Dismutation of a molybdenum(IV) acetonitrile-NNH2 complex to a molybdenum(IV) ethylimido complex + N2: Mechanistic implications on the protonation of coordinated nitriles at the β-carbon atom

Article abstract of DOI:10.1021/om050220r

Reaction of the Mo(0) depe complex [Mo(N₂H₂)(depe) ₂(CH₃CN)](OTf)₂ (3) (depe = 1,2-bis-(diethylphosphino)ethane) with base gives the ethylimido complex [Mo(depe)₂(CH₃CH₂N)CH₃-CN)](OTf) ₂ (10), whereas base treatment of the corresponding dppe complex [Mo(N₂H₂)-(dppe)₂(OTf)](OTf) (12; dppe = 1,2-bis(diphenylphosphino)ethane) leads to formation of [Mo(N ₂)(dppe)₂(CH₃CN)] (14). Reaction of dinitrogen complex 14 with HBF₄ gives the ethylimido complex [Mo(dppe) ₂(CH₃CH₂N)(F)](BF₄) (17). The protonation of coordinated acetonitrile in complexes 3 and 14 giving the corresponding ethylimido complexes is investigated. Experimental and theoretical evidence is presented for the hypothesis that acetonitrile is only activated toward protonation at the β-carbon if it is bound along with a Lewis base like triflate or fluoride to a Mo(0) center. The activation is further influenced by the phosphine coligands (depe > dppe). DFT geometry optimizations show that the acetonitrile ligand is bent in the activated complexes, exhibiting a lone pair at C_β. Protonation at this position first leads to Mo(II) methyl-azavinylidene intermediates (which cannot be isolated) and then to Mo(IV) ethylimido complexes.

Full text of DOI:10.1021/om050220r

Organometallics 2005, 24, 5393-5406
5393
Dismutation of a Molybdenum(IV) Acetonitrile-NNH2
Complex to a Molybdenum(IV) Ethylimido Complex +
N2: Mechanistic Implications on the Protonation of
Coordinated Nitriles at the â-Carbon Atom
Chinnappan Sivasankar, Natascha Bo¨res, Gerhard Peters, Carsten M. Habeck,
Felix Studt, and Felix Tuczek*
Institut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t Kiel,
D-24098 Kiel, Germany
Received March 22, 2005
Reaction of the Mo(0) depe complex [Mo(N2H2)(depe)2(CH3CN)](OTf)2 (3) (depe ) 1,2-bis-
(diethylphosphino)ethane) with base gives the ethylimido complex [Mo(depe)2(CH3CH2N)(CH3-
CN)](OTf)2 (10), whereas base treatment of the corresponding dppe complex [Mo(N2H2)-
(dppe)2(OTf)](OTf) (12; dppe ) 1,2-bis(diphenylphosphino)ethane) leads to formation of
[Mo(N2)(dppe)2(CH3CN)] (14). Reaction of dinitrogen complex 14 with HBF4 gives the
ethylimido complex [Mo(dppe)2(CH3CH2N)(F)](BF4) (17). The protonation of coordinated
acetonitrile in complexes 3 and 14 giving the corresponding ethylimido complexes is
investigated. Experimental and theoretical evidence is presented for the hypothesis that
acetonitrile is only activated toward protonation at the â-carbon if it is bound along with a
Lewis base like triflate or fluoride to a Mo(0) center. The activation is further influenced by
the phosphine coligands (depe > dppe). DFT geometry optimizations show that the
acetonitrile ligand is bent in the activated complexes, exhibiting a lone pair at Câ. Protonation
at this position first leads to Mo(II) methyl-azavinylidene intermediates (which cannot be
isolated) and then to Mo(IV) ethylimido complexes.
I. Introduction
tating protonation to the corresponding NNH2 species.
Remarkably, this protonation proceeds under retention
of the (dppe)2-nitrile ligation. This is in contrast to
corresponding bis(dinitrogen) complexes, where upon
treatment with acids HX, one N2 ligand is exchanged
against the conjugate base of the acid X, leading to
[M(NNH2)X(diphos)2]+ systems (M ) Mo, W; diphos )
dppe, depe).1,3-9 With respect to the construction of a
catalytic system for N2 fixation, this is unfavorable, as
it corresponds to 50% loss of activated substrate.
trans-Nitrile dinitrogen complexes thus have a double
advantage over their bis(dinitrogen) counterparts: higher
activation and retention of the trans ligand upon pro-
tonation. Unfortunately, however, the photochemical
method to generate the dppe trans-nitrile dinitrogen
systems does not appear to be applicable to the synthe-
sis of the corresponding depe complexes.10 Obviously the
quantum yield for the photosubstitution reaction of the
bis(dinitrogen) compounds leading to [M(N2)(RCN)-
(diphos)2] (M ) Mo, W; diphos ) dppe, depe) is much
lower for [M(N2)2(depe)2] than for [M(N2)2(dppe)2]. This
is possibly due to the fact that the relatively intense,
broad CT band at 380 nm characteristic of the Mo/W
dppe systems is lacking for the depe analogues.11 On
the other hand, the complexes [M(N2)(RCN)(depe)2] (M
The metal-centered conversion of dinitrogen to am-
monia under ambient conditions has been an issue of
long-standing interest.1 Before the discovery of a Mo-
(III) triamidoamine system providing the first truly
catalytic process of ammonia synthesis proceeding via
well-defined intermediates,2 this chemistry has been a
domain of Mo and W complexes with diphosphine (depe
or dppe) ligands.1,3,4 For [M(N2)2(dppe)2] complexes (M
) Mo, W), one dinitrogen can be exchanged in a
photochemical reaction against a nitrile ligand, leading
to trans-[M(N2)(RCN)(dppe)2] systems.5,6 As evident
from spectroscopic data and theoretical calculations, the
N2 ligand is more highly activated in the trans-nitrile
than in the parent bis(dinitrogen) complexes,5-7 facili-
* Corresponding author. E-mail: ftuczek@ac.uni-kiel.de. Fax: +49
431 880 1520.
(1) (a) MacKay, B. A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 385. (b)
Barriere, F. Coord. Chem. Rev. 2003, 236, 71. (c) Hidai, M. Coord.
Chem. Rev. 1999, 185-186, 99-108. (d) Hidai, M.; Mizobe, Y. Chem.
Rev. 1995, 95, 1115. (e) Leigh, G. J. Acc. Chem. Res. 1992, 25, 177. (f)
Henderson, R. A. Transition Met. Chem. 1990, 15, 330.
(2) (a) Studt, F.; Tuczek, F. Angew. Chem., Int. Ed. 2005, 44, 5639.
(b) Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103. (c)
Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock, A.
S.; Davis, W. M. J. Am. Chem. Soc. 2004, 126 (19), 6150. (d) Yandulov,
D. V.; Schrock, R. R. Science 2003, 301, 76. (e) Yandulov, D. V.; Schrock,
R. R.; Rheingold, A. L.; Cecciavelli, C.; Davis, W. M. Inorg. Chem. 2003,
42, 796. (f) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002,
124, 6252.
(7) Habeck, C. M.; Lehnert, N.; Na¨ther, C.; Tuczek, F. Inorg. Chem.
Acta 2002, 337, 11.
(3) Henderson, R. A.; Leigh, G. J.; Pickett, C. Adv. Inorg. Chem.
Radiochem. 1983, 27, 197.
(8) Dilworth, J. R.; Richards, R. L. Inorg. Synth. 1980, 20, 119.
(9) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc.,
Dalton Trans. 1976, 1520. (b) Chatt, J.; Pearman, A. J.; Richards, R.
L. J. Chem. Soc., Dalton Trans. 1977, 1852.
(4) Chatt, J.; Dilworth, J. R.; Richards, J. R. Chem. Rev. 1978, 78,
589.
(5) Chatt, J.; Leigh, G. J.; Neukomm, H.; Pickett, C. J.; Stanley, D.
R. J. Chem. Soc., Dalton Trans. 1980, 121.
(10) Habeck, C. M.; Bo¨res, N.; Tuczek, F. Unpublished work.
(11) Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1671.
(6) Tatsumi, T.; Hidai, M.; Uchida, Y. Inorg. Chem. 1975, 14, 2530.
10.1021/om050220r CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/24/2005

