Synthesis, characterization, selective catalytic activity, and reactivity of rare earth metal amides with different metal-nitrogen bonds

Article abstract of DOI:10.1021/om900191j

A series of neutral rare earth metal amides with different metal - nitrogen bonds were synthesized and characterized. The selective catalytic activity and reactivity of the complexes incorporating different metal - nitrogen (Ln - N, Ln = rare earth metal)

Full text of DOI:10.1021/om900191j

3882 Organometallics 2009, 28, 3882–3888
DOI: 10.1021/om900191j
Synthesis, Characterization, Selective Catalytic Activity, and Reactivity
of Rare Earth Metal Amides with Different Metal-Nitrogen Bonds
Xiancui Zhu,† Jiaxi Fan,† Yunjun Wu,† Shaowu Wang,*,†,‡ Lijun Zhang,† Gaosheng Yang,†
Yun Wei,† Chengwei Yin,† Hong Zhu,† Shihong Wu,† and Hongtao Zhang†
†Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based
Materials, Institute of Organic Chemistry, College of Chemistry and Materials Science, Anhui Normal
University, Wuhu, Anhui 241000, People’s Republic of China, and State Key Laboratory of Organometallic
‡
Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
People’s Republic of China
Received March 12, 2009
A series of neutral rare earth metal amides with different metal-nitrogen bonds were synthesized and
characterized. The selective catalytic activity and reactivity of the complexes incorporating different
metal-nitrogen (Ln-N, Ln = rare earth metal) bonds were studied. Treatment of (Me2Si)[(2,6-R1 -4-
2
R2-C6H2)NH]2 with [(Me3Si)2N]3LnIII(μ-Cl)Li(THF)3 in toluene produced the neutral rare earth metal
complexes {(Me2Si)[(2,6-R1 -4-R2-C6H2)N]2}LnN(SiMe3)2(THF) (R1 = iPr, R2 = H, Ln = Yb (1),
2
Y (2), Eu (3), Sm (4), Nd (5); R1 = R2 = H, Ln = Yb (6), Sm (7)) incorporating different Ln-N bonds
in good yields. Reaction of 6 or 7 with grease (Me2SiO)3 in toluene produced the selective insertion
products [C6H5N(Me2Si)N(C6H5)(Me2SiO)LnN(SiMe3)2]2 (Ln = Yb (8), Sm (9)). The structures of
complexes 1, 2, 4, 5, 8, and 9 were additionally determined by single-crystal X-ray analyses. Investigation
of catalytic activity of the complexes indicated that the complexes displayed a high selectivity on
cyclotrimerization or cyclodimerization toward isocyanates depending on the nature of the isocyanates.
These catalysts have the advantages of a high reactivity and good selectivity toward isocyanates, easy
preparation, low catalyst loading, and high conversion, as well as mild reaction conditions.
Introduction
other polar monomers.1,2 Reports on these transformations
catalyzed by rare earth metal amides including hydroamina-
tion,3 monocoupling reactions of isocyanides with terminal
alkynes,4 Cannizzaro-type disproportionation of aromatic
aldehydes,5 guanylation of amines,6 amidination of alkyne,7
It has been demonstrated that rare earth metal complexes
exhibit versatile catalytic activity on polyolefin functionali-
zation with heteroatoms.1 Recent interest in the use of amido
ligands for the preparation of early transition metal com-
plexes stems from the recognition that the electronic proper-
ties and steric effects of the ligands can be modified
by variation of the substituents of nitrogen atoms and
that complexes incorporating the amido ligands exhibited
catalytic activity in various transformations of polyolefins
and the ring-opening polymerization of δ-caprolactone and
::
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T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (b) Nobis, M.; Driessen-
::
Holscher, B. Angew. Chem., Int. Ed. 2001, 40, 3983. (c) Brunet, J. J.;
Neibecker, D. In Catalytic Heterofunctionalization from Hydroamina-
tion to Hydrozirconation; Togni, A., Grutzmacher, H., Eds.; Wiley-
::
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Muller, T. E. Angew. Chem., Int. Ed. 2003, 42, 2708. (e) Pohlki, F.;
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*Corresponding author. E-mail: swwang@mail.ahnu.edu.cn.
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::
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pubs.acs.org/Organometallics
Published on Web 05/28/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 13, 2009 3883
addition reactions of amines to nitriles,8 cross-Aldol
reactions,9 dimerization of terminal alkynes,10 hydrosilyla-
tion,11 hydroboration,12 Tishchenkoaldehydedimerization,13
and hydroalkoxylation14 have shown a wide application of the
rare earth metal amides as catalysts or precatalysts in organic
synthesis. Although stoichiometrically insertion of isocyanate
to the rare earth metal Ln-C, Ln-N, Ln-O, and Ln-S
bonds to produce the corresponding insertion products has
been extensively studied,15 lanthanocene complexes Cp2-
LnNiPr2(THF)16a (Cp0 = MeC5H4, Ln = Y, Er, Yb) or
Cp2LnCl/n-BuLi16b as catalysts or catalytic systems for cyclo-
trimerization of phenyl isocyanate are documented. Selective
transformation of the isocyanates to their corresponding
cyclodimerization or cyclotrimerization products with cyclo-
pentadienyl-free rare earth metal amides as catalysts is far less
explored.17
As we know, isocyanates can undergo cyclodimerization,
cyclotrimerization, and polymerization in the presence of
catalysts. Investigation of catalysts that can selectively
catalyze the cyclodimerization, cyclotrimerization, or poly-
merization of isocyanates is one active topic of modern
chemistry. Triaryl isocyanurates are known to improve
the stability of polyurethane networks with respect to
thermal resistance, flame retardation, chemical resistance,
and film-forming characteristics.18 Different isocyanate
Figure 1. Molecular structure of complex 1.
