Synthesis and molecular structure of a dilithium salt of the cis-diphenylcyclobutadiene dianion

Article abstract of DOI:10.1246/cl.2000.896

The cis-diphenyl-substituted cyclobutadiene cobalt complex (1), prepared from the reaction of 1,8-diphenyl-3,6-disila-1,7-octadiyne with CpCo(CO)₂, reacted with lithium metal in tetrahydrofuran (THF) to yield pale orange crystals of the dilithium salt of the cis-diphenylcyclobutadiene dianion (2). The structure was determined by spectroscopic methods and X-ray crystallography to reveal the aromatic character of the doubly-charged four-membered ring system.

Full text of DOI:10.1246/cl.2000.896

896
Chemistry Letters 2000
Synthesis and Molecular Structure of a Dilithium Salt of the cis-Diphenylcyclobutadiene Dianion
Tsukasa Matsuo, Takeru Mizue, and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571
(Received May 19, 2000; CL-000485)
The cis-diphenyl-substituted cyclobutadiene cobalt com-
plex (1), prepared from the reaction of 1,8-diphenyl-3,6-disila-
1,7-octadiyne with CpCo(CO)2, reacted with lithium metal in
tetrahydrofuran (THF) to yield pale orange crystals of the
dilithium salt of the cis-diphenylcyclobutadiene dianion (2).
The structure was determined by spectroscopic methods and X-
ray crystallography to reveal the aromatic character of the dou-
bly-charged four-membered ring system.
Phenyl groups are very useful for the stabilization of car-
banions with their large delocalization of negative charge.
Structures of various anions stabilized by phenyl groups both in
the solid state and in solution have been reported that include
both solvated and contact ion pair (CIP).1 As the phenyl-substi-
tuted cyclobutadiene dianion (CBD2–), the dilithium salt of 1,2-
diphenylbenzocyclobutadienediide2 and the dipotassium salt of
1,2,3,4-tetraphenylcyclobutadienediide3 have been reported,
however, the stabilization by six π-electron delocalization
remains unclear. The expected energy gain by delocalization of
the negative charge in the aromatic system is offset by electron
repulsion in the dianion.4 As a result, a large amount of nega-
tive charge is distributed on the substituents. The problem of
aromaticity in CBD2– and its derivatives still remains open,
both from experimental and theoretical points of view.5 Quite
recently, we have reported the first experimental evidence for
the aromaticity of CBD2– with a six π-electron system, the
dilithium salt of tetrakis(trimethylsilyl)cyclobutadiene dianion,
2[Li+]·[(Me3Si)4C42–].6 Herein, we report the synthesis, charac-
terization, and X-ray structure of the dilithium salt of the cis-
diphenylcyclobutadiene dianion, which reveals the aromatic
character of the doubly-charged four-membered ring system,
stabilized by both phenyl and silyl groups.
by X-ray crystallography (Figure 2).10 The molecule has a
crystallographic two-fold axis on the four-membered ring (C2
symmetry). The dilithium salt 2 is monomeric and forms contact
ion pairs (CIPs) in the crystal. One DME molecule is coordinat-
ed to each lithium atom. Li1 and Li1* are located above and
below the plane of the four-membered ring (C1–C2–C2*–C1*).
The two Li atoms are not located at the center of the four-mem-
bered ring, but are slightly shifted in the direction of the phenyl
groups, in contrast to 2[Li+]·[(Me3Si)4C42–] in which they are
located above and below the center of the four-membered ring.6
The distances between Li1 and the four carbon atoms (C1, C2,
C1*, and C2*) range from 2.186(6) to 2.256(6) Å (av 2.225(6) Å).
