Synthesis and characterization of siloxy, aminoxy, and oxo complexes from the reaction of a tantalum amide silyl complex with oxygen

Article abstract of DOI:10.1021/ic802380u

(Me₂N)₄Ta-SiButPh2 (1) reacts with O₂ to give (Me₂N)₄Ta(OSiButPh₂) (2), (Me2N)3Ta(ONMe₂)(OSiButPh₂) (3), and the unusual μ-oxo amino (Me₂N)₂(Ph

Full text of DOI:10.1021/ic802380u

Inorg. Chem. 2009, 48, 3073-3079
Synthesis and Characterization of Siloxy, Aminoxy, and Oxo Complexes
from the Reaction of a Tantalum Amide Silyl Complex with Oxygen
He Qiu,† Shu-Jian Chen,† Chang-Sheng Wang,‡,§ Yun-Dong Wu,‡ Ilia A. Guzei,| Xue-Tai Chen,⊥
and Zi-Ling Xue*,†
Department of Chemistry, The UniVersity of Tennessee, KnoxVille, Tennessee 37996, Department
of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong
Kong, China, Department of Chemistry, Liaoning Normal UniVersity, Dalian 116029, China,
Department of Chemistry, The UniVersity of Wisconsin, Madison, Wisconsin 53706-1396, and
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures,
School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China
Received December 13, 2008
(Me2N)4Ta-SiButPh2 (1) reacts with O2 to give (Me2N)4Ta(OSiButPh2) (2), (Me2N)3Ta(ONMe2)(OSiButPh2) (3), and
the unusual µ-oxo amino (Me2N)2(Ph2ButSiO)2(µ,η2-Me2NCH2NMe)2Ta2(µ-O)2 (4) containing two bridging chelating
(aminomethyl)amides -N(Me)CH2NMe2. The dimer 4 was characterized by X-ray crystallography. 2 also reacts with
O2 to give both 3 and 4. Reaction pathways in the formation of these complexes are discussed. In reactions of O2
with d0 1 and 2, oxidation of the ligands is the prevailing pathway.
Introduction
Reactions of d0 transition-metal complexes with O2 have
recently become one of the most important processes in the
preparation of metal oxide thin films as microelectronic gate
insulator materials.14-16 In the next generation of very large
scale integrated (VLSI) transistor devices, the thickness of
the gate insulator material needs to be less than 20 Å, and
thin films of metal oxides with large dielectric constants are
needed. Much research has been conducted to prepare oxides
of early transition metals such tantalum, zirconium, and
hafnium.14-18
Reactions of O2 with transition-metal complexes have been
widely studied in fundamental biological processes and the
design of oxidation catalysts. Most studies of these reactions
involve dn complexes.1-3 The presence of the valence d
electrons in the metal centers often leads to the oxidation of
the metals by O2, and this oxidation is believed to be a
driving force of the reactions. There are fewer studies of
the reactions of d0 complexes with O2. In these complexes,
d0 metal atoms at their highest oxidation states are unlikely
to be oxidized.4-14 In the reactions of O2 with d0 alkyl
complexes, oxygen insertion into M-R bonds has been
reported.4-6 A similar insertion into a M-Si bond was
observed in the reaction of Cp2Zr(SiMe3)Cl with O2.7
We have recently investigated reactions of O2 with d0
amides M(NMe2)4 (M ) Zr, Hf)19 and Ta(NMe2)5 to
20
provide an understanding of the reactions involving diradical
O2 and the pathway in the formation of metal oxides.
Reactions of M(NMe2)4 (M ) Zr, Hf) with O2 are fast,
yielding oxo aminoxo complexes M3(NMe2)6(µ-NMe2)3(µ3-
O)(µ3-ONMe2) (Scheme 1) and Me2N-NMe2.19 Reaction of
Ta(NMe2)5 with O2 is also fast, giving two aminoxy
complexes (Me2N)nTa(η2-ONMe2)5-n (n ) 4, 3), as well as
(Me2N)4Ta2[η2-N(Me)CH2NMe2]2(µ-O)2 and (Me2N)6Ta3[η2-
N(Me)CH2NMe2]2(η2-ONMe2)(µ-O)3 containing novel chelat-
ing (aminomethyl)amide -N(Me)CH2NMe2 ligands (Scheme
* To whom correspondence should be addressed. E-mail: xue@utk.edu.
†
The University of Tennessee.
The Hong Kong University of Science and Technology.
Liaoning Normal University.
The University of Wisconsin.
‡
§
|
⊥
Nanjing University.
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10.1021/ic802380u CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 7, 2009 3073
Published on Web 02/26/2009

Qiu et al.
