Quantitative geometric descriptions of the belt iron atoms of the iron-molybdenum cofactor of nitrogenase and synthetic iron(II) model complexes

Article abstract of DOI:10.1021/ic0609148

Six of the seven iron atoms in the iron-molybdenum cofactor of nitrogenase display an unusual geometry, which is distorted from the tetrahedral geometry that is most common in iron-sulfur clusters. This distortion pulls the iron along one C₃ axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of the Fe atoms in the iron-molybdenum cofactor with those of the presently and previously reported model complexes, as well as with those in polynuclear iron-sulfur compounds. This understanding of the iron coordination geometry is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.

Full text of DOI:10.1021/ic0609148

Inorg. Chem. 2007, 46, 60−71
Quantitative Geometric Descriptions of the Belt Iron Atoms of the
Iron Molybdenum Cofactor of Nitrogenase and Synthetic Iron(II) Model
Complexes
−
Javier Vela,† Jordi Cirera,‡ Jeremy M. Smith,† Rene J. Lachicotte,† Christine J. Flaschenriem,†
Santiago Alvarez,*,‡ and Patrick L. Holland*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Department
de Qu´ımica Inorga`nica, UniVersitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain
Received May 25, 2006
Six of the seven iron atoms in the iron−molybdenum cofactor of nitrogenase display an unusual geometry, which
is distorted from the tetrahedral geometry that is most common in iron
−sulfur clusters. This distortion pulls the iron
along one C3 axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is
rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in
iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to
trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of
the Fe atoms in the iron
as well as with those in polynuclear iron
is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.
−molybdenum cofactor with those of the presently and previously reported model complexes,
−sulfur compounds. This understanding of the iron coordination geometry
Introduction
Iron-molybdenum nitrogenases from Clostridium pas-
teurianium, Klebsiella pneumoniae, and Azotobacter Vine-
landii have been characterized by crystallography,4 and the
highest-resolution structure (1.16 Å) shows the constitution
of the FeMoco to be Fe7MoS9X(homocitrate), where X is a
central atom of the appropriate size and electron density to
be C, N, or O.5 There is substantial controversy over the
identity of X: ENDOR and ESEEM studies suggest that X
is not N,6 but a range of theoretical studies find that N is
most likely based on redox potentials, Mo¨ssbauer parameters,
and Fe-X distances.7 Identifying the site at which N2 binds
has also been controversial. N2 reactions are best known and
most efficient at molybdenum.8 However, the rates of
reduction of substrates in mutant enzymes are most sugges-
Nitrogenase: Evidence for the Importance of Belt Iron
Atoms. Nitrogenase enzymes perform the only known
biological transformation of the N2 molecule.1 There are three
very similar nitrogenase enzymes expressed by azatrophic
microorganisms, and the major difference between these
enzymes is in the metal composition, with iron-molybdenum
nitrogenases better understood and more active than iron-
vanadium and iron-only nitrogenases.2 Each enzyme has an
eight-metal cluster at which N2 binds and is reduced with
addition of protons, and the available evidence suggests that
the clusters in the different enzymes are similar except for
substitution of the heterometal (Fe7Mo in the “FeMoco,”
Fe7V in the “FeVco,” and Fe8 in the “FeFeco”).3
(4) (a) Kim, J.; Rees, D. C. Science 1992, 257, 1677-82. (b) Kim, J.;
Woo, D.; Rees, D. C. Biochemistry 1993, 32, 7104-7115. (c) Howard,
J. B.; Rees, D. C. Chem. ReV. 1996, 96, 2965-2982. (d) Mayer, S.
M.; Lawson, D. M.; Gormal, C. A.; Roe, S. M.; Smith, B. E. J. Mol.
Biol. 1999, 292, 871-891. (e) Einsle, O.; Tezcan, F. A.; Andrade, S.
L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science
2002, 297, 1696-1700. (f) Tezcan, F. A.; Kaiser, J. T.; Mustafi, D.;
Walton, M. Y.; Howard, J. B.; Rees, D. C. Science 2005, 309, 1377-
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(5) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida,
M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696-1700.
(6) Yang, T.-C.; Maeser, N. K.; Laryukhin, M.; Lee, H.-I.; Dean, D. R.;
Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2005, 127, 12804-
12805.
* To whom correspondence should be addressed.
† University of Rochester.
‡ Universitat de Barcelona.
(1) (a) Burgess, B. K.; Lowe, D. J. Chem. ReV. 1996, 96, 2983-3011.
(b) Howard, J. B.; Rees, D. C. Chem. ReV. 1996, 96, 2965-2982. (c)
Seefeldt, L. C.; Dance, I. G.; Dean, D. R. Biochemistry 2004, 43,
1401-1409.
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60 Inorganic Chemistry, Vol. 46, No. 1, 2007
10.1021/ic0609148 CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/14/2006

Trigonal Pyramidal Iron
tive of substrate binding at the central (“belt”) iron atoms.9
The most convincing evidence in this regard is the fact that
reducing the size of the amino acid Val-70 allows binding
of substituted acetylenes with substituents that engage in
hydrogen bonding to His-195 (residue numbering from the
a subunit of A. Vinelandii protein).10,11 Because acetylene
reduction is inhibited by N2 in these mutants, this binding
site is probably the same as that for N2.
Because of the likely role of the six belt iron atoms of the
FeMoco in catalysis, chemists desire to understand them in
detail. In the isolated, reduced cofactor, they exist as part of
a mixed-valence Fe2+/Fe3+ cluster (MN),12 and the crystal
structures invariably show that the geometry of each belt
iron atom is distorted away from tetrahedral, with the iron
atom near the plane of the three sulfur ligands. The
Mo¨ssbauer parameters of the belt iron atoms suggests strong
bonding to the three bridging sulfides, but a weak ionic
interaction with X.13 We have speculated that further
reduction of the cofactor could disrupt the Fe-X interaction,
leading to N2 binding.14-16 Computational studies have not
reached a consensus regarding the structural effects of
reduction on the core of the cofactor.7 Spectroscopic studies
using extended X-ray absoption fine structure (EXAFS)
indicate that the aVerage Fe-Fe distance contracts upon
reduction but (because of the complexity of the cofactor)
Figure 1. Structure of the iron-molybdenum cofactor of molybdenum-
iron nitrogenase.
do not give more detailed insight into bond forming/
breaking.17 The geometry, distortions, and cooperative move-
ments of the iron atoms in the FeMoco are likely to play an
important role in the observed reactivity.
Structurally Analogous Synthetic Complexes. Numerous
synthetic iron complexes have been studied in order to
provide a comparative basis for understanding the iron-
molybdenum cofactor.18 However, no detailed analysis of
the stereochemistry of its metal atoms has been undertaken,
and that is a relevant piece of information that must be taken
into account when trying to mimic the chemical activity of
the active site in model complexes. We thus need to focus
both in the local coordination geometry of the Fe atoms (that
can be modeled by mononuclear compounds) and on the
global shape of the Fe7Mo entity that can be modeled with
polynuclear compounds. Because the belt iron atoms are
coordinated to three bridging sulfides, in addition to a weak
interaction with X, models of the local coordination geometry
should include three strong donor ligands and one weaker
bond. Complexes of this type are rare in the synthetic
literature because iron(II) chemistry is dominated by com-
plexes with four or more strong donor ligands.
Iron-sulfur clusters make up one class of candidate
compounds, and they are well known in synthetic chemis-
try.19 In Fe4(µ3-S)4 clusters, iron atoms are coordinated by
three sulfides, and the fourth position is typically occupied
by a strongly bound ligand (e.g., chloride). In some cases,
use of phosphines has led to “prismane” and “basket” clusters
in which some iron atoms have a geometry that approach a
trigonal pyramid.20 Power has also synthesized a tris-
thiolatoiron(II) complex with no strong fourth donor but
which may have an agostic interaction of a C-H bond to
the iron.21 Another interesting family is the octanuclear M2-
Fe6 complexes, with M ) Mo or V and sulfido or thiolato
bridges.22 In these clusters, the Fe atoms are four-coordinate
and the global topology resembles that of FeMoco, with two
M atoms capping an Fe6 core.
(7) (a) Dance, I. Chem. Commun. 2003, 324-325. (b) Hinnemann, B.;
Norskøv, J. K. J. Am. Chem. Soc. 2003, 125, 1466-1467. (c) Lovell,
T.; Liu, T.; Case, D. A.; Noodleman, L. J. Am. Chem. Soc. 2003,
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Chem. Soc. 2003, 125, 15772-15778. (e) Vrajmasu, V.; Munck, E.;
Bominaar, E. L. Inorg. Chem. 2003, 42, 5974-5988. (f) Huniar, U.;
Ahlrichs, R.; Coucouvanis, D. J. Am. Chem. Soc. 2004, 126, 2588-
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Chem. 2005, 103, 344-353.
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Physiol. Plant Mol. Biol. 2001, 52, 269-295. (b) Seefeldt, L. C.;
Dance, I. G.; Dean, D. R. Biochemistry 2004, 43, 1401-1409. (c)
Dos Santos, P. C.; Igarashi, R. Y.; Lee, H.-I.; Hoffman, B. M.; Seefeldt,
L. C.; Dean, D. R. Acc. Chem. Res. 2005, 38, 208-214.
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D. R.; Seefeldt, L. C. J. Biol. Chem. 2004, 279, 34770-34775.
(11) Similar mutations have enabled the trapping of some substrates on
the FeMoco: (a) Barney, B. M.; Laryukhin, M.; Igarashi, R. Y.; Lee,
H.-I.; Dos Santos, P. C.; Yang, T.-C.; Hoffman, B. M.; Dean, D. R.;
Seefeldt, L. C. Biochemistry 2005, 44, 8030-8037. (b) Barney, B.
M.; Yang, T.-C.; Igarashi, R. Y.; Dos Santos, P. C.; Laryukhin, M.;
Lee, H.-I.; Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. J. Am. Chem.
Soc. 2005, 127, 14960-14961.
(17) Christiansen, J.; Tittsworth, R. C.; Hales, B. J.; Cramer, S. P. J. Am.
Chem. Soc. 1995, 117, 10017-24.
(18) Lee, S. C.; Holm, R. H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3595-
3600.
(19) Holm, R. H. Electron Transfer: Iron-Sulfur Clusters. In ComprehensiVe
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Elsevier: Oxford, 2004; Vol. 8, pp 61-90.
(20) Snyder, B. S.; Holm, R. H. Inorg. Chem. 1990, 29, 274-279 and
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(12) Yoo, S. J.; Angove, H. C.; Papaefthymiou, V.; Burgess, B. K.; Mu¨nck,
E. J. Am. Chem. Soc. 2000, 122, 4926-4936.
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5974-5988.
(14) Holland, P. L. Can. J. Chem. 2005, 83, 296-301.
(15) Evidence for flexibility of the FeMoco comes from EPR studies:
George, G. N.; Prince, R. C.; Bare, R. E. Inorg. Chem. 1996, 35, 434-
8.
(16) Computational studies support flexibility of the FeMoco: (a) Schimpl,
J.; Petrilli, H. M.; Blo¨chl, P. E. J. Am. Chem. Soc. 2003, 125, 15772-
15778. (b) Huniar, U.; Ahlrichs, R.; Coucouvanis, D. J. Am. Chem.
Soc. 2004, 126, 2588-2601. (c) Igarashi, R. Y.; Dos Santos, P. C.;
Niehaus, W. G.; Dance, I. G.; Dean, D. R.; Seefeldt, L. C. J. Biol.
Chem. 2004, 279, 34770-34775. (d) Dance, I. J. Am. Chem. Soc.
2004, 126, 11852-11863. (e) Dance, I. J. Am. Chem. Soc. 2005, 127,
10925-10942. (f) Ka¨stner, J.; Hemmen, S.; Blo¨chl, P. E. J. Chem.
Phys. 2005, 123, 074306/1-07430.6/8. (g) Dance, I. Biochemistry
2006, 45, 6328-6340.
(21) (a) MacDonnell, F. M.; Ruhlandt-Senge, K.; Ellison, J. J.; Holm, R.
H.; Power, P. P. Inorg. Chem. 1995, 34, 1815-22. (b) Evans, D. J.;
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Chem. 2004, 43, 674-682. (c) Osterloh, F.; Achim, C.; Holm, R. H.
Inorg. Chem. 2001, 40, 224-232. (d) Osterloh, F.; Sanakis, Y.; Staples,
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Chem. 2006, 45, 1997-2007.
Inorganic Chemistry, Vol. 46, No. 1, 2007 61

