Synthesis and reactivity of alkynyl niobocene complexes

Article abstract of DOI:10.1016/S0022-328X(02)02150-2

The synthesis of alkynyl containing niobocene complexes with differing auxiliary ligands is described. The niobium(III) derivatives of the general formula [Nb(η⁵-C₅H₄ SiMe₃)₂(C≡CR)(L)] where L is either carbonyl, phosphine, phosphite, or isocyanide, were prepared by the reaction of the bis(alkynyl)magnesium reagent with the corresponding cloro-niobocene precursor. In a similar manner the niobium(V) imido compounds, of the general formula, [Nb(=NR′)(η⁵- C₅H₄R″)₂(C≡CR)] were prepared. The characterization of these complexes is discussed. The reactivity of the alkynyl compounds towards oxidation and protonation has been studied.

Full text of DOI:10.1016/S0022-328X(02)02150-2

Journal of Organometallic Chemistry 670 (2003) 123ꢀ131
/
www.elsevier.com/locate/jorganchem
Synthesis and reactivity of alkynyl niobocene complexes
Antonio Antinolo a, Cristina Garc´ıa-Yebra b, Mariano Fajardo c, Isabel del Hierro c,
˜
Carmen Lo´pez-Mardomingo b, Isabel Lo´pez-Solera a, Antonio Otero a,*,
Yolanda Pe´rez c, Sanjiv Prashar c
a Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Universidad de Castilla-La Mancha, Facultad de Qu´ımicas, Campus Universitario,
13071 Ciudad Real, Spain
b Departamento de Tecnolog´ıa Qu´ımica, Ambiental y de los Materiales, E.S.C.E.T., Universidad Rey Juan Carlos, 28933 Mostoles, Madrid, Spain
c Departamento de Qu´ımica Orga´nica, Campus Universitario, Universidad de Alcala´, 28871 Alcala de Henares, Madrid, Spain
Received 9 October 2002; received in revised form 28 November 2002; accepted 30 November 2002
Abstract
The synthesis of alkynyl containing niobocene complexes with differing auxiliary ligands is described. The niobium(III)
derivatives of the general formula [Nb(h5-C5H4SiMe3)2(Cꢀ
/
CR)(L)] where L is either carbonyl, phosphine, phosphite, or isocyanide,
were prepared by the reaction of the bis(alkynyl)magnesium reagent with the corresponding cloro-niobocene precursor. In a similar
CR)], were prepared. The
manner the niobium(V) imido compounds, of the general formula, [Nb(ꢁ
/
NR?)(h5-C5H4Rƒ)2(Cꢀ
/
characterization of these complexes is discussed. The reactivity of the alkynyl compounds towards oxidation and protonation has
been studied.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Alkynyl; Niobocene; Protonation; Imido; Vinylidene
1. Introduction
compared with our reactivity studies of group 5 metal
alkynyl complexes, discussed in this paper.
The chemistry of d-block transition metals with
alkynyl [1], vinylidene [2], and alkyne [3] ligands have
been well documented with the main focus being on the
late transition elements. Early transition metal alkynyl
chemistry has centred on the group 4 elements titanium
and zirconium. The reactivity studies have been con-
centrated on ‘tweezer complexes’ and the formation by
During the past few years several families of alkyne
niobocene complexes have been reported by our group
[5]. Following from this work, we have expanded our
research into the synthesis and reactivity of alkynyl
containing niobocene complexes and in this paper we
present a review of our work in this field [6]. In addition,
we include the synthesis and characterization of some
previously unpublished alkynyl niobocene complexes.
coordination to the unsaturated alkynyl Cꢂ
/
C triple
bonds of late transition metals to give hetero early-late
dinuculear compounds. Lang et al. have recently
published an extensive review in this field [4]. Despite
this abundance of group 4 metal alkynyl complexes,
little work has been carried out into the reactivity taking
2. Results and discussion
2.1. Synthesis of alkynyl niobocene complexes
place (i.e. protonation) directly at the Cꢂ
/
C triple bond
and coupled with the limited redox chemistry of the
group 4 compounds means that they cannot be directly
The reaction of the Grignard reagent BrMg(Cꢀ
with [Nb(ꢁ
NBut)(h5-C5H5)2Cl] gave the alkynyl com-
plex [Nb(ꢁ CMe)] (1) as the unique
NBut)(h5-C5H5)2(Cꢀ
/
CMe)
/
/
/
product (Eq. (1)). However, this was the only case in
which the reaction yielded the desired alkynyl product
and when this substitution reaction was repeated with
* Corresponding author.
E-mail address: Antonio.Otero@uclm.es (A. Otero).
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(02)02150-2

