Isocyanide insertion reactivity of dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [{Nb(η5 -C5H4SiMe3)(CH2Ph) 2}2(μ-1,4-NC6H4N)]

Article abstract of DOI:10.1016/j.jorganchem.2004.01.029

The reactivity of dinuclear niobium and tantalum imido complexes with the isocyanide compound 2,6-Me₂C₆ H₃NC has been studied. The trialkyl complexes [{NbR₃(CH₃CN)}₂ (μ-1,3-NC₆H₄N)], [{NbR₃ (CH₃CN)}₂(μ-1,4-NC₆H₄N)] and [{TaR₃(CH₃CN)}₂ (μ-1,4-NC₆H₄N)] (R=CH₂ SiMe₃) gave [{Nb(η²-RC=NAr) ₂R}₂(μ-1,3-NC₆H₄N)] (1), [{Nb(η²-RC=NAr)₂R}₂ (μ-1,4-NC₆H₄N)] (2) and [{Ta(η²-RC=NAr)₂R}₂ (μ-1,4-NC₆H₄N)] (3) (R=CH₂ SiMe₃; Ar=2,6-Me₂C₆H₃), from the isocyanide insertion in two of the metal alkyl carbon bonds. The reaction of the isocyanide reagent with the di-alkyl mono-cyclopentadienyl derivatives [{Nb(η⁵-C₅H₄ SiMe₃)R₂}₂(μ-1,3-NC₆ H₄N)] R=Me, CH₂Ph, CH₂SiMe₃), [{Nb(η⁵-C₅H₄SiMe₃) R₂}₂(μ-1,4-NC₆H₄N)] (R=Me, CH₂Ph (4), CH₂SiMe₃) and [{Ta(η⁵-C₅Me₅)(CH₂ SiMe₃)₂}₂(μ-1,4-NC₆ H₄N)] yielded [{Nb(η⁵-C₅ H₄SiMe₃)(η²-RC=NAr)R}₂ (μ-1,3-NC₆H₄N)] (R=Me (5), CH₂Ph (6), CH₂SiMe₃ (7)), [{Nb(η⁵ -C₅H₄SiMe₃)(η²-RC=NAr) R}₂(μ-1,4-NC₆H₄N)] (R=Me (8), CH₂Ph (9), CH₂SiMe₃ (10)) and [{Ta(η⁵-C₅Me₅)(η² -Me₃SiCH₂C=NAr) CH₂SiMe₃} ₂(μ-1,4-NC₆H₄N)] (11) (Ar=2,6-Me₂C₆H₃), respectively, from a single insertion process. The reaction with the mono-alkyl complex [{Nb(η⁵-C₅H₄SiMe₃) (Me)Cl}₂(μ-1,4-NC₆H₄N)] gave [{Nb(η⁵-C₅H₄SiMe₃) (η²-MeC=NAr)Cl}₂(μ-1,4-NC₆ H₄N)] (12), produced from the isocyanide insertion in the metal-alkyl carbon bond. The alkyl-amido complex [{Nb(η⁵-C₅H₄SiMe₃) (Me)NMe₂}₂(μ-1,4-NC₆H₄N)] gave, from the preferential isocyanide insertion in the metal-amide nitrogen bond, [{Nb(η⁵-C₅H₄ SiMe₃)(η²-Me₂NC=NAr)Me}₂ (μ-1,4-NC₆H₄N)] (13). The molecular structure of one of the alkyl precursors, [{Nb(η⁵ -C₅H₄SiMe₃)(CH₂Ph) ₂}₂(μ-1,4-NC₆H₄N)] (4), has been determined.

Full text of DOI:10.1016/j.jorganchem.2004.01.029

Journal of Organometallic Chemistry 689 (2004) 1304–1314
www.elsevier.com/locate/jorganchem
Isocyanide insertion reactivity of dinuclear niobium and
tantalum imido complexes: X-ray crystal structure of
[{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)]
a,
Sanjiv Prashar , Mariano Fajardo , Andres Garces , Ivan Dorado ,
a
a
a
*
ꢀ
ꢀ
ꢀ
Antonio Antinolo , Antonio Otero , Isabel Lopez-Solera ,
b
b,
b
*
~
ꢀ
c
ꢀ
Carmen Lopez-Mardomingo
a
Departamento de Tecnologꢀıa Quꢀımica, Ambiental y de los Materiales, E.S.C.E.T., Universidad Rey Juan Carlos, 28933 Mꢀostoles (Madrid), Spain
b
Departamento de Quꢀımica Inorgꢀanica, Orgꢀanica y Bioquꢀımica, Universidad de Castilla-La Mancha, Facultad de Quꢀımicas,
Campus Universitario, 13071 Ciudad Real, Spain
c
Departamento de Quꢀımica Orgaꢀnica, Campus Universitario, Universidad de Alcalꢀa, 28871 Alcalꢀa de Henares (Madrid), Spain
Received 16 December 2003; accepted 29 January 2004
Abstract
The reactivity of dinuclear niobium and tantalum imido complexes with the isocyanide compound 2,6-Me2C6H3NC has been
studied. The trialkyl complexes [{NbR3(CH3CN)}2(l-1,3-NC6H4N)], [{NbR3(CH3CN)}2(l-1,4-NC6H4N)] and [{TaR3(CH3CN)}2-
(l-1,4-NC6H4N)] (R ¼ CH2SiMe3) gave [{Nb(g2-RC@NAr)2R}2(l-1,3-NC6H4N)] (1), [{Nb(g2-RC@NAr)2R}2(l-1,4-NC6H4N)]
(2) and [{Ta(g2-RC@NAr)2R}2(l-1,4-NC6H4N)] (3) (R ¼ CH2SiMe3; Ar ¼ 2,6-Me2C6H3), from the isocyanide insertion in two of
the metal alkyl carbon bonds. The reaction of the isocyanide reagent with the di-alkyl mono-cyclopentadienyl derivatives [{Nb(g5-
C5H4SiMe3)R2}2(l-1,3-NC6H4N)] (R ¼ Me, CH2Ph, CH2SiMe3), [{Nb(g5-C5H4SiMe3)R2}2(l-1,4-NC6H4N)] (R ¼ Me, CH2Ph
(4), CH2SiMe3) and [{Ta(g5-C5Me5)(CH2SiMe3)2}2(l-1,4-NC6H4N)] yielded [{Nb(g5-C5H4SiMe3)(g2-RC@NAr)R}2(l-1,3-
NC6H4N)] (R ¼ Me (5), CH2Ph (6), CH2SiMe3 (7)), [{Nb(g5-C5H4SiMe3)(g2-RC@NAr)R}2(l-1,4-NC6H4N)] (R ¼ Me (8), CH2Ph
(9), CH2SiMe3 (10)) and [{Ta(g5-C5Me5)(g2-Me3SiCH2C@NAr)CH2SiMe3}2(l-1,4-NC6H4N)] (11) (Ar ¼ 2,6-Me2C6H3), respec-
tively, from a single insertion process. The reaction with the mono-alkyl complex [{Nb(g5-C5H4SiMe3)(Me)Cl}2(l-1,4-NC6H4N)]
gave [{Nb(g5-C5H4SiMe3)(g2-MeC@NAr)Cl}2(l-1,4-NC6H4N)] (12), produced from the isocyanide insertion in the metal-alkyl
carbon bond. The alkyl-amido complex [{Nb(g5-C5H4SiMe3)(Me)NMe2}2(l-1,4-NC6H4N)] gave, from the preferential isocyanide
insertion in the metal-amide nitrogen bond, [{Nb(g5-C5H4SiMe3)(g2-Me2NC@NAr)Me}2(l-1,4-NC6H4N)] (13). The molecular
structure of one of the alkyl precursors, [{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)] (4), has been determined.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Imido; Niobium; Tantalum; Insertion reactions; Isocyanides
1. Introduction
plications in the design of low-dimensional, polymeric
materials with novel electrical and/or magnetic prop-
erties [1]. Several such complexes have been described
previously [2].
