Intramolecular C-O Insertion of a Germanium(II) Salicyl Alcoholate: A Combined Experimental and Theoretical Study

Article abstract of DOI:10.1002/ejic.201501109

The synthesis of germanium(II) 2-tert-butyl-4-methyl-6-(oxidomethyl)phenolate (1) starting from Ge[N(SiMe₃)₂]₂ and the corresponding salicyl alcohol is reported. Compound 1 undergoes an intramolecular oxidative insertion reaction of germanium into a C-O bond to result in a cyclic germanium(IV) tetraoxidogermocane (2). Addition of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol to either compound 1 or 2 gave a spirocyclic monoorgano dioxagermine (3). The results of ¹H NMR spectroscopic studies and DFT-D calculations are in agreement with the proposed reaction cascade in which the novel germylene 1 is first converted into the germocane 2 followed by reaction with 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol to finally provide compound 3. Addition of 4-(dimethylamino)pyridine to a solution of germylene 1 resulted in the formation of an air-stable monomeric 1:1 complex (4). The characterization of compounds 1-4 by single-crystal X-ray diffraction analysis, thermal analysis, and ¹H NMR, ¹³C{¹H} NMR, and ATR-FTIR spectroscopy is presented. Germylene 1 underwent an intramolecular oxidative insertion of Ge into a C-O bond in solution to give cyclic germocanes 2. Addition of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol to 1 or 2 gave spirocyclic dioxagermine 3. ¹H NMR analysis and DFT calculations concur with the proposed reaction cascade. Intermolecular stabilization of 1 upon addition of DMAP provided a stable monomeric 1:1 complex.

Full text of DOI:10.1002/ejic.201501109

FULL PAPER
DOI:10.1002/ejic.201501109
Intramolecular C–O Insertion of a Germanium(II) Salicyl
Alcoholate: A Combined Experimental and Theoretical
Study
Philipp Kitschke,[a] Tobias Rüffer,[b] Marcus Korb,[b]
Heinrich Lang,[b] Wolfgang B. Schneider,[c] Alexander A. Auer,[c] and
Michael Mehring*[a]
Keywords: Germanium / Germylenes / Insertion / Density functional calculations
The synthesis of germanium(II) 2-tert-butyl-4-methyl-6-(ox- the novel germylene 1 is first converted into the germocane
idomethyl)phenolate (1) starting from Ge[N(SiMe3)2]2 and
the corresponding salicyl alcohol is reported. Compound 1
undergoes an intramolecular oxidative insertion reaction of
germanium into a C–O bond to result in a cyclic germani-
um(IV) tetraoxidogermocane (2). Addition of 3-tert-butyl-2-
hydroxy-5-methylbenzyl alcohol to either compound 1 or 2
gave a spirocyclic monoorgano dioxagermine (3). The results
of 1H NMR spectroscopic studies and DFT-D calculations are
in agreement with the proposed reaction cascade in which
2 followed by reaction with 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol to finally provide compound 3. Addition of
4-(dimethylamino)pyridine to a solution of germylene 1 re-
sulted in the formation of an air-stable monomeric 1:1 com-
plex (4). The characterization of compounds 1–4 by single-
crystal X-ray diffraction analysis, thermal analysis, and 1H
NMR, 13C{1H} NMR, and ATR-FTIR spectroscopy is pre-
sented.
been a prerequisite to obtaining stable compounds.[6] Alk-
oxide-based germylenes possessing less bulky ligands have
to be stabilized by additional intramolecular coordination,
for example, by using amines, as reported by Huang and
co-workers as well as recently by Heidemann and Mathur.[7]
Following the concept of intermolecular donor stabiliza-
tion, Wetherby et al. were able to synthesize the cyclic
germanium(II) aryloxide (S)-[Ge{O2C20H10(SiMe2Ph)2-
3,3Ј}{NH3}].[8] In addition, cyclic germanium(II) arylox-
ides possessing additional intramolecular coordination of
oxygen atoms have been reported for calixarene deriva-
tives.[9]
Introduction
Since Harris and Lappert reported on the synthesis of
Ge[N(SiMe3)2]2 in 1974[1] an ongoing and increasing inter-
est in low-valent germanium(II) molecules has been ob-
served, mainly attributed to their analogous carbene char-
acter. Recent studies on germylenes have focused on their
use as ligands in transition-metal complexes,[2] their poten-
tial in catalysis,[3] and their application as model com-
pounds in mechanistic studies of oxidative addition reac-
tions.[2d,4] The majority of these studies follow the concept
of Harris and Lappert, which is based on applying amide
ligands, usually possessing bulky substituents, to stabilize
the low-valent metal.[2e,5] Studies on germylenes based on
alkoxides have been less frequently reported in the litera-
ture. The use of a sterically demanding backbone in the
alkoxide/aryloxide ligand, mainly based on phenolates, has
Germylenes lacking additional stabilization are known
to undergo oxidative insertion reactions according to their
electron deficiency at the germanium atom. Thus, insertion
into a diverse range of bond types, such as H–X (X = H,[10]
CH2R,[11] CN,[4d] NH2,[10] N3,[4d] and PH2[4c]), C–Y (Y =
Cl,[12] Br,[13] and I[7b,13,14]), N–Br,[15] P–Cl,[16] and E–E (E =
[a] Technische Universität Chemnitz, Fakultät für
Naturwissenschaften, Institut für Chemie, Professur
Koordinationschemie,
S[7b,14a,14c] and O[14c]), as well as the oxidative addition of
[7b,14a]
Br2
have been reported. Noteworthy, to the best of
Straße der Nationen 62, 09111 Chemnitz, Germany
E-mail: michael.mehring@chemie.tu-chemnitz.de
www.tu-chemnitz.de/chemie/koord/
our knowledge, the insertion into a C–H bond mediated by
a Lewis acid to give a germaindane starting from bis(2,4,6-
tri-tert-butylphenyl)germylene is the only reported example
of an intramolecular insertion reaction for germyl-
enes.[11c,17]
Herein we report on the synthesis and reactivity of ger-
manium(II) 2-tert-butyl-4-methyl-6-(oxidomethyl)phenolate
(1), which is the first example of an unsymmetrical cyclic
[b] Technische Universität Chemnitz, Fakultät für
Naturwissenschaften, Institut für Chemie, Professur
Anorganische Chemie,
Straße der Nationen 62, 09111 Chemnitz, Germany
[c] Max-Planck-Institut für Chemische Energiekonversion,
Stiftsstraße 34–36, 45470 Mülheim an der Ruhr, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201501109.
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germanium(II) compound exhibiting both an aryloxide and = 2, 4) according to the quantity and multiplicity of these
an alkoxide moiety (Scheme 1). The dynamic coordination signals (see Figure S2). The single-crystal X-ray diffraction
behavior and the reactivity of compound 1 in the presence analysis revealed the formation of a trimer of germylene 1
as well as in the absence of 3-tert-butyl-2-hydroxy-5-meth- in the solid state [2(1)3·Et2O] that crystallizes in the mono-
ylbenzyl alcohol and 4-(dimethylamino)pyridine were clinic space group P21/c. The molecular structure is given
studied, revealing an intramolecular oxidation reaction that in Figure 1 and selected bond lengths and angles are pre-
leads to a cyclic tetraoxidogermocane (2), a spirocyclic di- sented in the caption. Details of the structure determination
oxagermine (3), and a 4-(dimethylamino)pyridine complex are summarized in Table 2.
1
(4; Scheme 1). H NMR spectroscopic studies and DFT-D
calculations (B3LYP-D3/def2-TZVPP level of theory) were
carried out to identify the reaction paths that compounds
1 and 2 follow to form the monoorgano germanium(IV)
alcoholate 3.
Figure 1. Molecular structure of (1)3 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms
have been omitted and aromatic moieties are depicted in wireframe
style for clarity. Selected bond lengths [Å]: Ge1–O1 1.945(2), Ge1–
O2 1.862(2), Ge1–O6 2.003(2), Ge2–O1 2.044(2), Ge2–O3 2.019(2),
Ge2–O4 1.890(2), Ge3–O3 2.015(2), Ge3–O5 1.829(3), Ge3–O6
1.956(2); selected bond angles [°]: Ge1–O1–Ge2 127.73(11), Ge3–
O3–Ge2 132.29(11), Ge3–O6–Ge1 112.79(11), O2–Ge1–O1
91.88(9), O2–Ge1–O6 97.24(9), O1–Ge1–O6 90.65(9), O4–Ge2–O3
91.32(9), O4–Ge2–O1 96.00(9), O3–Ge2–O1 83.22(9), O5–Ge3–O6
92.69(9), O5–Ge3–O3 95.95(9), O6–Ge3–O3 85.21(9).
Scheme 1. Synthesis of compounds 1–4. Germylene 1 forms a tri-
mer (n = 3) in the solid state, whereas an equilibrium (n = 2, 3, 4)
is observed in solution.
