Unexpected formation of (dimethylaminomethylene)methylamide complexes from the reactions between metal chlorides and lithium dimethylamide

Article abstract of DOI:10.1021/om701173m

Reactions of MCl₅ (M = Nb, Ta) with LiNMe₂ have been found to yield M(NMe₂)₄(η²-MeNCH ₂NMe₂) (M = Nb, 2a; Ta, 2b) containing a chelating ligand (dimethylaminomethylene)methylamide, as confirmed by NMR spectroscopy, DFT calculations, and their reactivity studies.

Full text of DOI:10.1021/om701173m

1338
Organometallics 2008, 27, 1338–1341
Unexpected Formation of (Dimethylaminomethylene)methylamide
Complexes from the Reactions between Metal Chlorides and
Lithium Dimethylamide
Xin-Hao Zhang,† Shu-Jian Chen,‡ Hu Cai,# Hee-Jung Im,‡ Tianniu Chen,‡ Xianghua Yu,‡
Xuetai Chen,4 Zhenyang Lin,*,† Yun-Dong Wu,*,† and Zi-Ling Xue*,‡
Department of Chemistry, Hong Kong UniVersity of Science and Technology, Hong Kong, China,
Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996, and State Key Laboratory
of Coordination Chemistry, College of Chemistry and Chemical Engineering, Nanjing UniVersity,
Nanjing 210093, China
ReceiVed NoVember 22, 2007
Scheme 1
Summary: Reactions of MCl5 (M ) Nb, Ta) with LiNMe2 haVe
been found to yield M(NMe2)4(η2-MeNCH2NMe2) (M ) Nb, 2a;
Ta, 2b) containing a chelating ligand (dimethylaminomethyl-
ene)methylamide, as confirmed by NMR spectroscopy, DFT
calculations, and their reactiVity studies.
Introduction
Metathesis between transition metal halides and lithium
amides is the most common route to amide complexes.1,2 In
particular, homoleptic amides complexes M(NR2)n are almost
exclusively prepared by the process shown in (1),1a and they
have been used as precursors in the preparation of microelec-
tronic thin films of metal3 and oxides.4 Although most metath-
Scheme 2
* Address correspondence to this author. E-mail: chzlin@ust.hk (Z.L.);
chydwu@ust.hk (Y.-D.W.); xue@utk.edu (Z.-L.X.).
† Hong Kong University of Science and Technology.
‡ University of Tennessee.
# Current address: Department of Chemistry, Nanchang University,
Nanchang 336000, China.
4 Nanjing University.
(1) (a) Lappert, M. F.; Power, P. P.; Sanger A. R.; Srivastava, R. C.
Metal and Metalloid Amides; Ellis Horwood Ltd: Chichester, UK, 1980; p
470. (b) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273.
(c) Cotton, S. A. Annu. Rep. Prog. Chem., A: Inorg. Chem 1992, 89, 107.
(d) Kempe, R. Angew. Chem. Int. Ed 2000, 39, 468. (e) Gade, L. H.;
Mountford, P. Coord. Chem. ReV. 2001, 216–217, 65.
esis reactions give amide complexes, there have been a few
reports of ꢀ- and γ-H abstraction reactions yielding imine5 and
four-membered-ring metallaheterocyclic6 complexes (Sch-
eme 1).
(2) (a) Bradley, D. C.; Gitlitz, M. H. J. Chem. Soc. (A) 1969, 980. (b)
Bradley, D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 1355. (c) Bradley,
D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 449. (d) Heath, C. E.;
Hursthouse, M. B. Chem. Commun. 1971, 143. (e) Bott, S. G.; Hoffman,
D. M.; Rangarajan, S. P. Inorg. Chem. 1995, 34, 430.
MCln + nLiNR2 f M(NR2)n + nLiCl
(1)
In our preparation of M(NMe2)5 (M ) Nb, 1a; Ta, 1b) and
Ta(NMe2)4Cl from MCl5 and LiNMe2, we were surprised to
isolate 2a and 2b as well in 5–10% yields (Scheme 2). 2b was
also observed in the reactions of (Me2N)3TaCl2 with 2 equiv of
LiNMe2 and the reactions of (Me2N)4TaCl with 1 equiv of
LiNMe2 in 0.9% and 2.9% yields, respectively. Our character-
(3) See, e.g.: (a) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Chem.
