N-Aryl substituted heterocyclic silylenes

Article abstract of DOI:10.1039/b905905a

Two N-aryl substituted five-membered heterocyclic silylenes and their nickel complexes have been isolated and structurally characterized. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b905905a

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www.rsc.org/dalton | Dalton Transactions
N-Aryl substituted heterocyclic silylenes†
Lingbing Kong, Jianying Zhang, Haibin Song and Chunming Cui*
Received 24th March 2009, Accepted 11th May 2009
First published as an Advance Article on the web 21st May 2009
DOI: 10.1039/b905905a
Two N-aryl substituted five-membered heterocyclic silylenes
and their nickel complexes have been isolated and structurally
characterized.
sponding dichloride 26a in THF at low temperature (from -78
to -30 ◦C) after various reducing conditions had been examined
(Scheme 1). Similar reduction of 36b yielded 5 as yellow crystals
in ca. 64% yield while reduction with six equiv. of KC8 resulted in
the formation of extremely air-sensitive orange materials, which
have not been identified definitely at present. 5 was previously
obtained by reduction of the corresponding silicon dichloride,
but has not been structurally characterized.6a We found that
easily available 3 is an excellent precursor for 5,6b giving good
and reproducible yields. The formation of 5 might involve the
elimination of (SiCl2)n. However, we were unable to identify the
insoluble materials due to the complicated composition. Previous
unsuccessful efforts for the synthesis of crystalline N-aryl (phenyl,
p-tolyl and mesityl) substituted heterocyclic silylenes3b suggested
that the reduction conditions were important in the isolation of
these N-aryl substituted silylenes. Both 4 and 5 do not decompose
at 80 ◦C in C6D6 solution. The two compounds were fully
Since the first stable N-heterocyclic silylene (A) was reported
by West in 1994,1 several other types of cyclic silylenes have
also been reported (Chart 1).2,3 Among them, the five-membered
heterocyclic silylenes, silicon analogues of the “Ardueugo type”
carbenes, are particularly attractive synthetic targets due to their
fundamental interest and potential applications in transition
metal catalysis3 like N-heterocyclic carbenes (NHCs).4 However,
compared to a large number of isolable NHCs,5 only a handful of
analogous silylenes with tert-butyl and neopentyl substituents on
the ring nitrogen atoms have been fully characterized. It appears
surprising that few successful efforts6 have been dedicated to the
synthesis, structural characterization and reactivity of other
N-substituted heterocyclic silylenes, probably due to the lack of
reliable synthetic methods.3b Nevertheless, there is still a great
demand for new N-heterocyclic silylenes for the understanding
of the effects of steric and electronic factors on the stability,
structure and reactivity of this family of silylenes.3a,b We, therefore,
proposed to extend the synthesis and structural characterization
of this series of N-aryl substituted silylenes since the steric and
electronic effects on the phenyl rings can be tuned as required
as shown by N-aryl substituted NHCs. The substituents on the
phenyl rings and their different orientations have substantial
effects on the properties of the carbenes themselves and their metal
complexes.5b Herein, we report the synthesis and structures of the
N-heterocyclic silylenes 4 and 5 with bulky aryl substituents on the
ring nitrogen atoms and their corresponding nickel cyclooctadiene
complexes. They represent the first structurally characterized five-
membered heterocyclic silylenes with flanking N-aryl groups.
1
characterized by H, 13C and 29Si NMR spectroscopy, elemental
1
analysis and X-ray single crystal analysis. The H NMR spectra
of 4 and 5 show a resonance for the five-membered ring protons
at d 6.29 and 6.47 ppm, respectively. Their 29Si resonances in C6D6
fall at d 77.8 ppm for 4 and 76.7 for 5, quite close to that for A
(d = 78.3 ppm), indicating the formation of the N-heterocyclic
silylenes.
Chart 1
The aryl-substituted heterocyclic silylene 4‡ was finally obtained
as yellow crystals in ca. 43% yield by reduction of the corre-
Scheme 1
Single crystals of 4 were obtained from n-hexane at -40 ◦C. The
structure is shown in Fig. 1 with selected bond parameters.