Vela et al.
Recently, bulky â-diketiminate ligands (abbreviated LMe
and LtBu, Scheme 1) have been used for stabilizing three-
coordinate iron(II) complexes.23 In many cases, a fourth
ligand can bind weakly.23e,f,h,i,k The bridging sulfide complex
LtBuFe(µ-S)FeLtBu, which mimics a part of the cofactor,
coordinates certain N-donor ligands such as acetonitrile,
ammonia, and substituted hydrazines.24 However, it is not
immediately clear how to compare the geometry of these
synthetic complexes with the belt iron atoms of the FeMoco.
This paper describes our efforts to quantitatively describe
the geometries of four-coordinate iron(II) complexes in order
to provide a basis for comparison to the belt iron atoms of
the FeMoco. This is intended to facilitate spectroscopic
investigations that compare the iron-molybdenum cofactor
to other compounds. Two methods are described: a simple
but limited method based on L-M-L angles, and continuous
shape measures that are adapted for effective evaluation of
geometries near a trigonal pyramid. In order to provide a
large set of synthetic complexes for comparison, we also
report some new iron(II) â-diketiminate complexes.
Continuous Shape Measures. A brief description of
continuous shape measures is presented in this section. More
detailed information on this stereochemical tool and its
applications to transition metal compounds can be found in
elsewhere.25 Continuous shape measures were proposed by
Avnir and co-workers26 to provide a quantitative evaluation
of the degree of distortion of a set of atoms (e.g., the
coordination sphere of a transition metal) from a given ideal
polyhedral shape. In short, the proposed method consists in
finding the ideal structure having the desired shape that is
closest to the observed structure. The ideal and real polyhedra
are superimposed in such a way as to minimize the
expression in eq 1, the value of which is the shape measure
of the investigated structure Q relative to the ideal shape P,
where bqi are N vectors that contain the 3N Cartesian
coordinates of the problem structure Q, bpi contain the
coordinates of the ideal polyhedron P, and bq0 is the position
vector of the geometric center that is chosen to be the same
for the two polyhedra.
N
2
|bq - bp |
∑
i
i
i)1
S(Q, P) ) min
× 100
(1)
N
2
[
]
|bq - bq |
∑
i
0
i)1
S(Q, P) ) 0 corresponds to a structure Q fully coincident in
shape with the reference polyhedron P, regardless of size
and orientation. The maximum allowed value is S(Q, P) )
100, although in practice the values found for severely
distorted chemical structures are very rarely larger than 40.
For the purpose of the present work, we can use shape
measure S(tetrahedron) as a measure of the distortion of the
Fe(ligand)4 groups from the tetrahedron,27 and S(vTBP) to
measure the deviation of a structure to a vacant trigonal
bipyramid (vTBP), defined as a trigonal pyramid with axial
ligand-metal-basal ligand bond (see Scheme 2f). We can
also use shape measures to analyze the geometry of Fe6 cores
in synthetic M2Fe6 complexes and compare them with that
of the Fe7Mo group in the iron-molybdenum cofactor of
nitrogenase.
An additional advantage of using the shape measures
approach is that the minimal distortion pathway between two
ideal polyhedra is analytically defined,28 and one can also
calculate the distance of a given structure to such a pathway
via a path deViation function. In the present case, we will
be able to tell whether a Fe(ligand)4 distorted tetrahedral
structure falls along the pyramidalization path that takes a
tetrahedron to a vTBP or by how much it deviates from such
a path. Finally, the extent of transformation from one
polyhedral shape to another is quantified through the use of
a generalized interconVersion coordinate, a percentage that
shows how far a given structure has gone along the
polyhedral interconversion path.27b
(23) (a) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun.
2001, 1542-1543. (b) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.;
Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J.
Am. Chem. Soc. 2001, 123, 9222-9223. (c) Smith, J. M.; Lachicotte,
R. J.; Holland, P. L. Organometallics 2002, 21, 4808-4814. (d)
Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
R. J. J. Am. Chem. Soc. 2002, 124, 14416-14424. (e) Smith, J. M.;
Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003, 125, 15752-
15753. (f) Eckert, N. A.; Smith, J. M.; Lachicotte, R. J.; Holland, P.
L. Inorg. Chem. 2004, 43, 3306-3321. (g) Vela, J.; Vaddadi, S.;
Cundari, T. R.; Smith, J. M.; Gregory, E. A.; Lachicotte, R. J.;
Flaschenriem, C. J.; Holland, P. L. Organometallics 2004, 23, 5226-
5239. (h) Gregory, E. A.; Lachicotte, R. J.; Holland, P. L. Organo-
metallics 2005, 24, 1803-1805. (i) Vela, J.; Smith, J. M.; Yu, Y.;
Ketterer, N. A.; Flaschenriem, C. J.; Lachicotte, R. J.; Holland, P. L.
J. Am. Chem. Soc. 2005, 127, 7857-7870. (j) Eckert, N. A.; Stoian,
S.; Smith, J. M.; Bominaar, E. L.; Mu¨nck, E.; Holland, P. L. J. Am.
Chem. Soc. 2005, 127, 9344-9345. (k) Smith, J. M.; Sadique, A. R.;
Cundari, T. R.; Rodgers, K. R.; Lukat-Rodgers, G.; Lachicotte, R. J.;
Flaschenriem, C. J.; Vela, J.; Holland, P. L. J. Am. Chem. Soc. 2006,
128, 756-769. (l) Panda, A.; Stender, M.; Wright, R. J.; Olmstead,
M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909-3916.
(m) Sciarone, T. J. J.; Meetsma, A.; Hesssen, B.; Teuben, J. H. Chem.
Commun. 2002, 1580-1581.
Results
Synthesis of Four-Coordinate Iron(II) Diketiminate
Complexes. The synthesis and characterization of some of
the four-coordinate iron(II) diketiminate complexes analyzed
here have been reported.23 These four-coordinate iron(II)
diketiminate complexes are conveniently prepared by the
addition of a donor ligand L′ (L′ ) substituted pyridine or
nitrile) to the appropriate free three-coordinate (LRFeX) or
dimeric four-coordinate iron(II) precursor ([LRFe(µ-X)]2), (LR
) bulky â-diketiminate ligand, X ) halide, amide, hydro-
carbyl, hydride; Scheme 1a).
(24) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck, E.; Holland, P. L. J.
Am. Chem. Soc. 2004, 126, 4522-4523.
(25) (a) Zabrodsky, H.; Peleg, S.; Avnir, D. J. Am Chem. Soc. 1992, 114,
7843-7851. (b) Pinsky, M.; Avnir, D. Inorg. Chem. 1998, 37, 5575-
5582. (c) Alvarez, S.; Alemany, P.; Casanova, D.; Cirera, J.; Llunell,
M.; Avnir, D. Coord. Chem. ReV. 2005, 249, 1693-1708.
(26) Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M. New J. Chem. 2002,
26, 996-1009.
(27) (a) Cirera, J.; Alemany, P.; Alvarez, S. Chem.sEur. J. 2004, 10, 190-
207. (b) Cirera, J.; Ruiz, E.; Alvarez, S. Chem.sEur. J. 2006, 12,
3162-3167.
(28) Casanova, D.; Cirera, J.; Llunell, M.; Alemany, P.; Avnir, D.; Alvarez,
S. J. Am. Chem. Soc. 2004, 126, 1755-1763.
62 Inorganic Chemistry, Vol. 46, No. 1, 2007

