Improved Synthesis of Unsymmetrical N-Aryl-N′-alkylpyridyl ?-Diketimines Using Molecular Sieves and their Lithium Complexes

Article abstract of DOI:10.1002/ejic.201701383

The synthesis of unsymmetrical β-diketimines with N-aryl-N′-alkyl substitution require at least two condensation steps and tedious purification procedures. An improvement in the preparation of four unsymmetrical N-aryl-N′-alkylpyridyl β-diketimine ligands

Full text of DOI:10.1002/ejic.201701383

A Journal of
Accepted Article
Title: Improved Synthesis of Unsymmetrical N-aryl-N'-alkylpyridyl ß-
Diketimines using Molecular Sieves and their Lithium Complexes
Authors: Kuldeep Chand, Chen-Long Tsai, Hsuan-Ying Chen, Wei-Min
Ching, Sung-Po Hsu, James R. Carey, and Sodio Chin Neng
Hsu
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701383
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10.1002/ejic.201701383
European Journal of Inorganic Chemistry
FULL PAPER
Improved Synthesis of Unsymmetrical N-aryl-N’-alkylpyridyl ß-
Diketimines using Molecular Sieves and their Lithium Complexes
Kuldeep Chand,[a] Cheng-Long Tsai,[a] Hsuan-Ying Chen,[a] Wei-Min Ching,[b] Sung-Po Hsu,*[c] James R.
Carey,*[a,d] Sodio C. N. Hsu,*[a,e]
Abstract: The synthesis of unsymmetrical β-diketimines with N-
aryl-N’-alkyl substitution require at least two condensation steps
and tedious purification procedures. An improvement for the
preparation of four unsymmetrical N-aryl-N’-alkylpyridyl β-
diketimine ligands (HL1-HL4) using 5 Å molecular sieves was
reported. Synthesis of four Li complexes (LiL1, LiL2, LiL3, and
LiL4) containing N-aryl-N’-alkylpyridyl β-diketimines ligands is
also presented. All four Li complexes were fully characterized by
1H NMR, 13C NMR, and elemental analysis. X-ray structure
analysis and spectroscopic results indicate that the Li β-
diketimine structures and ligand binding modes are governed by
the bulkiness of the N-aryl substituent and the length of pyridyl
arm of the N-aryl-N’-alkyl β-diketiminate ligand.
synthesis of symmetrical β-diketimines is the use of molecular
sieves that may serve as a catalysts.[29-30] In light of the above, a
simple procedure is needed to synthesize unsymmetrical N-aryl-
N’-alkylpyridyl β-diketimines (UNAADpy). Herein, we report a
straightforward procedure for the synthesis of UNAADpy using 5
Å molecular sieves (Scheme 1) in high yield without further
purification. This high yield preparation may lead to an increase
in the application of unsymmetrical complexes such as UNAADpy.
Introduction
During past several decades, β-diketimines have been widely
used as chelating and sterically crowded spectator ligands.[1-7]
Introducing hemilabile pendant pyridyl donors in unsymmetrical
N-aryl-N’-alkyl β-diketimines (UNAAD) in the preparation of
unsymmetrical N-aryl-N’-alkylpyridyl β-diketimines (UNAADpy)
may introduce new electronic and steric binding features which
can aid in the stabilization and to the ability to modulate the
reactivity of unsaturated metallated complexes.[8-11] In order to
understand the coordination abilities of pendant donor of
UNAADpy, we recently reported several UNAADpy ligands and
their corresponding Zn(II) and Cu(I) complexes.[11-12] However the
isolated yields of UNAADpy were low (~50%) and required
several purification steps.[9-10] β-diketimines are usually prepared
by the condensation of acetylacetone with two equivalents of
primary amine in the presence of acid.[13-18] It is worthwhile to
mention that most β-diketimines have symmetrical N, N’-aryl or N,
N’-alkyl substituents,[13, 17, 19-22] but only a few unsymmetrical N-
β-diketiminato Li complexes are often used as ligand
transfer reagents in the preparation of transition metal
coordination complexes due to their high yield lithiation and
31-45]
transmetalation activities.[6,
For example, metal β-
diketiminate complexes with rare-earth or d-block transition
metals can be synthesized by reaction of β-diketiminato Li
complexes with their corresponding metal halides.[1, 5, 46-51] Indeed,
there are few studies describing the preparation of Li complexes
bearing unsymmetrical N-aryl-N’-alkyl β-diketimines (UNAAD).[52-
53]
In order to explore the lithiation chemistry of UNAADpy, we
synthesized four UNAADpy Li complexes (LiL1, LiL2, LiL3, and
LiL4) and investigated their binding modes in solution as well as
in the solid state.
