Characterization of structurally unusual diiron NxHy complexes

Article abstract of DOI:10.1021/ja903967z

(Figure Presented) A series of fascinating diiron complexes featuring bridging N_xH_y ligands stabilized by tris (phosphine)borate ([PhB(CH₂PR₂)₃] = [PhBP^R ₃]) ligands have been ch

Full text of DOI:10.1021/ja903967z

Published on Web 07/14/2009
Characterization of Structurally Unusual Diiron NxHy Complexes
Caroline T. Saouma, Peter Mu¨ller, and Jonas C. Peters*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received May 15, 2009; E-mail: jcpeters@mit.edu
Scheme 1
Mechanistic proposals concerning the pathway of N2 reduction
in biology at the MoFe-cofactor of nitrogenase continue to be
advanced.1 In addition to nitrogen, hydrazine2 and diazene1a are
nitrogenase substrates, and recent DFT calculations and spectro-
scopic studies suggest that whereas initial N2 binding may occur
at one iron center, diiron pathways may be involved at certain NxHy
intermediate stages en route to ammonia formation.1a,3 In this broad
context, recent work has explored the synthesis and spectroscopic
characterization of structurally unusual mono- and bimetallic iron
complexes featuring nitrogenous ligand functionalities.4,5 The
demand for such model complexes continues in light of recent
ENDOR and ESEEM spectroscopic data that has been obtained
under turnover conditions at the cofactor.1a,2,6 To date, there are
few synthetic iron systems that feature parent hydrazine (N2H4),7
hydrazido (N2H22-), diazene (N2H2),8 amide (NH2-),9 and imide
(NH2- 4d ligands. Herein we describe the synthesis and character-
)
ization of a series of structurally distinct diiron complexes that
feature each of these ligand types.
using 15N2H4 (1JNH ) 59.5 Hz for N2H22-; 1JNH ) 72.0 Hz for N2H4).
The broadness of the doublets precludes the resolution of higher-
order N-H and H-H coupling.11 The corresponding 15N NMR
Entry into the NxHy chemistry of present interest is realized with
the iron(II) alkyl precursors [PhBPR ]FeMe {[PhBPR ] ) PhB-
3
3
(CH2PR2)3-; R ) Ph, CH2Cy}. These high-spin complexes are
prepared either by addition of excess Me2Mg to a benzene solution
of [PhBPPh3]FeCl or by addition of MeLi to a thawing THF solution
2-
spectrum shows a doublet at -10.0 ppm for N2H2 and a triplet
at 58.37 ppm for N2H4,12 both of which collapse into singlets upon
1H decoupling. Complex 4 displays similar 15N NMR characteristics
(N2H22-: 0.86 ppm, doublet, J ) 45.2 Hz; N2H4: 50.86 ppm, triplet,
J ) 68.9 Hz).
of [PhBPCH Cy3]FeCl. The room-temperature addition of 1 equiv of
2
hydrazine to yellow THF solutions of either [PhBPPh3]FeMe (1) or
[PhBPCH Cy3]FeMe (2) results in the immediate release of methane
2
Whereas Sellmann previously reported a few examples of diiron
µ-η1:η1-HNdNH species,8 the bimetallic iron cores of 3 and 4 are
unique. Schrock has characterized a ditungsten (µ-η1:η1-N2H4)(µ-
η2:η2-N2H2) complex whose core is closely related to those in 3
and 4.13
and clean conversion to red and diamagnetic {[PhBPPh3]Fe}2(µ-η1:
η1-N2H4)(µ-η2:η2-N2H2) (3) or purple and diamagnetic {[PhBPCH Cy3]-
2
Fe}2(µ-η1:η1-N2H4)(µ-η2:η2-N2H2) (4), respectively (Scheme 1). The
solid-state structures of the diiron cores of 3 and 4 are nearly
identical [see the Supporting Information (SI)]; that of 3 is shown
Whereas solutions of 3 are stable for days at 60 °C, purple
solutions of 4 are thermally unstable even at 22 °C (t1/2 ) 4 h) and
decay quantitatively to the isolable green diamagnetic product
2-
in Figure 1. Its two iron centers are bridged by N2H4 and N2H2
ligands. We assign the end-on bridging ligand as N2H4 and the side-
10
2-
{[PhBPCH Cy3]Fe}(µ-η1:η1-N2H2)(µ-NH2)2 (5). Signature NMR data
bonded bridging ligand as N2H2
.
