Lanthanide-imido complexes and their reactions with benzonitrile

Article abstract of DOI:10.1002/anie.200461939

Highly reactive imido ligands, even when in a bridging mode as in 1, distinguish lanthanide-imido complexes from their d-block counterparts. For example, imido species 1 can undergo nucleophilic addition to benzonitrile to give 2. Catalytic cyclotrimerization of benzonitrile occurs in the presence of 1 or 2 by a unique combination of C-N bond-formation, hydrogen-transfer, and C-N bond-cleavage reactions (see scheme).

Full text of DOI:10.1002/anie.200461939

Angewandte
Chemie
Cyclotrimerization
Lanthanide–Imido Complexes and Their
Reactions with Benzonitrile**
Dongmei Cui, Masayoshi Nishiura, and Zhaomin Hou*
Transition-metal–imido complexes have received much cur-
=
rent interest because of the ability of the M N group to
undergo a wide range of reactions[1] such as metathesis of
imines, aldehydes, and carbodiimides,[2] metallacycle forma-
tion with alkenes and alkynes,[3] hydroamination of alkynes
[5]
and allenes,[4] and C H bond activation. In contrast to the
À
extensive studies on the rich chemistry of transition-metal–
imido complexes, reports on well-defined lanthanide–imido
complexes are scarce[6] and the reactions of lanthanide–imido
complexes have remained almost unexplored to date. This is
mainly due to the lack of suitable complexes for reactivity
studies.[7]
During our recent studies on lanthanide–polyhydrido
complexes,[8] we found that reactions of the tetranuclear
lanthanide polyhydrido complexes [{Cp’Ln(m-H)2}4(thf)]
[*] Dr. D. Cui, Dr. M. Nishiura, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory
RIKEN (The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
and
PRESTO
Japan Science and Technology Agency (JST) (Japan)
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 14078224, “Reaction Control of
Dynamic Complexes”) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. We are also grateful to the
Japan Society for the Promotion of Science (JSPS) for a Postdoctoral
Fellowship for D.C.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 959 –962
DOI: 10.1002/anie.200461939
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
959

Communications
(Cp’ = h5-C5Me4SiMe3; Ln = Lu, Y) with nitriles RCN can
the amido nitrogen atom is bonded terminally to one Lu atom
À
offer a convenient route to the corresponding imido com-
(Lu N 2.352(4) ꢀ). The formation of 2 apparently proceeded
ꢀ
À
plexes [{Cp’Ln(m3-NCH2R)}4] by double addition of the Ln H
by nucleophilic addition of the imido unit to the C N group of
[8a]
ꢀ
units across the C N bond. The imido species formed in
benzonitrile, which demonstrates that a lanthanide–imido
bond, even in a bridging form, is very reactive. This is in
contrast with what was observed previously for bonds
between transition metals and bridging imido ligands, which
are usually robust and unreactive.[1–5] Moreover, the resultant
product 2 represents the first metal complex of an amidinate
dianion, in contrast to the well-known complexes of various
metals with amidinate monoanions [RNC(R’)NR]À as
ligands.[10]
Interestingly, the reaction of the ethylimido complex
[{Cp’Y(m3-NCH2CH3)}4] (1c) with benzonitrile did not afford
an insertion product, but instead gave the mixed benzaldimi-
nato/ethylimido complex 4 (Scheme 1). X-ray analysis estab-
lished that 4 adopts a tetranuclear crownlike core structure, in
which the four Y atoms are unsymmetrically bridged by four
this way have a methylene substituent at the N atom and
adopt a novel Ln4N4 cubane core structure with m3-N-Ln
bridges, a structural feature that has not been observed
previously. Here we report that this new class of imido
complexes can show unprecedented reactivity. They not only
exhibit strong nucleophilicity but can also transfer a-hydro-
gen atoms.
Reaction of benzylimido–lutetium complex [{Cp’Lu(m3-
NCH2Ph)}4] (1a) with 4 equivalents of benzonitrile at room
temperature in toluene gave the benzamidinate dianion
complex 2 in 73% yield after recrystallization from THF
(Scheme 1).[9] X-ray analysis showed that 2 has a dimeric
structure (Figure 1) in which the two Lu atoms are bridged by
À
the ketimido nitrogen atom (Lu N 2.149(4), 2.326(4) ꢀ), and
À
m2-benzaldiminato and two m3-ethylimido ligands (Y N(m2-
iminato) 2.298(7)–2.532(7) ꢀ, Y N(m3-imido) 2.116(6)–
2.418(6) ꢀ, mean C N(iminato) 1.248(10) ꢀ, mean
À
À
À
C
N(imido) 1.473(7) ꢀ; Figure 1). In this reaction, four mole-
cules of benzonitrile were hydrogenated to give four benzald-
iminato species by abstraction of four methylene hydrogen
atoms from two of the four ethylimido units in 1c with the
release of two molecules of acetonitrile, while the other two
ethylimido units remained unchanged. Such formation of an
iminato species through hydrogen transfer from an imido
species to a nitrile is, as far as we are aware, unprecedented.
When 10 equivalents of benzonitrile were allowed to react
with the benzylimido complexes 1a and 1b, the benzonitrile
tetramerization products 3a and 3b were obtained, respec-
tively (Scheme 1). The reaction of the benzamidinate dianion
complex 2 with 3 equivalents of benzonitrile also afforded 3a
in high yield, and this suggests that 2 is an intermediate in the
formation of 3a from 1a. The structures of 3a and 3b were
confirmed by X-ray analysis (Figure 1).
Although the formation of 2 from 1a is straightforward,
the formation of 3a from 2 apparently requires both the
insertion of two molecules of benzonitrile into a Lu–amido
bond in 2 and hydrogen transfer from the methylene group to
the two incoming benzonitrile molecules. The formation of 3a
Scheme 1. Nucleophilic addition and hydrogen transfer of lanthanide–
imido complexes to benzonitrile.
Figure 1. X-ray crystal structures of 2, 3a, and 4 with 30% probability ellipsoids. Hydrogen atoms and lattice solvent molecules are omitted for
clarity.
960
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 959 –962