Mater. 1990, 2, 235. (b) Ossola, F.; Maury, F. Chem. Vap. Deposition 1997,
3, 137. (c) Dubois, L. H.; Zegarski, B. R.; Girolami, G. S. J. Electrochem.
Soc. 1992, 139, 3603. (d) Lewkebandara, T. S.; Sheridan, P. H.; Heeg, M. J.;
Rheingold, A. L.; Winter, C. H. Inorg. Chem. 1994, 33, 5879. (e) Cale,
T. S.; Chaara, M. B.; Raupp, G. B.; Raaijmakers, I. J. Thin Solid Films
1993, 236, 294. (f) Liu, X.-Z.; Wu, Z.-Z.; Peng, Z.-H.; Wu, Y.-D.; Xue,
Z.-L. J. Am. Chem. Soc. 1999, 121, 5350. (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. (h) Lehn, J.-S. M.; van der Heide, P.;
Wang, Y.; Suh, S.; Hoffman, D. M. J. Mater. Chem 2004, 14, 3239.
(4) (a) Bastianini, A.; Battiston, G. A.; Gerbasi, R.; Porchia, M.; Daolio,
S. J. Phys. IV 1995, 5, C5. (b) Hendrix, B. C.; Borovik, A. S.; Xu, C.;
Roeder, J. F.; Baum, T. H.; Bevan, M. J.; Visokay, M. R.; Chamber, J. J.;
Rotondaro, A. L. P.; Bu, H.; Colombo, L. Appl. Phys. Lett. 2002, 80, 2362.
(c) 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. (d) 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. (e) 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.
(5) (a) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159. (b)
Scoles, L.; Ruppa, K. B. P.; Gambarotta, S. J. Am. Chem. Soc. 1996, 118,
2529. (c) Cai, H.; Chen, T.-N.; Wang, X.-P.; Schultz, A. J.; Koetzle, T. F.;
Xue, Z.-L. Chem. Commun. 2002, 230. (d) Tsai, Y.-C.; Johnson, M. J. A.;
Mindiola, D. J.; Cummins, C. C.; Klooster, W. T.; Koetzle, T. F. J. Am.
Chem. Soc. 1999, 121, 10426. (e) Rothwell, I. P. Polyhedron 1985, 4, 177.
(f) Nugent, W. A.; Ovenall, D. W.; Holmes, S. J. Organometallics 1983,
2, 161. (g) Ahmed, K. J.; Chisholm, M. H.; Folting, K.; Huffman, J. C.
J. Am. Chem. Soc. 1986, 108, 989. (h) Takahashi, Y.; Onoyama, N.;
Ishikawa, Y.; Motojima, S.; Sugiyama, K. Chem. Lett. 1978, 525. (i) Airoldi,
C.; Bradley, D. C.; Vuru, G. Transition Met. Chem 1979, 4, 64.
(6) (a) Cai, H.; Yu, X.-H.; Chen, T.-N.; Chen, X.-T.; You, X.-Z.; Xue,
Z.-L. Can. J. Chem. 2003, 81, 1398. (b) Herzon, S. B.; Hartwig, J. F. J. Am.
Chem. Soc. 2007, 129, 6690.
10.1021/om701173m CCC: $40.75
2008 American Chemical Society
Publication on Web 02/19/2008

Notes
Organometallics, Vol. 27, No. 6, 2008 1339
Scheme 3
Scheme 4
are imine complexes (3b or 3b′). In addition, the 13C NMR
resonance of the -CH2- moiety at 82.7 ppm is much shifted
from those of coordinated imine C atoms around 140 ppm.9b,10
There is no characteristic imine (NdC) band around 1600 cm-1
in the IR spectrum of 2b.7 Hydrolysis of a solution of 2b in
benzene-d6 was conducted to confirm its structure.7 A GC-MS
and NMR analysis of the volatile products showed the formation
of Me2NCH2NMe2, H2NMe, and HNMe2. The formation of
Me2NCH2NMe2 and H2NMe is perhaps not surprising, given
that alkyl amines are known to undergo catalytic alkyl ex-
changes.11 The hydrolysis may initially yield HMeNCH2NMe2
and HNMe2 which then undergo the exchange in eq 2 catalyzed
by Ta oxide/hydroxide. Although we cannot rule out the
possibility that Me2NCH2NMe2 and H2NMe are the products
of the hydrolysis of Ta(NMe2)5(CH2dNMe) (3b), the observa-
tion of Me2NCH2NMe2 as a hydrolysis product as well as
spectroscopic and DFT studies discussed below strongly suggest
that 2b is Ta(NMe2)4(η2-MeNCH2NMe2) containing the chelat-
ing (aminomethyl)amide ligand.
