Activation of ammonia by a Si - O double bond and formation of a unique pair of sila-hemiaminal and silanoic amide tautomers

Article abstract of DOI:10.1021/ja1031024

The new silanone complex 3 is accessible in 82% yield and is capable of undergoing addition of ammonia under mild conditions, yielding the sila-hemiaminal 4 and, at the same time, its unique tautomer 5, the first silanoic amide. The unexpected formation of 3 is due to the presence of the basic exocyclic methylene group in the C₃N₂ ligand backbone. Strikingly, the tautomers 4 and 5 are in equilibrium in solution and can be cocrystallized in benzene or THF solutions having a SiOH...O Si hydrogen bond as confirmed by single-crystal X-ray diffraction analysis.

Full text of DOI:10.1021/ja1031024

Published on Web 05/04/2010
Activation of Ammonia by a SidO Double Bond and Formation of a Unique
Pair of Sila-Hemiaminal and Silanoic Amide Tautomers
Yun Xiong,† Shenglai Yao,† Robert Mu¨ller,‡ Martin Kaupp,‡ and Matthias Driess*,†
Institute of Chemistry: Metalorganics and Inorganic Materials, Technische UniVersita¨t Berlin, Strasse des 17.
Juni 135, Sekr. C2, D-10623 Berlin, Germany, and Institute of Physical and Theoretical Chemistry,
UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received April 13, 2010; E-mail: matthias.driess@tu-berlin.de
Scheme 1. DMAP Stabilized Silanone 3 and Its Reactivity Toward
Currently, the activation of ammonia is attracting considerable
interest because of its promising role for the development of facile
transformations into value-added products. Striking examples
comprise N-H bond splitting by a transition metal,1 N-heterocyclic
carbenes (NHCs)2 and even by heavier group 14 element car-
bene analogues,3,4 including ylide-like silylene 1, LSi: (L )
N(R)C(dCH2)CHdC(Me)N(R), R ) 2,6-iPr2C6H3)5 and its
Ni(CO)3 complex.6 In spite of recent progress, the activation of
N-H bonds of ammonia by coordinatively unsaturated sites
(nonmetals or metals) is still surprisingly rare. Perhaps the earliest
precedent reported for N-H activation of ammonia and primary
organoamines by Lewis acidic substrates is their nucleophilic
addition to CdO double bonds in aldehydes and ketones (R2CdO)
to give hemiaminals followed by an elimination step to form imines
and water.7 Since hemiaminals are generally unstable and imines
with a NsH moiety spontaneously polymerize, they need special
environments to be detectable.8 These classical features inspired
us to investigate whether ammonia reacts with a SidO double bond
to give isolable sila-hemiaminals (R2Si(OH)NH2) and/or iminosi-
lanes (R2SidNH). To the best of our knowledge, the chemistry of
silanones toward ammonia is currently unknown. Since silicon
analogues of ketones are elusive species, it seems appealing to
explore the chemistry of SidO double bonds by using donor-
stabilized silanone precursors. Recently, we reported the synthesis
and isolation of the first N-donor supported silanoic ester
(L′Si(dO)OR; L′ ) LH)9a and remarkably stable NHC-silanone
complexes (NHCfSi(L)dO),9b,c,10 starting from silylene 1.5b We
now learned that the NHC donor in the latter system can be replaced
by the weaker coordinated p-dimethylaminopyridine (DMAP)
ligand, leading to the corresponding silanone precursor 3 (Scheme
1) via oxidation of the silylene-DMAP adduct 2 (see Supporting
Information). Strikingly and in contrast to related NHC-stabilized
SidO complexes, 3 reacts readily with ammonia under mild
conditions. Herein we report for the first time the addition of
ammonia to the SidO double bond of 3, affording a unique pair of
tautomer products.
Ammonia
solution. According to NMR spectroscopy and ESI-MS measure-
ments of the resulting clear solution, DMAP in 3 is completely
released and one equimolar amount of ammonia has been added to
the SidO subunit, presumably via intermediate A. Furthermore,
the 1H NMR spectrum reveals that the expected adduct, sila-
hemiaminal 4, is in equilibrium with its striking tautomer, silanoic
amide 5 (Scheme 1), owing to the pronounced basicity of the
exocyclic methylene group on the C3N2 ligand backbone.5a,11
The equilibrium between 4 and 5 depends both on the polarity of
the solvent and on concentration. Although the amount of 5
increases in more concentrated solution (in both d8-THF and C6D6),
in all solutions the amount of 4 always dominates over 5 especially
in polar solvents such as d8-THF. While 4 shows only one set of
resonances at room temperature, by cooling of the solutions to -60
°C, the signals of 4 split completely into two sets of signals, but
the resonance signals of 5 remain as one set. The two sets of signals
for 4 coalesce at 0 °C. This observation suggests the presence of a
SisOsH· · · OdSi hydrogen bond between 4 and 5, which results
in two different sets of resonances for 4 in the 1H NMR spectrum:
one set from the hydrogen-bonded 4, the other set from “free” 4.
