Controlled synthesis of lanthanide-lithium inverse crown ether complexes

Article abstract of DOI:10.1016/j.inoche.2010.09.013

An ytterbium-lithium inverse crown ether complex stabilized by amine-bridged bis(phenolate) ligands (LYbBr)₂(μ₄-O) (μ₃-Li) (1) [L = Me₂NCH₂CH ₂N{CH₂-(2-O- C₆H₂-Bu ^t₂-3,5)}₂] was isolated as a byproduct from the reaction of LYbCl(THF) with CH₃Li in THF in a very low isolated yield. Further study revealed that the inverse crown ether complexes can be synthesized in a controlled manner by the reaction of bis(phenolate) lanthanide chloride with an in situ mixture of n-BuLi with water in THF. A second inverse crown ether complex (L′YbCl)₂(μ₄-O) (μ₃-Li) (2) [L′ = Me₂NCH₂CH ₂N{CH₂-(2-O-C₆H₂-Bu ^t-3-Me-5)}₂] was prepared in high isolated yield and well characterized.

Full text of DOI:10.1016/j.inoche.2010.09.013

Inorganic Chemistry Communications 13 (2010) 1566–1568
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Controlled synthesis of lanthanide–lithium inverse crown ether complexes
⁎
Xin-Hua Lu, Meng-Tao Ma, Ying-Ming Yao , Yong Zhang, Qi Shen
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
a r t i c l e i n f o
a b s t r a c t
Article history:
An ytterbium–lithium inverse crown ether complex stabilized by amine-bridged bis(phenolate) ligands
(LYbBr)2(μ4-O)(μ3-Li) (1) [L=Me2NCH2CH2N{CH2-(2-O- C6H2-But2-3,5)}2] was isolated as a byproduct from
the reaction of LYbCl(THF) with CH3Li in THF in a very low isolated yield. Further study revealed that the
inverse crown ether complexes can be synthesized in a controlled manner by the reaction of bis(phenolate)
lanthanide chloride with an in situ mixture of n-BuLi with water in THF. A second inverse crown ether
complex (L′YbCl)2(μ4-O)(μ3-Li) (2) [L′=Me2NCH2CH2N{CH2-(2-O-C6H2-But-3-Me-5)}2] was prepared in
high isolated yield and well characterized.
Received 3 August 2010
Accepted 8 September 2010
Available online 17 September 2010
Keywords:
Inverse crown ether complex
Bis(phenolate) ligand
Lanthanide complex
Synthesis
© 2010 Elsevier B.V. All rights reserved.
In recent years, the synthesis and characterization of inverse
crown ether complexes (II, Chart 1) have received considerable
attention because of their interesting structural feature and wide
applications in homogeneous catalysis [1–6]. In most cases, inverse
crown ether complexes are heterobimetallic complexes (M1≠M2) or
special ate complexes. Generally, M1 is an alkali metal, such as Li, Na
or K, and M2 is a main group or transition metal, focused on Zn, Mg
and Mn etc. Up to date, reports of inverse crown ether complexes
based on rare-earth metals are rare. To the best of our knowledge,
three structurally characterized inverse crown ether complexes
containing rare-earth metals were synthesized. Aspinall et al.
reported, for the first time, the yttrium-lithium inverse crown ether
cluster, which was isolated from the reaction of anhydrous YCl3 with
lithium triaminoamide [7]. Subsequently, Gambarotta et al. [8] and
Martins et al. [9] reported the syntheses and characterization of
samarium-lithium and yttrium-lithium inverse crown ether com-
plexes, respectively. However, all of these complexes are unexpected
products, and their formation mechanism is unclear to date. In all of
these cases, the oxygen source of the oxo anionic core was proposed
to come from the THF solvent, e.g. the in situ C–O cleavage of THF. But
this hypothesis cannot explain the fact that THF is commonly used as
solvent in the preparation of lanthanide amido or alkyl complexes by
the reaction of (organo)lanthanide halide with lithium amides or
lithium alkyls. So it was believed that the oxygen is from other source
presented in the reactions instead of THF. Herein we reported a
controlled synthesis of ytterbium–lithium inverse crown ether
complexes stabilized by amino-amino bis(phenolate) ligands.
Chart 1. Structures of crown ether complex and inverse crown ether complex.
