Reactivity studies of N-heterocyclic carbene complexes of germanium(II)

Article abstract of DOI:10.1021/om100059z

The chemistry of three N-heterocyclic carbene (NHC) complexes of GeR ₂, where R = Cl (1), O^tBu (2), Mes (3) (Mes = 2,4,6-trimethylphenyl), toward 2,3-dimethylbutadiene (DMB), 3,5-di-tert- butylorthoquinone, methyl iodide, pivalic acid, and benzophenone is reported. Upon heating, 2 and 3 cyclize with DMB to yield a germacyclopentene and free NHC. In contrast, 1 does not react with DMB. PB1PBE/6-311+G(d,p) model chemistry shows that the cycloaddition reactions of NHC-GeX₂ (X = F, Cl) with butadiene are not thermodynamically favorable. 3,5-Di-tert-butylorthoquinone reacts rapidly with 1-3 to form cyclic products; in the case of 1 and 2, the NHC remains coordinated to the germanium, resulting in a hypervalent species. Compounds 1-3 react with methyl iodide by displacement of I^-; in each case [NHC-GeR₂Me]⁺ is produced. Only compound 3 reacts in a controlled fashion with pivalic acid; both 1:1 and 1:2 adducts were characterized. Benzophenone failed to react with 1 or 2 but did undergo cycloaddition with 3. In comparison with uncomplexed GeR₂ species, the NHC-GeR₂ complexes are less reactive. The prospect of using NHC-GeR₂ as a synthon for GeR₂ appears to be reaction specific.

Full text of DOI:10.1021/om100059z

Organometallics 2010, 29, 4871–4881 4871
DOI: 10.1021/om100059z
Reactivity Studies of N-Heterocyclic Carbene Complexes of
Germanium(II)†
Paul A. Rupar, Viktor N. Staroverov, and Kim M. Baines*
Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
Received January 24, 2010
The chemistry of three N-heterocyclic carbene (NHC) complexes of GeR2, where R=Cl (1), OtBu (2),
Mes (3) (Mes = 2,4,6-trimethylphenyl), toward 2,3-dimethylbutadiene (DMB), 3,5-di-tert-butylortho-
quinone, methyl iodide, pivalic acid, and benzophenone is reported. Upon heating, 2 and 3 cyclize with
DMB to yield a germacyclopentene and free NHC. In contrast, 1 does not react with DMB. PB1PBE/
6-311þG(d,p) model chemistry shows that the cycloaddition reactions of NHC-GeX2 (X = F, Cl) with
butadiene are not thermodynamically favorable. 3,5-Di-tert-butylorthoquinone reacts rapidly with 1-3 to
form cyclic products; in the case of 1 and 2, the NHC remains coordinated to the germanium, resulting in a
hypervalent species. Compounds 1-3 react with methyl iodide by displacement of I-; in each case [NHC-
GeR2Me]þ is produced. Only compound 3 reacts in a controlled fashion with pivalic acid; both 1:1 and 1:2
adducts were characterized. Benzophenone failed to react with 1 or 2 but did undergo cycloaddition with 3.
In comparison with uncomplexed GeR2 species, the NHC-GeR2 complexes are less reactive. The prospect
of using NHC-GeR2 as a synthon for GeR2 appears to be reaction specific.
Introduction
of electron density to the empty p orbital, either by a donor
ligand or by π donation, reduces the Lewis acidity while simul-
taneously increasing the nucleophilicity of the electron lone pair.
As such, electronically stabilized germylenes often react primar-
ily through their electron lone pair rather than as Lewis acids.1
The chemistry of intermolecularly stabilized germylenes, with
Simple germylenes (GeR2) are amphoteric because of their
unoccupied p orbital and lone pair of electrons. The addition
† Part of the Dietmar Seyferth Festschrift. In honor of Professor Dietmar
Seyferth, in recognition of his outstanding contributions as the founding
Editor-in-Chief of Organometallics.
the exception of the substitution chemistry of GeCl2 (dioxane),
3
is poorly studied.2 We have utilized strong Lewis bases,3 with a
focus on N-heterocyclic carbenes,4-6 for the intermolecular sta-
bilization of GeR2.7 Since N-heterocyclic carbenes are among
the strongest known neutral donors,8 they are expected to signi-
ficantly alter the reactivity of GeR2 upon complexation: NHC-
GeR2 species are anticipated to be more nucleophilic and less
electrophilic in comparison with noncoordinated germylenes.9
Although NHC-GeR2 complexes have already proven to
be valuable precursors for the synthesis of unprecedented
*To whom correspondence should be addressed. E-mail: kbaines2@
uwo.ca.
(1) (a) Neumann, W. P. Chem. Rev. 1991, 91, 311. (b) Barrau, J.; Rima, G.
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Inorg. Chem. 2007, 46, 7713. (b) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. J. Am.
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Segmueller, T.; Schlueter, P. A.; Drees, M.; Schier, A.; Nogai, S.; Mitzel, N. W.;
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A. V.; Hahn, F. E.; Pape, T.; Hepp, A. Organometallics 2007, 26, 1972. (f) Hahn,
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V. A.; Skatova, A. A.; Cherkasov, V. K.; Fukin, G. K.; Baranov, E. V. Russ. Chem.
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2037. (n) Schoepper, A.; Saak, W.; Weidenbruch, M. J. Organomet. Chem. 2006,
691, 809. (o) Stanciu, C.; Richards, A. F.; Stender, M.; Olmstead, M. M.; Power,
P. P. Polyhedron 2006, 25, 477.
(3) Rupar, P. A.; Jennings, M. C.; Baines, K. M. Can. J. Chem. 2007,
85, 141.
(4) Rupar, P. A.; Jennings, M. C.; Ragogna, P. J.; Baines, K. M.
Organometallics 2007, 26, 4109.
(5) Rupar, P. A.; Jennings, M. C.; Baines, K. M. Organometallics
2008, 27, 5043.
(6) Rupar, P. A.; Staroverov, V. N.; Ragogna, P. J.; Baines, K. M.
J. Am. Chem. Soc. 2007, 129, 15138.
(7) Others have reported NHC complexes of GeR2. See: (a) Arduengo,
A. J., III; Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Inorg. Chem.
1993, 32, 1541. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Dalton
Trans. 2000, 3094. (c) Thimer, K. C.; Al-Rafia, S. M. I.; Ferguson, M. J.;
McDonald, R.; Rivard, E. Chem. Commun. 2009, 7119. (d) Sidiropoulos, A.;
Jones, C.; Stasch, A.; Klein, S.; Frenking, G. Angew. Chem., Int. Ed. 2009, 48,
9701.
(8) For a recent comparison of different neutral donors and their
Lewis basicity see: Gusev, D. G. Organometallics 2009, 28, 763.
(9) Recently the oxidation of a N-heterocyclic carbene-N-hetero-
cyclic germylene complex with N2O was reported to form a base-
stabilized germanone. The corresponding N-heterocyclic germylene,
without carbene coordination, is unreactive towards N2O. See: Yao,
S.; Xiong, Y.; Driess, M. Chem. Commun. 2009, 6466.
r
2010 American Chemical Society
Published on Web 07/08/2010
pubs.acs.org/Organometallics

4872 Organometallics, Vol. 29, No. 21, 2010
Rupar et al.
Chart 1
Scheme 1
cationic germanium(II) complexes supported by cryptands,10
crown ethers,11 or carbenes,6 a more systematic study of their
reactivity is needed. In this report, the reactivity of three NHC-
GeII species is examined (Chart 1). Compounds 1-3 were
chosen because they are representative of NHC complexes of
three different germylenes with a range of reactivities. 1 is a
complex of dichlorogermylene; dihalogermylenes are intrinsi-
cally stable and among the least reactive GeR2 compounds.
Compound 2 is a complex of a dialkoxygermylene, which is
intermediate in its reactivity. Compound 3 is a complex of a
highly reactive and transient diarylgermylene.