The use of imido groups in the organometallic
chemistry of group 5 elements is widespread [3]. A re-
view of our contribution to this field has recently been
published [4]. Part of our research work has been fo-
cussed on the incorporation of di-imido ligands that
bridge two metal centres and we have previously
Transition metal complexes in which the metal
centres are linked by a bridging ligand that has a de-
localized p-system are well known and have been the
subject of intense research due to their potential ap-
* Corresponding author. Tel.: +34-914887186; fax: +34-914888143.
E-mail address: s.prashar@escet.urjc.es (S. Prashar).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.01.029

S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
1305
Table 1
1H NMR data for 1–3 and 5–13a;b
Compound
M–R
NC–R
Me2C6H3
NC6H4N, Me2C6H3
and Ph
Cp
[{Nb(g2-Me3SiCH2-
C@NAr)2CH2SiMe3}2
(l-1,3-NC6H4N)] (1)
0.26 (s, 18H,
CH2SiMe3), 0.69
(s, 4H, CH2SiMe3)
0.07 (s, 36H,
1.91 (s, 12H), 2.42
(s, 12H)
6.59 (m, 3H,
NC6H4N),
CH2SiMe3), 2.77
(4H), 2.86 (4H)
(d, CH2SiMe3)
2J(1H–1H) 10.1 Hz
0.03 (s, 36H,
6.95–7.07 (m, 13H,
NC6H4N and
Me2C6H3)
6.92–6.97
(m, 16H, NC6H4N
and Me2C6H3)
[{Nb(g2-Me3SiCH2-
C@NAr)2CH2SiMe3}2
(l-1,4-NC6H4N)] (2)
0.25 (s, 18H,
CH2SiMe3), 0.57
(s, 4H, CH2SiMe3)
1.86 (s, 12H), 2.32
(s, 12H)
CH2SiMe3),
2.71 (4H), 2.79 (4H)
(d, CH2SiMe3)
2J(1H–1H) 10.4 Hz
0.02 (s, 36H,
[{Ta(g2-Me3SiCH2-
C@NAr)2CH2SiMe3}2
(l-1,4-NC6H4N)] (3)
0.24 (s, 18H,
CH2SiMe3), 0.32
(s, 4H, CH2SiMe3)
1.89 (s, 12H), 2.33
(s, 12H)
6.80–7.00
(m, 16H, NC6H4N
and Me2C6H3)
CH2SiMe3), 2.82
(4H), 3.84 (4H)
(d, CH2SiMe3)
2J(1H–1H) 10.0 Hz
1.94 (6H), 1.97 (6H) 1.59 (s, 12H)*, 2.15
(s, Me) (s, 12H)*
[{Nb(g5-C5H4SiMe3)
(g2-MeC@NAr)Me}2
(l-1,3-NC6H4N)] (5)
0.88 (s, 12H, Me)*
6.54 (m, 6H,
NC6H4N)*,
6.88–6.94
0.35 (18H), 0.36
(18H)
(s, C5H4SiMe3), 5.57
(4H), 5.88 (4H), 6.03
(4H), 6.27 (4H) (m,
C5H4)*
0.23 (18H), 0.26
(18H)
(m, 14H, NC6H4N
and Me2C6H3)
[{Nb(g5-C5H4SiMe3)
(g2-PhCH2C@NAr)
CH2Ph}2
3.07 (4H), 3.33 (4H) 3.65 (4H), 3.77 (4H) 1.65 (s, 6H), 1.66
(d, CH2Ph)* 2J
6.47–7.26
(s, 6H), 1.78 (s, 6H), (m, 60H, NC6H4N,
(d, CH2Ph)* 2J
(1H–1H) 15.7 Hz
(1H–1H) 10.1 Hz
1.81 (s, 6H)
Me2C6H3,
ArNCCH2Ph and
Nb–CH2Ph)*
(s, C5H4SiMe3), 5.38
(4H)*, 5.56 (2H),
5.59 (2H), 6.06
(4H)*, 6.07 (4H)*
(m, C5H4)
(l-1,3-NC6H4N)] (6)
[{Nb(g5-C5H4SiMe3)
(g2-Me3SiCH2C@NAr)
CH2SiMe3}2
0.26 (18H), 0.28
0.00 (18H), 0.01
1.71 (s, 6H), 1.72
6.55–6.98
0.24 (18H), 0.25
(18H)
(18H) (s, CH2SiMe3), (18H) (s, CH2SiMe3), (s, 6H), 2.49 (s, 6H), (m, 20H, NC6H4N
0.63 (2H), 0.64 (2H), 2.43 (2H), 2.44 (2H), 2.51 (s, 6H)
0.86 (2H), 0.88 (2H) 2.55 (2H), 2.57 (2H)
and Me2C6H3)*
(s, C5H4SiMe3), 5.97
(4H), 6.17 (4H), 6.44
(4H), 6.52 (4H) (m,
C5H4)*
(l-1,3-NC6H4N)] (7)
(d, CH2SiMe3)
2J(1H–1H) 12.4 Hz
0.87 (s, 12H, Me)*
(d, CH2SiMe3)
2J(1H–1H) 10.8 Hz
1.89 (6H), 1.90 (6H) 1.59 (s, 12H)*, 2.10
[{Nb(g5-C5H4SiMe3)
(g2-MeC@NAr)Me}2
(l-1,4-NC6H4N)] (8)
6.82–6.91
0.31 (18H), 0.32
(s, Me)
(s, 12H)*
(m, 12H, Me2C6H3)*, (18H)
6.79 (4H), 6.80 (4H) (s, C5H4SiMe3), 5.55
(s, NC6H4N)
(4H), 5.85 (4H), 6.01
(4H), 6.23 (4H) (m,
C5H4)*
[{Nb(g5-C5H4SiMe3)
(g2-PhCH2C@NAr)
CH2Ph}2
3.02 (4H), 3.29 (4H) 3.58 (4H), 3.69 (4H) 1.65 (s, 6H), 1.66
(d, CH2Ph)* 2J
6.69–7.26 (m,
(s, 6H), 1.69 (s, 6H), 52H, Me2C6H3,
0.20 (18H), 0.21
(18H)
(d, CH2Ph)* 2J
(1H–1H) 15.7 Hz
(1H–1H) 10.1 Hz
1.70 (s, 6H)
ArNCCH2Ph and
Nb–CH2Ph)*, 6.73
(4H), 6.74 (4H)
(s, NC6H4N)
(s, C5H4SiMe3), 5.37
(4H), 5.62 (4H), 6.01
(4H), 6.02 (4H) (m,
C5H4)*
0.20 (18H), 0.21
(18H)
(l-1,4-NC6H4N)] (9)
[{Nb(g5-C5H4SiMe3)