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A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ131
/
˜
other starting niobocene chloride complexes, and with a
variety of Grignard alkynyl reagents the reaction proved
to be unsuccessful. However, good results were obtained
NBut)(h5-C5H5)2(Cꢀ
CH2Ph (4)), [Nb(ꢁ
(Rꢁ
SiMe3 (5), But (6)), [Nb(ꢁ
Me3)2(CꢀCR)] (Rꢁ
SiMe3 (7), But (8)), [Nb(ꢁ
C5H4SiMe3)2(CꢀCR)] (Rꢁ
NC6H4Me-4)(h5-C5H4SiMe3)2(Cꢀ
But (12)), [Nb(ꢁNC6H4OMe-4)(h5-C5H4SiMe3)2(Cꢀ
CR)] (Rꢁ
SiMe3 (13), But (14)), which were obtained,
after appropriate work up, as air-sensitive yellow
crystalline solids uncontaminated by starting materials
(Eq. (2)).
In order to prepare ansa-niobocenes containing
simultaneously both imido and alkynyl ligands, we
have carried out analogous reactions to those described
/
CR)] (Rꢁ
/
SiMe3 (2), But (3),
/
NC6H4Me-4)(h5-C5H5)2(Cꢀ
/CR)]
/
/
NBut)(h5-C5H4Si-
when bis-alkynyl magnesium derivatives, Mg(Cꢀ
/CR)2,
/
/
/
NPh)(h5-
were used in this process. The magnesium bis-alkynyl
compounds are normally more reactive than their
Grignard counterparts and in the synthesis of the
alkynyl derivatives this factor is critical to avoid
contamination of the desired product with unreacted
starting material. We have observed that when the
reactions were carried out in refluxing THF the process
did not always go to completion and a mixture of both
starting material and the corresponding alkynyl-con-
taining complex was isolated. This is probably due to
/
/
Ph (9), But (10)), [Nb(ꢁ
/
/
CR)] (Rꢁ
/
SiMe3 (11),
/
/
/
the partial deactivation of Mg(Cꢀ
/
CR)2 caused by THF
in the preparation of 2ꢀ
appropriate starting materials with the corresponding
CR)2 reagents gave rise to the isolation of
/
14. Thus the reactions of the
coordination. This problem was overcome by conduct-
ing all the reactions in a non-coordinating solvent, such
as toluene. In this solvent, the corresponding reactions
Mg(Cꢀ
complexes
(Rꢁ
SiMe3 (15), But (16)), (Eq. (3)).
/
[Nb(ꢁ
/
NBut){Me2Si(h5-C5H4)2}(Cꢀ
/CR)]
at 100 8C afford the alkynyl complexes, [Nb(ꢁ
/
/
ð1Þ
ð2Þ