Germylenes are valuable building blocks for the synthesis
of germanium-containing compounds. Unfortunately, their
potential utility is often limited by their nonspecific reactiv-
ity. The NHC-GeR2 complexes may act as synthons of GeR2
while being easier to isolate and handle. Therefore, the reac-
tivity of 1-3 will be compared to the reactivity of uncoordi-
nated germylenes and the potential use of 1-3 as synthetic
equivalents of GeR2 will be evaluated.
Scheme 2
react with DMB it must be dissociated from any neutral donors,
then the difference in reactivity between GeCl2 (dioxane) and 1
3
toward DMB can be attributed to the much stronger coordina-
tion of the NHC to GeCl2 compared to 1,4-dioxane. Under
these conditions, the dissociation of GeCl2 from the carbene in 1
is apparently not favored kinetically. The reaction may also not
be thermodynamically favorable: GeCl2 may prefer to coordi-
nate with the NHC rather than form a germacyclopentene.
Interestingly, when 6 was added to a solution of the free
NHC 5, complex 7 was isolated from the reaction mixture
(Scheme 3). The structure of complex 7 consists of a molecule
of 6 coordinated by the carbene (Figure 1). Given that the
germanium center in 6 has two electron-withdrawing chlor-
ide substituents, it is not surprising that the germanium is
able to form a hypercoordinated species.16 When a solution
of 7 in THF was heated in a sealed tube for 3 days, DMB and
Results and Discussion
Reaction with 2,3-Dimethylbutadiene. 2,3-Dimethylbuta-
diene (DMB) is commonly used as a trapping reagent for
transient12 and stable13 germylenes since, in general, germy-
lenes undergo cycloaddition with DMB to form a germa-
cyclopentene in high yield.1,14
1
1 were formed, as determined by H NMR spectroscopy.
Thus, the formation of 6 by the reaction between 1 and DMB
appears to be thermodynamically unfavorable.
As was previously reported, complex 3 acts as a synthetic
equivalent of GeMes2: when 3 was heated with DMB, germa-
cyclopentene 4 was isolated (Scheme 1).4 We proposed that,
upon heating, uncoordinated GeMes2 was released from 3,
which then rapidly cyclized with DMB to give 4 and the NHC
5. To ascertain the generality of the reaction of DMB with
NHC-GeII species, the reactivity of 1 and 2 with DMB was
examined.
Heating a solution of complex 2and excess DMB at 80 °Cfor
18 h resulted in the formation of 8 and carbene 5 (Scheme 4).17
Unlike the case for 6, a coordination complex between 5 and 8
was not observed and heating 8 in the presence of 5 did not
result in retrocyclization.
On the basis of the results illustrated in Schemes 1-4, the
favorability of the reaction of a NHC-GeR2 complex with
DMB appears to be strongly substituent-dependent: both the
Mes- and OtBu-substituted compounds form the correspond-
ing germacyclopentene, while the germanium dichloride com-
plex favored coordination to NHC 5. Using PBE1PBE/
6-311þG(d,p) as the model chemistry, the energetics of a mo-
del system were examined computationally to gain further
insight into the reaction of butadiene with a series of NHC-
GeR2 complexes.18,19
A solution of 1 and DMB did not undergo any observable
reaction, as determined by 1H NMR spectroscopy, even after
prolonged heating in the presence of excess DMB (Scheme 2). In
contrast, GeCl2 (dioxane) readily reacts with DMB to form 6
3
under similar conditions.15 If it is assumed that for GeCl2 to
(10) Rupar, P. A.; Staroverov, V. N.; Baines, K. M. Science 2008, 322,
1360.
(11) Rupar, P. A.; Bandyopadhyay, R.; Cooper, B. F. T.; Stinchcombe,
M. R.; Ragogna, P. J.; Macdonald, C. L. B.; Baines, K. M. Angew. Chem.,
Int. Ed. 2009, 48, 5155.
(12) (a) Hurni, K. L.; Baines, K. M. Can. J. Chem. 2007, 85, 668. (b)
Neumann, W. P.; Schriewer, M. Tetrahedron Lett. 1980, 21, 3273. (c)
(16) Baukov, Y. I; Tandura, S. N. In The Chemistry of Organic
Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; Wiley: West
Sussex, England, 2002; Vol. 2, pp 963-1239.
(17) Compound 8 can also be synthesized by the reaction of 2 equiv of
Riviere, P.; Castel, A.; Satge, J. J. Organomet. Chem. 1982, 232, 123. (d)
KOtBu with 6.
ꢀ
Schriewer, M.; Neumann, W. P. J. Am. Chem. Soc. 1983, 105, 897. (e)
Baines, K. M.; Cooke, J. A.; Dixon, C. E.; Liu, H. W.; Netherton, M. R.
Organometallics 1994, 13, 631. (f) Tokitoh, N.; Okazaki, R. Coord. Chem.
Rev. 2000, 210, 251. (g) Jenkins, R. L.; Kedrowski, R. A.; Elliott, L. E.;
Tappen, D. C.; Schlyer, D. J.; Ring, M. A. J. Organomet. Chem. 1975, 86,
347. (h) Lollmahomed, F.; Leigh, W. J. Organometallics 2009, 28, 3239.
(13) Bonnefille, E.; Mazieres, S.; El Hawi, N.; Gornitzka, H.; Couret,
C. J. Organomet. Chem. 2006, 691, 5619.
(14) Nag, M.; Gaspar, P. P. Organometallics 2009, 28, 5612.
(15) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem.
Soc. 2004, 126, 16105.
(18) Calculations using standard density functionals such as B3LYP,
BP86, B98, and PW91 have been endorsed as an excellent method for
predicting bond lengths and bond dissociation energies of NHC com-
plexes of metals. See: Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.;
Cavallo, L. Coord. Chem. Rev. 2009, 253, 687.
(19) A previous computational study examined substituent effects on
the bonding in NHC-GeR2 complexes. Both MP2/6-311þG(d,p) and
PBE1PBE/6-311þG(d,p) model chemistries were used and found to give
very similar results. Therefore, only PBE1PBE/6-311þG(d,p) was used
in the present study; see: Ruddy, A. J.; Rupar, P. A.; Bladek, K. J.; Allan,
C. J.; Avery, J. C.; Baines, K. M. Organometallics 2010, 29, 1362.
ꢁ

Article
Organometallics, Vol. 29, No. 21, 2010 4873
Scheme 3
Chart 2
Chart 3
Figure 1. Thermal ellipsoid plot (50% probability surface) of 7.
˚
Hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (deg): Ge1-C1=1.965(2); Ge1-C14=1.942(2); Ge1-
C17=1.943(3); Ge1-Cl2=2.4007(7); Ge1-Cl3=2.5093(7); Cl2-
Ge1-Cl3 = 169.09(3); C14-Ge1-C17 = 96.87(11); C1-Ge-
Cl2 = 87.16(7); C1-Ge-Cl3 = 82.06(7).
Table 1. Relative Energies for the Reaction of NHC-GeR2
Complexes with Butadiene
Scheme 4
rel ΔG°298 K (kJ/mol)
substituent R on Ge
system A
system B
system C
H
0
0
0
0
0
0
0
not stable
12.3
not stable
not stable
5.6
-30.8
-13.0
-44.3
-88.9
30.8
25.0
-64.3
OH
NH2
Me
F
Cl
Ph
To reduce computational costs, simplified analogues of
the carbene complexes were studied (complexes 9-15), and
butadiene was used in place of DMB (Chart 2). The ener-
getics of the reactions of the NHC complexes of GeR2 with
butadiene was examined by comparing the three systems
shown in Chart 3. The total energy of system A, the NHC-
GeR2 complex plus butadiene, was used as the reference
point. System B, modeled after 7 (Scheme 3, Figure 1), con-
sists of a complex of the NHC with the germacyclopentene.
System C is the germacyclopentene with free carbene.
The results are tabulated in Table 1 and reveal a number of
interesting trends. With five of the seven substitution pat-
terns (R=OH, H, NH2, Me, Ph), system C is the most stable.
However, when the substituent on germanium is either fluo-
rine or chlorine, system A is energetically preferred. System B
is stable only when electronegative substituents (R=F, Cl,
OH) are on germanium.16 With less electron-withdrawing
substituents on germanium (R=H, NH2, Me, Ph), system B
is not stable and, upon geometry optimization, separates into
uncoordinated carbene and germacyclopentene (system C).