(g2-Me3SiCH2C@NAr)
CH2SiMe3}2
0.26 (18H),
0.27 (18H)
)0.05 (18H), )0.04
(18H) (s, CH2SiMe3), (s, 6H), 2.31 (s, 6H), Me2C6H3)*, 6.85
1.70 (s, 6H), 1.71
6.86–6.92 (m, 12H,
(s, CH2SiMe3),
0.60 (2H),
0.61 (2H),
2.40 (2H), 2.41 (2H), 2.33 (s, 6H)
2.50 (2H), 2.51 (2H)
(s, 8H, NC6H4N)*
(s, C5H4SiMe3), 5.91
(4H), 5.95 (4H), 6.35
(4H), 6.48 (4H) (m,
C5H4)*
(l-1,4-NC6H4N)] (10)
(d, ArNCCH2SiMe3)
2J(1H–1H) 10.8 Hz
0.83 (2H),
0.84 (2H)
(d, CH2SiMe3)
2J(1H–1H) 12.4 Hz
0.37 (18H),
[{Ta(g5-C5Me5)
0.00 (18H), 0.01
1.70 (s, 12H)*, 2.34
6.85–6.91 (m, 12H,
Me2C6H3)*, 6.95
(4H), 6.96 (4H)
(s, NC6H4N)
1.96 (30H), 1.97
(30H) (s, C5Me5)
(g2-Me3SiCH2C@NAr)
CH2SiMe3}2
0.38 (18H)
(18H) (s, CH2SiMe3), (s, 12H)*
2.12 (4H), 2.63 (4H)
(d, CH2SiMe3)*
2J(1H–1H) 10.8 Hz
(s,CH2SiMe3),
)0.30 (4H),
0.18 (4H)
(l-1,4-NC6H4N)] (11)
(d, Ta-CH2SiMe3)*
2J(1H–1H) 14.0 Hz

1306
S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
Table 1 (continued)
Compound
M–R
NC–R
Me2C6H3
NC6H4N,
Cp
Me2C6H3 and Ph
[{Nb(g5-C5H4SiMe3)
(g2-MeC@NAr)Cl}2
(l-1,4-NC6H4N)] (12)
1.73 (s, 12H, Me)*
2.01 (s, 6H), 2.02
6.81–6.90
0.37 (18H), 0.39
(s, 6H), 2.03 (s, 6H), (m, 12H, Me2C6H3)*, (18H)
2.04 (s, 6H) 6.78 (4H), 6.79 (4H) (s, C5H4SiMe3), 5.62
(s, NC6H4N)
(4H), 6.05 (4H), 6.38
(4H), 6.41 (4H) (m,
C5H4)*
[{Nb(g5-C5H4SiMe3)
(g2-Me2NC@NAr)Me}2
(l-1,4-NC6H4N)] (13)
0.73 (s, 12H, Me)*
1.94 (6H), 1.96 (6H), 1.86 (s, 12H)*, 2.16
2.80 (6H), 2.82 (6H) (s, 12H)*
(s, NMe2)
6.86–6.94
0.33 (18H), 0.35
(m, 12H, Me2C6H3)*, (18H)
6.82 (4H), 6.83 (4H) (s, C5H4SiMe3), 5.54
(s, NC6H4N)
(4H), 5.99 (4H), 6.22
(4H), 6.41 (4H) (m,
C5H4)*
a 300 MHz, C6D6, chemical shifts in d.
b
ꢁ
indicates that the signals for the two diastereoisomers coincide.
reported the preparation of metallocene, monocyclo-
pentadienyl, alkyl, hydride and benzamidinate deriva-
tives [5]. Following from this we now report our studies
into the reactivity of various di-imido group 5 com-
plexes towards isocyanide insertion processes in order to
synthesize new types of dinuclear niobium and tantalum
imido complexes linked by conjugated p systems.
1–3 were characterized by IR and NMR spectros-
copy. In addition to the band assigned to the imido
group (Nb@N) at ca. 1330 cmꢀ1, a band was observed
for the C@N of the g2-iminoacyl group at ca. 1570 cmꢀ1
in the IR spectra. The 1H NMR spectra (Table 1)
showed an AB spin system for the diastereoscopic
methylene protons of the inserted alkyl at ca. d 2.7 and a
singlet for the methylene group of the uninserted alkyl
groups at ca. d 0.6. The 13C NMR spectra (Table 2)
showed a downfield shift of the signals corresponding to
the methylene carbons of the inserted alkyl groups (ca. d
30) with respect to the methylene carbon of the unin-
serted alkyl moiety (ca. d 22). In addition the iminoacyl
quaternary carbon atom signal was observed at ca. d 240
in agreement with the data previously reported for g2-
iminoacyl systems [7].
2. Results and discussion
The migratory insertion of alkyl groups towards
isocyanide ligands allow the introduction of iminoacyl
groups which are present in different coordination
modes. For high valent oxophilic early transition metals,
the iminoacyl group typically adopts an g2-coordination
mode through both the nitrogen and carbon atoms [6].
Similarly an g2-coordination mode is proposed for the
iminoacyl ligand in 1–3 and 5–13 on the basis of IR and
13C NMR spectroscopy which show the characteristic
stretching vibration m(C@N) at ca. 1600 cmꢀ1 and imi-
noacyl quaternary carbon atom signal at ca. d 250, re-
spectively [6,7].
