Synthesis, characterisation of carbon-bridged (diphenolato)lanthanide complexes and their catalytic activity for Diels-Alder reactions

Article abstract of DOI:10.1002/ejic.200400519

The synthesis and structures of new lanthanide complexes supported by the carbon-bridged diphenolato ligand 2,2′-methylenebis(6-tert-butyl-4- methylphenolato) (MBMP^2-) are described. Reactions of anhydrous lanthanide trichlorides with Na₂MBMP in a 1:2 molar ratio in THF at room temperature afforded the corresponding "ate" (diphenolato) lanthanide complexes [(THF)_nLn(MBMP)₂Na(THF)₂] [Ln = Nd (1), Sm (2), n = 2; Ln = Yb, n = 1 (3)]. Recrystallisation of complexes 1-3 from toluene in the presence of DME gave the discrete ion-pair complexes [(MBMP)₂Ln(THF)₂][Na(DME) ₂(THF)₂] [Ln = Nd (4), Sm (5), Yb (6)]. These complexes have been fully characterised. The single-crystal structural analyses of 1, 3 and 6 revealed that the coordination geometries of the lanthanide ions can be best described as distorted octahedral in complexes 1 and 6 and distorted trigonal-bipyramidal in complex 3. It was found that these lanthanide complexes are able to act as Lewis acids to catalyse the Diels-Alder reactions of cyclopentadiene with substituted dienophiles with good activity and stereoselectivity. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200400519

FULL PAPER
Synthesis, Characterisation of Carbon-Bridged (Diphenolato)lanthanide
Complexes and Their Catalytic Activity for Diels–Alder Reactions
[a]
[a]
[a]
[a]
[a]
Xiaoping Xu, Mengtao Ma, Yingming Yao,* Yong Zhang, and Qi Shen*
Keywords: Lanthanides / O ligands / Structure elucidation / Catalysis / Diels–Alder reaction / Cyclopentadiene
The synthesis and structures of new lanthanide complexes
been fully characterised. The single-crystal structural analy-
ses of 1, 3 and 6 revealed that the coordination geometries
of the lanthanide ions can be best described as distorted oc-
tahedral in complexes 1 and 6 and distorted trigonal-bipy-
ramidal in complex 3. It was found that these lanthanide
complexes are able to act as Lewis acids to catalyse the Di-
els–Alder reactions of cyclopentadiene with substituted dien-
supported by the carbon-bridged diphenolato ligand 2,2Ј-
2–
methylenebis(6-tert-butyl-4-methylphenolato) (MBMP ) are
described. Reactions of anhydrous lanthanide trichlorides
with Na
ture afforded the corresponding “ate” (diphenolato)lantha-
nide complexes [(THF) Ln(MBMP) Na(THF) ] [Ln = Nd (1),
2
MBMP in a 1:2 molar ratio in THF at room tempera-
n
2
2
Sm (2), n = 2; Ln = Yb, n = 1 (3)]. Recrystallisation of com- ophiles with good activity and stereoselectivity.
plexes 1–3 from toluene in the presence of DME gave the
discrete ion-pair complexes [(MBMP)
2
Ln(THF)
2
][Na(DME)
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
2
(THF) ] [Ln = Nd (4), Sm (5), Yb (6)]. These complexes have
2
Introduction
of these carbon-bridged diphenolato ligands in transition
and main-group metal complexes has attracted considerable
attention in recent years and some of these complexes have
shown interesting catalytic activity. For example, the cor-
responding titanium complexes are able to catalyse not only
Of the possible alternatives to the traditional ancillary
bis(cyclopentadienyl) ligand set in lanthanide chemistry,
alkoxides (aryloxides) have received much attention and be-
come increasingly popular since they are easily available,
tuneable and even potentially recyclable ancillary ligand
[7]
the polymerisation of α-olefins in the presence of
[7a,7c,7d]
MAO
but also the living anionic polymerisation of
[
1]
sets for mediating the reactivity of electropositive cations.
The synthesis and structural characterisation of a variety
[7b]
ε-caprolactone. The aluminium complexes can effectively
catalyse the controllable polymerisation of propylene ox-
[
2]
[3]
of di- and trivalent lanthanide complexes supported by
monodentate aryloxides have been published and some of
them have shown good catalytic activities for the polymeris-
ation of polar and nonpolar monomers.[ To date, the che-
late diphenolates have been seldom used as ancillary ligands
in lanthanide chemistry.[
[7f]
[7g,7h,7k]
ide and ε-caprolactone
as well as MPV hydrogen
[7i]
transfer reactions. In view of this, it can be anticipated
that organolanthanide complexes supported by carbon-
bridged diphenolato ligands could lead to the activation of
a range of small molecules and act as new types of Lewis
acid catalysts for organic synthesis. However, these kinds
of ligands have not been used in lanthanide coordination
chemistry except for our recent report of the synthesis of
4]
5]
Chelate ligands have played a remarkable role in the de-
velopment of coordination chemistry. They may stabilise
complexes either by thermodynamic or kinetic means and
they may serve in the exploration of reaction mechanisms.
