Synthesis and reactions of (rer/-butyliniido)bis(r|-cyclopentadienyl)niobium cations: NMR evidence for d° olefin cations [NbKn-CsH52XlBu (η-C2HOltBCC5)4] and [Nb{(ii-C5H4)CMe2(i1

Article abstract of DOI:10.1039/b004062m

The new (/m-butylimido)n-cyclopentadienylniobium compounds [Nb(Ti-C₅H₅)₂(N'Bu)(OjSCFj)] 1, [Nb(n-C₅H₅)₂(N'Bu)(PMe₃)][O ₃SCF₃] 2, [Nb(n-C₅H_5<

Full text of DOI:10.1039/b004062m

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Synthesis and reactions of (tert-butylimido)bis(ꢀ-cyclopenta-
dienyl)niobium cations: NMR evidence for d0 olefin cations
[Nb{(ꢀ-C5H5)2}(NtBu)(ꢀ-C2H4)][B(C6F5)4] and [Nb{(ꢀ-C5H4)-
CMe2(ꢀ-C5H4)}(NtBu)(ꢀ-C2H3Me)][B(C6F5)4]
Martin J. Humphries, Richard E. Douthwaite and Malcolm L. H. Green*
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
Received 22nd May 2000, Accepted 5th July 2000
Published on the Web 9th August 2000
The new (tert-butylimido)η-cyclopentadienylniobium compounds [Nb(η-C5H5)2(NtBu)(O3SCF3)] 1*, [Nb(η-C5H5)2-
(NtBu)(PMe3)][O3SCF3] 2*, [Nb(η-C5H5)2(NtBu)][MeB(C6F5)3] 3, [Nb(η-C5H5)2(NtBu)][B(C6F5)4] 4, [Nb(η-C5H5)2-
(NtBu)(η-C2H4)][B(C6F5)4] 5, [Nb{(η-C5H4)2CMe2}(NtBu)(O3SCF3)] 6, [Nb{(η-C5H4)2CMe2}(NtBu)][B(C6F5)4] 7,
[Nb{(η-C5H4)2CMe2}(NtBu)(η-C2H4)][B(C6F5)4] 8 and [Nb{(η-C5H4)2CMe2}(NtBu)(η-C3H6)][B(C6F5)4] 9 have been
prepared and characterised; * indicates the crystal structure has been determined.
Introduction
The imido ligand has found widespread use in the chemistry of
the early transition metals both due to its isoelectronic relation-
ship with the cyclopentadiene ligand and the ease with which
derivatives of differing bulk and electronic properties can be
synthesized by variation of the group attached to the nitrogen
atom. As such, the literature concerning these complexes is
extensive, however it has been reviewed within the past few
years.1 Of particular interest to us were those complexes in
which co-ordinatively unsaturated imido complexes of van-
adium,2,3 tantalum4 and zirconium5–7 have displayed reactivi-
ties including C–H activation.
The molecular fragments [Zr(η-C5H5)2(NtBu)] and the cation
[Nb(η-C5H5)2(NtBu)]ϩ are isoelectronic. The neutral transient
[Zr(η-C5H5)2(NtBu)] has been associated with the activation of
the C–H bond in benzene and cycloaddition reactions with
alkenes and alkynes.6,7 The presence of a co-ordinatively
Fig. 1 The molecular structure of [Nb(η-C5H5)2(NtBu)(O3SCF3)] 1.
unsaturated, highly electrophilic niobium atom in the cation
Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have
[Nb(η-C5H5)2(NtBu)]ϩ could lead to high reactivities in com-
been omitted for clarity.
parable reactions and the greater electrophilicity of the metal
centre could lead to the cationic niobium compounds being
more reactive than their neutral zirconium analogues.
it is coupled to three equivalent fluorine atoms. There is a large
chemical shift difference (∆δ = 41.6 ppm) between the quater-
nary (Cα) and methyl (Cβ) carbons of the t-butyl group on the
imido ligand. This chemical shift difference is thought to give a
During the course of the work described below two ansa-
bridged compounds [Nb{(η-C5H4)2CMe2}(HNMe2)(NtBu)]-
[BPh4]8 and [Nb{(η-C5H4)CMe2(η3-C9H7)}(HNMe2)(NtBu)]-
qualitative indication of the degree of electron density donated
[BPh4]9 have been reported, as well as the tantalum compound
by the nitrogen atom to the metal.13 A ∆δ value of 41.6 ppm lies
[Ta(η-C5Me5)2(NtBu)(THF)][B(C6F5)4].10 Here we describe fur-
at the high end of the range previously reported for t-butyl-
ther studies on derivatives of the [Nb(η-C5H5)2(NtBu)]ϩ cation.
imido species, and is higher than any so far reported for a
niobium compound.11,13–18 This indicates a low electron density
on the nitrogen atom due to a high degree of nitrogen pπ to
metal dπ donation. The electron impact mass spectrum con-
Results and discussion
Treatment of [Nb(η-C5H5)2(NtBu)Cl]11,12 with AgO3SCF3
yielded [Nb(η-C5H5)2(NtBu)(O3SCF3)] 1 as slightly air- and
moisture-sensitive orange crystals that were soluble in all com-
mon non-protic organic solvents. The analytical and spectro-
scopic data which characterise the compound 1, and all the
other new compounds described in this work, are given in Table
1. The 1H and 13C-{1H} NMR spectra are consistent with both
of the possible formulations, [Nb(η-C5H5)2(NtBu)(O3SCF3)]
and [Nb(η-C5H5)2(NtBu)][O3SCF3], with the resonances of the
nuclei in the cyclopentadienyl and t-butyl groups being
observed as singlets. The 13C-{1H} NMR spectrum also shows
the resonance of the carbon of the triflate group as a quartet as
tained an envelope assignable to the molecular ion at m/z = 443.
The crystal structure of [Nb(η-C5H5)2(NtBu)(O3SCF3)] 1 has
been determined; the molecular structure is given in Fig. 1 and
selected interatomic distances and angles are listed in Table 2.