R
R
M
H3CCN
R
N
N
R
M
NCCH3
R
R
The reaction of the trialkyl dinuclear bis imido com-
plexes, [{Nb(CH2SiMe3)3(CH3CN)}2(l-1,3-NC6H4N)],
4ArNC
Ar
[{Nb(CH2SiMe3)3(CH3CN)}2(l-1,4-NC6H4N)]
and
[{Ta(CH2SiMe3)3(CH3CN)}2(l-1,4-NC6H4N)] with the
isocyanide reagent, 2,6-C6H3Me2NC, gave the products,
[{Nb(g2-Me3SiCH2C@NAr)2CH2SiMe3}2(l-1,3-NC6H4N)]
N
R
R
C
C
R
N
Ar
(1),
[{Nb(g2-Me3SiCH2C@NAr)2CH2SiMe3}2(l-1,4-
M
N
N
NC6H4N)] (2) and [{Ta(g2-Me3SiCH2C@NAr)2CH2-
SiMe3}2(l-1,4-NC6H4N)] (3) (Ar ¼ 2,6-Me2C6H3),
respectively (Eq. (1)). The final products originate from
the insertion of the isocyanide molecules into two of the
niobium carbon r-bonds. There was no evidence for the
formation of the single insertion product even when
the stoichiometry was adjusted to favour this, i.e. when
the reaction was carried out in deficiency of the isocya-
nide reagent.
N
R
R
C
Ar
M
N
C
Ar
R
R = CH2SiMe3
Ar = 2,6-Me2C6H3
M = Nb; 1,3-NC6H4N (1)
M = Nb; 1,4-NC6H4N (2)
M = Ta; 1,4-NC6H4N (3)
ð1Þ

Table 2
13C{1H}NMR data for 1–3 and 5–13a;b
Compound
M–R
NC–R
Me2C6H3,
NC6H4N
Me2C6H3 and Ph
Cp
CN
[{Nb(g2-
Me3SiCH2C@NAr)2
CH2SiMe3}2
3.9 (CH2SiMe3),
22.0 (CH2SiMe3)
0.0 (CH2SiMe3),
32.2 (CH2SiMe3)
19.1, 19.5
117.9, 119.0, 127.1,
158.1 (ipso-C)
126.2, 128.4, 128.9, 129.0,
131.1, 144.9 (Me2C6H3)
239.4
(l-1,3-NC6H4N)] (1)
[{Nb(g2-
Me3SiCH2C@NAr)2
CH2SiMe3}2
3.7 (CH2SiMe3),
21.9 (CH2SiMe3)
)0.2 (CH2SiMe3),
32.1 (CH2SiMe3)
19.0, 19.1
124.0, 153.1 (ipso-C) 126.1, 128.9, 129.0, 131.0,
144.9, 153.1 (Me2C6H3)
239.6
(l-1,4-NC6H4N)] (2)
[{Ta(g2-
Me3SiCH2C@NAr)2
CH2SiMe3}2
3.8 (CH2SiMe3),
27.0 (CH2SiMe3)
)0.2 (CH2SiMe3),
32.6 (CH2SiMe3)
18.9, 19.0
124.2, 154.3 (ipso-C) 126.3, 128.3, 128.9, 129.0,
131.2, 144.4 (Me2C6H3)
252.1
(l-1,4-NC6H4N)] (3)
[{Nb(g5-C5H4SiMe3)
(g2-MeC@NAr)Me}2
(l-1,3-NC6H4N)] (5)
9.2 (Me)*
22.9 (Me)*
17.8*, 18.0*
18.2*, 18.4*
118.2, 118.3, 119.1,
119.2, 128.0, 128.1,
157.6 (ipso-C),
126.1, 127.9, 128.6, 129.4,
130.8, 141.5 (Me2C6H3)*
0.5 (C5H4SiMe3)*,
111.6, 111.7, 114.3,
114.5, 118.3 (ipso-C)
(C5H5)*
0.5 (C5H4SiMe3)*
108.0, 108.1, 110.2,
110.3, 114.1, 114.3,
115.0 (ipso-C),
229.0*
230.8*
157.7 (ipso-C)
[{Nb(g5-C5H4SiMe3)
(g2-PhCH2C@NAr)
CH2Ph}2
39.3 (CH2Ph)*
43.2 (CH2Ph)*
118.6, 118.7, 119.7,
119.8, 128.0, 128.1,
157.5 (ipso-C),
121.1(*), 126.3(*), 127.3(*),
127.8(*), 128.3(*), 128.9(*),
129.0(*), 129.7(*), 130.0(*),
131.1(*), 135.7, 135.8, 141.2,
141.3, 154.0(*) (Me2C6H3
ArNCCH2Ph and Nb–CH2Ph)
125.8, 126.3, 129.2, 130.4,
132.6, 141.7 (Me2C6H3)*
(l-1,3-NC6H4N)] (6)
157.6 (ipso-C)
115.1 (ipso-C),
115.4, 115.7 (C5H5)
0.5 (C5H4SiMe3)*,
108.2, 108.4, 108.7,
108.8, 111.6, 111.7,
113.3 (ipso-C),
[{Nb(g5-C5H4SiMe3)
(g2-Me3SiCH2C@NAr)
CH2SiMe3}2(l-1,3-
NC6H4N)] (7)
3.8 (CH2SiMe3)*,
13.6 (CH2SiMe3)*
)0.2 (CH2SiMe3)*,
32.0 (CH2SiMe3)*
19.0*, 19.3*
117.7, 177.8, 118.6,
118.7, 128.0, 128.1,
157.8 (ipso-C),
228.2*
157.9 (ipso-C)
113.4 (ipso-C),
114.4, 114.6 (C5H5)
0.5 (C5H4SiMe3)*,
108.6, 109.1, 111.5
(ipso-C), 112.7,
[{Nb(g5-C5H4SiMe3)
(g2-MeC@NAr)Me}2
(l-1,4-NC6H4N)] (8)
9.1 (Me)*
22.8 (Me)*
17.8*, 17.9*
18.0*, 18.4*
123.9*, 153.1*
(ipso-C)
126.0, 127.8, 128.6, 129.3,
130.8, 141.5 (Me2C6H3)*
228.9*.
