Double deprotonation of coordinated ethylimide to CH3CN: Molecular mechanism and relevance to the chemistry of Mo and W organoimides

Article abstract of DOI:10.1039/b602038k

Reaction of [MCl(NEt)(dppe)₂]Cl (M = Mo, W) with n-BuLi in tert-butyl methyl ether under an N₂ atmosphere yields the M(0) bis(dinitrogen) complexes [M(N₂)₂(dppe)₂] and acetonitrile. A mechanism is proposed for this reaction which involves an anionic chloro-acetonitrile intermediate. The implications of these findings to the chemistry of Mo and W organoimides are discussed. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602038k

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www.rsc.org/dalton | Dalton Transactions
Double deprotonation of coordinated ethylimide to CH3CN: molecular
mechanism and relevance to the chemistry of Mo and W organoimides
Chinnappan Sivasankar and Felix Tuczek*
Received 10th February 2006, Accepted 31st March 2006
First published as an Advance Article on the web 21st April 2006
DOI: 10.1039/b602038k
Reaction of [MCl(NEt)(dppe)2]Cl (M = Mo, W) with n-BuLi in tert-butyl methyl ether under an N2
atmosphere yields the M(0) bis(dinitrogen) complexes [M(N2)2(dppe)2] and acetonitrile. A mechanism is
proposed for this reaction which involves an anionic chloro–acetonitrile intermediate. The implications
of these findings to the chemistry of Mo and W organoimides are discussed.
diethyl ether to give analytically pure [MoCl(NEt)(dppe)2]Cl.4 The
corresponding tungsten system was prepared analogously.
Introduction
The derivatization of N-containing ligands in order to introduce
nitrogen into organic molecules is an issue of continuing interest.1
Nitrido complexes of the type [MX(N)(diphos)2] (M = Mo, W;
Reaction of [MoCl(NEt)(dppe)2]Cl with n-BuLi
diphos = dppe, depe) are well suited for this purpose as their
n-Butyllithium (1.6 M hexane solution) was added to a sus-
preparation is relatively straightforward and the nitrido group is
nucleophilic.2 These systems in particular react with alkyl halides
to give the corresponding alkyl imido complexes.3 The ligand-
centered chemistry of the latter compounds has been intensively
investigated, and reactivity patterns have emerged.1,4 Release of
the final N-functionalized products (i.e., substituted amines) can
be effected electrochemically, as demonstrated for the synthesis of
amino acids.5 Pickett and coworkers have identified the interme-
diates occuring in these transformations.6 Specifically, they found
that two-electron reduction of alkylimides [MoX(NR)(dppe)2]+
(X = halide, R = alkyl) leads to the five-coordinate Mo(II)
complexes [Mo(NR)(dppe)2]. In the course of attempts to generate
such species by chemical reduction we detected that treatment
of [MCl(NEt)(dppe)2]+ (M = Mo, W) with n-BuLi leads to the
Mo/W dinitrogen complexes and acetonitrile, which must involve
a double deprotonation of the ethylimido ligand.
pension of [MoCl(NEt)(dppe)2]Cl in tert-butyl methyl ether
and stirred for 0.5 h to give an orange–yellow solution. The
solution was concentrated and n-hexane was added; upon cooling
to −25 ◦C a bright orange solid precipitated. The solid was
filtered off and washed with n-hexane to give analytically pure
[Mo(N2)2(dppe)2] (69% yield). The same reaction was performed
with [WCl(NEt)(dppe)2]Cl giving [W(N2)2(dppe)2] in 67% yield.
The products were characterized by comparison of their spectro-
scopic properties with the literature.8
Results and discussion
The starting compounds [MCl(NEt)(dppe)2]Cl (1; M = Mo/W)
were prepared by reaction of the mixed acetonitrile–N2 com-
plexes [M(N2)(CH3CN)(dppe)2] (M = Mo, W) with HCl in
benzene (Scheme 1).7 Under these conditions, the acetoni-
trile ligands are protonated to yield the corresponding M(IV)
ethylimido complexes.7,9 Recently we investigated this reaction
in more detail and found that in complexes of the type
[M(N2)(CH3CN)(diphos)2] (M = Mo, W; diphos = dppe, depe)
the nitrile ligand is activated towards protonation at the sp
carbon center after exchange of dinitrogen by a Lewis base
X, in general the conjugated base of the acid HX employed
for protonation.10 In the context of the present study, X =
Cl and diphos = dppe (Scheme 1). The corresponding anionic
species [MCl(CH3CN)(dppe)2]− (M = Mo, W; 3) promote electron
transfer to the nitrile ligand, making it susceptible to protic attack
at Cb. Under the described reaction conditions (vide supra) the
subsequent protonation steps leading to 2 and 1 are irreversible.
This ensures that the whole reaction proceeds unidirectionally
although the reactive, anionic intermediates 3 may only be present
in small concentrations.
Experimental
All syntheses were carried out in atmospheres of dinitrogen or
argon using standard Schlenk techniques. All solvents were dried
over appropriate drying agents under inert gas atmospheres.
