Synthesis and characterization of bridged bis(amidato) rare earth metal amides and their applications in C-N bond formation reactions

Article abstract of DOI:10.1039/c5dt04217h

Based on three bisamide proligands H₂Lⁿ (n = 1-3) (H₂L¹ = [(Me₃C₆H₂CONHCH₂)₂CH₂], H₂L² = [(Me₃C₆H₂CONHCH₂)₂C(CH₃)₂], H₂L³ = [Me₃C₆H₂CONH(CH₂)₂]₂NCH₃), eight bis(amidato) trivalent rare-earth metal amides {LⁿRE[N(TMS)₂]}₂ (n = 1, RE = La (1), Sm (2), Nd (3), Y (4); n = 2, RE = La (5), Nd (6); n = 3, RE = La (7), Nd (8); TMS = SiMe₃) were successfully synthesized by treatment of H₂Lⁿ with RE[N(TMS)₂]₃ in a 1:1 molar ratio. Complexes 3, and 5-8 were characterized by single-crystal X-ray diffraction, and NMR characterization was carried out for the La complexes 1, 5, 7 and the Y complex 4. These complexes exhibited high catalytic activities in both the direct amidation of aldehydes and the addition of amines with carbodiimine. It was found that the bis(amidato) rare earth metal amides bearing different linkers have different effects on the transformations and lanthanum and neodymium complexes performed better than others.

Full text of DOI:10.1039/c5dt04217h

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DOI: 10.1039/C5DT04217H
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ARTICLE
Synthesis and characterization of bridged bis(amidato) rare earth
metal amides and their applications in C-N bond formation
reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Bei Zhao*, Yang Xiao, Dan Yuan, Chengrong Lu, Yingming Yao*
DOI: 10.1039/x0xx00000x
Based on three bisamide proligands H2Ln (n = 1-3) (H2L1 = [(Me3C6H2CONHCH2)2CH2], H2L2 = [(Me3C6H2CONHCH2)2C(CH3)2],
H2L3 = [Me3C6H2CONH(CH2)2]2NCH3), eight bis(amidato) trivalent rare-earth metal amides {LnRE[N(TMS)2]}2 (n = 1, RE = La
(1), Sm (2), Nd (3), Y (4)); n = 2, RE = La (5), Nd (6)); n = 3, RE = La (7), Nd (8); TMS = SiMe3) were successfully synthesized by
treatment of H2Ln with RE[N(TMS)2]3 in 1:1 molar ratio. Complexes 3, and 5-8 were characterized by single-crystal X-ray
diffraction, and NMR characterization was carried out for La complexes 1, 5, 7 and Y complex 4. These complexes exhibited
high catalytic activities in both the direct amidation of aldehydes and the addition of amines with carbodiimine. It was
found that the bis(amidato) rare earth metal amides bearing different linkers have different effects on the transformations
and lanthanum and neodymium complexes performed better than others.
www.rsc.org/
Introduction
Organolanthanide amides are important rare earth metal La[N(TMS)2]3, a simple and easily available lanthanide amide,
complexes with rich reactivities[1]. In general, the catalytic without using any oxidants, bases, heat or light[6]. This finding
performance of the organolanthanide amides can be evoked chemists to pay constant attention to this
effectively controlled by changing the metal center or auxiliary transformation and many kinds of rare-earth metal amides
ligand[2]. In recent years, amidato groups, whose steric and with different ancillary ligands were found to be the efficient
electric effects can be easily tuned, attracted great attention in catalysts[7]. However, a relative large catalyst loading (5 mol%)
lanthanide chemistry[3]. Several amidato rare-earth metal and the relative narrow substrate scope were usually observed
amides have been successfully prepared and proved to be and are in need of improvement. Recently, Schafer’s group
useful catalysts for ring opening polymerization of lactones[3a-c] found yttrium amidato complexes can catalyze this conversion
and some valuable transformations including C-C bond[3d-e], C- in a very short time, but the yields of many products were still
[3f]
[3g-h, j]
P bond
and C-N bond
formation. To further explore not good enough[3h]. Thus, our new bridged bis(amidato)
the application of auxiliary amidato ligand in homogenous lanthanide amides were tested, which were found to be
rare-earth metal catalysts, the synthesis and characterization efficient catalysts in the direct amidation of aldehydes.
of bridged bis(amidato) rare-earth metal amides and their
The addition of amines and carbodiimine is a popular atom
applications were investigated and up to now, only four economic method to construct guanidines. However, the
successful cases were reported in the field[3b, e, i, j]. Thus, three synthesis of the guanidine compounds need harsh conditions
new series of bridged bis(amidato) rare-earth metal amides in the absence of catalyst[8]. Over the past decade, a large
were designed, prepared and well characterized in this report.
