Very large changes in bond length and bond angle in a heavy group 14 element alkyne analogue by modification of a remote ligand substituent

Article abstract of DOI:10.1021/ja0637090

The synthesis and characterization of the compound Me₃Si-4-Ar′SnSnAr′-4-SiMe₃ (Ar′-4-SiMe₃ = C₆H₂-2,6-(C₆H₃-2,6-i-Pr₂)₂-4-SiMe₃) shows that it has a Sn-Sn bond length = 3.066(1) A and a Sn-Sn-C bending angle of 99.25(14)°. These parameters differ by about0.4 A and about 26° from those previously reported for the closely related Ar′SnSnAr′ (Ar′ = C₆H₃-2,6-(C₆H₃-2,6-i-Pr₂)₂). The results show that, in accordance with the theoretical predictions by Nagase and Takagi, very small amounts of energy (ca. 5 kcal mol^-1) separate structural isomers of distannynes that have large differences in their bonding parameters. Copyright

Full text of DOI:10.1021/ja0637090

Published on Web 08/11/2006
Very Large Changes in Bond Length and Bond Angle in a Heavy Group 14
Element Alkyne Analogue by Modification of a Remote Ligand Substituent
Roland C. Fischer,‡ Lihung Pu,† James C. Fettinger,‡ Marcin A. Brynda,‡ and Philip P. Power*,‡
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616, and
Department of Chemistry, California State UniVersity, Dominguez Hills, 1000 East Victoria Street,
Carson, California 90747
Received May 26, 2006; E-mail: pppower@ucdavis.edu
Recent experimental work1-8 has resulted in the syntheses and
Scheme 1. Schematic Drawing of Ar*, Ar′, and Ar′-4-SiMe3
structural characterization of several examples of stable heavier
group 14 element alkyne analogues of formula RMMR (R ) large
aryl or silyl substituent, M ) Si, Ge, Sn, Pb). X-ray crystallographic
studies have shown that they have a trans-bent, planar,1-3,7 or almost
planar8 core arrangement with angles at M that range from 137.44-
(4)° (M ) Si)7 to 94.26(4)° (M ) Pb).1 With the exception of the
lead derivative, the M-M distances fall in the range expected for
an M-M bond order between 2 and 3. For the lead species
Ar*PbPbAr* (Ar* ) C6H3-2,6-(C6H2-2,4,6-i-Pr3)2, Scheme 1), the
Pb-Pb bond length is 3.1881(1) Å and there is a strongly bent
(Pb-Pb-C ) 94.26(4)°) core structure consistent with the repre-
sentation
Scheme 2. Synthetic Routes to 1-4
in which there is single-bonding and a nonbonding pairs of electrons
at each lead. Calculations by Frenking and co-workers have shown
that the Ar* ligand plays a crucial role in stabilizing the observed
molecular configuration in comparison to others that would be more
stable (by <10 kcal mol-1) with less crowding ligands.9 Calculations
by Nagase and Takagi10 for the Ge and Sn species RMMR (R )
penalty, and this unusual result has been confirmed by calculations
Ar* or Tbt (Tbt ) C6H2-2,4,6-{CH(SiMe3)2}3) predicted structures
on simpler RMMR (M ) Si-Pb; R ) H13 or Me14) models.
for Ar*GeGeAr* (Ge-Ge ) 2.277 Å, Ge-Ge-C ) 123.2°)11 and
We now supply experimental evidence to support this prediction
by the synthesis and characterization of the alkyne analogue 4-Me3-
Si-Ar′SnSnAr′-4-SiMe3 (1). This compound employs the modified
terphenyl ligand C6H2-2,6-(C6H3-2,6-i-Pr2)2-4-SiMe3 (Ar′-4-SiMe3,
TbtGeGeTbt (Ge-Ge ) 2.231 Å, Ge-Ge-C ) 121.8°) that were
quite similar to those experimentally measured for Ar′GeGeAr′ (Ar′
3
) C6H3-2,6-(C6H3-2,6-i-Pr2)2 , Ge-Ge ) 2.2850(6) Å, Ge-Ge-C
) 128.27(8)°) and more recently by Tokitoh and co-workers for
Scheme 1) in place of Ar′. This results in a species that has a much
BbtGeGeBbt (Bbt ) C6H2-2,6-{CH(SiMe3)2}2-4-C(SiMe3)3, Ge-
longer Sn-Sn distance and a narrower Sn-Sn-C angle than those
Ge ) 2.22 Å avg, Ge-Ge-C ) 131° avg).8 These bond distances
found in Ar′SnSnAr′.
were in the range observed for the Ge-Ge double bonds in
The ligand precursor 1-I-C6H2-2,6-(C6H3-2,6-i-Pr2)2-4-SiMe3 (4)
was isolated by the addition of 2 equiv of BrMgC6H3-2,6-i-Pr2
digermenes.12 The stronger bonding in BbtGeGeBbt was rational-
ized on the basis of a lower ∆D-Q for the GeBbt fragment, which
(BrMgDipp) to 1-Li-2,6-Cl2-C6H2-4-SiMe3, with subsequent quench-
leads to a stronger Ge-Ge interaction.8 In contrast, the calculations
ing with I2 by a standard route15 (Scheme 2). In a manner similar
predicted that a “multiple-bonded” Ar*SnSnAr* should have a more
trans-bent structure (Sn-Sn-C ) 111.0°, a C-Sn-Sn-C torsion
to the preparation of [Ar′Li]2,16 3 was synthesized by reaction of 4
with n-BuLi.
angle of 125.3°) and a relatively long Sn-Sn bond of 2.900 Å.
Subsequent reaction of 3 with excess SnCl2 in diethyl ether and
These values differed considerably from those experimentally
crystallization from the same solvent at -20 °C yielded [1-ClSnC6H2-
2,6-(C6H3-2,6-i-Pr2)2-4-SiMe3]2 (2). Reduction of 2 with potassium
measured for Ar′SnSnAr′ (Sn-Sn ) 2.6675(4) Å, Sn-Sn-C )
125.249(2)°). The calculations also predicted that the singly-bonded
in diethyl ether and subsequent crystallization afforded 1 as dark
Ar*SnSnAr* isomer (analogous to the Ar*PbPbAr* structure above)
green, air- and moisture-sensitive crystals.17 X-ray crystallography
with Sn-Sn ) 3.087 Å and Sn-Sn-C ) 99.0° differed in energy
by only 4.8 kcal mol-1 from “multiple-bonded” Ar*SnSnAr*.
Seemingly, these large structural changes carry only a small energy
showed that 1 has a trans-bent structure in the solid state18 with
geometric (Figure 1) parameters that differ dramatically (Figure 2)
from those previously reported for Ar′SnSnAr′.
In 1, the Sn-Sn bond length is 3.066(1) Å, which is about 0.4
Å longer than the 2.6675(4) Å in Ar′SnSnAr′. In addition, the Sn-
‡ University of California, Davis.
† California State University, Dominguez Hills.
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J. AM. CHEM. SOC. 2006, 128, 11366-11367
10.1021/ja0637090 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
their ∆D-Q energies that, when added together in the dimerized
product, results in a 4 kcal mol-1 difference.19 However it should
be borne in mind that these energy differences are sufficiently small
to be in the range of packing forces.
Acknowledgment. We are grateful to the National Science
Foundation for financial support. R.C.F. thanks the Max Kade
foundation for a postdoctoral fellowship.
Supporting Information Available: X-ray data (CIF) for 1.
This material is available free of charge via the Internet at
http:\\www.pubs.acs.org.
References
(1) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524.
(2) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 2002, 124, 5930.
Figure 1. Thermal ellipsoidal plot of 1 (30% probability) without H atoms.
Selected bond distances (Å) and angles (°): Sn(1)-Sn(1a), 3.066(1); Sn-
(1)-C(1), 2.208(5); Si(1)-C(4), 1.878(6); C(1)-Sn(1)-Sn(1a), 99.25(14);
C(2)-C(1)-Sn(1), 125.6(4); C(6)-C(1)-Sn(1), 115.0(4); C(2)-C(1)-
C(6), 118.3(5); C(1)-C(2)-C(7), 121.6(5); C(3)-C(2)-C(7), 118.5(5).
(3) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 1785.
(4) Wiberg, N.; Niedermayer, M.; Fischer, G.; No¨th, H.; Suter, M. Eur. J.
Inorg. Chem. 2002, 1066.
(5) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626.
(6) Wiberg, N.; Vasisht, S. K.; Fischer, G.; Mayer, P. Z. Anorg. Allg. Chem.
2004, 630, 1823.
(7) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755.
(8) Sugiyama, Y.; Sasamori, T.; Hosoi, Y.; Furukawa, Y.; Takagi, N.; Nagase,
S.; Tokitoh, N. J. Am. Chem. Soc. 2006, 128, 1023.
(9) Chen, Y.; Hartmann, M.; Diedenhofen, M.; Frenking, G. Angew. Chem.,
Int. Ed. 2001, 40, 2052.
(10) Takagi, N.; Nagase, S. Organometallics 2001, 20, 5498.
(11) Structure details of Ar*GeGeAr* and Ar*SnSnAr* remain unknown.
(12) Tokitoh, N.; Okazaki, R. In The Chemistry of Organic Germanium, Tin
and Lead Compounds; Rappoport, Z., Ed.; John Wiley & Sons: Chich-
ester, 2002; Vol. 2, p 843.
(13) Lein, M.; Krapp, A.; Frenking, G. J. Am. Chem. Soc. 2005, 127, 6290.
(14) Jung, Y.; Brynda, M.; Power, P. P.; Head-Gordon, M. J. Am. Chem. Soc.
2006, 128, 7185-7192.
(15) Twamley, B.; Hardman, N. J.; Power, P. P. Acta Crystallogr. 2000, C56,
514.
Figure 2. Comparison of core geometries for 4-SiMe3Ar′SnSnAr′-4-SiMe3
1 and Ar′SnSnAr′, flanking aryl groups are not shown for clarity.
(16) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2150.
(17) At room temperature and under strictly anhydrous and anaerobic condi-
tions, a solution of 0.850 g (1.36 mmol) [1-ClSn-C6H2-2,6-(C6H3-2,6-i-
Pr2)2-4-SiMe3]2 (prepared in a fashion similar to previously reported
procedures)5 in diethyl ether (25 mL) was added to a diethyl ether
suspension of 0.059 g (1.51 mmol) of finely dispersed potassium with
rapid stirring. The reaction mixture quickly adopted a deep green color,
and stirring was continued for 24 h, after which the precipitated material
and unreacted potassium were allowed to settle. The solution was filtered
through a filter-tipped cannula and concentrated in vacuo to incipient
crystallization (ca. 10 mL). Storage at -20 °C yielded 0.41 g (0.328 mmol,
48% yield) of dichroic green-dark orange crystals of 1‚Et2O. Anal. Calcd
for 1‚Et2O C70H100OSi2Sn2: C, 67.20; H, 8.06. Found: C, 66.79; H, 8.22.
Mp 183-185 dec. UV-vis λmax (nm, ꢀ [L mol-1cm-1]): 416 (4700),
608 (1200). 1H NMR (C6D6, 599.814 MHz, 25 °C): -0.28 (s, 18H,
(CH3)3Si), 1.12 (t, 6H, (CH3CH2)2O), 1.16 (d, 24H, 3JHH ) 6.8 Hz, o-CH-
(CH3)(CH3)), 1.38 (d, 24H, 3JHH ) 6.8 Hz, o-CH(CH3)(CH3)), 2.94 (septet,
Sn-C bond angle is 99.25(14)°, a decrease of about 26° in
comparison to the 125.24(7)° in Ar′SnSnAr′. Hence, the structural
parameters resemble those of Ar*PbPbAr* (Pb-Pb ) 3.1881(1)
Å, Pb-Pb-C ) 94.26(4)°) more than those of Ar′SnSnAr′ and
are consistent with Sn-Sn single bonding. Another striking
difference between the solid-state structures of 1 and Ar′SnSnAr′
is the perpendicular arrangement of the ligand’s central aryl rings
relative to the C-Sn-Sn-C in contrast to the parallel orientation
in Ar′SnSnAr′, where the central aryl rings lie in the plane with
the central structural unit. The dihedral angles Sn-Sn-C-C in 1
are 91.04 and -101.08°, but are 176.99° and 3.09° in Ar′SnSnAr′,
whereas the lead derivative Ar*PbPbAr* exhibits torsional angles
of 95.16° and -88.98°.