ization as well as experimental and theoretical studies of 2a,b
are reported.
Results and Discussion
Sublimation at 70 °C (0.05 mmHg) of the product mixtures
gave M(NMe2)5 (1a,b). Subsequent sublimation at 90 °C yielded
crude 2a,b as brown and pale yellow solids.7 Recrystallization
in Et2O or pentane at -30 °C gave crystalline solids of 2a,b.
1
The H resonances of MeNCH2-, -CH2-, and -CH2NMe2
of 2b were observed as singlets at 3.13, 3.98, and 2.16 ppm
(benzene-d6), respectively. The four -NMe2 ligands appeared
at 3.30 ppm. The peak (2.16 ppm) of -CH2NMe2, which forms
a NfTa dative bond, is upfield-shifted from those of -NMe2
and MeNCH2-. The 13C peaks of the -NMe2, -CH2NMe2,
and MeNCH2- were observed at 47.89, 47.29, and 39.13 ppm,
respectively. DEPT-135 and HMQC experiments confirmed the
assignment. In the1H-coupled 13C NMR spectrum, the -CH2-
resonance was a triplet at 82.67 ppm (1JC-H ) 141.1 Hz). The
crystal structures of 2a and 2b are isomorphous and disordered.7
They are given in the Supporting Information. They are also
isomorphous to that of W(NMe2)6.8 Because of the disorder with
the Th symmetry, the presence of the η2-MeNCH2NMe2 ligands
in 2a,b could not be confirmed by single-crystal X-ray diffrac-
tion. C, H, and N elemental analyses of 2a,b are consistent with
their compositions.8 It should be noted that, in the reaction of
MCl5 (M ) Nb, Ta) with 5 equiv of LiNMe2, the total yields
of M(NMe2)5 (1a,b) and 2a,b, based on MCl5, are 55% and
54%, respectively. Thus it is conceivable that the use of LiNMe2
to give M(NMe2)4(η2-MeNCH2NMe2) (2a,b), which consumes
at least 6 equiv of LiNMe2, may have contributed to the lower
yields of M(NMe2)5 (1a,b).
(2)
Experiments have been conducted to verify if Ta(NMe2)5 (1b)
reacts with freshly generated imine CH2dNMe to yield 2b.
CH2dNMe was prepared from the thermal cracking of its
trimer,12 and was added directly to Ta(NMe2)5 (1b) in toluene-
d8 at -45 °C. Variable-temperature NMR spectra were moni-
tored. Several tests failed to show the formation of 2b. In many
cases, only the polymerization of CH2dNMe was observed.12a
The results support the conclusions from computations that will
be discussed below. It is not clear at the present time how 2b
is formed in the reaction between TaCl5 and LiNMe2. It is likely
that a -NMe2 ligand undergoes the ꢀ-H abstraction reaction
with another amide or chloride ligand to yield a Ta(III) imine
complex (or its resonance, Scheme 4).6b This reaction is perhaps
parallel to the metathesis between MCl5 and LiNMe2 to give
M(NMe2)5 (1a,b). Subsequent oxidation by adventitious mo-
lecular oxygen perhaps returns the Ta(III) imine complex back
to d0 Ta(V) that is eventually converted to 2b through the
coupling of the imine and an amide ligand. There have been
precedents in the use of molecular oxygen in the preparation
of high-oxidation metal complexes.8b,13,14 Chisholm and co-
workers have reported the preparation of d0 Mo(NMe2)6 from
the reaction of d2 Mo(NMe2)4 with O2.13 Wilkinson and co-
workers found that WMe6 could be obtained from the reaction
of WCl6 with 3 equiv of LiMe in the presence of adventitious
molecular oxygen.14 Adventitious molecular oxygen was also
Studies were also conducted to rule out that 2a,b are imine
complexes (Scheme 3).5,9 In either σ-bonded 3b or π-bonded
3b′, Ha and Hb in the imine complexes are expected to be
chemically inequivalent. Only one peak (e.g., 3.98 ppm for 2b)
1
was observed for the -CH2- moiety, and the H-coupled 13C
resonance split into a triplet, indicating that the two H atoms
are chemically equivalent. This observation rules out that 2a,b
(7) See Supporting Information for details.