Scheme 1. Reactions of M(NMe2)n (n ) 4, M ) Zr, Hf; n )5, M )
Scheme 2. Reactions of O2 with an Equilibrium Mixture of d0
19-21
Ta) and 5 with O2
Tungsten Silyl Alkylidyne and Silyl Bis-alkylidene Complexes22,24k
Silyl amide (Me2N)4Ta-Si(SiMe3)3 (5) reacts with O2 to give
O2-stable (Me2N)3Ta(η2-ONMe2)[OSi(SiMe3)3] (6, Scheme
1).21 Oxygen insertion into the Ta-Si and one Ta-N bond
occurs in the reaction. We have also found that the reaction
of O2 with a mixture of d0 tungsten silyl alkylidyne and its
bis-alkylidene
tautomer
gives
(ButCH2)2W()O)-
[)CBut(SiButPh2)] where the silyl ligand has migrated to
the alkylidyne ligand (Scheme 2).22
In the current studies, (Me2N)4Ta-SiButPh2 (1), an
analogue of 5, was treated with O2. This reaction gives 2, 3,
and the oxo amide
4
containing two chelating
-N(Me)CH2NMe2 ligands (Scheme 3). 2 and 3 were also
prepared from the reactions of (Me2N)4TaCl (7) with
LiOSiButPh2 (8) and (Me2N)3TaCl2 (9) with 8 and LiONMe2,
respectively. The reaction between 2 and O2 gives 3 and 4
as well. Possible reaction pathways giving 2-4 are discussed.
1).20 These studies followed our earlier work in the inves-
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3074 Inorganic Chemistry, Vol. 48, No. 7, 2009

Siloxy, Aminoxy, and Oxo Complexes
Scheme 3. Formation of 2-4 from the Reaction of 1 with O2
Results and Discussion
and 3.62 (s, 6H, Ta-NMeeMef) ppm. A peak at 1.35 ppm
was assigned to the butyl group of the siloxy Ph2ButSiO-
ligand.
Preparation of 2, 3, and 4 from the Reaction of 1
with O2. d0 amide silyl 1 was prepared earlier from the
Crystals of 4 were obtained from both CH2Cl2 and toluene
solutions. They were found to contain solvent molecules.
4·CH2Cl2 was characterized by single crystal X-ray diffrac-
tion. Molecular drawing, crystallographic data, selected bond
distances and angles of 4 · CH2Cl2 are given in Figure 1 and
Tables 1 and 2, respectively. The dimeric 4 contains two
bridging oxo ligands. Each Ta center is coordinated by six
ligands in a pseudo octahedral geometry. The -OSiButPh2
and -NMe2 ligands and the two bridging oxo atoms are in
equatorial positions. The chelating -MeNCH2NMe2 ligands
are in axial positions, where the -NMe- unit is bound to a
Ta atom with a regular σ bond, and the NMe2 group in the
ligand forms a dative NfTa bond through the lone pair
electrons on the N atom. The Ta(1)-N(1) and Ta(1)-N(2)
σ bond distances of 2.034(4)-2.016(3) Å are similar to those
of Ta-NMe2 bonds in 1 [1.979(10)-2.029(10) Å] and
(Me2N)3TaCl(SiButPh2) [1.926(11)-1.989(11) Å].21 The da-
tive Ta(1)-N(4) bond length of 2.509(3) Å is significantly
reaction of (Me2N)4TaCl (7) with
1
equiv of
Li(THF)2SiButPh2 (10).21 After 1 was extracted from the
reaction mixture, it was directly used in the reaction with
1.5 equiv of O2. The deep brown solution of 1 slowly turned
to light yellow at -60 °C. A white precipitate was observed.
After removal of the volatiles, extraction of the residue by
CH2Cl2 followed by cooling gave colorless crystals of
4 · CH2Cl2 (4% yield). The residue was also extracted by
toluene to give crystals of 4 · toluene upon cooling which
were used for elemental analysis. When the reaction mixture
was analyzed by 1H NMR, it was mostly 2, 3, and HSiButPh2.
The presence of 4 was not clear. The solubility of 4 in non-
polar solvents is low. The solubilities of both 2 and 3 are
high. In fact, 2 is apparently a liquid at 23 °C, as discussed
below.
When the reaction was conducted with less than 1 equiv
of O2, it was not possible to obtain 4 through crystallization.
An excess of O2 (1.5 equiv) is important for the preparation
of 4. 4 was also prepared from the reaction of 2 with O2, as
discussed below.
The reaction of 1 with about 0.9 equiv of O2 was
monitored by NMR. 1H and 13C NMR spectra of the mixture
after 20 min showed it was a mixture of 2 (34% yield), 3
(5% yield), and unreacted 1 (16%). It is not clear if 4 was
produced from this reaction involving excess 1.