The triflate group is bound to the metal centre, as suggested by
the solubility of 1. The imido ligand is almost linear (Nb(1)–
N(2)–C(3) 176.4(2)Њ), implying that the nitrogen is sp hybrid-
ised and acting as a four-electron donor. The Nb(1)–N(2) bond
length of 1.758(2) Å lies in the range of those in other niobium
imido compounds in which the imido ligand formally donates
four electrons to the metal centre.1,8,19–21 The distances from the
niobium atom to the carbons of the cyclopentadienyl ring vary
2952
J. Chem. Soc., Dalton Trans., 2000, 2952–2959
This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b004062m

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Table 1 Analytical and spectroscopic data
Compound and analytical dataa
NMR datab
[Nb(η-C5H5)2(NtBu)(O3SCF3)] 1
Orange needles
C 40.6 (40.6), H 4.3 (4.3), N 3.1 (3.2)
EI MS: m/z = 443, Mϩ, 12%; 428, [M Ϫ Me]ϩ, 100%
[Nb(η-C5H5)2(NtBu)(PMe3)][O3SCF3] 2
Yellow crystals
1Hc in C6D6: 5.79 (s, 10 H, C5H5), 0.96 (s, 9 H, NC(CH3)3)
13C-{1H}d in C6D6: 120.09 (q, CF3), 112.49 (C5H5), 71.10 (NC(CH3)3), 29.47
(NC(CH3)3)
19Fe in C6D6: Ϫ77.40 (O3SCF33)
1H in C4D8O: 6.29 (d, 10 H, JP-H = 2, C5H5), 1.99 (d, 9 H, JP-H = 10, P(CH3)3), 1.09
2
(s, 9 H, NC(CH3)3)
C 41.7 (41.6), H 5.4 (5.4), N 2.7 (2.7)
13C-{1H} in C4D8O: 110.31 (C5H5), 68.11 (NC(CH3)3), 31.61 (NC(CH3)3), 19.20 (d,
1JP-C = 30.1, P(CH3)3)
ES MS: m/z = 370, Mϩ, 100%
19F in C4D8O: Ϫ80.56 (O3SCF3)
[Nb(η-C5H5)2(NtBu)][MeB(C6F5)3] 3
Red-brown
1H: 5.65 (s, 10 H, C5H5), 1.11 (br s, 3 H, CH3B(C6F5)3), 0.71 (s, 9 H, NC(CH3)3)
13C-{1H}: 150.16 (br d, 1JC-F ≈ 242, C6F5), 139.14 (br d, 1JC-F ≈ 240, C6F5), 138.17 (br d,
1JC-F ≈ 240, C6F5), not found (ipso C6F5), 114.43 (C5H5), 75.38 (NC(CH3)3), 31.17
(NC(CH3)3), 12.61 (br, CH3B(C6F5)3)
19F: Ϫ136.41 (m, o-F of C6F5), Ϫ168.34 (m, p-F of C6F5), Ϫ170.93 (m, m-F of C6F5)
11B: f Ϫ14.12 (CH3B(C6F5)3)
[Nb(η-C5H5)2(NtBu)][B(C6F5)4] 4
Red-brown
1H: 5.68 (s, 10 H, C5H5), 0.71 (s, 9 H, NC(CH3)3)
13C-{1H}: 150.44 (br d, 1JC-F ≈ 241, C6F5), 140.20 (br d, 1JC-F ≈ 248, C6F5), 138.22 (br d,
1JC-F ≈ 235, C6F5), 126.20 (br s, ipso-C of C6F5), 114.30 (C5H5), 75.01 (NC(CH3)3), 29.90
(NC(CH3)3)
19F: Ϫ134.84 (m, o-F of C6F5), Ϫ164.56 (m, p-F of C6F5), Ϫ168.54 (m, m-F of C6F5)
11B: Ϫ15.88 (B(C6F5)4)
[Nb(η-C5H5)2(NtBu)(η-C2H4)][B(C6F5)4] 5
Red-brown
1H: 5.40 (s, 10 H, C5H5), 4.10 (s, 4 H, C2H4), 0.71 (s, 9 H, NC(CH3)3)
13C-{1H}: 150.26 (br d, 1JC-F ≈ 238, C6F5), 140.09 (br d, 1JC-F ≈ 231, C6F5), 138.27 (br d,
1JC-F ≈ 243, C6F5), 126.20 (br s, ipso-C of C6F5), 112.45 (C5H5), 82.97 (C2H4), not found
(NC(CH3)3), 31.50 (NC(CH3)3)
19F: Ϫ134.81 (m, o-F of C6F5), Ϫ164.60 (m, p-F of C6F5), Ϫ168.59 (m, m-F of C6F5)
11B: Ϫ15.91 (B(C6F5)4)
[Nb{(η-C5H4)2CMe2}(NtBu)(O3SCF3)] 6
Orange needles
C 44.8 (44.7), H 5.0 (4.8), N 2.8 (2.9)
EI MS: m/z = 483, Mϩ, 15%; 468, [M Ϫ Me]ϩ, 100%;
412 [M Ϫ NtBu]ϩ, 32%
1H in C6D6: δ 6.47 (m, 2 H, C5H4), 5.83 (m, 2 H, C5H4), 5.63 (m, 2 H, C5H4), 5.39 (m,
2 H, C5H4), 1.60 (s, 3 H, C(CH3)2), 0.99 (s, 3 H, C(CH3)2), 0.93 (s, 9 H, NC(CH3)3)
13C-{1H} in C6D6: 144.62 (ipso-C of C5H4), 117.72 (C5H4), 112.80 (C5H4), 103.58
(C5H4), 95.68 (C5H4), 70.57 (NC(CH3)3), 29.10 (NC(CH3)3), 25.70 (C(CH3)2), 22.04
(C(CH3)2), 21.02 (C(CH3)2)
19F in C6D6: Ϫ77.38 (O3SCF3)
[Nb{(η-C5H4)2CMe2}(NtBu)][B(C6F5)4] 7
Red-brown
1H: 6.04 (m, 2 H, C5H4), 5.79 (m, 4 H, C5H4), 4.90 (m, 2 H, C5H4), 1.27 (s, 3 H,
C(CH3)2), 0.96 (s, 3 H, C(CH3)2), 0.71 (s, 9 H, NC(CH3)3)
1
13C-{1H}: 150.01 (br d, JC-F ≈ 236, C6F5), 147.23 (ipso-C of C5H4), 139.91 (br d,
1
1JC-F ≈ 243, C6F5), 138.00 (br d, JC-F ≈ 234, C6F5), 126.20 (br s, ipso-C of C6F5),
116.46 (C5H4), 115.96 (C5H4), 104.52 (C5H4), 100.38 (C5H4), 74.42 (NC(CH3)3), 38.26
(C(CH3)2), 31.31 (NC(CH3)3), 24.22 (C(CH3)2), 22.95 (C(CH3)2)
19F: Ϫ134.86 (m, o-F of C6F5), Ϫ164.55 (m, p-F of C6F5), Ϫ168.59 (m, m-F of C6F5)