5394 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
Scheme 1
evolution of N2 was observed upon addition of base, this
reaction in fact corresponds to a dismutation of an
NNH2-CH3CN complex to N2 + coordinated ethylimide.
We further found that this reaction does not require
stoichiometric, but only catalytic amounts of base. It
even proceeds spontaneously in the absence of base,
however at a slower rate.
Protonation of coordinated nitriles at the â-carbon
atom to generate alkylimido complexes has been evi-
denced before.12,14,15 In particular, Field et al. in the
above-mentioned study showed that [W(N2)(CH3CN)-
(dppe)2] can be protonated with HBF4 to the ethylimido
complex [WF(CH3CH2N)(dppe)2]BF4. The same observa-
tion has been made by Hidai et al. for Mo and W
complexes with various nitrile ligands.15 These authors
also provided a mechanism for the protonation of
coordinated â-ketonitriles and their subsequent C-N
cleavage. To the best of our knowledge, however, no
mechanism for the protonation of simple aliphatic and
aromatic nitriles on transition-metal complexes has
been given. The purpose of the present paper therefore
is to explain the observed base-catalyzed dismutation
of the Mo NNH2-acetonitrile complex to Mo(IV) eth-
ylimide + N2 in the framework of a general mechanism
for the protonation of coordinated nitriles at the â-atom.
To this end, the dismutation reaction is presented first,
providing extensive characterization of the correspond-
ing intermediates. Then, various mechanistic scenarios
are explored to account for the experimental observa-
tions, and the most probable reaction pathway is identi-
fied with the help of DFT. The corresponding mecha-
nism is finally applied to the protonation of a mixed
acetonitrile-N2 complex with HBF4. The general im-
plications of these results are discussed.
) Mo, W) potentially are of high interest to synthetic
nitrogen fixation, as the dinitrogen ligand should be
even more activated than in the corresponding dppe
compounds.7 It therefore would be desirable to have a
general (possibly non-photochemical) method for the
synthesis of trans nitrile-dinitrogen complexes that is
applicable to both dppe and depe systems.
2. Experimental Results
2.1. Synthesis of [Mo(N2H2)(depe)2(OTf)](OTf) (2)
and Reaction with Acetonitrile. The NNH2 complex
[Mo(N2H2)(depe)2(OTf)](OTf) (2) was prepared by reac-
tion of the bis(dinitrogen) complex [Mo(N2)2(depe)2] (1)
with triflic acid (Scheme 1).16 The 1H NMR spectrum of
complex 2 shows peaks at 1.24 and 2.06 ppm for the
CH3 and CH2 protons of the depe ligand, respectively;
a peak at 8.46 ppm corresponds to the NH2 protons
(Table 1). 31P NMR of complex 2 displays a single peak
at 47.06 ppm for the depe phosphorus atoms, indicating
a trans geometry of the complex. 19F NMR of complex 2
shows two peaks, one at -78.89 ppm for the coordi-
nated, and one (1:1 ratio) at -79.65 ppm for the
noncoordinated OTf fluorine atom. The IR spectrum
exhibits two bands for the NH2 stretches at 3097 and
3251 cm-1 (Table 2).
In this respect the finding of Field et al. is remarkable
in which the complex trans-[W(N2)(CH3CN)(dppe)2] can
in fact be obtained via a sequence of thermal reaction
steps.12 Of key importance in this reaction route is the
choice of triflic acid, HOTf, to protonate [W(N2)2(dppe)2],
generating [W(NNH2)(OTf)(dppe)2](OTf). The triflate
ligand in this NNH2 complex is easily displaced in
acetonitrile, leading to [W(NNH2)(RCN)(dppe)2](OTf)2.
Deprotonation of this complex with base (NEt3 or KOt-
Bu) then generates the acetonitrile-N2 complex [W(N2)-
(CH3CN)(dppe)2]. The first objective of the present paper
was to apply this method to generate the corresponding
complexes with depe ligands, i.e., [M(N2)(RCN)(depe)2]
(M ) Mo, W). For practical reasons, we focused our
attention on the Mo system.13 Up to the complex [Mo-
(NNH2)(CH3CN)(depe)2](OTf)2 (3) we found the Mo depe
chemistry to be analogous to that of the W dppe system
(Scheme 1). Surprisingly, however, treatment of 3 with
base did not lead to the corresponding N2-CH3CN
complex but to formation of the Mo(IV) ethylimido
complex [Mo(CH3CH2N)(CH3CN)(depe)2](OTf)2 (10). As
(14) (a) So´nia, M. P.; Cunha, R. M.; Fa´tima, M.; Guedes da Silva,
C.; Pombeiro, A. J. L. Inorg. Chem. 2003, 42, 2157. (b) Guedes da Silva,
M. F. C.; Frau´sto da Silva, J. J. R.; Pombeiro, A. J. L. Inorg. Chem.
2002, 41, 219. (c) Frau´sto da Silva, J. J. R.; Guedes da Silva, M. F. C.;
Henderson, R. A.; Pombeiro, A. J. L.; Richards, R. L. J. Organomet.
Chem. 1993, 461, 141. (d) Pombeiro, A. J. L. New J. Chem. 1994, 18,
163. (e) Pombeiro, A. J. L. Inorg. Chim. Acta 1992, 198-200, 179. (f)
Pombeiro, A. J. L.; Hughes, D. L.; Richards, R. L. J. Chem. Soc., Chem.
Commun. 1988, 1052.
(15) (a) Tanabe, Y.; Seino, H.; Ishii, Y.; Hidai, M. J. Am. Chem. Soc.
2000, 122, 1690. (b) Seino, H.; Tanabe, Y.; Ishii, Y.; Hidai, M. Inorg.
Chim. Acta 1998, 280, 163.
(16) Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett, C. J.; Rankin, D.
A. J. Chem. Soc., Dalton Trans. 1984, 1703.
(12) Field, L. D.; Jones, N. G.; Turner, P. Organometallics 1998, 17,
2394.
(13) (a) Tuczek, F.; Horn, K. H.; Lehnert, N. Coord. Chem. Rev. 2003,
245, 107. (b) Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1659.

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5395
Figure 1. UV-visible spectroscopic monitoring of the conversion of [Mo(depe)2(NNH2)(OTf)]+(2) to [Mo(depe)2(NNH2)(CH3-
CN)]2+ (3).
Table 1. 1H NMR, 31P NMR, and 19F NMR Spectral Data (δ, ppm) of Complexes 2, 3, 10, 14, and 17
1H NMR
CH3 CH2 CH2
system solvent acetonitrile depe depe dppe
31P NMR
CH3
Ph
dppe
NH2
CH2
CH3
NEt
depe,
dppe
NNH2 NEt
19F NMR
2
3
10
14
17
THF-d8
CD3CN
CD3CN
C6D6
1.24 2.06
1.16 1.92
1.20 2.05
47.06
47.27
45.96
59.9
-78.89 (bonded) -79.65 (free)
-79.60 (free)
2.18
1.9
0.98
7.43
0.86
1.0
2.35 6.89-7.57
2.60 6.86-7.43
2.71
CD2Cl2
1.83 -0.19
46.45
-148.31
(Mo-F)
-77.86
(BF4)
Table 2. IR and Raman Spectral Data (cm-1) of
Complexes 2, 3, 10, and 14
In analogy with the dppe complex [W(NNH2)(dppe)2-
(OTf)](OTf),12 the coordinated triflate of 2 is replaced
by acetonitrile upon stirring in CH3CN at room tem-
perature, leading to [Mo(NNH2)(depe)2(CH3CN)](OTf)2
(3) (Scheme 1). The ligand exchange reaction can be
followed by UV-visible spectroscopy (Figure 1). Com-
plex 2 shows three absorption bands, at 340, 434, and
540 nm. After 25 min these bands disappear, giving rise
to new bands at 479 and 600 nm, which indicate
formation of complex 3. 19F NMR of the product 3 shows
a single peak at -79.6 ppm corresponding to noncoor-
dinated OTf and thus reveals that the bound OTf anion
has been replaced by acetonitrile (Table 1). The 31P
NMR spectrum of 3 displays a single peak at 47.27 ppm,
indicating that all four depe phosphorus atoms are
equivalent and a trans configuration of the NNH2 and
stretching frequences
system
CtN
NtN
NH2
2
3097 (IR)
3251 (IR)
3095 (IR)
3253 (IR)
3
10
14
2278 (IR)
1326 (IR)
(NdN)
2280 (IR)
2272 (Raman)
2199 (IR)
1919 (IR)
1913 (Raman)
2210 (Raman)
(10) under evolution of N2 (Scheme 1). Successive
reduction of the added amount of NEt3 shows that the
base is only required to start this raction. In fact, the
reaction was also found to occur spontaneously in the
absence of NEt3. The identity of reaction product 10 was
proven by various spectroscopic methods. 1H NMR
shows peaks at 0.86 and 1.0 ppm for the CH3 and CH2
protons of NCH2CH3, respectively, and a peak at 1.9
ppm for the methyl protons of acetonitrile, confirming
the simultaneous presence of acetonitrile and alkylimide
(Figure S2 (a), Table 1). The peaks at 1.2 and 2.05 ppm
are due to the CH3 and CH2 protons of depe, respec-
tively. 31P NMR of 10 displays a single peak at 45.96
ppm (Figure S2 (b)), indicating that all four phosphorus
atoms from depe are equivalent and the complex has a
trans configuration. The IR spectrum of 10 exhibits a
band at 2280 cm-1 for the CtN stretch (Table 2); a band
for the MotN stretch cannot be assigned unambigu-
ously. Although complex 10 crystallizes well from a
1
CH3CN ligands applies. H NMR of complex 3 shows
peaks at 1.16 and 1.92 ppm for the CH3 and CH2 protons
of depe, respectively, a peak at 7.43 ppm for the NNH2
protons, and a peak at 2.18 ppm for the CH3 protons of
acetonitrile (Table 1, Figure S1). The IR spectrum of 3,
finally, exhibits a band at 1326 cm-1 for the NdN
stretch and bands at 3095 and 3253 cm-1 for N-H
vibrations of the NNH2 group (Table 2). The CtN
stretch of bound acetonitrile appears at 2278 cm-1
slightly higher in energy than in the free ligand.
,
2.2. Reaction of [Mo(N2H2)(depe)2(CH3CN)](OTf)2
(3)withBase.AdditionofNEt3 to[Mo(N2H2)(depe)2(CH3-
CN)](OTf)2 (3) in acetonitrile at room temperature leads
to formation of [Mo(depe)2(NCH2CH3)(CH3CN)](OTf)2