The five-membered SiN2C2 central ring is essentially planar (the
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai
University, Tianjin, 300071, People’s Republic of China. E-mail: cmcui@
nankai.edu.cn; Fax: +86-22-23503461; Tel: +86-22-23503461
† CCDC reference numbers 719928 (4), 719926 (5) and 719927 (7).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b905905a
˚
average deviation from the plane = 0.0015 A) with the Si–N bond
˚
lengths of 1.750(2) and 1.743(2) A, comparable to those found
in A. The flanking mesityl rings are substantially twisted with
5444 | Dalton Trans., 2009, 5444–5446
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synthetic routes were available. It is evident that the donation of
the lone pairs on the nitrogen atoms to the divalent silicon atom
is predominantly responsible for the stability of 4 and 5.
It is anticipated that the steric effects afforded by N-substituents
would be crucial to the geometry and coordination environments
of the corresponding metal silylene complexes. It has been shown
that the reactions of A, B and C with Ni(COD)2 resulted in
the formation of homoleptic trigonal planar Ni(A)3 and Ni(B)3
and tetrahedral Ni(C)4 complexes, respectively.7 To access the
coordinative properties of the N-aryl heterocyclic silylenes, their
reactions with Ni(COD)2 were investigated for comparison. The
reactions in THF at ambient temperature yielded 6 and 7 as yellow
crystals, irrespective of the amount of the silylenes employed.
The formation of 6 and 7 indicates that the steric bulkiness
of the flanking aryl substituents of 4 and 5 may prevent the
third silylene approaching the nickel atom. Complexes 6 and 7
Fig. 1 Ortep drawing of 4 with 30% probability. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (A) and an-
˚
gles (deg): Si1–N1 1.750(2), Si1–N2 1.743(2), N1–C10 1.390(3),
N1–C1 1.442(3), N2–C11 1.388(3), N2–C12 1.442(3), C10–C11
1.339(3)N1–Si1–N(2) 87.53(10), C10–N1–C1 120.96(19), C10–N1–Si1
113.30(16), C1–N1–Si1 125.70(16), C11–N2–C12 121.29(19), C11–N2–Si1
113.56(16), C12–N2–Si1 125.15(16).
respect to the SiN2C2 ring with the interplane angles of 89.0◦ and
105.3◦. The N–C and C–C distances in the SiN2C2 ring are in the
range of their representative values in aromatic systems, indicating
the electron delocalization in the C2N2 plane. The N–Si–N angle
of 87.53(10)◦ is slightly open in comparison with the calculated
value for the N-phenyl substituted silylene (86.0◦).8 The wide
Si1–N1–C1 and Si1–N2–C12 angles (125.70(16) and 125.15(16)◦)
leave the Si atom unprotected, suggesting that the p-electron
donation from the nitrogen lone pairs is a major stabilizing factor.
Compound 5 crystallizes in the monoclinic system space group
C2. The structure was modelled as consisting of disordered central
C2N2Si atoms. The structure of 5 is shown in Fig. 2. The Si–N bond
1
were characterized by H and 13C NMR spectroscopy, elemental
analysis and X-ray single-crystal analysis in the case of 7.7 The
1H, 13C NMR spectra of 6 and 7 clearly indicate the existence of
one COD ligand and two silylene ligands. Single crystals of 7 were
obtained from toluene at -40 ◦C. The structure of 7 was shown in
Fig. 3 with selected bond lengths and angles. The molecule has a
crystallographic C2 axis. The two silylene ligands are coordinated
to the Ni atom with the Si–Ni–Si angle of 101.91(4)◦. The Ni–Si
˚
bond lengths (2.1395(8) A) are shorter than those in Ni(A)2(CO)2
˚
(2.207(2) and 2.216(2) A) and slightly shorter than those
˚
found in trigonal Ni(A)3 and Ni(B)3 (2.144–2.175 A. The Ni–C
˚
lengths (1.761(4) and 1.774(4) A) are very close to those found in
˚
distances (2.130–2.146 A) are very close to those in Ni(COD)2
4, and longer than those in the corresponding dichloride (1.701(1)
9
˚
(2.11–2.13 A).
