Stabilized terphenyl-substituted digermene derivatives of simple organic groups and their halide precursors: Preference for symmetrically bonded structures

Article abstract of DOI:10.1021/om001077z

The reaction of 1 equiv of MeMgBr, EtMgBr, or LiPh with Ge(Cl)C₆H₃-2,6-Trip₂ (1; Trip = C₆H₂-2,4,6-i-Pr₃) in diethyl ether solution afforded the dimers {Ge(R)C₆H₃-2,6-Trip₂}₂ (R = Me (2), Et (3), Ph (4)), which have trans-bent Ge-Ge-bonded structures that are maintained in solution. The compounds 2 and 3 are rare examples of digermene species having alkyl substituents. The previously described halide precursor 1 was found to crystallize simultaneously as both a V-shaped monomer and weakly Ge-Ge-bonded dimer {Ge(Cl)C₆H₃-2,6-Trip₂}₂ (5), which dissociated readily to monomers in hydrocarbon solution. This species reacted readily with pyridine (py) to form the monomeric 1:1 adduct py·Ge-(Cl)C₆H₃-2,6-Trip₂ (7), which has pyramidal coordination at germanium. The dimeric structures found for 2-4 were in sharp contrast with recently published results for their tin and lead analogues, which displayed either unsymmetric dimeric structures as in 2,6-Trip₂H₃C₆(Me)₂ SnS?nC₆H₃-2,6-Trip₂ or monomeric structures as in Pb(Me)C₆H₃-2,6-Trip₂. The chloro derivatives {Ge(Cl)C₆H₃-2,6-Trip₂}_n (n = 1, 2; i.e. 1 and 5) also differed from their tin congeners in that the corresponding tin dimer is associated through chloride bridging, whereas 5, and the related dimer {Ge(Cl)C₆H₃-2,6-Mes₂}₂ (6), are associated through weak element-element bonding. The experimental results are in general agreement with earlier theoretical data on the hypothetical M₂H₄ (M = Ge, Sn, Pb) model compounds which predicted a stable, symmetric, dimeric, Ge-Ge-bonded structure for germanium but an unsymmetric methylmethylene analogue structure for tin.

Full text of DOI:10.1021/om001077z

1820
Organometallics 2001, 20, 1820-1824
Sta bilized Ter p h en yl-Su bstitu ted Diger m en e Der iva tives
of Sim p le Or ga n ic Gr ou p s a n d Th eir Ha lid e P r ecu r sor s:
P r efer en ce for Sym m etr ica lly Bon d ed Str u ctu r es
Matthias Stender, Lihung Pu, and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue,
Davis, California 95616
Received December 19, 2000
The reaction of 1 equiv of MeMgBr, EtMgBr, or LiPh with Ge(Cl)C6H3-2,6-Trip2 (1; Trip
) C6H2-2,4,6-i-Pr3) in diethyl ether solution afforded the dimers {Ge(R)C6H3-2,6-Trip2}2 (R
) Me (2), Et (3), Ph (4)), which have trans-bent Ge-Ge-bonded structures that are
maintained in solution. The compounds 2 and 3 are rare examples of “digermene” species
having alkyl substituents. The previously described halide precursor 1 was found to
crystallize simultaneously as both a V-shaped monomer and weakly Ge-Ge-bonded dimer
{Ge(Cl)C6H3-2,6-Trip2}2 (5), which dissociated readily to monomers in hydrocarbon solution.
This species reacted readily with pyridine (py) to form the monomeric 1:1 adduct py‚Ge-
(Cl)C6H3-2,6-Trip2 (7), which has pyramidal coordination at germanium. The dimeric
structures found for 2-4 were in sharp contrast with recently published results for their
tin and lead analogues, which displayed either unsymmetric dimeric structures as in 2,6-
Trip2H3C6(Me)2SnS¨nC6H3-2,6-Trip2 or monomeric structures as in Pb(Me)C6H3-2,6-Trip2. The
chloro derivatives {Ge(Cl)C6H3-2,6-Trip2}n (n ) 1, 2; i.e. 1 and 5) also differed from their tin
congeners in that the corresponding tin dimer is associated through chloride bridging,
whereas 5, and the related dimer {Ge(Cl)C6H3-2,6-Mes2}2 (6), are associated through weak
element-element bonding. The experimental results are in general agreement with earlier
theoretical data on the hypothetical M2H4 (M ) Ge, Sn, Pb) model compounds which predicted
a stable, symmetric, dimeric, Ge-Ge-bonded structure for germanium but an unsymmetric
methylmethylene analogue structure for tin.
