On the synthesis, structure, and reactivity of tetramesityldigermene

Article abstract of DOI:10.1021/om7005358

Tetramesityldigermene (1) can be synthesized cleanly and quantitatively by photolysis of hexamesitylcyclotrigermane (2) in THF at low temperature. A solution of tetramesityldigermene (1) in THF is stable for several weeks at room temperature; the digermene does not dissociate or rearrange. The molecular structure of tetramesityldigermene (1) has been determined using X-ray crystallography. The effectiveness of the new protocol for the synthesis of tetramesityldigermene (1) and its derivatives has been demonstrated in the study of the addition of carboxylic acids to tetramesityldigermene (1).

Full text of DOI:10.1021/om7005358

Organometallics 2007, 26, 5569-5575
5569
On the Synthesis, Structure, and Reactivity of
Tetramesityldigermene
Krysten L. Hurni, Paul A. Rupar, Nicholas C. Payne, and Kim M. Baines*
Department of Chemistry, UniVersity of Western Ontario, London, Ontario, Canada N6A 5B7
ReceiVed May 30, 2007
Tetramesityldigermene (1) can be synthesized cleanly and quantitatively by photolysis of hexamesi-
tylcyclotrigermane (2) in THF at low temperature. A solution of tetramesityldigermene (1) in THF is
stable for several weeks at room temperature; the digermene does not dissociate or rearrange. The molecular
structure of tetramesityldigermene (1) has been determined using X-ray crystallography. The effectiveness
of the new protocol for the synthesis of tetramesityldigermene (1) and its derivatives has been demonstrated
in the study of the addition of carboxylic acids to tetramesityldigermene (1).
thermolysis (Scheme 1).4 The silane is necessary to react
irreversibly with dimesitylgermylene (3) (also formed during
the reaction), which drives the reaction to completion. As a
consequence, chromatographic separation of dimesityl(trieth-
ylsilyl)germane (4) and the product(s) derived from digermene
1 is usually required.
Surprisingly, co-photolysis of 2 and triethylsilane at -70 °C
in THF did not produce silylgermane 4 as a product; only
products derived from digermene 1 were isolated.5 If indeed
tetramesityldigermene (1) can be produced cleanly and reliably
using this protocol, several advantages may be realized. Since
the photolysis of 1 equiv of 2 should result in the formation of
1.5 equiv of 1, the reaction is more atom economical in
comparison to the photolysis in toluene. Furthermore, since a
co-product is not formed, chromatographic separation is likely
unnecessary and, thus, may result indirectly in better yields of
the derivatives of digermene 1. Given that the development of
more efficient routes for the preparation of digermenes is of
continuing interest, our previous results compelled us to further
examine the general usefulness of this method for the preparation
of 1 and its derivatives.
Introduction
The synthesis of solid, stable derivatives of digermenes a
quarter century ago was significant for two key reasons. The
X-ray structures of the solid derivatives greatly facilitated the
understanding of the bonding in these and related compounds.1
Furthermore, digermenes, like alkenes, serve as excellent
substrates for the synthesis of numerous organogermanium
derivatives.1 More recently, there has been much interest in the
reactivity of dimers on germanium surfaces. It is our hypothesis
that the reactivity of molecular digermenes can guide the
understanding of the reactivity of the dimers, and particularly
the symmetrical dimers found on the Ge(100)-p(2×1) surface.2
For these reasons, new and/or improved syntheses of digermenes
are of ongoing importance.
Our group has had a long-standing interest in the chemistry
of digermenes and employs tetramesityldigermene (1) as a
prototypical substrate.3 The mesityl substituents are bulky
enough to kinetically stabilize the digermene, but not too bulky
so as to facilitate dissociation of the digermene 1 to germylenes.
Digermene 1 is most easily synthesized by the photolysis3a of
hexamesitylcyclotrigermane (2) in the presence of triethylsilane
in toluene at low temperature, but can also be produced by
Results and Discussion
* To whom correspondence should be addressed. E-mail: kbaines2@
uwo.ca.
To enable us to directly compare the previously established
method for the synthesis of 1 (method A: photolysis of
cyclotrigermane 2 at -70 °C in toluene in the presence of Et3-
SiH) and the new protocol (method B: photolysis of cyclo-
trigermane 2 at -70 °C in THF), 1 was converted to known
isolable adducts. Methanol and chloroform were selected as the
test reagents because the additions of these reagents to 1 have
been reported to proceed cleanly, albeit in low isolated yields.6,7
Low yields are typical for adducts of 1 produced by method A
because of the chromatographic separation required and the
nonquantitative conversion of 2 to 1. By comparing the isolated
yields of the adducts, we were able to qualitatively evaluate
the relative effectiveness of the two methods for the preparation
of derivatives of 1.
(1) For reviews on digermene chemistry, see: (a) Barrau, J.; Escudie´,
J.; Satge´, J. Chem. ReV. 1990, 90, 283. (b) Tsumuraya, T.; Batcheller, S.
A.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (c) Escudie´,
J.; Couret, C.; Ranaivonjatovo, H.; Satge´, J. Coord. Chem. ReV. 1994, 130,
427. (d) Baines, K. M.; Stibbs, W. G. AdV. Organomet. Chem. 1996, 39,
275. (e) Chaubon, M. A.; Ranaivonjatovo, H.; Escudie´, J.; Satge´, J. Main
Group Metal Chem. 1996, 19, 145. (f) Escudie´, J.; Ranaivonjatovo, H. AdV.
Organomet. Chem. 1999, 44, 113. (g) Weidenbruch, M. Eur. J. Inorg. Chem.
1999, 373. (h) Tokitoh, N.; Okazaki, R. In The Chemistry of Organic
Germanium, Tin, and Lead Compounds, Vol. 2; Rappoport, Z., Ed.; John
Wiley and Sons Ltd.: Chichester, 2002; pp 843-901. (i) Weidenbruch,
M. J. Organomet. Chem. 2002, 646, 39. (j) Weidenbruch, M. Organome-
tallics 2003, 22, 4348.
(2) Selected examples: (a) Hamers, R. J.; Coulter, S. K.; Ellison, M.
