Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido and β-diketiminato ligands

Article abstract of DOI:10.1039/c1dt10202h

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N^tBu)][X] (BDI = [Ar]NC(CH ₃)CHC(CH₃)N[Ar], Ar = 2,6-ⁱPr₂C ₆H₃; M = Nb, Ta; X = MeB(C₆F₅) ₃, B(C₆F₅)₄] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R₃PC(O)Me)M(N ^tBu)][B(C₆F₅)₄] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me₂-C ₆H₃) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylNC(Me))Nb(N^tBu)][B(C₆F₅) ₄], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe ₂)Nb(N^tBu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph₃SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.

Full text of DOI:10.1039/c1dt10202h

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This article is published as part of the Dalton Transactions themed issue entitled:
d0 organometallics in catalysis
Guest Editors John Arnold (UC Berkeley) and Peter Scott (University of Warwick)
Published in issue 30, 2011 of Dalton Transactions
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PERSPECTIVES:
Half-titanocenes for precise olefin polymerisation: effects of ligand substituents and some
mechanistic aspects
Kotohiro Nomura and Jingyu Liu
Dalton Trans., 2011, DOI: 10.1039/C1DT10086F
ARTICLES:
Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides
Michael S. Hill, Marina Hodgson, David J. Liptrot and Mary F. Mahon
Dalton Trans., 2011, DOI: 10.1039/C1DT10171D
Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido
and β-diketiminato ligands
Neil C. Tomson, John Arnold and Robert G. Bergman
Dalton Trans., 2011, DOI: 10.1039/ C1DT10202H
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PAPER
Synthesis and reactivity of cationic niobium and tantalum methyl complexes
supported by imido and b-diketiminato ligands†
Neil C. Tomson, John Arnold* and Robert G. Bergman*
Received 6th February 2011, Accepted 4th April 2011
DOI: 10.1039/c1dt10202h
The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes
[(BDI)MeM(NtBu)][X] (BDI = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-iPr2C6H3; M = Nb, Ta; X =
MeB(C6F5)3, B(C6F5)4] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but
formed phosphine-trapped acyl complexes [(BDI)(R3PC(O)Me)M(NtBu)][B(C6F5)4] (R = Et, Cy) in the
presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb
complex with XylNC (Xyl = 2,6-Me2-C6H3) resulted in irreversible formation of the iminoacyl complex
[(BDI)(XylN C(Me))Nb(NtBu)][B(C6F5)4], which did not bind phosphines but would add a methide
group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe2)Nb(NtBu).
Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide,
amidinate, and ketimide complexes; ii) protonolysis reactions with Ph3SiOH to form thermally robust
cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the
cationic Nb methyl complex.
reactive, the imido functionality is well-known to effect a range of
[2 + 2], C–H activation, and nucleophilic substitution reactions
Introduction
Cationic complexes of the d0 early-metal alkyls have shown
increased reactivity compared to their neutral counterparts. This
effect has been attributed variously to (i) the formation of lower-
coordinate complexes, which have an increased number of metal-
based orbitals available for binding and activating a substrate;
(ii) a decrease in the formal electron count at the metal, which
enhances the metal-based electrophilicity; and (iii) the polarization
of the M–C bond.1–7 Study of the metallocenium alkyl complexes
has contributed greatly to the development of these principles,
largely due to an interest in understanding their activity as olefin
polymerization catalysts.8–11
Recent research in Ziegler-type olefin polymerization has
focused on the development of ligand sets that may impart
steric or electronic effects different from those observed for the
metallocenes.12–20 The imido ligand has been used in this context,
leading to imido-supported early-metal alkyl cations with olefin
polymerization activities rivalling those of the metallocenes.3,21–23
Despite these advances in catalytic systems, the stoichiometric
chemistry of these complexes has remained relatively unexplored.
One question of particular interest to the imido-supported
cations is the behavior of the imido group. In contrast to the
metallocene systems in which the Cp ligands are generally non-
when bound to the more electropositive early-metals.24–32 This
reactivity creates a question of site-selectivity when early-metal
imido compounds have alkyl groups bound to them, since early-
metal alkyl groups may also react with unsaturated and protic
reagents to form insertion and protonolysis products.
We2 and others33–37 have initiated studies aimed at understanding
this reaction site selectivity, including how a cationic charge at
such imido-supported alkyl complexes may affect the observed
reactivity. The present work details our investigations into the
synthesis and reactivity of well-defined cationic alkyl complexes
of the heavier Group 5 metals supported by the tert-butylimido
(NtBu) ligand and the bulky, bidentate N,N¢-diaryl-b-diketiminate
(BDI, aryl = 2,6-iPr2-C6H3) ligand set.
Experimental
General considerations
Unless otherwise noted, all reactions were performed using
standard Schlenk line techniques or in an MBraun dry box under
an atmosphere of nitrogen (<1 ppm O2/H2O). All glassware,
cannulae, and Celite were stored in an oven at ca. 425 K. Pentane,
Et2O, and THF were purified by passage through a column of
activated alumina and degassed prior to use.38 C6H5F and C6H5Cl
Department of Chemistry, University of California, Berkeley, Califor-
nia, USA 94720. E-mail: arnold@berkeley.edu, rbergman@berkeley.edu;
Tel: +1–510–643–5181 (JA) and +1–510–642–2156 (RGB)
† Complete crystallographic data for compounds 2a, 4, and 5 is available.
CCDC reference numbers 675085, 808678 and 808679. For crystallo-
graphic data in CIF format see DOI: 10.1039/c1dt10202h
˚
were distilled from CaH2 and stored over activated 4 A molec-
ular sieves. Deuterated solvents were vacuum-transferred from
sodium/benzophenone (benzene) or calcium hydride (chloroben-
zene). NMR spectra were recorded on Bruker AV-300, AVQ-400
7718 | Dalton Trans., 2011, 40, 7718–7729
This journal is
The Royal Society of Chemistry 2011
©

1
and DRX-500 spectrometers. 1H and 13C{ H} chemical shifts are
corrected for Lorentz and polarization effects, but no correction
for crystal decay was applied. Data were analyzed for agreement
and possible absorption using XPREP.45 An empirical absorption
correction based on comparison of redundant and equivalent
reflections was applied using SADABS.46 Structures were solved by
direct methods with the aid of successive difference Fourier maps
and were refined against all data using the SHELXTL 5.0 software
package. Thermal parameters for all non-hydrogen atoms were
refined anisotropically, except in solvent molecules disordered
over multiple partially occupied positions. ORTEP diagrams were
created using the ORTEP-3 software package.47
given relative to residual solvent peaks; 19F chemical shifts are
referenced to external CFCl3 at 0.0 ppm; and 31P{ H} chemical
1
shifts are given relative to an external standard of P(OMe)3
set to 1.67 ppm. Proton and carbon NMR assignments were
routinely confirmed by H–1H (COSY) or H–13C (HMQC and
HMBC) experiments. Infrared (IR) samples were prepared as
Nujol mulls and were taken between KBr disks. The following
chemicals were purified prior to use: TaCl5 by sublimation;
1
1
˚
acetophenone was stored over 4 A molecular sieves, which had
been dried under vacuum for 12 h at 120 ◦C prior to use; B(C6F5)3
was purified by sublimation. Li(BDI)·OEt2,39 py2Cl3Ta(NtBu),40
[(Et2O)2H][B(C6F5)4],41 and (BDI)Me2Nb(NtBu)42 (1) were pre-
pared using the literature procedures. All other reagents were
acquired from commercial sources and used as received. Elemental
analyses were determined at the College of Chemistry, University
of California, Berkeley. The X-ray structural determinations were
performed at CHEXRAY, University of California, Berkeley.
[(BDI)MeNb(NtBu)][B(C6F5)4] (2a) from [(Et2O)2H][B(C6F5)4]
A solution of [(Et2O)2H][B(C6F5)4] (200 mg, 0.24 mmol) in Et2O
(6 mL) was added to a solution of 1 (148 mg, 0.24 mmol) in Et2O
(4 mL) at room temperature. The color immediately turned from
pale yellow to bright yellow and the solution effervesced rapidly.
After 10 min at room temperature, the solution was concentrated
to ca. 3 mL and filtered to give a clear yellow solution. The filtrate
was stored at -35 ◦C for 24 h during which time a crystalline
material formed. The solid was collected by filtration and dried
under vacuum. Yield: 251 mg, 81%. While 2a proved to be more
thermally robust than 1, isolated samples of 2a were stored in a
Crystallographic analysis
Single crystals of 2a, 4, and 5 were coated in Paratone-N oil,
mounted on a Kaptan loop, transferred to a Siemens SMART
diffractometer or a Bruker APEX CCD area detector,43 centered
in the beam, and cooled by a nitrogen flow low-temperature
apparatus that had been previously calibrated by a thermocouple
placed at the same position as the crystal. Preliminary orientation
matrices and cell constants were determined by collection of 60 10 s
frames, followed by spot integration and least-squares refinement.
