Lewis acid-mediated β-hydride abstraction reactions of divalent M(C(SiHMe2)3)2THF2 (M = Ca, Yb)

Article abstract of DOI:10.1021/ja9070865

(Chemical Equation Presented) The divalent calcium and ytterbium compounds M(C(SiHMe₂)₃)₂THF₂ contain A-agostic SiH groups, as determined by spectroscopy and crystallography. Upon thermolysis, HC(SiHMe₂

Full text of DOI:10.1021/ja9070865

Published on Web 10/01/2009
Lewis Acid-Mediated ꢀ-Hydride Abstraction Reactions of Divalent
M(C(SiHMe2)3)2THF2 (M ) Ca, Yb)
KaKing Yan, Brianna M. Upton, Arkady Ellern, and Aaron D. Sadow*
Department of Chemistry and U.S. Department of Energy Ames Laboratory, Iowa State UniVersity,
Ames, Iowa 50011-3111
Received August 21, 2009; E-mail: sadow@iastate.edu
Transition-metal alkyl compounds containing ꢀ-hydrogen and at
Essentially, the IR spectra and structural data suggest that 1a and 1b
access the configuration required for ꢀ-elimination. Additionally, the
IR and variable-temperature NMR spectra and the NMR coupling
constants suggest that the agostic interactions in 1a and 1b are weaker
than in Y(C(SiHMe2)3)3 (less ground-state stabilization),3h and thus,
we expected ꢀ-elimination to be more facile.
least one empty orbital in an open cis coordination site are susceptible
to elimination. These general rules also apply to elimination reactions
in main-group and rare-earth organometallics. While ꢀ-eliminations
of alkyllithiums tend to require forcing conditions (130-150 °C in
hydrocarbon solvent), the heavier congeners react more rapidly.1
Dialkylmagnesium compounds containing ꢀ-hydrogen eliminate olefin
upon thermolysis, but very little is known about eliminations of the
heavier analogues.2 Likewise, coordinatively unsaturated organolan-
thanides react via ꢀ-elimination.3 For example, isobutylene extrusion
from Cp2ErCMe3(THF) is faciliated by the addition of LiCl, presum-
ably to open a coordination site,3e whereas the lutetium analogue
decomposes at 70-80 °C.3c The bridged dimers [Cp*OArLu]2(µ-H)
(µ-CH2CH2R) are coordinatively saturated and robust toward elim-
ination.3g Importantly, unsaturated organolanthanides are highly reac-
tive, even mediating C-C bond cleavage through ꢀ-Me eliminations.3d,f
In contrast, the tris(alkyl)yttrium compound Y(C(SiHMe2)3)3 does
not undergo ꢀ-elimination, even though it is (at least formally) low-
coordinate, contains nine ꢀ-SiH groups, and forms labile agostic
interactions.4 ꢀ-Eliminations, as well as other reactivity, might be
inhibited by the three large C(SiHMe2)3 ligands around the Y(III)
center (ionic radius 0.9 Å).5 To test this idea, we prepared
tris(dimethylsilyl)methyl calcium(II) and ytterbium(II) compounds
(1.00 and 1.02 Å radii, respectively)5 that might be more reactive.
The desired dialkyl compounds M(C(SiHMe2)3)2THF2 [1; M )
Ca (1a), Yb (1b)] were prepared in THF from MI2 and 2 equiv of
Figure 1. ORTEP diagram of Yb(C(SiHMe2)3)2THF2 (1b). Dashed bonds
illustrate short Yb1-Si1 and Yb1-H1s distances. Distances (Å): Yb-C1,
2.596(4); Yb-Si1, 3.181(2); Yb-H1s, 2.50(3). Bond angles (deg):
Yb-C1-Si1, 90.6(2); C1-Yb-C1, 131.5(2). Torsion angles (deg):
Yb1-C1-Si2-H2s, -46.93; Yb1-C1-Si3-H3s, -59.18°.
4
However, 1a and 1b are unchanged after weeks in solution at
room temperature under an inert atmosphere. Extended thermolysis
(1a, >120 h; 1b, 96 h; 393 K in benzene-d6) formed HC(SiHMe2)3
quantitatively (1H NMR spectroscopy); the ꢀ-elimination product
disilacyclobutane (see below) was not detected. Because 1a and
1b possess structural and electronic features consistent with
ꢀ-elimination but lack that reactivity, we searched for conditions
to facilitate eliminations. Cationic species could be more likely to
undergo ꢀ-elimination, given their enhanced reactivity for insertion
(the microscopic reverse of ꢀ-elimination).9
KC(SiHMe2)3 according to eq 1:
Compounds 1a and 1b are essentially isostructural (see Figure 1
for an ORTEP diagram and key structural data for 1b). The most
important structural features are consistent with ꢀ-agostic SiH
interactions, including short M-Si distances [Ca-Si(2), 3.216(2)
Å], small M-C-Si angles [Ca, 90.7(3)°], short M-H distances