number of studies showed that many rare earth metal organic
Amides are one type of important nitrogen-containing complexes with diverse structures can catalyze the addition of
compounds, have unique physiological activities and are key amines and carbodiimides, including rare earth metal alkyl,
structural elements in the study of drug chemistry and amino and alkyl (aryl) oxy compounds with different ancillary
biochemistry[4]. Moreover, amide derivatives can also serve as ligands, such as cyclopentadienyl[9a-c], indenyl[9d-f], amino[9g-i]
ancillary ligands in the field of coordination chemistry[5]. In pyrrolyl[9j], guanidine[9k], N-heterocyclic carbene[9l], salen[9m]
, -
,
2008, Marks found that the direct amidation of aldehydes and diketiminato[9n], and amidine group[9o-p]. Very recently, our
amines to yield amides can be catalyzed by neutral group found that cyclohexyl bridged bis(amidato) trivalent rare
earth metal amides were efficient catalysts in the addition
o
reaction of amines and carbodiimine at 60 C under solvent-
free conditions[3j]. To get insight into the structure-activity
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow
University, Suzhou 215123, People’s Republic of China
*To whom correspondence should be addressed. Email: zhaobei@suda.edu.cn;
yaoym@suda.edu.cn
relationship of the bridged bis(amidato) rare earth metal
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amides in the guanidination, new bridged bis(amidato) These data suggest that the whole molecule of complex
DOI: 10.1039/C5DT04217H
more compact and the surrounding of the metal center may
lanthanide amides were also tested in this study.
be more open due to the flexible unbranched C3 linker in the
bis(amidato) ligand. This structural characters of complex
3
Results and discussion
probably result in improved catalytic reactivity (vide infra).
Syntheses and characterization of bridged bis(amidato) rare-earth
metal amides.
To further explore the reactivity of bridged bis(amidato) rare
earth metal amides, eight target complexes {LnRE[N(TMS)2]}2
(n = 1, RE = La (
1
), Sm (
2
), Nd (3), Y (4)); n = 2, RE = La (5), Nd
(6
)); n = 3, RE = La (
7
), Nd (8)) bearing different bridged
bis(amidato) groups were successfully synthesized by
treatment of suitable rare earth metal precursors
RE[N(SiMe3)2]3 with bridged bis(amidato) proligands[10] H2Ln (n
=
1-3) (H2L1
=
[(Me3C6H2CONHCH2)2CH2], H2L2
=
[(Me3C6H2CONHCH2)2(CH3)2C],
H2L3=
[Me3C6H2CONH(CH2)2]2NCH3) (Scheme 1) according to
literature methods[11]. Disappearance of the corresponding N-
1
H signals of proligands in the H NMR spectra of lanthanum
and yttrium complexes confirms the occurrence of metathesis.
All complexes are air and moisture sensitive, and complexes 1-
6
8
are soluble in both toluene and THF, while complexes
are soluble in THF, and slightly soluble in toluene.
7 and
Crystals of complex
obtained from toluene solution at room temperature. The
solid structure of complex is depicted in Figure 1 and the
3 suitable for X-ray diffraction were
3
details of the crystallographic data are summarized in TS 1 (See
ESI, Table 1). It is a centrosymmetric binuclear structure with
one toluene molecule in each unit cell. The Nd center is six
coordinated by two oxygen atoms and two nitrogen atoms
from the carbon bridged bis(amidato) ligand L1, one oxygen
atom from a second ligand L1 and one nitrogen atom from -
N(TMS)2 group. The bridged bis(amidato) group adopts a
coordination mode named4-(N,O): -O, in which ligand L1 not

only chelates one metal center but also bridges two metal
centers via an oxygen atom. When taking the metal center Nd
as the spiro atom, each amidato subunit in one bridged ligand
constitutes a planar quadrilateral since the sum of these bond
angles of Nd1-O1-C1(93.1(5)o), O1-C1-N1(117.2(8)o), C1-N1-
Nd1(97.4(5)o) and N1-Nd1-O1(51.7(2)o) is 359.5o, and the same
for the other metal center Nd2. The spatial configuration of
the metal center Nd can be described as a distorted pentagon
cone for N3 is the vertex. The average bond lengths of C-O and
C-N are 1.308 Å and 1.269 Å, respectively, indicating the
electron delocalization in amidato subunit. In comparison, the
Scheme
bis(amidato) ligands
1. Preparation of rare earth metal amides supported by bridged
Crystals of complexes
1
,
2
and
4
were obtained from
toluene solution using the same method as that for complex 3.
