Synthesis of tantalum and niobium complexes that contain the diamidoamine ligand, [(3,4,5-F3C6H2NCH2CH2)2NMe]2-, and the triamidoamine ligand, [(3,5-Cl2C6H<

Article abstract of DOI:10.1016/j.ica.2006.03.044

Several niobium and tantalum compounds were prepared that contain either the diamidoamine ligand, [(3,4,5-F₃C₆H₂NCH₂CH₂)₂NMe]^2- ([F₃N₂NMe]^2-), or

Full text of DOI:10.1016/j.ica.2006.03.044

Inorganica Chimica Acta 359 (2006) 4730–4740
www.elsevier.com/locate/ica
Synthesis of tantalum and niobium complexes that contain the
diamidoamine ligand, [(3,4,5-F3C6H2NCH2CH2)2NMe]2ꢀ, and the
triamidoamine ligand, [(3,5-Cl2C6H3NCH2CH2)3N]3ꢀ
*
Lourdes Pia H. Lopez, Richard R. Schrock , Peter J. Bonitatebus Jr.
The Department of Chemistry 6-331, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
Received 12 February 2006; accepted 6 March 2006
Available online 17 June 2006
Dedicated to Professor Dr. Dr. h.c. mult. Wolfgang A. Herrmann.
Abstract
Several niobium and tantalum compounds were prepared that contain either the diamidoamine ligand, [(3,4,5-F3C6H2N-
CH2CH2)2NMe]2ꢀ ([F3N2NMe]2ꢀ), or the triamidoamine ligand, [(3,5-Cl2C6H3NCH2CH2)3N]3ꢀ ([Cl2N2NMe]3ꢀ). The former include
[F3N2NMe]TaCl3, [F3N2NMe]NbCl3, [F3N2NMe]TaMe3, [F3N2NMe]NbMe3, [(F3N2NMe)TaMe2][MeB(C6F5)3], [F3N2NMe]Ta(CHSi-
Me3)(CH2SiMe3), [F3N2NMe]Ta(CH2-t-Bu)Cl2, [F3N2NMe]Ta(CH-t-Bu)(CH3), and [F3N2NMe]Ta(g2-C2H4)(CH2CH3). The latter
include [Cl2N2NMe]TaCl2, [Cl2N2NMe]TaMe2, [Cl2N2NMe]Ta(g2-C2H4), and [Cl2N2NMe]Ta(g2-C2H2).X-ray diffraction studies were
carried out on [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3), [F3N2NMe]Ta(g2-C2H4)(CH2CH3), and [Cl2N2NMe]TaMe2..
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Tantalum; Niobium; Amide; Alkylidene
1. Introduction
have published only two papers that concern diamido/
donor ligands on tantalum [9,11]. We report here an explo-
ration of d0 niobium and tantalum complexes that contain
the electron-withdrawing ligands [(3,4,5-F3C6H2NCH2-
CH2)NMe]2ꢀ or [(3,5-Cl2C6H3NCH2CH2)3N]3ꢀ complexes.
We undertook this investigation with an interest primar-
ily in alkylidene and olefin complexes, and the possibility
that an olefin could be induced to rearrange to an alkylidene.
The first reported reaction of this type was conversion of
[(Me3SiNCH2CH2)3N]Ta(g2-C2H4) to [(Me3SiNCH2-
CH2)3N]Ta(CHMe) in the presence of catalytic amount of
PhPH2 [12,13]. It was proposed that PhPH2 attacked
[(Me3SiNCH2CH2)3N]Ta(g2-C2H4) to yield intermediate
[(Me3SiNCH2CH2)3N]Ta(CH2CH3)(PHPh), which then
decomposed to yield [(Me3SiNCH2CH2)3N]Ta(CHMe)
and PhPH2. The strong agostic interaction in [(Me3-
SiNCH2CH2)3N]Ta(CHMe) was proposed to be the reason
why the ethylidene was favored over the ethylene complex.
Wolczanski and coworkers published an extensive study of
In the last several years various multidentate amido
ligands have helped uncover much new chemistry involving
metals in groups 4, 5, and 6 [1,2]. We have focused on
triamidoamine [3] ligands for metals in group 5 and group
6 and diamido/donor ligands primarily for metals in group
4 [4,5], but also to some extent for metals in group 5 and
group 6 [6–9]. We have been attracted to ligands that con-
tain aryl substituents on the amido nitrogens, especially
aryls that are not as electron-withdrawing as pentafluor-
ophenyl and that have no ortho substituents that can lead
to side reactions or other complications. Examples are ste-
rically demanding 3,5-terphenyl groups in triamidoamine
ligands [10] and 3,5-dichlorophenyl [9] and 3,4,5-trifluoro-
phenyl groups [8] groups in diamidoamine ligands. We
*
Corresponding author. Tel.: +1 617 253 1596; fax: +1 617 253 7670.
E-mail address: rrs@mit.edu (R.R. Schrock).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.03.044

Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
4731
the rearrangement of olefins in (t-Bu3SiO)3M (olefin)
(M = Nb or Ta) complexes to yield the tautomeric alkylid-
ene complexes [17]. In the (t-Bu3SiO)3M (olefin) systems
the intermediate is also proposed to be an alkyl, but one
formed as a consequence of CH activation of a methyl group
in the Silox ligand. Although we have not yet observed olefin
to alkylidene rearrangement in the new Nb and Ta com-
plexes that we prepared, they nevertheless are inherently
interesting examples of d0 Nb and Ta complexes that contain
relatively robust multidentate amido ligands.
give 2.59 g (74%) of orange crystals. The product could be
recrystallized from THF or dichloromethane. 1H NMR
(C6D6): d 7.17 (m, 4), 4.14 (m, 2), 3.56 (q, 2), 3.12 (m, 2),
2.09 (s, 3), 1.80(q, 2). 19F NMR: d ꢀ129 (dd, 4), ꢀ157 (tt,
2). Anal. Calc. for TaN3F6Cl3C17H15: C, 30.81; H, 2.28; N,
6.34; Cl, 16.05. Found: C, 30.48; H, 2.28; N, 6.26; Cl, 15.96%.
2.2.2. [(3,4,5-F3C6H2NCH2CH2)2NMe]NbCl3
A cold (ꢀ30 ꢁC) solution of H2[(3,4,5-F3C6H2NCH2-
CH2)2NMe] (2.00 g, 5.30 mmol) in 10 mL toluene was
added to a rapidly stirred slurry of NbCl5 (1.43 g,
5.30 mmol) in 15 mL of toluene. To the resulting orange
suspension was added 2.2 equiv. of triethylamine
(1.63 mL, 11.7 mmol). The reaction mixture was then
heated to 80 ꢁC and left to stir for 6 h. The toluene was
removed in vacuo and the residue was dissolved in 50 mL
of THF. The resulting red-orange suspension was then
allowed to cool to ꢀ30 ꢁC for 15 min to ensure complete
precipitation of triethylammonium chloride. The mixture
was then filtered through a bed of Celite. The filtrate was
collected and concentrated to 5–10 mL in vacuo, at which
point brick red microcrystals crashed out of solution. The
solids were collected on a frit, washed with pentane, and
dried in vacuo to give 2.4 g (80%) of the brick red product.
2. Experimental
2.1. General
All air-sensitive work was carried out in a Vacuum Atmo-
spheres dry box under a dinitrogen atmosphere or by stan-
dard Schlenk techniques. Chemicals were purchased from
Strem Chemicals, Inc., or Aldrich. Solid reagents were used
as received unless otherwise stated. Liquid reagents were dis-
tilled from CaH2 under dinitrogen. Diethyl ether, pentane,
toluene, benzene and tetrahydrofuran (THF) were sparged
with dinitrogen and passed through columns of activated
alumina. Dichloromethane was distilled from calcium
hydride. All deuterated solvents were freeze–pump–thaw
1
The product could be recrystallized from THF: H NMR
˚
degassed. All listed solvents were stored over 4 A molecular
(C6D6): d 7.26 (m, 4), 3.87 (m, 2), 3.25 (m, 4), 2.08 (s, 3),
1.77 (q, 2); 19F NMR: d ꢀ129 (dd, 4), ꢀ155 (tt, 2). Anal.
