A low-spin d5 iron imide: Nitrene capture by low-coordinate iron(I) provides the 4-coordinate Fe(III) complex [PhB(CH2PPh2)3]Fe≡N-p-tolyl

Article abstract of DOI:10.1021/ja028448i

Entry into [PhBP₃]Fe chemistry affords a rare, pseudotetrahedral iron(I) complex, [PhBP₃]Fe(PPh₃), with an S = ₃/₂ ground state. This precursor undergoes rapid oxidation by aryl azide to produce the d⁵ imide [PhBP₃]FeNAr (Ar=p-tolyl). The Fe (III) imide is significant in that it is low-spin and represents the first mononuclear imide of iron. Doublet [PhBP ₃]FeN≡Ar reacts rapidly and quantitatively with CO at room temperature to release isocyanate and [PhBP₃] Fe(CO)₂. The [PhBP₃]Fe(CO)₂ byproduct is also precursor to [PhBP₃]FeN≡Ar upon addition of aryl azide. Copyright

Full text of DOI:10.1021/ja028448i

Published on Web 12/13/2002
A Low-Spin d5 Iron Imide: Nitrene Capture by Low-Coordinate Iron(I)
Provides the 4-Coordinate Fe(III) Complex [PhB(CH2PPh2)3]FetN-p-tolyl
Steven D. Brown, Theodore A. Betley, and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received September 6, 2002 ; E-mail: jpeters@caltech.edu
While metalloenzymes such as nitrogenases and hydrogenases
are far from mechanistically well understood, it seems plausible
that low-valent, sulfur-ligated Fe(II) and Fe(I) species play a critical
role in their small-molecule reduction chemistry.1 The careful
preparation and study of the redox reactivity of low-coordinate Fe-
(I) species is therefore of concern.2,3 Iron(I) is an unusual oxidation
state whose coordination chemistry, particularly for low-coordinate
examples (4-coordinate or lower), is poorly developed. This point
was underscored recently in Parkin’s report of the 4-coordinate Fe-
(I) complex [PhTpBut]Fe(CO).4 Knowledge of the elementary
reactivity patterns of Fe(I) will guide mechanistic postulates for
its possible role in biological systems.
This work describes the synthesis of a rare, 4-coordinate Fe(I)
complex, [PhBP3]Fe(PPh3) (4) ([PhBP3] ) [PhB(CH2PPh2)3]-), that
serves as a synthetic surrogate to the tripodal “[PhBP3]Fe(I)”
subunit. Of particular interest is that this system undergoes rapid
oxidation by aryl azide to produce a mononuclear, terminally
bonded Fe(III) imide, [PhBP3]Fe(N-p-tolyl) (5). Synthetically rare
iron oxo and imide/nitrene species continue to receive intense
scrutiny due to their postulated role in catalysis.5-7 Recent work
by Lee and co-workers provided important precedent for the
“FedNR” multiple-bond linkage.5b However, Lee’s system is
synthetically ill-defined and electronically complex owing to its
cluster nature, which features four magnetically coupled iron
centers. The title complex reported herein provides a distinctly
simple, mononuclear 4-coordinate Fe(III) imide. Its S ) 1/2 ground-
state electronic configuration is also distinct in that, to the best of
our knowledge, low-spin examples of tetrahedral or pseudotetra-
hedral Fe(III) were previously unknown.
produce the yellow, 4-coordinate high-spin (S ) 2) halide deriva-
tives [PhBP3]FeX (1-3, respectively).8 The chloride derivative 1
underwent reduction by sodium/mercury amalgam (THF) in the
presence of 3 equiv of PPh3 to give the desired iron(I) precursor 4
as an orange, crystalline solid in 62% yield. Complexes 1 and 4
were each characterized by both X-ray crystallography (Scheme
1) and SQUID magnetometry (Figure 1). Their solid-state structures
establish a 4-coordinate, pseudotetrahedral geometry for each case.9
The magnetization data for 1 and 4 were rigorously temperature-
independent between 10 to 310 K and provided effective moments
at 290 K of 5.00 and 4.11 µB, respectively (See Figure 1a). These
moments suggest a high spin S ) 2 ground state in the case of
3
chloride 1 and an S ) /2 ground state for 4. The bromide and
iodide derivatives 2 and 3 are also high-spin (Evans method, 295
K).10 Complex 4 underwent clean oxidation by p-toyl azide at room
temperature in benzene to provide the forest-green, d5 imido
complex 5, which could be crystallized (50% isolated) by vapor
diffusion of petroleum ether into the reaction solution. The overall
1
reaction appeared to be quantitative by H and 31P NMR spec-
troscopies: a stoichiometric equivalent of p-tolyl-NdPPh3 was
produced as the only detectable byproduct. Complex 5 was
thoroughly characterized by X-ray diffraction (Scheme 1), SQUID
magnetization (Figure 1a), EPR spectroscopy (Figure 1b), and cyclic
voltammetry (Figure 1c). The Fe-N(1)-C(46) bond angle in 5 is
169.96(2)° and the Fe-N(1) bond distance of 1.6578(2) Å is much
shorter than typical Fe-amido linkages and is similar to that