Carbon-bridged diphenols such as 2,2Ј-methylenebis(6-tert-
butyl-4-methylphenol) and 2,2Ј-ethylid-
[5i]
divalent lanthanide complexes. Here we report the syn-
[
6]
thesis and characterisation of trivalent lanthanide com-
2–
plexes supported by the MBMP group and their catalytic
(MBMPH2)
activity for Diels–Alder reactions.
enebis(4,6-di-tert-butylphenol) (EDBPH ) have been found
2
to be able to act as dianionic ligands which have the advan-
tage of avoiding ligand redistribution reactions and provid-
ing a stereochemically rigid framework for the metal centre
which could affect stereospecific transformations. The use
Results and Discussion
Synthesis and Characterisation of Carbon-Bridged
(
Diphenolato)lanthanide Complexes
[
a]
Key Laboratory of Organic Synthesis of Jiangsu Province, De-
partment of Chemistry and Chemical Engineering, Suzhou
University,
Since the organolanthanide or organolanthanoid chlo-
rides are important precursors for lanthanide derivatives,
we tried to synthesise the mono(diphenolato)neodymium
complex [(MBMP)NdCl(THF) ] by the general salt metath-
Suzhou 215006, P. R. China
Fax: +86-512-65112371
E-mail:yaoym@suda.edu.cn
x
676
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400519
Eur. J. Inorg. Chem. 2005, 676–684

Carbon-Bridged (Diphenolato)lanthanide Complexes
FULL PAPER
esis reaction of anhydrous NdCl with MBMPNa in a 1:1 the bidentate DME has a relatively stronger coordinating
3
2
molar ratio in THF at ambient temperature but this attempt ability than the bridging phenolate.[
3e]
was unsuccessful. The resultant complex, isolated in moder- Complexes 1–3 are moderately air- and moisture-sensi-
ate yield, was characterised as a ligand-redistributed pro- tive. The crystals can be exposed to air for a few hours
duct [(THF) Nd(MBMP) Na(THF) ] (1). The reaction was without apparent decomposition but the colours of the
2
2
2
repeated with the stoichiometry appropriate for complex 1 solutions changed gradually in a few minutes. However,
and a yield of Ͼ78% was obtained. Thus, the isostructural complexes 4–6 are air- and moisture-sensitive and the crys-
complexes [(THF) Ln(MBMP) Na(THF) ] [Ln = Sm (2), n tals decompose in a few minutes when exposed to air. These
n
2
2
=
2; Yb (3), n = 1] were synthesised by the reaction of complexes are freely soluble in donor solvents such as THF
LnCl with MBMPNa in 1:2 molar ratio in THF as shown and DME, slightly soluble in toluene and benzene but in-
3
2
in Scheme 1.
soluble in hexane. The neodymium complexes (1 and 4) did
not provide any resolvable H NMR spectra due to the
1
strong paramagnetism of the neodymium ion.
Crystal Structure Analyses
Although there are many structurally characterised (di-
naphtholato)lanthanide complexes in the literature,[
there are only a few examples of structurally characterised
lanthanide complexes supported by diphenolato ligands.
The diphenolato ligand [1,1Ј-(2-OC H -tBu -3,5)] , in
5c,10]
Scheme 1.
6
2
2
2
These complexes gave satisfactory elemental analyses which two phenols are linked directly, was first used as an
[
5a]
and the lanthanide and sodium analyses indicated a molar ancillary ligand in organolanthanide chemistry. Recently,
ratio of 1:1 for Ln and Na in these complexes. The defin- other (diphenolato)lanthanide complexes containing donor-
itive structures of complexes 1 and 3 were determined by functionalised linkers have been reported.[5c–5h] The crystal
X-ray diffraction studies which revealed them to be “ate” structures of trivalent lanthanide complexes supported by
complexes in which a sodium atom is connected to one oxy- carbon-bridged diphenolate have not been reported until
[
5i]
gen atom from each of the diphenolato ligands.
now. To elucidate the influence of the carbon-bridged di-
It is interesting that recrystallisation of complexes 1–3 phenolates on the lanthanide coordination spheres, the X-
from toluene in the presence of DME gave the discrete ion- ray crystal structures of complexes 1, 3 and 6 were deter-
pair complexes [(MBMP) Ln(THF) ][Na(DME) (THF) ] mined.