11B: Ϫ15.90 (B(C6F5)4)
[Nb{(η-C5H4)2CMe2}(NtBu)(η-C2H4)][B(C6F5)4] 8
1H: 6.11 (m, 2 H, C5H4), 5.21 (m, 2 H, C5H4), 5.19 (m, 2 H, C5H4), 4.98 (m, 2 H, C5H4),
4.87 (s, 4 H, C2H4), 1.28 (s, 3 H, C(CH3)2), 0.99 (s, 3 H, C(CH3)2), 0.74 (s, 9 H,
NC(CH3)3)
Red-brown
1
13C-{1H}: 150.08 (br d, JC-F ≈ 240, C6F5), 140.59 (ipso-C of C5H4), 140.73 (br d,
1JC-F ≈ 243, C6F5), 137.99 (br d, 1JC-F ≈ 236, C6F5), 126.20 (br s, ipso-C of C6F5), 115.96
(C5H4), 115.21 (C5H4), 114.82 (C5H4), 100.89 (C5H4), 94.02 (C2H4), not found
(NC(CH3)3), 37.74 (C(CH3)2), 31.38 (NC(CH3)3), 24.22 (C(CH3)2), 22.85 (C(CH3)2)
19F: Ϫ134.79 (m, o-F of C6F5), Ϫ164.57 (m, p-F of C6F5), Ϫ168.55 (m, m-F of C6F5)
11B: Ϫ15.90 (B(C6F5)4)
[Nb{(η-C5H4)2CMe2}(NtBu)(η-C3H6)][B(C6F5)4] 9
Red-brown
1H: 6.67 (m, 1 H, H2CCH(CH3)), 6.03 (m, 1 H, C5H4 A), 5.94 (m, 1 H, C5H4 B), 5.53 (m,
1 H, C5H4 A), 5.41 (m, 1 H, C5H4 B), 5.39 (m, 1 H, C5H4 B), 5.18 (m, 1 H, C5H4 A), 5.00
2
3
(m, 1 H, C5H4 A), 4.84 (m, 1 H, C5H4 B), 4.77 (dd, JC-H = 3, JC-H = 17.5, 1 H, trans
2
3
H2CCH(CH3)), 4.60 (dd, JC-H = 3, JC-H = 9, 1 H, cis H2CCH(CH3)), 1.60 (d, 3 H,
H2CCH(CH3)), 1.29 (s, 3 H, C(CH3)2), 1.02 (s, 3 H, C(CH3)2), 0.75 (s, 9 H, NC(CH3)3)
1
13C-{1H}: 150.26 (br d, JC-F ≈ 242, C6F5), 141.08 (ipso-C of C5H4 A, B), 140.17
1
1
(H2CCHCH3), 140.16 (br d, JC-F ≈ 243, C6F5), 136.56 (br d, JC-F ≈ 238, C6F5), 126.20
(br s, ipso-C of C6F5), 116.86 (C5H4 A), 116.36 (C5H4 B), 115.63 (C5H4 B), 114.79 (C5H4
A), 102.26 (C5H4 B), 101.47 (C5H4 A), 98.79 (C5H4 A, B), 82.78 (H2CCHCH3), not
found (NC(CH3)3), 37.74 (C(CH3)2), 31.46 (NC(CH3)3), 24.69 (H2CCHCH3), 24.31
(C(CH3)2), 24.10 (C(CH3)2)
19F: Ϫ134.84 (m, o-F of C6F5), Ϫ164.58 (m, p-F of C6F5), Ϫ168.54 (m, m-F of C6F5)
11B: Ϫ15.87 (B(C6F5)4)
A, B signify the two inequivalent cyclopentadienyl rings
a Given as % found (calc.). b Given as: chemical shift (δ) (multiplicity, rel. intensity, J/Hz, assignment), etc. In C6D5Br unless otherwise stated. c At 500
MHz. d At 125 MHz. e At 470.4 MHz. f At 160.4 MHz.
considerably, from 2.402 to 2.565 Å, with the longest bond
lengths being those trans to the imido ligand (Nb(1)–C(15)
2.565(4), Nb(1)–C(16) 2.516(4) Å). This leads to the niobium–
ring centroid distances being relatively long, at 2.188 and
2.201 Å. The intra-ring C–C distances are also irregular,
although with no obvious pattern.
ring are similar to those noted in the previously reported
compounds [Mo(η-C5H4Me)2(NtBu)] and [Mo(η-C5H4Me)2-
(NtBu)Me][BF4],22 [Nb(η-C5H5)2(η1-C5H5)(NtBu)]15 and [Nb-
(η-C5H5)2(NtBu)Cl].11 These bis(η-cyclopentadienyl)imido
compounds formally have a 20 electron configuration, however
the structural data suggest that the lone pair on the imido
ligand causes some destabilisation of the bonding in the ring
These features of the bonding of the cyclopentadienyl
J. Chem. Soc., Dalton Trans., 2000, 2952–2959
2953

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sidered linear,1 hence the nitrogen is still sp hybridised. The
Table 2 Selected interatomic distances (Å) and angles (Њ) for [Nb-
(η-C5H5)2(NtBu)(O3SCF3)]
Nb(1)–N(2) distance of 1.779(2) Å is greater than that of the
neutral parent compound, however it is still in the region
expected for a Nb–N distance where the nitrogen is considered
to act as a four electron donor. The Nb–P bond length
(2.5974(7) Å) is at the low end of the range of values found for
previously reported niobium() compounds (2.577–2.649
Å).14,23–27 The bis(η-cyclopentadienyl)niobium ring system is
destabilised in a manner similar to that observed in 1; however
the cyclopentadienyl rings are even further from the metal
centre in 2 than in 1. Thus the Nb–C distances are in the range
2.479–2.570 Å, as opposed to 2.402–2.565 Å, leading to the
niobium–ring centroid distances also being longer than in 1, at
2.192 and 2.211 Å.
