A new type of lanthanide complex-two divalent ytterbium species assembled from cation-π interactions

Article abstract of DOI:10.1002/ejic.201101199

A new divalent ytterbium complex, [{(L^Ph)Yb^II(Et ₂O)(thf)}₂(μ-KI)] {L^Ph = Ph ₂Si(NAr)₂, Ar = 2,6-iPr₂C₆H ₃} (3), was synthesized by the reaction

Full text of DOI:10.1002/ejic.201101199

SHORT COMMUNICATION
DOI: 10.1002/ejic.201101199
A New Type of Lanthanide Complex – Two Divalent Ytterbium Species
Assembled from Cation–π Interactions
Cheng-Ling Pan,*[a,b] Shao-Ding Sheng,[a] Chang-Min Hou,[b] Yu-Song Pan,[a] Jing Wang,[a]
and Yong Fan*[b]
Keywords: Ytterbium / Cation–π interactions / Amides / Dimers / Synthesis design
A new divalent ytterbium complex, [{(LPh)YbII(Et2O)(thf)}2(μ-
KI)] {LPh = Ph2Si(NAr)2, Ar = 2,6-iPr2C6H3} (3), was synthe-
sized by the reaction of [(LPh)YbIIII(thf)2] (2) and an excess
of potassium. The bimetallic ytterbium(II) species with bulky
diamido ligands are bridged by a KI molecule with cation–π
interactions. These two complexes have been fully charac-
terized, particularly by X-ray crystallography.
Introduction
N(SiMe3)2.[13,14] In this respect, it is noteworthy that the
stabilizing properties of LPh [LPh = Ph2Si(NAr)2, Ar = 2,6-
iPr2C6H3] appear to be similar to those of bulky, N,NЈ-
Ever since Kagan’s report on SmI2, the chemistry of lan-
thanide complexes has been increasingly attractive. How-
ever, the chemistry of divalent lanthanide complexes is
largely limited, because of destabilizing factors.[1] For a long
time, the coordination chemistry of divalent lanthanide ions
chelating
β-diketiminates
[{(R1)NC(R2)}2C(R3)]–
(nacnac–)[15,16] and guanidinate ligands [(ArN)2CN-
(C6H11)2]– (Giso–),[17] which have been utilized in the prepa-
ration and characterization of homoleptic four-coordinate
Ln2+ complexes.
has been limited to that of the classical three elements Eu2+
,
Yb2+, and Sm2+ [2] These Ln2+ ions have the configuration
.
of 6s05d04fn, with n = 6 (Sm), 7 (Eu), and 14 (Yb). The
In our previous work, we obtained planar four-coordi-
nate LnII diamido complexes [(LPh)2Ln{K(solv.)x}2] (Ln =
Sm, Yb, and Eu), which represent the first example of lan-
thanides bearing heteroleptic sandwich structures.[18–19]
These results inspired us to extend the coordination chemis-
try of bulky diamido ligands to ytterbium(II) by taking ad-
vantage of related synthetic routes and cation–π interac-
tions. We have explored the coordination of amido ligand
reduction potentials for the trivalent ions Ln3+ + e– = Ln2+
are Eu3+/Eu2+ (–0.35 V), Yb3+/Yb2+ (–1.15 V), and Sm3+
/
Sm2+ (–1.55 V), similar to those of one-electron reducing
agents in organic and inorganic synthesis.[3] In contrast, the
reduction potential for potassium is much more negative
[K/K+ (–2.92 V)].[4] Therefore, with a properly useful li-
gand, we should be able to stabilize these ions with oxi-
dation state +2 in solution by alkali metal reduction of their
trivalent precursors. However, the synthesis of a wide
variety of low-valent lanthanide complexes with N,NЈ-
ligands remains sparsely studied.[5]
H2(LPh [20] [LPh = Ph2Si(NAr)2, Ar = 2,6-iPr2C6H3] (1) to
)
stabilize the ytterbium(II) complex, [{(LPh)YbII (thf)-
(Et2O)}2(μ-KI)] (3), by the reaction of [(LPh)YbIIII(thf)2] (2)
The accessibility and applications of divalent lanthanide
complexes can potentially be significantly broadened if they
can be readily prepared.[6–9] In addition to the success in
their synthesis, it is still important to explore alternative
ligands that are able to satisfy the coordination require-
ments of relatively large lanthanide cations.[10,11] In contrast
to
the popular cyclopentadienyl (Cp) ligands,
relatively little effort has been devoted to Cp-free ligands.[12]
The lanthanide chemistry of complexes with simple amido
ligands is limited primarily to an amido ligand, namely
[a] School of Materials Science and Engineering, Anhui University
of Science and Technology,
Huainan 232001, Anhui, P. R. China
E-mail: chengling_pan@126.com
[b] State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University,
Changchun 130012, P. R. China
E-mail: mrfy@jlu.edu.cn
Scheme 1.
