Synthesis and characterization of scandium silyl complexes of the type Cp*2ScSiHRR′. σ-bond metathesis reactions and catalytic dehydrogenative silation of hydrocarbons

Published on Web 12/21/2004

Synthesis and Characterization of Scandium Silyl Complexes

of the Type Cp*₂ScSiHRR′. σ-Bond Metathesis Reactions and

Catalytic Dehydrogenative Silation of Hydrocarbons

Aaron D. Sadow and T. Don Tilley*

Contribution from the Department of Chemistry, UniVersity of California, Berkeley,

Berkeley, California 94720-1460, and Chemical Sciences DiVision, Lawrence Berkeley National

Laboratory, 1 Cyclotron Road, Berkeley, California 94720

Received May 21, 2004; E-mail: tdtilley@socrates.berkeley.edu

Abstract: The scandium dihydrosilyl complexes Cp*₂ScSiH₂R (R ) Mes (4), Trip (5), SiPh₃(6), Si(SiMe₃)₃

(7); Mes ) 2,4,6-Me₃C₆H₂, Trip ) 2,4,6-ⁱPr₃C₆H₂) and Cp*₂ScSiH(SiMe₃)₂(8) were synthesized by addition

of the appropriate hydrosilane to Cp*₂ScMe (1). Studies of these complexes in the context of hydrocarbon

activation led to discovery of catalytic processes for the dehydrogenative silation of hydrocarbons (including

methane, isobutene and cyclopropane) with Ph₂SiH₂via σ-bond metathesis.

Introduction

methane have been primarily associated with selective oxidations

involving noble metal catalysts in highly acidic media.⁹

The rational design of catalysts for the selective functional-

ization of hydrocarbons represents an important challenge in

chemistry. Efforts directed toward the development of homo-

geneous catalytic hydrocarbon conversions have identified

several mechanisms by which transition metal centers may

activate C-H bonds.^1-4In addition, a few recent examples of

homogeneous catalysis involving reactions of alkanes and arenes

have been reported, including olefin hydroarylation,⁵hydrocar-

bon borylation,⁶and alkane dehydrogenation.⁷However, only

a few homogeneous catalytic derivatizations of methane (the

cheapest and most naturally abundant hydrocarbon) are

known.^8-10Recent advances in the catalytic chemistry of

Although these oxidative processes are promising, they suffer

from the limitation that the reaction products are more readily

oxidized than methane itself.^8,9Also, the harsh conditions

(highly acidic conditions, >150 °C) for many such reactions

make them unattractive for use in routine syntheses.⁹Methane

oxidations by metal oxo species in conjunction with an external

oxidant (O₂, peroxides) have also been investigated, and this

approach appears promising based on the known chemistry of

metalloenzymes such as methane monooxygenase.¹⁰However,

synthetic metal oxo catalysts have not yet provided the

selectivity and activity that is exhibited by metalloenzymes such

as methane monooxygenase and cytochrome P450.¹⁰Stoichio-

metric methane activations involving oxidative addition²or

concerted bond cleavage (e.g., 1,2 addition across a MdN

double bond or σ-bond metathesis)^3,4may provide alternative

strategies for catalytic methane conversions. For example, the

CsH bond activation of methane (and other light hydrocarbons)

by a silica-supported tantalum hydride may involve four-

centered transition states.¹¹

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9

J. AM. CHEM. SOC. 2005, 127, 643-656

643

A R T I C L E S

Sadow and Tilley

Scheme 1. General Catalytic Cycle for the Dehydrosilation of

complexes of the type Cp*₂MR (M ) Sc, Lu, Y; R ) H, CH₃)

are reactive toward unactivated C-H bonds under mild condi-

tions.^12,13Investigations have shown that these C-H bond

activations, including those with methane, proceed through

concerted, four-centered, electrocyclic transition states. Although

C-H bond activations of this type were reported over 20 years

ago,¹²only one example of a productive, selective, catalytic

hydrocarbon conversion via C-H σ-bond metathesis has been

reported (the alkylation of pyridine derivatives with [Cp₂ZrH-

(THF)][BPh₄]).¹⁵Activations of methane via well-defined

σ-bond metathesis steps have not yet been incorporated into

catalytic cycles. On the other hand, several catalytic processes

that involve the related activation of Si-H bonds via four-

centered transition states have been reported (e.g., hydrosilane

dehydropolymerization,^16,17olefin hydrosilation,¹⁸organosilane

hydrogenolysis¹⁹).

A strategy for the design of catalysts for hydrocarbon

transformations is suggested by the recent observation of arene

activation by the electrophilic cationic hafnium hydrosilyl

complex Cp₂Hf(η²-SiHMes₂)(µ-Me)B(C₆F₅)₃.²⁰Based on this

reaction and known chemistry for related hafnium alkyl and

hydride complexes,²¹a cycle for benzene dehydrosilylation was

proposed (Scheme 1). This proposed three-step cycle would

involve (1) a C-H bond activation reaction by a metal

hydrosilyl complex, (2) the transfer of an organic group from

the transition metal center to silicon, and (3) the dehydrocoupling

of M-H and Si-H bonds to reform the metal hydrosilyl

complex. Precedent for silicon-carbon bond formation (step

2) is provided by stoichiometric reactions of hydrosilanes with

hydrocarbyl complexes of d⁰and fⁿd⁰metals,²²and this step

Hydrocarbons

has been proposed for olefin hydrosilations catalyzed by Group

3 and lanthanide complexes.¹⁸Dehydrocouplings of M-H (M

) Zr, Hf, Y, Sm, Lu) and Si-H bonds to produce metal-silicon

bonds (step 3) have also been established.^19,23,24Unfortunately,

Cp₂Hf(η²-SiHMes₂)(µ-Me)B(C₆F₅)₃does not appear to be a

suitable catalyst for arene dehydrosilation.^20,26

A search for more reactive d⁰metal silyl complexes was

prompted by the possibility that such complexes might activate

C-H bonds and serve as catalysts for hydrocarbon dehydrosi-

lation. In particular, scandium silyl complexes were suggested

by the similar covalent radii of hafnium and scandium,^13aas

well as the isoelectronic relationship between complexes of the

types Cp₂ScR and Cp₂HfR⁺.²⁷Additionally, complexes of the

type Cp*₂ScR (R ) hydrido, alkyl) have been shown to be

highly reactive toward the C-H bonds of hydrocarbons such

as methane, benzene, and styrene.¹³A high reactivity for

scandium-silicon bonded compounds was also suggested by

the weaker nature of M-Si bonds in comparison to M-H and

M-C bonded analogues (M ) d⁰transition metal or f-element

center).²⁸Furthermore, M-Si bonds react more rapidly than

M-C bonds in σ-bond metathesis reactions with silanes and

hydrogen, and metal silyl species have been shown to participate

in a number of catalytic cycles (e.g., those involving the

dehydropolymerization of silanes).²³

The few reported compounds containing a scandium-silicon

bond are limited to the THF-stabilized complexes Cp₂Sc(SiR₃)-

(THF) (SiR₃) Si(SiMe₃)₃, Si^tBuPh₂, SiPh₃, and Si(SiMe₃)₂-

Ph).²⁹Although these complexes are extremely reactive toward

polar, unsaturated organic substrates such as CO, xylyl isocya-

nide, and CO₂, the only σ-bond metathesis process observed

for them was in the reaction of Cp₂Sc[Si(SiMe₃)₃](THF) with

HCtCPh to form the dimeric acetylide [Cp₂ScCtCPh]₂.^29c

Given the rich reactivity associated with 14-electron Cp*₂Sc-

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2896-2905. (d) Voskoboynikov, A. Z.; Shestakova, A. K.; Beletskaya, I.

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9

644 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

1

69%). H NMR (500 MHz): δ 2.91 (s, 2 H, SiH₂Si(SiMe₃)₃, J_SiH

)

derivatives and the expected enhanced reactivity toward σ-bond

metathesis for more coordinatively unsaturated complexes,³we

focused initial efforts on the synthesis of base-free silyl

complexes of the type Cp*₂ScSiR₃. As described herein,

complexes of this type may be prepared by reactions of Cp*₂-

ScMe (1) with certain hydrosilanes. Related σ-bond metathesis

reactions in this system have been investigated, including the

more commonly observed alkyl transfer from the metal to

silicon, which occurs for many primary and secondary silanes.

Finally, σ-bond metathesis reactions of the new scandium

hydrosilyl complexes with benzene and methane have been

investigated. On the basis of these studies, we have developed

a catalytic system for the dehydrosilation of hydrocarbons (eq

1) and describe its application in catalytic methane functional-

ization.³⁰

124 Hz), 1.88 (s, 30 H, C₅Me₅), 0.543 (s, 27 H, SiH₂Si(SiMe₃)₃). ¹³C-

{¹H} NMR: δ 123.56 (C₅Me₅), 12.71 (C₅Me₅), 3.66 [Si(SiMe₃)₃]. ²⁹Si-

{¹H} NMR: δ -8.28 [SiH₂Si(SiMe₃)₃], 124.48 [SiH₂Si(SiMe₃)₃]. IR

(KBr, cm^-1): 2949 (s), 2897 (s), 2023 (m), 1439 (w), 1381 (w), 1240

(s), 941 (m), 833 (s), 685 (m), 623 (m), 430 (m). Anal. Calcd for C₂₉H₅₉-

ScSi₅: C, 58.72; H, 10.03. Found: C, 58.82; H, 10.09. Mp 146.5-

149 °C.

Cp*₂ScSiH₂SiPh₃(7). To a 100-mL Schlenk flask containing Cp*₂-

ScMe (0.240 g, 0.726 mmol) and Ph₃SiSiH₃(0.212 g, 0.731 mmol)

was added ca. 20 mL of pentane. Toluene (40 mL) was added to the

slurry to dissolve all the solids. The resulting yellow solution was

filtered, concentrated to 10 mL, and cooled to -30 °C. A yellow

microcrystalline solid was isolated, washed with pentane, and dried in

vacuo, to yield 0.242 g of 7 (0.401 mmol, 55.2%). ¹H NMR (500

MHz): δ 7.94 (d, 6 H, Ph), 7.26 (t, 6 H, Ph), 7.23 (m, 3 H, Ph), 3.47

(s, 2 H, SiH, J_SiH) 128 Hz), 1.77 (s, 30 H, C₅Me₅). ¹³C{¹H} NMR

1

(125 MHz): δ 141.33 (C₆H₅), 137.31 (C₆H₅), 128.93 (C₆H₅), 128.67

(C₆H₅), 123.74 (C₅Me₅), 12.40 (C₅Me₅). ²⁹Si{¹H} NMR (99 MHz): δ

-3.75 (SiH₂SiPh₃), -113.83 (SiH₂SiPh₃). IR (KBr, cm^-1): 3064 (w),

2960 (w), 2902 (m), 2856 (w), 2031 (m), 2021 (m), 1427 (m), 1261

(w), 1095 (s), 1024 (m) 935 (m), 700 (s), 517 (m). Anal. Calcd for

C₃₈H₄₇ScSi₂: C, 75.34; H, 7.83. Found: C, 75.58; H, 7.97. Mp 175-

176 °C.