(24) For other related d0 silyl complexes, see, e.g. ,(a) Xue, Z.; Li, L.;
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X.-Z.; Li, L.-T.; Diminnie, J. B.; Yap, G. P. A.; Rheingold, A. L.;
Xue, Z.-L. Organometallics 1998, 17, 4597. (i) Wu, Z.-Z.; Diminnie,
J. B.; Xue, Z.-L. Organometallics 1999, 18, 1002. (j) Wu, Z.-Z.;
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Choi, S.-H.; Lin, Z.-Y.; Xue, Z.-L. Organometallics 1999, 18, 5488.
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J. R.; Diminnie, J. B.; Chen, T.-N.; Wiltz, A. M.; Xue, Z.-L.
Organometallics 2001, 20, 5542. (n) Blanton, J. R.; Chen, T.-N.;
Diminnie, J. B.; Cai, H.; Wu, Z.-Z.; Li, L.-T.; Sorasaenee, K. R.;
Quisenberry, K. T.; Pan, H.-J.; Wang, C.-S.; Choi, S.-H.; Wu, Y.-D.;
Lin, Z.-Y.; Guzei, I. A.; Rheingold, A. L.; Xue, Z.-L. J. Mol. Catal.
A Chem. 2002, 190, 101. (o) Chen, T.-N.; Sorasaenee, K. R.; Wu,
Z.-Z.; Diminnie, J. B.; Xue, Z.-L. Inorg. Chim. Acta 2003, 345, 113.
(p) Yu, X.-H.; Cai, H.; Guzei, I. A.; Xue, Z.-L. J. Am. Chem. Soc.
2004, 126, 4472. (q) Yu, X.-H.; Bi, S.-W.; Guzei, I. A.; Lin, Z.-Y.;
Xue, Z.-L. Inorg. Chem. 2004, 43, 7111. (r) Yu, X.-H.; Xue, Z.-L.
Organometallics 2005, 24, 4190. (s) Woods, J. B.; Yu, X.-H.; Chen,
T.-N.; Xue, Z.-L. Organometallics 2004, 23, 5910. (t) Qiu, H.; Cai,
H.; Woods, J. B.; Wu, Z.-Z.; Chen, T.-N.; Yu, X.-H.; Xue, Z.-L.
Organometallics 2005, 24, 4190. (u) Qiu, H.; Chen, S.-J.; Xue, Z.-L.
Inorg. Chem. 2007, 46, 6178. (v) Cai, H.; Yu, X.; Chen, S.; Qiu, H.;
Guzei, I. A.; Xue, Z.-L. Inorg. Chem. 2007, 46, 8071.
1
Characterization and Crystal Structure of 4. The H
NMR spectrum of 4 at 23 °C is consistent with the structural
assignment. There is an inversion center in 4. The two H
atoms in the -CH2- group are diastereotopic, and the bridging
η2-MeNCHaHbNMe2 ligand thus gives three methyl peaks
and two doublets for Ha and Hb, respectively. These two
1
doublets were found at 5.49 and 4.87 ppm in the H NMR
spectrum (2JH-H ) 9.8 Hz, Supporting Information). The two
methyl groups in -NMe2 in the -MeNCHaHbNMecMed ligand
are inequivalent, thus giving three methyl resonances at 3.36
(MeNCHaHbNMecMed), 2.88 (MeNCHaHbNMecMed) and
2.78 ppm (MeNCHaHbNMecMed) in a 1:1:1 ratio for the
ligand. The Ta-NMe2 amide ligand apparently does not
undergo a free rotation around the Ta-N bond. Thus, the
two methyl groups in Ta-NMe2 are not chemically equiva-
lent, and they were observed at 3.75 (s, 6H, Ta-NMeeMef)
Inorganic Chemistry, Vol. 48, No. 7, 2009 3075

Qiu et al.
Scheme 4. Direct Preparation of 2 and Its Reaction with O2
Figure 1. Molecular Structure of 4 in 4·CH2Cl2 Showing 30% Probability
Thermal Ellipsoids.
