Facile synthesis of alkynyl- and vinylidene-niobocene complexes. Unexpected η1-vinylidene-η2-alkyne isomerization

Article abstract of DOI:10.1021/om990957e

Treatment of Nb(η⁵-C₅H₄SiMe₃) ₂(Cl)(L) (1) with Mg(C=CR)₂ in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η⁵-C₅H₄SiMe₃) ₂(C=CR)(L) (2: L = CO, R = Ph (2a); L = CO, R = SiMe₃ (2b); L = CO, R = ^tBu (2c); L = PMe₂Ph, R = Ph (2d); L = P(OEt)₃, R = Ph (2e)). The alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give the corresponding cation-radical alkynyl complexes [Nb(η⁵-C₅H₄SiMe₃) ₂(C=CR)(L)].⁺[BPh₄]^- (3: L = CO, R = Ph (3a); L = CO, R = ^tBu (3c); L = PMe₂Ph, R = Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d² niobocene species [Nb(η₅-C₅H₄-SiMe₃) ₂(=C=CHR)(L)][BPh₄] (4: L = CO, R = Ph (4a); L = CO, R = ^tBu (4c); L = PMe₂Ph, R = Ph (4d)) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d² niobocene complex [(η⁵-C₅H₄SiMe₃) ₂(CO)Nb=C=C(Ph)(Ph)C=C=Nb(CO)(η⁵-C₅H ₄SiMe₃)₂][BPh₄]₂ (4a′) from a competitive ligand-ligand coupling process. Complexes 4 were also prepared by an alternative procedure in which the corresponding complexes 2 were reacted with HBF₄. Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η²-alkyne derivatives [Nb(η⁵-C₅H₄SiMe₃) ₂(η²(C,C)-HC≡ CR)(CO)]⁺ (5: R = Ph (5a); R = ^tBu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp₂(=C=CHCH₃)(L)]⁺/[NbCp ₂(HC≡ CCH₃)(L)]⁺ (Cp = η⁵-C₅H₅; L = CO, PH₃; exo, endo) model systems in order to explain the η¹-vinylidene-η²-alkyne rearrangement observed. Calculations have shown that in both carbonyl-niobocene and phosphine-niobocene systems the η¹-vinylidene and the η²-alkyne complexes are isoenergetic, in marked contrast with the systems previously considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift mechanism where η²(C,H)-alkyne species are involved. The energy barrier for the isomerization process in the phosphine-containing niobocene systems is almost 10 kcal mol^-1 higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation barrier indicates that the different experimental behavior between 4a, 4c, and 4d has a kinetic rather than a thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.

Full text of DOI:10.1021/om990957e

Organometallics 2000, 19, 1749-1765
1749
F a cile Syn th esis of Alk yn yl- a n d Vin ylid en e-Niobocen e
Com p lexes. Un exp ected η1-Vin ylid en e-η2-Alk yn e
Isom er iza tion
Cristina Garc´ıa-Yebra,† Carmen Lo´pez-Mardomingo,† and Mariano Fajardo‡
Departamento de Quı´mica Orga´nica and Departamento de Quı´mica Inorga´nica,
Campus Universitario, Universidad de Alcala´, 28871 Alcala´ de Henares (Madrid), Spain
Antonio Antin˜olo, Antonio Otero,* and Ana Rodr´ıguez
Departamento de Quı´mica Inorga´nica, Orga´nica y Bioquı´mica, Facultad de Quı´mica,
Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
Alain Vallat, Dominique Lucas, and Yves Mugnier
Laboratoire de Synthe`se et d’Electrosynthe`se Organometalliques associe´ au CNRS (URA 1685),
Faculte´ des Sciences, 6 Bd. Gabriel, 21000 Dijon, France
J orge J uan Carbo´ and Agust´ı Lledo´s*
Departament de Quı´mica, Universitat Auto`noma de Barcelona, 08193 Bellaterra,
Barcelona, Spain
Carles Bo
Departament de Quı´mica Fı´sica i Inorga`nica, Universitat Rovira i Virgili,
Pl. Imperial Tarraco, 1, 43005 Tarragona, Spain
Received December 6, 1999
Treatment of Nb(η5-C5H4SiMe3)2(Cl)(L) (1) with Mg(CtCR)2 in toluene, under appropriate reaction
conditions, leads to the alkynyl complexes Nb(η5-C5H4SiMe3)2(CtCR)(L) (2: L ) CO, R ) Ph (2a ); L
t
) CO, R ) SiMe3 (2b); L ) CO, R ) Bu (2c); L ) PMe2Ph, R ) Ph (2d ); L ) P(OEt)3, R ) Ph (2e)).
The alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give
the corresponding cation-radical alkynyl complexes [Nb(η5-C5H4SiMe3)2(CtCR)(L)]•+[BPh4]- (3: L )
CO, R ) Ph (3a ); L ) CO, R ) tBu (3c); L ) PMe2Ph, R ) Ph (3d )). These complexes, under different
experimental conditions, give rise to the mononuclear vinylidene d2 niobocene species [Nb(η5-C5H4-
t
SiMe3)2(dCdCHR)(L)][BPh4] (4: L ) CO, R ) Ph (4a ); L ) CO, R ) Bu (4c); L ) PMe2Ph, R ) Ph
(4d )) with a hydrogen atom by abstraction from the solvent or, for 3a , the binuclear divinylidene d2
niobocene complex [(η5-C5H4SiMe3)2(CO)NbdCdC(Ph)(Ph)CdCdNb(CO)(η5-C5H4SiMe3)2][BPh4]2 (4a ′)
from a competitive ligand-ligand coupling process. Complexes 4 were also prepared by an alternative
procedure in which the corresponding complexes 2 were reacted with HBF4. Finally, in solution the
CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization
process to give the η2-alkyne derivatives [Nb(η5-C5H4SiMe3)2(η2(C,C)-HCtCR)(CO)]+ (5: R ) Ph (5a );
R ) tBu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations
were carried out on [NbCp2(dCdCHCH3)(L)]+/[NbCp2(HCtCCH3)(L)]+ (Cp ) η5-C5H5; L ) CO, PH3;
exo, endo) model systems in order to explain the η1-vinylidene-η2-alkyne rearrangement observed.
Calculations have shown that in both carbonyl-niobocene and phosphine-niobocene systems the η1-
vinylidene and the η2-alkyne complexes are isoenergetic, in marked contrast with the systems previously
considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift
mechanism where η2(C,H)-alkyne species are involved. The energy barrier for the isomerization process
in the phosphine-containing niobocene systems is almost 10 kcal mol-1 higher than in the analogous
process for the carbonyl-containing niobocene system. This increase in activation barrier indicates
that the different experimental behavior between 4a , 4c, and 4d has a kinetic rather than a
thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.
In tr od u ction
documented. During the past few years several families
of alkyne-containing halobis(trimethylsilylcyclopen-
The chemistry of d-block transition metals with
alkynyl,1 vinylidene,2 and alkyne3 ligands has been well-
(1) (a) Nast, R. Coord. Chem. Rev. 1982, 47, 89. (b) Bruce, M. I.
Chem. Rev. 1983, 83, 203. (c) Manna, J .; J onh, K. D.; Hopkins, M. D.
Adv. Organomet. Chem. 1996, 79.
† Departamento de Qu´ımica Organica, Universidad de Alcala´.
‡ Departamento de Qu´ımica Inorganica, Universidad de Alcala´.
10.1021/om990957e CCC: $19.00 © 2000 American Chemical Society
Publication on Web 04/04/2000

1750 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
Sch em e 1
tadienyl)niobium complexes have been reported by our
group,4 and in particular, several cationic complexes
were isolated in good yields as their nitrile or isonitrile
adducts, [Nb(η5-C5H4SiMe3)2(η2(C,C)-RCtCR′)(L)]+, by
one-electron oxidation of the corresponding alkyne-
containing niobium(IV) species, Nb(η5-C5H4SiMe3)2(η2-
(C,C)-RCtCR′).5 These results encouraged us to explore
general synthetic methods for the preparation of alky-
nyl- and vinylidene-containing niobocene complexes,
which are rarely encountered in the literature. The
alkynyl and vinylidene species were successfully pre-
pared by reaction of the corresponding haloniobium(III)
complexes with the appropriate alkynylmagnesium
compounds, Mg(CtCR)2, followed by one-electron oxida-
tion of the isolated alkynylniobium species and a
hydrogen atom abstraction from the solvent. However,
surprising behavior was found for some vinylidene-
niobocene compounds, which isomerize easily in solution
to give the more stable alkyne-containing complexes.
The 1-alkyne to vinylidene rearrangement in the coor-
dination sphere of transition metals has proved to be a
useful entry to vinylidene complexes,6 vinylidene ap-
parently being stabilized in many transition-metal
complexes, and several catalytic cycles have been pro-
posed involving the participation of the η1-vinylidene-
η2-alkyne transformation.7 Despite the relative impor-
tance of this phenomenon, few examples of this type of
rearrangement have been described for group 5 ele-
ments.8 This paper will focus on the synthesis and
structural characterization of alkynyl-, vinylidene-, and
alkyne-containing niobocene complexes and an η1-vi-
nylidene-η2-alkyne conversion process. The study of the
vinylidene-alkyne rearrangement by means of DFT
calculations will also be discussed.
Resu lts a n d Discu ssion
P r ep a r a tion a n d Ch a r a cter iza tion of Alk yn yl-
Niobocen e Com p lexes. The starting niobium com-
plexes Nb(η5-C5H4SiMe3)2(Cl)(L) (1) reacted with the
appropriate dialkynylmagnesium reagents, in toluene,
to generate the corresponding alkynylniobocene com-
plexes 2 in good yields (Scheme 1). The use of a
noncoordinating solvent, such as toluene, has proved to
be essential in this process, due to the fact that the
reactivity of the magnesium reagent is increased. At-
tempts to carry out similar reactions with lithium
alkynyl reagents or the analogous Grignard alkynyl,
MgX(CtCR), in ethereal solvents have proved unsuc-
cessful. Previous efforts to prepare similar complexes
by reacting the halo derivatives M(η5-C5Me5)2Cl2 (M )
Nb, Ta) with LiCtCR were also unsuccessful.8 As far
as we are aware, this is the first general procedure to
prepare σ-alkynyl-niobocene complexes. Tantalum hy-
drido alkynyl species, Ta(η5-C5Me5)2(H)(CtCR), have
been proposed as the intermediates in the conversion
of η2-alkyne complexes, Ta(η5-C5Me5)2(η2(C,C)-HCtCR),
to the corresponding vinylidene species, Ta(η5-C5Me5)2-
(2) (a) Bruce, M. I.; Swincer, A. G. Adv. Organomet. Chem. 1983,
22, 59. (b) Bruce, M. I. Chem. Rev. 1991, 91, 197. (c) Antonova, A. B.;
J ohansson, A. A. Russ. Chem. Rev. (Engl. Transl.) 1989, 58, 693.
(3) (a) Ittel, S. D.; Ibers, J . A. Adv. Organomet. Chem. 1976, 14, 33.
(b) Otsuka, S.; Nakamura, A. Adv. Organomet. Chem. 1976, 14, 245.
(4) (a) Antin˜olo, A.; Go´mez-Sal, P.; Mart´ınez de Ilarduya, J . M.;
Otero, A.; Royo, P.; Mart´ınez-Carrera, S.; Garc´ıa-Blanco, S. J . Chem.
Soc., Dalton Trans. 1987, 975. (b) Antin˜olo, A.; Fajardo, M.; J alo´n, F.;
Lo´pez-Mardomingo, C.; Otero, A.; Sanz-Bernabe´, C. J . Organomet.
Chem. 1989, 369, 187. (c) Antin˜olo, A.; Fajardo, M.; Gil-Sanz, R.; Lo´pez-
Mardomingo, C.; Otero, A.; Lucas, D.; Chollet, H.; Mugnier, Y. J .
Organomet. Chem. 1994, 481, 27. (d) Antin˜olo, A.; Mart´ınez-Ripoll, M.;
Mugnier, Y.; Otero, A.; Prashar, S.; Rodr´ıguez, A. M. Organometallics
1996, 15, 3241.
(7) (a) Landon, S. J .; Shulman, P. M.; Geoffroy, G. L. J . Am. Chem.
Soc. 1985, 107, 6739. (b) Mache´, R.; Sasaki, Y.; Bruneau, C.; Dixneuf,
P. H. J . Org. Chem. 1989, 54, 1518. (c) Trost, B. M.; Dyker, G.;
Kulawiec, R. J . J . Am. Chem. Soc. 1990, 112, 7809. (d) Bianchini, C.;
Peruzzini, M.; Zanobini, F.; Frediani, P.; Albinati, A. J . Am. Chem.
Soc. 1991, 113, 5453.
(5) Antin˜olo, A.; Otero, A.; Fajardo, M.; Garc´ıa-Yebra, C.; Lo´pez-
Mardomingo, C.; Mart´ın, A.; Go´mez-Sal, P. Organometallics 1997, 16,
2601.
(6) See for example: Werner, H. Angew. Chem., Int. Ed. Engl. 1990,
29, 1077.
(8) Gibson, V. C.; Parkin, G.; Bercaw, J . E. Organometallics 1991,
10, 220.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1751
Sch em e 2
(dCdCH(R)), but such species could not be isolated or
identified.8 Alkynyl-containing cationic tantalocene com-
plexes, [Ta(η5-C5Me5)2(NHR′)(CtCR)][B(C6F5)4], have
recently been isolated from carbon-hydrogen bond
activation reactions of [Ta(η5-C5Me5)2(dNR′)(THF)]-
[B(C6F5)4] with propyne or phenylacetylene.9
containing complexes 2 were characterized by standard
spectroscopic methods. The IR spectra of these com-
plexes show a characteristic band at ca. 2000-2100
cm-1 (νCtC) which corresponds to the coordinated alky-
nyl unit and, in addition, in complexes 2a -c and 2f,g
one band at 1890-1950 cm-1 corresponding to νCtO. The
13C NMR spectra of these complexes exhibit two char-
acteristic resonances for the alkynyl CR and Câ carbons
near 125 and 103 ppm, respectively. Furthermore, in
the carbonyl-containing complexes the resonance of the
carbonyl carbon atom appears at ca. 250 ppm (see
Experimental Section).
The general procedure described for the preparation
of alkynyl-niobocene complexes did not, however, allow
the synthesis of terminal alkynyl species (R ) H), since
the preparation of the reagent Mg(CtCH)2 was not
possible. Different lithium reagents, such as HCtCLi‚
H2NCH2CH2NH2, or Grignard reagents, such as MgCl-
(CtCH) in THF, were employed as alternatives, but
their reactions with 1 under a variety of experimental
conditions led to the starting material as the only
organometallic product to be isolated from the reaction.
To overcome this problem, we sought to desilylate the
alkynyl ligand of complex 2b by reaction with Bu4NF
in THF.10 However, the complex Nb(η5-C5H5)2(Ct
CSiMe3)(CO) (2f), in which the cyclopentadienyl rings
had been desilylated, was isolated instead (Scheme 1).
A further desilylation of 2f using K2CO3 in methanol11
was successful, giving the desired terminal alkynyl-
containing complex Nb(η5-C5H5)2(CtCH)(CO) (2g)
(Scheme 1). Attempts to prepare 2g by the direct
reaction of 2b with K2CO3/CH3OH were also unsuccess-
ful, and unreacted 2b (80-90%), contaminated with as
yet unidentified products, was isolated. The alkynyl-
Oxidation P r ocesses of Alkyn yl-Con tain in g Niob-
ocen e Com p lexes. Oxidation reactions of complexes
2 have been carried out by chemical and electrochemical
methods. First, we considered the chemical oxidation
processes of complexes 2 with the ferrocenium salt
[FeCp2][BPh4] and, in the course of these studies, we
observed that the nature of the resulting products
depended dramatically on the substituent R on the
alkynyl ligand, the ancillary ligand L, and the experi-
mental conditions employed (temperature and solvent).
Thus, 2a ,c,d were reacted with the ferrocenium salt in
a 1:1 molar ratio, in CH2Cl2 at -30 °C, to give the
radical cationic alkynyl complexes [Nb(η5-C5H4SiMe3)2-
(CtCR)(L)]•+[BPh4]- (3) (Scheme 2). The species 3a ,c
were unstable, giving rise to the corresponding monovi-
nylidene complexes [Nb(η5-C5H4SiMe3)2(dCdCHR)(L)]-
[BPh4] (4) (Scheme 2), which result from a hydrogen
atom abstraction from the solvent. Compound 4a was
isolated as a 1:1 mixture of both exo and endo isomers
and 4c as a single isomer. To confirm this abstraction
process, the oxidation of 2a was carried out in dry CD2-
Cl2 at -40 °C and the deuterated complex [Nb(η5-C5H4-
(9) Blake, R. E.; Antonelli, D. M.; Henling, L. M.; Schaefer, W. P.;
Hardcastle, K. I.; Bercaw, J . E. Organometallics 1998, 17, 718.
(10) Wong, A.; Kang, P. C. W.; Tagge, C. D.; Leon, D. R. Organo-
metallics 1990, 9, 1992.
(11) This method was employed to desilylate the complex [Re(η5-
C5Me5)(-CtCCtCSiMe3)(NO)(PMe3)]: Weng, W.; Bartie, T.; Gladysz,
J . A. Angew. Chem., Int. Ed. Engl. 1994, 33, 2199.

