Zwitterionic base-stabilized digermadistannacyclobutadiene and tetragermacyclobutadiene

Article abstract of DOI:10.1002/chem.201300447

The syntheses of a zwitterionic base-stabilized digermadistannacyclobutadiene and tetragermacyclobutadiene supported by amidinates and low-valent germanium amidinate substituents are described. The reaction of the amidinate Ge^I dimer, [LGe:]₂ (1, L=PhC(NtBu)₂), with two equivalents of the amidinate tin(II) chloride, [LSnCl] (2), and KC₈ in tetrahydrofuran (THF) at room temperature afforded a mixture of the zwitterionic base-stabilized digermadistannacyclobutadiene, [L₂Ge₂Sn₂L ₂] (3; L=LGe:), and the bis(amidinate) tin(II) compound, [L ₂Sn:] (4). Compound 3 can also be prepared by the reaction of 1 with [L^ArSnCl] (5, L^Ar=tBuC(NAr)₂, Ar=2,6-iPr ₂C₆H₃) in THF at room temperature. Moreover, the reaction of 1 with the "onio-substituent transfer" reagent [4-NMe₂-C₅H₄NSiMe₃]OTf (8) in THF and 4-(N,N-dimethylamino)pyridine (DMAP) at room temperature afforded a mixture of the zwitterionic base-stabilized tetragermacyclobutadiene, [L ₄Ge₆] (9), the amidinium triflate, [PhC(NHtBu) ₂]OTf (10), and Me₃SiSiMe₃ (11). X-ray structural data and theoretical studies show conclusively that compounds 3 and 9 have a planar and rhombic charge-separated structure. They are also nonaromatic.

Full text of DOI:10.1002/chem.201300447

FULL PAPER
DOI: 10.1002/chem.201300447
Zwitterionic Base-Stabilized Digermadistannacyclobutadiene and
Tetragermacyclobutadiene
Hui-Xian Yeong,[a] Hong-Wei Xi,[b] Yongxin Li,[a] Sophy Bhasi Kunnappilly,[a]
Bozhen Chen,[c] Kai-Chung Lau,[d] Hajime Hirao,[a] Kok Hwa Lim,[b] and
Cheuk-Wai So*[a]
Abstract: The syntheses of a zwitter-
ionic base-stabilized digermadistanna-
cyclobutadiene and tetragermacyclobu-
tadiene supported by amidinates and
low-valent germanium amidinate sub-
stituents are described. The reaction of
the amidinate GeI dimer, [LGe:]2 (1,
diene, [L2Ge2Sn2L’2] (3; L’=LGe:),
and the bis(amidinate) tin(II) com-
pound, [L2Sn:] (4). Compound 3 can
also be prepared by the reaction of 1
reagent [4-NMe2-C5H4NSiMe3]OTf (8)
in THF and 4-(N,N-dimethylamino)pyr-
idine (DMAP) at room temperature af-
forded a mixture of the zwitterionic
with [LArSnCl] (5, LAr =tBuC
A
base-stabilized
diene, [L4Ge6] (9), the amidinium tri-
flate, [PhC(NHtBu)2]OTf (10), and
tetragermacyclobuta-
Ar=2,6-iPr2C6H3) in THF at room
temperature. Moreover, the reaction of
1 with the “onio-substituent transfer”
AHCTUNGTRENNUNG
L=PhC
N
Me3SiSiMe3 (11). X-ray structural data
and theoretical studies show conclu-
sively that compounds 3 and 9 have a
planar and rhombic charge-separated
structure. They are also nonaromatic.
