Efficient generation of stable adducts of Si(ii) dihydride using a donor-acceptor approach

Article abstract of DOI:10.1039/c2cc17101e

A stable Si(ii) dihydride complex, IPr·SiH₂· BH₃ (IPr = [(HCNDipp)₂C:]; Dipp = 2,6-ⁱPr ₂C₆H₃), was synthesized and preliminary reactivity involving this source of encapsulated silylene is reported. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17101e

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COMMUNICATION
Efficient generation of stable adducts of Si(II) dihydride using a
donor–acceptor approachwz
S. M. Ibrahim Al-Rafia, Adam C. Malcolm, Robert McDonald, Michael J. Ferguson and
Eric Rivard*
Received 16th November 2011, Accepted 7th December 2011
DOI: 10.1039/c2cc17101e
A stable Si(II) dihydride complex, IPrꢀSiH2ꢀBH3 (IPr =
[(HCNDipp)2C:]; Dipp = 2,6-iPr2C6H3), was synthesized and
preliminary reactivity involving this source of encapsulated
silylene is reported.
formation of the halosilylene adduct IPrꢀSiCl2ꢀBH3 in high yield.9
The reluctance of the Si–Cl bonds in IPrꢀSiCl2ꢀBH3 to undergo
hydride replacement chemistry with Li[BH4] mirrored our obser-
vations with the silagermene adduct IPrꢀCl2SiGeCl2ꢀW(CO)5,
wherein Si–H bond formation required the use of Li[AlH4] as a
hydride source.3d
The ability to intercept reactive main group species in the
presence of strong electron pair donors, such as N-heterocyclic
carbenes (NHCs), has had a profound influence in advancing
modern inorganic chemistry.1 Motivated by these break-
throughs, our group has recently employed a related donor–
acceptor strategy2 to access stable adducts of :GeH2 and :SnH2,3
and various inorganic congeners of ethylene, H2SiEH2 (E = Ge
and Sn).3 In this communication, we introduce an efficient route
to a stable bis adduct of the highly sought-after parent silylene,
:SiH2,4,5 and explore initial coordination chemistry involving
this novel Group 14 dihydride.
Our initial attempts to generate IPrꢀSiH2ꢀBH3 (2) from the
reaction of IPrꢀSiCl2ꢀBH3 with Li[AlH4] in ethereal solvents
led to the formation of the known alane, IPrꢀAlH3, as a major
product,10 with only trace amounts (o5%) of the desired
silylene adduct 2 observed by NMR spectroscopy. In order
to mitigate IPrꢀAlH3 formation, we decided to repeat the
hydride transfer reaction in a solvent mixture of lower polarity
(toluene/ether mixture) while concurrently decreasing the
reaction time to 1.5 h. Fortunately these alterations in the
reaction parameters led to the clean formation of the desired
Si(II) dihydride adduct IPrꢀSiH2ꢀBH3 (2) as a colorless solid in
a moderate isolated yield of 55% (Scheme 1).
The structure of IPrꢀSiH2ꢀBH3 (2) is presented in Fig. 1,11
and bears geometric features that closely resemble those found
within the germanium congener IPrꢀGeH2ꢀBH3 (1).3a The
dative CIpr–Si bond length in 2 [1.9284(15) A] is slightly
shorter than the related CIpr–Si interactions found within the
SiH2 adduct, IPrꢀSiH2ꢀBH2–SiH(B2H7)ꢀIPr [1.934(4) and
1.944(4) A], and in IPrꢀSiCl2ꢀBH3 [1.937(2) A].8,9 The adjacent
Si–B distance in 2 [1.992(2) A] lies in the range reported
for previously known BH3 adducts involving formal Si(II)
donor sites [1.965(2) to 1.996(4) A].4b,8,9 Each of the hydrogen
atoms bound to Si and B in 2 were located in the electron
difference map and refined isotropically to expected Si–H and
B–H distances.