(8) Additional studies were conducted to rule out the possibility that a
small amount of W(NMe2)6 was present in the samples of 2b, and
preferentially crystallized, giving the observed crystal structure. Resonances
of W(NMe2)6 were not observed in the 1H and 13C NMR spectra of 2b. In
addition, the color of the crystals of 2b is pale yellow, in contrast to the
reported red color for W(NMe2)6.8a,b Analyses by X-ray photoelectron
spectroscopy (XPS) of solids of 2b, Ta(NMe2)5, and W(NMe2)6 [containing
a small amount of W2(NMe2)6] were conducted, and the XPS spectra suggest
that 2b contains tantalum and no tungsten.7 (a) Bradley, D. C.; Chisholm,
M. H.; Heath, C. E.; Hursthouse, M. B. Chem. Commun. 1969, 1261. (b)
Bradley, D. C.; Chisholm, M. H.; Extine, M. W. Inorg. Chem. 1977, 16,
1791.
(10) It is likely that the 13C resonance for η2-imine C atoms is around
80–90 ppm,5c and the stretching band of the η2-imine C-N band may appear
below 1600 cm-1. In addition the IR υC)N bands for σ-imine complexes
could be difficult to observe.9b
(11) (a) Ballantine, J. A.; Purnell, J. H.; Rayanakorn, M.; Williams, K. J.;
Thomas, J. M. J. Mol. Catal. 1985, 30, 373. (b) Murahashi, S.; Yoshimura,
N.; Tsumiyama, T.; Kojima, T. J. Am. Chem. Soc. 1983, 105, 5002. (c)
Shvo, Y.; Laine, R. M. Chem. Commun. 1980, 753.
(9) See, e.g.: (a) Calligaris, M.; Randaccio, L. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A.,
Eds.; Pergamon: New York, 1987; Chapter 20.1. (b) Stark, G. A.; Gladysz,
J. A. Inorg. Chem. 1996, 35, 5509. (c) Galakhov, M. V.; Gómez, M.;
Jiménez, G.; Royo, P.; Pellinghelli, M. A.; Tiripicchio, A. Organometallics
1995, 14, 1901. (d) Coles, N.; Harris, M. C. J.; Whitby, R. J.; Blagg, J.
Organometallics 1994, 13, 190.
(12) (a) Anderson, J. L. U.S. Patent 2,729,679, 1956. (b) Hinze, J.; Curl,
R. F., Jr J. Am. Chem. Soc. 1964, 86, 5068.
(13) Chisholm, M. H.; Hammond, C. E.; Huffman, J. C. J. Chem. Soc.,
Chem. Commun. 1987, 1423.
(14) (a) Shortland, A. J.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1973, 872. (b) Gaylor, L.; Mertis, K.; Wilkinson, G. J. Organomet. Chem.
1975, 85, C37.

