Intermolecular β-hydrogen abstraction in ytterbium, calcium, and potassium tris(dimethylsilyl)methyl compounds

Article

pubs.acs.org/Organometallics

Intermolecular β‑Hydrogen Abstraction in Ytterbium, Calcium,

and Potassium Tris(dimethylsilyl)methyl Compounds

KaKing Yan, George Schoendorﬀ, Brianna M. Upton, Arkady Ellern, Theresa L. Windus,

and Aaron D. Sadow*

Department of Chemistry and U.S. Department of Energy Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States

S

* Supporting Information

ABSTRACT: A series of organometallic compounds contain-

ing the tris(dimethylsilyl)methyl ligand are described. The

potassium carbanions KC(SiHMe₂)₃and {KC(SiHMe₂)₃-

TMEDA}₂are synthesized by deprotonation of the hydro-

carbon HC(SiHMe₂)₃with potassium benzyl. {KC(SiHMe₂)₃-

TMEDA}₂crystallizes as a dimer with two types of three-center−

two-electron K−H−Si interactions: side-on coordination of SiH

(∠K−H−Si = 102(2)°) and more obtuse K−H−Si structures

(∠K−H−Si ≈ 150°). The divalent calcium and ytterbium

compounds M{C(SiHMe₂)₃}₂L (M = Ca, Yb; L = 2THF, TMEDA) are prepared from MI₂and 2 equiv of KC(SiHMe₂)₃. Low ¹J_SiH

coupling constants in the NMR spectra, low-energy ν_SiHbands in the IR spectra, and short M−Si distances and small M−C−Si angles

in the crystal structures suggest β-agostic interactions on each C(SiHMe₂)₃ligand. The IR assignments of M{C(SiHMe₂)₃}₂L

(L = 2THF, TMEDA) are supported by DFT calculations. The compounds M{C(SiHMe₂)₃}₂L react with 1 or 2 equiv of B(C₆F₅)₃to

give the 1,3-disilacyclobutane {Me₂SiC(SiHMe₂)₂}₂and MC(SiHMe₂)₃HB(C₆F₅)₃L or M{HB(C₆F₅)₃}₂L, respectively. In addition,

M{C(SiHMe₂)₃}₂L compounds react with BPh₃to give β-H abstracted products. The compounds M{C(SiHMe₂)₃}₂THF₂react with

SiMe₃I to yield Me₃SiH and disilacyclobutane as the products of β-H abstraction, while M{C(SiHMe₂)₃}₂TMEDA and Me₃SiI form a

mixture of Me₃SiH and the alkylation product Me₃SiC(SiHMe₂)₃in a 1:3 ratio.

M−C bonds that give strongly basic ligands, potentially open

coordination sites resulting from ﬂexible coordination geometries,

labile metal−ligand interactions, and often highly Lewis acidic

metal centers.

INTRODUCTION

■

Isolable and thermally robust organotransition-metal compounds

tend to lack β-hydrogen-containing alkyl ligands, as these

groups are susceptible to intramolecular reaction path-

ways, including β-hydrogen elimination and β-hydrogen

abstraction.¹However, this easily identiﬁed structural feature

is not the only requirement for classical intramolecular β-H

elimination; at least one vacant orbital or coordination site

must be located cis to the alkyl ligand, the accepting orbital

must have the appropriate energy, the symmetry of the orbitals

involved should be matched throughout the reaction, and the

overall thermodynamics must favor elimination and/or

subsequent products. The pathway for β-hydrogen elimination

of an alkyl ligand is the microscopic reverse of oleﬁn insertion,

and thus both the forward and reverse directions of these

reactions are chemically important.²

Likewise, there are speciﬁc requirements for β-hydrogen

abstraction. Intramolecular β-hydrogen abstraction requires a

suﬃciently basic X⁻ligand, also located cis to the alkyl ligand, to

allow the conjugate acid HX to be a leaving group. Note that

β-elimination and intramolecular β-abstraction are electronically

dissimilar in terms of the formal polarization of the β-hydrogen: a

hydride is transferred to the metal center in the former reac-

tion, whereas a proton is transferred in the latter transformation.

In an interesting contrast to transition-metal chemistry, many

main-group and rare-earth-metal alkyls are reticent to undergo

β-hydrogen eliminations and abstractions despite highly polarized

For example, β-hydrogen elimination of alkyllithiums takes

place at 130−150 °C in reﬂuxing hydrocarbons;³the sodium

congeners react more rapidly, requiring less forcing conditions,⁴

while compounds such as tert-butylpotassium readily eliminate

isobutylene as one of a number of reaction pathways.⁵Like-

wise, β-H-containing dialkylmagnesium compounds form oleﬁns

upon thermolysis; however, n-butylcalcium chloride persists in

reﬂuxing tetrahydrofuran.⁶Coordinatively unsaturated organo-

lanthanides also undergo β-elimination less readily than

organotransition-metal analogues.⁷In a representative example,

isobutylene elimination from isolable Cp₂ErCMe₃(THF) is

facilitated by LiCl at elevated temperatures.⁸Despite the hints

that group 1, group 2, and rare-earth-metal alkyls contain-

ing β-hydrogen are metastable, most main-group- and rare-

earth-metal organometallic compounds still avoid gratuitous

β-hydrogen in their ligands.⁹Thus, β-hydrogen-free benzyl,¹⁰

allyl,¹¹CH₂SiMe₃,¹²CH(SiMe₃)₂,¹³and the ultrabulky trisyl

C(SiMe₃)₃ligands¹⁴and their derivatives are commonly

Special Issue: Recent Advances in Organo-f-Element Chemistry

Received: October 30, 2012

Published: February 22, 2013

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Scheme 1. Main-Group-Metal- and Rare-Earth-Metal-Catalyzed (A) Meerwein−Ponndorf−Verley Reduction/Oppenauer

Oxidation and (B) Tishchenko Ester Formation That Are Proposed To Involve β-Hydrogen Abstractions

employed in starting material syntheses and in the preparation

of homoleptic alkyl compounds. Such ligands have allowed the

preparation of dialkyl calcium and ytterbium compounds, such

as Ca{C(SiMe₃)₃}₂,¹⁵Yb{C(SiMe₃)₃}₂,¹⁶Ca{C(SiMe₃)₂Ph}₂,¹⁰

Ca{CH(SiMe₃)₂}₂(C₄H₂O₂)₂,¹⁷and Yb{C(SiMe₃)₂SiMe₂X}₂

(X = OMe, CH₂CH₂OEt, CHCH₂, hexahydro-2H-pyrimido-

[1,2-a]pyrimidine).¹⁸

However, β-hydrogens are important in chemical transformations,

particularly catalytic reactions involving β-hydrogen abstraction such

as Meerwein−Ponndorf−Verley carbonyl reductions, Oppenauer

alcohol oxidations, and Tishchenko ester syntheses (Scheme 1).¹⁹

These reactions involve β-hydrogen on main-group- and

rare-earth-metal alkoxides rather than alkyls. The important

point here is that β-hydrogens are particularly reactive, even

though the elimination pathway is not facile in such main-

group- and rare-earth-metal alkoxides. Second, β-agostic alkyl

compounds provide key information as models for inter-

mediates in oleﬁn insertion reactions.²⁰β-Hydrogen-containing

silazides, such as N(SiHMe₂)t-Bu and N(SiHMe₂)₂, have rich

rare-earth- and early-transition-metal chemistry that centers on

the spectroscopic and structural features and reactivity of the

SiH moiety.²¹⁻²³Thus, a signiﬁcant amount of chemistry

comes from β-SiH-containing amido, β-CH-containing alkoxide,

and β-CH-containing alkyl compounds.

described above, in that the hydrogen is formally removed as a

hydride rather than as a proton.

A few examples of intermolecular β-hydrogen abstractions

from alkyl ligands have been reported, particularly for

aluminum and zinc alkyls.²⁸Recently, β-hydrogen abstractions

from zinc alkyls were shown to be favored by pre-coordination

of the organometallic compound and the Lewis acid.²⁹There-

fore, we wanted to explore the structures of starting materials

and β-abstraction products as well as the eﬀects of the Lewis

acid and ancillary ligands in our calcium and ytterbium system.

We have observed interesting structural and spectroscopic

eﬀects in these organometallic compounds containing the

C(SiHMe₂)₃ligands, and we have discovered that the M−C

bond (rather than the β-SiH) reactivity is enhanced by

TMEDA as an ancillary ligand.

RESULTS AND DISCUSSION

■

1. Synthesis of KC(SiHMe₂)₃and {KC(SiHMe₂)₃TMEDA}₂.

The reaction of HC(SiHMe₂)₃and potassium benzyl in THF

for 18 h provides KC(SiHMe₂)₃(1) as a red solid (eq 1). The

red material is insoluble in pentane, and the compound

solidiﬁes upon washing with that solvent.

Fewer alkyl groups contain β-SiH moieties, and therefore we

targeted ligands for rare-earth- and main-group-metal com-

pounds that contain M−C bonds and β-SiH groups. Lappert

described the preparation and the reaction of Me₂HSiCH₂-

MgBr and RhCl(PPh₃)₃that gives RhH(PPh₃)₄.²⁴Eaborn has

prepared analogues of trisyl (HC(SiMe₃)₃) containing dimethyl-

silyl groups such as HC(SiHMe₂)₃and HC(SiHMe₂)(SiMe₃)₂.²⁵

Ladipo demonstrated that the central CH of HC(SiHMe₂)₃is

acidic and readily deprotonated by lithium diisopropylamide.²⁶

Recently, we reported the homoleptic tris(alkyl)yttrium

complex Y{C(SiHMe₂)₃}₃and bis(alkyl)calcium(II) and

-ytterbium(II) compounds M{C(SiHMe₂)₃}₂THF₂(M = Ca, Yb)

containing the C(SiHMe₂)₃ligand.²⁷These molecules contain

spectroscopic and structural signatures associated with β-agostic

Si−H−M interactions but do not undergo β-H elimination

even though they are (at least formally) coordinatively

unsaturated. Upon thermolysis to 100 °C, only HC(SiHMe₂)₃

is observed. Although the classical intramolecular β-hydrogen

elimination is inhibited in the sterically hindered −C(SiHMe₂)₃,

addition of external Lewis acids results in abstraction of the

β-hydrogen rather than the C(SiHMe₂)₃group. This abstraction

is distinguished from the intramolecular β-hydrogen abstraction

1

The H NMR spectrum of KC(SiHMe₂)₃contained reso-

nances at 4.59 ppm (¹J_SiH= 154 Hz) and 0.46 ppm (³J_HH

=

3.2 Hz) assigned to SiH and SiMe groups, respectively,

the ¹³C{¹H} NMR spectrum contained signals at 2.61 and

5.19 ppm, and the ²⁹Si{¹H} NMR spectrum contained one signal

at −23.3 ppm. Signals for residual or coordinated THF are not

detected, and combustion analysis is consistent with the formu-

lation KC(SiHMe₂)₃. Thus, NMR spectroscopy is consistent

with the presence of a C₃axis that relates the three SiHMe₂

1

groups in 1. The unexpected red color, the slightly low J_SiH

coupling constant, and the IR spectrum with ν_SiHbands at 2108

and 1973 cm⁻¹hint at an interesting structure. A UV−vis

spectrum of 1 dissolved in benzene contained a stronger band

(λ_max352 nm, ε = 450 L mol⁻¹cm⁻¹) and a broad and weaker

band better described as a shoulder that tails from 450 to

550 nm (ε = 32.1 L mol⁻¹cm⁻¹at 480 nm).

{KC(SiHMe₂)₃TMEDA}₂({1·TMEDA}₂) is prepared by

the addition of TMEDA to KC(SiHMe₂)₃in benzene, and this

compound is a red solid that is soluble in pentane and benzene.

The solution-phase NMR spectra are incommensurate with

the results of an X-ray structure determination (the solid-phase

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structure is shown in eq 2), and this discrepancy suggests a

ﬂuxional solution-phase structure.

coordinated by a bidentate TMEDA ligand, the central carbon

and one “side-on” Si−H moiety from the C(SiHMe₂)₃ligand

and two hydrogens from two H−Si groups in the second

C(SiHMe₂)₃group of the dimer.

The potassium−carbon distance of 3.030(5) Å is shorter

than the related distance in potassium trisyl KC(SiHMe₂)₃of

3.10(1) Å; the structure of KC(SiMe₃)₃consists of linear chains

of alternating C(SiMe₃)₃groups and K atoms.^14cThe K−C

distance in chains of KC(SiHMe₂)(SiPhMe₂)₂of 3.167(8) Å is

also longer;^25bin that compound, there are close potassium

contacts to the silicon (K···Si, 3.457 Å) and hydrogen (K···H,

2.57(9) Å). The potassium in the literature compound is also

coordinated by two phenyl groups of the next alkylpotassium

repeat unit. The K−C interatomic distances in these

compounds are within the sum of van der Waals radii of C

and K (4.45 Å).³⁰The central carbons are typically planar in

tris(silyl) carbanions as well as phenyl-substituted carbanions;³¹

thus, the carbon and three substituents are nearly planar in

(1·TMEDA)₂, and the sum of the Si−C−Si angles is 358.8°.

There are three non-equivalent close contacts between

potassium and hydrogen (bonded to silicon) in {1·TMEDA}₂.