Results and Discussion
Ligand synthesis.
aryl-N’-alkyl β-diketimines (UNAAD)[23-26] have been reported due
to their tedious synthetic procedures.[8-11,
Symmetric β-
27]
β-diketimines exist as symmetrical and unsymmetrical ligands
due to the variability of the substitutions on the backbone or
enamine functional groups. Symmetrical β-diketimines can be
synthesized in a one pot reaction from acetylacetone and 2
equivalents of amine using acid catalysts such as TsOH/HCl (or
mixtures of both) in 80~95% yield.[17] Current preparations of
unsymmetrical N-aryl-N’-alkyl β-diketimines (UNAAD; in 50-60%
yield) requires a two-step procedure starting from acetylacetone
diketimines can be synthesized under several conditions with or
without the azeotropic removal of water or with stoichiometric or
17, 28]
catalytic amounts of acid.[4,
Another approach for the
[a]
[b]
[c]
[d]
[e]
Department of Medicinal and Applied Chemistry, Kaohsiung
Medical University, Kaohsiung 807, Taiwan.
Email: sodiohsu@kmu.edu.tw
Instrumentation Center, Department of Chemistry, National
Taiwan Normal University, Taipei 11677, Taiwan.
Email: seesaw037@gmail.com
Department of Physiology, School of Medicine, College of
Medicine, Taipei Medical University, Taipei 110, Taiwan.
Email: sphsu@tmu.edu.tw
Department of Applied Chemistry, National University of
Kaohsiung, Kaohsiung 804, Taiwan.
Email: jcarey@nuk.edu.tw
Department of Medical Research, Kaohsiung Medical
University Hospital, Kaohsiung 807, Taiwan.
Supporting information for this article is available on the
WWW under htttp://
and arylamine followed by second condensation with another
10]
alkylamine.[8,
Recently, our group reported the synthesis of
UNAADpy ligands HL1-HL4 using catalytic quantities of TsOH.
1
The reaction was monitored for two days using H NMR (Table
1).[11-12] We found that using one equivalent of TsOH gave low
product yields that required further purification. From a rational
synthetic standpoint, UNAAD ligands could be synthesized from
aryliminoketone or alkyliminoketone with alkyl- or aryl-amine,
respectively. According to literature precedence, the order of aryl-
17]
or alkyl-amine addition is important.[10,
Reaction between
aryliminoketone with alkylamine is usually preferred as compared
to reaction of alkyliminoketone with arylamine. As determined by
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10.1002/ejic.201701383
European Journal of Inorganic Chemistry
FULL PAPER
1, pyridylalkyliminoketone (alkyliminoketone) with one or two
equivalents of aryl amine forms symmetrical N,N’-diaryl
Table 1. Influence of molecular sieves in the preparation of N-
aryl-N’-alkylpyridyl ß-diketimines (UNAADpy)
substituted ß-diketimine as the major product with a minor Ligands
statistical mixture (e.g. N-aryl-N’-alkyl β-diketimines and N,N’-
dialkyl β-diketimines) and free pyridylalkylamine. Therefore,
Yield using 5 Å Yield
using Reaction time (h)
Molecular Sieves TSOH (%) [b]
(%)[a]
UNAADpy ligands HL1-HL4 should be synthesized from
aryliminoketone HLA and HLB with a suitable pyridylalkylamine
(Scheme 1).
HL1
HL2
HL3
HL4
96
95
98
95
57
55
56
66
48
48
48
48
The synthesis of diketimines and enamines are
accomplished using condensation reactions.[54-58] It is thought that
molecular sieves drive condensation reactions in the forward
direction by behaving as activating or dehydrating agents.[29-30, 57,
59] Another benefit of using molecular sieves that they are easily
removed after filtration.[59] According to 1H NMR monitoring,
preparation of UNAADpy (HL1-HL4) is achieved in 48 hrs using 5
Å molecular sieves (Table 1; Scheme 1). Confirmation of the
desired HL1-HL4 products was accomplished using 1H NMR
spectroscopy (see the supporting information) The purity (>98%)
was determined by comparison to our previous data.[11]
[a] Isolated yield of final product in presence of molecular sieves
5 Å. [b] Isolated yields of final product in presence of TsOH.
