Twice silicon-induced C-H activation and tautomerisation of a ?2-diketiminato ligand and formation of new types of N-heterocyclic silanes

Article abstract of DOI:10.1039/b820285k

Unexpected formation of the novel dianionic N,C-chelate ligand L _D^2- {HC[C(Me)-NR][C(NHR)=CH]}^2- occurs by a twice silicon-assisted C-H bond activation of a terminal methyl group on a ?2-diketiminato ligand backbone, start

Full text of DOI:10.1039/b820285k

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Twice silicon-induced C–H activation and tautomerisation of a b-diketiminato
ligand and formation of new types of N-heterocyclic silanes†
Yun Xiong, Shenglai Yao and Matthias Driess*
Received 9th October 2008, Accepted 21st November 2008
First published as an Advance Article on the web 8th December 2008
DOI: 10.1039/b820285k
Unexpected formation of the novel dianionic N,C-chelate
complexes containing the new related ligands LB-, [LB-H]2- and
LC2-, respectively (Scheme 1).4 Recently, we described a simple
one-pot synthesis of a N-heterocyclic dibromosilane LCSiBr2
2-
2-
ligand LD {HC[C(Me)-NR][C(NHR)=CH]} occurs by a
twice silicon-assisted C–H bond activation of a terminal
methyl group on a b-diketiminato ligand backbone, starting
from the corresponding lithium b-diketiminide and silicon
tetrabromide to give the new N-heterocyclic dibromosilanes
2 and 3.
2-
containing the dianionic ligand LC (R = 2,6-diisopropylphenyl,
DIPP).5a The latter dibromosilane results from the reaction
of the corresponding LALi salt with SiBr4 in the presence of
tetramethylethylenediamine (TMEDA) as an auxiliary base. Its
debromination leads to a new type of stable silylene LCSi:5a with
zwitterionic character owing to the contribution of the resonance
form (LC2-)’ on the electronic ground state (Scheme 1).4f,5
-
Monoanionic, chelating b-diketiminato ligands LA (Scheme 1)
are valuable supporting ligands for the development of new
molecular catalysts containing metals even in unusual low oxi-
dation states.1,2 The fascinating variety of the latter complexes
stems from the relatively strong coordination ability of the
We now learned that sterically less demanding substituted
lithium b-diketiminides LALi react with SiBr4 in the absence of
TMEDA to give a new type of N-heterocyclic dibromosilane
2-
containing the novel dianionic N,C-ligand LD (Scheme 1). In
-
resonance-stabilised ß-diketiminato ligand LA to metal centres
fact, reaction of the lithium b-diketiminide LALi 1 (R = 2,6-
dimethylphenyl, DMP) with SiBr4 in the molar ratio of 1:1 in
diethyl ether at -40 ◦C furnishes the new dibromosilanes 2 and 3,
containing the N,C-chelate ligand LD2-, along with 4 which is the
HBr adduct of LAH (Scheme 2). In contrast, the similar conversion
in the presence of TMEDA affords the dibromosilane 5 which can
along with their tuneable steric congestion. Additionally, the
b-diketiminato ligand can undergo rearrangements in its metal
complexes such as metal-mediated ring contraction or C–H bond
activation. Striking examples for ring contractions are shown by
the partial reductive C–N bond fission of the C3N2 backbone in
b-diketiminato complexes,3a–e which more recently led to the isola-
tion of a cyclopentadienide analogue with divalent germanium.3f
Furthermore, unexpected C–H activation on a methyl group of
the backbone of the b-diketiminato ligand afforded remarkable
1
be isolated in the form of colorless crystals in 63% yield. Its H,
13C and 29Si NMR spectroscopic data6 show similar features with
those observed for the LCSiBr2 derivative reported previously.5a
In addition, the structure of 5 has been confirmed by a single
crystal diffraction analysis (Fig. 1).7 Despite of the presence of the
less bulkier DMP group at nitrogen in 2, the metric features are
practically identical with those of LCSiBr2.5a
Scheme 1 ß-Diketiminato (LA-) and the related mono- and dianionic
2-
chelate ligands LB-, LC2-/(LC2-)’, and LD
.