R-H + H-SiR′₃^catalyst8 R-SiR′₃+ H-H

Experimental Section

(1)

General. All manipulations were performed under an atmosphere

of argon using Schlenk techniques and/or a glovebox. Dry, oxygen-

free solvents were employed throughout. Removal of thiophenes from

benzene and toluene was accomplished by washing each with H₂SO₄

and saturated NaHCO₃followed by drying over MgSO₄. Olefin

impurities were removed from pentane by treatment with concentrated

H₂SO₄, 0.5 N KMnO₄in 3 M H₂SO₄, saturated NaHCO₃, and then the

drying agent MgSO₄. All solvents were distilled from sodium ben-

zophenone ketyl, with the exception of benzene-d₆and cyclohexane-

d₁₂, which were purified by vacuum distillation from Na/K alloy. The

compounds Cp*₂ScCH₃(1), Cp*₂ScH (2), Cp*₂ScPh (3) were prepared

according to literature procedures.^13bSilanes were prepared by reduction

of the appropriate chlorosilane with LiAlH₄or LiAlD₄. Elemental

analyses were performed by the microanalytical laboratory at the

University of California, Berkeley. Infrared spectra were recorded using

a Mattson FTIR spectrometer at a resolution of 4 cm^-1. All NMR

spectra were recorded at room temperature in benzene-d₆unless

otherwise noted, using a Bruker AMX-300 spectrometer at 300 MHz

(¹H) or 75.5 MHz (¹³C) or a Bruker AM-400 spectrometer at 400 MHz

(¹H) or 377 MHz (¹⁹F) and a Bruker DRX-500 at 500 MHz (¹H), 125

MHz (¹³C), or 100 MHz (²⁹Si).

Cp*₂ScSiH(SiMe₃)₂(8). A pentane solution of (Me₃Si)₂SiH₂(0.27

g, 1.53 mmol, ca. 10 mL total volume) was added to Cp*₂ScMe (0.339

g, 1.02 mmol) dissolved in 20 mL of pentane. The resulting solution

turned pale green over 1 h. The solution was concentrated until the

green color intensified and was then cooled to -30 °C. An analytically

pure solid precipitated from solution, yielding 0.266 g of Cp*₂ScSiH-

1

(SiMe₃)₂(0.543 mmol, 52.2%). H NMR (500 MHz): δ 2.53 (s, 1 H,

1

SiH, J_SiH) 115 Hz), 1.87 (s, 15 H, C₅Me₅), 1.86 (s, 15 H, C₅Me₅),

0.54 (s, 18 H, SiMe₃). ¹³C{¹H} NMR (100 MHz): δ 123.65 (C₅Me₅),

123.28 (C₅Me₅), 12.75 (C₅Me₅), 12.63 (C₅Me₅), 5.94 (SiMe₃). ²⁹Si-

{¹H} NMR (100 MHz): δ -6.41 (SiMe₃), -108.23 (SiH). IR (KBr,

cm^-1): 2941 (s), 2906 (s), 2006 (m), 1439 (w), 1381 (w), 1236 (s),

1024 (w), 829 (s), 754 (w), 679 (m), 615 (m), 426 (m). Anal. Calcd

for C₂₆H₄₉ScSi₃: C, 63.62; H, 10.06. Found: C, 63.93; H, 9.95. Mp

163-166 °C.

Methane Dehydrosilation Catalysis. A cyclohexane solution of

Cp*₂ScMe (1) or Cp*₂ScH (2), Ph₂SiH₂(5-20 equiv), and a cyclooc-

tane standard were placed in a glass liner, which was placed in a Parr

high-pressure vessel. The Parr vessel was charged with methane (ca.

150 atm) and heated to 80 °C in a constant temperature oil bath for

1-7 days. The reaction mixture was allowed to cool, and the vessel

was slowly vented. The cyclohexane solution was quenched with H₂O

and filtered through Celite. The amount of Ph₂MeSiH formed was

determined using a Hewlett-Packard 6890 Series gas chromatograph.

Kinetic Measurements. Reactions were monitored by ¹H NMR

spectroscopy, with a Bruker DRX500 spectrometer, using 5-mm

Wilmad NMR tubes. The samples prepared by dissolution of 1 in

toluene-d₈containing a known concentration of C₈H₁₆standard were

cooled to -78 °C followed by addition of the silane via a syringe. The

NMR tube was quickly placed in the probe, which was precooled to

the required temperature. The probe temperature was calibrated using

a neat methanol sample and was monitored with a thermocouple. Single

scan spectra were acquired automatically at preset time intervals. The

peaks were integrated relative to cyclooctane as an internal standard.

Rate constants were obtained by nonweighted linear least-squares fit

of the integrated second-order rate law, ln{[Ph₂SiH₂]_t/[1]} ) ln{[Ph₂-

SiH₂]₀/[1]₀} + k∆₀t.

Cp*₂ScSiH₂Mes (4). Addition of neat MesSiH₃(0.120 g, 0.81 mmol)

to solid Cp*₂ScMe (0.049 g, 0.147 mmol) resulted in vigorous bubbling.

The resulting bright yellow solid was washed with cold pentane (3 ×

2 mL) to give 0.030 g of Cp*₂ScSiH₂Mes (0.065 mmol, 44.1%). The

filtrate was cooled to -30 °C to provide 3 as a crystalline solid in

1

58.8% total yield. H NMR (500 MHz): δ 6.99 (s, 2 H, C₆H₂Me₃),

4.35 (s, 2 H, SiH), 2.58 (s, 6 H, o-C₆H₂Me₃), 2.30 (s, 3 H, p-C₆H₂Me₃),

1.81 (s, 30 H, C₅Me₅). ¹³C{¹H} NMR (125 MHz): δ 160.70 (C₆H₂-

Me₃), 143.98 (C₆H₂Me₃), 141.56 (C₆H₂Me₃), 135.83 (C₆H₂Me₃), 123.00

(C₅Me₅), 26.45 (o-C₆H₂Me₃), 21.71 (p-C₆H₂Me₃), 11.86 (C₅Me₅). ²⁹Si-

{¹H} NMR (99 MHz): δ -71.0 (¹J_SiH) 135 Hz). IR (KBr, cm^-1):

2952 (s), 2908 (s), 2858 (s), 2014 (s), 1601 (w), 1487 (w), 1447 (m),

1378 (m), 1023 (w), 993 (w), 935 (m), 850 (w), 788 (m), 753 (w), 698

(s), 430 (m). Anal. Calcd for C₂₉H₄₃ScSi: C, 74.96; H, 9.33. Found:

C, 74.98; H, 9.31. Mp 175-176 °C.

Cp*₂ScSiH₂Si(SiMe₃)₃(6). Cp*₂ScMe (0.275 g, 0.832 mmol) and

(Me₃Si)₃SiSiH₃(0.237 g, 0.851 mmol) were separately dissolved in

15 mL of pentane. The solution of (Me₃Si)₃SiSiH₃was added to the

solution of Cp*₂ScMe, producing a bright yellow solution over 20 min.

The reaction solution was allowed to stand for 2 h and was then

concentrated to 20 mL and cooled to -78 °C. Yellow crystals of Cp*₂-

ScSiH₂Si(SiMe₃)₃were isolated by filtration (0.341 g, 0.574 mmol,

Results and Discussion

Initial attempts to synthesize compounds of the type Cp*₂-

ScSiR₃involved salt metathesis reactions under conditions

analogous to those used for the syntheses of Cp₂Sc(SiR₃)THF

(30) Sadow, A. D.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42, 803-805.

9

J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 645

A R T I C L E S

Sadow and Tilley

complexes.²⁹Cp*₂ScCl was treated with the silyl anion reagents

(THF)₃LiSi(SiMe₃)₃,^31aKSi(SiMe₃)₃,^31b(THF)₃LiSiPh₃,^31c(THF)₃-

LiSi^tBuPh₂,²⁹or (THF)_2.5LiSiMes₂H^31d(Mes ) 2,4,6-Me₃C₆H₂)

in both benzene-d₆and THF-d₈. However, at room temperature

no reactions occurred, and extended reaction times (>1 day)

and heating (∼80 °C) yielded only HSiR₃and unidentified

scandium products. Presumably, unfavorable steric interactions

between the Cp* ligands and the bulky tertiary silyl groups

prevent the formation of stable compounds.

An alternative approach has precedent in a few reactions of

lanthanide alkyl and hydride complexes with hydrosilanes,

which produce metal silyl complexes with concurrent elimina-

tion of alkane or dihydrogen.^23-25For example, the reaction of

(Me₃Si)₂SiH₂with Cp*₂SmCH(SiMe₃)₂gives Cp*₂SmSiH-

(SiMe₃)₂and (Me₃Si)₂CH₂.²⁴However, this transformation

proceeds via a mechanism involving hydrogenolysis of Cp*₂-

SmCH(SiMe₃)₂to Cp*₂SmH and (Me₃Si)₂CH₂followed by

dehydrocoupling of the samarium hydride with (Me₃Si)₂SiH₂.²⁴

More commonly, early metal and lanthanide hydrocarbyl

complexes react with silanes via carbon transfer to silicon and

formation of Si-C and M-H bonds. For example, the reaction

of CpCp*HfMe(µ-Me)B(C₆F₅)₃with PhSiH₃gives PhMe₂SiH

and the hafnium hydride CpCp*HfH(µ-H)B(C₆F₅)₃.²¹

quantitatively produced 2 and Ph₂MeSiH. Linear plots of ln-

{[1]₀/[1]} vs time indicated that the reaction is first-order in 1.

A plot of k_obsvs [Ph₂SiH₂] yielded a straight line that intercepted

the x-axis at the origin, demonstrating that the reaction is also

first-order in Ph₂SiH₂and that only one pathway is operative.