An arbitrary hemisphere of data was collected, and the raw data
were integrated using SAINT.44 Cell dimensions reported were
calculated from all reflections with I > 10 (Table 1). The data were
◦
1
-35 C freezer to prevent decomposition. H NMR (400 MHz,
C6D5Cl): d 7.25–7.05 (m, 6H, Ar), 5.86 (s, 1H, HC(C(Me)NAr)2),
2.13 (sept, 2H, CHMe2), 1.88 (s, 6H, HC(C(Me)NAr)2), 1.82 (sept,
2H, CHMe2), 1.21 (d, 6H, CHMe2), 1.06 (d, 6H, CHMe2), 1.00
(d, 6H, CHMe2), 0.95 (s, 3H, NbMe), 0.88 (d, 6H, CHMe2), 0.71
(s, 9H, Bu). 13C{ H} NMR (125.7 MHz, C6H5F/ 10% C6D6,
298 K): d 171.6, 142.5, 140.4, 139.8, 129.3, 125.9, 124.9, 96.3,
76.5, 48.5, 32.8, 31.2, 29.2, 25.7, 24.9, 24.5, 24.2, 23.8. 19F NMR
t
1
Table 1 Crystallographic data for compounds 2a, 4, and 5
Compound
2a·2Et2O
4·0.5hexane
5
Formula
Formula weight
C66H73BF20N3NbO2
1423.99
P1
C41H62Cl2N4Ta
862.80
P1
C34.86H55.58 Cl0.14N3Ta
702.65
P21/c
10.4872(15)
13.1367(19)
24.997(4)
90
93.375(2)
90
3437.8(9)
4
1.421
1508
0.33
¯
¯
Space Group
˚
a (A)
13.616(3)
16.444(3)
17.040(3)
114.348(2)
102.282(3)
91.108(3)
3371.6(11)
2
1.403
1464
0.28
0.815713
12 998
7758
0.0334
5193
838
0.0513, 0.1153
0.0935
1.054
10.4181(10)
12.5964(12)
16.7145(16)
84.9620(10)
76.5520(10)
74.7490(10)
2057.4(3)
2
1.393
886
0.28
0.840323
11 261
7455
0.0128
6827
433
0.0248, 0.0619
0.0286
1.267
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
Z
rcalcd (g cm-3)
F000
m (cm-1)
T
min/Tmax
0.75191
21 789
8300
0.0491
6457
No. rflns measured
No. indep. rflns
Rint
No. obs. (I > 2.00s(I))
No. variables
363
a
R1, wR2
0.0371, 0.0739
0.0540
0.991
R1 (all data)
GoF
Res. peak/hole (e /A )
-
3
˚
0.885/-0.437
2.594/-0.641
1.406/-0.615
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = (|Fo| - |Fc|)/ (|Fo|); wR2 = [ {w(Fo - Fc2)2}/ {w(Fo2)2}]1/2
.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7718–7729 | 7719
©

(376.5 MHz, C6D5Cl, 298 K): d -130.70 (s, o-F, B(C6F5)4), -161.43
(t, m-F, B(C6F5)4), -165.24 (t, p-F, B(C6F5)4). Anal. Calcd for
C58H53BF20N3Nb: C, 54.60; H, 4.19; N, 3.29. Found: C, 54.53, H
4.08; N, 3.24.
3.0 M solution in Et2O) was added dropwise by syringe. The
color immediately turned pale yellow from bright orange as a
white precipitate developed from the solution. The cold bath was
removed and the slurry was stirred at room temperature for 10 min.
Removal of the volatile materials under vacuum resulted in a
pale yellow solid. The product was extracted with hexane (2 ¥
15 mL) and the resulting solution was◦filtered and concentrated
to 10 mL. Storing the solution at -40 C induced crystallization
of the product within 12 h. The crystalline material was collected
[(BDI)MeNb(NtBu)][B(C6F5)4] (2a) from [Ph3C][B(C6F5)4]
A solution of [Ph3C][B(C6F5)4] (15 mg, 0.016 mmol) in C6D5Cl
(0.5 mL) was added to a solution of 1 (10 mg, 0.016 mmol) in
C6D5Cl (0.5 mL). The color of the solution immediately turned
bright yellow. The sample was analyzed spectroscopically, without
purification; 1H and 19F NMR data indicated a mixture of 2a and
1,1,1-triphenylethane.
1
by filtration and dried under vacuum. Yield: 202 mg, 74%. H
NMR (500 MHz, C6D6, 293 K): d 7.13 (br s, 6H, Ar), 5.22
(s, 1H, HC(C(Me)NAr)2), 3.08 (br m, 4H, CHMe2), 1.63 (s,
6H, HC(C(Me)NAr)2), 1.37 (br d, 12H, CHMe2), 1.14 (d, 12H,
t
1
CHMe2), 1.10 (s, 9H, Bu), 0.59 (s, 6H, NbMe2). 13C{ H} NMR
(125.7 MHz, C6D6, 298 K): d 167.1, 141.9, 126.7, 124.7, 103.3,
64.6 (tBu, Ca) 58.3 (TaMe2), 32.2 (tBu, Cb), 29.1, 26.8, 25.1, 25.0.
Anal. Calcd for C35H56N3Ta: C, 60.07; H, 8.07; N, 6.00. Found: C,
59.90, H, 8.21; N, 5.96.
[(BDI)MeNb(NtBu)][MeB(C6F5)3] (2b)
A solution of B(C6F5)3 (101 mg, 0.12 mmol) in Et2O (5 mL) was
added to a solution of 1 (75 mg, 0.12 mmol) in Et2O (5 mL)
at room temperature. The color of the solution immediately
turned from pale yellow to bright yellow. Removal of the volatile
materials under vacuum provided a yellow oil, from which a
solid deposited on addition of pentane (10 mL). The solid was
collected, washed again with pentane (2 ¥ 10 mL), and dried under
vacuum to give a pale yellow powder. Yield: 80 mg, 87%. 1H NMR
(400 MHz, C6D5Cl): d 7.20 (m, 2H, Ar), 7.08 (m, 4H, Ar), 5.93
[(BDI)MeTa(NtBu)][B(C6F5)4] (6)
Solid [(Et2O)2H][B(C6F5)4] (993 mg, 1.20 mmol) was added in
portions to a solution of 5 (847 mg, 1.21 mmol) in fluorobenzene
(10 mL). The solution rapidly effervesced as the color turned
slightly more golden yellow. The solution was stirred for 10 min
at room temperature, then the volatile materials were removed
under vacuum. The resulting oil was washed with pentane (2 ¥
10 mL) and the remaining solvent was removed under vacuum
to give a foam, which was crushed and washed thoroughly with
pentane (2 ¥ 10mL). The residualsolventwas againremovedunder
vacuum, leaving a yellow solid, which was crushed into a fine pale
green–yellow powder and collected. Yield: 1.50 g, 91%. 1H NMR
(300 MHz, C6H5Cl/ 10% C6D6): d 5.81 (s, 1H, HC(C(Me)NAr)2),
2.18 (sept, 2H, CHMe2), 2.06 (sept, 2H, CHMe2), 1.88 (s, 6H,
HC(C(Me)NAr)2), 1.25 (d, 6H, CHMe2), 1.08 (d, 6H, CHMe2),
1.02 (d, 6H, CHMe2), 0.89 (d, 6H, CHMe2), 0.81 (s, 3H, TaMe),
i
(s, 1H, HC(C(Me)NAr)2), 2.20 (sept, 2H, Pr-CH), 1.92 (s, 6H,
i
i
HC(C(Me)NAr)2), 1.88 (sept, 2H, Pr-CH), 1.25 (d, 9H, Pr-Me
i
i
and MeB), 1.10 (d, 6H, Pr-Me), 1.05 (d, 6H, Pr-Me), 1.01 (s,
3H, NbMe), 0.92 (d, 6H, Pr-Me), 0.75 (s, 9H, Bu). 19F NMR
(376.5 MHz, C6D5Cl): d -130.70 (d, MeB(C6F5)3, o-F), -163.20
(t, MeB(C6F5)3, m-F), -165.64 (t, MeB(C6F5)3, p-F). Anal. Calcd
for C53H56BF15N3Nb: C, 56.65; H, 5.02; N, 3.74. Found: C, 56.55,
H, 5.18; N, 4.05.
i
t
(BDI)pyCl2Ta(NtBu) (4)
THF (20 mL) was added to a flask containing a solid mixture of
py2Cl3Ta(NtBu) (500 mg, 0.97 mmol) and LiBDI·OEt2 (483 mg,
0.97 mmol) at room temperature. The resulting solution was stirred
and heated to 65 ◦C for 12 h; the color turned progressively orange
throughout the course of the reaction. The volatile materials were
removed under vacuum and the product was extracted with Et2O
(2 ¥ 15 mL) and filtered to give a clear orange filtrate. The
solution was concentrated to 10 mL and stored at -80 ◦C for
1 week. The orange crystals that precipitated from solution were
collected by filtration and dried under vacuum. Yield: 360 mg,
45%. X-ray quality crystal◦s were grown from a saturated hot
1
0.70 (s, 9H, tBu). 13C{ H} NMR (125.7 MHz, C6H5F/ 10% C6D6,
298 K): d 173.5, 142.0, 141.0, 140.6, 130.6, 126.0, 125.0, 95.7,
71.7 (tBu, Ca) 54.0 (TaMe), 32.7, 32.5 (tBu, Cb), 29.3, 25.5, 25.1,
24.4, 24.0, 23.6. 19F NMR (376.5 MHz, C6H5Cl/10% C6D6): d
-130.90 (s, o-F, B(C6F5)4), -161.61 (t, m-F, B(C6F5)4), -165.42 (t,
p-F, B(C6F5)4). Anal. Calcd for C58H53BF20N3Ta: C, 51.08; H, 3.92;
N, 3.08. Found: C, 50.79, H 4.04; N, 3.18.