A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ
/
131
125
˜
ð3Þ
In a similar manner the niobocene (III) alkynyl
[Li(Cꢀ
/
CH)(H2NCH2CH2NH2)], or Grignard reagents,
complexes [Nb(h5-C5H4SiMe3)2(Cꢀ
/
CR)(L)] (Rꢁ
CO (18); Rꢁ
But, Lꢁ
PMe2Ph (20); RꢁPh, LꢁP(OEt)3 (21);
CNC6H3Me2-2,6 (22); Rꢁ
But, Lꢁ
/
Ph,
such as MgCl(Cꢀ
/CH) in THF, were employed as
Lꢁ
(19); Rꢁ
RꢁSiMe3, Lꢁ
/
CO (17); Rꢁ
/
SiMe3, Lꢁ
/
/
/CO
alternatives, but their reaction with [Nb(h5-C5H4Si-
Me3)2Cl(L)] 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 18 by reaction with Bu4NF in THF [7]. However, the
complex, [Nb(h5-C5H5)2(Cꢀ
which the cyclopentadienyl rings have been desilylated,
was isolated instead (Eq. (6)). A further desilylation of
/
Ph, Lꢁ
/
/
/
/
/
/
/
CNC6H3Me2-2,6 (23)) were prepared (Eq. (4)) (See
Section 3 for preparation and characterization of the
new compounds 22 and 23). The contrasting electron
effects exerted by the different types of ligands should be
the primary factor in the reactivity of the alkynyl
complexes.
/CSiMe3)(CO)] (24), in
ð4Þ
Alternatively the synthesis of the alkynyl compounds
17 and 19 can be achieved by the treatment of the
cationic alkyne niobium complexes with KOBut (Eq.
(5)).
24 using K2CO3 in methanol [8] was successful, giving
the terminal alkynyl-containing complex [Nb(h5-
C5H5)2(Cꢀ
/
CH)(CO)] (25) (Eq. (6)).
The general procedures described for the preparation
of alkynyl niobocene complexes does not, however,
2.2. Characterization of alkynyl niobocene complexes
allow the synthesis of terminal alkynyl species (Rꢁ
since the preparation of the reagent Mg(CꢀCH)2 is
not possible. Different lithium reagents, such as
/
H),
1ꢀ
troscopy showed a band, at 2050ꢀ
to the n(CꢀC) of the alkynyl ligand and for the imido
/
25 were characterized spectroscopically. IR spec-
2110 cmꢁ1, assigned
/
/
/

126
A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ
/
131
˜
ð5Þ
ð6Þ
16, a band between 1230 and 1300 cmꢁ1
distance of 1.20(1) A which is typical for a Cꢂ
bond and the NbꢂC(1)ꢂ
linear nature confirms that the C(1) and C(2) atoms are
/
C triple
˚
complexes, 1ꢀ
/
characteristic of the terminal imido ligand [9]. In
addition the carbonyl containing complexes, 17, 18, 24
and 25, one band was observed in the IR spectra
between 1890 and 1950 cmꢁ1 corresponding to n(CO)
and for the isocyanide derivates 22 and 23 at 2279 and
/
/
C(2) angle of 177.9(6)8 whose
˚
N bond distance of 1.804(5) A,
sp hybridized. The Nbꢂ
/
is at the upper limit for those observed for niobium
˚
1.80 A) and is indicative of a
imido complexes (1.73ꢀ
/
2034 cmꢁ1, respectively, corresponding to n(Cꢀ
1H NMR spectra of the unsubstituted cyclopentadienyl
complexes 1ꢀ6 exhibited the expected singlet for the
cyclopentadienyl ligand. In addition for 1ꢀ3, 5 and 6
singlets were observed for the alkynyl substituents Me,
CH2Ph a
/
N). The
NbꢂN triple bond [10].
/
The near linear nature of the imide ligand indicates
that it acts as a four electron donor and thus the formal
electron count of 9 is 20 electrons with the excess 2
electrons probably being located in a non-bonding
orbital similar to that proposed by Green et al. for
/
/
SiMe3 or But and in the case of 4 where Rꢁ
/
[Mo(ꢁ
NBut)(h5-C5H5)2] [11].
/
singlet was observed for the CH2 protons and various
multiplets for the phenyl ring. When the trimethylsilyl
substituted cyclopentadienyl complexes 7ꢀ
/
14 and 17ꢀ23
/
2.3. Reactivity of alkynyl niobocene complexes
are considered their 1H NMR spectra are similar to
those of their unsubstituted analogues. The Cs symmetry
We have considered the chemical oxidation processes
of complexes 17, 19 and 20 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 when 17, 19, and 20 were reacted with the
ferrocenium salt in a 1:1 molar ratio, in CH2Cl2 at
of 7ꢀ
/
14 and 17ꢀ23 results in the appearance of four
/
multiplets for the C5 ring protons. In addition, the 13C
NMR spectra of all these complexes show two char-
acteristic resonances for the alkynyl Ca and Cb carbons
at ca. 110 and 128 ppm, respectively. Furthermore, in
the carbonyl and isocyanide containing complexes the
resonance of the carbonyl carbon atom appears at ca.
250 ppm and for the isocyanide carbon at ca. 230 ppm.
The molecular structure of [Nb(ꢁ
/
NPh)(h5-
ꢁ30 8C, the reaction gave the radical cationic alkynyl
/
+ ꢂ
complexes [Nb(h5-C5H4SiMe3)2(Cꢀ
/
CR)(L)] [BPh4]ꢁ
Ph, Lꢁ
PMe2Ph (28)) (Eq. (7)). The species 26 and 27 were
C5H4SiMe3)2(CꢀCPh)] (9) was established by X-ray
/
(RꢁPh, LꢁCO (26); Rꢁ CO (27); Rꢁ
/
/
/
But, Lꢁ
/
/
/
crystal diffraction studies and is the only example of a
group 5 alkynyl complex. The molecular structure is
shown in Fig. 1. 9 presents a typical bent metallocene
structure with two additional ligands. Both cyclopenta-
unstable, giving rise to the corresponding monovinyli-
dene complexes [Nb(h5-C5H4SiMe3)2(ꢁ
/
Cꢁ
/
CHR)(L)]-
But, Lꢁ
result from a hydrogen abstraction from the solvent (Eq.
dienyl rings are h5 bonded to the metal with the Centꢀ
NbꢀCent angle of 128.28. The alkynyl ligand is sigma
bonded to the metal as indicated by the C(1)ꢂC(2)
/
[BPh4] (Rꢁ
/
Ph, Lꢁ
/
CO (29); Rꢁ
/
/CO (30), which
/
/
(8)). 29 was isolated as a 1:1 mixture of both the exo and