The side-on interaction involves short K···H and short K···Si

interatomic distances of 2.80(5) and 3.450(2) Å. For com-

parison, the sum of H and K van der Waals radii is 3.84 Å, and

the sum of H and K covalent radii is 2.34 Å.³²Likewise, the

sum of Si and K van der Waals radii is 4.85 Å, and the sum of Si

and K covalent radii is 3.23 Å. The side-on interaction is further

identiﬁed by an acute K1−C1−Si3 angle of 87.2(2)°, while the

other K1−C1−Si1 and K1−C1−Si2 angles are greater than

90° at 95.8(2) and 98.1(2)°. As a result, the plane of the CSi₃

moiety is tilted toward the agostic SiH with respect to the

potassium−carbon vector.

As in 1, the ¹H, ¹³C{¹H}, and ²⁹Si NMR spectra of

(1·TMEDA)₂suggest a 3-fold rotation axis that relates the

SiHMe₂groups in the alkyl ligand. Thus, the ¹H NMR

spectrum of (1·TMEDA)₂contained resonances at 4.80 ppm

(¹J_SiH= 154 Hz) and 0.52 ppm (³J_SiH= 3.5 Hz), the ¹³C{¹H}

NMR spectrum contained signals at 5.32 and 5.19 ppm,

assigned to SiMe and KC groups, respectively, and the ²⁹Si{¹H}

NMR spectrum contained one signal at −23.7 ppm. As in

1

the TMEDA-free potassium alkyl, the H NMR data (¹J_SiH

=

154 Hz) and FTIR ν_SiHbands (2105, 2035, 1962 cm⁻¹in KBr)

suggest potassium−silylhydride interactions. Assignment of the

ν_SiHsignals was facilitated by the corresponding spectrum of

(1-d₂·TMEDA)₂, in which new peaks at 1533, 1462, and

1414 cm⁻¹were observed and the bands assigned to ν_SiHwere

1

absent. However, H NMR spectra acquired even at 185 K

were broad, and a spectrum consistent with a static structure

was not observed.

X-ray-quality crystals are grown from concentrated pentane

solution, and a solution from the diﬀraction study reveals that

(1·TMEDA)₂is dimeric in the solid state (Figure 1). The two

KC(SiHMe₂)₃units of the dimer are related by a crystallo-

graphically imposed inversion center. Each potassium center is

Four K···H−Si end-on interactions connect the two

KC(SiHMe₂)₃units to form the dimeric structure of

(1·TMEDA)₂, and two of these interactions are equivalent to

the two other interactions by crystallographically imposed

symmetry. Thus, the K1−Si1# distance is 3.983(2) Å, and the

K1−Si2# distance is 4.096(2) Å. The bridging hydrogen atoms

were located in the electron-density map, and the related

K1−H1s# and K1−H2s# distances are 2.68(5) and 2.85(5) Å.

The K1−H1s#−Si1# angle of 156(3)° is greater than the K1−

H2s#−Si2# angle of 149(3)°. Thus, the potassium compound

(1·TMEDA)₂contains two types of potassium−hydrosilyl

interactions that are best described as SiH analogues of agostic

and anagostic structures.³³The two potassium atoms of the

dimer are separated by a distance of 4.890(2) Å, which is within

the sum of van der Waals radii (5.50 Å) but longer than the

sum of covalent radii (4.06 Å); a K−K bond is not chemically

reasonable in (1·TMEDA)₂.

The connectivity of this structure contrasts with that of

LiC(SiHMe₂)₃THF₂and LiC(SiMe₃)₃TMEDA, which are

disproportionated Li dimers [Li(L)₄][Li(C(SiHMe₂)₃)₂].^14b,26

The potassium alkyl KC(SiHMe₂)(SiMe₂Ph)₂contains a similar

K(η²-SiH) interaction with a K···H distance of 2.57(9) A.^25b

2. Synthesis of M{C(SiHMe₂)₃}₂(THF)₂and M{C(SiHMe₂)₃}₂-

TMEDA (M = Ca, Yb). Bis(tris(dimethylsilyl)methyl)calcium

and -ytterbium compounds are synthesized by salt metathesis

reactions of MI₂and KC(SiHMe₂)₃. Reactions in tetrahydro-

furan provide the THF adducts Ca{C(SiHMe₂)₃}₂(THF)₂

(2·2THF, 58.7%) and Yb{C(SiHMe₂)₃}₂(THF)₂(3·2THF,

49.6%).^27bThe diamine adducts Ca{C(SiHMe₂)₃}₂TMEDA

(2·TMEDA, 23.7%) and Yb{C(SiHMe₂)₃}₂TMEDA

Figure 1. ORTEP diagram of {KC(SiHMe₂)₃TMEDA}₂

({1·TMEDA}₂). Ellipsoids are plotted at 35% probability. Hydrogen

atoms bonded to carbon are not illustrated. Hydrogen atoms bonded

to silicon were located objectively in the Fourier diﬀerence map.

Signiﬁcant interatomic distances (Å): K1−C1, 3.030(5); K1−H3s,

2.80(5); K1−Si3, 3.450(2); K1−Si1#, 3.983(2); K1−H1s#, 2.68(5);

K1−Si2#, 4.096(2); K1−H2s#, 2.85(4). Signiﬁcant interatomic angles

(deg): K1−C1−Si1, 95.8(2); K1−C1−Si2, 98.1(2); K1−C1−Si3,

87.2(2); K1−H1s#−Si1#, 156(3); K1−H2s#−Si2#, 149(3); K1−

H3s−Si3, 102(2).

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(3·TMEDA, 29.8%) are prepared from MI₂, 2 equiv of

KC(SiHMe₂)₃, and excess TMEDA in benzene (eq 3).

Table 1. Infrared Spectroscopic Data for

Tris(dimethylsilyl)methylpotassium, -calcium, and

-ytterbium with THF and TMEDA Ligands (KBr, cm⁻¹)

high

ν_SiH

intermediate low

compd

ν_SiH

25a

HC(SiHMe₂)₃

HC(SiDMe₂)₃

2090

n.a.

1534

2108

1532

n.a.

KC(SiHMe₂)₃(1)

KC(SiDMe₂)₃(1-d₃)

n.a.

1973

1417

1962

1414

n.a.

{KC(SiHMe₂)₃TMEDA}₂({1·TMEDA}₂) 2105

2035

1462

In general, the spectroscopic properties of M{C(SiHMe₂)₃}₂-

TMEDA and M{C(SiHMe₂)₃}₂THF₂are similar, but a few

features associated with the Si−H groups appear to be inﬂu-

{KC(SiDMe₂)₃TMEDA}₂

1533

({1-d₃·TMEDA}₂)

Ca{C(SiHMe₂)₃}₂THF₂(2·2THF)

Ca{C(SiDMe₂)₃}₂THF₂(2-d₆·2THF)

2107

1530

2066

1493

1905

1409

1

enced by the donor ligand and metal center. The H NMR

spectra for M{C(SiHMe₂)₃}₂TMEDA (M = Ca, Yb) each

Ca{C(SiHMe₂)₃}₂THF₂

contained one SiH resonance (Ca, δ 4.81, ¹J_SiH= 154 Hz; Yb, δ

(2_calc·2THF C₁)

1

sym

2087

2050

2065

1492

1887

1881

1890

1410

4.76, J_SiH= 148 Hz), and these spectral features are similar to

asym

2090

those of the THF analogues M{C(SiHMe₂)₃}₂THF₂(Ca, δ

1

Yb{C(SiHMe₂)₃}₂THF₂(3·2THF)

Yb{C(SiDMe₂)₃}₂THF₂(3-d₆·2THF)

Yb{C(SiHMe₂)₃}₂THF₂

(3_calc·2THF C₁)

2101

4.78, J_SiH= 152 Hz; Yb, δ 4.78, J_SiH= 150 Hz). The lowest

¹J_SiHcoupling constant was detected for the ytterbium

compound 3·TMEDA, while the largest J_SiHwas observed

1506 sh

1

for the calcium compound 2·TMEDA. Although these values

are likely aﬀected by several time-averaged factors resulting

from ﬂuxional exchange, 3·TMEDA consistently exhibits the

extreme of spectroscopic values (see below). The ²⁹Si NMR

spectra of TMEDA and bis(tetrahydrofuran) compounds 3 and

4 each contained a single resonance at ∼−20 ppm. Addition-

sym

asym

2074

2047

2051

2038

1493

2038

1494

2042

n.a.

1893

1878

1861

1411

1846

1380

1957

1921

1918

1899

2090

2·TMEDA

2-d₆·TMEDA

3·TMEDA

3-d₆·TMEDA

CaC(SiHMe₂)₃HB(C₆F₅)₃THF₂(4·2THF) 2077

YbC(SiHMe₂)₃HB(C₆F₅)₃THF₂(5·THF)

2105

1510 sh

2080

1505 sh

1

ally, singlet resonances in both H and ¹³C{¹H} NMR spectra

of 2·TMEDA and 3·TMEDA, assigned to the N-methyl and

methylene groups of the TMEDA ligand, suggested bidentate

coordination of TMEDA to ytterbium and calcium metal

centers, respectively.

2074

2094

4·TMEDA

5·TMEDA

2026

2027

Three slightly broad bands were observed in the IR spectra of

2·TMEDA and 3·TMEDA in the region associated with SiH

stretching modes (Table 1). Three bands are in contrast to

the two ν_SiHbands for KC(SiHMe₂)₃. The IR spectra of

2-d₆·TMEDA and 3-d₆·TMEDA provide further support for

the ν_SiHassignments (see Table 1). Additionally, the NMR

spectra suggest equivalent SiHMe₂groups; the comparison of

IR and NMR spectra indicates ﬂuxionality on the NMR time

scale. We assigned the two higher energy bands to terminal and

weakly activated Si−H, whereas the lowest energy bands are

associated with the Si−H bonds of silicon and hydrogen atoms

that most closely approach the metal centers. These assign-

ments are supported by DFT calculations (see below). The

frequencies of the terminal SiH (high ν_SiH) are approximately

constant for all compounds, but the low-energy ν_SiHbands vary

signiﬁcantly. The signals are similar for THF-coordinated

Ca and Yb compounds (∼1900 cm⁻¹), lower energy in

Ca{C(SiHMe₂)₃}₂TMEDA (1861 cm⁻¹), and lowest in Yb{C-

(SiHMe₂)₃}₂TMEDA (1846 cm⁻¹). On the basis of the

inequivalent SiH bands in the IR spectra, we attempted to

interactions (Tables 2 and 3) interestingly correlate with

spectroscopic trends (Table 1) for the THF and TMEDA

adducts.

The formally four-coordinate ytterbium center adopts a

distorted-tetrahedral geometry containing an acute ∠N−Yb−N

angle of 71.5(3)° and an obtuse ∠C−Yb−C angle of 121.8(3)°.

All of the SiHMe₂groups in the C(SiHMe₂)₃ligands are

oriented with hydrogen directed toward the interior of the

molecule and the methyl groups pointing outward. The hy-

drogen atoms bonded to silicon were located objectively in the

diﬀerence Fourier map; the positions of the SiMe₂atoms and

the electron density map provide a reasonable estimate of the

hydrogen positions, subject to the normal limitations associated

with X-ray diﬀraction. Still, the angles and distances of the

C(SiHMe₃)₃ligands show distortions associated with β-agostic

type structures. In particular, three categories of Yb−Si distances

are easily identiﬁed as long (∼4 Å), intermediate (3.4−3.5 Å), and

short (3.14−3.19 Å). For comparison, the Yb−Si distance in

Cp*YbSi(SiMe₃)₃is 3.032(2) Å.³⁴The Yb−C−Si angles in

3·TMEDA may also be categorized as obtuse (∼120°),

intermediate (95−102), and acute (86−88°), and these angles

correlate with the Yb−Si distances. However, the C−Si dis-

tances in the CSi₃moieties (C7−Si1, C7−Si2, C7−Si3 and

C14−Si4, C14−Si5, C14−Si6) are identical within 3σ error.

In 3·TMEDA, the SiHMe₂groups associated with the most

acute M−C−Si angles, Yb−C7−Si2 and Yb−C14−Si6, also

have small Yb−C−Si−H torsion angles, showing that the

Yb−C and Si−H bonds are coplanar. In particular, the Yb1−

C14−Si6−H6s torsion angle is 3.75° and Yb1−C7−Si2−H2s is

3.31°. The other Yb−C−Si−H torsion angles are greater than

1

resolve the H NMR spectra of 2·2THF, 3·2THF, 2·TMEDA,

and 3·TMEDA with variable-temperature measurements from

185 to 298 K in toluene-d₈. However, spectra acquired even at

195 K contained equivalent and broad resonances associated

with the SiHMe₂groups.

Recrystallization of Yb{C(SiHMe₂)₃}₂TMEDA from a

concentrated pentane solution at −30 °C provides X-ray-

quality crystals, and an ORTEP diagram is shown in Figure 2.

Although the previously reported THF adducts 2·2THF and

3·2THF have structures similar to the ytterbium diamine

adduct described here, the features of the M−C(SiHMe₂)₃

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Figure 2. ORTEP diagram of Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA). Carbon atoms on the TMEDA are plotted as points, and hydrogen atoms on

TMEDA and silyl methyl groups are not illustrated. Interatomic distances (Å): Yb1−C7, 2.68(1); Yb1−C14, 2.67(1); Yb1−Si1, 3.374(4); Yb1−Si2,

3.191(4); Yb1−Si3, 3.977(4); Yb1−Si4, 3.559(4); Yb1−Si5, 4.002(4); Yb1−Si6, 3.142(4); Yb1−H2s, 2.4(1); Yb1−H6s, 2.5(1). Interatomic angles

(deg): N1−Yb1−N2, 71.5(3); C7−Yb1−C14, 121.8(3); Si1−C7−Si2, 116.7(6); Si1−C7−Si3, 117.2(6); Si2−C7−Si3, 114.4(6); Si4−C14−Si5,

114.0(6); Si4−C14−Si6, 113.2(2); Si5−C15−Si6, 114.8(6); Yb1−H2s−Si2, 44(4); Yb1−H6s−Si6, 52(4).

