Preparation of a stable disilane amidinium heterocycle and attempted syntheses of an inorganic N-heterocyclic carbene

Article abstract of DOI:10.1016/j.jorganchem.2013.04.029

The cyclic disilane amidinium salt [(Me₂SiNDipp) ₂CH]OTf (Dipp = 2,6-ⁱPr₂C₆H ₃; OTf = OSO₂CF₃^-) was obtained in high yield, and attempts to generate the N-heterocyclic carbene [(Me ₂SiNDipp)₂C:] via deprotonation with numerous bases exclusively gave ring-opened products of the general form: R-SiMe ₂-SiMe₂N(Dipp)-CHNDipp (R = carbon-, nitrogen- or oxygen-based nucleophiles). Although this approach was not directly successful in generating silicon-based N-heterocyclic carbenes, judicious manipulation of the ring-bound substituents might enable the future synthesis of new inorganic carbenes to transpire.

Full text of DOI:10.1016/j.jorganchem.2013.04.029

Journal of Organometallic Chemistry 739 (2013) 26e32
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Preparation of a stable disilane amidinium heterocycle and attempted
syntheses of an inorganic N-heterocyclic carbene
S.M. Ibrahim Al-Rafia, James T. Goettel, Paul A. Lummis, Robert McDonald,
*
Michael J. Ferguson, Eric Rivard
Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr., Edmonton, Alberta, Canada T6G 2G2
a r t i c l e i n f o
a b s t r a c t
The cyclic disilane amidinium salt [(Me2SiNDipp)2CH]OTf (Dipp ¼ 2,6-iPr2C6H3; OTf ¼ OSO2CF3ꢀ) was
obtained in high yield, and attempts to generate the N-heterocyclic carbene [(Me2SiNDipp)2C:] via
deprotonation with numerous bases exclusively gave ring-opened products of the general form: R
eSiMe2eSiMe2N(Dipp)eCH]NDipp (R ¼ carbon-, nitrogen- or oxygen-based nucleophiles). Although
this approach was not directly successful in generating silicon-based N-heterocyclic carbenes, judicious
manipulation of the ring-bound substituents might enable the future synthesis of new inorganic car-
benes to transpire.
Article history:
Received 3 April 2013
Received in revised form
17 April 2013
Accepted 18 April 2013
Keywords:
N-Heterocyclic carbenes
Silicon
Ó 2013 Elsevier B.V. All rights reserved.
Ring-opening
Amidinium salts
1. Introduction
In this paper we describe our efforts to obtain an N-heterocyclic
carbene with a disilane backbone (compound E in Chart 1). The
Since their discovery [1], N-heterocyclic carbenes (NHCs) have
adopted a prominent role as versatile ligands in coordination
chemistry and are now replacing more traditional electron pair
donors, such as phosphines, for various applications [2]. As a result,
there has been considerable progress in modifying the electronic
and steric properties of this general ligand class by changing the
substituents at either the intraring carbon or nitrogen atoms,
initial goal of this project was to promote increased s-donor ability
relative to pre-existing NHCs via the placement of electron-
donating silicon and tin atoms within a carbene heterocycle. Of
note, we recently reported the heavier Group 14 element analogs of
the target carbene E, wherein the carbene carbon in the heterocycle
is replaced by Ge or Sn [i.e., (Me2SiNDipp)2E; E ¼ Ge and Sn;
Dipp ¼ 2,6-iPr2C6H3] [6,7].
including the development of carbene systems where the s-donor
site is found at alternate positions along the N-heterocyclic carbene
backbone [3].
2. Results and discussion
A less explored method of tuning the
s-donor/p-acceptor
2.1. Preparation of an amidinium heterocycle containing a disilane
backbone
strength of an N-heterocyclic carbene is to incorporate inorganic
elements within an NHC framework. Important breakthroughs in
this area include the synthesis of phosphorus-containing cyclic
carbenes, such as heterocycle A by Grubbs et al. [4], and the
preparation of boron-containing carbenes by the groups of Ber-
trand and Roesler (BeD) (Chart 1) [5]. Despite these advances, the
development of new inorganic element-based NHCs has been
somewhat slow due to the significant synthetic challenges stem-
ming from the high degrees of reactivity inherent to many inor-
ganic compounds.
The incorporation of inorganic elements within the backbone of
an N-heterocyclic carbene (NHC) represents a relatively new
concept, and provides an added dimension with respect to tuning
the donor/acceptor properties of an N-heterocyclic carbene [4,5,8].
In order to progress toward a cyclic carbene with a tetramethyldi-
silane (eMe2SieSiMe2e) backbone, we modeled our synthetic
strategy after a general method used previously to generate NHCs
with heterocycles containing phosphorus and boron atoms [4,5].
The requisite disilane amidinium salt [(Me2SiNDipp)2CH]OTf
(OTf ¼ OSO2CFꢀ3 ) (1) was prepared from the cyclocondensation
of ClSiMe2SiMe2Cl with the silylformamidine, DippN]CHN(Dipp)
* Corresponding author. Tel.: þ1 7804924255; fax: þ1 780492823.
E-mail address: erivard@ualberta.ca (E. Rivard).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.04.029

S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
27
Chart 1. Various known inorganic element-containing NHCs (AeD) and the target species (E) discussed in this paper; Dipp ¼ 2,6-iPr2C6H3.
SiMe3, in the presence of Me3SiOTf (Eq. (1)). The potential NHC
precursor, 1, was isolated as a thermally stable white solid
(Mp ¼ 245e250 ꢁC) in a nearly quantitative yield and was struc-
turally authenticated by single-crystal X-ray crystallography
(Fig. 1).
geometries [N(1)eSi(1)eSi(2)eN(2) torsion angle ¼ 5.08(6)ꢁ in 1]
with narrow intraring NeSieSi angles of ca. 90ꢁ. The adjacent
N(1)eC(1)eN(2) angle in the imidazolium cation was determined
to be 120.46(14)ꢁ, suggesting the presence of sp2-hybridization at
C(1); significant NeCeN
p-bonding is also present within this
imidazolium heterocycle as manifested by average CeN bond
ꢀ
lengths [1.326(3) A] that are shorter than typical CeN single bonds.