1752 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
SiMe3)2(dCdCDR)(CO)][BPh4] was isolated (established
by a 2H NMR spectrum), indicating that in fact the
solvent is the source of the H• (or D•) radical. Compound
3d , however, was stable under these experimental
conditions and was isolated as a deep red crystalline
material after appropriate workup. Solutions of 3d in
CH2Cl2 evolve slowly at room temperature to give, after
several weeks, the monovinylidene complex 4d in low
yield. In the case of 2b, the oxidation process with the
ferrocenium salt and the subsequent transformation of
the radical cationic alkynyl species in CH2Cl2 gave, in
low yield, the unsubstituted monovinylidene complex
[Nb(η5-C5H4SiMe3)2(dCdCH2)(CO)][BPh4] (4h ), with
loss of the SiMe3 group of the alkynyl ligand. Related
mononuclear vinylidene tantalocene and niobocene com-
plexes have previously been described.8,12
prevent the transformation to the corresponding vi-
nylidene complexes 4. This process of hydrogen atom
abstraction has been established in the evolution of
several organometallic radicals.16
In addition, a ligand-ligand coupling reaction to give
divinylidene species, versus hydrogen atom abstraction
to form monovinylidene species, must be considered as
a competitive reaction path in the evolution of the
radical alkynyl species, especially in light of the results
obtained by reacting complex 2a in THF under a variety
of reaction conditions. It is tempting to speculate that,
due to the presence of the more bulky (tBu) substituent,
the radical 3c cannot undergo the ligand-ligand cou-
pling to give the corresponding divinylidene complex,
although electronic factors, such as the capacity of the
ligand to delocalize electron density, could play an
important role in the reaction of the radical species.
However, 3d was especially stable and electronic factors
(the presence of a phosphine instead of a carbonyl as
ancillary ligand) and steric factors (a more bulky phos-
phine as ancillary ligand) could be responsible for this
behavior.
Furthermore, complexes 4 could alternatively be
prepared by the protonation of complexes 2 with HBF4
(Scheme 2), although the same reaction for 2b gave an
intractable mixture of products that could not be identi-
fied. This protonation reaction is reversible, and treat-
ment of 4 with KOtBu quantitatively gives 2. The
spectroscopic data support the proposed formulations
for complexes 3 and 4. The radical cationic species 3c,d
were spectroscopically characterized by their ESR spec-
tra (see below). The vinylidene R- and â-carbons in 4
exhibit characteristic resonances at ca. 380 and 120
ppm, respectively, in their 13C NMR spectra, which are
entirely consistent with the presence of a vinylidene
unit.2b
Electr och em ica l Stu d ies. Electrochemical studies
on complexes 2a ,c,d have been carried out. The simplest
voltammetric profile is obtained for 2d (Figure 1a and
Table 1), which undergoes reversible (ip,O/ip,O′ ≈ 1),
diffusion-controlled (ip,O′/v1/2 is nearly constant for scan
rates varying between 50 and 200 mV s-1) one-electron
oxidation to give the corresponding radical cation 3d .
When the controlled-potential electrolysis of 2d was
carried out, at 0 V and -30 °C, it resulted in a coulo-
metric consumption of nearly 1 F mol-1 (nexp ) 0.9). The
voltammogram of the electrolyzed solution shows a
peak, O (Figure 1), indicating that 3d was quantitatively
generated and was stable on the CV and electrolysis
time scale. ESR spectroscopic analysis of this solution
confirmed the formation of 3d as well as its stability
(see below). The complex 3d prepared by chemical
oxidation exhibits the same cyclic voltammetric fea-
tures.
The voltammetric behavior of complexes 2a ,c is more
complicated. Thus, the cyclic voltammogram of 2c at
room temperature (see Figure 2a) displays, in addition
to the O/O′ system (ip,O/ip,O′ ≈ 0.96), a small peak, R, at
lower cathodic potential (see Table 1 for potential
values). The latter peak becomes less intense as the
sweep rate is made higher, and lowering the tempera-
ture to -30 °C results in its complete disappearance (see
However, when the oxidation of 2a was carried out
in THF solution at room temperature, an alternative
product, the divinylidene complex [(η5-C5H4SiMe3)2(CO)-
NbdCdC(P h )(P h )CdCdNb(CO)(η5-C5H 4SiMe3)2]-
[BPh4]2 (4a ′),5 was obtained as the major product
(Scheme 2), along with a small proportion of the
monovinylidene complex 4a . In THF solution at low
temperature, the divinylidene is formed as the minor
product. Surprisingly, the formation of the correspond-
ing divinylidene species from compounds 2c,d has never
been observed, even though the oxidation has been
attempted under a wide variety of reaction conditions.
The formation of the divinylidene species 4a ′ can be
envisaged as being the result of a ligand-ligand cou-
pling reaction from the radical cationic alkynyl species
3a instead of the alternative process of hydrogen atom
abstraction. The chemistry of 17-electron organometallic
radicals is well-documented, and their often character-
istic chemical properties continue to be reported.13 It is
well-known that several organometallic radicals tend
to dimerize and that dimerization through the metal
center is most often observed. Nevertheless, the ligand-
ligand coupling could be favored if the metal center is
sterically protected and if one of the ligands possesses
a π-system to enable the delocalization of the spin
density. Thus, Lapinte and co-workers have demon-
strated that the iron-ethynyl complex [Fe(η5-C5Me5)2-
(CtCH)(dppe)] reacts with [FeCp2][PF6]to give a divi-
nylidene complex through a ligand-ligand coupling
step.14 However, they observed that the analogous
substituted alkynyl complexes [Fe(η5-C5Me5)2(CtCR)-
t
(dppe)] (R ) Ph, Bu) did not undergo dimerization or
hydrogen atom abstraction, and the 17-electron radical
cationic alkynyl complexes [Fe(η5-C5Me5)2(CtCR)-
(dppe)]•+[PF6]- could be isolated as air-stable solids. The
low reactivity of these complexes was explained on the
basis of the steric hindrance of the substituent attached
to the alkynyl carbon atom.15 In our case, the 17-electron
radical species 3a ,c were unstable, and this could be
due to the fact that the substituents on the alkynyl
group did not provide sufficient steric protection to
(12) (a) Van Asselt, A.; Burger, B. J .; Gibson, V. C.; Bercaw, J . E. J .
Am. Chem. Soc. 1986, 108, 5347. (b) Fermin, M. C.; Bruno, J . W. J .
Am. Chem. Soc. 1993, 115, 7511.
(13) Kuksis, I.; Baird, M. C. J . Organomet. Chem. 1997, 527, 137
and references therein.
(14) Le Narvor, N.; Toupet, L.; Lapinte, C. J . Am. Chem. Soc. 1995,
117, 7129.
(15) Connelly, N. G.; Gamasa, M. P.; Gimeno, J .; Lapinte, C.; Lastra,
E.; Maher, J . P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J . Chem.
Soc., Dalton Trans. 1993, 2575.
(16) See, for example: Iyer, R. S.; Selegue, J . P. J . Am. Chem. Soc.
1987, 109, 910.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1753
divinylidene species, or solvent-based hydrogen transfer,
giving rise to monovinylidene compounds. To determine
which mechanism operates in the course of the electro-
chemical oxidation, kinetic information has been ex-
tracted from the change in the voltammetric profile with
the concentration of 2a . Figure 3a displays the cyclic
voltammogram of 2a in THF at room temperature at c
) 0.8 mM, and the only reduction peak O is observed
on the reverse sweep. In contrast, when the concentra-
tion is raised to a much higher value (c ) 3.7 mM), then
peak R appears (Figure 3b). This result is in accordance
with the dimerization pathway (a bimolecular reaction
following second-order kinetics).18a Concentration has
no effect on the relative peak intensities (R versus O)
in an EC-type mechanism if the associated chemical
reaction kinetic is first order,18b as is the case in the
pathway involving hydrogen atom abstraction. As de-
finitive proof, we measured the cyclic voltammogram of
a chemically prepared sample of the divinylidene 4a ′,
which exhibits the expected peak R. However, we also
confirmed that the cyclic voltammogram of a chemically
prepared sample of 4c exhibits the peak R, indicating,
as was previously mentioned for the chemical oxidation
of 2c, that the cation radical alkynyl species 3c evolves
exclusively to give 4c by hydrogen atom abstraction
from the solvent.
F igu r e 1. Cyclic voltammograms of 2d (3.5 mM) in THF/
0.2 M NBu4ClO4 on a vitreous carbon disk electrode: (a)
before electrolysis at room temperature; (b) after electroly-
sis at 0 V at -30 °C. Scan rate: 200 mV s-1. Initial
potential: -1.6 V (a) and 0 V (b).
Complexes 3c,d were characterized by ESR spectros-
copy. Isotropic g and splitting constant values are listed
in Table 2. The spectrum of 3c exhibits a characteristic
10-line shape arising from coupling between the un-
paired spin and the niobium nucleus (100% natural
abundance, I ) 9/2). In the case of 3d , the spectrum (see
Figure 4) consists of a decet of doublets due to additional
superhyperfine coupling with phosphorus (aP ) 22.07
G). This last value compares quite well with that found
for the analogous chloro complex [Nb(η5-C5H4SiMe3)2-
(Cl)(P(OMe)3)]+ (aP ) 21.85 G).19 The outer lines are
poorly resolved relative to the inner lines due to a
considerable increase in line width at the ends of the
spectrum, a trend that has been well-established for this
type of radical.20 More informative are the values of the
isotropic hyperfine splitting constant, which are nearly
identical for both complexes (3c, aNb ) 67.88 G; 3d , aNb
) 70.97 G). This similarity reflects the degree of metal
character of the singly occupied molecular orbital.
Typical niobocene derivatives NbCp2X2 (X ) alkyl,
halogeno) exhibit values that lie in the range 80-120
G,21 whereas complexes with σ-donor-π-acceptor ligands
in the equatorial plane NbCp2(L) (L ) ketenimine,22
ketene,23 acetylene,24 aldehyde25) exhibit smaller aNb
values (8-20 G) due to appreciable delocalization of the
Ta ble 1. Cyclic Volta m m etr ic Da ta a for th e
Com p lexes Nb(η5-C5H4SiMe3)2(L)(CtCR)
b
compd
L
R
E1/2,O′ (V/ECS)
Ep,R (V/ECS)
2a
2c
2d
CO
CO
PMe2Ph
Ph
tBu
Ph
+0.27
+0.20
-0.43
-0.91
-1.04
a
In THF/0.2 M NBu4PF6 on vitreous carbon disk electrode at a
b
scan speed of 200 mV s-1
.
Taken as the half-sum of Ep,O′ and
17
Ep,O
.
Figure 2b), as ip,O/ip,O′ becomes nearly equal to 1. These
changes are strongly indicative of an EC-type mecha-
nism where the corresponding electrogenerated cation
radical alkynyl species 3c undergoes a chemical evolu-
tion.
With the aim of stabilizing 3c, an exhaustive elec-
trolysis was performed at -30 °C in THF with NBu4-
ClO4 as the supporting electrolyte. Complex 3c con-
sumes nearly 1 equiv of electron (working electrode
potential, +0.4 V vs ECS; nexp ) 0.95), and the cyclic
voltammogram of the electrolyzed solution (still regis-
tered at low temperature) does show peak O to some
extent but mainly consists of peak R (see Figure 2c)
together with small ill-defined peaks that appear in the
range 0-1 V. The ESR spectrum of this electrolyzed
solution indicates the presence of 3c (see below). When
the temperature is raised to room temperature, peak O
disappears (see Figure 2d) and, at the same time, the
solution becomes ESR silent. An electrochemical study
of complex 2a gives similar results: oxidation leads to
a diamagnetic complex through the transient cation
radical alkynyl 3a . This last product can be reduced
near -1 V (see Table 1). However, a question remains:
what is the nature of the final diamagnetic products
that are reduced at peak R? The chemical oxidation
experiments have proven that, starting from the radical
cationic alkynyl complexes 3, there are two reaction
pathways: either ligand-ligand coupling, to give the
(17) Noel, M.; Vasu, K. I. Cyclic Voltammetry and the Frontiers of
Electrochemistry; Aspect Publications: London, 1990.
(18) (a) Olmstead, M. L.; Hamilton, R. G.; Nicholson, R. S. Anal.
Chem. 1969, 41, 260. (b) Nicholson, R. S.; Shain, I. Anal. Chem. 1964,
36, 706.
(19) Lucas, D.; Mugnier, Y.; Antin˜olo, A.; Otero, A.; Fajardo, M. J .
Organomet. Chem. 1992, 435, C3-C7.
(20) Savaranamuthu, A.; Bruce, A. E.; Bruce, M. R. M.; Fermin, M.
C.; Hneihen, A. S.; Bruno, J . W. Organometallics 1992, 11, 2190 and
references therein.
(21) (a) Manzer, L. E. Inorg. Chem. 1977, 16, 525. (b) Broussier, R.;
Normand, H.; Gautheron, B. J . Organomet. Chem. 1978, 155, 337. (c)
Al-Mowali, A.; Kuder, W. A. A. J . Organomet. Chem. 1978, 155, 337.
(d) Hitchcock, P. B.; Lappert, M. F.; Milne, C. R. C. J . Chem. Soc.,
Dalton Trans. 1981, 180. (e) Bottomley, F.; Keizer, P. N.; White, P. S.;
Preston, K. F. Organometallics 1990, 9, 1916. (f) Brunner, H.; Gehart,
G.; Meier, W.; Wachter, J .; Riedel, A.; Elkrami, S.; Mugnier, Y.; Nuber,
B. Organometallics 1994, 13, 134.