of the amidinate tin(II) chloride,
[LSnCl] (2), and KC8 in tetrahydrofur-
an (THF) at room temperature afford-
ed a mixture of the zwitterionic base-
Keywords: cyclobutadienes ·
germanium
zwitterions
· N ligands · tin ·
stabilized
digermadistannacyclobuta-
Introduction
3,3,5,5-tetramethyl-s-hydrindacen-4-yl) comprises a planar
rhombic Si4 ring with a charge-separated structure, which
disfavors 4p electron delocalization, with the result that no
ring current can be observed.[2] A square planar tetrasilacy-
clobutadiene was reported to be stabilized as a transition-
metal complex.[3] Moreover, Power et al. reported the syn-
thesis of a 1,2-digermacyclobutadiene in which the planar
Cyclobutadiene (C4H4) is the smallest annulene containing
conjugated double bonds. The antiaromaticity and strained
geometry account for its high reactivity.[1] Recently, several
research groups have demonstrated that the introduction of
heavier Group 14 elements into a cyclobutadiene skeleton
has enabled the tuning of its stability and electronic struc-
tures.[2–5,19,24,27,28] Tamao et al. reported that the tetrasilacy-
clobutadiene [(EMind)4Si4] (EMind=1,1,7,7-tetraethyl-
trapezoidal Ge2C2 ring comprises of a C=C double bond,
[4]
ꢀ
two C Ge single bonds and a weak Ge=Ge double bond.
Additionally, we have demonstrated that the incorporation
of both silicon and germanium atoms into a cyclobutadiene
skeleton stabilized by amidinate ligands leads to a puckered
Si3Ge four-membered ring, which features an ylide structure
with considerable electron delocalization.[5] These results il-
lustrate that the geometric and electronic structure of heavi-
er cyclobutadiene analogues are governed by the heavier
Group 14 elements and supporting substituents. However,
tetragermacyclobutadiene and tetrastannacyclobutadiene
derivatives are still unknown. As such, we were interested in
incorporating germanium and/or tin atoms into a cyclobuta-
diene skeleton supported by amidinate ligands and there-
after elucidating its bonding properties. In this paper, we de-
scribe the synthesis and characterization of the zwitterionic
base-stabilized digermadistannacyclobutadiene and tetrager-
macyclobutadiene supported by amidinates and low-valent
germanium amidinate substituents.
[a] H.-X. Yeong, Dr. Y. Li, Dr. S. B. Kunnappilly, Prof. Dr. H. Hirao,
Prof. Dr. C.-W. So
Division of Chemistry and Biological Chemistry
Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
Fax : (+65)6791-1961
E-mail: CWSo@ntu.edu.sg
[b] Dr. H.-W. Xi, Prof. Dr. K. H. Lim
Division of Chemical and Biomolecular Engineering
Nanyang Technological University
62 Nanyang Drive, Singapore 637459 (Singapore)
[c] Prof. Dr. B. Chen
College of Chemistry and Chemical Engineering
Graduate University of Chinese Academy of Sciences
19A Yuquan Road, Beijing 10049 (P. R. China)
[d] Prof. Dr. K.-C. Lau
Department of Biology and Chemistry
City University of Hong Kong
Tat Chee Avenue, Hong Kong (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300447.
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Results and Discussion
dimer, [LSi:]2, to form a germanium(I)-silicon(I) intermedi-
ate [LGe-SiL], which then undergoes further rearrangement
to form the base-stabilized germatrisilacyclobutadiene
ylide.[5] It was anticipated that compound 3 could be synthe-
sized by a similar strategy. The reaction of 1 with [LArSnCl]
Synthesis and characterization of the zwitterionic base-stabi-
lized digermadistannacyclobutadiene: The reaction of the
ACTHNUTRGNEUNG
amidinate GeI dimer, [LGe:]2 (1, L=PhC(NtBu)2),[6] with
two equivalents of the amidinate tin(II) chloride, [LSnCl]
(2),[7] and KC8 in tetrahydrofuran (THF) at room tempera-
ture afforded a mixture of the zwitterionic base-stabilized
digermadistannacyclobutadiene [L2Ge2Sn2L’2] (3; L’=LGe:)
and the bis(amidinate) tin(II) compound [L2Sn:][7] (4;
ACTHNGUTERNNUG
(5; LAr =tBuC(NAr)2, Ar=2,6-iPr2C6H3)[10] in THF at room
temperature afforded a mixture of 3, [LAr2Sn:] (6),[11] and
[LGeCl] (7),[6] which was confirmed by 119Sn{1H} and
1H NMR spectroscopic analysis (Scheme 2). Volatiles were
Scheme 1), which was confirmed by 119Sn{1H} and H NMR
1
Scheme 2. Synthesis of compounds 3, 6, and 7.
removed and the residue was extracted with 1,2-dimethoxy-
ethane (DME). Concentration of the filtrate afforded com-
pound 3 in higher yield (16.8%) than with the aforemen-
tioned method. Although the mechanism is unknown as yet,
we propose that the reaction proceeds through the insertion
Scheme 1. Synthesis of compounds 3 and 4.
