Base-mediated conversion of hydrazine to diazene and dinitrogen at an iron center

Article abstract of DOI:10.1021/ic801856q

The treatment of the hydrazine complex c/s-[Fe(N₂H ₄)(dmpe)₂]²⁺ with base afforded the diazene complex c/s-[Fe(N₂H₂)(dmpe)₂]. This reaction is reversed by the treatment of the di

Full text of DOI:10.1021/ic801856q

Inorg. Chem. 2009, 48, 5-7
Base-Mediated Conversion of Hydrazine to Diazene and Dinitrogen at an
Iron Center
Leslie D. Field,* Hsiu L. Li, and Alison M. Magill
School of Chemistry, UniVersity of New South Wales, New South Wales 2052, Australia
Received September 29, 2008
The treatment of the hydrazine complex cis-[Fe(N2H4)(dmpe)2]2+
Scheme 1
with base afforded the diazene complex cis-[Fe(N2H2)(dmpe)2]. This
reaction is reversed by the treatment of the diazene complex with
a mild acid, while treatment of the hydrazine complex with a mixture
of KOBut and ButLi afforded the dinitrogen complex
[Fe(N2)(dmpe)2].
Biological or industrial conversion of dinitrogen to am-
monia, as mediated by transition metals, must proceed
through a series of intermediate stages.1 The nature of the
metal-bound intermediates provides vital information about
the mechanism of the transformation. The formation of
ammonia and hydrazine from iron dinitrogen complexes has
been reported,2,3 although few mechanistic studies have been
undertaken.4 Iron complexes of hydrazine and diazene are
possible intermediates in the reduction of coordinated dini-
trogen in such systems.
relatively poor yield (17%). We now report that this iron(0)
diazene complex can also be synthesized in significantly
improved yield (55%) by deprotonation of the dichloride salt
of 1 with KOBut (Scheme 1). The purity of the product was
also improved in that no cis-[FeH2(dmpe)2] was formed as
a reaction byproduct using this route.
In an analogous reaction, the treatment of a solution of
trans-[FeCl2(dmpe)2] and 1,2-diphenylhydrazine with KOBut
afforded the iron(0) azobenzene complex cis-
[Fe(PhNdNPh)(dmpe)2] (3). No reaction was observed
between the dichloride starting material with 1,2-diphenyl-
hydrazine in the absence of KOBut, indicating that depro-
tonation of the disubstituted hydrazine is probably the first
step in the reaction sequence.
We recently reported the syntheses of a side-on-bound
hydrazine complex, cis-[Fe(N2H4)(dmpe)2]2+ (1), as well as
the first side-on-bound diazene complex, cis-
[Fe(NHdNH)(dmpe)2] [2; dmpe ) 1,2-bis(dimethylphos-
phino)ethane].5 The diazene complex was previously isolated
by reduction of 1 (formed in situ by the treatment of trans-
[FeCl2(dmpe)2] with hydrazine) with potassium graphite in
*
To whom correspondence should be addressed. E-mail:
Such a reaction gives rise to the question of whether 2
should be considered an iron(II) hydrazido(2-) complex or
an iron(0) diazene π complex. A transition-metal complex
bearing an alkene or an alkyne ligand may be considered as
either (i) a metallacyclic complex where the metal has two
anionic σ donors or (ii) a donor-acceptor π complex where
bonding arises from electron donation from a filled p orbital
of the ligand into a suitably directed vacant metal orbital
and back-donation from an occupied metal d orbital into the
antibonding π* orbital of the ligand.6,7 While these two
models are often regarded as complementary, in reality, there
is a continuum between the two extremes.8 Theoretically,
l.field@unsw.edu.au.
(1) (a) Barney, B. M.; Lee, H. I.; Dos Santos, P. C.; Hoffmann, B. M.;
Dean, D. R.; Seefeldt, L. C. Dalton Trans. 2006, 2277–2284. (b)
Kozak, C. M.; Mountford, P. Angew. Chem., Int. Ed. 2004, 43, 1186–
1189. (c) Schlogl, R. Angew. Chem., Int. Ed. 2003, 42, 2004–2008.
(d) Schrock, R. R. Chem. Commun. 2003, 2389–2391. (e) Pool, J. A.;
Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 14326–
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78, 589–625. (g) Leigh, G. J. Acc. Chem. Res. 1992, 25, 177–181.
(2) (a) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem.
