Syntheses and structures of homoleptic lanthanide complexes with chelating o-dimethylaminobenzyl ligands: Key precursors in lanthanide chemistry

Article abstract of DOI:10.1021/om049327p

Reaction of o-Me₂N-C₆H₄CH₂K with YCl₃ in THF yielded (o-Me₂-C₆H ₄CH₂)₃Y (5-Y) in the form of light yellow crystalline plates (59% yield). The crystal structure shows three bidendate benzyl ligands bound to Y, which has a prismatic coordination sphere. The La analogue (5-La) was prepared similarly (41% crystalline yield) and is isostructural to 5-Y, but shows more extensive multihapto bonding of the benzyl ligand to the larger metal (short aryl - La interactions). Attempted synthesis of (o-Me₃Si-C₆H₄CH₂)₃La from o-Me₃Si-C₆H₄-CH₂K and LaCl ₃ unexpectedly gave [(o-Me₃Si-C₆H ₄CH₂)₄La^-][Li ⁺·(THF)₄] (32% crystalline yield), the source of the Li⁺ ion being impure o-Me₃Si-C₆H ₄CH₂K. Crystal structure determination revealed the (o-Me₃Si-C₆H₄CH₂)₄La ^- ion in which La has a distorted tetrahedral coordination sphere. The complexes (o-Ma₂N-C₆H₄CH₂) ₃Y and (o-Me₂N-C₆H₄-CH ₂)₃La show extraordinary thermal stability, but also sufficient reactivity in deprotonation of 9-t-BuN(H)SiMe₂-fluorene. The product, (S-t-BuNSiMe₂-fluorenyl)(o-Me ₂N-benzyl)Y·(THF) (7; 85% crystalline yield), shows a monomeric crystal structure with allylic coordination of the fluorenyl ring and bidentate coordination of the benzyl ligand. This complex was successfully hydrogenated with molecular H₂ (10 bar) to yield the dimer [(9-t-BuNSiMe₂-fluorenyl)YH·(THF)₂]₂ (8; 84% crystalline yield), in which the fluorenyl part of the ligand is bound to Y only in η¹-fashion.

Full text of DOI:10.1021/om049327p

Organometallics 2005, 24, 373-379
373
Syntheses and Structures of Homoleptic Lanthanide
Complexes with Chelating o-Dimethylaminobenzyl
Ligands: Key Precursors in Lanthanide Chemistry
Sjoerd Harder*
Universita¨t Duisburg-Essen, Universita¨tsstrasse 5, 45117 Essen, Germany
Received August 30, 2004
Reaction of o-Me2N-C6H4CH2K with YCl3 in THF yielded (o-Me2N-C6H4CH2)3Y (5-Y) in
the form of light yellow crystalline plates (59% yield). The crystal structure shows three
bidendate benzyl ligands bound to Y, which has a prismatic coordination sphere. The La
analogue (5-La) was prepared similarly (41% crystalline yield) and is isostructural to 5-Y,
but shows more extensive multihapto bonding of the benzyl ligand to the larger metal (short
aryl-La interactions). Attempted synthesis of (o-Me3Si-C6H4CH2)3La from o-Me3Si-C6H4-
CH2K and LaCl3 unexpectedly gave [(o-Me3Si-C6H4CH2)4La-][Li+‚(THF)4] (32% crystalline
yield), the source of the Li+ ion being impure o-Me3Si-C6H4CH2K. Crystal structure
determination revealed the (o-Me3Si-C6H4CH2)4La- ion in which La has a distorted
tetrahedral coordination sphere. The complexes (o-Me2N-C6H4CH2)3Y and (o-Me2N-C6H4-
CH2)3La show extraordinary thermal stability, but also sufficient reactivity in deprotonation
of 9-t-BuN(H)SiMe2-fluorene. The product, (9-t-BuNSiMe2-fluorenyl)(o-Me2N-benzyl)Y‚(THF)
(7; 85% crystalline yield), shows a monomeric crystal structure with allylic coordination of
the fluorenyl ring and bidentate coordination of the benzyl ligand. This complex was
successfully hydrogenated with molecular H2 (10 bar) to yield the dimer [(9-t-BuNSiMe2-
fluorenyl)YH‚(THF)2]2 (8; 84% crystalline yield), in which the fluorenyl part of the ligand is
bound to Y only in η1-fashion.
Introduction
the small methyl anion is certainly not big enough to
saturate the large coordination sphere of Ln3+, and
Me3Ln therefore forms a stable adduct with the MeLi
precursor.
Highly reactive tris-alkyllanthanide compounds are
extremely useful precursors in the syntheses of various
lanthanide complexes and are the key to many lan-
thanide catalysts.1 Their syntheses, however, are not
straightforward and are limited to a few alkyl groups.
These problems arise from the relatively high Lewis
Reaction of the bulkier tert-butyllithium with LnCl3
resulted in formation of an “ate”-complex (2), which
shows that even three tert-butyl anions cannot saturate
the Ln coordination sphere.6
acidity of the +3-charged lanthanide cations (Ln3+
)
combined with their respectable ionic sizes: the largest
lanthanide ion, La3+ (1.03 Å), is comparable in size to
Ca2+ (1.00 Å) and the smallest ion, Lu3+ (0.86 Å), is still
much larger than Mg2+ (0.66 Å).2 This combination of
relatively high Lewis acidity and a large coordination
sphere makes them on one hand very useful in catalysis,
however, on the other hand creates problems in their
syntheses.
Reaction of MeLi with several LnCl3 salts led to the
formation of a series of mixed-metal aggregates (1):3-5
* Tel: 49-201-1834510. Fax: 49-201-1832621. E-mail: sjoerd.harder@
uni-essen.de.
(1) Review articles: (a) Evans, W. J. Adv. Organomet. Chem. 1985,
24, 131-177. (b) Anwander, R.; Herrmann, W. A. Top. Curr. Chem.
1996, 179, 1-32. (c) Edelmann, F. T.; Freckmann, D. M. M.; Schu-
mann, H. Chem. Rev. 2002, 102, 1851-1896. (d) Arndt, S.; Okuda, J.
