Selective assembly of trinuclear rare-earth alkyl hydrido clusters supported by amidopyridinate ligands

Article abstract of DOI:10.1021/om800364b

The reactions of the bis(alkyl) complexes [Ap*Ln-(CH ₂SiMe₃)₂(thf)] (Ln = Y, Lu) with both PhSiH₃ and H₂ result in selective assembly of the novel trinuclear rare-earth alkyl hydrido clusters [(Ap*Ln)

Full text of DOI:10.1021/om800364b

Organometallics 2008, 27, 2905–2907
2905
Selective Assembly of Trinuclear Rare-Earth Alkyl Hydrido
Clusters Supported by Amidopyridinate Ligands
Dmitrii M. Lyubov,† Christian Do¨ring,‡ Georgii K. Fukin,† Anton V. Cherkasov,†
Andrei S. Shavyrin,† Rhett Kempe,*,‡ and Alexander A. Trifonov*,†
G.A. RazuVaeV Institute of Organometallic Chemistry of the Russian Academy of SciencesTropinina 49,
GSP-445, 603950 Nizhny NoVgorod, Russia, and Lehrstuhl Anorganische Chemie II, UniVersita¨t Bayreuth,
95440 Bayreuth, Germany
ReceiVed April 25, 2008
Scheme 1. Synthesis of 1 and 2
Summary: The reactions of the bis(alkyl) complexes [Ap*Ln-
(CH2SiMe3)2(thf)] (Ln ) Y, Lu) with both PhSiH3 and H2 result
in selectiVe assembly of the noVel trinuclear rare-earth alkyl
hydrido clusters [(Ap*Ln)3(µ2-H)3(µ3-H)2(CH2SiMe3)(thf)2]. Both
cluster compounds are single-component ethylene polymerization
catalysts.
Rare-earth-metal hydrides possess an intriguing variety of
unique structural and chemical properties.1 The rapid develop-
ment of this area, stimulated by the promising catalytic activity
of hydrido complexes, has resulted in considerable contributions
to organolanthanide chemistry.2 Until recently rare-earth-metal
hydrides were represented exclusively by sandwich-1 and half-
sandwich-type (“constrained geometry”)3 monohydride com-
plexes, and very few classes of their non-cyclopentadienyl
analogues are known.4 Assembly of the anionic trinuclear
tetrahydride lanthanide species {[Cp2LnH]3H}{Li(THF)4} was
reported by Evans in the early 1980s.1b–d The first “mono(cy-
clopentadienyl) dihydrido” complexes were reported in 2001,
and their stoichiometric and catalytic chemistry was developed
by Hou and co-workers.5 The synthesis of rare-earth polyhydrido
species in coordination environments alternative to that of
cyclopentadienyl still remains a challenge.4 Sterically demanding
amidopyridinato ligands6 were successfully used as a suitable
coordination environment for the stabilization of monomeric
lanthanide species, and our work has been aimed at the synthesis
of related polyhydrido complexes. Herein we report on the
selective formation, structure, and properties of trinuclear rare-
earth alkyl hydrido clusters.
Bulky (2,6-diisopropylphenyl)[6-(2,4,6-triisopropylphenyl)py-
ridin-2-yl]amine (Ap*-H) was used as the ligand precursor for
the preparation of the aminopyridinato dichloride, dialkyl, and
alkyl hydrido complexes of yttrium and lutetium. Reactions of
anhydrous LnCl3 (Ln ) Y, Lu) with an equimolar amount of
Ap*Li(Et2O)6b in THF at 20 °C afforded the ate complexes
[Ap*LnCl(thf)(µ-Cl)2Li(thf)2] (Ln ) Y (1), Lu (2)) (Scheme
1), which were isolated after recrystallization from THF-hexane
mixtures as pale yellow crystals in 78 and 85% yields,
* To whom correspondence should be addressed. A.A.T.: e-mail,
trif@iomc.ras.ru; fax, (+7)8314621497. R.K.: e-mail, kempe@
uni-bayreuth.de; fax, (+49)921552157.
† G.A. Razuvaev Institute of Organometallic Chemistry of the Russian
Academy of Sciences.
‡ Universita¨t Bayreuth.
