Diiron amido-imido complex [(Cp*Fe)2(μ2- NHPh)(μ2-NPh)]: Synthesis and a net hydrogen atom abstraction reaction to form a bis(imido) complex

Article abstract of DOI:10.1021/ic060744z

A reaction between [Cp*FeCl]_x and LiNHPh (1 equiv to Fe) produces a new paramagnetic Fe(II)-Fe(III) μ₂-amido- μ₂-imido complex [(Cp*Fe)₂(μ₂-NHPh) (μ₂-NPh)] (1), which, upon interaction with 2,2′-azobis(2,4- dimethylvaleronitrile), undergoes a net N-H hydrogen atom abstraction reaction to give a diamagnetic Fe(III)-Fe(III) μ₂-imido dimer [Cp*Fe(μ₂-NPh)]₂ (2). The molecular structures of 1 and 2 have been determined by single-crystal X-ray diffraction.

Full text of DOI:10.1021/ic060744z

Inorg. Chem. 2006, 45, 4871−4873
Diiron Amido−Imido Complex [(Cp*Fe)2(µ2-NHPh)(µ2-NPh)]: Synthesis
and a Net Hydrogen Atom Abstraction Reaction To Form a Bis(imido)
Complex
Shin Takemoto,† Shin-ichiro Ogura,† Ho Yo,† Yuko Hosokoshi,‡ Ken Kamikawa,† and
Hiroyuki Matsuzaka*,†
Department of Chemistry and Department of Physical Science, Graduate School of Science, Osaka
Prefecture UniVersity, Sakai, Osaka 599-8531, Japan
Received May 1, 2006
Scheme 1
A reaction between [Cp*FeCl]x and LiNHPh (1 equiv to Fe)
produces a new paramagnetic Fe(II)
complex [(Cp*Fe)2( 2-NHPh)( 2-NPh)] (1), which, upon interaction
with 2,2 -azobis(2,4-dimethylvaleronitrile), undergoes a net N
hydrogen atom abstraction reaction to give a diamagnetic Fe(III)
Fe(III) 2-imido dimer [Cp*Fe( 2-NPh)]2 (2). The molecular
−Fe(III) µ2-amido−µ2-imido
µ
µ
′
−H
−
µ
µ
structures of 1 and 2 have been determined by single-crystal X-ray
diffraction.
Amido and imido complexes of late transition metals have
attracted intense interest because of their importance in
understanding and developing catalytic transformations of
nitrogenous compounds.1 Pentamethylcyclopentadienyl (Cp*)
metal fragments have extensively been utilized in the study
on the nature of late metal-amido and -imido bonds in both
monomeric2 and bridged3 compounds. However, the chem-
istry of the corresponding Fe derivatives is far less developed,2d
although a growing number of Fe amido and imido com-
plexes have recently been reported for non-Cp Fe systems.4,5
We have recently shown the unique reactivity of the
diruthenium bridging amido complex [Cp*Ru(µ2-NHPh)]2,
including its conversion into imido complexes and C-N
bond formation with an alkyne.3h These findings prompted
us to develop a synthetic route to analogous Cp*Fe amido
and imido derivatives. Herein we report the synthesis of a
paramagnetic Fe(II)-Fe(III) µ2-amido-µ2-imido complex
[(Cp*Fe)2(µ2-NHPh)(µ2-NPh)] (1) and its transformation into
a diamagnetic bis(µ2-imido) complex [Cp*Fe(µ2-NPh)]2 (2).6
The interaction of [Cp*FeCl]x7 with LiNHPh (1 equiv to
Fe) did not give the expected Fe(II) µ2-amido complex
[Cp*Fe(µ2-NHPh)]2 but instead resulted in the formation of
the Fe(II)-Fe(III) µ2-amido-µ2-imido complex 1.8 After
recrystallization from hexanes, the product was isolated as
dark-green-brown crystals (44% yield based on Fe) and has
been characterized by spectroscopic [infrared, mass spec-
* To whom correspondence should be addressed. E-mail: matuzaka@
c.s.cias.osakafu-u.ac.jp.
† Department of Chemistry.