2
2
2
2
[
Ln = Nd (4), Sm (5), Yb (6)] which were confirmed by
Crystals of complex 1 suitable for an X-ray diffraction
1
elemental analysis and H NMR spectroscopy as well as by study were grown by cooling a concentrated toluene solu-
a structural determination in the case of 6.[ In the anionic tion to –10 °C. An ORTEP diagram with the atom-number-
8]
(
aryloxo)lanthanide complexes, most of the anions are con- ing scheme of 1 is shown in Figure 1, selected bond lengths
nected to the cations by bridging ligands and the ion-pair and angles are given in Table 1. Complex 1 has a C -sym-
complexes are rare. Previously reported ion-pair (aryloxo) metric dinuclear structure. The overall molecular geometry
2
lanthanide complexes are limited to [Na(DME) ][Ln- consists of a six-coordinate neodymium metal centre which
3
(
OC H -Ph -2,6) ],
[Na(diglyme) ][Ln(OC H -Ph -2,6)
is coordinated by the four oxygen atoms of the two
MBMP groups and two oxygen atoms from the THF
6
3
2
4
2
6
3
2
4
3e]
[
2–
[
[
diglyme = bis(2-methoxyethyl) ether, Ln = Nd, Er],
Nd(OC H -tBu -2,6-Me-4) ][Na(THF) ]
[3d]
and [(C Me )- molecules in a distorted octahedron as well as a four-coor-
6
3
2
4
6
5
5
[9]
Y(OC H -Me -2,6) ][Na(THF) ]. Due to the competition dinate sodium cation which is coordinated by two oxygen
6
3
2
3
6
between coordinating solvents and oxygen atoms in the atoms from the MBMP2– groups and two THF ligands in
2
–
MBMP groups for complexation of the sodium atom, ad- a distorted tetrahedron. The coordination geometry around
dition of DME broke the bridges between the sodium atom the neodymium atom is similar to that of the yttrium
and the diphenolato ligands and formed a sodium cation atom in [(2,6-Me -C H O) Y(THF) (µ-OC H -Me -2,6) -
coordinated by two DME groups and two THF groups as (THF)3]. The terminal phenolate [O(2), O(2_2)], one
shown in Scheme 2. This can be attributed to the fact that bridging phenolate [O(1)] and one THF molecule [O(3)] can
be viewed as occupying equatorial positions within the
2
6
3
2
2
6
3
2
2
[
9]
octahedron about the neodymium centre with Σ(O–Nd–O)
=
360.53°. The oxygen atom from another bridging phenol-
ate ligand [O(1_2)] and one oxygen atom from the THF
O(3_2)] occupy axial positions and the O(1_2)–Nd–O(3–2)
angle is slightly distorted away from the idealised 180° to
[
172.466(4)°.
The terminal Nd–O(Ar) bond lengths are 2.278(2) Å
Scheme 2.
which compare well with the previously reported terminal
Eur. J. Inorg. Chem. 2005, 676–684
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
677

X. Xu, M. Ma, Y. Yao, Y. Zhang, Q. Shen
FULL PAPER
Figure 1. ORTEP diagram of 1 showing atom-numbering scheme. Thermal ellipsoids are drawn at the 10% probability level and hydrogen
atoms are omitted for clarity.
[
5a]
Table 1. Selected bond lengths [Å] and angles [°] for complex 1.
(SiMe ) }(THF) ] [88.1(3)°]
reflecting the fact that ad-
3
2
3
dition of a methylene linker between two phenolates in-
creases the flexibility of the diphenolato ligand. Nd(1),
Na(1), O(1) and O(1_2) are exactly coplanar (plane 1) as
required by the crystallographic symmetry. The dihedral
angles between plane 1 and plane 2 [C(1) to C(6)], and be-
tween plane 1 and plane 3 [C(7) to C(12)] are 90.64(8) and
Nd(1)–O(1)
Nd(1)–O(3)
C(1)–O(1)
2.315(2)
2.559(3)
1.352(4)
2.305(2)
Nd(1)–O(2)
Nd(1)–Na(1)
C(7)–O(2)
2.278(2)
3.543(2)
1.339(4)
2.287(3)
Na(1)–O(1)
Na(1)–O(4)
O(1)–Nd(1)–O(2)
O(3)–Nd(1)–O(2_2)
O(1_2)–Nd(1)–O(3_2) 172.466(4) Nd(1)–O(1)–C(1)
Nd(1)–O(2)–C(7) 158.5(2)
95.51(8)
77.92(6)
O(2)–Nd(1)–O(3)
O(1)–Nd(1)–O(2_2) 97.61(6)
127.3(2)
90.09(8)
81.79(9)°, respectively, indicating that the orientations of
plane 1 and the arene rings of the diphenolato ligands are
approximately perpendicular. The dihedral angle between
two arene rings of the MBMP2– group is 68.9(1)°.
The sodium cation is coordinated to two THF oxygen
atoms and an oxygen atom from each of the two MBMP2–
groups. This coordination geometry is different from that
Nd–O(Ar) distances when the difference in coordination
number is considered.[
3d]
The bridging Nd–O(Ar) bond
length is only 0.04 Å longer than the terminal Nd–O(Ar)
bond length which is comparable with that in
in
[(THF)La(OC H -iPr -2,6) (µ-OC H -iPr -2,6) Na-
6 3 2 2 6 3 2 2
[3e]
(THF) ], in which one carbon atom from one of the bridg-
[
Na{Nd(OC H -Ph -2,6) }] but apparently shorter than
2
6
3
2
4
ing aryloxide ligands is directed to the fifth coordination
that in [(THF)La(OC H -iPr -2,6) (µ-OC H -iPr -2,6) Na-
6
3
2
2
6
3
2
2
[
3]
[
3]
site of the sodium cation, as well as in [Na{Nd(OC H -
(THF)2].
The O(2)–Nd–O(2_2) bond angle between
6 3
Ph -2,6) }], in which the sodium atom is surrounded by
the two terminal phenolato ligands is 162.90(6)° whereas
the O(1)–Nd–O(1_2) angle between the two bridging phe-
nolato ligands is dramatically smaller at 79.62(5)°. The
Nd(1)–O(1)–C(1) bond angle for the bridging phenolato li-
gand is rather acute at 127.3(2)° reflecting its interaction
with the sodium cation while the Nd(1)–O(2)–C(7) bond
angle for the terminal phenolato ligand is more obtuse
2
4
three bridging aryloxide oxygen atoms and three phenyl
[
3e]
groups.