cyclotrimerization catalysts have been reported with a ma-
jority of the conventional catalysts being anions or neutral
Lewis bases.18 Some metal-based catalysts have been de-
scribed, and a Lewis acid pathway could be envisioned for
these systems.19 Conventional catalysts for isocyanurate for-
mation suffer from low activity, necessitating severe condi-
tions, and poor selectivity, resulting in byproducts and
difficulties in separating the catalysts from the products.
However, lanthanide complexes have displayed advantages
for this transformation.16,17 Selective transformation of iso-
cyanates to substituted ureas after elimination of CO from the
cyclodimerization products still needs further exploration.
We have observed that rare earth metal complexes sup-
ported by a diamido ligand with a CH2Me2Si link are
excellent catalysts for the cyclotrimerization of aryl isocya-
nates to yield triaryl isocyanurates with a high selectivity.17
In an effort to elucidate the generality of this observation and
the features that dictate the activity and selectivity in this
reaction, we have extended the complexes employed in this
reaction to include rare earth metal complexes incorporating
dimethylsilylene-bridged diamido ligands. We report herein
the synthesis, characterization, and catalytic activity of a
series of rare earth metal amides, and the selective reactivity
of different metal-nitrogen bonds will also be reported for
the first time.
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Results and Discussion
Synthesis and Characterization of Rare Earth Metal Com-
plexes with Different Metal-Nitrogen Bonds. Treatment
of (Me2Si)[(2,6-R21-4-R2-C6H2)NH]2 with [(Me3Si)2N]3L-
n
III(μ-Cl)Li(THF)3 in toluene, after workup, produced the
rare earth metal complexes {(Me2Si)[(2,6-R12-4-R2-C6H2)-
N]2}LnN(SiMe3)2(THF) (R1 = iPr, R2 = H, Ln = Yb (1),
Y(2), Eu (3), Sm (4), Nd (5); R1 =R2 =H,Ln=Yb(6), Sm (7))
in good yields. The structures of the complexes 1, 2, 4,
and 5 were additionally determined by single-crystal X-ray
diffraction study. These complexes are sensitive to moisture,
but they are stable in an inert atmosphere for months.
They are soluble in THF, DME, CH2Cl2, and toluene, but
complexes 6 and 7 are only slightly soluble in n-hexane.
X-ray analyses revealed that complexes 1, 2, 4, and 5 are
neutral compounds supported by a dimethylsilylene-bridged
diamido ligand, an amido ligand N(SiMe3)2, and a coordi-
nated THF molecule (the representative structure of the
complexes is shown in Figure 1; figures for complexes 2, 4,
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3711. (b) Zhou, X.; Zhang, L.; Zhu, M.; Cai, R.; Weng, L. Organome-
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4931. (c) Spirkova, M.; Kubin, M.; Spacek, P.; Krakovsky, I.; Dusek, K.
J. Appl. Polym. Sci. 1994, 53, 1435. (d) Spirkova, M.; Kubin, M.; Spacek,
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Nambu, Y.; Endo, T. J. Org. Chem. 1993, 58, 1932. (f) Mizura, J.;
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references therein.

3884 Organometallics, Vol. 28, No. 13, 2009
Table 1. Selected Bond Lengths (A) and Bond Angles (deg) of Complexes 1, 2, 4, 5, 8, and 9
Zhu et al.
