Controlled growth of dichlorogermanium oligomers from lewis basic hosts

Article abstract of DOI:10.1002/anie.201302767

To branch or not to branch: A mild stepwise route to various linear and branched (GeCl₂)_x oligogermylenes supported by Lewis bases is reported, including the carbene-bound Ge₄ complex NHC×GeCl₂Ge(GeCl₃)₂ (see picture). Dipp=2,6-iPr₂C₆H₃, NHC=N-heterocyclic carbene. Copyright

Full text of DOI:10.1002/anie.201302767

Angewandte
Chemie
Communications
Main Group Chemistry
S. M. I. Al-Rafia, M. R. Momeni,
R. McDonald, M. J. Ferguson, A. Brown,
E. Rivard*
&&&& — &&&&
Controlled Growth of
Dichlorogermanium Oligomers from
Lewis Basic Hosts
To branch or not to branch: A mild
stepwise route to various linear and
branched (GeCl2)x oligogermylenes sup-
ported by Lewis bases is reported,
including the carbene-bound Ge4 com-
plex NHC·GeCl2Ge(GeCl3)2 (see picture).
Dipp=2,6-iPr2C6H3, NHC=N-hetero-
cyclic carbene.
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DOI: 10.1002/anie.201302767
Main Group Chemistry
Controlled Growth of Dichlorogermanium Oligomers from Lewis
Basic Hosts**
S. M. Ibrahim Al-Rafia, Mohammad R. Momeni, Robert McDonald, Michael J. Ferguson,
Alex Brown, and Eric Rivard*
The concept of catenation is widely exploited by synthetic
chemists to construct new polymeric/oligomeric materials
with desirable properties. As illustrated by the polyolefin
industry, control over macromolecular topology (for example,
branching vs. linear) is a key design criterion for the develop-
ment of advanced materials.[1] Amongst the inorganic
Group 14 tetrel elements, it has been shown that catenation
leads to species of the general form (R2E)n (E = Si, Ge, Sn,
and Pb); these materials display novel optoelectronic proper-
ties as a result of increasing s–s* conjugation, both as the
length of the chains is extended and as the core element
becomes heavier.[2] Consequently, polysilanes and their
heavier element congeners are now being actively explored
as photoresist materials.[3]
methylene and ethylene, EH2 and H2EE’H2 (E and E’ = Si,
Ge, and/or Sn), complexes.[10,11] Knowing that the GeII adduct
IPrGeCl2 (IPr= [(HCNDipp)2CD]; Dipp = 2,6-iPr2C6H3) con-
tains a nucleophilic lone pair at Ge,[10a] we decided to explore
whether this complex would interact with further equivalents
of Lewis acidic GeCl2 as a method to gain access to new
carbene-stabilized oligomers IPr·(GeCl2)x (x ꢀ 2). We began
our studies by combining IPr·GeCl2 with Cl2Ge·dioxane
(1 equiv) in toluene, resulting in the formation of a sparingly
soluble colorless solid [Eq. (1)]. This product was recrystal-
lized from CH2Cl2 to give the linear tetrachlorodigermene
adduct IPr·GeCl2GeCl2 (1) as pale yellow crystals in a 75%
yield (Figure 1).[12,13]
In general, polytetrelanes (R2E)n are synthesized under
harsh reducing conditions, such as Wurtz coupling, which
leads to uncontrolled polymer growth.[4,5] Drawn by this
challenge and the uncertainty associated with the structures
of the metastable halides (SiCl2)n and (GeCl2)n in the solid
state,[6] we focused our efforts towards developing an efficient
bottom–up synthesis of related oligomers and polymers
(ECl2)x (x ꢀ 2) in the presence of Lewis basic (LB) hosts.