5396 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
Scheme 2
Figure 2. UV-visible spectrum in acetonitrile at room
temperature.
Figure 3. Cyclic voltammogram of complex 10 (1 × 10-4
mol) in acetonitrile at room temperature.
mixture of acetonitrile and diethyl ether, the X-ray
structure refinement indicates significant thermal dis-
order, precluding a single-crystal structure determina-
tion.
([Mo(dppe)2(NR)(X)]+) in DMF (R ) Et, X ) Cl),17 which
appears to be due to the absence of a strong donor in
trans position to the ethylimido ligand in 10.
The base-catalyzed conversion of complex 3 to 10 was
also monitored by UV-visible absorption spectroscopy.
Electronic spectra recorded before and after addition of
base to the acetonitrile solution of complex 3 are shown
in Figure 2. Complex 3 exhibits absorption bands
centered around 470 and 605 nm. After addition of base
these bands disappear immediately and new bands
appear at 415 and 548 nm due to the formation of an
intermediate (possibly 9, cf. Scheme 2). Over a period
of 0.5 h this intermediate is completely converted to the
final product, complex 10, as indicated by a shift of the
415 nm band to 440 nm. It should be noted that complex
10 is also obtained directly from the reaction of 2 with
base in acetonitrile.
Complex 10 was also characterized by cyclic voltam-
metery performed on a 1 × 10-4 M solution in freshly
prepared, degassed acetonitrile containing 0.1 M [NBu4]-
(BF4) as supporting electrolyte with a Pt electrode as
anode. As evident from Figure 3, reduction takes place
at -1.5 V and oxidation at -1.0 V versus ferrocenium-
ferrocene. The observed potentials are 0.71 V less
negative than values reported for the similar system
2.3. Mechanism of the Formation of [Mo(depe)2-
(NCH2CH3)(CH3CN)](OTf)2 (10). Scheme 2 displays
possible pathways for the reaction of the Mo(IV) aceto-
nitrile-NNH2 complex 3 with base. In analogy with the
corresponding dppe complex 13 (see below) and other
NNH2systemstheadditionofbaseto[Mo(N2H2)(depe)2(CH3-
CN)](OTf)2 (3) in acetonitrile primarily leads to forma-
tion of the dinitrogen complex 4 with acetonitrile in
trans position. In the case of the dppe system the
acetonitrile-dinitrogen complex 14 can be isolated after
base treatment of the Mo NNH2 complex (see below and
ref 12), whereas for the depe system protonation of
acetonitrile and loss of N2 occur (vide supra). From DFT
calculations on a model of 3, however, the acetonitrile
ligand does not appear to be activated for protonation
at the â-carbon, as this atom carries a fairly large
positive charge (+0.56, see below), rather rendering it
susceptible to nucleophilic attack.18
For the possible activation of bound acetonitrile
toward protonation at a Mo(0) center, three scenarios
(17) Alias, Y.; Ibrahim, S. K.; Queiros, M. A.; Fonseca, A.; Talarmin,
J.; Volant, F.; Pickett, C. J. J. Chem. Soc., Dalton Trans. 1997, 4807.

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5397
Scheme 3
thus can be envisaged (Scheme 2): (i) loss of N2 leading
to the five-coordinate Mo(0) monoacetonitrile complex
5, (ii) exchange of N2 against acetonitrile, leading to the
Mo(0) bis(acetonitrile) complex 6; and (iii) exchange of
N2 against triflate, leading to [Mo(CH3CN)(OTf)(depe)2]-
(7). As shown by DFT, the positive charge on the
acetonitrile â-carbon is not changed much in the five-
coordinate and bis(acetonitrile) intermediates 5 and 6,
respectively (see below); that is, acetonitrile is not
activated for protonation in these systems either. An
appreciable reduction of the positive charge on the
â-carbon atom of acetonitrile is only observed for the
triflate-bound complex 7, where also a bending of this
ligand occurs. Both of these findings indicate an activa-
tion of the nitrile toward protonation. Attachment of a
proton to the â-carbon atom in a DFT simulation in fact
leads to formation of a C-H bond at this position (see
below). We therefore conclude that the anionic, base-
coordinated intermediate 7 (Scheme 2) is essential for
the observed formation of the ethylimido complex 10.
Importantly, complex 10 was also directly obtained from
the reaction of 2 with base in acetonitrile (vide supra).
In this case, deprotonation of 2 by the added base and
exchange of dinitrogen against nitrile also leads to the
acetonitrile-triflate complex 7, which reacts as shown
in Scheme 2. The formation of complex 7 is even possible
without base from deprotonation of the NNH2 complex
2 by triflate, followed by replacement of dinitrogen by
OTf; this route applies to the spontaneous reaction of
the NNH2-triflate complex 2 to the alkylimido complex
10.
the solvent acetonitrile, and upon crystallization, the
alkylimido-acetonitrile complex 10 is obtained. As the
reaction occurs with very small amounts of base or even
spontaneously, the protonating agent must be the NNH2
complex 3 itself. The whole reaction therefore can be
described as a dismutation of 3, involving an intermo-
lecular proton transfer from the NNH2 group to coor-
dinated acetonitrile under formation of ethylimide and
gaseous N2. A different visualization of the correspond-
ing mechanism is given in Scheme 3. It shows that the
reaction is started by addition of very small amounts of
base and exchange of N2 against triflate, generating the
activated acetonitrile-triflate complex 7. This is the
catalytically active species, which is protonated by the
parent Mo-NNH2 acetonitrile complex 3, generating the
alkylimido species 9 and an acetonitrile-N2 complex 4,
which re-forms the activated complex by exchange of
N2 against triflate. As this complex is constantly regen-
erated by the educt, the whole reaction can also be
described as autocatalytic.
An attempt to deprotonate the final product 10 with
NEt3 was unsuccessful, suggesting that the protonation
of the coordinated acetonitrile is irreversible. This is in
sharp contrast to the protonation of coordinated N2 to
NNH2, which can be reversed by the addition of base
(Scheme 2). Importantly, the irreversibility of the nitrile
protonation ensures that the overall reaction proceeds
quantitatively, although the catalytically active species
may be present only in small amounts.
2.4. Reaction of [Mo(N2H2)(dppe)2(OTf)](OTf)
(13) with Base. Reaction of the NNH2 dppe complex
[Mo(N2H2)(dppe)2(OTf)](OTf) (13) with base (e.g., Et3N)
in acetonitrile leads to formation of the dinitrogen
complex [Mo(N2)(CH3CN)(dppe)2] (14), in which aceto-
nitrile is coordinated trans to the dinitrogen ligand
(Scheme 1). This complex was isolated and characterized
using various spectroscopic techniques. The infrared
spectrum of 14 shows intense bands at 2199 cm-1 for
the CtN stretch of coordinated acetonitrile and at 1919
After double protonation of the coordinated nitrile
ligand at Câ ultimately leading to 9 (Scheme 2), the
bound OTf anion undergoes an exchange reaction with
(18) (a) Wagner, G.; Haukka, M.; Frau´sto da Silva, J. J. R.;
Pombeiro, A. J. L.; Kukushkin, V. Yu. Inorg. Chem. 2001, 40, 264. (b)
Wagner, G.; Pombeiro, A. J. L.; Kukushkin, V. Yu. J. Am. Chem. Soc.
2000, 122, 3104. (c) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord.
Chem. Rev. 1996, 147, 299. (d) Hegedus, Louis S.; Mulhern, T. A.;
Asada, H. J. Am. Chem. Soc. 1986, 108, 6224.

