Stabilization of a transient diorganogermylene by an N-heterocyclic carbene

Article abstract of DOI:10.1021/om7006198

The synthesis of a carbene-stabilized transient germylene is presented; the resulting complex liberates free germylene upon heating, forms a stable adduct with BH₃, and reacts with MeLi to displace the carbene.

Full text of DOI:10.1021/om7006198

Organometallics 2007, 26, 4109-4111
4109
Stabilization of a Transient Diorganogermylene by an
N-Heterocyclic Carbene
Paul A. Rupar, Michael C. Jennings, Paul J. Ragogna, and Kim M. Baines*
Department of Chemistry, The UniVersity of Western Ontario, London, Ontario N6A 5B7, Canada
ReceiVed June 21, 2007
Chart 1
Summary: The synthesis of a carbene-stabilized transient ger-
mylene is presented; the resulting complex liberates free
germylene upon heating, forms a stable adduct with BH3, and
reacts with MeLi to displace the carbene.
Germylenes, divalent germanium(II) compounds, are highly
reactive species and, unless stabilized by extreme steric bulk
or electronic effects, are short-lived.1 The reactivity of ger-
mylenes can be moderated by the introduction of a Lewis base,
which results in the formation of a donor-acceptor complex
by the transfer of electron density from the base into the empty
π orbital on germanium. Many examples of intramolecularly
base-stabilized germylenes are known, and their reactivity has
been well studied;1e however, far less is known about intermo-
lecularly stabilized germylenes, particularly diorganogermylenes.
Most intermolecularly base-stabilized :GeR2 compounds are
short-lived and are difficult to characterize.1a,e,2 On the other
hand, stable inorganic germanium(II) derivatives, such as GeCl2‚
(dioxane), have proven to be versatile reagents for the prepara-
tion of a variety of germanium compounds.1 We believe that
isolable, intermolecularly base-stabilized diorganogermylenes
will have useful applications as ligands in coordination
chemistry,1f,3 as new thermal precursors for germylenes, and in
the novel synthesis of germanium polymers.
of the heavy carbene analogue-germylene complexes and the
successful use of N-heterocyclic carbenes (NHCs) in the
stabilization of other main-group compounds7-11 prompted us
to explore the use of the NHC 312 for the base stabilization of
transient germylenes. Only two NHC-GeR2 species have been
13
structurally characterized: NHC-GeI2 and an NHC-N-
heterocyclic germylene complex.14 In both cases, the uncoor-
dinated, free germanium(II) compounds are intrinsically stable
and have been characterized independent of coordination. We
now report the synthesis and characterization of the NHC-
dimesitylgermylene complex 4, which is the first example of
base stabilization of a transient :GeR2 species by a carbene.
The germanium center has three carbon single bonds, has a lone
pair of electrons, and forms a stable adduct with BH3, an
example of an “in-series” coordination complex.15 Finally, 4
reacts with MeLi, demonstrating that the carbene can be
displaced from the germanium by a stronger base.
Recently, two groups reported the synthesis of anionic gallium
carbenoid-germanium(II) complexes. The first example, 1, was
4,5
with Lappert’s stable germylene Ge[CH(SiMe3)2]2 and the
second, 2, with dimesitylgermylene (Chart 1).6 The isolation
* To whom correspondence should be addressed. E-mail: kbaines2@
uwo.ca.
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10.1021/om7006198 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/14/2007

4110 Organometallics, Vol. 26, No. 17, 2007
Communications
Scheme 1
Two equivalents of carbene 3 was added to a yellow solution
Figure 1. Thermal ellipsoid plot (50% probability surface) of 4.
