Hydrogen-substituted osmium silylene complexes: Effect of charge localization on catalytic hydrosilation

Article abstract of DOI:10.1021/ja057494q

Addition of bulky primary silanes to the osmium benzyl compound, Cp*(ⁱPr₃P)OsCH₂Ph, afforded two neutral hydrogen-substituted silylene complexes via activation of two Si-H bonds. These species have been structurally charac

Full text of DOI:10.1021/ja057494q

Published on Web 12/15/2005
Hydrogen-Substituted Osmium Silylene Complexes: Effect of Charge
Localization on Catalytic Hydrosilation
Paul G. Hayes,† Chad Beddie,‡ Michael B. Hall,*,‡ Rory Waterman,† and T. Don Tilley*,†
Departments of Chemistry, UniVersity of California, Berkeley, California 94720-1460, and
Texas A&M UniVersity, College Station, Texas, 77843
Received November 2, 2005; E-mail: tdtilley@berkeley.edu; hall@mail.chem.tamu.edu
Scheme 1
Metal silylene complexes have been proposed as intermediates
in numerous catalytic reactions.1 While several routes to MdSi
complexes have been established,2 the direct generation of silylenes
from silanes (by oxidative addition of Si-H, followed by 1,2-
hydrogen migration to the metal) appears to be crucial to their
involvement in catalytic reactions.2c We recently reported evidence
for a new hydrosilation mechanism catalyzed by the cationic ruthe-
nium silylene complex [Cp*(iPr3P)(H)2RudSiHPh‚Et2O][B(C6F5)4].3
The proposed pathway involves activation of two Si-H bonds to
form a hydrogen-substituted silylene complex, followed by direct
addition of an sp2 Si-H bond to the alkene, H-migration from the
metal center, and finally, reductive elimination of the hydrosilation
product. To further our mechanistic understanding of this catalysis,
and to explore the influence of electronic charge on reactivity of
the Si-H group, neutral osmium silylene complexes were targeted.
The pathway to neutral osmium silylene complexes is outlined
in Scheme 1. The 16-electron complex Cp*(iPr3P)OsBr (1) reacted
with 0.5 equiv of Mg(CH2Ph)2(THF)2 to cleanly generate the corres-
ponding benzyl species Cp*(iPr3P)OsCH2Ph (2). Complex 2 is ther-
mally unstable, and definitive NMR assignments could not be ob-
adopt an approximately cis geometry with a H(1)-Os-Si-H(2)
dihedral angle of 53.2(4)°.
As reported previously,3 the cationic analogue of 4,
[Cp*(iPr3P)(H)2OsdSiH(trip)][B(C6F5)4] (8), rapidly reacts with
alkenes at -78 °C via insertion into the Si-H bond. This reaction
appears to model a key step in the catalytic hydrosilation of alkenes
by [Cp*(iPr3P)(H)2RudSiHPh]+. To probe the mechanism of this
tained due to fluxional behavior in solution. Thus, additional evi-
transformation, the kinetic isotope effect (KIE) was determined
dence for the identity of 2 was obtained by its conversion to the 18-
by a competition experiment involving reaction of 8 and
[Cp*(iPr3P)(D)2OsdSiD(trip)][B(C6F5)4] (8-d3) with 0.5 equiv of
electron CO adduct 3 (νCO ) 1883 cm-1) in 42% yield. The 1H NMR
spectrum of 3 is consistent with the presence of an η1-benzyl group.
Compound 2 readily activates the Si-H bonds of silanes. Treat-
ment of a toluene solution of 2 with sterically encumbered MesSiH3
(Mes ) mesityl) afforded 1 equiv of toluene and the ortho-methyl-
1-hexene. This experiment established an inverse KIE of kH/kD
)
0.8(1), which indicates significant sp2 f sp3 hybridization at silicon
during approach to the transition state for insertion into the Si-H
bond.4
activated silyl complex Cp*(iPr3P)(H)OsCH2C6H2(SiH2)Me2 in 84%
yield as identified by 1H, 13C, 29Si, and 31P NMR and IR spectros-
copy. More sterically demanding primary silanes were used to bias
the formation of silylene complexes over C-H activation. Reaction
of 2 with tripSiH3 (trip ) 2,4,6-iPr3-C6H2) led to exclusive formation
of the desired primary silylene complex Cp*(iPr3P)(H)OsdSiH(trip)
(4), which was isolated as analytically pure orange crystals in 69%
yield. Similarily, reaction of 2 with dmpSiH3 (dmp ) 2,6-Mes2-
C6H3) provided Cp*(iPr3P)(H)OsdSiH(dmp) (5) as a crystalline
orange solid. Complexes 4 and 5 were also prepared by the two-
step reaction sequence involving oxidative addition of the silane
to 1, followed by addition of Mg(CH2Ph)2(THF)2 (Scheme 1).3
Interestingly, the neutral silylene complex 4 is much less reactive
than 8 toward olefins. Thus, no reactions were observed between
4 and ethylene, 1-hexene, cyclohexene, and 2-butene over the course
of 120 h at room temperature in benzene-d6. Upon heating a toluene-
d8 solution of 4 and 1-hexene to 80 °C, 4 was observed to
decompose after 72 h, with no consumption of the alkene. This
dramatic difference in reactivity between 4 and its protonated
analogue 8 prompted an investigation of these Si-H bond additions
using computational methods.
Calculations5 utilized the DFT B3LYP approach and targeted
the model Os complexes Cp(H3P)(H)OsdSiH2 (A) and [Cp(H3P)(H)2-
1
Complex 4 exhibits characteristic H NMR shifts at 12.1 ppm
(SiH) and -16.0 (OsH), and a downfield 29Si NMR shift of 229
ppm. A very low 2JSiH value of 7.7 Hz suggests that any interaction
between the silicon and the metal hydride is minimal. Analogous