˚
1
2
4
5
Ln-Nav
2.181(14)
2.286(16)
73.0(5)
131.8(5)
135.9(5)
108.2(3)
106.5(5)
98.7(5)
2.213(4)
2.314(4)
2.268(3)
2.397(3)
2.295(2)
2.439(2)
69.44(7)
132.60(8)
137.01(9)
106.31(7)
107.52(7)
100.14(8)
70.23(8)
138.68(9)
136.93 (9)
103.69(10)
106.56(10)
96.50(10)
Ln-Oav
N(2)-Ln(1)-N(1)
N(3)-Ln(1)-N(1)
N(3)-Ln(1)-N(2)
N(1)-Ln(1)-O(1)
N(2)-Ln(1)-O(1)
N(3)-Ln(1)-O(1)
N(5)-Ln(2)-N(4)
N(6)-Ln(2)-N(4)
N(6)-Ln(2)-N(5)
N(4)-Ln(2)-O(2)
N(5)-Ln(2)-O(2)
N(6)-Ln(2)-O(2)
72.55(13)
135.94(14)
134.58(14)
107.37(16)
106.11(15)
97.27(15)
70.83(12)
138.59(12)
137.16(12)
103.39(12)
106.05(13)
96.33(13)
70.18(11)
137.15(13)
133.39(11)
106.90(10)
106.50(10)
99.14(11)
76.7(5)
136.3(5)
130.1(6)
104.6(6)
110.7(6)
96.6(5)
8
9
Yb-Nav
2.175(3)
2.232(3)
2.659(3)
Sm-Nav
2.272(7)
2.326(5)
2.792(6)
110.2(2)
112.3(2)
111.1(2)
101.3(2)
74.21(19)
140.3(2)
63.8(2)
Yb-Oav
Sm-Oav
Yb(1)-N(2)
Sm(1)-N(2)
N(1)-Yb(1)-O(1)#1
N(1)-Yb(1)-N(3)
O(1)#1-Yb(1)-N(3)
N(1)-Yb(1)-O(1)
O(1)#1-Yb(1)-O(1)
N(3)-Yb(1)-O(1)
N(1)-Yb(1)-N(2)
O(1)#1-Yb(1)-N(2)
N(3)-Yb(1)-N(2)
O(1)-Yb(1)-N(2)
107.01(11)
109.47(13)
108.52(11)
102.87(11)
75.01(10)
144.27(11)
67.27(12)
134.34(10)
116.09(11)
63.42(9)
N(1)-Sm(1)-O(1)#1
N(1)-Sm(1)-N(3)
O(1)#1-Sm(1)-N(3)
N(1)-Sm(1)-O(1)
O(1)#1-Sm(1)-O(1)
N(3)-Sm(1)-O(1)
N(1)-Sm(1)-N(2)
O(1)#1-Sm(1)-N(2)
N(3)-Sm(1)-N(2)
O(1)-Sm(1)-N(2)
131.63(18)
115.2(2)
61.57(18)
and 5 are in the Supporting Information). The angles around
the central metal are in the range of 73.0(5)° to 135.9(5)° in
1, 72.55(13)° to 135.94(14)° in 2, 70.83(12)° to 138.59(12)° in
4, and 69.44(7)° to 137.01(9)° in 5 (Table 1), deviating
from the ideal tetrahedron, so the coordination geometry
around the central metal can be described as a distorted
tetrahedral.
corresponding dimethylsilylene-bridged diamido anionic
or other neutral lanthanide amido complexes.22-25
The N(2)-Ln(1)-N(1) angles (formed by the nitrogen
atoms of the bridged diamido ligand and the center metal)
(69.44(7)° to 73.0(5)°, Table 1) found in the above complexes
are smaller than the corresponding N(2)-Ln(1)-N(1) angles
90.0(2)° to 92.1(5)° found in the complexes {(CH2SiMe2)-
[(2,6-iPr2C6H3)N]2}LnN(SiMe3)2(THF) (Ln = Yb, Y, Dy,
Sm, Nd)17 and LYbCl2(THF)2 (80.2(1)°) (L = N,N-bis(2,6-
dimethylphenyl)-2,4-pentanediiminate),25 but these angles
are comparable with those found in dimethylsilylene-bridged
diamido ligand complexes,23,24 suggesting different strain
force in the ring.
As indicated in Figure 1 and Scheme 2, there are different
metal-nitrogen bonds in the above complexes. One type
of metal-nitrogen bond is from the metal to the nitrogen
atom of the N(SiMe3)2, and the other type of metal-nitrogen
bond is from the metal to the nitrogen atoms of the bridged
ligands. To search for the selective reactivity of these
different metal-nitrogen bonds, the reactions of complexes
6 and 7 with (Me2SiO)3 were studied for the stoichiometric
reactions of the lanthanide amides with aromatic isocyanates
giving the unclear results.
The usual lanthanide contraction on moving from Nd3+
to Yb3+ is reflected in the average Ln-N bond distances of
2.295(2) A in 5, 2.268(3) A in 4, 2.213(4) A in 2, and 2.181(14)
˚
˚
˚
˚
A in 1 and the Ln-O bond distances found in the above
complexes (Table 1). These average Ln-N bond distances in
the above complexes are comparable with those found in
{(CH2SiMe2)[(2,6-iPr2C6H3)N]2}LnN(SiMe3)2(THF) (2.178
˚
˚
˚
˚
(4) A for Yb, 2.194(12) A for Y, 2.219(3) A for Dy, 2.263(7) A
17
for Sm, 2.294(5) A for Nd), but they are slightly shorter
˚
than the average Ln-N bond distances of 2.342(4) and 2.299
˚
(2) A for the corresponding yttrium and lutetium complexes
in [(NPNPh)Ln(CH2SiMe3)2(THF)] (NPNPh:N(Ph)PPh2 =
NC6H2Me3-2,4,6),2m even if the ionic radii differences
between the two types of complexes were taken into
account.20 It is found that the distances of the central metal
to the nitrogen atoms of the bridged ligand are generally
shorter than those of metal to the nitrogen atom of the
N(SiMe3)2 ligand (Table 1).