Results and Discussion
Syntheses and Characterization
The trimer (1)3 shows a distorted boat conformation of
Compound 1 was synthesized starting from Ge[N(SiMe3)2]2 the six-membered –[Ge1–O1–Ge2–O3–Ge3–O6]– ring. The
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol in n- germanium atoms are tricoordinated by one terminal phen-
pentane and isolated as a colorless solid in 83% yield olic oxygen atom and two bridging benzylic oxygen atoms.
(Scheme 1).
The germanium oxygen bond lengths [OAryl–Ge bonds:
Compound 1 is soluble in all common polar and nonpo- 1.829(2)–1.890(2) Å, OAlkyl–Ge bonds: 1.945(2)–2.044(2) Å]
lar organic solvents. Crystallization from a saturated diethyl are in the typical ranges reported for these types of bonds
ether solution gave crystals within 1 hour suitable for single- in compounds with a three-fold pyramidal coordination of
crystal X-ray diffraction analysis. 1H and 1H–13C{1H} low-valent germanium.[6b,6c,9a] The Ge1–O1 [1.945(2) Å],
HSQC NMR spectroscopic analyses of a freshly prepared Ge2–O3 [2.019(2) Å], and Ge3–O6 [1.956(2) Å] bond
solution of either crystalline or amorphous 1 in CDCl3 at lengths are shorter than the Ge1–O6 [2.003(2) Å], Ge2–O1
ambient temperature gave resonance signals (three sets of [2.044(2) Å], and Ge3–O3 [2.015(2) Å] bond lengths, respec-
signals assigned to the tBu, Me, and CH2 groups, respec- tively, which indicates that the secondary bonding of
tively, and six signals assigned to aromatic CH groups) that O1ǞGe2, O3ǞGe3, and O6ǞGe1 stabilize the low-valent
are in agreement with the trimeric structure observed in the species. The small O–Ge–O bond angles of the salicylic
solid state (Figure 1). One set of additional resonance sig- moieties [O2–Ge1–O1 91.88(9)°, O4–Ge2–O3 91.32(9)°,
nals of lower intensity possessing similar chemical shifts and O5–Ge3–O6 92.69(9)°] as well as the angles [ЄO1–O3–
and integral ratios was also observed, which indicates the Ge2–O4 89.146(2)°, ЄO3–O5–Ge3–O6 90.580(2)°, and
presence of at least one additional oligomer of germylene 1 ЄO6–O2–Ge1–O1 95.567(2)°] between the additionally co-
(see Figure S1 in the Supporting Information). Cooling the ordinating benzylic oxygen atoms (O1ǞGe2, O3ǞGe3, and
CDCl3 solution of 1 to –60 °C resulted in two independent O6ǞGe1) and the planes spanned by the germanium atoms
sets of additional resonance signals of lower intensity, and the oxygen atoms of the salicylic moiety (O3–Ge2–O4,
which were assigned to symmetrical oligomers of 1 (e.g., n O5–Ge3–O6, and O2–Ge1–O1) are in agreement with low-
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valent germanium possessing a stereochemically active lone
pair of electrons. The bridging benzylic oxygen atoms do-
nate electron density into the vacant p orbitals. The larger
deviation of the angle ЄO6–O2–Ge1–O1 [95.567(2)°] from
the ideal value of 90° might result from repulsion effects of
the sterically demanding salicylic moieties that are in close
proximity to each other.
Attempts to crystallize compound 1 from a 1,4-dioxane
solution at ambient temperature gave [2·(C4H8O2)2]·
3C4H8O2 in an overall yield of 80% after 2 weeks. Similarly,
compound 2 was also isolated starting from a solution of
germylene 1 in diethyl ether in 67% yield after 5 days
1
(Scheme 1). H NMR spectroscopic analysis of 2, which is
soluble in common organic solvents, gave multiple reso-
nance signals at chemical shifts centered at δ = 1.4, 2.2, 2.5,
and 6.9 ppm with integral ratios of 9:3:2:2 assigned to the
tBu, Me, CH2, and CH groups, respectively. The carbon
atoms of the methylene groups are located at δ = 15.3 ppm,
1
as determined by H–13C{1H} HSQC and 13C{1H} NMR
spectroscopy. Analysis of the thermal behavior of the pow-
der obtained from crystalline [2·(C4H8O2)2]·3C4H8O2 on
drying under an inert atmosphere (see Figures S3 and S4 in
the Supporting Information) revealed that two molecules of
1,4-dioxane are released at 98 °C followed by melting with
an onset temperature of 159 °C and decomposition at
260 °C by endothermic processes. Compound [2·(C4H8O2)2]·
Figure 2. Molecular structure of [2·(C4H8O2)2] in the solid state.
Thermal ellipsoids are drawn at the 50% probability level.
Hydrogen atoms have been omitted for clarity. The two weakly
coordinated solvent molecules and the aromatic moieties are de-
picted in wireframe style for clarity. Selected bond lengths [Å]:
Ge1–O1 1.832(6), Ge1–O2 1.755(6), Ge1–O4ЈЈ 1.761(5), Ge1–O8
2.447(6), Ge1–C7 1.913(8), Ge2–O2 1.730(6), Ge2–O3 1.805(6),
Ge2–O4 1.744(6), Ge2–C19 1.921(8); selected bond angles [°]: O1–
Ge1–O2 102.9(3), O1–Ge1–O4ЈЈ 103.4(3), O1–Ge1–C7 92.9(3),
O2–Ge1–O4ЈЈ 109.2(3), O2–Ge1–C7 122.2(3), O4ЈЈ–Ge1–C7
120.5(3), O1–Ge1–O5 173.4(2), O2–Ge2–O3 103.3(3), O2–Ge2–O4
110.0(3), O2–Ge2–C19 120.2(3), O3–Ge2–O4 107.3(3), O3–Ge2–
C19 95.0(3), O4–Ge2–C19 117.9(3), Ge1–O2–Ge2 133.4(3). Sym-
metry transformation used to generate equivalent atoms: Ј –x +
2, –y, –z + 1; ЈЈ –x + 2, –y + 2, –z.
¯
3C4H8O2 crystallizes in the triclinic space group P1. Its mo-
lecular structure is given in Figure 2 and selected bond
lengths and angles are presented in the caption. Details of
the structure determination are summarized in Table 2.
Compound 2 is monomeric in the solid state, possessing
a corrugated eight-membered –[Ge1–O2–Ge2–O4–Ge1Ј–
O2Ј–Ge2ЈЈ–O4ЈЈ]– ring. Germasesquioxanes and cyclic di-
organo germanium oxides have been reported to exhibit
similar structural motifs of eight-membered –[Ge–O]4– cy- (Ph2GeO)4,[18a]
(tBu2GeO)3,[19]
2tBu2Ge(OH)2·(tBu2-
cles.[18] The molecular structure of [2·(C4H8O2)2] exhibits Ci GeOH)2O·H2O,[20] [Ge(L1)2]2 (L1 = cis-1,2-cyclopentanedi-
symmetry with the center of inversion located in the plane olate), and Ge(OCH3)(L2)(L3)[21] (L2 = cis-oxolane-3,4-di-
spanned by the four germanium atoms. The equivalent ger- olate, L3 = cis-oxolane-3-ol-4-olate). Note that the distances
manium atoms Ge1 and Ge1Ј are pentacoordinated, show- between the oxygen atoms of the coordinating 1,4-dioxane
ing a distorted trigonal-bipyramidal coordination sphere by molecules and the germanium atoms [Ge1–O5 2.447(6) Å]
bonding to two bridging oxygen atoms and a carbon atom are significantly shorter than the sum of their van der Waals
in the equatorial positions, and a phenolic oxygen atom and radii (3.63 Å),[22] and thus heat treatment at temperatures
an additional oxygen atom from 1,4-dioxane in the axial above 98 °C is necessary to remove the coordinated mol-
positions, respectively. The second set of equivalent germa- ecules.
nium atoms (Ge2 and Ge2ЈЈ) are surrounded by two bridg-
The formation of compound 3 starting from germylene
ing oxygen atoms, an oxygen, and a carbon atom of the 1 was detected by NMR spectroscopic monitoring of a
aromatic moiety resulting in a distorted tetrahedral coordi- solution of 1 in CDCl3 over several days without rigorous
nation. The germanium–oxygen bond lengths of the bridg- exclusion of moisture. The dioxagermine 3 results from an
ing oxygen atoms are slightly shorter for the oxygen atoms intramolecular oxidative insertion reaction of the germylene
bound to Ge2 [Ge2–O2 1.730(6) Å, Ge2–O4 1.744(6) Å] 1 and subsequent reaction with 3-tert-butyl-2-hydroxy-5-
than for those bound to Ge1 [Ge1–O2 1.755(6) Å, Ge1– methylbenzyl alcohol. The formation of 3-tert-butyl-2-
O4ЈЈ 1.761(5) Å]. The latter holds for the germanium–oxy- hydroxy-5-methylbenzyl alcohol results from the partial
gen bonds connected to the aromatic moiety [Ge1–O1 hydrolysis of 1 due to the presence of traces of moisture.