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C.-L. Pan, S.-D. Sheng, C.-M. Hou, Y.-S. Pan, J. Wang, Y. Fan
SHORT COMMUNICATION
and an excess of potassium, as illustrated in Scheme 1. bly influence the structure of the usually formed mononu-
Herein, we report the synthesis and structural studies of clear or X-bridged dinuclear complexes “[Cp2Ln-
these complexes. To the best of our knowledge, ytter- (μ-X)]2”.[22] However, there is a growing interest in synthetic
bium(II) dimer complexes of amido ligands assembled from applications of σ-bonded organolanthanide complexes of
cation–π interactions are rare in lanthanide chemistry.
composition R2LnX (Ln = Sm, Eu, Yb; X = Cl, Br, I).[23]
X-ray crystallographic results indicated that complex 3
contains two ytterbium(II) units bridged by a KI molecule,
as shown in Figure 2. The LPh ligands coordinate in a che-
lating fashion [Yb1–N1 2.322(6) Å, Yb1–N2 2.345(5) Å,
Yb2–N3 2.346(6) Å, and Yb2–N4 2.323(6) Å], these Yb–N
distances are apparently longer than those observed in re-
lated trivalent ytterbium species {e.g., five-coordinate
[Yb(NHC6H3iPr2-2,6)3(thf)2] 2.17(2)–2.20(2) Å[12] and
complex 2}. The Yb–O (Et2O or thf) bond lengths are
Yb1–O1 2.411(7) Å, Yb1–O2 2.375(7) Å, Yb2–O3
2.373(8) Å, and Yb2–O4 2.450(9) Å. The KI is just at the
center of two ytterbium(II) groups with bond lengths Yb1–
I1 3.235(1) Å, Yb2–I1 3.156(1) Å, and K1–I1 3.329(2) Å,
and K···C bond lengths range from 2.957(7) to 3.484(10) Å.
To the best of our knowledge, there are no previously re-
ported examples of complexes containing a ytterbium(II)
dimer assembled from cation–π interactions.
Results and Discussion
The use of a dry-box and access to single crystals for X-
ray diffraction techniques made it possible to safely handle
and characterize these complexes. The crystal structure of
2 shows that the five-coordinate ytterbium center is coordi-
nated by a chelating diamido ligand, two thf molecules, and
an iodine atom, forming a distorted square-pyramidal ge-
ometry. The N1, N2, O1, and O2 atoms form the basal
plane, whilst the I1 occupies the apical position, as shown
in Figure 1. The Yb1–N1, Yb1–N2, Yb1–O1, and Yb1–O2
bond lengths are 2.165(2), 2.179(2), 2.319(2), and
2.339(2) Å, respectively. The Yb1–I1 bond length is
2.9151 Å. However, the YbII–N bond lengths of similar
complex [(LPh)YbII(thf)3] in our previous work fall within
the range 2.346(4)–2.394(4) Å.[18] The observed YbIII–N
bond lengths of 2 are obviously shorter than the corre-
sponding bond lengths of the ytterbium(II) complex, prob-
ably because of the smaller ionic radius of YbIII as com-
pared to YbII. As a result of the paramagnetic nature of 2,
well-resolved NMR spectra could not be obtained for the
trivalent ytterbium(III) complex.