This strategy is predicated on the propensity of strong
electron-pair donors to bind/stabilize SiCl2 and GeCl2 in the
form of stable molecular adducts LB·ECl2 (E = Si and Ge).[7]
It is hoped that by forming well-defined higher oligomers of
(ECl2)x (x ꢀ 2) that productive halide replacement chemistry
could later afford substituted (R2E)x analogues with tailored
optoelectronic properties,[2] as well as generating precursors
with suitable decomposition kinetics for chemical deposition
processes.[8] In pursuit of this goal, we herein report the mild
and sequential synthesis of Lewis base supported germanium
dichloride oligomers (GeCl2)x (x ꢀ 2) that form thermody-
namically favored branched structures upon increasing Ge
content (a principle that is well known for hydrocarbons).[9]
Our group reported the use of N-heterocyclic carbenes
(NHCs) to facilitate the isolation of the parent inorganic
Figure 1. Thermal ellipsoid plot (30% probability level) of IPr·-
GeCl2GeCl2 (1); all hydrogen atoms have been omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: C(1)–Ge(1) 2.032(5), Ge(1)–
Ge(2) 2.6304(9), Ge(1)–Cl(1) 2.1811(16), Ge(1)–Cl(2) 2.1780(15),
Ge(2)–Cl(3) 2.2568(16), Ge(2)–Cl(4) 2.2844(15); C(1)-Ge(1)-Ge(2)
125.04(14), Cl(1)-Ge(1)-Cl(2) 103.38(7), Cl(3)-Ge(2)-Cl(4) 96.22(2);
torsion angle=Cl(1)-Ge(1)-Ge(2)-Cl(3) 22.85(7).
The CIPr–Ge bond length in IPr·GeCl2GeCl2 (1) is
2.032(5) ꢀ, which is shorter than the related distance in
IPr·GeCl2 (2.112(2) ꢀ),[10a] whereas the two GeCl2 units in
1 are linked in nearly eclipsed arrangements when viewed
down the Ge–Ge bond vector (for example, Cl1-Ge1-Ge2-Cl3
[*] Dr. S. M. I. Al-Rafia, M. R. Momeni, Dr. R. McDonald,
Dr. M. J. Ferguson, Prof. Dr. A. Brown, Prof. Dr. E. Rivard
Department of Chemistry, University of Alberta
11227 Saskatchewan Dr., Edmonton, Alberta, T6G 2G2 (Canada)
E-mail: erivard@ualberta.ca
Homepage: http://www.chem.ualberta.ca/~erivard/
[**] We thank NSERC of Canada, Alberta Innovates-Technology Futures,
and the Canada Foundation for Innovation (CFI) for financial
support and Compute/Calcul Canada for computational resources.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302767.
2
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torsion angle = 22.85(7)8). The Ge–Ge bond distance in 1 is
2.6304(9) ꢀ and longer than most typical Ge–Ge single bonds
(ca. 2.40–2.50 ꢀ);[14] moreover, the germanium atom within
the terminal GeCl2 group contains a lone pair (bond angle
sum at Ge = 276.36(9)8). The Cl3-Ge2-Cl4 angle [96.22(6)8] is
appreciably narrower than the related Cl-Ge-Cl angle at the
four-coordinate Ge(1) center [103.39(7)8], and is consistent
with the presence of a high degree of p character in the
terminal Ge–Cl bonds.
features Ge centers in formal oxidation states of 0, + 2, and
+ 3 and can be regarded as an adduct between IPr and the
perhaloisobutylene congener Cl2GeGe(GeCl3)2.