Scheme 2 Synthesis of compounds 2–5.
Technische Universita¨t Berlin, Institute of Chemistry: Metalorganics and
Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135, D-10623,
Berlin, Germany. E-mail: matthias.driess@tu-berlin.de; Fax: +49(0)30-
314-29732; Tel: +49(0)30-314-29731
† Electronic supplementary information (ESI) available: Additional ex-
perimental details. CCDC reference numbers 705079–705081. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b820285k
The isolation of 2 and 3 has been achieved by fractional
crystallisation in n-hexane at -20 ◦C. Since the bromide salt 4
is insoluble in hydrocarbon solvents it can be easily separated
from 2 and 3 and isolated in 21% yield. Compound 2 results in the
form of colorless crystals in 38% yield, whereas 3 is accessible as
colourless crystals in only 9% yield. Apparently, LALi has a dual
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Fig. 1 Molecular structure of 5. Thermal ellipsoids are drawn at
50% probability level. Hydrogen atoms, except for those at C1, are
omitted for clarity. Selected bond lengths [pm] and angles [◦]: Br1-Si1
220.7(1), Br2-Si1 221.7(1), Si1-N2 169.2(3), Si1-N1 169.7(3), C1-C2
136.9(5), C2-C3 145.5(5), C3-C4 133.1(5), C4-C5 149.0(6), C2-N1 141.9(5),
C4-N2 143.8(5), N2-Si1-N1 107.14(15), N2-Si1-Br1 110.0(1), N1-Si1-Br1
113.67(13), N2-Si1-Br2 114.0(1), N1-Si1-Br2 110.16(11), Br1-Si1-Br2
102.02(4).
Fig. 2 Molecular structure of 2. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms, except for those at C1, C3, C5 and
N2, are omitted for clarity. Selected bond lengths [pm] and angles [◦]:
Si1-N1 171.8(3), Si1-C5 177.7(3), Si1-Br1 222.0(1), Si1-Br2 223.8(1),
N1-C2 140.1(4), N2-C4 137.5(4), C1-C2 149.0(4), C2-C3 135.4(4), C3-C4
143.9(4), C4-C5 136.5(4), N1-Si1-C5 106.41(14), N1-Si1-Br1 110.82(9),
C5-Si1-Br1 115.00(11), N1-Si1-Br2 111.14(9).
function and acts as a nucleophile and a base (HBr scavenger)
at the same time which is evident by formation of [LAH HBr] 4.
Thus, a larger molar ratio of LALi (mole ratio of LALi:SiBr4 = 3:2)
increases the yield of 2 to 43%. Correspondingly, the yield of 3 is
somewhat reduced owing to the higher reactivity of SiBr4 towards
the stronger nucleophile LALi versus 2.
The composition and constitution of the compounds 2 and
3 were proven by elemental analysis and NMR spectroscopy
(1H, 13C, and 29Si).6 The 1H NMR spectrum of 2 reveals two
4
characteristic doublets at 4.25 and 4.76 ppm with JHH = 2.7 Hz
corresponding to the a- and the g-H atoms on the C4NSi ring,
respectively, while the broad singlet at 4.47 ppm belongs to the
N-H proton of the terminal amino group. Likewise, two doublets
are observed in the 1H NMR spectrum of 3 at 5.22 and 5.96 ppm
(4JHH = 3.0 Hz) which can be assigned to the a- and the g-H
atoms on the C4NSi ring, respectively. The 1H-decoupled 29Si
NMR spectrum of 2 shows a singlet at -31.6 ppm, while that of 3
exhibits two singlets at -31.2 and -66.1 ppm corresponding to the
Si atom in the C4NSi ring versus that of the exocyclic SiBr3 group.
