A tris(alkyl)yttrium compound containing six β-agostic Si-H interactions

Article abstract of DOI:10.1039/b818630h

The new homoleptic rare earth compound [Y(C(SiHMe₂) ₃)₃] (2) is prepared in 82% yield by salt metathesis of YCl₃ and 3 equivalents of [KC(SiHMe₂)₃] (1); two β-agostic Y?(H-Si) interactions

Full text of DOI:10.1039/b818630h

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A tris(alkyl)yttrium compound containing six b-agostic Si–H
interactionsw
KaKing Yan, Andrew V. Pawlikowski, Chris Ebert and Aaron D. Sadow*
Received (in Berkeley, CA, USA) 22nd October 2008, Accepted 20th November 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b818630h
The new homoleptic rare earth compound [Y(C(SiHMe2)3)3] (2)
is prepared in 82% yield by salt metathesis of YCl3 and 3
equivalents of [KC(SiHMe2)3] (1); two b-agostic Yꢀ ꢀ ꢀ(H–Si)
interactions are observed for each C(SiHMe2)3 ligand in 2,
giving six agostic interactions per yttrium(III) center.
coordinate metal alkyl compounds.11 Monomeric tris(alkyl)
2 contains two agostic SiH groups in each –C(SiHMe2)3 alkyl
ligand for a total of six interactions per yttrium(III) center.
Compound 2 does not contain alkali metal cations, and the
presence of LiCl significantly alters the spectroscopic properties
of 2 by disrupting the agostic interactions. Additionally, the NMR
chemical shift and coupling constant data for 2 suggest substantial
electron delocalization in each tris(dimethylsilyl)methyl ligand.
We found that reactions of the known lithium carbanion
[LiC(SiHMe2)3(THF)2]10a and YCl3 in benzene provided
the desired yttrium alkyl compound [Y(C(SiHMe2)3)3] in low
yield. In hopes that the potassium congener would be a more
powerful alkylating agent, we prepared the THF-free
potassium salt [KC(SiHMe2)3] (1) by reaction of HC(SiHMe2)3
and 1 equiv. of [KCH2Ph] (eqn (1)).
b-Agostic alkyl structures are intermediate between metal
hydrido(olefin) and alkyl ligands, and their characteristic infrared
(nCH B 2250–2800 cmꢁ1) and NMR (1JCH B 50–100 Hz)
spectroscopic features support this picture.1,2 More specifically,
in d0 agostic compounds, the interaction is better described by
delocalization of the M–Ca bonding electrons to include the
b-carbon.3 Several common features are observed in the
bonding of b-agostic alkyl groups: the delocalized interactions
are typically weak (o 10 kcal molꢁ1)2 and easily disrupted by
the coordination of stronger donor ligands such as tetrahydro-
furan.4 Usually only one metal–carbon(b) interaction occurs per
alkyl ligand. Additionally, only one alkyl ligand is agostic per
metal center, although large organo-f-element compounds and
bimetallic species may have multiple interactions.5 For example,
[Ln(CH(SiMe3)2)3] (Ln = Sc, Y, Lu, La, Ce) contains one
b-agostic C–Si per bis(trimethylsilyl)methyl ligand giving three
total.6
ð1Þ
Compound 1 is obtained as a spectroscopically pure red solid
after removing all volatile materials. The material, which is
sufficiently pure to be used in further syntheses, is typically a
gummy solid, but thermally stable, air- and moisture-sensitive
red needles are obtained by careful recrystallization from
toluene at ꢁ30 1C. The red color is unexpected, since the
alkyllithium [Li(C(SiHMe2)3(THF)2]10a is off-white, and
[KC(SiMe3)3]12a and [KC(SiHMe2)(SiMe3)2]12b are colorless.
The red color of [K(C(SiHMe2)3] is not quenched by addition
of substoichiometric quantities of t-BuOH in benzene, suggesting
that the color is not due to a minor impurity such as [KCH2Ph].
Compound 1 does not sublime upon heating to 170 1C at 0.1 torr.