230.6*
113.7 (C5H5)*
[{Nb(g5-C5H4SiMe3)
(g2-PhCH2C@NAr)
CH2Ph}2(l-1,4-NC6H4N)]
(9)
39.3 (CH2Ph)*
43.0 (CH2Ph)*
124.4*, 153.0*
(ipso-C)
121.0(*), 126.3(*), 127.2(*),
127.7(*), 128.3(*), 128.8(*),
129.0(*), 129.6(*), 130.0(*),
131.0, 131.1, 135.7, 135.8,
0.4 (C5H4SiMe3)*,
107.8, 107.9, 110.0,
110.1, 114.5, 114.6,
114.7 (ipso-C), 114.8
141.2(*), 154.1, 154.2 (Me2C6H3 (ipso-C), 115.0,
ArNCCH2Ph and Nb–CH2Ph)
125.8, 126.2, 129.0, 129.1,
132.4, 141.7 (Me2C6H3)*
115.1 (C5H5)
[{Nb(g5-C5H4SiMe3)
(g2-Me3SiCH2C@NAr)
CH2SiMe3}2(l-1,4-
3.6 (CH2SiMe3)*,
13.2 (CH2SiMe3)*
)0.5 (CH2SiMe3)*,
31.7 (CH2SiMe3)*
18.7*, 18.8*
19.1*, 19.9*
123.6*, 153.2*
(ipso-C)
0.6 (C5H4SiMe3)*,
108.4, 109.3, 111.1,
112.9 (ipso-C),
114.1 (C5H5)*
12.0 (C5Me5)*,
113.2 (C5Me5)*
228.5*
241.8*
NC6H4N)] (10)
[{Ta(g5-C5Me5)(g2-
Me3SiCH2C@NAr)CH2Si-
Me3}2(l-1,4-NC6H4N)] (11)
4.9 (CH2SiMe3)*,
17.9 (CH2SiMe3)*
0.6 (CH2SiMe3)*,
30.0 (CH2SiMe3)*
124.5*, 152.4*
(ipso-C)
125.8, 126.1, 129.1, 129.4,
132.2, 141.8 (Me2C6H3)*

1308
S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
1–3 can be considered as six coordinate complexes
with an octahedral disposition or as a pseudo-tetra-
hedral structure where the g2-iminoacyl groups oc-
cupy one coordination site each. The latter hypothesis
has been observed in similar g2-iminoacyl group 5
metal complexes which show, in their X-ray struc-
tures, that the g2-iminoacyl group occupies one co-
ordination site [8]. Rothwell and co-workers have
prepared similar group 4 compounds, [M(OPh-2,6-
But2)2(g2-RNCR0)2]
(M ¼ Hf,
R ¼ Ph,
R0 ¼ Me;
M ¼ Zr, R ¼ But, R0 ¼ CH2Ph), where pseudo tetra-
hedral geometries were also observed [7c]. Taking into
account these previous reports, we propose that 1–3
also adopt a pseudo tetrahedral conformation. The
final disposition of the two g2-iminoacyl ligands will
allow for the possibility of cis or trans arrangements.
NMR spectroscopy, however, showed the presence of
a solitary final product, although this may be ex-
plained by the rapid interchange, in solution, in the
orientation of the carbon and nitrogen atoms of the
g2-iminoacyl groups. Indeed this type of behaviour is
quiet typical in these systems with the transformation
being carried out via rotation in the MCN plane of
the g2-iminoacyl ligand or via g2–g1–g2 coordination
[6].
We have also studied the insertion reactions of half
sandwich bis-alkyl imido complexes to compare and
contrast their behaviour with their electron deficient
trialkyl analogues. Previous to this we carried out
the X-ray single crystal structure determination of one
of the dialkyl monocyclopentadienyl precursors, namely
[{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)] (4).
The molecular structure of 4 and atomic numbering
scheme are shown in Fig. 1, selected bond lengths and
angles are given in Table 3.
Fig. 1. Molecular structure and atom-labeling scheme for [{Nb(g5-
C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)] (4), with thermal ellipsoids
at 30% probability.

S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
1309
Table 3
out of the plane defined by N(1), C(1), C(2) and C(3), by
ꢁ
Selected bond lengths (A) and angles (°) for 4
ꢁ
0.45(2) and 0.22(2) A.
The niobium–nitrogen distances of 1.79(1) and
4a
4b
ꢁ
1.75(1) A lie within the range expected for a triple
ꢁ
Bond lengths (A)
Nb(1)–N
1.79(1)
2.156
1.75(1)
2.161
bond [9,10]. The Nb(1)–N(1)–C(1) angles of 159(1)°
and 168(1)° deviate slightly from linear. Such devia-
tions from linear are normally in the range of 12–15°
(see Table 4). In the case of [Nb(@NPh)(g5-
Nb(1)–Cp(1)
Nb(1)–C(4)
Nb(1)–C(5)
Nb(1)–C(6)
Nb(1)–C(7)
Nb(1)–C(8)
Nb(1)–C(9)
Nb(1)–C(1)0
N(1)–C(1)
2.468(9)
2.54(2)
2.45(1)
2.42(2)
2.45(2)
2.22(2)
2.29(1)
1.38(2)
2.53(2)
2.51(2)
2.44(2)
2.43(2)
2.43(2)
2.25(1)
2.22(2)
1.38(2)
€
C5H4SiMe3)2Cl] extended-Huckel molecular-orbital
calculations established that the imido bond angles for
‘‘linear imide’’ in this class of complex can vary up to
140° with little effect on the Nb–N bonding [11].
Therefore we propose that in 4 the nitrogen atom is sp
hybridized.
Bond angles (°)
N(1)–Nb(1)–C(9)
N(1)–Nb(1)–C(10)
C(9)–Nb(1)–C(10)
Nb(1)–N(1)–C(1)
Cp(1)–Nb(1)–C(9)
Cp(1)–Nb(1)–C(10)
Cp(1)–Nb(1)–N(1)
101.4(6)
96.9(5)
122.9(7)
159(1)
109.0
97.2(6)
96.9(6)
123.1(7)
168(1)
108.7
ꢁ
The distances of 2.58(1) and 2.52(1) A between the
ipso carbon atom C(11) of the benzyl ligand and the
niobium atom indicates a weak interaction between
these two atoms. This interaction only occurs with one
of the benzyl groups with the other being situated at a
greater distance to the niobium atom (Nb(1)–C(21)
108.4
118.4
109.5
121.8
2
ꢁ
Cp(1) is the centroid of C(4)–C(8).
3.22(1) and 3.17(1) A). This type of g coordinative
behaviour of the benzyl ligand in d0 early transition
metal complexes is not uncommon and several examples
have previously been reported [12].
Suitable X-ray quality crystals of 4 were obtained by
the slow evaporation of a toluene solution. The com-
pound crystallizes in P1 space group, triclinic and with
two distinct molecules in the asymmetric unit. The
structure of 4 consists of a dinuclear complex where the
metal atoms are bound to the cyclopentadienyl ring in
an g5 mode. In addition, the niobium atoms are linked
to two methylene groups from CH2Ph and to the ni-
trogen of the imido group. 4 adopts a three-legged pia-
no-stool conformation about the metal centres. The
cyclopentadienyl rings are in a trans disposition with
respect to each other and in a typical eclipsed disposition
with respect to the imido group [9]. The metal atom is
In a similar manner to the synthesis of 1–3, the half
sandwich complexes, [{Nb(g5-C5H4SiMe3)(g2-RC@
NAr)R}2(l-1,3-NC6H4N)] (R ¼ Me (5), CH2Ph (6),
CH2SiMe3 (7)), [{Nb(g5-C5H4SiMe3)(g2-RC@NAr)
R}2(l-1,4-NC6H4N)] (R ¼ Me (8), CH2Ph (9),
CH2SiMe3 (10)) and [{Ta(g5-C5Me5)(g2-Me3SiCH2C@
NAr)CH2SiMe3}2(l-1,4-NC6H4N)] (11) (Ar ¼ 2,6-
Me2C6H3), were prepared via isocyanide reaction with
the corresponding bis-alkyl precursors (Eq. (2)). At-
tempts to insert a second isocyanide molecule were un-
successful.
