Synthesis and reactivity of a base-free n-heterocyclic silanimine

Article abstract of DOI:10.1021/om1005676

Reaction of the N-heterocyclic silylene (HCNDipp)₂Si (1, Dipp = 2,6-iPr₂C₆H₃) with the terphenyl azide ArN ₃ (Ar = 2,6-Mes₂C₆H₃, Mes =2,4,6-Me₃C₆

Full text of DOI:10.1021/om1005676

5738 Organometallics 2010, 29, 5738–5740
DOI: 10.1021/om1005676
Synthesis and Reactivity of a Base-Free N-Heterocyclic Silanimine†
Lingbing Kong and Chunming Cui*
State Key Laboratory and Institute of Element-Organic Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Received June 9, 2010
Summary: Reaction of the N-heterocyclic silylene(HCNDipp)2-
theoretical calculations on the model compound H2SidNSiH3
predict a very similar geometry as a result of the electropositive
silyl group on the nitrogen atom.6 However, there are no
examples of base-free silanimines with a SidN-C skeleton that
have been isolated and structurally characterized for compar-
ison. This situation may be largely related to the highly polar-
ized Si-N double bond and limited synthetic approaches.
The successful isolation of stable N-heterocyclic silylenes
opens a new and facile way for the generation of doubly
bonded silicon species. Thus, reactions of the stable silylenes
A-E (Chart 1) toward various organic azides, such as
Me3SiN3, Ph3CN3, PhN3, p-MeC6H4-N3, Ph3SiN3, and AdN3
(Ad = adamantyl), have been investigated.7 There are four
types of final products (Chart 2) that have been prepared
through the silylene-azide reactions, indicating that steric
effects of both silylenes and azides are the main factors for
the control of reaction products. Notably, a stable silanimine
with a THF molecule strongly coordinated to the silicon
center was obtained and structurally characterized by employ-
ing the bulky azide Ph3SiN3 and the silylene A.7a Herein, we
report on the isolation and structural characterization of the
base-free silanimine (HCNDipp)2SidNAr (2) prepared by
the reaction of our recently reported N-aryl-substituted
silylene 18 with the bulky terphenyl azide 2,6-Ar2C6H3N3
(Ar = 2,4,6-Me3C6H2).9 Interestingly, the N-heterocycle of
the silanimine undergoes a 1,3-cycloaddition reaction with
sulfur to give a bicyclic donor-supported silanimine.
Si (1, Dipp=2,6-iPr2C6H3) with the terphenyl azide ArN3 (Ar=
2,6-Mes2C6H3, Mes =2,4,6-Me3C6H2) in THF yielded the
base-free silanimine (HCNDipp)2SidNAr (2) with the almost
linear SidNC geometry in high yield. Reaction of 2 with sulfur
and H2O resulted in the 1,3-addition of S2 to the C2N2Si ring
and 1,2-addition of H2O to the SidN bond, respectively.
Since the first isolable silene and disilene were reported by
Brook and West in 1981,1 multiply bonded group 14 elemental
species have been some of the main focuses in main-group
chemistry.2 Silanimines, doubly bonded silicon-nitrogen
species, have attracted much attention as the heavy ana-
logues of imines. The first silanimines were reported indepen-
dently by Wiberg and Klingebiel in 1986.3 Although a fair
number of stable silanimines have been isolated and structu-
rally characterized, the majority of them are stabilized by the
coordination of a Lewis base, such as THF, pyridine and
other donors,4,5,7a to the silicon center, and stable donor-free
silaimines with a three-coordinate silicon atom remain extre-
mely rare in number. To the best of our knowledge, there are
only two donor-free silanimines, namely But2SidN-SiBut3
and But2SidN-SiBut2Ph, that have been structurally char-
aterized.3b,5 The most interesting structural feature of the two
compounds is their almost linear SidN-Si angle. Subsequent
† Dedicated to Prof. Herbert W. Roesky on the occasion of his 75th birthday.
*To whom correspondence should be addressed. E-mail: cmcui@
nankai.edu.cn.
(1) (a) West, R.; Fink, M. J.; Michl, J. Science 1984, 225, 1109.