1340 Organometallics, Vol. 27, No. 6, 2008
Notes
believed to play a role in the formation of d0 W(NMe2)6 from
reactions of d1 WBr5 or d2 WCl4(THF)2 with LiNMe2.8b
A
process involving disproportionation of the Ta(III) imine species
into Ta(V) and new Ta species at a lower oxidation seems
unlikely, as XPS analyses of the solid residue from the
sublimation of Ta(NMe2)4(η2-MeNCH2NMe2) (2b) suggest that
there was unlikely Ta(0) species in the residue.7
Another possible pathway leading to the formation of the
(aminomethylene)amide -N(Me)CH2NMe2 ligand in 2b is
perhaps similar to that in the reaction of d0 Ta(NMe2)5 (1b)
with O2,4e giving -N(Me)CH2NMe2 ligands in (Me2N)4Ta2[η2-
N(Me)CH2NMe2]2(µ-O)2 and (Me2N)6Ta3[η2-N(Me)CH2N-
Me2]2(η2-ONMe2)(µ-O)3. DFT studies indicate that the reaction
of Ta(NMe2)5 (1b) with O2 yields a peroxide complex
(Me2N)4Ta(η2-O-O-NMe2). The peroxide ligand plays an im-
portant role, including oxidizing an amide to an imine ligand
through the abstraction of a hydride. Insertion of MesNdCH2
into a Ta-amide bond yields the unusual -N(Me)CH2NMe2
ligands.4e There might be a similar pathway involving adventi-
tious molecular oxygen in the reactions of TaCl5 with LiNMe2,
leading to the formation of the (aminomethyl)amide -N-
(Me)CH2NMe2 ligand in 2b.
In the current work, 2b was found to be inert to molecular
oxygen, although it is easily hydrolyzed. Exposure of a solution
of 2b in benzene-d6 to 1 atm of O2 at 23 °C for 17 h showed
no decay of 2b.7 Thus a trace amount of 2b in Ta(NMe2)5 (1b)
does not participate in the reaction with O2.4e It should be noted
that hexacoordinated Mo(NMe2)6 reacts with O2 to give
unknown species,13 while W(NMe2)6 is inert to O2.15
Figure 1. Calculated structures and NMR chemical shifts of 2b
and 3b (bond distances in Å; NMR chemical shifts in ppm). Most
H atoms are omitted for clarity.
DFT calculations16 have been conducted in the current work
to help elucidate the structures of 2b and 3b. The optimized
structures are shown in Figure 1. Among the six ligands in 3b,
the imine coordinates to the Ta center with a long dative bond
(2.554 Å). The average length of the six Ta-N bonds is 2.142
Å, longer than the average [2.075(6) Å] in the crystal structure.7
Each of the five amide planes is neither parallel nor perpen-
dicular to the octahedral skeleton. The tilt angle of each amide
plane with repsect to the octehdral skelton is in the range of
10–25°. 2b adopts almost Cs symmetry. The tilt angles of the
amide planes to the octahedral skeleton are smaller, ca. 3–12°.
However, the octahedral skelton is much more distorted, with
a small N-Ta-N angle of 58.5° in the Ta(η2-MeNCH2NMe2)
moiety. The Namine-Ta distance is 2.629 Å, resulting in an
average Ta-N bond length of 2.146 Å, also longer than the
average [2.075(6) Å] in the crystal structure.7
NMR chemical shifts for 2b and 3b have been calculated
with the DFT method and are given in Figure 1. They show
that experimental values are close to calculated shifts of 2b,
and are different from those of 3b. The calculated chemical shifts
of Ha and Hb in 3b are at ca. 7.3–7.4 ppm and the dCHaHb are
at 153.3 ppm. Both are much downfield shifted from the
observed values.
a feasible process that has a low barrier and is exothermic
(Scheme 5). Both 1b + imine and 2b are ca. 14 kcal/mol more
stable than the imine complex 3b in free energy, suggesting
that 3b is not a stable intermediate to be isolated. Even if 3b is
a possible intermediate, the calculation results show that its
imine ligand likely departs from the metal center or undergoes
a coupling reaction with an amide ligand, yielding the η2-
MeNCH2NMe2 ligand in 2b. The calculation results given in
Scheme 5 show that the formation of 2b via 3b from the reaction
of 1b with imine requires a very high barrier of ca. 37 kcal/
mol in free energy, suggesting that 2b may not directly come
from the complexation of 1b with imine. These results are
consistent with the experimental observations described above
that Ta(NMe2)5 (1b) does not react with imine CH2dNMe.
It is not clear at the present time how the η2-MeNCH2NMe2
ligands in 2a,b are formed in the metathesis reactions between
metal halides and lithium amides. The current work shows that
the metathesis may be complex. The formation of 2a,b helps
shed light on the nature of the metathesis reactions, the most
common route to homoleptic metal amides.
