Cleavage of carbene-stabilized disilicon

Article abstract of DOI:10.1021/ja203208t

Reaction of carbene-stabilized disilicon L:Si=Si:L (L: =:C{N(2,6-Pr ⁱ₂C₆H₃)CH}₂, 1) with BH₃?THF results in facile cleavage of the silicon-silicon double bond and the formation of two quite

Full text of DOI:10.1021/ja203208t

COMMUNICATION
pubs.acs.org/JACS
Cleavage of Carbene-Stabilized Disilicon
Mariham Y. Abraham, Yuzhong Wang, Yaoming Xie, Pingrong Wei, Henry F. Schaefer, III, Paul v. R. Schleyer,
and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The University of Georgia, Athens, Georgia 30602-2556,
United States
S Supporting Information
b
ABSTRACT: Reaction of carbene-stabilized disilicon L:
SidSi:L (L: = :C{N(2,6-Pri2C6H3)CH}2, 1) with BH3
3
THF results in facile cleavage of the siliconꢀsilicon
double bond and the formation of two quite different
“pushꢀpull” stabilized products with borane- and carbene-
coordinated silylene moieties: 2, containing a parent
silylene (:SiH2); and 3, containing a unique three-mem-
bered cyclosilylene.
concomitant with cleavage of siliconꢀsilicon bond and boron-
to-silicon hydrogen transfer reactions. These processes could
eventually afford 2 via BH2 insertion between two silicon(II)
atoms. The formation of 3 may involve the cycloaddition of
the highly reactive Si(0) center of an L:Si(BH3) intermediate
to the CdC bond of an imidazole ring in a NHC:BH3
fragment. Indeed, similar reactions of silicon(0) atoms with
π-conjugated systems have been observed in matrix isolation
studies.20ꢀ22 Notably, the presence of a free NHC ligand in
the reaction system appears to be required for the formation of
3, where the NHC ligand essentially functions as a “trapping
agent” for the “pushꢀpull” stabilized Si(0) atom.
he intervening decades between the seminal discovery of the
T
first disilene1 in 1981 and that of the first disilyne2 in 2004
underscore the challenges, as well as the rewards, of siliconꢀ
silicon multiple bond chemistry. The recent realization of an
N-heterocyclic carbene-stabilized compound containing a siliconꢀ
silicon double bond, L:SidSi:L(L:=:C{N(2,6-Pri2C6H3)CH}2, 1),3
extends this series to disilicon(0) derivatives.
The X-ray structure of 2 (Figure 1) suggests its formation from
1 by a complex sequence of reactions involving four BH3
molecules. During this process, three borane hydrogen atoms
could have been transferred to the two silicon atoms of 1 leading
eventually to 2 via SidSi bond cleavage by BH2 insertion. Note
that electron pair donation from the Si(1) atom of 2 helps stabilize
the neutral three-membered B3H7 ring as in analogous L:B3H7
complexes (where L: = NH3,23 phosphine,24 and CO25). While
“pushꢀpull” N-heterocyclic carbeneꢀtransition metal carbonyl
complexed GeH2 and SnH2 analogues (4) have recently been
reported,26,27 2 is the first example of a “pushꢀpull” stabilized
parent silylene (:SiH2) persistent at room temperature.28 The
L:SiH(BH2)(B3H7) fragment in 2 mimics the Lewis acid
Compounds with siliconꢀsilicon multiple bonds are intriguing
not only because of the unique bonding nature of silicon, but also due
to the unusual reactivity of unsaturated SiꢀSi fragments.4ꢀ12 In
contrast to extensively explored hydroboration reactions of car-
bonꢀcarbon multiple bonds,13 the experimental hydroboration of