Table 1. Crystal Data and Structure Refinement for 4·CH2Cl2
empirical formula (fw)
crystal system
space group
C46H76Cl4N6O4Si2Ta2 (1337.01)
triclinic
P1
j
unit cell dimensions
a ) 10.965(3) Å, R ) 82.008(4)°
b ) 14.209(3) Å, ꢀ ) 75.331(3)°
c ) 18.356(4) Å, γ ) 89.577(3)°
2738.6(11) Å3
volume
Z
2
density (calculated)
absorption coefficient
F(000)
crystal size
theta range for data collection
index ranges
1.621 Mg/m3
4.276 mm-1
Scheme 5. Preparation of 3 from the Reaction of 9 with 8 and
LiONMe2
1336
0.38 × 0.27 × 0.10 mm3
1.16 to 26.38°
-13 e h e 13, -17 e k e 17,
-22 e l e 22
reflections collected
26802
independent reflections
completeness to theta ) 25.00°
absorption correction
max. and min. transmission
refinement method
10983 [R(int) ) 0.0297]
99.5%
semiempirical from equivalents
0.6744 and 0.2933
full-matrix least-squares on F2
10983/0/593
(Me2N)3Ta(η2-ONMe2)[OSi(SiMe3)3] [1.991(8) Å].21 The
Ta(1)-O(2)-Ta(2) angle of 99.19(12)° in the Ta(1)-O(2)-
Ta(2)-O(3) four-member ring is much larger than the
O(2)-Ta(1)-O(3) bond angle of 80.90(11)°.
data/restraints/parameters
goodness-of-fit on F2
0.969
final R indices [I > 2σ(I)]a
R indices (all data)a
R1 ) 0.0280, wR2 ) 0.0649
R1 ) 0.0434, wR2 ) 0.0723
1.639 and -0.671 e Å-3
2
4 was found to be thermally unstable in solution at 23
°C, but stable in the solid state. Decomposition of 4 gave
Me2NCH2NMe2 and an unknown Ta species. The formation
largest diff. peak and hole
a
2
wR2 ) {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2; R ) ∑||Fo| - |Fc||/∑|Fo|; w
) 1/[σ2(Fo ) + (aP)2 + bP]; P ) [2Fc2 + max(Fo , 0)]/3.
2
2
28
1
of Me2NCH2NMe2 was confirmed by H and 13C NMR.
Table 2. Selected Bond Distances (Å) and Angles (deg) in 4·CH2Cl2
Me2NCH2NMe2 is perhaps a product of a methyl exchange
between MeHNCH2NMe2 and HNMe2 that may have yielded
H2NMe as well.28
Ta(1)-O(2)
1.927(3)
1.964(3)
2.016(3)
2.509(3)
2.034(4)
1.482(5)
1.895(4)
160.98(11)
80.90(11)
95.06(11)
102.81(12)
85.75(13)
90.24(13)
170.38(12)
98.93(15)
77.99(11)
162.59(17)
80.90(11)
Ta(1)-N(4)
N(2)-C(20)
Si(1)-O(1)
N(3)-C(20)
N(2)-C(19)
N(3)-C(22)
2.509(3)
1.453(5)
1.623(3)
1.504(5)
1.474(5)
1.466(6)
Ta(1)-O(1)
Ta(1)-N(2)
Ta(1)-N(4)
The chelating ligand -N(Me)CH2NMe2 forms one σ bond
and one dative bond with a Ta atom. Such chelating ligand
is rare. In the reaction of MCl5 (M ) Nb, Ta) with LiNMe2,
M(NMe2)4(η2-MeNCH2NMe2) (M ) Nb, Ta) containing this
chelating ligand was isolated as a minor product.28 In the
reaction of d0 Ta(NMe2)5 with O2 (Scheme 1), this ligand
was observed in two products (Me2N)4Ta2[µ,η2-N(Me)-
CH2NMe2]2(µ-O)2 and (Me2N)6Ta3[η2-N(Me)CH2NMe2]2(η2-
ONMe2)(µ-O)3.20 Mechanistic considerations of the forma-
tion of the chelating -N(Me)CH2NMe2 ligand in 4 are
discussed below.
