Tribenzyl Tantalum(V) Complexes of Amine Bis(phenolate) Ligands: Investigation of α-Abstraction vs Ligand Backbone β-Abstraction Paths

Article abstract of DOI:10.1021/om0499761

The synthesis, structure, and reactivity of tribenzyl Ta(V) complexes of amine bis(phenolate) ligands were investigated as a function of the ligands' structure: i.e., the steric bulk of the phenolate substituents and the presence of a side arm donor. The tribenzyl complexes were prepared by toluene-elimination reactions of Ta(CH₂Ph)₅ with the ligand precursors. They were found to be of octahedral mer geometry, with the side-arm donor (if present) unbound. These complexes underwent two well-defined toluene elimination reactions determined by the structural features of the ligands. An α-abstraction reaction, leading to alkyl-alkylidene complexes, occurred only if the phenolate ortho substituents were small (H, Cl) and a side-arm donor was present. Bulkier phenolate substituents (Me) or the lack of a side-arm donor led to a β-abstraction process from the ligand backbone, forming a dibenzyl complex with a metallaazacyclopropane ring. The side-arm donor, if present, was found to bind to the metal in both paths. A bulkier ligand precursor having t-Bu groups on the two phenolate rings did not react with Ta(CH₂Ph)₅ at room temperature, whereas an asymmetric half-bulky ligand precursor carrying a Me group and a t-Bu group on the two rings led directly to the β-abstraction product.

Full text of DOI:10.1021/om0499761

1880
Organometallics 2004, 23, 1880-1890
Tr iben zyl Ta n ta lu m (V) Com p lexes of Am in e
Bis(p h en ola te) Liga n d s: In vestiga tion of r-Abstr a ction
vs Liga n d Ba ck bon e â-Abstr a ction P a th s
Stanislav Groysman,† Israel Goldberg,† Moshe Kol,*,† Elisheva Genizi,‡ and
Zeev Goldschmidt*,‡
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Tel Aviv 69978, Israel, and Department of Chemistry,
Bar-Ilan University, Ramat Gan 52900, Israel
Received J anuary 5, 2004
The synthesis, structure, and reactivity of tribenzyl Ta(V) complexes of amine bis-
(phenolate) ligands were investigated as a function of the ligands’ structure: i.e., the steric
bulk of the phenolate substituents and the presence of a “side arm” donor. The tribenzyl
complexes were prepared by toluene-elimination reactions of Ta(CH2Ph)5 with the ligand
precursors. They were found to be of octahedral mer geometry, with the side-arm donor (if
present) unbound. These complexes underwent two well-defined toluene elimination reactions
determined by the structural features of the ligands. An R-abstraction reaction, leading to
alkyl-alkylidene complexes, occurred only if the phenolate ortho substituents were small
(H, Cl) and a side-arm donor was present. Bulkier phenolate substituents (Me) or the lack
of a side-arm donor led to a â-abstraction process from the ligand backbone, forming a
dibenzyl complex with a metallaazacyclopropane ring. The side-arm donor, if present, was
found to bind to the metal in both paths. A bulkier ligand precursor having t-Bu groups on
the two phenolate rings did not react with Ta(CH2Ph)5 at room temperature, whereas an
asymmetric “half-bulky” ligand precursor carrying a Me group and a t-Bu group on the two
rings led directly to the â-abstraction product.
In tr od u ction
The use of predesigned ligand systems may offer a
considerable advantage in this kind of study. Recently,
we have been investigating the family of the amine bis-
(phenolate) ligands for group IV transition metals.6 This
family, comprised of dianionic [ONO] tridentate and
[ONXO] tetradentate ligands, which differ from one
another by the presence of an additional, so-called “side
arm” donor, led to the formation of penta- and hexaco-
ordinate complexes, respectively, with the [ONO] core
donors in a mer arrangement and the two labile groups
in a cis relationship. These complexes led to R-olefin
polymerization catalysts exhibiting a broad spectrum
of activities. We found that the existence of the side-
arm donor had a dramatic influence on the reactivity
of these complexes. The Zr and Hf complexes of ligands
with a side-arm donor were found to lead to highly
active polymerization catalysts,6c,d while the corre-
sponding Ti catalysts exhibited a prolonged living
character.6a,b,e-g In contrast, the five-coordinate Zr
H-abstraction processes are fundamental steps in
organometallic chemistry. Among these, the R- and â-H
abstractions are the most common, being the major
pathways for the decomposition of metal-alkyl single
bonds. In particular, the R-H abstraction reaction, which
leads to a metal-carbon double bond (along with alkane
formation), has been extensively studied since its initial
discovery in the early 1970s.1-3 Studies concerning R-H
abstraction were performed mainly on group V (Ta) and
group VI (Mo, W) metal complexes. Di- and trialkyl-
metal complexes have been employed as the main
substrates for the R-H abstraction reaction, leading to
the alkylidene and the alkyl-alkylidene complexes,
respectively. The alkyl group should be free of â-hydro-
gens; otherwise, a competitive â-H elimination pathway
becomes viable, resulting in the formation of a metal-
olefin complex instead of the metal-alkylidene complex.
Such basic studies are of major significance, since they
may provide mechanistic insight and lead to the design
of new ligand systems that may be ultimately employed
in well-defined olefin metathesis catalysis.4,5
(4) Wallace, K. C.; Liu, A. H.; Dewan, J . C.; Schrock, R. R. J . Am.
Chem. Soc. 1988, 110, 4964.
(5) For a recent review see: Fu¨rstner, A. Angew. Chem., Int. Ed.
2000, 39, 3012.
(6) (a) Tshuva, E. Y.; Versano, M.; Goldberg, I.; Kol, M.; Weitman,
H.; Goldschmidt, Z. Inorg. Chem. Commun. 1999, 2, 371. (b) Tshuva,
E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Inorg. Chem. Commun.
2000, 3, 611. (c) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z.
Organometallics 2001, 20, 3017. (d) Tshuva, E. Y.; Groysman, S.;
Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2002, 21, 662.
(e) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Chem.
Commun. 2001, 2120. (f) Groysman, S.; Goldberg, I.; Kol, M.; Genizi,
E.; Goldschmidt, Z. Inorg. Chim. Acta 2003, 345, 137. (g) Groysman,
S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2003, 22,
3013.
* To whom correspondence should be addressed. E-mail: moshekol@
post.tau.ac.il (M.K.); goldz@mail.biu.ac.il (Z.G.).
† Tel Aviv University.
‡ Bar-Ilan University.
(1) Schrock, R. R. Chem. Rev. 2002, 102, 145 and references therein.
(2) (a) Schrock, R. R. J . Organomet. Chem. 1976, 122, 209. (b)
Malatesta, V.; Ungold, K. U.; Schrock, R. R. J . Organomet. Chem. 1978,
152, C53. (c) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.
(3) Li, L.; Hung, M.; Xue, Z. J . Am. Chem. Soc. 1995, 117, 12746.
10.1021/om0499761 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/09/2004

Tantalum(V) Amine Bis(phenolate) Complexes
Organometallics, Vol. 23, No. 8, 2004 1881
Ta ble 1. Cr ysta llogr a p h ic Exp er im en ta l Deta ils
3a
6a
2b
3b
5b
8b
formula
C
39H45Cl4N2O2Ta‚
C
42H48NO2Ta
C29H62NO2Ta‚
C7H8
C
32H31Cl4N2O2Ta‚
C7H8
C
35H36NO3Ta‚
C
42H55N2O2Ta‚
C5H12
1/2C4H8O
1/2C4H8O
fw
a (Å)
b (Å)
c (Å)
932.57
779.76
12.1480(3)
12.5780(3)
23.2350(7)
90.00
94.790(1)
90.00
monoclinic
P21/c
743.89
890.47
10.9600(1)
14.1730(2)
23.5480(3)
90.00
92.136(1)
90.00
monoclinic
P21/n
737.67
872.98
14.0190(2)
17.1160(3)
17.7430(3)
90.00
94.011(1)
90.00
monoclinic
P21/n
11.6050(2)
13.2170(2)
14.6460(3)
114.999(1)
98.716(1)
94.798(1)
triclinic
P1h
8.2910(3)
13.0580(4)
17.2200(7)
90.442(1)
98.517(1)
94.348(3)
triclinic
P1h
15.8330(3)
10.6670(2)
19.8790(4)
90.00
100.676(1)
90.00
monoclinic
P21/a
3299.26(11)
1.485
R (deg)
â (deg)
γ (deg)
cryst syst
space group
V (Å3)
1984.95(6)
1.560
3.077
3537.85(16)
1.464
3.143
1838.10(11)
1.344
3.021
3655.31(8)
1.618
3.336
4246.99(12)
1.365
2.626
Dc (g cm-3
)
µ (cm-1
)
3.369
4
Z
2
4
2
4
4
no. of measd rflns
no. of rflns with
I > 2σ(I)
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
GOF
9339
8189
8310
4357
8594
6092
8745
7284
7830
5389
9887
7332
0.0395
0.0851
1.015
0.0717
0.0828
0.963
0.0737
0.1863
1.051
0.0300
0.0662
1.048
0.0434
0.0834
0.999
0.0395
0.0851
1.015
in a nitrogen-filled glovebox. Ta(CH2Ph)5,7a Lig1H2,6c Lig2H2,6a
Sch em e 1. Syn th esis of Tr iben zylta n ta lu m
Com p lexes
Lig4H2,6c and Lig6H2 were prepared according to previously
6a
published procedures. TaCl5, BnMgCl (1.0 M in ether), and
hexamethyldisiloxane (98%) were obtained from Aldrich Inc.