‡ Department of Physical Science.
(3) (a) Dobbs, D. A.; Bergman, R. G. Organometallics 1994, 13, 4594-
4605. (b) Holland, P. L.; Andersen, R. A.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 1092-1104. (c) Blake, R. E., Jr.; Heyn, R.
H.; Tilley, T. D. Polyhedron 1992, 11, 709-710. (d) Kuhlman, R.;
Folting, K.; Caulton, K. G. Organometallics 1995, 14, 3188-3195.
(e) Nakajima, Y.; Suzuki, H. Organometallics 2005, 24, 1860-1866.
(f) Matsuzaka, H.; Kamura, T.; Ariga, K.; Watanabe, Y.; Okubo, T.;
Ishii, T.; Yamashita, M.; Kondo, M.; Kitagawa, S. Organometallics
2000, 19, 216-218. (g) Matsuzaka, H.; Ariga, K.; Kase, H.; Kamura,
T.; Kondo, M.; Kitagawa, S.; Yamasaki, M. Organometallics 1997,
16, 4514-4516. (h) Takemoto, S.; Kobayashi, T.; Matsuzaka, H. J.
Am. Chem. Soc. 2004, 126, 10802-10803. (i) Takemoto, S.; Oshio,
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M.; Okimura, H.; Yamashita, M.; Miyasaka, H.; Ishii, T.; Yamashita,
M. Organometallics 2004, 23, 3587-3589.
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10.1021/ic060744z CCC: $33.50
Published on Web 06/03/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 13, 2006 4871

COMMUNICATION
1
trometry (MS), H NMR, and electron paramagnetic reso-
nance (EPR)], magnetometric (SQUID), and X-ray crystal-
lographic studies. The IR spectrum of 1 (THF) shows a
ν(N-H) band at 3296 cm-1, and the fast atom bombardment
(FAB)-MS spectrum gives an isotope cluster of the molecular
1
ion 1+ at m/z 565. The H NMR spectrum of 1 shows
paramagnetically shifted broad resonances. Although an
intense signal assignable to the Cp* group is observed at δ
3.76 ppm, other signals are not safely assigned because of
the unreliable integration of some extremely broad reso-
nances. The rhombic EPR spectrum of 1 (toluene, 77 K) is
1
consistent with the expected S ) /2 ground state, which is
also supported by the low-temperature SQUID magnetometry
(µeff ) 1.73 µB at 2 K). The moment increased as the
temperature was raised (2.85 µB at 300 K), indicating an
Figure 1. Molecular structure of 1. Ellipsoids are drawn at a 35% prob-
ability level, and hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)-Fe(2), 2.4916(6); Fe(1)-N(1), 1.990-
(4); Fe(2)-N(1), 2.007(4); Fe(1)-N(2), 1.751(3); Fe(2)-N(2), 1.747(4);
N(1)-C(1), 1.480(8); N(2)-C(7), 1.463(8); Fe(1)-N(1)-Fe(2), 77.12(15);
Fe(1)-N(1)-C(1), 114.4(6); Fe(2)-N(1)-C(1), 115.6(6); Fe(1)-N(2)-
Fe(2), 90.86(17); Fe(1)-N(2)-C(7), 130.1(5); Fe(2)-N(2)-C(7), 131.5(4).
(4) (a) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem.
Soc. 2006, 128, 5302-5303. (b) Thomas, C. M.; Mankad, N. P.; Peters,
J. C. J. Am. Chem. Soc. 2006, 128, 4956-4957. (c) Brown, S. D.;
Mehn, M. P.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 13146-
13147. (d) Duncan, J. S.; Nazif, T. M.; Verma, A. K.; Lee, S. C. Inorg.
Chem. 2003, 42, 1211-1224. (e) Link, H.; Decker, A.; Fenske, D. Z.
Anorg. Allg. Chem. 2000, 626, 1567-1574. (f) Barr, M. E.; Bjarnason,
A.; Dahl, L. F. Organometallics 1994, 13, 1981-1991. (g) Song, J.
S.; Geoffroy, G. L.; Rheingold, A. L. Inorg. Chem. 1992, 31, 1505-
1509. (h) Mu¨ller, J.; Sonn, I.; Alkhnoukh, T. J. Organomet. Chem.
