ansa-η-Cyclopentadienylimide derivatives of niobium

Article abstract of DOI:10.1039/a700713b

Treatment of the ansa-bridged η-cyclopentadienylimide compound [Nb{η⁵,κN-C₅H₄(CH₂) ₃N}Cl₂] 1 with MgMeBr gave the binuclear ansa-bridged η-cyclopentadienylimide derivative [MeNb{μ-C₅

Full text of DOI:10.1039/a700713b

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ansa-ç-Cyclopentadienylimide derivatives of niobium*
David M. Antonelli, Pedro T. Gomes, Malcolm L. H. Green, Ana M. Martins and
Philip Mountford
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
Treatment of the ansa-bridged η-cyclopentadienylimide compound [Nb{η5,κN-C5H4(CH2)3N}Cl2] 1 with
MgMeBr gave the binuclear ansa-bridged η-cyclopentadienylimide derivative [MeNb{µ-C5H4(CH2)3N}-
1
{µ-NCH(CH2)2C5H4}NbMe2] 2. The crystal structure of 2 has been determined. Detailed H and 13C NMR
studies showed that 2 undergoes an interesting fluxional process involving interchange of two methyl groups
under an ansa arch. Reaction of 1 with Na(C5H5) gave [Nb{η5,κN-C5H4(CH2)3N}(η5-C5H5)(η1-C5H5)] 3, which is
also fluxional and undergoes diverse dynamic processes including exchange between η1- and η5-cyclopentadienyl
rings. The compounds [Nb{η5,κN-C5H4(CH2)3N}(CH2Ph)2] and [Nb{η5,κN-C5H4(CH2)3N}{CH2C(Me)CH2}2]
have also been prepared.
Recently we reported the synthesis of the compound C5H4-
(SiMe3)(CH2)3N(SiMe3)2 and showed that it reacted readily
with niobium pentachloride to give the ansa-bridged η-cyclo-
pentadienylimide derivative [Nb{η5,κN-C5H4(CH2)3N}Cl2] 1.1,2
We have also reported the synthesis of
a series of
cyclopentadienyl-imide and -amide derivatives of early transi-
tion metals2 having in view the current interest in such Group 4
compounds as homogeneous catalysts for α-olefin poly-
merisation.3–6 We have explored further the synthetic potential
of the ligand precursor C5H4(SiMe3)(CH2)3N(SiMe3)2, and here
describe the preparation of alkyl derivatives of 1. It was envis-
aged that the dialkyl derivatives [Nb{η5,κN-C5H4(CH2)3N}R2],
where R = methyl or benzyl, would be precursors for the
monoalkyl cations [Nb{η5,κN-C5H4(CH2)3N}R]ϩ and that
these cations might have catalytic activity. A preliminary
account of part of this work has been published.2
Fig. 1 A CAMERON 7 plot of [MeNb{µ-C5H4(CH2)3N}{µ-NCH-
(CH2)2C5H4}NbMe2] 2. Thermal ellipsoids are drawn at the 20%
probability level and H atoms are omitted for clarity
Results and Discussion
Synthesis
Treatment of [Nb{η5,κN-C5H4(CH2)3N}Cl2] 1 with an excess
of MgMeBr (3–6 equivalents) in diethyl ether affords orange
crystals of the binuclear compound [MeNb{µ-C5H4-
(CH2)3N}{µ-NCH(CH2)2C5H4}NbMe2] 2 (ca. 60% yield). This
compound decomposes when in contact with chlorinated sol-
vents. It could not be isolated from the reaction between 1 and 2
equivalents of MgMeBr.
are somewhat longer than those observed in [(η5-C5Me4Et)-
Cl2Ta(µ-NCHMe)(µ-Cl)(µ-H)TaCl(η5-C5Me4Et)] (2.195 and
2.059 Å, respectively),12 in niobium–η2-iminoacyl systems
(≈2.18 and 2.16 Å, respectively)17 or in niobium–η2(C,N)-
ketenimine fragments (≈2.16 and 2,14 Å, respectively).18 This
increase reflects geometrical constraints arising from strain in
the bridging ansa system. The C(21)᎐N(2) distance [1.34(1) Å]
is also larger than for other co-ordinated alkenylideneamides
(1.26–1.28 Å),9,13 is close to those found in niobium–η2-
iminoacyl and niobium–η2(C,N)-ketenimine compounds (1.23–
1.33 Å)17,18 but is shorter than in the bridging alkenylideneam-
ide tantalum compound (1.435 Å).12 The imide nitrogen atom
N(1) attached to the non-deprotonated chain is sp2 hybridised
and bridges the two metal centres. The corresponding bond
lengths Nb(2)᎐N(1) [2.134(7) Å] and Nb(1)᎐N(1) [1.901(8) Å]
point to a double-bond character in the latter, as a consequence
of the nitrogen lone-pair donation to the Nb(1) centre. The
Nb(2)᎐Nb(1) distance [2.929(1) Å] is similar to the values found
for other imide-bridged niobium dimers,19 it is slightly larger
than in niobium metal (2.85 Å) and substantially larger than for
Nb᎐Nb double (2.70–2.74 Å) and triple bonds (2.61–2.63 Å).20
The two niobium, two nitrogen, and two carbon atoms C(11)
and C(21) are nearly coplanar; the maximum deviation from
the best fit Nb(1), Nb(2), N(1), N(2), C(11), C(21) least-squares
plane is 0.06 Å. The saturated (CH2)3 chain linking the bridging
imide group to the η5-C5H4 ring co-ordinated to Nb(1) appears
The crystal structure of compound 2 has been determined
and the molecular structure is shown in Fig. 1. Selected dis-
tances and angles are given in Table 1. The structure contains
only three methyl groups and interestingly a hydrogen has been
lost from a CH2 group adjacent to an imide nitrogen. There is a
virtually linear alkenylideneamide ligand 8–12 bridging the two
niobium atoms. This is the second structurally characterised
example of a bridging η2,κN-C(H)᎐N alkenylideneamide sys-
᎐
tem.12 It is bound through a Nb(1)᎐N(2) double bond the
length [1.847(8) Å] of which is typical of co-ordinated linear
alkenylideneamides (1.85–2.01 Å),9,12,13 lying in-between the
normal ranges observed for Nb᎐N single (2.16–2.33 Å)14 and
triple bonds (1.73–1.79 Å).1,15,16 The almost linear C(21)᎐
N(2)᎐Nb(1) geometry [168.0(7)Њ] indicates a sp-hybridised
nitrogen which is bonding to Nb(1). The C(H)᎐N double bond
᎐
is clearly η2-co-ordinated to the Nb(2) centre. However, the dis-
tances Nb(2)᎐C(21) [2.32(1) Å] and Nb(2)᎐N(2) [2.225(7) Å]
* Non-SI unit employed: bar = 105 Pa.