Ar
I
I
ꢀ
of [L SnCl] into the Ge Ge bond of 1 to form a transient
germanium(I)–tin(I) complex [LGe–SnLAr] , which then un-
dergoes a disproportionation to form 3 and 6.
spectroscopic analysis. The reaction mixture was filtered and
Et2O was added to the filtrate. Concentration of the filtrate
afforded compound 3 as a highly air- and moisture-sensitive
dark-red crystalline solid in very low yield (5.6%). Although
the mechanism for the formation of 3 and 4 is unknown as
yet, on the basis of experimental results, we propose that
the reaction appears to proceed through the reduction of
Compound 3 is the first zwitterionic base-stabilized heavi-
er cyclobutadiene analogue containing both germanium and
tin atoms. It has been characterized by NMR spectroscopy
and X-ray crystallography, and was also investigated by the-
oretical studies. Compound 3 exhibits high solubility in
THF, DME, and Et2O, and is sparingly soluble in toluene. It
is stable in the solid state at room temperature in an inert
C
[LSnCl] to form a transient tin(I) radical [LSn ] or a transi-
ent tin(I) dimer [LSn:]2, which undergoes a disproportiona-
tion with compound 1 to form 3 and 4. The evidence for the
disproportionation reaction can be illustrated by the reduc-
tion of 2 with KC8 in THF, which afforded 4 and elemental
tin.[8] Recently, Jones et al. reported that the lighter ana-
atmosphere. When crystals of
3
were dissolved in
[D6]benzene and [D8]THF at room temperature, the com-
pound decomposed slowly in the deuterated solvents within
two days to give black solids. In this regard, a freshly pre-
pared solution of 3 in [D8]THF was characterized by NMR
C
ACTHNUGTRNEU{GN CtBuNAr}2Ge ],
logue, stable germanium(I) radical [HC
was synthesized by reduction of the corresponding amidi-
nate germanium(II) chloride with Na
(C10H8) in THF.[9a]
Moreover, Jurkschat et al. reported that the organotin(I)
dimer [4-tBu-2,6-{P(OiPr)2 =O}2C6H2Sn:]2 can undergo a
disproportionation to form elemental tin and the diorgano-
stannylene [{4-tBu-2,6-{P
(OiPr)2 =O}2C6H2}2Sn:].[9b]
Recently, we reported that amidinate gemanium(II) chlor-
ide [LGeCl] can undergo an insertion with the amidinate SiI
1
spectroscopy. The H NMR spectrum of 3 shows two singlets
G
at d=0.96 and 1.40 ppm for the tBu substituents and a mul-
tiplet at d=7.19–7.28 ppm for the phenyl protons of the
amidinate ligands. The 119Sn{1H} NMR spectrum shows a
signal at d=ꢀ90.9 ppm, which lies between that of the 1,3-
diaza-2,4-distannacyclobutane-2,4-diyl[12] (d=ꢀ84 ppm) and
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
the stannyl anion [nBu3SnLiACHTGNUTRENNUNG
UV/Vis spectrum of 3 in THF at room temperature shows
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Group 14 Cylobutadienes
FULL PAPER
an absorption band at 508 nm in the visible light region. Ac-
cording to time-dependent DFT (TD-DFT) calculations on
3, the absorption maximum at 508 nm can be assigned to
the HOMO!LUMO transition (see below).
geometry. The sum of bond angles at the Ge1 atom is
245.238, which is comparable to that of the three-coordinate
germanium atoms in 1 (266.048, 267.648).[6] This geometry is
consistent with a stereoactive lone pair at the Ge1 atom.