Treatment of {(Me2Si)[(C6H5)N]2}LnN(SiMe3)2(THF)
(Ln = Yb (6), Sm (7)) with grease (Me2SiO)3 respectively
produced the selective insertion products [C6H5N(Me2Si)N-
(C6H5)(Me2SiO)LnN(SiMe3)2]2 (Ln = Yb (8), Sm (9))
(Scheme 2). Complexes 8 and 9 are sensitive to moisture
and are soluble in THF and DME, but they are only slightly
soluble in n-hexane. Their structures were determined by
˚
The average Yb-N bond distance of 2.181(14) A found
in 1 is comparable with the average Yb-N bond distance
of 2.174(1) A found in the cyclic binuclear Yb(III) complex
˚
[{μ-2-p-(Me3SiN)2C6H4}YbCl(THF)2]2.21 The correspond-
˚
˚
ing average Ln-N distances of 2.181(14) A in 1, 2.268(3) A in
4, and 2.295(2) A in 5 are shorter than those found in
˚
(23) Zhou, L. Y.; Sheng, H. T.; Yao, Y. M.; Zhang, Y.; Shen, Q.
J. Organomet. Chem. 2007, 692, 2990.
(24) Zhou, L. Y.; Yao, Y. M.; Li, C.; Zhang, Y.; Shen, Q. Organo-
metallics 2006, 25, 2880.
(20) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(21) Zhao, B.; Li, H.; Shen, Q.; Zhang, Y.; Yao, Y. M.; Lu, C. R.
Organometallics 2006, 25, 1825.
(22) Zhou, S.; Wang, S.; Yang, G.; Liu, X.; Sheng, E.; Zhang, K.;
Cheng, L.; Huang, Z. Polyhedron 2003, 22, 1019.
(25) Xue, M. Q.; Yao, Y. M.; Shen, Q.; Zhang, Y. J. Organomet.
Chem. 2005, 690, 4685.

Article
Organometallics, Vol. 28, No. 13, 2009 3885
Table 2. Optimal Conditions for the Cyclotrimerization of Phenyl
Isocyanate
entry cat. (mol %)a
solvent
temp (°C) time (h) yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
1
1
1
2
1
1
1
1
THF
toluene
Et2O
room temp
room temp
room temp
12
12
12
12
12
3
6
12
6
12
12
12
12
12
12
12
97
27
44
40
24
80
93
97
80
97
97
98
21
19
29
22
n-hexane room temp
CH2Cl2
THF
THF
THF
THF
THF
THF
THF
toluene
toluene
room temp
60
40
40
60
60
40
40
40
60
0.5
0.5
0.25
0.25
0.25
0.25
0.25
n-hexane 40
n-hexane 60
a Catalyst: samarium complex 4, catalyst loading: mole percentage
(mol %). b Isolated yield.
significantly longer than the corresponding Ln-N(1) and
Ln-N(3) distances, suggesting that the bonds between
the metal and N(2) are coordination bonds. The average
Yb-N (the anionic nitrogen atoms) distance of 2.178(3)
Figure 2. Molecular structure of complex 8.
Scheme 1
˚
˚
A found in 8 is shorter than that of 2.272(7) A found in
9, which is in agreement with the lanthanide contraction. The
˚
average Yb-N distance of 2.178(3) A found in 8 is compar-
able with that of 2.181(14) A found in 1, and the average
˚
˚
Sm-N distance of 2.272(7) A found in 9 is comparable with
˚
that of 2.268(3) A found in 4 and those of lanthanide
dinuclear complexes26 if the ionic difference between
different coordination numbers of the metals is taken into
account.20
Catalytic Cyclotrimerization of Aromatic Isocyanates. As
we know that isocyanates could undergo cyclodimerization,
cyclotrimerization, and polymerization reactions, the cata-
lytic activities of the complexes 1-7 on selective cyclotrimer-
ization or cyclodimerization of isocyanates were investigated
(Scheme 3). The samarium complex 4 was used to search for
catalytic activity. The results are listed in Table 2. From the
table, we can see that the complex exhibited catalytic activity
for the selective cyclotrimerization of phenyl isocyanate
(based on HR-MS and 1H NMR analytical results) in several
solvents such as THF, toluene, Et2O, n-hexane, and CH2Cl2
at room temperature (Table 2, entries 1 to 5); it exhibited the
highest activity in THF. When the reaction temperature was
raised to 40 °C, a 93% yield of the cyclotrimerization product
can be obtained in the presence of 1 mol % catalyst in THF in
6 h (Table 2, entry 7), and the conversion of the phenyl
isocyanate to cyclotrimer can be raised to 97% when the
reaction time was prolonged to 12 h. An even higher (98%)
yield can be obtained by carrying out the reaction in THF at
40 °C in the presence of 0.25 mol % catalyst loading for 12 h
(Table 1, entry 12). However, the results of the reactions in
other solvents under the same conditions were not improved
(Table 2, entries 13 to 16), indicating solvents effects on the
catalytic activity of the complexes.
Scheme 2
single-crystal X-ray diffraction study (Figure 2). X-ray ana-
lyses revealed that the Me2SiO group selectively inserted to
the Ln-N bond of the metal to the nitrogen atom of the
bridged ligand, not the Ln-N bond of the metal to the
N(SiMe3)2 group, indicating a high selective reactivity of
the metal-nitrogen bonds (Scheme 2). These results sug-
gested that the Ln-N bonds in the strained four-membered
ring are more reactive than the other Ln-N(SiMe3)2) bond.
The representative structure of the complexes is presented in
Figure 2. From the figure, we can see that they are central
symmetrical dimeric complexes with two central metals
bridged by the oxygen atoms of the Me2SiO group. Each
of the central metals adopts a distorted square-pyramidal
geometry, and the line formed by O and O#1 atoms are
shared by the two distorted pyramids.