The four-membered ring is planar and forms an almost
square structure, as confirmed by the internal bond angles
(C1*–C1–C2, 89.8(1); C1–C2–C2*, 90.2(1)°) and the sum of
the bond angles (360.0°). The average of the C–C distances in
the four-membered ring is 1.490 Å (C1–C2, 1.480(5); C1–C1*,
1.506(6); C2–C2*, 1.493(6) Å). These structural features of 2
correspond well to the criteria of aromaticity; e.g., 1) the planari-
As a precursor, we designed the cis-diphenyl-substituted
cyclobutadiene cobalt complex (1), which was prepared by the
reaction of 1,8-diphenyl-3,6-disila-1,7-octadiyne with
CpCo(CO)2 in refluxing octane in 63% yield.7 The molecular
structure of 1 is shown in Figure 1.8
Crystals of 1 (36 mg, 0.076 mmol) and lithium metal (30
mg, 4.3 mmol) were placed in a reaction tube with a magnetic
stirrer. Dry, oxygen-free THF (1.2 mL) was introduced by vac-
uum transfer, and the mixture was stirred at room temperature
to give a dark brown solution containing 2 within 24 h. After
the solvent was removed in vacuo, degassed hexane (5 mL) was
introduced by vacuum transfer. Then, after the lithium and
insoluble dark materials were removed, the solution was cooled
to afford air- and moisture-sensitive pale orange crystals of 2
(THF ligand) in a quantitative yield (Scheme 1).9 After ligand
exchange on the Li+ ions from THF to 1,2-dimethoxyethane
(DME), crystallization from heptane at –30 °C afforded fine
crystals of 2 containing two molecules of DME.
The molecular structure of 2 was unequivocally determined
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
897
tive charge on the phenyl ring. The 29Si signal of 2 (δ = –21.3)
is also shifted to higher field relative to that of 1 (δ = –7.0).
The present experimental results clearly show that the negative
charge is stabilized by both silyl and phenyl groups, but stabi-
lization by the phenyl group is more effective.
ty of the four-membered ring; and 2) the lack of bond alternation.
The C1–C3 distance in 2 (1.448(5) Å) is slightly shortened
in comparison with the corresponding distances in 1 (av 1.468(2)
Å), due to the delocalization of the negative charge on the phenyl
ring. However, the quinoid structure of the phenyl ring is not
found (C3–C4, 1.419(5); C3–C8, 1.401(5); C4–C5, 1.387(5);
C5–C6, 1.392(5); C6–C7, 1.387(5); C7–C8, 1.404(5) Å). The
Si1–C2 distance in 2 (1.828(4) Å) is also shorter than the corre-
sponding distances in 1 (av 1.861(1) Å) due to the pπ–σ* conju-
gation.11 The positions of the Si atoms and the ipso-carbon
atoms deviate up (Si1 and C3) and down (Si1* and C3*) about
the plane of the four-membered ring (C1–C2–C2*–C1*/C2–Si1,
12.14(2); C1–C2–C2*–C1*/C1–C3, 9.40(2)°).
References and Notes
1
2
For reviews, see: a) K. Müllen, Chem. Rev., 84, 603 (1984).
b) W. N. Setzer and P. v. R. Schleyer, Adv. Organomet.
Chem., 24, 390 (1985).
a) G. Boche, H. Etzrodt, M. Marsch, and W. Thiel, Angew.
Chem., Int. Ed. Engl., 21, 132 (1982). b) G. Boche and H.
Etzrodt, Tetrahedron Lett., 24, 5477 (1983). c) G. Boche, H.
Etzrodt, W. Massa, and G. Baum, Angew. Chem., Int. Ed.
Engl., 24, 863 (1985).
3
4
G. Boche, H. Etzrodt, M. Marsch, and W. Thiel, Angew.
Chem., Int. Ed. Engl., 21, 133 (1982).
a) G. Maier, Angew. Chem., Int. Ed. Engl., 27, 309 (1988).
b) A.-M. Sapse and P. v. R. Schleyer, “Lithium Chemistry,
A Theoretical and Experimental Overview,” Wiley-
Interscience, New York (1995).
5
a) T. Clark, D. Wilhelm, and P. v. R. Schleyer, Tetrahedron
Lett., 23, 3547 (1982). b) B. A. Hess, Jr., C. S. Ewig, and L.
J. Schaad, J. Org. Chem., 50, 5869 (1985). c) A. Skancke,
Nouv. J. Chim., 9, 577 (1985). d) G. v. Zandwijk, R. A. J.
Janssen, and H. M. Buck, J. Am. Chem. Soc., 112, 4155
(1990). e) M. Balci, M. L. Mckee, and P. v. R. Schleyer, J.