Nb(1)–N(2)
Nb(1)–O(7)
Nb(1)–C(16)
Nb(1)–C(18)
Nb(1)–C(20)
Nb(1)–C(22)
Nb(1)–C(24)
1.758(2)
2.142(2)
2.516(4)
2.402(4)
2.462(3)
2.564(3)
2.472(3)
N(2)–C(3)
1.450(3)
2.565(4)
2.435(4)
2.462(4)
2.533(3)
2.455(3)
Nb(1)–C(15)
Nb(1)–C(17)
Nb(1)–C(19)
Nb(1)–C(21)
Nb(1)–C(23)
N(2)–Nb(1)–O(7)
Cpcent–Nb(1)–Cpcent
97.95(9)
126.0
Nb(1)–N(2)–C(3)
176.4(2)
Table 3 Selected interatomic distances (Å) and angles (Њ) for [Nb-
(η-C5H5)2(NtBu)(PMe3)][O3SCF3] 2
The addition of a solution of B(C6F5)3 in d5-bromobenzene
to a solution of [Nb(η-C5H5)2(NtBu)Me] in d5-bromobenzene
caused an immediate change from yellow to red-brown. The
1H NMR spectrum of [Nb(η-C5H5)2(NtBu)Me] in d5-bromo-
benzene shows singlets at δ 5.55, 0.90 and 0.77 for the protons
on the cyclopentadienyl ring, t-butyl and methyl group in a
10:9:3 ratio respectively. Addition of B(C6F5)3 in d5-bromo-
benzene gives a new spectrum with the chemical shifts assign-
able to the protons of the cyclopentadienyl ring and t-butyl
group being δ 5.65 and 0.71 respectively. The methyl protons
of the [MeB(C6F5)3]Ϫ anion are observed as a broad singlet at
δ 1.11. In addition a small amount of decomposition products
is observed. The 11B NMR spectrum of 3 shows a singlet at
δ Ϫ14.12, in the region expected for a four-co-ordinate boron
species, consistent with the formation of the [MeB(C6F5)3]Ϫ
anion. The 19F NMR spectrum contained three multiplets at
δ Ϫ136.41, Ϫ168.34 and Ϫ170.93. A study of compounds con-
taining the [MeB(C6F5)3]Ϫ anion found that a good indication
of its degree of interaction with a metal centre is given by the
difference in chemical shifts between the meta- and para-
fluorines. A value greater than 3 ppm indicates a significant
interaction between the two ions whilst a value of less than
3 ppm implies that this is not the case.28 These data suggest
the product to be the compound [Nb(η-C5H5)2(NtBu)][MeB-
(C6F5)3] 3. The separation of the resonances of the α and
β carbons of the t-butyl group, ∆δ, is 44.2 ppm. This is
even greater than that observed for compound 1, reflecting the
greater degree of nitrogen pπ to metal dπ donation required
to stabilise the cationic metal centre in 3. It is likely that the
cationic fragment is heavily solvated rather than existing as the
naked cation.
Nb(1)–N(2)
Nb(1)–P(15)
Nb(1)–C(20)
Nb(1)–C(22)
Nb(1)–C(24)
Nb(1)–C(26)
Nb(1)–C(28)
1.779(2)
2.5974(7)
2.486(3)
2.517(3)
2.517(3)
2.530(3)
2.488(3)
N(2)–C(3)
1.436(3)
2.479(3)
2.562(3)
2.485(3)
2.570(3)
2.431(3)
Nb(1)–C(19)
Nb(1)–C(21)
Nb(1)–C(23)
Nb(1)–C(25)
Nb(1)–C(27)
P(15)–Nb(1)–N(2)
Cpcent–Nb(1)–Cpcent
90.71(7)
126.8
Nb(1)–N(2)–C(3)
171.90(19)
Fig. 2 Molecular structure of [Nb(η-C5H5)2(NtBu)(PMe3)][O3SCF3] 2.
Details as in Fig. 1.
system. They are thus best considered to be 18 ϩ 2 electron
compounds, with eighteen bonding electrons, plus two in a
ligand-based non-bonding orbital.
The reaction between [Nb(η-C5H5)2(NtBu)Me] and [Ph3C]-
[B(C6F5)4] in d5-bromobenzene affords a compound which
NMR data suggest to be the salt [Nb(η-C5H5)2(NtBu)]-
[B(C6F5)4] 4, in around 80% yield, along with 1,1,1-
triphenylethane. The NMR peaks assignable to the protons of
the cyclopentadienyl ring and t-butyl group of 4 are observed as
singlets at δ 5.68 and 0.71 respectively, in much the same posi-
tions as those of 3, as would be expected if the two both contain
the base-free cation [Nb(η-C5H5)2(NtBu)]ϩ. The remaining
20% of the product consists of a side-product that could not be
identified, but which did not participate in any of the reactions
discussed below. The 13C-{1H} NMR spectrum is also consist-
ent with the formation of the ion pair 4, along with the minor
side-product and Ph3CMe. The value of ∆δ for the t-butyl
group of the imido ligand (45.1 ppm) of 4 is even greater
than that of 3 (44.2 ppm). The 11B NMR spectrum shows a
single peak at δ Ϫ15.88, typical of a four-co-ordinate boron
species, with the 19F NMR spectrum showing three multiplets at
δ Ϫ134.84, Ϫ164.56 and Ϫ168.54.
The compound [Nb(η-C5H5)2(NtBu)(O3SCF3)] 1 in benzene
was treated with a solution of trimethylphosphine in benzene to
give yellow crystals of [Nb(η-C5H5)2(NtBu)(PMe3)][O3SCF3] 2,
which is soluble in THF but decomposed slowly in dichloro-
1
methane. The H NMR spectrum of 2 in d8-THF is consistent
with displacement of the triflate group from the niobium centre
to form an ion pair. A singlet was observed for the protons of
the t-butyl group, a doublet for the CH3 groups in PMe3 and a
doublet for the protons of the cyclopentadienyl due to coupling
to the phosphorus atom of the phosphine. The difference in
chemical shift between the α and β carbons of the t-butyl group,
∆δ, is 36.5 ppm, reflecting the increased electron density on the
niobium centre arising from the replacement of the triflate
group by the more electron-donating phosphine group, despite
the formal positive charge on the metal centre. The electrospray
mass spectrum (positive ion) contained the molecular ion at
m/z 370 as the 100% peak.
The crystal structure of [Nb(η-C5H5)2(NtBu)(PMe3)]-
[O3SCF3] 2 has been determined. The molecular structure is
shown in Fig. 2 and selected interatomic distances and angles
are given in Table 3. The compound 2 forms an ion pair. The
Nb(1)–N(2)–C(3) linkage is slightly more bent than in 1, how-
ever the angle of 171.90(19)Њ is still in the range that is con-
All further reactions were performed using complex 4 as
opposed to 3 as it was found that the [B(C6F5)4]Ϫ anion was less
co-ordinating than the [MeB(C6F5)3]Ϫ anion.
A sample of the salt [Nb(η-C5H5)2(NtBu)][B(C6F5)4] 4 in d5-
bromobenzene in a NMR tube fitted with a Teflon valve was
freeze–pump–thaw degassed and placed under an atmosphere
2954
J. Chem. Soc., Dalton Trans., 2000, 2952–2959

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Fig. 3 Possible equilibrium between complexes 5a and 5b.
