Bora-amidinate complexes of lanthanide(II) metals

Article abstract of DOI:10.1002/ejic.201000094

Reaction of (2-Me₂N-α-Me₃Si-benzyl) ₂Ln·(thf)₂ (Ln = Yb, Eu and Sm) with one equivalent of DIPP(H)N-BH-N(H)DIPP (^DIPPNBN-H₂, DIPP = 2,6-iPr-phenyl) gave the first Ln^II boraamidinate complexes which crystallized in the presence of THF as ^DIPPNBN·Ln·(thf) ₄. Although only the Sm and Eu complex crystallize isomorphic, all three show similar structures. The very small N-Ln-N' bite angles of 59.74(6)63.28(9)° leave a large part of the metal coordination sphere available for neutral thf ligands.

Full text of DOI:10.1002/ejic.201000094

FULL PAPER
DOI: 10.1002/ejic.201000094
Bora-Amidinate Complexes of Lanthanide(II) Metals
Sjoerd Harder*[a] and Dominik Naglav[a]
Keywords: Lanthanides / Europium / Ytterbium / Samarium
Reaction of (2-Me2N-α-Me3Si-benzyl)2Ln·(thf)2 (Ln = Yb, Eu
complex crystallize isomorphic, all three show similar struc-
tures. The very small N–Ln–NЈ bite angles of 59.74(6)–
63.28(9)° leave a large part of the metal coordination sphere
available for neutral thf ligands.
and Sm) with one equivalent of DIPP(H)N-BH-N(H)DIPP
(
DIPPNBN-H2, DIPP = 2,6-iPr-phenyl) gave the first LnII bora-
amidinate complexes which crystallized in the presence of
THF as DIPPNBN-Ln·(thf)4. Although only the Sm and Eu
Introduction
The chemistry of the lanthanide (Ln) metals in their hot
oxidation +2 is surprisingly similar to that of the heavier
alkaline-earth metal ions Ca2+ and Sr2+ [1]
This is not only
.
due to equal oxidation states but also to very similar metal–
ligand bond character: bonds to these metals should be
considered highly ionic.[2] In addition, the ionic radii for
Sm2+ and Eu2+ compare well to that of Sr2+ whereas the
size of Yb2+ is similar to that of Ca2+ (ionic radii for six-
coordinate ions in Å: Ca2+ 1.00, Yb2+ 1.02, Sr2+ 1.18, Eu2+
1.17, Sm2+ 1.22).[3] For this reason LnII chemistry faces the
same problems as the chemistry of the heavier alkaline-
earth metals. E.g. the considerable size of the metal ions
invokes the necessity to use large ligands that saturate the
metal’s coordination sphere in order to prevent formation
of insoluble coordination polymers. From this point of
view, it is not surprising that bulky cyclopentadienide (Cp)
ligands are popular in the organometallic chemistry of
LnII.[4] The milestone synthesis of Cp*2Sm[4a] (Cp* =
pentamethylcyclopentadienyl) initiated numerous investi-
gations on its use as an one-electron reduction agent.[5] Ex-
tensive developments in this field have led to many applica-
tions of SmII complexes in organic syntheses or in polymeri-
zation chemistry.[6] Despite the versatility of LnII reagents
only few homoleptic non-Cp complexes have been pre-
pared. These complexes are generally based on bulky li-
gands: e.g. silylated alkyl anions (1),[7] silylated amide
anions (2),[8] bulky amidinate (and guanidinate) anions
(3),[9] bulky β-diketiminate anions (4)[1] or bis-
(phosporanoimino)methanide anions (5).[10] While bulky li-
gands offer valuable protection for the LnII center, for cata-
lytic application less demanding ligands would be favour-
able. This initiated investigations on heteroleptic complexes
[a] Anorganische Chemie, Universität Duisburg–Essen,
Universitätsstraße 5–7, 45117 Essen, Germany
E-mail: sjoerd.harder@uni-due.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000094.