D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell,
J. N., Jr. Acc. Chem. Res. 2000, 33, 617. (b) Bent, S. F. Surf. Sci. 2002,
500, 879. (c) Buriak, J. M. Chem. ReV. 2002, 102, 1271. (d) Loscutoff, P.
W.; Bent, S.F. Annu. ReV. Phys. Chem. 2006, 57, 467.
(3) (a) Dixon, C. E.; Liu, H. W.; Vander Kant, C. M.; Baines, K. M.
Organometallics 1996, 15, 5701. (b) Dixon, C. E.; Cooke, J. A.; Baines,
K. M. Organometallics 1997, 16, 5437. (c) Dixon, C. E.; Netherton, M.
R.; Baines, K. M. J. Am. Chem. Soc. 1998, 120, 10365. (d) Fujdala, K. L.;
Gracey, D. W. K.; Wong, E. F.; Baines, K. M. Can. J. Chem. 2002, 80,
1387. (e) Samuel, M. S.; Baines, K. M. J. Am. Chem. Soc. 2003, 125, 12792.
(f) Rupar, P. A.; Jennings, M. C.; Baines, K. M. Can. J. Chem. 2007, 85,
141.
In a typical procedure for method B, a solution of 2 in
THF was irradiated (350 nm) at -70 °C, followed by addition
(4) Ando, W.; Tsumuraya, T. J. Chem. Soc., Chem. Commun. 1989, 770.
(5) Samuel, M. S.; Jennings, M. C.; Baines, K. M. J. Organomet. Chem.
2001, 636, 130.
(6) Baines, K. M.; Cooke, J. A. Organometallics 1991, 10, 3419.
(7) Samuel, M. S.; Jennings, M. C.; Baines, K. M. Organometallics 2001,
20, 590.
10.1021/om7005358 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/02/2007

5570 Organometallics, Vol. 26, No. 23, 2007
Hurni et al.
Scheme 1
known to be stabilized by Lewis bases. In fact, germylene-
Lewis base complexes have been previously characterized in
solution,9,10 in matrixes,11 and in the solid state.3f,12 Furthermore,
several intramolecularly base-stabilized germylenes have been
synthesized and characterized.13 Moreover, it has been demon-
strated that the reactivity of the Lewis acid-base complex of
diphenylgermylene and THF is less than the free germylene.
The kinetics of the reaction of diphenylgermylene with CCl4
was examined by laser flash photolysis. Diphenylgermylene
exhibited an extended lifetime when the reaction was performed
in THF compared to when it was carried out in hexanes.9b,c
Although absolute rate constants for the reactions of germylenes
in THF have not been determined, rate constants for reactions
of silylenes have been reported to decrease when the reactions
are performed in donating solvents, such as THF, compared to
nondonating solvents, such as hydrocarbons.14 Silylenes also
show an increase in their lifetimes in solvents with which they
form Lewis acid-base complexes, compared to nondonating
solvents.14
Scheme 2
A kinetic study of the complexation of dimesitylgermylene
(3) with THF was recently reported.9f The rate constant for the
complexation of 3 with THF was estimated to be 108-109 M-1
s-1 at 25 °C.9f The rate constant for the addition of 3 to Et3SiH
in hexanes, 1.1 × 105 M-1 s-1 15
, is much smaller than the rate
constant for complexation with THF. Moreover, by variation
of the concentration of THF, the equilibrium constant, Keq,
between free 3 and THF-complexed 3 was determined to be
1.1 ( 0.2 M-1 9f
. The dimerization of 3 to produce 1 occurs
of the reagent to the cold solution (dry ice/acetone) of 1 (Scheme
2). If the characteristic bright yellow color of 1 did not dissipate
within a few hours after addition, the solution was allowed to
warm to room temperature. The results are summarized in Table
1, along with the reported isolated yields using method A.
Although the crude product of each reaction was fairly clean
by 1H NMR spectroscopy, purification of the adducts by
chromatography was still necessary to obtain acceptable purities.
Nevertheless, an improvement in the isolated yields (∼10%)
for both products using the new protocol (method B) was
consistently achieved.
The decision to add the reagent to the solution of 1 at low
temperature stems from our earlier observation that 1 rearranges
to the isomeric mesityl(trimesitylgermyl)germylene (5) under
mild conditions (lower than room temperature) when
Et3SiH is present (Scheme 3).8 However, when 2 was photolyzed
at -70 °C in THF (in the absence of a germylene trap) and the
solution was subsequently warmed to room temperature, the
distinctive yellow color of 1 persisted, even after 6 h. When
methanol was added to this solution at room temperature, the
color immediately faded. Analysis of the 1H NMR spectrum of
the crude product revealed complete and clean conversion of 1
to 1,1,2,2-tetramesitylmethoxydigermane,6 indicating that 1 is
in fact stable at room temperature in THF in the absence of a
trap. No evidence for the formation of products derived from
the addition of methanol to either 3 or 5 was observed; 1 retains
its structural integrity in THF at room temperature. The clean
conversion of the digermene 1 to the final product at room
temperature negated the need for a chromatographic separation.
As a result, the isolated yield of the product improved
dramatically (see Table 1, method C). Analogous results were
obtained when chloroform was added to 1 under these condi-
tions.
quickly with a rate constant of 5 × 109 M-1 s-1 9a,f and was
observed to afford digermene 1 in 93% yield.9a Significantly,
an increase in the UV-vis absorption of 1 in the presence of a
low concentration of THF was observed and was attributed to
the influence of THF on the suppression of a decay pathway of
1: most likely, the addition of free 3 to 1.9f
Our results can be understood on the basis of the kinetic
studies. The irradiation of cyclotrigermane 2 produces both 1
and 3 (eq 1, Scheme 4).15 When 2 is photolyzed in THF, the
complexation of 3 with THF appears to lower the reactivity of
3 sufficiently to effectively prevent the addition of 3 to
digermene 1 (reverse reaction, eq 1, Scheme 4) or to Et3SiH
(eq 2, Scheme 4). Dimerization of 3 to 1, on the other hand,
appears to persist under the reaction conditions (eq 3, Scheme
(9) (a) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem.
Soc. 2004, 126, 16105. (b) Leigh, W. J.; Harrington, C. R. J. Am. Chem.
