Competitive insertion of isocyanide and carbon dioxide into niobium- and silicon-amido bonds

Article abstract of DOI:10.1021/om0103963

Reaction of the amido-niobium complex [Nb{η⁵-C₅H₄SiMe₂ (NH^tBu)}(N^tB)Cl (NH^tBu)] (2) with 1 equiv of CNAr (Ar = 2,6-Me₂C₆H₃) in hexane gave the im

Full text of DOI:10.1021/om0103963

Organometallics 2001, 20, 4623-4631
4623
Com p etitive In ser tion of Isocya n id e a n d Ca r bon Dioxid e
in to Niobiu m - a n d Silicon -Am id o Bon d s
M. Isabel Alcalde, M. Pilar Go´mez-Sal, and Pascual Royo*
Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Alcala´,
Campus Universitario, E-28871 Alcala´ de Henares, Spain
Received May 14, 2001
Reaction of the amido-niobium complex [Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] (2)
with 1 equiv of CNAr (Ar ) 2,6-Me2C6H3) in hexane gave the iminocarbamoyl compound
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl{η2-C(NHtBu)dNAr}] (5) in quantitative yield at room
temperature, whereas heating to 120 °C was required when similar reactions were carried
out with the bridged silyl amido compounds [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)Cl] (3) and
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(NHtBu)] (4) to give the bridging [Nb{η5-C5H4SiMe2(NtBu)-
η2-(CdNAr)}(NtBu)Cl] (6) and terminal iminocarbamoyl [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)-
{η2-C(NHtBu)dNAr}] (7) compounds, respectively. Benzylation of 6 with Mg(CH2Ph)2‚2THF
provided the benzyl η2-iminocarbamoyl compound [Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)-
(CH2Ph)] (8), which rearranges to the silyl-η1-amido η2-iminoacyl derivative [Nb{η5-C5H4-
SiMe2(η1-NtBu)}(NtBu){η2-C(CH2Ph)dNAr}] (9) on heating its C6D6 solution to 145 °C.
Reaction of compound 2 with CO2 gave the dicarbamato complex [Nb{η5-C5H4SiMe2(η1-
OOCNHtBu)}(η2-OOCNHtBu)(NtBu)Cl] (10), whereas the same reaction with [Nb{η5-C5H4-
SiMe2(NHtBu)}(NtBu)Cl2] (1) gave an unidentified mixture of products, which were
transformed after heating to 130-140 °C into the oxo derivative [{Nb(NtBu)Cl2}2{η5-C5H4-
SiMe2-O-SiMe2-η5-C5H4}] (11), which was further benzylated to give [{Nb(NtBu)(CH2Ph)2}2-
{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (12). Similar reaction of 3 with CO2 in toluene gave the
oxo compounds [Nb(NtBu)Cl-µ-(η5-C5H4SiMe2-η-O)]2 (13) and [{Nb(NtBu)Cl}2(µ-O){η5-C5H4-
SiMe2-O-SiMe2-η5-C5H4}] (14). The molecular structure of 13 was determined by X-ray
diffraction methods. Formation of the oxo complexes 11, 13, and 14 also resulted from
hydrolysis of [Nb(η5-C5H4SiMe2Cl)(NtBu)Cl2] in the presence of NEt3.
In tr od u ction
chelated η5-cyclopentadienyl-η-amido metal derivatives
when they are treated with alkali amides or amines
Structural, synthetic, and catalytic aspects of the
early transition metal cyclopentadienyl compounds have
received considerable attention in recent years, because
they are useful compounds that have found applications
as reagents in organic chemistry and particularly as
soluble Ziegler-Natta catalysts.1 The cyclopentadienyl
ring can be more than a mere spectator ligand, with
different substituents introduced to modify the steric
and electronic requirements of the central metal atom,
particularly when such substituents contain reactive
functional groups. Many studies on silyl-substituted ring
compounds have been reported2 comparing the reactiv-
ity of the Si-Cl bond of chlorosilyl-cyclopentadienyl
complexes with the M-Cl bonds of group 4,3 5,4 and 65
metal derivatives. Moreover, some chlorosilyl-cyclo-
pentadienylmetal compounds have been used to isolate
under different conditions.3b,f,6
Cyclopentadienyl complexes of transition metals also
containing imido ligands have been extensively re-
ported,7 and the reactivity and applications of those
containing coordinatively unsaturated metal centers
have been particularly studied.8
We recently4 reported the isolation of the four types
of amino-substituted cyclopentadienyl-imido niobium
complexes shown in Scheme 1, which contain respec-
(3) (a) Ciruelos, S.; Cuenca, T.; Go´mez-Sal, P.; Manzanero, A.; Royo,
P. Organometallics 1995, 14, 177. (b) Ciruelos, S.; Cuenca, T.; Go´mez,
R.; Go´mez-Sal, P.; Manzanero, A.; Royo, P. Organometallics 1996, 15,
5577. (c) Ciruelos, S.; Cuenca, T.; Go´mez, R.; Go´mez-Sal, P.; Manza-
nero, A.; Royo, P. Polyhedron 1998, 17, 1055. (d) Ciruelo, G.; Cuenca,
T.; Go´mez, R.; Go´mez-Sal, P.; Mart´ın, A.; Rodr´ıguez, G.; Royo, P. J .
Organomet. Chem. 1997, 547, 287. (e) Royo, B.; Royo, P.; Cadenas, L.
M. J . Organomet. Chem. 1998, 551, 293. (f) Go´mez, R.; Go´mez-Sal, P.;
Mart´ın, A.; Nu´n˜ez, A.; Del Real, P. A.; Royo, P. J . Organomet. Chem.
1998, 564, 93.
(4) (a) Alcalde, M. I.; Go´mez-Sal, P.; Mart´ın, A.; Royo, P. Organo-
metallics 1998, 17, 1144. (b) Alcalde, M. I.; Go´mez-Sal, P.; Royo, P.
Organometallics 1999, 18, 546.
(5) De la Mata, F. J .; Giner, P.; Royo, P. J . Organomet. Chem. 1999,
572, 155.
* Corresponding author. Tel: 34-1-8854765. Fax: 34-1-8854683.
E-mail: pascual.royo@uah.es.
(1) (a) Davies, S. G. Organotransition Metal Chemistry: Applications
to Organic Synthesis; Pergamon Press: New York, 1989. (b) Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds. Comprehensive Organometallic
Chemistry II; Pergamon Press: New York, 1987. (c) Togni, A.,
Halterman, R. L., Eds. Metallocenes. Synthesis, Reactivity, Applica-
tions; Wiley-VCH: Weinheim, 1998; Vol. 1.
(6) Okuda, J .; Eberle, T.; Spaniol, T. P. Chem. Ber. 1997, 130, 209.
(7) (a) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31,
123. (b) Nugent, W. A. Inorg. Chem. 1983, 22, 965. (c) Wigley, D. E.
Prog. Inorg. Chem. 1994, 42, 239.
(2) Cuenca, T.; Royo, P. Coord. Chem. Rev. 1999, 193-195, 447, and
references therein.
10.1021/om0103963 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/05/2001

4624 Organometallics, Vol. 20, No. 22, 2001
Alcalde et al.
Sch em e 1
Sch em e 2
reactivity of terminal Si-N, Nb-N, and bridging Si-
N-Nb bonds leading to new µ-oxo dinuclear compounds.