6a
˚
and 1.703(1) A). The short N1–C25 and N2–C26 bond length
˚
(1.420(4) and 1.379(4) A) and the long C25–C26 double bond
˚
length (1.391(7) A) indicate the delocalization of the p-electrons
on the central N1C25C26N2 plane. The N(1)–Si(1)–N(2) angle
(87.50(17)◦) is in line with that in 4. Similar to 4, 5 also features
the wide Si–N–C(aryl) angles (127.8(3) and 127.4(2)◦).
Fig. 3 Ortep drawing of 7 with 30% probability. §Hydrogen atoms
and one toluene molecule have been omitted for clarity. Selected bond
˚
length and angles Selected bond lengths (A) and angles (deg): Ni1–Si1
2.1395(8), Ni1–C27 2.130(3), Ni1–C30 2.146(2), Si1–N1 1.754(2), Si1–N2
1.754(2), N1–C13 1.410(3), N2–C14 1.394(3), N1–C1 1.452(3), C13–C14
1.330(3)Si1*–Ni1–Si1 101.91(4), C27–Ni1–C27* 99.89(13), C27–Ni1–Si1
98.40(7), C27–Ni1–C30 84.59(9), N2–Si1–N1 88.08(10), C13–N1–Si1
112.36(15), C1–N1–Si1 130.84(16), C14–N2–Si1 112.86(16), C15–N2–Si1
126.76(17). * 1 - x, y, 1.5 - z.
Fig. 2 Ortep drawing of 5 with 30% probability. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (A) and angles
(deg): Si1–N1 1.761(4), Si1–N2 1.774(4), C25–N1 1.420(4), C26–N2
1.379(4), C25–C26 1.391(7)N1–Si1–N2 87.50(17), C25–N1–Si1 113.4(3),
C26–N2–Si1 114.9(3), C25–C26–N2 111.9(3), C26–C25–N1 112.3(3),
C1–N1–C25 118.9(3), C1–N1–Si1 127.8(3), C13–N2–Si1 127.4(2).
˚
In summary, we have established suitable conditions for the
reliable synthesis of the two N-aryl substituted heterocyclic
silylenes 4 and 5 in good yield and structurally characterized them
for the first time. The synthetic conditions may be applied to the
other N-substituted silylenes so that the steric and electronic effects
on the stability of this family of silylenes could be experimentally
The twists of the aryl groups and wide Si–N–C(aryl) angles
observed in 4 and 5 indicate that the divalent silicon atom is not
efficiently protected by the flanking aryl groups. These structural
features imply that the other silylenes of this type with less
hindered groups might also be accessible provided that suitable
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realized. Initial examination shows that the two silylenes could
be used as ligands for transition metal complexes. These results
showed that N-substituents may have significant effects on the
structure of the silylene metal complexes. The reactivity studies
of 4 and 5, especially their transition metal complexes, and the
synthesis of the other N-substituted heterocyclic silylenes are
currently in progress.
refined anisotropically and hydrogen atoms by a riding model (SHELXL-
97).11 Crystal data for4: C20H24N2Si, Fw = 320.50, monoclinic, space group
◦
˚
P21/c, a = 8.2595(17), b = 13.842(3), c = 16.171(3) A, b = 97.06(3) , V =
1834.9(6) A , Z = 4, rcalcd = 1.160 g cm-3 13 920 reflections, 3234 unique
3
˚
(Rint = 0.0642)R1 = 0.0631 (I > 2s(I)), wR2 = 0.1694 (all data). 5:
C26H36N2Si, Fw = 404.66, monoclinic, space◦group C2, a = 20.089(4), b
3
˚
˚
= 6.4762(13), c = 20.030(4) A, b = 102.71(3) , V = 2576.3 (9) A , Z = 4,
rcalcd = 1.043 g cm-3 15 860 reflections, 6011 unique (Rint = 0.0408) R1 =
0.0489 (I > 2s(I)), wR2 = 0.1315 (all data). 7·C7H8: C67H92N4NiSi2, Fw =
1068.34, orthorhombic, space group Pbcn, a = 13.558(3), b = 18.331(4),
c = 24.290(5) A, V = 6037(2) A , Z = 4, rcalcd = 1.175 g cm-3 38 920
reflections, 5323 unique (Rint = 0.0684) R1 = 0.0684 (I > 2s(I)), wR2 =
0.1772 (all data).
