Synthesis and characterization of three new thermally stable N-heterocyclic germylenes

Article abstract of DOI:10.1016/j.jorganchem.2008.12.042

Three new stable germylenes, rac-1,3-di-tert-butyl-4,5-dimethyl-1,3-diaza-2-germacyclopentane-2-ylide (1), 1,3-di-tert-butyl-4,4-dimethyl-1,3-diaza-2-germacyclopentane-2-ylide (2), and rac-1,3,4-tri-tert-butyl-1,3-diaza-2-germacyclopentane-2-ylide (3) hav

Full text of DOI:10.1016/j.jorganchem.2008.12.042

Journal of Organometallic Chemistry 694 (2009) 2122–2125
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis and characterization of three new thermally stable
N-heterocyclic germylenes
*
Adam C. Tomasik, Nicholas J. Hill, Robert West
Organosilicon Research Center, Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, WI 53706, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new stable germylenes, rac-1,3-di-tert-butyl-4,5-dimethyl-1,3-diaza-2-germacyclopentane-
2-ylide (1), 1,3-di-tert-butyl-4,4-dimethyl-1,3-diaza-2-germacyclopentane-2-ylide (2), and rac-1,3,4-tri-
tert-butyl-1,3-diaza-2-germacyclopentane-2-ylide (3) have been synthesized by the reaction of their
corresponding germyl dichlorides with elemental lithium. Full synthetic procedures and characteriza-
tions are described.
Received 5 December 2008
Received in revised form 17 December 2008
Accepted 18 December 2008
Available online 25 December 2008
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Germylene
Germanium
Organogermanium
1. Introduction
The stabilization and isolation of compounds featuring p-block
elements in unusual or low oxidation states has been a common
theme in contemporary main group chemistry. Since their initial
disclosure in 1991 [1], the divalent dicoordinate Arduengo-type
N-heterocyclic carbenes (NHCs) A have dominated this field,
primarily on account of their utility as robust ligands for organo-
metallic coordination complexes [2] and, more recently, as organ-
ocatalysts [3]. However, the isolation of the first NHGe system by
Veith (B) actually predates that of the NHC by almost a decade,
[4] and Si- and Ge-analogues of Arduengo NHCs (C-E) were first
described in 1994 and 1992, respectively [5,6]. Since these initial
reports, the chemistry of thermally stable N-heterocyclic silylenes
and germylenes (NHSi and NHGe respectively) of type A has
developed apace [7–10].
Most of the five-membered NHGe species reported in the liter-
ature are based upon the 1,4-diazabuta-1,3-diene (DAB) frame-
work, and are prepared by salt metathesis reactions of the stable
and commercially available Ge(II) source [GeCl2(dioxane)] with a
dimetallated DAB [6]. Germylenes prepared by this procedure typ-
ically display increased thermal stability as compared to their sat-
urated analogues. Experimental and theoretical studies suggest
employed as substituents at the nitrogen atoms, a number of
unsaturated NHGe systems bearing bulky aromatic N-groups have
been reported recently [13–16].
that this is due to
r-electron withdrawal/-electron donation by
the two nitrogen atoms bound to the germanium atom in combina-
tion with electron delocalization in the heterocyclic 6-electron sys-
tem of the supporting DAB framework [12]. While bulky alkyl
groups (such as tert-butyl and neo-pentyl) have been commonly
Saturated NHGes such as E lack stabilization by aromatic elec-
tron delocalization but may be stabilized by -donation from the
nitrogen atom into the vacant p-orbital on germanium as well as
-withdrawal of electron density from the germanium to the nitro-
gen atoms [12]. Synthetic routes to saturated NHGes involve either
the transamination of a diamine ligand with the acyclic Ge(II) com-
pound [(Me3Si)2N]2Ge (F) [11], or salt metathesis of [GeCl2 Á (diox-
* Corresponding author.
E-mail address: west@chem.wisc.edu (R. West).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.12.042