General procedure for the synthesis of
[(BDI)(g2-OC(Me)(PR3))M(NtBu)] [B(C6F5)4] (7a, 7b, 8a, 8b)
1
hexane solution (+65/+25 C). H NMR (500 MHz, C6D6, 298
The methyl-bound cation (0.15 mmol) and phosphine (0.45
mmol) were dissolved in fluorobenzene (10 mL) and added to
a 50 mL Schlenk flask. The flask was cooled to -40 ◦C, then the
headspace was evacuated for 2 min and refilled with CO. The
flask was allowed to attain room temperature and the solution
was stirred for 4 h. The volatile materials were removed under
vacuum and the residue was washed thoroughly with pentane
(3 ¥ 15 mL). Compounds 7a, 7b, and 8b were analyzed without
further purification due to the low crystallinity of the products.
Compound 8a was crystallized from an Et2O solution layered with
pentane and stored at room temperature. Yield of 8a: 164 mg, 74%
of a yellow solid.
K): d 8.60 (d, 2H, py), 7.19 (br s, 3H, Ar), 7.09 (br m, 1H, Ar),
7.02 (br m, 2H, Ar), 6.62 (br s, 1H, py), 6.26 (br s, 2H, py),
5.22 (s, 1H, HC(C(Me)NAr)2), 3.86 (br s, 2H, CHMe2), 3.70
(br s, 2H, CHMe2), 1.80 (br s, 6H, CHMe2), 1.78 (br s, 3H,
HC(C(Me)NAr)2), 1.63 (br s, 3H, HC(C(Me)NAr)2), 1.28 (br s,
6H, CHMe2), 1.04 (br s, 6H, CHMe2), 0.99 (br s, 6H, CHMe2),
0.68 (s, 9H, tBu).
(BDI)Me2Ta(NtBu) (5)
A solution of 4 (320 mg, 0.39 mmol) in Et2O (25 mL) was cooled
to 0 ◦C and stirred vigorously as MeMgBr (0.26 mL, 0.78 mmol,
7720 | Dalton Trans., 2011, 40, 7718–7729
This journal is
The Royal Society of Chemistry 2011
©

[(BDI)(g2-OC(Me)(PEt3))Nb(NtBu)][B(C6F5)4] (7a)
1
to broad, overlapping Cy signals. 13C{ H} NMR (100.61 MHz,
C6H5F/10% C6D6): d 98.74 (d, JCP = 48.5 Hz). 31P{ H} NMR
1
1
1H NMR (500 MHz, C6H5F/10% C6D6, 298 K), major isomer:
d 5.62 (s, 1H, HC(C(Me)NAr)2), 3.27 (br m, 2H, CHMe2),
2.44 (br m, 2H, CHMe2), 1.83 (s, 6H, HC(C(Me)NAr)2), 1.17
(161.98 MHz, C6H5F/10% C6D6): d 32.65 (d, 1JCP = 48.6 Hz).
[(BDI)(g2-XylN CMe)Nb(NtBu)][B(C6F5)4] (9)
t
1
(s, 9H, Bu), 0.86 (t, 9H, PEt3), 0.62 (m, 6H, PEt3). H NMR
(500 MHz, C6H5F/10% C6D6, 298 K), minor isomer: d 5.70 (s,
1H, HC(C(Me)NAr)2), 3.00 (sept, 1H, CHMe2), 2.84 (sept, 1H,
CHMe2), 2.72 (sept, 1H, CHMe2), the remainder of the spectrum
could not be positively identified. Ratio or major to minor isomer
XylNC (15.9 mg, 0.12 mmol) in Et2O (3 mL) was added dropwise
to a stirred solution of 2a (156 mg, 0.12 mmol) in Et2O (7 mL)
at room temperature. The solution immediately turned dark red,
then golden yellow after a few seconds. After 5 min at room
temperature, the volatile materials were removed under vacuum
and the resulting pale orange solid was washed thoroughly with
pentane (2 ¥ 10 mL). The residual solvent was removed under
1
6.4 : 1. 13C{ H} NMR (125.8 MHz, C6H5F/10% C6D6, 298 K): d
86.76 (d, 1JCP = 55.6 Hz, major isomer), 78.80 (d, 1JCP = 53.2 Hz,
1
minor isomer). 31P{ H} NMR (202.5 MHz, C6H5F/10% C6D6,
1
298 K): d 36.00 (br s, Dn1/2 = 286 Hz), major isomer), 39.10 (d,
1JCP = 53.3 Hz, minor isomer).
vacuum to yield a pale yellow powder. Yield: 152 mg, 88%. H
NMR (500 MHz, C6D5Cl, 298 K): d 5.72 (s, HC(C(Me)NAr)2
of major isomer), 5.71 (s, HC(C(Me)NAr)2 of minor isomer). 1H
NMR (500 MHz, C6D5Cl, 343 K): d 7.3–6.9 (m, Ar), 5.74 (s,
1H, HC(C(Me)NAr)2), 3.05 (br m, 2H, CHMe2), 2.47 (br m, 2H,
CHMe2), 2.17 (br s, 3H, Xyl), 2.06 (br s, 3H, Xyl), 1.87 (br s, 3H,
HC(C(Me)NAr)2), 1.78 (br s, 3H, HC(C(Me)NAr)2), 1.50 (br s,
3H, XylN CMe), 1.3–0.6 (br m, CHMe2 and tBu). Anal. Calcd
for C67H62BF20N4Nb: C, 57.20; H, 4.44; N, 3.98. Found: C, 57.49,
H 4.68; N, 3.91. IR (KBr, nujol, cm-1): 1642 (m, nCN iminoacyl).
[(BDI)(g2-OC(Me)(PCy3))Nb(NtBu)][B(C6F5)4] (7b)
1H NMR (400 MHz, C6H5F/10% C6D6): d 5.62 (s, 1H,
HC(C(Me)NAr)2), 3.60 (sept, 1H, CHMe2), 3.29 (sept, 1H,
CHMe2), 2.60 (sept, 1H, CHMe2), 2.51 (sept, 1H, CHMe2), the
remainder of the spectrum could not be positively identified due
1
to broad, overlapping Cy signals. 13C{ H} NMR (100.61 MHz,
1
1
C6H5F/10% C6D6): d 89.75 (d, JCP = 53.4 Hz). 31P{ H} NMR
(161.98 MHz, C6H5F/10% C6D6): d 33.86 (d, 1JCP = 53.3 Hz).
[(BDI)(PhMe2CO)Nb(NtBu)][B(C6F5)4] (11)
[(BDI)(g2-OC(Me)(PEt3))Ta(NtBu)][B(C6F5)4] (8a)
Acetophenone (14.2 mL, 0.12 mmol) was added by syringe to a
solution of 2a (156 mg, 0.12 mmol) in Et2O (5 mL). The color of the
solution lightened slightly, and the solution was stirred for 30 min.
A pale yellow solid was obtained by removing the solvent in vacuo
and washing the resulting oil with several portions of pentane,
followed by removing the residual solvent under vacuum. Yield:
156 mg, 91%. 1H NMR (400 MHz, C6D5Cl): d 7.06–7.30 (m, 11H,
Ar), 6.19 (s, 1H, HC(C(Me)NAr)2), 2.34 (sept, 2H, iPr-CH), 2.05
1H NMR (400 MHz, C6H5Cl/10% C6D6, 298 K), major isomer:
d 5.68 (s, 1H, HC(C(Me)NAr)2), 3.52 (br m, 2H, CHMe2), 2.40
(br m, 2H, CHMe2), 1.82 (s, 3H, HC(C(Me)NAr)2), 1.77 (s, 3H,
HC(C(Me)NAr)2), 1.31 (d, 3H, CHMe2), 1.26 (d, 3H, CHMe2),
1.19 (m, 12H), 1.14 (m, 15H), 1.00 (m, 3H), 0.86 (t, 9H, PEt3), 0.61
(m, 6H, PEt3). 1H NMR (400 MHz, C6H5Cl/10% C6D6, 298 K),
minor isomer: d 5.68 (s, 1H, HC(C(Me)NAr)2), 3.21 (sept, 1H,
CHMe2), 3.09 (sept, 1H, CHMe2), 3.00 (sept, 1H, CHMe2), 2.27
(sept, 1H, CHMe2), 1.89 (s, 3H, HC(C(Me)NAr)2), 1.66 (s, 3H,
HC(C(Me)NAr)2), the remaining signals could not be positively
i
(sept, 2H, Pr-CH), 1.95 (s, 6H, HC(C(Me)NAr)2), 1.66 (s, 6H,
Me2C(Ph)ONb), 1.22 (d, 6H, iPr-Me), 1.13 (d, 6H, iPr-Me), 0.94
(d, 6H, iPr-Me), 0.72 (d, 6H, iPr-Me), 0.68 (s, 9H, tBu). 13C NMR
(125 MHz, C6D5Cl): d 172.07 (HC(C(Me)NAr)2), 149.50 (C6F5),
147.60 (C6F5), 146.57 (Ar), 141.76 (Ar), 140.18 (Ar), 139.87 (Ar),
137.50 (C6F5), 135.70 (C6F5), 129.58 (Ar), 128.85 (Ar), 125.17
(Ar), 124.61 (Ar), 123.99 (Ar), 99.35 (HC(C(Me)NAr), 86.66
(OCMe2Ph), 73.95 (tBu, Ca), 32.26 (OCMe2Ph), 30.81 (tBu, Cb),
30.00 (CHMe2), 28.68 (CHMe2), 25.37 (HC(C(Me)NAr)2), 24.29
(CHMe2), 24.23 (CHMe2), 23.76 (CHMe2), 23.71 (CHMe2). 19F
NMR (376.5 MHz, C6D5Cl): d -130.7 (s, o-F), -161.43 (t, m-F),
-165.24 (t, p-F). Anal. Calcd for C66H61BF20N3NbO: C, 56.79; H,
4.40; N, 3.01. Found: C, 56.94, H 4.58; N, 2.89.
