Reductive N-N bond cleavage of diphenylhydrazine and azobenzene induced by coordinatively unsaturated Cp*Fe{N(SiMe3)2}

Article abstract of DOI:10.1021/om0602963

The coordinatively unsaturated iron(II) amido complex Cp* Fe{N(SiMe₃)₂} (1) serves as an efficient precursor for the cleavage of the N-N bond of diphenylhydrazine. The N=N bond of azobenzene was also found to be activated by 1 in the

Full text of DOI:10.1021/om0602963

Organometallics 2006, 25, 3111-3113
3111
Reductive N-N Bond Cleavage of Diphenylhydrazine and
Azobenzene Induced by Coordinatively Unsaturated
Cp*Fe{N(SiMe3)2}
Yasuhiro Ohki,* Yuko Takikawa, Tsubasa Hatanaka, and Kazuyuki Tatsumi*
Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya
UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
ReceiVed April 4, 2006
Scheme 1
Summary: The coordinatiVely unsaturated iron(II) amido com-
plex Cp*Fe{N(SiMe3)2} (1) serVes as an efficient precursor for
the cleaVage of the N-N bond of diphenylhydrazine. The NdN
bond of azobenzene was also found to be actiVated by 1 in the
presence of pinacolborane.
The unique structures and function of nitrogenase active sites
have inspired chemists to synthesize molecular models capable
of cleaving N-N bonds.1 Iron is a common transition element
to the active sites of MoFe-, VFe-, and Fe-nitrogenases,2 and
thus reactions of iron toward nitrogenase-related substrates such
as N2, hydrazines, and azo compounds are of interest. Various
dinitrogen complexes of iron have been synthesized,3,4 and Fe-
(DMeOPrPE)2(N2) (DMeOPrPE ) 1,2-bis(bis(methoxypropyl)-
phosphino)ethane), which was spectroscopically characterized
in solution, was recently demonstrated to promote the partial
Bis(trimethylsilyl)amide on transition metals is known to
serve as a base. Indeed, treatment of Cp*Fe{N(SiMe3)2} (1)
with 1 equiv of N-methylaniline gave rise to a dimeric
methylphenylamido complex, Cp*2Fe2(µ-NMePh)2 (2), in 79%
yield via deprotonation of methylaniline (Scheme 1). Complex
+
conversion of the iron-bound N2 to N2H5 and ammonium
cation.4e Whereas the N-N bond cleavage of hydrazines has
been known to occur with early-transition-metal complexes and
some ruthenium complexes,5 there have only been a few
examples of such reactions induced by iron.6 We became
interested in the activation of N-N and NdN bonds by
organometallic iron complexes, in connection with our studies
on modeling the active sites of nitrogenase.7 We herein report
that a coordinatively unsaturated monomeric iron(II) amido
complex reported by Siemeling et al, Cp*Fe{N(SiMe3)2} (1),8
serves as an efficient precursor for the cleavage of N-N bond
of diphenylhydrazine. The NdN bond of azobenzene was also
found to be activated by 1 in the presence of 4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (pinacolborane).
1
2 is paramagnetic at ambient temperature, and the H NMR
spectrum features by broad signals at δ 91.7 (Cp*), at δ 72.1
(N-Me), and at δ 84.6, -28.8, and -33.7 (N-Ph). The X-ray
diffraction study confirmed the formulation of 2 as shown in
Figure 1 (left).9 Complex 2 has an inversion center at the
midpoint between the two iron atoms, and the amide nitrogens
and irons are crystallographically coplanar. The Fe-Fe distance
is 2.7925(8) Å, and the Fe-N distances (2.063(4) and 2.087(4)
(5) (a) Schrock, R. R.; Glassman, T. E.; Vale, M. G.; Kol, M. J. Am.
Chem. Soc. 1993, 115, 1760-1772. (b) Kuwata, S.; Mizobe, Y.; Hidai, M.
Inorg. Chem. 1994, 33, 3619-3620. (c) Malinak, S. M.; Coucouvanis, D.
J. Am. Chem. Soc. 1995, 117, 3126-3133. (d) Scholhammer, P.; Petillon,
F. Y.; Poder-Guillou, S.; Saillard, J. Y.; Talarmin, J.; Muir, K. W. Chem.
Commun. 1996, 2633-2634. (e) Heaton, B. T.; Jacob, C.; Page, P. Coord.