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444
2435

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conform to the solid-state structure shown in Fig. 1. The data
suggest that at 233 K the structure of 2 in solution adopts a
different conformation whereby there has been a clockwise
rotation about the CfЈ᎐CeЈ bond (see Scheme 1).
Table 1 Selected bond lengths (Å) and angles (Њ) for [MeNb{µ-C5H4-
(CH2)3N}{µ-NCH(CH2)2C5H4}NbMe2]
2 with estimated standard
deviations (e.s.d.s) in parentheses
The chemical shift data for compound 2 are unexceptional,
Nb(1)᎐N(1)
Nb(1)᎐Nb(2)
Nb(1)᎐N(2)
Nb(1)᎐C(1)
Nb(2)᎐N(1)
Nb(2)᎐N(2)
1.901(8)
2.929(1)
1.847(8)
2.21(1)
2.134(7)
2.225(7)
Nb(2)᎐C(2)
Nb(2)᎐C(3)
Nb(2)᎐C(21)
N(1)᎐C(11)
N(2)᎐C(21)
2.26(1)
2.28(1)
2.32(1)
1.46(1)
1.34(1)
1
except those for the alkenylideneamide group. The H and 13C
resonances of the CH᎐N group occur at δ 5.10 and 109.1,
᎐
respectively, and they are strongly shifted upfield compared to
the values for linear alkenylideneamide transition-metal com-
pounds, for example, δ 8.39 and 164.9 for [Sc(η-C5Me5)2-
{N᎐C(H)CMe }],8 δ 9.38 and 154.4 (J = 161 Hz) for [Sc(η-
᎐
3
CH
Nb(2)᎐N(1)᎐Nb(1)
Nb(2)᎐N(2)᎐Nb(1)
C(11)᎐N(1)᎐Nb(2) 133.0(6)
C(11)᎐N(1)᎐Nb(1) 133.7(6)
C(21)᎐N(2)᎐Nb(1) 168.0(7)
C(21)᎐N(2)᎐Nb(2)
C(12)᎐C(11)᎐N(1)
C(13)᎐C(12)᎐C(11) 115.1(10)
92.9(3)
91.5(3)
C(14)᎐C(13)᎐C(12) 114.9(9)
C Me ) N᎐C(H)C H OMe}],8 δ 7.61 and 166.2 (J = 160 Hz)
᎐
5
5
2
6
4
CH
C(12)᎐C(11)᎐N(1)
N(2)᎐C(21)᎐Nb(2)
C(22)᎐C(21)᎐N(2)
C(22)᎐C(21)᎐Nb(2) 116.0(8)
C(23)᎐C(22)᎐C(21) 107.9(9)
C(24)᎐C(23)᎐C(22) 107.9(8)
114.6(9)
69.1(5)
118.0(9)
for [Ti(η-C Me ) {N᎐C(H)CMe }Cl]11 and δ 170–186 for the
᎐
5
5
2
3
compounds [M(η-C H ) {N᎐C(Me)R}L]BPh , where M᎐Ti9
᎐
᎐
5
5
2
4
or Zr, R = alkyl or Ph, L = NCR or tetrahydrofuran,10 but less
76.6(5)
114.6(9)
than in the µ-C(H)᎐N alkenylideneamide tentalum compound
᎐
(δ 3.89).12 The NMR features of this group in 2 are consistent
with its strained η2-C᎐N co-ordination to Nb(2), as observed in
᎐
the molecular structure.
A typical temperature dependence of the 1H NMR spectra of
compound 2 is shown in Fig. 3. The interchange of the methyl
groups Meb and Mec reaches the slow-exchange-limit spectrum
at 193 K and shows two singlets at δ 0.70 and 1.27. These
resonances broaden as the temperature approaches 225 K and
coalesce at Tc = 259 K. At 333 K a single narrow resonance
shows the fast-exchange-limit region has been attained. All
other 1H resonance lines remain largely unaffected by the
exchange process and remain sharp throughout the range of
temperatures studied. Below 233 K the Meb 1H resonance
sharpens to about the same value of the Mea linewidth
to be strained as suggested by the angles subtended at the
three formally sp3 carbons of the (CH2)3 linkage (average
114.9Њ). The bonding in 2 indicated by the molecular structure
suggests Nb(1) and Nb(2) are 18- and 16-electron centres,
respectively.
The mechanism of formation of compound 2 is unknown
but two observations are pertinent. First, a gas chromato-
graphic-mass spectrometric analysis of the gas-phase products
of the reaction between 1 and MgMeBr found CH4 as the only
volatile product and secondly, the formation of 2 in good yields
requires a minimum stoichiometric ratio Nb :MgMeBr of 1:3.
It is possible there is an initial extraction of a proton from the
₁
(∆ν = 3.5 Hz, at 213 K), whilst the Mec line sharpening is not so
₂
₁
pronounced (∆ν = 6 Hz, at 213 K). This broadening can be
Ϫ
₂
relatively acidic CH2N hydrogens in compound 1 by CH3 of
attributed to a faster relaxation due to a stronger interaction
the Grignard reagent giving methane. The resulting compound
would be a chloromagnesium ketimide niobium derivative,
93
9
–
with the quadrupolar moment of the Nb nucleus (I = 2). At
193 K the whole spectrum broadens due to increasing solvent
viscosity. Identical features are present in a similar variable-
[Nb{C H (CH ) C(H)᎐N(MgBr)}Cl ], which upon reaction
᎐
5
4
2
2
2
with a second molecule of 1 would substitute one chlorine
giving rise to the chlorinated precursor of 2, namely [ClNb-
{µ-C5H4(CH2)3N}{µ-NCH(CH2)2C5H4}NbCl2], and elimin-
ation of MgCl2. A similar reaction occurs21 when lithium
ketimides (LiNCR2) react with [M(η-C5H5)2Cl2], where
M = Ti or Zr. Further reaction of the chloro analogue of 2
with MgMeBr would lead to 2.
1
temperature H NMR study performed at 500 MHz, in which
the coalescence temperature is now Tc = 277 K. At this tem-
¹
24
²
perature the rate constant k = 2 π(∆ν) and the free energy of
activation (∆G‡) of the exchange process can be estimated as
1391 sϪ1 and 51.0 kJ molϪ1, respectively.
