Diarylstannylene activation of hydrogen or ammonia with arene elimination

Article abstract of DOI:10.1021/ja805358u

Treatment of the stannylenes SnAr′₂ (Ar′ = C₆H₃-2,6(C₆H₃-2,6-Prⁱ₂)₂), SnAr₂^# (Ar^# = C₆H₃-2,6-(C₆H

Full text of DOI:10.1021/ja805358u

Published on Web 08/21/2008
Diarylstannylene Activation of Hydrogen or Ammonia with Arene Elimination
Yang Peng, Bobby D. Ellis, Xinping Wang, and Philip P. Power*
Department of Chemistry, UniVersity of California DaVis, One Shields AVenue, DaVis, California 95616
Received July 21, 2008; E-mail: pppower@ucdavis.edu
Activation of H2 by transition metal complexes has been known
and studied for many years.1-5 Since 1983 the synthesis and
characterization of stable H2 complexes, of which the first was
Mo(CO)3(PPri3)(η2-H2),6 has permitted the mechanism of the activation
process to be studied in great detail. This generally occurs by either a
homolytic or heterolytic pathway. The former involves side-on
σ-donation by the H-H σ-bond and back-donation by the metal into
the H-H σ* orbital.6-13 For heterolytic cleavage, the η2-H2 unit
becomes polarized as in Hδ--Hδ+ to afford hydride transfer to the
metal with inter- or intramolecular H+ migration to a Lewis base.
Surprisingly the related activation of N-H bonds via transition metal
complexes is relatively rare,14 and the first example of NH3 addition
to afford a product that contained both terminal M-NH2 and M-H
groups as in eq 1 was only recently reported.15
from which orange crystals of 1 were obtained.20a X-ray crystal-
lographic examination of the product 1 showed that it had an
identical Sn(II) bridging hydride structure to those recently re-
1
ported.21 Examination of the H NMR spectrum also revealed a
significantly deshielded signal at ca. 9.13 ppm which matched the
Sn-H chemical shifts in Sn(II) hydrides synthesized by reduction
using boron or aluminum hydrides. The 1H NMR spectrum of the
reaction mixture also displayed signals due to Ar′H.20f Subsequent
workup of the solution gave crystals of the arene which were shown
to be identical to those of an authentic sample.
In contrast to these studies the corresponding chemistry for stable
main group molecular compounds is virtually nonexistent. In 2005,
the first example, which involved the activation of H-H bonds by
the germanium species Ar′GeGeAr′ (Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2
or C6H3-2,6-Dipp2), was reported.16 In 2006, the reversible activa-
tion of H-H bonds by phosphine-boranes was described by Stephan
and co-workers,17 and facile splitting of hydrogen and ammonia
by stable carbenes was published by Bertrand and co-workers in
2007.18 In the latter case the reactivities of several carbenes with
H2 and NH3 were investigated and it was found that mono(amino)
carbenes readily activate both H2 and NH3. The hypothesis that
activation of H2 occurred via the route described in Scheme 1 was
supported by the calculations of Schoeller.18 We now report the
activation of H2 and NH3 by the heavier group 14 element carbene
analogue SnAr′2 (Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2) which occurs
under mild conditions with arene elimination. We also show that
the reactivity of stannylenes toward H2 is dependent on the tin
substituents.
The same procedure as that employed for 1, using D2 instead of
H2 (eq 3), afforded the deuteride 2.20b X-ray crystallography showed
that the orange crystalline product had almost the same unit cell
parameters in which the length and volume are within 3 standard
1
deviations of those measured for 1. The H NMR spectrum of 2
revealed no trace of the Sn-H signal at 9.13 ppm seen for 1. The
deuterium (2H) NMR spectrum of 2 displayed a broad signal at
8.95 ppm, which is consistent with the formation of [Ar*Sn(µ-
D)]2.22 Further workup of the mother liquor afforded Ar′D examined
by 1H and 2H spectroscopy which suggests that the reaction
proceeds by the initial addition of H2 or D2 (as in Scheme 1, E )
Sn) to afford Ar′2SnH2 or Ar′2SnD2 which eliminates Ar′H or Ar′D
due to the steric pressure in the molecule (eq 5).20f,g
Scheme 1. Schematic Representation of the Activation of by a
18
Divalent Group 14 Molecule, E ) C
Similar experiments involving the reaction of SnAr# 23 (Ar# )
2
24
C6H3-2,6(C6H2-2,4,6-Me3)2) or Sn{N(SiMe3)2}2 with H2 under
identical conditions yielded no evidence of hydrogenation after
several days.20d,e The electronic spectra of SnAr′2 (λmax ) 600
nm),19 SnAr# (λmax ) 553 nm),23 and Sn{N(SiMe3)2}2 (λmax
)
2
487 nm) suggest an inverse correlation between the HOMO-LUMO
gap and reactivity.19 This supports the view that the reactivity of
SnAr′2 may involve enhanced triplet character corresponding to its
wide C-Sn-C angle (117.6(8)°).
