N=N Bond Cleavage by a Low-Coordinate Iron(II) Hydride Complex

Full text of DOI:10.1021/ja038152s

Published on Web 11/27/2003
NdN Bond Cleavage by a Low-Coordinate Iron(II) Hydride Complex
Jeremy M. Smith,*,† Rene J. Lachicotte, and Patrick L. Holland*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received August 27, 2003; E-mail: holland@chem.rochester.edu
There is interest in low-coordinate transition-metal complexes1
because of their characteristically high reactivity, which includes
C-H activation, CtO cleavage, and NtN cleavage.2 The ubiquity
of transition-metal hydrides in bond transformations, as well as their
broad reactivity patterns,3 suggests that the chemistry of low-
coordinate transition-metal hydride complexes would be particularly
interesting. Furthermore, biological interest in low-coordinate
hydride complexes stems from the proposal that metal hydride
species are reactive intermediates in the catalytic cycle of the
enzyme nitrogenase.4 Here, we report a low-coordinate iron(II)
hydride complex and its ability to reduce and cleave multiple bonds.
Scheme 1
Figure 1. Crystal structure of [LFeH]2. Thermal ellipsoids shown at 50%
probability. In all figures, carbon-bound hydrogen atoms are omitted for
clarity. All iron-bound hydrogens were refined with isotropic thermal
parameters. Fe1-Fe1′ 2.624(2) Å, Fe1-N1 1.990(5) Å, Fe1-N2 2.022(6)
Å, N1-Fe1-N2 95.3(2)°.
intensity of a complicated series of peaks decreases; these signals
are assigned to the dimer [LFeH]2.9
At higher temperature or lower concentration, the color of the
solution changes from dark red to orange. The orange color is
associated with a visible band at 512 nm, as found in orange three-
coordinate â-diketiminate iron alkyl complexes.7a The magnetic
moment of toluene solutions increases with temperature and
parallels the increase in proportion of the monomer LFeH. The
solution magnetic moment at high temperature (µeff ≈ 4.8 µB) is
consistent with monomeric high-spin Fe(II), and the lower magnetic
moment at low temperature presumably results from antiferromag-
netic coupling in the dimer.10 In support of the monomer formula-
tion, the molecular weight of a solution at 80 °C (determined from
freezing-point depression of a 1.0% w/w solution of [LFeH]2 in
naphthalene) is 480(80) g/mol (calcd for LFeH ) 558 g/mol).
The hydride monomer is trapped with 4-tert-butylpyridine to give
LFe(H)(4-tBupy) (Figure 2), a rare example of a four-coordinate
transition-metal hydride.11 The hydride ligand was refined to an
Fe-H distance of 1.74(2) Å. No agostic interactions are evident
(all Fe‚‚‚H-C contacts are >3.0 Å), and the iron has a trigonal
pyramidal geometry with an axial pyridine.
Reaction of the three-coordinate complex LFeCl,5 where L is
the bulky â-diketiminate ligand6 in Scheme 1, with KBEt3H led to
isolation of dark red crystals in 78% yield. The X-ray crystal struc-
ture of the product revealed a dimer in the solid state, with iron
atoms bridged by two hydride ligands (Figure 1). The hydrides were
located in the Fourier difference map. Steric hindrance within the
dimer causes the â-diketiminate rings to twist into a distorted boat
conformation, in which the N-Fe-N plane is at an angle of 34.8-
(8)° to the C-C-C plane of the â-diketiminate ligand backbone.
Spectroscopic studies provide evidence for a monomer in
equilibrium with the crystallographically characterized dimer. The
1H NMR spectra of toluene-d8 solutions of the hydride complex
show reversible temperature-dependent behavior. Increasing the
temperature leads to an increase in the intensity of a set of signals
that have similar paramagnetic shifts as the analogous signals in
solutions of three-coordinate iron â-diketiminate complexes LFeX
(X ) Cl, hydrocarbyl, NHR).5,7 Because the chemical shifts in
paramagnetic complexes are very sensitive to the electronic and
geometric structure at iron, the spectral similarity strongly suggests
that the high-temperature spectrum corresponds to a planar three-
coordinate hydride complex LFeH.8 As these peaks intensify, the
The reactivity of the hydride complex with unsaturated hydro-
carbons is typical of metal hydrides and consistent with the presence
of a three-coordinate monomer. Reaction of [LFeH]2 with 3-hexyne
in Et2O at room temperature gives the three-coordinate vinyl
1
complex LFeC(Et)dC(H)Et, characterized by H NMR spectros-
copy and X-ray crystallography. The ethyl groups are cis, showing
that Fe and H add to the same face of the alkyne. The rate of
reaction is first-order in [LFeH]2 and zero-order in [hexyne], with
activation parameters ∆Hq ) 99(7) kJ mol-1 and ∆Sq ) 32(10) J
K-1 mol-1 (277-299 K). The rate of reaction with 2-butyne is
similar.12 These data suggest a mechanism in which rate-determining
dimer cleavage is followed by alkyne insertion into the iron-hydride
bond of monomeric LFeH.