1.832(6) Å compared with Ge2–O3 1.805(6) Å], whereas the Pristine 3 was obtained as colorless needles after stirring
germanium–carbon bonds Ge1–C7 and Ge2–C19 were de- compound 1 under reflux for 21 h in 1,4-dioxane, work-up,
termined to have bond lengths of 1.913(8) and 1.921(8) Å, and crystallization from 1,4-dioxane/CH2Cl2. The equi-
respectively. However, all the bond lengths are in agreement molar addition of 3-tert-butyl-2-hydroxy-5-methylbenzyl
with those found in germanium(IV) compounds, such as alcohol to a solution of germylene 1 in diethyl ether stored
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over molecular sieves gave compound 3 in 70% yield. Simi- agreement with the respective positions of the bridging oxy-
larly, the reaction of germocane 2 with 4 equiv. of 3-tert- gen atoms (axial position with longer bond length; equato-
butyl-2-hydroxy-5-methylbenzyl alcohol in diethyl ether rial position with shorter bond length). The same trend is
stored over molecular sieves provided the spirocyclic com- observed on comparing the bond lengths between the phen-
pound 3 in 41% yield (Scheme 1). Two sets of characteristic olic oxygen atoms and the germanium atom; the Ge1–O3
AB resonance signals for the diastereotopic methylene pro- bond [1.8324(15) Å, axial position] is significantly longer
tons of compound 3 at δ = 2.59–2.64 and 5.04–5.12 ppm than the Ge1–O2 bond [1.7779(15) Å, equatorial position].
assigned to CH2Ge and CH2O, respectively, are present in A similar structural motif exhibiting an equivalent ordering
its 1H NMR spectrum. The respective methylene carbon of the oxygen–germanium bond lengths has been reported
atoms resonate at chemical shifts of δ = 13.1 (CH2Ge) and for [Ge(L1)2]2 by Klüfers and Vogler.[21] All bond lengths
67.2 ppm (CH2O), as determined by 1H–13C{1H} HSQC are within typical ranges for germanium(IV) compounds
and 13C{1H} NMR spectroscopy. Note that compound 3 possessing Ge–C and Ge–O bonds, as reported for
exhibits an equilibrium between its monomer and dimer (tBu2GeO)3,[19]
2tBu2Ge(OH)2·(tBu2GeOH)2O·H2O,[20]
(3)2 in solution, as determined by temperature-dependent [Ge(L1)2]2, and Ge(OCH3)(L2)(L3).[21]
NMR spectroscopy (see Figure S5 in the Supporting Infor-
As outlined above, germylene 1 is not stable in solution
mation). Analysis of the thermal behavior of 3 (see Fig- and hence undergoes intramolecular C–O insertion. How-
ures S3 and S4) revealed an exothermic decomposition in- ever, we believed that it might be stabilized by the addition
stantly after melting at 213 °C. Compound 3, which is solu- of donor ligands, as reported for other germylenes.[5c,23]
ble in common organic solvents, crystallizes from 1,4-diox- Thus, stoichiometric amounts of 4-(dimethylamino)pyr-
ane/CH2Cl2 as a dimer in the monoclinic space group idine were added to a solution of 1 and finally compound
P21/n. The molecular structure of (3)2 is given in Figure 3 4, as the corresponding 4-(dimethylamino)pyridine complex
and selected bond lengths and bond angles are presented (1:1), was isolated in 75% yield (Scheme 1). Compound 4
in the caption. Details of the structure determination are neither decomposes in air in the solid state for at least
summarized in Table 2.
1 week, as determined by ATR-FTIR spectroscopy, nor
does it undergo any reaction in solution (diethyl ether and/
or CDCl3) at ambient temperature, even in the presence of
stoichiometric amounts of 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol. However, it decomposes in toluene solu-
tion at elevated temperatures (100 °C) to give 4-(dimeth-
ylamino)pyridine and germocane species such as 2. The
presence of moisture in CDCl3 solutions of complex 4 re-
sulted in the formation of spirocyclic germanium(IV) com-
pound 3. NMR spectroscopic analysis of the germylene
1
complex 4 gave 8 and 14 resonance signals in the H and
13C{1H} NMR spectra, respectively. The multiplicity
pattern, in addition to the small widths of half signal height
in the 1H NMR spectrum, indicates that the complex is
monomeric in solution, in accordance with its molecular
structure in the solid state. Analysis of the thermal behavior
(see Figures S3 and S4 in the Supporting Information) re-
vealed an exothermic decomposition instantly after melting
at 143 °C. Germanium(II) complex 4 crystallizes from di-
ethyl ether in the monoclinic space group P21/n. Its molecu-
lar structure is given in Figure 4 and selected bond lengths
and bond angles are presented in the caption. Details of the
structure determination are summarized in Table 2.
Figure 3. Molecular structure of (3)2 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms
have been omitted and aromatic moieties are depicted in wireframe
style for clarity. Selected bond lengths [Å]: Ge1–O1 2.1248(15),
Ge1–O2 1.7779(15), Ge1–O3 1.8324(15), Ge1–O1Ј 1.8311(15),
Ge1–C19 1.935(2); selected bond angles [°]: O1–Ge1–O2 88.45(6),
O1–Ge1–O3 170.35(6), O1–Ge1–O1Ј 74.81(7), O1–Ge1–C19
93.68(8), O2–Ge1–O3 95.09(7), O2–Ge1–O1Ј 107.04(7), O2–Ge1–
C19 123.25(8), O3–Ge1–O1Ј 95.56(6), O3–Ge1–C19 91.87(8), C19–
Ge1–O1Ј 128.19(9). Symmetry transformation used to generate
equivalent atoms: Ј –x + 1, –y, –z + 1.
Complex 4 is monomeric in the solid state, possessing
pyramidal geometry at the germanium atom. The Ge1–O1
[1.8151(16) Å], Ge1–O2 [1.8765(15) Å], and Ge1–N1
[2.0990(18) Å] bond lengths are in good agreement with val-
The dimer (3)2 exhibits Ci symmetry with the center of ues reported for the germylene complex [Py(CH2C-
inversion located in the plane spanned by the Ge1, O1, Ph2O)(CH2CMe2O)Ge] [Ge–O(CMe2CH2) 1.827(1) Å, Ge–
Ge1Ј, and O1Ј atoms. The equivalent benzylic oxygen atoms O(CPh2CH2) 1.881(1) Å, and Ge–N 2.110(1) Å].[7c] The re-
O1 and O1Ј bridge the germanium atoms, which possess a cently reported 4-(dimethylamino)pyridine complex of
distorted trigonal-bipyramidal coordination formed of four GeCl2 exhibits a similar Ge–Npyridine bond length of
oxygen atoms and one carbon atom in an equatorial posi- 2.028(2) Å.[23a] The Ge1–O1 bond (monodentate benzylic
tion. The Ge1–O1 [2.1248(15) Å] bond is significantly oxygen) is shorter than the Ge1–O2 bond (monodentate
longer than the Ge1–O1Ј [1.8311(15) Å] bond, which is in phenolic oxygen) and the corresponding bonds of the
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emplified by the isolation of germocane 2. In the presence
of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol, which
may result from partial hydrolysis, reaction to give the spi-
rocyclic germanium(IV) compound 3 was observed. To
identify the reaction paths by which 1 is converted into
compounds 2 and/or 3, 1H NMR spectroscopic studies and
DFT-D calculations were carried out.
1H NMR Spectroscopic Studies
1
Four H NMR spectroscopic experiments were carried
Figure 4. Molecular structure of the germylene complex 4 in the
solid state. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted and aromatic moieties
are depicted in wireframe style for clarity. Selected bond lengths
[Å]: Ge1–O1 1.8151(16), Ge1–O2 1.8765(15), Ge1–N1 2.0990(18);
selected bond angles [°]: O1–Ge1–O2 96.04(7), O1–Ge1–N1
94.73(7), O2–Ge1–N1 88.75(7).
out to study the reactions. Freshly prepared [D8]toluene
solutions of i) pure germylene 1, ii) an equimolar mixture of
1 and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol, iii) a
mixture of germocane 2 and 4 equiv. of 3-tert-butyl-2-hy-
droxy-5-methylbenzyl alcohol, and iv) a solution of com-
pound 3 in water-saturated CDCl3 were monitored for at
least 136 h (Scheme 2).
germylene (1)3 [Ge1–O1 1.945(2) Å, Ge2–O3 2.019(2) Å,
Experiment i) revealed a slow conversion of germylene 1
and Ge3–O6 1.956(2) Å] possessing bridging benzylic oxy- into germocane species (98%) within 190 h (see Figure S6
gen atoms. The bond angles ЄO1–Ge1–O2 [96.04(7)°], in the Supporting Information). The formation of minor
ЄO1–Ge1–N1 [94.73(7)°], and ЄO2–Ge1–N1 [88.75(7)°; amounts of compound 3 (2%) was also determined in ex-
Σ 279.52(21)°] are in agreement with a stereochemically periment i) due to the high sensitivity of germylene 1
active lone pair of electrons at germanium and donation of towards hydrolysis, which gives 3-tert-butyl-2-hydroxy-5-
additional electron density from the pyridine nitrogen atom methylbenzyl alcohol. The latter reacts with compound 1
into the vacant p-type orbital.
to give the spirocyclic compound 3 (see above). Analysis of
In summary, the germylene 1 is trimeric in the solid state the evolution of the resonance signals assigned to com-
and shows dynamic coordination behavior in solution. It is pounds 1–3 revealed that 3 is initially formed at the same
quite stable in the presence of donors, but smoothly con- time as the germocanes. However, the formation of com-
verts into the germanium(IV) species by intramolecular in- pound 3 ceased as the conversion of germylene 1 into germ-
sertion of germanium into the benzylic C–O bonds, as ex- ocanes progressed (see Figure S6).