A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ
/
131
127
˜
Fig. 1. Molecular structure of [Nb(ꢁ
/
NPh(h5-C5H4SiMe3)2(Cꢀ
CPh)] (9).
/
endo isomers and 30 as a single isomer. To confirm this
abstraction process, the oxidation of 17 was carried out
(Eq. (8)). Some related mononuclear vinylidene tanta-
locene and niobocene complexes have previously been
described [6c,12].
However, when the oxidation of 17 was carried out in
THF at room temperature, an alternative product, the
in dry CD2Cl2 at ꢁ
/
40 8C and the deuterated complex
CꢁCDPh)(CO)][BPh4] (31) (Eq.
[Nb(h5-C5H4SiMe3)2(ꢁ
/
/
2
(9)) was isolated (as established by a H NMR spectro-
scopy), proving that the solvent is the source of the H+
(or D+) radical.
divinylidene complex [(h5-C5H4SiMe3)2(CO)Nbꢁ
/
Cꢁ
/
C(Ph)(Ph)Cꢁ
/
Cꢁ
/
Nb(CO)(h5-C5H4SiMe3)2)][BPh4]2 (33)
Compound 28, however, was stable under these
experimental conditions and was isolated as a deep red
crystalline material after appropriate workup. Solutions
of 28 in CH2Cl2 evolve slowly at room temperature to
give, after several weeks, the monovinylidene complex
was obtained (Eq. (10)). Surprisingly, the formation of
the corresponding divinylidene species from compounds
19 and 20 has never been observed, even though the
oxidation has been attempted under a wide variety of
conditions. The formation of the divinylidene species 33
[Nb(h5-C5H4SiMe3)2(ꢁ
/
Cꢁ/CHPh)(PMe2Ph)][BPh4] (32)
can be envisaged as being the result of a ligandꢀ
/
ligand
ð7Þ