Angewandte
Chemie
À
À
from 2 could be explained by the reaction steps shown in
Scheme 2. The coordination of one molecule of benzonitrile
to each Lu atom in 2 should yield A. Hydrogen transfer from
a methylene group to coordinated PhCN in A (step a)[11]
(intramolecular C N bond formation) and C N bond cleav-
age.
A possible mechanism for the catalytic cyclotrimerization
of benzonitrile by 3a or 3b is shown in Scheme 4. The
coordination of one molecule of benzonitrile to a metal center
in 3a or 3b could give F. Methine hydrogen transfer (step a)
to coordinated benzonitrile would afford G. Intramolecular
nucleophilic addition of the amido unit to the newly formed
imine moiety (step b) could yield H. Aromatization of the
À
resultant six-membered ring in H through step c (Ln N bond
=
À
cleavage and C N bond formation) and step d (C N bond
À
cleavage and Ln N bond formation) should release the
triazine and generate I. An insertion of benzonitrile into I
would give J, which resembles intermediate B in Scheme 2,
and could regenerate 3a or 3b as described above.
Scheme 2. Possible mechanism for the formation of 3a.
should afford B. Nucleophilic addition of the newly formed
benzaldiminato unit to the aldimine group (step b) could yield
C, which on coordination of one molecule of PhCN gives D.
Migration of the methine hydrogen atom to the PhCN unit in
D (step c) followed by addition of the resulting aldiminato
unit to the aldimine group in E (step d) should give 3a.
It is also noteworthy that only one of the two benzami-
dinate dianion units in 2 formally participated in the reaction
with benzonitrile. No reaction was observed between the
remaining benzamidinate dianion unit and the benzonitrile
ligand in 3a or 3b even at elevated temperatures (808C).
Surprisingly to us, when an excess of benzonitrile was added
to a solution of 3a or 3b in toluene, the benzonitrile
cyclotrimerization product 2,4,6-triphenyl-1,3,5-triazine was
formed selectively, while 3a (or 3b) was recovered. The
polyhydrido complexes [{Cp’Ln(m-H)2}4(thf)] 1a and 1b and
benzamidinate dianion complex 2 also showed similar cata-
lytic activity for the cyclotrimerization of benzonitrile
(Scheme 3). Here, the formation of 3a (or 3b) was also
observed after completion of the reaction which strongly
suggests that 3a (or 3b) is the true catalytic species. The
formation of 2,4,6-triphenyl-1,3,5-triazine, a cyclic benzoni-
trile trimer, from 3a or 3b (containing a linear benzonitrile
tetramer as a ligand) clearly must involve both cyclization
Scheme 4. Possible mechanism for the catalytic cyclotrimerization of
benzonitrile.
In summary, tetranuclear m3-N-bridged lanthanide–imido
complexes 1a–c undergo not only nucleophilic addition but
also hydrogen-transfer reactions with benzonitrile to afford a
new series of structurally characterizable multinuclear lan-
thanide complexes 2–4 that have novel structures. The
À
combination of C N bond-formation, hydrogen-transfer,
À
and C N bond-cleavage steps constitutes a unique catalytic
cycle for the cyclotrimerization of benzonitrile.[12] This work
demonstrates for the first time that lanthanide–imido com-
plexes can show rich and unique chemistry that differs from
that of d-block transition metal analogues.
Received: September 10, 2004
Published online: December 21, 2004
Keywords: cyclotrimerization · hydrogen transfer · lanthanides ·
.
Scheme 3. Catalytic cyclotrimerization of benzonitrile.
N ligands · nitriles
Angew. Chem. Int. Ed. 2005, 44, 959 –962
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
961