For comparison, the related intraring CeN bond lengths within the
ꢀ
imidazolium triflate salt [(HCNDipp)2CH]OTf average to 1.329(5) A
[9]. Lastly, the aryl groups in 1 are significantly canted forward to
form a steric pocket about the proximal imidazolium carbon cen-
ters, C(1). The 19F{1H} NMR spectrum of 1 in CDCl3 is consistent
with the presence of a weakly interacting OTf anion in solution
(1)
(d
ꢀ78.3 ppm; c.f. [nBu4N]OTf,
d
ꢀ78.7 ppm) [10]. However signif-
icant Si/O contacts between the imidazolium ring in 1 and the
OTf anion were detected in the solid state [2.8090(16) and
2.8534(17) A] with the triflate anion located directly below the
As shown in Fig. 1, [(Me2SiNDipp)2CH]OTf (1) contains a cationic
NSiSiNC heterocycle with a nearly planar arrangement of the con-
stituent ring atoms. Interestingly, both SiMe2 units within the tet-
ramethyldisilane backbone in 1 are locked in mutually eclipsed
ꢀ
cationic ring; these SieO distances lie within the sum of the van der
ꢀ
Waals radii for O and Si (3.52 A) [11]. The observed metrical pa-
rameters in 1 attest to a low degree of steric protection provided by
the methyl substituents attached to the silicon atoms, thus enabling
weak cationeanion contacts to exist.
2.2. Attempted deprotonation of 1 to afford an N-heterocyclic
carbene
With the desired cyclic disilane amidinium salt 1 in hand, we
explored various methods of accessing the target carbene
[(Me2SiNDipp)2C:] via deprotonation of the ring-bound hydrogen
atom at C(1) (Fig. 1). Our first deprotonation attempt involved
treatmentof 1 with the commonly used hindered baseK[N(SiMe3)2].
This reaction yielded a new product as a colorless and moisture-
sensitive solid, however 1H NMR spectroscopic analysis revealed
that a downfield-positioned singlet at 7.39 ppmwas present in C6D6,
indicating that the N]CHeN unit remained intact. In addition, the
expected resonance for a carbenic carbon of an NHC was not located
in the 13C{1H} NMR spectrum (>180 ppm) [1]. These data, in
conjunction with the appearance of two inequivalent SiMe2 envi-
ronments in the 1H and 13C{1H} NMR spectra, suggested that
nucleophilic attack of the ring structure in 1 by the [N(SiMe3)2]
anion had transpired. Single-crystal X-ray crystallography later
identified the product as the ring-opened species (Me3Si)2NSiMe2
SiMe2N(Dipp)C(H)]NDipp (2) (Scheme 1, Fig. 2).
Fig. 1. Molecular structure (30% probability level) of the [(Me2SiNDipp)2CH]þ cation in
1 with all hydrogen atoms except theꢁimidazolium proton at C(1) omitted for clarity.
ꢀ
Selected bond lengths [A] and angles [ ]: Si(1)eSi(2) 2.2976(7), Si(1)eN(1) 1.8485(14),
The metrical parameters in 2 are consistent with the molecular
structure presented in Scheme 1. Specifically, planar coordination
environments are observed about N(1) and N(3) [angle
sum ¼ 359.0(3) and 359.94(17)ꢁ, respectively], while the SieN
Si(2)eN(2) 1.8496(15), N(1)eC(1) 1.330(2), N(2)eC(1) 1.323(2), Si(1)/O(2) 2.8090(16),
Si(1)/O(2)0 2.8534(17); N(1)eC(1)eN(2) 120.46(14), N(1)eSi(1)eSi(2) 90.71(5), N(2)e
Si(2)eSi(1) 90.34(5). O(2) is related to the triflate oxygen O(2)0 by the symmetry
operation ꢀx, ꢀy, 1 ꢀ z through the 0, 0, 1/2 inversion center.

28
S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
Scheme 1. Attempted deprotonation of [(Me2SiNDipp)2CH]OTf (1).
ꢀ
distances in 2 [1.7543(19) to 1.7999(18) A] are in the typical range
diethyl ether. As with K[N(SiMe3)2], nucleophilic ring-opening
of the heterocycle in 1 by Li[NHDipp] occurred to give the N-dis-
ilylamino-substituted formamidine chain (DippNH)SiMe2Si-
Me2N(Dipp)C(H)]NDipp (3) in nearly quantitative yield, with
concomitant formation of LiOTf as a by-product (Scheme 1, Fig. 3).
The refined metrical parameters for 3 derived by X-ray crystallog-
raphy were similar to 2, thus no further discussionwill be made here.
Positing that amides such as the [N(SiMe3)2]ꢀ and [NHDipp]ꢀ
were too nucleophilic to participate in clean deprotonation chem-
istry with 1, we attempted to prepare the disilane-bridged NHC
for single bonds with appreciable CeN bond length alternation
ꢀ
noted along the chain [C(1)eN(1)
¼
1.368(2) A; C(1)e
ꢀ
N(2) ¼ 1.274(3) A]; the latter bond lengths correspond to localized
CeN single and double bonds, respectively.