Table 2. Signiﬁcant Interatomic Distances (Å) from the Single-Crystal Diﬀraction Studies of Ca{C(SiHMe₂)₃}₂THF₂(2·2THF),

Yb{C(SiHMe₂)₃}₂THF₂(3·2THF), Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA), CaC(SiHMe₂)₃HB(C₆F₅)₃THF₂(4·2THF), and

YbC(SiHMe₂)₃HB(C₆F₅)₃THF₂(5·2THF), as well as from Density Functional Theory Modeling of Ca{C(SiHMe₂)₃}₂THF₂

(2_calc·2THF) and Yb{C(SiHMe₂)₃}₂THF₂(3_calc·2THF)

compd

agostic M−Si

intermediate M−Si

long M−Si

M−C bond

Ca{C(SiHMe₂)₃}₂THF₂(2·2THF)

2_calc·2THF

Ca1−Si2: 3.216(2)

3.209

Ca1−Si3: 3.571(2)

3.660

Ca1−Si1: 3.642(3)

3.721

Ca1−C7: 2.616(7)

2.579

Yb{C(SiHMe₂)₃}₂THF₂(3·2THF)

3_calc·2THF C₁

Yb1−Si1: 3.180(1)

3.272, 3.286

Yb1−Si3: 3.515(2)

3.624, 3.654

Yb1−Si2: 3.617(2)

3.785, 3.699

Yb1−C1: 2.596(4)

2.602, 2.598

Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA)

Yb1−Si6: 3.142(4)

Yb1−Si2: 3.191(4)

Ca1−Si3: 3.097(1)

Yb1−Si1: 3.1016(7)

Yb1−Si1: 3.374(4)

Yb1−Si4: 3.559(4)

Ca1−Si2: 3.097(1)

Yb1−Si2: 3.0925(7)

Yb1−Si3: 3.977(4)

Yb1−Si5: 4.002(4)

Ca1−Si1: 3.912(1)

Yb1−Si3: 3.937(1)

Yb1−C7: 2.68(1)

Yb1−C14: 2.67(1)

Ca1−C1: 2.566(3)

Yb1−C27: 2.593(2)

CaC(SiHMe₂)₃HB(C₆F₅)₃THF₂(4·2THF)

YbC(SiHMe₂)₃HB(C₆F₅)₃THF₂(5·2THF)

Table 3. Signiﬁcant Interatomic Angles (deg) from the Single-Crystal Diﬀraction Studies of Ca{C(SiHMe₂)₃}₂THF₂(2·2THF),

Yb{C(SiHMe₂)₃}₂THF₂(3·2THF), Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA), CaC(SiHMe₂)₃HB(C₆F₅)₃THF₂(4·2THF), and

YbC(SiHMe₂)₃HB(C₆F₅)₃THF₂(5·2THF), as well as from Density Functional Theory Modeling of Ca{C(SiHMe₂)₃}₂THF₂

(2_calc·2THF) and Yb{C(SiHMe₂)₃}₂THF₂(3_calc·2THF)

compd

agostic M−C−Si

intermediate M−C−Si

obtuse M−C−Si

Ca{C(SiHMe₂)₃}₂THF₂(2·2THF)

Ca{C(SiHMe₂)₃}₂THF₂(2_calc·2THF)

Yb{C(SiHMe₂)₃}₂THF₂(3·2THF)

Yb{C(SiHMe₂)₃}₂THF₂(3_calc·2THF) C₁

Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA)

Ca1−C7−Si2: 90.7(3)

90.730

Ca1−C7−Si3: 105.1(3)

109.358

Ca1−C7−Si1: 108.4(3)

111.747

Yb1−C1−Si1: 90.6(1)

92.407, 93.037

Yb1−C1−Si3: 103.9(2)

106.949, 108.099

Yb1−C1−Si2: 108.7(2)

113.896, 110.556

Yb1−C14−Si6: 86.2(4)

Yb1−C7−Si2: 88.1(4)

Ca1−C1−Si3: 88.2(1)

Yb1−C27−Si1: 87.43(9)

Yb1−C7−Si1: 95.1(5)

Yb1−C14−Si4: 102.0(5)

Ca1−C1−Si2: 88.1(1)

Yb1−C27−Si2: 87.22(9)

Yb1−C7−Si3: 120.7(5)

Yb1−C14−Si5: 123.3(5)

Ca1−C1−Si1: 125.2(1)

Yb1−C27−Si3: 125.3(1)

CaC(SiHMe₂)₃HB(C₆F₅)₃THF₂(4·2THF)

YbC(SiHMe₂)₃HB(C₆F₅)₃THF₂(5·2THF)

20°. Together, short Yb−Si distances and acute Yb−C−Si

angles, along with small Yb−C−Si−H torsion angles and short

Yb−H distances, provide structural support for two mono-

agostic C(SiHMe₂)₃ligands bonded to the ytterbium center in

3·TMEDA. Similar features are observed in 2·2THF and

3·2THF.

In fact, comparisons with THF adducts of Ca and Yb reveal

that the ytterbium TMEDA adduct contains shorter M−Si

distances, even though the ionic radius of six-coordinate Ca(II)

(1.00 Å) is 0.02 Å shorter than that of Yb(II) (1.02 Å).³⁵For

example, the shortest Yb−Si distance in 3·TMEDA is 3.142(4)

Å, whereas the shortest M−Si distances in THF adducts are

3.216(2) Å (Ca) and 3.180(1) Å (Yb). Likewise, the most acute

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M−C−Si angle is found in the ytterbium TMEDA adduct of

86.2(4)° in comparison to the Ca value of 90.7(3)° and Yb

angle of 90.6(1)° in the THF adducts. These structural features

nicely parallel the trends observed in the IR, where 3·TMEDA

contains the lowest energy ν_SiH. The M−C distances, however,

are long in the TMEDA adduct (Yb−C7, 2.68(1) Å; Yb−C14,

2.67(1) Å). These interatomic distance are longer by ∼0.2 Å

than those found in the corresponding C(SiMe₃)₃compound

Yb{C(SiMe₃)₃}₂(2.490(8) and 2.501(9) Å)¹⁶and even ∼0.1 Å

longer than in Yb{C(SiHMe₂)₃}₂THF₂(2.596(4) Å).

Clearly, the donor ligands THF and TMEDA inﬂuence

the spectroscopic and structural properties of compounds 2

and 3, particularly the characteristics associated with the

tris(dimethylsilyl)methyl−metal interaction. We were therefore

curious which donor would bind preferentially. Reaction of

1 equiv of TMEDA and 2·2THF in benzene-d₆quantitatively

provides 2·TMEDA and 2 equiv of THF (eq 4).

(3·2THF) were modeled using density functional theory

(DFT) employing the B3LYP functional. DFT optimizations

were performed for gas-phase species and compared with solid-

state results to help elucidate the locations of the hydrogens.

The calculations also facilitate assignments of infrared bands to

particular structural motifs; indeed, the calculations show that

the SiH groups with short M−SiH distances also have low-

energy ν_SiHbands. The large size of the compounds limits the

methods available for the calculations; X-ray coordinates were

used as the starting geometries for optimizations. Our ﬁrst cal-

culations employed the constraint of C₂symmetry. C₂-optimized

structures contain very small imaginary frequencies in the

Hessian calculations (2_calc·2THF, −19, −28 cm⁻¹; 3_calc·2THF,

−20, −24, −38 cm⁻¹). These imaginary frequencies correspond

to a rotational motion of the SiH(CH₃)₂groups around the

C−Si bonds that results in breaking of the C₂symmetry. The Yb

complex 3_calc·2THF, reoptimized without symmetry constraints

(C₁symmetry) to examine the eﬀect of this rotation, retains

the imaginary frequencies (−35, −23, and −18 cm⁻¹). Fur-

ther geometry optimizations were not performed because the

magnitudes of the imaginary frequencies are small and the

computational cost for these calculations is great. Furthermore,

the small magnitude of the imaginary frequencies indicates that

this region of the potential energy surface is relatively ﬂat; thus,

a structure with a positive deﬁnite Hessian is diﬃcult to locate.

Also, because the potential energy surface (PES) is relatively

ﬂat, the imaginary modes introduce only a small error in the

calculated energy. Finally, the C₁-optimized Yb complex and

C₂-optimized 3_calc·2THF have similar ν_SiHvalues, similar Si−H

distances, and similar Si−C distances.

In contrast, addition of excess THF (>5 equiv) to 3·TMEDA

provides the mixed donor compound {Yb[C(SiHMe₂)₃]₂-

THF}₂TMEDA ({3·THF}₂TMEDA), which is also isolated

in quantitative yield (eq 5).

Despite these minor diﬃculties, the optimized gas-phase

structures reproduce the general features of the structures ob-

tained from crystallography (see Tables 2 and 3 for crystallo-

graphic and DFT-calculated distances and angles). Each

C(SiHMe₂)₃ligand contains one SiHMe₂group with short

M−H and M−Si distances and acute M−C−Si angles con-

sistent with a β-agostic SiH. The optimized Ca−C distance of

2.58 Å is shorter than the experimental distance in 2·2THF of

2.616(7) Å (see Table 2), whereas the calculated Yb−C dis-

tance (2.60 Å) is similar to the experimental distance 2.596(4) Å.

The shortest optimized M−Si distances in each molecule are

reproduced well for Ca and are slightly longer than experiment for

Yb (Ca−Si, calcd 3.21 Å, exptl 3.216(2) Å; Yb−Si, calcd 3.27 Å,

exptl 3.180(1) Å).

The DFT calculations support a relationship between Si−H

and Yb−H distances (Table 4). The calculation of 2_calc·2THF

clearly shows a long Si−H distance is associated with a short

Ca−H distance; likewise, 3_calc·2THF contains long Si−H

distances for the hydrogen atoms that have close contacts to

the ytterbium center. Furthermore, the calculated vibrational

frequencies (see Table 1) show that the longest Si−H distances

are the moieties with the lowest energy ν_SiH. However, the

Si−H distances in the X-ray structures are identical within 3σ

error. Although 3·2THF contains a seemingly long Si−H

distance for the hydrogen that has a short Yb−H distance, that

observation may be fortuitous.

The thermal stabilities of 3·TMEDA and 3·2THF were

examined to further compare the eﬀects of THF and TMEDA

as ligands in these bis(alkyl) divalent metal compounds.

Thermolysis of both compounds at 120 °C in benzene-d₆

1

yields the hydrocarbon HC(SiHMe₂)₃as the only H NMR

spectroscopically active material; however, 3·TMEDA is

consumed in 6 h, whereas the THF adduct requires 96 h to

form HC(SiHMe₂)₃. The source of the hydrogen HC(SiHMe₂)₃

is unknown. The deuterated solvent is dried and distilled from

NaK alloy prior to thermolysis, and the interior surface of a

Teﬂon-sealed J. Young style NMR tube is silylated with CHCl₃/

Me₃SiCl. These compounds are highly air- and moisture

sensitive, and benzene-d₆solutions are 90% decomposed to

HC(SiHMe₂)₃upon 30 s of air exposure. Their persistence

in sealed NMR tubes at room and moderate temperatures

(60 °C) argues against adventitious hydrolysis in the high-

temperature reactions. Thermolysis of 3·2THF-d₈provides

HC(SiHMe₂)₃, ruling out THF as the source of hydrogen.

Thermolysis of Yb{C(SiDMe₂)₃}₂THF₂aﬀords HC-

(SiDMe₂)₃; this experiment rules out the silicon hydride as the

source of hydrogen by intramolecular or intermolecular

β-hydrogen abstraction. A related decomposition of Cp*La(CH-

(SiMe₃)₂)₂THF produces H₂C(SiMe₃)₂, in which the proton

source is proposed to be the C₅Me₅ligand; that is not a

possibility in this case.³⁶

For comparison, in-depth studies of monoagostic and diag-

ostic tetramethyldisilazide compounds of lanthanum, lutetium,

yttrium, and scandium reveal ∼0.05 Å lengthened Si−H dis-

tances.^23dIn computationally modeled compounds such as

{H₂Si(C₅H₄)₂}LaN(SiHMe₂)₂, the β-diagostic interaction is

described as a donor−acceptor pair, with both the Si and H

3. Density Functional Theory Calculations of Ca{C-

(SiHMe₂)₃}₂THF₂and Yb{C(SiHMe₂)₃}₂THF₂. Both Ca{C-

(SiHMe₂)₃}₂THF₂(2·2THF) and Yb{C(SiHMe₂)₃}₂THF₂

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from the central carbon to the silicon centers, followed by Si−C

bonding to the methyl groups. The Si−H bonding orbitals

are still lower in energy. The Kohn−Sham canonical orbitals

corresponding to terminal silicon−hydrogen bonding are

grouped by energy into two sets (−0.4790 and −0.4779

hartree; −0.3147 and −0.3136 hartree); the Si−H bonds

associated with the β-agostic structures are barely stabilized

versus the latter set (−0.3231 and −0.3175 hartree) and have

signiﬁcantly higher energy than the former.

4. β-Hydrogen Abstraction Reactions. The identity of

the organic thermolysis product as HC(SiHMe₂)₃argues against

β-hydrogen elimination as the kinetically favored reaction

pathway. While β-hydrogen eliminations are notoriously slow

for rare-earth- and main-group-metal compounds as noted in the

Introduction, 2·2THF, 3·2THF, 2·TMEDA, and 3·TMEDA

contain (presumably) a hydridic β-hydrogen bonded to silicon

and Lewis acid sites on the metal that might act as

intramolecular hydride acceptors via β-elimination or through

intermolecular reactions involving β-hydride abstraction.