Trigonal Pyramidal Iron
Scheme 1
Figure 2. Examples of four-coordinate iron(II) diketiminate complexes
with trigonal pyramidal distortions. Molecular structures of the new
t
complexes LMeFeiBu(tBuCN) (1) (a) and LMeFeCH2 Bu(tBuCN) (2) (b).
Thermal ellipsoids are shown at 50% probability with hydrogen atoms
omitted for clarity. The pseudoapical (axial) position in each complex is
occupied by the neutral ligand trimethylacetonitrile (L′).
Four new tetracoordinate iron(II) complexes LMeFeiBu(t-
BuCN) (1), LMeFeCH2tBu(tBuCN) (2), LMeFeCl(4-tBu-py) (3)
and LtBuFeCl(CH3CN) (4) have been prepared by the route
shown in Scheme 1a. The neutral donor (trimethylacetoni-
trile, 4-tert-butylpyridine, or acetonitrile) was mixed with a
three-coordinate hydrocarbyl complex, chloride complex LMe-
Fe(µ-Cl)2Li(THF)2 (Scheme 1b), or chloride complex LtBu
-
FeCl. Thermal-ellipsoid plots of the X-ray crystal structures
of 1-4 are presented in Figures 2 and 3 (see also Table 1).
Structurally characterized tetracoordinate iron complexes
with hydrocarbyl ligands are rare.29-32 The iron-carbon bond
distances for 1 (2.040(2) Å) and 2 (2.053(2) Å) are similar
to those observed in other four-coordinate complexes of iron-
(II) with sp3-hydrocarbyl ligands, which range between 2.032
and 2.120 Å.29 The iron-carbon bond distances in 1 and 2
are similar to those in the three-coordinate analogues (2.003-
2.060 Å),23 despite the higher coordination number.
The Fe-Cl distances in the four-coordinate chloride
complexes 3 (2.237(1), 2.238(1) Å) and 4 (2.222(2), 2.247-
(2) Å) are intermediate between the iron-chloride bond
distance in the three-coordinate complex LtBuFeCl 2.172(1)
Å and the Fe-Cl bond distances in the chloride-bridged four-
Figure 3. Molecular structures of the new four-coordinate diketiminate
iron(II) chloride complexes LMeFeCl(4-tBu-py) (3) and LtBuFeCl(CH3CN)
(4). Thermal ellipsoids are drawn at 50% probability with hydrogen atoms
omitted for clarity. The pseudoapical (axial) positions are occupied by 4-tert-
butylpyridine and acetonitrile, respectively.
coordinate complexes LMeFe(µ-Cl)2Li(ether)2 (2.324-2.343
Å, ether ) Et2O or THF), [LMeFe(µ-Cl)]2 (2.3583(5), 2.4045-
(5) Å), and [Mg(THF)4][LMeFeCl(µ-Cl)]2 (Fe-(µ-Cl) 2.377-
(1), Fe-Cl 2.267(1) Å).23a Thus, the iron-chloride bond
distance is severely affected by both the coordination number
at iron and the hapticity of the chloride ligand itself.
A number of four-coordinate iron(II) diketiminate com-
plexes containing two virtually identical ligands, e.g., LMe-
FeCl2Li(THF)2, [LMeFe(µ-Cl)]2 and [LMeFe(µ-F)]2 (Scheme
1b, c) have been reported. In new work, the chelated complex
(29) CSD version 5.27 + 1 update, 2005: Allen, F. H. Acta Crystallogr.
B 2002, 58, 380.
-
(30) Tetracoordinate iron complexes with sp3 hybridized hydrocarbyl
ligands: (a) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2004, 23, 237-246. (b)
Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1998, 120,
10561-10562. (c) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-
oka, Y.; Akita, M. Organometallics 2001, 20, 3582-3598. (d) Akita,
M.; Shirasawa, N.; Hikichi, S.; Moro-oka, Y. Chem. Commun. 1998,
973-974. (e) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Retting, S.
J.; Young, Jr., G. Organometallics 1998, 17, 2313-2323. (f) Hermes,
A. R.; Girolami, G. S. Organometallics 1987, 6, 763-768.
(31) Tetracoordinate iron complexes with sp2-hybridized hydrocarbyl
ligands: (a) Muller, H.; Seidel, W.; Gorls, H. Z. Anorg. Allg. Chem.
1996, 622, 1269-1273. (b) Magill, C. P.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Inorg. Chem. 1994, 33, 1928-1933. (c) Klose, A.;
Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re, N. J. Am.
Chem. Soc. 1994, 116, 9123-9135. (d) Klose, A.; Solari, E.; Ferguson,
R.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Z. Anorg. Allg. Chem.
1993, 12, 2414-2416. (f) Bazhenova, T. A.; Lobkovskaya, R. M.;
Shibaeva, R. P.; Shilova, A. K.; Gruselle, M.; Leny, G.; Deschamps,
E. J. Organomet. Chem. 1983, 244, 375-382.
LtBuFe(η2-OTf) (5, η2-OTf ) O,O-η2-O3SCF3 ) (Scheme 1d)
has been synthesized as described in the Experimental
Section and crystallographically characterized. The molecular
structure is shown in Figure 4.
The triflate (trifluoromethanesulfonate) complex LtBuFe-
(η2-OTf) (6, OTf ) O3SCF3, Figure 4) can be made in low
yields by the metathesis reaction of LtBuFeCl with LiOTf or
by the disproportionation reaction between LtBuFeNNFeLtBu
and LtBuFe(OTf)2 in a 1:2 ratio. However, the highest-yielding
synthesis of 3 involves direct deprotonation of lutidinium
triflate (2,6-(CH3)2C6H3NHOTf) with the hydride complex
[LtBuFe(µ-H)]2 (Scheme 1d). Because the triflate anion is
weakly coordinating, it is an excellent leaving group. Thus,
triflate complexes such as 5 are potentially useful in catalysis
and as synthetic precursors to other complexes.33 There are
no crystal structures reported for iron complexes where
triflate acts as a bidentate (dihapto) ligand.29,34 Moreover,
none of the reported iron triflate structures is four-coordinate.
(32) Tetracoordinate iron complexes with sp-hybridized carbyl ligands
(carbon monoxide excluded): (a) Riese, U.; Harms, K.; Pebler, J.;
Dehnicke, K. Z. Anorg. Allg. Chem. 1999, 625, 746-754. (b) Sydora,
O. L.; Wolczanski, P. T.; Lobkovsky, E. B. Angew. Chem., Int. Ed.
2003, 42, 2685-2687.
Inorganic Chemistry, Vol. 46, No. 1, 2007 63

