Synthesis, structure, and spectroscopy of an oxodiiron(II) complex

Article abstract of DOI:10.1021/ja0436704

Bridging oxo species are important in the synthetic and biological chemistry of iron, and are found with iron oxidation states from +2 to +4. We report the first oxodiiron(II) complex that has been crystallographically characterized. It has been examined

Full text of DOI:10.1021/ja0436704

Published on Web 06/09/2005
Synthesis, Structure, and Spectroscopy of an Oxodiiron(II) Complex
Nathan A. Eckert,† Sebastian Stoian,‡ Jeremy M. Smith,† Emile L. Bominaar,*,‡ Eckard Mu¨nck,*,‡ and
Patrick L. Holland*,†
Departments of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Carnegie Mellon UniVersity,
Pittsburgh, PennsylVania 15213
Received October 18, 2004; Revised Manuscript Received May 23, 2005; E-mail: holland@chem.rochester.edu
Diiron units with bridging oxygen ligands (i.e. O2-, OH-, or
H2O) are present in metalloenzymes such as methane monooxy-
genase, ∆9-desaturase, and ribonucleotide reductase.1 In these
enzymes, the Fe2 unit is bridged by one or two oxygen-based
ligands.1 As these enzymes activate O2 and transfer oxygen atoms
to various substrates, the iron centers pass through several oxidation
states from FeIIFeII to FeIVFeIV.1
To better understand the mechanisms of these enzymes, chemists
have synthesized oxo-bridged diiron complexes in a variety of
oxidation states.2,3 Oxodiiron(III) complexes are very common,
Figure 1. Thermal ellipsoid plot of [LtBuFe]2O‚OEt2 (50% probability
ellipsoids), viewed along the C2 axis. Hydrogen atoms and unassociated
diethyl ether are omitted for clarity. Relevant distances (Å) and angles
(deg): Fe-O 1.7503(4), Fe-N1 2.005(2), Fe-N2 1.966(2), Fe-O-Fe
167.55(14), N1-Fe-N2 98.22(7).
while their reduced analogues (i.e. FeIIFeIII or FeIIFeII) are rare.2
For example, there are only two isolable and fully characterized
FeIIFeIII oxo compounds.4 Others have been inferred from electro-
chemical studies.5 There is no published oxodiiron(II) compound
that is stable under ambient conditions.6 Here we report the
synthesis, structure, and spectroscopic properties of [LtBuFe]2O (LtBu
) ArNC(tBu)CHC(tBu)Ar-, where Ar ) 2,6-diisopropylphenyl),
an isolable oxo-bridged diferrous compound. Although the trigonal
planar coordination of the iron atoms in this compound is different
from the geometries in iron enzymes,1 the ability to stabilize this
unusual oxidation level is notable because it demonstrates a new
direction in oxodiiron chemistry.
Solid-state FTIR spectra of [LtBuFe]2O show an intense band at
868 cm-1 that shifts to 833 cm-1 when H218O is used to prepare
the oxo complex (Figure 2). The 35 cm-1 shift is consistent with
that expected for an Fe-O harmonic oscillator. We assign this peak
to the asymmetric stretching vibration of the Fe-O-Fe core, on
the basis of the correlation between Fe-O-Fe stretching frequen-
cies and Fe-O-Fe bond angles in oxodiiron(III) complexes.14 No
isotope-sensitive bands are observed in solution resonance Raman
spectra, despite excitation into electronic absorptions at 380 and
560 nm.