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10.1021/om800364b CCC: $40.75
2008 American Chemical Society
Publication on Web 06/10/2008

2906 Organometallics, Vol. 27, No. 13, 2008
Communications
Scheme 3. Synthesis of 5 and 6
Scheme 2. Synthesis of 3 and 4
respectively. Complex 1 has been characterized by an X-ray
diffraction study, which revealed its monomeric structure (see
the Supporting Information).
Alkylation of complexes 1 and 2 with 2 equiv of LiCH2SiMe3
in hexane at 0 °C allowed synthesis of the salt-free dialkyl
complexes [Ap*Ln(CH2SiMe3)2(thf)] (Ln ) Y (3), Lu (4)),
which were obtained after recrystallization from pentane (3) or
hexane (4) at -20 °C in 68 and 75% yields, respectively
(Scheme 2). Complexes 36c and 4 were also synthesized through
alkane elimination from trialkyl complexes and parent ami-
nopyridine in hexane at 0 °C.
Crystallization of 3 and 4 by slow cooling of their concen-
trated pentane or hexane solutions to -20 °C resulted in single
crystals of solvates containing one molecule of solvent per one
molecule of complex. X-ray crystal structure investigations have
revealed that 3 and 4 are isostructural monomeric complexes
(Figure 1). The coordination sphere of the metal atom consists
of two nitrogen atoms of the bidentate amidopyridinate ligand,
two carbon atoms of the alkyl groups, and one oxygen atom of
the THF molecule, resulting in a formal coordination number
of 5.
The Y-C bond lengths in complex 3 are slightly longer
compared to the appropriate distances in five-coordinated dialkyl
yttrium compounds7a–c and are very close to the values reported
for a related five-coordinated complex supported by a bulky
amidinate ligand (2.374(4), 2.384(4) Å).7d In complex 4, which
is a rather rare example of a five-coordinated dialkyl lutetium
complex, the Lu-C bond lengths are close to the distances
previously reported for an analogue containing an anilido-
pyridine-imine ligand (2.329(6), 2.349(6) Å).7e Complex 4,
despite the low coordination number of its central metal atom,
is surprisingly stable at room temperature in C6D6 solution: no
evidence of decomposition has been observed over 1 month.
The stability of complex 3 is somewhat lower: under similar
conditions over 1 week, ∼10% of the compound was decom-
1
posed. In the H NMR spectrum of complex 4 at 20 °C the
hydrogen atoms of methylene groups attached to the lutetium
atom appear as a singlet at -0.63 ppm; in the 13C{1H} NMR
spectrum the appropriate carbons give rise to a singlet at 46.1
ppm.
The most common synthetic route to lanthanide hydrido
complexes is σ-bond metathesis reaction of parent alkyls under
treatment with dihydrogen2c,f or phenylsilane.8 Hou and co-
workers have demonstrated that hydrogenolysis of the cyclo-
pentadienyl-supported dialkyl complexes Cp′Ln(CH2SiMe3)2-
(thf) (Cp′ ) C5Me4SiMe3, Ln ) Sc, Y, Gd, Dy, Ho, Er, Tm)
affords the tetranuclear polyhydrido clusters [Cp′Ln(µ-
H)2]4(thf)n, while the reaction with PhSiH3 in the case of
lutetium results in the formation of the dimeric alkyl hydrido
complex [Cp′Lu(µ-H)(CH2SiMe3(thf)]2.5 We have found that
the reactions of 3 and 4 with both PhSiH3 (1:2 molar ratio, 0 °C)
and H2 (5 atm, 15 °C, 24 h) smoothly occur in hexane under
the aforementioned conditions and result in formation of the
unusual trinuclear alkyl hydrido clusters [(Ap*Ln)3(µ2-H)3(µ3-
H)2(CH2SiMe3)(thf)2] (Ln ) Y (5), Lu (6)), which were isolated
after recrystallization from hexane at -20 ° C in 58 and 64%
yields, respectively (Scheme 3). Surprisingly, all attempts to
remove the remaining alkyl group and to obtain polyhydrido
clusters consisting of Ap*LnH2 units failed: the use of a 10-
fold molar excess of PhSiH3 or an increase in the reaction time
with H2 afforded only complexes 5 and 6. Until recently very
few examples of dimeric alkyl hydrido rare -earth complexes
have been described,5c,8,9 and to the best of our knowledge
complexes 5 and 6 present the first examples of alkyl hydrido
clusters.
Complexes 5 and 6 crystallize from hexane as solvates with
one molecule of the solvent per unit. Exposure of complexes 5
and 6 at room temperature to dynamic vacuum (1 h) allowed
us to remove hexane and to obtain nonsolvated compounds.