Vela et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for New Tetracoordinate Iron(II) Complexes
LMeFeiBu(tBuCN) (1)
LMeFeCH2 Bu(tBuCN) (2)
LMeFeCl(4-tBupy) (3)a
LtBuFeCl(CH3CN) (4)a
LtBuFe(η2-OTf) (5)
t
compound
Fe-X
2.040(2)
2.163(2)
2.023(1), 2.026(1)
2.053(2)
2.151(2)
2.027(1), 2.029(1)
2.237(1), 2.238(1)
2.106(2), 2.110(3)
2.004(2), 2.007(2)
1.999(2), 2.010(3)
0.525(1), 0.522(1)
120.5(2), 119.5(2)
120.7(3), 119.5(3)
94.5(1), 93.5(1)
2.222(2), 2.247(2)
2.089(6), 2.082(6)
1.996(5), 2.001(4)
2.000(4), 2.002(5)
0.469(2), 0.534(2)
126.3(5), 125.1(5)
2.131(3)
2.213(3)
1.959(3), 1.961(3)
Fe-L′
Fe-N(diketiminate)
(N2X)plane‚‚‚Fe b
0.3680(9)
119.5(1), 120.8(1)
0.4737(9)
121.0(1), 119.0(1)
0.501(2), 0.404(2)
128.7(3), 125.9(3)
C(Ar)-N-CR(LMe
)
c
N-Fe-N (bite angle)
Σ( L-Fe-L)d
92.74(5)
646.8(3)
89.62(4)
92.81(5)
648.3(3)
89.96(5)
98.0(2), 96.5(2)
653(1), 657(1)
88.1(1), 82.2(1)
95.6(1)
658.3(7)
84.2(1)
654.7(5), 654.5(5)
89.91(7), 89.63(7)
(LRFe)-(FeXL′)e
a Two independent molecules were found in the unit cell. b Minimum distance between iron and the plane formed by the three basal ligands. c Angle
around the imine nitrogen. d Sum of angles around iron. e Twist angle between the C3N2Fe plane (iron(II) diketiminate plane) and the FeXL′ plane.
LtBu. Accordingly, this angle is 119-121° in 1-3 (containing
LMe), and 125-129° in 3-5 (containing LtBu) (Table 1).
Perhaps the most striking structural feature among all the
aforementioned four-coordinate iron(II) complexes is the
flexibility of the coordination geometry around iron. Thus,
compounds with two identical monodentate ligands such as
[LMeFe(µ-F)]2, [LMeFe(µ-Cl)]2, or LMeFe(µ-Cl)2Li(Et2O)2 ap-
pear to have structures that differ from tetrahedral only by
virtue of the chelating ligand (Scheme 2b). In contrast, in
some four-coordinate adducts such as LtBuFeH(4-tBu-py),
LMeFeF(4-tBu-py), and LMeFeCl(tBupy) (3), the iron center
deviates substantially from a tetrahedral geometry. Their
idealized geometry is closer to a trigonal pyramid (Scheme
2e) characterized by axial ligand-metal-basal ligand bond
angles R and basal ligand-metal-basal ligand bond angles
â, retaining an approximate C3V symmetry. Different degrees
of pyramidalization, gauged by the average R values, can
be anticipated for the experimental structures, between 109.5°
for the tetrahedron and 90° when the metal atom is coplanar
with the three basal donor atoms. Therefore, we can consider
the extreme of such distortion to be a trigonal bipyramid
with one vacant axial position (vTBP, Scheme 2f).
On the other hand, we must not forget that the chelating
nature of the diketiminate ligands imposes a small N-Fe-N
bond angle compared to the ideal tetrahedral angle (ω,
Scheme 2b). In the absence of the umbrella distortion leading
to a trigonal pyramid, chelation results in a distortion to a
C2V structure, independently of the values adopted by the
bite angle ω. In real structures, both the umbrella and
chelating distortions of the tetrahedron may appear simul-
taneously (Scheme 2c), with the eventual addition of a
rocking distortion (Scheme 2d). Therefore, some quantitative
measures are necessary to distinguish whether the tetrahedron
or the vTBP best represents the coordination geometry in a
given complex and which angular parameters (R and â, or
ω and γ) can better describe a particular structure.
Figure 4. Thermal ellipsoid plot of the X-ray crystal structure of LtBu
-
Fe(η2-OTf) (5). Thermal ellipsoids are shown at 50% probability. Hydrogen
atoms are omitted for clarity.
Thus, complex 5 is an unprecedented example of η2-triflate
ligation to low-coordinate iron. The molecular structure of
5 (Figure 4) reveals that the FeO2 core is somewhat
asymmetric, with Fe-O distances that differ by 0.082(4) Å
(Table 1).
All of the four-coordinate iron(II) diketiminate complexes
contain a high-spin iron(II) center, consistent with the broad,
1
paramagnetically shifted signals in their H NMR spectra.
Their solution magnetic moments (Evans)35 are 4.5-5.5 µB,
in agreement with a high-spin d6 electronic configuration
having four unpaired electrons and a spin state of S ) 2.
In compounds 1-5, the structural parameters of the
diketiminate ligand closely resemble those observed for other
iron(II) diketiminate complexes: the diketiminate bite angles
(N-Fe-N) range from 92° to 98° and the Fe-N(diketimi-
nate) bond distances range from 1.95 to 2.03 Å (Table 1).
The C(Ar)-N-CR angle around the diketiminate nitrogen
atoms strongly depends on the bulk of the diketiminate,
typically being around 120° for LMe and ∼7-9° wider for
(33) Recent examples on the use of iron triflates in synthesis: (a) Seidel,
G.; Laurich, D.; Fu¨rstner, A. J. Org. Chem. 2004, 69, 3950-3952.
(b) Watahiki, T.; Akabane, Y.; Mori, S.; Oriyama, T. Org. Lett. 2003,
5, 3045-3048. (c) Zamojski, A.; Jarosz, S. Curr. Org. Chem. 2003,
7, 1-12. (d) Suda, K.; Baba, K.; Nakajima, S.; Takanami, T. Chem.
Commun. 2002, 21, 2570-2571. (e) Miesch, L.; Gateau, C.; Morin,
F.; Franck-Neumann, M. Tetrahedron Lett. 2002, 43, 7635-7638.
(34) Selected examples of η1-(trifluoromethanesulfonate)iron complexes:
(a) Britovsek, G. J. P.; England, J.; Spitzmesser, S. K.; White, A. J.
P.; Williams, D. J. Dalton Trans. 2005, 945-955. (b) Hagen, K. S.
Inorg. Chem. 2000, 39, 5867-5869.
Discussion
Quantitative Measures of the Geometry at Four-
Coordinate Metals. Traditional methods of analyzing ge-
ometries of metal centers are based on bond angles and
mathematical manipulations thereof. This strategy led some
of us to introduce an angle-based parameter τ.36,37 In four-
coordinate complexes, τ is the normalized difference between
the sum of the basal ligand-basal ligand angles and the sum
(35) (a) Schubert, E. M. J. Chem. Ed. 1992, 69, 62. (b) Evans, D. F. J.
Chem. Soc. 1959, 2003-2005.
64 Inorganic Chemistry, Vol. 46, No. 1, 2007

Trigonal Pyramidal Iron
Scheme 2
of the basal ligand-axial ligand angles:
tetrahedron, while the second one has τ ) 1 even if it is not
a vTBP. Although these are extreme cases, they tell us that
structures that are intermediate between the tetrahedron and
one of these shapes will have τ values that do not reflect a
degree of pyramidalization of a tetrahedron. These limitations
led us to seek a more general yet accurate method for
defining four-coordinate geometries.
(basal-M-basal) - (basal-M-axial)
∑
∑
τ )
(2)
90°
Using eq 2, an ideal tetrahedron has τ ) 0 and an ideal
vacant trigonal bipyramid has τ ) 1. To identify which is
the axial ligand, τ is calculated individually as if each of the
four ligands were axial, and the largest value is used. This
strategy is useful in defining a particular ligand as axial,
which is effective if one choice of τ is larger by 0.1 or more.
The parameter τ was used to describe where a structure
falls along the interconversion path between a tetrahedral
geometry (R ) â and τ ) 0) and a vTBP geometry (R )
90°, â ) 120°, and τ ) 1) shown in Scheme 2 (a, e, and f).
Using this measure, the belt iron atoms of the FeMoco have
τ ) 0.46 ( 0.03, with X in the axial position (throughout
the paper, we use PDB 1M1N, with 1.16 Å resolution,4e in
which there are four crystallographically independent
FeMoco clusters, giving 24 belt iron atoms). This indicates
that the belt iron atoms are about halfway between a
tetrahedron and an axially vacant trigonal bipyramid.
In our earlier work, τ values were given for a number of
model complexes.24 However, the τ parameter is limited: it
is only appropriate for structures that lie near the intercon-
version path between a perfect tetrahedron and a perfect
vTBP, and does not account for distortions that are not
symmetric about the C3 axis of a tetrahedron, such as the
chelation shown in Scheme 2b. As an example, consider two
extreme cases: the square planar and sawhorse structures.
The first one would have τ ) 0 but clearly is not a
Using Shape Measures To Define the Geometry of
Four-Coordinate Complexes. A more discerning method
for defining four-coordinate geometries is through the use
of continuous shape measures described above. For the
specific case of four-coordinate MX4 groups, the tetrahedral
shape measure, S(tetrahedron), indicates how much that
group deviates from a perfect tetrahedron. Hence, S(tetra-
hedron) values close to zero indicate nearly perfect tetrahedra,
and higher values reflect increasingly strong distortions.
Although the stereochemistry of four-coordinate metal
complexes has been the subject of much concern in the past
decades,38 the recent application of continuous shape mea-
sures has provided a new perspective and a more compre-
hensive description of the relationship between stereochem-
istry and electron configuration.27a Because square-planar and
tetrahedral geometries are most common for four-coordinate
complexes, it has been convenient to graphically describe
the geometry of a given four-coordinate complex by plotting
its shape measures with respect to a planar square, S(square),
and to a tetrahedron, S(tetrahedron).27,39 A number of related
complexes are plotted in this way, together with model
structures, to give a shape map. Figure 5 shows shape maps
for several families of four-coordinate Fe centers. It is evident
that most of the experimental structures are very far from
the square-planar geometry. On the other hand, it can be
(36) This concept is based on the τ used for five-coordinate complexes:
Addison, A. W.; Rao, Y. N.; Reedijk, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc. 1984, 1349-1356.
(37) Kaim and coworkers have used a related method, based on a sum of
the six L-M-L angles: for a tetrahedron, the sum of angles around
the metal is 657°, whereas in a trigonal pyramid, the sum of angles is
630°. (a) Stange, A. F.; Klein, A.; Klinkhammer, K.-W.; Kaim, W.
Inorg. Chim. Acta. 2001, 324, 336-341. (b) Schwach, M.; Hausen,
H.-D.; Kaim, W. Chem.sEur. J. 1996, 2, 446-451. (c) Titze, C.;
Kaim, W. Z. Naturforsch., B 1996, 51, 981-988.
(38) (a) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058-1076. (b)
Burdett, J. K. Molecular Shapes. Theoretical Models of Inorganic
Stereochemistry; Wiley: New York, 1980. (c) Kepert, D. L.; Inorganic
Stereochemistry; Springer: Heidelberg, 1982. (d) Albright, T. A.;
Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry;
Wiley: New York, 1985.
(39) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 7148-
7165.
Inorganic Chemistry, Vol. 46, No. 1, 2007 65