1754 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
F igu r e 2. Cyclic voltammograms of 2c.
F igu r e 3. Cyclic voltammograms of 2a .
Ta ble 2. ESR Da ta a for th e Com p lexes
localization of the unpaired spin density on the alkynyl
ligand.
[Nb(η5-C5H4SiMe3)2(L)(CtCR)]+
Isom er iza tion of η1-Vin ylid en e- to η2-Alk yn e-
Niobocen e Com p lexes. Although stable in the solid
state, the aforementioned vinylidene complexes 4a ,c
undergo a rearrangement in THF or acetonitrile solu-
compd
g
aNb (G)
aP (G)
3c
3d
1.9979
1.9966
67.88
70.97
22.07
a
Hyperfine coupling constants and isotropic g factors are all
corrected to second order using the Breit-Rabi equation.
(23) (a) Halfon, S. E.; Fermin, M. C.; Bruno, J . W. J . Am. Chem.
Soc. 1989, 111, 8738. (b) Antin˜olo, A.; Fajardo, M.; Lopez-Mardomingo,
C.; Otero, A.; Lucas, D.; Mugnier, Y.; Lanfranchi, M.; Pellinghelli, M.
A. J . Organomet. Chem. 1992, 435, 55.
(24) Antin˜olo, A.; Fajardo, M.; Lopez-Mardomingo, C.; Otero, A.;
Lucas, D.; Mugnier, Y.; Galakhov, M.; Gil-Sanz, R.; Chollet, H. J .
Organomet. Chem. 1994, 481, 27.
unpaired electron onto the organic ligand, as has been
demonstrated by theoretical calculations.26 The aNb
values found for 3c,d are intermediate between the two
sets of aNb values described above and denote a partial
(25) Antin˜olo, A.; Fajardo, M.; Otero, A.; Lucas, D.; Mugnier, Y.;
Chollet, H. J . Organomet. Chem. 1992, 426, C4.
(26) Antin˜olo, A.; Fajardo, M.; Otero, A.; De J esus, E.; Mugnier, Y.
J . Organomet. Chem. 1994, 470, 127.
(22) Antin˜olo, A.; Fajardo, M.; Lopez-Mardomingo, C.; Otero, A.;
Mourad, Y.; Mugnier, Y.; Sanz-Aparicio, J .; Fonseca, I.; Florencio, F.
Organometallics 1990, 9, 2919.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1755
F igu r e 4. ESR spectrum (X band) of 3d (solvent: THF,
room temperature).
F igu r e 5. ORTEP drawing of complex 5a .
Ta ble 3. Selected Bon d Len gth s a n d An gles for
Com p lex 5a
recover quantitatively the corresponding η1-alkynyl
complexes 2a ,c, respectively (Scheme 2). Although η1-
vinylidene-η2-alkyne isomerizations are rare, a few
examples have recently been described.27 In an elegant
and exhaustive work, Bly and co-workers27c have ex-
plained the isomerization of complexes [Cp(CO)2FedCd
CR1R2]+OTf- to the corresponding η2-alkyne derivatives
[Cp(CO)2Fe(η2-R1CtCR2)]+OTf -, through a 1,2-shift of
an alkyl group from the â-carbon to the R-carbon,
favored by both the extreme electrophilicity of the
vinylidene R-carbon and the high π-acidity of the
auxiliary carbonyl ligand, which gives rise to an electron-
deficient metal center. In the same way, Connelly and
co-workers27a,d,g have extensively studied the η1-vi-
nylidene-η2-alkyne isomerization induced by one-
electron oxidation of the 18-electron complexes [(η6-
C6R6)(CO)2CrdCdCR1R2]+. The facile isomerization
process observed for complexes 4a ,c could be explained
on the basis of analogous arguments. Thus, the presence
of a strong π-acid carbonyl ligand in the cationic d2 Nb-
(III) species 4a ,c would explain the higher electrophi-
licity of the vinylidene R-carbon (13C NMR spectroscopic
data indicate that this carbon atom in 4a is more
deshielded than that in 4d , ca. 378 ppm vs 368 ppm,
respectively) in these complexes in comparison with the
behavior of the R-carbon in complex 4d , and conse-
quently, the aforementioned η1-vinylidene-η2-alkyne
rearrangement would be especially favored. This aspect
will be analyzed in the next section by means of
theoretical calculations.
Distances (Å)
Nb(1)-C(5)
Nb(1)-C(7)
Nb(1)-C(6)
Nb(1)-C(101)
2.094(7)
2.222(5)
2.278(6)
2.0856(7
Nb(1)-C(102)
O(4)-C(5)
C(6)-C(7)
C(7)-C(8)
2.082(6)
1.120(7)
1.219(7)
1.467(7)
Angles (deg)
C(5)-Nb(1)-C(7)
C(5)-Nb(1)-C(6)
C(7)-Nb(1)-C(6)
102.6(2)
71.3(2)
31.4(2)
C(6)-C(7)-C(8)
O(4)-C(5)-Nb(1)
Nb(1)-C(7)-C(8)
146.2(6)
178.7(6)
137.0(4)
tions at room temperature to give, quantitatively, the
more stable η2-alkyne complexes [Nb(η5-C5H4SiMe3)2-
(η2(C,C)-HCtCR)(CO)]+ (5) (Scheme 2). Thus, the mix-
ture of endo and exo isomers of 4a gave the η2-alkyne-
containing complex 5a , isolated as the pure exo isomer.
Similar treatment of 4c led to a 1:1 mixture of isomers
of 5c. In contrast with this behavior, solutions of 4d
were stable even when a CD3CN solution in a sealed
NMR tube was heated to 180 °C. Particularly diagnostic
of the coordinated alkyne unit in complexes 5 is the
presence of a characteristic νCtC band in the IR spectra
(ca. 1772 cm-1) and resonances for the nonequivalent
carbon atoms in the 13C NMR spectra (ca. 111 and 132
ppm), which are in agreement with the previously
reported data for alkyne-containing cationic niobocene
complexes.5 The crystal structure of 5a consists of an
-
organometallic cation and a BF4 anion without any
particular cation-anion interaction. Selected bond
lengths and angles are depicted in Table 3. The molec-
ular structure of the cation represents a wedgelike
sandwich with an angle of 45.7° between the near-
planar cyclopentadienyl rings. The relative orientation
of the Cp′ rings is intermediate between eclipsed and
staggered, as indicated by the Si-C101-C102-Si3
angle of 88° (C(101) and C(102) are the centroids of the
Cp′ rings).
Figure 5 clearly indicates the exo geometry for alkyne
coordination with respect to the carbonyl ligand; the
phenyl group of the alkyne is located in the exo position
to release the steric interaction Nb-CO. The Nb(1)-
C(6) distance (2.278(6) Å) is slightly longer than the Nb-
(1)-C(7) length (2.222(5) Å). The C(6), C(7), and C(5)
atoms are coplanar with the Nb atom, a situation that
is in accord with general structural features of penta-
coordinated niobocene derivatives. Finally, treatment
of 5a ,c with KOtBu in THF solution allowed us to
Th eor etica l Stu d y. Rela tive Sta bility of th e Vi-
n ylid en e a n d Acetylen e Isom er s. The first point to
be addressed in the theoretical study was the relative
stabilities of the η1-vinylidene and η2-alkyne complexes.
The geometries of the exo (x) and endo (n ) isomers of
the model complexes [NbCp2(dCdCH(CH3))(L)]+ (L )
2
CO, 4CO; L ) PH3, 4PH ) and [NbCp2(η (C,C)-HCt
3
(27) (a) Connelly, N. G.; Orpen, G.; Rieger, A. L.; Rieger, P. H.; Scott,
C. J .; Rosair, G. M. J . Chem. Soc., Chem. Commun. 1992, 1293. (b)
Bly, R. S.; Raja, M.; Bly, R. K. Organometallics 1992, 11, 1220. (c)
Bly, R. S.; Zhong, Z.; Kane, C.; Bly, R. K. Organometallics 1994, 13,
889. (d) Connelly, N. G.; Geiger, W. E.; Lagunas, M. C.; Metz, R.;
Rieger, A. L.; Rieger, P. H.; Shaw, M. J . J . Am. Chem. Soc. 1995, 117,
12202. (e) Nombel, P.; Lugan, N.; Mathieu, R. J . Organomet. Chem.
1995, 503, C22. (f) Gamasa, M. P.; Gimeno, J .; Gonza´lez-Bernardo,
C.; Borge, J .; Garc´ıa-Granda, S. Organometallics 1997, 16, 2483. (g)
Bartlett, I. M.; Connelly, N. G.; Martin, A. J .; Orpen, A. G.; Paget, T.
J .; Rieger, A. L. J . Chem. Soc., Dalton Trans. 1999, 691.