Ta(1)-N(1)
N(3)-C(21)
Si(1)-C(1)
O(2)-Ta(1)-O(1)
O(2)-Ta(1)-O(3)
O(1)-Ta(1)-O(3)
O(1)-Ta(1)-N(2)
O(3)-Ta(1)-N(2)
O(2)-Ta(1)-N(1)
O(3)-Ta(1)-N(1)
N(2)-Ta(1)-N(1)
O(2)-Ta(1)-N(4)
Si(1)-O(1)-Ta(1)
O(2)-Ta(1)-O(3)
O(1)-Ta(1)-N(4)
O(3)-Ta(1)-N(4)
N(2)-Ta(1)-N(4)
N(1)-Ta(1)-N(4)
O(2)-Ta(1)-Ta(2)
O(1)-Ta(1)-Ta(2)
N(2)-Ta(1)-Ta(2)
N(1)-Ta(1)-Ta(2)
N(2)-C(20)-N(3)
Ta(1)-O(2)-Ta(2)
83.04(11)
81.73(11)
166.63(13)
92.80(13)
41.03(8)
132.30(8)
90.59(10)
131.17(10)
113.3(3)
99.19(12)
larger than the Ta-N dative bonds in 6 [2.254(9) Å]21 and
[TaCl(µ-Cl)(NBut)(NHBut)(NH2But)]2 [2.23(3) Å].26 It is not
clear why the dative Ta(1)-N(4) bond is longer than other
reported NfTa bonds. The Ta-O bonds lengths [1.927(3)-
1.964(3) Å] are close to those in other Ta complexes:
Ta(CH2SiMe3)(O2CNMe2)4 [1.932(5)-2.132(5) Å],27a
[(Me2N)2(Me2NH)TaCl2]2O [1.917(6)-1.928(6) Å]27b and
Preparation of
2
from
7
and
8
and Its
Characterization.22b 2 was also prepared by the reaction
in Scheme 4 and characterized to confirm its formation in
the reaction of 1 with O2 and study its reactivities. This
substitution reaction between 7 and 8 was found to be
relatively slow, and requires overnight stirring at 23 °C to
give an oil of 2 in 77% yield. When 2 was pure, it was a
3076 Inorganic Chemistry, Vol. 48, No. 7, 2009

Siloxy, Aminoxy, and Oxo Complexes
Scheme 6. Proposed Pathways in the Reaction of 1 with O2
liquid at 23 °C with a high solubility in non-polar solutions
such as pentane and benzene.
NMR spectrum of 3, the -NMe2 and -ONMe2 resonances at
47.1 and 49.6 ppm are close to those in (Me2N)3Ta(η2-
1
The NMR spectra of 2 are consistent with the structural
ONMe2)[OSi(SiMe3)3] (6). Both H and 13C resonances of
1
the methyl groups in the -OSiButPh2 ligand in 3 are close to
those in 2.
Unlike 6, 3 is not stable to O2, and the reaction gives
unknown species. One interpretation is that the -OSiButPh2
ligand in 3 is not as bulky as the -OSi(SiMe3)3 ligand in 6
and is thus not effective in protecting the metal center in 3
from O2 attack.
assignment. The H NMR spectrum of 2 shows only one
resonance for the four -NMe2 ligands at 3.17 ppm and one
signal for the But group of the siloxy ligand at 1.21 ppm.
The 13C{1H} NMR spectrum shows a single amide resonance
at 46.0 ppm and the But resonances at 27.9 (CMe3) and 20.0
(CMe3) ppm. These observations suggest that amide ligands
in 2 undergo a fast exchange.21
Once 2 is formed in the reaction of 1 with O2, does it
react with O2 further to give 3 and 4?
Mechanistic Considerations of the Reaction between
1 and O2. Our recent density-functional theory (DFT) studies
of the reaction between Ta(NMe2)5 and O2 show that O2
The reaction of 2 with O2 gives 3 and 4 (Scheme 4). Not
surprisingly no detectable amount of HSiButPh2 was in the
reaction mixture. The yield of 3 (77%) is higher than that in
the reaction of 1 with O2. The yields of 4 in the two reactions
are, however, similar. Attempts to isolate 3 from the reaction
mixture were unsuccessful. Recrystallization yielded 4 as
crystals. This is perhaps not surprising, as 3 was found to
have high solubility in most organic solvents.
inserts into
a
Ta-NMe2 bond, forming (Me2N)4-
Ta-O-O-NMe2.20 The R-O atom in this species either
abstracts a hydride from a neighboring -N(CH3)2 ligand,
yielding (Me2N)2Ta(ONMe2)(OH)(MeN)CH2), or inserts
into another Ta-NMe2 bond to give (Me2N)3Ta(O-NMe2)2.20
The imine ligand MeN)CH2 in (Me2N)2Ta(ONMe2)-
(OH)(MeN)CH2) then inserts into a Ta-NMe2 bond to give
the chelating -N(Me)CH2NMe2 ligand that is also observed
in 4 in the current studies. DFT calculations reveal exchanges
between Ta-NMe2 and Ta-ONMe2 ligands, and such an
exchange was observed in the following reaction: Ta(NMe2)5
Preparation of 3 from 8, 9, and LiONMe2 and Its
Characterization. Reaction of (Me2N)3TaCl2 (9) with 8 and
LiONMe2 also led to the formation of 3 (Scheme 5) which
was purified for spectroscopic characterization and elemental
analysis.