The structures of 2 and 3 were established by single-crystal X-
ray diffraction analyses as shown in Fig. 2 and Fig. 3, respectively.7
Both structures consist of an almost planar six-membered SiNC4
ring, featuring an exocyclic amino group with the nitrogen atom
nearly within the plan of the six-membered SiNC4 ring. Despite of
the difference of the amino groups, the interatomic parameters
of the SiNC4 ring in 2 and 3 are practically identical. The
endocyclic C–C bond distance alternation in 2 [135.4(4) (C2-C3),
143.9(4) pm (C3-C4), 136.5(4) pm (C4-C5)] suggests only little p-
conjugation, and the same is true for the corresponding butadiene-
like moiety in 3. Interestingly, the C4-C5 distance in 2 and 3 is much
shorter than the corresponding one in the related LBPCl+ complex
[149.5(5) pm],4e indicating reduced C–C p-bond character in the
latter.
Fig. 3 Molecular structure of 3. Thermal ellipsoids are drawn at
50% probability level. Hydrogen atoms, except for those at C1, C3,
˚
and C5, are omitted for clarity. Selected bond lengths [A] and angles
[◦]:Br1-Si1 222.2(2), Br2-Si1 222.8(2), Br3-Si2 219.0(2), Br4-Si2 218.6(2),
Br5-Si2 218.6(2), Si1-N1 171.7(6), Si1-C5 179.6(7), Si2-N2 170.1(6),
N1-C2 141.0(9), N2-C4 144.8(8), C1-C2 149.1(9), C2-C3 136.0(10),
C3-C4 142.9(9), C4-C5 134.4(9), N1-Si1-C5 105.0(3), N1-Si1-Br1
111.4(2), C5-Si1-Br1 112.8(3), N1-Si1-Br2 112.5(2), C5-Si1-Br2 113.8(3),
Br1-Si1-Br2 101.68(9).
angle of the six-membered b-diketiminate ring are mostly respon-
sible for this outcome.9a In a next step, intramolecular N→Si
coordination in the bis-imine I favours in the presence of the bases
LALi and/or LAH deprotonation of a b-methyl group affording
the corresponding azaallylide-like intermediate II (Scheme 3). The
latter undergoes rearrangement and subsequent Br- elimination to
give the N-heterocyclic dibromosilane III with a terminal imino
group. A similar rearrangement of an phosphorus analogue with
C,N-chelation has recently been reported by Cowley and co-
workers.4e,9d Remarkably, 1,3-H migration (tautomerisation) from
the a-C atom of the CH2-Si ring moiety to the imido nitrogen
How could one explain the formation of 2 and 3? Although
their mechanism is still unknown, it is reasonable to assume that
the initial step is a nucleophilic substitution at silicon to give LiBr
and the SiBr3-substituted bis-imine I as an reactive intermediate
(Scheme 3). Precedents for related species with substitution at
the g-C ring atom have previously been reported for reactions
employing germanium8 and phosphorus chlorides.9 It has been
suggested that both steric congestion and the unfavourable bite
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6 Spectroscopic data for 2: 1H NMR (200.13 MHz, C6D6, 298 K): d
1.40 (s, 3 H, NCMe), 2.11 (s, 6 H, 2,6-Me2C6H3), 2.31 (s, 6 H, 2,6-
Me2C6H3), 4.25 (d, 1 H, 4JHH = 2.7 Hz, a-CH), 4.47 (s, 1 H, NH), 4.76
(d, 1H,4JHH = 2.7 Hz, g-CH), 6.91–7.02 (m, br, 6 H, 2,6-Me2C6H3).