Chem. ReV. 1996, 154, 193-229. (f) Jahncke, M.; Neels, A.; Stoeckli-
Evans, H.; Vale, M. G.; Su¨ss-Fink, G. J. Organomet. Chem. 1998, 565,
97-103. (g) Scholhammer, Y.; Guenin, E.; Petillon, F. Y.; Talarmin, J.;
Muir, K. W. Inorg. Chem. 1999, 38, 1954-1955. (h) Nakajima, Y.; Suzuki,
H. Organometallics 2003, 22, 959-969. (i) Seino, H.; Masumori, T.; Hidai,
M.; Mizobe, Y. Organometallics 2003, 22, 3424-3431. (j) Nakajima, Y.;
Inagaki, A.; Suzuki, H. Organometallics 2004, 23, 4040-4046. (k) Shaver,
M. P.; Fryzuk, M. D. J. Am. Chem. Soc. 2005, 127, 500-501. (l) Nakajima,
Y.; Suzuki, H. Organometallics 2005, 24, 1860-1866.
* To whom correspondence should be addressed. E-mail: i45100a@
nucc.cc.nagoya-u.ac.jp (K.T.); ohki@mbox.chem.nagoya-u.ac.jp (Y.O.).
Fax: int. code +81-52-789-2943.
(1) (a) Barrie´re, F. Coord. Chem. ReV. 2003, 236, 71-89. (b) Kozak, C.
M.; Mountford, P. Angew. Chem., Int. Ed. 2004, 43, 1186-1189.
(2) (a) Howard, J. B.; Rees, D. C. Chem. ReV. 1996, 96, 2965-2982.
(b) Burgess, B, K.; Lowe, D. J. Chem. ReV. 1996, 96, 2983-3011. (c) Eady,
R. R. Chem. ReV. 1996, 96, 3013-3030.
(3) For reviews, see: (a) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem.
ReV. 1978, 78, 589-625. (b) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95,
1115-1133. (c) Richards, R. L. Coord. Chem. ReV. 1996, 154, 83-97. (d)
Hidai, M. Coord. Chem. ReV. 1999, 185-186, 99-108. (e) Fryzuk, M. D.;
Johnson, S. A. Coord. Chem. ReV. 2000, 200-202, 379-409. (f) Schrock,
R. R. Chem. Commun. 2003, 2389-2391. (g) MacKay, B. A.; Fryzuk, M.
D. Chem. ReV. 2004, 104, 385-401. (h) Schrock, R. R. Acc. Chem. Res.
2005, 38, 955-962.
(4) For recent examples, see: (a) Smith, J. M.; Lachicotte, R. J.; Pittard,
K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L.
J. Am. Chem. Soc. 2001, 123, 9222-9223. (b) Betley, T. A.; Peters, J. C.
J. Am. Chem. Soc. 2003, 125, 10782-10783. (c) Betley, T. A.; Peters, J.
C. J. Am. Chem. Soc. 2004, 126, 6252-6253. (d) Bart, S. C.; Lobkovsky,
E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794-13807. (e) Gilbertson,
J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 129, 10184-
10185.
(6) (a) Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 1999, 121, 10838-
10839. (b) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem.
Soc. 2003, 125, 15752-15753.
(7) (a) Ohki, Y.; Sunada, Y.; Honda, M.; Katada, M.; Tatsumi, K. J.
Am. Chem. Soc. 2003, 125, 4052-4053. (b) Ohki, Y.; Sunada, Y.; Tatsumi,
K. Chem. Lett. 2005, 34, 172-173. (c) Takuma, M.; Ohki, Y.; Tatsumi, K.
Organometallics 2005, 24, 1344-1347. (d) Arikawa, Y.; Kawaguchi, H.;
Kashiwabara, K.; Tatsumi, K. Inorg. Chem. 2002, 41, 513-520. (e)
Kawaguchi, H.; Yamada, K.; Ohnishi, S.; Tatsumi, K. J. Am. Chem. Soc.
1997, 119, 10871-10872.
(8) Siemeling, U.; Vorfeld, U.; Neumann, B.; Stammler, H.-G. Orga-
nometallics 1998, 17, 483-484.