The catalytic activities of other rare earth metal amides on
the selective cyclotrimerization of phenyl isocyanate were
(26) (a) Wang, Q.; Xiang, L.; Zi., G. J. Organomet. Chem. 2008, 693,
68–76. (b) Wang, Q.; Xiang, L.; Song, H.; Zi., G. J. Organomet. Chem.
2009, 694, 691–696. (c) Wang, Q.; Xiang, L.; Song, H.; Zi., G. Inorg.
Chem. 2008, 47, 4319–4328.
From Table 1, we can see that the Ln-N(2) distances of
2.659(3) A found in 8 and 2.792(6) A found in 9 are
˚
˚

3886 Organometallics, Vol. 28, No. 13, 2009
Zhu et al.
Table 3. Catalytic Activity of Different Rare Earth Metal Amides
on the Cyclotrimerization of Phenyl Isocyanate
Scheme 3. Catalytic Cyclodimerization or Cyclotrimerization of
Isocyanates
cat. loading
(mol %)
entry cat.
temp (°C) time (h) solvent yield (%)a
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8b
0.25
0.25
0.25
0.25
0.25
1.0
40
40
40
40
40
60
60
40
12
12
12
12
12
24
24
12
THF
THF
THF
THF
THF
THF
THF
THF
98
97
96
98
92
95
98
96
1.0
0.25
a Isolated yield. b Catalyst: [(Me3Si)2N]3Yb(μ-Cl)Li(THF)3 as catalyst.
Scheme 4. Proposed Catalytic Mechanism
Table 4. Data for the Cyclodimerization or Cyclotrimerization of
Different Isocyanates
entry catalyst a
R-
temp (°C)/time yield (%) b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
1
2
4
1
2
4
1
2
4
1
2
4
1
2
4
1
2
4
1
benzyl
benzyl
benzyl
phenyl
phenyl
phenyl
4-isopropylphenyl
4-isopropylphenyl
4-isopropylphenyl
4-nitrophenyl
4-nitrophenyl
4-nitrophenyl
cyclohexyl
cyclohexyl
cyclohexyl
isopropyl
isopropyl
isopropyl
allyl
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
40/12 h
93
97
94
98
97
98
96
97
94
0
0
0
43
61
48
51
63
49
0
a Catalyst loading: 0.25 mol %. b Isolated yield by running the
reaction in THF.
also investigated. The results are given in Table 3. It can
be seen that selective cyclotrimerization products could
be isolated in near-quantitative yield in the presence of
complexes 1-5 as catalysts with low catalyst loading
(0.25 mol %) (Table 3, entries 1 to 5). High yields of the
selective cyclotrimerization products could also be isolated
in the presence of complexes 6 and 7 as catalysts with 1 mol %
catalyst loading in THF at 60 °C (Table 3, entries 6 and 7).
As shown in Table 3, complexes 1-5 exhibited a similar
catalytic activity for the cyclotrimerization of phenyl iso-
cyanate, and the amido complex [(Me3Si)2N]3Yb(μ-Cl)Li-
(THF)3 also displayed a similar catalytic activity to complexes
1-5 (Table 3, entry 8), so some of these complexes were
selected as catalysts to study their catalytic activity on other
isocyanates. The results are presented in Table 4.
As shown in Table 4, all the complexes (1, 2, and 4)
examined exhibited high catalytic activity for the cyclotrimer-
ization of aromatic isocyanates such as phenyl isocyanate,
benzyl isocyanate, 4-isopropylbenzyl isocyanate (Table 4,
entries 1-9). However, 4-nitrophenylisocyanate cannot be
catalyzed to its corresponding cyclotrimerization product
by all the complexes examined under the same conditions
(Table 4, entries 10-12). This result is similar to our previous
report,17 which needs to be further investigated. It is notable
that other byproducts such as cyclodimerization products
or polymerization products were not observed in the reac-
tions, indicating a high selectivity of the catalysts.
When we tried to cyclotrimerize the aliphatic isocyanates
such as the isopropyl isocyanate, cyclohexyl isocyanate, and
allyl isocyanate to their corresponding cyclotrimerization
products using the above complexes as catalysts under the
same conditions, no cyclotrimerization products of the
corresponding aliphatic isocyanates were isolated from the
HR-MS and 1H NMR results of the reaction products;
instead, substituted ureas such as N,N0-diisopropylurea
and N,N0-dicyclohexylurea were isolated from the corre-
sponding reactions (Scheme 3, Table 4, entries 13 to 18).
The formation of the substituted ureas is believed to proceed
through elimination of CO from the corresponding isocya-
nate cyclodimerization products. These results are comple-
tely different from our previous work on the catalytic activity
of lanthanide amides {(CH2SiMe2)[(2,6-iPr2C6H3)N]2}LnN-
(SiMe3)2(THF) (Ln = Yb, Y, Dy, Sm, Nd), which can
selectively catalyze the corresponding aliphatic isocyanate
cyclotrimerization,17 suggesting ligand effects on the selec-
tivity of the catalysts. It was found that the yttrium complex
exhibited higher catalytic activity than the other complexes
examined (complexes 1 and 4) (Table 4, entries 13 to 18).