Resu lts a n d Discu ssion
tively a single terminal NHtBu group bound to silicon,
1; two terminal NHtBu groups bound to both silicon and
niobium, 2; one bridging NtBu group between silicon and
niobium, 3; and the same complex containing one
additional NHtBu group bound to niobium, 4. While
insertion of isocyanides into Si-N bonds is not known
due to the thermodynamic stability of Si-N compared
with Si-C bonds, a few examples of insertion reactions
of isocyanides into metal-amido bonds giving the first
η2-iminocarbamoyl derivatives were reported, initially
for uranium9 and molybdenum10 complexes. More re-
cently similar reactions have been reported for cyclo-
pentadienyl titanium, zirconium, and tantalum com-
pounds.11 In contrast, the insertion of carbon dioxide
into both silicon-amido and transition metal-amido
bonds to afford carbamato compounds is well known.11a,12
Similar CO2 insertion reactions with η5-cyclopentadi-
enyl-η-amido titanium and zirconium complexes to give
oxo derivatives with elimination of the corresponding
isocyanate have been reported.3b,13
In ser tion of Isocya n id e. The reaction of 1 equiv of
the isocyanide CNAr (Ar ) 2,6-Me2C6H3) with the amido
niobium compound [Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)-
Cl(NHtBu)] (2) proceeds immediately in hexane at room
temperature. The η2-iminocarbamoyl derivative [Nb{η5-
C5H4SiMe2(NHtBu)}(NtBu)Cl{η2-C(NHtBu)dNAr}] (5)
resulting from insertion of the isocyanide into the
niobium-amido bond (Scheme 2) was isolated as a
brown oil, which was characterized by elemental analy-
sis and IR and NMR spectroscopy.
In contrast, insertion of CNAr into the bridging
silylamido-niobium bond of [Nb{η5-C5H4SiMe2(η1-
NtBu)}(NtBu)Cl] (3) (Scheme 3) required heating the
toluene solution at 120 °C for 3 days. The isolated dark
red product was the bridged silyl-η2-iminocarbamoyl
derivative [Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)-
Cl] (6), which was characterized by elemental analysis
and IR and NMR spectroscopy. When a mixture of
compound [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(NHtBu)]
(4) and 1 equiv of isocyanide was heated in hexane at
120 °C, an even slower transformation was observed
(Scheme 3). In this case, insertion into the terminal
niobium-amido bond gave, after 5 days, a 75% yield of
the η2-iminocarbamoyl derivative [Nb{η5-C5H4SiMe2(η1-
NtBu)}(NtBu){η2-C(NHtBu)dNAr}] (7), as a dark red oil.
No insertion into the bridged silylamido-niobium bond
was observed.
In this paper we report our results from insertion
reactions of 2,6-dimethylphenylisocyanide made to de-
termine the comparative reactivity of terminal Nb-N
and bridging Nb-N-Si bonds in compounds 1-4 to give
new iminocarbamoyl derivatives. Analogous insertion
reactions of carbon dioxide were studied to compare the
(8) (a) Lee, S. Y.; Bergman, R. G. J . Am. Chem. Soc. 1996, 118, 6396.
(b) Schaller, C. P.; Cummins, C. C.; Wolczansky, P. T. J . Am. Chem.
Soc. 1993, 32, 131. (c) de With, J .; Horton, A. D. Organometallics 1996,
15, 591. (d) Blake, R. E.; Antonelli, D. M.; Henling, L. M.; Schaefer,
W. P.; Hardcastle, K. I.; Bercaw, J . E. Organometallics 1995, 14, 1901.
(e) Royo, P.; Sa´nchez-Nieves, J . J . Organomet. Chem. 2000, 597, 61.
(f) Castro, A.; Galakhov, M. V.; Go´mez, M.; Sa´nchez, F. J . Organomet.
Chem. 1999, 580, 161. (g) Antin˜olo, A.; Fajardo, M.; Huertas, C.; Otero,
A.; Prashar, S.; Rodr´ıguez, A. M. J . Organomet. Chem. 1999, 585, 154.
(h) Humphries, M. J .; Green, M. L. H.; Leech, M. A.; Gibson, V. C.;
J olly, M.; Williams, D. N.; Elsegood, M. R. J .; Clegg, W. J . Chem. Soc.,
Dalton Trans. 2000, 4044. (i) Humphries, M. J .; Green, M. L. H.;
Douthwaite, R. E.; Rees, L. H. J . Chem. Soc., Dalton Trans. 2000, 4555.
(9) (a) Zanella, P.; Brianese, N.; Casellato, U.; Ossola, F.; Porchia,
M.; Rosseto, G.; Graziani, R. J . Chem. Soc., Dalton Trans. 1987, 2039.
(b) Dormond, A.; Aaliti, A.; Mo¨ıse, C. J . Chem. Soc., Chem. Commun.
1985, 1231.
The values of the chemical shift difference (∆δ)
between the quaternary and methyl carbons of the Bu
t
groups of different imido and amido ligands are given
in Table 1. These differences may provide information
about the π-bond contribution of these ligands.7,8 The
values of ∆δ found in complexes 1-4 for the niobium-
bonded imido ligand indicate that its π-donor character
decreases in the order 1 > 2 > 3 > 4. Therefore, the
imido ligand is a weaker π-donor when the number of
competitive amido ligands (terminal and bridging)
increases (4), consistent with what should be expected.
It is also less π-donating with one bridging (3) than with
one terminal (2) amido group. It can also be concluded
that the π-character of the Nb-NHtBu bonds is higher
than that of the Si-NHtBu bonds, consistent with the
more electrophilic character of Nb. Similarly, the π-char-
acter of the Nb-NHtBu bond is higher when no com-
petitive amido ligand is present (2 > 4). In each case
∆δ is higher when the amido NtBu group bridges two
acidic centers (Si and Nb).
(10) Chisholm, M. H.; Hammond, C. E.; Huffman, D.; Ho, J . C. J .
Am. Chem. Soc. 1986, 108, 7860.
(11) (a) Palacios, F. Doctoral Thesis, Universidad de Alcala´, 1998.
(b) Galakhov, M. V.; Go´mez-Sal, P.; Mart´ın, A.; Mena, M.; Ye´lamos,
C. Eur. J . Inorg. Chem. 1998, 1319. (c) Sa´nchez-Nieves, J .; Royo, P.;
Pellinghelli, M. A.; Tiripicchio, A. Organometallics 2000, 19, 3169. (d)
Thirupathi, N.; Yap, G. P. A.; Richeson, D. S. Organometallics 2000,
19, 2573.
(12) (a) Yin, X.; Moss, J . R. Coord. Chem. Rev. 1999, 181, 27. (b)
Pandey, K. K. Coord. Chem. Rev. 1995, 140, 37. (c) Behr, A. Angew.
Chem., Int. Ed. Engl. 1988, 27, 661. (d) Chisholm, M. H.; Extine, M.
W. J . Am. Chem. Soc. 1977, 99, 782. (e) Chisholm, M. H.; Extine, M.
W. J . Am. Chem. Soc. 1977, 99, 792. (f) Chisholm, M. H.; Tan, L.-S.;
Huffman, J . C. J . Am. Chem. Soc. 1982, 104, 4879. (g) Go´mez-Sal, P.;
Irigoyen, A. M.; Mart´ın, A.; Mena, M.; Monge, M.; Ye´lamos, C. J .
Organomet. Chem. 1995, 494, C19.