2836
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 2836–2840

Bora-Amidinate Complexes of Lanthanide(II) Metals
of general formula Cp*LnER (ER = OR, NR2, SR).[11] On
account of the less demanding ER unit, these complexes
are much more open for substrate coordination. At the
same time, however, the strongly electron-donating Cp* li-
gand warrants an electron-rich LnII center for efficient re-
duction chemistry. Even more open LnII complexes have
been prepared by linking Cp* and ER units (6),[11] i.e., by
making use of the constrained geometry ligand well-known
in LnIII and group 4 chemistry.[12]
Results and Discussion
In a first series of experiments we attempted to depro-
tonate DIPPNBN-H2 with an equimolar amount of the com-
monly used LnII base Ln[(NSiMe3)2]2·(thf)2 (Ln = Sm, Eu
or Yb)[8] in benzene at 60 °C. Although distinct colour
changes were indicative for conversion we have never been
able to isolate well-defined products from these solutions.
It is likely that the acid-base equilibrium is not fully at the
product side. Although there is no information on the acid-
ity of bora-amidine ligands we estimate the pKa1 value of
DIPPNBN-H2 to be lower than that of (Me3Si)2NH (pKa =
25.8).[16] Relevant for the second deprotonation, however, is
the pKa2 value which might be in the same region. For
that reason we decided to use benzyllanthanide reagents
for deprotonation: (2-Me2N-α-Me3Si-benzyl)2Ln·(thf)2
(Scheme 2). These reagents, which are known for all three
lanthanide metals that generally occur in the 2+ oxidation
state (Sm,[1] Eu[17] and Yb[1]), are easily accessible in good
yields and crystalline purity. They are among the strongest
bases hitherto known in LnII chemistry and should be con-
sidered as convenient precursors.
Serendipitously, we recently found a very convenient one-
pot synthesis of a hitherto unknown bora-amidinate ligand
(abbreviated as bam ligand).[13] The ligand DIPPNBN-H2
can be obtained quantitatively from the (nBu)2Mg-cata-
lyzed reaction of two equivalents of 2,6-diisopropylaniline
with one equivalent of Me2S·BH3 (Scheme 1).[14] This li-
gand can be deprotonated twofold to give DIPPNBN2–
which, by isolobal replacement of BH– for CH, is iso-
electronic to the corresponding amidinate anion HC(N-
–
DIPP)2 . However, on account of its double negative
charge, it could substitute two amidinate ligands and
should therefore be regarded as a very compact highly elec-
tron-donating ligand. Whereas this ligand would be opti-
mally suited to stabilize complexes with metals in unusually
high oxidation states, its use in low valent Ln chemistry
could be equally rewarding. Complexes like DIPPNBN-LnII
(7) would feature reactive LnII centers that are unusually
exposed. At the same time, such compounds are isolobal to
an (amidinate)LnII+ cation (8) which hitherto has not been
prepared (7 could be seen as a complex in which the non-
coordinating anion is incorporated in the chelate ligand).
The ambiguity of an electron-rich LnII metal in a complex
that is isoelectronic with a cationic species could have inter-
esting consequences in reduction chemistry. In here we de-
scribe our preliminary investigations and introduce synthe-
ses and crystal structures of the first LnII bora-amidinate
complexes DIPPNBN-LnII (Ln = Yb, Eu, and Sm).[15]
Addition of DIPPNBN-H2 to a solution of (2-Me2N-α-
Me3Si-benzyl)2Yb·(thf)2 in benzene resulted already at
room temperature in an immediate colour change from
orange to red. Soon after, yellow rectangular crystals of
DIPPNBN-Yb·(thf)4 started to grow. In order to complete
the reaction, the mixture was heated overnight to
60 °C. After cooling a crop of large uniform crystals
could be isolated (yield: 88%). A similar procedure also
gave the Eu and Sm analogues in yields of 84%
and 68%, respectively. The Sm complex is extremely
sensitive towards oxidation and difficult to handle. The Yb
and Eu complexes were found to be much less sensitive.