In tr od u ction
valence isomer (PbC6H3-2,6-Trip2)2.9 In addition, the
reactions of terphenyl element(II) halides with organ-
otransition-metal anions have led to species with the
first stable triple bonds to germanium, as in (η5-C5H5)-
(CO)2MtGe-Ar (M ) Cr, Mo, W).10 In contrast, their
reactions with Grignard, organolithium, and related
reagents are expected to yield the more straightforward,
but relatively rare, diorgano compounds of the formula
M(R)Ar (R ) organic or related substituent). This has
proven to be the case for the reactions of Pb(Br)C6H3-
2,6-Trip2 with MeMgBr, MeLi, PhLi, or t-BuLi, which
afforded the simple monomeric species Pb(R)C6H3-2,6-
Trip2 (R ) Me, Ph, t-Bu) in good yields.5 However, the
corresponding reactions of MeLi or PhLi with Sn(Cl)-
C6H3-2,6-Trip2 yield a variety of different and unex-
pected products, among which are the lithium salts
LiSn(Me)ArSn(Me)2Ar11 and the distannylstannanediyl
species Sn{Sn(Ph)2Ar}2 (Ar ) C6H3-2,6-Trip2),12 as well
as an unsymmetric methylmethylene valence isomer
Sterically encumbered terphenyl element(II) halide
derivatives of the heavier group 14 elements (i.e., M(X)-
Ar: M ) Ge,1,2 Sn,1-4 Pb;5 X ) halide; Ar ) terphenyl)
have proven to be useful starting materials for several
new classes of compounds.6 The latter were generally
synthesized by reduction of the terphenyl element halide
precursors by a variety of routes which yielded stable
•
radical species such as (GeC6H3-2,6-Mes2)3 (Mes )
C6H2-2,4,6-Me3)7 or [(SnC6H3-2,6-Trip2)2]- (Trip )
C6H3-2,4,6-i-Pr3),3 the doubly bonded dianions [(MC6H3-
2,6-Trip2)2]2- (M ) Ge, Sn),8 or the neutral alkyne
(1) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organo-
metallics 1997, 16, 1920.
(2) Pu, L.; Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organo-
metallics 1998, 17, 5602.
(3) Olmstead, M. M.; Simons, R. S.; Power, P. P. J . Am. Chem. Soc.
1997, 119, 11705.
(4) Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron, in
press.
(5) Pu, L.; Twamley, B.; Power, P. P. Organometallics 2000, 19, 2874.
(6) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet.
Chem. 1999, 44, 1. For an account of some uses of this ligand in gallium
chemistry see: Robinson, G. H. Acc. Chem. Res. 1999, 32, 733. For
recent extensions to the lanthanide elements, see: Rabe, G. W.;
Steissel, L. M.; Liable-Sands, T. E.; Concolino, T. E.; Rheingold, A. L.
Inorg. Chem. 1999, 38, 3445. Heckmann, M.; Niemeyer, M. J . Am.
Chem. Soc. 2000, 122, 4427.
(8) Pu, L.; Senge, M. O.; Olmstead, M. M.; Power, P. P. J . Am. Chem.
Soc. 1998, 120, 12682.
(9) Pu, L.; Twamley, B.; Power, P. P. J . Am. Chem. Soc. 2000, 122,
3524.
(10) Simons, R. S.; Power, P. P. J . Am. Chem. Soc. 1996, 118, 11966.
Pu, L.; Twamley, B.; Haubrich, S. T.; Olmstead, M. M.; Mork, B. V.;
Simons, R. S.; Power, P. P. J . Am. Chem. Soc. 2000, 122, 650.
(11) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 32, 5444.
(12) Eichler, B. E.; Stanciu, C.; Power, P. P. Unpublished work.
(7) Olmstead, M. M.; Pu, L.; Simons, R. S.; Power, P. P. Chem.
Commun. 1997, 1595.
10.1021/om001077z CCC: $20.00 © 2001 American Chemical Society
Publication on Web 03/31/2001

Terphenyl-Substituted Digermene Derivatives
Organometallics, Vol. 20, No. 9, 2001 1821
Ge: C, 78.23; H, 9.33. Found: C, 79.10; H, 9.68. 1H NMR
(C6D6): δ 0.63 (q, 2H, CH2CH3), 3J ) 8.0 Hz; 0.84 (t, 3H,
CH2CH3), 3J ) 8.0 Hz; 1.07 (d, 12H, o-CH(CH3)2), 3J ) 6.9 Hz;
1.21 (d, 12H, o-CH(CH3)2), 3J ) 6.9 Hz; 1.39 (d, 12H, p-CH-
(CH3)2), 3J ) 6.9 Hz; 2.88 (sept, 2H, o-CH(CH3)2), 3J ) 6.9 Hz;
2.95 (sept, 1H, p-CH(CH3)2), 3J ) 6.9 Hz. 13C{1H} NMR: δ
6.50 (Ge-CH2CH3); 11.58 (Ge-CH2CH3); 22.75 (o-CH(CH3)2);
24.55 (o-CH(CH3)2); 25.92 (p-CH(CH3)2); 31.05 (p-CH(CH3)2);
34.29 (o-CH(CH3)2); 121.18, 122.03, 127.28, 128.00, 130.02,
137.33, 146.80, 148.08, 150.13 (Ar, C’s).