The ease of insertion into the amido-niobium bond
of complexes 2-4 is governed not only by the availability
of an empty orbital to accept isocyanide coordination but
also by the degree of steric congestion due to the number
of NtBu groups bound to the metal. In addition, inser-
(13) Kloppenburg, L.; Petersen, J . L. Organometallics 1996, 15, 7.

Niobium- and Silicon-Amido Bonds
Organometallics, Vol. 20, No. 22, 2001 4625
Sch em e 3
Ta ble 1. Va lu es of ∆δ for Va r iou s Im id o a n d Am id o Liga n d s
∆δ
Nb-OC-NHtBu
compound
NbdNtBu Nb-NHtBu Nb-NtBu-Si Si-NHtBu Nb-C(NAr)-NtBu
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl2] (1)4b
36.3
33.4
32.1
30.4
33.6
34.3
30.2
31.8
30.4
38.9
19.3
18.0
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] (2)4b
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)Cl] (3)4b
23.6
20.7
25.7
21.0
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(NHtBu)] (4)4b
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl{η2-C(NHtBu)dNAr}] (5)
[Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)Cl] (6)
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-C(NHtBu)dNAr}] (7)
[Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)(CH2Ph)] (8)
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-C(CH2Ph)dNAr}] (9)4b
[Nb{η5-C5H4SiMe2(η1-OOCNHtBu)}(η2-OOCNHtBu)(NtBu)Cl]
(10)
18.1
21.7
27.7
22.0
27.2
21.4
21.7
21.2; 21.6
[{Nb(NtBu)Cl2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (11)
[{Nb(NtBu)(CH2Ph)2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (12)
[Nb(NtBu)Cl-µ-(η5-C5H4SiMe2-η-O)]2 (13)
39.2
34.7
36.6
tion occurs more easily for terminal than for bridging
amido ligands. Consequently, insertion into the terminal
Nb-NHtBu group in compound 2 is easier (immediate
at room temperature) than into the bridging silylamido
ligand in compound 3 (3 days at 120 °C) and much
easier (5 days at 120 °C) than in compound 4, which
contains three NtBu systems. After the formation of the
18-electron compound 7, which contains the η2-imino-
carbamoyl ligand, no further insertion reaction occurs.
Benzylation of complex 6 with Mg(CH2Ph)2‚2THF in
toluene/hexane afforded the benzyl η2-iminocarbamoyl
derivative [Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)-
(CH2Ph)] (8), isolated in high yield as a brown solid
(Scheme 3). Complex 8 is a rare example of a niobium
compound containing a benzyl and an imino-carbamoyl
ligand. It is unlikely that 8 could be prepared by
reaction of the amido benzyl niobium derivative with
CNAr, due to preferential insertion of the isocyanide
into the niobium-benzyl bond to give the thermody-
namically more stable iminoacyl compound [Nb{η5-
C5H4SiMe2(η-NtBu)}(NtBu){η2-C(CH2Ph)dNAr}] (9), re-
ported previously.4b Complex 8 therefore offered a
unique opportunity to examine the comparative stability
of iminoacyl and iminocarbamoyl species. When a C6D6
solution of 8 was heated at 145 °C in a Teflon-valved
NMR tube, almost quantitative formation of the re-
ported iminoacyl 9 was observed (Scheme 3). Whereas
deinsertion of CO and CO2 from acyl and carbamato
ligands, respectively, is a well-documented process,14
deinsertion reactions of isocyanide from iminoacyl com-
plexes have received less attention.11a,15 Deinsertion of
isocyanide was demonstrated under these conditions
by the formation of an intermediate amido benzyl
compound [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(CH2Ph)],
which was observed together with the same decomposi-
tion products identified when a sample of free isocyanide
was heated at the same temperature. The amido benzyl
species was the major component of the mixture ob-
tained at higher temperature (160 °C), because the
dissociated isocyanide completely decomposed during
the reaction. However, 9 was the major component when
free isocyanide was added before heating. It can thus
be surmised that the reaction takes place by deinsertion
of the isocyanide followed by its reinsertion with benzyl
ligand migration.
The NMR data for all of the new η2-iminocarbamoyl
compounds described are consistent with that expected
1
for asymmetric molecules. The H NMR spectra show
resonances consistent with an ABCD spin system for
the cyclopentadienyl ring protons, two singlets for both
nonequivalent silicon methyls, and one singlet for each
type of tert-butyl group. The 13C NMR spectra are
particularly useful to differentiate the various types of
tert-butyl group. The 13C resonance of the quaternary
tert-butyl carbon is observed between δ 64-68 when this
group is bound to the imido ligand, at δ 49.7 when bound
(14) (a) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059. (b)
Chisholm, M. H.; Extine, M. W. J . Am. Chem. Soc. 1977, 99, 782. (c)
Chisholm, M. H.; Extine, M. W. J . Am. Chem. Soc. 1977, 99, 792 (d)
Chisholm, M. H.; Tan, L.-S.; Huffman, J . C. J . Am. Chem. Soc. 1982,
104, 4879.
(15) (a) Andre´s, R.; Galakhov, M.; Go´mez-Sal, M. P.; Mart´ın, A.;
Mena, M.; Santamar´ıa, C. Chem. Eur. J . 1998, 4, 1206. (b) Pizzano,
A.; Sa´nchez, L.; Altman, M.; Monge, A.; Ruiz, C.; Carmona, E. J . Am.
Chem. Soc. 1995, 117, 1759. (c) Fandos, R.; Meetsma, A.; Teuben, J .
H. Organometallics 1991, 10, 2665.

4626 Organometallics, Vol. 20, No. 22, 2001
Alcalde et al.
Sch em e 4
tion. Interactions of this type have been reported for
related organic substrates16 and organometallic com-
pounds.17
A similar insertion reaction should be expected for
complex 1, which contains only one Si-NHtBu bond.
However, the reaction of 1 with CO2 at room tempera-
ture resulted in the formation of a mixture of unidenti-
fied compounds, which on heating at 130-140 °C
afforded the µ-oxo dinuclear complex [{Nb(NtBu)Cl2}2-
{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (11). This transfor-
mation takes place with evolution of CO2 and simulta-
neous elimination of N,N′-di(tert-butyl)urea, (tBuNH)2CO,
which was identified by comparing its spectroscopic data
with those of a known sample.18
Ta ble 2. Ch em ica l Sh ifts of th e Ca r ba m a to P r oton
of 10
NHtBu (δ, C6D6)
carbamato group
bidentate
monodentate
25 °C
30 °C
40 °C
50 °C
4.72
4.72
4.73
4.75
4.74
4.72
4.72
5.52
5.36
5.21
5.08
4.97
5.52
4.82
A mechanism involving the formation of an unstable
intermediate carbamato compound resulting from inser-
tion of CO2 into the Si-NHtBu bond with further
intermolecular decomposition explains the behavior
observed (Scheme 5). An alternative method to synthe-
size 11 is described below. Complex 11 shows the NMR
data expected for a C2v symmetric molecule. The 1H
NMR spectrum exhibits an AA′BB′ spin system for the
ring protons and two singlets due to equivalent meth-
ylsilyl and tert-butyl groups. The IR absorption observed
at 1080 cm-1 due to the Si-O-Si bridge and the 13C
spectrum which showed the typical low-field resonance
(δ 69.5) of the quaternary tert-butyl imido carbon
confirmed this formulation. The value of ∆δ ) 39.2
observed for this compound is slightly lower than that
found4a for the related compound bearing the chlo-
rodimethylsilyl moiety (∆δ ) 40.1). This formulation
was also confirmed by preparing the tetrabenzyl deriva-
tive by treatment of a C6D6 solution of 11 in a NMR
tube with 4 equiv of Mg(CH2Ph)2‚2THF. The resulting
product [{Nb(NtBu)(CH2Ph)2}2{η5-C5H4SiMe2-O-SiMe2-
60 °C
3.6 × 10-2
M
M
7.2 × 10-3
to the Si-NHtBu amido ligand (5), and between δ 54
and 58 when bound to the Nb-NHtBu amido ligand and
is displaced to high field at δ 52.5 (5, 7) when the NHtBu
group migrates to give the π-delocalized system of the
η2-iminocarbamoyl ligand. However, no shift was ob-
served for the tert-butyl group bound to the bridging
iminocarbamoyl ligand in complexes 6 and 8 compared
with the values found4b for the bridging silyl-amido
ligand of related complexes, despite chain lengthening
and possible π-delocalization in the iminocarbamoyl
ligand. This chain lengthening is clearly evident in the
29Si NMR spectra, with a downfield shift of the silicon
resonance from δ -17.1 for 3 to δ 8.8 for 6. Similar
comparison of the silicon resonance observed for com-
plexes 4 (δ -20.8) and 7 (δ -22.0) confirms that
insertion of isocyanide into the terminal Nb-NHtBu
bond occurred, rather than insertion into the bridging
silyl-amido-Nb bond, which would resonate at ca. δ 8.0
as in complex 6. The iminocarbamoyl carbon resonance
is observed between δ 190.0 and 199.0 for all the
compounds bearing this ligand.