Hydrogen atoms are omitted for clarity. Selected bond distances
(Å) and angles (deg): Ge-C1 ) 2.078(3), Ge-C21 ) 2.0649-
(18), Ge-C31 ) 2.0713(16), C1-N2 ) 1.359(4), C1-N5 ) 1.357-
(4); C1-Ge-C21 ) 109.18(11), C1-Ge-C31 ) 95.93(12), C21-
Ge-C31 ) 112.61(19).
of tetramesityldigermene (Scheme 1).16 No visible change was
1
observed. H and 13C{1H} NMR spectroscopic analysis of the
yellow residue, obtained after removal of the solvent, indicated
quantitative conversion of the starting materials to a single
1
product.17 The H NMR spectrum of the product revealed the
carbene and dimesitylgermylene moieties to be in a 1:1 ratio,
and the 13C signal attributable to the carbenic carbon shifted
upfield from 206 to 176 ppm, which is indicative of carbene
coordination. Crystals suitable for X-ray crystallographic analy-
sis were grown from a concentrated toluene solution at -30
°C. The molecular structure of the product was unambiguously
determined to be 4 by single-crystal X-ray diffraction analysis
(Figure 1).18
Scheme 2
Scheme 3
The carbenic carbon-germanium bond length of 2.078(3) Å
is consistent with a carbon-germanium single bond, and the
germanium center is pyramidal, as expected, due to the presence
of a stereochemically active lone pair of electrons. The same
trends were observed in the related NHC-tin(II) and -lead(II)
complexes.10,11
Compound 4 can be considered as either a base-stabilized
germylene or a zwitterionic germane. 2,3-Dimethylbutadiene
(DMB), a well-known germylene trap, is often used to verify
the presence of reactive germylenes; the diene undergoes rapid
formal [2 + 4] cycloaddition with the germylene to give a
germacyclopentene.19-21 Addition of DMB to a THF solution
of 4 at room temperature resulted in no reaction, indicating that
the carbene-germanium bond is stable at room temperature.
Heating the THF solution to 70 °C in a sealed tube resulted in
the quantitative formation of DMB-trapped germylene 5, along
with a stoichiometric equivalent of the free NHC 3 (Scheme
2).22 We can conclude that 4 dissociates to the free carbene
and the free germylene under these conditions. Thus, 4 is best
represented as a base-stabilized germanium(II) species.
(16) Hurni, K. L.; Rupar, P. A.; Payne, N. C.; Baines, K. M. Submitted
for publication.
The germanium center in 4 has three bonds to carbon and a
lone pair and, thus, is an isovalent analogue of phosphine
(R3Ge:- cf. R3P:). To evaluate the potential of 4 to act as a
Lewis base, 1 equiv of BH3‚THF was added to a THF solution
of 4 (Scheme 3), resulting in the formation of a clear and
colorless solution.23 Removal of the solvent in vacuo gave a
white, air-stable powder. The 1H NMR spectrum of the powder
indicated that the carbene and dimesitylgermylene moieties
remained in a 1:1 ratio. In addition, a broad signal, which was
integrated for three hydrogens, was observed at 1.9 ppm. The
FT-IR spectrum of the powder showed a series of signals
centered at 2300 cm-1, which is in the expected range for
boron-hydrogen bond vibrations. High-resolution mass spec-
trometric analysis of the sample revealed an M+ ion consistent
with a BH3 adduct of 4. A single crystal suitable for X-ray
diffraction was grown by slow evaporation of a benzene solution
of the reaction product and was confirmed to be 6.
(17) Synthesis of 4: to a yellow solution of Mes4Ge2 (0.161 mmol, from
the photolysis of 100 mg of Mes6Ge3)16 dissolved in THF (5 mL) was added
the NHC 3 (0.32 mmol, 0.06 g) dissolved in THF (5 mL). The reaction
mixture was stirred for 5 min. The solvent was removed under vacuum,
giving a yellow powder of essentially pure 4 in 96% yield (0.15 g, 0.31
mmol). Crystals suitable for X-ray analysis were grown from a concentrated
toluene solution stored at -30 °C. Mp: 144-146 °C. 1H NMR: δ 0.96 (d,
3JHH ) 7 Hz, 12 H), 1.50 (s, 6 H), 2.29 (s, 6 H), 2.59 (s, 12 H), 5.73 (sept,
3JHH ) 7 Hz, 2 H), 6.93 (s, 4 H). 13C{1H} NMR: δ 9.99, 20.79, 21.20,
25.54, 51.91, 125.88, 128.51, 134.37, 143.97, 152.31, 176.06. EI-MS: m/z
311 [Mes2Ge, 6%], 180 [C{[N(i-Pr)C(CH3)]2}, 34%], 138 [C{[N(i-Pr)C-
(CH3)]2} - i-Pr, 40%]. Although 4 was clean by 1H NMR analysis (see
the Supporting Information), the elemental analysis of 4 was unsatisfactory,
most likely due to the air and moisture sensitivity of the compound. Selected
crystal data for 4: [C32.5H36GeN2], Mr ) 537.30, triclinic, space group P1h,
a ) 8.8091(9) Å, b ) 13.7282(17) Å, c ) 14.061(2) Å, R ) 63.736(5)°,
â ) 80.017(7)°, γ ) 81.165(8)°, V ) 1496(3) Å3, Z ) 2, T ) 150(2) K,
µ ) 1.046 mm-1, 12 668 reflections collected, 4990 independent (Rint
0.053), R1 ) 0.0678, wR2 ) 0.1347 (all data).