β-diketiminato ligand L1- or L2-. On the other hand, the LiL3 ligand
has a binuclear structure with an unusual Li coordination mode.
Attempts to grow crystals of LiL4 from a saturated hexane solution
were unsuccessful (results not shown). Vapour diffusion of
pentane into a solution of LiL4 in THF gave X-ray quality crystals
yellow crystals that revealed a THF solvated LiL4·(THF) product.
During our investigation of LiL1-LiL4 complexes, we found that
LiL1-LiL4 are quite soluble in non-polar solvents such as hexane,
pentane, and benzene. These complexes are stable in dry air but
decompose in the presence of moisture. The synthetic procedure
and crystallization conditions for the four Li complexes are shown
in Scheme 2.
Synthesis and Characterization of Li Complexes.
UNAADpy (HL1, HL2, HL3, and HL4) were treated with an
equimolar amount of n-butyllithium providing the desired Li
complexes LiL1, LiL2, LiL3, and LiL4, respectively (Scheme 2).
The fact that n-butyllithium can deprotonate the β-diketimine
ligands gives clues regarding the coordination ability of Li within
these metal complexes. The absence of downfield N-H
resonances in the 1H NMR spectrum of the Li complexes
suggests that the β-diketimines are deprotonated to their anionic
form. The 1H NMR spectra of the four Li complexes (L1-L4) show
a single type of resonance associated with the β-diketiminato
backbone and the chelated pyridyl arm, indicating that the solution
structures of LiL1, LiL2, LiL3, and LiL4 are rather similar. All Li
complexes were characterized by 1H NMR, 13C NMR, X-ray
diffraction, and elemental analysis. X-ray diffraction quality
crystals of complexes LiL1, LiL2, and LiL3 were obtained from
saturated hexane solutions (Table S1). The crystallographic
results of LiL1 and LiL2 show that the Li cation is chelated by the
Molecular Structures of Li Complexes.
The molecular structures of all four Li complexes are shown in
Figure 1. LiL1 crystallized with two crystallographically
independent but structurally similar molecules in the asymmetric
unit. Figure 1a shows only one LiL1 molecule in the asymmetric
unit due to identical coordination geometries, bond lengths, and
bond angles for the 2nd molecule in the unit cell. The anionic
tridentate ligand L1- displays a slightly distorted T-shaped
coordination geometry with a slight dislocation of Li (0.086 Å) from
the least-squares-plane (LSP) derived from the three nitrogen
atoms. The pyridylmethyl arm of the LiL1 forms a nearly planar
five-member ring about the Li centre, and connects with the planar
six-membered diazapentadienyl bidentate chelate ring by a Li
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10.1002/ejic.201701383
European Journal of Inorganic Chemistry
FULL PAPER
bridge. In contrast to LiL1, the Li centre of LiL2 (Figure 1b)
exhibits a non-planar distorted Y-shaped coordination geometry
and is positioned below the LSP derived from the three nitrogen
atoms by approximately 0.379 Å. This distortion may be due to
the longer pyridylethyl arm that is chelated to the Li center. Indeed,
the pyridylethyl arm of the LiL2 forms a six-member ring in the
twist boat conformation that is connected to the planar six-
membered diazapentadienyl bidentate chelate ring by a Li bridge.
The molecular structure of LiL3, in contrast to LiL1, is
binuclear with an unusual Li coordination mode (Figure 1c). The
unusual dinuclear structure of LiL3, as compared to LiL1, may due
to the decreased steric influence of the ortho position of N-aryl
substituent (2,4,6-trimethylphenyl) of the β-diketiminato ligand L3-.