Soc. 2005, 127, 5084. (c) Harrington, C. R.; Leigh, W. J.; Chan, B. K.;
Gaspar, P. P.; Zhou, D. Can. J. Chem. 2005, 83, 1324. (d) Leigh, W. J.;
Dumbrava, I. G.; Lollmahomed, F. Can. J. Chem. 2006, 84, 934. (e) Leigh,
W. J.; Lollmahomed, F.; Harrington, C. R. Organometallics 2006, 25, 2055.
(f) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald, J. M.
Organometallics 2006, 25, 5424.
(10) Huck, L. A.; Leigh, W. J. Organometallics 2007, 26, 1339.
(11) (a) Collins, S.; Murakami, S.; Snow, J. T.; Masamune, S. Tetrahe-
dron Lett. 1985, 26, 1281. (b) Ando, W.; Tsumuraya, T.; Sekiguchi, A.
Chem. Lett. 1987, 317. (c) Ando, W.; Itoh, H.; Tsumuraya, T.; Yoshida, H.
Organometallics 1988, 7, 1880. (d) Ando, W.; Tsumuraya, T. Organome-
tallics 1989, 8, 167. (e) Ando, W.; Itoh, H.; Tsumuraya, T. Organometallics
1989, 8, 2759.
(12) (a) Rivie`re, P.; Satge´, J.; Castel, A. C. R. Acad. Sci. Paris, Ser. C
1975, 281, 835. (b) Neumann, W. P. Chem. ReV. 1991, 91, 311. (c) Tian,
X.; Pape, T.; Mitzel, N. W. Heteroat. Chem. 2005, 16, 361.
(13) Selected examples: (a) Bender, J. E., IV; Banaszak Holl, M. M.;
Kampf, J. W. Organometallics 1997, 16, 2743. (b) Bibal, C.; Mazieres,
H.; Gornitzka, H.; Couret, C. Polyhedron 2002, 21, 2827. (c) Khrustalev,
V. N.; Portnyagin, I. A.; Zemlyansky, N. N.; Borisova, I. V.; Nechaev, M.
S.; Ustynyuk, Y. A.; Antipin, M. Y.; Lunin, V. J. Organomet. Chem. 2005,
690, 1172. (d) Ionkin, A. S.; Marshall, W. J.; Fish, B. M. Organometallics
2006, 25, 4170.
The irreversible conversion of 2 to 1 is likely assisted by the
complexation of 3 with THF. Transient germylenes are well-
(14) Belzner, J.; Ihmels, H. AdV. Organomet. Chem. 1999, 43, 1.
(15) Toltl, N. P.; Leigh, W. J.; Kollegger, G. M.; Stibbs, W. G.; Baines,
K. M. Organometallics 1996, 15, 3732.
(8) Baines, K. M.; Cooke, J. A.; Vittal, J. J. J. Chem. Soc., Chem.
Commun. 1992, 1484.

Synthesis, Structure, and ReactiVity of Tetramesityldigermene
Organometallics, Vol. 26, No. 23, 2007 5571
Table 1. Isolated Yields of Adducts of 1 Using Methods A, B, and C
yield
yield
yield
reagent X-Y
MeOH
CHCl3
t-BuCOOH
MesCOOH
(CPhHCH2)CHCOOH
X
Y
method Aa
method Bb
method Cc
MeO
CHCl2
H
Cl
H
H
H
22%
30%
35%
8a: 24%
8b: 42%
8c: 20%e
88%
88%
8a: 53%
8b: 55%
8c: 55%
25%d
t-BuCOO
MesCOO
(CPhHCH2)CHCOO
a Method A: Photolysis (350 nm) of 2 at -70 °C in toluene in the presence of Et3SiH, followed by the addition of the reagent to the solution of 1 cooled
in a dry ice/acetone bath. b Method B: Photolysis (350 nm) of 2 at -70 °C in THF, followed by the addition of the reagent to the solution of 1 cooled in
a dry ice/acetone bath. c Method C: Photolysis (350 nm) of 2 at -70 °C in THF, followed by the addition of the reagent to the solution of 1 at room
temperature. d Reference 7. e The H2O and RCOOH adducts of 1 were not separable.
Scheme 3
Scheme 5
Scheme 4
Scheme 6
noticeable color change. The solution was allowed to stir at
room temperature and monitored periodically by 1H NMR
spectroscopy. Over the course of 9 days, in the presence of the
diene, the formation of mesityl(trimesitylgermyl)germa-
cyclopentene (6) was observed (Scheme 6). The formation of
germacyclopentene 6 is best explained by a 1,2-shift of a mesityl
group in 1 to give mesityl(trimesitylgermyl)germylene (5),
followed by subsequent trapping with the diene.16d The rate of
formation of 6 was increased by heating the reaction mixture
to reflux in a sealed tube. The formation of decomposition
products (presumably oligomers) was also observed; however,
there was no evidence for the formation of dimesitylgermacy-
clopentene 7.16b,d The co-photolysis of cyclotrigermane 2 and
2,3-dimethylbutadiene in THF at -70 °C was also examined.
The irradiation produced germacyclopentene 716b,d and unidenti-
fied decomposition products.17 These results demonstrate that
once formed, germylene 3 can be trapped by a diene in THF.
As a result, we can confidently state that there is no evidence
for dissociation of 1 to germylene 3 in THF solution at
temperatures less than 60 °C. In the absence of dissociation of
digermene 1 to germylene 3, the mechanism for the formation
of cyclotrigermane 2 from 1 (Vide supra) is puzzling. We
postulate that, in the absence of a diene trap, 2 is formed via
4). As such, our specific reaction conditions (irradiation of 2 in
THF at -70 °C) allow for full and clean conversion of
cyclotrigermane 2 to digermene 1. Furthermore, the solution
of digermene 1 can be warmed to room temperature without
reversion to the cyclotrigermane 2.
