Oxo and imido/imido exchange and C-H activation reactions based on pentamethylcylopentadienyl imido tantalum complexes

Article abstract of DOI:10.1016/S0022-328X(99)00593-8

Reactions of [TaCp*Cl₄] with two, three and four equivalents of LiNH^tBu give the halo- and amido-imido complexes [TaCp*Cl₂(N^tBu)] (1a), [TaCp*Cl(N^tBu)(NH^tBu)] (2) and [TaCp*(N^tBu)(NH^tBu)₂] (3), respectively. The related complex [TaCp*Cl₂{N(2,6-Me₂C₆H₃)}] (1b) is prepared by a similar reaction using two equivalents of Li[NH(2,6-Me₂C₆H₃)]. Complex 3 can be transformed into 2 and further into 1a by reaction with SiClMe₃. Complex 1a reacts with CN^tBu to give the 18-electron adduct [TaCp*Cl₂(N^tBu)(CN^tBu)] (4) whereas addition of excess CN(2,6-Me₂C₆H₃) results in reductive elimination of the carbodiimide ^tBuN=C=N(2,6-Me₂C₆H₃) (5) to give [TaCp*Cl₂{CN(2,6-Me₂C₆H ₃)}₃]. However complex 1b does not react with any of the isocyanide ligands. Both complexes 1a and 1b react with PhCHO undergoing imido/oxo exchange to give the imines PhCH=NR (R=^tBu, 2,6-Me₂C₆H₃ (6)) and dimeric [TaCp*Cl₂(O)]₂ or trimeric [(TaCp*Cl)₃(μ₂-Cl)(μ₂-O) ₃(μ₃-O)] oxo-complexes, whereas only 1a reacts with CO₂, PhCH=NR′ (R′=Ph, Me) and (2,6-Me₂C₆H₃)N=C=N^tBu producing ^tBuN=CO, PhCH=N^tBu and ^tBuN=C=N^tBu, respectively and the corresponding oxo or imido tantalum derivative. None of the complexes reacts with CO or NCR (R=Me, Ph). The complex [TaCp*Me(N^tBu)(NH^tBu)] activates C-H bonds when heated in benzene and toluene affording [TaCp*Ph(N^tBu)(NH^tBu)] (7) and a mixture of [TaCp*(m-MeC₆H₄)(N^tBu)(NH^tBu)] 8a and [TaCp*(p-MeC₆H₄)(N^tBu)(NH^tBu)] (8b). All of the reported organic compounds and tantalum complexes were characterized by ¹H- and ¹³C-NMR spectroscopy.

Full text of DOI:10.1016/S0022-328X(99)00593-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 597 (2000) 61–68
Oxo and imido/imido exchange and C–H activation reactions
based on pentamethylcylopentadienyl imido tantalum complexes
Pascual Royo *, Javier Sa´nchez-Nieves
Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Alcala´, Edificio de Farmacia, Campus Uni6ersitario, E-28871 Alcala´ de Henares, Spain
Received 16 June 1999; received in revised form 21 September 1999
This paper is dedicated to Professor S. Pasynkiewicz on the occasion of his 70th birthday
Abstract
Reactions of [TaCp*Cl4] with two, three and four equivalents of LiNHtBu give the halo- and amido-imido complexes
[TaCp*Cl2(NtBu)] (1a), [TaCp*Cl(NtBu)(NHtBu)] (2) and [TaCp*(NtBu)(NHtBu)2] (3), respectively. The related complex
[TaCp*Cl2{N(2,6-Me2C6H3)}] (1b) is prepared by a similar reaction using two equivalents of Li[NH(2,6-Me2C6H3)]. Complex 3
can be transformed into 2 and further into 1a by reaction with SiClMe3. Complex 1a reacts with CNtBu to give the 18-electron
adduct [TaCp*Cl2(NtBu)(CNtBu)] (4) whereas addition of excess CN(2,6-Me2C6H3) results in reductive elimination of the
carbodiimide tBuNꢀCꢀN(2,6-Me2C6H3) (5) to give [TaCp*Cl2{CN(2,6-Me2C6H3)}3]. However complex 1b does not react with any
of the isocyanide ligands. Both complexes 1a and 1b react with PhCHO undergoing imido/oxo exchange to give the imines
PhCHꢀNR (R=tBu, 2,6-Me2C6H3 (6)) and dimeric [TaCp*Cl2(O)]2 or trimeric [(TaCp*Cl)3(m2-Cl)(m2-O)3(m3-O)] oxo-complexes,
t
whereas only 1a reacts with CO2, PhCHꢀNR% (R%=Ph, Me) and (2,6-Me2C6H3)NꢀCꢀNtBu producing BuNꢀCO, PhCHꢀNtBu
t
and BuNꢀCꢀNtBu, respectively and the corresponding oxo or imido tantalum derivative. None of the complexes reacts with
CO or NCR (R=Me, Ph). The complex [TaCp*Me(NtBu)(NHtBu)] activates C–H bonds when heated in benzene and tol-
uene affording [TaCp*Ph(NtBu)(NHtBu)] (7) and a mixture of [TaCp*(m-MeC6H4)(NtBu)(NHtBu)] 8a and [TaCp*(p-
1
MeC6H4)(NtBu)(NHtBu)] (8b). All of the reported organic compounds and tantalum complexes were characterized by H- and
13C-NMR spectroscopy. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Oxo/imido imido/imido exchange; C–H activation; Cyclopentadienyl imido tantalum complexes
1. Introduction
cesses [4] of great interest in organic synthesis using
imido complexes of the late and early transition metals.