Under vacuum, all complexes can lose part of the thf li-
gands.
Crystallographic studies revealed that all three complexes
crystallize as DIPPNBN-Ln·(thf)4: i.e. a monomer in which
four thf ligands are needed to saturate the coordination
sphere of the central metal (Figure 1, a). The crystal struc-
tures of DIPPNBN-Sm·(thf)4 and DIPPNBN-Eu·(thf)4 are iso-
morphic (crystal data are given in Table 1) and the asym-
metric unit contains one molecule. The complex with the
somewhat smaller metal, DIPPNBN-Yb·(thf)4, crystallizes in
a different crystal system with half a molecule in the asym-
Scheme 1.
Scheme 2.
Eur. J. Inorg. Chem. 2010, 2836–2840
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S. Harder, D. Naglav
FULL PAPER
Table 1. Crystal data.
Compound
DIPPNBN-Yb·(thf)4
DIPPNBN-Eu·(thf)4
DIPPNBN-Sm·(thf)4
Formula
MW
Size [mm]
Colour
Crystal system
Space group
C40H67BN2O4Yb·C4H8O
C40H67BN2O4Eu
802.73
0.4ϫ0.3ϫ0.2
yellow
monoclinic
P21/n
C40H67BN2O4Sm
801.12
0.3ϫ0.2ϫ0.1
black
monoclinic
P21/n
895.91
0.2ϫ0.05ϫ0.05
yellow
orthorhombic
C2221
a [Å]
b [Å]
c [Å]
α [°]
11.9023(8)
16.4066(8)
23.2940(14)
90
10.8223(5)
18.4564(8)
20.8239(9)
90
10.8185(4)
18.4750(4)
20.8417(8)
90
β [°]
90
90
4548.8(5)
4
1.308
97.968(2)
90
4119.2(3)
4
1.294
1.561
96.132(1)
90
4125.3(3)
4
1.290
1.462
γ [°]
V [Å3]
Z
ρ [gcm–3]
µ(Mo-Kα) [mm–1]
2.098
T [°C]
θ(max.) [°]
–120
28.4
–100
28.3
–100
29.3
Number of total/unique reflections
Rint
27241/5668
0.042
75701/10231
0.033
79921/11077
0.053
Observed reflections [IϾ2σ(I)]
Parameters
R1
4947
247
0.0275
0.0608
1.04
8510
445
0.0302
0.0688
1.11
–1.24/1.25
8308
445
0.0362
0.0848
1.07
–1.14/1.08
wR2
GOF
min./max. residual electron density [eÅ–3] –0.45/1.19
metric unit (the complex is crystallographically C2-symmet- aryl substituents make angles between 69.2(3)° and 87.2(3)°
ric). Selected bond lengths and angles are summarized in with the N–B–N plane. The Ln–N and Ln–O distances de-
Table 2.
crease along the row of lanthanide contraction (Sm Ͼ Eu
ϾYb), however, this decrease is less than expected. In going
from Sm to Yb the Ln–N and Ln–O distances decrease by
0.109(3) and 0.148(3) Å, respectively, which is less than the
0.20 Å decrease for their ionic radii. All complexes display
similar thf coordination: two of the thf ligands typically re-
side in the N–B–N plane. The average O–Ln–O’ angles for
neighbouring ligands (cis) are in the range of 79.69(8)–
82.44(7)° whereas those in trans position display average O–
Ln–OЈ angles between 128.58(8)° and 131.48(7)°. The B–
N bond lengths are in the very narrow range of 1.420(3)–
1.424(3) Å. The average value of 1.422(3) Å compares well
to that of 1.429(1) Å in borazine (BHNH)3.[18] Also the
group 2 metal complex PhB(N-DIPP)2Mg·(OEt2)2 shows a
similar B–N bond length of 1.444(5) Å.[19] There is not
much variation in the N–B–N angles: 118.7(2)–119.5(3)°.