However, in the case of allyl isocyanate, no cyclotrimerization
or substituted urea was isolated under the same conditions.
The catalytic reaction mechanism is proposed as follows
(Scheme 4): Coordination of the isocyanate to the central
metal under liberation of the coordinated THF molecule

Article
Organometallics, Vol. 28, No. 13, 2009 3887
gave the intermediate I, insertion of the isocyanate into the
metal-nitrogen bond of the metal to the bridged ligand leads
to the formation of the six-memebered intermediate II
to release the stain force of the four-membered ring,15
the intermediate II then reacted with another molecule of
isocyanate to give the eight-membered intermediate III. The
intermediate III either eliminated the cyclodimerization
product followed by giving off a molecule of carbon mon-
oxide (CO), producing the urea when the isocyanates are the
aliphatic isocyanates and thus furnishing the catalytic cycle,
or further reacted with isocyanate, producing the cyclotri-
merization products and the intermediate I when the iso-
cyanates are aromatic isocyanates and thus furnishing
a catalytic cycle. The selectivity of these complexes on
different isocyanates may be due to the electronic properties
of different isocyanates causing a different reactivity of the
intermediate III. However, other possibilities such as reac-
tion of the Ln-N(SiMe3)2 bond in the intermediate II with
isocyanate to give the catalytic products as proposed in our
previous work17 cannot be ruled out.
spectrometer (KBr pellet). Mass spectra were performed on a
Micromass GCT-MS spectrometer. Melting points were deter-
mined in capillaries and were uncorrected.
Preparation of {(Me2Si)[(2,6-iPr2C6H3)N]2}YbN(SiMe3)2-
(THF) (1). To a toluene (10.0 mL) solution of (Me2Si)-
[(2,6-iPr2C6H3)NH]2 (1.20 g, 2.92 mmol) was added a toluene
(40.0 mL) solution of [(Me3Si)2N]3YbIII(μ-Cl)Li(THF)3 (2.67 g,
2.92 mmol) at room temperature. After the reaction mixture was
stirred at room temperature for 6 h, the mixture was then
refluxed for 24 h, and the color of the solution gradually
changed from yellow to red. The solvent was evaporated under
reduced pressure. The residue was extracted with n-hexane (2 ꢀ
15.0 mL). The extractions were combined and concentrated to
about 10.0 mL. The red crystals were obtained by cooling the
concentrated solution at 0 °C for several days (1.92 g, 81%
yield). IR (KBr pellet, cm-1): ν 3063 (m), 3028 (m), 2962 (m),
2866 (m), 1620 (s), 1512(s), 1458(m), 1361 (s), 1330 (w), 1254 (s),
1196 (s), 1111 (w), 1045 (w), 957 (s), 879 (s), 829 (s), 686 (m), 628
(m). Anal. Calcd for C36H66N3OSi3Yb: C, 53.10; H, 8.17; N,
5.16. Found: C, 53.43; H, 7.99; N, 4.98.
Preparation of {(Me2Si)[(2,6-iPr2C6H3)N]2}YN(SiMe3)2-
(THF) (2). This compound was prepared as white crystals in
74% (1.21 g) yield by treatment of (Me2Si)[(2,6-iPr2C6H3)NH]2
(0.92 g, 2.24 mmol) with [(Me3Si)2N]3YIII(μ-Cl)Li(THF)3
(1.86 g, 2.24 mmol) using procedures similar to those described
above for the preparation of 1. 1H NMR (C6D6, ppm):
δ 7.32-7.12 (m, 6H) (C6H3), 3.93 (m, 4H, OC4H8), 3.59 (m, 4H,
CH(CH3)2), 1.43 (m, 4H, OC4H8), 1.24-1.19 (m, 24H, CH3),
0.35-0.09 (s, 24H, Si(CH3)3). 13C NMR (C6D6, ppm): δ 147.9,
141.4, 122.9, 120.1, 28.0, 27.3, 24.4, 23.2, 3.2, 0.8. IR (KBr pellet,
cm-1): ν 3063 (m), 2928 (m), 2866 (m), 1527 (s), 1458 (m), 1438
(m), 1381(s), 1361 (s), 1330 (w), 1253 (s), 1195 (s), 1099 (w), 1041
(w), 906 (s), 879 (s), 806 (m), 786 (m), 748 (w), 675(m). Anal.
Calcd for C36H66N3OSi3Y: C, 59.22; H, 9.11; N, 5.76. Found:
C, 58.88; H, 9.09; N, 5.67.