It appears from the above chemistry that the disilane backbone in
1 is prone to nucleophilic attack, thus posing a competing reaction
pathway to the desired deprotonation of the imidazolium CeH
group. Unfortunately we observed
a
similar ring-opening
process when 1 was treated with the lithium amide Li[NHDipp] in
Fig. 3. Molecular structure of (DippNH)NSiMe2SiMe2N(Dipp)C(H)]NDipp (3) at a 30%
Fig. 2. Molecular structure of (Me3Si)2NSiMe2SiMe2N(Dipp)C(H)]NDipp (2) at a 30%
probability level with all carbon-bound hydrogen atoms [except at C(5A)] and solvate
ꢁ
ꢀ
probability level with all hydrogen atoms except for theꢁformamidine proton at C(1)
molecules omitted for clarity. Selected bond lengths [A] and angles [ ] with values for
the second molecule of 3 in the asymmetric unit listed in square brackets: Si(1)eSi(2)
2.3553(8) [2.3518(8)], Si(1)eN(1) 1.7728(19) [1.7792(19)], Si(2)eN(3) 1.7408(19)
[1.7386(19)], N(1)eC(5) 1.376(3) [1.383(3)], N(2)eC(5) 1.287(3) [1.275(3)]; Si(2)eSi(1)e
N(1) 114.94(6) [114.54(6)], Si(1)eSi(2)eN(3) 112.13(7) [112.06(7)], Si(1)eN(2)eC(5)
120.25(14) [120.61(12)], N(1)eC(5)eN(2) 122.6(2) [123.14(19)].
ꢀ
omitted for clarity. Selected bond lengths [A] and angles [ ]: C(1)eN(1) 1.368(2), C(1)e
N(2) 1.274(3), N(1)eSi(1) 1.7999(18), Si(1)eSi(2) 2.3774(9), Si(2)eN(3) 1.7744(18),
N(3)eSi(3) 1.7543(19), N(3)eSi(4) 1.7565(19); N(1)eC(1)eN(2) 122.18(18), C(1)eN(1)e
Si(1) 119.70(14), N(1)eSi(1)eSi(2) 114.25(6), Si(1)eSi(2)eN(3) 107.89(6), Si(2)eN(3)e
Si(3) 123.11(10), Si(2)eN(3)eSi(4) 118.80(10), Si(3)eN(3)eSi(4) 118.03(10).

S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
29
[(Me2SiNDipp)2C:] (E) from the reaction of 1 with the recently
synthesized hindered base IPr]CH2 (IPr ¼ [(HCNDipp)2C]) [12e14].
Our studies were further motivated by the demonstrated role of
IPr]CH2 as a neutral Brønsted base in phosphazene [P(V)eN]
chemistry [15]. When IPr]CH2 was combined with a suspension of
1 in toluene, the immediate clearing of the mixture and the for-
mation of a yellow solution was noted. Analysis of the reaction
mixture revealed the presence of a new IPrCH2-containing product
that had spectral properties inconsistent with the formation of an
[IPreCH3]þ salt [15]; one would expect [IPreCH3]OTf to be formed
as a byproduct if successful deprotonation of 1 had occurred.
Recrystallization of the solid from a THF/hexanes mixture at ꢀ35 ꢁC
afforded large colorless blocks that were identified as the ring-
opened product [(IPrCH2)SiMe2SiMe2N(Dipp)C(H)]NDipp]OTf (4)
(Scheme 1, Fig. 4).
of the intraring NeC bonds in 4). The OTf counteranion in 4 is well
separated from the cationic chain in the solid state with no SieOTf
ꢀ
contacts noted within 6 A.
Given the widespread use of K[OtBu] to deprotonate imidazo-
lium cations and form stable N-heterocyclic carbenes [16], com-
pound 1 was combined with an equivalent of K[OtBu]. A white, free
flowing solid was obtained upon work-up which gave 1H, 13C{1H}
and 29Si{1H} NMR data consistent with the formation of the ring-
opened product [(tBuO)SiMe2SiMe2N(Dipp)C(H)]NDipp]
5
(Scheme 1). Specifically, two distinct SiMe2 environments were
located by NMR spectroscopy and characteristic 1H and 13C{1H}
NMR resonances for the tBuO residue were also detected. Moreover,
an intact formamidine CH group was found at 7.32 ppm in the 1H
NMR spectrum of 5 along with an accompanying imine 13C{1H}
NMR resonance at 157.1 ppm (in C6D6). Unfortunately attempts to
obtain crystals of 5 of suitable quality for single-crystal X-ray
crystallography were unsuccessful [17], however satisfactory
elemental analyses (C, H and N) were obtained. Lastly, attempts to
deprotonate 1 using stoichiometric quantities of sodium metal or
sodium naphthalenide gave the known aryl-substituted for-
mamidine DippN]C(H)eNH(Dipp) [5a] along with unknown
SiMe2-containing products.
The above reactions indicate a high propensity of the SieN
linkages in 1 to undergo nucleophilic cleavage. The significant steric
pocket created by the flanking Dipp groups about the imidazolium
proton at C(1) in 1 (Fig.1) likely directs the reactivity away from this
unit and toward the electron deficient silicon centers positioned at
the opposite end of the CN2Si2 ring. Another factor likely promoting
ring-opening versus deprotonation is the ring strain in the imida-
zolium precursor 1 as illustrated by the N(Dipp)-SieSi angles of ca.
90ꢁ; these angles become much wider in the ring-opened products
2e4 where angles in the range of 113.4(4)e118.0(10)ꢁ are observed.
Thus it appears that either less encumbered groups will be have to
present at the nitrogen atoms in the disilane amidinium ring,
and/or that increased steric bulk will have to be incorporated along
the disilane backbone in order to induce successful deprotonation
chemistry. We are currently exploring the synthesis of the hindered
analogs of 1 which contain increased steric bulk at silicon in order
suppress the ring-opening chemistry [18] (e.g., in the form of a
tetraisopropyldisilane group, eiPr2SiSiiPr2e, as part of an NHC unit).
