Crystal structure of TaCl(NMe2)4 and its reactions with lithium amides and water. Indirect observation of an equilibrium among TaCl(NMe2)4 Ta(NMe2)5 and ta 2(μcl)2(nme

Article abstract of DOI:10.1021/om800362t

Reactions of TaCl(NMe₂)₄ (2) with LiNR₂ (R = SiMe₃, Et), yielding Ta(NMe₂)₄(NR ₂) (R = SiMe₃, 4; Et, 7), Ta(NMe₂)₅ (1), (Me₂N)₃

Full text of DOI:10.1021/om800362t

Organometallics 2009, 28, 167–171
167
Crystal Structure of TaCl(NMe2)4 and Its Reactions with Lithium
Amides and Water. Indirect Observation of an Equilibrium among
TaCl(NMe2)4, Ta(NMe2)5 and Ta2(µ-Cl)2(NMe2)6Cl2
Shu-Jian Chen,† Hu Cai,†,‡ and Zi-Ling Xue*,†
Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600, and Department of
Chemistry, Nanchang UniVersity, Nanchang 336000, China
ReceiVed April 24, 2008
Reactions of TaCl(NMe2)4 (2) with LiNR2 (R ) SiMe3, Et), yielding Ta(NMe2)4(NR2) (R ) SiMe3, 4;
Et, 7), Ta(NMe2)5 (1), (Me2N)3TaN(SiMe3)SiMe2CH2 (5), and Ta(NMe2)3(NEt2)2 (8), respectively, suggest
the presence of an equilibrium among TaCl(NMe2)4 (2), Ta(NMe2)5 (1), and Ta2(µ-Cl)2(NMe2)6Cl2 (3):
2 2 h 1 + 0.5 3. New products Ta(NMe2)4(NR2) have been characterized. Ta(NMe2)4(NEt2) (7), Ta(NMe2)5
(1), and Ta(NMe2)3(NEt2)2 (8) are in a slow exchange: 2 7 h 1 + 8 with an equilibrium constant of Keq
) 0.25(0.01) at 90 °C. The following are also reported: X-ray crystal structure and a one-step synthesis
of TaCl(NMe2)4 (2), reaction of 2 with H2O giving Ta2(µ-Cl)2(µ-O)(NMe2)6 (9), X-ray structure of 9,
and preparation of LiNEt2 solid.
Chisholm and co-workers reported the crystal structure of
TaCl2(NMe2)3 as a dimer Ta2(µ-Cl)2(NMe2)6Cl2 (3).7 It is not
clear if TaCl(NMe2)4 (2) is a monomer or dimer. In our current
Introduction
Early transition metal amides are of intense interest as
precursors to microelectronic materials.1 They have been used
in chemical vapor deposition (CVD) processes to make metal
oxide thin films as gate insulators1a-d and metal nitride
(M-Si-N ternary) films as diffusion barriers.1e-g Ta(NMe2)5
(1), a CVD precursor to Ta2O5 films,2 was first reported by
Bradley in 1962. 3 TaCl(NMe2)4 (2), a precursor to Ta(NMe2)5
(1), was later prepared by Chisholm and co-workers using a
metathesis reaction between Ta(NMe2)5 (1) and Ta2(µ-
Cl)2(NMe2)6Cl2 (3).4 Monochloride TaCl(NMe2)4 (2) is a
precursor to amide, alkyl,4 silyl,5 guanidinate,6 and aminoxy3
derivatives. Such derivatives are often precursors to microelec-
tronic materials or intermediates in the formation of these
materials.3 The crystal structure of 2 remains unknown in large
part because of its high solubility in many organic solvents.
3,5,8
studies of the reactions of metal amides complexes with O2
and silanes,1g,9 we have prepared 2 through reaction of TaCl5
with 4 equiv of LiNMe2 and obtained crystals of 2 through
sublimation at 85 °C. Its X-ray crystal structure revealed that it
is a monomer in pseudo trigonal bipyramidal geometry with an
axial Cl ligand. The reaction of TaCl(NMe2)4 (2) with LiN-
(SiMe3)2 yields Ta(NMe2)4[N(SiMe3)2] (4) as well as Ta(NMe2)5
(1) and (Me2N)3TaN(SiMe3)SiMe2CH2 (5),10 suggesting an
equilibrium among 2, 1 and 3 (Scheme 1). 5 has been prepared
from Ta2(µ-Cl)2(NMe2)6Cl2 (3) and LiN(SiMe3)2.10 Additional
studies revealed that the reaction of TaCl(NMe2)4 (2) with
LiNEt2 gave Ta(NMe2)4(NEt2) (7), Ta(NMe2)5 (1) and
Ta(NMe2)3(NEt2)2 (8). 7, 1, and 8 are involved in a slow
exchange (Scheme 2). Our studies of 2 and its reactivities are
reported here.