ꢀ
The molecular structure of 3 is shown in Figure 1. The
Ge2Sn2 core is planar and rhombic, with the endocyclic Sn1-
Ge2-Sn1A and Ge2-Sn1-Ge2A angles being 110.38(2)8 and
69.62(2)8, respectively. The sum of the internal bond angles
is 360.08. The results are in contrast to the amidinate-stabi-
The Ge1 Sn1 bond (2.7247(7) ꢁ) is longer than the endocy-
ꢀ
clic Sn Ge bonds and comparable to that in the germylene-
stannylene
[HCACTHNGUTRENU(NG CMeNAr)2Sn-Ge{C(Me)CHC(Me)NAr}]
(2.7210(4) ꢁ).[15d]
Synthesis and characterization of the zwitterionc base-stabi-
lized tetragermacyclobutadiene: The reaction of 1 with
the
“onio-substituent
transfer”
reagent
[4-NMe2-
C5H4NSiMe3]OTf (8)[16] in THF and 4-(N,N-dimethylamino)-
pyridine (DMAP)[17] at room temperature afforded a mix-
ture of the zwitterionic base-stabilized tetragermacyclobuta-
diene [L4Ge6] (9), amidinium triflate [PhCACHTUNTRGNE(UNG NHtBu)2]OTf
(10), and Me3SiSiMe3 (11; Scheme 3), which was confirmed
Figure 1. ORTEP drawing of compound 3 (50% thermal ellipsoids). Dis-
order in phenyl and tBu substituents and hydrogen atoms are omitted for
ꢀ
clarity. Selected bond lengths [ꢁ] and angles [8]: Sn1 Ge1 2.7247(7),
ꢀ
ꢀ
ꢀ
ꢀ
Sn1 Ge2 2.6863(6), Sn1 Ge2A 2.6799(6), Ge1 N1 2.026(4), Ge1 N2
ꢀ
ꢀ
2.029(4), Ge2 N3 2.007(4), Ge2 N4 1.981(4), Sn1-Ge2-Sn1A 110.38(2),
Ge2-Sn1-Ge2A 69.62(2), Ge1-Sn1-Ge2 99.40(2), Ge1-Sn1-Ge2A
98.85(2), N3-Ge2-N4 66.31(16), N1-Ge1-N2 64.15(16), N1-Ge1-Sn1
90.20(13), N2-Ge1-Sn1 90.88(13).
lized germatrisilacyclobutadiene ylide, in which the Si3Ge
four-membered ring is puckered.[5] The amidinate ligands
are bonded in a N,N’-chelate fashion to the Ge2 and Ge2A
atoms, which adopt a distorted tetrahedral geometry. Each
endocyclic Sn atom in compound 3 is bonded with a low-
valent germanium amidinate substituent. The Sn1 and Sn1A
atoms adopt a distorted trigonal pyramidal geometry (the
sum of bond angles around the Sn atoms being 267.878).
This geometry is consistent with a stereoactive lone pair on
Scheme 3. Synthesis of compounds 9–11.
by NMR spectroscopy and X-ray crystallography (9:
Figure 2; 10: Figure S2). The reaction mixture was filtered
and the filtrate was concentrated to afford 9 as a highly air-
and moisture-sensitive red crystalline solid in 25.8% yield.
When volatiles of the reaction mixture were removed in
vacuo and the crude product was extracted with toluene, a
mixture of red crystals of 9, colorless crystals of 10, and crys-
tals of DMAP were afforded after concentration of the fil-
trate. The results are in contrast to the reaction of the light-
er congener [LSi:]2 with 8, which afforded the silyliumyli-
ꢀ
ꢀ
the Sn atoms. The Sn1 Ge2 (2.6863(6) ꢁ) and Sn1 Ge2A
(2.6799(6) ꢁ) bonds are longer than the Ge=Sn
(2.5065(5) ꢁ) double bond in a stannagermene,[14] and are
ꢀ
comparable to reported Sn Ge single bonds (2.599(3)–
2.7210(4) ꢁ).[15] This suggests that the endocyclic Sn Ge
ꢀ
bonds in 3 are single bonds. The results indicate that the
Ge2Sn2 core has a charge-separated structure, which was
also confirmed by theoretical studies (see below). Moreover,
the amidinate ligand is bonded in a N,N’-chelate fashion to
the Ge1 and Ge1A atoms, which adopt a trigonal pyramidal
dene cation [LSi
ACHTUNGTRENN(UNG DMAP)]OTf supported by an amidinate
and DMAP.[18] Although the mechanism for the formation
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C.-W. So et al.