Table 4
Selected structural data of some group 5 metal imido complexes
ꢁ
M–N (A)
M–N–C (°)
Reference
This work
[{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)] (4)
1.79(1)
1.75(1)
1.776(8)
1.783(5)
1.794(5)
1.793
159(1)
168(1)
160.9(7)
166.4(5)
167.4(5)
168.5
[{Nb(g5-C5H4SiMe3)Cl2}2(l-1,4-NC6H4N)]
[{Ta(g5-C5Me5)Cl2}2(l-1,4-NC6H4N)]
[{Ta(g5-C5Me5)(CH2SiMe3)2}2(l-1,4-NC6H4N)]
[Nb(@NAr)(g5-C5H5)(NHBut)Cl]a
[Nb(@NAr)(g5-C5H5)Cl2]a
[5d]
[5d]
[5d]
[10b]
[9]
1.761
1.763
165.6
166.6
[Nb(@NAr)(g5-C5H5)Cl2]b
[10c]
[10d]
[3]
[Nb(@NAr)(g5-C5H5)(g2-NHBut)(CH2But)H]a
[Ta(@NAr)(g5-C5Me5)Cl2]a
1.787
1.780
174.6
171.3
[Ta(@NAr)(g5-C5Me5)Cl2]b
1.774
1.812
1.821
169.9
171.4
171.8
[10a]
[10e]
[10f]
[Ta(@NAr)(g5-C5Me5)(CO){Si(SiMe3)3}H]a
[Ta(@NAr)(g5-C5H5)(PMe3)(SiMe2Cl)H]a
a Ar ¼ 2,6-Pr2i C6H3.
b Ar ¼ 2,6-Me2C6H3.

1310
S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
SiMe3
R
SiMe3
SiMe3
R
R
Nb
Nb
Nb
R
C R
N Ar
N
N
C
N
Nb
N
N
R
Ar
R'
R
N-outside
N-inside
Me3Si
2ArNC
Ar = 2,6-Me2C6H3
SiMe3
R
Fig. 2. Two possible conformations for the iminoacyl metal complexes
5–10.
R
C
Nb
N
stereoisomers that arise from the two niobium chiral
centres (Fig. 3). The chemical shifts for the isomers are
very similar and in many cases the signals from the two
isomers overlap (see Tables 1 and 2).
R
C
N
Ar
N
Nb
N
R
Ar
[{Nb(g5-C5H4SiMe3)(g2-MeC@NAr)Cl}2(l-1,4-
NC6H4N)] (12) was prepared in a similar manner to 5–
11 (Eq. (3)). The characterization of 12 by spectroscopic
methods revealed that it was similar to complexes 8–10
in all aspects (see Tables 1 and 2).
Me3Si
Ar = 2,6-Me2C6H3
1,3-NC6H4N
R = Me (5)
R = CH2Ph (6)
R = CH2SiMe3 (7)
1,4-NC6H4N
R = Me (8)
R = CH2Ph (9)
R = CH2SiMe3 (10)
ð2Þ
SiMe3
The IR spectra of 5–11 showed bands at ca. 1315 and
1630 cmꢀ1 assigned to the imido group and the C@N of
the g2-iminoacyl group, respectively. The chiral nature
of the niobium atoms in 5–10 means that for the cy-
clopentadienyl protons, four multiplets, corresponding
to an ABCD system, are observed in the 1H NMR
spectra. The presence of the g2-iminoacyl was confirmed
by the existence of a signal at ca. 230 ppm characteristic
Cl
Nb
N
Me
N
Me
Nb
Cl
Me3Si
2ArNC
1
for this group [7]. For 6, 7, 8, 10 and 11 the H NMR
ð3Þ
SiMe3
Cl
spectra showed an AB spin system for the methylene
protons of the inserted alkyl displaced downfield with
respect to that observed for the AB system of the dia-
stereopic protons of the remaining alkyl group (see
Table 1). The restricted rotation of the isocyanide phe-
nyl ring leads to the inequivalency of its two methyl
substituents and thus two signals were observed in the
1H NMR spectra.
Me
C
Nb
N
Me
C
N
Ar
N
N
Nb
Cl
Ar
Me3Si
If we assume that the g2-iminoacyl group occupies
one coordination site then we can consider that the
complexes 5–11 will adopt a pseudo tetrahedral geom-
etry. The iminoacyl group can position itself in two
distinct conformations with respect to the uninserted
alkyl group, the ‘‘proximal’’ or ‘‘N-outside’’ and the
‘‘distal’’ or ‘‘N-inside’’ configurations (see Fig. 2), which
would lead to the possibility of various isomers in the
final product. However, it is more likely that either rapid
exchange occurs between the two iminoacyl confirma-
tions as proposed in 1–3 or that the final product ob-
tained is that from thermodynamic control with the
iminoacyl group adopting only the N-inside configura-
tion as seen in similar metallocene complexes [13]. NMR
spectroscopy showed only the presence of the two dia-
12
Ar = 2,6-Me2C6H3
The reaction of the alkyl amido complex, [{Nb(g5-
C5H4SiMe3)(Me)(NMe2)}2(l-1,4-NC6H4N)], with the
isocyanide reagent only occurred at elevated tempera-
tures. The insertion reaction took place preferentially at
the Nb–NMe2 bond to give the g2-iminocarbamoyl
derivative
[{Nb(g5-C5H4SiMe3)(g2-Me2NC@NAr)
Me}2(l-1,4-NC6H4N)] (13) (Eq. (4)). This selective mi-
gration of the amide rather than the methyl group is in
agreement with that previously reported for similar ni-
obium and tantalum complexes [14]. However this be-
haviour is unusual, as shown in the reactivity of alkyl
amide complexes of titanium [15], zirconium [16],

S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
1311
SiMe3
R
thwarted by the great difficulty encountered in obtain-
ing crystals of sufficient quality for X-ray diffraction
analysis.
R
C
Nb
N
R
C
N
Ar
N
Nb
N
R
3. Conclusions
Ar
(a)
Me3Si
In this paper we report the synthesis of new iminoacyl
derivatives of dinuclear niobium and tantalum imido
complexes via isocyanide insertion reactions of their
alkyl precursors. The molecular structure of one of these
alkyl precursors has been determined by single crystal
X-ray diffraction studies and shows that one of the
benzyl ligands adopts an g2 coordination mode with
respect to the metal atom.
