zuccaccia,
C. J. Organomet. Chem. 2006, 691, 3881–3888.
(16) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
Disubstituted complexes 4 and 5 exhibit interesting
solution behavior as detected by infrared spectroscopy.
When freshly dissolved in CH2Cl2, both complexes show
two strong CO bands at 2043 and 2002 cm-1 for 4 and
2044 and 2004 cm-1 for 5 (see Figure 2A and Figure S2A
in the Supporting Information). After several minutes, a

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11285
Scheme 1
Table 1. Infrared Spectroscopic (v(CO)) Data For FeI2(CO)3L, FeI2(CO)2N2,
and FeI(CO)2LPyS Derivatives (Reported in CH2Cl2 Solution unless Otherwise
Noted)
compound
geometry
v(CO) (cm-1
)
(1) FeI2(CO)3IMes
(2) FeI2(CO)3SIMes
(3) FeI2(CO)3IMe
(4) FeI2(CO)2bipy
(5) FeI2(CO)2phen
(6) FeI2(CO)3PMe3
(7) FeI2(CO)3PPh3
(8) FeI2(CO)3PCy3
(9) FeI2(CO)3P(OEt)3
(10) FeI2(CO)3Py
mer
mer
mer
cis
cis
fac
mer
mer
mer
fac
2087 (w), 2040 (s), 2018 (m)
2082 (w), 2039 (s), 2019 (m)
2082 (w), 2034 (s), 2023 (sh)
2043 (s), 2002 (s)
2044 (s), 2003 (s)
2093 (s), 2047 (s), 2022 (s)
2093 (w), 2045 (s), 2032 (m)
2081 (w), 2031 (vs), 2017 (s, sh)
2104 (w), 2053 (s, br)
2095 (s), 2050 (ms), 2041 (ms)
2019 (s), 1971 (s)
2031 (s), 1983 (s)
Figure 2. Infrared spectral monitoring of CH2Cl2 solution of complex 5
at 22 °C: (A) immediately after dissolving complex 5; (B) 2 h later; (C)
after overnight (RT and in the dark).
(11) FeI(CO)2PCy3(PyS)a cis
(12) FeI(CO)2PPh3(PyS)a cis
Hmd
CO-Hmd
cis
fac
2011 (s), 1944 (s)a
2074 (s), 2020 (s), 1981 (s)a
diffraction study was of the thermodynamically favored
product, the trans-iodo form.
a Spectra recorded in THF.
The solution behavior described above for complexes 4
and 5 is also observed in THF solvent, and the results are
identical under Ar and N2 atmospheres, thus excluding
the possibility of dinitrogen adducts. Interestingly, in a
mixture of complex 4 isomers, analogous to spectrum B in
Figure 2, only the (assumed) cis-iodo isomer undergoes
13CO exchange (presumably via a CO dissociation/read-
dition process), Figure S3. This selectivity would appear
to offer an example of the cis-labilization effects of iodide
in that the operative pentacoordinate, square pyramidal
intermediate {cis-(I)-Fe(phen)(CO)(I)2} with an open site
trans to the poorer π-donating amine donor, would arise
preferentially from the cis-(CO),cis-(I)-Fe(phen)(CO)2-
(I)2 complex.
The above analysis relies on the two-isomeric-forms
hypothesis. The possibility of iodide dissociation yielding
a pentacoordinate intermediate susceptible to CO uptake
and exchange cannot at this stage be completely ruled out.