Vela et al.
Figure 5. Shape maps that display the range of geometries available to a type of complex; the x coordinate of each point represents how close it is to an
idealized tetrahedron, and the y coordinate represents how close it is to an idealized square planar geometry. The three plots show the Fe atoms of the
Fe-Mo cofactor (a), sulfur-bridged Fen clusters (b), and mononuclear diketiminate Fe complexes (c). The continuous line represents the minimal distortion
path for the interconversion of the tetrahedron and the square.
Figure 7. Schematic depiction of the structure of the Fe7Mo site in
FeMoco, showing the trigonal prism formed by the belt Fe atoms (the sticks
do not represent chemical bonds) and the local XFeS3 coordination sphere
of one of these atoms, with the other Fe-S and Fe-X bonds omitted for
simplicity. Fe ) orange, S ) yellow, Mo ) blue.
Figure 6. Paths in the tetrahedron-vTBP shape map corresponding to
(a) the umbrella distortion that interconverts those two polyhedra (Scheme
2e and f), (b) the decrease in the basal N-Fe-N bond angle ω for a structure
90°. If we close only one of the basal bond angles (N-Fe-
N) keeping a constant degree of pyramidalization (Scheme
2e-c) as would happen in the presence of a chelating
diketiminate ligand, the structures follow a line approxi-
mately perpendicular to the minimal distortion path (b in
Figure 6), while opening one basal angle results in a
practically coincident line in the shape map. Then, if we add
a bond distance distortion to a structure with a given degree
of pyramidalization and with a fixed chelating angle, to
represent the situation for the belt Fe atoms of the FeMo
cofactor with three longer (Fe-S) and one shorter (Fe-X)
bonds, a displacement in the direction c results.
with R ) 102° (R ) average basal-metal-apical angle, Scheme 2c and e)
and (c) a distortion leading to one short and three long bond distances, in
a 1:1.26 ratio.
seen that the families of iron-sulfur clusters and of iron
diketiminate complexes (Figure 5b, c) present a wider range
of distortions than the belt iron atoms in FeMoco (Figure
5a). While the latter seem to concentrate near the tetrahedral
shape, many members of the two other families present
distortions close to the spread or twist paths that take a
tetrahedron to the square planar or chelated rectangular
geometries, respectively.40 Since the iron sites in FeMoco
do not follow the path to the square, but rather seem to
present an umbrella distortion (Scheme 2e) that would
ultimately lead to a vTBP shape (Scheme 2f), we choose an
alternative shape map in which the two reference shapes are
the regular tetrahedron and the vTBP.
Before analyzing the experimental structural data, let us
briefly describe what can we expect in such a shape map,
with the help of model tetracoordinate structures (Figure 6).
The points shown on the two coordinate axes correspond to
the ideal tetrahedral and vTBP shapes (with all metal-ligand
bond distances the same), as indicated, and the curve
connecting those two points (a in Figure 6) represents the
minimum distortion interconVersion path that corresponds
to varying the Lax-M-Lbasal bond angles R from 109.5° to
Using Shape Measures To Describe the Belt Iron Atoms
of the FeMoco. The Fe7Mo cluster has the shape of an Fe6
trigonal prism, capped on its triangular faces by one Fe and
one Mo atom (Figure 7).41 The inner iron atoms can be
separated into two subgroups, depending on whether they
are closer to a capping Mo or Fe atom. All of them form
FeXS3 trigonal pyramids (X being the atom that centers the
trigonal prism) with X-Fe-S bond angles between 101°
and 103°. The position of these Fe atoms in the shape map
(Figure 8) clearly show them to be approximately halfway
(between 55 and 62%, according to the generalized polyhe-
dral interconversion coordinates described elsewhere27b
)
(41) Using a trigonal prismatic shape measure on the six belt iron atoms
gives a value of 0.02 for each of the four crystallographic sites,
indicating that the Fe6 group has a nearly perfect trigonal prismatic
geometry.
(40) Alvarez, S.; Avnir, D. Dalton Trans. 2003, 562-569.
66 Inorganic Chemistry, Vol. 46, No. 1, 2007

Trigonal Pyramidal Iron
Figure 8. Tetrahedron-vTBP shape map showing the position of the
coordination spheres of the belt Fe atoms in FeMoco. The blue circles
correspond to the belt Fe atoms at the Mo side, empty triangles to those at
the capping Fe side. The continuous line represents the minimal distortion
path between the tetrahedron and the vTBP, and the dotted line corresponds
to the closing of one basal S-Fe-S bond angle in a XFeS3 model with R
) 102° (Scheme 2e and c).
Figure 9. Scatter plot of the tetrahedral shape measures and the deviation
from the tetrahedron-vacant trigonal bipyramid path for the FeXS3 groups
corresponding to belt Fe atoms (blue circles correspond to the Mo side,
black squares to the Fe side of the MoFe7 bicapped trigonal prism in the
FeMoco).
Information). The S-Fe-S and S-Fe-X bond angles are
correlated for the sites at the Mo side of the cluster but not
for those at the Fe side.
along the interconversion path between the tetrahedron and
the vTBP (continuous line in Figure 8, i.e., path a of Figure
6), in agreement with the description provided by the τ
parameter. However, we notice also that the coordination
geometries of the Fe atoms progressively deviate from the
pyramidalization path as the distortion of the tetrahedron
increases (i.e., from bottom to top in Figure 8). Such a
deviation can be attributed mostly to the combined effects
of a chelating distortion (Scheme 2b) and bond distance
inequality that show up as deviations from the path a along
directions parallel to b and c in Figure 6. Effectively, analysis
of the structure of the iron-molybdenum cluster shows that
one basal edge of each Fe-centered pyramid is constrained
by the capping FeS2 or MoS2 groups to S-Fe-S bond angles
in the range 105-111°, while the other two basal angles are
more flexible and can adapt to the larger values required by
a pyramidalization distortion (115° < S-Fe-S < 123°). The
belt Fe atoms have slightly different geometries on the Fe-
capped and Mo-capped sides of the cofactor, with those on
the Mo side showing a greater variety of shape measures,
apparently following a well-defined trend.
There is an excellent correlation between the tetrahedral
shape measure and the deviation from the tetrahedron-vTBP
path (Figure 9). The belt sites at the Fe (black squares) and
Mo (blue circles) sides of the cluster follow essentially the
same trend, but the former present larger deviations both
from the tetrahedron and from the tetrahedron-vTBP path,
whereas the latter present smaller values but span a wider
range of geometries. This suggests that the main difference
between the different belt iron atoms is not associated to
changes in the pyramidalization, in keeping with the small
variation of the average X-Fe-S angles (101-103°).
Instead, the variation between belt iron atoms arises largely
from different basal bond angles. This can be seen by
analyzing the dependence of the deviation from the tetra-
hedron-vTBP path on the S-Fe-S angle (see Supporting
All the trends discussed indicate that shape variability is
associated with two correlated geometrical parameters that
are (at least for the Fe atoms at the Mo side) the S-Fe-S
angle of the FeMoS2 rhomb and the average axial-basal
bond angle R. The latter two seem to be induced by the
capping effect of the Mo terminal atom. It would have been
very difficult to discern these differences without the use of
shape measures.
Using Shape Measures To Describe Literature Iron-
Sulfur Complexes. Many Fe4S4 cuboidal clusters are known,
and sulfur bridged four-coordinate iron atoms are found in
a host of homo- and hetero-polynuclear complexes. In 465
fragments found in a database search29 for such compounds,
most are concentrated close to the tetrahedron (Figure 10),
even if showing varying degrees of distortion. Figure 10
highlights three Fe atoms (filled circles) that appear in the
same region of the shape map as those of FeMoco: (a)
[Fe4(SCH2Ph)6Br4],42 (b) [Tp2Mo2Fe6S9(SPh)4],3-43 and (c)
[Fe7S6Cl3(PEt3)4].44,45 In each case, only one or two iron
atoms of the cluster have the pyramidalized geometry that
should most closely mimic the FeMoco belt iron atoms. The
iron atom that most closely mimics the FeMoco belt, labeled
(c) in Figure 10, can be derived from an Fe6 “prismane”
structure by capping three sulfur atoms with a FeL fragment
(Scheme 3). The apical-basal bond angles of the capping
iron (bold in Scheme 3) are 99.9°, practically halfway (45%
(42) Whitener, M. A.; Bashkin, J. K.; Hagen, K. S.; Girerd, J.-J.; Gamp,
E.; Edelstein, N.; Holm, R. H. J. Am. Chem. Soc. 1986, 108, 5607-
5720.
(43) Zhang, Y.; Holm, R. H. Inorg. Chem. 2004, 43, 674-682.
(44) Noda, I.; Snyder, B. S.; Holm, R. H. Inorg. Chem. 1986, 25, 3851-
3853.
(45) Interestingly, the arrangement of the six belt Fe atoms in [Tp2Mo2Fe6S9
(SPh)4]3- and [Fe7S6Cl3(PEt3)4] are far from being a trigonal prism,
as revealed by their trigonal prismatic shape measures (5.0 and 21.7,
compared to 0.02 for the FeMoco).
Inorganic Chemistry, Vol. 46, No. 1, 2007 67