(b) Brooke, A. G.; Nyburg, S. C.; Abdesaken, F.; Gutekunst, B.; Gutekunst,
G.; Kallury, R. K. M. R.; Poon, Y. C.; Chang, Y. M.; Wong-Ng, W. J. Am.
Chem. Soc. 1982, 104, 5667.
(2) For reviews, see: (a) Driess, M. Coord. Chem. Rev. 1995, 145, 1.
(b) Driess, M. Adv. Organomet. Chem. 1996, 39, 193. (c) Power, P. P. Chem.
Rev. 1999, 99, 3463. (d) Okazaki, R.; Tokitoh, N. Acc. Chem. Res. 2000, 33,
625. (e) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479.
(f) Kira, M. Chem. Commun. 2010, 46, 2893.
(3) (a) Wiberg, N.; Schurz, K.; Fischer, G. Angew. Chem., Int. Ed.
1985, 24, 1053. (b) Wiberg, N.; Schurz, K.; Reber, G.; M€uller, G. J. Chem.
Soc., Chem. Commun. 1986, 591. (c) Hesse, M.; Klingebiel, U. Angew.
Chem., Int. Ed. 1986, 25, 649.
Treatment of 1 with 2,6-Ar2C6H3N3 in THF resulted in
immediate and rapid evolution of nitrogen. The base-free
silanimine 2 was obtained as yellow crystals in ca. 85% yield
after workup. 2 has been fully characterized by 1H, 13C, and
29Si NMR and IR spectroscopy, elemental analysis, and an
X-ray single-crystal analysis. The 29Si resonance of 2 in
CDCl3 occurs at δ -49.0 ppm, slightly deshielded compared
to that in the solvated silanimine (HCNBut)2Si(THF)-
NC(C6H5)3 (δ -66.6 ppm)7a because of the electron deficiency
of the low-coordinate silicon atom of 2.
Single crystals of 2 suitable for X-ray single-crystal anal-
ysis were obtained from n-hexane at -40 °C. The structure
is shown in Figure 1 with selected bond parameters.10 The
(4) Coordinated silanimine references: (a) Wiberg, N.; Wagner, G.
Chem. Ber. 1986, 119, 1467. (b) Wiberg, N.; Karampatses, P.; Kim, Ch.-K.
Chem. Ber. 1987, 120, 1213. (c) Wiberg, N.; Preiner, G.; Karampatses, P.; Kim,
Ch.-K.; Schurz, K. Chem. Ber. 1987, 120, 1357. (d) Wiberg, N.; Schurz, K.
Chem. Ber. 1988, 121, 581. (e) Wiberg, N.; Schurz, K. J. Organomet. Chem.
1988, 341, 145. (f) Wiberg, N.; Schurz, K.; M€uller, G.; Riede, J. Angew. Chem.,
Int. Ed. 1988, 27, 935. (g) Walter, S.; Klingebiel, U.; Schmidt, B, D.
J. Organomet. Chem. 1991, 412, 319. (h) Corriu, R.; Lanneau, G.; Priou, C.
Angew. Chem., Int. Ed. 1991, 30, 1130. (i) Junge, K.; Popowski, E.; Michalik,
M. Z. Anorg. Allg. Chem. 1999, 625, 1532. (j) Lerner, H. W.; Wiberg, N.; Bats,
J. W. J. Organomet. Chem. 2005, 690, 3898. (k) Lerner, H, W.; Bolte, M.;
Schurz, K.; Wiberg, N.; Baum, G.; Fenske, D.; Bats, J, W.; Wagner, M. Eur. J.
Inorg. Chem. 2006, 4998.
(6) Schleyer, P. v. R.; Stout, P. O. J. Chem. Soc., Chem. Commun.
1986, 1373.
(7) (a) Denk, M.; Hayashi, R.; West, R. J. Am. Chem. Soc. 1994, 116,
10813. (b) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (c) Hill,
N. J.; Moser, D. F.; Guzei, I. A.; West, R. Organometallics 2005, 24, 3346.
(d) Gehrhus, B.; Hitschcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem.
2001, 627, 1048. (e) Xiong, Y.; Yao, S.; Driess, M. Chem. ur. J. 2009, 15,
8542. (f) Tomasik, A. C.; Mitra, A.; West, R. Organometallics 2009, 28, 378.
