Preparation of iron amido complexes via putative Fe(IV) imido intermediates

Article abstract of DOI:10.1021/ja052952g

The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X-H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at λ_max ≈ 600 nm and extinction coefficients between 2000 and 3000 M^-1 cm^-1. These complexes are hygroscopic, reacting with 1 equiv of water to produce the corresponding Fe(III)-OH complexes and p-toluidine. Copyright

Full text of DOI:10.1021/ja052952g

Published on Web 08/02/2005
Preparation of Iron Amido Complexes via Putative Fe(IV) Imido Intermediates
Robie L. Lucas, Douglas R. Powell, and A. S. Borovik*
Department of Chemistry, UniVersity of Kansas, 2010 Malott Hall, 1251 Wescoe Hall DriVe,
Lawrence, Kansas 66045
Received May 5, 2005; E-mail: aborovik@ku.edu
This report describes the preparation and properties of iron(III)
amido complexes (FeIII-NHR) obtained from iron(II) precursors
and aryl azides. Their isolation is facilitated with ureate/amidate
tripodal ligands that create varied hydrogen bond (H-bond) networks
around the FeIII-N unit. Results from reactivity studies suggest
involvement of Fe(IV) imido intermediates.
amido and imido complexes were unsuccessful. These observations
and those reported by others4,11 suggested to us that hybrid tripods,
H41 and H52, with varied cavity architectures and H-bond networks,
may be more conducive in producing stable monomeric imido and
amido iron complexes.
The preparative route to the Fe(III) amido complexes is illustrated
in Scheme 1 for [FeIIIH22(NHTol)]-.13 Treating a dimethylacetamide
Research on metal complexes with terminal imido and amido
ligands has been stimulated by their proposed roles as intermediates
in various chemical transformations.1 For instance, metal imido
complexes have been implicated as the key intermediates in metal-
mediated group transfer reactions.2 In addition, intermediary species
with terminal amido ligands are linked to catalytic processes
involved in N-C bond formation.3 However, there are relatively
few examples of late 3d transition metal complexes with terminal
imido ligands,4 particularly those of iron. Lee reported a stable site-
differentiated 3Fe(III)/Fe(IV) cubane, where the Fe(IV) center has
a terminally bonded imido ligand.5 Peters reported the first
structurally characterized monomeric Fe(III) imido (FeIIItNR)
complexes using a bulky trisphosphinoborate ligand.6 Preparative
routes to these FeIIItNR complexes occurred by reaction of an Fe-
(I) precursor with organic azides. Hydrogenolysis of FeIIItNR
complexes with H2 in benzene yielded the Fe amido complex,
FeII-NHR.7 Wieghardt has described a low-spin Fe(III) amido
species with triazacyclononane,8 while Holland recently reported
the preparation of several Fe amido complexes using â-diketiminato
ligands.9 In addition, Bergman has isolated an FeII-NH2 complex
using bisphosphine ligands.10
Scheme 1
(DMA) solution of the high-spin iron(II) complex, [FeIIH22]-, with
p-tolyl azide (TolN3) results in a rapid color change from amber to
dark black-green with concomitant gas evolution that is attributed
to the expulsion of N2.14 K[FeIIIH22(NHTol)] is isolated as a dark
green solid with X-band EPR g-values of 8.65 and 4.15, values
5
that are indicative of a monomeric Fe(III) species with an S ) /2
spin state. An identical reaction sequence was carried out with
[FeIIH1]- to yield the corresponding Fe(III) amido complex
[FeIIIH1(NHTol)]- that exhibited spectroscopic properties similar
to those of [FeIIIH22(NHTol)]-. The complexes are hygroscopic:
for instance, allowing [FeIIIH1(NHTol)]- to react with 1 equiv of
water produces the Fe(III)-OH complex, [FeIIIH1(OH)]-, and
p-toluidine in 95 and 60% yields as determined by optical
spectroscopy (Scheme 1).13,15,16
Single-crystal X-ray diffraction study on K[FeIIIH22(NHTol)]
shows that the iron center is five-coordinate with trigonal bipyra-
midal geometry (Figure 2).13 The two ureas and isopropyl group
appended from N4 surround the p-tolyl amido ligand.
Analysis of the above findings suggests that during synthesis
the formal oxidation states at the Fe centers range from 1+ to 3+.
Que has recently provided evidence for the formation of an Fe(III)
amido complex, prepared by treating an Fe(II) complex with phenyl-
N-tosylimidoiodinane (PhIdNTs) and proposes an Fe(IV) imido
species as an intermediate.11
We have introduced the symmetrical urea- and amide-based
tripodal ligands, H30 and H63 (Figure 1), whose Fe(II) complexes
Figure 1. Tripodal ligands used in this study.
cleave O2 to form monomeric Fe(III) species with terminal oxo or
hydroxo ligands.12 Mechanistic studies on complexes of [FeIIH33]-