analogue of an alkene, Ar(Me)2SnS¨nAr.11 The isolation
of the latter species, in particular, instead of its sym-
metric alkene-like isomer Ar(Me)SnSn(Me)Ar, has
prompted the investigation of the corresponding reac-
tions of the organogermanium(II) halide Ge(Cl)C6H3-
2
2,6-Trip2 with a variety of small organolithium or
Grignard reagents. The results of these investigations
are now presented, and it is shown that in accordance
with theoretical calculations13-15 on the simple species
M2H4 (M ) Ge, Sn), the compounds formed for the
germanium and tin species differ fundamentally in their
bonding and structure.
{Ge(P h )C6H3-2,6-Tr ip 2}2 (4). A solution of LiPh‚Et2O17
(0.18 g, 1.12 mmol) in hexane/diethyl ether (1:1, 30 mL) was
added to a rapidly stirred solution of Ge(Cl)C6H3-2,6-Trip2 (0.66
g, 1.12 mmol) in hexane (20 mL) at room temperature. The
reaction mixture underwent little color change, and it was
stirred for a further 1 h at room temperature. The solvent was
removed under reduced pressure, and the orange residue was
extracted with benzene (35 mL). The orange benzene solution
was stirred for 16 h at room temperature. After filtration
through Celite, the filtrate was reduced to incipient crystal-
lization and stored in a ca. 4 °C refrigerator to give 4 as yellow
crystals: yield 0.31 g, 44%; mp 221-225 °C (darkens at ca.
155 °C). Anal. Calcd for C42H55Ge: C, 79.76; H, 8.77. Found:
C, 80.12; H, 8.99. 1H NMR (C6D6): δ 0.91 (d, 12H, o-CH(CH3)2),
3J ) 6.8 Hz; 1.05 (d, 12H, o-CH(CH3)), 3J ) 6.8 Hz; 1.27 (d,
12H, p-CH(CH3)2), 3J ) 6.8 Hz; 2.70 (sept, 2H, p-CH(CH3)), 3J
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were carried out
by using modified Schlenk techniques under an atmosphere
of N2 or in a Vacuum Atmospheres HE-43 drybox. All solvents
were distilled from Na/K alloy and degassed twice prior to use.
The compounds Ge(Cl)C6H3-2,6-Trip2 (1) and LiPh16 were
2
synthesized according to literature procedures. Pyridine (py)
was dried by distillation from CaH2. CH3MgBr in ether
solution was purchased commercially and used as received.
1H and 13C NMR spectra were recorded at 25 °C on a Varian
400 MHz spectrometer at 399.77 and 100.53 MHz, respectively,
and referenced to deuterated solvent.
3
) 6.8 Hz; 3.05 (sept, 4H, o-CH(CH3)2), J ) 6.8 Hz; 6.8-7.25
(m, Ar H). 13C{1H} NMR: δ 22.74 (o-CH(CH3)2); 24.41 (o-CH-
(CH3)2); 26.1 (p-CH(CH3)2); 30.9 (p-CH(CH3)2); 31.2 (o-CH-
(CH3)2); 121.1, 124.1, 126.5, 128.5, 128.6, 130.9, 136.1, 137.18,
138.8, 146.6, 146.9 (Ar, C’s).
{Ge(Me)C6H3-2,6-Tr ip 2}2 (2). A diethyl ether solution of
CH3MgBr (3 M, 0.77 mL, 2.31 mmol) was added to a solution
of Ge(Cl)C6H3-2,6-Trip2 (1.37 g, 2.32 mmol) in hexane (80 mL)
at ca. 0 °C with constant stirring. The reaction mixture, which
had assumed a yellow-orange color, was stirred until the ice
bath had thawed to room temperature. The solvent was then
removed under reduced pressure, and the yellow residue was
extracted with toluene (50 mL). After the orange solution was
filtered through Celite, its volume was reduced to incipient
crystallization, and storage in a ca. -20 °C freezer gave 2 as
yellow crystals: yield 0.51 g, 39%; mp 215-217 °C dec. Anal.