The computational results are consistent with the experi-
mental results. The formation of a dichlorogermacyclopen-
tene was not observed in the reaction between 1 and DMB;
the reaction of dichlorogermacyclopentene (6) with NHC 5
8.6
not stable
produced the hypercoordinate 7. On the basis of the compu-
tational results, complex 7 is expected to be thermodynami-
cally unstable toward the release of butadiene. Indeed, 7
dissociates upon heating by releasing DMB (Scheme 3) and
forming 1.
The computations indicate that system B may be experi-
mentally accessible when R=OH; however, in the R=OtBu
system, the corresponding pentacoordinated complex was
not observed (Scheme 4). Presumably, the increased steric
bulk of the OtBu substituent compared to that of the OH
group disfavors the formation of a pentacoordinate germa-
nium species.
Both the experimental and computational results show
that a dihalogenated germylene prefers to be coordinated to
a NHC rather than form an adduct with butadiene, whereas
the dialkoxy- and diorganogermylenes prefer the formation
of a germacyclopentene.
Reactions with an Orthoquinone. As with DMB, 3,5-di-
tert-butylorthoquinone reacts rapidly in high yield with
germylenes and, therefore, can be used as a trapping reagent

4874 Organometallics, Vol. 29, No. 21, 2010
Rupar et al.
Scheme 5
Scheme 6
Figure 2. Thermal ellipsoid plot (50% probability surface) of 17.
˚
Hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (deg): C1-Ge=1.995(2); Ge-Cl1=2.2738(10); Ge-
Cl2 = 2.1541(9); Ge-O14 = 1.8136(16); Ge-O15 = 1.8906(17);
O14-Ge-O15=86.86(7); O14-Ge-C1=132.33(8); O15-Ge-
C1=87.42(8); O14-Ge-Cl2=113.95(6); O15-Ge-Cl2=94.33(6);
C1-Ge-Cl2 = 113.66(7); O14-Ge-Cl1 = 85.58(6); O15-Ge-
Cl1 = 168.48(5); C1-Ge-Cl1 = 91.25(7).
uncoordinated, were not visible in the 1H NMR spectrum of the
crude reaction mixture. Under the reaction conditions, the NHC
appears to be reacting with the quinone; however, attempts to
determine the fate of the NHC failed. The direct reaction of the
NHC with 3,5-di-tert-butylorthoquinone also resulted in a visu-
ally similar deep blue solution. The 1H NMR spectrum of the
solution exhibited a multitude of signals, indicating a complex
mixture of products. Efforts to identify any of the products
derived from the reaction between the carbene and 3,5-di-tert-
butylorthoquinone were not successful. Possibly, the quinone is
abstracting an electron from the NHC, leading to the formation
of a radical anion/cation pair which then undergoes further
chemistry. The formation of a NHC radical cation upon expo-
sure of 5 to oxidants has been reported previously.22
for reactive divalent germanium compounds (Scheme 5).20
Due to the formation of two germanium-oxygen bonds and
the aromatization of the quinone (Scheme 5), the reaction of
3,5-di-tert-butylorthoquinone with complexes 1-3 is ex-
pected to be more thermodynamically favorable compared
to the analogous reactions with DMB.
In summary, 3,5-di-tert-butylorthoquinone reacts readily
with 1-3 to give a cycloadduct in a manner similar to that
observed with the corresponding free germylenes. The rapid
rate at which the orthoquinone reacts with the NHC com-
plexes suggests that 3,5-di-tert-butylorthoquinone is able to
react directly with germanium while it is still complexed to
the NHC. This is in contrast to DMB, which reacted slowly
with 2 and 3, even after extended periods of heating.
Reactions with Methyl Iodide. The reaction of germylenes
with methyl iodide has been reported and usually results in the
insertion of the germylene into the carbon-iodine bond and the
formation of tetravalent germanium.13,23 If the germylene is
stabilized by an intramolecular donor, nucleophilic attack of the
germylene on MeI can result in the formation of a cationic
germanium complex.24 Since intermolecularly stabilized Ge(II)
species have been less well studied, reports of their reactivity
Addition of the red 3,5-di-tert-butylorthoquinone to a
colorless solution of 1 resulted in rapid discoloration of the
quinone (Scheme 6). A white solid was isolated and was
identified as 17 by X-ray crystallography. Figure 2 shows the
solid-state structure of the cycloadduct: notably, the NHC
remains coordinated to the germanium.16
The reaction of 2 and 3,5-di-tert-butylorthoquinone be-
haved in exactly the same manner as with 1 (Scheme 6). A
white solid was isolated, and analysis by 1H NMR spectro-
scopy confirmed the formation of a 1:1 adduct of the quinone
with the NHC-coordinated germylene (18). Attempts to
grow crystals of 18 suitable for single-crystal X-ray diffrac-
tion were not successful.
The addition of 3,5-di-tert-butylorthoquinone to a yellow
solution of 3 resulted in the formation of a deep blue reaction
mixture. The 1H NMR spectrum of the solution was complex
but clearly showed the presence of 19 (Scheme 6), which was
subsequently isolated and characterized.21 Signals that could
be clearly attributed to an NHC moiety, either coordinated or
(22) Radical cations derived from 5 were formed by one-electron
oxidation of the NHC by tetracyanoethylene and ferrocenium. See:
Ramnial, T.; McKenzie, I.; Gorodetsky, B.; Tsang, E. M. W.; Clyburne,
J. A. C. Chem. Commun. 2004, 1054.
ꢁ
ꢀ
(20) (a) Riviere, P.; Castel, A.; Satge, J.; Guyot, D. J. Organomet.
(23) Weinert, C. S.; Fenwick, A. E.; Fanwick, P. E.; Rothwell, I. P.
ꢁ
ꢁ
ꢀ
Chem. 1986, 315, 157. (b) Mazieres, S.; Lavayssiere, H.; Dousse, G.; Satge,
J. Inorg. Chim. Acta 1986, 252, 25. (c) Michels, E.; Neumann, W. P.
Tetrahedron Lett. 1986, 27, 2455.
Dalton Trans. 2003, 532.
(24) (a) Jutzi, P.; Keitemeyer, S.; Neumann, B.; Stammler, H.-G.
Organometallics 1999, 18, 4778. (b) Schmidt, H.; Keitemeyer, S.; Neumann,
B.; Stammler, H.-G.; Schoeller, W. W.; Jutzi, P. Organometallics 1998, 17, 2149.
(21) Compound 19 has been reported previously.20a

Article
Organometallics, Vol. 29, No. 21, 2010 4875
Scheme 7
Scheme 8
Figure 3. Thermal ellipsoid plot (50% probability surface) of 20þ.
The iodide counteranion and hydrogen atoms are omitted for
˚
clarity. Selected bond lengths (A) and angles (deg): Ge-C1 =
1.994(9); Ge-C20 = 1.930(8); Ge-I1 = 2.5405(10), Ge-I3 =
2.5299(13); C20-Ge-C1 = 112.9(4); C20-Ge-I2 = 108.1(3);
C1-Ge-I2 = 118.2(3); C20-Ge-I1 = 111.3(3); C1-Ge-I1 =
101.3(2); I2-Ge-I1 = 104.57(4).
toward MeI or related electrophiles are limited. Previously it was
demonstrated that the lone pair of electrons on Ge in 3 is
chemically active by coordination of 3 to BH3.4 The reactions of
methyl iodide with 1-3 are now presented.
Scheme 9
Addition of an excess of MeI to a solution of 1in C6H6 resulted
in the appearance of several new signals in the 1HNMRspectrum
of the residue obtained after removal of the solvent, consistent
with the formation of methylated adducts of 1. ESI-MS (positive
mode) of the reaction mixture showed signals attributable to
the expected adduct, as well as signals attributable to species in
which one or both of the chlorides were replaced with iodides
(Scheme 7). Also evident in both the mass spectrograph and the
1H NMR spectrum were signals attributable to the methylated
NHC cation, 5-Meþ.25 The origin of 5-Meþ is not entirely clear
but could arise from the elimination of GeI2 from 20þ.