5398 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
Scheme 4
cm-1 for the N2 stretch (Table 2). The simultaneous
presence of acetonitrile and dinitrogen in the complex
is also evidenced by the Raman spectrum, which shows
a strong signal at 2210 cm-1 for the CtN stretch and
another broad signal at 1913 cm-1 for the N-N stretch
(Table 2). Proton NMR of 14 in C6D6 shows a single peak
at δ 0.98 for the methyl protons of CH3CN, a peak at
2.35 ppm for the CH2 protons of dppe, and peaks
ranging from 6.89 to 7.67 ppm for the aromatic protons
of dppe (Table 1 and Figure S3 (a)). These spectral data
are similar to those of [W(N2)(CH3CN)(dppe)2].12 31P-
{1H} NMR of 14 in C6D6 shows a single peak at δ 59.9
ppm for the four equivalent phosphorus atoms of dppe
(Figure S3 (b)), low-field shifted with respect to the
corresponding tungsten complex (33.6 ppm).12
of fluoride increases the charge donation from the
central atom to acetonitrile (vide infra). In analogy with
the OTf system the transferred electron density is
localized on the â-carbon of the ligand, initiating pro-
tonation to form the Mo(IV) ethylimido complex 17. This
reaction proceeds via the Mo(II) azavinylidene complex
16, in which two-electron transfer from the metal has
occurred. In the final, diprotonated complex 17 the
metal has transferred four electrons to the acetonitrile
ligand. The proposed mechanism is supported by theo-
retical calculations presented in the following sections.
Reaction of the tungsten analogue of 14, [W(N2)(CH3-
CN)(dppe)2] (14a), with triflic acid produced the corre-
sponding NNH2 complex, showing that in this case the
dinitrogen ligand of 14a and not the acetonitrile ligand
is protonated. This result indicates that the activation
of nitrile in the putative [W(CH3CN)(OTf)(dppe)2]-
intermediate (analogous to the depe complex 7) is not
sufficient to ensure its protonation. Thus dinitrogen
protonation prevails in this system.
2.5. Reaction of [M(N2)(CH3CN)(dppe)2] (M ) Mo,
W) with HBF4 and CF3SO3H. Reaction of [Mo(N2)(CH3-
CN)(dppe)2] (14) with tetrafluoroboric acid in benzene
leads to formation of the ethylimido complex [MoF-
(NCH2CH3)(dppe)2][BF4] (17) (Scheme 1), in analogy
with the W system investigated by Field et al.12 The
formation of complex 17 was confirmed by various
spectroscopic techniques. Proton NMR in CD2Cl2 shows
a triplet at -0.19 ppm for CH3CH2CN, a quartet at 1.83
ppm for CH3CH2CN, and peaks at 2.60 and 2.71 ppm
for the CH2 protons of the dppe ligand. Peaks corre-
sponding to the phenyl protons from the dppe ligands
range from 6.86 to 7.43 ppm (Table 1 and Figure S4
(a)). 31P{1H} NMR of 17 shows a peak at δ 46.45 ppm
for the four phosphorus atoms of the dppe ligand (Table
1 and Figure S4 (b)). 19F NMR indicates a chemical shift
of δ -148.31 for the coordinated fluorine atom and a
3. Electronic Structure Calculations
3.1. [Mo(H2P-CH2-CH2-PH2)2(CH3CN)(N2)] (4′)
and Derived Complexes. To theoretically study the
protonation of acetonitrile on a Mo center, we first
consider the starting complex [Mo(depe)2(CH3CN)-
(NNH2)]2+ (3), in which acetonitrile is coordinated trans
to the NNH2 ligand. DFT calculations carried out for
the model system [Mo(CH3CN)(NNH2)(PH3)4]2+ showed
that the â-carbon of the acetonitrile in the optimized
geometry carries a positive charge (+0.56), and there
is no significant change in bond distances (CtN ) 1.175
Å, Câ-C ) 1.461 Å) and angles (N-Câ-C ) 179.8°) of
acetonitrile when compared to the free ligand;7 hence
it is not activated in complex 3 (Tables 3 and 4). Similar
results are obtained for model system 4′ of the N2
complex [Mo(depe)2(CH3CN)(N2)] (4), in which acetoni-
trile is coordinated trans to dinitrogen (Figure 4, Table
3). The third possibility is that [Mo(depe)2(CH3CN)(N2)]
(4) loses dinitrogen to give the square pyramidal inter-
mediate [Mo(depe)2(CH3CN)] (5). The charge at Câ
(+0.28) in the corresponding model system 5′ (Figure
-
chemical shift of -77.86 ppm for the unbound BF4
counterion. The coordinated and noncoordinated fluo-
rine atoms are less shielded than in the [WF(NCH2-
CH3)(dppe)2][BF4] complex, where they are observed at
-174.8 and -149.9 ppm, respectively.12
In analogy with the depe system, formation of the
alkylimido complex 17 by protonation of the acetoni-
trile-N2 complex 14 with HBF4 can be explained as
follows (Scheme 4): Attack of HBF4 on complex 14 leads
to replacement of N2 by fluoride, forming the anionic
intermediate [MoF(NCCH3)(dppe)2]- (15). The binding