(g) Iwamoto, T.; Ohnishi, N.; Gui, Z.; Ishida, S.; Isobe, H.; Maeda, S.; Ohno,
K.; Kira, M. New J. Chem. 2010, 34, 1637.
€
(5) Niesmann, J.; Klingebiel, U.; Schafer, M.; Boese, R. Organome-
(8) Kong, L.; Zhang, J.; Song, H.; Cui, C. Dalton Trans. 2009, 5444.
(9) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549.
tallics 1998, 17, 947.
r
pubs.acs.org/Organometallics
Published on Web 10/11/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 22, 2010 5739
Chart 1
Chart 2
Figure 1. Ortep drawing of 2 with 30% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): Si1-N1 = 1.7040(16), Si1-N2 =
silicon atom in 2 is three-coordinated and adopts a trigonal-
planar geometry (sum of the bond angles around the silicon
359.93(9)°). The Si1-N3 bond distance (1.5329(16) A) is the
shortest one observed so far compared to those in the struc-
1.6990(16), Si1-N3 = 1.5329(16), N1-C13 = 1.413(2), N2-
C14 = 1.413(2), N3-C27 = 1.375(2), C13-C14 = 1.323(3); N1-
Si1-N2 = 93.23(8), N1-Si1-N3 = 133.50(9), N2-Si1-N3 =
133.20(9), C27-N3-Si1 = 176.69(15), C1-N1-C13 = 119.75(15),
C13-N1-Si1 = 109.38(12), C14-N2-Si1 = 109.38(12), C1-
N1-Si1 = 130.81(12), C15-N2-Si1 = 132.63(13).
˚
turally characterized free and base-stabilized silanimines
3a,5
ꢀ
˚
(1.568-1.665 A).
The most notable structural feature
of 2 is the almost linear Si1-N3-C27 angle (176.69(15)°).
A similar linear Si-N-Si angle (177.8(2)°) was previously
observed in the N-silyl-substituted silanimine But2SidNSi-
But3, which is in accord with ab initio calculations (6-31G*
basis set) on H2SidNSiH3 (175.6°). On the other hand, the
same basis set calculations on H2SidNH predicted a bent
Si-N-H geometry (126.6°), with the low linearization
barrier of ca. 6.0 kcal/mol at MP4/6-31G*.6,11 Moreover,
theoretical predictions on H2SidNMe with a 6-31G(d) basis
set also indicated the bent Si-N-C geometry (132.6°).12 We
reasoned that the almost linear Si1-N3-C27 angle in 2 may
result from the steric effects afforded by the Ar and Dipp
groups as well as the crystal packing (some close intermolec-
ular contacts among the aryl carbon atoms are present in
Scheme 1
˚
the unit cell: 3.629, 3.730, 3.801 A). The five-membered SiN2C2
ring is essentially planar (average deviation from the plane
CDCl3 occurs at δ -72.9 ppm, which is shifted to high field
compared to that for 2 but close to that for (HCNBut)2Si-
(THF)NC(C6H5)3 (δ -66.6 ppm).7a The 1H NMR spectrum
of 3 shows a single peak for the SiN2C2 ring protons
(d8-toluene, CD2Cl2, and CDCl3) at 298 K, and the 13C NMR
spectrum for the ring carbon also exhibits only one broad
singlet (105.1 ppm, CDCl3), indicating its fluxional behavior
due to the S2 swinging between the two carbon atoms. The
variable-temperature 1H NMR spectra showed that the single
peak broadened when the temperature was lowered to 233 K
and disappeared as the temperature dropped to 193 K. The
same phenomena were observed in the 13C NMR spectra at
these temperatures. However, the single peak reappeared in
˚
0.0028 A). The two endocyclic Si-N distances (Si1-N1 =
˚
˚
1.7040(16) A; Si1-N2 = 1.6990(16) A) are very close to the
corresponding Si-N bond lengths in (HCNDipp)2SiCl2
(1.702 A). The N3-C27 bond length of 1.375(2) A is short-
ened, probably due to the sp hybrid N3 atom.