Therefore, reactions of 2·2THF, 3·2THF, 2·TMEDA, and

3·TMEDA with the Lewis acids B(C₆F₅)₃, BPh₃, and Me₃SiI

were explored to test the nucleophilicity of the central

carbanion versus that of the peripheral β-hydrogen.

Table 4. M−H and Si−H Distances (Å) in 2·2THF,

2_calc·2THF, 3·2THF, 3_calc·2THF, and 3·TMEDA

compd

2·2THF

Si−H

M−H

Si1−H1s: 1.42(6)

Si2−H2s: 1.39(6)

Si3−H3s: 1.46(6)

1.541

Ca1−H1s: 3.37(2)

Ca1−H2s: 2.53(7)

Ca1−H3s: 3.86(6)

2.562

2_calc·2THF (C₂sym)

3·2THF

1.507

3.936

1.500

3.744

Si1−H1s: 1.48(3)

Si2−H2s: 1.36(4)

Si3−H3s: 1.42(4)

1.541, 1.538

Yb1−H1s: 2.50(3)

Yb2−H2s: 3.56(4)

Yb3−H3s: 3.59(4)

2.651, 2.679

3_calc·2THF (C₁sym)

3·TMEDA

1.500, 1.502

3.881, 3.665

1.506, 1.507

3.916, 3.991

Si1−H1s: 1.6(1)

Si4−H4s: 1.4(1)

Si2−H2s: 1.5(1)

Si6−H6s: 1.6(1)

Si3−H3s: 1.4(1)

Si5−H5s: 1.6(1)

Yb1−H1s: 2.9(1)

Yb1−H4s: 3.4(1)

Yb1−H2s: 2.4(1)

Yb1−H6s: 2.5(1)

Yb1−H3s: 4.2(1)

Yb1−H5s: 4.0(1)

contributing to the donor orbitals and the lanthanum s and d

orbitals as the acceptor.

Reactions of M{C(SiHMe₂)₃}₂L₂and B(C₆F₅)₃. Reactions of

2·2THF, 3·2THF, and B(C₆F₅)₃occur through β-hydrogen

abstraction to form MC(SiHMe₂)₃HB(C₆F₅)₃THF₂(M = Ca

(4·2THF); Yb (5·2THF)), as previously communicated.^27b

Similarly, the TMEDA adducts and B(C₆F₅)₃react to give

MC(SiHMe₂)₃HB(C₆F₅)₃TMEDA (M = Ca (4·TMEDA); Yb

(5·TMEDA)) and 0.5 equiv of 1,3-disilacyclobutane {Me₂SiC-

(SiHMe₂)₂}₂, which is the head-to-tail dimer of the silene

Me₂SiC(SiHMe₂)₂(eq 6).

In the present system, the Kohn−Sham orbitals show that

the HOMO and HOMO-1 (−0.1828 and −0.1919 hartree) are

largely centered on the carbon bonded to the calcium center.

An Edminston−Ruedenberg energy localization of the orbitals

provides a localized orbital picture (the HOMO is shown in

Figure 3a).³⁷The next lower set of orbitals involves bonding

These reactions are best monitored initially by ¹¹B NMR

spectroscopy, in which sharp upﬁeld doublet resonances

1

(4·2THF, −21.3 ppm, J_BH= 76.2 Hz; 4·TMEDA, −21.8 ppm,

¹J_BH= 75.0 Hz; 5·2THF, −20.8 ppm, ¹J_BH= 72.8 Hz; 5·TMEDA,

−21.4 ppm, ¹J_BH= 71.6 Hz) are consistent with formation of an

anionic, four-coordinate boron center that is bonded to hydrogen.

In the ¹H{¹¹B} NMR spectra, singlets were assigned to the boron

hydride (4·2THF, 2.63 ppm; 4·TMEDA, 2.64 ppm; 5·TMEDA,

1

3.22 ppm), and cross-peaks in H−¹¹B HMQC experiments

supported these assignments. The general NMR features of the

C(SiHMe₂)₃ligand were similar in 4·2THF, 5·2THF, 4·TMEDA,

and 5·TMEDA. In the ¹H NMR spectra, upﬁeld doublets

(∼0.3 ppm, ³J_SiH≈ 3 Hz) were assigned to methyl groups of the

SiHMe₂, whereas the SiH multiplets were characterized by signals

from 4.4 to 4.6 ppm (¹J_SiH= 145−149 Hz). Table 5 gives the SiH

Figure 3. Rendered images of localized orbitals of the HOMO (A) and

occupied Si−H bonding orbital associated with the β-agostic SiH (B).

Ca, Si, C, O, and H are shown as light green, teal, gray, red, and white,

respectively. The localized orbital is shown in green.

1

resonances and J_SiHcoupling constants for the dialkyl

compounds and the (alkyl)hydridoborate complexes for compar-

ison. The signals in the zwitterionic compounds are ca. 0.2 ppm

upﬁeld in comparison to those of the dialkyl compounds, and the

coupling constants are reduced by ∼5 Hz.

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example, the Ca1−Si3 distance of 3.097(1) Å (associated with

the proposed β-agostic structure) is ∼0.1 Å shorter than that of

the agostic Ca1−Si2 distance in 2·2THF (3.216(2) Å). The

hydrogen bonded to silicon (H3s) was located in the Fourier

diﬀerence map at the position expected for a roughly tetra-

hedral silicon center. SiH is directed toward the Ca center such

that the Ca1−C1−Si3−H3s torsion angle is −3(1)°. Further-

more, a second short calcium−silicon distance (Ca1−Si2 =

3.097(1) Å) is also observed; however, in this case, the torsion

angle is 37(1)°.

Table 5. Spectroscopic Features of Dialkyl and Monoalkyl

Ytterbium and Calcium Compounds

δ(SiH),

ppm

¹J_SiH

Hz

,

δ(SiH),

ppm

¹J_SiH

,

Hz

compd

2·2THF

3·2THF

2·TMEDA

3·TMEDA

4.78

4.81

4.76

152.1

150.4

154

4·2THF

5·3THF

4·TMEDA

5·TMEDA

4.55

4.61

4.44

4.51

148.9

146.4

144.9

147.0

148

The Ca center is further coordinated by two ortho-F atoms from

two C₆F₅groups in a bidentate HB(C₆F₅)₃ligand, with Ca−F

distances of 2.412(2) and 2.437(2) Å. The hydrogen bonded to

boron was located objectively in the diﬀerence Fourier map,

and its position is consistent with that expected for a tetrahedral

boron center. From this, the H1g−Ca1 distance of 2.45(3) Å is

signiﬁcantly (0.45 Å) longer than the sum of covalent radii of

Ca and H (1.99 Å), suggesting that the Ca−H interaction is

likely weak. The ν_BHvalues in this series of compounds (Table 6)

Interestingly, the trend in the IR spectra is somewhat

opposite to that of the NMR, in that the ν_SiHbands in the

zwitterionic compounds appeared at higher energy relative to

their dialkyl precursors (see Table 1). Three ν_SiHsignals were

observed in 4·2THF, 4·TMEDA, and 5·TMEDA, but only two

bands were observed for 5·2THF. As in the dialkyl compounds,

the lowest energy bands were assigned to the β-agostic SiH.

The spectrum for the ytterbium compound 5·TMEDA

contained the lowest energy band, and the trend of stretching

frequency for the zwitterionic compounds 5·TMEDA <

4·TMEDA ≈ 5·2THF < 4·2THF is similar to that of the

dialkyl compounds, 3·TMEDA < 2·TMEDA < 3·2THF <

2·2THF.

Table 6. Comparison of B−H Stretching Bands (KBr, cm⁻¹)

from IR Spectra of Mono- and Bis(hydridoborate)

Compounds

X-ray-quality crystals were obtained for 4·2THF and

5·2THF, and the general structural features and connectivities

of these zwitterionic compounds are similar (see Figure 4 for

the ORTEP diagram of 4·2THF).^27bOn the basis of the similar

structural features of these two crystallographically characterized

compounds and the similar overall spectroscopic features for

4·2THF, 5·2THF, 4·TMEDA, and 5·TMEDA, the connectivity

of these compounds is assigned as shown in eq 6.

The solid-state structure of 4·2THF conﬁrms that calcium

interacts with one HB(C₆F₅)₃group and one tris(dimethylsilyl)-

methyl ligand, as well as two THF ligands. The Ca−C distance

for 4·2THF (2.566(3) Å) is 0.05 Å shorter than for the neutral

compound 2·2THF (2.616(7) Å). This is in contrast to the pair

of Yb analogues, in which the Yb−C distances are identical in the

neutral and zwitterionic compounds. Additionally, the zwitter-

ionic nature of 4·2THF aﬀects Ca−Si and Ca−O distances. For

MC(SiHMe₂)₃{HB(C₆F₅)₃}L₂

ν_BH

M{HB(C₆F₅)₃}₂L₂

ν_BH

4·2THF

5·2THF

4·TMEDA

5·TMEDA

2329

2310

2302

2293

6·2THF

7·2THF

6·TMEDA

7·TMEDA

2308

2377

2383

2305

also suggested weak M···H···B interactions (see below). A similar

structural motif was reported for the coordination of HB(C₆F₅)₃

to a samarium(III) center in Cp*₂Sm{κ³-H,F,F-HB(C₆F₅)₃}.³⁸In

that compound, the Sm−H distance is 2.45(5) Å, the B−H

distance is 1.18(5) Å, and the infrared spectrum contains a ν_BH

band at 2290 cm⁻¹.

Reactions of 2·2THF or 3·2THF with 2 equiv of B(C₆F₅)₃in

benzene provide M{HB(C₆F₅)₃}₂(THF)₂(M = Ca (6·2THF), Yb

(7·2THF)) along with 1 equiv of the 1,3-disilacyclobutane

Figure 4. ORTEP diagram of CaC(SiHMe₂)₃{κ³-H,o-F,o-F-HB(C₆F₅)₃}(THF)₂(4·2THF). See Tables 2−4 for signiﬁcant interatomic distances and

angles.

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Figure 5. ORTEP diagram of Yb{κ³-HB(C₆F₅)₃}₂TMEDA (7·TMEDA). Ellipsoids are plotted at 35% probability, and hydrogen atoms on the

TMEDA ligand are not illustrated for clarity.

{Me₂SiC(SiHMe₂)₂}₂(eq 7). The “dizwitterionic” TMEDA

adducts 6·TMEDA and 7·TMEDA are prepared analogously

from 2·TMEDA and 3·TMEDA. These products are consistent

with β-abstraction of one hydrogen from each C(SiHMe₂)₃

ligand. Expectedly, the monoalkyl compounds 4 and 5 react

with 1 equiv of B(C₆F₅)₃to provide 6 and 7.

The TMEDA adducts of 6 and 7 precipitate from benzene,

whereas {Me₂SiC(SiHMe₂)₂}₂is soluble, thus allowing facile

separation of products. However, the poor solubility of

6·TMEDA and 7·TMEDA in aromatic and aliphatic solvents

limits solution-phase characterization.

The ν_BHstretching frequencies in monoalkyl hydridoborate

compounds 4 and 5 and bis(hydridoborates) 6 and 7 vary from

2293 to 2383 cm⁻¹(Table 6). For comparison, the ν_BHvalue

in Cp*₂Sc(κ²-C₆F₅)BH(C₆F₅)₂is 2410 cm⁻¹,³⁹and in that

compound an X-ray structural analysis indicates that the

boron−hydrogen distance is to long to allow a scandium−

hydrogen bonding interaction. The ν_BHvalues are generally

blue-shifted in the bis(hydridoborate) compounds versus the

mono(hydridoborate) compounds. The exception is 6·2THF,

in which the ν_BHvalue is 21 cm⁻¹lower in energy. The variation

in the ν_BHvalues suggests that there is some M···H···B inter-

action in these compounds (which is also suggested by X-ray

crystallography). However, the ﬂuxionality of these compounds

limits further conclusions on the strength of the interaction.

Both bis(hydridoborate) Yb complexes 7·2THF and

7·TMEDA crystallize from benzene solution. X-ray crystallog-

raphy reveals that the Yb center is coordinated to four o-F

atoms from four C₆F₅rings (two from each HB(C₆F₅)₃ligand)

as well as a bidentante TMEDA ligand or two THF ligands in

the two compounds (Figure 5). As in the mono(hydridoborate)

structures discussed above, the boron−hydrogen group points

toward the ytterbium centers. The crystallographically charac-

terized calcium analogue 6·2THF is isostructural with 7·2THF.

The SiH groups in 2·2THF, 3·2THF_,2·TMEDA, and

3·TMEDA have suﬃcient nucleophilic character to react with

the strong Lewis acid B(C₆F₅)₃, but the SiH moieties do not

react with calcium(II) or ytterbium(II) centers in any of the

neutral, cationic, or dicationic compounds 2·2THF−7·2THF or

3·TMEDA−7·TMEDA even though the THF and TMEDA

ligands are suﬃciently labile to undergo substitution. The only

hint of possible β-elimination comes from the thermolysis of

4·2THF, which gives HC(SiHMe₂)₃and the disilacyclobutane

species in a 4:3 ratio after 5 days at 80 °C. Disilacyclobutane

could form through a β-elimination process, although

dissociation of B(C₆F₅)₃from (Me₂HSi)₃CCaHB(C₆F₅)₃

followed by β-abstraction is more likely.