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5399
Table 3. Optimized Bond Distances (Å) and Bond Angles (deg) of the Model Systems 4′-7′ and 15′-19′a
model system
Mo-Nan
CtNan
Câ-C
Mo-N
Mo-P
Nan-Câ-C
Mo-N-Câ
N-Mo-Nan
[Mo(PH3)4(NNH2)(CH3CN)]2+ (ref 7)
2.275
1.175
1.160
1.169
1.461
1.461
1.459
2.609
179.82
180.0
179.73
CH3CN
[Mo(L)2(CH3CN)(N2)] (4′)
2.133
2.039
1.133
(NtN)
2.434
179.72
179.48
179.86
[Mo(L)2(CH3CN)] (5′)
2.044
2.113
1.990
1.174
1.172
1.202
1.459
1.458
2.395
Mo-O
1.456
1.535
1.493
2.422
2.424
2.443
179.36
179.82
139.63
179.54
179.82
176.56
[Mo(L)2(CH3CN)2] (6′)
Mo(L)2(CH3CN)(OTf)]- (7′)
175.94
O-Mo-N
[Mo(L)2 (CH3CN)(CH3-CH2N)]2+ (10′)
[Mo(L)2(F)(CH3CN)]- (15′)
2.237
1.726
1.99
1.160
1.438
1.21
2.545
179.87
112.79
133.92
179.78
179.47
176.28
2.228
Mo-F
2.078
[Mo(L)2(F)(CH3CHN)] (16′)
1.890
1.283
1.573
2.472
123.64
173.22
177.99
F-Mo-N
179.29
[Mo(L)2(F)(CH3CH2N)]+ (17′)
[Mo(PH3)4(CH3CNH)]+ (18′)
1.756
1.433
1.535
1.995
2.533
2.463
112.61
133.38
179.60
72.04
2.133
1.246
1.510
2.113
(Mo-Câ)
[Mo(PH3)4(CH3CHNH)]2+ (19′)
2.109
1.296
1.485
2.553
2.473
125.44
133.30
a L ) H2P-(CH2)2-PH2, an ) acetonitrile, numbers in italics are for Mo-NEt.
Table 4. NPA Charge Distribution of Model Systems 4′-7′ and 15′-17′a
system
Mo
N
Câ
C
X
ref
[Mo(PH3)4(NCCH3)(NNH2)]2+
-0.02
-0.50
0.56
NR -0.10
Nâ -0.58
NR -0.02
Nâ -0.08
7
*
[Mo(L)2(NCCH3)(N2)] (4′)
-1.10
-0.33
0.35
-0.78
[Mo(L)2(NCCH3)] (5′)
[Mo(L)2(NCCH3)2] (6′)
-0.88
-1.16
-0.32
-0.32
-0.32
-0.31
-0.21
-0.47
-0.36
-0.41
-0.35
0.28
0.33
0.33
0.11
-0.31
+0.52
0.05
-0.08
-0.30
-0.77
-0.77
-0.77
-0.74
-0.68
-0.81
-0.73
-0.71
-0.68
*
*
[Mo(L)2(NCCH3)(OTf)]- (7′)
-0.93
-0.07
-0.82 (OTf)
*
*
[Mo(L)2 (CH3CN)(CH3-CH2N)]2+ (10′)
[Mo(L)2(NCCH3)(F)]- (15′)
[Mo(L)2(NCHCH3)(F)] (16′)
[Mo(L)2(NCH2CH3)(F)]+ (17′)
-0.83
-0.34
0.07
-0.63 (F)
-0.58 (F)
-0.56 (F)
*
*
*
a X ) ligand trans to the acetonitrile. *This work. Numbers in italics are for acetonitrile. L ) H2P-CH2-CH2-PH2.
4) shows that there is a small charge transfer from the
metal center to acetonitrile, which is correlated with a
reduction of charge density on the Mo center from -1.10
to -0.88 (Table 4). This small reduction of positive
charge on the â-carbon is certainly not sufficient for
protonation of this atom. In a fourth possibility the
acetonitrile from the reaction medium can occupy the
vacant site in the five-coordinate intermediate 5 to give
the six-coordinated complex [Mo(depe)2(CH3CN)2] (6).
NPA charge analysis of the corresponding model system
6′ (Figure 4) shows that there is again a positive charge
(+0.33) on the â-carbon of acetonitrile (Table 4); there-
fore this ligand is not activated. Thus the â-carbon of
actonitrile is not activated in all of the above four
possible intermediates. This is supported by the struc-
tural parameters collected in Table 3.
ligand bends at Câ (N-Câ-C angle 139.63°), which has
not been observed for any other model system (Table
3). The geometry change of acetonitrile from linear to
bent reflects a charge transfer from the central ion to
the ligand, changing the hybridization of the â-carbon
of acetonitrile from sp to sp2.
As shown in Section 2.5, the reaction of HBF4 with
[Mo(dppe)2(CH3CN)(N2)] (14) in benzene generates the
acetonitrile-protonated and not the dinitrogen-proto-
nated product. In analogy with the OTf system, anion
binding should be the reason for this finding. To check
this assumption, the OTf ligand in model system 7′ was
replaced by fluoride, leading to structure [Mo(H2P-
CH2-CH2-PH2)2(CH3CN)(F)]- (15′). Initially the bond
angle at Câ was set to 180° and bond distances and
angles for rest of the atoms were set to the values of
the parent dinitrogen complex (14). In the course of
optimization the Mo-N bond distance of acetonitrile
was found to shorten from 2.379 Å to 1.99 Å and the
NtC bond distance to elongate from 1.161 Å to 1.21 Å.
Most importantly, the bond angle of CH3-CtN was
reduced from 180.0° to 133.0° (Table 3, Figure 4), similar
to what is observed for the OTf complex 7′.
The final possibility is that OTf replaces the dinitro-
gen ligand of 4 to give [Mo(depe)2(CH3CN)(OTf)]- (7),
in which acetonitrile is coordinated trans to the OTf
ligand (Scheme 2). This intermediate is examined in the
next section.
3.2. Mo(H2P-CH2-CH2-PH2)2(CH3CN)(OTf)]- (7′)
and [Mo(H2P-CH2-CH2-PH2)2(F)(CH3CN)]- (15′).
Geometry optimization of model system [Mo(H2P-CH2-
CH2-PH2)2(CH3CN)(OTf)]- (7′) shows that the Mo-Nan
(an ) acetonitrile) bond distance (1.990 Å) gets shorter
than a Mo-Nan single bond (compare the other model
systems in Table 3), whereas the C-N triple bond is
elongated to 1.202 Å. More importantly, the acetonitrile
Natural population analysis (NPA, Table 4) shows
that the positive charge on the â-carbon of nitrile
decreases from +0.35 in the dinitrogen complex to +0.11
(OTf complex 7′) and +0.05 (F complex 15′), whereas

5400 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
Figure 4. Optimized structures of the model systems (L ) H2P-CH2-CH2-PH2).
Figure 5. MO scheme of [Mo(H2P-CH2-CH2-PH2)2(CH3CN)(F)]- (15′).
the nitrogen atom of nitrile carries a charge of -0.31
(OTf complex) and -0.35 (F complex). The molecular
orbital calculation of model system 15′ reveals that the
HOMO has metal-fluoride and â-carbon lone-pair
character (13%; Table 5 and Figure 5). In contrast, the
contribution of NR in the highest occupied orbitals of
15′ is small; the same applies to the OTf complex 7′. In
the following, the fluoro complex [Mo(H2P-CH2-CH2-