1
η5-C5H4}] (12) was characterized by H and 13C NMR
spectroscopy. The 1H NMR spectrum shows the presence
of the expected two doublets due to the diastereotopic
benzylic protons of two equivalent benzyl groups bound
to each metal center. The reaction of the bridged silyl-
amido complex 3 with CO2, monitored at room temper-
ature by 1H NMR spectroscopy in C6D6, showed the
presence of tert-butylisocyanate tBuNCO, characterized
by comparing its spectroscopic data with those of a pure
commercial sample. The same reaction was carried out
in toluene at a preparative scale (Scheme 6) to give,
after 6 days at room temperature, a solution from which
the dinuclear compound [Nb(NtBu)Cl-µ-(η5-C5H4SiMe2-
η-O)]2 (13) was isolated as a yellow crystalline solid,
identified by elemental analysis, NMR and IR spectros-
copy, and an X-ray diffraction study on a single crystal.
Figure 1 shows a view of the molecular structure of 13
with the atomic labeling scheme, and Table 3 sum-
marizes selected bond distances and angles.
In ser tion of Ca r bon Dioxid e. A reaction flask
containing a hexane solution of complex 2 was placed
under an atmosphere of carbon dioxide. An immediate
reaction occurred at room temperature to give the di-
carbamato compound [Nb{η5-C5H4SiMe2(η1-OOCNH-
tBu)}(η2-OOCNHtBu) (NtBu)Cl] (10) as a white solid in
good yield (Scheme 4). Under these conditions the
insertion of carbon dioxide occurred in both the Si-
NHtBu and Nb-NHtBu bonds. The carbamato carbon
resonance appeared at δ 153.3 in the 13C NMR spectrum
of 10, confirming η1-coordination of the silicon-bound
carbamato substituent and supported by the ν(COO) IR
absorption observed at 1694 cm-1. In contrast, the 13C
NMR signal corresponding to the η2-coordinate car-
bamato ligand bound to niobium was observed at lower
field (δ 167.4), with a ν(COO) IR absorption at 1587
(16) (a) Gennari, G.; Gude, M.; Potenza, D.; Piarulli, U. Chem. Eur.
J . 1998, 4, 1924. (b) Smith, M. D.; Claridge, T. D. W.; Tranter, G. E.;
Sansom, M. S. P.; Fleet, G. W. J . Chem. Commun. 1998, 18, 2041. (c)
Allott, C.; Adams, H.; Bernad, P. L., J r.; Hunter, C. A.; Rotger, C.;
Thomas, J . A. Chem. Commun. 1998, 22, 2449.
(17) (a) Biradha, K.; Desiraju, G. R.; Braga, D.; Grepioni, F.
Organometallics 1996, 15, 1284. (b) Brammer, L.; Mareque Rivas, J .
C.; Zhao, D. Inorg. Chem. 1998, 37, 5512. (c) Okamura, T.; Sakauye,
K.; Ueyama, N.; Nakamura. A. Inorg. Chem. 1998, 37, 6731. (d) Carr,
J . D.; Coles, S. J .; Hassan, W. W.; Hursthouse, M. B.; Abdul Malik, K.
M.; Tucker, J . H. R. J . Chem. Soc., Dalton Trans. 1999, 57.
(18) Legzdins, P.; Rettig, S. J .; Ross, K. J . Organometallics 1994,
13, 569.
1
cm-1. The H NMR spectrum shows that the chemical
shift of the amino proton of the monodentate carbamato
ligand is temperature dependent (see Table 2), indicat-
ing a hydrogen bridge interaction between this proton
and the oxygen of a second carbamato group. This
resonance was displaced to higher field as the concen-
tration of complex 10 was systematically reduced,
evidence for the intermolecular nature of this interac-

Niobium- and Silicon-Amido Bonds
Organometallics, Vol. 20, No. 22, 2001 4627
F igu r e 1. Perspective view of complex 13 showing the atom-labeling scheme.
Sch em e 5
Sch em e 6
The molecular structure of 13 corresponds to a di-
nuclear compound in which the two moieties [Nb(η5-
C5H4SiMe2)(NtBu)Cl] are related by an inversion center
and are bonded by two oxygen bridges between silicon
and niobium atoms from different units. This bonding
arrangement is similar to that observed for the related
dinuclear titanium complex [TiCl2-µ-(η5-C5H4SiMe2-η-
O)]2.3a The coordination around the niobium center in

4628 Organometallics, Vol. 20, No. 22, 2001
Alcalde et al.
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 13
the shorter 2.412(2) and 2.428(2) Å corresponding to the
two carbon atoms trans to the silyl group to the longer
2.543(2) Å corresponding to the carbon bearing the silyl
group. The Si atom is almost located in the Cp plane,
with a distance to this plane of 0.011 Å, as expected for
a Si-Csp2 bond. It is important to notice that the Cp-
Nb-L and L-Nb-L′ angles are very similar to those
found for the related monomeric [Nb{η5-C5H4-SiMe2-
(NHtBu)}Cl2(NtBu)],4b showing that the spatial distri-
bution of the ligands is not affected by the formation of
the dinuclear structure.