The resulting rate law, rate ) k[1][Ph₂SiH₂], is consistent with

a mechanism in which the alkyl group is transferred to silicon

in a single, concerted step. The slopes of second-order plots of

ln{[Ph₂SiH₂]/[1]} vs time were used to determine the second-

order rate constants (e.g., k ) 6.78(6) × 10^-5M^-1s^-1, -81

°C; k ) 2.24(6) × 10^-4M^-1s^-1, -67 °C; k ) 2.07(6) × 10^-3

M^-1s^-1, -36 °C).³²

For the reaction of 1 with Ph₂SiH₂, a primary kinetic isotope

effect of k_H/k_D) 1.15(5) was determined by measuring rates

for the reaction with Ph₂SiD₂. This small isotope effect is

consistent with an early transition state in which the Si-H(D)

bond is not significantly broken. The activation parameters,

determined from plots of ln(k/T) vs 1/T over a temperature range

of -80 °C to -35 °C, are ∆H^q) 6.66(2) kcal/mol and ∆S^q)

-42.5(7) eu.³²The low enthalpy of activation indicates that bond

cleavage represents a minor component of the reaction barrier,

and that loss of translational entropy contributes substantially

to the overall activation energy.

Reactions of Cp*₂ScMe (1) with Primary and Secondary

Hydrosilanes to Produce Si-C Bonds. Reactions of 1 with

unhindered primary silanes (PhSiH₃, C₆H₁₁SiH₃, p-MeC₆H₄SiH₃,

3,5-Me₂C₆H₃SiH₃, SiH₄, MeSiH₃) and small secondary silanes

(Ph₂SiH₂, PhMeSiH₂, Me₂SiH₂, Et₂SiH₂) yield 2 and the

products of Si-C bond formation (RMeSiH₂or R₂MeSiH,

respectively). For example, the addition of PhSiH₃(1 equiv, 2

h, room temperature) to a benzene-d₆solution of 1 produced a

yellow solution containing PhMeSiH₂and the scandium hydride

2 (eq 2). A ¹H NMR spectrum of the reaction mixture indicated

The R-agostic SiH interaction in Cp₂Hf(η²-SiHMes₂)(µ-

Me)B(C₆F₅)₃appears to activate the Hf-Si bond toward

cleavage of the C-H bonds of arenes. In this system, the rates

of reaction of benzene with Cp₂Hf(η²-SiHMes₂)⁺and Cp₂Hf-

(η²-SiDMes₂)⁺are identical [k_H/k_D) 1.1(1)],²⁰implying that

the agostic interaction is maintained throughout the C-H

activation process. Compound 1 does not have an R-agostic

ground-state structure,¹³but R-agostic assistance in the transition

state of its reaction with Ph₂SiH₂seemed possible. The

secondary isotope effect was therefore determined by measuring

the rates of reaction for Cp*₂ScCD₃and Cp*₂ScCH₃. The small

value observed (k_H/k_D) 0.91(5) per H) suggests that R-agostic

assistance does not play a significant role in this σ-bond

metathesis reaction.

The reaction of 1 with Ph₂SiH₂, involving the transfer of a

methyl group from scandium to silicon, is relevant to the

catalytic dehydrosilation of methane, as discussed below. In

addition, the apparent formation of scandium silyl complexes

as minor products in reactions of 1 with unhindered primary

silanes suggested that via modification of the reaction conditions,

it might be possible to produce significant quantities of scandium

silyl complexes.

Reactions of Cp*₂ScMe (1) with Hindered Primary Silanes

to Produce Scandium Silyl Complexes. Surprisingly, the

reaction of 1 with MesSiH₃(1.2 equiv, cyclohexane-d₁₂, room

temperature, 3 h; Mes ) 2,4,6-Me₃C₆H₂) produced methane and

a bright yellow solution of Cp*₂ScSiH₂Mes (4, eq 3).

that several minor products also formed, including Cp*₂ScPh-

d₅(3-d₅) and CH₄(ca. 10% relative to PhMeSiH₂). Complex

3-d₅presumably forms via the metalation of benzene-d₆by 2.¹³

Methane may form as a byproduct in the direct reaction of 1

and PhSiH₃to yield the scandium silyl complex Cp*₂ScSiH₂-

Ph. Additionally, the yellow-colored reaction solution suggests

the presence of Cp*₂ScSiH₂Ph, since isolated scandium silyl

complexes are similarly colored (vide infra). Unfortunately, the

¹H NMR spectrum is complicated by rapid ScH/SiH exchange

that broadens and obscures the SiH resonances of PhSiH₃,

PhMeSiH₂, and the putative Cp*₂ScSiH₂Ph. Although this silyl

complex could not be isolated, its formation is suggested by

related reactions of 1 with hindered primary silanes which yield

isolable scandium silyl complexes (vide infra).

cyclohexane-d₁₂

Cp*₂ScCH

₃+ MesSiH₃

8

1

Cp* ScSiH Mes

+ CH₄(3)

2

Under pseudo-first-order conditions (5-15 equiv; -81 to -35

°C, in toluene-d₈), the reaction between 1 and Ph₂SiH₂

4

The ¹H NMR spectrum of 4 contains resonances corresponding

to the SiH₂(s, 3.83 ppm, 2 H), Cp*, and mesityl groups.

(31) (a) Gutekunst, G.; Brook, A. G. J. Organomet. Chem. 1980, 225, 1-3. (b)

Kayser, C.; Marschner, C. Monatsh. Chem. 1999, 130, 203-206. (c) Woo,

H.-G.; Freeman, W. P.; Tilley, T. D. Organometallics 1992, 11, 2198-

2205. (d) Roddick, D. M.; Heyn, R. H.; Tilley, T. D. Organometallics 1989,

8, 324-330.

(32) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;

McGraw-Hill: New York, 1995; p 155.

9

646 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

Table 2. Selected Bond Distances (Å) and Angles (deg) for

Table 1. Spectroscopic Data for 4, 6-8, and the Respective

Silanes

Cp*₂ScSiH₂SiPh₃(7)

¹J_SiH

(Hz)

²⁹Si

{

¹H

}

Bond Distances

Sc-Si1

2.797(1)

1.39(3)

Si1-Si2

Si1-H2

2.364(2)

1.47(3)

1.893(4)

2.1741(6)

compound

δ

(SiH)

(

R−Si) NMR

ν(SiH)

Si1-H1

MesSiH₃

4.32

4.35

3.52

3.47

3.65

2.90

3.01

2.53

198

135

188

128

191

124

166

115

-77.61

-72.00

-99.20

-124.48

-99.02

-124.48

-104.24

-108.23

2155

2014

2121

2023

2128

2031

2089

2006

Si2-C21

Si2-C33

Sc-Cp*_cent

1.896(4)

1.893(4)

2.1741(7)

Si2-C27

Sc-Cp*_cent

Cp*₂ScSiH₂Mes (4)

(Me₃Si)₃SiSiH₃

Cp*₂ScSiH₂Si(SiMe₃)₃(6)

Ph₃SiSiH₃

Cp*₂ScSiH₂SiPh₃(7)

(Me₃Si)₂SiH₂

Cp*₂ScSiH(SiMe₃)₂(8)

Bond Angles

Sc-Si1-Si2

122.43(5)

113(1)

Sc-Si1-H1

119(1)

98(1)

102(1)

113.6(1)

107.99(3)

142.96(4)

Sc1-Si1-H2

H1-Si1-H2

Si1-Si2-C21

Si1-Si2-C33

Cp*_cent-Sc-Si1

Si2-Si1-H1

96(1)

Si2-Si1-H2

111.2(1)

113.2(1)

108.77(3)

Si1-Si2-C27

Cp*_cent-Sc-Si1

Cp*_cent-Sc-Cp*_cent

Addition of excess MesSiH₃(2-5 equiv) to a pentane solution

of 1, followed by cooling to -30 °C produced 4 as a yellow

powder in 44% yield. A series of dihydrosilylscandium com-

plexes Cp*₂ScSiH₂R (R ) Trip (5), Si(SiMe₃)₃(6), SiPh₃(7);

Trip ) 2,4,6-ⁱPr₃C₆H₂) were also synthesized via this methane

elimination route. When fewer than 1.2 equiv of silane were

used, the products were contaminated with small amounts of

Cp*₂ScH and RMeSiH₂. Compound 5 could not be isolated free

of the excess TripSiH₃starting material, but 4, 6, and 7 were

purified by crystallization from pentane at -30 °C.

Table 3. Crystallographic Data for Compounds 7 and 8

7

8

empirical formula

formula weight

crystal color, habit

crystal size (mm)

crystal system

space group

a (Å)

ScSi₂C₃₈H₃₇

594.84

yellow, blocks

ScSi₃C₂₆H₄₉

490.89

yellow, blocks

0.23 × 0.23 × 0.15 0.30 × 0.21 × 0.16

monoclinic

P2₁/n (#14)

9.9125(8)

20.937(2)

16.232(1)

90

triclinic

P1h(#2)

9.1426(6)

10.4602(6)

17.096(1)

94.074(1)

105.507(1)

103.560(1)

1515.9(2)

4330, 3.5-45.0

1

b (Å)

c (Å)

The SiH H NMR resonances for the scandium silyl com-

plexes are upfield-shifted vs those for the corresponding free

silanes, and this is characteristic for hydrosilyl complexes of

electropositive d⁰metals (Table 1).^29d,31dThe ¹J_SiHvalues (125-

135 Hz) for 4-7 are lower than those for the corresponding

free silanes (188-200 Hz) and related 16-electron hydrosilyl

complexes of zirconium and hafnium (145-155 Hz)^23,31dbut

similar to those observed for lanthanide hydrosilyl species.^18,25c

The possibility of an R-agostic SiH is suggested by the detection

of a strong interaction of this type in the isoelectronic cationic

R (deg)

â (deg)

95.289

90

3354.3(4)

4204, 3.5-45.0

γ (deg)

V (Å³)

orientation reflections:

number, 2θ range (deg)

Z

4

2

D

_calc(g/cm³)

1.178

1256.00

3.14

SMART

Mo KR

(λ ) 0.710 69 Å)

graphite

monochromated

135

ω (0.3° per frame) ω (0.3° per frame)

10.0 s per frame

49.4

total: 13104

unique: 5548

0.044

T_max) 0.95

T_min) 0.51

direct methods

(SIR92)

1.075

F₀₀₀

536.00

µ (Mo KR) (cm^-1

diffractometer

radiation

)

3.71

SMART

Mo KR

(λ ) 0.710 69 Å)

graphite

monochromated

132

hafnium complex Cp₂Hf(η²-SiHMes₂)(µ-Me)B(C₆F₅)₃(¹J_SiH

)