Chem. Rev. 2002, 102, 1953-1976. (e) Yasuda, H. J. Organomet. Chem.
2002, 647, 128-138.
Reaction of LaCl3 with LiCH(SiMe3)2 resulted in the
formation of the mixed-metal species tris(alkyl)La‚(LiCl)
(3).7 Even a tris-alkyllanthanide with the bulky CH-
(SiMe3)2 group, so-called “big R”, tends to saturate the
metal coordination sphere by retaining the side-product
LiCl. True homoleptic tris-alkyllanthanide complexes
(2) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(3) Schumann, H.; Mu¨ller, J. Angew. Chem. 1978, 90, 307; Angew.
Chem., Int. Ed. 1978, 17, 276.
(4) Schumann, H.; Mu¨ller, J. Angew. Chem. 1981, 93, 127; Angew.
Chem., Int. Ed. 1981, 20, 120.
(5) Schumann, H.; Mu¨ller, J.; Bruncks, N.; Lauke, H.; Pickardt, J.
Organometallics 1984, 3, 69.
(6) Wayda, A. L.; Evans, W. J. J. Am. Chem. Soc. 1978, 100, 7119.
(7) Atwood, J. L.; Lappert, M. F.; Zhang, H. J. Chem. Soc., Chem.
Commun. 1988, 1308.
10.1021/om049327p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/04/2005

374 Organometallics, Vol. 24, No. 3, 2005
Harder
were prepared by reaction of LiCH(SiMe3)2 with the
sterically hindered lanthanide-phenoxide (2,6-tert-Bu2-
C6H3O)3Ln.8 The steric saturation of the lanthanide
metal is partly achieved by the bulk of the ligand, but
also via agostic Siδ--Μeδ+‚‚‚Ln3+ interactions.
the coordination sphere is the key to success. Therefore
syntheses of the benzyl complexes 4 and 5 were pursued.
Complex 4 would be stabilized not only by steric
shielding of the lanthanides coordination sphere but also
by possible agostic Siδ--Meδ+‚‚‚Ln3+ interactions. Com-
plex 5 would be stabilized by intramolecular coordina-
tion via the ortho-NMe2 substituent, similar to that
described for heteroleptic benzyllanthanide19 and aryl-
lanthanide17 complexes. The complexes 4 and 5 still
feature an unstabilized and therefore highly basic (or
nucleophilic) benzylic CH2 functionality.
Although several [(Me3Si)2CH]3Ln complexes have
been prepared, their reactivity is tempered by 2-fold
substitution of the carbanion with stabilizing silyl
groups. Syntheses and structural characterization of the
tris-alkyllanthanide complexes with the smaller, less
stabilized Me3SiCH2 group could be achieved for the
smaller lanthanide metals (Lu, Yb, Er, Sm, and Y);9-12
however, their THF-solvated complexes are not only air-
sensitive but extremely thermally sensitive as well (a
recent report showed that complexation with 12-crown-4
increases their stabilities significantly).13 Although their
syntheses from LnCl3 and Me3SiCH2Li are straightfor-
ward, the preparation of the Li precursor from Me3-
SiCH2Cl and fresh Li suspension is tedious and requires
sublimation.14 Recently a surprisingly simple one-step
procedure has been described15 for tris(alkyl)Yb com-
plexes: 8 Yb + 12 RI f 2 R3Yb + 6 YbI2 + 3 R-R (R )
-CH2SiMe3 or -CH2CMe3). This method circumvents
use of the Li precursor but is as yet only limited to Yb
and not very economical on the lanthanide metal.
All homoleptic tris-alkyllanthanide complexes R3Ln
known so far (R ) (Me3Si)2CH-, Me3SiCH2-, t-
BuCH2-, and (Ph2P)2CH-)8-12,15,16 have one thing in
common: the lack of a â-H atom, which prevents
decomposition via â-H elimination. In this light, it is
odd that neutral homoleptic benzyllanthanide complexes
received little attention. Only the preparation of a tris-
benzyl complex of the lightest group 3b metal, the
lanthanide-like Sc, has been described: (o-Me2N-C6H4-
CH2)3Sc.17 Reactions of PhCH2Li with LnCl3 (Ln ) Y
and Nd) resulted in mixtures of unusual complexes.18
It is proposed that the products (PhCH2)3Ln decompose
by an R-elimination pathway producing Schrock-car-
bene-like products (PhCH2LndCHPh), which can react
further to carbyne-like species (LntCPh).18
A further necessity seemed the use of benzylpotas-
sium instead of benzyllithium reagents in the reaction
with the LnCl3. This would produce the extremely
insoluble KCl instead of the well-soluble LiCl, which has
the advantage of bringing the reaction to completion as
well as circumventing the formation of mixed-metal
complexes such as R3Ln‚(KCl) or [R3LnCl-][K+].
The attempted preparation of 4-La by reacting 3
equiv of o-Me3Si-C6H4CH2K with LaCl3 in THF unex-
pectedly yielded the ate-complex [(o-Me3Si-C6H4CH2)4-
La-][Li+‚(THF)4] (6) in the form of large red crystalline
blocks (32%). The only explanation for the presence of
the Li+ ion is that the precursor o-Me3Si-C6H4CH2K,
which was prepared by reacting o-Me3Si-C6H4CH3 with
the superbase BuLi/KOC(Et)2Me, still contains consid-
erable quantities of o-Me3Si-C6H4CH2Li (at least 10%).
This could be due to either incomplete Li-K exchange
or insufficient removal of the LiOC(Et)2Me from o-Me3-
Si-C6H4CH2K (which seems reasonable considering the
very sticky nature of the reaction product).
This work presents a simple route to the syntheses
of homoleptic benzyl complexes of the lanthanides and
shows their crystal structures and their usefulness as
precursors in lanthanide chemistry.