The structure of the cationic silylamine chain [(IPrCH2)SiMe2
-
SiMe2N(Dipp)C(H)]NDipp]þ in 4 is shown in Fig. 4, and as in the
ring-opened products 2 and 3, an intact intrachain NeC(H)]N unit
is present. The nucleophilic IPrCH2 group is terminally bound to a
SiMe2-residue via C(5), and the corresponding SieC distance
ꢀ
[1.935(3) A] is considerably longer than the SieC bond lengths
involving the methyl groups in the adjacent Me2SiSiMe2 array [e.g.,
ꢀ
Si(1)eC(1) ¼ 1.869(4) A]. Notably, the eSiMe2eN(Dipp)C(H)]
NDipp unit is crystallographically disordered over two positions
and thus a higher degree of error is associated with the refined
metrical parameters involving this segment. Upon binding of the
IPr]CH2 donor to a Si center to form 4, a substantial lengthening of
ꢀ
the CIPreCH2 bond [C(6)eC(5) ¼ 1.472(4) A] occurs relative to the
free N-heterocyclic olefin IPr]CH2 which contains a bona fide C]C
ꢀ
double bond [1.331(8) A] [14]. Moreover, the analogous CIPreCH2
bond in the IPrCH2 adduct IPrCH2$SnH2$W(CO)5 [14] is slightly
ꢀ
shorter [1.446(2) A] than the related linkage in 4, but points toward
a similar bonding environment within the IPrCH2 donors in each
complex. The structural data for 4 is in line with the donation of
p-electron density from the terminal olefin group in IPr]CH2 to
silicon, with a concomitant increase in NeC -bonding within the
p
central heterocycle of the IPrCH2 donor (as evidenced by shortening
3. Conclusions
The potential N-heterocyclic carbene precursor [(HCNDipp)2CH]
OTf (1) was prepared and structurally characterized. Attempts to
generate a new inorganic carbene via deprotonation of the disilane-
containing precursor 1 with a variety of common sterically
encumbered bases invariably led to nucleophilic attack of the base
on the ring backbone to form ring-opened species with intact for-
mamidine CeH groups (compounds 2e5). Despite the lack of suc-
cessful deprotonation in this study, the preparation of a new class of
imidazolium rings with inorganic backbones is significant as future
modification of the steric bulk at the backbone-positioned Si cen-
ters might still allow for access to new carbene derivatives with
considerably altered electronic properties relative to their ubiqui-
tous organic counterparts.
4. Experimental
Fig. 4. Molecular structure of the [(IPrCH2)SiMe2SiMe2N(Dipp)C(H)]NDipp]þ cation
in 4 at a 30% probability level. All hydrogen atoms except for the formamidineꢁproton
4.1. General procedures
ꢀ
at C(9A) have been omitted for clarity. Selected bond lengths [A] and angles [ ] with
values belonging to
a disordered eSiMe2eN(Dipp)]CHN(Dipp) unit in square
All reactions were performed using standard Schlenk tech-
niques under an atmosphere of nitrogen or in a nitrogen-filled
glove box (Innovative Technology, Inc.). Solvents were dried using
a Grubbs-type solvent purification system [19] manufactured by
brackets: Si(1)eSi(2A) 2.357(7) [2.346(15)], Si(1)eC(5) 1.935(3), Si(2A)eN(3A)
1.800(12) [1.76(2)], N(3A)eC(9A) 1.376(17) [1.34(4)], N(4A)eC(9A) 1.281(17) [1.22(4)];
Si(2A)eSi(1)eC(5) 98.84(19) [102.0(4)], Si(1)eSi(2A)eN(3A) 113.4(4) [118.0(10)],
Si(2A)eN(3A)eC(9A) 119.7(8) [115.8(17)], N(3A)eC(9A)eN(4A) 121.5(12) [125(3)].

30
S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
Innovative Technology, Inc., and degassed (freeze-pump-thaw
method) and stored under an atmosphere of nitrogen prior to
use. Dichlorotetramethyldisilane and trimethylsilyl triflate
were purchased from Gelest and used as received. Potassium
bis(trimethylsilyl)amide, potassium tert-butoxide, and sodium
metal were purchased from Aldrich and used as received. DippN]
C(H)NDipp(SiMe3) (Dipp ¼ 2,6-iPr2C6H3) [4a], Li[NHDipp] [20], and
IPr]CH2 (IPr ¼ [(HCNDipp)2C]) [14] were prepared according to
literature procedures. 1H, 13C{1H}, 19F{1H} and 29Si{1H} NMR spectra
were recorded on a Varian iNova-400 spectrometer and referenced
(s, 1H, N]CHeN). 13C{1H} NMR (CDCl3):
d
ꢀ2.9 (SiCH3), 23.4
(CH(CH3)2), 27.2 (CH(CH3)2), 28.7 (CH(CH3)2), 125.2 (ArC), 129.9
(ArC), 133.1 (ArC), 145.5 (ArC), 168.9 (N]CHeN). 19F{1H} NMR
(376 MHz, CDCl3):
C30H47F3N2O3SSi2: C, 57.29; H, 7.53; N, 4.45; S, 5.10. Found: C, 57.14;
H, 7.13; N, 4.35; S, 4.49.
d
ꢀ78.3 (br). Mp (ꢁC): 245e250. Anal. Calcd. for
4.3.2. (Me3Si)2NSiMe2SiMe2N(Dipp)C(H)]NDipp (2)
To a mixture of K[N(SiMe3)2] (73 mg, 0.37 mmol) and 1 (173 mg,
0.275 mmol) was added 10 mL of benzene. A yellow mixture
formed within a few minutes and the mixture was stirred for 1 h.
