A Sulfido-Bridged Diiron(II) Compound and Its Reactions with Nitrogenase-Relevant Substrates

Article abstract of DOI:10.1021/ja049417l

The active site iron?molybdenum cofactor of nitrogenase has sulfide-bridged pairs of redox-active, trigonal pyramidal iron atoms that are postulated to be the site of N₂ transformation. A synthetic compound is described in which two three-coordinate iron(II) ions are bridged similarly by sulfide. The compound binds nitrogen donors to become trigonal pyramidal and cleaves the N?N bond of phenylhydrazine with oxidation of iron(II) to iron(III). Copyright

Full text of DOI:10.1021/ja049417l

Published on Web 03/20/2004
A Sulfido-Bridged Diiron(II) Compound and Its Reactions with
Nitrogenase-Relevant Substrates
Javier Vela,† Sebastian Stoian,‡ Christine J. Flaschenriem,† Eckard Mu¨nck,*,‡ and
Patrick L. Holland*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Department of Chemistry,
Carnegie Mellon UniVersity, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213
Received February 2, 2004; E-mail: holland@chem.rochester.edu
The iron-molybdenum cofactor (FeMoco, Figure 1a) is the site
at which nitrogenase reduces N2, small nitriles, isocyanides, and
alkynes.1 Recent X-ray studies suggest the presence of a central
atom X (X ) C, N, or O) in the center of the FeMoco;2,3 six “belt”
iron atoms have trigonal pyramidal geometries (see below) with X
in the axial positions. These symmetrically disposed iron atoms
are implicated in reactivity by ab initio calculations,3 the spectros-
copy and activity of mutants,4 and the comparable activity of iron-
only nitrogenases.1,5 Because X does not interact strongly with the
Figure 1. (a) FeMoco of nitrogenase.2 (b) Molecular structure of 1 (50%
“belt” iron atoms,6,7 and X is hypervalent, it is likely that Fe-X
ellipsoids, H atoms omitted). The sulfide bridge is shown in one of the two
half-occupancy, symmetry-equivalent bridging positions. See the Supporting
bonds are disrupted during catalysis.3e Three-coordinate iron
complexes therefore mimic possible reactive forms of the FeMoco
with broken Fe-X bonds. However, synthetic iron-sulfur clusters
containing three-coordinate iron are not known.8 Very few com-
plexes with three-coordinate iron have sulfur ligands, and their
reactivity has not been reported.9 Here, we report a low-coordinate,
sulfide-bridged diiron complex that resembles a fragment of the
FeMoco belt and describe its reactivity toward nitrogenase-relevant
substrates.
Information for structural details.
Addition of sulfur to [(LFe)2(µ-N2)]10 (L ) [HC(CMeN[2,6-
diisopropylphenyl])2]-) in diethyl ether gave the dinuclear diiron-
(II) sulfide LFeSFeL (1) (Figure 1b) in 82% yield. The crystal
structure of 1 reveals that the molecule is highly bent at the bridging
atom (Fe-S-Fe ) 101.70(7)°). The FeSFe core has Fe-S bond
distances of 2.250(2) and 2.346(2) Å. Iron atoms bridged by a single
sulfide are known only in a few FeS clusters and in five-coordinate
species.11,12 The 4.2 K zero-field Mo¨ssbauer spectrum of 1 in
benzene exhibits a single quadrupole doublet with ∆EQ ) 0.58
mm/s and isomer shift δ ) 0.86 mm/s; spectra recorded in applied
fields up to 7.0 T show that the two Fe(II) ions are antiferromag-
netically coupled to give a diamagnetic ground state (for details
on X-ray structures and Mo¨ssbauer spectra of 1, see the Supporting
Information).13
Figure 2. (a) Job plot for CH3CN binding to 1 in C6D6. (b) Molecular
structure of 1‚CH3CN (50% ellipsoids, H and aryl groups omitted).
Compound 1 is stable under dry nitrogen, but forms adducts with
nitrogen donors. In C6D6 solution, acetonitrile binds to 1 (Kassoc
)
340 ( 60 M-1), with rapid exchange on the NMR time scale. The
binding is 1:1, as shown by an NMR titration (Figure 2a) and the
crystal structure of the adduct (1‚CH3CN). Compound 1‚CH3CN
(Figure 2b) contains three- and four-coordinate iron atoms in close
proximity (Fe-Fe ) 3.8365(9) Å). The three-coordinate iron in
1‚CH3CN does not interact with the nitrile triple bond. Methyl-
hydrazine and 1,1-dimethylhydrazine bind end-on to the sulfide in
a 2:1 ratio (Figures 3a and S-2). Hydrazine-bound species have
been postulated as part of the catalytic cycle of nitrogenase.14
Ammonia binds to the sulfide to form 1‚2NH3, with a structure
like that of the other 2:1 adducts (Figure S-2).
Figure 3. Molecular structures of (a) 1‚2Me2NNH2 and (b) 2 (50%
ellipsoids, C-bound H atoms and aryl groups omitted). (c) X-band EPR
1
spectrum (toluene, 9 K, 9.622 GHz) of 2, with S )
/ simulation. (d)
2