Experimental Section
All manipulations, unless noted, were performed under a dry
N2 atmosphere with the use of Schlenk techniques. All solvents
were purified by distillation from K/benzophenone ketyl. Benzene-
For the energetic consideration, the computational studies
reveal that the complexation between 1b and imine (to give
3b) is an unfavorable process and the conversion of 3b to 2b is
1
d6 was dried over molecular sieves and stored under N2. H and
13C{1H} NMR spectra were recorded on a Bruker AC-250 or
AMX-400 spectrometer. They were referenced with residual protons
and solvent, respectively. FT-IR spectra of KBr pellets were
recorded on a Bio-Rad FTS-60A spectrometer. A Hewlett-Packard
6890 gas chromatograph (GC) with a 5793 mass selective (MS)
detector (MSD) was used to obtain GC/MS data. Elemental analyses
were performed by E+R Microanalytical Laboratory, Corona, New
York.
(15) Bradley, D. C.; Chisholm, M. H.; Extine, W.; Stager, M. E. Inorg.
Chem. 1977, 16, 1794.
(16) All calculations were performed with Gaussian 03. Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B., et al. Gaussian 03, Revision B.03; Gaussian,
Inc.: Wallingford, CT, 2004. A complete author list is given in the
Supporting Information.
(17) Sheldrick, G. M. SADABS, A Program for Empirical Absorption
Correction of Area Detector Data; University of Göttingen: Göttingen,
Germany, 2000. (b) Sheldrick, G. M. SHELXL-97, A Program for the
Refinement of Crystal Structures; University of Göttingen: Göttingen,
Germany, 1997.
Preparation of (Me2N)4Nb(η2-MeNCH2NMe2) (2a). NbCl5
(8.07 g, 29.9 mmol) in THF (30 mL) was added dropwise to 5

Notes
Organometallics, Vol. 27, No. 6, 2008 1341
Scheme 5. Calculated Activation Energies (free energies) and Reaction Energies (free energies) of the Complexation of Imine to
1b and the Transformation of 3b to 2b
equiv of LiNMe2 (7.62 g, 149 mmol) in THF (50 mL) at -78 °C
with vigorous stirring. The dark brown solution was warmed to
room temperature in 40 min. The mixture was stirred at room
temperature for another 23 h. All volatiles were removed in vacuo,
and the solid was extracted with hexanes. The solvent was then
removed from the filtrate in vacuo. Sublimation at 60 °C gave 5.77 g
of a mixture of 92 mol % Nb(NMe2)5 (1a) and 8 mol %
(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.17b In addition the global
refinements for the unit cells and data reductions of the two
structures were performed with the Saint program (version 6.02).
All calculations were performed with the SHELXTL (version 5.1)
proprietary software package.17b
1
(Me2N)4Nb(η2-MeNCH2NMe2) (2a), as revealed by the H NMR
spectrum of the mixture. Initial recrystallization of the mixture in
n-pentane at -30 °C yielded crystals of 1a (4.82 g. 15.3 mmol,
51.0% yield). Afterward, the supernatant solution was concentrated
and cooled to -30 °C to give brown crystals of 2a (0.385 g, 1.08
1
mmol, yield 3.6%). H NMR (benzene-d6, 399.9 MHz, 23 °C) δ
Computational Details
3.76 (s, 2H, Me2NCH2NMe), 3.20 (s, 24 H, NMe2), 3.09 (s, 3H,
Me2NCH2NMe), 2.13 (s, 6H, Me2NCH2NMe). 13C NMR (benzene-
d6, 100.6 MHz, 23 °C) δ 84.0 (Me2NCH2NMe), 48.5 (NMe2), 47.6
(Me2NCH2NMe), 40.4 (Me2NCH2NMe). Anal. Calcd for
C12H35N6Nb: C, 40.45; H 9.90. Found: C, 40.21; H 9.83. In addition,
EA shows no lithium within experimental error, suggesting that
2a is not π-bonded Li2[Nb(NMe2)5(η2-CH2dNMe)].