Trapping reactions of the imidozirconocene complex
[(h5-C5H5)2Zr(NtBu)], its reactions with isocyanides and
isobutylene oxide and many applications in the
metathesis of ketones, imines, isocyanates, [2+2] cy-
cloadditions of olefins and alkynes and [2+3] cycload-
ditions of azides have been extensively reported [5].
Studies with related molybdenum complexes have also
been reported for the methathesis of carbodiimides and
isocyanates [6a,b] and more recently for the catalytic
metathesis of imines [6c,d], trying to avoid competing
dimerization processes that would deactivate the zirco-
nium catalyst or high concentrations of imine that
would inhibit it. Imido titanium [7] and zirconium [5c,8]
complexes have been shown to activate the carbon–hy-
drogen bond of hydrocarbons. Fewer studies have been
reported for [2+2] cycloaddition or carbon–hydrogen
Interest in the synthesis and isolation of transition
metal imido [1] complexes was prompted by the use of
imido substituents as ‘protecting ligands’ in many early
transition metal alkylidene complexes, extensively ap-
plied as catalysts in olefin metathesis [2] and ring-open-
ing metathesis polymerization [3] processes. However,
the imido group is not only an unreactive spectator
ligand but a highly nucleophilic center which can react
with metal coordinated unsaturated molecules to which
the imido group is easily transferred. This reaction is
the basis for many catalytic and stoichiometric pro-
* Corresponding author. Tel.: +34-1-885-4765; fax: +34-1-885-
4683.
E-mail address: proyo@inorg.alcala.es (P. Royo)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00593-8

62
P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
bond activation reactions of imido vanadium [9] [10]
and tantalum [11] complexes. We reported [12] several
reactions giving imido tantalum complexes and the
conversion of an imido acyl complex into the oxo
iminoacyl derivative.
[TaCp*Cl(NtBu)(NHtBu)] (2), isolated as a spectro-
scopically pure yellow oil, whereas addition of only two
or one equivalents of LiNHtBu afforded a mixture
containing variable amounts of 2, [TaCp*Cl2(NtBu)]
(1a) and small amounts of other unidentified products.
The same reaction using different lithium amides was
reported to afford other monocyclopentadienyl imi-
dotantalum complexes [15]. We also used this proce-
dure to isolate the previously reported [12c] [TaCp*-
Cl2{N(2,6-Me2C6H3)}] (1b) from the reaction of an
ethyl ether suspension of [TaCp*Cl4] with two equiva-
lents of Li[NH(2,6-Me2C6H3)] to give 1b as an orange
solid in 81% yield.
This article describes the results of our work on the
reactivity of imido tantalum complexes [TaCp*Cl2-
(NR)] (Cp*=h5-C5Me5; R=tBu (1a), 2,6-Me2C6H3
(1b)) with various unsaturated compounds including
isocyanides CNR (R=tBu, 2,6-Me2C6H3), benzalde-
hyde PhCHO, imines PhCHNR (R=Ph, Me), carbodi-
t
imides BuNCN(2,6-Me2C6H3) and CO2, as well as the
carbon–hydrogen bond activation of benzene and
toluene by the methyl complex [TaCp*Me(NtBu)-
(NHtBu)].
1
A preliminary reaction monitored by H-NMR spec-
troscopy revealed that addition of one equivalent of
SiClMe3 transformed the diamido complex [TaCp*-
(NtBu)(NHtBu)2] (3) into the chloro-amido derivative
[TaCp*Cl(NtBu)(NHtBu)] (2) with elimination of
SiMe3(NHtBu), and further addition of a second equiv-
alent quantitatively converted 2 into the dichloro com-
pound [TaCp*Cl2(NtBu)] (1a), which was stable in the
presence of excess SiClMe3.
2. Results and discussion
2.1. Synthesis of imido complexes
We were looking for a convenient method to prepare
a series of chloro-amido imidotantalum complexes
avoiding the method used to isolate [TaCp*Cl2(NtBu)]
[13], which is rather tedious when bulk quantities of
this product are required. We decided to go in the
reverse direction starting from the totally amidated
compound and exchanging amido for chloro sub-
stituents [14].