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5401
Table 5. Charge Decomposition and Energies for Important Molecular Orbitals of the Model Systems
15′-17′
orbital decompositions
system
orbital no.
68
orbital description
energ (a u)
%Mo
11.56
%N
%Câ
0.59
%Cγ
0.31
%F
%P
15′
-0.134
5.85
58.77
15.64
Mo(0) d6
69
70
71
72
73
74
75
76
77
78
68
-0.120
-0.105
-0.021
0.013
10.97
5.75
1.82
1.34
0.42
1.67
1.37
0.27
3.05
2.80
1.72
2.75
9.84
1.35
0.91
1.01
12.89
8.44
0.50
2.14
1.60
0.63
4.46
0.50
0.07
0.13
0.01
4.08
2.16
0.21
12.94
4.77
1.27
1.23
0.33
31.97
35.46
0.31
4.09
5.33
0.20
0.67
0.08
1.98
0.82
66.26
45.04
45.60
9.69
10.00
8.72
23.48
21.62
19.32
36.98
18.01
5.98
dxy
59.39
48.81
55.01
56.70
50.79
64.40
14.92
47.12
12.23
dxz-px(Câ)
HOMO,dyz-py(Câ)
0.019
0.119
0.130
0.136
0.149
0.165
16′
-0.292
Mo(II) d4
69
70
71
72
73
74
75
76
77
78
68
-0.256
-0.253
-0.209
-0.172
-0.130
-0.020
0.002
8.16
5.34
5.91
4.26
34.15
0.35
0.26
15.78
0.20
1.59
0.25
0.02
0.01
0.62
3.31
0.34
0.97
18.40
2.37
0.52
0.18
0.75
0.04
2.98
2.40
0.26
7.74
0.02
0.24
9.94
0.14
0.18
0.43
0.01
0.01
12.04
25.72
12.26
0.07
8.52
3.82
0.28
2.23
0.18
0.00
0.01
58.41
51.72
17.40
5.64
17.21
70.72
57.34
43.94
43.83
8.84
53.99
24.43
23.94
dxy
HOMO,dxz-px(Câ)
LUMO,dyz-py(Câ)
4.56
10.26
28.58
33.56
30.83
68.12
49.40
0.015
0.016
0.027
17′
-0.436
Mo(IV) d2
69
70
71
72
73
74
75
76
77
78
-0.412
-0.404
-0.385
-0.380
-0.330
-0.167
-0.159
-0.134
-0.125
-0.120
15.62
17.09
15.37
12.23
80.64
41.43
46.26
31.35
25.79
32.90
15.32
23.04
21.39
16.93
0.32
21.02
21.15
0.11
4.94
0.94
2.60
0.79
0.66
0.66
1.00
0.14
2.02
0.40
2.18
4.30
0.60
2.24
0.02
2.03
5.35
0.01
0.37
0.41
1.44
0.16
21.53
18.46
0.01
3.75
3.61
0.24
3.29
0.11
43.47
38.72
28.60
40.06
3.38
18.42
13.93
61.43
36.76
31.36
px(N)-dxz
py(N)-dyz
HOMO, dxy
LUMO, dxz-px(N)
dyz-py(N)
0.16
0.78
PH2)2(F)(CH3CN)]- (15′) is treated as a generic model
system to study the stepwise protonation of organoni-
triles.
on the Câ atom of the ligand. The lone pair at Câ thus
can interact with another proton to give the doubly
protonated product 17′.
3.3. Monoprotonated Intermediate [MoII(H2P-
CH2-CH2-PH2)2(F)(CH3CHN)] (16′). Structure [Mo-
(H2P-CH2-CH2-PH2)2(F)(CH3CHN)] (16′) of the mono-
protonated intermediate (methyl-azavinylidene complex)
was optimized at the same level of theory as employed
in the previous calculations. The optimized structural
parameters are given in Table 3. In 16′ the Mo-F and
Mo-N interactions become stronger than in 15′, as
evident from the shortening of these two bonds in
comparison to the parent structure. The N-C bond
distance, in contrast, is elongated and is now compa-
rable to a double bond (Table 3). The N-C-C bond
angle is slightly reduced to 123.64° and the F-Mo-N
bond angle stays close to linear (Table 3).
3.4. Fluoro Ethylimido Complex [MoIV(H2P-
CH2-CH2-PH2)2(F)(CH3CH2N)]+ (17′) and Aceto-
nitrile Ethylimido Complex [Mo(H2P-CH2-CH2-
PH2)2(NCH2CH3)(CH3CN)]2+ (10′). The geometry of
the (doubly protonated) ethylimido model system 17′
was optimized at the same level as the other structures;
important bond distances and angles are given in Table
3. The Mo-N (acetonitrile) bond distance now is close
to a MotN distance, and the N-Câ distance is elongated
to a value close to a N-C single bond. There is a slight
change in the Mo-F bond distance as compared to
model system 16′ (Table 3). Selected molecular orbitals
are shown in Figure 7. The HOMO is the nonbonding
orbital dxy. The LUMO corresponds to the HOMO of the
monoprotonated complex 16′, reflecting another two-
electron oxidation of the molybdenum center to Mo(IV)
(d2). LUMO and LUMO+1 are metal-centered orbitals
(dxz and dyz) that have an antibonding interaction with
the px and py orbitals of the â-carbon of acetonitrile,
respectively (Figure 7 and Table 5). Similar results are
obtained for the complex [Mo(depe)2(NCH2CH3)(CH3-
CN)](OTf)2 (10), which is generated from 17′ through
replacement of OTf by acetonitrile (Schemes 1 and 2).
Selected bond distances and angles of 10′ are listed in
Table 3; contours are shown in Figure 8. As observed
for model system 17′, the HOMO is the nonbonding dxy
orbital. LUMO and LUMO+1 are dxz and dyz orbitals
Natural population analysis (NPA) of 16′ (Table 4)
shows that the â-carbon now carries a charge of -0.08.
Addition of one proton on the â-carbon of acetonitrile
thus has increased the negative charge on this center,
concomitant with a reduction of electron density on the
metal from -0.83 to -0.34. Selected orbitals of 16′ are
shown in Figure 6. The HOMO ( 73 ) is a Mo dxz orbital
(57.34% Mo, 8.52% F) with a contribution of an out-of-
plane p orbital (18.40%) at the â-carbon. The LUMO
corresponds to the HOMO (dyz) of the acetonitrile
complex 15′, reflecting a two-electron oxidation of the
molybdenum center to Mo(II) (d4) (Table 5). The electron
density transferred from the metal center is localized

5402 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
Figure 6. MO scheme of [Mo(H2P-CH2-CH2-PH2)2(NCHCH3)(F)] (16′). A Mo(II) d4 configuration applies.
Figure 7. MO scheme of the [Mo(H2P-CH2-CH2-PH2)2(NCH2CH3)(F)]+ (17′). A Mo(IV) d2 configuration applies.
that have an antibonding interaction with NEt and a
bonding interaction with the acetonitrile ligand, again
reflecting a Mo(IV) oxidation state of the central
atom.
3.5. Energetics of the Proposed Reaction Path.
To check the feasibility of the proposed reaction mech-
anism, an energy profile was determined (Figure 9 and
Table 6). Loss of N2 from the acetonitrile-dinitrogen
complex 4′ is associated with a standard free enthalpy
change of +18.7 kcal/mol (Figure 9, left). The subse-
quent bonding of triflate to the five-coordinate inter-
mediate 5′ leading to 7′ is also endergonic (+16.9 kcal/

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5403
Figure 8. Molecular orbital scheme of the model system [Mo(H2P-CH2-CH2-PH2)2(CH3CN)(CH3CH2N)]+ (10′).
latter step ensures the irreversibility of the entire
reaction course.
3.6. Examination of Primary Protonation at Nr.
As an alternative to the proposed scenarios, initial
protonation of the coordinated nitrile could in principle
also occur at NR (Scheme 5). We theoretically examined
this pathway based on the five-coordinate complex 5.
Protonation of the nitrile nitrogen atom first leads to
the side-on-coordinated intermediate 18 (cf. Supporting
Information). Further protonation of 18 at Câ then forms
the coordinatively unsaturated ketimine complex 19,
which adds another ligand X (OTf or fluoride), leading
to 20. To generate the observed alkylimido products 9
and 17, respectively, this complex has to rearrange,
either by a direct proton shift from the imine N to Câ or
(more probably) by a deprotonation/reprotonation pro-
cess via the azavinylidene intermediates 8 and 16,
respectively. Although this scenario is in principle
possible, it does not explain the differences observed
between the depe system 3 and the dppe system 13 upon
reaction with base (nitrile protonation vs lack of nitrile
activation; cf. Scheme 1). Furthermore, the different
protonation pathways observed for the acetonitrile-
dinitrogen complexes [M(dppe)2(N2)(CH3CN)] (M ) Mo,
Figure 9. Protonation of triflate (left) and fluoro (right)
acetonitrile complexes. All energies correspond to standard
free enthalpies and are in kcal/mol.
mol), but the subsequent protonation of Câ is strongly
exergonic (-67.1 kcal/mol). For the corresponding fluoro
complex 15′, the first protonation is exergonic by -129.7
kcal/mol (Figure 9, right). The apparent barrier for this
process (+35.6 kcal/mol for the triflate system, cf. Figure
9, left) is effectively lowered by the fact that 4′ loses
dinitrogen to the atmosphere, generating (under argon)
a sufficient concentration of the five-coordinate inter-
mediate 5′.28 This will bind triflate in a moderately
endergonic reaction, leading to the monoprotonated
intermediate 8′. The strongly exergonic character of the
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-
combe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.04; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(19) Horn, K. H.; Lehnert, N.; Tuczek, F. Inorg. Chem. 2003, 42,
1076.
(20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(21) Dunning, T. H., Hay, P. J., Schaefer, H. F, Eds. Modern
Theoretical Chemistry; Plenum: New York, 1976.
(22) Petersson, G. A.; Al-Laham. A. J. Chem. Phys. 1991, 94, 6081.
(23) (a) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992,
97, 2571. (b) Schaefer, A.; Hubner, C.; Ahlrichs, R. J. Chem. Phys. 1994,
100, 5829.
(24) (a) Cance`s, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997,
107, 3032. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem.
Phys. Lett. 1998, 286, 253. (c) Mennucci, B.; Tomasi, J. J. Chem. Phys.
1997, 106, 5151.
(25) Reed, A. E.; Curtis, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899. (b) Reed, A. E.; Weinhold, F. J. Am. Chem. Soc. 1986, 108, 3586.
(c) Reed, A. E.; Weinstock, R. B.; Weinhold. F. J. Chem. Phys. 1985,
83, 735. (d) Reed, A. E.; Winhold, F. J. Chem. Phys. 1983, 83, 1736.
(27) Neese, F. ORCA, version 2.2; Max-Planck Institut fu¨r Bioan-
organische Chemie: Mu¨hlheim/Ruhr, Germany, 2004.
(28) This hypothesis is supported by the fact that the reaction
proceeds slower under dinitrogen than under argon.