Ether and tetrahydrofuran were purified by reflux and distil-
lation under a dry argon atmosphere from Na/benzophenone.
Pentane was washed with HNO3/H2SO4 prior to distillation
from Na/benzophenone. Toluene was refluxed over molten Na
and distilled under Ar. NMR data were collected on Bruker
AC-200 and Bruker AC-400 spectrometers and referenced to
protio impurities in benzene-d6 (δ 7.15 ppm) and to the 13C
chemical shift of benzene (δ 128.70 ppm). Routine character-
ization of metal complexes consisted of 1H, BB, and DEPT-
135 13C NMR experiments, performed in C6D6. Kinetic data
were obtained via variable-temperature (VT) 1H NMR experi-
ments, using hexamethyldisiloxane as an internal standard.
VT NMR experiments for the decomposition of 2a and 3a were
performed in C6D6 (327 K). VT NMR experiments for the
decomposition of 5a , 6a , and 7a were performed in toluene-d8
(338, 344, and 349 K). The uncertainty of the kinetic data is
estimated to be 10%. X-ray diffraction measurements were
performed on a Nonius Kappa CCD diffractometer system,
using Mo KR (λ ) 0.7107 Å) radiation. The analyzed crystals
were embedded within a drop of viscous oil and freeze-cooled
to ca. 110 K. The structures were solved by a combination of
direct methods and Fourier techniques using DIRDIF software
and were refined by full-matrix least squares with SHELXL-
97. The crystal and experimental data are summarized in
Table 1. Elemental analyses were performed in the microana-
lytical laboratory at the Hebrew University of J erusalem. The
analyzed complexes consistently showed a low carbon percent-
age due to their extreme air sensitivity.
complexes featured fast deactivation, while the Ti
catalysts lacking that donor led to oligomerization.6b,c
Furthermore, we demonstrated that the organo-
metallic chemistry of the amine bis(phenolate) ligands
is not limited to group IV metals.7a Our preliminary
results have shown that Ta(CH2Ph)5 undergoes a fast
toluene elimination upon reaction with an [ONXO]
amine bis(phenolate) ligand precursor at room temper-
ature, affording an octahedral tribenzyl Ta(V) complex
in which the side-arm donor was not bound to the metal
(Scheme 1). Thus, at first sight, the role of a side-arm
donor seemed much less evident in the Ta(V) chemistry.
Having developed this straightforward synthesis to the
tribenzyltantalum complexes, we turned to investigate
their reactivity and, especially, evaluate the structural
parameters (i.e. side-arm donor and phenolate ring
substituents) that might favor the desired R-abstraction
path. Herein we describe these reactivity studies. We
demonstrate that, in addition to the R-abstraction route
leading to benzyl-benzylidene complexes, there exists
a competitive ligand backbone â-abstraction route. The
structural parameters that promote each of these routes
are discussed as well.
P r ep a r a tion of th e Am in e Bis(p h en ola te) Liga n d s
Lig3
H2, Lig5H2, Lig7H2, Lig8H2. P r ep a r a tion of Lig3H2. A
solution of 2,4-dichlorophenol (5.0 g, 30.7 mmol), 2-(dimethy-
lamino)ethylamine (1.68 mL, 15.3 mmol), and 36% aqueous
formaldehyde (3.6 mL, 43.2 mmol) in methanol (10 mL) was
stirred and refluxed for 24 h. The mixture was cooled, and the
product was filtered and washed with ice-cold methanol, to
give the colorless product (3.4 g, 50.7%). Mp: 169 °C. Anal.
Calcd for C18H20Cl4N2O2: C, 49.34; H, 4.60; Cl, 32.36; N, 6.39.
Found: C, 49.34, H, 4.90; Cl, 32.06; N, 6.25. 1H NMR (CDCl3,
200 MHz): δ 7.27 (d, J ) 2.0 Hz, 2H), 6.92 (d, J ) 2.0 Hz,
2H), 3.66 (s, 4H), 2.68 (m, 4H), 2.41 (s, 6H). 13C NMR (CDCl3,
200 MHz): δ 151.7 (2C), 129.6 (2CH), 128.3 (2CH), 124.4 (2C),
123.5 (2C), 122.6 (2C), 56.0 (CH2), 55.7 (CH2), 49.1 (CH2), 44.8
(2CH3).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations of the metal
complexes were performed under a dry nitrogen atmosphere
(7) (a) Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Orga-
nometallics 2003, 22, 3793. (b) Groysman, S.; Segal, S.; Shamis, M.;
Goldberg, I.; Kol, M.; Goldschmidt, Z.; Hayut-Salant, E. J . Chem. Soc.,
Dalton Trans. 2002, 3425.
P r ep a r a tion of Lig5H2. A solution of 2,4-dimethylphenol

1882 Organometallics, Vol. 23, No. 8, 2004
Groysman et al.
(5.0 g, 40.9 mmol), 2-methoxyethylamine (1.78 mL, 20.5 mmol),
and 36% aqueous formaldehyde (3.4 mL, 40.9 mmol) in
methanol (10 mL) was stirred and refluxed for 24 h. The
solution was cooled, and the product was filtered and washed
with cold methanol (3 × 10 mL), affording colorless crystals
(4.7 g, 67% yield). Mp: 22 °C. Anal. Calcd for C21H29NO3: C,
(CH), 128.43 (CH), 128.35 (CH), 119.82 (CH), 122.66 (C), 57.16
(CH2), 53.62 (CH2), 46.57 (CH3), 44.06 (CH2), 20.32 (CH3), 19.59
(CH3).
3a . Anal. Calcd for C39H39Cl4N2O2Ta: C, 52.60; H, 4.41; N,
1
3.15. Found: C, 51.39; H, 4.60; N, 3.21. H NMR (C6D6, 298
K, 200 MHz): δ 7.19 (d, J ) 2.3 Hz, 2H), 7.11, 7.02 (two
overlapping br s, 12 H), 6.75 (br s, 3H), 6.63 (d, J ) 2.3 Hz,
2H), 3.59 (d, J ) 14.1, 2H), 3.31 (br s, 6H), 3.03 (d, J ) 14.2,
1
73.44; H, 8.51; N, 4.08. Found: C, 73.38; H, 8.51; N, 4.08. H
NMR (CDCl3, 200 MHz): δ 6.88 (d, J ) 2.0 Hz, 2H), 6.68 (d,
J ) 2.0 Hz, 2H), 3.79 (s, 4H), 3.60 (t, J ) 5.0 Hz, 2H), 3.47 (s,
3H), 2.77 (t, J ) 5.0 Hz, 2H) 2.24 (s, 6H), 2.21 (s, 6H). 13C
NMR (CDCl3, 50.29 MHz): δ 152.2 (C), 131.4 (CH), 128.5 (CH),
128.1 (C), 125.3 (C), 120.7 (C), 71.1 (CH2), 59.1 (CH3), 57.1
(CH2), 51.0 (CH2), 20.4 (CH3), 16.1 (CH3).
1
2H), 1.46 (s, 6H), 1.36 (m, 3H), 1.04 (m, 2H). H NMR (C6D6,
323 K, 200 MHz): δ 7.23 (d, J ) 2.5 Hz, 2H), 7.11 (d, J ) 7.4
Hz, 6H), 7.00 (t, J ) 7.4 Hz, 2H), 6.74 (t, J ) 7.2, 3H), 6.63 (d,
J ) 2.5 Hz, 2H), 3.56 (d, J ) 14.2 Hz, 2H), 3.30 (s, 6H), 3.07
(d, J ) 14.1 Hz, 2H), 1.49 (s, 6H), 1.45 (m, 2H), 1.14 (m, 2H).
13C NMR (C6D6, 298 K, 50.29 MHz): δ 153.34 (C), 130.89 (CH),
129.89 (CH), 128.98 (CH), 131.96 (C), 127.70 (C), 127.61 (C),
125.03 (CH), 124.75 (C), ca. 85 (br s, CH2), 56.58 (CH2), 53.98
(CH2), 46.22 (CH3), 43.74 (CH2).
P r ep a r a tion of Lig7H2. A solution of 2,4-dichlorophenol
(5.1 g, 30 mmol), 1-aminopropane (0.95 g, 15 mmol), and 36%
aqueous formaldehyde (2.5 mL, 30 mmol) in methanol (10 mL)
was stirred and refluxed for 22 h. After the mixture was cooled,
the volatiles were removed under vacuum to give the crude
product as a heavy brown oil that was purified first by column
chromatography (silica gel, 5-10% ethyl acetate in hexane),
followed by recrystallization from hexane to give the pure
product as white crystals (2.3 g, 38% yield). Mp: 115-116 °C.
Anal. Calcd for C17H17Cl4NO2: C, 49.91; H, 4.19; N, 3.42.
Found: C,50.01;H,4.36;N,3.49.HRMS (DCI-I-Bu): 407.001 228
4a . The preparation and NMR characterization were re-
ported previously.7a Anal. Calcd for C43H51N2O2Ta: C, 63.86;
H, 6.36; N, 3.46. Found: C, 64.20; H, 6.44; N, 3.39.