spectroscopic features were observed for 5. The molecular structure
of 4 (Figure 1) features an unusually short Os-Si separation of
2.219(2) Å. Planarity at silicon is indicated by the summation of
angles about this atom (359.8°). The osmium- and silicon-bound
hydrogen atoms, which were located in the Fourier difference map,
Figure 1. ORTEP diagram of 4 with thermal ellipsoids shown at the 50%
probability level. All carbon-bound hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): Os-Si 2.219(2), Os-P 2.293(1), Os-H
1.871(5), Si-H 1.477(5). Sum of angles at silicon ) 359.8°.
† University of California, Berkeley.
‡ Texas A&M University.
9
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J. AM. CHEM. SOC. 2006, 128, 428-429
10.1021/ja057494q CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2 a
Scheme 3
is hypothesized that the difference in reactivity stems from the
importance of the resonance contributor that establishes a positive
charge at silicon (Scheme 3), rendering the silyl fragment isoelec-
tronic with a boryl group. It is well established that boryl
functionalities readily hydroborate unsaturated molecules by direct
addition of a B-H bond.8
a Solvent (C6H6)-corrected relative free energies are provided in paren-
theses based on A + ethylene ) 0.0 kcal/mol for the neutral complexes
and D + ethylene ) 0.0 kcal/mol for the cationic complexes.
The results reported above concern an interesting type of
transformation, in which a transition metal center activates one
substrate toward direct reaction with another, without prior
coordination of the second substrate to the metal center.9 In
particular, a dramatic example of the influence of charge distribution
on the reactivity of a metal silylene complex has been observed.
This information should be of use in the design of additional
catalytic reactions that may occur via direct addition of a metal-
activated element-hydrogen bond to an unsaturated substrate.
Acknowledgment. This work was supported by the NSF (CHE
0518074 (M.B.H.), DMS 0216275 (M.B.H.), and No. 0132099
(T.D.T.)). P.G.H. thanks NSERC of Canada for a fellowship (PDF).
The authors acknowledge the Miller Institute for a Research
Fellowship (R.W.) and Professorship (T.D.T.). C.B. and M.B.H.
also thank the Welch Foundation (A-0648).
Table 1. Selected Bond Lengths and Mulliken Charge
Distributions
Os
−
Si
Si
−
C
charge
on Os
fragmenta
charge
on Si
bond length
(Å)
bond length
(Å)
fragmentb
A
B
TS1
C
D
E
TS2
F
2.23
2.24
2.30
2.24
2.26
2.33
2.35
2.27
n/a
-0.09
-0.14
-0.30
-0.22
0.48
0.32
0.31
0.34
0.09
0.14
0.30
0.22
0.52
0.68
0.69
0.66
3.41
2.08
1.90
n/a
2.42
2.03
1.87
a Refers to the sum of the Mulliken charges of the Cp(H3P)(H)nOs
fragment; n ) 1, 2. b Refers to the sum of the Mulliken charges on the
SiH2 and C2H4 fragments.
Supporting Information Available: Crystallographic data (tables
and CIF) for 4; experimental and computational details, calculated
structures for ethylene coordination to osmium, JIMP representations
of the calculated structures, figures depicting all resonance structures
from Scheme 3, and GaussView illustrations of the LUMO of A and
D. This material is available free of charge via the Internet at http://
pubs.acs.org.
OsdSiH2]+ (D). Analysis of the coordination and insertion path-
ways revealed exergonic processes similar to those found for
[Cp(H3P)(H)2RudSiH2]+.6,7 However, coordination of ethylene to
A is ca. 8 kcal mol-1 higher in energy than the coordination of
ethylene to D. Additionally, the transition state for insertion into
the Si-H bond is 18 kcal mol-1 higher in energy for A vs D
(Scheme 2). These values suggest that there is a significant kinetic
barrier to both olefin coordination and insertion for the neutral
silylene species.
Orbital analyses of A and D demonstrated that the LUMOs for
both species are primarily silicon p-orbitals, and the LUMO of D
(-0.232 eV) is much closer in energy than the LUMO of A (-0.051
eV) to the HOMO of ethylene (-0.267 eV). This small HOMO-
LUMO gap allows for enhanced binding of ethylene to the cationic
species D (Table 1). The large reorganization energy associated
with insertion of ethylene into the Si-H bond of A arises in part from
the substantial electronic and structural changes required for the
transformation of B to TS1 (Table 1). In particular, the difference
between the Si-C bond lengths in B and TS1 was determined to be
1.33 Å, whereas there was only a change of 0.39 Å between E and
TS2. In the case of D, these smaller geometric and electronic changes
give rise to a lower transition-state energy (TS2 ) 6.3 kcal mol-1).7
Increased positive charge on the Si(H)2(C2H4) fragments is
generated by shifting electron density to Cp(H3P)(H)nOs by reducing
the Os-Si double bond character, as illustrated by the resonance
structures in Scheme 3 (see Supporting Information for additional
details), and is indicated by increased Os-Si bond lengths. It should
be noted that the increase in positive charge on the silicon fragment
corresponds to an increase in Os2+ character for the neutral
complexes and Os4+ character for the cationic complexes. Thus, it
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