1991, 414, 381-391.
(5) (a) Lucas, R. L.; Powell, D. R.; Borovik, A. S. J. Am. Chem. Soc.
2005, 127, 11596-11597. (b) Eckert, N. A.; Smith, J. M.; Lachicotte,
R. J.; Holland, P. L. Inorg. Chem. 2004, 43, 3306-3321. (c) Brown,
S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538-4539. (d)
Stokes, S. L.; Davis, W. M.; Odom, A. L.; Cummins, C. C.
Organometallics 1996, 15, 4521-4530. (e) Olmstead, M. M.; Power,
P. P.; Shoner, S. C. Inorg. Chem. 1991, 30, 2547-2551. (f) Power,
P. P. Comments Inorg. Chem. 1989, 8, 177-202. (g) Andersen, R.
A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert, M. F.; Leung,
W.-P.; Rypdal, K. Inorg, Chem. 1988, 27, 1782-1786. (h) Eller, P.
G.; Bradley, D. C.; Hursthouse, M. B.; Meek, D. W. Coord. Chem.
ReV. 1977, 24, 1-95.
antiferromagnetic coupling between the two high-spin Fe
centers. Similar behavior has been observed for other Fe-
(II)-Fe(III) complexes.4c
The single-crystal X-ray structure of 1 is shown in Figure
1.9 The molecule contains a nonplanar Fe2N2 core in which
the two FeN2 planes make a dihedral angle of 146.5°.10 The
two bridging N moieties are significantly dissimilar in their
geometries; the N(1) atom constitutes a distorted pyramid
with Fe(1), Fe(2), and C(1) atoms [sum of the bond angles
around N(1) ) 307°]11 and exhibits relatively long Fe-N
bond lengths [1.990(4) and 2.007(4) Å], while the N(2) atom
is nearly trigonal planar [sum of the bond angles around N(2)
) 353°] and shows very short Fe-N(2) bond distances
[1.747(4) and 1.751(3) Å]. These features are consistent with
the assignment of N(1) as an amido N and N(2) as an imido
N. The short Fe-Fe distance [2.4916(6) Å] may suggest an
Fe-Fe multiple bond.12
The formation of the mixed-valence complex 1 is rather
unexpected and markedly contrasts to the near-quantitative
formation of the Ru(II) dimer [Cp*Ru(µ2-NHPh)]2 from
[Cp*RuCl]4 and LiNHPh.3h The mechanism of the partial
oxidation of the Fe center in the synthesis of 1 is currently
unknown, but some redox reaction between the intermediate
Fe(II) amido species and the residual [Cp*FeCl]x may be a
possibility.
(6) Tatsumi and co-workers independently reported the synthesis of 2,
which involves a quite different approach from those reported herein:
Ohki, Y.; Takikawa, Y.; Hatanaka, T.; Tatsumi, K. Organometallics
2006, ASAP article.
(7) (a) Shintani, R.; Fu, G. Org. Lett. 2002, 4, 3699-3702. (b) Stephan,
M.; Mu¨ller, P.; Zenneck, U.; Pritzkow, H.; Siebert, W.; Grimes, R.
N. Inorg. Chem. 1995, 34, 2058-2067.
(8) Preparation of [(Cp*Fe)2(µ2-NHPh)(µ2-NPh)] (1). To a stirred solution
of 1,2,3,4,5-pentamethylcyclopentadiene (0.72 mL, 4.6 mmol) in 12
mL of THF was added n-BuLi (1.85 mL, 2.5 M solution in n-hexane)
at 0 °C. The resulting white suspension was cooled to -80 °C and
then transferred via a cannula to a cooled (-80 °C) suspension of
anhydrous FeCl2 (583 mg, 4.6 mmol) in THF (15 mL). After the
transfer was complete, the mixture was stirred for 30 min, while the
temperature was maintained at -80 °C. Separately, a THF solution
of LiNHPh was prepared by adding n-BuLi (1.85 mL, 2.5 M solution
in n-hexane) to a solution of aniline (0.43 mL, 4.6 mmol) in THF (10
mL) at -80 °C followed by warming to room temperature. The
solution of LiNHPh was cooled again to -80 °C and then transferred
via a cannula to the cooled suspension of [Cp*FeCl]x. After the
addition was complete, the mixture was allowed to warm slowly to
room temperature and stirred for 16 h. The dark-greenish-brown
solution obtained was evaporated to dryness, and the residue was
extracted with hexanes (70 mL). The extract was concentrated to ca.