The reaction of [TaCp*Cl4] with four equivalents of
LiNHtBu in ethyl ether at room temperature provides
a good method to isolate [TaCp*(NtBu) (NHtBu)2]
(3) as a colorless solid in 85% yield (see Scheme 1). The
same reaction carried out with three equivalents
of LiNHtBu gave the chloro amido complex
These observations were used to develop a conve-
nient method to prepare [TaCp*Cl2(NtBu)] (1a) by
reacting an ethyl ether suspension of [TaCp*Cl4] with
more than two equivalents of LiNHtBu, followed by
the addition of excess SiClMe3 to give 1a as a yellow oil
in 72% yield.
The presence of the amido and imido groups in
compounds 1–3 can be easily assigned from the 13C-
NMR resonance due to the quaternary carbon of the
tBu group, which was observed at ca. l 55 for the
amido NHtBu and at ca. l 65 for the imido NtBu
groups. Moreover, the 1H-NMR spectra showed the
resonance due to the amido proton for 2 and 3 at l
5.64 and l 3.01 (two protons) respectively, indicating a
higher p-bonding contribution from the amido group in
2 with its more electronegative chloro substituent.
All of the complexes 1–3 were thermally stable and
water sensitive although they could be stored unaltered
for long periods under an inert atmosphere.
2.2. Reactions with isocyanides
The imido complex [TaCp*Cl2(NtBu)] (1a) coordi-
nated the isocyanide CNtBu to give the 18-electron
complex [TaCp*Cl2(NtBu)(CNtBu)] (4) as a yellow
solid, stable at room temperature under an inert atmo-
sphere and also stable in solution. The formation of
complex 4 and its structural behavior is analogous to
that reported for related niobium and tantalum adducts
[16]. The isocyanide ligand dissociated on heating a
hexane solution of 4 to 140°C, leaving the starting
compound 1a and an unidentified residue.
However, a different rapid reaction took place when
Scheme 1.
complex 1a was treated with four equivalents of

P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
63
Scheme 2.
CN(2,6-Me2C6H3) to afford the tantalum(III) complex
[TaCp*Cl2{CN(2,6-Me2C6H3)}3] [17] with simultaneous
elimination of the carbodiimide BuNꢀCꢀN(2,6-Me2-
cyanates with elimination of CO2, using phosphine
oxides [18] and imido vanadium [10] or tungsten [6a]
compounds as catalysts.
t
C6H3) (5) (see Scheme 2). This behavior reveals that the
higher electrophilic character of the coordinated aryl
isocyanide carbon atom favors nucleophilic attack on
the tert-butyl imido nitrogen to give an intermediate
h2-coordinated carbodiimide complex similar to those
reported for related zirconium [5c] and iridium [5b]
complexes, although its formation could not be de-
tected in solution by 1H-NMR spectroscopy. In the
presence of three equivalents excess of the isocyanide,
the diimide is reductively eliminated to give the final
18-electron tantalum(III) complex. The same reaction
using one equivalent of the isocyanide in the presence
of three equivalents of other ligands (THF, PMe3) or
using less than three equivalents of isocyanide gave
mixtures containing varying amounts of the unreacted
starting complex and final products. No reaction was
observed when the starting complex 1a was added to a
solution containing the tantalum(III) complex. Car-
bodiimides have also been synthesized by coupling iso-
The aryl imido complex [TaCp*Cl2{N(2,6-Me2-
C6H3)}] (1b), which has a much less nucleophilic imido
nitrogen atom due to the aromatic aryl substituent, did
not react with any of the isocyanide ligands, probably
also due to the protecting bulkier aryl group which
hinders an interaction with the vacant metal orbital
[16c].
The chemical behavior of complexes 1a and 1b was
also studied in reactions with CO and RCN (R=Me,
Ph). No reaction was observed after heating benzene-d6
solutions of both complexes in sealed tubes at 240°C
for several hours, the unchanged starting reagents being
recovered.
2.3. Reactions with benzaldehyde (PhCHO)
Both imido complexes 1a and 1b reacted with one
equivalent of PhCHO to give the [2+2] cycloaddition
[19] products, which could not be detected in solution

64
P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
as they were rapidly transformed by elimination of
imine PhCHꢀNR (R=tBu [20], 2,6-Me2C6H3 (6)) and
transfer of the oxo group to the metal (Scheme 2). The
reaction of complex 1a with the more nucleophilic
tert-butyl imido ligand was complete after 4 h at room
temperature eliminating PhCHꢀNtBu and the resulting
‘[TaCp*Cl2(O)]’ fragment dimerized to give [TaCp*-
Cl2(O)]2 [21], whereas complex 1b with a less nucle-
ophilic aryl imido ligand reacts more slowly, the reac-
tion being complete after 12 h at room temperature (or
1 h at 90°C). Under these conditions the resulting
‘[TaCp*Cl2(O)]’ fragment was hydrolytically trans-
formed into the trimer [(TaCp*Cl)3(m2-Cl)(m2-O)3(m3-O)]
[21,22]. Both imines were recovered by chromatography
and characterized by NMR spectroscopy.
was used as solvent. An equilibrium was observed when
PhCHꢀNMe was used, leading after 3 days at 100°C in
C6D6 to a mixture containing the starting materials, the
amido complex [TaCp*Cl2(NMe)] [24] and the imine
PhCHꢀNtBu [20]. CDCl3 is not recommended because
overlapping resonances in the NMR spectra militate
against adequate spectral analysis.