5404 Organometallics, Vol. 24, No. 22, 2005
Table 6. Energies and Thermochemical Data of the Model Systems
Sivasankar et al.
c
ZPE + thermal
correction [hartree]
entropy
[cal/mol K]
∆Gsolv
G°
[kcal/mol]
system
energy [hartree]
[kcal/mol]
∆G° [kcal/mol]
[Mo(CH3CN)(N2)] (4′)
[Mo(CH3CN)] (5′)
[Mo(CH3CN)(OTf)]- (7′)
[Mo(CH3CHN)(OTf)]- (8′)
[Mo(CH3CH2N)(OTf)]+ (9′)
N2
-5747.66857494
-5638.05229405
-6599.84194930
-6600.41808753
-6600.83158869
-109.56669572
-961.767733
0.267178
0.256376
0.293095
0.307779
0.321379
0.008472
0.035384
148.706
145.504
190.040
185.630
180.697
45.885
+6.60
+6.83
-3606586.99
-3537807.32
-4141357.98
-4141679.63
-4141959.57
-68761.00
0a
+18.67a
+35.59a
-31.46a
-56.80a
-21.37
+7.98
-22.50
+1.59
-45.74
OTf-
85.515
-603567.58
HOTf f OTf-
∆G ) 254.60
HOTf
-962.253602
-5738.01181798
-5738.61965457
-5739.05892992
-233.749360
0.045640
0.257620
0.272423
0.287569
0.143810
86.893
149.889
151.433
151.220
79.769
-1.47
-18.64
+0.72
-16.09
-0.87
-603822.18
-3600558.87
-3600912.11
-3601195.00
-146614.50
[Mo(CH3CN)(F)]- (15′)
[Mo(CH3CHN)(F)] (16′)
[Mo(CH3CH2N)(F)]+ (17′)
ether
0b
-129.66b
-188.97b
etherH+ f ether
∆G ) 223.58
etherH+
-234.076363
0.157085
82.408
-26.79
-146838.08
a Relative to 4′ + free OTf-. b Relative to 15′ taking consumed etherH+ molecules into account. When employing a solution of HBF4 in
ether, the protonating agent is a protonated ether. c In all calculations involving triflic acid solv ) acetonitrile; in all calculations involving
HBF4, solv ) benzene.
Scheme 5
the corresponding depe systems. Upon addition of base
to the NNH2-acetonitrile complex 3, however, the
expected dinitrogen complex 4 was not formed, but
instead the ethylimido complex 10 was obtained. By
reduction of the amount of base it was then established
that only catalytic amounts of base are necessary to
start the reaction. Upon treatment with base, the [Mo-
(NNH2)(CH3CN)(dppe)2] complex 13, in contrast, was
found to give the corresponding dinitrogen complex 14,
which could not be isolated in the depe system. Subse-
quent addition of HBF4 to complex 14 then generates
the ethylimido complex 17, in agreement with the
literature.12,15
The observed protonation of coordinated acetonitrile
at a Mo(0)-N2 fragment with diphos coligands is
remarkable, as DFT predicts a fairly large positive
charge at the â-carbon of acetonitrile (+0.35), which
should render protonation of this atom rather unfavor-
able. This even more applies to the NNH2 complex [Mo-
(NNH2)(CH3CN)(dppe)2]2+ (+0.56; Table 4). In this
respect, it is noteworthy that the NNH2 depe complexes
2 ([Mo(NNH2)(OTf)(depe)2](OTf)) and 3 ([Mo(NNH2)(CH3-
CN)(depe)2](OTf)2) also spontaneously convert to the
ethylimido complex 10 + N2 in acetonitrile. These
autocatalytic conversions, however, take more time for
completion than the base-catalyzed reaction, indicating
that deprotonation of the NNH2 complexes to give the
dinitrogen intermediate 4 is essential for the reaction
(Schemes 2 and 4). However, acetonitrile does not
appear to be activated in this complex either (vide
supra). N2 therefore has to displaced or exchanged
against a stronger activating ligand. By exclusion of all
other possibilities, this can only be triflate, leading to
the six-coordinate, anionic Mo(0) complex 7. On the
basis of the significantly lowered positive charge on Câ
(+0.11) and the bending at Câ, acetonitrile in fact
appears to be activated toward protonation in 7. The
feasibility of the proposed reaction pathway was dem-
onstrated by an energy profile that shows that the
involved intermediates are thermally accessible. In a
strongly exergonic reaction, the activated complex 7
abstracts a proton from the parent NNH2 complex 3 to
give a monoprotonated Mo(II) azavinylidene complex 8,
W; HBF4: nitrile protonation; CF3SO3H: dinitrogen
protonation) would not be understandable. These ex-
perimental observations rather suggest a synergistic
activation of nitrile by the anionic Lewis base (triflate,
fluoride) and the phosphine ligation (dppe, depe), which
is not needed in Scheme 5, but is necessary in the
â-protonations described Schemes 2-4. We thus con-
clude that initial protonation at NR is not important in
the conversion of nitriles to alkylimides.
4. Discussion
In the preceding sections the conversion of a Mo(IV)
acetonitrile-NNH2 complex with depe coligands (3) to
a Mo(IV) ethylimido complex (10) + N2 has been
described. The reaction was discovered when we tried
to adapt the method of Field et al.12 for the preparation
of trans W dppe dinitrogen-acetonitrile complexes to

