A generic one-pot route to acyclic two-coordinate silylenes from silicon(IV) precursors: Synthesis and structural characterization of a silylsilylene

Article abstract of DOI:10.1002/anie.201208554

Si in sight: A one-pot, single-step synthesis of an acyclic silylsilylene, Si{Si(SiMe₃)₃}{N(SiMe₃)Dipp} (Dipp=2,6-iPr ₂C₆H₃), from a silicon(IV) starting material is reported, together with evidence for a mechanism involving alkali metal silylenoid intermediates. Copyright

Full text of DOI:10.1002/anie.201208554

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DOI: 10.1002/anie.201208554
Silicon Chemistry
A Generic One-Pot Route to Acyclic Two-Coordinate Silylenes from
Silicon(IV) Precursors: Synthesis and Structural Characterization of
a Silylsilylene**
Andrey V. Protchenko, Andrew D. Schwarz, Matthew P. Blake, Cameron Jones,*
Nikolas Kaltsoyannis,* Philip Mountford,* and Simon Aldridge*
By comparison with their heavier Group 14 congeners,
divalent silicon compounds are typically highly labile species
with short lifetimes at ambient temperature.[1] Consequently,
the observation by West et al. in 1981 of the transient silylene,
SiMes2 (Mes = 2,4,6-Me3C6H2), and its subsequent dimeriza-
=
tion at 77 K to give the disilene, Mes2Si SiMes2, represented
Figure 1. Recently reported acyclic two-coordinate silylenes. Dipp=2,6-
iPr2C6H3, Trip=2,4,6-iPr3C6H2.
a breakthrough in the evolution of modern main group
chemistry.[2] From that groundbreaking discovery until very
recently, only silylene systems stabilized by incorporation into
a cyclic framework, or by an augmented coordination number
at silicon, had been reported.[3–6] In early 2012, however, near
simultaneous reports from our program and that of Power
identified acyclic silylene systems that are stable at room
temperature and above (Figure 1).[7–9]
Both systems exploit the kinetic stabilization imparted by
extremely bulky peripheral substituents, a factor which
contributes to decomposition temperatures in excess of
1308C. On the other hand, the contrasting potential of these
two compounds for further reactivity is signaled by disparate
X-Si-X angles (]B-Si-N = 109.7(1)8; ]S-Si-S = 90.52(2)8)
and HOMO–LUMO gaps (197.2 and 411.0 kJmolÀ1, respec-
tively).[7,8] Consistently, the unsymmetrical boryl/amido
attribute of this compound is its direct formation from the SiIV
precursor Si{N(SiMe3)Dipp}Br3 (1; Dipp = 2,6-iPr2C6H3) and
two equivalents of (thf)2Li{B(NDippCH)2}.[10] In effect, the
boryllithium reagent accomplishes the dual roles of reducing
agent and source of the strongly sigma donating (formally
anionic) [B(NDippCH)2]À substituent. To better understand
this method and its scope for the direct conversion of SiIV
halides into acyclic silylenes, we have investigated the
reactivity of 1 towards other sterically encumbered anionic
main group and transition metal nucleophiles. In doing so we
have synthesized the first example of a stable two-coordinate
silylsilylene.[11]
Key synthetic steps are outlined in Scheme 1; the syn-
thesis of SiII systems by this route is dependent on the use of
an anionic reagent that is not only a good nucleophile, but
also 1) sterically bulky and 2) a strong reducing agent. In the
absence of sufficient nucleophilicity, as in the case of KC8,
dehalogenative coupling occurs to give the disilane 2 (see the
Supporting Information). With more nucleophilic, but insuf-
ficiently bulky anions, the possibility also exists for simple
substitution chemistry to occur at the silicon center, which
À
system is uniquely capable of activating E H bonds (E =
H,C) close to room temperature.[7] A convenient synthetic
[*] Dr. A. V. Protchenko, Dr. A. D. Schwarz, M. P. Blake,
Prof. P. Mountford, Prof. S. Aldridge
Inorganic Chemistry Laboratory, Department of Chemistry
University of Oxford, South Parks Rd, Oxford, OX13QR (UK)
E-mail: simon.aldridge@chem.ox.ac.uk
Homepage: http://users.ox.ac.uk/~quee1989/
Prof. N. Kaltsoyannis
Department of Chemistry, University College London, Christopher
Ingold Laboratories, 20 Gordon Street, London, WC1H 0AJ (UK)
Prof. C. Jones
School of Chemistry, PO Box 23, Monash University
Melbourne, VIC, 3800 (Australia)
[**] We thank the Leverhulme Trust (F/08699/E), the Australian
Research Council and the EPSRC (EP/F019181/1, EP/F055412/1,
and computational resources through its National Service for
Computational Chemistry Software). We also thank UCL for
computing resources through the Research Computing “Legion”
cluster and associated services.