35 mL and then stored in a freezer (-25 °C). After 2 days, dark-
green-brown crystals of 1 were collected by filtration and dried in
vacuo. Yield: 565 mg, 44%. Anal. Calcd for C32H41N2Fe2: C, 67.98;
H, 7.31; N, 4.95. Found: C, 67.26; H, 7.66; N, 4.32. The low C
percentage could be due to the extreme O sensitivity of the compound.
Repeated analyses failed to give the expected results. IR (THF): 3296
cm-1 [ν(N-H)]. MS (FAB): m/z 565 [M]+. 1H NMR (toluene-d8):
δ 16.1 (br, 2H), 7.53 (br, 2H), 4.39 (br, 1H), 3.76 (br, 30H), 0.20 (br,
2H), -34.48 (br, ∼2H), -43.43 (br, ∼2H). The spectral pattern retains
over the temperature range of +80 to -20 °C. The signals became
extremely broad at lower temperature. The EPR and SQUID data are
provided in the Supporting Information.
(9) Crystal data for 1: C32H41N2Fe2, fw ) 565.37, orthorhombic, space
group Pnn2, a ) 12.0653(17) Å, b ) 15.218(2) Å, c ) 15.969(3) Å,
V ) 2939.0(8) Å3, T ) 296 K, Z ) 4, µ(Mo KR) ) 1.011 mm-1
,
27 158 reflections measured, 6715 unique (Rint ) 0.0654), R1 )
0.0485, wR2 ) 0.1309, GOF ) 1.008. The refinement was successful
with the acentric Pnn2 as a racemic twin (Flack parameter 0.33), while
no workable solution was obtained with centrosymmetric Pnnm.
Crystal data for 2: C32H40N2Fe2, fw ) 1075.03, monoclinic, space
group C2/c, a ) 17.918(6) Å, b ) 11.820(3) Å, c ) 13.751(3) Å, â
) 93.07(2)°, V ) 2908.1(14) Å3, T ) 296 K, Z ) 4, µ(Mo KR) )
1.020 mm-1, 13 725 reflections measured, 3308 unique (Rint ) 0.0478),
R1 ) 0.0373, wR2 ) 0.0943, GOF ) 0.993.
(10) The deviation from planarity is smaller than that for the Ru2N2 core
of [Cp*Ru(µ2-NHPh)]2 (121.6° between RuN2 planes). See ref 4e.
(11) The N-H hydrogen atom was not found in the Fourier synthesis and,
hence, was located at the calculated position.
(12) For example, see: (a) Ohki, Y.; Kojima, T.; Oshima, M.; Suzuki, H.
Organometallics 2001, 20, 2654-2656. (b) Scha¨ufele, H.; Pritzkow,
H.; Zenneck, U. Angew. Chem., Int. Ed. 1988, 27, 1519-1521.
4872 Inorganic Chemistry, Vol. 45, No. 13, 2006

COMMUNICATION
(18) Å]. These features indicate strong π donation from imido
N atoms to the coordinatively unsaturated Fe centers.
The imido dimer 2 can also be prepared via a cationic
Fe(III)-Fe(III) complex [(Cp*Fe)2(µ2-NHPh)(µ2-NPh)]OTf
(1+OTf-), which is cleanly produced upon oxidation of 1
with ferrocenium triflate (Scheme 1).18 The salt 1+OTf-,
which is soluble in acetone and acetonitrile, moderately
soluble in THF, but insoluble in toluene and hexanes, was
isolated in 70% yield as an olive-green microcrystalline solid
and has been characterized by elemental analysis and NMR
(1H and 13C) spectroscopy. Deprotonation of 1+OTf- with
NaN(SiMe3)2 affords the bis(µ2-imido) complex 2 in 51%
isolated yield,19 while no reaction took place with a weaker
base, NEt3.