Interest in :SiH2 stems from its postulated existence during
the thermolytic synthesis of Si films from SiH4,6 while metallo-
silylenes (MQSiR2) have been implicated as intermediates
in hydrosilane polymerization and in the synthesis of halo-
methylsilanes.7 Of particular relevance to the title work,
Robinson and coworkers have synthesized a compound bearing
a formal :SiH2 unit, IPrꢀSiH2ꢀBH2–SiH(B3H7)ꢀIPr, via a complex
borane-induced Si–Si cleavage reaction involving the disilene bis
adduct IPrꢀSiQSiꢀIPr (IPr
= [(HCNDipp)2C:]; Dipp =
2,6-iPr2C6H3).8
Previously we showed that the germylene donor–acceptor
complex, IPrꢀGeH2ꢀBH3 (1), could be prepared from the
reaction of the germanium(II) halide precursor, IPrꢀGeCl2,
with 2 equiv. of Li[BH4].3a However a later attempt by Roesky
and coworkers to apply a similar strategy toward the synthesis
of the Si(II) analogue IPrꢀSiH2ꢀBH3 (2) was not successful.
Consistent with the abovementioned crystallographic data,
the IR spectrum of IPrꢀSiH2ꢀBH3 (2) exhibited a sharp band at
2096 cmꢁ1 due to coincident nsym and nasym Si–H stretching
modes, along with diagnostic n(10/11B–H) stretching vibrations
1g
Specifically, treatment of IPrꢀSiCl2 with Li[BH4] in THF
resulted in an unusual LiH elimination reaction and the
Department of Chemistry, University of Alberta, Edmonton, Alberta,
Canada T6G 2G2. E-mail: erivard@ualberta.ca;
Tel: +1 780-492-4255
w This article is part of the ChemComm ‘‘Frontiers in Molecular Main
Group Chemistry’’ web themed issue.
z Electronic supplementary information (ESI) available: Full experi-
mental details. CCDC 853340–853342 (2–4). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc17101e
Scheme 1 Synthesis of the Si(II) dihydride adducts 2 and 3.
c
1308 Chem. Commun., 2012, 48, 1308–1310
This journal is The Royal Society of Chemistry 2012

Fig. 2 Thermal ellipsoid plot (30% probability) of IPrꢀSiH2ꢀW(CO)5
(3). IPr-bound hydrogen atoms and hexane solvent are omitted for
clarity; Si–H distances were constrained to equal values during the
refinement. Selected bond lengths [A] and angles [1]: C(6)–Si 1.928(13),
Si–W 2.573(4), W–C(1) 1.966(15), W–C(2–5) 1.997(16) to 2.059(17),
Si–H 1.32(9); C(6)–Si–W 121.4(4), Si–W–C(1) 176.2(4), Si–W–C(2–5)
85.0(4) to 91.6(4).
Fig. 1 Thermal ellipsoid plot (30% probability) of IPrꢀSiH2ꢀBH3 (2).
IPr-bound hydrogen atoms are omitted for clarity. Selected bond
lengths [A] and angles [1]: C(1)–Si 1.9284(15), Si–B 1.992(2), Si–H
1.409(18) and 1.439(18), B–H 1.02(2), 1.02(3) and 1.12(2); C(1)–Si–B
112.11(8), H–Si–H 102.0(10).
from 2238 to 2345 cmꢁ1. The 29Si{1H} NMR spectrum
featured a well-resolved quartet pattern centered at ꢁ55.6 ppm
[1JSi–B = 46 Hz], while the –BH3 acceptor unit in 2 appeared as
a quartet in the 11B NMR spectrum [d = ꢁ46.2; 1JBH = 93 Hz].
The isotopomer IPrꢀSiD2ꢀBH3 (2D) was also synthesized and
yielded a n(Si–D) IR band at 1522 cmꢁ1, congruent with the
expected change in Si–H/D harmonic oscillator strength on going
from 2 to 2D. Theoretical calculations on the silylene–borane
ImMe2ꢀSiH2ꢀBH3 {ImMe2 = [(HCNMe)2C:]}11 gave optimized
structural and IR spectroscopic parameters that closely matched
those of 2. Moreover, NBO analysis indicated the presence of a
polar dative CIpr–Si interaction [77% bonding density on C] and
a relatively non-polar Si–B single bond [Wiberg Bond Index
(WBI) = 0.989].