Thus, firmer evidence is needed as to the identity of the
species X of ν(CO) = 2077 and 2033 cm-1 prior to further
new set of two CO bands at 2077 and 2033 cm-1, species
X, were observed to grow in the ν(CO) IR spectra for both
4 and 5 ((see Figure 2B and Figure S2B), shifting posi-
tively by 30 cm-1 relative to the original bands. While the
identity of species X is not known with certainty, isomeric
forms of Ru(bpy)(CO)2(Cl)2 have been structurally char-
acterized as cis-(CO),trans-(Cl)-Ru(bpy)(CO)2(Cl)2 and
cis-(CO),cis-(Cl)-Ru(bpy)(CO)2(Cl)2.17 Therefore, a rea-
sonable assumption is that the new CO bands reported in
Figure 2 are due to iodo-iron analogues. Density func-
tional theory (DFT) computations find the cis-(CO),
trans-(I)-Fe(phen)(CO)2(I)2 complex to be thermodyna-
mically favored by 5.6 kcal/mol over the cis-(CO),cis-(I)-
Fe(phen)(CO)2(I)2 isomer. The calculations also show
that the two IR bands for the carbonyls of the cis-(CO),
cis-(I)-Fe(phen)(CO)2(I)2 are ca. 10-12 cm-1 lower than
those of the trans-iodo isomer. While the computational
ν(CO) band position values, and their shift from one
isomer to another, are smaller than experimental values,
they are in sufficient agreement to lend confidence to the
following assumption. We assume that the cis-iodo form
was the kinetic product which was isolated, and when first
dissolved, spectrum A, Figure 2, proceeded to isomerize
into the more stable trans-iodo isomer that shows a 2-
band ν(CO) IR spectrum at higher wavenumbers, spectra
B and C, Figure 2. Note that by this analysis, we would
also conclude that the crystal that was chosen for X-ray
speculation.
€
18
Mossbauer Spectroscopy. Complexes 1, 6, and 7 ex-
hibit sharp quadrupole doublets at 6 K (Figure 3), with
isomer shifts ranging from 0.007 to 0.090 mm/s, and small
quadrupole splittings, consistent with hexacoordinate
low-spin FeII carbonyls.19 The isomer shifts of low-spin
(18) Bancroft, G. M.; Libbey, E. T. Dalton Trans. 1973, 2103–2111.
€
(19) Parish, R. V. Mossbauer Spectroscopy. The Organic Chemistry of
Iron, Volume 1. Organometallic Chemistry Series; Koerner von Gustorf, E. A.,
Grevels, F. W., Fischler, I., Eds.; Academic Press: London, England; 1978; pp
175-211.
(17) Haukka, M.; Kiviaho, J.; Ahlgren, M.; Pakkanen, T. A. Organome-
tallics 1995, 14, 825–833.

11286 Inorganic Chemistry, Vol. 48, No. 23, 2009
Li et al.
Scheme 2
synthetic analogues (in contrast to the vast literature that
has grown up as biomimetics of the [FeFe]-H2ase active
site).4,11,20-23 Complexes 7-9 have been utilized as pre-
cursors to more faithful models of the active site of [Fe]-
H2ase, specifically derivatives with N and S donor sites,
Scheme 2. On treatment of complexes 7 and 8 with
PyS-Naþ, complexes 11 and 12, respectively, are ob-
tained. Infrared spectra of these two complexes feature
two ν(CO) bands of equal intensity at 2019 and 1991 cm-1
for 11 and at 2031 and 1983 cm-1 for 12, indicating that
cis-carbonyls are in these molecules. The X-ray diffrac-
tion study of 12 (vide infra) confirmed this conclusion.
Thus, complexes 11 and 12 contain four of the first
coordination sphere donors of the active site, including
a nitrogen from pyridine. The I- ligand may be consid-
ered as the analogue of H2O (or actual open site) in the
enzymatic site since I- abstraction can generate a poten-
tial open (or active) site.
X-ray Diffraction Studies and Molecular Structures of
Complexes 1, 2, 3, 4, 10, and 12. Complexes 1-4, 10, and
12 were studied by X-ray diffraction; full structural files in
CIF format are available as described in the Supporting
Information. Molecular structures for all are given as
thermal ellipsoid plots in Figure 4; selected metric para-
meters for complexes 1-3 are listed in Table 2. All are
6-coordinate and octahedral. Only in the case of the
diiododicarbonyl complex 4 are the iodides in the trans
position; in all other complexes, a cis-I2Fe arrangement
prevails. The Fe-I distances are in a narrow range of
€
Figure 3. 6 K Mossbauer spectra for complexes 1, 6, and 7 in an applied
field of 0.03 T.