Scheme 2. Experiments monitored by 1H NMR spectroscopy in [D8]toluene for reactions (1)–(3) and CDCl3 for reaction (4) with the
final product ratios determined after 1) 190 h, 2) 185 h, 3) 16 min, and 4) 19 min.
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A slow conversion of germylene 1 in the presence of 3-
tert-butyl-2-hydroxy-5-methylbenzyl alcohol into germo-
cane species such as 2 (4%) and 3 (16%) was observed in
experiment ii) within 185 h (see Figure S5 in the Supporting
Information). As the resonance signals assigned to germo-
cane species and compound 3 increased in intensity, the in-
tensities of the signals assigned to compound 1 and 3-tert-
butyl-2-hydroxy-5-methylbenzyl alcohol decreased. Com-
parison of the intensities of the resonance signals centered
at δ = 2.2 (CH2Ge groups of germocanes and 3) and
4.8 ppm (CH2O groups of 3) after different intervals of time
indicated that the germocane species and compound 3 must
be formed on similar timescales. Note, the ratio of 3-tert-
butyl-2-hydroxy-5-methylbenzyl alcohol first decreased to
47% at 79 h and then increased continuously up to 66%
after 185 h. In addition, resonance signals assigned to water
appeared and increased steadily as the reaction mixture was
monitored. The final mixture of experiment ii) consisted of
germocane species (4%), compound 3 (16%), 3-tert-butyl-
2-hydroxy-5-methylbenzyl alcohol (66%), and water (14%).
Note that the addition of 2 equiv. of 3-tert-butyl-2-hydroxy-
5-methylbenzyl alcohol qualitatively gave a similar pro-
gression of resonance signals and a final mixture of germo-
cane species (2%), compound 3 (16%), 3-tert-butyl-2-hy-
?>droxy-5-methylbenzyl alcohol (70%), and water (12%),
which is indicative of an equilibrium (see the discussion below).
Germocane 2 reacted immediately to give 3 in the pres-
ence of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol, as
determined in experiment iii). The reaction mixture from
experiment iii) consisted of germocane species (46%), com-
Scheme 3. Proposed reaction sequence for the formation of dif-
ferent germocane species. a) Reaction of 2 with 3-tert-butyl-2-
hydroxy-5-methylbenzyl alcohol to form 3, b) hydrolysis of 3 to
give the oxagermolediol A, and c) condensation of A to give germo-
cane species.
The generated water subsequently reacts with the germa-
nium alcoholates [compounds 1 and 3 or at least 3 in the
case of experiment iii), Scheme 3, b] by fast hydrolysis.[24]
Thus, 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol is re-
covered, which explains the significantly larger proportion
of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol in the
product ratios, even larger than in the initial reaction mix-
ture of experiment ii). This is further supported by the ob-
servation that the content of 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol initially decreases, but recovers over the
time of experiment ii). Moreover, the hydrolysis of 3 leads
to 7-(tert-butyl)-5-methylbenzo[d][1,2]oxagermole-2,2(3H)-
diol (A), which undergoes condensation to result again in
the formation of germocane species (Scheme 3, c). Hence-
forward, these germocanes may also react with 3-tert-butyl-
2-hydroxy-5-methylbenzyl alcohol to give compound 3,
causing equilibration between the reactions illustrated in
Scheme 3. The latter is supported by the observation that
repetitions of experiments ii) and iii) providing an excess of
3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol led only to a
modification of the ratios of the products but did not
change the compositions of the final mixtures.
pound
3 (10%), 3-tert-butyl-2-hydroxy-5-methylbenzyl
alcohol (36%), and water (8%). It is noteworthy that the
addition of 6 equiv. of 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol gave a mixture of germocane species
(41%), compound 3 (12%), 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol (41%), and water (8%).
The spirocyclic germanium(IV) compound 3 was hy-
drolyzed in the presence of water with the simultaneous for-
mation of germocanes, as observed in experiment iv). The
reaction mixture consisted of germocane species (26%),
compound 3 (18%), 3-tert-butyl-2-hydroxy-5-methylbenzyl
alcohol (38%), and water (18%) after 19 min. The composi-
tion of the mixture was only marginally altered later on,
which is again indicative of an equilibrium.
In summary, germylene 1 is converted into germocane
species such as 2 in the absence of 3-tert-butyl-2-hydroxy-5-
methylbenzyl alcohol. The formation of water was observed
upon the conversion of either germylene 1 or germocane 2
DFT-D Calculations
DFT-D calculations at the B3LYP-D3/def2-TZVPP level
into compound 3 in the presence of 3-tert-butyl-2-hydroxy- of theory were carried out to study the reactions of com-
5-methylbenzyl alcohol, with germocane species remaining pound 1 to give 2–4. The relative energies of 2–4 with re-
in the mixtures. In addition, the formation of germocanes spect to 1 and the proposed intermediates of two reaction
was detected as compound 3 underwent hydrolysis. These paths that may explain the formation of the Ge–CH2 bond
observations may be explained as follows.
and consequently the formation of compounds 2 and 3
First, germocanes are initially formed by the slow con- starting from 1 were examined (Schemes 4, 5, and Table 1).
version reaction of germylene 1. Water is formed by the It is assumed for reaction path I that monomeric 1 reacts
reaction of germocane species, for example, in the reaction to give a germanone (B) by intramolecular insertion
of compound 2 with 3-tert-butyl-2-hydroxy-5-methylbenzyl (Scheme 5, a) to form the Ge–CH2 bond. Then intermedi-
alcohol to give compound 3 (Scheme 3, a).
ate B reacts to give 2 or, in the presence of 3-tert-butyl-2-
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Scheme 5. Illustration of the proposed reaction paths with the rela-
tive energies of compounds 1–4, the oligomers (1)n (n = 1–5), ger-
manone B, species (C)n (n = 1–5; compound 2, n = 4), and the
dimer (3)2 calculated at the B3LYP-D3/def2-TZVPP level of theory
to study the conversion of 1 into 2–4.
Scheme 4. Illustration of presumed rearrangement processes (blue
arrows) that explain the formation of the Ge–CH2 bond by intra-
molecular C–O insertion reaction of a) germylene 1 to give ger-
manone B and b) (1)4 to give germocane (C)4 = 2.
Table 1. Relative energies (ΔE) and structures of compounds 1–4, the oligomers (1)n (n = 1–5), germanone B, species (C)n (n = 1–5;
compound 2, n = 4), and the dimer (3)2 calculated at the B3LYP-D3/def2-TZVPP level of theory.
[a] Relative energies (ΔE) calculated by taking into account the total energies of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol and water
calculated at the B3LYP-D3/def2-TZVPP level of theory. [b] Relative energy (ΔE) calculated by taking into account the total energy of
4-(dimethylamino)pyridine calculated at the B3LYP-D3/def2-TZVPP level of theory.
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hydroxy-5-methylbenzyl alcohol, 3. It is assumed for reac- plex 4 upon heating into 4-(dimethylamino)pyridine and
tion path II that monomeric 1 first oligomerizes to give germylene 1, which may further react to give germocane
(1)n (n = 1–5) before Ge–CH2 bonds are formed by intra- (C)4, as determined experimentally (see above), is quite
molecular insertion reactions [exemplarily shown in likely in view of its moderate energy of formation
Scheme 4 (b) for the conversion of (1)4 into (C)4 = 2] to (–91.0 kJmol–1).
result in species (C)n (n = 1–5). In the case of the conversion
of 1 into 2, no further reaction occurs [on the condition of
tetramer formation, (C)4 = 2]. The formation of 3 is ex-
Conclusions
plained by the condensation of species (C)n with 3-tert-
butyl-2-hydroxy-5-methylbenzyl alcohol.
