Reactivity of Ar′GeGeAr′ (Ar′ = C6H 3-2,6-Dipp2, Dipp = C6H 3-2,6-iPr2) toward Alkynes: Isolation of a Stable Digermacyclobutadiene

Article abstract of DOI:10.1021/ja0318481

The first reactions of the digermyne Ar′GeGeAr(Ar′ = C₆H₃-2,6-Dipp₂, Dipp = C₆H₃-2,6-iPr₂) with alkynes are reported. 1 reacts with 1 equiv of H₅C₆CCC₆H₅ to afford the 1,2-digermacyclobutadiene 2 in high yield, while it reacts with 2 equiv of the less hindered alkyne Me3SiCCH to yield an unexpected bicyclic compound 3. Molecular structures of 2 and 3 were determined by X-ray crystallography. A possible mechanism for the formation of 3 is discussed. The high reactivity of 1, even at room temperature, emphasizes the fundamental differences between the GeGe and CC multiple bonds. Copyright

Full text of DOI:10.1021/ja0318481

Published on Web 03/31/2004
Reactivity of Ar′GeGeAr′ (Ar′ ) C6H3-2,6-Dipp2, Dipp ) C6H3-2,6-iPr2) toward
Alkynes: Isolation of a Stable Digermacyclobutadiene
Chunming Cui, Marilyn M. Olmstead, and Philip P. Power*
Department of Chemistry, UniVersity of California at DaVis, One Shields AVenue, DaVis, California 95616
Received December 19, 2003; E-mail: pppower@ucdavis.edu
Strained rings incorporating heavier group 14 elements have
attracted considerable attention in recent years due to their unique
structures and reaction patterns.1 Several three- and four-membered
rings with heavier main group elements have been synthesized and
characterized. However, stable cyclobutadiene rings containing Si-
Pb atoms remain unknown, although in a few cases they have been
postulated as reactive intermediates.2 This is, of course, consistent
with the instability of cyclobutadiene itself for which calculations
have indicated an antiaromatic destabilization of 30-60 kcal/mol.3
Moreover, recent experimental results also showed that the cyclic
4π-electron system is destabilized by 55 kcal/mol.4 For heavier
element derivatives, a few calculations have been carried out on
Figure 1. Thermal ellipsoid drawing of 2 (50% probability). Hydrogen
atoms and Dipp rings (except ipso carbon atoms) are not shown. Selected
bond lengths (Å) and angles (deg): Ge1-Ge2 2.4708(9), Ge1-C61
2.022(5), Ge2-C68 2.027(5), C61-C68 1.365(7), Ge1-C1 2.004(5), Ge2-
C2 2.003(5); C61-Ge1-Ge2 74.00(15), C68-Ge2-Ge1 74.22(15), Ge1-
C61-C68 106.2(4), C61-C68-Ge2 105.4(4), C61-Ge1-C1 109.2(2),
C1-Ge1-Ge2 134.8(15), Ge1-Ge2-C31 132.04(14).
SiC3H4,5 Si2C2H4,6 and their isomers. For Si2C2H4, they have shown
that the silacyclopropene HCC(SiH3)Si structure is the most stable
of the numerous possible isomers with 1,2-disilacyclobutadiene ca.
46 kcal/mol higher in energy. Accordingly, the synthesis and
characterization of a stable cyclobutadiene ring containing a heavy
group 14 element is of fundamental interest and a formidable
synthetic challenge. We previously reported the synthesis and
structure of the stable germanium alkyne analogue Ar′GeGeAr′ (1,
Ar′ ) C6H3-2,6-Dipp2, Dipp ) C6H3-2,6-iPr2).7 It has a trans-bent
geometry in contrast to that of the linear alkynes. The lone pair
character and consequent bending at the Ge atoms suggest
heightened reactivity, but little is known of the chemistry of these
heavier group 14 alkyne analogues. We now report the reaction of
the “digermyne” 1 with alkynes and the formation of the first stable
cyclobutadiene incorporating heavier group 14 elements as shown
in Scheme 1.