Figure 1. Molecular structure of 2. POV-RAY illustration, 30%
thermal ellipsoids, all hydrogen atoms omitted for clarity. Selected
bond lengths [Å] and angles [°]: Yb1–N1 2.165(2), Yb1–N2
2.179(2), Yb1–O1 2.319(2), Yb1–O2 2.339(2), Yb1–I1 2.915(1),
Si1–N1 1.722(2), Si1–N2 1.738(2); N1–Yb1–N2 72.65(8).
Figure 2. Molecular structure of 3, hydrogen atoms, Et2O, and thf
molecules, omitted for clarity. Selected bond lengths [Å] and angles
[°]: Yb1–N1 2.322(6), Yb1–N2 2.345(5), Yb2–N3 2.346(6), Yb2–
N4 2.323(6), Yb1–O1 2.411(7), Yb1–O2 2.375(7), Yb2–O3
2.373(8), Yb2–O4 2.450(9), Yb1–Si1 2.997(2), Yb2–Si2 2.972(2),
Yb1–I1 3.235(1), Yb2–I1 3.156(1), I1–K1 3.329(2); N1–Yb1–N2
68.36(19); N4–Yb2–N3 69.2(2).
Bis(cyclopentadienyl) complexes of type “Cp2LnX”, sim-
ilar to complex 2, have attracted increased attention, and
derivatives of the latter are meanwhile established homo-
The ligand [Ph2Si(2,6-iPr2C6H3N)2]2– (LPh) seems an ex-
geneous catalysts.[21] Heteroligands X (e.g., H–, X–) and the cellent candidate for the stabilization of divalent lanthanide
presence of neutral donor ligands (e.g., thf, OEt2) remarka- complexes by means of cation–π interactions.[18,19] Most
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Eur. J. Inorg. Chem. 2012, 779–782

Divalent Ytterbium Species Assembled from Cation–π Interactions
Cp-free divalent ytterbium complexes are mononuclear.
Gambarotta et al. discovered an octameric macrocyclic
structure consisting of eight [Ph2C(C4H3N)2YbII] units with
σ and π bonds and a flat tetrametallic unit with mixed-
valent ytterbium.[24] A few years ago, a binuclear ytter-
bium(II) hydrido complex of a bulky hydrotris(pyrazolyl)-
borate ligand was reported.[25] Monoamide and guanidinate
Experimental Section
General Procedures: All operations were performed under an inert
atmosphere (Ͻ5 ppm oxygen or water) in a nitrogen-filled dry-box
or by using standard Schlenk techniques. Solvents thf and n-hexane
were freshly distilled from sodium benzophenone ketyl prior to use.
All other chemicals were purchased from either Aldrich or Acros
Chemical Co. and were used as received unless otherwise indicated.
ligands with ytterbium(II) atoms were bridged by halogen 1H and 13C NMR spectra were recorded with a Bruker DPX300
atoms to form bimetallic complexes.[17,26] Therefore, the use
spectrometer at 300.13 and 75.47 MHz, respectively.
of divalent lanthanide ions for the construction of oligo-
mers is more difficult than the use of d-block metal ana-
logues.[22,27] In complex 3, the interaction between the alkali
metal cation and the π-face of neutral aromatic system is
an important binding force, which helps to stabilize the di-
valent ytterbium metal center. The construction of
lanthanide–alkali chains,[28] layers,[29] and wheels[30] with π
interactions has been a field of rapid growth, because of
the formation of fascinating structures and their potential
applications.[31]
Syntheses
[(LPh)YbIIII(thf)2] (2): A solution of [Ph2Si(2,6-iPr2C6H3NH)2][20]
(0.77 g, 1.5 mmol) in thf (15 mL) was slowly added to a stirred
suspension of KH (0.16 g, 4 mmol), and the resulting mixture was
stirred for 2 h at room temperature. The yellow solution was then
filtered to remove the excess of KH, and then a yellow solution of
the potassium amido compound (1.5 mmol) in thf (15 mL) was
treated with YbI3 (0.83 g, 1.5 mmol) at room temperature for 4 h.
The solvent was removed under reduced pressure, followed by ad-
dition of Et2O (40 mL). The Et2O extract of the solid was filtered,
reduced to approximately 5 mL, and kept for several days at room
temperature, after which single crystals of 2 suitable for X-ray crys-
tallographic studies were obtained to give the title compound 2 as
red crystals (69%). C44H60IN2O2SiYb (977.00): calcd. C 54.09, H
6.19, N 2.87; found C 53.78, H 5.94, N 2.49.