Vela et al.
Figure 11. Shape measures of iron atoms in diketiminate complexes
represented in the tetrahedron-vTBP shape map. The curves plotted
correspond to the umbrella distortion path of the tetrahedron (continuous
line) and of the chelated tetrahedron (dashed line), while the straight dotted
line corresponds to closing a basal N-Fe-N bond angle in a structure with
R ) 102°. Empty red squares, previously published structures (CSD); blue
circles, structures reported in this work; black triangles, iron(III) com-
pounds.23
Figure 10. Tetrahedron-vTBP shape map for iron atoms in synthetic
iron-sulfur clusters (circles) and diketiminate complexes (red squares). The
black circles correspond to the Fe atoms that appear closer to the position
of the belt Fe atoms in FeMoco (see Figure 8). The continuous line
represents the minimum distortion path between the tetrahedron and the
vTBP, the dashed curve corresponds to the related pyramidalization path
between a chelated tetrahedron and a chelated vTBP (with one ligand-
metal-ligand bond angle of 95°), and the dotted straight line corresponds
to closing one basal bond angle of an intermediate FeX4 shape with R )
102°.
angle asymmetry is larger in these complexes than in the
FeMoco belt iron atoms.
Using Shape Measures To Describe Iron â-Diketimi-
nate Complexes. The shape measures for previously reported
and current four-coordinate diketiminate-iron complexes are
shown in Figure 11. It is immediately evident that these
complexes offer a wide range of geometries, in which the
degree of pyramidalization R varies between 99° and 110°.
In spite of the geometrical variability, comparison with the
ideal vTBP shape within the continuous shape measures
approach allows us to unequivocally identify the atom that
corresponds to the axial position of the vTBP (Table 2).
Although the diketiminate usually lies in the basal plane,
some complexes approach a vTBP in which one of the
diketiminate N atoms would occupy an axial position. The
compounds that present a degree of pyramidalization com-
parable to those found for the belt Fe sites of FeMoco (101-
103°) are 2, a, b, e, f, and the Fe11 atom in 4.
Most of the diketiminate complexes, though, show strong
deviations from the umbrella path. Although one may suspect
such deviations to be due to the small chelate angles imposed
by the diketiminate ligand (89.8-99.1°), we did not find a
correlation between the bite angle and the path deviation
function. However, it is striking that the behavior of the Fe3+
and Fe2+ complexes is quite different: Figure 11 shows that
the iron(III) complexes (triangles) are substantially more
tetrahedral than the iron(II) complexes (squares and circles).
Let us first analyze the smaller deviations of the iron(III)
from the tetrahedron. In high-spin d5 complexes, we do not
expect electronically driven distortions of the tetrahedron.
Hence, it is not surprising to find that their deviations from
the ideal tetrahedron, gauged by the corresponding shape
measures, appear to increase with decreasing bite angles (see
Supporting Information). Moreover, it is seen that there is a
good negative correlation between the bite angle and the
opposed bond angle (γ, Scheme 2g). We conclude that the
Scheme 3
according to the generalized interconversion coordinate)
between the tetrahedral and vTBP reference shapes. It differs
from the Fe atoms in the FeMoco because it has crystallo-
graphically imposed trigonal symmetry, with three identical
S-Fe-S bond angles of 117°. The presence of trigonal
symmetry is reflected by its position on the minimal
distortion interconversion path of the shape map (Figure 10)
and the corresponding negligible value of the path deviation
function (0.51%).
Other iron atoms that deviate greatly from a tetrahedral
geometry appear at the upper left corner of the shape map.
These correspond to [Fe4MoS6Cl(PEt3)4] and [Fe4VS6Cl-
(PEt3)4],46 as well as to the lower Fe atoms (Fe2) in the
structure of [Fe7S6Cl3(PEt3)4] shown in Scheme 3.44 Those
Fe atoms are characterized by pyramidalization angles rather
close to vTBP (92.4°, 92.7°, and 94.7°, respectively) and by
a marked basal angle asymmetry (differences of 8-12°
between the smallest and the largest angles). The location
of these complexes in the shape map indicates that this basal
(46) Nordlander, E.; Lee, S. C.; Cen, W.; Wu, Z. Y.; Natoli, C. R.; Di
Cicco, A.; Filipponi, A.; Hedman, B.; Hodgson, K. O.; Holm, R. H.
J. Am. Chem. Soc. 1993, 115, 5549-5558.
68 Inorganic Chemistry, Vol. 46, No. 1, 2007