Protonation of Coordinated Nitriles at the â-Carbon
Organometallics, Vol. 24, No. 22, 2005 5405
which, however, cannot be isolated.29 Further protona-
tion of 8 leads to the Mo(IV) alkylimido complex 9.
Deprotonation of the NNH2 complex by these reactions
leads to the dinitrogen complex 4. The dinitrogen ligand
in 4 can again be replaced by OTf, re-forming complex
7; in this way a cyclic reaction course ensues (Scheme
4). The OTf ligand in complex 9 readily undergoes
replacement with acetonitrile to give complex 10, which
is isolated as the final product.
three-necked round-bottomed flask; 100 mL of acetonitrile was
added and the mixture stirred for 6 h. The solution was
concentrated to 10 mL, and diethyl ether was added. Complex
3 was precipitated out in a quantitative yield. IR (KBr): 1326
((s), NdNH2), 2278 (s, CtN), 3095, 3253 (w(br), NH2) cm-1
.
1H NMR (C6D6): δ 2.13 (m, PCH2CH2P), 2.18 (s, CH3CN), 7.52
(s, 2H, NH2) ppm. 31P{1H} NMR (C6D6): δ 44.5. Anal. Found:
C, 32.52; H, 5.86; N, 4.49. Calcd for C24H53N3P4S2O3F6Mo: C,
32.84; H, 6.08; N, 4.78.
Synthesis of [Mo(depe)2(NCH2CH3)(CH3CN)](OTf)2 (10).
A catalytic amount of NEt3 was added to 0.300 g of [Mo-
(N2H2)(depe)2(CH3CN)](OTf)2 (3) complex in 100 mL of aceto-
nitrile solution. This solution was stirred for 1 h and concen-
trated to 10 mL under vacuum. Diethyl ether was added to
this solution, and complex 10 was precipitated out. Yield: 94%.
IR (KBr): 2280 (w(br), CtN). Raman: 2272 (w(br), CtN). 1H
NMR (C6D6): δ 1.20 (t, 3H, CH3CH2N), 1.81 (m, 4H, PCH2C-
H2P), 2.1 (s, CH3CN), 1.94 (q, 2H, CH3CH2N) ppm. 31P{1H}
NMR (C6D6): δ 59.9. Anal. Found: C, 34.82; H, 6.08; N, 3.08.
Calcd for C26H56N2P4 S2O3F6Mo: C, 35.06; H, 6.29; N, 3.14.
Reaction of [Mo(N2H2)(dppe)2(OTf)](OTf) (12) with
Base. Et3N (0.23 mmol) was added to a solution of [Mo(N2H2)-
(dppe)2(OTf)](OTf) (12) (0.2 mmol) complex in acetonitrile (20
mL), and the reaction mixture was stirred at room tempera-
ture for 30 min. The resulting dark red precipitate was
separated by filtration, washed with acetonitrile, and dried
in vacuo to give [Mo(N2)(CH3CN)(dppe)2] (14) as a red solid
Whereas the depe dinitrogen-acetonitrile complex 4
is unstable toward protonation of coordinated acetoni-
trile, it is possible to isolate the dinitrogen-acetonitrile
dppe complex [Mo(dppe)2(N2)(CH3CN)] (14) by depro-
tonation of its NNH2 analogue 13 (vide supra). It can
be anticipated that the [Mo(dppe)2(N2)(CH3CN)] (14)
complex undergoes the same ligand displacement reac-
tions as its depe counterpart. In particular, an activated
acetonitrile-triflate complex like 7 may form if OTf- is
present in solution, but for dppe this species does not
appear to mediate protonation of the bound acetonitrile
to generate the corresponding alkylimido complex. This
finding would once more reflect the differences in the
electronic donor/acceptor properties of depe and dppe;
that is, depe is a stronger σ-donor than dppe, whereas
the latter is a stronger π-acceptor. As a consequence,
the triflate depe complex 7 mediates protonation of the
bound acetonitrile, whereas the activation of nitrile
provided by the analogous triflate dppe complex is not
sufficient. Nevertheless, the actonitrile-N2 dppe com-
plex 14 allows protonation of the coordinated acetoni-
trile using HBF4 as acid (Scheme 4). In the framework
of the proposed reaction scheme, this suggests that
binding of a stronger Lewis base than triflate (such as
fluoride) is required to activate acetonitrile on a Mo(0)
dppe complex. This is supported by DFT calculations,
which indicate a stronger charge transfer to acetonitrile
and a stronger exergonicity of the protonation reactions
by [MoF(diphos)2]- than by [Mo(OTf)(diphos)2]-. On the
other hand, if triflic acid is used to protonate the
tungsten dppe complex [W(dppe)2(N2)(CH3CN)], then
not acetonitrile but dinitrogen is protonated. These
findings demonstrate that the activation of nitrile is a
combined effect of the phosphine ligation and the Lewis-
basic coligand.
(73%). IR (KBr): 2199 (w(br), CtN), 1919 (w(br), N2) cm-1
Raman: 2201 (w(br), CtN), 1913 (w(br), N2) cm-1 1H NMR
.
.
(C6D6): δ 0.98 (s, 3H, CH3CN), 2.35 (m, 8H, PCH2CH2P), 6.89-
7.67 (m, 40H, P(C6H5)2) ppm. 31P{1H} NMR (C6D6): δ 59.9.
Anal. Found: C, 67.13; H, 5.13; N, 4.21. Calcd for C54H51N3P4-
Mo: C, 67.43; H, 5.34; N, 4.36.
Reaction of [Mo(N2)(CH3CN)(dppe)2] (14) with HBF4.
Tetrafluoroboric acid (0.28 mmol) was added to a solution of
[Mo(N2)(CH3CN)(dppe)2] (14) (0.24 mmol) in benzene (20 mL),
and the reaction mixture was stirred at room temperature for
2 h and filtered through a sintered-glass frit. The filtrate was
concentrated, and the addition of ether precipitated a yellow
solid, which was separated by filtaration and washed with
diethyl ether (2 × 10 mL) to give a yellow solid of [MoF(NCH2-
CH3)(dppe)2][BF4] (17) (62%). 1H NMR (CDCl3): δ -0.19 (t,
3H, CH3CH2N), 1.83 (dq, 2H, CH3CH2N), 2.60 (m, 4H,
PCH2CH2P), 2.71 (m, 4H, PCH2CH2P), 6.86-7.43 (m, 40H,
P(C6H6)2) ppm. 31P{1H} NMR (CDCl3): δ 46.45. 19F NMR
(CDCl3): δ -148.31 (1F, MoF), -77.86 (4F, BF4-) ppm. Anal.
Found: C, 62.04; H, 5.03; N, 1.13. Calcd for C54H53BF5NP4-
Mo: C, 62.26; H, 5.12; N, 1.34.
Reaction of [W(N2)(CH3CN)(dppe)2] (14a) with CF3S-
O3H. Triflic acid (0.37 g, 0.27 mmol) was added to a solution
of [W(N2)(CH3CN)(dppe)2] (0.12 g, 0.11 mmol) in benzene (25
mL), and the reaction mixture was stirred at room tempera-
ture for 3 h and filtered through a sintered-glass frit. The
filtrate was concentrated, and upon addition of ether an
orange-yellow solid precipitated. The solid was separated by
filtration (G4) and washed with diethyl ether (3 × 10 mL) to
give an orange-yellow mixture of [W(NNH2)(OTf)(dppe)2][OTf]
(20) and [W(NNH2)(CH3CN)(dppe)2][OTf]2 (21) (57%, 3:1,
respectively). The ratio of the complexes is identified on the
basis of NMR peak integrations. Complex 20: 1H NMR
(CDCl3): δ 2.59 (m, 4H, PCH2CH2P), 3.18 ((m, 4H, PCH2CH2P)),
6.18 (b, NH2), 7.10-7.39 (m, 40H, P(C6H6)2) ppm. 31P{1H} NMR
(CDCl3): δ 42.21. 19F NMR (CDCl3): δ -78.75 (3F, OTf),
-76.67 (3F, Mo-OTf). Complex 21: IR (KBr): 2269 (w(br),
CtN), 3235, 3344 (w(br), NH2) cm-1. 1H NMR (CDCl3): δ 2.15
(s, CH3CN), 2.59 (m, 4H, PCH2CH2P), 2.81 ((m, 4H, PCH2C-
H2P)), 6.18 (b, NH2), 7.10-7.39 (m, 40H, P(C6H6)2) ppm. 31P-
{1H} NMR (CDCl3): δ 38.95. 19F NMR (CDCl3): δ -78.75 (6F,
OTf).
5. Experimental Section
General Procedures. All syntheses were carried out in an
atmosphere of dinitrogen or argon using standard Schlenk
techniques. All solvents were dried over appropriate drying
agents under an inert atmosphere. MoCl5, dppe, and depe were
purchased from Aldrich. [Mo(N2H2)(depe)2(OTf)](OTf) (2) and
[Mo(N2H2)(dppe)2(OTf)](OTf) (12) were synthesized using lit-
erature procedures.16 MIR spectra were recorded from KBr
pellets using a Mattson Genesis Type I spectrometer. Raman
spectra were recorded on a Bruker IFS 66/CS NIR spectrom-
eter. NMR spectra were recorded on a Bruker Avance 400
pulse Fourier transform spectrometer operating at a 1H
frequency of 400.13 MHz, 31P 161.98 MHz, and 19F 376.50 MHz
using a 5 mm inverse triple-resonance probe head. References
as substitutive standards: H3PO4 85% pure, δ (31P) ) 0 ppm,
and CFCl3/CDCl3, δ (19F) ) 0 ppm, were used.
Synthesis of [Mo(N2H2)(depe)2(CH3CN)](OTf)2 (3). A
0.300 g sample of complex 2 was transferred into a 250 mL
DFT Calculations. The structures of model systems 4′-
10′ and 15′-19′ have been fully optimized using Becke’s three-
(29) The azavinylidene product could be isolated and characterized
in a [Re(I)Cl(dppe)2] complex; see ref 14b.

5406 Organometallics, Vol. 24, No. 22, 2005
Sivasankar et al.
parameter hybrid functional with the LYP correlation func-
tional of Lee, Yang, and Parr.20 The LANL2DZ basis set was
used for the metal center, and 6-31G* was used for the
nonmetal centers.21,22 N2, HOTf, OTf-, Et-O-Et, and Et-
OH-Et+ were optimized using the B3LYP functional together
with the TZVP23 basis set. Vibrational frequency calculations
have been performed on these optimized structures to obtain
the corresponding entropy, ZPE, and thermal correction data.
A solvent correction calculation (acetonitrile and benzene,re-
spectively) was also performed using the polarized continuum
model (PCM).24 Charges were calculated using the natural
bond orbital (NBO) formalism.25 All these computational
procedures were used as they are implemented in the Gauss-
ian-03 package.26 Wave functions were plotted using Gauss-
view 03. To obtain reliable single-point energies for 4′-5′, 7′-
9′, and 15′-17′, single-point calculations have been performed
on the optimized structures using the B3LYP functional
together with the larger TZVP basis set as implemented in
ORCA.27
Acknowledgment. F.T. and C.S. thank DFG Tu58/
12 and FCI for funding of this research.
Supporting Information Available: 1H NMR and 31P
NMR spectra of complexes 3, 10, 14, and 17 as well as
optimized Cartesian coordinates of the model systems. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM050220R