2086 and 2107 cmꢁ1 that were assigned as symmetric and
asymmetric Si–H stretching modes, respectively, on the basis
of DFT studies involving the model complex, ImMe2ꢀSiH2ꢀ
W(CO)5.11 The frequencies of the Si–H IR vibrations in 3
were of similar value as in the Si(II) hydride adduct,
[tBuNC(Ph)NtBu]SiHꢀBH3 [2107 cmꢁ1].4b The trans disposed
carbonyl ligand in 3 (relative to the Si donor) gave a char-
acteristic A1 vibration at 2044 cmꢁ1; this value is slightly lower
than the related stretching frequencies in the heavier element
analogues IPrꢀEH2ꢀW(CO)5 (E = Ge and Sn; n(CO)trans
=
2047 cmꢁ1 in both cases).3b These data suggest that the IPrꢀ
SiH2 group is a marginally stronger electron donor than
either of the Ge and Sn derivatives. Of further note, West’s
N-heterocyclic silylene [(HCNtBu)2Si:] does not form an
adduct with BH3, thus implying that the IPrꢀSiH2 unit is a
stronger Lewis base than this classic Si(II) heterocyclic donor.5c,12
DFT studies on ImMe2ꢀSiH2ꢀW(CO)5 identified a similar dative
The SiH2 complex 2 appears to be significantly more
stable than its Ge analogue IPrꢀGeH2ꢀBH3 (1). For example,
IPrꢀSiH2ꢀBH3 (2) is stable up to ca. 230 1C in the solid state
and remains unaltered in hot toluene (100 1C, 12 h). By
comparison, complex 1 has a much lower Tdec of 130 1C in
the solid state, while decomposition to Ge metal, H2 and IPrꢀ
BH3 is rapid at ca. 100 1C in toluene.3a Moreover, IPrꢀSiH2ꢀ
BH3 is unreactive towards Cy3P at room temperature, whereas
the GeH2 adduct 1 reacts with Cy3P to give the known
phosphine–borane adduct Cy3PꢀBH3, IPrꢀBH3 and the imida-
zole aminal [(HCNDipp)2CH2] as soluble products (ca. 25%
conversion after 24 h).11 These observations are in line with
the expected increase in both the electron donating and
accepting abilities of the SiH2 unit relative to GeH2,5b leading
to a higher degree of stability for IPrꢀSiH2ꢀBH3.
d
CIPr ꢁ–Sid+ bond [WBI = 0.725] as in ImMe2ꢀSiH2ꢀBH3;
moreover the SiH2 groups in both adducts have considerable
hydridic character (Hdꢁ). Lastly, the Si–W bonding electron
density in ImMe2ꢀSiH2ꢀW(CO)5 was moderately polarized
towards Si (61%; WBI = 0.791).11
In an attempt to better gauge the electron donating ability
of the IPrꢀSiH2 unit, and gain access to novel late metal
silylene chemistry,7 we attempted to prepare the Rh carbonyl
complex [IPrꢀSiH2ꢀRh(CO)2Cl]. Complexes of the general
form [LꢀRh(CO)2Cl] (L = 2eꢁ donor) are used to benchmark
relative ligand donor strengths by monitoring changes in the
n(CO) stretching frequencies.13 We initially investigated the
chemistry between IPrꢀSiH2ꢀBH3 and [Rh(CO)2Cl]2, and in
each case observed the rapid formation of Rh metal and the
generation of a large number of inseparable carbene-containing
products. A stable Rh–silylene complex was obtained when IPrꢀ
SiCl2 was reacted with [Rh(CO)2Cl]2. In place of forming the
target monometallic complex, [IPrꢀSiCl2ꢀRh(CO)2Cl], its coordi-
nation isomer trans-[(IPrꢀSiCl2)2Rh(CO)2]cis-[Rh(CO)2Cl2] (4)
was obtained as an orange crystalline solid (eqn (1)); a higher
An interesting silylene group transfer reaction was noted
when IPrꢀSiH2ꢀBH3 (2) was combined with THFꢀW(CO)5. In