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A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ131
/
˜
ð8Þ
ð9Þ
ð10Þ
coupling reaction from the radical cationic alkynyl
species 26 instead of the alternative process of hydrogen
atom abstraction. The chemistry of 17-electron organo-
metallic radicals is well-documented, and their often
characteristic chemical properties continue to be re-
ported [13]. It is well-known that several organometallic
radicals tend to dimerize and that dimerization through
the metal centre is most often observed. Nevertheless,
C5Me5)2(Cꢀ
/
CR)(dppe)] (Rꢁ
/
Ph, But) did not undergo
dimerization or hydrogen atom abstraction, and the 17-
electron radical cationic alkynyl complexes [Fe(h5-
+ꢂ
C5Me5)2(Cꢀ
/
CR)(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 26 and 27 were
unstable, and this could be due to the fact that the
substituents on the alkynyl group did not provide
sufficient steric protection to prevent the transformation
to the corresponding vinylidene complexes 4. This
process of hydrogen atom abstraction has been estab-
lished in the evolution of several organometallic radicals
[16].
the ligandꢀligand coupling could be favoured if the
/
metal centre is sterically protected and if one of the
ligands possesses a p-system to enable the delocalization
of the spin density. Thus, Lapinte and co-workers have
demonstrated that the ironꢀ
C5Me5)2(CꢀCH)(dppe)] reacts with [FeCp2][PF6] to give
a divinylidene complex through a ligandꢀligand cou-
/
ethynyl complex [Fe(h5-
/
/
pling step [14]. However, they observed that the
analogous substituted alkynyl complexes [Fe(h5-
Furthermore, complexes 29, 30 and 32 could alter-
natively be prepared by the protonation with HBF4 of

A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ
/
131
129
˜
complexes 17, 19 and 20, respectively (Eq. (11)). This
protonation reaction is reversible, and treatment of 29,
30 or 32 with KOBut quantitatively gives 17, 19 or 20,
respectively (Eq. (11)).
species [Nb(NHBut)(h5-C5H5)2(Cꢀ
[Nb(NHPh)(h5-C5H4SiMe3)2(Cꢀ
plexes were isolated as red crystalline very air-sensitive
/
CSiMe3)]ꢂ (34) and
/
CPh)]ꢂ (35). The com-
solids after appropriate work-up. 34ꢀ35 were character-
/
ð12Þ
ð11Þ
1
The reactivity of the alkynyl imido complexes 1ꢀ
/
16
ized by IR and H NMR spectroscopy. Bands at 2090
cmꢁ1 for 34 and 2071 cmꢁ1 for 35, assigned to the n(Cꢀ
with the protic reagents CF3COOH and HBF4 has been
studied. Protonation of these complexes may occur at
the alkynyl and/or the imido moiety as observed
previously in complexes containing only one of these
ligands [6a,17]. In nearly all cases the final product
could not be identified due probably to the evolution of
the desired protonated species although a residual broad
/
C) of the alkynyl ligand, were observed in the IR
spectra. Broad signals at 8.55 ppm for 34 and 8.22
ppm for 35, due to the proton bonded to the nitrogen
1
atom, were also observed in the H NMR spectra. In
addition in 34 the signal due to the But group (1.45 ppm)
is shifted downfield with respect to the signal observed
in the parent complex (1.01 ppm) thus reflecting the
change from imide to amide of the nitrogen containing
ligand. The normally greater Bronsted basicity of the
imido ligand relative to the alkynyl ligand may explain
the selective nature of the protonation reaction of these
signal, that may correspond to its Nꢂ
/
H proton, was
observed in the 1H NMR spectrum indicating that
amide-containing species may have initially been
formed. However, in the reactions of 2 and 9 with
CF3COOH and HBF4, respectively, the protonated
compounds could be isolated (Eq. (12)). It was found
that in these imido/alkynyl complexes protonation takes
place preferentially at the nitrogen atom of the imido
group leading to the formation of the cationic amide
alkynylꢀimido niobocene complexes.
/
In conclusion we present the first general synthetic
procedure for (s-alkynyl) niobocene complexes. These
complexes have been oxidized, and we have observed