The Na–O(Ar) bond length of 2.305(2) Å lies
within the range previously reported for Na–O(Ar) bond
[
3]
lengths. The O(1)–Na–O(1_2) bond angle between the
bridging phenolato ligands is 80.02(7)° while the O(4)–Na–
O(4_2) bond angle between two THF molecules is
88.40(9)°.
[
158.5(2)°] and is comparable with the corresponding bond
[3i]
Crystals of complex 3 suitable for an X-ray diffraction
angles in K[Nd(OC H -iPr -2,6) ]. The bite angle O(1)–
6
3
2
4
study were obtained from a concentrated toluene solution
at room temperature. The molecular structure of complex 3
is shown in Figure 2 with selected bond lengths and bond
angles listed in Table 2. The difference in the molecular
Nd–O(2) of 95.51(8)° for the diphenolato ligand is appar-
ently larger than those found in [{Ln[P1,1Ј-(2-OC H-tBu-
6
3
8
-Me -5,6) ][N(SiHMe ) ](THF)} ] [Ln = Y 88.79(6)°; La
2
2
5b]
2 2
2
[
8.83(7)°]
and [La{1,1Ј-(2-OC H -tBu -3,5) }{CH-
6 2 2 2
678
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 676–684

Carbon-Bridged (Diphenolato)lanthanide Complexes
FULL PAPER
Figure 2. ORTEP diagram of 3 showing atom-numbering scheme. Thermal ellipsoids are drawn at the 10% probability level and hydrogen
atoms are omitted for clarity.
Table 2. Selected bond lengths [Å] and angles [°] for complexes 3 the bridging ligands are, as expected, somewhat longer at
and 6.
2.147(3) Å (av.). The Na–O(Ar) and Na–O(THF) bond
lengths are comparable with those found in complex 1. As
previously observed in the structure of complex 1, the O–
Yb–O angle between the bridging phenolato ligands [O(1)–
Yb(1)–O(3) = 82.2(1)°] is apparently smaller than that be-
tween the two terminal phenolato ligands [O(2)–Yb(1)–
O(4) = 112.6(1)°]. The average Yb–O–C bond angle for the
bridging phenolato ligands is 135.1(3)° while the Yb–O–C
angles for the terminal phenolato ligands average 150.6(2)°
which is comparable with the corresponding bond angles in
complex 1. The dihedral angles between two arene rings in
the MBMP2– groups are quite different, i.e. 57.4(1)° for the
ligand containing O(1) and O(2) but 94.8(2)° for that con-
taining O(3) and O(4).
3
6a
6b
Yb(1)–O(1)
Yb(1)–O(2)
Yb(1)–O(3)
Yb(1)–O(4)
Yb(1)–O(5)
Yb(1)–O(6)
C(1)–O(1)
C(7)–O(2)
C(24)–O(3)
C(30)–O(4)
2.160(3)
2.059(3)
2.134(3)
2.088(3)
2.333(3)
–
1.350(5)
1.340(5)
1.349(5)
1.351(5)
2.157(5)
2.130(5)
2.156(5)
2.119(5)
2.419(5)
2.427(5)
1.330(8)
1.317(8)
1.331(9)
1.331(8)
2.148(5)
2.107(4)
2.161(5)
2.118(4)
2.421(5)
2.399(5)
1.328(8)
1.325(8)
1.333(8)
1.324(8)
Yb(1)–O(1)–C(1)
Yb(1)–O(2)–C(7)
Yb(1)–O(3)–C(24)
Yb(1)–O(4)–C(30)
135.8(3)
151.6(2)
134.4(3)
149.6(3)
152.1(5)
154.2(5)
147.1(5)
156.7(5)
149.9(5)
156.0(5)
151.7(4)
155.7(5)
Crystals of complex 6 suitable for an X-ray diffraction
study were obtained from a toluene/DME solution at –5 °C.
This complex is composed of a discrete six-coordinate
–
+
structures of complexes 3 and 1 is that only one THF mole- [(MBMP) Yb(THF) ] anion and an [Na(DME) (THF) ]
2 2 2 2
cule is coordinated to the central metal atom in the former. cation. Complex 6 crystallises with two crystallographically
Thus, the ytterbium centre is five-coordinate by four oxygen independent but chemically similar molecules (6a and 6b)
2
–
atoms from the two MBMP groups and one oxygen atom in the unit cell. The selected bond lengths and angles are
from a THF molecule in a somewhat distorted trigonal- provided in Table 2 for both molecules. The structure of the
bipyramidal geometry. The oxygen atoms O(5) and O(3) oc- anion of complex 6a with the atom-numbering scheme is
cupy axial positions and the oxygen atoms O(2), O(4) and shown in Figure 3.
O(1) can be considered to occupy equatorial positions. The
In the cation, the sodium atom is coordinated to four
overall molecular structure is similar to that of the pre- oxygen atoms from two DME molecules and two oxygen
viously reported [(THF)La(OC H -iPr -2,6) (µ-OC H - atoms from two THF molecules to form a slightly distorted
6
3
2
2
6
3
[3]
iPr -2,6) Na(THF) ]. The coordination geometry of the octahedral geometry. In the anion, the coordination geome-
2
2
2
sodium cation is identical to that observed in complex 1.
try around the ytterbium atom is different from that in
The terminal Yb–O(Ar) bond lengths are 2.059(3) and complex 3. The ytterbium atom is located in the centre of
2
.088(3) Å, respectively, giving an average of 2.073(8) Å an octahedron comprised of two MBMP2– groups and two
which is comparable with those previously reported ter- THF molecules in which the oxygen atoms from the
minal Yb–O(Ar) bond lengths.[3b] Yb–O(Ar) distances to MBMP groups [O(1), O(3), O(4)] and one THF molecule
2–
Eur. J. Inorg. Chem. 2005, 676–684
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
679

X. Xu, M. Ma, Y. Yao, Y. Zhang, Q. Shen
FULL PAPER
Figure 3. ORTEP diagram of the anion of 6a showing atom-numbering scheme. Thermal ellipsoids are drawn at the 10% probability
level and hydrogen atoms are omitted for clarity.