Trigonal Pyramidal Iron
Table 2. Stereochemical Parameters for Selected Iron(II) Diketiminato Complexes
compound
τ
axial
NCtBu
R
â
S(T)
S(vTBP)
S(chT)
S(chV)
∆
1
2
3
LMeFeiBu(NCtBu)
0.58
0.35
99.1
102.8
116.5
113.3
2.46
2.19
3.23
3.61
1.97
1.83
1.90
2.74
0.38
0.50
LMeFeCH2 Bu(NCtBu)
N(diketiminate)
t
LMeFeCl(tBupy)
(Fe12)
0.30
0.30
tBupy
tBupy
104.6
104.6
113.6
113.7
1.89
1.79
3.86
3.73
0.92
0.87
2.13
2.09
0.20
0.18
(Fe21)
4
LtBuFeCl(MeCN)
(Fe11)
0.40
0.25
0.40
0.45
0.43
0.62
0.34
0.45
0.50
0.13
0.03
MeCN
MeCN
O
102.6
105.8
103.6
102.0
102.5
99.0
104.2
102.2
101.0
107.7
109.3
114.9
113.4
115.8
115.6
115.3
117.6
114.4
115.8
116.1
111.6
110.3
1.65
2.10
7.75
1.38
1.32
2.24
1.27
1.37
1.10
1.39
2.23
2.92
4.10
8.98
3.01
3.19
2.83
3.32
2.95
2.60
4.35
4.83
1.21
1.11
4.35
0.75
0.68
1.43
0.61
0.74
0.86
0.46
0.78
1.89
2.52
6.34
1.91
2.12
1.21
2.34
2.08
2.13
3.46
4.24
0.23
0.31
1.20
0.11
0.13
0.14
0.15
0.14
0.18
0.26
0.46
(Fe22)
5
a
b
c
d
e
f
g
h
i
LtBuFeOTf
LMeFeF(CF3py)
LMeFeF(tBupy)
LMeFeNHDIPP(tBupy)
LtBuFeF(MeCN)
LtBuFeF(tBupy)
LtBuFeH(tBupy)
LtBuFe(OEt2)(FBF3)
LMeFeCl2Li(Et2O)2
LMeFeCl2Li(THF)2
(Fe11)
CF3py
tBupy
tBupy
NCMe
tBupy
tBupy
FBF3
N(diketiminate)
0.07
0.06
N(diketiminate)
N(diketiminate)
108.8
108.9
110.8
110.9
2.41
2.35
4.79
4.80
0.99
0.90
4.20
4.17
0.52
0.49
(Fe14)
j
(LMeFe)2(S)(H2NNPh)
(Fe1: Fe3+
(Fe2: Fe2+
)
)
0.19
0.19
N(diketiminate)
NH2NPh
106.7
106.8
112.4
112.5
1.51
2.32
3.70
4.63
0.40
0.98
3.32
2.48
0.22
0.27
k
l
(LMeFe)2(S)(Me2NNH2)2
(Fe1)
0.34
0.27
Me2N2H2
Me2N2H2
104.1
105.3
114.5
113.4
2.10
1.79
4.12
4.22
0.93
0.79
2.18
2.61
0.21
0.24
(Fe2)
(LMeFe)2(S)(MeNHNH2)2
(Fe1)
0.62
0.35
0.16
0.27
MeN2H3
MeN2H3
N(diketiminate)
N(diketiminate)
98.6
104.0
107.0
105.3
117.3
114.4
112.0
113.4
2.99
2.15
1.71
1.54
3.30
4.05
3.79
3.28
2.24
1.07
0.56
0.56
1.51
2.17
3.08
3.04
0.35
0.25
0.24
0.24
(Fe2)
m
n
(LMeFe)2(S)(MeCN)
[LMeFe(CH2CN)]2
a Pyramidality (R) and bite (â) angles, shape measures relative to the tetrahedron, the chelated tetrahedron and the chelated vTBP, as well as the path
deviation function (∆) relative to the interconversion between the chelated tetrahedron and the chelated vTBP (Scheme 2b and 2c). chT ) chelated tetrahedron,
chV ) chelated vacant trigonal bipyramid (bite angle of 95° in both cases); T ) tetrahedron; ∆ ) deviation from the chT-chV path
Fe3+ diketiminate complexes are chelated skew tetrahedra
(Scheme 2g), characterized by bond angles 95° e ω e 99°
and 120° g γ g 110°.
substituents at the ligands sterically disfavors the tetrahedral
geometry and a significant twist toward the planar square is
produced (torsion angles of 65° and 55°, respectively,
intermediate between the 90° of the ideal tetrahedron and
0° of the square), as shown by their position along the
interconversion path in Figure 5c (S(square) values of less
than 20).
Despite the clear presence of scissoring distortions in the
family of iron(II) diketiminate complexes, some relationship
seems to exist between S(vTBP) and the average pyramidality
angle R (figure provided as Supporting Information, regres-
sion coefficient r2 ) 0.75), indicating that the umbrella
distortion is also present in these compounds (Scheme 2c).
In order to better understand pyramidalization in these
compounds, we define a chelated tetrahedron and a chelated
VTBP by constraining one basal ligand-metal-basal ligand
angle to 95° (a typical bite angle for the iron diketiminates).
This allows the construction of shape maps showing the
interconversion of a chelated tetrahedron (Scheme 2b) and
a chelated vTBP (Scheme 2c, with R ) 90°). We characterize
the experimental structures by their deviations from the
High-spin d6 complexes are often nearly tetrahedral.39b
However, in this work we see that the iron(II) diketiminate
complexes are more distorted than their iron(III) analogues
and that the loss of tetrahedricity is not related to the
pyramidalization pathway (Figure 11). As expected for the
rather rigid bidentate ligand framework of diketiminates, the
bite angles of the iron(II) complexes vary in a narrow range
(89.5-98.5°). However, the opposing angle γ shows a much
wider variation (77-110°) than in the iron(III) complexes
(110-120°).23 Since tetrahedral high-spin d6 complexes have
3
an e3t2 configuration, they are Jahn-Teller unstable,47 and
simple symmetry analysis tells us that the umbrella mode
(belonging to the T2 symmetry representation) is ineffective
in splitting the e orbitals. In contrast, the scissoring (Scheme
2g: ω, γ < 109.5°) or plier (Scheme 2g: ω < 109.5° < γ)
distortion modes (of E symmetry) are expected to be
stabilizing, thus explaining the wide range of values found
for γ. The competing electronic demands of the axial and
basal sites of trigonal pyramidal iron(II) complexes have been
described.48,49 In a couple of cases,50,24 the presence of bulky
(51) We propose that the reference shapes and distortion paths should be
as symmetric as possible, especially because arbitrariness in the choice
of ideal shapes increases as the symmetry decreases. For instance,
the chelated umbrella, chelated sawhorse, and chelated twist paths have
been chosen with a bite angle of 95° to illustrate how the real structures
deviate from the ideal paths, but some specific structures would of
course fit better to alternative paths with different bite angles, and the
use of a large number of distortion paths “a la carte” would result in
the loss of simplicity and generality associated with shape and
symmetry measures.
(47) Vrajmasu, V. V.; Mu¨nck, E.; Bominaar, E. L. Inorg. Chem. 2004, 43,
4862-4866 and references therein.
(48) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365-374.
(49) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058-1076.
(50) Zhou, M.-S.; Huang, S.-P.; Weng, L.-H.; Sun, W.-H.; Liu, D.-S. J.
Organomet. Chem. 2003, 665, 237-245.
Inorganic Chemistry, Vol. 46, No. 1, 2007 69

Vela et al.
minimal distortion path (∆, Table 2) that converts a chelated
tetrahedron (with a N-Fe-N bond angle of 95°) into a
chelated vacant trigonal bipyramid while preserving C2V
symmetry.51
sense that they deviate from the tetrahedral structure toward
an axially vacant trigonal bipyramid. However, their stere-
ochemistry differs from that of the nitrogenase cofactor
because the bite angle of the diketiminato ligands is
significantly smaller than that of the capping MS2 groups in
the FeMoco. While iron(III) diketiminate complexes are
distorted from a tetrahedron only by the bite angle of the
chelating ligand, the iron(II) diketiminate complexes vary
over a wider range, in terms of both geometry and electron-
ics. The identification of larger electronic effects in iron(II)
than iron(III) systems implies that the iron-molybdenum
cofactor could be more distorted in the reduced states that
The results (Table 2) show that a number of the complexes
deviate substantially from the chelated pyramidalization path
(∆ > 0.3 for 1, 2, Fe22 in 4, 5, h, i, and Fe2 in l). The ones
that deviate most have either a chelating ligand (e.g., η2-
OTf and Cl2Li(solvent)n) or an extremely large ligand (e.g.,
iBu, neopentyl) that causes a lateral movement of the basal
ligand from the mirror plane. Structures 3, a, b, c, d, e, and
f are close to the chelated pyramidalization path (∆ e 0.2,
Table 2) and we can reasonably describe them as pyrami-
dalized chelated tetrahedra.
In summary, diketiminate-iron(III) complexes are dis-
torted from a tetrahedron mostly through (a) an umbrella
distortion (Scheme 2e) combined with (b) the restraint of
one angle by the chelating ligand (Scheme 2c). In the
corresponding iron(II) complexes, the structures found are
in general the result of the same distortions, supplemented
by (c) a Jahn-Teller-induced rocking distortion that tends
to preserve the C2V symmetry and (d) a sterically induced
twist of the non-diketiminate ligands that takes the geometry
toward the planar square.
52
bind N2 than in the crystallographically characterized MN
state, which is at a Fe2+4Fe3+3 oxidation level.12,13 It is hoped
that the stereoelectronic effects described in these studies
will assist in the continued design of synthetic complexes
that mimic the belt iron atoms of nitrogenase.
Experimental Section
Computations. Shape measures were calculated with the SHAPE
program (version 1.1: Llunell, M.; Casanova, D.; Cirera, J.; Bofill,
J. M.; Alemany, P.; Alvarez, S.; Pinsky, M.; Avnir, D., Barcelona,
2003).
Experimental Considerations. Syntheses and purifications were
performed under a nitrogen atmosphere by standard Schlenk
techniques or in an M. Braun Unilab N2-filled glove box maintained
at or below 1 ppm of O2 and H2O. Glassware was dried at 130 °C
Conclusions
1
overnight. H NMR data were recorded on a Bruker Avance 400
Comparison of biological metal sites with synthetic
analogues is assisted by quantitative shape measures that can
be used to objectively determine the coordination geometries
of the metal ions. In the particular case of the iron-
molybdenum cofactor of nitrogenase, the belt iron atoms are
especially problematic because they do not have a regular
coordination geometry. In this contribution, we analyze their
geometries using continuous shape measures, which give
greater geometric detail on the FeMoco belt sites, on the
iron sites of literature iron-sulfur clusters, and on the iron
sites in tetracoordinate diketiminate complexes.
Our data indicate that the geometry of each belt iron atom
of the FeMoco lies near the minimal distortion path between
a tetrahedron and a trigonal pyramid. They deviate from this
path because of a chelating MS2 unit in the basal plane, where
M is the capping Fe or Mo atom of the Fe7Mo cluster, and
because of the different lengths of the Fe-X and Fe-S
bonds. An interesting result of our analysis is that the belt
iron atoms at the Mo and Fe sides of the FeMoco trigonal
prism show distinct differences in their stereochemical
behavior. Few synthetic iron-sulfur clusters have iron atoms
with a geometry that lies in this region. A few are
pyramidalized, and one in particular, [Fe7S6Cl3(PEt3)4], is
identified that has an iron atom with a rather similar
geometry. However, unlike the set of belt iron atoms in the
FeMoco, which present a practically perfect trigonal pris-
matic arrangement, the inner Fe6 core of this and other
polynuclear complexes topologically related to the FeMoco
active site are far from that geometry.
MHz spectrometer at the specified temperature. 1H shifts are
reported in ppm (relative to residual C6HD5 at 7.13 ppm), and
relative integrations of peaks and assignments are given. Solution
magnetic susceptibilities were determined at 294 K by the Evans
method.35 Microanalyses were performed by Desert Analytics
(Tucson, AZ). Electronic spectra were recorded between 400 and
1100 nm with a Cary 50Bio UV-visible spectrophotometer, using
quartz cuvettes of 1 cm optical path length. Pentane, diethyl ether,
tetrahydrofuran (THF), toluene, and acetonitrile were purified by
passage through activated alumina and “deoxygenizer” columns
from Glass Contour Co. (Laguna Beach, CA). Deuterated benzene
was dried over CaH2, then over Na, and then vacuum-distilled into
a storage container. 4-tert-Butylpyridine and trimethylacetonitrile
were degassed and dried over activated molecular sieves or vacuum
distilled prior to use. Compounds LMeFe(µ-Cl)2Li(THF)2, LtBuFeCl,
t
LMeFe-iBu, LMeFeCH2 Bu, LtBuLi(THF),53 and [LtBuFeH]2 were
prepared by known procedures.23
LMeFeiBu(tBuCN) (1). Trimethylacetonitrile (52 µL, 471 µmol)
was added to a solution of LMeFeiBu (100 mg, 188 µmol) in diethyl
ether (4 mL). This solution was filtered through Celite and cooled
to -38 °C, after which yellow crystals were obtained (71 mg, 62%).
Anal. Found (Calcd) C, 73.94 (74.37), H, 9.13 (9.69), N, 7.02 (6.85).
µeff(C6D6, 21 °C) ) 5.1(5) µB. 1H NMR (C6D6, 21 °C): 81.4 (6H,
(CH3)2-iBu), 70.0 (1H, R-CH), 22.0 (6H, (CH3)2-L), 1.2 (9H, (CH3)3-
CN), -2.2 (4H, m-CH), -11.7 (12H, iPr-CH3), -62.5 (2H, p-CH),
-80.4 (12H, iPr-CH3), -96.0 (4H, iPr-CH). Vis (toluene/
trimethylacetonitrile, 10:1 v/v): 436 (970 M-1 cm-1), 487 (530
M-1 cm-1), 1089 nm (150 M-1 cm-1).
t
LMeFeCH2 Bu(tBuCN) (2). Prepared by a similar procedure to
t
1 from trimethylacetonitrile (49 µL, 445 µmol) and LMeFeCH2 Bu
(52) Kinetic evidence indicates that the cofactor must be reduced by at
least three electrons before binding of N2. For details, see ref 1.
(53) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485-1494.
The mononuclear iron diketiminate complexes reported
here and those previously published approach the coordina-
tion geometry of the inner Fe atoms in the FeMoco in the
70 Inorganic Chemistry, Vol. 46, No. 1, 2007