57 Hz; ²⁹Si NMR δ ) 158; ν(SiH) ) 1414 cm^-1).²⁰However,

the SiH resonance in the ¹H NMR spectrum of 7-d₁(vide infra)

is only slightly upfield-shifted (∼0.05 ppm) relative to the

chemical shift for the SiH group in 7.¹³These data suggest that

the scandium silyl complexes 4-7 do not contain R-agostic

interactions.

temperature (K)

scan type

scan rate

data collected, 2θ_max(deg)

reflections measured

10.0 s per frame

49.4

total: 7647

unique: 4799

0.036

T_max) 0.94

T_min) 0.42

direct methods

(SIR92)

The proposed structure of 7 was confirmed by an X-ray

crystallographic study (Figure 1; key bond lengths and angles

are listed in Table 2, and crystallographic data are listed in Table

3). The Sc-Si1-Si2 (122.43(5)°) and the Sc-Si1-H bond

angles (119(1)° and 113(1)°) are inconsistent with an R-agostic

R_int

transmission factors

structure solution

no. of observed data [I > 3σ(I)] 3349

3756

269

13.96

no. of parameters refined

reflections/parameter ratio

final residuals: R; R_w; R_all

goodness of fit indicator

max. shift/error final cycle

376

8.91

0.046; 0.056; 0.081 0.065; 0.090; 0.082

1.54

0.00

2.39

0.00

maximum and minimum peaks, 0.36, -0.50

0.68, -0.58

final diff. map (e^-/Å³)

structure (both R-hydrogens were located in the electron density

map, and their (x,y,z) coordinates were refined). The Sc-Si1

bond distance (2.797(1) Å) is shorter than the corresponding

bond length in the only other crystallographically characterized

scandium silyl complex, Cp₂Sc[Si(SiMe₃)₃](THF) (2.863(2)

Å).^29c,dThe bond angles around the scandium center in 7

(Cp*_cent-Sc-Si1 ) 107.99(3)°, 108.77(3)°; Cp*_cent-Sc-Cp*_cent

) 142.96(4)°) are similar to those reported for the 14-electron

Figure 1. ORTEP diagram of Cp*₂ScSiH₂SiPh₃(7). The hydrogen atoms,

with the exception of the two bonded to Si, were removed for clarity.

Thermal ellipsoids were drawn to 50% probability.

9

J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 647

A R T I C L E S

Sadow and Tilley

The more hindered secondary silane Mes₂SiH₂, and the

corresponding germane Mes₂GeH₂, did not react with 1 in

benzene-d₆or cyclohexane-d₁₂at room temperature over 1 week.

As in the absence of Mes₂SiH₂, heating cyclohexane-d₁₂

solutions of Cp*₂ScMe with Mes₂SiH₂(80 °C, 2 days) resulted

in the formation of [Cp*(η¹:η⁵-C₅Me₄CH₂)Sc]₂.

Mechanistic Experiments on the Formation of Scandium

Silyl Complexes. Three possible mechanisms for the formation

of scandium silyl compounds 4-7 are shown in Scheme 2. One

pathway involves the direct interaction of 1 with the Si-H bond

of a silane to produce a Sc-Si bond and methane. Related

second-order processes have been observed for reactions of

complexes of the type Cp*₂MCH₃(M ) Sc, Lu, Y) with various

hydrocarbons.^12,13An alternative mechanism is suggested by

mechanistic studies on reactions of Cp*₂MCH₃complexes with

methane and benzene, which involve a first-order process in

addition to the direct, second-order pathway.^12,13An analogous

pathway for the formation of scandium silyl complexes might

involve reaction of the intermediate [(η¹:η⁵-C₅Me₄CH₂)ScCp*]

with hydrosilanes. A third possible mechanism involves the

reaction of 1 with a catalytic amount of H₂to form hydride

complex 2 and methane. A subsequent dehydrocoupling reaction

of the scandium hydride with the silane substrate RSiH₃would

then produce Cp*₂ScSiH₂R and H₂. An analogous chain reaction

pathway was reported as the sole mechanism for the reaction

of Cp*₂SmCH(SiMe₃)₂with (Me₃Si)₂SiH₂to form Cp*₂SmSiH-

(SiMe₃)₂.²⁴

Figure 2. ORTEP diagram of Cp*₂ScSiH(SiMe₃)₂(8) illustrating the

molecular connectivity. Hydrogen atoms and the other half of the disordered

silyl ligand were omitted for clarity. Thermal ellipsoids are shown at 50%

probability.

scandium methyl complex 1 (Cp*_cent-Sc-C ) 108°, Cp*_cent

Sc-Cp*_cent) 144°).¹³

-

The thermal stability of the silyl complexes follows the trend

4 < 5 , 6 ≈ 7. Compounds 6 and 7 are stable in the solid

state at room temperature for >1 week, whereas 4 decomposed

over 12 h. The decompositions are much more rapid for the

aryl-substituted complexes (4 and 5; t_1/2≈ 24 h, room

temperature). For 6 and 7, the t_1/2values for thermal decomposi-

tion are much greater than 1 week.

The reaction of Cp*₂ScMe (1) with 1 equiv of (Me₃Si)₂SiH₂

(benzene-d₆, room temperature) yielded a 2:1 mixture of Cp*₂-

ScPh-d₅(3-d₅) and Cp*₂ScSiH(SiMe₃)₂(8). However, treatment

of 1 with excess (Me₃Si)₂SiH₂(pentane, room temperature) gave

the scandium hydrosilyl complex Cp*₂ScSiH(SiMe₃)₂(8, eq 4)

in 52% isolated yield.

The possible role of a pathway involving metalation of a Cp*

ligand was addressed with labeling experiments. Treatment of

1 with MesSiD₃quantitatively yielded Cp*₂ScSiD₂Mes and

CH₃D. Since no CH₄(by ¹H NMR spectroscopy) was detected

over the course of the reaction, we conclude that this mechanism

is not operative. Unfortunately, deuterium labeling studies

cannot distinguish between the direct, single-step reaction and

a mechanism involving H₂catalysis, because identically labeled

products would be formed via either pathway (the silane is the

source of H₂in the latter mechanism). Attempts to determine a

rate law for the reaction were unsuccessful, as plots of

ln{[MesSiH₃]/[1]} vs time were not linear for three half-lives

(MesSiH₃present in >5-fold excess, cyclohexane-d₁₂, -47 °C).

Note that the hydrogen-promoted mechanism (Scheme 2) can

yield clean pseudo first- and second-order kinetics, as long as

the total [metal hydride] + [H₂] concentration does not change

pentane

Cp* ScMe

Cp* ScSiH(SiMe )

+

3 2

+ 2(Me₃Si)₂SiH₂

8

2

1

8

CH₄+ (Me₃Si)₂MeSiH (4)

The ¹H NMR spectrum of 8 contains two Cp* resonances (1.87

and 1.85 ppm) and resonances corresponding to the SiMe₃and

SiH moieties (0.54 and 2.53 ppm, respectively). This spectrum

indicates a structure with C_ssymmetry and hindered rotation

about the Sc-Si bond. Similar spectroscopic features were

observed for the alkyl complexes Cp*₂LnCH(SiMe₃)₂(Ln )

Y, La, Nd, Sm, Lu).³³Interestingly, the lanthanide silyl

complexes Cp*₂LnSiH(SiMe₃)₂(Ln ) Y, Nd, Sm)^22a,24exhibit

1

over the course of the reaction.²⁴Analysis of the H NMR

spectra of reactions of 1 with MesSiH₃, TripSiH₃, or (Me₃-

Si)₃SiSiH₃revealed that the scandium hydride 2 was present,

in each case, in small quantities (∼5%).

A chain reaction involving Cp*₂ScH/H₂could be initiated

by the expected side reaction of 1 with RSiH₃, to form Cp*₂-

ScH and RMeSiH₂(vide supra). This H₂-mediated mechanism

could occur only if 2 reacts directly with the silanes to form

4-7, as was observed for the reaction of Cp*₂SmH with (Me₃-

Si)₂SiH₂.²⁴In fact, the reactions of 2 with RSiH₃(R ) Mes,

Si(SiMe₃)₃) produced the corresponding scandium silyl com-

1

equivalent Cp* and SiMe₃groups at room temperature (by H

NMR spectroscopy). The structure of 8 was determined by X-ray

crystallography (Figure 2). The structure is monomeric, the

silicon bonded to scandium is pyramidal, and the two Cp*

ligands are inequivalent. Unfortunately, the silicon was disor-

dered over two locations (above and below the plane of the

metallocene wedge), precluding meaningful discussion of bond

distances and angles. In contrast to 8, Cp*₂SmSiH(SiMe₃)₂is

dimeric in the solid state.²⁴

1

plexes in low yield (ca. 10% by H NMR spectroscopy, vida

infra). When the reactions of 1 with 1 equiv of RSiH₃(R )

Mes, Trip, (Me₃Si)₃Si, Ph₃Si) occurred in the presence of a

hydride trapping agent (benzene-d₆, which rapidly reacts with

2 to form 3-d₅¹³), the amount of the trapped product (3-d₅)

varied depending on the nature of the silane. Note that complex

(33) (a) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J.

J. Am. Chem. Soc. 1985, 107, 8103-8110. (b) 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, 1726-1733.

9

648 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

Scheme 2. Possible Mechanisms for Reaction of Cp*₂ScMe (1) with a Primary Silane

3 reacts much more slowly than 1 with the silanes MesSiH₃

and Ph₃SiSiH₃(vide infra), and at room temperature the reaction

between 1 and benzene is slow.¹³The addition of Ph₃SiSiH₃to

1 (1 equiv, in benzene-d₆) produced 7 in quantitative yield (¹H

NMR spectroscopy; Ph₃SiSiMeH₂, Cp*₂ScPh, and Cp*₂ScH

were not detected). However, treatment of 1 with (Me₃Si)₂SiH₂

(in benzene-d₆) led to the formation of a substantial quantity of

the scandium phenyl 3-d₅(along with (Me₃Si)₂MeSiH). The

relative amounts of 3 formed in the reaction of 1 with silanes

follows the trend: Ph₃SiSiH₃< (Me₃Si)₃SiSiH₃< MesSiH₃≈

TripSiH₃< (Me₃Si)₂SiH₂. Thus, for the secondary silane (Me₃-

Si)₂SiH₂the pathway involving the scandium hydride intermedi-

ate appears to be important for the formation of the silyl complex

8, whereas the direct interaction of 1 and Ph₃SiSiH₃via the

concerted, single-step reaction mechanism is probably respon-

sible for the formation of 7. Most likely, the reactions of 1 with

MesSiH₃, TripSiH₃, and (Me₃Si)₃SiSiH₃involve both pathways.