The first representative of an unsymmetrical cyclic
germylene possessing both an aryloxide as well as an alk-
oxide moiety as part of a heterocycle, namely germani-
um(II) 2-tert-butyl-4-methyl-6-(oxidomethyl)phenolate (1),
has been reported herein. The germylene 1 is not stable in
solution and converts into a cyclic germanium(IV) com-
The calculated formation energy of the intermediate B is
endothermic by 109.4 kJmol–1 (reaction path I), probably
due to the formation of an unfavorable Ge=O bond.[25] In
contrast, the reaction steps of reaction path II are all ener-
getically favored (Scheme 4 and Table 1). Therefore, we sup-
pose that reaction path I can be ruled out. In the case of
the first reaction step of reaction path II, the formation of
the tetramer (1)4 (–85.5 kJmol–1) was found to be energeti-
cally favored in comparison with the formation of the dimer
(1)2 (–55.9 kJmol–1), trimer (1)3 (–79.0 kJmol–1), and pen-
tamer (1)5 (–77.0 kJmol–1). Note, the small energy differ-
ences between the oligomers (1)n indicate that they may be
in chemical equilibrium in the solution phase. The equilib-
rium between the oligomers (1)n may depend on the specific
conditions of the solution (e.g., polarity of the solvent and/
or temperature). We assume that at least the tetramer (1)4
rather than the dimer (1)2 is in equilibrium with the trimer
(1)3 in CDCl3 solution based on our experimental results
(see above). Following reaction path II, the conversions of
the oligomers (1)n into (C)n exhibiting Ge–CH2 bonds are
exothermic. Species (C)5 (–144.4 kJmol–1) was calculated to
be the energetically slightly favored species. However,
(C)4 (–139.8 kJmol–1) is similar in energy and was obtained
experimentally [(C)4 Z germocane 2]. Please note that this
contradiction may be easily comprehensible bearing in
mind the level of theory applied. An estimate of the uncer-
tainty in the calculated energies easily amounts to 5–
10 kJmol–1, as the results were obtained by using DFT
methods and furthermore do not include the influence of
solvent. In addition, the conversion of tetramer (1)4 into
(C)4 by intramolecular oxidative insertion reactions may
have lower reaction barriers in comparison with the conver-
sion reactions of other oligomers [e.g., (1)5 Ǟ(C)5], which
may kinetically favor the formation of germocane (C)4. The
reaction of (C)4 (–139.8 kJmol–1) with 3-tert-butyl-2-
hydroxy-5-methylbenzyl alcohol to give monomeric 3
(–89.3 kJmol–1) is endothermic, but if the dimerization of
3 to give (3)2, as is observed in solution and in the solid
state (see above) is taken into account, the conversion of
(C)4 into 3 is energetically favorable. Thus, reaction path II
is in agreement with our experimental results. It is also
worth noting that the energy of compound 4 was calculated
to be –91.0 kJmol–1. Hence, the formation of complex 4
upon intermolecular donor stabilization is energetically fa-
vored compared with the oligomerization of 1 and thus pro-
hibits the conversion of the germylene 1 into the germocane
(C)4, in accordance with experimental results, at least at am-
pound,
2,4,6,8-tetrakis(3-tert-butyl-5-methyl-2-oxido-
phenyl)methanide-1,3,5,7,2,4,6,8-tetraoxidogermocane (2),
by intramolecular insertion of germanium into the benzylic
C–O bond to form a Ge–CH2 bond. In the presence of 3-
tert-butyl-2-hydroxy-5-methylbenzyl alcohol, the spirocyclic
germanium(IV) compound, 7,8Ј-di-tert-butyl-5,6Ј-dimethyl-
3H,4ЈH-spiro[benzo[d][1,2]oxagermole-2,2Ј-benzo[d][1,3,2]-
dioxagermine] (3), was obtained starting from either
germylene 1 or germocane 2. Compound 3, which exhibits a
Ge–CH2 bond in its molecular structure, is a condensation
product of germocane 2 with 3-tert-butyl-2-hydroxy-5-
methylbenzyl alcohol. Experimental studies and DFT-D
calculations were carried out to account for the formation
of germocane 2 and spirocyclic 3 starting from germylene
1. Based on our results, we suggest the following. 1) Com-
pound 1 forms oligomers with (1)3 being the major species
in chemical equilibrium in solution. 2) Intramolecular in-
sertion reactions of the oligomers (1)n slowly lead to cyclic
germocane species in solution with the formation of a tetra-
nuclear cyclic germocane –[Ge–O]4– species (2) favored.
3) The formation of –[Ge–O]4– (2) requires the intermediate
formation of (1)4, which is a minor species in solution. 4) In
the presence of 3-tert-butyl-2-hydroxy-5-methylbenzyl
alcohol, germocane 2 reacts by a condensation reaction
with the alcohol to give the spirocyclic compound 3.
5) Intermolecular donor stabilization of germylene 1 upon
addition of 4-(dimethylamino)pyridine prohibits oligomer-
ization and thus the conversion reaction, at least at ambient
temperature, as a result of the formation of the stable 1:1
complex 4. In conclusion, a novel, highly reactive germylene
has been accessed that shows unprecedented intramolecular
C–O insertion of germanium(II). The resulting monoor-
gano germocane is prone to condensation reactions with 3-
tert-butyl-2-hydroxy-5-methylbenzyl alcohol, finally re-
sulting in equilibration between germocanes, monoorgano
germanium
alkoxides,
3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol, and water. Similar reactivity might be ex-
pected for other salicyl alcoholates with elements in their
low-valent state.
Experimental Section
General: All reactions were performed under argon using Schlenk
bient temperature. In addition, the decomposition of com- techniques or in a glovebox. Solvents were purified and dried by
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applying standard techniques prior to use. All reactions were car-
ried out with freshly distilled, dried solvents. Experiments involving
freshly activated molecular sieves were performed without stirring.
1H, 13C{1H}, and 1H–13C{1H} HSQC NMR spectra were recorded
with a Bruker Avance III 500 spectrometer. CDCl3 was dried with
O), 1153 (ν C–C), 864, 815, 797 (δ Caryl–H, C6H2 backbone vi-
brations), 677, 602 (ν Ge–O), 556 (O–Ge–O) cm–1. C12H16GeO2
(264.85): calcd. C 54.41, H 6.09; found C 53.91, H 6.55. Single
crystals suitable for X-ray diffraction analysis were obtained by
evaporation of a saturated solution of compound 1 in diethyl ether
within 1 hour at ambient temperature.
1
molecular sieves. H NMR spectroscopic studies were carried out
under inert conditions in sealed NMR tubes using either freshly
prepared [D8]toluene dried with potassium or CDCl3. 1H NMR
spectra were recorded after preparation and at reasonable intervals.
ATR-FTIR spectra were recorded with a BioRad FTS-165 spec-
trometer. Melting points were determined with a B-540 melting
point apparatus from Büchi. CHN analyses were determined by
using a FlashEA 1112 NC Analyzer from Thermo Fisher Scientific.
DSC experiments were determined with a Mettler Toledo DSC 30
instrument using 40 μL aluminium crucibles. The measurements
were taken up to 400 °C at a heating rate of 10 Kmin–1 in N2 and
at a volume flow of 50 mLmin–1. TGA/DSC experiments were de-
termined with a Mettler Toledo TGA/DSC1 1600 system with an
MX1 balance in the range of 40–800 °C at a heating rate of
10 Kmin–1 in Ar and at a volume flow of 60 mLmin–1.
2,4,6,8-Tetrakis(3-tert-butyl-5-methyl-2-oxidophenyl)methanide-
1,3,5,7,2,4,6,8-tetraoxidogermocane (2)
Method a: A solution of germylene 1 (23.0 mg, 8.68ϫ10–2 mmol)
in 1,4-dioxane (5 mL) was stirred at ambient temperature for
30 min. Colorless crystals of [2·(C4H8O2)2]·3C4H8O2 were filtered
off after 2 weeks. These crystals were used for X-ray diffraction
analysis. Removal of the residual solvent of the filtrate under re-
duced pressure (10–2 mbar) gave [2·(C4H8O2)2] as a colorless solid,
1
yield 18.4 mg, 80%; m.p. 159–167 °C (decomp. at ca. 250 °C). H
NMR (500 MHz, CDCl3, 25 °C, TMS): δ = 1.38 (s, 9 H, tBu), 1.41
(s, 9 H, tBu), 1.42 (s, 9 H, tBu), 1.43 (s, 9 H, tBu), 2.22 (s, 3 H,
Me), 2.23 (s, 3 H, Me), 2.24 (s, 3 H, Me), 2.24 (s, 3 H, Me), 2.46–
2.62 (m, 8 H, CH2), 3.70 (s, 16 H, CH2, 1,4-dioxane), 6.93 (d, 4Jmeta
4
= 0.6 Hz, 4 H, C6H2), 6.94 (d, Jmeta = 0.6 Hz, 4 H, C6H2) ppm.