iron complexes are less sensitive to oxidation state
and coordination environment than those of high-spin
complexes, thus structure-spectroscopy correlations
based on isolated compounds are of little value. However,
as noted by Parish,19 examination of Mossbauer and
€
other spectroscopic data of a set of related compounds
leads to valuable correlations between structure and
spectroscopy. While, in general, introduction of CO
ligands has the effect of lowering the isomer shift, there
is an additional dependency on the nature of supplemen-
tary ligands that complete the coordination sphere in a
particular set of compounds. This series illustrates the
effect on the isomer shift of substitution of a phosphine
with the N-heterocyclic carbene IMes. The isomer shift of
complex 1, δ = 0.007 mm/s, is smaller than that of
complexes 6 and 7, consistent with the better σ-donating
ability of IMes compared to phosphines (PMe3 and
PPh3). The trend in isomer shifts parallels the observed
increase in CO stretching frequencies in the series. A
change in oxidation state has a more marked effect on
the isomer shift. Thus, the Fe0 complex (Fe(CO)4-
NHC, where NHC=IMesMe (1-(2,4,6-trimethylphenyl),
3-methylimidazol-2-ylidene) exhibits, at 5 K, an isomer
shift of -0.092 mm/s and ΔEQ = 2.01 mm/s (spectra not
shown).
˚
2.62-2.69 A with no obvious trends.
The NHC carbene substituted complexes 1-3 are of
meridianal geometry with the three CO ligands located in
a plane and each at ca. 90° to the carbene carbon donor of
the NHC. The structures of complexes 1 and 2 are
distinguishable only by the C-C distances in the NHC
five-membered rings. For both, the CN2C2 heterocycle
ligand plane is coincident with the CNHCFe(CO)2I co-
ordination plane. The mesityl groups flanking the car-
bene donor site exert a steric influence on the adjacent
COs resulting in a C1-Fe-C10 angle of 159.9 (3)° for
complex 1; the analogous C1-Fe-C2 angle in complex 2
is 160.55 (11)°. For complex 3, the plane of the carbene
bisects the cis ligands in the Fe(CO)3I plane with dihedral
angles of 33.5 and 55.6°. Again, the steric influence of the
NHC methyl substituents on N enforces a bend in the
C1-Fe-C2 angle of 166.6(2)°.
Reactivities Leading to Advanced Hmd Models. As the
active site of [Fe]-H2ase has been known with reasonable
certainty for only a short time, there are few reports of
Complex 10 has a facial-CO arrangement; two cis-CO
and two I- groups lie at the equatorial plane, and an
additional CO and the pyridine are positioned in trans-
axial sites. The angle between the two cis-COs is 92.1(2)°.
(20) Guo, Y. S.; Wang, H. X.; Xiao, Y. M.; Vogt, S.; Thauer, R. K.;
Shima, S.; Volkers, P. I.; Rauchfuss, T. B.; Pelmenschikov, V.; Case, D. A.;
Alp, E. E.; Sturhahn, W.; Yoda, Y.; Cramer, S. P. Inorg. Chem. 2008, 47,
3969–3977.
(21) Royer, A. M.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2008, 47,
395–397.
(22) Wang, X. F.; Li, Z. M.; Zeng, X. R.; Luo, Q. Y.; Evans, D. J.; Pickett,
C. J.; Liu, X. M. Chem. Commun. 2008, 3555–3557.
(23) Royer, A. M.; Rauchfuss, T. B.; Gray, D. L. Organometallics 2009,
3618–3620.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11287
Figure 4. Thermal ellipsoid representations of the molecular structures of monoiron complexes as indicated; hydrogen atoms omitted. Note: An absolute
structure (Flack) parameter of 0.50 for complex 12 indicates that the compound exists in the crystal as a racemic mixture; only one enantiomer is shown.
Table 2. Selected Metric Data for Complexes 1, 2, and 3
at an angle of 93.57(12)°. The bite angle of the NS ligand is
70.36 (7)°, reflecting severe strain in the NSFeC 4-mem-
complex
1
2
3
bered ring. The Fe-S and Fe-N distances are 2.362(1)
and 1.962(2) A, respectively. Analogous distances in the
˚
˚
˚
˚
Fe-I(1)
Fe-I(2)
Fe-CO trans to I(2) 1.792(7)
Fe-C(1)O(1)
Fe-CNHC
cis-CO angle (deg)
2.674(1) A
2.636(1)
2.695(1) A
2.641(1)
1.794(3)
1.859(3)
2.003(3)
2.665(1) A
2.667(1)
1.802(4)
1.824(4)
1.984(4)
early report of X-ray data for the Hmd active site were 2.4
˚
and 2.1 A for the Fe-S and Fe-N distances, respectively.