Given the stability of tetramesityldigermene (1) in THF at
room temperature, we questioned the need for photolysis at
-70 °C. Thus, we examined the photolysis of 2 in THF at
10 °C. Under these conditions, the photolysis did not go to
completion; a mixture of 1 and 2 in a 1:1 ratio was observed
by 1H NMR spectroscopy. Presumably, at this elevated reaction
temperature, the equilibrium constant for the formation of the
THF complex of 3 decreases, producing more of free 3. The
addition of 3 to 1 must compete effectively with the dimerization
of 3 at this temperature. If the mixture of 1 and 2 is allowed to
stir in THF at room temperature, 1 slowly reverts to the
cyclotrigermane 2 and some decomposition products. After
stirring for 3 weeks, the ratio of 1 to 2 changed slightly, from
1:1 to 1:1.27. If, instead, the THF solution of 1 and 2 is heated
to reflux, the loss of 1 occurs more quickly. After 2 weeks of
heating, the ratio of 1:2 had changed significantly to 1:7.14
(Scheme 5).
(16) (a) Neumann, W. P.; Schriewer, M. Tetrahedron Lett. 1980, 21,
3273. (b) Rivie`re, P.; Castel, A.; Satge´, J. J. Organomet. Chem. 1982, 232,
123. (c) Schriewer, M.; Neumann, W. P. J. Am. Chem. Soc. 1983, 105,
897. (d) Baines, K. M.; Cooke, J. A.; Dixon, C. E.; Liu, H. W.; Netherton,
M. R. Organometallics 1994, 13, 631. (e) Tokitoh, N.; Okazaki, R. Coord.
Chem. ReV. 2000, 210, 251.
(17) Decomposition products (likely oligomers) were also formed in small
quantities; however, they could not be separated cleanly to permit
identification by spectroscopic methods. Photolysis (350 nm) of a solution
of 2,3-dimethylbutadiene in THF at -70 °C in the absence of cyclotriger-
mane 2 also resulted in the formation of several oligomeric products.
To investigate the stability of digermene 1, we examined its
reactivity in the presence of a well-known germylene trap, 2,3-
dimethylbutadiene.12b,16 An excess of the diene was added to a
solution of 1 in THF at room temperature. There was no

5572 Organometallics, Vol. 26, No. 23, 2007
Hurni et al.
Scheme 7
Me3-6-t-Bu-C6H)-substituted digermene21 (Table 2). Interest-
ingly, the (2,3,4-Me3-6-t-Bu-C6H)-substituted digermene dis-
sociates in solution at room temperature to give the corresponding
germylene,21 whereas we have shown that tetramesityldigermene
(1) does not. As has previously been concluded, slight changes
in the spatial requirements of digermene substituents produce
remarkably different bond lengths and angles such that a
correlation between steric bulk about a digermene and its double-
bond length is not feasible.1g,i,j Thus, the rather long double bond
in 1, in comparison with the length of the Ge-Ge double bond
in the bulkier 2,6-diethylphenyl- and 2,4,6-triisopropylphenyl-
substituted derivatives, seems reasonable. With an improved
method for the synthesis of tetramesityldigermene (1) in hand,
we next wanted to demonstrate the usefulness of the procedure.
The kinetics of the addition of carboxylic acids to transient
digermenes has been examined; however, isolation of the
adducts was not practical.9a,b,d,e,10 Only one example of a
carboxylic acid adduct of a digermene has been reported: the
adduct between acetic acid and tetramesityldigermene (1).10 The
adduct, Mes2Ge(OAc)GeHMes2, was formed in good yield
based on a 1:1 stoichiometry (63%); however, it was noted that
the yield was lower in THF solution, although no details were
given.10 The generality of this reaction is still unknown. The
addition of (deuterated) acetic acid and formic acid to the
germanium dimers on the Ge(100)-2×1 surface has also been
investigated.22 The structure of the surface adduct(s) was
determined using a combination of spectroscopic and compu-
tational techniques. Given the recent interest in the addition of
carboxylic acids to digermenes, we selected this reaction to
showcase our new protocol for the synthesis of digermene
derivatives.
Figure 1. Structure of 1. Thermal ellipsoids are drawn at the 50%
probability level. All hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ge(1)-Ge(1A) )
2.2856(8), Ge(1)-C(11) ) 1.973(3), Ge(1)-C(1) ) 1.985(3),
C(11)-Ge(1)-C(1) ) 113.28(15), C(11)-Ge(1)-Ge(1A) ) 121.64-
(10), C(1)-Ge(1)-Ge(1A) ) 113.22(10).
Table 2. Selected Structural Data of Representative
Digermenes R2GedGeR2
trans-bent
angle (deg)
twist angle
(deg)
R
Ge-Ge (Å)
Mes
2.2856(8)
2.213(2)
2.213(1)
2.347(3)
2.2521(8)
33.4
12
12.3
37.9
0
2.9
10
13.7
not reported
20.4
2,6-diethylphenyla
2,4,6-triisopropylphenylb
(Et)(2,6-Trip2C6H3)c
2,3,4-Me3-6-t-Bu-C6Hd
a Reference 19a. b Reference 19b. c Trip)2,4,6-triisopropylphenyl, ref 20.
d Reference 21.
dimerization of the digermene with expulsion of a germylene
either from the resulting cyclotetragermane or from a possible
biradical intermediate. In the presence of a diene trap, however,
rearrangement to the germylgermylene 5 takes place preferen-
tially.
Crystals of digermene 1 were grown from a concentrated THF
solution, and the molecular structure of tetramesityldigermene
(1) was determined by X-ray crystallography (Figure 1).
Although bond lengths and angles of 1 have been calculated,9a
until now, there has been no experimental structural character-
ization of digermene 1. The Ge-Ge double-bond length of 1
was determined to be 2.2856(8) Å, and 1 has trans-bent and
twist angles of 33.4° and 2.9°, respectively. These values fall
within the known range of Ge-Ge bond lengths (2.21-2.46
Å),1g,18 trans-bent angles (0-45°),1g,h,18 and twist angles (0-
42°)18 in digermenes. The metrics obtained are also close to
the estimated values predicted by DFT calculations of 1 using
uncontracted Slater-type orbitals of triple-ú quality as the basis
set with the PW91 functionals.9a The Ge-Ge bond length of 1
is longer than that in tetrakis(2,6-diethylphenyl)digermene and
tetrakis(2,4,6-triisopropylphenyl)digermene, which both have a
Ge-Ge bond length of 2.213 Å19 despite the difference in the
steric bulk of the substituents (Table 2). While the bond length
of 1 is shorter than in the terphenyl-substituted digermenes,20
it is slightly longer than the Ge-Ge bond length of the (2,3,4-
The addition of carboxylic acids to 1 in THF was examined
at both room temperature and in the cold (dry ice/acetone bath).