Filtration of the mixture through Celite followed by removal of the
volatiles afforded a white solid that was recrystallized from a
toluene/hexanes mixture (ꢀ35 ꢁC) to give 2 as colorless crystals of
suitable quality for X-ray crystallography. Yield: 129 mg, 73%. 1H
externally to SiMe4 (1H, 13C{1H} and 29Si{1H}) and CFCl3 19F{1H}).
(
Elemental analyses were performed by the Analytical and Instru-
mentation Laboratory at the University of Alberta. Melting points
were obtained in sealed glass capillaries under nitrogen using a
MelTemp melting point apparatus and are uncorrected.
NMR (C6D6): d 0.28 (s, 18H, N(Si(CH3)3)2), 0.48 (s, 6H, Si(CH3)2), 0.69
4.2. X-ray crystallography
(s, 6H, Si(CH3)2), 1.08 (d, 12H, 3JHH ¼ 6.8 Hz, CH(CH3)2), 1.27 (d, 12H,
3JHH ¼ 6.8 Hz, CH(CH3)2), 3.42 (septet, 2H, 3JHH ¼ 6.8 Hz, CH(CH3)2),
3
Crystals of appropriate quality for X-ray diffraction studies were
removed from a vial in a glove box and immediately covered with a
thin layer of hydrocarbon oil (Paratone-N). A suitable crystal was
then selected, attached to a glass fiber, and quickly placed in a low-
temperature stream of nitrogen [21]. All data were collected using a
3.58 (septet, 2H, JHH ¼ 6.8 Hz, CH(CH3)2), 7.03e7.17 (m, 6H, ArH),
7.39 (s, 1H, N]CHeN). 13C{1H} NMR (C6D6):
d 0.1 (N(Si(CH3)3)2), 6.4
(Si(CH3)2), 6.9 (Si(CH3)2), 23.6 (CH(CH3)2), 25.0 (CH(CH3)2), 25.5
(CH(CH3)2), 27.9 (CH(CH3)2), 28.5 (CH(CH3)2), 123.5 (ArC), 123.9
(ArC), 124.2 (ArC), 140.8 (ArC), 147.6 (ArC), 157.6 (N]CHeN).
HR-MS, EI (m/z): Calcd. for [Mþ]: 639.42554. Found: 639.42471
Bruker APEX II CCD detector/D8 diffractometer using Mo Ka (1 and
2) and Cu K
a
(3 and 4) radiation with the crystals cooled to ꢀ100 ꢁC
(
D
ppm ¼ 1.3 ppm). Mp (ꢁC): 168e172. Anal. Calcd. for C35H65N3Si4:
(Table 1). The data were corrected for absorption through Gaussian
integration from indexing of the crystal faces. Structures were
solved using direct methods with the programs SHELXS-97 [22] (1),
SHELXD [23] (2e4), and refinements were completed using
SHELXL-97 [22]. Hydrogen atoms were assigned positions based on
the sp2 or sp3 hybridization geometries of their attached carbon or
nitrogen atoms, and were given thermal parameters 20% greater
than those of their parent atoms.
C, 65.66; H, 10.23; N, 6.56. Found: C, 65.28; H, 10.34; N, 6.76.
4.3.3. (DippNH)SiMe2SiMe2N(Dipp)C(H)]NDipp (3)
To a slurry of 1 (100 mg, 0.159 mmol) in 6 mL of toluene was
added a cold (ꢀ35 ꢁC) solution of Li[NHDipp] (29 mg, 0.16 mmol) in
5 mL of Et2O. A pale yellow mixture was formed immediately and
the reaction mixture was stirred overnight at room temperature
and filtered through Celite. Removal of the volatiles from the filtrate
followed by washing of the product with 3 mL of hexanes gave 3 as
a white powder. Yield: 103 mg, 98%. Crystals suitable for X-ray
crystallography were grown from a saturated THF solution layered
4.2.1. Special refinement conditions
Compound 4: Attempts to refine peaks of residual electron
density as disordered or partial-occupancy tetrahydrofuran solvent
molecules were unsuccessful. The data were corrected for disor-
dered electron density through use of the SQUEEZE procedure [24]
as implemented in PLATON [25]. A total solvent accessible void
volume of 2237.1 Å3 with a total electron count of 555 (consistent
with 12 molecules of solvent THF, or 3 molecules per asymmetric
unit) was found in the unit cell. Moreover, the distances within two
of the disordered isopropyl groups were restrained during the
with hexanes at ꢀ35 ꢁC. 1H NMR (C6D6):
d 0.38 (s, 6H, Si(CH3)2),
3
0.39 (s, 6H, Si(CH3)2), 1.09 (d, 6H, JHH ¼ 6.8 Hz, CH(CH3)2), 1.16 (d,
12H, 3JHH ¼ 6.8 Hz, CH(CH3)2), 1.22 (d, 12H, 3JHH ¼ 6.8 Hz, CH(CH3)2),
1.24 (d, 6H, 3JHH ¼ 6.8 Hz, CH(CH3)2), 3.39 (s, 1H, NH), 3.48 (septet,
3
3
2H, JHH ¼ 6.8 Hz, CH(CH3)2), 3.61 (septet, 4H, JHH ¼ 6.8 Hz,
CH(CH3)2), 7.02e7.13 (m, 9H, ArH), 7.42 (s, 1H, N]CHeN). 13C{1H}
NMR (C6D6, peak assignment based on gHSQC and 13C{1H} DEPT
experiments):
d
ꢀ1.4 (Si(CH3)2), 0.7 (Si(CH3)2), 23.9 (CH(CH3)2),
refinement as: d(C57AeC58A)
¼
d(C57AeC59A)
¼
d(C77Ae
24.2 (CH(CH3)2), 24.9 (CH(CH3)2), 25.1 (CH(CH3)2), 28.0 (CH(CH3)2),
28.3 (CH(CH3)2), 28.4 (CH(CH3)2), 123.4 (ArC), 123.7 (ArC), 124.2
(ArC), 124.3 (ArC), 124.4 (ArC), 127.8 (ArC), 138.4 (ArC), 139.5 (ArC),
140.7 (ArC), 145.3 (ArC), 146.3 (ArC), 147.9 (ArC), 158.2 (N]CHeN).