Calc. for NbN3F6Cl3C17H15: C, 35.54; H, 2.63; N, 7.31;
Cl, 18.15. Found: C, 35.63; H, 2.56; N, 7.41; Cl, 18.22%.
1
sieves. H NMR spectra were obtained on an instrument
operating at 300 MHz unless otherwise stated. 13C NMR
spectra were obtained on an instrument operating at
125 MHz, while 19F NMR spectra were obtained on a
282 MHz instrument. All spectra were recorded at or near
22 ꢁC. 1H and 13C NMR data are listed in parts per million
downfield from tetramethylsilane and were referenced using
the residual protonated solvent resonance. 19F NMR shifts
are reported relative to C6F6 used as an external reference.
Microanalyses were performed by Kolbe Microanalytical
2.2.3. [3,4,5-F3C6H2N(CH2CH2)2NMe]TaMe3
To a cooled (ꢀ30 ꢁC) suspension of 250 mg of [(3,4,5-
F3C6H2NCH2CH2)2NMe]TaCl3 (0.380 mmol) in 10 mL
diethyl ether was added 3.3 equiv. of methylmagnesium
chloride (1.2 mmol, 0.42 mL, 3.0 M in THF). The suspen-
sion was allowed to warm at room temperature as it was
stirred, during which time the color changed from orange
to bright yellow. After 1 h, 1,4-dioxane (0.30 mL) was
added. The mixture was filtered through Celite, and the fil-
trate was concentrated to dryness in vacuo. The yellow res-
idue was rinsed with pentane (15 mL) and dried in vacuo to
yield a yellow powder; yield 0.14 g (60%). 1H NMR
(C6D6): d 6.37 (m, 4), 3.05 (m, 2), 2.13 (q, 2), 1.93 (s, 3),
1.79 (q, 2), 0.93 (s, 9). 19F NMR: d ꢀ131 (dd, 4), ꢀ164
(tt, 2). Anal. Calc. for TaN3F6C20H24: C, 39.95; H, 4.02;
N, 6.99. Found: C, 40.06; H, 3.95; N, 6.87%.
Laboratories (Muhlheim an der Ruhr, Germany). TaCl Me
¨
2
3
[14] and H2[(3,4,5-F3C6H2NCH2CH2)2NMe] [8] were pre-
pared as reported in the literature.
2.2. Syntheses
2.2.1. [(3,4,5-F3C6H2NCH2CH2)2NMe]TaCl3
TaCl5 (1.90 g, 5.30 mmol) was added in portions to a cold
(ꢀ30 ꢁC) solution of H2[(3,4,5-F3C6H2NCH2CH2)2-
NMe] (2.00 g, 5.30 mmol) in 25 mL of dichloromethane.
To the resulting orange suspension was added 2.2 equiv. of
triethylamine (1.63 mL, 11.7 mmol). The reaction mixture
was left to stir for 5 h. The volatiles were removed in vacuo
and the residue was dissolved in 30 mL of THF. The result-
ing red-orange suspension was then allowed to cool to
ꢀ30 ꢁC for 15 min to ensure complete precipitation of trie-
thylammonium chloride. The mixture was then filtered
through a bed of Celite. The filtrate was collected and con-
centrated to 5–10 mL in vacuo, at which point bright orange
microcrystals formed in the solution. The solids were col-
lected on a frit, washed with pentane, and dried in vacuo to
2.2.4. [(3,4,5-F3C6H2NCH2CH2)2NMe]NbMe3
To a suspension of 200 mg (0.348 mmol) of [(3,4,5-
F3C6H2NCH2CH2)2NMe]NbCl3 in 10 mL of ether was
added dropwise 1.2 mL (1.2 mmol, 3.3 equiv.) of 1.0 M
methylmagnesium chloride in diethyl ether. The mixture
was allowed to warm to room temperature and stir for
60 min. To the resulting dark red brown solution was
added 0.24 mL 1,4-dioxane. The resulting suspension was
filtered through Celite and diethyl ether was removed from
the filtrate in vacuo. The product was obtained as maroon

4732
Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
1
powder in 61% yield. H NMR (C6D6): d 6.29 (m, 4), 3.08
(m, 2), 2.18 (m, 2), 1.93 (s, 3), 1.80 (q, 2), 1.48 (bs, 9). 19F
NMR: d ꢀ131 (dd, 4), ꢀ164 (tt, 2). Anal. Calc. for
NbN3F6C20H24: C, 46.80; H, 4.71; N, 8.19. Found: C,
46.94; H, 4.68; N, 8.08%.
The yellow residue was washed with diethyl ether. The fil-
trate was collected and the solvent was removed in vacuo.
The residue was triturated with 3 mL of pentane and the
solid was collected on a frit, washed with (3 · 2 mL) pen-
tane, and dried in vacuo; yield 0.121 g (46%) of a yellow
1
powder. H NMR (C6D6): d 7.22 (m, 4), 3.92 (m, 2), 3.60
2.2.5. {[(3,4,5-F3C6H2NCH2CH2)2NMe]TaMe2}-
[MeB(C6F5)3]
(q, 2), 2.80 (m, 2), 2.16 (s, 2), 1.95 (s, 3), 1.60 (q, 2), 0.61
(s, 9). 19F NMR: d ꢀ130 (dd, 4), ꢀ158 (tt, 2). Anal. Calc.
for TaN3F6Cl2C22H26: C, 37.84; H, 3.75; N, 6.02; Cl,
10.15. Found: C, 38.05; H, 3.68; N, 5.88; Cl, 10.25%.
B(C6F5)3 (170 mg, 0.333 mmol) was added to cooled
(ꢀ30 ꢁC) solution of 200 mg [(3,4,5-F3C6H2NCH2CH2)2-
NMe]TaMe3 (0.333 mmol) in 5 mL dichloromethane. The
mixture was allowed to warm to room temperature while
it was stirred. After 15 min, the solvent was removed in
vacuo. The residue was triturated with pentane and filtered
off and dried in vacuo to give a pale yellow powder in
2.2.8. [(3,4,5-F3C6H2NCH2CH2)2NMe]-
Ta(CH-t-Bu)(CH3)
Methylmagnesium chloride (0.13 mL, 3.0 M in THF,
0.38 mmol) was added dropwise to a stirred suspension
1
quantitative yield (363 mg). H NMR (C6D6): d 7.13 (m,
of
[(3,4,5-F3C6H2NCH2CH2)2NMe]Ta(CH2-t-Bu)Cl2
4), 4.72 (m, 2), 4.33 (m, 2), 3.61 (q, 4), 2.65 (s, 3, NMe),
1.35 (s, 3, TaMe), 1.15 (s, 3, TaMe), 0.468 (broad s, 3,
BMe). 19F NMR: ꢀ126.35 (dd, 4), ꢀ129.65 (d, 6),
ꢀ154.32 (tt, 2), ꢀ161.48 (t, 3), ꢀ164.17 (t, 6). Anal. Calc.
for TaN3F21BC38H24: C, 40.99; H, 2.17; N, 3.77. Found:
C, 41.11; H, 2.21; N, 3.73%.