Nb(1)-N(1)
Nb(1)-O(1)
Nb(1)-Cl(1)
Nb(1)-C(4)
Nb(1)-C(3)
Nb(1)-C(5)
Nb(1)-C(2)
Nb(1)-C(1)
Si(1)-O(1)
Si(1)-C(11)
Si(1)-C(10)
Si(1)-C(1a)
N(1)-C(6)
1.7530(19)
1.9089(15)
2.3849(7)
2.412(2)
2.428(2)
2.463(2)
2.497(2)
2.543(2)
1.6375(16)
1.847(3)
1.859(3)
1.868(2)
1.456(3)
2.156
The 1H and 13C NMR spectra of C6D6 solutions of
compound 13 always show the presence of a second
isomer in a molar ratio of ca. 1:3. Both isomers show
Nb(1)-Cp(1)
1
N(1)-Nb(1)-O(1)
N(1)-Nb(1)-Cl(1)
O(1)-Nb(1)-Cl(1)
Si(1)-O(1)-Nb(1)
C(6)-N(1)-Nb(1)
N(1)-Nb(1)-Cp(1)
Cl(1)-Nb(1)-Cp(1)
O(1)-Nb(1)-Cp(1)
104.55(8)
100.19(7)
107.46(5)
156.32(11)
171.77(17)
118.94
similar sets of H and 13C NMR signals (see Experi-
mental Section). Despite having two chiral centers, this
second component cannot be identified as a diastereo-
mer of 13 with the two chloro and NtBu groups oriented
to the same side in an eclipsed disposition, because this
would be an asymmetric molecule with a very different
NMR behavior. According to the 1H and 13C NMR
spectra observed, this isomer has to be the dinuclear
compound [{Nb(NtBu)Cl}2(µ-O)}2{η5-C5H4SiMe2-O-SiMe2-
η5-C5H4}] (14). Simultaneous insertion of CO2 into either
the Nb-N or the Si-N bonds would lead to a mixture
of two intermediates A and B (Scheme 6), which are
further transformed by elimination of tBuNdCdO
through an intermolecular reaction. Reaction between
two A or two B units affords compound 13, whereas
formation of a mixture of 13 and 14 could result from
the reaction of one A and one B unit. Therefore,
complexes 13 and 14 could be generated during the
insertion process, although concentration and crystal-
lization only allows 13 to be recovered. This result could
also be explained by accepting that only one of the two
possible insertions into either the Nb-N or the Si-N
bonds takes place to give 13 as the unique reaction
product, which in solution could be transformed into 14
by the intramolecular rearrangement represented in
Scheme 6. Actually, the NMR spectrum observed for a
sample prepared with a selected crystal of 13 showed
the presence of both 13 and 14 in approximately the
same molar ratio 1:3. Moreover, when a mixture of both
13 and 14 was heated in the presence of ClSiMe3, a
clean reaction occurred to give pure 11. As 11 can only
be formed by reaction of 14 with ClSiMe321 and not from
13, it can be concluded that conversion of 13 into 14
must happen in solution. Nevertheless none of these
results negates the possibility that 14 is formed during
the insertion reaction.
111.23
112.86
each fragment is typical of half-sandwich imido com-
plexes and could be described as a three-legged piano-
stool structure with the bridging oxygen, the imido
nitrogen, and the chloro ligand being the other three
leg substituents. If the centroids of the silyl-substituted
rings are considered as coordination sites, the central
core of the dinuclear structure appears as an eight-
membered cycle, with the two niobium and the two
bridging oxygen atoms as the seat of a chair conforma-
tion. It is apparent from the bond distances that Nb-O
and Si-O π-bond contributions exist. The Nb-O bond
length (1.908(1) Å) is very similar to those found for
Nb-O-Nb systems in dinuclear niobium(V) compounds
without any other additional bridging groups, such as
19a
[(NbCp2Cl)2(µ-O)][BF4]2 (1.876 Å), [Nb{CpCl3(H2O)}2-
(µ-O)]19b (1.915 Å), and [Nb{(C5H4Me)Cl3(H2O)}2(µ-O)]19d
(1.926 Å). The Si-O distance (1.637(1) Å) is larger than
that found in the titanium derivative3a and similar to
those found in siloxanes. The Nb-O-Si angle (156.3-
(1)°) is slightly smaller than that found for Ti-O-Si
(160.2(1)°), and it is between those found for Si-O-Si
(ca. 145°) and Nb-O-Nb (ca. 170-180°) in similar types
of dinuclear compound.19 The imido ligand is located
trans to the silyl group and is in a staggered disposition
similar to that in [Nb{η5-C5H4-SiMe2(NHtBu)}Cl2-
(NtBu)],4b rather than the usually eclipsed orientation
found for most compounds of this type.8g,h,20 The Nb-
(1)-N(1) distance of 1.7530(19) Å is in the range
expected for a four-electron donor imido ligand and
similar to those found in other imido niobium com-
pounds. The imido group adopts an almost linear
disposition (C(6)-N(1)-Nb(1), 171.7(1)°). A remarkable
feature is the partial loss of the η5-character of the Cp
ligand, as evidenced by Nb-C distances, ranging from
Both oxo complexes 13 and 14 contain two identical
fragments related by one inversion center 13 and one
plane of symmetry 14 each containing one chiral metal
center, which makes the two methylsilyl groups and the
four ring protons nonequivalent (ABCD spin system).
The 1H and 13C NMR spectra (see experimental) always
contain a mixture of both components in a molar ratio
of ca. 1:3 and show two similar sets of signals which
are consistent with this formulation, confirmed in the
case of complex 13 by the X-ray diffraction study.
Complex 11 was also isolated by addition of 0.5 mol
of water to a toluene solution of [Nb {η5-C5H4SiMe2Cl}-
(19) (a) Prout, K.; Cameron, T. S.; Forder, R. A.; Critchley, S. R.;
Denton, B.; Rees, G. V. Acta Crystallogr., Sect. B 1974, 30, 2290. (b)
Bottomley, F.; Keizer, P. N.; White, P. S.; Preston, K. F. Organome-
tallics 1990, 9, 1916. (c) Kirilova, N. I.; Lemenovskii, D. A.; Baukova,
T. V.; Struchkov, Yu. T. Koord. Khim. 1977, 3, 1600. (d) Prout, K.;
Daran, J . C. Acta Crystallogr., Sect. B 1979, 35, 2882. (e) Green, M. L.
H.; Hughes, A. K.; Mountford, P. J . Chem. Soc., Dalton Trans. 1991,
1407.
(20) Willians, D. N.; Mitchell, J . P.; Poole, A. D.; Siemeling. U.;
Clegg, W.; Hockless, D. C. R.; O’Neil, P. A.; Gibson, V. C. J . Chem.
Soc., Dalton Trans. 1992, 739.
(21) Antin˜olo, A.; Carrillo-Hermosilla, F.; Ferna´ndez-Baeza, J .;
Lanfranchi, M.; Lara-Sa´nchez, A.; Otero, A.; Palomares, E.; Pelling-
helli, M. A.; Rodr´ıguez, A. M. Organometallics 1998, 17, 3015.

Niobium- and Silicon-Amido Bonds
Organometallics, Vol. 20, No. 22, 2001 4629
two µ-oxo dinuclear isomers, [Nb(NtBu)Cl-µ-(η5-C5H4-
SiMe2-η-O)]2 and [{Nb(NtBu)Cl}2(µ-O)}{η5-C5H4SiMe2-
O-SiMe2-η5-C5H4}]. Both isomers were always present
in solution; however X-ray diffraction methods identified
the former complex as the unique solid reaction product.