The formation of a digermyl ester, 8a-c, was observed in all
cases (Scheme 7). Despite rigorous attempts to remove all traces
of water from the carboxylic acid prior to addition, the formation
of the water adduct of 1 was often observed. This necessitated
separation of the products by preparative thin-layer chroma-
tography, and thus, the isolated yields were modest. All products
were characterized by NMR and IR spectroscopy and mass
spectrometry; the isolated yields of the products are given in
Table 1.
The mass spectra of 8a-c contained signals that were
consistent with the formation of a 1:1 adduct between 1 and
1
the carboxylic acid. The H NMR spectra of 8a-c revealed a
characteristic singlet between 6.4 and 6.8 ppm, assigned to the
Ge-H moiety. The IR spectra of 8a-c showed absorptions
between 2030 and 2092 cm-1, which were assigned to the
stretching vibration of the Ge-H bond.3c,d,6,8,10,16d The 13C NMR
spectra of 8a-c contained signals (170-180 ppm) located in
the typical chemical shift range of ester carbonyl carbons. In
(18) Power, P. P. Chem. ReV. 1999, 99, 3463.
(19) (a) Snow, J. T.; Murakami, S.; Masamune, S.; Williams, D. J.
Tetrahedron Lett. 1984, 25, 4191. (b) Scha¨fer, H.; Saak, W.; Weidenbruch,
M. Organometallics 1999, 18, 3159.
(21) Weidenbruch, M.; Stu¨rmann, M.; Kilian, H.; Pohl, S.; Saak, W.
Chem. Ber. 1997, 130, 735.
(22) Filler, M. A.; Van Deventer, J. A.; Keung, A. J.; Bent, S. F. J. Am.
Chem. Soc. 2006, 128, 770.
(20) Stender, M.; Pu, L.; Power, P. P. Organometallics 2001, 20, 1820.

Synthesis, Structure, and ReactiVity of Tetramesityldigermene
Organometallics, Vol. 26, No. 23, 2007 5573
kinetic studies of the reaction of carboxylic acids with
digermenes9a,b,d,e,10 and are consistent with the structure of the
acetic acid adduct of 1, which was determined by NMR and IR
spectroscopy and mass spectrometry.10
Conclusion
In summary, we have evaluated a new protocol for the
synthesis of tetramesityldigermene (1): photolysis of cyclot-
rigermane 2 in THF at -70 °C. Digermene 1 was shown to be
stable in THF at room temperature; no evidence for dissociation
to dimesitylgermylene (3) or rearrangement to germylgermylene
5 was observed. Using the new protocol, we were able to
characterize digermene 1 by NMR spectroscopy and mass
spectrometry and determine its molecular structure by X-ray
crystallography. The benefits of our new protocol were dem-
onstrated in a study of the reaction between digermene 1 and
three carboxylic acids. Moreover, it was shown that the reactivity
between molecular digermenes and carboxylic acids mirrors the
reactivity of the symmetric Ge dimers found on the Ge(100)-
2×1 surface. It is our belief that digermenes can be used as
molecular models of these surface dimers and can assist in the
understanding of surface functionalization reactions.
Figure 2. Structure of 8a. Thermal ellipsoids are drawn at the
50% probability level. All hydrogen atoms, with the exception of
the Ge-H, are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Ge(1)-Ge(2) ) 2.4829(5), Ge(1)-C(11) ) 1.981-
(4), Ge(1)-C(21) ) 1.983(4), Ge(2)-C(31) ) 1.968(4), Ge(2)-
C(41) ) 1.981(4), Ge(1)-O(1) ) 1.863(3), O(1)-C(2) ) 1.317(5),
C(2)-O(3) ) 1.210(5); C(11)-Ge(1)-C(21) ) 111.67(15), C(11)-
Ge(1)-Ge(2) ) 116.18(11), C(31)-Ge(2)-C(41) ) 107.33(15),
C(41)-Ge(2)-Ge(1) ) 121.15(11), C(21)-Ge(1)-Ge(2) ) 120.67-
(10), C(31)-Ge(2)-Ge(1) ) 113.39(11), O(1)-Ge(1)-C(21) )
97.05(13), O(1)-Ge(1)-Ge(2) ) 103.72(8), O(3)-C(2)-O(1) )
122.6(4).
Experimental Section
General Procedures. Unless otherwise indicated, all reactions
were carried out in flame-dried glassware under an inert atmosphere
of argon using a dual-manifold vacuum line or prepared under a
nitrogen atmosphere using an MBraun Labmaster 130 glovebox.
THF was purged with nitrogen and then passed through an alumina
column (Innovative Technology Ltd.) prior to use. Toluene was
purged with nitrogen and then passed through an alumina column
and copper catalyst (Innovative Technology Ltd.) prior to use.
Methanol was distilled from magnesium and chloroform was
distilled from phosphorus pentoxide prior to use. Triethylsilane was
distilled from LiAlH4 prior to use. 2,3-Dimethylbutadiene was
purchased from the Aldrich Chemical Co. and used without further
purification. 2,4,6-Trimethylbenzoic acid and trans-2-phenylcyclo-
propane-1-carboxylic acid were purchased from the Aldrich Chemi-
cal Co.; pivalic acid was purchased from the Eastman Kodak Co.