Reactions of Cp*₂ScH (2) with Silanes. Given the potential

role of hydride 2 in catalytic reactions involving hydrocarbons

and silanes, and the possible intermediacy of 2 in the synthesis

of scandium silyl complexes, reactions of 2 with hydrosilanes

were investigated in detail. Hydrosilanes react rapidly with the

scandium hydride 2 via ScH/SiH exchange (eq 5, H′ ) H, D).

Primary silanes containing bulky substituents (e.g., MesSiH₃,

(Me₃Si)₃SiSiH₃, and Ph₃SiSiH₃) were observed to react with

Cp*₂ScH to form the corresponding scandium silyl only upon

removal of H₂(cyclohexane-d₁₂; one freeze/pump/thaw cycle

in a J. Young NMR tube rapidly gave the scandium silyl in ca.

10% yield at room temperature), but Si-Si dehydrocoupling

was not observed (by H NMR spectroscopy). The hindered

secondary silane Mes₂SiH₂did not react in the presence of 2

(cyclohexane-d₁₂, 1 week).

1

For less hindered silanes, the scandium hydride 2 slowly

catalyzes redistribution via Si-C bond activation.^19,25For

example, over 1 week (80 °C, cyclohexane-d₁₂), the reaction

of 2 with PhSiH₃produced Ph₂SiH₂(ca. 20% yield at 40%

conversion of PhSiH₃, GC-MS). With PhMeSiH₂, the scandium

hydride-catalyzed redistribution reactions exhibited an unusual

selectivity for Si-CH₃versus Si-C₆H₅bond activation. Heating

cyclohexane-d₁₂solutions (50 °C, 3 d) of PhMeSiH₂and 2

yielded a mixture of PhSiH₃and PhMe₂SiH (ca. 30% each) in

addition to the starting material PhMeSiH₂(ca. 40%, by GC-

MS); Ph₂SiH₂was not observed in the reaction mixture. This

selectivity for Si-CH₃bond activation is likely due to steric

factors. The silane Ph₂SiH₂appears to interact with compound

2 only via the ScH/SiH exchange reaction, since after several

days at room temperature PhSiH₃, Ph₃SiH, and oligosilanes were

1

not detected by H NMR spectroscopy (after H₂O quench) or

GC-MS. In comparison to the rapid redistribution reaction of

PhSiH₃and Ph₂SiH₂with early metal and lanthanide hydride

catalysts (e.g., Cp*₂MH; M ) Sm, Lu, Y, and [CpCp*HfH]-

[B(C₆F₅)₄]),^19,21,25the Cp*₂ScH-catalyzed redistribution of Ph-

SiH₃is slow.

This process is rapid for primary and smaller secondary silanes

at room temperature, as indicated by the presence of broad SiH

resonances in ¹H NMR spectra. Within 24 h the SiH resonances

disappear, presumably as a result of an H/D exchange process

involving benzene-d₆. In the presence of 2, the SiH resonances

of bulky secondary silanes (Mes₂SiH₂and ^tBu₂SiH₂) and tertiary

silanes (HSiPh₃, HSiEt₃, HSiPhMe₂, HSiPh₂Me) are sharp, and

³J_HHcoupling is observed at room temperature (for HSiEt₃,

HSiPhMe₂, and HSiPh₂Me). However, within 4 days at room

temperature the SiH resonances disappear, indicating that

deuterium exchange occurs at a very slow rate.

Reactions of Cp*₂ScPh (3) with Hydrosilanes. The reactiv-

ity of 3 toward hydrosilanes was of interest for several reasons.

First, it was thought that such reactions might provide an

alternative route to scandium silyl complexes. Second, the

transfer of a phenyl group from scandium to silicon represents

a potentially important step in catalytic benzene dehydrosila-

tions. In addition, this type of phenyl transfer would presumably

be required for the scandium-catalyzed redistribution of phe-

nylsilanes. Note that redistribution at silicon could compete with

a benzene dehydrosilation, if coupled to Si-Ph bond activation

9

J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 649

A R T I C L E S

Sadow and Tilley

by a Sc-H species.^19,21,25However, the latter process appears

to be very slow for Cp*₂ScH (vide supra).

Scheme 3. Density Functional Theory (DFT) Calculations of the

Gas-Phase Gibbs Free Energies of the Two Pathways for the

a

Reaction of Cp₂ScCH₃with SiH₄

The scandium phenyl 3 reacted rapidly with excess PhSiH₃

(1.5-2 equiv, benzene-d₆, room temperature) to produce a bright

yellow solution. The ¹H NMR spectrum of the reaction mixture

after 5 min indicated that 3 had been completely consumed and

that benzene had quantitatively formed. Interestingly, the major

scandium product was Cp*₂ScH (ca. 60%, eq 6).

Cp* ScPh

Cp* ScH

+ PhSiH₃f Ph-H +

+ Ph₂SiH₂(6)

2

3

2

The yellow color of the reaction solution indicated that scandium

silyl species were present, since the related scandium silyl

complexes 4-7 exhibit this color. Indeed, the ¹H NMR spectrum

contains a resonance at 4.08 ppm that is tentatively assigned to

the SiH₂group of Cp*₂ScSiH₂Ph. Over 12 h, the SiH resonances

of the complex disappeared, apparently as a result of H/D

exchange with benzene-d₆. However, prior to complete deu-

teration, a 1:1:1 triplet corresponding to the SiH resonance of

Cp*₂ScSiHDPh was detected (4.10 ppm). Note that this process

was also observed for the related complex 7 (vide supra). After

2 days at room temperature, the reaction mixture contained only

a small amount of Ph₂SiH₂(ca. 5%, by GC-MS after an

aqueous quench). Interestingly, PhH₂SiSiH₂Ph and other higher

molecular weight dehydrocoupling products were not detected

^aThe sum of the energies of the reactants have been arbitrarily set to

0.0 kcal/mol.

with d⁰transition metal and lanthanide complexes.^34-37These

theoretical investigations implicate concerted transition states

involving four-centered (2σ + 2σ) electrocyclic structures^35-37

but have not compared the barriers for the two pathways

described above. Given the large number of atoms and variables

associated with the Cp* ligand, model scandium species were

based on the (η⁵-C₅H₅)₂Sc fragment. The geometric structures

were minimized at the B3LYP/LACVP**++ level using the

Jaguar 4.0 suite,^38-40and the ground-state structures of Cp₂-

ScCH₃, Cp₂ScH, Cp₂ScSiH₃, Cp₂ScSiH₂CH₃, SiH₄, CH₄, H₂,

H₃SiCH₃, and H₂Si(CH₃)₂were verified by frequency calcula-

tions as containing exactly zero imaginary normal modes. All

transition states contained exactly one imaginary frequency

corresponding to a first-order saddle point on the potential

energy surface. The electronic energies were adjusted for the

zero-point energies (ZPE) and the Gibbs free energy (at 298.15

K) corrections obtained from the normal-mode analyses. The

calculated values for entropy and enthalpy obtained from the

frequency calculations indicate that entropic factors contribute

significantly to the activation barriers.

1

by GC-MS or H NMR spectroscopy.

Complex 3 reacted with MesSiH₃(1 equiv, in benzene-d₆,

room temperature) to slowly form equivalent amounts of Cp*₂-

ScSiH₂Mes (4) and benzene (t_1/2> 24 h, eq 7).

Cp* ScPh

Cp* ScSiH Mes

+ C₆H₆(7)

2 2

+ MesSiH₃f

2

3

4

This transformation is reversible in the absence of excess

MesSiH₃(i.e., 4 and benzene react to give 3 and MesSiH₃, vide

infra). Only a trace amount of MesPhSiH₂was observed (<5%),

which was identified by comparison of the ¹H NMR spectrum

to that of an independently prepared sample (by reaction of

MesSiH₂Cl with PhLi). The reaction of 3 with MesSiH₃is

significantly slower than the corresponding reaction of 1.

Compound 3 did not react with Ph₂SiH₂at room temperature

The two pathways for the reaction of Cp₂ScCH₃with SiH₄

are shown in Scheme 3, and the transition states for these

pathways are illustrated in Figures 3 and 4. The sum of the

energies of Cp₂ScCH₃and SiH₄(both electronic and Gibbs free

energies) were set at 0.0 kcal/mol; the energies of the transition

states and products are reported relative to this arbitrary level.

The activation barrier for Sc-Si and Si-C bond formation in

1

over the course of 2 days (benzene-d₆). A H NMR spectrum

of a benzene-d₆solution of 3 and Ph₂SiH₂heated to 70 °C for

several hours contained resonances due to the Cp* group of

3-d₅and benzene. Also, the SiH resonance of Ph₂SiH₂disap-

peared due to deuterium incorporation from benzene-d₆.

Density Functional Theory (DFT) Calculations on Reac-

tions of Cp₂ScCH₃with Hydrosilanes. In the reactions of 1

with hydrosilanes, subtle factors appear to control the selectivity

for Si-Me (and Sc-H) vs Me-H (and Sc-Si) bond formations.

It is of interest to control this reactivity, since it currently offers

the only route to 14-electron scandocene silyl complexes.

Perhaps more importantly, control over the selectivity for alkyl

transfer to silicon could allow the design of catalytic cycles for

hydrocarbon dehydrosilations. With these considerations in

mind, two primary pathways observed for the reaction of 1 with

hydrosilanes (scandium-silicon and silicon-carbon bond for-

mation) were modeled using density functional theory (DFT).

Several groups have used computational methods, including

extended Hu¨ckel MO analysis and DFT studies, to study σ-bond

metathesis reactions of dihydrogen, hydrocarbons, and silanes

(34) Rabaaˆ, H.; Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1986, 108,

4327-4333.

(35) (a) Steigerwald, M. L.; Goddard, W. A. J. Am. Chem. Soc. 1984, 106,

308-311. (b) Rappe´, A. K. Organometallics 1990, 9, 466-475. (c) Rappe´,

A. K.; Upton, T. H. J. Am. Chem. Soc. 1992, 114, 7507-7517.

(36) (a) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. Soc. 1993, 115, 636-

646. (b) Ziegler, T.; Folga, E. J. Organomet. Chem. 1994, 478, 57-65. (c)

Ustynyuk, Y. A.; Ustynyuk, L. Y.; Laikov, D. N.; Lunin, V. V. J.