a disilyne (RSitSiR, R = SiPri[CH(SiMe3)2]2) has recently been
reported to yield boryl-substituted disilenes.14,15 Hydroboration of
disilenes, on the other hand, has only been examined
computationally.16 We now report the unexpected siliconꢀsilicon
double bond cleavage and borane insertion reactions of 1 when
combined with BH3 THF and describe the formation,17 molecular
3
structures,17 and computational investigations18 of 2 and 3, the novel
ensuing products. The “pushꢀpull”stabilization of the parent silylene
(:SiH2) unit in 2 is significant since SiH2 has been observed as an
important intermediate in the chemical vapor deposition (CVD) of
silicon film by pyrolysis of silane (SiH4).19
Colorless crystals of 2 (72% yield) were obtained by
reaction of pure 1 with BH3 THF (1:4) in toluene. However,
3
in the presence of NHC (N-heterocyclic carbene) impurity in
1 (NHC:1 = 1:5), both 2 and 3 were isolated in a moderate
yield (30% yield for 2, 28% yield for 3) from the BH3 THF
3
reaction. Although the mechanistic details are unclear, struc-
tural and computational studies suggest that the formation of
2 may involve multiple complexations of boranes to 1,
Received: April 7, 2011
Published: May 19, 2011
r
2011 American Chemical Society
8874
dx.doi.org/10.1021/ja203208t J. Am. Chem. Soc. 2011, 133, 8874–8876
|

Journal of the American Chemical Society
COMMUNICATION
Figure 1. Molecular structure of 2. Thermal ellipsoids represent 30%
probability. (Hydrogen atoms at boron and silicon atoms were located in
the difference Fourier map. All the hydrogen atoms on carbon have been
omitted for clarity. The disorder in B(1)H2 unit is not shown (see Figure
S1, Supporting Information)17). Selected bond distances (Å) and angles
(deg) for (2): Si(1)ꢀC(1) 1.944(4), Si(2)ꢀC(28) 1.934(4), B(1)ꢀSi-
(1) 1.994(8), B(1)ꢀSi(2) 1.922(7), Si(2)ꢀH(4D) 1.49(2), Si(2)ꢀH-
(5D) 1.42(4), B(1)ꢀH(3D) 1.119(19), B(1)ꢀH(2D) 1.145(19),
Si(1)ꢀH(1D) 1.49(4), B(2)ꢀSi(1) 1.965(7); C(1)ꢀSi(1)ꢀB(1)
107.2(2), B(1)ꢀSi(2)ꢀC(28) 117.7(3), Si(1)ꢀB(1)ꢀSi(2) 109.1(4)
B(2)ꢀSi(1)ꢀB(1) 122.2(3), B(2)ꢀSi(1)ꢀC(1) 107.7(2), B(2)ꢀSi-
(1)ꢀH(1D) 111.0(14), H(2D)ꢀB(1)ꢀH(3D) 106(3), H(4D)ꢀSi-
(2)ꢀH(5D) 101(3).
Figure 2. Molecular structure of 3. Thermal ellipsoids represent 30%
probability. Some hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg) for (3): Si(1)ꢀB(2)
1.996(4), Si(1)ꢀC(28) 1.938(3), Si(1)ꢀC(2) 1.903(3), Si(1)ꢀC(3)
1.888(3), C(2)ꢀC(3) 1.530(4), C(1)ꢀB(1) 1.593(5); C(2)ꢀSi(1)ꢀ
B(2) 134.88(17), C(3)ꢀSi(1)ꢀB(2) 133.06(18), C(28)ꢀSi(1)ꢀB(2)
116.25(17), C(2)ꢀSi(1)ꢀC(28) 102.27(13), C(3)ꢀSi(1)ꢀC(28)
104.04(13), C(3)ꢀSi(1)ꢀC(2) 47.62(12), C(3)ꢀC(2)ꢀSi(1) 65.68(16),
C(2)ꢀC(3)ꢀSi(1) 66.70(16).
an electron pair from the carbene ligand. Although five and six-
membered cyclosilylenes32,33 are well-known, three-membered
cyclic silylenes based on C2H4Si: have only been studied by
matrix-isolation techniques.20 The three-membered silylene
ring in 3 has a 1.53 Å C(2)ꢀC(3) single bond and two ∼1.89
Å SiꢀC bonds, marginally shorter than the dative Si(1)ꢀC-
(28) bond length (1.938(3) Å). The ꢀ35.0 ppm (1J = 87 Hz)
quartet resonance of the BH3 at the C(1) carbene in the 1H-
coupled 11B-NMR spectrum of 3 is like that reported for L:
behavior of R groups in L:E(H)2R (E = Ge, Sn; R = BH3,
W(CO)5) (4).