identified due to overlap with the major isomer. 13C{ H} NMR
1
1
(100.61 MHz, C6H5F/10% C6D6, 298 K): d 93.68 (d, JCP
=
56.3 Hz, major isomer), 84.72 (d, 1JCP = 50.3 Hz, minor isomer).
1
31P{ H} NMR (161.98 MHz, C6H5F/10% C6D6, 298 K): d 36.9
(br d, 1JCP = 56 Hz, major isomer), 41.12 (d, 1JCP = 50.3 Hz, minor
isomer). 1H NMR (400 MHz, C6H5Cl/10% C6D6, 343 K): d 5.75
(s, 1H, HC(C(Me)NAr)2), 3.59 (sept, 1H, CHMe2), 2.47 (sept, 1H,
CHMe2), 2.46 (m, 2H, CHMe2), 1.90 (s, 3H, HC(C(Me)NAr)2),
1.84 (s, 3H, HC(C(Me)NAr)2), 1.52 (br s, 3H), 1.42 (br s, 3H), 1.29
(d, 3H), 1.24 (d, 3H), 1.15 (m, 21H), 1.01 (d, 3H), 0.92 (d, 3H), 0.69
1
(m, 6H, PEt3). 13C{ H} NMR (125.8 MHz, C6H5Cl/10% C6D6,
[(BDI)(PhMe2CO)Ta(NtBu)][B(C6F5)4] (12)
1
1
343 K): d 93.62 (d, JCP = 53.8 Hz). 31P{ H} NMR (202.5 MHz,
1
Compound 12 was prepared following the procedure given for
11, using 120 mg (0.088 mmol) of 6, 10.3 mL of acetophenone,
and fluorobenzene as the solvent instead of Et2O. The product
was isolated as an orange powder. Yield: 123 mg, 94%. 1H NMR
(500 MHz, C6H5F/10% C6D6, 298 K): d 7.25–7.10 (m, 11H, Ar),
6.15 (s, 1H, HC(C(Me)NAr)2), 2.37 (sept, 2H, CHMe2), 2.14
(sept, 2H, CHMe2), 1.94 (s, 6H, HC(C(Me)NAr)2), 1.65 (s, 6H,
PhMe2COTa), 1.15 (d, 6H, CHMe2), 1.11 (d, 6H, CHMe2), 0.92
(d, 6H, CHMe2), 0.71 (d, 6H, CHMe2), 0.65 (s, 9H, tBu). 13C NMR
(125 MHz, C6H5F/10% C6D6): d 175.2 (HC(C(Me)NAr)2), 150.4
C6H5Cl/10% C6D6, 343 K): d 36.80 (br d, JCP = 54 Hz). Anal.
Calcd for C65H68BF20N3OPTa: C, 51.70; H, 4.54; N, 2.78. Found:
C, 51.43, H 4.16; N, 3.13.
[(BDI)(g2-OC(Me)(PCy3))Ta(NtBu)][B(C6F5)4] (8b)
1H NMR (400 MHz, C6H5F/10% C6D6): d 5.71 (s, 1H,
HC(C(Me)NAr)2), 3.67 (sept, 1H, CHMe2), 3.61 (sept, 1H,
CHMe2), 2.61 (sept, 1H, CHMe2), 2.41 (sept, 1H, CHMe2), the
remainder of the spectrum could not be positively identified due
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7718–7729 | 7721
©

(C6F5), 148.5 (C6F5), 147.2 (Ar), 141.7 (Ar), 141.5 (Ar), 141.1
(Ar), 138.2 (C6F5), 136.3 (C6F5), 130.4 (Ar), 129.5 (Ar), 128.6
(Ar), 125.9 (Ar), 125.3 (Ar), 124.7 (Ar), 98.8 (HC(C(Me)NAr),
87.5 (OCMe2Ph), 69.9 (tBu, Ca), 32.7 (tBu, Cb), 32.7 (OCMe2Ph),
30.7 (CHMe2), 29.3 (CHMe2), 25.8, 25.2, 24.7, 24.2, 24.1. Anal.
Calcd for C66H61BF20N3OTa: C, 53.42; H, 4.14; N, 2.83. Found: C,
53.59, H 4.14; N, 2.94.
the color lightened to pale yellow. After 10 min, the volatile
materials were removed under vacuum, and the resulting oil was
washed with pentane (3 ¥ 5 mL) and dried under vacuum to yield
1
a pale yellow powder. Yield: 98 mg, 87%. H NMR (500 MHz,
C6H5F/10% C6D6): d 7.70 (m, 6H, SiPh3), 7.30 (m, 9H, SiPh3), 6.46
(s, 1H, HC(C(Me)NAr)2), 2.38 (sept, 2H, CHMe2), 2.09 (s, 6H,
HC(C(Me)NAr)2), 1.94 (sept, 2H, CHMe2), 1.18 (d, 6H, CHMe2),
0.96 (d, 6H, CHMe2), 0.89 (d, 6H, CHMe2), 0.72 (s, 9H, tBu), 0.32
1
(d, 6H, CHMe2). 13C{ H} NMR (125.7 MHz, C6H5F/10% C6D6,
[(BDI){j2-N2-MeC(NCy)2}Nb(NtBu)][B(C6F5)4] (13)
298 K): d 172.9, 142.3, 141.5, 140.4, 135.4, 134.7, 131.5, 129.1,
125.8, 125.2, 99.4, 76.2, 31.1, 31.0, 29.5, 26.1, 24.8, 24.5, 24.2,
23.6. 29Si NMR (99.4 MHz, C6H5F/10% C6D6): d -13.36. Anal.
Calcd for C75H65BF20N3NbOSi: C, 58.64; H, 4.27; N, 2.74. Found:
C, 58.80, H 4.44; N, 3.02.
A solution of CyN
C
NCy (20 mg, 0.08 mmol) in chloroben-
zene (2 mL) was added to a solution of 2a (104 mg, 0.08 mmol)
in chlorobenzene (6 mL). The solution turned from yellow–
orange to deep red–orange over 3 h, after which time the volatile
materials were removed under vacuum. The resulting red–orange
oil solidified after washing with pentane (3 ¥ 10 mL) and removing
the residual solvent under vacuum. Yield: 102 mg, 84%. 1H
NMR (500 MHz, C6D5Cl): d 7.25–7.00 (m, 6H, Ar), 5.90 (s,
1H, HC(C(Me)NAr)2), 3.22 (sept, 2H, CHMe2), 2.64 (pent, 2H,
NCH), 2.26 (sept, 2H, CHMe2), 1.82 (s, 6H, HC(C(Me)NAr)2),
1.82 (s, 3H, NC(Me)N), 1.5 (br m, 8H, Cy), 1.20 (d, 6H, CHMe2),
[(BDI)(Ph3SiO)Ta(NtBu)][B(C6F5)4] (16)
Compound 16 was prepared following the procedure given for
15, using 100 mg (0.07 mmol) of 6 and 20.3 mg (0.07 mmol)
of Ph3SiOH. The product was obtained as a white powder. Yield:
120 mg, 91%. 1H NMR (500 MHz, C6H5Cl/10% C6D6): d 7.70 (m,
6H, SiPh3), 7.33 (m, 9H, SiPh3), 6.43 (s, 1H, HC(C(Me)NAr)2),
2.37 (sept, 2H, CHMe2), 2.07 (s, 6H, HC(C(Me)NAr)2), 1.93 (sept,
2H, CHMe2), 1.18 (d, 6H, CHMe2), 0.95 (d, 6H, CHMe2), 0.88
t
1.14 (d, 6H, CHMe2), 1.14 (s, 9H, Bu), 1.10 (d, 6H, CHMe2),
1.07 (br m, 8H, Cy), 0.87 (d, 6H, CHMe2), 0.80 (br m, 4H, Cy).