Although separation of the reaction products was not suc-
cessful, 20[I] could also be formed by the reaction of 216 with
excess MeI (Scheme 8). Again, the formation of 5-Me[I] was ob-
served. Pale green crystals of 20[I] were mechanically separated
by inspection under an optical microscope. The structure of 20[I]
was confirmed by single-crystal X-ray diffraction (Figure 3); as
expected, a methyl group occupies the empty coordination site
that was evident in the structure of 21. The germanium complex
is cationic; the cation is separated from the iodide counterion,
Qualitatively, compounds 1-3 react at noticeably different
rates with methyl iodide. In solution, a precipitate (23[I]) was
observed instantly upon addition of MeI to a solution of 3. The
reaction of MeI and 2 was also quick, with precipitate forma-
tion occurring within a couple of minutes. Finally, 1 reacted
very slowly with methyl iodide. The reaction took days to go to
completion, even in the presence of excess MeI; furthermore,
the chemistry of 1 and MeI was complicated by halogen
exchanges (Scheme 7). Examination of the calculated energies
of the HOMOs of model compounds (Table 2) shows a good
correlation between the energy of the HOMO and the reactivity
of the related experimental systems toward MeI.
Complex 23[I] reacts rapidly with CDCl3, resulting in dis-
sociation of the carbene moiety and the quantitative formation
of 24,27 which was subsequently characterized (Scheme 10).28
Overall, the methylation of 3followed by chlorination to give 24
is the synthetic equivalent of the insertion of GeMes2 into MeCl,
˚
with the closest Ge-I approach being 4.305(1) A. The Ge-C1
and Ge-I bond lengths are contracted in comparison to those in
21,6 which can be understood given the conversion of the
electron lone pair on germanium to a bonding electron pair
and the cationic charge.
Complexes 2 and 3 both react rapidly with methyl iodide
(Scheme 9). In each case, a white powder formed upon addition
of a stoichiometric amount of methyl iodide to a solution of
either 2 or 3. The precipitates were identified as 22[I] and 23[I],
1
respectively, by H NMR spectroscopy, mass spectrometry,
and single-crystal X-ray diffraction (Figures 4 and 5).
(25) 5-Meþ has been reported before and was synthesized by the
€
direct reaction of 5 with MeI. See: Kuhn, N.; Gohner, M.; Steimann, M.
Z. Naturforsch., B 2002, 57, 631.
(26) Using 12 (R = CH3) as a model for the mesityl-substituted 3 is a
fairly poor approximation. It would be expected that the HOMO in 3
would receive additional stabilization as a result of the sp2 hybridization
of the ipso mesityl carbon substituents.
(27) 24 was previously reported and was synthesized by the addition
of MeLi to Mes2GeCl2. See: Duverneuil, G.; Mazerolles, P.; Perrier, E.
Appl. Organomet. Chem. 1995, 9, 37.
(28) 5 is believed to form a salt of Cl2, as was observed previously in
the reaction of 5 with hexachloroethane. See: Kuhn, N.; Abu-Rayyan,
€
A.; Gohner, M.; Steimann, M. Z. Anorg. Allg. Chem. 2002, 628, 1721.

4876 Organometallics, Vol. 29, No. 21, 2010
Rupar et al.
Table 2. Calculated PBE1PBE/6-311þG(d,p) Energies of the
HOMO of Model Compounds 10, 14, and 15 and the Qualitative
Reaction Rate of Related Experimental Systems
qualitative
reaction rate
with MeI
substitution
of model
compound
HOMO energy (eV)
of the model compound
(lone pair on Ge)
compound
1 (R = Cl)
2 (R = OtBu)
3 (R = Mes)26
slow
fast
fastest
14 (R = Cl)
10 (R = OH)
15 (R = Ph)
-6.21
-5.23
-4.80
Scheme 10
Figure 4. Thermal ellipsoid plot (50% probability surface) of 22þ.
The iodide counteranion and hydrogen atoms are omitted for cla-
˚
rity. Selected bond lengths (A) and angles (deg): Ge-C1=1.998(5);
Scheme 11
Ge-O1=1.764(3); Ge-O2=1.762(4); Ge-C22=1.924(5); O2-
Ge-O1 = 109.18(18); O2-Ge-C22 =106.8(2); O1-Ge-C22 =
119.7(2); O2-Ge-C1 = 106.71(19); O1-Ge-C1 = 105.46(19);
C22-Ge-C1 108.3(2).
addition to the conjugate acid of the carbene (5-Hþ). Com-
pound 27 is the same compound expected from the reaction of
pivalic acid with free dimesitylgermylene; compound 28 was not
anticipated as a product, and the mechanism for its formation
was not immediately clear.30
The ratio of 27 to 28 varied with the stoichiometry used
in the reaction. Compound 27 was formed exclusively when 3
was added dropwise to a solution containing an excess of
pivalic acid. Conversely, when 1 equiv of pivalic acid was
slowly added to a solution of 3, 28 was the only germanium-
containing compound detected by 1H NMR spectroscopy.
On the basis of these observations, it appears as if com-
pound 28 is formed by the reaction of 27 with 3. Indeed, when
27 and 3 were combined in solution, both 28 and 5 were
detected as products by 1H NMR spectroscopy (Scheme 13).
Previous work has demonstrated that the mechanism for the
addition of transient organogermylenes to carboxylic acids
proceeds initially by complexation of the carbonyl oxygen to
the germanium followed by proton transfer.29 However, this
mechanism is probably not operative in the formation of 27,
since the formally empty porbital on the NHC-GeR2 complex is
occupied by the carbene and, therefore, the Lewis acidity is
greatly diminished. Complex 3 is a Lewis base,4 and thus, the
formation of 27 through initial proton transfer, followed by
displacement of the carbene by pivalate, is proposed.
Figure 5. Thermal ellipsoid plot (50% probability surface) of 23þ.
The iodide counteranion and hydrogen atoms are omitted for
clarity. Selected bond lengths (A) and angles (deg): Ge1-C1 =
2.014(5); Ge1-C32=1.962(5); Ge1-C23=1.971(5); Ge1-C14=
1.977(5); C32-Ge1-C23=109.4(2); C32-Ge1-C14=107.0(2);
C23-Ge1-C14=116.4(2);C32-Ge1-C1=105.3(2); C23-Ge1-
C1=110.0(2).
˚
an otherwise difficult transformation to perform with a tran-
sient germylene. The reaction of 3 with ethyl iodide was also
examined. Ethyl iodide reacts with 3, forming the expected
ethylated species 25[I]. Cation 25þ also underwent a similar
chlorination reaction to give 26 (Scheme 11).
Reaction with Benzophenone. The stable germylene Ge[CH-
(SiMe3)2]2 reacts rapidly with phenones at room temperature
to yield conjugated trienes.31 Therefore, the reactivity of 1-3
with benzophenone was examined to see if the NHC-stabilized
germylenes react in the same manner as Ge[CH(SiMe3)2]2.
Reaction with Pivalic Acid. Germylenes react with a car-
boxylic acid by insertion into the oxygen-hydrogen bond,
resulting in the generation of a germyl ester.29 To determine if
NHC-coordinated germylenes will behave in the same manner,
the reactivity of 1-3 toward a carboxylic acid was investigated.
Addition of pivalic acid to solutions of 1 and 2 formed com-
1
(30) Adduct 28 has been previously reported and was formed by the
direct reaction of tetramesityldigermene with pivalic acid: Hurni, K. L.;
Rupar, P. A.; Payne, N. C.; Baines, K. M. Organometallics 2007, 26,
5569.
(31) (a) Sweeder, R. D.; Edwards, F. A.; Miller, K. A.; Banaszak
Holl, M. M.; Kampf, J. W. Organometallics 2002, 21, 457. (b) Sweeder,
R. D.; Cygan, Z. T.; Banaszak Holl, M. M.; Kampf, J. W. Organometallics
2003, 22, 4613.
plex mixtures, as ascertained by H NMR spectroscopy. At-
tempts to identify any of the products were not successful. In
contrast, 3 reacted cleanly with pivalic acid to form two different
germanium-containing compounds, 27 and 28 (Scheme 12), in
(29) Huck, L. A.; Leigh, W. J. Organometallics 2007, 26, 1339.