[
O(6)] can be considered as occupying equatorial positions sterically less demanding than the two monodentate arylox-
–
–
with Σ(O–Yb–O) = 359.0°. Two oxygen atoms from one ide groups [OC H -tBu -2,6-Me-4] , [OC H -Ph -2,6] and
MBMP group [O(2)] and the THF molecule [O(5)] occupy [OC H -iPr -2,6] but is comparable with two less bulky ar-
axial positions.
6
3
2
6
3
2
2
–
–
6
3
2
–
yloxide groups [OC H -Me -2,6] . It is because of the steri-
6 3 2
The Yb–O(Ar) bond lengths range from 2.119(5) to cally less demanding nature of MBMP2– that the desired
.157(5) Å, giving an average of 2.140(6) Å which is com- (diphenolato)lanthanide chloride [(MBMP)LnCl(THF)x]
2
parable with the terminal Yb–O(Ar) bond lengths in com- could not be achieved by the general salt metathesis reac-
plex 3 when the difference in coordination number is con- tion.
sidered but it is slightly longer than that in [Na(THF)
[9]
6]
[(C Me )Y(OC H -Me -2,6) ] (2.094 Å). The bite angles
5 5 6 3 2 3
Diels–Alder Reactions Catalysed by the (Diphenolato)
lanthanide Complexes
of O–Yb–O in complex 6a are 90.45(19)° and 95.72(19)°,
respectively, which are comparable with that in complex 1.
The average Yb–O–C bond angle is 152.5(5)° which is sim-
ilar to the corresponding bond angles in complexes 1 and
In recent years, as a result of the importance of Diels–
Alder reactions in the synthesis of natural products and
physiologically active molecules, increased interest has been
directed towards the development of efficient methods for
3
. The dihedral angles between the arene rings in the two
2
–
MBMP groups are 53.9(3)° and 56.5(2)°, respectively.
A comparison of the structural geometry of the (diphe-
nolato)lanthanide complexes with those of the structurally
characterised monodentate aryloxide examples enables an
2
–
evaluation of the MBMP ligand set compared with two
–
ArO ligands. The yttrium centre is six-coordinate in [(2,6-
Me -C H O) Y(THF) (µ-OC H -Me -2,6) (THF) ]
and
whereas the
lanthanum centre is only five-coordinate in [(THF)La-
2
6
3
2
2
6
3
2
2
9]
3
[
[
Na(THF) ][(C Me )Y(OC H -Me -2,6) ]
6 5 5 6 3 2 3
[3]
(
OC H -iPr -2,6) (µ-OC H -iPr -2,6) Na(THF) ] and the
6 3 2 2 6 3 2 2 2
neodymium centre is four-coordinate in [Na(THF) ]-
6
[
3d]
[
(
Nd(OC H -tBu -2,6-Me-4) ],
[Na(diglyme)][Nd-
6
3
2
4
[3e]
OC H -Ph -2,6) ],
Na[Nd(OC H -Ph -2,6) ]
and
However, in these (diphe-
nolato)lanthanide complexes, the central metal atom is five-
or six-coordinate. Thus, the diphenolato ligand MBMP2– is Scheme 3.
6
3
2
4
6
3
2
4
[3i]
K[Nd(OC H -iPr -2,6) ].
6
3
2
4
680
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Eur. J. Inorg. Chem. 2005, 676–684

Carbon-Bridged (Diphenolato)lanthanide Complexes
Table 3. Diels–Alder reactions of cyclopentadiene with dienophiles catalysed by carbon-bridged (diphenolato)lanthanide complexes.
FULL PAPER
Run
Cat.
Dienophile
[Cat.]/[Dienophile]
t [h]
Yield [%]
endo/exo
1
2
3
4
5
6
7
8
9
–
1
1
1
2
2
3
3
3
–
1
1
–
1
1
–
1
1
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl acrylate
methyl methacrylate
methyl methacrylate
methyl methacrylate
acrylonitrile
–
24
4
8
24
4
8
67
59
78
93
54
69
14
35
94
25
22
58
36
31
54
46
35
65
2.7
4.6
4.3
4.6
4.3
4.3
4.0
3.9
4.0
0.45
2.2
2.2
1.2
1.6
1.6
–
1:10
1:10
1:10
1:10
1:10
1:10
1:10
1:10
–
1:10
1:10
–
1:10
1:10
–
2
4
24
10
4
10
10
4
8
10
4
10
11
12
13
14
15
16
17
18
acrylonitrile
acrylonitrile
N-phenyl maleimide
N-phenyl maleimide
N-phenyl maleimide
1:10
1:10
35
35
8
the purpose of improving the reaction rates and/or stereo- inorganic reagents can increase the yield of the endo stereo-
selectivities of cycloaddition reactions.[ As Lewis-acidic isomer.