The N3-N4 bond distance
2
for 5 are as follows: Its 1H NMR spectrum (THF-d8) features three
broad singlets at 16.21, -1.65, and -3.85 ppm, corresponding to
of 1.429(3) Å in 3 is consistent with an N-N single bond, and
hence, this ligand is best assigned as N2H22- rather than HNdNH.
1
-
The H NMR spectrum (THF-d8) of 3 reveals a solution structure
bound HNdNH and diastereotopic NH2 protons, respectively.
Each of these 1H NMR resonances is split into a doublet upon 15
N
similar to that observed in the solid state. A N2H4 resonance is
2-
labeling. Its 15N NMR spectrum contains a doublet at 419.1 ppm
(1JNH ) 65.1 Hz) that collapses to a singlet upon decoupling of the
proton resonance at 16.21 ppm. The NH2- nitrogens are observed
as a triplet at -58.1 ppm (J ) 59.5 Hz).
noted at 2.54 ppm and a N2H2 resonance at 2.59 ppm. Each of
these peaks is split into an apparent doublet when 3 is prepared
The solid-state structure of 5 is shown in Figure 2. Both the
Fe-N1 bond distance of 1.882(8) Å and the N1-N1′ bond distance
of 1.283(15) Å are similar to those found in Sellmann’s Fe2(µ-η1:
η1-HNdNH) complex.8 In addition, the Fe-N bond-distance
contraction of 0.17 Å that occurs upon oxidation of hydrazine to
diazene is also consistent with other structurally characterized
diazene complexes8,14 and is likely due to a combination of the
smaller covalent radii of sp2-hybridized nitrogen relative to sp3-
hybridized nitrogen and modest back-bonding. Notable in the
structure of 5 is the presence of cis-diazene. Whereas monosub-
Figure 1. Solid-state molecular structures of the core atoms of 3 and 6.
See the SI for complete structures and details.
9
10358 J. AM. CHEM. SOC. 2009, 131, 10358–10359
10.1021/ja903967z CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
stituted diazenes (HNdNR) are known to bind transition metals in
a cis conformation,15 to our knowledge all known complexes of
HNdNH show trans ligation. Hence, 5 appears to be structurally
distinct in this context.8
of 1.84 Å is ∼0.17 Å longer than that found in {[PhBPPh3]Fe2(µ-
NH)(µ-H)}{Na}.4d The average Fe-P distance of 2.33 Å is ∼0.075
Å longer than those found in 3 and 6, consistent with its assignment
as an antiferromagnetically coupled diiron(III) complex.
The addition of 2.2 equiv of Pb(OAc)4 to red 3 leads to oxidation
of the bound N2H416 to afford green and diamagnetic {[PhBPPh3]-
Fe}2(µ-η1:η1-N2H2)(µ-η2:η2-N2H2) (6) as the major product along with
release of acetic acid. The solid-state structure of 6 is shown in Figure
1. Most striking is the presence of both an η1:η1-bridging cis-HNdNH
ligand and a µ-η2:η2 HN-NH2- ligand. To our knowledge, 6 is the
only transition-metal complex featuring the N2H2 ligand in each of its
limiting states of oxidation. The N1-N2 bond distance of 1.281(5) Å
for the ligand assigned as HNdNH is similar to that found for the
bridging HNdNH ligand of 5. The N3-N4 bond distance of 1.458(5)
Å for the ligand assigned as N2H22- is slightly elongated compared
with that of the N2H22- ligand of 3. The average Fe-Ndiazene bond
distance of 1.89 Å is appreciably shorter than the average
In conclusion, we have characterized a series of structurally
fascinating diiron NxHy species that contain hydrazine, hydrazido,
and cis-diazene bridges. Thermal and oxidative transformations also
lead to unusual examples of diiron species featuring µ-NH2 and
µ-NH ligands. In certain instances, low-temperature experiments
revealed the presence of intermediate species. Ongoing work
concerns detailed vibrational characterization of the species de-
scribed herein, in addition to mechanistic studies.