The variable-temperature 13C-{1H} NMR spectra of com-
pound 2 in the range 193–293 K are shown in Fig. 4. No 13C
resonance assignable to Mea could be observed; this absence
may be attributed to a strong scalar coupling interaction with
the quadrupolar 93Nb nucleus.25 We note the Nb(1)᎐C(1) bond
distance is approximately 0.06 Å shorter than the other two
Nb᎐Me distances. The 13C-{1H} spectra clearly show a methyl
exchange process. At 193 K the somewhat broad resonance
observed at δ 41.5 corresponds to Mec and the sharp one at δ
38.6 to Meb. At higher temperatures these lines coalesce and
finally merge into a single peak (δ 39.3) at 293 K. All the other
resonances remain sharp with increasing temperature except
that of the CgЈ alkenylidene carbon (δ 108.2 at 193 K and 109.1
at 293 K), which broadens considerably and almost disappears
at 293 K. This broadening is occurring in the same temperature
range as for the methyl exchange and, although no correspond-
ing broadening is observed for the HgЈ resonance (even at higher
temperatures, see Fig. 3), it seems reasonable to assume there is
a common underlying cause for the two observations.
Attempts to synthesize [Nb{η5,κN-C5H4(CH2)3N}Me2] or
the analogous [Nb{η5,κN-C5H4(CH2)3N}(CH2SiMe3)2], by the
addition of either stoichiometric amounts or an excess of LiMe
or the less reducing ZnMe2 or LiCH2SiMe3 to [Nb{η5,κN-C5H4-
(CH2)3N}Cl2] 1, were unsuccessful and gave complex decom-
position products.
Dynamic NMR studies on compound 2
The 1H and 13C NMR spectra of compound 2 have been
studied in detail. There are 22 inequivalent hydrogens groups
and all the 19 carbon nuclei of the asymmetric compound (C1
symmetry) are inequivalent. Using the NMR experiments22
13C-{1H} distortionless enhancement by polarisation transfer
1
(DEPT), H᎐1H shift-correlated two-dimensional spectroscopy
(COSY) and 13C᎐1H shift correlation, the spectra may be
fully assigned as shown in Table 2. Variable-temperature NMR
studies show 2 is fluxional involving the interchange of the two
methyl groups Meb and Mec. A phase-sensitive 1H᎐1H dipolar-
correlated two-dimensional (PSEXSY) NMR spectrum 23 at
233 K and using a mixing time τm = 0.5 s provided some spatial
assignments for 2 (Fig. 2). The observation of negative cross-
peaks confirmed that at 233 K Meb and Mec were slowly
exchanging, and further that these groups were the only ones
undergoing exchange. The positive NOE cross-peak correlating
the alkenylideneamide proton HgЈ (δ 5.12) with Mec (δ 1.14)
allows its spatial distinction from Meb (δ 0.52). The strong
A possible mechanism for the fluxional process is shown in
Scheme 2(a). A stereochemically non-rigid niobium site could
be envisaged undergoing some geometrical isomerisation
process26,27 possibly induced by dissociation of the strained
alkenylideneamide ligand from the niobium atom bearing Meb
and Mec. During this rearrangement, the two methyl ligands
would be able to exchange co-ordination positions by rotation,
under the ansa arch. In this equally populated A3B3 mutual
exchange process28 the planar inversion of the sp nitrogen in
the alkenylideneamide ligand may also occur [Scheme 2(b)].29,30
cross-peak obtained for HfЈ (δ 2.59) with HaЈ (δ 5.48) does not
1
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J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444

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1
Fig. 2 Two-dimensional H᎐1H PSEXSY NMR spectrum for compound 2 acquired at 233 K. Some of the most important nuclear Overhauser
effect (NOE) correlations (the heavier black lines indicate positive peaks) and the Meb /Mec chemical exchange process are indicated (the negative
peaks are magenta)
gous to the non-ansa bridged [Nb(η5-C5H5)2(η1-C5H5)(NBut)].31
The NMR data show compound 3 is fluxional in a manner
similar to that observed for [Nb(η5-C5H5)2(η1-C5H5)(NBut)].31
There are three fluxional processes, namely rotation of the η1-
C5H5 ligand about the Nb᎐Ci bond, migration of the niobium
centre amongst the carbon atoms of a η1-C5H5 ligand, and
chemical exchange between the η5- and η1-C5H5 ligands.
e1
a
e1
f1
f1
f2
e2
e2
g1
g1
g2
g2
f2
Meb
Meb
a
b'
a'
d
d
N
N
Mec
b'
c'
Nb
Nb
Nb
Nb
c'
rotation
about
Ce´ - Cf´
b
b
c
c
d'Mec
d'
N
N
Mea
a'
Mea
C
C
g'
e2'
g'
f2'
f1'
e1'
Variable-temperature 1H and 13C-{1H} NMR spectra of
compound 3 are shown in Figs. 5 and 6, respectively. The
techniques22 DEPT, 13C᎐1H shift correlation and selective
1H-decoupled 13C, were used to assign the majority of the
'
e2'
f1'
f2
e1'
N
C
Cp´
Hf1´
Hg´
C
Cp´
Hf2´
1
Hf2´
resonances. At 193 K the H NMR spectrum shows a single
Hf1´
Hg´
N
resonance for the η5-C5H5 ring at δ 6.02 and five resonances for
a ‘static’ η1-C5H5 ligand, as found for [Nb(η5-C5H5)2(η1-C5H5)-
(NBut)]. The full assignments are given in Table 2. The band
assigned to Ha(d) is a broad signal which may be attributed to a
faster relaxation due to the quadrupolar moment of the 93Nb
nucleus. Similarly, this Ha(d) line broadening is observed in the
13C-{1H} spectrum. The lowest-temperature (first) fluxional
process, namely, rotation of the η1-C5H5 ligand about the
rotation
about
Ce´ - Cf´
He1´
He2´
He1´
He2´
Scheme 1 Solid-state structure (left) and solution conformation at
Ϫ40 ЊC (right) of compound 2
Synthesis of compound 3 and dynamic NMR study
1
Treatment of [Nb{η5,κN-C5H4(CH2)3N}Cl2] 1 with sodium
cyclopentadienide gives the yellow tris(cyclopentadienyl) com-
pound [Nb{η5,κN-C5H4(CH2)3N}(η5-C5H5)(η1-C5H5)] 3 in 80%
yield. This formally 20-valence-electron compound is analo-
Nb᎐Ci bond, is better observed in the 500 MHz H and 125.7
MHz 13C-{1H} NMR spectra (in the range 183–223 K). The
olefinic protons of the η1-C5H5 ligand show pairwise coales-
cence while Hi remains at the same chemical shift. Above 223 K
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444
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Fig. 3 Variable-temperature 300 MHz 1H NMR spectra of compound 2 in [2H8]toluene. Asterisk denotes the residual CHD2 solvent resonance
the three η1-C5H5 resonances broaden and coalesce, and a broad
resonance emerges from the baseline and sharpens progres-
sively. Such behaviour is attributed to migration of the niobium
metal centre amongst the carbon atoms of the η1-C5H5 ligand,
through a 1,2-shift mechanism. Above 263 K, broadening and
coalescence (around 293 K) of the bands due to the η1- and η5-
C5H5 groups indicate that the two groups are slowly exchanging.