The molecular structure of 9, which is isostructural with
compound 3, is shown in Figure 2. The Ge4 core is planar
and rhombic, and the amidinate ligands are bidentate
bonded to the Ge3 and Ge3A atoms, which adopt a distort-
ed tetrahedral geometry. The Ge2 and Ge2A atoms are
bonded with a low-valent germanium amidinate substituent
and adopt a distorted trigonal pyramidal geometry (the sum
of bond angles around the Ge atoms being 283.278) that is
consistent with a stereoactive lone pair on them. The endo-
ꢀ
cyclic Ge Ge bonds (2.4577(6), 2.4568(6) ꢁ) are compara-
[20]
ꢀ
ble to the Ge Ge single bonds (average 2.44 ꢁ). They are
also comparable to those in the digermacyclobutadiene
(2.4708(9) ꢁ).[4] The results indicate that the Ge4 core has a
charge-separated structure, which is comparable to that of 3.
The Ge1 and Ge1A atoms adopt a trigonal pyramidal geom-
etry (the sum of bond angles around the Ge atoms being
251.268), which indicate the presence of electron lone pairs.
ꢀ
ꢀ
The Ge1 Ge2 and Ge1A Ge2A bonds (2.5461(6) ꢁ) are
comparable to that in 1 (2.569(5) ꢁ).[6] The results imply
that these bonds do not have any multiple bond character.
Figure 2. ORTEP drawing of compound 9 (50% thermal ellipsoids). Hy-
drogen atoms and THF molecule are omitted for clarity. Selected bond
Theoretical studies of the zwitterionic base-stabilized diger-
madistannacyclobutadiene and tetragermacyclobutadiene:
To understand the bonding nature in compounds 3 and 9,
they were investigated by means of density functional calcu-
ꢀ
ꢀ
lengths [ꢁ] and angles [8]: Ge1 Ge2 2.5461(6), Ge2 Ge3 2.4568(6),
ꢀ
ꢀ
ꢀ
ꢀ
Ge2 Ge3A 2.4577(6), Ge1 N1 2.020(3), Ge1 N2 2.026(3), Ge3 N3
ꢀ
1.989(3), Ge3 N4 1.976(3), N1-Ge1-N2 64.43(12), N1-Ge1-Ge2 93.68(9),
N2-Ge1-Ge2 93.15(9), Ge1-Ge2-Ge3 105.03(2), Ge1-Ge2-Ge3A
106.47(2), Ge3A-Ge2-Ge3 71.77(2), Ge2-Ge3-Ge2A 108.23(2), N3-Ge3-
N4 66.16(12).
lations.[21] The optimized geometries of
3 (UB3PW91/
LANL08d for Sn, 6-31+G(d) for Ge and N, and 6-31G(d)
for C and H; Figure S3) and 9 (UB3PW91/6-31+G(d); Fig-
ure S5) are in good agreement with the X-ray crystallo-
graphic data.
of 9 is unknown as yet, on the basis of the aforementioned
experimental results, we propose that the reaction of 1 and
The HOMO of 3 (Figure S4) and 9 (Figure S6) are com-
prised of two exocyclic Ge1-E and Ge1A-E s orbitals (3:
E=Sn; 9: E=Ge). The HOMO-1 of 3 and 9 mainly arise
from the combinations of lone pair orbitals at the three-co-
ordinate E atoms of the Ge2E2 ring. The HOMO-1 of 3 also
involves the lone pair orbitals at the exocyclic Ge atoms.