Reactions of M{C(SiHMe₂)₃}₂L₂and 1 equiv of BPh₃. Com-

pounds 2·2THF, 3·2THF_,2·TMEDA, and 3·TMEDA also

react with the much weaker Lewis acid BPh₃. This observa-

tion is unexpected because, as noted above, the calcium(II) and

ytterbium(II) centers in 2−7 are not suﬃcient Lewis acids to

mediate β-hydrogen elimination or abstraction. The M(HBPh₃)

and MHB(C₆F₅)₃products from the abstractions have dis-

similar connectivities in the solid state (see below). BPh₃

abstractions aﬀord a mixture of species, whereas B(C₆F₅)₃

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Figure 6. ORTEP diagram of Yb(HBPh₃)₂TMEDA (9·TMEDA). Ellipsoids are plotted at 35% probability, and hydrogen atoms bonded to C₆H₅

and TMEDA groups are not drawn for clarity. Selected interatomic distances (Å): Yb1−H1b, 2.22(5); Yb1−H2b, 2.39(4); Yb1−C6, 2.772(4);

Yb1−C25, 2.667(4); Yb1−C26, 2.843(4); B1−H1b, 1.19(5); B2−H2b, 1.22(5); Yb1−B1, 2.930(5); Yb1−B2, 3.065(4); B1−C6, 1.643(6); B1−C7,

1.621(6); B1−C13, 1.600(6); B2−C19, 1.623(6); B2−C25, 1.648(6); B2−C31, 1.624(6).

Figure 7. ORTEP diagram of Yb(HBPh₃)₂THF (9·THF). Ellipsoids are plotted at 35% probability, and hydrogen atoms bonded to C₆H₅and THF

groups are not drawn for clarity. Selected interatomic distances (Å): Yb1−H1b, 2.45(3); Yb1−H2b, 2.22(3); Yb1−B1, 2.675(3); Yb1−B2, 2.675(3);

Yb1−C5, 2.871(3); Yb1−C6, 2.793(3); Yb1−C18, 2.614(3); Yb1−C24, 2.705(3); Yb1−C36, 2.780(3); B1−C6, 1.642(4).

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provides a single product. In addition, the product distributions

from reactions of BPh₃and 2·TMEDA and 3·TMEDA in

comparison to 2·2THF and 3·2THF and BPh₃are diﬀerent.

For example, 3·TMEDA and BPh₃react in benzene to give

0.5 equiv of Yb(HBPh₃)₂TMEDA (9·TMEDA), leaving 0.5 equiv

of 3·TMEDA unreacted (eq 8). In contrast, 2·2THF and 3·2THF

and 1 equiv of BPh₃react to give a mixture of dialkyl starting

material, MC(SiHMe₂)₃(HBPh₃)(THF)_n(10·nTHF and

11·nTHF), and M(HBPh₃)₂THF (8·THF and 9·THF) in a

1:2:1 ratio, along with disilacyclobutane (eq 9).

The products 8·TMEDA and 9·TMEDA precipitate or

crystallize from the reaction mixtures and are isolated by

ﬁltration. Unfortunately, the compounds 8·TMEDA and

9·TMEDA are insoluble in pentane, benzene, toluene, and

THF, precluding characterization in those solvents. In contrast to

the IR spectra of the HB(C₆F₅)₃compounds, the IR spectra of

8·TMEDA and 9·TMEDA contained four ν_BHbands

(8·TMEDA 2059, 2027, 2008, and 1943 cm⁻¹; 9·TMEDA,

2054, 2024, 2006, and 1940 cm⁻¹). Despite the relatively simple

X-ray structure (see below), these signals suggest there are

multiple types of M···H···B interactions in these two compounds

in the solid state.

which contains a Ce−H−B angle of 139(1)°, a Ce···B distance

of 3.423(3) Å, and a B−H distance of 1.26 Å.⁴²Additionally,

the ruthenium compound Ru(η⁶-C₆H₅)(BHPh₃)(PMe₃)₃con-

tains a terminal boron hydrogen; the IR in that case reveals a

43

band at 2270 cm⁻¹assigned to ν_BH

.

−

Additionally, a number of BPh₄-containing ytterbium(II)

and calcium(II) compounds are known,⁴⁴and BPh₄is often

−

non-coordinating in these compounds. However, BPh₄is

known to coordinate to Yb(II) in Yb(η⁶-C₆H₅)(η⁴-C₆H₅)BPh₂-

type structures, where the shortest Yb−C distance is 2.833(3) Å.⁴⁵

In Yb{N(SiMe₃)(SiMe₂CH₂BPh₂(η²-C₆H₅)₂)}THF₂, short

Yb−C_ipsodistances (2.635(3) and 2.792(3) Å) are similar to

the Yb^II−C distance in 9·TMEDA.⁴⁵Deacon pointed out that

these Yb^II−C_ipsodistances are in the range observed for bridg-

ing Yb−Ph−Yb.⁴⁶

The monoalkyl compounds 10·nTHF and 11·nTHF are not

1

isolable but are characterized in solution by H NMR signals

that are distinct from those of the isolable and fully charac-

1

terized 2·2THF and 3·2THF. In particular, the H NMR spec-

trum of calcium 10·nTHF contained resonances at 4.51 ppm

(¹J_SiH= 152.7 Hz) and 0.41 ppm (³J_HH= 3.5 Hz), assigned to

the SiH and SiMe₂groups, respectively.

X-ray-quality crystals of 9·TMEDA are obtained from the

reaction mixture, and an X-ray diﬀraction study conﬁrmed the

identity of the product (Figure 6). The structure of 9·TMEDA

contains a bidentate TMEDA ligand and two HBPh₃ligands

that have short distances to ytterbium through hydrogen and

boron as well as ipso- and o-aryl carbons. Interestingly, the

related THF adduct 9·THF contains only one molecule of

THF coordinated to ytterbium (Figure 7). The HBPh₃−

ytterbium close interatomic contacts in 9·THF include the H

on boron (Yb1−H1b, 2.45(3) Å; Yb1−H2b, 2.22(3) Å), the

boron center (Yb1−B1 and Yb1−B2, 2.675(3) Å), and two of the

three ipso carbons in each HBPh₃group (Yb1−C6, 2.793(3) Å;

Yb1−C18, 2.614(3) Å; Yb1−C24, 2.750(3) Å; Yb1−C36,

2.780(3) Å). Coordination of an anion to BPh₃tends to lengthen

the B−C bonds from 1.58 Å in BPh₃by 0.05 Å.⁴⁰⁻⁴²This

lengthening eﬀect is further enhanced for the phenyl groups of

9·TMEDA and 9·THF, in which there is a Yb−C_ipsoclose contact.

In 9·TMEDA, for example, the pendant phenyl group has a B−C13

distance of 1.600(6) Å, whereas the B−C6 distance for the

coordinated phenyl is 1.643(6) Å. Likewise, in 9·THF, coordinated

(B2−C36) and noncoordinated (B2−C30) phenyls have distances

of 1.648(4) and 1.608(4) Å, respectively.

The species 8·THF and 9·THF are independently prepared

by the reactions of 2 equiv of BPh₃and 3·2THF and 4·2THF

(eq 10). Unlike the insoluble TMEDA adducts, 8·THF and

9·THF are soluble in benzene.

The best procedure to isolate 8·THF and 9·THF involves

evaporation of the reaction mixture followed by pentane washes

to remove the 1,3-disilacyclobutane {Me₂SiC(SiHMe₂)₂}₂

byproduct. One broad resonance was observed in the ¹¹B

NMR spectra of 8·THF (−6.0 ppm), while a broad doublet was

detected for 9·THF (−4.0 ppm, ¹J_BH= 56 Hz). In the ¹H{¹¹B}

NMR spectrum, singlet resonances were assigned to the boron

hydride (8·THF, 3.18 ppm; 9·THF, 3.66 ppm), and cross-peaks

1

in H−¹¹B HMQC experiments supported these assignments.

Two ν_BHbands (2021 and 1936 cm⁻¹) were observed in the IR

spectrum for 8·THF, but only one ν_BHband (2003 cm⁻¹) was

detected for 9·THF.

Reaction of M{C(SiHMe₂)₃}₂L₂and Me₃SiI. As with borane

Lewis acids, reactions of 2·2THF or 3·2THF and 1 equiv of

Me₃SiI in benzene-d₆results in β-hydrogen abstraction, yielding

HSiMe₃and the disilacyclobutane (eq 11). The HSiMe₃is

characterized by a multiplet at 4.18 ppm and a doublet at

0.01 ppm in ¹H NMR spectra of reaction mixtures. Only half of

the dialkyl starting material is consumed in the reaction, and a

white precipitate is formed that is likely a THF adduct of YbI₂

We know of only two other structurally characterized

compounds that contain the [HBPh₃] unit. Reaction of

(C₅H₃-t-Bu₂)₂CeH and BPh₃gives (C₅H₃-t-Bu₂)₂CeHBPh₃,

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Figure 8. ORTEP diagram of K{HB(C₆F₅)₃}TMEDA₂(13·2TMEDA). Ellipsoids are plotted at 35% probability, and hydrogen atoms on the

TMEDA ligands are not illustrated for clarity.

on the basis of the reaction stoichiometry and the observations

for the transformations involving BPh₃.

the ytterbium case, TMEDA adduct contained shorter Yb−Si

distances and a longer Yb−C distance than the corresponding

THF adducts. Furthermore, the lowest energy ν_SiHbands were

observed with 3·TMEDA in comparison to 2·2THF and

3·2THF. However, 2·TMEDA also gives Si−C bond formation,

even through that compound contains distances and spectro-

scopic features similar to those of the THF adducts.

Reaction of KC(SiHMe₂)₃and B(C₆F₅)₃. The bonding

interactions between the tris(dimethylsilyl)methyl ligand and

calcium(II) or ytterbium(II) have signiﬁcant polarity. The potas-

sium compound 1 is also highly polar, and its reactions with

B(C₆F₅)₃and Me₃SiI were investigated for comparison with the

calcium and ytterbium compounds.

Notably, neither Me₃SiC(SiHMe₂)₃nor the silyl ether iodide

Me₃SiO(CH₂)₄I product from THF ring opening are formed in

these reactions. Alternatively, Cp*La{CH(SiMe₃)₂}₂THF and

Me₃SiI were reported to react to give Me₃SiO(CH₂)₄I and

Cp*La{CH(SiMe₃)₂}₂.³⁶The observation of HSiMe₃and di-

silacyclobutane indicates that Me₃SiI reacts with the β-H rather

than the carbanion C(SiHMe₃)₃through β-H abstraction, similar

to reactions with boron-based Lewis acids.

Interestingly, the reaction of Me₃SiI and 2·TMEDA or

3·TMEDA provides Me₃SiC(SiHMe₂)₃as the major organic

product (eq 12). HSiMe₃and the disilacyclobutane

{Me₂SiC(SiHMe₂)₂}₂are formed as minor products (5:2:1),

β-Hydrogen abstraction readily occurs in the reaction of

B(C₆F₅)₃and KC(SiHMe₂)₃to yield the benzene-insoluble

pink solid KHB(C₆F₅)₃(13) and disilacyclobutane {Me₂SiC-

(SiHMe₂)₂}₂(eq 13). The product readily dissolves in THF,

and 13 was characterized in that solvent. Resonances at 3.71

1

as determined by integration of H NMR spectra of reaction

mixtures. Again, an insoluble white precipitate is formed

(likely MI₂), and only half of the dialkyl starting material is

consumed.

Thus, the central carbon of the −C(SiHMe₂)₃ligands in the

TMEDA adducts are accessible for interaction with Me₃SiI. In

1

and −27.3 ppm (¹J_BH= 93 Hz) in the H{¹¹B} and ¹¹B NMR

spectra were assigned to the hydrogen bonded to boron and

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Scheme 2. β-Hydrogen Elimination and Intermolecular β-Abstraction

the boron center, respectively, and these data are consistent

with a hydridoborate moiety. In the IR spectrum, a peak at

2382 cm⁻¹was detected that is similar to the boron−hydrogen

stretching frequencies in the calcium and ytterbium hydrido-

borate compounds described above.

Furthermore, reaction of KHB(C₆F₅)₃and 2 equiv of TMEDA

provides the monomeric species KHB(C₆F₅)₃TMEDA₂

(13·2TMEDA). The TMEDA ligand gives solubility to the

potassium salt, allowing characterization in benzene-d₆. The

β-SiH abstraction product is evident, as the ¹¹B NMR spectrum

revealed a doublet resonance at −24.7 ppm (¹J_BH= 85 Hz),

while the hydrogen bonded to boron was assigned to a reso-

nance at 3.46 ppm in the ¹H NMR spectrum. The ν_BHband at

2381 cm⁻¹is almost identical with that of the TMEDA-free 13.

The monomeric nature of 13·2TMEDA in the solid state has

been unambiguously demonstrated by an X-ray crystallographic

structure determination (Figure 8).

The K center is coordinated to two o-F from the two C₆F₅

rings to form a zwitterionic complex. The structural features

of the metal−hydridoperﬂuorophenylborate interaction to

ytterbium, calcium, and potassium are similar in dicationic

7·TMEDA, monoalkyl 5·2THF, and 13·2TMEDA. Thus, the

reactivity of B(C₆F₅)₃and ionic (tris(dimethylsilyl)methyl)

metal compounds involves abstraction of β-hydrogen to give

the κ³-H,F,F-HB(C₆F₅)₃M compounds.

Because the ytterbium compound 3·TMEDA and the lithium

alkyl LiC(SiHMe₂)₃provide silicon−carbon bond formation

upon reaction with Me₃Sil or Me₃SiCl,²⁶respectively, and

KC(SiHMe₂)₃reacts with MI₂to give M−C bond formations,

the reaction of the potassium alkyl and Me₃SiI was investigated.