In summary, this work has provided access to new
dinuclear Cp*Fe bridging amido and imido complexes. The
reaction between [Cp*FeCl]x and LiNHPh proceeded via
partial oxidation of the Fe(II) centers to yield the paramag-
netic Fe(II)-Fe(III) µ2-amido-µ2-imido complex 1. The
mixed-valence complex 1 can be transformed into the Fe-
(III) µ2-imido dimer 2 either directly by interaction with an
azobis(isobutyronitrile)-like azo compound or in a stepwise
manner via 1+OTf-. The former reaction indicated the ability
of the Fe-bound amido NH moiety to undergo homolytic
N-H bond cleavage. Works are now in progress to find out
the further reactivity of these complexes as well as their
applications to catalysis.
Figure 2. Molecular structure of 2. Ellipsoids are drawn at a 35%
probability level, and hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)-Fe(1)*, 2.44492(9); Fe(1)-N(1),
1.8022(19); Fe(1)*-N(1), 1.8100(18); N(1)-C(1), 1.402(3); Fe(1)-N(1)-
Fe(1)*, 85.38(8); Fe(1)-N(1)-C(1), 135.63(15); Fe(1)*-N(1)-C(1),
138.79(15).
Treatment of 1 with a radical precursor 2,2′-azobis(2,4-
dimethylvaleronitrile) produces a diamagnetic bis(µ2-imido)
complex 2 (Scheme 1).13 Homolytic N-H hydrogen atom
abstraction by the 2-cyano-4-methylpentyl radical is a likely
mechanism,14-17 though we cannot rule out the possibility
of a stepwise oxidation-deprotonation pathway. Complex
2 crystallizes as dark-brown plates (45% yield) from hexanes
1
and has been characterized by H NMR spectroscopy and
single-crystal X-ray diffraction (Figure 2). The molecule is
composed of the two inversion-related “Cp*FeNPh” units,
which constitute a planar Fe2N2 ring with an Fe-Fe
separation of 2.44492(9) Å. The bridging imido moieties are
planar at N (sum of the bond angles around N ) 360°) and
exhibit short Fe-N bond distances [1.8022(19) and 1.8100-
Acknowledgment. We are grateful for the financial
support from Toyota Motor Corp. and the Ministry of
Education, Science, Sports, and Culture of Japan (Grants
17750057 and 17036059 Research on Priority Area “Chem-
istry of Coordination Space”). We also thank Prof. Satoshi
Kawata at Osaka University for his assistance in EPR
measurement.
(13) Preparation of [Cp*Fe(µ2-NPh)]2 (2). Method A: To a stirred solution
of 1 (162 mg, 0.287 mmol) in 10 mL of hexanes was added 2,2′-
azobis(2,4-dimethylvaleronitrile) (71 mg, 0.287 mmol). The mixture
was stirred at 50 °C for 24 h, at which time the dark-brown mixture
was filtered hot. Allowing the filtrate to stand at -25 °C afforded
dark-brown plates, which were collected by filtration, washed twice
with cold hexanes (3 mL, -25 °C), and then dried in vacuo. Yield:
73 mg, 45%. Method B: To a stirred suspension of 1+OTf- (97 mg,
0.136 mmol) in 10 mL of THF was added NaN(SiMe3)2 (0.15 mL,
1.1 M solution in THF, 0.165 mmol) at room temperature. After stirring
for 2 h, the mixture was evaporated to dryness and the residue extracted
with hexanes (15 mL). Concentration to ca. 5 mL and cooling to -25
°C afforded dark-brown plates, which were collected by filtration and
dried in vacuo. Yield: 39 mg, 51%. 1H NMR (C6D6): δ 7.98 (m, 4H,
Ph), 7.28 (m, 2H, Ph), 6.62 (m, 4H, Ph), 2.48 (s, 30H, Cp*). Anal.
Calcd for C32H40N2Fe2: C, 68.10; H, 7.14; N, 4.96. Found: C, 67.01;
H, 7.31; N, 5.05. Repeated analyses failed to give the expected results.