10.1021/om0602963 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/19/2006

3112 Organometallics, Vol. 25, No. 13, 2006
Communications
Figure 1. Molecular structures of 2-4, with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angles (deg):
2, Fe-Fe′ ) 2.7925(8), Fe-N ) 2.063(4), Fe-N′ ) 2.087(4), Fe-N-Fe′ ) 84.58(19); 3, Fe-Fe′ ) 2.4389(5), Fe-N ) 1.804(1), Fe-N′
) 1.807(1), Fe-N-Fe′ ) 84.99(5); 4, Fe-Fe′ ) 2.5353(6), Fe-N ) 1.979(2), Fe-N-Fe′ ) 79.69(9).
Scheme 2
cooperation of two iron centers must be important for both
substrate binding and successive N-N bond cleavage, because
two electrons are needed for the reductive cleavage of hydrazine.
The requirement of multiple iron centers is of possible relevance
to the proposal that multiple iron centers could be the site of
N2 activation,12 whereas the possibility of molybdenum-
coordinated N2 is also described.13
The isolation of 3 prompted us to examine activation of the
NdN bond of azobenzene by 1. It was anticipated that
azobenzene would not be activated by 1 alone, because unlike
the case for diphenylhydrazine there are no protons in azoben-
zene to react with the amide of 1. In fact, treatment of 1 with
azobenzene ended up with mere recovery of the reactants. We
sensed that azobenzene might be activated if external proton or
Lewis acid generates “Cp*Fe” or “Cp*2Fe2” from 1 as a reactive
moiety, in light of the possible intermediate A for the reaction
between 1 and diphenylhydrazine to give 3. Thus, we carried
out an NMR-tube experiment in benzene-d6 for the reaction of
1 with azobenzene in the presence of 1 equiv of pinacolborane
(HBpin), to find that 3 was generated in 29% yield. Unfortu-
nately, this reaction is not very clean, and Cp*2Fe2(µ-H){µ-
N(SiMe3)2} (4) and Cp*2Fe were also detected in 17% and 25%
yields, respectively. Since the solubility of 3 in common organic
solvents is similar to that of 4 and Cp*2Fe, isolation of 3 from
the reaction mixture has not been successful. Nonetheless, it is
interesting to find that 1 can act as a precursor of active species
for the NdN bond cleavage of azobenzene.
To verify the formation of the dinuclear hydride/amide 4,
the amide complex 1 was treated with 1/2 equiv of HBpin. From
this reaction, 4 was isolated as dark brown crystals in 24% yield,
and its crystal structure was determined. As shown in Figure 1
(right), two iron atoms of 4 are bridged by one amide and one
hydride with an Fe-Fe distance of 2.5353(6) Å, where the
Fe-N distance (1.979(2) Å) is between those of 2 and 3. The
formation of 4 indicates that HBpin served as a hydride source
to replace the amide of 1,14 generating the aminoborane (Me3-
Si)2N-Bpin and the putative hydride compound “Cp*FeH”,
Å) are similar to those in the related amido-bridged complexes
of iron(II).10
Interestingly, when 1 was reacted with 1,2-diphenylhydrazine,
not only deprotonation of hydrazine but also N-N bond
cleavage took place. Addition of 1,2-diphenylhydrazine into a
toluene solution of 1 led to a dark brown solution, from which
the dinuclear ferric imido complex Cp*2Fe2(µ-NPh)2 (3) was
obtained in 59% yield as a brown solid (Scheme 2). The X-ray-
derived molecular structure is given at the center of Figure 1.
Complex 3 is diamagnetic according to the 1H NMR spectrum,
and the two iron(III) centers are magnetically coupled through
the short Fe-Fe (2.4389(5) Å) bond. The bridging imido
nitrogens donate π-electrons into the vacant Fe dπ orbitals
perpendicular to the Fe2N2 plane, and the consequence is the
very short Fe-N distance (1.804(1), 1.807(1) Å). The reaction
to give 3 was presumably initiated by a deprotonation step of
diphenylhydrazine with 2 equiv of 1, and the N-N bond
cleavage could have proceeded via the resulting azobenzene-
bridged dinuclear intermediate Cp*2Fe2(µ-N2Ph2) (A in Scheme
3). The related dinuclear iron fragments have been reported to
bind hydrazines, azobenzene, and molecular nitrogen.4,11 The
(11) (a) Doedens, R. J.; Ibers, J. A. Inorg. Chem. 1969, 8, 2709-2714.