SiMe3
R
Nb
R
N
R
C
N
Nb
N
SiMe3
R
C
Ar
N
(b)
Ar
Fig. 3. (a) Proposed structure for meso-[{Nb(g5-C5H4SiMe3)(g2-
RC@NAr)R}2(l-NC6H4N)]. (b) Proposed structure for rac-[{Nb(g5-
C5H4SiMe3)(g2-RC@NAr)R}2(l-NC6H4N)].
4. Experimental
4.1. Materials and procedures
All reactions were performed using standard Schlenk-
tube techniques under an atmosphere of dry nitrogen.
Solvents were distilled from appropriate drying agents
and degassed before use. 2,6-Me2C6H3NC was pur-
chased from Aldrich and used directly. The compounds
[{NbR3}2(l-1,3-NC6H4N)], [{NbR3}2(l-1,4-NC6H4N)],
[{TaR3}2(l-1,4-NC6H4N)] (R ¼ CH2SiMe3), [{Nb(g5-
C5H4SiMe3)R2}2(l-1,3-NC6H4N)] (R ¼ Me, CH2Ph,
CH2SiMe3), [{Nb(g5-C5H4SiMe3)R2}2(l-1,4-NC6H4N)]
(R ¼ Me, CH2Ph (4), CH2SiMe3), [{Ta(g5-C5Me5)
(CH2SiMe3)2}2(l-1,4-NC6H4N)], [{Nb(g5-C5H4SiMe3)
(Me)Cl}2(l-1,4-NC6H4N)] and [{Nb(g5-C5H4SiMe3)
(Me)NMe2}2(l-1,4-NC6H4N)] were prepared as de-
scribed earlier [5d]. IR spectra were recorded on a Per-
tungsten [17], uranium [18], tantalum [19] and niobium
[20], where the isocyanide insertion takes place at the
M–C(alkyl) bond. The presence of the g2-iminocarba-
moyl in 13 was confirmed by 13C NMR spectroscopy.
The signal corresponding to the carbon of the g2-im-
inocarbamoyl group was observed at d 200 shifted up-
field with relation to that observed in g2-iminoacyl
complexes (ca. d 230).
SiMe3
Me
Nb
N
NMe2
1
kin–Elmer PE 883 IR spectrophotometer. H and 13C
N
Me2N
Nb
spectra (Tables 1 and 2) were recorded on a Varian FT-
300 spectrometer and referenced to the residual deuter-
ated solvent. Microanalyses were carried out with a
Perkin–Elmer 2400 microanalyzer.
Me
Me3Si
2ArNC
ð4Þ
SiMe3
Me
4.2. Synthesis of [{Nb(g2-Me3SiCH2C@NC6H3Me2-2,
6)2(CH2SiMe3)}2(l-1,3-NC6H4N)] (1)
NMe2
C
Nb
N
Me2N
C
A hexane solution (20 ml) of 2,6-Me2C6H3NC (0.21
g, 1.60 mmol) was added dropwise to [{Nb(CH3CN)
(CH2SiMe3)3}2(l-1,3-NC6H4N)] (0.37 g, 0.41 mmol) in
hexane (100 ml) at room temperature and stirred for 20
min. The resulting suspension was filtered and solvent
removed, from the filtrate, in vacuo to give a yellow
solid corresponding to the title complex (0.40 g, 73%).
IR (Nujol/CsI): mNb¼N 1344, mC@N 1562 cmꢀ1. Anal.
Calc. for C66H106N6Nb2Si6: C, 59.25; H, 7.99; N, 6.28.
Found C, 59.43; H, 8.11; N, 6.66%.
N
Ar
N
N
Nb
Me
Ar
Me3Si
13
Ar = 2,6-Me2C6H3
Our numerous attempts to determine the molecular
structure of the isocyanide insertion products, 1–3 and
5–13, were always unsuccessful. Our efforts were

1312
S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
4.3. Synthesis of [{Nb(g2-Me3SiCH2C@NC6H3Me2-
2,6)2(CH2SiMe3)}2(l-1,4-NC6H4N)] (2)
4.8. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-MeC@
NC6H3Me2-2,6)Me}2(l-1,4-NC6H4N)] (8)
The preparation of 2 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.200 g, 1.52 mmol) and
[{Nb(CH3CN)(CH2SiMe3)3}2(l-1,4-NC6H4N)] (0.350
g, 0.39 mmol). Yield: 0.38 g, 72%. IR (Nujol/CsI): mNb¼N
The preparation of 8 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.17 g, 1.28 mmol) and
[{Nb(g5-C5H4SiMe3)Me2}2(l-1,4-NC6H4N)] (0.40 g,
0.64 mmol). Yield: 0.52 g, 93%. IR (Nujol/CsI): mNb¼N
1317, mC@N 1642 cmꢀ1. Anal. Calc. for C44H60N4Nb2Si2:
C, 59.58; H, 6.82; N, 6.32. Found C, 59.39; H, 6.75; N,
6.36%.
1316, mC@N 1571 cmꢀ1. Anal. Calc. for C66H106
-
N6Nb2Si6: C, 59.25; H, 7.99; N, 6.28. Found C, 59.21;
H, 7.98; N, 6.36%.
4.4. Synthesis of [{Ta(g2-Me3SiCH2C@NC6H3Me2-
2,6)2(CH2SiMe3)}2(l-1,4-NC6H4N)] (3)
4.9. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-PhCH2C@
NC6H3Me2-2,6)CH2Ph}2(l-1,4-NC6H4N)] (9)
The preparation of 3 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.10 g, 0.75 mmol) and
[{Ta(CH3CN)(CH2SiMe3)3}2(l-1,4-NC6H4N)] (0.20 g,
0.19 mmol). Yield: 0.22 g, 78%. IR (Nujol/CsI): mTa¼N
The preparation of 9 was carried out in an identi-
cal manner to 1. 2,6-Me2C6H3NC (0.15 g, 1.12 mmol)
and [{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,4-NC6H4N)]
(0.53 g, 0.56 mmol). Yield: 0.61 g, 91%. IR (Nujol/CsI):
mNb¼N 1315, mC@N 1627 cmꢀ1. Anal. Calc. for C68H76
N4Nb2Si2: C, 68.56; H, 6.43; N, 4.70. Found C, 68.30;
H, 6.47; N, 4.67%.
1331, mC@N 1565 cmꢀ1. Anal. Calc. for C66H106
-
N6Ta2Si6: C, 52.36; H, 7.06; N, 5.55. Found C, 52.21; H,
7.02; N, 5.60%.