Fig. 4 A Van’t Hoff plot of ln(Kc) vs. 1/T for the equilibria between
of ethene at room temperature, and the NMR spectrum
recorded. The 1H spectrum showed the original cation still to be
present, along with new resonances at δ 5.40, 4.10 and 0.71,
with integrals in the ratio 10:4:9, although the overlap of the
third peak with the t-butyl peak of the original cation meant
that an accurate integral was difficult to obtain. These peaks are
attributed to the d0 η-ethene compound [Nb(η-C5H5)2(NtBu)-
(C2H4)][B(C6F5)4] 5. Removal of the ethene by placing the
sample under reduced pressure removes the new peaks associ-
ated with 5 leaving only those of the compound 4. A room
temperature 13C-{1H} NMR spectrum of the ethene saturated
sample was recorded, along with a 13C–1H correlation. This
showed the peak at δ 4.10 in the 1H NMR spectrum to correlate
with a carbon resonance at δ 82.97. The 11B and 19F NMR
spectra of the solution of 4 in the presence of ethene were
closely similar to those of the sample prior to addition, showing
that the anion is unaffected by the addition of ethene.
complexes 5 and 4 in the presence of free ethene, in both d5-bromo-
benzene and d5-chlorobenzene.
type (gradient selected heteronuclear quantum correlations),
1
and the JC-H coupling constant of the sp2 C–H bonds of free
ethene was 160 Hz in the same sample. The azametallacyclo-
1
butane species 5b would be expected to have a lower JC-H
coupling constant due to the decreased s character in the
bonding between the carbon and protons of the ethene ligand.
Finally, we note that the chemical shifts of the η-ethene
hydrogens in the d0 cationic [V{η-C5H4(CH2)2NiPr}(NtBu)-
(η-C2H4)][MeB(C6F5)3]31 and [W(᎐CC H ){OCH C(CH ) } -
᎐
4
8
2
3 3 2
Br(η-C7H12)][GaBr4]32 lie within 1 ppm of the resonance of the
1
free olefin, and the JC-H coupling constants are slightly higher
than those of the unco-ordinated olefin. In comparison the pro-
ton chemical shifts arising from the co-ordinated olefin in the
four azametallacyclobutane species described above are shifted
between 2 and 3.5 ppm downfield of the free olefin and the 1JC-H
coupling constants are measured to be somewhat lower than for
the free olefin (145–150 vs. 160 Hz). Compound 5 is present in
equilibrium with the unco-ordinated or solvated cation and free
ethene. The equilibrium constant was determined by measuring
the integrals of the appropriate peaks in the 1H NMR spectrum
using the integral of the methyl group of Ph3CMe as an internal
standard. If the concentration of the solvent was taken to be
constant, the equilibrium constant, Kc = [Nb(C5H5)2(NtBu)-
(C2H4)]/[Nb(C5H5)2(NtBu)][C2H4], can be calculated to be
5.0 0.3 dm3 molϪ1 at 20 ЊC.
These NMR data are consistent with two compounds, the d0,
η2-ethene compound [Nb(η-C5H5)2(NtBu)(C2H4)][B(C6F5)4] 5a,
or the azametallacyclobutane compound [Nb(η-C5H5)2(CH2-
CH2NtBu)][B(C6F5)4] 5b, formed by cycloaddition of the
ethene molecule to the Nb᎐N bond. For example, the moieties
᎐
[Zr(η-C5H5)2(NtBu)],7 [Ti(η-C5Me5)2(NPh)]29 and [Ti(NSi-
tBu3)(OSitBu)2]30 react with ethene to give rise to azametal-
lacyclobutanes. Also, the displacement of diethyl ether from
[V(NSitBu3)2(NHSitBu3)(Et2O)] by ethene yields an intermedi-
ate azametallacyclobutane derivative, which undergoes C–H
activation to yield the alkenyl compound [V(NSitBu3)(NHSi-
tBu3)2(CHCH2)].3
For both compounds 5a and 5b the observation of a sharp
singlet for the CH2CH2 hydrogens would require a fluxional
equilibrium, as shown in Fig. 3. The reaction of [Nb(η-C5H5)2-
(NtBu)][MeB(C6F5)3] 3 with ethene in d5-bromobenzene was
found to give the same cationic fragment as the reaction of 4.
Variable temperature studies of complex 5 to Ϫ30 ЊC, the
freezing point of bromobenzene, at 500 MHz showed the peak
assigned to the four CH2CH2 protons to remain as a singlet.
The compound 5 was prepared in d5-chlorobenzene, which
has a freezing point of Ϫ45 ЊC, and on cooling to Ϫ43 ЊC the
singlet assigned to the CH2CH2 protons first broadened and
then resolved into a doublet with peak separation of 2 Hz but
no fine structure could be discerned. The corresponding reson-
ance in the 13C-{1H} NMR spectrum at Ϫ43 ЊC still showed
only a broadened singlet full width at half-height, (f.w.h.h. = 12
The equilibrium is temperature dependent, and shifts in
favour of the formation of the ethene compound 5a as the
temperature decreases, giving an equilibrium constant of
14.9 0.8 dm3 molϪ1 at Ϫ30 ЊC. Variable temperature NMR
studies gave the equilibrium constants at various temperatures
and a Van’t Hoff plot of ln K vs. 1/T is shown in Fig. 4. The
data show the difference between the binding enthalpy of the
ethene ligand and the solvent to [Nb(C5H5)2(NtBu)]ϩ is
∆HЊ = Ϫ10.1 3.3 kJ molϪ1. This value is notably lower than
the experimental, or calculated, values for other d0 ethene com-
plexes. For comparison, the position of the equilibrium of the
compound [W(᎐CC H ){(OCH C(CH ) ) }Br(η2-C H )]ϩ with
᎐
4
8
2
3
3
2
7
12
the solvated cation at various temperatures in d2-dichloro-
methane gave ∆HЊ = Ϫ13.6 0.5 kcal molϪ1 (Ϫ56.8 kJ
molϪ1).32 Also the equilibrium for the reaction of [Ti(NSitBu3)-
(OSitBu)2] with ethene in toluene gave ∆HЊ for the formation of
a d0 ethene complex to be ca. Ϫ11 kcal molϪ1 (Ϫ46 kJ molϪ1).30
Density functional calculations performed upon the fragment
[V{η-C5H4(CH2)2η-NH}(NH)(η2-C2H4)]ϩ, used as a model for
[V{η-C5H4(CH2)3NiPr}(NtBu)(η2-C2H4)]ϩ, gave a theoretical
ethene binding enthalpy of Ϫ31 kcal molϪ1 (129.6 kJ molϪ1).31
The equilibrium data in Fig. 4 show a greater degree of
ethene association in d5-chlorobenzene as compared to d5-
bromobenzene, giving Kc = 64.9 3.3 dm3 molϪ1 at 20 ЊC,
increasing to 216 11 dm3 molϪ1 at Ϫ30 ЊC. A Van’t Hoff plot
using these data allowed the binding enthalpy of the ethene
Hz); the lack of resolution may reflect a broadening effect
9
–
of the spin 2 niobium nucleus, or simply be due to the increased
viscosity of the solvent.