The HOMO-2 of 3 and 9 comprise of the endocyclic Ge-E s
orbitals. The LUMO of 3 and 9 involve: 1) the interaction of
p orbitals between the four coordinate Ge atoms, 2) the exo-
cyclic Ge-E s* orbitals, and 3) the p* orbitals of the phenyl
rings. The LUMO+1 of 3 comprises p* orbitals of the
phenyl rings and the exocyclic Ge-Sn p orbitals. The
LUMO+1 and LUMO+2 of 9 comprise of the p* orbitals
8 proceeds through the oxidation of 1 or the transient ger-
II
C
manium(I) radical [LGe ] with 8 to form a Ge cation inter-
mediate [LGe]OTf and 11. Subsequently, the highly electro-
philic GeII cation intermediate [LGe]OTf undergoes a dis-
proportionation with 1 to form 9 and 10. Recently, Driess
et al. proposed a similar mechanism for the reaction of the
amidinate silicon(II) chloride [LSiCl] with Cp*ZrCl3 (Cp*=
C5Me5) and concluded that it proceeds through the forma-
tion of the transient amidinate silicon(II) cation [LSi]
[Zr2Cl7Cp*], which undergoes a disproportionation with
[LSiCl] to form the base-stabilized tetrasilacyclobutadiene
dication, LH+Clꢀ and LSiCl3.[19]
of the phenyl rings. Moreover, the CASSCFACTHNUTRGENU(GN 4,6) of 3 and 9
Compound 9 is the first example of a zwitterionic base-
stabilized tetragermacyclobutadiene. It has been character-
ized by NMR spectroscopy and X-ray crystallography. It
was also investigated by theoretical studies. Compound 9 is
soluble in THF, DME, and pyridine, and is stable in the
solid state at room temperature in an inert atmosphere. The
1H and 13C NMR spectra display resonances due to the ami-
dinate ligands. The UV/Vis spectrum of 9 in THF at room
temperature shows an absorption band at 524 nm in the visi-
ble light region. According to time-dependent DFT (TD-
DFT) calculations on 9, the absorption maximum at 524 nm
is assigned to a mixture of HOMO!LUMO and HOMO!
LUMO+2 transitions (see below).
show that they have no biradicaloid character.
The Wiberg bond index (WBI)[22] indicates that the bond
ꢀ
orders of the endocyclic Ge E bonds (3: WBI: 0.90; 9:
WBI: 0.99) are equal and single bonds. Moreover, the NPA
charges of 3 (Ge2: 0.60, Sn1: ꢀ0.13; Table S3) and 9 (Ge2,
Ge2A: ꢀ0.54; Ge3, Ge3A: 0.89; Table S6) show that the
Ge2E2 four-membered ring has a charge-separated structure.
Furthermore, the nucleus-independent chemical shift
(NICS)[23] of 3 [NICS(1): 0.75, 0.96, NICS(0): 4.00] and 9
[NICS(1): ꢀ1.10, NICS(0): 2.48] suggest that there is no de-
localization of p electrons in the Ge2E2 four-membered ring.
Thus, compounds 3 and 9 are nonaromatic. The results are
in contrast to the amidinate-stabilized germatrisilacyclobuta-
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Group 14 Cylobutadienes
FULL PAPER
diene ylide[5] and 1,3-diphospha-2,4-disilacyclobutadiene,[24]
which display electron delocalization within the four-mem-
bered ring structure.
and Biological Chemistry, Nanyang Technological University. Melting
points were measured in sealed glass tubes and are not corrected.
Synthesis of 3: THF (20 mL) was added to a mixture of 1 (0.312 g,
0.513 mmol), 2 (0.398 g, 1.03 mmol), and KC8 (0.144 g, 1.07 mmol) at am-
bient temperature, and the resulting red mixture was stirred, overnight.
The insoluble precipitate was filtered off and Et2O was added to the fil-
trate. After filtration and concentration of the filtrate, 3 (0.021 g, 5.6%)
was obtained as dark-red crystals.