Article
Organometallics, Vol. 29, No. 21, 2010 4877
Scheme 12
Scheme 13
Scheme 14
Scheme 15
Both 1 and 2 did not show any reactivity toward benzo-
phenone even at elevated temperatures; however, complex 3
was found to react slowly with benzophenone over 24 h at
100 °C to form 29, which was isolated as a colorless powder
after purification by chromatography (Scheme 14). Integra-
1
tion of the H NMR spectrum of 29 clearly showed the
formation of a 1:1 adduct of the Mes2Ge moiety with
benzophenone. Mass spectrometric data were also consistent
with the formation of a 1:1 adduct.
The structure of 29 was determined by 1D and 2D NMR
techniques. In the 1H and 13C NMR spectrum of 29, signals
attributable to two different mesityl and two different phenyl
moieties were detected. The 1H-13C gHSQC spectrum of 29
was consistent with a 1,2-substitution pattern on one of the
aromatic rings originating from benzophenone. The other
phenone phenyl ring remained monosubstituted. The pre-
sence of the doubly benzylic proton was confirmed by
gCOSY and 1H-13C gHMBC spectroscopy.
On the basis of experimental evidence, the reaction of
Ge[CH(SiMe3)2]2 with phenones was proposed to occur via a
concerted [4 þ 2] cycloaddition.31 A similar mechanism is
likely operative in the formation of 29. As was previously
proposed, 3 dissociates to Mes2Ge and 5 (Scheme 15)
at elevated temperature (70-80 °C). Dimesitylgermylene
can then react with benzophenone, presumably via [4 þ 2]
cycloaddition. Subsequent rearomatization of the ring by a
transfer of a hydrogen atom results in the formation of 29.
Attempts to observe the postulated triene intermediate by 1H
NMR spectroscopy were unsuccessful; the hydrogen shift is
most likely catalyzed by the NHC 5, a strong base.32 The
reactions of the related R2Si with benzophenone have also
been studied; the formation of both conjugated trienes (R =
C5Me5)33 and rearomatized products (R = NR02, Me)34-36
have been reported.
Reactions That Did Not Proceed or Resulted in Intractable
Mixtures. In addition to the chemistry described above, the
reactions of 1-3 with a number of additional reagents were
explored. The results are summarized in Table 3. Essentially,
1 and 2 were found to be unreactive toward many reagents
under the reaction conditions examined. The NHC complex
3 was found to react with a wider array of reagents; however,
the product mixtures were often complex. Typically, the 1H
NMR spectra of the crude reaction mixtures displayed either
broad peaks indicative of the formation of polymeric ma-
terial (benzaldehyde, P4) or a large number of peaks suggest-
ing a multitude of products. Attempts to separate the
€
(33) Jutzi, P.; Eikenberg, D.; Bunte, E.-A.; Mohrke, A.; Neumann,
B.; Stammler, H.-G. Organometallics 1998, 15, 1930.
(34) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Organometallics
1997, 16, 4861.
(35) Ando, W.; Ikeno, M.; Sekiguchi, A. J. Am. Chem. Soc. 1977, 99,
6447.
(32) As a reference, the pKa of the conjugate acid of the NHC used in
this study has been calculated as approximately 36 in acetonitrile. See:
Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126,
8717.
(36) Xiong, Y.; Yao, S.; Driess, M. Chem. Eur. J. 2009, 15, 5545.

4878 Organometallics, Vol. 29, No. 21, 2010
Rupar et al.
Table 3. Summary of the Outcome of Reactions between
NHC-GeR2 and Various Reagentsa
toward 1-3. Treatmentof23[I] or25[I] withCDCl3 chlorinated
the germanium to give 24 and 26, respectively.
Pivalic acid formed a complex product mixture upon
addition with 1 and 2. In the mesityl system 3, two products
were formed and subsequently isolated and characterized.
The expected germylene/pivalic acid adduct 27 was isolated,
along with the unexpected 28, which can also be formed by
the addition of 3 to 27. The formation of either 27 or 28 can
be favored by manipulation of the reaction conditions.
Benzophenone was found to be unreactive with both 1 and
2 under the reaction conditions examined. Upon prolonged
heating, 3 reacted with benzophenone to give 29, which likely
arises from a [4 þ 2] cycloaddition between dimesitylgermyl-
ene and benzophenone, followed by a hydrogen shift. The
reactivity of 3 toward benzophenone is similar to what was
reported for uncoordinated germylenes and silylenes.
The reactivity of several additional reagents toward 1-3
was examined. In general, 1 and 2 were found to be un-
reactive. The mesityl-substituted 3 did react in some cases,
but the identities of the products were not determined
because of the complexity of the reaction mixtures.
reagent
1
2
3
TEMPO38
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
N/R
decb,c
decb,c
N/R
benzaldehyde36,39
bis(trimethylsilyl)acetylene
phenylacetylene40
triethylsilane1
P4
C-H activation with phenyl iodide42
a N/R = no reaction. Reactions were performed at 70 °C unless
otherwise noted. b Attempted at room temperature in THF. c Attempted
at -30 °C.
decb,c
N/R
41
decb,c
N/R
products through selective crystallization, selective precipi-
tation, or chromatography were not successful.
One of the contributing factors that may be leading to the
complicated reaction mixtures is that NHC 5 can be released
from germanium. Since NHCs are versatile organic catalysts
for a wide range of reactions, the presence of free NHC 5 may
lead to undesirable side reactions.37
In general, the substituent effects on the reactivity of uncoor-
dinated germylenes are similar to those observed for the NHC
germylene complexes. As a result of the intrinsic stability of the
corresponding dichlorogermylene and a HOMO stabilized by
the electronegative chlorines on the germanium center, 1was the
least reactive of the complexes. Compound 3 was the most
reactive, likely because of the inherent instability of the related
uncoordinated germylene (Mes2Ge) and the higher energy of its
HOMO as a result of having less electronegative carbon sub-
stituents on the germanium center. The reactivity of the OtBu-
substituted 2 was intermediate between that of 1 and 3.
After examination of the reactivity of 1-3, and a comparison
of the reactivity of the complexed GeR2 compounds to their
uncomplexed analogues, the likelihood of using NHC-GeR2 as
synthons for GeR2 appears to be situation-specific. The release
of carbene 5 is a concern, given the strongly basic nature of the
NHC which may lead to undesired side reactions.
Conclusions
In summary, the chemistry of 1-3 toward a variety of
reagents was explored. In some cases, the NHC-GeR2 com-
plexes formed reaction products similar to those of uncoor-
dinated germylenes, while in other situations, their behavior
was quite different from that of uncoordinated germylenes.
Compounds 2 (R = OtBu) and 3 (R = Mes) reacted with
DMB to give germacyclopentenes 4 and 8 in a manner identical
with that of uncoordinated germylenes. The dichloro derivative
1 did not react with DMB. PBE1PBE/6-311þG(d,p) calcula-
tions suggest that dichlorogermylene thermodynamically pre-
fers to be coordinated by the NHC rather than the diene.
3,5-Di-tert-butylorthoquinone was found to react quickly
with 1-3 to produce a cycloadduct. The rapid reaction of the
quinone with 1-3 suggests that the reaction occurs while the
NHC remains coordinated to the germanium; in the case of 1
and 2, the NHC ligand remained coordinated to germanium
even after cycloaddition. With the mesityl-substituted system 3,
the NHC was released from the germanium upon reaction with
the orthoquinone. The uncoordinated NHC reacted rapidly
with available 3,5-di-tert-butylorthoquinone, producing a com-
plex reaction mixture.