All carboxylic acids were dissolved in THF (1 mL) and allowed to
sit over 4 Å molecular sieves for 24 h prior to use. Hexamesityl-
cyclotrigermane (2) was prepared according to the literature
procedure.24 If solutions of 2 were not prepared in an inert
atmosphere glovebox, the solutions were degassed by successive
freeze-pump-thaw cycles.
the IR spectra of 8a-c, bands attributable to the stretching
vibration of a carbonyl group of a germyl ester were also
identified (1670-1690 cm-1). None of the aforementioned 1H
NMR signals assigned to the Ge-H revealed a cross-peak with
1
the 13C signals assigned to the carbonyl group in the H-13C
gHMBC spectra for adducts 8a-c, and thus, the framework of
8a-c was assigned as GeO(CdO)R and not Ge(CdO)OR. In
addition, correlations between the signals assigned to the R
1
groups in 8a and 8c in the H dimension and their respective
CdO signals in the 13C dimension were observed, providing
further evidence for the GeO(CdO)R arrangement. The frame-
work of the digermyl ester 8a was unambiguously confirmed
by X-ray crystallography. Crystals of 8a were grown by slow
evaporation of C6D6; the molecular structure of adduct 8a is
shown in Figure 2. As expected, the Ge-Ge bond length of 8a
has increased to 2.4829(5) Å from the bond length of 2.2856-
(8) Å observed in digermene 1. This is consistent with a change
in bond order from two to one. The Ge-Ge bond length is
slightly elongated from what is typically observed.1d,h The
Ge-O bond length of 1.863(3) Å is representative of this type
of bond.5,23
NMR spectra were recorded on Varian Mecury 400 or Inova
400 or 600 NMR spectrometers. The standards used were as
1
follows: residual C6D5H (7.15 ppm) for H NMR spectra; C6D6
The structures of 8a-c are consistent with those proposed
to be formed on the Ge(100)-2×1 surface.22 Bent et al.
monitored the addition of acetic acid and formic acid to the
Ge(100)-2×1 surface by IR spectroscopy; they recorded the IR
spectra of the surface periodically during the addition of the
carboxylic acid and after the reaction was complete. Absorptions
corresponding to Ge-H moieties were observed, in addition to
those resulting from the carbonyl and C-O functional groups.22
Using density functional theory calculations, Bent et al.
considered several possibilities for the structure of the surface
adduct. The digermyl ester was found to be the most thermo-
dynamically stable product of the addition.22 Thus, the reactivity
of the molecular digermene 1 exactly mirrors that of the Ge
surface dimer. Our results are also consistent with the digermyl
ester structures proposed to be formed in laser flash photolysis
central transition (128.00 ppm) for 13C NMR spectra. J values are
reported in hertz. EI- and GC-mass spectra were recorded on a
Finnigan MAT model 8400 mass spectrometer with an ionizing
voltage of 70 eV. IR spectra were recorded on a Perkin-Elmer
System 2000 FT IR spectrometer. Photolyses were performed in a
Rayonet photochemical reactor (Southern New England Co.)
equipped with 12 bulbs. Samples were cooled by circulating
methanol through a vacuum-jacketed quartz or Pyrex immersion
well using an Endocal model ULT-70 low-temperature external bath
circulator. Melting point data are uncorrected.
Single-crystal X-ray analyses of 1 and 8a were performed using
an Enraf-Nonius Kappa CCD X-ray diffractometer. The structures
were solved and refined using SHELXTL/PC V6.14 software. A
(24) (a) Ando, W.; Tsumuraya, T. J. Chem. Soc., Chem. Commun. 1987,
1514. (b) Tsumuraya, T.; Kabe, Y.; Ando, W. J. Organomet. Chem. 1994,
482, 131.
(23) Baines, K. M.; Stibbs, W. G. Coord. Chem. ReV. 1995, 145, 157.

5574 Organometallics, Vol. 26, No. 23, 2007
Hurni et al.
Table 3. Crystallographic Data for 1 and 8a
1
8a
empirical formula
fw
C36H44Ge2‚C4H8O
694.00
C41H54Ge2O2
724.02
temperature (K)
wavelength (Å)
cryst syst
space group
a (Å)
150(2)
0.71073
orthorhombic
Pbcn
11.816(2)
14.413(3)
20.729(4)
90
3530.4(12)
4
1.306
1.732
150(2)
0.71073
monoclinic
P2(1)/c
12.8332(4)
14.0900(3)
20.2794(6)
94.320(13)
3656.50(18)
4
1.315
1.677
1520
0.47 × 0.23 × 0.08
2.04-27.48
15 532
b (Å)
c (Å)
â (deg)
volume (Å3)
Z
calcd density (g/cm3)
absorp coeff (mm-1
F(000)
)
1456
cryst size (mm3)
0.15 × 0.14 × 0.05
θ range for data collection (deg)
no. of reflns collected
2.04-27.48
25 363
no. of indep reflns
completeness to θ ) 25.03° (%)
absorp corr
max. and min. transmn
no. of data/restraints/params
goodness-of-fit on F2
4049 [Rint ) 0.063]
8310 [Rint ) 0.076]
99.9
99.0
semiempirical from equivalents
0.9184 and 0.7812
4049/0/189
semiempirical from equivalents
0.8845 and 0.5030
8310/18/414
1.058
1.043
final R indices [I > 2 σ(I)]
R1 ) 0.0565, wR2 ) 0.1582
R1 ) 0.0825, wR2 ) 0.1778
2.309 and -0.912
R1 ) 0.0522, wR2 ) 0.1243
R1 ) 0.0944, wR2 ) 0.1414
0.896 and -0.920
R indices (all data)
largest diff peak and hole (e Å-3
)
summary of the crystal data collection and refinement for 1 and
8a is presented in Table 3. Complete X-ray experimental details
are given in the Supporting Information.
Addition of Chloroform to 1. Addition of chloroform (1 mL,
excess) to the yellow solution of 1 (0.17 mmol) in THF (8 mL) at
room temperature resulted in the complete loss of color within 2
min. The solvent was removed by rotary evaporation to yield
1,1,2,2-tetramesityl-1-chloro-2-(dichloromethyl)digermane7 (111
mg, 88%).
Preparation of Tetramesityldigermene (1). A clear, colorless
solution of 2 (100 mg, 0.11 mmol) in THF (8 mL) was prepared
in a Pyrex Schlenk tube. The solution was irradiated (350 nm) at
-70 °C for 18 h, at which point the solution was bright yellow in
color.25 THF was removed in Vacuo from 1 to yield a yellow
powder. Analysis of the powder by 1H NMR spectroscopy revealed
that the conversion of 2 to 1 was complete. The melting point
of 1 was obtained under an inert (N2) atmosphere. 1: Mp: 124-
Addition of 2,3-Dimethylbutadiene to 1. 2,3-Dimethylbutadiene
(0.1 mL, excess) was added to a yellow solution of 1 (0.17 mmol)
in THF (8 mL). After 9 days of stirring at room temperature, the
color of the solution had faded to pale yellow. The solvent and
excess 2,3-dimethylbutadiene were removed by rotary evaporation
to yield a white powder identified as 1-mesityl-3,4-dimethyl-1-
(trimesitylgermyl)-1-germacyclopent-3-ene16d (6) contaminated with
a small amount of decomposition products17 (96.4 mg, 85% of 6).