At the same time a similar process occurs for the diastereotopic
He and Hg chain protons since the four resonances of the
(CH2)3 chain broaden then collapse. It is interesting that the Hf
have near identical chemical shifts at all temperatures.
symmetry from C1 to Cs, since there is an apparent mirror plane
formed by the nitrogen atom, the chain carbon atoms, the sub-
stituted cyclopentadienyl ring centroid and the Nb atom. At
this temperature the process has not attained the fast-exchange
limit since the peaks are still broadened. The 13C-{1H} reson-
ance of the ipso-carbon (CY) of the C5H4 group at δ 124.7
remains sharp with increasing temperature, while the other four
carbon resonances are fully collapsed at 298 K. These features
reflect the averaging effect of the two fast exchanging C5H5
groups, and the static nature of the ansa-bridged C5H4 ring. The
exchange between the two C5H5 rings may involve η3–η5 equi-
libria.31 A summary of the fluxional processes observed for 3 is
presented in Scheme 3.
At 328 K a resonance at δ 6.00 may be assigned to coales-
cence of the signals of the two C5H5 rings; there is one reson-
ance at δ 5.18 for Hb and Hc, and one resonance for Ha and Hd,
the latter being hidden under the C5H5 peak. This spectrum
Synthesis and characterisation of compounds 4 and 5
confirms there is a fast η5-C5H5
η1-C5H5 exchange
which, in consequence, causes a shift in the apparent molecular
Treatment of compound 1 with 2, or more, equivalents of
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J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444

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Table 2 Analytical and spectroscopic data
Compound a
1 [Nb{η5,κN-C5H4(CH2)3N}Cl2]
NMR data b
1H (r.t.):c 6.57 (t, 2 H, JH H = 2.5, C5H4), 5.79 (t, 2 H, JH H = 2.5, C5H4), 3.50 (t, 2 H, JH H = 5.5, CH2N), 2.68
(m, 2 H, C5H4CH2), 2.27 (m, 2 H, C5H4CH2CH2)
C, 34.9 (33.8); H, 3.8 (3.6); N, 4.6 (4.9)
1H (r.t.):d 5.93 (t, 2 H, JH H = 2.4, C5H4), 5.04 (t, 2 H, JH H = 2.4 C5H4), 2.67 (t, 2 H, JH H = 5.4, CH2N), 1.69
(m, 2 H, C5H4CH2), 1.33 (m, 2 H, C5H4CH2CH2)
Mass spectrum: m/z 283 (Mϩ)
13C-{1H} (r.t.):c 124.0 (ipso-C of C5H4), 117.5 (CH of C5H4), 107.4 (CH of C5H4), 58.1 (CH2N), 32.3
(C5H4CH2CH2), 24.9 (C5H4CH2)
2 [MeNb{µ-C5H4(CH2)3N}{µ-NCH-
(CH2)2C5H4}NbMe2]
1H (r.t.):e 6.05 (q ϩ q, 2 H, Ha), 5.99 (q ϩ q, 2 H, JH H = 2.4, Hb ϩ HbЈ), 5.52 (q, 1 H, JH H = 2.4, HaЈ), 5.19 (q,
1 H, Hc), 5.10 (s, 1 H, HgЈ), 4.90 (q, 1 H, JH H = 2.4, HcЈ), 4.15 (m, 1 H, Hg1), 4.02 (q, 1 H, JH H = 2.4, Hd),
3.90 (m, 1 H, Hg2), 3.86 (q, 1 H, JH H = 2.4, HdЈ), 3.12 (m, 1 H, HfЈ2), 2.61 (m, 1 H, HfЈ1), 2.47 (m, 1 H, He),
2.22 (m, 1 H, HeЈ), 1.96 (m, 1 H, Hf), 1.72–1.58 (m, 3 H, He ϩ Hf ϩ HeЈ), 0.65 (br s, 6 H, Meb ϩ Mec),
Ϫ0.54 (s, 3 H, Mea)
C, 48.3 (48.5); H, 6.1 (6.0); N, 5.9 (6.0)
1H (r.t.):d 6.05 (q ϩ q, 2 H, Ha ϩ HbЈ), 5.99 (q, 1 H, JH H = 2.4, Hb), 5.55 (q, 1 H, JH H = 2.4, HaЈ), 5.18 (q ϩ s,
2 H, Hc ϩ HgЈ), 4.93 (q, 1 H, JH H = 2.4, HcЈ), 4.18 (m, 1 H, Hg1), 4.03 (q, 1 H, JH H = 2.4, Hd), 3.92 (m, 1 H,
Hg2), 3.90 (q, 1 H, JH H = 2.4, HdЈ), 3.14 (m, 1 H, HfЈ2), 2.62 (m, 1 H, HfЈ1), 2.43 (m, 1 H, He), 2.20 (m, 1 H,
HeЈ), 1.94 (m, 1 H, Hf), 173–1.55 (m, 3 H, He ϩ Hf ϩ HeЈ), 0.80 (br s, 6 H, Meb ϩ Mec), Ϫ0.47 (s, 3 H, Mea)
e1
f1
f2
e2
g1
g2
Meb
a
d
N
Mec
c'
b'
Nb
Nb
13C (r.t.):e 124.7f (ipso-C of C5H4), 121.3 (ipso-C of C5H4) 113.5 (JCH = 181, Ca or CbЈ), 112.9 (JCH = 171, Ca