1
5a . H NMR (C6D6, 298 K, 200 MHz): δ 7.15-6.97 (Ar H,
12H), 6.85 (d, J ) 1.5 Hz, 2H), 6.78 (br s, 3H), 6.57 (d, J ) 1.8
Hz, 2H), 3.82 (d, J ) 13.5, 2H), 3.42 (d, J ) 13.6 Hz, 2H), 3.3
(br s, 4H), 3.09 (br s, 2H), 2.83 (t, J ) 4.7 Hz, 2H), 2.71 (s,
3H), 2.29 (s, 6H), 2.12 (s, 6H), 1.66 (t, J ) 5.0 Hz, 2H). 1H
NMR (C6D6, 323 K, 200 MHz): δ 7.11-6.97 (Ar H, 12H), 6.88
(d, J ) 1.1 Hz, 2H), 6.76 (t, J ) 6.9 Hz, 3H), 6.57 (d, J ) 1.3
Hz, 2H), 3.78 (d, J ) 13.7 Hz, 2H), 3.42 (d, J ) 13.8 Hz, 2H),
3.19 (s, 6H), 2.88 (t, J ) 5.0 Hz, 2H), 2.73 (s, 3H), 2.29 (s, 6H),
2.12 (s, 6H), 1.72 (t, J ) 5.0 Hz, 2H). 13C NMR (C6D6, 298 K,
50.29 MHz): δ 155.17 (C), 132.73 (CH), 132.25 (CH), 131.37
(CH), 129.97 (C), 129.57 (CH), 128.99 (CH), 128.59 (CH),
127.16 (C), 125.06 (C), ca. 80 (br s), 69.09 (CH2), 59.04 (CH3),
58.03 (CH2), 45.74 (CH2), 21.43 (CH3), 16.89 (CH3).
1
(C17H17Cl4NO2; M+). H NMR (CDCl3, 200 MHz): δ 7.26 (d, J
) 2.0 Hz, 2H), 6.98 (d, J ) 2.0 Hz, 2H), 5.90 (bs, 2H), 3.72 (s,
4H), 2.48 (m, 2H), 1.63 (m, 2H), 0.89 (t, J ) 7.5 Hz, 3H). 13C
NMR (CDCl3, 50.29 MHz): δ 150.74 (C), 128.54 (CH), 128.48
(CH), 124.75 (C), 124.31 (C), 121.24 (C), 55.54, (CH2), 55.26 (2
CH2), 18.87 (CH2), 11.65 (CH3).
P r ep a r a tion of Lig8H2. A solution of 2,4-dimethylphenol
(1.83 g, 15.0 mmol), 2,4-di-tert-butylphenol (3.1 g, 15.0 mmol),
2-(dimethylamino)ethylamine (1.65 mL, 15.0 mmol), and 36%
aqueous formaldehyde (3.5 mL, 42.0 mmol) in methanol (10
mL) was stirred and refluxed for 18 h. The mixture was cooled
and decanted. TLC of the remaining solid (10% methanol-
90% hexane) showed clearly the formation of three products:
the two symmetric bis(phenols) and the cross-coupling product.
A single crystallization of the mixture from methanol (100 mL)
gave selectively the symmetrical 2,4-dimethyl derivative (2 g,
mp 167 °C, 37% yield). The symmetrical 2,4-di-tert-butyl
derivative (0.96 g, mp 130 °C, 13% yield) crystallized from the
mother liquor. Finally, column chromatography (silica, chlo-
roform) of the remaining mixture gave the pure crossed amine
bis(phenol) as colorless crystals (1.86 g, 28.1% yield). Mp: 117
°C. Anal. Calcd for C28H44N2O2: C, 76.32; H, 10.06; N, 6.36.
Found: C, 76.16, H, 10.24, N, 6.44. HRMS: calcd, 441.348 10
(MH+); found, 441.350 00. 1H NMR (CDCl3, 200 MHz): δ 7.18
(d, J ) 2.0 Hz, 2H), 6.85 (d, J ) 2.0 Hz, 2H), 6.85 (s, 1H), 6.68
(s, 1H), 3.64 (s, 2H), 3.56 (s, 2H), 2.58 (s, 4H), 2.32 (s, 6H)
2.19 (s, 6H), 1.38 (s, 9H), 1.36 (s, 9H). 13C NMR (CDCl3, 200
MHz): δ 153.2 (C), 152.7 (C), 140.4 (C), 135.9 (C), 131.1 (CH),
128.6 (CH), 127.3 (C), 125.6 (C), 124.6 (CH), 123.3 (CH), 121.6
(C), 57.3 (CH2), 56.1 (CH2), 55.2 (CH2), 48.9 (CH2), 44.9 (CH3),
35.0 (C), 34.1 (C), 31.7 (CH3), 29.6 (CH3), 20.3 (CH3), 16.2
(CH3).
6a . 1H NMR (C6D6, 298 K, 200 MHz) δ 7.23-6.56 (Ar H,
19H), 3.81 (d, J ) 14.0, 2H), 3.39 (br s, 4H), ca. 3.0 (br s, 2H),
2.96 (d, J ) 14.1, 2H), 1.96 (s, 12H), 1.35 (m, 2H), 0.86 (m,
2H), 0.03 (t, J ) 7.2 Hz, 3H). 13C NMR (C6D6, 298 K, 50.29
MHz): δ 157.02 (C), 139.19 (C), 131.55 (C), 131.50 (CH), 131.38
(C), 131.22 (CH), 128.47 (CH), 124.51 (CH), 122.45 (C), 119.92
(CH), 80.0 (br s, CH2), 56.37 (CH2), 48.96 (CH2), 20.38 (CH3),
19.68 (CH3), 11.86 (CH2), 11.33 (CH3).
7a . 1H NMR (C6D6, 298 K, 200 MHz): δ 7.18 (d, J ) 2.5 Hz,
2H), 7.10, 7.02, and 6.76 (3 overlapping br s, 15 H), 6.51 (d, J
) 2.4, 2H), 3.45 and ca. 3.4 (overlapping peaks: d, J ) 14.3
Hz, and br s, 8H), 2.52 (d, J ) 14.3 Hz, 2H), 0.96 (m, 2H),
0.45 (m, 2H), -0.13 (t, J ) 7.2 Hz, 3H). 13C NMR (C6D6, 298
K, 100.62 MHz): δ 153.60 (C), 131.23 (CH), 129.98 (C), 129.117
(C), 128.66 (CH), 128.54 (CH), 127.92 (C), 127.26 (CH), 126.34
(C), 125.31 (CH), 124.99 (C), 55.61 (CH2), 49.03 (CH2), 11.82
(CH2), 10.95 (CH3).
P r ep a r a tion a n d Ch a r a cter iza tion of th e Ben zyl-
Ben zylid en e (2b, 3b) a n d Diben zyl Com p lexes (5b -8b).
P r ep a r a tion of 2b. 2a (207 mg, 0.256 mmol) was dissolved
in toluene (4 mL) and was heated to 85 °C for 1 h. The reaction
mixture was cooled, and the solvent was removed in vacuo,
leading to a yellow-orange crude product. 1H NMR of the crude
product indicated the formation of an essentially pure mixture
of two alkyl-alkylidene products in an approximately 3:1 ratio,
as revealed from the TadC(Ph)H peaks at 9.28 and 9.17 ppm
and the Ta-CH2Ph peaks at 2.94 and 2.89 ppm. The major
product (2b) is obtained upon crystallization from cold ether
in 22% yield (20 mg of 2b from 90 mg of the crude mixture).
1H NMR (C6D6, 298 K, 200 MHz): δ 9.17 (s, 1H, TadC(Ph)H),
7.50 (m, 4H), 7.28 (d, J ) 8.0 Hz, 2H), 7.11 (t, J ) 7.8 Hz,
2H), 6.79 (m, 4H), 6.62 (s, 2H), 4.13 (d, J ) 13.6 Hz, 2H), 2.94
(s, 2H), 2.67 (d, J ) 13.6 Hz, 2H), 2.05 (s, 6H), 1.97 (s, 6H),
1.94 (br s, 8H), 1.37 (m, 2H). 13C NMR (C6D6, 298 K, 50.29
MHz): δ 247.25 (TadC(Ph)H), 157.53 (C), 154.41 (C), 146.97
(C), 139.14 (C), 132.03 (CH), 130.03 (CH), 129.48 (C), 128.80
(CH), 127.85 (CH), 125.25 (CH), 123.85 (C), 121.48 (CH),
P r ep a r a tion of th e Tr iben zyl Com p lexes 2a -7a . Gen -
er a l P r oced u r e. Lig2H2-Lig7H2 were dissolved in ca. 1 mL
of toluene and added dropwise to a stirred red-brown solution
of Ta(CH2Ph)5 (1 equiv) in ca. 1 mL of toluene. The resulting
dark yellow solution was stirred at room temperature. After
2 h the solvent was removed in vacuo, leading to the yellow-
orange 2a -7a quantitatively.