Sch em e 7
Exp er im en ta l Section
(NtBu)Cl2] in the presence of NEt3 (Scheme 7), demon-
strating that selective hydrolysis of the Si-Cl rather
than the Nb-Cl bond occurs. This result is consistent
with similar behavior found for reaction with a protic
Gen er a l Com m en ts. All manipulations were performed
under an inert atmosphere of argon using standard Schlenk
or drybox techniques. Solvents used were previously dried and
freshly distilled under argon: toluene from sodium, hexane
from sodium-potassium amalgam. Unless otherwise stated,
reagents were obtained from commercial sources and used as
received. Mg(CH2Ph)2‚2THF22 and 1-44 were prepared by
t
reagent such as NH2 Bu4b and contrasting with the
reactivity observed for [Nb{η5-C5H4SiMe2Cl}Cl4], when
hydrolysis is not selective and takes place at both Nb-
Cl and Si-Cl bonds simultaneously.4a The same hy-
drolysis carried out using a 1:1 compound 11/H2O molar
ratio resulted in formation of a mixture of compounds
11, 13, and 14. Two possible pathways can be proposed
to explain the formation of 13, namely, the selective
conversion of all the Si-Cl into Si-OH bonds and their
intermolecular reaction with the Nb-Cl bond of a
second molecule with elimination of HCl, or a selective
intermolecular hydrolysis to give 11 which requires 0.5
mol of water. Further intramolecular hydrolysis of the
Nb-Cl bonds of 11 with a second 0.5 mol of water gave
13 and 14.
1
reported methods. H and 13C NMR spectra were recorded on
Varian Unity FT-300 and Varian FT-500 Unity Plus instru-
ments, and chemical shifts were measured relative to the
residual 1H and 13C resonances of benzene-d6 used as solvent,
δ 7.15 (1H) and 128 (13C). IR spectra were recorded on a Perkin-
Elmer 583 spectrophotometer (4000-200 cm-1). C, H, and N
analyses were carried out with a Perkin-Elmer 240 C analyzer.
The elemental analysis for liquid compound 7, which partially
decomposed during sublimation, gave unacceptable deviation.
P r ep a r a tion of [Nb{η5-C5H4SiMe2(NHtBu )}(NtBu )Cl{η2-
C(NHtBu )dNAr }] (5) (Ar ) 2,6-Me2C6H3). An equimolar
mixture of[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] (2)
(0.80 g, 1.72 mmol) and isocyanide (0.22 g, 1.72 mmol) in
hexane (50 mL) was stirred at room temperature for 12 h, and
the solvent was then removed under vacuum to give a brown
oil in quantitative yield, which was characterized as 5. 1H
NMR (300 MHz, C6D6, δ): 0.56 (s, 3H, SiMe2), 0.62 (s, 3H,
SiMe2), 0.98 (s, 9H, CMe3), 1.22 (s, 9H, CMe3), 1.23 (s, 9H,
CMe3), 2.04 (s, 3H, Me2C6H3NdC), 2.09 (br s, 1H, NHSi), 2.34
(s, 3H, Me2C6H3NdC), 5.43 (br s, 1H, NHC), 5.96 (m, 1H,
C5H4), 6.33 (m, 1H, C5H4), 6.45 (m, 1H, C5H4), 6.80 (m, 1H,
C5H4), 6.90-6.98 (m, Ph). 13C NMR (75 MHz, C6D6, δ): 3.2,
3.8 (SiMe2), 19.0, 19.1 (Me2C6H3NdC), 30.8 (CNHCMe3), 31.6
(SiNHCMe3), 33.9 (NbNCMe3), 52.5 (CNHCMe3), 49.7 (SiNH-
CMe3), 67.5 (NbNCMe3), 105.5, 107.4, 115.9, 118.3 (C2-C5,
C5H4), 121.9 (Cipso C5H4), 126.0-140.0 (Ph, C6H3), 190.5 (Nd
C-NHCMe3). IR (ν cm-1): 3323, 1598, 1570, 1350. Anal. Calcd
for C28H48ClN4SiNb: C, 56.30; H, 8.12; N, 9.38. Found: C,
55.73; H, 7.86; N, 8.62.
Con clu sion s
Insertion of isocyanide CNAr (Ar ) 2,6-Me2C6H3) into
the terminal Nb-N bonds of amido complexes [Nb{η5-
C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] and [Nb{η5-C5H4-
SiMe2(η1-NtBu)}(NtBu)(NHtBu)] is more favorable than
insertion into the bridging Nb-N-Si amido system of
compound [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)Cl]. The
ease of insertion is controlled by the steric demands of
the number of NtBu groups present in the molecule.
These insertions lead to the formation of terminal
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl{η2-C(NHtBu)dN-
Ar}], [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-C(NHtBu)d
NAr}], and bridging [Nb{η5-C5H4SiMe2(NtBu)-η2-(Cd
NAr)}(NtBu)Cl] η2-iminocarbamoyl derivatives, respec-
tively. The benzyl η2-iminocarbamoyl-amidosilyl com-
pound [Nb{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)-
(CH2Ph)] is transformed into the η2-iminoacyl-η1-amido-
silyl derivative [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-
C(CH2Ph)dNAr}] by heating its C6D6 solution at 145
°C, one rare example of deinsertion of isocyanide.
Insertion of carbon dioxide takes place easily at room
temperature into the terminal Si-N amido bond of
compound [Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl2] to give
an unidentified mixture, which is transformed by heat-
ing at 130-140 °C into the µ-oxo dinuclear complex
[{Nb(NtBu)Cl2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] as a
unique reaction product with elimination of tert-butyl-
urea. Similar insertion takes place simultaneously into
the terminal Nb-N and Si-N amido bonds of [Nb{η5-
C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] to give the di-
carbamato derivative [Nb{η5-C5H4SiMe2(η1-OOCNH-
tBu)}(η2-OOCNHtBu)(NtBu)Cl]. A much slower insertion
of CO2 was observed into the bridging Si-amido-Nb
system of compound [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)-
P r ep a r a tion of [Nb {η5-C5H4SiMe2(NtBu )-η2-(CdNAr )}-
(NtBu )Cl] (6) (Ar ) 2,6-Me2C6H3). A mixture of [Nb{η5-C5H4-
SiMe2(η1-NtBu)}(NtBu)Cl], (3) (0.75 g, 1.91 mmol) and isocy-
anide (0.25 g, 1.91 mmol) in toluene (50 mL) was warmed to
120 °C for 3 days in a Teflon-valved Schlenk. The solvent was
removed under vacuum, and the resulting dark oil was washed
with hexane (2 × 10 mL) to give a dark red solid characterized
as 6. Concentration of the mother liquor and subsequent
cooling to -30 °C gave a second crop of 6 (0.85 g, 1.82 mmol,
1
85% yield). H NMR (300 MHz, C6D6, δ): 0.39 (s, 3H, SiMe2),
0.41 (s, 3H, SiMe2), 0.92 (s, 9H, CMe3), 1.19 (s, 9H, CMe3), 1.89
(s, 3H, Me2C6H3NdC), 2.37 (s, 3H, Me2C6H3NdC), 5.12 (m, 1H,
C5H4), 6.23 (m, 1H, C5H4), 6.35 (m, 1H, C5H4), 6.61 (m, 1H,
C5H4), 6.90-7.00 (m, Ph). 13C NMR (75 MHz, C6D6, δ): 0.0,
3.0 (SiMe2), 19.3, 19.5 (Me2C6H3NdC), 30.0 (SiNCMe3), 32.0
(NbNCMe3), 57.7 (SiNCMe3), 66.3 (NbNCMe3), 99.8 (C2-C5,
C5H4), 103.5 (Cipso C5H4), 109.1, 109.3, 125.5 (C2-C5, C5H4),
126.4-142.7 (Ph, C6H3) (NdC-NHCMe3 not observed). 29Si
NMR (C6D6, 25 °C, δ): 8.8. IR (ν cm-1): 1650, 1586, 1368. Anal.
Calcd for C24H37ClN3SiNb: C, 55.00; H, 7.13; N, 8.02. Found:
C, 55.96; H, 7.11; N, 7.88.