The crystal structure of [(o-Me3Si-C6H4CH2)4La-]-
[Li+‚(THF)4] (Figure 1, Table 1) shows a pseudo-S4-
symmetric (Me3Si-C6H4CH2)4La- ion in which four
benzyl ligands are bound to the La metal in a distorted
tetrahedral fashion. Two of the C-La bonds (La-C11
2.668(5) Å, La-C31 2.650(5) Å) are distinctively shorter
than the other two (La-C1 2.712(4) Å, La-C21 2.729-
(5) Å). The benzyl ligands with the longer C-La bonds,
however, show a tendency toward η2-bonding: short
distances (2.887(5) and 2.891(4) Å) between Cipso and
the La metal are observed. The Cipso atoms of the other
benzyl ligands are nearly 3 Å away from La. Also
distances between La and the ortho-carbon atoms are
rather long (range 3.0-3.1 Å). The benzylic hydrogens
could not be located and have been calculated; therefore
no conclusions on the hybridization states of the benzylic
carbons were drawn.
Results and Discussion
To prepare benzyl complexes of even the largest
lanthanide metal (La), steric or electronic saturation of
(8) Hitchcock, P. B.; Lappert, M. F.; Smith, R. G.; Bartlett, R. A.;
Power P. P. J. Chem. Soc., Chem. Commun. 1988, 1007.
(9) Lappert, M. F.; Pearce R. J. Chem. Soc., Chem. Commun. 1973,
126.
(10) Schumann, H.; Mu¨ller J. J. Organomet. Chem. 1978, 146, C5.
(11) Hultzsch, K. C.; Voth, P.; Beckerle, K.; Spaniol, T. P.; Okuda,
J. Organometallics 1984, 3, 69.
(12) Schumann, H.; Freckmann, D. M. M.; Dechert, S. Z. Anorg. Allg.
Chem. 2002, 628, 2422.
(13) Arndt, S.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J.; Honda,
M.; Tatsumi, K. J. Chem. Soc., Dalton Trans. 2003, 3622.
(14) Tessier-Youngs, C.; Beachley, O. T., Jr. Inorg. Synth. 1986, 24,
95.
(15) (a) Niemeyer, M. Z. Anorg. Allg. Chem. 2000, 626, 1027. (b)
Niemeyer, M. Acta Crystallogr. 2001, E57, m553.
(16) Karsch, H. H.; Appelt, A.; Mu¨ller, G. Angew. Chem. 1986, 98,
832; Angew. Chem., Int. Ed. 1986, 25, 823.
(17) Manzer, L. E. J. Am. Chem. Soc. 1978, 100, 8068.
(18) (a) Guzman, I. S.; Chigir, N. N.; Sharaev, O. K.; Bondarenko,
G. N.; Tinyakova, E. I.; Dolgoplosk, B. A. Dokl. Akad. Nauk SSSR 1979,
249, 860. (b) Chigir, N. N.; Guzman, I. S.; Sharaev, O. K.; Tinyakova,
E. I.; Dolgoplosk, B. A. Dokl. Akad. Nauk SSSR 1982, 263, 375.
1H NMR data show that the approximate S4-sym-
metry of the (Me3Si-C6H4CH2)4La- ion is likely retained
(19) (a) Wayda, A. L. Organometallics 1983, 2, 565. (b) Wayda, A.
L. Organometallics 1984, 3, 939. (c) Wayda, A. L.; Rogers, R. D.
Organometallics 1985, 4, 1440.

Homoleptic Benzyllanthanide Complexes
Organometallics, Vol. 24, No. 3, 2005 375
Table 1. Selected Interatomic Distances (Å) and Angles (deg) for 5-Y, 5-La, 6, 7, and 8
(o-Me2N-benzyl)3Y (5-Y)
Y-C11
Y-C21
Y-C31
Y-N1
Y-N2
Y-N3
2.471(3)
2.487(3)
2.458(3)
2.555(3)
2.535(2)
2.609(3)
Y-C12
Y-C17
Y-C22
Y-C27
Y-C32
Y-C37
2.754(3)
C11-Y-N2
C11-Y-C31
N2-Y-C31
N1-Y-C21
C21-Y-N3
N1-Y-N3
89.53(9)
89.7(1)
91.14(9)
92.72(9)
88.32(9)
97.70(9)
C11-Y-N1
C21-Y-N2
C31-Y-N3
68.6(1)
69.1(1)
68.6(1)
2.802(3)
2.788(3)
2.808(3)
2.861(3)
2.919(3)
(o-Me2N-benzyl)3La (5-La)
La-C11
La-C21
La-C31
La-N1
La-N2
La-N3
2.635(3)
2.661(3)
2.638(3)
2.695(2)
2.697(2)
2.705(2)
La-C12
La-C17
La-C22
La-C27
La-C32
La-C37
2.853(2)
C11-La-N2
C11-La-C31
N2-La-C31
N1-La-C21
C21-La-N3
N1-La-N3
89.42(8)
87.99(9)
90.66(8)
97.15(7)
96.46(8)
102.40(7)
C11-La-N1
C21-La-N2
C31-La-N3
64.64(8)
64.58(7)
64.87(8)
2.881(3)
2.886(2)
2.899(2)
2.913(2)
2.928(2)
[(o-Me3Si-benzyl)4La-][Li+‚(THF)4] (6)
La-C1
2.712(4)
2.668(5)
2.729(5)
2.650(5)
La-C2
2.887(5)
2.982(5)
2.891(4)
2.982(5)
C1-La-C11
C1-La-C21
C1-La-C31
C11-La-C21
118.9(2)
78.4(1)
C11-La-C31
C21-La-C31
95.3(1)
117.1(2)
La-C11
La-C21
La-C31
La-C12
La-C22
La-C32
124.0(2)
126.2(2)
(9-t-BuNSiMe2-fluorenyl)(o-Me2N-benzyl)Y‚(THF) (7)
Y-C1
Y-C10
Y-N1
Y-N2
2.409(3)
2.522(2)
2.660(2)
2.233(2)
Y-O
2.357(2)
2.733(2)
2.763(3)
2.957(3)
3.106(3)
C1-Y-N1
C1-Y-O1
C1-Y-N2
N2-Y-C10
67.58(9)
115.80(8)
100.9(1)
69.52(7)
N1-Y-O1
N1-Y-N2
O1-Y-N2
83.22(6)
163.19(7)
91.53(7)
Y-C11
Y-C22
Y‚‚‚C2
Y‚‚‚C7
[(9-t-BuNSiMe2-fluorenyl)YH(THF)2]2 (8)
Y-C1
Y-N1
Y-O1
Y-O2
2.541(9)
2.196(8)
2.404(5)
2.351(7)
Y-H
2.333
C1-Y-N1
O1-Y-N1
O1-Y-O2
O1-Y-C1
O2-Y-N1
69.9(3)
157.1(2)
83.1(2)
O2-Y-C1
H-Y-H′
O1-Y-H
O1-Y-H′
127.4(3)
78.3
Y-H′
2.396
Y‚‚‚C13
Y‚‚‚C22
Y‚‚‚Y′
3.019(8)
3.415(9)
3.663(3)
82.7
89.3(3)
92.8
101.7(3)
Figure 1. Crystal structure of [(o-Me3Si-benzyl)4La-]-
[Li+‚(THF)4] (6). All hydrogens (except the benzylic H’s)
have been omitted for clarity.