or CbЈ), 109.1 (JCH = 161, br, CgЈ), 107.4 (JCH = 174, Cb), 105.5 (JCH = 179, CdЈ), 102.1 (JCH = 174, Cc),
101.6 (JCH = 175, Cd), 101.2 (JCH = 173, CaЈ), 97.1 (JCH = 171, CcЈ), 61.3 (JCH = 135. Cg), 39.3 (br,
Meb ϩ Mec, JCH = 131) (Mea missing), 38.3 (JCH = 130, Cf), 37.5 (JCH = 129, CfЈ), 27.3 (JCH = 130, Ce),
24.4 (JCH = 129, CeЈ)
b
c
d'
N
a'
Mea
C
g'
e2'
f1'
13C-{1H} (r.t.):d 124.7 (ipso-C of C5H4), 121.3 (ipso-C of C5H4), 113.6 (Ca or CbЈ), 112.6 (Ca or CbЈ), 109.0
(br, CgЈ), 107.4 (Cb), 105.4 (CdЈ), 102.2 (c), 101.6 (Cd), 101.2 (CaЈ), 97.1 (CcЈ), 61.3 (Cg), 39.2 (br, Meb ϩ Mec)
(Mea missing), 38.3 (Cf), 37.6 (CfЈ), 27.2 (Ce), 24.6 (CeЈ)
'
f2
e1'
3 [Nb{η5,κN-C5H4(CH2)3N}(η5-C5H5)-
(η1-C5H5)]
1H (20 ЊC):g vbr peaks
1H (Ϫ40 ЊC):g 6.24 (vbr, 5 H, η1-C5H5), 5.99 (q, 1 H, JH H = 2.4, Hd(a)), 5.95 (s, 5 H, η5-C5H5), 5.58 (q, 1 H,
JH H = 2.4, Hc(b)), 4.92 (q, 1 H, JH H = 2.4, Hb(c)), 3.71 (m, 1 H, CHHN), 3.62 (q, 1 H, JH H = 2.4, Ha(d)), 3.42
(m, 1 H, CHHN), 2.53 (m, 1 H, C5H4CHH), 2.22 (m, 1 H, C5H4CHH), 1.80 (m, 2 H, C5H4CH2CH2)
1H (Ϫ80 ЊC):g 6.65 (br s, 1 H, Hα(αЈ)), 6.35 (br s, 1 H, HαЈ(α)), 6.26 (br s, 1 H, Hβ(βЈ)), 6.19 (br s, 1 H, HβЉ(β)),
6.02 (br s, 6 H, η5-C5H5 and Hd(a)), 5.71 (br s, 1 H, Hc(b)), 5.22 (br s, 1 H, Hi), 4.90 (br s, 1 H, Hb(c)), 3.73 (br
m, 1 H, CHHN), 3.43 (br m, 1 H, CHHN), 3.28 (vbr, 1 H, Ha(d)), 2.56 (br m, 1 H, C5H4CHH), 2.12 (br m,
1 H, C5H4CHH), 1.79 (br m, 2 H, C5H4CH2CH2)
C, 62.7 (63.0); H, 5.8 (5.9); N, 4.1 (4.1)
Mass spectrum: m/z 343 (Mϩ)
d(a)
c(b)
13C-{1H} Ϫ40 ЊC:g δ 124.7 (CY), 116.2 (Ca(d)), 114.5 (Cc(b)), 110.4 (η5-C5H5), 107.8 (Cn(c)), 92.8 (Cd(a)), 59.2
(Cg), 28.5 (Cf), 25.8 (Ce).
Y
e
b(c)
a(d)
13C-{1H}(Ϫ80 ЊC):g 139.1 (CαЈ(α)), 137.8 (Cα(αЈ)), 124.7 (CY), 120.2 (CβЈ(β)), 120.0 (Cβ(βЈ)), 117.9 (vbr, Ca(d)),
114.5 (Cc(b)), 110.4 (η5-C5H5), 106.8 (Cb(c)), 92.4 (Cd(a)), 59.2 (Cg), 57.1 (Ci), 27.7 (Cf), 25.8 (Ce)
f
Nb
N
i
g
α′(α)
β′(β)
α(α′)
β(β′)
4 [Nb{η5,κN-C5H4(CH2)3N}(CH2Ph)2]
1H (r.t.):e 7.00 (m, 2 H, p-H of Ph), 6.98 (m, 4 H, m-H of Ph), 6.67 (m, 4 H, o-H of Ph), 5.19 (t, 2 H,
JH H = 2.5, C5H4), 4.90 (t, 2 H, JH H = 2.5, C5H4), 2.96 (t, 2 H, JH H = 5.5, CH2N), 2.00 (m, 2 H, C5H4CH2),
1.69 (d, 2 H, 2JH H = 7.8, NbCHHPh), 1.44 (m, 2 H, C5H4CH2CH2), 1.41 (d, 2 H, 2JH H = 7.8, NbCHHPh)
13C (r.t.):e 140.5 (ipso-C of Ph), 129.0 (JCH = 162, o- and m-H of Ph), 124.2 (JCH = 151, p-H of Ph), 117.3
(ipso-C of C5H4), 107.9 (JCH = 178, CH of C5H4), 101.9 (JCH = 175, CH of C5H4), 55.8 (JCH = 136,
CH2N), 40.7 (JCH = 135, NbCH2Ph), 32.9 (JCH = 128, C5H4CH2CH2), 25.3 (JCH = 127, C5H4CH2)
C, 67.2 (66.8); H, 6.3 (6.1); N, 3.5 (3.7)
5 [Nb{η5,κN-C5H4(CH2)3N}{η-CH2C-
(Me)CH2}2]h
1H (18 ЊC):e [CH2C(CH3)CH2 absent], 5.34 (br t, 2 H, JH H = 2.3, C5H4), 4.77 (br s, 2 H, C5H4), 2.79 (t, 2 H,
JH H = 5.5, CH2N), 2.24 (m, 2 H, C5H4CH2), 1.76 [br s, CH2C(CH3)CH2], 1.60 (m, 2 H, C5H4CH2CH2)
1H (100 ЊC):e 5.38 (t, 2 H, JH H = 2.3, C5H4), 4.65 (t, 2 H, JH H = 2.3, C5H4), 3.13 [vbr, 8 H, CH2C(CH3)CH2],
2.80 (t, 2 H, JH H = 5.5, CH2N), 2.29 (m, 2 H, C5H4CH2), 1.71 [s, 6 H, CH2C(CH3)CH2], 1.58 (m, 2 H,
C5H4CH2CH2)
a
b
Analytical data given as: found (calculated) %. 1H NMR at 300 MHz, 13C at 75 MHz. Data given as: chemical shift (δ), multiplicity (s = singlet,
d = doublet, t = triplet, m = multiplet; br = broad, v = very), relative intensity, coupling constant (in Hz) and assignment. c In CD2Cl2. d In [2H6]ben-
zene. e In [2H8]toluene. f Hidden under the solvent peak. g In [2H8]thf. h Characterised by NMR spectra only.