However, the reaction of KC(SiHMe₂)₃and Me₃SiI gives

HSiMe₃and disilacyclobutane as the soluble products; Me₃Si−

C(SiHMe₂)₃is not formed.

with bulky Lewis acids (i.e., B(C₆F₅)₃). Recently, we have ob-

served a similar eﬀect in the reactions of β-hydrogen containing

silazides such as LiN(SiHMe₂)₂and LiN(SiHMe₂)-t-Bu with

zirconium halides, where the sterically most hindered reaction

partners give zirconium hydride products rather than silazido

zirconium species.⁴⁷

There are similarities between these intermolecular abstrac-

tion reactions and intramolecular β-elimination: both form

metal hydrides, and both result in the expulsion of an unsatu-

rated organic fragment from the metal alkyl. In the current

transformations, calcium and ytterbium hydridoborates are the

products, and dissociation of B(Aryl)₃from the M−H−

B(Aryl)₃species would provide a metal hydride, to give ﬁnal

products equivalent with intramolecular β-elimination. Fur-

thermore, it is worth noting that the B(C₆F₅)₃and BPh₃Lewis

acids provide the needed empty orbital which is apparently not

present in the calcium or ytterbium alkyl compounds that

would allow intramolecular β-elimination or a bimolecular

β-abstraction. Considering these points, this abstraction of a

β-hydrogen by an external Lewis acid can also be described as a

Lewis acid mediated β-hydrogen elimination (Scheme 2).

Thus, structural models for intermediates in these β-abstraction

reactions are important for understanding the reaction path-

way. Intramolecular three-center−two-electron interactions be-

tween the β-CH bond and metal centers (i.e., β-agostic

interactions) are proposed to provide insight into the pathway

for β-elimination and its microscopic reverse, oleﬁn insertion.

As noted in the Introduction, these abstraction reactions have

features similar to those of intramolecular elimination reactions,

as they provide metal hydride and unsaturated organic prod-

ucts. Thus, it is interesting to consider the role of the β-agostic

interactions in the potassium, calcium, and ytterbium

tris(dimethylsilyl)methyl compounds in the intermolecular

β-abstraction reaction. The bridging Si−H groups in the dimeric

potassium compound {KC(SiHMe₂)₃TMEDA}₂, where a third

SiH group approaches a potassium center in a side-on β-agostic

type interaction, provides a structural model for an arrested

intermolecular β-hydrogen abstraction. The “intramolecular”

β-agostic interaction in this structure does not interact with

the second potassium center, and this may suggest that the reac-

tions of Lewis acids with the ytterbium and calcium alkyl com-

pounds involve abstraction of the terminal SiH groups, rather

than the SiH’s involved in side-on coordination. The presence of

the intramolecular silicon−hydrogen−calcium close contacts in

this structure, however, may be signiﬁcant, either for increasing

the nucleophilicity of the remaining SiH groups or simply serving

as a spectroscopic and structural marker that shows signiﬁcant

nucleophilic character of all the SiH groups in these hydro-

silylalkyl ligands.

CONCLUSION

■

The β-hydrogen in the tris(dimethylsilyl)methide ligand inﬂu-

ences the reaction pathways in reactions of its organometallic

compounds with Lewis acids. Two pathways have been

observed: β-hydrogen abstraction and ligand group transfer.

The favored pathway depends strongly on the identity of the

Lewis acid center, but it is also inﬂuenced by ancillary ligands

and the metal center(s) involved. Thus, reactions of KC-

(SiHMe₂)₃and MI₂salts (M = Ca, Yb) result in ligand trans-

fer through salt metathesis. M{C(SiHMe₂)₃}₂TMEDA and

Me₃SiI give a mixture of alkylation and β-hydrogen abstraction,

whereas M{C(SiHMe₂)₃}₂THF₂and Me₃SiI react solely

through the β-abstraction pathway. Borane electrophiles, such

as B(C₆F₅)₃and BPh₃, react with the SiH. The alkylation

pathways appear most facile when the partners are sterically less

hindered (i.e., MI₂salts), and β-hydrogen abstraction occurs

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the resulting gummy solid was extracted with pentane (10 mL). The

pentane extract was concentrated and cooled to −30 °C for

recrystallization to obtain yellow needles of {KC(SiHMe₂)₃TMEDA}₂

EXPERIMENTAL SECTION

■

General Procedures. All reactions were performed under a dry

argon atmosphere using standard Schlenk techniques or under a

nitrogen atmosphere in a glovebox unless otherwise indicated. Dry,

oxygen-free solvents were used throughout. Benzene, toluene, pentane,

and tetrahydrofuran were degassed by sparging with nitrogen, ﬁltered

through activated alumina columns, and stored under N₂. Benzene-d₆,

toluene-d₈, and THF-d₈were vacuum -transferred from Na/K alloy

and stored under N₂in the glovebox. Anhydrous CaI₂was purchased

from Aldrich and used as received. All organic reagents were purchased

1

({1·TMEDA}₂; 0.093 g, 0.271 mmol, 35.2%). H NMR (benzene-d₆,

400 MHz, 25 °C): δ 4.80 (m, 3 H, ¹J_SiH= 154 Hz, SiH), 1.78 (s, 4 H,

3

NCH₂), 1.74 (s, 12 H, NMe₂), 0.52 (d, 18 H, J_HH= 3.5 Hz, SiMe₂).

¹³C{¹H} NMR (benzene-d₆, 100 MHz, 25 °C): δ 57.33 (CH₂), 45.68

(NCH₃), 14.69 (KC), 5.33 (SiCH₃). ²⁹Si{¹H} NMR (benzene-d₆, 79.5

MHz, 25 °C): δ −23.7 (SiHMe₂). IR (KBr, cm⁻¹): 2946 s, 2828 m,

2105 m (ν_SiH), 2035 m br (ν_SiH), 1962 m (ν_SiH), 1580 w, 1469 m,

1361 w, 1296 w, 1250 s, 1154 w, 1000 s br, 895 s, 781 s, 699 w, 665 w.

Anal. Calcd for C₁₃H₃₇KSi₃N₂: C, 45.28; H, 10.82; N, 8.12. Found: C,

44.95; H, 11.32; N, 7.28. Mp: 47−53 °C.

from Aldrich. Anhydrous YbI₂,⁴⁸B(C₆F₅)₃,⁴⁹and HC(SiHMe₂)₃

25a

were prepared as described in literature procedures. We previously

reported the following compounds in the Supporting Information

of ref 27 in initially communicated work: KC(SiHMe₂)₃(1), Ca{C-

(SiHMe₂)₃}₂THF₂(2·2THF), Yb{C(SiHMe₂)₃}₂THF₂(3·2THF),

CaC(SiHMe₂)₃{HB(C₆F₅)₃}THF₂(4·2THF), YbC(SiHMe₂)₃{HB-

(C₆F₅)₃}THF₂(4·2THF), Ca{HB(C₆F₅)₃}₂THF₂(6·2THF), Yb{HB-

(C₆F₅)₃}₂THF₂(7·2THF), and Yb(HBPh₃)₂THF (9·THF). 1,1,3,3-

Tetramethyl-2,2,4,4-tetrakis(dimethylsilyl)-1,3-disilacyclobutane was

identiﬁed by comparison with literature values²⁶and X-ray crys-

tallography (see the Supporting Information of ref 27b). ¹H, ¹³C{¹H},

¹¹B, ¹⁹F, and ²⁹Si NMR spectra were collected on Agilent MR-400,

Bruker DRX-400, Bruker AVIII 600, and Bruker AVII 700 spectrom-

eters. ¹⁵N chemical shifts were determined by ¹H−¹⁵N HMBC experi-

ments on a Bruker AVII 600 spectrometer with a Bruker Z-gradient

{KC(SiDMe₂)₃TMEDA}₂({1-d₃·TMEDA}₂). The procedure for

{KC(SiHMe₂)₃TMEDA}₂was followed, using KC(SiDMe₂)₃(0.168 g,

0.724 mmol) and TMEDA (0.33 mL, 2.18 mmol) to give KC-

(SiDMe₂)₃TMEDA (0.144 g, 0.414 mmol, 57.5%). ²H NMR

(benzene-H₆, 93.0 MHz, 25 °C): δ 4.83 (br s, SiD). IR (KBr, cm⁻¹):

1533 (ν_SiD), 1462 (ν_SiD), 1414 (ν_SiD).

Ca{C(SiHMe₂)₃}₂TMEDA (2·TMEDA). CaI₂(0.384 g, 1.31 mmol)

and KC(SiHMe₂)₃(0.598 g, 2.62 mmol) were suspended in benzene

(10 mL), and TMEDA (0.39 mL, 2.62 mmol) was added. The mixture

was stirred at room temperature for 12 h. Evaporation of the benzene,

extraction of the residue with pentane (2 × 10 mL), concentration in

vacuo, and cooling to −30 °C overnight provided yellow crystals of

Ca{C(SiHMe₂)₃}₂TMEDA (2·TMEDA; 0.166 g, 0.310 mmol, 23.7%).

Alternatively, 2·TMEDA can be prepared from CaI₂(0.165 g, 0.562

mmol) and 1·TMEDA (0.388 g, 1.124 mmol) in benzene (10 mL);

stirring for 12 h followed by an identical workup gave 51.7% yield

1

inverse TXI H/¹³C/¹⁵N 5 mm cryoprobe. ²⁹Si{¹H} NMR spectra

were recorded using DEPT experiments, and assigments were

veriﬁed by ¹H COESY, ¹H−¹³C HMQC, ¹H−¹³C HMBC, and

¹H−²⁹Si HMBC experiments. UV−vis spectral data were measured on

a Shimadzu 3101 PC spectrophotometer. Elemental analysis was

performed using a Perkin-Elmer 2400 Series II CHN/S by the Iowa

State Chemical Instrumentation Facility.

1

(0.156 g, 0.291 mmol). H NMR (benzene-d₆, 600 MHz, 25 °C): δ

4.81 (m, 3 H, ¹J_SiH= 154 Hz, SiHMe₂), 1.80 (s, 12 H, NCH₃), 1.76 (s,

3

4 H, NCH₂), 0.53 (d, 18 H, J_HH= 3.3 Hz, SiCH₃). ¹³C{¹H} NMR

(benzene-d₆, 150 MHz, 25 °C): δ 57.3 (NCH₂), 45.7 (NCH₃), 5.34

(¹J_SiC= 47 Hz, SiCH₃), 2.48 (CaC). ²⁹Si{¹H} NMR (benzene-d₆,

119.3 MHz, 25 °C): δ −23.6 (SiHMe₂). IR (KBr, cm⁻¹): 2949 s br,

2896 s br, 2828 m, 2105 m br (ν_SiH), 2038 m br (ν_SiH), 1861 m br

(ν_SiH), 1599 w, 1468 s, 1297 m, 1249 s, 942 s br, 889 s br, 836 s br,

775 s, 670 s. Anal. Calcd for C₂₀H₅₈Si₆N₂Ca: C, 44.88; H, 10.92; N,

5.23. Found: C, 44.77; H, 10.83; N, 5.23. Mp 138−140 °C.

Ca{C(SiDMe₂)₃}₂THF₂(2-d₆·2THF). The procedure was similar to

that for the preparation of Ca{C(SiHMe₂)₃}₂THF₂,^27busing CaI₂

(0.157 g, 0.535 mmol) and KC(SiDMe₂)₃(0.248 g, 1.07 mmol) to

give Ca{C(SiDMe₂)₃}₂THF₂(0.205 g, 0.360 mmol, 67.4%). ²H NMR

(benzene-H₆, 93.0 MHz, 25 °C): δ 4.71 (br s, SiD). IR (KBr, cm⁻¹):

1530 (ν_SiD), 1493 (ν_SiD), 1409 (ν_SiD).

Yb{C(SiDMe₂)₃}₂THF₂(3-d₆·2THF). The procedure was similar to

that for Yb{C(SiHMe₂)₃}₂THF₂,^27busing YbI₂(0.145 g, 0.340 mmol)

and KC(SiDMe₂)₃(0.158 g, 0.680 mmol) to give Yb{C-

(SiDMe₂)₃}₂THF₂(0.163 g, 0.234 mmol, 68.9%). ²H NMR

(benzene-H₆, 93.0 MHz, 25 °C): δ 4.77 (br s, SiD). IR (KBr, cm⁻¹):

1506 (ν_SiD), 1492 (ν_SiD), 1410 (ν_SiD).

Computational Details. All electronic structure calculations were

performed with the NWChem computational chemistry software suite.⁵⁰

The 6-311++G** basis set was used for H, C, N, O, and Ca.⁵¹The small-

core Stuttgart relativistic eﬀective core potential (RECP) and associated

basis set were used for Yb,⁵²and the large-core Stuttgart RECP and

associated basis set were used for Si.⁵³Density functional theory with the

B3LYP⁵⁴functional was used for both the geometry optimizations and

the Hessian (frequency) calculations. The vibrational frequencies were

calculated with the harmonic oscillator approximation. C₁and C₂

symmetries were used in the geometry optimization calculations.

HC(SiDMe₂)₃. Lithium aluminum deuteride (1.548 g, 36.88 mmol)

was suspended in diethyl ether (50 mL) in a 100 mL Schlenk ﬂask,

and the reaction vessel was cooled to 0 °C. A diethyl ether solution

(20 mL) of HC(SiBrMe₂)₃(5.249 g, 12.29 mmol) was added slowly.

