Synthesis of a base-stabilized (Chlorogermyl)metallogermylene and its photochemical conversion to a (Chlorogermyl)germylyne complex

Article abstract of DOI:10.1246/cl.2013.43

An NHC-stabilized (chlorogermyl)metallogermylene [Cp- (CO) ₃WGe{GeCl(Mes)₂}(MeIiPr)] (NHC: N-heterocyclic carbene, MeIiPr: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Mes: mesityl) was synthesized by the reaction of [Na(dme)2]- [CpW(CO)₃] (dme: 1,2-dimethoxyethane) with (chlorogermyl)- chlorogermylene [GeCl{GeCl(Mes) ₂}(MeIiPr)]. Irradiation to a solution of the metallogermylene with an Hg lamp afforded an NHC-stabilized (chlorogermyl)germylyne complex [Cp- (CO)₂W=Ge{GeCl(Mes)₂}(MeIiPr)].

Full text of DOI:10.1246/cl.2013.43

doi:10.1246/cl.2013.43
Published on the web December 22, 2012
43
Synthesis of a Base-stabilized (Chlorogermyl)metallogermylene
and Its Photochemical Conversion to a (Chlorogermyl)germylyne Complex
Shin-ya Tashita, Takahito Watanabe, and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578
(Received October 9, 2012; CL-121036; E-mail: tobita@m.tohoku.ac.jp)
An NHC-stabilized (chlorogermyl)metallogermylene [Cp-
(CO)3WGe{GeCl(Mes)2}(MeIiPr)] (NHC: N-heterocyclic car-
bene, MeIiPr: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene,
Mes: mesityl) was synthesized by the reaction of [Na(dme)2]-
[CpW(CO)3] (dme: 1,2-dimethoxyethane) with (chlorogermyl)-
chlorogermylene [GeCl{GeCl(Mes)2}(MeIiPr)]. Irradiation to a
solution of the metallogermylene with an Hg lamp afforded
an NHC-stabilized (chlorogermyl)germylyne complex [Cp-
(CO)2W¸Ge{GeCl(Mes)2}(MeIiPr)].
Germylene 1 was prepared by a slightly modified method of
Baines et al.3 They originally synthesized germylene 1 by the
reaction of [GeMes2(MeIiPr)] with [GeCl2(MeIiPr)] in THF at
room temperature in 25% yield.3 In our modified method,
[GeCl2(dioxane)] was used instead of [GeCl2(MeIiPr)] and the
reaction temperature was raised to 50 °C, which provided
germylene 1 in 87% isolated yield (eq 1).5
ð1Þ
Transition-metal vinylidene complexes have been of great
interest in the fields of organometallic chemistry and organic
synthesis because of their unique reactivity as catalysts caused
by the cumulative double bonds.1 A large number of vinylidene
complexes have been synthesized and their synthetic methods
are now well established.1 In contrast, the heavier group 14
congeners [M] = E = ER2 ([M] = transition-metal fragment; E =
Si, Ge, Sn, and Pb) are still elusive, although some theoretical
studies of disilavinylidene complexes have been reported.2
A reason why development of the chemistry of heavier
vinylidene complexes has been slow is obviously the absence of
appropriate precursors. Recently, Baines and co-workers report-
ed the synthesis of a (chlorogermyl)chlorogermylene [GeCl-
{GeCl(Mes)2}(MeIiPr)] (1).3 We found that 1 can become a good
precursor for the synthesis of a digermavinylidene complex. Our
strategy for synthesizing the digermavinylidene complex from 1
is shown in Scheme 1; (i) synthesis of a metallogermylene A or
a germylyne complex B containing a chlorogermyl group by
metallation of 1, (ii) synthesis of a cationic digermavinylidene
Subsequent treatment of germylene 1 with 1.5 equivalent
of [Na(dme)2][CpW(CO)3] in THF gave an NHC-coordinated
metallogermylene [Cp(CO)3WGe{GeCl(Mes)2}(MeIiPr)] (2) in
50% yield (crude) (eq 2).5 Substitution of chloride occurred only
on the less hindered divalent germanium. Complex 2 was
thermally unstable in solution and slowly decomposed even at
room temperature, which made the purification of 2 difficult.6
However, we fortunately obtained a single crystal of 2 by
recrystallization from its toluene/hexane solution at ¹30 °C,
which enabled the structure determination of 2 by X-ray
crystallography (Figure 1).7
ð2Þ
¹
complex C by Cl abstraction. N-Heterocyclic carbenes (NHCs)
could stabilize all these metallogermylene, germylyne com-
plexes, and cationic digermavinylidene complex by coordination
to the low-valent ¡-Ge atoms. In fact, a number of low-valent
germanium species have been stabilized by NHCs and isolated.4
In this paper, we report the synthesis and characterization of an
NHC-stabilized germylene having a [Cp(CO)3W] group and a
chlorogermyl group, and its photoinduced conversion to a
germylyne complex (step (i) in Scheme 1).
Figure 1. ORTEP drawing of 2¢n-hexane with 50% thermal ellipsoids.
The crystal solvent n-hexane and all hydrogen atoms are omitted for clarity.
Selected bond distances (¡) and angles (°): W-Ge(1) 2.8127(8), W-C(6)
1.974(8), W-C(7) 1.963(9), W-C(8) 1.953(7), Ge(1)-Ge(2) 2.5493(10),
Ge(1)-C(27) 2.082(6), Ge(2)-C(9) 2.001(6), Ge(2)-C(18) 2.005(8), Ge(2)-
Cl 2.2598(18), Ge(1)-W-C(6) 78.9(2), C(6)-W-C(7) 76.7(3), C(7)-W-
C(8) 77.7(3), C(8)-W-Ge(1) 72.8(2), W-Ge(1)-C(27) 106.4(2), C(27)-
Ge(1)-Ge(2) 107.16(19), Ge(2)-Ge(1)-W 112.45(3).
Scheme 1. Synthetic strategy for a cationic digermavinylidene complex C.
Chem. Lett. 2013, 42, 43-44
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