2a . 1H NMR (C6D6, 298 K, 200 MHz): δ 7.21 (d, J ) 7.4 Hz,
6H), 7.07 (t, J ) 7.4 Hz, 6H), 6.80 (m, 5H), 6.64 (s, 2H), 3.84
(d, J ) 13.9 Hz, 2H), 3.35 (d, J ) 13.9 Hz, 2H), ca. 3.3 (br s,
6H), 1.99 (s, 12H), 1.76 (t, J ) 5.8 Hz, 2H), 1.62 (s, 6H), 1.54
(t, J ) 6.0 Hz, 2H); 13C NMR (C6D6, 298 K, 50.29 MHz): δ
156.92 (C), 132.11 (CH), 131.49 (CH), 131.05 (CH), 128.97

Tantalum(V) Amine Bis(phenolate) Complexes
Organometallics, Vol. 23, No. 8, 2004 1883
120.39 (CH), 64.72 (CH2), 63.25 (CH2), 60.49 (CH2), 53.07
(CH2), 50.69 (N(CH3)2), 20.30 (Ar-CH3), 19.60 (Ar-CH3).
F or m a tion of th e Alk yl-Alk ylid en e 3b/3c Mixtu r e
u p on 3a Decom p osition . A 15 mg portion of 3a was
dissolved in ca. 0.5 mL of C6D6 and the solution heated to 80
°C for 1 h in an NMR tube. According to the 1H NMR spectrum,
the resulting crude product consists of an essentially pure
mixture of the two alkyl-alkylidene products 3b and 3c in a
ca. 1.5:1 ratio (C6D6, δ, selected absorptions): 8.95 and 8.80
ppm for TadC(Ph)H, 2.93 and 2.91 ppm for Ta-CH2Ph, 1.86
and 1.68 ppm for N(CH3)2.
La r ge-Sca le P r ep a r a tion of 3b. A 397 mg portion (0.45
mmol) of 3a was dissolved in 7 mL of toluene and the solution
heated for 1 h to 85-90 °C. After the concentration of the
solvent to ca. 3 mL, the crude mixture was left in the glovebox
freezer (-35 °C). Four crops (143 mg) of red-orange crystals
were collected, giving pure 3b in 44% yield. Anal. Calcd for
133.92 (CH), 131.77 (CH), 131.50 (CH), 131.43 (CH), 130.01
(C), 129.42 (C), 129.00 (C), 128.48 (C), 125.48 (CH), 123.84
(CH), 123.01 (C), 119.34 (CH), 115.81 (CH), 80.81 (CH), 74.23
(CH2), 62.40 (CH2), 56.25 (CH2), 54.64 (CH2), 20.55 (CH3),
20.33 (CH3), 20.13 (CH2), 19.74 (CH3), 19.60 (CH3), 12.51 (CH3).
P r ep a r a tion of 7b. A 91 mg amount (0.105 mmol) of 7a in
4 mL of toluene was heated to ca. 90 °C for 4.5 h. The reaction
was monitored by 1H NMR spectroscopy, which indicated that
7b was the only organometallic (diamagnetic) product formed.
After toluene removal, the resulting yellow-brown solid was
extracted with ca. 3 mL of diethyl ether. Cooling in the
glovebox freezer (-35 °C) for 24 h yielded yellow microcrystals
of 7b (11 mg, 0.014 mmol, 14% yield). Subsequent workup of
the ether solution, which was comprised of ether evaporation
and pentane washings, yielded another 27 mg of essentially
pure 7b. 1H NMR (C6D6, 298 K, 200 MHz): δ 7.17 (m, 2H),
7.02 (d, J ) 7.3 Hz, 4H), 6.91 (t, J ) 7.7 Hz, 4H), 6.76 (t, J )
7.2 Hz, 2H), 6.47 (d, J ) 2.5 Hz, 1H), 6.42 (d, J ) 2.3 Hz, 1H),
3.43 (AB system, J ) 14.4 Hz, 2H), 2.50 (m, 5H), 2.36 (m, 2H),
1.10 (m, 1H), 0.80 (m, 1H), 0.19 (t, J ) 7.4 Hz, 3H). 13C NMR
+ DEPT-135 (C6D6, 298 K, 50.29 MHz): δ 132.97 (CH), 131.11
(CH), 130.87 (CH), 130.46 (CH), 130.22 (CH), 129.98 (C),
129.89 (CH), 129.12 (CH), 129.02 (CH), 128.88 (C), 128.39 (C),
127.80 (CH), 126.88 (CH), 126.35 (C), 125.69 (CH), 77. 05 (CH),
71.81 (CH2), 64.75 (CH2), 56.25 (CH2), 55.03 (CH2), 20.09 (CH2),
12.25 (CH3).
C
32H31Cl4 N2O2Ta‚C7H8: C, 52.60; H, 4.41; N, 3.15. Found: C,
1
51.12; H, 4.26; N, 3.43. H NMR (C6D6, 298 K, 200 MHz): δ
8.81 (s, 1H), 7.46 (d, J ) 7.3 Hz, 2H), 7.37 (t, J ) 7.3 Hz, 2H),
7.19 (d, J ) 2.5 Hz, 2H), 7.07 (d, J ) 1.6 Hz, 2H), 7.05 (s, 2H),
6.76 (m, 2H), 6.55 (d, J ) 2.5 Hz, 2H), 3.71 (d, J ) 13.9 Hz,
2H), 2.91 (s, 2H), 2.21 (d, J ) 14.0 Hz, 2H), 1.86 (s, 6H), 1.48
(m, 2H), 1.14 (m, 2H). 13C NMR (C6D6, 298 K, 50.29 MHz): δ
1
249.31 (TadC(Ph)H, J C-H ) 106 Hz), 153.99 (C), 153.22 (C),
145.69 (C), 131.82 (CH), 131.22 (CH), 129.97 (C), 129.29 (CH),
128.99 (C), 128.53 (CH), 128.39 (CH), 127.42 (CH), 126.56
(CH), 126.34 (C), 126.12 (C), 125.32 (C), 122.01 (CH), 65.87
(CH2), 62.40 (CH2), 60.12 (CH2), 52.81 (CH2), 50.46 (CH3).
P r ep a r a tion a n d Ch a r a cter iza tion of 5b. 5a (73 mg,
0.10 mmol) in toluene (3 mL) was heated to 80 °C for 1 h. The
solvent was removed in vacuo, leading to a yellow-orange crude
product. A 53 mg amount of the crude mixture was extracted
with ca. 2 mL of ether and kept at -35 °C for several days,
giving pure 5b as yellow crystals (18 mg, 34%). 1H NMR (C6D6,
298 K, 200 MHz): δ 7.57 (d, J ) 7.1 Hz, 2H), 7.38 (t, J ) 7.4
Hz, 2H), 7.02 (m, 3H), 6.88 (d, J ) 8.1 Hz, 2H), 7.70 (m, 3H),
6.61 (d, J ) 2.1 Hz, 1H), 6.25 (d, J ) 2.0 Hz, 1H), 3.50 (d, J )
14.4 Hz, 1H), 3.49 (s, 1H), 3.38 (d, J ) 11.9 Hz, 1H), 3.28 (d,
J ) 11.1 Hz, 1H), 3.18 (d, J ) 14.4 Hz, 1H), ca. 3.0 (m, 2H),
2.94 (d, J ) 11.9 Hz, 1H), 2.65 (d, J ) 11.2 Hz, 1H), 2.57 (s,
3H), 2.36 (m, 2H), 2.30 (s, 3H), 2.26 (s, 3H), 2.20 (s, 3H), 2.06
(s, 3H). 13C NMR (C6D6, 298 K, 50.29 MHz): δ 150.13 (C),
149.75 (C), 132.40 (CH), 131.74 (CH), 130.86 (C), 129.98 (CH),
129.50 (C), 129.40 (CH), 129.01 (CH), 128.86 (CH), 128.52 (C),
128.44 (CH), 128.36 (C), 126.53 (CH), 126.36 (C), 126.22 (C),
125.19 (C), 123.68 (C), 123.24 (CH), 123.11 (CH), 81.89 (CH),
76.28 (CH2), 75.72 (CH2), 63.54 (CH2), 59.46 (CH3), 51.43 (CH2),
21.51 (CH3), 21.22 (CH3), 16.89 (CH3), 16.70 (CH3).