EXAFS refinement data for the Hmd active site are closer
to those from the complex 12 model, 2.335(4) and
1.848(5)
2.002(6)
C(1)-Fe-C(2) C(1)-Fe-C(3) C(1)-Fe-C(3)
10
˚
2.052(9) A, respectively.
96.1(1)
trans-CO angle (deg) 159.9(3)
I(2)-Fe-CO (trans) 178.4(2)
94.6(1)
160.6(1)
178.2(1)
90.6(2)
166.6(2)
175.6(1)
Conclusion
The structural and spectroscopic information gleaned
from the series of mono- and disubstituted complexes,
FeI2(CO)xL4-x suggest them to be rudimentary structural
and spectroscopic models of the [Fe]-H2ase or Hmd
active site. The dicarbonyl complex 11 best matches the
ν(CO) IR values of Hmd while the tricarbonyl complex 6
mimics the CO-inhibited state, CO-Hmd; see Table 1.
The structures of complexes 1-3 and 10 together with
their ν(CO) IR vibration patterns provide structural
references for phosphine derivatives 6-9 for which,
despite many attempts, the crystal structures were not
obtained. Depending on the size of the phosphines,
complexes 6-9 adopt mer or fac geometries.
Complex 4 was found in C2v symmetry with two trans-
I- ligands at axial positions; the phenanthroline bidentate
ligand and the two cis-CO ligands occupy the equatorial
plane. The angle between the cis-CO0s is 90.09 (17)°,
consistent with the ν(CO) pattern in the solution spectra.
Complex 12 models the equatorial ligand set of the
Hmd active site with one S, one N, and two cis-CO ligands
€
In terms of the Mossbauer parameters, the tricarbonyl
complex 6 (Figure 3) provides the best match of Hmd
(in its dicarbonyl form, δ = 0.06 mm/s; ΔEQ = 0.65
mm/s).6 These data beg the question of the usefulness
€
of Mossbauer parameters in biomimetics of superficially
similar coordination spheres as the enzyme active site that
they portend to mimic.

11288 Inorganic Chemistry, Vol. 48, No. 23, 2009
Li et al.
Metalloenzyme active sites have complex coordination
spheres, improved by long-term evolution, which provide
fine-tuning of the electron density at the metal through
second coordination sphere effects and intricate hydrogen
bonding that controls solvent effects and substrate binding.
While synthetic chemists have successfully implemented
gross features of nature’s design of ligands and, in some
cases, unusual stereochemistry,24,25 control of finer effects
from the outer coordination sphere by use of inner sphere
abiological ligand constructs is still lacking.26 However, this
challenge is important for bioinspired catalysis as subtle
effects influence electronic structure and spectroscopy in
ways that, most probably, have not been fully appreciated.27
It is well-known that for high spin iron complexes
literature values. 31P NMR shifts are referenced to 100% H3PO4
(0 ppm). Solution IR spectra were recorded on a Bruker Tensor
27 FTIR spectrometer using 0.1 mm NaCl sealed cells.
€
Mossbauer spectra were collected with a spectrometer model
CCR4K, in constant acceleration mode, cooled to cryogenic
temperatures by a closed-cycle refrigerator and fitted with a
permanent 300 Gauss magnet. The spectra were analyzed with
the program WMOSS (Thomas Kent, SeeCo.us, Edina, Min-
nesota).