1
13C{ H} NMR (125.7 MHz, C6D5Cl): d 185.6, 172.6, 148.8, 147.7,
1
142.9, 142.1, 140.7, 137.5, 135.5, 129.0, 124.8, 108.8, 74.5, 60.0,
34.1, 32.1, 31.7, 29.3, 27.4, 26.0, 25.5, 25.0, 24.7, 24.6, 23.1, 22.4,
(d, 6H, CHMe2), 0.70 (s, 9H, tBu), 0.25 (d, 6H, CHMe2). 13C{ H}
NMR (125.7 MHz, C6H5F/10% C6D6, 298 K): d 175.3, 141.9,
141.4, 140.9, 135.4 134.4, 131.6, 129.2, 128.9, 125.9, 125.2, 99.0,
71.0 (tBu, Ca), 32.5 (tBu, Cb), 31.2, 29.4, 26.1, 25.3, 24.5, 24.2, 23.6.
29Si NMR (99.4 MHz, C6H5F/10% C6D6): -12.15. Anal. Calcd for
C75H65BF20N3OSiTa: C, 55.46; H, 4.03; N, 2.59. Found: C, 55.65,
H 4.26; N, 2.57.
20.6, 13.9. IR: nC (cm-1) = 1643 (amidinate), 1513 (BDI). Anal.
N
Calcd for C71H75BF20N5Nb: C, 57.54; H, 5.10; N, 4.73. Found: C,
57.34, H 5.22; N, 5.10.
[(BDI)(tBuMeC N)Nb(NtBu)][B(C6F5)4] (14)
tBuCN (13.4 mL, 0.12 mmol) was added by syringe to a solution of
2a (156 mg, 0.12 mmol) in Et2O (10 mL). The solution immediately
turned red–orange. After 3 h at room temperature, the solution was
concentrated under vacuum to ca. 2 mL, at which point an orange
microcrystalline material precipitated from the solution. The solid
was collected following isolation from the mother liquor by
filtration and removal of the residual solvent under vacuum. Yield:
137 mg, 83%. 1H NMR (500 MHz, C6D5Cl, 298 K): d 7.1–6.8 (m,
Ar), 5.49 (br s, 1H, HC(C(Me)NAr)2), 2.62 (br m, 2H, CHMe2),
2.43 (br m, 2H, CHMe2), 1.56 (br s, 6H, HC(C(Me)NAr)2), 1.31 (s,
3H, tBuMeC N), 1.09 (br s, 12 H, CHMe2), 1.02 (br s, 9H, tBu),
0.89 (br s, 12H, CHMe2), 0.71 (s, 9H, tBu). 1H NMR (500 MHz,
C6H5Cl(C6D6), 343 K): (Aryl resonances were obscured by solvent
peaks) d 5.69 (s, 1H, HC(C(Me)NAr)2), 2.55 (br s, 2H, CHMe2),
2.43 (br s, 2H, CHMe2), 1.74 (s, 6H, HC(C(Me)NAr)2), 1.34 (s,
3H, tBuMeC N), 1.18 (d, 12H, CHMe2), 1.10 (br s, 6H, CHMe2),
Polymerization of ethylene with 2b
A solution of 2b (21.2 mg, 0.019 mmol) in 15 mL of C6H5Cl was
added to a 100 mL Schlenk flask. The solution was frozen, and
the headspace was evacuated and replaced with an atmosphere of
ethylene (85 mL, 3.5 mmol). The flask was sealed and allowed to
warm to room temperature. The solution was stirred for 12 h, over
which time a solid deposited. Methanol (5 mL) was added, and
the solid was collected on a frit, washed with HF(aq) (0.5 M, 10 mL)
then water (50 mL) and dried under vacuum to give a white solid.
Yield: 95 mg, 92%. M.p. 122 ◦C.
Results and discussion
Synthesis and characterization of four-coordinate niobium cations
Published synthetic routes to early-metal alkyl cations involve
alkyl or halide group abstraction, alkyl group protonolysis,
oxidation of d-electron containing species, or oxidation of metal–
carbon bonds. In the systems described here, we found alkyl group
abstraction and alkyl group protonolysis reactions to be the most
effective.
t
t
1
0.96 (br s, 6H, CHMe2), 0.86 (s, 9H, Bu), 0.85 (s, 9H, Bu). H
NMR (500 MHz, C6D5Cl, 223 K): d 5.63 (s, HC(C(Me)NAr)2 of
minor isomer), 5.43 (s, HC(C(Me)NAr)2 of major isomer). The
remaining signals were still unresolved at this temperature. Ratio
of major : minor isomers = 2.5. Anal. Calcd for C63H62BF20N4Nb:
C, 55.68; H, 4.60; N, 4.12. Found: C, 55.80, H 4.89; N, 4.50.
Within the category of alkyl anion group removal, several routes
were available for generating stable, monomeric cations from our
previously described dimethyl complex (BDI)Me2Nb(NtBu) (1).
The complex [(BDI)MeNb(NtBu)][B(C6F5)4] (2a) was prepared
by addition of a solution of either [(Et2O)2H][B(C6F5)4] in Et2O
or [Ph3C][B(C6F5)4] in chlorobenzene to a solution of 1 in Et2O
[(BDI)(Ph3SiO)Nb(NtBu)][B(C6F5)4] (15)
Ph3SiOH (20.3 mg, 0.07 mmol) was dissolved in fluorobenzene
(2 mL) and added to a solution of 2a (93 mg, 0.07 mmol) in
fluorobenzene at room temperature. The solution effervesced as
7722 | Dalton Trans., 2011, 40, 7718–7729
This journal is
The Royal Society of Chemistry 2011
©

Scheme 1
or chlorobenzene, respectively (Scheme 1). Reaction with either
reagent occurs within seconds at room temperature and results
in a color change from pale yellow to golden yellow. Use of
Jutzi’s acid, [(Et2O)2H][B(C6F5)4],41 was found to be the preferable
method because of the ease of preparation of the acid from
KB(C6F5)4, its crystallinity, and its formation of volatile by-
products (Et2O and CH4) on reaction with a methide group. The
solution rapidly effervesces in the case of [(Et2O)2H][B(C6F5)4],
Fig. 1 Molecular structure of the cationic portion of 2a as determined
by a single crystal X-ray diffraction study. The hydrogen atoms, the
two co-crystallized molecules of Et2O, and the B(C6F5)4 anion were
omitted for clarity; the thermal ellipsoids were set at the 50% proba-
˚
bility level. Selected bond lengths (A): Nb(1)–C(1) 2.163(5), Nb(1)–N(1)
1
1.737(4), Nb(1)–N(2) 2.038(4), Nb(1)–N(3) 2.036(4), N(2)–C(19) 1.341(6),
C(19)–C(20) 1.419(7), C(20)–C(21) 1.405(7), N(3)–C(21) 1.354(6). Se-
lected bond angles (◦): N(1)–Nb(1)–N(3) 108.22(18), N(1)–Nb(1)–N(2)
108.59(19), N(3)–Nb(1)–N(2) 96.13(16), N(1)–Nb(1)–C(1) 108.0(2),
N(3)–Nb(1)–C(1) 116.0(2), N(2)–Nb(1)–C(1) 119.13(19).
and 1,1,1-triphenylethane may be observed by H NMR when
[Ph3C][B(C6F5)4] is used.
1
A H NMR spectrum of the product taken in chlorobenzene-
d5 indicates that the molecule has averaged Cs symmetry in
solution. The spectrum has sharp lines at room temperature,
and the observed symmetry◦ in solution does not change on
heating or cooling (+70/-40 C) the sample in the NMR probe.
The spectrum has one singlet at 1.01 ppm corresponding to the
remaining Nb-bound methyl group, and a 19F NMR spectrum of
the crystallized product displays the expected resonances for the
[B(C6F5)4]- group. The resonance due to the Nb–CH3 carbon in
the 13C NMR spectrum is not shifted appreciably from the signal
for the two Nb–Me groups of the starting material, but the Ddab
value for 2a increased to 45.3, compared to 35.1 (avg.) for the
neutral dimethyl,42 suggesting that the imido nitrogen of 2a is less
shielded than that of 1.25 Since both compounds 1 and 2a have
orbitals available that can accommodate Nb–N triple bonding
(1s, 2p),48 this increase in the Ddab value indicates a change
in the electrophilicity of the metal center, not simply an orbital
rehydridization about the metal center to accommodate formerly
symmetry-forbidden p-bonding from the imido nitrogen. Used in
this qualitative sense for comparing two closely-related complexes,
the Ddab values provide a useful point of comparison of the relative
electron deficiency of the metal centers.49
The cationic Nb center has a distorted tetrahedral geometry,
constrained by the bite angle of the BDI ligand (N(2)–Nb(1)–
N(3) = 96.11◦), which has opened up considerably from that of
1 (N(2)–Nb(1)–N(3) = 83.33◦). The structural changes induced
by the lower coordination number and the cationic charge at the
metal center are reflected by a contracted Nb–Me bond length
˚
˚
of 2.164 A compared to that in 1 (2.177 and 2.187 A) and a
˚
˚
shorter Nb–Nimido bond length (1.736 A vs. 1.777 A for 1). The non-
crystallographically imposed local mirror symmetry of the cation
results in statistically indistinguishable Nb–NBDI bond lengths of
˚
2.036(4) and 2.038(4) A, whereas the trans-influence of the imido
group in complex 1 lengthens the Nb–NBDI bond for the nitrogen
˚
˚
trans to the imido to 2.357 A, compared to 2.136 A for the
equatorial Nb–NBDI bond.