(120 mg, 0.174 mmol) in 3 mL diethyl ether at ꢀ30 ꢁC. The
reaction mixture slowly became yellow-brown over a period
of 1 h. 1,4-Dioxane (0.20 mL) was added and the mixture
was stirred for an additional 10 min. The mixture was filtered
and the solid was rinsed with ether (3 · 2 mL). The filtrate
was concentrated to dryness and the residue was triturated
with pentane (5 mL). The solid was filtered off, rinsed with
pentane (5 mL), and dried in vacuo to yield a pale yellow
powder; 81 mg (73% yield). 1H NMR (C6D6): d 6.90 (m, 4),
3.30 (m, 2), 2.83 (m, 2), 2.40 (s, 1), 2.14 (m, 2), 1.75 (m, 2),
1.32 (s, 3), 0.72 (s, 9), 0.49 (s, 3); 19F NMR: d ꢀ134 (dd, 4),
169 (tt, 2). 1H-coupled 13C NMR: d 242 (d, JCH = 90), 178
(s), 151(d), 135 (d), 105 (d), 58 (t), 54 (t), 45 (s), 40 (t), 34
(q), 33 (q). Anal. Calc. for TaN3F6C23H28: C, 43.07; H,
4.40; N, 6.55. Found: C, 42.88; H, 4.38; N, 6.45%.
2.2.6. [(3,4,5-F3C6H2NCH2CH2)2NMe]-
Ta(CHSiMe3)(CH2SiMe3)
Trimethylsilylmethylmagnesium chloride (1.00 mL,
0.997 mmol) was added dropwise to a stirred suspension
of [(3,4,5-F3C6H2NCH2CH2)2NMe]TaCl3 (200 mg, 0.302
mmol) in 10 mL of diethyl ether at ꢀ30 ꢁC. The reaction
mixture slowly turned yellow-orange. After 60 min,
0.24 mL (3.0 mmol) of 1,4-dioxane was added. The mixture
was stirred for an additional 10 min and filtered. The solid
was rinsed with (3 · 2 mL) ether. The filtrate was concen-
trated to dryness in vacuo. The oily yellow-orange residue
was dissolved in 5–10 mL toluene and the solution was
heated to 50 ꢁC for 3 h. Volatile components were then
removed in vacuo. The residue was triturated with pentane
(5 mL), and the solid was filtered off, rinsed with pentane
(5 mL), and dried in vacuo to yield a yellow powder; yield
0.15 g (67% yield). X-ray quality crystals were grown from
2.2.9. [(3,4,5-F3C6H2NCH2CH2)2NMe]-
Ta(CH-t-Bu)(CH2-t-Bu)
To a cooled suspension of 250 mg (0.380 mmol) [(3,4,5-
F3C6H2NCH2CH2)2NMe]TaCl3 in 10 mL ether was added
0.31 mL of 4.0 M neopentylmagnesium chloride (in ether)
(1.2 mmol). The mixture was allowed to warm to room tem-
perature and was stirred for 6 h. 1,4-Dioxane (0.20 mL) was
added and the mixture was left to stir for 10 more minutes
before being filtered through Celite. The yellow residue
was washed with ether (5 mL). The filtrate was collected
and the ether was removed in vacuo. The residue was tritu-
rated with 3 mL pentane and the solid was collected on a frit,
washed with pentane (3 · 2 mL), and dried in vacuo to give
1
toluene at ꢀ30 ꢁC over a period of two days. H NMR
(C6D6): d 7.05 (m, 4), 5.82 (s, 1), 3.22 (m, 2), 2.93 (q, 2),
2.23 (q, 2), 1.77 (q, 2), 1.28 (s, 3), 0.28 (s, 2), ꢀ0.007 (s,
9), ꢀ0.19 (s, 9); 19F NMR: d ꢀ134 (dd, 4), ꢀ169 (tt, 2).
1H-coupled 13C NMR: 243 (d, JCH = 97), 155 (s), 151
(d), 135 (d), 105 (d), 60 (t), 58 (t), 54 (t), 37 (q), 3.5 (q),
3.3 (q). Anal. Calc. for TaN3F6Si2C25H36: C, 41.15; H,
4.97; N, 5.76. Found: C, 41.23; H, 5.06; N, 5.78%.
1
0.57 g (22%) of a yellow powder. H NMR (C6D6): d 6.73
(m, 4), 3.13 (m, 2), 2.95 (m, 2), 2.12 (m, 2), 1.83 (m, 2), 1.80
(s, 3), 1.35 (s, 9), 0.82 (s, 2), 0.75 (s, 9), 0.39 (s, 1); 19F
NMR: d ꢀ134 (dd, 4), 168 (tt, 2). 1H-coupled 13C NMR: d
228 (d, JCH = 74), 157 (s), 151 (d), 136(d), 107 (d), 67 (t),
66 (t), 46 (s), 45 (q), 38 (s), 36 (t), 35 (q), 33 (q).
2.2.7. [(3,4,5-F3C6H2NCH2CH2)2NMe]Ta(CH2-t-Bu)Cl2
To a cooled suspension of 250 mg (0.380 mmol) [(3,4,5-
F3C6H2NCH2CH2)2NMe]TaCl3 in 10 mL ether was added
0.31 mL of 4.0 M neopentylmagnesium chloride (in diethyl
ether; 1.2 mmol). The mixture was allowed to warm to
room temperature and was stirred for 3 h. 1,4-Dioxane
(0.30 mL) was added and the mixture was left to stir for
10 more minutes before it was filtered through Celite.
2.2.10. [(3,4,5-F3C6H2NCH2CH2)2NMe]-
Ta(g2-CH2CH2)(CH2CH3)
Ethylmagnesium bromide (1.00 mL, 0.997 mmol) was
added dropwise to a stirred solution of 200 mg of
[(3,4,5-F3C6H2NCH2CH2)2NMe]TaCl3 (0.302 mmol) in

Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
4733
10 mL of THF at ꢀ30 ꢁC. The bright orange solution
immediately became pale orange, then slowly turned
red-orange as more ethylmagnesium bromide was added.
The mixture was allowed to warm to room temperature
as it was being stirred. After 2 h, all solvents were
removed in vacuo. Diethyl ether (10 mL) was added to
the brown residue, followed by 0.20 mL of 1,4-dioxane.
The mixture was stirred for 10 more minutes and fil-
tered. The solid was rinsed with diethyl ether (3 · 2 mL)
and the brown filtrate was concentrated to dryness in
vacuo. The brown residue was triturated with 3 mL pen-
tane and the solid was collected on a frit, rinsed with
(3 · 2 mL) pentane, and dried in vacuo to give 91 mg of
(17 mmol) of triethylamine. The resulting deep red-orange
mixture was left to stir at room temperature for 16 h. The
volatiles were removed in vacuo and the residue was taken
up in 50 mL THF. The suspension was cooled to ꢀ30 ꢁC
before filtering off the triethylammonium chloride. The
deep red filtrate was then concentrated until the volume
was 5–10 mL. The solids that precipitated out of solution
was collected on a frit, washed with diethyl ether (10 mL)
and pentane (10 mL), and dried in vacuo. The product
was obtained as brick red microcrystals (1.8 g) in 70%
1
yield. H NMR (C6D6): d 7.04 (d, 6), 6.83 (t, 3), 3.47 (t,
6), 2.26 (t, 6). Anal. Calc. for C24H21N4Cl8Ta: C, 34.73;
H, 2.55; Cl, 34.17; N, 6.75. Found: C, 34.51; H, 2.64; Cl,
34.05; N, 6.73%.
1
brown powder which is ꢁ80% pure by NMR. H NMR
(C6D6): d 5.98 (m, 4), 3.31 (m, 2), 2.61 (m, 4), 2.35 (s,
3), 2.05 (m, 2), 1.81 (q, 2), 1.69 (t, 3), 1.53 (s, 4). 19F
NMR: d ꢀ131 (dd, 4), 164 (tt, 2). 1H-coupled 13C
NMR: d 151 (d), 145 (s), 137 (d), 105 (d), 67 (t), 56 (t),
55 (t), 54(t), 48 (q), 13 (q).