19
Reaction of a dark blue solution of SnAr′2 with H2 gas at
60-70° for 1 h (eq 2) resulted in a color change to dark green
9
12268 J. AM. CHEM. SOC. 2008, 130, 12268–12269
10.1021/ja805358u CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
(12) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201.
(13) Heinekey, D. M.; Lledas, A.; Lluch, J. M. Chem. Soc. ReV. 2004, 33, 175.
(14) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. Inorg. Chem. 1987, 26,
971.
(15) Zhao, J.; Goldman, A. S.; Hartwig, J. S. Science 2005, 307, 1080.
(16) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127,
12232.
(17) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science
2006, 314, 1124.
(18) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G.
Science 2007, 316, 439.
(19) Spikes, G. H.; Peng, Y.; Fettinger, J. C.; Power, P. P. Z. Anorg. Allg. Chem.
2006, 632, 1005.
(20) All manipulations were carried out under anaerobic and anhydrous
conditions. (a) 1. [Ar′Sn(µ-H)]2: A solution of SnAr′2 (1.15 g, 1.26 mmol)
in toluene (50 mL) was stirred at 65° for 2 h under a H2 atmosphere to
give a dark green solution. The mixture was concentrated to ca. 10 mL
under vacuum which afforded orange crystals of 1 upon cooling to ca.-
16°C. Yield: 39%. 1H NMR (C6D6): 0.93 (d, 6H, 3JHH ) 6.6 Hz, CH(CH3)2,
Figure 1. Thermal ellipsoid (30%) drawing of 3. Hydrogen atoms, except
those at nitrogen, are not shown for clarity.
3
3
The addition of an excess of dry ammonia to a dark blue solution
of SnAr′2 in toluene rapidly discharged the color. Concentration of
the solution produced colorless crystals of the new species 3 (eq 4).20c
X-ray crystallography25 afforded a dimeric structure as illustrated in
Figure 1. The tin centers were symmetrically bridged by the NH2
ligands in which the two hydrogens were located in the electron density
map. The rhombohedral Sn2N2 core is planar with the angles NSnN
) 76.38(5)° and SnNSn ) 103.62(5)°. The tins have terminally bound
Ar′ ligands which yield trigonal pyramidal coordination as shown by
the sum of the angles at tin of 266.16°. The Sn-N bridging distances
(2.1913(13) and 2.1918(13) Å) are in good agreement with the reported
Sn-N distance 2.21 Å in {Ar*Sn(µ-NH2)}2 (Ar* ) C6H3-2,6(C6H2-
2,4,6-Pri3)2)26 which differs from 3 in that it has a bulkier terphenyl
ligand. The IR spectrum displayed two weak sharp bands at 3357 and
3260 cm-1 that are due to the two N-H stretching modes of the -NH2
groups. These frequencies are close to those at 3370 and 3290 cm-1
observed for [Ar*Sn(µ-NH2)]2.26 The reaction probably proceeds in
the same manner as that described in eq 5 to generate an Ar′2Sn(H)NH2
intermediate which eliminates Ar′H to afford 3.
1.02 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2, 1.04 (d, 6H, JHH ) 6.6 Hz,
CH(CH3)2, 1.11 (d, 6H, 3JHH ) 6.6 Hz, CH(CH3)2), 3.00 (overlap septets,
4H, CH(CH3)2), 7.03 (d, 4H,3 JHH ) 7.5 Hz, m-DippH), 7.10 (m, overlap
3
1
ArH), 7.30 (t, 2H, JHH ) 7.5 Hz, p-DippH), 9.13 (s, 1H, JSn-H ) ca. 89
Hz, Sn-H). 119Sn{1H} NMR: δ 657. (b) 2. [Ar′Sn(µ-D)]2: A solution of
SnAr′2 (1.05 g, 1.15 mmol) in toluene (50 mL) was stirred at 65 °C for 2 h
under a D2 atmosphere to give a dark green solution. The mixture was
concentrated to ca. 10 mL under vacuum which afforded orange crystals
of 2 upon cooling to ca.-16 °C. Yield: 45%. 1H NMR (C6D6): 0.93 (d,
3
3
6H, JHH ) 6.6 Hz, CH(CH3)2, 1.02 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2),
3
3
1.04 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2), 1.11 (d, 6H, JHH ) 6.6 Hz,
CH(CH3)2), 3.00 (overlap septets, 4H, CH(CH3)2), 7.03 (d, 4H, 3JHH ) 7.5
Hz, m-DippH 7.10-7.28 (m, overlap ArH),) 2H NMR (C7H8): 8.98 (s, 1D).
119Sn{1H} NMR (C7D8): δ 610. (c) 3. [Ar′Sn(µ-NH2)]2: To a deep blue
solution of SnAr′2 (0.45g, 0.5 mmol) in toluene (50 mL) at-78° was added
several drops of liquid ammonia. The solution became light yellow.