* To whom correspondence should be addressed. E-mail:
xue@ion.chem.utk.edu.
† University of Tennessee.
‡ Nanchang University.
Results and Discussion
(1) (a) Wallace, R. M.; Wilk, G. D. Crit. ReV. Solid State Mater. Sci.
2003, 28, 231. (b) Smith, R. C.; Ma, T.; Hoilien, N.; Tsung, L. Y.; Bevan,
M. J.; Colombo, L.; Roberts, J.; Campbell, S. A.; Gladfelter, W. L. AdV.
Mater. Opt. Electron. 2000, 10, 105. (c) Hendrix, B. C.; Borovik, A. S.;
Xu, C.; Roeder, J. F.; Baum, T. H.; Bevan, M. J.; Visokay, M. R.; Chambers,
J. J.; Rotondaro, A. L. P.; Bu, H.; Colombo, L. Appl. Phys. Lett. 2002, 80,
2362. (d) Son, K.-A.; Mao, A. Y.; Sun, Y.-M.; Kim, B. Y.; Liu, F.; Kamath,
A.; White, J. M.; Kwong, D. L.; Roberts, D. A.; Vrtis, R. N. Appl. Phys.
Lett. 1998, 72, 1187. (e) Raaijmakers, I. J. Thin Solid Films 1994, 247, 85.
(f) Custer, J. S.; Smith, P. M.; Fleming, J. G.; Roherty-Osmun, E. Inorganic
Materials Synthesis; Winter, C. H., Hoffman, D. M., Eds.; ACS Symposium
Series; American Chemical Society: Washington, D.C., 1999; Vol. 727, p
86. (g) Liu, X.-Z.; Wu, Z.-Z.; Cai, H.; Yang, Y.-H.; Chen, T.-N.; Vallet,
C. E.; Zuhr, R. A.; Beach, D. B.; Peng, Z.-H.; Wu, Y.-D.; Concolino, T. E.;
Rheingold, A. L.; Xue, Z.-L. J. Am. Chem. Soc. 2001, 123, 8011.
(2) Chen, S.-J.; Zhang, X.-H.; Yu, X.; Qiu, H.; Yap, G. P. A.; Guzei,
I. A.; Lin, Z.; Wu, Y.-D.; Xue, Z.-L. J. Am. Chem. Soc. 2007, 129, 14408.
(3) Bradley, D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 1355.
(4) Chisholm, M. H.; Tan, L.-S.; Huffman, J. C. J. Am. Chem. Soc.
1982, 104, 4879.
TaCl(NMe2)4 (2) had been prepared earlier from TaCl5 in
three steps.4 In the current work, it was prepared in one step in
66% yield from the reaction of TaCl5 with 4 equiv of LiNMe2.
Our earlier, repeated attempts to grow crystals of 2 from its
solution failed, either because of its high solubility in most
solvents or because of the noncyrstalline nature of its solid
obtained from the solvents. Pale-yellow crystals of 2 were later
obtained on a coldfinger during the sublimation of 2 at 85 °C
from the reaction between TaCl5 and 4 equiv of LiNMe2. The
(7) Chisholm, M. H.; Huffman, J. C.; Tan, L.-S. Inorg. Chem. 1981,
20, 1859.
(8) Wang, R.-T.; Zhang, X.-H.; Chen, S.-J.; Yu, X.-H.; Wang, C.-S.;
Beach, D. B.; Wu, Y.-D.; Xue, Z.-L. J. Am. Chem. Soc. 2005, 127, 5204.
(9) (a) Liu, X.-Z.; Wu, Z.-Z.; Peng, Z.-H.; Wu, Y.-D.; Xue, Z.-L. J. Am.
Chem. Soc. 1999, 121, 5350. (b) Cai, H.; Chen, T.-N.; Wang, X.-P.; Schultz,
A. J.; Koetzle, T. F.; Xue, Z.-L. Chem. Comm. 2002, 230.
(10) Cai, H.; Yu, X.; Chen, T.; Chen, X.-T.; You, X.-Z.; Xue, Z. Can.
J. Chem. 2003, 81, 1398.
(5) Wu, Z.; Cai, H.; Yu, X.; Blanton, J. R.; Diminnie, J. B.; Pan, H.-J.;
Xue, Z. Organometallics 2002, 21, 3973.
(6) Tin, M. K. T.; Thirupathi, N.; Yap, G. P. A.; Richeson, D. S. J. Chem.
Soc., Dalton Trans. 1999, 2947.
10.1021/om800362t CCC: $40.75
2009 American Chemical Society
Publication on Web 11/20/2008

168 Organometallics, Vol. 28, No. 1, 2009
Chen et al.