11]
[15b,15c]
Recently, there has been one example re-
catalysts, lanthanide complexes are well established and ported in which this exo-selective reaction can be converted
have been found to be effective catalysts in numerous Diels– to an endo-selective process in chloroaluminate ionic li-
[
12]
[15d]
Alder reactions. We found that these carbon-bridged (di- quids.
Complex 1 showed poor activity and stereoselec-
phenonato)lanthanide complexes can also catalyse the tivity for the reaction of cyclopentadiene with acrylonitrile,
Diels–Alder reactions of cyclopentadiene with methyl acry- although the result was similar to those reported in the lit-
[
16]
late, methyl methacrylate, acrylonitrile and N-phenylmale- erature. For the symmetrically substituted dienophile N-
imide with relatively good activities and stereoselectivities phenylmaleimide, excellent stereoselectivity was obtained.
as shown in Scheme 3 with preliminary results listed in The reaction with N-phenylmaleimide using complex 1 as
Table 3.
catalyst gave the endo stereoisomer as the major product
As shown in Table 3, these (diphenolato)lanthanide com- but the selectivity was lower than those of montmorillonite
plexes accelerate the Diels–Alder reactions of cyclopentadi- and alumina.[17]
ene with these dienophiles and the product yields increased
as a function of time whereas the stereoselectivities re-
mained essentially the same at all reaction times. Reactions
with substituted dienophiles produced a mixture of endo
Conclusions
In summary, we have successfully synthesised a series of
and exo isomers although the stereoselectivity (endo/exo ra-
new soluble carbon-bridged (diphenolato)lanthanide com-
tio) is apparently dependent on the dienophile. For reac-
plexes and characterised some of their structural features
tions with asymmetrically substituted dienophiles, the reac-
by X-ray crystallography. Moreover, it was found that coor-
tion with methyl acrylate afforded the best stereoselectivity.
dinated solvents dramatically affect the solid-state struc-
The endo/exo ratio of 4.6 can be achieved for this reaction
at 40 °C in the presence of 10 mol-% of complex 1 as the
catalyst (the endo/exo ratio is 2.7 for the blank reaction,
runs 1 and 2). In comparison with other lanthanide cata-
lysts, the activities and stereoselectivities exhibited by the
tures of the (diphenolato)lanthanide complexes. Crystallis-
ation from toluene in the presence of THF afforded the
ate” complexes while in presence of DME discrete ion-pair
complexes were formed. Preliminary results revealed that
these “ate” (diphenolato)lanthanide complexes have good
activities and stereoselectivities for the Diels–Alder reac-
tions of cyclopentadiene with some substituted dienophiles.
Further studies of these reactions are in progress in our
laboratory.
“
(diphenolato)lanthanide complexes for the reaction of cy-
clopentadiene with methyl acrylate are higher than that of
[
13a]
anhydrous lanthanide chlorides
and are comparable
with those of SmI2[
13b]
and Sc(OTf) if toluene is used as
3
solvent (using scCO as solvent, the endo/exo ratio is up to
2
13c]
1
0)[
but are lower than those of other Lewis acid cata-
lysts such as aluminium,
[14a–14c]
[14d]
copper
and rutheni-
um[
14e]
complexes. It is worthy to note that the reaction of Experimental Section
cyclopentadiene with methyl methacrylate catalysed by
complex 1 is endo-selective and gave products with an endo/
exo ratio of 2.2. The Diels–Alder reaction of cyclopentadi-
General Procedures: All manipulations were performed under ar-
gon using standard Schlenk techniques. THF, DME and toluene
were distilled from sodium benzophenone ketyl before use. Anhy-
ene with methyl methacrylate normally gave a higher drous lanthanide trichlorides were prepared according to the litera-
amount of the exo stereoisomer[
15a]
and some organic and ture procedures and Na
[18]
(MBMP) was prepared from the reac-
2
Eur. J. Inorg. Chem. 2005, 676–684
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 681

X. Xu, M. Ma, Y. Yao, Y. Zhang, Q. Shen
FULL PAPER
tion of MBMPH
2
with Na in THF [MBMPH
2
= 2,2Ј-methyl-
C
70
H112NaO12Nd (1312.88): calcd. C 64.04, H 8.60, Nd 10.99, Na
enebis(6-tert-butyl-4-methylphenol)]. Lanthanide analyses were
1.75; found C 63.74, H 8.39, Nd 10.72, Na 1.68.
performed by EDTA titration with xylenol orange indicator and
Synthesis of [(THF) Sm(MBMP) ][Na(DME) (THF) ] (5): To a
2
2
2
2
[
19]
hexamine buffer
and chloride analyses were carried out using
solution of complex 2 (2.84 g, 2.5 mmol) in toluene (50 mL) was
added DME (1 mL). After stirring the solution for about 30 min,
pale-yellow microcrystals were obtained upon concentration and
subsequent cooling of the solution to –5 °C for 2 d. Yield: 2.83 g,
the Volhard method. The sodium content was determined with a
Hitachi 180-80 polarised Zeeman atomic absorption spectropho-
tometer. Carbon and hydrogen analyses were performed by direct
combustion with a Carlo–Erba EA-1110 instrument. IR spectra
were recorded with a Nicolet-550 FTIR spectrometer as KBr pel-
lets. Uncorrected melting points of crystalline samples in sealed
capillaries (under argon) are reported as ranges. Methyl acrylate,
methyl methacrylate and acrylonitrile were washed with diluted
86%. M.p. 98–99 °C (dec.). IR (KBr): ν˜ = 2951 (s), 2921 (s), 2870
(
(
m), 1605 (w), 1466 (s), 1431 (s), 1385 (m), 1292 (s), 1084 (s), 860
–
1 1
m) cm . H NMR (C
6
D
6
): δ = 1.47 [36 H, C(CH
), 2.78–3.61 (br., 40 H, DME, THF and
), 7.08 (4 H, ppm.