2.2.13. [(3,5-Cl2C6H3NCH2CH2)3N]Ta(C2H4)
Method A: Ethylmagnesium chloride (0.38 mL, 2.0 M in
THF) was added dropwise to a cooled (ꢀ30 ꢁC) solution of
300 mg (0.361 mmol) [(3,5-Cl2C6H3NCH2CH2)3N]TaCl2 in
10 mL THF. The reaction mixture was stirred for 6 h at
room temperature. All volatiles were then removed in
vacuo. The product was taken up in 5 mL toluene and
the solution filtered through a bed of Celite. The filtrate
was concentrated to dryness, and the red brown residue
was triturated within pentane, collected on a frit, and dried
in vacuo. The product was obtained as an orange-brown
powder in 84% yield (240 mg).
2.2.11. H3[(3,5-Cl2C6H3NCH2CH2)3N]
The dichloromethane and hexane used in this reaction
were analytical grade and used as obtained from the
supplier. The Pd catalyst was pre-formed by dissolving
1.5 g (2.4 mmol) rac-BINAP in 80 mL toluene with vigor-
ous stirring and heating. Pd2DBA3 (0.82 g, 0.90 mmol)
was added to the solution, the solution was stirred for
60 min, and the mixture was filtered to remove Pd(0).
The red-orange solution was then added to a 350-mL
Schlenk tube charged with 8.77 g (60.0 mmol) (H2N-
CH2CH2)3N, 1-bromo-3,5-dichlorobenzene (40.7 g, 180
mmol), sodium tert-butoxide (20.2 g, 210 mmol) and
150 mL toluene. The reaction mixture was heated to
90 ꢁC under a dinitrogen atmosphere for 16 h. The mix-
ture was then cooled to room temperature and filtered
through Celite, and volatiles were removed from the fil-
trate on a rotary evaporator. The resulting brown residue
was taken up in 100 mL of dichloromethane. The solution
was washed with water (2 · 100 mL) and saturated NaCl
solution (2 · 100 mL) and dried over MgSO4. MgSO4 was
filtered off, and the filtrate was concentrated in vacuo to
leave a red-brown oil. The oil was redissolved in
100 mL dichloromethane and 100 mL hexane was added.
The flask was stored at 0 ꢁC for several days over which
time solids formed. Two crops of pale yellow powder
were obtained to give a combined yield of 17 g (50%).
1H NMR (C6D6): d 6.78 (t, 3), 6.21 (d, 6), 3.52 (t, 3),
2.32 (q, 6), 1.90 (t, 6). Anal. Calc. for C24H24Cl6N4: C,
49.60; H, 4.16; Cl, 36.60; N, 9.64. Found: C, 49.65; H,
4.21; Cl, 36.56; N, 9.49%.
Method B: A solution of 100 mg (0.172 mmol) H3[(3,5-
Cl2C6H3NCH2CH2)3N] in 3 mL toluene was cooled to
ꢀ30 ꢁC, and 61.6 mg (0.172 mmol) of TaCl5was added.
The reaction mixture was stirred for 15 min at room tem-
perature and cooled to ꢀ30 ꢁC. Ethylmagnesium chloride
(0.44 mL, 2.0 M in diethyl ether) was then added drop-
wise. The reaction mixture was stirred for 6 h at room
temperature. The resulting deep red-brown mixture was
filtered through Celite, and the filtrate was concentrated
to dryness. The residue was triturated with pentane and
the solids were collected on a frit and dried in vacuo to
give 108 mg (80% yield) of orange-brown powder. 1H
NMR (C6D6): d 6.84 (t, 3), 6.57 (d, 6), 3.29 (t, 6), 2.28
1
(t, 6), 1.66 (s, 4). H-coupled 13C NMR: d 157 (s), 135
(m), 124 (dt), 123 (dt), 73 (t, JCH = 143), 58 (t), 54 (t).
Anal. Calc. for TaN4Cl6C26H25: C, 39.67; H, 3.20; Cl,
27.02; N, 7.12. Found: C, 39.38; H, 3.24; Cl, 27.18; N,
6.97%.
2.2.14. [(3,5-Cl2C6H3NCH2CH2)3N]TaMe2
Method A: To a cooled (ꢀ30 ꢁC) suspension of 100 mg
(0.120 mmol) [(3,5-Cl2C6H3NCH2CH2)3N]TaCl2 in 3 mL
toluene was added 2.1 equiv. of MeMgCl (0.08 mL, 3 M
in THF). The reaction mixture was allowed to warm to
room temperature as it was stirred, during which time it
changed from a brick-red to a brown-yellow color. After
60 min, the mixture was filtered through Celite, and the
filtrate was then taken to dryness in vacuo. The residue
was rinsed with diethyl ether (3 · 2 mL) and pentane
2.2.12. [(3,5-Cl2C6H3NCH2CH2)3N]TaCl2
To a rapidly stirred suspension of TaCl5 (1.23 g,
3.44 mmol) in 100 mL dichloromethane at ꢀ30 ꢁC was
added (in portions) 2.00 g of solid H3[(3,5-Cl2C6H3-
NCH2CH2)3N]. The mixture, which turned orange immedi-
ately, was stirred for 15 min before adding 2.4 mL

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Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
(3 · 2 mL), then dried to give 75 mg of bright yellow pow-
der in 79% yield.
could be synthesized similarly and isolated as brick-red
microcrystals
Method B: A solution of 100 mg (0.172 mmol) H3[(3,5-
Cl2C6H3NCH2CH2)3N] in 3 mL toluene was cooled to
ꢀ30 ꢁC, and 61.6 mg (0.172 mmol) TaCl5 was added. The
reaction mixture was stirred for 15 min at room tempera-
ture then cooled to ꢀ30 ꢁC. Methyl lithium (0.63 mL,
1.4 M in diethyl ether) was added dropwise to the reaction
mixture, which was then left to stir at room temperature for
60 min. The resulting suspension of dark precipitate in a
yellow-brown liquid was filtered through Celite, and the fil-
trate was concentrated to dryness. The residue was tritu-
rated with pentane, collected on a frit, and dried in vacuo
to give 88 mg (65% yield) of yellow powder. Single crystals
suitable for X-ray diffraction were grown from concen-
trated toluene solution at ꢀ30 ꢁC. 1H NMR (C6D6): d
6.90 (t, 3), 6.80 (d, 6), 3.22 (t, 6), 2.09 (t, 6), 1.23 (s, 6). Anal.
Calc. for TaN4Cl6C26H27: C, 39.57; H, 3.45; N, 7.10.
Found: C, 39.46; H, 3.41; N, 7.16%.