The N–B–N angle in DIPPNBN-Ln·(thf)4 [average:
119.1(3)°] is considerable wider than that in complexes con-
taining the PhB(N-DIPP)2 ligand [N–B–N range: 109.4(6)–
112.5(3)°].[13] This widening is due to the presence of a
small H-substituent in the backbone of DIPPNBN2–. Despite
a rather wide N–B–N angle, the ligand bite angles in
DIPPNBN-Ln·(thf)4 are among the most acute reported so
far. This is related to the long Ln–N bond lengths: hitherto
the Sm–N bond length of 2.466(2) Å is the longest reported
metal–N bond length to a bam ligand. The N–Ln–NЈ bite
angle decreases with increasing metal size and in DIPPNBN-
Sm·(thf)4 an extremely small bite-angle of less than 60° is
found. As exemplified in the space-filling model for the
DIPPNBN-Sm fragment (Figure 1, b), this leaves ample
Figure 1. a) Crystal structure of DIPPNBN-Yb·(thf)4; all hydrogen
atoms, except that at B, have been omitted for clarity. The Sm and
Eu complexes are isostructural. b) Spacefilling model for the Sm
analogue, the complex with the largest metal and smallest N–Ln–
N bite-angle. Neutral thf ligands have been omitted in order to
show the space around the metal.
In all structures the DIPPNBN ligands coordinate to the
metal in a N, NЈ-chelating fashion. The metal is situated
close to the plane spanned by the N–B–N backbone [the
maximum out-of-plane deviation is 0.043(1) Å found in
DIPPNBN-Sm·(thf)4]. Least-squares planes defined for the
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Eur. J. Inorg. Chem. 2010, 2836–2840

Bora-Amidinate Complexes of Lanthanide(II) Metals
Table 2. Selected bond lengths [Å] and angles [°]. Average values are given between brackets.
Compound
Yb
Eu
Sm
M–N
2.346(3)
2.436(2)–2.462(2)
Ͻ2.449(2)Ͼ
2.443(2)–2.466(2)
Ͻ2.455(2)Ͼ
M–O
B–N
2.429(2)–2.505(3)
Ͻ2.467(3)Ͼ
1.424(3)
2.586(2)–2.636(2)
Ͻ2.602(2)Ͼ
1.416(3)–1.424(3)
Ͻ1.420(3)Ͼ
2.599(2)–2.647(2)
Ͻ2.615(2)Ͼ
1.416(3)–1.427(3)
Ͻ1.422(3)Ͼ
N–B–NЈ
N–M–NЈ
119.5(3)
63.28(9)
119.0(2)
59.97(7)
118.7(2)
59.74(6)
O–Yb–OЈ cis
76.65(8)–82.72(8)
Ͻ79.69(8)Ͼ
117.49(8)–139.67(9)
Ͻ128.58(8)Ͼ
0.000(1)
78.70(7)–88.64(7)
Ͻ82.44(7)Ͼ
123.65(7)–139.13(6)
Ͻ131.39(7)Ͼ
0.039(1)
78.34(7)–82.31(7)
Ͻ80.51(7)Ͼ
124.42(7)–138.54(6)
Ͻ131.48(7)Ͼ
0.043(1)
O–Yb–OЈ trans
Deviation M
N–B–N plane [Å]
Dihedral angle
DIPP/NBN [°]
69.2(3)
77.7(3)–87.1(3)
Ͻ82.4(3)Ͼ
77.4(3)–87.2(3)
Ͻ82.3(3)Ͼ
space around the metal for coordination of four thf ligands. ture of these complexes could lead to interesting redox
For comparison, the metal in the constrained geometry cat- chemistry and/or catalytic activity. We are currently explor-
alyst 6 (Ln = Yb, R = Ph) contains three thf ligands in its ing the redox-chemistry and catalytic potential of these
coordination sphere. Although there are examples for LnII novel systems.
