Isolable p - And m -[(tBu2MeSi)2Si] 2C6H4: Disilaquinodimethane vs triplet bis(silyl radical)

Article abstract of DOI:10.1021/ja2014746

Isomeric p- and m-disilaquinodimethanes 2 and 4 were synthesized by the reductive dehalogenation of the corresponding p- and m-bis(halosilyl)benzenes 1 and 3, respectively, and were isolated and structurally characterized. The X-ray diffraction and solid-

Full text of DOI:10.1021/ja2014746

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Isolable p- and m-[(tBu2MeSi)2Si]2C6H4: Disilaquinodimethane vs
Triplet Bis(silyl radical)
Takeshi Nozawa, Michiyo Nagata, Masaaki Ichinohe, and Akira Sekiguchi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
S Supporting Information
b
In this paper, we report the synthesis and structural identification
ABSTRACT: Isomeric p- and m-disilaquinodimethanes 2
and 4 were synthesized by the reductive dehalogenation of
the corresponding p- and m-bis(halosilyl)benzenes 1 and 3,
respectively, and were isolated and structurally character-
ized. The X-ray diffraction and solid-state NMR studies of 2
revealed its singlet quinodimethane structure featuring two
exocyclic SidC double bonds with some singlet biradical
contribution. In contrast, the X-ray crystallography and EPR
measurements of 4 disclosed its biradical nature, described
as a triplet ground state bis(silyl radical).
of the isomeric p- and m-disilaquinodimethanes 2 and 4, of which
the former is classified as having a predominantly singlet ground-
state p-disilaquinodimethane form featuring two exocyclic SidC
bonds, whereas the latter was found to represent a triplet ground-
state bis(silyl radical).
1,4-Bis(bromosilyl)benzene derivative 1 was reduced with 2
equiv of KC8 in dry THF at ꢀ78 °C, forming 3,6-bis[bis(di-tert-
butylmethylsilyl)silylidene]cyclohexa-1,4-diene (2), which was
isolated from pentane as air-sensitive purple crystals in 23% yield
(Scheme 1). Likewise, the meta-isomer, 1,3-bis[bis(di-tert-butyl-
methylsilyl)silyl]benzene-10,100-diyl (4), was prepared by reduc-
tion of the corresponding precursor, 1,3-bis(iodosilyl)benzene
derivative 3, and isolated by recrystallization from pentane as air-
sensitive yellow crystals in 29% yield (Scheme 1).13,14 The
electronic spectra of isomeric 2 and 4 were distinctly different
(see the Supporting Information). Thus, the longest wavelength
absorption of 2 was remarkably red-shifted compared with that of
4 (2, 555 nm (ε 6800) vs 4, 414 nm (ε 870), 433 nm (ε 780)),14
pointing to different electronic structures for 2 and 4. Such a
distinction was demonstrated by X-ray crystallographic studies,
which revealed that in their crystalline form these two isomers
have quite different geometries. Thus, 2 exhibited a disilaquino-
dimethane structure with trigonal-planar configuration at the Si1
and Si1# atoms (sum of the bond angles 359.96°) and coplanar
alignment of the 3p(Si) and 2p(C) (aromatic ring) orbitals,
allowing their effective π-conjugation (Figure 1, left).14 The
Si1ꢀC1 bond of 1.8174(14) Å is notably shorter than typical
SiꢀC(aryl) single bonds (1.879 Å),15 although longer than the
SidC bonds in previously reported silenes (1.702ꢀ1.775 Å).16
Within the six-membered ring, CꢀC bond alternation character-
istic of the cyclohexa-1,4-diene system was observed, C1ꢀC2 =
1.423(2) Å, C1ꢀC3# = 1.422(2) Å, C2ꢀC3 = 1.3700(19) Å,
although the extent of such alternation (Δ ≈ 0.05 Å) was smaller
than that of the carbon analogue, tetraphenylparaquinodimethane