C6F3H2
C6F3H2
2.2 eq NEt3
N
H
N
Cl
CH2Cl2/ 22˚C
TaCl5
Ta
Cl
Cl
MeN
+
Me
N
- 2 Et3NHCl
H
N
N
C6F3H2
C6F3H2
ð1Þ
The reaction between [F3N2NMe]TaCl3 and 3.3 equiv.
of methylmagnesium chloride afforded the trimethyl spe-
cies, [F3N2NMe]TaMe3. An alternate route to [F3N2NMe]-
TaMe3 consists of the reaction between K2[F3N2NMe] and
TaCl2Me3 [14], as shown:
C6F3H2
C6F3H2
N
K
N
Me
ether/ 22˚C
TaMe3Cl2
Ta
Me
Me
MeN
+
Me
N
K
N
N
C6F3H2
C6F3H2
2.2.15. [(3,5-Cl2C6H3NCH2CH2)3N]Ta(g2-C2H2)
ð2Þ
To a rapidly stirred solution of [(3,5-Cl2C6H3NCH2-
CH2)3N]TaCl2 (100 mg, 0.120 mmol) in 10 mL THF
(ꢀ30 ꢁC) was added dropwise 0.25 mL of vinylmagnesium
bromide (0.25 mmol, 1.0 M in THF). After stirring the
mixture for 3 h, 0.25 mL of 1,4-dioxane was added, and
the reaction mixture was allowed to stir for additional
15 min. The volatile components were removed in vacuo
and the residue was taken up in 5 mL toluene. The
solution was filtered through Celite and the filtrate was
concentrated to dryness. The residue was triturated in
3 mL pentane, collected on frit, and dried to give the
product in 68% yield (64 mg). 1H NMR (500 MHz,
C6D6): d 11.90 (s, 2), 6.82 (t, 3), 6.53 (d, 6), 3.25 (t, 6),
The room temperature 1H NMR spectrum of
[F3N2NMe]TaMe3 reveals that the methyl ligands are
1
magnetically equivalent on the H NMR timescale, giving
rise to a singlet at 0.93 ppm in toluene-d8. At ꢀ80 ꢁC the
methyl ligands are all distinguishable as singlet reso-
nances at 1.45, 0.97, and 0.61 ppm, similar to what has
been observed in [(C6F5NCH2CH2)2NMe]TaMe3 [11]
and [(3,5-Cl2C6H3NCH2CH2)2NMe]TaMe3 [9]. At
ꢀ50 ꢁC only two resonances are observed at 1.15 and
0.59 ppm with relative areas of 6:3. Therefore, we pro-
pose that [F3N2NMe]TaMe3 assumes the mer configura-
tion at low temperatures, as shown in Eq. (2), and that
two of the three methyl groups equilibrate before all
three become equivalent on the NMR time scale. Details
of the equilibration process are not known and we can-
not rule out dissociation of the amine donor to give a
fluxional five-coordinate intermediate. [F3N2NMe]TaMe3
is relatively stable at room temperature, unlike
[(C6F5NCH2CH2)2NH]TaMe3 [11], which evolves meth-
ane even in the solid state to give the triamido complex,
[(C6F5NCH2CH2)2N]TaMe2.
2.23 (t, 6). 1H-coupled 13C NMR:
d
226 (d,
JCH = 181 Hz), 161 (s), 135 (t), 123 (dt), 123 (dt), 57 (t),
53 (t).
3. Results and discussion
3.1. Complexes that contain the [(3,4,5-
F3C6H2NCH2CH2)NMe]2ꢀ ligand
The analogous niobium compound, [F3N2NMe]NbMe3,
was prepared by treating [F3N2NMe]NbCl3 with 3 equiv.
of methylmagnesium chloride. [F3N2NMe]NbMe3 is also
fluxional at room temperature with the three methyl
ligands giving rise to a broad singlet at 1.36 ppm in the
1H NMR spectrum at room temperature in toluene-d8.
The variable-temperature behavior of [F3N2NMe]NbMe3
is similar to that observed for [F3N2NMe]TaMe3. The
methyl resonances in [F3N2NMe]NbMe3 are broader than
they are in [F3N2NMe]TaMe3, a typical consequence of the
onset of coupling of the methyl protons to Nb.
Reduction of (C6F5NHCH2CH2)2NMe with LiAlH4
produces (3,4,5-F3C6H2NHCH2CH2)2NMe (H2[F3NMe])
in good yield [8]. The reaction between H2[F3N2NMe]
and TaCl5 in the presence of triethylamine in dichloro-
methane at room temperature gave [F3N2NMe]TaCl3 as
bright orange microcrystals (Eq (1)). The presence of only
two resonances at ꢀ129 ppm and ꢀ157 ppm in the 19F
NMR spectrum in C6D6 and the sharp multiplet in the
1
aromatic region of the H NMR spectrum are consistent
with equivalent, rapidly rotating trifluorophenyl rings. On
the basis of an X-ray structure of [(3,5-C6Cl2H3NCH2-
CH2)2NMe]TaMe3 [9], we assume that the ligand adopts
(approximately) the mer configuration, as shown in Eq
(1). The analogous niobium species, [F3N2NMe]NbCl3,
A methyl ligand can be abstracted from [F3N2NMe]-
TaMe3 with B(C6F5)3 to yield [(F3N2NMe)TaMe2]-
[MeB(C6F5)3] (Eq. (3)) as a yellow powder in 100% yield

Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
4735
+
C6F3H2
C6F3H2
N
N
Me
B(C6F5)3
CH2Cl2
Me
[MeB(C6F5)3]-
Me
Me
Ta
Ta
N
Me
N
Me
N
Me
N
C6F3H2
C6F3H2
ð3Þ
The 1H and 19F NMR spectra of [(F3N2NMe)TaMe2]-
[MeB(C6F5)3] are consistent with the proposed mirror
symmetry of the trigonal bipyramidal structure. The triflu-
orophenyl rings are equivalent and rotating rapidly on the
NMR time scale at room temperature. The methyl ligands
are observed as two singlets at 1.35 and 1.15 ppm in
CD2Cl2. The observed high field shift for the meta- and
para-fluorine substituents of the anion, as well as the small
Dd (dm ꢀ dp) of 2.69 ppm in the 19F NMR spectrum are
indicative of solvent-separated, relatively non-coordinating
[MeB(C6F5)3]ꢀ ion in solution [15,16]. A similar cationic
complex, {[(C6F5NCH2CH2)2NH]TaMe2}[MeB(C6F5)3],
has been synthesized and structurally characterized [11].
The [MeB(C6F5)3]ꢀ anion is clearly separated from
{[(C6F5NCH2CH2)2NH]TaMe2}+ in {[(C6F5NCH2CH2)2-
NH]TaMe2}[MeB(C6F5)3].
Fig. 1. The solid-state structure of [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3).
Table 1
˚
Selected intramolecular distances (A) and bond angles (ꢁ) of
The reaction between [F3N2NMe]TaCl3 and trim-
ethylsilylmethylmagnesium chloride afforded [F3N2NMe]-
Ta(CHSiMe3)(CH2SiMe3) as a yellow powder
[F3N2NMe]Ta(CHSiMe3)(CH2SiMe3)
Ta–C(1)
Ta–C(5)
Ta–N(1)
Ta–N(2)
Ta–N(3)
1.906(13)
2.124(12)
2.116(11)
2.094(10)
2.239(11)
C(1)–Ta–N(3)
N(2)–Ta–C(5)
N(1)–Ta–C(5)
C(5)–Ta–N(3)
N(2)–Ta–N(1)
136.4(5)
101.6(4)
100.5(4)
115.5(5)
146.9(4)
C6F3H2
C6F3H2
1. 3.3 Me3SiCH2MgCl
Et2O, -30 o
N
N
Cl
C
CHSiMe3
CH2SiMe3
Cl
Cl
Ta
Ta
Me
N
Me
N
Si(1)–C(1)–Ta
Si(2)–C(5)–Ta
C(1)–Ta–C(5)
C(1)–Ta–N(1)
C(1)–Ta–N(2)
144.8(8)
N(2)–Ta–N(3)
N(1)–Ta–N(3)
C(14)–N(1)–Ta
C(20)–N(2)–Ta
74.1(4)
74.3(4)
133.4(9)
127.1(8)
2. 45 oC in toluene, 3h
127.9(6)
108.0(5)
100.3(5)
95.8(5)
N
N
C6F3H2
C6F3H2
ð4Þ
It is not clear whether the reaction proceeds via a-hydro-
gen abstraction in putative [F3N2NMe]Ta(CH2SiMe3)3 or
[F3N2NMe]Ta(CH2SiMe3)2Cl, or whether [F3N2NMe]-
Ta(CH2SiMe3)2Cl is dehydrohalogenated by the third
equivalent of lithium reagent. Mechanistic questions of
this type typically have not been answered in any tanta-
lum alkylidene system, including the first of its type,
Ta(CHCMe3)(CH2CMe3)3 [17]. The alkylidene carbon
resonance in [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3) is
position with the substituent (Si(1)) pointing away from
C(5). The alkylidene ligand lies approximately in the
C(1)/Ta/C(5)/N(3) plane. The difference between the
˚
˚
Ta–C(1) (1.906(13) A) and Ta–C(5) (2.124(12) A) bonds
is typical of that between an alkyl and an alkylidene
[17,19]. The relatively short Ta–C(1) (Ta@Ca) bond dis-
1
˚
observed at 243 ppm with JCH = 97 Hz and the corre-
tance (1.906(13) A) and large Si(1)–C(1)–Ta (Ta@Ca–Si)
sponding proton resonance is found at 5.82 ppm, both
in C6D6. The high-field chemical shift for Ha combined
with a low value for JCH suggests that there is a signifi-
cant agostic interaction between the tantalum and the Ca–
Ha bond [17–19].
