A bridging hexazene (RNNNNNNR) ligand from reductive coupling of azides

Article abstract of DOI:10.1021/ja801375g

This communication reports the first examples of transition metal complexes containing an RNNNNNNR^2- ligand. Addition of 1-azidoadamantane to the diiron(I) synthon L^RFeNNFeL^R (L^R = HC[C(R)N(2,6-iPr₂C₆H₃)]₂; R = methyl, tert-butyl) leads to the diiron complexes L^RFe(μ-η²:η²-AdN₆Ad)FeL^R, which are surprisingly thermally stable. Magnetic, Mssbauer, and crystallographic data are consistent with pairs of high-spin iron(II) ions antiferromagnetically coupled through a dianionic AdN₆Ad^2- bridge. Copyright

Full text of DOI:10.1021/ja801375g

Published on Web 04/18/2008
A Bridging Hexazene (RNNNNNNR) Ligand from Reductive
Coupling of Azides
Ryan E. Cowley,† Je´roˆme Elha¨ık,† Nathan A. Eckert,† William W. Brennessel,†
Eckhard Bill,‡ and Patrick L. Holland*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and
Max-Planck-Institut fu¨r Bioanorganische Chemie, D-45470 Mu¨lheim an der Ruhr, Germany
Received February 24, 2008; E-mail: holland@chem.rochester.edu
Hexaaza-1,5-dienes1,2 (Figure 1a) are unusual high-energy
molecules that are often shock-sensitive. Only two have been
crystallographically characterized.3 They decompose thermally and
photochemically,4 and the products can be triazene and azide,3 N,C-
disubstituted tetrazoles,6 or diazo compounds.7 Their oligomers and
polymers have also been studied.4a,5 Hexaza-1,5-dienes have been
synthesized via two routes: N-N coupling of two aryldiazonium
equivalents with a diacylhydrazine,6 and coupling of two in situ
generated triazenyl radicals.2,7 No coordination chemistry of
hexaazadienes has been reported. Here we describe iron complexes
of the 1,6-bis(1-adamantyl)hexaaza-1,5-dienyl (“hexazene”) ligand
(Figure 1b) that show surprising thermal and photolytic stability.
In addition to structural and spectroscopic characterization of the
complexes, we show that the hexazene ligand mediates magnetic
coupling between high-spin metal centers.
Figure 1. (a) General structure of hexaaza-1,5-dienes. (b) The “hexazene”
ligand reported here.
We recently reported that the metastable imidoiron(III) complex
L
MeFe(NAd)(tBupy) (LMe ) HC[C(Me)N(2,6-iPr2C6H3)]2) can be
generated from addition of 1-adamantyl azide (N3Ad) to an iron(I)
source in the presence of tert-butylpyridine.8,9 However, an
analogous reaction in the absence of coordinating ligands does not
give an imidoiron complex. Thus, addition of 2.0 equiv of N3Ad
to LMeFeNNFeLMe in pentane immediately produces an orange
precipitate in 74% yield. The solid is poorly soluble in common
solvents, preventing reliable solution characterization. Crystal-
lography reveals that the product is LMeFe(µ-η2:η2-AdN6Ad)FeLMe
.
Figure 2. The solid-state structure of LMeFe(µ-η2:η2-AdN6Ad)FeLMe, using
50% thermal ellipsoids. Hydrogen atoms and a cocrystallized molecule of
CH2Cl2 are omitted for clarity. The thermal ellipsoid plot of LtBuFe(µ-η2:η2-
AdN6Ad)FeLtBu is shown in Figure S-1. Selected bond lengths for both
complexes are listed in Table 1.
A similar hexazene complex supported by LtBu (LtBu ) HC[C-
(tBu)N(2,6-iPr2C6H3)]2) is obtained by reaction of LtBuFeNNFeLtBu
with 2.0 equiv of N3Ad in hexanes at -78 °C. The solid-state
structures (LMe, Figure 2; LtBu, Figure S-1) show that a new N-N
bond exists between two N3Ad units, with the resulting AdN6Ad
ligand connected to two four-coordinate Fe centers.
Table 1. Bond Lengths (Å) in LMeFe(µ-η2:η2-AdN6Ad)FeLMe and
LtBuFe(µ-η2:η2-AdN6Ad)FeLtBu
The metrical parameters of LMeFe(µ-η2:η2-AdN6Ad)FeLMe and
LMe complex
LtBu complex
L
tBuFe(µ-η2:η2-AdN6Ad)FeLtBu are similar to one another (Table
Fe(1)-N(11)
Fe(1)-N(21)
Fe(1)-N(1)
Fe(1)-N(3)
N(1)-N(2)
N(2)-N(3A)
N(3)-N(3A)
2.016(1)
2.021(1)
2.059(1)
2.003(1)
1.305(2)
1.305(2)
1.411(2)
2.018(2)
2.043(2)
2.061(2)
2.013(2)
1.307(2)
1.318(2)
1.408(3)
1), and so only the LMe complex is discussed in detail here. Each
iron atom has a roughly tetrahedral geometry and is bound to both
halves of the AdN6Ad unit and to a diketiminate ligand. There is
a crystallographic inversion center between N3 and N3A that
