A monomeric, donor-free lithium complex with a new overcrowded β-diketiminato ligand

Article abstract of DOI:10.1246/cl.2004.134

A monomeric, donor-free complex of lithium with a new overcrowded β-diketiminato ligand, [Li{TbtNC(Me)CHC(Me)-NMes}] (Tbt = 2,4,6-[CH(SiMe₃)₂]₃C₆H₂, Mes = mesityl), was synthesized and isolated as col

Full text of DOI:10.1246/cl.2004.134

134
Chemistry Letters Vol.33, No.2 (2004)
A Monomeric, Donor-free Lithium Complex with a New Overcrowded ꢀ-Diketiminato Ligand
Nobuhiro Takeda, Hirofumi Hamaki, and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received November 4, 2003; CL-031047)
A monomeric, donor-free complex of lithium with a new
overcrowded ꢀ-diketiminato ligand, [Li{TbtNC(Me)CHC(Me)-
NMes}] (Tbt = 2,4,6-[CH(SiMe3)2]3C6H2, Mes = mesityl),
was synthesized and isolated as colorless crystals. Its unique
structure was revealed by X-ray structural analysis to be com-
pared with those of donor-coordinated lithium ꢀ-diketiminates.
of the mixture yielded 3 as a pure material in 85% yield.5
The X-ray structural analysis of 3 showed that there is no in-
termolecular contact between neighboring molecules (the shortest
ꢀ
Li–C intermolecular distance: 3.718(6) A) and no solvent
coordinates to the lithium atom of 3 (Figure 1).6 To the best of
our knowledge, this is the first structural analysis of monomeric,
donor-free lithium ꢀ-diketiminate. It should be noted that
[Li{[DipNC(Me)]2CH}],3
[Li{(Me3Si)NC(R1)CHC(R2)N-
(SiMe3)}] (R1 = R2 = Ph;7 R1 = Ph, R2 = t-Bu;8 R1 = R2 =
Recently, much attention has been paid to the chemistry of ꢀ-
diketiminato ligands.1 In particular, ꢀ-diketiminato ligands hav-
ing bulky substituents on the nitrogen atoms have been success-
fully applied to the synthesis of various types of complexes of
main group elements and transition metals.2 Lithium ꢀ-diketimi-
nates have been most widely used as ligand sources for the prep-
aration of ꢀ-diketiminato complexes, and their properties have
been studied extensively. However, there is no example for X-
ray structural analysis of monomeric, donor-free lithium ꢀ-dike-
timinates, since they have a tendency to dimerize or oligomelize.
Even [Li{[DipNC(Me)]2CH}] bearing bulky Dip (2,6-diisopro-
pylphenyl) groups reportedly exists as a dimer or a dodecamer
in the crystalline states.3 On the other hand, we have succeeded
in the synthesis and isolation of a variety of reactive species con-
taining a low-coordinated heavier main group element by taking
advantage of 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt)
group, which efficiently protects them from the dimerization
and oligomerization.4 In this paper, we present the synthesis of
a monomeric, donor-free lithium complex with a new ꢀ-diketimi-
nato ligand bearing a Tbt group and also the elucidation of its
ability at the complexation with solvents.
Refluxing of a toluene solution of TbtNH2, acetylacetone (10
equiv.), and HCl/Et2O (0.5 equiv.) resulted in the almost quanti-
tative formation of enaminoketone 1 via monocondensation reac-
tion (Scheme 1). Further condensation with MesNH2 in the pres-
ence of TiCl4 (0.7 equiv.) gave an unsymmetrically substituted
enaminoimine 2 in 94% yield. An ether solution of 2 was treated
with 2.5 molar amounts of n-butyllithium at 0 ꢁC, and then the re-
action mixture was stirred at room temperature for 8 h. The sub-
sequent exchange of the solvents to dry hexane resulted in the for-
mation of colorless, moisture-sensitive precipitates of lithium ꢀ-
diketiminate 3. Since 3 was insoluble in hexane, simple filtration
9
NMe2 ), and [Li{(Me3Si)NC(R1)C(R2)(C5H4N-2)}] (R1 = t-
Bu, R2 = H; R1 = Ph, R2 = SiMe3)10 reportedly exist as dimers
or dodecamers in the crystalline states. This difference is most
likely due to the steric hindrance of the extremely bulky Tbt group
on the nitrogen atom in 3. It should be additional interest for the
newly obtained complex 3 that 3 has two intramolecular LiꢂꢂꢂCH3
interactions as judged by the LiꢂꢂꢂC(15) and LiꢂꢂꢂC(29) distances.
The LiꢂꢂꢂCH interaction has been reported for some lithium
amides, alkyllithiums, and ate complexes.11 The C3N2Li ring of
3 is almost planar and the two sets of Li–N, N–C, and C–C dis-
tances in the C3N2Li ring are almost equivalent to each other.