2.6. Reaction with carbodiimide
[NtBuꢀCꢀN(2,6-Me2C6H3)]
Complex 1a reacted with one equivalent of (2,6-
Me2C6H3)NꢀCꢀNtBu in a sealed NMR tube with ex-
change of the imido groups to give the imido complex
t
1b and BuNꢀCꢀNtBu after 12 h at 190°C. Formation
of the intermediate tantalum derivative containing the
triazamethylenemethane ligand [23a,25] formed by a
[2+2] cycloaddition of the carbodiimide could not be
observed as spontaneous metathesis gave the imido
complex containing the less-basic imido group.
2.4. Reaction with carbon dioxide (CO2)
Reactions of C6D6 solutions of complexes 1a and 1b
were studied in sealed NMR tubes filled with dry CO2
1
and monitored by H-NMR spectroscopy. Reaction of
complex 1a with the more electrophilic imido ligand
2.7. Reaction with amine RNH2 (R=2,6-Me2C6H3)
started at 90°C and was complete after heating the tube
1
to 140°C for 12 h to give a H-NMR spectrum with
Reaction of complex 1a containing the more-basic
NtBu imido group with the less basic RNH2 amine at
135°C for 8 days resulted in a similar exchange [26]
with elimination of the more basic BuNH2 amine to
give complex [TaCp*Cl2{N(2,6-Me2C6H3)}] (1b), con-
signals due to the oxotantalum trimer [22] and the
isocyanate BuNꢀCꢀO, formed by addition of CO2 to
the Ta–N bond [11b,23] (see Scheme 2). Formation of
the intermediate O,O%-carbamate complex [23a] was not
observed. The same reaction using the aryl imido com-
plex 1b was slower and not observed below 80°C,
always giving a mixture of components which could not
t
t
taining the less basic imido substituent (see Scheme 2).
2.8. C–H bond acti6ation reactions
1
be determined by the H-NMR spectra.
The formally 16-electron imido complexes 1a and 1b
cannot activate C–H bonds. However, the methyl
derivative [TaCp*Me(NtBu)(NHtBu)], obtained by re-
action of the chloro methyl complex with LiNHtBu
[27], gave the aryl compound [TaCp*Ph(NtBu)-
(NHtBu)] (7), and a mixture of [TaCp*R(NtBu)-
(NHtBu)] (R=m-MeC6H4 (8a), p-MeC6H4 (8b)) when
heated at 220°C in benzene and toluene, respectively,
with evolution of methane (see Scheme 3). These prod-
ucts resulted from activation of one ring Csp2–H bond
by the bis-imido derivative ‘[TaCp*(NtBu)2]’, which
could not be trapped by addition of bases such as THF,
pyridine or Et2O and whose formation could not be
detected.
No C–H bond activation was observed when cyclo-
hexane was used as solvent [28], although the bis-amido
compound [TaCp*(NtBu)(NHtBu)2] (3) was the major
reaction product, probably resulting from intermolecu-
lar rearrangement and decomposition in a dimethyl
complex [TaCp*Me2(NtBu)] simultaneously formed.
The new m- and p-toluoyl complexes were obtained
as an unresolvable mixture containing 8a/8b in a molar
ratio of ca. 1/4, deduced from the relative intensity of
the methyl and ring proton signals observed in its
2.5. Reaction with N-benzylideneimines (PhCHꢀNR,
R=Ph, Me)
The reaction of a C6D6 solution of complex 1a with
one equivalent of PhCHꢀNPh was monitored in a
1
sealed NMR tube by H-NMR spectroscopy (Scheme
2). No reaction was observed at room temperature, and
there was a very slow reaction on heating to 165°C
which was complete after 4 days. The resulting 1H-
NMR spectrum showed characteristic resonances due
to the known [17] imido complex [TaCp*Cl2(NPh)] and
the reported N-benzylidenetert-butylamine PhCHꢀ
NtBu [20]. It follows that the reaction comprised ex-
change of a more-donating by a less-donating TaꢀNR
imido group through [2+2] cycloaddition of the start-
ing imine, leading to the intermediate formation of a
2,4-diazametallacyclobutane [5d,e,6c,d], which could
not be observed. When excess N-benzylideneaniline
was used, the reaction was complete at 120 and
100°C when the molar ratios were 1/7 and 1/30,
respectively.
The temperature and time required to complete the
exchange reaction decreased (100°C, 48 h) when CDCl3

P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
65
1H-NMR spectrum in C6D6. The presence of the corre-
4. Experimental
sponding o-toluoyl complex was not observed.