1756 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
destabilization (1.7 kcal/mol) of the endo form of 5PH is
3
found, showing that the alkyne isomers are more
influenced by steric effects than the corresponding
vinylidene analogues. The alkyne substituent prefer-
entially occupies the outside site. It can be expected that
energy differences found in model systems increase in
real systems, due to the steric effects caused by the
presence of bulkier substituents in alkyne and phos-
phine ligands. In the following discussion we will
compare the energies of the exo isomers.
The most striking result of the thermodynamic study
is the very similar energies of the vinylidene- and
alkyne-containing niobocenes. 4xCO, 4xPH , and 4xCl are
3
calculated to be only 0.8, 1.3, and 1.4 kcal/mol more
stable than their respective η2-alkyne-containing niob-
ocene isomers. This is in marked contrast with the
systems previously considered in theoretical studies.28
The effect of ligand substitution on the relative stabili-
ties seems minor, although it always appears that
replacement of a strong π-acid CO ligand by a less π-acid
PH3 ligand favors the vinylidene form. The energetic
results suggest that both the vinylidene- and alkyne-
niobocene complexes could be prepared experimentally.
The energy differences between the two forms that were
found are within the limits of accuracy of the modeling
and the methodology employed, but a definitive answer
with regard to which is the most stable species cannot
be established. We described in the preceding section
that 4a ,c isomerize to the corresponding η2-alkyne-
containing species 5a ,c. However, when CO is replaced
by a phosphine ligand in 4d , the analogous isomeriza-
tion process does not take place. This fact has been
related to the higher electrophilicity of the vinylidene
R-carbon (CR) in cationic complexes with CO ligands.27c
Computed NBO charges for the R-carbon are -0.12e in
4xCl, -0.08e in 4xPH , and -0.03 in 4xCO. In agreement
3
with the 13C NMR data, the replacement of CO by PH3
causes an increase in the electron density at the
R-carbon. In line with previous arguments,27c the ther-
modynamics of the η1-vinylidene-η2-alkyne isomeriza-
tion in niobocene complexes follows the same trend as
the vinylidene CR charges; therefore, the relative stabil-
ity of η1-vinylidene decreases as the electron density at
CR diminishes. However, small differences in the rela-
tive energies found do not seem to justify the opposite
experimental behavior exhibited by 4a ,c and 4d . To
account for this phenomenon, we will focus in the next
section on the kinetics of the isomerization.
F igu r e 6. Optimized B3LYP structures (Å, deg) of reac-
tants and products for isomerization of [NbCp2(dCd
CHCH3)(L)]+ to [NbCp2(HCtCCH3)(L)]+. Hydrogen atoms
of Cp rings are omitted for simplicity.
Mech an ism of th e η1-Vin yliden e-η2-Alkyn e Isom -
er iza tion . Two intramolecular mechanisms are well-
documented in the literature for the η2-alkyne f η1-
vinylidene rearrangement in the coordination sphere of
transition-metal complexes: the 1,2-hydrogen shift and
the 1,3-hydrogen shift.2 We considered these two in-
tramolecular mechanisms for the isomerization of the
η1-vinylidene complexes [NbCp2(η1-CCHCH3)(L)]+ (L )
CO, PH3) to the corresponding η2-alkyne derivatives.
Starting from the vinylidene species, in the first mech-
anism a 1,2-hydrogen shift from the â- to R-carbon takes
place, followed by a slippage process of the alkyne ligand
from a σ η2(C,H) coordination to a π η2(C,C) coordina-
CCH3)(L)]+ (L ) CO, 5CO; L ) PH3, 5PH ) were fully
3
optimized under the constraints of the Cs symmetry
group (Figure 6). For comparative purposes we also
considered the exo η1-vinylidene and η2-alkyne niobo-
cenes with L ) Cl (4xCl and 5xCl, respectively). The
calculated Nb-Cl, CtC, and Nb-C distances in 5xCl
(2.565, 1.288, and 2.135 and 2.178 Å, respectively) agree
well with the distances determined by X-ray diffraction
in Nb(η5-C5H4SiMe3)2(η2(C,C)-PhCtCPh)Cl (2.538(2),
1.27(1), and 2.185(9) and 2.171(8) Å, respectively).4a
In all cases the exo isomer appears as the most stable
one, but the energy difference between the exo and endo
isomers of all complexes is very small. The calculated
energy differences for the vinylidene and alkyne com-
plexes with L ) CO and the vinylidene complex with L
) PH3 are less than 0.1 kcal/mol. A more marked
(28) (a) Wakatsuki, Y.; Koga, N.; Yamazaki, H.; Morokuma, K. J .
Am. Chem. Soc. 1994, 116, 8105. (b) Wakatsuki, Y.; Koga, N.; Werener,
H.; Morokuma, K. J . Am. Chem. Soc. 1997, 119, 360. (c) Stegmann,
R.; Frenking, G. Organometallics 1998, 17, 2089.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1757
tion. Early qualitative theoretical studies29 supported
this mechanism. MP2 calculations have proved that the
alkyne-vinylidene rearrangement in the RuX2(PR3)2 +
HCtCHR′ systems takes place with this mechanism.28a
The second mechanism consists of the 1,3-hydrogen shift
from the â-carbon to the metal, giving rise to an
(alkynyl)hydridometal intermediate that undergoes a
reductive elimination to form the alkyne-containing
species. There is also experimental evidence to suggest
that isomerization can proceed via an (alkynyl)hydri-
dometal intermediate.30-33 Recent theoretical work has
shown that the η2-alkyne f η1-vinylidene rearrange-
ment in [RhCl(PH3)2(HCCH)] takes place by an inter-
molecular process via the hydrido intermediate [RhHCl-
(PH3)2(CCH)].28b Our study is concerned with the
intramolecular mechanisms, because steric repulsions
caused by the Cp-substituted rings may work against
an intermolecular mechanism in the niobocene systems
considered here.
We have explored the reaction path for both exo and
endo isomers. Possible intermediates (Figure 7) and
transition states (Figure 8) associated with each mech-
anism were located and the activation barriers deter-
mined (Figures 9 and 10).
(i) Rea ction In ter m ed ia tes. For the isomerization
of 4CO and 4PH two kinds of intermediates have been
3
found in the potential energy surfaces: hydrido alkynyl
species A, which are related to the 1,3-hydrogen shift
mechanism, and η2(C,H) alkyne complexes B, which are
involved in the 1,2-hydrogen shift mechanism.
For the niobocene system with a carbonyl ligand we
located two hydrido alkynyl minima (AxCO and An CO
;
see Figure 7). In AxCO the hydrido is cis to the carbonyl
ligand, whereas in AnCO the alkynyl is cis to the
carbonyl ligand. Both hydrido alkynyl complexes are
very destabilized with respect to the reactant. AxCO and
An CO are found 38.3 and 32.3 kcal/mol above the
vinylidene complexes 4xCO and 4n CO, respectively. Two
similar minima have been found in the niobocene
system with the phosphine ligand (AxPH and An PH ).
3
3
The cis hydrido phosphine AxPH and the cis alkynyl
3
phosphine An PH isomers lie 38.2 and 25.6 kcal/mol
3
PH3
above 4xPH and 4n , respectively. The highest stabil-
3
ity of the hydrido alkynyl complex with the hydrido in
a lateral site could be related to a stronger Nb-H bond
for this disposition of the ligands. The Nb-H distance
is notably shorter in the An isomers than in the Ax
ones.
The four η2(C,H)-alkyne-containing reaction interme-
diates [NbCp2(L)(η2(C,H)-(HCCCH3)]+ (L ) CO, exo
F igu r e 7. Optimized B3LYP structures (Å, deg) of inter-
mediates for isomerization of [NbCp2(dCdCHCH3)(L)]+ to
[NbCp2(HCtCCH3)(L)]+. Hydrogen atoms of Cp rings are
omitted for simplicity.
(BxCO) and endo (Bn CO); L ) PH3, exo (BxPH ) and endo
3
(Bn PH ); see Figure 7) were located in the potential
3
(29) Silvestre, J .; Hoffmann, R. Helv. Chim. Acta 1985, 65, 1461.
(30) (a) Nesmeyanov, A. N.; Aleksandrov, G. G.; Antonova, A. B.;
Anisimov, K. N.; Kolobova, N. E.; Struchkov, Yu.-T. J . Organomet.
Chem. 1976, 110, C36. (b) Antonova, A. B.; Kolobova, N. E.; Petrovsky,
P. V.; Lokshin, B. V.; Obeyzuk, N. S. J . Organomet. Chem. 1977, 137,
55.
(31) (a) Wolf, J .; Werner, H.; Serhadli, O.; Ziegler, M. L. Angew.
Chem., Int. Ed. Engl. 1983, 22, 414. (b) Garcia Alonso, F. J .; Hoehn,
A.; Wolf, J .; Otto, H.; Werener, H. Angew. Chem., Int. Ed. Engl. 1985,
24, 406. (c) Werner, H.; Garcia Alonso, F. J .; Otto, H.; Wolf, J . Z.
Naturforsch. 1988, 43B, 722. (d) Dziallas, M.; Werener, H. J . Chem.
Soc., Chem. Commun. 1987, 852.
energy surfaces. They are situated between 20 and 30
kcal/mol above their corresponding vinylidene forms.
The lengthening of the C-H distances with respect to
their values in the η2(C,C)-alkyne complexes, as well
as the distortion of the C-C-H angle to values between
132.5 and 138.2°, are in accordance with the η2(C,H)
nature of these species and indicate that an interaction
of the C-H bond with the metal center contributes to
the stabilization of the σ-alkyne intermediates. σ η2-
(C,H)-alkyne complexes similar to those presented here
have also been found in theoretical studies of the 1,2-
(32) Bianchini, C.; Peruzzini, M.; Vacca, A.; Zanobini, F. Organo-
metallics 1991, 10, 3697.
(33) De los Rios, I.; J ime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P.
J . Am. Chem. Soc. 1997, 119, 6529.

1758 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
F igu r e 8. Optimized B3LYP structures (Å, deg) of transition states for isomerization of [NbCp2(dCdCHCH3)(L)]+ to
[NbCp2(HCtCCH3)(L)]+. Hydrogen atoms of Cp rings are omitted for simplicity.
mechanism for the alkyne f vinylidene rearrange-
ment.28,34 The existence of the metal-CH interaction
was confirmed by a Bader analysis35 of the electron
C2H2 moiety was found. Recently, Stegmann and Fren-
king have characterized a nonplanar transition state
structure for the 1,2-rearrangement on the coordination
sphere of the F4W metallic fragment.28c The nonplanar
structure was explained in terms of the highly anionic
character of the C2H2 fragment in the tungsten com-
plex.28c This is not the case in the niobocene systems
under consideration; therefore, we restricted our study
to the in-plane migration of H.
density of Bn PH . Two bond critical points, one between
3
Nb and H and the other between Nb and C and a ring
critical point between Nb, C, and H, were found.
In the niobocene carbonyl system the exo and endo
η2(C,H)-alkyne-containing intermediates display similar
energies. BxCO and Bn CO are found 21.1 and 22.2 kcal/
mol above the vinylidene-containing compounds 4xCO
and 4n CO, respectively. Both are manifestly more stable
Energies and geometries of the transition structures
TS1xCO and TS1n CO are very similar. TS1xCO is
reached from 4xCO with an energy barrier of 30.0 kcal/
mol, and TS1n CO lies 30.5 kcal/mol above 4n CO. These
transition states have a product-like nature, with the
migrating H almost transferred to CR. The topological
analysis of the electron density confirms this fact. A
bond critical point was found between the R-carbon and
the hydrogen atom, whereas such a bond critical point
does not appear between the â-carbon and the hydrogen
atom.
than the hydrido alkynyl complexes AxCO and An CO
,
respectively. In the niobocene phosphine systems, the
intermediates BxPH and Bn PH are situated 28.5 and
3
3
23.9 kcal/mol above the corresponding vinylidene com-
PH3
plexes 4xPH and 4n , respectively. When a phosphine
3
is present as an ancillary ligand, the endo η2(C,H)-
alkyne Bn PH and endo hydrido alkynyl An
interme-
PH3
3
diates become very close in energy. Bn PH is found only
3
1.7 kcal/mol below the hydrido alkynyl An PH
.
3
(ii) Rea ction P a th for th e η1-Vin ylid en e f η2-
Alk yn e Isom er iza tion . During the course of this study
we looked for the transition states that connect the
different minima. For the niobocene system with the
carbonyl ligand, a transition state for the 4CO f BCO
transformation was found for each isomer (exo and
The next step in the 1,2-mechanism might be the
slippage of the η2(C,H)-alkyne-containing intermediate
to form the more stable π-alkyne-containing product
(BCO f 5CO). Indeed, we located a transition state for
this process (TS2CO; Figure 8). Because of the very small
differences found between the exo and endo isomers in
the niobocene carbonyl compounds, we only considered
the alkyne reorganization for the slightly more stable
exo isomer. The transition state TS2xCO is found only
0.7 kcal/mol above the intermediate. It is worth noting
that in Figure 8 the geometry of TS2xCO shows a less
distorted alkyne fragment than in BxCO. The Nb-CR
and particularly the Nb-H distances are lengthened on
going from B to TS2. A bond critical point between Nb
and H was not found in TS2xCO, but such a point was
located for the Nb-CR bond. Thus, this transition state
might be described as containing a σ η1-alkyne struc-
endo). These transition structures (TS1xCO and TS1n CO
)
correspond to the 1,2-hydrogen shift from the vinylidene
Câ to the CR. Their geometries (Figure 8) are similar to
those theoretically determined for the 1,2-mechanism
in the rearrangement η2(C,H)-alkyne f vinylidene in
Ru(II)28a and Os(II)34 metallic systems. In both systems
a transition state with a planar arrangement of the
(34) Olivan, M.; Clot, E.; Eisenstein, O.; Caulton, K. G. Organome-
tallics 1998, 17, 3091.
(35) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: New York, 1990.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1759
similar to those found for the carbonyl system, there is
a significant difference between the carbonyl- and
PH3
phosphine-containing niobocenes: TS1xPH and TS1n
3
lie 40.0 and 38.5 kcal/mol above the vinylidene struc-
tures, whereas TS1CO species are reached with an
energy barrier of 30 kcal/mol. The replacement of the
carbonyl ligand by a less π-acid ligand appreciably
increases the energy barrier of the 1,2-migration. We
have already discussed the fact that when the CO ligand
is replaced by PH3, the electron density at the vinylidene
CR atom increases due to the enhancement of the metal
back-donation. During the migration process this in-
teraction must vanish to form the new C-H bond. Thus,
the strongest interaction determines that the hydrogen
migration would be more unfavorable. We did not
determine the transition state for the next step of the
1,2-mechanism: the slippage process of the alkyne
F igu r e 9. Potential energy profile (kcal mol-1) for isomer-
ization of exo-[NbCp2(dCdCHCH3)(CO)]+ to exo-[NbCp2-
(HCtCCH3)(CO)]+. Values in parentheses are calculated
energies for endo isomers.
ligand (BPH f5PH ). It is expected that this last step
will take place with a low energy barrier, as found for
the carbonyl system, and it should not be the rate-
determining step in this mechanism.
3
3
In the potential energy surface of the phosphine-
containing niobocene system the endo hydrido alkynyl
intermediate An PH was found to be energetically very
3
close to the η2(C,H)-alkyne-containing intermediate and
more than 10 kcal/mol below the transition state
TS1PH . Thus, in this case the 1,3-mechanism could be
3
operative. The search for a transition state that defines
the 1,3-hydrogen migration led to the structure TS3n PH
3
(Figure 8), which is found 52.2 kcal/mol above the
vinylidene species. This value clearly indicates that the
PH3
4PH fA
process is kinetically unfavorable with re-
3
spect to the 1,2-migration. The TS3n PH structure was
unequivocally assigned as a transition state by a nu-
merical calculation of its Hessian matrix.
3
The geometry of the transition state TS3n PH is
F igu r e 10. Potential energy profile (kcal mol-1) for
isomerization of endo-[NbCp2(dCdCHCH3)(PH3)]+ to endo-
[NbCp2(HCtCCH3)(PH3)]+. Values in parentheses are
calculated energies for exo isomers.
3
different from those of the previously reported transition
states for the 1,3-hydrogen shift.28b,c The main difference
is found in the metal-CR-Câ angle (77.4° in TS3n PH
,
3
162.3° in RhCl(PH3)2(CdCH2)28b). In the Rh(I) transition
state the migrating H interacts simultaneously with Rh,
CR, and Câ. A close look at the geometric parameters in
Figure 8 and the topological analysis of the charge
ture. The very low energy barrier calculated for the
process η2(C,H)-alkyne f η2(C,C)-alkyne shows that the
B species are not very stable minima and suggests that
the existence or nonexistence of the σ-coordinated η2-
(C,H)-alkyne as a reaction intermediate in the 1,2-
mechanism will depend on the particular nature of the
system. A slight destabilization of these intermediates
causes the 1,2-mechanism to pass from a two-step
process to a concerted reaction.
density shows that in TS3n PH the hydrogen interacts
3
only with the Câ atom but the metal interacts simulta-
neously with both CR and Câ atoms. If we investigate
PH3
the reverse process, i.e., from APH to 4 , it could very
3
well be considered as an insertion of the alkynyl ligand
into the Nb-H bond rather than a 1,3-hydrogen shift.
It is interesting to note that a CR-P interaction is
The calculated potential energy profile for the η1-
vinylidene f η2-alkyne rearrangement in the niobocene
carbonyl system is presented in Figure 9. The energies
of the hydrido alkynyl intermediates are above those of
the transition structures TS1CO; thus, the 1,3-mecha-
nism can be discarded for the 4CO f 5CO isomerization
process. For this reason, we did not explore the reaction
path for the 1,3-hydrogen migration.
developing in TS3n PH , characterized by a bond critical
3
point in the charge density. This interaction is probably
involved in the stabilization of the transition state
structure.
The very close energies of the hydrido alkynyl An PH
3
and η2(C,H)-alkyne-containing Bn PH intermediates
3
prompted us to study their interconversion. A transition
state for this process, TS4n PH (Figure 8), was located
3
The energetic profile obtained for the phosphine-
containing niobocene system is depicted in Figure 10.
With regard to the 1,2-mechanism, the two transition
only 1.4 kcal/mol above An PH . The topological analysis
3
of the electron density shows that in this transition state
the hydrogen atom is interacting simultaneously with
the metal atom and the R-carbon. The very low barrier
found for the oxidative addition (and its reverse reduc-
tive elimination) of the C-H bond in this system
states (TS1xPH and TS1n PH ; Figure 8) for the forma-
tion of η2(C,H)-alkyne intermediates from the corre-
3
3
sponding vinylidene complexes (4PH f BPH ) were
3
3
located. Although the geometries of both are very