Mo¨ssbauer spectrum of 2 (solid, 140 K), with simulation for two doublets
(representing 86% of the total Fe) using parameters quoted in the text. The
bracket indicates the doublet for the ferric site, and the shoulder at +0.8
mm/s belongs to a contaminant (14%).
In constrast, treatment of 1 with phenylhydrazine gave the mixed-
valence FeIIFeIII complex 2, which was formed in ∼90% yield and
isolated in 41% yield. Its crystal structure (Figure 3b) shows that
2 contains a formally anionic, bridging phenylhydrazido ligand.
† University of Rochester.
‡ Carnegie Mellon University.
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J. AM. CHEM. SOC. 2004, 126, 4522-4523
10.1021/ja049417l CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Geometric Parameters
compound Fe‚‚‚Fe (Å)
N-N bonds, it shows great promise for mimicking key features of
biological nitrogen fixation.
Fe−S−Fe (deg)
τa
0.16
1
3.5644(9)
3.8365(9)
3.4498(9)
101.70(7)
119.25(10)
101.50(6)
Acknowledgment. The authors are grateful for funding from
the University of Rochester and the A.P. Sloan Foundation (P.L.H.),
a Hooker Fellowship (J.V.), and the National Science Foundation
(E.M.). We thank Prof. Joe Dinnocenzo for the use of a GC
instrument.
1‚CH3CN
2
0.19 FeIII
0.10 FeII
0.31
1‚2Me2NNH2
1‚2NH3
4.1864(9)
3.9782(7)
4.1158(10)
2.60(2)
141.91(5)
126.56(5)
136.53(8)
71.2(4)
0.33
0.48
0.46
0.8-1.0
1.18
1‚2MeNHNH2
FeMoco2
Supporting Information Available: Synthetic, characterization,
structural, binding, and spectroscopic data (PDF and CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
basket clusters11
[(iPrNCOCH2)3N)Fe]- 17
2.6-2.8
74-76
a τ ) [∑( Lbasal-Fe-Lbasal) - ∑( Lbasal-Fe-Laxial)]/90. See the
Supporting Information for details.
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mm/s) sites (Figure 3c). Studies in applied fields up to 7.0 T show
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ferric and ferrous sites are negative and positive, respectively,
consistent with antiferromagnetic coupling.15
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of phenylhydrazine, and aniline (GC/MS) and ammonia (up to 0.5
equiv detected by the indophenol method) are produced. A balanced
equation for the formation of 2 consistent with these observations
is shown in eq 1.
2(1) + 3PhNHNH2 f 2(2) + PhNH2 + NH3
(1)
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ground state has a small paramagnetic admixture, indicated by a positive
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1.5 and 140 K. Mo¨ssbauer studies of mononuclear Fe(II) diketiminate
complexes by Andres et al. have revealed essentially unquenched orbital
Because the key “belt” sites of the FeMoco appear to be distorted
from tetrahedral toward a trigonal pyramidal17 geometry, it is of
interest to quantify pyramidal distortions of tetrahedral iron(II) in
these synthetic compounds. In Table 1, we introduce a normalized
measure of pyramidalization τ, which describes the distortion of
tetrahedral coordination along a pseudo-C3 axis (τ ) 0 for
tetrahedron, τ ) 1 for trigonal pyramid with flat base). As expected,
the FeMoco belt iron atoms are fairly pyramidalized (τ ) 0.46 (
0.03). The iron sites in FeS clusters are tetrahedral (τ e 0.3), except
for the “basket” clusters and close relatives (see the Supporting
Information).11 The iron atoms in 1‚CH3CN and 2 are nearly
tetrahedral (τ ) 0.10-0.19), but the alkylhydrazine and ammonia
adducts of 1 are substantially pyramidalized (τ ) 0.31-0.48).
Thus, the belt iron atoms of FeMoco have significant pyramidal
distortion, and this work shows that comparable distortions can be
achieved in synthetic complexes without the aid of constraining
ligands. The adducts of 1 have τ values that do not correlate with
ligand size, suggesting that pyramidalization may be electronically
controlled. The diketiminate-supported FeSFe core is quite flexible,
with Fe‚‚‚Fe distances ranging from 3.45 to 4.19 Å and Fe-S-Fe
angles varying from 101.5° to 141.9°. Theoretical studies indicate
that diketiminate ligands have “soft” electronic properties commonly
associated with sulfide/thiolate ligands.13,18 Because the synthetic
sulfide complex described here binds nitrogen donors and cleaves
angular momentum and very large zero-field splittings, |D| > 100 cm-1
.
A realistic spin coupling model will have to take these unusual features
into account. See: Andres, H.; Bominaar, E.; Smith, J. M.; Eckert, N.
A.; Holland, P. L.; Mu¨nck, E. J. Am. Chem. Soc. 2002, 124, 3012.
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