Calcd for C37H52Ge: C, 78.04; H, 9.21. Found: C, 77.69; H,
9.46. 1H NMR (C6D6): δ -0.16 (s, 3H, Ge-CH3); 1.19 (d, 12H,
p y‚Ge(Cl)C6H3-2,6-Tr ip 2 (7). Pyridine (0.23 mL, 2.84
mmol) was added dropwise via syringe to an orange solution
of Ge(Cl)C6H3-2,6-Trip2 (1.64 g, 2.78 mmol) in hexane (80 mL)
at ca. 25 °C with constant stirring. The solution, which had
turned yellow, was stirred at ca. 25 °C for 1 h. The volume of
the solution was then reduced to incipient crystallization and
stored in a refrigerator at ca. 4 °C to give the product 1 as
yellow crystals: yield 1.45 g, 80.1%; mp 160-161 °C dec. Anal.
Calcd for C41H54ClGeN: C, 73.62; H, 8.14; N, 2.09. Found: C,
74.01; H, 8.15; N, 1.99. 1H NMR (C6D6): δ 1.08 (d, 12H, p-CH-
3
3
o-CH(CH3)2), J ) 6.8 Hz; 1.28 (d, 12H, o-CH(CH3)), J ) 6.8
Hz; 1.35 (d, 12H, p-CH(CH3)2), 3J ) 6.8 Hz; 2.87 (sept, 2H,
p-CH(CH3)2), 3J ) 6.8 Hz; 2.94 (sept, 4H, o-CH(CH3)2), 3J )
6.8 Hz; 7.15 (s, m-Trip); 7.1-7.2 (m, o- and p-C6H3). 13C{1H}
NMR: δ -2.1 (Ge-CH3); 22.64 (o-CH3(CH3)2); 24.13 (o-CH-
(CH3)2); 25.78 (p-CH(CH3)2); 30.81 (p-CH(CH3)2); 34.63 (o-CH-
(CH3)2); 120.5 (m-Trip); 137.65 (p-C6H3); 138.61 (m-C6H3);
143.74 (o-C6H3); 146.65 (i-Trip); 147.1 (p-Trip); 148.63 (o-Trip).
{Ge(Et)C6H3-2,6-Tr ip 2}2 (3). A diethyl ether solution of
EtMgBr, which was generated from the addition of CH3CH2-
Br (0.08 mL, 1.05 mmol) to Mg (0.027 g, 1.11 mmol) in Et2O
(10 mL), was added to a solution of Ge(Cl)C6H3-2,6-Trip2 (0.62
g, 1.05 mmol) in hexane (80 mL) at ca. 25 °C with constant
stirring. The reaction mixture, which had assumed a yellow
color, was stirred for a further 16 h at room temperature. The
solvent was removed under reduced pressure, and the yellow
residue was extracted with benzene (30 mL). After the red
solution was filtered through Celite, its volume was reduced
to incipient crystallization. Storage in a ca. 4 °C refrigerator
afforded 3‚2C6H6 as yellow crystals: yield of 3 0.37 g, 52%;
3
3
(CH3)2), J ) 6.7 Hz; 1.21 (d, 12H, o-CH(CH3)2), J ) 6.7 Hz;
1.41 (d, 12H, o-CH(CH3)2), 3J ) 6.7 Hz; 2.74 (sept, 2H,
p-CH(CH3)2), 3J ) 6.7 Hz; 3.34 (sept, 4H, o-CH(CH3)2), 3J )
6.7 Hz; 6.26 (2H, m-py); 6.59 (1H, p-py); 7.19 (s, 4H, m-Trip);
7.26 (1H, p-C6H3); 7.93 (2H, m-C6H3); 7.98 (2H, o-py). 13C{1H}
NMR: δ 22.94 (o-CH(CH3)2); 24.37 (o-CH(CH3)2); 26.33 (p-CH-
(CH3)2); 31.24 (p-CH(CH3)2); 34.53 (o-CH(CH3)2); 121.16 (m-
Trip); 123.94 (m-py); 130.32 (p-py); 131.25 (p-C6H3); 137.10 (m-
C6H3); 146.60 (i-Trip); 147.40 (p-Trip); 148.58 (o-py); 148.61
(o-C6H3); 148.62 (o-Trip), 159.51 (i-C6H3). UV-vis (hexane):
λmax ) 380, ꢀ ) 800 L mol-1cm-1
.