Compound 2 is a rare example of a species containing an
extended perhalogermane moiety[19] and is, to our knowledge,
the first polygermane synthesized through the controlled
sequential addition of germanium halides. X-ray crystallog-
raphy[12] (Figure 2) revealed that the dative CIPr–Ge inter-
action in 2 is quite similar in length (2.0024(5) ꢀ) to that
found in the digermene adduct 1, wherein all three core Ge–
In accordance with the lengthened Ge–Ge bond in
IPr·GeCl2GeCl2 (1), compound 1 decomposes in THF solvent
by Ge–Ge bond scission to give IPr·GeCl2 and presumably
Cl2Ge·THF (Scheme 1).[15] Attempts to prepare the donor–
acceptor digermene adduct IPr·GeH2GeH2·BH3 through
treatment of 1 with Li[BH4], gave the known GeII dihydride
[10a]
adduct IPr·GeH2·BH3
as the only soluble product.[16] In
a further demonstration of the lability of the terminal GeCl2
group, reaction of 1 with 2,3-dimethyl-1,3-butadiene cleanly
afforded the cycloadduct Cl2Ge(CH2CMe)2 and IPr·GeCl2
(Scheme 1).[17] Despite the tendency of the Ge–Ge bond in
1 to cleave in solution, compound 1 is stable in the solid state
under N2 up to ca. 1308C.
Figure 2. Thermal ellipsoid plot (30% probability level) of IPr·GeCl2Ge-
(GeCl3)2 (2); all hydrogen atoms and CH2Cl2 solvate have been omitted
for clarity. Selected bond lengths [ꢀ] and angles [8]: C(1)–Ge(1)
2.0024(5), Ge(1)–Cl(1) 2.1506(15), Ge(1)–Cl(2) 2.1734(16), Ge(1)–
Ge(2) 2.4983(8), Ge(2)–Ge(3) 2.4870(8), Ge(3)–Ge(4) 2.4987(8),
Ge(3)–Cl(3-5) 2.1549(18) to 2.1792(15), Ge(4)-Cl(6-8) 2.1525(19) to
2.1675(17); C(1)-Ge(1)-Ge(2) 117.46(14), Ge(1)-Ge(2)-Ge(3) 90.90(3),
Ge(1)-Ge(2)-Ge(4) 91.27(3), Ge(3)-Ge(2)-Ge(4) 89.20(3).
Ge bond lengths in 2 lie in the narrow range of 2.4870(8) to
2.4987(8) ꢀ. These latter distances are significantly con-
tracted with respect the Ge–Ge bond found in IPr-
·GeCl2GeCl2 (1; 2.6304(9) ꢀ) and are approaching the Ge–
Ge distances present in the neopentyl-shaped species
(Cl3Ge)4Ge (avg. of 2.420(6) ꢀ).[19d] The central germanium
atom of the Ge4 branch in 2 (Ge2) adopts a significantly
pyramidalized geometry (ꢀ8Ge(2) = 271.71(3)8) due to the
presence of a lone pair, while the average Ge-Ge-Ge angles at
Ge2 (90.46(3)8) indicate that a very high degree of p character
resides in these bonds.
Treatment of 2 with IPr, a strong s-donor, instigated an
unusual halide migration/Ge–Ge bond cleavage reaction to
regenerate IPr·GeCl2GeCl2 (1; Scheme 2). The detailed
mechanism of this transformation is unknown at this time,
but our preliminary theoretical investigations (see below)
suggest that isomerization of 2 to form related species, such as
the linear isomer IPr·(GeCl2)4, might be feasible. Attempts to
further grow the Ge chain by combining 2 with additional
equivalents of Cl2Ge·dioxane failed to yield any discernable
reaction. Our efforts to install hydride functionality onto the
Ge chain in 2 (to form germanium polyhydrides and/or
clusters)[8,20] by treatment with Li[BH4] led to the formation
of IPr·GeH2·BH3[10a] and germanium metal. We are currently
exploring related hydride transfer chemistry, as our prior
experiences have shown that the nature of the hydride source
Scheme 1. Representative chemistry of IPr·GeCl2GeCl2 (1). Dipp=2,6-
iPr2C6H3.
Given the successful synthesis of a carbene-supported
GeCl2–GeCl2 array,[13,18] we decided to investigate the syn-
thesis of higher germanium dichloride oligomers using
a similar strategy. When IPr·GeCl2GeCl2 (1) was combined
with either one or two additional equivalents of
GeCl2·dioxane in toluene, the branched Ge4 adduct,
IPr·GeCl2Ge(GeCl3)2 (2) was isolated in low to moderate
yield as a pale yellow solid (Scheme 2). Notably, compound 2
Scheme 2. Synthesis of IPr·GeCl2(GeCl3)2 (2) and carbene-induced
reversion to IPr·GeCl2GeCl2 (1). Dipp=2,6-iPr2C6H3, IPr=[(HCN-
Dipp)2CD].