X-ray Structure Determinations. For all reported structures, a
Bausch and Lomb 10ꢀ microscope was used to identify suitable
crystals of the same habit. Each crystal was coated in paratone,
affixed to a Nylon loop, and placed under streaming nitrogen
(110 K) in a Bruker SMART 1000 CCD or Bruker-D8 Adv
GADDS diffractometer (See details in the .cif file of the
Supporting Information). The space groups were determined
on the basis of systematic absences and intensity statistics. The
structures were solved by direct methods and refined by full-
matrix least-squares on F2. Anisotropic displacement para-
meters were determined for all nonhydrogen atoms. Hydrogen
atoms were placed at idealized positions and refined with fixed
isotropic displacement parameters. The following is a list of
programs used: data collection and cell refinement, SMART
WNT/2000 version 5.63231 or FRAMBO version 4.1.0532
(GADDS); data reductions, SAINTPLUS version 6.63;33 ab-
sorption correction, SADABS;34 structural solutions,
SHELXS-97;35 structural refinement, SHELXL-97;35 graphics
and publication materials, X-Seed version 1.5.36
€
Mossbauer parameters exist in dramatically different ranges
depending on the oxidation state of iron.28-30 Single ligand
effects are often detectable, but they are smaller compared to
those due to oxidation state changes. This is not the case for
low-spin complexes, where the isomer shift is less sensitive to
oxidation state and ligand sphere. However, within the above
series of low-spin tricarbonyl complexes 1, 6, and 7, the
isomer shifts correlate well to the donating ability of the
ancillary ligands; specifically to changes in one ligand, PR3,
within the coordination sphere. As compared to the high-spin
complexes, these differences are smaller. Thus sets of com-
plexes with systematic changes are needed in order to docu-
ment them for the newly discovered bio-organo-iron
chemistry as is found in the hydrogenases, particularly in
the binuclear settings with mixed metals and mixed oxidation
states.11 If such a methodical approach is synthetically
accessible in model complexes, useful conclusions can be
Synthetic Procedures. Note: All complexes (1-12) are light
and heat sensitive; therefore, preparations, isolation, and ma-
nipulations were conducted in the dark. The isolated complexes
were stored under argon at -40 °C in a freezer within the
glovebox.
FeI2(CO)3IMes. (1). 1,3-Bis-(2,4,6-trimethylphenyl) imida-
zolium chloride (750 mg, 1.8 mmol), KOtBu (500 mg, 3.6 mmol),
and THF (30 mL) were added to a 50 mL flask and stirred at
22 °C for 1 h. The solvent was removed under reduced pressure,
and the residual solid was extracted with hexane (30 mL). The
resulting suspension was filtered through Celite, and the filtrate
of free carbene was added to a hexane (20 mL) solution of
FeI2(CO)4 (750 mg, 1.8 mmol). The reaction mixture was stirred
for 1 h at 22 °C in dark. The product precipitated, and the solid
was collected by filtration, washed with two 25 mL portions of
hexane, and dried under vacuum to give a dark red powder.
Crude yield: 1.086 g (86.4%); recrystallization from Et2O gave
analytically pure crystals used for analysis and X-ray diffraction
€
drawn from Mossbauer spectroscopy as to the subtle tuning
of electron density at low-spin iron within the evolutionarily
perfected diverse ligation environment.
Experimental Section
Materials and Techniques. All reactions and operations were
carried out on a double manifold Schlenk vacuum line under N2
atmosphere with rigorous exclusion of light. Hexane, CH2Cl2,
CH3CN, MeOH, and diethyl ether were freshly purified by an
MBraun manual solvent purification system packed with Alcoa
F200 activated alumina desiccant. The purified solvents were
stored with molecular sieves under N2 for no more than 1 week
before use. The known complexes including FeI2(CO)4 were
prepared according to literature procedures.1 The following
materials were of reagent grade and were used as purchased
from Sigma-Aldrich: Fe(CO)5, 1,3-bis(2,4,6-trimethylphenyl)
imidazolium chloride, 1,3-bis(2,4,6-trimethylphenyl) imidazoli-
nium chloride, 1,3-dimethylimidazolium iodode, triphinopho-
sphine, trimethylphosphine, pyridine, 2,20-bipyridine, and 1,10-
phenanthroline. The NMR spectra were measured on a Varian
1
studies. H NMR (ppm, acetone-d6): δ = 7.68 (s, 2H, NCH),
7.21 (s, 4H, m-Mes), 2.39 (d, 6H, p-Mes), 2.24 (s, 12H, o-Mes).
IR (CH2Cl2, cm-1): 2087 (w), 2040 (s), 2018 (m). Anal. calcd for
C24H24FeI2O3N2: C, 41.29; H, 3.47; N, 4.01. Found: C, 40.78;
H, 3.21; N, 3.61.