44
Complex 2 is the first example of NHC-coordinated
metallogermylene. This complex adopts a four-legged piano-
stool geometry consisting of a cyclopentadienyl ligand, a
(chlorogermyl)germylene ligand, and three carbonyl ligands.
The ¡-Ge atom is strongly pyramidalized (the sum of bond
angles around ¡-Ge: 326.0°) owing to the existence of a
stereochemically active lone pair. The Ge(1)-Ge(2) bond length
(2.5493(10) ¡) of 2 is comparable with the Ge-Ge bond length
of 1 (2.5355(19) ¡),3 while the Ge(1)-C(27) bond length
(2.082(6) ¡) is shorter than the Ge-C(NHC) bond length of 1
(2.147(12) ¡).3 The W-Ge bond length (2.8127(8) ¡) of 2 is
significantly longer than that of a base-free metallogermylene
[Cp(CO)3WGeArtip] [Artip: 2,6-bis(2,4,6-triisopropylphenyl)-
phenyl] (2.681(3) ¡).8 The W-Ge bond of the latter complex
is considered to have a substantial multiple bond character due
Scheme 2. Possible resonance structures for complex 3.
equivalently in contrast with the signals for two Mes groups
in complex 2 that appeared inequivalently. This feature is
consistent with the achiral structure of 3. In the IR spectrum of 3,
the wavenumbers of CO stretching bands (1801 and 1874 cm
are remarkably lower than those of a base-free germylyne
¹1
)
¹1
complex [Cp(CO)2W¸Ge{C(SiMe3)3}] (1860 and 1924 cm
)
to the ³-back donation from the d orbital of the W to the vacant
and are slightly higher than those of its NHC adduct
[Cp(CO)2WGe{C(SiMe3)3}(MeIMe)] (MeIMe: 1,3,4,5-tetrameth-
ylimidazol-2-ylidene) (1779 and 1854 cm¹1).11 These data
suggest a significant contribution of a zwitterionic canonical
structure E containing an anionic tungsten center and a cationic
NHC (Scheme 2).
³
p-orbital of the Ge.9 In the case of 2, this ³-back donation is
considerably reduced by the competitive electron donation from
the coordinated MeIiPr to the p-orbital of ¡-Ge. This reduction of
³-back donation is probably the main reason for the elongation
of the W-Ge bond. It, at the same time, makes the tungsten
center more electron-rich and results in the increase of ³-back
donation to the carbonyl ligands. This tendency is reflected in
the ¯CO frequencies of 2 (1859-1957 cm¹1) that are lower than
those of [Cp(CO)3WGeArtip] (1885-2022 cm¹1).8 Steric repul-
sion between CO ligands and MeIiPr moiety also possibly causes
the elongation of the W-Ge bond. Indeed, the interatomic
distance (3.409(12) ¡) between O(2) of a CO ligand and C(35)
In summary, we synthesized and characterized the NHC-
coordinated metallogermylene 2 and germylyne complex 3
having a chlorogermyl group on the ¡-Ge atoms. These
complexes are promising precursors for unprecedented diger-
mavinylidene complexes. The synthesis of a digermavinylidene
¹
complex by Cl abstraction from 2 or 3 (step (ii) in Scheme 1)
is under active investigation.
i
of a Pr group of the MeIiPr moiety is shorter than the sum
(3.52 ¡) of the van der Waals radii of oxygen (1.52 ¡)10a and a
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 22350024 and 23750054) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. We