1
13C{ H} NMR (100.61 MHz, C6D6, 298 K): d 18.2–22.6 (Me), 78.12(a-
1
C), 100.4 (g-C), 125.6–155.2 (NCMe, NCCH2, 2,6-Me2C6H3). 29Si{ H}
NMR (79.49 MHz, C6D6, 298 K): d -31.4 (s). EI-MS: m/z (%): 492.3(5,
[M]+), 476.9 (6, [M - Me]+, 371.8 (7, [M - NHR]+, R = 2,6-Me2C6H3),
306.2 (35, [M - SiBr2]+), 291.1 (30.0, [M - SiBr2 - Me]+), 187.1(100,
[HSiBr2]+). Elemental analysis (%) calcd for C21H24N2SiBr2: C, 51.23;
H, 4.91; N, 5.69. Found: C, 50.82; H, 4.70; N, 5.44. Spectroscopic
data for 3: 1H NMR (200.13 MHz, C6D6, 298 K): d 1.36 (s, 3 H,
NCMe), 2.21 (s, 6 H, 2,6-Me2C6H3), 2.28 (s, 6 H, 2,6-Me2C6H3), 5.22
4
(d, 1 H,4JHH = 3.0Hz, a-CH), 5.96 (d, 1H, JHH = 3.0Hz, g-CH),
1
6.81–6.90 (m, br, 6 H, 2,6-Me2C6H3). 13C{ H} NMR (100.61 MHz,
C6D6, 298 K): d 19.3–22.7 (Me), 92.7 (a-C), 101.1 (g-C), 128.6–
153.7 (NCMe, NCCH2, 2,6-Me2C6H3). 29Si{ H} NMR (79.49 MHz,
1
Scheme 3 Proposed mechanism for the formation of 2 via the reactive
intermediates I, II, and III.
C6D6, 298 K): d -31.2 (s, SiBr2), -66.1 (s, SiBr3). EI-MS: m/z (%):
759.6(55, [M]+), 744.6 (21, [M - Me]+, 679.7 (35, [M-Br]+), 491.9 (14,
[M-SiBr3]+), 477.0(13, [M-SiBr3-Me]+), 371.7(13, [M-SiBr3-NHR]+,
R = 2,6-Me2C6H3). Elemental analysis (%) calcd for C21H23N2Si2Br5: C,
33.23; H, 3.05; N, 3.69. Found: C, 33.05; H, 3.21; N, 3.70. Spectroscopic
data for 4: 1H NMR (200.13 MHz, C6D6, 298 K):d 1.89 (s, 12 H, 2,6-
Me2C6H3), 2.61 (s, 6 H, NCMe), 4.10 (s, 1 H, g-CH), 6.72–6.98 (m,
br, 6 H, 2,6-Me2C6H3), 11.45 (s, 2H, NH); EI-MS: m/z (%): 307.1 (9,
[M - Br]+), 187.1 (100, [M - NC6H3Me2]+. Spectroscopic data for 5: 1H
NMR (200.13 MHz, C6D6, 298 K): d 1.32 (s, 3 H, NCMe), 2.39 (s, 6
H, 2,6-Me2C6H3), 2.49 (s, 6 H, 2,6-Me2C6H3), 3.56 (s, 1 H, NCCH2),
3.97 (s, 1 H, NCCH2), 5.36 (s, 1 H, g-CH), 6.87–7.14 (m, br, 6 H, 2,6-
centre in III is favoured by the a-Si effect10 leading to the formation
of 2. In other words, the formation of 2 involves a double C–H
bond activation induced by silicon. Apparently, 3 results from
silylation of 2 with SiBr4 under HBr elimination.
In summary, silylation of LALi 1 with SiBr4 in the presence or
absence of TMEDA as auxiliary base leads to remarkably different
products: While the N-heterocyclic dibromosilane 5 is formed in
the presence of TMEDA, the same conversion without TMEDA
affords the distinct product 2 and its SiBr3-derivative 3 along with
the HBr adduct of LAH. Both 2 and 3 are promising precursors for
novel isolable N-heterocyclic silylenes. Respective investigations
are in progress.
1
Me2C6H3). 13C{ H} NMR (100.61 MHz, C6D6, 298 K):d 20.1, 20.2,
21.2 (NCMe, 2,6-Me2C6H3), 87.5 (NCCH2), 105.9 (g-C), 129.2–144.7
1
(NCMe, NCCH2, 2,6-Me2C6H3). 29Si{ H} NMR (79.49 MHz, C6D6,
298 K):d -55.1 (s). EI-MS: m/z (%): 492.8 (7, [M]+), 491.8 (20, [M - H]+,
476.9 (100, [M - Me]+). Elemental analysis (%) calcd for C21H24N2SiBr2:
C, 51.23; H, 4.91; N, 5.69. Found: C, 51.08; H, 4.96; N, 5.99.