FeI2(CO)3SIMes. (2). Deprotonation of the imidazolium salt
and isolation of the free carbene was performed precisely as
described above for (1) using 342.9 mg, 1 mmol of 1,3-bis(2,4,
6-trimethylphenyl) imidazolinium chloride and 224 mg, 2 mmol,
of KOtBu. Reaction with FeI2(CO)4 (421.6 mg, 1 mmol) and
1
Mercury or Unity þ300 MHz NMR spectrometer. H NMR
shifts are referenced to residual solvent resonances, according to
(31) SMART, V5.632 Program for Data Collection on Area Detectors;
BRUKER AXS Inc.: Madison, WI, 2005.
(32) FRAMBO:FRAME Buffer Operation, version 41.05 Program for Data
Collection on Area Detectors; BRUKER AXS Inc.: Madison, WI, 2001.
(33) SAINT, V6.63 Program for Reduction of Area Detector Data; BRUCK-
ER AXS Inc.: Madison, WI, 2007.
(34) Sheldrick, G. M. SADABS, Program for Absorption Correction of Area
Detector Frames; Brucker AXS.: Madison, WI, 2001.
(35) Sheldrick, G. SHELXS-97, Program for Crystal Structure Solution;
(24) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008–
7009.
(25) Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darensbourg,
M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492–9495.
(26) Kubas, G. J. Chem. Rev. 2007, 107, 4152–4205.
(27) Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62–
72.
€
(28) Christner, J. A.; Munck, E.; Janick, P. A.; Siegel, W. M. J. Biol.
€
€
€
Instit€utf€ur Anorganische Chemie der Universitat Gottingen: Gottingen, Germany,
1997.
Chem. 1981, 256, 2098.
(29) Christner, J. A.; Muenck, E.; Kent, T. A.; Janick, P. A.; Salerno,
J. C.; Siegel, L. M. J. Am. Chem. Soc. 1984, 106, 6786.
(30) Gutlich, P.; Link, R.; Trautwein, A. Mossbauer Spectroscopy and
Transition Metal Chemistry; Springer Verlag: Berlin, 1979.
(36) Sheldrick, G. SHELXL-97, Program for Crystal Structure Refinement;
€
€
€
Instit€ut f€ur Anorganische Chemie der Universitat Gottingen: Gottingen, Germany,
1997.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11289
workup as described above gave a gray powder. Crude yield: 500
mg (72%); analytically pure crystals were obtained as for
complex 1. 1H NMR (ppm, acetone-d6): δ 7.13 (s, 4H, m-Mes),
4.73 (s, 1H, NCH), 4.59 (s, 1H, NCH), 4.14 (s, 2H, NCH), 2.47
(s, 12H, o-Mes), 2.34 (s, 6H, p-Mes). IR (CH2Cl2, cm-1: 2082
(w), 2039 (s), 2019 (m). Anal. calcd for C24H26FeI2O3N2: C,
41.17; H, 3.74; N, 4.00. Found: C, 40.89; H, 3.76; N, 3.57.
FeI2(CO)3IMe. (3). As described above, the 1,3-dimethylimi-
dazolium iodide (224 mg, 1 mmol) salt was deprotonated with
KOtBu (224 mg, 2 mmol), and the free carbene was added to a
hexane (20 mL) solution of FeI2(CO)4 (421.6 mg, 1 mmol). A red
powder was ultimately obtained. Crude yield: 250 mg (50%);
FeI2(CO)3PCy3 (8). Yield: 3.0 g (94%). 1H NMR (ppm,
acetone-d6): 2.58 (br, 3H, -CH-), 1.93 (br, 12 H, -CH2-),
1.76 (br, 12 H, -CH2-), 1.36 (br, 6 H, -CH2-). 31P NMR
(ppm, acetone-d6): 76.25 (s). IR (THF, cm-1): 2081 (w), 2031
(vs), 2017 (s, sh). Anal. calcd for C21H33FeI2O3P: C, 37.42; H,
4.93. Found: C, 37.26; H, 4.90.
1
FeI2(CO)3P(OEt)3 (9). Yield: 1.8 g (68%). H NMR (ppm,
acetone-d6): 4.36 (m, 6 H, OCH2CH3), 1.40 (t, 9 H, OCH2CH3).
31P NMR (ppm, acetone-d6): 159.31 (s). IR (THF, cm-1): 2104
(w), 2053 (s). Anal. calcd for C9H15FeI2O6P: C, 19.31; H, 2.70.