Scheme 1. Equilibria among 2, 1, 3 (Solution), and 3 (Solid) as Well as Reaction of 2 with LiN(SiMe3)2 To Give 4, 1, and 5
Scheme 2
longer than those of the axial Ta-Cl bond [2.454(4) Å] in
(Me2N)3Ta(SiPh2But)Cl5 and the terminal Ta-Cl bond in 3
[2.463(1) Å].7 In 2, the axial Ta(1)-N(4) bond [2.008(4) Å] is
longer than equatorial Ta-N bonds [1.974(4)-1.988(3) Å].
Such a difference was also observed in the structures of
(Me2N)3Ta(SiPh2But)Cl
and
(Me2N)3Ta[Si(SiMe3)3]Cl/
(Me2N)3Ta[Si(SiMe3)3]Br.5 These Ta-N bond lengths are
similar to those [1.965(5)-2.038(8) Å] in Ta(NMe2)5 (1).11 The
Cl(1)-Ta(1)-N(4) angle of 178.85(11)° shows that the two
axial ligands are nearly linear. The angles of the axial chloride
to equatorial N atoms are 87.06(11)-88.58(11)°, smaller than
those of the axial N(4) to the N atoms [90.38(16)-93.87(15)°],
probably as a result of the repulsion by bulkier axial -NMe2
ligand.
crystals were carefully removed from the coldfinger, and one
was cut into a suitable size for X-ray diffraction. The crystals
of 2 were very sensitive to air, decomposing readily in paratone
oil which was used to coat crystals for protection.
Direct reaction of Ta(NMe2)5 with Me3SiCl gave Ta2(µ-
Cl)2(NMe2)6Cl2 (3), rather than TaCl(NMe2)4 (1), because of
the facile ligand redistribution and the lower solubility of 3.7
Our studies further reveal that there is an equilibrium among
TaCl(NMe2)4 (2), Ta(NMe2)5 (1), and Ta2(µ-Cl)2(NMe2)6Cl2 (3)
in toluene-d8. In 1H NMR at 23 °C, the equilibrium 2 2 h 1 +
0.5 3 is not directly observed.12 Although 3 is not observed, its
presence is inferred from the formation of 5 (Scheme 1) and
HN(SiMe3)2, when LiN(SiMe3)2 is added to 2. Reaction of 3
with LiN(SiMe3)2 is known to give 5 and HN(SiMe3)2, with
Ta(NMe2)3[N(SiMe3)2]Cl as an intermediate.10 Other products
from the reaction between TaCl(NMe2)4 (2) and LiN(SiMe3)2
at 23 °C are Ta(NMe2)4[N(SiMe3)2] (4), Ta(NMe2)5 (1) with a
molar ratio of 4: 1: 5: HN(SiMe3)2 ≈ 5: 1: 1: 1.
Crystal structure of Ta2(µ-Cl)2(NMe2)6Cl2 (3),7 reported by
Chrisholm and co-workers, displays a dimer, where two
chlorides bridge two Ta atoms to give a four-member ring with
two other chlorides and all amides as terminal ligands. In
comparison, crystal structure of 2 reveals a monomer (Figure
1) as a distorted trigonal bipyramid. This suggests monomer is
more thermally and sterically stable for the monochloride 2. It
should be noted that Ta(NMe2)5 (1) and Ta(NEt2)5 adopt a
distorted square pyramidal and trigonal bipyramidal structure,
respectively.11 The chloride in TaCl(NMe2)4 (2) takes the axial
position with a Ta-Cl bond length of 2.5077(13) Å, slightly
The observation that significant amounts of both 1 and 5 are
yielded in the reaction of 2 with LiN(SiMe3)2 suggests that the
reaction of 3 with LiN(SiMe3)2 is faster than that of 2 with
LiN(SiMe3)2, thus shifting the equilibrium to the formation of
1 and 3. In other words, although the concentration of 3 is below
the 1H NMR detection limit, its fast reaction with LiN(SiMe3)2
eventually increases concentrations of 1, 5 and HN(SiMe3)2 to
20% of that of 4 at the end of the reaction. The indirect
observation of the equilibrium 2 2 h 1 + 0.5 3 here is unusual.
(11) (a) Ta(NMe2)5 (1): Batsanov, A. S.; Churakov, A. V.; Howard,
J. A. K.; Hughes, A. K.; Johnson, A. L.; Kingsley, A. J.; Neretin, I. S.;
Wade, K. J. Chem. Soc., Dalton Trans. 1999, 3867. (b) TaNEt2)5: Davies,
H. O.; Jones, A. C.; McKinnell, E. A.; Raftery, J.; Muryn, C. A.; Afzaalb,
M.; O’Brien, P. J. Mater. Chem. 2006, 16, 2226.