(Δ = 0.10 Å).3b These structural features of 2 imply that its structure
is best described as the predominant contribution of the closed-shell
quinoid form with some (but by no means full) contribution of the
singlet bis(silyl radical) character. The solid-state 13C and 29Si NMR
spectra of 2 are also in accord with its formulation as the p-
disilaquinodimethane derivative: the resonances observed at 156.3
ppm (13C NMR) and 91.1 ppm (29Si NMR) are typical for the
doubly bonded C and Si atoms of stable silenes SidC.16
rganic free radicals are open-shell species featuring an
O
unpaired electron and are ubiquitous in a variety of
chemical transformations. To date, a number of free radicals
have been classified as persistent or even isolable species, in
which the highly reactive odd-electron centers are stabilized by
either thermodynamic or kinetic means.1 On the other hand, the
high-spin molecules possessing several radical centers in a single
molecule and serving as model compounds for the design of
organic ferromagnetic materials1,2 are more challenging, espe-
cially from the viewpoint of potential communication between
the radical centers. Among the most representative examples of
such species, one can mention p- and m-quinodimethane deri-
vatives, of which the para-isomer is known to possess a singlet
ground state with a closed-shell quinoid form,3 whereas the meta-
isomer was shown to manifest a robust triplet ground state.4
The heavy analogues of organic free radicals featuring an
unpaired electron on a silicon, germanium, tin, or lead atom as
the key reactive intermediates in a number of organometallic
transformations represent another important synthetic challenge.5
Although the first persistent radicals of the type [(Me3Si)2-
CH]3E• (E = Si, Ge, Sn) were reported by Lappert and co-
workers as early as 1973,6 the isolation and structural characteriza-
tion of the first stable cyclotrigermenyl radical was reported by
Power et al. in 1997,7a and then the stable silyl radical was achieved
in 2001 with the synthesis of a cyclotetrasilenyl radical.7b Since
then, several other representatives of isolable silyl,8 germyl,8,9
stannyl,10 and plumbyl11 radicals have been reported, including
neutral acyclic nonconjugated radicals, cyclic radicals, and
charged radical species (such as silylene anion-radical, disilene
anion-radical and cation-radical, and disilyne anion-radical) as
well as singlet biradicaloid species.12 However, despite such
recent advances in the field, oligo-radicals with two (or more)
heavy group 14 element radical centers have remained elusive.
In contrast, in 4 the 3p(Si) and 2p(C) (aromatic ring) orbitals
are orthogonal to each other, an arrangement that prevents their
Received: February 16, 2011
Published: March 28, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja2014746 J. Am. Chem. Soc. 2011, 133, 5773–5775
|

Journal of the American Chemical Society
COMMUNICATION
Scheme 1. Synthesis of the Disilaquinodimethane 2 and
Bis(silyl radical) 4
Figure 2. EPR spectrum of 4 in 3-methylpentane at 80 K. The peaks
marked by ꢁ are due to a monosilyl radical impurity.
Scheme 2. Reactivity of the Disilaquinodimethane 2 and
Bis(silyl radical) 4
Figure 1. (Left) ORTEP drawing of 2 (50% probability level). One of
the tert-butyl groups on Si3 is disordered, and the minor contribution
(occupancy factor 0.299) and hydrogen atoms are omitted for clarity. #
indicates the following symmetry transformation: ꢀx, ꢀy þ 1, ꢀz.