Experimental Section
Reactions were performed under an inert atmosphere of
nitrogen using standard techniques. Solvents were purified
according to literature procedures43 and stored over 4 A molec-
˚
ular sieves under N2. All NMR spectra were acquired using
C6D6, THF-d8, or CD3CN as the solvent. 1H NMR spectra were
referenced to residual C6D5H (7.15 ppm), residual CD2HCN
(1.94 ppm), or the upfield signal of THF-d7 (3.58 ppm). Melting
points were determined under a N2 atmosphere and are un-
corrected. FT-Raman spectra were acquired on bulk samples
sealed in a melting point tube under nitrogen. Compounds 1,6
2,5 3,4 and 216 were synthesized by following literature proce-
dures. All other chemicals were purchased from commercial
suppliers. Pivalic acid was dried prior to use by first dissolving it
Like intramolecularly stabilized germylenes, complexes 1-3
acted as nucleophiles toward methyl iodide by quaternizing the
germanium and forming cationic complexes. The qualitative
rate of the reaction was inversely proportional to the energy of
the HOMO of model germanium compounds. The alkylation
reaction is limited to unhindered alkyl iodides as substrates.
Alkyl chlorides and alkyl bromides were found to be unreactive
(37) (a) Marion, N.; Di’ez-Gonza’lez, S.; Nolan, S. P. Angew. Chem.,
Int. Ed. 2007, 46, 2988. (b) Enders, D.; Niemeier, O.; Henseler, A. Chem.
Rev. 2007, 107, 5606.
(38) Iwamoto, T.; Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M. J.
Organomet. Chem. 2004, 689, 1337.
˚
in THF and storing over 4 A molecular sieves under N2. 2,3-
˚
Dimethylbutadiene was dried under N2 over 4 A molecular
sieves prior to use. Elemental analyses were performed at
Guelph Chemical Laboratories, Guelph, Ontario, Canada.
Attempted Reaction of 1 with DMB. In a screw-cap vial filled
with THF (2 mL) was added 1 (0.05 g, 0.155 mmol) and DMB
(0.113 mL, 1 mmol). The reaction mixture was stirred for 18 h at
(39) Baines, K. M.; Cooke, J. A.; Vittal, J. J. Heteroat. Chem. 1994, 5,
293.
€
(40) Yao, S.; van Wullen, C.; Driess, M. Chem. Commun. 2008, 5393.
(41) Xiong, Y.; Yao, S.; Brym, M.; Driess, M. Angew. Chem., Int. Ed.
1
2007, 46, 4511.
room temperature. Analysis of an aliquot by H NMR spec-
troscopy showed that no reaction had occurred. The screw
(42) Walker, R. H.; Miller, K. A.; Scott, S. L.; Cygan, Z. T.; Bartolin,
J. M.; Kampf, J. W.; Holl Banaszak, M. M. Organometallics 2009, 28,
2744.
(43) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
cap vial was sealed and heated to 100 °C for 3 days, after which
analysis of an aliquot by H NMR spectroscopy showed no
reaction.
1

Article
Synthesis of 7. To a solution of 6 (0.1 g, 0.44 mmol) in C6H6
Organometallics, Vol. 29, No. 21, 2010 4879
solution of 3. During the addition, the color of the reaction mixture
turned from yellow to dark blue. The reaction mixture was
extracted with an NH4Cl aqueous solution (10 mL). The aqueous
layer was extracted with Et2O (10 mL ꢀ 3). The organic layers were
combined, dried over MgSO4, and filtered. Evaporation of the
solvent yielded compound 19 (0.11 g, 65%) as a white residue which
was identified by 1H NMR spectroscopy and EI/MS.21
Reaction of 1 with Methyl Iodide. To a solution of 1 (0.10 g,
0.31 mmol) in C6H6 (4 mL) was added excess methyl iodide (0.19
mL, 3.1 mmol). The solution was stirred overnight, after which
time it was pale green. Hexanes (10 mL) was added to the
reaction mixture, causing a pale yellow solid to precipitate. The
precipitate was collected, redissolved in THF (3 mL), and
analyzed by ESI/MS (positive mode).
Reaction of 21 with Methyl Iodide. To a solution of 21 (0.08 g,
0.15 mmol) in C6H6 (6 mL) was added MeI (80 μL, 1.2 mmol). The
reaction mixture was stirred for 3 days. Hexanes (10 mL) was
added to induce precipitation of a pale green solid which was
collected. Crystals suitable for single-crystal X-ray diffraction were
acquired by the slow diffusion of Et2O into a saturated CH3CN
solution. Both yellow and pale green single crystals were grown.
The yellow crystals were identified to be 5-Meþ by comparison of
the unit cell of the crystals to the reported literature values for
5-Meþ.25 The pale green crystals were analyzed by single-crystal
X-ray diffraction and found to be 20[I].
(3 mL) was added 5(0.08 g, 0.44 mmol). The solution was stirred for
10 min. Hexanes (10 mL) was added to induce the formation of a
white precipitate. The precipitate was identified as 7 (0.14 g, 78%).
Crystals suitable for single-crystal X-ray diffraction were grown by
the slow diffusion of pentane into a concentrated solution of 7 in
C6H6. Mp: 136-142 °C. 1HNMR(C6D6):1.28(d,3JHH =7Hz, 12
H, iPr-Me), 1.36 (s, 6 H, dC(N)Me), 1.87 (s, 6 H, dC(CH2)Me)),
3
i
2.97 (s, 4 H, CH2), 5.13 (sept, JHH = 7 Hz, 2 H, Pr-CH). FT-
Raman (cm-1):137(m), 161(w), 250(m), 460(w), 526(w), 581(w),
695 (s), 780 (w), 891 (w), 1166 (w), 1305 (w), 1394 (m), 1447 (m),
1625 (m), 2915 (s), 2944 (s), 2984 (s). Anal. Calcd for C17H31Cl2-
GeN2: C, 50.17; N, 6.88; H, 7.68. Found: C, 49.79; N, 6.99; H, 7.86.
Thermolysis of 7. A solution of 7 (0.02 g, 0.11 mmol) dissolved
in C6H6 (5 mL) was heated in a sealed screw-cap bottle for 3
days. Analysis of an aliquot by 1H NMR spectroscopy showed
the quantitative formation of 1 and DMB.
Reaction of 2 with DMB. To a solution of 2 (0.05 g, 0.13 mmol)
in THF (2 mL) was added excess DMB (1 mL, 8.8 mmol). The
reaction mixture was placed in a sealed tube and heated to 70 °C
1
for 4 days. Analysis of an aliquot by H NMR spectroscopy
showed the quantitative formation of 8 and 5.
Synthesis of 8. To a solution of 6 (0.1 g, 0.44 mmol) dissolved
in THF (3 mL) was added KOtBu (0.1 g, 0.88 mmol). The
reaction mixture was stirred overnight. The solvent was re-
moved under vacuum, yielding a colorless residue. The residue
was taken up in Et2O (10 mL). A white suspension, presumed to
be KCl, was removed by centrifugation. The solvent was re-
moved under vacuum to yield a colorless liquid that was iden-
tified as 8 (0.11 g, 85%). 1H NMR (C6D6): δ 1.38 (s, 18 H, tBu),
1.60 (s, 6 H, dCMe), 1.73 (s, 4 H, CH2). EI/MS : m/z 302 [Mþ,
29%], 287 [Mþ - Me, 18%], 205 [þGe(OtBu)2 - Me, 100%], 147
[GeOtBu, 85%], 82 [þDMB, 35%]. High-resolution EI/MS for
C14H2874GeO2: m/z calcd 302.1303, found 302.1292.
Reaction of 1 with 3,5-Di-tert-butylorthoquinone. 3,5-Di-tert-
butylorthoquinone (0.07 g, 0.31 mmol) dissolved in THF (5 mL)
was added dropwise over 2 min to a solution of 1(0.10 g, 0.31 mmol)
in THF (2 mL). During the addition, the red color of the ortho-
quinone quickly faded. After the addition was complete, the solvent
was evaporated under high vacuum to yield an off-white powder.
The powder was washed with hexanes (2 mL) to give a brilliant
white solid identified as 17 (0.16 g, 94%). Crystals suitable for
single-crystal X-ray diffraction were acquired by the slow diffusion
of Et2O into a saturated solution of 17 in C6H6. Mp: 196-202 °C.
1H NMR (C6D6): δ 1.06 (d, 3JHH = 7 Hz, 12 H, iPr-Me), 1.28 (s, 6
H, dCMe), 1.37 (s, 9 H, tBu), 1.71 (s, 9 H, tBu), 5.67 (sept, 3JHH =7
Hz, 2 H, iPr-CH), 7.06 (s, 1 H, ArCH), 7.26 (s, 1 H, ArCH). FT-
Raman (cm-1): 109 (s), 177 (w), 243 (s), 270 (w), 319 (m), 381 (m),
547 (w), 642 (w), 812 (w), 888 (w), 915 (m), 1029 (w), 1103 (w), 1201
(w), 1292 (w), 1330 (w), 1424 (s), 1447 (s), 1581 (w), 1598 (w), 1625
(m), 2874 (s), 2942 (s), 2986 (s). Anal. Calcd for C29H42N2GeCl2:C,
55.08; N, 5.14; H, 7.58. Found: C, 55.29; N, 4.90; H, 7.85.
Reaction of 2 with 3,5-Di-tert-butylorthoquinone. 3,5-Di-tert-
butylorthoquinone (0.04 g, 0.18 mmol) dissolved in hexanes (5 mL)
was added dropwise over 2 min to a solution of 2 (0.07 g, 0.18 mmol)
in hexanes (5 mL). During the addition, the color of the orthoqui-
none solution (green) quickly faded. After the addition was com-
plete, the solvent was evaporated under high vacuum, leaving behind
an off-white powder. The powder was determined to be 18 (0.10 g,
91%). Mp: 120-122 °C. 1H NMR (C6D6): δ 1.13 (d, 3JHH = 7 Hz,
12 H, iPr-Me), 1.41 (s, 6 H, dCMe), 1.42 (s, 9 H, tBu), 1.66 (s, 18 H,
OtBu), 1.82 (s, 9 H, tBu), 5.56 (sept, 3JHH = 7 Hz, 2 H, iPr-CH), 6.98
(d, 4JHH = 2 Hz, 1 H, ArCH), 7.10 (d, 4JHH = 2 Hz, 1 H, Ar-CH).
Raman (cm-1): 138 (w), 229 (m), 271 (w), 451 (w), 599 (m), 780 (w),
831 (w), 887 (w), 918 (w), 1108 (w), 1202 (m), 1238 (m), 1331 (w),
1448 (s), 1597 (w), 1636 (w), 2700 (w), 2924 (s), 2967 (s).
Reaction of 2 with Methyl Iodide. To a solution of 2 (0.05 g,
0.13 mmol) in C6H6 (2 mL) was added methyl iodide (8 μL, 0.13
mmol). After 5 min, a white precipitate formed. The solution was
stirred for an additional 10 min. Hexanes (10 mL) was added to the
reaction solution to complete the precipitation. The white precipi-
tate was collected and identified as 22[I] (0.06 g, 86%). Crystals
suitable for single-crystal X-ray diffraction were acquired by the
slow diffusion of Et2O into a saturated CH3CN solution of 22[I].
Mp: 160-165 °C. 1H NMR (CD3CN): δ 1.30 (s, 3 H, Me), 1.35 (s,
t
3
i
18 H, Bu), 1.56 (d, JHH = 7 Hz, 12 H, Pr-Me), 2.35 (s, 6 H,
3
i
dCMe), 5.37 (s, JHH = 7 Hz, 2 H, Pr-CH). ESI-MS (positive
mode): m/z 415 [22þ, 100%]. Raman (cm-1): 597 (m), 1293 (w),
1447 (m), 1459 (m), 1629 (m), 2910 (s), 2973 (s).
Reaction of 3 with Methyl Iodide. To a yellow solution of 3
(0.08 g, 0.16 mmol) in C6H6 (5 mL) was added MeI (10 μL, 0.16
mmol). A white precipitate formed immediately. Hexanes (10 mL)
was added to the reaction solution to complete the precipitation of
1
23[I] (0.05 g, 50%). The H NMR spectrum of 23[I] taken in
CD3CN was complicated at room temperature with numerous
broad signals and was difficult to interpret. As the temperature
was decreased, the spectrum changed but was still complicated.
High-temperature NMR experiments were also attempted but
resulted in compound decomposition. Crystals suitable for single-
crystal X-ray diffraction were grown by diffusing pentane into a
concentrated THF solution of 23[I]. Mp: 198-202 °C. 1H NMR
(CD3CN, room temperature): 1.29 (s)-1.31 (bs, 15 H total, iPr-Me
and Me), 2.13 (bs, 12 H, Mes-o-Me), 2.28, 2.36 (both s, 6 H, Mes-p-
i
Me and dCMe), 4.57 (bs, 2 H, Pr-CH), 6.78 (s, 4 H, Mes-CH).
Raman (cm-1): 106 (w), 229 (w), 557 (m), 596 (m), 887 (w), 1047
(w), 1292 (m), 1384 (m), 1450 (m), 1604 (m), 1629 (w), 2736 (w),
2927 (s), 2982 (s). ESI-MS (positive mode): m/z 507 [23þ, 100%].
Anal. Calcd for C30H45GeIN2: C, 56.90; N, 4.42; H, 7.16. Found: C,
56.78; N, 4.29; H, 7.29.
Reaction of 23[I] with CDCl3. 23[I] (2.00 g, 0.32 mmol) was
dissolved in CDCl3 (2 mL), resulting in a colorless solution.
After 10 min the reaction mixture turned brown. The solvent
was extracted with a saturated NH4Cl solution (10 mL). The
aqueous layer was extracted with CH2Cl2 (3 ꢀ 10 mL). The
organic layers were combined, dried over MgSO4, and filtered.
Removal of the solvent yielded a brown residue. The residue was
redissolved in hexanes and passed through a short silica plug.
Removal of the hexanes yielded a colorless residue identified as
24 (0.08 g, 70%). The identity of 24 was confirmed by compar-
ison of the 1H NMR spectral data and EI/MS data to the
literature values.27
Reaction of 3 with 3,5-Di-tert-butylorthoquinone. Compound
3 (0.16 g, 0.32 mmol) was dissolved in THF (10 mL), resulting
in a yellow solution. 3,5-Di-tert-butylorthoquinone (0.07 g, 0.32
mmol), dissolved in THF (5 mL), was added dropwise to the THF

4880 Organometallics, Vol. 29, No. 21, 2010
Table 4. Crystallographic Data for Compounds 7, 17, 20[I], 22[I], and 23[I]
Rupar et al.
param
7
17
20[I]
22[I]
23[I]
empirical formula
formula wt
cryst syst
C
405.92
monoclinic
P21/c
15.9298(5)
8.3530(2)
16.5558(5)
90
115.8120(14)
90
17H30Cl2GeN2
C25H40Cl2GeN2O2
544.10
triclinic
C12H23GeI3N2
648.61
C20H41GeIN2O2
541.06
monoclinic
P21/c
11.821(2)
13.765(3)
15.378(3)
90.00
92.14(3)
90.00
C30H45GeIN2
633.19
monoclinic
P21
11.251(2)
18.190(4)
15.163(3)
90
107.22(3)
90
orthorhombic
P212121
10.246(2)
12.522(3)
14.850(3)
90
90
90
1905.2(7)
4
space group
˚
P1
a (A)
8.5449(17)
8.8873(18)
18.880(4)
78.89(3)
79.44(3)
81.13(3)
1372.5(5)
2
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
1983.15(10)
4
2500.6(9)
4
2964.1(11)
4
no. of data/restraints/params
goodness of fit
R1 (I > 2σ(I))
wR2 (all data)
4554/0/204
1.066
0.0409
0.1084
0.570, -0.738
6266/0/301
1.043
0.0380
0.0958
0.544, -0.696
5559/0/171
0.994
0.0526
0.1357
1.266, -1.759
5678/0/248
1.155
0.0526
0.1582
3.340, -1.171
13 307/1/641
1.057
0.0428
0.1042
0.732, -0.979
-3
˚
largest diff peak, hole (e A
)
Reaction of 3 with Ethyl Iodide. To a yellow solution of 3 (0.17
g, 0.32 mmol) in C6H6 (5 mL) was added EtI (26 μL, 0.32 mmol).