Mg(CH2Ph)Cl gives pale yellow [Nb{η5,κN-C5H4(CH2)3N}-
(CH2Ph)2] 4 in 92% yield. Compound 4 is an analogue of the
non-ansa-bridged [Nb(η5-C5Me5)(NC6H3Pri2-2,6)(CH2Ph)2]32
and apparently has isostructural relationships with the ansa-
bridged zirconocene derivative [Zr(ebthi)(CH2Ph)2] [ebthi =
rac-ethylenebis(tetrahydroindenyl)]. This 16-electron zirconium
compound has two η1-benzyl ligands.33 However, it is well
known that, depending on the degree of participation of the
phenyl π system in the metal–ligand bonding, benzyl groups are
capable of adopting a variety of co-ordination modes ranging
from η1 to η7.34,35 Among the π-benzyl d0 early-transition-metal
complexes the η2 co-ordination mode is the most common.35 It
has been noted that η2-benzyl ligands (three-electron donor)
can be differentiated from the η1-benzyls by their NMR
features.4,34,36 The η2-benzyl ligands show higher-field shifts of
the ortho 1H resonances (δ < 6.8), a higher-field shift of the CH2
13C resonance (δ < 75) together with large methylene 1JCH coup-
ling constants (>130 Hz) and smaller CH2 geminal coupling
2
constants (4 < JH H < 7 Hz), compared with those for η1-benzyl
2
groups (8 < JH H < 2 Hz).
1
The H NMR spectrum of compound 4 shows three reson-
ances for the methylene chain protons (δ 1.44, 2.00 and 2.96)
and an A2B2 pattern for the C5H4 ring protons (δ 4.90 and 5.19)
indicating a Cs symmetry, with a mirror plane containing the
nitrogen, the chain carbons, the η-C5H4 ring centroid and
the Nb atom. Two equivalent benzyl ligands are present, each
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444
2439

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Fig. 4 Variable-temperature 75.43 MHz 13C NMR spectra of compound 2 in [2H8]toluene. Asterisks denote the solvent residual resonances
possessing two diastereotopic CH2Ph protons. These appear as
an AB pattern at δ 1.69 and 1.41 with geminal JH H = 7.8 Hz. A
multiplet at δ 6.67 can be assigned to the shielded o-protons. In
the 13C NMR spectrum the CH2Ph resonance appears at a
relatively high field (δ 40.7) with a C᎐H coupling (JCH = 135 Hz)
indicating some sp2 character. According to these NMR data,
which are similar to those found for the isoelectronic zir-
conium cyclopentadienylamide analogue [Zr{η5,κN-C5H4-
(CH2)3NMe}(CH2Ph)2],4 two situations are possible. Either
both benzyl ligands are co-ordinated in a η2 fashion or there is
one η2- and one η1-co-ordinated benzyl undergoing rapid
exchange. Variable-temperature NMR experiments [room tem-
perature (r.t.) to Ϫ80 ЊC] were inconclusive since niobium
quadrupolar broadening effects operate at low temperatures.
However, no loss of symmetry was observed for 4 down to
Ϫ60 ЊC. If two η2-benzyl groups were bound to Nb then this
would give a formally 20-electron compound. As noted for
[Nb{η5,κN-C5H4(CH2)3N}(η5-C5H5)(η1-C5H5)] 3 and its non-
ansa-bridged analogue31 this is not an unprecedented situation
for cyclopentadienylimide niobium compounds.
Meb
Nb
N
Mec
Nb
N
Mea
C
‡
Mec
Nb
Me
N
N
Meb
Nb
Me
Nb
Nb
N
N
Mea
Mea
C
C
(a)
The NMR data for [Nb(η-C5Me5)(NC6H3Pri2-2,6)(CH2-
Ph)2]32 which is a close analogue of 4 suggest that in this com-
pound both the benzyl ligands are η1-co-ordinated. Thus, the
13C resonance of the CH2 group appears at δ 57.68 with a
geminal coupling JH H = 10.8 Hz and this is too large for an η2
co-ordination. The absence of η2 co-ordination by the benzyl
groups of [Nb(η-C5Me5)(NC6H3Pri2-2,6)(CH2Ph)2] may reflect
the more sterically demanding η-C5Me5 group and/or the lack
of strain introduced by the ansa-bridging propyl chain. The
latter effect could facilitate the interaction of the benzyl Ph
Nb
N
Nb
N
Nb
N
Nb
Nb
N
N
C
C
C
C
C
H
CH2
H
CH2
H
CH2
H
CH2
H
CH2
(b)
Scheme 2 (a) Proposed mechanism for the interconversion of the
methyl groups Meb and Mec of compound 2. (b) The proposed inver-
sion about the planar nitrogen in the alkenylidene ligand
2440
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444

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Fig. 5 Variable-temperature 300 MHz 1H NMR spectra of compound 3 in [2H8]thf. Asterisks denote the solvent resonances; cp = C5H5
rings with the laterally projecting lowest unoccupied molecular
orbital (LUMO) in compound 4.16
ring. The data confirm a Cs symmetry for this compound. A
broad signal at δ 1.76 is assigned to methyl groups of the
allylic ligands, whereas all the CH2 allyl protons are collapsed
into the baseline. This is a situation similar to that observed
for 3, and corresponds to chemical exchange between the two
allyl ligands at an intermediate rate. As the temperature is
raised to 100 ЊC there is a marked sharpening of the methyl
and C5H4 resonances, the latter acquiring the typical multi-
plicities of an A2B2 pattern. Simultaneously, it is observed
that a broad resonance emerges from the baseline at δ 3.13
and sharpens out progressively while approaching the fast-
exchange limit. At this temperature this resonance integrates
for eight protons, being assigned to all the anti- and syn-
protons in both of the now fast-exchanging 2-methylallyl
groups. Correspondingly, the Me peak integrates for six pro-
tons. However, at lower temperatures (e.g. Ϫ40 ЊC) very com-
plex spectra are obtained due to gradual slowing of allyl flux-
ionality.27,38 This complexity arises from the several possible
Attempts to polymerise ethylene with compound 4 were
unfruitful. The addition of B(C6F5)3 or [CPh3][B(C6F5)4], or an
excess of mao (mao = methylaluminoxane) to a toluene solu-
tion of 4 in the presence of ethylene gave no polyethylene. We
note that the compound [Nb{η5,κN-C5H4(CH2)3N}Cl2] 1 is
isoelectronic and an isolobal analogue of the compound [Zr(η5-
C5H5)2Cl2] which is a well known precursor to a homogeneous
Ziegler–Natta catalyst. However, all the attempts to poly-
merise ethylene using 1 in the presence of mao (1:1000) under
the conditions37 used for [Zr(η5-C5H5)2Cl2] were unsuccessful.