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ꢁ
has a profound impact in dictating the success of E H bond
2.001(4) ꢀ and are slightly shorter than the respective
IPrCH –Ge distance in IPrCH2·GeCl2·W(CO)5
formation.[10,11]
C
2
Another dimension of reactivity to explore would be the
role of the Lewis base in directing Ge oligomer growth;
accordingly, we investigated germylene oligomerization in the
(2.056(3) ꢀ).[21] The less-hindered donor site in IPrCH2
relative to IPr enables three IPrCH2·GeCl2 units to bind to
a single Ge center, with added stabilization in the form of
lattice enthalpy resulting from the formation of a salt. The
formation of 4 shows that the Lewis basic scaffold has an
important role in dictating the outcome of germylene
oligomerization.
The above chemistry implies that, as the Ge content is
increased, branched (GeCl2)x structures become more stable
relative to linear systems. Given the wealth of data that
supports the thermodynamic preference for branching in
extended hydrocarbons,[9] we were eager to further explore
our Ge systems theoretically. We initiated our computational
studies by determining the relative energies of a series of
carbene-capped linear and branched oligomers, IPr·(GeCl2)x
(x = 2–4), both in the gas phase and with a CH2Cl2 solvation
model.[22] The results are summarized in Figure 4; in each
instance, there is a clear energetic preference for forming
branched structures once three Ge atoms are bound in
sequence.
As noted above, the Ge–Ge linkage in IPr·GeCl2GeCl2 (1;
2.6304(9) ꢀ) is much longer than the Ge-Ge bonds in
IPr·GeCl2Ge(GeCl3)2 (2; avg. of 2.4947(14) ꢀ). Accordingly,
natural bond orbital (NBO) analysis shows that the Wiberg
bond index (WBI) for the Ge–Ge bond in 1 is 0.69, whereas
much higher indices (0.90 to 0.92) are found in the branched
Ge4 array in 2; thus, branching appears to be partially driven
by the formation of stronger Ge–Ge bonds. In both adducts,
the CIPr–Ge bonds have similar WBI values (0.55 and 0.59,
presence of the nucleophilic olefin IPr CH2.[21] Starting from
=
the new adduct IPrCH2·GeCl2 (3),[22] we targeted the prep-
aration of higher polygermanes by the addition of
Cl2Ge·dioxane to 3. Instead of isolating the expected tetra-
chlorodigermene adduct, IPrCH2·GeCl2GeCl2, the unusual
donor-capped Ge4 dication [(IPrCH2·GeCl2)3Ge]2+ was
formed as part of the bis(trichlorogermate) salt 4 in 97%
yield [Eq. (2)].
As
illustrated
in
Figure 3,[12]
the
dicationic
[(IPrCH2·GeCl2)3Ge]2+ portion of compound 4 has a trigonal
pyramidal geometry at the central Ge1 atom derived from
three capping IPrCH2·GeCl2 groups and a lone pair. The
empirical formula of 4 consists of one IPrCH2 unit per two
GeCl2 fragments, and thus 4 can be considered a structural
isomer of 1 with the IPr donor replaced by IPrCH2. The
constituent Ge–Ge bond lengths within the Ge4 core are
nearly equivalent (avg. of 2.488(6) ꢀ) and similar in length to
the Ge–Ge distances in 2 (avg. of 2.4947(14) ꢀ); for
comparison, the Ge–Ge lengths in the hindered germyl
anion
[(Me3Ge)3Ge]ꢁ
were
found
to
average
2.4412(12) ꢀ.[23] The CIPrCH –Ge bonds in 4 average to
2
Figure 3. Thermal ellipsoid plot (30% probability level) of the
[(IPrCH2·GeCl2)3Ge]2+ dication in 4; selected hydrogen atoms, the
[GeCl3]ꢁ anions, and CH2Cl2 solvate have been omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: C(1)–Ge(2) 2.004(4), C(5)–
Ge(3) 1.996(4), C(9)–Ge(4) 2.004(4), Ge(1)–Ge(2) 2.4899(6), Ge(1)–
Ge(3) 2.4743(6), Ge(1)–Ge(4) 2.5005(6); Ge(2)-Ge(1)-Ge(3) 98.75(2),
Ge(2)-Ge(1)-Ge(4) 88.38(2), Ge(3)-Ge(1)-Ge(4) 95.61(2).