We previously reported that the reaction of amino-amino bridged
bis(phenolate) ytterbium chloride LYbCl(THF) [L=Me2NCH2CH2N
{CH2-(2-O-C6H2-But2-3,5)}2] with 1 equiv. of LiCH3 in THF gave an
ytterbium methyl complex in 50% isolated yield [10]. A further study
revealed that, except the desired product, an unexpected byproduct
can be isolated from this reaction in a low yield (b5%), which was
characterized to be an ytterbium–lithium inverse crown ether
complex (LYbBr)2(μ4-O)(μ3-Li) (1) by X-ray structure determination
(Scheme 1) [11,12]. It was believed that the bromide in complex 1
came from the LiBr in the LiCH3 solution through chloride-bromide
exchange reaction.
The molecular structure of complex 1 with selected bond lengths
and angles is shown in Fig. 1. Complex 1 is an ytterbium–lithium
heterobimetallic species, and has centrosymmetric structure. Two
ytterbium atoms, two lithium atoms and four oxygen atoms consist of
an eight-membered ring, and one oxygen atom lies in the middle of
the ring in μ4 mode to form an inverse crown ether structure. The
⁎
Corresponding author. College of Chemistry, Chemical Engineering and Materials
Science, Dushu Lake Campus, Soochow University, Suzhou, 215123, People's Republic of
China. Tel.: +86 512 65882806; fax: +86 512 65880305.
E-mail address: yaoym@suda.edu.cn (Y.-M. Yao).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.013

X.-H. Lu et al. / Inorganic Chemistry Communications 13 (2010) 1566–1568
1567
Scheme 1. Synthesis of amine bridged bis(phenolate) ytterbium methyl complex and inverse crown ether complex 1.
Fig. 1. Structure of complex 1 drawn with 20% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Yb(1)–O(1) 2.175(4), Yb
(1)–O(2) 2.179(4), Yb(1)–O(3) 2.0918(2), Yb(1)–N(1) 2.453(5), Yb(1)–N(2) 2.515(5), Yb(1)–Br(1) 2.7170(7), Li(1)–O(1) 1.882(11), Li(1A)–O(2) 1.854(12), Li(1)–O(3) 1.975(11);
O(1)–Yb(1)–O(2) 157.6(2), O(3)–Yb(1)–N(2) 168.8(1), Br(1)–Yb(1)–O(1) 95.7(1), O(1)–Yb(1)–N(1) 81.0(2), N(1)–Yb(1)–O(2) 83.7(2), O(2)–Yb(1)–Br(1) 103.5(1), O(1)–Li(1)–
O(2A) 169.7(8), O(1)–Li(1)–O(3) 94.9(5), O(3)–Li(1)–O(2A) 94.9(5), Li(1)–O(3)–Li(1A) 180.0(2).
ytterbium center is six-coordinate and bound to three oxygen atoms
and two nitrogen atoms and one bromide to form a slightly distorted
octahedral coordination geometry, in which O(1), O(2), N(1) and Br
(1) can be considered to occupy the equatorial positions with the sum
of the bond angles of 363.9(1)°, and N(2) and O(3) to occupy the axial
positions. The average Yb–O(Ar) bond distance of 2.177(4) Å is
obviously longer than that in LYbCl(THF) (2.111(4) Å) [10] because of
the formation of bridge bonding, but it compares well with the
corresponding bond distance in (LYCH2SiMe3)2(μ4- O)(μ3-Li) (2.230
(3) Å) [9] when the difference in ionic radii is considered. The lithium
Scheme 2. Synthesis of inverse crown ether complex 2.

1568
X.-H. Lu et al. / Inorganic Chemistry Communications 13 (2010) 1566–1568
Research Project of the Natural Science Foundation of the Jiangsu
Higher Education Institutions (Project 07KJA15014), and the Qing Lan
Project is gratefully acknowledged.
Appendix A. Supplementary material
CCDC 787033 and 787034 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data to this article can be
found online at doi:10.1016/j.inoche.2010.09.013.
References
[1] R.E. Mulvey, Organometallics 25 (2006) 1060.
[2] M. Westerhausen, Dalton Trans. (2006) 4755.
[3] R.E. Mulvey, Acc. Chem. Res. 42 (2009) 743.
[4] A.R. Kennedy, J. Klett, R.E. Mulvey, S. Newton, D. Wright, Chem. Commun. (2008)
308.
[5] N.M. Clark, P. Garcia-Alvarez, A.R. Kennedy, C.T. O'Hara, G.M. Robertson, Chem.
Commun. (2009) 5835.