complexes with even five thf ligands in its coordination
sphere, LnI2·(thf)5 (Ln = Sm or Eu) and Eu(C6F5)2·
(thf)5,[20] the anionic ligands coordinated symmetrically
Experimental Section
around the metal. In contrast, the small bite angle of the
General: All manipulations were performed under a dry and oxy-
N–B–N ligands in DIPPNBN-Ln·(thf)4 is responsible for the
gen-free argon atmosphere using Schlenk line and glove box tech-
very open nature of DIPPNBN-Ln·(thf)4 complexes.
niques and freshly dried solvents. Following complexes have been
The Sm and Eu complexes are paramagnetic but diamag-
prepared according to literature procedures: DIPPNBN-H2,[14] (2-
netic DIPPNBN-Yb·(thf)4 could be completely characterized
Me2N-α-Me3Si-benzyl)2Ln·(thf)2 (Ln = Sm,[1] Eu,[17] Yb[1]).
by 1H, 13C and 11B NMR spectroscopy. The 1H NMR spec-
General Synthetic Route: Equimolar quantitities of DIPPNBN-H2
(0.32 mmol) and (2-Me2N-α-Me3Si-benzyl)2Ln·(thf)2 (0.32 mmol)
were dissolved in a mixture of 1.9 mL of benzene and 0.05 mL of
THF and heated for 16 h at 60 °C. Slowly cooling to 5 °C gave
trum of the complex dissolved in C6D6 shows only one
doublet for the iPr-Me groups and one septet for the iPr-
CH groups. Also in 13C NMR spectrum single resonances
are observed for the iPr-Me and iPr-CH groups. This indi-
crystals of the product in the following yields: Ln = Yb 232 mg
cates that the DIPP substituents can rotate freely around
the N-DIPP axis and is due to the small hydride substituent
in the boron backbone. In complexes with the PhB(N-
DIPP)2 ligand rotation of the DIPP substituent is blocked
by the Ph group and the iPr substituents give various NMR
signals.[19] The 11B NMR resonance for DIPPNBN-Yb·(thf)
(88%, yellow), Ln = Eu 216 mg (84%, yellow) and Ln = Sm 175 mg
(68%, black). The complexes DIPPNBN-Eu·(thf)4 and DIPPNBN-
Sm·(thf)4 are paramagnetic but DIPPNBN-Yb·(thf)4 has been fully
characterized by NMR spectroscopy. 1H NMR (300 MHz, [D6]-
3
benzene, 25 °C): δ = 1.29 [d, J(H,H) = 5.8 Hz, 24 H, CHMe2], 1.31
(b, 16 H, thf), 3.45 (b, 16 H, thf), 3.59 [sept, 3J(H,H) = 5.8 Hz, 4 H,
CHMe2], 5.39 (b, 1 H, BH), 6.95 [t, 4J(H,H) = 6.9 Hz, 2 H, Ar], 7.15
[d, 4J(H,H) = 6.9 Hz, 4 H, Ar] ppm. 13C{1H} NMR (75 MHz, [D6]-
benzene, 25 °C): δ = 25.6 (CHMe2), 26.1 (thf), 29.0 (CHMe2), 68.3
(thf), 119.1 (Ar), 123.6 (Ar), 141.2 (Ar), 154.6 (Ar) ppm. 11B{1H}
NMR (160.5 MHz, [D6]benzene, 25 °C): δ = 32.0 (s, BH)ppm.