1
The H-NMR spectrum of the aryl complex 7 shows
4.1. General methods
the amido proton resonance at l 5.71, whereas two
resonances were observed at l 5.59 and l 5.60 for 8a
and 8b, respectively, and a unique w(NH) IR absorption
appeared at 3320 cm−1. The 13C-NMR spectra of the
aryl complexes 7–8 show the expected resonances due
All manipulations were performed under inert atmo-
sphere of argon using standard Schlenk techniques or a
dry box. Solvents used were previously dried and
freshly distilled under argon: tetrahydrofuran (THF)
from sodium benzophenone ketyl; toluene from
sodium; hexane and pentane from sodium–potassium
alloy. Unless otherwise stated, reagents were obtained
from commercial sources and used as received. Thermal
reactions were carried out in a Roth autoclave with a
regulated temperature between 20 and 300°C.
t
to the quaternary Bu carbon of the amido and imido
groups at ca. l 55 and l 65, respectively, and the signal
due to the tantalum bound Cipso between l 181.3 and l
185.9.
3. Conclusions
IR spectra were recorded in KBr pellets, over the
range 4000–200 cm−1 on a Perkin–Elmer 583 spec-
trophotometer. 1H- and 13C-NMR spectra were
recorded on a Varian Unity VXR-300. Chemical shifts,
The imido tantalum complex [TaCp*Cl2(NtBu)]
(Cp*=h5-C5Me5) coordinates CNtBu to give the 18-
electron adduct [TaCp*Cl2(NtBu)(CNtBu)] but reacts
with CN(2,6-Me2C6H3) with reductive transfer of the
1
in ppm, are measured relative to residual H and 13C
resonances of solvents: 7.15 (1H, C6D6), 7.24 (1H,
CDCl3), and 128.0 (13C, C6D6) and 77.0 (13C, CDCl3),
and coupling constants are in Hz. C, H and N analyses
for all new compounds were carried out with a Perkin–
Elmer 240 C microanalyzer.
t
imido moiety to give the carbodiimide BuNꢀCꢀN(2,6-
Me2C6H3) and the tantalum(III) derivative [Ta(h5-
C5Me5)Cl2{CN(2,6-Me2C6H3)}3]. Similar reactions did
not take place with the related complex [TaCp*Cl2-
{N(2,6-Me2C6H3)}] containing a less nucleophilic imido
ligand. The imido complexes also react with PhCHO,
CO2 and PhCHNR (R=Ph, Me) through a [2+2]
cycloaddition to give respectively the keteneimine and
benzimine products resulting from exchange of the oxo
by the imine group and the imine by a more electroneg-
ative imine substituent under appropriate conditions.
The less-basic N(2,6-Me2C6H3) imido group is much
less reactive.
4.2. Synthesis of [Ta(p5-C5Me5)Cl2(NtBu)] (1a)
A solution of LiNHtBu (1.19 g, 15.06 mmol) in Et2O
(40 ml) was added slowly at room temperature (r.t.) to
a suspension of [TaCp*Cl4] (3.00 g, 6.55 mmol) in Et2O
(30 ml) and the mixture was stirred for 12 h. SiClMe3
(one equivalent) was then added and after 12 h of
stirring the solvent was removed under vacuum and the
residue was extracted into hexane (2×50 ml). The
solution was filtered and the volatiles were removed to
yield 1a as a yellow oil (2.16 g, 72%). 1H-NMR (CDCl3,
l, ppm): 1.21 (s, 9H, NtBu), 2.21 (s, 15H, C5Me5).
The methyl complex [TaCp*Me(NtBu)(NHtBu)] acti-
vates benzene and toluene C–H bonds when heated at
220°C to give the aryl compounds [TaCp*R(NtBu)
(NHtBu)] (R=Ph, m-MeC6H4, p-MeC6H4). This reac-
tivity is lower than that found for related more-elec-
trophilic tantalum complexes.
Scheme 3.

66
P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
4.3. Synthesis of [Ta(p5-C5Me5)Cl2{N(2,6-Me2C6H3)}]
(1b)
solvent was filtered off to give 4 as a yellow solid, which
was washed with pentane and dried under vacuum (0.24
g, 84%). IR (KBr, w, cm−1): 2200 (s, CN), 1350 (m,
1
t
A solution of LiNH(2,6-Me2C6H3) (1.67 g, 13.10
mmol) in Et2O (50 ml) was added slowly at r.t. to a
suspension of [TaCp*Cl4] (3.00 g, 6.55 mmol) in Et2O
(30 ml) and the mixture was stirred for 12 h. The
solution was filtered and volatiles were removed to give
1b as an orange solid, which was washed with hexane
(30 ml) and dried under vacuum (2.68 g, 81%). 1H-
NMR (CDCl3, l, ppm): 2.17 (s, 15H, C5Me5), 2.35 (s,
6H, Me2C6H3), 6.62 (t, J=7.65 Hz, 1Hpara, Me2C6H3),
6.96 (d, J=7.65 Hz, 2Hmeta, Me2C6H3).