X-r a y Cr ysta llogr a p h ic Stu d ies. The crystals of 2-5 and
7 were removed from the Schlenk tube under a stream of N2
and immediately covered with a layer of hydrocarbon oil. A
suitable crystal was selected, attached to a glass fiber, and
immediately placed in the low-temperature nitrogen stream.18
All data were collected near 130 K using Bruker SMART 1000
(Mo KR radiation and a CCD area detector) equipment. The
SHELXTL version 5.03 program package was used for struc-
ture solutions and refinements.19 Absorption corrections were
applied using the SADABS program.20 The structures were
solved by direct methods and refined by full-matrix least-
squares procedures. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in the refine-
mp 234-237 °C (darkens at ca. 210 °C). Anal. Calcd for C38H54
-
(13) Trinquier, G. J . Am. Chem. Soc. 1990, 112, 2130.
(14) Trinquier, G. J . Am. Chem. Soc. 1991, 113, 144.
(15) A reviewer suggested that the observation of the unsymmetric
methyl isomer in the case of tin11 may be due to the lower barrier to
rearrangement in the tin system. Calculations for the process H2-
GeGeH2 f H3GeGeH indicate a barrier of 12-14 kcal mol-1: Grev, R.
S.; Schaefer, H. F. Organometallics 1992, 11, 3489. Significantly higher
barriers are expected for the methyl-substituted analogue. The barrier
for the corresponding methyl-tin system is currently unknown.
(16) Schlosser, M.; Ladenberger, V. J . Organomet. Chem. 1967, 8,
195.
(17) Hope, H.; Power, P. P. J . Am. Chem. Soc. 1983, 105, 5320.
(18) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(19) SHELXTL version 5.03; Bruker AXS, Madison, WI, 1998.
(20) SADABS is an empirical absorption correction program that is
part of the SAINT Plus NT version 5.0 package: Bruker AXS, Madison,
WI, 1998.

1822 Organometallics, Vol. 20, No. 9, 2001
Ta ble 1. Cr ysta llogr a p h ic Da ta for Com p ou n d s 2-5 a n d 7
Stender et al.
2
3‚2C6H6
4
5
7
formula
fw
color, habit
cryst syst
space group
a, Å
C
37H52Ge
C88H120Ge2
1323.02
C42H55Ge
632.45
orange, block
monoclinic
P21/n
12.4506(8)
12.6854(9)
22.626(2)
94.482(1)
3562.6(4)
4
C72H98Cl2Ge2
1179.58
orange, block
monoclinic
P21/c
14.003(1)
15.9072(1)
18.003(1)
109.817(2)
3772.9(5)
2
C41H54ClGeN
668.89
569.38
yellow-green, cube
monoclinic
P21/c
13.8080(6)
16.0161(7)
17.5669(8)
108.771(1)
3678.3(3)
4
yellow parallelepiped
monoclinic
P21/c
14.1210(7)
15.9066(8)
18.1521(9)
110.814(1)
3811.2(3)
2
yellow, parallelepiped
monoclinic
P21/c
12.7657(4)
17.6140(5)
16.9179(5)
105.204(1)
3670.9(2)
4
b, Å
c, Å
â, deg
V (Å3)
Z
dcalcd (Mg/m3)
θ range, deg
1.028
1.77-31.51
0.852
1.153
1.54-25.00
0.832
1.179
1.81-35.51
0.887
1.038
1.76-31.53
0.901
1.210
1.70-31.5
0.935
µ (mm-1
)
no. of obsd data, I > 2σ(I)
R1
8473
0.0398
3411
0.0486
6718
0.0542
4105
0.1194
9359
0.0341
wR2
0.0973
0.1110
0.1481
0.3083
0.0982
Ta ble 2. Selected Str u ctu r a l P a r a m eter s for Com p ou n d s 1-7
Ge-Ge (Å)
Ge-C6H3-2,6-Trip2 (Å)
Ge-L (Å)
C-Ge-L (deg)
∑(Ge) (deg)d
δ (deg)e
Ge(Cl)C6H3-2,6-Trip2 (1)
1.989(5)a
1.9705(15)a
1.995(3)a
1.997(2)a
1.973(7)a
2.000(6)a
2.0476(12)a
2.2026(19)b
1.9836(17)a
1.913(5)a
101.31(15)
111.77(7)
106.8(2)
116.08(10)
118.03(19)
109.1(2)
{Ge(Me)C6H3-2,6-Trip2}2 (2)
{Ge(Et)C6H3-2,6-Trip2}2 (3)
{Ge(Ph)C6H3-2,6-Trip2} (4)
{Ge(Cl)C6H3-2,6-Trip2}2 (5)
{Ge(Cl)C6H3-2,6-Mes2}2 (6)
py‚Ge(Cl)C6H3-2,6-Trip2 (7)
2.3173(3)
2.347(3)
2.3183(5)
2.363(2)
2.443(2)
342.89
342.98
348.35
346.59
332.49
290.65
39.7
37.9
33.7
36.8
39.0
1.950(3)a
2.209(2)b
2.120(2)b
2.3021(4)c
2.1403(1)b
93.24(3)
a
b
d
Ge-C bond. Ge-Cl bond. c Ge-N bond. Sum of angles at Ge. e Out-of-plane angle defined by
ment at calculated positions using a riding model in the
SHELXTL program. The ethyl derivative 3 crystallized from
benzene as the solvate 3‚2C6H6. Some details of the data
collection and refinement are given in Table 1. Selected bond
distances and angles are given in Table 2. Further details are
provided in the Supporting Information.