P r ep a r a tion of 6b (On e P ot, fr om Lig6H2 a n d Ta -
(CH2P h )5). A 37 mg amount (0.11 mmol) of Lig6H2 in ca. 1
mL of toluene was added dropwise to a red-brown solution of
Ta(CH2Ph)5 (72 mg, 0.11 mmol) in ca. 1 mL of toluene. After
it was stirred for 2 h at room temperature, the reaction
mixture was heated to 90 °C for an additional 2 h. The solvent
was removed in vacuo, and the crude product was extracted
with ca. 4 mL of pentane. Cooling to -35 °C for 1 h and
filtering off the solid impurities led to a bright yellow pentane
solution. Solvent removal yielded 35 mg (0.05 mmol, 46% yield)
of 6b as a bright yellow solid. 1H NMR + COSY + HMQC
(C6D6, 298 K, 400 MHz): δ 7.43 (d, J ) 7.7 Hz, 2H), 7.28 (t, J
) 7.7 Hz, 2H), 7.05 (t, 7.3, 1H), 6.94 (d, J ) 7.9 Hz, 2H), 6.85
(t, 7.8 Hz, 2H), 6.57 (m, 2H), 6.36 (s, 1H), 6.33 (s, 1H), 6.32 (s,
1H), 3.92 (d, J ) 13.7 Hz, 1H), 3.82 (d, J ) 13.8 Hz, 1H), 3.31
(d, J ) 11.9 Hz, 1H), 3.16 (d, J ) 12.0 Hz, 1H), 2.75 (s, 1H,
PhCH(N)Ta), 2.69 (m, 2H, NCH2CH2CH3), 2.60 (d, J ) 11.1
Hz, 1H), 2.48 (d, J ) 11.1 Hz, 1H), 1.99 (s, 3H, Ar-CH3), 1.94
(s, 3H, Ar-CH3), 1.92 (s, 3H, Ar-CH3), 1.88 (s, 3H, Ar-CH3),
1.43 (m, 1H, NCH2CH2CH3), 1.03 (m, 1H, NCH2CH2CH3), 0.29
(t, J ) 7.4 Hz, 3H, NCH2CH2CH3). 13C NMR (C6D6, 298 K,
50.29 MHz): δ 158.50 (C), 144.17 (C), 136.61 (C), 136.23 (C),
P r ep a r a tion of 8b. A 58 mg portion (0.13 mmol) of Lig8H2
was dissolved in ca. 1 mL of toluene and slowly added to a
red-brown solution of Ta(CH2Ph)5 (86 mg, 0.13 mmol) in ca. 1
mL of toluene. The resulting mixture was stirred for 2 h at
room temperature, after which the solvent was removed in
vacuo. The crude 8b was washed once with pentane (3 mL),
leading to 52 mg (0.065 mmol, 50% yield) of pure 8b. X-ray-
quality crystals were obtained from cold (-35 °C) ether. 1H
NMR (C6D6, 298 K, 200 MHz): δ 7.57 (d, J ) 7.7 Hz, 2H),
7.40 (t, J ) 7.8 Hz, 2H), 7.33 (d, J ) 2.2 Hz, 1H), 6.98 (m, 4
H), 6.67 (m, 4H), 6.23 (br s, 1H), 3.61 (d, J ) 14.8 Hz, 1H),
3.29 (m, 2H), 3.24 (d, J ) 15.1 Hz, 1H), 2.71 and 2.82
(overlapping AB system and m, J ) 11.6 Hz for the AB system,
3H), 2.41 (m, 1H), 2.13 (s, 3H, Ar-CH3), 2.06 (s, 3H, Ar-CH3),
2.00 (s, 6H, N(CH3)2), 1.53 (s, 9H, Ar-C(CH3)3), 1.39 (s, 9H,
Ar-C(CH3)3). 13C NMR (C6D6, 298 K, 50.29 MHz): δ 161.51
(C), 154.95 (C), 153.01 (C), 142.32 (C), 135.32 (C), 135.05 (CH),
132.56 (CH), 131.06 (C), 129.01 (C), 128.94 (CH), 128.59 (CH),
127.99 (CH), 127.79 (C), 127.68 (CH), 125.96 (CH), 123.55
(CH), 122.58 (CH), 121.89 (CH), 110.09 (Ta-CH), 76.27 (CH2),
75.85 (CH2), 64.82 (CH2), 64.63 (CH2), 53.79 (CH2), 50.10 (CH3),
35.27 (C), 32.80 (CH3), 32.71 (C), 31.49 (CH3), 21.29 (CH3),
16.99 (CH3).
Kin etic Exp er im en ts. 1H NMR Mon itor in g of 2a De-
com p osition . A 27 mg portion (0.033 mmol) of 2a was
dissolved in benzene-d6 (containing hexamethyldisiloxane).
The final volume of the solution was 0.40 mL. The resulting
solution was heated to 327(1) K in the NMR probe for 140 min,
1
being sampled (by H NMR) every 10 min.
1H NMR Mon itor in g of 3a Decom p osition . A 17 mg
portion (0.074 mmol) of 3a was dissolved in benzene-d6 (0.60
mL total volume), resulting in a 0.123 M 3a solution. The
resulting solution was heated to 327(1) K in the NMR probe
1
for 180 min, being sampled (by H NMR) every 5 min.
1H NMR Mon itor in g of 5a Decom p osition . Since no
decomposition of 5a was observed at 327(1) K in benzene-d6
(for 40 min), higher temperature experiments were attempted.
A 15 mg portion (0.019 mmol) of 5a in toluene-d8 (0.40 mL,
0.047 M) was reacted at 338(1) K for 240 min, with sampling
every 10 min. A 17 mg portion (0.021 mmol) in toluene-d8 (0.38
mL, 0.055 M) was heated to 344(1) K for 180 min (until 5a
reached ca. 1/10 of its initial concentration), with sampling
every 10 min. A 17 mg portion (0.021 mmol) of 5a (0.40 mL,
0.053 M) was heated to 349(1) K for 120 min, with sampling
every 5 min.

1884 Organometallics, Vol. 23, No. 8, 2004
Groysman et al.
F igu r e 2. ORTEP drawing of 3a (50% probability el-
lipsoids). The hydrogen atoms and the crystallization
solvent were omitted for clarity. Selected bond distances
(Å) and angles (deg): Ta1-O2 ) 1.916(2), Ta1-O3 )
1.895(2), Ta1-N4 ) 2.506(3), Ta1-C5 ) 2.224(4), Ta1-
F igu r e 1. Amine bis(phenolate) ligands employed in this
study.
6a /7a Decom p osition Exp er im en ts. The kinetic experi-
ment monitoring 6a decomposition was performed at 349(1)
K. A 20 mg portion (0.026 mmol) of 6a in toluene-d8 (0.44 mL,
0.059 M) was heated to the desired temperature for 180 min,
being sampled by 1H NMR every 10 min. When 7a was heated
to the same (349(1) K) temperature, no sign of decomposition
was detected for at least 50 min.
C6
)
2.267(3), Ta1-C7
) 2.246(3); O3-Ta1-O2
153.12(10), C6-Ta1-N4 ) 158.29(11), C5-Ta1-C7 )
148.30(12).
to the overall Cs symmetry on the NMR time scale,
whereas the benzyl ligands are highly fluxional at room
temperature, giving rise to two broad signals in the
aliphatic region. Slight heating (to ca. 40 °C) of 2a -7a
coalesces the benzyl signals into one broad singlet, while
cooling to ca. -50 °C (measured for 4a ) resolves three
distinct benzyl systems, as expected for a Cs-sym-
metrical complex having three different benzyl groups
lying on a mirror plane. The identical behavior of 2a -
7a in solution at or near room temperature implies that
the side-arm donor, present in complexes 2a -5a , is not
coordinating to the metal center in solution. To gain
further structural information regarding these types of
compounds, we carried out X-ray structure determina-
tions for 3a , 4a , and 6a (Figures 2-4, respectively). As
expected, the Ta(V) metal in all three structures is
hexacoordinate, with the side-arm donor unbound. The
geometry around the metal may be defined as distorted
octahedral. The tridentate [ONO] cores of the ligands
bind to the metal in a mer fashion, resulting in a mer
disposition of the three benzyl groups. The Ta-O bond
lengths lie in the relatively narrow range of 1.876(3)-
1.916(2) Å and closely resemble those normally observed
for early-transition-metal phenolate complexes.4,6-10 The
nitrogen-to-metal bond lengths in all three structures
are longer than previously observed for related group
IV complexes (2.506(3)-2.548(6) Å).6,11 This trend may
be attributed to the presence of the three electron-
Resu lts a n d Discu ssion
The amine bis(phenolate) ligands employed in this
study are presented in Figure 1. Lig1H2-Lig4H2 are all
[ONNO]-type ligands bearing a dimethylamino donor
on a side arm and differ in their phenolate substituents.
Lig1H2 is the bulkiest, featuring tert-butyl groups in the
ortho positions of the phenolate oxygens, whereas
Lig2H2 is the least bulky, featuring H atoms in ortho
positions. Lig3H2 features electron-withdrawing chloro
groups, and Lig4H2 has an intermediate steric bulk
featuring o-methyl substituents. Lig5H2 is analogous to
Lig4H2, except for having a methoxy side-arm donor. In
contrast, the [ONO]-type Lig6H2 and Lig7H2 have no
side-arm donor. Finally, the reactivity of the “half-bulky”
Lig8H2, bearing different substituents (t-Bu and Me) on
the phenolate rings (Figure 1), toward Ta(CH2Ph)5 was
investigated. All ligands were prepared by a single-step
Mannich condensation among the suitable phenol,
amine, and formaldehyde, as previously described,6
except for the asymmetric Lig8H2, which was synthe-
sized from a mixture of two different phenols and was
purified by consecutive crystallizations and chromatog-
raphy.
In contrast to their reaction with group IV tetrabenzyl
complexes, the reaction of the ligand precursors with
Ta(CH2Ph)5 was found to depend strongly on the bulk
of the aromatic ring substituents: the ligand bearing
t-Bu groups in the ortho position of both phenolates
(Lig1H2) did not show any appreciable reactivity toward
pentabenzyltantalum precursor at room temperature for
hours. On the other hand, the ligand precursors Lig2H2-
Lig7H2 reacted with pentabenzyltantalum at room tem-
perature, giving the tribenzyl complexes 2a -7a in high
yields. Complexes 2a -7a exhibit a somewhat dual
character in solution at room temperature: The amine
bis(phenolate) ligand-to-metal binding is rigid, according
to the AB pattern of the ligand methylene protons and
(8) Kim, Y.; Kapoor, P. N.; Verkade, J . G. Inorg. Chem. 2002, 41,
4834.