The zero-field Mo¨ssbauer spectrum of solid [LtBuFe]2O at 4.2 K
(Figure 3A) exhibits a quadrupole doublet with ∆EQ ) 1.42(2)
mm/s and δ ) 0.64(2) mm/s (vs Fe metal at 298 K). In frozen
toluene solution we observe ∆EQ ) 1.44(2) mm/s and δ ) 0.79(2)
mm/s.15 Given the low coordination numbers, the iron(II) sites are
expected to be high-spin, like the three-coordinate Fe(II) diketimi-
nate complexes reported previously.16
7
The reaction of equimolar amounts of [LtBuFeH]2 and H2O in
THF gives orange-red [LtBuFe]2O in 71% yield.8 The same com-
pound can also be accessed in 53% yield by adding 1 equiv of
Me3NO to LtBuFeNNFeLtBu.9 The solid-state structure of [LtBuFe]2O
is shown in Figure 1.8 There is a crystallographically imposed C2
axis that renders the two LtBuFe units equivalent. The molecule is
planar at iron (sum of bond angles ) 359.4(1)°), as found in other
three-coordinate iron diketiminate compounds.7,9,10 The Fe-O-Fe
angle of 167.55(14)° is only slightly bent, and the two diketiminate
planes are nearly perpendicular (dihedral angle of 70.09(4)°).
The Fe-O distance of 1.7503(4) Å is difficult to put into context
because the only iron(II) compounds with oxo ligands are part of
mixed-valence dimers that do not have localized valence on the
crystallographic time scale.4 However, there are many examples
of crystallographically characterized oxodiiron(III) compounds (Fe-
O, av 1.774 Å, std dev 0.029 Å).11 Because the ionic radius of FeII
(0.77 Å) is substantially larger than that of FeIII (0.63 Å),12 it is
noteworthy that the two Fe-O bonds in [LtBuFe]2O are as short as
those observed for Fe(III)-oxo bridges. They are substantially
shorter than the Fe-O distances in iron(II) hydroxides.4b,c,13 Density-
functional calculations show that reduction from the hypothetical
diiron(III) complex [LtBuFe]2O2+ to diiron(II) places two electrons
2
in dz orbitals, where the z axes are oriented normal to the
diketiminate planes. As these orbitals have nonbonding character
with respect to the ligands, the Fe-O distances calculated for the
diiron(II) species (Fe-O ) 1.76 Å) are very close to those
calculated for diiron(III) (Fe-O ) 1.77 Å).
Figure 2. Infrared spectra of KBr pellets of [LtBuFe]2O and its 18O iso-
topomer. Vertical scaling of the subtraction has been increased for clarity.
† University of Rochester.
‡ Carnegie Mellon University.
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J. AM. CHEM. SOC. 2005, 127, 9344-9345
10.1021/ja0436704 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: Synthetic, spectroscopic, and
crystallographic data, and computational details (PDF, CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625. (b) Solomon,
E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S. K.; Lehnert,
N.; Neese, F.; Skulan, A. J.; Yang, Y. S.; Zhou, J. Chem. ReV. 2000, 100,
235.
(2) (a) Kurtz, D. M. Chem. ReV. 1990, 90, 585 and references therein. (b)
Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987.
(3) Reviews and recent examples: (a) Fontecave, M.; Me´nage, S.; Duboc-
Toia, C. Coord. Chem. ReV. 1998, 178-180, 1555. (b) Du Bois, J.;
Mizoguchi, T. J.; Lippard, S. J. Coord. Chem. ReV. 2000, 200-202, 443.
(c) Costas, M.; Chen, K. C.; Que, L. Coord. Chem. ReV. 2000, 200-202,
517. (d) Liu, C.; Yu, S.; Li, D.; Liao, Z.; Sun, X.; Xu, H. Inorg. Chem.
2002, 41, 913. (e) Tshuva, E. Y.; Lee, D.; Bu, W.; Lippard, S. J. J. Am.
Chem. Soc. 2002, 124, 2416. (f) Tolman, W. B.; Que, L. J. Chem. Soc.,
Dalton Trans. 2002, 5, 653. (g) Raffard-Pons y Moll, N.; Banse, F.; Miki,
K.; Nierlich, M.; Girerd, J. Eur. J. Inorg. Chem. 2002, 8, 1941. (h) Marlin,
D. S.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2003, 42, 1681.
(4) (a) Arena, F.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc.,
Chem. Commun. 1986, 1369. (b) Cohen, J. D.; Payne, S. C.; Hagen, K.