All calculations were performed by using the Gaussian 03
package16 with density functional theory at the B3LYP level. Full
geometry optimizations were carried out with the basis set LanL2DZ
with f polarization functions18 for Ta and 6-31G* for the rest of
the elements. Vibration frequency calculations were performed for
all the calculated structures. The calculated relative energies shown
in the text have been corrected with zero-point energy (ZPE). NMR
shielding tensors were calculated by using the Gauge-Independent
Atomic Orbital (GIAO) method19 with the same DFT method used
in the geometry optimization. The NMR shielding tensor of
tetramethylsilane (TMS) was calculated as the reference.
Preparation of Ta(NMe2)4(η2-MeNCH2NMe2) (2b). To a slurry
of LiNMe2 (5.36 g, 105 mmol) in hexanes (100 mL) was added
TaCl5 (7.16 g, 20 mmol) slowly at -30 °C with stirring. The
mixture was stirred overnight at room temperature, and volatiles
were removed in vacuo to give dark brown solid. The solid was
sublimed in vacuo at 70 °C to give Ta(NMe2)5 (3.51 g, 8.74 mmol,
43.8%) on a coldfinger at -5 °C. The residue was then further
sublimed in vacuo at 90 °C to give crude 2b as a pale yellow solid
(0.89 g, 2.0 mmol, 10%). The solid was crystallized in Et2O at
Acknowledgment. This work is supported by the Research
Grants Council of Hong Kong (N-HKUST 623/04 and
HKUST 602304), the National Science Foundation (CHEM-
0516928), the British Royal Society Kan Tong Po Visiting
Professorship, and the Changjiang Lecture Professor Pro-
gram. The authors thank Drs. Glenn P. A. Yap and Jeffrey
C. Bryan for help with the crystal structures, and Profs.
Malcolm H. Chisholm and Sandro Gambarotta for helpful
discussions.
1
-30 °C to give pale yellow crystals of 2b. H NMR (benzene-d6,
250.1 MHz, 23 °C) δ 3.98 (s, 2H, CH2), 3.30 (s, 24H, NMe2), 3.13
(s, 3H, MeNCH2), 2.16 (s, 6H, CH2NMe2). 13C NMR (benzene-d6,
1
62.9 MHz, 23 °C) δ 82.7 (CH2, JC-H ) 141.1 Hz), 47.9 (NMe2,
1
1JC-H ) 131.5 Hz), 47.3 (CH2NMe2, JC-H ) 135.5 Hz), 39.1
1
(MeNCH2, JC-H ) 131.8 Hz). HMQC NMR of 1b is consistent
with the assignments. Anal. Calcd for C12H35N6Ta: C, 32.43; H,
7.94; N, 18.91. Found: C, 32.27; H, 7.86; N, 18.73.
Supporting Information Available: Experimental section for
XPS studies; DFT calculations, including calculated total energies
and structures with Cartesian coordinates; crystallographic data and
ORTEP views of 2a,b; XPS and IR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
Hydrolysis of 2b. To 2b (37.5 mg, 0.084 mmol) in benzene-d6
in a Young’s tube was added 6 equiv of H2O (9 µL, 0.504 mmol).
The products were immediately analyzed by 1H NMR, 13C NMR,
1H-coupled 13C NMR, HMQC, and GC/MS. In a separate test, 2
equiv of H2O was added to a solution of 2b in benzene-d6.
Unhydrolyzed 2b, Me2NCH2NMe2, HNMe2, and H2NMe were
OM701173M
1
observed in H NMR.
(18) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth,
A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
Chem. Phys. Lett. 1993, 208, 111.
(19) (a) London, F. J. Phys. Radium 1937, 8, 397. (b) McWeeny, R.
Phys. ReV. 1962, 126, 1028. (c) Wolinski, K.; Hilton, J. F.; Pulay, P. J. Am.
Chem. Soc. 1990, 112, 8251.
Determination of X-ray Crystal Structures of 2a,b. The data
for the X-ray crystal structures of 2a,b were collected on a Smart
1000 X-ray diffractometer equipped with a CCD area detector fitted
with an upgraded Nicolet LT-2 low-temperature device. The data
were obtained with use of a graphite-monochromated Mo source