Acknowledgment. We acknowledge the NIH (GM-070757).
Funding for the MIT Department of Chemistry Instrumentation
Facility has been provided in part by the NSF (CHE-0234877).
C.T.S. is grateful for an NSF graduate fellowship.
2-
Fe-Nhydrazido bond distance of 1.99 Å.
Supporting Information Available: Detailed experimental proce-
dures and characterization data for 1-7 and crystallographic details
for 3-7 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
As was observed for 3 and 4, the structure of 6 is preserved in
solution. In the H NMR spectrum (C6D6) of 6, broad singlets at
13.20 and 4.16 ppm are assigned to the HNdNH and N2H2
1
2-
protons, respectively; both of these peaks are split into apparent
broad doublets when samples of 6 are prepared using 15N-enriched
6. The 15N NMR spectrum of 6 contains a doublet at 58.0 ppm
(1JNH ) 58.1 Hz) for the N2H22- nitrogens and a doublet of doublets
at 407.5 ppm (1JNH ) 69.2 Hz, J ≈ 20 Hz) corresponding to the
HNdNH ligand.
References
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(7), along with p-hydrobenzoquinone as a byproduct. In order to
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with a Toepler pump apparatus, and the evolution of 1 equiv of N2
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1
of 7 displays a single resonance at 32.5 ppm at 22 °C. The H
NMR spectrum shows a single set of [PhBPPh3] resonances, with
an additional singlet at 25.3 ppm that integrates to one proton per
[PhBPPh3]; when 7 is prepared using isotopically enriched 3, the
singlet at 25.04 ppm is split into a doublet (1JNH ) 64.0 Hz). The
15N NMR spectrum displays a doublet at 563.5 ppm. On the basis
of these NMR data as well as IR data (see the SI), the solution
structure of 7 is assigned as diferric {[PhBPPh3]Fe}2(µ-NH)2. The
spectroscopic data for the bridging NH ligand in 7 is similar to
that previously reported for {([PhBPPh3]Fe)2(µ-NH)(µ-H)}{Na}.4d
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(10) For all of the structures presented herein, the protons on the NxHy ligands
were located in the difference Fourier map and refined semi-freely with
the help of restraints on the N-H distances and the Fe-N-H angles while
constraining the Uiso value of each H atom to-1.2 times the Ueq value of
the N atom connected to that proton.
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Engl. 1986, 25, 383. (b) Smith, M. R., III; Cheng, T. Y.; Hillhouse, G. L.
J. Am. Chem. Soc. 1993, 115, 8638.
(12) All of the 15N NMR chemical shifts are reported relative to liquid NH3.
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8316.
(14) Cheng, T. Y.; Ponce, A.; Rheingold, A. L.; Hillhouse, G. L. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 657.
Figure 2. Solid-state molecular structures of the core atoms of 5 and 7.
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See the SI for complete structures and details.
Crystals of 7 can be grown from a THF/cyclopentane solution, and
its solid-state structure is shown in Figure 2. The bridging imides are
disordered over three positions (see the SI) and modeled satisfactorily
with a population ratio of 0.93:0.75:0.32. The average Fe-N distance
(16) Smith, M. R., III; Cheng, T. Y.; Hillhouse, G. L. J. Am. Chem. Soc. 1993,
115, 8638.
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