Communications
[1] Recent reviews on early-transition-metal–imido complexes:
a) A. P. Duncan, R. G. Bergman, Chem. Rec. 2002, 2, 431 – 445;
b) P. Mountford, Chem. Commun. 1997, 2127 – 2134.
[2] For examples, see: a) T. Ong, G. A. Yap, D. S. Richeson, Chem.
Commun. 2003, 2612 – 2613; b) R. L. Zuckerman, S. W. Krska,
R. G. Bergman, J. Am. Chem. Soc. 2000, 122, 751 – 761; c) R. L.
Zuckerman, R. G. Bergman, Organometallics 2000, 19, 4795 –
4809; d) W. Wang, J. H. Espenson, Organometallics 1999, 18,
5170 – 5175; e) S. Y. Lee, R. G. Bergman, J. Am. Chem. Soc.
1996, 118, 6396 – 6406.
[3] a) R. L. Zuckerman, R. G. Bergman, Organometallics 2001, 20,
1792 – 1807; b) P. J. Walsh, F. J. Hollander, R. G. Bergman, J.
Am. Chem. Soc. 1988, 110, 8729 – 8731; c) J. L. Polse, R. A.
Anderson, R. G. Bergman, J. Am. Chem. Soc. 1998, 120, 13405 –
13414; d) P. L. McGrane, M. Jensen, T. Livinghouse, J. Am.
Chem. Soc. 1992, 114, 5459 – 5460.
[4] For recent examples, see: a) Y. Li, Y. Shi, A. L. Odom, J. Am.
Chem. Soc. 2004, 126, 1794 – 1803; b) C. Cao, Y. Shi, A. L.
Odom, J. Am. Chem. Soc. 2003, 125, 2880 – 2881; c) L. Acker-
mann, R. G. Bergman, R. Loy, J. Am. Chem. Soc. 2003, 125,
11956 – 11963.
[5] a) H. M. Hoyt, M. E. Michael, R. G. Bergman, J. Am. Chem.
Soc. 2004, 126, 1018 – 1019; b) J. L. Bennett, P. T. Wolczanski, J.
Am. Chem. Soc. 1997, 119, 10696 – 10719.
[6] a) G. R. Giesbrecht, J. C. Gordon, Dalton Trans. 2004, 16, 2387 –
2393; b) D. J. Beetstra, A. Meetsma, B. Hessen, J. H. Teuben,
Organometallics 2003, 22, 4372 – 4374; c) J. C. Gordon, G. R.
Giesbrecht, D. L. Clark, P. J. Hay, D. W. Keogh, R. Poli, B. L.
Scott, J. G. Watkin, Organometallics 2002, 21, 4726 – 4734; d) H.-
S. Chan, H.-W. Li, Z. Xie, Chem. Commun. 2002, 652 – 653; e) S.
Wang, Q. Yang, T. C. W. Mak, Z. Xie, Organometallics 1999, 18,
5511 – 5517; f) Z. Xie, S. Wang, Q. Yang, T. C. W. Mak, Organo-
metallics 1999, 18, 1578 – 1579; g) A. A. Trifonov, M. N. Boch-
karev, H. Schumann, J. Loebel, Angew. Chem. 1991, 103, 1170 –
1171; Angew. Chem. Int. Ed. Engl. 1991, 30, 1149 – 1150.
[7] The deprotonation approaches used successfully for the gener-
ation of transition-metal–imido species[1–5] did not seem to work
well for the analogous lanthanide–imido complexes.[6] When
main group metal–alkyl compounds, such as RLi or AlR3, were
used as a base to deprotonate primary amido–lanthanide
complexes, the main group metal ions were often incorporated
into the resultant imido products.[6c,d]
[8] a) D. Cui, O. Tardif, Z. Hou, J. Am. Chem. Soc. 2004, 126, 1312 –
1313; b) O. Tardif, D. Hashizume, Z. Hou, J. Am. Chem. Soc.
2004, 126, 8080 – 8081; c) O. Tardif, M. Nishiura, Z. Hou,
Organometallics 2003, 22, 1171 – 1173; d) Z. Hou, Y. Zhang, O.
Tardif, Y. Wakatsuki, J. Am. Chem. Soc. 2001, 123, 9216 – 9217.
[9] The products reported in this paper have all been fully
characterized. See Supporting Information for experimental
details.
[10] L. Bourget-Merle, M. F. Lappert, J. R. Severn, Chem. Rev. 2002,
102, 3031 – 3065.
[11] The formation of 4 in the reaction of 1c with PhCN should
proceed by a similar hydrogen-transfer process.
[12] Although benzonitrile cyclotrimerization is known, the mecha-
nistic aspects of this process are not well understood. For
examples, see: a) J. H. Forsberg, V. T. Spaziano, T. M. Balasu-
bramanian, G. K. Liu, S. A. Kinsley, C. A. Duckworth, J. J.
Poteruca, P. S. Brown, J. L. Miller, J. Org. Chem. 1987, 52, 1017 –
1021; b) F. Xu, J. H. Sun, H. B. Yan, Q. Shen, Synth. Commun.
2000, 30, 1017 – 1022.
962
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 959 –962