Unfortunately, unsatisfactory results from X-ray diffraction
were obtained and only the skeletons of the molecular
structures of
spectra of complexes
results of complexes
1
,
2
and
4
could be determined. However, NMR
and and the elemental analysis
and are informative. Thus, it can be
1
4
1
,
2
4
deduced that the molar ratio of the ligand L1 to the metal
center and the amido group is 1:1:1. Therefore, it is presumed
bond length of Nd1-
than the bond length of Nd-
amidate rare earth
Li(THF)Nd[C6H5C(O)NC6H3 Pr2]4; while the bond length of Nd1-
-O2 (2.397(6) Å) is very close to the bond length of Nd- -O4
(2.385 (5) Å) in the above anionic complex[3c]. The bond
lengths of Nd1- -N1 (2.491(7) Å), Nd1- -O1 (2.554(5)Å), Nd1A-
-O1 (2.384(6) Å) in complex were slightly shorter than the
corresponding bond lengths of Nd1- -N2 (2.526 (5)Å), Nd1-
O2 (2.593(4)Å) and Nd1A- -O2(2.443(6) Å) in the cyclohexyl

-N2 (2.430(8) Å) was significantly shorter
-N4 (2.694 (5)Å) in anionic
metals complex

that the solid structures of complexes
Crystals of complexes and suitable for X-ray diffraction
were obtained from THF-hexane solutions after several days at
room temperature. The molecular structure of complex was
monoclinic, P12/c1 space group with two free THF molecules
in every unit cell, while isostructural complex belongs to
P1c1 space group with two n-hexane molecules in every unit
cell. The molecular structure of complex is a representative
1-4 are isostructural.
5
6
i


5


6

3

-
5

example, which is shown in Figure 2, and details of the
crystallographic data are summarized in TS 1 (See ESI, Table 1).
bridged
bis(amidato)
neodymium
amide
t
[3e]
{tBuC6H4CONH(C6H10)NHCOC6H4 Bu]Nd[N(TMS)2]THF}2
.
2 | J. Name., 2012, 00, 1-3
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It is shown that the metal center is six coordinated with two
ARTICLE
The different coordination modes of the same ligand in
View Article Online
DOI: 10.1039/C5DT04217H
7 and 8 may be attributed to the different ionic
amidato groups, one silylamido ligand and a bridged oxygen complexes
atom from the other amidato group. Though the coordination radii between La and Nd and the relative bulky bridged
modes of bridged bis(amidato) group L2 are almost the same bis(amidato) donor H2L3.
as those in complex 3, the location of silylamido groups is on
the same side of the square plane of La1-O2-La1A-O2A. The
bond lengths of La1-N1 bond (2.492 (3) Å) and La1-O1 bond
(2.471 (3) Å) of amidato ligand adopting -(N,O) coordination
mode are shorter than the bond lengths of La1-N2 bond (2.706
(3) Å) and La1-O2 bond (2.579 (3) Å) of amidato ligand
adopting
with the bond lengths of complex
complex are similar to the corresponding values in complex
taking into account the difference in ionic radii.

-(N,O):μ-O coordination mode, which is consistent
3
. The bond parameters in
6
5,
Aminolysis of Ln[N(TMS)2]3 (Ln = La, Nd) with proligand H2L3
[Me3C6H2CONH(CH2)2]2NCH3 yielded two desired complexes
{L3Ln[N(TMS)2]}2 (Ln = La(
7), Nd(8)). Suitable crystals for X-ray
diffraction determination were obtained from toluene solution
at room temperature and details of the crystallographic data
are collected in TS 1 (See ESI, Table 1). The solid structure of
Figure 1. Molecular structure of complex 3toluene. Hydrogen atoms and toluene
molecules are omitted for clarity.
complex
bridged bis(amidato) groups in complex
those in complexes . It crystallizes in the monoclinic crystal
7
is depicted in Fig. 3. The coordination modes of
7
are consistent with
1-6
system, P21/n space group. The spatial configuration of the
metal center can be described as a distorted pentagonal
pyramid, with N3 as the vertex. Similar to complexes
5 and 6,
silylamido groups in complex 7 are located at the same side of
the coordination plane of La1-O2-La1A-O2A. It is noteworthy
that the bridging nitrogen atoms do not coordinate to the
lanthanum centers.
Though neodymium complex
ligand L3 as lanthanum complex
8
is supported by the same
7
, it crystallizes in the triclinic
crystal system, P-1 space group, and has a binuclear
centrosymmetric structure with a coordination number of six.