Figure 1. Crystal structure of TaCl(NMe2)4 (2). Selected bond
lengths (Å) and angles (deg): Ta(1)-N(1) 1.951(4), Ta(1)-N(2)
1.974(4), Ta(1)-N(3) 1.988(3), Ta(1)-N(4) 2.008(4), Ta(1)-Cl(1)
2.5077(13), N(1)-Ta(1)-N(2) 122.05(17), N(1)-Ta(1)-N(3)
118.09(17), N(2)-Ta(1)-N(3) 119.46(14), N(1)-Ta(1)-N(4)
90.38(16),N(2)-Ta(1)-N(4)92.14(18),N(3)-Ta(1)-N(4)93.87(15),
N(1)-Ta(1)-Cl(1) 88.58(11), N(2)-Ta(1)-Cl(1) 87.99(13),
N(3)-Ta(1)-Cl(1) 87.06(11), N(4)-Ta(1)-Cl(1) 178.85(11).
(12) (a) Both 1H and 13C -NMe2 peaks of Ta(NMe2)5 (1) and
TaCl(NMe2)4 (2) overlap in toluene-d8. The presence of 1 in the equilibrium
mixture could not be independently confirmed. (b) A small amount of Ta2(µ-
Cl)2(µ-O)(NMe2)6 (9) is often present in the equilibrium mixture, and its
-NMe2 peak in 1H NMR at 3.57 ppm is close to that of Ta2(µ-Cl)2(NMe2)6Cl2
(3) at 3.56 ppm in toluene-d8.

Crystal Structure of TaCl(NMe2)4
Organometallics, Vol. 28, No. 1, 2009 169
Reverse reaction in the equilibrium 2 2 h 1 + 0.5 3 (Scheme
1) was also observed. Mixing Ta(NMe2)5 (1) and 0.5 equiv of
Ta2(µ-Cl)2(NMe2)6Cl2 (3) in toluene-d8 at 23 °C led to the
formation of 2, although the low solubility (1.5 mg/mL estimated
by 1H NMR) and slow dissolvation of 3 in toluene-d8 [3
(solution) h 3 (solid), Scheme 1] at 23 °C make the formation
of 2 slow. Heating the solution to 80 °C increases the solubility
of 3 and makes the formation of TaCl(NMe2)4 (2) faster. After
the formation of 2, the solution showed no detectable amount
of Ta2(µ-Cl)2(NMe2)6Cl2 (3) by 1H NMR,12 indicating the
equilibrium 2 2 h 1 + 0.5 3 (Scheme 1) is shifted exclusively
to the left at 23 °C.
words, the exchange of the starting materials 2 2 h 1 + 0.5 3
led to the observation of the three products 7, 1 and 8 in this
reaction.
The new complex Ta(NMe2)4[N(SiMe3)2] (4) was character-
1
ized by H and 13C NMR and elemental analysis. Repeated
attempts to obtain the crystal structure of 4 were not successful.
Crystal-like solids gave poor reflections on an X-ray diffrac-
tometer.
Ta(NMe2)4[N(SiMe3)2] (4) does not convert to 1 and
(Me2N)3TaN(SiMe3)SiMe2CH2 (5) in toluene-d8 at 23 °C for a
few days. Heating the solution at 86 °C for 24 h led to the
decomposition of 4 to unknown species. The result rules out a
pathway in which 4 is yielded initially in the reaction of
TaCl(NMe2)4 (2) with LiN(SiMe3)2, and then it converts to 1
and 5.
Complex TaCl(NMe2)4 (2) in THF was found to react with
Reactions of TaCl(NMe2)4 (2) with LiN(SiMe3)2 was found
to be complete in 4 h at 10 °C and in 2.5 h at 15 °C, respectively.
Slow formation of 1 and 5 was observed in the process. Ta2(µ-
Cl)2(NMe2)6Cl2 (3) was not, however, detected during monitor-
ing of the reaction, suggesting that the equilibrium 2 2 h 1 +
0.5 3 (Scheme 1) is shifted exclusively to the left. Reaction of
3 with LiN(SiMe3)2 further decreases the likeliness of the
observation of 3 in the reaction.
Figure 2. Crystal structure of Ta2(µ-Cl)2(µ-O)(NMe2)6 (9). Selected
bond lengths (Å) and angles (deg): Cl(1)-Ta(1) 2.7571(15),
Cl(1)-Ta(2) 2.8214(15), Cl(2)-Ta(2) 2.7128(17), Cl(2)-Ta(1)
2.7618(16), N(1)-Ta(1) 1.965(7), N(2)-Ta(1) 1.995(7), N(3)-Ta(1)
1.943(7), N(4)-Ta(2) 1.951(7), N(5)-Ta(2) 1.993(7), N(6)-Ta(2)
1.961(7), O-Ta(1) 1.931(5), O-Ta(2) 1.936(5), Ta(1)-Cl(1)-Ta(2)
73.92(4), Ta(2)-Cl(2)-Ta(1) 75.57(4), Ta(1)-O-Ta(2) 120.3(3),
O-Ta(1)-Cl(1) 74.04(16), O-Ta(1)-Cl(2) 72.96(16).