7 Crystal data for 2:† C21H24Br2N2Si, M = 492.33, monoclinic, space
˚
group P21/a, a = 14.9450(4), b = 8.2720(2), c = 17.7852(4) A, b =
◦
104.903(3) . V = 2124.73(9) A , Z = 4, rcalc = 1.539 Mg m-3, m(Mo
3
Acknowledgements
˚
Ka) = 3.879 mm-1, 13787 collected reflections, 3731 crystallographi-
cally independent reflections [Rint = 0.0370], 2656 reflections with I >
2s(l), qmax = 25.00◦, R(Fo) = 0.0305 (I > 2s(l)), wR(Fo2) = 0.0708
(all data), 240 refined parameters. Crystal data for 3: C21H23Br5N2Si2,
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
M = 759.14, orthorhombic, space group P212121, a = 11.1177(5),
3
˚
˚
b = 13.9333(7), c = 16.9301(4) A. V = 2622.6(2) A , Z = 4, rcalc
=
Notes and References
1.923 Mg m-3, m(Mo Ka) = 7.766 mm-1, 13453 collected reflections,
4586 crystallographically independent reflections [Rint = 0.0687], 3015
reflections with I > 2s(l), qmax = 25.00◦, R(Fo) = 0.0410 (I > 2s(l)),
wR(Fo2) = 0.0690 (all data), 276 refined parameters, Flack parameter:
005(14). Crystal data for 5: C21H24Br2N2Si, M = 492.33, orthorhombic,
1 For review, see: L. Bourget-Merle, M. F. Lappert and J. R. Severn,
Chem. Rev., 2002, 102, 3031.
2 Examples for stabilization of unusual oxidation states, see and the cited
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2008, 27, 457; (b) S. P. Green, C. Jones and A. Stasch, Science, 2007,
318, 1754.
˚
space group Pna21, a = 18.7680(4), b = 7.8976(2), c = 14.2285(5) A.
3
V = 2108.98(10) A , Z = 4, rcalc = 1.551 Mg m-3, m(Mo Ka) =
˚
3 See, for example: (a) F. Basuli, U. J. Kilgore, D. Brown, J. C. Huffman
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3.908 mm-1, 11209 collected reflections, 3592 crystallographically
independent reflections [Rint = 0.0431], 2875 reflections with I > 2s(l),
qmax = 25.00◦, R(Fo) = 0.0294 (I > 2s(l)), wR(Fo2) = 0.0521 (all data),
240 refined parameters, Flack parameter: 008(8). Crystals of 2, 3, and
5 were each mounted on a glass capillary in perfluorinated oil and
measured in a cold N2 flow. The data of compounds 2, 3 and 5 were
collected on an Oxford Diffraction Xcalibur S Sapphire at 150 K (Mo-
˚
Ka radiation, l = 0.71073 A). CCDC 705079–705081.
8 B. Ra¨ke, F. Zu¨lch, Y. Ding, J. Prust, H. W. Roesky, M. Noltemeyer and
H.-G. Schmidt, Z. Anorg. Allg. Chem., 2001, 627, 836.
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McDonald and M. J. Ferguson, Inorg. Chem., 2004, 43, 734; (c) P. B.
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10 a-Si effect: a-Silyl-substituted hydrocarbons contain activated C–
H bonds (C-–H+) through negative hyperconjugation. See: A. R.
Bassindale and P. G. Taylor, Activating and directive effects of silicon,
in The Chemistry of Organic Silicon Compounds, ed. S. Patai and
Z. Rappoport, Wiley, New York, 1989, part 2, chapter 14, pp. 893–
963, and cited references therein.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 421–423 | 423
©