Germanium(IV) chloride and 1,1,1,3,3,3-hexamethyldisilazane
were purchased from ABCR GmbH & Co KG. n-Butyllithium
(2.5 m) and 2-tert-butyl-4-methylphenol were purchased from
Merck Schuchardt OHG and Thermo Fisher Scientific, respec-
tively. 4-(Dimethylamino)pyridine was purchased from Alfa
13C{1H} NMR (125 MHz, CDCl3, 25 °C, TMS): δ = 15.3 (CH2),
20.9 (Me), 29.5 (Cp, tBu), 34.8 (Cq, tBu), 67.1 (CH2, 1,4-dioxane),
121.5 (C6H2), 126.3 (C6H2), 126.9 (C6H2), 128.5 (C6H2), 136.1
1
(C6H2), 153.8 (C6H2) ppm. H–13C{1H} HSQC NMR (125 MHz,
CDCl3, 25 °C, TMS): δ = 1.23/29.3 (Cp, tBu), 1.26/29.4 (Cp, tBu),
1.30/29.4 (Cp, tBu), 1.35/29.3 (Cp, tBu), 2.44/15.5 (CH2), 2.50/15.0
(CH2), 2.54/15.2 (CH2), 2.16/20.8 (Me), 3.63/67.0 (CH2, 1,4-diox-
Aesar GmbH
&
Co KG. 3-tert-Butyl-2-hydroxy-5-methyl-
benzyl alcohol,[26] GeCl2·1,4-dioxane,[27] LiN(SiMe3)2,[27] and
[1]
Ge[N(SiMe3)2]2 were synthesized according to literature pro-
ane), 6.81/126.6 (C H ), 6.85/126.3 (C H ) ppm. FTIR (ATR): ν =
˜
6
2
6
2
cedures.
3023 (ν Caryl–H), 2967 (ν CH3/CH2), 2923 (ν CH3/CH2), 2857 (ν
CH3/CH2), 1636 (ν C=C), 1492 (δ CH3), 1391 (δ CH3/CH2), 1248
(ν C–O), 1221 (ν C–O), 1134 (ν C–C), 1086 (ν C–C), 853, 830, 808
(δ Caryl–H and C6H2 backbone vibrations), 658, 602 (ν Ge–O),
532(ν Ge–C) cm–1. C56H80Ge4O12, [2·(C4H8O2)2] (1235.68): calcd.
C 54.43, H 6.53; found C 54.25, H 6.62.
Germanium(II) 2-tert-Butyl-4-methyl-6-(oxidomethyl)phenolate (1):
A
solution of 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol
(318.0 mg, 1.638 mmol) in n-pentane (7 mL) was added dropwise to
a solution of Ge[N(SiMe3)2]2 (644.0 mg, 1.638 mmol) in n-pentane
(5 mL) at ambient temperature. The pale-yellow solution was
stirred for 10 min. The volatile solvent (approximately 4/5 of the
solvent volume) was removed by slow evaporation under reduced
pressure (10–1 mbar). A colorless precipitate was formed during
evaporation of the solvent. The colorless solid was filtered off and
washed with n-pentane (thrice with 1 mL each) to give compound
1 after evaporating all volatile residues under reduced pressure
(10–2 mbar), yield 361.0 mg, 83%. TGA, DSC, and melting-point
determination (see Figures S3 and S4 in the Supporting Infor-
mation) revealed that compound 1 decomposes at 133 °C by an
Method b: A solution of compound 1 (438.4 mg, 1.655 mmol) in
diethyl ether (45 mL) was stirred at ambient temperature for 132 h.
Removal of all volatiles under reduced pressure (10–2 mbar) yielded
a colorless solid (311.7 mg). The product was composed of com-
pound 2 (88%; yield 0.278 mmol, 67%) and compound 3 (12%;
1
yield 0.038 mmol, 9%), as determined by H NMR analysis.
7,8Ј-Di-tert-butyl-5,6Ј-dimethyl-3H,4ЈH-spiro[benzo[d][1,2]oxa-
germole-2,2Ј-benzo[d][1,3,2]dioxagermine] (3)
1
exothermic process. H NMR (500 MHz, CDCl3, 25 °C, TMS): δ
Method a: A solution of germylene 1 (249.8 mg, 0.9431 mmol) in
1,4-dioxane (50 mL) was heated at reflux with stirring for 21 h.
Removal of the solvent under reduced pressure (10–2 mbar) at
50 °C gave a colorless solid. The product was suspended in diethyl
ether (10 mL) and insoluble byproducts were removed by filtration.
= 1.36 (s, 9 H, tBu), 1.44 (s, 9 H, tBu), 1.45 (s, 9 H, tBu), 2.19 (s,
2
3 H, Me), 2.23 (s, 3 H, Me), 2.33 (s, 3 H, Me), 4.28 (d, JCH2
=
2
12.6 Hz, 1 H, HA/CH2), 4.31 (d, JCH2 = 12.6 Hz, 1 H, HB/CH2),
4.40 (d, 2JCH2 = 12.0 Hz, 1 H, HAЈ/CH2), 4.46 (d, 2JCH2 = 12.6 Hz,
1 H, HAЈЈ/CH2), 4.57 (d, 2JCH2 = 12.6 Hz, 1 H, HBЈЈ/CH2), 4.73 (d, Compound 3 was obtained after removal of all volatiles under re-
2JCH2 = 12.0 Hz, 1 H, HBЈ/CH2), 5.82 (d, Jmeta = 1.3 Hz, 1 H,
moved pressure (10–2 mbar), yield 50.0 mg, 24.0%; m.p. 213–
4
C6H2), 6.66 (d, 4Jmeta = 1.0 Hz, 1 H, C6H2), 6.75 (d, 4Jmeta = 1.3 Hz, 216 °C. H NMR (500 MHz, CDCl3, 25 °C, TMS): δ = 1.36 (s, 18
1
4
4
2
1 H, C6H2), 7.05 (d, Jmeta = 1.0 Hz, 1 H, C6H2), 7.07 (d, Jmeta
=
H, tBu), 2.27 (s, 3 H, Me), 2.30 (s, 3 H, Me), 2.59 (d, JCH2
=
4
2
1.3 Hz, 1 H, C6H2), 7.15 (d, Jmeta = 1.3 Hz, 1 H, C6H2) ppm. 17.6 Hz, 1 H, HAЈ/CH2Ge), 2.64 (d, JCH2 = 17.6 Hz, 1 H, HBЈ/
13C{1H} NMR (125 MHz, CDCl3, 25 °C, TMS): δ = 20.9 (Me), CH2Ge), 5.04 (d, 2JCH2 = 13.1 Hz, 1 H, HXЈ/CH2O), 5.12 (d, 2JCH2
4
29.8 (Cp, tBu), 30.0 (Cp, tBu), 30.1 (Cp, tBu), 128.5 (C6H2) ppm.