TaN). H-NMR (CDCl3, l, ppm): 1.18 (s, 9H, Bu),
1.55 (s, 9H, CNtBu), 2.14 (s, 15H, C5Me5). 13C-NMR
(CDCl3, l, ppm): 12.15 (C5Me5), 29.82 (CMe3), 32.16
(CMe3), 57.67 (CMe3), 67.46 (CMe3), 119.10 (C5Me5),
151.15 (tBuNC). Anal. Calc. for C19H33Cl2N2Ta: C,
42.15; H, 6.16; N, 5.18. Found: C, 42.20; H, 6.14; N,
5.09%.
4.7. Reaction of [TaCp*Cl2(NtBu)] with CN(2,6-Me2-
C6H3). Synthesis of [TaCp*Cl2{CN(2,6-Me2C6H3)}3]
t
and BuNꢀCꢀN(2,6-Me2C6H3) (5)
4.4. Synthesis of [Ta(p5-C5Me5)Cl(NtBu)(NHtBu)] (2)
Isocyanide CN(2,6-Me2C6H3) (1.20 g, 9.15 mmol)
was added to a solution of [TaCp*Cl2(NtBu)] (1a) (1.00
g, 2.18 mmol) in hexane (25 ml) and the mixture was
stirred for 1 h at r.t. The color changed from yellow to
dark red leaving a red solid, which was separated by
filtration and washed with hexane (2×20 ml) to be
characterized as the reported tantalum(III) complex
A solution of LiNHtBu (0.51 g, 6.55 mmol) in Et2O
(25 ml) was added slowly at r.t. to a suspension of
[TaCp*Cl4] (1.00 g, 2.18 mmol) in Et2O (15 ml) and the
mixture was stirred overnight. The solvent was removed
under vacuum and the residue was extracted into hex-
ane (40 ml) to give a solution which was then filtered.
Complex 2 was isolated as a yellow oil after removing
the volatiles under vacuum. IR (KBr, w, cm−1): 3330
1
(1.53 g, 90%). H-NMR (C6D6, l, ppm): 2.14 (s, 15H,
C5Me5), 2.51 (s, 6H, 2,6-Me2C6H3), 2.53 (s, 12H, 2,6-
Me2C6H3), 6.60–6.90 (m, 9H, 2,6-Me2C6H3). The solu-
tion was passed through a column of silica gel and then
eluted with THF to give a solution, which after removal
of the solvent under reduced pressure rendered a color-
less solid identified as tBuNꢀCꢀN(2,6-Me2C6H3) (5)
1
(w, N–H), 1269 (s, TaꢀN). H-NMR (CDCl3, l, ppm):
1.22 (s, 18 H, TaꢀNtBu and NHtBu), 2.07 (s, 15 H,
C5Me5), 5.64 (s, 1 H, NH). 13C-NMR (CDCl3, l, ppm):
11.6 (C5Me5), 33.3 (CMe3), 34.3 (CMe3), 55.7
(NHCMe3), 65.0 (TaꢀNCMe3), 116.8 (C5Me5). Anal.
Calc. for C18H34ClN2Ta: C, 43.68; H, 6.94; N, 5.66%.
Found: C, 42.98; H, 6.51; N, 5.33%.
1
(0.35 g, 79%). H-NMR (CDCl3, l, ppm): 1.36 (s, 9H,
CMe3), 2.34 (s, 6H, 2,6-Me2C6H3), 6.70–6.80 (m, 3H,
2,6-Me2C6H3). 13C-NMR (CDCl3, l, ppm): 19.03 (2,6-
Me2C6H3), 31.33 (CMe3), 45.06 (CMe3), 123.96,
128.04, 132.31, 137.01 (2,6-Me2C6H3), 296.85 (NCN).
4.5. Synthesis of [Ta(p5-C5Me5)(NtBu)(NHtBu)2] (3)
A solution of LiNHtBu (1.42 g, 18.00 mmol) in Et2O
(50 ml) was added slowly at r.t. to a suspension of
[TaCp*Cl4] (2.00 g, 4.37 mmol) in Et2O (20 ml) and the
mixture was stirred overnight. The solvent was removed
under vacuum and the residue was extracted into hex-
ane (50 ml). After filtration the volatiles were removed
to yield 3 as a white solid (1.97 g, 85%). IR (KBr, w,
cm−1): 3326 (w, N–H), 1264 (s, TaꢀN). 1H-NMR
(CDCl3, l, ppm): 1.20 (s, 18H, NHtBu), 1.27 (s, 9H,
TaꢀNtBu), 1.99 (s, 15H, C5Me5), 3.01 (s, 2H, NH).