to the weakness of the Sn-Sn “double” bond, which is
composed of two relatively weak21 polar-dative interac-
tions,22 in comparison to the relatively strong (usually
Resu lts a n d Discu ssion
The reaction of 1 equiv of MeMgBr, EtMgBr, or LiPh
with Ge(Cl)C6H3-2,6-Trip2 in diethyl ether solution
afforded the alkyl derivatives 2 and 3 and the aryl
species 4, in moderate yield in accordance with eq 1.
ca. 40 kcal mol-1) Sn-Sn covalent single bond.23 Trin-
quier has shown, in calculations13,14 on the hypothetical
hydrogen derivatives M2H4 (M ) Si, Ge,15 Sn, Pb), that
the unsymmetric structure H3MM¨ H is more stable than
the trans-bent symmetric structure H2MMH2 for M )
Sn, Pb but vice versa for M ) Si, Ge. With the crowding
R substituents normally required for stability, a sym-
metric structure similar to (b) is invariably observed for
the Sn2R4 species, since an unsymmetric R3SnS¨nR
structure similar to (a) is disfavored, owing to the steric
conflict of the three large R groups at one of the tins.
However, the C6H3-2,6-Trip2 substituent allows struc-
tures such as (a) to be isolated with methyl groups as
coligands since two methyl ligands and one terphenyl
ligand do not unduly crowd the tin environment.
In contrast, the monomeric structure depicted in (c)
is probably a result of the overall weakness of the Pb-
Pb interaction in comparison to the Sn-Sn and Ge-Ge
bonds. A similar picture emerges from the synthesis of
phenyl derivative 4, whose symmetric formulation,
The symmetric, Ge-Ge-bonded, dimeric formulas of
these compounds are in sharp contrast to those observed
for their tin11 and lead4 methyl analogues, which have
either the unsymmetric structure (a), (rather than the
symmetric structure (b)) or the monomeric structure (c).
For tin it is believed that (a) is preferred over (b), owing
(21) For example: Zilm, K. W.; Lawless, G. A.; Merrill, R. M.; Millar,
J . M.; Webb, G. J . Am. Chem. Soc. 1987, 109, 7236.
(22) Lappert, M. F. Adv. Chem. Ser. 1976, No. 150, 256.
(23) Simoes, J . A. M.; Liebman, J . F.; Slayden, S. W. Thermochem-
istry of Organometallic Compounds of Germanium, Tin and Lead. In
The Chemistry of Organic Germanium, Tin and Lead Compounds;
Patai, S., Ed.; Wiley: Chichester, U.K., 1995; Chapter 4.

Terphenyl-Substituted Digermene Derivatives
Organometallics, Vol. 20, No. 9, 2001 1823
dimerized through chloride bridging with no Sn-Sn
bonding. The different structures seen for the germa-
nium and tin dimers reflect the weaker metal-metal
bonding of tin.
The monomeric structure observed for 12 was origi-
nally accounted for on the basis of the large size of
-C6H3-2,6-Trip2 substituent, which prevented associa-
tion.24 This steric explanation appeared plausible since,
with the less bulky -C6H3-2,6-Mes2 substituent, weak
association occurred to afford the Ge-Ge-bonded dimer
6.1 However, the isolation of the analogous -C6H3-2,6-
Trip2-substituted dimer 5, whose Ge-Ge bond distance
is significantly shorter than that in 6, essentially
vitiates this explanation. The longer Ge-Ge bond in the
apparently less crowded 6 can be rationalized on the
basis of secondary interactions between the germanium
and one of the o-mesityl rings, which effectively increase
its coordination number and weaken the Ge-Ge inter-
action (see Figure 5 of ref 1). In the bulkier -C6H3-2,6-
Trip2 analogue 5, however, significant interactions of
this type do not occur. This permits a shorter Ge-Ge
bond in 5 despite the larger size of the organic substitu-
ent. Nonetheless, the Ge-Ge bond in 5 is quite weak,
since the compound is dissociated to the monomer 1 in
solution. Closer examination of the monomeric structure
of 1 also shows that there exists close intra- and
intermolecular contracts between the germanium and
various C-H moieties of the isopropyl substituents of
-C6H3-2,6-Trip2 groups. The closest Ge‚‚‚H approaches
are in the range 2.86-3.32 Å. The corresponding
interactions in 5 are all greater than 3.4 Å, however.