1760 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
F igu r e 11. Partially optimized B3LYP structures (Å, deg) for rotation of vinylidene and alkyne ligands in [NbCp2(dCd
CHCH3)(CO)]+ and [NbCp2(HCtCCH3)(CO)]+, respectively.
suggests that hydrido alkynyl and η2(C,H)-alkyne-
containing species coexist and interconvert easily.
Our theoretical study has shown that in both the
niobocene-carbonyl and the niobocene-phosphine sys-
tems the vinylidene f alkyne isomerization takes place
by the 1,2-mechanism. The different experimental be-
haviors of 4a ,c and 4d should be understood on the basis
of kinetic effects rather than thermodynamics, as we
suggested in the previous section. The difference of
almost 10 kcal/mol found in the activation barrier of the
vinylidene f η2(C,H)-alkyne step justifies the experi-
mental findings. Moreover, the calculated energy barrier
of the rate-determining step (30 kcal/mol) is in ac-
cordance with the experimental conditions under which
the 4a ,c f 5a ,c isomerization occurs (1-2 days, room
temperature).
exo f en d o In ter con ver sion . Up to now we have
considered the isomerization reaction of the exo (endo)
vinylidene to the exo (endo) alkyne species. It remains
to be clarified as to how the exo and endo isomers are
able to interconvert. With this aim in mind, we per-
formed an estimation of the rotational barrier of the
vinylidene and alkyne carbonyl species. The structures
of the two complexes with the vinylidene and alkyne
ligands rotated 90° from the orientation they adopt in
the minima were partially optimized, with the ligands
forced to remain in the rotated orientation (4r otCO and
5r otCO, respectively; Figure 11).
The vinylidene-rotated structure is found 27.9 kcal/
mol above the minimum 4xCO. This value gives an
estimation of the rotational barrier. The high energetic
cost of the rotation does not seem to be related to steric
effects, since the methyl substituent is relatively far
from the cyclopentadienyl rings. Thus, electronic factors
should be responsible for this high rotation barrier.
Vinylidene holds two nonequivalent π-acceptor orbit-
als: an empty π orbital in the CCH plane and a π*
orbital, which is higher in energy, in the perpendicular
plane.36 The only occupied d orbital of the d2 [NbCp2-
(CO)]+ metallic fragment lies in the plane that contains
the vinylidene ligand and is perpendicular to the axis
that joints the two Cp rings.37 This orbital back-donates
electron density to the π orbital in the minima 4CO and
to π* in the rotated structure 4r otCO. The π back-
donation diminishes with the rotation, as indicated by
the evolution of the geometrical parameters (Figures 6
and 11). The same effect causes the high rotational
barrier found for the dihydrogen ligand in dihydrogen
niobocene complexes.38
To estimate the rotational barrier for the alkyne-
containing complex, we started the optimization with
an η2(C,C)-alkyne structure rotated by 90°. However,
the geometry optimization process ended in the 5r otCO
structure. It is worth noting that the coordination of the
alkyne ligand in 5r otCO (Figure 11) could be better
described as an η1-alkyne species. The instability of the
η2(C,C)-alkyne-rotated structure clearly has a steric
origin and is caused by the repulsion between the
methyl substituent of the alkyne and the Cp rings of
the metallic fragment. The alkyne ligand is forced to
change its coordination mode when it rotates in order
to minimize the steric contacts. 5r otCO is found 20.9
kcal/mol above the alkyne minimum. The energy and
the geometry of this structure suggests that it could be
the transition state for the rotation of the η2(C,H)-alkyne
intermediates, BCO. In fact, the optimization of BxCO
rotated by 90° around the C-H bond leads to a structure
that is almost identical with that of 5r otCO. A three-
step mechanism for the rotation of the η2(C,C)-alkyne
complex can be envisaged. In the first step the π-alkyne
complex rearranges to the σ η2(C,H)-alkyne complex.
Then the η2(C,H)-alkyne complex rotates with a very
low barrier, and finally, the slippage of the σ η2(C,H)-
alkyne complex gives the other isomer of the η2(C,C)-
alkyne species.
The study of the rotational processes has shown that
the rotational barriers for the vinylidene and alkyne
ligands in niobocene systems are high. For the vi-
nylidene (27.9 kcal/mol), the value is in the range of the
activation barrier for the vinylidene f alkyne rear-
rangement. For the alkyne, the reaction intermediates,
i.e., η2(C,H)-alkyne species, are involved in the rotation.
The exo and endo isomers of reactants and products do
not interconvert directly but can interconvert along the
vinylidene f η2-alkyne reaction path. The η2(C,H) f
η2(C,C) slippage is the key step in the interconversion
between the exo and endo isomers.
(36) Kostic, N. M.; Fenske, R. F. Organometallics 1982, 1, 974.
(37) Lauher, J . W.; Hoffmann, R. J . J . Am. Chem. Soc. 1976, 98,
1729.
(38) Antin˜olo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; Garcia-Yuste,
S.; Otero, A.; Camanyes, S.; Maseras, F.; Moreno, M.; Lledo`s, A.; Lluch,
J . M. J . Am. Chem. Soc. 1997, 119, 6107.

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1761
with a Perkin-Elmer 240 B microanalyzer. NMR spectra were
recorded on a Varian Unity FT-300 instrument (1H at 300
MHz, 13C at 75 MHz). IR spectra were recorded as Nujol mulls
between CsI plates or as a KBr mull (in the region 4000-200
cm-1) on a Perkin-Elmer PE 833 IR spectrophotometer. Mass
spectroscopic analyses were performed on a VG Autospec
instrument (FAB techniques using NBA as matrix) or a
Hewlett-Packard 5988A (m/z 50-1000) (chemical ionization
techniques). Cyclic voltammetry was carried out in a standard
three-electrode Tacussel UAP4 unit cell. The reference elec-
trode was saturated calomel (SCE), separated from the solu-
tion by a sintered-glass disk. The auxiliary electrode was a
platinum wire. For all voltammetric measurements the work-
Con clu d in g Rem a r k s
In conclusion, the first general procedure to prepare
(σ-alkynyl)niobocene complexes has been described.
These complexes were chemically and electrochemically
oxidized, and we have observed that the nature of the
resulting products, radical alkynyl, vinylidene, or divi-
nylidene species, depends on the substituent R on the
alkynyl ligand, the nature of the ancillary ligand L, and
the experimental conditions (temperature and solvent).
The noteworthy feature of this work is the observation
of a new vinylidene to alkyne isomerization in the
coordination sphere of an early transition metal. Theo-
retical calculations have shown that in both niobocene-
carbonyl and niobocene-phosphine systems, vinylidene
and alkyne complexes are isoenergetic, a situation that
is in marked contrast with the systems previously
considered in theoretical studies. Although in niob-
ocene-phosphine systems the η2-alkyne form is slightly
more destabilized than in niobocene-carbonyl systems,
this fact does not justify the different experimental
behavior observed between 4a , 4c, and 4d . An inspec-
tion of the factors governing the relative stability of
vinylidene-alkyne complexes suggests that while the
electrophilicity of the vinylidene R-carbon plays a key
role in a given metal system, other factors may be
responsible for the relative stability when different
transition-metal systems are compared. A theoretical
study of the possible intramolecular mechanisms for the
isomerization has shown that it takes place by a two-
step 1,2-hydrogen shift mechanism. In the first step a
σ η2(C,H)-alkyne is formed by means of a 1,2-hydrogen
migration from the â- to the R-carbon. The next step is
the slippage process of the alkyne ligand from a σ η2-
(C,H) coordination to an η2(C,C) coordination. The
different experimental behaviors exhibited by 4a , 4c,
and 4d are explained in terms of kinetic effects. The
difference of almost 10 kcal/mol found in the activation
barrier of the η1-vinylidene f η2(C,H)-alkyne step found
on going from the carbonyl-niobocene to the phos-
phine-niobocene system justifies the experimental find-
ings. Moreover, the theoretical study indicates that
η2(C,H)-alkyne intermediates are involved in the exo f
endo interconversion. The exo and endo isomers of
reactants and products do not interconvert directly but
can easily interconvert along the η1-vinylidene f η2-
alkyne reaction path. This fact suggests a three-step
mechanism for rotation of the terminal alkyne: (i)
rearrangement of η2(C,C)-alkyne toward an η2(C,H)-
alkyne species, (ii) exo-endo interconversion with a low
rotational barrier of the η2(C,H)-alkyne species, and (iii)
a new slippage of the η2(C,H)-alkyne complex to the
other η2(C,C)-alkyne isomer.
ing electrode was either
a vitreous carbon or platinum
electrode. For controlled-potential electrolysis, a mercury pool
was used as the anode and a platinum plate as the cathode,
the latter being separated from the solution by a sintered-glass
disk. In all cases the electrolyte was a 0.2 M solution of
tetrabutylammonium hexafluorophosphate or perchlorate in
tetrahydrofuran. The electrolyses were performed with an
Amel 552 potentiostat coupled to an Amel 721 electronic
integrator. Electron spin resonance spectra were taken at room
temperature on a Bruker ESP300 spectrometer. Field calibra-
tions were made with DPPH (g ) 2.0034).
Syn t h esis of Nb (η5-C5H 4SiMe3)2(CtCP h )(CO) (2a ).
Meth od 1. To a mixture of Nb(η5-C5H4SiMe3)2(Cl)(CO) (180
mg, 0.41 mmol) and Mg(CtCPh)2 (140 mg, 0.62 mmol) was
added toluene and the resulting suspension stirred at room
temperature for 7 h. The solvent was subsequently removed
in vacuo and the residue extracted with hexane. The resulting
green solution was evaporated to dryness to give an oily
product that was identified as the alkynyl complex 2a (169
mg, 0.34 mmol, 83%).
Meth od 2. To a mixture of both endo and exo isomers of
the vinylidene complex [Nb(η5-C5H4SiMe3)2(dCdCHPh)(CO)]-
[BPh4] (4a [BPh4]; 30 mg, 0.04 mmol) and KOtBu (4.12 mg, 0.04
mmol) was added THF at 0 °C. The color of the reaction
mixture changed from yellow to green. After it was stirred for
30 min at this temperature, the reaction mixture was stirred
for a further 30 min at room temperatue and evaporated to
dryness and pentane added. The resulting solution was again
evaporated to dryness to give the alkynyl complex Nb(η5-C5H4-
SiMe3)2(CtCPh)(CO) (18 mg, 0.04 mmol, 99%). N.B.: depro-
tonation of [Nb(η5-C5H4SiMe3)2(dCdCHPh)(CO)][BF] (4a [BF4])
and [Nb(η5-C5H4SiMe3)2(η2(C,C)-HCtCPh)(CO)]+ (5a ) under
the same conditions, also gave the alkynyl complexes in good
1
yields. IR: νCO 1920, νCtC 2076 cm-1. H NMR (C6D6): δ 0.19
(s, 18H, SiMe3), 4.70-4.80 (2H), 4.80-4.86 (2H), 4.90-5.00
(2H), 5.20-5.30 (2H) (m, C5H4SiMe3), 6.90-7.00 (m, 1H,
p-C6H5 of PhCtC), 7.04-7.12 (m, 2H, m-C6H5 of PhCtC),
7.50-7.60 (m, 2H, o-C6H5 of PhCtC). 13C{1H} NMR (C6D6):
δ 0.18 (SiMe3), 92.26, 94.13, 98.59, 99.34, 101.34 (C5H4SiMe3),
113.21 (PhCtC-Nb), 122.07 (PhCtC-Nb), 123.53 (C-4),
124.67 (C-3 and C-5), 129.63 (C-1), 130.84 (C-2 and C-6)
(phenyl group of PhCtC), 255.87 (CO). MS chemical ionization
(m/e (relative intensity)): 496 (7) (M+, [Nb(η5-C5H4SiMe3)2(Ct
CPh)(CO)]+), 468 (100) (M+ - CO), 395 (39) (M+ - CtCPh),
367 (67) (M+ - CO - CtCPh).
Syn th esis of Nb(η5-C5H4SiMe3)2(CtCSiMe3)(CO) (2b).
To a mixture of Nb(η5-C5H4SiMe3)2(Cl)(CO) (210 mg, 0.49
mmol) and Mg(CtCSiMe3)2 (210 mg, 0.97 mmol) was added
toluene at room temperature. The resulting solution was
stirred at 80 °C for 7 h, with the color of the reaction mixture
changing from purple to dark green. The solvent was then
evaporated in vacuo and the residue extracted with pentane.
The resulting solution was again evaporated to dryness to give
an oily product, which was identified as complex 2b (230 mg,
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Nb(η5-C5H4SiMe3)2(Cl)(CO) and
Nb(η5-C5H4SiMe3)2Cl(PMe2Ph) were prepared by published
methods.4a The synthesis of the alkynylmagnesium compounds
Mg(CtCR)2 was performed by following a previously reported
method.39 All reactions were carried out under an inert
atmosphere (argon) using standard Schlenk techniques. Sol-
vents were freshly distilled from appropriate drying agents
and degassed before use. Elemental analyses were performed
0.47 mmol, 96%). IR: νCO 1927, νCtC 2015 cm-1 1H NMR
.
(C6D6): δ 0.17 (s, 18H, C5H4SiMe3), 0.32 (s, 9H, CtCSiMe3),
4.68-4.78 (4H), 4.86-4.94 (2H), 5.16-5.22 (2H) (m, C5H4-
SiMe3). 13C{1H} NMR (C6D6): δ 0.15 (C5H4SiMe3), 1.80 (Ct
(39) Eisch, J . J .; Sanchez, R. J . Organomet. Chem. 1985, 296, C-27.