The natural-bond-orbital (NBO)[25] analyses of
3
(Table S1) and 9 (Table S4) also show that the Lewis struc-
tures of the Ge2E2 rings have four occupied s orbitals for
ꢀ
the endocyclic Ge E bonds [electron occupancy: 1.80 e (3),
Alternative method: THF (10 mL) was added to a mixture of 1 (0.304 g,
0.501 mmol) and 5 (0.287 g, 0.501 mmol) at ambient temperature, and the
resulting red mixture was stirred, overnight. Volatiles were removed in
vacuo and the residue was extracted with DME. After filtration and con-
centration of the filtrate, 3 (0.0305 g, 16.8%) was obtained as dark-red
crystals. M.p. 2908C (dec); 1H NMR (399.5 MHz, [D8]THF, 22.28C): d=
0.96 (s, 36H; tBu), 1.40 (s, 36H; tBu), 7.19–7.28 ppm (m, 20H; Ph);
13C NMR (100.6 MHz, [D8]THF, 23.38C): d=29.15, 32.91 (CMe3), 51.50,
53.28 (CMe3), 125.83, 128.12, 128.22, 128.70, 128.94, 129.46, 138.23, 141.40
(Ph), 152.93, 153.83 ppm (NCN); 119Sn{1H} NMR (149.1 MHz, THF,
24.78C): d=ꢀ90.9 ppm; UV/Vis (THF): lmax (e, molꢀ1 dm3 cmꢀ1)=221
(7884), 241 (6063), 246 (5774), 252 (5594), 258 (5603), 270 (6225), 354
(3867), 508 nm (1952); elemental analysis calcd (%) for C60H92Ge4N8Sn2:
C 49.58, H 6.38; N 7.71; found: C 49.24, H 6.32, N 7.43.
1.82 e (9)]. Moreover, there are two lone pair orbitals on
the three-coordinate E atoms (3: sp50.9 hybrids; 9: p orbital),
which are stabilized by geminal hyperconjugation with the
endocyclic and exocyclic Ge-E s* orbitals [the sum of
second order perturbation stabilizing energies for each lone
pair orbital: 417.1 kcalmolꢀ1 (3, Table S2); 91.2 kcalmolꢀ1
(9, Table S5)].
Furthermore, the NBO analyses show that the lone pair
electrons at the Ge1 and Ge1A atoms in 3 and 9 are high in
s-character, with some directionality (3: sp0.22, occupancy:
1.96; 9: sp0.24, occupancy: 1.95). The WBI shows that the
ꢀ
exocyclic GeACHTUNGTRENNUNG(1/1A) E bonds [WBI: 0.84 (3); 0.86 (9)] are
Crystal data for 3: [C60H92Ge4N8Sn2]; M=1453.16; monoclinic; space
group P21/n; a=9.2404(4), b=19.1831(9), c=18.9287(10) ꢁ; b=
99.302(4)8; V=3311.2(3) ꢁ3; Z=2; 1calcd =1.458 mgmꢀ3; 48844 measured
reflections; 10587 independent reflections; 432 refined parameters; R1 =
0.0501, wR2 =0.1059 (I>2s(I)).
single bonds.
Overall, the results show that compounds 3 and 9 have a
planar rhombic charge-separated Ge2E2 ring. Moreover, in a
recent review, Bertrand showed that when a l5-phosphacy-
clobutadiene contains a phosphorus atom without any p or-
bitals available for the p system of a cyclobutadiene, the 4p
electron four-membered heterocycle is not antiaromatic and
features strong ylide character or interrupted cyclic p deloc-
alization.[26] The results are comparable to those obtained
with compounds 3 and 9, which are nonaromatic.
Synthesis of 9: A solution of 8 (0.176 g, 0.511 mmol) in THF (2 mL) was
added to a stirring solution of 1 (0.309 g, 0.509 mmol) and DMAP
(0.128 g, 1.05 mmol) in THF (10 mL) at ambient temperature, and the re-
sulting solution was stirred for 12 h. After filtration and concentration of
the filtrate, 9 (0.0493 g, 25.8%) was obtained as red crystals. M.p. 2288C
(dec); 1H NMR (395.9 MHz, [D8]THF, 23.48C): d=1.16 (s, 72H; tBu),
7.28–7.40 ppm (m, 20H; Ph); 13C NMR (100.5 MHz, [D5]Pyr, 23.68C):
d=32.44, 32.50 (CMe3), 53.51, 53.84 (CMe3), 128.42, 129.11, 129.73,
130.38, 130.75, 131.26, 131.97, 137.65 (Ph), 154.95, 158.35 ppm (NCN);
UV/Vis (DME): lmax (e, molꢀ1 dm3 cmꢀ1)=225 (272), 237 (270), 252 (287),
258 (298), 275 (331), 348 (376), 524 nm (414); elemental analysis calcd
(%) for C60H92Ge6N8: C 52.92, H 6.81, N 8.23; found: C 52.78, H 6.53, N
8.15.