The coalescence of these two sets of signals above 0 °C can be
rationalized by their rapid exchange (“free” vs hydrogen-bonded
4) at the NMR time scale.
Similar to procedures previously reported,9,10 3 is simply
accessible in 83% yield by oxygenation of 2 with N2O in toluene.
It has been fully characterized, including by X-ray diffraction
analysis. The spectroscopic and structural features of 3 are similar
to those of related NHC-silanone adducts (see Supporting Informa-
tion).
In fact, crystallization in benzene and THF solutions leads to
single crystals of a 1:1 mixture of the tautomers 4 and 5. Its
molecular structure has been established by an X-ray diffraction
analysis (Figure 1). Two independent molecules of tautomeric pairs
are present in the asymmetric unit. In each pair the Si(O)NH2
moieties of 4 and 5 prefer a gauche conformation and are connected
by an OH · · · O hydrogen bridge with O · · · O distances of 2.570
and 2.589 Å, which fall in the common range for hydrogen bonds.
This is in contrast to the structural features of the only known sila-
Crystals of 3 are even stable in dry air, marginally soluble in
hydrocarbons and ethereal solvents, but very soluble in dichlo-
romethane. Exposure of a pale yellow suspension of 3 in toluene
to dry ammonia gas at room temperature leads to a clear colorless
† Technische Universita¨t Berlin.
‡ Universita¨t Wu¨rzburg.
9
6912 J. AM. CHEM. SOC. 2010, 132, 6912–6913
10.1021/ja1031024  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Energies from Full Optimizations at the RI-BP86/
def2-TZVP Level of Theory of the Reaction Products of Compound
3 with Ammonia To Give 4, 5, and Its Hydrogen-Bonded Pair
Figure 1. Molecular structure of the H-bonded adduct between 4 and 5.
Thermal ellipsoids are drawn at 50% probability level. H atoms (except
those at C1, N3, N6, O1) and solvent benzene molecules are omitted for
clarity. The H atoms at O1, N3, and N6 were located from the difference
electron density map and refined. There are two pairs of molecules (see
Supporting Information); only one tautomeric pair is shown here. Selected
interatomic distances (Å) and angles (deg): Pair 1: Si1-O1 1.607(2),
Si2-O2 1.545(2), C1-C2 1.383(3), C4-C5 1.463(3), C30-C31 1.499(4),
C33-C34 1.516(4), Si1-N1 1.742(2), Si1-N2 1.736(2), Si1-N3 1.681(3),
Si2-N6 1.677(2), Si2-N5 1.793(2), Si2-N4 1.817(2), O2 · · ·H17 1.721,
O1-H17 0.872, O1-O2 2.570; O1-H17-O2 172.9; Pair 2: Si1-O1
1.607(2), Si2-O2 1.546(2), C1-C2 1.404(4), C4-C5 1.432(3), C30-C31
1.494(4), C33-C34 1.508(4), Si1-N11.746(2), Si1-N2 1.737(2), Si1-N3
1.677(2), Si2-N6 1.683(2), Si2-N5 1.795(2), Si2-N4 1.805(2), O2 · · ·H17
1.720, O1-H17 0.854, O1-O2 2.589; O1-H17-O2 173.2.
accordance with DFT calculations, the latter can readily undergo
tautomerization to furnish the unprecedented silanoic amide deriva-
tive 5. Compounds 4 and 5 are in equilibrium in solution and can
undergo intermolecular stabilization in solution and in the solid
state via SiOH · · · OdSi interaction. The relatively weakly coordi-
nated DMAP ligand in 4 calls for future studies of EsH bond
activation of small molecules by SidO double bonds.