One of the Li's in the dinuclear LiL3 complex displays a distorted
tetrahedral environment that is coordinated by a bidentate
diazapentadienyl chelate ring and two pyridyl moieties. The other
Li atom in LiL3 is coordinated by three nitrogen atoms giving rise
to a trigonal pyramidal coordination mode. The trigonal pyramidal
coordination mode involves a bidentate diazapentadienyl chelate
ring and a bridged amide group from another diazapentadienyl
chelate ring. The three coordinate Li ion sits 0.512 Å below the
LSP of three nitrogen atoms. Using THF as a crystallization
solvent gave a THF solvated product LiL4·(THF) (Figure 1d). The
anionic tridentate ligand L4- exhibits distorted tetrahedral
coordination geometry with a coordinated THF molecule. The
pyridylethyl arm of the LiL4·(THF) forms a six-membered ring in
the twist boat conformation that connects to the planar six-
membered diazapentadienyl bidentate chelate ring by a Li bridge.
In contrast to LiL2, the Li of LiL4·(THF) sits out of the LSP of three
nitrogen atoms by 0.706 Å.
due to the hemilabile nature of the pyridylmethyl arm during the
crystallization packing process.
Complexation of LiL3 and LiL4 with THF.
In order to understand the THF binding ability of LiL3 and LiL4, an
1H NMR titration was performed by the sequential addition of THF
to a C6D6 solutions of LiL3 and LiL4 (Fig. S9-S10 respectively).
The NMR signals assigned to THF increasingly moved to lower-
field upon the gradual addition of THF. These titration results are
similar to a previous 1H NMR titration study of symmetrical N, N’-
aryl β-diketiminato Li complexes that suggest a rapid equilibrium
at 298K between THF coordination and de-coordination in the
NMR time scale.[52] Therefore, a similar behaviour of rapid
equilibrium between THF coordination and de-coordination in the
1H NMR time scale for LiL3 and LiL4 is likely. It is reasonable to
conclude that THF binds rather weakly to LiL3 and LiL4.
In common with all known structures of N,N’-aryl substituent
β-diketiminato Li complexes,[2, 60] the NCCCN backbone atoms
are coplanar, indicating a delocalized electronic structure. The
pyridyl group of the LiL1-LiL4 complexes coordinates to the Li ion
with Li-Npy bond lengths of 2.061~2.170 Å. These bonds are
longer than the Li-Namide bond lengths of 1.864~2.008 Å of the
NCCCN backbones. For comparison of LiL1-LiL4 to the free
ligand HL1, selected bond distances and angles, are listed in
Table 2. Considering the crystallographic results of all of the Li
complexes discussed above, there are two factors that apparently
affect the structure of the Li complexes containing the UNAADpy
ligand. First, the bulky 2,6-bisisopropylphenyl substituent of L1-
and L2- may encourage the LiL1 and LiL2 complexes to form
tridentate three coordinated mononuclear compounds. On the
other hand, the less bulky 2,4,6-trimethylphenyl substituents of
HL3 and HL4 may lead to an increase in vacancy around the
metal center allowing for increased coordination. Indeed, LiL3
and LiL4·(THF) both display four coordinate environments about
the Li center. Second, the length of pyridyl arm in UNAADpy
ligands, may also affect the metal center geometry and the
behaviour of the tridentate chelation or bridged dimerization
binding modes. In general, the chelating pyridylmethyl arms form
five-membered rings with anionic β-diketiminato moieties L1- and
L3-, and the pyridylethyl arms form six-membered rings with the
anionic β-diketiminato moieties L2- and L4-. In a chelating ring
stability study of corresponding Zn(II) and Cu(I) complexes,[11-12]
we found that the five-membered chelating ring (pyridylmethyl
arm) have stronger tendency to becomes a pendent arm than the
six-membered chelating ring (pyridylethyl arm) in solution.
Therefore, the unusual dinuclear structure formation of LiL3 may
Conclusions
We successfully synthesized a series of unsymmetrical N-aryl-N’-
alkylpyridyl β-diketimines (UNAADpy; HL1-HL4) using molecular
sieves 5 Å as a catalyst with improved yield compared to using
acid (TsOH) catalysts. The lithiation products of LiL1-LiL4 were
obtained by treatment of HL1-HL4 with nBuLi, respectively. X-ray
structure analysis and spectroscopic results reveals three types
binding modes, monomer (LiL1, LiL2), dimer (LiL3), and THF
adduct (LiL4·THF). LiL3 exists as dimer due to the less bulky N-
aryl substituent and shorter pyridylmethyl arm, while LiL4 exists
as a THF-adduct due to the bulkier N-aryl substituent and longer
pyridylethyl arm. In summary, we obtained high yields of ligands
HL1-HL4 and the observed variable binding modes in LiL1-LiL4
due to the bulkiness of the N-aryl substituent and the length of
pyridyl arm.