Soc. 2005, 127, 10184–10185. (b) George, T. A.; Rose, D. J.; Chang,
Y. D.; Chen, Q.; Zubieta, J. Inorg. Chem. 1995, 34, 1295–1298. (c)
Jezowska-Trzebiatowska, B.; Sobota, P. J. Organomet. Chem. 1972,
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Shilov, A. E.; Ukhin, L. Y. J. Chem. Soc., Chem. Commun. 1971,
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3541.
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10.1021/ic801856q CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 1, 2009 5
Published on Web 12/01/2008

COMMUNICATION
Table 1. CDA Results for 2
basis seta
d
b
r
∆
BS2
BS3
0.540
0.547
0.324
0.323
-0.415
-0.417
-0.047
-0.044
a BS2: 6-311G(2d,p) on iron and 6-31G(d) on other atoms. BS3:
6-311G(2d,p) and LANL2DZ ECP on iron and 6-31G(d) on other atoms;
d ) electron donation; b ) back-donation; r ) repulsive polarization; ∆ )
rest term.
the question of whether a metal-ligand bond should be
classified as covalent or of the donor-acceptor type may be
answered by charge decomposition analysis (CDA),9 which
has been used to determine the nature of bonding interactions
in both transition-metal6-8,10,11 and main-group12 complexes.
CDA considers the bonding in a complex in terms of the
(fragment) molecular orbital interactions between two closed-
shell fragments, in this case Fe(dmpe)2 and NHdNH. CDA
allows the relative amount of electron donation (d), back-
donation (b), and the interaction between the occupied
orbitals of both fragments leading to repulsive polarization
(r) to be calculated. In addition, the rest term, ∆, resulting
from the mixing of unoccupied orbitals on the two fragments
is also determined. ∆ is a highly sensitive indicator that may
be used to determine the nature of the bonding interaction
between two fragments: in donor-acceptor complexes, this
term should be virtually zero.6
The bond lengths calculated with the B3LYP density
functional and basis set BS113 are in good agreement with
the experimental bond distances for 2, and a comparison
shows that the average difference for core bond lengths is
0.045 Å. The results of CDA calculations, using this
optimized geometry with two different basis sets,14 are given
in Table 1.
Figure 1. Donation (left) and back-donation (right) orbitals for 2. Hydrogen
atoms on phosphine ligands have been omitted for clarity.
Table 2. AIM Results for 2
F(rc)a
32F(r)a
Fe-N1
Fe-N2
N1-N2
N1-Fe-N2
Fe-P6
0.095
0.094
0.377
0.473
0.077
0.089
0.089
0.083
0.350
0.349
-0.573
0.219
0.204
0.210
0.227
Fe-P7
Fe-P8
Fe-P9
a F(rc) ) electron density; 32F(r) ) Laplacian of electron density.
complex 2 is a donor-acceptor π complex between diazene
and iron(0) rather than a hydrazido fragment binding to an
iron(II) metal center.
Further evidence that 2 should be considered as a
donor-acceptor complex is provided by Bader’s Atoms In
Molecules (AIM) theory.16 AIM theory calculates the
electron density, F(r), and the Laplacian of the electron
density, 32F(r), which yields information regarding the nature
of the bonding.11,16,17 The results of the AIM analysis are
shown in Table 2. For all of the Fe-N and Fe-P bonds, the
values of the Laplacian are typical of systems with closed-
shell, donor-acceptor interactions.11,18
Quite remarkably, the deprotonation of the hydrazine
complex 1 to form the diazene complex 2 is reversible, and
the reaction of 2 with the weak acid 2,6-lutidinium triflate
reforms the hydrazine complex 1. This was proven by
labeling the hydrazine complex with 15N and monitoring the
reaction mixture with NMR spectroscopy. Upon treatment
of the poorly soluble orange dichloride salt of hydrazine
complex 1 with KOBut in tetrahydrofuran-d8 (THF-d8), a
yellow solution formed and a 15N NMR signal at -314.4
ppm for diazene complex 2 was observed (Figure 2a).
Residual uncoordinated hydrazine (A) at -335.8 ppm was
also present. After the addition of 2,6-lutidinium triflate, a
color change to orange and two broad doublets at 5.06 and
4.21 ppm (1JH-N ) 79 Hz) for the protons on the hydrazine
ligand of 1 were observed in the 1H NMR spectrum as well
as a 15N signal at -388.8 ppm (Figure 2b), indicating that
the initial deprotonation was indeed reversible. A broad
signal for hydrazinium (NH3NH32+, B) at -331.5 ppm from
The donation to back-donation ratio (d/b), ca. 1.7, shows
that there is significant back-donation from the metal center
to the unoccupied orbitals on the ligand. The orbitals involved
in donation and back-donation are shown in Figure 1. The
rest term ∆ is virtually zero,15 which clearly shows that
(9) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352–9362.