As shown in Fig. 4, the neodymium atom is coordinated with
two nitrogen atoms from an amidato group, a nitrogen atom
that bridges two amidato groups, two oxygen atoms from the
other amidato ligand, and a nitrogen atom from the silylamido
group. The bis(amidato) group adopts a bridging mode in
Figure 2. Molecular structure of complex 52THF. Hydrogen atoms, and THF molecules
are omitted for clarity.
complex
8, that is, the nitrogen atom coordinate to one
neodymium atom, while the oxygen atom in the same
fragment coordinate to the other metal center in the unit cell.
The bridging fashion is one of the possible coordination modes
of amidato ligands in organometallics[12] , and to the best of
our knowledge, this is the first case reported in lanthanide
chemistry. The average bond lengths of C-O and C-N are 1.291
Å and 1.301 Å, which shows significant electron delocalization
in the amidato fragment. The average bond length of Nd-O
bond is 2.346 Å, which is shorter than Nd-
complex (2.397 Å) and complex (2.400 Å). The Nd-N
average bond length of 2.500 Å is slightly longer than the Nd-
-N bond lengths of complex (2.430 Å) and complex (2.463
-O bond lengths of
3
6

3
6
Figure 3. Molecular structure of one independent molecule of complex 72toluene.
Å). The spatial configuration of the metal center can be
described as a distorted triangular prism and the dihedral
angle of the planes consisting of N1A, N2A, N3A and N4A, O1,
O3 is 9.6o .
Hydrogen atoms and toluene molecules are omitted for clarity.
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17). Finally, the optimal condition was determined to be 1 mol%
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complex 1
as the catalyst in THF at roomDtOemI: 1p0e.1r0a3t9u/rCe5DfoTr04321h7rHs
(Table 1, entry 12).
Table 1. Condition screening of the amidation of 9a with 10a catalyzed by various
bridged bis(amidato) rare earth amidesa
O
CHO
Catalyst
N
N
+
H
r.t., Sol.
9a
10a
11a
Molar ratio
(9a/10a)
Yield(%)c
Catalyst loading
(mol%)b
Time/h
entry
Solvent
Cat.
46
80
92
1:1
1:2
1:3
3
3
3
1
1
1
THF
THF
THF
3
3
3
1
2
Figure 4. Molecular structure of complex 82toluene. Hydrogen atoms and toluene
molecules are omitted for clarity.
3
4
5
6
Catalytic activities of complexes 1-5 and 7 in the amidation of
amines and aldehydes.
72
85
93
88
1:3
1:3
1:3
1:3
3
3
3
3
1
1
1
1
THF
THF
0.5
1
Direct reaction of amine and aldehyde has attracted increasing
attention because of its high efficiency in the formation of
amide bonds[13]. In 2006, Wang’s group successfully introduced
THF
-
6
3
7
LnCl3 and Ln[N(TMS)2]3(-Cl)Li(THF)3 to catalyze the reaction of
45
8
1:3
1:3
1:3
3
3
3
1
CH2Cl2
3
aldehydes and amides, though the substrate scope was
limited[14]. Before long, Marks developed neutral Ln[N(TMS)2]3
as the single component catalyst, which showed limitations in
catalyzing reactions of aliphatic amines[6]. Recently, Schafer’s
contributions in this transformation using yttrium amidate
complexes as the catalyst shortened the reaction time to only
5 min, and catalytic activities of rare earth metal amides are
found to largely depend on the auxiliary ligands[3h]. Despite so
many reports, no lanthanide bimetallic complex has been
applied in catalyzing the amidation reactions. To explore the
potential bimetallic synergistic effect of the bis(amidato) rare
earth metal amides, their performance as catalysts for the
amidation of N-methyl benzylamine and benzaldehyde was
examined and the results are listed in Table 1.
9
2.5
2.5
91
94
98
Tol
3
3
10
Et2O
1:3
3
11
12
2.5
THF
3
3
98
1:3
1:3
1:3
1
2
4
1
1
1
THF
THF
THF
91
90
13
14
3
3
1:3
5
96
68
92
1
THF
THF
THF
3
3
3
15
16
17
1:3
1:3
7
1
1
0.5
a Amine was first added to the solution of catalyst, and aldehyde was added 30
min later. The concentration of amine was 0.8 mol∙L-1. Based on the catalyst
b
amount. cGC yield based on the amount of amine.