1762 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
CSiMe3), 92.30, 93.87, 98.76, 99.11, 101.32 (C5H4SiMe3), 102.80
(Me3SiCtC-Nb), 129.28 (Me3SiCtC-Nb), 255.64 (CO). MS
468 (18) (M+ - PMe2Ph), 367 (50) (M+ - PMe2Ph - CtCPh).
Anal. Calcd for C32H42Si2NbP: C, 63.35; H, 6.98. Found: C,
63.20; H, 6.90.
chemical ionization (m/e (relative intensity)): 493 (27) (M+
+
1), 492 (18) (M+, [Nb(η5-C5H4SiMe3)2(CtCSiMe3)(CO)]+), 477
Syn th esis of Nb(η5-C5H4SiMe3)2(CtCP h )(P (OEt3)) (2e).
To a mixture of Nb(η5-C5H4SiMe3)2(Cl)(P(OEt)3)(Cl) (560 mg,
0.98 mmol) and Mg(CtCPh)2 (660 mg, 2.90 mmol) was added
toluene and the resulting suspension stirred at room temper-
ature for 3 h. The solvent was removed in vacuo and the
residue extracted with pentane. The resulting red solution was
evaporated to dryness to give an oily red solid. The solid was
washed with pentane at -80 °C to give a red oily product,
which was identified as 2d (600 mg, 0.94 mmol, 96.5%). IR
(KBr): νCtC 2049 cm-1. 1H NMR (C6D6): δ 0.29 (s, 18H, SiMe3),
(33) (M+ - Me), 464 (18) (M+ - CO), 395 (100) (M+
CtCSiMe3), 367 (21) (M+ - CO - CtCSiMe3).
-
Syn t h esis of Nb (η5-C5H 4SiMe3)2(CtCtBu )(CO) (2c).
Meth od 1. The synthesis of the alkynyl complex 2c was
carried out by following the same method as described for 2a
and 2b above. Thus, 320 mg (0.73 mmol) of Nb(η5-C5H4SiMe3)2-
(Cl)(CO) was treated with 260 mg (1.39 mmol) of Mg(CtCtBu)2
in toluene and the reaction mixture stirred at 80 °C for 7 h.
The solvent was then removed in vacuo and the resulting solid
washed twice with pentane at 0 °C. The combined washings
were then evaporated to dryness to give the alkynyl complex
2c as a green oil (330 mg, 0.69 mmol, 94%).
1.06 [t, 2J H-H ) 6.96 Hz, 9H, P(OCH2CH3)3], 4.04 [dq, 3J P-H
)
6.96 Hz, 2J H-H ) 6.96 Hz, 6H, P(OCH2CH3)3], 4.85-4.90 (2H),
4.90-5.00 (2H), 5.05-5.15 (2H), 5.50-5.60 (2H) (m, C5H4-
SiMe3), 6.85-6.95 (m, 3H), 7.30-7.40 (m, 2H) (C6H5 group of
PhCtC). 13C{1H} NMR (C6D6): δ 0.46 (SiMe3), 16.42 [d, 3J P-C
) 5.49 Hz, P(OCH2CH3)3], 61.26 [d, 2J P-C ) 5.49 Hz, P(OCH2-
CH3)3], 91.19, 92.33, 94.01, 97.46, 103.36 (C5H4SiMe3), 100.78
(PhCtC), 104.40 (broad, PhCtC-Nb), 123.09 (C-4), 127.05
(C-3 and -5), 129.33 (C-2 and -6), 133.90 (C-1) (phenyl group
of PhCtC). FAB MS (m/e (relative intensity)): 635 (33) (M+
+ 1), 634 (11) (M+, [Nb(η5-C5H4SiMe3)2(CtCPh)(P(OEt)3)]+),
468 (19) [M+ - P(OEt)3]; 367 (68) [M+ - P(OEt)3 - CtCPh].
Syn th esis of Nb(η5-C5H5)2(CtCSiMe3)(CO) (2f). To a
solution of the alkynyl complex Nb(η5-C5H4SiMe3)2(CtCSiMe3)-
(CO) (2b; 120 mg, 0.24 mmol) in THF at 0 °C was added 19.7
µL (15.6 mg, 0.48 mmol) of dry, freshly distilled MeOH and
121.80 µL (0.12 mmol) of a 1 M solution of the desilylating
agent NBu4F in THF. The mixture was stirred at 0 °C for 1 h
and then for a further 1 h 45 min at room temperature. The
solvent was then removed in vacuo and the red residue
extracted with a mixture of toluene and diethyl ether (80/20).
The resulting solution was left to stand overnight at -35 °C
to precipitate various impurities. Subsequent filtration at this
temperature gave a green solution, which was evaporated to
dryness to give a green solid identified as Nb(η5-C5H5)2(Ct
CSiMe3)(CO) (70 mg, 0.20 mmol, 80%). IR (KBr): νCO 1897,
Meth od 2. To an equimolar mixture of [Nb(η5-C5H4SiMe3)2-
(dCdCHtBu)(CO)][BF4] (4c[BF4]; 100 mg, 0.18 mmol) and
KOtBu (20 mg, 0.18 mmol) was added 10 mL of THF at 0 °C.
A rapid color change from yellow to green was observed. The
reaction mixture was stirred for 30 min at 0 °C and then for
a further 30 min at room temperature. The solvent was then
removed in vacuo and the residue extracted with pentane. The
resulting solution was then evaporated to dryness to give the
alkynyl complex 2c (81 mg, 0.17 mmol, 95%). N.B.: when the
same reaction was carried out using a mixture of both endo
and exo isomers of [Nb(η5-C5H4SiMe3)2(η2(C,C)-HCtCtBu)-
(CO)]+ (5c), the same result was obtained and 2c was isolated
1
in good yield. IR: νCO 1910, νCtC 2093 cm-1. H NMR (C6D6):
δ 0.20 (s, 18H, SiMe3), 1.35 (s, 9H, CtCCMe3), 4.70-4.77 (2H),
4.77-4.82 (2H), 4.92-5.00 (2H), 5.22-5.28 (2H), (m, C5H4-
SiMe3). 13C{1H} NMR (C6D6): δ 0.19 (SiMe3), 29.67 [Ct
CC(CH3)3], 32.94 [CtCC(CH3)3], 91.88, 93.60, 98.81, 98.81,
101.98 (C5H4SiMe3), 112.50 (tBuCtC-Nb), 129.75 (tBuCtC-
Nb), 257.85 (CO). MS chemical ionization (m/e (relative
intensity)): 477 (44) (M+ + 1), 476 (29) (M+, [Nb(η5-C5H4-
SiMe3)2(CtCtBu)(CO)]+), 461 (42) (M+ - Me), 448 (63) (M+
-
CO), 395 (100) (M+ - CtCtBu), 367 (17) (M+ - CO - CtCtBu).
Syn th esis of Nb(η5-C5H4SiMe3)2(CtCP h )(P Me2P h ) (2d ).
Meth od 1. To a mixture of Nb(η5-C5H4SiMe3)2(Cl)(PMe2Ph)
(130 mg, 0.24 mmol) and Mg(CtCPh)2 (110 mg, 0.49 mmol)
was added toluene and the resulting suspension stirred at
room temperature for 4 h. The solvent was removed in vacuo
and the residue extracted with pentane. The resulting red
solution was evaporated to dryness to give an oily red solid
that was subsequently washed with pentane at -80 °C. The
red precipitate that was formed was identified as 2d (100 mg,
0.16 mmol, 71%).
νCtC 2004 cm-1 1H NMR (C6D6): δ 0.34 (s, 9H, CtCSiMe3),
.
4.65 (s, 10H, C5H5). 13C{1H} NMR (C6D6): δ 1.96 [CtCSi-
(CH3)3], 92.28 (C5H5), 92.16 (Me3SiCtC-Nb), 129.02 (Me3SiCt
C-Nb). The CO carbon is not observed. MS chemical ionization
(m/e (relative intensity)): 348 (83) (M+, [Nb(η5-C5H5)2(Ct
CSiMe3)(CO)]+), 333 (100) (M+ - Me), 320 (70) (M+ - CO),
251 (38) (M+ - CtCSiMe3), 223 (40) (M+ - CO - CtCSiMe3).
Anal. Calcd for C16H19NbOSi: C, 55.18; H, 5.50. Found: C,
54.91; H, 5.54.
Meth od 2. To a mixture of both endo and exo isomers of
the vinylidene complex [Nb(η5-C5H4SiMe3)2(dCdCHPh)(PMe2-
Ph)][BF4] (4d ; 30 mg, 0.04 mmol) and KOtBu (4.12 mg, 0.04
mmol) was added THF at 0 °C, with the color changing from
yellow to dark red. The reaction mixture was stirred for 30
min at 0 °C and then for a further 30 min at room temperature.
The mixture was evaporated to dryness and the residue
extracted several times with pentane. The resulting solution
was again evaporated to dryness to give the alkynyl complex
Nb(η5-C5H4SiMe3)2(CtCPh)(PMe2Ph) (2d ; 21.22 mg, 0.035
mmol, 88%). IR (KBr): νCtC 2035 cm-1. 1H NMR (C6D6): δ 0.20
Syn th esis of Nb(η5-C5H5)2(CtCH)(CO) (2g). To a mixture
of the alkynyl complex Nb(η5-C5H5)2(CtCSiMe3)(CO) (2f; 210
mg, 0.60 mmol) and K2CO3 (50 mg, 0.36 mmol) was added 3
mL of dry, freshly distilled methanol at 0 °C. The mixture was
stirred at this temperature for 30 min and then for a further
1 h at room temperature. The mixture was then left to stand
for 30 min and filtered to separate the remaining K2CO3. The
solvent was evaporated in vacuo and the resulting green solid
washed with cold diethyl ether (-30 to -40 °C). N.B.: the
green product often appears contaminated with an orange solid
that is insoluble in toluene at -40 °C and which has yet to be
identified. Thus, when the impure compound 2g is dissolved
in the minimum amount of toluene and left to stand at -40
°C for 12 h, a red precipitate and a green solution are obtained.
After filtration and evaporation of the filtrate to dryness, the
terminal alkynyl complex 2g is obtained as a green micro-
crystalline solid (64 mg, 0.23 mmol, 40%). IR (KBr): νCO 1892,
2
(s, 18H, SiMe3), 1.36 [d, J P-H ) 7.69 Hz, 6H, P(CH3)2Ph],
4.40-4.50 (2H), 4.55-4.70 (4H), 5.30-5.40 (2H) (m, C5H4-
SiMe3), 6.90-7.30 (m, 6H), 7.43-7.55 (m, 4H) (C6H5 groups
of PhCtC and PMe2Ph). 13C{1H} NMR (C6D6): δ 0.52 (SiMe3),
1
18.28 [d, J P-C ) 23.8 Hz, P(CH3)2Ph], 90.34, 91.79, 95.27,
97.19, 101.96 (C5H4SiMe3), 100.82 (PhCtC), 104.58 (broad,
PhCtC-Nb), 123.18 (C-4), 128.96 (C-3 and -5), 129.47 (C-2
and -6), 129.70 (C-1) (phenyl group of PhCtC), 128.88 (d, 3J P-C
νCtC 2044, νtC-H 3264 cm-1 1H NMR (C6D6): δ 2.53 (s, 1H,
.
CtCH), 4.72 (s, 10H, C5H5). 13C{1H} NMR (C6D6): δ 92.16
(C5H5), 92.31 (HCtC-Nb), 107.85 (HCtC-Nb). The CO
carbon is not observed. FAB MS (m/e (relative intensity)): 277
(81) (M+ + 1), 276 (13) (M+, [Nb(η5-C5H5)2(CtCH)(CO)]+), 248
(50) (M+ - CO), 251 (13) (M+ - CtCH), 223 (100) (M+ - CO
2
) 11.90 Hz, C-3 and -5), 129.47 (s, C-4), 130.32 (d, J ) 8.24
Hz, C-2 and -6), 142.38 (d, 1J P-C ) 29.30 Hz, C-1) (phenyl group
of PMe2Ph). FAB MS (m/e (relative intensity)): 607 (77) (M+
+ 1), 606(22) (M+, [Nb(η5-C5H4SiMe3)2)(CtCPh)(PMe2Ph)]+),