1
127 °C (dec). H NMR (ppm): 6.72 (s, 8H, Mes CH), 2.39 (s,
24H, Mes o-CH3), 2.06 (s, 12H, Mes p-CH3). 13C NMR (ppm):
143.57 (i-MesC), 142.24 (o-MesC), 138.21 (p-MesC), 128.58 (Mes
CH), 25.46 (Mes o-CH3), 21.09 (Mes p-CH3). High-resolution EI-
MS: Exact mass calcd for C36H4570Ge2 (M+ + H) 617.201, found
617.199.26
Co-photolysis of 2,3-Dimethylbutadiene and 2. A solution of
2,3-dimethylbutadiene (0.1 mL, excess) and 2 (50 mg, 0.05 mmol)
in THF (4 mL) was irradiated (350 nm) for 16 h. The solvent and
excess 2,3-dimethylbutadiene were removed by rotary evaporation
to yield 1,1-dimesityl-3,4-dimethyl-1-germacyclopent-3-ene16b,d (7)
and decomposition products17 (54.8 mg) as determined by 1H NMR
spectroscopy.
Addition of Methanol to 1. Method C. Addition of methanol
(1 mL, excess) to the yellow solution of 1 (0.17 mmol) in THF (8
mL) at room temperature resulted in an instantaneous and complete
loss of color. The solvent was removed by rotary evaporation to
yield 1,1,2,2-tetramesitylmethoxydigermane6 (97.7 mg, 88%).
Method A. A solution of 2 (50 mg, 0.05 mmol) and Et3SiH (1
mL, excess) dissolved in toluene (4 mL) was prepared in a Pyrex
Schlenk tube. The solution was irradiated (350 nm) at -70 °C for
16 h, at which point the solution was bright yellow in color.
Methanol (1 mL, excess) was added to the cooled solution (dry
ice/acetone), and the mixture was slowly warmed to room temper-
ature, resulting in complete loss of color. The solvent was removed
by rotary evaporation to yield a pale yellow oil. The product mixture
was purified by preparative thin-layer chromatography (silica gel;
70/30 CH2Cl2/hexanes) to give (hydroxydimesitylgermyl)-
dimesitylgermane,16d dimesityl(triethylsilyl)germane,8 and 1,1,2,2-
tetramesitylmethoxydigermane6 (7.29 mg, 22%).
Addition of Pivalic Acid to 1. A solution of pivalic acid (25.3
mg, 0.25 mmol) dissolved in THF (1 mL) was added to the bright
yellow solution of 1 (0.17 mmol) in THF (8 mL). After 5 min at
room temperature, the color of the solution had faded to pale yellow.
The solvent was removed by rotary evaporation to yield an oily,
yellow residue. The product mixture was purified by preparative
thin-layer chromatography (silica gel; 70/30 CH2Cl2/hexanes) to
give
excess
pivalic
acid,
(hydroxydimesitylgermyl)-
dimesitylgermane,16d and 8a (65.1 mg, 53%) as colorless crystals.
8a: Mp: 190-192 °C (dec). IR (cm-1): 2964 (m), 2959 (m), 2923
(m), 2870 (m), 2085 (w), 2033 (w), 1685 (s), 1559 (s), 1458 (s),
1290 (m), 1177 (s), 1028 (m), 848 (m), 738 (m), 668 (m), 648
(m), 546 (m). 1H NMR (ppm): 6.69 (s, 8H, Mes CH), 6.51 (s, 1H,
GeH), 2.39 (s, 12H Mes o-CH3), 2.36 (s, 12H, Mes o-CH3), 2.07
(s, 6H, Mes p-CH3), 2.04 (s, 6H, Mes p-CH3), 1.20 (s, 9H, C(CH3)3).
13C NMR (ppm): 179.27 (CdO), 143.85, 143.17 (o-MesC), 138.91,
138.42 (p-MesC), 138.03, 135.62 (i-MesC), 129.76, 129.09 (Mes
CH), 39.78 (C(CH3)3), 27.91 (C(CH3)3), 24.98, 24.41 (Mes o-CH3),
20.97, 20.93 (Mes p-CH3). EI-MS (m/z): 724 (M+ 72Ge74Ge, 3),
623 (M+ - O(CO)t-Bu, 3), 605 (M+ - Mes, 1), 502 (Mes372Ge2H,
(25) For the UV-visible spectrum of 1, see refs 9f, 11c,e, and 15.
(26) The low-resolution mass spectrum of 1 revealed the presence of
oxidation product(s) likely formed during the introduction of the sample
into the mass spectrometer. EI-MS (m/z): 654 (M+ + O2, 72Ge74Ge, 34),
72
638 (M+ + O, 72Ge74Ge, 17), 623 (M+ + H, 72Ge74Ge, 23), 535 (Mes3
-
-
Ge74GeO2, 47), 517 (Mes372Ge2O, 47), 429 (Mes372Ge, 58), 415 (Mes2
72
Ge2O2H, 44), 329 (Mes274GeOH, 100), 311 (Mes272GeH, 83), 192 (Mes72-
GeH, 55), 119 (Mes, 87).

Synthesis, Structure, and ReactiVity of Tetramesityldigermene
1), 431 (Mes374Ge, 18), 413 (M+ - Mes272GeH, 100), 311 (Mes2
Organometallics, Vol. 26, No. 23, 2007 5575
72
-
raphy (silica gel; 70/30 CH2Cl2/hexanes) to give excess trans-2-
phenylcyclopropane-1-carboxylic acid, (hydroxydimesitylgermyl)-
dimesitylgermane,16d and 8c (72.8 mg, 55%) as a pale yellow oil.
8c: IR (cm -1): 2964 (m), 2960 (m), 2922 (m), 2859 (m), 2078
(w), 2033 (w), 1688 (s), 1682 (s), 1602 (s), 1462 (m), 1451 (m),
GeH, 28), 293 (M+ - Mes374Ge, 5), 192 (Mes72GeH, 15), 119 (Mes,
8). High-resolution EI-MS: Exact mass calcd for C41H5472Ge74-
GeO2 724.256, found 724.254.