Reaction of TaCl(NMe2)4 (2) with 1 equiv of LiNEt2 was
also investigated. This reaction at 23 °C was found to yield
Ta(NMe2)4(NEt2) (7) as well as Ta(NMe2)3(NEt2)2 (8) and
Ta(NMe2)5 (1) in a molar ratio of about 2.8:1:1 (Scheme 2).
Ta(NMe2)3(NEt2)2 (8) has been reported earlier in a patent by
Chen and co-workers.13 Ta(NMe2)4(NEt2) (7) is a new product,
1
and it was characterized by H and 13C NMR and elemental
H2O (0.5 equiv in THF), yielding Ta2(µ-Cl)2(µ-O)(NMe2)6 (9)
and HNMe2 (eq 1). One -NMe2 ligand in 2 was removed as
HNMe2 in the reaction, followed by dimerization to give 9. The
X-ray structure of 9 in Figure 2 reveals that, of the two bridging
Ta-Cl-Ta bonds, Cl(1)-Ta(1) bond length [2.7571(15) Å] is
smaller than that of the Cl(1)-Ta(2) bond [2.8214(15) Å].
However,bothbridgingTa-Clbondlengths[2.7128(17)-2.8214(15)
Å] in 9 are longer than that of the terminal Ta-Cl bond
[2.5077(13) Å] in 2. The difference here is similar to those in
Ta2(µ-Cl)2(NMe2)6Cl2 (3)7 and Cl(Me2N)2Zr(µ-Cl)2(µ-
NMe2)Zr(NMe2)2(THF).14 In 3, the bridging Ta-Cl bonds
[2.586(1) and 2.635(2) Å] are longer than that of the terminal
Ta-Cl bond [2.463(1) Å].7 The bridging Zr-Cl bond lengths
of 2.6130(12)-2.7214(12) Å in the Zr complex14 are also longer
than that of the terminal Zr-Cl bond [2.4409(12) Å]. The
Ta(1)-O-Ta(2) bond angle [120.3(3)°] is much larger than two
Ta(1)-Cl-Ta(2) bond angles [73.92(4) and 75.57(4)°] in 9.
analysis. 7 is a pale-yellow solid at 23 °C. One set of peaks
were observed for both -NMe2 and -NEt2 ligands in 1H and 13
C
NMR spectra. The structure of 7 is not clear. If it is trigonal
bipyramidal, the NMR observations suggest there is perhaps a
fast exchange of the axial and equatorial ligands in 7.
We also found that there is an exchange among 7, 1 and 8:
2 7 h 1 + 8 (Scheme 2).
Two tests were conducted. In the first test, 1 was added to a
mixture of 7 and 8 in toluene-d8 at 23 °C, and peaks of 7 slowly
grew up while peaks of 1 and 8 dropped. In the absence of 1,
no change was observed of a mixture of 7 and 8 in toluene-d8
at 23 °C for 9 days. In a second test, a solution of
Ta(NMe2)4(NEt2) (7) in toluene-d8 was monitored and found
to yield Ta(NMe2)3(NEt2)2 (8) and Ta(NMe2)5 (1). The exchange
2 7 h 1 + 8 (Scheme 2) at 90 °C is faster, but it still took 4-5
days to reach an equilibrium with Keq ) 0.25(0.01) at 90 °C.
Given that the exchange of the products 2 7 h 1 + 8 (Scheme
2) is very slow, it is unlikely that this exchange would change
the ratio of the three products (7: 1: 8 ) 2.8: 1: 1) from the
reaction of TaCl(NMe2)4 (2) with 1 equiv of LiNEt2. In other
(13) Chen, T.; Xu, C.; Baum, T. H. U.S. Patent 2005079290,2005,
“Tantalum amide complexes for depositing tantalum-containing films, and
method of making same.” Preparation, NMR chemical shifts, and elemental
analysis of Ta(NMe2)3(NEt2)2 (8) were given.
(14) Wu, Z.-Z.; Diminnie, J. B.; Xue, Z.-L. Inorg. Chem. 1998, 37, 2570.
(15) (a) Sheldrick, G. M. SADABS, A Program for Empirical Absorption
Correction of Area Detector Data; University of Go¨ttingen: Go¨ttingen,
Germany, 2000. (b) Sheldrick, G. M. SHELXL-97, A Program for the
Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
Chisholm and co-workers have also reported a Ta dimeric
complex Ta2(µ-O)Cl2(NMe2)6(HNMe2)2 (10) containing a single
bridging oxo ligand.7 It is interesting to compare 9 with 10.