= 13.1 Hz, 1 H, HYЈ/CH2O), 6.78 (d, Jmeta = 1.9 Hz, 1 H, C6H2),
1H–13C{1H} HSQC NMR (125 MHz, CDCl3, 25 °C, TMS): δ = 6.96 (d, Jmeta = 1.3 Hz, 1 H, C6H2), 6.98 (d, Jmeta = 1.3 Hz, 1 H,
4
4
4
1.28/29.9 (tBu), 1.30/29.3 (tBu), 1.37/29.8 (tBu), 2.15/20.7 (Me),
C6H2), 7.09 (d, Jmeta = 1.9 Hz, 1 H, C6H2) ppm. 13C{1H} NMR
2.22/20.8 (Me), 2.25/20.8 (Me), 4.22/61.9 (CH2), 4.32/61.8 (CH2), (125 MHz, CDCl3, 25 °C, TMS): δ = 13.1 (CH2Ge), 20.8 (Me),
4.38/63.5 (CH2), 4.49/63.7 (CH2), 4.64/61.8 (CH2), 5.74/127.2
(C6H2), 6.58/126.5 (C6H2), 6.66/126.2 (C6H2), 6.97/128.5 (C6H2),
20.9 (Me), 29.4 (Cp, tBu), 29.8 (Cp, tBu), 34.7 (Cq, tBu), 34.8 (Cq,
tBu), 67.2 (CH2O), 121.4 (C6H2), 126.0 (C6H2), 126.4 (C6H2), 126.9
6.99/128.9 (C H ), 7.07/128.5 (C H ) ppm. FTIR (ATR): ν = 3021
(C6H2), 127.7 (C6H2), 128.5 (C6H2), 128.7 (C6H2), 130.2 (C6H2),
˜
6
2
6
2
1
(ν Caryl–H), 2946 (ν CH3/CH2), 2909 (ν CH3/CH2), 2859 (ν CH3/ 136.6 (C6H2), 139.6 (C6H2), 151.9 (C6H2), 153.8 (C6H2) ppm. H–
CH2), 1607 (ν C=C), 1470 (δ CH3), 1441 (δ CH3/CH2), 1225 (ν C– 13C{1H} HSQC NMR (125 MHz, CDCl3, 25 °C, TMS): δ = 1.29/
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29.4 (tBu), 2.19/20.7 (Me), 2.22/20.7 (Me), 2.54/12.8 (CH2Ge), 4.98/ CDCl3, 25 °C, TMS): δ = 20.8 (Me), 29.8 (Cp, tBu), 34.6 (Cq, tBu),
67.0 (CH2O), 5.03/67.0 (CH2O), 6.70/126.0 (C6H2), 6.88/126.4 39.3 (Me), 63.1 (CH2), 106.6 (C5H4N), 125.8 (C6H2), 126.1 (C6H2),
(C6H2), 6.90/126.5 (C6H2), 7.01/127.5 (C6H2) ppm. NMR analysis
126.4 (C6H2), 131.3 (C6H2), 138.6 (C6H2), 145.5 (C5H4N), 152.8
1
data for (3)2: H NMR (500 MHz, CDCl3, 25 °C, TMS): δ = 0.83
(C5H4N), 155.7 (C6H2) ppm. 1H–13C{1H} HSQC NMR (125 MHz,
2
(d, JCH2 = 17.6 Hz, 1 H, HA/CH2), 1.48 (s, 9 H, tBu), 1.53 (s, 9 CDCl3, 25 °C, TMS): δ = 1.32/29.4 (tBu), 2.18/20.8 (Me), 3.00/
2
H, tBu), 1.80 (s, 3 H, Me), 2.24 (s, 3 H, Me), 2.32 (d, JCH2
=
39.3 (Me), 4.41/62.8 (CH2), 6.46/106.2 (C5H4N), 6.63/126.1 (C6H2),
2
17.6 Hz, 1 H, HB/CH2), 4.72 (d, JCH2 = 12.6 Hz, 1 H, HAЈ/CH2), 6.95/126.1 (C H ), 8.08/144.2 (C H N) ppm. FTIR (ATR): ν =
˜
6
2
5
4
2
4.99 (d, JCH2 = 12.6 Hz, 1 H, HBЈ/CH2), 5.90 (s, 1 H, C6H2), 6.65 3068 (ν Caryl–H), 2994 (ν Caryl–H), 2956 (ν CH3/CH2), 2906 (ν
(s, 1 H, C6H2), 6.92 (s, 1 H, C6H2), 7.06 (s, 1 H, C6H2) ppm. CH3/CH2), 2857 (ν CH3/CH2), 2834 (ν CH3/CH2), 1619 (ν C=C/
13C{1H} NMR (125 MHz, CDCl3, 25 °C, TMS): δ = 16.0 (CH2), C=N), 1536 (ν C=C/C=N), 1462 (δ = CH3), 1439 (δ CH3/CH2),
20.4 (Me), 20.9 (Me), 29.6 (Cp, tBu), 29.8 (Cp, tBu), 34.8 (Cq, tBu), 1391 (ν Caryl–N), 1244 (ν C–O), 1217 (ν C–N), 1150 (ν C–C), 860,
34.9 (Cq, tBu), 66.8 (CH2O), 121.7 (C6H2), 125.4 (C6H2), 126.1 843, 808 (δ Caryl–H and C6H2 backbone vibrations), 606, 569 (ν
(C6H2), 126.4 (C6H2), 127.2 (C6H2), 127.5 (C6H2), 128.6 (C6H2), Ge–O), 540(ν Ge–N) cm–1. C19H26GeN2O2 (387.02): calcd. C
130.4 (C6H2), 135.1 (C6H2), 141.2 (C6H2), 152.8 (C6H2), 153.7 58.96, H 6.77, N 7.24; found C 58.49, H 6.77, N 7.12. Single crys-
1
(C6H2) ppm. H–13C{1H} HSQC NMR (125 MHz, CDCl3, 25 °C, tals suitable for X-ray diffraction analysis were obtained by slow
TMS): δ = 0.74/15.8 (CH2), 1.41/29.3 (tBu), 1.45/29.4 (tBu), 1.73/
20.1 (Me), 2.16/20.7 (Me), 2.22/15.8 (CH2), 4.62/66.7 (CH2O), 4.90/
66.7 (CH2O), 5.82/127.0 (C6H2), 6.57/126.1 (C6H2), 6.85/125.2
evaporation of the solvent at ambient temperature of a saturated
solution of complex 4 in diethyl ether.
1H NMR Spectroscopic Study i): Compound
1
(9.8 mg,
(C H ), 6.97/127.4 (C H ) ppm. FTIR (ATR): ν = 3012 (ν C –
˜
3.7ϫ10–2 mmol) was dissolved in [D8]toluene (0.5 mL). The solu-
tion was stored under inert conditions in a sealed NMR tube at
ambient temperature. 1H NMR spectra were recorded within 237 h
of preparation. The composition of the mixture did not change
after 190 h. The final mixture consisted of germocane species
(98%) and compound 3 (2%), as determined by 1H NMR analysis.
6
2
6
2
aryl
H), 2965 (ν CH3/CH2), 2944 (ν CH3/CH2), 2909 (ν CH3/CH2), 2863
(ν CH3/CH2), 1607 (ν C=C), 1466 (δ CH3), 1439 (δ CH3/CH2),
1227 (ν C–O), 978 (ν C–C), 940 (ν C–C), 862, 841, 808 (δ Caryl–H
and C6H2 backbone vibrations), 673, 617 (ν Ge–O), 575, 542 (ν
Ge–C) cm–1. C24H32Ge2O6 (561.69): calcd. C 65.34, H 7.31; found
C 64.95, H 7.45. Single crystals of (3)2 suitable for X-ray diffraction
analysis were obtained by crystallization from 1,4-dioxane/CH2Cl2
solution.
1H NMR Spectroscopic Study ii)
Method a: A mixture of compound 1 (8.0 mg, 3.0ϫ10–2 mmol)
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (5.8 mg,
3.0ϫ10–2 mmol) was dissolved in [D8]toluene (0.5 mL). The solu-
tion was stored under inert conditions in a sealed NMR tube at
ambient temperature. 1H NMR spectra were recorded within 231 h
of preparation. The composition of the mixture did not change
after 185 h. The final mixture consisted of germocane species (4%),
compound 3 (16%), 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol
Method b: A solution of germylene 1 (100.0 mg, 0.3775 mmol)
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (73.3 mg,
0.3775 mmol) in diethyl ether (21 mL) was prepared in a Schlenk
tube equipped with freshly activated molecular sieves at ambient
temperature. The mixture was aged for 159 h. A precipitate formed
that was dissolved by the addition of THF. Filtration and removal
of the solvent under reduced pressure (10–2 mbar) gave a colorless
solid (128.8 mg). The product was composed of compound 3 (81%;
yield 0.265 mmol, 70%) and 3-tert-butyl-2-hydroxy-5-methylbenzyl
alcohol (19%; yield 0.062 mmol), as determined by 1H NMR
analysis.
1
(66%), and water (14%), as determined by H NMR analysis.
Method b: A mixture of compound 1 (4.2 mg, 1.6ϫ10–2 mmol)
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (6.2 mg,
3.2ϫ10–2 mmol) was dissolved in [D8]toluene (0.5 mL). The solu-
tion was stored under inert conditions in a sealed NMR tube at
ambient temperature. 1H NMR spectra were recorded within 231 h
of preparation. The composition of the mixture did not change
after 185 h. The final mixture consisted of germocane species (2%),
compound 3 (16%), 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol
Method c: A solution of germocane 2 (8.3 mg, 7.8ϫ10–3 mmol)
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (6.1 mg,
3.13ϫ10–2 mmol) in diethyl ether (15 mL) was prepared in a
Schlenk tube equipped with freshly activated molecular sieves at
ambient temperature and allowed to age for 2 d. Filtration and
removal of the solvent under reduced pressure (10–2 mbar) yielded
a colorless solid (6.1 mg). The product was composed of compound
3 (87%; yield 1.30ϫ10–2 mmol, 41%) and 3-tert-butyl-2-hydroxy-
5-methylbenzyl alcohol (13%; 1.9ϫ10–3 mmol) as determined by
1H NMR analysis.
1
(70%), and water (12%), as determined by H NMR analysis.
1H NMR Spectroscopic Study iii)
Method a: A mixture of compound 2 (6.1 mg, 5ϫ10–3 mmol)
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (4.2 mg,
2.2ϫ10–2 mmol) was dissolved in [D8]toluene (0.5 mL). The solu-
tion was stored under inert conditions in a sealed NMR tube at
ambient temperature. 1H NMR spectra were recorded within 137 h
of preparation. The composition of the mixture did not change
[4-(Dimethylamino)pyridine][germanium(II) 2-tert-Butyl-4-methyl-6-
(oxidomethyl)phenolate] (4): A solution of germylene 1 (292.3 mg,
1.104 mmol)
and
4-(dimethylamino)pyridine
(134.8 mg,
1
after its first analysis by H NMR spectroscopy within 16 min of
1.104 mmol) in diethyl ether (30 mL) was stirred at ambient tem-
perature for 16 h. Removal of the solvent under reduced pressure
(10–2 mbar) gave a colorless solid that was washed with n-pentane
(3ϫ 3 mL) to give the germylene complex 4 as a colorless solid
after evaporating volatile residues under reduced pressure
preparation. The final mixture consisted of germocane species
(46%), compound 3 (10%), 3-tert-butyl-2-hydroxy-5-methylbenzyl
1
alcohol (36%), and water (8%), as determined by H NMR analy-
sis.
(10–2 mbar), yield 322.9 mg, 75%; m.p. 143–145 °C. 1H NMR Method b: A mixture of compound 2 (4.9 mg, 4ϫ10–3 mmol)
(500 MHz, CDCl3, 25 °C, TMS): δ = 1.40 (s, 9 H, tBu), 2.25 (s, 3
and 3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol (4.6 mg,
2.4ϫ10–2 mmol) was dissolved in [D8]toluene (0.5 mL). The solu-
tion was stored under inert conditions in a sealed NMR tube at
ambient temperature. 1H NMR spectra were recorded within 137 h
of preparation. The composition of the mixture did not change
3
H, Me), 3.07 (s, 6 H, Me), 4.48 (s, 2 H, CH2), 6.52 (dd, Jortho
=
4
4
5.7, Jmeta = 1.6 Hz, 2 H, C5H4N), 6.70 (s, Jmeta = 2.2 Hz, 1 H,
4
3
C6H2), 7.01 (d, Jmeta = 2.2 Hz, 1 H, C6H2), 8.14 (dd, Jortho
=
5.7, 4Jmeta = 1.6 Hz, 2 H, C5H4N) ppm. 13C{1H} NMR (125 MHz,
Eur. J. Inorg. Chem. 2015, 5467–5479
5476
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
1
after its first analysis by H NMR spectroscopy within 15 min of
prepared solution and after heating the solution at 60 °C for 6 h.
preparation. The final mixture consisted of germocane species
The solution was not stored under an inert atmosphere during
1
(41%), compound 3 (12%), 3-tert-butyl-2-hydroxy-5-methylbenzyl heating. H NMR spectroscopic analysis of the solution after 6 h
1
alcohol (39%), and water (8%), as determined by H NMR analy-
sis.
revealed a composition of germocane species (5%), compound 3
(28%) and the complex 4 (67%).
1H NMR Spectroscopic Study iv): Compound
3 (14.0 mg, Reaction of Complex 4 at Elevated Temperature: Complex 4
3.1ϫ10–2 mmol) was dissolved in water-saturated CDCl3 (0.5 mL).
The solution was stored under inert conditions in a sealed NMR
tube at ambient temperature. 1H NMR spectra were recorded
within 121 h of preparation. The composition of the mixture did
(171.4 mg, 4.42ϫ10–1 mmol) was dissolved in toluene (50 mL) and
stirred at 100 °C for 23 h. Removal of the solvent under reduced
pressure (10–2 mbar) yielded a colorless solid. The product was
composed of germocane species (10%), compound 3 (3%), and 4-
(dimethylamino)pyridine (87%), as determined by 1H NMR analy-
sis. Note that the product should theoretically consist of germocane
2 (20%) and 4-(dimethylamino)pyridine (80%), respectively, if
clean conversion of complex 4 into germocane 2 and 4-(dimeth-
ylamino)pyridine is assumed.
1
not change after its first analysis by H NMR spectroscopy within
19 min of preparation. The final mixture consisted of germocane
species (26%), compound 3 (18%), 3-tert-butyl-2-hydroxy-5-meth-
ylbenzyl alcohol (38%), and water (18%), as determined by 1H
NMR analysis.
1H NMR Spectroscopic Study of the Mixture of Complex 4 and 3-
tert-Butyl-2-hydroxy-5-methylbenzyl Alcohol in CDCl3: A mixture
of compound 4 (5.1 mg, 1.3ϫ10–2 mmol) and 3-tert-butyl-2-hy-
droxy-5-methylbenzyl alcohol (2.6 mg, 1.3ϫ10–2 mmol) was dis-
Single-Crystal X-ray Diffraction Analyses: Crystallographic data of
(1)3·Et2O, [2·(C4H8O2)2]·3C4H8O2, (3)2, and 4 were collected with
an Oxford Gemini S diffractometer (CrysAlis RED Version
1.171.32.5 from Oxford Diffraction Ltd.) by using Mo-Kα (λ =
0.71073 Å) or Cu-Kα radiation (λ = 1.54184 Å) at 100 or 110 K.
The structures were solved by direct methods using SHELXS-2013
and refined by full-matrix least-square procedures on F2 using
SHELXL-2013.[28] Absorption corrections were semi-empirical
from equivalents. All non-hydrogen atoms were refined anisotropi-
cally and a riding model was employed in the refinement of
hydrogen atom positions. The crystallographic data are presented
in Table 2.
1
solved in CDCl3 (0.5 mL). H NMR spectra were recorded of the
freshly prepared solution as well as of the solution stored under
inert conditions and at ambient temperature within 48 h of prepa-
ration. All recorded 1H NMR spectra were identical by their chemi-
cal shifts as well as their intensities of resonance signals, which
indicating that compound 4 does not undergo any reaction with 3-
tert-butyl-2-hydroxy-5-methylbenzyl alcohol.
1H NMR Spectroscopic Study of Complex 4 in CDCl3 at Elevated
Temperature: Compound 4 (6.4 mg, 1.7ϫ10–2 mmol) was dissolved CCDC-1057425 [for 2(1)3·Et2O], -1057424 {for [2·(C4H8O2)2]·
1
in CDCl3 (0.5 mL). H NMR spectra were recorded of the freshly
3C4H8O2}, -1056308 [for (3)2], and -1057426 (for 4) contain the
Table 2. Crystallographic and experimental data of the single-crystal X-ray diffraction analyses of 2(1)3·Et2O, [2·(C4H8O2)2]·3C4H8O2,
(3)2, and 4.
2(1)3·Et2O
[2·(C4H8O2)2]·
3C4H8O2
(3)2
4
Formula
C76H106Ge6O13
1663.14
110
C68H104Ge4O18
1499.87
100
C48H64Ge2O6
882.17
100
C19H26GeN2O2
387.01
110
Molecular mass [gmol–1]
Temperature [K]
Wavelength [Å]
Crystal system
0.71073
monoclinic
P21/c
0.30ϫ 0.10ϫ 0.04
15.0464(3)
11.3329(3)
23.1488(5)
1.54184
triclinic
P1
0.71073
monoclinic
P21/n
0.40ϫ 0.40ϫ 0.20
15.1462(5)
7.0767(2)
21.7259(7)
0.71073
monoclinic
P21/n
0.40ϫ 0.30ϫ 0.30
8.8986(4)
20.5100(9)
10.5958(5)
¯
Space group
Crystal size [mm3]
0.12ϫ 0.04ϫ 0.02
9.0781(10)
13.2094(13)
15.2661(15)
100.391(8)
95.373(8)
98.029(9)
1769.7(3)
1
a [Å]
b [Å]
c [Å]
α [°]
β [°]
103.963(2)
107.190(4)
94.277(4)
γ [°]
V [Å3]
3830.68(15)
2
1.442
2224.67(12)
2
1.317
1928.46(15)
4
1.333
Z
Density, calcd. [Mgm–3]
1.407
2.510
μ [mm–1]
2.382
1.398
1.601
F (000)
1716
784
928
808
θ range for data collection [°]
Index ranges
2.938–25.00
–17ՅhՅ15
–9ՅkՅ13
–27ՅlՅ27
15872
4.08–62.08
–10ՅhՅ8
–12ՅkՅ15
–17ՅlՅ15
9835
2.92–25.25
–12ՅhՅ18
–8ՅkՅ8
–25ՅlՅ26
10811
3.036–24.993
–9ՅhՅ10
–15ՅkՅ24
–11ՅlՅ12
7800
Reflections collected
Independent reflections
6710
[R(int.) = 0.0309]
6710
5466
[R(int.) = 0.0786]
5466
0.933
R1 = 0.0998, wR2 =
0.2618
4012
[R(int.) = 0.0332]
4012
1.059
R1 = 0.0341, wR2 =
0.0883
3383
[R(int.) = 0.0228]
3383
1.041
R1 = 0.0302, wR2 =
0.0721
Data
Goodness-of-fit on F2
1.036
Final R indices [I Ͼ 2σ(l)], ωR2(F2) (all R1 = 0.0349, wR2 =
data)
0.0884
0.890 and –0.607
Largest diff. peak and hole [eÅ–3]
2.434 and –1.041
0.423 and –0.580
0.373 and –0.369
Eur. J. Inorg. Chem. 2015, 5467–5479
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FULL PAPER
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Received: September 29, 2015
Published Online: October 28, 2015
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