The SiꢀCcarbene bond distances, 1.934(4) and 1.944(4) Å in 2 as
well as 1.938(3) Å in 3, are close to those reported for 1 (1.9271
(15) Å).3 The central boron atom (B(1)) environment in 2 is
disordered;18 the average B(1)ꢀSi(1) (1.980 Å) and B(1)ꢀSi(2)
(1.902 Å) bond distances are comparable to the 1.965(7) Å of
B(2)ꢀSi(1). The B3H7 ring in 2 gives rise to the broad 1H NMR
singlet resonance at ꢀ0.29 ppm and the 11B NMR multiplet at ꢀ30.0
ppm. The silylene coordination shifts the 1H NMR resonance of B3H7
to higher field (ꢀ0.29 ppm) than that of NH3B3H7 (1.62ppm)23 and
of L:0B3H7 (L:0 = PH3, CH3PH2, (CH3)2PH, (CH3)3P) adducts
(1.23ꢀ0.72 ppm),24 indicating the stronger electron-donating
ability of silylene than phosphine or ammonia. Although the BH2
proton resonances were not observed in the 1H NMR
spectrum,29,30 they appeared clearly in the proton-coupled
11B NMR as a triplet at ꢀ50.4 ppm, a position comparable to the
reported ꢀ43.0 ppm signal of [Tbt(Mes)SiHBH2CNAr] (Tbt =
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl).30 While the 29Si
resonance of the silicon atoms in 2 could not be observed due
to the strong line-broadening caused by quadrupolar boron
1
BH3 (ꢀ35.4 ppm; J = 83 Hz).34 The ꢀ47.0 ppm broad
doublet resonance of BH3 coordinating to the central silicon
atom is close to the ꢀ43.4 ppm (1J = 84 Hz) of potassium
tris(trimethylsilyl)silylboranate.31 As in 2, the 29Si resonance
of the silicon atom in 3 could not be observed.31
The carbene-stabilized disilicon compound, 1, exhibits un-
usual reactivity with BH3, affording two unique silylene compounds,
2 and 3, which contain a parent silylene (:SiH2) and a three-
membered silylene ring, respectively. The driving force for the
formation of 2 is provided by its stabilizing B3H7 structural feature.18
’ ASSOCIATED CONTENT
S
Supporting Information. Full details of the syntheses,
b
1
nuclei,31 the H NMR spectrum of 2 gave a SiH doublet (2.90
computations, and X-ray crystal structure determinations
(including CIF files); complete ref 18. This material is available
free of charge via the Internet at http://pubs.acs.org.
ppm, J = 7.0 Hz)29 and a SiH2 triplet (3.13 ppm, J = 12.0 Hz).
The bonding of 2 was further probed by DFT analysis at the
B3LYP/DZP level.18 The 0.953, 0.999, and 0.869 Wiberg bond
indices of B(1)ꢀSi(1), B(1)ꢀSi(2), and B(2)ꢀSi(1), respectively,
support their single bond essence. The positive natural charges on
the silicon atoms in 2, þ0.95 for Si(1) and þ0.90 for Si(2), and the
negative charges of the B(1) atom (ꢀ0.92) and the B3H7 ring
(ꢀ1.03) are consistent with the dative character of the B(1)ꢀSi(2)
and B(2)ꢀSi(1) bonding.
’ AUTHOR INFORMATION
Corresponding Author
robinson@chem.uga.edu
’ ACKNOWLEDGMENT
The silicon atom of the central three-membered silylene ring, a
significant structural feature of 3 (Figure 2), exhibits simultaneous
nucleophilic and electrophilic (“push-pull”) bonding character by
donating its electron pair to a BH3 while—simultaneously—accepting
We are grateful to the National Science Foundation for
support: CHE-0953484 (G.H.R., Y.W.), CHE-0608142 (G.H.R),
CHE-0749868 (H.F.S.), and CHE-0716718 (P.v.R.S.).