Supporting information for this article, including synthetic, spec-
troscopic, and crystallographic data for compounds 2, 4, 5, 6, and
{CpFe(CO)}2(m-CO)(m-SiBr{N(SiMe3)Dipp}, as well as details of the
DFT calculations on 3b, and CIFs for 2, 3b, 4, 5, 6, and
{CpFe(CO)}2(m-CO)(m-SiBr{N(SiMe3)Dipp}, is available on the
WWW under http://dx.doi.org/10.1002/anie.201208554.
Scheme 1. Syntheses of acyclic silylenes 3a and 3b. Key reagents:
a) KC8 (1.0 equiv), thf, À788C to room temperature, ca. 20%;
b) (thf)2K[Si(SiMe3)3] (2.0 equiv), hexane, room temperature, 5 min, ca.
50% (for 3b); see Ref. [7] for 3a.
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would consequently remain tetravalent. Thus, although the
state structure of 3a). DFT calculations reproduce well the
reaction of 1 with K[CpFe(CO)2] primarily yields the coupling
observed pattern of NMR signals for both isomers, and also
imply that the energetic difference between these two
rotamers is very small (2.7 kJmolÀ1).
At a more general level, quantum chemical input indicates
that the HOMO for 3b (43.0% Si(1) 3p, 13.1% Si(1) 3 s;
Figure 3) is separated from the LUMO (70.3% Si(1) 3p, 9.3%
À
product 2, a small quantiy of the Fe Si containing system
{CpFe(CO)}2(m-CO)(m-SiBr{N(SiMe3)Dipp}), was also iso-
lated (see the Supporting Information).[12] In the case of
bulkier nucleophiles, redox properties are important; thus,
although 1 is unreactive towards sterically encumbered alkali
metal amides, such as Li{N(SiMe3)Dipp} or K{N(SiMe3)2}, it
reacts readily with two equivalents of (thf)2Li{B(NDippCH)2}
or (thf)2K[Si(SiMe3)3], or three equivalents of K[CPh3],
generating BrB(NDippCH)2,[7] BrSi(SiMe3)3, and Gombergꢀs
dimer, respectively.
In the case of K[CPh3], the silicon-containing product is
a labile species that gives rise to a 29Si signal at dSi =
442.6 ppm, in the region characteristic of two-coordinate
silylenes (dSi = 439.7 ppm for Si{B(NDippCH)2}{N(SiMe3)-
Dipp}, 3a).[7] With (thf)2K[Si(SiMe3)3], however, a thermally
stable (Td = ca. 1408C) purple crystalline material can be
isolated, which has been characterized by standard spectro-
scopic techniques and by single crystal X-ray diffraction
(Figure 2).[13] The monomeric structure of the silylene prod-
uct, Si{Si(SiMe3)3}{N(SiMe3)Dipp} (3b), which is implied by
Figure 3. a) HOMO (À4.684 eV) and b) LUMO (À2.693 eV) of silylsily-
lene 3b.
N 2p) by 192.1 kJmolÀ1 and that the energetic gap between
the singlet ground state and triplet first excited state is
103.7 kJmolÀ1. These figures are very similar to those
determined for 3a (197.2 and 103.9 kJmolÀ1, respectively),[7]
and consistent with the patterns of reactivity delineated for
the borylsilylene system; 3b also 1) reacts with dihydrogen at
room temperature in hexane solution to give the correspond-
ing dihydrosilane 4 (which gives rise to diagnostic new
1
resonances at dH = 5.17 and dSi = À35.2 ppm, JSiÀH
=
195 Hz),[15] and 2) undergoes intramolecular C H activation
in [D6]benzene over a period of 5 days at 808C to give sila-
indoline 5 (Scheme 2, see also the Supporting Information).
À
Figure 2. Molecular structure of 3b. Hydrogen atoms omitted for
clarity; thermal elipsoids set at the 35% probability level. Key bond
lengths [ꢀ] and angles [8]: Si(1)–N(1) 1.720(1), Si(1)–Si(2) 2.386(1);
N(1)-Si(1)-Si(2) 116.91(5), Si(1)-Si(2)-Si(3) 101.32(2), Si(1)-Si(2)-Si(4)
92.25(2), Si(1)-Si(2)-Si(5) 138.14(3).