+
(Me2N)3Ta(O-NMe2)2
h
2(Me2N)4TaO-NMe2.20
1H and 13C{1H} NMR spectra of 3 suggest that the
structure of 3 is similar to that of 6.21 Three resonances were
observed in the 1H NMR spectrum of 3. The -NMe2
resonance (3.28 ppm) and -ONMe2 signal (2.36 ppm) are
only slightly upfield-shifted from those in 6. In the 13C{1H}
(Me2N)4Ta(SiButPh2) (1) and (Me2N)4Ta-Si(SiMe3)3 (5) are
similar to Ta(NMe2)5 except that the former contain an
anionic silyl ligand. The reactions of 1, 5,21 and
Cp2Zr(SiMe3)Cl7 with O2 yield products with O insertion
into M-Si bonds. Given that silicon is known to be
oxophilic, it is reasonable to assume that the initial step in
the reaction of 1 with O2 involves O2 insertion into the Ta-Si
bond, forming Ta peroxide species (Me2N)4Ta(O-O-
SiButPh2) (A1) in Scheme 6. A1 then undergoes reactions
as shown in the two pathways in Scheme 6. In Path I, the
R-O atom in A1 abstracts a hydride from a neighboring
-N(CH3)2 ligand to give A2 containing an OH and an imine
MeN)CH2 ligand. Two reactions subsequently occur: elimi-
(25) (a) Yu, X.-H.; Morton, L. A.; Xue, Z.-L. Organometallics 2004, 23,
2210. (b) Xue, Z.-L. Comments Inorg. Chem. 1996, 18, 223. (c) Chen,
T.-N.; Xue, Z.-L. Chin. J. Inorg. Chem. 1999, 15, 413.
(26) Jones, T. C.; Nielson, A. J.; Rickard, E. F. Chem. Commun. 1984,
205.
(27) (a) Chisholm, M. H.; Tan, L.-S.; Huffman, J. C. J. Am. Chem. Soc.
1982, 104, 4879. (b) Chisholm, M. H.; Huffman, J. C.; Tan, L.-S.
Inorg. Chem. 1981, 20, 1859.
(28) Zhang, X.-H.; Chen, S.-J.; Cai, H.; Im, H.-J.; Chen, T.; Yu, X.; Chen,
X.; Lin, Z.; Wu, Y.-D.; Xue, Z.-L. Organometallics 2008, 27, 1338.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3077

Qiu et al.
Reaction of 1 with O2 in an NMR Tube. 7 (0.041 g, 0.110
mmol) and 10 (0.047 g, 0.120 mmol) were mixed in a Young NMR
tube (benzene-d6, 0.6 mL) to yield 1. Bibenzyl (0.018 g, 0.100
mmol) was used as an internal standard. O2 (0.10 mmol) was added
as described above, and the solution shaken for 10 min at 23 °C.
The brown solution gradually turned into light yellow during this
period. 1H and 13C NMR spectra of the mixture after 20 min of the
reaction showed that it was a mixture of 2 (34% yield), 3 (5%
yield) and unreacted 1 (16%).
nation of HNMe2 from A2, yielding A3, and an imine
insertion into a Ta-NMe2 bond to give A4 containing the
-N(Me)CH2NMe2 ligand. Dimerization of A4 yields 4
containing two oxo and two -N(Me)CH2NMe2 bridging
ligands (Figure 1). If the hydroxyl ligand in A2 reacts with
1, it may lead to the formation of HSiButPh2 (or HNMe2)
and decomposition of 1.
In a parallel pathway in Scheme 6 (Path II), the R-O atom
in Ta peroxide A1 may insert into another Ta-NMe2 bond,
yielding 3. An exchange between the aminoxide
Ta-O-NMe2 in 3 and the amide Ta-NMe2 in 1 gives 2.
This exchange is similar to the exchange Ta(NMe2)5 +
(Me2N)3Ta(O-NMe2)2 h 2(Me2N)4TaO-NMe2 observed
earlier.20
Reaction of 1 with O2 to Give 2-4. 10 (1.622 g, 4.160 mmol)
in Et2O (20 mL) at -30 °C was added dropwise to 7 (1.502 g,
3.830 mmol) in Et2O (30 mL) at -30 °C. The mixture was warmed
with stirring to 23 °C in 2 h. The volatiles were removed. Pentane
(20 mL) was added, and the solution was then filtered. After
filtration, O2 (5.520 mmol) was added. The yellow solution was
stirred overnight at 23 °C. A white precipitate was observed in the
mixture. All volatiles were removed in vacuo to leave a yellow oil
containing a white solid. This mixture was then dissolved in CH2Cl2,
and the solution was cooled to -30 °C to grow colorless crystals
It is perhaps not surprising that reactions of both 1 and 2
with O2 give 3 and 4 containing O-NMe2 and chelating
-N(Me)CH2NMe2 ligands, given that 2 contains four amide
ligands.20 Pathways similar to those in Scheme 6 could be
postulated for the reaction of 2 with O2.