These variable temperature NMR data do not distinguish
between the two possible structures a or b for complex 5.
However, circumstantial evidence supports the structure a with
the η-ethene ligand. Thus, the peak assigned to the CH2CH2
protons occurs at δ 4.10, and this value is relatively close to
that for free ethene (δ 5.23). Further, the 1JC-H coupling constant
of the protons of CH2CH2 is 167 Hz, as determined from
a carbon coupled proton–carbon correlation of the gHSQC
J. Chem. Soc., Dalton Trans., 2000, 2952–2959
2955

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The position of the equilibrium between complexes 7 and 8
was measured at different temperatures by means of variable
temperature NMR studies. The values of Kc are of the same
order as those of the non-bridged analogue, varying from
5.7 0.3 to 12.1 0.6 dm3 molϪ1 over the temperature range 20
to Ϫ10 ЊC. A Van’t Hoff plot of ln(Kc) vs. 1/T (Fig. 5) gave
the value of ∆HЊ for the binding of the ethene ligand as
Ϫ16.0 1.2 kJ molϪ1. This value is similar to that of the
non-bridged analogue.
The introduction of propene to a sample of complex 7 leads
to the production of extremely complex NMR spectra. In the
1H NMR spectrum, in addition to the peaks that have previ-
ously been assigned to 7, a number of new proton resonances
were observed, which could be attributed to the η-propene
compound [Nb{(η-C5H4)2CMe2}(NtBu)(C3H7)][B(C6F5)4] 9,
however a number of resonances proved to be coincident,
complicating the assignment. The 13C-{1H} NMR spectrum is
better resolved, however there are still many very closely spaced
peaks.
Fig. 5 A Van’t Hoff plot of ln(K) vs. 1/T (KϪ1) for the temperature
dependence of the equilibrium between complexes 8 and 7 in the
presence of ethene in d5-bromobenzene.
ligand to be calculated to be ∆HЊ = Ϫ14.7 1.4 kJ molϪ1. This
value is not significantly different from that obtained when d5-
bromobenzene was used as the solvent, being within the bounds
of experimental error.
The introduction of an atmosphere of propene to a NMR
sample of complex 4 in the same manner as that used for ethene
did not show any peaks arising from co-ordination of the
propene. This is not surprising since it is well known that in
olefin–metallocene compounds ethene bonds more readily than
the bulkier propene.
The presence of an ansa bridge can alter the chemistry of
the ansa-metallocenes as compared to that of non-bridged
analogues.33–36 The introduction of a single carbon bridge
between the two cyclopentadienyl rings causes the angle
between the two ring centroid–metal bonds to decrease by
approximately 10Њ. It was decided to compare the reactivity
of [Nb(η-C5H5)2(NtBu)Cl] with that of the ansa analogue
[Nb{(η-C5H4)2CMe2}(NtBu)Cl].
1
The H NMR spectrum in d5-bromobenzene of the solution
of compound 9 shows resonances characteristic of the protons
of propene species. Of these, four were consistent with the pres-
ence of free propene in d5-bromobenzene, at δ 5.62, 4.86, 4.82
and 1.49, whilst others could be assigned to the three vinylic
hydrogens of a co-ordinated propene. The internal vinylic pro-
ton was shifted 1 ppm downfield from that of free propene,
whilst the resonance of the methyl group was also shifted
downfield by approximately 0.1 ppm. The peaks due to the two
methylene protons were separated by 0.18 ppm as opposed to
0.05 ppm for free propene, and occurred upfield of the reson-
ances of the free olefin.
In the region containing the resonances of the cyclopenta-
dienyl ring protons, six new singlets were observed which were
assigned to the protons of the now inequivalent cyclopenta-
dienyl rings. The coupling to other hydrogens on the ring was
unresolved. In addition to these, two other resonances that were
The reaction between [Nb{(η-C5H4)2CMe2}(NtBu)Cl]9 and
AgO3SCF3 in benzene gave the compound [Nb{(η-C5H4)2-
CMe2}(NtBu)(O3SCF3)] 6 as slightly air- and moisture-sensitive
orange needle crystals. These were soluble in non-polar organic
solvents. The value of ∆δ for the t-butyl group was measured to
be 41.5 ppm, virtually identical to that of the non-bridged
species. The electron impact mass spectrum of 6 showed the
molecular ion at m/z 483, along with fragments resulting from
loss of a methyl group and also from loss of the imido group.
Addition of a solution of [Ph3C][B(C6F5)4] in d5-bromo-
benzene to a solution of [Nb{(η-C5H4)2CMe2}(NtBu)Me]9 in
d5-bromobenzene gave a red-brown solution which the NMR
data suggest to be the compound [Nb{(η-C5H4)2CMe2}-
1
coupled to these singlets were found via a H–1H correlation
spectrum. One of these was coincident with the resonance
of a cyclopentadienyl proton of complex 7, whilst the other
was coincident with a resonance of the methylene protons of
free propene. The unresolved coupling of these new cyclo-
pentadienyl peaks may be due to a dynamic process involving
rotation of the co-ordinated propene molecule.
Peaks that could be assigned to the proton resonances of the
methyl groups on the backbone of an ansa-cyclopentadienyl
ligand and to the t-butyl group of an imido ligand of a new
species were also observed.
The 13C-{1H} NMR spectrum was assigned with the aid of
both one-bond (HSQC) and two-bond (heteronuclear multiple
bond correlation, HMBC) 13C–1H correlation spectra. This
also showed the presence of a new species in addition to free
propene and the solvated cation. Peaks at δ 140.17, 82.78 and
24.69 were found to correlate with the peaks of the second
1
(NtBu)][B(C6F5)4] 7. The H NMR spectrum shows the chem-
ical shift of the peak due to the protons of the t-butyl to be
close to that of the non-bridged compound 4. The backbone
methyl groups occur as two singlets, consistent with the pseudo
tetrahedral symmetry expected for 7. The protons of the
cyclopentadienyl ring give rise to three multiplets, integrating as
2, 4 and 2 protons respectively. The 13C-{1H} NMR spectrum
was also consistent with the proposed formulation of the
product, with all of the peaks expected for 7 being observed.