reported by Lee.5b These structural data are consistent with strong
π-bonding in the Fe-N(1) linkage: 5 is best represented with a
triple bond as shown in Scheme 1. The axial π-interactions
destabilize two orbitals of dxz and dyz parentage to provide the low-
High-yield (>80%) entry to the “[PhBP3]Fe” system proceeded
from metallation of FeX2 salts (X ) Cl, Br, I) by Tl[PhBP3] to
1
spin S ) /2 configuration observed for 5, as indicated from its
Scheme 1
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J. AM. CHEM. SOC. 2003, 125, 322-323
10.1021/ja028448i CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
(Scheme 1) and has been independently generated by reaction of 4
with excess CO. In light of the recent nickel chemistry reported by
Hillhouse and Mindiola,13 well-defined examples of nitrene transfer
to CO with release of isocyanate are now established for first row
systems including Fe(III), Co(III), and Ni(II). Particular to the
present system is that dicarbonyl 6 is itself a precursor to imide 5.
An NMR tube experiment showed that the addition of 2 equiv of
p-tolyl azide to 6 slowly (rt, C6D6, 44 h) effected its conversion
back to imide 5, along with the release of free isocyanate (80%,
external standard). Efforts are underway to realize a catalytic Fe-
(I)/Fe(III) group transfer cycle based on 5 or 6.
In summary, this work shows that Fe(I) supported by a strong
field and tripodal phosphine donor can undergo a facile, multielec-
tron group transfer process to accept a strongly π-donating imide
ligand. The imide can be subsequently released to the acceptor
substrate CO. The method of formation of imide 5, its reactivity
with CO, and its regeneration from 6 underscore the viability of a
well-defined, Fe(I) f Fe(III)/Fe(III) f Fe(I) group transfer loop.
Whether parallels to such group transfer processes occur in
biological systems, such as the reducing FeS-clusters of certain
metalloenzymes, is worth considering.
Acknowledgment. We thank the ACS PRF and the Dreyfus
Foundation for financial support, and David M. Jenkins, Dr. Michael
W. Day, and Dr. Angel J. Di Bilio for assistance.
Supporting Information Available: Detailed experimental pro-
cedures, characterization data (PDF); crystallographic information (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 1. (a) SQUID magnetization data for complexes 1, 4, and 5. (b)
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2.0. While the three components g1, g2, and g3 are well resolved at
30 K, g2 and g3 were not resolved in a 77 K EPR spectrum.
Assignment of the electronic configuration of 5 can be made
cautiously by assuming approximate three-fold symmetry and
placing the molecular z-axis along the Fe-N bond vector. Using
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(6) Mansuy and co-workers were the first to report heme-supported [FeN-
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2
2
2
slightly above two lower-lying and filled dx -y and dz orbitals to
2
2
2
1
0
0 11
2
2
provide the ground-state configuration (dz ) (dx -y ) (dxy) (dxz) (dyz) .
This electronic model suggests that it should be possible to reduce
5 by one electron, in accord with its cyclic voltammetry. A well-
behaved and fully reversible FeII/III couple is observed at -1.35
V, whereas an irreversible oxidation is observed at ∼-300 mV in
the CV (Figure 1c). That 5 can be reversibly reduced to the d6
anion “[[PhBP3]FetNAr]-” is plausible, given the stability of its
isolobal and isoelectronic d6 relative [PhBP3]CotNAr.12
(7) Wieghardt and Meyer have provided spectroscopic evidence at 77 K for
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is well-resolved. See Supporting Information.
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Preliminary reactivity studies with imide 5 highlight its reactive
nature by comparison to its cobalt congener [PhBP3]CotNAr,
which required forcing conditions to release its “NAr” group to
CO (70 °C, 12 days).12 Complex 5 reacted immediately and at room
temperature upon CO addition to quantitatively release isocyanate
(OdCdN-p-tolyl, external integration standard) and form the
golden dicarbonyl byproduct [PhBP3]Fe(CO)2 (6). Complex 6 has
been structurally characterized as a distorted square pyramid
2
(11) Whether the predominantly dz orbital (with admixed s and pz character)
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resolve this issue are underway.
(12) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124,
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(13) (a) Mindiola, D. J.; Hillhouse, G. L. Chem. Commun. 2002, 1840. (b)
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