In addition, the N–C and C–C bond lengths are intermediate val-
ꢀ
ues between the typical single [C–N: av 1.469; C–C: av 1.530 A]
and double [C–C=N–C: av 1.279; (C)HCsp2=Csp2(C)2: av
12
1.326 A] bonds. These features observed in the C3N2Li ring
ꢀ
of 3 clearly indicate that the ꢁ-bondings in the C3N2Li ring are
delocalized as well as those of some donor-coordinated lithium
ꢀ-diketiminates, e.g. [Li{(2-NC5H4)2CH}(thf)2],13 [Li{[DipNC-
(t-Bu)]2CH}(thf)],14 [Li{[DipNC(Me)]2CH}(solv)] (solv = thf
or OEt2),3 and [Li{[Et2N(CH2)2NC(Me)]2CH}],15 and the dimer
and dodecamer of [Li{[DipNC(Me)]2CH}].3 The Li–N distances
are close to those of lithium ꢀ-diketiminates having weak Li–O or
LiꢂꢂꢂCH interactions such as [Li{[DipNC(t-Bu)]2CH}(thf)]
14
ꢀ
ꢀ
[1.927(6) A],
[Li{[DipNC(Me)]2CH}(Et2O)]
[1.917(4),
3
1.912(4) A], and [Li{[DipNC(Me)]2CH}]n [n ¼ 2 or 12:
3
1.892(7)–1.919(7) A], and shorter than those of strong donor-co-
ꢀ
ordinated lithium ꢀ-diketiminates, e.g. [Li{[(Me3Si)NC(Ph)]Þ2-
8
ꢀ
CH}(thf)2] [av 2.02(2) A],
[Li{[DipNC(Me)]2CH}(thf)]
C(15)
O
O
TbtNH2, HCl/Et2O
aq NaHCO3
C(29)
N(1)
C(1)
Li(1)
toluene, reflux, 8 h
99%
N(2)
C(3)
Tbt
Tbt
Mes
MesNH2, TiCl4
NH
O
NH N
toluene, reflux, 16 h
94%
C(2)
1
2
Figure 1. ORTEP drawing of 3 (50% thermal ellipsoids). Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond distances (A) and angles (deg): Li(1)–N(1)
Li
n
-BuLi (2.5 eq)
Tbt
Mes
R
R
N
N
Tbt: R = CH(SiMe3)2
Mes: R = Me
1.917(6), Li(1)–N(2) 1.915(6), N(1)–C(1) 1.322(4), N(2)–C(3) 1.328(4),
C(1)–C(2) 1.425(4), C(2)–C(3) 1.418(4), Li(1)ꢂꢂꢂC(15) 2.835(6), Li(1)ꢂꢂꢂC(29)
2.670(5), N(2)–Li(1)–N(1) 103.8(3), Li(1)–N(1)–C(1) 119.0(2), Li(1)–N(2)–
C(3) 117.9(3), N(1)–C(1)–C(2) 123.7(3), C(1)–C(2)–C(3) 130.1(3), N(2)–
C(3)–C(2) 125.1(3).
Et2O
0 °C → rt, 8 h
85% (isolated)
R
3
Scheme 1. Synthesis of lithium ꢀ-diketiminate 3.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.2 (2004)
135
3
[1.958(5) A], and [Li{[Et2N(CH2)2NC(Me)]2CH}] [1.968(3),
ꢀ
the coordinated THF molecule. In addition, the small deviation
of the O(1) atom from the C3N2Li ring plane in the opposite di-
rection toward the C(18) atom may be explained in terms of the
intramolecular LiꢂꢂꢂCH3 interaction between the Li(1) atom and
the C(18)H3 moiety. The bond distance of O(1)–Li(1) for 5 is
15
ꢀ
1.974(3) A].
Interestingly, the solvent-free complex 3 was obtained, when
the lithiation reaction of 2 was performed in Et2O. Furthermore,
the addition of Et2O to a C6D6 solution of 3 showed very little
1
7
change in the H and Li NMR spectra, and the evaporation of
an ether solution of 3 gave only solvent-free 3. By contrast, the
addition of THF instead of Et2O lead to the different results,
which are most likely attributable to the greater ability of THF
as a donor in comparison to Et2O.3 When 1.3 molar amount of
much longer than those of [Li{[DipNC(R)]2CH}(thf)] (R =
3
ꢀ
ꢀ 14
Me: 1.790(7) A, t-Bu: 1.910(6) A ), strongly suggesting the
weaker coordination of THF to the lithium in 5. In addition, the
N(2)–Li(1) bond length of 5 is close to those of 3, while the
N(1)–Li(1) bond length being longer. This result indicates that
the coordination of THF to the lithium atom of 5 mainly affects
the distance of the N(1)–Li(1) bond situated at the pseudo-
trans-position of the O(1)–Li(1) bond, rather than that of the
N(2)–Li(1) bond at the pseudo-cis-position.