Experimental Section
All manipulations involving Li complexes were carried out in an
atmosphere of purified dinitrogen in a dry box, or using standard
Schlenk line techniques. Chemical reagents were purchased from
Aldrich Chemical Co. Ltd., Lancaster Chemicals Ltd., or Fluka Ltd.
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10.1002/ejic.201701383
European Journal of Inorganic Chemistry
FULL PAPER
Table 2. Selected bond lengths (Å) and angles (deg) for HL1, LiL1, LiL2, LiL3 and LiL4 complexes
HL1
LiL1
LiL2
LiL3
LiL4(THF)
Molecule A
1.940(10)
Molecule B
1.898(10)
Li-Namide(aryl)
1.892(9)
1.968(17)c;
1.997(16)d
1.992(4)
Li- Namide(alkyl)
Li-Npy
1.872(10)
2.061(11)
1.858(10)
2.074(10)
1.874(8)
2.034(9)
2.002(19)c;
2.011(16)d
2.046(17)d;
2.104(16)d
1.979(4)
2.172(4)
C-C(NCCCN backone)
1.431(5)
1.367(5)
1.393(7)
1.410(7)
1.401(7)
1.406(7)
1.413(6)
1.417(6)
1.404(12)c;
1.414(12)c
1.407(3)
1.413(3)
1.407(11)d;
1.400(12)d
0.010c; 0.007d
0.006
a
0.064
0.017
0.005
0.004
C-C
C-N(NCCCN backone)
1.305(4)
1.323(6)
1.334(6)
1.328(5)
1.314(10)c;
1.322(10)c
1.326(3)
1.350(4)
0.045
1.316(6)
1.314(6)
1.322(5)
1.321(10)d;
1.293(10)d
1.315(3)
0.008c; 0.028d
0.011
a
0.007
0.020
0.006
C-N
Namide-Li-Namide
Namide(aryl)-Li-Npy
99.2(5)
172.6(6)
100.1(5)
173.3(6)
102.0(4)
142.6(5)
97.8(8)c; 89.6(7)d
103.1(7)d; 140.6(8)d 131.7(2)
96.38(19)
Namide(alkyl)-Li-Npy
86.0(4)
72.8
84.8(4)
71.29
102.5(4)
77.15
137.5(8)d; 83.4(6)d
84.90c; 88.26d
93.93(17)
85.09
∠NCCCN, arylb
81.44
a Bond length difference of NCCCN backbone. b∠angle between the NCCCN backbone plane and N-aryl ring. c Structural parameters
around three coordinated Li ion. d Structural parameters around four coordinated Li ion.
HL2-HL4: Following a similar procedure as described for HL1,
HL2-HL4 were obtained as reddish brown, dark brown, and
glittering yellow oil in yield 95-98% respectively. The 1H NMR data
(details in supporting information) regarding NH, Py-H, backbone-
CH protons, and ESI-MS m/z data are in agreement as discussed
in our previous literature data reports.[11]
All reagents were used without further purification, apart from all
solvents, which were dried over Na (Et2O, THF) or CaH2 (CH2Cl2,
CH3CN) and then thoroughly degassed before use. Molecular
sieves (5 Å, 8-12 mesh) were purchased from Acros Ltd. 2-((2,6-
diisopropylphenyl)imido)-2-penten-4-one[61]
trimethylphenyl)imido)-2-penten-4-one[62]
and
were
2-((2,4,6-
synthesized
following published procedures. Molecular sieves (5 Å, 8-12
mesh) dried under vacuum overnight at 250°C prior to use. 1H
NMR and 13C NMR spectra were acquired using a Varian Gemini-
200 proton/carbon FT NMR, 400 MHz or a Varian Gemini-500
proton/carbon FT NMR spectrometer. ESI mass spectra were
collected using a Waters ZQ 4000 mass spectrometer. Elemental
analyses were performed using a Heraeus CHN-OS Rapid
Elemental Analyzer.