Mp (ꢁC): 119e121. Anal. Calcd. for C41H65N3Si2: C, 75.05; H, 9.98; N,
6.40. Found: C, 75.33; H, 10.18; N, 6.26.
C79A) ¼ d(C57BeC58B) ¼ d(C57BeC59B) ¼ d(C77BeC78B) ¼
ꢀ
d(C77BeC79B) ¼ 1.52(1) A; d(C58A/C59A) ¼ d(C78A/C79A) ¼
ꢀ
d(C58B/C59B) ¼ d(C78B/C79B) ¼ 2.49(1) A.
4.3. Synthetic procedures
4.3.1. [(Me2SiNDipp)2CH]OTf (1)
4.3.4. [(IPrCH2)SiMe2SiMe2N(Dipp)C(H)]NDipp]OTf (4)
DippN]CHeN(SiMe3)(Dipp) (5.409 g, 12.38 mmol) was mixed
with ClSiMe2SiMe2Cl (2.30 mL, 2.31 g, 12.4 mmol) in 60 mL of
CH2Cl2 at room temperature. After 1 h, Me3SiOTf (2.24 mL, 2.75 g,
12.4 mmol) in 10 mL of CH2Cl2 was added to the reaction mixture
dropwise at 0 ꢁC. The reaction mixture was then allowed to warm
to room temperature and stirred for 2 h, and the volatiles were
removed under dynamic vacuum to yield pure 1 as a fine white
powder. Yield: 7.41 g, 95%. Recrystallization of the product from
toluene/CH2Cl2 yielded 1 as colorless crystals of suitable quality for
To a mixture of 1 (100 mg, 0.159 mmol) and IPr]CH2 (64 mg,
0.16 mmol) was added 10 mL of toluene to give a pale yellow solution
that was stirred overnight at room temperature. Removal of the
volatiles from the reaction mixture gave a white solid (137 mg, 84%)
which was then recrystallized from a THF/hexanes mixture at ꢀ35 ꢁC
to afford 4 as colorless blocks suitable for X-ray crystallography. 1H
NMR (C6D6):
d
ꢀ0.14 (s, 6H, Si(CH3)2), 0.02 (s, 6H, Si(CH3)2), 0.90 (d,
12H, 3JHH ¼ 7.0 Hz, CH(CH3)2), 1.03 (d, 12H, 3JHH ¼ 7.0 Hz, CH(CH3)2),
3
3
1.09 (d, 12H, JHH ¼ 7.0 Hz, CH(CH3)2), 1.13 (d, 12H, JHH ¼ 7.0 Hz,
3
X-ray crystallographic analysis. 1H NMR (CDCl3):
d
0.67 (s, 12H,
CH(CH3)2), 2.15 (s, 2H, IPrCH2e), 2.46 (septet, 4H, JHH ¼ 7.0 Hz,
3
Si(CH3)2), 1.12 (d, 12H, JHH ¼ 6.8 Hz, CH(CH3)2), 1.35 (d, 12H,
3JHH ¼ 6.8 Hz, CH(CH3)2), 2.91 (septet, 4H, 3JHH ¼ 6.8 Hz, CH(CH3)2),
7.26 (d, 4H, 3JHH ¼ 8.0 Hz, ArH), 7.39 (t, 2H, 3JHH ¼ 8.0 Hz, ArH), 7.56
CH(CH3)2), 3.05 (septet, 2H, 3JHH ¼ 7.0 Hz, CH(CH3)2), 3.13 (septet, 2H,
3JHH ¼ 7.0 Hz, CH(CH3)2), 6.90 (d, 4H, 3JHH ¼ 8.0 Hz, ArH), 7.04 (t, 2H,
3JHH ¼ 8.0 Hz, ArH), 7.07 (s, 1H, N]CHeN), 7.11 (d, 4H, 3JHH ¼ 8.0 Hz,

S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
31
Table 1
Crystallographic data for compounds 1e4.
1
2
3b
4$3THF
Empirical formula
fw
C37H55F3N2O3SSi2
721.07
C35H65N3Si4
640.26
C41H65N3Si2
656.14
0.42 ꢂ 0.16 ꢂ 0.10
Orthorhombic
Pca21
C70H109F3N4O6SSi2
1247.85
0.22 ꢂ 0.19 ꢂ 0.04
Monoclinic
P21/c
Cryst. dimens. (mm3)
Cryst. syst.
0.41 ꢂ 0.17 ꢂ 0.10
0.47 ꢂ 0.32 ꢂ 0.20
Triclinic
Triclinic
Space group
P1
P1
Unit cell dimensions
ꢀ
a (A)
12.0453(6)
12.9148(6)
14.8650(7)
107.5129(6)
90.7948(6)
113.3687(5)
2000.79(17)
2
12.608(2)
12.784(3)
13.411(3)
85.364(2)
74.799(2)
73.697(2)
2002.2(7)
2
21.2311(3)
19.8205(3)
19.3515(3)
90
90
90
8143.3(2)
8
1.070
22.8618(4)
15.9609(3)
20.8615(4)
90
98.9915(9)
90
7518.7(2)
4
1.102
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
3
ꢀ
V (A )
Z
r
(g cmꢀ3
)
1.197
0.190
1.062
0.174
Abs. coeff. (mmꢀ1
)
1.001
1.132
T (K)
2
Total data
173(1)
55.10
17873
173(1)
53.00
16260
173(1)
140.10
53094
173(1)
135.98
49,007
qmax (ꢁ)
Unique data (Rint
Obs. data [I > 2s(I)]
)
9151 (0.0256)
6856
490
0.0474
0.1382
8274 (0.0276)
6141
389
0.0492
0.1509
15096 (0.0317)
14,242
838
0.0431
0.1230
0.784/ꢀ0.267
13,063 (0.0420)
10,225
809
0.0883
0.2456
Params
R1 [I > 2
s
(I)]a
wR2 [all data]a
Max/min Dr (e
ꢀ ꢀꢀ3
A
)
0.488/ꢀ0.487
0.539/ꢀ0.444
0.377/ꢀ0.390
a
R1 ¼ SkFor ꢀ rFck/SrFor; wR2 ¼ [Sw(F2o ꢀ Fc2)2/Sw(Fo4)]1/2
.
b
Flack parameter ¼ 0.264(17).