7
THF was added to the C6D6 solution of 3, the Li NMR signal
for the mixture was shifted to up-field region (1.89 ppm) com-
pared with that of 3 (2.33 ppm). In the 1H NMR spectrum of
the mixture, the signals assigned to the THF part were observed
at lower field than those of free THF. The addition of 9.6 molar
amounts of THF to the solution of 3 in C6D6 resulted in the ap-
The unique coordination of THF in 5 implies that transition
metal and main group element complexes having this ꢀ-diketimi-
nato ligand would show unique properties. Further investigation
on the ꢀ-diketiminato lithium complex 3 and the application of
this ligand to the syntheses of other metal complexes are currently
in progress.
This work was partially supported by Grants-in-Aid for COE
Research on Elements Science (No. 12CE2005), Scientific Re-
search (Nos. 14078213 and 15750031), and the 21 COE on Kyoto
University Alliance for Chemistry from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan.
7
pearance of the Li signal at much higher field (1.69 ppm) along
with the 1H NMR resonances for the THF moiety being close to
those of free THF. These results suggest the rapid equilibrium be-
tween the free complex 3 plus THF and the THF adduct
[Li{TbtNC(Me)CHC(Me)NMes}(thf)] (4) at room temperature
within the time scale of NMR spectroscopy. That is, the addition
of a large amount of THF may lead to the increase in the ratio of 4
compared to 3 together with the augmentation in the ratio of free
THF relative to the coordinated THF in 4. When the C6D6 solu-
tion containing 3 and THF was fully evaporated, the lithium ꢀ-di-
ketiminate coordinated by one THF molecule 4 was isolated as
colorless solids.16 Although a single crystal of 4 suitable for X-
ray structural analysis has not been obtained yet, we have suc-
ceeded in the crystallographic analysis of the analogously pre-
pared lithium complex having Tbt and Ph groups on the N-termi-
nals, [Li{TbtNC(Me)CHC(Me)NPh}(thf)] (5).6
References and Notes
1
L. Bourget-Merle, M. F. Lappert, and J. R. Severn, Chem. Rev., 102, 3031
(2002).
2
Recent papers, see: P. G. Hayes, W. E. Piers, and M. Parvez, J. Am. Chem. Soc.,
125, 5622 (2003); F. Basuli, B. C. Bailey, J. Tomaszewski, J. C. Huffman, and D.
J. Mindiola, J. Am. Chem. Soc., 125, 6052 (2003).
3
4
M. Stender, R. J. Wright, B. E. Eichler, J. Prust, M. M. Olmstead, H. W. Roesky,
and P. P. Power, J. Chem. Soc., Dalton Trans., 2001, 3465.
Recent papers, see: N. Nakata, N. Takeda, and N. Tokitoh, Angew. Chem., Int.
Ed., 42, 115 (2003); A. Shinohara, N. Takeda, and N. Tokitoh, J. Am. Chem.
Soc., 125, 10804 (2003).
Spectral data of 3: 1H NMR (300 MHz, C6D6): ꢂ 0.06 (s, 18H), 0.18 (s, 18H),
0.22 (s, 18H), 1.43 (s, 1H), 1.76 (s, 3H), 1.90 (s, 3H), 2.20 (s, 2H), 2.23 (s,
3H), 2.27 (s, 6H), 4.91 (s, 1H), 6.58 (br s, 2H), 6.92 (s, 2H); 7Li NMR
Figure 2 shows that 5 has a planar C3N2Li ring, in which the
two sets of C–C and N–C distances are very similar to each other,
indicating the delocalization of the ꢁ-bondings, as well as that of
3. The LiN2O moiety has nonplanar structure (the sum of angles
around Li(1): 353.8ꢁ), and the coordinated O(1) atom leans to the
side of phenyl group as judged by the remarkably larger N(1)–
Li(1)–O(1) bond angle than the N(2)–Li(1)–O(1) bond angle.
By contrast, the reported lithium ꢀ-diketiminates coordinated
with one THF molecule, [Li{[DipNC(R)]2CH}(thf)] (R = Me,3
t-Bu14), have a trigonal planar geometry around the lithium atom
with a narrower N–Li–N bond angle. This largely distorted T-
shaped structure at the lithium atom of 5 is probably due to the
severe repulsion between the extremely bulky Tbt group and
5
6
(116 MHz, C6D6) ꢂ 2.33.