Addition of 2,4,6-Trimethylbenzoic Acid to 1. A solution of
2,4,6-trimethylbenzoic acid (41.6 mg, 0.25 mmol) dissolved in THF
(1 mL) was added to the bright yellow solution of 1 (0.17 mmol)
in THF (8 mL). The color of the reaction mixture faded to pale
yellow within 1 min at room temperature. The solvent was removed
by rotary evaporation to yield an oily, yellow residue. The product
mixture was purified by preparative thin-layer chromatography
(silica gel; 70/30 CH2Cl2/hexanes) to give excess 2,4,6-trimethyl-
benzoic acid, (hydroxydimesitylgermyl)dimesitylgermane,16d and
8b (73.2 mg, 55%) as a white solid. 8b: Mp: 197-198 °C (dec).
IR (cm-1): 3021 (m), 2964 (m), 2924 (m), 2857 (m), 2733 (m),
2092 (w), 2030 (w), 1678 (s), 1674 (s), 1668(s), 1602 (s), 1468
(s), 1462 (s), 1451 (s), 1446 (s), 1286 (s), 1173 (s), 1087 (s), 1029
1
1446 (m), 1402 (m), 1196 (m), 848 (m), 698 (m), 546 (m). H
NMR (ppm): 7.06-6.96 (m, 3H, m,p-PhH), 6.86-6.83 (m, 2H,
o-PhH), 6.71 (s, 4H, Mes CH), 6.69 (s, 4H, Mes CH), 6.38 (s, 1H,
GeH), 2.55 (ddd, 1H, CHPh, J ) 9.1, 6.1, 4.1 Hz), 2.42 (s, 6H,
Mes o-CH3), 2.40 (s, 6H Mes o-CH3), 2.38 (s, 12H, Mes o-CH3),
2.07 (s, 3H, Mes p-CH3), 2.06 (s, 3H, Mes p-CH3), 2.050, 2.046
(each s, 6H Mes p-CH3), 1.97 (ddd, 1H, CHCOOGe, J ) 8.4, 5.2,
4.0 Hz), 1.56 (ddd, 1H, CH2, J ) 9.2, 5.2, 4.2 Hz), 0.98 (ddd, 1H,
CH2, J ) 8.3, 6.3, 4.3 Hz). 13C NMR (ppm): 173.66 (CdO),
144.03, 143.91, 143.14, 143.05 (o-MesC), 141.11 (i-PhC), 139.02,
139.00, 138.53 (p-MesC), 137.62, 137.47, 135.11 (i-MesC) 129.85,
129.83, 129.15 (Mes CH), 128.60 (m-PhC), 126.45 (o-PhC), 126.32
(p-PhC), 26.63 (CHCOOGePh), 26.16 (CHPh), 24.84, 24.80, 24.27,
24.20 (Mes o-CH3), 20.98, 20.97, 20.94 (Mes p-CH3), 16.94 (CH2).
EI-MS (m/z): 784 (M+ 72Ge74Ge, 8), 769 (M+ - CH3, 1), 665
(M+ - Mes, 2), 622 (M+ - HO(CO)CHCH2CHPh, 3), 549 (M+
1
(m), 848 (s), 740 (s), 668 (m), 633 (m) 546 (m). H NMR (ppm):
6.78 (s, 1H, GeH), 6.70 (s, 8H, Mes CH), 6.64 (s, 2H, acid Mes
CH), 2.43 (s, 12H, Mes o-CH3), 2.42 (s, 12H, Mes o-CH3), 2.19
(s, 6H, acid Mes o-CH3), 2.071, 2.067 (each s, 12H, Mes p-CH3),
2.03 (s, 3H, acid Mes p-CH3). 13C NMR (ppm): 171.95 (CdO),
143.99, 143.56 (o-MesC), 139.14, 138.47 (p-MesC), 138.32 (i-
MesC), 138.00 (acid p-MesC), 135.81 (acid o-MesC), 135.69 (i-
MesC), 133.48 (acid i-MesC), 129.79, 129.16 (Mes CH), 128.88
(acid Mes CH), 25.11, 24.86 (Mes o-CH3), 20.98, 20.96 (Mes p-CH3
and acid Mes p-CH3), 20.41 (acid Mes o-CH3). EI-MS (m/z): 786
(M+ 72Ge74Ge, 13), 771 (M+ - CH3, 4), 667 (M+ - Mes, 1), 622
-
74GeO(CO)CHCH2CHPh, 1), 502 (Mes372Ge2H, 1), 473 (M+
-
Mes272GeH, 100), 431 (Mes374Ge, 14), 355 (Mes272GeO(CO)H, 56),
312 (Mes274Ge, 58), 192 (Mes72GeH, 79), 119 (Mes, 34). High-
resolution EI-MS: Exact mass calcd for C46H5472Ge74GeO2 784.256,
found 784.254.
Acknowledgment. We thank Dr. Michael Jennings (UWO)
for collection of crystallographic data and the Natural Sciences
and Engineering Research Council of Canada, the University
of Western Ontario, and the Ontario Photonics Consortium for
financial support. We also thank Teck Cominco Ltd. for a
donation of GeCl4.
74
(M+ - HO(CO)Mes, 3), 475 (M+ - Mes272GeH, 100), 431 (Mes3
-
Ge, 14), 356 (M+ - Mes372GeH, 13), 312 (Mes274Ge, 31), 192
(Mes72GeH, 22), 119 (Mes, 18). High-resolution EI-MS: Exact
mass calcd for C46H5672Ge74GeO2 786.272, found 786.269.
Addition of trans-2-Phenylcyclopropane-1-carboxylic Acid to
1. A solution of trans-2-phenylcyclopropane-1-carboxylic acid (40.0
mg, 0.25 mmol) dissolved in THF (1 mL) was added to the bright
yellow solution of 1 (0.17 mmol) in THF (8 mL). The solution
was allowed to stir at room temperature for 5 min, after which the
color of the solution had completely faded to pale yellow. The crude
product mixture was purified by preparative thin-layer chromatog-
Supporting Information Available: 1H NMR spectra of
compounds 1 and 8a-c. X-ray experimental details and a CIF file
giving crystallographic information for 1 and 8a. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM7005358