A.C. Tomasik et al. / Journal of Organometallic Chemistry 694 (2009) 2122–2125
2123
ane)] with a dilithiated diamine. Hahn et al. recently employed
such procedures to prepare the first examples of bidentate chelat-
ing benzannelated bis(germylenes) [17,18].
Germylene 2 is substituted with two methyl groups on the
same backbone carbon. Compound 2 was prepared by reducing
its corresponding dichlorogermane 10 with elemental lithium as
shown in Scheme 2. First, a-bromoisobutyryl bromide 6 was trea-
ted with tert-butylamine to give the bromo amide 7 in 96% yield
[21,22]. The tertiary bromide position was substituted with tert-
butylamine during a subsequent step in the presence of powdered
NaOH in 85% yield to give the amido amine 8 [23]. Finally the dia-
mine ligand 9 was obtained by reducing 8 with LiAlH4 in 2-meth-
yltetrahydrofuran in 87% yield [23]. The diamine was then
converted to the dichlorogermane 10 after reaction with germa-
nium tetrachloride in the presence of triethylamine in 67% yield.
The dichlorogermane was reduced with elemental lithium at room
temperature and the germylene 2 was produced in 76% yield. The
germylene was purified by short-path distillation at 90 °C
(0.1 Torr).
Germylene 3 has one tert-butyl group in the backbone. It is syn-
thesized by the same general method as 1 and 2 as shown in
Scheme 3. N,N’-di-tert-butylethylenediimine 11 was treated with
tert-butyllithium to produce the iminoamine 12 in 97% yield [20]
which was then reduced to the diamine 13 with AlH3ÁNMe3 in
96% yield [24]. The diamine was then converted to the dichloroger-
mane 14 by reaction with germanium tetrachloride in the presence
of triethylamine in 65% yield. When the dichlorogermane was re-
duced with elemental lithium at room temperature, the germylene
3 was obtained in 75% yield. The germylene was purified by distil-
lation at 85 °C (0.1 Torr).
2. Results and discussion
Herein we report the synthesis and characterization of rare
examples of saturated N-heterocyclic germylenes derived from
simple diamines: compound 1 (as a racemic mixture), the germa-
nium analogue of the previously reported racemic N-heterocyclic
silylene Me2[NN]Si, [19] and compounds 2 and 3, the latter also a
racemic mixture. The structures of 1, 2, and 3 are similar to that
of E but with substitution of alkyl groups at the backbone carbon
atoms. Germylenes 1, 2, and 3 are monomeric and isolated as pale
yellow liquids. All compounds were characterized by 1H, and
13C{1H} NMR, high-resolution mass spectrometry, and elemental
analysis.
In conclusion, three new N-heterocyclic saturated germylenes,
1, 2, and 3 have been synthesized. It appears that the divalent Ge
atom is quite tolerant to various backbone ligands and its +2 oxida-
tion state is stabilized by the nitrogen atoms in the backbone.
These compounds may lead to new N-heterocyclic ligands (or chi-
ral ligands) for metal complex catalysis.
The new germylenes 1, 2, and 3. The S,S isomer of 1 is not shown.
The racemic germylene 1 was prepared by the reduction of the
dichlorogermane compound 5 with lithium metal in THF at room
temperature as shown in Scheme 1. First N,N’-di-tert-butylethyl-
enediimine, obtained by the reaction of glyoxal with tert-butyl-
amine, was treated with methyllithium to produce a racemic
mixture of the diamine compound 4 in 94% yield [20]. No meso
diamine compound was observed in this reaction, as the second
nucleophilic addition of CH3À to a C@N double bond is evidently
highly stereoselective. The diamine 4 was then converted to the
corresponding dichlorogermane 5 after reaction with germanium
tetrachloride in the presence of 1,8-diazabicyclo [2.2.2] octane
(DABCO). When 5 was reduced with metallic lithium in THF at
room temperature the racemic germylene 1 was produced in 71%
yield. Germylene 1 was purified by short-path Kugelrohr distilla-
tion at 80 °C (0.1 Torr).
3. Experimental
3.1. General
3.1.1. Chemicals and reagents
All reactions and manipulations were conducted under a dry ar-
gon atmosphere using standard Schlenk techniques. DABCO, tert-
butylamine,
a-bromoisobutyrl bromide, sodium hydroxide, lith-
ium aluminum hydride, and tert-butyllithium were obtained from
Sigma Aldrich and used as received. AlH3 Á NMe3 [25] and N,N’-di-
Scheme 1. The synthesis of germylene 1. Compounds 4, 5, and 1 are shown as pairs of enantiomers.

2124
A.C. Tomasik et al. / Journal of Organometallic Chemistry 694 (2009) 2122–2125
Scheme 2. The synthesis of germylene 2.
Scheme 3. The synthesis of germylene 3.
tert-butylethylenediimine [26] were prepared via literature meth-
ods. Germanium tetrachloride was obtained from Gelest Inc. and
distilled and degassed immediately before use. Lithium metal
was purchased in mineral oil and washed with pentane prior to
use. All solvents were dried using either sodium/potassium alloy
or sodium benzophenone radical.
(125 mL) was stirred at 0 °C. GeCl4 (11.0 g, 51.3 mmol) in 25 mL
of hexane was added dropwise to the cooled solution. The reaction
mixture was then fitted with a reflux condenser and refluxed over-
night after which it was cooled, filtered, and the solvent removed.
The crude yellow oil was distilled in a Kugelrohr oven (80 °C,
0.1 Torr) to give a viscous pale yellow oil (11.28 g, 76.9%).
1H NMR (C6D6): d 1.07 (d, 6H, J = 6.3 Hz); 1.22 (s, 18H); 2.73 (q,
2H, J = 6.3 Hz). 13C{1H} NMR (C6D6): d 25.27; 31.60; 54.55; 58.07.
High resolution mass spectrometry: calculated for [C12H26N2-
GeCl2]+ 338.0711, found 338.0715. Anal. Calc. for C12H26N2GeCl2:
C, 42.16; H, 7.67; N, 8.19. Found: C, 42.43; H, 7.69; N, 8.22%.
3.1.2. Instrumentation
1H NMR and 13C{1H} NMR spectra were recorded at room tem-
perature on a Bruker AC-300 spectrometer and were referenced to
internal TMS or residual solvent resonances. Mass spectral analy-
ses were performed using a Shimadzu QP 5000. Elemental analyses
were performed by Robertson Microlit Laboratories Madison, NJ,
USA and Chemisar Laboratories Inc. Guelph, ON, Canada.
3.3. rac-1,3-Di-tert-butyl-4,5-dimethyl-1,3-diaza-2-
germacyclopentane-2-ylide, 1
3.2. rac-1,3-Di-tert-butyl-2,2-dichloro-4,5-dimethyl-1,3,2-
To a 200 mL Schlenk flask was added lithium wire cut into small
diazagermolidine, 5
pieces (0.35 g, 50.6 mmol) and THF (100 mL). Then, rac-1,3-di-tert-
butyl-2,2-dichloro-4,5-dimethyl-1,3,2-diazagermolidine
(5)
A solution of the diamine (4) (8.60 g, 42.9 mmol), 1,8-diazabicy-
clo[2.2.2]octane (DABCO) (9.65 g, 86.0 mmol), and hexane
(8.05 g, 23.6 mmol) dissolved in 15 mL THF was added via cannula.
The reaction was monitored by 1H NMR until all the starting