4.5. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-MeC@
NC6H3Me2-2,6)Me}2(l-1,3-NC6H4N)] (5)
4.10. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-Me3Si-
CH2C@NC6H3Me2-2,6)CH2SiMe3}2(l-1,4-NC6H4N)]
(10)
The preparation of 5 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.42 g, 3.20 mmol) and
[{Nb(g5-C5H4SiMe3)Me2}2(l-1,3-NC6H4N)] (1.00 g,
1.60 mmol). Yield: 1.33 g, 94%. IR (Nujol/CsI): mNb¼N
1316, mC@N 1620 cmꢀ1. Anal. Calc. for C44H60N4Nb2Si2:
C, 59.58; H, 6.82; N, 6.32. Found C, 59.41; H, 6.77; N,
6.34%.
The preparation of 10 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.31 g, 2.36 mmol) and
[{Nb(g5-C5H4SiMe3)(CH2SiMe3)2}2(l-1,4-NC6H4N)]
(1.08 g, 1.18 mmol). Yield: 1.30 g, 93%. IR (Nujol/CsI):
mNb¼N 1320, mC@N 1617 cmꢀ1. Anal. Calc. for C56H92-
N4Nb2Si6: C, 57.21; H, 7.89; N, 4.77. Found C, 57.11;
H, 7.96; N, 4.66%.
4.6. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-PhCH2C@
NC6H3Me2-2,6)CH2Ph}2(l-1,3-NC6H4N)] (6)
4.11. Synthesis of [{Ta(g5-C5Me5)(g2-Me3SiCH2C@
NC6H3Me2-2,6)CH2SiMe3}2(l-1,4-NC6H4N)] (11)
The preparation of 6 was carried out in an identi-
cal manner to 1. 2,6-Me2C6H3NC (0.24 g, 1.82 mmol)
and [{Nb(g5-C5H4SiMe3)(CH2Ph)2}2(l-1,3-NC6H4N)]
(0.85 g, 0.91 mmol). Yield: 0.99 g, 91%. IR (Nujol/CsI):
. Anal. Calc. for
C68H76N4Nb2Si2: C, 68.56; H, 6.43; N, 4.70. Found C,
68.37; H, 6.45; N, 4.76%.
The preparation of 11 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.10 g, 0.76 mmol) and
[{Ta(g5-C5Me5)(CH2SiMe3)2}2(l-1,4-NC6H4N)] (0.41
g, 0.38 mmol). Yield: 0.41 g, 80%. IR (Nujol/CsI): mTa¼N
1325, mC@N 1620 cmꢀ1. Anal. Calc. for C60H96N4Si4Ta2:
C, 53.47; H, 7.18; N, 4.16. Found C, 53.30; H, 7.13; N,
4.21%.
mNb¼N 1314, mC@N 1633 cmꢀ1
4.7. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-Me3Si-
CH2C@NC6H3Me2-2,6)CH2SiMe3}2(l-1,3-NC6H4N)]
(7)
4.12. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-MeC@
NC6H3Me2-2,6)Cl}2(l-1,4-NC6H4N)] (12)
The preparation of 7 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.29 g, 2.18 mmol) and
[{Nb(g5-C5H4SiMe3)(CH2SiMe3)2}2(l-1,3-NC6H4N)]
(1.00 g, 1.09 mmol). Yield: 1.21 g, 94%. IR (Nujol/CsI):
mNb¼N 1316, mC@N 1625 cmꢀ1. Anal. Calc. for C56H92
N4Nb2Si6: C, 57.21; H, 7.89; N, 4.77. Found C, 57.04;
H, 7.88; N, 4.81%.
The preparation of 12 was carried out in an identical
manner to 1. 2,6-Me2C6H3NC (0.26 g, 1.96 mmol) and
[{Nb(g5-C5H4SiMe3)(Me)Cl}2(l-1,4-NC6H4N)] (0.65 g,
0.98 mmol). Yield: 0.45 g, 49%. IR (Nujol/CsI): mNb¼N
1316, mC@N 1663 cmꢀ1. Anal. Calc. for C42H60-
N4Cl2Nb2Si2: C, 54.37; H, 5.87; N, 6.04. Found C,
54.50; H, 5.90; N, 5.99%.

S. Prashar et al. / Journal of Organometallic Chemistry 689 (2004) 1304–1314
1313
4.13. Synthesis of [{Nb(g5-C5H4SiMe3)(g2-Me2NC@
NC6H3Me2-2,6)Me}2(l-1,4-NC6H4N)] (13)
refined with an overall isotropic temperature factor us-
ing a riding model. Weights were optimized in the final
cycles. The crystal data and details of data collection
and structure analyses are summarized in Table 5.
The preparation of 13 was carried out in an identi-
cal manner to 1. 2,6-Me2C6H3NC (0.37 g, 2.78 mmol)
and [{Nb(g5-C5H4SiMe3)(Me)NMe2}2(l-1,4-NC6H4N)]
(0.95 g, 1.39 mmol). Yield: 1.15 g, 88%. IR (Nujol/CsI):
5. Supplementary material
mNb¼N 1315, mC@N 1639 cmꢀ1
. Anal. Calc. for
C46H66N6Nb2Si2: C, 58.46; H, 7.04; N, 8.89. Found C,
58.81; H, 7.07; N, 8.92%.
Crystallographic data for the structural analysis of 4
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC-205741. Copies of this in-
formation may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44-1223-336033; E-mail: de-
posit@ccdc.cam.ac.uk or http//www.ccdc.cam.ac.uk).
4.14. X-ray structure determination of [{Nb(g5-
C5H4SiMe3)(CH2Ph2}2(l-1,4-NC6H4N)] (4)
Intensity data were collected on a NONIUS-MACH3
diffractometer equipped with a graphite monochroma-
ꢁ
tor (Mo Ka radiation, k ¼ 0:71073 A) using an x=2h
scan technique. The specimens diffracted only weakly.
The final unit cell parameters were determined from 25
well-centered reflections and refined by the least-squares
method. The space group was determined from the
systematic absences and this was vindicated by the
success of the subsequent solutions and refinements.
Two standard reflections, measured every 98 reflections,
showed an intensity decay of 28% at the end of the
process. The structure was solved by direct methods
using the SHELXS computer program [21] and refined in
F 2 by full-matrix least-squares (SHELXL-97) [22]. All
non-hydrogen atoms were refined with anisotropic
thermal parameters for all compounds. The hydrogen
atoms were included in calculated positions and were
Acknowledgements
We gratefully acknowledge financial support from the
ꢀ
Ministerio de Ciencia y Tecnologıa, Spain (Grant no.
BQU2002-04638-C02-01), the Junta de Comunidades de
ꢀ
ꢀ
Castilla-La Mancha (PAC-02-003), the Direccion Gen-
ꢀ
ꢀ
eral de Investigacion de la Consejerıa de Educacion de la
Comunidad Autonoma de Madrid (Grant no.
ꢀ
07N-0036/98) and the Universidad Rey Juan Carlos
(PPR-2003-54).
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