3
˚
˚
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (20725205) and 111 plan for support of this work.
1 M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P. Verne,
A. Haaland, M. Wagner and N. Metzler, J. Am. Chem. Soc., 1994, 116,
2691.
2 (a) R. West and M. Denk, Pure Appl. Chem., 1996, 68, 785; (b) B.
Gehrhus, M. F. Lappert, J. Heinicke, R. Boese and D. Blaser, J. Chem.
Soc., Chem. Commun., 1995, 1931; (c) J. Heinicke, A. Oprea, M. K.
Kindermann, T. Karpati, L. Nyulaszi and T. Veszprem, Chem.–Eur. J.,
1998, 4, 541; (d) M. Kira, S. Ishida, T. Iwamoto and C. Kabuto, J. Am.
Chem. Soc., 1999, 121, 9722; (e) M. Driess, S. Yao, M. Brym, C. van
Wullen and D. Lentz, J. Am. Chem. Soc., 2006, 128, 9628; (f) C. W.
So, H. W. Roesky, J. Magull and R. B. Oswald, Angew. Chem., Int. Ed.,
2006, 45, 3948.
3 (a) For reviews on silylenes and silylene metal complexes, see: M. Haaf,
T. A. Schmedake and R. West, Acc. Chem. Res., 2000, 33, 704; (b) N. J.
Hill and R. West, J. Organomet. Chem., 2004, 689, 4165; (c) B. Gehrus
and M. F. Lappert, J. Organomet. Chem., 2001, 617–618, 209; (d) M.
Kira, J. Organomet. Chem., 2004, 689, 4475; (e) R. Waterman, P. G.
Hayes and T. D. Tilley, Acc. Chem. Res., 2007, 40, 712.
4 (a) A. J. Arduengo, III, H. V. R. Dias, R. L. Harlow and M. Kline,
J. Am. Chem. Soc., 1992, 114, 5530; (b) A. J. Arduengo III, S. F. Gamper,
J. C. Calabrese and F. Davidson, J. Am. Chem. Soc., 1994, 116, 4391;
(c) L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet. Chem.,
2000, 606, 49; (d) A. J. Arduengo, III, R. Krafczyk and R. Schmutzler,
Tetrahedron, 1999, 55, 14523.
5 (a) A. J. Arduengo III, Acc. Chem. Res., 1999, 32, 913; (b) D. Bourissou,
O. Guerret, F. P. Gabba and G. Bertrand, Chem. Rev., 2000, 100, 39;
(c) S. Wu¨rtz and F. Glorius, Acc. Chem. Res., 2008, 41, 1523; (d) F. E.
Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122.
6 (a) H. H. Karsch, P. A. Schluter, F. Bienlein, M. Herker, E. Witt, A.
Sladek and M. Heckel, Z. Anorg. Allg. Chem., 1998, 624, 295; (b) R. J.
Baker, C. Jones, D. P. Mills, G. A. Pierce and M. Waugh, Inorg. Chim.
Acta, 2008, 361, 427.
Notes and references
‡ 4: A solution of 2 (1.95 g, 5.0 mmol) in THF (70 mL) was added to
a suspension of potassium graphite (1.35 g, 10.0 mmol) at -78 ◦C. The
reaction mixture was stirred for 4 d at low temperature. The volatiles
were removed, and the residue was extracted with n-hexane (80 mL).
After filtration, the yellow filtrate was concentrated and stored at -40 ◦
C
overnight to yield yellow crystals of 4 (0.69 g, 43.1%). M.p. 110–112 ◦C.
1H NMR (C6D6): d 2.17 (s, 6H, p-CH3), 2.25 (s, 12H, o-CH3), 6.29 (s,
2H, NCH), 6.83 (m, 4H, ArH), 13C NMR (C6D6): d 18.5 (p-CH3), 21.0
(o-CH3), 124.4 (HCCH), 129.3, 134.8, 135.8, 140.3 (ArC). 29Si NMR
(C6D6): d 77.8. Anal. found for C20H24N2Si: C 74.92, H 7.41, N 8.68. Calc.
C 74.95, H 7.55, N 8.74. 5: It was prepared similarly to 4 and obtained
as yellow crystals (0.49 g, 60.8%). M.p. 131 ◦C. 29Si NMR (C6D6): d 76.7.