Trigonal Pyramidal Iron
(97 mg, 178 µmol). Yellow crystals: 61 mg, 55%. Anal. Found
(Calcd) C, 74.19 (74.62), H, 9.54 (9.79), N, 6.43 (6.14). µeff(C6D6,
p-CH), -92.5 (12, iPr-CH3), -113.1 (4, iPr-CH). Vis (toluene):
565 nm (410 M-1 cm-1).
1
21 °C) ) 4.9(5) µB. H NMR (C6D6, 21 °C): 88.9 (9H, (CH3)3-t-
X-ray Structures. Crystalline samples were grown in the glove
box from pentane or ether solutions at -38 °C. Each sample was
rapidly mounted under Paratone-8277 onto a glass fiber and
immediately placed in a cold nitrogen stream at -80 °C on the
X-ray diffractometer. X-ray intensity data were collected on a
standard Bruker-axs SMART CCD area detector system equipped
with a normal focus molybdenum-target X-ray tube operated at 2.0
kW (50 kV, 40 mA). A total of 2424 frames of data were collected
using a narrow frame method with scan widths of 0.3° in ω. Frames
were integrated to a maximum 2θ angle of 56.6° with SAINT. The
final unit cell parameters (at -80 °C) were determined from the
least-squares refinement of three-dimensional centroids of >4000
reflections for each crystal. Data were corrected for absorption with
the SADABS54 program. The space groups were assigned using
XPREP, and the structures were solved by direct methods and
refined employing full-matrix least-squares on F2 (Bruker-AXS,
SHELXTL-NT,55 version 5.10). All non-H atoms were refined with
anisotropic thermal parameters, and hydrogen atoms were refined
with riding isotropic thermal parameters. The structures refined to
goodness of fit values and final residuals found in the Supporting
Information.
Bu), 54.4 (1H, R-CH), 13.8 (6H, (CH3)2-L), 1.2 (9H, (CH3)3CN),
i
-3.8 (4H, m-CH), -11.2 (12H, Pr-CH3), -59.0 (2H, p-CH),
-63.6 (12H, iPr-CH3), -97.2 (4H, iPr-CH). Vis (toluene/
trimethylacetonitrile, 10:1 v/v): 433 (810 M-1 cm-1), 486 (470
M-1 cm-1), 1094 nm (110 M-1 cm-1).
LMeFeCl(4-tBu-py) (3). To a solution of LMeFeCl2Li(THF)2 (112
mg, 158 µmol) in Et2O (6 mL) was added 4-tert-butylpyridine (25
µL, 160 µmol). After being stirred at room temperature for 10 min,
the soluble fraction was filtered through Celite. This solution was
cooled to -38 °C, and light orange crystals were obtained: 82 mg,
83% (in three crops). Anal. Found (Calcd) C, 71.06 (71.65), H,
8.36 (8.22), N, 6.39 (6.35). µeff(C6D6, 21 °C) ) 5.3(5) µB. 1H NMR
(C6D6, 21 °C): 41.5, 22.0, 19.2, 10.0, 1.1, -2.4, -3.9, -9.3, -12.0,
-28.6, -78.1, -85.7. Vis (toluene): 420 (2890 M-1 cm-1), 11006
nm (230 M-1 cm-1).
LtBuFeCl(CH3CN) (4). To a solution of LtBuFeCl (214 mg, 360
µmol) in Et2O (6 mL) was added acetonitrile (50 µL, 950 µmol).
The solution was then filtered through Celite and cooled to -38
°C, and yellow crystals were obtained: 121 mg, 53% (single crop).
Several attempts to obtain elemental analysis of this compound were
unsuccessful, possibly because the dry solid loses acetonitrile: upon
standing for 2 h at room temperature, yellow crystals of 4 became
red colored as in LtBuFeCl. µeff(C6D6, 21 °C) ) 5.6(5) µB. 1H NMR
Acknowledgment. Funding for this work has been
provided by the U.S. National Institutes of Health (GM-
65313 to P.L.H.), the A. P. Sloan Foundation (Research
Fellowship to P.L.H.), the University of Rochester (Hooker
and Messersmith fellowships to J.V.), the Direccio´n General
de Investigacio´n (MEC project CTQ2005-08123-C02-
02BQU to S.A.), and Comissio´ Interdepartamental de Cie`ncia
i Tecnologia (CIRIT, grant 2005SGR-00036 to S.A.). J.C.
thanks MEC for an FPU grant (reference AP2002-2236).
i
(C6D6, 21 °C): 23.8 (12H, Pr-CH3), 9.4 (4H, m-CH), 1.8 (3H,
i
CH3CN), -8.6 (12H, Pr-CH3), -52.3 (18H, (CH3)3-L), -96.8
(4H, iPr-CH). Vis (toluene/acetonitrile, 1:1 v/v): 445 (1200 M-1
cm-1), 1142 nm (180 M-1 cm-1).
LtBuFe(η2-O3SCF3) (5). Lutidinium triflate (5.2 mg, 4.7 µmol),
[LtBuFeH]2 (11.4 mg, 10.2 µmol) and C6D6 (0.5 mL) were placed
in a resealable J. Young NMR tube. Immediate bubbling ensued
upon mixing, and after 2-5 min at room temperature, a bright red
solution was evident. Complete conversion to 3 was observed
(100% yield by 1H NMR). When this reaction was repeated starting
from 231 mg of [LtBuFeH]2 in diethyl ether (5 mL), 68 mg (23%
yield) of 6 as red crystals were obtained after cooling the resulting
solution to -38 °C for 2 days (one crop was collected). Anal. Found
(Calcd) C, 60.56 (61.18), H, 7.80 (7.56), N, 4.53 (3.96). µeff(C6D6,
21 °C) ) 5.8(5) µB. 1H NMR (C6D6, 21 °C): 42.4 (1, R-CH), 35.9
(18, (CH3)3C-L), 13.9 (4, m-CH), -21.3 (12, iPr-CH3), -27.9 (2,
Supporting Information Available: Graphs of correlations and
crystallographic details (PDF, CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
IC0609148
(54) The SADABS program is based on the method of Blessing: see
Blessing, R. H. Acta Crystallogr. A 1995, 51, 33-38.
(55) SHELXTL NT: Structure Analysis Program, version 5.10; Bruker-
AXS: Madison, WI, 1995.
Inorganic Chemistry, Vol. 46, No. 1, 2007 71