Figure 2. Crystal structure of (o-Me2N-C6H4CH2)3Y
(5-Y). All hydrogens (except the benzylic H’s) have been
omitted for clarity. The complex (o-Me2N-C6H4CH2)3La
(5-La) is isostructural but shows more extensive multi-
hapto bonding to the metal (shorter aryl-metal interac-
tions) than 5-Y.
in solution as well. An unusual high-field shift is
observed for the aromatic proton in ortho-position of the
benzylic CH2 substituent (δ ) 4.76 ppm). Partially
negatively charged benzyl ligands normally show the
largest high-field shift for the para-H.20 The extraordi-
nary high-field shift for the ortho-H in (Me3Si-C6H4-
CH2)4La- can be explained by the shielding ASIS effect21
of the neighboring aromatic ring. In the S4-symmetric
structure of (Me3Si-C6H4CH2)4La-, all ortho-H’s are
positioned over an aromatic ring: the crystal structure
shows H‚‚‚Arylcenter distances in the range 2.49-2.62 Å
and C-H‚‚‚Arylcenter angles varying from 165° to 167°.
Reaction of 3 equiv of o-Me2N-C6H4CH2K with YCl3
in THF yielded (o-Me2N-C6H4CH2)3Y (5-Y) in the form
of light yellow crystalline plates (59% yield). The La
analogue (5-La) could be prepared by a similar proce-
dure and was obtained in the form of light orange
crystals (41% yield).
The crystal structure of 5-Y (Figure 2, Table 1) shows
a paddle-wheel structure in which Y displays a pris-
matic coordination. The bidentate C,N ligand has a bite
angle of 68.8(1)°. The 3-fold symmetry is broken by the
“upside down” coordination of one of the benzyl ligands.
This is probably due to steric hindrance between the
Me2N substituents as can be deduced from the bond
angles between the ligands. The C-Y-C and C-Y-N
bond angles within the triangular faces of the prism
(20) Feil, F.; Harder, S. Organometallics 2000, 19, 5010.
(21) Haigh, C. W.; Mallion, R. B. Prog. Nucl. Magn. Reson. Spectrosc.
1980, 13, 303.

376 Organometallics, Vol. 24, No. 3, 2005
Harder
(N1-C21-N3 and C11-N2-C31) are close to 90°,
whereas the N1-Y-N3 angle of 97.7(1)° is significantly
larger. The bond distances between Y and the benzylic
carbons vary between 2.458(3) and 2.487(3) Å and are
significantly shorter than the distances between Y and
Cipso or Cortho (range: 2.754(3)-2.919(3) Å).
The crystal structure of 5-La (Figure 2, Table 1) is
isostructural to that of the Y-analogue. The N-metal
bonds in 5-La are on average 0.133 Å longer than those
in 5-Y. This value corresponds well with the 0.132 Å
difference in the ionic radii of both metals (the ionic radii
for Y3+ and La3+ are respectively 0.90 and 1.032 Å).2
The CH2-metal bonds in 5-La, however, are on average
0.173 Å longer than those in 5-Y and thus longer than
expected. On the other hand, the distances of La to the
Cipso and Cortho atoms are on average only 0.078 Å longer
than the corresponding distances in 5-Y, therefore
shorter than expected. This demonstrates a more pro-
nounced multihapto bonding in 5-La. The average bite
angle of 64.7(1)° for the bidentate C,N ligand in 5-La is
smaller than in 5-Y, which is due to the longer metal-
ligand bond distances.
In both structures all hydrogen atoms could be located
and refined. This allows a comparison of the hybridiza-
tion at the benzylic carbons. The sums of the H-C-H
and C-C-H angles for the three different benzylic
carbons in 5-Y are 344(2)°, 344(2)°, and 345(2)°. Sum-
mation of these angles in 5-La gives 347(2)°, 347(2)°,
and 348(2)°. Both benzyl ligands show a hybridization
of the CH2 group between sp2 and sp3, and the benzylic
carbon in 5-La seems slightly more planar (although
standard deviations are high, the reproducibility of the
values within the complexes is good).
Figure 3. Crystal structure of (9-t-BuNSiMe2-fluorenyl)-
(o-Me2N-C6H4CH2)Y‚(THF) (7). All hydrogens (except the
benzylic H’s) have been omitted for clarity.
decomposition of the compounds. The remarkable sta-
bility of 5-Y and 5-La is likely due to the stabilizing
intramolecular coordination of the Me2N substituents.
Although the complexes 5-Y and 5-La are stabilized
by 3-fold intramolecular N-metal coordination, they are
still well-accessible and reactive precursors for the
syntheses of a variety of lanthanide complexes. For
example, it can be shown that the less reactive yttrium
complex 5-Y is able to deprotonate fluorenes and alkyl-
amines and could be an interesting precursor in the
synthesis of constrained geometry catalysts based on
lanthanides.23 Reaction with the ligand 9-t-BuN(H)-
SiMe2-fluorene in THF produced the heteroleptic ben-
zylyttrium complex 7 in the form of orange crystals in
85% yield (eq 1).