Selected bond lengths (Å) and angles (deg): Si1ꢀSi2 = 2.3675(6),
Si1ꢀSi3 = 2.3737(6), Si1ꢀC1 = 1.8174(14), C1ꢀC2 = 1.423(2),
C1ꢀC3# = 1.422(2), C2ꢀC3 = 1.3700(19). Si2ꢀSi1ꢀSi3 = 122.86(2),
Si2ꢀSi1ꢀC1 = 117.52(5), Si3ꢀSi1ꢀC1 = 119.58(5), Si1ꢀC1ꢀC2 =
120.66(11), Si1ꢀC1ꢀC3# = 124.39(11), C2ꢀC1ꢀC3# = 114.94(12),
C1ꢀC2ꢀC3 = 122.78(13), C2ꢀC3ꢀC1# = 122.29(13). (Right)
ORTEP drawing of 4 (50% probability level). Hydrogen atoms are
omitted for clarity. # indicates the following symmetry transformation:
ꢀx þ 1/2, ꢀy þ 1/2, z. Selected bond lengths (Å) and angles (deg):
Si1ꢀSi2 = 2.3554(14), Si1ꢀSi3 = 2.3524(14), Si1ꢀC2 = 1.9108(19),
C1ꢀC2 = 1.387(3), C1ꢀC2# = 1.387(3), C2ꢀC3 = 1.395(5),
C3ꢀC4 = 1.386(4), C4ꢀC3# = 1.386(4). Si2ꢀSi1ꢀSi3 = 122.21(5),
Si2ꢀSi1ꢀC2 = 117.15(5), Si3ꢀSi1ꢀC2 = 119.29(5), C2ꢀC1ꢀC2# =
123.2(4), Si1ꢀC2ꢀC1 = 120.8(3), Si1ꢀC2ꢀC3 = 121.73(17),
C1ꢀC2ꢀC3 = 117.5(2), C2ꢀC3ꢀC4 = 121.0(3), C3ꢀC4ꢀC3# =
119.7(4).
π-bonding, although the geometry at both Si1 and Si1# atoms is
still nearly trigonal-planar (sum of the bond angles 358.65°)
(Figure 1, right).14 Accordingly, the Si1ꢀC2 and Si1#ꢀC2#
bonds of 1.9108(19) Å were no longer shortened, and CꢀC
bond alternation was not observed within the six-membered ring,
pointing to its 6π-electron aromaticity. Such structural peculia-
rities of 4 permit its classification as a bis(silyl radical) bridged by
a m-phenylene group, and indeed computations showed the
preference for the triplet ground state of 4 over its singlet state by
∼20 kcal/mol.17 Such an assumption on the spin multiplicity of 4
was further corroborated by its EPR measurements. The EPR
spectrum of 4 (Figure 2) measured at 80 K in 3-methylpentane
showed characteristic signals at 335.2 mT (g = 2.0034). As is well
known, the EPR resonances of triplet biradicals are split by the
magnetic dipoleꢀdipole interaction between unpaired electrons
(zero-field splitting, ZFS).19 Accordingly, the EPR signal of 4 was
split into six lines with the ZFS parameters D = 6.4 ꢁ 10ꢀ3 cmꢀ1
(13.8 mT) and E = 0.80 ꢁ 10ꢀ3 cmꢀ1 (1.72 mT). The average
distance between the unpaired electrons in 4 was thus estimated
as 5.89 Å, assuming the point-dipole approximation, a value that
is close to the distance of 5.72 Å determined from the X-ray
crystal structure, indicating that in 4 the two unpaired electrons
reside mainly on the silicon atoms. A resonance at 167.4 mT
was observed in a characteristic half-field region corresponding
to a forbidden Δm = 2 transition. The temperature-dependent
EPR measurements of this signal intensity revealed that it
follows the Curie law (see the Supporting Information),18
indicating that 4 represents a triplet ground-state biradical,
despite the perpendicular arrangement of its m-phenylene
π-orbitals and 3p(Si) orbitals holding unpaired electrons, in
contrast to the coplanar conjugated system of these orbitals in the
carbon analogue.1,2
In accord with their differing structures, 2 and 4 react differently
with MeOH and 1,4-cyclohexadiene (Scheme 2). Thus, 2 is
reactive toward both MeOH (as a typical reagent for silenes) to
give the 1,6-adduct 5 and 1,4-cyclohexadiene (as a hydrogen
donor) to give the hydrogen-abstraction product 6 in quantita-
tive yields.14 These results indicate that 2 has both a SidC double
bond and bis(silyl radical) reactivity. On the other hand,
although 4 failed to react with MeOH, it quantitatively reacted
with 1,4-cyclohexadiene to form m-bis(hydrosilyl)benzene 7,14
indicating that 4 has only bis(silyl radical) reactivity.