[6] J.C. Wu, X.B. Pan, N. Tang, C.C. Lin, Inorg. Chem. 49 (2010) 5362.
[7] H.C. Aspinall, M.R. Tillotson, Inorg. Chem. 35 (1996) 2163.
[8] T. Dube, S. Gambarotta, G. Yap, Organometallics 17 (1998) 3967.
[9] S. Barroso, J.L. Cui, J.M. Carretas, A. Cruz, I.C. Santos, M.T. Duarte, J.P. Telo, N.
Marques, A.M. Martins, Organometallics 28 (2009) 3449.
Fig. 2. Structure of complex 2 drawn with 20% probability ellipsoids. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (deg): Yb(1)–O(1) 2.187
(2), Yb(1)–O(2) 2.179(2), Yb(1)–O(3) 2.1145(3), Yb(1)–N(1) 2.460(2), Yb(1)–N(2)
2.542(2), Yb(1)–Cl(1) 2.5464(8), Li(1A)–O(1) 1.853(6), Li(1)–O(2) 1.889(5), Li(1)–O
(3) 1.948(5); O(1)–Yb(1)–O(2) 158.97(7), O(3)–Yb(1)–N(2) 164.88(5), Cl(1)–Yb(1)–
O(1) 101.02(6), O(1)–Yb(1)–N(1) 84.32(7), N(1)–Yb(1)–O(2) 81.99(7), O(2)–Yb(1)–
Cl(1) 96.36(5), O(1A)–Li(1)–O(2) 167.6(3), O(2)–Li(1)–O(3) 95.9(2), O(3)–Li(1)–O
(1A) 96.2(2), Li(1)–O(3)–Li(1A) 180.0(1).
[10] Y.M. Yao, M.T. Ma, X.P. Xu, Y. Zhang, Q. Shen, W.T. Wong, Organometallics 24
(2005) 4014.
[11] Preparation of complex 1: During the synthesis of complex LYbMe(THF) [10], a
small amount of colorless crystals (complex 1) were isolated from a concentrated
toluene solution at –10 °C. IR (KBr pellet, cm–1): 2953 (s), 2900 (s), 2872 (s), 1625
(m), 1527 (m), 1540 (m), 1458 (s), 1416 (m), 1360 (m), 1226 (s), 1195 (s), 1048
(m), 981 (m), 928 (m), 795 (m), 722 (m), 639 (s), 553 (m), 502 (s).
[12] Crystal data determined at 193(2) K for complex 1·2C7H8: C82H124Br2Li2N4O5Yb2,
M=1765.63, monoclinic, space group P21/n, a=11.7904(8), b=21.6051(17),
c=26.9307(13) Å, β=91.358(2)°, V=4311.6(6) Å3, Z=2, Dc =1.360 g/cm3,
R1 =0.0529, wR2 =0.1297. Data were collected on a Rigaku Mercury CCD area
atom is coordinated by three oxygen atoms, and the bond distances of
Li–O are in the range usually found between Li cations and neutral
oxygen donor ligands.
The isolation and characterization of complex 1 encouraged us to
further explore its formation mechanism. The structure of complex 1
can be considered as two LYbBr moieties connected by one Li2O
molecule. So we tried to synthesize the inverse crown ether complex
by the reaction of L′YbCl(THF) [L′=Me2NCH2CH2N{CH2-(2-O-C6H2-
But-3-Me-5)}2] [13] with Li2O in THF. But the attempts were
unsuccessful because Li2O is insoluble in THF. We noticed that all of
the rare-earth metal containing inverse crown ether complexes were
isolated from the reactions of lanthanide chlorides with lithium amide
or lithium alkyls, whereas these lithium reagents are extremely
sensitive to moisture. Therefore, it was postulated that the Li2O might
be from the in situ hydrolysis reaction of the lithium reagent by
moisture. To prove this hypothesis, a reaction was designed. Firstly, n-
BuLi was mixed with 0.5 equiv of water in THF to give a transparent
solution. To this solution was added a THF solution of L′YbCl(THF).
After workup, the desired inverse crown ether complex (L′YbCl)2(μ4-
O)(μ3-Li) was isolated from a concentrated toluene solution in high
yield as colorless crystals (Scheme 2) [14].