ArH), 7.23 (t, 2H, 3JHH ¼ 8.0 Hz, ArH), 8.40 (s, 2H, NeCHe in IPrCH2).
Faculty Award to E.R.; graduate fellowship to S.M.I.A.) and Suncor
Energy Inc. (Petro-Canada Young Innovator Award to E.R.). We
would also like to thank Prof. Roland Roesler (University of Calgary)
for helpful discussions. David Xu and Daniel Golec are also
acknowledged for their experimental assistance.
13C{1H} NMR (C6D6, peak assignment based on gHSQC and 13C{1H}
DEPT experiments):
d
ꢀ2.6 (coincidental Si(CH3)2 groups), 14.4
(IPrCH2e), 22.4 (CH(CH3)2), 23.6 (CH(CH3)2), 24.6 (CH(CH3)2), 25.3
(CH(CH3)2), 26.1 (CH(CH3)2), 27.9 (CH(CH3)2), 28.3 (CH(CH3)2), 29.5
(CH(CH3)2), 123.5 (eNeCHe in IPrCH2), 124.3 (ArC), 124.4 (ArC),
125.9 (ArC), 127.2 (ArC), 130.4 (ArC), 132.7 (ArC), 137.1 (ArC), 140.3
(ArC), 145.6 (ArC), 147.4 (ArC), 150.2 (ArC), 157.4 (N]CHeN). 19F{1H}
Appendix A. Supplementary material
NMR (C6D6):
d
ꢀ77.5 (s, eCF3). Mp (ꢁC): 193e195. Anal. Calcd. for
CCDC 931778e931782, NMR spectra for all reported compounds
(1e5) along with crystallographic details for 6, X-ray data (com-
pounds 1e4 and 6) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
C58H85F3N4O3SSi2: C, 67.53; H, 8.31; N, 5.43; S, 3.11. Found: C, 67.10;
H, 8.24; N, 5.22; S, 2.68.
4.3.5. [(tBuO)SiMe2SiMe2N(Dipp)C(H)]NDipp] (5)
To a mixture of 1 (100 mg, 0.158 mmol) and K[OtBu] (18 mg,
0.161 mmol) was added 6 mL of THF. The reaction mixture was
stirred overnight at room temperature to give a clear solution.
Removal of the volatiles from the resulting solution afforded a white
solid, which was then extracted with 6 mL of toluene and filtered
through Celite to obtain a colorless filtrate. Removal of the volatiles
from the filtrate yielded 5 as a white solid. Yield: 94 mg, 90%. 1H
Appendix B. Supplementary material
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.04.029.
NMR (C6D6): d 0.43 (s, 6H, Si(CH3)2), 0.55 (s, 6H, Si(CH3)2),1.10 (d, 6H,
3JHH ¼ 7.0 Hz, CH(CH3)2), 1.17 (s, 9H, OC(CH3)3), 1.26 (d, 9H,
3JHH ¼ 7.0 Hz CH(CH3)2), 1.27 (d, 9H, 3JHH ¼ 7.0 Hz, CH(CH3)2), 3.51
(septet, 4H, 3JHH ¼ 7.0 Hz, CH(CH3)2), 7.02e7.17 (m, 6H, ArH), 7.32 (s,
References
[1] (a) A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361;
(b) For an attempt to prepare a stable NHC, see: H.-W. Wanzlick, H.-J. Kleiner
Angew. Chem. 73 (1961) 493;
1H, N]CHeN). 13C{1H} NMR (C6D6):
d
ꢀ1.8 (Si(CH3)2), 3.8 (Si(CH3)2),
(c) For an early report of a stable acyclic carbene, see: A. Igau, H. Grutzmacher,
A. Baceiredo, G. Bertrand J. Am. Chem. Soc. 110 (1988) 6463.
[2] (a) A.J. Arduengo III, Acc. Chem. Res. 32 (1999) 913;
(b) D. Bourissou, O. Guerret, F.P. Gabbaï, G. Bertrand, Chem. Rev. 100 (2000) 39;
(c) T.M. Trnka, R.H. Grubbs, Acc. Chem. Res. 34 (2001) 18;
(d) F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122;
(e) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612;
(f) Y. Wang, G.H. Robinson, Inorg. Chem. 50 (2011) 12326.
[3] (a) S. Gründemann, A. Kovacevic, M. Albrecht, J.W. Faller, R.H. Crabtree, Chem.
Commun. (2001) 2274;
23.9 (CH(CH3)2), 24.8 (CH(CH3)2), 25.5 (CH(CH3)2), 27.9 (OC(CH3)3),
28.3 (CH(CH3)2), 32.3 (OC(CH3)3), 123.4 (ArC), 123.5 (ArC), 124.2
(ArC), 127.9 (ArC), 128.1 (ArC), 128.3 (ArC), 140.6 (ArC), 147.8 (ArC),
157.1 (N]CHeN). 29Si{1H} NMR (C6D6):
d
ꢀ8.4 (s, Si(CH3)2), 3.5 (s,
Si(CH3)2). Mp (ꢁC): 96e98. Anal. Calcd. for C35H65N3Si4: C, 71.68; H,
10.21; N, 5.07. Found: C, 71.94; H, 10.18; N, 5.38.