Warming to room temperature produced a colorless solution, which was
concentrated to ca. 30 mL under reduced pressure to give colorless crystals
that were identified as 3 on the basis of NMR spectroscopy and X-ray
crystallography. Yield: 55%. Mp: 120-125°. 1H NMR (C7D8): δ 0.72 (s,
2H, NH2), 1.48 (d, 12H, 3JHH ) 6.6 Hz, CH(CH3)2), 1.63 (d, 12H, 3JHH
)
6.6 Hz, CH(CH3)2), 3.42 (septets, 4H, CH(CH3)2), 7.48 (t, 2H, 3JHH ) 6.6
3
3
Hz, p-DippH), 7.58 (t, 1H, JHH ) 6.6 Hz, p-C6H3), 7.65 (d, 4H, JHH
)
6.6 Hz, m-Dipp)), 7.68 (d, 2H,3 JHH )7.5 Hz, m-C6H3). 13C{1H}
NMR(C7D8): 24.31 (CH(CH3)2), 26.55 (CH(CH3)2), 31.22 (CH(CH3)2),
126.45 (p-C6H3), 128.91 (p-Dipp), 129.54 (i-Dipp), 130.24 (m-C6H3), 138.17
(i-Dipp), 147.15 (o-C6H3), 157.42 (i-C6H3). 119Sn{1H} NMR (C7D8): δ
280.26. IR (Nujol): ν 3357, 3260 cm-1 (ν NH2, weak). (d) 4. SnAr#2: A
purple solution of SnAr#2 (0.75 g, 1 mmol) in toluene (50 mL) was stirred
at 70° for 6 days under a H2 atmosphere. No color change was observed.
The solvent was pumped off, and 1H and 119Sn NMR spectroscopy showed
that it was the reactant SnAr#2. Recovered yield >80%. 1H NMR (C6D6):
1.90 (s, 12H, o-CH3), 2.21 (s, 6H, p-CH3), 6.77 (s, 4H, m-Mes), 6.80 (d,
2H, JHH ) 7.5 Hz, m-C6H3), 7.12 (t, 1H, JHH ) 7.8 Hz, p-C6H3). 119Sn
{1H} NMR: δ 635.23. (e) 5. Sn{N(SiMe3)2}2: An orange solution of
Sn{N(SiMe3)2}2 (1.09 g, 2.5 mmol) in toluene (50 mL) was stirred at 70°
for 3 days under a H2 atmosphere. No color change was observed. The
solvent was pumped off, and 1H, 119Sn NMR spectroscopy indicated
unchanged Sn{N(SiMe3)2}2. Recovered yield >85%. 1H NMR (C6D6): 0.28
(s, 18H, -CH3). 119Sn{1H} NMR: δ 767.27a,b(f) Ar′H (from mother liquor):
In summary we have shown that the stannylenene SnAr′219 reacts
with H2 or NH3 to afford the products 1 or 3. The corresponding
lack of reactivity of H2 toward SnAr#2 or Sn{N(SiMe3)2}2 suggests
that the ability of SnAr′2 to activate H2 may be associated with
increased triplet character in its ground state. The reaction differs
from that of carbenes in that an arene is eliminated. However the
initial step probably proceeds according to Scheme 1 (E ) Sn).
Acknowledgment. We thank the U.S. Department of Energy
Office of Basic Energy Sciences DE-FG02-07ER46475 for financial
support. B.D.E. thanks the NSERC of Canada for a postdoctoral
fellowship. We thank Dr. Jeff De Ropp and Prof. R. A. Andersen
for helpful discussions.
3
1H NMR (C6D6): δ 1.11 (d, 6H, JHH ) 6.6 Hz, CH(CH3)2), 1.14 (d, 6H,
3JHH ) 6.6 Hz, CH(CH3)2), 2.90 (septets, 4H, CH(CH3)2), 6.89 (s, i-C6H3),
3
7.04 (s, i-Dipp), 7.09 (d, 4H, m-C6H3), 7.10 (d, 4H, JHH ) 6.6 Hz,
m-Dipp)), 7.22 (t, 1H, 3JHH ) 7.5 Hz, p-C6H3), 7.31 (t, 4H, 3JHH ) 6.6 Hz,
p-Dipp). (g) Ar′D (mother liquor): 1H NMR (C6D6): δ 1.11 (d, 6H, 3JHH
)
6.6 Hz, CH(CH3)2), 1.14 (d, 6H, 3JHH ) 6.6 Hz, CH(CH3)2), 2.89 (septets,
3
4H, CH(CH3)2), 7.04 (s, i-Dipp), 7.09 (d, 4H, m-C6H3), 7.10 (d, 4H, JHH
) 6.6 Hz, m-Dipp)), 7.22 (t, 1H, 3JHH ) 7.5 Hz, p-C6H3), 7.31 (t, 4H, 3JHH
Supporting Information Available: Crystallographic data for 3
(CIF), unit cell parameters for 1 and 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
) 6.6 Hz, p-Dipp). 2H NMR (C6H14): 6.98 (s, i-C6H3).
(21) Rivard, E.; Fischer, R. C.; Wolf, R.; Peng, Y.; Merrill, W. A.; Schley,
N. D.; Zhu, Z.; Pu, L.; Fettinger, J. C.; Teat, S. J.; Nowik, I.; Herber, R. H.;
Takagi, N.; Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2007, 129, 16197.
(22) Eichler, B. E.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 8785.
(23) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics
1997, 16, 1920.
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