130
A. Antinolo et al. / Journal of Organometallic Chemistry 670 (2003) 123ꢀ
/
131
˜
that the nature of the resulting products, radical alkynyl,
vinylidene, or divinylidene species depend on the sub-
stituent R, on the alkynyl ligand, the nature of the
ancillary ligand L, and the experimental conditions. In
addition protonation processes of the alkynyl niobocene
imido complexes indicate that the reaction takes place
preferentially at the imido group, giving rise to the
corresponding amido complexes.
2.37 (s, 6H, C6H3Me2), 4.80, 5.07, 5.18, 5.49 (m, 2H,
C5H4). 13C{1H} NMR (300 MHz, C6D6): d 1.9 (SiMe3),
20.0 (C6H3Me2), 33.3 (CC(CH3)3), 33.5 (C(CH3)3), 93.5,
97.2, 99.5, 104.1 (C5H4), 91.2 (C ꢀ
/
CNb), 128.8 (Cꢀ
/
CNb), 126ꢀ132 (C6H3Me2). Anal. Calc. for
/
C31H44NNbSi2: C, 64.22; H, 7.65; N, 2.42. Found C,
64.00; H, 7.59; N, 2.38.
Acknowledgements
3. Experimental
We gratefully acknowledge financial support from the
Direccio´n General de Ensenanza Superior e Investiga-
3.1. Materials and procedures
˜
cio´n Cient´ıfica, Spain (Grants. No. PB 98-0159-C02-01-
02, No. BQU2000-0463) and the Universidad Rey Juan
Carlos (projects PIPR-02-09 and PIPR-02-02).
All reactions were performed using standard Schlenk-
tube techniques in an atmosphere of dry nitrogen.
Solvents were distilled from appropriate drying agents
and degassed before use. The synthesis and detailed
characterization data of the compounds 1ꢀ
[6b], 17 [6c], 18ꢀ21 and 24ꢀ33 [6a] can be found in our
previous publications. IR spectra were recorded on a
Perkin-Elmer PE 883 IR spectrophotometer. 1H and 13
spectra were recorded on a Varian FT-300 spectrometer
and referenced to the residual deuteriated solvent.
Microanalyses were carried out with a Perkin-Elmer
2400 microanalyzer.
/
16, 34ꢀ35
/
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/
/
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/
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/
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˜
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/
(0.45 g, 85%). IR (Nujol mull): nCN 2034 cmꢁ1; nCꢀC
2279 cmꢁ1. 1H NMR (300 MHz, C6D6): d 0.23 (s, 18H,
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˜
SiMe3), 0.30 (s, 9H, Cꢀ
4.77, 5.01, 5.15, 5.45 (m, 2H, C5H4). 13C{1H} NMR (300
MHz, C6D6): d 1.1 (SiMe3), 1.9 (CꢀCSiMe3), 20.0
(C6H3Me2), 94.0, 97.5, 99.1, 103.1 (C5H4), 91.6 (C ꢀ
CNb), 126ꢀ132 (C6H3Me2). Anal.
/CSiMe3), 2.37 (s, 6H, C6H3Me2),
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/
/
I. Orive, A. Otero, S. Prashar, J. Organomet. Chem. 655 (2002)
63.
CNb), 128.8 (Cꢀ
/
/
Calc. for C30H28NNbSi3: C, 60.47; H, 7.44; N, 2.35.
Found C, 60.28; H, 7.36; N, 2.33.
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3.3. Synthesis of [Nb(h5-C5H4SiMe3)2(Cꢀ
/
CBut)(CNC6H3Me2-2,6)] (22)
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The synthesis of 22 was carried out in an identical
CBut)2 (0.20 g, 1.07 mmol)
Lo´pez-Mardomingo, A. Mart´ın, P. Go´mez-Sal, Organometallics
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manner to that of 21. Mg(Cꢀ
/
and [Nb(h5-C5H4SiMe3)2Cl(CNC6H3Me2-2,6)] (0.41 g,
0.77 mmol). Yield 0.46 g, 80%. IR (Nujol mull): nCN
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