Found: C, 18.84; H, 2.74.
FeI2(CO)3Py. (10). To a solution of FeI2(CO)4 (1.0 g, 2.4
mmol) in 30 mL hexane was added a solution of pyridine (0.2
mL, 2.4 mmol) in 30 mL of hexane. The reaction mixture was
stirred for 1 h at 22 °C in the dark. The precipitated solid product
was collected by filtration, washed with 2 ꢀ 25 mL hexane, and
dried under vacuum to give a gray powder. Yield: 0.88 g (77%);
crystals for analyses and X-ray diffraction were obtained as
1
recrystallization from ether gave analytically pure crystals. H
NMR (ppm, acetone-d6): δ = 7.45 (d, 2H, NCH), 3.95 (d, 6H,
NMe). IR (CH2Cl2, cm-1): 2082 (w), 2034 (s), 2023 (sh). Anal.
calcd for C8H8FeI2O3N2: C, 19.62; H, 1.65; N, 5.72. Found: C,
19.45; H, 2.09; N, 5.31.
FeI2(CO)2bipy. (4). To a solution of FeI2(CO)4 (1.0 g, 2.4
mmol) in 30 mL of hexane was added a solution of 2,20-
bipyridine (375 mg, 2.4 mmol) in 25 mL of hexane. The reac-
tion mixture was stirred for 1 h at 22 °C in the dark. The result-
ing precipitate was collected by filtration, washed with two
25 mL portions of hexane, and dried under vacuum to give a
dark red powder. Yield: 1.12 g (89.4%); crystals were grown by
layering a CH2Cl2 solution with hexane. 1H NMR (ppm,
CD3CN): δ 8.50 (d, 2 H), 8.10 (m, 2 H), 7.39 (d, 4 H). IR
(CH2Cl2, cm-1) of cis-(CO),cis-(I)-FeI2(CO)2bipy: 2043 (s),
2002 (s). IR (CH2Cl2, cm-1) of (presumed, see discussion above)
cis-(CO),trans-(I)-FeI2(CO)2bipy: 2077 (s), 2025 (s). ESI-MS
(CH2Cl2, m/z): 521 [M - H]-, 495 [M - CO þ H]þ. Anal. calcd
for C12H8FeI2O2N2: C, 27.62; H, 1.55; N, 5.37. Found: C, 26.59;
H, 1.81; N, 5.84.
1
described for complex 1. H NMR (ppm, acetone-d6): 9.63 (s,
2H), 8.12 (s, 1H), 7.65 (s, 2H). IR (CH2Cl2, cm-1): 2095 (s), 2050
(ms), 2041 (ms). Anal. calcd for C8H5FeI2O3N: C, 20.32; H,
1.07; N, 2.96. Found: C, 20.80; H, 1.39 ; N, 3.47.
FeI(CO)2PPh3PyS (11). To a solution of FeI2(CO)3PPh3 (400
mg, 0.61 mmol) in 20 mL THF was added a solution of
PyS-Naþ (81 mg, 0.61 mmol) in 10 mL THF. The reaction
mixture was stirred for 12 h at 22 °C in the dark. After removal
of solvent, the residue solid was extracted with 50 mL Et2O.
After concentration of the filtrate, a red powder precipitated
upon addition of 50 mL hexane. Yield: 156 mg (42%); recrys-
tallization from ether gave analytically pure crystals. 1H NMR
(ppm, acetone-d6): 8.17 (d, 1H, PyS), 7.45 (m, 15H, PPh3), 7.15
(t, 1H, PyS), 6.75 (t, 1H, PyS), 6.13 (d, 1H, PyS). 31P NMR
(ppm, acetone-d6): δ = 77.69 (s). IR (THF, cm-1): 2031 (s), 1983
(s). Anal. calcd for C25H19FeINO2PS: C, 49.13; H, 3.13; N, 2.29.
Found: C, 49.07; H, 3.11; N, 2.08.