It is found that the amidation was smoothly catalyzed by
complexes 1-5 and 7 in THF at room temperature. Even though
Subsequently, the reaction of various amines and
aldehydes were investigated and the results are shown in
Table 2. It can be seen that electronically varied aldehydes
underwent the transformation with N-methyl benzylamine
smoothly and the yield of the desired amides were excellent
except for the substrate p-methoxy benzaldehyde (Table 2,
the main side reaction named Tishchenko reaction was also
accelerated under the same reaction conditions, the increasing
amount of benzaldehyde was beneficial to amidation, as the
yield of the desired amide increased from 46 to 92% (Table 1,
entries 1-3). The reaction time and the solvent had a great
influence on the reaction outcome and under optimized
conditions, the reaction in THF for 3 hours was the best choice
(Table 1, entries 3-11). The central metal had somewhat effect
11a
tetrahydropyrrole, and piperidine were investigated and the
yields were satisfying, ranging from 71-94% (Table 2, 11e 11m),
-11d). Several secondary cyclic amines, such as morpholine,
-
which are comparable with previous reports[7]. Though the
chlorine atom adjacent to carbonyl group in o-
chlorobenzaldehyde decelerated the reaction process, the
yield of the corresponding amide increased when the amount
of catalyst loading was doubled (Table 2, 11j). If N-methyl
benzylamine was replaced with aniline, the yield of amide
dramatically dropped to 52% (Table 2, 11a and 11n),
meanwhile the main byproduct imine was observed in GC
determination. It may be ascribed to the less nucleophilic
nature of aniline comparing to that of aliphatic amine. With
benzopyrrole and its derivatives as the substrate amine, the
on the reaction and the La complex
1 exhibited the highest
activity (Table 1, entries 3 and 12-14). The ligand effect was
also observed in this catalytic system. For the same lanthanum
amides, the yield of the reaction catalyzed by complex
7
bearing the relative bulky ligand L3 was only 68%, while the
reaction was basically complete when complexes 1 and 5
bearing the amidato ligands L1 and L2 were used as the
catalysts (Table 1, entries 12 and 15-16). It is presumed that
the relative crowded surrounding of lanthanum in complex
7
hindered the approaching of aldehyde to the metal center. The
attempt to reduce the amount of catalyst by half failed
because the yield dropped apparently (Table 1, entries 12 and
yields were better (Table 2, 11o-11q). To our delight, the
4 | J. Name., 2012, 00, 1-3
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o
, at 60 C for 15 min under
results of bisamidation of piperazine were excellent in the follows: 0.25 mol% catalyst
presence of 2 mol% catalyst , which was less reported in solvent-free conditions.
previous work (Table 2, 11r 11t).
3
View Article Online
DOI: 10.1039/C5DT04217H
1
o
-
Various amines were screened at 60 C under solvent-free
conditions for appropriate reaction time in the presence of
Table 2. Substrate scope for the amidationa,b
catalyst
3 and the results are shown in Table 4. It was found
O
that the electronic effect of less sterically hindered
substituents, such as halides, methyl and methoxy groups had
no significant influence on the guanidination of substituted
anilines and DIC. The target guanidine can be easily obtained in
CHO
R'
H
R'
1 mol% Cat. 1
THF, r.t., 3 h
N
N
+
R"
R''
9
R
R
10
11
O
N
O
O
O
N
N
excellent yields in a time as short as 15 min (Table 4, 13a-13i).
N
O
F
However, the reaction time needs to be prolonged to 24 h for
o-tert-butylaniline, and 1 h for pyridine-2-amine to afford the
corresponding guanidine in 90% and 95% yields, respectively
(Table 4, 13j and 13k). As for the substrates bearing the strong
electron withdrawing group, for example, nitro and
trifluoromethyl group, or the ortho disubsititued substrate 2,6-
methylaniline, the yields of products dropped to 61-76% in the
11b 77%
11c
11d 93%
98%
O
11a 96%
O
O
O
N
N
N
N
O
O
O
N
O
F
Cl
N
11f 88%
11g 94%
11h 89%
11e 90%
O
O
Cl
O
O
O
Cl
N
N
N
O
O
O
O2N
11j 79%c
11k 90%
11i 90%
11l 71%
presence of 0.5 mol% catalyst
catalytic system was also suitable for some cyclic secondary
amines, such as pyrrole and morpholine (Table 4, 13o 13r). It
also provides a convenient access to bisguanidine in a high
yield using 0.5 mol% catalyst for 24 h (Table 4, 13s). All
3 for 24 h (Table 4, 13l-13n). The
11m 82%
O
O
N
O
O
N
N
H
N
-
11n 52%
11o 79%
11p 68%
11q 73%
3
O
O
O
Cl
N
N
N
results presented here are comparable to those of reactions
catalyzed by cyclohexyl bridged bis(amidato) neodymium
amides[3j]
.
N
N
N
Cl
O
O
O
11s 88%c
11r 92%c
11t 93%c
Table 3. Condition screening of the guanidination of DIC and aminesa
a
Reaction conditions: amine (1.0 mmol), aldehydes (3.0 mmol), catalyst 1 (0.01
mmol), THF (0.8 mL). b Isolated yield based on amine. c Catalyst 1 (0.02 mmol).