Acknowledgment. We are grateful to the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie for financial
support.
hemiaminal, tBu2Si(OH)NH2 characterized structurally which forms
also a dimer but possesses NH · · ·O and OH · · ·N instead of OH · · ·O
bridges.12 The Si atoms in the subunits 4 and 5 are tetrahedral
coordinated and embedded in slightly puckered six-membered
C3N2Si rings. The most noteworthy structural feature is the SisO
distance (1.607(2) Å) in 4 and the much shorter SisO distance
(1.545(2), 1.546(2) Å) in 5, suggesting a SisO single bond in 4
and significant SidO double bond character in 5.13 Accordingly,
one of the exocyclic CsC bonds in 4 [C1sC2 (1.383(3), 1.404(4)
Å)] represents a CdC double bond, whereas both exocyclic CsC
bonds in 5 [C30-C31 (1.499(4), 1.508(4) Å) and C33-C34
(1.516(4), 1.494(4) Å)] are CsC single bonds. All SisNH2
distances, ranging from 1.676(3) to 1.683(2) Å, are shorter than
the endocyclic SisN distances (1.736(2)-1.746(2) Å).
To gain insight into the thermodynamic features of these systems,
DFT calculations (RI-BP86/def2-TZVP level of theory) have been
performed for activation of ammonia by compound 3 (Scheme 2).
In addition to the results in the gas phase, a continuum solvent
model has been applied for the conversions in toluene. Consistent
with the experimental observations, the calculations revealed that
displacement of the DMAP donor and addition of ammonia to the
SidO double bond in 3 can lead to both tautomers 4 and 5 in
different ratios and can be dependent on the presence of a solvent
and on concentration (Scheme 2). Likewise, our calculations support
the formation of the tautomer pair 4-5 via a hydrogen bond.
In summary, we reported here the unique reactivity of a SidO
double bond toward ammonia. In contrast to NHC-stabilized
silanones, the DMAP-stabilized SidO precursor 3 is capable
undergoing addition of ammonia under mild conditions and
extrusion of the DMAP ligand to give the sila-hemiaminal 4. In
Supporting Information Available: Experimental details for the
syntheses and spectroscopic data of starting materials 2 (precursors for
3), 3, 4, and 5 (PDF) and crystallographic data for 2, 3, and the adduct
4-5 (CIF), respectively, as well as computational details and coordi-
nates of DFT-optimized structures. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) (a) Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080. (b)
Braun, T. Angew. Chem., Int. Ed. 2005, 44, 5012.
(2) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G.
Science 2007, 316, 439.
(3) Peng, Y.; Ellis, B. D.; Wang, X.; Power, P. P. J. Am. Chem. Soc. 2008,
130, 12268.
(4) Chase, P. A.; Stephan, D. W. Angew. Chem. 2008, 120, 7543.
(5) (a) Driess, M.; Yao, S.; Brym, M.; van Wu¨llen, C.; Lentz, D. J. Am. Chem.
Soc. 2006, 128, 9628. (b) Jana, A.; Schulzke, C.; Roesky, H. W.
J. Am.Chem. Soc. 2009, 131, 4600.
(6) Meltzer, A.; Inoue, S.; Pra¨sang, C.; Driess, M. J. Am. Chem. Soc. 2010,
132, 3038.
(7) (a) Liebig, J. Julius Liebigs Ann. Chem. 1835, 14, 133. (b) Schiff, H. Ann.
1864, 118, 131. (c) Chudek, J. A.; Foster, R.; Young, D. J. Chem. Soc.,
Perkin Trans. 2 1985, 1285, and cited references therein.
(8) Iwasawa, T.; Hooley, R. J.; Rebek, J., Jr. Science 2007, 317, 493.
(9) (a) Yao, S.; Xiong, Y.; Brym, M.; Driess, M. J. Am. Chem. Soc. 2007,
129, 7268. (b) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2009,
131, 7562. (c) Xiong, Y.; Yao, S.; Mueller, R.; Kaupp, M.; Driess, M.
Nature Chem. 2010, in press; DOI: 10.1038/nchem.666.
(10) (a) Yao, S.; Xiong, Y.; Driess, M. Chem.sEur. J. 2010, 16, 1281. (b) Xiong,
Y.; Yao, S.; Driess, M. Chem. Asian. J. 2010, 5, 322.
(11) Driess, M.; Yao, S.; Brym, M.; von Wu¨llen, C. Angew. Chem., Int. Ed.
2006, 45, 6730.
(12) Reiche, C.; Kliem, S.; Klingebiel, U.; Noltemeyer, M.; Voit, C.; Herbst-
Irmer, R.; Schmatz, S. J. Organomet. Chem. 2003, 667, 24.
(13) Epping, J.-D.; Yao, S.; Karni, M.; Apeloig, Y.; Driess, M. J. Am. Chem.
Soc. 2010, 132, 5443.
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