Figure 4. Computed [M06-2X/cc-pVDZ] relative electronic energies
(kcalmolꢁ1) for a series of carbene-bound (GeCl2)x oligomers (x=2–4)
showing a preference for Ge-chain branching at higher Ge content;
values in parentheses have been determined using a CH2Cl2 solvation
model.[22]
4
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respectively) and similar bond polarities (ca. 80% electron
density towards the donor C atoms), which is consistent with
dative CIPr–Ge interactions. In line with the data shown,
a high degree of p character is present within the Ge–Ge
bonds in 1, 2, and [(IPrCH2·GeCl2)3Ge]2+ leading to the
narrow bond angles observed.
development of novel Group 14 materials through bottom–up
methods.
Received: April 3, 2013
Published online: && &&, &&&&
Keywords: carbenes · density functional calculations ·
We further analyzed 1, 2, and the dication in 4 by AIM
(atoms in molecules). A nice correlation between the higher
WBI values noted for the Ge–Ge bonds in 2 with an increase
in the value of 1(r), which corresponds to the minimum
electron density along a bond path, was noted (Supporting
Information, Figure S6).[22] Moreover, a noticeable increase in
covalent character of the Ge–Ge bonds, which is related to
.
germanium · main group elements · oligomerization
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·GeCl2GeCl2GeCl2GeCl2, displayed
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what is observed for their ubiquitous hydrocarbon analogues.
The ability to readily form polyhalogermane structures in
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Figure 5. Computed [M06-2X/cc-pVDZ] free energy barrier of halide
migration/isomerization involving ImMe2·GeCl2GeCl2 in the gas phase,
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paper. These data can be obtained free of charge from The
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B(C6F5)3 (5); see the Supporting Information for more details.
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[22] See the Supporting Information for additional details. Three
different functionals were investigated, B3LYP, PBE0, and M06-
2X, with the M06-2X/cc-pVDZ functional/basis set combination
giving the most consistent results with respect to the exper-
imental data and (limited)CCSD(T)/cc-pVDZ results.
[23] J. Hlina, J. Baumgartner, C. Marschner, Organometallics 2010,
29, 5289.
2
[18] For
a
formal valence isomer of 1, NHC·GeCl–GeMes2Cl
[24] In AIM, covalent bonds have negative 5 1 values, whereas ionic
(NHC = N-heterocyclic carbene), see: a) P. A. Rupar, M. C.
Jennings, K. M. Baines, Organometallics 2008, 27, 5043; b) S.
Tashita, T. Watanabe, H. Tobita, Chem. Lett. 2013, 42, 43.
bonds adopt positive values: M. D. Esrafili, J. Mol. Model. 2012,
18, 2003.
[25] a) G. Trinquier, J. Am. Chem. Soc. 1990, 112, 2130; b) A. F.
Richards, A. D. Phillips, M. M. Olmstead, P. P. Power, J. Am.
Chem. Soc. 2003, 125, 3204.
[19] For halogenated polygermane chains, see: a) A. A. Espenbetov,
Yu. T. Struchkov, S. P. Kolesnikov, O. M. Nefedov, J. Organomet.
Chem. 1984, 275, 33; b) H. H. Karsch, B. Deubelly, J. Riede, G.
6
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