Complex 9 is a derivative of 10 without the HNMe2 ligand.

170 Organometallics, Vol. 28, No. 1, 2009
Chen et al.
A
pale-yellow solid (0.700 g, 1.630 mmol, 27.5%) of
The Ta atoms in 10 adopt an octahedron coordination sphere
using terminal chloride and HNMe2 as ligands. Loss of lone
pair electrons in HNMe2 in 9 leads to the formation of the two
bridging chloride ligands so that their lone pair electrons could
be used to remediate electron deficiency in 9. The Ta-O bond
length of 1.928(6) Å in 10 is nearly identical to those [1.931(5)
and 1.936(5) Å] in 9. The Ta-O-Ta bond [174.3(3)°] in 10 is
nearly linear, while that [120.3(3)°] in Ta2(µ-Cl)2(µ-O)(NMe2)6
(9) is, however, much smaller. The two Ta atoms in 9 are closer
[Ta(1)-to-Ta(2) distance: 3.3545 Å] than those in, e.g., Ta2(µ-
Cl)2(NMe2)6Cl2 (3) [Ta(1)-to-Ta(2) distance: 4.1 Å]7 probably
to accommodate one oxo and two bridging ligands. In addition
to the smaller Ta-O-Ta bond angle in 9 (than in 10), the
Ta-Cl-Ta [73.92(4) and 75.57(4)°] bond angles in 9 are also
much smaller than that [103.7(0)°] in Ta2(µ-Cl)2(NMe2)6Cl2 (3).7
The Ta-N bond lengths [1.943(7)-1.995(7) Å] are close to
those in, e.g., TaCl(NMe2)4 (2) [1.951(4) to 2.008(4) Å].
Ta(NMe2)4(NEt2) (7) was isolated in one day. 1H NMR (benzene-
d6, 399.79 MHz, 23 °C) δ 3.49 (q, 4H, NCH2CH3), 3.28 (s, 24H,
NCH3), 1.12 (t, 6H, NCH2CH3). 13C NMR (benzene-d6, 100.53
MHz, 23 °C) δ 47.57 (s, NCH2CH3), 46.60 (s, NCH3), 17.01 (s,
NCH2CH3). Anal. calcd for C12H34N5Ta: C, 33.57; H, 7.98. Found:
C 33.36, H 7.84.
Heating of Ta(NMe2)4[N(SiMe3)2] (4) at 86 °C. The solution
of Ta(NMe2)4[N(SiMe3)2] (4) (16.7 mg, 0.0323 mmol) and bibenzyl
(internal standard, 11.3 mg, 0.0620 mmol) in toluene (0.46 mL) in
a Young tube was heated at 86 °C for 50 h. 1H NMR spectrum of
the solution revealed that 4 had decomposed to unknown species.
Indirect Observation of the Equilibrium among
TaCl(NMe2)4 (2), Ta(NMe2)5 (1), and Ta2(µ-Cl)2(NMe2)6Cl2
(3). TaCl(NMe2)4 (2, 35.7 mg, 0.0914 mmol), LiN(SiMe3)2 (15.3
mg, 0.0914 mmol), and bibenzyl (internal standard, 15.9 mg, 0.0872
mmol) were dissolved in toluene-d8 in a Young tube. The reaction
yielded Ta(NMe2)4[N(SiMe3)2] (4, 0.0592 mmol, 65% yield),
Ta(NMe2)5 (1, 0.0123 mmol, 13% yield), 6 (0.0113 mmol, 12%
yield), and HN(SiMe3)2 (0.0115 mmol, 13% yield), suggesting that
there is an equilibrium as shown in Scheme 1.
Experimental Section
General Procedures. All manipulations were carried out under
a dry and oxygen-free nitrogen atmosphere with the use of glovebox
or Schlenk techniques. All solvents were purified by distillation
from potassium/benzophenone ketyl. Benzene-d6 and toluene-d8
were dried and stored over activated molecular sieves under
nitrogen. TaCl5 (Strem), LiNMe2 (Aldrich), and LiN(SiMe3)2
Reaction of TaCl(NMe2)4 (2) with 1 equiv of LiN(SiMe3)2
at 10 and 15 °C. Reaction at 10 °C. LiN(SiMe3)2 (12.7 mg, 0.0760
mmol) and bibenzyl (internal standard, 20.4 mg, 0.112 mmol) were
dissolved in toluene-d8 in a Young tube and the solution was cooled
to -78 °C. TaCl(NMe2)4 (2, 29.7 mg, 0.0760 mmol) was added at
1
-78 °C, and the solution was then warmed to 10 °C. H NMR
1
(Aldrich) were used as received. H and 13C NMR spectra were
was used to follow the reaction, and it revealed that the reaction
was completed in 4 h.
recorded on a Bruker AMX-400 Fourier transform spectrometer.