Compound 1 was treated with 2 equivalents of Mg(C3H4-
Me-2)Cl giving [Nb{η5,κN-C5H4(CH2)3N}{CH2C(Me)CH2}2] 5
1
as a colourless oil in 76% yield. The room-temperature H
NMR spectrum of 5 showed bands at δ 2.80, 2.29 and 1.58
assignable to the three methylene protons of the ansa chain
and there were two somewhat broad resonances for the C5H4
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444
2441

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Fig. 6 Variable-temperature 75.43 MHz 13C NMR spectra of compound 3 in [2H8]thf. Asterisk denotes the solvent resonance; cp = C5H5
d(a)
c(b)
b(c)
i
Y
e
Me
Nb
N
a(d)
f
Nb
Me
Nb
Nb
N
H
N
Me
N
g
α′(α)
β′(β)
C
α(α′)
H
2
3
β(β′)
(i )
(ii )
(a)
5
1
1
3
5
[Nb]
[Nb]
5
4
3
4
[Nb]
2
1
2
3
4
Nb
H
H
N
Cl
2
Cl
(b)
(iv )
1
(iii )
Nb
Nb
Nb
N
N
N
Nb
Nb
N
N
(c)
5
4
Scheme 3 Fluxional processes proposed for compound 3: (a) rotation
of the η1-C5H5 ligand about the Nb᎐Cipso bond; (b) a 1,2 shift mechan-
ism for migration of the niobium centre around the η1-C5H5 ring,
[Nb] = Nb{η5,κN-C5H4(CH2)3N}(η-C5H5); and (c) η5-C5H5 ←→ η 1 -
C 5 H 5 ring exchange
Scheme 4 Reactions of [Nb{η5,κN-C5H4(CH2)3N}Cl2] with some
alkylating agents. (i) MgMeBr (excess) in diethyl ether, room tem-
perature, 12 h; (ii) Na(C5H5) (2 equivalents) in diethyl ether–thf (1:1),
room temperature, 12 h; (iii) Mg(CH2Ph)Cl (2 equivalents) in diethyl
ether, Ϫ40 ЊC to room temperature, 4 h; (iv) Mg(C3H4Me-2)Cl (2
equivalents) in diethyl ether, Ϫ80 ЊC to room temperature, 12 h
combinations of η3 and η1 co-ordination of the two 2-methyl-
allyl groups in 5, namely as in [Nb{η5,κN-C5H4(CH2)3N}(η1-
C3H4Me-2)2],
[Nb{η5,κN-C5H4(CH2)3N}(η3-C3H4Me-2)(η1-
C3H4Me-2)] or [Nb{η5,κN-C5H4(CH2)3N}(η3-C3H4Me-2)2],
and/or from the possible existence of several allyl rotational
isomers (endo or exo).
the compounds 2–5 are shown in Scheme 4. No catalytic activ-
ity towards olefin polymerisation was found using these ansa-η-
cyclopentadienylimide compounds. It seems probable that the
lack of catalytic activity is due to the relevant frontier orbitals
In conclusion, the new reactions and structures proposed for
2442
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444

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being too dissimilar to those of the bent bis(η-cyclopenta-
dienyl) analogues. We note, however, that other mono- and bis-
(imido) complexes have been shown to be precursors to ethene
polymerisation catalysts.39–41
with pentane (50 cm3). The resulting yellow extract was filtered
from the off-white MgCl2ؒEt2O residue and the volatiles were
removed under reduced pressure. After extraction of the resi-
due with Et2O (60 cm3) and filtration, the filtrate was concen-
trated and cooled to Ϫ80 ЊC giving pale yellow crystals of 4. A
second crop of crystals was isolated by concentration of the
mother-liquor. Combined yield 0.31 g, 92%.
Experimental
All manipulations and reactions were carried out under an
atmosphere of dinitrogen (<10 ppm oxygen or water) using
standard Schlenk vessels and vacuum-line techniques or in an
inert-atmosphere box. Solvents were predried over activated 4
Å molecular sieves and then distilled under an atmosphere of
dinitrogen from potassium (tetrahydrofuran), sodium (tolu-
ene), sodium–potassium alloy [light petroleum (b.p. 40–60 ЊC)
and diethyl ether] and stored under dinitrogen. Deuteriated
NMR solvents were stored in ampoules over activated molecu-
lar sieves (C6D6, [2H8]toluene and CD2Cl2) or dried using Na/K
alloy ([2H8]tetrahydrofuran) and transferred by vacuum
distillation.
[Nb{ç5,êN-C5H4(CH2)3N}{CH2C(Me)CH2}2] 5. A suspension
of compound 1 (0.287 g, 1.01 mmol) in Et2O (30 cm3) was
cooled to Ϫ80 ЊC and a solution of Mg(C3H4Me-2)Cl in Et2O
(4.9 cm3 of a 0.42 mol dmϪ3 solution, 2.06 mmol) was added
dropwise. The mixture was allowed to warm to r.t. and stirred
overnight. The solvent was removed under reduced pressure
and the residue extracted with pentane (50 cm3). The resulting
colourless solution was filtered from the off-white MgCl2ؒEt2O
residue and the volatiles were removed under reduced pressure.
After extraction with Et2O (60 cm3) and filtration, the solvent
was removed under reduced pressure to give 5 as an off-white
oil. Yield 0.230 g, 76%.
The NMR spectra were recorded on a Brüker AM-300 [1H,
300 MHz; 13C, 75.43 MHz] or AM-500 [1H, 500 MHz; 13C,
1
125.72 MHz] and referenced internally using residual H and
Attempted polymerisation of ethylene
13C solvent resonances relative to tetramethylsilane (δ 0). Low-
resolution mass spectra were obtained on an AEI MS 302 mass
spectrometer, updated by a data-handling system supplied by
Mass Spectroscopy Services Ltd. Elemental analyses were per-
formed by the Analytical Laboratory in this Department.
Using the [Nb{ç5,êN-C5H4(CH2)3N}Cl2]–mao system. A solu-
tion of mao (2.311 g, 39.8 mmol) in toluene (15 cm3) at 25 ЊC
was equilibrated with ethylene at 2 bar relative pressure. A yel-
low toluene solution (5 cm3) of compound 1 (0.011 g, 0.039
mmol) was added to the mao solution and the pressure was
maintained at 2 bar. Initially the solution turned to pale orange.
After 26 h no polyethylene was observed and the deep orange
reaction mixture contained a small amount of a deep red pre-
cipitate. Work-up of the reaction mixture showed the absence
of polyethylene fractions.