Complexes 5 and 6 are extremely air- and moisture-sensitive
crystalline solids; they are highly soluble in hexane and pentane.
Complexes 5 and 6 can be kept in the solid state or in C6D6
solutions under dry argon or in sealed evacuated tubes at 20 °C
for several weeks without decomposition. Clear yellow single-
crystal samples of 5 suitable for an X-ray crystal structure
determination were obtained by slowly cooling its hexane
Figure 1. ORTEP drawing of 3 and 4 with 30% thermal ellipsoids.
The Me groups of Me3Si and CH2 groups of THF are omitted.
Selected bond lengths (Å) and angles (deg): for 3, M-N(1) )
2.316(4), M-N(2) ) 2.415(4), M-C(1) ) 2.370(5), M-C(5) )
2.383(5), M-O ) 2.337(3), C-Ln-C ) 113.20(19), N-Ln-N
) 57.33(14); for 4, M-N(1) ) 2.272(2), M-N(2) ) 2.371(2),
M-C(1) ) 2.320(3), M-C(5) ) 2.332(3), M-O ) 2.2907(19),
C-Ln-C ) 113.10(9), N-Ln-N ) 58.44(7).
(8) Voskoboinikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin,
K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics
1997, 16, 4690–4700.
(9) (a) Voskoboinikov, A. Z.; Shestakova, A. K.; Beletskaya, I. P.
Organometallics 2001, 20, 2794–2801. (b) Stern, D.; Sabat, M.; Marks,
T. J. J. Am. Chem. Soc. 1990, 112, 9558–9575.

Communications
Organometallics, Vol. 27, No. 13, 2008 2907
hydrido ligands. Apparently the signal at 9.08 ppm is due to
µ2-hydrido ligands, while the signals at 12.25 and 12.37 ppm
correspond to the µ3-hydrido ligands situated in the apical
positions of the trigonal bipyramid Lu3(µ3-H)2. The signals of
the hydrido ligands of 6 are substantially shifted to low field
compared to the positions of respective signals of the reported
cyclopentadienyl polyhydrido clusters (8.81 ppm),5c which
corresponds to the tendency observed in the series of yttrium
hydrides supported by cyclopentadienyl, cyclopentadienylamido,
1
amidinate, and guanidinate ligands.4b In the H NMR spectra
of 5 the µ2-hydrido ligands appear as a triplet of doublets at
5.66 ppm with intensity corresponding to three protons. The
multiplicity of this signal results from the coupling of each
hydrido ligand with two neighboring yttrium nuclei (1JYH
)
20.8 Hz) and with the third yttrium atom (1JYH ) 5.8 Hz)
situated across the planar Y3H3 core. Unfortunately, the signals
corresponding to the µ3-hydrido ligands cannot be assigned
unambiguously, since they overlap with signals of aromatic
protons. Nevertheless, the existence of cross-peaks in the COSY
spectrum of 5 between the triplet of doublets at 5.66 ppm and
the multiplet between 6.9 and 7.3 ppm gives evidence of the
location of these signals in the area 6.9-7.3 ppm. The protons
of the methylene group attached to the metal atom are
nonequivalent in both 5 and 6 and appear in the 1H NMR spectra
at 293 K as a set of two doublets (at -1.10 and -0.03 ppm
Figure 2. ORTEP drawing of 5 with 30% thermal ellipsoids. The
i-Pr groups of Ap* and the CH2 groups of THF are omitted. Selected
bond lengths (Å) and angles (deg): Y(1A)-N(1A) ) 2.323(2),
Y(1A)-O(1A))2.3496(19),Y(1A)-N(2A))2.459(2),Y(1A)-Y(1B)
) 3.4408(4), Y(1A)-Y(1C) ) 3.5158(4), Y(1B)-N(1B) ) 2.307(2),
Y(1B)-O(1B))2.3510(19),Y(1B)-N(2B))2.477(2),Y(1B)-Y(1C)
) 3.5058(4), Y(1C)-N(1C) ) 2.331(2), Y(1C)-C(33C) ) 2.402(3),
Y(1C)-N(2C) ) 2.503(2), Y(1B)-Y(1A)-Y(1C) ) 60.512(8),
N(1A)-Y(1A)-N(2A) ) 56.70(8), N(1B)-Y(1B)-N(2B) )
56.64(7), N(1C)-Y(1C)-N(2C) ) 56.40(7).