this process the IPrꢀSiH2 unit remained intact to give a
thermally stable tungsten complex IPrꢀSiH2ꢀW(CO)5 (3) in a
66% yield with concomitant loss of THFꢀBH3 (Scheme 1). The
NMR spectroscopic and X-ray crystallographic data11 (Fig. 2)
for 3 were in accordance with the formation of a tungsten-
bound silylene. The IR spectrum of 3 afforded sharp bands at
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1308–1310 1309

isolated yield of 4 (87%) was obtained with an excess of
[Rh(CO)2Cl]2.11
Notes and references
1 (a) N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005, 249, 829;
(b) Y. Wang and G. H. Robinson, Inorg. Chem., 2011, 50, 12326;
(c) R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking and
G. Bertrand, Science, 2011, 333, 610; (d) Y. Wang, Y. Xie, P. Wei,
R. B. King, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson,
Science, 2008, 321, 1069; (e) C. Jones, Chem. Commun., 2001, 2293;
(f) S. J. Bonyhady, D. Collis, G. Frenking, N. Holzmann, C. Jones
and A. Stasch, Nat. Chem., 2010, 2, 865; (g) R. S. Ghadwal,
H. W. Roesky, S. Merkel, J. Henn and D. Stalke, Angew. Chem.,
Int. Ed., 2009, 48, 5683; (h) A. C. Filippou, O. Chernov and
G. Schnakenburg, Angew. Chem., Int. Ed., 2009, 48, 5687;
(i) S. Yao, Y. Xiong and M. Driess, Chem.–Eur. J., 2010, 16, 1281;
(j) A. J. Arduengo III, J. C. Calabrese, A. H. Cowley, H. V. Rasika
Dias, J. R. Goerlich, W. J. Marshall and B. Riegel, Inorg. Chem.,
1997, 36, 2151; (k) J. L. Dutton and P. J. Ragogna, Inorg. Chem.,
2009, 48, 1722.
ð1Þ
2 (a) U. Vogel, A. Y. Timoshkin and M. Scheer, Angew. Chem., Int.
Ed., 2001, 40, 4409; (b) P. A. Rupar, M. C. Jennings, P. J. Ragogna
and K. M. Baines, Organometallics, 2007, 26, 4109; (c) Y. Xiong,
S. Yao and M. Driess, Angew. Chem., Int. Ed., 2010, 49, 6642;
(d) R. S. Ghadwal, R. Azhakar, H. W. Roesky, K. Propper,
¨
B. Dittrich, S. Klein and G. Frenking, J. Am. Chem. Soc., 2011,
133, 17552.
3 (a) K. C. Thimer, S. M. I. Al-Rafia, M. J. Ferguson, R. McDonald
and E. Rivard, Chem. Commun., 2009, 7119; (b) S. M. I. Al-Rafia,
A. C. Malcolm, S. K. Liew, M. J. Ferguson and E. Rivard, J. Am.
Chem. Soc., 2011, 133, 777; (c) S. M. I. Al-Rafia, A. C. Malcolm,
S. K. Liew, M. J. Ferguson, R. McDonald and E. Rivard, Chem.
Commun., 2011, 47, 6987; (d) S. M. I. Al-Rafia, A. C. Malcolm,
R. McDonald, M. J. Ferguson and E. Rivard, Angew. Chem., Int.
Ed., 2011, 50, 8354; (e) S. Inoue and M. Driess, Angew. Chem., Int.
Ed., 2011, 50, 5614.
4 For recent examples of Si(II) monohydrides, see: (a) S.-H. Zhang,
H.-X. Yeong, H.-W. Xi, K. H. Lim and C.-W. So, Chem.–Eur. J.,
2010, 16, 10250; (b) A. Jana, D. Leusser, I. Objartel, H. W. Roesky
and D. Stalke, Dalton Trans., 2011, 40, 5458; (c) R. Rodriguez,
D. Gau, Y. Contie, T. Kato, N. Saffon-Merceron and
A. Baceiredo, Angew. Chem., Int. Ed., 2011, 50, 11492;
Fig. 3 Thermal ellipsoid plot (30% probability) of the trans-
[(IPrꢀSiCl2)2Rh(CO)2]+ cation in 4. IPr-bound hydrogen atoms and
CH2Cl2 solvent are omitted for clarity. Selected bond lengths [A] and
angles [1]: Rh(1)–Si 2.3605(8), Rh(1)–C(1) 1.901(4), C(2)–Si
1.939(3); C(2)–Si–Rh(1) 123.41(10), Cl(1)–Si–Cl(2) 102.56(5),
Si–Rh(1)–C(1) 90.94(11), Si–Rh(1)–Si(A) 180.0.