P r ep a r a tion of [Nb {η5-C5H4SiMe2(η1-NtBu )}(NtBu ){η2-
C(NHtBu )dNAr }] (7) (Ar ) 2,6-Me2C6H3). An equimolar
mixture of [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(NHtBu)] (4)
t
Cl], which is transformed with elimination of BuNd
CdO in an intermolecular reaction to give a mixture of
(22) Schrock. R. R. J . Organomet. Chem. 1976, 122, 209.

4630 Organometallics, Vol. 20, No. 22, 2001
Alcalde et al.
Ta ble 4. Cr ysta l, Exp er im en ta l Da ta , a n d
Str u ctu r e Refin em en t P r oced u r es for
Com p ou n d 13
(0.90 g, 2.09 mmol) and isocyanide (0.27 g, 2.09 mmol) in
hexane (50 mL) was heated to 120 °C for 5 days in a sealed
tube. The volatiles were removed under reduced pressure, and
the residue was extracted into pentane (2 × 20 mL). The
solvent was removed under vacuum from the solution to give
a dark red oil characterized as 7 (0.88 g, 1.75 mmol, 75% yield).
1H NMR (300 MHz, C6D6, δ): 0.63 (s, 3H, SiMe2), 0.73 (s, 3H,
SiMe2), 1.02 (s, 9H, CMe3), 1.25 (s, 9H, CMe3), 1.26 (s, 9H,
CMe3), 2.09 (s, 3H, Me2C6H3NdC), 2.11 (s, 3H, Me2C6H3Nd
C), 5.29 (br s, 1H, NH), 5.84 (m, 1H, C5H4), 6.14 (m, 1H, C5H4),
formula
C22, H38, Cl2, O2,N2,Si2,Nb2
675.44
Mw
cryst habit
color
prismatic
yellow
cryst size
symmetry
unit cell dimens
a (Å)
0.22 × 0.25 × 0.30 mm
triclinic, P1h
7.437(1)
8.818(1)
11.819(1)
78.24(1)
80.97(1)
81.65(1)
744.25(15)
1
6.63 (m, 1H, C5H4), 6.70 (m, 1H, C5H4), 6.80-7.10 (m, Ph). 13
C
b (Å)
c (Å)
NMR (75 MHz, C6D6, δ): 4.2, 4.5 (SiMe2), 18.2, 19.4 (Me2C6-
H3NdC), 31.1 (CNHCMe3), 32.7 (Si,Nb-NCMe3), 35.4 (NbNC-
Me3), 52.5 (CNHCMe3), 54.7 (Si, Nb-NCMe3), 65.6 (NbNCMe3),
104.6, 109.4 (C2-C5, C5H4), 110.1 (Cipso C5H4), 114.5, 116.8 (C2-
C5, C5H4), 125.4-143.2 (Ph, C6H3), 196.7 (NdC-NHCMe3). 29Si
NMR (C6D6, 25 °C, δ): -22.0.
R (deg)
â (deg)
γ (deg)
V, Å 3
Z
D
calc, g cm-3
1.507
P r ep a r a tion of [Nb {η5-C5H4SiMe2(NtBu )-η2-(CdNAr )}-
(NtBu )(CH2P h )] (8) (Ar ) 2,6-Me2C6H3). A mixture of [Nb-
{η5-C5H4SiMe2(NtBu)-η2-(CdNAr)}(NtBu)Cl] (6) (0.50 g, 0.95
mmol) and Mg(CH2Ph)2‚2THF (0.67 g, 1.90 mmol) in hexane/
toluene (50:10 mL) was stirred at room temperature for 24 h.
After removal of the solvent under vacuum the residue was
extracted into hexane (2 × 20 mL) and the filtrate was
concentrated to 10 mL. Cooling to -30 °C gave a brown oily
product, which was characterized as 8 (0.41 g, 0.71 mmol, 75%
F(000)
344
10.50
µ, cm-1
scan mode
no. of reflns
measd
ω/2θ, 2.38 < θ < 24.97
2840
indep
2620
obsd
2471 (I > 2σ(I))
range of hkl
standard reflns
refinement method
final R indices all dataa
final R indices I > 2σ(I)
weighting scheme paramsb
x
0 < h <8, -10 < k <10, -13 < l <14
3 every 200 reflns
full matrix least squares on F2
R1 ) 0.0240, wR2 ) 0.0604
R1 ) 0.0219, wR2 ) 0.0591
1
yield). H NMR (300 MHz, C6D6, δ): 0.43 (s, 3H, SiMe2), 0.49
(s, 3H, SiMe2), 1.05 (s, 9H, CMe3), 1.20 (s, 9H, CMe3), 1.72 (s,
3H, Me2C6H3NdC), 2.20 (d, 1H, J HH ) 10.2 Hz, CH2Ph), 2.27
(s, 3H, Me2C6H3NdC), 3.22 (d, 1H, J HH ) 10.2 Hz, CH2Ph),
4.84 (m, 1H, C5H4), 5.72 (m, 1H, C5H4), 6.11 (m, 1H, C5H4),
6.21 (m, 1H, C5H4), 6.80-7.40 (m, Ph). 13C NMR (75 MHz,
C6D6, δ): 0.3, 3.1 (SiMe2), 19.1, 19.3 (Me2C6H3NdC), 30.1 (Si-
NCMe3), 32.8 (NbNCMe3), 38.2 (CH2Ph), 57.3 (Si-NCMe3), 64.6
(NbNCMe3), 99.9 (C2-C5, C5H4), 102.8 (Cipso C5H4), 106.8, 109.5,
119.8 (C2-C5, C5H4), 122.2-156.1 (Ph, C6H3), 198.6 (NdC-
NCMe3). IR (ν cm-1): 1648, 1570, 1368. Anal. Calcd for
0.0402
0.3461
0.88
-0.87
1.011
y
largest diff peak
and hole e/Å3
goodness of fit on F2
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w((Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
.
a
resulting solution was cooled to room temperature to give a
white crystalline solid, which was characterized as (NHtBu)2CO.
After filtration the solvent was removed under vacuum and
the residue was extracted into hexane (4 × 25 mL). Removal
of the solvent under vacuum gave a yellow solid, which was
C
31H44N3SiNb: C, 64.21; H, 7.66; N, 7.25. Found: C, 64.44;
H, 7.62; N, 7.23.
P r ep a r a tion of [Nb{η5-C5H4SiMe2(η1-OOCNHtBu )}(η2-
OOCNH t Bu )(Nt Bu )Cl] (10). A solution of [Nb{η5-C5H4-
SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] (2) (1.50 g, 3.22 mmol) in
hexane (50 mL) was stirred overnight at room temperature
under a CO2 atmosphere. The solution was concentrated (15
mL) and cooled to -30 °C to give a gelatinous product. After
filtration, pentane (5-10 mL) was added to give a white solid,
which was characterized as 10. Concentration of the mother
liquor and subsequent cooling to -30 °C gave a second crop of
product (1.51 g, 2.73 mmol, 85% yield). 1H NMR (300 MHz,
C6D6, δ): 0.72 (s, 3H, SiMe2), 0.74 (s, 3H, SiMe2), 1.12 (s, 9H,
CMe3), 1.13 (s, 9H, CMe3), 1.15 (s, 9H, CMe3), 4.72 (br s, 1H,
NH), 5.52 (br s, 1H, NH), 6.33 (m, 2H, C5H4), 6.64 (m, 1H,
C5H4), 6.90 (m, 1H, C5H4). 13C NMR (75 MHz, C6D6, δ): -0.9,
-0.5 (SiMe2), 28.7 (CNHCMe3), 29.1 (CNHCMe3), 30.4 (NbNC-
Me3), 49.9 (CNHCMe3), 50.7 (CNHCMe3), 69.3 (NbNCMe3),
105.7, 107.5, 116.4, 126.6 (C2-C5, C5H4) (Cipso not observed),
153.3, 167.4 (CO2NH). IR (ν cm-1): 3390, 3323, 1694, 1587,
1367. Anal. Calcd for C21H39ClN3O4SiNb: C, 45.52; H, 7.11;
N, 7.58. Found: C, 45.08; H, 7.23; N, 7.25.