C.; Sanders-Loehr, J. J. Am. Chem. Soc. 1997, 119, 2960. (c) Payne, S.
C.; Hagen, K. S. J. Am. Chem. Soc. 2000, 122, 6399.
(5) (a) Hartman, J. R.; Rardin, R. L.; Chaudhuri, P.; Pohl, K.; Wieghardt, K.;
Nuber, B.; Weiss, J.; Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S. J.
J. Am. Chem. Soc. 1987, 109, 7387. (b) Feig, A. L.; Masschelein, A.;
Bakac, A.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 334. (c) Davydov,
R. M.; Me´nage, S.; Fontecave, M.; Gra¨slund; Ehrenberg, A. J. Biol. Inorg.
Chem. 1997, 2, 242. (d) Musie, G.; Lai, C.; Reibenspies, J. H.; Sumner,
L. W.; Darensbourg, M. Y. Inorg. Chem. 1998, 37, 4086. (e) Kiani, S.;
Tapper, A.; Staples, R. J.; Stavropoulos, P. J. Am. Chem. Soc. 2000, 122,
7503.
Figure 3. Mo¨ssbauer spectra of solid [LtBuFe]2O at 4.2 K, recorded in
zero field (A) and a parallel applied field of 4.0 T (B). The solid line in (B)
is a computer simulation for ∆EQ ) (eQVzz/12)[(1 + (η2/3)]1/2 ) + 1.42
mm/s, η ) 0, δ ) 0.64 mm/s, and Beff,x ) (1 + Rx)B for Rx ) 0.2; details
are given in the Supporting Information. Absorption from a contaminant
with broad, unresolved features (ca. 20%) is indicated by arrows.17
We have recorded Mo¨ssbauer spectra for two solid samples and
for a toluene sample at temperatures from 4.2 to 150 K and applied
fields, B, up to 7.0 T. The applied-field spectra have shapes very
similar to those observed for diamagnetic compounds.18 However,
at all temperatures and for all fields, in the solid as well as in the
frozen solution, the magnetic splitting of the low-energy features
is slightly larger than accounted for by the applied field (see Figure
S-1, Supporting Information). The applied-field spectra yield ∆EQ
> 0 and a small asymmetry parameter, 0 e η e 0.4. Within the
resolution, the splitting of the low-field features can be modeled
by adding a positiVe magnetic hyperfine field, Bint, perpendicular
to the largest component of the electric field gradient, Vzz. Our
simulations reveal that Bint is proportional to the applied field, Bint
) RB, but independent of temperature up to 150 K. The simulation
in Figure 3B assumes that the effective field along x is given by
Beff,x ) (1 + Rx)B, with Rx ≈ +0.2 and Ry ) Rz ) 0 (see Supporting
Information for details). The observation of a positive and tem-
perature-independent Rx suggests substantial unquenched orbital
angular momentum, as observed by us in other three-coordinate
iron(II) complexes.16
Room-temperature 1H NMR spectra of [LtBuFe]2O in C6D6 exhibit
only seven paramagnetically shifted resonances, with relative
intensities as expected for the coordinated diketiminate ligands.19
This shows that the diketiminate ligands are equivalent on the NMR
time scale at room temperature, and the molecule has averaged D2h
or D2d symmetry. The observation of equivalent Fe sites agrees
with the single high-spin iron(II) environment seen by Mo¨ssbauer
spectroscopy in frozen toluene solution.
(6) In one case, electrochemical experiments imply transient stability of an
oxoiiron(II) compound. See ref 5d.
(7) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003,
125, 15752.
(8) Synthetic and crystallographic details are in the Supporting Information.
(9) 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.