Conclusions
We have successfully synthesized the diamido complex of
YbII by the potassium reduction of a new YbIII starting
material. This method proved to be efficient in the synthesis
of some divalent ytterbium complexes in lanthanide chemis-
try. Our studies revealed that diamido ligand can be used
as a ditopic linker to generate polymetallic complexes by
taking advantage of the cation–π interaction. The structural
versatility and the novelty of these ytterbium complexes are
a starting point for further investigations. The method
might be extended to the synthesis of other lanthanide ele-
ments. The work is in progress in our laboratory.
[{(LPh)YbII(thf)(Et2O)}2(μ-KI)] (3). A thf solution of complex 2
[(LPh)YbI(thf)2] (1.01 g, 1.0 mmol) was stirred with an excess of K
(0.06 g, 1.5 mmol) for 6 h at room temperature. The solvent was
removed under reduced pressure, followed by addition of Et2O
(40 mL). The Et2O extract of the solid was filtered, reduced to ap-
proximately 3 mL, and kept for several days at room temperature,
after which single crystals of 3 suitable for X-ray crystallographic
studies were obtained to give the title compound 3 as deep red
crystals (39%). 1H NMR (C6D6, 300 MHz): δ = 7.82 (d, J = 4.8 Hz,
4 H, ArH), 7.24–7.05 (m, 10 H, ArH), 6.99 (t, J = 7.2 Hz, 2 H,
ArH), 3.26–3.54 (m, 8 H, thf), 4.33 (sept., J = 6.9 Hz, 4 H,
Table 1. Crystal data and structure refinements for complexes 2 and 3.
Empirical formula
C44H60IN2O2SiYb
C88H124IKN4O4Si2Yb2
Formula weight
Temperature [K]
Crystal system
976.97
173(2)
orthorhombic
Pbca
1870.17
173(2)
orthorhombic
Pbca
Space group
a [Å]
b [Å]
19.9934(6)
18.4582(5)
23.4780(7)
8664.4(4)
8
1.498
2.935
23.6218(9)
21.3970(7)
34.5017(12)
17438.4(11)
8
1.425
2.610
c [Å]
Volume [Å3]
Z
Density (calculated) [Mgm–3]
Absorption coefficient [mm–1]
F(000)
3928
7616
Crystal size [mm]
θ range for data collection
Reflection collected
Independent reflections
Completeness to θ
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
Largest diff. peak and hole [ eÅ–3]
0.30ϫ0.20ϫ0.20
1.73 to 26.07°
46356
8572 [R(int) = 0.0304]
θ = 26.07°, 100.0%
0.5914 and 0.4730
Full-matrix least-squares on F2
8572/0/460
0.28ϫ0.23ϫ0.19
1.18 to 26.04°
94252
17202 [R(int) = 0.0619]
θ = 26.04°, 100.0%
0.6369 and 0.5285
Full-matrix least-squares on F2
17202/47/827
1.044
1.037
R1 = 0.0235, wR2 = 0.0565
R1 = 0.0330, wR2 = 0.0616
0.733 and –0.496
R1 = 0.0582, wR2 = 0.1341
R1 = 0.0995, wR2 = 0.1869
2.360 and –2.307
Eur. J. Inorg. Chem. 2012, 779–782
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C.-L. Pan, S.-D. Sheng, C.-M. Hou, Y.-S. Pan, J. Wang, Y. Fan
SHORT COMMUNICATION
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CHMe2), 1.25–1.44 (br., 8 H, thf), 1.11 (d, J = 6.9 Hz, 24 H,
CHMe2) ppm. 13C NMR (C6D6, 75.47 MHz): δ = 161.3, 158.2,
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Acknowledgments
We gratefully acknowledge financial support from Anhui Univer-
sity of Science and Technology, and we thank Prof. Hongjie Zhang
for kind suggestions. We thank for financial support from the State
Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University (No. 2012–27).
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Received: October 28, 2011
Published Online: January 11, 2012
782
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Eur. J. Inorg. Chem. 2012, 779–782