Acknowledgments
(b) V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand, Angew. Chem.
Int. Ed. 44 (2005) 5705;
(c) E. Aldeco-Perez, A.J. Rosenthal, B. Donnadieu, P. Parameswaran,
G. Frenking, G. Bertrand, Science 326 (2009) 556;
(d) M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 49 (2010)
8810;
This work was supported by the Natural Sciences and Engi-
neering Research Council (NSERC) of Canada, the Canada Founda-
tion for Innovation, Alberta Innovates-Technology Futures (New

32
S.M.I. Al-Rafia et al. / Journal of Organometallic Chemistry 739 (2013) 26e32
(e) D. Martin, N. Lassauque, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed.
51 (2012) 6172.
[12] A. Dumrath, X.-F. Wu, H. Neumann, A. Spannenberg, R. Jackstell, M. Beller,
Angew. Chem. Int. Ed. 49 (2010) 8988.
[4] (a) E. Despagnet-Ayoub, R.H. Grubbs, J. Am. Chem. Soc. 126 (2004) 10198;
(b) J.D. Masuda, C. Martin, C. Lyon-Saunier, A. Baceiredo, H. Gornitzka,
B. Donnadieu, G. Bertrand, Chem. Asian J. (2007) 178;
(c) G. Ung, D. Mendoza-Espinosa, J. Bouffard, G. Bertrand, Angew. Chem. Int.
Ed. 50 (2011) 4215.
[13] For earlier studies involving less hindered N-heterocyclic olefins: (a) N. Kuhn,
H. Bohnen, J. Kreutzberg, D. Bläser, R. Boese, J. Chem. Soc. Chem. Commun.
(1993) 1136;
(b) N. Kuhn, H. Bohnen, D. Bläser, R. Boese, Chem. Ber. 127 (1994) 1405.
[14] S.M.I. Al-Rafia, A.C. Malcolm, S.K. Liew, M.J. Ferguson, R. McDonald, E. Rivard,
Chem. Commun. 47 (2011) 6987.
[5] (a) K.E. Krahulic, G.D. Enright, M. Parvez, R. Roesler, J. Am. Chem. Soc. 127
(2005) 4142;
[15] S.M.I. Al-Rafia, M.J. Ferguson, E. Rivard, Inorg. Chem. 50 (2011) 10543.
[16] L. Jafarpour, E.D. Stevens, S.P. Nolan, J. Organomet. Chem. 606 (2000) 49.
(b) C. Präsang, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc. 127 (2005) 10182;
(c) T.D. Forster, K.E. Krahulic, H.M. Tuononen, R. McDonald, M. Parvez,
R. Roesler, Angew. Chem. Int. Ed. 45 (2006) 6356;
(d) Y. Ishida, B. Donnadieu, G. Bertrand, Proc. Natl. Acad. Sci. U. S. A. 103 (2006)
13585.
[17] Occasionally during the synthesis of 5, single crystals of
a byproduct
DippN]CHN(Dipp)eSiMe2SiMe2eN(Dipp)CH]NDipp 6 could be isolated
from the reaction mixture (few mg). This product was identified by X-ray
crystallography (see Fig. S6 and Table S1 in the Supporting information)
[6] S.K. Liew, S.M.I. Al-Rafia, J.T. Goettel, P.A. Lummis, S.M. McDonald, L.J. Miedema,
M.J. Ferguson, R. McDonald, E. Rivard, Inorg. Chem. 51 (2012) 5471.
[7] Cyclic stannylenes with disilane backbones have also been reported: (a)
K. Horchler, C. Stader, B. Wrackmeyer, Inorg. Chim. Acta 117 (1986) L39;
(b) B. Wrackmeyer, C. Stader, K. Horchler, H. Zhou, D. Schlosser, Inorg. Chim.
Acta 176 (1990) 205.
[8] (a) G.D. Frey, M. Song, J.-B. Bourg, B. Donnadieu, M. Soleilhavoup, G. Bertrand,
Chem. Commun. (2008) 4711;
(b) D. Martin, A. Baceiredo, H. Gornitzka, W.W. Schoeller, G. Bertrand, Angew.
Chem. Int. Ed. 44 (2005) 1700.
[9] E.D. Blue, T.B. Gunnoe, J.L. Petersen, P.D. Boyle, J. Organomet. Chem. 691
(2006) 5988.
however attempts to directly prepare
NC(H)]NDipp] [5a] with 1 yielded no discernable reaction; thus at this
time we are unsure how trace quantities of
synthesis of 5.
6 via the reaction of Li[(Dipp)
6 are formed during the
[18] K. Tamao, A. Kawachi, Y. Ito, Organometallics 12 (1993) 580.
[19] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J. Timmers, Organo-
metallics 15 (1996) 1518.
[20] J.T. Patton, S.C. Feng, K.A. Abboud, Organometallics 20 (2001) 3399.
[21] H. Hope, Prog. Inorg. Chem. 41 (1994) 1.
[22] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112.
[23] T.R. Schneider, G.M. Sheldrick, Acta Crystallogr. D58 (2002) 1772.
[24] P. Van Der Sluis, A.L. Spek, Acta Crystallogr. A46 (1990) 194.
[25] (a) A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7;
[10] D.A. Boersma, H.M. Goff, Inorg. Chem. 21 (1982) 581.
[11] N. Wiberg, A.F. Holleman, E. Wiberg (Eds.), Holleman-Wiberg’s Inorganic
Chemistry, Academic Press, New York, NY, 2001.
(b) PLATON
e A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands.