Reversible complexation of isocyanides by the distannyne Ar′SnSnAr′ (Ar′ = C6H3-2,6(C 6H3-2,6-iPr2)2)

Article abstract of DOI:10.1039/b919828h

The reaction of the distannyne Ar′SnSnAr′ (Ar′ = C ₆H₃-2,6(C₆H₃-2,6-ⁱPr ₂)₂) with tert-butyl or mesityl isocyanide afforded the bis-adducts Ar′SnSnAr′(CNBu^t)₂

Full text of DOI:10.1039/b919828h

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Reversible complexation of isocyanides by the distannyne Ar0SnSnAr0
(Ar0 = C6H3-2,6(C6H3-2,6-iPr2)2)w
Yang Peng, Xinping Wang, James C. Fettinger and Philip P. Power*
Received (in Berkeley, CA, USA) 23rd September 2009, Accepted 7th December 2009
First published as an Advance Article on the web 8th January 2010
DOI: 10.1039/b919828h
The reaction of the distannyne Ar0SnSnAr0 (Ar0 = C6H3-
2,6(C6H3-2,6-iPr2)2) with tert-butyl or mesityl isocyanide
afforded the bis-adducts Ar0SnSnAr0(CNBut)2 or Ar0SnSnAr0
(CNMes)2 in which the isonitriles are reversibly bound under
ambient conditions.
report the reactions of the distannyne Ar0SnSnAr0
with tBuNC and MesNC which yield
adducts
Ar0SnSnAr0(CNBut)2 and Ar0SnSnAr0(CNMes)2
1
2
: 1
2
3
exclusively. The most striking feature of the formation of 2
and 3 is that the reaction is reversible under ambient conditions
and has similarities to the reversible complexation of ethylene
by distannynes near room temperature.6
Several heavier group 14 element analogues of ketenimines
(which can also be viewed as isonitrile adducts of the heavier
element carbene congeners) have been isolated with the use of
bulky substituents.1 Their chemistry has been shown to differ
considerably from that of their carbon analogues. For exam-
ple, the reactions of isonitriles with alkynes generally afford
cyclopropenimine products derived from cycloadditions.2,3 In
Complex 2z was prepared by the dropwise addition of
excess ButNC to a concentrated toluene solution of 1. This
afforded a deep red solution which, upon cooling to ꢀ18 1C,
gave 2 as very dark red crystals in 70% yield. In a similar way
the reaction of MesNC with 1 gave dark red crystals of 3 in
67% yield (Scheme 2). Both 2 and 3 are stable in the solid state
but dissociate to 1 and the corresponding isocyanides when
redissolved in hydrocarbon solvents. No 1 : 1 complexes
could be detected by NMR spectroscopy.
contrast, the treatment of digermyne Ar0GeGeAr0 (Ar0
=
C6H3-2,6(C6H3-2,6-iPr2)2) with isonitriles afforded 1 : 1 or
2 : 1 isonitrile adducts in which one or both of the group 14
element atoms are complexed as in Ar0GeGeAr0(CNBut) or
Ar0GeGeAr0(CNMes)2 (Mes = C6H2-2,4,6-Me3).4 The structural
details of the latter species—strongly pyramidal germanium
coordination, short C–N multiple bonds and Ge–C(isonitrile)
distances consistent with single bonds—suggest that its bonding
is most accurately represented by structure C in Scheme 1.
A similar reaction between Me3SiNC and the disilyne
RSiSiR (R = SiiPr{CH(SiMe3)2}2) was reported by Sekiguchi
and co-workers to afford the 2 : 1 bis-adduct RSiSiR(CNSiMe3)2
which was proposed to have the silaketenimine structure A
with a significant zwitterionic contribution from B due to its
somewhat short Si–C (isonitrile) bond and pyramidalization
of the coordination geometry at the Si atoms.5 Herein we
When red crystals of 3 are dissolved in hexane at room
temperature, a green solution is obtained whose electronic
spectrum displays two absorptions at 410 and 597 nm which
are characteristic of the p - n+ and n - n+ transitions of 17
ꢀ
indicating that 3 is mainly dissociated to 1 and MesNC.
Cooling the solution to ca. ꢀ40 1C regenerated the red color
characteristic of 3 (Fig. 1). The UV-vis spectrum of the ꢀ40 1C
solution afforded an absorption at 514 nm. Warming the
solution to room temperature restored the absorptions at
410 and 597 nm with disappearance of the feature at 514 nm
Scheme 2 Reactions of Ar0SnSnAr0 with ButNC or MesNC.
Scheme 1 Resonance forms bis(isonitrile) complexes of RMMR
species (M = Si, Ge, Sn, Pb; R = bulky ligand; R0 = But, Mes, SiMe3).
Department of Chemistry, University of California, Davis, CA 95616,
USA. E-mail: pppower@ucdavis.edu; Fax: +1-530-752-8995;
Tel: +1-530-752-6913
w Electronic supplementary information (ESI) available: Crystallo-
graphic data for 2 and 3 (CIF), determination of association energy
of 2 and 3 by variable temperature 1H NMR and variable temperature
behavior of the UV-vis spectrum illustrating the dissociation of 3 to 1
and free MesNC. CCDC 748927, 748928. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b919828h
Fig. 1 Annealing solution of 3 at ca. ꢀ40 1C (red, left) which upon
warming to 25 1C (green, right) indicated dissociation to 1 and free
MesNC.
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(see ESIw). Complex 2 displays similar UV-vis spectral
characteristics to those of 3. The IR spectrum of 2 displays
bands at 2162 and 2175 cmꢀ1 which are shifted to higher
frequencies than the free ligand (ButNC, 2134 cmꢀ1). This
trend has also been observed in other metal isocyanide
complexes.8–10 The IR spectrum of 3 displays a strong broad
C–N stretching band at 2278 cmꢀ1 which is unusually high
for a transition metal isocyanide complex8–10 (cf. MesNC,
2118 cmꢀ1). The 119Sn NMR spectra of 2 and 3 display signals
at 181 and 381 ppm respectively. These shifts lie upfield and
downfield of the 335.1 ppm from Ar0SnSnAr0 in the solid
state.11 The reasons for the different spectroscopic properties
of 2 and 3 are unclear at present.
and Sn1 = 285.31 and Sn2 = 281.71 for 3. This indicates that
there is a non-bonded electron pair at each tin. The flatter
geometry in 2 is consistent with the more crowding nature of
the ButNC ligand, which may also be responsible for the fact
that the angles subtended by the isonitrile ligands with respect
to the tin–tin bond averages more than 201 greater in 2.
The C–N–C angle in the complexed isocyanide ligands is
ca. 1701 for 2 and 1751 for 3. The average Sn–C distances to
the ButNC and MesNC ligands, ca. 2.29 A, are very similar
and are ca. 0.05 A longer than the 2.24 A seen for the
Sn–C(ipso) bonds. The Sn–Sn bond lengths in 2 and 3 are
2.9282(6) and 3.0412(3) A, respectively, which are at the upper
end of the reported Sn–Sn single bond range.12 The N–C bond
distances within the isocyanide donors (1.159 A for 2 and 3)
are typical for the N–C triple bonds.13 There are bending
angles of 1541 for 2 and 1571 for 3 at the ligating carbons of
each isonitrile ligand. These crystal data thus support the
bonding model C shown in Scheme 1. The main difference
between the structures of 2 and 3 can be attributed to the fact
that in 3 MesNC has a flat, two-dimensional shape so that the
mesityl rings can be oriented parallel to each other. The
mesityl rings are partially eclipsed since the two MesNC
molecules complex on the same side of the molecule with an
acute torsion angle (C61Sn1Sn2C71) of ca. 711 between
the Sn–C(isocyanide) vectors. The mesityl ring planes are
separated by only ca. 4.0 A which is consistent with weak
p-interactions. The torsion angle (C1Sn1Sn2C31) between the
two large Ar0 substituents is also decreased from 1801 in the
precursor 1 to 81.91 in 3. Conversely, in 2, the large size of
ButNC ligands is reflected in the fact that each one coordinates
to the tin at different sides of Sn–Sn bond. The torsion angle
between the two isonitriles (C61Sn1Sn2C66 = 131.02(2)1) is
much wider than the corresponding angle (ca. 711) in 3. In
addition the torsion angle C1Sn1Sn2C31 (ca. 1581) is much
wider than the 81.91 in 3. The greater steric congestion lowers
torsion angles between the two MesNC and the two Ar0
groups in 3, which may be responsible for the fact that the
Sn–Sn distance is ca. 0.1 longer than it is in 2.
The crystal structuresy of 2 and 3 determined by X-ray
crystallography are shown in Fig. 2 and 3, where it can be seen
that each tin is complexed by a RNC: donor (R = But, Mes).
As a result, the tins are pyramidally coordinated with inter-
ligand angular sums at Sn1 = 310.31 and Sn2 = 305.81 for 2
Fig. 2 Crystal structure of 2. Hydrogen atoms are not shown for
clarity. Selected bond lengths (A) and angles (1): Sn1–Sn2 2.9282(6) A,
Sn1–C1 2.240(4) A, Sn1–C61 2.294(5) A, Sn2–C31 2.244(4) A, Sn2–C66
2.309(5) A, C61–N61 1.160(5) A, C66–N66, 1.157(6) A; C1–Sn1–C61
100.07(14)1, C1–Sn1–Sn2 110.29(10)1, C61–Sn1–Sn2 99.96(11)1,
C31–Sn2–Sn1 104.35(10)1, C31–Sn2–C66 100.29(15)1, C66–Sn2–Sn1
101.13(10)1, Sn1–C61–N61 154.8(4)1, C61–N61–C62 171.5(4)1,
Sn2–C66–N66 154.6(4)1, C66–N66–C67 168.4(4)1.
van’t Hoff analysis of variable temperature 1H NMR
spectrum affords reaction enthalpy DHassn = ꢀ25(3) kJ molꢀ1
for 2 and DHassn = ꢀ127(4) kJ molꢀ1 for 3 (see ESIw) and
indicates relatively weak isonitrile coordination. The free
energy DGassn is ca. ꢀ17(3) kJ molꢀ1 for 2 and ꢀ4.3(3) kJ molꢀ1
for 3 which indicates that the reaction is strongly affected by
the entropic factor TDS. Coordination occurs via the empty 5p
orbital perpendicular to the tin coordination plane. The ready
availability of the 5p orbital at each tin is supported by the
calculations of Takagi and Nagase,14,15 who showed that the
energy between the single bonded forms of Ar0SnSnAr0 which
contains an essentially empty 5p orbital and a non-bonded
electron pair at each tin and a multiple bonded form
Ar0SnSnAr0 is only ca. 5 kcal molꢀ1(Fig. 4).
Fig. 3 Crystal structure of 3. Hydrogen atoms are not shown
for clarity. Selected bond lengths (A) and angles (1): Sn1–Sn2
3.0412(3) A, Sn1–C1 2.244(3) A, Sn1–C61 2.294(3) A, Sn2–C31
2.234(3) A, Sn2–C71 2.284(3) A, C61–N61 1.158(4) A, C71–N71
1.160(4) A; C1–Sn1–C61 99.04(10)1, C1–Sn1–Sn2 107.93(7)1,
C61–Sn1–Sn2 78.34(7)1, C31–Sn2–C71 98.97(11)1, C31–Sn2–Sn1
106.68(7)1, C71–Sn2–Sn1 76.03(8)1, Sn1–C61–N61 159.7(3)1,
C61–N61–C62 178.2(3)1, Sn2–C71–N71 155.9(3)1, C71–N71–C72 171.9(3)1.