After the addition, the reaction mixture was stirred at room tem-

perature for 12 h and then heated to reﬂux for 2.5 h. Saturated

ammonium chloride solution (15 mL) was added slowly at 0 °C to

quench the reaction mixture. The resulting mixture was ﬁltered to

remove insoluble salts. The organic phase was separated, washed with

water (2 × 10 mL) and brine (1 × 10 mL), and dried with anhydrous

sodium sulfate. Evaporation of diethyl ether provided HC(SiDMe₂)₃

as a spectroscopically pure colorless oil (2.252 g, 11.6 mmol, 94.7%).

²H NMR (CDCl₃, 93.0 MHz, 25 °C): δ 4.15 (br s, SiD).

Ca{C(SiDMe₂)₃}₂TMEDA (2-d₆·TMEDA). The procedure was

similar to that for Ca{C(SiHMe₂)₃}₂TMEDA using CaI₂(0.143 g,

0.485 mmol), KC(SiDMe₂)₃(0.225 g, 0.970 mmol), and TMEDA

(0.15 mL, 0.970 mmol) to give Ca{C(SiDMe₂)₃}₂TMEDA (0.232 g,

2

0.434 mmol, 89.4%). H NMR (benzene-H₆, 93.0 MHz, 25 °C): δ

4.71 (br s, SiD). IR (KBr, cm⁻¹): 1510 sh (ν_SiD), 1493 (ν_SiD), 1411 (ν_SiD).

Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA). A mixture of KC(SiHMe₂)₃

(0.580 g, 2.54 mmol) and YbI₂(0.542 g, 1.27 mmol) in benzene was

allowed to react in the presence of TMEDA (0.38 mL, 2.5 mmol). A

workup procedure similar to that described above for 2·TMEDA

provided a deep red pentane extract; crystallization at −30 °C aﬀorded

red crystals of Yb{C(SiHMe₂)₃}₂TMEDA (3·TMEDA) (0.253 g,

0.379 mmol, 29.8%). 3·TMEDA can also be prepared from YbI₂

(0.339 g, 0.795 mmol) and 1·TMEDA (0.548 g, 1.590 mmol) in an

Tris(dimethylsilyl)methylpotassium-d₃. KC(SiDMe₂)₃(1-d₃).

The procedure for the synthesis of KC(SiHMe₂)₃was followed:²⁷

THF (30 mL) was added to a mixture of HC(SiDMe₂)₃(1.841 g, 9.51

mmol) and KCH₂Ph (1.239 g, 9.51 mmol) in a 100 mL Schlenk ﬂask.

The dark red mixture was stirred at room temperature for 12 h, and all

volatile materials were removed under reduced pressure. The result-

ing reddish brown gummy solid was dissolved in a minimal amount

of toluene and cooled to −30 °C to aﬀord KC(SiDMe₂)₃(1.910 g,

8.25 mmol, 86.7%) as red needles. ²H NMR (benzene-H₆, 93.0 MHz,

25 °C): δ 4.59 (br s, SiD). IR (KBr, cm⁻¹): 1532 (ν_SiD), 1417 (ν_SiD).

{KC(SiHMe₂)₃TMEDA}₂({1·TMEDA}₂). Excess TMEDA (0.35 mL,

2.33 mmol) was added to KC(SiHMe₂)₃(0.175 g, 0.77 mol) dissolved

in benzene. The red mixture was stirred for 30 min at room tem-

perature. The volatiles were evaporated under reduced pressure, and

1

improved yield of 70.5% (0.375 g, 0.560 mmol). H NMR (benzene-d₆,

600 MHz, 25 °C): δ 4.76 (m, 3 H, ¹J_SiH= 148 Hz, SiHMe₂), 1.99 (s, 12 H,

3

NMe), 1.72 (s, 4 H, NCH₂), 0.492 (d, 18 H, J_HH= 2.8 Hz, SiMe₂).

¹³C{¹H} NMR (benzene-d₆, 150 MHz, 25 °C): δ 57.38 (NCH₂),

47.11 (NMe), 11.94 (YbC), 4.82 (¹J_SiC= 50 Hz, SiMe₂). ²⁹Si{¹H}

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NMR (benzene-d₆, 119.3 MHz, 25 °C): δ −17.9 (SiHMe₂, J_YbSi

=

Anal. Calcd for BC₃₁F₁₅H₃₈Si₃N₂Yb: C, 37.54; H, 3.86; N, 2.82. Found:

C, 37.59; H, 3.61; N, 2.75. Mp: 120−125 °C.

9.1 Hz). IR (KBr, cm⁻¹): 2949 s br, 2895 s br, 2843 m, 2802 m, 2080 s

br (ν_SiH), 2038 s br (ν_SiH), 1846 s br (ν_SiH), 1584 w, 1468 s, 1248 s,

1029 s, 938 s br, 884 s br, 835 s br, 773 s, 670 s. Anal. Calcd for C₂₀H₅₈

Si₆N₂Yb: C, 35.95; H, 8.75; N, 4.19. Found: C, 35.81; H, 8.74; N, 4.42.

Mp 90−95 °C.

Ca{HB(C₆F₅)₃}₂TMEDA (6·TMEDA). Ca{C(SiHMe₂)₃}₂TMEDA

(0.095 g, 0.177 mmol) and B(C₆F₅)₃(0.190 g, 0.371 mmol) were

allowed to react in benzene (5 mL). As the mixture was stirred, a light

yellow solid precipitated, and the mixture was stirred for an additional

10 min. The solid was isolated by ﬁltration, washed with benzene (2 ×

4 mL) and pentane (1 × 4 mL), and dried under reduced pressure to

yield Ca{HB(C₆F₅)₃}₂TMEDA as a white solid (0.147 g, 0.124 mmol,

70.3%). ¹H NMR (THF-d₈, 600 MHz, 25 °C): δ 4.02−3.33 (br q, 1 H,

HB), 2.31 (br s, 4 H, NCH₂), 2.15 (br s, 12 H, NMe). ¹H{¹¹B} NMR

(THF-d₈, 600 MHz, 25 °C): δ 2.64 (br s, HB) (resonances assigned to

Yb{C(SiDMe₂)₃}₂TMEDA (3-d₆·TMEDA). The procedure was

similar to that for Yb{C(SiHMe₂)₃}₂TMEDA using YbI₂(0.143 g,

0.335 mmol), KC(SiDMe₂)₃(0.155 g, 0.670 mmol), and TMEDA

(0.10 mL, 0.670 mmol) to give Yb{C(SiDMe₂)₃}₂TMEDA (0.164 g,

2

0.244 mmol, 72.8%). H NMR (benzene-H₆, 93.0 MHz, 25 °C): δ

4.79 (br s, SiD). IR (KBr, cm⁻¹): 1505 (ν_SiD), 1494 (ν_SiD), 1380 (ν_SiD).

{Yb[C(SiHMe₂)₃]₂THF}₂TMEDA ({3·THF}₂TMEDA). Excess THF

(0.45 mL, 5.60 mmol) was added to a benzene solution (5 mL) of

3·TMEDA (0.375 g, 0.560 mol) at room temperature. The red mixture

was stirred for 30 min. Evaporation of the volatile materials provided a

1

TMEDA are identical in H and H{¹¹B} NMR spectra). ¹³C{¹H}

NMR (THF-d₈, 150 MHz, 25 °C): δ 150.05 (br, C₆F₅), 148.47 (br,

C₆F₅), 139.37 (br, C₆F₅), 138.14 (br, C₆F₅), 136.14 (br, C₆F₅), 58.86

(NCH₂), 46.23 (br, NMe). ¹¹B NMR (THF-d₈, 119.3 MHz, 25 °C):

δ −27.3 (d, ¹J_BH= 93.2 Hz). ¹⁹F NMR (THF-d₈, 376 MHz, 25 °C): δ

gummy solid of ({3·THF}₂TMEDA) (0.381 g, 0.558 mmol, 99.6%).

1

3

¹H NMR (benzene-d₆, 600 MHz, 25 °C): δ 4.70 (m, 6 H, J_SiH

=

−136.8 (d, J_FF= 20.4 Hz, 12 F, o-F), −169.5 (t, J_FF= 17.7 Hz, 6 F,

p-F), −172.1 (t, J_FF= 22.0 Hz, 12 F, m-F). IR (KBr, cm⁻¹): 2965 m,

3

148 Hz, SiHMe₂), 3.67 (br s, 4 H, OCH₂CH₂), 1.97 (s, 6 H, NMe),

1.77 (s, 2 H, NCH₂), 1.36 (br s, 4 H, OCH₂CH₂), 0.49 (d, 36 H,

³J_HH= 2.1 Hz, SiMe₂). ¹³C{¹H} NMR (benzene-d₆, 150 MHz, 25 °C):

δ 69.46 (OCH₂), 57.46 (NCH₂), 46.88 (NMe), 25.68 (OCH₂CH₂),

11.81 (YbC), 4.70 (¹J_SiC= 50 Hz, SiMe₂). ²⁹Si{¹H} NMR (benzene-d₆,

119.3 MHz, 25 °C): δ −18.4. IR (KBr, cm⁻¹): 2951 s, 2896 s, 2802 m,

2095 s br (ν_SiH), 2039 s br sh (ν_SiH), 1850 m br (ν_SiH), 1598 w, 1493

m, 1468 w, 1249 s, 1028 s br, 937 s br, 890 s br, 834 s br, 773 s, 670 s.

Anal. Calcd for C₂₁H₅₈Si₆NOYb: C, 36.97; H, 8.57; N, 2.05. Found: C,

37.04; H, 9.01; N, 1.95.

2383 m br (ν_BH), 1665 m, 1606 m, 1515 s, 1466 s br, 1373 m, 1274 s,

1113 s, 1086 s, 965 s br, 913 m, 828 m, 789 m, 768 m, 685 m, 666 m.

Anal. Calcd for B₂C₄₂F₃₀H₁₈N₂Ca: C, 42.67; H, 1.53; N, 2.37. Found:

C, 43.09; H, 1.79; N, 2.00. Mp: 205−211 °C.

Yb{HB(C₆F₅)₃}₂TMEDA (7·TMEDA). The procedure was similar to

that for 6·TMEDA, using Yb{C(SiHMe₂)₃}₂TMEDA (0.095 g, 0.143

mmol) and B(C₆F₅)₃(0.153 g, 0.299 mmol) to yield Yb(HB-

(C₆F₅)₃)₂TMEDA as an oﬀ-white solid (0.158 g, 0.120 mmol, 84.1%).

¹H NMR (THF-d₈, 600 MHz, 25 °C): δ 4.03−3.32 (br q, 1 H, HB),

1

2.37 (s, 4 H, NCH₂), 2.19 (s, 12 H, NMe). H{¹¹B} NMR (THF-d₈,

CaC(SiHMe₂)₃{HB(C₆F₅)₃}TMEDA (4·TMEDA). Benzene (5 mL)

was added to a mixture of Ca{C(SiHMe₂)₃}₂TMEDA (0.052 g, 0.097

mmol) and B(C₆F₅)₃(0.050 g, 0.097 mmol). The mixture was stirred

for 45 min, and then the volatile components were evaporated under

reduced pressure. The yellow solid was washed with pentane (3 × 5 mL)

and dried under vacuum to yield CaC(SiHMe₂)₃{HB(C₆F₅)₃}-

600 MHz, 25 °C): δ 2.64 (br s, HB) (resonances assigned to TMEDA

are identical in ¹H and ¹H{¹¹B} NMR spectra). ¹³C{¹H} NMR (THF-d₈,

150 MHz, 25 °C): δ 150.04 (br, C₆F₅), 148.51 (br, C₆F₅), 139.45

(br, C₆F₅), 138.10 (br, C₆F₅), 136.52 (br, C₆F₅), 58.55 (NCH₂), 46.05

(br, NMe). ¹¹B NMR (THF-d₈, 119.3 MHz, 25 °C): δ −27.3 (d, ¹J_BH

1

= 93.3 Hz). ¹⁹F NMR (THF-d₈, 376 MHz, 25 °C): δ −136.9 (d, ³J_FF

=

TMEDA as a white solid (0.055 g, 0.064 mmol, 65.7%). H NMR

1

3

(benzene-d₆, 600 MHz, 25 °C): δ 4.44 (m, J_SiH= 144.9 Hz, 3 H,

20.9 Hz, 12 F, o-F), −169.2 (t, J_FF= 20.2 Hz, 6 F, p-F), −171.9 (t,

³J_FF= 19.1 Hz, 12 F, m-F). IR (KBr, cm⁻¹): 3094 w, 2974 w, 2900 w,

2305 m br (ν_BH), 1647 m, 1606 m, 1517 s, 1466 s br, 1374 m, 1281 m,

1123 s br, 1083 s, 957 s, 898 m, 789 m, 769 m, 754 m, 685 m, 673 m.

Anal. Calcd for B₂C₄₂F₃₀H₁₈N₂Yb: C, 38.36; H, 1.38; N, 2.13. Found:

C, 38.69; H, 1.24; N, 1.94. Mp: 163−170 °C.

SiH), 3.00−2.27 (br q, 1 H, HB), 1.70 (br s, 12 H, NMe), 1.51 (br s, 4

3

1

H, NCH₂), 0.30 (d, J_HH= 3.4 Hz, 18 H, SiMe₂). H{¹¹B} NMR

(benzene-d₆, 600 MHz, 25 °C): δ 2.64 (br s, HB) (the other signals

1

were unchanged from coupled H NMR spectrum). ¹³C{¹H} NMR

(benzene-d₆, 150 MHz, 25 °C): δ 149.78 (br, C₆F₅), 148.30 (br,

C₆F₅), 138.83 (br, C₆F₅), 137.12 (br, C₆F₅), 56.72 (NCH₂), 46.37 (br,

NMe), 11.69 (CaC), 2.15 (SiMe₂). ¹¹B NMR (benzene-d₆, 119.3

MHz, 25 °C): δ −21.8 (d, ¹J_BH= 75.0 Hz). ¹⁹F NMR (benzene-d₈, 376

MHz, 25 °C): δ −134.9 (br, 6 F, o-F), −159.0 (br, 3 F, p-F), −163.2

(6 F, m-F). ²⁹Si{¹H} NMR (benzene-d₆, 119.3 MHz, 25 °C): δ −18.9.