)
(18) The exact mechanism for the formation of 4 is not known. We have
shown that tetramesityldigermene is stable in solution at room temperature
and have found no evidence for the spontaneous dissociation into 2 equiv
of dimesitylgermylene.6,16
(19) (a) Tsumuraya, T.; Kabe, Y.; Ando, W. J. Organomet. Chem. 1994,
482, 131. (b) Rivie`re, P.; Satge´, J. J. Organomet. Chem. 1982, 232, 123.
(20) Hurni, K. L.; Baines, K. M. Can. J. Chem., doi: 10.1139/V07-039.
(21) Baines, K. M.; Cooke, J. A.; Dixon, C. E.; Liu, H. W.; Netherton,
M. R. Organometallics 1994, 13, 631.
(22) The NHC 3 does not react with DMB under these conditions.

Communications
Organometallics, Vol. 26, No. 17, 2007 4111
Scheme 4
of electrons on the germanium center into a bonding pair of
electrons.26 The Ge-B bond length is 2.094(3) Å. Heating 4 in
the presence of Ph3P‚BH3 resulted in the formation of 6 and
the recovery of free PPh3, demonstrating that 4 is a stronger
donor than PPh3 (Scheme 3).
Finally, in an effort to displace the carbene from the ger-
manium, methyllithium was added to a solution of 4. After an
aqueous workup, compound 727 was isolated.28 The formation
of 7 is consistent with nucleophilic attack of the methyllithium
on the germanium center of 4 displacing carbene 3, resulting
in the formation of (dimesitylmethylgermyl)lithium, which is
protonated upon aqueous workup (Scheme 4). Preliminary
results show that when 0.5 equiv of methyllithium is added to
4, Mes2(Me)GeGe(H)Mes2 is formed after workup, presumably
by addition of Mes2Ge(Me)Li to 4.29 Thus, we have established
that 4 is a potential precursor for anionic germanium compounds.
To summarize, we have synthesized the first example of a
carbene-stabilized transient diorganogermylene, 4, from readily
available starting materials. Complex 4 acts as a strong Lewis
base toward BH3 to give 6, a rare example of an intermolecular
group 14 “in-series” coordination compound. The carbene-
germylene complex 4 is a thermal source of dimesitylgermylene
and reacts with MeLi to displace the carbene. Currently, we
are examining 4 as a germylene precursor for use in coordination
chemistry8g and as a precursor for anionic germanium com-
pounds.
Figure 2. Thermal ellipsoid plot (50% probability surface) of 6.
Hydrogen atoms are omitted for clarity. Selected bond distances
(Å) and angles (deg): C1-Ge ) 2.045(2), Ge-B ) 2.094(3),
Ge-C21 ) 2.005(2), Ge-C31 ) 2.001(2), C1-N2 ) 1.361(3),
C1-N5 ) 1.353(3); C1-Ge-B ) 104.6(1), C1-Ge-C21 )
102.67(9), C1-Ge-C31 ) 109.47(9), C21-Ge-C31 ) 112.98-
(19).
The molecular structure of 6 is shown in Figure 2. Complex
6 can be viewed as a carbene-germylene-borane “in-series”
coordination complex, where the germanium is simultaneously
an electron pair acceptor and an electron pair donor (Scheme
3). The metrics of 6 are similar to those of 4; however, the
NHC-Ge-Mes angles are slightly more obtuse24 and the
germanium-carbon bond lengths are somewhat shorter.25 Both
observations are consistent with the conversion of the lone pair
Acknowledgment. We thank the NSERC (Canada) and the
Ontario Photonics Consortium (OPC) for funding. We thank
Doug Hairsine for acquiring mass spectrometric data. We also
thank Teck Cominco Ltd. for a generous gift of GeCl4.
(23) Synthesis of 6: to a yellow solution of 4 (0.15 g, 0.31 mmol)
dissolved in THF (10 mL) was added a 1 M solution of BH3‚THF in THF
(0.31 mL, 0.31 mmol). The yellow solution faded to a clear and colorless
solution after 15 min. The solvent was removed under vacuum, giving a
white powder of pure 6 in quantitative yield. Crystals suitable for
X-ray analysis were grown by the slow evaporation of a saturated C6H6
solution. Mp: 155-162 °C dec. 1H NMR: δ 1.00 (d, 3JHH ) 7 Hz, 12 H),
1.51 (s, 6 H), 1.70-2.10 (broad, 3 H), 2.18 (s, 6 H), 2.52 (s, 12 H),
5.55 (broad, 2 H), 6.82 (s, 4 H). 13C{1H} NMR: δ 10.12, 21.04, 21.39,
26.02, 51.41, 127.55, 129.42, 136.97, 143.01, 144.46, 164.60. 11B NMR:
δ -28.49 (broad). IR: 847 (m), 1035 (s), 1374 (s), 1457 (broad, s), 1600
(m), 2268 (s), 2298 (s), 2349 (s), 2375 (m), 2731 (w), 2867 (s), 2921 (s),
2874 (s) cm-1. EI-MS: m/z 505 [M+, 5%], 492 [M+ - BH3, 100%], 373
[C{[N(i-Pr)C(CH3)]2}GeMes, 10%], 311 [Mes2Ge, 20%]. High resolution
EI-MS: m/z calcd for C29H4511B74GeN2 505.2818, found 505.2820. Anal.
Calcd for C29H45BGeN2: C, 68.96; H, 8.98. Found: C, 68.62; H, 9.45.
Selected crystal data for 6: [C32H48BGeN2], Mr ) 544.12, monoclinic, space
group P21/c, a ) 22.0634(5) Å, b ) 8.6076(2) Å, c ) 16.2745(2) Å, â )
Supporting Information Available: A CIF file giving crystal-
lographic data and text giving detailed experimental procedures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM7006198
(26) Similar bond angle shifts have been noted in a carbene-phosphin-
idene complex upon coordination to BH3. See: Arduengo, A. J., III; Carmalt,
C. J.; Clyburne, J. A. C.; Cowley, A. H.; Pyati, R. J. Chem. Soc., Chem.
Commun. 1997, 981.
(27) 7 was previously synthesized by a different method. See: Castel,
A.; Rivie`re, P.; Satge´, J.; Ko, Y. J. Organomet. Chem. 1988, 342, C1.
(28) The carbene is presumably converted to the imidazolium chloride,
which is removed in the aqueous phase.
93.0960(10)°, V ) 3086.23(14) Å3, Z ) 4, T ) 150(2) K, µ ) 1.014 mm-1
,
28 738 reflections collected, 5429 independent (Rint ) 0.057), R1 ) 0.0524,
wR2 ) 0.0924 (all data).
(24) The sum of the bond angles around Ge in 6 is 325°, and the sum of
the C-Ge-C bond angles around Ge in 4 is 318°.
(29) Mes2(Me)GeGe(H)Mes2 was previously synthesized by a different
method. See the Supporting Information and: Dixon, C. E.; Netherton, M.
R.; Baines, K. M. J. Am. Chem. Soc. 1998, 120, 10365.
(25) The sum of the C-Ge bond lengths in 6 is 6.063 Å, and the sum
of the C-Ge bond lengths in compound 4 is 6.215 Å.