In contrast to the low yielding reaction of YCl3 and
[LiC(SiHMe2)3(THF)2], a suspension of YCl3 and 3 equiv. of
[KC(SiHMe2)3] in benzene affords the homoleptic solvent-free
tris(alkyl)yttrium (2) in excellent yield (eqn (2)).
Multiple agostic interactions per metal center are more
common with amido (particularly silylamido) in comparison to
alkyl ligands. Early examples of bis-agostic disilylamide ligands
were observed in X-ray structures of Na[Ln(N(SiMe3)2)3]
(Ln = Eu, Yb) in which four methyl carbons have close contacts
with the lanthanide center and three carbons have close contacts
with sodium cations.7 Rare earth silylamido compounds with
b-hydrogens have multiple agostic interactions with a single
metal center; for example, [Eu(N(SiHMe2)tBu)3] contains three
mono-b-agostic ligands.8 Rare earth ansa-metallocenes such as
Me2Si(C5Me4)2YN(SiHMe2)2 exhibit a rare bis-b-SiH agostic
structure,9 but interestingly the bulkier Cp*2YN(SiHMe2)2 is
only mono-b-agostic.9c
Here we report the syntheses and spectroscopic charac-
terization of homoleptic potassium and yttrium compounds
KC(SiHMe2)3 (1) and [Y(C(SiHMe2)3)3] (2). Compound 2 is
the first example of a transition-metal compound containing
the bulky –C(SiHMe2)3 ligand,10 which is a smaller derivative
of the trisyl ligand –C(SiMe3)3 that has been used to isolate low
ð2Þ
The room temperature 1H NMR spectrum (benzene-d6) of
1
2 contained a multiplet at 3.85 ppm (Si–H, JSiH = 134 Hz)
3
and a doublet at 0.39 ppm (Si–CH3, JHH = 3.2 Hz). Two
absorptions at 2106 and 1845 cmꢁ1 (KBr) observed in the
infrared spectrum were attributed to SiH vibrations (2102 and
Iowa State University and Ames Laboratory, Ames, IA, USA.
E-mail: sadow@iastate.edu; Fax: +1 (0)515294 0105;
Tel: +1 (0)515 294 8069
w Electronic supplementary information (ESI) available: Experi-
mental procedures and characterization data for 1 and 2. See DOI:
10.1039/b818630h.
1844 cmꢁ1, THF). The IR data and low JSiH coupling
constant for 2 suggest agostic bonding involving the SiH
1
moieties.13
ꢂc
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656 | Chem. Commun., 2009, 656–658

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Chart 1 Possible structures of 2.
each ligand relates the two agostic SiHMe2 groups, and the
methyls in the non-agostic moiety are also mirror-related.
These data are consistent with either pyramidal C3v (A) or
planar C3h-symmetric (B) structures (Chart 1); we favor A
because compounds containing agostic interactions typically
exhibit distortions from VSEPR geometries and yttrium is
pyramidal in [Y(CH(SiMe3)2)3].1,2,6
Fig. 1 1H NMR spectra of 2 showing the splitting of Si-H resonances
into a 2 : 1 ratio as temperature is lowered.
Given the possibility that these interactions involve K+ ions in
‘ate’-type complexes,14 additional investigations were required.
The identity of 2 as a homoleptic, salt-free tris(alkyl) compound
is confirmed by elemental analysis and mass spectrometry
(the parent ion was observed at m/z = 656). ICP-MS indicated
that K+ and Clꢁ are present only in trace amounts.