An X-ray diffraction study of [F3N2NMe]Ta(CHSi-
Me3)(CH2SiMe3) revealed the structure shown in Fig. 1.
Selected distances and angles are listed in Table 1. The
complex is best described as a distorted square pyramid
in which the alkyl group (C(5)) occupies the axial posi-
tion. The [F3N2NMe]2ꢀ ligand coordinates to the metal
in the basal positions and the amido nitrogen atoms are
essentially planar, consistent with p-donation to the
metal. The alkylidene a carbon atom C(1) is in a basal
bond angle (144.8(8)ꢁ) are consistent with a significant
agostic interaction between the metal and Ha. These met-
rical parameters should be compared with those in two
previously crystallographically characterized tantalum
trimethylsilylmethyl trimethylsilylmethylidene complexes
[20,21]. The three nd orbitals that could be involved in
p-bonding framework of the complex are illustrated in
Fig. 2. The Ta@C p-bond employs the dxy orbital, while
overlap with the imido nitrogen out of phase p-orbitals
employs the dxz orbital. From this analysis the CHa ago-
stic interaction would seem to most likely involve the
metal dyz orbital.
1
Initial attempts to synthesize the neopentyl/neopentylid-
ene complex in an analogous fashion gave instead the

4736
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z
H
H
H
C
C
C
N
N
N
N
y
x
SiMe3
SiMe3
N
SiMe3
N
N
dxy
dxz
dyz
Fig. 2. Qualitative analysis of the p-bonding framework of [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3).
mononeopentyl complex, [F3N2NMe]Ta(CH2-t-Bu)Cl2, as
a yellow powder (Eq. (5)), even in the presence of 3.3 equiv.
of neopentylmagnesium chloride. No additional alkylation
took place in 3 h under the conditions employed. Steric
congestion at the metal center caused by the more bulky
neopentyl ligand is believed to hinder further substitution.
After 6 h, however, [F3N2NMe]Ta(CH-t-Bu)(CH2-t-Bu)
could be isolated in low yield (22%). The neopentylidene
ically neopentyl a-hydrogens are more activated as a conse-
quence of an increase in the Ta–Ca–Cb angle in a sterically
crowded circumstance [17].
The alkylidene proton and carbon resonances of the
three alkyl/alkylidene complexes are collected in Table 2.
All Ha resonances are found in the region 0–6 ppm and
alkylidene carbon resonances are found in the range 220–
245 ppm. The 1JCH values for the alkylidene carbon are uni-
formly low and well within the range expected for Ta(V)
alkylidene complexes exhibiting a-agostic interactions
1
proton was observed by H NMR in C6D6 at 0.39 ppm,
and the carbon atom was observed as a doublet at
1
1
228 ppm (JCH = 74 Hz) in the H-coupled 13C NMR spec-
[22,23]. The variation in JCH in the order C < B < A
trum. The chemical shift for the alkylidene proton and the
extremely low value for JCH again are characteristic of a
significant agostic interaction of the CHa electrons with
the metal
emphasizes the dramatic tendency for neopentylidenes to
engage in a-agostic interactions and the steric demands of
both neopentylidene and neopentyl groups [19].
The reaction between [F3N2NMe]TaCl3 and CH3CH2-
MgBr led to formation of [F3N2NMe]Ta(g2-C2H4)(CH2-
CH3) as a brown powder (Eq. (7)). There is no evidence
for formation of any significant amount of [F3NMe]-
Ta(CHCH3)(CH2CH3)
C6F3H2
C6F3H2
N
N
Cl
3.3 t-BuCH2MgCl
3 h
Cl
CH2-t-Bu
Ta
Cl
Cl
Ta
Me
N
Me
N
Cl
-30 oC to 22 o
C
N
N
C6F3H2
C6F3H2
C6F3H2
C6F3H2
N
N
3.3 CH3CH2MgBr
Cl
ð5Þ
Further alkylation of [F3N2NMe]Ta(CH2-t-Bu)Cl2 with
Ta
Ta
Me
N
Cl
Cl
Me
N
CH2CH3
C6F3H2
-30 oC to 22 o
2hrs
C
N
N
C6F3H2
2.2 equiv. of methylmagnesium chloride afforded a species
whose 1H NMR spectrum is consistent with it being
[F3N2NMe]Ta(CH-t-Bu)(CH3) (Eq. (6)). In addition to
the diamidoamine resonances, three singlets were observed
at 2.40, 0.72, and 0.49 ppm in a ratio of 1:9:3, correspond-
ing to the neopentylidene Ha, t-butyl, and methyl reso-
nances, respectively. The alkylidene Ca resonance was
ð7Þ
Although competition between a hydrogen abstraction and
b hydrogen abstraction is known in tantalum chemistry
that involves triamidoamine ligands [13,24], steric factors
in the intermediate leading to [F3NMe]Ta(CH-
CH3)(CH2CH3) are not likely to be significant enough to
block b hydrogen abstraction. NMR studies of F3N2NMe]-
Ta(g2-C2H4)(CH2CH3) reveal that all four protons of the
1
found as a doublet at 242 ppm (1JCH = 90 Hz) in the H-
coupled 13C NMR spectrum
C6F3H2
C6F3H2
N
N
Cl
2.2 MeMgCl
1 hr
CH3
Ta
CH2-t-Bu
Me
N
Ta
Me
N
Table 2
CH-t-Bu
Cl
N
-30 oC to 22 o
C
Relevant alkylidene Ha and Ca resonances of tantalum alkyl/alkylidene
complexes
N
C6F3H2
C6F3H2
Complex
1Ha
(ppm)a
13Ca
(ppm)b
1JCH
(Hz)c
ð6Þ
The neopentylidene ligand could form via dehalogenation
of [F3N2NMe]Ta(CH2-t-Bu)Cl2 to give [F3N2NMe]-
Ta(CH-t-Bu)Cl, followed by its rapid reaction with addi-
tional methylating agent to give the observed product. If
a-hydrogen abstraction were to take place in an intermedi-
ate such as [F3N2NMe]Ta(CH2-t-Bu)Me2, we would expect
a neopentylidene to be formed instead of a methylene. Typ-
[F3N2NMe]Ta(CHSiMe3)(CH2SiMe3) (A)
[F3N2NMe]Ta(CHCMe3)(CH3) (B)
[F3N2NMe]Ta(CHCMe3)(CH2CMe3) (C)
5.82
2.40
0.39
243
242
228
97
90
74
a
Solvent: C6D6 (300 MHz).
Toluene-d8 (125 MHz).
1JCH values were determined from the alkylidene doublet in 1H-cou-
b
c
pled 13C experiments.

Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
4737
Table 3
ethylene ligand give rise to a singlet at 1.53 ppm, even at
˚
Selected intramolecular distances (A) and bond angles (ꢁ) of [F3N2NMe]-
Ta(g2-C2H4)(CH2CH3)
ꢀ80 ꢁC. The ethylene carbons are likewise equivalent on
the 13C NMR time scale and give rise to a resonance at
56 ppm.
Ta–C(1)
Ta–C(2)
Ta–C(3)
C(1)–C(2)
C(3)–C(4)
Ta–N(1)
Ta–N(2)
Ta–N(3)
2.164(9)
2.166(9)
2.268(8)
1.508(13)
1.423(14)
1.997(7)
2.003(7)
2.367(6)
C(1)–Ta–N(3)
C(2)–Ta–C(3)
N(1)–Ta–C(2)
N(2)–Ta–C(2)
C(2)–Ta–N(3)
N(1)–Ta–C(3)
N(2)–Ta–C(3)
C(3)–Ta–N(3)
130.7(3)
126.5(3)
107.8(4)
108.1(4)
90.0(3)
94.3(3)
91.0(3)
143.4(3)
An X-ray study of [F3N2NMe]Ta(g2-C2H4)(CH2CH3)
revealed the structure shown in Fig. 3. If the bound ethyl-
ene is viewed as occupying a single coordination site, the
geometry about the tantalum center may be viewed as dis-
torted square pyramid with the ethylene ligand in the apical
position. At this stage it is not known if the fluxional pro-
cess that equilibrates the ethylene protons and carbon
atoms consists only of rotation of the ethylene about the
Ta-ethylene bond axis, or whether the core itself rearranges
at the same time.
C(4)–C(3)–Ta
C(1)–Ta–C(2)
C(1)–Ta–C(3)
N(1)–Ta–C(1)
N(2)–Ta–C(1)
110.6(6)
40.8(4)
85.8(3)
112.7(3)
116.9(4)
N(1)–Ta–N(2)
N(1)–Ta–N(3)
N(2)–Ta–N(3)
C(10)–N(1)–Ta
C(16)–N(2)–Ta
130.4(3)
74.8(3)
72.2(2)
123.3(6)
120.3(5)
Pertinent bond lengths and angles in [F3N2NMe]Ta-
(g2-C2H4)(CH2CH3) are listed in Table 3. The ethylene
moiety has relatively short and essentially equivalent Ta-
CH2CH2)3N]3ꢀ will be abbreviated as [Cl2N2NMe]3ꢀ in
discussions below.
˚
˚
C bond lengths (2.172(8) A and 2.174(8) A). The observed
˚
C(1)–C(2) bond distance (1.502 A) is only slightly shorter
The dichloride complex, [Cl2N2NMe]TaCl2, was pre-
pared in a reaction between H3[(3,5-Cl2C6H3NCH2-
CH2)3N] and TaCl5 in the presence of triethylamine (Eq.
(8)). The 1H NMR spectrum of brick red [(3,5-Cl2-
C6H3NCH2CH2)3N]TaCl2 is consistent with threefold
symmetry on the NMR time scale, as found in the related
species [(Me3SiNCH2CH2)3N]TaCl2 [29]
than typical C–C single bonds and much longer than typi-
cal C@C double bonds [25], indicative of extensive backb-
onding from the metal to the ethylene ligand. The
orientation of the ethylene ligand suggests that the orbital
that is used for p backbonding is the orbital normal to that
utilized in forming the alkylidene p-bond in [F3N2NMe]-
Ta(CHSiMe3)(CH2SiMe3). Evidently, these two possible
p bonding orbitals (dxy and dyz in Fig. 2) are relatively close
in energy. Exposure of [F3N2NMe]Ta(g2-C2H4)(CH2CH3)
to 1.5 equiv. of 13C-labeled ethylene at room temperature
gave no evidence of ethylene exchange.
ArCl
ArCl
5 eq NEt3
Cl
Cl
ArCl
CH2Cl2/ rt
N
N
3,5-Cl2C6H3N
TaCl5
Ta
N
+
N
3
NH2
- 3 Et3NHCl
ð8Þ
3.2. Tantalum (V) complexes that contain the
[(3,5-Cl2C6H3NCH2CH2)3N]3ꢀ ligand
The reaction between [(3,5-Cl2C6H3NCH2CH2)3N]-
TaCl2 and 2 equiv. of ethylmagnesium chloride gave the
ethylene complex, [(3,5-Cl2C6H3NCH2CH2)3N]Ta(g2-
C2H4), in good yield (Eq. (9)). [(3,5-Cl2C6H3NCH2CH2)3-
N]Ta(g2-C2H4) also could be obtained in one step by
treating a mixture of H3[(3,5-Cl2C6H3NCH2CH2)3N] and
TaCl5 with 5 equiv. of ethylmagnesium chloride
A palladium-catalyzed Buchwald–Hartwig C–N cou-
pling reaction [26–28] was employed in order to synthesize
H3[(3,5-Cl2C6H3NCH2CH2)3N] from (H2NCH2CH2)3N
and 3 equiv. of 1-bromo-3,5-dichlorobenzene in 50% yield
without complications. For convenience, [(3,5-Cl2C6H3N-
Fig. 3. The solid-state structure of [F3N2NMe]Ta(g2–C2H4)(CH2CH3).

4738
ArCl
Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
ArCl
ArCl
Cl
plexes. (Compare with the decomposition of [Cl2N2NMe]-
TaMe2 below.)
Alkylation of [Cl2N2NMe]TaCl2 with 2 equiv. of meth-
ylmagnesium chloride in toluene affords [Cl2N2NMe]-
TaMe2 as bright yellow crystals (Eq. (10)).
Cl
ArCl
ArCl
2 CH3CH2MgCl
THF
ArCl
N
N
Ta
N
Ta
N
N
N
N
N
ð9Þ
ArCl
ArCl
ArCl
ArCl
Cl
Me
Cl
Me
N
ArCl
ArCl
In the 1H NMR spectrum, the proton resonance for bound
ethylene is found as a singlet at 1.66 ppm (C6D6,
500 MHz). The ethylene carbons likewise are equivalent
on the 13C NMR time scale and give rise to a resonance
at 72.58 ppm (1JCH = 143 Hz). All spectroscopic data sug-
gest that [Cl2N2NMe]Ta(g2-C2H4) is C3-symmetric on the
NMR time scale, as found also for [(Me3SiNCH2CH2)3N]-
Ta(g2-C2H4) [24]. We assume that the structure of
[Cl2N2NMe]Ta(g2-C2H4) is analogous to that found for
[(Me3SiNCH2CH2)3N]Ta(g2-C2H4) [24]. In [(Me3SiNCH2-
CH2)3N]Ta(g2-C2H4) the C–C axis of the ethylene is lined
up with one of the amido Ta–N bonds. The ethylene ligand
rotates rapidly about the pseudo-C3 axis of the complex on
the NMR time scale as a consequence of degeneracy of the
2 eq MeMgCl
toluene
N
N
N
Ta
N
Ta
N
N
N
ð10Þ
[Cl2N2NMe]TaMe2 also can be obtained through addi-
tion of 5 equiv. of MeMgCl to a mixture of TaCl5 and
H3[Cl2N2NMe]. A resonance for the two methyl groups
in [Cl2N2NMe]TaMe2 is found as a singlet at 1.31 ppm in
1
the H NMR spectrum.