Scheme 1
Å) indicate single bonding. The Ge-Ge bond length (2.4708(9)
Å) is slightly longer than a single bond (2.44 Å)10a and is just above
the known range for “digermenes” (2.21-2.46 Å).10b,c The data
for 2 clearly establish the presence of a carbon-carbon double bond
and two germanium-carbon single bonds in the central four-
membered ring. In addition, the pyramidal Ge environments and
the Ge-Ge bond length are consistent with a weak Ge-Ge double
bond as seen in several acyclic digermenes.10b,c,11
Compound 3 crystallizes as two crystallographically independent
molecules which have similar bond lengths and angles. One of these
molecules is shown in Figure 2. The unusual structure results from
the addition of a Dipp ring from an Ar’ ligand to the postulated
diradical intermediate B in Scheme 1. The C61-C62 and C66-
C67 bond lengths are in the range for alkenes, while C38-C39
(1.555 Å) is close to that of a C-C single bond. The Ge atoms are
each coordinated to four carbon atoms and display a distorted
tetrahedral geometry. The Ge-Ge separation is 3.076(3) Å, which
underlines the breaking of the Ge-Ge bond during the reaction.
The reaction of 1 with PhCCPh or HCCSiMe3 under ambient
conditions underlines its high reactivity in comparison to alkynes.
A possible mechanism for the formation of 2 and 3 is shown in
Scheme 1. The 1,2-digermacyclobutadienes 2 and A are formed
initially by a [2+2] cycloaddition of 1 with PhCCPh or HCCSiMe3.
The species A is probably more reactive due to the lower steric
protection afforded by the hydrogen substituent on the carbon atom.
It reacts with another molecule of HCCSiMe3 to afford the 1,4-
Upon stirring an n-hexane solution of 1 and diphenylacetylene
at room temperature for 7 days, the orange red color became deep
red.8 Cooling the solution at 6 °C overnight afforded black-red
crystals of 2, a 1,2-digermacyclobutadiene, in high yield (Scheme
1). It displayed relatively high thermal stability as indicated by its
high melting point (178 °C). Nonetheless, it is extremely air-
sensitive both in solution and in the solid state. It was characterized
1
by H NMR, 13C NMR, IR, and UV-vis spectroscopies, and by
X-ray crystallography. The reaction of 2 with an excess of
diphenylacetylene under normal conditions did not result in further
addition of alkyne. However, the reaction of 1 with the less crowded
terminal alkyne HCCSiMe3 led to the unusual bicyclic compound
3 (Scheme 1), which was also spectroscopically and structurally
characterized.
The structure of 2 is shown in Figure 1.9 Its most prominent
feature is the four-membered Ge2C2 ring which is an almost planar
trapezoid (average deviation from the plane 0.026 Å; sum of the
internal angles 359.82°). The two terphenyl ligands are disposed
above and below the Ge2C2 ring such that the Ge centers have
pyramidal coordination (∑°Ge ) 318.0° and 317.3°, ClGe1Ge2C31
) 151.4°). The C61-C68 bond length (1.365(7) Å) is consistent
with C-C double bonding, and the Ge-C ring distances (ca. 2.00
9
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J. AM. CHEM. SOC. 2004, 126, 5062-5063
10.1021/ja0318481 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
(3) (a) Glukhovtsev, M. N.; Laiter, S.; Pross, A. J. Phys. Chem. 1995, 99,
6828. (b) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1983, 105,
7500. (c) Rogers, D. W.; Mclafferty, F. J.; Podosenin, A. V. J. Phys.
Chem. 1996, 100, 17148.
(4) Deniz, A. A.; Peters, K. S.; Snyder, G. J. Science 1999, 286, 1119.
(5) (a) Colvin, M. E.; Schaefer, H. F. Faraday Symp. 1984, 19, 39. (b)
Schriver, G. W.; Fink, M. J.; Gordon, M. S. Organometallics 1987, 6,
1977. (c) Maier, G.; Reisenauer, H. P.; Jung, J.; Pacl, H.; Egenolf, H.
Eur. J. Org. Chem. 1998, 1297.
(6) (a) Holme, T. A.; Gordon, M. S.; Yabushita, S.; Schmidt, M. W.
Organometallics 1984, 3, 583. (b) Maier, G.; Reisenauer, H. P.; Meudt,
A. Eur. J. Org. Chem. 1998, 1291.
(7) (a) Pu, L. H.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R.
S.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626.
(b) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 1785.