A.C. Tomasik et al. / Journal of Organometallic Chemistry 694 (2009) 2122–2125
2125
material disappeared (ꢀ1 h). The reaction mixture was filtered and
the solvent removed resulting in a dark brown oil. The oil was vac-
uum distilled (80 °C, 0.1 Torr) to give a very pale yellow liquid
(4.55 g, 71.2%).
C14H30N2GeCl2: C, 45.45; H, 8.17; N, 7.57. Found: C, 45.77; H, 8.45;
N, 7.58%.
3.7. rac-1,3,4-Tri-tert-butyl-1,3-diaza-2-germacyclopentane-2-ylide,
3
1H NMR (C6D6): d 1.11 (d, 6H, J = 6.0 Hz); 1.28 (s, 18H); 3.02 (q,
2H, J = 6.0 Hz). 13C{1H} NMR (C6D6): d 27.44; 33.87; 55.02; 63.02.
High resolution mass spectrometry: calculated for [C12H26N2Ge]+
268.1333, found 268.1335. Anal. Calc. for C12H26N2Ge: C, 53.19;
H, 9.67; N, 10.34. Found: C, 52.92; H, 9.86; N, 10.45%.
In a 100 mL Schlenk flask was placed lithium wire cut into
pieces (0.20 g, 28.8 mmol). To this flask 40 mL THF was added.
Then, rac-1,3,4-tri-tert-butyl-2,2-dichloro-1,3,2-diazagermolidine
(14) (5.18 g, 14.0 mmol) dissolved in 20 mL THF was added via
cannula. The reaction was monitored by 1H NMR until all the start-
ing material disappeared (ꢀ1 h). The reaction mixture was filtered
and the solvent removed resulting in a pale yellow oil. The yellow
oil was vacuum distilled (85 °C, 0.1 Torr) to give a pale yellow li-
quid (3.14 g, 75.1%).
3.4. 1,3-Di-tert-butyl-2,2-dichloro-4,4-dimethyl-1,3,2-
diazagermolidine, 10
A solution of the diamine (9) (6.17 g, 30.8 mmol), triethylamine
(12 mL), and hexane (125 mL) was stirred at 0 °C. GeCl4 (7.26 g,
33.9 mmol) in 25 mL of hexane was added dropwise to the cooled
solution. The reaction mixture was then fitted with a reflux con-
denser and refluxed overnight after which it was cooled, filtered,
and the solvent removed. The crude yellow oil was distilled in a
Kugelrohr oven (105 °C, 0.1 Torr) to give a pale yellow oil (7.00 g,
67.1%).
1H NMR (C6D6): d 0.92 (s, 9H); 1.28 (s, 9H); 1.34 (s, 9H); 3.2–3.4
(m, 3H). 13C{1H} NMR (C6D6): d 29.15; 32.34; 35.04; 35.60; 52.34;
55.60; 56.99; 69.11. High resolution mass spectrometry: calculated
for [C14H30N2Ge-C4H9]+ 239.0942, found 239.0952. Anal. Calcd. for
C14H30N2Ge: C, 56.23; H, 10.11; N, 9.37. Found: C, 55.88; H, 9.93; N,
9.04%.
1H NMR (C6D6): d 1.21 (s, 6H); 1.22 (s, 9H); 1.36 (s, 9H); 2.59 (s,
2H). 13C{1H} NMR (C6D6): d 29.63; 29.73; 34.15; 54.13; 56.79;
60.17; 61.24. High resolution mass spectrometry: calculated for
[C12H26N2GeCl2]+ 338.0711, found 338.0723. Anal. Calc. for
C12H26N2GeCl2: C, 42.16; H, 7.67; N, 8.19. Found: C, 41.82; H,
8.06; N, 8.42%.
Acknowledgements
We thank the National Science Foundation for support of this
research (Grant No. CHE-0451536).
Appendix A. Supplementary data
3.5. 1,3-Di-tert-butyl-4,4-dimethyl-1,3-diaza-2-germacyclopentane-
2-ylide, 2
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.12.042.
To a 100 mL Schlenk flask was added lithium wire cut into small
pieces (0.17 g, 24.3 mmol) and THF (40 mL). Then, 1,3-di-tert-bu-
tyl-2,2-dichloro-4,4-dimethyl-1,3,2-diazagermolidine (10) (4.06 g,
11.9 mmol) dissolved in 15 mL THF was added dropwise. The reac-
tion was monitored by 1H NMR until all the starting material dis-
appeared (ꢀ3 h). The reaction mixture was filtered and the
solvent removed resulting in a dark yellow oil. The yellow oil
was vacuum distilled (90 °C, 0.1 Torr) to give a very pale yellow
oil (2.44 g, 75.8%).
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A solution of the diamine (13) (10.0 g, 43.8 mmol), triethyl-
amine (20 mL), and hexane (125 mL) was stirred at 0 °C. GeCl4
(9.38 g, 43.8 mmol) in 25 mL of hexane was added dropwise to
the cooled solution. The reaction mixture was then fitted with a re-
flux condenser and refluxed overnight after which it was cooled,
filtered, and the solvent removed. The crude yellow oil was dis-
tilled in a Kugelrohr oven (105 °C, 0.1 Torr) to give a very viscous
yellow oil (10.45 g, 64.5%).
1H NMR (C6D6): d 0.98 (s, 9H); 1.23 (s, 9H); 1.25 (s, 9H); 2.7-3.0
(m, 3H). 13C{1H} NMR (C6D6): d 29.43; 29.66; 31.58; 36.61; 46.94;
54.58; 56.78; 63.23. High resolution mass spectrometry: calculated
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