(d) M. Stoelzel, C. Prasang, S. Inoue, S. Enthaler and M. Driess,
¨
The structure of 4 was verified by single-crystal X-ray
crystallography (Fig. 3),11 while 13C{1H} NMR spectroscopic
studies revealed the retention of two distinct carbonyl environ-
ments in solution.14 The ligation of two IPrꢀSiCl2 units to a
sole Rh center in 4 is a likely consequence of the reduced
proximal bulk at the donor site of this 2eꢁ ligand relative to
NHCs (which readily give the monosubstituted complexes,
[(NHC)Rh(CO)2Cl]).13 Unfortunately our attempts to generate
a Rh-bound silylene complex featuring reactive Si–H groups
via the reaction of 4 with various hydride sources exclusively led
to the formation of metallic Rh and complicated product
mixtures. The synthesis of a complex with stable Si(II)–Rh
interactions represents a new addition to the growing family of
metallosilylenes,7,15 and future work will focus on investigating
other methods to deliver SiH2 functionality onto metal centers.
In summary, we have uncovered an efficient synthetic pathway
for the preparation of stable donor–acceptor adducts of SiH2.
Future work will involve an in-depth study of the reactivity of the
IPrꢀSiH2 array including the exploration of potential Si–H bond
activation processes. These studies could add valuable insight
into the nature of metal-assisted catalysis involving silanes.4c,7
This work has been supported by NSERC of Canada
(E.R. and A.C.M.), the Canada Foundation for Innovation,
Alberta Innovates (E.R. and S.M.I.A.) and Suncor Energy
Inc. (Petro-Canada Young Innovator Award to E.R.).
Angew. Chem., Int. Ed., 2011, DOI: 10.1002/anie.201105722.
5 (a) W. Petz, Chem. Rev., 1986, 86, 1019; (b) Y. Mizuhata,
T. Sasamori and N. Tokitoh, Chem. Rev., 2009, 109, 3479;
(c) M. Haaf, A. Schmiedl, T. A. Schmedake, D. R. Powell,
A. J. Millevolte, M. Denk and R. West, J. Am. Chem. Soc.,
1998, 120, 12714 and references therein.
6 J. M. Jasinski and S. M. Gates, Acc. Chem. Res., 1991, 24, 9.
7 (a) R. Waterman, P. G. Hayes and T. D. Tilley, Acc. Chem. Res.,
2007, 40, 712; (b) C. Zybill and G. Muller, Angew. Chem., Int. Ed.
¨
Engl., 1987, 26, 669; (c) D. G. Gusev, F.-G. Fontaine, A. J. Lough
and D. Zargarian, Angew. Chem., Int. Ed., 2003, 42, 216;
(d) M. Ochiai, H. Hashimoto and H. Tobita, Angew. Chem., Int.
Ed., 2007, 46, 8192.
8 M. Y. Abrahim, Y. Wang, Y. Xie, P. Wei, H. F. Schaefer III, P. v.
R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2011,
133, 8874.
9 R. Azhakar, G. Tavcar, H. W. Roesky, J. Hey and D. Stalke, Eur.
J. Inorg. Chem., 2011, 475.
10 R. J. Baker, A. J. Davies, C. Jones and M. Kloth, J. Organomet.
Chem., 2002, 656, 203. The direct reaction between IPrꢀSiCl2 and
Li[AlH4] also gave IPrꢀAlH3 as the major product.
11 See the ESIz for full details.
12 N. Metzler and M. Denk, Chem. Commun., 1996, 2657.
13 D. M. Khramov, V. M. Lynch and C. W. Bielawski, Organometallics,
2007, 26, 6042.
14 (a) M. A. Garralda and L. Ibarlucea, J. Organomet. Chem., 1986,
311, 225; (b) J. J. Kim and H. Alper, Chem. Commun., 2005, 3059.
15 (a) R. S. Ghadwal, R. Azhakar, K. Propper, J. J. Holstein, B. Dittrich
¨
and H. W. Roesky, Inorg. Chem., 2011, 50, 358; (b) J. Li, S. Merkel,
J. Henn, K. Meindl, A. Doring, H. W. Roesky, R. S. Ghadwal and
¨
D. Stalke, Inorg. Chem., 2010, 49, 775.
c
1310 Chem. Commun., 2012, 48, 1308–1310
This journal is The Royal Society of Chemistry 2012