The homoleptic benzyl complex 5-Y is moderately
soluble in aromatic solvents, whereas 5-La is only
slightly soluble in these solvents (a similar solubility
was observed in THF). 1H and 13C NMR data show only
one set of signals for the o-Me2N-benzyl group, which
means either that the asymmetry of the crystal struc-
ture is not retained in solution or that the ligand-metal
coordination is dynamic on the NMR time scale. Cooling
of the solutions resulted in broadening of the signals,
but at -20 °C no splitting of the signals was observed
(precipitation of 5-Y from a diluted solutions in toluene
started at -20 °C).
1
The JC-H coupling constants in the benzylic CH2
groups are a sensitive probe for the hybridization of the
benzylic carbon atom.22 The coupling constant in 5-La
(142.8 Hz) is significantly higher than that in 5-Y (133.2
Hz), thus confirming the less pyramidal structure of the
benzylic carbon in 5-La as observed in the solid state.
This implies that the C-metal bond in 5-La is more
ionic than that in 5-Y.
The thermal stabilities of the complexes 5-Y and 5-La
are remarkable. Toluene solutions of these homoleptic
compounds can be stored at room temperature for over
four months with only negligible decomposition (ca. 3%
o-Me2N-toluene is observed). Solutions can also be
heated to 75 °C with only minor decomposition (ca. 5%
o-Me2N-toluene was observed after heating overnight).
Only overnight heating at 100 °C resulted in complete
The crystal structure (Figure 3, Table 1) shows an
allylic coordination mode for the fluorenyl ligand: the
(23) (a) Shapiro, P. J.; Bunel, E. E.; Schaefer, W. P.; Bercaw, J. E.
Organometallics 1990, 9, 867. (b) Mu, Y.; Piers, W. E.; MacQuarrie,
D. C.; Zawarotko, M. J.; Young, V. G., Jr. Organometallics 1996, 15,
2720. (c) Tian, S.; Arrendondo, V. M.; Stern, S. L.; Marks, T.
Organometallics 1999, 18, 2568. (d) Hultzsch, K. C.; Spaniol, T. P.;
Okuda, J. Angew. Chem. 1999, 111, 163; Angew. Chem., Int. Ed. 1999,
38, 227. (e) Hultzsch, K. C.; Voth, P.; Beckerle, K.; Spaniol, T. P.;
Okuda, J. Organometallics 2000, 19, 228. (f) Arndt, S.; Voth, P.;
Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 4690.
(22) Hoffmann, D.; Bauer, W.; Hampel; F.; van Eikema Hommes,
N. J. R.; Schleyer, P. von R.; Otto, P.; Pieper, U.; Stalke, D.; Wright,
D. S.; Snaith, R. J. Am. Chem. Soc. 1994, 116, 528.

Homoleptic Benzyllanthanide Complexes
Organometallics, Vol. 24, No. 3, 2005 377
bond distance Y-C10 (2.522(2) Å) is significantly shorter
than the distances to the neighboring C atoms (2.733-
(2) and 2.763(3) Å). The other two atoms in the five-
membered ring are too remote (>3.05 Å) to be consid-
ered bonding even though the fluorenyl ring substantially
bends toward the metal: the angle between the Si-C10
axis and the five-membered ring is 13.5(1)°. The bite
angle of the benzylic C,N ligand (67.58(9)°) is similar
to that in 5-Y. The benzylic CH2 group is pyramidal with
a hybridization close to sp3: the sum of the H-C-H
and C-C-H angles is 337.6°.
The heteroleptic benzylyttrium complex 7 is slightly
1
soluble in benzene at 20 °C. H and 13C NMR spectra
show one sharp signal for the diastereotopic Me sub-
stituents in the Me2Si bridge and one set of sharp
signals for the diastereotopic sides of the fluorenyl
group. Therefore the ligands bound to Y are in fast
exchange at room temperature (lower temperatures
could not be reached due to the insolubility of the
complex).
Figure 4. Crystal structure of [(9-t-BuNSiMe2-fluorenyl)-
YH‚(THF)2]2 (8). All hydrogens (except the hydrido H) have
been omitted for clarity.
set of two doublets and two triplets is observed for the
fluorenyl ring, while the Me2Si group is observed as a
broad singlet. Coalescence for the Me2Si signals is
reached at +40 °C (∆Gq ) 14.9 kcal mol-1).
The benzyl- and hydrido-lanthanide complexes (7 and
8) might be attractive species for numerous catalytic
processes. Their broader scope in catalysis is currently
under investigation.
The heteroleptic benzylyttrium complex 7 can easily
be converted in an yttrium-hydrido complex: hydroge-
nation of 7 with molecular hydrogen (10 bar) gave
crystals of the yttrium-hydrido complex 8 in 84%
isolated yield (eq 2). The crystals contain several
molecules of uncoordinated disordered THF molecules,
resulting in poor crystal quality. Nevertheless a struc-
ture could be determined and even tentative H atoms
were located in the difference Fourier map. The com-
pound crystallizes as a C2-symmetric dimer in which
the hydrogen atoms bridge the Y atoms more or less
symmetrically. The Y-H distances (2.33 and 2.39 Å) are
significantly longer than reported earlier (cf. 2.18(2) Å
for [(MeCp)2YH‚(THF)]2), however close to a Y-H bond
distance of 2.30(4) Å in a comparable constrained geom-
etry complex [Me4CpSi(Me)2CH2N(t-Bu)YH‚(THF)]2.24
The Y‚‚‚Y′ distance in 8 (3.663(3) Å) is comparable to
those in other dimeric yttrium-hydrides with con-
strained geometry ligands ([Me4CpSi(Me)2CH2N(t-Bu)-
YH‚(THF)]2: Y‚‚‚Y′ ) 3.7085(8) Å; [Me4CpSi(Me)2N-
(t-Bu)YH‚(THF)]2: Y‚‚‚Y′ ) 3.672(1) Å).23d,24 The ring
formed by Y and the hydride atoms is nearly planar,
with a small Y-H-Y′-H′ torsion angle of 5.3(1)°. The
fluorenyl ligand is coordinated to the Y in η1-fashion:
the neighboring carbon atoms are more than 3 Å away
from the metal and do not contribute significantly to
the bonding. However, the bite angle of the ansa-
fluorenyl/amide ligand (C1-Y-N ) 69.9(3)°) is similar
to that in 7. Complex 8 is only sparingly soluble at room
temperature in THF-d8. A sharp 1/2/1-triplet for the
hydride protons in the 1H NMR spectrum indicates that
the solid state dimer, in which the Y atoms are bridged
by two hydride protons, is also retained in solution. The
Conclusion
Neutral homoleptic tris(o-Me2N-benzyl)lanthanide
complexes can be prepared from the corresponding
benzylpotassium and LnCl3 and can be obtained in
crystalline purity in reasonable yields. Although these
compounds are stabilized by intramolecular Me2N‚‚‚Ln
coordination, the high deprotonating power of the benzyl
anion is useful in organolanthanide syntheses. Reaction
with C-H and N-H acidic ligands generates a hetero-
leptic benzyllanthanide compound, which can be con-
verted to a hydride.