3
8H, JHH ) 6.8 Hz, o-CH(CH3)(CH3)), 3.27 (q, 4H, (CH3CH2)2O), 7.07
3
3
(d, 8H, JHH ) 6.7 Hz, m-Dipp), 7.17 (t, 4H, JHH ) 6.7 Hz, p-Dipp),
7.98 (s, 4H, m-C6H2Si(CH3)3). 13C{1H} (C6D6, 150.823 MHz, 25 °C):
-0.6 ((H3C)3Si), 15.5 ((CH3CH2)2O), 26.9 (CH(CH3)(CH3)), 32.5 (CH-
(CH3)(CH3)), 35.9 (CH(CH3)(CH3)), 65.9 ((CH3CH2)2O), 125.2 (p-Dipp),
127.9 (m-Dipp), 138.0 (m-C6H2), 141.2 (o-Dipp), 143.2 (p-C6H2), 151.1
(i-Dipp), 161.4 (o-C6H2), 174.6 (i-C6H2). 29Si{1H} (C6D6, 119.165 MHz,
25 °C): -4.2. 119Sn{1H} (C6D6, 223.671 MHz, 25 °C): no signal
observed.
The UV-vis spectrum of 1 in hexanes displays two strong
absorptions at 416 (ꢀ ) 4700 L mol-1 cm-1) and 608 (ꢀ ) 1200 L
-1
mol
cm-1) nm and are slightly bathochromically shifted in
comparison to those of Ar′SnSnAr′ (410 and 597 nm) and
Ar*SnSnAr* (409 and 593 nm), suggesting similar, strongly bent
structures of the three compounds in solution.
(18) Crystal data for 1‚Et2O at 90 K with Mo KR (λ ) 0.71073 Å): a )
11.605(3) Å, b ) 24.573(5) Å, c ) 12.733(3) Å, â ) 114.407(4)°, V )
3306.5(12) Å3, M ) 1251.06 g mol-1, âcalcd ) 1.257 Mg m-3, F(000) )
1312, monoclinic, space group P2(1)/c, Z ) 2, R1 ) 0.0557 for 3272 (I
> 2(I)) data, wR2 ) 0.1246 for all 5975 data. Equipment: Bruker
SMART1000 CCD system. Absorption correction was performed using
SADABS.18a The structure was solved by direct methods (SHELXS-97),18b
and nonhydrogen atoms were refined anisotropically (full-matrix least-
squares on F2, SHELXL-97).18c (a) Sheldrick G. M. SADABS, version
2.10; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 2003. (b) Sheldrick,
G. M. Acta Crystallogr. 1990, A46, 467. (c) Sheldrick, G. M. SHELXS-
97 and SHELXL-97; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(19) Brynda, M. A.; Fischer, R. C.; Power, P. P. Unpublished work.
Our results vindicate the theoretical prediction10,13,14 that relatively
small amounts of energy separate two different bonding modes of
the tin analogues of alkynes. Modification of the known terphenyl
ligand Ar′ by the introduction of SiMe3 instead of H at the para-
position of the central aryl ring induces a single-bonded structure
without alteration of the steric crowding near the tin center.
Preliminary theoretical data on model moieties MC6H4-4-SiMe3 and
MC6H5 (M ) Ge, Sn) indicate about a 2 kcal mol-1 difference in
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