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1763
- CtCH). Anal. Calcd for C13H11NbO: C, 56.54; H, 3.98.
Found: C, 56.49; H, 4.05.
132.85 (NbdCdC), 213.74, 214.44 (CO), 377.97, 378.39 (Nbd
CdC). FAB MS (m/e (relative intensity)): 497 (100) (M+, [Nb-
(η5-C5H4SiMe3)2(CCHPh)(CO)]+), 469 (27) (M+ - CO), 395 (62)
(M+ - CCHPh), 367 (69) (M+ - CO - CCHPh). Anal. Calcd for
Syn th esis of [Nb(η5-C5H4SiMe3)2(CtCP h )(P Me2P h )]-
[BP h 4] (3d ). To a solution of Nb(η5-C5H4SiMe3)2(CtCPh)-
(PMe2Ph) (2d ; 80 mg, 0.13 mmol) in dichloromethane at -30
°C was added [Fe(η5-C5H5)2][BPh4] (66 mg, 0.13 mmol), with
the color of the solution quickly changing from dark to bright
light red. After the mixture was stirred for 45 min, pentane
was slowly added at -30 °C and a red crystalline solid
precipitated. The solid was filtered off and washed several
times with pentane at the same temperature and dried in
vacuo. The resulting red crystals were identified as the
paramagnetic complex 3d (60 mg, 0.065 mmol, 50%). IR
(KBr): νCtC 2034 cm-1. FAB MS (m/e (relative intensity)): 607
(57) (M•+ + 1), 606 (100) (M•+, [Nb(η5-C5H4SiMe3)2 (CtCPh)-
(PMe2Ph)]•+), 468 (41) (M•+ - PMe2Ph), 367 (31) (M•+ - PMe2-
Ph - CtCPh). Anal. Calcd for C56H62BNbPSi2: C, 72.64; H,
6.75. Found: C, 72.60; H, 6.85.
C
25H32BF4NbOSi2: C, 51.40; H, 5.48. Found: C, 51.38; H, 5.50.
Syn th esis of [Nb(η5-C5H4SiMe3)2(dCdCH2)(CO)][BP h 4]
(4h ). The oxidation of the Nb(III) complex 2b, described above,
always yielded an unexpected product that was difficult to
isolate. The best results were obtained when 180 mg (0.36
mmol) of 2b, Nb(η5-C5H4SiMe3)2(CtCSiMe3)(CO), was treated
with 182 mg (0.36 mmol) of [FeCp2][BPh4] in THF at -20 °C.
The reaction mixture was stirred for 10 min between -20 and
0 °C and for a further 15 min at this temperature. The
resulting red-brown solution was evaporated to dryness to give
an oily green residue, which was then washed several times
with diethyl ether at 0 °C. A gray solid was finally isolated
and identified as the terminal monovinylidene compound [Nb-
(η5-C5H4SiMe3)2(dCdCH2)(CO)][BPh4] (4e; 80 mg, 0.10 mmol,
30%). All attempts at recrystallization of this complex resulted
in decomposition. IR (Nujol): νCdC vinylidene 1620, νCO 2042
Syn th esis of [Nb(η5-C5H4SiMe3)2(dCdCHP h )(CO)]+ (4a).
Meth od 1. To an equimolar mixture of Nb(η5-C5H4SiMe3)2-
(CtCPh)(CO) (2a ; 170 mg, 0.34 mmol) and [FeCp2][BPh4] (173
mg, 0.34 mmol) was added 30 mL of CH2Cl2 at -30 °C. The
solution was stirred for 2 h 30 min while the temperature was
slowly increased to -10 °C. The mixture was then stirred for
a further 1 h between -10 and 10 °C. After this time the
solvent was evaporated to dryness to give an orange oily
residue that, after washing several times with diethyl ether,
gave a yellow solid, which was subsequently identified as a
1:1 mixture of both exo and endo isomers of [Nb(η5-C5H4-
SiMe3)2(dCdCHPh)(CO)][BPh4] (190 mg, 0.23 mmol, 68%).
cm-1 1H NMR (CD3CN): δ 0.25 (s, 18H, SiMe3), 5.94-6.01
.
(4H), 6.01-6.05 (2H), 6.08-6.12 (2H) (m, C5H4SiMe3), 6.14,
6.15 (s, 1H, H2CdCdNb), 6.79-6.88 (4H, p-C6H5), 6.94-7.04
(8H, m-C6H5), 7.21-7.32 (8H, o-C6H5) (phenyl groups of
BPh4-). 13C{1H} NMR (CD3CN): δ -0.10 (SiMe3), 104.39,
104.91, 106.08, 107.66, 109.79 (Cipso) (C5H4SiMe3), 118.64 (Nbd
CdCH2), 122.73, 126.56, 136.73 (phenyl groups of BPh4-),
-
13
11
164.78 (Cipso of phenyl groups of BPh4 , q, J
) 49 Hz).
C-
B
Signals corresponding to carbons of CO and NbdCdCH2 are
not observed. FAB MS (m/e (relative intensity)): 421 (100) (M+,
[Nb(η5-C5H4SiMe3)2(CCH2)(CO)]+), 395 (41) (M+ - CCH2), 393
(35) (M+ - CO), 367 (72) (M+ - CCH2 - CO). Simulation of
the isotopic composition of M+ was identical with the experi-
mental result. Anal. Calcd for C43H48BNbOSi2: C, 69.72; H,
6.53. Found: C, 69.77; H, 6.55.
Spectroscopic data for 3a [BPh4] are as follows. IR (Nujol): νCd
1
vinylidene 1620, νCO 2054 cm-1. H NMR (CD3CN): δ 0.20,
C
0.23 (s, 18H, SiMe3, both isomers), 5.92-5.98 (2H), 6.00-6.04
(2H), 6.04-6.08 (2H), 6.10-6.14 (2H), 6.14-6.18 (2H), 6.18-
6.26 (6H) (m, C5H4SiMe3, both isomers), 6.78-6.88 (m, 8H,
p-C6H5), 6.92-7.04 (m, 16H, m-C6H5), 7.20-7.32 (m, 16H,
o-C6H5) (phenyl group of BPh4-), 7.10-7.20 (m, 4H, m-C6H5-
CdCdNb), 7.32-7.42 (s, 1H, dCdCHPh, and m, 6H, p- and
o-C6H5-CdCdNb), 7.63 (s, 1H, dCdCHPh). Anal. Calcd for
Syn th esis of [Nb(η5-C5H4SiMe3)2(dCdCHtBu )(CO)]+ (4c).
Meth od 1. To an equimolar mixture of Nb(η5-C5H4SiMe3)2-
(CtCtBu)(CO) (2c; 70 mg, 0.15 mmol) and [FeCp2][BPh4] (74
mg, 0.15 mmol) was added 20 mL of CH2Cl2 at -30 °C. The
solution was then slowly warmed to -10 °C over 2 h 30 min
and then to 10 °C over a further 1 h. After this time the solvent
was removed in vacuo and the resulting oily brown mixture
washed twice with diethyl ether at 0 °C to give a pale brown
precipitate, which was subsequently identified as a single
isomer of the compound [Nb(η5-C5H4SiMe3)2(dCdCHtBu)(CO)]-
[BPh4] (4c[BPh4]; 56 mg, 0.07 mmol, 48%). Dimerization to
the divinylidene complex was not observed in any case, even
when the reaction was performed under different reaction
conditions (solvents, temperature, and concentration). Spec-
C
49H52BNbOSi2: C, 72.05; H, 6.42. Found: C, 71.90; H, 6.40.
Meth od 2. To a solution of Nb(η5-C5H4SiMe3)2(CtCPh)(CO)
(2a ; 120 mg, 0.24 mmol) in diethyl ether was added 35.6 µL
(39 mg, 0.24 mmol) of HBF4‚2Et2O (solution in Et2O) at -30
°C. The reaction mixture was stirred while the temperature
of the solution was slowly increased to 25 °C. The product
precipitated as a yellow solid, which was isolated by filtration
and washed twice with the same solvent. This solid was
subsequently identified as a 1:1 mixture of both exo and endo
isomers of the complex [Nb(η5-C5H4SiMe3)2(dCdCHPh)(CO)]-
[BF4] (4a [BF4]; 120 mg, 0.20 mmol, 86%). Spectroscopic data
for 4a [BF4] are as follows. IR (Nujol): νCdC vinylidene 1641,
1
troscopic data for 4c[BPh4] are as follows. H NMR (CD3CN):
δ 0.24 (s, 18H, SiMe3), 1.08 (s, 9H, tBu), 5.80-5.86 (2H), 5.88-
5.94 (2H), 5.94-6.00 (2H), 6.20-6.26 (2H) (m, C5H4SiMe3), 6.53
(s, 1H, dCdCHtBu), 6.78-6.88 (m, 4H, p-C6H5), 6.92-7.04 (m,
8H, m-C6H5), 7.20-7.30 (m, 8H, o-C6H5) (phenyl group of
BPh4-). Anal. Calcd for C47H56BNbOSi2: C, 70.84; H, 7.08.
Found: C, 70.89; H, 7.05.
ν
CO 2073 cm-1. 1H NMR (CD3CN): δ 0.20, 0.23 (s, 18H, SiMe3,
both isomers), 5.96-6.02 (2H), 6.02-6.10 (4H), 6.10-6.16 (2H),
6.16-6.20 (2H), 6.20-6.30 (6H) (m, C5H4SiMe3, both isomers),
7.10-7.22 (m, 4H, m-C6H5CdCdNb), 7.24-7.42 (m, 6H, p- and
o-C6H5CdCdNb), 7.38, 7.63 (s, 1H, dCdCHPh, both isomers).
1
The proportion of isomers initially observed when the H NMR
Met h od 2. To a solution of Nb(η5-C5H4SiMe3)2(CtCtBu)-
(CO) (2c; 200 mg, 0.42 mmol) in 20 mL of diethyl ether at -40
°C was added 62 µL (0.42 mmol) of HBF4‚2Et2O (solution in
Et2O). This solution was stirred for 2 h, with the color changing
from green to orange, until the temperature had reached -5
°C. The product precipitated as a yellow solid, which was
isolated by filtration and washed three times with the same
solvent. The resulting solid was identified as [Nb(η5-C5H4-
SiMe3)2(dCdCHtBu)(CO)][BF4] (4c[BF4]; 100 mg, 0.18 mmol,
42%). Spectroscopic data for 4c[BF4] are as follows. IR (KBr):
spectrum was acquired in CD3COCD3 at -70 °C was 2:5. All
the NMR data in this case are as follows. 1H NMR (CD3COCD3,
-70 °C, major isomer): δ 0.22 (s, 18H, SiMe3), 6.30-6.50 (4H),
6.50-6.70 (4H), (m, C5H4SiMe3), 7.20-7.44 (m, 5H, phenyl
1
group of C6H5-CdCdNb), 7.86 (s, 1H, dCdCHPh). H NMR
(CD3COCD3, -70 °C, minor isomer): δ 0.26 (s, 18H, SiMe3),
6.20-6.30 (2H), 6.30-6.50 (2H), 6.50-6.70 (2H), 6.70-6.90
(2H), (m, C5H4SiMe3), 7.20-7.44 [m, 5H, phenyl group of
(C6H5)HCdCdNb], 7.74 (s, 1H, dCdCHPh). 13C{1H} NMR
(CD3COCD3, -70 °C, mixture of both endo and exo isomers):
δ -1.32, -1.03 (SiMe3), 103.96, 104.32, 105.20, 105.53, 105.66,
106.90, 107.03, 107.64 (C5H4SiMe3), 126.58, 127.03 (p-C6H5),
127.99, 128.09 (m-C6H5), 128.98, 129.16 (o-C6H5), 133.98,
135.44 (Cipso) (phenyl groups of C6H5HCdCdNb), 132.35,
ν
CdC vinylidene 1626, νCO 2074 cm-1. 1H NMR (CD3CN), δ 0.24
t
(s, 18H, SiMe3), 1.08 (s, 9H, Bu), 5.82-5.88 (2H), 5.90-5.96
(2H), 5.94-6.02 (2H), 6.22-6.28 (2H) (m, C5H4SiMe3), 6.53 (s,
1H, dCdCHBut). 1H NMR (CD3COCD3, -70 °C): δ 0.31 (s,
18H, SiMe3), 1.13 (s, 9H, tBu), 6.14 (2H), 6.22 (2H), 6.32 (2H),