[Ca-H2s, 2.53(6) Å], and planar M-C-Si-H four-membered
rings (the Ca1-C7-Si2-H2s torsion angle is 2.79°). The M-C
distances, however, are long [Ca-C7, 2.616(7) Å]. For comparison,
the Ca-C bond distances in Ca(CH(SiMe3)2)2(dioxane)2 and
Ca(C(SiMe3)2)2 are much shorter [2.373(4) and 2.459(9) Å,
respectively].6,7 Also, the Yb-C distances in the compound
Yb(C(SiMe3)3)2 [2.490(8) and 2.501(9) Å]8 are shorter by ∼0.1 Å
than those in 1b.
Alkyl group abstraction by strong Lewis acids is a well-known
route to cationic d0 metal alkyl compounds.9c Reactions of B(C6F5)3
and 1a or 1b in benzene-d6 produced a disilacyclobutane (eq 2)
previously obtained from (THF)2LiC(SiHMe2)3 and SiCl4:10
Three bands assigned to νSiH in the IR spectra are consistent
with three inequivalent SiH groups distinguished by crystallography,
including the ꢀ-agostic SiH structure (Ca, 1905 cm-1; Yb, 1890 cm-1;
KBr). The SiHMe2 groups appear to be equivalent as a result of rapid
exchange on the 1H NMR time scale, even at 185 K (in toluene-d8).
The disilacyclobutane is a head-to-tail dimer of the silene
Me2SidC(SiHMe2)2, suggesting ꢀ-elimination. However, the other
products are not consistent with that pathway. The organometallic
product shown in eq 2 [M ) Ca (2a), Yb (2b)] contains a -C(Si-
9
15110 J. AM. CHEM. SOC. 2009, 131, 15110–15111
10.1021/ja9070865 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
HMe2)3 ligand and HB(C6F5)3 counterion, revealing that the silene is
formed through borane-mediated ꢀ-H abstraction rather than ꢀ-hydride
elimination from a cationic (or zwitterionic) intermediate. SiH abstrac-
tion from silanes by strong Lewis acids is known to provide reactive
silyl cations,11,12 and the selectivity for SiH abstraction rather than
carbanion abstraction to give [(Me2HSi)3CB(C6F5)3]- may result from
steric hindrance.13 In contrast, no interaction between HC(SiHMe2)3
and B(C6F5)3 could be detected at room temperature in benzene-d6.
Likewise, Zn(C(SiHMe2)3)2 and B(C6F5)3 do not react at 65 °C in
benzene-d6 over 1 day.
hydridic to react with the much weaker Lewis acid BPh3. Interest-
ingly, reaction of BPh3 and 1b gave a mixture of Yb(C(SiHMe2)3)-
(HBPh3)(THF)n (4), Yb(HBPh3)2THF (5), and starting dialkyl 1b
in addition to silacyclobutane (eq 4):
Reaction of 2 equiv of BPh3 and 1b gave 5 and disilacyclobutane
(see the SI).
The different νBH of 2 (a, 2329 cm-1; b, 2308 cm-1) in the IR
The SiH groups in 1 and 2 are clearly hydridic on the basis of their
reactions with Lewis acids. Compounds 1 and 2 contain open
coordination sites on Lewis acidic metal centers and accessible
ꢀ-hydrogens that form agostic interactions. However, these alkyls are
clearly deactivated against ꢀ-elimination. Most likely, the M · · ·Si
interactions and delocalization of charge on the C(SiHMe2)3 ligand,
as evidenced by X-ray structures and IR spectroscopy, increase the
barrier to ꢀ-hydride elimination with respect to other pathways.
spectra suggest a M · · · HB interaction. For comparison, νBH in
14
[Cp*2ZrH][HB(C6F5)3] is 2364 cm-1
.
Three (2a) and two (2b)
bands are attributed to νSiH (2077, 2042, and 1957 cm-1 in 2a; 2074
and 1921 cm-1 in 2b).
Acknowledgment. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-AC02-07CH11358) for
financial support.
Supporting Information Available: Experimental section and X-ray
crystallographic data for 1a, 1b, 2a, 2b, 3a, 5, and the 1,3-disilacy-
clobutane (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. ORTEP diagram of YbC(SiHMe2)3THF2(µ-H)(µ-F-C6F4)2BC6F5
(2b). THF is drawn using the ball-and-stick representation for clarity. Dashed
bonds represent close contacts between Yb and Si or H. Distances (Å):
Yb1-C27, 2.593(2); Yb1-Si1, 3.1016(7); Yb1-Si2, 3.0925(7). Angles
(deg): Yb1-C27-Si1, 87.42(9); Yb1-C27-Si2, 87.22(9).
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This experiment clearly shows that B(C6F5)3-mediated H abstraction
from zwitterionic 2 is feasible. The product 3 contains two
HB(C6F5)3 ligands coordinated in the same κ3-H,F,F-tridentate
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