Organomet. Chem. 2000, 597, 182-189.

(37) (a) Deelman, B.-J.; Teuben, J. H.; Macgregor, S. A.; Eisenstein, O. New.

J. Chem. 1995, 19, 691-698. (b) Maron, L.; Perrin, L.; Eisenstein, O. J.

Chem. Soc., Dalton Trans. 2002, 534-539. (c) Perrin, L.; Maron, L.;

Eisenstein, O. Inorg. Chem. 2002, 41, 4355-4362.

(38) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Volko, S. H.;

Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-1211. (c) Lee, C.; Yang,

W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.

(39) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. J. Chem. Phys. 1980, 72,

650-654.

(40) Jaguar 4.0, release 23; Schro¨dinger, Inc.: Portland, OR, 1998.

9

650 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

Ph₂SiH₂(eq 2, k_H/k_D) 0.91(5)) is also consistent with a

transition state that does not require R-agostic assistance.

To assess the potential importance of a chain reaction

involving a scandium hydride intermediate, DFT methods were

used to model this alternative two-step pathway (i.e., hydro-

genolysis of Cp₂ScCH₃and subsequent dehydrogenative meta-

lation of SiH₄by Cp₂ScH, eq 8).

H₂

SiH₄

Cp2ScCH₃

8 Cp₂ScH

8 Cp₂ScSiH₃

(8)

-CH₄

-H₂

The sum of the reactant molecules’ energies [Cp₂ScCH₃+ SiH₄

+ H₂] were again set to 0 kcal/mol; all the transition states,

intermediates, and product energies are reported relative to this

arbitrary value (Scheme 4). As expected, hydrogenolysis of the

Sc-C bond of Cp₂ScCH₃is exothermic by 14.4 kcal/mol, while

dehydrocoupling of Cp₂ScH and SiH₄is endothermic by 5.1

kcal/mol. In contrast, the dehydrocoupling of Cp₂LaH and SiH₄

to Cp₂LaSiH₃and H₂was calculated to be exothermic by 4.7

kcal/mol.^37cThe energetic barriers for both steps of the sequence

are lower than the barrier for the direct exchange reaction

discussed above, indicating that such a pathway is possible for

this ligand exchange reaction. The calculated ∆G^qfor the

hydrogenolysis of Cp₂ScCH₃is 19.52 kcal/mol (transition state

C), and the calculated activation barrier for the metalation of

SiH₄by Cp₂ScH is 21.54 kcal/mol (transition state D). Although

the calculated activation barrier for this hydrogen-mediated

process is lower than the barrier for the direct reaction of Cp₂-

ScCH₃with SiH₄, low concentrations of Cp*₂ScH or H₂in the

actual reaction mixture could easily cause the absolute rate of

this process to be slower than the one-step pathway.

Figure 3. Rendered illustration of the DFT-optimized transition state

structure (A) for the interaction of Cp₂ScCH₃with SiH₄which produces

Cp₂ScSiH₃and CH₄. The hydrogen atoms on the Cp rings were removed

for clarity.

C-H Bond Activation Reactions of Benzene and Methane

with Cp*₂ScSiH₂R Complexes. Given the possibility of

productive hydrocarbon activations based on σ-bond metathesis

(Scheme 1), we investigated the reactions of benzene and

methane with scandium silyl complexes. The silyl complex 4

reacted slowly with benzene (as the solvent) to form the

corresponding phenyl complex 3 and MesSiH₃in ∼ 90% yield

Figure 4. Rendered illustration of the DFT-optimized transition state

structure (B) for the reaction of Cp₂ScCH₃with SiH₄to form Cp₂ScH and

H₃CSiH₃. The hydrogen atoms on the Cp rings were removed for clarity.

1

by H NMR spectroscopy (room temperature, t_1/2≈ 24 h).

Heating benzene-d₆solutions of 4 at 65 °C produced 3-d₅in

ca. 70% yield after 60 min (90% conversion by ¹H NMR

spectroscopy). This reaction may occur via several mechanisms

(Scheme 5) which are analogous to the pathways considered

for formation of the scandium silyl compounds (vide supra,

Scheme 2). The direct interaction of the scandium-silicon bond

with a C-H bond of benzene, via a four-centered transition

state, is suggested by an analogous benzene metalation by the

isoelectronic hafnium hydrosilyl species, Cp₂Hf(SiHMes₂)(µ-

Me)B(C₆F₅)₃.²⁰Also, 1 is known to react with benzene primarily

via an analogous second-order reaction.¹³The second pathway,

involving intramolecular Cp* activation and MesSiH₃elimina-

tion as the first step, was proposed as a minor pathway for the

reaction of benzene with 1.¹³Finally, the possible participation

of the hydride 2 is suggested by its formation in the thermal

decomposition of 4 and by its direct reaction with benzene to

form 3.¹³There are several pathways by which 2 may form

during the reaction of 4 with benzene, including reactions of 4

with H₂or MesSiH₃, or with a second equivalent of 4.²³

this simplified system are remarkably similar (∆G^q) 26.7 and

27.4 kcal/mol, respectively; ∆E_tot) 14.0 and 11.6 kcal/mol,

q

respectively; E_tot) electronic energy). This similarity is

consistent with the experimental results, which show that both

types of reactions occur for 1 with relatively minor variations

in the structure of the silane. Note that the experimental value

for ∆G^qfor the formation of Ph₂MeSiH from 1 and Ph₂SiH₂is

19.3 kcal/mol (determined from ∆H^qand ∆S^qat 298.15 K).

The transition states shown in Figures 3 and 4 appear to involve

coordination of an R-CH bond to the scandium center in an

agostic fashion. However, a three-center two-electron R-agostic

interaction should lengthen the coordinated C-H bond distance.

Comparisons of the C-H bond lengths calculated for CH₄(1.09

Å), Cp₂ScCH₃(1.10 Å), the two nonbridging C-H bonds in

transition states A and B ([Cp₂ScCH₃‚SiH₄]^q, C-H_terminal, 1.09

and 1.10 Å), and the bridged Sc‚‚‚H‚‚‚C structures (C-H_bridge

for A is 1.11 Å, B is 1.12 Å) suggest the short Sc‚‚‚H distances

(A, 2.41 Å; B, 2.15 Å) merely result from the geometric

constraints imposed by a rigid, highly ordered transition state.

The secondary kinetic isotope effect for the reaction of 1 and

1

The H NMR spectrum of the reaction mixture of 4 and

benzene-d₆indicated that the conversion of 4 to 3 is quite

complicated. After 1 h at 65 °C this mixture contained the

9

J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 651

A R T I C L E S

Sadow and Tilley

Scheme 4. DFT Calculation of the H₂-Mediated Ligand Exchange Pathway^a

^aThe sum of the energies of Cp₂ScCH₃, SiH₄, and H₂has arbitrarily been set to 0.0 kcal/mol, and all of the products’ eneriges are reported relative to

the initial point. The barrier for the hydrogenolysis of Cp₂ScCH₃is 19.52 kcal/mol, and the barrier for metalation of SiH₄by Cp₂ScH is 21.54 kcal/mol.

Scheme 5. Possible Pathways for the Conversion of Cp*₂ScSiH₂Mes (4) to Cp*₂ScPh (3)

starting material 4 (ca. 16% of total scandium present, deter-

mined via H NMR spectroscopy by integration of Cp* peaks

slope of plots of ln[4] vs time), the overall reaction time for

formation of 3 from the scandium silyl complex is much less

than it is for reaction of the methyl complex (1) with benzene.

Upon addition of small amounts of Ph₂SiH₂to 4 in benzene-d₆

(∼0.1 equiv) to generate trace amounts of 2 and initiate the

hydride-mediated pathway, the overall rate of formation of 3

increased. Furthermore, the thermolysis of 4 in cyclohexane-

1

relative to a cyclooctane standard) and the products MesSiH₃-

d₁, 3-d₅(ca. 70%), and 2-d₁(ca. 12%). The presence of the

scandium hydride product complicates NMR analysis because

the SiH resonances of any primary and unhindered secondary

silanes are broadened and shifted by rapid ScH/SiH exchange

processes. This exchange reaction, in conjunction with a slower

H/D exchange process between benzene-d₆and the Sc-H of

2, results in deuteration of the hydrosilanes in the reaction

mixture. A ²H NMR spectrum indicates that MesD₂SiSiD₂Mes

(∼5%) is a product of this reaction, but there is no evidence

d₁₂at 50 °C yielded Cp*₂ScH in 95% yield (t_1/2) 8.5 h). Thus,

the metalation of benzene by the scandium hydride intermediate

appears to be the primary reaction pathway, although the direct

reaction between 4 and benzene may occur during the early

stages of the reaction.

1

2

for (Cp*-d_x)Cp*ScC₆D₅in either the H or H NMR spectra.

The absence of deuterium in the Cp* ligand rules out a pathway

involving activation of a Cp* ligand.

Interestingly, Cp*₂ScSiH₂SiPh₃(7) reacted with benzene-d₆

at 65 °C to form Cp*₂ScSiHDSiPh₃(7-d₁) and Cp*₂ScSiD₂-

SiPh₃(7-d₂) (t_1/2) 24 h for conversion to 7-d₂) prior to

formation of the scandium phenyl 3. Only trace amounts of 3-d₅

(<5%) and the scandium hydride 2 (<5%) were formed at this

temperature after 24 h. As in the reaction of 4 with benzene,

the rate of conversion of 7 to 7-d₁and 7-d₂increased as the

reaction proceeded. Given the presence of trace amounts of 2,

this H/D exchange could proceed without cleavage of the

scandium-silicon bond. Alternatively, the exchange might

involve the reaction of benzene-d₆with 7 to produce Ph₃-

Kinetic studies indicate that the conversion of 4 to 3 in

benzene is approximately two times faster than the rate of this

conversion in benzene-d₆. However, for the conversion of 4 to

3, pseudo-first-order plots of ln{[4]₀/[4]) vs time are not linear.

Plots of [Cp*₂ScPh] vs time are S-shaped (Figure 5), resulting

from an initially slow reaction which then speeds up (probably

via catalysis). Although the initial rates for the reactions of

benzene with 1 and 4 are similar (as determined from the initial

9

652 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

Figure 5. Plot of ln[7] vs time for the thermolysis of 3 in benzene-d₆. As the reaction proceeds, the slope of the curve becomes more steep, indicating an

autocatalytic rate enhancement.