A related cationic complex was prepared by addition of a
chlorobenzene solution containing 1.0 equiv of B(C6F5)3 to a
solution of 1 in chlorobenzene, resulting in methide group abstrac-
tion to form [(BDI)MeNb(NtBu)][MeB(C6F5)3] (2b, Scheme 2). A
1H NMR spectrum of the product in chlorobenzene-d5 revealed
that the transition metal-based portion of the molecule has an
identical composition to that in 2a. The boron-bound methyl
group resonates at 1.25 ppm, and the 19F NMR spectrum reveals
D(mF – pF) = 2.4 ppm, indicating that on the NMR time scale
the methylborate anion shows no directional interaction with
the metal center.51 The solubility of this complex (soluble in
Et2O, THF, chlorobenzene; insoluble in C6H6 and pentane) is
similar to that of the related cationic complex 2a, thus supporting
The product is insoluble in pentane and benzene and fully
soluble in ethers and haloarenes, which is consistent with a
molecule of considerably higher polarity than that of the starting
material. The formation of a dimer in solution is unlikely
considering the build-up of charge required by dimerization, and
notably, the high Ddab falls outside the range observed for amido-
t
type Bu groups.50 Derivatization of 2a by addition of 1.0 equiv
of MeMgBr to a solution of 2a in Et2O cleanly reforms 1, and
while the dimethyl complex decomposes thermally, compound 2a
is stable at room temperature under an inert atmosphere in the
solid state, decomposing appreciably only at 75 ◦C in solution.
These data all support the formulation of 2a as a monomeric, ion-
paired, tetrahedral methylniobium complex; a composition that
was also found in the solid state.
Single crystals of 2a were grown from Et2O, and a crys-
tallographic study reveals a unit cell containing the niobium
cation, B(C6F5)4 anion (Fig. 1) and four Et2O of crystallization.
Scheme 2
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7718–7729 | 7723
©

the formulation of 2b as an ion-paired niobium cation with a
methylborate anion.
Synthesis of tantalum dichloride, dimethyl and cationic
monomethyl complexes
Due partly to the stronger bonds between Ta and the lighter p-
block elements compared to the analogous Nb–p-block bonds,
the chemistry of Ta has received considerably more attention
than that of its lighter congener. This difference in reactivity
between the 4d and 5d metals has been used for forming stable
5d metal analogs of 4d metal complexes that are too reactive
to be investigated. Thus, for comparison of the Nb chemistry
with the better-known Ta chemistry, we prepared the dichloride,
dimethyl, and cationic monomethyl Ta complexes analogous to the
Nb complexes (BDI)pyCl2Nb(NtBu) (3), 1, and 2a, respectively.
Synthesis of the dichlorotantalum complex (BDI)-
pyCl2Ta(NtBu) (4, Scheme 3) proceeded in a manner similar to
that of the analogous niobium complex 3,42 though heating the
solution to reflux for 12 h was required to drive the reaction
to completion. The product was crystallized from a saturated
Et2O solution at -80 ◦C, although in lower yields than for 3. The
crystalline material obtained from this process quickly desolvates,
affording a bright yellow powder after drying under vacuum.
Fig. 2 Molecular structure of 4 as determined by a single crys-
tal X-ray diffraction study. The hydrogen atoms were omitted for
clarity; the thermal ellipsoids were set at the 50% probability
˚
level. Selected bond lengths (A): Ta(1)–N(1) 1.784(2), Ta(1)–N(2)
2.108(2), Ta(1)–N(3) 2.392(2), Ta(1)–N(4) 2.350(2), Ta(1)–Cl(1) 2.4128(7),
Ta(1)–Cl(2) 2.3993(7), N(2)–C(18) 1.378(4), C(18)–C(19) 1.370(4),
C(19)–C(20) 1.426(4), N(3)–C(20) 1.324(4). Selected bond angles (◦):
Ta(1)–N(1)–C(1) 164.9(2), Cl(1)–Ta(1)–Cl(2) 160.85(3), N(1)–Ta(1)–N(3)
174.34(9), N(2)–Ta(1)–N(4) 173.32(8), N(2)–Ta(1)–N(3) 83.59(9).
1
The H NMR spectrum of 4 has features similar to those of 3,
displaying averaged Cs symmetry with peak broadening due to
reversible pyridine dissociation in solution.
Fig. 3 Molecular structure of 5 as determined by a single crystal X-ray
diffraction study. The hydrogen atoms and Cl(1) were omitted for clarity;
the thermal ellipsoids were set at the 50% probability level. Selected
Scheme 3
Crystals suitable for X-ray analysis were grown from hot hexane
(+65/+20 ◦C). The molecular structure is illustrated in Fig. 2,
with selected metric parameters reproduced in the caption. The
metal center has a distorted octahedral geometry, with the two
axial chlorines angled toward the pyridine ligand (Cl(1)–Ta(1)–
Cl(2) = 160.85◦). Due to the effect of the lanthanide contraction,
the bond lengths about the metal center are similar to those for
the analogous Nb complex (3) described previously (maximum
deviation <1.2%).
The dimethyl tantalum complex was synthesized by addition of
2.0 equiv of MeMgBr to a solution of 4 in Et2O. The dimethyl
product (BDI)Me2Ta(NtBu) (5, Scheme 3) was crystallized from
a saturated pentane solution, yielding pale yellow blocks suitable
for crystallographic analysis. The overall geometry of 5 (Fig. 3) is
similar to that of 1, being trigonal bipyramidal (t = 0.71)52 with
˚
bond lengths (A): Ta(1)–C(1) 2.193(14), Ta(1)–C(2) 2.172(4), Ta(1)–N(1)
1.779(3), Ta(1)–N(2) 2.119(3), Ta(1)–N(3) 2.347(3), N(2)–C(20) 1.362(5),
C(20)–C(21) 1.381(5), C(21)–C(22) 1.405(5), N(3)–C(22) 1.317(5). Selected
bond angles (◦): N(2)–Ta(1)–C(2) 112.57(14), N(2)–Ta(1)–C(1) 129.6(4),
C(2)–Ta(1)–C(1) 115.7(4), N(1)–Ta(1)–N(3) 172.17(12), N(2)–Ta(1)–Cl(1)
136.3(8), C(3)–N(1)–Ta(1) 168.5(3).
˚
in place of the C(1) methyl group (d(Ta(1)–Cl(1)) = 2.379 A),
indicating that complex 5 has co-crystallized with the methyl
chloride complex (BDI)ClMeTa(NtBu). The formation of this
compound is assumed to occur via addition of an insufficient
amount of MeMgBr. Interestingly, attempts at forming mixed
alkyl chloride complexes in the Nb system failed.42
In contrast to the behavior of the Nb complex 1, reaction of
5 with B(C6F5)3 failed to yield the homologous methyltantalum
cation, presumably due to the stronger Ta–Me bond compared
to that in the Nb system. Nevertheless, on reaction of a slight
excess of 5 with either [Ph3C][B(C6F5)4] or [(Et2O)2H][B(C6F5)4] in
fluorobenzene, the solution turned golden yellow, and when the
protic acid was used the solution rapidly effervesced.
˚
equatorial methyl groups (Ta(1)–C(1) = 2.194 A; Ta(1)–C(2) =
˚
˚
2.172 A), an axial imido ligand (d(Ta(1)–N(1)) = 1.779 A), and a
BDI ligand spanning axial and equatorial coordination sites.
The crystallographic data fit more accurately with a model that
incorporates 15% occupancy of chlorine near the C(1) site and
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Removal of the volatile materials under vacuum followed
by multiple pentane washes yielded the cationic complex
[(BDI)MeTa(NtBu)][B(C6F5)4] (6, Scheme 4) in good yield and
high purity.
in M–C bonding energy and by a decrease in entropy. The higher
cationic charge at the metal center, shown crystallographically to
result in shorter Nb–C and Nb–Nimido bond lengths, may shift
the equilibrium toward the side of free CO despite the electron
deficiency at the metal center.
While NMR spectroscopic observation of a stable acyl
adduct was not possible, a transient insertion product was
trapped by phosphines to yield the tetrahedral Lewis base
adducts of an acyl. Addition of CO to solutions of either
2a or 6 in the presence of phosphines resulted in rapid reac-
2
tions to give the phosphine-trapped acyl complexes [(BDI)(h -
OC(Me)(PR2R¢))M(NtBu)][B(C6F5)4] (7a, R = R¢ = Et, M = Nb; 7b,
R = R¢ = Cy, M = Nb; 8a, R = R¢ = Et, M = Ta; 8b, R = R¢ = Cy, M =
Ta; Scheme 5). Compound 8a was isolated as a crystalline solid
in high yield, and the remaining compounds were characterized
by analysis of the reaction mixture by NMR spectroscopy, which
indicated that the products formed in >90% yield.