Thus, the key factors in determining the different
structures of 1 and 5 are the weak Ge-Ge, Ge-C, and
C-H‚‚‚Ge interactions. The weakness of the Ge-Ge
bonding in the structure of 5 is also supported by the
absence of significant changes in the Ge-C and Ge-Cl
distances upon dissociation to monomers.
F igu r e 1. Schematic drawing of the arylgermanium
chloride dimer 5. Selected bond distances and angles are
given in Table 2.
similar to those seen for 2 and 3, differs from the
products obtained12 for the tin and lead derivatives (d)
and (e).
(THF)LiSnPh2Ar
(d)
:Pb(Ph)Ar
(e)
Ar ) -C6H3-2,6-Trip2
It is clear that the sizes of the group 14 element and
its organic substituents, the stability of the lone pair,
as well as the strength of the M-M (M ) Ge, Sn, Pb)
interaction play a crucial role in the type of product
obtained. This effect is also seen in the aryl element
halide precursors 1, 5, and 6, which show a tendency
to form element-element-bonded dimers in the case of
germanium (see below) but are observed as halide-
bridged dimers with no metal-metal bonding for tin and
lead.1,5 However, this should not obscure the fact that
the Ge-Ge bonding in the arylgermanium halide dimers
5 and 6 is quite weak, as evidenced by the simultaneous
crystallization of Ge(Cl)C6H3-2,6-Trip2 as the monomer
12 or as the dimer 5. It is also notable that either 1 or
5 reacts readily with pyridine to give the 1:1 adduct 7,
which is also in keeping with the weakness of the Ge-
The trans-bent geometries of 5 and 6 and the rela-
tively long Ge-Ge “double bond” distances are indica-
tive of weak bonding which can be rationalized in terms
of the mixing of the Ge-Ge σ* and π orbitals,25 which
weakens the bond strength and produces lone pair
electronic character and pyramidal geometry at the
germaniums. In the case of 5 and 6 this interaction is
weakened further by the presence of electronegative
chloro substituents which enhance such mixing.22 The
dimeric structures of the tetraorganodigermanium com-
pounds 2-4 resemble those of the chloro derivatives 5
and 6 in that they all exist in the trans-bent E
stereoisomeric form. This finding may be contrasted
with the solid-state structure of {Ge(Mes)(C6H3-2,6-i-
Pr2)}2, which exists as the Z isomer.26 In solution,
however, it is in equilibrium with its E isomer. This type
of equilibrium is not observed for 2-4. It seems probable
that the large size of the -C6H3-2,6-Trip2 substituent
does not favor the formation of the Z isomer for steric
reasons. The Ge-Ge distances in 2-4 lie about midway
between the shortest (ca. 2.21 Å) and longest (ca. 2.46
1
Ge interaction. Solution H NMR spectroscopic studies
of 1 or 5 do not show the existence of a monomer-dimer
equilibrium at room temperature. In addition, 1H NMR
spectra of solutions of 2-4 also support the existence
of only one isomer in solution.
X-r a y Cr ysta l Str u ctu r es. The structures 2-5 and
7 were determined by single-crystal X-ray diffraction.
The structures of the monomer Ge(Cl)C6H3-2,6-Trip2 (1)2
and the Ge-Ge bonded dimer {Ge(Cl)C6H3-2,6-Mes2}2
(6)1 have already been described elsewhere. Key struc-
tural data for the series of compounds 1-7 are listed in
Table 2. The X-ray data for 1 and 5 show that Ge(Cl)-
C6H3-2,6-Trip2 can exist in the solid state in either the
monomeric or the dimeric form. The two structures
observed for 1 and 5 (Figure 1) in the crystalline phase
parallel recent findings4 for the corresponding tin
monomer and dimers Sn(Cl)C6H3-2,6-Trip2 and {Sn(µ-
Cl)C6H3-2,6-Trip2}2, which also crystallized simulta-
neously from solution. However, the tin species is
(24) Other crowded organogermanium(II) chloride species such as
Ge(Cl)C5Me5 and Ge(Cl)C6H2-2,4,6-t-Bu3 have also been synthesized:
Kohl, F. X.; J utzi, P. J . Organomet. Chem. 1987, 329, C17. J utzi, P.;
Leue, C. Organometallics 1994, 13, 2898.
(25) Grev, R. S. Adv. Organomet. Chem. 1991, 33, 125.