[M(N2)(CH3CN)(dppe)2] (M = Mo, W) was synthesized using
literature procedures.4,7
Reaction of [M(N2)(CH3CN)(dppe)2] with HCl
Hydrochloric acid was condensed onto a frozen solution of
[Mo(N2)(CH3CN)(dppe)2] (0.24 g, 0.22 mmol) in benzene (25 ml),
and the reaction mixture was stirred at room temperature for 2 h
and filtered through a sintered-glass frit. The filtrate was concen-
trated and upon addition of diethyl ether an orange yellow solid
precipitated. The solid was separated by filtration and washed with
Addition of an excess of n-BuLi (hexane solution) to suspen-
sions of [MCl(NEt)(dppe)2]Cl (M = Mo, W; 1) in tert-butyl
methyl ether afforded the dinitrogen complexes [M(N2)2(dppe)2]
(5) in about 70% yield (Scheme 2). Acetonitrile formed in this
reaction was detected by gas chromatography. Importantly, the
Institut fu¨r Anorganische Chemie University of Kiel, Otto-Hahn-Platz 6-
7/10, Kiel, Germany. E-mail: ftuczek@ac.uni-kiel.de; Fax: +49 431 880
1520; Tel: +49 431 880 1410
3396 | Dalton Trans., 2006, 3396–3398
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Scheme 1
ethylimido complex 1 leads to a chloro–azavinylidene intermediate
2 which in a second reaction with n-BuLi generates the anionic
M(0) chloro–acetonitrile species 3. Subsequent ligand exchange
reactions of acetonitrile and chloride for N2 generate the final
products; i.e., the M(0) bis(dinitrogen) complexes 5 (Scheme 3).
On the basis of Schemes 1 and 3 both the protonation
of coordinated acetonitrile and the deprotonation of coordi-
nated ethylimide thus would involve the anionic intermediates
[MCl(CH3CN)(dppe)2]− (3; M = Mo, W). Mechanistic evidence
for the existence of such species has been presented in our previous
study,10 where we showed that acetonitrile is only activated towards
protonation if it is coordinated to a zerovalent metal center in the
presence of an additional Lewis-basic ligand (vide supra).
Scheme 2
Double deprotonation of coordinated methylene imide groups
has not been known so far, except for Mo complexes with imide
esters; i.e., [MoCl(NCH2COOR)(dppe)2]+.4,11 These compounds
have been deprotonated with strong bases such as methoxide
or K[OBut], leading in the presence of N2 or CO to the free
deprotonation of coordinated ethylimide corresponds to a reversal
of the protonation described in Scheme 1. Based on a comparison
with the latter reaction, we propose the following mechanism for
the deprotonation: initial reaction of n-BuLi with the chloro–
Scheme 3
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cyanoformic acid esters NCCOOR and [Mo(N2)2(dppe)2] or
[Mo(CO)2(dppe)2]. A mechanism has been suggested which is
analogous to Scheme 3 and, in particular, also involves anionic
chlorocyanoformic acid ester intermediates.4 In these reactions,
however, the acidity of the methylene protons is increased due to
the vicinity of an electron-withdrawing ester group whereas in our
case such additional activation of C–H bonds is absent.12
imide ligands as well as on coordinated azavinylidene groups have
been known, it herewith has been demonstrated that two sequen-
tial deprotonations on non-activated methylene imide groups can
be achieved by employing n-BuLi as a base in these reactions.
Acknowledgements
Single deprotonation reactions of the active methylene group
of ethylimido complexes have been known as well. Thus, 10% for-
mation of the azavinylidene complex 2 has been observed during
the electrochemical reduction of 1.6 Moreover, electrochemistry
has been employed to show that 1 reacts quantitatively with
K[OBut] in thf to give 2.4 The latter species has further been
trapped by addition of MeI, leading to the isopropylimide cation
[MoCl(NCHMe2)(dppe)2]+, which again could be deprotonated to
give the corresponding azavinylidene complex.4 Deprotonation of
a d4 chloro–azavinylidene to a d6 chloro–nitrilo complex has also
been observed before. Thus, Re(I) chloro–arylnitrile complexes
could be prepared by treatment of the corresponding Re(III) chloro
arylazavinylidenecomplexes with [NBu4]OH.13 The corresponding
M(0) chloro–nitrile complexes 3 (M = Mo, W), in contrast,
cannot be isolated and in the presence of N2 are converted the
bis(dinitrogen) complexes 5.
Funding of this research by DFG Tu58-12 and FCI is gratefully
acknowledged.
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We have shown that treatment of the M(IV) ethylimido complexes
1 with n-BuLi causes a double deprotonation of the ligand to
generate acetonitrile and the M(0) bis(dinitrogen) complexes 5
(M = Mo, W), which constitutes the counterpart to the well-
known transition-metal mediated diprotonation of acetonitrile to
ethylimide. Whereas double deprotonation reactions on activated
and single deprotonation reactions on non-activated methylene
3398 | Dalton Trans., 2006, 3396–3398
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