The methyl resonances in the 13C{1H} NMR spectra were
used to model the exchange of agostic and non-agostic SiH
groups from 190 to 260 K because simulation of the 1H NMR
Low temperature NMR spectroscopy indicates that each
–C(SiHMe2)3 alkyl ligand interacts with the yttrium center
3
spectra were complicated by JHH. The peak-widths at half-
1
through two b-agostic interactions. H NMR spectra of 2 in
height (o1/2; e.g., 3.6 Hz at 190 K) for all three SiMe
resonances are identical within error, and the signals sharpen
to the same extent as the low temperature limit is approached.
gNMR17 was used to simulate the dynamic exchange of the
three methyl sites; in our model, all three sites were allowed to
exchange. Good correlation between simulated and experi-
mental spectra allowed the determination of rate constants for
the process. An Eyring plot provided activation parameters
(DHz = 13.4 ꢃ 0.1 kcal molꢁ1; DSz = 10.9 ꢃ 0.1 cal molꢁ1 K;
DGz = 11.3 ꢃ 0.2 kcal molꢁ1 at 190 K). The small positive
value for DSz is consistent with a dissociative mechanism
in which one or both b-agostic interactions are disrupted,
followed by –C(SiHMe2)3 rotation and re-formation of the
bis-b-agostic structure (Scheme 1). The small values for
the activation parameters suggest that exchange within one
–C(SiHMe2)3 group is not affected by the other two ligands
(i.e., the exchange processes in each ligand are not related).
The strengths of b-agostic interactions have been estimated
by calculations as o10 kcal molꢁ1 for dn (n a 0) complexes,2
and the interactions in d0 compounds are typically smaller
(e.g., the b-agostic ethyl in Me2YEt was calculated to be
2.4 kcal molꢁ1).18 Thus, the exchange process in 2 proceeds
with a small activation barrier. For comparison, the d2 com-
pounds Cp2M(Me2HSiCRCSiHMe2) (M = Ti, Zr) con-
tain one b-agostic and one non-agostic SiH that undergo
exchange; for this process, DGz values of 8.8 kcal molꢁ1 (Ti)
and 14 kcal molꢁ1 (Zr) at 190 K were measured.19 The small
toluene-d8 were recorded from 320 to 190 K (Fig. 1 shows the
SiH region). As the temperature was lowered, the SiH and
SiMe resonances were broadened to the coalescence point at
240 K. At 190 K, the 1H NMR spectrum was sufficiently
resolved for interpretation, and two SiMe2 and two SiH
resonances were detected. The downfield-shifted SiH at
4.71 ppm (1JSiH = 190 Hz, 3 H) corresponds to the terminal
SiH group, while the upfield chemical shift (3.40 ppm,
1JSiH = 108 Hz, 6 H) for the other SiH resonance is assigned
to the b-agostic Yꢀ ꢀ ꢀ(H–Si) in 2 based on the low coupling
constant. The 2 : 1 ratio corresponds to six non-classical
SiH groups and three terminal SiH moieties. At 190 K in
the 1H NMR spectrum, the two methyl resonances at 0.47
(36 H, coincident diastereotopic agostic SiMe2, assigned by a
1H COSY experiment) and 0.34 ppm (18 H) were too broad
for additional structural assignment (o1/2 = 14.9 Hz).
Therefore, variable temperature 13C{1H} NMR spectro-
scopy was needed to better characterize 2. Only one type of
yttrium-bonded carbon is detected as a doublet from ambient
conditions to low temperature (298 K: d 17.3, 1JYC = 9.8 Hz;
1
190 K: d 15.9, JYC = 9.9 Hz) in the 13C{1H} NMR
spectra (toluene-d8). Thus, the three –C(SiHMe2)3 ligands
are related by a C3-axis. A comparison with b-SiC agostic
1
[Y(CH(SiMe3)2)3] (d 50.0, JYC = 30.2 Hz)6c highlights the
unusually high field chemical shift and low JYC for the
1
1
yttrium-bound carbon in 2. The low JYC of 2 results from a
combination of electron delocalization in the b-agostic
alkyl15 and the electropositive Si substituents on carbon
(Bent’s rule).16
The SiMe2 region of the VT 13C{1H} NMR spectra is
also informative; as the temperature is cooled, the sharp signal
(d 3.96) broadens and begins to decoalesce at 240 K into three
separate SiMe resonances. The three 13C{1H} NMR SiMe
resonances were assigned using a 1H–13C HMQC experiment:
resonances at 4.39 and 2.91 correspond to the two diastereo-
topic methyl groups in the agostic SiHMe2 moieties, and the
third signal at 4.60 corresponds to the two methyl carbons in
the non-agostic SiHMe2. At all temperatures, all three
–C(SiHMe2)3 ligands are equivalent. A mirror plane bisecting
Scheme 1 Fischer projections illustrating two possible mechanisms
for exchange of agostic and non-agostic SiH groups.