IR (KBr, cm⁻¹): 2962 m, 2897 m, 2851 m, 2302 m br (ν_BH), 2094 m

br (ν_SiH), 2026 m br (ν_SiH), 1918 m br (ν_SiH), 1646 m, 1603 w, 1517 s

br, 1467 s br, 1373 m, 1283 s, 1253 m, 1124 s, 1078 s, 1024 m, 965 s

br, 835 s, 790 s. Anal. Calcd for BC₃₁F₁₅H₃₈Si₃N₂Ca: C, 43.36; H, 4.46;

N, 3.26. Found: C, 43.15; H, 4.38; N, 3.24. Mp: 137−140 °C.

YbC(SiHMe₂)₃{HB(C₆F₅)₃}TMEDA (5·TMEDA). The procedure

was similar to that for the calcium analogue 4·TMEDA above, with

Yb{C(SiHMe₂)₃}₂TMEDA (0.083 g, 0.124 mmol) and B(C₆F₅)₃

(0.063 g, 0.124 mmol) aﬀording 5·TMEDA as a yellow solid (0.075 g,

0.076 mmol, 61.2%). ¹H NMR (benzene-d₆, 600 MHz, 25 °C): δ 4.51

(m, ¹J_SiH= 147.0 Hz, 3 H, SiH), 3.67−2.82 (br q, 1 H, HB), 1.71 (br s,

Ca(HBPh₃)₂TMEDA (8·TMEDA). Benzene (3 mL) was added to a

mixture of Ca{C(SiHMe₂)₃}₂TMEDA (0.077 g, 0.144 mmol) and

BPh₃(0.070 g, 0.290 mmol). The colorless solution mixture was

thoroughly mixed and allowed to stand at room temperature for 10 h

to yield white crystals. The solution was decanted, and the oﬀ-white

crystals were washed with benzene (3 mL) and pentane (2 × 3 mL)

and dried under vacuum to give 8·TMEDA as a white, crystalline,

benzene-insoluble solid (0.060 g, 0.094 mmol, 65.2%). IR (KBr, cm⁻¹):

3056 m, 2998 m, 2059 m (ν_BH), 2027 m (ν_BH), 2008 m (ν_BH), 1943 s

(ν_BH), 1577 m, 1478 m br, 1428 m, 1284 m, 1168 m br, 1066 m, 1027 m,

788 m, 734 s, 707 vs br. Anal. Calcd for B₂C₄₂H₄₈N₂Ca: C, 78.51; H,

7.53; N, 4.36. Found: C, 77.97; H, 7.90; N, 3.60. Mp: 240−245 °C dec.

Yb(HBPh₃)₂TMEDA (9·TMEDA). The procedure followed that for

the calcium analogue 8·TMEDA, using Yb{C(SiHMe₂)₃}₂TMEDA

(0.087 g, 0.130 mmol) and BPh₃(0.063 g, 0.261 mmol) to give

9·TMEDA as a red, insoluble, crystalline solid (0.074 g, 0.095 mmol,

73.2%). IR (KBr, cm⁻¹): 3059 s, 3040 s, 2995 s, 2880 m, 2054 s (ν_BH),

2024 s (ν_BH), 2006 s (ν_BH), 1940 s (ν_BH), 1582 m, 1464 s, 1428 s,

1284 w, 1158 s br, 1065 m, 943 s, 786 m, 733 s br, 706 s br. Anal.

Calcd for B₂C₄₂H₄₈N₂Yb: C, 65.05; H, 6.24; N, 3.61. Found: C, 64.84;

H, 6.04; N, 3.59. Mp: 140−150 °C.

3

12 H, NMe), 1.55 (br s, 4 H, NCH₂), 0.33 (d, J_HH= 2.9 Hz, 18 H,

1

SiMe₂). H{¹¹B} NMR (benzene-d₆, 600 MHz, 25 °C): δ 3.22 (br s,

1

HB) (all other resonances were identical to the H NMR spectrum).

¹³C{¹H} NMR (benzene-d₆, 150 MHz, 25 °C): δ 149.88 (br, C₆F₅),

148.34 (br, C₆F₅), 140.84 (br, C₆F₅), 138.93 (br, C₆F₅), 137.17 (br,

C₆F₅), 56.65 (NCH₂), 46.18 (br, NMe), 17.41 (YbC), 2.15 (SiMe₂).

Ca(HBPh₃)₂THF (8·THF). The procedure followed that for the

calcium TMEDA adduct 8·TMEDA, with Ca{C(SiHMe₂)₃}₂THF₂

(0.072 g, 0.127 mmol) and BPh₃(0.062 g, 0.255 mmol) providing

8·THF as a white solid (0.057 g, 0.095 mmol, 74.5%). ¹H NMR

(benzene-d₆, 600 MHz, 25 °C): δ 7.64 (br, 12 H, m-C₆H₅), 7.24 (br,

12 H, o-C₆H₅), 7.16 (br, 6 H, p-C₆H₅), 3.23 (m, 4 H, CH₂CH₂O),

1

¹¹B NMR (benzene-d₆, 119.3 MHz, 25 °C): δ −21.4 (d, J_BH= 71.6

Hz). ¹⁹F NMR (benzene-d₆, 376 MHz, 25 °C): δ −134.7 (br, 6 F,

o-F), −159.3 (br, 3 F, p-F), −163.7 (6 F, m-F). ²⁹Si{¹H} NMR

(benzene-d₆, 119.3 MHz, 25 °C): δ −18.0. IR (KBr, cm⁻¹): 2961 m,

2895 m, 2849 m, 2293 m br (ν_BH), 2094 m br (ν_SiH), 2027 m br (ν_SiH),

1899 m br (ν_SiH), 1645 m, 1602 w, 1516 s, 1466 s br, 1373 m, 1326 vw,

1282 m, 1253 m, 1110 s, 1076 s, 1024 m, 965 s br, 894 s br, 837 s, 789 m.

1

1.15 (m, br, 4 H, CH₂CH₂O). H{¹¹B} NMR (benzene-d₆, 600 MHz,

25 °C): δ 3.18 (br, HB). ¹³C{¹H} NMR (benzene-d₆, 150 MHz,

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25 °C): δ 155.2 (ipso-CH), 139.3 (m-CH), o-CH and p-CH overlapped

with C₆D₆, 69.8 (CH₂CH₂O), 25.4 (CH₂CH₂O). ¹¹B NMR (benzene-

d₆, 119.3 MHz, 25 °C): δ −6.0 (br). IR (KBr, cm⁻¹): 3058 m, 2992 m,

2021 m br (ν_BH), 1936 m br (ν_BH), 1581 m, 1481 m, 1429 m, 1257 w,

1184 m br, 1021 m, 978 m, 880 m br, 737 s, 703 vs Anal. Calcd

for B₂C₄₀H₄₀OCa: C, 80.28; H, 6.74. Found: C, 76.09; H, 7.21. Mp:

113−116 °C.

ACKNOWLEDGMENTS

■

K.Y., G.S., and A.D.S. gratefully thank the NSF (CHE-0955635,

CRIF-0946687, MRI-1040098, and OCI-0749156) for ﬁnancial

support. This research was also supported by an allocation of

advanced computing resources provided by the National

Science Foundation (A.D.S. and T.L.W.). The computations

were performed on Kraken at the National Institute for Com-

putational Sciences. T.L.W. is supported by the U.S. Depart-

ment of Energy, Oﬃce of Basic Energy Sciences, Division of

Chemical Sciences, Geosciences, and Biosciences through the

Ames Laboratory Chemical Physics project. The Ames

Laboratory is operated for the U.S. Department of Energy by

Iowa State University under Contract No. DE-AC02-

07CH11358.

KHB(C₆F₅)₃(13). Benzene (5 mL) was added to a mixture of KC-

(SiHMe₂)₃(0.107 g, 0.469 mmol) and B(C₆F₅)₃(0.252 g, 0.492 mmol),

and the solution was stirred for 10 min. The orange color quickly faded

away, and a pale pink crystalline solid precipitated. The benzene solution

was ﬁltered, and the solid was washed with benzene (2 × 5 mL) and

pentane (1 × 4 mL). The volatiles were evaporated under reduced

pressure to yield a white solid (0.168 g, 0.303 mmol, 64.7%). ¹H NMR

(THF-d₈, 600 MHz, 25 °C): δ 4.04−3.38 (br q, 1 H, HB). ¹³C{¹H}

NMR (THF-d₈, 150 MHz, 25 °C): δ 150.1 (br, C₆F₅), 148.5 (br, C₆F₅),

139.5 (br, C₆F₅), 137.9 (br, C₆F₅), 136.5 (br, C₆F₅). ¹¹B NMR (THF-d₈,

1

119.3 MHz, 25 °C): δ −27.3 (d, J_BH= 92.5 Hz). ¹⁹F NMR (THF-d₈,

REFERENCES

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376 MHz, 25 °C): δ −136.7 (d, J_FF= 21.6 Hz, 6 F, o-F), −169.8

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3

(t, J_FF= 20.2 Hz, 3 F, p-F), −172.3 (t, J_FF= 18.9 Hz, 6 F, m-F). IR

(KBr, cm⁻¹): 3092 w, 3037 w, 2964 w, 2819 w, 2586 vw, 2382 s br

(ν_BH), 2224 vw, 2178 vw, 2132 vw, 2093 vw, 2027 vw, 1644 vs, 1606 m,

1511 vs, 1450 vs, 1407 m, 1324 vw, 1273 vs, 1114 vs, 1086 vs, 1021 m,

945 vs, 914 vs, 886 s, 843 w, 785 m, 769 s, 752 s, 733 w, 720 vw, 666 s.

Anal. Calcd for BC₁₈F₁₅HK: C, 39.16; H, 0.18. Found: C, 40.15; H,

0.26. Mp: 290−292 °C dec.

KHB(C₆F₅)₃TMEDA₂(14). KHB(C₆F₅)₃(0.090 g, 0.162 mmol)

was suspended in benzene (2 mL), and excess TMEDA (73 mL, 0.486

mmol) was added to yield a clear solution. The mixture was stirred for

10 min, and the volatiles were evaporated under reduced pressure to

give KHB(C₆F₅)₃TMEDA₂as a white solid (0.125 g, 0.159 mmol,

98.1%). X-ray-quality crystals can be grown from a toluene solution of

1

KHB(C₆F₅)₃TMEDA₂at −30 °C for 3 days. H NMR (benzene-d₆,

600 MHz, 25 °C): δ 3.84−3.08 (br q, 1 H, HB), 1.90 (s, 8 H, NCH₂),

1.82 (s, 24 H, NCH₃). ¹³C{¹H} NMR (benzene-d₆, 150 MHz, 25 °C):

δ 150.0 (br, C₆F₅), 148.5 (br, C₆F₅), 140.1 (br, C₆F₅), 138.5 (br,

C₆F₅), 136.9 (br, C₆F₅), 57.5 (NCH₂), 45.4 (NMe). ¹¹B NMR

1

(benzene-d₆, 119.3 MHz, 25 °C): δ −24.7 (d, J_BH= 83.1 Hz). ¹⁹F

3

NMR (benzene-d₆, 376 MHz, 25 °C): δ −135.9 (d, J_FF= 21.8 Hz,

3

6 F, o-F), −163.2 (t, J_FF= 20.6 Hz, 3 F, p-F), −170.0 (t, J_FF

=

21.8 Hz, 6 F, m-F). IR (KBr, cm⁻¹): 2948 m, 2872 m, 2832 m, 2792 m,

2712 m, 2381 m br (ν_BH), 1643 m, 1514 s, 1461 s, 1364 m, 1297 m,

1278 m, 1099 s, 969 s, 907 m, 782 m, 764 m. Calcd for BC₃₀F₁₅H₃₃K:

C, 45.93; H, 4.24; N, 7.14. Found: C, 46.02; H, 3.84; N, 6.87.

Mp: 90−92 °C.

ASSOCIATED CONTENT

■

S

* Supporting Information

CIF ﬁles giving X-ray crystallographic data for the compounds

{KC(SiHMe₂)₃TMEDA}₂({1·TMEDA}₂), Yb{C(SiHMe₂)₃}₂-

TMEDA (3·TMEDA), CaC(SiHMe₂)₃{HB(C₆F₅)₃}THF₂

(4·2THF),^27bYb{κ³-HB(C₆F₅)₃}₂THF₂(7·2THF), Yb{κ³-HB-

(C₆F₅)₃}₂TMEDA (7·TMEDA), Yb(HBPh₃)₂TMEDA

(9·TMEDA), Yb(HBPh₃)₂THF (9·THF), and KHB(C₆F₅)₃-

TMEDA₂(13·2TMEDA) and tables giving initial and

optimized geometries and minimized total energies of

Ca(C(SiHMe₂)₃)₂THF₂and Yb(C(SiHMe₂)₃)₂THF₂. This

material is available free of charge via the Internet at http://

pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail: sadow@iastate.edu.

Notes

The authors declare no competing ﬁnancial interest.

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Intermolecular β-hydrogen abstraction in ytterbium, calcium, and potassium tris(dimethylsilyl)methyl compounds

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