(10) (a) Dapprich, S.; Frenking, G. Angew. Chem., Int. Ed. Engl. 1995,
34, 354–357. (b) Boehme, C.; Frenking, G. Organometallics 1998,
17, 5801–5809. (c) Szilagyi, R. K.; Frenking, G. Organometallics
1997, 16, 4807–4815. (d) Dapprich, S.; Frenking, G. Organometallics
1996, 15, 4547–4551. (e) Vyboishchikov, S. F.; Frenking, G. Theor.
Chem. Acc. 1999, 102, 300–308.
(11) Decker, S. A.; Klobukowski, M. J. Am. Chem. Soc. 1998, 120, 9342–
9355.
(12) Frenking, G.; Dapprich, S.; Kohler, K. F.; Koch, W.; Collins, J. R.
Mol. Phys. 1996, 89, 1245–1263.
(13) Geometry optimization was undertaken using the B3LYP density
functional and BS1: 6-311+G(2d,p) basis set and LANL2DZ ECP
on iron, together with the 6-31G(d) basis set on all other atoms using
Gaussian03. CDA was performed with the CDA2.1 program developed
by S. Dapprich and G. Frenking (available from ftp.chemie.uni-
marburg.de) using the B3LYP/BS2 (6-311G(2d,p) basis set on iron,
together with the 6-31G(d) basis set on all other atoms) or the B3LYP/
BS3 (6-311G(2d,p) basis set and LANL2DZ ECP on iron, together
with the 6-31G(d) basis set on all other atoms). AIM calculations were
performed with AIMAll (version 08.05.04), Todd A. Keith, 2008
(available from aim.tkgristmill.com).
(16) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: Oxford, U.K., 1994.
(14) The inclusion of an all-electron basis set did not have any deleterious
effects on the CDA undertaken here, with the results being virtually
identical regardless of the basis set used.
(15) For donor-acceptor complexes, the absolute value of ∆ is generally
<0.05. For example, see: Frenking, G.; Fro¨hlich, N. Chem. ReV. 2000,
100, 717–774. Vyboishchikov, S. F.; Frenking, G. Chem.sEur. J.
1998, 8, 1428–1438, and ref 8.
(17) Frenking, G.; Sola, M.; Vyboishchikov, S. F. J. Organomet. Chem.
2005, 690, 6178–6204.
(18) By way of comparison, the Laplacian of the electron density in the
iron-alkyne bond of the donor-acceptor complex Fe(CO)4(C2H2) was
shown to be 0.358. In contrast, the Laplacian of the electron density
for the C-C bond (at -1.008) was found to be consistent with that
of shared, covalent interactions.11
6 Inorganic Chemistry, Vol. 48, No. 1, 2009

COMMUNICATION
Scheme 2
Figure 2. 15N{1H} NMR spectra (THF-d8, 298 K, 41 MHz, referenced to
nitromethane) of 1 (a) after the addition of KOBut and (b) after the addition
of 2,6-lutidinium triflate (A ) NH2NH2; B ) NH3NH32+).
spectroscopy (Scheme 2). An absorbance at 1928 cm-1 for
ν(15N2) matched exactly with that of an authentic sample of
[Fe(15N2)(dmpe)2] prepared by the reduction of trans-
[FeCl2(dmpe)2] with KC8 in hexane under nitrogen followed
by placement of the solution under an atmosphere of 15N2.
In the 15N NMR spectrum, two resonances at -47.7 and
-48.7 ppm were observed, which agree with those for the
authentic sample. The reaction probably proceeds via suc-
cessive deprotonation of the coordinated hydrazine to form
a coordinated diazene and then elimination of H2 to form
the dinitrogen species.25
We have demonstrated the chemistry of iron diazene and
hydrazine complexes, which now link iron dinitrogen,
diazene, hydrazine, and ammine complexes in a chain of
chemical interconversions. These results may lend further
insight into the mechanism of nitrogen reduction at early-
transition-metal centers.
protonation of residual hydrazine was also detected in the
reprotonation step. To the best of our knowledge, the
reversible interconversion of an iron(II) hydrazine to an
iron(0) diazene is unprecedented in the literature.
After several weeks, 15N NMR signals at -364.4 ppm for
19
+
ammonium (NH4 ) and at -417.6 ppm for an ammine
complex cis-[FeCl(NH3)(dmpe)2]+ (4) were detected in the
reaction mixture. The identity of 4 was confirmed by the
preparation of an authentic sample by the treatment of trans-
[FeCl2(dmpe)2] with a saturated solution of ammonia in
THF.20 The reaction of hydrazine complex 1 with 2,6-
lutidinium triflate to form NH4+ and ammine complex 4 has
been verified independently.