ꢂc
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positive DSz measured for 2 seems more consistent with
exchange via dissociation of only one agostic interaction,
although direct comparisons with other d0fn systems con-
taining exchanging b-agostic SiH groups are not available.
The large DHz (relative to the bond strength calculated for
Me2YEt) likely results from the combined effect of the
increased donor ability of a more polarizable Si–H13 bond in
2 compared to the b-C–H in Me2YEt and the barrier to
rotation of a bulky ligand containing an agostic SiH group.
In fact, the relatively strong bis-b-agostic interactions in
2 are not disrupted by THF. Its IR spectra in a KBr pellet and
as a THF solution are very similar (vide supra), and the room
temperature 1JSiH in benzene-d6 and THF-d8 are identical. The
inert nature of 2 likely results from both the steric bulk of the
–C(SiHMe2)3 ligands and their stabilizing agostic interactions.
However, addition of LiCl to a THF-d8 solution of 2 results in
a color change from yellow to pale orange, and several spectral
features indicate a clear interaction. There is a 0.7 ppm
downfield change in the SiH chemical shift to 4.40 ppm, and
Prof. Victor S.-Y. Lin is thanked for use of his IR spectro-
meter and Prof. Robert J. Angelici is acknowledged for many
useful discussions.
Notes and references
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1
the value of JSiH (room temperature) increases from 130 to
162 Hz. The SiMe resonances are also greatly affected by the
presence of LiCl, shifting 0.32 ppm upfield to ꢁ0.07 in
comparison to 2. The IR spectrum in THF contains only
one nSiH at 2106 cmꢁ1, indicating that the agostic interactions
are broken by the addition of LiCl. The interaction of 2 and
LiCl persists upon evaporation of the volatile materials and
addition of benzene-d6. In contrast, no change in the IR or
NMR spectra of 2 is observed upon addition of KCl. The
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1
of organo-rare earth compounds. For example, the H NMR
spectra of [Lu(CH(SiMe3)2)3] and [Lu(CH(SiMe3)2)3(m-Cl)K]
are identical, so the presence and quantity of the KCl-adduct is
1
not easily determined.6b The diagnostic nSiH and JSiH in 2 are
clearly useful for detecting interactions between alkali metal
halides and rare earth compounds.
Additionally, compound 2 is robust and does not undergo
b-hydride elimination. When a solution of 2 is heated at 60 1C
in benzene-d6, no change is observed after 1 day. After two
days at 100 1C, HC(SiHMe2)3 is formed as the only observed
product with 58% of 2 remaining (as determined by 1H NMR
spectroscopy). Two features of 2 inhibit b-hydride elimination:
first, two ‘ancillary’ bulky C(SiHMe2)3 ligands create a
crowded yttrium center. Second, the product of elimination
would give a silene (Me2HSi)2CQSiMe2 that is not stabilized
by p-donation from a metal center.20
In conclusion, we have shown that it is possible to prepare
metal alkyl compounds containing many b-SiH groups (there
are nine in 2), even one with a coordinatively and electro-
nically unsaturated trivalent yttrium center. The interesting
spectroscopic properties of compound 2 described here make
it a promising new starting material for organo-rare earth
chemistry and catalysis.
We thank the U.S. DOE office of Basic Energy Sciences,
through the Catalysis Science Grant No. AL-03-380-011.
20 T. D. Tilley, in The Silicon–Heteroatom Bond, eds. S. Patai and
Z. Rappoport, Wiley, Chichester, 1991.
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This journal is The Royal Society of Chemistry 2009
658 | Chem. Commun., 2009, 656–658