1
characterized as 11 (0.64 g, 0.88 mmol, 75% yield). H NMR
(300 MHz, C6D6, δ): 0.45 (s, 12H, SiMe2), 1.07 (s, 18H, CMe3),
6.08 (m, 4H, C5H4), 6.47 (m, 4H, C5H4). 13C NMR (75 MHz,
C6D6, δ): 1.6 (SiMe2), 30.3 (NbNCMe3), 69.5 (NbNCMe3), 109.9,
122.5 (C2-C5, C5H4), 125.3 (Cipso C5H4). IR (ν cm-1): 1370, 1080.
Anal. Calcd for C22H38Cl4N2Si2ONb2: C, 36.18; H, 5.24; N, 3.84.
Found: C, 36.22; H, 5.50; N, 4.03.
P r ep a r a tion of [{Nb(NtBu )(CH2P h )2}2{η5-C5H4SiMe2-O-
SiMe2-η5-C5H4}] (12). C6D6 was added to a mixture of [{Nb-
(NtBu)Cl2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (11) (0.020 g,
0.028 mmol) and Mg(CH2Ph)2‚2THF (0.039 g, 0.11 mmol) in a
Teflon-valved NMR tube. After 3 h at room temperature the
formation of 12 was demonstrated by the 1H and 13C NMR
1
spectra of the solution. H NMR (300 MHz, C6D6, δ): 0.38 (s,
12H, SiMe2), 1.12 (s, 18H, CMe3), 1.67 (d, 4H, J HH ) 8.1 Hz,
CH2Ph), 1.97 (d, 4H, J HH ) 8.1 Hz, CH2Ph), 5.55 (m, 4H, C5H4),
5.97 (m, 4H, C5H4), 6.85-7.50 (m, Ph). 13C NMR (75 MHz,
C6D6, δ): 2.13 (SiMe2), 31.8 (NbNCMe3), 41.2 (CH2Ph), 66.5
(NbNCMe3), 106.4, 113.5 (C2-C5, C5H4), 113.1 (Cipso C5H4),
125.0, 128.7, 130.3, 139.9 (Ph).
P r ep a r a tion of [{Nb(NtBu )Cl2}2{η5-C5H4SiMe2-O-SiMe2-
η5-C5H4}] (11). Meth od A. Water (25 µL, 1.27 mmol) was
added to a solution of [Nb(η5-C5H4SiMe2Cl)(NtBu)Cl2] (1.00 g,
2.55 mmol) and NEt3 (0.35 mL, 2.55 mmol) in toluene (50 mL).
The reaction mixture was stirred at room temperature for 12
h. After removal of the solvent under vacuum, the residue was
extracted into hexane (2 × 20 mL) and the solvent was again
removed to give a yellow solid characterized as 11 (0.65 g, 0.89
mmol, 70% yield).
P r epar ation of [Nb(NtBu )Cl-µ-(η5-C5H4SiMe2-η-O)]2 (13)
+ [{Nb(NtBu )Cl}2(µ-O)}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}]
(14). A solution of [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)Cl] (3)
(1.50 g, 3.82 mmol) in toluene (50 mL) was stirred for 6 days
at room temperature under CO2 atmosphere. The solvent was
removed under vacuum, and the residue was extracted into
hexane (3 × 25 mL). The filtrate was concentrated (15 mL)
under reduce pressure and cooled to -30 °C to give yellow
crystals, which were characterized as 13 (1.10 g, 1.62 mmol,
85% yield). Anal. Calcd for C22H38Cl2N2O2Si2Nb2: C, 39.11; H,
Meth od B. A solution of [Nb(η5-C5H4SiMe2NHtBu)(NtBu)-
Cl2] (1) (1.00 g, 2.33 mmol) in toluene (50 mL) was stirred for
6 h at 140 °C under CO2 atmosphere in a sealed tube. The

Niobium- and Silicon-Amido Bonds
Organometallics, Vol. 20, No. 22, 2001 4631
5.68; N, 4.15. Found: C, 38.84; H, 5.99; N, 4.10. Solutions of
13 contain a mixture of two isomers in a ratio 1:3. Data for
the major component: 1H NMR (300 MHz, C6D6, δ): 0.40 (s,
6H, SiMe2), 0.57 (s, 6H, SiMe2), 1.13 (s, 18H, CMe3), 6.16 (m,
2H, C5H4), 6.40 (m, 2H, C5H4), 6.47 (m, 2H, C5H4), 6.51 (m,
2H, C5H4). 13C NMR (75 MHz, C6D6, δ): 0.9, 1.9 (SiMe2), 31.3
(CMe3), 67.9 (CMe3), 107.2, 111.2, 117.5, 125.2 (C2-C5, C5H4),
123.8 (Cipso C5H4). Data for the minor component: 1H NMR
(300 MHz, C6D6, δ): 0.43 (s, 6H, SiMe2), 0.44 (s, 6H, SiMe2),
The structure was solved by direct methods (SHELXS 97)23
and refined by least squares against F2 (SHELXL 97).23 All
non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were introduced from geometrical calculations
and refined using a riding model with thermal parameters
equivalent to that of the C atom to which they were attached.
The final R values and other interesting X-ray structural
analysis data are presented in Table 2.
1.15 (s, 18H, CMe3), 6.29 (m, 4H, C5H4), 6.51 (m, 2H, C5H4
,
Ack n ow led gm en t. We thank the Direccio´n General
de Ensen˜anza Superior e Investigacio´n Cient´ıfica (DGE-
SIC) for financial support of this work (Project PB97-
0776).
overlapped), 6.55 (m, 2H, C5H4). 13C NMR (75 MHz, C6D6, δ):
1.1, 1.5 (SiMe2), 31.3 (CMe3) overlapped, 67.9 (CMe3) over-
lapped, 108.6, 113.4, 116.6, 124.6 (C2-C5, C5H4), 123.5, (Cipso
C5H4).
Cr ysta l Str u ctu r e Deter m in a tion of 13. Yellow crystals
of compound 13 were obtained by crystallization from toluene
at -30 °C, and a suitable sized crystal was sealed under argon
in a Lindemann capillary tube and mounted in an Enraf-
Nonius CAD 4 automatic four-circle diffractometer with
graphite-monochromated Mo ΚR radiation (λ ) 0.71073 Å).
Crystallographic and experimental details are summarized in
Table 4. Data were collected at room temperature. Intensities
were corrected for Lorentz and polarization effects in the usual
manner. No absorption or extinction corrections were made.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
parameters and estimated standard deviations, bond distances
and angles, anisotropic displacement parameters, and hydro-
gen coordinates for 13. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0103963
(23) Sheldrick, G. M. SHELX-97, Programs for Crystal Structure
Analysis (Release 97-2); Universita¨t Go¨ttingen: Go¨ttingen, Germany,
1998.