Catalytic activities of complexes 1-4, 6 and 8 in the guanidination
of DIC and amines.
Guanidine is an important structural unit of some drugs due to
their physiological activity in antibacterial, enzyme inhibitors,
antiviral and so on[15].Guanidine and its derivatives were also
commonly used as ligands in coordination chemistry[16]
.
Moreover, guanidine itself was found to be efficient catalyst in
organic synthesis[17]. Hence, great progress was made in the
synthetic method of guanidine and its derivatives[18]
.
Encouraged by our previous work[3j], the catalytic activities of
bis(amidato) rare earth metal amides and in the
1
-
4
,
6
8
addition of aniline and diisopropyl carbodiimine (DIC) were
investigated and the results are summarized in Table 4. The
addition gave rise to the expected guanidines, up to 96% yields
o
at 60 C after 15 min (Table 3, entries 1-6). The central metal
had little effect on the transformation, while the relative bulky
ligand, especially L3 had great influence on the outcome, as the
yield dramatically dropped to 52% (Table 3, entries 1-6). The
coordinating THF was not an ideal solvent and the optimal
condition is solvent free (Table 3, entries 1, 5, and 7-9). At
ambient temperature, the satisfying yield of 92% was only
a
Reaction conditions: amine (1.0 mmol), DIC (1.0 mmol), catalyst 3 (0.0025
mmol); b Isolated yield.
obtained after 3 h in the presence of easily prepared catalyst
(Table 3, entries 10 and 11). Even though the amount of
catalyst was reduced to half, the yield of guanidine
3
3
maintained (Table 3, entries 1 and 12). So, the optimal
condition for the model reaction of aniline and DIC is as
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
All experiments were performed by using standard Schlenk or
View Article Online
Table 4. Substrate scope for the guanidination
DOI: 10.1039/C5DT04217H
glove-box techniques, with rigorous exclusion of traces of
moisture and air. After being dried over, THF, toluene and n-
hexane
were
purified
by
distillation
from
sodium/benzophenoneketyl. Benzene-d6, THF-d8 and toluene-
d8 were dried with sodium sand. All commercially available
chemicals (aldehydes, amines, and DIC) were dried with CaH2
and distilled in vacuum or atmospheric pressure prior to use.
NMR spectra were recorded using a Bruker-400 MHz
spectrometer in CDCl3, benzene-d6, THF-d8 or toluene-d8 at
room temperature, unless otherwise stated. The crystal
structures were resolved by direct method equipped with
graphite-monochromatized Mo Кα ( = 0.71075 Å) radiation
after measuring and collecting data on a Rigaku Mercury CCD
X-ray diffractometer. All the non-hydrogen atomic coordinates
and anisotropic thermal parameters went on with correction
by the full matrix least squares method and refined by using
the SHELXL-1997 program[19].The C, H, and elemental analyses
of rare-earth metal complexes were performed in the element
analyzer of the Carlo Erba 1110 and determination of infrared
absorption spectrum was implemented in the vertex 70
Fourier transform infrared spectrometer, scanning range from
400-4000 cm-1.
Synthesis of {L1Ln[N(SiMe3)2]}2 (1-4)
To a solution of Ln[N(SiMe3)2]3 (1 mmol) in THF (10 mL), a THF
suspension (10 mL) of proligand H2L1 (1 mmol) was added
slowly at 25 °C and the mixture was stirred for 15 h under 60
oC. After THF was removed under vacuum, the solid residue
was washed with n-hexane to remove silylamine, which was
recrystallized in toluene. After several days, suitable crystals
were obtained in 61%-74% yield.
Conclusions
In summary, overall eight bis(amidato) rare earth metal
amides have been successfully prepared and well
characterized through the reactions of the different bridged
amide proligands H2Ln with appropriate RE[N(TMS)2]3 in good
yields. It is noteworthy that the spatial structures of these
complexes depend on the diverse structures of ligands and the
ionic radii of the metal centers. It is the first example that the
bridged bis(amidato) rare earth metal amides were
successfully introduced in the preparation of amides from the
direct amidation of amines with aldehydes. Aliphatic
secondary amines and diamines also underwent the
1
{L1La[N(SiMe3)2]}2 (1): colourless crystals, yield 61%; H NMR
(400 MHz, d8-Tol): 6.79-6.61 (m, 8H, ArH), 3.08-3.03 (m, 4H,
CH2N), 2.74-2.69 (m, 4H, CH2N), 2.41-2.12 (m, 40H, CH2, ArCH3),
0.32 (s, 30H, SiCH3), 0.08 (s, 6H, SiCH3); 13C NMR (100 MHz, d8-
THF): 177.89, 135.72, 134.35, 132.44, 127.10, 46.34, 33.34,
32.39, 19.65, 18.65, 3.77, 3.50, 1.22; IR (cm−1): 2957 (w),
2923 (w), 1627 (s), 1613 (s), 1544 (m), 1459 (m), 1435 (m),
1399 (m), 1377 (m), 1340 (m), 1251 (m), 1179 (m), 1143 (m),
931 (s), 847 (s); Anal. calcd for C58H92La2N6O4Si4 (1327.54
g∙mol−1): C 52.47, H 6.99, N 6.33, La 20.93; Found: C 52.19, H
6.84, N 6.13, La 20.78.