Scheme 4
The 1H NMR spectrum of 6 closely resembles that of 2a,
which indicates local, averaged Cs symmetry for the cation in
solution. The chemical shift of the Ta–Me group at 0.83 ppm
is shifted upfield from that for 2a, but like 2a, the 13C{ H} NMR
1
spectrum reveals a large Ddab value of 39.2, indicative of increased
donation of electron density from the imido nitrogen to the metal
center. Solutions of 6, like 2a, are stable in the presence of the
parent dimethyl complex, and addition of 1.0 equiv of MeMgBr
to a solution of 6 in Et2O regenerated the dimethyl complex 5
in quantitative yield. The stability of 6 in Et2O is limited, and
attempts at crystallizing the product from Et2O/pentane resulted
in a crystalline mixture of 6 and another unidentified product that
may have incorporated Et2O, based on NMR analysis.
Scheme 5
Insertion of CO and XylNC
1
The 13C{ H} NMR spectra obtained on samples of 7a, b and 8a,
We were initially interested in carbonylating 2a and 6 to form
cationic acyl complexes as an extension of our previous work
studying the carbonylation pathways available to the dimethyl
complex (BDI)Me2Nb(NtBu) (1).53 The complicated mixture of
products observed on carbonylation of the 1 was attributed
largely to the various reaction pathways available to the remaining
niobium-bound methyl group following formation of the transient
monoacyl intermediate. Accordingly, when the methyl bromide
complex (BDI)MeBrNb(NtBu) was exposed to carbon monoxide,
the compound cleanly inserted CO at room temperature to yield
an observable monoacyl.53 We consequently reasoned that the
cationic monomethyl compounds could serve as suitable entry
points into stoichiometric monoacyl chemistry and provide a
powerful tool for C–C bond formation.
Surprisingly, neither 2a nor 6 reacted with CO to yield an
observable complex in solution (1 atm CO, 20 ◦C). NMR spectra
of a solution of the cationic complexes under CO did not
indicate a change in the molecular constitution of the complexes,
and methylation of the material recovered from the attempted
carbonylation yielded only the parent dimethyl compounds. This
lack of reactivity was especially surprising considering the range of
products observed upon carbonylation of 1. Reversible carbonyla-
tion of early-metal alkyls is well-documented in the literature but
has generally been observed with metallocene systems which have
a limited number of vacant orbitals.54 In these cases, the facility
of the de-insertion reaction was found to depend on the donor
capability of the other, non-Cp ligand.55 In the current system,
the failure of 2a and 6 to insert CO indicates that any energetic
gain in M–O bond energy (the effect widely believed to drive the
formation of early-metal acyls) can be counteracted by a decrease
b synthesized with 13CO exhibit doublets in the 80–100 ppm range
with coupling constants (ca. 50 Hz) identical to those observed
for doublets in the 31P{ H} spectrum (20–45 ppm, Tables 2
1
1
and 3). Coupling constants in this range are indicative of JCP
interactions, and related acyl and silaacyl complexes exhibit simi-
larly upfield-shifted carbon signals for the metal-bound carbon
atom (relative to transition-metal acyls) and downfield-shifted
phosphorus resonances (relative to the free phosphine).56–66 These
signals are indicative of the alkoxide and phosphonium character
of the carbon and phosphorus atoms, respectively. Although
the crystallographic data were not sufficient to fully elucidate
the structure, a partial X-ray crystal structure of 8a supports
the proposed formulation of the products as the monomeric,
2
phosphine coordinated h -acyl complexes (Scheme 5).
While the mechanism of this reaction has not been determined,
it should be noted that, as observed with CO, no change in
the 1H NMR spectrum of the cation could be detected upon
Table 2 Chemical shifts (in ppm) of the signals in 13C{ H} and 31P{ H}
NMR spectra corresponding to the 13C labelled carbon and the phospho-
nium phosphorus
1
1
Isomer 1a
13C
Isomer 2a
13C
343 K
13C
Compound
31P
31P
31P
7a
7b
8a
8b
86.76
89.75
93.68
98.74
36.00
33.86
36.9
78.80
—
84.72
—
39.10
—
41.12
—
—
—
93.62
—
—
—
36.80
—
32.65
a Collected at 298 K.
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©

1
1
Table 3 JCP coupling constants (in Hz) for the signals in the 13C{ H}
and 31P{ H} NMR spectra corresponding to the 13C labelled carbon and
1
the phosphonium phosphorus
Isomer 1a
13C
Isomer 2a
13C
343 K
13C
Compound
31P
31P
31P
b,c
7a
7b
8a
8b
55.6
53.4
56.3
48.5
—
53.2
—
50.3
—
53.3
—
50.3
—
—
—
53.8
—
—
Fig. 4 Two diastereomers of 7a (M = Nb) and 8a (M = Ta) proposed to
form on coordination of PEt3 to the acyl carbon.
53.3
—
56c
54(1)c
—
48.6
temperature, resulting in a single doublet in each spectrum. The
chemical shifts of the doublets are close to the values of the major
isomer observed at room temperature but have shifted slightly in
the direction of the minor isomer (Tables 2 and 3).
a Collected at 298 K. b The signal appeared as a broad singlet at this
temperature. c Precision of measurement decreased due to line broadening
at these temperatures.
Compared to CO insertion reactions, the insertion of iso-
cyanides into early-metal alkyl bonds is more likely to result in
stable products.54 Accordingly, a rapid reaction (in the absence
of a phosphine) was observed between 1.0 equiv of XylNC
(Xyl = 2,6-Me2C6H3) and 2a in C6H5Cl to form the cationic
treatment with an excess of phosphine. Known methods for the
synthesis of related compounds have been performed in a stepwise
fashion, where CO addition to alkyl or silyl complexes led to
isolatable acyls; subsequent reactions with phosphines generated
the phosphine-coordinated (sila-)acyls.61–62
2
iminoacyl complex [(BDI)(h -XylN CMe)Nb(NtBu)][B(C6F5)4]
(9, Scheme 7). The reaction mixture immediately turned dark red
on addition of the isocyanide, then golden yellow within seconds.
A 1H NMR spectrum of the reaction mixture revealed the presence
of two products, as indicated by the appearance of two new BDI
methine resonances at 5.84 and 5.83 ppm. We attribute these
signals to the stereoisomers generated by the variable orientation
The electrophilicity of early-metal acyl carbons indicates that
direct intermolecular addition of nucleophiles to the acyl carbons
is a likely mechanism (Scheme 6, route a). While this could be
occurring during the formation of compounds 7a, b and 8a,
b, another possibility would be a “phosphine-first” mechanism
(Scheme 6, route b). In this case, phosphine coordination would
create a more electron-rich, pentacoordinate complex, similar to
the methyl bromide complex (BDI)MeBrNb(NtBu), which has
been observedtoformastableniobium acylatroomtemperature.53
CO coordination and insertion into the Ta–Me bond of the
phosphine-coordinated cation would lead to an acyl complex.
Subsequent phosphine migration to the acyl carbon would then
furnish the phosphine trapped acyl products 7a, b and 8a, b in
a mechanism reminiscent of methyl group migration to form the
intermediate ketone adduct upon carbonylation of 1.
2
of the h -iminoacyl ligand (Scheme 7). A VT NMR experiment
confirmed this assignment: on heating the sample to 343 K the
resonances for the BDI backbone methine coalesced into a single
signal. The remainder of the spectrum was broad and largely
unidentifiable at this temperature (and up to 398 K), but cooling
the sample back to room temperature caused the two original
Compounds 7b and 8b appear to form only one isomer in
solution, while 7a and 8a form as a mixture of isomers (Fig. 4). In
the latter cases, the major isomer displays considerable fluxional
behavior at room temperature. Heating a solution of 8a to 343 K
causes the two isomers to interconvert rapidly on the NMR
time scale, allowing for observation of a time-averaged species,
as determined by monitoring the coalescence of the BDI ligand
backbone methine signals. The 13C{ H} and 31P{ H} spectra of
the 13C-labelled compound similarly indicate that the two isomers
are in rapid equilibrium relative to the NMR timescale at elevated
1
1
Scheme 7
Scheme 6 Possible mechanisms leading from compound 2a(6) to compound 7(8).
7726 | Dalton Trans., 2011, 40, 7718–7729 This journal is The Royal Society of Chemistry 2011
©

resonances to reappear, as the isomers resolved relative to the
NMR timescale. An IR spectrum of 9 further supported the
proposed formulation. Due to the reduced N–C bond order within
the iminoacyl group, the N–C stretching frequency in the IR
spectrum shifts to lower frequencies. A stretch attributable to 9
was observed at 1642 cm-1.
Since the acyl and iminoacyl complexes are isolobal, we
were interested to see if the iminoacyl cations would coordinate
phosphines in a manner analogous to that observed for the acyl
compounds described above; however, no reaction was observed
between 9 and PEt3 at room temperature. This lack of reactivity
between the phosphine and the iminoacyl carbon compared to the
facile formation of the phosphine-trapped acyls is supported by
theoretical studies that have predicted a higher energy p*CN orbital
on iminoacyl ligands compared to that of the corresponding p*CO
orbital of an acyl,67 but the lack of reactivity may also reflect
the different steric properties imposed by the bulky 2,6-xylyl
substituent on the iminoacyl nitrogen.