Scheme 6. A Possible Catalytic Cycle for Methane

SiSiH₂D. Metalation of Ph₃SiSiHD-H with concurrent benzene-

Dehydrosilation via the Metalation of Methane by Cp*₂ScH to

Form 1 and Eliminate H₂, Followed by Transfer of the Methyl

Group from Scandium to Silicon

d₅elimination would yield 7-d₁. A third possibility involves

scandium-silyl hydrogenolysis to form 2 and Ph₃SiSiH₃,

followed by ScD/SiH exchange and then the dehydrocoupling

of Ph₃SiSiHD-H with 2.

The reaction of 4 with methane (ca. 7 atm, 14 equiv in

solution) in cyclohexane-d₁₂occurred slowly at room temper-

ature over 4 days to give MesSiH₃and Mes(Me)SiH₂(85% and

15%, respectively, by GC-MS), along with Cp*₂ScH (42%)

as the major scandium-containing product (eq 9).

Cp* ScSiH Mes

Cp* ScCH

+ CH₄h

+

2

3

4

1

Cp* ScH

MesSiH₃h

+ Mes(Me)SiH₂(9)

2

At intermediate stages of the reaction, the methyl complex 1

was observed in trace quantities. As described above, 1 reacted

with MesSiH₃to give 4 and CH₄as the primary kinetic products.

Taken together, these results indicate the existence of the

coupled equilibria in eq 9, for which the thermodynamic

products are Mes(Me)SiH₂and 2. However, just as in the

metalation of benzene discussed above, it is unclear whether

the C-H bond of methane is activated by 4 or 2. It is noteworthy

that unlike the reaction of 4 with methane, which produced small

but detectable amounts of the scandium methyl complex 1, the

metalation of CH₄by 2 to form 1 could not be directly observed.

For example, heating cyclohexane-d₁₂solutions of 2 under

7-150 atm of CH₄to 80 °C for 1-4 days, followed by release

of the pressure, did not produce observable quantities of 1 (by

¹H NMR spectroscopy). However, a reaction between Cp*₂-

ScD and CH₄is implied by the incorporation of deuterium into

methane in the presence of excess D₂or benzene-d₆.¹³Ad-

ditionally, the mechanism of this exchange likely involves

metalation of CH₄by 2 to form 1. Subsequent reaction of 1

with D₂would then produce CH₃D and 2-d₁.

Catalytic Hydrocarbon Dehydrosilation. Several observa-

tions suggested that catalytic methane functionalization via

dehydrosilation (eq 1, R ) CH₃) might be possible with a Cp*₂-

Sc-derived catalyst. For example, the individual steps of the

catalytic cycle of Scheme 1 (where R ) CH₃) have been

observed in the stoichiometric reactions of Cp*₂ScMe with

silanes (Si-CH₃bond formation), the dehydrocoupling of Cp*₂-

ScH with hydrosilanes (Sc-Si bond formation), and the reaction

of Cp*₂ScSiH₂R with hydrocarbons (C-H bond activation).

Additionally, C-H bond activations by 2 or (η¹:η⁵-C₅Me₄CH₂)-

ScCp* ¹³may participate in a catalytic cycle for hydrocarbon

dehydrosilation. A cycle involving methane metalation by 2,

followed by methyl transfer, is shown in Scheme 6.^18eA third

catalytic cycle involves the addition of methane to the Cp*-

metalated species (η¹:η⁵-C₅Me₄CH₂)ScCp*, which appears to

be involved in the activation of ¹³CH₄by 1 (Scheme 7).¹³The

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J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 653

A R T I C L E S

Sadow and Tilley

Scheme 7. A Possible Catalytic Cycle for Methane

Dehydrosilation by the Addition of CH₄Across the Sc-C Bond of

the Metalated Complex (η¹:η⁵-C₅Me₄CH₂)ScCp*, Followed by

Transfer of the Methyl Group to Silicon

Initial mechanistic investigations of the methane dehydrosi-

lation focused on determining the composition of the catalytic

reaction mixture. Therefore, the reactions were monitored by

¹H NMR spectroscopy in sealed NMR tubes at low methane

pressures (∼7 atm). Unfortunately, identification of the species

responsible for C-H bond activation was complicated by the

presence of several unidentified scandium species in addition

to 2 in the reaction mixture. However, as the concentration of

2 decreased over the course of the reaction due to slow

decomposition, the rate of formation of Ph₂MeSiH also de-

creased (as determined by ¹H NMR spectroscopy through

integration relative to a cyclohexane standard). This apparent

dependence of rate of product formation on the concentration

of 2 suggests that it participates in the catalytic cycle. The

pathway for the decomposition of 2 does not involve formation

of the insoluble complex [µ-(η¹:η⁵-C₅Me₄CH₂)ScCp*]₂,¹³since

the reaction mixtures (cyclohexane-d₁₂, 7 atm of methane, 10

equiv of Ph₂SiH₂) were homogeneous for at least 1 week.

Hydride 2 was stable at 80 °C in cyclohexane-d₁₂solution in

the presence of 1.2 equiv of Ph₂MeSiH under 150 atm of

methane for 4 days.

successful development of catalytic hydrocarbon dehydrosilation

via these cycles requires that possible dehydropolymerization

and redistribution side reactions of the silanes do not occur.

Significantly, the addition of Ph₂SiH₂or MesSiH₃to cyclohex-

ane-d₁₂solutions of 2 did not lead to formation of dehydropo-

lymerization and redistribution products at room temperature

over several days (vide supra).

Cyclohexane-d₁₂solutions of 2 and various silanes (in a J.

Young NMR tube) were frozen at -196 °C, and the headspace

was evacuated and then replaced with 0.5 atm of CH₄. The

solutions were carefully warmed to room temperature, shaken

vigorously to dissolve the maximum amount of methane, and

then heated to 50-80 °C. A cyclohexane-d₁₂solution of Cp*₂-

ScH and Ph₂SiH₂(10 equiv) reacted under ca. 7 atm of CH₄in

a J. Young tube at 80 °C to yield Ph₂MeSiH (by GC-MS and

¹H NMR spectroscopy) in substoichiometric quantities (∼0.4

equiv after 1 week). Although catalytic methane activation was

not initially observed, all of the Ph₂MeSiH product was derived

from methane. The reaction rate is dependent on methane

concentration as demonstrated by the production of 5 equiv of

Ph₂MeSiH when a cyclohexane solution of Ph₂SiH₂and 1 was

heated to 80 °C under 150 atm of methane (1 week, 1 equiv

from 1; eq 10).

The mechanism of Si-C bond formation could proceed via

a four-centered transition state in which a Sc-Me derivative

reacts with Ph₂SiH₂to yield Cp*₂ScH and Ph₂MeSiH (methyl

transfer) or by reaction of a scandium-silyl complex with

methane (silyl transfer). The former mechanism is more

attractive, since it does not require carbon to occupy the

â-position of the four-centered transition state^16band since this

transformation has been directly observed in the reaction of Ph₂-

SiH₂with 1 (vide supra). In addition, the reaction of 4 with a

large excess of methane as shown in eq 9 is significantly slower

and less efficient than the reaction of 1 with Ph₂SiH₂. These

observations lead us to favor methyl transfer from Sc to Si as

the Si-C bond-forming step in the catalysis of eq 10. This

would indicate that the other product of Si-CH₃bond formation,

Cp*₂ScH (2), must be involved in the catalytic cycle.

Study of the methane activation step is complicated by the

number of scandium species, including 2, Cp*₂ScSiHPh₂, and

(η¹:η⁵-C₅Me₄CH₂)ScCp*, that might be involved. So far, it has

not been possible to directly observe a stoichiometric C-H bond

activation that could model this step. For example, attempts to

directly observe the metalation of methane by Cp*₂ScH have

been unsuccessful, although this metalation is implied by

deuteration of CH₄catalyzed by 2.¹²Furthermore, studies of

the metalation of methane by Cp*₂ScCH₂CMe₃have shown that

this reaction is accelerated by the addition of Cp*₂ScH (i.e., a

hydride catalyst).⁴¹This result also implies that 2 reacts directly

with CH₄to form 1 and H₂. However, as suggested by

investigations of equilibria between scandium hydride and

scandium alkyl species,⁴²the metalation of methane by 2 may

be an equilibrium process which highly favors the scandium

hydride and methane reactants over the products (eq 11).

Furthermore, the reaction of Cp*₂ScMe with H₂is quite fast

(the microscopic reverse of methane metalation).¹³In summary,

the above observations suggest that the reaction of 2 with CH₄

produces a low equilibrium concentration of 1 (eq 11).

10% Cp*₂ScMe

Ph₂SiH₂+ 150 atm CH₄

8 Ph₂MeSiH (10)

C₆H₁₂, 80 °C, 7 days

Increasing the amount of Ph₂SiH₂to 20 equiv did not substan-

tially affect the rate of reaction, as the same amount of Ph₂-

MeSiH product (5 equiv) was observed after 1 week. These

observations indicate that the rate-limiting step in the catalytic

cycle is C-H bond activation.

Though the methane conversion is slow, the reaction is

reasonably selective with 75% of the Ph₂SiH₂consumed being

converted to Ph₂MeSiH. It seems possible that some of the Ph₂-

SiH₂was consumed by competitive Si-Ph hydrogenolysis^19,25

and silane dehydrocoupling,¹⁷but no other products were

observed by GC after removal of the catalyst by an aqueous

workup. Increasing the temperature to 100 °C decreased the

1

amount of Ph₂MeSiH produced (<1 turnover, by H NMR

(41) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 7971-7977.

(42) Bulls, A. R.; Bercaw, J. E.; Manriquez, J. M.; Thompson, M. E. Polyhedron

1988, 7, 1409-1428.

spectroscopy), apparently as a result of rapid decomposition of

the Cp*₂ScH catalyst at this temperature.