The treatment of 15N-labeled hydrazine complex 1 with a
mixture of KOBut and ButLi (Schlosser base)21 in hexane
afforded
the
end-on-bound
dinitrogen
complex
[Fe(N2)(dmpe)2] (5),3,22,23 as shown by IR and 15N NMR
Acknowledgment. We gratefully acknowledge financial
support from the Australian Research Council and the
Australian Partnership for Advanced Computing for super-
computing time.
(19) The 15N NMR chemical shift for ammonium was verified by a
comparison with an authentic sample of ammonium triflate prepared
in situ in THF.
(20) trans-[FeCl2(dmpe)2] (100 mg, 0.234 mmol) was dissolved in an
ammonia-saturated solution of THF (3 mL) under nitrogen and the
solution left to stand in a sealed flask for 9 days. The purple
precipitate formed was collected by filtration, dissolved in methanol,
and added to a solution of NaBPh4 (99 mg, 0.29 mmol) in methanol.
The flaky dark-pink crystalline solid was collected by filtration,
washed with methanol, and dried in vacuo (0.126 g, 71%). Elem
anal. Calcd for C36H55BClFeNP4 · CH4O (759.84): C, 58.5; H, 7.8;
N, 1.8. Found: C, 58.5; H, 7.9; N, 1.9%. 1H{31P} NMR (THF-d8,
400 MHz): δ 7.28 (m, 8H, o-Ph), 6.87 (m, 8H, m-Ph), 6.73 (m,
4H, p-Ph), 3.27 (s, 3H, CH3OH), 3.05 (br, 1H, CH3OH), 2.12-1.63
(m, 6H, CH2, overlap with THF-d8 residual), 1.55 (s, 3H, CH3),
1.52 (s, 3H, CH3), 1.48 (s, 3H, CH3), 1.47-1.36 (m, 5H, CH2 and
NH3), 1.44 (s, 3H, CH3), 1.15 (s, 3H, CH3), 1.07 (s, 6H, 2 × CH3),
0.87 (s, 3H, CH3). 31P{1H} NMR (THF-d8, 162 MHz): δ 73.0 (ddd,
Supporting Information Available: Experimental conditions,
spectroscopic data, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC801856Q
(22) Leigh, G. J.; Jimenez-Tenorio, M. J. Am. Chem. Soc. 1991, 113, 5862–
5863.
(23) The dinitrogen complex 5 is the major product in the reaction and is
formed in a yield of approximately 30% from 1 (by NMR spectros-
copy). Complex 5 is unstable and decomposes under reduced pres-
sure,3,22,24 and it was not possible to determine an isolated yield.
(24) Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J.; Rowley,
A. T. J. Chem. Soc., Dalton Trans. 1993, 3041.
(25) The possibility that the reaction proceeds by loss of diazene followed
by disproportionation to yield hydrazine and free dinitrogen, which
can be trapped by iron(0), can be discounted. The reaction is done
using 15N-labeled [Fe(N2H4)(dmpe)2]2+ 1 under a 14N atmosphere, and
there is little incorporation of 14N into the product Fe(N2)(dmpe)2 5
by IR spectroscopy.
2
2
2JP
) 32.8 Hz, JP
) 41.4 Hz, JP
) 41.2 Hz, 1P, PA),
A-PB
A-PC
A-PD
63.7 (ddd, 2JP
) 50.3 Hz, 2JP
) 146.0 Hz, 1P, PB), 60.9
B-PD
(ddd, 2JP
)B-3P1C.4 Hz, 1P, PC), 56.0 (ddd, 1P, PD). 15N{1H} NMR
C-PD
(THF-d8, 41 MHz, from HN-HSQC): δ -417.1 (correlation
1
with H δ 1.41, NH3). IR: 3537m, 3340m, 3321m, 3261w, 3232w,
3189w, 3055m, 3040m, 1620w, 1578m, 1558w, 1422s, 1302m,
1281m, 1266w, 1240s, 1186w, 1176w, 1131w, 1075w, 1033w,
1020s, 931s, 892s, 842s, 801w, 750s, 733s, 707s, 651m, 611s
cm-1
.
(21) Schlosser, M.; Strunk, S. Tetrahedron Lett. 1984, 25, 741–744.
Inorganic Chemistry, Vol. 48, No. 1, 2009 7