Elemental analysis was conducted by Complete Analysis Labora-
tories, Inc., Parsippany, NJ.
Synthesis of LiNEt2. HNEt2 was dried over NaOH pellets at 23
°C for 24 h and then distilled at 72 °C under nitrogen. HNEt2
(12.590 g, 0.172 mmol) in hexanes (30 mL) at 0 °C was treated
with 1 equiv of n-BuLi (0.173 mmol, 108.0 mL, 1.6 M in hexane)
over a period of 1 h. The mixture was stirred at 23 °C for additional
2.5 h. Volatiles were removed in vacuo to give white powders of
LiNEt2 (12.920 g, 0.163 mmol, 95% yield).
Reaction at 15 °C. LiN(SiMe3)2 (12.7 mg, 0.0760 mmol) and
bibenzyl (internal standard, 13.3 mg, 0.0729 mmol) were dissolved
in toluene-d8 in a Young tube and the solution was cooled to -78
°C. TaCl(NMe2)4 (2, 29.7 mg, 0.0760 mmol) was added at -78
°C, and the solution was then warmed to 15 °C. 1H NMR was used
to follow the reaction, and it revealed that the reaction was
completed in 2.5 h.
The reaction at both 10 and 15 °C gave the same products
Ta(NMe2)4[N(SiMe3)2] (4), Ta(NMe2)5 (1), 5, and HN(SiMe3)2 in
ratios of 4:1:1:1 and 3:1:1:1, respectively.
Synthesis of TaCl(NMe2)4 (2). Hexanes (100 mL) were added
to a solid mixture of TaCl5 (7.660 g, 21.38 mmol) and LiNMe2
(4.363 g, 85.52 mmol) at -40 °C with vigorous stirring. The
mixture was slowly warmed to 23 °C, and the solution was then
refluxed at 72 °C for 22 h. The deep brown solution was filtered,
and the residue was extracted with hexanes for 3 times (3 × 60
mL). Volatiles were removed from the filtrate in vacuo, and
subsequent sublimation at 85 °C yielded 5.567 g (14.18 mmol, 66%)
Reaction of TaCl(NMe2)4 (2) with 1 equiv of LiNEt2.
TaCl(NMe2)4 (2, 53.4 mg, 0.137 mmol) and LiNEt2 (10.8 mg, 0.137
mmol) were mixed in benzene-d6 (0.46 mL) in a Young tube and
the solution was shaken for 10 min. The reaction yielded
Ta(NMe2)4(NEt2) (7), Ta(NMe2)3(NEt2)2 (8), and Ta(NMe2)5 (1)
in a molar ratio 2.8:1:1.
1
of yellow crystalline TaCl(NMe2)4 (2). H NMR of 2(benzene-d6,
Exchange of Ta(NMe2)4(NEt2) (7), Ta(NMe2)3(NEt2)2 (8),
and Ta(NMe2)5 (1). Exchange at 23 °C. Ta(NMe2)5 (1, 22.3 mg,
0.0556 mmol) was added to a mixture of Ta(NMe2)4(NEt2) (7) and
Ta(NMe2)3(NEt2)2 (8, total 19.3 mg; 26 mol% 7 and 74 mol% 8)
and bibenzyl (internal standard, 10.6 mg, 0.0576 mmol) in toluene-
399.88 MHz, 23 °C) δ 3.24 (s, 24H, NMe2). 13C NMR (benzene-
d6, 100.55 MHz, 23 °C) δ 45.93 (s, NMe2). The crystals were used
directly in the subsequent X-ray diffraction to be discussed below.
Synthesis of Ta(NMe2)4[N(SiMe3)2] (4). Toluene (20 mL) was
added to a solid mixture of TaCl(NMe2)4 (2, 0.684 g, 1.74 mmol)
and LiN(SiMe3)2 (0.291 g, 1.74 mmol) at -78 °C with vigorous
stirring. The mixture was slowly warmed to 23 °C and stirred for
15 h. Volatiles were removed from the mixture in vacuo, and the
residue was extracted with n-pentane. Filtration and crystallization
yielded a pale-yellow solid of 4 (0.423 g, 0.817 mmol, 47% yield)
at -32 °C. 1H NMR of 4 (benzene-d6, 399.79 MHz, 23 °C) δ 3.26
(s, 24H, NMe2), 0.33 (s, 18H, SiMe3). 13C NMR (benzene-d6, 100.54
MHz, 23 °C) δ 48.28 (s, NMe2), 5.17 (s, SiMe3). Anal. calcd for
C14H42N5Si2Ta: C, 32.48; H, 8.18. Found: C, 32.41; H, 8.13.