Elemental analysis for DIPPNBN-Eu·(thf)4: calcd. (%) for
C40H67BEuN2O4 (802.76): calcd. C 59.85, H 8.41, N 3.49; found C
59.34, H 8.21, N 3.22. Elemental analysis for DIPPNBN-Yb·(thf)4:
calcd. for C40H67BN2O4Yb (823.84): calcd. C 58.32, H 8.20, N
3.40; found C 58.30, H 8.41, N 3.38. The extreme sensitivity of
DIPPNBN-Sm·(thf)4 towards oxidation disabled a satisfying elemen-
tal analysis. Crystals of DIPPNBN-Sm·(thf)4 were quenched in
CD3OD. The colourless solution was freed from Sm traces by filter-
is a broad signal at δ = 32.0 ppm [cf. for complexes con-
4
taining the ligand PhB(N-DIPP)2 chemical shifts in the
range of 28–30 ppm are observed].[19]
Conclusions
The first bora-amidinate complexes of LnII metals have
been prepared. Successful twofold deprotonation of
DIPPNBN-H2 could only be achieved by use of strongly ba-
sic LnII precursors: (2-Me2N-α-Me3Si-benzyl)2Ln·(thf)2.
The products all crystallize as complexes of composition
DIPPNBN-Ln·(thf)4 (Ln = Yb, Eu, Sm) and, although only
two of these complexes crystallize isomorphic, all three
show a similar structure. The very small bite angle of the
twofold negatively charged ligand DIPPNBN2– [range:
59.74(6)–63.28(9)°] leaves a large part of the metal cation
1
ing through Al2O3. H NMR analysis gave a ligand/thf ratio of 1/
3.8. The following melting points have been determined: DIPPNBN-
Sm·(thf)4 141 °C; DIPPNBN-Eu·(thf)4 143 °C; DIPPNBN-Yb·(thf)4
160 °C (dec).
Crystal Structure Determinations: All data were collected on a Sie-
exposed for coordination of neutral ligands. The open na- mens SMART CCD APEX II diffractometer (Table 2). The struc-
Eur. J. Inorg. Chem. 2010, 2836–2840
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Harder, D. Naglav
FULL PAPER
tures have been solved by direct methods (SHELXS-97)[21] and
were refined with SHELXL-97.[22] The geometry calculations and
graphics have been performed with PLATON.[23]
DIPPNBN-Yb·(thf)4: The asymmetric unit contains half a molecule
of non-coordinated THF in a void. This solvent molecule shows
slight disorder which was treated by refinement with large aniso-
tropic displacement parameter. The BH hydrogen atom has been
located in the difference Fourier map and was refined isotropically.
All other hydrogen atoms have been placed on calculated positions
and were refined in a riding mode.
DIPPNBN-Eu·(thf)4: The BH hydrogen atom has been located in
the difference Fourier map and was refined isotropically. All other
hydrogen atoms have been placed on calculated positions and were
refined in a riding mode.
DIPPNBN-Yb·(thf)4: The thf ligands show a slight ring-puckering
disorder typical for this ligand. This has been treated by refinement
with large anisotropic displacement parameter. The BH hydrogen
atom has been located in the difference Fourier map and was re-
fined isotropically. All other hydrogen atoms have been placed on
calculated positions and were refined in a riding mode.
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[15] a) Prof. Tristram Chivers kindly informed us of his concurrent
work on (bora-amidinate)LnIII complexes which was published
during the revision of this manuscript: A. M. Corrente, T. Chi-
vers, Inorg. Chem. 2010, 49, 2457–2463; b) Other ongoing work
from our own research group deals with bora-amidinate LnIII
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Acknowledgments
We acknowledge the Deutsche Forschungsgemeinschaft (DFG) for
support of this project within SPP1166 (Lanthanide-specific func-
tionalities in molecule and material). Prof. Dr. R. Boese and D.
Bläser (University of Duisburg-Essen) are thanked for collection of
X-ray diffraction data. We thank Prof. Dr. T. Chivers for informing
us on his concurrent work on bora-amidinate LnIII complexes and
for helpful discussions. S. Range is kindly acknowledged for techni-
cal assistance during part of this work.
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Published Online: May 19, 2010
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