(8) All manipulations were carried out under anaerobic and anhydrous
conditions. 2: A mixture of Ar′GeGeAr′7 (1, Ar′ ) C6H3-2,6-Dipp2, Dipp
) C6H3-2,6-iPr2, 0.140 g, 0.149 mmol) and diphenylacetylene (0.053 g,
0.30 mmol) in n-hexane (30 mL) was stirred at room temperature for 7
days. The resulting deep red solution was concentrated and stored at 6
°C overnight to give dark red crystals of 2 (0.148 g, 77.7%). Mp: 178
°C. 1H NMR (C6D6, 300.08 MHz): δ 0.77 (br, 24H, CHMe2), 1.07 (br,
24H, CHMe2), 3.04 (sept, 8H, J ) 6.6 Hz, CHMe2), 6.65 (br, 4H, Ar-
H), 6.71 (br, 6H, Ar-H), 6.98 (d, 6H, J ) 7.5 Hz, Ar-H), 7.09-7.21
(m, 6H, Ar-H), 7.29 (d, 4H, J ) 6.9 Hz, Ar-H). 13C NMR (C6D6, 100.52
MHz): δ 23.5 (CHMe2), 26.1 (CHMe2), 31.1 (CHMe2), 123.5, 126.4,
127.1, 129.1, 129.3, 130.8, 138.0, 140.2, 145.7, 146.8, 147.5, 159.2
(unsaturated carbon). IR (KBr, Nujol): 1605 (w), 1580 (w), 1570 (w),
855 (w), 757 (s), 735 (m), 711 (s), 688 (m), 452 (m), 367 (s) cm-1. UV-
vis (n-hexane): 375 nm (shoulder). 3: This compound was prepared
similarly to 2. After workup, yellow crystals of 3 were obtained at ca.
-20 °C from n-hexane (0.120 g, 84%). Mp: 225 °C (dec). 1H NMR (C6D6,
300.08 MHz): δ -0.21 (s, 9H, SiMe3), -0.15 (s, 9H, SiMe3), 0.24 (d,
6H, CHMe2), 0.90, 1.01, 1.19, 1.23-1.45 (m, 42H, CHMe2), 1.68 (br,
1H, C39, H), 2.03 (sept, 1H, J ) 6.7 Hz, CHMe2), 2.28 (sept, 1H, J )
6.7 Hz, CHMe2), 2.82 (sept, 2H, J ) 6.7 Hz, CHMe2), 3.05 (mult, 3H, J
) 6.7 Hz, CHMe2), 3.35 (sept, 1H, J ) 6.7 Hz, CHMe2), 5.38 (br s, 1H,
Vi-CH), 5.76 (br, s, 1H, Vi-CH), 6.58 (d, 2H, J ) 7.5 Hz, Ar-H), 6.98-
7.24 (m, 13H, Ar-H), 7.50 (d, 2H, J ) 7.4 Hz, Ar-H). 13C NMR (C6D6,
75.5 MHz): δ -1.58 (SiMe3), -1.28 (SiMe3), 12.95 (CHMe2), 17.84
(CHMe2), 19.64 (CHMe2), 21.64 (CHMe2), 24.41 (CHMe2), 29.71
(CHMe2), 32.78 (CHMe2), 34.57 (CHMe2), 50.05 (Vi-C), 121.67, 122.74,
124.6, 126.08, 130.46, 131.06, 140.48, 145.14, 147.11, 159.21, 163.55
(unsaturated carbon). IR (KBr, Nujol): 1591 (w), 1575 (w), 1554 (w),
1305 (m), 1248 (m), 1154 (m), 1056 (w), 967 (m), 934 (w), 893 (s), 875
Figure 2. Thermal ellipsoid drawing of 3 (50% probability). Hydrogen
atoms are not shown. Selected bond lengths (Å) and angles (deg): Ge1-
C1 1.978(6), Ge1-C39 2.002(6), Ge1-C61 1.945(7), Ge1-C66 1.957(6),
Ge2-C31 1.973(6), Ge2-C38 2.017(7), Ge2-C62 1.978(5), Ge2-C67
1.961(6), C66-C67 1.334(9), C61-C62 1.360(9), C38-C39 1.555(9);
C39-Ge1-C61 102.1(3), C39-Ge1-C66 104.3(3), C61-Ge1-C66
100.9(3), C1-Ge1-C39 119.8(3), C1-Ge1-C61 119.9(3), C1-Ge1-C66
107.3(3), Ge1-C39-C38 110.8((4), Ge1-C61-C62 117.8(4).
digermabenzene intermediate B,12 followed by the activation of one
of the flanking aryl rings on the terphenyl ligands to give 3. The
regioselectivity of this reaction may be attributed to steric effects
in which head-to-head coupling of the germanium center with the
less hindered carbon atom of the unsaturated C-C units is favored.