29Si NMR analysis (dSi = 438, 467 ppm) is thus confirmed
crystallographically. Although the Si–N and Si–Si distances
associated with two-coordinate Si(1) (1.720(1) and
2.386(1) ꢁ, respectively) are largely unremarkable,[14] the
N(1)-Si(1)-Si(2) angle of 116.91(5)8 is significantly wider than
the corresponding N-Si-B angle of 109.7(1)8 measured for
3a.[7] This difference is presumably at least partly steric in
origin, with the notable buttressing between the amido Dipp
substituent and the SiMe3 group centered around Si(5)
leading to asymmetry within the Si(SiMe3)3 (hypersilyl)
À
À
Scheme 2. Activation of H H and C H bonds by silylsilylene 3b. Key
reagents and conditions: a) H2 (1 atm), hexane, 208C, 2.5 h, >90% by
NMR spectroscopy (62% yield of isolated product); b) C6D6, 808C,
5 d, ca. 65% yield (see the Supporting Information for X-ray structures
of 4 and 5).
moiety, as manifested by
a
Si(1)-Si(2)-Si(5) angle of
From a mechanistic perspective, the syntheses of 3a and
3b outlined in Scheme 1 implicate the main group anion in
both the reduction and metathesis steps. Although a number
of observations, such as the lack of reactivity of 1 towards
bulky, but less reducing nucleophiles (such as amides), and
the fact that systems of the type X2Si{B(NDippCH)2}{N-
138.14(3)8 vs. 101.32(2) and 92.25(2)8 for Si(1)-Si(2)-Si(3/4).
Another consequence of the high degree of steric crowd-
ing in 3b is evident from multinuclear NMR data in
[D6]benzene solution. Two sets of signals in approximately
1:1 ratio are observed, resulting from conformers related by
À
À
restricted rotation about the Si(1) N(1) bond. Thus, one set
(SiMe3)Dipp} are stable with respect to B X reductive
of signals is derived from the isomer found in the solid state, in
which the Dipp substituent lies syn to the hypersilyl group,
whereas the other set is due to the corresponding anti isomer
(a conformation which is analogous to that found in the solid
elimination,[7] imply that metathesis is not the first mecha-
nistic step, we have sought to confirm that reduction is
a viable alternative. Such a mechanism would necessarily
involve silylenoid intermediates (Scheme 3); consistent with
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Mechanistically, both borylsilylene 3a and digallylsilyle-
noid 6 can be regarded as being derived from a common class
of bromosilylenoid intermediate of the type (LnM)SiBr-
{E(NDippCH)2}{N(SiMe3)Dipp} (E = B or Ga; M = alkali
metal). 3a is thus the product of the subsequent elimination of
LiBr, whereas 6 results from an alternative gallyl/bromide
metathesis step, presumably made possible by the longer
À
À
Si Ga bonds (vs. Si B) and the consequently reduced steric
demands of the gallyl fragment. The weaker donor capabil-
ities of the gallyl (vs. boryl) ligand also provide a rationale for
the reduced propensity to eliminate alkali metal halide.[21]
In summary, we have reported a one-pot, single-step
synthesis of a thermally stable acyclic silylsilylene, Si{Si-
(SiMe3)3}{N(SiMe3)Dipp} (3b), from a SiIV starting material,
together with evidence for the formation of alkali metal
silylenoid systems under related conditions. Systems of the
type RSi(SiR3) have previously been discussed in the context
[22,23]
=
of their rearrangement to isomeric disilenes, R2Si SiR2.
In the current study, no such transformation has been
À
identified for 3b, which appears to instead undergo C H
activation at elevated temperatures.[24] On the other hand,
alternative modes of reactivity have been demonstrated, such
À
À
as oxidative addition of H H and C H bonds, which have
ample precedent in d-block chemistry.
Scheme 3. Proposed mechanism for the one-pot formation of acyclic
silylenes 3a and 3b from 1 through silylenoid intermediates (M=Li or
K), and the structure of the digallylsilylenoid 6. Key bond lengths [ꢀ]
and angles [8]: Si(1)–N(5) 1.815(3), Si(1)–Ga(1) 2.438(1), Si(1)–Ga(2)
2.452(1), Si(1)···K(1) 3.470(1); N(5)-Si(1)-Ga(1) 110.3(1), N(5)-Si(1)-
Ga(2) 108.8(1), Ga(1)-Si(1)-Ga(2) 91.8(1), K(1)-Si(1)-Ga(1) 152.9(1),
K(1)-Si(1)-Ga(2) 99.5(1), K(1)-Si(1)-N(5) 89.3(1).