(9) (a) Chamberlain, L. R.; Rothwell, I. P.; Folting, K.; Huffman, J .
C. J . Chem. Soc., Dalton Trans. 1987, 155. (b) Vilardo, J . S.; Lockwood,
M. A.; Hanson, L. G.; Clark, J . R.; Parkin, B. C.; Fanwick, P. E.;
Rothwell, I. P. J . Chem. Soc., Dalton Trans. 1997, 3353. (c) Riley, P.
N.; Thorn, M. G.; Vilardo, J . S.; Lockwood, M. A.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1999, 18, 3016. (d) Scheiger, S. W.;
Salberg, M. M.; Pulvirenti, A. L.; Freeman, E. E.; Fanwick, P. E.;
Rothwell, I. P. J . Chem. Soc., Dalton Trans. 2001, 2020.
(10) Fox, P. A.; Bruck, M. A.; Gray, S. D.; Gruhn, N. E.; Grittini,
C.; Wigley, D. E. Organometallics 1998, 17, 2720.
(11) A tantalum tris(ethoxide) complex of an amine bis(phenolate)
ligand featured
Unpublished results).
a N-Ta bond length of 2.40 Å (Tshuva, E. Y.

Tantalum(V) Amine Bis(phenolate) Complexes
Organometallics, Vol. 23, No. 8, 2004 1885
Sch em e 2 r-Abstr a ction P r ocess Lea d in g to
Alk yl-Alk ylid en e Com p lexes
F igu r e 3. ORTEP drawing of 4a (50% probability el-
lipsoids). The hydrogen atoms and the crystallization
solvent were omitted for clarity. Selected bond distances
(Å) and angles (deg): Ta1-O2 ) 1.876(2), Ta1-O3 )
1.893(2), Ta1-N4 ) 2.513(3), Ta1-C5 ) 2.248(3), Ta1-
C6 ) 2.279(3), Ta1-C7 ) 2.250(3); O2-Ta1-O3
)
with amine bis(phenolate) ligands featuring t-Bu sub-
stituents on the two phenolate rings.
153.69(10), C6-Ta1-N4 ) 165.69(11), C5-Ta1-C7 )
152.17(13).
F or m a tion a n d Str u ctu r e of Alk ylid en e Com -
p lexes. The conditions known to facilitate alkylidene
formation via R-hydrogen abstraction are (1) sufficient
steric bulk around the metal and (2) addition of an
“external donor”, such as the small ligand PMe3.1,2c
A
priori, it would seem plausible that an “internal donor”,
covalently bonded to a ligand, will have the same, or
even greater, effect on the formation of an alkylidene
group. However, the role of such an internal donor has
been much less explored.12 The tribenzyl complexes 2a -
5a presented above contain such a “dormant” internal
donor, unattached to the metal. We anticipated that,
under certain conditions, this donor would be able to
trigger the R-hydrogen abstraction sequence and lead
to the alkyl-alkylidene complexes.
Upon heating of 2a or 3a to ca. 90 °C, a fast reaction
takes place, leading to a mixture of two alkyl-alky-
lidene products for both substrates (2b and 2c and 3b
and 3c, respectively; see Scheme 2), accompanied by
liberation of toluene. The relative ratios of the products
are different for 2a and 3a , being ca. 3:1 and 1.5:1,
respectively. The existence of a TadC double bond in
these species is clearly manifested by 1H NMR (9.18 and
9.28 ppm for 2b and 2c, 8.95 and 8.80 ppm for 3b and
3c), and 13C NMR (247.2 ppm for both 2b and 2c, 249.0
and 249.2 ppm for 3b and 3c). In addition to the
benzylidene signal, the 1H NMR spectrum of each
complex contains two types of ligand aromatic protons,
a single AB system for the Ar-CH2-N protons, and a
sharp singlet for the PhCH2 protons. According to the
1H and 13C NMR spectra, the two products resulting
from each starting material are structural isomers of
presumably Cs symmetry (on the NMR time scale at
room temperature). This isomerism could stem from
binding of the side-arm donor to the metal, with the
alkylidene ligand and the dimethylamino donor mutu-
ally cis or trans (see Scheme 2). There is no intercon-
F igu r e 4. ORTEP drawing of 6a (50% probability el-
lipsoids). The hydrogen atoms and the crystallization
solvent were omitted for clarity. Selected bond distances
(Å) and angles (deg): Ta1-O2 ) 1.893(5), Ta1-O3 )
1.877(5), Ta1-N4 ) 2.548(6), Ta1-C5 ) 2.299(7), Ta1-
C6 ) 2.234(7), Ta1-C7 ) 2.203(7); O3-Ta1-O2
)
)
152.6(2), C5-Ta1-N4
150.3(3).
)
164.2(2), C7-Ta1-C6
donating benzyl groups. It is possible that this weak
Ta-N bond undergoes detachment in solution, giving
rise to the formation of a pseudo-pentacoordinate spe-
cies, which facilitates the equilibration of the benzyl
groups.
The Ta-C bond lengths and the Ta-C-C bond angles
are in the ranges of 2.203(7)-2.299(7) Å and 100.5(2)-
125.8(5)°, supporting η1 binding. Interestingly, these
crystal structures are almost superimposable, except for
the slight deviation of the benzyl groups and the N
substituent. Noteworthy is the orientation of the “middle”
benzyl ligand, i.e., the one trans to the nitrogen donor,
which points toward the ortho substituent of the phe-
nolate ring. Probably, this steric interaction for a bulkier
substituent in the ortho position (e.g. t-Bu) is too severe,
thus preventing the formation of tribenzyl complexes
(12) Van Doorn, J . A.; van der Heijden, H.; Orpen, A. G. Organo-
metallics 1994, 13, 4271.

1886 Organometallics, Vol. 23, No. 8, 2004
Groysman et al.
Experimental Section for details) in an overall yield of
44%. X-ray structure determination was carried out for
both compounds (Figures 7 and 8).
Significantly, the major isomers 2b and 3b have the
same configuration. Both compounds are alkyl-alky-
lidene Ta(V) complexes, with the side-arm donor at-
tached to the metal, thus completing its octahedral
geometry. The benzyl ligand is located trans to the side-
arm donor, and the benzylidene ligand is trans to the
central nitrogen. Except for the attachment of the donor,
the coordination mode of the amine bis(phenolate)
ligand core ([ONO]) remained unchanged. The tantalum-
to-carbon single- and double-bond lengths differ mark-
edly, being 2.246(8) Å vs 1.968(10) Å for 2b and 2.256(3)
Å vs 1.992(4) Å for 3b. Interestingly, the side-arm
nitrogen is bound more tightly to the metal than the
central nitrogen (2b, 2.402(7) Å vs 2.567(10) Å; 3b,
2.385(2) Å vs 2.497(3) Å, respectively). This may be
attributed to a stronger trans effect of an alkylidene
relative to an alkyl ligand.14 The relatively small distor-
tion of the benzylidene ligand (TadC-C bond angle of
140.7(7)° for 2b and 143.0(3)° for 3b) is consistent with
F igu r e 5. Decomposition of 3a .
1
the relatively large value of 106 Hz for J C-H (measured
for 3b) and denotes a weak R-agostic interaction.1
The orientation of the benzylidene ligand deserves
some attention (see Figures 7 and 8). The C(8)-C(6)-
Ta unit of the benzylidene ligand in both structures lies
in the plane defined by the two phenolic oxygen donors
and the central nitrogen donor, with a slight “out of
plane” distortion for 3b. The preference for this orienta-
tion may be attributed to competition with the phenolate
oxygen lone pairs for empty Ta(dπ) orbitals to which
electron donation can take place: by turning “sideways”,
the alkylidene carbon p orbital may π-interact with a
specific Ta d orbital that cannot interact with the oxygen
lone pairs due to symmetry considerations, as shown
in Figure 9. The slight “out of plane” distortion of the
alkylidene ligand in 3b (see top view in Figure 8) may
result from steric interaction with the ortho-positioned
chloro group, which is consistent with a slight elongation
of the TadC bond in 3b.
F igu r e 6. Formation of the two benzylidene isomers (3b
and 3c) upon 3a decomposition.
version between the 2b-2c and 3b-3c isomers upon
heating: that is, they are stable on the laboratory time
scale.
To obtain further insight into the reaction mecha-
nism, we followed the decomposition of 3a at 54(1) °C
1
by means of H NMR spectroscopy, in the presence of
an internal standard compound (Me3Si-O-SiMe3). The
reaction (3a f 3b + 3c + toluene) follows a first-order
kinetics, as expected for the R-hydrogen abstraction
reaction (see Figure 5 for details).1-3 The observed rate
of decomposition of 3a , 1.6 × 10-4 s-1, is of the same
order of magnitude as the rate constant reported for the
decomposition of pentabenzyltantalum (0.4 × 10-4 s-1
at 40 °C).2b Significantly, the total concentration of the
reactant and both products is equal to the initial
concentration of 3a throughout the reaction, implying
that no other organometallic product is formed except
for the two alkyl-alkylidene complexes. The reaction
clearly obeys a C r A f B reaction sequence (Figure
6), giving further evidence that the isomers do not
interconvert under the reaction conditions employed and
yielding partial rate constants of k1 ) 0.9 × 10-4 s-1
and k2 ) 0.6 × 10-4 s-1, for 3b and 3c, respectively.13
Similarly, the observed decomposition of 2a at 54(1) °C
is also first order in [Ta], with a rate constant of 1.3 ×
Role of th e Or th o Su bstitu en ts: C-H Activa tion
of th e Liga n d Ba ck bon e. Upon heating, the reaction
1
takes different paths for 4a and 5a . H and 13C NMR
spectra of the crude reaction mixtures revealed that only
traces of alkylidene products have formed. Instead, the
major product in both reactions (4b and 5b) was a
dibenzyl complex of low (C1) symmetry, as evident from
four different Ar-Me groups.
So far, our attempts to obtain pure 4b have failed;
however, the crude reaction mixtures containing 4b and
5b are similar. Complex 5b could be isolated in a pure
form by crystallization from cold ether as deep yellow
plates. The crystals obtained were subjected to X-ray
diffraction study, revealing an unusual heptacoordinate
complex of approximately pentagonal-bipyramidal ge-
ometry around the Ta center (see Figure 10). A benzyl
carbon on the ligand backbone has been activated,
10-4 s-1
.
The major isomer of each reaction mixture (2b and
3b) could be isolated by partial crystallization. The yield
for the isolation of pure 2b is low (22%), despite the
favorable product ratio, perhaps due to its high solubil-
ity in hydrocarbon solvents. Fortunately, pure 3b is
obtained in good yield upon crystallization from toluene
solution (several crops). 3b may be synthesized directly,
in a one-pot synthesis, from Lig3H2 and Ta(CH2Ph)5 (see
(14) (a) Abbhenuis, H. C. L.; van Belzen, R.; Grove, D. M.; Klomp,
A. J . A.; van Mier, G. P. M.; Spek, A. L.; van Koten, G. Organometallics
1993, 12, 210. (b) Rietveld, M. H. P.; Teunissen, W.; Hagen, H.; van
der Water, L.; Grove, D. M.; van der Schaaf, P. A.; Mu¨hlebach, A.;
Kooijtman, H.; Smeets, W. J . J .; Veldman, N.; Spek, A. L.; van Koten,
G. Organometallics 1997, 16, 1675. (c) Abbhenuis, H. C. L.; Rietveld,
M. H. P.; Haarman, H. F.; Hogerheide, M. P.; Spek, A. L.; van Koten,
G. Organometallics 1994, 13, 3259.
(13) Alberty, R. A.; Silbey, R. J . Physical Chemistry; Wiley: New
York, 1991.

Tantalum(V) Amine Bis(phenolate) Complexes
Organometallics, Vol. 23, No. 8, 2004 1887
F igu r e 7. ORTEP drawings of 2b (side view (left) and top view (right); 30% probability ellipsoids). Only one conformation
of the disordered benzyl ligand is shown. Hatoms are omitted for clarity. The structure of 2b was of somewhat low quality,
because the diffraction data of the partly twinned crystals could not be well resolved. This is mostly reflected in the elongated
ellipsoids of the phenolate rings. Selected bond distances (Å) and angles (deg): Ta1-O2 ) 1.883(7), Ta1-O3 ) 1.894(8),
Ta1-N4 ) 2.402(7), Ta1-N5 ) 2.567(10), Ta1-C6 ) 1.968(10), Ta1-C7 ) 2.246(8); C8-C6-Ta1 ) 140.7(7).
F igu r e 8. ORTEP drawing of 3b (side view (left) and top view (right); 50% probability ellipsoids). H atoms and toluene
solvent are omitted for clarity. Selected bond distances (Å) and angles (deg): Ta1-O2 ) 1.935(2), Ta1-O3 ) 1.924(2),
Ta1-N4 ) 2.385(2), Ta1-N5 ) 2.497(3), Ta1-C6 ) 2.256(3), Ta1-C7 ) 1.992(4); O3-Ta1-O2 ) 157.79(9), C6-Ta1-N4
) 164.31(10), C7-Ta1-N5 ) 167.73(11), C8-C7-Ta1 ) 143.0(3), C14-C6-Ta1 ) 113.1(2), C20-O2-Ta1 ) 145.4(2),
C37-O3-Ta1 ) 145.75(19).
(Ta1-C6 ) 2.251(5) Å). This ring resembles those
reported for the Ta complexes with arylamine ligands.14,15
Both phenolate oxygen donors occupy axial positions
(O2-Ta1-O3 ) 164.95(15)°). The Ta-OAr bond lengths
are longer in 5b (1.944(3) and 1.980(3) Å) than in the
tribenzyl (1.876(2)-1.916(2) Å) complexes and the ben-
zyl-benzylidene (1.883(7)-1.935(2) Å) complexes, and
the Ta-O-C bond angles are substantially narrower
(124.9(3) and 131.8(3)°) than in the tribenzyl complexes
(144.9(2)-148.8(2)°) and the benzyl-benzylidene com-
plexes (143.5(9)-145.8(2)°). This may result from a
reduced O(pπ)-Ta(dπ) electron donation in the hepta-
coordinate 5b.9a,14c,16,17 The Ta-C bond lengths and Ta-
C-C bond angles in 5b lie in the normal ranges of
2.199(5)-2.271(5) Å and 111.8(4)-119.5(4)°.
The unique decomposition pathway leading to 5b (and
presumably to 4b), competing with the “normal” R-hy-
drogen abstraction process, may be described as “â-
hydrogen abstraction”, given the N-to-Ta coordination
(see Figure 11). The closest distance between a benzyl
carbon and a neighboring “â-H” is ca. 2.9 Å, as revealed
F igu r e 9. Sideways bending of the benzylidene group
dictated by a specific pπ-dπ interaction.
leading to pentacoordinate binding of the amine bis-
(phenolate) ligand to the metal center. This is our first
encounter with such an activation of the amine bis-
(phenolate) ligand backbone, which thus far seemed to
be highly robust. According to the bond lengths and
angles, the formed metallazzacyclopropane ring contains
the neutral nitrogen σ-donor tightly bound to the metal
(Ta1-N5 ) 2.188(4) Å), along with the alkyl donor
(15) De Castro, I.; Galakhov, M. V.; Go´mez, M.; Go´mez-Sal, P.; Royo,
P. Organometallics 1996, 15, 1362.
(16) Goffindaffer, T. W.; Rothwell, I. P.; Huffman, J . C. Inorg. Chem.
1983, 22, 2906.
(17) Wolczanski, P. T. Polyhedron 1995, 14, 3335.

1888 Organometallics, Vol. 23, No. 8, 2004
Groysman et al.
F igu r e 12. Decomposition of 5a , which leads to the
formation of 5b and 5c (65 °C; 5c is represented by the
bottom dotted line).
F igu r e 10. ORTEP drawing of 5b (50% probability el-
lipsoids). H atoms and disordered solvent are omitted for
clarity. Selected bond distances (Å) and angles (deg): Ta1-
O2 ) 1.980(3), Ta1-O3 ) 1.944(3), Ta1-O4 ) 2.361(4),
Ta1-N5 ) 2.188(4), Ta1-C6 ) 2.251(5), Ta1-C7 ) 2.199-
(5), Ta1-C8 ) 2.271(5); O3-Ta1-O2 ) 164.95(15), C7-
Ta1-C6 ) 87.63(19), N5-Ta1-C6 ) 38.92(16), N5-Ta1-
O4 ) 70.89(14), C8-Ta1-O4 ) 82.69(16), C21-O2-Ta1
) 124.9(3), C36-O3-Ta1 ) 131.8(3).
F igu r e 13. First-order decomposition of 5a at 65, 71, and
76 °C.
species. Along with the starting complex 5a and the
major product 5b, an additional product, albeit in a low
concentration, was detected throughout the experiment
(Figure 12). According to the recognizable signal at 9.2
F igu r e 11. Possible decomposition routes for the tribenzyl
complexes 2a -7a .
1
ppm in the H NMR spectrum, this additional product
is assumed to be an alkyl-alkylidene complex (of the
2b and 3b type).
from the solid-state structure of 4a , and is only slightly
longer than the distance between the active groups
leading to the R-hydrogen abstraction process: i.e., a
benzyl carbon and a proton of an adjacent benzyl group
of ca. 2.5 Å. Therefore, a competition between these two
processes may take place. We propose that this competi-
tion is governed by the size of the ortho substituents
on the phenolate rings. Notably, certain [N3N]Ta-
(CH2R)2-type complexes were found to decompose
through a similar â-hydrogen abstraction in the tria-
mido-amine ligand backbone.18 However, such a path-
way in that system triggered a C-N bond scission in
the ligand backbone, leading to the formation of tetraa-
mido-monoalkyl derivatives, which contain the Ta-
NRCHdCH2 fragment. Thus, it seems that the use of
the aromatic rings as the “spacers” between donor atoms
increases the C-N bond stability and leads to the
formation of a metallaazacyclopropane ring, instead.
We followed the decomposition of 5a by 1H NMR
spectroscopy. Since 5a did not exhibit any significant
signs of decomposition at 54 °C for 1 h, the NMR
experiments were performed at higher temperatures
(65(1), 71(1), and 76(1) °C) in toluene-d8. At all three
temperatures, the reaction mixture during the experi-
ment consisted of at least three distinct organometallic
The decompositions at all the temperatures occurred
with first-order kinetics, having rate constants of 1.1 ×
10-4, 2.0 × 10-4, and 4.5 × 10-4 s-1 at 65(1), 71(1), and
76(1) °C, respectively (Figure 13). The low concentration
of the presumed benzyl-benzylidene complex 5c, along
with the apparent formation of traces of additional
products, does not allow a more detailed kinetic study.
The preference for the â-H abstraction pathway,
derived from the steric bulk of an ortho substituent, is
even more pronounced in the reaction of Lig8H2 with
Ta(CH2Ph)5. As mentioned above, Lig1H2, the amine bis-
(phenolate) ligand precursor, bearing ortho t-Bu groups
on both phenolate rings, does not react with Ta(CH2-
Ph)5 at room temperature. The reaction of the “half-
bulky” Lig8H2 with Ta(CH2Ph)5, on the other hand, was
found to proceed smoothly, leading to a single product.
However, the reaction does not yield the expected
tribenzylamine bis(phenolate) Ta(V) complex 8a . Proton
and carbon spectra of the product suggest an asym-
metric dibenzyl product, possessing no alkylidene sig-
nals. The DEPT-135 spectrum displays five different
methylene signals, along with a single methine signal,
characteristic of the C-H activated product (110 ppm
for 8b, 106 ppm for 5b). The final confirmation of the
nature of 8b was obtained by means of X-ray crystal-
lography (Figure 14). As a whole, the molecular struc-
ture of 8b resembles the structure of 5b, including the
(18) (a) Freundlich, J . S.; Schrock, R. R.; Davis, W. M. J . Am. Chem.
Soc. 1996, 110, 4964. (b) Freundlich, J . S.; Schrock, R. R.; Davis, W.
M. Organometallics 1996, 15, 2777.

Tantalum(V) Amine Bis(phenolate) Complexes
Organometallics, Vol. 23, No. 8, 2004 1889
Sch em e 3. P r op osed Str u ctu r es of 6b a n d 7b
reaction is significantly slower for this type of com-
plexes. Keeping in mind the almost instantaneous
decomposition of 2a -5a at 90 °C, we were surprised to
find that ca. 25% of 7a had not reacted even after 4.5 h
at that temperature! The 1H NMR monitored decom-
position of 6a at 76(1) °C yielded a rate constant of 1.7
× 10-4 s-1, which is significantly lower than the
decomposition rate constant of 5a measured at the same
temperature (4.5 × 10-4 s-1). Complex 7a did not exhibit
any appreciable decomposition at 76(1) °C for ca. 1 h.
F igu r e 14. ORTEP drawing of 8b (50% probability el-
lipsoids). H atoms and the pentane molecule are omitted
for clarity. The elongated ellipsoids of one of the phenyl
rings reflect the conformational disorder. Selected bond
distances (Å) and angles (deg): Ta1-O2 ) 1.977(3), Ta1-
O3 ) 1.928(3), Ta1-N4 ) 2.463(4), Ta1-N5 ) 2.205(3),
Ta1-C6 ) 2.246(4), Ta1-C7 ) 2.288(4), Ta1-C8 )
Relatively pure 6b is obtained in moderate yield,
albeit after a somewhat tedious workup procedure (see
Experimental Section); pure 7b is obtained upon re-
crystallization from cold ether (as yellow microcrystals).
1H NMR spectra indicate C1-symmetrical complexes,
possessing three different AB systems (integrated each
to two protons), one singlet (integrated to a single
proton), and five different Me groups (in 6b). The 13C
NMR (DEPT) spectra feature a single nonaromatic C-H
resonance (80 ppm for 6b, 77 ppm for 7b). Again, the
low symmetry is confirmed by five different CH2 peaks
for both complexes, along with five CH3 signals for 6b
(a single CH3 signal for 7b). On the basis of the
structures of 5b and 8b, we propose that 6b and 7b are
C-H activated hexacoordinate complexes.
2.254(4); O3-Ta1-O2 ) 163.68(11), N5-Ta1-C6
)
38.71(12), C6-Ta1-C7 ) 165.47(15), N5-Ta1-N4 )
72.86(12).
orientation and bond lengths of the corresponding
benzyl groups and of the Ta-N-C metallacycle. As in
5b, the Ta-O bond lengths (1.928(3) and 1.977(3) Å)
and Ta-O-C bond angles (123.6(2) and 136.3(2)°) in
8b indicate a reduced O(pπ)-Ta(dπ) donation, likely due
to the existence of the additional donor in this hepta-
coordinate complex. Notably, the C-H activation takes
place at the more crowded (t-Bu carrying) ligand side.
The structure of 8b supports our proposal that the
reaction course is dictated, among other consideration,
by the size of the ortho substituents of the phenolate
rings. Whereas methyl groups prevent the formation of
an alkylidene function upon the thermal decomposition
of a tribenzyl precursor, an even stronger steric conges-
tion caused by a single t-butyl group leads directly to
the C-H activated product. The preference of “â-H
abstraction” pathway could be traced back to the
tendency of the benzylidene group to point toward the
ortho substituent, being hindered as the latter becomes
bulky.
Role of th e Sid e-Ar m Don or . Thus far, we observed
that the side-arm donor binds to the metal upon toluene
elimination. However, since that binding occurs in both
reaction pathways, i.e. R- or â-H abstraction, the role
of this “intramolecular” donor in promoting the forma-
tion of alkylidene complexes still has to be clarified.
Given the tendency of the tribenzyl Ta complexes of
amine bis(phenolate) ligands with “compact” ortho sub-
stituents (H in 2a , Cl in 3a ) to lead to benzylidene
complexes, the corresponding tribenzyl complexes of the
analogous ligands that lack an additional donor (6a and
7a , respectively) were investigated.
Although the â-H abstraction in these complexes is
evident, the exact location of the C-H activated carbon
needs confirmation. The Ph-CH2-N site seems to be
the most appropriate. However, the NCH2(CH2) position
cannot be totally excluded, according to the proton
spectrum of 6b and 7b. The specific location of the
activated C-H site was clarified by means of 2D NMR
spectroscopy. Since the spectra of 6b and 7b seem very
much alike, 2D experiments were performed for 6b only.
3J H-H correlation spectroscopy (COSY) unequivocally
demonstrated a full correlation pattern for NCH2CH2-
CH3, thereby verifying that the propyl unit remains
intact. The other correlations detected in the aliphatic
region are between diastereotopic CH2 protons that
appear as three AB patterns. These methylene protons
belong to two benzyl ligands and a single PhCH2N unit.
As expected, no correlation was detected for the CH
resonance. These results clearly support a “regular” â-H
abstraction pathway, which takes place at one of the
PhCH2N units. An additional support of the proposed
1
structure of 6b and 7b was obtained by a 2D J C-H
correlation experiment (HMQC technique), showing a
correlation between the proton singlet at 2.78 ppm and
the methine carbon at 80 ppm. Thus, one can safely
conclude that the tribenzyl complexes lacking the “in-
tramolecular donor” (6a and 7a ) tend to decompose via
the â-H abstraction route, as depicted in Scheme 3.
When subjected to thermolysis, 6a and 7a undergo
toluene elimination reactions, affording a single dibenzyl
product for each reactant (6b and 7b, respectively).
According to the 1H NMR spectra, no benzylidene
complex is detected upon decomposition of 7a and only
traces are observed on decomposition of 6a . Notably, the

1890 Organometallics, Vol. 23, No. 8, 2004
Groysman et al.
Con clu sion s
steric hindrance. Possible explanations for this depen-
dence may be either of a kinetic source, e.g. a require-
ment of side-arm coordination prior to R-hydrogen
abstraction, or a thermodynamic source, e.g. the insta-
bility of a five-coordinate benzyl-benzylidene tantalum
complex of this ligand system. We are currently inves-
tigating the reactivity of these types of complexes.
Our findings indicate that tribenzyltantalum com-
plexes of the chelating amine bis(phenolate) ligands may
undergo two well-defined toluene elimination reactions.
The preference for either of these two reaction paths is
dictated by the steric influence of the phenolate sub-
stituents and the presence of an extra donor on the side
arm. As the benzylidene group prefers to point at a
specific direction, it should come as no surprise that,
by hindering this specific direction via bulky ortho
substituents on the phenolate rings, the R-hydrogen
abstraction pathway is blocked and replaced by the
competitive â-hydrogen abstraction pathway.
Ack n ow led gm en t. This research was supported by
the Ministry of Science, Culture and Sport and by the
Israel Science Foundation, founded by the Israel Acad-
emy of Sciences and Humanities. S.G. thanks Sheba D.
Bergman for technical assistance.
The source of the side-arm donor dependence of the
R-hydrogen abstraction pathway is less obvious. The
trialkyl complexes of amine bis(phenolate) ligands lack-
ing the side-arm donor are substantially more stable,
and when they eventually do react, they choose the
â-hydrogen abstraction path, even in the absence of
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data as CIF files for 3a , 6a , 2b, 3b, 5b, and 8b. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM0499761