Lithiation: LiL1, LiL2, LiL3, and LiL4 were synthesized using
similar procedures. The 1H NMR and 13C NMR spectra of
compounds LiL1-LiL4 in C6D6 solution are shown in Fig. S7-S14
respectively. As a representative example the synthesis of LiL1 is
given in detail below.
LiL1: In an inert atmosphere, nBuLi (1.01 mL, 1.05 equiv, 1.60 M
in hexanes) was added dropwise to a stirred solution of HL1
(0.527 g, 1.51 mmol) in benzene (0.898 mL). The solution was
stirred for approximately 60 min producing a yellow precipitate.
The reaction was reduced in volume and placed at -20 °C
overnight. The solution was filtered under vacuum to give a cream
yellow solid (0.415 g, 77%). Single crystals of LiL1 suitable for X-
ray diffraction were obtained from several recrystallizations from
hexane solutions at -20°C. 1H NMR (C6D6, 200 MHz, 298K): δ
7.30 (d, J = 5.3 Hz,1H, Py1l1), 7.21-7.15 (m, 3H, Ar-Hj1, k1), 6.85
(td, J = 7.6 Hz, 1H, Py3i1), (d, J = 8 Hz, 1H, Py4h1), 6.32 (t, J
= 6.4 Hz, 1H, Py2g1), 5.09 (s, 1H, backone-CHf1), 4.51 (s, 2H,
CH2Pye1), 3.53 (septet, 2H, J = 7 Hz, ArCH(CH3)2, d1), 2.02 (s, 3H,
backone-CH3, c1), 1.98 (s, 3H, backone-CH3, b1), 1.30 (d, 6H, J = 7
Hz, ArCH(CH3)2, a1), 1.18 (d, 6H, J = 7 Hz, ArCH(CH3)2, a1). 13C
NMR (C6D6, 100 MHz, 298K): δ 165.11, 164.73, 162.79, 150.09,
Ligand Synthesis: The syntheses of HL1-HL4 were carried out
using similar procedures. As a representative example, the
synthesis of HL1 is given in detail below.
HL1: 2-(2,6-diisopropylphenylimido)-2-pentene-4-one (4.2 g,
0.39 mmol) and 2-(aminomethyl) pyridine (2.0 mL, 0.39 mmol)
were refluxed in toluene (50 mL) for 2 days in presence of 5 Å
molecular sieves (30 g). The resultant mixture was filtered and
evaporated under reduced pressure. The filtrate was extracted
1
with hexane to obtain a yellow solid (5.4 g, 96%). The H NMR
data (details in supporting information) regarding NH, Py-H,
backbone-CH protons, and ESI-MS m/z data are in agreement as
discussed in our previous literature data reports.[11]
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10.1002/ejic.201701383
European Journal of Inorganic Chemistry
FULL PAPER
148.16, 141.07, 136.41, 123.26, 122.92, 122.21, 120.95, 93.92,
55.03, 28.06, 25.18, 23.43, 22.88, 21.17. Anal. Calcd for
C23H30LiN3: C, 77.72; H, 8.51; N, 11.82. Found: C, 77.69; H, 8.55;
N, 11.81.