ꢁ
Crystal data of 3:
C
41H77LiN2Si6; Mr ¼ 773:53, triclinic, P1 (no. 2),
ꢀ
ꢀ
a ¼ 9:4485ð11Þ, b ¼ 13:2943ð11Þ A, c ¼ 20:5252ð18Þ A, ꢃ ¼ 99:097ð7Þ, ꢀ ¼
ꢁ
ꢀ 3
91:883ð8Þ, ꢄ ¼ 97:439ð8Þ , V ¼ 2520:6ð4Þ A , Z ¼ 2, T ¼ 103ð2Þ K, Dcalcd
¼
1:019 g cmꢃ1
,
independent reflections 8165 (Rint ¼ 0:0362), R1 ½I >
2ꢅIꢄ ¼ 0:0566, wR2 (all data) = 0.1239. CCDC-221891. Crystal data of 5:
C42H79LiN2OSi6; Mr ¼ 803:55, monoclinic, P21=c (no. 14), a ¼ 13:041ð2Þ,
ꢁ
ꢀ
ꢀ
ꢀ 3
b ¼ 21:711ð4Þ A, c ¼ 18:047ð3Þ A, ꢀ ¼ 98:7507ð19Þ , V ¼ 5050:2ð15Þ A ,
Z ¼ 4, T ¼ 93ð2Þ K, Dcalcd ¼ 1:057 g cmꢃ1
, independent reflections 8797
(Rint ¼ 0:0243), R1 ½I > 2ꢅIꢄ ¼ 0:0457, wR2 (all data) = 0.1169. CCDC-
221892.
P. B. Hitchcock, M. F. Lappert, and D.-S. Liu, J. Chem. Soc., Chem. Commun.,
1994, 1699.
7
8
9
P. B. Hitchcock, M. F. Lappert, M. Layh, D.-S. Liu, R. Sablong, and T. Shun, J.
Chem. Soc., Dalton Trans., 2000, 2301.
X. Chen, C. Du, J.-P. Guo, X.-H. Wei, and D.-S. Liu, J. Organomet. Chem., 655,
89 (2002).
O(1)
C(18)
10 B.-J. Deelman, M. F. Lappert, H.-K. Lee, T. C. W. Mak, W.-P. Leung, and P.-R.
Wei, Organometallics, 16, 1247 (1997).
11 M. A. Beswick and D. S. Wright, in ‘‘Comprehensive Organometallic Chemistry
II,’’ ed. by E. W. Abel, F. G. Stone, and G. Wilkinson, Pergamon, Oxford, New
York, Tokyo (1995), Vol. 1, p 1.
Li(1)
N(2)
N(1)
12 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen, and R. Taylor,
J. Chem. Soc., Perkin Trans. 2, 1987, S1.
13 H. Gornitzka and D. Stalke, Organometallics, 13, 4398 (1994).
14 P. H. M. Budzelaar, A. B. v. Oort, and A. G. Orpen, Eur. J. Inorg. Chem., 1998,
1485.
15 D. Neculai, H. W. Roesky, A. M. Neculai, J. Magull, R. Herbst-Irmer, B.
Walfort, and D. Stalke, Organometallics, 22, 2279 (2003).
16 Spectral data of 4: 1H NMR (300 MHz, C6D6): ꢂ 0.14 (s, 18H), 0.20 (s, 18H),
0.25 (s, 18H), 1.19 (m, 4H), 1.44 (s, 1H), 1.74 (s, 3H), 1.91 (s, 3H), 2.23 (s,
5H), 2.24 (s, 6H), 3.31 (m, 4 H), 4.91 (s, 1 H), 6.59 (br s, 2 H), 6.87 (s, 2 H);
7Li NMR (116 MHz, C6D6) ꢂ 2.01.
C(1)
C(3)
C(2)
Figure 2. ORTEP drawing of 5 (50% thermal ellipsoids). Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond distances (A) and angles (deg): Li(1)–N(1)
1.976(4), Li(1)–N(2) 1.923(4), Li(1)–O(1) 1.965(4), N(1)–C(1) 1.330(3),
N(2)–C(3) 1.326(3), C(1)–C(2) 1.413(3), C(2)–C(3) 1.404(3), Li(1)ꢂꢂꢂC(18)
2.809(4); N(1)–Li(1)–N(2) 100.9(2), N(1)–Li(1)–O(1) 148.5(2), N(2)–Li(1)–
O(1) 104.4(2), Li(1)–N(1)–C(1) 118.1(2), Li(1)–N(2)–C(3) 121.1(2), N(1)–
C(1)–C(2) 125.2(2), C(1)–C(2)–C(3) 130.6(2), N(2)–C(3)–C(2) 123.2(2).
Published on the web (Advance View) January 9, 2004; DOI 10.1246/cl.2004.134