Synthesis of Ta(NMe2)4(NEt2) (7). To TaCl(NMe2)4 (2, 2.330
g, 5.932 mmol) in n-pentane (15 mL) at 0 °C was added slowly 1
equiv of LiNEt2 (0.460 g, 5.932 mmol) in Et2O (15 mL). The
solution was then stirred at 0-13 °C for 2 h. The solution was
settled and filtered. Volatiles were removed from the filtrate in vacuo
and n-pentane was added to the residue. The solution was
concentrated and then put in a freezer (-32 °C) for crystallization.
1
d8 (0.46 mL). The solution was kept at 23 °C for over 160 h. H
NMR revealed that the peaks of 7 slowly grew up. A similar slow
exchange was observed with a solution of 1 (8.7 mg, 0.0218 mmol),
7 and 8 (total 18.8 mg; 29 mol% 7 and 71 mol% 8) and bibenzyl
(internal standard, 10.3 mg, 0.0565 mmol) in toluene-d8 (0.46 mL).
Exchange at 90 °C. A solution of 1 (9.1 mg, 0.0227 mmol), 7
and 8 (total 18.8 mg; 29 mol% 7 and 71 mol% 8) and bibenzyl
(internal standard, 10.0 mg, 0.0549 mmol) in toluene-d8 (0.46 mL)
was initially heated at 60 °C and then at 90 °C. The solution was
monitored by 1H NMR at 23 °C. The heating at 90 °C lasted about
4-5 days until no change in concentrations of the three complexes
was observed. Since the exchange at 23 °C was very slow and the
1
time needed for a H NMR spectrum was short (about 10 min),
cooling the solution to 23 °C for 10 min essentially “quenched”
the exchange. In a second test, a solution of Ta(NMe2)4(NEt2) (7,
12.3 mg, 0.0287 mmol) and bibenzyl (internal standard, 10.1 mg,
0.0554 mmol) in toluene-d8 was heated at 90 °C. The solution was

Crystal Structure of TaCl(NMe2)4
Organometallics, Vol. 28, No. 1, 2009 171
periodically cooled to 23 °C to take 1H NMR spectra. The heating
was found to yield Ta(NMe2)3(NEt2)2 (8) and Ta(NMe2)5 (1) over
a period of 4-5 days to reach an equilibrium.
When a mixture (19.9 mg total) of 7, 8 and bibenzyl (internal
standard, 10.5 mg, 0.0576 mmol) in toluene-d8 (0.46 mL) was kept
at 23 °C for 9 days, no change was observed.
detector fitted with an upgraded Nicolet LT-2 low temperature
device, and a graphite-monochromated Mo source (KR radiation,
0.71073 Å). Suitable crystals were coated with paratone oil (Exxon)
and mounted on loops under a stream of nitrogen at the data
collection temperature. The structures were solved by direct
methods. Non-hydrogen atoms were anisotropically refined. All
hydrogen atoms were treated as idealized contributions. Empirical
absorption correction was performed with SADABS.15a In addition
the global refinements for the unit cells and data reductions of the
two structures were performed using the Saint program (version
6.02). All calculations were performed using SHELXTL (version
5.1) proprietary software package.15b
Reactions of TaCl(NMe2)4 with H2O. Ta2(µ-Cl)2(µ-O)(NMe2)6
(9) has been isolated from the reaction of TaCl(NMe2)4 (2) with 1
equiv of H2O. To TaCl(NMe2)4 (2, 1.000 g, 2.546 mmol) in THF
(10 mL) with vigorous stirring at -30 °C was added slowly 1 equiv
of H2O (46.0 µL) in 5 mL of THF. After temperature was raised to
23 °C, solution was stirred for additional 1 h. Volatiles were
removed under reduced pressure and pale yellow residue was
extracted with hexanes. Filtrate was concentrated for crystallization
at -32 °C. Yellow crystals of 9 (0.463 g, 0.649 mmol, 51% yield)
Acknowledgment is made to the National Science Foun-
dation (CHE-051692). We thank Tara Williams for assistance.
1
were collected in a few days. H NMR of 9 (benzene-d6, 399.87
MHz, 23 °C) δ 3.57 (s, 36H, 6NMe2). 13C NMR (benzene-d6, 100.56
MHz, 23 °C) δ 47.36 (s, 6NMe2). Anal. calcd for C12H36N6OCl2Ta2:
C, 20.21; H, 5.09. Found: C, 20.14; H, 5.07.
Supporting Information Available: Crystallographic data of 2
and 9. This material is available free of charge via the Internet at
http://pubs.acs.org.
Determination of X-Ray Crystal Structures of 2 and 9. The
data were collected at -100 °C on a Bruker-AXS APEX diffrac-
tometer with a Kryoflex low temperature device, a CCD area
OM800362T