Complex 2 was well characterized including X-ray structure determi-
nation [15]. The molecular diagram is depicted in Fig. 2, with selected
bond lengths and angles. Complex 2 also has a square-like Yb2Li2 structure
linked by four oxygen atoms from two amino-amino bridged bis
(phenolate) ligands, and one μ4-O lies in the core of the ring to form an
inverse crown ether structure. Each of the ytterbium atoms is coordinated
by three oxygen atoms, two nitrogen atoms and one chloride to form a
distorted octahedral geometry. The Yb–O, Yb–N, and Li–O bond distances
accord with the corresponding bond distances in complex 1.
detector in
ω scan mode using graphite-monochromated Mo-Kα radiation
(λ=0.71070 Å). Of 47452 data collected, 9860 were unique reflections
(Rint =0.0394), and 8819 were observed reflections (IN2.0 σ(I)). The corrections
for Lp factors and empirical absorption were applied to the intensity data. The
structure was solved by direct methods with SHELX-97 program and expanded
with difference Fourier technique. The non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were located at the calculated positions.
The anisotropic thermal parameters for the non-hydrogen atoms were refined by
full-matrix least-squares techniques on F2. Crystallographic data for the structural
analysis have been deposited with the Cambridge Crystallographic Data Center
CCDC No. 787033.
[13] Preparation of L′YbCl(THF): A solution of LH2 (2.82 g, 6.40 mmol) in THF (20 mL)
was added dropwise to
a NaH suspension (14.02 mmol) in THF at room
temperature. After 14 h, the mixture was filtered. The resulting pale yellow
solution was added to a suspension of YbCl3 (1.79 g, 6.40 mmol) in THF (20 mL).
After the solution was stirred overnight at room temperature, the precipitation
was separated from the reaction mixture by centrifugation. The solvent was
removed under vacuum, and the residue was extracted with toluene. Pale yellow
crystals were obtained from a toluene solution at room temperature in a few days
(3.45 g, 75%). Mp: 183 185 °C (dec). Anal. Calcd. for C32H50ClN2O3Yb: C, 53.44; H,
7.01; N, 3.89; Cl, 4.93; Yb, 24.06. Found: C, 53.61; H, 7.06; N, 3.47; Cl, 4.81; Yb,
24.43. IR (KBr pellet, cm 1): 2956 (s), 2917 (s), 2871 (s), 1752 (m), 1605 (m), 1562
(m), 1451 (s), 1382 (m), 1027 (m), 864 (m), 826 (m), 524 (s).
[14] Preparation of complex 2: To a stirred THF solution containing water (1.08 mmol)
was added n-BuLi in hexane (1.48 ml, 2.16 mmol). The mixture was stirred at
room temperature for 6 hours, and then was added to a THF solution of L′YbCl
(THF) (1.55 g, 2.16 mmol) (50 mL). The solution was stirred overnight, and then
the solvent was evaporated to dryness under vacuum. The residual solid was
extracted with toluene. Colorless crystals were obtained at –10 °C in a few days
(1.05 g, 73%). Mp: 218 220 °C (dec). Anal. Calcd for C56H84Cl2Li2N4O5Yb2: C,
50.80; H, 6.39; N, 4.23; Cl, 5.35; Yb, 26.14. Found: C, 50.42; H, 6.12; N, 4.55; Cl,
5.03; Yb, 26.57. IR (KBr pellet, cm–1): 2964 (s), 2913 (s), 2849 (s), 1779 (m), 1592
(m), 1548 (m), 1463 (s), 1403 (m), 1391 (m), 1347 (m), 1245 (m), 1149 (m),
1028 (m), 865 (m), 815 (m), 770 (m), 718 (m), 535 (s).
In summary, we have synthesized and structurally characterized
two ytterbium–lithium inverse crown ether complexes. An efficient
and straightforward approach was developed for the controlled
synthesis of this kind of complexes.
[15] Crystal data determined at 193(2)
K for complex 2: C56H84Cl2Li2N4O5Yb2,
M=1324.13, monoclinic, space group P21/c, a=14.222(3), b=9.5773(15),
c=22.257(4) Å, β=102.013(4)°, V=2965.1(9) Å3, Z=4, Dc =1.483 g/cm3,
R1 =0.0259, wR2 =0.0595. Of 31157 data collected, 6779 were unique reflections
(Rint =0.0294), and 6422 were observed reflections (IN2.0 σ(I)). The structure
was solved as that described for complex 1. The CCDC No. is 787034.
Acknowledgements
Financial support from the National Natural Science Foundation of
China (Grants 20771078, 20972108 and 20632040), the Major Basic