1764 Organometallics, Vol. 19, No. 9, 2000
Garc´ıa-Yebra et al.
6.58 (2H), (m, C5H4SiMe3), 6.68 (s, 1H, dCdCHtBu). 13C{1H}
NMR (CD3COCD3, -70 °C): δ -0.13 (SiMe3), 30.48 [C(CH3)3],
37.80 [C(CH3)3], 104.85, 105.09, 105.96, 107.96, 112.50 (C5H4-
SiMe3), 144.09 (NbdCdC), 218.39 (CO). The signal corre-
sponding to (NbdCdC) is not observed. Anal. Calcd for
(CO)][BF4] (4c[BF4]; 40 mg, 0.07 mmol) in CD3CN was placed
in an NMR tube and left to stand at room temperature for 48
h. After this time the 1H NMR spectrum showed two new
compounds in a 1:1 ratio, which were identified as a mixture
of both endo and exo isomers of the corresponding η2-alkyne
complex. Spectroscopic data for the mixture of the two new
compounds are as follows. 1H NMR (CD3CN): isomer I, δ 0.18
(s, 18H, SiMe3), 1.34 (s, 9H, tCCMe3), 5.34-5.40 (2H), 5.94-
6.00 (2H), 6.00-6.04 (2H), 6.18-6.24 (2H), (m, C5H4SiMe3),
6.50 (s, 1H, CtCH); isomer II, δ 0.31 (s, 18H, SiMe3), 1.33 (s,
9H, tCCMe3), 5.14-5.22 (2H), 5.28-5.34 (2H), 5.70-5.76 (2H),
6.24-6.30 (2H) (m, C5H4SiMe3), 7.16 (s, 1H, CtCH). 13C{1H}
NMR (CD3CN; mixture of both isomers): δ -0.32, -0.60
(SiMe3), 30.79, 32.73 [C(CH3)3], 36.09, 38.40 [C(CH3)3], 84.32,
100.90, 103.21, 104.50, 104.87, 105.56, 108.79, 109.28, 111.91,
112.84 (C5H4SiMe3), 127.48, 133.03, 144.56, 145.24 (HCt
CtBu). The signal corresponding to CO is not observed. FAB
MS (m/e (relative intensity)): 477 (94) (M+, [Nb(η5-C5H4-
SiMe3)2(η2(C,C)-HCtCtBu)(CO)]+), 449 (26) (M+ - CO), 395
(100) (M+ - HCtCtBu), 367 (85) (M+ - CO - HCtCtBu).
Com p u ta tion a l Deta ils. Calculations were performed with
the GAUSSIAN 94 series of programs40 within the framework
of the density functional theory (DFT)41 using the B3LYP
functional.42 A quasi-relativistic effective core potential opera-
tor was used to represent the 28 innermost electrons of the
niobium atom.43 The basis set for the metal atom was that
associated with the pseudopotential,43 with a standard dou-
ble-ú LANL2DZ contraction.40 The 6-31G(d,p) basis set was
used for the P, CR, Câ, HR, and Cl atoms and the carbonyl
ligand, whereas the 6-31G basis set was used for the other
carbon and hydrogen atoms.44 To back up the methodology
employed, vinylidene/alkyne systems that have been studied
previously were reoptimized using the B3LYP functional and
basis set of the same quality as used for the niobocene
complexes: LANL2DZ basis set for metal atoms, 6-31G(d,p)
basis set for the atoms directly attached to the metal and the
acetylene and vinylidene ligands, and 6-31G basis set for the
other atoms.43,44 Our results for the RuCl2(PH3)2 and RhCl-
(PH3)2 systems (vinylidene isomer 13.7 and 6.3 kcal/mol more
stable than the alkyne, respectively) are in reasonable agree-
ment with those previously reported at the MP2 level (19.528a
and 7.8 kcal/mol,28b respectively). The higher stability of the
η2-alkyne isomer in F4W (8.1 kcal/mol) also agrees with
previous CCSD(T)//DFT calculations (10.4 kcal/mol).28c The
calculated relative energy for the free vinylidene with respect
to acetylene (41.8 kcal/mol) also reproduces previous theoreti-
cal45 and experimental46 results.
C
23H36BF4NbOSi2: C, 48.94; H, 6.43. Found: C, 48.90; H, 6.45.
Syn th esis of [Nb(η5-C5H4SiMe3)2(dCdCHP h )(P Me2P h )]-
[BF 4] (4d ). To a solution of Nb(η5-C5H4SiMe3)2(CtCPh)(PMe2-
Ph) (2d ; 100 mg, 0.165 mmol) in diethyl ether at -30 °C was
added 24.2 µL (27 mg, 0.165 mmol) of HBF4‚2Et2O (solution
in diethyl ether). The rapid formation of a yellow precipitate
was observed. The solution was stirred for 30 min at this
temperature and then left to stand. The yellow precipitate was
filtered off, washed twice with diethyl ether, and dried in
vacuo. The solid was identified as a 1:1 mixture of the endo
and exo isomers of the cationic monovinylidene complex [Nb-
(η5-C5H4SiMe3)2(dCdCHPh)(PMe2Ph)][BF4] (4d ; 70 mg, 0.10
mmol, 61%). IR (KBr): νCdC vinylidene 1622 cm-1
(CD3CN): δ 0.15, 0.21 (s, 18H, SiMe3), 1.79, 1.88 [d, J P-H
.
1H NMR
2
)
9.15 Hz, 6H, PPh(CH3)2], 5.26-5.34 (2H), 5.48-5.56 (2H),
5.70-5.78 (6H), 5.78-5.86 (2H), 5.98-6.06 (2H), 6.12-6.22
(2H) (m, C5H4SiMe3), 7.20-7.34 (m, 10H), 7.34-7.44 (m, 5H),
7.52-7.60 (m, 3H), 7.60-7.70 (m, 2H) [phenyl groups of
(C6H5)HCdCdNb and PMe2Ph], 7.45, 7.69 (s, 1H, dCdCHPh).
13C{1H} NMR (CD3CN, mixture of both endo and exo iso-
1
mers): δ 0.00 (SiMe3), 19.33 [d, J P-C ) 32.04 Hz, P(CH3)2-
1
Ph], 20.11 [d, J P-C ) 30.52 Hz, P(CH3)2Ph], 101.18, 103.35,
103.87, 105.63, 106.99, 107.23, 108.12, 108.89, 111.34, 112.63
(C5H4SiMe3), 128.01, 128.41 (C-4), 129.79, 129.95 (C-3 and -5),
131.49, 131.63 (C-2 and -6), 134.14, 136.36 (C-1) [phenyl groups
3
of (C6H5)HCdCdNb], 128.48 (d, J P-C ) 10.68 Hz, C-3 and
3
-5), 129.87 (d, J P-C ) 12.20 Hz, C-3 and -5), 130.19, 131.04
(d, 2J P-C ) 9.16 Hz, C-2 and -6), 131.49, 131.63 (s, C-4), 136.81,
1
138.29 (d, J P-C ) 41.90 Hz, C-1), (phenyl group of P(CH3)2-
Ph), 136.12, 136.48 (d, 3J P-C ) 3 Hz, NbdCdC), 366.55, 369.63
(broad, NbdCdC). Anal. Calcd for C32H43BF4NbPSi2: C, 55.34;
H, 6.24. Found: C, 53.99; H, 6.02.
Syn t h esis of [Nb (η5-C5H 4SiMe3)2(η2(C,C)-H CtCP h )-
(CO)]+ (5a ). The synthesis of this cationic terminal alkyne
complex was carried out through the facile conversion of the
monovinylidene [Nb(η5-C5H4SiMe3)2(dCdCHPh)(CO)]+ to the
title complex, 5a . Thus, a solution of [Nb(η5-C5H4SiMe3)2(d
CdCHPh)(CO)][BPh4] (4a [BPh4]; 1:1 mixture of both exo and
endo isomers (90 mg, 0.11 mmol)) in 5 mL of THF was stirred
for 24 h at room temperature. After the solvent was removed
in vacuo, an oily brown residue was obtained, which, after it
was washed twice with diethyl ether, gave a pale brown solid
identified as the exo isomer of [Nb(η5-C5H4SiMe3)2(η2(C,C)-
HCtCPh)(CO)][BPh4] (85 mg, 0.10 mmol, 95%). Formation of
the endo isomer was not observed. The product was recrystal-
lized from a mixture of acetonitrile and diethyl ether to give
pale yellow crystals. This reaction occurs readily at room
temperature, in solvents such as THF and CD3CN, to give the
alkyne complex 5a . The same reaction occurred when a
solution of [Nb(η5-C5H4SiMe3)2(dCdCHPh)(CO)][BF4] (4a[BF4])
Cs symmetry has been maintained in the geometry optimi-
zations. All stationary points were optimized with analytical
(40) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. A.; Petersson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; AndrJ s, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian
94; Gaussian Inc., Pittsburgh, PA, 1995.
(41) (a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms
and Molecules; Oxford University Press: Oxford, U.K., 1989. (b)
Ziegler, T. Chem. Rev. 1991, 91, 651.
(42) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(b) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (c) Stephens, P. J .;
Devlin, F. J .; Chabalowski, C. F.; Frisch, M. J . J . Phys. Chem. 1994,
98, 11623.
1
was stirred in THF. IR (KBr): νCtC 1772, νCO 2044 cm-1. H
NMR (CD3CN): δ 0.03 (s, 18H, SiMe3), 5.52-5.60 (2H), 5.62-
5.72 (2H), 6.30-6.38 (2H), 6.46-6.56 (2H), (m, C5H4SiMe3),
6.76-6.88 (m, 4H, p-C6H5), 6.92-7.04 (m, 8H, m-C6H5), 7.20-
7.32 (m, 8H, o-C6H5) (phenyl groups of BPh4-), 7.38 (s, 1H,
tCH), 7.40-7.60 (3H), 7.60-7.70 (2H) (m, C6H5 group of HCt
CC6H5). 13C{1H} NMR (CD3CN): δ -1.13 (SiMe3), 91.71,
103.06, 108.47, 108.76, 109.44 (C5H4SiMe3), 111.51 (HCt
CC6H5), 122.73, 126.54, 136.69 (phenyl groups of BPh4-),
126.61, 130.02, 131.13, 133.63 (phenyl group of the ligand HCt
(43) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(44) (a) Francl, M. M.; Pietro, W. J .; Hehre, W. J .; Binkley, J . S.;
Gordon, M. S.; Defrees, D. J .; Pople, J . A. J . Chem. Phys. 1982, 77,
3654. (b) Hehre, W. J .; Ditchfield, R.; Pople, J . A. J . Chem. Phys. 1972,
56, 2257. (c) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973,
28, 213.
CC6H5), 132.08 (HCtCC6H5), 164.74 (Cipso of phenyl groups of
-
13
11
BPh4 , q, J
) 49 Hz). A signal corresponding to the CO
C-
B
(45) See for example: (a) Gallo, M. M.; Hamilton, T. P.; Schaefer,
H. F. J . Am. Chem. Soc. 1990, 112, 8714. (b) Peterson, G. A.; Tensfeldt,
T. G.; Montgomery, J . A., J r. J . Am. Chem. Soc. 1992, 114, 6133. (c)
J ensen, J . H.; Morokuma, K.; Gordon, M. S. J . Chem. Phys. 1994, 100,
1981.
carbon was not observed. Anal. Calcd for C49H52BNbOSi2: C,
72.02; H, 6.37. Found: C, 71.95; H, 6.40.
Syn t h esis of [Nb (η5-C5H 4SiMe3)2(η2(C,C)-HCtCtBu )-
(CO)]+ (5c). A solution of [Nb(η5-C5H4SiMe3)2(dCdCHtBu)-

Alkynyl- and Vinylidene-Niobocene Complexes
Organometallics, Vol. 19, No. 9, 2000 1765
first derivatives. Transition states were located by means of
approximate Hessians and synchronous transit-guided quasi-
Newtonian methods.47 Most of the transition states located are
similar to those already characterized in previous theoretical
studies of the η2-alkyne f η1-vinylidene isomerization. For this
reason, and for computational limitations owing to the size of
the systems considered, we have not fully characterized them.
However, due to the differences between our transition state
for the 1,3-hydrogen migration and published transition states
for this mechanism, and also as an additional check, we did
reflections obeyed the condition I > 2σ(I). Data were corrected
in the usual fashion for Lorentz and polarization effects, and
a semiempirical absorption correction was applied.50 The
structure was solved by direct methods,51 and refinement on
F2 was carried out by full-matrix least-squares analysis.52
A
number of cycles of refinement with isotropic thermal param-
eters, followed by difference synthesis, enabled location of all
the non-hydrogen atoms. The SiMe3 groups are disordered, and
occupancies were refined initially and then fixed. Isotropic
temperature parameters were considered for these atoms and
anisotropic thermal parameters on the remaining atoms. The
hydrogen atoms were included in calculation positions but
were not refined. Weights were optimized in the final cycles.
Refinement converged at the following reliability values: R1
) 0.0577 (for reflections with I > 2σ(I)), S ) 0.898. Other
detailed data are supplied in the Supporting Information.
compute numerically the full Hessian for transition state
TS3n PH
.
3
Atomic charges have been calculated, maintaining the NBO
partitioning scheme.48 Bader’s topological analysis of the
electron density36 was performed using the XAIM 1.0 pro-
gram.49
X-r a y Da ta Collection for 5a . Crystals of 5a were grown
from a mixture of acetonitrile and diethyl ether. A suitable
crystal was sealed in a Lindeman capillary tube under dry
nitrogen and used for data collection. Reflections were collected
at 25 °C on a NONIUS-MACH3 diffractometer equipped with
a graphite-monochromated radiation source (λ ) 0.7107 Å).
Unit cell dimensions were obtained by a least-squares fit of
the 2θ values of 25 high-order reflections by using the MACH3
centering routines. Crystal are monoclinic, space group P21/c,
with Z ) 4: Mr ) 816.81, a ) 13.458(1) Å, b ) 13.544(9) Å, c
) 24.736(3) Å, â ) 94.51(1)°. The intensities of 10 826
reflections were collected (2 < θ < 28°); of these, only 3456
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Direccio´n General de Ensen˜an-
za Superior e Investigacio´n Cientif´ıca (Grant Nos. PB95-
0023-C01-C02 and PB95-0639-CO2) of Spain. The use
of computational facilities at the Centre de Supercom-
putacio` i Comunicacions de Catalunya (C4) is also
greatly appreciated.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, structure solution and refinement details, atomic coor-
dinates, bond lengths and angles, and anisotropic thermal
parameters for [Nb(η5-C5H4SiMe3)2(η2(C,C)-HCtCPh)(CO)]-
[BPh4] (5a [BPh4]). This material is available free of charge via
the Internet at http://pubs.acs.org.
(46) (a) Ervin, K. M.; Ho, J .; Lineberger, W. C. J . Chem. Phys. 1989,
91, 5974. (b) Ervin, K. M.; Gronert, S.; Barlow, S. E.; Gilles, M. K.;
Harrison, A. G.; Bierbaum, V. M.; De Puy, C. H.; Lineberger, W. C.;
Ellison, G. B. J . Am. Chem. Soc. 1990, 112, 5750. (c) Chen, Y.; J onas,
D. M.; Hamilton, C. E.; Green, P. G.; Kinsey, J . L.; Field, R. W. Ber.
Bunsen-Ges. Phys. Chem. 1988, 92, 329. (d) Chen, Y.; J onas, D. M.;
Kinsey, J . L.; Field, R. W. J . Chem. Phys. 1989, 91, 3976.
(47) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J . J . Comput.
Chem. 1996, 17, 49.
(48) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(49) This program was developed by J ose Carlos Ortiz and Carles
Bo, Universitat Rovira i Virgili, Tarragona, Spain.
OM990957E
(50) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351-359.
(51) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 435-
436.
(52) Sheldrick, G. M. Program for the Refinement of Crystal
Structures from Diffraction Data; University of Go¨ttingen, Go¨ttingen,
Germany, 1993.