13C-NMR (CDCl3, l, ppm): 11.6 (C5Me5), 33.9
(TaꢀNCMe3), 35.1 (NHCMe3), 53.7 (NHCMe3), 64.4
(TaꢀNCMe3), 114.3 (C5Me5). Anal. Calc. for
C22H44N3Ta: C, 49.70; H, 8.36; N, 7.91%. Found: C,
49.72; H, 8.11; N, 7.56%.
4.8. Reaction of [TaCp*Cl2(NtBu)] with PhCHO
A solution of [TaCp*Cl2(NtBu)] (1a) (0.50 g, 1.09
mmol) and PhCHO (1.30mmol) in hexane (15 ml) was
stirred for 4 h at r.t. The suspension was allowed to
settle and filtered, leaving a yellow solid that was
identified as [TaCp*Cl2(O)]2 (0.40 g, 90%). 1H-NMR
(C6D6, l, ppm): 2.22 (C5Me5).
The resulting solution was passed through a silica gel
column and eluted with THF. The volatile components
were removed under vacuum and PhCHꢀNtBu was
obtained as a white oil (0.15 g, 78%). 1H-NMR (CDCl3,
l, ppm): 1.36 (s, 9H, CMe3), 7.40–7.45 (m, 3H, C6H5),
7.78–7.84 (m, 2H, C6H5), 8.32 (s, 1H, NꢀCH).
4.6. Synthesis of [Ta(p5-C5Me5)Cl2(NtBu)(CNtBu)] (4)
4.9. Reaction of [TaCp*Cl2{N(2,6-Me2C6H3)}] with
PhCHO
t
Isocyanide BuNC (0.65 mmol) was added to a solu-
tion of [TaCp*Cl2(NtBu)] (1a) (0.25 g, 0.53 mmol) in
hexane (10 ml) at r.t. The solution quickly became
cloudy and after 2 h stirring was stopped and the
A mixture of [TaCp*Cl2{N(2,6-Me2C6H3)}] (1b)
(0.50 g, 0.99 mmol) and PhCHO (1.15 mmol) in toluene

P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
67
(15 ml) was stirred for 12 h at r.t. (or for 6 h at 90°C),
the color changed from orange to yellow. The solvent
was then removed under vacuum and n-hexane (15 ml)
was added. The suspension was allowed to settle and
filtered leaving a yellow solid, which was identified as
1H-NMR did not show a change until the tube was
heated at 135°C; the reaction was complete after 8 days
t
with the formation of BuNH2 and [TaCp*Cl2{N(2,6-
1
Me2C6H3)}] (1b) (\95% by NMR). H-NMR (C6D6,
l, ppm): 1.83 (s, 15H, C5Me5), 2.45 (s, 6H, 2,6-
Me2C6H3), 6.66 (t, 1H, C6H5), 6.96 (d, 2H, C6H5).
[Ta3Cp*Cl4O4] (0.30 g, 81%). 1H-NMR (CDCl3, l,
3
ppm): 2.23 (s, 15 H, C5Me5), 2.21 (s, 30 H, C5Me5).
The resulting solution was passed through a silica gel
column and eluted with THF. After removing the
volatile components PhCHꢀN(2,6-Me2C6H3) (6) was
obtained as a white oil (0.17 g, 85%). 1H-NMR (CDCl3,
l, ppm): 2.13 (s, 6H, 2,6-Me2C6H3), 6.91–7.05 (m, 3H,
2,6-Me2C6H3), 7.45 (m, 3H, C6H5), 7.88 (m, 2H, C6H5),
8.21 (s, 1H, NꢀCH). 13C-NMR (CDCl3, l, ppm): 18.2
(2,6-Me2C6H3), 127.2, 128.0, 128.6, 128.9, 129.70,
131.6, 134.4, 135.8 (2,6-Me2C6H3 and C6H5), 162.8
(C=N).
4.14. Reaction of [TaCp*Cl2(NtBu)] with
tBuNꢀCꢀN(2,6-Me2C6H3)
An NMR tube was charged with a solution of
[TaCp*Cl2(NtBu)] (1a) (0.010 g, 2.18×10−5 mol) and
tBuNꢀCꢀN(2,6-Me2C6H3) (2.20×10−5 mol) in C6D6.
The 1H-NMR did not show a change until the tube was
heated at 180°C in an autoclave; the reaction was
complete after 10 h at 190°C with the formation of
[TaCp*Cl2{N(2,6-Me2C6H3)] (1b) and tBuNꢀCꢀNtBu
(80% by NMR). 1H-NMR (C6D6, l, ppm): 1.16 (s,
4.10. Reaction of [TaCp*Cl2(NtBu)] with CO2
18H, Bu).
t
An NMR tube was charged with a solution of
[TaCp*Cl2(NtBu)] (1a) in C6D6 and sealed under CO2.