† Current address: Department of Chemistry and Biochemistry, New Mexico
State University, Las Cruces, NM 88003.
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J. AM. CHEM. SOC. 2003, 125, 15752-15753
10.1021/ja038152s CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
azobenzene is cleaved, suggesting that low-coordinate iron hydrides
are capable of performing difficult reactions reminiscent of those
in the catalytic cycle of nitrogenase.
Acknowledgment. We thank the University of Rochester and
the NSF (CHE-01345658) for funding, Sandip Sur for assistance
with NMR experiments, and Kent Rodgers and Gudrun Lukat-
Rodgers for valiant attempts to obtain resonance Raman spectra of
the hydride complexes.
Supporting Information Available: Syntheses, characterization,
equilibrium, and kinetics data (PDF) and crystallographic data (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
Figure 2. Crystal structure of LFe(H)(4-tBupy). Thermal ellipsoids shown
at 50% probability. Fe-H1 1.74(2) Å, Fe-N11 2.022(1) Å, Fe-N21 2.031-
(2) Å, Fe-N13 2.124(2) Å, N11-Fe-N21 97.36(6)°, N11-Fe-N13
100.00(6)°, N21-Fe-N13 99.53(6)°, N13-Fe-H1 103.5(6)°.
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Figure 3. Crystal structure of LFeN(Ph)NHPh. Thermal ellipsoids shown
at 50% probability; major disorder component shown. Fe-N11 1.980(2)
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which rapidly inserts into the iron-hydride bond to give the red
hydrazido complex LFeN(Ph)NHPh. This complex has trigonal-
planar geometry in the solid state (Figure 3). Its solution 1H NMR
spectrum suggests that rotation about the Fe-N(hydrazido) bond
is hindered because heating causes the four resonances for the
â-diketiminate isopropyl methyl groups to coalesce to two signals.
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1
At high temperature, the H NMR spectrum is analogous to those
of other three-coordinate â-diketiminate iron complexes.5,7
Extended heating of LFeN(Ph)NHPh (80 °C, 3 h) results in N-N
bond rupture, as evidenced by clean formation of the anilidoiron-
(II) complex LFeNHPh (Scheme 1).13 The overall sequence of reac-
tions therefore results in cleavage of the NdN bond of azobenzene
by [LFeH]2. This is a unique transformation for a mononuclear,
late transition-metal complex. Most mononuclear examples of NdN
bond cleavage are by low-valent early transition-metal complexes,
driven by oxidation of the metal.14 In contrast, the iron atom in
each complex in Scheme 1 remains in the +2 oxidation state, and
the reducing equivalents come from the hydride ligands.
(12) kobs (288 K, toluene-d8): 3-hexyne 3.1(3) × 10 -4 s-1; 2-butyne 5.4(1) ×
-4
10
s-1
.
(13) LFeNHPh was synthesized independently and characterized by 1H NMR
and crystallography. Azobenzene produced from isolated samples of
LFeN(Ph)NHPh was identified by 1H NMR and GC-MS. See the
Supporting Information for details.
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Preliminary kinetics studies of the decomposition of LFeN(Ph)-
NHPh show it to be first-order in [LFeN(Ph)NHPh], with kobs
)
4.2(3) × 10-4 s-1 at 358 K. While the mechanism of the
transformation is still under investigation, the intermediacy of LFeH
is unlikely because adding excess 3-hexyne as a trap does not affect
the course of the reaction.
In summary, we have prepared three- and four-coordinate iron
hydride complexes, whose low coordination number leads to
unusual bond cleavage reactivity. Notably, the NdN bond of
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J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003 15753