9

654 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Sc Silyl Complexes of the Type Cp*₂ScSiHRR

′

A R T I C L E S

(d⁰) transition metal and lanthanide complexes and include rapid

MH/SiH exchange and metal-mediated Si-C bond formation

(alkyl transfer). However, some aspects to this reactivity are

unusual, such as the interaction of methyl complex 1 with certain

silanes to produce methane and a Sc-Si bond. Also, it is

noteworthy that although Cp*₂Sc derivatives are generally quite

reactive in σ-bond metathesis processes, the hydride 2 is not

an efficient catalyst for the dehydropolymerization or redistribu-

tion of silanes. These unusual selectivities appear to allow

observation of the first catalytic methane dehydrosilation, which

apparently involves only σ-bond metathesis steps. The mech-

anism of this catalysis appears to be analogous to those

previously proposed for d⁰metal-catalyzed dehydropolymer-

izations of silanes^16b,17eand stannanes,⁴³as well as organosilane

redistribution and hydrogenolysis reactions.^19,21,25

Cp* ScH

Cp* ScMe

+ CH₄h

+ H₂

(11)

2

1

Methane activation by the Cp*-activated, monomeric species

(η¹:η⁵-C₅Me₄CH₂)ScCp* is suggested by studies of the C-H

bond activations of methane and benzene by 1, which revealed

a methane activation process that is zero-order in methane and

first-order in scandium.¹³Furthermore, metalation of the Cp*

methyl groups is indicated by the rapid incorporation of

deuterium into the Cp* methyl groups of 2 and 3 in the presence

of benzene-d₆.¹³However, heating cyclohexane-d₁₂suspensions

of [µ-(η¹:η⁵-C₅Me₄CH₂)(η⁵-Cp*)Sc]₂with CH₄(80 °C, 1 week)

did not produce detectable quantities of 1 (by ¹H NMR

spectroscopy), though the insolubility of the dimeric complex

may hinder its reactivity.¹³Additionally, the putative species

(η¹:η⁵-C₅Me₄CH₂)ScCp* could not be detected in the catalytic

reaction mixture or in reactions of 1 with benzene or methane

As with several other d⁰transition metal and f-element

hydride complexes,^17e,19,23,252 reacts with silanes via relatively

rapid ScH/SiH exchange. The rate of this process is very

sensitive to steric factors. Thus, for primary and less hindered

secondary silanes, the exchange is rapid and results in broad

(by H NMR spectroscopy).¹³

1

A third possibility is that Cp*₂ScSiHPh₂mediates the C-H

bond activation, as suggested by reactions of isoelectronic Cp₂-

Hf(SiHMes₂)(µ-Me)B(C₆F₅)₃with benzene and toluene.²⁰In

fact, reaction of the scandium silyl 3 with CH₄produces small

quantities of the methyl complex 1 and the methylsilane Mes-

(Me)SiH₂(vide supra). However, Cp*₂ScSiHPh₂could not be

observed under the catalytic reaction conditions. All attempts

to independently generate and characterize Cp*₂ScSiHPh₂failed,

including treatment of 3 with excess Ph₂SiH₂(2-10 equiv, in

benzene-d₆, cyclohexane-d₁₂) at room temperature over 1 week.

Addition of neat Ph₂SiH₂(10 equiv) to 1, 2, and 3, followed by

the addition of cyclohexane-d₁₂, also did not generate Cp*₂-

ScSiHPh₂(by ¹H NMR spectroscopy). These reactions produced

yellow solutions, but a scandium-silyl complex could not be

1

SiH resonances in H NMR spectra of the reaction mixtures.

Reactions between 2 and more hindered secondary silanes

(Mes₂SiH₂and ^tBu₂SiH₂) and tertiary silanes (Et₃SiH, Ph₃SiH,

and Ph₂MeSiH) are slow relative to the ¹H NMR time scale, as

evidenced by sharp SiH resonances which slowly disappear due

to deuterium incorporation from benzene-d₆. Similar results have

been observed for the Group 4 hydrides CpCp*HfHCl and Cp*₂-

HfH₂and for the lanthanide hydrides (Cp*₂LnH)₂(Ln ) Lu,

Sm, Y).^17e,19,23,25It was therefore somewhat surprising to observe

that 2 exhibits unusually low activities toward the metalation,

dehydrocoupling, and redistribution of hydrosilanes. For com-

parison, the isoelectronic Cp*₂LuH reacts rapidly with PhSiH₃

at room temperature to yield benzene and short polysilane

chains,¹⁹and the complex Cp*₂SmH reacts with PhSiH₃to form

mixtures of Ph₃SiH, Ph₂SiH₂, and SiH₄.^25cWhile these dehy-

drocoupling and redistribution reactions are catalyzed by 2, they

require forcing conditions (elevated temperatures or active

removal of dihydrogen) and only occur with relatively unhin-

dered silanes.

1

observed by H NMR spectroscopy.

Complex 2 also catalyzes the slow dehydrogenative silylation

of other hydrocarbons. For example, the reaction of Ph₂SiH₂(8

equiv) with isobutylene (18 equiv) in the presence of 2 produced

the vinyl silane Ph₂(Me₂CCH)SiH (50 °C, ca. 2 turnovers after

20 days in cyclohexane-d₁₂). With PhSiH₃and isobutylene, a

mixture of hydrosilation (Me₂HCCH₂SiH₂Ph) and dehydrosi-

lation (Me₂CCHSiH₂Ph) products were observed (3:2 ratio;

cyclohexane-d₁₂solvent), as well as minor amounts of dehy-

drocoupling products. The scandium hydride complex also

catalyzed the very slow dehydrocoupling of cyclopropane and

Ph₂SiH₂in cyclohexane-d₁₂at 80 °C (2.5 turnovers, 20 days).

Hydride 2 reacts directly with cyclopropane and isobutylene to

give the scandium complexes Cp*₂Sc^cPr and Cp*₂ScCHCMe₂,

respectively. These scandium complexes were also detected (by

¹H NMR spectroscopy) in the catalytic reaction mixtures. This

observation suggests that the dehydrosilations of cyclopropane

and isobutylene proceed via a two step mechanism involving

hydrocarbon metalation by 2, followed by organic group transfer

to silicon. Interestingly, benzene is not a suitable substrate for

this dehydrosilation process. Though the metalation of benzene

by 2 occurs easily, stoichiometric reactions of Cp*₂ScPh (3)

with hydrosilanes demonstrate that transfer of a phenyl group

from scandium to silicon is not facile (see above). Thus, in this

case, dehydrosilation is inhibited by slow phenyl transfer rather

than by the C-H bond activation step.

The reactions of Cp*₂ScMe (1) with RSiH₃(R ) Mes, Trip,

SiPh₃, Si(SiMe₃)₃) are unusual in that d⁰transition metal and

lanthanide alkyl complexes typically react with hydrosilanes to

form Si-C bonds and metal hydride complexes. However, the

lanthanide complexes Cp*₂LnR (Ln ) Sm, R ) Ph; Ln ) Lu,

R ) Me) react with o-MeOC₆H₄SiH₃to form the corresponding

lanthanide silyl complex,^19,25cand the reactions of Cp*₂LnCH-

(SiMe₃)₂(Ln ) Sm, Y, Nd) with (Me₃Si)₂SiH₂also afford the

corresponding lanthanide silyl complexes Cp*₂LnSiH(SiMe₃)₂.^22a

However, note that the latter reaction has been shown to proceed

via hydrogenolysis of the Ln-C bond to form Cp*₂LnH, which

then reacts with the silane (Me₃Si)₂SiH₂.²⁴

The reactions of 1 and 2 with silanes and hydrocarbons

provide a basis for the development of catalytic dehydrosilations.

This reactivity has led to the first dehydrosilation of methane

and one of the few examples of catalytic C-H bond activation

via σ-bond metathesis.^15,41Interestingly, although complexes

of the type Cp*₂MR (M ) Sc, Lu, Y; R ) H, Me) react with

Concluding Remarks

(43) (a) Imori, T.; Tilley, T. D. J. Chem. Soc., Chem. Commun. 1993, 1607-

1609. (b) Imori, T.; Lu, V.; Cai, H.; Tilley, T. D. J. Am. Chem. Soc. 1995,

117, 9931-9940.

Reactions of Cp*₂ScR (R ) hydrido, alkyl) derivatives with

silanes proceed via pathways that are well established for early

9

J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 655

A R T I C L E S

Sadow and Tilley

methane under mild conditions,^12,13the scandium complexes

are the least reactive of the series toward stoichiometric methane

activation. Thus, more active catalysts might be based on other

electrophilic, trivalent transition-metal or lanthanide centers.

However, selectivities in the reactions of metal hydride and

metal alkyl species with silanes is critical to the catalysis.

As mentioned previously, an important aspect of the chemistry

of Cp*₂ScH (2), which allows this complex to function as a

catalyst for the dehydrosilation of methane, is its relatively low

activity toward the dehydropolymerization and redistribution of

silanes. In contrast, the activations of Si-Ph bonds by (Cp*₂-

LuH)₂and (Cp*₂SmH)₂in redistribution and hydrogenolysis

processes are rapid.^19,25Presumably, these trends reflect dif-

ferences in the steric properties of the Cp*₂Sc, Cp*₂Lu, and

Cp*₂Sm fragments, as it is known that σ-bond metathesis

reactions are highly sensitive to steric factors. Apparently, steric

interactions in the Cp*₂Sc system are significant enough to

impact the selectivities in σ-bond metathesis but not severe

enough to prohibit the reactions of interest. Thus 2 reacts with

Ph₂SiH₂via ScH/SiH exchange, and 1 reacts with Ph₂SiH₂

selectively by methyl transfer to silicon.

metathesis. The search for more efficient processes of this type

would be facilitated by a deeper understanding of the factors

that influence selectivities. In this regard, the results reported

here are promising in that they demonstrate that selectivities in

σ-bond metathesis reactions may be readily manipulated via

modest changes in the catalyst structure and in the nature of

the substrates.

Acknowledgment. This work was supported by the National

Science Foundation and by the Director, Office of Energy

Research, Office of Basic Energy Sciences, Chemical Sciences

Division, of the U.S. Department of Energy under Contract DE-

AC03-76SF00098. We also thank Dr. John Bercaw, Dr. Richard

Andersen, and Dr. Mark Thompson for helpful discussions; Dr.

Benjamin King and Dr. Robert Bergman for assistance with

DFT calculations; and Dr. Allen G. Oliver and Dr. Fred

Hollander for assistance with the X-ray structure determination.

Supporting Information Available: CIF files for the X-ray

structures Cp*₂ScSiH₂SiPh₃(6) and Cp*₂ScSiH(SiMe₃)₂(8).

This material is available free of charge via the Internet at

http://pubs.acs.org.

Although the hydrocarbon conversions reported here are slow

and not yet synthetically useful, they suggest a new approach

for the development of catalytic processes based on σ-bond

JA040141R

9

656 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Article Doi

Synthesis and characterization of scandium silyl complexes of the type Cp*2ScSiHRR′. σ-bond metathesis reactions and catalytic dehydrogenative silation of hydrocarbons

DOI: 10.1021/ja040141r

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Article abstract of DOI:10.1021/ja040141r

Full text of DOI:10.1021/ja040141r

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