1
of 4 (0.092 g, 0.078 mmol, 4%). H NMR (benzene-d6, 399.87
MHz, 23 °C) δ 8.00-6.80 (m, C6H5), 5.49 (d, 2H, MeNCHaHb-
It is not clear why the current reaction of 1 with O2 gives
three products 2-4 and the reaction of another silyl complex
5 with O2 yields only 6 (Scheme 1).21 The -Si(SiMe3)3
ligand is known to be bulkier than the -SiButPh2
ligand.23a,b,24a,b,d,f,g,j,p,r It would not be surprising if steric
bulkiness in the peroxo intermediate (Me2N)4Ta[-O-
OSi(SiMe3)3] (similar to A1 in Scheme 6) prevents the R-O
atom from reaching a ꢀ-H atom of an amide ligand in
(Me2N)4Ta[-O-OSi(SiMe3)3], making Path I in Scheme 6
unfavorable for the -Si(SiMe3)3 analogue. Steric bulkiness
in 6 may also prevent it from undergoing a ligand exchange
with unreacted 5. In other words, kinetic barriers may have
limited the reactivities of bulkier 5 and 6.
2
NMe2, JH-H ) 9.8 Hz), 4.87 (d, 2H, MeNCHaHbNMe2), 3.75 (s,
6H, NMeeMef), 3.62 (s, 6H, NMeeMef), 3.36 (s, 6H, MeNCH2NMe2)
2.88 (s, 6H, MeNCH2NMecMed), 2.78 (s, 6H, MeNCH2NMecMed)
1.35 (s, 9H, CMe3). 13C{1H} NMR (benzene-d6, 100.57 MHz, 7
°C) δ 115-135 (C6H5), 87.6 (MeNCH2NMe2), 48.7 (NMeeMef),
48.6 (NMeeMef), 46.8 (MeNCH2NMecMed), 46.6 (MeNCH2NMecMed),
38.8 (MeNCH2NMe2), 27.5 (CMe3), 22.7 (CMe3). In obtaining
crystals of high purity for elemental analysis, toluene was also used
to grow the colorless crystals. Anal. Calcd for C51H80N6O4Si2Ta2
1
(4·toluene): C, 48.64; H, 6.40. Found: C, 48.56; H, 6.29. A H
NMR spectrum of the mixture in benzene-d6 showed that it was
mostly 2, 3, and HSiButPh2 (19% yield). The presence of 4 was
not clear in the spectrum.
Preparation of 2 from 7 and 8. A mixture of 7 (1.281 g, 3.260
mmol) and 8 (0.801 g, 3.05 mmol) was placed in a Schlenk flask
at 0 °C. Cold Et2O (30 mL) at 0 °C was added. The yellow solution
was slowly warmed to 23 °C overnight with stirring. After volatiles
were removed in vacuo, residue was extracted with pentane (2 ×
20 mL) and filtered. Removing volatiles in the filtrate gave a yellow
Experimental Section
All manipulations were performed under a dry nitrogen atmo-
sphere with the use of either a dry box or standard Schlenk
techniques. Solvents were purified by distillation from potassium/
benzophenone ketyl. Benzene-d6 and toluene-d8 were dried over
activated molecular sieves and stored under N2. TaCl5 (Strem) was
freshly sublimed. O2 (National Welders Supply Co.) was dried by
P2O5 before use. 7,27a 8,22a 9,27b and 1029 were prepared by the
literature procedures. 1H and 13C{1H} NMR spectra were recorded
on a Bruker AC-250 or AMX-400 spectrometer and referenced to
1
oil of 2 (1.438 g, 2.350 mmol, 77% yield). H NMR (benzene-d6,
300.09 MHz, 23 °C) δ 8.00-6.80 (m, C6H5), 3.17 (s, 24H, NMe2),
1.22 (s, 9H, CMe3). 13C{1H} NMR (benzene-d6, 75.46 MHz, 23
°C) δ 135-127 (C6H5), 46.0 (NMe2), 27.9 (CMe3), 20.0 (CMe3).
Anal. Calcd for C24H43N4SiOTa: C, 47.05; H, 7.07. Found: C, 47.02;
H, 6.85.
1
solvent (residual protons in H spectra). Elemental analyses were
Preparation of 3 from 8 and 9 or 2 and O2. Method 1. 8
(0.322 g, 1.23 mmol) in tetrahydrofuran (THF, 15 mL) was added
to 9 (0.472 g, 1.23 mmol) in THF (10 mL) at -78 °C with stirring.