(26) Batcheller, S. A.; Tsumuraya, T.; Tempkin, O.; Davis, W. M.;
Masamune, S. J . Am. Chem. Soc. 1990, 112, 9394.

1824 Organometallics, Vol. 20, No. 9, 2001
Stender et al.
F igu r e 3. Schematic drawing of py‚Ge(Cl)C6H3-2,6-Trip2
(7). Selected bond distances and angles are given in Table
2.
distance is 2.3021(4) Å, which is much greater than the
sum of atomic radii of germanium and nitrogen (ca. 1.95
Å).29 It is only 0.067 Å shorter than the 2.369(4) Å Sn-N
distance observed in the corresponding tin species py‚
Sn(Cl)C6H3-2,6-Trip2, whereas the difference in covalent
radii between these two elements is ca. 0.18 Å.29 The
unusual elongation of the Ge-N bond thus suggests
considerable steric crowding in this molecule. The
congestion is also reflected in a deviation of the Ge(1)-
N(1) vector by 15.5° from an idealized 90° with respect
to the C(1)-Ge(1)-Cl(1) plane. The plane of the pyridine
ring is also tilted by 12.6° from the Ge(1)-N(1) line. In
addition, the Ge(1)-C(1)-C(2) (132.49(9)°) and Ge(1)-
C(1)-C(2) (109.05(8)°) angles differ by over 23°, the
wider angle being associated with the pyridine-coordi-
nated side of the C(1), Ge(1), Cl(1) plane. The corre-
sponding angular differences in 2-6 are in the range
11-14°, and the two angles are identical in the structure
of the uncomplexed precursor 1.
F igu r e 2. Schematic drawing of {Ge(Et)C6H3-2,6-Trip2}2
(3). Selected bond distances and angles are given in Table
2.
Å) bonds observed for R2GeGeR2 species.27 The Ge-Ge
bond lengths in 2 (2.3173(3) Å) and 4 (2.3183(5) Å)
resemble the 2.301(1) Å found in {Ge(Mes)(C6H3-2,6-i-
Pr2)}2. The out of plane angles 39.7° (2) and 33.7° (4)
bracket the 36° observed for {Ge(Mes)(C6H3-2,6-i-Pr2)}2.
A somewhat longer Ge-Ge bond is observed for the
ethyl-substituted 3 (see the structure in Figure 2),
although the out-of-plane angle is less than that in 2.
The out-of-plane angles observed in 2-4 are at the high
end of the scale for tetraorganodigermanium com-
pounds.27 However, it is apparent from Table 2 that
there is no correlation between this parameter and the
Ge-Ge bond lengths. Finally, it is notable that the
methyl and ethyl derivatives 2 and 3 are rare examples
of digermenes with alkyl substituents, the only previous
example being the tetraalkyl [Ge{CH(SiMe3)2}2]2.28
Treatment of solutions of 1 or 5 with pyridine readily
affords the 1:1 pyridine complex 7. The structure of this
species (Figure 3) resembles those of its tin4 and lead5
analogues, in which the pyridine is coordinated through
the “empty” p orbital lying perpendicular to the M, Cl,
C(ipso) plane. This results in a pyramidal coordination
at germanium (sum of angles at Ge 290.65°). The Ge-N
Con clu sion . The germanium derivatives 2-4 display
a different bonding pattern from the corresponding tin
and lead species owing to stronger Ge-Ge multiple
bonds in comparison to those of tin and lead. The
cocrystallization of the halides 1 and 5 show that weak
Ge-C, Ge-H interactions play a role in determining
the strength of Ge-Ge bonding.
Ack n ow led gm en t. We are grateful to the National
Science Foundation for financial support.
(27) Mackay, K. M. Structural aspects of compounds containing C-E
(E ) Ge, Sn, Pb) bonds. In The Chemistry of Organic Germanium, Tin
and Lead Compounds; Patai, S., Ed.; Wiley: Chichester, U.K., 1995;
Chapter 4. Baines, K. M.; Stibbs, W. G. Adv. Organomet. Chem. 1996,
39, 325. Escudie´, J .; Ranaivonjatovo, H. Adv. Organomet. Chem. 1999,
44, 113. Power, P. P. Chem. Rev. 1999, 99, 3463.
(28) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J .; Thorne, A. J . J .
Chem. Soc., Chem. Commun. 1984, 480.
(29) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Claren-
don: Oxford, U.K., 1984; pp 788, 1279, 1280.
Su p p or tin g In for m a tion Ava ila ble: Tables giving full
details of the crystallographic data and data collection param-
eters, atom coordinates, bond distances, bond angles, aniso-
tropic thermal parameters, and hydrogen coordinates for 2-5
and 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM001077Z