FeI2(CO)2phen (5). To a solution of FeI2(CO)4 (1.5 g, 3.6
mmol) in 25 mL of CH2Cl2 was added a solution of 1,10-
phenanthroline (0.648 g, 3.6 mmol) in 25 mL of CH2Cl2. The
reaction mixture was stirred for 0.5 h at 22 °C. The solvent was
removed under reduced pressure. The residual solid was washed
with two 25 mL portions of hexane and dried under vacuum to
FeI(CO)2PCy3PyS (12). The preparation of 12 was identical
to that of 11. Yield: 140 mg (37%). 1H NMR (ppm, acetone-d6):
8.62 (d, 1H, PyS), 7.56 (t, 1H, PyS), 6.99 (t, 1H, PyS), 6.71 (d, 1H,
PyS), 2.38 (m, 3H, PCy3), 1.68 (br, 18H, PCy3), 1.21 (br, 12H,
PCy3). 31P NMR (ppm, acetone-d6): 76.44 (s). IR (THF, cm-1):
2019 (s), 1971 (s). Anal. calcd for C25H37FeINO2PS: C, 47.71; H,
5.93; N, 2.23. Found: C, 48.03; H, 6.41; N, 2.13.
1
give a dark red powder. Yield: 1.6 g (81%). H NMR (ppm,
CD3CN): δ 8.61 (d, 2 H), 8.26 (s, 2 H), 7.63 (m, 4 H). IR
(CH2Cl2, cm-1) of cis-(CO),cis-(I)-FeI2(CO)2phen: 2044 (s),
2003 (s). IR (CH2Cl2, cm-1) of (presumed, see discussion above)
cis-(CO),trans-(I)-FeI2(CO)2phen: 2077 (s), 2025 (s). Anal. calcd
for C14H8FeI2O2N2: C, 30.80; H, 1.48; N, 5.13. Found: C, 29.83;
H, 1.52; N, 5.18.
Acknowledgment. We acknowledge financial support from
the National Science Foundation (CHE-0616695 and CHE-
0910679) and contributions from the R. A. Welch Foundation
(A-0924). We appreciate helpful discussions with Professors
D. J. Darensbourg and M. B. Hall of TAMU. The synthetic
work was performed at TAMU, Department of Chemistry, by
B.L. and T.L. as equal contributors. The China Scholarship
Council provided salary for B.L. C.V.P., who acknowledges
support from the National Science Foundation (CHE-0421116)
Preparation of FeI2(CO)3P (P = PMe3 (6), PPh3 (7), PCy3
(8), and P(OEt)3 (9)). The typical procedure used to prepare
these complexes follows.13-15 To a solution of FeI2(CO)4 (2.0 g,
4.74 mmol) in 30 mL of hexane was added of a solution of the
phosphine (4.74 mmol) in 20 mL of hexane. The reaction
mixture was stirred for 1 h at room temperature in the dark.
The solvent was removed in vacuo to give a dark red residue. The
product was purified by washing with 2 ꢀ 5 mL portions of
hexane and dried under vacuum to give a dark brown solid.
€
for purchase of the Mossbauer spectrometer and the Summer
1
Fellows program at Ursinus College for summer research sup-
port for her and A.B., is the corresponding author for the
€
Mossbauer spectroscopy data.
FeI2(CO)3PMe3 (6). Yield: 460 mg (83%). H NMR (ppm,
acetone-d6): δ = 2.20 (d, 9H, PMe3). 31P NMR (ppm, acetone-
d6): δ = 30.7 (s). IR (CH2Cl2, cm-1): 2093 (s), 2047 (s), 2022 (s).
Anal. calcd for C6H9FeI2O3P: C, 15.34; H, 1.93. Found: C,
15.48; H, 1.86.
Supporting Information Available: Full structure files, in CIF
format, of complexes 1-4, 10, and 12; IR spectra of complexes
2-4, 7, 9, and 11; IR spectra of complex 4 under 13CO atmo-
sphere; DFT methods, optimized geometries, and IR frequen-
cies of isomers of complex 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
FeI2(CO)3PPh3 (7). Yield: 1.5 g (95%). 1H NMR (ppm,
acetone-d6): 7.80 (m, 6H, PPh3), 7.61 (m, 9H, PPh3). 31P NMR
(ppm, acetone-d6): 64.2 (s). IR (THF, cm-1): 2093 (w), 2045 (s),
2032 (m). Anal. calcd for C21H15FeI2O3P: C, 38.45; H, 2.30.
Found: C, 38.28; H, 2.41.