After the mixture was stirred for 22 h, LiONMe2 (0.082 g, 1.2
mmol) was added at -18 °C. After additional stirring for 24 h, all
volatiles were removed in vacuo. The residue was extracted with
hexanes and filtrated. The volatiles in the filtrate were removed in
vacuo to give light yellow powders of 3 (0.434 g, 0.690 mmol,
56% yield). 1H NMR (benzene-d6, 400.04 MHz) δ 8.00-7.00 (m,
4H, C6H5), 3.28 (s, 18H, NMe2), 3.36 (s, 6H, ONMe2) 1.25 (s, 9H,
CMe3). 13C NMR (benzene-d6, 100.59 MHz) δ 147.0-127.0 (C6H5),
49.4 (ONMe2), 46.9 (NMe2), 27.4 (CMe3), 28.7 (CMe3). Anal. Calcd
for C24H43N4SiO2Ta: C, 45.85; H, 6.89. Found: C, 45.68; H, 6.79.
Method 2. A solution of 2 (1.038 g, 1.700 mmol) in pentane
(30 mL) was added O2 (1.020 mmol) as described above. The
yellow solution was vigorously stirred overnight. A white precipitate
was observed. Volatiles were then removed in vacuo to give a
performed by Complete Analysis Laboratories Inc., Parsippany, NJ.
The addition of O2 was conducted by the following procedure.
The volume of an empty NMR tube or Schlenk flask (100-500
mL) was measured. After a fixed volume of solvent (benzene-d6
or pentane) and reactants were added, the solution was frozen by
liquid nitrogen, and the headspace was evacuated for 10 min on a
gas manifold system. The bottom of the NMR tube or flask
containing the solution was then kept at -60 °C, while the top
was kept at 23 °C. O2 was then introduced till the system reached
a preset pressure (100-760 torr). The ideal gas law PV ) nRT (T
) 296 K) was used to calculated the amount of O2 in the NMR
tube or Schlenk flask. The vapor pressures of benzene-d6 or pentane
at -60 °C were ignored in the calculation. The solution was then
slowly warmed to 23 °C with stirring.
(29) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993,
12, 2584.
3078 Inorganic Chemistry, Vol. 48, No. 7, 2009

Siloxy, Aminoxy, and Oxo Complexes
1
liquid. H NMR of the liquid showed no detectable amount of
formed with SADABS.30a In addition, the global refinements for
the unit cells and data reductions were performed using the Saint
program (version 6.02).30a All calculations were performed using
SHELXTL (version 5.1) proprietary software package.30b
HSiButPh2. Pentane (2 × 20 mL) was added to extract the liquid,
and the solution was filtered. After removal of the volatiles, crude
3 was obtained as a yellow solid (0.825 g, 1.31 mmol, 77% yield).
It should be noted that in a few times when 3 was prepared by this
reaction, a white precipitate of 4 was obtained.
Preparation of 4 from 2 and O2. A solution of 2 (1.369 g,
2.360 mmol) in pentane (30 mL) was added O2 (2.51 mmol) as
described above. The yellow solution was vigorously stirred
overnight. A white precipitate was observed. Then all volatiles were
removed in vacuo. The residue was extracted with pentane (2 ×
20 mL) and filtered. Pentane in the filtrate was removed in vacuo,
and the residue was dissolved in CH2Cl2 (3 mL). Recrystallization
at -30 °C yielded colorless crystals of 4·CH2Cl2 (0.074 g, 0.055
mmol, 5% yield).
Determination of the Structure of 4 by Single Crystal
X-ray Diffraction. The data were collected on a Bruker AXS Smart
1000 X-ray diffractometer equipped with a CCD area detector and
a graphite-monochromated Mo source (KR radiation, 0.71073 Å)
and fitted with an upgraded Nicolet LT-2 low temperature device.
A suitable crystal was coated with paratone oil (Exxon) and
mounted on a hairloop under a stream of nitrogen at -100(2) °C.
The structure was solved by direct methods. Non-hydrogen atoms
were anisotropically refined. All hydrogen atoms were treated as
idealized contributions. Empirical absorption correction was per-
Acknowledgment is made to the U.S. National Science
Foundation (CHE-0516928 to Z.X.), Research Grant Council
of Hong Kong (HKUST6083/02M to Y.D.W.), National
Basic Research Program of China (Nos. 2006CB806104 to
X.T.C.), and Changjiang (Cheung Kong) Lecture Professor-
ship (to Z.X.) for support of the research.
1
Supporting Information Available: H NMR spectrum of 4
showing two doublets of the η2-MeNCHaHbNMe2 ligand; a table
listing NMR resonances of 2, 3, and 6 in benzene-d6; crystal-
lographic data of 4·CH2Cl2. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC802380U
(30) (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.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3079