Conclusion
In summary, a series of four-coordinated neutral lantha-
nide amides {(Me2Si)[(2,6-R12-4-R2-C6H2)N]2}LnN(Si-
i
Me3)2(THF) (R1 = Pr, R2 = H, Ln = Yb (1), Y (2), Eu
(3), Sm (4), Nd (5); R1 = R2 = H, Ln = Yb (6), Sm (7))
having different metal-nitrogen bonds have been synthe-
sized and characterized. Results of reactions of the com-
plexes with (Me2SiO)3 indicated that the Ln-N bonds in the
complexes displayed different reactivity. Investigation of the
catalytic activity of complexes {(Me2Si)[(2,6-R12-4-R2-C6H2)
N]2}LnN(SiMe3)2(THF) indicated that these complexes ex-
hibited a high selectivity on cyclotrimerization or cyclodi-
merization of isocyanates depending on the nature of the
isocyanates. These complexes exhibited a high catalytic
activity and a high selectivity on cyclotrimerization of aro-
matic isocyanates; all the complexes showed a similar cata-
lytic activity. The complexes exhibited a good catalytic
activity and a high selectivity on cyclodimerization of ali-
phatic isocyanates; the yttrium complex had a higher cata-
lytic activity than other complexes examined. The complexes
not only have a high selective reactivity toward isocyanates
but also have the advantages of easy preparation, low
catalyst loading, high conversion, mild reaction conditions,
and compatibility with different substrates and solvents.
Preparation of {(Me2Si)[(2,6-iPr2C6H3)N]2}EuN(SiMe3)2-
(THF) (3). This compound was obtained as a red powder in
45% yield following the procedures described for complex
1 from the reaction of (Me2Si)[(2,6-iPr2C6H3)NH]2 (0.38 g,
0.93 mmol) with [(Me3Si)2N]3EuIII(μ-Cl)Li(THF)3 (0.82 g,
0.93 mmol). IR (KBr pellet, cm-1): ν 3028 (m), 2962(m), 1635
(s), 1462 (s), 1438 (m), 1361 (s), 1327 (w), 1257 (s), 1195 (s), 1057
(w), 910 (s), 802 (m), 744 (m). Anal. Calcd for C36H66N3OSi3Eu:
C, 54.51; H, 8.39; N, 5.30. Found: C, 54.25; H, 7.93; N, 5.18.
Preparation of {(Me2Si)[(2,6-iPr2C6H3)N]2}SmN(SiMe3)2-
(THF) (4). Following the procedures described for the prepara-
tion of complex 1, complex 4 was prepared as yellow crystals in
77% yield from the reaction of (Me2Si)[(2,6-iPr2C6H3)NH]2
(0.94 g, 2.28 mmol) with [(Me3Si)2N]3SmIII(μ-Cl)Li(THF)3
(2.03 g, 2.28 mmol). IR (KBr pellet, cm-1): ν 3063 (m), 2962
(m), 2928 (m), 2866 (m), 1624 (s), 1458 (m), 1438 (m), 1253 (s),
1195 (s), 1111 (w), 1045 (w), 933 (s), 906 (s), 879 (m), 786 (w),
748 (m), 678 (m). Anal. Calcd for C36H66N3OSi3Sm: C, 54.63;
H, 8.40; N, 5.31. Found: C, 54.77; H, 8.03; N, 5.02.
Experimental Section
Materials and Methods. All syntheses and manipulations of
air- and moisture-sensitive materials were performed under a
dry argon and oxygen-free atmosphere using standard Schlenk
techniques. All solvents were refluxed and distilled over sodium
benzophenone ketyl under argon prior to use unless otherwise
noted. [(Me3Si)2N]3LnIII(μ-Cl)Li(THF)3 (Ln = Yb, Y, Eu, Sm,
and Nd),22 (Me2Si)[(2,6-iPr2C6H3)NH]2,27 and (Me2Si)[(C6H5)-
NH]223 were prepared according to literature methods. Elemen-
tal analyses data were obtained on a Perkin-Elmer 2400 Series II
elemental analyzer. 1H NMR and 13C NMR spectra for analyses
of compounds were recorded on a Bruker AV-300 NMR
spectrometer (300 MHz for 1H; 75.0 MHz for 13C) in C6D6 for
lanthanide complexes and in CDCl3 for organic compounds.
Chemical shifts (δ) were reported in ppm. J values are reported
in Hz. IR spectra were recorded on a Shimadzu FTIR-8400s
Preparation of {(Me2Si)[(2,6-iPr2C6H3)N]2}NdN(SiMe3)2-
(THF) (5). This compound was prepared as blue crystals in
53% yield from the reaction of (Me2Si)[(2,6-iPr2C6H3)NH]2
(0.87 g, 2.12 mmol) with [(Me3Si)2N]3NdIII(μ-Cl)Li(THF)3
(1.88 g, 2.12 mmol) in toluene using procedures similar to those
described above for the preparation of 1. IR (KBr pellet, cm-1):
ν 2962 (m), 2870 (m), 1620 (s), 1462 (m), 1438 (m), 1384 (s),
1365(m), 1261 (s), 1207 (s), 1091 (w), 860 (m), 744 (m), 659 (m).
Anal. Calcd for C36H66N3NdOSi3: C, 55.05; H, 8.47; N, 5.35.
Found: C, 54.96; H, 8.12; N, 5.02.
Preparation of {(Me2Si)[(C6H5)N]2}YbN(SiMe3)2(THF) (6).
This complex was prepared as red crystals in 73% yield
from reaction of (Me2Si)[(C6H5)NH]2 (0.45 g, 1.87 mmol) with
[(Me3Si)2N]3YbIII(μ-Cl)Li(THF)3 (1.71 g, 1.87 mmol) in to-
luene using procedures similar to those described above for
the preparation of 1. IR (KBr pellet, cm-1): ν 2962 (m), 1600 (s),
(27) Hill, M. S.; Hitchcock, P. B. Organomatallics 2002, 3258–3262.