Experimental Section
General Comments. All experiments were carried out
under argon using predried solvents and Schlenk techniques.
o-Me2N-benzylpotassium was prepared according to our earlier
reported procedure.25 The ligand 9-t-BuN(H)SiMe2-fluorene
was prepared according to literature26 with the exception that
the intermediate 9-ClSiMe2-fluorene was isolated and crystal-
lized before reaction with t-BuNH2. o-Me3Si-toluene was
prepared according to the literature.27 The metal salts YCl3‚
(THF)3.5 and LaCl3‚(THF)3.5 were also prepared according to
literature procedures, and the THF content was determined
by C,H analysis.28,29
Synthesis of o-Me3Si-benzylpotassium. n-Butyllithium
(2.1 M, 10.0 mL, 21.0 mmol) was added dropwise to a solution
of o-Me3Si-toluene (4.28 g; 26.1 mmol) in THF (25 mL) and
potassium 3-methyl-3-pentanolate (3.13 g, 22.4 mmol) pre-
cooled at -70 °C (the BuLi was added such that drops float
1
coupling constant JY-H of 29.3 Hz is in the 27.0-35.3
Hz range reported for other dimeric yttrium hydrido
complexes.23e Broad signals for the diastereotopic sides
of the fluorenyl group and two broadened singlets for
the diastereotopic Me groups in the Me2Si bridge are
observed in the 1H NMR spectrum of 8. This means that
at room temperature ligand exchange is slow. Heating
the sample to +60 °C results in fast exchange of the
diastereopic groups and coalescence of their signals: a
(25) Harder, S.; Feil, F. Organometallics 2002, 21, 2268.
(26) Alt, H. G.; Fo¨ttinger, K.; Milius, W. J. Organomet. Chem. 1999,
572, 21.
(27) Bazant, V.; Chvalovsky, V.; Rathousky, J. Organosilicon Com-
pounds; Academic Press: New York and London, 1965; Part 2(1), pp
382 and 383.
(28) Bruin, P.; Booij, J. H.; Teuben, J. H.; Oskam, A. J. Organomet.
Chem. 1988, 350, 17.
(29) Den Haan, K. H.; de Boer, J. L.; Teuben, J. H.; Spek, A. L.;
Kojic-Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726.
(24) Trifonov, A. A.; Spaniol, T. P.; Okuda, J. Organometallics 2001,
20, 4869.

378 Organometallics, Vol. 24, No. 3, 2005
Table 2. Crystal Data for the Compounds 5-Y, 5-La, 6, 7, and 8
Harder
5-Y
5-La
6
7
8
formula
C
27H36N3Y1
C27H36N3La1
[C40H60La1Si4]-
[C16H32Li1O4]
1087.51
0.5 × 0.5 × 0.4
triclinic
P1h
C32H43N2O1Si1Y1C6H6
C54H80N2O4Si2Y2-
[C4H8O]4
1343.65
0.5 × 0.4 × 0.2
orthorhombic
P212121
13.341(2)
21.961(5)
12.815(3)
90
MW
491.50
0.6 × 0.6 × 0.1
monoclinic
P21/c
16.810(3)
9.4486(9)
16.615(3)
90
541.50
0.5 × 0.4 × 0.3
monoclinic
P21/c
17.102(3)
9.4466(9)
16.649(2)
90
666.79
0.5 × 0.5 × 0.4
monoclinic
P21/c
11.739(1)
15.907(1)
18.4633(2)
90
size (mm3)
cryst syst
space group
a (Å)
12.162(1)
12.485(1)
20.850(2)
97.414(9)
105.217(8)
92.930(9)
3017.4(6)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
111.163(13)
90
110.079(9)
90
98.877(9)
90
90
90
2461.0(7)
4
2526.3(6)
4
3406.5(6)
4
3755(1)
2
F (g.cm-3
)
1.327
1.424
1.197
1.300
1.188
µ(Mo KR) (mm-1
T (°C)
)
2.388
1.709
0.827
1.779
1.601
-120
-120
-90
-120
-120
θ(max)
26.1
26.5
25.4
27.0
25.0
no. of unique reflns, Rint
4867, 0.033
3582
5244, 0.015
4488
11 167, 0.044
8505
7470, 0.031
5057
4304, 0.068
2823
no. of obsvd reflns (I > 2σ(I))
no. of params
R1
424
424
609
584
295
0.034
0.022
0.049
0.036
0.068
wR2
GOF
0.086
0.053
0.143
0.085
0.193
1.04
1.06
0.99
1.01
0.97
max./min. resd (e Å-3
)
-0.62/0.62
-0.64/0.63
-1.40/1.37
-0.40/0.39
-0.74/0.74
along the cooled walls of the Schlenk tube). The solution, which
immediately turned dark red, was stirred at -70 °C for ca. 1
h. Subsequently, the solution was slowly warmed to 0 °C and
the solvents were removed under vacuum, allowing the tem-
perature to rise to 20 °C in the later stages. The sticky dark
red paste was washed five times with 40 mL portions of
hexane. The resulting extremely air-sensitive dark red powder
was dried under vacuum (40 °C, 1 Torr, 30 min). Yield: 5.10
g (96%). 1H NMR (C6D6/THF-d8, 250 MHz, 20 °C): 0.40 (s, 9H,
Me3Si); 2.97 (s, 1H, CH2); 3.10 (s, 1H, CH2); 5.21 (t, 6.3 Hz,
1H, aryl); 5.80 (d, 7.5 Hz, 1H, aryl); 6.35 (t, 7.0 Hz, 1H, aryl);
6.70 (d, 7.0 Hz, 1H, aryl). 13C NMR (C6D6/THF-d8, 62.86 MHz,
20 °C): -1.7 (Me3Si); 62.9 (CH2); 96.7; 108.4; 110.2; 112.3;
138.6; 152.9 (aromatics).