(2JHH ) 9.0 Hz) for 5 and at -1.35 and -0.42 ppm (2JHH
)
9.7 Hz) for 6). Thus, the 1H NMR spectra of 5 and 6 prove that
the trimeric structures of these compounds are retained in
solutions in noncoordinating solvents.
Complexes 5 and 6 catalyze ethylene polymerization (20 °C,
ethylene pressure 0.5 atm) but are inactive in styrene poly
merization. The ethylene polymerization activity of complex 5
was found to be 560 g mmol-1 bar-1 h-1, but the catalyst was
deactivated in 3 h. The lutetium complex 6 was less active (168
g mmol-1 bar-1 h-1) but did not demonstrate loss of the reaction
rate over 1 day.
In summary, it was found that the reactions of dialkyl
complexes [Ap*Ln(CH2SiMe3)2(thf)] (Ln ) Y, Lu) with both
PhSiH3 and H2 result in selective assembly of the novel
trinuclear rare-earth alkyl hydrido clusters [(Ap*Ln)3(µ2-H)3(µ3-
H)2(CH2SiMe3)(thf)2]. The yttrium complex has been structur-
ally characterized. Both compounds show moderate activity in
ethylene polymerization. Further studies on the synthesis and
reactivity of this novel family of alkyl hydrido clusters are
currently in progress.
solution to -20 °C. X-ray single-crystal structure analysis has
shown that 5 adopts a trimeric structure (Figure 2), where three
Ap*Y fragments are bound by three µ2-H and two µ3-H ligands,
while the alkyl group remains terminal. The coordination sphere
of two yttrium atoms is determined by two nitrogen atoms of
Ap* ligands, four hydrido ligands, and the oxygen atom of the
coordinated THF molecule. In the coordination environment of
the third yttrium atom there is no THF molecule, but it is
covalently bound to the CH2SiMe3 group. The hexanuclear Y3H3
core is nearly planar (the maximum deviation from the Y3H3
plane is 0.132 Å), and the two remaining hydrogen ligands are
situated above and below this plane (1.023 and 1.089 Å). The
Y-(µ2-H) distances are 2.08-2.16 Å, whereas the Y-(µ3-H)
distances are in the range of 2.19-2.42 Å. The Y-Y distances
in complex 5 (3.5158(4), 3.4408(4), and 3.5058(4) Å) are
noticeably shorter compared to the related distances in dimeric
hydrides supported by bulky guanidinate ligands (3.6522(5)10
and 3.6825(5) Å4b). The Y-C bond in 5 (2.402(5) Å) is slightly
elongated compared to that in the starting dialkyl derivative 3.
The Ap* ligands appear as complex sets of signals in the 1H
NMR spectra (C7D8, -80 to -60 °C); however, the fact that
the para protons of the pyridyl fragments give rise to three
Acknowledgment. This work has been supported by the
Russian Foundation for Basic Research (Grant Nos. 08-03-
00391-a, 06-03-32728), the RFBR-DFG grant (Grant No.
06-03-04001), and the DFG Schwerpunktprogramm 1166
“Lanthanoidspezifische Funktionalita¨ten in Moleku¨l und
Material”. A.A.T. thanks the Russian Foundation for Science
Support and R.K. the Fonds der Chemischen Industrie.
3
3
signals (5, 6.67, 6.78, and 6.83 ppm (dd, JHH ) 8.4 Hz, JHH
3
) 7.2 Hz); 6:, 6.62, 6.73, and 6.78 ppm (dd, JHH ) 8.5 Hz,
3JHH ) 7.2 Hz)) reflects that their nonequivalence resulted from
the unsymmetric structures of 5 and 6. Three slightly broadened
singlets (9.08, 12.25, and 12.37 ppm) with an integral intensity
Supporting Information Available: Text, tables, figures, and
CIF files giving detailed information on the synthesis and charac-
terization of the lanthanide complexes described here and crystal-
lographic details of the structures determined by X-ray crystal
structure analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
ratio of 3:1:1 in the H NMR spectrum of 6 correspond to the
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Antipin, M. Yu.; Pelce´, O.; Schappacher, M.; Guillaume, S. M.; Trifonov,
A. A. Eur. J. Inorg. Chem. 2008, 12, 2090–2098.
OM800364B