transformation smoothly using La complex
1 as the catalyst
and the target amides were obtained in moderate to excellent
yields with some substituent tolerance. Furthermore, the
catalytic system also performed well in the addition of amines
{L1Sm[N(SiMe3)2]}2 (2): colourless crystals, yield 66 %; IR
(
cm−1): 2954 (w), 2921 (w), 1625 (s), 1612 (s), 1540 (m),
1457 (m), 1438 (m), 1397 (m), 1374 (m), 1342 (m), 1249 (m),
1179 (m), 1140 (m), 930 (s), 845 (s); Anal. calcd for
C58H92Sm2N6O4Si4 (1354.47 g∙mol−1): C 51.58, H 6.87, N 6.22,
Sm 22.27; Found: C 51.44, H 6.68, N 6.16, Sm 20.78.
and DIC. Neodymium precatalyst
3 can be used to promote
reactions of electronically and sterically varied amines with DIC
under appropriate conditions. Direct comparison of other
previously published rare earth metal amido complexes with
{L1Nd[N(SiMe3)2]}2 (3): blue crystals, yield 74%; IR (cm−1):
2953 (w), 2919 (w), 1624 (s), 1612 (s), 1540 (m), 1457 (m),
1434 (m), 1397 (m), 1374 (m), 1342 (m), 1249 (m), 1177 (m),
1140 (m), 929 (s),845 (s); Anal. calcd for C58H92Nd2N6O4Si4
(1334.46 g∙mol−1): C 52.06, H 6.93, N 6.28, Nd 21.56; Found: C
52.33, H 6.78, N 5.94, Nd 21.19.
complexes
1
and
3
bearing carbon bridged bis(amidato) group
and are superior with
showed that the catalytic activities of
1
3
respect to the reduced amount of precatalysts and the relative
wide substrate scope. Many efforts to investigate catalytic
mechanisms and develop new efficient catalysts are still in
process in our lab.
1
{L1Y[N(SiMe3)2]}2 (4): colourless crystals, yield 64%; H NMR
(400 MHz, d8-THF): 6.81(s, 8H, ArH), 2.93-2.91(m, 8H, CH2N),
2.40-2.39(m, 4H, CH2), 2.30-2.20(m, 36H, ArCH3), 0.12 (s, 24H,
Experimental section
6 | J. Name., 2012, 00, 1-3
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SiCH3), 0.05 (s, 4H, SiCH3), 0.04 (s, 4H, SiCH3), -0.07 (s, 4H, and 0.8 mL of THF successively, then the mixture was stirred at
View Article Online
SiCH3); IR (cm−1): 2950 (w), 2916 (w), 1622 (s), 1612 (s), 1540 room temperature for half an hour. Aldehyde (3.0 mmol) was
DOI: 10.1039/C5DT04217H
(m), 1457 (m), 1430 (m), 1397 (m), 1371 (m), 1342 (m), 1249 added, and the mixture was continued to be stirred for
(m), 1177 (m), 1140 (m), 929 (s), 845 (s); Anal. calcd for another 3 h. The reaction was quenched by ethyl acetate and
C58H92Y2N6O4Si4 (1226.44 g∙mol−1): C 56.75, H 7.55, N 6.85, Y the target product was separated and purified by column
14.49; Found: C 56.40, H 7.43, N 6.77, Y 14.39.
Synthesis of {L2Ln[N(SiMe3)2]}2 (5 and 6)
chromatography.
General procedure for the guanidination of DIC and amines:
To a solution of Ln[N(SiMe3)2]3 (1 mmol) in THF (10 mL) , a Take complex
THF solution (10 mL) of H2L2 (1 mmol) was added slowly at aniline (1.0 mmol) was added into a Schlenk tube and stirred
25 °C and the mixture was stirred for 24 h under room for a while. After complex was dissolved in aniline, DIC (1.0
3 as an example. Complex 3 (0.0025 mmol) and
3
temperature. After THF was removed under vacuum, the solid mmol) was added by syringe. The resulting mixture was stirred
o
residue was washed with hexane to remove silylamine, which at 60 C for an appropriate time, then quenched by water (2
was recrystallized in hexane-THF. After several days, suitable mL), extracted with dichloromethane (3 × 10 mL). The organic
crystals were obtained in 72% yield for
5
and 75% yield for
6
.
layer was dried over anhydrous Na2SO4, filtered, and the
{L2La[N(SiMe3)2]}2 (5): colourless crystals, yield 72%; 1H solvent was removed under vacuum to give the target product.