Attempted Synthesis of (o-Me3Si-benzyl)3La (4-La). A
precooled (-50 °C) solution of o-Me3Si-benzylpotassium (1.08
g, 5.34 mmol) in THF (10 mL) was added at once to a
suspension of LaCl3‚(THF)3.5 (0.89 g, 1.78 mmol) in 10 mL of
THF of the same temperature. After slowly warming to 20 °C
and stirring for 2 h a dark red solution with a little precipitate
resulted. All THF was removed under vacuum, and 15 mL of
hexane was added. The resulting suspension was centrifuged
and the clear red mother liquor was slowly cooled to -30 °C,
resulting in the precipitation of a crop of large dark red
crystalline blocks. Concentration of the mother liquor resulted
in a second crop (combined yields: 0.46 g). X-ray diffraction
shows the composition: [(o-Me3Si-benzyl)4La-][Li+‚(THF)4] (6;
yield based on o-Me3Si-C6H4CH2K: 24%). 1H NMR (THF-d8,
250 MHz, 20 °C): 0.29 (s, 9H, Me3Si); 1.65 (sb, 2H, CH2); 1.76
(m, 16 H, THF); 3.64 (m, 16H, THF); 4.76 (d, 7.5 Hz, 1H, aryl);
6.15 (t, 7.2 Hz, 1H, aryl); 6.80 (t, 7.5 Hz, 1H, aryl); 7.02 (d, 7.0
Hz, 1H, aryl). 13C NMR (C6D6/THF-d8, 62.86 MHz, 20 °C): 0.3
(Me3Si); 26.3 (THF); 68.1 (THF); 65.9 (CH2); 112.3; 115.9;
127.5; 134.2; 137.9; 155.4 (aromatics). Anal. Calcd for C56H92-
LaLiO4Si4: C 61.85; H 8.53. Found: C 61.31; H 8.21.
times with 15 mL portions of a THF/hexane mixture (ratio 1:2).
The combined THF/hexane layers were concentrated to ca. 25
mL and slowly cooled to -30 °C. The product crystallized in
the form of light yellow plates. Concentration of the mother
liquor resulted in a second crop (combined yields: 2.88 g; 59%).
1H NMR (C6D6, 250 MHz, 20 °C): 1.63 (s, 2H, CH2); 2.09 (s,
6H, Me2N); 6.66 (t, 7.0 Hz, 1H, aryl); 6.82 (d, 7.5 Hz, 1H, aryl);
6.98 (t, 6.8 Hz, 1H, aryl); 7.06 (d, 6.5 Hz, 1H, aryl). 13C NMR
(C6D6, 62.86 MHz, 20 °C): 44.0 (Me2N); 45.5 (d, 1J(13C-89Y) )
21.0 Hz, CH2); 117.5; 120.3; 127.6; 129.0; 135.4; 142.1 (aromat-
ics). Anal. Calcd for C27H36N3Y: C 65.98; H 7.38. Found: C
65.68; H 7.28.
Synthesis of (o-Me2N-benzyl)3La (5-La). A precooled
(-50 °C) solution of o-Me2N-benzylpotassium (4.05 g, 30.0
mmol) in THF (30 mL) was added at once to a suspension of
LaCl3‚(THF)3.5 (4.98 g, 10.0 mmol) in THF (30 mL) of the same
temperature. The red color of the potassium precursor disap-
peared immediately and a yellow-green suspension resulted.
After warming to 20 °C and stirring for 1 h, the suspension
was centrifugated and the mother liquor was isolated. The
precipitate was extracted two times with 10 mL THF portions,
and the combined THF fractions were concentrated to ca. 20
mL. Hexane (ca. 5 mL) was added, and the solution was slowly
cooled to -30 °C: The product crystallized in the form of light
orange plates. Concentration of the mother liquor resulted in
a second crop (combined yields: 2.22 g; 41%). 1H NMR
(toluene-d8, 250 MHz, 45 °C): 1.76 (sb, 2H, CH2); 2.10 (s, 6H,
Me2N); 6.43 (t, 7.0 Hz, 1H, aryl); 6.75-6.87 (m, 3H, aryl). 13
C
NMR (toluene-d8, 62.86 MHz, 45 °C): 41.6 (Me2N); 57.8 (CH2);
115.0; 121.9; 127.5; 127.7; 132.0; 142.4 (aromatics). Anal. Calcd
for C27H36N3La: C 59.89; H 6.70. Found: C 59.72; H 6.48.