thank the Research and Analytical Center for Giant Molecules of
Tohoku University for elemental analysis.
Me group (2.0 ¡).10b
1
The H NMR spectrum of 2 shows two sets of signals for
two Mes groups on the ¢-Ge atom (¤ 2.10, 2.13 ( p-Me) and ¤
2.63, 2.94 (o-Me)). This is attributable to the diastereotopic
relationship of the two Mes groups caused by the chirality at the
pyramidalized ¡-Ge atom.
References and Notes
1
a) M. I. Bruce, Chem. Rev. 1991, 91, 197. b) J. P. Selegue, Coord. Chem. Rev.
2004, 248, 1543. c) H. Werner, Coord. Chem. Rev. 2004, 248, 1693. d) C.
Bruneau, P. H. Dixneuf, Angew. Chem., Int. Ed. 2006, 45, 2176. e) J. M.
Lynam, Chem.®Eur. J. 2010, 16, 8238.
UV irradiation to crude metallogermylene 2 in benzene
solution with periodic removal of CO under reduced pressure
gave an NHC-stabilized germylyne complex [Cp(CO)2W¸Ge-
{GeCl(Mes)2}(MeIiPr)] (3) in 5% isolated yield based on 1
(49% NMR yield based on 2) (eq 3).5 Unlike complex 2,
complex 3 was stable at room temperature in C6D6, but it
gradually decomposed at 50 °C. A similar photoreaction has
previously been reported by Power et al. for conversion of a
metallogermylene [Cp(CO)3WGeArtip] to a germylyne complex
[Cp(CO)2W¸GeArtip].8
2
a) R. Stegmann, G. Frenking, Organometallics 1995, 14, 5308. b) Y.
Kuramoto, N. Sawai, Y. Fujiwara, M. Sumimoto, Y. Nakao, H. Sato, S.
Sakaki, Organometallics 2005, 24, 3655.
3
4
P. A. Rupar, M. C. Jennings, K. M. Baines, Organometallics 2008, 27, 5043.
a) Y. Mizuhata, T. Sasamori, N. Tokitoh, Chem. Rev. 2009, 109, 3479, and
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J. C. Avery, K. M. Baines, Organometallics 2010, 29, 1362. g) N. Katir, D.
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ð3Þ
5
6
7
8
9
Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Keeping a C6D6 solution of 2 at room temperature for 1 day resulted in
formation of a complicated mixture (t1/2 = ca. 100 min).
The crystallographic data for 2¢n-hexane can be obtained free of charge from
the Cambridge Crystallographic Data Centre (CCDC No. 907619).
L. Pu, B. Twamley, S. T. Haubrich, M. M. Olmstead, B. V. Mork, R. S.
Simons, P. P. Power, J. Am. Chem. Soc. 2000, 122, 650.
Complex 3 was characterized by NMR and IR spectroscopy,
elemental analysis, and preliminary X-ray crystallography.5
Although the R-factor for the crystal structure analysis of 3
was not good enough, it clearly supported the structure of 3 as a
base-stabilized germylyne complex: The W-Ge bond length was
very short and the geometry around the ¡-Ge atom of 3 was
K. K. Pandey, M. Lein, G. Frenking, J. Am. Chem. Soc. 2003, 125, 1660.
10 a) A. Bondi, J. Phys. Chem. 1964, 68, 441. b) L. Pauling, The Nature of the
Chemical Bond, 3rd ed., Cornell University Press, Ithaca, New York, 1960,
pp. 257-264.
11 A. C. Filippou, K. W. Stumpf, O. Chernov, G. Schnakenburg, Organo-
metallics 2012, 31, 748.
1
almost completely planar.5 In the H NMR spectrum of 3, the
signals for two Mes groups on the ¢-Ge atom appeared
Chem. Lett. 2013, 42, 43-44
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/