The 1H-NMR showed no change until the tube was
heated at 100°C. The reaction was complete after 12 h
at 140°C giving [Ta3Cp3*Cl4O4] (78% by NMR) and
4.15. Synthesis of [TaCp*Ph(NtBu)(NHtBu)] (7)
A solution of [TaCp*Me(NtBu)(NHtBu)] (2.0 g, 4.22
mmol) in C6H6 (15 ml) was heated in an autoclave for
3 days at 220°C. The volatiles were then removed under
vacuum, hexane (2×30 ml) was added and the solution
was filtered. Removal of solvents to 5 ml and cooling at
−20°C yielded 7 as a white solid (1.72 g, 76%). IR
1
tBuNCO (92% by NMR). H-NMR (C6D6, l, ppm):
0.90 (CMe3).
4.11. Reaction of [TaCp*Cl2(NtBu)] with PhCHꢀNPh
(KBr, w, cm−1): 3318 (w, N–H), 1268 (s, TaꢀN). H-
1
NMR (CDCl3, l, ppm): 1.24 (s, 18H, CMe3), 1.93 (s,
15H, C5Me5), 5.71 (s, 1H, NH), 6.99 (t, J=6.9 Hz,
1Hpara, C6H5), 7.10 (m, 2Hmeta, C6H5), 7.41 (d, J=6.6
Hz, 2Hortho, C6H5). 13C-NMR (CDCl3, l, ppm): 11.5
(C5Me5), 33.6 (CMe3), 34.6 (CMe3), 55.6 (NHCMe3),
64.8 (TaꢀNCMe3), 115.3 (C5Me5), 124.4, 126.5, 141.4,
185.9 (C6H5). Anal. Calc. for C24H39N2Ta: C, 53.73; H,
7.33; N, 5.22. Found: C, 53.79; H, 7.16; N, 5.15%.
An NMR tube was charged with a solution of
[TaCp*Cl2(NtBu)] (1a) (0.010 g, 2.18×10−5 mol) and
1
PhCHꢀNPh (2.20×10−5 mol) in C6D6. The H-NMR
spectrum did not show a change until the tube was
heated at 165°C in an autoclave; the reaction was
complete after 5 days to give PhCHꢀNtBu and
1
[TaCp*Cl2(NPh)] (93% by NMR). H-NMR (C6D6, l,
ppm): 1.85 (s, 15H, C5Me5), 6.77 (t, 1H, C6H5), 6.96 (d,
2H, C6H5), 7.23 (t, 2H, C6H5).
4.16. Synthesis of [TaCp*R(NtBu)(NHtBu)]
(R=m-MeC6H4 (8a), p-MeC6H4 (8b))
4.12. Reaction of [TaCp*Cl2(NtBu)] with PhCHꢀNMe
A solution of [TaCp*Me(NtBu)(NHtBu)] (2.0 g, 4.22
mmol) in toluene (15 ml) was treated by the procedure
described in Section 4.15 to yield a white solid, which
was identified as a mixture of 8a and 8b (total yield 1.81
g, 78%). IR (KBr, w, cm−1): 3320 (w, N–H), 1272 (s,
TaꢀN). 1H-NMR (CDCl3, l, ppm): 1.24 (s, 36H,
CMe3), 1.92 (s, 30H, C5Me5), 2.24 (s, 6H, MeC6H4),
5.59 (s, 1H, NH), 5.60 (s, 1H, NH), 6.80 (8a, d, J=7.3
Hz, 1Hpara, MeC6H4), 7.00 (8a, m, 1Hmeta, MeC6H4),
7.19 (8a, d, J=7.3 Hz, 1Hortho, MeC6H4), 7.25 (8a, s,
An NMR tube was charged with a solution of
[TaCp*Cl2(NtBu)] (1a) (0.010 g, 2.18×10−5 mol) and
PhCH=NMe (2.20×10−5 mol) in C6D6. The 1H-
NMR spectrum did not show a change until the tube
was heated at 80°C, reaching an equilibrium mixture
of 1a, imine PhCHꢀNMe, [TaCp*Cl2(NMe)] and
PhCHꢀNtBu.
4.13. Reaction of [TaCp*Cl2(NtBu)] with
NH2(2,6-Me2C6H3)
1Hortho, MeC6H4), 6.92 (8b, d, J=7.5 Hz, 2Hmeta
,
MeC6H4), 7.29 (8b, d, J=7.5 Hz, 2Hortho, MeC6H4).
13C-NMR (CDCl3, l, ppm): 11.5 (C5Me5), 21.4
(MeC6H4), 21.8 (MeC6H4), 33.6 (CMe3), 34.6 (CMe3),
55.6 (NHCMe3), 64.8 (TaꢀNCMe3), 115.3 (C5Me5),
An NMR tube was charged with a solution of
[TaCp*Cl2(NtBu)] (1a) (0.010 g, 2.18×10−5 mol) and
NH2(2,6-Me2C6H3) (2.18×10−5 mol) in C6D6. The

68
P. Royo, J. Sa´nchez-Nie6es / Journal of Organometallic Chemistry 597 (2000) 61–68
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5.00%.
Acknowledgements
The authors acknowledge DGICYT (project PB97-
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MEC for a fellowship.
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