In related work reversible reactions of isonitrile with
mononuclear silylene or stannylene species, which are heavier
element carbene analogues, are known. For the silylenes, it
was shown that either silylene–Lewis base complex16 or
dialkylsilaketenimine product17 was obtained depending on
the isonitrile employed. The association–dissociation equili-
brium was qualitatively shown by NMR spectroscopy but no
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ca. ꢀ18 1C for one day, after which X-ray quality crystals were
obtained as dark red blocks (0.26 g, 67%). Mp: 182–185 1C. The dark
red crystals afforded a deep green colored solution when they were
redissolved in hexane or toluene. To obtain the electronic spectrum, a
cooled hexane solution of 3 (2.23 ꢂ 10ꢀ6 mol Lꢀ1) was prepared
on which the UV-vis spectrum was immediately obtained. lmax: nm
(e in molꢀ1 L cmꢀ1) = 502 (5400). For NMR studies, a deep green
C7D8 solution of 1 was prepared in a J. Young NMR tube to which
excess MesNC was added to result in a dark red solution and was then
exposed to ultrasonic frequencies for 5 minutes to ensure the reaction
completion. At 22 1C, 1H NMR (C7D8): d 1.12 (d, 6H, 3JHH = 6.6 Hz,
Fig. 4 Calculated energy difference between single and multiple
bonded isomers of Ar0SnSnAr0.14,15
3
CH(CH3)2), 1.17 (d, 6H, JHH = 6.6 Hz, CH(CH3)2), 1.28 (d, 6H,
3JHH = 6.6 Hz, CH(CH3)2), 1.50 (d, 6H, 3JHH = 6.6 Hz, CH(CH3)2),
1.87 (o-CH3 on the complexed MesNC), 1.92 (o-CH3 on the neutral
MesNC), 2.06 (p-CH3 on the complexed MesNC), 2.17 (p-CH3 on the
quantitatively measured association energies were determined
however. For a stannylene, a fully characterized reversible
complex of isocyanide with stannylene was reported
3
neutral MesNC), 2.89 (septets, 1H, JHH = 6.6 Hz, CH(CH33)2), 3.14
(septets, 2H, JHH = 6.6 Hz, CH(CH3)2), 3.22 (septets, 1H, JHH =
+
by Grutzmacher et al. in which the association energy
3
(DHassn = Bꢀ29.6 kJ molꢀ1) and 0.16 A longer distance of
Sn–C(isonitrile) than Sn–C(ipso) indicated quite weak inter-
action between the stannylene and isocyanide.18
6.6 Hz, CH(CH3)2), 6.36 (neutral MesNC), 7.06 (complexed MesNC),
6.89–7.31(Ar–H). 13C{1H} NMR (C7D8): 18.7 (neutral MesNC), 20.8
(neutral MesNC), 24.3 (CH(CH3)2), 24.5 (complex MesNC), 26.5
(CH(CH3)2), 30.8 (complexed MesNC), 31.1 (CH(CH3)2), 122.9,
123.7, 124.4, 128.5, 146.9, 1482 (Ar–C). Signal of i-C6H3 was not
observed, 134.7, 137.4, 138.6, 170.2 (neutral MesNC). 119Sn{1H}: 381.
In summary, distannyne 1 reacts reversibly with isonitriles
ButNC and MesNC to afford 2 : 1 bis-adducts 2 and 3
exclusively. The bonding occurs mainly by the carbon lone
pair Lewis base complexation via the 5p orbital at each tin.
The crystal data and variable temperature UV-vis spectrum
suggested that the weak interaction between the Sn and
isocyanide due to the steric effect might be the main
contributor to the reversible process.
n
NC(cmꢀ1): 2278 (br).
y Crystal data for 2 at T = 90(2) K with MoKa (l = 0.71073 A):
C70H92N2Sn2(2.5C7H8), M = 1429.17, monoclinic, space group P21/c,
a = 19.763(4) A, b = 16.802(4) A, c = 25.620(5) A, b = 113.081(3)1,
V = 7827(3) A3, Z = 4, m = 0.682 mmꢀ1, Rint = 0.0678, R1 = 0.0495
for 11 397 (I 42s(I)) reflections, wR2 = 0.1180 (all data). CCDC
748928. Crystal data for 3 at T = 90(2) K with MoKa (l = 0.71073 A):
C80H96N2Sn2(2C7H8), M = 1507.28, monoclinic, space group C/c,
a = 14.1189(9) A, b = 22.0924(13) A, c = 25.6881(16) A, b =
We thank the National Science Foundation (CHE-0641020)
for support of this work.