Conclusion
We have reported the syntheses of the first zwitterionic
base-stabilized digermadistannacyclobutadiene and tetrager-
macyclobutadiene by using simple procedures. X-ray struc-
tural data and theoretical studies show conclusively that
compounds 3 and 9 have a planar and rhombic charge-sepa-
rated skeleton. The Ge2Sn2 four-membered ring in 3 and the
Ge4 four-membered ring in 9 comprise four single bonds,
with lone-pair electrons at the three-coordinate Sn and Ge
atoms, respectively. Moreover, compounds 3 and 9 are non-
aromatic. The reactivities of compounds 3 and 9 are under
investigation and the results will be published in due course.
Crystal data for 9: [C68H108Ge6N8O2]; M=1505.16; monoclinic; space
group P21/c; a=11.8191(7), b=17.2043(11), c=18.0867(11) ꢁ; b=
93.573(3)8; V=3670.6(4) ꢁ3; Z=2; 1calcd =1.362 mgmꢀ3; 88284 measured
reflections; 12224 independent reflections; 391 refined parameters; R1 =
0.0580, wR2 =0.1400 (I>2s(I)).
X-ray data collection and structural refinement: Intensity data for com-
pounds 3, 6, 9, and 10 were collected with a Bruker APEX II diffracto-
AHCTUNGERTGmNNUN eter. The crystals were measured at 103(2) K. The structures were
solved by direct phase determination (SHELXS-97) and refined for all
data by full-matrix least squares methods on F2.[29] All non-hydrogen
atoms were subjected to anisotropic refinement. The hydrogen atoms
were generated geometrically and allowed to ride on their respective pa-
rents atoms; they were assigned appropriate isotopic thermal parameters
and included in the structure-factor calculations. CCDC-904992 (3),
904993 (6), 904994 (9), and 904995 (10) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
General procedure: All manipulations were carried out under an inert at-
mosphere of argon gas by using standard Schlenk techniques. THF,
DME, Et2O, and toluene were dried over and distilled over Na/K alloy
prior to use. [D6]Benzene and [D8]THF were dried over and distilled
over K metal prior to use. [D5]Pyr was dried over and distilled over
CaH2 prior to use. Compounds 1, 2, 5, and 8 were prepared as previously
Acknowledgements
This work was supported by the Academic Research Fund Tier 1 (RG
57/11).
1
described.[6,7,10,16] The H, 13C and 119Sn NMR spectra were recorded with
JEOL ECA 400 and Bruker Avance 400 spectrometers. The chemical
shifts d are reported relative to SiMe4 for 1H and 13C, and SnMe4 for
119Sn. Elemental analyses were performed by the Division of Chemistry
Chem. Eur. J. 2013, 00, 0 – 0
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C.-W. So et al.
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Received: February 4, 2013
Revised: June 27, 2013
Published online: && &&, 0000
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These are not the final page numbers!

Group 14 Cylobutadienes
FULL PAPER
Heavy elements: Zwitterionic base-sta-
bilized digermadistannacyclobutadiene
(1) and tetragermacyclobutadiene (2)
were synthesized by simple procedures
(see scheme). X-ray structural data
and theoretical studies show that these
compounds have a planar and rhombic
charge-separated structure. They are
also nonaromatic.
Group 14 Elements
H.-X. Yeong, H.-W. Xi, Y. Li,
S. B. Kunnappilly, B. Chen, K.-C. Lau,
H. Hirao, K. H. Lim,
C.-W. So* ...................... &&&& — &&&&
Zwitterionic Base-Stabilized Digerma-
distannacyclobutadiene and Tetrager-
macyclobutadiene
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!