As further confirmation of the proposed structure of 9, addition
of 1.0 equiv of MeMgBr in Et2O resulted in a rapid reaction to give
2
2
the h -ketimine complex (BDI)(h -XylN CMe2)Nb(NtBu) (10)
in near quantitative yield. Complex 10 was described previously
as the product resulting from the reaction of 1 with 1.0 equiv of
XylNC,53 and the synthesis of 10 from 9 supports the intermediacy
of the iminoacyl complex during the formation of 10 from 1
directly. This reaction parallels the addition of nucleophiles to
organic imines and ketones, whereby the transition metal can be
considered to act as both a Lewis acid binding to the N/O group
and as an electron-withdrawing carbon-bound leaving group,
activating the unsaturated carbon toward nucleophilic attack. In
the present system, the methyl group could add either to the
iminoacyl carbon or to the formally cationic metal center. In
the latter case, subsequent methyl group transfer to the iminoacyl
carbon would then furnish 10 (Scheme 8). This latter process would
mimic the assumed mechanism for the formation of the ketimine
complex from the reaction of 1 with XylNC, but the electrophilic
character of the iminoacyl may be sufficient to promote attack by
the incoming nucleophile directly. The direct addition mechanism
would be related to the attack of a nucleophile on an organic imine
or iminium carbon.
Related insertion reactions with 2a and 6
In contrast to the behavior of 1, the cation 2a underwent 1,2-
insertion reactions with several unsaturated organic substrates
at room temperature, but no reaction was observed between
2a and terminal or internal aryl alkynes, PhNCO, tBuNCS,
or propylene oxide. The stability of this complex toward such
reagents under these conditions (low-polarity solvents, 20 ◦C, 1
atm pressure) is reminiscent of a structurally related imidotitanium
BDI complex.68
Addition of stoichiometric amounts of acetophenone to so-
lutions of 2a or 6 in fluorobenzene cleanly led to the alkoxide
complexes [(BDI)(PhMe2CO)M(NtBu)][B(C6F5)4] (11, M = Nb;
12 M = Ta; Scheme 9) by insertion of acetophenone into the M–
Me bond. The 1H NMR spectra of the products indicate average
Cs-symmetry. A new, sharp singlet appears in the range of 1.60–
1.70 ppm for each compound, which integrates to 6H relative to
one equivalent of the BDI ligand. Assignment of this signal to the
equivalent methyl groups on the alkoxide ligand was supported
1
by an HMBC H-13C correlation experiment that revealed this
signal to be within three bonds of a carbon with a chemical shift
1
between 85 and 90 ppm. Only one 13C{ H} NMR resonance was
observed above 170 ppm for each compound, and those signals
are attributable to the two symmetry-related imine carbons of the
BDI ligand. Similarly, no IR stretches were observed in the ketone
region of the spectra of the isolated products.
On addition of the carbodiimide CyN
C
NCy to a solution
Scheme 8
of 2a in chlorobenzene, the solution turned from yellow to bright
Scheme 9
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©

2
red over the course of 3 h, affording the k -amidinate complex
Summary and conclusions
[(BDI){k -N2-MeC(NCy)2}Nb(NtBu)][B(C6F5)4] (13, Scheme 9).
2
1
The neutral dimethyl complexes 1 (Nb) and 5 (Ta) undergo clean
methide group protonolysis to yield the stable 12/14 e- cations 2a
and 6. These compounds have been thoroughly characterized and,
for 2a, confirmed structurally. The crystallographic data indicate a
shortening of the Nb–Me and Nb NtBu bonds relative to those
observed for 1, which is indicative of a complex with both a lower
coordination number and higher metal-based cationic charge.
In contrast to the divergent carbonylation chemistry observed
with the dimethyl compounds, the methyl cations were not found
to irreversibly bind CO. In the presence of phosphines, cationic
phosphine-trapped acyl species could be formed cleanly as either
a single (PCy3) or a mixture (PEt3) of isomers. When isotopically
enriched 13CO was used in the reaction, these complexes were
readily identified by diagnostic doublets (1JCP ª 50 Hz) in the
Complex 13 has averaged Cs symmetry in solution by H NMR
spectroscopy as judged by the appearance of a single pentet
integrating to 2H at 2.66 ppm that corresponds to the two N-
CH(c-C5H10) protons. A new singlet at 1.84 ppm is assigned to
1
the amidinate methyl group (MeCN2), and the 13C{ H} NMR
spectrum has both a new amidinate carbon resonance at 185.5 ppm
(NCN) and a new amidinate methyl resonance at 13.9 ppm
(NC(Me)N), which is shifted considerably upfield from that
observed for the cation. The IR spectrum of 13 shows a signal
at 1643 cm-1 due to the amidinate NCN functionality, consistent
with analogous bands in other early transition metal complexes.69
A related insertion process occurred upon the reaction
of 2a with tBuCN to form the cationic Nb ketimide
[(BDI)(tBuMeC N)Nb(NtBu)][B(C6F5)4] (14; Scheme 9). The 1H
NMR signal for the Nb–Me group shifts from 0.95 ppm for 2a to
1.31 ppm for 14.70 Notably, the 1H NMR spectrum for 14 displays
broad signals, indicating that interconversion of the two possible
ketimide regioisomers is occurring at room temperature at a rate
close to the NMR timescale. A variable temperature NMR study
in C6H5Cl reveals that the broadened signals sharpen upon heating
to 343 K, indicating the presence of a single complex with overall
Cs symmetry. On cooling to 223 K, the single broad resonance
observed at 5.55 ppm at room temperature for the BDI methine
proton decoalesces into two non-equivalent singlets in a 2.5 : 1
ratio, with resonances at 5.29 and 5.58 ppm, respectively.
1
1
13C{ H} and 31P{ H} NMR. The coupling constants and the
associated chemical shift ranges of the doublets coincide well with
those of analogous complexes.
A related insertion reaction with XylNC was observed to yield a
stable cationic iminoacyl species as a mixture of two isomers. This
compound was stable in solution up to 398 K and, in contrast to
the acyls, did not bind phosphines to yield tetrahedral phosphine-
trapped iminoacyls. The relative stability of these compounds,
and related iminoacyl complexes as a whole, compared to that of
their isoelectronic acyl counterparts has been documented in the
literature.54
The cationic complexes 2a and 6 underwent insertion reactions
with acetophenone to yield cationic alkoxide complexes, and 2a
t
inserted CyN
C
NCy and BuCN to yield the amidinate and
Reaction of cations 2a and 6 with Ph3SiOH
ketimide complexes, respectively. Both the Ta and Nb complexes
cleanly extruded methane on reaction with Ph3SiOH to yield the
siloxide complexes in high yields.
The stability of the cationic complexes toward a protic silanol
reagent was also investigated. On addition of a solution of 1.0
equiv of Ph3SiOH in fluorobenzene to a solution of either 2a
or 6 in fluorobenzene the solution rapidly effervesced, yielding
[(BDI)(Ph3SiO)M(NtBu)][B(C6F5)4] (15, M = Nb; 16, M = Ta;
Scheme 9) in quantitative yields as the product resulting from
clean protonolysis of the M–Me group. By comparison, the
methyl-bound cationic complexes were stable in the presence of
an excess of [(Et2O)2H][B(C6F5)4] at room temperature, indicating
that a coordinating counter-ion is needed to effect methane loss.
Similarly, the methyl-bound cations were found to be stable toward
reaction with H2 at elevated temperatures (75 ◦C for 2a and 135 ◦C
for 6). The siloxide complexes are thermally robust, persisting in
solution at 135 ◦C for days without decomposition.
Finally, while the Ta cation was not observed to polymerize ethy-
lene, the Nb complex 2b afforded high-density polyethylene in high
yield at room temperature under 1 atm of pressure. The niobium
cation failed to yield appreciable quantities of poly(a-olefins) when
polymerization reactions were screened under similar conditions
with both branched and linear a-olefins, indicating that the steric
requirements of the BDI ligand allow only the smallest substrates
to enter the coordination sphere of the metal.
Taken together, these data indicate that the Nb/Ta imido link-
ages for these complexes, when in the presence of a Nb/Ta alkyl
bond, do not react irreversibly with a range of unsaturated organic
molecules. Furthermore, complexes 2a, b and 6 are isoelectronic
analogues to similarly unreactive and structurally analogous Ti
complexes,68 suggesting that the early-metal (BDI)M(NR) frame-
work may provide a general, robust platform for the investigation
of non-metallocene, early-metal alkyl chemistry.
Polymerization of ethylene with 2b
Since early-transition metal imido-supported alkyl cations are
well-known to effect olefin polymerization,3,21–23 we tested our sys-
tems for this capability. The Ta cation 6 failed to give appreciable
quantities of polyethylene, but exposure of a solution of 2b (0.5
mol%, 0.002 M) in C6H5Cl to 200 equiv of ethylene (1 atm) at room
temperature caused a white solid to precipitate after stirring for
12 h. The solid that formed was collected (>90%, see experimental
section) and identified as high-density polyethylene, having a sharp
melting point at 122 ◦C. Other a-olefins (both branched and
linear) failed to yield polymer in appreciable quantities under these
conditions.
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