renders the two halves of the molecule equivalent. Each FeN4
metallacycle is planar (maximum deviation from the least-squares
plane is 0.0019(9) Å) and each iron resides out of thediketiminate
N2C3 plane (0.753(2) Å).
The bond distances within the hexazene unit give insight into the
electron localization within the complex. The equivalent N1-N2 and
N2-N3A distances are ca. 0.1 Å shorter than the N3-N3A single
bond.10 This is characteristic of π-electron delocalization between N(1),
N(2), and N(3A) and thus suggests resonance between allyl-like Lewis
structures (Figure 3). The presence of two negatively charged allyl-
like fragments indicates that AdN6Ad is dianionic, which in turn
implies a +2 oxidation state at each iron atom. To confirm this
hypothesis, we collected the zero-field Mo¨ssbauer spectrum of a solid
sample of LMeFe(µ-η2:η2-AdN6Ad)FeLMe at 80 K. The spectrum
(Figure 4) shows a single quadrupole doublet with δ ) 0.75 mm/s
and ∆EQ ) 3.12 mm/s. The iron sites of LMeFe(µ-η2:η2-
AdN6Ad)FeLMe are equivalent as in the X-ray crystal structure. These
values are consistent with high-spin iron(II), as found in other ferrous
ꢀ-diketiminate complexes,11 in four-coordinated complexes supported
† University of Rochester.
‡ Max-Planck-Institut fu¨r Bioanorganische Chemie.
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6074 J. AM. CHEM. SOC. 2008, 130, 6074–6075
10.1021/ja801375g CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
considerably from the data (dashed line). Thus the combined crystal-
lographic, spectroscopic, and magnetic data indicate that there are two
high-spin iron(II) centers, which are antiferromagnetically coupled to
give a diamagnetic ground state.
Because of the formal oxidation of iron(I) to iron(II) in the reaction
of LRFeNNFeLR to LRFe(µ-η2:η2-AdN6Ad)FeLR, we view the
hexazene ligand as being derived from reductive azide coupling. We
are not aware of any other published examples of reductive coupling
of two azides.
Figure 3. Two possible resonance structures for LRFe(µ-η2:η2-AdN6Ad)FeLR
To our knowledge, these are the first transition metal complexes
with a hexa-nitrogen R2N62- ligand. Although they contain a locally
high concentration of nitrogen atoms, they are remarkably inert as long
as they are protected from O2 and moisture. Under N2, the solid
complexes slowly decompose to a black residue only above 140 °C
in the solid state and do not explosively decompose when struck with
a hammer. This contrasts with several examples of explosive 1,4-
pentaazadien-3-ide complexes.15 Treatment of LMeFe(µ-η2:η2-
AdN6Ad)FeLMe with neutral donor ligands (tert-butylisocyanide,
4-tert-butylpyridine), irradiation (200 W mercury lamp for 70 min),16
or extended heating (100 °C for 5 days, slurry in C6D6) does not result
in a color change or the appearance of new paramagnetically shifted
peaks in the 1H NMR spectrum. This stability contrasts greatly with
metal-free hexazenes, which decompose within minutes upon heating
or irradiating.4a–c This difference demonstrates the importance of iron
coordination in stabilizing the catenated nitrogen atoms.
that assign the compound as a formal diiron(II) complex.
Figure 4. Mo¨ssbauer spectrum of LMeFe(µ-η2:η2-AdN6Ad)FeLMe. The majority
of the signal (98% of total iron) was fit with values of δ ) 0.75(1) mm/s and ∆EQ
) 3.12(2) mm/s, and the remaining impurity (2%) was fit with δ ) 0.06(4) mm/s
and ∆EQ ) 0.27(8) mm/s. The lines represent the Lorentzian doublets of the
subspectra and their superposition (red).
Acknowledgment. We thank the Petroleum Research Fund
(44942-AC to P.L.H.) and the National Science Foundation
(Graduate Research Fellowship to R.E.C. and CHE-0134658 to
P.L.H.) for funding. We thank Sebastian Stoian and Prof. Eckard
Mu¨nck for a preliminary Mo¨ssbauer spectrum.
Supporting Information Available: Synthetic, structural, and
crystallographic details. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Figure 5. Variable-temperature magnetic susceptibility of LMeFe(µ-η2:η2-
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paramagnetic species, as judged by 1H NMR spectroscopy.
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