(10) (a) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001,
1542. (b) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Organometallics
2002, 21, 4808. (c) Sciarone, T. J. J.; Meetsma, A.; Hessen, B.; Teuben,
J. H. Chem. Commun. 2002, 1580. (d) Vela, J.; Smith, J. M.; Lachicotte,
R. J.; Holland, P. L. Chem. Commun. 2002, 2886. (e) Gibson, V. C.;
Marshall, E. L.; Navarro-Llobet, D.; White, A. J. P.; Williams, D. J. Dalton
Trans. 2002, 23, 4321. (f) Panda, A.; Stender, M.; Wright, R. J.; Olmstead,
M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909. (g) Eckert,
N. A.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Inorg. Chem. 2004,
43, 3306. (h) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck, E.; Holland,
P. L. J. Am. Chem. Soc. 2004, 126, 4522.
(11) Allen, F. H. Acta Crystallogr. 2002, B58, 380. A search of the Cambridge
Structural Database provided 194 examples of crystallographically
characterized compounds containing an unsupported Fe-O-Fe unit (i.e.
no other bridging ligands). Only diiron compounds were included, and
all the compounds found in the search were diiron(III). Histograms of
the search results are included in the Supporting Information.
(12) Values given are for four-coordinate iron, the lowest coordination number
for which values are available. Shannon, R. D. Acta Crystallogr. 1976,
A32, 751.
(13) (a) Kitajima, N.; Tamura, N.; Tanaka, M.; Moro-oka, Y. Inorg. Chem.
1992, 31, 3342. (b) Lachicotte, R. J.; Kitaygorodskiy, A.; Hagen, K. S. J.
Am. Chem. Soc. 1993, 115, 8883. (c) Lee, D.; Pierce, B.; Krebs, C.;
Hendrich, M. P.; Huynh, B. H.; Lippard, S. J. J. Am. Chem. Soc. 2002,
124, 3993. (d) Stubna, A.; Jo, D.-H.; Costas, M.; Brenessel, W. W.;
Andres, H.; Bominaar, E. L.; Mu¨nck, E.; Que, L. Inorg. Chem. 2004, 43,
3067.
(14) Sanders-Loehr, J.; Wheeler, W. D.; Shiemke, A. K.; Averill, B. A.; Loehr,
T. M. J. Am. Chem. Soc. 1989, 111, 8084.
(15) This surprising difference in isomer shift (δ) implies that 1 has a different
In conclusion, we have synthesized and characterized a stable
oxodiiron(II) complex. It contains two identical high-spin, three-
coordinate Fe(II) centers supported by bulky diketiminate ligands.
Further studies on the reactivity, electronic structure, and magnetic
properties of this interesting complex are underway.
conformation in solution. See Supporting Information for details.
(16) Andres, H.; Bominaar, E. L.; Smith, J. M.; Eckert, N. A.; Holland, P. L.;
Mu¨nck, E. J. Am. Chem. Soc. 2002, 124, 3012.
(17) Our samples are prepared in a glovebox, and the sample holders are sealed
with a shrink fit cap (Delrin over brass). However, the intensity of the
contaminant signals increases over weeks during storage in liquid N2.
(18) Antiferromagnetic coupling of the ferrous ions in [LtBuFe]2O is supported
by density functional theory calculations (see Supporting Information)
that indicate an exchange coupling J ≈ 200-250 cm-1 (with H ) JS1‚
S2), in accord with a nearly linear Fe-O-Fe superexchange pathway.
Acknowledgment. The authors thank the National Science
Foundation (CHE-0134658, P.L.H.; MCB-0424494, E.M.), the
University of Rochester (Weissberger Memorial Fellowship, N.A.E.),
and the A. P. Sloan Foundation (P.L.H.) for funding, and Profs. K.
R. Rodgers and G. Lukat-Rodgers for resonance Raman measure-
ments.
(19) The two methyl groups of the ligand isopropyl groups are inequivalent
due to hindered rotation of the aryl moiety. See ref 10f and Supporting
Information.
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