8875
dx.doi.org/10.1021/ja203208t |J. Am. Chem. Soc. 2011, 133, 8874–8876

Journal of the American Chemical Society
COMMUNICATION
’ REFERENCES
(1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343–1344.
(2) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305,
1755–1757.
(3) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. Science 2008, 321, 1069–1071.
(4) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419–509.
(5) Okazaki, R.; West, R. Adv. Organomet. Chem. 1996, 39, 231–273.
(6) Kira, M.; Iwamoto, T. Adv. Organomet. Chem. 2006, 54, 73–148.
(7) Sekiguchi, A.; Ichinohe, M.; Kinjo, R. Bull. Chem. Soc. Jpn. 2006,
79, 825–832.
(8) Sekiguchi, A. Pure Appl. Chem. 2008, 80, 447–457.
(9) Wang, Y.; Robinson, G. H. Chem. Commun. 2009, 5201–5213.
(10) Scheschkewitz, D. Chem.—Eur. J. 2009, 15, 2476–2485.
(11) Murata, Y.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2010,
132, 16768–16770.
(12) Yamaguchi, T.; Sekiguchi, A.; Driess, M. J. Am. Chem. Soc. 2010,
132, 14061–14063.
(13) Mikhailov, B. M.; Bubnov, Y. N. Organoboron Compounds in
Organic Synthesis; Hardwood Academic: Amsterdam, 1984.
(14) Takeuchi, K.; Ikoshi, M.; Ichinohe, M.; Sekiguchi, A. J. Am.
Chem. Soc. 2010, 132, 930–931.
(15) Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. Organometallics 2011,
30, 2044–2050.
(16) Xu, Y.-J.; Zhang, Y.-F.; Li, J.-Q. Chem. Phys. Lett. 2006, 421,
36–41.
(17) See the Supporting Information for synthetic and crystallo-
graphic details.
(18) Computations: The structure of 2 was optimized at the
B3LYP/DZP DFT level with the Gaussian 94 and Gaussian 03
programs: Frisch, M. J.; et al. Gaussian 94, Revision B.3; Gaussian
Inc.: Pittsburgh, PA, 1995; Frisch, M. J.; et al. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004. See the Supporting
Information for the full citation.
(19) Koecher, T.; Kerst, C.; Friedrichs, G.; Temps, F. In Silicon
Chemistry; Jutzi, P., Schubert, U., Eds.; Wiley-VCH: Weinheim, 2003, pp
44ꢀ57.
(20) Maier, G.; Reisenauer, H. P.; Egenolf, H. Eur. J. Org. Chem.
1998, 1313–1317.
(21) Maier, G.; Reisenauer, H. P. Eur. J. Org. Chem. 2003, 488–491.
(22) Maier, G.; Reisenauer, H. P. Eur. J. Org. Chem. 2003, 479–487.
(23) Yoon, C. W.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc.
2009, 131, 855–864.
(24) Bishop, V. L.; Kodama, G. Inorg. Chem. 1981, 20, 2724–2727.
(25) Mebel, A. M.; Musaev, D. G.; Morokuma, K. Chem. Phys. Lett.
1993, 214, 69–76.
(26) Thimer, K. C.; Al-Rafia, S. M. I.; Ferguson, M. J.; McDonald, R.;
Rivard, E. Chem. Commun. 2009, 7119–7121.
(27) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.;
Rivard, E. J. Am. Chem. Soc. 2011, 133, 777–779.
(28) Inoue, S.; Driess, M. Angew. Chem., Int. Ed. 2011DOI: 10.1002/
anie.201101812.
(29) Kajiwara, T.; Takeda, N.; Sasamori, T.; Tokitoh, N. Organo-
metallics 2004, 23, 4723–4734.
(30) Takeda, N.; Kajiwara, T.; Tokitoh, N. Chem. Lett. 2001,
1076–1077.
(31) Arp, H.; Marschner, C.; Baumgartner, J. Dalton Trans. 2010,
39, 9270–9274.
(32) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165–4183.
(33) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc.
1999, 121, 9722–9723.
(34) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.;
Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc.
2007, 129, 12412–12413.
8876
dx.doi.org/10.1021/ja203208t |J. Am. Chem. Soc. 2011, 133, 8874–8876