112NaO12Sm (1319.01): calcd. C 63.76, H 8.56, Sm 11.41, Na
.74; found C 63.83, H 8.63, Sm 11.02, Na 1.62.
3 3
) ], 1.97 (16 H,
THF), 2.17 (12 H, ArCH
ArCH ), 6.93 (4 H,
C H
70
3
2
C
6
H
2
6 2
C H )
NaOH solution three times, dried with anhydrous NaSO
4
for 24 h
and distilled before use. N-Phenylmaleimide was prepared accord-
1
[
20]
ing to the literature.
Cyclopentadiene was freshly prepared by
thermally cracking dicyclopentadiene. Other reagents were com-
mercially available.
2
2
2
2
Synthesis of [(THF) Yb(MBMP) ][Na(DME) (THF) ] (6): To a
solution of complex 3 (3.21 g, 2.95 mmol) in toluene (50 mL) was
added DME (1 mL). After stirring the solution for about 30 min,
light-yellow microcrystals were obtained upon concentration and
subsequent cooling of the solution to –5 °C for 2 d. Yield: 2.28 g,
1%. M.p. 62–63 °C (dec.). IR (KBr): ν˜ = 2955 (s), 2920 (m), 2870
m), 1635 (w), 1466 (m), 1442 (s), 1234 (m), 1157 (m), 1049 (w),
Synthesis of [(THF)
suspension of NdCl
slowly added THF solution of Na
.50 mmol) at room temperature. After being stirred for 24 h, the
2
Nd(MBMP)
(1.63 g, 6.50 mmol) in THF (50 mL) was
(MBMP) (27.6 mL,
2 2
Na(THF) ] (1). Method A: To a
3
a
2
6
6
(
8
precipitate was separated from the reaction mixture using a centri-
fuge, the solvent was completely removed in vacuo and toluene was
added to extract the product. The dissolved portion was removed
with a centrifuge. Light-blue microcrystals were obtained from the
concentrated toluene solution. Yield: 2.41 g, 32%. M.p. 165–166 °C
–
1 1
64 (m) cm . H NMR (C
6
D
6
): δ = 1.45 [36 H, C(CH
), 3.12–3.85 (br., 40 H, THF DME
), 7.07 (4 H, ppm.
112NaO12Yb (1341.68): calcd. C 62.66, H 8.41, Yb 12.91, Na
.71; found C 63.03, H 8.81, Yb 12.98, Na 1.68.
3 3
) ], 1.87 (16
H, THF), 2.16 (12 H, ArCH
and ArCH ), 6.92 (4 H,
3
2
C
6
H
2
6 2
C H )
70
C H
1
(
dec.). IR (KBr): ν˜ = 2955 (s), 2871 (m), 1632 (m), 1474 (s), 1389
–1
(
m), 1055 (w), 861 (m) cm . C62
5.75, H 8.19, Nd 12.73, Na 2.03; found C 65.43, H 8.01, Nd 12.55,
Na 1.91. Method B: To a suspension of NdCl (1.28 g, 5.10 mmol)
in THF (40 mL) was slowly added a THF solution of Na (MBMP)
43.3 mL, 10.2 mmol) at room temperature. After the reaction solu-
H92NaO
8
Nd (1132.64): calcd. C
Diels–Alder Reactions Catalysed by the (Diphenolato)lanthanide
Complexes: In a typical run, methyl acrylate (0.25 mL, 2.7 mmol)
was added to a toluene solution of the catalyst (3.7 mL,
6
3
0.27 mmol). After stirring the solution at 40 °C for 5 min, freshly
cracked cyclopentadiene (0.30 mL, 3.3 mmol) was added with a sy-
ringe and stirring was continued for the desired time. Methanol
was added to the mixture to terminate the reaction and acetylben-
zene was immediately added as an internal standard for quantita-
tive analysis. The product yields and endo/exo ratios were deter-
mined using GC or NMR spectroscopy as described in the litera-
ture for different dienophiles.[
2
(
tion had been stirred for 24 h, complex 1 was isolated as described
above (4.53 g, 78%).
Synthesis of [(THF)
complex 2 was carried out as described above for complex 1
Method B) but anhydrous SmCl (1.3 g, 5.06 mmol) was used in-
stead of NdCl . Colourless microcrystals were obtained from tolu-
ene. Yield: 4.21 g, 73%. M.p. 144–145 °C (dec.). IR (KBr): ν˜ =
2 2 2
Sm(MBMP) Na(THF) ] (2): The synthesis of
(
3
13a,15c,16a,17]
3
X-ray Crystallography: Crystals of complexes 1, 3 and 6 suitable for
X-ray diffraction were sealed in thin-walled glass capillaries under
argon. Diffraction data were collected with a Rigaku Mercury
2
1
955 (s), 2920 (s), 2870 (m), 1604 (w), 1435 (s), 1385 (m), 1250 (s),
–1 1
087 (w), 860 (m) cm . H NMR (C
6
D
6
): δ = 1.47 [36 H, C(CH
), 3.66 (16 H, THF), 3.78
) ppm. C62 92NaO
1138.76): calcd. C 65.39, H 8.14, Sm 13.18, Na 2.02; found C
5.27, H 7.93, Sm 13.47, Na 1.98.