The reaction mixture was stirred for 2 h, over which time the
bright yellow solution of 3 faded to a pale straw color. Hexanes
(5 mL) was added to induce the precipitation of 25[I], which was
Reaction of 3 with Benzophenone. To a solution of 3 (0.08 g, 0.16
mmol) in THF (5 mL) was added benzophenone (0.03 g, 0.16 mmol)
in a screw-cap-sealed vial. The reaction mixture was heated to 80 °C
and stirred for 18 h. The reaction mixture was extracted with NH4Cl
(10 mL ꢀ 2), which in turn was extracted with Et2O (10 mL ꢀ 3).
The organic layers were combined and then dried over MgSO4 and
filtered. Removal of the solvent by evaporation under vacuum gave a
colorless residue which was purified by silica gel chromatography
using 90% CH2Cl2/10% hexanes as the eluent to give 29 (0.02 g,
1
collected as an off-white sticky residue. As with 22[I], the H
NMR spectrum of 25[I] was complicated at room temperature
with numerous broad signals. ESI-MS (positive mode): m/z 521
[Mþ, 35%], 209 [5-Etþ, 90%], 181 [5-Hþ, 100%].
1
Synthesis of 26. 25[I] (0.07 g, 0.1 mmol) was dissolved in
CDCl3 (2 mL), resulting in a colorless solution. After 10 min the
reaction mixture turned brown. The solvent was extracted with a
saturated NH4Cl solution (10 mL). The aqueous layer was ex-
tracted with CH2Cl2 (3 ꢀ 10 mL). The organic layers were com-
bined, dried over MgSO4, and filtered. Removal of the solvent
yielded a brown residue. The residue was redissolved in hexanes and
passed through a short silica plug. Removal of the hexanes yielded a
colorless residue identified as 26 (0.035 g, 94%). 1H NMR (C6D6):
δ 1.19 (t, 3JHH = 8 Hz, 3 H, CH2CH3), 1.64 (q, 3JHH = 7 Hz, 2 H,
CH2CH3), 2.04 (s, 6 H, Mes-p-Me), 2.38 (s, 12 H, Mes-o-Me), 6.64
25%). H NMR (CD2Cl2): δ 2.23 (s, 3H, C23H3), 2.28 (s, 3H,
C22H3), 2.41 (s, 6H, C20H3), 2.43 (s, 6H, C21H3), 6.20 (s, 1H, C19H),
6.81 (s, 2H, C12H), 6.86 (s, 2H, C13H), 7.05-7.07 (m, 1H, C10H),
7.15-7.24 (m, 5H, C17H þ C14H þ C16H), 7.27-7.31 (m, 2H, C11H
þ C15H), 7.84-7.86 (m, 1H, C18H). 13C NMR (CD2Cl2): δ 21.27
(C23), 21.39 (C22), 23.18 (C21), 24.21 (C20), 83.87 (C19), 125.16
(C18), 127.62 (C17), 127.81 (C16), 128.04 (C15), 128.86 (C14),
129.37 (C13), 129.59 (C12), 129.78 (C11), 132.28 (C10), 135.47
(C9), 136.23 (C8), 137.82 (C7), 139.90 (C6), 140.18 (C5), 143.05
(C4), 143.09 (C3), 146.38 (C2), 152.81 (C1). High-resolution EI/MS
for C31H3274GeO: m/z calcd 494.1665, found 494.1649.
(s, 4 H, Mes-CH). EI/MS: m/z 376 [Mþ, 18%], 347 [Mþ - Et,
70
100%], 311 [GeMes2 - H]. High-resolution MS/EI for C20H27
-
Ge35Cl: m/z calcd 372.1043, found 372.1028.
Reaction of 3 with Excess Pivalic Acid. To a solution of pivalic
acid (0.14 g, 1.4 mmol) in THF (2 mL) was added dropwise a
solution of 3 (0.08 g, 0.16 mmol) in THF (1 mL) over 5 min. The
rate of addition was such that the yellow color of 3 was allowed to
dissipate before the next drop was added. The solvent was removed
under vacuum, yielding a colorless residue. A 1HNMRspectrumof
the residue revealed the presence of 27, 5-Hþ, and pivalic acid. The
residue was suspended in Et2O (10 mL) and then extracted with a
concentrated NH4Cl solution (10 mL). The organic layer was
separated, dried over MgSO4, and evaporated under vacuum,
leaving a colorless waxy residue. The residue was placed under
high vacuum for 1 week to remove most of the pivalic acid;
however, it was not possible to completely remove all traces of
pivalic acid from 27. 1H NMR(C6H6):δ1.19 (s, 9 H, tBu), 2.04 (s, 6
H, Mes-p-Me), 2.44 (s, 12 H, Mes-o-Me), 6.67 (s, 4 H, Mes-CH),
7.33 (s, 1 H, GeH). EI-MS: m/z 413 [Mþ, 20%], High-resolution
MS/EI for C23H3174GeO2: m/z calcd 413.1539, found 413.1519.
Reaction of 3 with Limiting Pivalic Acid. Pivalic acid (8 mg,
0.08 mmol), dissolved in THF (0.45 mL), was added dropwise to a
solution of 3 (0.08 g, 0.16 mmol) in THF (10 mL) over 5 min.
During this time the yellow color of 3 faded to give a colorless
solution. After the addition was complete, the solvent was evapo-
rated under high vacuum, leaving behind a colorless residue. The
residue was redissolved in Et2O (10 mL). The Et2O solution was
extracted with NH4Cl (10 mL ꢀ 2) and then dried over MgSO4.
After filtration and removal of the solvent by evaporation under
vacuum, 28 was isolated (0.04 g, 67%). The identity of 28 was
confirmed by comparison with an authentic sample.30
Computational Details. The geometries of the model com-
pounds (systems B and C) were optimized with the GAUSSIAN
0344 program using the PBE1PBE density functional and the
6-311þG(d,p) basis set. Tight convergence criteria for the self-
consistent field (SCF=Tight) and ultrafine integration grids
(Int=Grid=Ultrafine) were used during the calculations. All
optimized geometries did not have any imaginary frequencies
and, therefore, are minima on the potential energy surface. The
optimized geometries are given in the Supporting Information.
Single-Crystal X-ray Diffraction Experimental Details. Data
were collected at low temperature (-123 °C) on a Nonius
Kappa-CCD area detector diffractometer with COLLECT.
The unit cell parameters were calculated and refined from the

Article
Organometallics, Vol. 29, No. 21, 2010 4881
full data set. Crystal cell refinement and data reduction were
carried out using HKL2000 DENZO-SMN.45 Absorption cor-
rections were applied using HKL2000 DENZO-SMN
(SCALEPACK). Crystallographic data for compounds 7, 17,
20[I], 22[I], and 23[I] are given in Table 4.
The SHELXTL/PC V6.14 suite of programs was used to solve
the structures by direct methods.46 Subsequent difference Fourier
syntheses allowed the remaining atoms to be located. All of the non-
hydrogen atoms were refined with anisotropic thermal parameters.
The hydrogen atom positions were calculated geometrically and
were included as riding on their respective carbon atoms.
The maximum/minimum electron difference peaks are large
for 20[I] and 22[I]. The analyses unambiguously establish the
structures of the two salts. The large maximum/minimum
electron difference peaks are due to poor absorption correction,
and in the latter case, also due to the large crystal size.
(44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.05; Gaussian, Inc., Wallingford, CT, 2004.
Acknowledgment. We thank the NSERC (Canada)
and the University of Western Ontario for funding. We
thank Doug Hairsine for acquiring the mass spectra,
Dr. P. J. Ragogna and members of the Ragogna group
and Dr. N. C. Payne for helpful discussions, and Teck
Cominco Ltd. for a generous gift of GeCl4.
Supporting Information Available: CIF files giving crystal-
lographic data, figures giving NMR spectra, and text and tables
giving computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
(45) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter,
C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276,
Macromolecular Crystallography, Part A, 307.
(46) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.