The value of ∆δ between the α and β carbons of the t-butyl
group was measured to be 43.1 ppm.
The introduction of an atmosphere of ethene into an NMR
sample of complex 7 leads to spectral changes consistent with
the formation of the ion pair [Nb{(η-C5H4)2CMe2}(NtBu)-
(η-C2H4)][B(C6F5)4] 8. The 1H NMR spectrum showed a
resonance at δ 4.87 assignable to the methylene protons of the
co-ordinated ethene. There are seven other new resonances that
can be assigned to the protons of the cyclopentadienyl ring,
backbone methyl groups and t-butyl group on the imido ligand.
The 13C-{1H} NMR spectrum is also consistent with the form-
ation of the ethene adduct, with the carbon resonance of the
co-ordinated olefin being observed as a singlet at δ 94.02.
1
propene molecule observed in the H NMR spectrum. These
are shifted from the resonances of free propene (δ 135.13,
117.56 and 20.91 in d5-bromobenzene). The terminal vinyl
carbon shows a substantial (Ϫ34.78 ppm) shift upfield, whilst
the internal vinyl carbon shows a small shift (ϩ5.04 ppm)
downfield. These features are similar to those observed for the
olefinic groups in the compounds [V{η-C5H4(CH2)3NiPr}-
(NtBu)(η2-C3H7)][B(C6F5)3Me] (δ Ϫ24.2 and ϩ2.2, averaged
over the two isomers)31 and [Zr(η-C5H5)2{O(CMe2)(CH2)3-
CH᎐CH }][B(C F ) Me] (δ Ϫ19.6 and ϩ18.4),37 in which a
᎐
2
6
5 3
substantial amount of olefin polarisation was suggested.
The NMR data are all consistent with an equilibrium
between complex 7 and propene giving the adduct [Nb{(η-
C5H4)2CMe2}(NtBu)(η-CH2CHMe)][B(C6F5)4] 9.
The formation of the η-propene compound 9 whilst the
analogous compound could not be seen with the non-ansa
analogue may be attributed to introduction of the single carbon
2956
J. Chem. Soc., Dalton Trans., 2000, 2952–2959

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Fig. 6 A Van’t Hoff plot of ln(Kc) vs. 1/T (KϪ1) for the temperature
dependence of the equilibrium between complexes 9 and 7 in the
presence of propene in d5-bromobenzene.
bridge in 9 which would cause an increase in the bending angle
between the cyclopentadienyl rings, and thereby reduce the
steric repulsion of the η-propene ligand by the cyclopenta-
dienyl rings. This increase of bending angle is clearly suggested
by consideration of the molecular structures of [Nb(η-C5H5)2-
(NtBu)Cl] (123.4Њ)11 and [Nb{(η-C5H4)2CMe2}(NtBu)Cl]
(113.3Њ).17
The position of the equilibrium between complex 7 and C3H6
and 9 was measured at different temperatures by means of
variable temperature NMR studies, and Kc found to vary from
5.0 0.3 to 12.0 dm3 molϪ1 over the temperature range 20 to
Ϫ30 ЊC. A Van’t Hoff plot of ln(Kc) vs. 1/T (Fig. 6) gave a value
of ∆HЊ of Ϫ10.3 1.4 kJ molϪ1
.
The new compounds, their reactions and proposed structures
are shown in Scheme 1.
Experimental
All preparations of air- and/or moisture-sensitive materials
were carried out under an inert atmosphere of dinitrogen using
standard Schlenk line techniques, or in an inert atmosphere dry
box containing dinitrogen. Dinitrogen was purified by passage
through columns filled with molecular sieves (4 Å) and either
manganese() oxide suspended on vermiculite for the vacuum
line or BASF catalyst for the dry box. Solvents and solutions
were transferred through stainless steel or Teflon cannulæ,
using a positive pressure of inert gas. All glassware and cannulæ
were thoroughly dried at 150 ЊC before use. Celite (Fluka) was
similarly dried.
All solvents were deoxygenated before use by repeated evacu-
ation followed by admission of dinitrogen. Solvents were pre-
dried over activated molecular sieves (4 Å) and then distilled
over potassium (THF, benzene, toluene, light petroleum bp
100–120 ЊC), sodium/potassium alloy (n-pentane, light petrol-
eum bp 40–60 ЊC). Deuteriated solvents were dried over
potassium (d6-benzene, d8-THF, d5-bromobenzene, d5-chloro-
benzene) and vacuum distilled prior to use.
NMR spectra were recorded on either a Varian UnityPlus
500 (1H, 11B, 13C and 19F at 499.988, 160.415, 125.718, and
470.374 MHz respectively) or a Varian Hg300 spectrometer
(1H, 13C and 19F at 300.178, 75.477 and 282.398 MHz respec-
tively), referenced internally using the residual protio solvent
(1H) and solvent (13C) resonances and measured relative to
tetramethylsilane (1H and 13C; δ 0) or referenced externally to
BF3ؒEt2O (11B; δ 0) or CFCl3 (19F; δ 0). Electron impact mass
spectra were recorded by the mass spectrometry service of the
Dyson Perrins Laboratory, electrospray mass spectra by Dr
Greg Mullen on a Micromass LC TOF ESI mass spectrometer.
We also thank Dr J. A. Ballantine and the Mass Spectroscopy
Service at Swansea.
Scheme 1 (i) AgO3SCF3, THF; (ii) PMe3, C6H6; (iii) B(C6F5)3,
C6D5Br; (iv) [Ph3C][B(C6F5)4], C6D5Br; (v) C2H4; (vi) AgO3SCF3, C6H6;
(vii) [Ph3C][B(C6F5)4], C6D5Br; (viii) C2H4; (ix) C3H6.
(NtBu)MeCl],16 [Nb{(η-C5H4)2CMe2}(NtBu)Cl]17 and [Nb-
{(η-C5H4)2CMe2}(NtBu)Me]17 were prepared according to
literature procedures. The previously reported compound
[Nb{(η-C5H5)2(NtBu)Me]13 was prepared via a variation on
the literature procedure, as described below. AgO3SCF3 was
purchased from the Avocado chemical company.