Phys. Chem., A, 104, 1246 (2000).
6
7
A. Sekiguchi, T. Matsuo, and H. Watanabe, J. Am. Chem.
Soc., 122, 5652 (2000).
M.p. 56 °C; 1H NMR (CDCl3, δ) 0.16 (s, 6 H, CH3), 0.42 (s,
6 H, CH3), 0.72–0.84 (m, 2 H, CHH), 0.90–1.01 (m, 2 H,
CHH), 4.65 (s, 5 H, CpH), 7.18–7.26 (m, 6 H, ArH),
7.40–7.44 (m, 4 H, ArH); 13C NMR (CDCl3, δ) –1.4 (CH3),
1.2 (CH3), 8.9 (CH2), 66.5 (C), 81.4 (CH), 83.7 (C), 125.6
(CH), 126.9 (CH), 128.1 (CH), 139.3 (C); 29Si NMR
(CDCl3, δ) –7.0; Anal. Calcd for C27H31CoSi2: C, 68.90; H,
6.64%. Found: C, 68.45; H, 6.62%.
8
9
Crystal data for 1 at 120 K: C27H31CoSi2, FW = 470.63, a =
9.770(2), b = 26.795(9), c = 10.188(4) Å, β = 113.093(2)°, V
= 2453(1) Å3, monoclinic, space group P21/c, Z = 4, ρ =
1.274 g·cm–3. The final R1 factor was 0.0348 for 5860 reflec-
tions with I > 2σ(I) (wR2 = 0.0936 for all data). GOF =
1.040.
We have also characterized the structure of 2 in solution on
the basis of NMR spectroscopy.9 Interestingly, in the 7Li NMR
spectrum of 2 in benzene-d6, one signal was found at δ = –4.21.
This considerable upfield shift is evidently caused by the strong
shielding effect of the diatropic ring current resulting from the
six π-electron system. This suggests that the molecular struc-
ture of 2 in the crystal is maintained in solution. However, the
signal of 2 (δ = –4.21) is slightly shifted to lower field com-
pared with that of 2[Li+]·[(Me3Si)4C42–] (δ = –5.07),6 due to the
decrease of the ring current by the introduction of phenyl
groups.
1H NMR (C6D6, 298 K, δ) 0.53 (s, 12 H, CH3), 1.19 (br.s,
THF), 1.22 (s, 4 H, CH2), 3.30 (br.s, THF), 6.74 (t, J = 7.2
Hz, 2 H, CH), 7.25 (t, J = 7.2 Hz, 4 H, CH), 7.62 (d, J = 7.2
Hz, 4 H, CH); 13C NMR (C6D6, 298 K, δ) 2.9 (CH3), 12.3
(CH2), 25.4 (THF), 68.3 (THF), 89.6 (C), 102.8 (C), 117.4
(CH), 121.3 (CH), 128.5 (CH), 142.8 (C); 29Si NMR (C6D6,
7
298 K, δ) –21.3; Li NMR (C6D6, 298 K, δ) –4.21 (LiCl in
MeOH, external).
10 Crystal data for 2 at 120 K: C30H46Li2O4Si2, FW = 540.73, a
The 13C NMR spectrum of 2 shows two signals for the
cyclobutadienediide ring carbons appearing at δ = 89.6 (PhC)
and 102.8 (SiC) together with the four signals for phenyl car-
bons (δ = 117.4 (para-), 121.3 (ortho-), 128.5 (meta-), and
142.8 (ipso-)). The 13C signals of ortho- and para-carbons are
shifted to higher field relative to those signals in 1 (δ = 126.9
(ortho-) and 125.6 (para-)) due to delocalization of the nega-
= 22.291(2), b = 10.426(2), c = 15.513(2) Å, β
=
117.085(7)°, V = 3209.9(8) Å3, monoclinic, space group
C2/c, Z = 4, ρ = 1.119 g·cm–3. The final R1 factor was
0.0757 for 3515 reflections with I > 2σ(I) (wR2 = 0.1943 for
all data). GOF = 1.101.
11 For a review, see: A. Sekiguchi and T. Matsuo, J. Syn.
Org. Chem. Jpn., 57, 53 (1999).