91.1690(10)1, V = 8011.0(9) A3, Z = 4, m = 0.670 mmꢀ1, Rint
=
0.0400, R1 = 0.0312 for 16 462 (I 42s(I)) reflections, wR2 = 0.0740
(all data). CCDC 748927.
Notes and references
1 J. Escudie, H. Ranaivonjatovo and L. Rigon, Chem. Rev., 2000,
´
100, 3639.
z All manipulations were carried out under anaerobic and anhydrous
conditions. 2: to a solution of Ar0SnSnAr0 1 (0.3 g, 0.29 mmol) in
toluene (10 ml) was added excess ButNC (0.1 ml, 0.89 mmol) at
ambient temperature. The color of the solution changed immediately
from deep green to dark red. Stirring was maintained for 1 h to ensure
complete conversion. The solution was cooled to ca. ꢀ18 1C for one
day, after which X-ray quality crystals were obtained as dark red
blocks (0.24 g, 70%). Mp: 156–157 1C. The dark red crystals afforded
a deep green colored solution when they were redissolved in hexane or
toluene. To obtain the electronic spectrum, a cooled hexane solution
of 2 (9.18 ꢂ 10ꢀ6 mol Lꢀ1) was prepared on which the UV-vis spectrum
was immediately obtained. lmax: nm (e in molꢀ1 L cmꢀ1) = 422 (300).
For NMR studies, a deep green C7D8 solution of 1 was prepared in a
J. Young NMR tube to which excess ButNC was added. This resulted
in a dark red solution and was then exposed to ultrasonic frequencies
2 T. R. Oakes, H. G. David and F. J. Nagel, J. Am. Chem. Soc.,
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for 5 minutes to ensure reaction completion. At 22 1C, 1H NMR
3
(C7D8): d 1.01 (excess neutral (CH3)3CNC), 1.06 (d, 12H, JHH
6.6 Hz, CH(CH3)2), 1.32 (s, 9H, coordinated (CH3)3CNC), 1.34
=
3
3
(d, 12H, JHH = 6.6 Hz, CH(CH3)2), 3.04 (septets, 4H, JHH
=
6.6 Hz, CH(CH3)2), 6.92–7.21(Ar–H). 13C{1H} NMR (C7D8): 24.2
(complex (CH3)3C–NC), 24.3 (CH(CH3)2), 25.6 (complexed (CH3)3C–NC),
26.5 (CH(CH3)2), 27.1 (CH(CH3)2), 31.5 (excess (CH3)3C–NC), 31.7
(excess (CH3)3C–NC), 47.8 (complexed (CH3)3C–NC), 54.3 (excess
(CH3)3C–NC)), 124.0, 124.3, 128–129 (overlap with C7D8), 147.7,
156.6 (Ar–C). Signal of i-C6H3 was not observed. 119Sn{1H}: 181.
n
NC(cmꢀ1): 2175, 2162. 3: to a solution of Ar0SnSnAr0 1 (0.3 g,
17 T. Abe, T. Iwamoto, C. Kabuto and M. Kira, J. Am. Chem. Soc.,
2006, 128, 4228.
0.29 mmol) in toluene (10 ml) was added excess MesNC (0.1 g,
0.69 mmol) at ambient temperature. The color of the solution changed
immediately from deep green to dark red. Stirring was maintained
for 1 h to ensure complete conversion. The solution was cooled to
+
18 H. Grutzmacher, S. Freitag, R. Herbst-Irmer and G. S. Sheldrick,
Angew. Chem., Int. Ed. Engl., 1992, 31, 437.
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