(14) The reaction may have some mechanistic relevance to the oxidative
deprotonation of coordinated amines observed in ruthenium porphyrin
systems: (a) Huang, J.-S.; Leung, S. K.-Y.; Zhou, Z.-Y.; Zhu, N.;
Che, C.-M. Inorg. Chem. 2005, 44, 3780-3788. (b) Huang, J.-S.; Sun,
X.-R.; Leung, S. K.-Y.; Cheung, K.-K.; Che, C.-M. Chem.sEur. J.
2000, 6, 334-344.
(15) Hydrogen atom abstraction from Fe and Mn hydroxo complexes is
known: Gupta, R.; Borovik, A. S. J. Am. Chem. Soc. 2003, 125,
13234-13242.
(16) Hydrogen atom transfer involving N-H bond cleavage: Mader, E.
A.; Larsen, A. S.; Mayer, J. M. J. Am. Chem. Soc. 2004, 126, 8066-
8067.
(17) Hydrogen atom addition to an imido N atom: (a) Thyagarajan, S.;
Shay, D. T.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J.
Am. Chem. Soc. 2003, 125, 4440-4441. (b) Kogut, E.; Wiencko, H.
L.; Zhang, L.; Cordeau, D. E.; Warren, T. H. J. Am. Chem. Soc. 2005,
127, 11248-11249. (c) Lucas, R. L.; Powell, D. R.; Borovik, A. S. J.
Am. Chem. Soc. 2005, 127, 11596-11597.
Supporting Information Available: Experimental details and
characterization data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
IC060744Z
(18) Preparation of [(Cp*Fe)2(µ2-NHPh)(µ2-NPh)]OTf (1+OTf-). To a
cooled (-80 °C) solution of 1 (359 mg, 0.635 mmol) in 20 mL of
THF was added [Cp2Fe]OTf (212 mg, 0.631 mmol), and the mixture
was allowed to warm to room temperature with stirring over 17 h to
form a green suspension. The mixture was evaporated to dryness, and
the residue was extracted with acetone (20 mL). Evaporation of the
solvent gave an olive-green solid, which was washed twice with diethyl
ether (15 mL) and dried in vacuo. Yield: 317 mg, 70%. Anal. Calcd
for C33H41N2O3F3SFe2: C, 55.48; H, 5.78; N, 3.92. Found: C, 55.06;
H, 5.37; N, 4.02. 1H NMR (acetone-d6): δ 8.82 (d, 1H, Ph), 7.62 (m,
2H, Ph), 7.55 (t, 2H, Ph), 7.34 (m, 1H, Ph), 7.27 (t, 1H, Ph), 7.12 (d,
1H, Ph), 6.71 (t, 1H, Ph), 3.82 (d, 1H, Ph), 3.76 (s, 1H, NH), 1.16 (s,
30H, Cp*). 13C{1H} NMR (acetone-d6): δ 171.39 (s, Ph), 164.09 (s,
Ph), 130.24 (s, Ph), 130.18 (s, Ph), 129.79 (s, Ph), 129.69 (s, Ph),
128.76 (s, Ph), 126.16 (s, Ph), 124.48 (s, Ph), 121.54 (q, CF3), 120.23
(s, Ph), 119.14 (s, Ph), 118.63 (s, Ph), 97.48 (s, C5Me5), 9.38 (s,
C5Me5). The 1H NMR data contain an unusually high-field-shifted
phenyl resonance (δ 3.82). The 1H-1H and 13C-1H COSY experi-
ments suggest that the high-field aryl proton would be one of the
o-phenyl protons (possibly of the amido phenyl). Because the 1JCH of
160 Hz disagrees with an agostic interaction, we tentatively assume
the effect of the Cp* ring current as a reason for such a high-field
shift.
(19) The synthetic strategy is similar to that employed for the conversion
of a Ni(I) amide to a Ni(II) imide: Midiola, D. J.; Hillhouse, G. L. J.
Am. Chem. Soc. 2001, 123, 4623-4624.
Inorganic Chemistry, Vol. 45, No. 13, 2006 4873