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Journal of the American Chemical Society
COMMUNICATION
In summary, we have succeeded in the synthesis, isolation, and
structural characterization of the first stable p- and m-disilaqui-
nodimethane derivatives, 2 and 4. Their noticeable differences
are the alignments of the 3p(Si) and 2p(C) (aromatic ring)
orbitals. The former exhibited a nearly planar arrangement,
giving a p-disilaquinodimethane structure, whereas the latter
exhibited an orthogonal arrangement, leading to a diradical
structure with a triplet ground state.
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’ ASSOCIATED CONTENT
(10) Sekiguchi, A.; Fukawa, T.; Lee, V. Ya.; Nakamoto, M. J. Am.
Chem. Soc. 2003, 125, 9250.
S
Supporting Information. Experimental procedures and
b
spectral data for compounds 1ꢀ7; tables of crystallographic data,
including atomic positional and thermal parameters, and CIF
files for 2 and 4; UVꢀvis spectral charts of 2 and 4; temperature-
dependent EPR spectral chart of 4. This material is available free
of charge via the Internet at http://pubs.acs.org.
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H.-J.; Kelm, H. Angew. Chem., Int. Ed. 2007, 46, 1156. (b) Becker, M.
F€orster, C.; Franzen, C.; Harthrath, J.; Kirsten, E.; Knuth, J.; Klinkhammer,
K. W.; Sharma, A.; Hinderberger, D. Inorg. Chem. 2008, 47, 9965.
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centered free radicals:(a) Power, P. P. Chem. Rev. 2003, 103, 789.
(b) Lee, V. Ya.; Sekiguchi, A. Eur. J. Inorg. Chem. 2005, 1209. (c) Lee, V.
Ya.; Sekiguchi, A. Acc. Chem. Res. 2007, 40, 410.(d) Lee, V. Ya.;
Sekiguchi, A. In Reviews of Reactive Intermediates Chemistry; Moss,
R. A., Platz, M. S., Jones, M., Jr., Eds.; Wiley: Hoboken, 2007; Chapter
2. (e) Lee, V. Ya.; Sekiguchi, A. Organometallic Compounds of Low-
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’ AUTHOR INFORMATION
Corresponding Author
sekiguch@chem.tsukuba.ac.ip
’ ACKNOWLEDGMENT
(13) Reduction of the corresponding 1,3-bis(bromosilyl)benzene
derivative with potassium graphite in THF failed to produce the desired
4.
This work was supported by a Grant-in-Aid for Scientific
Research (Nos. 19105001, 21350023, 21108502) from the
Ministry of Education, Science, Sports, and Culture of Japan,
JSPS Research Fellowship for Young Scientist (T.N.). We are
grateful to Prof. Yitzhak Apeloig and Dr. Boris Tumanskii
(Technion) for helpful discussions on the EPR spectrum of 4.
The authors are grateful to Dr. Vladimir Ya. Lee for valuable
discussions.
(14) For the synthesis as well as spectral and crystal data of new
compounds 1ꢀ7, see the Supporting Information.
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Organic Silicon Compounds, Vol. 2, Part 1; Rappoport, Z., Apeloig, Y.,
Eds.; Wiley: Chichester, 1998; Chapter 5.
(16) See ref 12e, Chapter 5.
(17) Theoretical calculations on the model compound
[(H3Si)2Si]2C6H4 were performed at the DFT UB3LYP/6-31 g(d)
level using the GAUSSIAN03 program package.
(18) For temperature-dependent EPR measurements of 4, see the
Supporting Information.
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