Syntheses
[Nb(ꢀ-C5H5)2(NtBu)Me]. A colourless solution of Mg(C5H5)2
(0.77 g, 5 mmol) in toluene (20 ml) was added to a yellow
solution of [Nb(η-C5H5)(NtBu)MeCl] (3.00 g, 10 mmol) in
toluene (30 ml) at room temperature. A precipitate formed over
the course of 5 min, with the solution becoming a darker
yellow. The reaction mixture was stirred for 15 min and then
filtered to remove MgCl2. Removal of the volatiles gave the
crude product as a dark yellow oil in good purity. The product
could further be purified by sublimation as described previ-
ously.11 Yield 2.40 g, 73% based on [Nb(η-C5H5)(NtBu)MeCl].
Elemental analyses were obtained by the elemental analysis
department of the Inorganic Chemistry Laboratory.
The compounds [Nb(η-C5H5)2(NtBu)Cl],11 [Nb(η-C5H5)-
J. Chem. Soc., Dalton Trans., 2000, 2952–2959
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[Nb(ꢀ-C5H5)2(NtBu)(O3SCF3)] 1. A colourless solution of
AgO3SCF3 (0.156 g, 0.6 mmol) in THF (15 ml) was added to a
yellow solution of [Nb(η-C5H5)2(NtBu)Cl] (0.200 g, 0.6 mmol)
in THF (15 ml) at room temperature. A grey-white solid
precipitated immediately, and the solution became orange.
The reaction mixture was stirred for 20 min, after which time
the precipitate of AgCl was allowed to settle and removed by
filtration. The volatiles were removed under reduced pressure
and the resulting orange solid was extracted with light petrol-
eum (bp 100–120 ЊC) (3 × 20 ml) to give an orange solution.
The solution was concentrated to 10 ml under reduced pressure,
and then cooled to Ϫ80 ЊC to yield the product as orange
crystals which were isolated by filtration, washed with cold
pentane and dried in vacuo. Yield 0.159 g, 59% based on
[Nb(η-C5H5)2(NtBu)Cl].
Table 4 Crystal structure data
1
2
Molecular formula
Formula weight
Crystal system
Space group
C15H19F3NNbO3S
C18H28F3NNbO3PS
519.36
Monoclinic
P21/n
8.868(1)
18.934(2)
13.641(2)
92.12(2)
2288.9(4)
4
443.28
Monoclinic
P21/n
a/Å
b/Å
c/Å
8.648(1)
24.018(3)
9.588(2)
112.71(2)
1837.1(4)
4
β/Њ
V/Å3
Z
T/K
180
0.78
180
0.70
µ/mmϪ1
Total data collected
9039
11019
4777
Unique data
3859
[Nb(ꢀ-C5H5)2(NtBu)(PMe3)][O3SCF3] 2. An orange solution
of [Nb(η-C5H5)2(NtBu)(O3SCF3)] (0.300 g, 0.68 mmol) in
benzene (5 ml) was placed in one side of an H bridge, and a
solution of trimethylphosphine (0.2 ml, 1.93 mmol) in benzene
(5 ml) placed in the other half. The apparatus was then sealed
and left to stand. Over the course of 2 days a yellow crystalline
solid formed from the benzene solution, which gradually
became colourless. The supernatant was decanted, and the solid
washed with pentane (2 × 5 ml) and dried in vacuo. Yield 0.340
g, 96% based on [Nb(η-C5H5)2(NtBu)(O3SCF3)].
Merging R (%)
R
Rw
0.02
0.0410
0.0488
0.032
0.0327
0.0396
Crystallography
Data were collected as previously described,38 the images being
processed with the DENZO and SCALEPACK programs.39 All
solution, refinement and graphical calculations were performed
using the CRYSTALS40
and CAMERON41,42 software pack-
ages. A Chebychev weighting scheme43 with the parameters
4.37, 2.57 and 2.99 was applied to the crystal structure of com-
[Nb(ꢀ-C5H5)2(NtBu)][MeB(C6F5)3] 3. A colourless solution
of B(C6F5)3 (26.4 mg, 5.2 × 10Ϫ2 mmol) in d5-bromobenzene
(0.5 ml) was added to a yellow solution of [Nb(η-C5H5)2-
(NtBu)Me] (16 mg, 5.2 × 10Ϫ2 mmol) in the same solvent (0.2
ml). The resulting red-brown solution was then transferred
to a NMR tube. Yield 95% (calculated from the 1H NMR
spectrum).
pound 1 giving a final R factor of 0.041 and Rw
= 0.0488 with a
maximum residual electron density of 0.52 e ÅϪ3. A similar
weighting scheme was applied to the structure of 2 using the
parameters 2.54, 0.640 and 2.06. This yielded a final R factor of
0.0327, Rw
= 0.0396 with a maximum residual electron density
of 0.83 e ÅϪ3. The crystallographic data are given in Table 4.
CCDC reference number 186/2082.
See http://www.rsc.org/suppdata/dt/b0/b004062m/ for crys-
tallographic files in .cif format.
[Nb(ꢀ-C5H5)2(NtBu)][B(C6F5)4] 4. An orange solution of
[Ph3C][B(C6F5)4] (48 mg, 5.2 × 10Ϫ2 mmol) in d5-bromobenzene
or d5-chlorobenzene (0.5 ml) was added to a yellow solution of
[Nb(η-C5H5)2(NtBu)Me] (16 mg, 5.2 × 10Ϫ2 mmol) in the same
solvent. The resulting red-brown solution was then transferred
to a NMR tube. Yield 80% (calculated from the 1H NMR
spectrum).
Acknowledgements
We thank the EPSRC for a grant (to M. J. H.) and St Johns
College, Oxford for partial support.
[Nb{(ꢀ-C5H4)2CMe2}(NtBu)(O3SCF3)] 6. A colourless solu-
tion of AgO3SCF3 (0.31 g, 1.22 mmol) in benzene (20 ml) was
added to a yellow solution of [Nb{(η-C5H4)2CMe2}(NtBu)Cl]
(0.45 g, 1.22 mmol) in benzene (15 ml) at room temperature. A
white precipitate formed immediately, and the solution became
orange. The reaction was then worked up as above. Concen-
tration of the solvent provided orange crystals which were
isolated by filtration, washed with cold pentane (2 × 5 ml) and
dried in vacuo. The filtrate was cooled to Ϫ20 ЊC to yield a
further crop of orange crystals. Yield 0.24 g, 53% based on
[Nb{(η-C5H4)2CMe2}(NtBu)Cl].
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