Experimental Section
Synthesis of 3b:
A solution of (thf)2K[Si(SiMe3)3] (113 mg,
0.26 mmol) in hexane (4 mL) was added to a solution of 1 (68 mg
0.13 mmol) in hexane (1 mL) at room temperature. The color
immediately changed to bright purple and copious precipitate
formed. The solution was filtered and stored at À808C for 3 d,
yielding purple crystals slightly contaminated with BrSi(SiMe3)3.[25]
Slow crystallization from hexanes at room temperature gave X-ray
quality crystals of 3b (35 mg, 0.067 mmol, 51%). Melting point:
decomposing (decolorizing) at 1408C and above. 1H NMR (C6D6,
293 K): d = 0.30 (s, 9H+27H, NSiMe3+SiSiMe3), 0.35 (s, 9H,
NSiMe3), 0.46 (s, 27H, SiSiMe3), 1.09 (d, 3J(H,H) = 7.0 Hz, 6H,
CHMe2), 1.17 (d, 3J(H,H) = 7.0 Hz, 6H, CHMe2), 1.23 (two over-
lapping d, 12H, CHMe2), 3.42 and 3.44 (two overlapping sept,
3J(H,H) = 7.0 Hz, 4H, CHMe2), 7.06 ppm (br s, 6H, CH of Ar).
13C NMR (C6D6, 293 K): d = 3.45 and 4.11 (NSiMe3), 4.28 and 4.37
(SiSiMe3), 14.35 (hexane CH3), 23.03 (hexane CH2), 23.58, 24.41,
25.29 and 26.24 (CHMe2), 28.58 and 29.07 (CHMe2), 31.94 (hexane
CH2), 124.11 and 124.18 (m-CH of Ar), 125.16 and 126.16 (p-CH of
Ar), 142.24 and 145.08 (o-C of Ar), 146.12 and 152.31 ppm (ipso-C of
Ar). 29Si NMR (C6D6, 293 K): d = À107.19 and À105.89 (SiSiMe3),
À8.33 (SiSiMe3), 5.77 and 9.73 (NSiMe3), 438.20 and 467.46 ppm
(central Si). Crystallographic data: C24H53NSi6, Mr 524.21, monoclinic,
C2/c, a = 22.4461(2), b = 9.6252(1), c = 32.0919(4) ꢁ, b = 104.481(1)8,
V= 6713.1(1) ꢁ3, Z = 8, 1c = 1.037 MgmÀ3, T= 150 K, l = 0.71073 ꢁ.
25749 reflections collected, 7636 independent [R(int) = 0.046], which
were used in all calculations. R1 = 0.0374, wR2 = 0.0363 for observed
unique reflections [F2 > 2s(F2)] and R1 = 0.0613, wR2 = 0.0470 for all
unique reflections. Max. and min. residual electron densities 0.34 and
À0.26 eꢁÀ3. CCDC 895809 (3b) contains the supplementary crystal-
lographic 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.
this proposal, Lee and co-workers have shown that SiRBr3
(where R = C(SiMe3)3) is susceptible to bromide/lithium
exchange in the presence of powerful lithium-containing
reductants, to give a silylenoid species, SiR(Br)2Li, which is
then amenable to substitution with anionic nucleophiles to
give SiR(Br)Ar(Li).[16]
In the current study, the viability of alkali metal silylenoid
species can be demonstrated explicitly; the reaction of 1 with
[(Et2O)K{Ga(NDippCH)2}](OEt2), which is a gallium ana-
logue of (thf)2Li{B(NDippCH)2},[17] generates the potassium
digallylsilylenoid {(Et2O)K}Si{Ga(NDippCH)2}2{N(SiMe3)-
À
Dipp}, 6, together with the Ga Ga bonded dimer {Ga-
(NDippCH)2}2 (Scheme 3).[18] Structurally, 6 features a silicon
center that is significantly distorted from tetrahedral (]K-Si-
Ga = 152.9(1), 99.5(1)8, ]K-Si-N = 89.3(1)8), together with
a potassium center ligated by a single ether donor and
engaged in weak interactions with Si(1)
(d(K···Si) = 3.470(1) vs. 3.14 ꢁ for the sum of the respective
covalent radii)[19] and with the flanking aryl p systems of the
amido and gallyl Dipp substituents, for example, d(K···C) =
3.156(4)–3.287(4) for the amido Dipp group. As such, 6 shares
key structural features with the alkyltin system
{(tmeda)K}Sn{Ga(NdippCH)2}2{CH(SiMe3)2}.[20] These and
related tin systems have been prepared from stannylene
precursors, and in a similar vein 6 can be formulated as
a Lewis base adduct of the putative gallylsilylene Si{Ga-
(NDippCH)2}{N(SiMe3)Dipp}.
Received: October 24, 2012
Published online: November 20, 2012
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1
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