13
˚
˚
Reaction of the silanimine 2 with elemental sulfur in toluene
at 60 °C for 2 days lead to the degradation of S8 to give the
donor-supported silanimine 3 containing an S2 subunit
in 88%yield(Scheme1).3has been fully characterized by multi-
nuclear NMR and IR spectroscopy, elemental analysis, and
an X-ray single-crystal analysis. The 29Si resonance of 3 in
(10) The X-ray data were collected on a Rigaku Saturn CCD
diffractometer using graphite-monochromated Mo KR radiation (λ =
both the H and 13C NMR spectra when the temperature
1
˚
rose to 233 K. It is quite possible that the signals are too
broad to be observed at low temperatures. The structure of
3 was finally confirmed by an X-ray single-crystal analysis.
Single crystals of 3 were obtained from n-hexane/THF
at -40 °C.14 There are two independent molecules in the
asymmetric unit, and only one of the molecules is shown in
0.710 73 A) at 113 K. The structure was solved by direct methods
(SHELXS-97)18 and refined by full-matrix least squares on F2. All
non-hydrogen atoms were refined anisotropically and hydrogen atoms
by a riding model (SHELXL-97).19 Crystal data for 2: C50H61N3Si, fw=
˚
˚
732.11, monoclinic, space group P21/c, a=12.956(3) A, b=13.282(3) A,
˚
3
˚
c=25.126(5) A, β=90.31(3)°, V=4323.6(15) A , Z=4, Fcalcd=1.125 g
=
cm-3, 24 981 reflections measured, 7612 unique reflections (Rint
0.0323), R1=0.0577 (I > 2σ(I)), wR2=0.1533 (all data).
(11) Avakyan, V. G.; Sidorkin, V. F.; Belogolova, E. F.; Guselnikov,
S. L.; Gusel’nikov, L. E. Organometallics 2006, 25, 6007.
(14) Crystal data for 3: C103H129N6S4Si2, fw = 1635.54, triclinic,
˚
˚
˚
(12) Metail, V.; Joanteguy, S.; Chrostowska-Senio, A.; Pfister-Guillouzo,
G.; Systermans, A.; Ripoll, J. L. Inorg. Chem. 1997, 36, 1482.
(13) Karsch, H. H.; Schluter, P. A.; Bienlein, F.; Herker, M.; Witt, E.;
space group P1, a=10.9167(19) A, b=19.022(3) A, c=23.425(3) A, β=
3
83.910(9)°, V = 4659.3(12) A , Z = 2, Fcalcd = 1.166 g cm-3; 30 789
˚
reflections measured, 17 892 unique reflections (Rint = 0.0323), R1 =
0.0558 (I > 2σ(I)), wR2=0.1470 (all data).
Sladek, A.; Heckel, M. Z. Anorg. Allg. Chem. 1998, 624, 295.

5740 Organometallics, Vol. 29, No. 22, 2010
Kong and Cui
quite possible that S8 attacks the partially positive silicon
center to result in the fragmentation of S8 and formation
of the electrophilic sulfur center Si-S-Sδþ, which could
either be attacked by the CdC bond or the negatively
charged nitrogen atom.16 Since the formation of the five-
membered ring in 3 is favorable, the reaction exclusively
yielded 3.
The reaction of 2 with water resulted in the formation of
the amino-silanol 4 via the 1,2-addition of H2O to the SidN
double bond.3a,4c-4e,17 Compound 4 has been fully charac-
terized by multiple nuclear NMR and IR spectroscopy and
elemental analysis. The H2O addition product 4 displays two
single peaks at δ 2.46 and 3.96 ppm, indicating the formation
of N-H and Si-OH bonds.
In conclusion, the first stable base-free silanimine, 2, with
an almost linear SidN-C skeleton has been prepared and
structurally characterized. Reaction of 2 with elemental
sulfur yielded the novel imine-supported silanimine 3. 2 is a
new addition to the rare examples of well-defined base-free
silanimines. The structural analysis of 2 alone with the
known But2SidN-SiBut3 disclosed that both the steric
and electronic effects of the substituents on the unsaturated
nitrogen atom are responsible for the central geometry of
unsupported silanimines. Initial examinations of the reactiv-
ity of 2 demonstrated that either the five-membered hetero-
cycle or the SidN double bond may be involved in chemi-
cal reactions, as shown by the 1,3-addition of S2 to the
SiN2C2 ring and 1,2-addition of H2O to the SidN double
bond. Further studies on the reactivity of 2 and theoretical
calculations on 2 and the related systems are currently in
progress.