structures were solved by direct methods and were refined on F2
by the full-matrix least-squares method using the SHELXL-2014/7
program.[63] All non-hydrogen atoms were refined anisotropically,
whereas hydrogen atoms were placed at the calculated positions
and included in the final stage of refinements with fixed
parameters. Further details are given in Table S1 in the
supporting information. CCDC No. 1814931 (for LiL1), 1814932
(for LiL2), 1814933 (for LiL3), and 1814934 (for LiL4) contain the
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
LiL2: In an inert atmosphere, nBuLi (1.01 mL, 1.05 equiv, 1.60 M
in hexanes) was added dropwise to a stirred solution of HL2
(0.548 g, 1.51 mmol) in benzene (0.898 mL). LiL2 was isolated
from hexane to give orange crystals (0.367 g, 66 %). 1H NMR
(C6D6, 200 MHz, 298K): δ 7.66 (d, J = 5.3 Hz, 1H, Py1n2), 7.30-
7.15 (m, 3H, Ar-Hk2-m2), 6.85 (td, J= 7.6 Hz, 1H, Py3j2),(d, J=
8 Hz, 1H, Py4i2), 6.32 (t, J = 6.4 Hz, 1H, Py2h2), 4.95 (s, 1H,
backone-CHg2), 3.50 (septet, J = 7 Hz, 2H, ArCH(CH3)2, f2), 3.66
(t, J = 7 Hz, 2H, CH2CH2Pye2), 2.63 (t, J = 7 Hz, 2H, CH2CH2Pyd2),
1.96 (s, 3H, backone-CH3, c2), 1.93 (s, 3H, backone-CH3, b2), 1.29
(d, J = 7 Hz, 6H, ArCH(CH3)2, a2), 1.18 (d, J = 7 Hz, 6H,
ArCH(CH3)2, a2). 13C NMR (C6D6, 100 MHz, 298K): δ 165.32,
162.65, 162.18, 150.12, 148.41, 141.25, 137.86, 124.33, 123.11,
122.78, 121.34, 93.35, 49.47, 40.93, 27.98, 25.01, 23.34, 23.07,
21.27. Anal. Calcd for C24H32LiN3: C, 78.02; H, 8.73; N, 11.37.
Found: C, 77.99; H, 8.74; N, 11.40.
Acknowledgements.
We gratefully acknowledge financial support from the Ministry of
Science and Technology of Taiwan (MOST 102-2113-M-037-008-
MY3; MOST 104-2632-M-037-001), and Kaohsiung Medical
University (Aim for the Top University Grant, grant No. KMU-
TP105PR12) for generous funding. We thank Mr. Ting-Shen Kuo
from the National Taiwan Normal University for X-ray structural
determinations and Mr. Min-Yuan Hung from the Center for
Research Resources and Development of KMU for their facilities.
LiL3: In an inert atmosphere, nBuLi (1.04 mL, 1.08 equiv, 1.60 M
in hexanes) was added dropwise to a stirred solution of HL3
(0.463 g, 1.51 mmol) in hexane (0.928 mL). This complex was
isolated from hexane at to give yellow crystals (0.269 g, 57%).1H
NMR (C6D6, 400MHz, 298K): δ 7.21 (s, 1H, Py1k3), 6.98 (s, 2H,
Ar-Hj3), 6.85(td, J = 7.6 Hz, 1H, Py3i3), (d, J = 8 Hz, 1H, Py4h3),
6.32 (t, J = 6.4 Hz, 1H, Py2g3), 5.00 (s, 1H, backone-CHf3), 4.51
(s, 2H, CH2Pye3), 2.32 (s, 6H, ortho PhCH3, d3), 2.29 (s, 3H, para
PhCH3, c3), 2.00 (s, 3H, backone-CH3, b3), 1.95 (s, 3H, backone-
CH3, a3). 13C NMR (C6D6, 100 MHz, 298K): δ165.77, 165.51,
163.72, 151.61, 149.07, 137.10, 131.17, 131.10, 129.75, 122.86,
121.53, 94.77, 56.12, 23.76, 23.70, 22.26, 21.69. Anal. Calcd for
C20H24LiN3: C, 76.66; H, 7.72; N, 13.41. Found: C, 76.86; H, 7.67;
N, 13.65.
Keywords: β-Diketiminate ligand • Lithium • Acetylacetone •
Chelation
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Entry for the Table of Contents
FULL PAPER
ß-Diketiminato Lithium Complexes
Kuldeep Chand, Cheng-Long Tsai, Hsuan-
Ying Chen, Wei-Min Ching, Sung-Po Hsu,
James R. Carey, Sodio C. N. Hsu
Page No. – Page No.
Improved Synthesis of Unsymmetrical
N-aryl-N’-alkylpyridyl
ß-Diketimines
using Molecular Sieves and their
Lithium Complexes
Four unsymmetrical N-aryl-N’-alkylpyridyl β-diketiminate ligands HL1-HL4 and were
synthesized in the presence of 5 Å molecular sieves in high yield. Deprotonation of
unsymmetrical N-aryl-N’-alkylpyridyl β-diketimines using nBuLi yields corresponding
stable Li complexes. These lithium complexes were characterized using single crystal X-
ray diffraction. The observed binding mode differences of LiL1, LiL2, LiL3, and LiL4 are
rationalized by considering the steric interference of the N-aryl substituent and the length
of pyridyl arm of the N-aryl-N’-alkyl β-diketiminate ligand.
*Lithiation Chemistry
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