(b) Doedens, R. J. Inorg. Chem. 1970, 9, 429-436. (c) Little, R. G.;
Doedens, R. J. Inorg. Chem. 1972, 11, 1392-1397. (d) Wucherer, E. J.;
Tasi, M.; Hansert, B.; Powell, A. K.; Garland, M.-T.; Halet, J.-F.; Saillard,
J.-Y.; Vahrenkamp, H. Inorg. Chem. 1989, 28, 3564-3572. (e) Zimniak,
A. J. Organomet. Chem. 2002, 645, 274-280.
(9) Crystal data for 2: triclinic, P1h, a ) 10.349(8) Å, b ) 12.101(4) Å,
c ) 12.420(3) Å, R ) 91.378(10)°, â ) 90.12(6)°, γ ) 99.34(4)°, V )
1534.3(14) Å3, Z ) 2, Fcalcd ) 1.287 g cm-3, 6745 reflections (2θ e 55°),
5290 observed with F > 2σ(F), 333 parameters, R ) 0.083, Rw ) 0.110,
GOF ) 1.47. Crystal data for 3: monoclinic, C2/c, a ) 17.794(3) Å, b )
11.639(2) Å, c ) 13.517(2) Å, â ) 93.709(2), V ) 2793.6(7) Å3, Z ) 8,
(12) (a) Huniar, U.; Ahlrichs, R.; Coucouvanis, D. J. Am. Chem. Soc.
2004, 126, 2588-2601. (b) Dance, I. J. Am. Chem. Soc. 2004, 126, 11852-
11863.
F
calcd ) 1.330 g cm-3, 3184 reflections (2θ e 55°), 3151 observed with F
> 0σ(F), 163 parameters, R ) 0.030, Rw ) 0.036, GOF ) 1.31. Crystal
data for 4: orthorhombic, Fddd, a ) 20.681(3) Å, b ) 22.191(3) Å, c )
25.607(3) Å, V ) 11751.4(2) Å3, Z ) 16, Fcalcd ) 1.229 g cm-3, 3377
reflections (2θ e 55°), 3361 observed with F > 0σ(F), 142 parameters, R
) 0.046, Rw ) 0.047, GOF ) 0.90.
(10) Olmstead, M. M.; Power, P. P.; Schoner, S. C. Inorg. Chem. 1991,
30, 2547-2551.
(13) (a) Gro¨nberg, K. L. C.; Gormal, C. A.; Durrant, M. C.; Smith, B.
E.; Heenderson, R. A. J. Am. Chem. Soc. 1998, 120, 10613-10621. (b)
Henderson, R. A. Chem. ReV. 2005, 105, 2365-2437.
(14) The related hydroboration of coordinated dinitrogen is known:
Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew. Chem.,
Int. Ed. 2002, 41, 3709-3712.

Communications
Organometallics, Vol. 25, No. 13, 2006 3113
Scheme 3
which then reacted with the remaining 1 to give the dinuclear
complex 4. The formation of (Me3Si)2N-Bpin was confirmed
by a 1H NMR spectrum in benzene-d6, where the methyl signals
of aminoborane appeared at δ 1.03 and 0.32. Complex 4 was
found not to react with azobenzene.
Taking the above results into consideration, we propose a
possible mechanism for the NdN bond cleavage of azobenzene
promoted by 1 and HBpin. As is the case of the reaction between
1 and 1/2 equiv of HBpin to give 4, the NdN activation reaction
probably proceeds by way of the hydride species “Cp*FeH” or
its dimerized form “Cp*2Fe2(µ-H)2”, which may be stabilized
with azobenzene, solvent, or aminoborane. The hydride com-
plex, in either a monomeric or a dimeric form, would take up
azobenzene, leading probably to the azobenzene/hydride-bridged
dinuclear intermediate Cp*2Fe2(µ-H)2(µ-N2Ph2) (B). Reductive
elimination of H2 from B supplies two electrons to the dinuclear
iron center, giving rise to Cp*2Fe2(µ-N2Ph2) (A), which may
in turn facilitate the reductive cleavage of the NdN bond. The
observation of a singlet signal at δ 4.45 in the NMR experiment
indicates the formation of H2, whereas a quantitative analysis
was not available. As mentioned earlier in this paper, A is also
a possible intermediate in the reaction of 1 with diphenylhy-
drazine.
Acknowledgment. This research was financially supported
by Grants-in-Aid for Scientific Research (Nos. 14078211 and
17036020) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
Supporting Information Available: Text, a table, and CIF files
giving details of the synthesis and characterization and crystal-
lographic data for 2-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0602963