Synthesis of (9-t-BuNSiMe2-fluorenyl)(o-Me2N-benzyl)-
Y‚(THF) (7). A solution of (o-Me2N-benzyl)3Y (2.34 g, 4.76
mmol) and 9-t-BuN(H)SiMe2-fluorene (1.42 g, 4.76 mmol) in
THF (20 mL) was stirred for 2 h at 68 °C. Subsequent removal
of all solvent under vacuum gave an orange oil, which solidified
after adding hexane. The solid was washed twice with 10 mL
portions of hexane and then dried under vacuum (0.01 Torr,
70 °C, 5 min). The orange powder was then dissolved in 20
mL of hot benzene. Slowly cooling to +5 °C gave large orange
crystals of the product, which crystallized with one C6H6
molecule (yield: 2.70 g, 85%). 1H NMR (C6D6, 250 MHz, 50
°C): 0.86 (s, 6H, Me2Si); 0.90 (m, 4H, THF); 1.32 (s, 6H, NMe2);
1.45 (s, 9H, tBu); 1.68 (d, 2J(1H-89Y) ) 1.2 Hz, 2H, CH2); 2.87
(m, 4H, THF); 6.21 (d, 7.5 Hz, 1H, aryl); 6.31 (t, 7.5 Hz, 1H,
Synthesis of (o-Me2N-benzyl)3Y (5-Y). A precooled (-50
°C) solution of o-Me2N-benzylpotassium (4.05 g, 30.0 mmol)
in THF (20 mL) was added at once to a suspension of YCl3‚
(THF)3.5 (4.48 g, 10.0 mmol) in THF (20 mL) of the same
temperature. The red color of the potassium precursor disap-
peared immediately, and a yellow-brown suspension resulted.
After warming to 20 °C and stirring for 1 h, the suspension
was concentrated to about one-third of its original volume and
25 mL of hexane was added. After centrifugation, the mother
liquor was isolated and the precipitate was extracted three

Homoleptic Benzyllanthanide Complexes
Organometallics, Vol. 24, No. 3, 2005 379
aryl); 6.71 (t, 7.2 Hz, 1H, aryl); 6.89 (d, 7.0 Hz, 1H, aryl); 7.22
(t, 7.8 Hz, 2H, fluorenyl); 7.44 (t, 7.9 Hz, 2H, fluorenyl); 8.16
(d, 7.5 Hz, 2H, fluorenyl); 8.25 (d, 7.5 Hz, 2H, fluorenyl). 13C
NMR (C6D6, 62.86 MHz, 50 °C): 6.1 (Me2Si); 25.0 (THF); 36.2
(tBu); 42.5 (Me2N); 46.8 (d, 1J(13C-89Y) ) 31.5 Hz, CH2); 54.4
(tBu); 71.5 (THF); 89.9; 117.8; 118.9; 119.5; 199.7; 121.2; 125.1;
127.3; 129.3; 130.9; 139.7; 140.9; 144.7 (aromatics). Anal. Calcd
for C32H43N2OSiY: C 65.29; H 7.36. Found: C 65.23; H 7.12.
Synthesis of [(9-t-BuNSiMe2-fluorenyl)YH‚(THF)2]2 (8).
A solution of (9-t-BuNSiMe2-fluorenyl)(o-Me2N-benzyl)Y‚(THF)
(7) (1.24 g, 2.08 mmol) in 25 mL of benzene was stirred for 7
h in an autoclave under H2-pressure (10 bar) at 70 °C. After
the hydrogenolysis, all solvent was removed and the yellow-
orange solid residue was washed with 15 mL of hexane and
subsequently dried under vacuum (0.01 Torr, 50 °C, 10 min).
The yellow-orange powder was dissolved in 8 mL of hot THF,
and the solution was slowly cooled to -30 °C. The product
crystallized in the form of large yellow plates, which were
crushed and subjected to vacuum (0.01 Torr, 10 min) to remove
THF enclosed in the crystal lattice (yield: 0.92 g, 84%). 1H
NMR (THF-d8, 250 MHz, 60 °C): 0.35 (sb, 6H, Me2Si); 1.57
(s, 9H, tBu); 1.75 (m, 8H, THF); 3.64 (m, 8H, THF); 6.36 (t,
1J(1H-89Y) ) 29.3 Hz, Y-H); 6.85 (t, 7.2 Hz, 2H, fluorenyl);
7.13 (t, 7.2 Hz, 2H, fluorenyl); 7.83 (d, 7.5 Hz, 2H, fluorenyl);
7.95 (d, 7.5 Hz, 2H, fluorenyl). 13C NMR data could not be
recorded due to the low solubility of the complex and broad
signals as a consequence of dynamic processes. Anal. Calcd
for C54H80N2O4Si2Y2: C 61.47; H 7.64. Found: C 61.82; H 7.83.
Crystal Structures. Crystal diffraction data were mea-
sured on an Enraf-Nonius CAD4 diffractometer (crystal data
are given in Table 2). All crystal structures were solved with
DIRDIF30 and refined with SHELXL-97.31 In all cases absorp-
tion correction was applied using the psi-scans method incor-
porated in PLATON,32 which was also used for geometry
calculations and graphics. For the structures 5-Y, 5-La, and
7, all hydrogen atoms have been observed and were refined
isotropically. All hydrogens in 6 and those in 8 (except for the
hydride hydrogen) have been calculated and were refined in
a riding mode.
The crystal structure of 6 contains a Li+‚(THF)4 ion in which
one of the THF ligands shows severe ring-puckering disorder,
which could not be resolved and was refined with large
anisotropical displacement parameters. The maximum and
minimum residual electron densities are in the area of the
disordered THF molecule.
Crystals of 8 are extremely brittle and contain large voids,
which are filled with disordered noncoordinating THF mol-
ecules (as confirmed by NMR analysis). The disorder of these
THF molecules could not be solved and was therefore treated
with the bypass method using the SQUEEZE procedure33
incorporated in PLATON. The asymmetric unit contained ca.
3.5 molecules of disordered THF (electron count ) 109).
Acknowledgment. The author is grateful to the
DFG for supporting this research within the AM2-
network (a German-Canadese cooperation on polymer-
ization chemistry).
Supporting Information Available: All crystallographic
information for 5-Y, 5-La, 6, 7, and 8 (bond distances and
angles, crystal data, refinement details, ORTEP plots) is
available free of charge via the Internet at http://pubs.acs.org.
OM049327P
(31) Sheldrick, G. M SHELXL-97, Programs for the Determination
of Crystal Structures; Universita¨t Go¨ttingen: Germany, 1997.
(32) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2000.
(33) van der Sluis, P. A.; Spek, A. L. Acta Crystallogr. 1990, A46,
194.
(30) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
de Gelder, R.; Israel, R. The DIRDIF Program System; Crystallography
Laboratory, University of Nijmegen: The Netherlands, 1994.