suggest the involvement of an Fe(IV) oxo intermediate, showing
that a two-electron process involving a high valent iron center is
viable. We reasoned that a similar process could occur to prepare
iron complexes with terminal imido and amido ligands. While
[FeII0]- and [FeIIH33]- react with organic azides, the initial products
of these reactions are too unstable, and repeated attempts to isolate
Figure 2. Thermal ellipsoid plot for [FeIIIH22(NHTol)]-. Thermal ellipsoids
are drawn at the 50% probability level, and carbon-bonded hydrogen atoms
are omitted for clarity.
The Fe-N7 length is 1.968(4) Å, and a nearly linear N1-Fe-
N7 angle of 173.10(15)° is observed; these findings are similar to
those reported for other Fe(III) amido complexes.9 Moreover, the
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C O M M U N I C A T I O N S
presence of intramolecular H-bonds is supported by the average
HN‚‚‚N7 distance of 2.990(6) Å. The aryl ring of the amido ligand
is positioned between the urea groups, resulting in an N2-Fe-N3
angle of 127.32(14)°. A similar increase of one trigonal angle is
intermediate undergoes intramolecular C-H bond insertion to an
appended phenyl group in the formation of an Fe(III)-anilido
complex. These differences in reactivity highlight the importance
of cavity architecture (i.e., secondary coordination sphere) around
reactive metal site(s). Our ability to tune cavity properties assisted
in isolating [FeIIIH1(NHTol)]- and [FeIIIH22(NHTol)]-. Additional
studies of these effects on metal-mediated processes are ongoing.
found in related M-OH complexes of [H33]3- 12a,c
This arrangement
.
of the tolyl moiety leaves the N-H bond of the amido ligand
pointed within the cavity toward the less bulky isopropyl group of
the amidate.
Examples of monomeric Fe(IV) species in synthetic systems are
rare,5,17 but we propose that the described Fe(III) amido complexes
form via H-atom abstraction by Fe(IV) imido intermediates. As
organic azides generally react with transition metal ions by a two-
electron process to yield imido complexes,1c,e it is expected that
starting from Fe(II) would result in formation of the Fe(IV) imido
species at some point along the reaction pathway (Figure 3).
Acknowledgment. Acknowledgment is made to the NIH
(GM50781) for financial support of this work and the NSF (CHE-
0079282) for funding of the X-ray diffraction instrumentation.
Supporting Information Available: Details for all experiments,
spectra for the water reactions (Figures S1-S3), and crystallographic
details for K[FeIIIH22(NHTol)] (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. Proposed mechanism involving an Fe(IV) imido intermediate.
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Conversion of this high valent species to the Fe(III) amido could
then take place by the formal transfer of a H-atom from the DMA
solvent.
We have shown that this proposal may be viable by using 1,2-
diphenylhydrazine (DPH) and 9,10-dihydroanthracene (DHA) as
external H-atom sources (eq 1). Reactions using DPH result in clean
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conversion to azobenzene, which is produced in yields of ∼90%
when used with either Fe(II) complex. Although reactions with
DHA do not give complete conversion to anthracene as a single
product, they demonstrate C-H bond activation. Addition of TolN3
to a solution of [FeIIH22]- in DMA containing 0.5 equiv of DHA
produces [FeIIIH22(NHTol)]- and a mixture of organic products
with DHA starting material recovered in 73% yield, along with
anthracene (14%) and the coupling product, 9,9′,10,10′-tetrahydro-
9,9′-bianthracene (8%). Considerably more DHA is converted when
carrying out the reaction to prepare [FeIIIH1(NHTol)]- from
[FeIIH1]-: anthracene is obtained in 49% yield, with the coupling
product (19%) and DHA (19%) also present.
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of X-H bond cleavage and complement results on iron oxo
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force to abstract H-atoms.18 Moreover, H-atom abstraction appears
to follow an intermolecular path in forming [FeIIIH1(NHTol)]- and
[FeIIIH22(NHTol)]-, which contrasts the intramolecular routes
involving ligand modification found in other systems.4e,f,11 In
particular, Que proposes that a trigonal bipyramidal Fe(IV) imido
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(15) Treating [FeIIIH22(NHTol)]- with 1 equiv of H2O afforded [FeIIIH22(OH)]-
and p-toluidine in yields of 50 and 30%.
(16) The properties of the Fe(III)-OH products match those obtained for pure
samples prepared by independent methods (unpublished results).
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