Syntheses
[Nb{ç5,êN-C5H4(CH2)3N}Cl2] 1. A solution of C5H4(SiMe3)-
1,2
(CH2)3N(SiMe3)2 (3.60 g, 10.60 mmol) in CH2Cl2 (100 cm3)
was added dropwise to a suspension of NbCl5 (2.863 g, 10.60
mmol) in CH2Cl2 (500 cm3) at Ϫ80 ЊC. The mixture was allowed
to warm to r.t. and was stirred for 3 d. The solvent was removed
under vacuum and the residue extracted with toluene (3 × 60
cm3) at 80 ЊC. The resulting dark brown extract was concen-
trated until a powdery precipitate separated. The mixture was
then cooled to Ϫ80 ЊC. After 3 d a yellow powder was obtained.
Yield 1.58 g, 53%.
Using
the
[Nb{ç5,êN-C5H4(CH2)3N}(CH2Ph)2]–[CPh3]-
[B(C6F5)4] system. A pale yellow solution of compound 4 (0.018
g, 0.046 mmol) in toluene (25 cm3) was transferred to a Fischer-
Porter pressure vessel. This solution at 25 ЊC was equilibrated
with ethylene at 2 bar relative pressure. Addition of an orange
solution of [CPh3][B(C6F5)4] (0.041 g, 0.046 mmol) in toluene
(25 cm3) and re-establishment of the ethylene pressure gave,
after 2 h, a deep red solution with a trace of a dark precipitate.
After 7 h the solution was greenish and a dark precipitate could
be observed. After 17 h the excess of ethylene was vented and
some drops of methanol were added which immediately
recovered a red colour. Subsequent addition of methanol
caused decomposition of the system. No polyethylene fractions
could be recovered after the mixture work-up.
[MeNb{ì-C5H4(CH2)3N}{ì-NCH(CH2)2C5H4}NbMe2] 2. A
suspension of compound 1 (0.38 g, 1.35 mmol) in Et2O (30 cm3)
was treated dropwise at r.t. with an excess of MgMeBr in Et2O
(2.20 cm3 of 1.87 mol dmϪ3 solution, 4.11 mmol). An initial
yellow-orange oily precipitate gradually transformed into an
orange solution and a pale MgBrClؒEt2O precipitate. This
mixture was stirred for 12 h at r.t. The volatiles were removed
under reduced pressure and the residue was extracted with
pentane (60 cm3). The solvent was removed from the filtrate
under reduced pressure and the residue redissolved in Et2O
(30 cm3). The resulting orange extract was filtered, concentrated
to about 10 cm3 and cooled to Ϫ80 ЊC giving orange crystals.
Yield 0.19 g, 60%.
Crystallography
Data collection and processing parameters for compound 2 are
given in Table 3. Data were collected on an Enraf-Nonius
CAD4 diffractometer, an empirical absorption correction
based on azimuthal ψ-scan data was applied and the data were
corrected for Lorentz-polarisation effects. Systematically absent
reflections were rejected and equivalent reflections were merged.
The heavy-atom positions were determined from a Patterson
synthesis. Subsequent Fourier-difference syntheses revealed the
positions of all other non-hydrogen atoms. Hydrogen atoms
could not be located from difference syntheses and were placed
in estimated positions (C᎐H 0.96 Å) with isotropic thermal
parameters 1.3 times that of the Uequiv of the supporting carbon
atom. The non-hydrogen atoms were refined against Fo using
full-matrix least-squares procedures with the hydrogen atoms
‘riding’ on their supporting carbon atoms. The data were cor-
rected for the effects of anomalous dispersion and isotropic
extinction in the final stages of refinement. Crystallographic
calculations were carried out using the CRYSTALS42 program
on a MicroVAX 3800 computer in the Chemical Crystal-
lography Laboratory, Oxford.
[Nb{ç5,êN-C5H4(CH2)3N}(ç5-C5H5)(ç1-C5H5)] 3. To a sus-
pension of compound 1 (0.31 g, 1.09 mmol) in Et2O (40 cm3)
was added dropwise a solution of Na(C5H5) (0.21 g, 2.38 mmol)
in thf (40 cm3) at r.t. The mixture changed from yellow to
orange in the first minutes and was stirred overnight. The sol-
vent was removed under reduced pressure and the residue
extracted with Et2O (70 cm3). Concentration and cooling of the
extracts to Ϫ80 ЊC gave two crops of yellow microcrystals of 3.
Yield 0.3 g, 80%.
[Nb{ç5,êN-C5H4(CH2)3N}(CH2Ph)2] 4.
A suspension of
compound 1 (0.24 g, 0.85 mmol) in Et2O (30 cm3) was cooled to
Ϫ40 ЊC and a solution of Mg(CH2Ph)Cl in Et2O (1.42 cm3 of a
1.2 mol dmϪ3 solution, 1.70 mmol) added dropwise. The mix-
ture was allowed to warm to r.t. and stirred for 4 h. The solvent
was removed under reduced pressure and the residue extracted
J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444
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Table 3 Data collection and processing parameters for compound 2
Formula
M
C19H28N2Nb2
470.26
Crystal size/mm
Crystal system
Space group
0.25 × 0.40 × 0.60
Orthorhombic
Pn21a
a/Å
b/Å
18.935(5)
7.354(8)
13.374(5)
1862.2
c/Å
U/Å3
Z
4
Dc /g cmϪ3
1.677
Radiation (λ/Å)
Mo-Kα (0.710 69)
11.79
µ/cmϪ1
F (000)
952
2θ Limits /Њ
2–50
Scan mode
ω–2θ
Total unique data collected
No. observations [I > 3σ(I )]
No. variables
1773
1572
209
Obs./variables
7.5
Weighting scheme
Maximum, minimum peaks in final
difference map/e ÅϪ3
r.m.s. shift/e.s.d. in final least-squares
cycle
Unit weights
0.48, Ϫ0.03
0.022
R a
0.032
0.037
¹
RЈb
R = Σ||Fo| Ϫ |Fc||/Σ|Fo|. b RЈ = [Σw(|Fo| Ϫ |Fc|)2/Σw|Fo|2]².
a
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Atomic coordinates, thermal parameters, and bond lengths
and angles have been deposited at the Cambridge Crystallo-
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CCDC for this material should quote the full literature citation
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25 M. L. H. Green, D. O’Hare and J. G. Watkin, J. Chem. Soc.,
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Acknowledgements
We thank NATO and Junta Nacional de Investigação Científica
e
Tecnológica (Portugal) for fellowships (to P. T. G.
and A. M. M., respectively), Wolfson College, Oxford for a
Junior Research Fellowship (to P. M.) and Drs. L.-L. Wong,
J. Claridge and J. R. Ascenso for helpful discussions.
31 M. L. H. Green, D. M. Michaelidou, P. Mountford, A. G. Suárez
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Received 30th January 1997; Paper 7/00713B
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J. Chem. Soc., Dalton Trans., 1997, Pages 2435–2444