Although [2 + 2] cycloadditions are common for reactions of
unsaturated heavier group 14 element species with unsaturated
hydrocarbons,13 reaction with a relatively unactivated benzene ring
is uncommon, suggesting that the reactive intermediate B has
considerable diradical character.
In summary, the reactions of PhCCPh or HCCSiMe3 with
Ar′GeGeAr′, 1, show that it is much more reactive than normal
alkynes. The heightened reactivity of 1 is very probably due to its
incipient diradical character and accessibility of the excited state
of this molecule.
(m), 840 (m), 760 (m), 748 (m), 660 (m), 644 (s), 605 (s), 475 (w) cm-1
.
(9) Crystal data for 2 at 130(2) K with Cu KR radiation (λ ) 1.54178 Å):
trigonal, space group R3h, a ) b ) 43.910(7) Å, c ) 19.208(7) Å, R ) â
) 90°, γ ) 120°, R1 ) 0.0586 for 7378 reflections (I > 2σ(I)), wR2 )
0.1705 (all data). Crystal data for 3 at 91(2) K with Mo KR (λ ) 0.71073
Å): monoclinic, space group Ia, a ) 33.299(8) Å, b ) 13.209(3) Å, c )
38.495(13) Å, â ) 104.211(6)°, R1 ) 0.073 for 30 438 reflections
(I > 2σ(I)), wR2 ) 0.2380 (all data).
(10) (a) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon:
Oxford, 1984; p 1280. (b) Baines, K. M.; Stubbs, W. G. AdV. Organomet.
Chem. 1996, 39, 275. (c) Escudie, J.; Ranaivonjatovo, H. AdV. Organomet.
Chem. 1999, 44, 114.
Acknowledgment. We thank the National Science Foundation
for support of this work.
Supporting Information Available: X-ray CIF data for 2 and 3.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(11) The weakness of the Ge-Ge double bond is underlined by the large torsion
(75.7°) and out-of-plane (46.5°) angular parameters. For a discussion of
these, see: Power, P. P. Dalton Trans. 1998, 2939.
(12) For a 1,4-disila or digerma(Dewar-benzene), see: Kabe, Y.; Ohkubo, K.;
Ishikawa, H.; Ando, W. J. Am. Chem. Soc. 2000, 122, 3775. Ohtaki, T.;
Ando, W. Organometallics 1996, 15, 3103.
References
(1) (a) Escudie, J.; Couret, C.; Ranaivonjatovo, H.; Satge, J. Coord. Chem.
ReV. 1994, 130, 427. (b) Lee, V. Y.; Sekiguchi, A. In The Chemistry of
Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.;
Wiley: Chichester, 2002; Vol. 2, Part 1, Chapter 14. (c) Sekiguchi, A.;
Lee, V. Y. Chem. ReV. 2003, 103, 1429.
(2) (a) Ohgaki, H.; Fukaya, N.; Ando, W. Organometallics 1997, 16, 4956.
(b) Kon, Y.; Ogasawara, J.; Sakamoto, K.; Kabuto, C.; Kira, M. J. Am.
Chem. Soc. 2003, 125, 9310. (c) Gee, J. R.; Howard, W. A.; McPherson,
G. L.; Fink, M. J. J. Am. Chem. Soc. 1991, 113, 5461. (d) Maier, G.;
Born, D. Angew. Chem., Int. Ed. Engl. 1989, 28, 1050.
(13) For example: (a) Batcheller, S. A.; Masamune, S. Tetrahedron Lett. 1988,
29, 3382. (b) Ando, W.; Tsumuraya, T. J. Chem. Soc., Chem. Commun.
1989, 12, 770. (c) Apeloig, Y.; Bravo-Zhivotovskii, D.; Zharov, I.; Panov,
V.; Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1998, 120, 1398. (d)
Bravozhivotovskii, D.; Zharov, I.; Kapon, M.; Apeloig, Y. J. Chem. Soc.,
Chem. Commun. 1995, 1625. (e) Fukaya, N.; Ichinohe, M.; Kabe, Y.;
Sekiguchi, A. Organometallics 2001, 20, 3364. (f) Fukaya, N.; Ichinohe,
M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2000, 39, 3881.
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