Reaction Mechanism of the CtN Triple Bond Cleavage of
-Ketonitriles on a Molybdenum(0) Center
1
Yoshiaki Tanabe,
†
Hidetake Seino,
‡
Youichi Ishii,
†
and Masanobu Hidai*
,†
Contribution from the Department of Chemistry and Biotechnology, Graduate School of Engineering,
The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Institute of Industrial Science,
The UniVersity of Tokyo, Roppongi, Minato-ku, Tokyo 106-8558, Japan
ReceiVed June 25, 1999
Abstract: The molybdenum dinitrogen complex trans-[Mo(N
2
)
2
(dppe)
2
](2) reacted with 2-2.5 equiv of various
-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR
1
-
COR
2
)(dppe)
2
](4;R
1
) H, R
2
) 4-MeOCOC
6
H
4
, 4-ClC
6
H
4
, 4-Tol, 4-MeOC
6
H
4
, 2-C
4
H
3
O, 2-C
4
H
3
S, Pr
i
;R
1
) CN, R
2
) Me, Ph; dppe ) Ph
2
PCH
2
CH
2
PPh
2
) via the CtN triple bond cleavage of the nitriles on the
molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room
temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH
2
COBu
t
)-
(NCCHCOBu
t
)(dppe)
2
](5k), which further underwent the cleavage of the CdN double bond of the
alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBu
t
)-
(dppe)
2
] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of
4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt
3
][OTf] (OTf ) OSO
2
CF
3
) resulted in
the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH
2
CH
2
CO(C
6
H
4
Cl-4)}{NCCHCO-
(C
6
H
4
Cl-4)}(dppe)
2
][OTf] (6c
+
[OTf]
-
). The solid-state structures of 4h‚1.5C
2
H
4
Cl
2
(R
1
) CN, R
2
) Me),
5k‚C
6
H
6
, and 6c
+
[OTf]
-
were determined by single-crystal X-ray analyses. The detailed NMR analysis of the
reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo-
(NCHCH
2
COR)(NCCHCOR)(dppe)
2
](5) act as the key intermediates for the CtN triple bond fission, and
the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation
with the Hammett σ
p
or σ
a
constants for the aroyl substituents, where positive F values were obtained (F
p
,
1.42; F
a
, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the
substitution of a dinitrogen ligand in 2 with a -ketonitrile molecule, (2) the fast protonation of the nitrile
ligand by a second -ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton
shift from the R-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an
enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido
complex 4 as the final product.
Introduction
Recent studies on the transition-metal complexes of organo-
nitriles have revealed that the coordinated nitriles are endowed
with rich reactivities.
2-6
Most of the metal-promoted reactions
of nitriles reported so far are concerned with the nitrile ligands
coordinated to electron-deficient metal centers through their
nitrogen atoms. In such complexes, the σ-donating character
of the nitrile ligands enhances the reactivity of the cyano group
toward nucleophiles or facilitates the deprotonation or oxidative
addition of the R-CH group. In contrast, the nitriles coordinated
to π-donating metal complexes are expected to be susceptible
to attack by electrophiles owing to their π-accepting character.
However, nitrile complexes with strongly π-donating low-valent
metal centers have been considerably limited in number, and
the reactivities of nitriles in such complexes remain to be
developed.
7-11
In the course of our continuing investigation into the
reactivities of the coordinated dinitrogen in low-valent molyb-
denum and tungsten dinitrogen complexes of the type [M(N
2
)
2
-
(P)
4
] (M ) Mo, W; P ) tertiary phosphine),
12-17
we previously
†
Department of Chemistry and Biotechnology.
‡
Institute of Industrial Science.
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10.1021/ja9921928 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/10/2000