Anal. found for C26H36N2Si: C, 76.51, H, 8.66, N, 6.81. Calc. C 77.17, H
8.97, N 6.92. 6: A solution of 4 (0.64 g, 2.0 mmol) and Ni(COD)2] (0.27 g,
1.0 mmol) in THF (50 mL) was stirred for 24 h at room temperature.
The solvents were removed, and the remaining solid was crystallized from
toluene at -40 ◦C to yield yellow crystals of 6 (0.43 g, 53%). M.p. 192 ◦
C
(dec). 1H NMR (C6D6): d 1.35 (m, 4H, CH2), 1.97 (m, 4H, CH2), 2.05 (s,
12H, p-CH3), 2.20 (s, 24H, o-CH3), 3.93 (s, 4H, CH), 6.18 (s, 4H, NCH),
6.79 (s, 8H, ArH). 13C NMR (C6D6): d 18.5 (p-CH3), 20.9 (o-CH3), 32.1
(CH2), 79.5 (CH), 123.3 (HCCH), 129.2, 135.3, 135.5, 141.1 (Ar–C). Anal.
found for C48H60N4NiSi2: C 71.52, H 7.86, N 6.92. Calc. C 71.36, H 7.49,
N 6.94. 7: it was prepa◦red similarly to 6 and obtained as yellow crystals
(0.68 g, 64%). M.p. 232 C (dec). 1H NMR (C6D6): d 0.36 (d, 6H, CHMe2),
1.04 (d, 6H, CHMe2), 1.23 (t, 12H, CHMe2), 1.32 (t, 12H, CHMe2), 1.32
(m, 4H, CH2), 1.38 (d, 12H, CHMe2), 2.10 (s, 3H, PhCH3), 2.26 (m, 2H,
CH2), 2.59 (m, 2H, CH2), 3.10 (m, 2H, CHMe2), 3.17 (m, 2H, CHMe2),
3.53 (m, 2H, CHMe2), 3.77 (m, 2H, CHMe2), 3.90 (m, 2H, CH), 4.51 (m,
2H, CH), 6.06 (d, 2H, HCCH), 6.16 (d, 2H, HCCH), 7.02–7.35 (m, 17H,
Ar-H). 13C NMR (C6D6): d 23.3, 23.8, 24.0, 26.7, 27.3, 27.6, 27.8, 28.1,
28.3, 28.5, 30.0, 30.2 (CHMe2), 34.0, 35.0 (CH2), 80.4, 80.9 (CH),21.4,
125.7, 128.5, 129.3, 137.9 (toluene-C), 123.6, 123.9, 124.0, 124.8 (HCCH),
125.1, 125.4, 140.8, 141.2, 146,3, 146,4, 147.1, 147.2 (Ar–C). Anal. found
C67H92N4NiSi2: C 75.34, H 8.74, N 5.22. Calc. C 75.32, H 8.68, N 5.24.
§ The X-ray data were collected on a Bruker Smart-CCD diffractometer
7 H. M. Tuononen, R. Roesler, J. L. Dutton and P. J. Ragogna, Inorg.
Chem., 2007, 46, 10693.
8 (a) T. A. Schmedake, M. Haaf, B. J. Paradise, D. Powell and R. West,
Organometallics, 2000, 19, 3263; (b) B. Gehrhus, P. B. Hitchcock, M. F.
Lappert and H. Maciejewski, Organometallics, 1998, 17, 5599.
9 P. Macchi, D. M. Proserpio and A. Sironi, J. Am. Chem. Soc., 1998,
120, 1447.
10 G. M. Sheldrick, SHELXS-90/96, Program for Structure Solution, Acta
Crystallogr., Sect. A, 1990, 46, 467.
11 G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Goettingen, Goettingen, Germany, 1997.
˚
using graphite-monochromated Mo Ka radiation (l = 0.71073 A) at
113 K. The structure was solved by direct methods (SHELXS-97)10 and
refined by full-matrix least squares on F2. All non-hydrogen atoms were
5446 | Dalton Trans., 2009, 5444–5446
This journal is
The Royal Society of Chemistry 2009
©