Single crystals of [Cl2N2NMe]TaMe2 suitable for X-ray
crystallographic studies were obtained from toluene solu-
tion at ꢀ30 ꢁC. The structure obtained (Fig. 4) shows it
to be a six coordinate species with the methyl ligands
within the bowl-like coordination pocket defined by the
ligand. The structure is related to that found for
[(Me3SiNCH2CH2)3N]Ta(Et)(Me) [24], in which the alkyl
ligands lie approximately in the plane formed by the donor
nitrogen, the tantalum center, and one of the imido nitro-
gen atoms. The Ta–C bond lengths in [Cl2N2NMe]TaMe2
1
dxz and dyz orbitals. Comparison of the ethylene H and
13C resonances of [Cl2N2NMe]Ta(g2-C2H4) with those
reported for [(R3SiNCH2CH2)3N]Ta(g2-C2H4) complexes
(R = Me or Et) are summarized in Table 4. Formation
of [(R0NCH2CH2)3N]Ta(g2-C2H4) (R0 = 3,5-Cl2C6H3,
Me3Si, Et3Si) complexes is believed to proceed via
b-abstraction in intermediate [(R0NCH2CH2)3N]Ta(CH2-
CH3)2. Only in the synthesis of [(Et3SiNCH2CH2)3N]-
Ta(g2-C2H4) is a-abstraction to give [(Et3SiNCH2CH2)3-
N]Ta(CHCH3), competitive with b-hydrogen abstraction
[13]. It was proposed that the sterically demanding triethyl-
silyl substituents in [(Et3SiNCH2CH2)3N]Ta(CHCH3)
force the Ta–Ca–Cb angles in the diethyl intermediate to
increase, thereby activating the a hydrogens toward a-
abstraction to give the alkylidene and at the same time pre-
venting activation of b hydrogens toward b-abstraction.
All attempts to convert [Cl2N2NMe]Ta(g2-C2H4) into
[Cl2N2NMe]Ta@CHMe in the presence of a catalytic
amount of PhPH2 were unsuccessful. No observable
change was noted when [Cl2N2NMe]Ta(g2-C2H4) was trea-
ted with catalytic amounts of PhPH2 in deuterated benzene
or toluene even after several days at room temperature.
The mixtures were not heated since [Cl2N2NMe]Ta(g2-
C2H4) was found to be unstable in solution at elevated
temperatures and to decompose in a manner analogous
to [(R3SiNCH2CH2)3N]Ta(g2-C2H4) (R = Me, Et) com-
˚
(ꢁ2.22 A) are essentially the same within experimental
error. The Ta–Naxial bond length in [Cl2N2NMe]TaMe2
˚
(2.318(6) A) is significantly shorter than the Ta-Naxial bond
˚
length in [(Me3SiNCH2CH2)3N]Ta(Et)(Me) (2.444(8) A).
A list of selected bond lengths and angles can be found
in Table 5.
[Cl2N2NMe]TaMe2 is stable in the solid state at room
temperature, but it evolves methane in solution at elevated
temperatures. The thermolysis product is believed to be anal-
ogous to the thermolysis product of [(R3SiNCH2CH2)3N]-
TaMe2 (R = Me or Et) [24,13] on the basis of a NMR
spectrum of the product analogous to that for MeTa[N-
(SiR3)(CH@CH2)][N(CH2CH2NSiR3)2]} where R = Me
Table 4
Comparison of ethylene
H
and
C
resonances of [(3,5-
Cl2C6H3NCH2CH2)3N]Ta(g2–C2H4) with reported [(R3SiNCH2CH2)3N]-
Ta(g2–C2H4) complexes
Complex
1H
13C
1JCH
(ppm)
(ppm)
(Hz)
[(3,5-Cl2C6H3NCH2CH2)3N]Ta(g2-C2H4)
[(Me3SiNCH2CH2)3N]Ta(g2-C2H4)
[(Et3SiNCH2CH2)3N]Ta(g2-C2H4)
1.66
2.14
2.10
72.6
62.6
62.7
143
144
141
Fig. 4. The solid-state structure of [(3,5-Cl2C6H3NCH2CH2)3N]TaMe2.

Lourdes Pia H. Lopez et al. / Inorganica Chimica Acta 359 (2006) 4730–4740
4739
Table 5
dyz in Fig. 2) are relatively close in energy and that sterics
appear to dictate the orientation of the alkylidene or olefin.
Tantalum complexes that contain the [Cl2N2NMe]3ꢀ
ligand are also readily prepared in a ‘‘direct’’ method. In
general they appear to be quite similar to [(Me3SiN-
CH2CH2)3N]3ꢀ complexes that have been reported,
although conversion of the ethylene to the ethylidene com-
plex in the presence of a catalytic amount of PhPH2 was
not observed. Although we had hoped that the backbone
would be more stable to N–C bond cleavage reactions than
in the [(Me3SiNCH2CH2)3N]3ꢀ ligand system, that does
not appear to be the case.
˚
Selected intramolecular distances (A) and bond angles (ꢁ) of [(3,5-
Cl2C6H3NCH2CH2)3N]TaMe2
Ta–C(1)
Ta–C(2)
2.224(8)
2.221(7)
2.085(6)
2.077(6)
2.006(6)
2.318(6)
77.8(3)
N(2)–Ta–N(1)
N(3)–Ta–N(2)
N(3)–Ta–N(1)
C(9)–N(1)–Ta
C(15)–N(2)–Ta
C(21)–N(3)–Ta
96.2(3)
116.3(3)
124.4(2)
125.2(5)
129.3(5)
121.7(5)
Ta–N(1)
Ta–N(2)
Ta–N(3)
Ta–N(4)
C(2)–Ta–C(1)
[24] or Et [13], as shown in Eq. (11). Isolation of this product
was not pursued
Acknowledgement
ArCl
ArCl
Me
Me
ArCl
ArCl
Me
N
ArCl
ArCl
We thank the National Science Foundation (CHE-
9988766) for supporting this research.
N
toluene-d8/110 o
C
Ta
N
N
N
Ta
N
N
N
- CH4
Appendix A. Supplementary material
ð11Þ
Fully labeled thermal ellipsoid drawing, atomic coordi-
nates, bond lengths and angles, anisotropic displacement
parameters, and hydrogen coordinates are available for
[F3N2NMe]Ta(CHSiMe3)(CH2SiMe3) (01199), [F3N2NMe]-
Ta(g2-C2H4)(CH2CH3) (02042), and [Cl2N2NMe]TaMe2.
(03047). These data are available to the public at http://
www.reciprocalnet.org/. The numbers in parentheses listed
above can be used to access each structure at this site.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2006.
03.044.
Addition of 2 equiv. of vinylmagnesium chloride to
[Cl2N2NMe]TaCl2 produced the acetylene complex,
[Cl2N2NMe]Ta(g2-C2H2) (Eq. (12))
ArCl
ArCl
ArCl
ArCl
Cl
Cl
ArCl
ArCl
2 eq H2C=CHMgBr
toluene
N
N
N
N
Ta
N
Ta
N
N
N
ð12Þ
Proton and carbon NMR established that this molecule
has pseudo-C3-symmetry with the acetylene proton reso-
nance at 11.90 ppm and the corresponding carbon reso-
nances at 226 ppm (JCH = 181 Hz). The structure of this
compound is likely to be closely related to that reported
for [(Me3SiNCH2CH2)3N]Ta(g2-C2H2) [24]. Note that in
the [(Et3SiNCH2CH2)3N]3ꢀ system, a dimeric alkylidene
complex, {[(Et3SiNCH2CH2)3N]Ta(=CHCH2)}2, was
obtained in combination with the [(Et3SiNCH2CH2)3N]-
Ta(g2-C2H2) when the reaction was performed at elevated
temperatures [13]; most likely the dimeric species is formed
through coupling of two d1 [(Et3SiNCH2CH2)3N]Ta(CH@
CH2) molecules. An analogous reaction to give the hypo-
thetical {[Cl2N2NMe]Ta(=CHCH2)}2 was not observed
when the reaction shown in Eq. (12) was performed at ele-
vated temperatures.
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