3888 Organometallics, Vol. 28, No. 13, 2009
Zhu et al.
1496(s), 1384 (s), 1288 (m), 1261 (s), 1091 (w), 1030 (m), 914 (w),
798 (s), 752 (m), 690 (w), 720 (m), 669 (m). Anal. Calcd for
C35H58N3O2Si3Yb (6+C7H8+THF): C, 51.89; H, 7.22; N,
5.19. Found: C, 51.67; H, 7.06; N, 4.94.
(w), 1847 (m), 1774 (m), 1600 (s), 1496 (s), 1388 (s), 1292 (m),
1257 (m), 1176 (s), 1153 (m), 1076 (m), 1030 (s), 995 (m), 914 (w),
887 (w), 829 (w), 798 (m), 752 (m), 694 (w), 659 (w). Anal. Calcd
for C65H104N6O2Si8Sm2 (9+3C7H8): C, 51.13; H, 6.86; N, 5.50.
Found: C, 51.77; H, 7.25; N, 5.56.
Preparation of {(Me2Si)[(C6H5)N]2}SmN(SiMe3)2(THF) (7).
This complex was prepared as yellow crystals in 67% yield
from reaction of (Me2Si)[(C6H5)NH]2 (0.55 g, 2.27 mmol) with
[(Me3Si)2N]3SmIII(μ-Cl)Li(THF)3 (2.03 g, 2.27 mmol) in to-
luene using procedures similar to those described above for
the preparation of 1. IR (KBr pellet, cm-1): ν 3086 (m), 3036
(m), 2962 (m), 2901 (m), 1600 (s), 1500 (s), 1388 (s), 1288 (m),
1257 (s), 1180 (w), 1153 (w), 1076 (m), 1030 (m), 995 (m), 887 (s),
614 (w). Anal. Calcd for C27H42N3Si3Sm (7+C7H8-THF): C,
50.41; H, 6.58; N, 6.53. Found: C, 50.06; H, 6.09; N, 6.55.
Preparationof [C6H5N(Me2Si)N(C6H5)(Me2SiO)YbN(SiMe3)2]2
(8). To a toluene solution of {(Me2Si)[(C6H5)N]2}YbN(SiMe3)2-
(THF) (6) (1.30 g, 2.00 mmol) was added grease (Me2SiO)3 (0.22
g, 1.00 mmol), and the reaction was refluxed for 12 h. The
solution was concentrated to about 10 mL. Orange crystals were
obtained by cooling the solution at -5 °C for several days (0.84
g, 55% yield). IR (KBr pellet, cm-1): ν 3036 (m), 2958 (m), 1921
(w), 1832 (m), 1774 (m), 1600 (s), 1498 (s), 1384 (s), 1288 (m),
1257 (m), 1176 (s), 1153 (s), 1111 (w), 1076 (m), 1030 (s), 995 (m),
914 (w), 883 (w), 829 (w), 798 (m), 752 (m), 694 (w), 659 (w).
Anal. Calcd for C58H96N6O2Si8Yb2 (8+2C7H8): C, 47.06; H,
6.54; N, 5.68. Found: C, 47.17; H, 5.98; N, 5.92.
General Procedure for the Cyclotrimerization or Cyclodimer-
ization of Isocyanates Catalyzed by {(Me2Si)[(2,6-R12-4-R2-
C6H2)N]2}LnN(SiMe3)2(THF) (10 as an example). A 30 mL
Schlenk tube under dried argon was charged with complex 1
(0.016 g, 0.02 mmol), and THF (5 mL) and phenyl isocyanate
(0.95 g, 8.0 mmol) were added. The resulting mixture was stirred
at room temperature for 12 h. After the solvent was removed
under reduced pressure, the residue was washed with hexane
(3 ꢀ 5 mL). The final product, 10, was isolated as colorless
crystals by recrystallization from THF (98%): 1H NMR
(CDCl3, ppm) δ 7.53-7.40 (m, 15H, aromatic CH); 13C NMR
(CDCl3, ppm) δ 148.4, 133.2, 129.1, 128.0.
X-ray crystallography section and crystallographic data can
be found in the Supporting Information.
Acknowledgment. This work was co-supported by the
National Natural Science Foundation of China (Grant
Nos. 20672002, 20832001) and grants from Anhui Pro-
vince (Nos. TD200707, 2007Z016).
Preparation of [C6H5N(Me2Si)N(C6H5)(Me2SiO)SmN(Si-
Me3)2]2 (9). This complex was prepared as yellow crystals by
reaction of {(Me2Si)[(C6H5)N]2}SmN(SiMe3)2(THF) (7) (1.12
g, 1.80 mmol) with (Me2SiO)3 (0.20 g, 0.90 mmol) following
procedures similar to those used for the preparation of 8 (0.78 g,
60% yield). IR (KBr pellet, cm-1): ν 3039 (m), 2958 (m), 1925
Supporting Information Available: Text for characterization
data of compounds 10-14, figures for compounds 2, 4, 5, 9, and
12-14, table giving bond lengths, crystallographic data, and
CIF files giving crystal data for complexes 1, 2, 4, 5, 9, and 12-
14. This material is available free of charge via the Internet at
http://pubs.acs.org.