Figure 2. Ortep drawing of one of the two independent mole-
cules of 3 with 30% probability ellipsoids. Hydrogen atoms and
solvent molecule have been omitted for clarity. Selected bond
˚
lengths (A) and angles (deg) for the molecule with the Si1 atom
(selected parameters for the molecule with Si2 atom are also given
for comparison): Si1-N1=1.892(2)/Si2-N4=1.892(2), Si1-
N2 = 1.7448(18)/Si2-N5 = 1.7410(18), Si1-N3 = 1.5709(19)/
Si2-N6 = 1.5866(19), Si1-S1 = 2.2183(8)/Si2-S3 = 2.2056(8),
C27-N3 = 1.373(3)/C77-N6 = 1.381(3), C1-N1 = 1.309(3),
C2-N2=1.398(3), C1-C2=1.473(3); N2-Si1-N3=121.94(10),
N1-Si1-N3=124.57(9), N1-Si1-N2=87.05(9), N3-Si1-S1=
124.77(7), N2-Si1-S1=97.29(7), N1-Si1-S1=91.56(6), C27-
N3-Si1 =157.28(16)/C77-N6-Si2 =145.40(16).
Figure 2 with selected bond lengths and angles. The silicon
atom is four-coordinate and is covalently coordinated to two
nitrogen atoms and one sulfur atom, and the other coordina-
tion site is occupied by an imine nitrogen atom. Conse-
quently, the SidN double bond lengths (1.5709(19) and
Acknowledgment. We are grateful to the National Natu-
ral Science Foundation of China (Grant No. 20725205) and
111 plan for the support of this work.
˚
1.5866(19) A) in the two independent molecules of 3 are
˚
slightly lengthened compared to that found in 2 (1.5329(16) A).
The central SidN-C angles in the two molecules of 3 (157.28
and 145.40°) are significantly contracted in comparison with
that in 2 due to the four-coordinate silicon atoms in 3. In the
SiN2C2 rings of 3, the two Si-N bonds in one molecule are
Supporting Information Available: Text and figures giving
experimental details for the synthesis and characterization of the
compounds reported in this paper and representive VT NMR
spectra of 3 and CIF files giving crystallographic data for 2 and
3. This material is available free of charge via the Internet at
http://pubs.acs.org.
˚
significantly different (1.892(2) vs 1.7448(18) A and 1.892(2)
˚
vs 1.740(18) A in the two independent molecules); the longer
bonds are consistent with a dative Si-N bond. Consequently,
˚
(16) (a) Andrianarison, M.; Couret, C.; Declercq, J. P.; Dubourg, A.;
Escudie, J.; Ranaivonjatovo, H.; Satge, J. Organometallics 1988, 7, 1545.
(b) Winkel, Y. V. D.; Bastiaans, H. M. M.; Bickelhaupt, F. J. Organomet.
Chem. 1991, 405, 183.
(17) (a) Grosskopf, D.; Klingebiel, U.; Belgardt, T.; Noltemeyer, M.
Phosphorus, Sulfur Silicon Relat. Elem. 1994, 91, 241. (b) Xiong, Y.; Yao,
S.; M€uller, R.; Kaupp, M.; Driess, M. J. Am. Chem. Soc. 2010, 132, 6912.
(18) Sheldrick, G. M. SHELXS-90/96, Program for Structure Solu-
tion. Acta Crystallogr., Sect. A 1990, 46, 467.
one C-N single bond (1.398(3)/1.410(3) A) and one C-N
doublebond (1.309(3)/1.297(3) A) are observed in the SiN2C2
˚
ring. The bond distances of Si-S, S-S, and S-C in 3 are in
good agreement with those reported in the literature.15
The mechanism for the formation of 3 is not clear at
this stage. Due to the highly polarized Si-N bond, it is
(15) Boudjouk, P.; Black, E.; Kumarathasan, R.; Samaraweera, U.;
Castellino, S.; Oliver, J. P.; Kampf, J. W. Organometallics 1994, 13, 3715.
(19) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of G€ottingen: G€ottingen, Germany, 1997.