3
)
], 2.01–2.58 (br., 28 H, THF and ArCH
4 H, ArCH ), 6.93–7.08 (4 H, C
3
CCD area detector. The structures were solved by direct methods
3
2
(
(
2
6
H
2
H
8
Sm and refined by full-matrix least-squares procedures based on |F| .
All non-hydrogen atoms were refined with anisotropic displace-
ment coefficients. Hydrogen atoms were treated as idealised contri-
butions. The structures were solved and refined using the CRYS-
6
Synthesis of [(THF)Yb(MBMP) Na(THF)
complex 3 was carried out as described for complex 1 but anhy-
drous YbCl (1.31 g, 4.69 mmol) was used instead of NdCl . Yellow
microcrystals were obtained from toluene. Yield: 4.07 g, 75%. M.p.
42–143 °C (dec.). IR (KBr): ν˜ = 2954 (s), 2909 (s), 2869 (m), 1605
2
2
] (3): The synthesis of
TALS (for 1 and 3) and SHELXS-97 (for 6) programs. Crystal and
refinement data are listed in Table 4. CCDC-239852 to -239854 for
, 3 and 6, respectively, contain the supplementary crystallographic
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.
3
3
1
1
–
1
(
m), 1526 (m), 1434 (s), 1240 (s), 1156 (s), 1050 (m), 862 (m) cm .
1
H NMR (C
ArCH ), 3.42–3.81 (br., 16 H, THF and ArCH
) ppm. C58 84NaO Yb (1089.33): calcd. C 63.95, H 7.77, Yb
5.88, Na 2.11; found C 63.56, H 7.74, Yb 15.60, Na 1.93.
6
D
6
): δ = 1.45 (48 H, C(CH
3
)
3
and THF), 2.17 (12 H,
3
2
), 6.93–7.08 (8 H,
C
6
H
2
H
7
Acknowledgments
1
The authors are grateful to the Chinese National Natural Science
Foundation (20272040 and 20472063) and the Key Laboratory of
Organic Synthesis of Jiangsu Province for financial support.
Synthesis of [(THF) Nd(MBMP) ][Na(DME) (THF) ] (4): To a
2
2
2
2
solution of complex 1 (3.27 g, 2.8 mmol) in toluene (50 mL) was
added DME (1 mL). After stirring the solution for about 30 min,
blue-purple microcrystals were obtained upon concentration and
subsequent cooling of the solution to –5 °C for 3 d. Yield: 3.11 g,
[
1] For reviews see: a) V. C. Gibson, S. K. Spitzmesser, Chem. Rev.
2003, 103, 283–315; b) G. J. P. Britovsek, V. C. Gibson, D. F.
Wass, Angew. Chem. 1999, 111, 448–468; Angew. Chem. Int.
8
1
5%. M.p. 109–110 °C (dec.). IR (KBr): ν˜ = 2951 (s), 2912 (s),
608 (w), 1466 (s), 1435 (s), 1385 (m), 1084 (s), 860 (m) cm .
–
1
682
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 676–684

Carbon-Bridged (Diphenolato)lanthanide Complexes
FULL PAPER
Table 4.Details of the crystallographic data and refinements for 1, 3 and 6.
1
3·3C
7
H
8
4 8
6·C H O
Empirical formula
Formula mass
T [K]
C
62
H
92NaNdO
8
C
79
H
108NaO
7
Yb
74
C H120NaO13Yb
1413.73
193(2)
1132.63
193(2)
1365.75
193(2)
λ [Å]
Crystal system
Space group
0.7107
monoclinic
C2/c
0.7107
triclinic
P1
0.7107
triclinic
P1
¯
¯
Unit-cell dimensions:
a [Å]
b [Å]
c [Å]
α [°]
18.421(2)
17.955(1)
18.099(2)
90
96.269(5)
90
5950.6(8)
4
1.264
0.930
2396
12.9822(4)
15.3241(5)
19.3690(5)
79.089(7)
80.662(7)
78.608(6)
3677.7(2)
2
13.7108(7)
17.1666(12)
36.308(3)
89.862(11)
80.019(9)
79.611(9)
8276.2(9)
4
β [°]
γ [°] 3
V [Å ]
Z
3
Density [Mgm ]
1.233
1.328
1438
1.135
1.187
2996
–1
Absorption coefficient [mm ]
F(000)
θ
max [°]
27.48
58952
6953
6046
372
0.0390
0.1110
1.004
27.48
36994
16575
14039
27.49
87226
35903
33673
Reflections collected
Independent reflections
Observed reflections
Parameters refined
Final R [I Ͼ 3σ(I)]
wR
796
1366
0.0450
0.1330
1.034
0.0862
0.2066
1.189
GoF on F2
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683

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Received: June 15, 2004
684
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 676–684