NMR (400 MHz, d8-THF): 6.78 (s, 8H, ArH), 2.73 (s, 8H, CH2),
2.24 (d, J = 10.5 Hz, 36H, ArCH3), 0.70 (s, 12H, CH3), 0.14 (s,
24H, Si-CH3), 0.04 (s, 12H, Si-CH3); 13C NMR (100 MHz, d8-THF):
Notes:
179.4, 135.7, 134.2, 132.6, 127.0, 66.7, 57.8, 35.1, 24.9, 24.8,
24.2, 24.0, 23.8, 23.6, 23.4, 19.6, 18.9, 3.5, 1.2; IR (
, cm−1):
Key Laboratory of Organic Synthesis of Jiangsu Province,
College of Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123, People’s
Republic of China. Fax: +86-512-65880305; Tel: +86-512-
65880305; E-mail: zhaobei@suda.edu.cn,† the informaꢀon
that supplementary of crystallographic data can be obtained
2951 (w), 2919 (w) , 1627 (s), 1612 (s), 1535 (m), 1437 (m),
1377 (m), 1341 (s), 1248 (m), 1212 (m), 1180 (m), 1141 (m),
930 (s), 846 (s); Anal. calcd for C62H100La2N6O4Si4 (1382.50
g∙mol−1): C 53.82, H 7.20, N 5.88, La 20.20; Found: C 53.58, H
7.13, N 5.89, La 19.92.
{L2Nd[N(SiMe3)2]}2 (6): light blue crystals, yield 75%; IR (
,
free
of
charge
via
cm−1): 2953 (w), 2919 (w), 1625 (s), 1612 (s), 1540 (m), 1457
(m), 1435 (m), 1397 (m), 1375 (m), 1343 (m), 1249 (m), 1177
(m), 1128 (m),930 (m), 846 (s); Anal. calcd for C62H100Nd2
N6O4Si4 (1390.51 g∙mol−1): C 53.41, H 7.23, N 6.03, Nd 20.69;
Found: C 53.01, H 6.97, N 5.80, Nd 20.40.
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or e-mail:
deposit@ccdc.cam.ac.uk. [CCDC: 1402613 (3), 1402611 (5),
1402612 (6), 1407220 (7), 1402610(8)]
Acknowledgments:
Synthesis of {L3Ln[N(SiMe3)2]}2 (7 and 8)
We gratefully acknowledge financial supports from the
National Natural Science Foundation of China (Grant Nos.
21172165, 21132002, 21372172 and 21572151), PAPD, the
Major Research Project of the Natural Science Foundation of
the Jiangsu Higher Education Institutions (Project
14KJA150007), and the Qing Lan Project.
To a solution of Ln[N(TMS)2]3 (1 mmol) in THF (10 mL), a
THF solution (10 mL) of H2L3 (1 mmol) was added slowly at
o
25 °C and the mixture was stirred for 24 h under 60 C. Then
THF was removed under vacuum, the residue was washed by
n-hexane for several times. After that, the powder was
dissolved in mixed toluene-hexane and recrystallized at room
temperature. The crystals of complexes
in 78% and 79% yield, respectively.
7 and 8 were isolated
{L3La[N(SiMe3)2]}2 (7): colourless crystals, yield 78%; 1H NMR
(400 MHz, C6D6): 6.85(s, 8H, ArH), 2.91-2.71 (m, 8H, CH2-N),
2.61-2.08 (m, 50H, CH2, ArCH3), 0.31 (s, 36H, Si-CH3); IR
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(0.01 mmol), N-methylbenzylamine (1.0 mmol)
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DOI: 10.1039/C5DT04217H
Graphic Abstract
Synthesis and characterization of bridged bis(amidato) rare earth metal amides and their
applications in C-N bond formation reactions
Bei Zhao*, Yang Xiao, Dan Yuan, Chengrong Lu, Yingming Yao*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical
Engineering and Materials Science,
Soochow University, Suzhou 215123, PR China
Eight bis(amidato) rare-earth metal amides were successfully synthesized and well characterized,
which exhibited high catalytic activities in both the direct amidation of aldehydes and the addition
of amines with carbodiimine.